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Sample records for archaean crustal evolution

  1. Stagnant lids and mantle overturns: Implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics

    Directory of Open Access Journals (Sweden)

    Jean H. Bédard

    2018-01-01

    probability that oceanic crustal segments could founder in an organized way, producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga. Plate tectonics today is constituted of: (1 a continental drift system that started in the Early Archaean, driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons; (2 a subduction-driven system that started near the end of the Archaean.

  2. Asteroids and Archaean crustal evolution: Tests of possible genetic links between major mantle/crust melting events and clustered extraterrestrial bombardments

    Science.gov (United States)

    Glikson, A. Y.

    1992-01-01

    Since the oldest intact terrestrial rocks of ca. 4.0 Ga and oldest zircon xenocrysts of ca. 4.3 Ga measured to date overlap with the lunar late heavy bombardment, the early Precambrian record requires close reexamination vis a vis the effects of megaimpacts. The identification of microtektite-bearing horizons containing spinals of chondritic chemistry and Ir anomalies in 3.5-3.4-Ga greenstone belts provides the first direct evidence for large-scale Archaean impacts. The Archaean crustal record contains evidence for several major greenstone-granite-forming episodes where deep upwelling and adiabatic fusion of the mantle was accompanied by contemporaneous crustal anatexis. Isotopic age studies suggest evidence for principal age clusters about 3.5, 3.0, and 2.7 (+/- 0.8) Ga, relics of a ca. 3.8-Ga event, and several less well defined episodes. These peak events were accompanied and followed by protracted thermal fluctuations in intracrustal high-grade metamorphic zones. Interpretations of these events in terms of internal dynamics of the Earth are difficult to reconcile with the thermal behavior of silicate rheologies in a continuously convecting mantle regime. A triggering of these episodes by mantle rebound response to intermittent extraterrestrial asteroid impacts is supported by (1) identification of major Archaean impacts from microtektite and distal ejecta horizons marked by Ir anomalies; (2) geochemical and experimental evidence for mantle upwelling, possibly from levels as deep as the transition zone; and (3) catastrophic adiabatic melting required to generate peridotitic komatites. Episodic differentiation/accretion growth of sial consequent on these events is capable of resolving the volume problem that arises from comparisons between modern continental crust and the estimated sial produced by continuous two-stage mantle melting processes. The volume problem is exacerbated by projected high accretion rates under Archaean geotherms. It is suggested that

  3. Crustal response to lithosphere evolution

    DEFF Research Database (Denmark)

    Artemieva, Irina; Thybo, Hans; Cherepanova, Yulia

    2012-01-01

    We present a new model for the structure of the crust in an area which stretches from the North Atlantic region in the west to the Verkhoyansk Ridge in the east and encompasses Greenland, Iceland, most of Europe, West Siberian basin, and the Siberian cratons. The model is based on critically asse......, thicknesses of different crustal layers, and Pn seismic velocities....... assessed results from various seismic studies, including reflection and refraction profiles and receiver function studies. The region includes a nearly continuous age record for crustal evolution over ca. 3.6-3.8 billion years. We present an analysis of the crustal structure heterogeneity in relation...

  4. Geodynamic evolution of the West and Central Pilbara Craton in Western Australia : a mid-Archaean active continental margin

    NARCIS (Netherlands)

    Beintema, K.A.

    2003-01-01

    The Archaean era lasted for about one third of the Earth's history, from ca 4.0 until 2.5 billion years ago. Because the Archaean spans such a long time, knowledge about this era is for understanding the evolution of the Earth until the present day, especially because it is the time offormation of

  5. Geodynamic evolution of the West and Central Pilbara Craton in Western Australia : a mid-Archaean active continental margin

    NARCIS (Netherlands)

    Beintema, K.A.

    2003-01-01

    The Archaean era lasted for about one third of the Earth's history, from ca 4.0 until 2.5 billion years ago. Because the Archaean spans such a long time, knowledge about this era is for understanding the evolution of the Earth until the present day, especially because it is the time

  6. MAGNETAR FIELD EVOLUTION AND CRUSTAL PLASTICITY

    International Nuclear Information System (INIS)

    Lander, S. K.

    2016-01-01

    The activity of magnetars is believed to be powered by colossal magnetic energy reservoirs. We sketch an evolutionary picture in which internal field evolution in magnetars generates a twisted corona, from which energy may be released suddenly in a single giant flare, or more gradually through smaller outbursts and persistent emission. Given the ages of magnetars and the energy of their giant flares, we suggest that their evolution is driven by a novel mechanism: magnetic flux transport/decay due to persistent plastic flow in the crust, which would invalidate the common assumption that the crustal lattice is static and evolves only under Hall drift and Ohmic decay. We estimate the field strength required to induce plastic flow as a function of crustal depth, and the viscosity of the plastic phase. The star’s superconducting core may also play a role in magnetar field evolution, depending on the star’s spindown history and how rotational vortices and magnetic fluxtubes interact.

  7. The precambrian crustal evolution and mineralization cycle of uranium in the northeast of norern China platform

    International Nuclear Information System (INIS)

    Guo Zhitian.

    1986-01-01

    According to the evolution history of the crust the region is divided into three Precambrian structural structural units: (1) Archaean craton; (2) Early Proterozoic zone of fold; (3) Middle-late Proterozoic depression zone. The Archaean-craton mainly consists of granite complex and metasediments. They form the first generation of uranium sources. Proterozoic is characterized by the obvious cycle of sedimentation which consists of the second generation of uranium source. There were multiplestage and congenetic nature in the formation of uranium deposit. The mineralization of uranium coincides with geotectonicdeveloping stage -- igneous activity -- metamorphism in their time. The formation of uranium deposits generally underwent the weathering and erosion of original uraniferous bodies-the migration, redeposition and reformed concentration by metamorphism and metamorphosed hydrothermal solution, and the mineralization was not only of intermittence, but also of inheritance. The evolutional process of forming uranium deposits undergoing various geological function of a structural cycle in the uranium geochemical anomalous area is called uranium mineralizational cycle. The Northeast of Northern China Platform had undergone multiple times structural movements causing migration and concentration of uranium and having mutiple cycle mineralizational character. Corresponding to the three main developing stages of the crustal evolution the Precambrian uranium mineralization in the Northeast of northern China platform area may be divided into three cycles: Late Archaeozoic mineralizational cycle, Early Proterozoic mineralizational cycle, and Middle Proterozoic mineralizational cycle. It is possible to search for potential uranium metallogenetic provinces to study the crustal evolution and the multiple cycle characters of uranium minerogenetic process in the Northern China platform

  8. Crustal evolution inferred from apollo magnetic measurements

    International Nuclear Information System (INIS)

    Dyal, P.; Daily, W.D.; Vanyan, L.L.

    1978-09-01

    Magnetic field and solar wind plasma density measurements were analyzed to determine the scale size characteristics of remanent fields at the Apollo 12, 15, and 16 landing sites. Theoretical model calculations of the field-plasma interaction, involving diffusion of the remanent field into the solar plasma, were compared to the data. The information provided by all these experiments shows that remanent fields over most of the lunar surface are characterized by spatial variations as small as a few kilometers. Large regions (50 to 100 km) of the lunar crust were probably uniformly magnetized during early crustal evolution. Bombardment and subsequent gardening of the upper layers of these magnetized regions left randomly oriented, smaller scale (5 to 10 km) magnetic sources close to the surface. The larger scale size fields of magnitude approximately 0.1 gammas are measured by the orbiting subsatellite experiments and the small scale sized remanent fields of magnitude approximately 100 gammas are measured by the surface experiments

  9. Effect of thicker oceanic crust in the Archaean on the growth of continental crust through time

    International Nuclear Information System (INIS)

    Wilks, M.E.

    1988-01-01

    Present crustal evolution models fail to account for the generation of the large volume of continental crust in the required time intervals. All Archaean plate tectonic models, whether invoking faster spreading rates, similar to today's spreading rates, or longer ridge lengths, essentially propose that continental crust has grown by island arc accretion due to the subduction of oceanic crust. The petrological differences that characterize the Archaean from later terrains result from the subduction of hotter oceanic crust into a hotter mantle. If the oceanic crust was appreciably thicker in the Archaean, as geothermal models would indicate, this thicker crust is surely going to have an effect on tectonic processes. A more valid approach is to compare the possible styles of convergence of thick oceanic crust with modern convergence zones. The best modern analog occurs where thick continental crust is colliding with thick continental crust. Oceanic crustal collision on the scale of the present-day Himalayan continental collision zone may have been a frequent occurrence in the Archaean, resulting in extensive partial melting of the hydrous underthrust oceanic crust to produce voluminous tonalite melts, leaving a depleted stabilized basic residuum. Present-day island arc accretion may not have been the dominant mechanism for the growth of the early Archaean crust

  10. Changing styles of crustal growth in Southern Africa: constraints from geochemical and Sr-Nd isotope studies in Archaean to Pan African terrains

    International Nuclear Information System (INIS)

    McDermott, F.; Hawkesworth, C.J.; Harris, N.B.W.

    1988-01-01

    Nd isotopic data was presented for southern Africa in support of episodic crustal growth. Over 50 percent of the continental crust there had formed before 2.5 Ga, and less than 10 percent was produced after about 1.0 Ga. The data imply a mean crustal age of about 2.4 Ga for southern Africa, and a higher rate of crustal growth than that derived from Australian shale data, particularly during the Proterozoic. Isotopic data from Damara metasediments imply that there is no need to invoke decoupling of the Rb-Sr and Sm-Nd systems in the continental crust

  11. Changing styles of crustal growth in Southern Africa: Constraints from geochemical and Sr-Nd isotope studies in Archaean to Pan African terrains

    Science.gov (United States)

    Mcdermott, F.; Hawkesworth, C. J.; Harris, N. B. W.

    1988-01-01

    Nd isotopic data was presented for southern Africa in support of episodic crustal growth. Over 50 percent of the continental crust there had formed before 2.5 Ga, and less than 10 percent was produced after about 1.0 Ga. The data imply a mean crustal age of about 2.4 Ga for southern Africa, and a higher rate of crustal growth than that derived from Australian shale data, particularly during the Proterozoic. Isotopic data from Damara metasediments imply that there is no need to invoke decoupling of the Rb-Sr and Sm-Nd systems in the continental crust.

  12. Evolution of the Gondwanaland Archaean Shield: ion microprobe zircon dating and southwestern Australia/Wilkes Land, Antartica

    International Nuclear Information System (INIS)

    Lovering, J.F.; Comaford, D.J.

    1979-01-01

    The ion microprobe has been used to study 207 Pb/ 206 Pb ages on 20μm-sized sites on single zircon grains from coastal rocks on either side of the rift in the Gondwanaland Archaean Shield between southwestern Australia and Wilkes Land, Antarctica. The ages on individual sites on zircon grains from a variety of rock types from southwestern Australia show a range from 1600 m.y. to about 3400 m.y., with an inverse dependence on the uranium abundance at each site. Ages of zircons from rocks from the Antartic region show a range from 1600 m.y. to 3100 m.y

  13. Preservation of an Archaean whole rock Re-Os isochron for the Venetia lithospheric mantle: Evidence for rapid crustal recycling and lithosphere stabilisation at 3.3 Ga

    Science.gov (United States)

    van der Meer, Quinten H. A.; Klaver, Martijn; Reisberg, Laurie; Riches, Amy J. V.; Davies, Gareth R.

    2017-11-01

    Re-Os and platinum group element analyses are reported for peridotite xenoliths from the 533 Ma Venetia kimberlite cluster situated in the Limpopo Mobile Belt, the Neoarchaean collision zone between the Kaapvaal and Zimbabwe Cratons. The Venetian xenoliths provide a rare opportunity to examine the state of the cratonic lithosphere prior to major regional metasomatic disturbance of Re-Os systematics throughout the Phanerozoic. The 32 studied xenoliths record Si-enrichment that is characteristic of the Kaapvaal lithospheric mantle and can be subdivided into five groups based on Re-Os analyses. The most pristine group I samples (n = 13) display an approximately isochronous relationship and fall on a 3.28 ± 0.17 Ga (95 % conf. int.) reference line that is based on their mean TMA age. This age overlaps with the formation age of the Limpopo crust at 3.35-3.28 Ga. The group I samples derive from ∼50 to ∼170 km depth, suggesting coeval melt depletion of the majority of the Venetia lithospheric mantle column. Group II and III samples have elevated Re/Os due to Re addition during kimberlite magmatism. Group II has otherwise undergone a similar evolution as the group I samples with overlapping 187Os/188Os at eruption age: 187Os/188OsEA, while group III samples have low Os concentrations, unradiogenic 187Os/188OsEA and were effectively Re-free prior to kimberlite magmatism. The other sample groups (IV and V) have disturbed Re-Os systematics and provide no reliable age information. A strong positive correlation is recorded between Os and Re concentrations for group I samples, which is extended to groups II and III after correction for kimberlite addition. This positive correlation precludes a single stage melt depletion history and indicates coupled remobilisation of Re and Os. The combination of Re-Os mobility, preservation of the isochronous relationship, correlation of 187Os/188Os with degree of melt depletion and lack of radiogenic Os addition puts tight constraints on

  14. Geodynamic evolution of the West Africa between 2.2 and 2 Ga: the Archaean style of the Birimian greenstone belts and the sedimentary basins in northeastern Ivory-Coast

    International Nuclear Information System (INIS)

    Vidal, M.; Pouclet, A.; Delor, C.; Simeon, Y.; Alric, G.

    1996-01-01

    The litho-structural features of Palaeo-proterozoic terrains of northeastern Ivory-Coast, greenstones belts and then sedimentary basin Birimian), are similar to those of Archaean terrains. Their early deformation is only voluminal deformation due to granitoid intrusions, mainly between 2.2 and 2.16 Ga. The shortening deformation (main deformation) is expressed by right folds and transcurrent shear zones ca 2.1 Ga. Neither thrust deformation nor high pressure metamorphic assemblages are known. This pattern of flexible and hot crust, at least between 2.2 and 2.16 Ga, is pole apart to a collisional pattern, proposed for West African Craton by some authors. The Archaean/Palaeo-proterozoic boundary would not represent a drastic change of the geodynamic evolution of the crust. (authors). 60 refs., 5 figs., 6 photos

  15. Isotope and trace element models of crustal evolution

    International Nuclear Information System (INIS)

    O'Nions, R.K.; Hamilton, P.J.

    1981-01-01

    Some of the isotopic constraints on the development of continental crust from about 3.8 Ga ago are reviewed. Particularly it is noted that Archaean granitic (sensu lato) rocks have initial 143 Nd/ 144 Nd ratios close to predicted values for the bulk Earth at the time before emplacement, whereas those Phanerozoic granites investigated so far diverge considerably from the bulk Earth and betray the existence of later continental crust in their provenance. Geochemical evidence for recycling of some continent-derived elements into the mantle is examined and the important distinction between selected element recycling and bulk return of continental material is emphasized. Various transport models that have been proposed to model the development of continental crust are examined and some of their differences and similarities, particularly with respect to implications for continental recycling, are highlighted. (author)

  16. New Rb-Sr isotopic ages and geochemistry of granitic gneisses from southern Bastar: implications for crustal evolution

    International Nuclear Information System (INIS)

    Sarkar, G.; Gupta, S.N.; Bishui, P.K.

    1994-01-01

    Deformed gneisses from the southern Bastar craton yield Rb-Sr whole-rock ages of 2560 Ma and 2659 Ma with initial Sr ratios ranging between 0.70899 and 0.70726 respectively. The isotopic data are found to be scattered even at the outcrop scale which possibly indicate large-scale reworking of the gneisses during the period. The high initial Sr ratios that associate with scattering of the isotopic data reflect reworking of older gneisses. Geochemically, these gneisses are considered to be derived from an amphibolitic or basaltic protolith. The 2095 Ma (initial Sr ratio of 0.74312) old leucocratic granite intrusive into these gneisses represent early Proterozoic magmatic activity. Based on the available isotopic and geochemical data, it is suggested that the Bastar craton represents a polyphase, multicomponent terrain developed by repeated magmatism at a much earlier, probably during mid-Archaean, time and was extensively reworked during the time span between end-Archaean and early Proterozoic period. This reworking may be synchronous with coalescing of smaller crustal components possibly during the end-Archaean time. (author). 21 refs., 5 figs., 2 tabs

  17. The geophysical character of southern Alaska - Implications for crustal evolution

    Science.gov (United States)

    Saltus, R.W.; Hudson, T.L.; Wilson, Frederic H.

    2007-01-01

    The southern Alaska continental margin has undergone a long and complicated history of plate convergence, subduction, accretion, and margin-parallel displacements. The crustal character of this continental margin is discernible through combined analysis of aeromagnetic and gravity data with key constraints from previous seismic interpretation. Regional magnetic data are particularly useful in defining broad geophysical domains. One of these domains, the south Alaska magnetic high, is the focus of this study. It is an intense and continuous magnetic high up to 200 km wide and ∼1500 km long extending from the Canadian border in the Wrangell Mountains west and southwest through Cook Inlet to the Bering Sea shelf. Crustal thickness beneath the south Alaska magnetic high is commonly 40–50 km. Gravity analysis indicates that the south Alaska magnetic high crust is dense. The south Alaska magnetic high spatially coincides with the Peninsular and Wrangellia terranes. The thick, dense, and magnetic character of this domain requires significant amounts of mafic rocks at intermediate to deep crustal levels. In Wrangellia these mafic rocks are likely to have been emplaced during Middle and (or) Late Triassic Nikolai Greenstone volcanism. In the Peninsular terrane, the most extensive period of mafic magmatism now known was associated with the Early Jurassic Talkeetna Formation volcanic arc. Thus the thick, dense, and magnetic character of the south Alaska magnetic high crust apparently developed as the response to mafic magmatism in both extensional (Wrangellia) and subduction-related arc (Peninsular terrane) settings. The south Alaska magnetic high is therefore a composite crustal feature. At least in Wrangellia, the crust was probably of average thickness (30 km) or greater prior to Triassic mafic magmatism. Up to 20 km (40%) of its present thickness may be due to the addition of Triassic mafic magmas. Throughout the south Alaska magnetic high, significant crustal growth

  18. Crustal evolution at mantle depths constrained from Pamir xenoliths

    Science.gov (United States)

    Kooijman, E.; Hacker, B. R.; Smit, M. A.; Kylander-Clark, A. R.; Ratschbacher, L.

    2012-12-01

    Lower crustal xenoliths erupted in the Pamir at ~11 Ma provide an exclusive opportunity to study the evolution of crust at mantle depths during a continent-continent collision. To investigate, and constrain the timing of, the petrologic processes that occurred during burial to the peak conditions (2.5-2.8 GPa, 1000-1100 °C; [1]), we performed chemical- and isotope analyses of accessory minerals in 10 xenoliths, ranging from eclogites to grt-ky-qtz granulites. In situ laser ablation split-stream ICPMS yielded 1) U-Pb ages, Ti concentrations and REE in zircon, 2) U/Th-Pb ages and REE in monazite, and 3) U-Pb ages and trace elements in rutile. In addition, garnet, and biotite and K-feldspar were dated using Lu-Hf and 40Ar/39Ar geochronology, respectively. Zircon and monazite U-(Th-)Pb ages are 101.9±1.8, 53.7±1.0, 39.1±0.8, 21.7±0.4, 18.2±0.5, 16.9±0.8, 15.1±0.3 (2σ) and 12.5-11.1 Ma; most samples showed several or all of these populations. The 53.7 Ma and older ages are xenocrystic or detrital. For younger ages, zircon and monazite in individual samples recorded different ages-although zircon in one rock and monazite in another can be the same age. The 39.1 Ma zircon and monazite mostly occur as inclusions in minerals of the garnet-bearing assemblage that represents the early, low-P stages of burial. Garnet Lu-Hf ages of 37.8±0.3 Ma support garnet growth at this time. Spinifex-like textures containing 21.7-11.1 Ma zircon and monazite record short-lived partial melting events during burial. Aligned kyanite near these patches indicates associated deformation. Zircons yielding ≤12.5 Ma exhibit increased Eu/Eu* and markedly decreased HREE concentrations, interpreted to record feldspar breakdown and omphacite growth during increasing pressure. Rutile U-Pb cooling ages are 10.8±0.3 Ma in all samples. This agrees with the weighted mean 40Ar/39Ar age of eight biotite, K-feldspar and whole rock separates of 11.00+0.16/-0.09 Ma. Rutile in eclogites provides Zr

  19. Crustal evolution in north-east and east Africa from model Nd ages

    International Nuclear Information System (INIS)

    Harris, N.B.W.; Hawkesworth, C.J.; Ries, A.C.

    1984-01-01

    The authors present the results of an Nd isotope study on the major rock units of the Pan-African (1,100-500 Myr BP) terrane. Charnockites from Jabel Uweinat, a basement inlier at the junction of Egypt, Libya and the Sudan, yield middle Archaean model Nd ages, whilst model ages of < 1,200 Myr have been obtained in a belt from the Eastern Desert of Egypt to north-west Kenya. Overall, the Pan-African rocks from north-east and east Africa and those from the Damara of Namibia exhibit a wide range of epsilonsub(Nd)(T) from +7.5 to -18.0 which reflects regional changes in tectonic style and is not readily reconciled with simple models for the evolution of average continental crust. (author)

  20. 207Pb-206Pb zircon ages of eastern and western Dharwar craton, southern India : Evidence for contemporaneous Archaean crust

    Science.gov (United States)

    Maibam, B.; Goswami, J. N.; Srinivasan, R.

    2009-04-01

    Dharwar craton is one of the major Archaean crustal blocks in the Indian subcontinent. The craton is comprised of two blocks, western and eastern. The western domain is underlain by orthogneisses and granodiorites (ca. 2.9-3.3 Ga) collectively termed as Peninsular Gneiss [e.g., 1] interspersed with older tracts of metasedimentary and metamorphosed igneous suites (Sargur Group and Dharwar Group; [2]). The eastern part of the craton is dominated by Late Archaean (2.50-2.75 Ga) granitoids and their gneissic equivalents. They are interspersed with schist belts (also of Sargur Group and Dharwar Group), which are lithologically similar to the Dharwar Supergroup in the western block, but are in different metamorphic dress. Here we report 207Pb-206Pb age of zircons separated from the metasedimentary and gneissic samples from the two blocks to constrain the evolution of the Dharwar craton during the early Archaean. Detrital zircons of the metasedimentary rocks from both the blocks show a wide range of overlapping ages between ~2.9 to >3.5 Ga. Zircon ages of the orthogneisses from the two blocks showed that most of the analysed grains of the eastern Dharwar block are found to be of the age as old as the western Dharwar gneisses. Imprints of younger events could be discerned from the presence of overgrowths in zircons from the studied samples throughout the craton. Our data suggest that crust forming cycles in the two blocks of the Dharwar craton occurred contemporaneously during the Archaean. References [1] Beckinsale, R.D., Drury, S.A., Holt, R.W. (1980) Nature 283, 469-470. [2] Swami Nath J., Ramakrishnan M., Viswanatha M.N. (1976) Rec. Geol. Surv. Ind., 107, 149-175.

  1. Drifting Continents and Wandering Poles. Crustal Evolution Education Project. Teacher's Guide [and] Student Investigation.

    Science.gov (United States)

    Stoever, Edward C., Jr.

    Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

  2. Drifting Continents and Magnetic Fields. Crustal Evolution Education Project. Teacher's Guide [and] Student Investigation.

    Science.gov (United States)

    Stoever, Edward C., Jr.

    Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

  3. Volcanoes: Where and Why? Crustal Evolution Education Project. Teacher's Guide [and] Student Investigation.

    Science.gov (United States)

    Stoever, Edward C., Jr.

    Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

  4. Hot Spots in the Earth's Crust. Crustal Evolution Education Project. Teacher's Guide [and] Student Investigation.

    Science.gov (United States)

    Stoever, Edward C., Jr.

    Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

  5. Structural observations and U-Pb mineral ages from igneous rocks at the Archaean-Palaeoproterozoic boundary in the Salahmi Schist Belt, central Finland: constraints on tectonic evolution

    Directory of Open Access Journals (Sweden)

    Pietikäinen, K.

    1999-06-01

    Full Text Available The study area in Vieremä, central Finland, contains part of Archaean-Palaeoproterozoic boundary. In the east, the area comprises Archaean gneiss and the Salahmi Schist Belt. The rocks of the schist belt are turbiditic metagreywackes, with well-preserved depositional structures, occurring as Proterozoic wedge-shaped blocks, and staurolite schists, the latter representing higher-strained and metamorphosed equivalents of the metagreywackes. In the west of the area there is an Archaean gneiss block, containing strongly elongated structures, and deformed Svecofennian supracrustal rocks, which are cut by deformed granitoids. These are juxtaposed with the schist belt. The boundaries of these tectonometamorphic blocks are narrow, highly strained mylonites and thrust zones. The metamorphic grade of the supracrustal rocks increases from east to west, the increase being stepwise across the mylonitic block boundaries. The rocks are more deformed from east to west with younger structures overprinting. In the staurolite schists of the Salahmi Schist Belt, the most prominent structure is a lineation (L2 that overprints the bedding and axial plane foliation. In Sorronmäki quarry, at the western boundary of the schist belt, this Palaeoproterozoic lineation dominates all the structures in tonalite gneiss, which gives a U-Pb age of 2731±6 Ma. Southeast of the quarry, at the same boundary, the Salahmi schists have been overturned towards the northeast, suggesting that the Archaean gneiss at Sorronmäki has been thrust towards the northeast over these rocks. In the western part of the study area, the Leppikangas granodiorite that intrudes the Svecofennian supracrustal rocks gives a U-Pb age of 1891+6 Ma. In the granodiorite, a strong lineation formed by the intersection of two foliations, which maybe L2 is associated with thrusting towards the northeast. The monazite age of the Archaean Sorronmäki gneiss is 1817+3 Ma, and the titanite age of the Svecofennian

  6. Average sedimentary rock rare Earth element patterns and crustal evolution: Some observations and implications from the 3800 Ma ISUA supracrustal belt, West Greenland

    Science.gov (United States)

    Dymek, R. F.; Boak, J. L.; Gromet, L. P.

    1983-01-01

    Rare earth element (REE) data is given on a set of clastic metasediments from the 3800 Ma Isua Supracrustal belt, West Greenland. Each of two units from the same sedimentary sequence has a distinctive REE pattern, but the average of these rocks bears a very strong resemblance to the REE pattern for the North American Shale Composite (NASC), and departs considerably from previous estimates of REE patterns in Archaean sediments. The possibility that the source area for the Isua sediments resembled that of the NASC is regarded as highly unlikely. However, REE patterns like that in the NASC may be produced by sedimentary recycling of material yielding patterns such as are found at Isua. The results lead to the following tentative conclusions: (1) The REE patterns for Isua Seq. B MBG indicate the existence of crustal materials with fractionated REE and negative Eu anomalies at 3800 Ma, (2) The average Seq. B REE pattern resembles that of the North American Shale Composite (NASC), (3) If the Seq. B average is truly representative of its crustal sources, then this early crust could have been extensively differentiated. In this regard, a proper understanding of the NASC pattern, and its relationship to post-Archaean crustal REE reservoirs, is essential, (4) The Isua results may represent a local effect.

  7. Crustal evolution of South American Platform based on Sm-Nd isotope geochemistry

    International Nuclear Information System (INIS)

    Sato, Kei

    1998-01-01

    Sm-Nd isotopic systematics is relevant to the topics of origin and evolution the of continental crust, where model ages refer to the time when crustal material was differentiated from the upper mantle. Alternative interpretations are due to a lack of adequate information on crustal processes and the variable composition of the mantle sources. The Sm-Nd methods are presented, and applied on rock materials from the South American Platform. The main conclusions indicate juvenile accretion with higher growth rates (peaks), around 3.7-3.5 Ga (∼ 0.5% in volume), 3.1 - 2.9 Ga (∼16%), 2.7 - 2.6 (∼ 9%), 2.2 - 1.9 (35%) and 1.3-1.0 (7%). The continental growth curve indicates that about 35 % of the crust was formed by 2.5 Ga, 88% by 1.8 Ga and 99% by 1.0 Ga, and the remaining ∼ 1 % was added in the Phanerozoic. Rapid crustal growth occurred between 2.2 and 1.9 Ga. The main period of continental crust formation occurred during the Paleoproterozoic, corresponding to 54 % in volume. Sm-Nd model ages, when compared with the crystallisation ages of granitoid rocks, furnish a rough estimate of juvenile vs. reworked material. Within the South American Platform about 45% of juvenile continental crust is still preserved within tectonic provinces of different ages. The remainder represents continental crust reworked in younger tectono-thermal events. In particular crustal reworking was predominating over juvenile accretion during Meso-Neoproterozoic. The Transbrasiliano Lineament is a megasuture, active in the Neoproterozoic, which separates a large northwestern mass, including the Amazonian and Sao Luis Cratons, from a southeastern mass, formed by a collage of cratonic fragments, of which the Sao Francisco and Rio de La Plata are the largest. The crustal evolutions of these two large continental masses are considered individually, and can be resumed following form: I - Old Archean rocks (>3.4 Ga) are found only within the south-eastern part (Gaviao Block, Contendas

  8. The distribution of radioelements in archaean granites of the Kaapvaal Craton, with implications for the source of uranium in the Witwatersrand Basin

    International Nuclear Information System (INIS)

    Robb, L.J.; Meyer, M.; Ferraz, M.F.; Drennan, G.R.

    1989-05-01

    Approximately 500 samples from the Archaean granitic basement of the southern Kaapvaal Craton have been analysed, for U and Th. When viewed in conjunction with geological relationships, the radioelement distribution patterns in the Archaean basement provide contraints regarding the origin of uranium in the Witwatersrand Basin. Granites in the Baberton region are sub-divided into three magnetic cycles, the earliest cycle comprising tonalite-trondhjemite gneisses, the intermediate cycle comprising literally extensive K-rich batholiths and the final stage consisting of discrete intrusive granitic plutons. Uranium and thorium contents vary as a function of age and rock type, an increase progressively from the first cycle through to the third cycle. Certain of the late granite plutons may have been S-type in origin, have relatively low Th/U ratios, high U contents, and are characterized by accessory minerals dominated by monazite-like phases. The late granite plutons with highest radioelement contents appear to have formed circa 2,8 Ga, an age which coincides with granulite facies metamorphism and uranium-thorium depletion in the lower crust, as recrorded in the Vredeford crustal profile. Uranium has been leached from portions of the regolith profile, but also concentrated into leucoxene-rich zones derived from the breakdown of pre-existing titanium-bearing phases. The widespread development of an uraniferous leucoxene protore in weathered source rocks of the Witwatersrand Basin has relevance to the genesis of authigenic U-Ti phases (brannerite) in the reefs themselves. The study of radioelement distribution in Archaean granites adjacent to the Witwatersrand Basin provides a framework within which considerations regarding the origin of the uranium deposits in the basin can be viewed. The secular evolution of the Archaean granitic basement, hydrothermal processes, and palaeoweathering all played a role in the formation of the Witwatersrand deposits

  9. Crustal evolution of South American Platform based on Sm-Nd isotope geochemistry; Evolucao crustal da plataforma sul americana com base na geoquimica isotopica Sm-Nd

    Energy Technology Data Exchange (ETDEWEB)

    Sato, Kei

    1998-07-01

    Sm-Nd isotopic systematics is relevant to the topics of origin and evolution the of continental crust, where model ages refer to the time when crustal material was differentiated from the upper mantle. Alternative interpretations are due to a lack of adequate information on crustal processes and the variable composition of the mantle sources. The Sm-Nd methods are presented, and applied on rock materials from the South American Platform. The main conclusions indicate juvenile accretion with higher growth rates (peaks), around 3.7-3.5 Ga ({approx} 0.5% in volume), 3.1 - 2.9 Ga ({approx}16%), 2.7 - 2.6 ({approx} 9%), 2.2 - 1.9 (35%) and 1.3-1.0 (7%). The continental growth curve indicates that about 35 % of the crust was formed by 2.5 Ga, 88% by 1.8 Ga and 99% by 1.0 Ga, and the remaining {approx} 1 % was added in the Phanerozoic. Rapid crustal growth occurred between 2.2 and 1.9 Ga. The main period of continental crust formation occurred during the Paleoproterozoic, corresponding to 54 % in volume. Sm-Nd model ages, when compared with the crystallisation ages of granitoid rocks, furnish a rough estimate of juvenile vs. reworked material. Within the South American Platform about 45% of juvenile continental crust is still preserved within tectonic provinces of different ages. The remainder represents continental crust reworked in younger tectono-thermal events. In particular crustal reworking was predominating over juvenile accretion during Meso-Neoproterozoic. The Transbrasiliano Lineament is a megasuture, active in the Neoproterozoic, which separates a large northwestern mass, including the Amazonian and Sao Luis Cratons, from a southeastern mass, formed by a collage of cratonic fragments, of which the Sao Francisco and Rio de La Plata are the largest. The crustal evolutions of these two large continental masses are considered individually, and can be resumed following form: I - Old Archean rocks (>3.4 Ga) are found only within the south-eastern part (Gaviao Block

  10. A coupled petrological-geodynamical model to investigate the evolution of crustal magmatic systems

    Science.gov (United States)

    Kaus, B. J. P.; Rummel, L.; White, R. W.

    2017-12-01

    The evolution of crustal magmatic systems can be analyzed from different physical and chemical perspectives. Most previous work focus either on the petrological side (considering thermal effects and ignoring mechanics), or on the mechanical evolution (assuming a fixed melt chemistry). Here, we consider both by combining a 2D finite element code, MVEP2, with a thermodynamic modelling approach (Perple_X). Density, melt fraction and the chemical composition of the liquid and solid phase are computed for different starting rock compositions and the evolving chemistry is tracked on markers via 10 main oxides (SiO2-TiO2-Al2O3-Cr2O3-MgO-FeO-CaO-Na2O-K2O-H2O). As soon as the local chemistry changes due to melt extraction, new phase diagrams are computed based on the residual solid chemistry for the deflated magma chamber or on the liquid chemistry for newly generated magma filled fractures. To investigate the chemical evolution in magma chambers and magma filled fractures, we inject mafic sills periodically at varying depth levels into the continental crust. The initial sill injections are focused in either one or two main zones in the crust and may interact with each other. The formation of magma filled fractures from this partially molten zone is tracked with a semi analytical dike initiation algorithm that forms new dikes as a function of the local stress field above the partially molten region and subsequently depletes and compacts the magma source region. Dike generation is thus affected by the background strain rate, amount and depth of melt accumulations as well as parameters that control the plastic and viscous behaviour of the crust (e.g. cohesion, viscous creep flow low etc.). Results show that magma filled fractures triggered by sill injections preferentially form under extensional conditions, particularly within the middle crust (in ca. 25 km depth). Magma chambers in the lower continental crust, on the other hand, are stable over a longer period of time due a

  11. Evolution of deep crustal magma structures beneath Mount Baekdu volcano (MBV) intraplate volcano in northeast Asia

    Science.gov (United States)

    Rhie, J.; Kim, S.; Tkalcic, H.; Baag, S. Y.

    2017-12-01

    Heterogeneous features of magmatic structures beneath intraplate volcanoes are attributed to interactions between the ascending magma and lithospheric structures. Here, we investigate the evolution of crustal magmatic stuructures beneath Mount Baekdu volcano (MBV), which is one of the largest continental intraplate volcanoes in northeast Asia. The result of our seismic imaging shows that the deeper Moho depth ( 40 km) and relatively higher shear wave velocities (>3.8 km/s) at middle-to-lower crustal depths beneath the volcano. In addition, the pattern at the bottom of our model shows that the lithosphere beneath the MBV is shallower (interpret the observations as a compositional double layering of mafic underplating and a overlying cooled felsic structure due to fractional crystallization of asthenosphere origin magma. To achieve enhanced vertical and horizontal model coverage, we apply two approaches in this work, including (1) a grid-search based phase velocity measurement using real-coherency of ambient noise data and (2) a transdimensional Bayesian joint inversion using multiple ambient noise dispersion data.

  12. Middle proterozoic supra crustal and brazilian orogeny in the southeast Ceara state: a mono cyclic evolution

    International Nuclear Information System (INIS)

    Sa, J.M.; Bezerra, F.H.R.; Freitas Macedo, M.H. de; Pereira, R.

    1988-01-01

    The Oros belt is situated in the southeastern part of Ceara state, Brazil, and geologically pertains to the Borborema province. This belt comprises a volcano-sedimentary sequence of middle proterozoic age resting unconformably upon basement of Archean/low proterozoic age. In the geological map of Ceara state, this belt displays an elongate shape towards N-S, turning to ENE-WSW in the south, and reaches 12 km wide in the central park. This paper describes the relationships between the country rocks and the supra crustal sequence, as well as the plutonic intrusions and their tectonic metamorphic evolution. New Rb-Sr whole-rock dates are presented which are very important to separate anorogenic and syn-orogenic granites. (author)

  13. Geochronological synthesis of Bahia state and the crustal evolution, based in evolution diagram of Sr and initial rate of Sr87/Sr86

    International Nuclear Information System (INIS)

    Sato, K.

    1986-01-01

    The crustal evolution of the ancient terrains of the State of Bahia, Brazil, is attempted with the aid of Sr isotopic results as natural tracers. Some Nd and Pb isotopic data are also available, and support the main conclusions based on Sr evolution diagrams. The analysis of the Sr evolution diagrams shows that the Archean Terrains are mainly formed by accretion from mantle-derived material, but crustal reworking is indicated by the high initial 87 Sr/ 86 Sr value of the Jequie Complex. The Transamazonian mobile belt include both types of materials, but the 87 Sr/ 86 Sr value, generally lower than those of the Jequie Complex, markes improbable a direct derivation. During Middle and Late Proterozoic, the continental crust was already well consolidated, and reworking of crustal material predominated within the Espinhaco and Brasiliano folded systems [pt

  14. Seismic studies of crustal structure and tectonic evolution across the central California margin and the Colorado Plateau margin

    Science.gov (United States)

    Howie, John Mark

    This thesis presents results from two integrated deep-crustal seismic-reflection and wide-angle-reflection/refraction studies that improve our understanding of crustal structure and tectonic evolution in two tectonically active areas of the western United States. A multi-faceted approach to the study of crustal structure includes the use of compressional and shear wave seismic data. Supplementing the controlled source seismic observations with seismicity, gravity, heat flow, laboratory measurements and available geologic information allows a much improved understanding of crustal structure and tectonic evolution than would be available from the seismic data alone. Chapter 1 introduces the data integration strategy applied to the studies completed. In Chapter 2, an integrated crustal-velocity model across the south-central California margin west of the San Adreas fault is presented. The crustal structure defines tectonostratigraphic terranes 15 to 20 km thick underlain by a 6-km-thick high-velocity layer (6.8-7.0 km/s) interpreted as tectonically underplated oceanic crust. Structures defined in the oceanic crust indicate significant compressional and strike-slip deformation within the oceanic crust that probably formed during the final stages of subduction from 24-16 Ma. In Chapter 3, the crustal model from Chapter 2 is used as a constraint for models of the tectonic evolution of the Pacific-North American transform plate boundary. By combining the crustal structure with thermal models for asthenospheric upwelling associated with a slab-free window, I find that the mantle lithosphere east of the coast beneath south-central California probably delaminated from the oceanic crust, stranding the oceanic crust beneath the margin. In Chapter 4, results from a high-resolution reflection experiment in central Arizona across the southwestern edge of the Colorado Plateau address the relationship between strength of the crust and localization of extensional tectonism. A low

  15. U-Pb and Lu-Hf systematics of zircons from Sargur metasediments, Dharwar Craton, Southern India: new insights on the provenance and crustal evolution

    International Nuclear Information System (INIS)

    Maibam, Bidyananda; Gerdes, Axel; Srinivasan, R.; Goswami, J.N.

    2017-01-01

    A study of U-Pb and Lu-Hf-Yb isotope data in zircons from metamorphosed psammopelite and quartzite from the type area of Archaean Sargur Group, Dharwar Craton, India is carried out. Two age populations are observed: an older population with concordant U-Pb ages between 2.7 and 2.8 Ga, and a younger population with ages in the 2.4 - 2.6 Ga age range. The εHf values of 0 to + 2.0 for the older zircon population suggest that they were derived from juvenile crust formed at 2.7- 2.8 Ga. Sub-chondritic εHf values for the younger population indicate metamorphism and/or crustal reworking at ∼2.5 Ga. Meta-sedimentary enclaves in the Sargur type area are therefore part of the gneiss-supracrustal complex of different antiquities and may not have an independent stratigraphic status. (author)

  16. Crustal stress evolution of last 700 years in North China and earthquake occurrences

    Science.gov (United States)

    Wan, Y.; Shen, Z.; Gan, W.; Li, T.; Zeng, Y.

    2004-12-01

    We simulate the evolution process of cumulative Coulomb failure stress change (Δ CFS) in North China since 1303, manifested by secular tectonic stress loading and occurrence of large earthquakes. Secular tectonic stress loading is averaged from crustal strain rates derived from GPS. Fault rupture parameters of historical earthquakes are estimated as follows: the earthquake rupture length and the amount of slip are derived based on their statistical relationships with the earthquake intensity distribution and magnitude, calibrated using parameters of instrumental measured contemporary earthquakes. The earthquake rake angle is derived based on geologically determined fault setting parameters and seismically estimated orientation of regional tectonic stresses. Assuming a layered visco-elastic medium, we calculate stress evolution resulted from secular tectonic loading and coseismic and postseismic deformation. 49 M¡Y6.5 earthquakes occurred in North China since 1303. Statistics shows that 39 out of the 48 subsequent events were triggered by positive Δ CFS, yielding a triggering rate of 81.3%. The triggering rate for M¡Y5 earthquakes after the 1976 Tangshan earthquake is 82.1%. The triggering rate is up to 90% if corrections are made for some aftershocks which were wrongly identified as occurred in stress shadow zones because of errors in parameter estimates of historical earthquakes. Our study shows very high correlation between positive Δ CFS and earthquake occurrences. Relatively high Δ CFS in North China at present time is concentrated around the Bohai Sea, the west segment of the Northern Qinling fault, western end of the Zhangjiakou-Bohai seismic zone, and the Taiyuan basin in Shanxi rift zone, suggesting relatively higher earthquake potential in these areas.

  17. Subsidence history, crustal structure and evolution of the Nogal Rift, Northern Somalia

    Science.gov (United States)

    Ali, M. Y.; Watts, A. B.

    2013-12-01

    Seismic reflection profile, gravity anomaly, and biostratigraphic data from deep exploration wells have been used to determine the tectonic subsidence, structure and evolution of the Nogal basin, Northern Somalia, one of a number of ENE-WSW trending early Mesozoic rifts that formed prior to opening of the Gulf of Aden. Backstripping of biostratigraphic data at the Nogal-1 and Kali-1 wells provides new constraints on the age of rifting, and the amount of crustal and mantle extension. The tectonic subsidence and uplift history at the wells can be generally explained as a consequence of two, possibly three, major rifting events. The first event initiated in the Late Jurassic (~156 Ma) and lasted for ~10 Myr. We interpret the rift as a late stage event associated with the break-up of Gondwana and the separation of Africa and Madagascar. The second event initiated in the Late Cretaceous (~80 Ma) and lasted for ~20 Myr. This event probably correlates with a rapid increase in spreading rate on the ridges separating the African and Indian and African and Antarctica plates and a contemporaneous slowing down of Africa's plate motion. The backstripped tectonic subsidence data can be explained by a multi-rift extensional model with a stretching factor, β, in the range 1.17-1.38. The third and most recent event occurred in the Oligocene (~32 Ma) and lasted for ~10 Myr. This rift only developed at the centre of the basin close to Nogal-1 well, and is related to the opening of the Gulf of Aden. The amount of crustal thinning inferred at the Kali-1 well is consistent with the results of Process-Oriented Gravity and Flexure (POGM) modelling, assuming an elastic thickness of ~30 km. The thinning at the Nogal-1 well, however, is greater by ~ 7 km than predicted suggesting that the basin may be locally underplated by magmatic material. Irrespective, POGM suggests the transition between thick crust beneath Northern Somalia to thin crust beneath the Indian Ocean forms a ~500 km wide

  18. Nd isotopes in French Phanerzoic shales: external vs. internal aspects of crustal evolution

    International Nuclear Information System (INIS)

    Michard, A.; Gurriet, P.; Soudant, M.; Alberede, F.; Ecole Nationale Superieure de Geologie Appliquee et de Prospection Miniere, 54 - Nancy

    1985-01-01

    Nd isotopic composition has been determined on shales of Paleozoic (Brittany and Languedoc) and Mesozoic (Lorraine) age. epsilonsub(Nd)(T) values range from -6 to -12 while Nd crustal residence ages are typically in excess over their stratigraphic ages by some 1.4 Ga. Exceptions to this rule are the sediments coeval with Hercynian, Caledonian and Cadomian orogenic events, the epsilonsub(Nd)(T) values of which suggest addition of mantle material to the sediment in the form of volcanoclastic detritus. In Brittany, this is confirmed by the local zircon chronology which provides upper intercepts of Concordia up to 800 m.y. in excess of Nd crustal residence ages. Comparison of crustal residence ages with stratigraphic ages through geologic time suggests a three stage history: a) for rocks older than 2 Ga, stratigraphic and crustal residence ages coincide, b) from 2 to 1 Ga, crustal residence ages level off at ca. 2 Ga and then c) decrease down to 1.4 Ga in recent sediments. Two extreme models can account for the observed variations: an internally driven model, in which variable quantities of mantle material are added to the crust, and an externally driven model, in which the rate of crustal recycling is low in the Archean but increases rapidly at the onset of the atmospheric oxygen buildup. (author)

  19. Lead isotopes in archaean plutonic rocks

    International Nuclear Information System (INIS)

    Oversby, V.M.

    1978-01-01

    Archaean intrusive rocks have initial Pb isotopic compositions which show a varied and complex history for the source regions of the rocks. Even the oldest rocks from Greenland indicate heterogenous U and Pb distribution prior to 3800 m.y. ago. Source regions with μ values less than 7 must have played a significant role in the early history of the earth. By late Archaean time U/Pb ratios of source regions had increased substantially. Data from Australia and North America show distinct regional differences, both within and between continents. (Auth.)

  20. Along Arc Structural Variation in the Izu-Bonin Arc and its Implications for Crustal Evolution Processes

    Science.gov (United States)

    Kodaira, S.; Sato, T.; Takahashi, N.; Ito, A.; Kaneda, Y.

    2005-12-01

    A continental-type middle crust having Vp = 6.1 - 6.3 km/s has been imaged at several oceanic island arcs (e.g. northern Izu, Mariana, Tonga, Kyushu-Palau ridge) since Suyehiro et al. (1996) has found a felsic middle crust in the northern Izu arc. A high velocity lower crust (Vp > 7.3 km/s) underlying the felsic middle crust has been also underlined as a characteristic structure in the northern Izu arc. A bulk composition of the crust in the Izu arc may indicate more mafic than that of a typical continental crust due to a large volume of the high velocity lower crust. Since a crust becomes more mature toward the north along the Izu-Bonin arc, investigating structural variation along the volcanic front has been believed to provide a fundamental knowledge for a crustal evolution process. In 2004 and 2005, Japan Agency for Marine-Earth Science and Technology has conducted two along arc wide-angle seismic surveys from the Sagami-bay to the Kita-Iwo jima, a total profile length of about 1000 km. Although data from the Bonin-part of the profile which were acquired this year has not been processed yet, a result from the Izu-part, from the Sagami-bay to Tori shima, shows significant structural variations along the volcanic front. The crustal thickness are varied with a wavelength of several tens of km, i.e., thickened up to 25-30 km around the volcanoes (the Miyake jama, Hachijo jima, Aoga sima, Sumisu jima), while thinned down to 20 km between them. The fine seismic velocity image obtained by refraction tomography as well as a wide-angle reflection migration shows that the variation of the crustal block having 6.0 - 6.7 km/s, which is a typical continental crustal velocity, is mainly responsible for the observed variation of the crustal thickness. The thickness of the high velocity lower crust is not significantly varied along the arc. Therefore, an average crustal seismic velocity (varied 6.6 to 7.0 km/s) represents a higher velocity that that of a typical continental

  1. Crustal evolution derived from the Izu-Bonin-Mariana arc velocity images

    Science.gov (United States)

    Takahashi, N.; Kodaira, S.; Tatsumi, Y.; Miura, S.; Sato, T.; Yamashita, M.; No, T.; Takahashi, T.; Noguchi, N.; Takizawa, K.; Kaiho, Y.; Kaneda, Y.

    2010-12-01

    The Izu-Bonin-Mariana arc is known as one of typical oceanic island arcs, which has developed by subduction between oceanic crusts producing continental materials. Japan Agency for Marine-Earth Science and Technology has carried out seismic surveys using a multi-channel reflection survey system (MCS) and ocean bottom seismographs (OBSs) in the Izu-Bonin-Mariana (IBM) arc since 2002, and reported these crustal images. As the results, we identified the structural characteristics of whole Izu-Bonin-Mariana arc. Rough structural characteristics are, 1) middle crust with Vp of 6 km/s, 2) upper part of the lower crust with Vp of 6.5-6.8 km/s, 3) lower part of the lower crust with Vp of 6.8-7.5 km/s, and 4) lower mantle velocity beneath the arc crusts. In addition, structural variation along the volcanic front, for example, thickness variation of andesitic layers was imaged and the distributions is consistent with those of rhyolite volcanoes, that is, it suggested that the cause the structural variation is various degree of crustal growth (Kodaira et al., 2007). Moreover, crustal thinning with high velocity lower crust across arc was also imaged, and it is interpreted that such crust has been influenced backarc opening (Takahashi et al., 2009). According to Tatsumi et al. (2008), andesitic middle crust is produced by differentiation of basaltic lower crust and a part of the restites are transformed to the upper mantle. This means that region showing much crustal differentiation has large volume of transformation of dense crustal materials to the mantle. We calculated volume profiles of the lower crust along all seismic lines based on the petrologic model, and compared them with observed real volumes obtained by seismic images. If the real volume of the lower crust is large, it means that the underplating of dense materials to the crustal bottom is dominant rather than transformation of dense materials to the upper mantle. According to obtained profiles to judge if the

  2. Reconnaissance isotopic investigations in the Namaqua mobile belt and implications for proterozoic crustal evolution - Namaqualand geotraverse

    International Nuclear Information System (INIS)

    Barton, E.S.

    1983-01-01

    Fourteen rock units in the western portion of the Namaqua mobile belt were investigated by some or all of the following isotopic methods: Rb-Sr, Pb-Pb, Th-Pb total-rock and Rb-Sr mineral age measurements. Two major tectogenetic events are recongnised. The early Orange River event is represented by the emplacement and metamorphism of the volcanic rocks of the Haib Subgroup and the emplacement of granitoids of the Vioolsdrif and Gladkop suites in the regions of the Richtersveld Province and the Steinkopf domain. The later Namaqua event is represented mainly by extensive granitoid emplacement and high-grade granulite facies metamorphism in the Okiep Copper District, with minor granitoid and dyke emplacement taking place in the Steinkopf domain. The polymetamorphism associated with these tectonic events had far-reaching effects in the form of long continued isotopic resetting of rock and mineral systems. In the instances where the regression of the total-rock isotopic data define errorchron results, geological disturbance is inferred. The Sr-isotopic results indicate substrantial crustal reworking and two periods of mantle differentiation producing continental crust at approximately 2 000 and approximately 1 200 Ma ago. The Pb-isotopic data for these rocks indicate μ 2 values for the source regions that are compatible with crustal reworking for the felsic rocks or a significant amount of crustal recycling into the mantle for the mafic rocks. On this basis a magmatic arc environment is envisaged for the rocks generated during the Orange River event. The deformation fabrics associated with the Orange River and Namaqua tectogenetic episodes are dated by the time of emplacement of syn- and post-tectonic granitic rocks. The regional fabric observed in the Steinkopf domain and in the Okiep Copper District was not synchronously developed

  3. Crustal Structure and Evolution of the Eastern Himalayan Plate Boundary System, Northeast India

    Science.gov (United States)

    Mitra, S.; Priestley, K. F.; Borah, Kajaljyoti; Gaur, V. K.

    2018-01-01

    We use data from 24 broadband seismographs located south of the Eastern Himalayan plate boundary system to investigate the crustal structure beneath Northeast India. P wave receiver function analysis reveals felsic continental crust beneath the Brahmaputra Valley, Shillong Plateau and Mikir Hills, and mafic thinned passive margin transitional crust (basement layer) beneath the Bengal Basin. Within the continental crust, the central Shillong Plateau and Mikir Hills have the thinnest crust (30 ± 2 km) with similar velocity structure, suggesting a unified origin and uplift history. North of the plateau and Mikir Hills the crustal thickness increases sharply by 8-10 km and is modeled by ˜30∘ north dipping Moho flexure. South of the plateau, across the ˜1 km topographic relief of the Dawki Fault, the crustal thickness increases abruptly by 12-13 km and is modeled by downfaulting of the plateau crust, overlain by 13-14 km thick sedimentary layer/rocks of the Bengal Basin. Farther south, beneath central Bengal Basin, the basement layer is thinner (20-22 km) and has higher Vs (˜4.1 km s-1) indicating a transitional crystalline crust, overlain by the thickest sedimentary layer/rocks (18-20 km). Our models suggest that the uplift of the Shillong Plateau occurred by thrust faulting on the reactivated Dawki Fault, a continent margin paleorift fault, and subsequent back thrusting on the south dipping Oldham Fault, in response to flexural loading of the Eastern Himalaya. Our estimated Dawki Fault offset combined with timing of surface uplift of the plateau reveals a reasonable match between long-term uplift and convergence rate across the Dawki Fault with present-day GPS velocities.

  4. Crustal structure and evolution of the Pyrenean-Cantabrian belt: A review and new interpretations from recent concepts and data

    Science.gov (United States)

    Teixell, A.; Labaume, P.; Ayarza, P.; Espurt, N.; de Saint Blanquat, M.; Lagabrielle, Y.

    2018-01-01

    . To the west, oceanic subduction of the Bay of Biscay under the North Iberian margin is supported by an upper plate thrust wedge, gravity and magnetic anomalies, and 3D inclined sub-crustal reflections. However, discrepancies remain for the location of continent-ocean transitions in the Bay of Biscay and for the extent of oceanic subduction. The plate-kinematic evolution during the Mesozoic, which involves issues as the timing and total amount of opening, as well as the role of strike-slip drift, is also under debate, discrepancies arising from first-order interpretations of the adjacent oceanic magnetic anomaly record.

  5. Avalonian crustal controls on basin evolution: implications for the Mesozoic basins of the southern North Sea

    Science.gov (United States)

    Smit, Jeroen; van Wees, Jan-Diederik; Cloetingh, Sierd

    2015-04-01

    Little is known of the Southern North Sea Basin's (SNSB) Pre-Permian basement due to a lack of outcrop and cores. The nature and structure of the East Avalonian crust and lithosphere remain even less constrained in the absence of deep seismic (refraction) lines. However, various studies have hinted at the importance of the Reactivation of the Early Carboniferous fault network during each consecutive Mesozoic and Cenozoic tectonic phase, demonstrating the key role of weak zones from the Early Carboniferous structural grain in partitioning of structural deformation and vertical basin motions at various scales. Although the older basin history and the basement attract increasing attention, the Pre-Permian tectonics of the SNSB remains little studied with most attention focused on the Permian and younger history. The strong dispersal of existing constraints requires a comprehensive study from Denmark to the UK, i.e. the East Avalonian microplate, bordered by the Variscan Rheïc suture, the Atlantic and Baltica. Based on an extensive literature study and the reinterpretation of publicly available data, linking constraints from the crust and mantle to stratigraphic-sedimentological information, we complement the map of Early Carboniferous rifting of East Avalonia and propose a new tectonic scenario. From the reinterpretation of the boundary between Avalonia and Baltica we propose a new outline for the Avalonian microplate with implications for the tectonics of the North German Basin. Furthermore, we highlight the nature and extent of the major crustal/lithospheric domains with contrasting structural behaviour and the major boundaries that separate them. Results shed light on the effects of long lived differences in crustal fabric that are responsible for spatial heterogeneity in stress and strain magnitudes and zonations of fracturing, burial history and temperature history. The geomechanical control of large crustal-scale fault structures will provide the constraints

  6. Crustal thickness controlled by plate tectonics

    DEFF Research Database (Denmark)

    Artemieva, Irina M.; Meissner, Rolf

    2012-01-01

    /gabbro–eclogite phase transition in crustal evolution and the links between lithosphere recycling, mafic magmatism, and crustal underplating. We advocate that plate tectonics processes, togetherwith basalt/gabbro–eclogite transition, limit crustal thickness worldwide by providing effective mechanisms of crustal...

  7. The forearc crustal evolution of Izu-Bonin (Ogasawara) region obtained by seismic reflection and refraction surveys

    Science.gov (United States)

    Yamashita, M.; Kodaira, S.; Takahashi, N.; Tatsumi, Y.; Kaneda, Y.

    2009-12-01

    The Izu-Bonin (Ogasawara)-Mariana (IBM) arc is known to the typical oceanic island arc, and it is the most suitable area to understand the growth process of island arc. By previous seismic survey and deep sea drilling, convex basements are distributed along North-South direction in present forearc region. The convex basements are reported to be formed during Oligocene and Eocene (Taylor, 1992). In IBM forearc region, the middle crust with 6 km/s is recognized by seismic survey using OBSs. In IBM region, four IODP drilling sites are proposed in order to understand comprehensive growth process of arc and continental crust evolution. Two of them are located in forearc region. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) carried out multi-channel seismic reflection survey using 7,800/12,000 cu.in. air gun and 5-6 km streamer with 444/204 ch hydrophones in the IBM region since 2004. We investigate the crustal structure beneath the Izu-Bonin forearc region for contribution of IBM drilling site along five long survey lines, which are across from present volcanic front to forearc basin. Seismic refraction survey is also conducted across forearc region using 84 OBSs every 1 km interval. Shallow crustal structure can be classified four units including basement which compared between previous drilling results and obtained seismic profiles. In IBM forearc region, thick sedimentary basin distribute from east side of volcanic front. Two convex basement peaks are indicated in across profile of forearc region. These peaks are estimated the top of paleoarc (Oligocene and Eocene) by previous ODP drilling. The half graben structure with major displacement is identified from west side of present volcanic front to the top of Oligocene arc. On the other hand, there is no displacement of sediments between the Oligocene arc and Eocene arc. This result shows the same origin of basement between the present volcanic front and Oligocene arc. There is long time difference of

  8. Why Wet Kaolin can be used as a Crustal Analog and its Application to Fault Evolution at Restraining Bends

    Science.gov (United States)

    Cooke, M. L.; van der Elst, N.; Schottenfeld, M. T.

    2010-12-01

    To simulate geologic deformation on observable time and length scales within the lab, a subset of analog modelers have used wet kaolin. Unlike the more often used sand, wet kaolin beautifully exhibits detailed fault structures. Furthermore, faults within the kaolin are more readily reactivated than those in sand. The low plasticity of kaolin (compared to other clays) gives it low shear strength. Consequently, the clay is a suitable analog material if we assume that the wet kaolin deforms by coulomb frictional failure. Koalin generally deforms as a Bingham solid and exhibits more complex deformation than the perfectly plastic behavior assumed with Coulomb failure. We performed fall cone and rheometric tests on wet kaolin to refine our quantitative understanding of its rheology. We use North American wet kaolin with density 1.65-1.7 g/cm3 and water content of 37.5-38.5%. The fall cone tests reveal that the undrained shear strength (100-160 Pa) is greater than previously measured with a viscometer. The rheometer tests show that the wet koalin exhibits many of the same properties of crustal materials including: 1) elastic behavior at low strains, 2) stress relaxation at near-failure strains, 3) creep under static load, 4) yield strength sensitive to strain rate and 5) rate and state dependent failure. Armed with quantitative values for this complex deformation, we can better scale the length and strain rate of the wet koalin experiments to specific crustal settings. Experiments of deformation around restraining bends show features very similar to those found in natural examples. The detailed fault structures produced in the wet kaolin can be analyzed to understand the evolution of active faulting at restraining bends.

  9. Fossil Microorganisms in Archaean deposits of Northern Karelia

    Science.gov (United States)

    Astafieva, M. M.; Hoover, R. B.; Rozanov, A. Y.; Vrevskiy, A. B.

    2005-01-01

    Newly found biomorphic microstructures from the Upper Archaean (lopian) rocks from Northern Karelia are described. The presence of various microorganisms of bacterial nature and even cyanobacteria (and possibly eukaryotic forms) is suggested. The necessity of employing methods of electron microscopy, as well as traditional methods, while studying the very early manifestations of life in Archaean and Early Proterozoic is noted.

  10. Subsidence history, crustal structure, and evolution of the Somaliland-Yemen conjugate margin

    Science.gov (United States)

    Ali, M. Y.; Watts, A. B.

    2013-04-01

    We have used biostratigraphic data from deep exploration wells to determine the tectonic subsidence history of the Somaliland (northwestern Somalia)-Yemen conjugate margin, a poorly known margin in the central part of the Gulf of Aden. Bathymetry and magnetic anomaly data suggest the Gulf of Aden is a young feature that formed following the rifting apart and breakup of the African and Arabian plates ~32 Ma. Our tectonic subsidence data suggest, however, that the present-day Gulf of Aden developed on an earlier Mesozoic rift system. The oldest episode of rifting initiated at ~156 Ma and lasted for ~10 Ma and had a NW-SE trend. We interpret the rift as a late stage event associated with the breakup of Gondwana and the separation of Africa and Madagascar. At ~80 Ma, there is evidence of an intermediate rift event which correlates with a rapid increase in spreading rate on the ridges separating the African and Indian and African and Antarctica plates and a contemporaneous slowing down of Africa's plate motion. The combined effect of all three rifting events has been to thin the crust and upper mantle by up to a factor of 2. The amount of thinning deduced from the wells is in accord with the crustal structure inferred from available seismic refraction data and process-oriented gravity and flexure modeling. The margin is asymmetric with a steeper gradient in the Moho on the Yemen side than the Somaliland side. The main discrepancy is on the Yemen side where the gravity-derived Moho is 10 km deeper than the well-derived Moho. We attribute the discrepancy to the addition of material at the base of the crust since rifting, possibly magma sourced from the Afar plume.

  11. Adakitic magmas: modern analogues of Archaean granitoids

    Science.gov (United States)

    Martin, Hervé

    1999-03-01

    Both geochemical and experimental petrological research indicate that Archaean continental crust was generated by partial melting of an Archaean tholeiite transformed into a garnet-bearing amphibolite or eclogite. The geodynamic context of tholeiite melting is the subject of controversy. It is assumed to be either (1) subduction (melting of a hot subducting slab), or (2) hot spot (melting of underplated basalts). These hypotheses are considered in the light of modern adakite genesis. Adakites are intermediate to felsic volcanic rocks, andesitic to rhyolitic in composition (basaltic members are lacking). They have trondhjemitic affinities (high-Na 2O contents and K 2O/Na 2O˜0.5) and their Mg no. (0.5), Ni (20-40 ppm) and Cr (30-50 ppm) contents are higher than in typical calc-alkaline magmas. Sr contents are high (>300 ppm, until 2000 ppm) and REE show strongly fractionated patterns with very low heavy REE (HREE) contents (Yb≤1.8 ppm, Y≤18 ppm). Consequently, high Sr/Y and La/Yb ratios are typical and discriminating features of adakitic magmas, indicative of melting of a mafic source where garnet and/or hornblende are residual phases. Adakitic magmas are only found in subduction zone environments, exclusively where the subduction and/or the subducted slab are young (subducted and where the adakitic character of the lavas correlates well with the young age of the subducting oceanic lithosphere. In typical subduction zones, the subducted lithosphere is older than 20 Ma, it is cool and the geothermal gradient along the Benioff plane is low such that the oceanic crust dehydrates before it reaches the solidus temperature of hydrated tholeiite. Consequently, the basaltic slab cannot melt. The released large ion lithophile element (LILE)-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting. Afterwards, the residue is made up of olivine+clinopyroxene+orthopyroxene, such that the partial melts are HREE-rich (low La/Yb and Sr

  12. Early Archaean collapse basins, a habitat for early bacterial life.

    Science.gov (United States)

    Nijman, W.

    For a better definition of the sedimentary environment in which early life may have flourished during the early Archaean, understanding of the basin geometry in terms of shape, depth, and fill is a prerequisite. The basin fill is the easiest to approach, namely from the well exposed, low-grade metamorphic 3.4 - 3.5 Ga rock successions in the greenstone belts of the east Pilbara (Coppin Gap Greenstone Belt and North Pole Dome) in West Australia and of the Barberton Greenstone Belt (Buck Ridge volcano-sedimentary complex) in South Africa. They consist of mafic to ultramafic volcanic rocks, largely pillow basalts, with distinct intercalations of intermediate to felsic intrusive and volcanic rocks and of silicious sediments. The, partly volcaniclastic, silicious sediments of the Buck Ridge and North Pole volcano-sedimentary complexes form a regressive-transgressive sequence. They were deposited close to base level, and experienced occasional emersion. Both North Pole Chert and the chert of the Kittys Gap volcano-sedimentary complex in the Coppin Gap Greenstone Belt preserve the flat-and-channel architecture of a shallow tidal environment. Thickness and facies distribution appear to be genetically linked to systems, i.e. arrays, of syn-depositionally active, extensional faults. Structures at the rear, front and bottoms of these fault arrays, and the fault vergence from the basin margin towards the centre characterize the basins as due to surficial crustal collapse. Observations in the Pilbara craton point to a non-linear plan view and persistence for the basin-defining fault patterns over up to 50 Ma, during which several of these fault arrays became superposed. The faults linked high-crustal level felsic intrusions within the overall mafic rock suite via porphyry pipes, black chert veins and inferred hydrothermal circulations with the overlying felsic lavas, and more importantly, with the cherty sediments. Where such veins surfaced, high-energy breccias, and in the

  13. Crustal evolution of Eocene paleo arc around Ogasawara region obtained by seismic reflection survey

    Science.gov (United States)

    Yamashita, M.; Takahashi, N.; Kodaira, S.; Miura, S.; Ishizuka, O.; Tatsumi, Y.

    2011-12-01

    The Izu-Bonin (Ogasawara)-Mariana (IBM) arc is known to the typical oceanic island arc, and it is the most suitable area to understand the growth process of island arc. The existence of two paleo arc which consists of Oligocene and Eocene paleo age is known in IBM forearc region by geological and geophysical studies. The Ogasawara ridge is also known to locate the initial structure of arc evolution from geologic sampling of research submersible. In this region, IODP drilling site: IBM-2 is proposed in order to understand the temporal and spatial change in arc crust composition from 50 to 40Ma magmatism. Site IBM-2 consists of two offset drilling holes (BON-1, BON-2). BON-1 designed to first encounter forearc basalt and will reach the sheeted dykes. BON-2 will start in boninites and finish in fore arc basalts. The purpose of these drilling is sampling the full volcanic stratigraphy from gabbro to boninite. There is no seismic data around BON-1 and BON-2, therefore it is need to conduct the multi-channel seismic reflection survey. Japan Agency for Marine-Earth Science and Technology carried out multi-channel seismic reflection survey and wide-angle reflection survey using 7,800 cu.in. air gun, 5 km streamer with 444 ch hydrophones and 40 OBSs in March 2011. We obtained two seismic reflection profiles of lines KT06 and KT07 along the paleo arc around Ogasawara ridge. Line KT06 located the north side of Ogasawara ridge. Line KT07 located the trench side of Ogasawara ridge. Lines KT06 is also deployed the OBSs every 5 km interval. Thin sediments are covered with basement in both survey lines. There are some sediment filled in depression topography. The low-frequency reflection from the top of subducting Pacific plate is recognized in line KT06. The continuity of this reflection is not clear due to the complicated bathymetry. The displacement of basement in northern side of Ogasawara ridge is identified along the lineament of bathymetry in Line 06. This structure is

  14. Temporal Evolution of the Upper Continental Crust: Implications for the Mode of Crustal Growth and the Evolution of the Hydrosphere

    Science.gov (United States)

    Rudnick, R. L.; Gaschnig, R. M.; Li, S.; Tang, M.; Qiu, L.; Valley, J. W.; Zurkowski, C.; McDonough, W. F.

    2014-12-01

    The upper continental crust (UCC), the interface between the atmosphere and solid Earth, is the site of weathering that produces sedimentary rocks, influences ocean chemistry through runoff of soluble elements, and affects climate through CO2 draw-down. The UCC also contains more than 50% of the crust's highly incompatible element budget (including K, Th, and U). Therefore, understanding its composition and evolution provides insight into how continents have formed, evolved, and interacted with the hydrosphere. New major and trace element compositions of >100 glacial diamictites and >100 Archean shales, plus δ7Li and δ18O for a subset of these samples, combined with data from the literature, show that the average composition of the UCC has changed through time, reflecting both the rise of atmospheric oxygen and its attendant effects on weathering, as well as the mode of crust formation and differentiation. Some changes that occur as a step function near the Archean/Proterozoic boundary (increased Th/U, decreased Mo/Pr, V/Lu) reflect the rise of oxygen at the great oxidation event (GOE) and its influence on chemical weathering signatures in the UCC. Other changes are more gradual with time (e.g., higher Th/Sc and δ18O, lower Ni/Co, La/Nb, Eu/Eu* and transition metal abundances) and reflect an UCC that has transitioned from a more mafic to a more felsic bulk composition, and which experienced increased interaction with the hydrosphere with time. The gradual nature of these compositional changes likely reflects the waning heat production of the Earth, rather than an abrupt change in tectonics or style of crust formation. These more gradual changes in crust composition, which contrast with the abrupt changes associated with the GOE, suggest that a fundamental change in the nature of crust differentiation is unlikely to be responsible for the rise of atmospheric oxygen (cf. Keller and Schoene, 2012). Indeed, it appears that the opposite may be true: that the rise of

  15. Spatial relationships between crustal structures and mantle seismicity in the Vrancea Seismogenic Zone of Romania: Implications for geodynamic evolution

    Science.gov (United States)

    Enciu, Dana-Mihaela

    Integration of active and passive-source seismic data is employed to study the relationships between crustal structures and seismicity in the SE Carpathian foreland of Romania, and the connection with the Vrancea Seismogenic Zone. Relocated crustal epicenters and focal mechanisms are correlated with industry seismic profiles Comanesti, Ramnicu Sarat, Braila and Buzau, the reprocessed DACIA PLAN profile and the DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) II and III profiles in order to understand the link between neo-tectonic foreland deformation and Vrancea mantle seismicity. Projection of crustal foreland hypocenters onto deep seismic profiles identified active crustal faults suggesting a mechanical coupling between sedimentary, crustal and upper mantle structures on the Trotus, Sinaia and newly observed Ialomita Faults. Seismic reflection imaging revealed the absence of west dipping reflectors in the crust and an east dipping to horizontal Moho in the proximity of the Vrancea area. These findings argue against both 'subduction-in-place' and 'slab break-off' as viable mechanisms for generating Vrancea mantle seismicity.

  16. Recycling of the Archaean continental crust: the case study of the Gavião, State of Bahia, NE Brazil

    Science.gov (United States)

    Pinto, M. Santos; Peucat, J. J.; Martin, H.; Sabaté, P.

    1998-09-01

    The Gavião block, located to the west of the São Francisco Craton (State of Bahia, NE Brazil), is the oldest crustal block so far recognised in South America—3.42 Ga. In its southern part, the Gavião block has been divided into three domains on the basis of 207Pb/ 206Pb dating on single zircons and monazites combined with Sr and Nd isotopic data and major and trace element geochemical modelling. These are: (1) an Archaean juvenile domain which consists of grey gneisses (Bernada massif) which evidence mantle extraction around 3.3 Ga; (2) an Archaean domain (3.24-3.16 Ga) either recycled, or juvenile with crustal contamination, consisting of trondhjemitic grey gneisses (Aracatu massif) and K-rich calc-alkaline granitic gneisses (Mariana and Serra do Eixo massifs); (3) a Paleoproterozoic recycled domain consisting mainly of the Umburanas granites, which yielded inherited zircons ages ranging from 3.1 to 2.8 Ga, whereas the monazite age is ca 2.0 Ga. The Aracatu and Mariana massifs are cut by granites at ca 2.0 Ga the same age of the Serra da Franga massif. The Gavião block is a type example of Archaean continental crust (3.2 Ga) that has been recycled through partial melting events mainly in Paleoproterozoic times during the Transamazonian orogeny (2.0-2.1 Ga). Brasiliano cooling ages are recorded by the Rb-Sr system of biotite-whole rock pairs ca 500 Ma.

  17. Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir

    Science.gov (United States)

    Barbara Carappa,; Mustapha, F.S.; Cosca, Michael A.; Gehrels, George E.; Schoenbhohm, L; Sobel, E.; DeCelles.P.,; Russell, Joellen; Goodman, Paul

    2014-01-01

    The Pamir is the western continuation of Tibet and the site of some of the highest mountains on Earth, yet comparatively little is known about its crustal and tectonic evolution and erosional history. Both Tibet and the Pamir are characterized by similar terranes and sutures that can be correlated along strike, although the details of such correlations remain controversial. The erosional history of the Pamir with respect to Tibet is significantly different as well: Most of Tibet has been characterized by internal drainage and low erosion rates since the early Cenozoic; in contrast, the Pamir is externally drained and topographically more rugged, and it has a strongly asymmetric drainage pattern. Here, we report 700 new U-Pb and Lu-Hf isotope determinations and >300 40Ar/39Ar ages from detrital minerals derived from rivers in China draining the northeastern Pamir and >1000 apatite fission-track (AFT) ages from 12 rivers in Tajikistan and China draining the northeastern, central, and southern Pamir. U-Pb ages from rivers draining the northeastern Pamir are Mesozoic to Proterozoic and show affinity with the Songpan-Ganzi terrane of northern Tibet, whereas rivers draining the central and southern Pamir are mainly Mesozoic and show some affinity with the Qiangtang terrane of central Tibet. The εHf values are juvenile, between 15 and −5, for the northeastern Pamir and juvenile to moderately evolved, between 10 and −40, for the central and southern Pamir. Detrital mica 40Ar/39Ar ages for the northeastern Pamir (eastern drainages) are generally older than ages from the central and southern Pamir (western drainages), indicating younger or lower-magnitude exhumation of the northeastern Pamir compared to the central and southern Pamir. AFT data show strong Miocene–Pliocene signals at the orogen scale, indicating rapid erosion at the regional scale. Despite localized exhumation of the Mustagh-Ata and Kongur-Shan domes, average erosion rates for the northeastern Pamir

  18. Granulite belts of Central India with special reference to the Bhopalpatnam Granulite Belt: Significance in crustal evolution and implications for Columbia supercontinent

    Science.gov (United States)

    Vansutre, Sandeep; Hari, K. R.

    2010-11-01

    The Central Indian collage incorporates the following major granulite belts: (1) the Balaghat-Bhandara Granulite Belt (BBG), (2) the Ramakona-Katangi Granulite Belt (RKG), (3) the Chhatuabhavna Granulite (CBG) of Bilaspur-Raigarh Belt, (4) the Makrohar Granulite Belt (MGB) of Mahakoshal supracrustals, (5) the Kondagaon Granulite Belt (KGGB), (6) the Bhopalpatnam Granulite Belt (BGB), (7) the Konta Granulite Belt (KTGB) and (8) the Karimnagar Granulite Belt (KNGB) of the East Dharwar Craton (EDC). We briefly synthesize the general geologic, petrologic and geochronologic features of these belts and explain the Precambrian crustal evolution in Central India. On the basis of the available data, a collisional relationship between Bastar craton and the EDC during the Paleo-Mesoproterozoic is reiterated as proposed by the earlier workers. The tectonic evolution of only few of the orogenic belts (BGB in particular) of Central India is related to Columbia.

  19. Role of upper-most crustal composition in the evolution of the Precambrian ocean-atmosphere system

    Science.gov (United States)

    Large, R. R.; Mukherjee, I.; Zhukova, I.; Corkrey, R.; Stepanov, A.; Danyushevsky, L. V.

    2018-04-01

    Recent research has emphasized the potential relationships between supercontinent cycles, mountain building, nutrient flux, ocean-atmosphere chemistry and the origin of life. The composition of the Upper-Most Continental Crust (UMCC) also figures prominently in these relationships, and yet little detailed data on each component of this complex relationship has been available for assessment. Here we provide a new set of data on the trace element concentrations, including the Rare Earth Elements (REE), in the matrix of 52 marine black shale formations spread globally through the Archean and Proterozoic. The data support previous studies on the temporal geochemistry of shales, but with some important differences. Results indicate a change in provenance of the black shales (upper-most crustal composition), from more mafic in the Archean prior to 2700 Ma, to more felsic from 2700 to 2200 Ma, followed by a return to mafic compositions from 2200 to 1850 Ma. Around 1850 to 1800 Ma there is a rapid change to uniform felsic compositions, which remained for a billion years to 800 Ma. The shale matrix geochemistry supports the assertion that the average upper-most continental source rocks for the shales changed from a mix of felsic, mafic and ultramafic prior to 2700 Ma to more felsic after 1850 Ma, with an extended transition period between. The return to more mafic UMCC from 2200 to 1850 Ma is supported by the frequency of Large Igneous Provinces (LIPs) and banded iron formations, which suggest a peak in major mantle-connected plume events and associated Fe-rich hydrothermal activity over this period. Support for the change to felsic UMCC around 1850 Ma is provided by previous geological data which shows that felsic magmas, including, A-type granites and K-Th-U-rich granites intruded vast areas of the continental crust, peaking around 1850 Ma and declining to 1000 Ma. The implications of this change in UMCC are far reaching and may go some way to explain the distinct

  20. Geochemical and biologic constraints on the Archaean atmosphere and climate – A possible solution to the faint early Sun paradox

    DEFF Research Database (Denmark)

    Rosing, Minik Thorleif; Brid, Dennis K.; Sleep, Norman H.

    into account the apparent growth of Earth continents (Collerson and Kamber 1999) and the absence of land vegetation during the Precambrian for the evolution of the surface albedo, and a model for the abundance and properties of clouds that takes into account the lower abundance of biogenic cloud condensation......There is ample geological evidence that Earth’s climate resembled the present during the Archaean, despite a much lower solar luminosity. This was cast as a paradox by Sagan and Mullen in 1972. Several solutions to the paradox have been suggested, mostly focusing on adjustments of the radiative...... properties of Earth’s atmosphere e.g. Kasting (1993), by increasing the mixing ratio of CO2 and/or adding various other greenhouse gasses. We have used banded iron formation (BIF), which are chemical sediments precipitated out of the Archaean ocean to characterize the composition of the atmosphere...

