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Sample records for juvenile continental crust

  1. Freshly brewed continental crust

    Science.gov (United States)

    Gazel, E.; Hayes, J. L.; Caddick, M. J.; Madrigal, P.

    2015-12-01

    Earth's crust is the life-sustaining interface between our planet's deep interior and surface. Basaltic crusts similar to Earth's oceanic crust characterize terrestrial planets in the solar system while the continental masses, areas of buoyant, thick silicic crust, are a unique characteristic of Earth. Therefore, understanding the processes responsible for the formation of continents is fundamental to reconstructing the evolution of our planet. We use geochemical and geophysical data to reconstruct the evolution of the Central American Land Bridge (Costa Rica and Panama) over the last 70 Ma. We also include new preliminary data from a key turning point (~12-6 Ma) from the evolution from an oceanic arc depleted in incompatible elements to a juvenile continental mass in order to evaluate current models of continental crust formation. We also discovered that seismic P-waves (body waves) travel through the crust at velocities closer to the ones observed in continental crust worldwide. Based on global statistical analyses of all magmas produced today in oceanic arcs compared to the global average composition of continental crust we developed a continental index. Our goal was to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust. We suggest that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone, a process probably more common in the Achaean where most continental landmasses formed, can produce the starting material necessary for juvenile continental crust formation.

  2. Raising the continental crust

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    Campbell, Ian H.; Davies, D. Rhodri

    2017-02-01

    The changes that occur at the boundary between the Archean and Proterozoic eons are arguably the most fundamental to affect the evolution of Earth's continental crust. The principal component of Archean continental crust is Granite-Greenstone Terranes (GGTs), with granites always dominant. The greenstones consist of a lower sequence of submarine komatiites and basalts, which erupted onto a pre-existing Tonalite-Trondhjemite-Granodiorite (TTG) crust. These basaltic rocks pass upwards initially into evolved volcanic rocks, such as andesites and dacites and, subsequently, into reworked felsic pyroclastic material and immature sediments. This transition coincides with widespread emplacement of granitoids, which stabilised (cratonised) the continental crust. Proterozoic supra-crustal rocks, on the other hand, are dominated by extensive flat-lying platform sequences of mature sediments, which were deposited on stable cratonic basements, with basaltic rocks appreciably less abundant. The siliceous TTGs cannot be produced by direct melting of the mantle, with most hypotheses for their origin requiring them to be underlain by a complimentary dense amphibole-garnet-pyroxenite root, which we suggest acted as ballast to the early continents. Ubiquitous continental pillow basalts in Archean lower greenstone sequences require the early continental crust to have been sub-marine, whereas the appearance of abundant clastic sediments, at higher stratigraphic levels, shows that it had emerged above sea level by the time of sedimentation. We hypothesise that the production of komatiites and associated basalts, the rise of the continental crust, widespread melting of the continental crust, the onset of sedimentation and subsequent cratonisation form a continuum that is the direct result of removal of the continent's dense amphibole-garnet-pyroxenite roots, triggered at a regional scale by the arrival of a mantle plume at the base of the lithosphere. Our idealised calculations suggest

  3. Continental crust generated in oceanic arcs

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    Gazel, Esteban; Hayes, Jorden L.; Hoernle, Kaj; Kelemen, Peter; Everson, Erik; Holbrook, W. Steven; Hauff, Folkmar; van den Bogaard, Paul; Vance, Eric A.; Chu, Shuyu; Calvert, Andrew J.; Carr, Michael J.; Yogodzinski, Gene M.

    2015-04-01

    Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intra-oceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70 Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10 Myr ago--coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone--a process probably more common in the Archaean--can produce juvenile continental crust.

  4. Palaeomagnetism and the continental crust

    Energy Technology Data Exchange (ETDEWEB)

    Piper, J.D.A.

    1987-01-01

    This book is an introduction to palaeomagnetism offering treatment of theory and practice. It analyzes the palaeomagnetic record over the whole of geological time, from the Archaean to the Cenozoic, and goes on to examine the impact of past geometries and movements of the continental crust at each geological stage. Topics covered include theory of rock and mineral magnetism, field and laboratory methods, growth and consolidation of the continental crust in Archaean and Proterozoic times, Palaeozoic palaeomagnetism and the formation of Pangaea, the geomagnetic fields, continental movements, configurations and mantle convection.

  5. Generation of continental crust in intra-oceanic arcs

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    Gazel, E.; Hayes, J. L.; Kelemen, P. B.; Everson, E. D.; Holbrook, W. S.; Vance, E.

    2014-12-01

    The origin of continental crust is still an unsolved mystery in the evolution of our planet. Although the best candidates to produce juvenile continental crust are intra-oceanic arcs these systems are dominated by basaltic lavas, and when silicic magmas are produced, the incompatible-element compositions are generally too depleted to be a good match for continental crust estimates. Others, such as the W. Aleutians, are dominated by andesitic melts with trace element compositions similar to average continental crust. In order to evaluate which intra-oceanic arcs produced modern continental crust, we developed a geochemical continental index (CI) through a statistical analysis that compared all available data from modern intra-oceanic arcs with global estimates of continental crust. Our results suggest that magmas from Costa Rica (100 have the least continent-like geochemical signatures. In these arcs the subducting plate is old (>100 Ma), not overprinted by enriched intraplate volcanism and the geochemistry may be dominated by slab-derived, aqueous fluids. We also found a strong correlation between the CI and average crustal P-wave velocity, validating the geochemical index with the available seismic data for intra-oceanic arcs. In conclusion, the production of young continental crust with compositions similar to Archean continental crust is an unusual process, limited to locations where there are especially voluminous partial melts of oceanic crust.

  6. Continental Crust Growth as a Result of Continental Collision: Ocean Crust Melting and Melt Preservation

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    Niu, Y.; Zhao, Z.; Zhou, S.; Zhu, D.; Dong, G.; Mo, X.; Xie, G.; Dong, X.

    2010-12-01

    The significance of the continental crust (CC) on which we live is self-evident. However, our knowledge remains limited on its origin, its way and rate of growth, and how it has acquired the “andesitic” composition from mantle derived magmas. Compared to rocks formed from mantle derived magmas in all tectonic settings, volcanic arc rocks associated with oceanic lithosphere subduction share some common features with the CC; both are relatively depleted in “fluid-insoluble” elements (e.g., Nb, Ta and Ti), but enriched in “fluid-soluble” elements (e.g., U, K and Pb). These chemical characteristics are referred to as the “arc-like signature”, and point to a genetic link between subduction-zone magmatism and CC formation, thus leading to the “island-arc” model widely accepted for the origin of the CC over the past 40 years. However, it has been recognized also that this “island-arc” model has several difficulties. These include (1) bulk arc crust (AC) is basaltic, whereas the bulk CC is andesitic [1]; (2) AC has a variably large Sr excess whereas the CC is Sr deficient [2]; and (3) AC production is mass-balanced by subduction-erosion and sediment recycling, thus contributing no new mass to CC growth, at least in the Phanerozoic [3,4]. Our data on magmatic rocks (both volcanic and intrusive) formed during the India-Asia continental collision (~65 - ~45Ma) [5] show a remarkable compositional similarity to the bulk CC with the typical “arc-like signature” [6]. Also, these syncollisional felsic rocks exhibit strong mantle isotopic signatures, implying that they were recently derived from a mantle source. The petrology and geochemistry of these syncollisional felsic rocks is most consistent with an origin via partial melting of upper oceanic crust (i.e., last fragments of underthrusting oceanic crust) under amphibolite facies conditions, adding net mantle-derived materials to form juvenile CC mass. This leads to the logical and testable hypothesis

  7. Making continental crust: The sanukitoid connection

    Institute of Scientific and Technical Information of China (English)

    Yoshiyuki TATSUMI

    2008-01-01

    The average continental crust possesses intermediate compositions that typify arc magmatism and as a result it is believed to have been created at ancient convergent plate boundaries. One possible mechanism for intermediate continental crust formation is the direct production of andesitic melts in the upper mantle. Sanukitoids, which characterize the Setouchi volcanic belt, SW Japan, include unusually high-Mg andesites (HMA). They were generated by slab melting and subsequent melt-mantle interactions under unusual tectonic settings such as where warm lithosphere subducts into hot upper mantle. Such conditions would have existed in the Archean. Hydrous HMA magmas are likely to have solidified within the crust to form HMA plutons, which were then remelted to produce differentiated sanukitoids. At present, generation and differentiation of HMA magmas may be taking place in the Izu-Bonin-Mariana arc-trench system (IBM), because (1) HMA magmatism characterizes the initial stages of the iBM evolution and (2) the IBM middle crust exhibits Vp identical to that of the bulk continental crust. Vp estimates for plutonic rocks with HMA compositions support this. However tonalitic composition for middle-crust-forming rocks cannot be ruled out, suggesting an alternative possibility that the continental crust has been created by differentiation of mantle-derived basaltic magmas.

  8. The Continental Crust: A Geophysical Approach

    Science.gov (United States)

    Christensen, Nikolas I.

    Nearly 80 years ago, Yugoslavian seismologist Andrija Mohorovicic recognized, while studying a Balkan earthquake, that velocities of seismic waves increase abruptly at a few tens of kilometers depth , giving rise to the seismological definition of the crust. Since that discovery, many studies concerned with the nature of both the continental and oceanic crusts have appeared in the geophysical literature.Recently, interest in the continental crust has cascaded. This is largely because of an infusion of new data obtained from major reflection programs such as the Consortium for Continental Reflection Profiling (COCORP) and British Institutions Reflection Profiling Syndicate (BIRPS) and increased resolution of refraction studies. In addition, deep continental drilling programs are n ow in fashion. The Continental Crust: A Geophysical Approach is a summary of present knowledge of the continental crust. Meissner has succeeded in writing a book suited to many different readers, from the interested undergraduate to the professional. The book is well documented , with pertinent figures and a complete and up-to-date reference list.

  9. The continental record and the generation of continental crust

    OpenAIRE

    Cawood, Peter Anthony; Hawkesworth, Chris; Dhuime, Bruno Philippe Marcel

    2013-01-01

    Continental crust is the archive of Earth history. The spatial and temporal distribution of Earth's record of rock units and events is heterogeneous; for example, ages of igneous crystallization, metamorphism, continental margins, mineralization, and seawater and atmospheric proxies are distributed about a series of peaks and troughs. This distribution reflects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, a...

  10. Cyclic growth in Atlantic region continental crust

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    Goodwin, A. M.

    1986-01-01

    Atlantic region continental crust evolved in successive stages under the influence of regular, approximately 400 Ma-long tectonic cycles. Data point to a variety of operative tectonic processes ranging from widespread ocean floor consumption (Wilson cycle) to entirely ensialic (Ampferer-style subduction or simple crustal attenuation-compression). Different processes may have operated concurrently in some or different belts. Resolving this remains the major challenge.

  11. Continental crust formation on early Earth controlled by intrusive magmatism

    Science.gov (United States)

    Rozel, A. B.; Golabek, G. J.; Jain, C.; Tackley, P. J.; Gerya, T.

    2017-05-01

    The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present-day Earth, two crust-mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism and the “Plutonic squishy lid” tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite-trondhjemite-granodiorite (TTG) continental crust but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.

  12. Continental crust formation on early Earth controlled by intrusive magmatism.

    Science.gov (United States)

    Rozel, A B; Golabek, G J; Jain, C; Tackley, P J; Gerya, T

    2017-05-18

    The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present-day Earth, two crust-mantle interaction modes differing in melt eruption efficiency have been proposed: the Io-like heat-pipe tectonics regime dominated by volcanism and the "Plutonic squishy lid" tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite-trondhjemite-granodiorite (TTG) continental crust but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat-pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG-like continental crust using self-consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism-dominated heat-pipe tectonics model results in cold crustal geotherms and is not able to produce Earth-like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data (low-, medium- and high-pressure TTG). Our study thus suggests that the pre-plate-tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io-like heat-pipe regime.

  13. Growth of the lower continental crust

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    Rudnick, Roberta L.

    1988-01-01

    One of the largest uncertainties in crustal composition and growth models is the nature of the lower continental crust. Specifically, by what processes is it formed and modified, and when is it formed, particularly in reference to the upper crust? The main reason for this lack of information is the scarcity of lower crustal rock samples. These are restricted to two types: rocks which outcrop in granulite facies terrains and granulite facies xenoliths which are transported to the earth's surface by young volcanics. The important conclusions arising from the xenolith studies are: the majority of mafic lower crustal xenoliths formed through cumulate process, resitic xenoliths are rare; and formation and metamorphism of the deep crust is intimately linked to igneous activity and/or orogeny which are manifest in one form or another at the earth's surface. Therefore, estimates of crustal growth based on surface exposures is representative, although the proportion of remobilized pre-existing crust may be significantly greater at the surface than in the deep crust.

  14. Continental Growth and Recycling in Convergent Orogens with Large Turbidite Fans on Oceanic Crust

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    Ben D. Goscombe

    2013-07-01

    Full Text Available Convergent plate margins where large turbidite fans with slivers of oceanic basement are accreted to continents represent important sites of continental crustal growth and recycling. Crust accreted in these settings is dominated by an upper layer of recycled crustal and arc detritus (turbidites underlain by a layer of tectonically imbricated upper oceanic crust and/or thinned continental crust. When oceanic crust is converted to lower continental crust it represents a juvenile addition to the continental growth budget. This two-tiered accreted crust is often the same thickness as average continental crustal and is isostatically balanced near sea level. The Paleozoic Lachlan Orogen of eastern Australia is the archetypical example of a tubidite-dominated accretionary orogeny. The Neoproterozoic-Cambrian Damaran Orogen of SW Africa is similar to the Lachlan Orogen except that it was incorporated into Gondwana via a continent-continent collision. The Mesozoic Rangitatan Orogen of New Zealand illustrates the transition of convergent margin from a Lachlan-type to more typical accretionary wedge type orogen. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of turbidite and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism recorded in these rocks reflects the crustal thickening phase, whereas post-tectonic magmatism constrains the timing of chemical maturation and cratonization. Cratonization of continental crust is fostered because turbidites represent fertile sources for felsic magmatism. Recognition of similar orogens in the Proterozoic and Archean is important for the evaluation of crustal growth models, particularly for those based on detrital zircon age patterns, because crustal growth by accretion of upper oceanic crust or mafic underplating does not readily result in the addition of voluminous zircon

  15. Density Sorting During the Evolution of Continental Crust

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    Kelemen, P. B.; Behn, M. D.; Hacker, B. R.

    2015-12-01

    We consider two settings - in addition to "delamination" of arc lower crust - in which dense, mafic eclogites founder into the convecting mantle while buoyant, felsic lithologies accumulate at the base of evolving continental crust. Arc processes play a central role in generating continental crust, but it remains uncertain how basaltic arc crust is transformed to andesitic continental crust. Dense, SiO2-poor products of fractionation may founder from the base of arc crust by "delamination", but lower arc crust after delamination has significantly different trace elements compared to lower continental crust (LCC). In an alternative model, buoyant magmatic rocks generated at arcs are first subducted, mainly via subduction erosion. Upon heating, these buoyant lithologies ascend through the mantle wedge or along a subduction channel, and are "relaminated" at
the base of overlying crust (e.g., Hacker et al EPSL 11, AREPS 15). Average buoyant lavas and plutons
for the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs fall within the range of estimated LCC major and trace elements. Relamination is more efficient in generating continental crust than delamination. Himalayan cross-sections show Indian crust thrust beneath Tibetan crust, with no intervening mantle. There is a horizontal Moho at ca 80 km depth, extending from thickened Indian crust, across the region where Tibetan crust overlies Indian crust, into thickened Tibetan crust. About half the subducted Indian crust is present, whereas the other half is missing. Data (Vp/Vs; Miocene lavas formed by interaction of continental crust with mantle; xenolith thermometry) indicate 1000°C or more from ca 50 km depth to the Moho since the Miocene. We build on earlier studies (LePichon et al Tectonics 92, T'phys 97; Schulte-Pelkum et al Nature 05; Monsalve et al JGR 08) to advance the hypothesis that rapid growth of garnet occurs at 70-80 km and 1000°C within subducting Indian crust. Dense eclogites founder

  16. Continental collision zones are primary sites for net continental crust growth — A testable hypothesis

    Science.gov (United States)

    Niu, Yaoling; Zhao, Zhidan; Zhu, Di-Cheng; Mo, Xuanxue

    2013-12-01

    The significance of the continental crust (CC) on which we live is self-evident. However, our knowledge remains limited on its origin, its way and rate of growth, and how it has acquired the "andesitic" composition from mantle derived magmas. Compared to rocks formed from mantle derived magmas in all geological environments, volcanic arc rocks associated with seafloor subduction share some common features with the CC; both are relatively depleted in "fluid-insoluble" elements (e.g., Nb, Ta and Ti), but enriched in "fluid-soluble" elements (e.g., U, K and Pb). These chemical characteristics are referred to as the "arc-like signature", and point to a possible link between subduction-zone magmatism and CC formation, thus leading to the "island arc" model widely accepted for the origin of the CC over the past 45 years. However, this "island-arc" model has many difficulties: e.g., (1) the bulk arc crust (AC) is basaltic whereas the bulk CC is andesitic; (2) the AC has variably large Sr excess whereas the CC is weakly Sr deficient; and (3) AC production is mass-balanced by subduction erosion and sediment recycling, thus contributing no net mass to the CC growth, at least in the Phanerozoic. Our recent and ongoing studies on granitoid rocks (both volcanic and intrusive) formed in response to the India-Asia continental collision (~ 55 ± 10 Ma) show remarkable compositional similarity to the bulk CC with the typical "arc-like signature". Also, these syncollisional granitoid rocks exhibit strong mantle isotopic signatures, meaning that they were recently derived from a mantle source. The petrology and geochemistry of these syncollisional granitoid rocks are most consistent with an origin via partial melting of the upper ocean crust (i.e., last fragments of underthrusting ocean crust upon collision) under amphibolite facies conditions, adding net mantle-derived materials to form juvenile CC mass. This leads to the logical and testable hypothesis that continental collision

  17. Estimating the global volume of deeply recycled continental crust at continental collision zones

    Science.gov (United States)

    Scholl, D. W.; Huene, R. V.

    2006-12-01

    CRUSTAL RECYCLING AT OCEAN MARGINS: Large volumes of rock and sediment are missing from the submerged forearcs of ocean margin subduction zones--OMSZs. This observation means that (1) oceanic sediment is transported beneath the margin to either crustally underplate the coastal region or reach mantle depths, and that (2) the crust of the forearc is vertically thinned and horizontally truncated and the removed material transported toward the mantle. Transport of rock and sediment debris occurs in the subduction channel that separates the upper and lower plates. At OMSZs the solid-volume flux of recycling crustal material is estimated to be globally ~2.5 km3/yr (i.e., 2.5 Armstrong units or AU). The corresponding rate of forearc truncation (migration of the trench axis toward a fix reference on the continent) is a sluggish 2-3 km/Myr (about 1/50th the orthogonal convergence rate). Nonetheless during the past 2.5 Gyr (i.e., since the beginning of the Proterozoic) a volume of continental material roughly equal to the existing volume (~7 billion cubic km) has been recycled to the mantle at OMSZs. The amount of crust that has been destroyed is so large that recycling must have been a major factor creating the mapped rock pattern and age-fabric of continental crust. RECYCLING AT CONTINENT/ARC COLLISIONS: The rate at which arc magmatism globally adds juvenile crust to OMSZs has been commonly globally estimated at ~1 AU. But new geophysical and dating information from the Aleutian and IBM arcs imply that the addition rate is at least ~5 AU (equivalent to ~125 km3/Myr/km of arc). If the Armstrong posit is correct that since the early Archean a balance has existed between additions and losses of crust, then a recycling sink for an additional 2-3 AU of continental material must exist. As the exposure of exhumed masses of high P/T blueschist bodies documents that subcrustal streaming of continental material occurs at OMSZs, so does the occurrence of exhumed masses of UHP

  18. Sulfur and metal fertilization of the lower continental crust

    Science.gov (United States)

    Locmelis, Marek; Fiorentini, Marco L.; Rushmer, Tracy; Arevalo, Ricardo; Adam, John; Denyszyn, Steven W.

    2016-02-01

    Mantle-derived melts and metasomatic fluids are considered to be important in the transport and distribution of trace elements in the subcontinental lithospheric mantle. However, the mechanisms that facilitate sulfur and metal transfer from the upper mantle into the lower continental crust are poorly constrained. This study addresses this knowledge gap by examining a series of sulfide- and hydrous mineral-rich alkaline mafic-ultramafic pipes that intruded the lower continental crust of the Ivrea-Verbano Zone in the Italian Western Alps. The pipes are relatively small (tectonic architecture of any given terrain, metals and volatiles stored in the lower continental crust may become available as sources for subsequent ore-forming processes, thus enhancing the prospectivity of continental block margins for a wide range of mineral systems.

  19. Does subduction zone magmatism produce average continental crust

    Science.gov (United States)

    Ellam, R. M.; Hawkesworth, C. J.

    1988-01-01

    The question of whether present day subduction zone magmatism produces material of average continental crust composition, which perhaps most would agree is andesitic, is addressed. It was argued that modern andesitic to dacitic rocks in Andean-type settings are produced by plagioclase fractionation of mantle derived basalts, leaving a complementary residue with low Rb/Sr and a positive Eu anomaly. This residue must be removed, for example by delamination, if the average crust produced in these settings is andesitic. The author argued against this, pointing out the absence of evidence for such a signature in the mantle. Either the average crust is not andesitic, a conclusion the author was not entirely comfortable with, or other crust forming processes must be sought. One possibility is that during the Archean, direct slab melting of basaltic or eclogitic oceanic crust produced felsic melts, which together with about 65 percent mafic material, yielded an average crust of andesitic composition.

  20. Continental crust composition constrained by measurements of crustal Poisson's ratio

    Science.gov (United States)

    Zandt, George; Ammon, Charles J.

    1995-03-01

    DECIPHERING the geological evolution of the Earth's continental crust requires knowledge of its bulk composition and global variability. The main uncertainties are associated with the composition of the lower crust. Seismic measurements probe the elastic properties of the crust at depth, from which composition can be inferred. Of particular note is Poisson's ratio,Σ ; this elastic parameter can be determined uniquely from the ratio of P- to S-wave seismic velocity, and provides a better diagnostic of crustal composition than either P- or S-wave velocity alone1. Previous attempts to measure Σ have been limited by difficulties in obtaining coincident P- and S-wave data sampling the entire crust2. Here we report 76 new estimates of crustal Σ spanning all of the continents except Antarctica. We find that, on average, Σ increases with the age of the crust. Our results strongly support the presence of a mafic lower crust beneath cratons, and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions, followed by magmatic underplating, or a model in which crust formation processes have changed since the Precambrian era.

  1. Radiogenic heat production in the continental crust

    Science.gov (United States)

    Jaupart, Claude; Mareschal, Jean-Claude; Iarotsky, Lidia

    2016-10-01

    The thermal structure and evolution of continents depend strongly on the amount and distribution of radioactive heat sources in the crust. Determining the contribution of crustal rocks beneath a superficial layer is a major challenge because heat production depends weakly on major element composition and physical properties such as seismic wavespeed and density. Enriched granitic intrusives that lie at the current erosion level have a large impact on the surface heat flux but little influence on temperatures in the deep crust. Many lower crustal rocks that are poor in heat producing elements are restites from ancient orogenic events, implying that enrichment of the upper crust was achieved at the expense of deeper crustal levels. For the same total heat production, concentrating heat sources in an upper layer acts to reduce temperatures in the lower crust, thereby allowing stabilization of the crust. The present-day structure of the crust is a consequence of orogeny and should not be adopted for thermal models of the orogenic event itself. This review summarizes information extracted from large data sets on heat flow and heat production and provides estimates of crustal stratification and heat production in several geological provinces. Analysis of global and regional data sets reveals the absence of a positive correlation between surface heat flow and crustal thickness, showing that the average crustal heat production is not constant. Differences of heat flow between geological provinces are due in large part to changes of crustal structure and bulk composition. Collating values of the bulk crustal heat production in a few age intervals reveals a clear trend of decrease with increasing age. This trend can be accounted for by radioactive decay, indicating that thermal conditions at the time of crustal stabilization have not changed significantly. For the average crustal thickness of 40 km, Moho temperatures are near solidus values at the time of stabilization

  2. Tectonic escape in the evolution of the continental crust

    Science.gov (United States)

    Burke, K.; Sengor, C.

    1986-01-01

    The continental crust originated by processes similar to those operating today and continents consist of material most of which originated long ago in arc-systems that have later been modified, especially at Andean margins and in continental collisions where crustal thickening is common. Collision-related strike-slip motion is a general process in continental evolution. Because buoyant continental (or arc) material generally moves during collision toward a nearby oceanic margin where less buoyant lithosphere crops out, the process of major strike-slip dominated motion toward a 'free-face' is called 'tectonic escape'. Tectonic escape is and has been an element in continental evolution throughout recorded earth-history. It promotes: (1) rifting and the formation of rift-basins with thinning of thickened crust; (2) pervasive strike-slip faulting late in orogenic history which breaks up mountain belts across strike and may juxtapose unrelated sectors in cross-section; (3) localized compressional mountains and related foreland-trough basins.

  3. Experimental constraints on phase relations in subducted continental crust

    Science.gov (United States)

    Hermann, Jörg

    2002-01-01

    Synthesis piston cylinder experiments were carried out in the range 2.0-4.5 GPa and 680-1,050 °C to investigate phase relations in subducted continental crust. A model composition (KCMASH) has been used because all major ultrahigh-pressure (UHP) minerals of the whole range of rock types typical for continental crust can be reproduced within this system. The combination of experimental results with phase petrologic constraints permits construction of a UHP petrogenetic grid. The phase relations demonstrate that the most important UHP paragenesis consists of coesite, kyanite, phengite, clinopyroxene, and garnet in subducted continental crust. Below 700 °C talc is stable instead of garnet. As most of these minerals are also stable at much lower pressure and temperature conditions it is thus not easy to recognize UHP metamorphism in subducted crust. A general feature, however, is the absence of feldspars at H2O-saturated conditions. Plagioclase is never stable at UHP conditions, but K-feldspar can occur in H2O-undersaturated rocks. Mineral compositions in the experiments are fully buffered by coexisting phases. The Si content of phengite and biotite increase with increasing pressure. At 4.0 GPa, 780 °C, biotite contains 3.28 Si per formula unit, which is most probably caused by solid solution of biotite with talc. Above 800 °C, the CaAl2SiO6 component in clinopyroxene buffered with kyanite, coesite and a Mg-phase increases with increasing temperature, providing a tool to distinguish between 'cold' and 'hot' eclogites. Up to 10% Ca-eskolaite (Ca0.5[]0.5AlSi2O6) in clinopyroxene has been found at the highest temperature and pressure investigated (>900 °C, 4.5 GPa). Garnet buffered with coesite, kyanite and clinopyroxene displays an increase of grossular component with increasing pressure for a given temperature. Although the investigated system represents a simplification with respect to natural rocks, it helps to constrain general features of subducted continental

  4. Continental crust under the southern Porcupine Seabight west of Ireland

    Science.gov (United States)

    Makris, J.; Egloff, R.; Jacob, A. W. B.; Mohr, P.; Murphy, T.; Ryan, P.

    1988-08-01

    Two new seismic refraction/wide-angle reflection profiles demonstrate that the crust beneath the southern Porcupine Seabight, out to water depths in excess of 4000 m, is of continental type. They also reveal the rifted margin of the Porcupine basin on its eastern side. Crustal thickness under the Seabight, inclusive of sediments which are up to 6 km thick, decreases from 23 km in the east to about 10 km at a sharp continent-ocean transition in the west.

  5. Formation and evolution of Precambrian continental crust in South China

    Institute of Scientific and Technical Information of China (English)

    ZHENG YongFei; ZHANG ShaoBing

    2007-01-01

    The occurrence of zircons with U-Pb ages of ~3.8 Ga and Hf model ages of ~4.0 Ga in South China suggests the existence of the Hadean crustal remnants in South China. Furthermore, a detrital zircon with a U-Pb age as old as 4.1 Ga has been found in Tibet. This is the oldest zircon so far reported in China. These results imply that continental crust was more widespread than previously thought in the late Hadean, but its majority was efficiently reworked into Archean continental crust. On the basis of available zircon U-Pb age and Hf isotope data, it appears that the growth of continental crust in South China started since the early Archean, but a stable cratonic block through reworking did not occur until the Paleoproterozoic. Thus the operation of some form of plate tectonics may occur in China continents since Eoarchean. The initial destruction of the South China craton was caused by intensive magmatic activity in association with the assembly and breakup of the supercontinent Rodinia during the Neoproterozoic. However, most of the Archean and Paleoproterozoic crustal materials in South China do not occur as surface rocks, but exist as sporadic crustal remnants. Nevertheless, the occurrence of Neoproterozoic magmatism is still a signature to distinguish South China from North China.

  6. The effect of thicker oceanic crust in the Archaean on the growth of continental crust through time

    Science.gov (United States)

    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.

  7. Lead isotopic evolution of Archean continental crust, Northern Tanzania

    Science.gov (United States)

    Bellucci, J. J.; McDonough, W. F.; Rudnick, R. L.; Walker, R. J.

    2010-12-01

    The continental crust is stratified in composition; the upper crust is generally enriched in highly incompatible trace elements relative to the lower crust [1]. The Western Granulite section of the Mozambique Belt of Northern Tanzania yields Archean Nd model ages and has zircons with U-Pb ages of ~2.6 Ga [2,3], but was strongly re-worked during the Pan-African Orogeny, ca. 560 Ma [2,3,4]. Here we use time-integrated Pb isotopic modeling for lower and middle crustal xenoliths, as well as upper crustal granulites to determine the timing of, and degree of intra-crustal differentiation. The Pb isotopic compositions of most feldspars in the lower crustal samples, measured via LA-MC-ICPMS, fall on the trend defined by the Tanzanian Craton [5] and therefore, were most likely extracted from the mantle at a similar time, ca. 2.7 Ga. However, some xenoliths fall off this trend and show enrichment in 207Pb/204Pb, which we interpret as reflecting derivation from more heterogeneous mantle than that sampled in the Tanzanian Craton. In contrast to lower crustal xenoliths from the Tanzanian Craton [5], we see no single feldspar Pb-Pb isochrons, which indicates complete re-homogenization of the Pb isotopic composition of the feldspars in the lower crust of the Mozambique Belt during the Pan-African Orogeny, and heating to > 600°C [5]. Using time integrated Pb modeling, the upper crust of the Western Granulites is enriched in U by ˜ 2.5 relative to that of the lower crust, which must have taken place around the time of mantle extraction (ca. 2.7 Ga). In addition, these calculations are consistent with a Th/U ratio of ˜ 4 for the bulk lower crust and ˜ 3 for the bulk upper crust. The common Pb isotopic composition of a single middle crustal xenolith implies a Th/U of 20, but is unlikely to be generally representative of the middle crust. [1] Rudnick, R. L. and Gao, S. (2003). In the Crust, vol. 3, Treatise on Geochemistry:1-64. [2] Mansur, A. (2008) Masters Thesis, University of

  8. Crustal redistribution, crust-mantle recycling and Phanerozoic evolution of the continental crust

    Science.gov (United States)

    Clift, Peter D.; Vannucchi, Paola; Morgan, Jason Phipps

    2009-12-01

    We here attempt a global scale mass balance of the continental crust during the Phanerozoic and especially the Cenozoic (65 Ma). Continental crust is mostly recycled back into the mantle as a result of the subduction of sediment in trenches (1.65 km 3/a), by the subduction of eroded forearc basement (1.3 km 3/a) and by the delamination of lower crustal material from orogenic plateaus (ca. 1.1 km 3/a). Subduction of rifted crust in continent-continent collision zones (0.4 km 3/a), and dissolved materials fixed into the oceanic crust (ca. 0.4 km 3/a) are less important crustal sinks. At these rates the entire continental crust could be reworked in around 1.8 Ga. Nd isotope data indicate that ca. 80% of the subducted continental crust is not recycled by melting at shallow levels back into arcs, but is subducted to depth into the upper mantle. Continent-continent collision zones do not generally form new crust, but rather cause crustal loss by subduction and as a result of their physical erosion, which exports crust from the orogen to ocean basins where it may be subducted. Regional sedimentation rates suggest that most orogens have their topography eliminated within 100-200 million years. We estimate that during the Cenozoic the global rivers exported an average of 1.8 km 3/a to the oceans, approximately balancing the subducted loss. Accretion of sediment to active continental margins is a small contribution to crustal construction (ca. 0.3 km 3/a). Similarly, continental large igneous provinces (flood basalts) represent construction of only around 0.12 km 3/a, even after accounting for their intrusive roots. If oceanic plateaus are accreted to continental margins then they would average construction rates of 1.1 km 3/a, meaning that to keep constant crustal volumes, arc magmatism would have to maintain production of around 3.8 km 3/a (or 94 km 3/Ma/km of trench). This slightly exceeds the rates derived from sparse seismic experiments in oceanic arc systems. Although

  9. Petrological and two-phase flow modelling of deep arc crust: insights on continental crust formation

    Science.gov (United States)

    Riel, Nicolas; Bouilhol, Pierre; van Hunen, Jeroen; Cornet, Julien

    2017-04-01

    The genesis of felsic crust is generally attributed to two main processes: the differentiation of primary magmas by crystallization within the crust or uppermost mantle and the partial melting of older crustal rocks. The Mixing/Assimilation/Hybridization of these magmas in the deep crust (MASH zone) and their subsequent segregation constitutes the principal process by which continents have become differentiated into a more mafic, residual lower crust and a more felsic and hydrated upper crust. Although this model describes qualitatively how continental crust forms, little is known on the physical and chemical mechanisms occurring at the root of volcanic arcs. To assess the dynamics of partial melting, melt injection and hybridization in the deep crust, a new 2-D two-phase flow code using finite volume method has been developed. The formulation takes into account: (i) melt flow through porosity waves/channels, (ii) heat transfer, assuming local thermal equilibrium between solid and liquid, (iii) thermodynamic modelling of stable phases and (iv) injection of mantle-derived melt at the Moho. Our parametric study shows that pressure, heat influx and melt:rock ratio are the main parameters controlling the volume and composition of differentiated magma. Overall the composition of segregated products scatters in two groups: felsic (80-68% SiO2) and intermediate (60-52% SiO2), with an average andesitic composition. The bimodal distribution is controlled by amphibole which buffer the composition of segregated products to high SiO2-content when stable. As the amphibole-out melting reaction is crossed segregated products become intermediate. When compared to available geological evidence, the liquid line of descent of mantle-derived magma do not fit the Mg# versus silica trends of exposed volcanic arcs. Instead our modelling results show that reactive flow of those same magma through a mafic crust is able to reproduce such trends.

  10. Formation of hybrid arc andesites beneath thick continental crust

    Science.gov (United States)

    Straub, Susanne M.; Gomez-Tuena, Arturo; Stuart, Finlay M.; Zellmer, Georg F.; Espinasa-Perena, Ramon; Cai, Yue; Iizuka, Yoshiyuki

    2011-03-01

    Andesite magmatism at convergent margins is essential for the differentiation of silicate Earth, but no consensus exists as to andesite petrogenesis. Models proposing origin of primary andesite melts from mantle and/or slab materials remain in deadlock with the seemingly irrefutable petrographic and chemical evidence for andesite formation through mixing of basaltic mantle melts with silicic components from the overlying crust. Here we use 3He/4He ratios of high-Ni olivines to demonstrate the mantle origin of basaltic to andesitic arc magmas in the central Mexican Volcanic Belt (MVB) that is constructed on ~ 50 km thick continental crust. We propose that the central MVB arc magmas are hybrids of high-Mg# > 70 basaltic and dacitic initial mantle melts which were produced by melting of a peridotite subarc mantle interspersed with silica-deficient and silica-excess pyroxenite veins. These veins formed by infiltration of reactive silicic components from the subducting slab. Partial melts from pyroxenites, and minor component melts from peridotite, mix in variable proportions to produce high-Mg# basaltic, andesitic and dacitic magmas. Moderate fractional crystallization and recharge melt mixing in the overlying crust produces then the lower-Mg# magmas erupted. Our model accounts for the contrast between the arc-typical SiO2 variability at a given Mg# and the strong correlation between major element oxides SiO2, MgO and FeO which is not reproduced by mantle-crust mixing models. Our data further indicate that viscous high-silica mantle magmas may preferentially be emplaced as intrusive silicic plutonic rocks in the crust rather than erupt. Ultimately, our results imply a stronger turnover of slab and mantle materials in subduction zones with a negligible, or lesser dilution, by materials from the overlying crust.

  11. Influence of Earth crust composition on continental collision style in Precambrian conditions: Results of supercomputer modelling

    Science.gov (United States)

    Zavyalov, Sergey; Zakharov, Vladimir

    2016-04-01

    A number of issues concerning Precambrian geodynamics still remain unsolved because of uncertainity of many physical (thermal regime, lithosphere thickness, crust thickness, etc.) and chemical (mantle composition, crust composition) parameters, which differed considerably comparing to the present day values. In this work, we show results of numerical supercomputations based on petrological and thermomechanical 2D model, which simulates the process of collision between two continental plates, each 80-160 km thick, with various convergence rates ranging from 5 to 15 cm/year. In the model, the upper mantle temperature is 150-200 ⁰C higher than the modern value, while the continental crust radiogenic heat production is higher than the present value by the factor of 1.5. These settings correspond to Archean conditions. The present study investigates the dependence of collision style on various continental crust parameters, especially on crust composition. The 3 following archetypal settings of continental crust composition are examined: 1) completely felsic continental crust; 2) basic lower crust and felsic upper crust; 3) basic upper crust and felsic lower crust (hereinafter referred to as inverted crust). Modeling results show that collision with completely felsic crust is unlikely. In the case of basic lower crust, a continental subduction and subsequent continental rocks exhumation can take place. Therefore, formation of ultra-high pressure metamorphic rocks is possible. Continental subduction also occurs in the case of inverted continental crust. However, in the latter case, the exhumation of felsic rocks is blocked by upper basic layer and their subsequent interaction depends on their volume ratio. Thus, if the total inverted crust thickness is about 15 km and the thicknesses of the two layers are equal, felsic rocks cannot be exhumed. If the total thickness is 30 to 40 km and that of the felsic layer is 20 to 25 km, it breaks through the basic layer leading to

  12. Formation of lower continental crust by relamination of buoyant arc lavas and plutons

    Science.gov (United States)

    Kelemen, Peter B.; Behn, Mark D.

    2016-03-01

    The formation of the Earth's continents is enigmatic. Volcanic arc magmas generated above subduction zones have geochemical compositions that are similar to continental crust, implying that arc magmatic processes played a central role in generating continental crust. Yet the deep crust within volcanic arcs has a very different composition from crust at similar depths beneath the continents. It is therefore unclear how arc crust is transformed into continental crust. The densest parts of arc lower crust may delaminate and become recycled into the underlying mantle. Here we show, however, that even after delamination, arc lower crust still has significantly different trace element contents from continental lower crust. We suggest that it is not delamination that determines the composition of continental crust, but relamination. In our conceptual model, buoyant magmatic rocks generated at arcs are subducted. Then, upon heating at depth, they ascend and are relaminated at the base of the overlying crust. A review of the average compositions of buoyant magmatic rocks -- lavas and plutons -- sampled from the Aleutians, Izu-Bonin-Marianas, Kohistan and Talkeetna arcs reveals that they fall within the range of estimated major and trace elements in lower continental crust. Relamination may thus provide an efficient process for generating lower continental crust.

  13. Thermochemical structure of the Earth's mantle and continental crust

    DEFF Research Database (Denmark)

    Guerri, Mattia

    task. In this thesis, I adopt a multidisciplinary approach geared towards the unification of geochemical and geophysical constraints within a framework provided by theoretical and experimental mineral and rock physics. In the first part, I focus on the Continental crust, assessing the relations between...... elastic properties and chemical composition taking into account the thermal and pressure effects. I found that even small amounts of H2O have an extremely high impact on the elastic properties of crustal rock. In addition, I show the potential that modifications in the stable mineralogical assemblage have...

  14. Potential mechanisms of pore-fluid movement from continental lithospheric mantle into upper continental crust

    Institute of Scientific and Technical Information of China (English)

    ZHAO Chong-bin; PENG Sheng-lin; LIU Liang-ming; B.E.HOBBS; A.ORD

    2008-01-01

    Through integrating the state of the art scientific knowledge in different research fields, some potential mechanisms of large-scale movements of underground pore-fluids such as H2O and CO2 in the continental lithosphere were presented and discussed. The results show that the generation and propagation of porosity waves are important mechanisms to transport mass and heat fluxes from the continental lithospheric mantle into the lower continental crust; the generation and propagation of porosity waves, pore-fluid flow focusing through lower and middle crustal faults, aclvection of pore-fluids through the lower and middle crust, and whole-crustconvection in some particular cases are important mechanisms to transport mass and heat fluxes from the lower into the upper continental crust; heat and mass transport through convective pore-fluid flow is the most effective mechanism of ore body formation and mineralization in hydrothermal systems; due to heat and mass exchange at the interface between the earth surface, hydrosphere and atmosphere, it is very important to consider the hydro-geological effect of the deep earth pore-fluids such as H2O and CO2 on the global warming and climate change in future investigations.

  15. Has 7% of Continental Crust been Lost since Pangea Broke Up?

    Science.gov (United States)

    Scholl, D. W.; Stern, R. J.

    2012-12-01

    After modern plate tectonics began, the net growth or loss of continental crust predominantly involved the mass balance at subduction zones (SZs) between the yin of adding mantle-sourced arc igneous rocks and the subtracting yang of recycling existing crust back into the mantle. Field observations suggest that during Mesozoic and Cenozoic time, a rough long-term balance existed at ocean-margin SZs (e.g., W. N. America, Andes). But a different picture, one of net loss, emerges when additions and losses at collisional or crust-suturing SZs (e.g., India-Tibet) are considered. GAINS AND LOSSES SINCE ~200 Ma Because Mesozoic and Cenozoic convergent margins can be field inspected, the net growth of continental crust after the breakup of Pangea at ~200 Ma can be estimated. Pangea breakup also marked the beginning of the present supercontinent cycle. Newly established (Eocene) ocean-margin SZs (e.g., IBM, Tonga-Kermadec) added juvenile arc crust for at least 10-15 Myr at rates ~10-15 times higher than later and elsewhere at long-established SZs (~30 km3/Myr/km). During the Cenozoic, at colliding SZs (e.g., Alps, India-Tibet, Arabia-Eurasia) tomographic and geological data document losses of subducted continental crust sustained for 15-50 Myr at rates ~15 times that typical from the upper plate at ocean-margin SZs (~70 km3/Myr/km). For additions, we considered that as the Atlantic opened in early Jurassic time, new, prodigiously productive SZs were initiated along the western margin of North and Middle America but not along western South America and the eastern margin of Eurasia. In the Cretaceous, new SZs formed along much of the northern margin of the Tethys, along western Sumatra and southern Java, and at the great arc of the Caribbean. In the early Eocene, in the offshore, a lengthy (~20,000 km) curtain of new, voluminously productive intra-oceanic SZs formed from the Aleutian Islands southward to the Kermadec Islands. For subtractions, we applied subduction losses (~70

  16. New Tracers of Gas Migration in the Continental Crust

    Energy Technology Data Exchange (ETDEWEB)

    Kurz, Mark D. [Woods Hole Oceanographic Inst., MA (United States)

    2015-11-01

    Noble gases are exceptional tracers in continental settings due to the remarkable isotopic variability between the mantle, crust, and atmosphere, and because they are inert. Due to systematic variability in physical properties, such as diffusion, solubility, and production rates, the combination of helium, neon, and argon provides unique but under-utilized indices of gas migration. Existing noble gas data sets are dominated by measurements of gas and fluid phases from gas wells, ground waters and hot springs. There are very few noble gas measurements from the solid continental crust itself, which means that this important reservoir is poorly characterized. The central goal of this project was to enhance understanding of gas distribution and migration in the continental crust using new measurements of noble gases in whole rocks and minerals from existing continental drill cores, with an emphasis on helium, neon, argon. We carried out whole-rock and mineral-separate noble gas measurements on Precambrian basement samples from the Texas Panhandle. The Texas Panhandle gas field is the southern limb of the giant Hugoton-Panhandle oil and gas field; it has high helium contents (up to ~ 2 %) and 3He/4He of 0.21 (± 0.03) Ra. Because the total amount of helium in the Panhandle gas field is relatively well known, crustal isotopic data and mass balance calculations can be used to constrain the ultimate source rocks, and hence the helium migration paths. The new 3He/4He data range from 0.03 to 0.11 Ra (total), all of which are lower than the gas field values. There is internal isotopic heterogeneity in helium, neon, and argon, within all the samples; crushing extractions yield less radiogenic values than melting, demonstrating that fluid inclusions preserve less radiogenic gases. The new data suggest that the Precambrian basement has lost significant amounts of helium, and shows the importance of measuring helium with neon and argon. The 4He/40Ar values are particularly useful

  17. Continental crust anisotropy measurements from tectonic tremor in Cascadia

    Science.gov (United States)

    Huesca-Pérez, Eduardo; Ortega, Roberto; Valenzuela, Raúl W.

    2017-05-01

    We present new observations of crustal anisotropy in the southern Cascadia fore arc from tectonic tremor. The abundance of tremor activity in Oregon and northern California during slow-slip events offers an enormous amount of information with which to measure and analyze anisotropy in the upper brittle continental crust. To accomplish this, we performed analyses of wave polarization and shear wave splitting of tectonic tremor signals by using three component broadband seismic stations. The splitting times range between 0.11 and 0.32 s and are consistent with typical values observed in the continental crust. Fast polarization azimuths are, in general, margin parallel and trend N-S, which parallels the azimuths of the maximum compressive stresses observed in this region. This pattern is likely to be controlled by the stress field. Comparatively, the anisotropic structure of fast directions observed in the northern section of the Cascadia margin is oblique with respect to the southern section of Cascadia, which, in general, trends E-W and is mainly controlled by active faulting and geological structures. Source distribution analysis using a bivariate normal distribution that expresses the distribution of tremors in a preferred direction shows that in northern California and Oregon, the population of tremors tends to distribute parallel to fast polarization azimuths and maximum compressive stresses, suggesting that both tremor propagation and anisotropy are influenced by the stress field. Results show that the anisotropy reflects an active tectonic process that involves the northward movement of the Oregon Block, which is rotating as a rigid body. In northern Cascadia, previous results of anisotropy show that the crust is undergoing a shortening process due to velocity differences between the Oregon Block and the North America plate, which is moving more slowly with respect to the Oregon Block, making it clash against Vancouver Island.

  18. Seismic lamination and anisotropy of the Lower Continental Crust

    Science.gov (United States)

    Meissner, Rolf; Rabbel, Wolfgang; Kern, Hartmut

    2006-04-01

    Seismic lamination in the lower crust associated with marked anisotropy has been observed at various locations. Three of these locations were investigated by specially designed experiments in the near vertical and in the wide-angle range, that is the Urach and the Black Forrest area, both belonging to the Moldanubian, a collapsed Variscan terrane in southern Germany, and in the Donbas Basin, a rift inside the East European (Ukrainian) craton. In these three cases, a firm relationship between lower crust seismic lamination and anisotropy is found. There are more cases of lower-crustal lamination and anisotropy, e.g. from the Basin and Range province (western US) and from central Tibet, not revealed by seismic wide-angle measurements, but by teleseismic receiver function studies with a P-S conversion at the Moho. Other cases of lamination and anisotropy are from exhumed lower crustal rocks in Calabria (southern Italy), and Val Sesia and Val Strona (Ivrea area, Northern Italy). We demonstrate that rocks in the lower continental crust, apart from differing in composition, differ from the upper mantle both in terms of seismic lamination (observed in the near-vertical range) and in the type of anisotropy. Compared to upper mantle rocks exhibiting mainly orthorhombic symmetry, the symmetry of the rocks constituting the lower crust is either axial or orthorhombic and basically a result of preferred crystallographic orientation of major minerals (biotite, muscovite, hornblende). We argue that the generation of seismic lamination and anisotropy in the lower crust is a consequence of the same tectonic process, that is, ductile deformation in a warm and low-viscosity lower crust. This process takes place preferably in areas of extension. Heterogeneous rock units are formed that are generally felsic in composition, but that contain intercalations of mafic intrusions. The latter have acted as heat sources and provide the necessary seismic impedance contrasts. The observed

  19. Nature of the crust under Afar: new igneous, not thinned continental

    Science.gov (United States)

    Mohr, Paul

    1989-10-01

    Thinned continental crust is considered absent from beneath Afar, except for isolated remnants such as comprise the Danakil Block. The Ethiopian Plateau sialic crust thins abruptly across the plateau-Afar margin to abut new igneous crust under Afar, generated during the early development of the Red Sea basin. Analyses of stretching and sea-floor spreading amounts elsewhere in the Red Sea and Gulf of Aden basins are employed to support this concept. The dual layering of the Afar crust, and the similarity of P-wave velocities in these layers to velocities in sialic crust, lead to the proposal that new continental crust can be generated at magmatic rift zones.

  20. Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons

    Science.gov (United States)

    Stevenson, Ross K.; Patchett, P. Jonathan

    1990-01-01

    Results from the fractionation of zircon by sedimentary processes into continental margin sandstone yield information on the preservation of preexisting continental crust in the form of zircon, making it possible to distinguish between the contrasting theories of gradual growth versus constant volume of continental crust over geologic time. In this work, Hf-176/Hf-177 ratios were determined for detrital zircon fractions from 2.0-2.5, 2.6-3.0, and pre-3.0 Gyr old sandstones from the Canadian-Shield, the North-Atlantic, the Wyoming, and the Kaapvaal Cratons. Results pointed to small amounts of continental crust prior to 3.0 Gyr ago and a rapid addition of continental crust between 2.5 and 3.0 Gyr ago, consistent with the gradual growth of continental crust, and giving evidence against no-growth histories.

  1. Physical processes in the growth of the continental crust

    Science.gov (United States)

    Schubert, G.

    1988-01-01

    Major mechanisms of crustal addition are volcanism and plutonism at plate boundaries and within plate interiors. One approach to deciding if island arc magmatism dominated ancient crustal growth is to assess the rate at which the process has operated in the recent past. The localized addition rates were found to be comparable to present day global rates. One physical observable that was used to constrain models of crustal growth is sea level. A simple physical model was developed to explore the consequences of constant freeboard (the height of the continents above sea level). Global geoid and sea floor topography data were used to identify and study oceanic plateaus and swells that have either continental crustal roots or anomalously thick ocean crusts.

  2. Generation and preservation of continental crust in the Grenville Orogeny

    Institute of Scientific and Technical Information of China (English)

    Christopher J. Spencer; Peter A. Cawood; Chris J. Hawkesworth; Anthony R. Prave; Nick M.W. Roberts; Matthew S.A. Horstwood; Martin J. Whitehouse; EIMF

    2015-01-01

    Detrital zircons from modern sediments display an episodic temporal distribution of U-Pb crystallization ages forming a series of ‘peaks’ and ‘troughs’. The peaks are interpreted to represent either periods of enhanced generation of granitic magma perhaps associated with mantle overturn and superplume events, or preferential preservation of continental crust during global collisional orogenesis. The close association of those peaks with the assembly of supercontinents implies a causal relationship between collisional orogenesis and the presence of zircon age peaks. Here these two end-member models (episodic periodicity of increased magmatism versus selective preservation during collisional orogenesis) are assessed using U-Pb, Hf, and O analysis of detrital zircons from sedimentary successions deposited during the w1.3e1.1 Ga accretionary, w1.1e0.9 Ga collisional, and < 0.9 Ga extensional collapse phases of the Grenville orogenic cycle in Labrador and Scotland. The pre-collisional, accretionary stage provides a baseline of continental crust present prior to orogenesis and is dominated by Archean and Paleoproterozoic age peaks associated with pre-1300 Ma Laurentian geology. Strata deposited during the Grenville Orogeny display similar Archean and Paleoproterozoic detrital populations along with a series of broad muted peaks from w1500 to 1100 Ma. However, post-collisional sedimentary successions display a dominant age peak between 1085 and 985 Ma, similar to that observed in modern North American river sediments. Zircons within the post-orogenic sedimentary successions have progressively lower 3Hf and higher d18O values from w1800 to w1200 Ma whereupon they have higher 3Hf and d18O within the dominant 1085e985 Ma age peak. Furthermore, the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material. The timing of this dominant age peak coincides with the peak of metamorphism

  3. Generation and preservation of continental crust in the Grenville Orogeny

    Directory of Open Access Journals (Sweden)

    Christopher J. Spencer

    2015-05-01

    Zircons within the post-orogenic sedimentary successions have progressively lower ɛHf and higher δ18O values from ∼1800 to ∼1200 Ma whereupon they have higher ɛHf and δ18O within the dominant 1085–985 Ma age peak. Furthermore, the Lu-Hf isotopic profile of the Grenville-related age peak is consistent with significant assimilation and contamination by older crustal material. The timing of this dominant age peak coincides with the peak of metamorphism and magmatism associated with the Grenville Orogeny, which is a typical collisional orogenic belt. The change from broad muted age peaks in the syn-orogenic strata to a single peak in the post-orogenic sedimentary successions and in the modern river sediments implies a significant shift in provenance following continental collision. This temporal change in provenance highlights that the source(s, from which detrital zircons within syn-orogenic strata were derived, was no longer available during the later stages of the accretionary and collisional stages of the orogenic cycle. This may reflect some combination of tectonic burial, erosion, or possibly recycling into the mantle by tectonic erosion of the source(s. During continental collision, the incorporated continental crust is isolated from crustal recycling processes operative at subduction margins. This tectonic isolation combined with sedimentary recycling likely controls the presence of the isotopic signature associated with the Grenville Orogeny in the modern Mississippi and Appalachian river sediments. These results imply that zircon age peaks, which developed in conjunction with supercontinents, are the product of selective crustal preservation resulting from collisional orogenesis.

  4. Subduction and exhumation of continental crust: insights from laboratory models

    Science.gov (United States)

    Bialas, Robert W.; Funiciello, Francesca; Faccenna, Claudio

    2011-01-01

    When slivers of continental crust and sediment overlying oceanic lithosphere enter a subduction zone, they may be scraped off at shallow levels, subducted to depths of up to 100-200 km and then exhumed as high pressure (HP) and ultra-high pressure (UHP) rocks, or subducted and recycled in the mantle. To investigate the factors that influence the behaviour of subducting slivers of continental material, we use 3-D dynamically consistent laboratory models. A laboratory analogue of a slab-upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank. A sliver of continental material, also composed of silicone putty, overlies the subducting lithosphere, separated by a syrup detachment. The density of the sliver, viscosity of the detachment, geometry of the subducting system (attached plate versus free ridge) and dimensions of the sliver are varied in 34 experiments. By varying the density of the sliver and viscosity of the detachment, we can reproduce a range of sliver behaviour, including subduction, subduction and exhumation from various depths and offscraping. Sliver subduction and exhumation requires sufficient sliver buoyancy and a detachment that is strong enough to hold the sliver during initial subduction, but weak enough to allow adequate sliver displacement or detachment for exhumation. Changes to the system geometry alter the slab dip, subduction velocity, pattern of mantle flow and amount of rollback. Shallower slab dips with more trench rollback produce a mantle flow pattern that aids exhumation. Steeper slab dips allow more buoyancy force to be directed in the up-dip direction of the plane of the plate, and aide exhumation of subducted slivers. Slower subduction can also aide exhumation, but if slab dip is too steep or subduction too slow, the sliver will subduct to only shallow levels and not exhume. Smaller slivers are most easily subducted and exhumed and influenced by the mantle flow.

  5. Deep observation and sampling of the earth's continental crust (DOSECC): Continental scientific drilling workshop

    Energy Technology Data Exchange (ETDEWEB)

    1985-01-01

    Research summaries are presented of ongoing or proposed deep drilling programs to explore hydrothermal systems, buried astroblemes, continental crust, magma systems, mountain belt tectonics, subduction zones, and volcanoes. Separate abstracts have been prepared for individual papers. (ACR)

  6. Can Seismology Detect Ductile Deformation in the Deep Continental Crust?

    Science.gov (United States)

    Lloyd, G. E.; Halliday, J. M.; Butler, R. W.; Casey, M.; Kendall, J.

    2005-12-01

    Various models exist to explain deformation in deep continental crust, broadly divisible into localized simple shear and distributed pure shear. Although seismic imaging is perhaps the most effective way to determine deformation at depth, the veracity of any interpretation depends critically upon knowledge of the seismic properties of the rocks. As a region of active crustal thickening and exhumation, the Nanga Parbat Massif (NPM), Pakistan Himalaya, offers insight into how localized thrust faults at the Earth's surface couple with distributed strain at depth. Gneissic, mylonitic and cataclastic rocks at the surface were sampled as proxies for lithologies and fabrics currently accommodating deformation at depth. The LPO of individual minerals were measured using SEM/EBSD, from which elastic constants, 3D seismic velocities and anisotropies were predicted. Micas contribute most to the bulk anisotropy; only when mica content is low and/or LPO is weak do other minerals contribute significantly. The anisotropy varies with deformation style: background gneiss samples have highest AVs anisotropy (9.5 percent) compared with highly strained gneiss samples (5.1-6.3 percent) and cataclasite (0.4 percent). This suggests that some shear zones are characterized by low anisotropy. Sample elastic constants were used to construct models, based on current ideas of NPM tectonics, through which P-, S- and converted waves were ray traced. Foliation orientation and intensity have dramatic impacts on seismic waves: rocks with 9.5 percent AVs and vertically aligned foliation persistent through 40km of crust induce ~1.5s AVs compared to horizontal alignment that induces only 0.2s AVs. Mode conversions and transverse component energy are also diagnostic of foliation intensity and orientation. The models indicate that it is possible to discriminate between different deformation processes active at depth using seismic measurements and we conclude that the dominant process in the NPM is

  7. Sm/Nd Evolution of Upper Mantle and Continental Crust:Constraints on Gowth Rates of the Continental Crust

    Institute of Scientific and Technical Information of China (English)

    李曙光

    1992-01-01

    A new approach to the investigation of the Sm/Nd evolution of the upper mantle directly from the data on lherzolite xenoliths is described in this paper.It is demonstrated that the model age TCHUR of an unmetasomatic iherzolite zenolith ca represent the mean depletion age of its mantle source, thus presenting a correlation trend between fSm/Nd and the mean depletion age of the upper mantle from the data on xenoliths.This correlation trend can also be derived from the data on river suspended loads as well as from granitoids.Based on the correlation trend mentioned above and mean depletion ages of the upper mantle at various geological times, an evolution curve for the mean fSm/Nd value of the upper mantle through geological time has been established.It is suggested that the upwilling of lower mantle material into the upper mantle and the recycling of continental crust material during the Archean were more active ,thus maintaining fairly constantfSm/Nd and εNd values during this time period. Similarly ,an evolution curve for the mean fSm/Nd value of the continental crust through geological time has also been established from the data of continental crust material.In the light of both evolution curves for the upper mantle and continental crust ,a growth curve for the continental crust has been worked out ,suggesting that :(1)about 30%(in volume )of the present crust was present as the continental crust at 3.8 Ga ago ;(2)the growth rate was much lower during the Archean ;and (3)the Proterozoic is another major period of time during which the continental crust wsa built up .

  8. Density constraints on the formation of the continental Moho and crust

    Science.gov (United States)

    Herzberg, C. T.; Carr, M. J.; Fyfe, W. S.

    1983-01-01

    As part of a general review of recent high-resolution seismological data, constraints are established for the distribution of mass in the continental crust and upper mantle of the earth. It is shown that the densities of mantle magmas such as MORB-like tholeiites, oicrites, and komatites at 10 kilobars are greater than densities in the continental crust. In order to explain the deviation, a mechanism is proposed for the control of mass transfer during igneous and metamorphic processes throughout geological time. Within the context of a description of the mechanism, the continental crust is characterized as a density filter through which only highly evolved magmas with H2O may pass. It is shown that the removal of less evolved magmas can lead to an over accretion (penetration) or an under accretion (underplating) of magmas in the continental Moho. Some implications of the proposed mechanism for current theoretical models of crust formation are discussed in detail.

  9. Large-scale subduction of continental crust implied by India-Asia mass-balance calculation

    Science.gov (United States)

    Ingalls, Miquela; Rowley, David B.; Currie, Brian; Colman, Albert S.

    2016-11-01

    Continental crust is buoyant compared with its oceanic counterpart and resists subduction into the mantle. When two continents collide, the mass balance for the continental crust is therefore assumed to be maintained. Here we use estimates of pre-collisional crustal thickness and convergence history derived from plate kinematic models to calculate the crustal mass balance in the India-Asia collisional system. Using the current best estimates for the timing of the diachronous onset of collision between India and Eurasia, we find that about 50% of the pre-collisional continental crustal mass cannot be accounted for in the crustal reservoir preserved at Earth's surface today--represented by the mass preserved in the thickened crust that makes up the Himalaya, Tibet and much of adjacent Asia, as well as southeast Asian tectonic escape and exported eroded sediments. This implies large-scale subduction of continental crust during the collision, with a mass equivalent to about 15% of the total oceanic crustal subduction flux since 56 million years ago. We suggest that similar contamination of the mantle by direct input of radiogenic continental crustal materials during past continent-continent collisions is reflected in some ocean crust and ocean island basalt geochemistry. The subduction of continental crust may therefore contribute significantly to the evolution of mantle geochemistry.

  10. The growth of the continental crust: Constraints from radiogenic isotope geochemistry

    Science.gov (United States)

    Taylor, Paul N.

    1988-01-01

    Most models for evolution of continental crust are expressed in the form of a diagram illustrating the cumulative crustal mass (normalized relative to the present crustal mass) as a function of time. Thus, geochronological data inevitably play a major role in either constructing or testing crustal growth models. For all models, determining the start-time for effective crustal accretion is of vital importance. To this end, the continuing search for, and reliable characterization of, the most ancient crustal rock-units remains a worthy enterprise. Another important role for geochronology and radiogenic isotope geochemistry is to assess the status of major geological events as period either of new crust generation or of reworking of earlier formed continental crust. For age characterization of major geological provinces, using the critieria outined, the mass (or volume) of crust surviving to the present day should be determinable as a function of crust formation age. More recent developments, however, appear to set severe limitations on recycling of crust, at least by the process of sediment subduction. In modeling crustal growth without recycling, valuable constaints on growth rate variations through time can be provided if variations in the average age of the continental crust can be monitored through geological history. The question of the average age of the exposed continental crust was addressed by determining Sm-Nd crustal residence model ages (T-CR) for fine-grained sediment loads of many of the world's major rivers.

  11. Granitic Perspectives on the Generation and Secular Evolution of the Continental Crust

    Science.gov (United States)

    Kemp, A. I. S.; Hawkesworth, C. J.

    2003-12-01

    Every geologist is acquainted with the principle of "uniformitarianism," which holds that present-day processes are the key to those that operated in the past. But the extent this applies to the processes driving the growth and differentiation of the Earth's continental crust remains a matter of debate. Unlike its dense oceanic counterpart, which is recycled back into the mantle by subduction within 200 Ma (see Chapter 3.13), the continental crust comprises buoyant quartzofeldspathic materials and is difficult to destroy by subduction. The continental crust is, therefore, the principal record of how conditions on the Earth have changed, and how processes of crust generation have evolved through geological time. It preserves evidence of secular variation in crustal compositions, and thus the way in which the crust has formed throughout Earth's history. Exploring the nature and origin of these variations is the focus of this chapter.Continental rocks are highly differentiated, and so the crust is enriched in incompatible components compared to the primeval chondritic composition (see Chapter 3.01). Of these, water is perhaps the most relevant, both for the origin and evolution of life, and also for many models of crust generation and differentiation. Similarly, the mass of continental crust is just 0.57% of the silicate Earth, and yet it contains ˜35% of the potassium (using the crustal composition estimates in Table 1). Continental rocks comprise the buoyant shell that was once thought to float on a basaltic substratum, inferred from the wide distribution of chemically similar continental flood basalts (von Cotta, 1858). The links with the adjacent oceans were perhaps unclear, "the greatest mountains confront the widest oceans" ( Dana, 1873). Yet, it has long been argued that the rock that has the most similar composition to the average continental crust, andesite, may be generated by fractional crystallization of basalt ( Daly (1914) and Bowen (1928); but see the

  12. Deep structure of the central Lesser Antilles Island Arc : relevance for the formation of continental crust

    OpenAIRE

    H. Kopp; Weinzierl, W.; Becel, A.; Charvis, Philippe; Evain, M.; Flueh, E. R.; Gailler, A.; Galve, A.; Hirn, A.; Kandilarov, A.; D. Klaeschen; M. Laigle; Papenberg, C.; L. Planert; Roux, E.

    2011-01-01

    Oceanic island arcs are sites of high magma production and contribute to the formation of continental crust. Geophysical studies may provide information on the configuration and composition of island arc crust, however, to date only few seismic profiles exist across active island arcs, limiting our knowledge on the deep structure and processes related to the production of arc crust. We acquired active-source wide-angle seismic data crossing the central Lesser Antilles island arc north of Domi...

  13. Scales of Heterogeneities in the Continental Crust and Upper Mantle

    OpenAIRE

    M. Tittgemeyer; F. Wenzel; Trond Ryberg; Fuchs, K

    1999-01-01

    A seismological characterization of crust and upper mantle can refer to large-scale averages of seismic velocities or to fluctuations of elastic parameters. Large is understood here relative to the wavelength used to probe the earth. In this paper we try to characterize crust and upper mantle by the fluctuations in media properties rather than by their average velocities. As such it becomes evident that different scales of heterogeneities prevail in different layers of crust mantle. Although ...

  14. Zinc Isotopic Signatures of the Upper Continental Crust

    Science.gov (United States)

    Xia, Y.; Zhang, X.; Zhang, H.; Huang, F.

    2016-12-01

    To examine the Zn isotope systematics within the Upper Continental Crust (UCC), and isotope fractionation during chemical weathering in large spatial and temporal scales, we analyzed Zn isotopic compositions of loess, glacial diamictites, river sediments, and igneous rocks (samples in total 77). The Zn isotopic compositions (δ66Zn relative to JMC-Lyon) of loess display a limited variation (0.17‰ to 0.29‰), which is negatively correlated with Zn content and proxies for chemical weathering (e.g. CIA values), reflect the impact of chemical weathering. Glacial diamictites have more variable δ66Zn (0.09‰ to 0.48‰), but the average δ66Zn (0.29±0.03‰, 2SD) is similar to loess. δ66Zn of glacial diamictites correlate roughly negatively with CIA values, but have no correlation with Zn content, implying source heterogeneity and effect from chemical weathering. δ66Zn of A-type (0.39‰ to 0.45‰) and S-type (0.28‰ to 0.35‰) granites are both homogeneous, but the latter have systematically lighter δ66Zn. This may reflect no Zn isotopic fractionation during magmatic processes and involvement of isotopically light meta-sedimentary into the sources of S-type granites. Furthermore, δ66Zn in riverine sediments display a small variation from 0.23‰ to 0.37‰, while δ66Zn of the the shales vary from 0.14‰ to 0.53‰, which could result from a combination of processes, such as biological cycling and chemical weathering. Overall, our data suggest that incipient chemical weathering can fractionate Zn isotopes significantly, meanwhile, during this process, heavy Zn are released preferentially. The UCC is estimated to have an average δ66Zn of 0.30 ±0.03‰ (2SD) with data collected in this study, which is similar to the estimated value of Bulk Silicate Earth (0.28±0.05‰)[1] and mean dissolved riverine flux (0.33‰)[2], but distinctly lighter than the bulk composition of dissolved Zn in the ocean (0.51‰)[2]. [1] Chen et al., Zinc isotope fractionation

  15. Crustal radiogenic heat production and the selective survival of ancient continental crust

    Science.gov (United States)

    Morgan, P.

    1985-01-01

    It is pointed out that the oldest terrestrial rocks have so far revealed no evidence of the impact phase of earth evolution. This observation suggests that processes other than impact were dominant at the time of stabilization of these units. However, a use of the oldest terrestrial rocks as a sample of the early terrestrial crust makes it necessary to consider the possibility that these rocks may represent a biased sample. In the present study, the global continental heat flow data set is used to provide further evidence that potassium, uranium, and thorium abundances are, on the average, low in surviving Archean crust relative to younger continental crust. An investigation is conducted of the implications of relatively low crustal radiogenic heat production to the stabilization of early continental crust, and possible Archean crustal stabilization models are discussed.

  16. Tectonic erosion, subduction accretion and arc collision as controls on the growth of the continental crust

    Science.gov (United States)

    Clift, P.; Vannucchi, P.; Draut, A.

    2003-04-01

    Subduction plate boundaries, at which tectonic erosion removes material from the overriding plate, account for 57% of the total length of the global subduction system and are favored where convergence rates exceed 7 cm/yr and where the sedimentary cover is less than 1 km. Accretion conversely preferentially occurs in regions of slow convergence (1 km. The slope gradients and taper angle of accretionary plate margins correlates with plate convergence rates, while erosive margin slopes appear to be independent of this. Rates of trench retreat do not appear to correlate with any simple characteristic of the plate interaction, but are largely a function of the history of seamount or ridge collisions. Mass balances of the global subduction system indicate that the entire volume of the continental crust can be recycled through the subduction system every 2.6 Ga. Even in accretionary margins a median of only 32% of the incoming sedimentary mass is accreted over time scales of 10 my or greater, resulting in long-term net loss of continental crust along continental active margins. Average magmatic productivity in the active margins must exceed 75 km3/my if the volume of the continental crust is to reach the slow growth rate indicated by isotopic and continental freeboard arguments. Geological arguments indicate that magmatic accretion rates must be faster in oceanic arcs (87-95 km3/my) and less in the continental arcs (65-83 km3/my). Mass balance arguments in oceanic arcs require that their crustal thicknesses must be Continental growth is principally achieved through the collision of oceanic island arcs to continental margins. Although oceanic arcs are chemically distinct from continental crust, the collision process involves the loss of mafic and ultramafic lower crust and the emplacement of voluminous, high silica, light rare earth element enriched melts, transforming the net composition into something more continental in character.

  17. Structure and composition of the continental crust in East China

    Institute of Scientific and Technical Information of China (English)

    高山; 骆庭川; 张本仁; 张宏飞; 韩吟文; 赵志丹; KERN; Hartmut

    1999-01-01

    Crustal structures of nine broad tectonic units in China, except the Tarim craton, are derived from 18 seismic refraction profiles including 12 geoscience transects. Abundances of 63 major, trace and rare earth elements in the upper crust in East China are estimated. The estimates are based on sampling of 11 451 individual rock samples over an area of 950 000 km~2, from which 905 large composite samples are prepared and analyzed by 13 methods. The middle, lower and total crust compositions of East China are also estimated from studies of exposed crustal cross sections and granulite xenoliths and by correlation of seismic data with lithologies. All the tectonic units except the Tarim craton and the Qinling orogen show a four-layered crustal structure, consisting of the upper, middle, upper lower, and lowermost crusts. P-wave velocities of the bulk lower crust and total crust are 6.8—7.0 and 6.4—6.5 km/s, respectively. They are slower by 0.2—0.4 km/s than the global averages. The bulk lower crust is su

  18. The arc delaminate: a geochemical reservoir twice the size of the continental crust

    Science.gov (United States)

    Jagoutz, O.; Schmidt, M. W.

    2012-12-01

    Most primitive melts in arcs are basaltic in composition but the continental crust or arc average is andesitic. To evolve from a primitive basalt to an andesitic composition, cumulates have to be fractionated and, if gravitationally unstable, can be delaminated. Such lower crustal cumulates are exposed in the Kohistan arc (N Pakistan) in a 10 km section through dunites, wehrlites, websterites, cpx-bearing garnetites and hornblendites, and garnet gabbros. We have compiled primitive melts for nine island arcs from the literature and fitted these with the bulk Kohistan arc [1] or average bulk continental crust [2] and the Kohistan cumulates. By average, ~15 wt% wehrlite + ~20% garnet hornblendite + ~35% garnet gabbro complement ~30% arc or continental crust and explain very well (r2~2) the evolution from a tholeiitic/calc-alkaline primitive high-Mg basalt to the continental crust. The bulk delaminate has 44-48 wt% SiO2, total alkalis of 1.1-1.4 wt% and an XMg of 0.67-0.69. Mass fractions derived from major elements were employed to compare trace elements: cumulates+crust deviate on average only by 25-30% from primitive melts, with the biggest deviations on the subduction-added traces. Relative to the continental or arc crust, the delaminate mass results to 1.8-2.5 times that of the continental crust. The delaminates have rho=3.2-3.5 g/cm3 and VP=7.9-8.2. At the base of the crust, they are thus difficult to distinguish seismically. Once reaching a critical thickness, they may sink into the deeper mantle where they form a geochemical reservoir twice the size of the continental crust. With respect to primitive mantle, the delaminate is enriched in Ba, K, Sr, and P and REE with LREEcrust increases the flux of primitive melt in arcs threefold. This places the magma

  19. Water in granulites: implications for the nature and evolution of the lower continental crust

    Institute of Scientific and Technical Information of China (English)

    YANG Xiaozhi; XIA Qunke; Etienne DELOULE; FAN Qicheng; HAO Yantao

    2007-01-01

    The lower continental crust is one of the most important sphere-layers in the deep earth, and is the direct place where the crust-mantle interactions occur. Granulites are the dominated rocks in the lower crust, and have critical implications for the knowledge of the composition, nature and evolution of the deep crust; fluids are important mediums influencing many geochemical, geophysical and geodynamical characteristics of the lower crust, and may also play a fundamental role in the petrogenesis of granulites and the formation of the lower crusts. In this paper, we review recent advances involved with the deep continental crust, granulites and fluids, and some longstanding debates. Combined with the Fourier-transform infrared spectroscopy (FTIR) analysis performed on the mineral assemblages (cpx, opx, plag and grt) in lower crustal granulite xenoliths and terrains (exposed section) from east China, it is suggested that structural water, dominated by OH, in these nominally anhydrous phases may constitute the most important water reservoir in the deep crust. This structual water may help to understand many lower crustal geological processes and phenomena (e. G. Seismic activities and electrical conductive anomalies), and influences from these water must be taken into consideration.

  20. Excavation and Melting of the Hadean Continental Crust by Late Heavy Bombardment

    CERN Document Server

    Shibaike, Yuhito; Ida, Shigeru

    2015-01-01

    No Hadean rocks have ever been found on Earth's surface except for zircons---evidence of continental crust, suggesting that Hadean continental crust existed but later disappeared. One hypothesis for the disappearance of the continental crust is excavation/melting by the Late Heavy Bombardment (LHB), a concentration of impacts in the last phase of the Hadean eon. In this paper, we calculate the effects of LHB on Hadean continental crust in order to investigate this hypothesis. Approximating the size-frequency distribution of the impacts by a power-law scaling with an exponent {\\alpha} as a parameter, we have derived semi-analytical expressions for the effects of LHB impacts. We calculated the total excavation/melting volume and area affected by the LHB from two constraints of LHB on the moon, the size of the largest basin during LHB, and the density of craters larger than 20 km. We also investigated the effects of the value of {\\alpha}. Our results show that LHB does not excavate/melt all of Hadean continental...

  1. Entropy-information perspective to radiogenic heat distribution in continental crust

    OpenAIRE

    Singh, R N; Manglik, A.

    2013-01-01

    Depth distribution of radiogenic heat sources in continental crust is an important parameter that controls its thermal structure as well as the mantle heat flow at the base of continental lithosphere. Various models for the depth distribution of radiogenic heat sources have been proposed. Starting from constant and exponential models based on linear heat flow–heat generation relationship the present-day layered models integrate crustal structure and laboratory measurements of radiogenic heat ...

  2. Radiogenic heat production, thermal regime and evolution of continental crust

    Science.gov (United States)

    Mareschal, Jean-Claude; Jaupart, Claude

    2013-12-01

    Heat flow and heat production data complement seismic information and provide strong constraints on crustal composition, thickness and evolution. They have helped understand the nature of the Mohorovicic discontinuity and the variations in seismic velocities below the Moho. Notably, heat flow studies have delineated the vertical distribution of heat producing elements throughout the crust and in the upper most mantle lithosphere. Analysis of global data sets on heat flow and crustal thickness demonstrate that there is no correlation between these two variables. This is due to the large spatial variations in crustal composition and heat production that exist within a single geological province. For a given crustal thickness, the Moho temperature varies within a wide range (≈ 300 K) depending on surface heat flux and crustal heat production. Thus one cannot use generic models based on a “type” crustal column to calculate crustal geotherms. In stable regions, lower crustal temperatures depend on the amount and vertical distribution of heat producing elements in the crust. These temperatures determine the conditions of crustal stability and impose a limit on the maximum thickness of a stabilized crust.

  3. No evidence for Hadean continental crust within Earth's oldest evolved rock unit

    Science.gov (United States)

    Reimink, J. R.; Davies, J. H. F. L.; Chacko, T.; Stern, R. A.; Heaman, L. M.; Sarkar, C.; Schaltegger, U.; Creaser, R. A.; Pearson, D. G.

    2016-10-01

    Due to the acute scarcity of very ancient rocks, the composition of Earth's embryonic crust during the Hadean eon (>4.0 billion years ago) is a critical unknown in our search to understand how the earliest continents evolved. Whether the Hadean Earth was dominated by mafic-composition crust, similar to today's oceanic crust, or included significant amounts of continental crust remains an unsolved question that carries major implications for the earliest atmosphere, the origin of life, and the geochemical evolution of the crust-mantle system. Here we present new U-Pb and Hf isotope data on zircons from the only precisely dated Hadean rock unit on Earth--a 4,019.6 +/- 1.8 Myr tonalitic gneiss unit in the Acasta Gneiss Complex, Canada. Combined zircon and whole-rock geochemical data from this ancient unit shows no indication of derivation from, or interaction with, older Hadean continental crust. Instead, the data provide the first direct evidence that the oldest known evolved crust on Earth was generated from an older ultramafic or mafic reservoir that probably surfaced the early Earth.

  4. Importance of Melt Weakening, Rheological Heterogeneity, and Strain Partitioning in Exhumed Sections of Deep Continental Crust

    Science.gov (United States)

    Dumond, G.; Mahan, K. H.; Andronicos, C.; Williams, M. L.; Hollister, L. S.

    2011-12-01

    Results from recent interdisciplinary studies of exposed deep continental crust in the Canadian Shield and Cordillera illustrate how melt weakening, rheological heterogeneity, and strain partitioning act as fundamental controls on the flow of continental lower crust. The importance of each of these processes has been inferred by remotely-based studies and modeling of orogenic plateaus and large hot orogens, e.g., the Himalayan-Tibetan system. Our field-based results emphasize (1) how melt-enhanced strain localization and melt weakening are facilitated by magmatic underplating of mantle-derived melts and partial melting of pre-existing middle to lower crust, (2) that exposed gneiss domes, sill complexes, and sub-horizontal tectonites represent viable records of middle to lower crustal flow coincident with episodes of crustal-scale transpression and transtension, and (3) how rheology of continental lithosphere evolves heterogeneously with space and time during orogeny, suggesting variable degrees of crust-mantle coupling and decoupling. The record gleaned from exposed Canadian deep crust highlights the inherently 3-dimenisional nature of lateral and vertical ductile flow of lower crust. In both examples, lower crustal flow was influenced by the occurrence of transcurrent sub-vertical shear zones that separate rheologically heterogeneous panels of continental crust. This heterogeneity is manifested in a spatially- and temporally-varying record of syn-kinematic metamorphic reactions that point to deep crustal regions that have undergone protracted records of isobaric cooling versus other regions that experienced isothermal decompression following melt weakening. We conclude that (1) underplating/intraplating of mantle-derived magmas in concert with dehydration melting of continental crust are important controls on melt weakening and crustal flow, in addition to other mechanisms such as partial melting due to radioactive self-heating of thickened crust [e.g., the

  5. Formation of Fe-Mn crusts within a continental margin environment

    Science.gov (United States)

    Conrad, Tracey A.; Hein, James R.; Paytan, Adina; Clague, David A.

    2017-01-01

    This study examines Fe-Mn crusts that form on seamounts along the California continental-margin (CCM), within the United States 200 nautical mile exclusive economic zone. The study area extends from approximately 30° to 38° North latitudes and from 117° to 126° West longitudes. The area of study is a tectonically active northeast Pacific plate boundary region and is also part of the North Pacific Subtropical Gyre with currents dominated by the California Current System. Upwelling of nutrient-rich water results in high primary productivity that produces a pronounced oxygen minimum zone. Hydrogenetic Fe-Mn crusts forming along the CCM show distinct chemical and mineral compositions compared to open-ocean crusts. On average, CCM crusts contain more Fe relative to Mn than open-ocean Pacific crusts. The continental shelf and slope release both Fe and Mn under low-oxygen conditions. Silica is also enriched relative to Al compared to open-ocean crusts. This is due to the North Pacific silica plume and enrichment of Si along the path of deep-water circulation, resulting in Si enrichment in bottom and intermediate waters of the eastern Pacific.The CCM Fe-Mn crusts have a higher percentage of birnessite than open-ocean crusts, reflecting lower dissolved seawater oxygen that results from the intense coastal upwelling and proximity to zones of continental slope pore-water anoxia. Carbonate fluorapatite (CFA) is not present and CCM crusts do not show evidence of phosphatization, even in the older sections. The mineralogy indicates a suboxic environment under which birnessite forms, but in which pH is not high enough to facilitate CFA deposition. Growth rates of CCM crusts generally increase with increasing water depth, likely due to deep-water Fe sources mobilized from reduced shelf and slope sediments.Many elements of economic interest including Mn, Co, Ni, Cu, W, and Te have slightly or significantly lower concentrations in CCM crusts relative to crusts from the Pacific

  6. Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust

    Science.gov (United States)

    Akinin, V.V.; Miller, E.L.; Wooden, J.L.

    2009-01-01

    Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep

  7. A plate tectonics oddity: Caterpillar-walk exhumation of subducted continental crust

    NARCIS (Netherlands)

    Tirel, C.; Brun, J.-P.; Burov, E.; Wortel, M.J.R.; Lebedev, S.

    2013-01-01

    Since plate tectonics began on Earth, grandiose "subduction factories" have continually shaped the continents, accreting continental blocks and new crust at the convergent plate boundaries. An enigmatic product of subduction factories is the high-pressure to ultrahigh-pressure (HP-UHP) metamorphic

  8. A plate tectonics oddity: Caterpillar-walk exhumation of subducted continental crust

    NARCIS (Netherlands)

    Tirel, C.; Brun, J.-P.; Burov, E.; Wortel, M.J.R.; Lebedev, S.

    2013-01-01

    Since plate tectonics began on Earth, grandiose "subduction factories" have continually shaped the continents, accreting continental blocks and new crust at the convergent plate boundaries. An enigmatic product of subduction factories is the high-pressure to ultrahigh-pressure (HP-UHP) metamorphic c

  9. A plate tectonics oddity: Caterpillar-walk exhumation of subducted continental crust

    NARCIS (Netherlands)

    Tirel, C.; Brun, J.-P.; Burov, E.; Wortel, M.J.R.; Lebedev, S.

    2013-01-01

    Since plate tectonics began on Earth, grandiose "subduction factories" have continually shaped the continents, accreting continental blocks and new crust at the convergent plate boundaries. An enigmatic product of subduction factories is the high-pressure to ultrahigh-pressure (HP-UHP) metamorphic c

  10. Modification of an oceanic plateau, Aruba, Dutch Caribbean: Implications for the generation of continental crust

    NARCIS (Netherlands)

    White, R.V.; Tarney, J.; Kerr, A.C.; Saunders, A.D.; Kempton, P.D.; Pringle, M.S.; Klaver, G.T.

    1999-01-01

    The generation of the continental crust may be connected to mantle plume activity. However, the nature of this link, and the processes involved, are not well constrained. An obstacle to understanding relationships between plume-related mafic material and associated silicic rocks is that later tecton

  11. Magnetic Structure of Continental Crust:Implications for Crustal Structure and Evolution

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    Magnetic structure of the continental crust is one of the important geophysical aspects of continental lithosphere. This paper reviews the achievements in the research into the magnetic structure and its significance for crustal tectonics, composition, metamorphic facies, crust-mantle interaction and magnetization of deep crust. Further studies are suggested according to the basic principles of rock and mineral magnetism in terms of petrology, geochemistry and structural geol ogy. Emphasis is placed on new geological ideas and synthetic studies of the relationship between deep geological processes and interpretation of gravity, magnetic, electrical and seismic data. The relationships between magnetic, density, electricity, velocity, geothermal structures and deep geodynamic processes are taken as a system for the research into the deep geology.

  12. Comparison of element abundance between the exposed crust of the continent of China and the global averaged upper continental crust: Constraints on crustal evolution and some speculations

    Institute of Scientific and Technical Information of China (English)

    WANG Yang

    2007-01-01

    Based on the results of a study of regional element abundance in eastern China and the 1:200 000 geochemical surveys in northern Xinjiang, the element geochemical characteristics of the exposed crust in 23 tectonic units of the continent of China are summarized.Compared with the global average abundance of the upper continental crust, the exposed crust of the continent of China is compositionally more evolved than the upper crust of the island arc, but less evolved than the mature Precambrian Canadian shield. The exposed crust of the North China and Yangtze platforms has a lower SiO2 content, but markedly higher CaO and MgO contents due to the presence of widespread carbonate strata, which suggests that we should not neglect the contribution of carbonate rocks in the study of the exposed crust and the element abundance of the upper crust, in comparison with two recently published average compositional models of the global upper continental crust,the exposed crust of the continent of China is depleted in Au,Hg, Mo, Sn, and W, which suggests that their abundance in the present global models is overestimated. The exposed crust of the North China plat form and the Qinling-Dabieshan fold belt to its south has lower μ(238U/204pb) values(8), but other regions of the continent of China exhibit much higher μ values, which implies that the low μ feature of the North China platform and its adjacent regions does not have global significance. Considering the apparent lateral variation in composition of the exposed crust for the tectonic units of the continent of China, there is no adequate reason to take the average upper crust compositional model of the North China platform and its adjacent regions as a reliable composition representative for Chinese and global upper continental crust composition.

  13. From a collage of microplates to stable continental crust - an example from Precambrian Europe

    Science.gov (United States)

    Korja, Annakaisa

    2013-04-01

    Svecofennian orogen (2.0-1.7 Ga) comprises the oldest undispersed orogenic belt on Baltica and Eurasian plate. Svecofennian orogenic belt evolved from a series of short-lived terrane accretions around Baltica's Archean nucleus during the formation of the Precambrian Nuna supercontinent. Geological and geophysical datasets indicate W-SW growth of Baltica with NE-ward dipping subduction zones. The data suggest a long-lived retreating subduction system in the southwestern parts whereas in the northern and central parts the northeasterly transport of continental fragments or microplates towards the continental nucleus is also documented. The geotectonic environment resembles that of the early stages of the Alpine-Himalayan or Indonesian orogenic system, in which dispersed continental fragments, arcs and microplates have been attached to the Eurasian plate margin. Thus the Svecofennian orogeny can be viewed as proxy for the initial stages of an internal orogenic system. Svecofennian orogeny is a Paleoproterozoic analogue of an evolved orogenic system where terrane accretion is followed by lateral spreading or collapse induced by change in the plate architecture. The exposed parts are composed of granitoid intrusions as well as highly deformed supracrustal units. Supracrustal rocks have been metamorphosed in LP-HT conditions in either paleo-lower-upper crust or paleo-upper-middle crust. Large scale seismic reflection profiles (BABEL and FIRE) across Baltica image the crust as a collage of terranes suggesting that the bedrock has been formed and thickened in sequential accretions. The profiles also image three fold layering of the thickened crust (>55 km) to transect old terrane boundaries, suggesting that the over-thickened bedrock structures have been rearranged in post-collisional spreading and/or collapse processes. The middle crust displays typical large scale flow structures: herringbone and anticlinal ramps, rooted onto large scale listric surfaces also suggestive

  14. Continental crust formation: Numerical modelling of chemical evolution and geological implications

    Science.gov (United States)

    Walzer, U.; Hendel, R.

    2017-05-01

    Oceanic plateaus develop by decompression melting of mantle plumes and have contributed to the growth of the continental crust throughout Earth's evolution. Occasional large-scale partial melting events of parts of the asthenosphere during the Archean produced large domains of precursor crustal material. The fractionation of arc-related crust during the Proterozoic and Phanerozoic contributed to the growth of continental crust. However, it remains unclear whether the continents or their precursors formed during episodic events or whether the gaps in zircon age records are a function of varying preservation potential. This study demonstrates that the formation of the continental crust was intrinsically tied to the thermoconvective evolution of the Earth's mantle. Our numerical solutions for the full set of physical balance equations of convection in a spherical shell mantle, combined with simplified equations of chemical continent-mantle differentiation, demonstrate that the actual rate of continental growth is not uniform through time. The kinetic energy of solid-state mantle creep (Ekin) slowly decreases with superposed episodic but not periodic maxima. In addition, laterally averaged surface heat flow (qob) behaves similarly but shows peaks that lag by 15-30 Ma compared with the Ekin peaks. Peak values of continental growth are delayed by 75-100 Ma relative to the qob maxima. The calculated present-day qob and total continental mass values agree well with observed values. Each episode of continental growth is separated from the next by an interval of quiescence that is not the result of variations in mantle creep velocity but instead reflects the fact that the peridotite solidus is not only a function of pressure but also of local water abundance. A period of differentiation results in a reduction in regional water concentrations, thereby increasing the temperature of the peridotite solidus and the regional viscosity of the mantle. By plausibly varying the

  15. Decoupled Rb-Sr and Sm-Nd isotopic evolution of the continental crust

    Science.gov (United States)

    Goldstein, S. L.

    1988-01-01

    Evidence was presented that the Rb-Sr and Sm-Nd isotopic systems are decoupled in crust-mantle evolution. Rare earth element (including Sm and Nd) residue principally in silicates, and are resistant to mobilization by weathering and metamorphism. In contrast, Rb and Sr are easily fractionated by crustal processes and residue in carbonates as well as in silicates. As a result, continental Sr, but not Nd, can be recycled into the mantle by exchange of seawater with basalt at spreading ridges and by subduction of carbonates associated with ridge processes. These effects result in mean Rb-Sr ages of the continental crust and of the upper mantle that are too young. Crustal growth curves based largely on Rb-Sr data, such that of Hurley and Rand, are therefore incorrect.

  16. Li Isotopic Composition and Concentration of the Upper Continental Crust: New Insights from Desert Loess

    Science.gov (United States)

    Sauzeat, L.; Rudnick, R. L.; Chauvel, C.