  1. Why Archaean TTG cannot be generated by MORB melting in subduction zones

    Science.gov (United States)

    Martin, Hervé; Moyen, Jean-François; Guitreau, Martin; Blichert-Toft, Janne; Le Pennec, Jean-Luc

    2014-06-01

    produced. Consequently, internal recycling of oceanic plateaus does not appear to be a suitable process for the genesis of Archaean continental crust. A possible alternative to this scenario is the subduction of oceanic plateaus. This hypothesis is supported by a present-day analog. In Ecuador, the Carnegie ridge, which is an oceanic plateau resulting from the Galapagos hot spot activity, is being subducted beneath the South American plate. Not only are the resulting magmas adakitic (TTG-like) in composition, but the volcanic productivity is several times greater than in other parts of the Andean volcanic arc. Above the location where the plateau is subducted, the arc is wide and the quaternary volcanoes numerous (about 80 active edifices). The volcanic productivity of each individual volcano also is more intense than away from the subduction focal point with an average output rate of about 0.4-0.5 km3·ka- 1 compared with only about 0.05-0.2 km3·ka- 1 for production rates at volcanoes erupting in the rest of the arc. Consequently, we infer that occasional subduction of oceanic plateaus throughout Earth's history can account for the episodic nature of crustal growth. Additionally, the generation by this mechanism of huge volumes of TTG-like magmas would readily dominate the crustal growth record.

  2. Lithospheric Structure, Crustal Kinematics, and Earthquakes in North China: An Integrated Study

    Science.gov (United States)

    Liu, M.; Yang, Y.; Sandvol, E.; Chen, Y.; Wang, L.; Zhou, S.; Shen, Z.; Wang, Q.

    2007-12-01

    The North China block (NCB) is geologically part of the Archaean Sino-Korean craton. But unusual for a craton, it was thermally rejuvenated since late Mesozoic, and experienced widespread extension and volcanism through much of the Cenozoic. Today, the NCB is characterized by strong internal deformation and seismicity, including the 1976 Tangshan earthquake that killed ~250,000 people. We have started a multidisciplinary study to image the lithospheric and upper mantle structure using seismological methods, to delineate crustal kinematics and deformation via studies of neotectonics and space geodesy, and to investigate the driving forces, the stress states and evolution, and seismicity using geodynamic modeling. Both seismic imaging and GPS results indicate that the Ordos plateau, which is the western part of the NCB and a relic of the Sino-Korean craton, has been encroached around its southern margins by mantle flow and thus is experiencing active cratonic destruction. Some of the mantle flow may be driven by the Indo-Asian collision, although the cause of the broad mantle upwelling responsible for the Mesozoic thinning of the NCB lithosphere remains uncertain. At present, crustal deformation in the NCB is largely driven by gravitational spreading of the expanding Tibetan Plateau. Internal deformation within the NCB is further facilitated by the particular tectonic boundary conditions around the NCB, and the large lateral contrasts of lithospheric strength and rheology. Based on the crustal kinematics and lithospheric structure, we have developed a preliminary geodynamic model for stress states and strain energy in the crust of the NCB. The predicted long-term strain energy distribution is comparable with the spatial pattern of seismic energy release in the past 2000 years. We are exploring the cause of the spatiotemporal occurrence of large earthquakes in the NCB, especially the apparent migration of seismicity from the Weihe-Shanxi grabens around the Ordos to

  3. Crustal-scale alpine tectonic evolution of the western Pyrenees - eastern Cantabrian Mountains (N Spain) from integration of structural data, low-T thermochronology and seismic constraint

    Science.gov (United States)

    DeFelipe, I.; Pedreira, D.; Pulgar, J. A.; Van der Beek, P.; Bernet, M.; Pik, R.

    2017-12-01

    The Pyrenean-Cantabrian Mountain belt extends in an E-W direction along the northern border of Spain and resulted from the convergence between the Iberian and European plates from the Late Cretaceous to the Miocene, in the context of the Alpine orogeny. The main aim of this work is to characterize the tectonic evolution at a crustal-scale of the transition zone from the Pyrenees to the Cantabrian Mountains, in the eastern Basque-Cantabrian Basin (BCB). We integrate structural work, thermochronology (apatite fission track and zircon (U-Th)/He) and geophysical information (shallow seismic reflection profiles, deep seismic refraction/wide-angle reflection profiles and seismicity distribution) to propose an evolutionary model since the Jurassic to the present. During the Albian, hyperextension related to the opening of the Bay of Biscay yielded to mantle unroofing to the base of the BCB. This process was favored by a detachment fault that connected the mantle in its footwall with the base of a deep basin in its hanging wall. During this process, the basin experienced HT metamorphism and fluid circulation caused the serpentinization of the upper part of the mantle. There is no evidence of seafloor mantle exhumation before the onset of the Alpine orogeny. The thermochronological study points to a N-vergent phase of contractional deformation in the late Eocene represented by the thin-skinned Leiza fault system followed in the early Oligocene by the S-vergent, thick-skinned, Ollín thrust. Exhumation rates for the late Eocene-early Oligocene are of 0.2-0.7 km/Myr. After that period, deformation continues southwards until the Miocene. The crustal-scale structure resultant of the Alpine orogeny consists of an Iberian plate that subducts below the European plate. The crust is segmented into four blocks separated by three S-vergent crustal faults inherited from the Cretaceous extensional period. The P-wave velocities in this transect show anomalous values (7.4 km/s) in the

  4. Archaean TTG of Vodlozero Terrain, Fennoscandian Shield

    Science.gov (United States)

    Chekulaev, Valery; Arestova, Natalia

    2014-05-01

    The Vodlozero terrain is the largest (about 270*240 km) early Archaean fragment of Fennoscandian Shield and composes its eastern part. The granitoids of TTG suite are predominant component of the terrain. The greenstone belts are placed along the margins of the terrain. Several stages of TTG formation can be distinguished in Achaean crust history. (1) The oldest TTG are trondhjemites and tonalities with age of 3240 Ma. They contain rare and small amphibolite inclusions of the same age. These TTG are characterized by high Sr (av. 412 ppm), Sr/Y (70), (La/Yb)n (54) and low Y (av. 7 ppm), Yb (0.32 ppm) and Nb (4 ppm). It was shown (Lobach-Zhuchenko et al., 2000), that the source of these TTG could be basic rocks, having composition similar with TH1 by K.Condie. (2) The tonalities and granodiorites with age of 3150 Ma are disposed near greenstone belts and contain compared to TTG of the first group less Sr (av. 250 ppm), Sr/Y (22), (La/Yb)n (18) and more K, Rb (av. 70 ppm), Ba (470 ppm), Y (11 ppm),Yb (1.16 ppm). TTG of both groups have identical T(DM)Nd (3250-3400 Ma) and differences in composition is evidently connected with lower level of source melting of the second group and also with K-metasomatism. The volcanics of the greenstone belts have age 3020 - 2940 Ma. Dykes of gabbro-amphibolites and andesites with the same age and composition cut TTG of the first and the second groups. The age of the third TTG group is about 2900 Ma ago. These rocks form leucosoma of migmatites within TTG of the second group. The composition of the third TTG and Nd isotope data suppose their origin by the melting of ancient TTG crust simultaneously with greenstone belt emplacement. The fourth TTG group with age 2780-2850 Ma forms a small intrusions, cutting older TTG and greenstone rocks. Their composition is similar to 3150 Ma TTG. Nd isotope data indicate that these TTG have younger (about 2850 Ma) source. Thus there are four TTG groups formed into interval more 400 Ma. The age and

  5. Petrology and geochemistry of charnockites (felsic ortho-granulites) from the Kerala Khondalite Belt, Southern India: Evidence for intra-crustal melting, magmatic differentiation and episodic crustal growth

    Science.gov (United States)

    Ravindra Kumar, G. R.; Sreejith, C.

    2016-10-01

    The Kerala Khondalite Belt (KKB) of the southern India encompasses volumetrically significant magmatic components. Among these, orthopyroxene-bearing, felsic ortho-granulites, popularly known as charnockites in Indian context, constitute an important lithology. In contrast to the well-known phenomena of arrested charnockitization, the geochemical characteristics and petrogenesis of these ortho-granulite suites remain poorly studied, leaving geodynamic models envisaged for the KKB highly conjectural. In this paper, we try to bridge this gap with detailed results on orthopyroxene-bearing, felsic ortho-granulites spread over the entire KKB and propose a new petrogenetic and crustal evolution model. Based on geochemical characteristics, the orthopyroxene-bearing, felsic ortho-granulites (charnockites sensu lato) of KKB are classified into (1) tonalitic (TC), (2) granitic (GC), and (3) augen (AC) suites. Members of the TC follow sodic (characterized by decreasing CaO/Na2O), whereas those of the GC and AC follow calc-alkaline trends of differentiation. Geochemical patterns of the TC resemble those of the Archaean tonalite-trondhjemite-granodiorite (TTG) suites, with slightly magnesian character (average Mg# = 33), moderate LREE (average LaN = 154), low HREE (average YbN = 6) and Y (1-53 ppm; average 11 ppm). The TC is also characterized by positive to slightly negative europium anomalies (Eu/Eu* = 0.7 to 1.67). The GC and AC suites, on the other hand, resemble post-Archaean arc-related granites. The GC displays ferroan nature (average Mg# = 22), low to moderate degrees of REE fractionation (average [La/Yb]N = 34.84), high contents of Y (5-128 ppm; average 68), and low Sr/Y (1-98) ratios. Significant negative Eu anomalies (Eu/Eu* = 0.18-0.91; average 0.50) and low Sr (65-690 ppm) are also noted in the GC. Similar chemical characteristics are shown by the AC, with ferroan nature (average Mg# = 21), low to moderate degrees of REE fractionation (average [La/Yb]N = 26), high

  6. Deep and persistent melt layer in the Archaean mantle

    Science.gov (United States)

    Andrault, Denis; Pesce, Giacomo; Manthilake, Geeth; Monteux, Julien; Bolfan-Casanova, Nathalie; Chantel, Julien; Novella, Davide; Guignot, Nicolas; King, Andrew; Itié, Jean-Paul; Hennet, Louis

    2018-02-01

    The transition from the Archaean to the Proterozoic eon ended a period of great instability at the Earth's surface. The origin of this transition could be a change in the dynamic regime of the Earth's interior. Here we use laboratory experiments to investigate the solidus of samples representative of the Archaean upper mantle. Our two complementary in situ measurements of the melting curve reveal a solidus that is 200-250 K lower than previously reported at depths higher than about 100 km. Such a lower solidus temperature makes partial melting today easier than previously thought, particularly in the presence of volatiles (H2O and CO2). A lower solidus could also account for the early high production of melts such as komatiites. For an Archaean mantle that was 200-300 K hotter than today, significant melting is expected at depths from 100-150 km to more than 400 km. Thus, a persistent layer of melt may have existed in the Archaean upper mantle. This shell of molten material may have progressively disappeared because of secular cooling of the mantle. Crystallization would have increased the upper mantle viscosity and could have enhanced mechanical coupling between the lithosphere and the asthenosphere. Such a change might explain the transition from surface dynamics dominated by a stagnant lid on the early Earth to modern-like plate tectonics with deep slab subduction.

  7. Stable isotope composition and volume of Early Archaean oceans

    DEFF Research Database (Denmark)

    Pope, Emily Catherine; Rosing, Minik Thorleif; Bird, Dennis K.

    via biogenic methanogenesis [2]. Mass balance considerations within the Earth system places a cumulative upper limit on elemental hydrogen loss to space of ~1.8x1022mol elemental hydrogen H, constraining maximum Archaean atmospheric methane levels at ~3.8Ga to

  8. the origin of late archaean granitoids in the sukumaland greenstone

    African Journals Online (AJOL)

    Mgina

    melting at the base of a late Archaean thickened sub-arc basaltic crust. Melting to form the Suite 1 granitoids occurred in the eclogite stability field whereas Suite 2 formed by melting at shallower ... of TTG in terms of slab melting processes.

  9. Archaean Gold Mineralization in an Extensional Setting: The Structural History of the Kukuluma and Matandani Deposits, Geita Greenstone Belt, Tanzania

    Directory of Open Access Journals (Sweden)

    Shimba D. Kwelwa

    2018-04-01

    Full Text Available Three major gold deposits, Matandani, Kukuluma, and Area 3, host several million ouncez (Moz of gold, along a ~5 km long, WNW trend in the E part of the Geita Greenstone Belt, NW Tanzania. The deposits are hosted in Archaean volcanoclastic sediment and intrusive diorite. The geological evolution of the deposits involved three separate stages: (1 an early stage of syn-sedimentary extensional deformation (D1 around 2715 Ma; (2 a second stage involving overprinting ductile folding (D2–4 and shearing (D5–6 events during N-S compression between 2700 and 2665 Ma, coeval with the emplacement of the Kukuluma Intrusive Complex; and (3 a final stage of extensional deformation (D7 accommodated by minor, broadly east-trending normal faults, preceded by the intrusion of felsic porphyritic dykes at ~2650 Ma. The geometry of the ore bodies at Kukuluma and Matandani is controlled by the distribution of magnetite-rich meta-ironstone, near the margins of monzonite-diorite bodies of the Kukuluma Intrusive Complex. The lithological contacts acted as redox boundaries, where high-grade mineralization was enhanced in damage zones with higher permeability, including syn-D3 hydrothermal breccia, D2–D3 fold hinges, and D6 shears. The actual mineralizing event was syn-D7, and occurred in an extensional setting that facilitated the infiltration of mineralizing fluids. Thus, whilst gold mineralization is late-tectonic, ore zone geometries are linked to older structures and lithological boundaries that formed before gold was introduced. The deformation-intrusive history of the Kukuluma and Matandani deposits is near identical to the geological history of the world-class Nyankanga and Geita Hill deposits in the central part of the Geita Greenstone Belt. This similarity suggests that the geological history of much of the greenstone belt is similar. All major gold deposits in the Geita Greenstone Belt lack close proximity to crustal-scale shear zones; they are associated

  10. Crustal contamination and crystal entrapment during polybaric magma evolution at Mt. Somma-Vesuvius volcano, Italy: Geochemical and Sr isotope evidence

    Science.gov (United States)

    Piochi, M.; Ayuso, R.A.; de Vivo, B.; Somma, R.

    2006-01-01

    New major and trace element analyses and Sr-isotope determinations of rocks from Mt. Somma-Vesuvius volcano produced from 25 ky BP to 1944 AD are part of an extensive database documenting the geochemical evolution of this classic region. Volcanic rocks include silica undersaturated, potassic and ultrapotassic lavas and tephras characterized by variable mineralogy and different crystal abundance, as well as by wide ranges of trace element contents and a wide span of initial Sr-isotopic compositions. Both the degree of undersaturation in silica and the crystal content increase through time, being higher in rocks produced after the eruption at 472 AD (Pollena eruption). Compositional variations have been generally thought to reflect contributions from diverse types of mantle and crust. Magma mixing is commonly invoked as a fundamental process affecting the magmas, in addition to crystal fractionation. Our assessment of geochemical and Sr-isotopic data indicates that compositional variability also reflects the influence of crustal contamination during magma evolution during upward migration to shallow crustal levels and/or by entrapment of crystal mush generated during previous magma storage in the crust. Using a variant of the assimilation fractional crystallization model (Energy Conservation-Assimilation Fractional Crystallization; [Spera and Bohrson, 2001. Energy-constrained open-system magmatic processes I: General model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation. J. Petrol. 999-1018]; [Bohrson, W.A. and Spera, F.J., 2001. Energy-constrained open-system magmatic process II: application of energy-constrained assimilation-fractional crystallization (EC-AFC) model to magmatic systems. J. Petrol. 1019-1041]) we estimated the contributions from the crust and suggest that contamination by carbonate rocks that underlie the volcano (2 km down to 9-10 km) is a fundamental process controlling magma compositions at Mt. Somma

  11. The Imaging and Evolution of Seismic Layer 2A Thickness from a 0-70 Ma Oceanic Crustal Transect in the South Atlantic

    Science.gov (United States)

    Estep, J. D.; Reece, R.; Kardell, D. A.; Christeson, G. L.; Carlson, R. L.

    2017-12-01

    Seismic layer 2A, the uppermost igneous portion of oceanic crust, is commonly used to refer to the seismic velocities of upper crust that are bounded below by a steep vertical velocity gradient. Layer 2A velocities are known to increase with crustal age, from 2.5 km/s in crust 15 Ma. Thickness of layer 2A has been shown to increase by a factor of 2 within 1 Ma at fast spreading ridges and then remain relatively constant, while layer 2A maintains a fairly consistent thickness, irrespective of age, at slow-intermediate spreading ridges. Layer 2A thickness and velocity evolution studies to date have been largely focused on young oceanic crust very proximal to a spreading center with little investigation of changes (or lack thereof) that occur at crustal ages >10 Ma. We utilize a multichannel seismic dataset collected at 30° S in the western South Atlantic that continuously images 0 - 70 Ma oceanic crust along a single flowline generated at the slow-intermediate spreading Mid-Atlantic Ridge. We follow the methods of previous studies by processing the data to image the layer 2A event, which is then used for calculating thickness. 1D travel time forward modeling at regularly spaced age intervals across the transect provides for the conversion of time to depth thickness, and for determining the evolution of velocities with age. Our results show layer 2A in 20 Ma crust is roughly double the thickness of that in crust 0-5 Ma (830 vs. 440 m), but thickness does not appear to change beyond 20 Ma. The layer 2A event is readily observable in crust 0-50 Ma, is nearly completely absent in crust 50-65 Ma, and then reappears with anomalously high amplitude and lateral continuity in crust 65-70 Ma. Our results suggest that layer 2A thickens with age at the slow-intermediate spreading southern Mid-Atlantic Ridge, and that layer 2A either continues to evolve at the older crustal ages, well beyond the expected 10-15 Ma "mature age", or that external factors have altered the crust at

  12. Crustal nature and origin of the Russian Altai: Implications for the continental evolution and growth of the Central Asian Orogenic Belt (CAOB)

    Science.gov (United States)

    Cai, Keda; Sun, Min; Buslov, M. M.; Jahn, Bor-ming; Xiao, Wenjiao; Long, Xiaoping; Chen, Huayong; Wan, Bo; Chen, Ming; Rubanova, E. S.; Kulikova, A. V.; Voytishek, E. E.

    2016-04-01

    The Central Asian Orogenic Belt is a gigantic tectonic collage of numerous accreted terranes. However, its geodynamic evolution has been hotly debated primarily due to incomplete knowledge on the nature of these enigmatic terranes. This work presents new detrital zircon U-Pb and Hf isotopic data to constrain the crustal nature and origin of the Russian Altai, a critical segment of Altai-Mongolian terrane. The youngest zircon 206Pb/238U ages of 470 Ma constrain that the Terekta Formation, previously envisaged as Precambrian basement, was actually deposited after the Middle Ordovician. As for the three more sedimentary sequences above the Terekta Formation, they have youngest zircon 206Pb/238U ages of 425 Ma, 440 Ma and 380 Ma, respectively, indicating their depositions likely in the Late Silurian to Devonian. From all analyses, it is noted that many zircon U-Pb ages cluster at ca. 520 Ma and ca. 800 Ma, and these zircons display oscillatory zoning and have subhedral to euhedral morphology, which, collectively, suggests that adjacent Neoproterozoic to Paleozoic igneous rocks were possibly dominant in the sedimentary provenance. Additionally, a few rounded Archean to Mesoproterozoic zircon grains are characterized by complex texture, which are interpreted as recycling materials probably derived from the Tuva-Mongolian microcontinent. Precambrian rocks have not been identified in the Russian Altai, Chinese Altai and Mongolian Altai so far, therefore, Precambrian basement may not exist in the Altai-Mongolian terrane, but this terrane probably represents a large subduction-accretion complex built on the margin of the Tuva-Mongolian microcontinent in the Early Paleozoic. Multiple episodes of ridge-trench interaction may have caused inputs of mantle-derived magmas to trigger partial melting of the newly accreted crustal materials, which contributed to the accretionary complex. During accretionary orogenesis of the CAOB, formation of such subduction-accretion complex is

  13. Neutron star crustal plate tectonics. I. Magnetic dipole evolution in millisecond pulsars and low-mass X-ray binaries

    International Nuclear Information System (INIS)

    Ruderman, M.

    1991-01-01

    Crust lattices in spinning-up or spinning-down neutron stars have growing shear stresses caused by neutron superfluid vortex lines pinned to lattice nuclei. For the most rapidly spinning stars, this stress will break and move the crust before vortex unpinning occurs. In spinning-down neutron stars, crustal plates will move an equatorial subduction zone in which the plates are forced into the stellar core below the crust. The opposite plate motion occurs in spinning-up stars. Magnetic fields which pass through the crust or have sources in it move with the crust. Spun-up neutron stars in accreting low-mass X-ray binaries LMXBs should then have almost axially symmetric magnetic fields. Spun-down ones with very weak magnetic fields should have external magnetic fields which enter and leave the neutron star surface only near its equator. The lowest field millisecond radiopulsars seem to be orthogonal rotators implying that they have not previously been spun-up in LMXBs but are neutron stars initially formed with periods near 0.001 s that subsequently spin down to their present periods. Accretion-induced white dwarf collapse is then the most plausible genesis for them. 29 refs

  14. Studies on Archaean Dharwar Tectonic Province 433

    Indian Academy of Sciences (India)

    age data on the geological evolution of southern India; Proc. workshop on deep continental crust of. South India, LPI Tech. Tept. No. 88-06, pp. 181-183. Viswanatha M. N, Ramakrishnan M and Swami Nath J 1982 Angular unconformity between Sargur and. Dharwar supracrustals in Shigegudda, Karnataka cration, South ...

  15. Crustal permeability

    Science.gov (United States)

    Gleeson, Tom; Ingebritsen, Steven E.

    2016-01-01

    Permeability is the primary control on fluid flow in the Earth’s crust and is key to a surprisingly wide range of geological processes, because it controls the advection of heat and solutes and the generation of anomalous pore pressures.  The practical importance of permeability – and the potential for large, dynamic changes in permeability – is highlighted by ongoing issues associated with hydraulic fracturing for hydrocarbon production (“fracking”), enhanced geothermal systems, and geologic carbon sequestration.  Although there are thousands of research papers on crustal permeability, this is the first book-length treatment.  This book bridges the historical dichotomy between the hydrogeologic perspective of permeability as a static material property and the perspective of other Earth scientists who have long recognized permeability as a dynamic parameter that changes in response to tectonism, fluid production, and geochemical reactions. 

  16. Geochronology and geochemistry of deep-seated crustal xenoliths in the northern North China Craton: Implications for the evolution and structure of the lower crust

    Science.gov (United States)

    Su, Yuping; Zheng, Jianping; Griffin, William L.; Huang, Yan; Wei, Ying; Ping, Xianquan

    2017-11-01

    The age and composition of the lower crust are critical in understanding the processes of continental formation and evolution, and deep-seated granulite xenoliths can offer direct information on the lower crust. Here, we report mineral chemistry, whole-rock major and trace elements, Sr-Nd isotopes and zircon U-Pb-Hf results for a suite of deep-seated crustal xenoliths, recently discovered in the Cenozoic basalts of the Nangaoya area in the northern part of the North China Craton (NCC). Based on the P-T estimates, these xenoliths including mafic, intermediate and felsic granulites and hornblendites were sampled from different levels of the lower crust. While a hornblendite has a flat REE pattern, all other xenoliths display LREE enrichment and depletion of Nb, Ta, Th and Ti. The mafic granulite xenolith has relatively high whole-rock εNd(t) value of - 13.37, and yields Mesozoic (188-59 Ma) zircons ages with high εHf(t) values from - 15.3 to - 9.2. The garnet-bearing intermediate granulite-facies rocks show low εNd(t) values from - 16.92 to - 17.48, and reveal both Paleoproterozoic (1948 Ma) and Mesozoic (222-63 Ma) zircon U-Pb ages. Their Mesozoic zircons have lower εHf(t) values (from - 18.4 to - 13.8) than those from the mafic xenolith. The remaining intermediate to felsic xenoliths show Paleoproterozoic zircon ages, and the lowest εNd(t) values (from - 20.78 to - 24.03). The mafic-intermediate granulites with Mesozoic zircons originated from the interaction of lower crust-derived magmas with mantle melts, with higher proportions of mantle magmas involved in the generation of mafic granulite, whereas intermediate to felsic xenoliths without Mesozoic zircons represent ancient Paleoproterozoic to Neoarchean deep crust. These deep-seated xenoliths reveal complicated crustal evolution processes, including crustal growth during Neoarchean (2.5-2.7 Ga), middle Paleoproterozoic (2.2-2.1 Ga) and Mesozoic, and reworking during early Paleoproterozoic, late

  17. Petrography and geochemistry of the Javaherdasht basalts (east of Guilan Province): The investigation of the role of crystal fractionation and crustal contamination in the magmatic evolution

    International Nuclear Information System (INIS)

    Haghnazar, Sh.; Malakotian, S.

    2009-01-01

    The Javaherdasht Basalts show compositional range from olivine basalts to quartz basaltic andesites. Petrographic studies indicate that the differentiation of clinopyroxene and olivine minerals has main role for lithologic variety of the basalts. The corrosion golf, crenated margins and lack of the same colour in the clinopyroxene phenochrysts margins with matrix Pyroxene grains express a nonequilibrium and are petrographic features for crustal contamination of the basalts.The positive correlation Ca O,Ca O/Al 2 O 3 and Cr with Mg and Ca O/Al 2 O 3 with Sc and the negative correlation Al 2 O 3 with Mg are geochemical characters for the differentiation of clinopyroxene and olivine in the magmatic evolution of the area.The high ratios of Ba/Zr and Pb/Nd and law ratio of Ce/Pb and positive correlation of SiO 2 and Rb with 87 Sr/ 86 Sr and negative correlation of Nd-Sr isotopes display the contamination of these basalts with continental crust.

  18. Provenance and tectonic setting of siliciclastic rocks associated with the Neoproterozoic Dahongliutan BIF: Implications for the Precambrian crustal evolution of the Western Kunlun orogenic belt, NW China

    Science.gov (United States)

    Hu, Jun; Wang, He; Wang, Min

    2017-10-01

    The Late Neoproterozoic Dahongliutan BIF is associated with siliciclastic rocks in the Tianshuihai terrane of the Western Kunlun orogenic belt (WKO), NW China. The sedimentary rocks have various weathering indices (e.g., CIA = 57-87, PIA = 61-96 and Th/U = 4.85-12.45), indicative of varying degrees of weathering in the source area. The rocks have trace element ratios, such as Th/Sc = 0.60-1.21 and Co/Th = 0.29-1.67, and light rare earth element (LREE) enriched chondrite-normalized REE patterns, suggesting that they were mainly sourced from intermediate and felsic rocks. Available U-Pb ages of detrital zircon from these rocks reveal that the detrital sources may have been igneous and metamorphic rocks from the WKO and the Tarim Block. Our study suggests that the Dahongliutan BIF and hosting siliciclastic rocks may have deposited in a setting transitional from a passive to active continental margin, probably related to the Late Neoproterozoic-Early Cambrian seafloor spreading and subduction of the Proto-Tethys Ocean. U-Pb dating of 163 detrital zircons defines five major age populations at 2561-2329 Ma, 2076-1644 Ma, 1164-899 Ma, 869-722 Ma and 696-593 Ma. These age groups broadly correspond to the major stages of supercontinent assembly and breakup events widely accepted for Columbia, Rodinia and Gondwana. Some zircons have TDM2 model ages of 3.9-1.8 Ga and negative εHf(t) values, suggesting that the Archean to Paleoproterozoic (as old as Eoarchean) crustal materials were episodically reworked and incorporated into the late magmatic process in the WKO. Some Neoproterozoic zircons have TDM2 model ages of 1.47-1.07 Ga and 1.81-1.53 Ga and positive εHf(t) values, indicating juvenile crustal growth during the Mesoproterozoic. Our new results, combined with published data, imply that both the Tianshuihai terrane in the WKO and the Tarim Block share the same Precambrian tectonic evolution history.

  19. Crustal structure and evolution of the NW Zagros Mountains (Iran): Insights from numerical modeling of the interplay between surface and tectonic processes

    Science.gov (United States)

    Saura, Eduard; Garcia-Castellanos, Daniel; Casciello, Emilio; Vergés, Jaume

    2014-05-01

    Protracted Arabia-Eurasia convergence resulted in the closure of the >2000 km wide Neo-Tethys Ocean from early Late Cretaceous to Recent. This process was controlled by the structure of the NE margin of the Arabian plate, the NE-dipping oceanic subduction beneath Eurasia, the obduction of oceanic lithosphere and the collision of small continental and volcanic arc domains of the SW margin of Eurasia. The evolution of the Zagros Amiran and Mesopotamian foreland basins is studied in this work along a ~700 km long transect in NW Zagros constrained by field, seismic and published data. We use the well-defined geometries and ages of the Amiran and Mesopotamian foreland basins to estimate the elastic thickness of the lithosphere and model the evolution of the deformation to quantitatively link the topographic, tectonic and sedimentary evolution of the system. Modelling results show two major stages of emplacement. The obduction (pre-collision) stage involves the thin thrust sheets of the Kermanshah complex together with the Bisotun basement. The collision stage corresponds to the emplacement of the basement duplex and associated crustal thickening, coeval to the out of sequence emplacement of Gaveh Rud and Imbricated Zone in the hinterland. The geodynamic model is consistent with the history of the foreland basins, with the regional isostasy model, and with a simple scenario for the surface process efficiency. The emplacement of Bisotun basement during obduction tectonically loaded and flexed the Arabian plate triggering deposition in the Amiran foreland basin. The basement units emplaced during the last 10 My, flexed the Arabian plate below the Mesopotamian basin. During this stage, material eroded from the Simply Folded belt and the Imbricated zone was not enough to fill the Mesopotamian basin, which, according to our numerical model results, required a maximum additional sediment supply of 80 m/Myr. This additional supply had to be provided by an axial drainage system

  20. Crustal evolution of granitoids and gneisses from the Cambaizinho belt, southern Brazil: Review zircon Pb-Pb evaporation ages and Pb-Nd-Sr isotopes

    International Nuclear Information System (INIS)

    Remus, M.V.D; Macambira, M.B; Hartmann, L.A.; Beilfuss, M

    2001-01-01

    Deformed granitoids and gneisses from the Cambai Complex (900-700 Ma) along Cambaizinho Creek and in the Vila Nova do Sul region, state of Rio Grande do Sul, Brazil, were formed in a remarkably short time, about 10 m.y., between 704±13 and 697±3 Ma. The data base of this work includes eighteen zircon Pb/Pb evaporation analyses, five Pb isotope in feldspar and whole rock. The oldest known rocks in the region are polydeformed dioritic gneisses dated by conventional U-Pb zircon at 704±13 Ma. New Pb-Pb zircon evaporation data on the late transcurrent, less deformed and more evolved granitoids (Sanga do Jobim Granitoids) yield a 697± Ma age and indicates that the evolution of the plutonic magmatism in the area was nearly contemporaneous. These data contrast with previous interpretations based on Rb-Sr data which considered that these rock associations were formed during a longer time period (700-640 Ma). All these granitoids intruded the supracrustal sequence. These granitoids yield a minimum age of about 700 Ma for the formation of the supracrustal sequence and its regional dynamothermal metamorphism. Lead isotope composition of K-feldspar from Sanga do Jobim Granitoids plot close to, but slightly below the lead isotope evolution curve of orogeny in the Zartmann and Doe model (1981). This indicates that the setting for these granitoids was that of a juvenile magmatic arc. These new data plus previous data in the region also corroborate that the crustal evolution involved juvenile crust accreted between 760-700 Ma. In contrast, the Cacapava and Sao Sepe Granites intruded the supracrustal sequences along the eastern side of the Sao Gabriel Block at 562 Ma and 550 Ma, respectively, and show Pb and Nd isotope signatures from an old basement. This evidence suggests that the juvenile terrane was thrusted over the older basement situated along the eastern part of the shield during the Dom Feliciano collisional orogeny at about 620-590 Ma (au)

  1. Active tectonics and drainage evolution in the Tunisian Atlas driven by interaction between crustal shortening and slab pull

    Science.gov (United States)

    Camafort, Miquel; Booth-Rea, Guillermo; Pérez-Peña, Jose Vicente; Melki, Fetheddine; Gracia, Eulalia; Azañón, Jose Miguel; Ranero, César R.

    2017-04-01

    Active tectonics in North Africa is fundamentally driven by NW-SE directed slow convergence between the Nubia and Eurasia plates, leading to a region of thrust and strike-slip faulting. In this paper we analyze the morphometric characteristics of the little-studied northern Tunisia sector. The study aimed at identifying previously unknown active tectonic structures, and to further understand the mechanisms that drive the drainage evolution in this region of slow convergence. The interpretation of morphometric data was supported with a field campaign of a selection of structures. The analysis indicates that recent fluvial captures have been the main factor rejuvenating drainage catchments. The Medjerda River, which is the main catchment in northern Tunisia, has increased its drainage area during the Quaternary by capturing adjacent axial valleys to the north and south of its drainage divide. These captures are probably driven by gradual uplift of adjacent axial valleys by reverse/oblique faults or associated folds like El Alia-Teboursouk and Dkhila faults. Our fieldwork found that these faults cut Holocene colluvial fans containing seismites like clastic dikes and sand volcanoes, indicating recent seismogenic faulting. The growth and stabilization of the axial Medjerda River against the natural tendency of transverse drainages might be caused by a combination of dynamic topography and transpressive tectonics. The orientation of the large axial Medjerda drainage that runs from eastern Algeria towards northeastern Tunisia into the Gulf of Tunis, might be the associated to negative buoyancy caused by the underlying Nubia slab at its mouth, together with uplift of the Medjerda headwaters along the South Atlassic dextral transfer zone.

  2. High-spatial resolution SIMS U-Pb zircon dating on Malay tin granites: New insights to crustal evolution of the Malaysian Peninsula

    Science.gov (United States)

    Ng, S.; Searle, M. P.; Whitehouse, M. J.; Chung, S.; Robb, L. J.; Ghani, A. A.; Sone, M.

    2012-12-01

    characteristics, such as in sodium, potassium, ferric and ferrous ion concentration. Exceptionally high rubidium concentration in some samples also hinders the classification. The ɛNd(T) values of most of the samples lie between -2.5 and -9.7, suggesting that crustal remelting played an important role in the granite petrogenesis. Likewise tin mineralization is not restricted to S-type granites throughout all provinces. Further field studies and continuation of the U-Pb dating programme through southern Malaysia to Singapore Island should result in tighter constraints on the magmatic and tectonic evolution of the region. References Cobbing, E. J., Mallick, D. I. J., Pitfield, P. E. J., and Teoh, L. H., 1986, The granites of the Southeast Asian Tin Belt: Journal of Geological Society, v. 143, p. 537-550. Searle, M. P., Whitehouse, M. J., Robb, L. J., Ghani, A. A., Hutchison, C. S., Sone, M., Ng, S. W.-p., Roselee, M. H., Chung, S.-l., and Oliver, G. J. H., 2012, Tectonic evolution of the Sibumasu-Indochina terrane collision zone in Thailand and Malaysia; constraints from new U-Pb zircon chronology of SE Asian tin granitoids: Journal of Geological Society, v. 169, p. 489-500.

  3. First record of 1.2 Ga quartz dioritic magmatism in the Archaean Yilgarn Craton, Western Australia, and its significance

    Energy Technology Data Exchange (ETDEWEB)

    Qiu, Y.; McNaughton, N.J.; Groves, D.I.; Dunphy, J.M. [University of Western Australia, Nedlands, WA (Australia). Centre for Strategic Mineral Deposits, Department of Geology and Geophysics

    1999-06-01

    Ion microprobe (SHRIMP) U-Pb zircon dating. Pb-Nd isotope tracer studies and major, trace and rare-earth element analyses have identified, for the first time. a Mesoproterozoic (1.2 Ga) quartz diorite intrusion in the central part of the Archaean Yilgarn Craton. Western Australia The quartz diorite is characterised by intergrowths of quartz and plagioclase, having low A/CNK (0 8). low K{sub 2}O (0.28 wt%), Ba (54 ppm), Rb (11 ppm), Sr (92 ppm), Pb (13 ppm), U (1.7 ppm) and Th (6 ppm) contents, high Nd (41 ppm), Sm (10.5 ppm), Zr (399 ppm). Nb (18.5 ppm). Y (57.5 ppm) and Sc (19 ppm) contents. a low Pb isotope two-stage model {mu} value (6.3), and a primitive initial e{sub Nd} value (+3.4) at 1.2 Ga. It is interpreted that the 1.2 Ga quartz diorite was derived from a predominantly mantle source, with minor crustal contamination, possibly from the surrounding Archaean monzogranites or their source region, during magma ascent. The age (1215 {+-} 11 Ma) of the intrusion overlaps with the timing of a major continental collisional orogeny in the Albany-Fraser Orogen, about 400km south, and is broadly coeval with the diamond-bearing Argyle lamproites in the east Kimberley Block. This study extends the history of granitoid magmatism in the central craton by more than 1.0 billion years (2.6 to 1.2 Ga), and has implications for isotopic data interpretations of tectonothermal events in the craton. Copyright (1999) Blackwell Science Pty Ltd 33 refs., 3 tabs., 6 figs.

  4. Shear Wave Velocity Structure of Southern African Crust: Evidence for Compositional Heterogeneity within Archaean and Proterozoic Terrains

    Energy Technology Data Exchange (ETDEWEB)

    Kgaswane, E M; Nyblade, A A; Julia, J; Dirks, P H H M; Durrheim, R J; Pasyanos, M E

    2008-11-11

    Crustal structure in southern Africa has been investigated by jointly inverting receiver functions and Rayleigh wave group velocities for 89 broadband seismic stations spanning much of the Precambrian shield of southern Africa. 1-D shear wave velocity profiles obtained from the inversion yield Moho depths that are similar to those reported in previous studies and show considerable variability in the shear wave velocity structure of the lower part of the crust between some terrains. For many of the Archaean and Proterozoic terrains in the shield, S velocities reach 4.0 km/s or higher over a substantial part of the lower crust. However, for most of the Kimberley terrain and adjacent parts of the Kheis Province and Witwatersrand terrain, as well as for the western part of the Tokwe terrain, mean shear wave velocities of {le} 3.9 km/s characterize the lower part of the crust along with slightly ({approx}5 km) thinner crust. These findings indicate that the lower crust across much of the shield has a predominantly mafic composition, except for the southwest portion of the Kaapvaal Craton and western portion of the Zimbabwe Craton, where the lower crust is intermediate-to-felsic in composition. The parts of the Kaapvaal Craton underlain by intermediate-to-felsic lower crust coincide with regions where Ventersdorp rocks have been preserved, and thus we suggest that the intermediate-to-felsic composition of the lower crust and the shallower Moho may have resulted from crustal melting during the Ventersdorp tectonomagmatic event at c. 2.7 Ga and concomitant crustal thinning caused by rifting.

  5. Recent crustal movements

    Science.gov (United States)

    Maelzer, H.

    Calculation of temporal height changes for the determination of recent vertical crustal movements in northern, western, and southern Germany is described. Precise geodetic measurements and their analysis for the determination of recent crustal movements in north-eastern Iceland, western Venezuela, and central Peru are described. Determination of recent vertical crustal movements by leveling and gravity data; geodetic modeling of deformations and recent crustal movements; geodetic modeling of plate motions; and instrumental developments in geodetic measuring are discussed.

  6. Rifting an Archaean Craton: Insights from Seismic Anisotropy Patterns in E. Africa

    Science.gov (United States)

    Ebinger, C. J.; Tiberi, C.; Currie, C. A.; van Wijk, J.; Albaric, J.

    2016-12-01

    Few places worldwide offer opportunities to study active deformation of deeply-keeled cratonic lithosphere. The magma-rich Eastern rift transects the eastern edge of the Archaean Tanzania craton in northeastern Tanzania, which has been affected by a large-scale mantle upwelling. Abundant xenolith locales offer constraints on mantle age, composition, and physical properties. Our aim is to evaluate models for magmatic fluid-alteration (metasomatism) and deformation of mantle lithosphere along the edge of cratons by considering spatial variations in the direction and magnitude of seismic anisotropy, which is strongly influenced by mantle flow patterns along lithosphere-asthenosphere topography, fluid-filled cracks (e.g., dikes), and pre-existing mantle lithosphere strain fabrics. Waveforms of teleseismic earthquakes (SKS, SKKS) recorded on the 39-station CRAFTI-CoLiBREA broadband array in southern Kenya and northern Tanzania are used to determine the azimuth and amount of shear-wave splitting accrued as seismic waves pass through the uppermost mantle and lithosphere at the craton edge. Lower crustal earthquakes enable evaluation of seismic anisotropy throughout the crust along the rift flanks and beneath the heavily intruded Magadi and Natron basins, and the weakly intruded Manyara basin. Our results and those of earlier studies show a consistent N50E splitting direction within the craton, with delay times of ca. 1.5 s, and similar direction east of the rift in thinner Pan-African lithosphere. Stations within the rift zone are rotated to a N15-35E splitting, with the largest delay times of 2.5 s at the margin of the heavily intruded Magadi basin. The short length scale of variations and rift-parallel splitting directions are similar to patterns in the Main Ethiopian rift attributed to melt-filled cracks or oriented pockets rising from the base of the lithosphere. The widespread evidence for mantle metasomatism and magma intrusion to mid-crustal levels suggests that

  7. In situ isotopic analyses of U and Pb in zircon by remotely operated SHRIMP II, and Hf by LA-ICP-MS: an example of dating and genetic evolution of zircon by {sup 176}Hf/{sup 177}Hf from the Ita Quarry in the Atuba Complex, SE, Brazil; Analises in situ de U e Pb em zircao por SRIMP II por controle remoto e de Hf por LA-ICP-MS: um exemplo de datacao e da evolucao genetica de zircao atraves da razao {sup 176}Hf/{sup 177} em amostra da Pedreira Ita no Complexo Atuba, SE, Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Sato, K.; Siga Junior, Oswaldo; McReath, Ian; Sproesser, Walter; Basei, Miguel Angelo Stipp [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias. Centro de Pesquisas Geocronologicas], e-mail: keisato@usp.br, e-mail: osigajr@usp.br, e-mail: ianmcr@usp.br, e-mail: wmspres@usp.br, e-mail: baseimas@usp.br; Silva, Josiane Aline da [Universidade de Sao Paulo (USP), SP (Brazil). Programa de Pos-graduacao em Geoquimica e Geotectonica; Dunyi, Liu [Institute of Geology, Beijing (China); Iizuka, Takafumi; Rino, Shuji; Hirata, Takafumi [Tokyo Institute of Technology, Tokyo (Japan)

    2009-10-15

    Remotely-operated SHRIMP dating of zircon is an interesting alternative for dating of zircon crystals. Although it does not represent any technical progress of the geochronological method using the U-Pb system in zircon it is a very useful and cheap facility. The procedure was first used for mass spectrometric analyses involving two international laboratories in Sao Paulo, Brazil and Beijing, China. It was applied to samples of three gneiss-migmatitic rocks from the Ita quarry in the Atuba Complex (located between the Luis Alves and the Apiai Domain) to test previous controversial hypotheses about its evolution. The presence of important archaean and paleo proterozoic components in the complex is confirmed by analyses of zircon found in probably neo proterozoic leucosomes. Diorite intrusion also occurred during the neo proterozoic, associated with the 0.6Ga continental collisions involved in the assembly of Gondwana. The determination of Hf isotope ratios by LA-ICP/MS represents a new option for checking the relative importance of mantle ({epsilon}{sub Hf} > 0) and crustal contributions (({epsilon}{sub Hf} < 0) during the growth of the zircon crystals. While the archaean component in the complex was derived from the mantle ({epsilon}{sub Hf} + 1.5 to + 8.7) the paleo proterozoic component had a crustal contribution ({epsilon}{sub Hf} - 9.1 to -10.1). (author)

  8. Metal availability and the expanding network of microbial metabolisms in the Archaean eon

    Science.gov (United States)

    Moore, Eli K.; Jelen, Benjamin I.; Giovannelli, Donato; Raanan, Hagai; Falkowski, Paul G.

    2017-09-01

    Life is based on energy gained by electron-transfer processes; these processes rely on oxidoreductase enzymes, which often contain transition metals in their structures. The availability of different metals and substrates has changed over the course of Earth's history as a result of secular changes in redox conditions, particularly global oxygenation. New metabolic pathways using different transition metals co-evolved alongside changing redox conditions. Sulfur reduction, sulfate reduction, methanogenesis and anoxygenic photosynthesis appeared between about 3.8 and 3.4 billion years ago. The oxidoreductases responsible for these metabolisms incorporated metals that were readily available in Archaean oceans, chiefly iron and iron-sulfur clusters. Oxygenic photosynthesis appeared between 3.2 and 2.5 billion years ago, as did methane oxidation, nitrogen fixation, nitrification and denitrification. These metabolisms rely on an expanded range of transition metals presumably made available by the build-up of molecular oxygen in soil crusts and marine microbial mats. The appropriation of copper in enzymes before the Great Oxidation Event is particularly important, as copper is key to nitrogen and methane cycling and was later incorporated into numerous aerobic metabolisms. We find that the diversity of metals used in oxidoreductases has increased through time, suggesting that surface redox potential and metal incorporation influenced the evolution of metabolism, biological electron transfer and microbial ecology.

  9. The geology of the Vaalputs radioactive waste disposal site, with implications for granite-charnockite relationships and crustal evolution in western Namaqualand

    International Nuclear Information System (INIS)

    Andreoli, M.A.G.; Brynard, H.J.; Anderson, N.J.B.; Hart, R.J.; Moore, J.M.; Welke, H.

    1990-01-01

    The Precambrian geology of the Valputs site is characterized by extensive sheets of Spektakel Suite syntectonic granite gneisses. Experimental methods used for the geological evaluation of the Vaalputs site included semi-regional and site-specific mapping. All main rock types were petrologically and geochemically classified. In addition, preliminary Rb-Sr whole-rock ages were calculated for the main lithotypes, together with Sm-Nd dating. The investigations for the licensing of the Vaalputs site have revealed numerous intriguing aspects of the crustal history in southwestern Namaqualand. 10 refs

  10. A geochemical and Pb, Sr isotopic study of the evolution of granite-gneisses from the Bastar craton, Central India

    International Nuclear Information System (INIS)

    Sarkar, G.; Paul, D.K.; Misra, V.P.; de Laeter, J.R.; Mc Naughton, N.J.

    1990-01-01

    Preliminary Pb-Pb and Rb-Sr geochronology of granitic and gneissic rocks from the Sukma area of the Bastar craton, Central India, provides important constraints on crustal evolution. Much of the craton is made up of felsic orthogneisses and younger granitic intrusives, compositionally ranging from tonalite to granite. Pb-Pb isotopic data suggest the presence of ca. 3.0 Ga old gneisses. Younger granitic intrusives have been dated at ca. 2.6 Ga which represents a widespread resetting and/or emplacement event. Comparison of the Pb-Pb and Rb-Sr whole rock ages suggests that the latter were more perturbed after the gneiss-forming or emplacement events. All rock suites show significant geological scatter of isotopic data probably because of sampling on a regional scale, and reflect multi-stage isotopic evolution in a complex terrain. The present isotopic data indicate the presence of Archaean rock in the Bastar craton and suggest temporal similarity with the oldest crustal rocks in the Singhbhum and Dharwar cratons. (author). 18 refs., 4 tabs., 8 figs

  11. REE, Sm-Nd and U-Pb zircon study of eclogites from the Alpine External Massifs (Western Alps): Evidence for crustal contamination

    International Nuclear Information System (INIS)

    Paquette, J.L.; Eidgenoessische Technische Hochschule, Zurich; Menot, R.P.; Peucat, J.J.

    1989-01-01

    A geochemical and geochronological study of the Alpine External Crystalline Massifs (AECM) of Aiguilles Rouges, Belledonne and Argentera was undertaken in order to constrain the geodynamic evolution of this segment of the Variscan foldbelt. Another aim of the study is to characterize the behaviour of isotopic markers, in particular the U-Pb zircon system, under high-grade metamorphic conditions. The whole-rock geochemistry of eclogites and amphibolites was investigated using major and trace element (including the REE) analytical techniques; isotopic studies were performed by application of the Sm-Nd whole-rock and U-Pb zircon methods. In terms of regional geological history, the early development of metamorphic and magmatic activity in the AECM is typical of the extensional tectonic regime observed throughout the Variscan foldbelt during the Cambro-Ordovician (i.e. basic magmatism dated at 475-450 Ma). The composition of the metabasic rocks is closely similar to tholeiites emplaced into thinned continental crust which are generally associated with the initial stages of oceanic rifting. The source regions for these metabasics are characterized by initial ε Nd values between +6 and +8, suggesting depleted mantle sources influenced by a weak crustal component and/or the existence of a metasomatised lithosphere. The multi-stage eclogite-facies metamorphism is dated at 425-395 Ma (i.e. Silurian). An application of the U-Pb method, associated with the artificial abrasion of zircon grains, has led to the recognition of a weak crustal contamination in the metabasic protoliths. This is implied by the Archaean and Lower Proterozoic upper intercepts on Concordia - devoid of geological significance - which reflect the presence of a pre-existing basement to the AECM. (orig./WL)

  12. Early Neoarchaean A-type granitic magmatism by crustal reworking ...

    Indian Academy of Sciences (India)

    29

    marginal part of the Singhbhum craton whose origin and role in crustal evolution are poorly ...... Lu-Hf and Sm-Nd isotope systematics of Archean komatiites; Earth Planet. ..... Association Commission on New Minerals and Mineral Names; Can.

  13. The anatomy of an andesite volcano: A time-stratigraphic study of andesite petrogenesis and crustal evolution at Ruapehu volcano, New Zealand

    DEFF Research Database (Denmark)

    Price, R.C.; Gamble, J.A.; Smith, I.E.M.

    2012-01-01

    Ruapehu, New Zealand’s largest active andesite volcano is located at the southern tip of the Taupo Volcanic Zone (TVZ), the main locus of subduction-related volcanism in the North Island. Geophysical data indicate that crustal thickness transitions from ... Ruapehu. The volcano is built on a basement of Mesozoic metagreywacke and geophysical evidence together with xenoliths contained in lavas indicates that this is underlain by oceanic, meta-igneous lower crust. The present-day Ruapehu edifice has been constructed by a series of eruptive events that produced...... and andesite. Dacite also occurs but only one basalt flow has been identified. There have been progressive changes in the minor and trace element chemistry and isotopic composition of Ruapehu eruptives over time. In comparison with rocks from younger formations, Te Herenga eruptives have lower K2O abundances...

  14. Evolution of the Dharwar Craton: a terrain of early Archean crustal stability, long term orogenic cycles and large scale palaeobiological activity

    International Nuclear Information System (INIS)

    Srinivasan, R.; Naqvi, S.M.

    1988-01-01

    Traceable history of the Dharwar Craton goes back to approximately 3400 m.y. old tonalitic to trondhjemitic fundamental gneiss whose REE composition indicates its derivation from a preexisting basalt which apparently had very short time of crustal residence. The fundamental gneiss is preserved only as rare relicts in the vast gneissic complex of the Indian Peninsula (the Peninsular Gneiss), and as pebbles in the conglomerates of the Archean Dharwar sequence. Study of these relicts, shows evidence of a deformation episode prior to the deposition of the high- and low-grade Dharwar supracrustal sequence. The Dharwar supracrustal sequence is briefly described. Geochemistry of the volcanic and sedimentary rocks of the Dharwar supracrustal belts are examined

  15. Nd isotopes and crustal growth rate

    International Nuclear Information System (INIS)

    Albarede, F.

    1988-01-01

    Sm/Nd isotopic constraints on crustal growth is discussed. In order to constrain Sm/Nd fractionation between continental crust and depleted mantle, an extensive data base of isotopic measurements (assumed to be adequately representative of continental crust) was compiled. The results imply that the evolution of depleted mantles was roughly linear, with no major discontinuities over the course of geologic time. This is different from other determinations of depleting mantle evolution, which show nonlinear behavior. The Sm/Nd evolution lines for continental crust and depleted mantle intersect between 3.8 to 4.0 Ga, which may indicate that the onset of continental growth was later than 4.5 Ga. A mathematical model is described, the results of which imply that time integrated crustal additions from the mantle are about 1.8 to 2.5 cu km/a, whereas crustal subtractions by sediment recycling are about 0.6 to 1.5 cu km/a. This results in a net time integrated crustal growth rate of about 1 cu km/a, which is similar to present day rates determined, for example, by Reymer and Schubert

  16. Magmatism during the accretion of the late Archaean Dharwar Craton (South India): sanukitoids and related rocks in their geological context.

    Science.gov (United States)

    Moyen, J.-F.; Martin, H.; Jayananda, M.; Peucat, J.-J.

    2003-04-01

    The South Indian Dharwar Craton assembled during the late-Archaean (ca. 2.5 Ga). This event was associated with intense granite genesis and emplacement. Based on petrography and geochemistry, 4 main types of late Archaean granitoids were distinguished: (1) Anatectic granites (and diatexites), formed by partial melting of TTG gneisses; (2) Classical TTGs; (3) Sanukitoids, generated by interaction between slab melts (TTG) and mantle peridotite; (4) The high HFSE Closepet granite, interpreted as derived from partial melting of a mantle metasomatized by slab melts (TTG). While the 3 later groups all are interpreted as resulting from slab melt/mantle wedge interactions, their differences are related to decreasing felsic melt/peridotite ratios during the ascent “slab melts” in the mantle wedge above an active subduction zone. Field data together with geochronology and isotope geochemistry allow to subdivide the Dharwar craton into three main domains: (1) The Western Dharwar Craton (WDC) is an old (3.3 2.9 Ga ), stable continental block with limited amounts of 2.5 Ga old anatectic granites. (2) The Eastern Dharwar Craton (EDC) is subdivided into two parts: (2a) West of Kolar Schist Belt, a region of 3.0-2.7 Ga old basement intruded by 2.5 Ga old anatectic granites; (2b) East of Kolar, an area featuring mainly 2.5 Ga old diatexites and granites, derived of partial melting of a newly accreted TTG crust. Anatectic granites are ubiquitous, and late in the cratonic evolution; they witnessed generalized melting of a juvenile crust. In contrast, deep-originated granites emplaced before this melting and are restricted to the boundaries between the blocks. This structure of distinct terranes separated by narrow bands operating as channels for deep-originated magmas provides independent evidences for a two-stage evolution: an arc accretion context for the TTG, sanukitoids and related rocks, immediately followed by high temperature reworking of the newly accreted craton

  17. Manganiferous minerals of the epidote group from the Archaean basement of West Greenland

    DEFF Research Database (Denmark)

    Katerinopoulou, Anna; Balic Zunic, Tonci; Kolb, Jochen

    2014-01-01

    The chemical compositions and crystal structures of Mn3+-containing minerals from the epidote group in Greenland rocks are investigated and described in detail. They occur in hydrothermally altered Archaean mafic sequences within the gneissic complex of the North Atlantic craton of West Greenland...

  18. Salinity of the Archaean oceans from analysis of fluid inclusions in quartz

    Science.gov (United States)

    Marty, Bernard; Avice, Guillaume; Bekaert, David V.; Broadley, Michael W.

    2018-05-01

    Fluids trapped in inclusions in well-characterized Archaean hydrothermal quartz crystals were analyzed by the extended argon-argon method, which permits the simultaneous measurement of chlorine and potassium concentrations. Argon and nitrogen isotopic compositions of the trapped fluids were also determined by static mass spectrometry. Fluids were extracted by stepwise crushing of quartz samples from North Pole (NW Australia) and Barberton (South Africa) 3.5-3.0-Ga-old greenstone belts. The data indicate that fluids are a mixture of a low salinity end-member, regarded as the Archaean oceanic water, and several hydrothermal end-members rich in Cl, K, N, and radiogenic parentless 40Ar. The low Cl-K end-member suggests that the salinity of the Archaean oceans was comparable to the modern one, and that the potassium content of the Archaean oceans was lower than at present by about 40%. A constant salinity of the oceans through time has important implications for the stabilization of the continental crust and for the habitability of the ancient Earth.

  19. Archean crustal evolution of the Narryer Gneiss Terrane, Western Australia, as revealed by the U-Pb age and Hf-isotope compositions of zircon from the granitic gneisses

    Science.gov (United States)

    Sylvester, P.; Souders, K.; Crowley, J. L.; Myers, J.

    2011-12-01

    The Narryer Gneiss Terrane of the Yilgarn Craton, Western Australia, is an important area for studies of early crustal evolution because of the preservation of (1) detrital zircons of Hadean to Archean age in the Jack Hills and Mt. Narryer metasedimentary belts, and (2) several widespread units of granitic gneisses emplaced between ca. 3.7 and 2.6 Ga. We have analyzed the U-Pb geochronology and Hf-isotope geochemistry of magmatic zircons from 38 samples of the granitic gneisses using laser ablation - (multicollector) - ICPMS. The sample suite is dominated by the Meeberrie gneiss, a banded quartz-microcline-oligoclase-biotite gneiss of monzogranite to granodiorite composition, and the Dugel gneiss, a leucocratic, pegmatite-layered syenogranite gneiss, but gneisses of dioritic to tonalitic composition, as well as less deformed granite sheets, are also represented. Magmatic zircons were identified on the basis of the preservation of oscillatory zoning in BSE and CL images, igneous Th/U ratios (>0.2), and concordant U-Pb isotopic systematics with low common Pb contents. The results indicate many of the gneisses are composed of the products of multiple magmatic events, as has been reported previously for samples of the Meeberrie gneiss (Kinny & Nutman, 1996, Precambrian Res. 78, 165-178). Major ages of magmatism preserved in the gneisses occurred at ca. 3685-3665 Ma, 3620-3565 Ma, 3495-3440 Ma, 3375-3330 Ma, and 3300-3260 Ma. The late granite sheets crystallized at 2710-2645 Ma. Hf-isotope compositions of the zircons trend to less radiogenic values with decreasing age, with ɛHf values of ca. 0 to -5 for 3.7-3.4 Ga gneisses, ca. -1 to -9 for 3.4-3.2 Ga gneisses and ca. -5 to -20 for the late granite sheets. The array of the Hf isotopic compositions with time for the entire sample set are fit well by a regression indicating a source reservoir with a 176Lu/177Hf of 0.022 extracted from the depleted mantle at 3.9 Ga. This suggests that the Narryer gneisses and late granite

  20. Zircon U-Pb ages and geochemistry of migmatites and granites in the Foping dome: Evidence for Late Triassic crustal evolution in South Qinling, China

    Science.gov (United States)

    Zhang, He; Li, Shuang-Qing; Fang, Bo-Wen; He, Jian-Feng; Xue, Ying-Yu; Siebel, Wolfgang; Chen, Fukun

    2018-01-01

    Migmatites provide a record of melt formation and crustal rheology. In this study we present zircon U-Pb ages and geochemical composition of migmatites from the Foping dome and granites from the Wulong pluton. U-Pb results from migmatite zircons indicate two episodes of partial melting. Rim domains from a leucosome in the Longcaoping area yield an age of ca. 209 Ma. Migmatites collected from the Foping dome yield U-Pb zircon ages of 2910 to 190 Ma, suggesting the involvement of meta-sedimentary source components. Rim domains of the zircons with low Th/U ratios (pluton can be subdivided into high Sr/Y and low Sr/Y granites. U-Pb zircon ages vary from 219 to 214 Ma for the high Sr/Y granites and from 214 to 192 Ma for the low Sr/Y granites. High Sr/Y granites have higher Na2O and Sr contents than the low Sr/Y granites. They also lack negative Eu anomalies and are depleted in HREE compared to the low Sr/Y granites. Initial 87Sr/86Sr ratios and εNd values of all the samples roughly overlap with those of Neoproterozoic basement rocks exposed in South Qinling. Including previous studies, we propose that the high and low Sr/Y granites formed by melting of thickened and normal crust, respectively. Close temporal-spatial relationship of the high and low Sr/Y granites with the two-stage migmatization events implies variation of crustal thickness and thermal overprints of the orogenic crust in post-collisional collapse. Following the collision of South Qinling and the Yangtze block prior to 219 Ma, partial melting of the deep crust occurred. The melts migrated upwards to form the high Sr/Y granites. This process occurred rapidly and caused collapse of the thickened crust and carried heat upwards, leading to further partial melting within the shallower crust and formation of the low Sr/Y granites.

  1. High but balanced sedimentation and subsidence rates (Moodies Group, Barberton Greenstone Belt), followed by basin collapse: Implication for Archaean tectonics

    Science.gov (United States)

    Heubeck, Christoph; Lowe, Donald R.; Byerly, Gary R.

    2010-05-01

    Archaean tectonophysical models distinguish between thick, rigid and thin, mobile crust; from these the major mechanisms and rates for continental growth are derived. Archaean sedimentary rocks, preserved in metamorphosed and highly deformed greenstone belts, can contribute to constrain these models by estimating subsidence rates, derived from the combination of facies changes and precise age dates. Largely siliciclastic strata of the Moodies Group form the topmost unit of the Barberton Supergroup of the Barberton Greenstone Belt (BGB), South Africa, represent one of the world's oldest unmetamorphosed quartz-rich sedimentary sequences, and reach ca. 3500m thick (Lowe and Byerly, 2007). Large parts of the Moodies Group were deposited in apparent sedimentary continuity in alluvial, fluvial, shoreline and shallow-marine environments (e.g., Eriksson, 1979; Heubeck and Lowe, 1994). Distinctive sources and variations in facies indicate that Moodies deposition occurred at times in several basins. In several now tectonically separated regions, a regional basaltic lava (unit MdL of Anhaeusser, 1968) separates a lower unit (ca. 2000m thick and possibly representing an extensional setting) from an upper unit (ca. 1500m thick and characterized by progressive unconformities, rapidly changing facies, thicknesses, and sandstone petrographic composition). Single zircons separated from a felsic air-fall tuff of the middle Moodies Group and immediately overlying the basaltic lava in the Saddleback Syncline were dated on the Stanford-USGS SHRIMP RG. Out of 24 dated grains, two near-concordant groups have mean ages of 3230,6+-6,1Ma (2σ; n=9) and 3519+-7 Ma (2σ; n=9), respectively. We interpret the former age as representing the depositional age of the tuff, the latter as representing inherited zircons from underlying Onverwacht-age basement. The interpreted depositional age of the Moodies tuff is indistinguishable from numerous similar ages from felsic and dacitic volcanics at the

  2. New insight on the paleoproterozoic evolution of the São Francisco Craton: Reinterpretation of the geology, the suture zones and the thicknesses of the crustal blocks using geophysical and geological data

    Science.gov (United States)

    Sampaio, Edson E. S.; Barbosa, Johildo S. F.; Correa-Gomes, Luiz C.

    2017-07-01

    The Archean-Paleoproterozoic Jequié (JB) and Itabuna-Salvador-Curaçá (ISCB) blocks and their tectonic transition zone in the Valença region, Bahia, Brazil are potentially important for ore deposits, but the geological knowledge of the area is still meager. The paucity of geological information restricts the knowledge of the position and of the field characteristics of the tectonic suture zone between these two crustal segments JB and ISCB. Therefore, interpretation of geophysical data is necessary to supplement the regional structural and petrological knowledge of the area as well as to assist mining exploration programs. The analysis of the airborne radiometric and magnetic data of the region has established, respectively, five radiometric domains and five magnetic zones. Modeling of a gravity profile has defined the major density contrasts of the deep structures. The integrated interpretation of the geophysical data fitted to the known geological information substantially improved the suture zone (lower plate JB versus upper plate ISCB) delimitation, the geological map of the area and allowed to estimate the thicknesses of these two blocks, and raised key questions about the São Francisco Craton tectonic evolution.

  3. Selection for Protein Kinetic Stability Connects Denaturation Temperatures to Organismal Temperatures and Provides Clues to Archaean Life.

    Directory of Open Access Journals (Sweden)

    M Luisa Romero-Romero

    Full Text Available The relationship between the denaturation temperatures of proteins (Tm values and the living temperatures of their host organisms (environmental temperatures: TENV values is poorly understood. Since different proteins in the same organism may show widely different Tm's, no simple universal relationship between Tm and TENV should hold, other than Tm≥TENV. Yet, when analyzing a set of homologous proteins from different hosts, Tm's are oftentimes found to correlate with TENV's but this correlation is shifted upward on the Tm axis. Supporting this trend, we recently reported Tm's for resurrected Precambrian thioredoxins that mirror a proposed environmental cooling over long geological time, while remaining a shocking ~50°C above the proposed ancestral ocean temperatures. Here, we show that natural selection for protein kinetic stability (denaturation rate can produce a Tm↔TENV correlation with a large upward shift in Tm. A model for protein stability evolution suggests a link between the Tm shift and the in vivo lifetime of a protein and, more specifically, allows us to estimate ancestral environmental temperatures from experimental denaturation rates for resurrected Precambrian thioredoxins. The TENV values thus obtained match the proposed ancestral ocean cooling, support comparatively high Archaean temperatures, and are consistent with a recent proposal for the environmental temperature (above 75°C that hosted the last universal common ancestor. More generally, this work provides a framework for understanding how features of protein stability reflect the environmental temperatures of the host organisms.

  4. Preliminary discussion on possible genesis of crustal rotation, its impact on geotectonic evolution and its relation to large-scale metallogeny in Hunan province and adjacent regions

    International Nuclear Information System (INIS)

    Shu Xiaojing

    2005-01-01

    Hunan province and adjacent regions show ring-form distribution features both on surface geologic structure and geophysical field. Such features might result from the rotation movement of the earth crust and exert serious impact on the geotectonic evolution and large-scale metallogeny in Hunan province and adjacent regions. This paper makes a preliminary discussion on the possible genesis of such rotation movement, as well as the associated series of geologic processes and its relation to large-scale metallogeny in Hunan province and adjacent regions. (authors)

  5. A hitherto unknown river type from the Archaean at Bhurkuli (Jharkhand, E India

    Directory of Open Access Journals (Sweden)

    Loon A.J. (Tom van

    2017-06-01

    Full Text Available The Archaean granitoid pluton of the Singhbhum craton in E India is overlain by Archaean to Palaeoproterozoic metasediments. These sediments are still poorly known and their stratigraphy is under debate. Several scattered, most probably Meso- to Neoarchaean, conglomerates are present in the state of Jharkhand that differ so much in characteristics that they are probably not related to each other. The sedimentology of a series of conglomerate patches and layers near Bhurkuli has been investigated, including the characteristics of the clasts. It is deduced on the basis of these characteristics and the sedimentological context that the Bhurkuli conglomerates represent the channel facies of a river system that differed from the types of fluvial systems that exist nowadays.

  6. Ideas and perspectives: hydrothermally driven redistribution and sequestration of early Archaean biomass - the "hydrothermal pump hypothesis"

    Science.gov (United States)

    Duda, Jan-Peter; Thiel, Volker; Bauersachs, Thorsten; Mißbach, Helge; Reinhardt, Manuel; Schäfer, Nadine; Van Kranendonk, Martin J.; Reitner, Joachim

    2018-03-01

    Archaean hydrothermal chert veins commonly contain abundant organic carbon of uncertain origin (abiotic vs. biotic). In this study, we analysed kerogen contained in a hydrothermal chert vein from the ca. 3.5 Ga Dresser Formation (Pilbara Craton, Western Australia). Catalytic hydropyrolysis (HyPy) of this kerogen yielded n-alkanes up to n-C22, with a sharp decrease in abundance beyond n-C18. This distribution ( ≤ n-C18) is very similar to that observed in HyPy products of recent bacterial biomass, which was used as reference material, whereas it differs markedly from the unimodal distribution of abiotic compounds experimentally formed via Fischer-Tropsch-type synthesis. We therefore propose that the organic matter in the Archaean chert veins has a primarily microbial origin. The microbially derived organic matter accumulated in anoxic aquatic (surface and/or subsurface) environments and was then assimilated, redistributed and sequestered by the hydrothermal fluids (hydrothermal pump hypothesis).

  7. Early tectonic history of the Marymia Inlier and correlation with the Archaean Yilgarn Craton, Western Australia

    International Nuclear Information System (INIS)

    Bagas, L.

    1999-01-01

    The Archaean granite-greenstone rocks of the Marymia Inlier outcrop within Proterozoic rocks forming the Capricorn Orogen. Five major deformation events are recognised in the rocks of the Plutonic Well and Baumgarten greenstone belts. The first two events were Late Archaean and synchronous with major epithermal gold mineralisation in the belts. Palaeoproterozoic extensional faulting was probably related to the early stages of the Capricorn Orogeny. The fourth event records a compressional phase of the Capricorn Orogeny associated with greenschist-facies metamorphism, whereas the last major event involved wrench faulting associated with minor folding. The Archaean tectonic history, rock types and timing of mineralisation strongly suggest that the Marymia Inlier is part of the Yilgarn Craton, and that each of the provinces in the craton experienced the same geological history since 2.72 Ga. The inlier is now interpreted to include two components, one is the eastern or northern extension of either the Narryer Terrane. Murchison Province or Southern Cross Province, and the other is the northwestern extension of the Eastern Goldfields Province. The Jenkin Fault, which was active in Proterozoic times, separates these two components. Copyright (1999) Blackwell Science Pty Ltd

  8. Primordial crustal growth in northern Nigeria: preliminary Rb-Sr and Sm-Nd constraints from Kaduna migmatite-gneiss complex

    International Nuclear Information System (INIS)

    Dada, S. S.; Briqueu, L.; Birck, J. L.

    1998-01-01

    The Kaduna Migmatite-Gneiss Complex in the central area of the Northern shield includes variably migmatised granitotrondhjemitic gneisses and amphibolite of hitherto unknown age. The amphibolite enclaves and dykes are metatholeiites with comparatively unfractionated rare-earth patterns. The two main rock units (TTG and amphibolite) exhibit complementary geochemical signatures in the normalised abundance patterns of relatively incompatible elements and suggest possible derivation of the gneisses from subduction related mafic material. Sm-Nd and Rb-Sr isotopic data document early Archaean crustal formation of new crust and its subsequent late Archaean differentiation. These preliminary results form an evidence for a more extended crustal history in the heart of the Pan-African domain (ca. 600 Ma.). They suggest the differentiation of juvenile crustal protolith from a chondritic reservoir about 3.5 Ga. for the gneiss-amphibolite bimodal suite. A tectonothermal event about 3.1-3.0 Ga led to the emplacement of an early gneiss as indicated from Rb-Sr and U-Ph zircon analyses. Subsequent differentiation and/or reworking around 2.8-2.7 Ga is coherent with the Liberian orogeny within the West African- Latino American subregion

  9. Decrease in oceanic crustal thickness since the breakup of Pangaea

    Science.gov (United States)

    van Avendonk, Harm J. A.; Davis, Joshua K.; Harding, Jennifer L.; Lawver, Lawrence A.

    2017-01-01

    Earth's mantle has cooled by 6-11 °C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7 km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15-20 °C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.

  10. Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites

    Science.gov (United States)

    Nisbet, E. G.; Cheadle, M. J.; Arndt, N. T.; Bickle, M. J.

    1993-09-01

    The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to Fo 93.6, implying eruption at around 1520°C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to Fo 94.1, implying eruption at 1560°C. However, inferences of erupted liquids containing 32-33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580°C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18-20% MgO and erupted at circa 1400°C. If 1580°C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200°C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900°C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300°C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200-300

  11. New isotopic and field evidence for the ages and distribution of Archaean rocks in east Antarctica

    International Nuclear Information System (INIS)

    Kinny, P.D.; Delor, C.P.

    1990-01-01

    The Precambrian shield of East Antarctica is composed of a number of recognised Archaean cratonic nuclei surrounded by Proterozoic metamorphic complexes. Poor exposure, inaccessibility and the effects of multiple tectonothermal overprints combine to confound the knowledge of the early history of these terranes. Against this, it is shown how recent advances in zircon geochronology allied with new petrological, geochemical and field observations have resulted in major revisions to the chronostratigraphy of several key areas, including Napier Complex of Enderby Land, Vestfold Hills and Rauer Group. 11 refs

  12. Analogue modelling of strike-slip fault propagation across a rheological/morphological crustal anisotropy: implications for the morphotectonic evolution of the Gloria Fault - Tore Madeira Rise area in NE Atlantic.

    Science.gov (United States)

    Tomás, Ricardo; Rosas, Filipe M.; Duarte, João C.; Terrinha, Pedro; Kullberg, Maria C.; Almeida, Jaime; Barata, Frederico; Carvalho, Bruno; Almeida, Pedro

    2015-04-01

    The Gloria Fault (GF) marks the E-W dextral transcurrent plate boundary between Eurasia and Africa in NE Atlantic, displaying complying high magnitude (historical and instrumental) seismic activity (e.g. M=7.1 in 1939 and M=8.4 in 1941, Bufforn et al., 1988), and cutting across a NNE-SSW 1000 km long bathymetric ridge: the so called Tore-Madeira Rise - TMR (rising in average 3km above the abyssal plain). The precise origin and tectono-magmatic evolution of the TMR is still not fully understood, although reported wide-angle refraction data points to a rheological configuration comprising an isostatically compensated thickened oceanic crust, possibly formed during a period of high accretion in the Mid-Atlantic Ridge (Pierce and Barton, 1991). Widespread evidence for volcanic activity has also been recognized, spanning from late Cretaceous to Present (Geldmacher et al. 2006, Merle et al. 2009), noticeably with the most recent volcanism (~500 Ky) occurring as tectonically aligned volcanic plugs, distributed along the E-W tectonic trend of the GF-related structures. To better understand the complex interference at play in this key area between the tectonic structures (essentially determined by the Gloria Fault system), the present and past magmatic activity and the resulting seafloor morphology, a series of dynamically scaled analogue modelling experiments have been conceived and carried out. The main focus of this experimental work was to decipher the potential influence of a rheological vs. morphological anisotropy (accounting for the TMR) on the lateral propagation of a major right-lateral strike-slip fault (representing the GF). The preliminary comparison of the obtained experimental results with the natural morphotectonic pattern in the study area reveals, not only a strong tectonic control of the ongoing volcanism, manifested by the observed preferred directions of aligned volcanic plugs, but also a so far unsuspected deflection/distributed pattern of several

  13. Archaean ultra-depleted komatiites formed by hydrous melting of cratonic mantle.

    Science.gov (United States)

    Wilson, A H; Shirey, S B; Carlson, R W

    2003-06-19

    Komatiites are ultramafic volcanic rocks containing more than 18 per cent MgO (ref. 1) that erupted mainly in the Archaean era (more than 2.5 gigayears ago). Although such compositions occur in later periods of Earth history (for example, the Cretaceous komatiites of Gorgona Island), the more recent examples tend to have lower MgO content than their Archaean equivalents. Komatiites are also characterized by their low incompatible-element content, which is most consistent with their generation by high degrees of partial melting (30-50 per cent). Current models for komatiite genesis include the melting of rock at great depth in plumes of hot, diapirically rising mantle or the melting of relatively shallow mantle rocks at less extreme, but still high, temperatures caused by fluxing with water. Here we report a suite of ultramafic lava flows from the Commondale greenstone belt, in the southern part of the Kaapvaal Craton, which represents a previously unrecognized type of komatiite with exceptionally high forsterite content of its igneous olivines, low TiO(2)/Al(2)O(3) ratio, high silica content, extreme depletion in rare-earth elements and low Re/Os ratio. We suggest a model for their formation in which a garnet-enriched residue left by earlier cratonic volcanism was melted by hydration from a subducting slab.

  14. Witwatersrand gold deposits formed by volcanic rain, anoxic rivers and Archaean life

    Science.gov (United States)

    Heinrich, Christoph A.

    2015-03-01

    The Witwatersrand Basin in South Africa is one of the best-preserved records of fluvial sedimentation on an Archaean continent. The basin hosts the worlds biggest gold resource in thin pebble beds, but the process for gold enrichment is debated. Mechanical accumulation of gold particles from flowing river water is the prevailing hypothesis, yet there is evidence for hydrothermal mobilization of gold by fluids invading the metasedimentary rocks after their burial. Earth's atmosphere three billion years ago was oxygen free, but already sustained some of the oldest microbial life on land. Here I use thermodynamic modelling and mass-balance calculations to show that these conditions could have led to the chemical transport and precipitation of gold in anoxic surface waters, reconciling the evidence for fluvial deposition with evidence for hydrothermal-like chemical reactions. I suggest that the release of sulphurous gases from large volcanic eruptions created acid rain that enabled the dissolution and transport of gold in surface waters as sulphur complexes. Precipitation of the richest gold deposits could have been triggered by chemical reduction of the dissolved gold onto organic material in shallow lakes and pools. I conclude that the Witwatersrand gold could have formed only during the Archaean, after the emergence of continental life but before the rise of oxygen in the Earth's atmosphere.