    2014-12-01

    The isotopic composition of lithium (δ7Li) is recognized to be an excellent proxy of near-surface fluid-rock reactions during weathering. Using Li as a tracer of these processes however requires constraints on the average Li composition of terrestrial reservoirs, in particular that of the upper continental crust. To date, only one value for the average δ7Li value of the upper continental crust, derived from periglacial loess, shales, and granites is available in the literature (7δLi = 0 ± 4 (2σ), Teng et al., 2004). Several values exist for the average [Li] of the upper crust, but they differ by more than 30%. We measured the Li isotopic composition of about 30 desert and periglacial loess (unweathered windblown sediments) from several parts of the world (Europe, Argentina, China and Tajikistan). We demonstrate that desert loess, which is more homogeneous and representative of larger portions of the Earth's surface, provides a better proxy for the average composition of the upper continental crust compared to periglacial loess. The Li isotopic compositions and concentrations of desert loess are controlled by eolian sorting, which can be quantified as a binary isotopic mixing between a weathered fine-grained end-member and an unweathered coarse-grained end-member. Using correlations between Li isotopic compositions, Li concentrations and trace element concentrations in desert loess, we estimate new average values for the upper continental crust: 1 ± 2 (2σ); [Li] = 35.3 ± 4.6 (2σ) ppm. This δ7Li value is slightly higher than that previously published in Teng et al. (2004), but overlaps within uncertainty, whereas the [Li] is identical to that of Teng et al. (2004: 35 ± 11, 2σ); both new estimates have lower uncertainty. Our new estimate of [Li], along with that of Teng et al. (2004), are higher than all previous estimates for the upper continental crust, raising the question as to whether the average concentrations of other mobile alkali metals such as

  17. Niobium-tantalum Fractionation During Slab Subduction:Implications for the Formation of Continental Crust

    Institute of Scientific and Technical Information of China (English)

    Yilin Xiao; Weidong Sun; Jochen Hoefs; Klaus Simon; Zeming Zhang; Shuguang Li; Albrecht W. Hofmann

    2007-01-01

    @@ Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the major way that shaped the chemical characteristics of the CC by most geologists, as the CC shares many characteristics with modern adakites, some geologists argued that Archean TTG was formed in the same way as modern arcs rather than slab melting, whereas others proposed that melting at the bottom of the thickened oceanic crust was more important. Recently,the debate is mainly focused on the unique subchondritic Nb/Ta value of the CC, and particularly, how Nb and Ta fractionated from each other and consequently how, in detail, the CC was built.

  18. Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius.

    Science.gov (United States)

    Ashwal, Lewis D; Wiedenbeck, Michael; Torsvik, Trond H

    2017-01-31

    A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U-Pb zircon ages recovered directly from 5.7 Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U-Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ∼700 km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India.

  19. Entropy-information perspective to radiogenic heat distribution in continental crust

    Directory of Open Access Journals (Sweden)

    R. N. Singh

    2013-06-01

    Full Text Available Depth distribution of radiogenic heat sources in continental crust is an important parameter that controls its thermal structure as well as the mantle heat flow at the base of continental lithosphere. Various models for the depth distribution of radiogenic heat sources have been proposed. Starting from constant and exponential models based on linear heat flow–heat generation relationship the present-day layered models integrate crustal structure and laboratory measurements of radiogenic heat sources in various exposed rocks representing crustal composition. In the present work, an extended entropy theory formalism is used for estimation of radiogenic heat sources distribution in continental crust based on principle of maximum entropy (POME. The extended entropy principle yields a constant heat generation model if only a constraint given by total radiogenic heat in the crust is used and an exponential form of radiogenic heat sources distribution if an additional constraint in the form of a second moment is used in the minimization of entropy.

  20. Constraints on the development of the early continental crust from isotopic data

    Science.gov (United States)

    Jacobson, S. B.

    1985-01-01

    Aspects of the origin and development of the early (AE) continential crust are addressed by radiogenic isotope and trace element studies. The most important ones are: (1) at what time did the earliest continental crust form; (2) what was its composition; (3) by what processes did it grow and by what processes was it destroyed; (4) what were the rates of production and destruction as a function of time during this time period? Nd is isotopic data on the oldest terrestrial rocks indicate that the mantle at this time had already suffered substantial depletion in incompatible elements due to earlier continent forming events. Isotopic data on young volcanic rocks derived from the depleted mantle show no evidence of this early history. The observed isotopic patterns of Nd, Sr, Hf and Pb through time together with the presently observed age spectrum of crustal rocks are considered. These patterns can be modelled by a transport model in which the continental growth and destruction rates are allowed to vary as a function of time. It is suggest that the mass of the continents at 3.8 AE ago was about 25% of the current continental mass. However, due to the very high recycling rates obtained in the early Archean only a few percent of this crust has been preserved up to the present.

  1. Detrital zircon evidence for Hf isotopic evolution of granitoid crust and continental growth

    Science.gov (United States)

    Iizuka, Tsuyoshi; Komiya, Tsuyoshi; Rino, Shuji; Maruyama, Shigenori; Hirata, Takafumi

    2010-04-01

    We have determined U-Pb ages, trace element abundances and Hf isotopic compositions of approximately 1000 detrital zircon grains from the Mississippi, Congo, Yangtze and Amazon Rivers. The U-Pb isotopic data reveal the lack of >3.3 Ga zircons in the river sands, and distinct peaks at 2.7-2.5, 2.2-1.9, 1.7-1.6, 1.2-1.0, 0.9-0.4, and uniformitarian secular change in Hf isotopes of granitoid crusts; Hf isotopic compositions of granitoid crusts deviate from the mantle evolution line from about 3.3 to 2.0 Ga, the deviation declines between 2.0 and 1.3 Ga and again increases afterwards. Consideration of mantle-crust mixing models for granitoid genesis suggests that the noted isotopic trends are best explained if the rate of crust generation globally increased in two stages at around (or before) 3.3 and 1.3 Ga, whereas crustal differentiation was important in the evolution of the continental crust at 2.3-2.2 Ga and after 0.6 Ga. Reconciling the isotopic secular change in granitoid crust with that in sedimentary rocks suggests that sedimentary recycling has essentially taken place in continental settings rather than active margin settings and that the sedimentary mass significantly grew through addition of first-cycle sediments from young igneous basements, until after ˜1.3 Ga when sedimentary recycling became the dominant feature of sedimentary evolution. These findings, coupled with the lack of zircons older than 3.3 Ga in river sands, imply the emergence of large-scale continents at about 3.3 Ga with further rapid growth at around 1.3 Ga. This resulted in the major growth of the sedimentary mass between 3.3 and 1.3 Ga and the predominance of its cannibalistic recycling later.

  2. Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust

    Science.gov (United States)

    Von Huene, R.; Scholl, D. W.

    1991-01-01

    At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (???19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-mediumsized prisms have formed (???16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (???8,200 km), roughly 70% of the incoming trench floor section is

  3. Geometries of hyperextended continental crust in northeastern continental brazilian margin: insights from potential field and seismic interpretation

    Science.gov (United States)

    Magalhães, José; Barbosa, José; Ribeiro, Vanessa; Oliveira, Jefferson; Filho, Osvaldo; Buarque, Bruno

    2016-04-01

    continental crust) to NE-SW (fracture zones in oceanic crust). In the gravimetric maps (Residual isostatic and Free-Air) and in 2D forward modeling the negative and positive gravity anomalies are related to grabens and horsts, respectively. The Pernambuco Basin shows depocentres oriented on NW-SE and structural-marginal highs with circular and elongated shape trending NNE-SSW. The basins in the north sector present elongated positive and negative gravimetric anomalies trending NNW-SSE that are confirmed by the seismic interpretation as basement highs and small grabens, respectively. From integration of results, the oblique rifting process promoted by thinning and exhumation deformation phases and the magmatic activity in the hyperextended domain of the Pernambuco Plateau, produced an anastomosing pattern between the two sets of shear zones (NE-SW and NW-SE) and a series of rhomb-shaped structures (lozenges). The northern Paraíba and Natal Platform basins, the orthogonal arrangement between Pre-Cambrian shear zones (NE-SW and E-W) and the rift axis favored these structures to initiate strait transforming faults from thinned continental crust to oceanic crust.

  4. Generation of new continental crust by sublithospheric silicic-magma relamination in arcs: A test of Taylor's andesite model

    NARCIS (Netherlands)

    Castro, Antonio; Vogt, Katharina; Gerya, Taras

    2013-01-01

    The paradox of the Earth's continental crust is that although this reservoir is generally regarded as having differentiated from the mantle, it has an andesitic bulk composition that contrasts with the intrinsic basaltic composition of mantle-derived melts. Classical models for new crust generation

  5. Formation of metamorphic core complexes in non-over-thickened continental crust: A case study of Liaodong Peninsula (East Asia)

    OpenAIRE

    Wang, Kun; Burov, Evgueni; Gumiaux, Charles; Chen, Yan; Lu, Gang; Mezri, Leila; Zhao, Liang

    2015-01-01

    International audience; Pre-thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes (MCCs). However, the discovery of MCCs in the Liaodong Peninsula, where the crust has a normal thickness (~ 35 km), challenges the universality of this scenario. Therefore, we implement a series of 2-D numerical thermo-mechanical modeling experiments in which we investigate the conditions of MCC formation in normal crusts, as well as th...

  6. Geochemical and Sr-Nd-Pb isotopic evidence for ancient lower continental crust beneath the Xi Ujimqin area of NE China

    Science.gov (United States)

    Gao, Xiaofeng; Guo, Feng; Xiao, Peixi; Kang, Lei; Xi, Rengang

    2016-05-01

    The Central Asian Orogenic Belt (CAOB) is the largest Phanerozoic accretionary orogen on Earth. The role that Precambrian continental microblocks played in its formation, however, remains a highly controversial topic. New zircon U-Pb age data and whole-rock geochemical and Sr-Nd-Pb isotopic studies on Permian (253-251 Ma) andesites from the Xi Ujimqin area provide the first evidence for the existence of a continental lower mafic crust in the eastern segment of the CAOB. These Permian lavas generally have chemical compositions similar to experimental melts of garnet pyroxenites. Based on Sr-Nd-Pb isotopic compositional differences, they can be further subdivided into two groups. Group 1 has moderately radiogenic Sr (87Sr/86Sr(i) = 0.7060-0.7062) and nonradiogenic Nd (εNd(t) = - 9.0-8.3) and Pb (e.g., 206Pb/204Pb = 17.18-17.23) isotopic compositions similar to the ancient lower mafic crust beneath the North China Craton (NCC). Compared with Group 1, Group 2 has less radiogenic Sr (87Sr/86Sr(i) = 0.7051-0.7055), and more radiogenic Nd (εNd(t) = - 0.2-+1.4) and Pb (e.g., 206Pb/204Pb = 18.04-18.20) isotopic compositions as observed in the Phanerozoic granitoids and felsic lavas of the CAOB. The combined geochemical and isotopic data indicate that Group 1 was derived from ancient lower mafic crust of the NCC affinity, with a residual assemblage of pyroxene + plagioclase + amphibole. The source for Group 2 was a mixture of ancient lower mafic crust and a juvenile crustal component, and melting left a residue of orthopyroxene + clinopyroxene + plagioclase + garnet + amphibole. Generation of these two types of late Permian andesites favors a model whereby breakoff of a subducted slab and subsequent lithospheric extension triggered extensive asthenospheric upwelling and melting of the continental mafic lower crust of the eastern CAOB. The discovery of ancient lower continental crust of the NCC affinity in the CAOB implies that the NCC experienced continental breakup during

  7. No significant production of continental crust prior to 3.8 Ga

    Science.gov (United States)

    Vervoort, J. D.; Kemp, T.; Fisher, C. M.

    2012-12-01

    In his final paper before his death in 1991 (The Persistent Myth of Crustal Growth), Dick Armstrong argued that continental crustal recycling has kept pace with crustal growth and that the volume of continental crust has been in an approximate steady state (no net growth) for at least the past 4 Ga. Although this has been a minority viewpoint (most scientists favor some manner of progressive crustal growth) Armstrong's no-growth model has had some vehement supporters over the years. Since this paper was published, most of the discussion in the literature has focused on the isotopic record—particularly that of the oldest preserved igneous rocks. Recently, attention has shifted to detrital zircons and, in particular, the Hf isotopic record in the oldest known zircons. Two main features from these data have been used to argue for widespread crust in the early Earth: 1) heterogeneous Hf isotopic compositions of the oldest zircons; and 2) Hf model ages in Archean detrital zircons that are much older than their crystallization ages. The dataset on which these observations have been based, however, is not robust. Many data are compromised by complexities in age and isotopic composition. Hf zircon model ages are even more problematic as they are loaded with assumptions and based on a depleted mantle evolution reference that is not well known, is most likely wrong prior to 3.8 Ga, and has uncertain relevance for understanding crustal growth. In order to provide a less ambiguous isotopic record, a better approach is needed: the integration of age (U-Pb zircon) and isotopic information (Hf-Nd whole rock, Hf-O in zircons) from the oldest, but also least complicated and altered, magmatic samples. Several lines of evidence suggest that formation of continental crust did not begin in earnest until ca. 3.8-3.5 Ga: 1) lack of older crust or inherited pre-4 Ga zircon cores in the geological record in shield areas; 2) conspicuous lack of pre-4 Ga zircons in nearly all Archean

  8. Insights into chemical weathering of the upper continental crust from the geochemistry of ancient glacial diamictites

    Science.gov (United States)

    Li, Su; Gaschnig, Richard M.; Rudnick, Roberta L.

    2016-03-01

    Glacial diamictites, with ages ranging from ∼2900 to 0.01 Ma, record the changing composition of the upper continental crust through time (Gaschnig et al., 2014). Li concentrations and isotopic compositions, combined with Pb isotopic compositions, chemical index of alteration (CIA) values and relative Sr concentrations are used here to assess the degree of chemical weathering recorded in these deposits and the origin of this signature. The δ7Li values of most of the diamictites (ranging from -3.9 to +3.5) are lower than those of mantle-derived basalts (+3.7 ± 2, 2σ), and the low δ7Li values are generally accompanied by high CIA and low Sr/Sr∗ values (or Sr depletion factor, Sr/Sr∗ = Sr/(Ce∗Nd)0.5), reflecting a weathering signature that may have derived from pre-depositional, syn-depositional, and/or post-depositional weathering processes. Profiles through three glacial diamictites with relatively high CIA (a fresh road cut of the Neoproterozoic Nantuo Formation (CIA = 62-69), and drill cores through the Paleoproterozoic Timeball Hill (CIA = 66-75) and Duitschland Formations (CIA = 84-91)) do not show evidence of significant post-depositional weathering. High Th/U, reflecting loss of uranium during oxidative weathering, is seen in all Paleozoic and Neoproterozoic diamictites and a few Paleoproterozoic deposits. Pb isotopic systematics suggest that this signature was largely inherited from preexisting crust, although a subset of samples (the Neoproterozoic Konnarock, Paleozoic Dwyka, and several of the Paleoproterozoic Duitschland samples) appears to have experienced post-depositional U loss. Modern glaciomarine sediments record little weathering (CIA = 47, Sr/Sr∗ = 0.7, δ7Li = +1.8), consistent with the cold temperatures accompanying glacial periods, and suggesting that limited syn-depositional weathering has occurred. Thus, the chemical weathering signature observed in ancient glacial diamictites appears to be largely inherited from the upper

  9. Influence of water on rheology and strain localization in the lower continental crust

    Science.gov (United States)

    Getsinger, A. J.; Hirth, G.; Stünitz, H.; Goergen, E. T.

    2013-07-01

    We investigated deformation processes within a lower crustal shear zone exposed in gabbros from Arnøya, Norway. Over a distance of ˜1 m, the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. With increasing proximity to the pegmatite, there is a significant increase in the abundance of amphibole and zoisite (which form at the expense of pyroxene and calcic plagioclase) and a slight increase in the strength of plagioclase lattice-preferred orientation, but there is little change in recrystallized plagioclase grain size. Phase diagrams, the presence of hydrous reaction products, and deformation mechanism maps all indicate that the water activity (aH2O) during deformation must have been high (˜1) in the sheared gabbro compared with the nonhydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn-deformational, water-consuming reactions, creating a rheological contrast between wet and dry lithologies that promote strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. These results provide an example of how fluid weakens lower continental crust lithologies at high pressures and temperatures.

  10. Felsic Magmatism through Intracrustal Melting of Previously Formed Volcanic-Arc Crust: Implications for Differentiation and Secular Evolution of the Continental Crust

    Science.gov (United States)

    G R, R. K.; C, S.

    2015-12-01

    The fundamental challenge in understanding the origin and evolution of the continental crust is to recognize how primary mantle source, and oceanic crust, which are essentially mafic to ultramafic in composition, could differentiate into a more or less felsic compositions. It is possible to understand growth and differentiation of the continental crust by constraining the interplay of magmatism, deformation, and high-grade metamorphism in the lower crust. Here, we apply this knowledge on the lower crustal granitoids of southern India and speculate on the variations in geochemistry as a consequence of differentiation and secular evolution of the continental crust.The major groups of granitoids of southern India are classified as metatonalites, comparable to typical Archaean TTGs with pronounced calc-alkaline affinity, and metagranites which are magmatic fractionation produced by reworking of early crust. Metatonalites are sodic-trondhjemites with slightly magnesian, moderate LREE (average LaN = 103) and low HREE (average YbN = 2) characerestics, where as metagranites are calc-alkaline ferroan types with enriched LREE (average LaN = 427) and HREE (average YbN = 23). Petrogenetic characteristics of granitoids illustrate continuous evolution of a primary crust into diverse magmatic units by multiple stages of intracrustal differentiation processes attributed to following tectonic scenarios: (1) formation of tonalitic magma by low- to moderate-degree partial melting of hydrated basaltic crust at pressures high enough to stabilize garnet-amphibole residue and (2) genesis of granite in a continental arc-accretion setting by an episode of crustal remelting of the tonalitic crust, within plagioclase stability field. The first-stage formed in a flat-subduction setting of an volcanic-arc, leading to the formation of tonalites. The heat budget required is ascribed to the upwelling of the mantle and/or basaltic underplating. Progressive decline in mantle potential temperature

  11. 陆壳的垂向增生%Vertical Accretion of Continental Crust

    Institute of Scientific and Technical Information of China (English)

    陈立辉

    2001-01-01

    Vertical accretion of continental crust is a recycling process,which mainly includes crust magmatic underplating and lithosphere delamination. Crust sinking might be another important process which made crust material enter mantle.%陆壳的垂向增生是幔源物质进入到下地壳和下地壳物质进入到地幔的双向过程,前者主要表现为壳下底侵作用,后者主要表现为岩石圈规模的拆沉作用,其中拆沉作用往往诱发了陆壳下大规模的底侵作用。下地壳部分熔融残余的超镁铁质岩沉入到岩石圈地幔的过程称为陆壳沉没作用,它可能是陆壳物质进入地幔的一种重要方式。

  12. Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

    Science.gov (United States)

    Molli, Giancarlo; Menegon, Luca; Malasoma, Alessandro

    2017-07-01

    The switching in deformation mode (from distributed to localized) and mechanisms (viscous versus frictional) represent a relevant issue in the frame of crustal deformation, being also connected with the concept of the brittle-ductile transition and seismogenesis. In a subduction environment, switching in deformation mode and mechanisms and scale of localization may be inferred along the subduction interface, in a transition zone between the highly coupled (seismogenic zone) and decoupled deeper aseismic domain (stable slip). However, the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as fundamental in some cases for the localization of deformation and shear zone development, thus representing a case in which switching deformation mechanisms and scale and style of localization (deformation mode) interact and relate to each other. This contribution analyses an example of a millimetre-scale shear zone localized by brittle precursor formed within a host granitic protomylonite. The studied structures, developed in ambient pressure-temperature (P-T) conditions of low-grade blueschist facies (temperature T of ca. 300 °C and pressure P ≥ 0. 70 GPa) during involvement of Corsican continental crust in the Alpine subduction. We used a multidisciplinary approach by combining detailed microstructural and petrographic analyses, crystallographic preferred orientation by electron backscatter diffraction (EBSD), and palaeopiezometric studies on a selected sample to support an evolutionary model and deformation path for subducted continental crust. We infer that the studied structures, possibly formed by transient instability associated with fluctuations of pore fluid pressure and episodic strain rate variations, may be considered as a small-scale example of fault behaviour associated with a cycle of interseismic creep and coseismic rupture or a new analogue for episodic tremors and slow

  13. Geological cycles and a two-stage history of the Continental Crust (Robert Wilhelm Bunsen Medal Lecture)

    Science.gov (United States)

    Hawkesworth, Chris J.; Cawood, Peter A.; Dhuime, Bruno

    2014-05-01

    The continental crust is the archive of Earth history, and the apparently cyclical nature of geological evolution is a feature of the geological record. The advent of radiometric ages has highlighted that the spatial and temporal distribution of the Earth's record of rock units and events is heterogeneous with distinctive peaks and troughs in the distribution of ages of igneous crystallization, metamorphism, continental margins and mineralization. It is argued that the temporal distribution largely reflects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. In contrast there are other signals, such as the Sr isotope ratios of seawater, mantle temperatures, and redox conditions on the Earth, where the records are regarded as primary because they are not sensitive to the numbers of samples of different ages that have been analysed. Models based on the U-Pb, Hf and O isotope ratios of detrital zircons suggest that at least ~60-70% of the present volume of the continental crust had been generated by 3 Ga. The sedimentary record is biased by preferential sampling of relatively young material in their source terrains. The implication is that there were greater volumes of continental crust in the Archaean than might be inferred from the compositions of detrital zircons and sediments. The growth of continental crust was a continuous rather than an episodic process, but the rates of continental growth were significantly higher before 3 Ga than subsequently. The time-integrated Rb/Sr ratios, and the average SiO2 contents, indicate that new continental crust was largely mafic over the first 1.5 Ga of Earth's evolution, and that significant volumes of pre-3 Ga crust may have been associated with intraplate magmatism. Since ~3 Ga there has been an increase in Rb/Sr, SiO2, and the inferred thickness of new crust, consistent

  14. Native iron in the continental lower crust: petrological and geophysical implications.

    Science.gov (United States)

    Haggerty, S E; Toft, P B

    1985-08-16

    Lower crustal granulite xenoliths recovered from a kimberlite pipe in western Africa contain native iron (Fe(0)) as a decomposition product of garnet and ilmenite. Magnetic measurements show that less than 0.1 percent (by volume) of iron metal is present. Data from geothermometry and oxygen geobarometry indicate that the oxide and metal phases equilibrated between iron-wüstite and magnetite-wüstite buffers, which may represent the oxidation state of the continental lower crust, and the depleted lithospheric upper mantle. Ferromagnetic native iron could be stable to a depth of approximately 95 kilometers and should be considered in the interpretation of long-wavelength static magnetic anomalies.

  15. Volcanic-plutonic parity and the differentiation of the continental crust.

    Science.gov (United States)

    Keller, C Brenhin; Schoene, Blair; Barboni, Melanie; Samperton, Kyle M; Husson, Jon M

    2015-07-16

    The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks-volcanic and plutonic rocks-and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global data set of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation.

  16. Processes of lithosphere evolution: New evidence on the structure of the continental crust and uppermost mantle

    Science.gov (United States)

    Artemieva, I.M.; Mooney, W.D.; Perchuc, E.; Thybo, H.

    2002-01-01

    We discuss the structure of the continental lithosphere, its physical properties, and the mechanisms that formed and modified it since the early Archean. The structure of the upper mantle and the crust is derived primarily from global and regional seismic tomography studies of Eurasia and from global and regional data on seismic anisotropy. These data as documented in the papers of this special issue of Tectonophysics are used to illustrate the role of different tectonic processes in the lithospheric evolution since Archean to present. These include, but are not limited to, cratonization, terrane accretion and collision, continental rifting (both passive and active), subduction, and lithospheric basal erosion due to a relative motion of cratonic keels and the convective mantle. ?? 2002 Elsevier Science B.V. All rights reserved.

  17. Tectonic controls on hydrothermal mineralisation in hot continental crust: Thermal modelling and spatial analysis

    Science.gov (United States)

    Gessner, K.; Porwal, A.

    2009-04-01

    Hydrothermal ore deposits provide a record of excess energy flux and mass transfer in the Earth's lithosphere. The heterogeneous distribution of ore deposits in space and time provides a challenge to uniformitarian geodynamic and tectonic concepts, but unusual thermal and structural events often coincide with high mineral endowment. In the Australian Proterozoic continental backarcs and intracratonic rifts host large resources of base metals, gold, and uranium. We present thermal models and spatial analyses of mineral occurrences within the Mount Isa Inlier, an inverted Mesoproterozoic rift in northwest Queensland, Australia, to demonstrate how thermal structure, tectonic style and crustal scale fluid flow are related. In the Mount Isa Inlier, radiogenic heat production contributes significantly to present day surface heat flow, and Mesoproterozoic geotherms of 40°C km-1 in the upper crust can be inferred from lithosphere-scale conductive models. The combination of thick continental crust and high temperatures implies that localization of deformation was limited to a thin upper crustal layer. During rifting mid-crustal rocks intruded by syn-extensional granites were exhumed as metamorphic core complexes in strike-parallel linear basement belts. The resulting horizontal strength contrast between sedimentary basins and shallow basement domains became a focus for deformation during subsequent crustal shortening. Our spatial analysis of mineral occurrences demonstrates that epigenetic copper mineralization at Mount Isa correlates positively with steep fault zones bounding linear basement domains, and granites within these domains. Mineralization potential is greatly increased, because high permeability along steep fault zones enables hydrothermal fluid flow between magmatic, metamorphic and sedimentary reservoirs. We argue that the deformation behavior of hot continental lithosphere generates a favorable environment for hydrothermal mineralization by linking shallow

  18. Ultra-deep Subduction of Continental Crust: Evidence from Natural Rocks and Experimental Investigations

    Science.gov (United States)

    Dobrzhinetskaya, L.; Green, H. W.

    2005-12-01

    Much of what we know about the deep subduction of continental crust is gained from study of the mineralogy of ultrahigh-pressure metamorphic (UHPM) rocks incorporated within continent-continent collision belts. UHPM rocks of metasedimentary origin were only recently recognized due to discovery of coesite, diamond, titanite inferred to have contained six-fold coordinated Si before exsolution of coesite, and TiO2 with alpha-PbO2 structure. Modern seismic tomography provides remarkable images that suggest lithospheric plates are subducted to the core-mantle boundary and may remain there stagnated during long geological times; the presence of continental material within such plates cannot be excluded. The mineral phase transformations possible within deeply subducted continental crust have been the subject of intensive laboratory experimentation during the last decade. Though many new UHP minerals were synthesized at P~ 6 to >20 GPa (e.g., wadeite, topaz-OH, phase Egg, K- and Na-hollandite) in the KNASH, KASH and ASH chemical systems, none except topaz-OH has yet been identified in UHPM rocks. The microstructural evidence of former majoritic garnet decompression is proven only for garnet peridotite, whereas no evidence of such structures has been reported yet from diamondiferous felsic rocks. It is not clear if this is because the UHPM minerals of felsic rocks are easily lost during retrograde metamorphism, or if this is because garnets crystallized in the felsic systems do not contain a large majoritic component at high pressures. There is no clear indication of what portion of subducted continental crust is returned back to Earth's surface, and what fraction may have become more dense than mantle rocks and sunk down to the mantle transition zone and even deeper. Is there any connection between mantle plumes and deeply subducted continental rocks? The UHPM discipline would also benefit from new experiments designed to reproduce decompression structures of UHP minerals

  19. Evolution of the depleted mantle and growth of the continental crust: improving on the imperfect detrital zircon record

    Science.gov (United States)

    Vervoort, J. D.; Kemp, A. I. S.; Patchett, P. J.

    2012-04-01

    One of the basic tenets of terrestrial geochemistry is that the continental crust has been extracted from the mantle leaving the latter depleted in incompatible elements. Nd and Hf isotopes have long shown that this process has been an essential feature of the Earth throughout its history. There is wide agreement on the general nature of this process, but the details of the isotopic record—and their implications for the depletion of the mantle and the extraction of continental crust—remain debated. Recently, much attention has been given to detrital zircons in both modern and ancient sediments. An advantage of this approach is the integration of the crystallization history of the zircon from the U-Pb chronometer with its Hf isotopic composition, which can provide important information on whether the zircons have been derived from juvenile or reworked crust. One essential requirement in this approach, however, is to unambiguously determine the crystallization ages of the zircons. We suggest that this represents an important—but generally ignored—source of uncertainty in the Hf isotopic record from detrital zircons. The quality filter most often used to assess the integrity of zircon U-Pb systematics is concordance; if a zircon is concordant, it is assumed that the U-Pb age is accurate. A concordance filter is less effective in old zircons, however, because ancient Pb loss, viewed today, parallels concordia. Without the benefit from the geological context of the host rock to the zircons, it is impossible to unambiguously determine it true crystallization age. Ancient Pb loss in zircons produces an apparent age less than the true magmatic age. The initial Hf isotopic composition of these zircons, as a result, will be calculated at the wrong age and will be anomalously low (by ~2.2 epsilon Hf units per 0.1 Ga). Hf model ages, calculated from these parameters, will be artificially old and spurious. The combination of unradiogenic Hf and Hf model ages > U-Pb ages

  20. Subduction Tectonic Erosion, Sediment Accretion and Arc Collisions in maintaining the Continental Crust

    Science.gov (United States)

    Clift, P.; Vannucchi, P.; Schouten, H.

    2007-12-01

    Estimates of modern continental crustal recycling in subduction zones can be made from plate convergence velocities, the thicknesses of trench sediments, volumes and ages of accretionary complexes together with rates of trench retreat. Plate convergence rates appear to be the primary control on crustal subduction, with convergence >7.5 cm/yr associated with tectonic erosion. Collision of aseismic ridges with trenches drives around two thirds of forearc tectonic erosion over periods >10 m.y.. Globally material subduction at least as deep as the magmatic roots of arc systems is around 3.0 Armstrong Units (1 AU = 1 km3/yr), of which 1.65 AU comprises subducted sediments, with 1.33 AU of eroded forearc crust. Recycling rates along a single margin may show strong temporal variation over 1 m.y. periods. Isotopic variations in Costa Rican tephra suggest that sediment accretion is the most common mode of tectonism, but this is separated by short periods of dramatic erosion that cause net crustal loss. Even where erosion is continuous this can operate in a fast steady-state mode or a slower temporary style. On the Central Andean margin tectonic erosion since 20 Ma has caused trench retreat, but slow subsidence under the coastal zone implies steepening of the forearc taper rather than large scale retreat. The Neogene mass loss rate of 13 km3/m.y./km is 5-10 times lower than the long-term average. Since 2 Ma this rate has slowed further due to underplating under the coastal zone. A climatic role in driving continental erosion and moving the margin into a more accretionary state has been suggested but is hard to demonstrate. Average global mass loss requires that Cenozoic arc productivity lies close to 75 km3/m.y./km if the volume of the continental crust is to be maintained. Efficient accretion of oceanic arc crust is essential in maintaining the total crustal volume. In the classic Taiwan-Luzon example local crustal mass balancing implies that ~90% of the igneous arc crust

  1. Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

    Science.gov (United States)

    Jonathan, Patchett P.; Kouvo, O.; Hedge, C.E.; Tatsumoto, M.

    1982-01-01

    We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from ??Hf=0 to +14, or about 60% of the variability of the present mantle. The approximate ??Hf=2??Nd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf=??? 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships. The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive ??Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle. ?? 1981 Springer-Verlag.

  2. Is there continental crust underneath the northern Natal Valley and the Mozambique Coastal Plains?

    Science.gov (United States)

    Leinweber, Volker Thor; Jokat, W.

    2011-07-01

    To draw conclusions about the crustal nature and history of the Natal Valley and the Mozambique Ridge, systematic potential field data were obtained during the AISTEK III cruise with R/V Pelagia in 2009. This paper presents and interprets the results of that expedition. The new magnetic data reveal a pattern of linear magnetic spreading anomalies, NW-SE trending in the southwestern part of the Mozambique Ridge and E-W trending on its central part. The Ariel Graben, which separates the Mozambique Ridge from the Northern Natal Valley, is represented by a pronounced negative anomaly in the magnetic- as well as the free-air gravity field. The Northern Natal Valley bears a complicated pattern of anomalies with mainly SW-NE trends. In the Northern Natal Valley, no indications for a continent-ocean-boundary between continental crust in the north and oceanic crust in the south exist, either in the free-air gravity or in the magnetic field. The magnetic wavelengths of the Mozambique Coastal Plains are similar to those of the Northern Natal Valley and the Mozambique Ridge. Particularly in the gravity data, the Mozambique coastal plains, the Northern Natal Valley and the Mozambique Ridge appear as one continuous geological province. We interpret the region from the Mateke-Sabi monocline in the north to the southwestern tip of the Mozambique Ridge as mainly being floored by thickened oceanic crust.

  3. On the relationship between sequential faulting, margin asymmetry and highly thinned continental crust

    Science.gov (United States)

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

    2014-05-01

    The architecture of magma-poor continental margins is remarkably variable. The width of highly thinned continental crust (with a thickness material; (2) Formation of a low viscosity exhumation channel adjacent to the rift centre that is generated by heat transfer from the upwelling mantle and enhanced by viscous strain softening. Rift migration takes place in a steady-state manner and is accomplished by oceanward-younging sequential faults within the upper crust and balanced through lower crustal flow. We demonstrate that the rate of extension has paramount control on margin width. Since higher velocities lead to elevated heat flow within the rift and hence to hot and weak lower crust, a larger low-viscosity exhumation channel is generated that facilitates rift migration leading to wider margins. The South Atlantic is an ideal test bed for the hypothesis of velocity-dependent margin width since rifting was fast in the south, but slow in the northern part. As predicted by our numerical models, the maximum present-day margin width increases almost linearly from the conjugate Equatorial margin segments to the Florianopolis/Walvis ridge. Even though the polarity of the magma-poor South Atlantic margins alternates, the asymmetry and the width of the wider margin are in very good agreement with our simulations. The described rift evolution has three fundamental implications: (1) It implies sustained transfer of material across the extensional plate boundary thereby predicting that large portions of a wide margin originate from its conjugate side. (2) Migration of the deformation locus causes faulting in the distal parts of the margin to postdate that of the proximal parts by as much as 10 million years. This means that syn-rift and post-rift phase are location-dependent. (3) Lateral movement of the rift centre generates drastically different peak heat flow and subsidence histories at the proximal and the distal margin.

  4. The fate of mafic and ultramafic intrusions in the continental crust

    Science.gov (United States)

    Roman, Alberto; Jaupart, Claude

    2016-11-01

    Geochemical and petrological data indicate that the bulk continental crust results from the fractionation of basaltic magmas followed by the foundering of residual mafic cumulates. Structural and geological evidence for foundering has been elusive and it is argued that it lies in the shapes of mafic intrusions that have been preserved in the crust. Numerical calculations of visco-elasto-plastic deformation induced by a dense intrusive body in continental crust have been carried out for a wide range of physical conditions. Three regimes are defined on the basis of the amount of dense material that remains at the original emplacement level as well as on the shape of the residual body. With strong encasing rocks, the intrusion deforms weakly in a sagging regime characterized by downwarping of the floor. At the other extreme, the intrusion sinks through weak surroundings, leaving behind a very small volume of material. In an intermediate regime, the intrusion does not sink wholesale and undergoes a dramatic change of shape. A residual body is preserved with a shape that depends on the aspect ratio of the initial intrusion. For aspect ratios of order one, the residual body is funnel-shaped above a thin and deep vertical extension. For the small aspect ratios that typify large igneous complexes such as the Bushveld, South Africa, the residual body is characterized by thick peripheral lobes with inward-dipping igneous layers and a thinner central area that has lost some of the basal cumulates. The transitions between these regimes depend on the rheology and temperature of encasing rocks.

  5. Subduction of European continental crust to 70 km depth imaged in the Western Alps

    Science.gov (United States)

    Paul, Anne; Zhao, Liang; Guillot, Stéphane; Solarino, Stefano

    2015-04-01

    The first conclusive evidence in support of the burial (and exhumation) of continental crust to depths larger than 90 km was provided by the discovery of coesite-bearing metamorphic rocks in the Dora Maira massif of the Western Alps (Chopin, 1984). Since then, even though similar outcrops of exhumed HP/UHP rocks have been recognized in a number of collisional belts, direct seismic evidences for subduction of continental crust in the mantle of the upper plate remain rare. In the Western Alps, the greatest depth ever recorded for the European Moho is 55 km by wide-angle seismic reflection (ECORS-CROP DSS Group, 1989). In an effort to image the European Moho at greater depth, and unravel the very complex lithospheric structure of the W-Alps, we have installed the CIFALPS temporary seismic array across the Southwestern Alps for 14 months (2012-2013). The almost linear array runs from the Rhône valley (France) to the Po plain (Italy) across the Dora Maira massif where exhumed HP/UHP metamorphic rocks of continental origin were first discovered. We used the receiver function processing technique that enhances P-to-S converted waves at velocity boundaries beneath the array. The receiver function records were migrated to depth using 4 different 1-D velocity models to account for the strongest structural changes along the profile. They were then stacked using the classical common-conversion point technique. Beneath the Southeast basin and the external zones, the obtained seismic section displays a clear converted phase on the European Moho, dipping gently to the ENE from ~35 km at the western end of the profile, to ~40 km beneath the Frontal Penninic thrust (FPT). The Moho dip then noticeably increases beneath the internal zones, while the amplitude of the converted phase weakens. The weak European Moho signal may be traced to 70-75 km depth beneath the eastern Dora Maira massif and the westernmost Po plain. At shallower level (20-40 km), we observe a set of strong

  6. Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

    Science.gov (United States)

    Molli, Giancarlo; Menegon, Luca; Malasoma, Alessandro

    2017-04-01

    The switching in deformation mode (from distributed to localized) and mechanism (viscous versus frictional) represent a relevant issue in the frame of processes of crustal deformation in turn connected with the concept of the brittle-"ductile" transition and seismogenesis. On the other hand the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as having a fundamental role in the localization of deformation and shear zone development, thus representing a case in which switching deformation mode and mechanisms interact and relate to each other. This contribution analyses an example of a crystal plastic shear zone localized by brittle precursor formed within a host granitic-mylonite during deformation in subduction-related environment. The studied sample come from the external Corsican continental crust units involved in alpine age subduction and characterized by a low grade blueschist facies peak assemblages. The blueschist facies host rock is cut by a thin (preserves domains with a cataclastic microstructure overprinted by mylonitic deformation. Blue amphibole is stable in the shear zone foliation, which therefore formed under HP/LT metamorphic conditions in a subduction environment. Quartz microstructure in the damage zone flanking the brittle-viscous shear zone shows evidence of both microcracking and dislocation glide, with limited recrystallization localized in intracrystalline bands. In the mylonite portion of the shear zone, quartz forms polycrystalline ribbons of dynamically recrystallized grains with a crossed-girdle c-axis CPO. Extrapolation of laboratory-derived flow laws indicates strain rate of ca. 3.5 * 10-12 s-1 during viscous flow in the shear zone. The studied structures, possibly formed by transient instability related to episodic stress/strain rate variations, may be considered as a small scale example of fault behaviour associated with a cycle of interseismic creep

  7. Palaeoproterozoic high-pressure granulite overprint of the Archaean continental crust: evidence for homogeneous crustal thickening (Man Rise, Ivory Coast)

    Science.gov (United States)

    Pitra, Pavel; Kouamelan, Alain N.; Ballèvre, Michel; Peucat, Jean-Jacques

    2010-05-01

    well as coeval intrusion of juvenile magmas. Crustal shortening was mainly accommodated by transpressive shear zones and by lateral crustal spreading rather than large-scale thrust systems. The occurrence of high-pressure granulites does not imply thrust-dominated tectonics. Feybesse, J.-L. & Milési, J.-P., 1994. The Archean/Proterozoic contact zone in West Africa: a mountain belt of décollement thrusting and folding on a continental margin related to 2.1 Ga convergence of Archean cratons? Precambrian Research, 69, 199-227. Schulmann, K., Schaltegger, U., Ježek, J., et al. 2002. Rapid burial and exhumation during orogeny: thickening and synconvergent exhumation of thermally weakened and thinned crust (Variscan orogen in Western Europe). American Journal of Science, 302, 856-879. Schulmann, K., Lexa, O., Štípská, P. et al., 2008. Vertical extrusion and horizontal channel flow of orogenic lower crust: key exhumation mechanisms in large hot orogens? Journal of Metamorphic Geology, 26(2), 273-297.

  8. Early evolution of the continental crust, the oxygenated atmosphere and oceans, and the heterogeneous mantle

    Science.gov (United States)

    Ohmoto, H.

    2011-12-01

    The current paradigm for the evolution of early Earth is that, only since ~2.5 Ga ago, the Earth began to: (a) form a large granitic continental crust; (b) form an oxygenated atmosphere; (c) operate oxidative weathering of rocks on land; (d) form Fe-poor, but S-, U- and Mo-rich, oceans; (e) operate large-scale transfers of elements between oceans and oceanic crust at MORs; (f) subduct the altered oceanic crust; (g) create the mantle heterogeneity, especially in the concentrations and isotopic compositions of Fe(III), Fe(II), U, Pb, alkali elements, C, S, REEs, and many other elements; (h) create chemical and isotopic variations among OIB-, OPB-, and MORB magmas, and between I- and S-type granitoid magmas; and (i) create variations in the chemical and isotopic compositions of volcanic gas. Submarine hydrothermal fluids have typically developed from seawater-rock interactions during deep (>2 km) circulation of seawater through underlying hot volcanic rocks. When the heated hydrothermal fluids ascend toward the seafloor, they mix with local bottom seawater to precipitate a variety of minerals on and beneath the seafloor. Thus, the mineralogy and geochemistry of submarine hydrothermal deposits and associated volcanic rocks can be used to decipher the chemistry of the contemporaneous seawater, which in turn indicate the chemistry of the atmosphere and the compositions and size of the continental crust. The results of mineralogical and geochemical investigations by our and other research groups on submarine hydrothermal deposits (VMS and BIF) and hydrothermally-altered submarine volcanic rocks in Australia, South Africa, and Canada, ~3.5-2.5 Ga in ages, suggest that the above processes (a)-(i) had began by ~3.5 Ga ago. Supportive evidence includes, but not restricted to, the similarities between Archean submarine rocks and modern ones in: (1) the abundance of ferric oxides; (2) the Fe(III)/Fe(I) ratios; (3) the abundance of barite; (4) the increased Li contents; (5) the

  9. Fractional Crystallisation of Archaean Trondhjemite Magma at 12-7 Kbar: Constraints on Rheology of Archaean Continental Crust

    Science.gov (United States)

    Sarkar, Saheli; Saha, Lopamudra; Satyanarayan, Manavalan; Pati, Jayanta

    2015-04-01

    Fractional Crystallisation of Archaean Trondhjemite Magma at 12-7 Kbar: Constraints on Rheology of Archaean Continental Crust Sarkar, S.1, Saha, L.1, Satyanarayan, M2. and Pati, J.K.3 1. Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee-247667, Haridwar, India, 2. HR-ICPMS Lab, Geochemistry Group, CSIR-National Geophysical Research Institute, Hyderabad-50007, India. 3. Department of Earth and Planetary Sciences, Nehru Science Centre, University of Allahabad, Allahabad-211002, India. Tonalite-Trondhjemite-Granodiorite (TTGs) group of rocks, that mostly constitute the Archaean continental crusts, evolved through a time period of ~3.8 Ga-2.7 Ga with major episodes of juvenile magma generations at ~3.6 Ga and ~2.7 Ga. Geochemical signatures, especially HREE depletions of most TTGs conform to formation of this type of magma by partial melting of amphibolites or eclogites at 15-20 kbar pressure. While TTGs (mostly sodic in compositions) dominates the Eoarchaean (~3.8-3.6 Ga) to Mesoarchaean (~3.2-3.0 Ga) domains, granitic rocks (with significantly high potassium contents) became more dominant in the Neoarchaean period. The most commonly accepted model proposed for the formation of the potassic granite in the Neoarchaean time is by partial melting of TTGs along subduction zones. However Archaean granite intrusive into the gabbro-ultramafic complex from Scourie, NW Scotland has been interpreted to have formed by fractional crystallization of hornblende and plagioclase from co-existing trondhjemitic gneiss. In this study we have studied fractional crystallization paths from a Mesoarchaean trondhjemite from the central Bundelkhand craton, India using MELTS algorithm. Fractional crystallization modeling has been performed at pressure ranges of 20 kbar to 7 kbar. Calculations have shown crystallization of garnet-clinopyroxene bearing assemblages with progressive cooling of the magma at 20 kbar. At pressure ranges 19-16 kbar, solid phases

  10. Crustal basement controls granitoid magmatism, and implications for generation of continental crust in subduction zones: A Sr-Nd-Hf-O isotopic study from the Paleozoic Tongbai orogen, central China

    Science.gov (United States)

    Wang, Hao; Wu, Yuan-Bao; Yang, Jin-Hui; Qin, Zheng-Wei; Duan, Rui-Chun; Zhou, Lian; Yang, Sai-Hong

    2017-06-01

    Ascertaining the petrogenesis of granitoid rocks in subduction zones holds the key for understanding the processes of how continental crust is produced. The synchronous Taoyuan and Huanggang plutons occur in two different geological units of the Paleozoic Tongbai orogen of central China. They provide an optimal opportunity for a study to address the role of the crustal basement in generating voluminous granitoid magmatism in subduction zones. The Taoyuan and Huanggang plutons have identical U-Pb zircon crystallization ages of 440-444 Ma, which are temporally related to northward subduction of the Paleotethyan Ocean. The Taoyuan samples show high SiO2 (73.36-79.16%) and low Al2O3 (12.00-13.45%) contents, Mg numbers (20.6-38.2), and Sr/Y (2.04-10.1) and (La/Yb)N (2.34-7.32) ratios with negative Eu anomalies (Eu/Eu* = 0.33-0.93). They yielded positive εNd(t) (+ 3.0 to + 6.7) and εHf(t) (+ 11.8 to + 13.2) values, elevated initial Sr isotopic ratios (0.7040-0.7057) and relatively low zircon δ18O values of 4.62-5.39‰. These suggest that they were produced through partial melting of hydrothermally altered lower crust of the accreted Erlangping oceanic arc. In contrast, the Huanggang samples exhibit variable whole-rock geochemical and isotopic compositions with SiO2 contents of 57.01-64.42 wt.%, initial Sr isotopic ratios of 0.7065-0.7078, and εNd(t) values of - 5.7 to - 9.4. Additionally, they have high zircon δ18O values of 7.57-8.45‰ and strongly negative zircon εHf(t) values of - 14.4 to - 10.5. They were suggested to have been mainly derived from ancient continental crust of the Kuanping crustal unit with the addition of 20-40% juvenile, mantle-derived material. Accordingly, the granitoids in both oceanic and continental arcs are likely to be mainly derived from intracrustal melting of their crustal basement. It is revealed by the Huanggang pluton that little net continental crust growth occurs in continental arcs, and addition of new volume of continental

  11. Tectonic juxtaposition of crust and continental growth during orogenesis: Example from the Rengali Province, eastern India

    Directory of Open Access Journals (Sweden)

    Sankar Bose

    2015-07-01

    Full Text Available The southern boundary of the Singhbhum Craton witnessed multiple orogenies that juxtaposed thin slice of granulite suite of the Rengali Province against the low-grade granite-greenstone belt of the craton along the E–W trending Sukinda Thrust. The strong southerly dipping mylonitic foliation within the granulites along with the prominent down-dip mineral lineation, suggest a northerly-verging thrusting. Mylonitized charnockite at the contact zone contains enclaves of mafic and ultramafic granulite, whereas granitoid gneiss contains enclaves of pelitic granulite. Mafic granulite enclaves preserve an early (S1M foliation that formed during D1M deformation. This rock, along with the host charnockite, were intensely deformed by the D2M thrusting event and resulting S2M foliation development in both rock suites. Geothermobarometric and pseudosection analyses show that the garnet-clinopyroxene-plagioclase-orthopyroxene-ilmenite-quartz assemblage in mafic granulite was stabilized at high-pressure and temperature conditions (10−12 kbar, 860 °C and was overprinted by a fine-grained assemblage of clinopyroxene-plagioclase ± hornblende that developed during decompression (down to 5.5–7.5 kbar. Matrix hornblende shows incipient breakdown to garnet-clinopyroxene-quartz intergrowth due to a granulite facies reworking. A contrasting P-T history is preserved in the pelitic granulite. The peak assemblage garnet-orthopyroxene-cordierite-quartz-rutile was stabilized at ∼6.0 kbar, 730 °C which resulted from heating of the mid crust magma during the D2M thrusting. The contrasting P-T histories could result from the tectonic juxtaposition of lower- and mid-crustal section during the D2M event. Evidences of an early orogenic imprint within the mafic granulite imply involvement of deep continental crust during southward growth of the Singhbhum Craton.

  12. The influence of partial melting and melt migration on the rheology of the continental crust

    Science.gov (United States)

    Cavalcante, Geane Carolina G.; Viegas, Gustavo; Archanjo, Carlos José; da Silva, Marcos Egydio

    2016-11-01

    The presence of melt during deformation produces a drastic change in the rheological behavior of the continental crust; rock strength is decreased even for melt fractions as low as ∼7%. At pressure/temperature conditions typical of the middle to lower crust, melt-bearing systems may play a critical role in the process of strain localization and in the overall strength of the continental lithosphere. In this contribution we focus on the role and dynamics of melt flow in two different mid-crustal settings formed during the Brasiliano orogeny: (i) a large-scale anatectic layer in an orthogonal collision belt, represented by the Carlos Chagas anatexite in southeastern Brazil, and (ii) a strike-slip setting, in which the Espinho Branco anatexite in the Patos shear zone (northeast Brazil) serves as an analogue. Both settings, located in eastern Brazil, are part of the Neoproterozoic tectonics that resulted in widespread partial melting, shear zone development and the exhumation of middle to lower crustal layers. These layers consist of compositionally heterogeneous anatexites, with variable former melt fractions and leucosome structures. The leucosomes usually form thick interconnected networks of magma that reflect a high melt content (>30%) during deformation. From a comparison of previous work based on detailed petrostructural and AMS studies of the anatexites exposed in these areas, we discuss the rheological implications caused by the accumulation of a large volume of melt "trapped" in mid-crustal levels, and by the efficient melt extraction along steep shear zones. Our analyses suggest that rocks undergoing partial melting along shear settings exhibit layers with contrasting competence, implying successive periods of weakening and strengthening. In contrast, regions where a large amount of magma accumulates lack clear evidence of competence contrast between layers, indicating that they experienced only one major stage of dramatic strength drop. This comparative

  13. Processes and timescales of melt metasomatism in the continental lower crust of West Greenland

    Science.gov (United States)

    Smit, Matthijs; Waight, Tod; Nielsen, Troels

    2017-04-01

    The lower continental crust is infiltrated by mantle-derived melts that are at extreme chemical disequilibrium with this reservoir. The chemical effect, and the degree and extent of melt infiltration within the lower crust are important parameters to investigating the role of this process in modifying local and regional crustal composition. However, in spite of ground-breaking contributions in this field, the processes of melt metasomatism within the lower crust are not entirely clear. Challenges lie in obtaining primary petrological, geochemical and chronological information from deep crustal rocks that are difficult to sample and are often repeatedly recrystallized and chemically modified. In this study, we investigated mafic granulite xenoliths from the crustal root of the Archean North Atlantic Craton (Sarfartôq location) and the adjoining Paleoproterozoic Nagssugtôqidian Orogen (Sisimiut location), West Greenland. These rocks were chosen because: 1) they are xenolith and hence are unaffected by tectonic overprinting, 2) they show a rich petrological record of melt infiltration formed in the run up to their scavenging by the host aillikite magmas, 3) they represent two locations with similar crustal composition and architecture, but different underlying lithospheric mantle enabling teasing-out of the role of melt-mantle interaction. We used various imaging techniques and in-situ chemical analysis by electron-probe micro-analyzer to characterize the different melt pockets within the samples, their textural relationships, as well as their composition. In addition, we performed thermometry and multi-method speedometry (Fe-in-rutile, majors-in-garnet) to assess the conditions and duration of melt infiltration for melts of different texture and composition. The primary granular assemblage of the xenoliths has been infiltrated by three different types of melts, each of which is found in grain boundaries, melt-corroded cracks, and voids formed by melt

  14. Nd isotopic variation of Paleozoic-Mesozoic granitoids from the Da Hinggan Mountains and adjacent areas, NE Asia: Implications for the architecture and growth of continental crust

    Science.gov (United States)

    Yang, Qidi; Wang, Tao; Guo, Lei; Tong, Ying; Zhang, Lei; Zhang, Jianjun; Hou, Zengqian

    2017-02-01

    There is a long-standing controversy regarding the tectonic division, composition and structure of the continental crust in the Da Hinggan Mountains and adjacent areas, which are mainly part of the southeastern Central Asian Orogenic Belt (CAOB). This paper approaches these issues via neodymium isotopic mapping of Paleozoic-Mesozoic (480 to 100 Ma) granitoids. On the basis of 943 published and 8 new whole-rock Nd isotopic data, the study area can be divided into four Nd isotopic provinces (I, II, III and IV). Province I (the youngest crust, Nd model ages (TDM) = 0.8-0.2 Ga) is a remarkable region of Phanerozoic crustal growth, which may reflect a major zone for closures of the Paleo-Asian Ocean. Province II (slightly juvenile crust, TDM = 1.0-0.8 Ga), the largest Nd isotopic province in the southeastern CAOB, is considered to reflect the recycling of the initial crustal material produced during the early stage (Early Neoproterozoic) evolution of the Paleo-Asian Ocean. Province III (slightly old crust, TDM = 1.6-1.1 Ga) is characterized by ancient crustal blocks, such as the central Mongolian, Erguna, Dariganga and Hutag Uul-Xilinhot blocks, which represent micro-continents and Precambrian basements in the southeastern CAOB. Several parts of Province III are located along the northern margin of the North China Craton (NCC), which is interpreted as a destroyed cratonic margin during the Paleozoic and Mesozoic. Province IV (the oldest crust, TDM = 2.9-1.6 Ga) mainly occurs within the NCC and reflects its typical Precambrian nature. These mapping results indicate that the boundary between Provinces II and III (the northern margin of the NCC) along the Solonker-Xar Moron Fault can be regarded as the lithospheric boundary between the CAOB and NCC. Provinces I and II account for 20% and 44% of the area of the southeastern CAOB, respectively, and therefore the ratio of continental growth is 64% from the Neoproterozoic to the Mesozoic, which is typical for this part of the

  15. What governs the enrichment of Pb in the continental crust? An answer from the Mexican Volcanic Belt

    Science.gov (United States)

    Goldstein, S. L.; Lagatta, A.; Langmuir, C. H.; Straub, S. M.; Martin-Del-Pozzo, A.

    2009-12-01

    One of Al Hofmann’s many important contributions to our understanding of geochemical cycling in the Earth is the observation that Pb behaves like the light rare earth elements Ce and Nd during melting to form oceanic basalts, but is enriched in the continental crust compared to the LREE by nearly an order of magnitude (Hofmann et al. 1986). This is unusual behavior, and has been called one of the Pb paradoxes, since in most cases, the ratios of elements are effectively the same in the continental crust and oceanic basalts if they show similar mantle melting behavior. One of several mechanisms suggested to mediate this special enrichment is hydrothermal circulation at ocean ridges, which preferentially transports Pb compared to the REE from the interior of the ocean crust to the surface. We confirm the importance of hydrothermal processes at the East Pacific to mediate Pb enrichment at the Trans-Mexican Volcanic Belt (TMVB, through comparison of Pb isotope and Ce/Pb ratios of TMVB lavas with sediments from DSDP Site 487 near the Middle America trench. The lavas of the Trans-Mexican Volcanic Belt include “high Nb” alkali basalts (HNAB), whose trace element patterns lack subduction signatures. The HNAB basalts and hydrothermally affected sediments from DSDP 487, form end-members that bound calcalkaline lavas from volcanoes Colima, Toluca, Popocatépetl, and Malinche in Ce/Pb versus Pb isotope space. The HNAB represent the high Ce/Pb and high Pb-isotope end-member. The hydrothermal sediments have Pb isotopes like Pacific MORB but Ce/Pb ratios typical of the arcs and the continental crust, and an order of magnitude lower than MORB. No analyzed calcalkaline lavas are have compositions outside of the bounds formed by the HNAB and the hydrothermal sediments. The Ce/Pb and Pb isotope ratios show that the calcalkaline lava compositions are inconsistent with contributions from HNAB and EPR MORB, rather the contributions are from HNAB upper mantle and subducted

  16. Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites

    Science.gov (United States)

    Gaschnig, Richard M.; Rudnick, Roberta L.; McDonough, William F.; Kaufman, Alan J.; Valley, John W.; Hu, Zhaochu; Gao, Shan; Beck, Michelle L.

    2016-08-01

    The composition of the fine-grained matrix of glacial diamictites from the Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic, collected from four modern continents, reflects the secular evolution of the average composition of the upper continental crust (UCC). The effects of localized provenance are present in some cases, but distinctive geochemical signatures exist in diamictites of the same age from different localities, suggesting that these are global signatures. Archean UCC, dominated by greenstone basalts and to a lesser extent komatiites, was more mafic, based on major elements and transition metal trace elements. Temporal changes in oxygen isotope ratios, rare earth elements, and high field strength elements indicate that the UCC became more differentiated and that tonalite-trondhjemite-granodiorite suites became less important with time, findings consistent with previous studies. We also document the concentrations of siderophile and chalcophile elements (Ga, Ge, Cd, In, Sn, Sb, W, Tl, Bi) and lithophile Be in the UCC through time, and use the data for the younger diamictites to construct a new estimate of average UCC along with associated uncertainties.

  17. The composition of the modern juvenile arc crust and the nature of crustal delaminates in arcs (Invited)

    Science.gov (United States)

    Jagoutz, O. E.; Schmidt, M. W.

    2010-12-01

    The intraoceanic Kohistan arc, northern Pakistan, exposes a complete crustal section encompassing infracrustal cumulates formed at ≥ 55 km depth, a broadly basaltic/gabbroic lower crust, a 26 km thick calc-alkaline batholith, and 4 km of a volcanoclastic/sedimentary sequence. The bulk composition of the Kohistan arc crust has been approximated by estimating the relative volumes of exposed rocks through detailed field observations in combination with geobarometric constrains of the units' thicknesses and satellite images for lateral extension. We separated the arc in 3 major lower, mid-, and mid- to upper crustal units containing 14 subunits which compositions were derived from averaging 562 whole rock analyses. The details of the resulting bulk composition depend slightly on the method of integration, but all models yield andesitic bulk supra MOHO compositions. The Kohistan bulk arc composition results very similar to global continental crust estimates indicating that modern arc activity is the dominant process that formed the (preserved) continental crust. Fitting the bulk Kohistan arc crust and the ultramafic cumulates exposed at base of the arc (dunites, wehrlites, websterites, cpx-bearing garnetites and hornblendites, and garnet gabbros) to primitive arc melts with calc-alkaline/tholeiitic, alkaline, and boninitic affinity from various island arcs demonstrates that delamination of wehrlites + garnet hornblendites ± garnet gabbros perfectly explains the evolution from a tholeiitic/calc-alkaline primitive high-Mg basalt to the continental crust. Mass balance demonstrates that volumes of delaminate similar to the continental crust are required. Compared to depleted mantle, the delaminate is enriched in K, Cs, Ba and Pb and depleted in Zr and Th. It has further a subchondritic Nb/Ta and the high Pb and low U concentrations lead to a very unradiogenic Pb isotopy that may compensate for the depleted mantle Our results document that infra arc processes even in a

  18. LITHOPROBE: a scientific update-new images of the continental crust

    Science.gov (United States)

    Clowes, Ron M.

    1990-02-01

    Seismic line run across the continental margin northeast of Newfoundland help determine the nature of extensional tectonics and associated sedimentary basin evolution on the rifted margin. The GLIMPCE Transect has enabled a glimpse of the mid-continent crust by recording marine reflection data in the Great Lakes. The sections from Lake Superior reveal — 10 km of sediments overlying a 20 km thick sequence of strongly reflective volcanics and inter-flow sediments, perhaps representing the greatest vertical extent of intra-continental rift deposits on Earth.The original Archean/Early Proterozoic crust is greatly thinned beneath the rift basin. In the Lake Huron/Georgian Bay profile, the Grenville front is imaged as the westernmost reflection of a spectacular 70 km wide band of southeast-dipping reflections that clearly truncates prominent sub-horizontal crustal reflections of the Superior granite-rhyolite terrane. The Kapuskasing Structure Zone is a sequence of lower crustal rocks that have been thrust upward along the Ivanhoe Lake cataclastic zone and are now exposed at the surface. As part of the Kapuskasing Transect, 340 km of regional deep reflection data and 20 km of " high resolution" data (20 m station spacing, 20 m source interval and a higher sweep range) over the cataclastic zone were recorded in 1987-1988. The Abitibi sub-province of Ontario and Quebec contains mineralized belts for which it is famous. As a preliminary program for the Abitibi-Grenville Transect, 130 km of regional and 65 km of high-resolution Seismic profiles also were recorded in 1987-1988. These were centred over major fault zones with which mineralization is related. Data quality is excellent and the data are in the process of being interpreted.

  19. Adakite-type sodium-rich rocks in Awulale Mountain of west Tianshan: Significance for the vertical growth of continental crust

    Institute of Scientific and Technical Information of China (English)

    2001-01-01

    The sodium-rich dacites and albite porphyries of Permian in the Awulale Mountain of west Tianshan have unique chemical and trace element signatures identical to adakite. These intermediate-acidic igneous rocks are characterized by high Na2O, Al2O3 and Sr contents and high Sr/Y and La/Y ratios (>40 and >20, respectively), and low Y and Yb contents, and strong depletion in HREE, and posi- tive Eu anomaly. The (143Nd/144Nd)i is in the range from 0.51236 to 0.51248 and the e Nd(t) is positive value (+0.79- +3.11); the (87Sr/86Sr)i is in the range from 0.7052 to 0.7054. These Nd and Sr isotopic composition features indicate that the source rocks of these adakite-type rocks are from a weakly depleted mantle, or a depleted mantle, but was con-taminated by the crustal materials. These adakite-type rocks were most likely derived from the partial melting of new underplated basaltic rocks under the conditions of amphibo-lite to eclogite transition in the postcollisional environment of North Xinjiang during the Permian Period. They are not only the Phanerozoic juvenile crust materials, but also are probably animportant symbol of the underplating of mantle- derived basaltic magmas and the vertical growth of continen-tal crust in the west Tianshan area during the postcollision of Late Paleozoic.

  20. Construction of mass-age curves for the continental crust: An empirical model and an example from the western US

    Science.gov (United States)

    Depaolo, D. J.; Schubert, G. S.; Linn, Ann

    1988-01-01

    The methodology of determining crustal mass-age curves is discussed. Problems in doing this include determination of model ages and accounting for mixing of materials of different ages during crust forming processes. These difficulties can be overcome with some reasonable assumptions and estimates of rock volumes based on areal proportions. This technique was used to construct a reasonably well constrained mass-age curve for the southwestern U.S. based on isotopic measurements on over 100 samples. The results imply that the crust in this area grew episodically at 2.8, 1.8, and 0.1 Ga. It is estimated that it would take on the order of 10000 to 100000 individual Sm-Nd isotopic measurements to carry out a similar exercise for continental crust world wide.

  1. Intrusion of granitic magma into the continental crust facilitated by magma pulsing and dike-diapir interactions: Numerical simulations

    Science.gov (United States)

    Cao, Wenrong; Kaus, Boris J. P.; Paterson, Scott

    2016-06-01

    We conducted a 2-D thermomechanical modeling study of intrusion of granitic magma into the continental crust to explore the roles of multiple pulsing and dike-diapir interactions in the presence of visco-elasto-plastic rheology. Multiple pulsing is simulated by replenishing source regions with new pulses of magma at a certain temporal frequency. Parameterized "pseudo-dike zones" above magma pulses are included. Simulation results show that both diking and pulsing are crucial factors facilitating the magma ascent and emplacement. Multiple pulses keep the magmatic system from freezing and facilitate the initiation of pseudo-dike zones, which in turn heat the host rock roof, lower its viscosity, and create pathways for later ascending pulses of magma. Without diking, magma cannot penetrate the highly viscous upper crust. Without multiple pulsing, a single magma body solidifies quickly and it cannot ascent over a long distance. Our results shed light on the incremental growth of magma chambers, recycling of continental crust, and evolution of a continental arc such as the Sierra Nevada arc in California.

  2. Prolonged post-rift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea)

    Science.gov (United States)

    Zhao, Fang; Alves, Tiago M.; Wu, Shiguo; Li, Wei; Huuse, Mads; Mi, Lijun; Sun, Qiliang; Ma, Benjun

    2016-07-01

    Three-dimensional (3D) seismic, borehole and geochemical data reveal a prolonged phase of post-rift magmatism on highly extended crust of the Baiyun Sag, South China Sea. Two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the South China Sea. Biostratigraphic data from exploration wells BY7-1 and BY2, complemented by K-Ar datings from core samples, confirm that magmatic activity in the Baiyun Sag occurred in two main stages: (1) a first episode at the base of the Miocene (23.8 Ma); and (2) a second episode occurring at the end of the Early Miocene (17.6 Ma). The relative location of volcanic complexes in the Baiyun Sag, and their stratigraphic position, reveals prolonged magmatism inboard of the ocean-continent transition zone during continental breakup. We suggest that magmatism in the Baiyun Sag reflects progressive continental breakup in the South China Sea, with the last volcanic episode marking the end of a breakup sequence representing the early post-rift tectonic events associated with the continental breakup process. Seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup.

  3. The Origin of Voluminous Dacite (vs. Andesite) at Mature, Thick Continental Arcs: A Reflection of Processes in the Deep Crust

    Science.gov (United States)

    Lange, R. A.

    2013-12-01

    An outstanding question is why some continental arc segments are characterized by voluminous eruptions of dacite (65-70 wt% SiO2), whereas others erupt more andesite (58-64 wt% SiO2) than any other magma type. An example of the former is the Altiplano-Puna region of the central Andean arc, which has erupted a predominance of dacite over all magma types 10-1 Ma (de Silva, 1989). In contrast, a 200-km arc segment of the Mexican volcanic arc (Michoacán-Guanajuato arc segment) has erupted ~75% andesite, ~26% basaltic andesite and 20%) of hornblende-rich (~40%) gabbronorite in the deep crust, driven by mantle-derived basalt intrusions at depths of 30-40 km. The absence of any dacite or rhyolite along this arc segment indicates that interstitial liquid from crystal-rich andesites never segregated to form eruptible magma. Thus, little upper-crust differentiation occurred along this arc segment. On the basis of phase-equilibrium experiments in the literature (e.g., Sisson et al., 2005), it is proposed that rhyolite and dacite did form during partial melting of the lower arc crust, but at melt fractions too low (≤15%) to permit efficient transport to the upper crust (Vigneresse and Tikoff, 1999). It is further proposed that the reason why dacite is so abundant at mature thick continental arcs (e.g., Altiplano-Puno complex) may be because mantle-derived basalts are primarily emplaced at similar depths (~30-40 km) in continental arc crustal columns. If so, in the central Andean arc, a depth of 30-40 km is within the middle dioritic crust (Graeber and Asch, 1999). Partial melts of hornblende diorite (vs. hornblende gabbro) are predicted to be dacitic (vs. andesitic) at melt fractions of 20-25%, which permits transport to the upper crust. It is therefore proposed that it is deep crustal processes that determine whether andesite or dacite is the most voluminous magma type emplaced into the upper crust and erupted at arcs.

  4. The Pakistan Himalaya: Tectonics at the NW corner of exposed Indian continental crust (Invited)

    Science.gov (United States)

    Dipietro, J. A.

    2009-12-01

    Lithotectonic units in the Himalaya maintain structural continuity from Nepal to Zanskar where they are deformed across a broad, arc-parallel, anticlinorium that plunges northwest such that the Tethyan sequence on the north limb, although truncated by the Indus Suture zone and Ladakh batholith, appears to wrap around the nose of the fold to form the Kashmir Tethys on the south limb. From a wide syn-metamorphic thrust zone in Nepal, the MCT southwest of the Kashmir Tethys narrows to form a late- to post-metamorphic thrust that shallows with the plunge of the anticlinorium. Structural continuity is disrupted by middle Miocene to active transverse fault zones that form the western margins of the Nanga Parbat and Hazara syntaxes (the Raikot-Diamir and Jhelum-Balakot fault zones respectively). It is here that Lesser, Greater, and Tethys Himalayan units lose their identity as defined and understood in the Central Himalaya. Both transverse fault zones are east-side-up and associated with antiforms that bring Lower Proterozoic rock to the surface. The MCT is folded around the nose of the Hazara antiform where it steepens to include a complete Lower Proterozoic to Mesozoic rock sequence in its hanging wall block. The Pakistan Himalaya west of the syntaxis, including the Swat area, forms the NW corner of exposed Indian continental crust. Metamorphism and major deformation are associated with two opposing thrust systems that occurred nearly synchronously prior to and possibly during the metamorphic peak. The earliest is associated with underthrusting beneath southwestward advancing ophiolitic mélange of the Indus suture zone consistent with early fold vergence and stretching lineations in Swat, and with kinematic indicators east of the syntaxis in Zanskar. This was followed by underthusting beneath southeastward advancing Nawagai ophiolitic mélange consistent with kinematic indicators in the West Pakistan fold belt. Both mélange units are metamorphosed with Indian plate

  5. MORB mantle hosts the missing Eu (Sr, Nb, Ta and Ti) in the continental crust: New perspectives on crustal growth, crust-mantle differentiation and chemical structure of oceanic upper mantle

    Science.gov (United States)

    Niu, Yaoling; O'Hara, Michael J.

    2009-09-01

    protolith consists of ~ 66.4% amphibole, ~ 29.2% plagioclase and 4.4% ilmenite. In terms of simple modal melting models, the bulk distribution coefficient ratios D2Eu/(Sm + Gd) = 1.21, D2Sr/(Pr + Nd) = 1.04, DNb/Th = 44, DTa/U = 57, DTi/Sm = 3.39 and DNb/Ta = 1.30 readily explains the small but significant negative Eu and Sr anomalies, moderate negative Ti anomaly and huge negative Nb and Ta anomalies as well as the more sub-chondritic Nb/Ta ratio in the syncollisional andesitic melt that is characteristic of and contributes to the continental crust mass. These results support the hypothesis that continental collision zones are primary sites of net continental crust growth, whereas the standard "island arc" model has many more difficulties than certainties. That is, it is the continental collision (vs. "island arc magmatism" or "episodic super mantle avalanche events") that produces and preserves the juvenile crust, and hence maintains net continental growth. The data also allow us to establish the robust composition of depleted and most primitive (or "primary") MORB melt with 13% MgO. This, together with the estimated positive Eu and Sr anomalies in the DMM, further permits estimation that the DMM may occupy the uppermost ~ 680 km of the convective mantle following the tradition that the DMM lies in the shallowest mantle. However, the tradition may be in error. The seismic low velocity zone (LVZ) may be compositionally stratified with small melt fractions concentrated towards the interface with the growing lithosphere because of buoyancy. Such small melt fractions, enriched in volatiles and incompatible elements, continue to metasomatize the growing lithosphere before it reaches the full thickness after ~ 70 Myrs. Hence, the oceanic mantle lithosphere is a huge enriched geochemical reservoir. On the other hand, deep portions of the LVZ, which are thus relatively depleted, become the primary source feeding the ridge because of ridge-suction-driven lateral material supply to

  6. Quantifying the Relationship between Strike-slip Fault Spacing and Brittle Crust Thickness in Continental Settings based on Sandbox Experiments

    Science.gov (United States)

    Lin, J. C.; Zuza, A. V.; Yin, A.

    2015-12-01

    Parallel and evenly spaced strike-slip faults occur widely in continental settings. The fault spacing varies from 10s of km along transform fault systems (e.g., southern California and New Zealand) to 200-400 km in continental interiors (e.g., central Tibet, central Asia, and North China plains). In order to understand the role of the brittle crust rheology and thickness in controlling the fault spacing in continental settings, we performed a series of sandbox experiments using medium-grained dry sand under strike-slip simple-shear conditions. With a self-built sliding device, we determined the yield strength of the dry sand to follow a Coulomb fracture relationship: Tn = 0.5173Sn + 15.475(Pa), with R2 = 0.936, where Tn is the shear stress, Sn is the normal stress, R is the coefficient of linear correlation, and 15.475 Pa is the cohesive strength. In our experiments, we created parallel Riedel shears in a simple shear zone with sand layer thickness varying from 1 cm to 6 cm. The relationship between the fault spacing (S) and the sand layer thickness (T) fits a linear equation of S = 0.5528T + 4.765, with R2 = 0.975. Assuming that the cohesive strength of the continental crust is 1-5 MPa and neglecting the density difference between rock and sand, the scaling relationship between sand and crustal thickness can be approximated as 1:1 X 105 to 1:5 X 105, which means that our experiments simulate a range of crustal thicknesses from 1 km to 30 km. The relevance of the S-T relationship obtained from this study will be tested in areas of active strike-slip tectonics, where the thickness of the brittle crust can be determined by the thickness of the seismogenic zones.

  7. The Significance of Crust Structure and Continental Dynamics Inferred from Receiver Functions in West Yunnan

    Institute of Scientific and Technical Information of China (English)

    HE Chuansong; ZHU Lupei; WANG Qingcai

    2009-01-01

    In our study we collected the teleseismic record of 31 broadband stations and 9 PASSCAL stations in West Yunnan, as well as extracted more than a million receiver functions. Using the waveform model and stacking techniques, we calculated the earth crust thicknesses and V_p/V_s ratios below the stations and obtained 35 valid data points. At the same time, we evenly stacked the receiver functions at the same station and superimposed the two profiles' cross sections of the main tectonic units. The results show a clear difference between the crust thicknesses of different tectonic units. Because of the magma underplatting and delimanition of the lower crust in the role of deep process, the West Yunnan's crust can be divided two kinds-mafic-uitramafic and feidspathic crusts. The research also shows that the mafic-ultramafic crust corresponds to a good background of mineralization. The delamination of the lower crust is one of the leading causes for moderate to strong earthquake prone in central Yunnan. The thinner crust and high velocity ratio as well as the muitimodal structure of P_s in the Tengchong volcanic area confirms existence of a deep process of the strong magma underplating. Due to the basic crust structure and nature, it is believed that the Honghe fault is a main suture of the Gondwana and Eurasia continents.

  8. H2O-fluid-saturated melting of subducted continental crust facilitates exhumation of ultrahigh-pressure rocks in continental subduction zones

    Science.gov (United States)

    Labrousse, L.; Duretz, T.; Gerya, T.

    2015-10-01

    We present two-dimensional numerical models of plate subduction and collision inspired by the Scandinavian Caledonian orogeny to investigate the possible impact of continental crust partial melting on the exhumation of ultra-high pressure metamorphic rocks. Three possible reactions were tested: low temperature solidus representing H2O-fluid-saturated partial melting, and two end-member reaction curves for dehydration melting. Thermo-mechanical effects of partial melting were implemented as (1) a viscosity decrease as a determined rheologically critical melt percentage was reached (here 0.1), (2) a change in effective heat capacity and adiabatic heating/cooling accounting for a latent heat term in the heat equation. Among the 3 tested reactions, only H2O-fluid-saturated partial melting drastically modifies the collision dynamics from the non-melting reference model holding all other parameters constant. A substantially low general viscosity truncation (here 1017 Pa s) is needed to properly resolve the effect of partial melting on deep collision processes. Low temperature melting indeed induces the development of a low viscosity buoyant plume prior to slab detachment, where migmatites exhume from UHP conditions at rates and with pressure-temperature paths similar to the natural values acknowledged for the Norwegian Caledonides. High temperature melting has no drastic influence on early collision dynamics. While positive buoyancy remains the first order driver for the exhumation of buried continental rocks, exhumation initiates in these cases with eduction subsequent to slab detachment. Melting and formation of a migmatite plume can later occur along decompression path while continental crust undergoes thermal reequilibration at temperatures above 900 °C. Some of the partially molten material can also relaminate in the overriding plate rather than exhume within the collision zone. Even if minor in terms of amount of magma produced, H2O-fluid-saturated partial melting

  9. The interplay and effects of deformation and crystallized melt on the rheology of the lower continental crust, Fiordland, New Zealand

    Science.gov (United States)

    Miranda, Elena A.; Klepeis, Keith A.

    2016-12-01

    Microstructural, electron backscatter diffraction (EBSD), and misorientation analyses of a migmatitic granulite-facies orthogneiss from the exhumed lower crust of a Cretaceous continental arc in Fiordland, New Zealand show how deformation was accommodated during and after episodes of melt infiltration and high-grade metamorphism. Microstructures in garnet, omphacite, plagioclase, and K-feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase, with subsequent deformation becoming partitioned into plagioclase. Continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite, where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding, most likely accommodated by diffusion creep, in fine recrystallized plagioclase grains. Our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc, where initial weakening was primarily achieved by deformation of both omphacite and plagioclase. Subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite, and that changes in deformation mechanisms within plagioclase, and an evolution of its strength, primarily control the rheology of the lower crust during and after episodes of melting and magma addition.

  10. Thermo-mechanical two-phase flow models of magma ascent in the continental crust with and without extension

    Science.gov (United States)

    Schmeling, Harro; Maruqart, Gabriele; Weinberg, Roberto; Cruden, Sandy

    2017-04-01

    Melting within the lower continental crust with and without extension and subsequent ascent of silicic melts is modelled by a thermo-mechanical two-phase flow approach. The approach is based on the conservation equations of mass, momentum, and energy for melt and solid, respectively, and includes a simplified binary melting model, as well as compaction / decompaction of the solid matrix. The rheology is based on dislocation creep of quartzite or granite, and includes plasticity. 2D models are carried out for cases without and with differential melt-matrix flow. As control parameter the heat flow is varied between 75 and 90 mW m-2 at the base of a thickened continental crust. In the case of no differential flow (batch melting) the model predicts episodic melting, rise and freezing of partially molten magmatic bodies. The recurrence time inversely scales with the bottom heat flux. In the case of allowing for melt migration, no such episodicity is observed anymore. Melt accumulates within melt rich layers and bodies, which subsequently rise through the crust by a combination of diapirism and decompaction related sinking of solid material through the melt rich layer. Final emplacement depths are between 30 and 15 km, shapes of the resulting plutons are visualized by the evolved enrichment and depletion fields. They show a strong dependence on the applied bottom heat fluxes.

  11. Making Earth's earliest continental crust - an analogue from voluminous Neogene silicic volcanism in NE-Iceland

    Science.gov (United States)

    Berg, Sylvia E.; Troll, Valentin R.; Burchardt, Steffi; Riishuus, Morten S.; Deegan, Frances M.; Harris, Chris; Whitehouse, Martin J.; Gústafsson, Ludvik E.

    2014-05-01

    Borgarfjörður Eystri in NE-Iceland represents the second-most voluminous exposure of silicic eruptive rocks in Iceland and is a superb example of bimodal volcanism (Bunsen-Daly gap), which represents a long-standing controversy that touches on the problem of crustal growth in early Earth. The silicic rocks in NE-Iceland approach 25 % of the exposed rock mass in the region (Gústafsson et al., 1989), thus they significantly exceed the usual ≤ 12 % in Iceland as a whole (e.g. Walker, 1966; Jonasson, 2007). The origin, significance, and duration of the voluminous (> 300 km3) and dominantly explosive silicic activity in Borgarfjörður Eystri is not yet constrained (c.f. Gústafsson, 1992), leaving us unclear as to what causes silicic volcanism in otherwise basaltic provinces. Here we report SIMS zircon U-Pb ages and δ18O values from the region, which record the commencement of silicic igneous activity with rhyolite lavas at 13.5 to 12.8 Ma, closely followed by large caldera-forming ignimbrite eruptions from the Breiðavik and Dyrfjöll central volcanoes (12.4 Ma). Silicic activity ended abruptly with dacite lava at 12.1 Ma, defining a ≤ 1 Myr long window of silicic volcanism. Magma δ18O values estimated from zircon range from 3.1 to 5.5 (± 0.3; n = 170) and indicate up to 45 % assimilation of a low-δ18O component (e.g. typically δ18O = 0 ‰, Bindeman et al., 2012). A Neogene rift relocation (Martin et al., 2011) or the birth of an off-rift zone to the east of the mature rift associated with a thermal/chemical pulse in the Iceland plume (Óskarsson & Riishuus, 2013), likely brought mantle-derived magma into contact with fertile hydrothermally-altered basaltic crust. The resulting interaction triggered large-scale crustal melting and generated mixed-origin silicic melts. Such rapid formation of silicic magmas from sustained basaltic volcanism may serve as an analogue for generating continental crust in a subduction-free early Earth (e.g. ≥ 3 Ga, Kamber et

  12. The composition of the foundered complement to the continental crust and a re-evaluation of fluxes in arcs

    Science.gov (United States)

    Jagoutz, O.; Schmidt, M. W.

    2013-06-01

    Most primitive arc melts are basaltic in composition, yet the bulk continental crust, thought to be generated in arcs, is andesitic. In order to produce an andesitic crust from primitive arc basalts, rocks complementary to the andesitic crust have to be fractionated and subsequently removed, most likely through density sorting in the lower arc crust. The Kohistan Arc in northern Pakistan offers a unique opportunity to constrain the composition and volume of material fluxes involved in this process. In a lower crustal section >10 km cumulates (dunites, wehrlites, websterites, clinopyroxene-bearing garnetites and hornblendites, and garnet-gabbros) are exposed that are 0.1-0.3 g/cm3 denser than the underlying mantle. The cumulates combine with the andesitic bulk Kohistan Arc crust to reproduce the major and trace element composition of primitive basaltic arc melts. Our petrochemical analysis suggests that fractionation and subsequent foundering of wehrlites+ultramafic hornblende-garnet-clinopyroxene cumulates+garnet-gabbros is a viable mechanism for producing andesitic crust from a calc-alkaline/tholeiitic primitive high-Mg basalt. The mass of the foundered material is approximately twice that of the arc crust generated. For an overall andesitic arc composition, we estimate a magma flux into the arc (11-15 km3/yr) about three times the rate of arc crust production itself. Foundering fluxes of cumulates (6.4-8.1 km3/yr) are one third to half those of the globally subducted oceanic crust (~19 km3/yr). Hence, the delaminate forms a volumetrically significant, albeit refractory and depleted geochemical reservoir in the mantle. Owing to its low U/Pb and high Lu/Hf the foundered material evolves with time to a reservoir characterized by unradiogenic Pb and highly radiogenic Hf isotopes, unlike any of the common mantle endmembers defined by OIB chemistry. The unradiogenic Pb of the foundered arc cumulates could counterbalance the radiogenic Pb composition of the depleted

  13. Origin of granulite terranes and the formation of the lowermost continental crust.

    Science.gov (United States)

    Bohlen, S R; Mezger, K

    1989-04-21

    Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystallization of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites.

  14. The Rock Record Has It About Right—No Significant Continental Crust Formation Prior to 3.8 Ga

    Science.gov (United States)

    Vervoort, J. D.; Kemp, A. I.; Fisher, C.; Bauer, A.; Bowring, S. A.

    2015-12-01

    Although limited in its exposed extent and by its quality of preservation, the geologic record through the first two billion years of Earth's history provides surprisingly tight constraints on the growth and evolution of continental crust. The magmatic zircon record for this period is dominated by Neoarchean U-Pb ages, with greatly diminishing abundance of older rocks and no known zircon bearing rocks much older than 4.0 Ga. A similar pattern exists for the ages of detrital zircons but with important addition of zircons as old as 4.4 Ga, mostly from the Jack Hills metaconglomerate. It has been suggested that this represents an artefact of preservation rather than the actual production rate of older crust, with the implication that large volumes of older crust have been destroyed by various recycling processes. This undoubtedly has happened to some extent, but there is considerable uncertainty as to how much has been destroyed and the nature of the early-formed crust. Here is where the long-lived radiogenic isotopic record, particularly Lu-Hf, can provide important information on the sources of the zircons by integrating age and tracer isotopic information in not only the same sample or zircon but even in the same domain of zircon. Using the most robust data from zircon and whole rock samples, excluding those with unconstrained ages and mixed-domain analyses, the most radiogenic Hf isotope compositions are characterized by ~ chondritic Hf isotopic compositions from 4.4 to ~ 3.8 Ga and a nearly linear evolution trend from epsilon Hf of 0 at 3.8 Ga to ~ epsilon Hf of +16 at present. There remains no evidence from the Hf isotope record for widespread mantle depletion prior to 3.8 Ga. Excluding the Jack Hills zircons, there is a conspicuous lack of pre 3.9 Ga zircons in even the oldest sediments1. This indicates that crust prior to 3.8 Ga was likely small in volume and/or effectively recycled back into the mantle on short time scales and did not result in significant

  15. Subduction or obduction of continental crust in the northern Norwegian Caledonides? An example from the Nordmannvik Nappe

    Science.gov (United States)

    Faber, Carly; Stünitz, Holger; Jeřábek, Petr; Gasser, Deta; Konopásek, Jiří; Kraus, Katrin

    2017-04-01

    The debate about how and why continental crust is subducted is ongoing (Ingalls et al., 2016). This work uses the tectonmetamorphic history of a the Nordmannvik nappe in the northern Scandinavian Caledonides to discuss mid- to lower-crustal processes involved in the subduction of continental crust during the Caledonian Orogeny. The Nordmannvik Nappe, together with the underlying Kåfjord and Vaddas nappes, constitutes the Reisa Nappe Complex (RNC). The RNC overlies continental rocks of the Kalak Nappe Complex (KNC), and a clear oceanic suture between Baltican basement, the KNC and the RNC is missing. The RNC consists mainly of paragneisses of mostly unknown depositional age. Rare fossils in the Vaddas Nappe indicate that it at least partly consists of Ordovician-Silurian (>460 Ma) metasediments (Binns and Gayer, 1980). Both the Nordmannvik and Vaddas Nappes were intruded by gabbroic melt around 439 Ma at 9 kbar (c. 30 km) (Getsinger et al., 2013). Therefore, the host and intrusive rocks were already buried to positions far deeper than oceanic crust prior to nappe stacking. Nordmannvik nappe rocks show at least two distinct metamorphic fabrics; 1) an early high-grade kyanite-present migmatitic fabric and 2) a pervasive mylonitic fabric. Based on microstructural observations and pseudosection modeling these two fabrics are estimated to have formed at 770-800 °C and 9.4-11 kbar and 580-630 °C and 8-9.8 kbar, respectively. The presence of sillimanite in garnet cores (confirmed by Raman spectra) and garnet core compositions also suggest that an earlier, less well constrained, history exists with metamorphism around 815 °C and 8.7 kbar, similar to that recognized in the KNC, where it is dated to be pre-Caledonian. The lack of ocean floor rocks between the Nordmannvik Nappe and the Baltica basement suggests that the Nordmannvik Nappe and nappe units below were fairly proximal to Baltica prior to the Caledonian Orogeny. Their position below the Lyngen Nappe (Iapetus

  16. Archean Arctic continental crust fingerprints revealing by zircons from Alpha Ridge bottom rocks

    Science.gov (United States)

    Sergeev, Sergey; Petrov, Oleg; Morozov, Andrey; Shevchenko, Sergey; Presnyakov, Sergey; Antonov, Anton; Belyatsky, Boris

    2015-04-01

    nowadays iceberg formation. Moreover, no Grenvillian-age zircons were revealed in studied sample. The nearest areas of Paleo- and Mesoarchean rocks appearance are deeply inland territories like Acasta Gneiss Complex, Uivak Gneisses, the Superior Province, Anabar and Aldan Shields, Northern China craton (Anshan complex), thus, a only possibility to bring some fragments of basement rocks from these areas to oceanic coast is transcontinental river transportation. The second possibility - this gneissic clast has a local provenance and has undergone a submarine weathering, shallow marine avalanche, proximal transportation by alongside ocean flows, tidal waves abrasion, and as so, reflects local bedrock geology, i.e. adjacent Alpha Ridge rock composition. Additional confirmation of this can be seen in a wide distribution of Qu-sandstones with Paleo-Mezoarchean zircons, and finds of similar allochtonous zircons in dolerites along Alpha-Mendeleev Ridge profile. The studied fragment is a unique evidence for the possible existence of Paleoarchean continental crust within the submarine Alpha-Mendeleev Ridge in Arctic Ocean.

  17. Anatexis of accretionary wedge, Pacific-type magmatism, and formation of vertically stratified continental crust in the Altai Orogenic Belt

    Science.gov (United States)

    Jiang, Y. D.; Schulmann, K.; Sun, M.; Å típská, P.; Guy, A.; Janoušek, V.; Lexa, O.; Yuan, C.

    2016-12-01

    Granitoid magmatism and its role in differentiation and stabilization of the Paleozoic accretionary wedge in the Chinese Altai are evaluated in this study. Voluminous Silurian-Devonian granitoids intruded a greywacke-dominated Ordovician sedimentary succession (the Habahe Group) of the accretionary wedge. The close temporal and spatial relationship between the regional anatexis and the formation of granitoids, as well as their geochemical similarities including rather unevolved Nd isotopic signatures and the strong enrichment of large-ion lithophile elements relative to many of the high field strength elements, may indicate that the granitoids are product of partial melting of the accretionary wedge rocks. Whole-rock geochemistry and pseudosection modeling show that regional anatexis of fertile sediments could have produced a large amount of melts compositionally similar to the granitoids. Such process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, which can be the major reason for the gravity high over the Chinese Altai. Our results show that melting and crustal differentiation can transform accretionary wedge sediments into vertically stratified and stable continental crust. This may be a key mechanism contributing to the peripheral continental growth worldwide.

  18. 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

  19. Partial melting of deeply subducted continental crust and the formation of quartzofeldspathic polyphase inclusions in the Sulu UHP eclogites

    Institute of Scientific and Technical Information of China (English)

    ZENG LingSen; LIANG FengHua; ASIMOW Paul; CHEN FangYuan; CHEN Jing

    2009-01-01

    Two types of quartzofeldspathic inclusions hosted by omphacite and garnet were identified in the Sulu UHP eclogites.The first consists of albite,quartz,and various amounts of K-feldspar.In contrast,the second consists predominantly of K-feldspar and quartz without any albite.The presence of quartzofeldspathic inclusions within the UHP mafic eclogites indicates that partial melting occurred in deeply subducted continental crust via mica dehydration melting reactions at an early stage of rapid exhumation.Such a melting event generated hydrous Na-K-Al-Si melts.These melts infiltrated into the mafic eclogite and were captured by recrystallizing garnet or omphacite,which together followed by dehydration and crystallization to form feldspar-bearing polyphase inclusions.Formation of silicate melts within the deeply subducted continental slab not only provides an excellent medium to transport both mobile (LILE) and immobile (HFSE) elements over a large distance,but also induces effective changes in the physical properties of the UHP slab.This process could be a major factor that enhances rapid exhumation of a deeply subducted continental slab.