  15. Heat flow, heat generation and crustal thermal structure of the northern block of the South Indian Craton

    Science.gov (United States)

    Gupta, Mohan L.; Sharma, S. R.; Sundar, A.

    Heat flow values and heat generation data calculated from the concentration of heat producing radioactive elements, U, Th and K in surface rocks were analyzed. The South Indian Craton according to Drury et al., can be divided into various blocks, separated by late Proterozoic shear belts. The northern block comprises Eastern and Western Dharwar Cratons of Rogers (1986), Naqvi and Rogers (1987) and a part of the South Indian granulite terrain up to a shear system occupying the Palghat-Cauvery low lands. The geothermal data analysis clearly demonstrates that the present thermal characteristics of the above two Archaean terrains of the Indian and Australian Shields are quite similar. Their crustal thermal structures are likely to be similar also.

  16. Heat flow, heat generation and crustal thermal structure of the northern block of the South Indian Craton

    Science.gov (United States)

    Gupta, Mohan L.; Sharma, S. R.; Sundar, A.

    1988-01-01

    Heat flow values and heat generation data calculated from the concentration of heat producing radioactive elements, U, Th and K in surface rocks were analyzed. The South Indian Craton according to Drury et al., can be divided into various blocks, separated by late Proterozoic shear belts. The northern block comprises Eastern and Western Dharwar Cratons of Rogers (1986), Naqvi and Rogers (1987) and a part of the South Indian granulite terrain up to a shear system occupying the Palghat-Cauvery low lands. The geothermal data analysis clearly demonstrates that the present thermal characteristics of the above two Archaean terrains of the Indian and Australian Shields are quite similar. Their crustal thermal structures are likely to be similar also.

  17. Did the formation of D″ cause the Archaean-Proterozoic transition?

    Science.gov (United States)

    Campbell, Ian H.; Griffiths, Ross W.

    2014-02-01

    The MgO content of the highest MgO plume-related komatiites and picrites remained constant at 32±2.5% between 3.5 and 2.7 Ga, then fell to 21±3% by ca. 2.0 Ga, a value similar to the present day value. Because there is a linear relationship between the liquidus temperature of dry magmas and their MgO content this observation implies that the temperature of mantle plumes changed little between 3.5 and 2.7 Ga, and then fell by 200-250 °C between 2.7 and 2.0 Ga to a temperature similar to that of present plumes. We suggest that Archaean plumes originate from the core-mantle boundary and that their temperature remained constant because the temperature of the outer core was buffered by solidification of the Fe-Ni inner core. At about 2.7 Ga dense former basaltic crust began to accumulate at the core and eventually covered it to produce an insulating layer that reduced the heat flux out of the core and lowered the temperature of mantle plumes. The temperature of mantle plumes fell as the dense layer above the core thickened until it exceeded the critical thickness required for convection. Because heat is transferred rapidly across the convecting part of the insulating layer, any further increase in its thickness by the addition more basaltic material has no influence on the temperature at the top of the layer, which is the source of Post-Archaean mantle plumes. We equate the dense layer above the core with the seismically identified layer D″. Our analyses suggest the drop in plume temperatures produced by a dense insulating layer above the core will be about 40% once it starts to convect, which is consistent with the observed drop inferred from the decrease in the MgO content of komatiites and picrites at that time.

  18. Crustal Ages of the Ocean Floor - Poster

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The Crustal Ages of the Ocean Floor Poster was created at NGDC using the Crustal Ages of the Ocean Floor database draped digitally over a relief of the ocean floor...

  19. The crustal thickness of Australia

    Science.gov (United States)

    Clitheroe, G.; Gudmundsson, O.; Kennett, B.L.N.

    2000-01-01

    We investigate the crustal structure of the Australian continent using the temporary broadband stations of the Skippy and Kimba projects and permanent broadband stations. We isolate near-receiver information, in the form of crustal P-to-S conversions, using the receiver function technique. Stacked receiver functions are inverted for S velocity structure using a Genetic Algorithm approach to Receiver Function Inversion (GARFI). From the resulting velocity models we are able to determine the Moho depth and to classify the width of the crust-mantle transition for 65 broadband stations. Using these results and 51 independent estimates of crustal thickness from refraction and reflection profiles, we present a new, improved, map of Moho depth for the Australian continent. The thinnest crust (25 km) occurs in the Archean Yilgarn Craton in Western Australia; the thickest crust (61 km) occurs in Proterozoic central Australia. The average crustal thickness is 38.8 km (standard deviation 6.2 km). Interpolation error estimates are made using kriging and fall into the range 2.5-7.0 km. We find generally good agreement between the depth to the seismologically defined Moho and xenolith-derived estimates of crustal thickness beneath northeastern Australia. However, beneath the Lachlan Fold Belt the estimates are not in agreement, and it is possible that the two techniques are mapping differing parts of a broad Moho transition zone. The Archean cratons of Western Australia appear to have remained largely stable since cratonization, reflected in only slight variation of Moho depth. The largely Proterozoic center of Australia shows relatively thicker crust overall as well as major Moho offsets. We see evidence of the margin of the contact between the Precambrian craton and the Tasman Orogen, referred to as the Tasman Line. Copyright 2000 by the American Geophysical Union.

  20. Highly refractory Archaean peridotite cumulates: Petrology and geochemistry of the Seqi Ultramafic Complex, SW Greenland

    Directory of Open Access Journals (Sweden)

    Kristoffer Szilas

    2018-05-01

    large-scale melting event(s, which resulted in the ultra-depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth's first continents and the corresponding SCLM. Keywords: North Atlantic Craton, Archaean, Dunite, Platinum-group elements, Ultra-depleted mantle, Fiskefjord

  1. Percolation of diagenetic fluids in the Archaean basement of the Franceville basin

    Science.gov (United States)

    Mouélé, Idalina Moubiya; Dudoignon, Patrick; Albani, Abderrazak El; Cuney, Michel; Boiron, Marie-Christine; Gauthier-Lafaye, François

    2014-01-01

    The Palaeoproterozoic Franceville basin, Gabon, is mainly known for its high-grade uranium deposits, which are the only ones known to act as natural nuclear fission reactors. Previous work in the Kiéné region investigated the nature of the fluids responsible for these natural nuclear reactors. The present work focuses on the top of the Archaean granitic basement, specifically, to identify and date the successive alteration events that affected this basement just below the unconformity separating it from the Palaeoproterozoic basin. Core from four drill holes crosscutting the basin-basement unconformity have been studied. Dating is based on U-Pb isotopic analyses performed on monazite. The origin of fluids is discussed from the study of fluid inclusion planes (FIP) in quartz from basement granitoids. From the deepest part of the drill holes to the unconformable boundary with the basin, propylitic alteration assemblages are progressively replaced by illite and locally by a phengite + Fe chlorite ± Fe oxide assemblage. Illitic alteration is particularly strong along the sediment-granitoid contact and is associated with quartz dissolution. It was followed by calcite and anhydrite precipitation as fracture fillings. U-Pb isotopic dating outlines three successive events: a 3.0-2.9-Ga primary magmatic event, a 2.6-Ga propylitic alteration and a late 1.9-Ga diagenetic event. Fluid inclusion microthermometry suggests the circulation of three types of fluids: (1) a Na-Ca-rich diagenetic brine, (2) a moderately saline (diagenetic + meteoric) fluid, and (3) a low-salinity fluid of probable meteoric origin. These fluids are similar to those previously identified within the overlying sedimentary rocks of the Franceville basin. Overall, the data collected in this study show that the Proterozoic-Archaean unconformity has operated as a major flow corridor for fluids circulation, around 1.9 Ga. highly saline diagenetic brines; hydrocarbon-rich fluids derived from organic matter

  2. Glacial rebound and crustal stress in Finland

    International Nuclear Information System (INIS)

    Lambeck, K.; Purcell, A.

    2003-11-01

    The last ice age of Fennoscandinavia continues to have geological repercussions across Finland despite the last ice having retreated almost 10,000 years ago: land uplift, shoreline retreat, and the stress state of the crust continues to evolve. This report focusses on the glacial rebound signals for Finland and the Gulf of Bothnia and explores the consequences of the ongoing deformation. The rebound signals include the geological evidence as well as instrumental observations: the tide gauge and lake-level measurements of the past century, the changes in geodetic levels recorded in the repeat levelling surveys of the region and the direct measurement of crustal deformation (radial and horizontal) using high-precision space-geodesy measurements. These signals provide constraints on the Earth's rheology, its elasticity and viscosity, and the glacial history of the region. Once observationally constrained, the rebound models are used to predict both the ongoing evolution of shorelines and the changing state of stress within the crust. This report covers: (i) A review of glacial rebound modelling for Scandinavia (Sections 2 and 3). (ii) Review of observational evidence relating to sea-level change and crustal rebound (Section 4). (iii) New earth and ice-sheet model results from the inversion of the geological evidence for sea-level change, including models of shoreline evolution (Sections 5 and 6). (iv) Earth-model results from the inversion of the geodetic evidence for sea-level change (Section 7). (v) Development of crustal stress models for past and present stress states (Section 8). (vi) Conclusions and recommendations (Section 9). Specific conclusions reached pertain to: (i) Thickness of ice cover over Scandinavia since the Last Glacial Maximum, particularly for the Lateglacial period. (ii) Sea-level change and shoreline evolution for the Baltic area since the time the region became ice-free for the last time. (iii) The predicted rates of present-day crustal

  3. Deep Crustal Melting and the Survival of Continental Crust

    Science.gov (United States)

    Whitney, D.; Teyssier, C. P.; Rey, P. F.; Korchinski, M.

    2017-12-01

    Plate convergence involving continental lithosphere leads to crustal melting, which ultimately stabilizes the crust because it drives rapid upward flow of hot deep crust, followed by rapid cooling at shallow levels. Collision drives partial melting during crustal thickening (at 40-75 km) and/or continental subduction (at 75-100 km). These depths are not typically exceeded by crustal rocks that are exhumed in each setting because partial melting significantly decreases viscosity, facilitating upward flow of deep crust. Results from numerical models and nature indicate that deep crust moves laterally and then vertically, crystallizing at depths as shallow as 2 km. Deep crust flows en masse, without significant segregation of melt into magmatic bodies, over 10s of kms of vertical transport. This is a major mechanism by which deep crust is exhumed and is therefore a significant process of heat and mass transfer in continental evolution. The result of vertical flow of deep, partially molten crust is a migmatite dome. When lithosphere is under extension or transtension, the deep crust is solicited by faulting of the brittle upper crust, and the flow of deep crust in migmatite domes traverses nearly the entire thickness of orogenic crust in Recognition of the importance of migmatite (gneiss) domes as archives of orogenic deep crust is applicable to determining the chemical and physical properties of continental crust, as well as mechanisms and timescales of crustal differentiation.

  4. The elusive Hadean enriched reservoir revealed by 142Nd deficits in Isua Archaean rocks.

    Science.gov (United States)

    Rizo, Hanika; Boyet, Maud; Blichert-Toft, Janne; O'Neil, Jonathan; Rosing, Minik T; Paquette, Jean-Louis

    2012-11-01

    The first indisputable evidence for very early differentiation of the silicate Earth came from the extinct (146)Sm-(142)Nd chronometer. (142)Nd excesses measured in 3.7-billion-year (Gyr)-old rocks from Isua (southwest Greenland) relative to modern terrestrial samples imply their derivation from a depleted mantle formed in the Hadean eon (about 4,570-4,000 Gyr ago). As dictated by mass balance, the differentiation event responsible for the formation of the Isua early-depleted reservoir must also have formed a complementary enriched component. However, considerable efforts to find early-enriched mantle components in Isua have so far been unsuccessful. Here we show that the signature of the Hadean enriched reservoir, complementary to the depleted reservoir in Isua, is recorded in 3.4-Gyr-old mafic dykes intruding into the Early Archaean rocks. Five out of seven dykes carry (142)Nd deficits compared to the terrestrial Nd standard, with three samples yielding resolvable deficits down to -10.6 parts per million. The enriched component that we report here could have been a mantle reservoir that differentiated owing to the crystallization of a magma ocean, or could represent a mafic proto-crust that separated from the mantle more than 4.47 Gyr ago. Our results testify to the existence of an enriched component in the Hadean, and may suggest that the southwest Greenland mantle preserved early-formed heterogeneities until at least 3.4 Gyr ago.

  5. A Field Trip to the Archaean in Search of Darwin’s Warm Little Pond

    Directory of Open Access Journals (Sweden)

    Bruce Damer

    2016-05-01

    Full Text Available Charles Darwin’s original intuition that life began in a “warm little pond” has for the last three decades been eclipsed by a focus on marine hydrothermal vents as a venue for abiogenesis. However, thermodynamic barriers to polymerization of key molecular building blocks and the difficulty of forming stable membranous compartments in seawater suggest that Darwin’s original insight should be reconsidered. I will introduce the terrestrial origin of life hypothesis, which combines field observations and laboratory results to provide a novel and testable model in which life begins as protocells assembling in inland fresh water hydrothermal fields. Hydrothermal fields are associated with volcanic landmasses resembling Hawaii and Iceland today and could plausibly have existed on similar land masses rising out of Earth’s first oceans. I will report on a field trip to the living and ancient stromatolite fossil localities of Western Australia, which provided key insights into how life may have emerged in Archaean, fluctuating fresh water hydrothermal pools, geological evidence for which has recently been discovered. Laboratory experimentation and fieldwork are providing mounting evidence that such sites have properties that are conducive to polymerization reactions and generation of membrane-bounded protocells. I will build on the previously developed coupled phases scenario, unifying the chemical and geological frameworks and proposing that a hydrogel of stable, communally supported protocells will emerge as a candidate Woese progenote, the distant common ancestor of microbial communities so abundant in the earliest fossil record.

  6. The Archaean Granny Smith gold deposit, western Australia: age and Pb-isotope tracer studies

    International Nuclear Information System (INIS)

    Ojala, V.J.; McNaughton, N.J.; Groves, D.I.; Ridley, J.R.; Fanning, C.M.

    1997-01-01

    The Granny Smith gold deposits are situated within a greenstone sequence in the Laverton-Leonora area of the Northeastern Goldfields Province of the Archaean Yilgarn Block, Western Australia. The greenstone sequence (U-Pb zircon age of 2677±6 Ma, felsic pyroclastic rock) was intruded by the Granny Smith Granodiorite at 2665±4 Ma. Gold mineralisation is located along a reactivated N-S Stricking, thrust which wraps around the granitoid intrusion, and within the granitoid intrusion. Initial lead-isotope compositions of the Granny Smith Granodiorite and ore-fluid Pb, estimated from K-feldspar and galena and lead tellurides, respectively, are slightly different. Calculations based on Pb isotope data for the host rocks, and the U-Pb zircon age of the Granny Smith Granodiorite, suggest that ore-fluid Pb was derived from a source with a similar initial lead-isotopic composition to the source of the Granny Smith Granodiorite but about 30 million years after the intrusion of the granitoid. The Pb-isotope data for granitoids of the Northeastern Goldfields fall in a distinct field different to that of the granitoids of the Norseman area and those from Kambalda to Menzies. (authors)

  7. Crustal Seismic Anisotropy: Implications for Understanding Crustal Dynamics

    Science.gov (United States)

    Meltzer, A.; Christensen, N.; Okaya, D.

    2003-12-01

    The Nanga Parbat - Haramosh massif, in the core of the western syntaxis of the Himalaya, represents a unique exposure of mid-lower continental crust from beneath a collisional orogen. The exhumed core of the massif forms a large scale antiformal structure with axial orientation of N10E and associated lineation directed north-south with near-vertical dips. Laboratory measurements of seismic velocity on a suite of quartzofeldspathic gneisses from the massif show a relatively strong degree of anisotropy, up to 12.5% for compressional waves and up to 21% for shear waves. The degree of velocity anisotropy is primarily a function of mica content and rock fabric strength. The strong anisotropy measured in these rocks should be observable in recorded seismic field data and provides a means of mapping rock fabric at depth provided the rock fabric is coherent over appropriate length scales. An IRIS/PASSCAL deployment of 50 short period instruments recorded local and regional earthquakes to characterize seismicity and determine crustal structure beneath the massif as part of a multidisciplinary NSF Continental Dynamics study investigating the active tectonic processes responsible for exhumation and crustal reworking at Nanga Parbat. Microseismicity at Nanga Parbat is distributed along strike beneath the massif but exhibits a sharp drop-off laterally into adjacent terranes and with depth. This data set is ideal for studying crustal seismic anisotropy because the raypaths are restricted to the crust, sharp onsets in P and S allow for clear identification of arrivals, and source-receiver geometries sample a range of azimuths with respect to structure. Preliminary analysis indicates that the majority of local events exhibit some degree of splitting and that splitting patterns, while complicated, are coherent. While splitting delay normally increases with distance traveled through anisotropic material, the range of delay times can be due to heterogeneity in composition, lateral

  8. Crustal and upper mantle structure of Siberia from teleseismic receiver functions

    DEFF Research Database (Denmark)

    Soliman, Mohammad Youssof Ahmad; Thybo, Hans; Artemieva, Irina

    2015-01-01

    ). With this method, we determine seismic P- and S-velocities that are comparable to the results of teleseismic body wave and surface wave tomography techniques. The RF model shows variations in the crustal thickness between 35 and 55 km. Intracrustal structures are identified, in particular using the high......This study presents seismic images of the crustal and lithospheric structure in Siberia based on the available broadband seismic data using teleseismic receiver functions (RFs). We invert P- and S-RFs jointly. The inversion technique is carried out by approach described by Vinnik et al. (2004....... The current results of RF analysis of the crustal and mantle structure will help to build a model for tectonic and geodynamic evolution of different provinces of Siberia. We compare our results to the recent detailed models of crustal structure in the area and with seismic models for similar geodynamic...

  9. A Geodynamic Study of Active Crustal Deformation and Earthquakes in North China

    Science.gov (United States)

    Yang, Y.; Liu, M.

    2005-12-01

    North China is part of the Archaean Sino-Korean craton, yet today it is a region of intense crustal deformation and earthquakes, including 21 M >=7.0 events since 512 AD. More than half of the large events occurred within the Fen-Wei rift system surrounding the stable Ordos plateau; the largest events (M >=7.3) show a sequential southward migration along the rift. However, since 1695 the Fen-Wei rift has became seismically dormant, while seismicity seems having shifted eastward to the North China plain, marked by the 1996 Tangshan earthquake (M=7.8). We have developed a 3D viscoelastic geodynamic model to study the cause of seismicity and its spatial-temporal pattern in North China. Constrained by crustal kinematics from GPS and neotectonic data, the model shows high deviatoric stress in the North China crust, resulting mainly from compression of the expanding Tibetan Plateau and resistance from the stable Siberian block. Within North China seismicity is largely controlled by lateral heterogeneity of lithospheric structures, which explains the concentration of seismicity in the Fen-Wei rift. Our results show that stress triggering may have contributed to the sequential migration of large events along the rift, and the release and migration of stress and strain energy from these large events may partially explain the intense seismicity in the North China plain in the past 300 years. Comparing the predicted long-term spatial pattern of strain energy with seismic energy release provides some insights of potential earthquake risks in North China.

  10. Crustal structure of Central Sicily

    Science.gov (United States)

    Giustiniani, Michela; Tinivella, Umberta; Nicolich, Rinaldo

    2018-01-01

    We processed crustal seismic profile SIRIPRO, acquired across Central Sicily. To improve the seismic image we utilized the wave equation datuming technique, a process of upward or downward continuation of the wave-field between two arbitrarily shaped surfaces. Wave equation datuming was applied to move shots and receivers to a given datum plane, removing time shifts related to topography and to near-surface velocity variations. The datuming procedure largely contributed to attenuate ground roll, enhance higher frequencies, increase resolution and improve the signal/noise ratio. Processed data allow recognizing geometries of crust structures differentiating seismic facies and offering a direct image of ongoing tectonic setting within variable lithologies characterizing the crust of Central Sicily. Migrated sections underline distinctive features of Hyblean Plateau foreland and above all a crustal thinning towards the Caltanissetta trough, to the contact with a likely deep Permo-Triassic rifted basin or rather a zone of a continent to oceanic transition. Inhomogeneity and fragmentation of Sicily crust, with a distinct separation of Central Sicily basin from western and eastern blocks, appear to have guided the tectonic transport inside the Caltanissetta crustal scale syncline and the accumulation of allochthonous terrains with south and north-verging thrusts. Major tectonic stack operated on the construction of a wide anticline of the Maghrebian chain in northern Sicily. Sequential south-verging imbrications of deep elements forming the anticline core denote a crust wedge indenting foreland structures. Deformation processes involved multiple detachment planes down to decoupling levels located near crust/mantle transition, supporting a presence of high-density lenses beneath the chain, interrelated to a southwards push of Tyrrhenian mantle and asthenosphere.

  11. Determined Initial lead for South Of Isua (SOI) terrain suggests a single homogeneous source for it and possibly other archaean rocks

    Science.gov (United States)

    Tera, F.

    2011-12-01

    A Thorogenic-Uranogenic Lead Isotope Plane (TULIP), which entails plotting 206/208 (or its reverse) vs 207/208 (or its reverse), was applied to the Pb data on South of Isua (SOI) by Kamber et al., (1). When the data on 20 samples of these rocks and feldspars are plotted in pairs (each pair is a rock and its feldspar) on TULIP, they fall on 10 mixing lines that converge on a single spot (Fig. 1). This is the end member initial lead (EMIL). The 206/208 & 207/208 so determined are 0.3675 and 0.43525, respectively. From these values one calculates 207/206 = 1.1843 ± 0.0007, for EMIL. This pattern requires either: A) each pair has a singular kappa, K = 232Th/238U, different from others, or B) a pair's in situ decay Pb was homogenized in recent times. On 204/206 vs 207/206 diagram, the whole rocks of SOI define a 3.776 Ga isochron (2). From this and EMIL's 207/206, one obtains: 206/204 = 10.977, 207/204 = 12.974; and 208/204 = 29.756. This singularity of initial Pb contrasts with a deduced variability by the original authors (1). EMIL's radiogenic *(207/206) = 1.6220, gives a single-stage age = 5.9 Ga, indicating inapplicability of its evolution in one stage. Also, the μ calculated from 238U-206Pb for the single stage is different from that inferred from 235U-207Pb, confirming disqualification of this scenario. Reconciliation of the two decay schemes necessitates assumption of EMIL evolution in a minimum of two stages. Starting at 4.563 Ga, five scenarios were assumed: First stage ends and second starts at 4.55, 4.54, 4.53, 4.52 or 4.51 Ga. Second stages end at 3.776 Ga. The calculated μ1 for the first stage are 106, 59.5, 44.6, 36.3 and 30.9 respectively. For μ2 the change is limited, from 5.45 to 5.28. Only an average calculated K for both stages is possible. For the five outlined scenarios it ranges from 1.118 to 1.111. Earlier, Tera (3) observed that initial Pb of the oldest terrestrial reservoir requires evolution in two stages. There too μ1 >> μ2. Data on

  12. Ideas and perspectives: hydrothermally driven redistribution and sequestration of early Archaean biomass – the “hydrothermal pump hypothesis”

    Directory of Open Access Journals (Sweden)

    J.-P. Duda

    2018-03-01

    Full Text Available Archaean hydrothermal chert veins commonly contain abundant organic carbon of uncertain origin (abiotic vs. biotic. In this study, we analysed kerogen contained in a hydrothermal chert vein from the ca. 3.5 Ga Dresser Formation (Pilbara Craton, Western Australia. Catalytic hydropyrolysis (HyPy of this kerogen yielded n-alkanes up to n-C22, with a sharp decrease in abundance beyond n-C18. This distribution ( ≤  n-C18 is very similar to that observed in HyPy products of recent bacterial biomass, which was used as reference material, whereas it differs markedly from the unimodal distribution of abiotic compounds experimentally formed via Fischer–Tropsch-type synthesis. We therefore propose that the organic matter in the Archaean chert veins has a primarily microbial origin. The microbially derived organic matter accumulated in anoxic aquatic (surface and/or subsurface environments and was then assimilated, redistributed and sequestered by the hydrothermal fluids (hydrothermal pump hypothesis.

  13. Bouguer gravity trends and crustal structure of the Palmyride Mountain belt and surrounding northern Arabian platform in Syria

    Science.gov (United States)

    Best, John A.; Barazangi, Muawia; Al-Saad, Damen; Sawaf, Tarif; Gebran, Ali

    1990-12-01

    This study examines the crustal structure of the Palmyrides and the northern Arabian platform in Syria by two- and three-dimensional modeling of the Bouguer gravity anomalies. Results of the gravity modeling indicate that (1) western Syria is composed of at least two different crustal blocks, (2) the southern crustal block is penetrated by a series of crustal-scale, high-density intrusive complexes, and (3) short-wavelength gravity anomalies in the southwest part of the mountain belt are clearly related to basement structure. The crustal thickness in Syria, as modeled on the gravity profiles, is approximately 40 ±4 km, which is similar to crustal thicknesses interpreted from refraction data in Jordan and Saudi Arabia. The different crustal blocks and large-scale mafic intrusions are best explained, though not uniquely, by Proterozoic convergence and suturing and early Paleozoic rifting, as interpreted in the exposed rocks of the Arabian shield. These two processes, combined with documented Mesozoic rifting and Cenozoic transpression, compose the crustal evolution of the northern Arabian platform beneath Syria.

  14. Bouguer gravity trends and crustal structure of the Palmyride Mountain belt and surrounding northern Arabian platform in Syria

    Energy Technology Data Exchange (ETDEWEB)

    Best, J.A.; Barazangi, M. (Cornell Univ., Ithaca, NY (USA)); Al-Saad, D.; Sawaf, T.; Gebran, A. (Syrian Petroleum Company, Damascus (Syria))

    1990-12-01

    This study examines the crustal structure of the Palmyrides and the northern Arabian platform in Syria by two- and three-dimensional modeling of the Bouguer gravity anomalies. Results of the gravity modeling indicate that (1) western Syria is composed of at least two different crustal blocks, (2) the southern crustal block is penetrated by a series of crustal-scale, high-density intrusive complexes, and (3) short-wavelength gravity anomalies in the southwest part of the mountain belt are clearly related to basement structure. The crustal thickness in Syria, as modeled on the gravity profiles, is approximately 40{plus minus}4 km, which is similar to crustal thicknesses interpreted from refraction data in Jordan and Saudi Arabia. The different crustal blocks and large-scale mafic intrusions are best explained, though not uniquely, by Proterozoic convergence and suturing and early Paleozoic rifting, as interpreted in the exposed rocks of the Arabian shield. These two processes, combined with documented Mesozoic rifting and Cenozoic transpression, compose the crustal evolution of the northern Arabian platform beneath Syria.

  15. The dehydration, rehydration and tectonic setting of greenstone belts in a portion of the northern Kaapvaal Craton, South Africa

    Science.gov (United States)

    Vanreenen, D. D.; Barton, J. M., Jr.; Roering, C.; Vanschalkwyk, J. C.; Smit, C. A.; Debeer, J. D.; Stettler, E. H.

    1986-01-01

    High-grade gneiss terranes and low-grade granite-greenstone terranes are well known in several Archaean domains. The geological relationship between these different crustal regions, however, is still controversial. One school of thought favors fundamental genetic differences between high-grade and low-grade terranes while others argue for a depth-controlled crustal evolution. The detailed examination of well-exposed Archaean terranes at different metamorphic grades, therefore, is not only an important source of information about the crustal levels exposed, but also is critical to the understanding of the possible tectonic and metamorphic evolution of greenstone belts with time. Three South African greenstone belts are compared.

  16. Tropics in Antarctica? Crustal Evaluation Education Project. Teacher's Guide [and] Student Investigation.

    Science.gov (United States)

    Stoever, Edward C., Jr.

    Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

  17. Trace element characteristics of mafic and ultramafic meta-igneous rocks from the 3.5 Ga. Warrawoona group: evidence for plume-lithosphere interaction beneath Archaean continental crust

    International Nuclear Information System (INIS)

    Bolhar, R.; Hergt, J.; Woodhead, J.

    1999-01-01

    Full text: Magnesian- to Fe-rich tholeiitic basalts represent the dominant lithology in the Marble Bar Greenstone Belt, E-Pilbara Craton, and are locally associated with komatiitic basalts and rare komatiitic cumulates. Based on trace element characteristics, the extrusive and intrusive rocks from all three major stratigraphic units can be subdivided into LREE enriched and unfractionated to weakly LREE depleted groups. The former group is characterized by La/Sm pm = 1.7-4.6, Gd/Yb pm = 1.23.2 and Nb/Th pm 0.1-0.5, while the latter rocks possess ratios of La/Sm pm = 0.5-1.7, Gd/Yb pm = 0.8-1.9 and Nb/Th pm = 0.4-1.3. Nb/La -Nb/Th relationships in the LREE enriched samples indicate 7-28% contamination by crustal material similar in composition to Pilbara granitoids. LREE enrichment and strong negative HFSE anomalies, along with MgO = 2.2-22.0 wt% and SiO 2 = 39.2-63.5 wt%, have been observed in numerous Archaean greenstone belts, and can be successfully modeled in this study by AFC processes. In contrast, strong HFSE depletion combined with unfractionated to slightly depleted LREE in rocks of the latter group require different processes. Melting of mantle material previously depleted by melt extraction, enrichment of LILE and LREE relative to the HFSE in an arc-like environment and HFSE fractionation as a result of garnet retention in the melting source cannot account for negative Nb, Ta, Ti, P and strong positive Pb anomalies. Introduction of small amounts of crustal material into a depleted or primitive mantle, as possibly indicated by Nb/Ta ratios between 12 and 18, also fails to reproduce the trace element abundances of the second group of rocks. Recycling of oceanic crust previously processed through a subduction zone (low Th/Nb, La/Nb) and sub-arc lithospheric mantle (high Th/Nb, La/Nb), and subsequent mixing into the Archaean mantle has been recently invoked by several workers (e.g. Kerrich et al., EPSL, 168, 101-115; 1999) to explain the origin of

  18. ADMAP-2: The second generation Antarctic crustal magnetic anomaly map.

    Science.gov (United States)

    Ferraccioli, F.; Golynsky, A.; Golynsky, D.; Young, D. A.; Eagles, G.; Damaske, D.; Finn, C.; Aitken, A.; von Frese, R. R. B.; Ghidella, M. E.; Kim, H. R.; Hong, J.

    2017-12-01

    ADMAP-2 is the second generation crustal magnetic anomaly compilation for the Antarctic region south of 60°S. It was produced from more than 3.5 million line-km of near-surface terrestrial, airborne and marine magnetic observations collected since the International Geophysical Year 1957/58 through 2013. The data were edited, IGRF corrected, profile levelled and gridded at a 1.5-km interval on a polar stereographic projection using the minimum curvature technique. Given the ubiquitous polar cover of snow, ice and sea water, the magnetic anomaly compilation offers important constraints on the global tectonic processes and crustal properties of the Antarctic. It also links widely separated areas of outcrop to help unify disparate geologic studies, and provides insights on the lithospheric transition between Antarctica and adjacent oceans, as well as the geodynamic evolution of the Antarctic lithosphere in the assembly and break-up of the Gondwana, Rodinia, and Columbia supercontinents and key piercing points for reconstructing linkages between the protocontinents. The magnetic data together with ice-probing radar and gravity information greatly facilitate understanding the evolution of fundamental large-scale geological processes such as continental rifting, intraplate mountain building, subduction and terrane accretion processes, and intraplate basin formation.

  19. Geochemical evidence for subduction in the early Archaean from quartz-carbonate-fuchsite mineralization, Isua Supracrustal Belt, West Greenland

    DEFF Research Database (Denmark)

    Pope, Emily Catherine; Rosing, Minik Thorleif; Bird, Dennis K.

    Quartz, carbonate and fuchsite (chromian muscovite) is a common metasomatic assemblage observed in orogenic gold systems, both in Phanerozoic convergent margin settings, and within supracrustal and greenstone belts of Precambrian rocks. Geologic and geochemical observations in younger orogenic...... systems suggest that ore-forming metasomatic fluids are derived from subduction-related devolitilization reactions, implying that orogenic Au-deposits in Archaean and Proterozoic supracrustal rock suites are related to subduction-style plate tectonics beginning early in Earth history. Justification...... with Phanerozoic orogenic deposits and that this type of metasomatism is a unique result of subduction-related processes. Fuchsite from the ISB has a δ18O and δD of 7.7 to 17.9‰ and -115 to -61‰, respectively. δ18O of quartz from the same rocks is between 10.3 and 18.6‰. Muscovite-quartz oxygen isotope thermometry...

  20. NASA plan for international crustal dynamics studies

    Science.gov (United States)

    1979-01-01

    The international activities being planned as part of the NASA geodynamics program are described. Methods of studying the Earth's crustal movements and deformation characteristics are discussed. The significance of the eventual formalations of earthquake predictions methods is also discussed.

  1. Crustal balance and crustal flux from shortening estimates in the Central Andes

    Science.gov (United States)

    Hindle, David; Kley, Jonas; Oncken, Onno; Sobolev, Stephan

    2005-01-01

    The Central Andes of South America form the second largest high elevation plateau on earth. Extreme elevations have formed on a noncollisional margin with abundant associated arc magmatism. It has long been thought that the crustal thickness necessary to support Andean topography was not accounted for by known crustal shortening alone. We show that this may in part be due to a two-dimensional treatment of the problem. A three-dimensional analysis of crustal shortening and crustal thickness shows that displacement of material towards the axis of the bend in the Central Andes has added a significant volume of crust not accounted for in previous comparisons. We find that present-day crustal thickness between 12°S and 25°S is accounted for (∼-10% to ∼+3%)with the same shortening estimates, and the same assumed initial crustal thickness as had previously led to the conclusion of a ∼25-35% deficit in shortening relative to volume of crustal material. We suggest that the present-day measured crustal thickness distribution may not match that predicted due to shortening, and substantial redistribution of crust may have occurred by both erosion and deposition at the surface and lower crustal flow in regions of the thermally weakened middle and lower crust.

  2. "Storms of crustal stress" and AE earthquake precursors

    Directory of Open Access Journals (Sweden)

    G. P. Gregori

    2010-02-01

    Full Text Available Acoustic emission (AE displays violent paroxysms preceding strong earthquakes, observed within some large area (several hundred kilometres wide around the epicentre. We call them "storms of crustal stress" or, briefly "crustal storms". A few case histories are discussed, all dealing with the Italian peninsula, and with the different behaviour shown by the AE records in the Cephalonia island (Greece, which is characterized by a different tectonic setting.

    AE is an effective tool for diagnosing the state of some wide slab of the Earth's crust, and for monitoring its evolution, by means of AE of different frequencies. The same effect ought to be detected being time-delayed, when referring to progressively lower frequencies. This results to be an effective check for validating the physical interpretation.

    Unlike a seismic event, which involves a much limited focal volume and therefore affects a restricted area on the Earth's surface, a "crustal storm" typically involves some large slab of lithosphere and crust. In general, it cannot be easily reckoned to any specific seismic event. An earthquake responds to strictly local rheological features of the crust, which are eventually activated, and become crucial, on the occasion of a "crustal storm". A "crustal storm" lasts typically few years, eventually involving several destructive earthquakes that hit at different times, at different sites, within that given lithospheric slab.

    Concerning the case histories that are here discussed, the lithospheric slab is identified with the Italian peninsula. During 1996–1997 a "crustal storm" was on, maybe elapsing until 2002 (we lack information for the period 1998–2001. Then, a quiet period occurred from 2002 until 26 May 2008, when a new "crustal storm" started, and by the end of 2009 it is still on. During the 1996–1997 "storm" two strong earthquakes occurred (Potenza and

  3. Geological evolution of the Antongil Craton, NE Madagascar

    Science.gov (United States)

    Schofield, D.I.; Thomas, Ronald J.; Goodenough, K.M.; De Waele, B.; Pitfield, P.E.J.; Key, R.M.; Bauer, W.; Walsh, G.J.; Lidke, D.J.; Ralison, A.V.; Rabarimanana, M.; Rafahatelo, J.-M.; Randriamananjara, T.

    2010-01-01

    The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U-Pb zircon analyses to provide an event history for its evolution.The oldest rocks in the Antongil Craton form a nucleus of tonalitic gneiss, characteristic of Palaeo-Mesoarchaean cratons globally, including phases dated between 3320 ?? 14. Ma to 3231 ?? 6. Ma and 3187 ?? 2. Ma to 3154 ?? 5. Ma. A series of mafic dykes was intruded into the Mesoarchaean tonalites and a sedimentary succession was deposited on the craton prior to pervasive deformation and migmatisation of the region. The age of deposition of the metasediments has been constrained from a volcanic horizon to around 3178 ?? 2. Ma and subject to migmatisation at around 2597 ?? 49. Ma. A subsequent magmatic episode generated voluminous, weakly foliated granitic rocks, that also included additions from both reworked older crustal material and younger source components. An earlier granodiorite-dominated assemblage, dated between 2570 ?? 18. Ma and 2542 ?? 5. Ma, is largely exposed in xenoliths and more continuously in the northern part of the craton, while a later monzogranite-dominated phase, dated between 2531 ?? 13. Ma and 2513 ?? 0.4. Ma is more widely developed. Together these record the stabilisation of the craton, attested to by the intrusion of a younger dyke swarm, the age of which is constrained by a sample of metagabbro dated at 2147 ?? 6. Ma, providing the first evidence for Palaeoproterozoic rocks from the Antongil Craton.The youngest events recorded in the isotopic record of the Antongil Craton are reflected in metamorphism, neocrystallisation and Pb-loss at 792 ?? 130. Ma to 763 ?? 13. Ma and 553 ?? 68. Ma. These events are

  4. Magma oceanography. II - Chemical evolution and crustal formation. [lunar crustal rock fractional crystallization model

    Science.gov (United States)

    Longhi, J.

    1977-01-01

    A description is presented of an empirical model of fractional crystallization which predicts that slightly modified versions of certain of the proposed whole moon compositions can reproduce the major-element chemistry and mineralogy of most of the primitive highland rocks through equilibrium and fractional crystallization processes combined with accumulation of crystals and trapping of residual liquids. These compositions contain sufficient Al to form a plagioclase-rich crust 60 km thick on top of a magma ocean that was initially no deeper than about 300 km. Implicit in the model are the assumptions that all cooling and crystallization take place at low pressure and that there are no compositional or thermal gradients in the liquid. Discussions of the cooling and crystallization of the proposed magma ocean show these assumptions to be disturbingly naive when applied to the ocean as a whole. However, the model need not be applied to the whole ocean, but only to layers of cooling liquid near the surface.

  5. Late Archaean tectonics and sedimentation of the South Rand area, Witwatersrand basin

    International Nuclear Information System (INIS)

    Spencer, R.M.

    1992-01-01

    The sedimentary fill of the southern part of the northeastern Witwatersrand basin consists of four unconformity bounded mega sequences. Early sedimentation took place in a stable, epi continental basin characterized by amphidromic flow. Gradual transgression to distal shelf facies was followed by gradual emergence to intertidal facies. Unconformity Bounded Mega sequence 2 shows that the basin underwent regression, in which discrete uplifts provided a source of granite-greenstone-derived sediment to associated braid plain aprons. Thereafter the basin subsided into a system almost identical to that in which Unconformity Bounded Mega sequence 1 developed. Unconformity Bounded Mega sequence 3 was deposited in a similar marine environment, on an angular unconformity in the east. Regional uplift occurred to the northwest of the basin. Unconformity Bounded Mega sequence 4 records progradation of a perennial braid plain controlled by uplift in the east, and by the minor influence of an uplift to the northwest. Rapid transgression resulted in submarine fan facies development, after which rapid emergence was controlled by uplift in the east, and to a lesser extent, the north. The braid plain was the site of extrusion of komatiitic lavas of the lower Ventersdorp Supergroup and was subsequently smothered by the sustained outpouring of a two kilometer-thick pile of basalts. Crustal extension climaxed after extrusion of felsic volcanics. This extension is antithetic to regional down-to-the-northwest, lower Ventersdorp Supergroup rifting. The last conspicuous phase of Precambrian tectonics is the superposition of a right-lateral wrench system on the early structural framework, after deposition of the lower Transvaal Sequence. Analysis of the samples was carried out by X-ray fluorescence spectrometry. 243 refs., 119 figs., 8 tabs

  6. Estimating Crustal Properties Directly from Satellite Tracking Data by Using a Topography-based Constraint

    Science.gov (United States)

    Goossens, S. J.; Sabaka, T. J.; Genova, A.; Mazarico, E. M.; Nicholas, J. B.; Neumann, G. A.; Lemoine, F. G.

    2017-12-01

    The crust of a terrestrial planet is formed by differentiation processes in its early history, followed by magmatic evolution of the planetary surface. It is further modified through impact processes. Knowledge of the crustal structure can thus place constraints on the planet's formation and evolution. In particular, the average bulk density of the crust is a fundamental parameter in geophysical studies, such as the determination of crustal thickness, studies of the mechanisms of topography support, and the planet's thermo-chemical evolution. Yet even with in-situ samples available, the crustal density is difficult to determine unambiguously, as exemplified by the results for the Gravity and Recovery Interior Laboratory (GRAIL) mission, which found an average crustal density for the Moon that was lower than generally assumed. The GRAIL results were possible owing to the combination of its high-resolution gravity and high-resolution topography obtained by the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter (LRO), and high correlations between the two datasets. The crustal density can be determined by its contribution to the gravity field of a planet, but at long wavelengths flexure effects can dominate. On the other hand, short-wavelength gravity anomalies are difficult to measure, and either not determined well enough (other than at the Moon), or their power is suppressed by the standard `Kaula' regularization constraint applied during inversion of the gravity field from satellite tracking data. We introduce a new constraint that has infinite variance in one direction, called xa . For constraint damping factors that go to infinity, it can be shown that the solution x becomes equal to a scale factor times xa. This scale factor is completely determined by the data, and we call our constraint rank-minus-1 (RM1). If we choose xa to be topography-induced gravity, then we can estimate the average bulk crustal density directly from the data

  7. Investigation of lunar crustal structure and isostasy. Final technical report

    International Nuclear Information System (INIS)

    Thurber, C.H.

    1987-07-01

    The lunar mascon basins have strongly free air gravity anomalies, generally exceeding 100 milligals at an elevation of 100 km. The source of the anomalies is a combination of mantle uplift beneath the impact basins and subsequent infilling by high-density mare basalts. The relative contribution of these two components is still somewhat uncertain, although it is generally accepted that the amount of mantle uplift greatly exceeds the thickness of the basalts. Extensive studies have been carried out of the crustal structure of mare basins, based on gravity data, and their tectonic evolution, based on compressive and extensional tectonic features. The present study endeavored to develop a unified, self-consistent model of the lunar crust and lithosphere incorporating both gravity and tectonic constraints

  8. Crustal thickness and Vp/Vs beneath the southeastern United States: Constraints from receiver function stacking

    Science.gov (United States)

    Yang, Q.; Gao, S. S.; Liu, K. H.

    2017-12-01

    To provide new constraints on crustal structure and evolution models beneath a collage of tectonic provinces in the southeastern United States, a total of 10,753 teleseismic receiver functions recorded by 125 USArray and other seismic stations are used to compute crustal thickness and Vp/Vs values. The resulting crustal thicknesses range from 25 km at the coast to 51 km beneath the peak of the southern Appalachians with an average of 36.2 km ± 5.5 km. The resulting crustal thicknesses correlate well with surface elevation and Bouguer gravity anomalies. Beneath the Atlantic Coastal Plain, the crustal thicknesses show a clear eastward thinning with a magnitude of 10 km, from about 40 km beneath the western margin to 30 km beneath the coast. The Vp/Vs values for the entire study area range from 1.71 to 1.90 with a mean value of 1.80 ± 0.04. The mean Vp/Vs value is 1.82±0.035 in the southern Appalachian Mountain. The slightly larger than normal crustal Vp/Vs for this area might be the result of significant erosion of the felsic upper crust over the past 300 million years. Alternatively, it could also suggest the existence of pervasive magmatic intrusion into the Appalachian crust. The Vp/Vs measurements in the Atlantic Coastal Plain increase toward the east, ranging from 1.75 to 1.82, probably indicating a gradual increase of mafic magmatic intrusion into thinner crust during the development of the passive continental margin.

  9. Geodynamic evolution and crustal growth of the central Indian Shield

    Indian Academy of Sciences (India)

    R. Narasimhan (Krishtel eMaging) 1461 1996 Oct 15 13:05:22

    mantle interaction in the generation of the granitoids. The low values of Mg#, Ni, Cr, Sr and high values of SiO2 in the gneisses in turn overrules such melt-mantle interaction. 1. Introduction. The formation and growth of continental crust.

  10. Archean crustal evolution in the central Minto block, northern Quebec

    International Nuclear Information System (INIS)

    Skulski, T.; Percival, J.A.; Stern, R.A.

    1996-01-01

    The central Minto block contains three volcano-sedimentary successions. Near Lake Qalluviartuuq, an isotopically primitive ( 2.83 Ga ε Nd +3.8 to +2.3) 2.83 Ga volcano-plutonic sequence comprises depleted tholeiitic basalts, anorthositic gabbro, and diorite-granodiorite that is unconformably overlain by 2.76 Ga ε Nd +1.8) calc-alkaline sequence of pillow basalts, andesites, and peridotite cut by 2.73 Ga diorite. To the west, and in inferred tectonic contact, the sediment-dominated Kogaluc sequence includes both isotopically evolved calc-alkaline rocks ( 2.76 Ga ε Nd +1.6 to -0.1) including 2.78Ga ε Nd Nd 2.725Ga ε Nd - 1. 6). (author). 19 refs., 4 tabs., 5 figs

  11. Archean crustal evolution in the central Minto block, northern Quebec

    Energy Technology Data Exchange (ETDEWEB)

    Skulski, T; Percival, J A; Stern, R A [Geological Survey of Canada, Ottawa, ON (Canada)

    1997-12-31

    The central Minto block contains three volcano-sedimentary successions. Near Lake Qalluviartuuq, an isotopically primitive ({sup 2.83} {sup Ga}{epsilon}{sub Nd} +3.8 to +2.3) 2.83 Ga volcano-plutonic sequence comprises depleted tholeiitic basalts, anorthositic gabbro, and diorite-granodiorite that is unconformably overlain by <2.77 Ga conglomerates. Overlying the conglomerate is a more evolved ({sup 2.76} {sup Ga}{epsilon}{sub Nd} +1.8) calc-alkaline sequence of pillow basalts, andesites, and peridotite cut by 2.73 Ga diorite. To the west, and in inferred tectonic contact, the sediment-dominated Kogaluc sequence includes both isotopically evolved calc-alkaline rocks ({sup 2.76} {sup Ga}{epsilon}{sub Nd} +1.6 to -0.1) including <2.76 Ga rhyolitic tuff, pillowed andesites, and 2.76 Ga quartz-feldspar porphyry, and less abundant, depleted tholeiitic basalts (2.76 GaF-Nd +2.4). These are interlain with sedimentary rocks including banded iron-formation, quartzite, and metagreywacke. Calc-alkaline batholiths include 2.78 Ga pyroxene-bearing intermediate and felsic plutons ({sup 2.78Ga}{epsilon}{sub Nd} <+2.7) and younger, peraluminous tonalites ({epsilon}{sub Nd} <+1.3). Late, 2.73 Ga peraluminous granitoids are isotopically evolved ({sup 2.725Ga}{epsilon}{sub Nd} - 1. 6). (author). 19 refs., 4 tabs., 5 figs.

  12. The Crustal Structure of the North-South Earthquake Belt in China Revealed from Deep Seismic Soundings and Gravity Data

    Science.gov (United States)

    Zhao, Yang; Guo, Lianghui; Shi, Lei; Li, Yonghua

    2018-01-01

    The North-South earthquake belt (NSEB) is one of the major earthquake regions in China. The studies of crustal structure play a great role in understanding tectonic evolution and in evaluating earthquake hazards in this region. However, some fundamental crustal parameters, especially crustal interface structure, are not clear in this region. In this paper, we reconstructed the crustal interface structure around the NSEB based on both the deep seismic sounding (DSS) data and the gravity data. We firstly reconstructed the crustal structure of crystalline basement (interface G), interface between upper and lower crusts (interface C) and Moho in the study area by compiling the results of 38 DSS profiles published previously. Then, we forwardly calculated the gravity anomalies caused by the interfaces G and C, and then subtracted them from the complete Bouguer gravity anomalies, yielding the regional gravity anomalies mainly due to the Moho interface. We then utilized a lateral-variable density interface inversion technique with constraints of the DSS data to invert the regional anomalies for the Moho depth model in the study area. The reliability of our Moho depth model was evaluated by comparing with other Moho depth models derived from other gravity inversion technique and receiver function analysis. Based on our Moho depth model, we mapped the crustal apparent density distribution in the study area for better understanding the geodynamics around the NSEB.

  13. Late Archaean-early Proterozoic source ages of zircons in rocks from the Paleozoic orogen of western Galicia, NW Spain

    Energy Technology Data Exchange (ETDEWEB)

    Kuijper, R P; Priem, H N.A. [Laboratorium voor Isotopen-Geologie, Amsterdam (Netherlands); Den Tex, E [Rijksuniversiteit Utrecht (Netherlands). Inst. voor Aardwetenschappen

    1982-08-01

    U-Pb data are reported for nine suites of zircons and three monazites from the Paleozoic orogen in western Galicia: one paragneiss and six orthogneisses from the early Paleozoic basement, and two Carboniferous (ca. 310 Ma old) intrusions of two-mica granite. New whole-rock Rb-Sr analyses, along with earlier data, indicate an age of ca. 470-440 Ma (Ordovician) for the emplacement of the granitic precursors of the orthogneisses. Monazite from the paragneiss also yields an U-Pb age of ca. 470 Ma. From the high initial /sup 87/Sr//sup 86/Sr ratios an involvement of Precambrian continental crust material is evident in the generation of the early Paleozoic suite of granites, while the zircon U-Pb data give evidence of the presence of about 3.0-2.0 Ga old (late Archaean-early Proterozoic) components in the source material. Zircons from the oldest sedimentary rocks in the area, now present as catazonal paragneisses and a likely source for the granites, likewise reveal a provenance age of 3.0-2.0 Ga.

  14. Crustal structure of the Khartoum Basin, Sudan

    CSIR Research Space (South Africa)

    El Tahir, N

    2013-05-01

    Full Text Available Basin ranges between 33 and 37 km, with an average of 35 km, and that the crustal Vp/Vs ratio ranges from 1.74 to 1.81, with an average of 1.78. From the joint inversion of receiver functions and Rayleigh wave group velocities,we obtained similar results...

  15. How Does Heat Flow Vary in the Ocean Floor? Crustal Evaluation Education Project. Teacher's Guide [and] Student Investigation.

    Science.gov (United States)

    Stoever, Edward C., Jr.

    Crustal Evolution Education Project (CEEP) modules were designed to: (1) provide students with the methods and results of continuing investigations into the composition, history, and processes of the earth's crust and the application of this knowledge to man's activities and (2) to be used by teachers with little or no previous background in the…

  16. Estimating the Crustal Power Spectrum From Vector Magsat Data: Crustal Power Spectrum

    Science.gov (United States)

    Lowe, David A. J.; Parker, Robert L.; Purucker, Michael E.; Constable, Catherine G.

    2000-01-01

    The Earth's magnetic field can be subdivided into core and crustal components and we seek to characterize the crustal part through its spatial power spectrum (R(sub l)). We process vector Magsat data to isolate the crustal field and then invert power spectral densities of flight-local components along-track for R(sub l) following O'Brien et al. [1999]. Our model (LPPC) is accurate up to approximately degree 45 (lambda=900 km) - this is the resolution limit of our data and suggests that global crustal anomaly maps constructed from vector Magsat data should not contain features with wavelengths less than 900 km. We find continental power spectra to be greater than oceanic ones and attribute this to the relative thicknesses of continental and oceanic crust.

  17. Constraints on the formation of the Martian crustal dichotomy from remnant crustal magnetism

    Science.gov (United States)

    Citron, Robert I.; Zhong, Shijie

    2012-12-01

    The Martian crustal dichotomy characterizing the topographic difference between the northern and southern hemispheres is one of the most important features on Mars. However, the formation mechanism for the dichotomy remains controversial with two competing proposals: exogenic (e.g., a giant impact) and endogenic (e.g., degree-1 mantle convection) mechanisms. Another important observation is the Martian crustal remnant magnetism, which shows a much stronger field in the southern hemisphere than in the northern hemisphere and also magnetic lineations. In this study, we examine how exogenic and endogenic mechanisms for the crustal dichotomy are constrained by the crustal remnant magnetism. Assuming that the dichotomy is caused by a giant impact in the northern hemisphere, we estimate that the average thickness of ejecta in the southern hemisphere is 20-25 km. While such a giant impact may cause crustal demagnetization in the northern hemisphere, we suggest that the impact could also demagnetize the southern hemisphere via ejecta thermal blanketing, impact demagnetization, and heat transfer from the hot layer of ejecta, thus posing a challenge for the giant impact model. We explore how the pattern of magnetic lineations relates to endogenic theories of dichotomy formation, specifically crustal production via degree-1 mantle convection. We observe that the pattern of lineations roughly corresponds to concentric circles about a single pole, and determine the pole for the concentric circles at 76.5° E and 84.5° S, which nearly overlaps with the centroid of the thickened crust in the southern hemisphere. We suggest that the crustal magnetization pattern, magnetic lineations, and crustal dichotomy (i.e., thickened crust in the highlands) can be explained by a simple endogenic process; one-plume convection causes melting and crustal production above the plume in the southern hemisphere, and strong crustal magnetization and magnetic lineations are formed in the southern

  18. Crustal parameters in the Iberian Peninsula

    Science.gov (United States)

    Banda, E.

    1988-06-01

    The structure of the crust in the Iberian Peninsula has been investigated for the last 15 years by Spanish and Portuguese groups in close collaboration with other European institutions. The first experiments were carried out in Portugal (Mueller et al., 1973) with the aim of investigating the crustal structure of the Hercynian belt in the southwest corner of the Iberian peninsula. Other experiments have been subsequently realized to study different aspects of the crust in various regions of Portugal. In Spain the main effort has been focused in Alpine areas, with the first experiments in the Alboran Sea and the Betic Cordilleras (Working Group for Deep Seismic Sounding in Spain, 1974-1975, 1977; Working Group for Deep Seismic Sounding in the Alboran Sea, 1974-1975, 1978). Follow-up experiments until 1981 completed the work in the Betic Cordillera. Extensive experiments were carried out in the Pyrenees in 1978. Further surveys covered the Balearic Islands in 1976, the Valencia Trough in 1976 and 1983, and the Celtiberian Chain (or Iberic system) in 1981. The Hercynian belt has only been studied in detail in the northwest corner of Spain in 1982, with smaller studies in the central Iberian Massif in 1976 and 1986. Mostaanpour (1984) has compiled some crustal parameters (crustal thickness, average crustal velocity and Pn velocity) for western Europe. Meanwhile, more complete data are available for the Iberian Peninsula. The results presented here were derived from a large number of seismic refraction experiments which have been carried out mostly along or close to coastal areas of the Iberian Peninsula. Offshore explosions of various sizes were used as the energy source in most cases, in addition to some quarry blasts. Unfortunately this leaves most of the inner part of the Iberian Peninsula unsurveyed. Our purpose is to summarize some of the crustal parameters obtained so far and to detail the appropriate literature for the interested reader.

  19. Fine-scale crustal structure of the Azores Islands from teleseismic receiver functions

    Science.gov (United States)

    Spieker, K.; Rondenay, S.; Ramalho, R. S.; Thomas, C.; Helffrich, G. R.

    2016-12-01

    The Azores plateau is located near the Mid-Atlantic Ridge (MAR) and consists of nine islands, most of which lie east of the MAR. Various methods including seismic reflection, gravity, and passive seismic imaging have been used to investigate the crustal thickness beneath the islands. They have yielded thickness estimates that range between roughly 10 km and 30 km, but until now models of the fine-scale crustal structure have been lacking. A comparison of the crustal structure beneath the islands that lie west and east of the MAR might give further constraints on the evolution of the islands. For example, geochemical studies carried out across the region predict the existence of volcanic interfaces that should be detected seismically within the shallow crust of some of the islands. In this study, we use data from ten seismic stations located on the Azores Islands to investigate the crustal structure with teleseismic P-wave receiver functions. We query our resulting receiver functions for signals associated with the volcanic edifice, the crust-mantle boundary, and potential underplated layers beneath the various islands. The islands west of the MAR have a crustal structure comprising two discontinuities - an upper one at 1-2 km depth marking the base of the volcanic edifice, and a lower one at 10 km depth that we interpret as crust-mantle boundary. The islands east of the MAR can be subdivided into two groups. The central islands that are closer to the MAR exhibit a crustal structure similar to that of the western islands, with a volcanic edifice reaching a depth of 2 km and an average crust-mantle boundary at around 12 km depth. The easternmost islands, located on the oldest lithosphere, exhibit a more complex crustal structure with evidence for a mid-crustal interface and an underplated layer, yielding an effective crust-mantle boundary at >15 km depth. The difference in structure between proximal and distal islands might be related to the age of the plate at the

  20. Crustal growth of the Izu-Ogasawara arc estimated from structural characteristics of Oligocene arc

    Science.gov (United States)

    Takahashi, N.; Yamashita, M.; Kodaira, S.; Miura, S.; Sato, T.; No, T.; Tatsumi, Y.

    2011-12-01

    Japan Agency for Marine-Earth Science and Technology (JAMSTEC) carried out seismic surveys using a multichannel reflection system and ocean bottom seismographs, and we have clarified crustal structures of whole Izu-Ogasawara (Bonin)-Marina (IBM) arc since 2002. These refection images and velocity structures suggest that the crustal evolution in the intra-oceanic island arc accompanies with much interaction of materials between crust and mantle. Slow mantle velocity identified beneath the thick arc crusts suggests that dense crustal materials transformed into the mantle. On the other hand, high velocity lower crust can be seen around the bottom of the crust beneath the rifted region, and it suggests that underplating of mafic materials occurs there. Average crustal production rate of the entire arc is larger than expected one and approximately 200 km3/km/Ma. The production rate of basaltic magmas corresponds to that of oceanic ridge. Repeated crustal differentiation is indispensable to produce much light materials like continental materials, however, the real process cannot still be resolved yet. We, therefore, submitted drilling proposals to obtain in-situ middle crust with P-wave velocity of 6 km/s. In the growth history of the IBM arc, it is known by many papers that boninitic volcanisms preceded current bimodal volcanisms based on basaltic magmas. The current volcanisms accompanied with basaltic magmas have been occurred since Oligocene age, however, the tectonic differences to develop crustal architecture between Oligocene and present are not understood yet. We obtained new refraction/reflection data along an arc strike of N-S in fore-arc region. Then, we estimate crustal structure with severe change of the crustal thickness from refraction data, which are similar to that along the volcanic front. Interval for location of the thick arc crust along N-S is very similar to that along the volcanic front. The refection image indicates that the basement of the fore

  1. Crustal tomographic imaging and geodynamic implications toward south of Southern Granulite Terrain (SGT), India

    Science.gov (United States)

    Behera, Laxmidhar

    2011-09-01

    The crustal structure toward southern part of SGT is poorly defined leaving an opportunity to understand the tectonic and geodynamic evolution of this high-grade granulite terrain surrounded by major shear and tectonically disturbed zones like Achankovil Shear Zone (AKSZ) and Palghat Cauvery Shear Zone (PCSZ). To develop a geologically plausible crustal tectonic model depicting major structural elements, a comprehensive tomographic image was derived using deep-seismic-sounding data corroborated by Bouguer gravity modeling, coincident-reflection-seismic, heat-flow and available geological/geochronological informations along the N-S trending Vattalkundu-Kanyakumari geotransect. The final tectonic model represents large compositional changes of subsurface rocks accompanied by velocity heterogeneities with crustal thinning (44-36 km) and Moho upwarping from north to south. This study also reveals and successfully imaged anomalous zone of exhumation near AKSZ having transpression of exhumed rocks at mid-to-lower crustal level (20-30 km) with significant underplating and mantle upwelling forming a complex metamorphic province. The presence of shear zones with high-grade charnockite massifs in the upper-crust exposed in several places reveal large scale exhumation of granulites during the Pan-African rifting (~ 550 Ma) and provide important insights of plume-continental lithosphere interaction with reconstruction of the Gondwanaland.

  2. Thinned crustal structure and tectonic boundary of the Nansha Block, southern South China Sea

    Science.gov (United States)

    Dong, Miao; Wu, Shi-Guo; Zhang, Jian

    2016-12-01

    The southern South China Sea margin consists of the thinned crustal Nansha Block and a compressional collision zone. The Nansha Block's deep structure and tectonic evolution contains critical information about the South China Sea's rifting. Multiple geophysical data sets, including regional magnetic, gravity and reflection seismic data, reveal the deep structure and rifting processes. Curie point depth (CPD), estimated from magnetic anomalies using a windowed wavenumber-domain algorithm, enables us to image thermal structures. To derive a 3D Moho topography and crustal thickness model, we apply Oldenburg algorithm to the gravity anomaly, which was extracted from the observed free air gravity anomaly data after removing the gravity effect of density variations of sediments, and temperature and pressure variations of the lithospheric mantle. We found that the Moho depth (20 km) is shallower than the CPD (24 km) in the Northwest Borneo Trough, possibly caused by thinned crust, low heat flow and a low vertical geothermal gradient. The Nansha Block's northern boundary is a narrow continent-ocean transition zone constrained by magnetic anomalies, reflection seismic data, gravity anomalies and an interpretation of Moho depth (about 13 km). The block extends southward beneath a gravity-driven deformed sediment wedge caused by uplift on land after a collision, with a contribution from deep crustal flow. Its southwestern boundary is close to the Lupar Line defined by a significant negative reduction to the pole (RTP) of magnetic anomaly and short-length-scale variation in crustal thickness, increasing from 18 to 26 km.

  3. NASA Space Geodesy Program: GSFC data analysis, 1992. Crustal Dynamics Project VLBI geodetic results, 1979 - 1991

    Science.gov (United States)

    Ryan, J. W.; Ma, C.; Caprette, D. S.

    1993-01-01

    The Goddard VLBI group reports the results of analyzing 1648 Mark 3 data sets acquired from fixed and mobile observing sites through the end of 1991, and available to the Crustal Dynamics Project. Two large solutions were used to obtain Earth rotation parameters, nutation offsets, radio source positions, site positions, site velocities, and baseline evolution. Site positions are tabulated on a yearly basis for 1979 to 1995, inclusive. Site velocities are presented in both geocentric Cartesian and topocentric coordinates. Baseline evolution is plotted for 200 baselines, and individual length determinations are presented for an additional 356 baselines. This report includes 155 quasar radio sources, 96 fixed stations and mobile sites, and 556 baselines.

  4. The development and evolution of landform based on neotectonic ...

    Indian Academy of Sciences (India)

    Lingmin Zhong

    2018-02-14

    Feb 14, 2018 ... involving integration of data from the aspects of structural geology ... regions is very sensitive to crustal movement such as folding and faulting ...... drainage network evolution in the upper Narmada Valley: Implication to ...

  5. Rock strength measurements on Archaean basement granitoids recovered from scientific drilling in the active Koyna seismogenic zone, western India

    Science.gov (United States)

    Goswami, Deepjyoti; Akkiraju, Vyasulu V.; Misra, Surajit; Roy, Sukanta; Singh, Santosh K.; Sinha, Amalendu; Gupta, Harsh; Bansal, B. K.; Nayak, Shailesh

    2017-08-01

    Reservoir triggered earthquakes have been occurring in the Koyna area, western India for the past five decades. Triaxial tests carried out on 181 core samples of Archaean granitoids underlying the Deccan Traps provide valuable constraints on rock strength properties in the Koyna seismogenic zone for the first time. The data include measurements on granite gneiss, granite, migmatitic gneiss and mylonitised granite gneiss obtained from boreholes KBH-3, KBH-4A, KBH-5 and KBH-7 located in the western and eastern margins of the seismic zone. Salient results are as follows. (i) Increase of rock strength with increasing confining pressure allow determination of the linearized failure envelopes from which the cohesive strength and angle of internal friction are calculated. (ii) Variable differential stresses at different depths are the manifestations of deformation partitioning in close association of fault zone(s) or localized fracture zones. (iii) Fractures controlled by naturally developed weak planes such as cleavage and fabric directly affect the rock strength properties, but the majority of failure planes developed during triaxial tests is not consistent with the orientations of pre-existing weak planes. The failure planes may, therefore, represent other planes of weakness induced by ongoing seismic activity. (iv) Stress-strain curves confirm that axial deformation is controlled by the varying intensity of pre-existing shear in the granitoids, viz., mylonite, granite gneiss and migmatitic gneiss. (v) Frequent occurrences of low magnitude earthquakes may be attributed to low and variable rock strength of the granitoids, which, in turn, is modified by successive seismic events.

  6. Crustal Structure beneath Alaska from Receiver Functions

    Science.gov (United States)

    Zhang, Y.; Li, A.

    2017-12-01

    The crustal structure in Alaska has not been well resolved due to the remote nature of much of the state. The USArray Transportable Array (TA), which is operating in Alaska and northwestern Canada, significantly increases the coverage of broadband seismic stations in the region and allows for a more comprehensive study of the crust. We have analyzed P-receiver functions from earthquake data recorded by 76 stations of the TA and AK networks. Both common conversion point (CCP) and H-K methods are used to estimate the mean crustal thickness. The results from the CCP stacking method show that the Denali fault marks a sharp transition from thick crust in the south to thin crust in the north. The thickest crust up to 52 km is located in the St. Elias Range, which has been formed by oblique collision between the Yakutat microplate and North America. A thick crust of 48 km is also observed beneath the eastern Alaska Range. These observations suggest that high topography in Alaska is largely compensated by the thick crust root. The Moho depth ranges from 28 km to 35 km beneath the northern lowlands and increases to 40-45 km under the Books Range. The preliminary crustal thickness from the H-K method generally agrees with that from the CCP stacking with thicker crust beneath high mountain ranges and thinner crust beneath lowlands and basins. However, the offshore part is not well constrained due to the limited coverage of stations. The mean Vp/Vs ratio is around 1.7 in the Yukon-Tanana terrane and central-northern Alaska. The ratio is about 1.9 in central and southern Alaska with higher values at the Alaska Range, Wrangell Mountains, and St. Elias Range. Further data analyses are needed for obtaining more details of the crustal structure in Alaska to decipher the origin and development of different tectonic terranes.

  7. Interaction between mantle and crustal detachments: a non-linear system controlling lithospheric extension

    Science.gov (United States)

    Rosenbaum, G.; Regenauer-Lieb, K.; Weinberg, R. F.

    2009-12-01

    We use numerical modelling to investigate the development of crustal and mantle detachment faults during lithospheric extension. Our models simulate a wide range of rift systems with varying values of crustal thickness and heat flow, showing how strain localization in the mantle interacts with localization in the upper crust and controls the evolution of extensional systems. Model results reveal a richness of structures and deformation styles, which grow in response to a self-organized mechanism that minimizes the internal stored energy of the system by localizing deformation at different levels of the lithosphere. Crustal detachment faults are well developed during extension of overthickened (60 km) continental crust, even when the initial heat flow is relatively low (50 mW/m2). In contrast, localized mantle deformation is most pronounced when the extended lithosphere has a normal crustal thickness (30-40 km) and an intermediate (60-70 mW/m2) heat flow. Results show a non-linear response to subtle changes in crustal thickness or heat flow, characterized by abrupt and sometime unexpected switches in extension modes (e.g. from diffuse rifting to effective lithospheric-scale rupturing) or from mantle- to crust-dominated strain localization. We interpret this non-linearity to result from the interference of doming wavelengths. Disharmony of crust and mantle doming wavelengths results in efficient communication between shear zones at different lithospheric levels, leading to rupturing of the whole lithosphere. In contrast, harmonious crust and mantle doming inhibits interaction of shear zones across the lithosphere and results in a prolonged rifting history prior to continental breakup.

  8. Interaction between mantle and crustal detachments: A nonlinear system controlling lithospheric extension

    Science.gov (United States)

    Rosenbaum, Gideon; Regenauer-Lieb, Klaus; Weinberg, Roberto F.

    2010-11-01

    We use numerical modeling to investigate the development of crustal and mantle detachments during lithospheric extension. Our models simulate a wide range of extensional systems with varying values of crustal thickness and heat flow, showing how strain localization in the mantle interacts with localization in the upper crust and controls the evolution of extensional systems. Model results reveal a richness of structures and deformation styles as a response to a self-organized mechanism that minimizes the internal stored energy of the system by localizing deformation. Crustal detachments, here referred as low-angle normal decoupling horizons, are well developed during extension of overthickened (60 km) continental crust, even when the initial heat flow is relatively low (50 mW m-2). In contrast, localized mantle deformation is most pronounced when the extended lithosphere has a normal crustal thickness (30-40 km) and an intermediate heat flow (60-70 mW m-2). Results show a nonlinear response to subtle changes in crustal thickness or heat flow, characterized by abrupt and sometimes unexpected switches in extension modes (e.g., from diffuse extensional deformation to effective lithospheric-scale rupturing) or from mantle- to crust-dominated strain localization. We interpret this nonlinearity to result from the interference of doming wavelengths in the presence of multiple necking instabilities. Disharmonic crust and mantle doming wavelengths results in efficient communication between shear zones at different lithospheric levels, leading to rupturing of the whole lithosphere. In contrast, harmonic crust and mantle doming inhibits interaction of shear zones across the lithosphere and results in a prolonged history of extension prior to continental breakup.

  9. Time-scale invariant changes in atmospheric radon concentration and crustal strain prior to a large earthquake

    Directory of Open Access Journals (Sweden)

    Y. Kawada

    2007-01-01

    Full Text Available Prior to large earthquakes (e.g. 1995 Kobe earthquake, Japan, an increase in the atmospheric radon concentration is observed, and this increase in the rate follows a power-law of the time-to-earthquake (time-to-failure. This phenomenon corresponds to the increase in the radon migration in crust and the exhalation into atmosphere. An irreversible thermodynamic model including time-scale invariance clarifies that the increases in the pressure of the advecting radon and permeability (hydraulic conductivity in the crustal rocks are caused by the temporal changes in the power-law of the crustal strain (or cumulative Benioff strain, which is associated with damage evolution such as microcracking or changing porosity. As the result, the radon flux and the atmospheric radon concentration can show a temporal power-law increase. The concentration of atmospheric radon can be used as a proxy for the seismic precursory processes associated with crustal dynamics.

  10. Crustal Magnetic Field Anomalies and Global Tectonics

    Science.gov (United States)

    Storetvedt, Karsten

    2014-05-01

    A wide variety of evidence suggests that the ruling isochron (geomagnetic polarity versus age) hypothesis of marine magnetic lineations has no merit - undermining therefore one of the central tenets of plate tectonics. Instead, variable induction by the ambient geomagnetic field is likely to be the principal agent for mega-scale crustal magnetic features - in both oceanic and continental settings. This revitalizes the fault-controlled susceptibility-contrast model of marine magnetic lineations, originally proposed in the late 1960s. Thus, the marine magnetic 'striping' may be ascribed to tectonic shearing and related, but variable, disintegration of the original iron-oxide mineralogy, having developed primarily along one of the two pan-global sets of orthogonal fractures and faults. In this way, fault zones (having the more advanced mineral alteration) would be characterized by relatively low susceptibility, while more moderately affected crustal sections (located between principal fault zones) would be likely to have less altered oxide mineralogy and therefore higher magnetic susceptibility. On this basis, induction by the present geomagnetic field is likely to produce oscillating magnetic field anomalies with axis along the principal shear grain. The modus operandi of the alternative magneto-tectonic interpretation is inertia-driven wrenching of the global Alpine age palaeo-lithosphere - triggered by changes in Earth's rotation. Increasing sub-crustal loss to the upper mantle during the Upper Mesozoic had left the ensuing Alpine Earth in a tectonically unstable state. Thus, sub-crustal eclogitization and associated gravity-driven delamination to the upper mantle led to a certain degree of planetary acceleration which in turn gave rise to latitude-dependent, westward inertial wrenching of the global palaeo-lithosphere. During this process, 1) the thin and mechanically fragile oceanic crust were deformed into a new type of broad fold belts, and 2) the continents

  11. Structural and metamorphic evolution of the Mid-Late Proterozoic Rayner Complex, Cape Bruce, East Antarctica

    International Nuclear Information System (INIS)

    Dunkley, D.J.; Clarke, G.L.; White, R.W.

    2002-01-01

    Granulite to transitional granulite facies gneisses exposed at Cape Bruce, Rayner Complex, East Antarctica, record three main orogenic/magmatic phases: (1) intrusion of c. 1000-980 Ma felsic orthogneisses into Mid-Proterozoic metasediments, contemporary with the development of north-trending reclined to recumbent folds; (2) extensive c. 980-900 Ma felsic magmatism, including equivalents of the Mawson Charnockite, which accompanied the development of upright, east-northeast-trending folds; and (3) ultramylonite zones of uncertain age. The first two phases are known as the Rayner Structrual Episode, the effects of which are similar in rocks to the east of Cape Bruce, at Mawson, and in the northern Prince Charles Mountains. Archaean rocks immediately to the west of Cape Bruce were tectonically reworked during the Rayner Structural Episode. The first orogenic phase is inferred to represent the collision between a wedge-shaped Proterozoic block comprising rocks of the Mawson Coast and Eastern Ghats Province, with the Archaean Napier Complex. The second orogenic phase included a major period of crustal growth through emplacement of the Mawson Charnockite and equivalents. (author). 41 refs., 6 figs., 1 tab

  12. The Mars Crustal Magnetic Field Control of Plasma Boundary Locations and Atmospheric Loss: MHD Prediction and Comparison with MAVEN

    Science.gov (United States)

    Fang, Xiaohua; Ma, Yingjuan; Masunaga, Kei; Dong, Chuanfei; Brain, David; Halekas, Jasper; Lillis, Robert; Jakosky, Bruce M.; Connerney, Jack; Grebowsky, Joseph; hide

    2017-01-01

    We present results from a global Mars time-dependent MHD simulation under constant solar wind and solar radiation impact considering inherent magnetic field variations due to continuous planetary rotation. We calculate the 3-D shapes and locations of the bow shock (BS) and the induced magnetospheric boundary (IMB) and then examine their dynamic changes with time. We develop a physics-based, empirical algorithm to effectively summarize the multidimensional crustal field distribution. It is found that by organizing the model results using this new approach, the Mars crustal field shows a clear, significant influence on both the IMB and the BS. Specifically, quantitative relationships have been established between the field distribution and the mean boundary distances and the cross-section areas in the terminator plane for both of the boundaries. The model-predicted relationships are further verified by the observations from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Our analysis shows that the boundaries are collectively affected by the global crustal field distribution, which, however, cannot be simply parameterized by a local parameter like the widely used subsolar longitude. Our calculations show that the variability of the intrinsic crustal field distribution in Mars-centered Solar Orbital itself may account for approx.60% of the variation in total atmospheric loss, when external drivers are static. It is found that the crustal field has not only a shielding effect for atmospheric loss but also an escape-fostering effect by positively affecting the transterminator ion flow cross-section area.

  13. Crustal structure, and topographic relief in the high southern Scandes, Norway

    Science.gov (United States)

    Stratford, W.; Thybo, H.; Frassetto, A.

    2010-05-01

    Resolving the uplift history of southern Norway is hindered by the lack of constraint available from the geologic record. Sediments that often contain information of burial and uplift history have long since been stripped from the onshore regions in southern Norway, and geophysical, dating methods and geomorphological studies are the remaining means of unraveling uplift history. New constraints on topographic evolution and uplift in southern Norway have been added by a recent crustal scale refraction project. Magnus-Rex (Mantle investigation of Norwegian uplift Structure, refraction experiment) recorded three ~400 km long active source seismic profiles across the high southern Scandes Mountains. The goal of the project is to determine crustal thickness and establish whether these mountains are supported at depth by a crustal root or by other processes. The southern Scandes Mountains were formed during the Caledonian Orogeny around 440 Ma. These mountains, which reach elevations of up to ~2.5 km, are comprised of one or more palaeic (denudation) surfaces of rolling relief that are incised by fluvial and glacial erosion. Extreme vertical glacial incision of up to 1000 m cuts into the surfaces in the western fjords, while the valleys of eastern Norway are more fluvial in character. Climatic controls on topography here are the Neogene - Recent effects of rebound due to removal of the Fennoscandian ice sheet and isostatic rebound due to incisional erosion. However, unknown tectonic uplift mechanisms may also be in effect, and separating the tectonic and climate-based vertical motions is often difficult. Sediment and rock has been removed by the formation of the palaeic surfaces and uplift measurements cannot be directly related to present elevations. Estimates so far have indicated that rebound due to incisional erosion has a small effect of ~500 m on surface elevation. Results from Magnus-Rex indicate the crust beneath the high mountains is up to 40 km thick. This

  14. Crustal structure in the Kiruna area, northern Sweden, based on seismic reflection profiling

    Science.gov (United States)

    Juhojuntti, Niklas; Bergman, Stefan; Olsson, Sverker

    2013-04-01

    Northernmost Sweden is currently one of the most active mining areas in Europe. In order to better understand the regional three-dimensional crustal structure and to support deep ore exploration, we have acquired a 74 km long seismic reflection profile in the Kiruna area. The upper crust in this area is largely composed of various supracrustal units, which are dominated by metabasalts, acidic metavolcanics and clastic metasedimentary rocks, resting on an Archaean metagranitoid complex. All of these units have been intruded by plutonic rocks, and to variable degrees folded, sheared and metamorphosed, during the Svecokarelian orogeny. The profile crosses several steep ductile shear zones, some of which extend for hundreds of kilometres along strike. Many of the lithological contacts and deformation zones are expected to be seismically reflective. The profile is located only a few kilometres from the world's largest underground iron-ore mine in Kiruna, and closer to the profile there are several known ore bodies, some of which are active exploration targets. For the seismic recording we used approximately 350 geophones in split-spread configuration, at a separation of 25 m. The main seismic source was the Vibsist system (an impact source), which normally was employed at every geophone station. We also fired explosive charges (8-16 kg) at a few locations distributed along the profile to image deeper structures, although at very low resolution. Wireless seismometers were placed along and to the side of the profile, mainly in order to achieve better velocity control and to study out-of-the-plane reflections. Some mining blasts in Kiruna were also recorded. The upper crust in the area is quite reflective, most clearly demonstrated by the dynamite shot records. Some of the reflections appear to originate from steeply dipping structures. The dynamite shot records show a set of reflections at 3-4 s twt, corresponding to a depth of roughly 10 km, the explanation for which is

  15. Modes of continental extension in a crustal wedge

    KAUST Repository

    Wu, Guangliang

    2015-07-01

    © 2015 Elsevier B.V. We ran numerical experiments of the extension of a crustal wedge as an approximation to extension in an orogenic belt or a continental margin. We study the effects of the strength of the lower crust and of a weak mid-crustal shear zone on the resulting extension styles. A weak mid-crustal shear zone effectively decouples upper crustal extension from lower crustal flow. Without the mid-crustal shear zone, the degree of coupling between the upper and the lower crust increases and extension of the whole crust tends to focus on the thickest part of the wedge. We identify three distinct modes of extension determined by the strength of the lower crust, which are characterized by 1) localized, asymmetric crustal exhumation in a single massif when the lower crust is weak, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone when the lower crust is strong. A frictionally stronger mid-crustal shear zone does not change the overall model behaviors but extension occurred over multiple rolling-hinges. The 3 modes of extension share characteristics similar to geological models proposed to explain the formation of metamorphic core complexes: 1) the crustal flow model for the weak lower crust, 2) the rolling-hinge and crustal flow models when the lower crust is intermediate and 3) the flexural uplift model when the lower crust is strong. Finally we show that the intensity of decoupling between the far field extension and lower crustal flow driven by the regional pressure gradient in the wedge control the overall style of extension in the models.

  16. Modes of continental extension in a crustal wedge

    KAUST Repository

    Wu, Guangliang; Lavier, Luc L.; Choi, Eunseo

    2015-01-01

    © 2015 Elsevier B.V. We ran numerical experiments of the extension of a crustal wedge as an approximation to extension in an orogenic belt or a continental margin. We study the effects of the strength of the lower crust and of a weak mid-crustal shear zone on the resulting extension styles. A weak mid-crustal shear zone effectively decouples upper crustal extension from lower crustal flow. Without the mid-crustal shear zone, the degree of coupling between the upper and the lower crust increases and extension of the whole crust tends to focus on the thickest part of the wedge. We identify three distinct modes of extension determined by the strength of the lower crust, which are characterized by 1) localized, asymmetric crustal exhumation in a single massif when the lower crust is weak, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone when the lower crust is strong. A frictionally stronger mid-crustal shear zone does not change the overall model behaviors but extension occurred over multiple rolling-hinges. The 3 modes of extension share characteristics similar to geological models proposed to explain the formation of metamorphic core complexes: 1) the crustal flow model for the weak lower crust, 2) the rolling-hinge and crustal flow models when the lower crust is intermediate and 3) the flexural uplift model when the lower crust is strong. Finally we show that the intensity of decoupling between the far field extension and lower crustal flow driven by the regional pressure gradient in the wedge control the overall style of extension in the models.

  17. A basin on an unstable ground: Correlation of the Middle Archaean Moodies Basin, Barberton Greenstone Belt, South Africa

    Science.gov (United States)

    Ohnemueller, Frank; Heubeck, Christoph; Kirstein, Jens; Gamper, Antonia

    2010-05-01

    time and immediately predates the initiation of basin shortening. Basin compartmentalization was likely due to the movement along a group of major faults (Sheba, Haki, Barbrook, Saddleback Faults) between the present Saddleback and Eureka Synclines, creating at least two subbasins in late Moodies time. Even though sediment provenance thus became localized, intensive Archaean weathering likely contributed to generate petrographically similar quartz-rich sandstones in fault-bounded minibasins. The late-Moodies minibasins may have become connected occasionally, allowing concurrent deposition of thin BIFs. A similar phase of movement along the major transcurrent Inyoka Fault may be responsible for the distinct petrographic character of Moodies sandstones south of that fault.

  18. Protoliths of enigmatic Archaean gneisses established from zircon inclusion studies: Case study of the Caozhuang quartzite, E. Hebei, China

    Directory of Open Access Journals (Sweden)

    Allen P. Nutman

    2014-07-01

    Full Text Available A diverse suite of Archaean gneisses at Huangbaiyu village in the North China Craton, includes rare fuchsite-bearing (Cr-muscovite siliceous rocks – known as the Caozhuang quartzite. The Caozhuang quartzite is strongly deformed and locally mylonitic, with silica penetration and pegmatite veining common. It contains abundant 3880–3600 Ma and some Palaeoarchaean zircons. Because of its siliceous nature, the presence of fuchsite and its complex zircon age distribution, it has until now been accepted as a (mature quartzite. However, the Caozhuang quartzite sample studied here is feldspathic. The shape and cathodoluminescence petrography of the Caozhuang quartzite zircons show they resemble those found in immature detrital sedimentary rocks of local provenance or in Eoarchaean polyphase orthogneisses, and not those in mature quartzites. The Caozhuang quartzite intra-zircon mineral inclusions are dominated by quartz, with lesser biotite, apatite (7% and alkali-feldspar, and most inclusions are morphologically simple. A Neoarchaean orthogneiss from near Huangbaiyu displays morphologically simple inclusions with much more apatite (73%, as is typical for fresh calc-alkaline granitoids elsewhere. Zircons were also examined from a mature conglomerate quartzite clast and an immature feldspathic sandstone of the overlying weakly metamorphosed Mesoproterozoic Changcheng System. These zircons have oscillatory zoning, showing they were sourced from igneous rocks. The quartzite clast zircons contain only rare apatite inclusions (<1%, with domains with apatite habit now occupied by intergrowths of muscovite + quartz ± Fe-oxides ± baddeleyite. We interpret that these were once voids after apatite inclusions that had dissolved during Mesoproterozoic weathering, which were then filled with clays ± silica and then weakly metamorphosed. Zircons in the immature feldspathic sandstone show a greater amount of preserved apatite (11%, but with petrographic

  19. Shear wave velocity structure of the lower crust in southern Africa: evidence for compositional heterogeneity within Archaean and Proterozoic terrains

    CSIR Research Space (South Africa)

    Kgaswane, EM

    2009-12-01

    Full Text Available locations are shown with solid triangles, and event locations are shown with white circles. Most of the events are located outside of the area shown in the map. B12304 KGASWANE ET AL.: CRUSTAL STRUCTURE OF SOUTHERN AFRICA 6 of 19 B12304 velocity profiles... supergroup (white line) taken from van der Westhuizen et al. [2006] and the location of lower crust xenoliths obtained from Pretorius and Barton [2003] and Schmitz and Bowring [2003a]. The number labels 1–15 represent the names of the kimberlites: 1...

  20. Crustal Growth: In Defense of the Dogma

    Science.gov (United States)

    Albarede, F.; Blichert-Toft, J.; Guitreau, M.

    2012-12-01

    Plate tectonics was not even in its teens when Armstrong suggested that mantle and crust have interacted at steady-state over Earth's history. With the help of new geochemical tools and large-scale compilations, the concept of steady-state crust (as opposed to continuous crustal growth) is being revived with the implications that the equivalent of several volumes of present-day crust (PDCV) may have been subducted through geological times. Here we argue --or recall-- that four different lines of evidence invalidate this model. (i) The subduction filter must be particularly efficient for argon, even more so than for LILE and most other volatile elements. Atmosphere collects 40Ar degassed from both the extant crust and the crust dragged down at subduction zones over geological time. Regardless of the residence time of the crust at the surface, the amount of atmospheric 40Ar limits subduction of continental crust into the mantle to < 30% of the PDCV [1]. (ii) EM II, the only component that undoubtedly represents subducted continental crust in oceanic basalts, is extremely uncommon. (iii) Crustal age histograms are irrepressibly episodic. It has been argued that erosion selectively removes the crust with the elusive ages [2]. Ages of detrital zircons, which in the selective erosion conjecture should fill the voids, do not support this view [3]. Episodicity is difficult to reconcile with a continental protolith isolated by the common geological processes working either at mid-ocean ridges or subduction zones. A role may be recognized for Wilson cycles, if they can be shown to have prevailed for the entire history of the Earth. Geochemistry demonstrates that superplume material makes up the crustal protolith of all the major juvenile provinces. (iv) The residence time in the mantle of the elements distinctive of the crust is similar to the age of the Earth or even longer [4]. Continental crust finds its source in the instabilities of the lower mantle and the irreversible

  1. Mechanisms for strain localization within Archaean craton: A structural study from the Bundelkhand Tectonic Zone, north-central India

    Science.gov (United States)

    Sarkar, Saheli; Patole, Vishal; Saha, Lopamudra; Pati, Jayanta Kumar; Nasipuri, Pritam

    2015-04-01

    The transformation of palaeo-continents involve breakup, dispersal and reassembly of cratonic blocks by collisional suturing that develop a network of orogenic (mobile) belts around the periphery of the stable cratons. The nature of deformation in the orogenic belt depends on the complex interaction of fracturing, plastic deformation and diffusive mass transfer. Additionally, the degree and amount of melting during regional deformation is critical as the presence of melt facilitates the rate of diffusive mass transfer and weakens the rock by reducing the effective viscosity of the deformed zone. The nature of strain localization and formation of ductile shear zones surrounding the cratonic blocks have been correlated with Proterozoic-Palaeozoic supercontinent assembly (Columbia, Rodinia and Gondwana reconstruction). Although, a pre-Columbia supercontinent termed as Kenorland has been postulated, there is no evidence that supports the notion due to lack of the presence of shear zones within the Archaean cratonic blocks. In this contribution, we present the detailed structural analysis of ductile shear zones within the Bundelkhand craton. The ductlile shear zone is termed as Bundelkhand Tectonic Zone (BTZ) that extends east-west for nearly 300 km throughout the craton with a width of two-three kilometer . In the north-central India, the Bundelkhand craton is exposed over an area of 26,000 sq. The craton is bounded by Central Indian Tectonic zone in the south, the Great Boundary fault in the west and by the rocks of Lesser Himalaya in the north. A series of tonalite-trondjhemite-granodiorite gneiss are the oldest rocks of the Bundelkhand craton that also contains a succession of metamorphosed supracrustal rocks comprising of banded iron formation, quartzite, calc-silicate and ultramafic rocks. K-feldspar bearing granites intrude the tonalite-trondjhemite-granodiorite and the supracrustal rocks during the time span of 2.1 to 2.5 Ga. The TTGs near Babina, in central

  2. Crustal permeability: Introduction to the special issue

    Science.gov (United States)

    Ingebritsen, Steven E.; Gleeson, Tom

    2015-01-01

    The topic of crustal permeability is of broad interest in light of the controlling effect of permeability on diverse geologic processes and also timely in light of the practical challenges associated with emerging technologies such as hydraulic fracturing for oil and gas production (‘fracking’), enhanced geothermal systems, and geologic carbon sequestration. This special issue of Geofluids is also motivated by the historical dichotomy between the hydrogeologic concept of permeability as a static material property that exerts control on fluid flow and the perspective of economic geologists, geophysicists, and crustal petrologists who have long recognized permeability as a dynamic parameter that changes in response to tectonism, fluid production, and geochemical reactions. Issues associated with fracking, enhanced geothermal systems, and geologic carbon sequestration have already begun to promote a constructive dialog between the static and dynamic views of permeability, and here we have made a conscious effort to include both viewpoints. This special issue also focuses on the quantification of permeability, encompassing both direct measurement of permeability in the uppermost crust and inferential permeability estimates, mainly for the deeper crust.

  3. Oblique reactivation of lithosphere-scale lineaments controls rift physiography - the upper-crustal expression of the Sorgenfrei-Tornquist Zone, offshore southern Norway

    Science.gov (United States)

    Phillips, Thomas B.; Jackson, Christopher A.-L.; Bell, Rebecca E.; Duffy, Oliver B.

    2018-04-01

    Pre-existing structures within sub-crustal lithosphere may localise stresses during subsequent tectonic events, resulting in complex fault systems at upper-crustal levels. As these sub-crustal structures are difficult to resolve at great depths, the evolution of kinematically and perhaps geometrically linked upper-crustal fault populations can offer insights into their deformation history, including when and how they reactivate and accommodate stresses during later tectonic events. In this study, we use borehole-constrained 2-D and 3-D seismic reflection data to investigate the structural development of the Farsund Basin, offshore southern Norway. We use throw-length (T-x) analysis and fault displacement backstripping techniques to determine the geometric and kinematic evolution of N-S- and E-W-striking upper-crustal fault populations during the multiphase evolution of the Farsund Basin. N-S-striking faults were active during the Triassic, prior to a period of sinistral strike-slip activity along E-W-striking faults during the Early Jurassic, which represented a hitherto undocumented phase of activity in this area. These E-W-striking upper-crustal faults are later obliquely reactivated under a dextral stress regime during the Early Cretaceous, with new faults also propagating away from pre-existing ones, representing a switch to a predominantly dextral sense of motion. The E-W faults within the Farsund Basin are interpreted to extend through the crust to the Moho and link with the Sorgenfrei-Tornquist Zone, a lithosphere-scale lineament, identified within the sub-crustal lithosphere, that extends > 1000 km across central Europe. Based on this geometric linkage, we infer that the E-W-striking faults represent the upper-crustal component of the Sorgenfrei-Tornquist Zone and that the Sorgenfrei-Tornquist Zone represents a long-lived lithosphere-scale lineament that is periodically reactivated throughout its protracted geological history. The upper-crustal component of

  4. Crustal melting and recycling: geochronology and sources of Variscan syn-kinematic anatectic granitoids of the Tormes Dome (Central Iberian Zone). A U-Pb LA-ICP-MS study

    Science.gov (United States)

    López-Moro, F. J.; López-Plaza, M.; Gutiérrez-Alonso, G.; Fernández-Suárez, J.; López-Carmona, A.; Hofmann, M.; Romer, R. L.

    2018-04-01

    In this study, we report U-Pb Laser Ablation ICP-MS zircon and ID-TIMS monazite ages for peraluminous granitoid plutons (biotite ± muscovite ± cordierite ± sillimanite) in the Tormes Dome, one of the gneiss-cored domes located in the Central Iberian Zone of the Variscan belt of northern Spain. Textural domains in zircon, interpreted to represent the magmatic crystallization of the granitoids (and one monazite fraction in the Ledesma pluton) yielded ages around 320 Ma, in agreement with other geochronological studies in the region. This age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the Variscan chain of western Europe. In addition, there are several populations of inherited (xenocrystic) zircon: (1) Carboniferous zircon crystals (ca. 345 Ma) as well as one of the monazite fractions in the coarse-grained facies of the Ledesma pluton that also yielded an age of ca. 343 Ma. (2) Devonian-Silurian zircon xenocrysts with scattered ages between ca. 390 and 432 Ma. (3) Middle Cambrian-Ordovician (ca. 450-511 Ma). (4) Ediacaran-Cryogenian zircon ages (ca. 540-840 Ma). (5) Mesoproterozoic to Archaean zircon (900-2700 Ma). The abundance of Carboniferous-inherited zircon shows that crustal recycling/cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence. In our case study, it seems plausible that a "crustal layer" of ca. 340 Ma granitoids/migmatites was recycled, partially or totally, only 15-20 My after its emplacement.

  5. Crustal Structure and Mantle Transition Zone Thickness beneath the Central Mongolia from Teleseismic Receiver Functions

    Science.gov (United States)

    He, J.; Wu, Q.; Gao, M.; Munkhuu, U.; Demberel, S. G.

    2013-12-01

    The Mongolian Plateau (northern Asia) is situated between the Gobi-Altai range and the Siberian craton. In order to understand the crustal and mantle structure environmental characteristics, we use the teleseismic data recorded by 69 broadband stations located in the Central Mongolia(103.5°-111.5°E, 42°-50°N). The teleseismic events are selected from the global earthquakes between Aug. 2011 and Dec. 2013 with magnitude >5.5and the epicentral distance range from 30° to 95° to the center of the network. Lateral variations of the crustal thicknesses H and Vp/Vs ratios are obtained by using receiver function method. The crust thins gradually from northwest to southeast in the studying field. We found that the thinnest crust is ~37.5km in the southeast which is Gobi. The distribution of Vp/Vs ratios are between 1.68 and 1.84, which shows the heterogeneity. There are three high-anomaly areas: the Gobi range which is the Later Paleozoic Orogeny; the Khentei Mountains which is in the Jurassic-Cretaceous Reactive Continental Margin; the northwest area which is granite. Our research not only reveals the powerful evident of the crustal formation and evolution mechanism, but also provides some constraints on the mechanism of uplift of the Mongolian Plateau.This study was supported by the international cooperation project of the Ministry of Science and Technology of China (2011DFB20120).

  6. Crustal thickness and Moho sharpness beneath the Midcontinent rift from receiver functions

    Directory of Open Access Journals (Sweden)

    Moikwathai Moidaki

    2013-02-01

    Full Text Available The Mesoproterozoic Midcontinent rift (MCR in the central US is an approximately 2000 km long, 100 km wide structure from Kansas to Michigan. During the 20-40 million years of rifting, a thick (up to 20 km layer of basaltic lava was deposited in the rift valleys. Quantifying the effects of the rifting and associated volcanic eruptions on the structure and composition of the crust and mantle beneath the MCR is important for the understanding of the evolution of continental lithosphere. In this study we measure the crustal thickness (H, and the sharpness of the Moho (R at about 24 portable and permanent stations in Iowa, Kansas, and South Dakota by stacking Pto- S converted waves (PmS and their multiples (PPmS and PSmS. Under the assumption that the crustal mean velocity in the study area is the same as the IASP91 earth model, we find a significantly thickened crust beneath the MCR of about 53 km. The crustal Vp/Vs ratios increases from about 1.80 off rift to as large as 1.95 within the rift, which corresponds to an increase of Poisson’s ratio from 0.28 to 0.32, suggesting a more mafic crust beneath the MCR. The R measurements are spatially variable and are relatively small in the vicinity of the MCR, indicating the disturbance of the original sharp Moho by the rifting and magmatic intrusion and volcanic eruption.

  7. Simulations of tremor-related creep reveal a weak crustal root of the San Andreas Fault

    Science.gov (United States)

    Shelly, David R.; Bradley, Andrew M.; Johnson, Kaj M.

    2013-01-01

    Deep aseismic roots of faults play a critical role in transferring tectonic loads to shallower, brittle crustal faults that rupture in large earthquakes. Yet, until the recent discovery of deep tremor and creep, direct inference of the physical properties of lower-crustal fault roots has remained elusive. Observations of tremor near Parkfield, CA provide the first evidence for present-day localized slip on the deep extension of the San Andreas Fault and triggered transient creep events. We develop numerical simulations of fault slip to show that the spatiotemporal evolution of triggered tremor near Parkfield is consistent with triggered fault creep governed by laboratory-derived friction laws between depths of 20–35 km on the fault. Simulated creep and observed tremor northwest of Parkfield nearly ceased for 20–30 days in response to small coseismic stress changes of order 104 Pa from the 2003 M6.5 San Simeon Earthquake. Simulated afterslip and observed tremor following the 2004 M6.0 Parkfield earthquake show a coseismically induced pulse of rapid creep and tremor lasting for 1 day followed by a longer 30 day period of sustained accelerated rates due to propagation of shallow afterslip into the lower crust. These creep responses require very low effective normal stress of ~1 MPa on the deep San Andreas Fault and near-neutral-stability frictional properties expected for gabbroic lower-crustal rock.

  8. Yellowstone-Snake River Plain seismic profilling experiment: Crustal structure of the eastern Snake River Plain

    International Nuclear Information System (INIS)

    Braile, L.W.; Smith, R.B.; Ansorge, J.; Baker, M.R.; Sparlin, M.A.; Prodehl, C.; Schilly, M.M.; Healy, J.H.; Mueller, S.; Olsen, K.H.

    1982-01-01

    Seismic refraction profiles recorded along the eastern Snake River Plain (ESRP) in southeastern Idaho during the 1978 Yellowstone-Snake River Plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation (Q-1) structure of the ESRP. Travel-time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the ESRP is highly anomalous. Approximately 3 to 6 km of volcanic rocks (with some interbedded sediments) overlie an upper-crustal layer (compressional velocity approx. =6.1 km/s) which thins southwestward along the ESRP from a thickness of 10 km near Island Park Caldera to 2 to 3 km beneath the central and southwestern portions of the ESRP. An intermediate-velocity (approx. =6.5 km/s) layer extends from approx. =10 to approx. =20 km depth. a thick (approx. =22 km) lower crust of compressional velocity 6.8 km/s, a total crustall thickness of approx. =42 km, and a P/sub n/ velocity of approx. =7.9 km/s is observed in the ESRP, similar to the western Snake River Plain and the Rocky Mountains Provinces. High attenuation is evident on the amplitude corrected seismic data due to low-Q values in the volcanic rocks (Q/sub p/ = 20 to 200) and throughout the crust (Q/sub p/ = 160 to 300). Based on these characteristics of the crustal structure and volcanic-age progression data, it is suggested that the ESRP has resulted from an intensitive period of intrusion of mantle-derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse. This activity began about 15 m.y. ago in southwestern Idaho and has migrated northeast to its present position at Yellowstone. Subsequent cooling of the intruded upper crust results in the 6.5 km/s velocity intermediate layer. Crustal subsidence and periodic basaltic volcanism as represented by the ESRP complete the sequence of crustal evolution

  9. Seismically constrained two-dimentional crustal thermal structure of ...

    Indian Academy of Sciences (India)

    Cambay basin; P-wave velocity; heat flow; heat generation; 2-D modelling; crustal thermal structure; Mohodepth; Curie isotherm. ... This work deals with the two-dimensional thermal modelling to delineate the crustal thermal structure along a 230 km long Deep Seismic Sounding (DSS) profile in the north Cambay basin.

  10. Magmatic and Crustal Differentiation History of Granitic Rocks from Hf-O Isotopes in Zircon

    Science.gov (United States)

    Kemp, , A. I. S.; Hawkesworth, , C. J.; Foster, , G. L.; Paterson, , B. A.; Woodhead, , J. D.; Hergt, , J. M.; Gray, , C. M.; Whitehouse, M. J.

    2007-02-01

    Granitic plutonism is the principal agent of crustal differentiation, but linking granite emplacement to crust formation requires knowledge of the magmatic evolution, which is notoriously difficult to reconstruct from bulk rock compositions. We unlocked the plutonic archive through hafnium (Hf) and oxygen (O) isotope analysis of zoned zircon crystals from the classic hornblende-bearing (I-type) granites of eastern Australia. This granite type forms by the reworking of sedimentary materials by mantle-like magmas instead of by remelting ancient metamorphosed igneous rocks as widely believed. I-type magmatism thus drives the coupled growth and differentiation of continental crust.

  11. Fossil Microorganisms in Archaean

    Science.gov (United States)

    Astafleva, Marina; Hoover, Richard; Rozanov, Alexei; Vrevskiy, A.

    2006-01-01

    Ancient Archean and Proterozoic rocks are the model objects for investigation of rocks comprising astromaterials. The first of Archean fossil microorganisms from Baltic shield have been reported at the last SPIE Conference in 2005. Since this confeence biomorphic structures have been revealed in Archean rocks of Karelia. It was determined that there are 3 types of such bion structures: 1. structures found in situ, in other words microorganisms even-aged with rock matrix, that is real Archean fossils biomorphic structures, that is to say forms inhabited early formed rocks, and 3. younger than Archean-Protherozoic minerali microorganisms, that is later contamination. We made attempt to differentiate these 3 types of findings and tried to understand of burial of microorganisms. The structures belongs (from our point of view) to the first type, or real Archean, forms were under examination. Practical investigation of ancient microorganisms from Green-Stone-Belt of Northern Karelia turns to be very perspective. It shows that even in such ancient time as Archean ancient diverse world existed. Moreover probably such relatively highly organized cyanobacteria and perhaps eukaryotic formes existed in Archean world.

  12. Basalt generation at the Apollo 12 site. Part 2: Source heterogeneity, multiple melts, and crustal contamination

    Science.gov (United States)

    Neal, Clive R.; Hacker, Matthew D.; Snyder, Gregory A.; Taylor, Lawrence A.; Liu, Yun-Gang; Schmitt, Roman A.

    1994-01-01

    The petrogenesis of Apollo 12 mare basalts has been examined with emphasis on trace-element ratios and abundances. Vitrophyric basalts were used as parental compositions for the modeling, and proportions of fractionating phases were determined using the MAGFOX prograqm of Longhi (1991). Crystal fractionation processes within crustal and sub-crustal magma chambers are evaluated as a function of pressure. Knowledge of the fractionating phases allows trace-element variations to be considered as either source related or as a product of post-magma-generation processes. For the ilmenite and olivine basalts, trace-element variations are inherited from the source, but the pigeonite basalt data have been interpreted with open-system evolution processes through crustal assimilation. Three groups of basalts have been examined: (1) Pigeonite basalts-produced by the assimilation of lunar crustal material by a parental melt (up to 3% assimilation and 10% crystal fractionation, with an 'r' value of 0.3). (2) Ilmenite basalts-produced by variable degrees of partial melting (4-8%) of a source of olivine, pigeonite, augite, and plagioclase, brought together by overturn of the Lunar Magma Ocean (LMO) cumulate pile. After generation, which did not exhaust any of the minerals in the source, these melts experienced closed-system crystal fractionation/accumulation. (3) Olivine basalts-produced by variable degrees of partial melting (5-10%) of a source of olivine, pigeonite, and augite. After generation, again without exhausting any of the minerals in the source, these melts evolved through crystal accumulation. The evolved liquid counterparts of these cumulates have not been sampled. The source compositions for the ilmenite and olivine basalts were calculated by assuming that the vitrophyric compositions were primary and the magmas were produced by non-modal batch melting. Although the magnitude is unclear, evaluation of these source regions indicates that both be composed of early- and

  13. Fluvial archives, a valuable record of vertical crustal deformation

    Science.gov (United States)

    Demoulin, A.; Mather, A.; Whittaker, A.

    2017-06-01

    The study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms, but also for identifying the nature of crustal deformation and its history. In recent decades, geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens. Here, we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions (the effects of horizontal deformation are not addressed). Channel long profiles have received considerable attention in the literature, and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives, before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady-state characteristics. Then, we review the literature dealing with the transient response of rivers to tectonic perturbation, through the production of knickpoints propagating through the drainage network. Inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories. Finally, we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes. As well as the erosional imprint of tectonic forcing, sedimentary deposits, such as fluvial terrace staircases, are also considered as a classical component of tectonic geomorphology. We show that these studies have recently benefited from rapid advances in dating techniques, allowing more reliable reconstruction of incision histories and estimation of incision rates. The combination of progress in the understanding of transient river profiles and larger, more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace

  14. Crustal and deep seismicity in Italy (30 years after

    Directory of Open Access Journals (Sweden)

    G. Selvaggi

    1997-06-01

    recent important achievement of the prolonged detailed seismic monitoring of our territory, which will provide in the future more and more precise indications of where earthquakes will strike. In addition, the accurate location of hundreds of intermediate and deep earthquakes beneath the two arcs has recently provided (together with seismic tomography results new hints on the tectonic setting of Italy and its evolution over time, on the relations between deep processes and crustal stress, and ultimately on the mechanisms of earthquake generation in our country.

  15. The Kovdor-2015 experiment: study of the parameters of a conductive layer of dilatancy-diffusion nature (DD Layer) in the Archaean crystalline basement of the Baltic Shield

    Science.gov (United States)

    Zhamaletdinov, A. A.; Velikhov, E. P.; Shevtsov, A. N.; Kolobov, V. V.; Kolesnikov, V. E.; Skorokhodov, A. A.; Korotkova, T. G.; Ivonin, V. V.; Ryazantsev, P. A.; Birulya, M. A.

    2017-06-01

    This paper addresses the Kovdor-2015 Experiment involving frequency electromagnetic soundings of the Archaean basement of the Earth's crust in the southwestern part of the Kola Peninsula. Eleven soundings were carried out using two transmitting arrangements, 85 km apart. Each arrangement consisted of two mutually orthogonal grounded electric dipoles of 1.5 km long. The distances between the source and the receiver were 25 and 50 km. Interpretation of the results took into account the influence of displacement currents and static distortions. It is found that there is an intermediate conductive layer of the dilatancy-diffusion nature (DD layer) with a longitudinal conductivity of about one siemens at depths ranging from 1.5-2 to 5-7 km. The results are interpreted in the terms of geodynamics.

  16. On the metamorphic history of an Archaean granitoid greenstone terrane, East Pilbara, Western Australia, using the 40Ar/39Ar age spectrum technique

    International Nuclear Information System (INIS)

    Wijbrans, J.R.; McDougall, I.

    1987-01-01

    Age spectrum analyses of blue-green hornblendes from amphibolites from the Western Shaw Belt, East Pilbara, Western Australia, indicate an age of at least 3200 Ma for early regional metamorphism. Ages on hornblende and muscovite from the narrow contact zone with the adjacent Yule Batholith probably data updoming of the granitoid gneiss terranes at 2950 Ma. Hornblendes from within the Shaw Batholith and from a contact zone of a post-tectonic granitoid yield ages of 2840-2900 Ma, indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post-tectonic granitoids. The preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the Late Archaean, suggesting that updoming occurred during a period of rapid tectonism. (orig.)

  17. On the metamorphic history of an Archaean granitoid greenstone terrane, East Pilbara, Western Australia, using the /sup 40/Ar//sup 39/Ar age spectrum technique

    Energy Technology Data Exchange (ETDEWEB)

    Wijbrans, J.R.; McDougall, I.

    1987-07-01

    Age spectrum analyses of blue-green hornblendes from amphibolites from the Western Shaw Belt, East Pilbara, Western Australia, indicate an age of at least 3200 Ma for early regional metamorphism. Ages on hornblende and muscovite from the narrow contact zone with the adjacent Yule Batholith probably data updoming of the granitoid gneiss terranes at 2950 Ma. Hornblendes from within the Shaw Batholith and from a contact zone of a post-tectonic granitoid yield ages of 2840-2900 Ma, indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post-tectonic granitoids. The preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the Late Archaean, suggesting that updoming occurred during a period of rapid tectonism.

  18. GPS-derived crustal deformation in Azerbaijan

    Science.gov (United States)

    Safarov, Rafig; Mammadov, Samir; Kadirov, Fakhraddin

    2017-04-01

    Crustal deformations of the Earth's crust in Azerbaijan were studied based on GPS measurements. The GPS velocity vectors for Azerbaijan, Iran, Georgia, and Armenia were used in order to estimate the deformation rates. It is found that compression is observable along the Greater Caucasus, in Gobustan, the Kura depression, Nakhchyvan Autonomous Republic, and adjacent areas of Iran. The axes of compression/contraction of the crust in the Greater Caucasus region are oriented in the S-NE direction. The maximum strain rate is observed in the zone of mud volcanism at the SHIK site (Shykhlar), which is marked by a sharp change in the direction of the compression axes (SW-NE). It is revealed that the deformation field also includes the zones where strain rates are very low. These zones include the Caspian-Guba and northern Gobustan areas, characterized by extensive development of mud volcanism. The extension zones are confined to the Lesser Caucasus and are revealed in the Gyadabei (GEDA) and Shusha (SHOU) areas. The analysis of GPS data for the territory of Azerbaijan and neighboring countries reveals the heterogeneous patterns of strain field in the region. This fact suggests that the block model is most adequate for describing the structure of the studied region. The increase in the number of GPS stations would promote increasing the degree of detail in the reconstructions of the deformation field and identifying the microplate boundaries.It is concluded that the predominant factor responsible for the eruption of mud volcanoes is the intensity of gasgeneration processes in the earth's interior, while deformation processes play the role of a trigger. The zone of the epicenters of strong earthquakes is correlated to the gradient zone in the crustal strain rates.

  19. Fractal behavior in continental crustal heat production

    Directory of Open Access Journals (Sweden)

    N. Vedanti

    2011-02-01

    Full Text Available The distribution of crustal heat production, which is the most important component in the elucidation of continental thermal structure, still remains a theoretical assumption. In general the heat production values must decrease with depth, but the form of decrease of heat production in the crust is not well understood. The commonly used heat production models are: "block model", in which heat production is constant from the surface to a given depth and the "exponential model", in which heat production diminishes as an exponential function of depth. The exponential model is more widely used wherein sources of the errors are heterogeneity of rock and long wavelength changes due to changes in lithology and tectonic elements, and as such exponential distribution does not work satisfactorily for the entire crust. In the present study, we analyze for the first time, deep crustal heat production data of six global areas namely Dharwar craton (India, Kaapvaal craton (South Africa, Baltic shield (Kola, Russia, Hidaka metamorphic belt (Japan, Nissho pluton (Japan and Continental Deep Drilling site (KTB, Germany. The power spectrum of all the studied data sets exhibits power law behaviour. This would mean slower decay of heat production with depth, which conforms to the known geologic composition of the crust. Minimum value of the scaling exponent has been found for the KTB borehole, which is apparently related to higher heat production of gneisses, however for other study areas, scaling exponent is almost similar. We also found that the lower values of scaling exponents are related to higher heat production in the crust as is the case in KTB. Present finding has a direct relevance in computation of temperature-depth profiles in continental regions.

  20. Fractal behavior in continental crustal heat production

    Science.gov (United States)

    Vedanti, N.; Srivastava, R. P.; Pandey, O. P.; Dimri, V. P.

    2011-02-01

    The distribution of crustal heat production, which is the most important component in the elucidation of continental thermal structure, still remains a theoretical assumption. In general the heat production values must decrease with depth, but the form of decrease of heat production in the crust is not well understood. The commonly used heat production models are: "block model", in which heat production is constant from the surface to a given depth and the "exponential model", in which heat production diminishes as an exponential function of depth. The exponential model is more widely used wherein sources of the errors are heterogeneity of rock and long wavelength changes due to changes in lithology and tectonic elements, and as such exponential distribution does not work satisfactorily for the entire crust. In the present study, we analyze for the first time, deep crustal heat production data of six global areas namely Dharwar craton (India), Kaapvaal craton (South Africa), Baltic shield (Kola, Russia), Hidaka metamorphic belt (Japan), Nissho pluton (Japan) and Continental Deep Drilling site (KTB, Germany). The power spectrum of all the studied data sets exhibits power law behaviour. This would mean slower decay of heat production with depth, which conforms to the known geologic composition of the crust. Minimum value of the scaling exponent has been found for the KTB borehole, which is apparently related to higher heat production of gneisses, however for other study areas, scaling exponent is almost similar. We also found that the lower values of scaling exponents are related to higher heat production in the crust as is the case in KTB. Present finding has a direct relevance in computation of temperature-depth profiles in continental regions.

  1. The crustal structures from Wuyi-Yunkai orogen to Taiwan orogen: the onshore-offshore wide-angle seismic experiment of TAIGER and ATSEE projects

    Science.gov (United States)

    Kuochen, H.; Kuo, N. Y. W.; Wang, C. Y.; Jin, X.; Cai, H. T.; Lin, J. Y.; Wu, F. T.; Yen, H. Y.; Huang, B. S.; Liang, W. T.; Okaya, D. A.; Brown, L. D.

    2015-12-01

    The crustal structure is key information for understanding the tectonic framework and geological evolution in the southeastern China and its adjacent area. In this study, we integrated the data sets from the TAIGER and ATSEE projects to resolve onshore-offshore deep crustal seismic profiles from the Wuyi-Yunkai orogen to the Taiwan orogen in southeastern China. Totally, there are three seismic profiles resolved and the longest profile is 850 km. Unlike 2D and 3D first arrival travel-time tomography from previous studies, we used both refracted and reflected phases (Pg, Pn, PcP, and PmP) to model the crustal structures and the crustal reflectors. 40 shots, 2 earthquakes, and about 1,950 stations were used and 15,319 arrivals were picked among three transects. As a result, the complex crustal evolution since Paleozoic era are shown, which involved the closed Paleozoic rifted basin in central Fujian, the Cenozoic extension due to South China sea opening beneath the coastline of southern Fujian, and the on-going collision of the Taiwan orogen.

  2. The Glacial BuzzSaw, Isostasy, and Global Crustal Models

    Science.gov (United States)

    Levander, A.; Oncken, O.; Niu, F.

    2015-12-01

    The glacial buzzsaw hypothesis predicts that maximum elevations in orogens at high latitudes are depressed relative to temperate latitudes, as maximum elevation and hypsography of glaciated orogens are functions of the glacial equilibrium line altitude (ELA) and the modern and last glacial maximum (LGM) snowlines. As a consequence crustal thickness, density, or both must change with increasing latitude to maintain isostatic balance. For Airy compensation crustal thickness should decrease toward polar latitudes, whereas for Pratt compensation crustal densities should increase. For similar convergence rates, higher latitude orogens should have higher grade, and presumably higher density rocks in the crustal column due to more efficient glacial erosion. We have examined a number of global and regional crustal models to see if these predictions appear in the models. Crustal thickness is straightforward to examine, crustal density less so. The different crustal models generally agree with one another, but do show some major differences. We used a standard tectonic classification scheme of the crust for data selection. The globally averaged orogens show crustal thicknesses that decrease toward high latitudes, almost reflecting topography, in both the individual crustal models and the models averaged together. The most convincing is the western hemisphere cordillera, where elevations and crustal thicknesses decrease toward the poles, and also toward lower latitudes (the equatorial minimum is at ~12oN). The elevation differences and Airy prediction of crustal thickness changes are in reasonable agreement in the North American Cordillera, but in South America the observed crustal thickness change is larger than the Airy prediction. The Alpine-Himalayan chain shows similar trends, however the strike of the chain makes interpretation ambiguous. We also examined cratons with ice sheets during the last glacial period to see if continental glaciation also thins the crust toward

  3. Petrographic, geochemical and isotopic evidence of crustal assimilation processes in the Ponte Nova alkaline mafic-ultramafic massif, SE Brazil

    Science.gov (United States)

    Azzone, Rogério Guitarrari; Montecinos Munoz, Patricio; Enrich, Gaston Eduardo Rojas; Alves, Adriana; Ruberti, Excelso; Gomes, Celsode Barros

    2016-09-01

    Crustal assimilation plus crystal fractionation processes of different basanite magma batches control the evolution of the Ponte Nova cretaceous alkaline mafic-ultramafic massif in SE Brazil. This massif is composed of several intrusions, the main ones with a cumulate character. Disequilibrium features in the early-crystallized phases (e.g., corrosion and sieve textures in cores of clinopyroxene crystals, spongy-cellular-textured plagioclase crystals, gulf corrosion texture in olivine crystals) and classical hybridization textures (e.g., blade biotite and acicular apatite crystals) provide strong evidence of open-system behavior. All samples are olivine- and nepheline-normative rocks with basic-ultrabasic and potassic characters and variable incompatible element enrichments. The wide ranges of whole-rock 87Sr/86Sri and 143Nd/144Ndi ratios (0.70432-0.70641 and 0.512216-0.512555, respectively) are indicative of crustal contribution from the Precambrian basement host rocks. Plagioclase and apatite 87Sr/86Sr ratios (0.70422-0.70927) obtained for the most primitive samples of each intrusion indicate disequilibrium conditions from early- to principal-crystallization stages. Isotope mixing-model curves between the least contaminated alkaline basic magma and heterogeneous local crustal components indicate that each intrusion of the massif is differentiated from the others by varied degrees of crustal contribution. The primary mechanisms of crustal contribution to the Ponte Nova massif involve the assimilation of host rock xenoliths during the development of the chamber environment and the assimilation of partial melts from the surrounding host rocks. Thermodynamic models using the melts algorithm indicate that parental alkaline basic magmas can be strongly affected by contamination processes subsequently to their initial stages of crystallization when there is sufficient energy to assimilate partial melts of crustal host rocks. The assimilation processes are considered to

  4. Quantitative tectonic reconstructions of Zealandia based on crustal thickness estimates

    Science.gov (United States)

    Grobys, Jan W. G.; Gohl, Karsten; Eagles, Graeme

    2008-01-01

    Zealandia is a key piece in the plate reconstruction of Gondwana. The positions of its submarine plateaus are major constraints on the best fit and breakup involving New Zealand, Australia, Antarctica, and associated microplates. As the submarine plateaus surrounding New Zealand consist of extended and highly extended continental crust, classic plate tectonic reconstructions assuming rigid plates and narrow plate boundaries fail to reconstruct these areas correctly. However, if the early breakup history shall be reconstructed, it is crucial to consider crustal stretching in a plate-tectonic reconstruction. We present a reconstruction of the basins around New Zealand (Great South Basin, Bounty Trough, and New Caledonia Basin) based on crustal balancing, an approach that takes into account the rifting and thinning processes affecting continental crust. In a first step, we computed a crustal thickness map of Zealandia using seismic, seismological, and gravity data. The crustal thickness map shows the submarine plateaus to have a uniform crustal thickness of 20-24 km and the basins to have a thickness of 12-16 km. We assumed that a reconstruction of Zealandia should close the basins and lead to a most uniform crustal thickness. We used the standard deviation of the reconstructed crustal thickness as a measure of uniformity. The reconstruction of the Campbell Plateau area shows that the amount of extension in the Bounty Trough and the Great South Basin is far smaller than previously thought. Our results indicate that the extension of the Bounty Trough and Great South Basin occurred simultaneously.

  5. Global variations in gravity-derived oceanic crustal thickness: Implications on oceanic crustal accretion and hotspot-lithosphere interactions

    Science.gov (United States)

    Lin, J.; Zhu, J.

    2012-12-01

    We present a new global model of oceanic crustal thickness based on inversion of global oceanic gravity anomaly with constrains from seismic crustal thickness profiles. We first removed from the observed marine free-air gravity anomaly all gravitational effects that can be estimated and removed using independent constraints, including the effects of seafloor topography, marine sediment thickness, and the age-dependent thermal structure of the oceanic lithosphere. We then calculated models of gravity-derived crustal thickness through inversion of the residual mantle Bouguer anomaly using best-fitting gravity-modeling parameters obtained from comparison with seismically determined crustal thickness profiles. Modeling results show that about 5% of the global crustal volume (or 9% of the global oceanic surface area) is associated with model crustal thickness 8.6 km and is interpreted to have been affected by excess magmatism. The percentage of oceanic crustal volume that is associated with thick crustal thickness (>8.6 km) varies greatly among tectonic plates: Pacific (33%), Africa (50%), Antarctic (33%), Australia (30%), South America (34%), Nazca (23%), North America (47%), India (74%), Eurasia (68%), Cocos (20%), Philippine (26%), Scotia (41%), Caribbean (89%), Arabian (82%), and Juan de Fuca (21%). We also found that distribution of thickened oceanic crust (>8.6 km) seems to depend on spreading rate and lithospheric age: (1) On ocean basins younger than 5 Ma, regions of thickened crust are predominantly associated with slow and ultraslow spreading ridges. The relatively strong lithospheric plate at slow and ultraslow ridges might facilitate the loading of large magmatic emplacements on the plate. (2) In contrast, crustal thickness near fast and intermediately fast spreading ridges typically does not exceed 7-8 km. The relatively weak lithosphere at fast and intermediately fast ridges might make it harder for excess magmatism to accrete. We further speculate that

  6. Satellite measurements of the earth's crustal magnetic field

    Science.gov (United States)

    Schnetzler, C. C.

    1989-01-01

    The literature associated with the Magsat mission has evaluated the capabilities and limitations of satellite measurements of the earth's crustal magnetic field, and demonstrated that there exists a 300-3000 km magnetic field, related to major features in the earth's crust, which is primarily caused by induction. Due to its scale and sensitivity, satellite data have been useful in the development of models for such large crustal features as subduction zones, submarine platforms, continental accretion boundaries, and rifts. Attention is presently given to the lack of agreement between laboratory and satellite estimates of lower crustal magnetization.

  7. Tectonic evolution of Mars

    International Nuclear Information System (INIS)

    Wise, D.U.; Golombek, M.P.; McGill, G.E.

    1979-01-01

    Any model for the tectonic evolution of Mars must account for two major crustal elements: the Tharsis bulge and the topographically low and lightly crated northern third of the planet. Ages determined by crater density indicate that both of these elements came into existence very early in Martian history, a conclusion that holds no matter which of the current crater density versus age curves is used. The size of these two major crustal elements and their sequential development suggest that both may be related to a global-scale internal process. It is proposed that the resurfacing of the northern third of Mars is related to subcrustal erosion and isostatic foundering during the life of a first-order convection cell. With the demise of the cell, denser segregations of metallic materials began to coalesce as a gravitatively unstable layer which finally overturned to form the core. In the overturn, lighter crustal materials was shifted laterally and underplated beneath Tharsis to cause rapid and permanent isostatic rise. This was followed by a long-lived thermal phase produced by the hot underplate and by the gravitative energy of core formation slowly making its way to the surface to produce the Tharsis volcanics

  8. Evolution of the Pine Creek Geosyncline

    International Nuclear Information System (INIS)

    Stuart-Smith, P.G.; Crick, I.H.; Needham, R.S.; Wills, K.

    1980-01-01

    An interpretation of the evolution of the geosyncline is described in four main stages. Stage 1. The geosyncline began to develop in the Early Proterozoic probably during incipient rifting and subsidence of Archaean basement. Continental and shallow marine sediments were deposited at the margins of the geosyncline while in its centre a thicker sequence of fine clastic and chemical sediments, and volcanics accumulated. Stage 2. Uplift of Archaean basement resulted in partial erosion of Stage 1 sediments and an influx of clastic material into the geosyncline. Sandstone and conglomerate formed extensive alluvial fans flanking the uplifted areas and graded distally into littoral and subtidal deposits which later transgressed over them. Stage 3. The third stage started with a tectonic event which caused uplift, folding and subsequent peneplanation of Stage 1 and 2 sediments. A new phase of sedimentation, dominated by chemical and organic sediments and felsic volcanics, accompanied a shallow marine transgression. A major shift in the locus of tectonic activity within the geosyncline from the centre to the west resulted in faulting and volcanism on the western margin and caused an influx of flysch-type sediment into the geosyncline to form an eastwards-thickening wedge. At the close of sedimentation sills of tholeiitic basalt were emplaced into the Early Proterozoic sediments. Stage 4. Continued subsidence of the geosyncline resulted in deformation, metamorphism and partial anatexis of the Early Proterozoic sediments, and some subsequent igneous activity. The sediments were uplifted, eroded and overlain by flat-lying Middle Proterozoic, younger Proterozoic, Palaeozoic and Mesozoic rocks, indicating continued tectonic stability since early Middle Proterozoic times

  9. Early Neoarchaean A-type granitic magmatism by crustal reworking ...

    Indian Academy of Sciences (India)

    29

    understand their petrogenesis and tectonic setting. .... crystallize from magmas with temperatures significantly higher than those of other intracrustal ...... blanketing by greenstone belt volcanic rocks, crustal thickening and hot subduction or a. 1.

  10. Crustal structure and tectonic model of the Arctic region

    DEFF Research Database (Denmark)

    Petrov, Oleg; Morozov, Andrey; Shokalsky, Sergey

    2016-01-01

    We present a new model of the crustal and tectonic structure of the Arctic region north of 60° N latitude, constrained as a part of the international Atlas of Geological Maps of the Circumpolar Arctic under the aegis of the Commission for the Geological Map of the World. The region is largely...... formed by (i) Archean-Paleoproterozoic shields and platforms, (ii) orogenic belts of the Neoproterozoic to the Late Mesozoic ages overlain by platform and basin sediments, (iii) Cenozoic rift structures formed in part as a consequence of seafloor spreading in the North East Atlantic Ocean...... and thickness of the sedimentary cover and presents tectonic regionalization based on 18 major crustal types (oceanic, transitional, and continental) recognized in the Arctic. A 7600. km-long crustal geotransect across the region illustrates the details of its crustal and tectonic structure. We discuss...

  11. Continental crustal formation and recycling: Evidence from oceanic basalts

    Science.gov (United States)

    Saunders, A. D.; Tarney, J.; Norry, M. J.

    1988-01-01

    Despite the wealth of geochemical data for subduction-related magma types, and the clear importance of such magmas in the creation of continental crust, there is still no concensus about the relative magnitudes of crustal creation versus crustal destruction (i.e., recycling of crust into the mantle). The role of subducted sediment in the formation of the arc magmas is now well documented; but what proportion of sediment is taken into the deeper mantle? Integrated isotopic and trace element studies of magmas erupted far from presently active subduction zones, in particular basaltic rocks erupted in the ocean basins, are providing important information about the role of crustal recycling. By identifying potential chemical tracers, it is impossible to monitor the effects of crustal recycling, and produce models predicting the mass of material recycled into the mantle throughout long periods of geological time.

  12. Crustal Structure of Active Deformation Zones in Africa: Implications for Global Crustal Processes

    Science.gov (United States)

    Ebinger, C. J.; Keir, D.; Bastow, I. D.; Whaler, K.; Hammond, J. O. S.; Ayele, A.; Miller, M. S.; Tiberi, C.; Hautot, S.

    2017-12-01

    The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow-moving African continent, which last experienced orogeny during the Pan-African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas-rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises 20% new magmatic material ponded in the lower crust and intruded as sills and dikes at shallower depths. In the Main Ethiopian Rift, intrusions comprise 30% of the crust below axial zones of dike-dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges. Our comparisons suggest that transitional crust, including seaward dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15-20 km thick crust. In the 30 Ma Recent CVL, which lacks a hot spot age progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hot spot toward the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology, and contributing significantly to global carbon and water fluxes.

  13. Ionospheric precursors for crustal earthquakes in Italy

    Directory of Open Access Journals (Sweden)

    L. Perrone

    2010-04-01

    Full Text Available Crustal earthquakes with magnitude 6.0>M≥5.5 observed in Italy for the period 1979–2009 including the last one at L'Aquila on 6 April 2009 were considered to check if the earlier obtained relationships for ionospheric precursors for strong Japanese earthquakes are valid for the Italian moderate earthquakes. The ionospheric precursors are based on the observed variations of the sporadic E-layer parameters (h'Es, fbEs and foF2 at the ionospheric station Rome. Empirical dependencies for the seismo-ionospheric disturbances relating the earthquake magnitude and the epicenter distance are obtained and they have been shown to be similar to those obtained earlier for Japanese earthquakes. The dependences indicate the process of spreading the disturbance from the epicenter towards periphery during the earthquake preparation process. Large lead times for the precursor occurrence (up to 34 days for M=5.8–5.9 tells about a prolong preparation period. A possibility of using the obtained relationships for the earthquakes prediction is discussed.

  14. The crustal dynamics intelligent user interface anthology

    Science.gov (United States)

    Short, Nicholas M., Jr.; Campbell, William J.; Roelofs, Larry H.; Wattawa, Scott L.

    1987-01-01

    The National Space Science Data Center (NSSDC) has initiated an Intelligent Data Management (IDM) research effort which has, as one of its components, the development of an Intelligent User Interface (IUI). The intent of the IUI is to develop a friendly and intelligent user interface service based on expert systems and natural language processing technologies. The purpose of such a service is to support the large number of potential scientific and engineering users that have need of space and land-related research and technical data, but have little or no experience in query languages or understanding of the information content or architecture of the databases of interest. This document presents the design concepts, development approach and evaluation of the performance of a prototype IUI system for the Crustal Dynamics Project Database, which was developed using a microcomputer-based expert system tool (M. 1), the natural language query processor THEMIS, and the graphics software system GSS. The IUI design is based on a multiple view representation of a database from both the user and database perspective, with intelligent processes to translate between the views.

  15. Cooperative research in space geodesy and crustal dynamics

    Science.gov (United States)

    1994-01-01

    This research grant, which covered the period of July 1991 to August 1994, was concerned with a variety of topics within the geodesy and crustal dynamics fields. The specific topics of this grant included satellite tracking and gravity field determinations and crustal dynamics (this concentrated of space geodetic site stability for VLBI sites). Summaries of the specific research projects are included along with a list of publications and presentations supported by this research grant.

  16. Lower crustal earthquakes in the North China Basin and implications for crustal rheology

    Science.gov (United States)

    Yuen, D. A.; Dong, Y.; Ni, S.; LI, Z.

    2017-12-01

    The North China Basin is a Mesozoic-Cenozoic continental rift basin on the eastern North China Craton. It is the central region of craton destruction, also a very seismically active area suffering severely from devastating earthquakes, such as the 1966 Xingtai M7.2 earthquake, the 1967 Hejian M6.3 earthquake, and the 1976 Tangshan M7.8 earthquake. We found remarkable discrepancies of depth distribution among the three earthquakes, for instance, the Xingtai and Tangshan earthquakes are both upper-crustal earthquakes occurring between 9 and 15 km on depth, but the depth of the Hejian earthquake was reported of about 30 72 km, ranging from lowermost crust to upper mantle. In order to investigate the focal depth of earthquakes near Hejian area, we developed a method to resolve focal depth for local earthquakes occurring beneath sedimentary regions by P and S converted waves. With this method, we obtained well-resolved depths of 44 local events with magnitudes between M1.0 and M3.0 during 2008 to 2016 at the Hejian seismic zone, with a mean depth uncertainty of about 2 km. The depth distribution shows abundant earthquakes at depth of 20 km, with some events in the lower crust, but absence of seismicity deeper than 25 km. In particular, we aimed at deducing some constraints on the local crustal rheology from depth-frequency distribution. Therefore, we performed a comparison between the depth-frequency distribution and the crustal strength envelop, and found a good fit between the depth profile in the Hejian seismic zone and the yield strength envelop in the Baikal Rift Systems. As a conclusion, we infer that the seismogenic thickness is 25 km and the main deformation mechanism is brittle fracture in the North China Basin . And we made two hypotheses: (1) the rheological layering of dominant rheology in the North China Basin is similar to that of the Baikal Rift Systems, which can be explained with a quartz rheology at 0 10 km depth and a diabase rheology at 10 35 km

  17. CRUSTAL THICKNESS VARIATIONS AND SEISMICITY OF NORTHWESTERN SOUTH AMERICA

    Directory of Open Access Journals (Sweden)

    Woo Kim Jeong

    2007-06-01

    Full Text Available Any uncompensated mass of the northern Andes Mountains is presumably under pressure to adjust within the Earth to its ideal state of isostatic equilibrium. Isostasy is the ideal state that any
    uncompensated mass seeks to achieve in time. These pressures interact with the relative motions between adjacent plates that give rise to earthquakes along the plate boundaries. By combining the
    gravity MOHO estimates and crustal discontinuities with historical and instrumental seismological catalogs the correlation between isostatically disturbed terrains and seismicity has been established.
    The thinner and thicker crustal regions were mapped from the zero horizontal curvature of the crustal thickness estimates. These boundaries or edges of crustal thickness variations were compared to
    crustal discontinuities inferred from gravity and magnetic anomalies and the patterns of seismicity that have been catalogued for the last 363 years. The seismicity is very intense along the Nazca-North
    Andes, Caribbean-North American and North Andes-South American collision zones and associated with regional tectonic compressional stresses that have locally increased and/or diminished by
    compressional and tensional stress, respectively, due to crustal thickness variations. High seismicity is also associated with the Nazca-Cocos diverging plate boundary whereas low seismicity is associated with the Panama-Nazca Transform Fault and the South American Plate.

  18. Greenland from Archaean to Quaternary, Descriptive text to the 1995 Geological Map of Greenland 1:2 500 000, 2nd edition

    Directory of Open Access Journals (Sweden)

    Kalsbeek, Feiko

    2009-11-01

    Full Text Available The geological development of Greenland spans a period of nearly 4 Ga, from Eoarchaean to the Quaternary. Greenland is the largest island on Earth with a total area of 2 166 000 km2, but only c. 410 000 km2 are exposed bedrock, the remaining part being covered by a major ice sheet (the Inland Ice reaching over 3 km in thickness. The adjacent offshore areas underlain by continental crust have an area of c. 825 000 km2. Greenland is dominated by crystalline rocks of the Precambrian shield, which formed during a succession of Archaean and Palaeoproterozoic orogenic events and stabilised as a part of the Laurentian shield about 1600 Ma ago. The shield area can be divided into three distinct types of basement provinces: (1 Archaean rocks (3200–2600 Ma old, with local older units up to >3800Ma that were almost unaffected by Proterozoic or later orogenic activity; (2 Archaean terrains reworked during the Palaeoproterozoic around 1900–1750 Ma ago; and (3 terrains mainly composed of juvenile Palaeoproterozoic rocks (2000–1750 Ma in age.Subsequent geological developments mainly took place along the margins of the shield. During the Proterozoic and throughout the Phanerozoic major sedimentary basins formed, notably in North and North-East Greenland, in which sedimentary successions locally reaching 18 km in thickness were deposited. Palaeozoic orogenic activity affected parts of these successions in the Ellesmerian fold belt of North Greenland and the East Greenland Caledonides; the latter also incorporates reworked Precambrian crystalline basement complexes. Late Palaeozoic and Mesozoic sedimentary basins developed along the continent–ocean margins in North, East and West Greenland and are now preserved both onshore and offshore. Their development was closely related to continental break-up with formation of rift basins. Initial rifting in East Greenland in latest Devonian to earliest Carboniferous time and succeeding phases culminated with the

  19. N-S crustal shear system in the Bundelkhand massif: A unique crustal evolution signature in the northern Indian peninsula

    Science.gov (United States)

    Singh, S. P.; Bhattacharya, A. R.

    2017-12-01

    The Bundelkhand massif, located in the northern part of the Indian shield, is a poly-deformed and poly-metamorphic terrain. This paper reports a new shear system developed throughout the massif in the form of N-S trending quartz veins that are sometimes quartzo-feldspathic and rarely granitic in composition. The veins are vertical and commonly occur in conjugate sets. This tectono-magmatic event appears to represent the youngest shear system of the massif as it cross-cuts all the earlier shear systems (E-W, NE-SE and NW-SE). Emplacement of this N-S vein system may have taken place due to extensional processes that developed some cracks along which siliceous magma was vertically emplaced. The complete absence of signature of the N-S event from the surrounding sedimentary cover of Vindhyan Supergroup, Bijawar and Gwalior Groups suggests that this shear system is pre-tectonic to the nearly E-W trending passive basins developed at the margins of the Bundelkhand craton. Further, several workers have considered the Bundelkhand massif as a part of the Aravalli craton. However, due to the absence of N-S, as well as the other (i.e., E-W, NW-SE and NW-SE), tectonic fabrics of the Bundelkhand massif in other cratons of the Peninsular India, and vice versa, makes the Bundelkhand block a separate and unique craton of its own and is not part of the Aravalli craton.

  20. Effective stress, friction and deep crustal faulting

    Science.gov (United States)

    Beeler, N.M.; Hirth, Greg; Thomas, Amanda M.; Burgmann, Roland

    2016-01-01

    Studies of crustal faulting and rock friction invariably assume the effective normal stress that determines fault shear resistance during frictional sliding is the applied normal stress minus the pore pressure. Here we propose an expression for the effective stress coefficient αf at temperatures and stresses near the brittle-ductile transition (BDT) that depends on the percentage of solid-solid contact area across the fault. αf varies with depth and is only near 1 when the yield strength of asperity contacts greatly exceeds the applied normal stress. For a vertical strike-slip quartz fault zone at hydrostatic pore pressure and assuming 1 mm and 1 km shear zone widths for friction and ductile shear, respectively, the BDT is at ~13 km. αf near 1 is restricted to depths where the shear zone is narrow. Below the BDT αf = 0 is due to a dramatically decreased strain rate. Under these circumstances friction cannot be reactivated below the BDT by increasing the pore pressure alone and requires localization. If pore pressure increases and the fault localizes back to 1 mm, then brittle behavior can occur to a depth of around 35 km. The interdependencies among effective stress, contact-scale strain rate, and pore pressure allow estimates of the conditions necessary for deep low-frequency seismicity seen on the San Andreas near Parkfield and in some subduction zones. Among the implications are that shear in the region separating shallow earthquakes and deep low-frequency seismicity is distributed and that the deeper zone involves both elevated pore fluid pressure and localization.

  1. Intensity attenuation for active crustal regions

    Science.gov (United States)

    Allen, Trevor I.; Wald, David J.; Worden, C. Bruce

    2012-07-01

    We develop globally applicable macroseismic intensity prediction equations (IPEs) for earthquakes of moment magnitude M W 5.0-7.9 and intensities of degree II and greater for distances less than 300 km for active crustal regions. The IPEs are developed for two distance metrics: closest distance to rupture ( R rup) and hypocentral distance ( R hyp). The key objective for developing the model based on hypocentral distance—in addition to more rigorous and standard measure R rup—is to provide an IPE which can be used in near real-time earthquake response systems for earthquakes anywhere in the world, where information regarding the rupture dimensions of a fault may not be known in the immediate aftermath of the event. We observe that our models, particularly the model for the R rup distance metric, generally have low median residuals with magnitude and distance. In particular, we address whether the direct use of IPEs leads to a reduction in overall uncertainties when compared with methods which use a combination of ground-motion prediction equations and ground motion to intensity conversion equations. Finally, using topographic gradient as a proxy and median model predictions, we derive intensity-based site amplification factors. These factors lead to a small reduction of residuals at shallow gradients at strong shaking levels. However, the overall effect on total median residuals is relatively small. This is in part due to the observation that the median site condition for intensity observations used to develop these IPEs is approximately near the National Earthquake Hazard Reduction Program CD site-class boundary.

  2. The crustal structure of Ellesmere Island, Arctic Canada—teleseismic mapping across a remote intraplate orogenic belt

    Science.gov (United States)

    Schiffer, Christian; Stephenson, Randell; Oakey, Gordon N.; Jacobsen, Bo H.

    2016-03-01

    Ellesmere Island in Arctic Canada displays a complex geological evolution. The region was affected by two distinct orogenies, the Palaeozoic Ellesmerian orogeny (the Caledonian equivalent in Arctic Canada and Northern Greenland) and the Palaeogene Eurekan orogeny, related to the opening of Baffin Bay and the consequent convergence of the Greenland plate. The details of this complex evolution and the present-day deep structure are poorly constrained in this remote area and deep geophysical data are sparse. Receiver function analysis of seven temporary broad-band seismometers of the Ellesmere Island Lithosphere Experiment complemented by two permanent stations provides important data on the crustal velocity structure of Ellesmere Island. The crustal expression of the northernmost tectonic block of Ellesmere Island (˜82°-83°N), Pearya, which was accreted during the Ellesmerian orogeny, is similar to that at the southernmost part, which is part of the Precambrian Laurentian (North America-Greenland) craton. Both segments have thick crystalline crust (˜35-36 km) and comparable velocity-depth profiles. In contrast, crustal thickness in central Ellesmere Island decreases from ˜24-30 km in the Eurekan fold and thrust belt (˜79.7°-80.6°N) to ˜16-20 km in the Hazen Stable Block (HSB; ˜80.6°-81.4°N) and is covered by a thick succession of metasediments. A deep crustal root (˜48 km) at ˜79.6°N is interpreted as cratonic crust flexed beneath the Eurekan fold and thrust belt. The Carboniferous to Palaeogene sedimentary succession of the Sverdrup Basin is inferred to be up to 1-4 km thick, comparable to geologically-based estimates, near the western margin of the HSB.

  3. Lithospheric controls on crustal reactivation and intraplate mountain building in the Gobi Corridor, Central Asia

    Science.gov (United States)

    Cunningham, D.

    2017-12-01

    This talk will review the Permian-Recent tectonic history of the Gobi Corridor region which includes the actively deforming Gobi Altai-Altai, Eastern Tien Shan, Beishan and North Tibetan foreland. Since terrane amalgamation in the Permian, Gobi Corridor crust has been repeatedly reactivated by Triassic-Jurassic contraction/transpression, Late Cretaceous extension and Late Cenozoic transpression. The tectonic history of the region suggests the following basic principle for intraplate continental regions: non-cratonized continental interior terrane collages are susceptible to repeated intraplate reactivation events, driven by either post-orogenic collapse and/or compressional stresses derived from distant plate boundary convergence. Thus, important related questions are: 1) what lithospheric pre-conditions favor intraplate crustal reactivation in the Gobi Corridor (simple answer: crustal thinning, thermal weakening, strong buttressing cratons), 2) what are the controls on the kinematics of deformation and style of mountain building in the Gobi-Altai-Altai, Beishan and North Tibetan margin (simple answer: many factors, but especially angular relationship between SHmax and `crustal grain'), 3) how does knowledge of the array of Quaternary faults and the historical earthquake record influence our understanding of modern earthquake hazards in continental intraplate regions (answer: extrapolation of derived fault slip rates and recurrence interval determinations are problematic), 4) what important lessons can we learn from the Mesozoic-Cenozoic tectonic history of Central Asia that is applicable to the tectonic evolution of all intraplate continental regions (simple answer: ancient intraplate deformation events may be subtly expressed in the rock record and only revealed by low-temperature thermochronometers, preserved orogen-derived sedimentary sequences, fault zone evidence for younger brittle reactivation, and recognition of a younger class of cross-cutting tectonic

  4. Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes

    Science.gov (United States)

    Rudnick, R. L.; Liu, X. M.

    2012-04-01

    The continental crust has an "intermediate" bulk composition that is distinct from primary melts of peridotitic mantle (basalt or picrite). This mismatch between the "building blocks" and the "edifice" that is the continental crust points to the operation of processes that preferentially remove mafic to ultramafic material from the continents. Such processes include lower crustal recycling (via density foundering or lower crustal subduction - e.g., relamination, Hacker et al., 2011, EPSL), generation of evolved melts via slab melting, and/or chemical weathering. Stable isotope systems point to the influence of chemical weathering on the bulk crust composition: the oxygen isotope composition of the bulk crust is distinctly heavier than that of primary, mantle-derived melts (Simon and Lecuyer, 2005, G-cubed) and the Li isotopic composition of the bulk crust is distinctly lighter than that of mantle-derive melts (Teng et al., 2004, GCA; 2008, Chem. Geol.). Both signatures mark the imprint of chemical weathering on the bulk crust composition. Here, we use a simple mass balance model for lithium inputs and outputs from the continental crust to quantify the mass lost due to chemical weathering. We find that a minimum of 15%, a maximum of 60%, and a best estimate of ~40% of the original juvenile rock mass may have been lost via chemical weathering. The accumulated percentage of mass loss due to chemical weathering leads to an average global chemical weathering rate (CWR) of ~ 1×10^10 to 2×10^10 t/yr since 3.5 Ga, which is about an order of magnitude higher than the minimum estimates based on modern rivers (Gaillardet et al., 1999, Chem. Geol.). While we cannot constrain the exact portion of crustal mass loss via chemical weathering, given the uncertainties of the calculation, we can demonstrate that the weathering flux is non-zero. Therefore, chemical weathering must play a role in the evolution of the composition and mass of the continental crust.

  5. Anomalous uranium concentration in Archaean basement Shear at Dhani Basri and its significance on Southern Margin of Alwar sub-basin, Rajasthan

    International Nuclear Information System (INIS)

    Panigrahi, B.; Shaji, T.S.; Sharma, G.S.; Yadav, O.P.; Nanda, L.K.

    2008-01-01

    Prominent shear zones cutting through the basement and cover rocks of Delhi Supergroup have been recognized in Dhani Basri - Ramewala sector of Dausa district, Rajasthan. One such shear zone traversing the granite gneiss (Archaean basement) has been observed at Dhani Basri. The sheared rock is exposed in the form of a small hump and gives appearance of quartzite due to intense silicification. Grab samples collected from the shear zone rock analysed upto 93 ppm U 3 O 8 and <10 ppm ThO 2 , which is anomalous in comparison to unsheared rock which analysed 51 ppm eU 3 O 8 , upto 5 ppm U 3 O 8 and 80 ppm ThO 2 . Gamma-ray logging of boreholes drilled by GSI across this shear zone indicated uranium mineralization of the order of 0.030% eU 3 O 8 x 5.40 m and the primary radioactive mineral has been identified as uraninite. The extension of Dhani Basri shear zone inside the cover rocks of Meso-Proterozoic Delhi Supergroup of rocks of Alwar sub-basin is of paramount importance in locating unconformity related as well as hydrothermal vein type uranium mineralization. (author)

  6. An Archaean Tonalite-Trondhjemite-Granodiorite Association of the Kursk Block (Voronezh Massif): Composition, Age, and Correlation with the Ukrainian Shield

    Science.gov (United States)

    Savko, K. A.; Samsonov, A. V.; Larionov, A. N.; Korish, E. Kh.; Bazikov, N. S.

    2018-01-01

    Framing of the Archaean greenstone belts of the Kursk Block (KB) of the East Sarmatia preserves rocks of the TTG association: those do not form massifs with distinct boundaries, but occur as fields gradually transiting into gneisses and migmatites. According to Sm-Nd isotope-geochemical data, the TTG are characterized by positive values of ɛNd(2960) = +0.3…+1.6 and protolith model ages of T Nd( DM) = 3100-3200 Ma. Magmatic protoliths of the Kursk Block TTG were formed about 2960 Ma by melting from a juvenile basite source. These age estimates are significantly younger than heterochronous (3.19, 3.13 and 3.07 Ga) TTGs of the Middle Dnieper granite-greenstone terrane. On the other hand, the similarity of ɛNd(T) implies a single source of their protoliths. Consequently, the KB TTGs, apparently, are a result of transformation of an older sial crust preserved within the Middle Dnieper Block.

  7. New Crustal Thickness for Djibouti, Afar, Using Seismic Techniques

    Science.gov (United States)

    Dugda, Mulugeta; Bililign, Solomon

    2008-10-01

    Crustal thickness and Poisson's ratio for the seismic station ATD in Djibouti, Afar, has been investigated using two seismic techniques (H-κ stacking of receiver functions and a joint inversion of receiver functions and surface wave group velocities). Both techniques give consistent results of crustal thickness 23±1.5 km and Poisson's ratio 0.31±0.02. We also determined a mean P-wave velocity (Vp) of ˜6.2 km/s but ˜6.9-7.0 km/s below a 2 - 5 km thick low velocity layer at the surface. Previous studies of crustal structure for Djibouti reported that the crust is 6 to 11 km thick while our study shows that the crust beneath Djibouti is between 20 and 25 km. This study argues that the crustal thickness values reported for Djibouti for the last 3 decades were not consistent with the reports for the other neighboring region in central and eastern Afar. Our results for ATD in Djibouti, however, are consistent with the reports of crustal thickness in many other parts of central and eastern Afar. We attribute this difference to how the Moho (the crust-mantle discontinuity) is defined (an increase of Vp to 7.4 km/s in this study vs. 6.9 km/s in previous studies).

  8. Crustal Poisson’s ratio anomalies in the eastern part of North China and their origins

    Directory of Open Access Journals (Sweden)

    Wencai Yang

    2011-07-01

    Full Text Available Seismic tomography can provide both fine P-wave and S-wave velocity structures of the crust and upper mantle. In addition, with proper computation, Poisson’s ratio images from the seismic velocities can be determined. However, it is unknown whether Poisson’s ratio images have any advantages when compared with the P-wave and S-wave velocity images. For the purposes of this study, high-resolution seismic tomography under the eastern part of North China region was used to determine detailed 3-D crustal P- and S-wave seismic velocities structure, as well as Poisson’s ratio images. Results of Poisson’s ratio imaging show high Poisson’s ratio (high-PR anomalies located in the Hengshan-North Taihang-Zhangjiakou (H-NT-Z region, demonstrating that Poisson’s ratio imaging can provide new geophysical constraints for regional tectonic evolution. The H-NT-Z region shows a prominent and continuous high-PR anomaly in the upper crust. Based on Poisson’s ratio images at different depths, we find that this high-PR anomaly is extending down to the middle crust with thickness up to about 26 km. According to rock physical property measurements and other geological data, this crustal Poisson’s ratio anomaly can be explained by Mesozoic partial melting of the upper mantle and basaltic magma underplating related to the lithospheric thinning of the North China craton.

  9. Thermal and petrologic constraints on lower crustal melt accumulation under the Salton Sea Geothermal Field

    Science.gov (United States)

    Karakas, Ozge; Dufek, Josef; Mangan, Margaret T.; Wright, Heather M.; Bachmann, Olivier

    2017-06-01

    In the Salton Sea region of southern California (USA), concurrent magmatism, extension, subsidence, and sedimentation over the past 0.5 to 1.0 Ma have led to the creation of the Salton Sea Geothermal Field (SSGF)-the second largest and hottest geothermal system in the continental United States-and the small-volume rhyolite eruptions that created the Salton Buttes. In this study, we determine the flux of mantle-derived basaltic magma that would be required to produce the elevated average heat flow and sustain the magmatic roots of rhyolite volcanism observed at the surface of the Salton Sea region. We use a 2D thermal model to show that a lower-crustal, partially molten mush containing Salton Trough, and are consistent with seismic observations. Our results indicate limited melting and assimilation of pre-existing rocks in the lower crust. Instead, we find that basalt fractionation in the lower crust produces derivative melts of andesitic to dacitic composition. Such melts are then expected to ascend and accumulate in the upper crust, where they further evolve to give rise to small-volume rhyolite eruptions (Salton Buttes) and fuel local spikes in surface heat flux as currently seen in the SSGF. Such upper crustal magma evolution, with limited assimilation of hydrothermally altered material, is required to explain the slight decrease in δ18 O values of zircons (and melts) that have been measured in these rhyolites.

  10. Along-axis crustal structure of the Porcupine Basin from seismic refraction data modelling

    Science.gov (United States)

    Prada, Manel; Watremez, Louise; Chen, Chen; O'Reilly, Brian; Minshull, Tim; Reston, Tim; Wagner, Gerlind; Gaws, Viola; Klaschen, Dirk; Shannon, Patrick

    2016-04-01

    The Porcupine Basin is a tongue-shaped offshore basin SW of Ireland that formed during the opening of the North Atlantic Ocean. Its history of development involved several rifting and subsidence phases during the Late Paleozoic and Cenozoic, with a particular major rift phase occurring in Late Jurassic-Early Cretaceous times. Previous work, focused on subsidence analysis, showed that stretching factors (β) in the northern part of the basin are 6. However, recent studies based on seismic reflection and refraction profiles concluded that β in places along the basin axis were significantly higher, and suggested the presence of major crustal faulting and uppermost mantle serpentinization in the basin. Constraining β and the processes related to the formation of the basin will provide insights into aspects such as the tectonic response to lithospheric extension and the thermal evolution of the basin. Here we present the tomography results of five wide-angle seismic (WAS) profiles acquired across and along the basin axis. We used a travel time inversion method to model the WAS data and obtain P-wave velocity (Vp) models of the crust and uppermost mantle, together with the geometry of the main geological interfaces along each of these lines. Coincident seismic reflection profiles to each WAS line were also used to integrate the tectonic structure with the Vp model. These results improved constrains on the location of the base of the crust and allow to estimate maximum β (βmax) along each profile. The analysis shows that βmax values in the northern part of the basin are 5-6 times larger than estimates based on subsidence analysis. Towards the south, βmax increases up to 10, but then rapidly decreases to 3.3 southwards. These values are well within the range of crustal extension at which the crust becomes entirely brittle at magma-poor margins allowing the formation of major crustal faulting and serpentinization of the mantle. In agreement with this observation, Vp

  11. Topography of the Betics: crustal thickening, dynamic topography and relief inheritance

    Science.gov (United States)

    Janowski, Marianne; Loget, Nicolas; Bellahsen, Nicolas; Husson, Laurent; Le Pourhiet, Laetitia; Meyer, Bertrand

    2017-04-01

    The main mechanism that explains high orogenic topographies is the isostatic adjustment due to crustal thickening. However in the Betic Cordillera (South Spain), the present-day elevation and crustal thickness are not correlated. That is at odds with the general premise of isostasy and requires reappraising the question of the driving mechanisms leading to the current topography. The Betics are located at the western edge of the alpine Mediterranean belt. Its Cenozoic orogenic building was disrupted by a major crustal thinning event induced by a slab rollback in the internal zones (Alboran domain) during Neogene. Topography was largely levelled and flooded by the sea during Neogene extension, and then has been folded since the Late Tortonian inversion. The present-day topography shows flat summits still preserved from fluvial regression in the internal zones (central and eastern Betics). These low-relief surfaces may be inherited from the Neogene planation toward sea-level as rocks cooling histories inferred from low-temperature thermochronology seem to point it out. Post-Tortonian shortening estimated thanks to a crustal-scale N-S cross-section in the eastern Betics (at the Sierra Nevada longitude) does not exceed few kilometers which is much lower than the shortening required by isostatic equilibrium, and is thus insufficient to explain the post-Tortonian topography building. We tested the hypothesis that mantle dynamics could in fact be an important mechanism that explains the topography of the Betics. We first computed the residual topography (i.e. the non-isostatic component of the elevation) using the most recent published Moho mapping of the area. In the western Betics, our results show important negative residual topography (down to -3 km) possibly associated with the west-Alboran slab suction. In the eastern Betics however, positive residual topography is important (up to +3 km) and can be explained by the dynamic mantle support of the topography, possibly

  12. Crustal Viscosity Structure Estimated from Multi-Phase Mixing Theory

    Science.gov (United States)

    Shinevar, W. J.; Behn, M. D.; Hirth, G.