  20. Formation of metamorphic core complexes in non-over-thickened continental crust: A case study of Liaodong Peninsula (East Asia)

    Science.gov (United States)

    Wang, Kun; Burov, Evgueni; Gumiaux, Charles; Chen, Yan; Lu, Gang; Mezri, Leila; Zhao, Liang

    2015-12-01

    Pre-thickened hot orogenic crust is often considered a necessary condition for the formation of continental metamorphic core complexes (MCCs). However, the discovery of MCCs in the Liaodong Peninsula, where the crust has a normal thickness (~ 35 km), challenges the universality of this scenario. Therefore, we implement a series of 2-D numerical thermo-mechanical modeling experiments in which we investigate the conditions of MCC formation in normal crusts, as well as the relationships between the underlying mechanisms and the syn-rift basin evolution. In these experiments, we explore the impact of the lithostratigraphic and thermo-rheological structure of the crust. We also examine the lithosphere thickness, strain softening, extension rate, and surface erosion/ sedimentation processes. The experiments demonstrate that high thermal gradients and crustal heterogeneities result only in a symmetric spreading dome, which is geometrically incompatible with the observations of the MCCs in the Liaodong Peninsula. According to our further findings, the strain softening should play a key role in the development of asymmetric strain localization and domal topography uplift, while synchronous surface erosion controls the polarity of the syn-rift basin. The synthetic model data are compatible with the geological observations and cooling history based on the thermo-chronology for the eastern part of the East Asia during the late Mesozoic to the early Cenozoic. The model-predicted P-T-t paths are essentially different from those inferred for the other known MCCs, confirming the exceptional character of the MCC formation in the wide rift system of the East Asia.

  1. Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8-3.1 Ga detrital zircons

    Science.gov (United States)

    Bolhar, Robert; Hofmann, Axel; Kemp, Anthony I. S.; Whitehouse, Martin J.; Wind, Sandra; Kamber, Balz S.

    2017-10-01

    Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9-2.8 Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon δ18O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In εHf-time space, 3.8-3.6 Ga grains define an array consistent with reworking of a mafic reservoir (176Lu/177Hf ∼0.015) that separated from chondritic mantle at ∼3.9 Ga. Crustal domains formed after 3.6 Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35 Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral

  2. Application program of CRUST-1 10km continental scientific drilling rig in SK-2 scientific drilling well

    Science.gov (United States)

    Sun, Youhong; Gao, Ke; Yu, Ping; Liu, Baochang; Guo, Wei; Ma, Yinlong; Yang, Yang

    2014-05-01

    SK-2 Well is located in DaQing city,where is site of the largest oil field in China,Heilongjiang province, north-east of China.The objective of SK-2 well is to obtain full cores of cretaceous formation in Song Liao basin,and to build the time tunnel of Cretaceous greenhouse climate change,and to clarify the causes,processes and results of the formations of DaQing oil field. This will ensure to achieve our ultimate goals,to test the CRUST-1 drilling rig and improve China's deep scientific drilling technology,to form the scientific drilling technology,method and system with independent intellectual property rights,and to provide technical knowledge and information for China's ten kilometers super-deep scientific drilling technical resources.SK-2 Well is at 6400 meter depth, where the drilling inclination is 90 degree and the continuous coring length is 3535 meter that from 2865 to 6400 meter,the recovery rate of the core is greater or equal to 95 percent with 100 millimeters core diameter and 3.9 degree per 100 meter geothermal gradient.The CRUST-1 rig is designated with special drilling equipment for continental scientific drilling combined to the oil drilling equipment ability with advanced geological drilling technology which is highly automatic and intelligent. CRUST-1 drilling ability is 10000 meter with the maximum hook load 700 tons, the total power is 4610 Kilowatt.CRUST-1 will be integrated with a complete set of automation equipment,including big torque hydraulic top drive,high accuracy automatic drilling rod feeding system, suspended automatic drill string discharge device,hydraulic intelligent iron roughneck,and hydraulic automatic catwalk to fully meet the drilling process requirements of SK-2.Designed with advanced drilling technique for 260 degree in the bottom of SK-2 well and hard rock,including the drilling tools of high temperature hydraulic hammer,high temperature resistance and high strength aluminum drill pipe,high temperature preparation of mud

  3. Mesozoic adakitic rocks from the Xuzhou-Suzhou area, eastern China: Evidence for partial melting of delaminated lower continental crust

    Science.gov (United States)

    Xu, Wen-Liang; Wang, Qing-Hai; Wang, Dong-Yan; Guo, Jing-Hui; Pei, Fu-Ping

    2006-09-01

    Adakitic rocks in the Xuzhou-Suzhou area, eastern China, consist of dioritic and monzodioritic porphyries and were dated at 131-132 Ma by the SHRIMP U-Pb zircon method. These rocks have high MgO content (1.47-5.73%), high Mg # values (0.49-0.61), and high La/Yb and Sr/Y ratios. These features are similar to rocks derived from partial melting of a subducted oceanic slab. However, their high initial 87Sr/ 86Sr (0.7053-0.7075) and low ɛNd( t) values (-4.43˜-13.14) are inconsistent with the origin from slab melting. These rocks often contain garnet residual crystals and eclogite, garnet clinopyroxenite, and garnet amphibolite xenoliths. Petrographical characteristics and estimated P- T conditions of these xenoliths indicate that they were once deeply subducted and subsequently underwent rapid exhumation in the early Mesozoic. Garnet residual crystals from the porphyries show similar chemical compositions to garnets from garnet clinopyroxenite and garnet amphibolite xenoliths. Ages of the inherited zircons of the xenoliths and their host rocks likely indicate that sources for the adakitic magma and protoliths of the eclogite and garnet clinopyroxenite xenoliths in the study area were from Precambrian basement of the North China Craton. The data also suggest that the lower continental crust in the eastern North China Craton was thickened during the early Mesozoic and delaminated in the early Cretaceous. The high-Mg adakitic magma resulted from partial melting of this delaminated lower continental crust and its subsequent interaction with the mantle during upward transport, leaving garnet as the residual phase.

  4. Mesozoic adakitic rocks from the Xuzhou Suzhou area, eastern China: Evidence for partial melting of delaminated lower continental crust

    Science.gov (United States)

    Xu, Wen-Liang; Wang, Qing-Hai; Wang, Dong-Yan; Guo, Jing-Hui; Pei, Fu-Ping

    2006-07-01

    Adakitic rocks in the Xuzhou-Suzhou area, eastern China, consist of dioritic and monzodioritic porphyries and were dated at 131-132 Ma by the SHRIMP U-Pb zircon method. These rocks have high MgO content (1.47-5.73%), high Mg # values (0.49-0.61), and high La/Yb and Sr/Y ratios. These features are similar to rocks derived from partial melting of a subducted oceanic slab. However, their high initial 87Sr/ 86Sr (0.7053-0.7075) and low ɛNd( t) values (-4.43 to -13.14) are inconsistent with the origin from slab melting. These rocks often contain garnet residual crystals and eclogite, garnet clinopyroxenite, and garnet amphibolite xenoliths. Petrographical characteristics and estimated P-T conditions of these xenoliths indicate that they were once deeply subducted and subsequently underwent rapid exhumation in the early Mesozoic. Garnet residual crystals from the porphyries show similar chemical compositions to garnets from garnet clinopyroxenite and garnet amphibolite xenoliths. Ages of the inherited zircons of the xenoliths and their host rocks likely indicate that sources for the adakitic magma and protoliths of the eclogite and garnet clinopyroxenite xenoliths in the study area were from Precambrian basement of the North China Craton. The data also suggest that the lower continental crust in the eastern North China Craton was thickened during the early Mesozoic and delaminated in the early Cretaceous. The high-Mg adakitic magma resulted from partial melting of this delaminated lower continental crust and its subsequent interaction with the mantle during upward transport, leaving garnet as the residual phase.

  5. Porosity evolution at the brittle-ductile transition in the continental crust: Implications for deep hydro-geothermal circulation.

    Science.gov (United States)

    Violay, M; Heap, M J; Acosta, M; Madonna, C

    2017-08-09

    Recently, projects have been proposed to engineer deep geothermal reservoirs in the ductile crust. To examine their feasibility, we performed high-temperature (up to 1000 °C), high-pressure (130 MPa) triaxial experiments on granite (initially-intact and shock-cooled samples) in which we measured the evolution of porosity during deformation. Mechanical data and post-mortem microstuctural characterisation (X-ray computed tomography and scanning electron microscopy) indicate that (1) the failure mode was brittle up to 900 °C (shear fracture formation) but ductile at 1000 °C (no strain localisation); (2) only deformation up to 800 °C was dilatant; (3) deformation at 900 °C was brittle but associated with net compaction due to an increase in the efficiency of crystal plastic processes; (4) ductile deformation at 1000 °C was compactant; (5) thermally-shocking the granite did not influence strength or failure mode. Our data show that, while brittle behaviour increases porosity, porosity loss is associated with both ductile behaviour and transitional behaviour as the failure mode evolves from brittle to ductile. Extrapolating our data to geological strain rates suggests that the brittle-ductile transition occurs at a temperature of 400 ± 100 °C, and is associated with the limit of fluid circulation in the deep continental crust.

  6. Melting of subducted continental crust: Geochemical evidence from Mesozoic granitoids in the Dabie-Sulu orogenic belt, east-central China

    Science.gov (United States)

    Zhao, Zi-Fu; Liu, Zhi-Bin; Chen, Qi

    2017-09-01

    Syn-collisional and postcollisional granitoids are common in collisional orogens, and they were primarily produced by partial melting of subducted continental crust. This is exemplified by Mesozoic granitoids from the Dabie-Sulu orogenic belt in east-central China. These granitoids were emplaced in small volumes in the Late Triassic (200-206 Ma) and the Late Jurassic (146-167 Ma) but massively in the Early Cretaceous (111-143 Ma). Nevertheless, all of them exhibit arc-like trace element distribution patterns and are enriched in Sr-Nd-Hf isotope compositions, indicating their origination from the ancient continental crust. They commonly contain relict zircons with Neoproterozoic and Triassic U-Pb ages, respectively, consistent with the protolith and metamorphic ages for ultrahigh-pressure (UHP) metaigneous rocks in the Dabie-Sulu orogenic belt. Some granitoids show low zircon δ18O values, and SIMS in-situ O isotope analysis reveals that the relict zircons with Neoproterozoic and Triassic U-Pb ages also commonly exhibit low δ18O values. Neoproterozoic U-Pb ages and low δ18O values are the two diagnostic features that distinguish the subducted South China Block from the obducted North China Block. Thus, the magma source of these Mesozoic granitoids has a genetic link to the subducted continental crust of the South China Block. On the other hand, these granitoids contain relict zircons with Paleoproterozoic and Archean U-Pb ages, which are present in both the South and North China Blocks. Taken together, the Mesozoic granitoids in the Dabie-Sulu orogenic belt and its hanging wall have their magma sources that are predominated by the continental crust of the South China Block with minor contributions from the continental crust of the North China Block. The Triassic continental collision between the South and North China Blocks brought the continental crust into the thickened orogen, where they underwent the three episodes of partial melting in the Late Triassic, Late

  7. The crust and upper mantle of central East Greenland - implications for continental accretion and rift evolution

    Science.gov (United States)

    Schiffer, Christian; Balling, Niels; Ebbing, Jörg; Holm Jacobsen, Bo; Bom Nielsen, Søren

    2016-04-01

    The geological evolution of the North Atlantic Realm during the past 450 Myr, which has shaped the present-day topographic, crustal and upper mantle features, was dominated by the Caledonian orogeny and the formation of the North Atlantic and associated igneous activity. The distinct high altitude-low relief landscapes that accompany the North Atlantic rifted passive margins are the focus of a discussion of whether they are remnant and modified Caledonian features or, alternatively, recently uplifted peneplains. Teleseismic receiver function analysis of 11 broadband seismometers in the Central Fjord Region in East Greenland indicates the presence of a fossil subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of hydrated mantle peridotite. This model is generally consistent with gravity and topography. It is shown that the entire structure including crustal thickness variations and sub-Moho heterogeneity gives a superior gravity and isostatic topographic fit compared to a model with a homogeneous lithospheric layer (1). The high topography of >1000 m in the western part of the area is supported by the c. 40 km thick crust. The eastern part requires buoyancy from the low velocity/low density mantle wedge. The geometry, velocities and densities are consistent with structures associated with a fossil subduction zone. The spatial relations with Caledonian structures suggest a Caledonian origin. The results indicate that topography is isostatically compensated by density variations within the lithosphere and that significant present-day dynamic topography seems not to be required. Further, this structure is suggested to be geophysically very similar to the Flannan reflector imaged north of Scotland, and that these are the remnants of the same fossil subduction zone, broken apart and separated during the formation of the North Atlantic in the early Cenozoic (2). 1) Schiffer, C., Jacobsen, B.H., Balling, N., Ebbing, J. and Nielsen, S

  8. Tectonic juxtaposition of crust and continental growth during orogenesis:Example from the Rengali Province, eastern India

    Institute of Scientific and Technical Information of China (English)

    Sankar Bose a; Susmita Guha a; Gautam Ghosh a; Kaushik Das b; Joydip Mukhopadhyay a; c

    2015-01-01

    The southern boundary of the Singhbhum Craton witnessed multiple orogenies that juxtaposed thin slice of granulite suite of the Rengali Province against the low-grade granite-greenstone belt of the craton along the EeW trending Sukinda Thrust. The strong southerly dipping mylonitic foliation within the granulites along with the prominent down-dip mineral lineation, suggest a northerly-verging thrusting. Mylonitized charnockite at the contact zone contains enclaves of mafic and ultramafic granulite, whereas granitoid gneiss contains enclaves of pelitic granulite. Mafic granulite enclaves preserve an early (S1M) foliation that formed during D1M deformation. This rock, along with the host charnockite, were intensely deformed by the D2M thrusting event and resulting S2M foliation development in both rock suites. Geothermobarometric and pseudosection analyses show that the garnet-clinopyroxene-plagioclase-orthopyroxene-ilmenite-quartz assemblage in mafic granulite was stabilized at high-pressure and temperature conditions (10?12 kbar, 860 ?C) and was overprinted by a fine-grained assemblage of cli-nopyroxene-plagioclase ? hornblende that developed during decompression (down to 5.5e7.5 kbar). Matrix hornblende shows incipient breakdown to garnet-clinopyroxene-quartz intergrowth due to a granulite facies reworking. A contrasting P-T history is preserved in the pelitic granulite. The peak assemblage garnet-orthopyroxene-cordierite-quartz-rutile was stabilized at w6.0 kbar, 730 ?C which resulted from heating of the mid crust magma during the D2M thrusting. The contrasting P-T histories could result from the tectonic juxtaposition of lower- and mid-crustal section during the D2M event. Evidences of an early orogenic imprint within the mafic granulite imply involvement of deep continental crust during southward growth of the Singhbhum Craton.

  9. Gold distribution in Archean continental crust: Evaluating the effects of intracrustal differentiation in the Tanzanian Craton

    Science.gov (United States)

    Long, K.; Rudnick, R. L.; McDonough, W. F.; Manya, S.

    2013-12-01

    We have evaluated the vertical distribution of gold in variably metamorphosed igneous rocks in the Tanzanian Craton: 2.6 Ga upper-crustal greenschist-facies greenstone belt basalts and andesites from the Lake Victoria Gold Field of northern Tanzania, and compositionally similar 2.6 Ga lower-crustal mafic granulite-facies xenoliths that were carried in rift-related basalts that erupted nearby. We implemented the preconcentration method of Pitcairn et al. (2006), which utilizes chromatographic separation of gold from acid-digested rocks using diisobutyl ketone (DIBK), followed by standard addition ICP-MS to determine the distribution of gold in the crust. Repeat analyses of the certified reference material TDB-1, a whole-rock powder diabase dike from Tremblay Lake, Saskatchewan, Canada (certified gold concentration = 6.3 × 1.0 ng/g), yielded an average gold concentration of 6.5 × 1.1 ng/g. Results were reproducible to within 17% for rock powder aliquots between 200-600 mg (n=38), where 400 mg sample aliquots were reproducible to within 6% (n=9), and 600 mg aliquots were reproducible to within 4.5% (n=4). Better reproducibility for the greater sample aliquots likely reflects the 'nugget' effect. Rock samples in the 0.1-0.8 ng/g gold concentration range reproduced to within 27% for 400-600 mg sample aliquots. Although the lavas come from an area containing gold deposits, all were more than 5 km from any gold mine. The Tanzanian greenstone belt basalts have the highest gold concentrations (9 ng/g to 62 μg/g, ave. = 40 (+68/-25) ng/g, 1σ (n=10)), followed by the greenstone belt andesites (0.4 to 120 ng/g, ave. = 1.1 (+0.9/-0.5) ng/g, 1σ (n=14)). The lowest concentrations were observed in the granulite-facies lower-crustal xenoliths (0.1 to 3.3 ng/g, ave. = 0.3 (+0.3/-0.1) ng/g, 1σ (n=21)). Gold is incompatible in silicates and can partition into hydrothermal and/or magmatic fluid or vapour during high-grade metamorphic dehydration reactions or partial melting

  10. Brittle versus ductile deformation as the main control of the deep fluid circulation in continental crust

    Science.gov (United States)

    Violay, Marie; Madonna, Claudio; Burg, Jean-Pierre

    2016-04-01

    The Japan Beyond-Brittle Project (JBBP) and the Taupo Volcanic Zone-Deep geothermal drilling project in New Zealand (TVZ-DGDP) proposed a new concept of engineered geothermal development where reservoirs are created in ductile rocks. This system has several advantages including (1) a simpler design and control of the reservoir due to homogeneous rock properties and stress states in the ductile domain ,(2) possible extraction of supercritical fluids (3) less probability for induced earthquakes. However, it is at present unknwon what and how porosity and permeability can be engineered in such environments. It has been proposed that the magmatic chamber is surrounded by a hot and ductile carapace through which heat transfer is conductive because the plastic behaviour of the rock will close possible fluid pathways. Further outward, as temperature declines, the rock will encounter the brittle-ductile transition with a concomitant increase in porosity and permeability. The thickness of the conduction-dominated, ductile boundary zone between the magmatic chamber and the convecting geothermal fluid directly determines the rate of heat transfer. To examine the brittle to ductile transition in the context of the Japanese crust, we conducted deformation experiments on very-fine-grain granite in conventional servocontrolled, gas-medium triaxial apparatus (from Paterson instrument). Temperature ranged from 600° C to 1100° C and effective confining pressure from 100 to 150 MPa. Dilatancy was measured during deformation. The method consisted in monitoring the volume of pore fluid that flows into or out of the sample at constant pore pressure. Permeability was measured under static conditions by transient pressure pulse method. Mechanical and micro-structural observations at experimental constant strain rate of 10-5 s-1 indicated that the granite was brittle and dilatant up to 900 ° C. At higher temperatures the deformation mode becomes macroscopically ductile, i

  11. Creep of mafic dykes infiltrated by melt in the lower continental crust (Seiland Igneous Province, Norway)

    Science.gov (United States)

    Degli Alessandrini, G.; Menegon, L.; Malaspina, N.; Dijkstra, A. H.; Anderson, M. W.

    2017-03-01

    A dry mafic dyke from a continental lower-crustal shear zone in the Seiland Igneous Province (northern Norway) experienced syn-kinematic melt-rock interaction. Viscous shearing occurred at T ≈ 800 °C, P ≈ 0.75-0.95 GPa and was coeval with infiltration of felsic melt from adjacent migmatitic metapelites. The dyke has a mylonitic microstructure where porphyroclasts of orthopyroxene, clinopyroxene and plagioclase are wrapped by a fine-grained (4-7 μm) polyphase mixture of clinopyroxene + orthopyroxene + plagioclase + quartz + ilmenite ± K-feldspar ± apatite. Microstructural observations and electron backscatter diffraction analysis indicate that the porphyroclasts deformed by a combination of dislocation glide and fracturing, with only a limited record of dislocation creep, recovery and dynamic recrystallization. We identified diffusion creep as the dominant deformation mechanism in the mixture based on the small grain size, phase mixing and weak crystallographic preferred orientation of all phases (interpreted as the result of oriented grain growth during viscous flow). The polyphase mixture did not form by dynamic recrystallization or by mechanical fragmentation of the porphyroclasts, but rather by melt-rock interaction. Thermodynamic models indicate that the syn-kinematic mineral assemblage results from the chemical interaction between a pristine mafic dyke and ca. 10 vol.% of felsic melt infiltrating from the adjacent partially molten metapelites. Extrapolation of laboratory-derived flow laws to natural conditions indicates that the formation of interconnected layers of fine-grained reaction products deforming by diffusion creep induces a dramatic weakening in the mafic granulites, with strain rates increasing up to 2-3 orders of magnitude. The reaction weakening effect is more efficient than the weakening associated with melt-assisted diffusion creep in the presence of up to 10 vol.% of infiltrated melt without formation of fine-grained reaction products

  12. Detachment within subducted continental crust and multi-slice successive exhumation of ultrahigh-pressure metamorphic rocks: Evidence from the Dabie-Sulu orogenic belt

    Institute of Scientific and Technical Information of China (English)

    LIU YiCan; LI ShuGuang

    2008-01-01

    Although tectonic models were presented for exhumation of ultrahigh-pressure (UHP) metamorphic rocks during the continental collision, there is increasing evidence for the decoupling between crustal slices at various depths within deeply subducted continental crust. This lends support to the multi-slice successive exhumation model of the UHP metamorphic rocks in the Dabie-Sulu orogen. The available evidence is summarized as follows: (1) the low-grade metamorphic slices, which have geotectonic af-finity to the South China Block and part of them records the Triassic metamorphism, occur in the northern margin of the Dabie-Sulu UHP metamorphic zone, suggesting decoupling of the upper crust from the underlying basement during the initial stages of continental subduction; (2) the Dabie and Sulu HP to UHP metamorphic zones comprise several HP to UHP slices, which have an increased trend of metamorphic grade from south to north but a decreased trend of peak metamorphic ages corre-spondingly; and (3) the Chinese Continental Science Drilling (CCSD) project at Donghai in the Sulu orogen reveals that the UHP metamorphic zone is composed of several stacked slices, which display distinctive high and low radiogenic Pb from upper to lower parts in the profile, suggesting that these UHP crustal slices were derived from the subducted upper and middle crusts, respectively. Detachment surfaces within the deeply subducted crust may occur either along an ancient fault as a channel of fluid flow, which resulted in weakening of mechanic strength of the rocks adjacent to the fault due to fluid-rock interaction, or along the low-viscosity zones which resulted from variations of geotherms and lithospheric compositions at different depths. The multi-slice successive exhumation model is different from the traditional exhumation model of the UHP metamorphic rocks in that the latter as-sumes the detachment of the entire subducted continental crust from the underlying mantle lithosphere and its

  13. Arc-continent collision and the formation of continental crust: A new geochemical and isotopic record from the Ordovician Tyrone Igneous Complex, Ireland

    Science.gov (United States)

    Draut, Amy E.; Clift, Peter D.; Amato, Jeffrey M.; Blusztajn, Jerzy; Schouten, Hans

    2009-01-01

    Collisions between oceanic island-arc terranes and passive continental margins are thought to have been important in the formation of continental crust throughout much of Earth's history. Magmatic evolution during this stage of the plate-tectonic cycle is evident in several areas of the Ordovician Grampian-Taconic orogen, as we demonstrate in the first detailed geochemical study of the Tyrone Igneous Complex, Ireland. New U-Pb zircon dating yields ages of 493 2 Ma from a primitive mafic intrusion, indicating intra-oceanic subduction in Tremadoc time, and 475 10 Ma from a light rare earth element (LREE)-enriched tonalite intrusion that incorporated Laurentian continental material by early Arenig time (Early Ordovician, Stage 2) during arc-continent collision. Notably, LREE enrichment in volcanism and silicic intrusions of the Tyrone Igneous Complex exceeds that of average Dalradian (Laurentian) continental material that would have been thrust under the colliding forearc and potentially recycled into arc magmatism. This implies that crystal fractionation, in addition to magmatic mixing and assimilation, was important to the formation of new crust in the Grampian-Taconic orogeny. Because similar super-enrichment of orogenic melts occurred elsewhere in the Caledonides in the British Isles and Newfoundland, the addition of new, highly enriched melt to this accreted arc terrane was apparently widespread spatially and temporally. Such super-enrichment of magmatism, especially if accompanied by loss of corresponding lower crustal residues, supports the theory that arc-continent collision plays an important role in altering bulk crustal composition toward typical values for ancient continental crust. ?? 2009 Geological Society of London.

  14. Lu-Hf systematics of the ultra-high temperature Napier Complex, East Antarctica: evidence for the early Archean formation of continental crust

    Science.gov (United States)

    Choi, S.; Mukasa, S. B.; Andronikov, A. V.; Osanai, Y.; Harley, S. L.; Kelly, N. M.

    2009-12-01

    The Napier Complex in East Antarctica comprises some of the oldest rocks on earth (~3.8 billion years old), overprinted by an ultra-high temperature (UHT) metamorphic event near the Archean-Proterozoic boundary. Garnet, orthopyroxene, sapphirine, osumilite, rutile and a whole rock representing an equilibrated assemblage from this belt yield a Lu-Hf isochron age of 2,403 ± 43 Ma. Preservation of the UHT mineral assemblage in the rock analyzed suggests rapid cooling with closure likely to have occurred for the Lu-Hf system at post-peak UHT conditions near a temperature of ~800C. Individual zircon grains from Gage Ridge within the Napier Complex yielded a remarkably uniform range of 176Hf/177Hf values between 0.280433 ± 7 and 0.280505 ± 10, corresponding to ɛHf > +5.6 at 3.85 Ga relative to the chondritic uniform reservoir (CHUR). Because of their exceedingly low Lu/Hf values (<0.001), the grains are effectively recording the initial Hf isotope composition of the magmatic systems from which the gneiss protoliths crystallized. These results indicate that (1) the source of the crustal materials that formed the Napier Complex at 3.85 Ga were depleted relative to the CHUR. The extent of depletion involved is higher than has been predicted by extrapolation from the Lu-Hf isotopic evolution inferred for the source of Proterozoic and Phanerozoic basalts, judging from an fLu/Hf value of 0.51, (2) the depleted mantle reservoir has been in existence since very early in Earth’s history, in agreement with the early differentiation of the Earth that the latest core formation models require, and (3) an extremely depleted source also mean that the bulk of continental crust was extracted from the mantle by ~3.8 Ga. Moreover, the results demonstrate that even the oldest silicic rocks in the complex are not likely to have formed from remobilized older crustal materials, but were instead juvenile products of mantle melting. In addition, zircons with metamorphic rims have a similar

  15. Generation of continental crust in the northern part of the Borborema Province, northeastern Brazil, from Archaean to Neoproterozoic

    Science.gov (United States)

    de Souza, Zorano Sérgio; Kalsbeek, Feiko; Deng, Xiao-Dong; Frei, Robert; Kokfelt, Thomas Find; Dantas, Elton Luiz; Li, Jian-Wei; Pimentel, Márcio Martins; Galindo, Antonio Carlos

    2016-07-01

    This work deals with the origin and evolution of the magmatic rocks in the area north of the Patos Lineament in the Borborema Province (BP). This northeastern segment of NE Brazil is composed of at least six different tectonic blocks with ages varying from late-Archaean to late-Palaeoproterozoic. Archaean rocks cover ca. 5% of the region. They were emplaced over a period of 700 Ma, with at least seven events of magma generation, at 3.41, 3.36, 3.25, 3.18, 3.12, 3.03, and 2.69 Ga. The rocks are subalkaline to slightly alkaline, with affinity to I- and M-type magmas; they follow trondhjemitic or potassium calc-alkaline differentiation trends. They have epsilon Nd(t) of +1.4 to -4.2 and negative anomalies for Ta-Nb, P and Ti, consistent with a convergent tectonic setting. Both subducted oceanic crust and upper mantle (depleted or metasomatised) served as sources of the magmas. After a time lapse of about 350 m y., large-scale emplacement of Paleoproterozoic units took place. These rocks cover about 50% of the region. Their geochemistry indicates juvenile magmatism with a minor contribution from crustal sources. These rocks also exhibit potassic calc-alkaline differentiation trends, again akin to I- and M-type magmas, and show negative anomalies for Ta-Nb, Ti and P. Depleted and metasomatised mantle, resulting from interaction with adakitic or trondhjemitic melts in a subduction zone setting, is interpreted to be the main source of the magmas, predominanting over crustal recycling. U-Pb ages indicate generation of plutonic rocks at 2.24-2.22 Ga (in some places at about 2.4-2.3 Ga) and 2.13-2.11 Ga, and andesitic volcanism at 2.15 Ga. Isotopic evidence indicates juvenile magmatism (epsilon Nd(t) of +2.9 to -2.9). After a time lapse of about 200 m y. a period of within-plate magmatic activity followed, with acidic volcanism (1.79 Ga) in Orós, granitic plutonism (1.74 Ga) in the Seridó region, anorthosites (1.70 Ga) and A-type granites (1.6 Ga) in the Transverse Zone

  16. Brittle/Ductile deformation at depth during continental crust eclogitization (Mont-Emilius klippe, Western Internal Alps).

    Science.gov (United States)

    Hertgen, Solenn; Yamato, Philippe; Morales, Luiz; Angiboust, Samuel

    2016-04-01

    Eclogitic rocks are important for understanding tectonics at large scale as they provide key constraints on both the evolution (P-T-t-ɛ paths) and the deformation modes of the crust along the subduction interface. We herein focus our study on eclogitized mafic dykes remnants exposed within granulites from the continental basement silver of the Mt. Emilius klippe (Western Internal Alps, Italy). These eclogites exhibit highly deformed garnetite and clinopyroxenite levels. In some places, these rocks with a ± mylonitic aspect can be found as clasts within meter-thick brecciated fault rocks formed close to metamorphic peak conditions in eclogite facies. Especially, the garnet-rich levels tend to behave in a brittle fashion while deformation within clinopyroxene-rich levels is mostly accommodated by creep. This is evidenced by the presence of elongated grains, subgrain boundaries and intense grain size reduction close to rigid garnets. Crystallographic preferred orientation (CPO) measurements in garnets indicate a quasi-random distribution. In most of the clinopyroxenes levels nevertheless, the CPO is relatively strong, with multiples of uniform distribution varying from 4 to 5.5 (value of 1 is random texture). This CPO is characterized by a strong alignment of poles (001) parallel to the lineation and (100) and [010] distributed along girdles cross-cutting the foliation plane. Our study thus attests that the materials along the subduction interface at P~2.0-2.5 GPa and T~500-550°C can locally be brittle where deformation is classically envisioned as ductile. In addition to this deformation analysis, we present a petrological study of these eclogites, from the outcrop to the microscopic scale, tracking the chemical evolution associated to the observed deformation. Based on all these data, we finally propose a tectono-metamorphic history for these rocks allowing to explain the co-existence of ductile and brittle features developed in the same metamorphic facies, and

  17. Petromagnetic Properties of Granulite-Facies Rocks from the Northern North China Craton:Implications for Magnetic and Evolution of the Continental Lower Crust

    Institute of Scientific and Technical Information of China (English)

    Qingsheng Liu; Hongcai Wang; Jianping Zheng; Qingli Zeng; Qingsong Liu

    2013-01-01

    This paper studies magnetic properties and composition of granulite-facies rocks of both the Neogene and Archean continental lower crust in the Neogene xenolith-bearing Hannuoba(汉诺坝) alkaline basalt and the exposed lower crustal section in the Archean Huai'an(淮安) terrain (Wayaokou (瓦窑口)-Manjinggou(蔓菁沟) profile),the northern North China Craton.It provides a unique opportunity for a comparative study of magnetic properties and composition of both the Archean and Neogene continental lower crust.We measure magnetic parameters (susceptibility kand magnetic hysteresis parameters,such as saturation magnetization Js,saturation isothermal remanent magnetization Jrs,and intrinsic coercivity Hc) of eleven Hannuoba lower crustal xenoliths and nine terrain granulites from the Archean Huai'an terrain.Results indicate that the average values of k,Js and Jrs of Archean granulites are 4 122×10-6 Sl,523.1 A/m and 74.9 A/m,respectively,which are generally higher than those of granulite-facies xenoliths (1 657×10-6 SI,163.9 A/m and 41.9 A/m,respectively).These two types of granulites contain ilmenite,(titano) magnetite,minor hematite and some "magnetic silicates" (clinopyroxene,plagioclase and biotite).The Mg-rich ilmenite in granulite-facies xenolith is relatively higher than that in terrain granulites.We observe a more evolved character as higher magnetic as well as lower Sr/Nd,Cr/Nd,Ni/Nd,Co/Nd and V/Nd ratios in terrain granulites.These differences in magnetic characteristics reflect their different origins and evolutions.The high magnetization of granulites in the Huai'an terrain represents magnetic properties of the Archean continental lower crust,and low magnetization of granulite-facies xenoliths represents magnetic properties of the Cenozoic lower crusts in the northern North China Craton.

  18. Report of the 8th International Symposium on the Observation of the Continental Crust Through Drilling; Dai 8 kai tairiku kagaku kussaku kokusai symposium ni sankashite

    Energy Technology Data Exchange (ETDEWEB)

    Yoshida, K. [Super Deep Core Drilling Study Group, Japan, Tokyo (Japan)

    1996-11-29

    This report relates to the 8th International Symposium on the Observation of the Continental Crust Through Drilling, convened at Agency of Industrial Science and Technology, Tsukuba City, on February 26, 1996. The symposium was represented by approximately 200 people from the U.S., Canada, Britain, Germany, France, Russia, China, and some others, who discussed active faults, drilling and logging, transfer of fluids and heat in the crust, history of the earth and climate, ICDP (International Continental Scientific Drilling Program) and international cooperation under this program in the future, etc. In reference to ultradeep drilling in the world, drillings by Germany`s KTB (Kontinentales Tiefbohrprogramm)(9,101m deep) and Russia at Kola Peninsula (l2,261m) were reviewed. Concerning the efforts of U.S. Continental Scientific Drilling Program during the previous 11-year period, it was reported that it had cost a total of $84,000,000; that investigations had been made into volcanos and geotherm, fault tectonics, sedimentary basins, holes due to meteorites, and metal ore deposits; and that 61 holes (total length: 31,310m and maximum depth: 3,510m) had been drilled and investigated. 6 figs., 3 tabs.

  19. Post-collisional granitoids from the Dabie orogen: New evidence for partial melting of a thickened continental crust

    Science.gov (United States)

    He, Yongsheng; Li, Shuguang; Hoefs, Jochen; Huang, Fang; Liu, Sheng-Ao; Hou, Zhenhui

    2011-07-01

    The geological implications of granitoid magmas with high Sr/Y and La/Yb are debated because these signatures can be produced by multiple processes. This study presents comprehensive major and trace element compositions and zircon SHRIMP U-Pb age data of 81 early Cretaceous granitoids and 4 mafic enclaves from the Dabie orogen to investigate partial melting of the thickened lower continental crust (LCC). On the basis of Sr/Y ratios, granitoids can be grouped into two magma series: (i) high Sr/Y granitoids (HSG) and (ii) normal granitoids with low Sr/Y. Relative to normal granitoids, HSG display the following distinct chemical features: (1) at given SiO 2 and CaO contents, the HSG have significantly higher Sr than normal granitoids, defining two different trends in Sr versus SiO 2, CaO diagrams; (2) highly depleted heavy rare earth element (REE) relative to middle and light REE with (Dy/Yb) N and (La/Yb) N up to 3.2 and 151, respectively; (3) variable and higher Nb/Ta; and (4) positive correlations among Sr/Y, (Dy/Yb) N, (La/Yb) N, and Nb/Ta. High Sr/Y, (La/Yb) N, (Dy/Yb) N, and Sr/CaO of HSG do not correlate with major elements (e.g., SiO 2). Large variations in these ratios at a given SiO 2 content indicate that these features do not reflect magma mixing or fractionation. HSG have higher Sr at a given CaO content and larger variation of (Dy/Yb) N than old crustal rocks (including exposed basement, global mafic LCC xenoliths, high Sr/Y TTG suites, and adakites in modern arcs). This precludes inheritance of the HSG chemical features from these source rocks. Instead, the chemical features of the HSG are best explained by partial melting of the thickened LCC with garnet-dominant, plagioclase-poor, and rutile-present residual lithologies. The coupled chemical features of the HSG are not observed in post-collisional granitoids younger than ca.130 Ma, indicating removal of the eclogitic source and/or residuum of HSG underneath the orogen. These characteristic chemical

  20. Geothermal modelling of faulted metamorphic crystalline crust: a new model of the Continental Deep Drilling Site KTB (Germany)

    Science.gov (United States)

    Szalaiová, Eva; Rabbel, Wolfgang; Marquart, Gabriele; Vogt, Christian

    2015-11-01

    The area of the 9.1-km-deep Continental Deep Drillhole (KTB) in Germany is used as a case study for a geothermal reservoir situated in folded and faulted metamorphic crystalline crust. The presented approach is based on the analysis of 3-D seismic reflection data combined with borehole data and hydrothermal numerical modelling. The KTB location exemplarily contains all elements that make seismic prospecting in crystalline environment often more difficult than in sedimentary units, basically complicated tectonics and fracturing and low-coherent strata. In a first step major rock units including two known nearly parallel fault zones are identified down to a depth of 12 km. These units form the basis of a gridded 3-D numerical model for investigating temperature and fluid flow. Conductive and advective heat transport takes place mainly in a metamorphic block composed of gneisses and metabasites that show considerable differences in thermal conductivity and heat production. Therefore, in a second step, the structure of this unit is investigated by seismic waveform modelling. The third step of interpretation consists of applying wavenumber filtering and log-Gabor-filtering for locating fractures. Since fracture networks are the major fluid pathways in the crystalline, we associate the fracture density distribution with distributions of relative porosity and permeability that can be calibrated by logging data and forward modelling of the temperature field. The resulting permeability distribution shows values between 10-16 and 10-19 m2 and does not correlate with particular rock units. Once thermohydraulic rock properties are attributed to the numerical model, the differential equations for heat and fluid transport in porous media are solved numerically based on a finite difference approach. The hydraulic potential caused by topography and a heat flux of 54 mW m-2 were applied as boundary conditions at the top and bottom of the model. Fluid flow is generally slow and

  1. Extension velocity partitioning, rheological crust-mantle and intra-crustal decoupling and tectonically inherited structures: consequences for continental rifting dynamics.

    Science.gov (United States)

    Wang, Kun; Mezri, Leila; Burov, Evgueni; Le Pourhiet, Laetitia

    2015-04-01

    We implemented series of systematic thermo-mechanical numerical models testing the importance of the rheological structure and extension rate partitioning for continental rift evolution. It is generally assumed that styles of continental rifting are mainly conditioned by the initial integrated strength of the lithosphere. For example, strong plates are expected to undergo extension in narrow rifting mode, while weak lithospheres would stretch in wide rifting mode. However, we show that this classification is largely insufficient because the notion of the integrated strength ignores the internal rheological structure of the lithosphere that may include several zones of crust-mantle or upper-crust-intermediate (etc) crust decoupling. As well, orogenic crusts characterizing most common sites of continental extension may exhibit inverted lithological sequences, with stronger and denser formerly lower crustal units on top of weaker and lighter upper crustal units. This all may result in the appearance of sharp rheological strength gradients and presence of decoupling zones, which may lead to substantially different evolution of the rift system. Indeed, strong jump-like contrasts in the mechanical properties result in mechanical instabilities while mechanical decoupling between the competent layers results in overall drop of the flexural strength of the system and may also lead to important horizontal flow of the ductile material. In particular, the commonly inferred concept of level of necking (that assumes the existence of a stationary horizontal stretching level during rifting) looses its sense if necking occurs at several distinct levels. In this case, due to different mechanical strength of the rheological layers, several necking levels develop and switch from one depth to another resulting in step-like variations of rifting style and accelerations/decelerations of subsidence during the active phase of rifting. During the post-rifting phase, initially decoupled

  2. Crustal structure of the Altiplano from broadband regional waveform modeling: Implications for the composition of thick continental crust

    Science.gov (United States)

    Swenson, Jennifer L.; Beck, Susan L.; Zandt, George

    2000-01-01

    We have modeled the full waveforms from six intermediate-depth and two shallow earthquakes recorded at regional distances by the BANJO Broadband Andean Joint Experiment (BANJO) and Seismic Exploration of the Deep Altiplano (SEDA) portable seismic networks in the central Andes. In this study we utilize data from those BANJO and SEDA stations located within the Altiplano and Eastern Cordillera. We used reflectivity synthetic seismograms and a grid search to constrain four parameters of the Altiplano-Eastern Cordillera lithosphere: crustal thickness, average crustal velocity (Vp), and crustal and upper mantle Poisson's ratios (σcrust and σmantle). Using our grid search, we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths. Robust models for the Altiplano that provide the best overall fit between the data and synthetic seismograms are characterized by an average Vp of 5.75-6.25 km/s, crustal thicknesses of 60-65 km, σcrust = 0.25, and σmantle = 0.27-0.29. We find a north-south variation in the structure of the Altiplano, with the crust south of the BANJO transect characterized by either lower than average crustal P wave velocities or a slightly higher σcrust relative to crust north of the BANJO transect. These results are consistent with a model of crustal thickening caused predominantly by tectonic shortening of felsic crust, rather than by underplating or magmatic intrusion from the mantle.

  3. Evaluating Complex Magma Mixing via Polytopic Vector Analysis (PVA in the Papagayo Tuff, Northern Costa Rica: Processes that Form Continental Crust

    Directory of Open Access Journals (Sweden)

    Guillermo E. Alvarado

    2013-08-01

    Full Text Available Over the last forty years, research has revealed the importance of magma mixing as a trigger for volcanic eruptions, as well as its role in creating the diversity of magma compositions in arcs. Sensitive isotopic and microchemical techniques can reveal subtle evidence of magma mixing in igneous rocks, but more robust statistical techniques for bulk chemical data can help evaluate complex mixing relationships. Polytopic vector analysis (PVA is a multivariate technique that can be used to evaluate suites of samples that are produced by mixing of two or more magma batches. The Papagayo Tuff of the Miocene-Pleistocene Bagaces Formation in northern Costa Rica is associated with a segment of the Central American Volcanic Arc. While this segment of the arc is located on oceanic plateau, recent (<8 Ma ignimbrites bear the chemical signatures of upper continental crust, marking the transition from oceanic to continental crust. The Papagayo Tuff contains banded pumice fragments consistent with one or more episodes of mixing/mingling to produce a single volcanic deposit. The PVA solution for the sample set is consistent with observations from bulk chemistry, microchemistry and petrographic data from the rocks. However, without PVA, the unequivocal identification of the three end-member solution would not have been possible.

  4. Continental crust subducted deeply into lithospheric mantle: the driving force of Early Carboniferous magmatism in the Variscan collisional orogen (Bohemian Massif)

    Science.gov (United States)

    Janoušek, Vojtěch; Schulmann, Karel; Lexa, Ondrej; Holub, František; Franěk, Jan; Vrána, Stanislav

    2014-05-01

    The vigorous Late Devonian-Early Carboniferous plutonic activity in the core of the Bohemian Massif was marked by a transition from normal-K calc-alkaline, arc-related (~375-355 Ma), through high-K calc-alkaline (~346 Ma) to (ultra-)potassic (343-335 Ma) suites, the latter associated with mainly felsic HP granulites enclosing Grt/Spl mantle peridotite bodies. The changing chemistry, especially an increase in K2O/Na2O and 87Sr/86Sri with decrease in 143Nd/144Ndi in the basic end-members, cannot be reconciled by contamination during ascent. Instead it has to reflect the character of the mantle sources, changing over time. The tectonic model invokes an oceanic subduction passing to subduction of the attenuated Saxothuringian crust under the rifted Gondwana margin (Teplá-Barrandian and Moldanubian domains). The deep burial of this mostly refractory felsic metaigneous material is evidenced by the presence of coesite/diamond (Massonne 2001; Kotková et al. 2011) in the detached UHP slices exhumed through the subduction channel and thrusted over the Saxothuringian basement, and by the abundance of felsic HP granulites (> 2.3 GPa), some bearing evidence for small-scale HP melt separation, in the orogen's core (Vrána et al. 2013). The subduction channel was most likely formed by 'dirty' serpentinites contaminated by the melts/fluids derived from the underlying continental-crust slab (Zheng 2012). Upon the passage through the orogenic mantle, the continental crust-slab derived material not only contaminated the adjacent mantle forming small bodies/veins of pyroxenites (Becker 1996), glimmerites (Becker et al. 1999) or even phlogopite- and apatite-bearing peridotites (Naemura et al. 2009) but the felsic HP-HT granulites also sampled the individual peridotite types at various levels. Eventually the subducted felsic material would form an (U)HP continental wedge under the forearc/arc region, to be later redistributed under the Moldanubian crust by channel flow and crustal

  5. What Hf isotopes in zircon tell us about crust-mantle evolution

    Science.gov (United States)

    Iizuka, Tsuyoshi; Yamaguchi, Takao; Itano, Keita; Hibiya, Yuki; Suzuki, Kazue

    2017-03-01

    The 176Lu-176Hf radioactive decay system has been widely used to study planetary crust-mantle differentiation. Of considerable utility in this regard is zircon, a resistant mineral that can be precisely dated by the U-Pb chronometer and record its initial Hf isotope composition due to having low Lu/Hf. Here we review zircon U-Pb age and Hf isotopic data mainly obtained over the last two decades and discuss their contributions to our current understanding of crust-mantle evolution, with emphasis on the Lu-Hf isotope composition of the bulk silicate Earth (BSE), early differentiation of the silicate Earth, and the evolution of the continental crust over geologic history. Meteorite zircon encapsulates the most primitive Hf isotope composition of our solar system, which was used to identify chondritic meteorites best representative of the BSE (176Hf/177Hf = 0.282793 ± 0.000011; 176Lu/177Hf = 0.0338 ± 0.0001). Hadean-Eoarchean detrital zircons yield highly unradiogenic Hf isotope compositions relative to the BSE, providing evidence for the development of a geochemically enriched silicate reservoir as early as 4.5 Ga. By combining the Hf and O isotope systematics, we propose that the early enriched silicate reservoir has resided at depth within the Earth rather than near the surface and may represent a fractionated residuum of a magma ocean underlying the proto-crust, like urKREEP beneath the anorthositic crust on the Moon. Detrital zircons from world major rivers potentially provide the most robust Hf isotope record of the preserved granitoid crust on a continental scale, whereas mafic rocks with various emplacement ages offer an opportunity to trace the Hf isotope evolution of juvenile continental crust (from εHf[4.5 Ga] = 0 to εHf[present] = + 13). The river zircon data as compared to the juvenile crust composition highlight that the supercontinent cycle has controlled the evolution of the continental crust by regulating the rates of crustal generation and intra

  6. Continental hyperextension, mantle exhumation, and thin oceanic crust at the continent-ocean transition, West Iberia: New insights from wide-angle seismic

    Science.gov (United States)

    Davy, R. G.; Minshull, T. A.; Bayrakci, G.; Bull, J. M.; Klaeschen, D.; Papenberg, C.; Reston, T. J.; Sawyer, D. S.; Zelt, C. A.