    2014-12-01

    Estimates of lower crustal viscosity are typically constrained by analyses of isostatic rebound, post seismic creep, and laboratory-derived flow laws for crustal rocks and minerals. Here we follow a new approach for calculating the viscosity structure of the lower continental crust. We use Perple_X to calculate mineral assemblages for different crustal compositions. Effective viscosity is then calculated using the rheologic mixing model of Huet et al. (2014) incorporating flow laws for each mineral phase. Calculations are performed along geotherms appropriate for the Basin and Range, Tibetan Plateau, Colorado Plateau, and the San Andreas Fault. To assess the role of crustal composition on viscosity, we examined two compositional gradients extending from an upper crust with ~67 wt% SiO2 to a lower crust that is either: (i) basaltic with ~53 wt% SiO2 (Rudnick and Gao, 2003), or (ii) andesitic with ~64% SiO2 (Hacker et al., 2011). In all cases, the middle continental crust has a viscosity that is 2-3 orders of magnitude greater than that inferred for wet quartz, a common proxy for mid-crustal viscosities. An andesitic lower crust results in viscosities of 1020-1021 Pa-s and 1021-1022 Pa-s for hotter and colder crustal geotherms, respectively. A mafic lower crust predicts viscosities that are an order of magnitude higher for the same geotherm. In all cases, the viscosity calculated from the mixing model decreases less with depth compared to single-phase estimates. Lastly, for anhydrous conditions in which alpha quartz is stable, we find that there is a strong correlation between Vp/Vs and bulk viscosity; in contrast, little to no correlation exists for hydrous conditions.

  13. New Interpretation of Crustal Extension Evidences on Mars

    Science.gov (United States)

    Grin, E. A.

    The record of early evolution of life on Earth has been obscured by extensive surface activity. On the opposite, large fractions of the martian surface date back to an early clement epoch favorable to the needs of biological systems [1]. The upper martian surface reflects a wide variety of modifying processes which destroy the geological context. However, due to endogenic causes acting after the end of the primordial bombardment, abundant extensional structures display vertical sequences of stratigraphic units from late Noachian to early Hesperian periods [2]. Deep structural incisions in the upper crust provide unaltered strata, open flanks, and slope deposits that favor the use of an autonomous lander-rover-penetrator The strategy for an exobiology search of such an optimum site should be guided by the recent attention devoted to extensional structures and their global significance [4]. Geological evidence supporting the martian crustal extension is suggested by abundant fractures associated with the dichotomy boundary northland-south upland, i.e., Aeolis Region, and peak igneous activity (Elysium bulge). As pointed out by [5], the system of fractures correlates with the endogenic origin of the dichotomy, as related to a major difference in the thicknessof the crust. Perpendicular to this boundary, fractures of deep graben testify to a general tectonic crust relaxation. The opening of the graben, joined with compressive wrinkles, is the signature of a dynamical pervasive stress regime that implies a large scale roll-over of the upper crust over the ductile interface of a more dense mantle. This general motion is not a transport of material, as there is no thickening on the boundary of the dichotomy. The horizontal movement is due to the gravitational mechanism and differential thermal convection cells in the upper crust over the slope of the anti-flexure rigid interface consequential to Elysium bulge. The fracturation occurs as the neutral zone of the crust rises

  14. Crustal fraction of moment of inertia in pulsars

    International Nuclear Information System (INIS)

    Atta, Debasis; Mukhopadhyay, Somnath; Basu, D.N.

    2015-01-01

    In the present work, stability of the β-equilibrated dense nuclear matter is analyzed with respect to the thermodynamic stability conditions. Based on the density dependent M3Y (DDM3Y) effective nucleon-nucleon (NN) interaction, the location of the inner edge of neutron star crusts and core-crust transition density and pressure are calculated and crustal fraction of moment of inertia is determined. These results for pressure and density at core-crust transition together with the observed minimum crustal fraction of the total moment of inertia provide a new limit for the radius of the Vela pulsar

  15. Crustal structure and active tectonics in the Eastern Alps

    DEFF Research Database (Denmark)

    Brückl, E.; Behm, M.; Decker, K.

    2010-01-01

    fragment (PA), was interpreted and a triple junction was inferred. The goal of this study has been to relate these deep crustal structures to active tectonics. We used elastic plate modeling to reconsider the Moho fragmentation. We interpret subduction of EU below AD and PA from north to south......During the last decade, a series of controlled source seismic experiments brought new insight into the crustal and lithospheric structure of the Eastern Alps and their adjacent tectonic provinces. A fragmentation of the lithosphere into three blocks, Europe (EU), Adria (AD), and the new Pannonian...

  16. Extreme Mesozoic crustal thinning in the Eastern Iberia margin: The example of the Columbrets Basin (Valencia Trough)

    Science.gov (United States)

    Mohn, G.; Etheve, N.; Frizon de Lamotte, D.; Roca, E.; Tugend, J.; Gómez-Romeu, J.

    2017-12-01

    Eastern Iberia preserves a complex succession of Mesozoic rifts partly or completely inverted during the Late Cretaceous and Cenozoic in relation with Africa-Eurasia convergence. Notably, the Valencia Trough, classically viewed as part of the Cenozoic West Mediterranean basins, preserves in its southwestern part a thick Mesozoic succession (locally »10km thick) over a highly thinned continental basement (locally only »3,5km thick). This sub-basin referred to as the Columbrets Basin, represents a Late Jurassic-Early Cretaceous hyper-extended rift basin weakly overprinted by subsequent events. Its initial configuration is well preserved allowing us to unravel its 3D architecture and tectono-stratigraphic evolution in the frame of the Mesozoic evolution of eastern Iberia. The Columbrets Basin benefits from an extensive dataset combining high resolution reflection seismic profiles, drill holes, refraction seismic data and Expanding Spread Profiles. Its Mesozoic architecture is controlled by interactions between extensional deformation and halokinesis involving the Upper Triassic salt. The thick uppermost Triassic to Cretaceous succession describes a general synclinal shape, progressively stretched and dismembered towards the basin borders. The SE-border of the basin is characterized by a large extensional detachment fault acting at crustal scale and interacting locally with the Upper Triassic décollement. This extensional structure accommodates the exhumation of the continental basement and part of the crustal thinning. Eventually our results highlight the complex interaction between extreme crustal thinning and occurrence of a pre-rift salt level for the deformation style and tectono-stratigraphic evolution of hyper-extended rift basins.

  17. Radiogenic isotopes: the case for crustal recycling on a near-steady-state no-continental-growth Earth

    International Nuclear Information System (INIS)

    Armstrong, R.L.

    1981-01-01

    The proposition that continental crust is recycled into the mantle and that the Earth is in a near-steady state with essentially constant volumes of oceans and crust through geological time is defended. Constancy of continental freeboard and uniformity of thickness of stable continental crust with age are the only two quantitative measures of crustal volume through time and these imply negligible crustal growth since 2.9 Ga B.P. Planetary analogies, Pb isotopes, atmospheric evolution, and palaeomagnetism also argue for early terrestrial differentiation. Rates of crustal growth and recycling are sufficient to reach a near-steady state over the first 1 Ga of Earth history, before widespread cratonization. Pb, Sr and Nd isotopic compositions of igneous rocks from the mantle are explainable in terms of a near-steady-state model. The recycling process can be observed on the Earth today. The observed escape of primordial 3 He from the mantle is not evidence for continuing continental differentiation or against early differentiation of the Earth. Even if nearly complete equilibrium chemical differentiation occurred at 4.6 Ga B.P., some 3 He would remain dissolved in the interior and would escape as recycling continued. (U.K.)

  18. Crustal structure beneath the Paleozoic Parnaíba Basin revealed by airborne gravity and magnetic data, Brazil

    Science.gov (United States)

    de Castroa, David L.; Fuck, Reinhardt A.; Phillips, Jeffrey D.; Vidotti, Roberta M.; Bezerra, Francisco H. R.; Dantas, Elton L.

    2014-01-01

    The Parnaíba Basin is a large Paleozoic syneclise in northeastern Brazil underlain by Precambrian crystalline basement, which comprises a complex lithostructural and tectonic framework formed during the Neoproterozoic–Eopaleozoic Brasiliano–Pan African orogenic collage. A sag basin up to 3.5 km thick and 1000 km long formed after the collage. The lithologic composition, structure, and role in the basin evolution of the underlying basement are the focus of this study. Airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the Parnaíba Basin. Results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding Neoproterozoic fold belts in the basement. Triangular-shaped basement inliers are geophysically defined in the central region of this continental-scale Neoproterozoic convergence zone. A 3-D gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36–40.5 km deep crustal root, with borders defined by a high-density and thinner crust. Forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by Brasiliano shear zones, representing lithospheric sutures of the Amazonian and São Francisco Cratons, Tocantins Province and Parnaíba Block. In addition, coincident residual gravity, residual magnetic, and pseudo-gravity lows indicate two complex systems of Eopaleozoic rifts related to the initial phase of the sag deposition, which follow basement trends in several directions.

  19. Upper crustal structure of Madeira Island revealed from ambient noise tomography

    Science.gov (United States)

    Matos, Catarina; Silveira, Graça; Matias, Luís; Caldeira, Rita; Ribeiro, M. Luísa; Dias, Nuno A.; Krüger, Frank; Bento dos Santos, Telmo

    2015-06-01

    We present the first image of the Madeira upper crustal structure, using ambient seismic noise tomography. 16 months of ambient noise, recorded in a dense network of 26 seismometers deployed across Madeira, allowed reconstructing Rayleigh wave Green's functions between receivers. Dispersion analysis was performed in the short period band from 1.0 to 4.0 s. Group velocity measurements were regionalized to obtain 2D tomographic images, with a lateral resolution of 2.0 km in central Madeira. Afterwards, the dispersion curves, extracted from each cell of the 2D group velocity maps, were inverted as a function of depth to obtain a 3D shear wave velocity model of the upper crust, from the surface to a depth of 2.0 km. The obtained 3D velocity model reveals features throughout the island that correlates well with surface geology and island evolution.

  20. Application of SAR interferometry to low-rate crustal deformation fields

    Science.gov (United States)

    Vincent, Paul

    Differential SAR interferometry is applied to the study of low-rate interseismic crustal deformation fields along three regions of the San Adreas fault system: Salton Sea (southernmost region), Pinto Mountain fault (south-central region), and San Francisco Bay (northern region). New techniques are developed to analyze and model these low-rate deformation fields including constrained horizontal-vertical component deconvolution, deformation phase pattern analysis and strain field evolution modeling. Several new active faults were discovered as well as unmeasured activity on existing faults in the process of this SAR interferometry study. The feasibility and limitations of InSAR as a tool to study low-rate deformation fields is also addressed.

  1. Crustal-Scale Fault Interaction at Rifted Margins and the Formation of Domain-Bounding Breakaway Complexes: Insights From Offshore Norway

    Science.gov (United States)

    Osmundsen, P. T.; Péron-Pinvidic, G.

    2018-03-01

    The large-magnitude faults that control crustal thinning and excision at rifted margins combine into laterally persistent structural boundaries that separate margin domains of contrasting morphology and structure. We term them breakaway complexes. At the Mid-Norwegian margin, we identify five principal breakaway complexes that separate the proximal, necking, distal, and outer margin domains. Downdip and lateral interactions between the faults that constitute breakaway complexes became fundamental to the evolution of the 3-D margin architecture. Different types of fault interaction are observed along and between these faults, but simple models for fault growth will not fully describe their evolution. These structures operate on the crustal scale, cut large thicknesses of heterogeneously layered lithosphere, and facilitate fundamental margin processes such as deformation coupling and exhumation. Variations in large-magnitude fault geometry, erosional footwall incision, and subsequent differential subsidence along the main breakaway complexes likely record the variable efficiency of these processes.

  2. Crustal Gravitational Potential Energy Change and Subduction Earthquakes

    Science.gov (United States)

    Zhu, P. P.

    2017-05-01

    Crustal gravitational potential energy (GPE) change induced by earthquakes is an important subject in geophysics and seismology. For the past forty years the research on this subject stayed in the stage of qualitative estimate. In recent few years the 3D dynamic faulting theory provided a quantitative solution of this subject. The theory deduced a quantitative calculating formula for the crustal GPE change using the mathematic method of tensor analysis under the principal stresses system. This formula contains only the vertical principal stress, rupture area, slip, dip, and rake; it does not include the horizontal principal stresses. It is just involved in simple mathematical operations and does not hold complicated surface or volume integrals. Moreover, the hanging wall vertical moving (up or down) height has a very simple expression containing only slip, dip, and rake. The above results are significant to investigate crustal GPE change. Commonly, the vertical principal stress is related to the gravitational field, substituting the relationship between the vertical principal stress and gravitational force into the above formula yields an alternative formula of crustal GPE change. The alternative formula indicates that even with lack of in situ borehole measured stress data, scientists can still quantitatively calculate crustal GPE change. The 3D dynamic faulting theory can be used for research on continental fault earthquakes; it also can be applied to investigate subduction earthquakes between oceanic and continental plates. Subduction earthquakes hold three types: (a) crust only on the vertical up side of the rupture area; (b) crust and seawater both on the vertical up side of the rupture area; (c) crust only on the vertical up side of the partial rupture area, and crust and seawater both on the vertical up side of the remaining rupture area. For each type we provide its quantitative formula of the crustal GPE change. We also establish a simplified model (called

  3. MEVTV Workshop on Early Tectonic and Volcanic Evolution of Mars

    International Nuclear Information System (INIS)

    Frey, H.

    1988-01-01

    Although not ignored, the problems of the early tectonic and volcanic evolution of Mars have generally received less attention than those later in the evolution of the planet. Specifically, much attention was devoted to the evolution of the Tharsis region of Mars and to the planet itself at the time following the establishment of this major tectonic and volcanic province. By contrast, little attention was directed at fundamental questions, such as the conditions that led to the development of Tharsis and the cause of the basic fundamental dichotomy of the Martian crust. It was to address these and related questions of the earliest evolution of Mars that a workshop was organized under the auspices of the Mars: Evolution of Volcanism, Tectonism, and Volatiles (MEVTV) Program. Four sessions were held: crustal dichotomy; crustal differentiation/volcanism; Tharsis, Elysium, and Valles Marineris; and ridges and fault tectonics

  4. Upper mantle and crustal structure of the East Greenland Caledonides

    DEFF Research Database (Denmark)

    Schiffer, Christian; Balling, N.; Jacobsen, B. H.

    The East Greenland and Scandinavian Caledonides once formed a major coherent mountain range, as a consequence of the collision of the continents of Laurentia and Baltica. The crustal and upper mantle structure was furthermore influenced by several geodynamic processes leading to the formation of ...

  5. Mars - Crustal structure inferred from Bouguer gravity anomalies.

    Science.gov (United States)

    Phillips, R. J.; Saunders, R. S.; Conel, J. E.

    1973-01-01

    Bouguer gravity has been computed for the equatorial region of Mars by differencing free air gravity and the gravity predicted from topographic variations. The free air gravity was generated from an eighth-order set of spherical harmonic coefficients. The gravity from topographic variations was generated by integrating a two-dimensional Green's function over each contour level. The Bouguer gravity indicates crustal inhomogeneities on Mars that are postulated to be variations in crustal thickness. The Tharsis ridge is a region of thick continental type crust. The gravity data, structural patterns, topography, and surface geology of this region lead to the interpretation of the Tharsis topographic high as a broad crustal upwarp possibly associated with local formation of lower-density crustal material and subsequent rise of a thicker crust. The Amazonis region is one of several basins of relatively thin crust, analogous to terrestrial ocean basins. The Libya and Hellas basins, which are probable impact features, are also underlain by thin crust and are possible regions of mantle upwelling.

  6. Seismically constrained two-dimensional crustal thermal structure of ...

    Indian Academy of Sciences (India)

    The temperature field within the crust is closely related to tectonic history as well as many other geological processes inside the earth. Therefore, knowledge of the crustal thermal structure of a region is of great importance for its tectonophysical studies. This work deals with the two-dimensional thermal modelling to ...

  7. An Approach to the Crustal Thickness Inversion Problem

    Science.gov (United States)

    De Marchi, F.; Di Achille, G.

    2017-12-01

    We describe a method to estimate the crustal thickness of a planet and we apply it to Venus. As in the method of (Parker, 1972), modified by (Wieczorek & Phillips, 1998), the gravity field anomalies of a planet are assumed to be due to the combined effect of topography and relief on the crust-mantle interface. No assumptions on isostasy are necessary. In our case, rather than using the expansion of the powers of the relief in Taylor series, we model the gravitational field of topography/relief by means of a large number of prism-shaped masses covering the whole surface of the planet. Under the hypothesis that crustal and mantle densities are the same everywhere, we solve for the relief depths on the crust-mantle interface by imposing that observed and modeled gravity field at a certain reference spherical surface (external to the planet) must be equal. This method can be extended to the case of non-uniform densities. Finally, we calculate a map of the crustal thickness of Venus and compare our results with those predicted by previous work and with the global distribution of main geological features (e.g. rift zones, tesserae, coronae). We discuss the agremeent between our results and the main geodynamical and crustal models put forth to explain the origin of such features and the applicability of this method in the context of the mission VOX (Venus Origins Explore), proposed for NASA's NF4 call.

  8. Crustal rifting and magmatic underplating in the Izu-Ogasawara (Bonin) intra-oceanic arc detected by active source seismic studies

    Science.gov (United States)

    Takahashi, N.; Kodaira, S.; Yamashita, M.; Miura, S.; Sato, T.; No, T.; Tatsumi, Y.; Kaneda, Y.

    2009-12-01

    Rift has thicker crust than that beneath recent volcanic front, although crustal thinning with high velocity lower crust was detected beneath advanced rifted region. This suggests that the magmatic underplating play a role to make open the crust. The magmatic underplating accompanied with the initial rifting is one of important issues to discuss the crustal evolution.

  9. Crustal structure along the west flank of the Cascades, western Washington

    Science.gov (United States)

    Miller, K.C.; Keller, Gordon R.; Gridley, J.M.; Luetgert, J.H.; Mooney, W.D.; Thybo, H.

    1997-01-01

    Knowledge of the crustal structure of the Washington Cascades and adjacent Puget Lowland is important to both earthquake hazards studies and geologic studies of the evolution of this tectonically active region. We present a model for crustal velocity structure derived from analysis of seismic refraction/wide-angle reflection data collected in 1991 in western Washington. The 280-km-long north-south transect skirts the west flank of the Cascades as it crosses three tectonic provinces including the Northwest Cascades Thrust System (NWCS), the Puget Lowland, and the volcanic arc of the southern Cascades. Within the NWCS, upper crustal velocities range from 4.2 to 5.7 km s-1 and are consistent with the presence of a diverse suite of Mesozoic and Paleozoic metasediments and metavolcanics. In the upper 2-3 km of the Puget Lowland velocities drop to 1.7-3.5 km s-1 and reflect the occurrence of Oligocene to recent sediments within the basin. In the southern Washington Cascades, upper crustal velocities range from 4.0 to 5.5 km s-1 and are consistent with a large volume of Tertiary sediments and volcanics. A sharp change in velocity gradient at 5-10 km marks the division between the upper and middle crust. From approximately 10 to 35 km depth the velocity field is characterized by a velocity increase from ???6.0 to 7.2 km s-1. These high velocities do not support the presence of marine sedimentary rocks at depths of 10-20 km beneath the Cascades as previously proposed on the basis of magnetotelluric data. Crustal thickness ranges from 42 to 47 km along the profile. The lowermost crust consists of a 2 to 8-km-thick transitional layer with velocities of 7.3-7.4 km s-1. The upper mantle velocity appears to be an unusually low 7.6-7.8 km s-1. When compared to velocity models from other regions, this model most closely resembles those found in active continental arcs. Distinct seismicity patterns can be associated with individual tectonic provinces along the seismic transect. In

  10. The Crustal Structure and Seismicity of Eastern Venezuela

    Science.gov (United States)

    Schmitz, M.; Martins, A.; Sobiesiak, M.; Alvarado, L.; Vasquez, R.

    2001-12-01

    Eastern Venezuela is characterized by a moderate to high seismicity, evidenced recently by the 1997 Cariaco earthquake located on the El Pilar Fault, a right lateral strike slip fault which marks the plate boundary between the Caribbean and South-American plates in this region. Recently, the seismic activity seems to migrate towards the zone of subduction of the Lesser Antilles in the northeast, where a mb 6.0 earthquake occurred in October 2000 at 120 km of depth. Periodical changes in the seismic activity are related to the interaction of the stress fields of the strike-slip and the subduction regimes. The seismic activity decreases rapidly towards to the south with some disperse events on the northern edge of the Guayana Shield, related to the Guri fault system. The crustal models used in the region are derived from the information generated by the national seismological network since 1982 and by microseismicity studies in northeastern Venezuela, coinciding in a crustal thickness of about 35 km in depth. Results of seismic refraction measurements for the region were obtained during field campains in 1998 (ECOGUAY) for the Guayana Shield and the Cariaco sedimentary basin and in 2001 (ECCO) for the Oriental Basin. The total crustal thickness decreases from about 45 km on the northern edge of the Guayana Shield to some 36 km close to El Tigre in the center of the Oriental Basin. The average crustal velocity decreases in the same sense from 6.5 to 5.8 km/s. In the Cariaco sedimentary basin a young sedimentary cover of 1 km thickness with a seismic velocity of 2 km/s was derived. Towards the northern limit of the South-American plate, no deep seismic refraction data are available up to now. The improvement of the crustal models used in that region would constitute a step forward in the analysis of the seismic hazard. Seismic refraction studies funded by CONICIT S1-97002996 and S1-2000000685 projects and PDVSA (additional drilling and blasting), recording equipment

  11. High resolution 2D numerical models from rift to break-up: Crustal hyper-extension, Margin asymmetry, Sequential faulting

    Science.gov (United States)

    Brune, Sascha; Heine, Christian; Pérez-Gussinyé, Marta; Sobolev, Stephan

    2013-04-01

    Numerical modelling is a powerful tool to integrate a multitude of geological and geophysical data while addressing fundamental questions of passive margin formation such as the occurrence of crustal hyper-extension, (a-)symmetries between conjugate margin pairs, and the sometimes significant structural differences between adjacent margin segments. This study utilises knowledge gathered from two key examples of non-magmatic, asymmetric, conjugate margin pairs, i.e. Iberia-New Foundland and Southern Africa-Brazil, where many published seismic lines provide solid knowledge on individual margin geometry. While both margins involve crustal hyper-extension, it is much more pronounced in the South Atlantic. We investigate the evolution of these two margin pairs by carefully constraining our models with detailed plate kinematic history, laboratory-based rheology, and melt fraction evaluation of mantle upwelling. Our experiments are consistent with observed fault patterns, crustal thickness, and basin stratigraphy. We conduct 2D thermomechanical rift models using the finite element code SLIM3D that operates with nonlinear stress- and temperature-dependent elasto-visco-plastic rheology, with parameters provided by laboratory experiments on major crustal and upper mantle rocks. In our models we also calculate the melt fraction within the upwelling asthenosphere, which allows us to control whether the model indeed corresponds to the non-magmatic margin type or not. Our modelling highlights two processes as fundamental for the formation of hyper-extension and margin asymmetry at non-magmatic margins: (1) Strain hardening in the rift center due to cooling of upwelling mantle material (2) The formation of a weak crustal domain adjacent to the rift center caused by localized viscous strain softening and heat transfer from the mantle. Simultaneous activity of both processes promotes lateral rift migration in a continuous way that generates a wide layer of hyper-extended crust on

  12. Compositional stratigraphy of crustal material from near-infrared spectra

    International Nuclear Information System (INIS)

    Pieters, C.M.

    1987-01-01

    An Earth-based telescopic program to acquire near-infrared spectra of freshly exposed lunar material now contains data for 17 large impact craters with central peaks. Noritic, gabbroic, anorthositic and troctolitic rock types can be distinguished for areas within these large craters from characteristic absorptions in individual spectra of their walls and central peaks. Norites dominate the upper lunar crust while the deeper crustal zones also contain significant amounts of gabbros and anorthosites. Data for material associated with large craters indicate that not only is the lunar crust highly heterogeneous across the nearside, but that the compositional stratigraphy of the lunar crust is nonuniform. Crustal complexity should be expected for other planetary bodies, which should be studied using high spatial and spectral resolution data in and around large impact craters

  13. Compositional stratigraphy of crustal material from near-infrared spectra

    Science.gov (United States)

    Pieters, Carle M.

    1987-01-01

    An Earth-based telescopic program to acquire near-infrared spectra of freshly exposed lunar material now contains data for 17 large impact craters with central peaks. Noritic, gabbroic, anorthositic and troctolitic rock types can be distinguished for areas within these large craters from characteristic absorptions in individual spectra of their walls and central peaks. Norites dominate the upper lunar crust while the deeper crustal zones also contain significant amounts of gabbros and anorthosites. Data for material associated with large craters indicate that not only is the lunar crust highly heterogeneous across the nearside, but that the compositional stratigraphy of the lunar crust is nonuniform. Crustal complexity should be expected for other planetary bodies, which should be studied using high spatial and spectral resolution data in and around large impact craters.

  14. A new model of crustal structure of Siberia

    DEFF Research Database (Denmark)

    Cherepanova, Yulia; Artemieva, Irina; Thybo, Hans

    2010-01-01

    to the Verkoyansk Ridge/Lena river in the east, and from the Arctic shelf in the north to the Tien Shan and Altay-Sayans mountains in the south. The new crustal model is based on our new ("from scratch") compilation of all available reliable seismic data and includes the results of seismic reflection, refraction...... orientation. Low surface heat flow (on average around 20-22 microW/m3) and the absence of the high-velocity (Vp>7.2 km/s) lowercrustal layer in the block with the thick crust suggest that eclogitization in the crustal root was subdued, thus allowing preservation of the ultra thick, seismically distinguishable...

  15. Geophysical evidence for the crustal variation and distribution of magmatism along the central coast of Mozambique

    Science.gov (United States)

    Mueller, Christian Olaf; Jokat, Wilfried

    2017-08-01

    For our understanding of the timing and geometry of the initial Gondwana break-up, still a consistent image of the crustal composition of the conjugated margins of central Mozambique and Antarctica and the location of their continent-ocean boundaries is missing. In this regard, a main objective is the explanation for the source of the different magnetic signature of the conjugate margins. Based on a revised investigation of wide-angle seismic data along two profiles across the Mozambican margin by means of an amplitude modelling, this study presents the crustal composition across and along the continental margin of central Mozambique. Supported by 2D magnetic modelling, the results are compared to the conjugate margin in Antarctica and allow new conclusions about their joined tectonic evolution. An observed crustal diversity between the north-eastern and south-western parts of the central Mozambican margin, testifies to the complex break-up history of this area. Conspicuous is the equal spatial extent of the HVLCB along the margin of 190-215 km. The onset of oceanic crust at the central Mozambican margin is refined to chron M38n.2n (164.1 Ma). Magnetic modelling supports the presence of reversed polarized SDRs in the continent-ocean transition that were mainly emplaced between 168.5 and 166.8 Ma (M42-M40). Inferred SDRs in the Riiser-Larsen Sea might be emplaced sometime between 166.8 and 164.1 Ma (M39-M38), but got overprinted by normal polarized intrusions of a late stage of rift volcanism, causing the opposite magnetic signature of the conjugate margins. The distribution of the magmatic material along the central coast of Mozambique clearly indicates the eastern extension of the north-eastern branch of the Karoo triple rift along the entire margin. The main magmatic phase affecting this area lasted for at least 12 Myr between 169 and 157 Ma, followed by the cease of the magmatism, perhaps due to the relative southwards motion of the magmatic centre.

  16. Application of FE software Elmer to the modeling of crustal-scale processes

    Science.gov (United States)

    Maierová, Petra; Guy, Alexandra; Lexa, Ondrej; Cadek, Ondrej

    2010-05-01

    place during the late stage of the Variscan orogeny in the area of the Bohemian Massif and they are well documented in the geological record. Extensive geological data are thus available and they can be compared with the results of our numerical simulations. Firstly, we model the indentation of a stiff block into a thick and hot crustal root and the consequent flow of the orogenic crust. For the development of the flow, the free surface deformation and erosion are essential. The importance of plastic deformation varies with the thermal structure of the domain. Secondly, we show an influence of thermal, density and viscosity structure of the crust on the time evolution and the final geometry of diapirs. The importance of the strain-rate dependence of viscosity, which is neglected in some numerical models, is discussed.

  17. The nature of crustal reflectivity at the southwest Iberian margin

    Science.gov (United States)

    Buffett, G. G.; Torne, M.; Carbonell, R.; Melchiorre, M.; Vergés, J.; Fernàndez, M.

    2017-11-01

    Reprocessing of multi-channel seismic reflection data acquired over the northern margin of the Gulf of Cádiz (SW Iberian margin) places new constraints on the upper crustal structure of the Guadalquivir-Portimão Bank. The data presented have been processed with optimized stacking and interval velocity models, a better approach to multiple attenuation, preserved amplitude information to derive the nature of seismic reflectivity, and accurate time-to-depth conversion after migration. The reprocessed data reveal a bright upper crustal reflector just underneath the Paleozoic basement that spatially coincides with the local positive free-air gravity high called the Gulf of Cádiz Gravity High. To investigate the nature of this reflector and to decipher whether it could be associated with pieces of mantle material emplaced at upper crustal levels, we calculated its reflection coefficient and compared it to a buried high-density ultramafic body (serpentinized peridotite) at the Gorringe Bank. Its reflection coefficient ratio with respect to the sea floor differs by only 4.6% with that calculated for the high-density ultramafic body of the Gorringe Bank, while it differs by 35.8% compared to a drilled Miocene limestone unconformity. This means that the Gulf of Cádiz reflector has a velocity and/or density contrast similar to the peridotite at the Gorringe Bank. However, considering the depth at which it is found (between 2.0 and 4.0 km) and the available geological information, it seems unlikely that the estimated shortening from the Oligocene to present is sufficient to emplace pieces of mantle material at these shallow levels. Therefore, and despite the similarity in its reflection coefficient with the peridotites of the Gorringe Bank, our preferred interpretation is that the upper crustal Gulf of Cádiz reflector represents the seismic response of high-density intracrustal magmatic intrusions that may partially contribute to the Gulf of Cádiz Gravity High.

  18. Investigating Microbial Biofilm Formations on Crustal Rock Substrates

    Science.gov (United States)

    Weiser, M.; D'Angelo, T.; Carr, S. A.; Orcutt, B.

    2017-12-01

    Ocean crust hosts microbial life that, in some cases, alter the component rocks as a means of obtaining energy. Variations in crust lithology, included trace metal and mineral content, as well as the chemistry of the fluids circulating through them, provide substrates for some microbes to metabolize, leading to formation of biofilm community structures. Microbes have different parameters for the situations in which they will form biofilms, but they must have some source of energy in excess at the site of biofilm formation for them to become stationary and form the carbohydrate-rich structures connecting the cells to one another and the substrate. Generally, the requirements for microbes to form biofilms on crustal minerals are unclear. We designed two experiments to test (1) mineral preference and biofilm formation rates by natural seawater microbial communities, and (2) biofilm development as a function of phosphate availability for an organism isolated from subseafloor ocean crust. In Experiment 1, we observed that phyric basalt groundmass is preferentially colonized over aphyric basalt or metal sulfides in a shallow water and oxic seawater environment. In experiment 2, tests of the anaerobic heterotroph Thalassospira bacteria isolated from oceanic crustal fluids showed that they preferentially form biofilms, lose motility, and increase exponentially in number over time in higher-PO4 treatments (50 micromolar), including with phosphate-doped basalts, than in treatments with low phosphate concentrations (0.5 micromolar) often found in crustal fluids. These observations suggest phosphate as a main driver of biofilm formation in subsurface crust. Overall, these data suggest that the drivers of microbial biofilm formation on crustal substrates are selective to the substrate conditions, which has important implications for estimating the global biomass of life harbored in oceanic crust.

  19. Parallel Fast Multipole Boundary Element Method for crustal dynamics

    International Nuclear Information System (INIS)

    Quevedo, Leonardo; Morra, Gabriele; Mueller, R Dietmar

    2010-01-01

    Crustal faults and sharp material transitions in the crust are usually represented as triangulated surfaces in structural geological models. The complex range of volumes separating such surfaces is typically three-dimensionally meshed in order to solve equations that describe crustal deformation with the finite-difference (FD) or finite-element (FEM) methods. We show here how the Boundary Element Method, combined with the Multipole approach, can revolutionise the calculation of stress and strain, solving the problem of computational scalability from reservoir to basin scales. The Fast Multipole Boundary Element Method (Fast BEM) tackles the difficulty of handling the intricate volume meshes and high resolution of crustal data that has put classical Finite 3D approaches in a performance crisis. The two main performance enhancements of this method: the reduction of required mesh elements from cubic to quadratic with linear size and linear-logarithmic runtime; achieve a reduction of memory and runtime requirements allowing the treatment of a new scale of geodynamic models. This approach was recently tested and applied in a series of papers by [1, 2, 3] for regional and global geodynamics, using KD trees for fast identification of near and far-field interacting elements, and MPI parallelised code on distributed memory architectures, and is now in active development for crustal dynamics. As the method is based on a free-surface, it allows easy data transfer to geological visualisation tools where only changes in boundaries and material properties are required as input parameters. In addition, easy volume mesh sampling of physical quantities enables direct integration with existing FD/FEM code.

  20. Crustal structure and regional tectonics of SE Sweden and the Baltic Sea

    International Nuclear Information System (INIS)

    Milnes, A.G.; Gee, D.G.; Lund, C.E.

    1998-11-01

    In this desk study, the available geophysical and geological data on the crustal structure and regional tectonics of the wider surroundings of the Aespoe site (SE Sweden and adjacent parts of the Baltic Sea) are compiled and assessed. The aim is to contribute to the knowledge base for long-term rock mechanical modeling, using the Aespoe site as a proxy for a high-level radioactive waste repository site in Swedish bedrock. The geophysical data reviewed includes two new refraction/wide-angle reflection seismic experiments carried out within the EUROBRIDGE project, in addition to the numerous earlier refraction seismic profiles. The BABEL normal-incidence deep seismic profile is also considered. New geological data, presented at EUROBRIDGE workshops, and in recent SGU publications, are reviewed for the same area. In combination with the seismic data, these provide a base for interpreting the present composition and structure, and the Palaeoproterozoic-Mesoproterozoic evolution, of the crustal segment within which the Aespoe site lies - the Smaaland mega-block. This is characterized by having undergone little regionally significant deformation or magmatism since Neoproterozoic times (the last 1000 million years). It is shown that, at this scale of observation (of the order of 100 km), the long-term rheology of the lithosphere can be argued from a relatively tight observational network, when combined with the results of earlier SKB studies (seismo-tectonics, uplift patterns, state of stress, heat flow) and published research. Although many uncertainties exist, the present state of knowledge would suffice for first exploratory calculations and sensitivity studies of long-term, large-scale rock mechanics

  1. Crustal structure and regional tectonics of SE Sweden and the Baltic Sea

    Energy Technology Data Exchange (ETDEWEB)

    Milnes, A.G. [Bergen Univ. (Norway). Dept. of Geology; Gee, D.G.; Lund, C.E. [Uppsala Univ. (Sweden). Dept. of Earth Sciences

    1998-11-01

    In this desk study, the available geophysical and geological data on the crustal structure and regional tectonics of the wider surroundings of the Aespoe site (SE Sweden and adjacent parts of the Baltic Sea) are compiled and assessed. The aim is to contribute to the knowledge base for long-term rock mechanical modeling, using the Aespoe site as a proxy for a high-level radioactive waste repository site in Swedish bedrock. The geophysical data reviewed includes two new refraction/wide-angle reflection seismic experiments carried out within the EUROBRIDGE project, in addition to the numerous earlier refraction seismic profiles. The BABEL normal-incidence deep seismic profile is also considered. New geological data, presented at EUROBRIDGE workshops, and in recent SGU publications, are reviewed for the same area. In combination with the seismic data, these provide a base for interpreting the present composition and structure, and the Palaeoproterozoic-Mesoproterozoic evolution, of the crustal segment within which the Aespoe site lies - the Smaaland mega-block. This is characterized by having undergone little regionally significant deformation or magmatism since Neoproterozoic times (the last 1000 million years). It is shown that, at this scale of observation (of the order of 100 km), the long-term rheology of the lithosphere can be argued from a relatively tight observational network, when combined with the results of earlier SKB studies (seismo-tectonics, uplift patterns, state of stress, heat flow) and published research. Although many uncertainties exist, the present state of knowledge would suffice for first exploratory calculations and sensitivity studies of long-term, large-scale rock mechanics 101 refs, 22 figs

  2. Crustal Structure in the Western Part of Romania from Local Seismic Tomography

    Science.gov (United States)

    Zaharia, Bogdan; Grecu, Bogdan; Popa, Mihaela; Oros, Eugen; Radulian, Mircea

    2017-12-01

    The inner part of the Carpathians in Romania belongs to the Carpathians-Pannonian system bordered by the Eastern Carpathians to the north and east, Southern Carpathians to the south and Pannonian Basin to the west. It is a complex tectonic region with differential folding mechanisms, post-collisional kinematics, rheology and thermal properties, including within its area the Apuseni Mountains and the Transylvanian Basin. The purpose of this study is to map the 3-D structure of the crust over this region on the basis of local earthquake data. Input data were recorded during the South Carpathian Project (2009-2011), a successful collaboration between the Institute of Geophysics and Tectonics of the University of Leeds and the National Institute for Earth Physics (NIEP), Romania. A temporary array of 32 broadband seismic stations (10 CMG-40T, 8 CMG-3T and 14 CMG-6TD) was installed across the western part of Romania (spaced at 40 to 50 km intervals) during the project. In addition, 25 stations deployed in the eastern Hungary and Serbia was considered. P- and S-wave arrivals are identified for all the selected events (minimum 7 phases per event with reasonable signal/noise ratio). All the events are first relocated using Joint Hypocentre Determination (JHD) technique. Then the well-located events were inverted to determine the crustal structure using LOTOS algorithm. The lateral variations of the crustal properties as resulted from the tomography image are interpreted in correlation with the station corrections estimated by JHD algorithm and with the post-collisional evolution of the Carpathians-Pannonian system.

  3. Lower Crustal Seismicity, Volatiles, and Evolving Strain Fields During the Initial Stages of Cratonic Rifting

    Science.gov (United States)

    Lambert, C.; Muirhead, J.; Ebinger, C. J.; Tiberi, C.; Roecker, S. W.; Ferdinand-Wambura, R.; Kianji, G.; Mulibo, G. D.

    2014-12-01

    The volcanically active East African rift system in southern Kenya and northern Tanzania transects thick cratonic lithosphere, and comprises several basins characterized by deep crustal seismicity. The US-French-Tanzania-Kenya CRAFTI project aims to understand the role of magma and volatile movement during the initiation and evolution of rifting in cratonic lithosphere. Our 38-station broadband network spans all or parts of fault-bounded rift segments, enabling comparison of lithospheric structure, fault kinematics, and seismogenic layer thickness with age and proximity to the deeply rooted Archaen craton. Seismicity levels are high in all basins, but we find profound differences in seismogenic layer thickness along the length of the rift. Seismicity in the Manyara basin occurs almost exclusively within the lower crust, and in spatial clusters that have been active since 1990. In contrast, seismicity in the ~ 5 My older Magadi basin is localized in the upper crust, and the long border fault bounding the west side of the basin is seismically inactive. Between these two basins lies the Natron rift segment, which shows seismicity between ~ 20 and ~2 km depth, and high concentrations at Oldoinyo Lengai and Gelai volcanoes. Older volcanoes on the uplifted western flank (e.g., Ngorongoro) experience swarms of activity, suggesting that active magmatism and degassing are widespread. Focal mechanisms of the frequent earthquakes recorded across the array are spatially variable, and indicate a stress field strongly influenced by (1) Holocene volcanoes, (2) mechanical interactions between adjacent rift basins, and (3) a far-field ESE-WNW extensional stress regime. We explore the spatial correlation between zones of intense degassing along fault systems and seismicity, and examine the influence of high gas pressures on lower and upper crustal seismicity in this youthful cratonic rift zone.

  4. Crustal thickness of Antarctica estimated using data from gravimetric satellites

    Science.gov (United States)

    Llubes, Muriel; Seoane, Lucia; Bruinsma, Sean; Rémy, Frédérique

    2018-04-01

    Computing a better crustal thickness model is still a necessary improvement in Antarctica. In this remote continent where almost all the bedrock is covered by the ice sheet, seismic investigations do not reach a sufficient spatial resolution for geological and geophysical purposes. Here, we present a global map of Antarctic crustal thickness computed from space gravity observations. The DIR5 gravity field model, built from GOCE and GRACE gravimetric data, is inverted with the Parker-Oldenburg iterative algorithm. The BEDMAP products are used to estimate the gravity effect of the ice and the rocky surface. Our result is compared to crustal thickness calculated from seismological studies and the CRUST1.0 and AN1 models. Although the CRUST1.0 model shows a very good agreement with ours, its spatial resolution is larger than the one we obtain with gravimetric data. Finally, we compute a model in which the crust-mantle density contrast is adjusted to fit the Moho depth from the CRUST1.0 model. In East Antarctica, the resulting density contrast clearly shows higher values than in West Antarctica.

  5. Crustal thickness of Antarctica estimated using data from gravimetric satellites

    Directory of Open Access Journals (Sweden)

    M. Llubes

    2018-04-01

    Full Text Available Computing a better crustal thickness model is still a necessary improvement in Antarctica. In this remote continent where almost all the bedrock is covered by the ice sheet, seismic investigations do not reach a sufficient spatial resolution for geological and geophysical purposes. Here, we present a global map of Antarctic crustal thickness computed from space gravity observations. The DIR5 gravity field model, built from GOCE and GRACE gravimetric data, is inverted with the Parker–Oldenburg iterative algorithm. The BEDMAP products are used to estimate the gravity effect of the ice and the rocky surface. Our result is compared to crustal thickness calculated from seismological studies and the CRUST1.0 and AN1 models. Although the CRUST1.0 model shows a very good agreement with ours, its spatial resolution is larger than the one we obtain with gravimetric data. Finally, we compute a model in which the crust–mantle density contrast is adjusted to fit the Moho depth from the CRUST1.0 model. In East Antarctica, the resulting density contrast clearly shows higher values than in West Antarctica.

  6. Boron isotope fractionation in magma via crustal carbonate dissolution.

    Science.gov (United States)

    Deegan, Frances M; Troll, Valentin R; Whitehouse, Martin J; Jolis, Ester M; Freda, Carmela

    2016-08-04

    Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ(11)B values down to -41.5‰, reflecting preferential partitioning of (10)B into the assimilating melt. Loss of (11)B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports (11)B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ(11)B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

  7. Precambrian crustal history of the Nimrod Group, central Transantarctic Mountains

    International Nuclear Information System (INIS)

    Goodge, J.W.; Fanning, C.M.

    2002-01-01

    High-grade metamorphic and igneous rocks of the Nimrod Group represent crystalline basement to the central Transantarctic Mountains. Despite metamorphism and penetrative deformation during the Ross Orogeny, they preserve a deep record of Precambrian geologic history in this sector of the East Antarctic shield. A review of available U-Pb geochronometric data reveals multiple geologic events spanning 2.5 b.y. of Archean to Early Paleozoic time, including: (1) juvenile Archean crust production by magmatism between 3150 and 3000 Ma; (2) crustal stabilisation and metamorphism between 2955 and 2900 Ma; (3) ultra-metamorphism or anatexis at c. 2500 Ma; (4) deep-crustal metamorphism and magmatism between 1720 and 1730 Ma, redefining the Nimrod Orogeny; (5) post-1700 Ma sedimentation; and (6) basement reactivation involving high-grade metamorphism, magmatism, and penetrative deformation during the Ross Orogeny between 540 and 515 Ma. A strong regional metamorphic and deformational Ross overprint, dated by U-Pb and Ar thermochronology, had pronounced thermomechanical effects on the basement assemblage, yet rocks of the Nimrod Group retain robust evidence of their Precambrian ancestry. The zircon U-Pb record therefore demonstrates that primary crustal lithosphere of the East Antarctic shield extends to the central Transantarctic Mountains, and that it has undergone multiple episodes of reactivation culminating in the Ross Orogeny. (author). 48 refs., 2 figs., 1 tab

  8. Boron isotope fractionation in magma via crustal carbonate dissolution

    Science.gov (United States)

    Deegan, Frances M.; Troll, Valentin R.; Whitehouse, Martin J.; Jolis, Ester M.; Freda, Carmela

    2016-08-01

    Carbon dioxide released by arc volcanoes is widely considered to originate from the mantle and from subducted sediments. Fluids released from upper arc carbonates, however, have recently been proposed to help modulate arc CO2 fluxes. Here we use boron as a tracer, which substitutes for carbon in limestone, to further investigate crustal carbonate degassing in volcanic arcs. We performed laboratory experiments replicating limestone assimilation into magma at crustal pressure-temperature conditions and analysed boron isotope ratios in the resulting experimental glasses. Limestone dissolution and assimilation generates CaO-enriched glass near the reaction site and a CO2-dominated vapour phase. The CaO-rich glasses have extremely low δ11B values down to -41.5‰, reflecting preferential partitioning of 10B into the assimilating melt. Loss of 11B from the reaction site occurs via the CO2 vapour phase generated during carbonate dissolution, which transports 11B away from the reaction site as a boron-rich fluid phase. Our results demonstrate the efficacy of boron isotope fractionation during crustal carbonate assimilation and suggest that low δ11B melt values in arc magmas could flag shallow-level additions to the subduction cycle.

  9. Lower crustal seismic activity in the Adana Basin (Eastern Mediterranean): Possible connection to gravitational flexure

    Science.gov (United States)

    Ergin, Mehmet; Aktar, Mustafa

    2018-04-01

    High quality broadband data, together with the application of the double difference relocation technique, has been used to study the characteristics of the lower crustal seismicity in the Adana Basin, in southwestern Turkey. Deep events are clearly seen to be restricted only to the Adana Basin and never extend outside its boundaries. Furthermore, the seismogenic zone is observed to align roughly with the main axis of the basin and plunges steadily in the SSW-direction, following the basement trend of the Adana Basin. Similarities between geometries of the basin evolution and the deep seismic production suggest that both processes are closely related. A flexure process is proposed related to the subsidence of the Adana Basin. The seismogenic zone, originally at a shallow depth, is assumed to have been displaced vertically into the lower crust, by flexure. The temperature evolution of the crust during the flexure has been studied in detail using finite difference modeling, with amplitude and duration parameters taken from earlier studies. It has been concluded that the physical conditions for brittle fracturing remained unchanged for an extended period of time after the flexure. The brittle layers originally at shallow depths, preserved their original thermal properties after the subsidence and will continue to produce earthquakes at considerable depths. Numerical tests using inferred parameters imply a total vertical shift of 7-8 km for the seismogenic zone. Discussions for additional processes, which may further contribute to the cooling of the crust, are also included.

  10. West-directed thrusting south of the eastern Himalayan syntaxis indicates clockwise crustal flow at the indenter corner during the India-Asia collision

    Science.gov (United States)

    Haproff, Peter J.; Zuza, Andrew V.; Yin, An

    2018-01-01

    Whether continental deformation is accommodated by microplate motion or continuum flow is a central issue regarding the nature of Cenozoic deformation surrounding the eastern Himalayan syntaxis. The microplate model predicts southeastward extrusion of rigid blocks along widely-spaced strike-slip faults, whereas the crustal-flow model requires clockwise crustal rotation along closely-spaced, semi-circular right-slip faults around the eastern Himalayan syntaxis. Although global positioning system (GPS) data support the crustal-flow model, the surface velocity field provides no information on the evolution of the India-Asia orogenic system at million-year scales. In this work, we present the results of systematic geologic mapping across the northernmost segment of the Indo-Burma Ranges, located directly southeast of the eastern Himalayan syntaxis. Early research inferred the area to have experienced either right-slip faulting accommodating northward indentation of India or thrusting due to the eastward continuation of the Himalayan orogen in the Cenozoic. Our mapping supports the presence of dip-slip thrust faults, rather than strike-slip faults. Specifically, the northern Indo-Burma Ranges exposes south- to west-directed ductile thrust shear zones in the hinterland and brittle fault zones in the foreland. The trends of ductile stretching lineations within thrust shear zones and thrust sheets rotate clockwise from the northeast direction in the northern part of the study area to the east direction in the southern part of the study area. This clockwise deflection pattern of lineations around the eastern Himalayan syntaxis mirrors the clockwise crustal-rotation pattern as suggested by the crustal-flow model and contemporary GPS velocity field. However, our finding is inconsistent with discrete strike-slip deformation in the area and the microplate model.

  11. New constraints on the crustal structure beneath northern Tyrrhenian Sea

    Science.gov (United States)

    Levin, V. L.; Park, J. J.

    2009-12-01

    We present new seismological data on the seismic structure beneath the Tyrrhenian Sea between Corsica and the coast of Italy. Teleseismic receiver functions from two Tyrrhenian islands (Elba and Gorgona) identify clear P-to-S mode-converted waves from two distinct interfaces, at ~20 and ~45 km depth. Both interfaces are characterized by an increase of seismic wavespeed with depth. Using a summation of direct and multiply-reflected body waves within the P wave coda we estimate the mean ratio of compressional and shear wave speeds above the 45 km interface to be 1.75-1.80. Using reflectivity computations in 1D layered models we develop a model of seismic wavespeed distribution that yields synthetic seismograms very similar to those observed. We apply a Ps-multiple summation procedure to the synthetic waveforms to further verify the match between observed and predicted wavefields. The lower layer of our model, between 20 and 45 km, has Vp ~ 7.5 km/sec, a value that can be ascribed to either very fast crustal rocks or very slow upper mantle rocks. The Vp/Vs ratio is ~1.8 in this intermediate layer. On the basis of a well-constrained downward increase in seismic wave speed beneath this second layer, we interpret it as the magmatically reworked lower crust, a lithology that has been proposed to explain high-Vp layers in the crustal roots of island-arc terranes and volcanically altered continental margins, as well as lower-crustal high-Vp features sometimes seen beneath continental rifts. The presence of a thick layer of high-Vp, but crustal, lithology beneath the Tyrrhenian Sea differs considerably from previous estimates that interpreted the interface at ~20 km as the Moho. Our new interpretation obviates a need for a crustal thickness change of over 20 km at the crest of the Apennines orogen. We propose an alteration in the properties of the lower crust instead. We argue that ongoing convergent subduction of the Adriatic lithospehre is not required beneath northern

  12. Polyphase Neoproterozoic orogenesis within the east Africa- Antarctica orogenic belt in central and northern Madagascar

    Science.gov (United States)

    Key, R.M.; Pitfield, P.E.J.; Thomas, Ronald J.; Goodenough, K.M.; Waele, D.; Schofield, D.I.; Bauer, W.; Horstwood, M.S.A.; Styles, M.T.; Conrad, J.; Encarnacion, J.; Lidke, D.J.; O'connor, E. A.; Potter, C.; Smith, R.A.; Walsh, G.J.; Ralison, A.V.; Randriamananjara, T.; Rafahatelo, J.-M.; Rabarimanana, M.

    2011-01-01

    Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa-Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820-760 Ma, 630-595 Ma and 560-520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid- Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global 'Terminal Pan-African' event (560-490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana. ?? The Geological Society of London 2011.

  13. Crustal structure of Tolfa domes complex (northern Latium - Italy) inferred from receiver functions analysis: an interplay between tectonics and magmatism

    Science.gov (United States)

    Buttinelli, M.; Bianchi, I.; Anselmi, M.; Chiarabba, C.; de Rita, D.; Quattrocchi, F.

    2010-12-01

    this low Vs layer, we find some interesting features corresponding to: - a low Vs shallow and 2 km thick layer of Liguride and Plio-Pleistocene units (z = 0-2 km of depth) - a high Vs 4-5 km thick anisotropic layer of limestones (z = 2-7 km of depth) - a very high Vs (3.8 km/s) 4 km thick layer probably corresponding to the metamorphic basement. The analysis of the geometry of the velocity changes between these layers (from the surface to the bottom of metamorphic basement), also yield evidence of crustal block tilting, due to the development of the eastern continental passive margin of the Tyrrhenian sea. The general crustal setting observed between the TDC and the Argentario areas is also consistent with the simple shear models suggested for back-arc basins opening. Comparison of RF’s TDC models with MAON station data also led to important considerations confirming the initial evolutive phase of the Tyrrhenian sea opening, in association with the first occurrences of intrusive magmatism in these areas.

  14. The geotectonic evolution of southern part of Sao Francisco Craton, based in geochronologic interpretation

    International Nuclear Information System (INIS)

    Teixeira, W.

    1985-01-01

    Interpretation of available radiometric data from poly metamorphic terranes of southern part of the Sao Francisco Craton demonstrates the importance of geochronology as a tool in the study of ancient crustal evolution. In addition, radiometric study of basic intrusive magmatism helps define the most important epochs of crustal rifting during the Proterozoic. The definition of the southern border of the cratonic area based on distinctive age patterns of the geochronological provinces is also discussed. Finally, the geochronologic evolution of the Bambui platform cover is presented. Approximately 250 radiometric age determinations (Rb-Sr, K-Ar and Pb-Pb methods) were interpreted principally through the use of iso chronic diagrams. The geologic history tectonomagnetic events identified in this study is compared to the crustal evolution of similar segments of the Sao Francisco Craton and elsewhere. (author)

  15. Subduction initiation and recycling of Alboran domain derived crustal components prior to the intra-crustal emplacement of mantle peridotites in the Westernmost Mediterranean: isotopic evidence from the Ronda peridotite

    Science.gov (United States)

    Varas-Reus, María Isabel; Garrido, Carlos J.; Bosch, Delphine; Marchesi, Claudio Claudio; Acosta-Vigil, Antonio; Hidas, Károly; Barich, Amel

    2014-05-01

    During Late Oligocene-Early Miocene different domains formed in the region between Iberia and Africa in the westernmost Mediterranean, including thinned continental crust and a Flysch Trough turbiditic deposits likely floored by oceanic crust [1]. At this time, the Ronda peridotite likely constituted the subcontinental lithospheric mantle of the Alboran domain, which mantle lithosphere was undergoing strong thinning and melting [2] [3] coevally with Early Miocene extension in the overlying Alpujárride-Maláguide stacked crust [4, 5]. Intrusive Cr- rich pyroxenites in the Ronda massif records the geochemical processes occurring in the subcontinental mantle of the Alboran domain during the Late Oligocene [6]. Recent isotopic studies of these pyroxenites indicate that their mantle source was contaminated by a subduction component released by detrital crustal sediments [6]. This new data is consistent with a subduction setting for the late evolution of the Alboran lithospheric mantle just prior to its final intracrustal emplacement in the early Miocene Further detailed structural studies of the Ronda plagioclase peridotites-related to the initial stages of ductile emplacement of the peridotite-have led to Hidas et al. [7] to propose a geodynamic model where folding and shearing of an attenuated mantle lithosphere occurred by backarc basin inversion followed by failed subduction initiation that ended into the intracrustal emplacement of peridotite into the Alboran wedge in the earliest Miocene. This hypothesis implies that the crustal component recorded in late, Cr-rich websterite dykes might come from underthrusted crustal rocks from the Flysch and/or Alpujárrides units that might have been involved in the earliest stages of this subduction initiation stage. To investigate the origin of crustal component in the mantle source of this late magmatic event recorded by Cr-pyroxenites, we have carried out a detail Sr-Nd-Pb-Hf isotopic study of a variety of Betic

  16. Nitrogen evolution within the Earth's atmosphere-mantle system assessed by recycling in subduction zones

    Science.gov (United States)

    Mallik, Ananya; Li, Yuan; Wiedenbeck, Michael

    2018-01-01

    Understanding the evolution of nitrogen (N) across Earth's history requires a comprehensive understanding of N's behaviour in the Earth's mantle - a massive reservoir of this volatile element. Investigation of terrestrial N systematics also requires assessment of its evolution in the Earth's atmosphere, especially to constrain the N content of the Archaean atmosphere, which potentially impacted water retention on the post-accretion Earth, potentially causing enough warming of surface temperatures for liquid water to exist. We estimated the proportion of recycled N in the Earth's mantle today, the isotopic composition of the primitive mantle, and the N content of the Archaean atmosphere based on the recycling rates of N in modern-day subduction zones. We have constrained recycling rates in modern-day subduction zones by focusing on the mechanism and efficiency of N transfer from the subducting slab to the sub-arc mantle by both aqueous fluids and slab partial melts. We also address the transfer of N by aqueous fluids as per the model of Li and Keppler (2014). For slab partial melts, we constrained the transfer of N in two ways - firstly, by an experimental study of the solubility limit of N in melt (which provides an upper estimate of N uptake by slab partial melts) and, secondly, by the partitioning of N between the slab and its partial melt. Globally, 45-74% of N introduced into the mantle by subduction enters the deep mantle past the arc magmatism filter, after taking into account the loss of N from the mantle by degassing at mid-ocean ridges, ocean islands and back-arcs. Although the majority of the N in the present-day mantle remains of primordial origin, our results point to a significant, albeit minor proportion of mantle N that is of recycled origin (17 ± 8% or 12 ± 5% of N in the present-day mantle has undergone recycling assuming that modern-style subduction was initiated 4 or 3 billion years ago, respectively). This proportion of recycled N is enough to

  17. The PROTEUS Experiment: Active Source Seismic Imaging of the Crustal Magma Plumbing Structure of the Santorini Arc Volcano

    Science.gov (United States)

    Hooft, E. E. E.; Morgan, J. V.; Nomikou, P.; Toomey, D. R.; Papazachos, C. V.; Warner, M.; Heath, B.; Christopoulou, M. E.; Lampridou, D.; Kementzetzidou, D.

    2016-12-01

    The goal of the PROTEUS seismic experiment (Plumbing Reservoirs Of The Earth Under Santorini) is to examine the entire crustal magma plumbing system beneath a continental arc volcano and determine the magma geometry and connections throughout the crust. These physical parameters control magma migration, storage, and eruption and inform the question of how physical and chemical processing of magma at arc volcanoes forms the andesitic rock compositions that dominate the lower continental crust. These physical parameters are also important to understand volcanic-tectonic interactions and geohazards. Santorini is ideal for these goals because the continental crust has been thinned by extension and so the deep magmatic system is more accessible, also it is geologically well studied. Since the volcano is a semi-submerged, it was possible to collect a unique 3D marine-land active source seismic dataset. During the PROTEUS experiment in November-December of 2015, we recorded 14,300 marine sound sources from the US R/V Langseth on 89 OBSIP short period ocean bottom seismometers and 60 German and 5 Greek land seismometers. The experiment was designed for high-density spatial sampling of the seismic wavefield to allow us to apply two state-of-the-art 3D inversion methods: travel time tomography and full waveform inversion. A preliminary travel time tomography model of the upper crustal seismic velocity structure of the volcano and surrounding region is presented in an accompanying poster. We also made marine geophysical maps of the seafloor using multi-beam bathymetry and of the gravity and magnetic fields. The new seafloor map reveals the detailed structure of the major fault system between Santorini and Amorgos, of associated landslides, and of newly discovered volcanic features. The PROTEUS project will provide new insights into the structure of the whole crustal magmatic system of a continental arc volcano and its evolution within the surrounding tectonic setting.

  18. Gravity anomaly and crustal structure characteristics in North-South Seismic Belt of China

    Science.gov (United States)

    Shen, Chongyang; Xuan, Songtbai; Yang, Guangliang; Wu, Guiju

    2017-04-01

    reflects basic frame work of the regional crust structure. The earth's crust basically present three layer structure, nearly horizontally distributes, undulation of Moho is obvious, which is consistent with the results of seismic sounding and seismic array detection; in the local area, there are lower density layer zonal distribution in the earth's crust what accelerates the lateral movement in up and middle crust; when the substance of the Tibetan plateau spreads around, the integrity in up and middle crust is well, and it is basically a coupling movement together; in the lower crust, the thickness of the Tibetan plateau is outward gradually thinning, there is decoupling phenomenon in crust-mantle; The results of the gravity and the crustal density structure show that the research area can be divided into several part such as Qinghai-Tibet Plateau, Sichuan-Yunnan block, Ordos block and Alxa block, the transitional zones of the Qinghai-Tibet Plateau and Sichuan basin, and Alxa and Ordos are complex, and Moho slope is bigger, where is the part of strong tectonic activity and strong earthquakes occur easily. The research is of great significance for study the crustal deep structure, geodynamic evolution process and environment of earthquake gestation of the NSSB region.

  19. Crustal heterogeneity and seismotectonics of the region around Beijing, China

    Science.gov (United States)

    Huang, Jinli; Zhao, Dapeng

    2004-07-01

    A detailed three-dimensional (3-D) P-wave velocity model of the crust and uppermost mantle under the Chinese capital (Beijing) region is determined with a spatial resolution of 25 km in the horizontal direction and 4-17 km in depth. We used 48,750 precise P-wave arrival times from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by a new digital seismic network consisting of 101 seismic stations equipped with high-sensitivity seismometers. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new insights into the geological structure and tectonics of the region, such as the lithological variations and large fault zones across the major geological terranes like the North China Basin, the Taihangshan and the Yanshan mountainous areas. The velocity images of the upper crust reflect well the surface geological and topographic features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocities, respectively. The Taihangshan and Yanshan mountainous regions are generally imaged as broad high-velocity zones, while the Quaternary intermountain basins show up as small low-velocity anomalies. Velocity changes are visible across some of the large fault zones. Large crustal earthquakes, such as the 1976 Tangshan earthquake ( M=7.8) and the 1679 Sanhe earthquake ( M=8.0), generally occurred in high-velocity areas in the upper to middle crust. In the lower crust to the uppermost mantle under the source zones of the large earthquakes, however, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes.

  20. Effect of Crustal Density Structures on GOCE Gravity Gradient Observables

    Directory of Open Access Journals (Sweden)

    Robert Tenzer Pavel Novák

    2013-01-01

    Full Text Available We investigate the gravity gradient components corrected for major known anomalous density structures within the Earth¡¦s crust. Heterogeneous mantle density structures are disregarded. The gravimetric forward modeling technique is utilized to compute the gravity gradients based on methods for a spherical harmonic analysis and synthesis of a gravity field. The Earth¡¦s gravity gradient components are generated using the global geopotential model GOCO-03s. The topographic and stripping gravity corrections due to the density contrasts of the ocean and ice are computed from the global topographic/bathymetric model DTM2006.0 (which also includes the ice-thickness dataset. The discrete data of sediments and crust layers taken from the CRUST2.0 global crustal model are then used to apply the additional stripping corrections for sediments and remaining anomalous crustal density structures. All computations are realized globally on a one arc-deg geographical grid at a mean satellite elevation of 255 km. The global map of the consolidated crust-stripped gravity gradients reveals distinctive features which are attributed to global tectonics, lithospheric plate configuration, lithosphere structure and mantle dynamics (e.g., glacial isostatic adjustment, mantle convection. The Moho signature, which is the most pronounced signal in these refined gravity gradients, is superimposed over a weaker gravity signal of the lithospheric mantle. An interpretational quality of the computed (refined gravity gradient components is mainly limited by a low accuracy and resolution of the CRUST2.0 sediment and crustal layer data and unmodeled mantle structures.

  1. Improved H-κ Method by Harmonic Analysis on Ps and Crustal Multiples in Receiver Functions with respect to Dipping Moho and Crustal Anisotropy

    Science.gov (United States)

    Li, J.; Song, X.; Wang, P.; Zhu, L.

    2017-12-01

    The H-κ method (Zhu and Kanamori, 2000) has been widely used to estimate the crustal thickness and Vp/Vs ratio with receiver functions. However, in regions where the crustal structure is complicated, the method may produce uncertain or even unrealistic results, arising particularly from dipping Moho and/or crustal anisotropy. Here, we propose an improved H-κ method, which corrects for these effects first before stacking. The effect of dipping Moho and crustal anisotropy on Ps receiver function has been well studied, but not as much on crustal multiples (PpPs and PpSs+PsPs). Synthetic tests show that the effect of crustal anisotropy on the multiples are similar to Ps, while the effect of dipping Moho on the multiples is 5 times that on Ps (same cosine trend but 5 times in time shift). A Harmonic Analysis (HA) method for dipping/anisotropy was developed by Wang et al. (2017) for crustal Ps receiver functions to extract parameters of dipping Moho and crustal azimuthal anisotropy. In real data, the crustal multiples are much more complicated than the Ps. Therefore, we use the HA method (Wang et al., 2017), but apply separately to Ps and the multiples. It shows that although complicated, the trend of multiples can still be reasonably well represented by the HA. We then perform separate azimuthal corrections for Ps and the multiples and stack to obtain a combined receiver function. Lastly, the traditional H-κ procedure is applied to the stacked receiver function. We apply the improved H-κ method on 40 CNDSN (Chinese National Digital Seismic Network) stations distributed in a variety of geological setting across the Chinese continent. The results show apparent improvement compared to the traditional H-κ method, with clearer traces of multiples and stronger stacking energy in the grid search, as well as more reliable H-κ values.

  2. Testing Predictions of Continental Insulation using Oceanic Crustal Thicknesses

    Science.gov (United States)

    Hoggard, Mark; Shorttle, Oliver; White, Nicky

    2016-04-01

    The thermal blanketing effect of continental crust has been predicted to lead to elevated temperatures within the upper mantle beneath supercontinents. Initial break-up is associated with increased magmatism and the generation of flood basalts. Continued rifting and sea-floor spreading lead to a steady reduction of this thermal anomaly. Recently, evidence in support of this behaviour has come from the major element geochemistry of mid-ocean ridge basalts, which suggest excess rifting temperatures of ˜ 150 °C that decay over ˜ 100 Ma. We have collated a global inventory of ˜ 1000 seismic reflection profiles and ˜ 500 wide-angle refraction experiments from the oceanic realm. Data are predominantly located along passive margins, but there are also multiple surveys in the centres of the major oceanic basins. Oceanic crustal thickness has been mapped, taking care to avoid areas of secondary magmatic thickening near seamounts or later thinning such as across transform faults. These crustal thicknesses are a proxy for mantle potential temperature at the time of melt formation beneath a mid-ocean ridge system, allowing us to quantify the amplitude and duration of thermal anomalies generated beneath supercontinents. The Jurassic break-up of the Central Atlantic and the Cretaceous rifting that formed the South Atlantic Ocean are both associated with excess temperatures of ˜ 50 °C that have e-folding times of ˜ 50 Ma. In addition to this background trend, excess temperatures reach > 150 °C around the region of the Rio Grande Rise, associated with the present-day Tristan hotspot. The e-folding time of this more local event is ˜ 10 Ma, which mirrors results obtained for the North Atlantic Ocean south of Iceland. In contrast, crustal thicknesses from the Pacific Ocean reveal approximately constant potential temperature through time. This observation is in agreement with predictions, as the western Pacific was formed by rifting of an oceanic plate. In summary

  3. Lower crustal intrusions beneath the southern Baikal Rift Zone

    DEFF Research Database (Denmark)

    Nielsen, Christoffer; Thybo, Hans

    2009-01-01

    centre. The BEST (Baikal Explosion Seismic Transect) project acquired a 360-km long, deep seismic, refraction/wide-angle reflection profile in 2002 across southern Lake Baikal. The data from this project is used for identification of large-scale crustal structures and modelling of the seismic velocities....../s and 7.9 km/s. We interpret this feature as resulting from mafic to ultra-mafic intrusions in the form of sills. Petrological interpretation of the velocity values suggests that the intrusions are sorted by fractional crystallization into plagioclase-rich low-velocity layers and pyroxene- and olivine...

  4. Time-Dependent Crustal Deformation Associated With the 2004 Chuetsu and the 2007 Chuetsu-Oki Earthquakes

    Science.gov (United States)

    Meneses Gutierrez, A.; Sagiya, T.

    2013-12-01

    There is an ongoing concentrated deformation along the Japan Sea coast, which has been identified as Niigata Kobe Tectonic Zone (Sagiya et al., 2000). Large historical earthquakes have occurred in this area, and in recent years, Niigata has suffered the impact of two important events, known as the 2004 Mid-Niigata Prefecture earthquake (M 6.8) and The 2007 Niigata-ken Chuetsu-Oki earthquake (M 6.6), which considerately affected the crustal deformation pattern. For this reason, we review temporal variation of crustal deformation pattern in the mid Niigata region based on daily coordinates of 28 GPS sites from the GEONET network for three time windows: before 2004, 2004-2007 and after 2007 until March 2011, to avoid effects of crustal deformation associated with the 2011 Tohoku-Oki earthquake. We observed a migration of the deformation pattern in the East-West direction through the contraction belts for the above time windows. Before 2004, we recognize a clear shortening of 0.3ppm/yr in the area between the source regions of 2004 and 2007 quakes. After the 2004 Chuetsu earthquake, this shortening rate decreased. On the other hand, an accelerated contraction occurred to the east of this region, around the source region of the 2004 earthquake. After the 2007 earthquake, another contraction zone appeared to the northwest, near the 2007 source region. These time-dependent behaviors suggest there exists strong interaction between parallel fault segments in this area. It is crucially important to reveal such interaction to understand crustal deformation and seismogenesis in this region. We construct kinematic deformation models to interpret the time-dependent deformation pattern for each time period and to investigate mechanical interaction of coseismic as well as probably aseismic fault slips. Optimal faults parameters were established using a grid search, and computing the 95% confidence interval for each model parameter using the normalized Chi-squared distribution to

  5. The T-Reflection and the deep crustal structure of the Vøring Margin offshore Mid-Norway

    Science.gov (United States)

    Abdelmalak, M. M.; Faleide, J. I.; Planke, S.; Gernigon, L.; Zastrozhnov, D.; Shephard, G. E.; Myklebust, R.

    2017-12-01

    Volcanic passive margins are characterized by massive occurrence of mafic extrusive and intrusive rocks, before and during plate breakup, playing major role in determining the evolution pattern and the deep structure of magma-rich margins. Deep seismic reflection data frequently provide imaging of strong continuous reflections in the middle/lower crust. In this context, we have completed a detailed 2D seismic interpretation of the deep crustal structure of the Vøring volcanic margin, offshore mid-Norway, where high-quality seismic data allow the identification of high-amplitude reflections, locally referred to as the T-Reflection (TR). Using the dense seismic grid we have mapped the top of the TR in order to compare it with filtered Bouguer gravity anomalies and seismic refraction data. The TR is identified between 7 and 10 s. Sometimes it consists of one single smooth reflection. However, it is frequently associated with a set of rough multiple reflections displaying discontinuous segments with varying geometries, amplitude and contact relationships. The TR seems to be connected to deep sill networks and locally located at the continuation of basement high structures or terminates over fractures and faults. The spatial correlation between the filtered positive Bouguer gravity anomalies and the TR indicates that the latter represents a high impedance boundary contrast associated with a high-density/velocity body. Within an uncertainty of ± 2.5 km, the depth of the mapped TR is found to correspond to the depth of the top of the Lower Crustal Body (LCB), characterized by high P-wave velocities (>7 km/s), in 50% of the outer Vøring Margin areas, whereas different depths between the TR and the top LCB are estimated for the remaining areas. We present a tectonic scenario, where a large part of the deep structure could be composed of preserved upper continental basement and middle to lower crustal lenses of inherited and intruded high-grade metamorphic rocks. Deep

  6. Crustal Structure of the Tengchong Intra-plate Volcanic Area

    Science.gov (United States)

    Qian, Rongyi; Tong, Vincent C. H.

    2015-09-01

    We here provide an overview of our current understanding of the crustal structure of Tengchong in southwest China, a key intra-plate volcanic area along the Himalayan geothermal belt. Given that there is hitherto a lack of information about the near-surface structure of intra-plate volcanic areas, we present the first seismic reflection and velocity constraints on the shallow crust between intra-plate volcanoes. Our near-surface seismic images reveal the existence of dome-shaped seismic reflectors (DSRs) in the shallow crust between intra-plate volcanic clusters in Tengchong. The two DSRs are both ~2 km wide, and the shallowest parts of the DSRs are found at the depth of 200-300 m. The velocity model shows that the shallow low-velocity layer (<4 km/s) is anomalously thick (~1 km) in the region where the DSRs are observed. The presence of DSRs indicates significant levels of intra-plate magmatism beneath the along-axis gap separating two volcano clusters. Along-axis gaps between volcano clusters are therefore not necessarily an indicator of lower levels of magmatism. The seismic images obtained in this technically challenging area for controlled-source seismology allow us to conclude that shallow crustal structures are crucial for understanding the along-axis variations of magmatism and hydrothermal activities in intra-plate volcanic areas.

  7. Seismotectonics and Crustal Thickness of Northwest Mindoro, Philippines

    Science.gov (United States)

    Chen, P. F.; Olavere, E. A.; Lee, K. M.; Bautista, B.; Solidum, R., Jr.; Huang, B. S.

    2015-12-01

    Mindoro Island locates where the Palawan Continental Block (PCB) indented into the Philippine Mobile Belt (PMB) during the Early Miocene and where the Manila Trench terminates, having ceased convergence due to collision. On the transition from subduction to collision, Northwest Mindoro exhibits vigorous seismic activity and has been debated about its affiliation being PCB or PMB. Here, we use data from both the EHB and Global Centroid Moment Tensor catalogues to study the regional seismotectonics. We also deployed five broadband stations to probe the crustal thickness beneath NW Mindoro using receiver function analysis. Results show that, following the southeasterly reduction of convergence rates at the southern termination of the Manila Trench, the slab dipping angles steepen, were initiated at depth (~200 km) and propagate upwards. The horizontal distances of the trench and slab, as measured from the Wadati-Benioff zone at 200 km depth, also reduce in a southeasterly direction. Observations of intermediate-depth earthquakes that exhibit predominantly down-dip extensional stress patterns attest that the steepening of slab dipping angles is due to the negative buoyancy of the slab. Preliminary results of receiver function analysis suggest that the crustal thickness beneath NW Mindoro is about 40 km and is probably PCB affiliated.

  8. Glacio-Seismotectonics: Ice Sheets, Crustal Deformation and Seismicity

    Science.gov (United States)

    Sauber, Jeanne; Stewart, Iain S.; Rose, James

    2000-01-01

    The last decade has witnessed a significant growth in our understanding of the past and continuing effects of ice sheets and glaciers on contemporary crustal deformation and seismicity. This growth has been driven largely by the emergence of postglacial rebound models (PGM) constrained by new field observations that incorporate increasingly realistic rheological, mechanical, and glacial parameters. In this paper, we highlight some of these recent field-based investigations and new PGMs, and examine their implications for understanding crustal deformation and seismicity during glaciation and following deglaciation. The emerging glacial rebound models outlined in the paper support the view that both tectonic stresses and glacial rebound stresses are needed to explain the distribution and style of contemporary earthquake activity in former glaciated shields of eastern Canada and Fennoscandia. However, many of these models neglect important parameters, such as topography, lateral variations in lithospheric strength and tectonic strain built up during glaciation. In glaciated mountainous terrains, glacial erosion may directly modulate tectonic deformation by resetting the orogenic topography and thereby providing an additional compensatory uplift mechanism. Such effects are likely to be important both in tectonically active orogens and in the mountainous regions of glaciated shields.

  9. Crustal structure of the Eastern Alps and their foreland

    DEFF Research Database (Denmark)

    Grad, M.; Brückl, E.; Majdanski, M.

    2009-01-01

    The subject of this paper concerns the seismic modelling of the crustal structure in the transition zone from the Bohemian Massif, across the Molasse basin and the Eastern Alps to the Southern Alps, mainly on the territory of Austria. The CEL10/Alp04 profile crosses the triple point of the European......) are distinct up to 60-90 km offset and are characterized by large variations in apparent velocity and amplitude. The contact between the Molasse basin and the Eastern Alps represents a barrier for seismic waves. Mid-crustal reflections (Pc) are usually recorded at short distance intervals (20-50 km......, was undertaken using a ray-tracing technique. The P-wave velocity in the crystalline upper crust of the Bohemian Massif and Molasse basin is about 6.15 km s-1, which is slightly higher than in the Alpine area (about 6.0 km s-1). Below the northern accretionary wedge of the Eastern Alps low-velocity sediments...

  10. Lower crustal xenoliths, Chinese Peak lava flow, central Sierra Nevada.

    Science.gov (United States)

    Dodge, F.C.W.; Calk, L.C.; Kistler, R.W.

    1986-01-01

    This assemblage of pyroxenite, peridotite and mafic granulite xenoliths in the toe of a 10 m.y. trachybasalt flow remnant overlying late Cretaceous granitic rocks, indicates the presence of a mafic-ultramafic complex beneath this part of central California; orthopyroxenites, websterites and clinopyroxenites are dominant. A few of the xenoliths contain ovoid opaque patches that are apparently pseudomorphs after garnet and have pyralspite garnet compositions; using a garnet-orthopyroxene geobarometer, they indicate a lower crustal depth of approx 40 km. Abundant mafic granulites can be subdivided into those with Al2O3 = or 15% and showing considerable scatter on oxide variation diagrams. The high-alumina granulite xenoliths have relatively low 87Rb/86Sr but high 87Sr/86Sr, whereas the low-alumina and ultramafic xenoliths have a wide range of 87Rb/86Sr, but lower 87Sr/86Sr; the isotopic data indicate roughly the same age as that of overlying granitic plutons (approx 100 m.y.). However, the granitic rocks have initial 87Sr/86Sr ratios intermediate between those of the high-alumina and ultramafic xenoliths, suggesting that they result from the mixing of basaltic magma (represented by the ultramafic rocks) and crustal materials, with subsequent crystal fractionation.-R.A.H.

  11. Correlation of Crustal Structures and Seismicity Patterns in Northern Appalachians

    Science.gov (United States)

    Yang, X.; Gao, H.

    2017-12-01

    The earthquake distributions in northern Appalachians are bounded by major geologically-defined terrane boundaries. There is a distinct seismic gap within Taconic Belt between the Western Quebec Seismic Zone (WQSZ) to the west and the seismically active Ganderia terrane to the east. It is not clear, however, what crustal structures control the characteristics of earthquake clustering in this region. Here we present a newly constructed crustal shear velocity model for the northern Appalachians using Rayleigh wave data extracted from ambient noises. Our tomographic model reveals strongly heterogeneous seismic structures in the crust. We observe multiple NW-dipping patches of high-velocity anomalies in the upper crust beneath the southeastern WQSZ. The upper crust shear velocities in the Ganderia and Avalonia region are generally lower than those beneath the WQSZ. The middle crust has relatively lower velocities in the study area. The earthquakes in the study area are constrained within the upper crust. Most of the earthquake hypocenters within the WQSZ are concentrated along the NW-dipping boundaries separating the high-velocity anomalies. In contrast, most of the earthquake hypocenters in the Ganderia and Avalonia region are diffusely distributed without clear vertical lineaments. The orientations of maximum compressive stresses change from W-E in the Ganderia and Avalonia region to SW-NE in the WQSZ. The contrasts in seismicity, velocity, and stress field across the Taconic Belt indicate that the Taconic Belt terrane may act as a seismically inactive buffer zone in northern Appalachians.

  12. Electrical structure and its implication across the lower- and upper-crustal settings of South India

    Science.gov (United States)

    Raval, U.

    1988-01-01

    Measurements of a large scale MMA experiment covering both the granulite and greenstone terrains of Archeans in the southern part of India is re-visited and re-analyzed. The induced field variations contain the signatures of crustal and subcrustal electrical conductivities, although substantially distorted by the sea-land interfaces and cenozoic sediments. However, through a selection of some reconnaissance profiles and temporal variations, an attempt is made to deduce whether: (1) significant differences exist between the electrical structures of the high and low grade complexes (i.e., if the electrical conductivity of the lower crust is due to minerological composition or is intrinsic to the positioning at depths greater than 15 km); (2) the probable seaward extension of the continental crust and its transition to oceanic type may also contribute (through intracrustal DC-like telluric sheets) to the induction field in addition to or rather than the sharply localized zones; (3) the observed parameters are indicative of a formal anisotropy and/or undulations in the deep crust; and (4) the postulate of relatively hotter Indian shield is reflected particularly with regard to differential metamorphism. In the last case, the crust-mantle coupling in this region - unlike other similar areas - seems to be markedly affected by the evolution of Ne-plate velocity field.

  13. Alps, Carpathians and Dinarides-Hellenides: about plates, micro-plates and delaminated crustal blocks

    Science.gov (United States)

    Schmid, Stefan

    2014-05-01

    Before the onset of Europe-Africa continental collision in the Dinarides-Hellenides (around 60Ma) and in the Alps and Western Carpathians (around 35 Ma), and at a large scale, the dynamics of orogenic processes in the Mediterranean Alpine chains were governed by Europe-Africa plate convergence leading to the disappearance of large parts of intervening oceanic lithosphere, i.e. the northern branch of Neotethys along the Sava-Izmir-Ankara suture and Alpine Tethys along the Valais-Magura suture (Schmid et al. 2008). In spite of this, two major problems concerning the pre-collisional stage are still poorly understood: (1) by now we only start to understand geometry, kinematics and dynamics of the along-strike changes in the polarity of subduction between Alps-Carpathians and Dinarides-Hellenides, and (2) it is not clear yet during exactly which episodes and to what extent intervening rifted continental fragments such as, for example, Iberia-Briançonnais, Tisza, Dacia, Adria-Taurides moved independently as micro-plates, and during which episodes they remained firmly attached to Europa or Africa from which they broke away. As Europe-Africa plate convergence slowed down well below 1 cm/yr at around 30 Ma ago these pre-collisional processes driven by plate convergence on a global scale gave way to more local processes of combined roll-back and crustal delamination in the Pannonian basin of the Carpathian embayment and in the Aegean (as well as in the Western Mediterranean, not discussed in this contribution). In the case of the Carpathian embayment E-directed roll back totally unrelated to Europe-Africa N-S-directed convergence, started at around 20 Ma ago, due to the presence relict oceanic lithosphere in the future Pannonian basin that remained un-subducted during collision. Due to total delamination of the crust from the eastward rolling back European mantle lithosphere the anticlockwise rotating ALCAPA crustal block, consisting of Eastern Alps and Western Carpathian

  14. Guangxi crustal structural evolution and the formation and distribution regularities of U-rich strata

    International Nuclear Information System (INIS)

    Kang Zili.

    1989-01-01

    Based on summing up Guangxi geotectonic features and evolutionary regularities, this paper discusses the occurrence features, formation conditions and time-space distribution regularities of various U-rich strata during the development of geosyncline, platform and diwa stages, Especially, during diwa stage all those U-rich strata might be reworked to a certain degree and resulted in the mobilization of uranium, then enriching to form polygenetic composite uranium ore deposits with stratabound features. This study will be helpful for prospecting in the region

  15. Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia

    Science.gov (United States)

    Şengör, A. M. C.; Natal'in, B. A.; Burtman, V. S.

    1993-07-01

    A new tectonic model, postulating the growth of giant subduction-accretion complexes along a single magmatic arc now found contorted between Siberia and Baltica, shows that Asia grew by 5.3 million square kilometres during the Palaeozoic era. Half of this growth may have occurred by the addition of juvenile crust newly extracted from the mantle, supporting models of considerable continental growth continuing throughout the Phanerozoic eon.

  16. Crustal structure, evolution, and volcanic unrest of the Alban Hills, Central Italy

    Science.gov (United States)

    Chiarabba, C.; Amato, A.; Delaney, P.T.

    1997-01-01

    The Alban Hills, a Quaternary volcanic center lying west of the central Apennines, 15-25 km southeast of Rome, last erupted 19ka and has produced approximately 290 km3 of eruptive deposits since the inception of volcanism at 580 ka. Earthquakes of moderate intensity have been generated there at least since the Roman age. Modern observations show that intermittent periods of swarm activity originate primarily beneath the youngest features, the phreatomagmatic craters on the west side of the volcano. Results from seismic tomography allow identification of a low-velocity region, perhaps still hot or partially molten, more than 6 km beneath the youngest craters and a high-velocity region, probably a solidified magma body, beneath the older central volcanic construct. Thirty centimeters of uplift measured by releveling supports the contention that high levels of seismicity during the 1980s and 1990s resulted from accumulation of magma beneath these craters. The volume of magma accumulation and the amount of maximum uplift was probably at least 40 ?? 106 m3 and 40 cm, respectively. Comparison of newer levelings with those completed in 1891 and 1927 suggests earlier episodes of uplift. The magma chamber beneath the western Alban Hills is probably responsible for much of the past 200 ka of eruptive activity, is still receiving intermittent batches of magma, and is, therefore, continuing to generate modest levels of volcanic unrest. Bending of overburden is the most likely cause of the persistent earthquakes, which generally have hypocenters above the 6-km-deep top of the magma reservoir. In this view, the most recent uplift and seismicity are probably characteristic and not precursors of more intense activity.

  17. Precambrian continental crustal evolution of Southeastern Sao Paulo state - Brazil: based on isotopic evidences

    International Nuclear Information System (INIS)

    Tassinari, C.C.G.

    1988-01-01

    The isotopic studies on granitic intrusions, orthogneissic rocks and migmatitic terranes in the Southeastern Sao Paulo are presented, indicating the age and the geochemical nature of the continental crust of this area. Approximately 300 Ar, Sr and Pb isotopic age determinations are included in this paper, categorized as to their reliability and significance. Looking for the continental crust growth related to the geological time, at the end of the lower Proterozoic, at least 85% of the continental crust has already accreted and differentiated. (C.G.C.)

  18. Crustal structure and evolution of the Arctic Caledonides: Results from controlled-source seismology

    Science.gov (United States)

    Aarseth, Iselin; Mjelde, Rolf; Breivik, Asbjørn Johan; Minakov, Alexander; Faleide, Jan Inge; Flueh, Ernst; Huismans, Ritske S.

    2017-10-01

    The continuation of the Caledonides into the Barents Sea has long been a subject of discussion, and two major orientations of the Caledonian deformation fronts have been suggested: NNW-SSE striking and NE-SW striking. A regional NW-SE oriented ocean bottom seismic profile across the western Barents Sea was acquired in 2014. In this paper we map the crust and upper mantle structure along this profile in order to discriminate between different interpretations of Caledonian structural trends and orientation of rift basins in the western Barents Sea. Modeling of P-wave travel times has been done using a ray-tracing method, and combined with gravity modeling. The results show high P-wave velocities (4 km/s) close to the seafloor, as well as localized sub-horizontal high velocity zones (6.0 km/s and 6.9 km/s) at shallow depths which are interpreted as magmatic sills. Refractions from the top of the crystalline basement together with reflections from the Moho give basement velocities from 6.0 km/s at the top to 6.7 km/s at the base of the crust. P-wave travel time modeling of the OBS profile indicate an eastwards increase in velocities from 6.4 km/s to 6.7 km/s at the base of the crystalline crust, and the western part of the profile is characterized by a higher seismic reflectivity than the eastern part. This change in seismic character is consistent with observations from vintage reflection seismic data and is interpreted as a Caledonian suture extending through the Barents Sea, separating Barentsia and Baltica. Local deepening of Moho (from 27 km to 33 km depth) creates ;root structures; that can be linked to the Caledonian compressional deformation or a suture zone imprinted in the lower crust. Our model supports a separate NE-SW Caledonian trend extending into the central Barents Sea, branching off from the northerly trending Svalbard Caledonides, implying the existence of Barentsia as an independent microcontinent between Laurentia and Baltica.

  19. The effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low-angle normal faults

    KAUST Repository

    Wu, Guangliang; Lavier, Luc L.

    2016-01-01

    To investigate the formation of core complexes and low-angle normal faults, we devise thermomechanical simulations on a simplified wedge-like orogenic hinterland that has initial topography, Moho relief, and a preexisting midcrustal shear zone that can accommodate shear at very low angles (<20°). We mainly vary the strength of the lower crust and the frictional strength of the preexisting midcrustal shear zone. We find that the strength of the lower crust and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes. With increasing lower crustal strength, we recognize varying extensional features with decreasing exhumation rate: these are characterized by bivergent metamorphic massifs, classic Cordilleran metamorphic core complexes, multiple consecutive core complexes (or boudinage structures), and a flexural core complex underlined by a large subsurface low-angle detachment fault with a small convex curvature. Topographic loading and mantle buoyancy forces, together with divergent boundaries, drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading. The detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust. Though the regional stress is largely Andersonian, we find non-Andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient. Our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve.

  20. The effects of lower crustal strength and preexisting midcrustal shear zones on the formation of continental core complexes and low-angle normal faults

    KAUST Repository

    Wu, Guangliang

    2016-08-22

    To investigate the formation of core complexes and low-angle normal faults, we devise thermomechanical simulations on a simplified wedge-like orogenic hinterland that has initial topography, Moho relief, and a preexisting midcrustal shear zone that can accommodate shear at very low angles (<20°). We mainly vary the strength of the lower crust and the frictional strength of the preexisting midcrustal shear zone. We find that the strength of the lower crust and the existence and strength of a preexisting shear zone significantly affect the formation and evolution of core complexes. With increasing lower crustal strength, we recognize varying extensional features with decreasing exhumation rate: these are characterized by bivergent metamorphic massifs, classic Cordilleran metamorphic core complexes, multiple consecutive core complexes (or boudinage structures), and a flexural core complex underlined by a large subsurface low-angle detachment fault with a small convex curvature. Topographic loading and mantle buoyancy forces, together with divergent boundaries, drive a regional lower crustal flow that leads to the exhumation of the lower crust where intensive upper crustal faulting induces strong unloading. The detachment fault is a decoupling zone that accommodates large displacement and accumulates sustained shear strain at very low angle between upper and lower crust. Though the regional stress is largely Andersonian, we find non-Andersonian stress in regions adjacent to the preexisting shear zone and those with high topographic gradient. Our new models provide a view that is generally consistent with geological and geophysical observations on how core complexes form and evolve.

  1. Anomalous Structure of Oceanic Lithosphere in the North Atlantic and Arctic Oceans: A Preliminary Analysis Based on Bathymetry, Gravity and Crustal Structure

    Science.gov (United States)

    Barantsrva, O.

    2014-12-01

    We present a preliminary analysis of the crustal and upper mantle structure for off-shore regions in the North Atlantic and Arctic oceans. These regions have anomalous oceanic lithosphere: the upper mantle of the North Atlantic ocean is affected by the Iceland plume, while the Arctic ocean has some of the slowest spreading rates. Our specific goal is to constrain the density structure of the upper mantle in order to understand the links between the deep lithosphere dynamics, ocean spreading, ocean floor bathymetry, heat flow and structure of the oceanic lithosphere in the regions where classical models of evolution of the oceanic lithosphere may not be valid. The major focus is on the oceanic lithosphere, but the Arctic shelves with a sufficient data coverage are also included into the analysis. Out major interest is the density structure of the upper mantle, and the analysis is based on the interpretation of GOCE satellite gravity data. To separate gravity anomalies caused by subcrustal anomalous masses, the gravitational effect of water, crust and the deep mantle is removed from the observed gravity field. For bathymetry we use the global NOAA database ETOPO1. The crustal correction to gravity is based on two crustal models: (1) global model CRUST1.0 (Laske, 2013) and, for a comparison, (2) a regional seismic model EUNAseis (Artemieva and Thybo, 2013). The crustal density structure required for the crustal correction is constrained from Vp data. Previous studies have shown that a large range of density values corresponds to any Vp value. To overcome this problem and to reduce uncertainty associated with the velocity-density conversion, we account for regional tectonic variations in the Northern Atlantics as constrained by numerous published seismic profiles and potential-field models across the Norwegian off-shore crust (e.g. Breivik et al., 2005, 2007), and apply different Vp-density conversions for different parts of the region. We present preliminary results

  2. Crustal deformation mechanism in southeastern Tibetan Plateau: Insights from numerical modeling

    Science.gov (United States)

    Li, Y.; Liu, S.; Chen, L.

    2017-12-01

    The Indo-Asian collision developed the complicated crustal deformation around the southeastern Tibetan plateau. Numerous models have proposed to explain the crustal deformation, but the mechanism remains controversial, especially the increasing multi-geophysics data, which demonstrate the existence of lower velocity, lower resistivity and high conductivity, implying that lower crustal flow is responsible for the crustal deformation, arguing for the lower crust flow model. To address the relations between the crust flow and the surface deformation, we employ a three-dimensional viscoelastic finite model to investigate the possible influence on the surface deformation, and discuss the stress field distribution under the model. Our preliminary results suggest that lower crustal flow plays an important role in crustal deformation in southeastern Tibetan plateau. The best fitting is achieved when the flow velocity of the lower crust is approximately 10-11 mm/a faster than that of the upper crust. Crustal rheological properties affect regional crustal deformation, when the viscosity of the middle and lower crust in the South China block reaches 1022 and 1023 Pa.s, respectively; the predicted match observations well, especially for the magnitude within the South China block. The maximum principal stress field exhibits clear zoning, gradually shifting from an approximately east-west orientation in the northern Bayan Har block to southeast in the South China block, southwest in the western Yunnan block, and a radially divergent distribution in the Middle Yunnan and Southern Yunnan blocks.

  3. Comparison of publically available Moho depth and crustal thickness grids with newly derived grids by 3D gravity inversion for the High Arctic region.

    Science.gov (United States)

    Lebedeva-Ivanova, Nina; Gaina, Carmen; Minakov, Alexander; Kashubin, Sergey

    2016-04-01

    deep Arctic Ocean: results of a 3D gravity modeling Russian Geology and Geophysics 54, 247-262. Jakobsson M, Mayer L, Coakley B, Dowdeswell JA, Forbes S, Fridman B, Hodnesdal H, Noormets R, Pedersen R, Rebesco M, Schenke HW, Zarayskaya Y, Accettella D, Armstrong A, Anderson RM, Bienhoff P, Camerlenghi A, Church I, Edwards M, Gardner JV, Hall JK, Hell B, Hestvik O, Krist-offersen Y, Marcussen C, Mohammad R, Mosher D, Nghiem SV, Pedrosa MT, Travaglini PG, Weatherall P (2012). The international bathymetric chart of the Arctic Ocean (IBCAO) version 3.0. Geophys Res Lett 39, L12609. Laske, G., Masters., G., Ma, Z. and Pasyanos, M. (2013). Update on CRUST1.0 - A 1-degree Global Model of Earth's Crust, Geophys. Res. Abstracts, 15, Abstract EGU2013-2658, 2013. Minakov A, Faleide JI, Glebovsky VY, Mjelde R (2012) Structure and evolution of the northern Barents-Kara Sea continental margin from integrated analysis of potential fields, bathymetry and sparse seismic data. Geophys J Int 188, 79-102. Petrov O., Smelror M., Shokalsky S., Morozov A., Kashubin S., Grikurov G., Sobolev N., Petrov E., (2013). A new international tectonic map of the Arctic (TeMAr) at 1:5 M scale and geodynamic evolution in the Arctic region. EGU2013-13481. Reguzzoni, M., & Sampietro, D. (2014). GEMMA: An Earth crustal model based on GOCE satellite data. International Journal of Applied Earth Observation and Geoinformation Spasojevic S. & Gurnis M., (2012). Sea level and vertical motion of continents from dynamic earth models since the late Cretaceous. American Association of Petroleum Geologists Bulletin, 96, pp. 2037-2064.

  4. Density heterogeneity of the upper mantle beneath Siberia from satellite gravity and a new regional crustal model

    DEFF Research Database (Denmark)

    Herceg, Matija; Thybo, Hans; Artemieva, Irina

    2013-01-01

    We present a new regional model for the density structure of the upper mantle below Siberia. The residual mantle gravity anomalies are based on gravity data derived from the GOCE gravity gradients and geopotential models, with crustal correction to the gravity field being calculated from a new...... on regional and global crustal models. We analyze how uncertainties and errors in the crustal model propagate from crustal densities to mantle residual gravity anomalies and the density model of the upper mantle. The new regional density model for the Siberian craton and the West Siberian Basin complements...... regional crustal model. This newly compiled database on the crustal seismic structure, complemented by additional constraints from petrological analysis of near-surface rocks and lower crustal xenoliths, allows for a high-resolution correction of the crustal effects as compared to previous studies based...

  5. Kinematics and significance of a poly-deformed crustal-scale shear zone in central to south-eastern Madagascar: the Itremo-Ikalamavony thrust

    Science.gov (United States)

    Giese, Jörg; Schreurs, Guido; Berger, Alfons; Herwegh, Marco

    2017-09-01

    Across the crystalline basement of Madagascar, late Archaean rocks of the Antananarivo Block are tectonically overlain by Proterozoic, predominantly metasedimentary units of the Ikalamavony and Itremo Groups of the Southwest Madagascar Block. The generally west-dipping tectonic contact can be traced for more than 750 km from NW to SE and is referred to here as the Itremo-Ikalamavony thrust. The basal units of the SW Madagascar Block comprise metasedimentary quartzites with the potential to preserve a multistage deformation history in their microstructures. Previous studies suggest contrasting structural evolutions for this contact, including eastward thrusting, top-to-the-west directed extension and right-lateral strike-slip deformation during the late Neoproterozoic/Ediacaran. In this study, we integrate remote sensing analyses, structural and petrological fieldwork, as well as microstructural investigations of predominantly quartz mylonites from the central southern segment of the contact between Ankaramena and Maropaika. In this area, two major phases of ductile deformation under high-grade metamorphic conditions occurred in latest Neoproterozoic/early Phanerozoic times. A first (Andreaba) phase produces a penetrative foliation, which is parallel to the contact between the two blocks and contemporaneous with widespread magmatism. A second (Ihosy) phase of deformation folds Andreaba-related structures. The investigated (micro-)structures indicate that (a) juxtaposition of both blocks possibly already occurred prior to the Andreaba phase, (b) (re-)activation with top-to-the-east thrusting took place during the latest stages of the Andreaba phase, (c) the Ihosy phase resulted in regional-scale open folding of the tectonic contact and (d) reactivation of parts of the contact took place at distinctively lower temperatures post-dating the major ductile deformations.

  6. Combined Gravimetric-Seismic Crustal Model for Antarctica

    Science.gov (United States)

    Baranov, Alexey; Tenzer, Robert; Bagherbandi, Mohammad

    2018-01-01

    The latest seismic data and improved information about the subglacial bedrock relief are used in this study to estimate the sediment and crustal thickness under the Antarctic continent. Since large parts of Antarctica are not yet covered by seismic surveys, the gravity and crustal structure models are used to interpolate the Moho information where seismic data are missing. The gravity information is also extended offshore to detect the Moho under continental margins and neighboring oceanic crust. The processing strategy involves the solution to the Vening Meinesz-Moritz's inverse problem of isostasy constrained on seismic data. A comparison of our new results with existing studies indicates a substantial improvement in the sediment and crustal models. The seismic data analysis shows significant sediment accumulations in Antarctica, with broad sedimentary basins. According to our result, the maximum sediment thickness in Antarctica is about 15 km under Filchner-Ronne Ice Shelf. The Moho relief closely resembles major geological and tectonic features. A rather thick continental crust of East Antarctic Craton is separated from a complex geological/tectonic structure of West Antarctica by the Transantarctic Mountains. The average Moho depth of 34.1 km under the Antarctic continent slightly differs from previous estimates. A maximum Moho deepening of 58.2 km under the Gamburtsev Subglacial Mountains in East Antarctica confirmed the presence of deep and compact orogenic roots. Another large Moho depth in East Antarctica is detected under Dronning Maud Land with two orogenic roots under Wohlthat Massif (48-50 km) and the Kottas Mountains (48-50 km) that are separated by a relatively thin crust along Jutulstraumen Rift. The Moho depth under central parts of the Transantarctic Mountains reaches 46 km. The maximum Moho deepening (34-38 km) in West Antarctica is under the Antarctic Peninsula. The Moho depth minima in East Antarctica are found under the Lambert Trench (24

  7. Magma Supply of Southwest Indian Ocean: Implication from Crustal Thickness Anomalies

    Science.gov (United States)

    Chiheng, L.; Jianghai, L.; Huatian, Z.; Qingkai, F.

    2017-12-01

    The Southwest Indian Ridge (SWIR) is one of the world's slowest spreading ridges with a full spreading rate of 14mm a-1, belonging to ultraslow spreading ridge, which are a novel class of spreading centers symbolized by non-uniform magma supply and crustal accretion. Therefore, the crustal thickness of Southwest Indian Ocean is a way to explore the magmatic and tectonic process of SWIR and the hotspots around it. Our paper uses Residual Mantle Bouguer Anomaly processed with the latest global public data to invert the relative crustal thickness and correct it according to seismic achievements. Gravity-derived crustal thickness model reveals a huge range of crustal thickness in Southwest Indian Ocean from 0.04km to 24km, 7.5km of average crustal thickness, and 3.5km of standard deviation. In addition, statistics data of crustal thickness reveal the frequency has a bimodal mixed skewed distribution, which indicates the crustal accretion by ridge and ridge-plume interaction. Base on the crustal thickness model, we divide three types of crustal thickness in Southwest Indian Ocean. About 20.31% of oceanic crust is 9.8km thick as thick crust. Furthermore, Prominent thin crust anomalies are associated with the trend of most transform faults, but thick crust anomalies presents to northeast of Andrew Bain transform fault. Cold and depleted mantle are also the key factors to form the thin crust. The thick crust anomalies are constrained by hotspots, which provide abundant heat to the mantle beneath mid-ocean ridge or ocean basin. Finally, we roughly delineate the range of ridge-plume interaction and transform fault effect.

  8. Crustal structure and sedimentation history over the Alleppey platform, southwest continental margin of India: Constraints from multichannel seismic and gravity data

    Directory of Open Access Journals (Sweden)

    P. Unnikrishnan

    2018-03-01

    Full Text Available The Alleppey Platform is an important morphological feature located in the Kerala-Konkan basin off the southwest coast of India. In the present study, seismic reflection data available in the basin were used to understand the sedimentation history and also to carry out integrated gravity interpretation. Detailed seismic reflection data in the basin reveals that: (1 the Alleppey Platform is associated with a basement high in the west of its present-day geometry (as observed in the time-structure map of the Trap Top (K/T boundary, (2 the platform subsequently started developing during the Eocene period and attained the present geometry by the Miocene and, (3 both the Alleppey platform and the Vishnu fracture zone have had significant impact on the sedimentation patterns (as shown by the time-structure and the isochron maps of the major sedimentary horizons in the region. The 3-D sediment gravity effect computed from the sedimentary layer geometry was used to construct the crustal Bouguer anomaly map of the region. The 3-D gravity inversion of crustal Bouguer anomaly exhibits a Moho depression below the western border of the platform and a minor rise towards the east which then deepens again below the Indian shield. The 2-D gravity modelling across the Alleppey platform reveals the geometry of crustal extension, in which there are patches of thin and thick crust. The Vishnu Fracture Zone appears as a crustal-scale feature at the western boundary of the Alleppey platform. Based on the gravity model and the seismic reflection data, we suggest that the basement high to the west of the present day Alleppey platform remained as a piece of continental block very close to the mainland with the intervening depression filling up with sediments during the rifting. In order to place the Alleppey platform in the overall perspective of tectonic evolution of the Kerala-Konkan basin, we propose its candidature as a continental fragment.

  9. Ultrasound monitoring of applied forcing, material ageing, and catastrophic yield of crustal structures

    Directory of Open Access Journals (Sweden)

    G. P. Gregori

    2007-11-01

    Full Text Available A new kind of data analysis is discussed – and a few case histories of actual application are presented – concerning the physical information attainable by acoustic emission (AE records in geodynamically active or volcanic areas. The previous analyses of such same kind of observations were reported in several papers appeared in the last few years, and here briefly recalled. They are concerned with the inference of the forcing ("F" acting on the physical system, and on the ageing ("T" or fatigue of its "solid" structures. The new analysis here discussed deals with the distinction between a state of applied stress ("hammer regime", compared to state of "recovery regime" of the system while it seeks a new equilibrium state after having been perturbed. For instance, in the case of a seismic event – and according to some kind of almost intuitive argument – the "hammer regime" is the phenomenon leading to the main shock, while the "recovery regime" deals with the well known aftershocks. Such same intuitive inference, however, can be investigated by a much more formal algorithm, aimed at envisaging the minor changes of the behaviour of the system, during its history and during its present dynamic evolution. As a demonstrative application, detailed consideration is given of AE records – each one lasting for a few years – collected on the Italian peninsula vs. records collected on the Kefallinìa Island (western Greece. Such two areas are well known being characterised by some great comparative difference in their respective tectonic setting. When considering planetary scale phenomena, they appear comparatively very close to each other. Hence, they are likely being presumably affected by similar large-scale external actions, although they ought to be expected to respond in some completely different way. Such facts are clearly manifested by some substantially different AE responses of the local crustal

  10. New aerogeophysical views of crustal architecture in the Recovery frontier of East Antarctica

    Science.gov (United States)

    Ferraccioli, Fausto; Forsberg, Rene; Jordan, Tom; Matsuoka, Kenichi; Olsen, Arne; King, Owen; Ghidella, Marta

    2014-05-01

    East Antarctica is the least known continent on Earth, despite being regarded as a keystone in Gondwana, Rodinia and possibly Columbia supercontinents. Significant progress has however been made in recent years in the exploration of East Antarctica using airborne geophysical techniques. Spurred by the International Polar Year major collaborative aerogeophysical campaigns have been performed over the Wilkes Subglacial Basin, the Aurora Subglacial Basin and the Gamburtsev Subglacial Mountains. Analyses of these recent datasets is providing fundamental new glimpses into the crustal architecture in interior East Antarctica, as well as several new interpretations regarding its linkages with tectonic and geodynamic evolution from the Precambrian to the Mesozoic. Here we present the first results of a major reconnaissance aerogeophysical survey over the largely unexplored Recovery ice stream catchment in East Antarctica, flown during the IceGRAV 2012-13 field season, as part of a new international Danish, Norwegian, UK and Argentine collaboration. Over 29,000 line km of new radio-echo sounding, laser altimetry, gravity and magnetic data were acquired using a British Antarctic Survey Twin Otter. We will focus primarily on presenting the new potential field datasets and discuss the anomaly patterns seen in aeromagnetic anomaly maps, free air, Bouguer and isostatic residual maps. The aerogeophysical datasets we will present provide a new foundation to address a cascade of open questions regarding this part of East Antarctica, including: i) Where are and what is the nature of the major tectonic boundaries separating the Coast block, the Shackleton Range and the Dronning Maud Land crustal provinces? Specifically is there new geophysical evidence in support of a Pan-African age suture zone in the Shackleton Range linked to Gondwana assembly?; ii) is there evidence in support of an older Grenvillian-age orogenic belt, extending across the interior of East Antarctica?; Or, is

  11. Crustal and upper mantle investigations of the Caribbean-South American plate boundary

    Science.gov (United States)

    Bezada, Maximiliano J.

    The evolution of the Caribbean --- South America plate boundary has been a matter of vigorous debate for decades and many questions remain unresolved. In this work, and in the framework of the BOLIVAR project, we shed light on some aspects of the present state and the tectonic history of the margin by using different types of geophysical data sets and techniques. An analysis of controlled-source traveltime data collected along a boundary-normal profile at ˜65°W was used to build a 2D P-wave velocity model. The model shows that the Caribbean Large Igenous Province is present offshore eastern Venezuela and confirms the uniformity of the velocity structure along the Leeward Antilles volcanic belt. In contrast with neighboring profiles, at this longitude we see no change in velocity structure or crustal thickness across the San Sebastian - El Pilar fault system. A 2D gravity modeling methodology that uses seismically derived initial density models was developed as part of this research. The application of this new method to four of the BOLIVAR boundary-normal profiles suggests that the uppermost mantle is denser under the South American continental crust and the island arc terranes than under the Caribbean oceanic crust. Crustal rocks of the island arc and extended island arc terranes of the Leeward Antilles have a relatively low density, given their P-wave velocity. This may be caused by low iron content, relative to average magmatic arc rocks. Finally, an analysis of teleseismic traveltimes with frequency-dependent kernels produced a 3D P-wave velocity perturbation model. The model shows the structure of the mantle lithosphere under the study area and clearly images the subduction of the Atlantic slab and associated partial removal of the lower lithosphere under northern South America. We also image the subduction of a section of the Caribbean plate under South America with an east-southeast direction. Both the Atlantic and Caribbean subducting slabs penetrate the

  12. Physics of Earthquake Disaster: From Crustal Rupture to Building Collapse

    Science.gov (United States)

    Uenishi, Koji

    2018-05-01

    Earthquakes of relatively greater magnitude may cause serious, sometimes unexpected failures of natural and human-made structures, either on the surface, underground, or even at sea. In this review, by treating several examples of extraordinary earthquake-related failures that range from the collapse of every second building in a commune to the initiation of spontaneous crustal rupture at depth, we consider the physical background behind the apparently abnormal earthquake disaster. Simple but rigorous dynamic analyses reveal that such seemingly unusual failures actually occurred for obvious reasons, which may remain unrecognized in part because in conventional seismic analyses only kinematic aspects of the effects of lower-frequency seismic waves below 1 Hz are normally considered. Instead of kinematics, some dynamic approach that takes into account the influence of higher-frequency components of waves over 1 Hz will be needed to anticipate and explain such extraordinary phenomena and mitigate the impact of earthquake disaster in the future.

  13. PIXEL: Japanese InSAR community for crustal deformation research

    Science.gov (United States)

    Furuya, M.; Shimada, M.; Ozawa, T.; Fukushima, Y.; Aoki, Y.; Miyagi, Y.; Kitagawa, S.

    2007-12-01

    In anticipation of the launch of ALOS (Advanced Land Observation Satellite) by JAXA (Japan Aerospace eXploration Agency), and in order to expand and bolster the InSAR community for crustal deformation research in Japan, a couple of scientists established a consortium, PIXEL, in November 2005 in a completely bottom-up fashion. PIXEL stands for Palsar Interferometry Consortium to Study our Evolving Land. Formally, it is a research contract between JAXA and Earthquake Research Institute (ERI), University of Tokyo. As ERI is a shared institute of the Japanese universities and research institutes, every scientist at all Japanese universities and institutes can participate in this consortium. The activity of PIXEL includes information exchange by mailing list, tutorial workshop for InSAR software, research workshop, and PALSAR data sharing. After the launch of ALOS, we have already witnessed several earthquakes and volcanic activities using PALSAR interferometry. We will briefly show and digest some of those observation results.

  14. Sub-crustal seismic activity beneath Klyuchevskoy Volcano

    Science.gov (United States)

    Carr, M. J.; Droznina, S.; Levin, V. L.; Senyukov, S.

    2013-12-01

    Seismic activity is extremely vigorous beneath the Klyuchevskoy Volcanic Group (KVG). The unique aspect is the distribution in depth. In addition to upper-crustal seismicity, earthquakes take place at depths in excess of 20 km. Similar observations are known in other volcanic regions, however the KVG is unique in both the number of earthquakes and that they occur continuously. Most other instances of deep seismicity beneath volcanoes appear to be episodic or transient. Digital recording of seismic signals started at the KVG in early 2000s.The dense local network reliably locates earthquakes as small as ML~1. We selected records of 20 earthquakes located at depths over 20 km. Selection was based on the quality of the routine locations and the visual clarity of the records. Arrivals of P and S waves were re-picked, and hypocentral parameters re-established. Newl locations fell within the ranges outlined by historical seismicity, confirming the existence of two distinct seismically active regions. A shallower zone is at ~20 km depth, and all hypocenters are to the northeast of KVG, in a region between KVG and Shiveluch volcano. A deeper zone is at ~30 km, and all hypocenters cluster directly beneath the edifice of the Kyuchevskoy volcano. Examination of individual records shows that earthquakes in both zones are tectonic, with well-defined P and S waves - another distinction of the deep seismicity beneath KVG. While the upper seismic zone is unquestionably within the crust, the provenance of the deeper earthquakes is enigmatic. The crustal structure beneath KVG is highly complex, with no agreed-upon definition of the crust-mantle boundary. Rather, a range of values, from under 30 to over 40 km, exists in the literature. Similarly, a range of velocity structures has been reported. Teleseismic receiver functions (RFs) provide a way to position the earthquakes with respect to the crust-mantle boundary. We compare the differential travel times of S and P waves from deep

  15. Contrasting magmatic structures between small plutons and batholiths emplaced at shallow crustal level (Sierras de Córdoba, Argentina)

    Science.gov (United States)

    Pinotti, Lucio P.; D'Eramo, Fernando J.; Weinberg, Roberto F.; Demartis, Manuel; Tubía, José María; Coniglio, Jorge E.; Radice, Stefania; Maffini, M. Natalia; Aragón, Eugenio

    2016-11-01

    Processes like injection, magma flow and differentiation and influence of the regional strain field are here described and contrasted to shed light on their role in the formation of small plutons and large batholiths their magmatic structures. The final geometric and compositional arrangement of magma bodies are a complex record of their construction and internal flow history. Magma injection, flow and differentiation, as well as regional stresses, all control the internal nature of magma bodies. Large magma bodies emplaced at shallow crustal levels result from the intrusion of multiple magma batches that interact in a variety of ways, depending on internal and external dynamics, and where the early magmatic, growth-related structures are commonly overprinted by subsequent history. In contrast, small plutons emplaced in the brittle-ductile transition more likely preserve growth-related structures, having a relatively simple cooling history and limited internal magma flow. Outcrop-scale magmatic structures in both cases record a rich set of complementary information that can help elucidate their evolution. Large and small granitic bodies of the Sierra Pampeanas preserve excellent exposures of magmatic structures that formed as magmas stepped through different rheological states during pluton growth and solidification. These structures reveal not only the flow pattern inside magma chambers, but also the rheological evolution of magmas in response to temperature evolution.

  16. Crustal structure of Australia from ambient seismic noise tomography

    Science.gov (United States)

    Saygin, Erdinc; Kennett, B. L. N.

    2012-01-01

    Surface wave tomography for Australian crustal structure has been carried out using group velocity measurements in the period range 1-32 s extracted from stacked correlations of ambient noise between station pairs. Both Rayleigh wave and Love wave group velocity maps are constructed for each period using the vertical and transverse component of the Green's function estimates from the ambient noise. The full suite of portable broadband deployments and permanent stations on the continent have been used with over 250 stations in all and up to 7500 paths. The permanent stations provide a useful link between the various shorter-term portable deployments. At each period the group velocity maps are constructed with a fully nonlinear tomographic inversion exploiting a subspace technique and the Fast Marching Method for wavefront tracking. For Rayleigh waves the continental coverage is good enough to allow the construction of a 3D shear wavespeed model in a two stage approach. Local group dispersion information is collated for a distribution of points across the continent and inverted for a 1D SV wavespeed profile using a Neighbourhood Algorithm method. The resulting set of 1D models are then interpolated to produce the final 3D wavespeed model. The group velocity maps show the strong influence of thick sediments at shorter periods, and distinct fast zones associated with cratonic regions. Below the sediments the 3D shear wavespeed model displays significant heterogeneity with only moderate correlation with surface tectonic features. For example, there is no evident expression of the Tasman Line marking the eastern edge of Precambrian outcrop. The large number of available inter-station paths extracted from the ambient noise analysis provide detailed shear wavespeed information for crustal structure across the Australian continent for the first time, including regions where there was no prior sampling because of difficult logistics.

  17. Shallow and deep crustal velocity models of Northeast Tibet

    Science.gov (United States)

    Karplus, M.; Klemperer, S. L.; Mechie, J.; Shi, D.; Zhao, W.; Brown, L. D.; Wu, Z.

    2009-12-01

    The INDEPTH IV seismic profile in Northeast Tibet is the highest resolution wide-angle refraction experiment imaging the Qaidam Basin, North Kunlun Thrusts (NKT), Kunlun Mountains, North and South Kunlun Faults (NKT, SKT), and Songpan-Ganzi terrane (SG). First arrival refraction modeling using ray tracing and least squares inversion has yielded a crustal p-wave velocity model, best resolved for the top 20 km. Ray tracing of deeper reflections shows considerable differences between the Qaidam Basin and the SG, in agreement with previous studies of those areas. The Moho ranges from about 52 km beneath the Qaidam Basin to 63 km with a slight northward dip beneath the SG. The 11-km change must occur between the SKF and the southern edge of the Qaidam Basin, just north of the NKT, allowing the possibility of a Moho step across the NKT. The Qaidam Basin velocity-versus-depth profile is more similar to the global average than the SG profile, which bears resemblance to previously determined “Tibet-type” velocity profiles with mid to lower crustal velocities of 6.5 to 7.0 km/s appearing at greater depths. The highest resolution portion of the profile (100-m instrument spacing) features two distinct, apparently south-dipping low-velocity zones reaching about 2-3 km depth that we infer to be the locations of the NKF and SKF. A strong reflector at 35 km, located entirely south of the SKF and truncated just south of it, may be cut by a steeply south-dipping SKF. Elevated velocities at depth beneath the surface location of the NKF may indicate the south-dipping NKF meets the SKF between depths of 5 and 10 km. Undulating regions of high and low velocity extending about 1-2 km in depth near the southern border of the Qaidam Basin likely represent north-verging thrust sheets of the NKT.

  18. Electrical imaging of deep crustal features of Kutch, India

    Science.gov (United States)

    Sastry, R. S.; Nagarajan, Nandini; Sarma, S. V. S.

    2008-03-01

    A regional Magnetotelluric (MT) study, was carried out with 55 MT soundings, distributed along five traverses, across the Kutch Mainland Unit (KMU), on the west coast of India, a region characterized by a series of successive uplifts and intervening depressions in the form of half graben, bounded by master faults. We obtain the deeper electrical structure of the crust beneath Kutch, from 2-D modelling of MT data along the 5 traverses, in order to evaluate the geo-electrical signatures, if any, of the known primary tectonic structures in this region. The results show that the deeper electrical structure in the Kutch region presents a mosaic of high resistive crustal blocks separated by deep-rooted conductive features. Two such crustal conductive features spatially correlate with the known tectonic features, viz., the Kutch Mainland Fault (KMF), and the Katrol Hill Fault (KHF). An impressive feature of the geo-electrical sections is an additional, well-defined conductive feature, running between Jakhau and Mundra, located at the southern end of each of the five MT traverses and interpreted to be the electrical signature of yet another hidden fault at the southern margin of the KMU. This new feature is named as Jakhau-Mundra Fault (JMF). It is inferred that the presence of JMF together with the Kathiawar Fault (NKF), further south, located at the northern boundary of the Saurashtra Horst, would enhance the possibility of occurrence of a thick sedimentary column in the Gulf of Kutch. The region between the newly delineated fault (JMF) and the Kathiawar fault (NKF) could thus be significant for Hydrocarbon Exploration.

  19. Chemical cycles and health risks of some crustal nuclides

    International Nuclear Information System (INIS)

    McKone, T.E.

    1981-01-01

    This dissertation describes and utilizes an approach for assessing long term health risks due to dispersion of naturally occurring radionuclide series and chemical toxins by normal and altered landscape chemical cycles. In particular, the health risks resulting from geochemical mobilizations of arsenic, lead, uranium and radium are considered. Based on a review of toxic waste hazard-measures and risk assessment studies, a general expression is developed for quantifying health risks imposed by the introduction of toxic materials to components of the total environment. This general measure deals with long term interactions within and between the internal human environment and the external biogeochemical environment. Health hazards are expressed as dose factors which convert environmental concentrations into a corresponding dose field (organ doses in rad for radionuclides; daily intake for toxic elements). The dose field is translated into population health risk expressed as lifetime cancer risk for carcinogens and average blood levels for other toxins. The landscape cell (or prism) is presented as a tool for visualizing and mapping toxic material cycles near the crustal surface. The overall process is incorporated in the GEOTOX code which is a geochemical systems model for describing the dynamics of crustal toxins within a landscape and the resulting health risks. GEOTOX is used to investigate the response of regional landscapes to increased soil and rock inventories of 238 U, 226 Ra, arsenic and lead. It is found that each decay series of element imposes a hazard by its behavior in the total environment that can not be quantified by a similar measure of toxicity

  20. Variation in the crustal structure across central Iceland

    Science.gov (United States)

    Du, Zhijun; Foulger, G. R.

    2001-04-01

    We determine the crustal structures beneath 12 broad-band seismic stations deployed in a swath across central Iceland along and around the ICEMELT explosion seismic profile by combining teleseismic receiver functions, surface wave dispersion curves and the waveforms of a large, local event in Iceland. By using teleseisms that approach from different backazimuths, we study lateral structural variability out of the line of the ICEMELT profile. Beneath Tertiary areas, the thickness of the upper crust, as defined by the 6.5kms-1 velocity horizon, is ~8km and the depth to the base of the lower crust, as defined by the 7.2kms-1 velocity horizon, is ~29-32km. Beneath the currently active rift zone the upper crust thins to ~6.0km and the depth to the base of the lower crust increases to ~35-40km. A substantial low-velocity zone underlies the Middle Volcanic Zone in the lower crust, which may indicate anomalously high geothermal gradients there. This suggests that the large-scale thermal centre of the hotspot may be more westerly than northwest Vatnajokull, where it is generally assumed to lie. Simplified description of the results notwithstanding, there is substantial variability in the overall style of crustal structure throughout Iceland, and a clear, tripartite division into upper and lower crusts and a sharp Moho is poorly supported by many of our results. The nature, distinctiveness and continuity of the Moho is variable and in many areas the crust-mantle transition is a zone with enhanced velocity gradients several kilometres thick.

  1. The Evolution of Eastern Himalayan Syntaxis of Tibetan Plateau

    Science.gov (United States)

    Zhang, S.; Wu, T.; Li, M.; Zhang, Y.; Hua, Y.; Zhang, B.

    2017-12-01

    Indian plate has been colliding with Eurasian plate since 50Ma years ago, resulting in the Tethys extinction, crust shortening and Tibetan plateau uplift. But it is still a debate how the Tibetan Plateau material escaped. This study tries to invert the distributions of dispersion phase velocity and anisotropy in Eastern Himalayan Syntaxis (EHS) based on the seismic data. We focused on the seven sub-blocks around EHS region. Sub-block "EHS" represents EHS corner with high velocity anomalies, significantly compressed in the axle and strike directions. Sub-blocks "LSD", "QTB" and "SP-GZB" are located at its northern areas with compressions also, and connected with low-velocity anomalies in both crustal and upper mantle rocks. Sub-block "ICB" is located at its southern area with low velocity anomaly, and connected with Tengchong volcano. Sub-blocks "SYDB" and "YZB" are located at its eastern areas with high velocity anomalies in both crustal and upper mantle rocks. Our results demonstrated that significant azimuthal anisotropy of crust (t£30s) and upper mantle (30s£t£60s). Crustal anisotropy indicates the orogenic belt matched well with the direction of fast propagation, and upper mantle anisotropy represents the lattic-preferred orientation (LPO) of mantle minerals (e.g. olivine and basalt), indicating the features of subducting Indian plate. Besides, Red River fault is a dextral strike fault, controlling the crustal and mantle migration. There is a narrow zone to be the channel flow of Tibetan crustal materials escaping toward Yunnan area. The evolution of EHS seems constrained by gravity isostatic mechanism. Keywords: Tibetan Plateau; Eastern Himalayan Syntaxis; Red River fault; crustal flow; surface wave; anisotropy

  2. Density heterogeneity of the North American upper mantle from satellite gravity and a regional crustal model

    DEFF Research Database (Denmark)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans

    2014-01-01

    -density conversion and (ii) uncertainties in knowledge of the crustal structure (thickness and average Vp velocities of individual crustal layers, including the sedimentary cover). In this study, we address both sources of possible uncertainties by applying different conversions from velocity to density...... and by introducing variations into the crustal structure which corresponds to the uncertainty of its resolution by highquality and low-quality seismic models. We examine the propagation of these uncertainties into determinations of lithospheric mantle density. Given a relatively small range of expected density...

  3. Regional Crustal Deformation and Lithosphere Thickness Observed with Geodetic Techniques

    Science.gov (United States)

    Vermeer, M.; Poutanen, M.; Kollo, K.; Koivula, H.; Ahola, J.

    2009-04-01

    The solid Earth, including the lithosphere, interacts in many ways with other components of the Earth system, oceans, atmosphere and climate. Geodesy is a key provider of data needed for global and environmental research. Geodesy provides methods and accurate measurements of contemporary deformation, sea level and gravity change. The importance of the decades-long stability and availability of reference frames must be stressed for such studies. In the future, the need to accurately monitor 3-D crustal motions will grow, both together with increasingly precise GNSS (Global Navigation Satellite System) positioning, demands for better follow-up of global change, and local needs for crustal motions, especially in coastal areas. These demands cannot yet be satisfied. The project described here is a part of a larger entity: Upper Mantle Dynamics and Quaternary Climate in Cratonic Areas, DynaQlim, an International Lithosphere Project (ILP) -sponsored initiative. The aims of DynaQlim are to understand the relations between upper mantle dynamics, mantle composition, physical properties, temperature and rheology, to study the postglacial uplift and ice thickness models, sea level change and isostatic response, Quaternary climate variations and Weichselian (Laurentian and other) glaciations during the late Quaternary. We aim at studying various aspects of lithospheric motion within the Finnish and Fennoscandian area, but within a global perspective, by the newest geodetic techniques in a multidisciplinary setting. The studies involve observations of three-dimensional motions and gravity change in a multidisciplinary context on a range of spatial scales: the whole of Fennoscandia, Finland, a regional test area of Satakunta, and the local test site Olkiluoto. Objectives of the research include improving our insight into the 3-D motion of a thick lithosphere, and into the gravity effect of the uplift, using novel approaches; improving the kinematic 3-D models in the

  4. A numerical model for dynamic crustal-scale fluid flow

    Science.gov (United States)

    Sachau, Till; Bons, Paul; Gomez-Rivas, Enrique; Koehn, Daniel

    2015-04-01

    Fluid flow in the crust is often envisaged and modeled as continuous, yet minimal flow, which occurs over large geological times. This is a suitable approximation for flow as long as it is solely controlled by the matrix permeability of rocks, which in turn is controlled by viscous compaction of the pore space. However, strong evidence (hydrothermal veins and ore deposits) exists that a significant part of fluid flow in the crust occurs strongly localized in both space and time, controlled by the opening and sealing of hydrofractures. We developed, tested and applied a novel computer code, which considers this dynamic behavior and couples it with steady, Darcian flow controlled by the matrix permeability. In this dual-porosity model, fractures open depending on the fluid pressure relative to the solid pressure. Fractures form when matrix permeability is insufficient to accommodate fluid flow resulting from compaction, decompression (Staude et al. 2009) or metamorphic dehydration reactions (Weisheit et al. 2013). Open fractures can close when the contained fluid either seeps into the matrix or escapes by fracture propagation: mobile hydrofractures (Bons, 2001). In the model, closing and sealing of fractures is controlled by a time-dependent viscous law, which is based on the effective stress and on either Newtonian or non-Newtonian viscosity. Our simulations indicate that the bulk of crustal fluid flow in the middle to lower upper crust is intermittent, highly self-organized, and occurs as mobile hydrofractures. This is due to the low matrix porosity and permeability, combined with a low matrix viscosity and, hence, fast sealing of fractures. Stable fracture networks, generated by fluid overpressure, are restricted to the uppermost crust. Semi-stable fracture networks can develop in an intermediate zone, if a critical overpressure is reached. Flow rates in mobile hydrofractures exceed those in the matrix porosity and fracture networks by orders of magnitude

  5. Seismically constrained two-dimensional crustal thermal structure of ...

    Indian Academy of Sciences (India)

    oil bearing sedimentary basins of India is situated at ... related to the evolution of the western margin of India ... of approximately 46 mW/m2 for stable continental areas. This makes the region interesting from ... for hydrocarbon accumulation.

  6. Crustal volumes of the continents and of oceanic and continental submarine plateaus

    Science.gov (United States)

    Schubert, G.; Sandwell, D.

    1989-01-01

    Using global topographic data and the assumption of Airy isostasy, it is estimated that the crustal volume of the continents is 7182 X 10 to the 6th cu km. The crustal volumes of the oceanic and continental submarine plateaus are calculated at 369 X 10 to the 6th cu km and 242 X 10 to the 6th cu km, respectively. The total continental crustal volume is found to be 7581 X 10 to the 6th cu km, 3.2 percent of which is comprised of continental submarine plateaus on the seafloor. An upper bound on the contintental crust addition rate by the accretion of oceanic plateaus is set at 3.7 cu km/yr. Subduction of continental submarine plateaus with the oceanic lithosphere on a 100 Myr time scale yields an upper bound to the continental crustal subtraction rate of 2.4 cu km/yr.

  7. Crustal seismicity and active fault system in the SE of Romania

    International Nuclear Information System (INIS)

    Raileanu, V; Bala, A.; Radulian, M.; Popescu, E.; Mateciuc, D.; Popa, M.; Dinu, C.; Diaconescu, V.

    2007-01-01

    Romania is known as a country with a high seismicity located in the Vrancea region where 2-3 strong intermediate depth earthquakes/century generate great damages and casualties. A moderate crustal seismicity is also observed in other zones of the country, with events having a moderate magnitude but sometimes with important economic and social effects on the locale scale. The crustal seismogenic zones are located in front of the Eastern Carpathian Bend, South Carpathians, Dobrogea, Banat, Crisana and Maramures regions. The SE part of Romania comprises some of the most active crustal seismic sources that generated earthquakes up to Mw=6.5 concentrated in more zones, namely: Vrancea crustal domain, E Vrancea zone that is overlapped on the Focsani basin, Barlad and Predobrogean depressions along with the North Dobrogea Orogen, Intramoesian and Shabla (Bulgaria) zones and Fagaras-Campulung-Sinaia zone. (authors)

  8. 2-D Crustal thermal structure along Thuadara–Sindad DSS profile ...

    Indian Academy of Sciences (India)

    Thuadara–Sindad Deep Seismic Sounding (DSS) profile which runs almost in the N–S direction ... These studies include four Deep Seis- ... Geology and tectonic frame work ..... alous high-velocity layer at shallow crustal depths in the.

  9. Crustal structure and tectonics of the Ninetyeast Ridge from seismic and gravity studies

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Neprochnov, Y.P.; Rao, D.G.; Grinko, B.N.

    Seismic reflection and refraction, gravity, and bathymetric data across and along the central part of the Ninetyeast Ridge were analyzed to determine the crustal structure of the ridge and to understand its tectonics. The ridge in the study area...

  10. The Chaotic Terrains of Mercury: A History of Large-Scale Crustal Devolatilization

    Science.gov (United States)

    Rodriguez, J. A. P.; Domingue, D. L.; Berman, D. C.; Kargel, J. S.; Baker, V. R.; Teodoro, L. F.; Banks, M.; Leonard, G.

    2018-05-01

    Approximately 400 million years after the Caloris basin impact, extensive collapse formed Mercury's chaotic terrains. Collapse likely resulted from regionally elevated heat flow devolatilizing crustal materials along NE and NW extensional faults.

  11. Further Mapping of Mercury's Crustal Magnetic Field Using MESSENGER Magnetometer Data

    Science.gov (United States)

    Hood, L. L.; Oliveira, J. S.; Spudis, P. D.; Galluzzi, V.

    2018-05-01

    Further mapping of Mercury's crustal magnetic field shows that anomalies are associated with some impact craters but not others. Differences in impactor composition (e.g., iron content) may be indicated by this new observation.

  12. Using crustal thickness and subsidence history on the Iberia-Newfoundland margins to constrain lithosphere deformation modes during continental breakup

    Science.gov (United States)

    Jeanniot, Ludovic; Kusznir, Nick; Manatschal, Gianreto; Mohn, Geoffroy

    2014-05-01

    contribution of buoyancy driven upwelling and their spatial and temporal evolution including lateral migration are determined by using a series of numerical experiments, tested and calibrated against observations of crustal thicknesses and water-loaded subsidence. Pure-shear widths exert a strong control on the timing of crustal rupture and melt initiation; to satisfy OCT architecture, subsidence and mantle exhumation, we need to focus the deformation from a broad to a narrow region. The lateral migration of the deformation flow axis has an important control on the rupture of continental crust and lithosphere, melt initiation, their relative timing, the resulting OCT architecture and conjugate margin asymmetry. The numerical models are used to predict margin isostatic response and subsidence history.

  13. A preliminary investigation of vertical crustal movements in the United Kingdom in the context of subsurface nuclear waste isolation for the Department of the Environment

    International Nuclear Information System (INIS)

    1981-03-01

    The report falls under the headings: vertical crustal movements - a manifestation of intraplate geological processes (reports of vertical crustal movements observed by geodetic levelling in intraplate environments; vertical crustal movements and the geological record; possible causes of vertical crustal movements); an investigation of recent crustal movements in a test area of the United Kingdom by comparison of geodetic levelling records; vertical crustal movements and the isolation of nuclear waste in intraplate geological systems (conventional methods of site appraisal - a perspective of geological hazard assessment; the role of vertical crustal movements as a tool for rationalisation of hazard assessment, site selection and the assessment of future geological change); research options. (U.K.)

  14. Seismic cycle feedbacks in a mid-crustal shear zone

    Science.gov (United States)

    Melosh, Benjamin L.; Rowe, Christie D.; Gerbi, Christopher; Smit, Louis; Macey, Paul

    2018-07-01

    Mid-crustal fault rheology is controlled by alternating brittle and plastic deformation mechanisms, which cause feedback cycles that influence earthquake behavior. Detailed mapping and microstructural observations in the Pofadder Shear Zone (Namibia and South Africa) reveal a lithologically heterogeneous shear zone core with quartz-rich mylonites and ultramylonites, plastically overprinted pseudotachylyte and active shear folds. We present evidence for a positive feedback cycle in which coseismic grain size reduction facilitates active shear folding by enhancing competency contrasts and promoting crystal plastic flow. Shear folding strengthens a portion of a shear zone by limb rotation, focusing deformation and promoting plastic flow or brittle slip in resulting areas of localized high stress. Using quartz paleopiezometry, we estimate strain and slip rates consistent with other studies of exhumed shear zones and modern plate boundary faults, helping establish the Pofadder Shear Zone as an ancient analogue to modern, continental-scale, strike-slip faults. This feedback cycle influences seismicity patterns at the scale of study (10s of meters) and possibly larger scales as well, and contributes to bulk strengthening of the brittle-plastic transition on modern plate boundary faults.

  15. Magmatism and crustal extension: Constraining activation of the ductile shearing along the Gediz detachment, Menderes Massif (western Turkey)

    Science.gov (United States)

    Rossetti, Federico; Asti, Riccardo; Faccenna, Claudio; Gerdes, Axel; Lucci, Federico; Theye, Thomas

    2017-06-01

    The Menderes Massif of western Turkey is a key area to study feedback relationships between magma generation/emplacement and activation of extensional detachment tectonics. Here, we present new textural analysis and in situ U-(Th)-Pb titanite dating from selected samples collected in the transition from the undeformed to the mylonitized zones of the Salihli granodiorite at the footwall of the Neogene, ductile-to-brittle, top-to-the-NNE Gediz-Alaşheir (GDF) detachment fault. Ductile shearing was accompanied by the fluid-mediated sub-solidus transformation of the granodiorite to orthogneiss, which occurred at shallower crustal levels and temperatures compatible with the upper greenschist-to-amphibolite facies metamorphic conditions (530-580 °C and P system ages during fluid-assisted syn-tectonic re-crystallisation in the transition from magma crystallization and emplacement (at 16-17 Ma) to the syn-tectonic, solid-state shearing (at 14-15 Ma). A minimum time lapse of ca. 1-2 Ma is then inferred between the crustal emplacement of the Salihli granodiorite and nucleation of the ductile extensional shearing along the Gediz detachment. The reconstruction of the cooling history of the Salihli granodiorite documents a punctuated evolution dominated by two episodes of rapid cooling, between 14 Ma and 12 Ma ( 100 °C/Ma) and between 3 and 2 Ma ( 105 °C/Ma). We relate the first episode to nucleation and development of post-emplacement of ductile shearing along the GDF and the second to brittle high-angle faulting, respectively. Our dataset suggests that in the Menderes Massif the activation of ductile extension was a consequence, rather than the cause, of magma emplacement in the extending crust.

  16. Incremental growth of an upper crustal, A-type pluton, Argentina: Evidence of a re-used magma pathway

    Science.gov (United States)

    Alasino, Pablo H.; Larrovere, Mariano A.; Rocher, Sebastián; Dahlquist, Juan A.; Basei, Miguel A. S.; Memeti, Valbone; Paterson, Scott; Galindo, Carmen; Macchioli Grande, Marcos; da Costa Campos Neto, Mario

    2017-07-01

    Carboniferous igneous activity in the Sierra de Velasco (NW Argentina) led to the emplacement of several magmas bodies at shallow levels (relationships) intrusive units are: (1) the Asha unit (340 ± 7 Ma): a tabular to funnel-shaped intrusion emplaced during a regional strain field dominated by WSW-ENE shortening with contacts discordant to regional host-rock structures; (2) the San Blas unit (344 ± 2 Ma): an approximate cylindrical-shaped intrusion formed by multiple batches of magmas, with a roughly concentric fabric pattern and displacement of the host rock by ductile flow of about 35% of shortening; and (3) the Hualco unit (346 ± 6 Ma): a small body with a possible mushroom geometry and contacts concordant to regional host-rock structures. The magma pulses making up these units define two groups of A-type granitoids. The first group includes the peraluminous granitic rocks of the Asha unit generated mostly by crustal sources (εNdt = - 5.8 and εHft in zircon = - 2.9 to - 4.5). The second group comprises the metaluminous to peraluminous granitic rocks of the youngest units (San Blas and Hualco), which were formed by a heterogeneous mixture between mantle and crustal sources (εNdt = + 0.6 to - 4.8 and εHft in zircon = + 3 to - 6). Our results provide a comprehensive view of the evolution of an intrusive complex formed from multiple non-consanguineous magma intrusions that utilized the same magmatic plumbing system during downward transfer of host materials. As the plutonic system matures, the ascent of magmas is governed by the visco-elastic flow of host rock that for younger batches include older hot magma mush. The latter results in ductile downward flow of older, during rise of younger magma. Such complexes may reflect the plutonic portion of volcanic centers where chemically distinct magmas are erupted.

  17. Seismological Constraints on Lithospheric Evolution in the Appalachian Orogen

    Science.gov (United States)

    Fischer, K. M.; Hopper, E.; Hawman, R. B.; Wagner, L. S.

    2017-12-01

    Crust and mantle structures beneath the Appalachian orogen, recently resolved by seismic data from the EarthScope SESAME Flexible Array and Transportable Array, provide new constraints on the scale and style of the Appalachian collision and subsequent lithospheric evolution. In the southern Appalachians, imaging with Sp and Ps phases reveals the final (Alleghanian) suture between the crusts of Laurentia and the Gondwanan Suwannee terrane as a low angle (Kellogg, 2017) isostatic arguments indicate crustal thicknesses were 15-25 km larger at the end of the orogeny, indicating a thick crustal root across the region. The present-day residual crustal root beneath the Blue Ridge mountains is estimated to have a density contrast with the mantle of only 104±20 kg/m3. This value is comparable to other old orogens but lower than values typical of young or active orogens, indicating a loss of lower crustal buoyancy over time. At mantle depths, the negative shear velocity gradient that marks the transition from lithosphere to asthenosphere, as illuminated by Sp phases, varies across the Appalachian orogen. This boundary is shallow beneath the northeastern U.S. and in the zone of Eocene volcanism in Virginia, where low velocity anomalies occur in the upper mantle. These correlations suggest recent active lithosphere-asthenosphere interaction.

  18. Rb/Sr geochronology in whole rocks and minerals of the Cumaru granodiorite, Serra dos Gradaus, Para state, Brazil

    International Nuclear Information System (INIS)

    Lafon, J.M.; Scheller, T.; Pereira, E.D.; Macambira, J.B.

    1990-01-01

    The Cumaru granodiorite occurs in the Serra dos Gradaus region, southeastern part of the Metallogenic Province of Carajas, Para. Rb-Sr systematics have been provided in whole rocks and minerals for samples of the Cumaru granodiorite thus an age of 2543 ± 53 Ma, with an initial isotopic ratio of 0.70311 ± 34 (MSWD+1.87) was obtained for whole rocks samples. Taking in account that these rocks are not affected by metamorphism and/or deformation, we consider the age of 2543 ± 53 Ma as an emplacement age corresponding to the crystallization of the body. Such an age confirms the existence of a late Archaean plutonic event in the Serra dos Gradaus area and the interpretation of the Cumaru granodiorite as a contemporaneous and cogenetic body of the Juruena type granites (Ca. 2000 Ma old), as proposed previously, must be definitively abandoned. Therefore, Archaean ages for the greenstone belt sequence (Gradaus group) as well as for the Xingu complex in this area are also confirmed, although by indirect evidence. The age obtained implies that the latter represents an Archaean metamorphic basement in the Serra dos Gradaus region rather than the reworking of the late archaean granitics rocks during the Transmazonian orogenic event. The initial isotopic ratio of 0.70311 ± 34 is close to a mantellic or low time of crustal residence source material ratios at the end of Archaean times. Therefore, comparison with isotopic initial ratios of other granitic rocks which occur in the Rio Maria region identifies an evolution line with a Rb-Sr ratio of 0.25 for a crustal source material that would have separated from mantle about 2.8 Ga ago. (author)

  19. Tracing crustal contamination along the Java segment of the Sunda Arc, Indonesia

    Science.gov (United States)

    Jolis, E. M.; Troll, V.; Deegan, F.; Blythe, L.; Harris, C.; Freda, C.; Hilton, D.; Chadwick, J.; Van Helden, M.

    2012-04-01

    Arc magmas typically display chemical and petrographic characteristics indicative of crustal input. Crustal contamination can take place either in the mantle source region or as magma traverses the upper crust (e.g. [1]). While source contamination is generally considered the dominant process (e.g. [2]), late-stage crustal contamination has been recognised at volcanic arcs too (e.g. [3]). In light of this, we aim to test the extent of upper crustal versus source contamination along the Java segment of the Sunda arc, which, due its variable upper crustal structure, is an exemplary natural laboratory. We present a detailed geochemical study of 7 volcanoes along a traverse from Anak-Krakatau in the Sunda strait through Java and Bali, to characterise the impact of the overlying crust on arc magma composition. Using rock and mineral elemental geochemistry, radiogenic (Sr, Nd and Pb) and, stable (O) isotopes, we show a correlation between upper crustal composition and the degree of upper crustal contamination. We find an increase in 87Sr/86Sr and δ18O values, and a decrease in 143Nd/144Nd values from Krakatau towards Merapi, indicating substantial crustal input from the thick continental basement present. Volcanoes to the east of Merapi and the Progo-Muria fault transition zone, where the upper crust is thinner, in turn, show considerably less crustal input in their isotopic signatures, indicating a stronger influence of the mantle source. Our new data represent a systematic and high-resolution arc-wide sampling effort that allows us to distinguish the effects of the upper crust on the compositional spectrum of individual volcanic systems along the Sunda arc. [1] Davidson, J.P, Hora, J.M, Garrison, J.M & Dungan, M.A 2005. Crustal Forensics in Arc Magmas. J. Geotherm. Res. 140, 157-170; [2] Debaille, V., Doucelance, R., Weis, D., & Schiano, P. 2005. Geochim. Cosmochim. Acta, 70,723-741; [3] Gasparon, M., Hilton, D.R., & Varne, R. 1994. Earth Planet. Sci. Lett., 126, 15-22.

  20. Schumpeter's Evolution

    DEFF Research Database (Denmark)

    Andersen, Esben Sloth

    reworking of his basic theory of economic evolution in Development from 1934, and this reworking was continued in Cycles from 1939. Here Schumpeter also tried to handle the statistical and historical evidence on the waveform evolution of the capitalist economy. Capitalism from 1942 modified the model...

  1. Galactic evolution

    International Nuclear Information System (INIS)

    Pagel, B.

    1979-01-01

    Ideas are considered concerning the evolution of galaxies which are closely related to those of stellar evolution and the origin of elements. Using information obtained from stellar spectra, astronomers are now able to consider an underlying process to explain the distribution of various elements in the stars, gas and dust clouds of the galaxies. (U.K.)

  2. Darwinian evolution

    NARCIS (Netherlands)

    Jagers op Akkerhuis, Gerard A.J.M.; Spijkerboer, Hendrik Pieter; Koelewijn, Hans Peter

    2016-01-01

    Darwinian evolution is a central tenet in biology. Conventionally, the defi nition of Darwinian evolution is linked to a population-based process that can be measured by focusing on changes in DNA/allele frequencies. However, in some publications it has been suggested that selection represents a

  3. Crustal deformation and volcanism at active plate boundaries

    Science.gov (United States)

    Geirsson, Halldor

    Most of Earth's volcanoes are located near active tectonic plate boundaries, where the tectonic plates move relative to each other resulting in deformation. Likewise, subsurface magma movement and pressure changes in magmatic systems can cause measurable deformation of the Earth's surface. The study of the shape of Earth and therefore studies of surface deformation is called geodesy. Modern geodetic techniques allow precise measurements (˜1 mm accuracy) of deformation of tectonic and magmatic systems. Because of the spatial correlation between tectonic boundaries and volcanism, the tectonic and volcanic deformation signals can become intertwined. Thus it is often important to study both tectonic and volcanic deformation processes simultaneously, when one is trying to study one of the systems individually. In this thesis, I present research on crustal deformation and magmatic processes at active plate boundaries. The study areas cover divergent and transform plate boundaries in south Iceland and convergent and transform plate boundaries in Central America, specifically Nicaragua and El Salvador. The study is composed of four main chapters: two of the chapters focus on the magma plumbing system of Hekla volcano, Iceland and the plate boundary in south Iceland; one chapter focuses on shallow controls of explosive volcanism at Telica volcano, Nicaragua; and the fourth chapter focuses on co- and post-seismic deformation from a Mw = 7.3 earthquake which occurred offshore El Salvador in 2012. Hekla volcano is located at the intersection of a transform zone and a rift zone in Iceland and thus is affected by a combination of shear and extensional strains, in addition to co-seismic and co-rifting deformation. The inter-eruptive deformation signal from Hekla is subtle, as observed by a decade (2000-2010) of GPS data in south Iceland. A simultaneous inversion of this data for parameters describing the geometry and source characteristics of the magma chamber at Hekla, and

  4. Data base and seismicity studies for Fagaras, Romania crustal earthquakes

    International Nuclear Information System (INIS)

    Moldovan, I.-A.; Enescu, B. D.; Pantea, A.; Constantin, A.; Bazacliu, O.; Malita, Z.; Moldoveanu, T.

    2002-01-01

    Besides the major impact of the Vrancea seismic region, one of the most important intermediate earthquake sources of Europe, the Romanian crustal earthquake sources, from Fagaras, Banat, Crisana, Bucovina or Dobrogea regions, have to be taken into consideration for seismicity studies or seismic hazard assessment. To determine the characteristics of the seismicity for Fagaras seismogenic region, a revised and updated catalogue of the Romanian earthquakes, recently compiled by Oncescu et al. (1999) is used. The catalogue contains 471 tectonic earthquakes and 338 induced earthquakes and is homogenous starting with 1471 for I>VIII and for I>VII starting with 1801. The catalogue is complete for magnitudes larger than 3 starting with 1982. In the studied zone only normal earthquakes occur, related to intracrustal fractures situated from 5 to 30 km depth. Most of them are of low energy, but once in a century a large destructive event occurs with epicentral intensity larger than VIII. The maximum expected magnitude is M GR = 6.5 and the epicenter distribution outlines significant clustering in the zones and on the lines mentioned in the tectonic studies. Taking into account the date of the last major earthquake (1916) and the return periods of severe damaging shocks of over 85 years it is to be expected very soon a large shock in the area. That's why a seismicity and hazard study for this zone is necessary. In the paper there are studied the b parameter variation (the mean value is 0.69), the activity value, the return periods, and seismicity maps and different histograms are plotted. At the same time there are excluded from the catalogue the explosions due to Campulung quarry. Because the catalogue contains the aftershocks for the 1916 earthquake for the seismicity studies we have excluded these shocks. (authors)

  5. Shallow Crustal Thermal Structures of Central Taiwan Foothills Region

    Directory of Open Access Journals (Sweden)

    Shao-Kai Wu

    2013-01-01

    Full Text Available Crustal thermal structures are closely related to metamorphism, rock rheology, exhumation processes, hydrocarbon maturation levels, frictional faulting and other processes. Drilling is the most direct way to access the temperature fields in the shallow crust. However, a regional drilling program for geological investigation is usually very expensive. Recently, a large-scale in-situ investigation program in the Western Foothills of Central Taiwan was carried out, providing a rare opportunity to conduct heat flow measurements in this region where there are debates as to whether previous measured heat flows are representative of the thermal state in this region. We successfully collected 28 geothermal gradients from these wells and converted them into heat flows. The new heat flow dataset is consistent with previous heat flows, which shows that the thermal structures of Central Taiwan are different from that of other subduction accretionary prisms. We then combine all the available heat flow information to analyze the frictional parameters of the Chelungpu fault zone that ruptured during the 1999, Chi-Chi, Taiwan, earthquake. The heat flow dataset gave consistent results compared with the frictional parameters derived from another independent study that used cores recovered from the Chelungpu fault zone at depth. This study also shows that it is suitable for using heat-flow data obtained from shallow subsurface to constrain thrusting faulting parameters, similar to what had been done for the strike-slip San Andreas Fault in California. Additional fieldworks are planned to study heat flows in other mountainous regions of Taiwan for more advanced geodynamic modeling efforts.

  6. Solitary Waves of Ice Loss Detected in Greenland Crustal Motion

    Science.gov (United States)

    Adhikari, S.; Ivins, E. R.; Larour, E. Y.

    2017-12-01

    The annual cycle and secular trend of Greenland mass loading are well recorded in measurements of solid Earth deformation. While bedrock vertical displacements are in phase with loading as inferred from space observations, horizontal motions have received almost no attention. The horizontal bedrock displacements can potentially track the spatiotemporal detail of mass changes with great fidelity. Our analysis of Greenland crustal motion data reveals that a significant excitation of horizontal amplitudes occurs during the intense Greenland melting. A suite of space geodetic observations and climate reanalysis data cannot explain these large horizontal displacements. We discover that solitary seasonal waves of substantial mass transport traveled through Rink Glacier in 2010 and 2012. We deduce that intense summer melting enhanced either basal lubrication or shear softening, or both, causing the glacier to thin dynamically. The newly routed upstream sublglacial water was likely to be both retarded and inefficient, thus providing a causal mechanism for the prolonged ice transport to continue well into the winter months. As the climate continues to produce increasingly warmer spring and summer, amplified seasonal waves of mass transport may become ever more present in years of future observations. Increased frequency of amplified seasonal mass transport may ultimately strengthen the Greenland's dynamic ice mass loss, a component of the balance that will have important ramifications for sea level rise. This animation shows a solitary wave passing through Rink Glacier, Greenland, in 2012, recorded by the motion of a GPS station (circle with arrow). Darker blue colors within the flow indicate mass loss, red colors show mass gain. The star marks the center of the wave. Credit: NASA/JPL-Caltech

  7. Mechanism of crustal extension in the Laxmi Basin, Arabian Sea

    Directory of Open Access Journals (Sweden)

    Anju Pandey

    2015-11-01

    Full Text Available Continental rifting and magmatism has been extensively studied worldwide as it is believed that continental rifting, break up of continents and associated magmatism lead to genesis of new oceanic crust. However, various regions of the world show that these processes may lead to genesis of other types of crust than the oceanic crust. Laxmi Basin in the western continental margin of the India is one such region with an enigmatic crust. Due to its extreme strategic significance for the palaeogeographic reconstruction of continents during Cretaceous continental breakup of India, this basin has attracted various workers for more than two decades. However, still the issue of nature of crust in the basin remains controversial. In this contribution, in order to identify nature of crust, mechanism of continental extension in the Laxmi Basin has been studied for the first time through newly acquired seismic data from the basin. Here, we propose a plausible mechanism of crustal extension in the Laxmi Basin which eventually constrains the nature of crust of the Laxmi Basin. We have demonstrated that the crust in the Laxmi Basin can be categorised in two zones of stretched and transitional crust. In the stretched zone several fault bounded horst and graben structures are identified which preserve syn- and post-rift sediments along with different periods of hiatus in sedimentations as unconformities. These faults are identified as listric faults in the upper crust which sole out in the detachment faults. Detachment faults decouples the upper brittle and lower ductile crust. The transitional crust is identified as heavily intruded by sills and basaltic volcanic which were emplaced due to melting of subcontinental mantle (SCM after hyper-stretching of crust and serpentinisation of the SCM. Panikkar Ridge is proposed to be one such basaltic volcanic body derived from melting of lower part of the SCM.

  8. Crustal anisotropy across northern Japan from receiver functions.

    Science.gov (United States)

    Bianchi, I; Bokelmann, G; Shiomi, K

    2015-07-01

    Northern Japan is a tectonically active area, with the presence of several volcanoes, and with frequent earthquakes among which the destructive M w  = 8.9-9.0 Tohoku-oki occurred on 11 March 2011. Tectonic activity leaves an imprint on the crustal structures, on both the upper and the lower layers. To investigate the crust in northern Japan, we construct a receiver function data set using teleseismic events recorded at 58 seismic stations belonging to the Japanese National (Hi-net) network. We isolate the signals, in the receiver function wavelet, that witness the presence of anisotropic structures at depth, with the aim of mapping the variation of anisotropy across the northern part of the island. This study focuses on the relation among anisotropy detected in the crust, stresses induced by plate convergence across the subduction zone, and the intrinsic characteristics of the rocks. Our results show how a simple velocity model with two anisotropic layers reproduces the observed data at the stations. We observe a negligible or small amount of signal related to anisotropy in the eastern part of the study area (i.e., the outer arc) for both upper and lower crust. Distinct anisotropic features are observed at the stations on the western part of the study area (i.e., the inner arc) for both upper and lower crust. The symmetry axes are mostly E-W oriented. Deviation from the E-W orientation is observed close to the volcanic areas, where the higher geothermal gradient might influence the deformation processes.