    2016-05-01

    Hyperextension of continental crust at the Deep Galicia rifted margin in the North Atlantic has been accommodated by the rotation of continental fault blocks, which are underlain by the S reflector, an interpreted detachment fault, along which exhumed and serpentinized mantle peridotite is observed. West of these features, the enigmatic Peridotite Ridge has been inferred to delimit the western extent of the continent-ocean transition. An outstanding question at this margin is where oceanic crust begins, with little existing data to constrain this boundary and a lack of clear seafloor spreading magnetic anomalies. Here we present results from a 160 km long wide-angle seismic profile (Western Extension 1). Travel time tomography models of the crustal compressional velocity structure reveal highly thinned and rotated crustal blocks separated from the underlying mantle by the S reflector. The S reflector correlates with the 6.0-7.0 km s-1 velocity contours, corresponding to peridotite serpentinization of 60-30%, respectively. West of the Peridotite Ridge, shallow and sparse Moho reflections indicate the earliest formation of an anomalously thin oceanic crustal layer, which increases in thickness from ~0.5 km at ~20 km west of the Peridotite Ridge to ~1.5 km, 35 km further west. P wave velocities increase smoothly and rapidly below top basement, to a depth of 2.8-3.5 km, with an average velocity gradient of 1.0 s-1. Below this, velocities slowly increase toward typical mantle velocities. Such a downward increase into mantle velocities is interpreted as decreasing serpentinization of mantle rock with depth.

  7. Compression-extension transition of continental crust in a subduction zone: A parametric numerical modeling study with implications on Mesozoic-Cenozoic tectonic evolution of the Cathaysia Block

    Science.gov (United States)

    Chan, Lung Sang; Gao, Jian-Feng

    2017-01-01

    The Cathaysia Block is located in southeastern part of South China, which situates in the west Pacific subduction zone. It is thought to have undergone a compression-extension transition of the continental crust during Mesozoic-Cenozoic during the subduction of Pacific Plate beneath Eurasia-Pacific Plate, resulting in extensive magmatism, extensional basins and reactivation of fault systems. Although some mechanisms such as the trench roll-back have been generally proposed for the compression-extension transition, the timing and progress of the transition under a convergence setting remain ambiguous due to lack of suitable geological records and overprinting by later tectonic events. In this study, a numerical thermo-dynamical program was employed to evaluate how variable slab angles, thermal gradients of the lithospheres and convergence velocities would give rise to the change of crustal stress in a convergent subduction zone. Model results show that higher slab dip angle, lower convergence velocity and higher lithospheric thermal gradient facilitate the subduction process. The modeling results reveal the continental crust stress is dominated by horizontal compression during the early stage of the subduction, which could revert to a horizontal extension in the back-arc region, combing with the roll-back of the subducting slab and development of mantle upwelling. The parameters facilitating the subduction process also favor the compression-extension transition in the upper plate of the subduction zone. Such results corroborate the geology of the Cathaysia Block: the initiation of the extensional regime in the Cathaysia Block occurring was probably triggered by roll-back of the slowly subducting slab. PMID:28182640

  8. Nature of the crust in the Laxmi Basin (14°-20°N), western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Rao, D.G.; Sar, D.

    ; Courtillot et al., 1988; White and McKenzie, 1989). The Deccan basalts erupted by the R?;union hotspot (68.5-62 Ma) are found as continental flood basalts on western Indian shield as well as on the Praslin Island in the Seychelles micro-continent (Devey.... The boundary is well expressed in the shelf region and was interpreted by Chaubey et al. (2002) as Miocene unconformity. The unconformity was also identified on our seismic profiles that extend in deep offshore regions (Figure 2). In the Western Basin thin...

  9. Coupling of Oceanic and Continental Crust During Eocene Eclogite-Facies Metamorphism: Evidence From the Monte Rosa Nappe, Western Alps, Italy

    Science.gov (United States)

    Lapen, T. J.; Johnson, C. M.; Baumgartner, L. P.; Skora, S.; Mahlen, N. J.; Beard, B. L.

    2006-12-01

    Subduction of continental crust to HP-UHP metamorphic conditions requires overcoming density contrasts that are unfavorable to deep burial, whereas exhumation of these rocks can be reasonably explained through buoyancy-assisted transport in the subduction channel to more shallow depths. In the western Alps, both continental and oceanic lithosphere has been subducted to eclogite-facies metamorphic conditions. The burial and exhumation histories of these sections of lithosphere bear directly on the dynamics of subduction and the stacking of units within the subduction channel. We address the burial history of the continental crust with high precision U-Pb rutile and Lu-Hf garnet geochronology of the eclogite-facies Monte Rosa nappe (MR), western Alps, Italy. U-Pb rutile ages from quartz-carbonate-white mica-rutile veins that are hosted within eclogite and schist of the MR, Gressoney Valley, Italy, indicate that it was at eclogite-facies metamorphic conditions at 42.6 +/- 0.6 Ma. The sample area (Indren glacier, Furgg zone; Dal Piaz, 2001) consists of eclogite boudins that are surrounded by micaceous schist. Associated with the eclogite and schist are quartz-carbonate-white mica-rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist. Intrusion of the veins occurred at eclogite-facies metamorphic conditions (480-570°C, >1.3-1.4 GPa) based on textural relations, oxygen isotope thermometry, and geothermobarometry. Lu-Hf geochronology of garnet from a chloritoid-talc-garnet-phengite-quartz-calcite-pyrite - chalcopyrite bearing boudin within talc-chloritoid whiteschists of the MR, Val d'Ayas, Italy (Chopin and Monie, 1984; Pawlig, 2001) yields an age of 40.54 +/- 0.36 Ma. The talc-chloritoid whiteschists from the area record pressures and temperatures of 1.6-2.4 GPa and 500-530°C (Chopin and Monie, 1984; Le Bayon et al., 2006) indicating near UHP metamorphic conditions. Based on the age, P-T, and textural

  10. Assessment of resource selection models to predict occurrence of five juvenile flatfish species (Pleuronectidae) over the continental shelf in the western Gulf of Alaska

    Science.gov (United States)

    Wilson, Matthew T.; Mier, Kathryn L.; Cooper, Dan W.

    2016-05-01

    According to the nursery size hypothesis, flatfish recruitment is constrained by nursery area. Thus, if resource selection models can be shown to accurately predict the location and geographic extent of flatfish nursery areas, they will become important tools in the management and study of flatfish population dynamics. We demonstrate that some resource selection models derived previously to predict the presence and absence of juvenile flatfishes near shore were applicable to the broader continental shelf. For other age-species groups, derivation of new models for the continental shelf was necessary. Our study was conducted in the western Gulf of Alaska (GoA) during October 2011 on four groups of age-0 juvenile flatfishes: Pacific halibut (Hippoglossus stenolepis), arrowtooth flounder (Atheresthes stomias), northern rock sole (Lepidopsetta polyxystra), and flathead sole (Hippoglossoides elassodon); and three groups of age-1 juvenile flatfishes: northern rock sole, flathead sole, and yellowfin sole (Limanda aspera). Sampling occurred at 33 sites across the continental shelf. Fish were collected using a 3-m beam trawl, and a midwater trawl. Environmental data were collected on sediment composition and water temperature and depth. Many of the age-species groups co-occurred in the Shumagin and Barnabus sea valleys; however, age-0 arrowtooth flounder occurred at more locations than other juveniles, perhaps due to a relatively broad tolerance of environmental conditions and to the utilization of midwater habitat. Thus, the large nursery area of arrowtooth flounder may be one reason why they are currently the most abundant GoA flatfish. In fact, among all species, mean recruitment at age 3 increased with the percent occurrence of age-0 juveniles at the 33 sites, a proxy for relative nursery area, in accordance with the nursery size hypothesis, suggesting that mean recruitment among GoA flatfishes is structured by nursery size.

  11. 87Sr/86Sr Ratios in Carbonate From the Red Lake and Steep Rock Groups in Canada Suggest Rb-enriched Continental Crust was Influencing Seawater Chemistry Prior to 3.0 Ga

    Science.gov (United States)

    Satkoski, A.; Fralick, P. W.; Beard, B. L.; Johnson, C.

    2015-12-01

    Previous work has suggested that prior to 2.5 Ga, Sr isotope compositions of seawater were essentially mantle buffered and the effects of continental weathering on seawater chemistry were negligible. To test this, we collected Sr isotope data from 2.93 and 2.80 Ga carbonates that are part of the Red Lake and Steep Rock groups (Canada), respectively. To better understand carbonate formation and any post-depositional alteration, Sr isotopes are considered with O isotopes and REEs, as well as Rb and Sr contents, including correction for decay of 87Rb. All samples have Y/Ho ratios higher than chondrite and have positive La anomalies, which, combined with low Rb contents suggests that clastic contamination is negligible. Samples we consider near pristine have δ18O (VSMOW) values >20‰. Samples with the highest Y/Ho ratios and largest La anomalies from Red Lake and Steep Rock have initial 87Sr/86Sr ratios of 0.7018-0.7020. This Sr isotope composition is significantly more radiogenic than contemporaneous mantle (0.7011-0.7012), especially at a time when the isotopic difference between the crust and mantle was much less than today. This implies that radiogenic continental crust was emergent and shed detritus into the world's oceans prior to 3.0 Ga, in contrast with proposals for submerged continental crust, but in line with new estimates that continental crust at 3.0 Ga was 60-70% of current volume. We contend that this large amount of crust combined with enhanced Archean weathering could account for the radiogenic Sr isotope compositions we report here, and suggests a significant impact from continental weathering on ocean chemistry during the Archean.

  12. Iron and Zinc isotope fractionation during magmatism in the continental crust: Evidence from bimodal volcanic rocks from Hailar basin, NE China

    Science.gov (United States)

    Xia, Ying; Li, Shuangqing; Huang, Fang

    2017-09-01

    This study presents Fe-Zn isotope data for a suite of well-characterized bimodal volcanic rocks from Hailar Basin, northeast China to understand the mechanism of Fe isotope fractionation in highly differentiated igneous rocks. The samples range from basaltic trachyandesites to trachytes-rhyodacites, and rhyolites. The δ56Fe values increase with increasing SiO2 contents with the rhyolites having the highest δ56Fe (up to 0.64 ± 0.02‰) among the previously reported data for igneous rocks at a similar SiO2. The lack of correlation between δ56Fe and Rb/La argues against the effect of fluid exsolution on Fe isotopes. The δ56Fe do not show a clear correlation with δ66Zn and radiogenic isotopes, suggesting that thermal diffusion or crustal contamination cannot produce the high δ56Fe in Hailar volcanic rocks. Fe isotopic variation in Hailar volcanic rocks can be explained by two steps of magmatism. During the first step, partial melting of basaltic trachyandesites with an average δ56Fe of 0.09 ± 0.14‰ produced trachytes-rhyodacites with an average δ56Fe of 0.24 ± 0.27‰. Modelling using rhyolite-MELTS shows that Fe isotopes can be fractionated by preferential partitioning of isotopically different Fe3+ and Fe2+ between the solid residue and partial melt. The second step involves formation of rhyolites with significantly high δ56Fe through partial melting or extensive crystallization of crust materials, during which isotopically heavy Fe preferentially partition into the rhyolitic melt. Therefore, fractionation of Fe isotopes between melts and minerals can result in high δ56Fe in SiO2-rich igneous rocks and apparent Fe isotope heterogeneity within the continental crust.

  13. Deformation Mechanism and Rheological Property of Granulite in the Continental Lower Crust: A Review%大陆下地壳麻粒岩的流变学研究进展

    Institute of Scientific and Technical Information of China (English)

    李丽敏; 刘祥文; 谢战军

    2011-01-01

    大险下地壳麻粒岩的流变学研究可以解释地壳变形、壳幔物质交换以及岩石圈深部动力学过程等科学问题.前人通过研究各矿物的显微构造变形特征与变形机制,运用广义混合流变律探讨多矿物复合岩石的流变性质,结合水与流体对岩石变形强度的弱化作用,阐明在大陆下地壳变形环境下复矿麻粒岩的塑性变形和韧性流变性质.目前人们致力于对天然变形岩石和实验变形矿物的高温高压实验研究,通过实脸室模拟大陆下地壳变形条件下岩石矿物的变形、熔融过程,探讨大陆下地壳复矿麻粒岩流变学特征.大陆下地壳麻粒岩流变学研究在宏观和微观构造上存在着密切的联系,以青藏高原下地壳流动与麻粒岩高温塑性流变的关系为例,阐述区城构造(造山带深部物质的挤出)与大陆下地壳流变学的关系及其制约因素研究的进展.复矿麻粒岩流变学研究是探讨大陆下地壳流动变形和变形强度的科学基础,由于该项研究在国内外开展时间还不算长,许多科学问题尚待深入研究.%The rheological study of granulite from continental lower crust is one of the scientific problems,which reveals the crustal deformation, crust-mantle exchange, and deep lithosphere dynamic process. This paper integrates the previous work on microstructure and deformation mechanisms of multiphase granulite, and clarifies the plastical deformation mechanism and rheological properties of multiphase granulite under the deformation conditions of lower continental crust by the application of generalized mixed flow law and combining the effect of water weakening or fluid on the deformation strength of the lower crust. At present, researchers are working on the high-temperature and high-pressure deformation experiments of natural and experimental deformed minerals, and explore the rheological property of multiphase granulite in the continental lower crust by

  14. High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the north Pyrenean paleo-passive margin

    Directory of Open Access Journals (Sweden)

    C. Clerc

    2015-02-01

    Full Text Available An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a dataset of more than one hundred peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM. This dataset is completed by previous PT estimates based on mineral assemblages, and new Ar–Ar (amphibole, micas and U–Pb (titanite ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key-localities of the Mauléon Basin, Arguenos-Moncaup, Lherz, Boucheville and the Bas-Agly, we analyse the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps, and into two detailed maps of the Arguenos-Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been

  15. Pressure-dependent seismic reflection amplitude changes in crystalline crust: lessons learned at the Continental Deep Drilling Site (KTB)

    Science.gov (United States)

    Beilecke, T.; Bram, K.; Buske, S.

    2010-01-01

    We conducted an active seismic experiment aimed at measuring changes in seismic reflection amplitudes as a consequence of fresh water injection and corresponding pressure changes at the German Continental Deep Drilling site (KTB). The injection took place at the bottom of the 4-km-deep pilot borehole in the SE2 fault zone in crystalline rock units between the springs of 2004 and 2005. Prior to the experiment, theoretical calculations indicated a possible increase in the compressional wave reflection coefficient as a result of an injection-induced reduction of the seismic velocities within the fault zone. Despite good repeatability of the emitted source signals, the experiment suffered from missing the clear reflection signals expected from the fault zone with regard to seismic data from past experiments. Applying various data-processing steps did not enhance the signals enough to obtain clear reflections or even pressure-dependent reflection amplitude changes. The signal-to-noise ratio remains smaller than the effects under observation. Provided that reflections are present in the data, the error bar of the recorded signals is of the order of 100 per cent. Therefore, we conclude that the experiment was not successful in seismically measuring pressure variations. However, important lessons for land seismic time-lapse measurements in crystalline environments have been learned: (i) The source should be capable of emitting frequencies below 30 Hz. (ii) The detector array setup proved to be partly questionable because in a scattering environment like the crystalline rocks at the KTB site, the incidence of a plane wave precondition might be violated for high-frequency signals. (iii) Near-surface variations of elastic properties likely influence seismic monitoring. (iv) Using a step function, that is a first-order pressure discontinuity, to model the subsurface pressure build-up, is very likely too simple an approach.

  16. Ancient plate kinematics derived from the deformation pattern of continental crust: Paleo- and Neo-Tethys opening coeval with prolonged Gondwana-Laurussia convergence

    Science.gov (United States)

    Kroner, Uwe; Roscher, Marco; Romer, Rolf L.

    2016-06-01

    The formation and destruction of supercontinents requires prolonged convergent tectonics between particular plates, followed by intra-continental extension during subsequent breakup stages. A specific feature of the Late Paleozoic supercontinent Pangea is the prolonged and diachronous formation of the collisional belts of the Rheic suture zone coeval with recurrent continental breakup and subsequent formation of the mid-ocean ridge systems of the Paleo- and Neo-Tethys oceans at the Devonian and Permian margins of the Gondwana plate, respectively. To decide whether these processes are causally related or not, it is necessary to accurately reconstruct the plate motion of Gondwana relative to Laurussia. Here we propose that the strain pattern preserved in the continental crust can be used for the reconstruction of ancient plate kinematics. We present Euler pole locations for the three fundamental stages of the Late Paleozoic assembly of Pangea and closure of the Rheic Ocean: (I) Early Devonian (ca. 400 Ma) collisional tectonics affected Gondwana at the Armorican Spur north of western Africa and at the promontory of the South China block/Australia of eastern Gondwana, resulting in the Variscan and the Qinling orogenies, respectively. The Euler pole of the rotational axis between Gondwana and Laurussia is positioned east of Gondwana close to Australia. (II) Continued subduction of the western Rheic Ocean initiates the clockwise rotation of Gondwana that is responsible for the separation of the South China block from Gondwana and the opening of Paleo-Tethys during the Late Devonian. The position of the rotational axis north of Africa reveals a shift of the Euler pole to the west. (III) The terminal closure of the Rheic Ocean resulted in the final tectonics of the Alleghanides, the Mauritanides and the Ouachita-Sonora-Marathon belt, occurred after the cessation of the Variscan orogeny in Central Europe, and is coeval with the formation of the Central European Extensional

  17. Impact of melt migration on the evolution of major and trace element composition in a crystalline mush: Implications for chemical differentiation in the continental crust

    Science.gov (United States)

    Jackson, M.; Solano, J.; Sparks, R. S.; Blundy, J.

    2013-12-01

    Migration of melt through a crystalline mush is common within the continental crust, occurring in magma chambers and lava flows. Mush formation and associated migration of the buoyant melt along grain boundaries is inevitable during melting of initially solid rock, or cooling and crystallisation of magma. Because there is efficient exchange of heat and mass between melt and solid phases, they remain in local thermal and chemical equilibrium. The composition of the melt therefore evolves as it migrates through the mush and, to properly capture this, models are required that include chemical reaction. However, although reactive transport models have been developed for the mantle, none have yet been presented that are applicable to the continental crust. Models developed for the mantle are not directly applicable to the crust, because the initial and boundary conditions are different. We present the first quantitative model of heat, mass and both major and trace element transport in a mush undergoing compaction which accounts for component transport and chemical reaction during melt migration and which is applicable to crustal systems. The model describes the phase behaviour of binary systems (both eutectic and solid solution), with melt and solid composition determined from phase diagrams using the local temperature and bulk composition. Trace element concentration is also determined. The results demonstrate that component transport and chemical reaction generates compositional variation in both major and trace elements that is not captured by existing geochemical models. Even for the simplest case of a homogenous, insulated column that is instantaneously melted then allowed to compact, we find that component transport and reaction leads to spatial variations in major element composition, and produces melt that is more enriched in incompatible elements than predicted by batch melting. In deep crustal hot zones (DCHZ), created by the repeated intrusion of hot, mantle

  18. Upper Limit for Rheological Strength of Crust in Continental Subduction Zone:Constraints Imposed by Laboratory Experiments

    Institute of Scientific and Technical Information of China (English)

    Zhou Yongsheng; Zhong Dalai; He Changrong

    2004-01-01

    The transitional pressure of quartz-coesite under the differential stress and highly-strained conditions is far from the pressure of the stable field under the static pressure. Therefore, the effect of the differential stress should be considered when the depth of petrogenesis is estimated about ultrahigh pressure metamorphic (UHPM) rocks. The rheological strength of typical ultrahigh pressure rocks in continental subduction zone was derived from the results of the laboratory experiments. The results indicate the following three points. (1) The rheological strength of gabbro, similar to that of eclogite, is smaller than that of clinopyroxenite on the same condition. (2) The calculated strength of rocks (gabbro, eclogite and clinopyroxenite) related to UHPM decreases by nearly one order of magnitude with the temperature rising by 100 ℃ in the range between 600 and 900 ℃. The calculated strength is far greater than the faulting strength of rocks at 600 ℃, and is in several hundred to more than one thousand mega-pascals at 700-800 ℃, which suggests that those rocks are located in the brittle deformation region at 600 ℃, but are in the semi-brittle to plastic deformation region at 700-800 ℃. Obviously, the 700 ℃ is a brittle-plastic transition boundary. (3) The calculated rheological strength in the localized deformation zone on a higher strain rate condition (1.6×10-12 s-l) is 2-5 times more than that in the distributed deformation zone on a lower strain rate condition (1.6×10-14 s-1). The average rheological stress (1 600 MPa) at the strain rate of 10-12 s-1 stands for the ultimate differential stress of UHPM rocks in the semi-brittle flow field, and the average rheological stress (550-950 MPa) at the strain rate of l0-14 -10-13 s-l stands for the ultimate differential stress of UHPM rocks in the plastic flow field, suggesting that the depth for the formation of UHPM rocks is more than 20-60 km below the depth estimated under static pressure condition

  19. Sudbuction-related tectonic mixing between serpentinized mantle and continental crust in the internal Western Alps: exhumed portion of a marble cake?

    Science.gov (United States)

    Spalla, M.; Zucali, M.; Cantù, M.; Roda, M.; Marotta, A.

    2011-12-01

    The pre-Alpine continental crust of the Western Alps is widely affected by subduction-related high-pressure low-temperature (HP-LT) metamorphism and the Sesia-Lanzo Zone (SLZ) represents the widest crustal complex re-equilibrated under HP conditions during Late-Cretaceous (early-Alpine) times. Its Alpine tectonic evolution is compatible with an uplift during active oceanic lithosphere subduction (e.g. Spalla et al., 1996; Meda et al., 2010; Zucali & Spalla, 2011). In the SLZ three main lithologic complexes have been recognized (e.g. Compagnoni et al., 1977): i) Gneiss Minuti complex (GMC); ii) Eclogitic Micaschists complex (EMC); iii) II Dioritic-Kinzigitic Zone (IIDK). IIDK consists of kilometer-size lenses of pelitic and mafic granulites that escaped eclogite facies re-equilibration. EMC and GMC, both pervasively eclogitised, strongly differ in the volume percentage of greenschist facies re-equilibration (Stuenitz, 1989; Spalla et al., 1991). In the southern SLZ the metamorphic complex of Rocca Canavese thrust sheets (RCT; Pognante, 1989 a; Pognante, 1989b) has been recognised, on the ground of its strongly contrasted Alpine metamorphic evolution, characterized by a P-climax recorded under very LT conditions. A new multi-scale structural analysis, performed to acquire new detailed petrologic data and to refine the tectonic trajectory of such a peculiar metamorphic complex, pointed out that RCT is a tectonic mixing of serpentinized mantle and continental rocks deformed together under high pressure conditions. Field analysis reports a superposed grid of metamorphic foliations allowing regional scale correlation of structural and metamorphic stages, after micro-structural identification of mineral assemblages marking successive fabrics in each rock type. Chemical data on variations of mineral compositions in different micro-structural sites, led to the reconstruction of a PTdt (Pressure-Temperature-relative deformation time) path and opened therefore the possibility

  20. Vertical tectonics at a continental crust-oceanic plateau plate boundary zone: Fission track thermochronology of the Sierra Nevada de Santa Marta, Colombia

    Science.gov (United States)

    Villagómez, Diego; Spikings, Richard; Mora, AndréS.; GuzmáN, Georgina; Ojeda, GermáN.; CortéS, Elizabeth; van der Lelij, Roelant

    2011-08-01

    The topographically prominent Sierra Nevada de Santa Marta forms part of a faulted block of continental crust located along the northern boundary of the South American Plate, hosts the highest elevation in the world (˜5.75 km) whose local base is at sea level, and juxtaposes oceanic plateau rocks of the Caribbean Plate. Quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary, and the driving forces of rock uplift along the active margin. The Sierra Nevada Province of the southernmost Sierra Nevada de Santa Marta exhumed at elevated rates (≥0.2 Km/My) during 65-58 Ma in response to the collision of the Caribbean Plateau with northwestern South America. A second pulse of exhumation (≥0.32 Km/My) during 50-40 Ma was driven by underthrusting of the Caribbean Plate beneath northern South America. Subsequent exhumation at 40-25 Ma (≥0.15 Km/My) is recorded proximal to the Santa Marta-Bucaramanga Fault. More northerly regions of the Sierra Nevada Province exhumed rapidly during 26-29 Ma (˜0.7 Km/My). Further northward, the Santa Marta Province exhumed at elevated rates during 30-25 Ma and 25-16 Ma. The highest exhumation rates within the Sierra Nevada de Santa Marta progressed toward the northwest via the propagation of NW verging thrusts. Exhumation is not recorded after ˜16 Ma, which is unexpected given the high elevation and high erosive power of the climate, implying that rock and surface uplift that gave rise to the current topography was very recent (i.e., ≤1 Ma?), and there has been insufficient time to expose the fossil apatite partial annealing zone.

  1. The formation of granulite and its contribution to evolution of the continental crust.%麻粒岩的形成及其对大陆地壳演化的贡献

    Institute of Scientific and Technical Information of China (English)

    翟明国; 刘文军

    2001-01-01

    The granulite is one of the most important rocks in the earth. Its formation was controlled mainly by the condition of terrestrial heat. Their main formation models are related to continental collision, continental extension and continental arc. In some extent, granulite is the synonym of the lower continental crust. By research of granulite terranes and granulite xenoliths in volcanic rocks, composing and structure of deep substance in the earth could be realized directly. Research of granulite is a key point to understand formation and evolution of continental crust.%麻粒岩是地球中最重要的岩石类型之一。它的形成主要受地热条件的控制。麻粒岩的形成可以与大陆碰撞、大陆拉张以及大陆弧模式相联系。麻粒岩在某种程度上,还是下地壳的同义词。通过麻粒岩地体以及火山岩中麻粒岩捕掳体的研究,使人们对地球深部的物质组成和结构有了直接的了解。麻粒岩在陆壳的形成和演化中具有举足轻重的地位。陆核说、陆壳垂直增生或横向增生说,都以麻粒岩作为重要的岩石学依据。

  2. Dynamics of Pre-3 Ga Crust-Mantle Evolution

    Science.gov (United States)

    Patchett, P. J.; Chase, C. G.; Vervoort, J. D.

    2004-05-01

    During 3.0 to 2.7 Ga, the Earth's crust underwent a non-uniformitarian change from a pre-3.0 Ga environment where long-term preservation of cratons was rare and difficult, to post-2.7 Ga conditions where cratons were established and new continental crust generation took place largely at craton margins. Many models view the Earth's surface during pre-3 Ga time as broadly equivalent to the post 2.7 Ga regime. Any such uniformitarian or gradual evolution cannot explain the conundrum that only a tiny amount of pre-3 Ga crust is preserved today coupled with the fact that very little pre-3 Ga crust was incorporated into the large amount of new craton that came into existence during 3.0-2.7 Ga. If large volumes of pre-3 Ga continental crust existed, it disappeared either just prior to 3 Ga, or during 3.0-2.7 Ga. To explain sudden appearance of surviving but dominantly juvenile continental crust in a model where continents were large prior to 3 Ga, it would be necessary either that pre-3 Ga continent was recycled into the mantle at sites systematically different from those where new 3.0-2.7 Ga crust was made, or that widespread continent destruction preceded the 3.0-2.7 Ga crustal genesis. From expected mantle overturn in response to the heat budget, it is likely that most pre-3 Ga crust was both more mafic and shorter-lived than after 3 Ga. Although Nd and Hf ratios for pre-3 Ga rocks are uncertain due to polymetamorphism, it appears that depleted upper mantle was widespread by 2.7 Ga, even pre-3 Ga. Depletion may have been largely achieved by formation, subduction and storage of mafic crust for periods of 200-500 m.y. The rapid change to large surviving continents during 3.0-2.7 Ga was due to declining mantle overturn, and particularly to development of the ability to maintain subduction in one zone of the earth's surface for the time needed to allow evolution to felsic igneous rock compositions. In as much as storage of subducted slabs is probably occurring today, and

  3. Rb–Sr and Sm–Nd isotope systematics and geochemical studies on metavolcanic rocks from Peddavura greenstone belt: Evidence for presence of Mesoarchean continental crust in easternmost part of Dharwar Craton, India

    Indian Academy of Sciences (India)

    M Rajamanickam; S Balakrishnan; R Bhutani

    2014-07-01

    Linear, north–south trending Peddavura greenstone belt occurs in easternmost part of the Dharwar Craton. It consists of pillowed basalts, basaltic andesites, andesites (BBA) and rhyolites interlayered with ferruginous chert that were formed under submarine condition. Rhyolites were divided into type-I and II based on their REE abundances and HREE fractionation. Rb–Sr and Sm–Nd isotope studies were carried out on the rock types to understand the evolution of the Dharwar Craton. Due to source heterogeneity Sm–Nd isotope system has not yielded any precise age. Rb–Sr whole-rock isochron age of 2551 ± 19 (MSWD = 1.16) Ma for BBA group could represent time of seafloor metamorphism after the formation of basaltic rocks. Magmas representing BBA group of samples do not show evidence for crustal contamination while magmas representing type-II rhyolites had undergone variable extents of assimilation of Mesoarchean continental crust (< 3.3 Ga) as evident from their initial Nd isotope values. Trace element and Nd isotope characteristics of type I rhyolites are consistent with model of generation of their magmas by partial melting of mixed sources consisting of basalt and oceanic sediments with continental crustal components. Thus this study shows evidence for presence of Mesoarchean continental crust in Peddavura area in eastern part of Dharwar Craton.

  4. Hybrid accretionary/collisional mechanism of Paleozoic Asian continental growth

    Science.gov (United States)

    Schulmann, Karel; Lexa, Ondrej; Janousek, Vojtech; Pavla, Stipska; Yingde, Jiang; Alexandra, Guy; Min, Sun

    2016-04-01

    Continental crust is formed above subduction zones by well-known process of "juvenile crust growth". This new crust is in modern Earth assembled into continents by two ways: (i) short-lived collisions of continental blocks with the Eurasian continent along the "Alpine-Himalayan collisional/interior orogens" in the heart of the Pangean continental plates realm; and (ii) long lived lateral accretion of ocean-floor fragments along "circum-Pacific accretionary/peripheral orogens" at the border of the Pacific oceanic plate. This configuration has existed since the late Proterozoic, when the giant accretionary Terra Australis Orogen developed at periphery of an old Palaeo-Pacific ocean together with collisional Caledonian and Variscan orogens. At the same time, the large (ca. 9 millions km2) Central Asian Orogenic Belt (CAOB) developed in the NE part of the Pangea. This orogen reveals features of both peripheral and interior orogens, which implies that the generally accepted "peripheral-accretionary" and "interior- collisional" paradigm is not applicable here. To solve this conundrum a new model of unprecedented Phanerozoic continental growth is proposed. In this model, the CAOB precursor evolved at the interface of old exterior and young interior oceans. Subsequently, the new lithospheric domain was transferred by advancing subduction into the interior of the Pangean mostly continental realm. During this process the oceanic crust was transformed into continental crust and it was only later when this specific lithosphere was incorporated into the Asian continent. If true, this concept represents revolutionary insight into processes of crustal growth explaining the enigma of anchoring hybrid lithosphere inside a continent without its subduction or Tibetan-type thickening.

  5. Seismic and magnetic susceptibility anisotropy of middle-lower continental crust: Insights for their potential relationship from a study of intrusive rocks from the Serre Massif (Calabria, southern Italy)

    Science.gov (United States)

    Punturo, Rosalda; Mamtani, Manish A.; Fazio, Eugenio; Occhipinti, Roberta; Renjith, A. R.; Cirrincione, Rosolino

    2017-08-01

    We investigated the relationships between fabric, seismic and magnetic anisotropy on lithotypes representative of a continental crust exposed in the Serre Massif (Southern Italy). In particular, from five granitoids and one metagabbro, cubes were cut according to the fabric elements and seismic properties up to 400 MPa confining pressure were measured with a triaxial multi-anvil apparatus; we also calculated the elastic properties based on the mineral content and composition. In granitoids, measured average compressional wave velocity (Vp) of the fracture-free aggregate at 400 MPa is 6.2 km/s, whereas average shear wave velocity (Vs) is 3.6 km/s, with Poisson's ratio ranging from 0.240 to 0.257, related to the modal proportions of quartz. In metagabbro, average Vp and Vs at 400 MPa are 6.9 km/s and 3.7 km/s, respectively. Results showed that intrinsic velocity distribution, after microcracks closure, depends on progressive alignment of anisotropic minerals such as biotite, amphibole and pyroxene, with maxima velocities localized within the foliation plane. Mean magnetic susceptibility, Km, of the granitic rocks is units, indicating that paramagnetic minerals such as biotite and amphibole control the intensity of magnetic anisotropy. Comparison of seismic and magnetic anisotropies highlighted the different role of constituting minerals over the petrophysical properties. Moreover, a positive correlation between seismic and magnetic anisotropy has been recognized, indicating that biotite and amphibole contribute to the petrophysical and textural anisotropy in the middle crust. Conversely, in metagabbro, the anisotropy of magnetic susceptibility (AMS) is controlled by magnetite and pyrrhotite although these form < 10% of the rock, which dominantly comprises paramagnetic minerals such as biotite and orthopyroxene. Unlike granitoids, in metagabbro the petrophysical properties are controlled by the paramagnetic minerals, while the magnetic anisotropy is controlled by the

  6. Zircon U-Pb geochronology and petrogenesis of metabasites from the western Beihuaiyang zone in the Hong'an orogen, central China: Implications for detachment within subducting continental crust at shallow depths

    Science.gov (United States)

    Liu, Yi-Can; Liu, Li-Xiang; Li, Yuan; Gu, Xiao-Feng; Song, Biao

    2017-09-01

    Deformed low-grade metabasites from the western segment of the Beihuaiyang zone in the Hong'an orogen, central China can be divided into two types, i.e. meta-gabbro and meta-basalt. These lithologies have been studied by using whole-rock elemental and Sr-Nd-Pb isotopic analyses, and zircon SHRIMP U-Pb dating. Concordant zircon U-Pb ages of 631 ± 5 Ma and 623 ± 14 Ma are obtained for the meta-gabbros, consistent with a previously reported U-Pb age of 635 ± 5 Ma. The meta-basalt was dated to have a protolith age of middle Neoproterozoic (∼750 Ma) and a metamorphic age at ca. 240 Ma. The all studied metabasites occur as block or slice within a metamorphosed Ordovician volcanic zone (originally named as the Dingyuan Formation) and are in tectonic contact to each other. The gabbro and basalt emplaced at ∼630 Ma and ∼750 Ma, respectively in a continental rifting setting, whereas their present country rocks were erupted at ∼465 Ma in an arc setting. The Pb-isotope compositions of the low-grade meta-gabbros and meta-basalts are similar to those from the Dabie ultrahigh-pressure (UHP) meta-igneous rocks with an upper continental crust affinity. The protolith ages of the studied relatively low-grade meta-basic rocks are in good agreement not only with ages for two episodes of middle and late Neoproterozoic mafic and felsic magmatism in the Suizhou to Zaoyang areas at the northern margin of the South China Block, but are also in agreement with the protolith ages of UHP meta-igneous rocks in the Dabie-Sulu orogenic belt. Therefore, these Neoproterozoic low-grade metabasites are considered to be exotic and they may have been detached and offscraped from the subducting upper crust of the South China Block at shallow depths during continental collision in the Triassic. They were subsequently exhumed in the initial stage of continental subduction, and thrusted over the Paleozoic metamorphosed rocks in the southern margin of the North China Block or as foreign slices

  7. Enrichments of the mantle sources beneath the Southern Volcanic Zone (Andes) by fluids and melts derived from abraded upper continental crust

    DEFF Research Database (Denmark)

    Holm, Paul Martin; Søager, Nina; Dyhr, Charlotte Thorup

    2014-01-01

    mantle by means of subduction erosion in response to the northward increasingly strong coupling of the converging plates. Both types of enrichment had the same Pb isotope composition in the TSVZ with no significant component derived from the subducting oceanic crust. Pb–Sr–Nd isotopes indicate a major...

  8. The giant Dexing porphyry Cu-Mo-Au deposit in east China: product of melting of juvenile lower crust in an intracontinental setting

    Science.gov (United States)

    Hou, Zengqian; Pan, Xiaofei; Li, Qiuyun; Yang, Zhiming; Song, Yucai

    2013-12-01

    The Dexing porphyry Cu-Mo-Au deposit in east China (1,168 Mt at 0.45 % Cu) is located in the interior of the South China Craton (SCC), made up of two lithospheric blocks, the Yangtze and Cathaysia blocks. The Cu-Mo-Au mineralization is associated with mid-Jurassic granodioritic porphyries with three high-level intrusive centers, controlled by a series of lineaments at the southeastern edge of the Yangtze block. Available age data define a short duration (172-170 Ma) of the felsic magmatism and the mineralization (171 ± 1 Ma). The deposit shows broad similarities with deposits in volcanoplutonic arcs, although it was formed in an intracontinental setting. Porphyries associated with mineralization are mainly granodiorites, which contain abundant phenocrysts (40-60 %) and carry contemporaneous microgranular mafic enclaves (MMEs). They are mainly high-K calc-alkaline and show geochemical affinities with adakite, characterized by relatively high MgO, Cr, Ni, Th, and Th/Ce ratios. The least-altered porphyries yielded relatively uniform ɛ Nd( t) values from -0.9 to +0.6, and wide (87Sr/86Sr)i range between 0.7046 and 0.7058 partially overlapping with the Sr-Nd isotopic compositions of the MMEs and mid-Jurassic mafic rocks in the SCC. Zircons from the porphyries have positive ɛ Hf( t) values (3.4 to 6.9), and low δ18O values (4.7 to 6.3 ‰), generally close to those of depleted mantle. All data suggest an origin by partial melting of a thickened juvenile lower crust involving mantle components (e.g., Neoproterozoic mafic arc magmas), triggered by invasion of contemporaneous mafic melts at Dexing. The MMEs show textural, mineralogical, and chemical evidence for an origin as xenoliths formed by injection of mafic melts into the felsic magmas. These MMEs usually contain magmatic chalcopyrite, and have original, variable contents of Cu (up to 500 ppm). Their geochemical characteristics suggest that they were derived from an enriched mantle source, metasomatized by

  9. Modification of the Continental Crust by Subduction Zone Magmatism and Vice-Versa: Across-Strike Geochemical Variations of Silicic Lavas from Individual Eruptive Centers in the Andean Central Volcanic Zone

    Directory of Open Access Journals (Sweden)

    Gary S. Michelfelder

    2013-11-01

    Full Text Available To better understand the origin of across-strike K2O enrichments in silicic volcanic rocks from the Andean Central Volcanic Zone, we compare geochemical data for Quaternary volcanic rocks erupted from three well-characterized composite volcanoes situated along a southeast striking transect between 21° and 22° S latitude (Aucanquilcha, Ollagüe, and Uturuncu. At a given SiO2 content, lavas erupted with increasing distance from the arc front display systematically higher K2O, Rb, Th, Y, REE and HFSE contents; Rb/Sr ratios; and Sr isotopic ratios. In contrast, the lavas display systematically lower Al2O3, Na2O, Sr, and Ba contents; Ba/La, Ba/Zr, K/Rb, and Sr/Y ratios; Nd isotopic ratios; and more negative Eu anomalies toward the east. We suggest that silicic magmas along the arc front reflect melting of relatively young, mafic composition amphibolitic source rocks and that the mid- to deep-crust becomes increasingly older with a more felsic bulk composition in which residual mineralogies are progressively more feldspar-rich toward the east. Collectively, these data suggest the continental crust becomes strongly hybridized beneath frontal arc localities due to protracted intrusion of primary, mantle-derived basaltic magmas with a diminishing effect behind the arc front because of smaller degrees of mantle partial melting and primary melt generation.

  10. A statistical assessment of seismic models of the U.S. continental crust using Bayesian inversion of ambient noise surface wave dispersion data

    Science.gov (United States)

    Olugboji, T. M.; Lekic, V.; McDonough, W.

    2017-07-01

    We present a new approach for evaluating existing crustal models using ambient noise data sets and its associated uncertainties. We use a transdimensional hierarchical Bayesian inversion approach to invert ambient noise surface wave phase dispersion maps for Love and Rayleigh waves using measurements obtained from Ekström (2014). Spatiospectral analysis shows that our results are comparable to a linear least squares inverse approach (except at higher harmonic degrees), but the procedure has additional advantages: (1) it yields an autoadaptive parameterization that follows Earth structure without making restricting assumptions on model resolution (regularization or damping) and data errors; (2) it can recover non-Gaussian phase velocity probability distributions while quantifying the sources of uncertainties in the data measurements and modeling procedure; and (3) it enables statistical assessments of different crustal models (e.g., CRUST1.0, LITHO1.0, and NACr14) using variable resolution residual and standard deviation maps estimated from the ensemble. These assessments show that in the stable old crust of the Archean, the misfits are statistically negligible, requiring no significant update to crustal models from the ambient noise data set. In other regions of the U.S., significant updates to regionalization and crustal structure are expected especially in the shallow sedimentary basins and the tectonically active regions, where the differences between model predictions and data are statistically significant.

  11. U and Th content in the Central Apennines continental crust: a contribution to the determination of the geo-neutrinos flux at LNGS

    CERN Document Server

    Coltorti, M; Mantovani, F; Morsilli, M; Fiorentini, G; Riva, A; Rusciadelli, G; Tassinari, R; Tomei, C; Di Carlo, G; Chubakov, V

    2011-01-01

    The regional contribution to the geo-neutrino signal at Gran Sasso National Laboratory (LNGS) was determined based on a detailed geological, geochemical and geophysical study of the region. U and Th abundances of more than 50 samples representative of the main lithotypes belonging to the Mesozoic and Cenozoic sedimentary cover were analyzed. Sedimentary rocks were grouped into four main "Reservoirs" based on similar paleogeographic conditions and mineralogy. Basement rocks do not outcrop in the area. Thus U and Th in the Upper and Lower Crust of Valsugana and Ivrea-Verbano areas were analyzed. Based on geological and geophysical properties, relative abundances of the various reservoirs were calculated and used to obtain the weighted U and Th abundances for each of the three geological layers (Sedimentary Cover, Upper and Lower Crust). Using the available seismic profile as well as the stratigraphic records from a number of exploration wells, a 3D modelling was developed over an area of 2^{\\circ}x2^{\\circ} dow...

  12. The ordering mode of geochemical differentiation index of elements in continental crust.%大陆地壳中元素地球化学分异指数排序模式

    Institute of Scientific and Technical Information of China (English)

    岑况; 陈媛

    2011-01-01

    compatibility. Logarithm values of these indexes are arranged from high to low to make up a sequence of geochemical differentiation intensity of elements in the upper crust: at the left side of the sequence are strongly positive differentiation elements, namely strongly incompatible elements, enriched in the continental crust; at the right are strongly negative differentiation elements, namely strongly compatible elements, relatively depleted in the continental upper crust; the enrichment or depletion of elements whose differentiation index value are close to 1 is not obvious in the crust and they are called transition elements. According to this sequence, we propose a new geochemical classification of elements: strongly positive differentiation elements distributed in the left of the sequence are crustphilic; strongly negative differentiation elements distributed in the right of the sequence are mantlephilic; elements located in the middle of the sequence are crustphilic-man-tlephilic transition elements. The element positions and the law of their distribution in the periodic table of elements according to above categories, the variation of differentiation intensity index and its relationship with the properties of corresponding elements in the periodic table of elements are analyzed in this paper.