  9. Fragmentation of wall rock garnets during deep crustal earthquakes

    NARCIS (Netherlands)

    Austrheim, Håkon; Dunkel, Kristina G.; Plümper, Oliver; Ildefonse, Benoît; Liu, Yang; Jamtveit, Bjørn

    2017-01-01

    Fractures and faults riddle the Earth's crust on all scales, and the deformation associated with them is presumed to have had significant effects on its petrological and structural evolution. However, despite the abundance of directly observable earthquake activity, unequivocal evidence for seismic

  10. Evidence for crustal low shear-wave speed in western Saudi Arabia from multi-scale fundamental-mode Rayleigh-wave group-velocity tomography

    KAUST Repository

    Tang, Zheng

    2018-05-15

    We investigate the crustal and upper-mantle shear-velocity structure of Saudi Arabia by fundamental-mode Rayleigh-wave group-velocity tomography and shear-wave velocity inversion. The seismic dataset is compiled using ∼140 stations of the Saudi National Seismic Network (SNSN) operated by the Saudi Geological Survey (SGS). We measure Rayleigh-wave group-velocities at periods of 8–40 s from regional earthquakes. After obtaining 1-D shear-wave velocity models by inverting group-velocities at each grid node, we construct a 3-D shear-velocity model for Saudi Arabia and adjacent regions by interpolating the 1-D models. Our 3-D model indicates significant lateral variations in crustal and lithospheric thickness, as well as in the shear-wave velocity over the study region. In particular, we identify zones of reduced shear-wave speed at crustal levels beneath the Cenozoic volcanic fields in the Arabian Shield. The inferred reductions of 2–5% in shear-wave speed may be interpreted as possibly indicating the presence of partial melts. However, their precise origin we can only speculate about. Our study also reveals an upper-mantle low velocity zone (LVZ) below the Arabian Shield, supporting the model of lateral mantle flow from the Afar plume. Further geophysical experiments are needed to confirm (or refute) the hypothesis that partial melts may exist below the Cenozoic volcanism in western Saudi Arabia, and to build a comprehensive geodynamic–geological model for the evolution and present state of the lithosphere of the Arabian Plate and the Red Sea.

  11. Deformation processes and weakening mechanisms within the frictional viscous transition zone of major crustal-scale faults: insights from the Great Glen Fault Zone, Scotland

    Science.gov (United States)

    Stewart, M.; Holdsworth, R. E.; Strachan, R. A.

    2000-05-01

    The Great Glen Fault Zone (GGFZ), Scotland, is a typical example of a crustal-scale, reactivated strike-slip fault within the continental crust. Analysis of intensely strained fault rocks from the core of the GGFZ near Fort William provides a unique insight into the nature of deformation associated with the main phase of (sinistral) movements along the fault zone. In this region, an exhumed sequence of complex mid-crustal deformation textures that developed in the region of the frictional-viscous transition (ca. 8-15 km depth) is preserved. Fault rock fabrics vary from mylonitic in quartzites to cataclastic in micaceous shear zones and feldspathic psammites. Protolith mineralogy exerted a strong control on the initial textural development and distribution of the fault rocks. At lower strains, crystal-plastic deformation occurred in quartz-dominated lithologies to produce mylonites simultaneously with widespread fracturing and cataclasis in feldspar- and mica-dominated rocks. At higher strains, shearing appears to increasingly localise into interconnected networks of cataclastic shear zones, many of which are strongly foliated. Textures indicative of fluid-assisted diffusive mass transfer mechanisms are widespread in such regions and suggest that a hydrous fluid-assisted, grainsize-controlled switch in deformation behaviour followed the brittle comminution of grains. The fault zone textural evolution implies that a strain-induced, fluid-assisted shallowing and narrowing of the frictional-viscous transition occurred with increasing strain. It is proposed that this led to an overall weakening of the fault zone and that equivalent processes may occur along many other long-lived, crustal-scale dislocations.

  12. Evidence for crustal low shear-wave speed in western Saudi Arabia from multi-scale fundamental-mode Rayleigh-wave group-velocity tomography

    KAUST Repository

    Tang, Zheng; Mai, Paul Martin; Chang, Sung-Joon; Zahran, Hani

    2018-01-01

    We investigate the crustal and upper-mantle shear-velocity structure of Saudi Arabia by fundamental-mode Rayleigh-wave group-velocity tomography and shear-wave velocity inversion. The seismic dataset is compiled using ∼140 stations of the Saudi National Seismic Network (SNSN) operated by the Saudi Geological Survey (SGS). We measure Rayleigh-wave group-velocities at periods of 8–40 s from regional earthquakes. After obtaining 1-D shear-wave velocity models by inverting group-velocities at each grid node, we construct a 3-D shear-velocity model for Saudi Arabia and adjacent regions by interpolating the 1-D models. Our 3-D model indicates significant lateral variations in crustal and lithospheric thickness, as well as in the shear-wave velocity over the study region. In particular, we identify zones of reduced shear-wave speed at crustal levels beneath the Cenozoic volcanic fields in the Arabian Shield. The inferred reductions of 2–5% in shear-wave speed may be interpreted as possibly indicating the presence of partial melts. However, their precise origin we can only speculate about. Our study also reveals an upper-mantle low velocity zone (LVZ) below the Arabian Shield, supporting the model of lateral mantle flow from the Afar plume. Further geophysical experiments are needed to confirm (or refute) the hypothesis that partial melts may exist below the Cenozoic volcanism in western Saudi Arabia, and to build a comprehensive geodynamic–geological model for the evolution and present state of the lithosphere of the Arabian Plate and the Red Sea.

  13. Geochemical evidence for Paleozoic crustal growth and tectonic conversion in the Northern Beishan Orogenic Belt, southern Central Asian Orogenic Belt

    Science.gov (United States)

    Yuan, Yu; Zong, Keqing; He, Zhenyu; Klemd, Reiner; Jiang, Hongying; Zhang, Wen; Liu, Yongsheng; Hu, Zhaochu; Zhang, Zeming

    2018-03-01

    The Beishan Orogenic Belt is located in the central southernmost part of the Central Asian Orogenic Belt (CAOB), which plays a key role in understanding the formation and evolution of the CAOB. Granitoids are the documents of crustal and tectonic evolution in orogenic belts. However, little is known regarding the petrogenesis and geodynamic setting of the widely distributed Paleozoic granitoids in the Northern Beishan Orogenic Belt (NBOB). The present study reveals significant differences concerning the petrogenesis and tectonic setting of early and late Paleozoic granitoids from the NBOB. The early Paleozoic granitoids from the 446-430 Ma Hongliuxia granite complex of the Mazongshan unit and the 466-428 Ma Shibanjing complex of the Hanshan unit show classic I-type granite affinities as revealed by the relative enrichment of LILEs and LREEs, pronounced depletions of Nb, Ta and Ti and the abundant presence of hornblende. Furthermore, they are characterized by strongly variable zircon εHf(t) values between - 16.7 and + 12.8 and evolved plagioclase Sr isotopic compositions of 0.7145-0.7253, indicating the involvement of both juvenile and ancient continental crust in the magma source. Thus, we propose that the early Paleozoic granitoids in the NBOB were generated in a subduction-related continental arc setting. In contrast, the late Paleozoic 330-281 Ma granitoids from the Shuangjingzi complex of the Hanshan unit exhibit positive zircon εHf(t) values between + 5.8 and + 13.2 and relatively depleted plagioclase Sr isotopic compositions of 0.7037-0.7072, indicating that they were mainly formed by remelting of juvenile crust. Thus, an intra-plate extensional setting is proposed to have occurred during formation of the late Paleozoic granitoids. Therefore, between the early and late Paleozoic, the magma sources of the NBOB granitoids converted from the reworking of both juvenile and ancient crusts during a subduction-induced compressional setting to the remelting of

  14. Estimates of volume and magma input in crustal magmatic systems from zircon geochronology: the effect of modelling assumptions and system variables

    Directory of Open Access Journals (Sweden)

    Luca eCaricchi

    2016-04-01

    Full Text Available Magma fluxes in the Earth’s crust play an important role in regulating the relationship between the frequency and magnitude of volcanic eruptions, the chemical evolution of magmatic systems and the distribution of geothermal energy and mineral resources on our planet. Therefore, quantifying magma productivity and the rate of magma transfer within the crust can provide valuable insights to characterise the long-term behaviour of volcanic systems and to unveil the link between the physical and chemical evolution of magmatic systems and their potential to generate resources. We performed thermal modelling to compute the temperature evolution of crustal magmatic intrusions with different final volumes assembled over a variety of timescales (i.e., at different magma fluxes. Using these results, we calculated synthetic populations of zircon ages assuming the number of zircons crystallising in a given time period is directly proportional to the volume of magma at temperature within the zircon crystallisation range. The statistical analysis of the calculated populations of zircon ages shows that the mode, median and standard deviation of the populations varies coherently as function of the rate of magma injection and final volume of the crustal intrusions. Therefore, the statistical properties of the population of zircon ages can add useful constraints to quantify the rate of magma injection and the final volume of magmatic intrusions.Here, we explore the effect of different ranges of zircon saturation temperature, intrusion geometry, and wall rock temperature on the calculated distributions of zircon ages. Additionally, we determine the effect of undersampling on the variability of mode, median and standards deviation of calculated populations of zircon ages to estimate the minimum number of zircon analyses necessary to obtain meaningful estimates of magma flux and final intrusion volume.

  15. Stellar evolution

    CERN Document Server

    Meadows, A J

    2013-01-01

    Stellar Evolution, Second Edition covers the significant advances in the understanding of birth, life, and death of stars.This book is divided into nine chapters and begins with a description of the characteristics of stars according to their brightness, distance, size, mass, age, and chemical composition. The next chapters deal with the families, structure, and birth of stars. These topics are followed by discussions of the chemical composition and the evolution of main-sequence stars. A chapter focuses on the unique features of the sun as a star, including its evolution, magnetic fields, act

  16. Spatial Relationship Between Crustal Structure and Mantle Seismicity in the Vrancea Seismogenic Zone of Romania

    Science.gov (United States)

    Knapp, C. C.; Enciu, D. M.; Knapp, J. H.

    2007-12-01

    Active crustal deformation and subsidence in the Southeast Carpathian foreland has previously been attributed to active foundering of thickened continental lithosphere beneath the Carpathian bend region (Knapp et al, 2005). The present study involves integration of active and passive-source seismic data in order to place constraints on the duration, timing, and scale of crustal deformation in the Carpathian foreland, and in particular to assess the genetic relationship with the Vrancea intermediate-depth seismogenic zone (VSZ). Relocated crustal earthquakes and focal mechanisms were correlated with four deep industry seismic profiles, the reprocessed DACIA PLAN deep seismic profile, and the DRACULA (Deep Reflection Acquisition Constraining Unusual Lithospheric Activity) II and III profiles. Projection of foreland crustal hypocenters onto the deep seismic lines correlates well with previously identified crustal faults such as the Trotus and Sinaia, as well as the newly identified Ialomita Fault. Specifically, results of this study (1) image the full crustal and uppermost mantle structure of the Focsani Basin in the close proximity of the VSZ, (2) show evidence for a sub-horizontal, slightly east-dipping Moho in the vicinity of the VSZ and thinning of the crust towards the Carpathian orogen, (3) illustrate the conspicuous absence of west-dipping fabrics or structures in the crust and across the Moho, (4) present evidence that the Trotus Fault is a crustal-scale active fault with a dextral sense of motion, (5) suggest that the Paleozoic age Peceneaga-Camena and Capidava-Ovidiu Faults have not been active in post-Paleozoic time, and (6) show evidence for a new active crustal scale sinistral fault, named the Ialomita fault. Both the seismogenic Vrancea body and deformation in the Focsani Basin appear to be concentrically bound by the Trotus Fault in the north and east and the Sinaia-Ialomita Fault in the south, suggesting a coupled deformation between the VSZ and the

  17. Qinghai-Tibet Plateau crustal thickness derived from EGM2008 and CRSUT2.0

    Directory of Open Access Journals (Sweden)

    Zhou Hao

    2014-11-01

    Full Text Available Qinghai-Tibet Plateau is the most complex region for crustal thickness inversion, while high-resolution earth gravity model (EGM makes it possible to obtain high precision gravity anomaly, which is a key parameter to depict the Earth’s inner structure in geodesy domain. On the basis of this principle, we calculated the Bouguer gravity anomalies in Qinghai-Tibet Plateau with EGM2008 and SRTM6. 0 by efficient high-degree spherical harmonic synthesis algorithm. In order to obtain the gravity anomaly caused by Moho density mutant, the noises caused by the topography was removed by wavelet details. Then, the crustal thickness was corrected on the basis of CRUST 2. 0 with the deep-large-scale single density interface formula. The inversion result indicates that the crustal thickness in Qinghai-Tibet Plateau is between 50 km and 75 km, which is in correspondence with the recent science research result. Compared with the 2 degree CRUST 2. 0 model, the spatial resolution of crustal thickness in our research can reach 40 arc minutes. In addition, there is a positive relationship between the inversed crustal thickness and topography, which can prove the effectiveness of Airy-Heiskanen isostatic model in gravity reduction.

  18. Influence of crustal layering and thickness on co-seismic effects of Wenchuan earthquake

    Directory of Open Access Journals (Sweden)

    Tan Hongbo

    2011-02-01

    Full Text Available Using the PSGRN/PSCMP software and the fault model offered by USGS and on the basis of finite rectangular dislocation theory and the local layered wave velocity structures of the crust-upper-mantle, the influences of crustal layering and thickness on co-seismic gravity changes and deformation of Wenchuan earthquake have been simulated. The results indicate that; the influences have a relationship with the attitude of faults and the relative position between calculated points and fault. The difference distribution form of simulated results between the two models is similar to that of co-seismic effect. For the per centum distribution, it’s restricted by the zero line of the co-seismic effects obviously. Its positive is far away from the zero line. For the crustal thickness, the effect is about 10% – 20%. The negative and the effect over 30% focus around the zero line. The average influences of crustal layering and thickness for the E-W displacement, N-S displacement, vertical displacement and gravity changes are 18.4%,18.0%, 15.8% and 16.2% respectively, When the crustal thickness is 40 km, they are 4.6%, 5.3%, 3.8% and 3.8%. Then the crustal thickness is 70 km, the average influences are 3.5%, 4.6%, 3.0% and 2.5% respectively.

  19. Obtaining Crustal Properties From the P Coda Without Deconvolution: an Example From the Dakotas

    Science.gov (United States)

    Frederiksen, A. W.; Delaney, C.

    2013-12-01

    Receiver functions are a popular technique for mapping variations in crustal thickness and bulk properties, as the travel times of Ps conversions and multiples from the Moho constrain both Moho depth (h) and the Vp/Vs ratio (k) of the crust. The established approach is to generate a suite of receiver functions, which are then stacked along arrival-time curves for a set of (h,k) values (the h-k stacking approach of Zhu and Kanamori, 2000). However, this approach is sensitive to noise issues with the receiver functions, deconvolution artifacts, and the effects of strong crustal layering (such as in sedimentary basins). In principle, however, the deconvolution is unnecessary; for any given crustal model, we can derive a transfer function allowing us to predict the radial component of the P coda from the vertical, and so determine a misfit value for a particular crustal model. We apply this idea to an Earthscope Transportable Array data set from North and South Dakota and western Minnesota, for which we already have measurements obtained using conventional h-k stacking, and so examine the possibility of crustal thinning and modification by a possible failed branch of the Mid-Continent Rift.

  20. Seismotectonics of Taiwan Shoal region in northeastern SCS: Insights from crustal structure

    Science.gov (United States)

    Kuiyuan, Wan; Jinlong, Sun; Shaohong, Xia; Xiaoling, Xie; Xiang, Zhang; Huilong, Xu; Jinghe, Cao

    2017-04-01

    A seismicity cluster and a great 16 September 1994 earthquake occur in the Taiwan Shoal region, outer rise of the Manila subduction zone. To understand what mechanisms control and generate the earthquake cluster, it is important to investigate the deep crustal structure of the Taiwan Shoal region. We present a 2-D seismic tomographic image of the crustal structure along the OBS2012 profile based on ocean bottom seismographic (OBS) data. The structure exhibits that a high velocity anomaly in the upper crust beneath the Taiwan Shoal is flanked by lower velocity anomalies. Based on the crustal structure, we study the 765 earthquakes, which occurred in the period 1991-2015. These epicenters, combined with the regional faults, and crustal structure, allow us to better understand the nature of the active tectonics in this region. The high velocity area is interpreted as representing stronger, defining major asperities where stress is concentrated corresponding to the location of the earthquake cluster. The earthquake cluster is influenced by the fault interactions. However, the 16 September 1994 earthquake is independents of the seismic activities but associated with the reactivation of the preexisting fault. In Taiwan region, the slab-pull was resisted by the exposed pre-collision accretionary prism and the resistive force caused the in-plane compressive stress accumulation. This condition may favor the triggering of future damaging earthquakes in this region. Key words: earthquake cluster; crustal structure; fault interactions; outer rise; Taiwan Shoal

  1. Crustal moment of inertia of glitching pulsars with the KDE0v1 Skyrme interaction

    Energy Technology Data Exchange (ETDEWEB)

    Madhuri, K.; Routray, T.R.; Pattnaik, S.P. [Sambalpur University, School of Physics, Jyotivihar (India); Basu, D.N. [Variable Energy Cyclotron Center, Kolkata (India)

    2017-07-15

    The mass, radius and crustal fraction of moment of inertia in neutron stars are calculated using β-equilibrated nuclear matter obtained from the Skyrme effective interaction. The transition density, pressure and proton fraction at the inner edge separating the liquid core from the solid crust of the neutron stars are determined from the thermodynamic stability conditions using the KDE0v1 set. The neutron star masses obtained by solving the Tolman-Oppenheimer-Volkoff equations using neutron star matter obtained from this set are able to describe highly massive compact stars ∝ 2M {sub CircleDot}. The crustal fraction of the moment of inertia can be extracted from studying pulsar glitches. This fraction is highly dependent on the core-crust transition pressure and corresponding density. These results for pressure and density at core-crust transition together with the observed minimum crustal fraction of the total moment of inertia provide a limit for the radius of the Vela pulsar, R ≥ 3.69 + 3.44M/M {sub CircleDot}. Present calculations suggest that the crustal fraction of the total moment of inertia can be ∝ 6.3% due to crustal entrainment caused by the Bragg reflection of unbound neutrons by lattice ions. (orig.)

  2. Crustal CO2 liberation during the 2006 eruption and earthquake events at Merapi volcano, Indonesia

    Science.gov (United States)

    Troll, Valentin R.; Hilton, David R.; Jolis, Ester M.; Chadwick, Jane P.; Blythe, Lara S.; Deegan, Frances M.; Schwarzkopf, Lothar M.; Zimmer, Martin

    2012-06-01

    High-temperature volcanic gas is widely considered to originate from ascending, mantle-derived magma. In volcanic arc systems, crustal inputs to magmatic gases mainly occur via subducted sediments in the mantle source region. Our data from Merapi volcano, Indonesia imply, however, that during the April-October 2006 eruption significant quantities of CO2 were added from shallow crustal sources. We show that prior to the 2006 events, summit fumarole gas δ13C(CO2) is virtually constant (δ13C1994-2005 = -4.1 ± 0.3‰), but during the 2006 eruption and after the shallow Yogyakarta earthquake of late May, 2006 (M6.4; hypocentres at 10-15 km depth), carbon isotope ratios increased to -2.4 ± 0.2‰. This rise in δ13C is consistent with considerable addition of crustal CO2 and coincided with an increase in eruptive intensity by a factor of ˜3 to 5. We postulate that this shallow crustal volatile input supplemented the mantle-derived volatile flux at Merapi, intensifying and sustaining the 2006 eruption. Late-stage volatile additions from crustal contamination may thus provide a trigger for explosive eruptions independently of conventional magmatic processes.

  3. Crustal processes of the Mid-Ocean Ridge

    Science.gov (United States)

    Ballard, Richard D.; Craig, H.; Edmond, J.; Einaudi, M.; Holcomb, R.; Holland, H.D.; Hopson, C.A.; Luyendyk, B.P.; Macdonald, K.; Morton, J.; Orcutt, J.; Sleep, N.

    1981-01-01

    Independent geological and geophysical investigations of the Mid-Ocean Ridge system have begun to focus on the nature of the magma chamber system underlying its central axis. Thermal models predict the existence of a steady-state chamber beneath a thin crustal lid ranging in thickness from 2 to 13 kilometers. The only aspect of the system that these models fail to account for is the extremely slow spreading rates. Seismological studies reveal the existence of a low-velocity zone beneath segments of the East Pacific Rise, which is thought to correspond to a chamber system having a half-width of approximately 5 to 10 kilometers. These estimates compare favorably with those derived separately through petrological investigations of deep-sea drilling results, various sampling programs, and field and laboratory studies of ophiolites. The chamber is thought to be wing-shaped and to remain continuously open; it is thought to be fed from the center while simultaneously solidifying at the sides as spreading carries the two halves apart. Progressive fractionation occurs by crystal settling coupled with repeated replenishment and magma mixing in an open steady-state system. Near-bottom studies reveal that the zone of extrusion above the chamber is narrow, but its eruptive history is cyclic in nature, in conflict with the predictions of a steady-state model. On-bottom gravity data at 21 ??N on the East Pacific Rise reveal a negative gravity anomaly that may be related to the uppermost part of the chamber. The anomaly is only 2 kilometers wide and 1 kilometer below the sea floor. This feature may be associated with a short-term upper magma reservoir. The cyclic volcanic activity is directly related to the active phase of hydrothermal circulation responsible for the observed negative thermal anomaly. The volume of water associated with this circulation is equal to the entire ocean volume passing through the accretion zone approximately every 8 million years. This is about 0

  4. Crustal Structure of Khövsgöl, Mongolia

    Science.gov (United States)

    Scott, A. M.; Meltzer, A.; Stachnik, J.; Russo, R.; Munkhuu, U.; Tsagaan, B.

    2017-12-01

    Mongolia is part of the Central Asian Orogenic Belt, an accretionary event that spanned 800 million years from the mid-Proterozoic to mid-Phanerozoic. As a result of the past collisional and rifting events, the modern Khövsgöl rift system of northern Mongolia contains a heterogeneous lithospheric structure. The current rift system has three parallel N-S trending basins that roughly align with terrane boundaries. Structures inherited during the accretionary events may be a factor influencing regional deformation. The forces that drive local deformation are not well understood, but varying processes have been proposed: far-field effects of India-Eurasian plate convergence, westward subduction of the Pacific plate, magmatic underplating at the base of the crust, mantle plume activity, and asthenospheric mantle convection. Determining the nature of crustal features within this poorly understood region may illuminate processes that control rifting within intracontinental settings. A network of 26 broadband seismic stations encompassing 200 square kilometers of the Khövsgöl rift system were deployed from August 2014 to June 2016. More than 2100 events were detected, and most earthquakes were concentrated near rift structures. Events between Busiin-Gol and Darkhad, the westernmost and central basins of the Khövsgöl rift system, are distributed within the crust. An active fault is outlined along the eastern border of the Darkhad basin. Khövsgöl earthquakes bound both sides of the rift. Along the northern border of Lake Khövsgöl, seismic events define a shallow active fault orthogonal to the basin. The largest event recorded within the network was a magnitude ml=5.2 located near the northeastern border of Lake Khövsgöl on 12-05-2014. The focal mechanism of this earthquake is predominantly strike-slip, but also includes an extensional component. This work focuses on earthquake relocation and calculating moment tensors and focal mechanisms of larger regional

  5. Crustal processes of the mid-ocean ridge.

    Science.gov (United States)

    1981-07-03

    Independent geological and geophysical investigations of the Mid-Ocean Ridge system have begun to focus on the nature of the magma chamber system underlying its central axis. Thermal models predict the existence of a steady-state chamber beneath a thin crustal lid ranging in thickness from 2 to 13 kilometers. The only aspect of the system that these models fail to account for is the extremely slow spreading rates. Seismological studies reveal the existence of a low-velocity zone beneath segments of the East Pacific Rise, which is thought to correspond to a chamber system having a half-width of approximately 5 to 10 kilometers. These estimates compare favorably with those derived separately through petrological investigations of deep-sea drilling results, various sampling programs, and field and laboratory studies of ophiolites. The chamber is thought to be wing-shaped and to remain continuously open; it is thought to be fed from the center while simultaneously solidifying at the sides as spreading carries the two halves apart. Progressive fractionation occurs by crystal settling coupled with repeated replenishment and magma mixing in an open steady-state system. Near-bottom studies reveal that the zone of extrusion above the chamber is narrow, but its eruptive history is cyclic in nature, in conflict with the predictions of a steady-state model. On-bottom gravity data at 21 degrees N on the East Pacific Rise reveal a negative gravity anomaly that may be related to the uppermost part of the chamber. The anomaly is only 2 kilometers wide and 1 kilometer below the sea floor. This feature may be associated with a short-term upper magma reservoir. The cyclic volcanic activity is directly related to the active phase of hydrothermal circulation responsible for the observed negative thermal anomaly. The volume of water associated with this circulation is equal to the entire ocean volume passing through the accretion zone approximately every 8 million years. This is about 0

  6. Mechanisms of unsteady shallow creep on major crustal faults

    Science.gov (United States)

    Jiang, J.; Fialko, Y. A.

    2017-12-01

    A number of active crustal faults are associated with geodetically detectable shallow creep, while other faults appear to be locked all the way to the surface over the interseismic period. Faults that exhibit shallow creep also often host episodic accelerated creep events. Examples include the Ismetpasa segment of the North Anatolian Fault (NAF) in Turkey and the Southern San Andreas and Superstition Hills (SHF) faults in Southern California. Recent geodetic observations indicate that shallow creep events can involve large fault sections (tens of km long) and persist throughout different stages of a seismic cycle. A traditional interpretation of shallow creep in terms of a velocity-strengthening (VS) layer atop the seismogenic velocity-weakening (VW) zone fails to explain episodic creep events. Wei et al. (2013) proposed that such events can be due to a thin VW layer within the VS shallow crust, implying rather special structural and lithologic conditions. We explore the rheologic controls on aseismic episodic slip and its implications for seismic faulting in the framework of laboratory rate-and-state friction. Observations of co-, post- and inter-seismic slip from the NAF and SHF are used to infer depth-dependent frictional properties in a 2D fault model. In particular, creep events with displacements on the order of millimeters and periods of months are reproduced in a model having monotonic depth variations in rate-and-state parameters. Such a model includes a velocity-neutral (VN) layer sandwiched between the surface layer with VS frictional properties, constrained by observed postseismic afterslip, and a deeper VW layer that largely controls the recurrence of major earthquakes. With the presence of the VN layer, the amount of surface-breaching coseismic slip critically depends on how dynamic weakening varies with depth in the seismogenic layer. Observations of limited surface slip during prior events on the NAF and SHF suggest that coseismic fault weakening is

  7. Provenance and tectonic setting of the supra-crustal succession of the Qinling Complex: Implications for the tectonic affinity of the North Qinling Belt, Central China

    Science.gov (United States)

    Shi, Yu; Huang, Qianwen; Liu, Xijun; Krapež, Bryan; Yu, Jinhai; Bai, Zhian

    2018-06-01

    The Qinling Complex lies in the Qinling orogenic belt of Central China and holds the key to understanding the evolution of this feature. The Qinling Complex comprises a basement complex composed of amphibolite and ecologite, overlain by a supra-crustal succession that has been metamorphosed to the upper greenschist facies at approximately 516-509 Ma. The protoliths of the meta-sedimentary rocks are graywackes, which are divided into lower, middle and upper units. Detrital zircons from nine samples of the supra-crustal succession have ages ranging from 1182 to 1158 Ma for the lower unit, 957 to 955 Ma for the middle unit and 917 to 840 Ma for the upper unit. The lower unit is intruded by a ca. 960 Ma pluton. The bulk compositions of these meta-sedimentary rocks and their detrital zircon ages clearly indicate derivation from Meso- and Neo-proterozoic granites. Thus, we suggest that the sedimentary succession was derived from an arc-related tectonic setting and that none of the detritus was sourced from the southern margin of the North China Block or from the northern and western margins of the South China Block. We conclude that the North Qinling Belt was an independent micro-continental block during the Meso- to Neo-proterozoic.

  8. The role of Mesozoic sedimentary basin tapers on the formation of Cenozoic crustal shortening structures and foredeep in the western Sichuan Basin, China

    Science.gov (United States)

    Wang, M.

    2017-12-01

    The foreland basin records important clues of tectonic and sedimentary process of mountain-building, thus to explore its dynamic mechanism on the formation is an important issue of the mountain-basin interaction. The Longmen Shan fold-and-thrust belt and its adjacent Sichuan basin located in the eastern margin of Tibetan Plateau, are one of the most-concerned regions of studying modern mountain-building and seismic process, and are also a natural laboratory of studying the dynamics of the formation and development of foreland basin. However, it still need further explore on the mechanics of the development of the Cenozoic foreland basin and thrust-belts in the western Sichuan Basin. The Longmen Shan thrust belt has experienced multi-stages of tectonics evolution, foreland basin formation and topography growth since Late Triassic, and whether the early formed basin architecture and large Mesozoic sedimentary basin taper can influence the formation and development of the Cenozoic foreland basin and thrust belts? To solve these issues, this project aim to focus on the Cenozoic foreland basin and internal crustal shortening structures in the western Sichuan basin, on the basis of growth critical wedge taper theory. We will reconstruct the shape of multi-phases of sedimentary basin tapers, the temporal-spatial distribution of crustal shortening and thrusting sequences, and analyze the control mechanism of Mesozoic sedimentary basin taper on the formation of Cenozoic foreland basins, and final explore the interaction between the tectonics geomorphology, stress field and dynamic propagation of foreland basin.

  9. Further constraints on neutron star crustal properties in the low-mass X-ray binary 1RXS J180408.9-342058

    Science.gov (United States)

    Parikh, A. S.; Wijnands, R.; Degenaar, N.; Ootes, L.; Page, D.

    2018-05-01

    We report on two new quiescent XMM-Newton observations (in addition to the earlier Swift/XRT and XMM-Newton coverage) of the cooling neutron star crust in the low-mass X-ray binary 1RXS J180408.9-342058. Its crust was heated during the ˜4.5 month accretion outburst of the source. From our quiescent observations, fitting the spectra with a neutron star atmosphere model, we found that the crust had cooled from ˜100 to ˜73 eV from ˜8 to ˜479 d after the end of its outburst. However, during the most recent observation, taken ˜860 d after the end of the outburst, we found that the crust appeared not to have cooled further. This suggested that the crust had returned to thermal equilibrium with the neutron star core. We model the quiescent thermal evolution with the theoretical crustal cooling code NSCool and find that the source requires a shallow heat source, in addition to the standard deep crustal heating processes, contributing ˜0.9 MeV per accreted nucleon during outburst to explain its observed temperature decay. Our high quality XMM-Newton data required an additional hard component to adequately fit the spectra. This slightly complicates our interpretation of the quiescent data of 1RXS J180408.9-342058. The origin of this component is not fully understood.

  10. Inferring regional vertical crustal velocities from averaged relative sea level trends: A proof of concept

    Directory of Open Access Journals (Sweden)

    Bâki Iz H.

    2017-02-01

    Full Text Available This study demonstrates that relative sea level trends calculated from long-term tide gauge records can be used to estimate relative vertical crustal velocities in a region with high accuracy. A comparison of the weighted averages of the relative sea level trends estimated at six tide gauge stations in two clusters along the Eastern coast of United States, in Florida and in Maryland, reveals a statistically significant regional vertical crustal motion of Maryland with respect to Florida with a subsidence rate of −1.15±0.15 mm/yr identified predominantly due to the ongoing glacial isostatic adjustment process. The estimate is a consilience value to validate vertical crustal velocities calculated from GPS time series as well as towards constraining predictive GIA models in these regions.

  11. Sensitivity analysis of crustal correction for calculation of lithospheric mantle density from gravity data

    DEFF Research Database (Denmark)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans

    2016-01-01

    for the crust and (ii) uncertainties in the seismic crustal structure (thickness and average VP velocities of individual crustal layers, including the sedimentary cover). We examine the propagation of these uncertainties into determinations of lithospheric mantle density and analyse both sources of possible......We investigate how uncertainties in seismic and density structure of the crust propagate to uncertainties in mantle density structure. The analysis is based on interpretation of residual upper-mantle gravity anomalies which are calculated by subtracting (stripping) the gravitational effect...... mantle, knowledge on uncertainties associated with incomplete information on crustal structure is of utmost importance for progress in gravity modelling. Uncertainties in the residual upper-mantle gravity anomalies result chiefly from uncertainties in (i) seismic VP velocity-density conversion...

  12. Thickening the outer margins of the Tibetan Plateau: The role of crustal shortening

    Science.gov (United States)

    Lease, R. O.; Burbank, D. W.

    2012-12-01

    One of the most direct consequences of the collision of two buoyant continents is large-scale crustal thickening that results in the upward and outward growth of high terrain. As the stronger Indian continent has collided with weaker Asia over at least the past 50 Myr, widespread crustal thickening has occurred over an area that is approximately 2.5 million km^2 at present. The resultant Tibetan crust is the thickest observed on Earth today with an average thickness of 65 km and a maximum that may reach 90 km in places. The mechanisms by which Tibetan crust has thickened, however, as well as the timing and distribution of these mechanisms across the plateau, remain debatable. Two of the most popular mechanisms for thickening the crust beneath the margins of the Tibetan Plateau are: 1) pure shear with faulting and folding in the upper crust and horizontal shortening below; and 2) flow and inflation of lower or middle crust without significant shortening of the upper crust. To help discriminate between the relative contributions of these two mechanisms, well-constrained estimates of upper crustal shortening are needed. Here we document the Cenozoic shortening budget across the northeastern Tibetan Plateau margin near 36°N 102.5°E with several 100- to 145-km-long balanced cross sections. Thermochronological and magnetostratigraphic data indicate that modest NNE-SSW shortening began in middle Eocene time, shortly after initial India-Asia collision. Accelerated east-west shortening, however, did not commence until ~35 Myr later. A five-fold acceleration in shortening rates in middle Miocene-to-Recent time accounts for more than half of the total Cenozoic crustal shortening and thickening in this region. Overall, the balanced cross sections indicate 11 ± 2 % east-west shortening since middle Miocene time, and ~9 ± 2 % NNE-SSW shortening between middle Eocene and middle Miocene times. Given the present-day crustal thickness of 56 ± 4 km in northeastern Tibet, crustal

  13. Modelling of crustal rock mechanics for radioactive waste storage in Fennoscandia - problem definition

    International Nuclear Information System (INIS)

    Stephansson, O.

    1987-05-01

    Existing knowledge of crustal stresses for Fennoscandia is presented. Generic, two-dimensional models are proposed for vertical and planar sections of a traverse having a direction NW-SE in Northern Fennoscandia. The proposed traverse will include the major neotectonic structures at Lansjaerv and Paervie, respectively, and also the study site for storage of spent nuclear fuel at Kamlunge. The influence of glaciation, deglaciation, glacial rebound on crustal rock mechanics and stability is studied for the modelling work. Global models, with a length of roughly 100 km, will increase our over all understanding of the change in stresses and deformations. These can provide boundary conditions for regional and near-field models. Properties of strength and stiffness of intact granitic rock masses, faults and joints are considered in the modelling of the crustal rock mechanics for any of the three models described. (orig./HP)

  14. Influence of mid-crustal rheology on the deformation behavior of continental crust in the continental subduction zone

    Science.gov (United States)

    Li, Fucheng; Sun, Zhen; Zhang, Jiangyang

    2018-06-01

    Although the presence of low-viscosity middle crustal layer in the continental crust has been detected by both geophysical and geochemical studies, its influence on the deformation behavior of continental crust during subduction remains poorly investigated. To illustrate the crustal deformation associated with layered crust during continental subduction, we conducted a suite of 2-D thermo-mechanical numerical studies with visco-brittle/plastic rheology based on finite-differences and marker-in-cell techniques. In the experiments, we established a three-layer crustal model with a quartz-rich middle crustal layer embedded between the upper and lower continental crust. Results show that the middle crustal layer determines the amount of the accreted upper crust, maximum subduction depth, and exhumation path of the subducted upper crust. By varying the initial effective viscosity and thickness of the middle crustal layer, the further effects can be summarized as: (1) a rheologically weaker and/or thicker middle crustal layer results in a larger percentage of the upper crust detaching from the underlying slab and accreting at the trench zone, thereby leading to more serious crustal deformation. The rest of the upper crust only subducts into the depths of high pressure (HP) conditions, causing the absence of ultra-high pressure (UHP) metamorphic rocks; (2) a rheologically stronger and/or thinner middle crustal layer favors the stable subduction of the continental crust, dragging the upper crust to a maximum depth of ∼100 km and forming UHP rocks; (3) the middle crustal layer flows in a ductile way and acts as an exhumation channel for the HP-UHP rocks in both situations. In addition, the higher convergence velocity decreases the amount of subducted upper crust. A detailed comparison of our modeling results with the Himalayan collisional belt are conducted. Our work suggests that the presence of low-viscosity middle crustal layer may be another possible mechanism for

  15. The Upper- to Middle-Crustal Section of the Alisitos Oceanic Arc, (Baja, Mexico): an Analog of the Izu-Bonin-Marianas (IBM) Arc

    Science.gov (United States)

    Medynski, S.; Busby, C.; DeBari, S. M.; Morris, R.; Andrews, G. D.; Brown, S. R.; Schmitt, A. K.

    2016-12-01

    The Rosario segment of the Cretaceous Alisitos arc in Baja California is an outstanding field analog for the Izu-Bonin-Mariana (IBM) arc, because it is structurally intact, unmetamorphosed, and has superior three-dimensional exposures of an upper- to middle-crustal section through an extensional oceanic arc. Previous work1, done in the pre-digital era, used geologic mapping to define two phases of arc evolution, with normal faulting in both phases: (1) extensional oceanic arc, with silicic calderas, and (2) oceanic arc rifting, with widespread diking and dominantly mafic effusions. Our new geochemical data match the extensional zone immediately behind the Izu arc front, and is different from the arc front and rear arc, consistent with geologic relations. Our study is developing a 3D oceanic arc crustal model, with geologic maps draped on Google Earth images, and GPS-located outcrop information linked to new geochemical, geochronological and petrographic data, with the goal of detailing the relationships between plutonic, hypabyssal, and volcanic rocks. This model will be used by scientists as a reference model for past (IBM-1, 2, 3) and proposed IBM (IBM-4) drilling activities. New single-crystal zircon analysis by TIMS supports the interpretation, based on batch SIMS analysis of chemically-abraded zircon1, that the entire upper-middle crustal section accumulated in about 1.5 Myr. Like the IBM, volcanic zircons are very sparse, but zircon chemistry on the plutonic rocks shows trace element compositions that overlap to those measured in IBM volcanic zircons by A. Schmitt (unpublished data). Zircons have U-Pb ages up to 20 Myr older than the eruptive age, suggesting remelting of older parts of the arc, similar to that proposed for IBM (using different evidence). Like IBM, some very old zircons are also present, indicating the presence of old crustal fragments, or sediments derived from them, in the basement. However, our geochemical data show that the magmas are

  16. Seismically imaged shallow and deep crustal structure and potential field anomalies across the Eastern Dharwar Craton, south Indian shield: Possible geodynamical implications

    Science.gov (United States)

    Pandey, O. P.; Chandrakala, K.; Vasanthi, A.; Kumar, K. Satish

    2018-05-01

    The time-bound crustal evolution and subsequent deformation of the Cuddapah basin, Nellore Schist Belt and Eastern Ghats terrain of Eastern Dharwar Craton, which have undergone sustained geodynamic upheavals since almost 2.0 billion years, remain enigmatic. An attempt is made here to integrate newly available potential field data and other geophysical anomalies with deep seismic structure, to examine the generative mechanism of major crustal features, associated with this sector. Our study indicates that the initial extent of the Cuddapah basin sedimentation may have been much larger, extending by almost 50-60 km west of Tadipatri during Paleoproterozoic period, which subsequently shrank due to massive erosion following thermal uplift, caused by SW Cuddapah mantle plume. Below this region, crust is still quite warm with Moho temperatures exceeding 500 °C. Similarly, Nallamalai Fold Belt rocks, bounded by two major faults and extremely low gravity, may have occupied a large terrain in western Cuddapah basin also, before their abrasion. No geophysical signatures of thrusting are presently seen below this region, and thus it could not be an alien terrain either. In contrast, Nellore Schist Belt is associated with strikingly high positive gravity, possibly caused by a conspicuous horst structure and up dipping mafic crustal layers underneath, that resulted due to India-east Antarctica collision after the cessation of prolonged subduction (1.6-0.95 Ga). Further, the crustal seismic and gravity signatures would confirm presence of a totally distinct geological terrain east of the Cuddapah basin, but the trace of Eastern Ghats Belt is all together missing. Instead, all the geophysical signatures, point out to presence of a Proterozoic sedimentary terrain, east of Nellore Schist Belt. It is likely that the extent of Prorerozoic sedimentation was much larger than thought today. In addition, presence of a seismically detected Gondwana basin over Nellore Schist Belt, apart

  17. Lithospheric strength in the active boundary between the Pacific Plate and Baja California microplate constrained from lower crustal and upper mantle xenoliths

    Science.gov (United States)

    Chatzaras, Vasileios; van der Werf, Thomas; Kriegsman, Leo M.; Kronenberg, Andreas; Tikoff, Basil; Drury, Martyn R.

    2017-04-01

    The lower crust is the most poorly understood of the lithospheric layers in terms of its rheology, particularly at active plate boundaries. We studied naturally deformed lower crustal xenoliths within an active plate boundary, in order to link their microstructures and rheological parameters to the well-defined active tectonic context. The Baja California shear zone (BCSZ), located at the western boundary of the Baja California microplate, comprises the active boundary accommodating the relative motion between the Pacific plate and Baja California microplate. The basalts of the Holocene San Quintin volcanic field carry lower crustal and upper mantle xenoliths, which sample the Baja California microplate lithosphere in the vicinity of the BCSZ. The lower crustal xenoliths range from undeformed gabbros to granoblastic two-pyroxene granulites. Two-pyroxene geothermometry shows that the granulites equilibrated at temperatures of 690-920 oC. Phase equilibria (P-T pseudosections using Perple_X) indicate that symplectites with intergrown pyroxenes, plagioclase, olivine and spinel formed at 3.6-5.4 kbar, following decompression from pressures exceeding 6 kbar. FTIR spectroscopy shows that the water content of plagioclase varies among the analyzed xenoliths; plagioclase is relatively dry in two xenoliths while one xenolith contains hydrated plagioclase grains. Microstructural observations and analysis of the crystallographic texture provide evidence for deformation of plagioclase by a combination of dislocation creep and grain boundary sliding. To constrain the strength of the lower crust and upper mantle near the BCSZ we estimated the differential stress using plagioclase and olivine grain size paleopiezomtery, respectively. Differential stress estimates for plagioclase range from 10 to 32 MPa and for olivine are 30 MPa. Thus the active microplate boundary records elevated crustal temperatures, heterogeneous levels of hydration, and low strength in both the lower crust and

  18. Crustal structure of north Peru from analysis of teleseismic receiver functions

    Science.gov (United States)

    Condori, Cristobal; França, George S.; Tavera, Hernando J.; Albuquerque, Diogo F.; Bishop, Brandon T.; Beck, Susan L.

    2017-07-01

    In this study, we present results from teleseismic receiver functions, in order to investigate the crustal thickness and Vp/Vs ratio beneath northern Peru. A total number of 981 receiver functions were analyzed, from data recorded by 28 broadband seismic stations from the Peruvian permanent seismic network, the regional temporary SisNort network and one CTBTO station. The Moho depth and average crustal Vp/Vs ratio were determined at each station using the H-k stacking technique to identify the arrival times of primary P to S conversion and crustal reverberations (PpPms, PpSs + PsPms). The results show that the Moho depth correlates well with the surface topography and varies significantly from west to east, showing a shallow depth of around 25 km near the coast, a maximum depth of 55-60 km beneath the Andean Cordillera, and a depth of 35-40 km further to the east in the Amazonian Basin. The bulk crustal Vp/Vs ratio ranges between 1.60 and 1.88 with the mean of 1.75. Higher values between 1.75 and 1.88 are found beneath the Eastern and Western Cordilleras, consistent with a mafic composition in the lower crust. In contrast values vary from 1.60 to 1.75 in the extreme flanks of the Eastern and Western Cordillera indicating a felsic composition. We find a positive relationship between crustal thickness, Vp/Vs ratio, the Bouguer anomaly, and topography. These results are consistent with previous studies in other parts of Peru (central and southern regions) and provide the first crustal thickness estimates for the high cordillera in northern Peru.

  19. Developing a Crustal and Upper Mantle Velocity Model for the Brazilian Northeast

    Science.gov (United States)

    Julia, J.; Nascimento, R.

    2013-05-01

    Development of 3D models for the earth's crust and upper mantle is important for accurately predicting travel times for regional phases and to improve seismic event location. The Brazilian Northeast is a tectonically active area within stable South America and displays one of the highest levels of seismicity in Brazil, with earthquake swarms containing events up to mb 5.2. Since 2011, seismic activity is routinely monitored through the Rede Sismográfica do Nordeste (RSisNE), a permanent network supported by the national oil company PETROBRAS and consisting of 15 broadband stations with an average spacing of ~200 km. Accurate event locations are required to correctly characterize and identify seismogenic areas in the region and assess seismic hazard. Yet, no 3D model of crustal thickness and crustal and upper mantle velocity variation exists. The first step in developing such models is to refine crustal thickness and depths to major seismic velocity boundaries in the crust and improve on seismic velocity estimates for the upper mantle and crustal layers. We present recent results in crustal and uppermost mantle structure in NE Brazil that will contribute to the development of a 3D model of velocity variation. Our approach has consisted of: (i) computing receiver functions to obtain point estimates of crustal thickness and Vp/Vs ratio and (ii) jointly inverting receiver functions and surface-wave dispersion velocities from an independent tomography study to obtain S-velocity profiles at each station. This approach has been used at all the broadband stations of the monitoring network plus 15 temporary, short-period stations that reduced the inter-station spacing to ~100 km. We expect our contributions will provide the basis to produce full 3D velocity models for the Brazilian Northeast and help determine accurate locations for seismic events in the region.

  20. Reworked crustal of early Paleozoic WuYi Orogen revealed by receiver function data

    Science.gov (United States)

    Wei, Y.; Duan, Y.; Tian, X.; Zhao, Y.

    2017-12-01

    Intraplate orogenic belt, which occurs at the rigid and undeformable plate interiors, is a distinct new type of orogen rather than an interplate or plate marginal orogenic belt, whose deformation occurs exclusively at plate margins. Therefore, intraplate orogenic belts are the most obvious exception to the plate-tectonic paradigm, they are uncommon in Earth's history. The early Paleozoic Wuyi orogen in South China is one of the few examples of intraplate orogen, and is a key to understanding the process of intraplate orogenesis and global early Paleozoic geodynamics. In this study, we select teleseismic records from 45 mobile linear seismic stations deployed in Wuyi Mountain and 58 permanent stations setting in Jiangxi and Fujian provinces, from January 2011 to December 2012, and calculate the crustal thickness and average crustal Vp/Vs ratio using the H-κ stacking method. The main results include the following: 1) the crustal average Poission's ratio shows an increase tendency from land to sea, the interior of Wuyi orogen belt with an low ration less than 0.23, and the coastline with high ration which is up to 0.28, which indicate a very heterogeneous crustal structure and composition in Wuyi orogen and coast belt. 2) the crustal thickness ranges 28-34 km and shows a tendency of thinning from inland to coast in the region of SE China margin, which maight mean the eastern Eurasia lithospheric is extension and thinning induced by the subducted paleo-Pacific slab. To conclusion, we assume that Wuyi orogen experienced upper crustal thickening, lower crust and lithosphere delamination during the early Paleozoic orogeny, and lithosphere extension in Mesozoic. This research is founded by the Natural Science Foundation of China (41174052 and 41604048).

  1. Seismic crustal structure of the North China Craton and surrounding area: Synthesis and analysis

    Science.gov (United States)

    Xia, B.; Thybo, H.; Artemieva, I. M.

    2017-07-01

    We present a new digital model (NCcrust) of the seismic crustal structure of the Neoarchean North China Craton (NCC) and its surrounding Paleozoic-Mesozoic orogenic belts (30°-45°N, 100°-130°E). All available seismic profiles, complemented by receiver function interpretations of crustal thickness, are used to constrain a new comprehensive crustal model NCcrust. The model, presented on a 0.25° × 0.25°grid, includes the Moho depth and the internal structure (thickness and velocity) of the crust specified for four layers (the sedimentary cover, upper, middle, and lower crust) and the Pn velocity in the uppermost mantle. The crust is thin (30-32 km) in the east, while the Moho depth in the western part of the NCC is 38-44 km. The Moho depth of the Sulu-Dabie-Qinling-Qilian orogenic belt ranges from 31 km to 51 km, with a general westward increase in crustal thickness. The sedimentary cover is 2-5 km thick in most of the region, and typical thicknesses of the upper crust, middle crust, and lower crust are 16-24 km, 6-24 km, and 0-6 km, respectively. We document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence, the depth of the Moho. There is no systematic regional pattern in the average crustal Vp velocity and the Pn velocity. We examine correlation between the Moho depth and topography for seven tectonic provinces in the North China Craton and speculate on mechanisms of isostatic compensation.

  2. Orphan Basin crustal structure from a dense wide-angle seismic profile - Tomographic inversion

    Science.gov (United States)

    Watremez, Louise; Lau, K. W. Helen; Nedimović, Mladen R.; Louden, Keith E.; Karner, Garry D.

    2014-05-01

    Orphan Basin is located on the eastern margin of Canada, offshore of Newfoundland and East of Flemish Cap. It is an aborted continental rift formed by multiple episodes of rifting. The crustal structure across the basin has been determined by an earlier refraction study using 15 instruments on a 550 km long line. It shows that the continental crust was extended over an unusually wide region but did not break apart. The crustal structure of the basin thus documents stages in the formation of a magma-poor rifted margin up to crustal breakup. The OBWAVE (Orphan Basin Wide-Angle Velocity Experiment) survey was carried out to image crustal structures across the basin and better understand the processes of formation of this margin. The spacing of the 89 recording stations varies from 3 to 5 km along this 500-km-long line, which was acquired along a pre-existing reflection line. The highest resolution section corresponds to the part of the profile where the crust was expected to be the thinnest. We present the results from a joint tomography inversion of first and Moho reflected arrival times. The high data density allows us to define crustal structures with greater detail than for typical studies and to improve the understanding of the processes leading to the extreme stretching of continental crust. The final model was computed following a detailed parametric study to determine the optimal parameters controlling the ray-tracing and the inversion processes. The final model shows very good resolution. In particular, Monte Carlo standard deviations of crustal velocities and Moho depths are generally Orphan Basin is the result of rifting of a non-homogeneous Avalon terrane where the lower crust is primarily ductile.

  3. Monitoring Vertical Crustal Deformation and Gravity Variations during Water Level Changes at the Three Gorges Reservoir

    Directory of Open Access Journals (Sweden)

    WANG Wei

    2017-06-01

    Full Text Available Monitoring vertical crustal deformation and gravity changes during water level changes at the Three Gorges reservoir is important for the safe operation of the Three Gorges Dam and for the monitoring and prevention of a regional geological disaster. In this study, we determined vertical crustal deformation and gravity changes during water level variations of the Three Gorges reservoir from direct calculations and actual measurements and a comprehensive solution. We used water areas extracted image data from the ZY-3 satellite and water level data to calculate gravity changes and vertical crustal deformation caused by every 5 m change in the water level due to storage and drainage of the Three Gorges reservoir from 145 m to 175 m. The vertical crustal deformation was up to 30 mm. The location of gravity change above 20 μ Gal(1 Gal=10-2 m/s2 was less than 2 km from the centerline of the Yangtze River. The CORS ES13 in Badong, near the reservoir, measured the vertical crustal deformation during water level changes. Because of the small number of CORS and gravity stations in the Three Gorges reservoir area, monitoring deformation and gravity related to changes in the Three Gorges reservoir water level cannot be closely followed. Using 26 CORS and some of the gravity stations in the Three Gorges area and based on loading deformation and the spherical harmonic analysis method, an integrated solution of vertical deformation and gravity variations during water level changes of the reservoir was determined, which is consistent with the actual CORS monitoring results. By comparison, we found that an integrated solution based on a CORS network can effectively enhance the capability of monitoring vertical crustal deformation and gravity changes during water level variations of the reservoir.

  4. Modeling the blockage of Lg waves from 3-D variations in crustal structure

    Science.gov (United States)

    Sanborn, Christopher J.; Cormier, Vernon F.

    2018-05-01

    Comprised of S waves trapped in Earth's crust, the high frequency (2-10 Hz) Lg wave is important to discriminating earthquakes from explosions by comparing its amplitude and waveform to those of Pg and Pn waves. Lateral variations in crustal structure, including variations in crustal thickness, intrinsic attenuation, and scattering, affect the efficiency of Lg propagation and its consistency as a source discriminant at regional (200-1500 km) distances. To investigate the effects of laterally varying Earth structure on the efficiency of propagation of Lg and Pg, we apply a radiative transport algorithm to model complete, high-frequency (2-4 Hz), regional coda envelopes. The algorithm propagates packets of energy with ray theory through large-scale 3-D structure, and includes stochastic effects of multiple-scattering by small-scale heterogeneities within the large-scale structure. Source-radiation patterns are described by moment tensors. Seismograms of explosion and earthquake sources are synthesized in canonical models to predict effects on waveforms of paths crossing regions of crustal thinning (pull-apart basins and ocean/continent transitions) and thickening (collisional mountain belts), For paths crossing crustal thinning regions, Lg is amplified at receivers within the thinned region but strongly disrupted and attenuated at receivers beyond the thinned region. For paths crossing regions of crustal thickening, Lg amplitude is attenuated at receivers within the thickened region, but experiences little or no reduction in amplitude at receivers beyond the thickened region. The length of the Lg propagation within a thickened region and the complexity of over- and under-thrust crustal layers, can produce localized zones of Lg amplification or attenuation. Regions of intense scattering within laterally homogeneous models of the crust increase Lg attenuation but do not disrupt its coda shape.

  5. A hybrid origin of the Martian crustal dichotomy: Degree-1 convection antipodal to a giant impact

    Science.gov (United States)

    Citron, Robert I.; Manga, Michael; Tan, Eh

    2018-06-01

    The Martian crustal dichotomy is the stark ∼5 km difference in surface elevation and ∼26 km difference in crustal thickness between the northern lowlands and southern highlands that originated within 100s of Myr of Mars' formation. The origin of the dichotomy has broad implications for the geodynamic history of Mars, but purely exogenic or endogenic theories so far cannot explain all of the large scale geophysical observations associated with dichotomy formation. A giant impact can produce the shape and slope of the dichotomy boundary, but struggles to explain Mars' remanent crustal magnetic signatures and the ultimate formation of Tharsis. Degree-1 mantle convection can relate the crustal dichotomy to the formation of Tharsis, but does not explain the elliptical dichotomy shape and must be initiated by a large pre-existing viscosity jump in the mantle. We propose a hybrid model of dichotomy formation in which a giant impact induces degree-1 convection with an upwelling antipodal to the impact site. In this scenario, a giant impact in the northern hemisphere excavates crust, creating an initial difference in crustal thickness and possibly composition between the two hemispheres. Over 10s to 100s of Myr, the dominant upwelling(s) would migrate to be under the thicker, insulating crust in the southern hemisphere, generating melt that further thickens the southern crust. We examine this process using 3-D mantle convection simulations, and find that a hemispherical difference in crustal thickness and composition caused by a giant impact can induce degree-1 convection with the upwelling(s) antipodal to the impact site in <100 Myr.

  6. Coulomb stress change of crustal faults in Japan for 21 years, estimated from GNSS displacement

    Science.gov (United States)

    Nishimura, T.

    2017-12-01

    Coulomb stress is one of the simplest index to show how the fault is close to a brittle failure (e.g., earthquake). Many previous studies used the Coulomb stress change (ΔCFS) to evaluate whether the fault approaches failure and successfully explained an earthquake triggered by previous earthquakes and volcanic sources. Most studies use a model of a half-space medium with given rheological properties, boundary conditions, dislocation, etc. to calculate ΔCFS. However, Ueda and Takahashi (2005) proposed to calculate DCFS directly from surface displacement observed by GNSS. There are 6 independent components of stress tensor in an isotropic elastic medium. On the surface of the half-space medium, 3 components should be zero because of no traction on the surface. This means the stress change on the surface is calculated from the surface strain change using Hooke's law. Although an earthquake does not occur on surface, the stress change on the surface may approximate that at a depth of a shallow crustal earthquake (e.g., 10 km) if the source is far from the point at which we calculate the stress change. We tested it by comparing ΔCFS from the surface displacement and that from elastic fault models for past earthquakes. We first estimate a strain change with a method of Shen et al.(1996 JGR) from surface displacement and then calculate ΔCFS for a targeted focal mechanism. Although ΔCFS in the vicinity of the source fault cannot be reproduced from the surface displacement, surface displacement gives a good approximation of ΔCFS in a region 50 km away from the source if the target mechanism is a vertical strike-slip fault. It suggests that GNSS observation can give useful information on a recent change of earthquake potential. We, therefore, calculate the temporal evolution of ΔCFS on active faults in southwest Japan from April 1996 using surface displacement at GNSS stations. We used parameters for the active faults used for evaluation of strong motion by the

  7. Prediction of long-term crustal movement for geological disposal of radioactive waste

    International Nuclear Information System (INIS)

    Sasaki, Takeshi; Morikawa, Seiji; Tabei, Kazuto; Koide, Hitoshi; Tashiro, Toshiharu

    2000-01-01

    Long-term stability of the geological environment is essential for the safe geological disposal of radioactive waste, for which it is necessary to predict the crustal movement during an assessment period. As a case study, a numerical analysis method for the prediction of crustal movement in Japan is proposed. A three-dimensional elastic analysis by FEM for the geological block structure of the Kinki region and the Awaji-Rokko area is presented. Stability analysis for a disposal cavern is also investigated. (author)

  8. Fault offsets and lateral crustal movement on Europa - Evidence for a mobile ice shell

    International Nuclear Information System (INIS)

    Schenk, P.M.; Mckinnon, W.B.

    1989-01-01

    An examination is conducted of Europa's cross-cutting structural relationships between various lineament types, in order to constrain the type of structure involved in each such case and, where possible, to also constrain the degree of extension across the lineaments. Evidence is adduced for significant lateral crustal movement, allowing alternative models and mechanisms for lineament formation to be discussed, as well as plausible lithospheric and crustal models. The question as to whether any of the water-ice layer has been, or currently is, liquid, is also treated in light of the evidence obtained. 53 refs

  9. Terrestrial spreading centers under Venus conditions - Evaluation of a crustal spreading model for Western Aphrodite Terra

    Science.gov (United States)

    Sotin, C.; Senske, D. A.; Head, J. W.; Parmentier, E. M.

    1989-01-01

    The model of Reid and Jackson (1981) for terrestrial spreading centers is applied to Venus conditions. On the basis of spreading rate, mantle temperature, and surface temperature, the model predicts both isostatic topography and crustal thickness. The model and Pioneer Venus altimetry and gravity data are used to test the hypothesis of Head and Crumpler (1987) that Western Aphrodite Terra is the location of crustal spreading on Venus. It is concluded that a spreading center model for Ovda Regio in Western Aphrodite Terra could account for the observed topography and line-of-sight gravity anomalies found in the Pioneer data.

  10. On the geographical distribution of induced time-varying crustal magnetic fields

    DEFF Research Database (Denmark)

    Thebault, E.; Hemant, K.; Hulot, G.

    2009-01-01

    A long standing question in geomagnetism is whether the time variation of the induced crustal field is a detectable quantity and, if so, at which spatial wavelengths. We tackle this problem with the help of a forward modeling approach using a vertically integrated susceptibility (VIS) grid...... of the Earth's crust. For spherical harmonic degrees 15-90, we estimate the root mean square of the crustal magnetic field secular variation to amount 0.06-0.12 nT/yr at the terrestrial surface between epochs 1960-2002.5. The geographical distribution of the signal shows absolute values reaching 0.65-1.30 n...

  11. Vertical crustal movements in Southern California, 1974 to 1978

    Science.gov (United States)

    Burford, R.O.; Gilmore, Thomas D.

    1984-01-01

    An extensive resurvey of most of the first-order leveling network in southern California, known as the Southern California Releveling Program (SCRP), was carried out during the first 5 months of 1978. The primary scientific purpose of these measurements was to rapidly update the vertical control record throughout a recently uplifted region of southern California in order to more thoroughly document the vertical component of tectonic movement and to provide a reliable base for comparison with future levelings. Analyses of historic first-order leveling results have clearly demonstrated that a broad crustal upwarping, largely contained within a region consisting of the Transverse Ranges province and an area along the intervening section of the San Andreas fault system, had developed between about 1959 and 1974. Unfortunately, there is strong evidence that parts of the 1978 SCRP data are contaminated by the effects of intrasurvey tectonic deformation, limited surficial failures, and, less certainly, magnetically induced systematic error associated with the use of automatic levels. However, any distortions in leveling results caused by these or other factors are not so serious as to render the SCRP data useless. In fact, the bulk of these data can be accepted at face value, and most of the remaining data can be incorporated with some caution to augment the more reliable parts of the network. The evaluation of the 1978 leveling is based on a combination of circuit-misclosures, local timing of the field observations, analysis of profiles of apparent height changes derived from comparisons with previous levelings, and an analysis of the position and orientation of the various routes in relation to the regional structural grain and the gradients of differential vertical motion established by previous investigations. Comparisons of the 1978 SCRP results with the latest of the previous surveys along each route retained in the analysis show that all but about one-third of the

  12. Faults, fluids and friction : Effect of pressure solution and phyllosilicates on fault slip behaviour, with implications for crustal rheology

    NARCIS (Netherlands)

    Bos, B.

    2000-01-01

    In order to model the mechanics of motion and earthquake generation on large crustal fault zones, a quantitative description of the rheology of fault zones is prerequisite. In the past decades, crustal strength has been modeled using a brittle or frictional failure law to represent fault slip at

  13. Faults, fluids and friction : effect of pressure solution and phyllosilicates on fault slip behaviour, with implications for crustal rheology

    NARCIS (Netherlands)

    Bos, B.

    2000-01-01

    In order to model the mechanics of motion and earthquake generation on large crustal fault zones, a quantitative description of the rheology of fault zones is prerequisite. In the past decades, crustal strength has been modeled using a brittle or frictional failure law to represent fault slip

  14. Crustal structure beneath the southern Korean Peninsula from local earthquakes

    Science.gov (United States)

    Kim, Kwang-Hee; Park, Jung-Ho; Park, Yongcheol; Hao, Tian-Yao; Kim, Han-Joon

    2017-05-01

    The 3-D subsurface structure beneath the southern Korean Peninsula is poorly known, even though such information could be key in verifying or rejecting several competing models of the tectonic evolution of East Asia. We constructed a 3-D velocity model of the upper crust beneath the southern Korean Peninsula using 19 935 P-wave arrivals from 747 earthquakes recorded by high-density local seismic networks. Results show significant lateral and vertical variations: velocity increases from northwest to southeast at shallow depths, and significant velocity variations are observed across the South Korea Tectonic Line between the Okcheon Fold Belt and the Youngnam Massif. Collision between the North and South China blocks during the Early Cretaceous might have caused extensive deformation and the observed negative velocity anomalies in the region. The results of the tomographic inversion, combined with the findings of previous studies of Bouguer and isostatic gravity anomalies, indicate the presence of high-density material in the upper and middle crust beneath the Gyeongsang Basin in the southeastern Korean Peninsula. Although our results partially support the indentation tectonic model, it is still premature to discard other tectonic evolution models because our study only covers the southern half of the peninsula.

  15. Animal evolution

    DEFF Research Database (Denmark)

    Nielsen, Claus

    This book provides a comprehensive analysis of evolution in the animal kingdom. It reviews the classical, morphological information from structure and embryology, as well as the new data gained from studies using immune stainings of nerves and muscles and blastomere markings, which makes it possi......This book provides a comprehensive analysis of evolution in the animal kingdom. It reviews the classical, morphological information from structure and embryology, as well as the new data gained from studies using immune stainings of nerves and muscles and blastomere markings, which makes...

  16. A review of porosity-generating mechanisms in crustal shear zones

    Science.gov (United States)

    Fusseis, F.; Regenauer-Lieb, K.; Revets, S.

    2009-04-01

    Knowledge of the spatiotemporal characteristics of permeability is critical for the understanding of fluid migration in rocks. In diagenetic and metamorphic rocks different porosity-generating mechanisms contribute to permeability and so influence fluid migration and fluid/rock interaction. However, little is known about their relative contributions to the porosity architecture of a rock in a tectono-metamorphic environment. This presentation reviews porosity-generating mechanisms that affect fluid migration in shear zones, the most important crustal fluid conduits, in the context of the tectonometamorphic evolution of rocks. Mechanisms that generate porosity can be classified in a) those that involve the direct action of a fluid, b) processes in which a fluid partakes or that are supported by a fluid or c) mechanism that do not involve a fluid. a) Hydraulic fracturing, where it happens through the formation of tensile fractures, occurs where pore fluid pressures equalize the combined lithostatic pressure and strength of the rock (Etheridge et al., 1984, Cox & Etheridge, 1989, Oliver, 1996). Here an internally released (devolatilisation reactions, e.g., Rumble, 1994, Hacker, 1997, Yardley, 1997 and references therein) or externally derived (infiltrating from metamorphic, magmatic or meteoric sources, Baumgartner et al., 1997, Jamtveit et al., 1997, Thompson, 1997, Gleeson et al., 2003) fluid directly causes the mechanical failure of a rock. Where a fluid is in chemical disequilibrium with a rock (undersaturated with regard to a chemical species) minerals will be dissolved, generating dissolution porosity. Rocks ‘leached' by the removal of chemical components by vast amounts of fluid are reported to lose up to 60% of their original volume (e.g., Kerrich et al., 1984, McCaig 1988). Dissolution porosity is probably an underrated porosity-generating mechanism. It can be expected along the entire metamorphic evolution, including diagenesis (Higgs et al., 2007) and

  17. Cell evolution and Earth history: stasis and revolution.

    Science.gov (United States)

    Cavalier-Smith, Thomas

    2006-06-29

    This synthesis has three main parts. The first discusses the overall tree of life and nature of the last common ancestor (cenancestor). I emphasize key steps in cellular evolution important for ordering and timing the major evolutionary innovations in the history of the biosphere, explaining especially the origins of the eukaryote cell and of bacterial flagella and cell envelope novelties. Second, I map the tree onto the fossil record and discuss dates of key events and their biogeochemical impact. Finally, I present a broad synthesis, discussing evidence for a three-phase history of life. The first phase began perhaps ca 3.5 Gyr ago, when the origin of cells and anoxic photosynthesis generated the arguably most primitive prokaryote phylum, Chlorobacteria (= Chloroflexi), the first negibacteria with cells bounded by two acyl ester phospholipid membranes. After this 'chlorobacterial age' of benthic anaerobic evolution protected from UV radiation by mineral grains, two momentous quantum evolutionary episodes of cellular innovation and microbial radiation dramatically transformed the Earth's surface: the glycobacterial revolution initiated an oxygenic 'age of cyanobacteria' and, as the ozone layer grew, the rise of plankton; immensely later, probably as recently as ca 0.9 Gyr ago, the neomuran revolution ushered in the 'age of eukaryotes', Archaebacteria (arguably the youngest bacterial phylum), and morphological complexity. Diversification of glycobacteria ca 2.8 Gyr ago, predominantly inhabiting stratified benthic mats, I suggest caused serial depletion of 13C by ribulose 1,5-bis-phosphate caboxylase/oxygenase (Rubisco) to yield ultralight late Archaean organic carbon formerly attributed to methanogenesis plus methanotrophy. The late origin of archaebacterial methanogenesis ca 720 Myr ago perhaps triggered snowball Earth episodes by slight global warming increasing weathering and reducing CO2 levels, to yield runaway cooling; the origin of anaerobic methane

  18. Structure of the Kaoko Belt, Namibia: progressive evolution of a classic transpressional orogen

    Science.gov (United States)

    Goscombe, Ben; Hand, Martin; Gray, David

    2003-07-01

    The Kaoko Belt portion of the Damara Orogen, Namibia, is the deeply eroded core of a sinistral transpressional orogen that has half-flower structure geometry centred on the major, 4-5-km-wide Purros Mylonite Zone. Formed between the Congo Craton in the east and Rio De La Plata Craton in Brazil, the Kaoko Belt represents the northern coastal arm of a triple junction within the Pan-African Orogenic System. Consisting of reworked Archaean, Palaeoproterozoic and Mesoproterozoic basement and a cover of Neoproterozoic Damara Sequence, the Kaoko Belt can be sub-divided structurally into three parallel NNW-trending zones. The Eastern Kaoko Zone comprises sub-greenschist facies shelf carbonates that have been uprightly folded. The Central Kaoko Zone contains a slope and deep basin facies succession that has experienced intense deformation, including pervasive reworking of basement into large-scale east-vergent nappes. The Western Kaoko Zone is predominantly deep basin facies of high metamorphic grade intruded by numerous granites. It has experienced intense wrench-style deformation with formation of upright isoclines and steep, crustal-scale shear zones. The Kaoko Belt evolved through three distinct phases of a protracted Pan-African Orogeny in the late Neoproterozoic to Cambrian. (1) An early Thermal Phase (M 1) was responsible for pervasive partial melting and granite emplacement in the Western Kaoko Zone from 656 Ma. (2) The Transpressional Phase produced the geometry of the belt by progressive sinistral shearing between 580 and 550 Ma. Deformation was continuously progressive through two stages and involved both temporal and spatial migration of deformation outwards towards the margin. The early strike-slip Wrench-Stage produced a high-strain L-S fabric by sub-horizontal transport. Deformation became progressively more transpressive, with high-angle convergence and flattening strains during the Convergent-Stage. In this stage, strike-slip movements evolved through

  19. landforms evolution in collisional-dominated settings: the case of Northern Sicily (Central Mediterranean)

    Science.gov (United States)

    Nigro, Fabrizio; Renda, Pietro; Favara, Rocco

    2010-05-01

    In the young mountain chains underwent to emersion, the different crustal blocks which compose the belt may be subjected to differentiate tilting during uplift. The tilting process may be revealed both by the stratal pattern of the syn-uplifting deposits or deduced by the function altitude/area ratio. The prevailing of the uplift rate with respect to the tilting rate (and vice versa) result from the shape of this function. So, in young mountains the hypsometric analysis may results a useful tool for decipher how the crustal blocks are underwent to uplift. An integrate analysis based on stratigraphy, structural and morphometric data represents the correctly approach for characterise the landform evolution in regions underwent to active tectonics. In the aim to evaluate the recent tectonic history from topography in regions underwent to active deformations, by deducing the effect of tectonisms on landforms, the definition of the boundary conditions (regarding the crustal deformation) is fundamental for morphometric analysis. In fact, the morphologic style and the morphometric pattern in tectonically active settings are closely related to the dominance of rock masses exceeding for uplift (or failure for subsidence) with respect to the exogenous erosional processes. Collisional geodynamic processes induce crustal growth for faulting and folding. In this earth's sectors, the uplift of crustal blocks is a very common effect of compressional deformation. It reflects for example fold amplification and thrusting, but it is a very common process also in settings dominated by crustal thinning, where the viscoelastic properties of the lithosphere induce tilting and localised uplift of normal-faulted crustal blocks. The uplift rate is rarely uniform for wide areas within the orogens on the passive margins, but it changes from adjacent crustal blocks as the effect of space-variation of kinematics conditions or density. It also may change within a single block, as the effect of

  20. The topography of a continental indenter: The interplay between crustal deformation, erosion, and base level changes in the eastern Southern Alps

    Science.gov (United States)

    Heberer, B.; Prasicek, G.; Neubauer, F.; Hergarten, S.

    2017-01-01

    Abstract The topography of the eastern Southern Alps (ESA) reflects indenter tectonics causing crustal shortening, surface uplift, and erosional response. Fluvial drainages were perturbed by Pleistocene glaciations that locally excavated alpine valleys. The Late Miocene desiccation of the Mediterranean Sea and the uplift of the northern Molasse Basin led to significant base level changes in the far field of the ESA and the Eastern Alps (EA), respectively. Among this multitude of mechanisms, the processes that dominate the current topographic evolution of the ESA and the ESA‐EA drainage divide have not been identified. We demonstrate the expected topographic effects of each mechanism in a one‐dimensional model and compare them with observed channel metrics. We find that the normalized steepness index increases with uplift rate and declines from the indenter tip in the northwest to the foreland basin in the southeast. The number and amplitude of knickpoints and the distortion in longitudinal channel profiles similarly decrease toward the east. Changes in slope of χ‐transformed channel profiles coincide spatially with the Valsugana‐Fella fault linking crustal stacking and uplift induced by indenter tectonics with topographic evolution. Gradients in χ across the ESA‐EA drainage divide imply an ongoing, north directed shift of the Danube‐ESA watershed that is most likely driven by a base level rise in the northern Molasse basin. We conclude that the regional uplift pattern controls the geometry of ESA‐EA channels, while base level changes in the far field control the overall architecture of the orogen by drainage divide migration. PMID:28344912

  1. Gravity evidence for shaping of the crustal structure of the Ameca graben (Jalisco block northern limit). Western Mexico

    Science.gov (United States)

    Alatorre-Zamora, Miguel Angel; Campos-Enríquez, José Oscar; Fregoso-Becerra, Emilia; Quintanar-Robles, Luis; Toscano-Fletes, Roberto; Rosas-Elguera, José

    2018-03-01

    The Ameca tectonic depression (ATD) is located at the NE of the Jalisco Block along the southwestern fringe of the NW-SE trending Tepic-Zacoalco Rift, in the west-central part of the Trans-Mexican Volcanic Belt, western Mexico. To characterize its shallow crustal structure, we conducted a gravity survey based on nine N-S gravity profiles across the western half of the Ameca Valley. The Bouguer residual anomalies are featured by a central low between two zones of positive gravity values with marked gravity gradients. These anomalies have a general NW-SE trend similar to the Tepic-Zacoalco Rift general trend. Basement topography along these profiles was obtained by means of: 1) a Tsuboi's type inverse modeling, and 2) forward modeling. Approximately northward dipping 10° slopes are modeled in the southern half, with south tilted down faulted blocks of the Cretaceous granitic basement and its volcano-sedimentary cover along sub-vertical and intermediate normal faults, whereas southward dipping slopes of almost 15° are observed at the northern half. According to features of the obtained models, this depression corresponds to a slight asymmetric graben. The Ameca Fault is part of the master fault system along its northern limit. The quantitative interpretation shows an approximately 500 to 1100 m thick volcano-sedimentary infill capped by alluvial products. This study has several implications concerning the limit between the Jalisco Block and the Tepic-Zacoalco Rift. The established shallow crustal structure points to the existence of a major listric fault with its detachment surface beneath the Tepic-Zacoalco Rift. The Ameca Fault is interpreted as a secondary listric fault. The models indicate the presence of granitic bodies of the Jalisco Block beneath the TMVB volcanic products of the Tepic-Zacoalco rift. This implies that the limit between these two regional structures is not simple but involves a complex transition zone. A generic model suggests that the

  2. The relationship between the seismic characteristics of crustal structure in Shikoku Basin and en-echelon arrangement

    Science.gov (United States)

    Yamashita, M.; Takahashi, N.; Kodaira, S.; No, T.; Takizawa, K.; Miura, S.; Kaiho, Y.; Sato, T.; Kaneda, Y.

    2007-12-01

    Detailed crustal structure information of a back-arc basin must be obtained to elucidate the mechanism of its opening. Especially, the Shikoku Basin, which occupies the northern part of the Philippine Sea Plate between the Kyushu-Palau Ridge and the Izu-Ogasawara Arc, is an important area to elucidate the evolution of the back-arc basins as a part of the growth process of the Philippine Sea. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) carried out multi-channel seismic reflection survey using 12,000 cu.in. air gun and streamer with 204 ch hydrophones in the Izu-Ogasawara region since 2004. The total length of survey lines is more than 10,000 km until 2006. We investigate the crustal structure beneath the Shikoku Basin along 10 survey lines, which are across to the strike of the en-echelon seamount chains in the rear arc. From the seismic profiles, some faults and intrusion structures are obtained in the Shikoku Basin. The deformation structure with acoustic basement is widely distributed between the Shikoku Basin and the Izu-Ogasawara arc. Some intrusions structure is identified in the Shikoku Basin are exposed on seafloor. The intrusions structure is assumed to locate in the extended region of the en-echelon arrangement. The strike-slip faults with flower structure cutting whole sediments are located in the arc-backarc transition zone in the northern Shikoku Basin, suggesting that this region is in share stress. On the other hand, these structures indicating the deformation and intrusions are not recognized in the eastern side of the Kyushu-Palau Ridge. The Izu-Ogasawara arc is colliding to the Japan Island arc in the Sagami Bay. In the Nankai Trough, the Philippine Sea plate is subducting to the Japan Island arc. Therefore, the strike-slip and reverse fault would be developed by the compression stress in the eastern side of Philippine Sea plate. If the en-echelon arrangement is developed along these faults, the intrusions structure obtained by

  3. Representing Evolution

    DEFF Research Database (Denmark)

    Hedin, Gry

    2012-01-01

    . This article discusses Willumsen's etching in the context of evolutionary theory, arguing that Willumsen is a rare example of an artist who not only let the theory of evolution fuel his artistic imagination, but also concerned himself with a core issue of the theory, namely to what extent it could be applied...

  4. Security Evolution.

    Science.gov (United States)

    De Patta, Joe

    2003-01-01

    Examines how to evaluate school security, begin making schools safe, secure schools without turning them into fortresses, and secure schools easily and affordably; the evolution of security systems into information technology systems; using schools' high-speed network lines; how one specific security system was developed; pros and cons of the…

  5. Cepheid evolution

    International Nuclear Information System (INIS)

    Becker, S.A.