  13. The Lost South Gobi Microcontinent: Protolith Studies of Metamorphic Tectonites and Implications for the Evolution of Continental Crust in Southeastern Mongolia

    Directory of Open Access Journals (Sweden)

    Matthew J. Heumann

    2013-08-01

    Full Text Available The Central Asian Orogenic Belt, or Altaids, is an amalgamation of volcanic arcs and microcontinent blocks that records a complex late Precambrian–Mesozoic accretionary history. Although microcontinents cored by Precambrian basement are proposed to play an integral role in the accretion process, a lack of isotopic data hampers volume estimates of newly produced arc-derived versus old-cratonic crust in southeastern Mongolia. This study investigates metamorphic tectonites in southern Mongolia that have been mapped as Precambrian in age, largely on the basis of their high metamorphic grade and high strain. Here we present results from microstructural analyses and U-Pb zircon geochronology on samples from Tavan Har (44.05° N, 109.55° E and the Yagan-Onch Hayrhan metamorphic core complex (41.89° N, 104.24° E. Our results show no compelling evidence for Precambrian basement in southeastern Mongolia. Rather, the protoliths to all tectonites examined are Paleozoic–Mesozoic age rocks, formed during Devonian–Carboniferous arc magmatism and subsequent Permian–Triassic orogenesis during collision of the South Mongolia arc with the northern margin of China. These results yield important insights into the Paleozoic accretionary history of southern Mongolia, including the genesis of metamorphic and igneous basement during the Paleozoic, as well as implications for subsequent intracontinental reactivation.

  14. Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental break-up

    Directory of Open Access Journals (Sweden)

    K. Becker

    2014-06-01

    Full Text Available High-velocity lower crust (HVLC and seaward dipping reflector sequences (SDRs are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal four times larger cross sectional areas of HVLC than at the South American margin, a finding that is in sharp contrast to the distribution of the flood basalts in the Paraná-Etendeka Large Igneous Provinces (LIP. Also, the position of the HVLC with respect to the seaward dipping reflector sequences varies consistently along both margins. Close to the Falkland-Agulhas Fracture Zone a small body of HVLC is not accompanied by seaward dipping reflectors. In the central portion of both margins, the HVLC is below the inner seaward dipping reflector wedges while in the northern area, closer to the Rio Grande Rise/Walvis Ridge, large volumes of HVLC extend far seawards of the inner seaward dipping reflectors. This challenges the concept of a simple extrusive/intrusive relationship between seaward dipping reflector sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and break-up process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple shear dominated extension.

  15. Eocene deep crust at Ama Drime, Tibet

    DEFF Research Database (Denmark)

    Kellett, Dawn; Cottle, John; Smit, Matthijs Arjen

    2014-01-01

    burial and exhumation of a cold subducted slab. The rocks instead resulted from crustal thickening during the early stages of continental collision, and resided in the lower-middle crust for >20 m.y. before they were exhumed and reheated. These new data provide solid evidence for the Indian crust having...

  16. He and N isotopes in thermal springs of the Mexican Pacific coast: subducting slab, continental crust and mantle contributions to fluids of a forearc zone.

    Science.gov (United States)

    Taran, Yuri; Inguaggiato, Salvatore; Varley, Nicholas; Ramirez Guzman, Alejandro

    2010-05-01

    Two oceanic plates are subducting beneath the continent along the Mexican Pacific coast: Cocos Plate south of Colima graben (~19°N) and a young Rivera Plate to the north of Colima graben. The trench is situated ~ 70 km from the shore line which is very close comparing with other continental margins. There are 26 groups of thermal springs between 16°N and 21°N, in a 30 km-wide zone along the coast. The temperature and salinity ranges are 40-90°C and 100-20,000 ppm, respectively. The springs are mainly of a low salinity ( 400, δ15N = +4.6‰, almost no CH4 ( 300, δ15N = +5‰ and 3He/4He = 0.4Ra. A number of hot and warm springs associated with Puerto Vallarta graben are characterized by high 3He/4He up to 4.5Ra, elevated N2/Ar and δ15N. The last group, Punta Mita hot springs (20° 46'N), are submarine vents, 10 m deep. Their gas has elevated CH4 content, high N2/Ar and 3He/4He = 0.4Ra. The results are discussed in several aspects: (1) Why this low heat flow zone is characterized by so high hydrothermal activity? (2) Does the elevated 3He/4He within Michoacan-Colima profile relate to the slab detachment associated with the contact between Cocos and Rivera plates? (3) Do high N2/Ar and δ15N above the Rivera Plate subduction indicate the forearc degassing of the accreted organic-rich oceanic sediments? (4) How to estimate the total flux of volatiles released in a forearc zone from the subducting slab?

  17. 3.30 Ga high-silica intraplate volcanic-plutonic system of the Gavião Block, São Francisco Craton, Brazil: Evidence of an intracontinental rift following the creation of insulating continental crust

    Science.gov (United States)

    Zincone, Stefano A.; Oliveira, Elson P.; Laurent, Oscar; Zhang, Hong; Zhai, Mingguo

    2016-12-01

    High-silica rhyolites having U-Pb zircon ages of 3303 ± 11 Ma occur along the eastern border of the Gavião Block (Brazil) associated with the Contendas-Mirante and Mundo Novo supracrustal belts. Unlike many Archean greenstone sequences, they are not interlayered with mafic to intermediate units. Instead, they belong to an inter-related plutonic-volcanic system, together with granitic massifs having similar zircon crystallization ages of ca. 3293 ± 3 Ma and 3328 ± 3 Ma and plotting along the same geochemical trends as the rhyolites. The rhyolites show well-preserved primary volcanic features such as magma flow textures and euhedral phenocrysts. High emplacement temperatures are indicated by petrographic evidence (β-quartz phenocrysts), zircon saturation temperatures (915-820 °C) and geochemical data, especially high SiO2 (74-79 wt.%) together with elevated Fe2O3(T) ( 3 wt.%), MgO (0.5-1.5 wt.%) and low Al2O3 (extraction and eruption of highly silicic residual liquid formed by crystallization of granitic magma in a relatively shallow (< 10 km) reservoir, now represented by the granite massifs. The granite magma was formed by melting or differentiation of material similar to the diorite gneiss that occurs regionally. The 3.30 Ga volcanic-plutonic systems formed after a period of crustal growth and stabilization of a thick continental lithosphere, represented by massive 3.40-3.33 Ga TTG and medium to high-K calk-alkaline magmatism in the Gavião Block. The 3.30 Ga-old rhyolites and granites would therefore have formed in an intracontinental tectonic setting after the formation and stabilization of new continental crust, and accordingly would represent the first stages of rifting and continental break-up. Intraplate magmatism and intracrustal differentiation processes took place on Earth at 3.3 Ga and produced magmas that were distinct from Archean TTGs, questioning the reliability (or at least the uniqueness) of "intraplate models" to explain the origin of the

  18. Element geochemistry of weathering profile of dolomitite and its implications for the average chemical composition of the upper-continental crust--Case studies from the Xinpu profile,northern Guizhou Province,China

    Institute of Scientific and Technical Information of China (English)

    2000-01-01

    Geochemical behavior of chemical elements is studied in a dolomitite weathering profile in upland of karst terrain in northern Guizhou.Two stages can be recognized during the process of in situ weathering of dolomitite:the stage of sedentary accumulation of leaching residue of dolomitite and the stage of chemical weathering evolution of sedentary soil.Ni,Cr,Mo,W and Ti are the least mobile elements with reference to Al.The geochemical behavior of REE is similar to that observed in weathering of other types of rocks.Fractionation of REE is noticed during weathering,and the two layers of REE enrichments are thought to result from downward movement of the weathering front in response to changes in the environment.It is considered that the chemistry of the upper part of the profile,which was more intensively weathered,is representative of the mobile components of the upper curst at the time the dolomitite was formed,while the less weathered lower profile is chemically representative of the immobile constitution.Like glacial till and loess,the "insoluble" materials in carbonate rocks originating from chemical sedimentation may also provide valuable information about the average chemical composition of the upper continental crust.

  19. Prograde infiltration of Cl-rich fluid into the granulitic continental crust from a collision zone in East Antarctica (Perlebandet, Sør Rondane Mountains)

    Science.gov (United States)

    Kawakami, Tetsuo; Higashino, Fumiko; Skrzypek, Etienne; Satish-Kumar, M.; Grantham, Geoffrey; Tsuchiya, Noriyoshi; Ishikawa, Masahiro; Sakata, Shuhei; Hirata, Takafumi

    2017-03-01

    input of Cl-rich fluids through tectonic boundaries during continental collision.

  20. Layered granitoids: Interaction between continental crust recycling processes and mantle-derived magmatism: Examples from the Évora Massif (Ossa-Morena Zone, southwest Iberia, Portugal)

    Science.gov (United States)

    Moita, Patrícia; Santos, José F.; Pereira, M. Francisco

    2009-08-01

    of the leucogranite magma. The Almansor outcrop is then interpreted as the remnants of a shear zone that operated as a pathway for melts that moved upward through the crust providing the locus for differentiation and mingling/mixing of magmas, whilst Alto de São Bento would correspond to the zone, at a higher crustal level, where magmas were trapped and forced to spread horizontally. At Valverde (the third area) foliated and non-foliated granitoids are spatially related and field criteria links these rocks to metamorphic protolith and anatectic melt, respectively. However, petrographic, geochemical and isotopic information shows that they all are compositionally identical trondhjemites with no evidence of metamorphic fabric. In the foliated rocks, mesoscopic features are interpreted as resulting from melt segregation structures formed in a crystallizing mush. In contrast to the previous areas, the Valverde trondhjemites probably do not belong to the main calc-alkaline plutonic suite of the Évora Massif, since they have a distinct Sr and Nd isotope signature.

  1. Active seismic monitoring of changes of the reflection response of a crystalline shear zone due to fluid injection in the crust at the Continental Deep Drilling Site, Germany

    Science.gov (United States)

    Beilecke, T.; Kurt, B.; Stefan, B.

    2005-12-01

    In theory and in the laboratory variations of the hydraulic pressure can be detected with seismic methods: A lowering of the hydraulic pressure leads to the closure of micro-cracks within the rock (increase of the differential or effective pressure). Subsequently, the seismic velocities increase. An increase of the hydraulic pressure leads to reverse seismic effects. Consequently, seismic impedance contrasts and associated reflection amplitudes vary in the case of a propagating fluid pressure front in a rock matrix with inhomogeneous permeability - as is the case at shear zones. The largest amplitude changes can be expected with vertical ray inclination on the impedance contrast. Generally, the expected effects are small however (Kaselow, 2004). The practical utilization of active seismics for the detection of pressure changes at large scale in hard rock is currently being studied at the Continental Deep Drilling Site (KTB). The injection of water (200 l/min) in a depth of about 4000 m into the so-called SE2 shear zone in the KTB pilot hole was monitored with active seismics between May 2004 and April 2005. The core of the experiment layout is a fixed 5-arm geophone array consisting of 24 3-component geophones, buried at about 70 cm depth. The source signal is a vertical vibrator sweep of 30 s length with the spectrum 30-120 Hz. The signal is sent into the ground 32 times during each cycle, detected with the array and recorded separately for each geophone channel, without prior correlation with the source signal. This allows maximum post-processing with seismic processing and analysis tools and especially permits the use of array properties to increase the signal-to-noise ratio. Critical parameters of the experiment are the repeatability of the source signal as well as the stability of the receiver properties. Another pivot is the hydraulic pressure and its distribution built up within the rock matrix. Estimations based on model calculations show that a change of

  2. Rocas Verdes Ophiolite Complexes in the Southernmost Andes: Remnants of the Mafic Igneous Floor of a Back-arc Basin that Rifted the South American Continental Crust in the Late Jurrassic and Early Cretaceous

    Science.gov (United States)

    Stern, C. R.

    2001-12-01

    The Rocas Verdes are an en echelon group of late Jurassic and early Cretaceous igneous complexes in the southernmost Andes. They consist of mafic pillow lavas, dikes and gabbros interpreted as the upper portions of ophiolite complexes formed along mid-ocean-ridge-type spreading centers. When secondary metamorphic affects are accounted for, the geochemistry of mafic Rocas Verdes rocks are similar to ocean-ridge basalts (MORB). The spreading centers that generated the Rocas Verdes rifted the southwestern margin of the Gondwana continental crust, during the start of break-up in the southern Atlantic, to form the igneous floor of a back-arc basin behind a contemporaneous convergent plate boundary magmatic arc. Late Jurassic and early Cretaceous sediments from both the magmatic arc on the southwest and the continental platform on the northeast of the basin were deposited in the Rocas Verdes basin, and these sediments are interbedded with mafic pillow lavas along the margins of the Rocas Verdes mafic complexes. Also, mafic dikes and gabbros intrude older pre-Andean and Andean lithologies along both flanks of the Rocas Verdes, and leucocratic country rocks are engulfed in the Rocas Verdes mafic complexes. These relations indicate that the Rocas Verdes complexes formed in place and are autochthonous, having been uplifted but not obducted, which may explain the lack of exposure of the deeper ultramafic units. Zircon U/Pb ages of 150+/-1 Ma for the Larsen Harbour Formation, a southern extension of the Rocas Verdes belt on South Georgia Island, and 138+/-2 Ma for the Sarmiento complex, the northernmost in the Rocas Verdes belt, indicate that this basin may have formed by "unzipping" from the south to the north, with the southern portion beginning to form earlier and developing more extensively than the northern portion of the basin. Paleomagnetic data suggest that the Rocas Verdes basin developed in conjunction with the displacement of the Antarctic Peninsula and opening of

  3. Short episodes of crust generation during protracted accretionary processes: Evidence from Central Asian Orogenic Belt, NW China

    Science.gov (United States)

    Tang, Gong-Jian; Chung, Sun-Lin; Hawkesworth, Chris J.; Cawood, P. A.; Wang, Qiang; Wyman, Derek A.; Xu, Yi-Gang; Zhao, Zhen-Hua

    2017-04-01

    Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (∼540-270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ∼400 Ma and 280 Ma for the Altai segment, and ∼430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (∼20-40 Ma), and variations in magma volumes and in Nd-Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting. The Junggar segment is characterized by high and uniform Nd-Hf isotope ratios (εNd (t) = + 5 to + 8; zircon εHf (t) = + 10 to + 16) and it appears to have formed in an intra-oceanic arc system. In the Altai and Chinese Tianshan segments, the Nd-Hf isotope ratios (εNd (t) = - 7 to + 8; zircon εHf (t) = - 16 to + 16) are lower, although they increase with decreasing age of the rock units. The introduction of a juvenile component into the Chinese Tianshan and Altai granitic rocks appears to have occurred in continental arc settings and it reflects a progressive reduction in the contributions from old continental lower crust and lithospheric mantle. Within the long-lived convergent margin setting (over ∼200 Ma), higher volumes of magma, and greater contributions of juvenile material, were typically emplaced over short time intervals of ∼20-40 Ma. These intervals were associated with higher Nb/La ratios, coupled with lower La/Yb ratios, in both the mafic and granitic rocks, and these episodes of increased magmatism from intraplate-like sources are therefore thought to have been in response to lithospheric extension. The trace element and Nd-Hf isotope data, in combination with estimates of granitic magma volumes, highlight

  4. Continental Crust Subduction and Ultrahigh Pressure Metamorphism

    Science.gov (United States)

    Rumble, D.; Liou, J. G.; Jahn, B. M.

    2003-12-01

    Chemical reactions in marine sediments and the resulting fluxes across the sediment-water interface influence the global carbon cycle and the pH of the sea and affect the abundance of CaCO3 and opal-forming plankton in the ocean. On very long timescales these diagenetic reactions control carbon burial in sedimentary rocks and the oxygen content of the atmosphere. Sedimentary deposits that remain after diagenesis are the geochemical artifacts used for interpreting past changes in ocean circulation, biogeochemical cycles, and climate. This chapter is about the processes of diagenesis and burial of the chemical elements that make up the bulk of the particulate matter that reaches the seafloor (organic matter, CaCO3, SiO2, Fe, Mn, and aluminosilicates).Understanding of sediment diagenesis and benthic fluxes has evolved with advances in both experimental methods and modeling. Measurements of chemical concentrations in sediments, their associated pore waters and fluxes at the sediment-water interface have been used to identify the most important reactions. Because transport in pore waters is usually by molecular diffusion, this medium is conducive to interpretation by models of heterogeneous chemical equilibrium and kinetics. Large chemical changes and manageable transport mechanisms have led to elegant models of sediment diagenesis and great advances in understanding of diagenetic processes.We shall see, though, that the environment does not yield totally to simple models of chemical equilibrium and chemical kinetics, and laboratory determined constants often cannot explain the field observations. For example, organic matter degradation rate constants determined from modeling are so variable that there are essentially no constraints on these values from laboratory experiments. In addition, reaction rates of CaCO3 and opal dissolution determined from modeling pore waters usually cannot be reproduced in laboratory experiments of these reactions. The inability to mechanistically understand reaction kinetics calculated from diagenesis models is an important uncertainty in the field today.Processes believed to be most important in controlling the preservation of organic matter have evolved from a focus on the lability of the substrate to the protective mechanisms of mineral-organic matter interactions. The specific electron acceptor is not particularly important during very early diagenesis, but the importance of oxygen to the degradation of organic matter during later stages of diagenesis has been clarified by the study of diagenesis in turbidites deposited on the ocean floor during glacial periods.Evolution of thinking about the importance of reactions between seawater and detrital clay minerals has come full circle since the mid-1960s. "Reverse weathering" reactions were hypothesized in very early chemical equilibrium (Sillen, 1961) and mass-balance ( Mackenzie and Garrels, 1966) models of the oceans. Subsequent observations that marine clay minerals generally resemble those weathered from adjacent land and the discovery of hydrothermal circulation put these ideas on the back burner. Recent studies of silicate and aluminum diagenesis, however, have rekindled awareness of this process, and it is back in the minds of geochemists as a potentially important process for closing the marine mass balance of some elements.

  5. Geophysical and geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia

    Science.gov (United States)

    Guy, Alexandra; Schulmann, Karel; Janoušek, Vojtech; Štípská, Pavla; Armstrong, Robin; Belousova, Elena; Dolgopolova, Alla; Seltmann, Reimar; Lexa, Ondrej; Jiang, Yingde; Hanžl, Pavel

    2016-04-01

    The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the "Khantaishir" arc (520-495Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ±20kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

  6. Divesidad y distribución de los crustáceos decápodos de la franja superior del talud superior continental (300-500 m. de profundidad en la parte norte del mar Caribe colombiano Diversity and distribution of the crustacean decapods of the upper continental slope (300-500m deep in the nort colombian Caribeanean sea

    Directory of Open Access Journals (Sweden)

    Campos Campos Néstor Hernando

    2000-06-01

    Full Text Available Como un primer paso para obtener la información acerca de la diversidad marina de Colombiaen aguas profundas, el INVEMAR desarrolló un proyecto para la caracterización de la ma-crofauna de la franja superior del talud continental del Mar Caribe colombiano. El trabajopresentado cubre la identificaciónn taxonómica, cuantificación, distribución y estructura dela comunidad de los crustáceos decápodos colectados en las ecoregiones Guajira, Palomino,Tayrona y Magdalena, localizadas entre La Guajira y Magdalena en el Mar Caribe colombiano.Para cada ecoregión se hicieron muestreos en seis estaciones, tres a 300 m y tres a 500 m deprofundidad. El material colectado fue identificado y depositado en la Colección de Referenciade Organismos Marinos del INVEMAR y en el Museo de Historia Natural del Instituto de Ciencias Naturales de la Universidad Nacional de Colombia. La información taxonómicapertinente a cada especie se ingresó a la base de datos del Sistema de Información sobreBiodiversidad Marina del INVEMAR e incluyó la información taxonómica de las familias, génerosy especies con imágenes de los ejemplares. La similaridad entre estaciones fue analizada mediantemétodos multivariados de ordenación y clasificación que permitieron agrupar las estaciones. Losgrupos obtenidos fueron comparados paramétricamente en cuanto a su abundancia, riqueza,diversidad, dominancia y equitatividad. Posteriormente se realizó un análisis inverso para definiry caracterizar el ensamblaje de especies de cada grupo de estaciones. Se colectaron en total 6.381ejemplares de crustáceos decápodos pertenecientes a 37 familias y 99 especies, de los cuales48% se registran por primera vez para el Mar Caribe colombiano, además se colectaronejemplares de dos especies que teniendo en cuenta las opiniones de los expertos y según labibliografía y material consultado son nuevas especies, una del género Cymonomoidesy otra delgénero Pyromaia. 77% de la abundancia

  7. Juvenile angiofibroma

    Science.gov (United States)

    Nasal tumor; Angiofibroma - juvenile; Benign nasal tumor; Juvenile nasal angiofibroma; JNA ... Juvenile angiofibroma is not very common. It is most often found in adolescent boys. The tumor contains ...

  8. Tibet and Beyond: Magmatic Records from CIA (Caucasus-Iran-Anatolia) and Southern Tibet with Implications for Asian Orogeny and Continental Growth

    Science.gov (United States)

    Chung, Sun-Lin

    2016-04-01

    This study, based on an ongoing joint research project "Tibet and Beyond", presents a synthesis of principal magmatic records from the CIA (Caucasus-Iran-Anatolia) and Tibet-Himalaya orogens resulting from the continental collisions of Arabia and India, respectively, with Eurasia. In both orogens, through this and other recent studies, the temporal and spatial variations in magmatism pre-, syn- and post-dating the collisions can now be much better defined, thus improving our understanding of collision zone magmatism that appears to have evolved with changes in the lithospheric structures over time and space by collisional processes. The two "collisional" Tethyan orogens were preceded by accretionary orogenic processes, which not only had produced a substantial amount of juvenile continental crust but also fulfill the "orogenic cycle" that evolved from an accretionary into a collisional system. Geochemical data reveal that in contrast to generating vast portions of juvenile crust in the early, accretionary stages of orogenic development, crustal recycling plays a more important role in the later, collisional stages. The latter, as exemplified in SE Turkey and southern Tibet, involves addition of older continental crust material back into the mantle, which subsequently melted and caused compositional transformation of the juvenile crust produced in the accretionary stages. Similar features are observed in young volcanic rocks from eastern Taiwan, the northern Luzon arc complex and part of the active subduction/accretion/collision system in Southeast Asia that may evolve one day to resemble the eastern Tethyan and central Asian orogenic belts by collision with the advancing Australian continent.

  9. The evolution of oceanic 87Sr/86Sr does not rule out early continental growth

    Science.gov (United States)

    Flament, N.; Coltice, N.; Rey, P. F.

    2010-12-01

    evolution of oceanic 87Sr/86Sr is not a suitable proxy for continental growth. We suggest that the delayed appearance of the differentiated reservoir in surface geochemical tracers reflects the emergence of the continents rather than a peak in extraction of juvenile continental crust from the mantle. Therefore, there could be no need for delayed continental growth models. Armstrong, R. L., 1981. Radiogenic isotopes: the case for crustal recycling on a near-steady-state no-continental-growth. Earth. Philos. Trans. R. Soc. London 301, 443-471. Flament, N., Coltice, N., and Rey, P. F., 2008. A case for late-Archaean continental emergence from thermal evolution models and hypsometry. Earth Planet. Sc. Lett. 275, 326-336. Labrosse, S., Jaupart, C., 2007. Thermal evolution of the Earth: Secular changes and fluctuations of plate characteristics. Earth Planet. Sc. Lett. 260, 465-481. Rey, P. F. and Coltice N., 2008. Neoarchean strengthening of the lithosphere and the coupling of the Earth's geochemical reservoirs. Geology 36, 635-638. Taylor, S. R. and McLennan, S. M., 1985. The continental crust: its composition and evolution. Blackwell Scientific Publications, 328 p.

  10. Continental growth seen through the sedimentary record

    Science.gov (United States)

    Dhuime, Bruno; Hawkesworth, Chris J.; Delavault, Hélène; Cawood, Peter A.

    2017-07-01

    Sedimentary rocks and detrital minerals sample large areas of the continental crust, and they are increasingly seen as a reliable archive for its global evolution. This study presents two approaches to model the growth of the continental crust through the sedimentary archive. The first builds on the variations in U-Pb, Hf and O isotopes in global databases of detrital zircons. We show that uncertainty in the Hf isotope composition of the mantle reservoir from which new crust separated, in the 176Lu/177Hf ratio of that new crust, and in the contribution in the databases of zircons that experienced ancient Pb loss(es), adds some uncertainty to the individual Hf model ages, but not to the overall shape of the calculated continental growth curves. The second approach is based on the variation of Nd isotopes in 645 worldwide fine-grained continental sedimentary rocks with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter referred to as K. This dimensionless parameter relates the proportions of younger to older source rocks in the sediment, to the proportions of younger to older source rocks present in the crust from which the sediment was derived. We suggest that a Hadean/Archaean value of K = 1 (i.e., no preferential erosion), and that post-Archaean values of K = 4-6, may be reasonable for the global Earth system. Models built on the detrital zircon and the fine-grained sediment records independently suggest that at least 65% of the present volume of continental crust was established by 3 Ga. The continental crust has been generated continuously, but with a marked decrease in the growth rate at 3 Ga. The period from > 4 Ga to 3 Ga is characterised by relatively high net rates of continental growth (2.9-3.4 km3 yr- 1 on average), which are similar to the rates at which new crust is generated (and destroyed) at the present time. Net growth rates are much lower since 3 Ga (0

  11. Eocene deep crust at Ama Drime, Tibet

    DEFF Research Database (Denmark)

    Kellett, Dawn; Cottle, John; Smit, Matthijs Arjen

    2014-01-01

    for burial of the lower Indian crust beneath Tibet reported from the central-eastern Himalaya. Granulite-facies overprinting followed at ca. 15–13 Ma, as indicated by U-Pb zircon ages. Unlike ultrahigh-pressure eclogites of the northwest Himalaya, the Ama Drime eclogites are not characteristic of rapid...... burial and exhumation of a cold subducted slab. The rocks instead resulted from crustal thickening during the early stages of continental collision, and resided in the lower-middle crust for >20 m.y. before they were exhumed and reheated. These new data provide solid evidence for the Indian crust having...

  12. Thermal models pertaining to continental growth

    Science.gov (United States)

    Morgan, Paul; Ashwal, Lew

    1988-01-01

    Thermal models are important to understanding continental growth as the genesis, stabilization, and possible recycling of continental crust are closely related to the tectonic processes of the earth which are driven primarily by heat. The thermal energy budget of the earth was slowly decreasing since core formation, and thus the energy driving the terrestrial tectonic engine was decreasing. This fundamental observation was used to develop a logic tree defining the options for continental growth throughout earth history.

  13. Dermatomyositis (Juvenile)

    Science.gov (United States)

    ... Am A Patient / Caregiver Diseases & Conditions Dermatomyositis (Juvenile) Dermatomyositis (Juvenile) Fast Facts Patients with JDM have varying ... What are common signs and symptoms of juvenile dermatomyositis? The most common signs and symptoms of JDM ...

  14. Retinoschisis (Juvenile)

    Science.gov (United States)

    ... here Home › Eye Conditions Listen Retinoschisis What is Juvenile Retinoschisis? Juvenile retinoschisis is an inherited disease diagnosed in childhood ... degeneration of the retina. What are the symptoms? Juvenile retinoschisis, also known as X-linked retinoschisis, occurs ...

  15. Early Carboniferous (˜357 Ma) crust beneath northern Arabia: Tales from Tell Thannoun (southern Syria)

    Science.gov (United States)

    Stern, Robert J.; Ren, Minghua; Ali, Kamal; Förster, Hans-Jürgen; Al Safarjalani, Abdulrahman; Nasir, Sobhi; Whitehouse, Martin J.; Leybourne, Matthew I.; Romer, Rolf L.

    2014-05-01

    Continental crust beneath northern Arabia is deeply buried and poorly known. To advance our knowledge of this crust, we studied 8 xenoliths brought to the surface by Neogene eruptions of Tell Thannoun, S. Syria. The xenolith suite consists of two peridotites, one pyroxenite, four mafic granulites, and one charnockite. The four mafic granulites and charnockite are probably samples of the lower crust, and two mafic granulites gave 2-pyroxene equilibration temperatures of 780-800 °C, which we take to reflect temperatures at the time of formation. Peridotite and pyroxenite gave significantly higher temperatures of ∼900 °C, consistent with derivation from the underlying lithospheric mantle. Fe-rich peridotite yielded T∼800 °C, perhaps representing a cumulate layer in the crust. Three samples spanning the lithologic range of the suite (pyroxenite, mafic granulite, and charnockite) yielded indistinguishable concordant U-Pb zircon ages of ∼357 Ma, interpreted to approximate when these magmas crystallized. These igneous rocks are mostly juvenile additions from the mantle, as indicated by low initial 87Sr/86Sr (0.70312 to 0.70510) and strongly positive initial εNd(357 Ma) (+4 to +9.5). Nd model ages range from 0.55 to 0.71 Ga. We were unable to unequivocally infer a tectonic setting where these melts formed: convergent margin, rift, or hotspot. These xenoliths differ from those of Jordan and Saudi Arabia to the south in four principal ways: 1) age, being least 200 Ma younger than the presumed Neoproterozoic (533-1000 Ma) crust beneath Jordan and Saudi Arabia; 2) the presence of charnockite; 3) abundance of Fe-rich mafic and ultramafic lithologies; and 4) the presence of sapphirine. Our studies indicate that northern Arabian plate lithosphere contains a significant proportion of juvenile Late Paleozoic crust, the extent of which remains to be elucidated. This discovery helps explain fission track resetting documented for rocks from Israel and provides insights into

  16. Evidence of a Neoproterozoic active continental margin - Geochemistry and isotope geology of high-grade paragneiss from the Ribeira Orogen, SE Brazil

    Science.gov (United States)

    Capistrano, G. G.; Schmitt, R. S.; Medeiros, S. R.; Fernandes, G. L. F.

    2017-08-01

    Ediacaran paragneisses from the Palmital Unit are located in a key region, between two major tectonic domains of the Ribeira Orogen (in Rio de Janeiro, SE Brazil): the Cabo Frio Tectonic Domain and the Oriental Terrane. We present here petrographic, geochemical and isotopic data in order to partially unravel the origin and tectonic nature of the protoliths from these metamorphic rocks. Litharenites interpreted as immature sediments, mostly derived from the erosion of felsic rocks (granites/rhyolites and diorites/andesites) are here described. Multi-elements patterns and trace elements ratios reinforce an upper continental crust nature for the composition of the protoliths. These were probably located close to the source area and accommodated in semi-arid climate and high topography conditions. Tectonic discrimination diagrams indicate that the Palmital basin developed in an active continental margin, corroborated by the zircon detrital spectra. The main population of detrital zircon (ca. 750-550 Ma) is partially coeval with the age of the Rio Negro continental magmatic arc, resident in the Oriental Terrane. The Palmital basin could represent a forearc environment with no oceanic crust material, but only a continuous sedimentation of turbidites derived from the arc, with gradational bedding signifying a subaqueous environment, without outside tectonic disturbances. On the other hand, TDM ages of 1.6-1.8 Ga suggest that these sediments are not juvenile, indicating also a contribution from an ancient crust. This recycled continental crust could come either from the basement of the Oriental Terrane (which was not identified yet) or from the basement of the Cabo Frio Tectonic Domain. In the last assumption the Palmital deposition would be concomitant with the initiation of continental collision and the subduction of the passive margin of the Cabo Frio Tectonic Domain towards west. This unit was subsequently metamorphosed/deformed during the ca. 540 Ma collision between

  17. Composition of the continental plates

    Science.gov (United States)

    Gilluly, J.

    1954-01-01

    The structures of continental plates and of oceanic basins suggested by several seismologists are utilized to estimate the relative volumes of sial and sima in the earth's crust. It seems that sial of the composition of the average igneous rock constitutes fully 26% and perhaps as much as 43% of the total crust. This ratio is far higher than seems likely if the sial had been entirely derived through fractional crystallization of a basaltic magma. The relative paucity of intermediate rocks as compared with granite and gabbro in the crust points in the same direction. The tentative conclusion is reached that the sial owes a large part of its volume to some process other than fractional crystallization of basalt-possibly to the emanation of low-melting constituents such as water, silica, potassa, soda, and alumina directly from the mantle to the crust. ?? 1954 Springer-Verlag.

  18. Developing the plate tectonics from oceanic subduction to continental collision

    Institute of Scientific and Technical Information of China (English)

    ZHENG YongFei; YE Kai; ZHANG LiFei

    2009-01-01

    The studies of continental deep subduction and ultrahigh-pressure metamorphism have not only promoted the development of solid earth science in China,but also provided an excellent opportunity to advance the plate tectonics theory.In view of the nature of subducted crust,two types of subduction and collision have been respectively recognized in nature.On one hand,the crustal subduction occurs due to underflow of either oceanic crust (Pacific type) or continental crust (Alpine type).On the other hand,the continental collision proceeds by arc-continent collision (Himalaya-Tibet type) or continent-continent collision (Dabie-Sulu type).The key issues in the future study of continental dynamics are the chemical changes and differential exhumation in continental deep subduction zones,and the temporal-spatial transition from oceanic subduction to continental subduction.

  19. The structural evolution of the deep continental lithosphere

    Science.gov (United States)

    Cooper, C. M.; Miller, Meghan S.; Moresi, Louis

    2017-01-01

    of the continental lithosphere through time and implications for the formation and preservation of deep structure. These are contrasted with the dynamical picture of modern day continental growth during lateral accretion of juvenile crust with reference to examples from the Australian Tasmanides and the Alaskan accretionary margin.

  20. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics.

    Science.gov (United States)

    Tang, Ming; Chen, Kang; Rudnick, Roberta L

    2016-01-22

    The Archean Eon witnessed the production of early continental crust, the emergence of life, and fundamental changes to the atmosphere. The nature of the first continental crust, which was the interface between the surface and deep Earth, has been obscured by the weathering, erosion, and tectonism that followed its formation. We used Ni/Co and Cr/Zn ratios in Archean terrigenous sedimentary rocks and Archean igneous/metaigneous rocks to track the bulk MgO composition of the Archean upper continental crust. This crust evolved from a highly mafic bulk composition before 3.0 billion years ago to a felsic bulk composition by 2.5 billion years ago. This compositional change was attended by a fivefold increase in the mass of the upper continental crust due to addition of granitic rocks, suggesting the onset of global plate tectonics at ~3.0 billion years ago. Copyright © 2016, American Association for the Advancement of Science.

  1. Relamination of mafic subducting crust throughout Earth's history

    Science.gov (United States)

    Maunder, Ben; van Hunen, Jeroen; Magni, Valentina; Bouilhol, Pierre

    2016-09-01

    Earth has likely cooled by several hundred degrees over its history, which has probably affected subduction dynamics and associated magmatism. Today, the process of compositional buoyancy driven upwelling, and subsequent underplating, of subducted materials (commonly referred to as ;relamination;) is thought to play a role in the formation of continental crust. Given that Archean continental crust formation is best explained by the involvement of mafic material, we investigate the feasibility of mafic crust relamination under a wide range of conditions applicable to modern and early Earth subduction zones, to assess if such a process might have been viable in an early Earth setting. Our numerical parametric study illustrates that the hotter, thicker-crust conditions of the early Earth favour the upward relamination of mafic subducting crust. The amount of relaminating subducting crust is observed to vary significantly, with subduction convergence rate having the strongest control on the volume of relaminated material. Indeed, removal of the entire mafic crust from the subducting slab is possible for slow subduction (∼2 cm/yr) under Archean conditions. We also observe great variability in the depth at which this separation occurs (80-120 km), with events corresponding to shallower detachment being more voluminous, and that relaminating material has to remain metastably buoyant until this separation depth, which is supported by geological, geophysical and geodynamical observations. Furthermore, this relamination behaviour is commonly episodic with a typical repeat time of approximately 10 Myrs, similar to timescales of episodicity observed in the Archean rock record. We demonstrate that this relamination process can result in the heating of considerable quantities of mafic material (to temperatures in excess of 900 °C), which is then emplaced below the over-riding lithosphere. As such, our results have implications for Archean subduction zone magmatism, for

  2. Early formation of evolved asteroidal crust.

    Science.gov (United States)

    Day, James M D; Ash, Richard D; Liu, Yang; Bellucci, Jeremy J; Rumble, Douglas; McDonough, William F; Walker, Richard J; Taylor, Lawrence A

    2009-01-08

    Mechanisms for the formation of crust on planetary bodies remain poorly understood. It is generally accepted that Earth's andesitic continental crust is the product of plate tectonics, whereas the Moon acquired its feldspar-rich crust by way of plagioclase flotation in a magma ocean. Basaltic meteorites provide evidence that, like the terrestrial planets, some asteroids generated crust and underwent large-scale differentiation processes. Until now, however, no evolved felsic asteroidal crust has been sampled or observed. Here we report age and compositional data for the newly discovered, paired and differentiated meteorites Graves Nunatak (GRA) 06128 and GRA 06129. These meteorites are feldspar-rich, with andesite bulk compositions. Their age of 4.52 +/- 0.06 Gyr demonstrates formation early in Solar System history. The isotopic and elemental compositions, degree of metamorphic re-equilibration and sulphide-rich nature of the meteorites are most consistent with an origin as partial melts from a volatile-rich, oxidized asteroid. GRA 06128 and 06129 are the result of a newly recognized style of evolved crust formation, bearing witness to incomplete differentiation of their parent asteroid and to previously unrecognized diversity of early-formed materials in the Solar System.

  3. Mantle plumes and continental tectonics.

    Science.gov (United States)

    Hill, R I; Campbell, I H; Davies, G F; Griffiths, R W

    1992-04-10

    Mantle plumes and plate tectonics, the result of two distinct modes of convection within the Earth, operate largely independently. Although plumes are secondary in terms of heat transport, they have probably played an important role in continental geology. A new plume starts with a large spherical head that can cause uplift and flood basalt volcanism, and may be responsible for regional-scale metamorphism or crustal melting and varying amounts of crustal extension. Plume heads are followed by narrow tails that give rise to the familiar hot-spot tracks. The cumulative effect of processes associated with tail volcanism may also significantly affect continental crust.

  4. Crust rheology, slab detachment and topography

    Science.gov (United States)

    Duretz, T.; Gerya, T. V.

    2012-04-01

    The collision between continents following the closure of an ocean can lead to the subduction of continental crust. The introduction of buoyant crust within subduction zones triggers the development of extensional stresses in slabs which eventually result in their detachment. The dynamic consequences of slab detachment affects the development of topography, the exhumation of high-pressure rocks and the geodynamic evolution of collision zones. We employ two-dimensional thermo-mechanical modelling in order to study the importance of crustal rheology on the evolution of spontaneous subduction-collision systems and the occurrence of slab detachment. The modelling results indicate that varying the rheological structure of the crust can results in a broad range of collisional evolutions involving slab detachment, delamination (associated to slab rollback), or the combination of both mechanisms. By enhancing mechanical coupling at the Moho, a strong crust leads to the deep subduction of the crust (180 km). These collisions are subjected to slab detachment and subsequent coherent exhumation of the crust accommodated by eduction (inversion of subduction sense) and thrusting. In these conditions, slab detachment promotes the development of a high (> 4.5 km) and narrow (delamination of the lithosphere, preventing slab detachment to occur. Further shortening leads to buckling and thickening of the crust resulting in the development of topographic bulging on the lower plate. Collisions involving rheologically layered crust are characterised by a decoupling level at mid-crustal depths. These initial condition favours the delamination of the upper crust as well as the deep subduction of the lower crust. These collisions are thus successively affected by delamination and slab detachment and both processes contribute to the exhumation of the subducted crust. A wide (> 200 km) topographic plateau develops as the results of the buoyant extrusion of the upper crust onto the foreland

  5. Juvenile Scleroderma

    Science.gov (United States)

    Juvenile Scleroderma INTRODUCTION Every parent will experience a moment of panic when told their child has scleroderma. ... in all their family members as well. CONCLUSION Juvenile scleroderma can be unsettling for the child and ...

  6. Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago.

    Science.gov (United States)

    Næraa, T; Scherstén, A; Rosing, M T; Kemp, A I S; Hoffmann, J E; Kokfelt, T F; Whitehouse, M J

    2012-05-30

    Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate

  7. U-Pb SHRIMP and Sm-Nd geochronology of the Silvânia Volcanics and Jurubatuba Granite: juvenile Paleoproterozoic crust in the basement of the Neoproterozoic Brasília Belt, Goiás, central Brazil

    Directory of Open Access Journals (Sweden)

    FISCHEL DANIELLE P.

    2001-01-01

    Full Text Available U-Pb SHRIMP and Sm-Nd isotopic ages were determined for felsic metavolcanic rocks from the Silvânia Sequence and Jurubatuba Granite in the central part of the Brasília Belt. Zircon grains from a metavolcanic sample yielded 2115 ± 23 Ma and from the granite yielded 2089 ± 14 Ma, interpreted as crystallization ages of these rocks. Six metavolcanic samples of the Silvânia Sequence yielded a six-point whole-rock Sm-Nd isochron indicating a crystallization age of 2262 ± 110 Ma and positive epsilonNd(T = +3.0 interpreted as a juvenile magmatic event. Nd isotopic analyses on samples from the Jurubatuba Granite have Paleoproterozoic T DM model ages between 2.30 and 2.42 Ga and epsilonNd(T values vary between -0.22 and -0.58. The oldest T DM value refers to a sedimentary xenolith in the granite. These results suggest crystallization ages of Silvânia volcanics and Jurubatuba Granite are the first evidence of a ca. 2.14-2.08 juvenile magmatic event in the basement of the central part of the Brasília Belt that implies the presence of arc/suture hidden in reworked basement of the Brasília Belt.

  8. Coupled zircon Lu-Hf and U-Pb isotopic analyses of the oldest terrestrial crust, the >4.03 Ga Acasta Gneiss Complex

    Science.gov (United States)

    Bauer, Ann M.; Fisher, Christopher M.; Vervoort, Jeffrey D.; Bowring, Samuel A.

    2017-01-01

    The Acasta Gneiss Complex of the Northwest Territories, Canada, contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust-forming processes on the early Earth. Here we report the results of a combined Lu-Hf and U-Pb isotopic study of zircons from predominantly felsic orthogneisses from the Acasta Gneiss Complex that crystallized between ∼4.0 and 2.9 Ga, many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and Hf isotopic domains. To ensure the reliability of the analyses and of subsequent geologic interpretations, we employed the laser ablation split-stream (LASS) technique to concurrently measure the Lu-Hf and U-Pb isotopic systems in zircon. Our results confirm prior findings of precursor Hadean crust (>4.0 Ga) in the source of these rocks and the continued involvement of this reservoir until ∼3.6 Ga. We present evidence for the input of relatively more juvenile material at ∼3.6 Ga, which we suggest corresponds to a fundamental change in the source of the magmas. This study extends the lower bound of the published Acasta Hf isotopic record from 3.6 Ga to 2.9 Ga and demonstrates that the ∼3.6 Ga-2.9 Ga interval is largely represented by reworking of relatively juvenile ∼3.6 Ga crust and the diminution of the >4.0 Ga crustal signal. Significantly, there is no evidence that rocks within the Acasta Gneiss Complex were derived from a strongly depleted mantle.

  9. Re-Os Isotopes Distinguish Crust vs. Slab Inputs to Northern Cascade Arc Basalts

    Science.gov (United States)

    Mullen, E.; Gannoun, A.; Nauret, F.; Schiano, P.; Weis, D.

    2015-12-01

    Delineating the relative contributions of mantle, slab, and crust to arc magmas is particularly challenging in the Cascades where the continental crust is juvenile and contrasts little with magmas in traditional radiogenic isotope systems (Sr-Nd-Hf-Pb). The Re-Os isotope system offers a sensitive technique for evaluating these contributions because even young crust has significantly higher Os ratios than the mantle. We analyzed Re-Os isotope ratios in 33 primitive basalts from 9 volcanic centers of the northern Cascade Arc (Garibaldi Volcanic Belt, GVB). Although GVB basalts have mantle-like Sr-Pb-Nd-Hf ratios (Mullen & Weis, 2015, EPSL), the range in 187Os/188Os is very large (0.13-0.99) with [Os] of Cinder Cone/Mt. Garibaldi, Salal Glacier, Mt. Meager, Indian Pass/Glacier Peak) has low Os isotopic ratios (0.13-0.19), only slightly elevated relative to global mantle wedge peridotites (≤0.16), indicating minimal crustal contamination. Group 1 samples lie on Os-Sr isotope mixing curves indicating variable sediment input to the mantle. Os ratios of Group 2 basalts (Silverthrone, Bridge River, Elaho, Cheakamus, Mt. Baker, Glacier Peak) extend to extremely high values, up to 0.99, and lie on different Os-Sr mixing curves indicating addition of a crustal contaminant. Sr-Nd-Hf-Pb isotopic ratios cannot be used to identify this crustal input because Groups 1 and 2 are identical in these isotope systems. Interaction with a mafic underplate from older Cenozoic or accreted Mesozoic arcs is likely, and Re-Os analyses of basement samples (in progress) will provide a test of this hypothesis. This study reveals that most primitive magmas in the Cascades have suffered variable crustal contamination, but only the Re-Os isotope system has the potential to delineate the extent of this involvement.