    1984-05-01

    A review of the phases of stellar evolution relevant to Cepheid variables of both Types I and II is presented. Type I Cepheids arise as a result of normal post-main sequence evolutionary behavior of many stars in the intermediate to massive range of stellar masses. In contrast, Type II Cepheids generally originate from low-mass stars of low metalicity which are undergoing post core helium-burning evolution. Despite great progress in the past two decades, uncertainties still remain in such areas as how to best model convective overshoot, semiconvection, stellar atmospheres, rotation, and binary evolution as well as uncertainties in important physical parameters such as the nuclear reaction rates, opacity, and mass loss rates. The potential effect of these uncertainties on stellar evolution models is discussed. Finally, comparisons between theoretical predictions and observations of Cepheid variables are presented for a number of cases. The results of these comparisons show both areas of agreement and disagreement with the latter result providing incentive for further research

  6. Venom Evolution

    Indian Academy of Sciences (India)

    IAS Admin

    Therefore, the platypus sequence was studied to quantify the role of gene duplication in the evolution of venom. ... Platypus venom is present only in males and is used for asserting dominance over com- petitors during the ... Certain toxin gene families are known to re- peatedly evolve through gene duplications. The rapidly ...

  7. The Crustal Thickness of the Philippine Sea Plate Derived from Gravity Data

    Directory of Open Access Journals (Sweden)

    Horng-Yuan Yen

    2015-01-01

    Full Text Available We constructed a new free-air gravity anomaly map of the Philippine Sea Plate (PSP using ship-tracked gravity data from the National Geophysical Data Center (NGDC. Our results show that the isogals trend correlates well with the tectonic structures in the PSP. After removing the gravity induced by sea water from the free-air gravity data, we obtained the regional Bouguer gravity anomaly, which is later used to compute the Moho geometry in the PSP by applying the Parker-Oldenburg iterative method. Our results indicate that in the southern part of the West Philippine Basin (WPB the crustal thickness is nearly homogeneous with a value of about 5 km, which implies that the WPB is quite stable. The low-amplitude and near-zero free-air gravity anomalies clearly indicate that the whole WPB, except at trenches and island arcs, is nearly in a state of isostatic equilibrium. The average crustal thickness of the Palau Kyushu Ridge (PKR is more than 10 km. In the eastern PSP the crustal thickness gradually increases eastward. Our results also imply that a relatively thin and low density mantle exists beneath the Parece Vela Basin (PVB as a consequence of back-arc spreading and serpentinized upwells of the thin crustal thickness.

  8. Crustal structure under the central High Atlas Mountains (Morocco) from geological and gravity data

    Science.gov (United States)

    Ayarza, P.; Alvarez-Lobato, F.; Teixell, A.; Arboleya, M. L.; Tesón, E.; Julivert, M.; Charroud, M.

    2005-05-01

    Seismic wide angle and receiver function results together with geological data have been used as constraints to build a gravity-based crustal model of the central High Atlas of Morocco. Integration of a newly acquired set of gravity values with public data allowed us to undertake 2-2.5D gravity modelling along two profiles that cross the entire mountain chain. Modelling suggests moderate crustal thickening, and a general state of Airy isostatic undercompensation. Localized thickening appears restricted to the vicinity of a north-dipping crustal-scale thrust fault, that offsets the Moho discontinuity and defines a small crustal root which accounts for the minimum Bouguer gravity anomaly values. Gravity modelling indicates that this root has a northeasterly strike, slightly oblique to the ENE general orientation of the High Atlas belt. A consequence of the obliquity between the High Atlas borders and its internal and deep structure is the lack of correlation between Bouguer gravity anomaly values and topography. Active buckling affecting the crust, a highly elevated asthenosphere, or a combination of both are addressed as side mechanisms that help to maintain the high elevations of the Atlas mountains.

  9. Papers presented to the Conference on Heat and Detachment in Crustal Extension on Continents and Planets

    Science.gov (United States)

    1985-01-01

    Several topics relative to heat and detachment in crustal extension on continents and planets are discussed. Rifting on Venus, heat flow and continental breakup, magnetism, the mountains and tectonic processes of Io, and the ductile extension of planetary lithospheres are among the topics covered.

  10. Millennia of magmatism recorded in crustal xenoliths from alkaline provinces in Southwest Greenland

    DEFF Research Database (Denmark)

    Smit, Matthijs; Waight, Tod Earle; Nielsen, Troels

    2016-01-01

    Neoproterozoic alkaline provinces in West Greenland: 1)Sarfartôq, which overlies Archean ultra-depleted SCLM and yielded ultra-deep mineral indicators, and 2)Sisimiut, where the SCLM is refertilized and deep xenoliths (>120km) are lacking. We focused on the rare and understudied crustal xenoliths, which preserve...

  11. Satellite gravity anomalies and crustal features of the Central Indian Ocean Basin

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, D.G.; Krishna, K.S.; Neprochnov, Y.P.; Grinko, B.N.

    -air anomalies due to volcanic constructs that trend in arcuate shape from north to south parallel to 82.5 degrees E are noted as southern continuity of the 85 degrees E Ridge, which terminates at the Afanasy Nikitin seamount. Change in rheology of the crustal...

  12. The crustal uplift determined at the Jakobshavn glacier (West Greenland) using ATM and GPS data

    DEFF Research Database (Denmark)

    Muresan, Ioana Stefania; Frumosu, Flavia Dalia; Khan, Shfaqat Abbas

    present both a predicted and observed crustal upliftfor the Jakobshavn glacier using ATM data (Airborne Topographic Mapper) from NASA ATM flights during 1997, 2005 and 2010 supplemented with data provided from continuous Global Positioning System (GPS), measurements made on bedrock between 2005...

  13. Existence of torsional surface waves in an earth's crustal layer lying ...

    Indian Academy of Sciences (India)

    This paper aims to study the dispersion of torsional surface waves in a crustal layer being sandwiched between a rigid boundary plane and a sandy mantle. In the mantle, rigidity and initial stress vary linearly while density remains constant. Dispersion relation has been deduced in a closed form by means of variable ...

  14. Revised crustal architecture of the southeastern Carpathian foreland from active and passive seismic data

    Science.gov (United States)

    Enciu, Dana M.; Knapp, Camelia C.; Knapp, James H.

    2009-08-01

    Integration of active and passive source seismic data is employed in order to study the nature of the relationships between crustal seismicity and geologic structures in the southeastern (SE) Carpathian foreland of Romania and the possible connection with the Vrancea Seismogenic Zone (VSZ) of intermediate-depth seismicity, one of the most active earthquake-prone areas in Europe. Crustal epicenters and focal mechanisms are correlated with four deep industry seismic profiles, the reprocessed Danube and Carpathian Integrated Action on Process in the Lithosphere and Neotectonics (DACIA PLAN) profile and the Deep Reflection Acquisition Constraining Unusual Lithospheric Activity II and III (DRACULA) profiles in order to understand the link between neotectonic foreland deformation and Vrancea mantle seismicity. Projection of crustal foreland hypocenters onto deep seismic profiles identifies several active crustal faults in the SE Carpathian foreland and suggests a mechanical coupling between the mantle located VSZ and the overlying foreland crust. The coupled associated deformation appears to take place on the Trotus Fault, the Sinaia Fault, and the newly detected Ialomita Fault. Seismic reflection imaging reveals the absence of west dipping reflectors in the crystalline crust and a slightly east dipping to horizontal Moho in the proximity of the Vrancea area. These findings argue against previously purported mechanisms to generate mantle seismicity in the VSZ including oceanic lithosphere subduction in place and oceanic slab break off, furthermore suggesting that the Vrancea seismogenic body is undetached from the overlying crust in the foreland.

  15. Magnetic crustal thickness in Greenland from CHAMP and Ørsted data

    DEFF Research Database (Denmark)

    Maule, Cathrine Fox; Purucker, Michael E.; Olsen, Nils

    2005-01-01

    and observatory data. After correcting for the remanent magnetization, we determine the vertically integrated magnetization of the crust. Making some simplifying assumptions about the susceptibility, the thickness of the magnetic crust is determined by iteratively improving an initial crustal thickness model...

  16. Geothermal modeling along a two-dimensional crustal profile in Southern Portugal

    Czech Academy of Sciences Publication Activity Database

    Correia, A.; Šafanda, Jan

    2002-01-01

    Roč. 34, č. 1 (2002), s. 47-61 ISSN 0264-3707. [Geothermics at the turn of the century. Evora, 03.04.2000-07.04.2000] Institutional research plan: CEZ:AV0Z3012916 Keywords : geothermal modeling * Southern Portugal * surface heat flow * crustal profile Subject RIV: DC - Siesmology, Volcanology, Earth Structure Impact factor: 1.058, year: 2002

  17. Global maps of the CRUST 2.0 crustal components stripped gravity disturbances

    NARCIS (Netherlands)

    Tenzer, R.; Hamayun, K.; Vajda, P.

    2009-01-01

    We use the CRUST 2.0 crustal model and the EGM08 geopotential model to compile global maps of the gravity disturbances corrected for the gravitational effects (attractions) of the topography and of the density contrasts of the oceans, sediments, ice, and the remaining crust down to the Moho

  18. Real-time and on-demand buoy observation system for tsunami and crustal displacement

    Science.gov (United States)

    Takahashi, N.; Imai, K.; Ishihara, Y.; Fukuda, T.; Ochi, H.; Suzuki, K.; Kido, M.; Ohta, Y.; Imano, M.; Hino, R.

    2017-12-01

    We develop real-time and on-demand buoy observation system for tsunami and crustal displacement. It is indispensable for observation of crustal displacement to understand changes of stress field related to future large earthquakes. The current status of the observation is carried out by using a vessel with an interval of a few times per a year. When a large earthquake occurs, however, we need dense or on-demand observation of the crustal displacement to grasp nature of the slow slip after the rupture. Therefore, we constructed buoy system with a buoy station, wire-end station, seafloor unit and acoustic transponders for crustal displacement, and we installed a pressure sensor on the seafloor unit and GNSS system on the buoy in addition to measurement of e distance between the buoy and the seafloor acoustic transponders. Tsunami is evaluated using GNSS data and pressure data sent from seafloor. Observation error of the GNSS is about 10 cm. The crustal displacement is estimated using pressure sensor for vertical and acoustic measurement for horizontal. Using current slack ratio of 1.58, the observation error for the measurement of the crustal displacement is about 10 cm. We repeated three times sea trials and confirmed the data acquisition with high data quality, mooring without dredging anchor in the strong sea current with a speed of 5.5 knots. Current issues to be resolved we face are removing noises on the acoustic data transmission, data transmission between the buoy and wire-end stations, electrical consumption on the buoy station and large observation error on the crustal displacement due to large slack ratio. We consider the change of the acoustic transmission for pressure data, replace of a GNSS data logger with large electrical consumption, and reduce of the slack ratio, and search method to reduce resistance of the buoy on the sea water. In this presentation, we introduce the current status of the technical development and tsunami waveforms recorded on our

  19. Crustal inheritance and arc magmatism: Magnetotelluric constraints from the Washington Cascades on top-down control

    Science.gov (United States)

    Bedrosian, P.; Peacock, J.; Bowles-martinez, E.; Schultz, A.; Hill, G.

    2017-12-01

    Worldwide, arc volcanism occurs along relatively narrow magmatic arcs, the locations of which are considered to mark the onset of dehydration reactions within the subducting slab. This `bottom-up' approach, in which the location of arc volcanism reflects where fluids and melt are generated, explains first-order differences in trench-to-arc distance and is consistent with known variations in the thermal structure and geometry of subducting slabs. At a finer scale, arc segmentation, magmatic gaps, and anomalous forearc and backarc magmatism are also frequently interpreted in terms of variations in slab geometry, composition, or thermal structure.The role of inherited crustal structure in controlling faulting and deformation is well documented; less well examined is the role of crustal structure in controlling magmatism. While the source distribution of melt and subduction fluids is critical to determining the location of arc magmatism, we argue that crustal structure provides `top-down' control on patterns or seismicity and deformation as well as the channeling and ascent of arc magmas. We present evidence within the Washington Cascades based upon correlation between a new three-dimensional resistivity model, potential-field data, seismicity, and Quaternary volcanism. We image a mid-Tertiary batholith, intruded within an Eocene crustal suture zone, and extending throughout much of the crustal column. This and neighboring plutons are interpreted to channel crustal fluids and melt along their margins within steeply dipping zones of marine to transitional metasedimentary rock. Mount St. Helens is interpreted to be fed by fluids and melt generated further east at greater slab depths, migrating laterally (underplating?) beneath the Spirit Lake batholith, and ascending through metasedimentary rocks within the brittle crust. At a regional scale, we argue that this concealed suture zone controls present-day deformation and seismicity as well as the distribution of forearc

  20. Modeling of periodic great earthquakes on the San Andreas fault: Effects of nonlinear crustal rheology

    Science.gov (United States)

    Reches, Ze'ev; Schubert, Gerald; Anderson, Charles

    1994-01-01

    We analyze the cycle of great earthquakes along the San Andreas fault with a finite element numerical model of deformation in a crust with a nonlinear viscoelastic rheology. The viscous component of deformation has an effective viscosity that depends exponentially on the inverse absolute temperature and nonlinearity on the shear stress; the elastic deformation is linear. Crustal thickness and temperature are constrained by seismic and heat flow data for California. The models are for anti plane strain in a 25-km-thick crustal layer having a very long, vertical strike-slip fault; the crustal block extends 250 km to either side of the fault. During the earthquake cycle that lasts 160 years, a constant plate velocity v(sub p)/2 = 17.5 mm yr is applied to the base of the crust and to the vertical end of the crustal block 250 km away from the fault. The upper half of the fault is locked during the interseismic period, while its lower half slips at the constant plate velocity. The locked part of the fault is moved abruptly 2.8 m every 160 years to simulate great earthquakes. The results are sensitive to crustal rheology. Models with quartzite-like rheology display profound transient stages in the velocity, displacement, and stress fields. The predicted transient zone extends about 3-4 times the crustal thickness on each side of the fault, significantly wider than the zone of deformation in elastic models. Models with diabase-like rheology behave similarly to elastic models and exhibit no transient stages. The model predictions are compared with geodetic observations of fault-parallel velocities in northern and central California and local rates of shear strain along the San Andreas fault. The observations are best fit by models which are 10-100 times less viscous than a quartzite-like rheology. Since the lower crust in California is composed of intermediate to mafic rocks, the present result suggests that the in situ viscosity of the crustal rock is orders of magnitude

  1. Three-dimensional Crustal Structure beneath the Tibetan Plateau Revealed by Multi-scale Gravity Analysis

    Science.gov (United States)

    Xu, C.; Luo, Z.; Sun, R.; Li, Q.

    2017-12-01

    The Tibetan Plateau, the largest and highest plateau on Earth, was uplifted, shorten and thicken by the collision and continuous convergence of the Indian and Eurasian plates since 50 million years ago, the Eocene epoch. Fine three-dimensional crustal structure of the Tibetan Plateau is helpful in understanding the tectonic development. At present, the ordinary method used for revealing crustal structure is seismic method, which is inhibited by poor seismic station coverage, especially in the central and western plateau primarily due to the rugged terrain. Fortunately, with the implementation of satellite gravity missions, gravity field models have demonstrated unprecedented global-scale accuracy and spatial resolution, which can subsequently be employed to study the crustal structure of the entire Tibetan Plateau. This study inverts three-dimensional crustal density and Moho topography of the Tibetan Plateau from gravity data using multi-scale gravity analysis. The inverted results are in agreement with those provided by the previous works. Besides, they can reveal rich tectonic development of the Tibetan Plateau: (1) The low-density channel flow can be observed from the inverted crustal density; (2) The Moho depth in the west is deeper than that in the east, and the deepest Moho, which is approximately 77 km, is located beneath the western Qiangtang Block; (3) The Moho fold, the directions of which are in agreement with the results of surface movement velocities estimated from Global Positioning System, exists clearly on the Moho topography.This study is supported by the National Natural Science Foundation of China (Grant No. 41504015), the China Postdoctoral Science Foundation (Grant No. 2015M572146), and the Surveying and Mapping Basic Research Programme of the National Administration of Surveying, Mapping and Geoinformation (Grant No. 15-01-08).

  2. Detailed crustal structure of the North China and its implication for seismicity

    Science.gov (United States)

    Jiang, Wenliang; Wang, Xin; Tian, Tian; Zhang, Jingfa; Wang, Donglei

    2014-02-01

    Since the Mesozoic-Cenozoic era the North China Craton has experienced an important tectonic transition and it has given rise to complicated crustal structure and strong earthquake activity. Based on the large-scale surface gravity data, we studied the detailed crustal structure and seismogenic mechanism of the North China. The results indicate that the North China presents typical characteristics of adjoining depression and uplift, alternating basins and hills, inhomogeneous density and also great differences in crustal structure and Moho topography. The upper and middle crustal structures are dominated by the NNE-striking tectonic units, with many faults cut down to the middle crust. The lower crust is characterized by the folding-structure, with high and low-density placed alternately from west to east, presenting lateral heterogeneous feature. Adjusted by the gravity isostasy, Moho topography of the North China fluctuates greatly. Compared with the North China Basin, crustal thickness in the Western Taihang, northern Yanshan and Luzhong areas are much thicker while those densities are lower than the North China Basin. The dominating tectonic direction of the Moho topography strikes NE to NNE and undulates alternately from west to east. The epicenters are mostly concentrated in the upper and middle crust, especially the transitional areas between the high and low-gravity anomalies. The Tancheng earthquake in 1668, Sanhe earthquake in 1673, Tangshan earthquake in 1976, and all other seismic tectonic zones of the North China are all distributed in area where magma moves strongly beneath the crust, which is considered to be related to the movement of the high density, unstable and heat flows along the deep passage from the uppermost and asthenosphere due to the subduction of the Pacific slab towards the Eurasian plate.

  3. Crustal insights from gravity and aeromagnetic analysis: Central North Slope, Alaska

    Science.gov (United States)

    Saltus, R.W.; Potter, C.J.; Phillips, J.D.

    2006-01-01

    Aeromagnetic and gravity data are processed and interpreted to reveal deep and shallow information about the crustal structure of the central North Slope, Alaska. Regional aeromagnetic anomalies primarily reflect deep crustal features. Regional gravity anomalies are more complex and require detailed analysis. We constrain our geophysical models with seismic data and interpretations along two transects including the Trans-Alaska Crustal Transect. Combined geophysical analysis reveals a remarkable heterogeneity of the pre-Mississippian basement. In the central North Slope, pre-Mississippian basement consists of two distinct geophysical domains. To the southwest, the basement is dense and highly magnetic; this basement is likely mafic and mechanically strong, possibly acting as a buttress to basement involvement in Brooks Range thrusting. To the northeast, the central North Slope basement consists of lower density, moderately magnetic rocks with several discrete regions (intrusions?) of more magnetic rocks. A conjugate set of geophysical trends, northwest-southeast and southwest-northeast, may be a factor in the crustal response to tectonic compression in this domain. High-resolution gravity and aeromagnetic data, where available, reflect details of shallow fault and fold structure. The maps and profile models in this report should provide useful guidelines and complementary information for regional structural studies, particularly in combination with detailed seismic reflection interpretations. Future challenges include collection of high-resolution gravity and aeromagnetic data for the entire North Slope as well as additional deep crustal information from seismic, drilling, and other complementary methods. Copyrights ?? 2006. The American Association of Petroleum Geologists. All rights reserved.

  4. Strain transformation between tectonic extrusion and crustal thickening in the growth of the Tibetan Plateau

    Science.gov (United States)

    Liu, M.; Li, Y.; Sun, Y.; Shen, X.

    2017-12-01

    The Indo-Eurasian continental collision since 50 Ma has thickened the crust to raise the Himalayan-Tibetan Plateau and driven lateral extrusion of Asian lithospheric blocks to affect Cenozoic tectonics in central and east Asia. The relative roles of crustal thickening and tectonic extrusion, and the strain partitioning between them over time and space, remain controversial. We have analyzed the strain rates using GPS velocities, and correlated the results with vertical motion derived from precise leveling. We found that tectonic extrusion largely transforms to crustal thickening near the margins of the Tibetan Plateau. Near the NW margin of the Tibetan Plateau, the shear stain transforms to compressive strain, consistent with neotectonic studies that indicate crustal shortening and uplift. Around the SE margin, shear stain largely terminates in the southern Yunnan province of China. The present-day crustal motion in SE Tibetan Plateau can be well explained by gravitational spreading without invoking plate-edge push as envisioned in the tectonic extrusion model. Using data collected from local seismic arrays, we derived receiver functions to image the lithospheric structures across the Tibetan Plateau and the Alashan block to its north and the Ordos block to its east. Our results indicate that the mantle lithosphere of these bounding Asian blocks has not been reworked by Tibetan tectonics; instead they have acted as restrictive walls to the growing Tibetan Plateau. Our finite element modeling shows that crustal deformation along the margins of the Tibetan Plateau are consistent with the notion that the east- and southeastward extrusion of the Tibetan lithosphere is largely confined to the Tibetan Plateau because of the restrictive bounding blocks of the Asian lithosphere. Thus the tectonic impact of the Indo-Eurasian collision on the Cenozoic Asian tectonics may not be as extensive as previously thought.

  5. Topside ionosphere of Mars: Variability, transient layers, and the role of crustal magnetic fields

    Science.gov (United States)

    Gopika, P. G.; Venkateswara Rao, N.

    2018-04-01

    The topside ionosphere of Mars is known to show variability and transient topside layers. In this study, we analyzed the electron density profiles measured by the radio occultation technique aboard the Mars Global Surveyor spacecraft to study the topside ionosphere of Mars. The electron density profiles that we used in the present study span between 1998 and 2005. All the measurements are done from the northern high latitudes, except 220 profiles which were measured in the southern hemisphere, where strong crustal magnetic fields are present. We binned the observations into six measurement periods: 1998, 1999-north, 1999-south, 2000-2001, 2002-2003, and 2004-2005. We found that the topside ionosphere in the southern high latitudes is more variable than that from the northern hemisphere. This feature is clearly seen with fluctuations of wavelengths less than 20 km. Some of the electron density profiles show a transient topside layer with a local maximum in electron density between 160 km and 210 km. The topside layer is more prone to occur in the southern hemispheric crustal magnetic field regions than in the other regions. In addition, the peak density of the topside layer is greater in regions of strong crustal magnetic fields than in other regions. The variability of the topside ionosphere and the peak density of the topside layer, however, do not show one-to-one correlation with the strength of the crustal magnetic fields and magnetic field inclination. The results of the present study are discussed in the light of current understanding on the topside ionosphere, transient topside layers, and the role of crustal magnetic fields on plasma motions.

  6. Evolution of uranogenic and thorogenic lead. Pt. 2

    International Nuclear Information System (INIS)

    Amov, B.G.

    1983-01-01

    On the basis of a dynamic model of a continuous Pb isotope evolution, the variations in the isotopic ratios of uranogenic and thorogenic Pb in a number of young ore deposits, ocean sediments and volcanic rocks from mid-ocean ridges, oceanic islands, island arcs and continents are evaluated. The deviations in the Th/U ratio from its model values are calculated. A model parameter T which indicates a more ancient enrichment in U/Pb (crustal-type Pb sources) or younger enrichment in U/Pb (mantle-type Pb sources) is also calculated. The relative change in the Th/U ratio is close to 1 for mantle-type Pb sources (lower T values) and decreases for crustal-type Pb sources (higher T values). An interpretation of these changes could be connected with the differences in the U and Th geochemical behaviours in metamorphic processes under the conditions of a deeper (mantle) or crustal source owing to the differences in their valence states. In some cases they have similar (U 4+ and Th 4+ ) migration abilities; in others (U 6+ and Th 4+ ), considerably different. (orig.)

  7. Nudging Evolution?

    OpenAIRE

    Katharine N. Farrell; Andreas Thiel

    2013-01-01

    This Special Feature, "Nudging Evolution? Critical Exploration of the Potential and Limitations of the Concept of Institutional Fit for the Study and Adaptive Management of Social-Ecological Systems," aims to contribute toward the development of social theory and social research methods for the study of social-ecological system dynamics. Our objective is to help strengthen the academic discourse concerning if, and if so, how, to what extent, and in what concrete ways the concept of institut...

  8. Community Evolution

    OpenAIRE

    Saganowski, Stanisław; Bródka, Piotr; Kazienko, Przemysław

    2016-01-01

    The continuous interest in the social network area contributes to the fast development of this field. The new possibilities of obtaining and storing data facilitate deeper analysis of the entire social network, extracted social groups and single individuals as well. One of the most interesting research topic is the network dynamics and dynamics of social groups in particular, it means analysis of group evolution over time. It is the natural step forward after social community extraction. Havi...

  9. Evolution of Neutron Stars and Observational Constraints

    Directory of Open Access Journals (Sweden)

    Lattimer J.

    2010-10-01

    Full Text Available The structure and evolution of neutron stars is discussed with a view towards constraining the properties of high density matter through observations. The structure of neutron stars is illuminated through the use of several analytical solutions of Einstein’s equations which, together with the maximally compact equation of state, establish extreme limits for neutron stars and approximations for binding energies, moments of inertia and crustal properties as a function of compactness. The role of the nuclear symmetry energy is highlighted and constraints from laboratory experiments such as nuclear masses and heavy ion collisions are presented. Observed neutron star masses and radius limits from several techniques, such as thermal emissions, X-ray bursts, gammaray flares, pulsar spins and glitches, spin-orbit coupling in binary pulsars, and neutron star cooling, are discussed. The lectures conclude with a discusson of proto-neutron stars and their neutrino signatures.

  10. Crustal anisotropy from Moho converted Ps wave splitting and geodynamic implications in Northeastern margin of Tibetan Plateau

    Science.gov (United States)

    Xie, Z.; Wu, Q.; Zhang, R.

    2017-12-01

    Collision between Indian and Eurasian result in intense deformation and crustal shortening in the Tibetan Plateau. NE margin of Tibetan Plateau experienced complex deformation between Qilian orogen and its adjacent blocks, Alxa Block in the north and Ordos Block in the east. We focus on if there any evidences exist in the NE margin of Tibetan Plateau, which can support crustal channel flow model. China Earthquake Administration had deployed temporary seismic array which is called ChinaArray Phase Ⅱ, dense seismic stations covered NE margin of Tibetan Plateau. Seismic data recorded by 81 seismic stations is applied in this research. We calculated receiver functions with time-domain deconvolution. We selected RFs which have clear Ps phase both in radial and transverse components to measure Ps splitting owing to crustal anisotropy, and 130 pairs of anisotropy parameters of 51 seismic stations were obtained. We would like to discuss about dynamic mechanism of this area using crustal anisotropy associated with the result of SKS-splitting and surface constrains like GPS velocity. The result can be summarized as follows. The large scale of delay time imply that the crustal anisotropy mainly derives from middle to lower crust rather than upper crust. In the southeastern part of the research area, crustal anisotropy is well agree with the result computed form SKS-splitting and GPS velocity directions trending NWW-SEE or E-W direction. This result imply a vertically coherent deformation in the area as the directions of crustal anisotropy trend to be perpendicular to the direction of normal stress. In the middle and north part of the research area, the fast polarization direction of crustal anisotropy is NEE-SWW or E-W direction, parallels with direction of GPS velocity, but differ to the direction of the result of SKS-splitting. This result may imply that decoupled deformation in this area associated with middle to lower crustal flow.

  11. Evaluation of six NEHRP B/C crustal amplification models proposed for use in western North America

    Science.gov (United States)

    Boore, David; Campbell, Kenneth W.

    2016-01-01

    We evaluate six crustal amplification models based on National Earthquake Hazards Reduction Program (NEHRP) B/C crustal profiles proposed for use in western North America (WNA) and often used in other active crustal regions where crustal properties are unknown. One of the models is based on an interpolation of generic rock velocity profiles previously proposed for WNA and central and eastern North America (CENA), in conjunction with material densities based on an updated velocity–density relationship. A second model is based on the velocity profile used to develop amplification factors for the Next Generation Attenuation (NGA)‐West2 project. A third model is based on a near‐surface velocity profile developed from the NGA‐West2 site database. A fourth model is based on velocity and density profiles originally proposed for use in CENA but recently used to represent crustal properties in California. We propose two alternatives to this latter model that more closely represent WNA crustal properties. We adopt a value of site attenuation (κ0) for each model that is either recommended by the author of the model or proposed by us. Stochastic simulation is used to evaluate the Fourier amplification factors and their impact on response spectra associated with each model. Based on this evaluation, we conclude that among the available models evaluated in this study the NEHRP B/C amplification model of Boore (2016) best represents median crustal amplification in WNA, although the amplification models based on the crustal profiles of Kamai et al. (2013, 2016, unpublished manuscript, see Data and Resources) and Yenier and Atkinson (2015), the latter adjusted to WNA crustal properties, can be used to represent epistemic uncertainty.

  12. Sensitivity analysis of crustal correction and its error propagation to upper mantle residual gravity and density anomalies

    DEFF Research Database (Denmark)

    Herceg, Matija; Artemieva, Irina; Thybo, Hans

    2013-01-01

    ) uncertainties in the velocity-density conversion and (ii) uncertainties in knowledge of the crustal structure (thickness and average Vp velocities of individual crustal layers, including the sedimentary cover). In this study, we address both sources of possible uncertainties by applying different conversions...... from velocity to density and by introducing variations into the crustal structure which corresponds to the uncertainty of its resolution by high-quality and low-quality seismic models. We examine the propagation of these uncertainties into determinations of lithospheric mantle density. The residual...

  13. A preliminary investigation of vertical crustal movements in the United Kingdom in the context of subsurface nuclear waste isolation

    International Nuclear Information System (INIS)

    1981-03-01

    Two types of change will influence the environment of a subsurface nuclear waste isolation facility: natural geological changes and changes caused by the construction of the waste repository and introduction of the waste. This report is concerned with vertical crustal movements, which are an expression of natural geological changes. Vertical crustal movements observed outside the United Kingdom are reviewed, and vertical movements in a test region of the UK investigated by comparison of geodetic levellings. The implications of vertical crustal movement to waste isolation facilities and some potentially valuable lines of research are discussed. (author)

  14. TopoGreenland: crustal structure in central-eastern Greenland along a new refraction profile

    Science.gov (United States)

    Shulgin, Alexey; Thybo, Hans; Field Team TopoGreenland

    2013-04-01

    We present the seismic structure in the interior of Greenland based on the first measurements by the seismic refraction/wide angle reflection method. Previous seismic surveys have only been carried out offshore and near the coast of Greenland, where the crustal structure is affected by oceanic break-up and may not be representative of the interior of the island. Acquisition of geophysical data in onshore Greenland is logistically complicated by the presence of an up to 3.4 km thick ice sheet, permanently covering most of the land mass. The seismic data was acquired by a team of six people during a two-month long experiment in summer of 2011 on the ice cap in the interior of central-eastern Greenland. The EW-trending profile extends 310 km inland from the approximate edge of the stable ice cap near Scoresby Sund across the center of the ice cap. The planned extension of the profile by use of OBSs and air gun shooting in Scoresbysund Fjord to the east coast of Greenland was unfortunately canceled, because navigation was prevented by ice drift. 350 Reftek Texan receivers recorded high-quality seismic data from 8 equidistant shots along the profile. Explosive charge sizes were 1 ton at the ends and ca. 500 kg along the profile, loaded with about 125 kg at 35-85 m depth in individual boreholes. Two-dimensional velocity model based on tomographic inversion and forward ray tracing modeling shows a decrease of crustal thickness from 47 km below the center of Greenland in the western part to 40 km in the eastern part of the profile. Earlier studies show that crustal thickness further decreases eastward to ca. 30 km below the fjord system, but details of the changes are unknown. Relatively high lower crustal velocities (Vp 6.8 - 7.3) in the western part of the TopoGreenland profile may indicate past collision tectonics or may be related or to the passage of the Iceland mantle plume. The origin of the pronounced circum-Atlantic mountain ranges in Norway and eastern Greenland

  15. Cluster evolution

    International Nuclear Information System (INIS)

    Schaeffer, R.

    1987-01-01

    The galaxy and cluster luminosity functions are constructed from a model of the mass distribution based on hierarchical clustering at an epoch where the matter distribution is non-linear. These luminosity functions are seen to reproduce the present distribution of objects as can be inferred from the observations. They can be used to deduce the redshift dependence of the cluster distribution and to extrapolate the observations towards the past. The predicted evolution of the cluster distribution is quite strong, although somewhat less rapid than predicted by the linear theory

  16. Crystallization of oxidized, moderately hydrous arc basalt at mid-to-lower crustal pressures

    Science.gov (United States)

    Blatter, D. L.; Sisson, T. W.; Hankins, W. B.

    2012-12-01

    basalts to rhyolites or granites (ASI/wt% SiO2: 0.012-0.014), and do not commonly become peraluminous until SiO2 exceeds 69 wt%. These relations are consistent with plagioclase accompanying mafic silicates during nearly all evolution of arc magmas, with little or no early crystallization interval dominated by augite, unlike these and other high-pressure crystallization results. Possible implications are: (1) Parental basaltic arc magmas generally have <3 wt% H2O, and so saturate with plagioclase earlier than in these and other investigators' experiments, (2) These and other investigators' basaltic starting compositions have appropriate H2O but insufficient normative plagioclase, and so crystallize excessive augite before saturating with plagioclase, (3) Common parental arc magmas are basaltic andesites, not basalts, and undergo early crystallization dominated by orthopyroxene that does not modify melt ASI, and/or (4) The spectrum of common arc magma compositions is dominantly due to mixing of mafic magmas with evolved crustal melts or residual liquids that are saturated with plagioclase, thereby causing the coupled and consistent increase in SiO2 with ASI. What can be stated with confidence is that the simple case of deep crystallization-differentiation of hydrous basalt produces daughter liquids that differ in important aspects from common arc magmas.

  17. Full 40 km crustal reflection seismic datasets in several Indonesian basins

    Science.gov (United States)

    Dinkelman, M. G.; Granath, J. W.; Christ, J. M.; Emmet, P. A.; Bird, D. E.

    2010-12-01

    Long offset, deep penetration regional 2D seismic data sets have been acquired since 2002 by GX Technology in a number of regions worldwide (www.iongeo.com/Data_Libraries/Spans/). Typical surveys consist of 10+ lines located to image specific critical aspects of basin structure. Early surveys were processed to 20 km, but more recent ones have extended to 40-45 km from 16 sec records. Pre-stack time migration is followed by pre-stack depth migration using gravity and in some cases magnetic modeling to constrain the velocity structure. We illustrate several cases in the SE Asian and Australasian area. In NatunaSPAN™ two generations of inversion can be distinguished, one involving Paleogene faults with Neogene inversion and one involving strike slip-related uplift in the West Natuna Basin. Crustal structure in the very deep Neogene East Natuna Basin has also been imaged. The JavaSPAN™ program traced Paleogene sediments onto oceanic crust of the Flores Sea, thus equating back arc spreading there to the widespread Eocene extension. It also imaged basement in the Makassar Strait beneath as much as 6 km of Cenozoic sedimentary rocks that accumulated Eocene rift basins (the North and South Makassar basins) on the edge of Sundaland, the core of SE Asia. The basement is seismically layered: a noisy upper crust overlies a prominent 10 km thick transparent zone, the base of which marks another change to slightly noisier reflectivity. Eocene normal faults responsible for the opening of extensional basins root in the top of the transparent layer which may be Moho or a brittle-ductile transition within the extended continental crust. Of particular significance is the first image of thick Precambrian basins comprising the bulk of continental crust under the Arafura Sea in the ArafuraSPAN™ program. Four lines some 1200 km long located between Australia and New Guinea on the Arafura platform image a thin Phanerozoic section overlying a striking Precambrian basement composed of

  18. Columbia River volcanism - The question of mantle heterogeneity or crustal contamination

    Science.gov (United States)

    Carlson, R. W.; Lugmair, G. W.; Macdougall, J. D.

    1981-01-01

    It is found that, although crystal fractionation played an important role in producing the chemical characteristics of Columbia River Province basalts displaying a wide range of chemical and isotopic compositions, the isotopic variability calls for the involvement of at least two isotopically distinct components. The major and trace element characteristics of the main volume of the basalts are not consistent with a metasomatized mantle source region, and the presence of a primordial mantel component is not supported by the chemical data. Models of simple binary mixing between a primary magma and Precambrian sialic crustal materials, while satisfying the observed Nd and Sr isotopic variations, fail to account for major trace element abundances. The combination of crustal assimilation and fractional crystalization is found to give a superior fit to the compositional data.

  19. The gravity field and crustal structure of the northwestern Arabian Platform in Jordan

    Science.gov (United States)

    Batayneh, A. T.; Al-Zoubi, A. S.

    2001-01-01

    The Bouguer gravity field over the northwestern Arabian Platform in Jordan is dominated by large variations, ranging from -132 to +4 mGal. A study of the Bouguer anomaly map shows that the gravity field maintains a general north-northeasterly trend in the Wadi Araba-Dead Sea-Jordan Riff, Northern Highlands and Northeast Jordanian Limestone Area, while the remainder of the area shows north-northwesterly-trending gravity anomalies. Results of 2-D gravity modeling of the Bouguer gravity field indicate that the crustal thickness in Jordan is ˜ 38 km, which is similar to crustal thicknesses obtained from refraction data in northern Jordan and Saudi Arabia, and from gravity data in Syria.

  20. Evidence for crustal recycling during the Archean: the parental magmas of the stillwater complex

    International Nuclear Information System (INIS)

    McCallum, I.S.

    1988-01-01

    The petrology and geochemistry of the Stillwater Complex, an Archean (2.7 Ga) layered mafic intrusion in the Beartooth Mountains of Montana, is discussed. Efforts to reconstruct the compositions of possible parental magmas and thereby place some constraints on the composition and history of their mantle source regions was studied. A high-Mg andesite or boninite magma best matches the crystallization sequences and mineral compositions of Stillwater cumulates, and represents either a primary magma composition or a secondary magma formed, for example, by assimilation of crustal material by a very Mg-rich melt such as komatiite. Isotopic data do not support the extensive amounts of assimilation required by the komatiite parent hypothesis, and it is argued that the Stillwater magma was generated from a mantle source that had been enriched by recycling and homogenization of older crustal material over a large area

  1. Crustal geomagnetic field - Two-dimensional intermediate-wavelength spatial power spectra

    Science.gov (United States)

    Mcleod, M. G.

    1983-01-01

    Two-dimensional Fourier spatial power spectra of equivalent magnetization values are presented for a region that includes a large portion of the western United States. The magnetization values were determined by inversion of POGO satellite data, assuming a magnetic crust 40 km thick, and were located on an 11 x 10 array with 300 km grid spacing. The spectra appear to be in good agreement with values of the crustal geomagnetic field spatial power spectra given by McLeod and Coleman (1980) and with the crustal field model given by Serson and Hannaford (1957). The spectra show evidence of noise at low frequencies in the direction along the satellite orbital track (N-S). indicating that for this particular data set additional filtering would probably be desirable. These findings illustrate the value of two-dimensional spatial power spectra both for describing the geomagnetic field statistically and as a guide for diagnosing possible noise sources.

  2. The use of satellite laser observations in studying the crustal movements

    Directory of Open Access Journals (Sweden)

    Gamal F. Attia

    2012-12-01

    Full Text Available The mutual tectonic displacements of the lithospheric blocks take place within the deep fracture dividing them into hundreds and thousands kilometers long. It is possible to suggest that the reason of the accumulation of considerable local shift deformations is the change of the velocity of the tectonic motion in some or other parts of fractures as a result of different physical, chemical and mechanical processes. Nowadays, the range precision of Satellite Laser Ranging (SLR technique reaches a few millimeters level. Therefore, the space geodesy technique becomes a very important tool in detecting and monitoring recent crustal movements. Regular repeated measurements of the baselines between some stations on different plates give the possibility to construct precise and detail models of crustal movements. In this paper, the length of four baselines between Helwan-SLR station and other four SLR stations are calculated using satellite geodetical technique.

  3. An evaluation of information on vertical crustal movements pertaining to deep disposal

    International Nuclear Information System (INIS)

    Gale, J.E.; Quinlan, G.; Rogerson, R.; Welhan, J.

    1986-03-01

    The geological and historical information on the magnitude and distribution of uplift and differential movements of rock masses as well as groundwater flow system transients that result from glacial unloading, erosion and tectonic stress have been reviewed. Data presented in the literature show that vertical crustal movements have occurred during the Cenozoic. In addition, the literature indicates significant transients exist in groundwater flow systems. The documented evidence of vertical crustal movements, plus supporting data on the stress-permeability constitutive relationships for discontinuities in fractured crystalline rocks, and three-dimensional modelling capability justifies a detailed analysis of the effects of vertical uplift on bedrock and on groundwater as they pertain to the deep disposal of radioactive waste. 159 annotated refs

  4. CHEMICAL EVOLUTION

    Energy Technology Data Exchange (ETDEWEB)

    Calvin, Melvin

    1965-06-01

    How did life come to be on the surface of the earth? Darwin himself recognized that his basic idea of evolution by variation and natural selection must be a continuous process extending backward in time through that period in which the first living things arose and into the period of 'Chemical Evolution' which preceded it. We are approaching the examination of these events by two routes. One is to seek for evidence in the ancient rocks of the earth which were laid down prior to that time in which organisms capable of leaving their skeletons in the rocks to be fossilized were in existence. This period is sometime prior to approximately 600 million years ago. The earth is believed to have taken its present form approximately 4700 million years ago. We have found in rocks whose age is about 1000 million years certain organic molecules which are closely related to the green pigment of plants, chlorophyll. This seems to establish that green plants were already fluorishing prior to that time. We have now found in rocks of still greater age, namely, 2500 million years, the same kinds of molecules mentioned above which can be attributed to the presence of living organisms. If these molecules are as old as the rocks, we have thus shortened the time available for the generation of the complex biosynthetic sequences which give rise to these specific hydrocarbons (polyisoprenoids) to less than 2000 million years.

  5. Northeastern Brazilian margin: Regional tectonic evolution based on integrated analysis of seismic reflection and potential field data and modelling

    Science.gov (United States)

    Blaich, Olav A.; Tsikalas, Filippos; Faleide, Jan Inge

    2008-10-01

    Integration of regional seismic reflection and potential field data along the northeastern Brazilian margin, complemented by crustal-scale gravity modelling, is used to reveal and illustrate onshore-offshore crustal structure correlation, the character of the continent-ocean boundary, and the relationship of crustal structure to regional variation of potential field anomalies. The study reveals distinct along-margin structural and magmatic changes that are spatially related to a number of conjugate Brazil-West Africa transfer systems, governing the margin segmentation and evolution. Several conceptual tectonic models are invoked to explain the structural evolution of the different margin segments in a conjugate margin context. Furthermore, the constructed transects, the observed and modelled Moho relief, and the potential field anomalies indicate that the Recôncavo, Tucano and Jatobá rift system may reflect a polyphase deformation rifting-mode associated with a complex time-dependent thermal structure of the lithosphere. The constructed transects and available seismic reflection profiles, indicate that the northern part of the study area lacks major breakup-related magmatic activity, suggesting a rifted non-volcanic margin affinity. In contrast, the southern part of the study area is characterized by abrupt crustal thinning and evidence for breakup magmatic activity, suggesting that this region evolved, partially, with a rifted volcanic margin affinity and character.

  6. SPECIALIZED MAPPING OF CRUSTAL FAULT ZONES. PART 1: BASIC THEORETICAL CONCEPTS AND PRINCIPLES

    Directory of Open Access Journals (Sweden)

    K. Zh. Seminsky

    2014-01-01

    Full Text Available Long-term studies of shear zones have included collection of data on fractures showing no indication of displacement which are termed as 'blank' fractures. A method aimed at mapping fault structures and stress fields has been developed on the basis of results of paragenetic analysis of measurements of abundant fractures. The method is termed as 'specialized mapping', firstly, due to its specific structural goal so that to distinguish it from the conventional geological mapping of regions in nature, and, secondly, because of the specific procedure applied to refer to fractures as references to decipher fault-block patterns of natural regions. In Part 1, basic theoretical concepts and principles of specialized mapping are described. Part 2 is being prepared for publication in one of the next issues of the journal; it will cover stages of the proposed method and describe some of the cases of its application.In terms of general organizational principles, specialized mapping is similar to other methods based on structural paragenetic analysis and differs from such methods in types of paragenesises viewed as references to reveal crustal fault zones. Such paragenesises result from stage-by-stage faulting (Fig 2 and Fig. 7 during which stress fields of the 2nd order are regularly changeable within the shear zone. According to combined experimental and natural data, a complete paragenesis of fractures in the shear zone includes a major (1st order fault plane and fractures of other seven types, R, R’, n, n’, t, t’ and T (2nd order (Fig. 4 and Fig 8. At the fracture level, each of them corresponds to a paragenesis including three nearly perpendicular systems of early ruptures (Fig. 1, which are based on two classical patterns of conjugated fractures, one of which is consistent with the position of the fault plane (Fig. 3. Taking into account that strike-slip, reverse and normal faults are similar in terms of mechanics (i.e. they are formed due to

  7. New aeromagnetic data of eastern Dronning Maud Land: implications for the spatial extent of a major Early Neoproterozoic juvenile crustal province

    Science.gov (United States)

    Ruppel, A. S.; Jacobs, J.; Eagles, G.; Läufer, A.; Jokat, W.

    2017-12-01

    A long-standing collaboration between Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and the Federal Institute for Geosciences and Natural Resources (BGR) aims to investigate the sub-ice crustal architecture and tectonic evolution of East Antarctica. Its main emphasis is on Dronning Maud Land (DML). During the austral summers 2014 and 2015, ca. 40.000 line kilometre of new magnetic, gravity and ice-penetrating radar data were collected with 10 km line spacing. Here, we report on magnetic anomaly data to the east and south of Sør Rondane (eastern DML), analysed with several filtering techniques. These data are integrated with exposure information from Sør Rondane, the Belgica Mts., and the Yamato Mts.. The study area covers the eastern part of a major, recently revealed Early Neoproterozoic juvenile crustal block, the Tonian Oceanic Arc Super Terrane (TOAST). The western extent of the TOAST is well defined by the Forster Magnetic Anomaly and characterized by a province of subdued SE-striking parallel positive magnetic anomalies in the mostly ice-covered region of south-eastern DML (the SE DML province). Geological investigations showed that this area can be correlated with exposures in Sør Rondane and scattered nunataks west of it. U-Pb ages of ca. 1000-900 Ma, are documented from zircons of gabbro-trondhjemite-tonalite-granodiorite (GTTG) suites in both areas. Further, geochemical analyses prove a juvenile character of the GTTGs, which are interpreted as oceanic arc complexes. Glacial drift from southern Sør Rondane points to an inland continuation of the TOAST, so far of unknown dimensions. The new magnetic data constrain the southern and eastern minimum extent of the TOAST, which we think has a minimum area of 450.000 km2. The spatial extent of this major juvenile crustal province has major significance for the tectonic reconstruction of East Antarctica and its involvement in Rodinia since it is suggested having evolved

  8. The influence of tectonic inheritance on crustal extension style following failed subduction of continental crust: applications to metamorphic core complexes in Papua New Guinea

    Science.gov (United States)

    Biemiller, J.; Ellis, S. M.; Little, T.; Mizera, M.; Wallace, L. M.; Lavier, L.

    2017-12-01

    The structural, mechanical and geometric evolution of rifted continental crust depends on the lithospheric conditions in the region prior to the onset of extension. In areas where tectonic activity preceded rift initiation, structural and physical properties of the previous tectonic regime may be inherited by the rift and influence its development. Many continental rifts form and exhume metamorphic core complexes (MCCs), coherent exposures of deep crustal rocks which typically surface as arched or domed structures. MCCs are exhumed in regions where the faulted upper crust is displaced laterally from upwelling ductile material along a weak detachment fault. Some MCCs form during extensional inversion of a subduction thrust following failed subduction of continental crust, but the degree to which lithospheric conditions inherited from the preceding subduction phase control the extensional style in these systems remains unclear. For example, the Dayman Dome in Southeastern Papua New Guinea exposes prehnite-pumpellyite to greenschist facies rocks in a smooth 3 km-high dome exhumed with at least 24 km of slip along one main detachment normal fault, the Mai'iu Fault, which dips 21° at the surface. The extension driving this exhumation is associated with the cessation of northward subduction of Australian continental crust beneath the oceanic lithosphere of the Woodlark Plate. We use geodynamic models to explore the effect of pre-existing crustal structures inherited from the preceding subduction phase on the style of rifting. We show that different geometries and strengths of inherited subduction shear zones predict three distinct modes of subsequent rift development: 1) symmetric rifting by newly formed high-angle normal faults; 2) asymmetric rifting along a weak low-angle detachment fault extending from the surface to the brittle-ductile transition; and 3) extension along a rolling-hinge structure which exhumes deep crustal rocks in coherent rounded exposures. We

  9. Crustal and mantle structure and anisotropy beneath the incipient segments of the East African Rift System: Preliminary results from the ongoing SAFARI

    Science.gov (United States)

    Yu, Y.; Reed, C. A.; Gao, S. S.; Liu, K. H.; Massinque, B.; Mdala, H. S.; moidaki, M.; Mutamina, D. M.; Atekwana, E. A.; Ingate, S. F.; Reusch, A.; Barstow, N.

    2013-12-01

    Despite the vast wealth of research conducted toward understanding processes associated with continental rifting, the extent of our knowledge is derived primarily from studies focused on mature rift systems, such as the well-developed portions of the East African Rift System (EARS) north of Lake Malawi. To explore the dynamics of early rift evolution, the SAFARI (Seismic Arrays for African Rift Initiation) team deployed 50 PASSCAL broadband seismic stations across the Malawi, Luangwa, and Okavango rifts of the EARS during the summer of 2012. The cumulative length of the profiles is about 2500 km and the planned recording duration is 2 years. Here we present the preliminary results of systematic analyses of data obtained from the first year of acquisition for all 50 stations. A total of 446 high-quality shear-wave splitting measurements using PKS, SKKS, and SKS phases from 84 teleseismic events were used to constrain fast polarization directions and splitting times throughout the region. The Malawi and Okavango rifts are characterized by mostly NE trending fast directions with a mean splitting time of about 1 s. The fast directions on the west side of the Luangwa Rift Zone are parallel to the rift valley, and those on the east side are more N-S oriented. Stacking of approximately 1900 radial receiver functions reveals significant spatial variations of both crustal thickness and the ratio of crustal P and S wave velocities, as well as the thickness of the mantle transition zone. Stations situated within the Malawi rift demonstrate a southward increase in observed crustal thickness, which is consistent with the hypothesis that the Malawi rift originated at the northern end of the rift system and propagated southward. Both the Okavango and Luangwa rifts are associated with thinned crust and increased Vp/Vs, although additional data is required at some stations to enhance the reliability of the observations. Teleseismic P-wave travel-time residuals show a delay of about

  10. Olivine-bearing lithologies on the Moon: Constraints on origins and transport mechanisms from M3 spectroscopy, radiative transfer modeling, and GRAIL crustal thickness

    Science.gov (United States)

    Corley, Laura M.; McGovern, Patrick J.; Kramer, Georgiana Y.; Lemelin, Myriam; Trang, David; Gillis-Davis, Jeffrey J.; Taylor, G. Jeffrey; Powell, Kathryn E.; Kiefer, Walter S.; Wieczorek, Mark; Zuber, Maria T.

    2018-01-01

    High-resolution hyperspectral data from Chandrayaan-1's Moon Mineralogy Mapper (M3) allow detection of olivine on the lunar surface. Olivine exposed at the surface may originate as mantle material or igneous products (intrusive or extrusive). Potential transport mechanisms include excavation of the mantle or lower crustal material by impacts that form basins and complex craters, differentiation of impact melt sheets, or magmatic emplacement of lavas, cumulates, or xenoliths. A sample of the lunar mantle, which has not been conclusively identified in the lunar sample collection, would yield fundamental new insights into the composition, structure, and evolution of the lunar interior. Olivine identified in remote spectral data is generally accepted to originate from the primary mantle, because abundant olivine is expected to exist in the mantle and lower crust, yet have sparse occurrences in the upper crust. In this study, we identified 111 M3 single-pixel spectra with characteristic absorption features consistent with olivine at Crisium, Nectaris, and Humorum basins and near the craters Roche and Tsiolkovsky. In an effort to determine the origins and transport mechanisms that led to these individual exposures, we estimated mineral abundances using radiative transfer modeling and examined crustal thickness estimates, topography and slope maps, and images from the Lunar Reconnaissance Orbiter Camera (LROC). At Crisium basin, where crustal thickness is near 0 km (Wieczorek et al., 2013), mantle olivine may have been exposed by basin-forming impact and deposited on the rim. Picard crater, which is superposed on the floor of Crisium, also exhibits potential mantle olivine in its ejecta. Within Nectaris basin, olivine exposures are confined to the rims of small craters on the mare, which are inferred to excavate a layer of olivine-rich mare basalt. Olivine occurrences on the rim of Humorum basin, including those located on a graben, are likely to be cumulates of shallow

  11. The Crustal Magnetization Mapping in the Ocean Basin of the South China Sea and its Tectonic Implications

    Science.gov (United States)

    Guo, L.; Meng, X.

    2015-12-01

    The South China Sea (SCS), surrounded by the Eurasia, Pacific and India-Australia plates, was formed by the interaction of the three plates and the Cenozoic seafloor spreading. Magnetic data is the crucial data for understanding tectonic evolution and seafloor spreading model in the SCS. Magnetization intensity is related closely to rock type and tectonics. Through magnetization mapping, the distribution of apparent magnetization in the subsurface will be obtained, benefiting in lithologic classification and geological mapping. Due to strong remanence presented in the oceanic crust, magma and seamounts in the SCS, the magnetization directions are complex and heterogeneous, quite different from the modern geomagnetic field directions. However, the routine techniques for magnetization mapping are based on negligence of remanence. The normalized source strength (NSS), one quantity transformed from the magnetic anomalies, is insensitive to remanence and responds well to the true locations of magnetic sources. The magnetization mapping based on the NSS will effectively reduce effects of remanence, benefitting in better geological interpretation. Here, we assembled high-resolution total magnetic intensity (TMI) data around the ocean basin of the SCS, and then transformed them into the NSS. Then we did magnetization mapping based on the NSS to obtain the crustal magnetization distribution in the studied area. The results show that the magnetization distribution inside of each subbasin is relatively homogeneous, but that of eastern subbasin is mostly strong with amplitude of 0.2A/m~4.2A/m, while that of southwestern subbasin is weak with amplitude of 0.2A/m~1.1A/m. It implies that magnetic structure and tectonic features in the crust are discriminative between both subbasins, and the tectonic boundary between both subbasins is roughly ranges from the northeastern edge of the Zhongsha Islands running in the southeast direction to the northeastern edge of the Reed Bank.

  12. Crustal and Upper Mantle Structure from Joint Inversion of Body Wave and Gravity Data

    Science.gov (United States)

    2012-09-01

    We use both free-air and Bouguer gravity anomalies derived from the global gravity model of the GRACE satellite mission. The gravity data provide...relocation analysis. We use both free-air and Bouguer gravity anomalies derived from the global gravity model of the GRACE satellite mission. The gravity...topographic relief this effect needs to be removed; thus, we converted free-air anomalies into Bouguer anomalies assuming a standard density for crustal rocks

  13. Elementary Theoretical Forms for the Spatial Power Spectrum of Earth's Crustal Magnetic Field

    Science.gov (United States)

    Voorhies, C.

    1998-01-01

    The magnetic field produced by magnetization in Earth's crust and lithosphere can be distinguished from the field produced by electric currents in Earth's core because the spatial magnetic power spectrum of the crustal field differs from that of the core field. Theoretical forms for the spectrum of the crustal field are derived by treating each magnetic domain in the crust as the point source of a dipole field. The geologic null-hypothesis that such moments are uncorrelated is used to obtain the magnetic spectrum expected from a randomly magnetized, or unstructured, spherical crust of negligible thickness. This simplest spectral form is modified to allow for uniform crustal thickness, ellipsoidality, and the polarization of domains by an periodically reversing, geocentric axial dipole field from Earth's core. Such spectra are intended to describe the background crustal field. Magnetic anomalies due to correlated magnetization within coherent geologic structures may well be superimposed upon this background; yet representing each such anomaly with a single point dipole may lead to similar spectral forms. Results from attempts to fit these forms to observational spectra, determined via spherical harmonic analysis of MAGSAT data, are summarized in terms of amplitude, source depth, and misfit. Each theoretical spectrum reduces to a source factor multiplied by the usual exponential function of spherical harmonic degree n due to geometric attenuation with attitude above the source layer. The source factors always vary with n and are approximately proportional to n(exp 3) for degrees 12 through 120. The theoretical spectra are therefore not directly proportional to an exponential function of spherical harmonic degree n. There is no radius at which these spectra are flat, level, or otherwise independent of n.

  14. Temperature and redox effect on mineral colonization in Juan de Fuca Ridge flank subsurface crustal fluids

    Directory of Open Access Journals (Sweden)

    Jean-Paul eBaquiran

    2016-03-01

    Full Text Available To examine microbe-mineral interactions in subsurface oceanic crust, we evaluated microbial colonization on crustal minerals that were incubated in borehole fluids for one year at the seafloor wellhead of a crustal borehole observatory (IODP Hole U1301A, Juan de Fuca Ridge flank as compared to an experiment that was not exposed to subsurface crustal fluids (at nearby IODP Hole U1301B. In comparison to previous studies at these same sites, this approach allowed assessment of the effects of temperature, fluid chemistry, and/or mineralogy on colonization patterns of different mineral substrates, and an opportunity to verify the approach of deploying colonization experiments at an observatory wellhead at the seafloor instead of within the borehole. The Hole U1301B deployment did not have biofilm growth, based on microscopy and DNA extraction, thereby confirming the integrity of the colonization design against bottom seawater intrusion. In contrast, the Hole U1301A deployment supported biofilms dominated by Epsilonproteobacteria (43.5% of 370 16S rRNA gene clone sequences and Gammaproteobacteria (29.3%. Sequence analysis revealed overlap in microbial communities between different minerals incubated at the Hole U1301A wellhead, indicating that mineralogy did not separate biofilm structure within the one-year colonization experiment. Differences in the Hole U1301A wellhead biofilm community composition relative to previous studies from within the borehole using similar mineral substrates suggest that temperature and the diffusion of dissolved oxygen through plastic components influenced the mineral colonization experiments positioned at the wellhead. This highlights the capacity of low abundance crustal fluid taxa to rapidly establish communities on diverse mineral substrates under changing environmental conditions such as from temperature and oxygen.

  15. Geochemistry of granitoid rocks from the western Superior Province: Evidence for 2- and 3-stage crustal evolution models

    Science.gov (United States)

    Beakhouse, G. P.; errane) are discussed.

    1986-01-01

    The Superior Province is divisible into subprovinces that can be classified as greenstone-tonalite, paragneiss, or batholitic terranes and are distinguished by differences in lithologic proportions, metamorphic grade, and structural style. The origin and significance of contrasting geochemical characteristics of plutonic rocks from the Winnipeg River subprovince (a batholithic terrane) and the Wabigoon subprovince (a greenstone-tonalite terrane) are discussed.

  16. Thermo-rheological aspects of crustal evolution during continental breakup and melt intrusion : The Main Ethiopian Rift, East Africa

    NARCIS (Netherlands)

    Lavecchia, Alessio; Beekman, Fred; Clark, Stuart R.; Cloetingh, Sierd A P L

    2016-01-01

    The Cenozoic-Quaternary Main Ethiopian Rift (MER) is characterized by extended magmatic activity. Although magmatism has been recognized as a key element in the process of continental breakup, the interaction between melts and intruded lithosphere is still poorly understood. We have performed a 2D

  17. Directed evolution and in silico analysis of reaction centre proteins reveal molecular signatures of photosynthesis adaptation to radiation pressure.

    Directory of Open Access Journals (Sweden)

    Giuseppina Rea

    2011-01-01

    Full Text Available Evolutionary mechanisms adopted by the photosynthetic apparatus to modifications in the Earth's atmosphere on a geological time-scale remain a focus of intense research. The photosynthetic machinery has had to cope with continuously changing environmental conditions and particularly with the complex ionizing radiation emitted by solar flares. The photosynthetic D1 protein, being the site of electron tunneling-mediated charge separation and solar energy transduction, is a hot spot for the generation of radiation-induced radical injuries. We explored the possibility to produce D1 variants tolerant to ionizing radiation in Chlamydomonas reinhardtii and clarified the effect of radiation-induced oxidative damage on the photosynthetic proteins evolution. In vitro directed evolution strategies targeted at the D1 protein were adopted to create libraries of chlamydomonas random mutants, subsequently selected by exposures to radical-generating proton or neutron sources. The common trend observed in the D1 aminoacidic substitutions was the replacement of less polar by more polar amino acids. The applied selection pressure forced replacement of residues more sensitive to oxidative damage with less sensitive ones, suggesting that ionizing radiation may have been one of the driving forces in the evolution of the eukaryotic photosynthetic apparatus. A set of the identified aminoacidic substitutions, close to the secondary plastoquinone binding niche and oxygen evolving complex, were introduced by site-directed mutagenesis in un-transformed strains, and their sensitivity to free radicals attack analyzed. Mutants displayed reduced electron transport efficiency in physiological conditions, and increased photosynthetic performance stability and oxygen evolution capacity in stressful high-light conditions. Finally, comparative in silico analyses of D1 aminoacidic sequences of organisms differently located in the evolution chain, revealed a higher ratio of residues

  18. Aeromagnetic study of the Hengshan-Wutai-Fuping region: Unraveling a crustal profile of the Paleoproterozoic Trans-North China Orogen

    Science.gov (United States)

    Zhang, Jian; Zhao, Guochun; Shen, Wenlue; Li, Sanzhong; Sun, Min

    2015-11-01

    An integrated crustal profile of the intervening Trans-North China Orogen (TNCO) is one of the key issues to understanding the tectonic evolution of the North China Craton. However, the existing geological studies focus only on the surface-mapping based petrological, geochemical and structural analysis, but lack subsurface geophysical evidence and thus make the crustal profile interpretations ambiguous. In contrast, the current geophysical data covers a very large-scale lithospheric mantle and fails to image the detailed structural pattern of the orogenic crust. To achieve this goal, we present high-resolution aeromagnetic data for the Hengshan-Wutai-Fuping region, the largest exposure of the central TNCO. The reduced-to-pole magnetic anomaly map firstly verifies the regional tectonic subdivision that the high-grade metamorphic terranes (i.e. Hengshan and Fuping Complexes) are consistent with high-magnetic responses and long-wavelength anomalies, intervened by a low-grade terrane (Wutai Complex) characterized by low-magnetic responses and short-wavelength anomalies. 3D Euler deconvolution reveals that the tendencies of the clustered solutions show large consistence with the major structural pattern of the region which is characterized by a fan-shaped doubly-vergent orogenic wedge. Upward continuation further shows that the northwest part of the orogen yields a thicker crust and is most likely located closer to the paleosubduction zone. The new aeromagnetic data, combined with structural, petrological and metamorphic data indicate that an eastward-dipping subduction zone was most possibly active before the collision of the Western and Eastern Blocks, leading to the formation of the TNCO and the final amalgamation of the North China Craton.

  19. Present-day crustal deformation and strain transfer in northeastern Tibetan Plateau

    Science.gov (United States)

    Li, Yuhang; Liu, Mian; Wang, Qingliang; Cui, Duxin

    2018-04-01

    The three-dimensional present-day crustal deformation and strain partitioning in northeastern Tibetan Plateau are analyzed using available GPS and precise leveling data. We used the multi-scale wavelet method to analyze strain rates, and the elastic block model to estimate slip rates on the major faults and internal strain within each block. Our results show that shear strain is strongly localized along major strike-slip faults, as expected in the tectonic extrusion model. However, extrusion ends and transfers to crustal contraction near the eastern margin of the Tibetan Plateau. The strain transfer is abrupt along the Haiyuan Fault and diffusive along the East Kunlun Fault. Crustal contraction is spatially correlated with active uplifting. The present-day strain is concentrated along major fault zones; however, within many terranes bounded by these faults, intra-block strain is detectable. Terranes having high intra-block strain rates also show strong seismicity. On average the Ordos and Sichuan blocks show no intra-block strain, but localized strain on the southwestern corner of the Ordos block indicates tectonic encroachment.

  20. Crustal structure of the SW Iberian passive margin: The westernmost remnant of the Ligurian Tethys?

    Science.gov (United States)

    Ramos, A.; Fernández, O.; Torne, M.; Sánchez de la Muela, A.; Muñoz, J. A.; Terrinha, P.; Manatschal, G.; Salas, M. C.

    2017-05-01

    At present, the SW Iberian margin is located along the convergent Iberia-Nubia plate boundary. In Mesozoic times, the margin was located at the triple junction of the Ligurian Tethys, Central Atlantic and Northern Atlantic. The characterization of its crustal structure has allowed us to propose a configuration for this triple junction and to determine the role that this transform margin played within the plate kinematic system. In this paper we present an integrated study based on the interpretation of a 2D regional multichannel seismic survey consisting of 58 profiles, tied with onshore geology and exploratory wells, and on gravimetric modeling performed over four NW-SE trending profiles. Integrated interpretation of MCS data combined with 2D gravity modeling reveals a complex pattern in the southward crustal thinning of SW Iberia and supports the possible presence of oceanic crust under the Gulf of Cadiz. The tapering of Iberian crust is characterized by steps with rapid changes in the thickness of the crust, and thinning to Bank. Margin inversion and the pre-existing extensional crustal structure are responsible for the areal distribution and amplitude of the prominent positive gravity anomaly observed in the Gulf of Cadiz.

  1. Distinct crustal isostasy trends east and west of the Rocky Mountain Front

    KAUST Repository

    Schmandt, Brandon

    2015-12-14

    © 2015. American Geophysical Union. All Rights Reserved. Seismic structure beneath the contiguous U.S. was imaged with multimode receiver function stacking and inversion of Rayleigh wave dispersion and ellipticity measurements. Crust thickness and elevation are weakly correlated across the contiguous U.S., but the correlation is ∼3-4 times greater for separate areas east and west of the Rocky Mountain Front (RMF). Greater lower crustal shear velocities east of the RMF, particularly in low-elevation areas with thick crust, are consistent with deep crustal density as the primary cause of the contrasting crust thickness versus elevation trends. Separate eastern and western trends are best fit by Airy isostasy models that assume lower crust to uppermost mantle density increases of 0.18 g/cm3 and 0.40 g/cm3, respectively. The former value is near the minimum that is plausible for felsic lower crust. Location of the transition at the RMF suggests that Laramide to post-Laramide processes reduced western U.S. lower crustal density.

  2. Martian crustal dichotomy: product of accretion and not a specific event?

    International Nuclear Information System (INIS)

    Frey, H.; Schultz, R.A.; Maxwell, T.A.

    1987-01-01

    Attempts to explain the fundamental crustal dichotomy on Mars range from purely endogenic to extreme exogenic processes, but to date no satisfactory theory has evolved. What is accepted is: (1) the dichotomy is an ancient feature of the Martian crust, and (2) the boundary between the cratered highlands and northern plains which marks the dichotomy in parts of Mars has undergone significant and variable modification during the observable parts of Martian history. Some ascribe it to a single mega-impact event, essentially an instantaneous rearrangement of the crustal structures (topography and lithospheric thickness). Others prefer an internal mechanism: a period of vigorous convection subcrustally erodes the northern one third of Mars, causing foundering and isostatic lowering of that part of Mars. The evidence for each theory is reviewed, with the conclusion that there is little to recommend either. An alternative is suggested: the formation of the crustal dichotomy on Mars was not a specific tectonic event but a byproduct of the accretionary process and therefore a primordial characteristic of the Martian crust, predating the oldest recognizable landforms

  3. Preliminary crustal deformation model deduced from GPS and earthquakes’ data at Abu-Dabbab area, Eastern Desert, Egypt

    Directory of Open Access Journals (Sweden)

    Abdel-Monem S. Mohamed

    2013-06-01

    From the seismic tomography study, the 3D Vp and Vp/Vs crustal models indicate high Vp/Vs values forms an elongated anomaly, in the central part of the study area, that extends from a depth of 12 km to about 1–2 km of depth is obtained. By using this crustal model in relocations all seismicity informed that most of the seismicity strongly tend to occur in a cluster manner exactly above the southern part of the study area. Based on the conducted source mechanism study, it is noticed that shallow earthquakes are associated by a high CLVD ratio (up to 40%. Furthermore, initiation of a high level seismic activity, without a large seismic main shock is observed in the Abu-Dabbab area. The distribution of micro-earthquakes tends to align in an ENE–WSW direction marking a zone of activity verse the Red Sea. The nucleation of the seismic activity beneath the southern part of the Abu-Dabbab crust is more consistent with the obtained crustal deformation result by increasing the crustal movement in the south part than the northern part. Then, based on the obtained results of the above mentioned studies; seismic tomography; source mechanisms, and crustal deformation we conclude that these seismic activities that are associated by crustal deformation are owing to some magma activity beneath the crust of the Abu-Dabbab area.

  4. Estimating crustal thickness and Vp/Vs ratio with joint constraints of receiver function and gravity data

    Science.gov (United States)

    Shi, Lei; Guo, Lianghui; Ma, Yawei; Li, Yonghua; Wang, Weilai

    2018-05-01

    The technique of teleseismic receiver function H-κ stacking is popular for estimating the crustal thickness and Vp/Vs ratio. However, it has large uncertainty or ambiguity when the Moho multiples in receiver function are not easy to be identified. We present an improved technique to estimate the crustal thickness and Vp/Vs ratio by joint constraints of receiver function and gravity data. The complete Bouguer gravity anomalies, composed of the anomalies due to the relief of the Moho interface and the heterogeneous density distribution within the crust, are associated with the crustal thickness, density and Vp/Vs ratio. According to their relationship formulae presented by Lowry and Pérez-Gussinyé, we invert the complete Bouguer gravity anomalies by using a common algorithm of likelihood estimation to obtain the crustal thickness and Vp/Vs ratio, and then utilize them to constrain the receiver function H-κ stacking result. We verified the improved technique on three synthetic crustal models and evaluated the influence of selected parameters, the results of which demonstrated that the novel technique could reduce the ambiguity and enhance the accuracy of estimation. Real data test at two given stations in the NE margin of Tibetan Plateau illustrated that the improved technique provided reliable estimations of crustal thickness and Vp/Vs ratio.

  5. Particle-In-Cell Simulations of the Solar Wind Interaction with Lunar Crustal Magnetic Anomalies: Magnetic Cusp Regions

    Science.gov (United States)

    Poppe, A. R.; Halekas, J. S.; Delory, G. T.; Farrell, W. M.

    2012-01-01

    As the solar wind is incident upon the lunar surface, it will occasionally encounter lunar crustal remanent magnetic fields. These magnetic fields are small-scale, highly non-dipolar, have strengths up to hundreds of nanotesla, and typically interact with the solar wind in a kinetic fashion. Simulations, theoretical analyses, and spacecraft observations have shown that crustal fields can reflect solar wind protons via a combination of magnetic and electrostatic reflection; however, analyses of surface properties have suggested that protons may still access the lunar surface in the cusp regions of crustal magnetic fields. In this first report from a planned series of studies, we use a 1 1/2-dimensional, electrostatic particle-in-cell code to model the self-consistent interaction between the solar wind, the cusp regions of lunar crustal remanent magnetic fields, and the lunar surface. We describe the self-consistent electrostatic environment within crustal cusp regions and discuss the implications of this work for the role that crustal fields may play regulating space weathering of the lunar surface via proton bombardment.

  6. A deep crustal fluid channel into the San Andreas Fault system near Parkfield, California

    Science.gov (United States)

    Becken, M.; Ritter, O.; Park, S.K.; Bedrosian, P.A.; Weckmann, U.; Weber, M.

    2008-01-01

    Magnetotelluric (MT) data from 66 sites along a 45-km-long profile across the San Andreas Fault (SAF) were inverted to obtain the 2-D electrical resistivity structure of the crust near the San Andreas Fault Observatory at Depth (SAFOD). The most intriguing feature of the resistivity model is a steeply dipping upper crustal high-conductivity zone flanking the seismically defined SAF to the NE, that widens into the lower crust and appears to be connected to a broad conductivity anomaly in the upper mantle. Hypothesis tests of the inversion model suggest that upper and lower crustal and upper-mantle anomalies may be interconnected. We speculate that the high conductivities are caused by fluids and may represent a deep-rooted channel for crustal and/or mantle fluid ascent. Based on the chemical analysis of well waters, it was previously suggested that fluids can enter the brittle regime of the SAF system from the lower crust and mantle. At high pressures, these fluids can contribute to fault-weakening at seismogenic depths. These geochemical studies predicted the existence of a deep fluid source and a permeable pathway through the crust. Our resistivity model images a conductive pathway, which penetrates the entire crust, in agreement with the geochemical interpretation. However, the resistivity model also shows that the upper crustal branch of the high-conductivity zone is located NE of the seismically defined SAF, suggesting that the SAF does not itself act as a major fluid pathway. This interpretation is supported by both, the location of the upper crustal high-conductivity zone and recent studies within the SAFOD main hole, which indicate that pore pressures within the core of the SAF zone are not anomalously high, that mantle-derived fluids are minor constituents to the fault-zone fluid composition and that both the volume of mantle fluids and the fluid pressure increase to the NE of the SAF. We further infer from the MT model that the resistive Salinian block

  7. Magmatism at different crustal levels in the ancient North Cascades magmatic arc

    Science.gov (United States)

    Shea, E. K.; Bowring, S. A.; Miller, R. B.; Miller, J. S.

    2013-12-01

    The mechanisms of magma ascent and emplacement inferred from study of intrusive complexes have long been the subject of intense debate. Current models favor incremental construction based on integration of field, geochemical, geochronologic, and modeling studies. Much of this work has been focused on a single crustal level. However, study of magmatism throughout the crust is critical for understanding how magma ascends through and intrudes surrounding crustal material. Here, we present new geochronologic and geochemical work from intrusive complexes emplaced at a range of crustal depths in the Cretaceous North Cascades magmatic arc. These complexes were intruded between 92 and 87 Ma at depths of at ≤5 -10 km, ~20 km, and ~25 km during this time. U-Pb CA-TIMS geochronology in zircon can resolve Jack-Entiat intrusive complex, a highly elongate amalgamation of intrusions recording two episodes of magmatism between~92-88 Ma and ~80-77 Ma. Each of these complexes provides a window into crustal processes that occur at different depths. Our data suggest assembly of the Black Peak intrusive complex occurred via a series of small (0.5-2 km2) magmatic increments from ~92 Ma to ~87 Ma. Field relations and zircon trace element geochemistry indicate each of these increments were emplaced and crystallized as closed systems-we find no evidence for mixing between magmas in the complex. However, zircon inheritance becomes more common in younger intrusions, indicating assimilation of older plutonic material, possibly during magma production or transport. The Seven-Fingered Jack intrusive complex, emplaced around 15-20 km, preserves a much more discontinuous record of intrusion than the Black Peak. Our data indicate major magmatism in the complex occurred between ~92.1-91.1 Ma. Inheritance in the Seven-Fingered Jack is common, particularly along contacts between intrusions. The Tenpeak intrusive complex, assembled between ~92 Ma and 89 Ma, represents one of the deepest exhumed

  8. Continental Delamination of the Romanian Eastern Carpathians: A Lower Crustal Origin of the Vrancea Seismogenic Zone?

    Science.gov (United States)

    Fillerup, M. A.; Knapp, J. H.; Knapp, C. C.

    2006-12-01

    Two lithosphere-scale, explosive-source seismic reflection profiles (DRACULA I and DACIA PLAN), inclusive of the hinterland and foreland of the Romanian Eastern Carpathians, provide new evidence for the geodynamic origin of the Vrancea Seismogenic Zone (VSZ) of Romania. These data, collected to evaluate existing subduction-related and delamination geodynamic models proposed to explain the intermediate depth seismicity associated with the Vrancea zone, show evidence of continental crust extending continuously above the VSZ from the Carpathian foreland well into the Transylvanian hinterland. Crustal thicknesses inferred from these data based on reflectivity show a 40-45 km crust below the Transylvanian basin abruptly shallowing to 32 km for ~120 km beneath the fold and thrust belt of the main Carpathian orogen and thickening again to 38-42 km crust in the foreland. This thinned crust outlines an apparent lower crustal sub-orogenic cavity that is overlain by a relatively subhorizontal reflective fabric absent of dipping reflectivity. The northwest dipping Vrancea seismogenic body, a 30x70x200 km volume of intermediate depth earthquakes, is located on the eastern flank of the apparently thin crust beneath the Carpathian orogen. Amplitude decay curves show penetration of seismic energy to a depth of ~60 km in the vicinity of the sub-orogenic cavity, implying this non- reflective zone is a geologic signature. Rotation of the VSZ about a hinge beneath the foreland basin at a depth of ~50 km restores to fill the lower-crustal cavity under the orogen, suggesting the VSZ represents a portion of brittle lower crust delaminated during continental lithospheric delamination which may have caused regional uplift of the Transylvanian basin. The lack of through-going, dipping crustal-scale boundaries along this composite lithospheric transect would appear to preclude subduction as an explanation for seismicity in the VSZ, consistent with abundant surface geologic data. These

  9. Evaluation of Earth's Geobiosphere Emergy Baseline and the Emergy of Crustal Cycling

    Science.gov (United States)

    De Vilbiss, Chris

    This dissertation quantitatively analyzed the exergy supporting the nucleosynthesis of the heavy isotopes, Earth's geobiosphere,