  10. Crustal growth at active continental margins: Numerical modeling

    NARCIS (Netherlands)

    Vogt, Katharina|info:eu-repo/dai/nl/370618947; Gerya, Taras; Castro, Antonio

    2012-01-01

    The dynamics and melt sources for crustal growth at active continental margins are analyzed by using a 2D coupled petrological–thermomechanical numerical model of an oceanic-continental subduction zone. This model includes spontaneous slab retreat and bending, dehydration of subducted crust, aqueous

  11. Juvenile Judge

    Institute of Scientific and Technical Information of China (English)

    1997-01-01

    SHANG Xiuyun was among the first sitting judges when the juvenile court was set up in Beijing 10 years ago. With enriched experience she has altered the way judges ask questions in court. She began the practice of inviting juvenile offenders, their parents, relatives, friends and teachers to the juvenile court to work hand in hand in dealing with cases: Facing their relatives and friends and hearing their heartfelt words, juvenile offenders would often be touched, thus bringing forth a positive attitude toward life.

  12. Moroccan crustal response to continental drift.

    Science.gov (United States)

    Kanes, W H; Saadi, M; Ehrlich, E; Alem, A

    1973-06-01

    The formation and development of a zone of spreading beneath the continental crust resulted in the breakup of Pangea and formation of the Atlantic Ocean. The crust of Morocco bears an extremely complete record of the crustal response to this episode of mantle dynamics. Structural and related depositional patterns indicate that the African margin had stabilized by the Middle Jurassic as a marine carbonate environment; that it was dominated by tensile stresses in the early Mesozoic, resulting in two fault systems paralleling the Atlantic and Mediterranean margins and a basin and range structural-depositional style; and that it was affected by late Paleozoic metamorphism and intrusion. Mesozoic events record the latter portion of African involvement in the spreading episode; late Paleozoic thermal orogenesis might reflect the earlier events in the initiation of the spreading center and its development beneath significant continental crust. In that case, more than 100 million years were required for mantle dynamics to break up Pangea.

  13. Evaluating the relative roles of crustal growth and recycling through continental arc magmatism in the Ross orogen, Antarctica

    Science.gov (United States)

    Hagen-Peter, Graham; Cottle, John

    2017-04-01

    There remains much debate about the mechanisms of the growth and differentiation of continental crust over geologic time, although the geochemical resemblance between continental crust and arc magmatism around the world make subduction-related magmatism a conspicuous candidate. It's clear that both juvenile magmatism and crustal recycling occur at convergent margins, but is it difficult to quantify the roles of these two end-member processes. This is particularly challenging in continental arc settings, where magmas ascend through and interact with thick continental lithosphere of variable—and usually unknown—age and composition. We assess the relative magnitudes of crustal growth and recycling in a 500-km-long segment of the Ross orogen of Antarctica—an archetypal example of a long-lived "Cordilleran-style" continental arc—utilizing an extensive set of zircon Hf (˜70 samples) and whole rock Nd and Sr (15 samples) isotopic data for igneous rocks ranging from gabbro to granite. Initial ɛNd and 87Sr/86Sr values range from +0.1 to -10.3 and ˜0.7044 to 0.7137, respectively. Initial ɛHf values (weighted means of individual analyses from each sample) are predominately negative—ranging from +3.5 to -12.3—potentially interpreted as reflecting a dominant crustal component in the source of the granitoids. However, inherited zircon domains provide evidence for significantly less radiogenic ancient crust in the unexposed basement of the orogen. Additionally, primitive samples (SiO2 69) range in initial ɛHf from approximately +0.8 to -4.0, representing juvenile magmatism with enriched isotopic compositions. While a broader range and lower ɛHf values (+3.5 to -12.3) in more evolved samples from large granitic plutons likely reflect variable degrees of crustal assimilation during differentiation, overlap with the isotopic compositions of primitive samples permits differentiation with relatively minor degrees of crustal assimilation. This qualitative

  14. Evolution of the earth's crust: Evidence from comparative planetology

    Science.gov (United States)

    Lowman, P. D., Jr.

    1973-01-01

    Geochemical data and orbital photography from Apollo, Mariner, and Venera missions were combined with terrestrial geologic evidence to study the problem of why the earth has two contrasting types of crust (oceanic and continental). The following outline of terrestrial crustal evolution is proposed. A global crust of intermediate to acidic composition, high in aluminum, was formed by igneous processes early in the earth's history; portions survive in some shield areas as granitic and anorthositic gneisses. This crust was fractured by major impacts and tectonic processes, followed by basaltic eruptions analogous to the lunar maria and the smooth plains of the north hemisphere of Mars. Seafloor spreading and subduction ensued, during which portions of the early continental crust and sediments derived therefrom were thrust under the remaining continental crust. The process is exemplified today in regions such as the Andes/Peru-Chile trench system. Underplating may have been roughly concentric, and the higher radioactive element content of the underplated sialic material could thus eventually cause concentric zones of regional metamorphism and magmatism.

  15. Filling in the juvenile magmatic gap: Evidence for uninterrupted Paleoproterozoic plate tectonics

    Science.gov (United States)

    Partin, C. A.; Bekker, A.; Sylvester, P. J.; Wodicka, N.; Stern, R. A.; Chacko, T.; Heaman, L. M.

    2014-02-01

    Despite several decades of research on growth of the continental crust, it remains unclear whether the production of juvenile continental crust has been continuous or episodic throughout the Precambrian. Models for episodic crustal growth have gained traction recently through compilations of global U-Pb zircon age frequency distributions interpreted to delineate peaks and lulls in crustal growth through geologic time. One such apparent trough in zircon age frequency distributions between ∼2.45 and 2.22 Ga is thought to represent a pause in crustal addition, resulting from a global shutdown of magmatic and tectonic processes. The ∼2.45-2.22 Ga magmatic shutdown model envisions a causal relationship between the cessation of plate tectonics and accumulation of atmospheric oxygen over the same period. Here, we present new coupled U-Pb, Hf, and O isotope data for detrital and magmatic zircon from the western Churchill Province and Trans-Hudson orogen of Canada, covering an area of approximately 1.3 million km2, that demonstrate significant juvenile crustal production during the ∼2.45-2.22 Ga time interval, and thereby argue against the magmatic shutdown hypothesis. Our data is corroborated by literature data showing an extensive 2.22-2.45 Ga record in both detrital and magmatic rocks on every continent, and suggests that the operation of plate tectonics continued throughout the early Paleoproterozoic, while atmospheric oxygen rose over the same time interval. We argue that uninterrupted plate tectonics between ∼2.45 and 2.22 Ga would have contributed to efficient burial of organic matter and sedimentary pyrite, and the consequent rise in atmospheric oxygen documented for this time interval.

  16. Dynamics of continental accretion.

    Science.gov (United States)

    Moresi, L; Betts, P G; Miller, M S; Cayley, R A

    2014-04-10

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.

  17. Juvenile Arthritis

    Science.gov (United States)

    Juvenile arthritis (JA) is arthritis that happens in children. It causes joint swelling, pain, stiffness, and loss of motion. It can affect any joint, but ... of JA that children get is juvenile idiopathic arthritis. There are several other forms of arthritis affecting ...

  18. Icelandic-type crust

    Science.gov (United States)

    Foulger, G.R.; Du, Z.; Julian, B.R.

    2003-01-01

    Numerous seismic studies, in particular using receiver functions and explosion seismology, have provided a detailed picture of the structure and thickness of the crust beneath the Iceland transverse ridge. We review the results and propose a structural model that is consistent with all the observations. The upper crust is typically 7 ?? 1 km thick, heterogeneous and has high velocity gradients. The lower crust is typically 15-30 ?? 5 km thick and begins where the velocity gradient decreases radically. This generally occurs at the V p ??? 6.5 km s-1 level. A low-velocity zone ??? 10 000 km2 in area and up to ??? 15 km thick occupies the lower crust beneath central Iceland, and may represent a submerged, trapped oceanic microplate. The crust-mantle boundary is a transition zone ???5 ?? 3 km thick throughout which V p increases progressively from ???7.2 to ???8.0 km s-1. It may be gradational or a zone of alternating high- and low-velocity layers. There is no seismic evidence for melt or exceptionally high temperatures in or near this zone. Isostasy indicates that the density contrast between the lower crust and the mantle is only ???90 kg m-3 compared with ???300 kg m-3 for normal oceanic crust, indicating compositional anomalies that are as yet not understood. The seismological crust is ???30 km thick beneath the Greenland-Iceland and Iceland-Faeroe ridges, and eastern Iceland, ???20 km beneath western Iceland, and ???40 km thick beneath central Iceland. This pattern is not what is predicted for an eastward-migrating plume. Low attenuation and normal V p/V s ratios in the lower crust beneath central and southwestern Iceland, and normal uppermost mantle velocities in general, suggest that the crust and uppermost mantle are subsolidus and cooler than at equivalent depths beneath the East Pacific Rise. Seismic data from Iceland have historically been interpreted both in terms of thin-hot and thick-cold crust models, both of which have been cited as supporting the plume

  19. [Juvenile scleroderma].

    Science.gov (United States)

    de Mâcedo, Patrícia Andrade; Shinjo, Samuel Katsuyuki; Goldenstein-Schainberg, Cláudia

    2008-01-01

    Juvenile scleroderma is a rare childhood condition characterized by fibrosis of the skin and internal organs. Clinical manifestations of childhood scleroderma are different from adult disease and early recognition, correct classification and treatment can improve long-term outcome. This review explores the most recent actualizations on clinical manifestations, classification criteria, treatment options and prognosis of juvenile scleroderma. There are two main forms of the disease: localized scleroderma and systemic sclerosis. Localized scleroderma is the most common form in children and mostly restricted to the skin. Juvenile diffuse systemic sclerosis is related to visceral involvement and cardiac disease which is the main cause of death in these patients. The outcome of juvenile systemic sclerosis is better compared with the adult form. Treatment remains a medical challenge and the EULAR task force proposed an approach to juvenile scleroderma treatment based on expert's opinion and guidelines used for the treatment of adults. Larger studies on childhood scleroderma are warranted.

  20. The continental lithosphere

    DEFF Research Database (Denmark)

    Artemieva, Irina

    2009-01-01

    The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle are consist......The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle...... are consistent with modern geophysical data. In the lithospheric mantle of the continents, seismic velocity variations of a non-thermal origin (calculated from global Vs seismic tomography data [Grand S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philosophical Transactions...... of the Royal Society of London. Series A, 360, 2475–2491.; Shapiro N.M., Ritzwoller M.H. 2002. Monte-Carlo inversion for a global shear velocity model of the crust and upper mantle. Geophysical Journal International 151, 1–18.] and lithospheric temperatures [Artemieva I.M., Mooney W.D., 2001. Thermal structure...

  1. Moho vs crust-mantle boundary: Evolution of an idea

    Science.gov (United States)

    O'Reilly, Suzanne Y.; Griffin, W. L.

    2013-12-01

    The concept that the Mohorovicic Discontinuity (Moho) does not necessarily coincide with the base of the continental crust as defined by rock-type compositions was introduced in the early 1980s. This had an important impact on understanding the nature of the crust-mantle boundary using information from seismology and from deep-seated samples brought to the surface as xenoliths in magmas, or as tectonic terranes. The use of empirically-constrained P-T estimates to plot the locus of temperature vs depth for xenoliths defined a variety of geotherms depending on tectonic environment. The xenolith geotherms provided a framework for constructing lithological sections through the deep lithosphere, and revealed that the crust-mantle boundary in off-craton regions commonly is transitional over a depth range of about 5-20 km. Early seismic-reflection data showed common layering near the Moho, correlating with the petrological observation of multiple episodes of basaltic intrusion around the crust-mantle boundary. Developments in seismology, petrophysics and experimental petrology have refined interpretation of lithospheric domains. The expansion of in situ geochronology (especially zircon U-Pb ages and Hf-isotopes; Os isotopes of mantle sulfides) has defined tectonic events that affected whole crust-mantle sections, and revealed that the crust-mantle boundary can change in depth through time. However, the nature of the crust-mantle boundary in cratonic regions remains enigmatic, mainly due to lack of key xenoliths or exposed sections. The observation that the Moho may lie significantly deeper than the crust-mantle boundary has important implications for modeling the volume of the crust. Mapping the crust using seismic techniques alone, without consideration of the petrological problems, may lead to an overestimation of crustal thickness by 15-30%. This will propagate to large uncertainties in the calculation of elemental mass balances relevant to crust-formation processes

  2. The nature of orogenic crust in the central Andes

    Science.gov (United States)

    Beck, Susan L.; Zandt, George

    2002-10-01

    The central Andes (16°-22°S) are part of an active continental margin mountain belt and the result of shortening of the weak western edge of South America between the strong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield. We have combined receiver function and surface wave dispersion results from the BANJO-SEDA project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure. The major results are as follows: (1) The crust supporting the high elevations is thick and has a felsic to intermediate bulk composition. (2) The relatively strong Brazilian lithosphere is underthrusting as far west (65.5°W) as the high elevations of the western part of the Eastern Cordillera (EC) but does not underthrust the entire Altiplano. (3) The subcrustal lithosphere is delaminating piecemeal under the Altiplano-EC boundary but is not completely removed beneath the central Altiplano. The Altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation, leading to lower crustal flow and flat topography. In contrast, in the high-relief, inland-sloping regions of the EC and sub-Andean zone, the upper crust is still strongly coupled across the basal thrust of the fold-thrust belt to the underthrusting Brazilian Shield lithosphere. Subcrustal shortening between the Altiplano and Brazilian lithosphere appears to be accommodated by delamination near the Altiplano-EC boundary. Our study suggests that orogenic reworking may be an important part of the "felsification" of continental crust.

  3. Early Mesozoic deep-crust reworking beneath the central Lhasa terrane (South Tibet): Evidence from intermediate gneiss xenoliths in granites

    Science.gov (United States)

    Zhou, Xiang; Zheng, Jian-Ping; Xiong, Qing; Yang, Jing-Sui; Wu, Yuan-Bao; Zhao, Jun-Hong; Griffin, William L.; Dai, Hong-Kun

    2017-03-01

    Understanding the rheological behavior of the Tibetan Plateau and its response to geodynamic processes requires a clear knowledge of the composition, evolution and lithological properties of the deep crust. Here we present U-Pb-Hf isotopes of zircons, bulk-rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host Early Mesozoic granites (205 ± 6 Ma) in the central Lhasa terrane to probe the deep crust beneath Southern Tibet. The xenoliths contain plagioclase, amphibole, titanite, allanite, quartz, biotite and muscovite, with accessory Fe-Ti oxides, apatite and zircon. Bulk-rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite-facies metamorphism (equilibration at pressures of 0.46-0.83 GPa and temperatures of 650 °C), before they were captured by the host granite at 205 Ma. Zircons in these xenoliths show complex microstructures, including inherited cores, magmatic or metamorphic bands, and high U-Th hydrothermal rims. Inherited zircon cores record U-Pb ages from 2277 Ma to 517 Ma. Igneous zircons show a range of concordant U-Pb ages, suggesting a protracted magmatism from 236 Ma to 203 Ma. Metamorphic zircon zones record the timing of amphibolite-facies metamorphism from 224 to 192 Ma, while the high U-Th hydrothermal rims show a subsequent fluid activity until 150 Ma. Unradiogenic Hf isotopic compositions of both xenoliths and host granites [xenolith εHf(t) = - 11.2 to 0; host granite εHf(t) = - 17.3 to - 3.3] indicate that the Early Mesozoic deep crust in the central Lhasa terrane originated mainly from ancient (i.e., Proterozoic) crust, with little or no interaction with juvenile magmas. This study suggests a possible continental differentiation mechanism during crustal reworking; progressive melting may initiate from the lower mafic crust (at ca. 236 Ma) and gradually migrate into the sediment-rich upper crust (until ca. 203 Ma). The reworking

  4. Juvenile Prostitution.

    Science.gov (United States)

    Csapo, Marg

    1986-01-01

    Recent research and Canadian government committee reports concerning juvenile prostitution are reviewed. Proposals are made in the realms of law and social policy; and existing programs are described. (DB)

  5. Juvenile Prostitution.

    Science.gov (United States)

    Csapo, Marg

    1986-01-01

    Recent research and Canadian government committee reports concerning juvenile prostitution are reviewed. Proposals are made in the realms of law and social policy; and existing programs are described. (DB)

  6. Lithospheric structure and continental geodynamics

    Institute of Scientific and Technical Information of China (English)

    许忠淮; 石耀霖

    2003-01-01

    This paper briefly reviews main progress in the research on lithospheric structure and continental geodynamics made by Chinese geophysicists during last 4 years since 22nd IUGG general assembly in July 1999. The research mainly covers the following fields: investigations on regional lithospheric structure, DSS survey of crust and upper mantle velocity structure, study on present-day inner movement and deformation of Chinese mainland by analyzing GPS observations, geodynamics of Qingzang plateau, geophysical survey of the Dabie-Sulu ultra-high pressure metamorphic belt and probing into its formation mechanism, geophysical observations in sedimentary basins and study on their evolution process, and plate dynamics, etc.

  7. Juvenile myasthenia

    Directory of Open Access Journals (Sweden)

    Knežević-Pogančev Marija

    2011-01-01

    Full Text Available Introduction. Juvenile myasthenia is a chronic autoimmune neuromuscular disease characterized by varying degrees of fluctuating, painless muscle weakness and rapid fatigue of any muscles under voluntary control. Juvenile myasthenia is a form of myasthenia appearing in adolescent age, representing 10% to 15% of all cases of myasthenia gravis. Juvenile myasthenia is presented by a defect in the transmission of nerve impulses to muscles, resulting from a breakdown in the normal communication between nerves and muscles. In myasthenia, antibodies produced by the body’s own immune system block, alter, or destroy the receptors for acetylcholine. Juvenile myasthenia is neither directly inherited nor is it contagious. Signs and Symptoms. The first noticeable symptoms may be eye muscle weakness, difficulty in swallowing, or slurred speech. Juvenile myasthenia usually affects muscles innervated by the cranial nerves (face, lips, tongue, neck and throat, but it can affect any muscle group. Symptoms vary in type and severity with typical periods of exacerbation interspersed with periods of remission. When the muscles necessary for breathing are affected, a patient is said to be in a myasthenic crisis, which is a life-threatening situation. Disease Outcome and Treatment. Juvenile myasthenia produces sporadic but progressive weakness and abnormal fatigability of striated (skeletal muscles, exacerbated by exercise and repeated movement, but improved by rest and anticholinesterase drugs. Juvenile myasthenia follows an unpredictable course of recurring exacerbations and periodic remissions. With current therapies, however, most cases of juvenile myasthenia are not as serious as the name implies. Although there is no known cure, drug treatment has improved prognosis and allows patients to lead relatively normal lives, except during exacerbations.

  8. Interpretation of free-air gravity anomaly data for determining the crustal structure across the continental margins and aseismic ridges: Some examples from Indian continental margins and deep-sea basins

    Digital Repository Service at National Institute of Oceanography (India)

    Ramana, M.V.

    of Alaska and Japan trench, east of the Japanese Islands. Gravity anomalies across continental margins Continental margins are at or near to the transition zone between continental and oceanic crusts. Passive margins are commonly in isostatic.... Generalized bathymetry map of the Indian continental margins After Mishra et al., (2004) The western and eastern margins of India are classified under passive/Atlantic type continental margins, and the structural architecture is similar to any...

  9. Tracking the thermal properties of the lower continental crust

    DEFF Research Database (Denmark)

    Ray, Labani; Förster, Hans-Jürgen; Förster, Andrea

    2015-01-01

    terms of sample size or physical-chemical-mechanical condition) are not available, bulk TC of high-grade metamorphic rocks with low anisotropy and porosity could be satisfactorily good assessed from modal mineralogy, using the data sets for mineral TC applied in this study. Further work is required......In this study, the bulk thermal conductivity (TC) of 26 rock samples representing different types of granulite-facies rocks, i.e., felsic, intermediate and mafic granulites, from the Southern Granulite Province, India, is measured at dry and saturated conditions with the optical-scanning method....... Thermal conductivity is also calculated from modal mineralogy (determined by XRD and EPMA), applying several mixing models commonly used in thermal studies. Most rocks are fine- to medium-grained equigranular in texture. All samples are isotropic to weakly anisotropic and possess low porosities (

  10. Workshop on the Deep Continental Crust of South India

    Science.gov (United States)

    Ashwal, Lewis D. (Editor)

    1988-01-01

    The various theories in the field on the Southern Indian granulite formation are discussed in this workshop proceeding. That some widely accepted ideas about the origin of granulites, particularly relating to the role of metamorphic fluids, will have to be modified, is one outcome of the workshop. Extended abstracts of the papers presented, summaries of the attendant discussions, up-to-date accounts of the geology of the South Indian Precambrian Shield, and detailed field trip guides to all areas visited are presented. It should serve as a convenient source of information and reference to those interested in the classic Precambrian high grade terrains on Earth.

  11. Mineralization through geologic time: Evolution of continental crust

    Science.gov (United States)

    Veizer, Jan; Laznicka, Peter; Jansen, S. L.

    1988-01-01

    In analogy to living systems, geologic entities (e.g., rocks, mineral deposits, tectonic realms and domains) are involved in the process of perpetual generation and destruction (birth/death cycles). This results in time distribution patterns akin to age structures in living populations and the systematics is amenable to treatment by the concepts of population dynamics. Utilizing this theoretical approach, the survivorship patterns for major realms of the plate tectonic system, for consitutent rocks, and for the entombed mineral resources are predicted. The present inventory encompasses global economic accumulations of metals by geologic age. The deposits of these metals were assigned to nine broad genetic categories, with an attempt to relate each category to tectonic setting within the framework of global plate tectonics.

  12. Thermochemical structure of the Earth's mantle and continental crust

    DEFF Research Database (Denmark)

    Guerri, Mattia

    A detailed knowledge of the Earth's thermal structure and chemical composition is fundamental in order to understand the processes driving the planet ormation and evolution. The inaccessibility of most of the Earth's interior makes the determination of its thermo-chemical conditions a challenging...... in determining crustal seismic discontinuities. In the second chapter, I deal about the possibility to disentangle the dynamic and isostatic contribution in shaping the Earth's surface topography. Dynamic topography is directly linked to mantle convection driven by mantle thermo-chemical anomalies, and can...... argue therefore that our understandings of the lithosphere density structure, needed to determine the isostatic topography, and of the mantle density and viscosity, required to compute the dynamic topography, are still too limited to allow a robust determination of mantle convection effects on the Earth...

  13. Mineralization through geologic time: Evolution of continental crust

    Science.gov (United States)

    Veizer, Jan; Laznicka, Peter; Jansen, S. L.

    In analogy to living systems, geologic entities (e.g., rocks, mineral deposits, tectonic realms and domains) are involved in the process of perpetual generation and destruction (birth/death cycles). This results in time distribution patterns akin to age structures in living populations and the systematics is amenable to treatment by the concepts of population dynamics. Utilizing this theoretical approach, the survivorship patterns for major realms of the plate tectonic system, for consitutent rocks, and for the entombed mineral resources are predicted. The present inventory encompasses global economic accumulations of metals by geologic age. The deposits of these metals were assigned to nine broad genetic categories, with an attempt to relate each category to tectonic setting within the framework of global plate tectonics.

  14. Juvenile idiopathic arthritis

    Science.gov (United States)

    Juvenile rheumatoid arthritis (JRA); Juvenile chronic polyarthritis; Still disease; Juvenile spondyloarthritis ... The cause of juvenile idiopathic arthritis (JIA) is not known. It ... illness . This means the body attacks and destroys healthy body ...

  15. Juvenile Polyposis Syndrome

    Science.gov (United States)

    ... Types of Cancer > Juvenile Polyposis Syndrome Request Permissions Juvenile Polyposis Syndrome Approved by the Cancer.Net Editorial Board , 12/2015 What is juvenile polyposis syndrome? Juvenile polyposis syndrome (JPS) is a ...

  16. Continental underplating after slab break-off

    Science.gov (United States)

    Magni, V.; Allen, M. B.; van Hunen, J.; Bouilhol, P.

    2017-09-01

    We present three-dimensional numerical models to investigate the dynamics of continental collision, and in particular what happens to the subducted continental lithosphere after oceanic slab break-off. We find that in some scenarios the subducting continental lithosphere underthrusts the overriding plate not immediately after it enters the trench, but after oceanic slab break-off. In this case, the continental plate first subducts with a steep angle and then, after the slab breaks off at depth, it rises back towards the surface and flattens below the overriding plate, forming a thick horizontal layer of continental crust that extends for about 200 km beyond the suture. This type of behaviour depends on the width of the oceanic plate marginal to the collision zone: wide oceanic margins promote continental underplating and marginal back-arc basins; narrow margins do not show such underplating unless a far field force is applied. Our models show that, as the subducted continental lithosphere rises, the mantle wedge progressively migrates away from the suture and the continental crust heats up, reaching temperatures >900 °C. This heating might lead to crustal melting, and resultant magmatism. We observe a sharp peak in the overriding plate rock uplift right after the occurrence of slab break-off. Afterwards, during underplating, the maximum rock uplift is smaller, but the affected area is much wider (up to 350 km). These results can be used to explain the dynamics that led to the present-day crustal configuration of the India-Eurasia collision zone and its consequences for the regional tectonic and magmatic evolution.

  17. FAST TRACK PAPER: Older crust underlies Iceland

    Science.gov (United States)

    Foulger, G. R.

    2006-05-01

    The oldest rocks outcropping in northwest Iceland are ~16 Myr old and in east Iceland ~13 Myr. The full plate spreading rate in this region during the Cenozoic has been ~2 cm a-1, and thus these rocks are expected to be separated by ~290 km. They are, however, ~500 km apart. The conclusion is inescapable that an expanse of older crust ~210 km wide underlies Iceland, submerged beneath younger lavas. This conclusion is independent of any considerations regarding spreading ridge migrations, jumps, the simultaneous existence of multiple active ridges, three-dimensionality, or subsidence of the lava pile. Such complexities bear on the distribution and age of the older crust, but not on its existence or its width. If it is entirely oceanic its maximum age is most likely 26-37 Ma. It is at least 150 km in north-south extent, but may taper and extend beneath south Iceland. Part of it might be continental-a southerly extension of the Jan Mayen microcontinent. This older crust contributes significantly to crustal thickness beneath Iceland and the ~40 km local thickness measured seismically is thus probably an overestimate of present-day steady-state crustal production at Iceland.

  18. Ca isotope fingerprints of early crust-mantle evolution

    Science.gov (United States)

    Kreissig, K.; Elliott, T.

    2005-01-01

    The utility of 40Ca/ 44Ca as a tracer of pre-existing crustal contributions in early Archaean cratons has been explored to identify traces of Hadean crust and to assess the style of continental growth. The relatively short half-life of 40K (˜1.3 Gy) means that its decay to 40Ca occurs dominantly during early Earth History. If Archaean crust had a significant component derived from a more ancient protolith, as anticipated by "steady state" crustal evolution models, this should be clearly reflected in radiogenic 40Ca/ 44Ca ratios (or positive initial ɛ Ca) in different Archaean cratons. A high precision thermal ionisation technique has been used to analyse the 40Ca/ 44Ca ratios of plagioclase separates and associated whole rocks in ˜3.6 Ga (early Archaean) samples from Zimbabwe and West Greenland. Three out of four tonalite, trondhjemite, granodiorite (TTG) suite samples from Zimbabwe display initial 40Ca/ 44Ca ratios indistinguishable from our measured modern MORB value (i.e., ɛ Ca(3.6) ˜ 0). Greenland samples, however, are very diverse ranging from ɛ Ca(3.7) = 0.1 in mafic pillow lavas and felsic sheets from the Isua supracrustal belt, up to very radiogenic signatures (ɛ Ca(3.7) = 2.9) in both mafic rocks of the Akilia association and felsic TTG from the coastal Amîtsoq gneisses. At face value, these results imply the Zimbabwe crust is juvenile whereas most Greenland samples include an earlier crustal component. Yet the west Greenland craton, as with many Archaean localities, has experienced a complex geological history and the interpretation of age-corrected initial isotope values requires great care. Both felsic and mafic samples from Greenland display ɛ Ca(3.7) so radiogenic that they are not readily explained by crustal growth scenarios. The presence of such radiogenic 40Ca/ 44Ca found in low K/Ca plagioclases requires Ca isotope exchange between plagioclase and whole rock during later metamorphic event(s). In addition the unexpectedly radiogenic Ca

  19. Juvenile Spondyloarthritis

    Science.gov (United States)

    Gmuca, Sabrina; Weiss, Pamela F.

    2015-01-01

    Purpose of review To provide a comprehensive update of the pathogenesis, diagnostic imaging, treatments, and disease activity measurements of juvenile spondyloarthritis (JSpA). Recent findings Genetic and microbiome studies have provided new information regarding possible pathogenesis of JSpA. Recent work suggests that children with JSpA have decreased thresholds for pain in comparison to healthy children. Additionally, pain on physical examination and abnormalities on ultrasound of the entheses are not well correlated. Treatment guidelines for juvenile arthritis, including JSpA, were published by the American College of Rheumatology and are based on active joint count and presence of sacroiliitis. Recent studies have established the efficacy of tumor necrosis factor inhibitors in the symptomatic treatment of axial disease, though their efficacy for halting progression of structural damage is less clear. Newly developed disease activity measures for JSpA include the Juvenile Arthritis Disease Activity Score and the JSpA Disease Activity index. In comparison to other categories of juvenile arthritis, children with JSpA are less likely to attain and sustain inactive disease. Summary Further microbiome and genetic research may help elucidate JSpA pathogenesis. More randomized therapeutic trials are needed and the advent of new composite disease activity measurement tools will hopefully allow for the design of these greatly needed trials. PMID:26002028

  20. Formation of Continental Fragments: The Tamayo Bank, Gulf of California

    Science.gov (United States)

    van Wijk, J.; Abera, R.; Axen, G. J.

    2015-12-01

    Potential field data are used to construct a two-dimensional crustal model along a profile through the Tamayo Trough and Bank in the Gulf of California. The model is constrained by seismic reflection and refraction data, and field observations. The potential field data do not fit a model where the crust of the Tamayo trough is continental, but they fit well with a model where the Tamayo trough crust is oceanic. This implies that the Tamayo Bank is entirely bounded by oceanic crust and is a microcontinent. The oceanic crust of the Tamayo trough that separates the Tamayo Bank from the mainland of Mexico is thin (~4 km), so oceanic spreading was probably magma-starved before it ceased. This led us to come up with a model that explains the formation of microcontinents that are smaller in size and are not found in the proximity of hotspots. At first, seafloor spreading commences following continental breakup. When the magma supply to the ridge slows down, the plate boundary strengthens. Hence, the ridge may be abandoned while tectonic extension begins elsewhere, or slow spreading may continue while a new ridge starts to develop. The old spreading ridge becomes extinct. An asymmetric ocean basin forms if the ridge jumps within oceanic lithosphere; a microcontinent forms if the ridge jumps into a continental margin. This model for formation of continental fragments is applicable to other regions as well, eliminating the need of mantle plume impingement to facilitate rifting of a young continental margin and microcontinent formation.

  1. Scales and effects of fluid flow in the upper crust.

    Science.gov (United States)

    Cathles, L M

    1990-04-20

    Two of the most important agents of geological change, solar energy and internal heat from the mantle, meet and battle for dominance in propelling aqueous and related fluids in the earth's upper crust. Which prevails and how they interact are subjects of active research. Recent work has demonstrated that both agents can propel fluids over nearly continental-scale distances in a fashion that influences a host of important geological processes and leaves a record in chemical alteration, mineral deposits, and hydrocarbon resources.

  2. Continental tectonics in the aftermath of plate tectonics

    Science.gov (United States)

    Molnar, Peter

    1988-01-01

    It is shown that the basic tenet of plate tectonics, rigid-body movements of large plates of lithosphere, fails to apply to continental interiors. There, buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity. The role of crustal blocks and of the detachment of crustal fragments in this process is discussed. Future areas of investigation are addressed.

  3. Continental tectonics in the aftermath of plate tectonics

    Science.gov (United States)

    Molnar, Peter

    1988-01-01

    It is shown that the basic tenet of plate tectonics, rigid-body movements of large plates of lithosphere, fails to apply to continental interiors. There, buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity. The role of crustal blocks and of the detachment of crustal fragments in this process is discussed. Future areas of investigation are addressed.

  4. Pre-collisional, Tonian (ca. 790 Ma) continental arc magmatism in southern Mantiqueira Province, Brazil: Geochemical and isotopic constraints from the Várzea do Capivarita Complex

    Science.gov (United States)

    Martil, Mariana Maturano Dias; de Fátima Bitencourt, Maria; Nardi, Lauro Valentim Stoll; Koester, Edinei; Pimentel, Márcio Martins

    2017-03-01

    This paper focuses on the pre-collisional mature arc magmatism (ca. 790 Ma) recorded in orthogneisses from the Várzea do Capivarita Complex (VCC), southern Mantiqueira Province (PM), Brazil. The complex comprises ortho- and paragneisses tectonically interleaved during a transpressive high grade regime (ca. 650 Ma), possibly related to oblique collision. The VCC orthogneisses are metaluminous to peraluminous calc-alkaline rocks, with high 87Sr/86Sr(i) ratios from 0.71628 to 0.72509 and εNd(790) values from - 7.19 to - 10.06. The VCC magmatism is correlated with other ca. 800 Ma arc sequences from southern PM, as the Porongos Metamorphic Complex (PMC) metavolcanic rocks and the orthogneisses from Cerro Bori (CB), Uruguay. All associations show signatures typical of accretionary orogens, TDM and Meso to Paleoproteroic inheritance ages, and strong evidence of crustal assimilation/contamination. Their high K contents, and the tendency to move toward the post-collisional field in geotectonic diagrams suggest that they were generated in thick-crust, mature arc environments. In contrast, the CB sequence exhibits a less mature continental-arc character, suggestive of thinner crust or shorter distance to the active margin. VCC and CB orthogneisses, and part of the PMC metavolcanic rocks may be interpreted as part of the same magmatism, or at least as fragments of similar magmatic arcs. However, VCC magmatism is distinct from continental arc sequences in the São Gabriel Block (ca. 700-750 Ma). Isotope signatures for this younger magmatism indicate major contribution of Neoproterozoic juvenile sources, with only little amounts of reworked, old continental crust. Geochemical and Sr-Nd signatures presented in this paper suggest that at least part of the PMC metavolcanic rocks are the protoliths of the VCC orthogneisses. This, together with the isotope evidence of similarity between VCC and PMC igneous and sedimentary fractions, corroborates the hypothesis that the VCC and PMC

  5. Implication of Flow in the Lower Crust on Strain Localization

    Science.gov (United States)

    Le Pourhiet, Laetitia

    2016-04-01

    A major difference between oceanic and continental crust is the capacity of the lower crust to flow. This has been the moto of the research group centered around Genia Burov over the last 15 years and I will try to summarize the results of number of numerical models run in different geodynamic setting to tackle the question of the rheology of the lithosphere and crust at the scale of plate tectonics. I will insist on how apriori very complex numerical models have helped the community to build our intuition on geodynamics processes and change the way of thinking the interactions between mantle process and crustal processes which are the core of plate tectonic and beyond. I will finally discuss what have we learn about the rheology of the lithosphere so far and how we intend to pursues evgeni's fundamental contribution to the field.

  6. Analog Modeling of Continental Lithosphere Subduction

    Science.gov (United States)

    Willingshofer, E.; Sokoutis, D.; Luth, S.; Beekman, F.; Cloetingh, S.

    2012-12-01

    Lithospheric-scale analog modeling sheds light on the consequences of decoupling within the continental lithosphere and along plate interfaces during continental collision. The model results provide valuable information in terms of strain localization, deformation of the subducting slab and the evolution and architecture of the overlying mountain belt and its topography. A weak layer has been implemented in three-layer models to simulate decoupling along the plate interface and at different levels of the lithosphere (brittle-ductile transition, entire lower crust, crust-mantle boundary). Additionally, varying the strength of the mantle lithosphere of both the upper as well as the lower plate regulated the degree of plate coupling. Plate boundaries were orthogonal to the convergence direction. All models emphasize that strong decoupling at the plate interface is a pre-requisite for the subduction of continental lithosphere. In addition, deformation of the subducting slab was found to be sensitive to the strength contrast between the subduction zone and the mantle lithosphere of the downgoing as well as the upper plate. As such, a low strength contrast between the plate interface and the lower plate leads to deformation of the subducting slab by thickening and the development of a shallow slab. Conversely, when the strength contrast is high, deep slabs evolve which undergo relatively less deformation. Furthermore, the level of decoupling in the downgoing plate governs how much continental crust is subducted together with the mantle lithosphere. Shallow decoupling, at the brittle-ductile transition, results in subduction of the lower crust whereas small amounts of lower crust are subducted when decoupling occurs at the level of the Moho. Weak plate coupling and a weak lower crust of the lower plate steer the evolution of mountain belts such that deformation propagates outward, in the direction of the incoming plate, by successive imbrication of upper crustal thrust

  7. The off-crust origin of granite batholiths

    Institute of Scientific and Technical Information of China (English)

    Antonio Castro

    2014-01-01

    Granitod batholiths of I-type features (mostly granodiorites and tonalites), and particularly those forming the large plutonic associations of active continental margins and intracontinental collisional belts, represent the most outstanding magmatic episodes occurred in the continental crust. The origin of magmas, however, remains controversial. The application of principles from phase equilibria is crucial to understand the problem of granitoid magma generation. An adequate comparison between rock com-positions and experimental liquids has been addressed by using a projected compositional space in the plane F(Fe þ Mg)eAnorthiteeOrthoclase. Many calc-alkaline granitoid trends can be considered cotectic liquids. Assimilation of country rocks and other not-cotectic processes are identified in the projected diagram. The identification of cotectic patterns in batholith implies high temperatures of magma segregation and fractionation (or partial melting) from an intermediate (andesitic) source. The com-parison of batholiths with lower crust granulites, in terms of major-element geochemistry, yields that both represent liquids and solid residues respectively from a common andesitic system. This is compatible with magmas being formed by melting, and eventual reaction with the peridotite mantle, of subducted mélanges that are finally relaminated as magmas to the lower crust. Thus, the off-crust generation of granitoids batholiths constitutes a new paradigm in which important geological implica-tions can be satisfactorily explained. Geochemical features of Cordilleran-type batholiths are totally compatible with this new conception.

  8. The off-crust origin of granite batholiths

    Directory of Open Access Journals (Sweden)

    Antonio Castro

    2014-01-01

    Full Text Available Granitod batholiths of I-type features (mostly granodiorites and tonalites, and particularly those forming the large plutonic associations of active continental margins and intracontinental collisional belts, represent the most outstanding magmatic episodes occurred in the continental crust. The origin of magmas, however, remains controversial. The application of principles from phase equilibria is crucial to understand the problem of granitoid magma generation. An adequate comparison between rock compositions and experimental liquids has been addressed by using a projected compositional space in the plane F(Fe + Mg–Anorthite–Orthoclase. Many calc-alkaline granitoid trends can be considered cotectic liquids. Assimilation of country rocks and other not-cotectic processes are identified in the projected diagram. The identification of cotectic patterns in batholith implies high temperatures of magma segregation and fractionation (or partial melting from an intermediate (andesitic source. The comparison of batholiths with lower crust granulites, in terms of major-element geochemistry, yields that both represent liquids and solid residues respectively from a common andesitic system. This is compatible with magmas being formed by melting, and eventual reaction with the peridotite mantle, of subducted mélanges that are finally relaminated as magmas to the lower crust. Thus, the off-crust generation of granitoids batholiths constitutes a new paradigm in which important geological implications can be satisfactorily explained. Geochemical features of Cordilleran-type batholiths are totally compatible with this new conception.

  9. Subduction to Continental Delamination: Insights From Laboratory Experiments

    Science.gov (United States)

    Gogus, O. H.; Corbi, F.; Faccenna, C.; Pysklywec, R. N.

    2009-05-01

    The evolution of the lithosphere through subduction-collision and delamination and its surface/crustal response (topography/deformation) is investigated in this work. We present a series of lithosphere scale two dimensional (2-D) and three dimensional (3-D) laboratory experiments to better understand such processes. In these experiments, an idealized viscously deforming crust-mantle lithosphere-mantle system is configured with silicone putty (representing lithospheric mantle and upper crust) and glucose syrup (representing the upper mantle and lower crust). The initial focus was to investigate the physical development of delamination versus continental subduction without plate convergence. Experiments show that the delamination or continental subduction is strongly dependent on the density of the crust (both crust and mantle lithosphere subducts when crust has a higher density, instead of delamination), while in the investigated range, the viscosity of the weak layer does not have much influence on the process. In all the experiments, the topography is asymmetric with subsidence above the delaminating hinge due to the dynamic vertical pulling driven by the delaminating slab, and uplift above the delaminated region due to the buoyancy of asthenosphere. Our investigation on the oceanic subduction with a convergence rate of ~ 3cm/year plate velocity suggests that subduction -collision - delamination is well defined and at the end, the delaminating crust from the lithosphere is overthrusted on top of the overriding plate. Our results provide integrated insights on the Alpine-Himalayan type orogenies, in particular the neotectonic evolution of Eastern Anatolian plateau.

  10. Juvenile Justice in Milwaukee

    Science.gov (United States)

    Williams, Gary L.; Greer, Lanetta

    2010-01-01

    Historically, there have been several attempts made to address issues surrounding juvenile delinquency. The Wisconsin Legislature outlines the objectives of the juvenile justice system in the Juvenile Justice Code in s. 939.01, ?to promote a juvenile justice system capable of dealing with the problem of juvenile delinquency, a system which will…

  11. Juvenile xanthogranuloma.

    Science.gov (United States)

    Singh, R; Ghazali, W

    1992-05-01

    Juvenile xanthogranuloma is a benign cutaneous growth presenting as papules or nodules. It is characterized by an intradermal collection of lipid-laden macrophages and varying degrees of fibroblastic proliferation. We have recently observed two patients with xanthogranulomas: one was found to have a papular type and the second patient had multiple nodular growths. We present these cases, which should be considered in the differential diagnosis of skin nodules.

  12. 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.

  13. Dermatomiositis juvenil

    OpenAIRE

    Goldaracena, Pablo; Pérez, Federico

    2008-01-01

    La dermatomiositis juvenil (DMJ) es una enfermedad multi sistémica de etiología desconocida, caracterizada por una vasculitis que ocasiona una inflamación no supurativa del músculo estriado y lesiones cutáneas distintivas. La cobertura de los criterios de Bohan y Peter establece el diagnóstico: exantema patognomónico junto a debilidad muscular proximal simétrica, elevación sérica de enzimas musculares, s...

  14. AFSC/ABL: Juvenile chum salmon allozyme stock identification, Gulf of Alaska 2000-2004

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Summer surveys (Julyb??August) of juvenile salmon ecology along the continental shelf of the Gulf of Alaska are conducted annually by scientists from the Ocean...

  15. Generation of continental adakitic rocks: Crystallization modeling with variable bulk partition coefficients

    Science.gov (United States)

    Dai, Hong-Kun; Zheng, Jianping; Zhou, Xiang; Griffin, W. L.

    2017-02-01

    The geochemical signatures (i.e., high Sr/Y and La/Yb ratios) of adakitic rocks in continental settings, which are derived from the continental lower crust rather than from subducted slabs, may reflect high-pressure melting in the lower crust or may be inherited from their sources. The North China Craton (NCC) is an ideal place for investigation of this type of adakites due to its ubiquitous distribution. As an example, we explore the petrogenesis of the Jurassic ( 163 Ma) adakitic rocks in western Liaoning, in the NE part of the NCC, using elemental and Sr-Nd isotopic analysis and crystallization modeling based on Rhyolite-MELTS. The modeling demonstrates that adakitic signatures can be generated by fractional crystallization of magmas within crust of normal thickness (i.e., 33 km). Partial-melting modeling based on the composition of the lower continental crust shows that only the adakitic rocks from orogenic belts require a thickened crust (i.e., 45 km). We suggest that continental adakitic rocks are not necessarily linked to high-pressure processes and their use as an indicator of thickened/delaminated continental crust should be regarded with caution.

  16. Physics of Neutron Star Crusts

    Directory of Open Access Journals (Sweden)

    Chamel Nicolas

    2008-12-01

    Full Text Available The physics of neutron star crusts is vast, involving many different research fields, from nuclear and condensed matter physics to general relativity. This review summarizes the progress, which has been achieved over the last few years, in modeling neutron star crusts, both at the microscopic and macroscopic levels. The confrontation of these theoretical models with observations is also briefly discussed.

  17. Rifted Continental Margins: The Case for Depth-Dependent Extension

    Science.gov (United States)

    Huismans, Ritske S.; Beaumont, Christopher

    2016-04-01

    Even though many basic properties of non-volcanic rifted margins are predicted by uniform extension of the lithosphere, uniform extension fails to explain other important characteristics. Particularly significant discrepancies are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I), where large tracts of continental mantle lithosphere are exposed at the seafloor, and at; 2) ultra-wide central South Atlantic margins (Type II) where continental crust spans wide regions below which it appears that lower crust and mantle lithosphere were removed. Neither corresponds to uniform extension in which crust and mantle thin by the same factor. Instead, either the crust or mantle lithosphere has been preferentially removed during extension. We show that the Type I and II styles are respectively reproduced by dynamical numerical lithospheric stretching models (Models I-A/C and II-A/C) that undergo depth-dependent extension. In this notation A and C imply underplating of the rift zone during rifting by asthenosphere and lower cratonic lithosphere, respectively. We also present results for models with a weak upper crust and strong lower crust, Models III-A/C, to show that lower crust can also be removed from beneath the rift zone by horizontal advection with the mantle lithosphere. From the model results we infer that these Types I, II, and III margin styles are controlled by the strength of the mid/lower crust, which determines the amount of decoupling between upper and lower lithosphere during extension and the excision of crust or mantle. We also predict the styles of sedimentary basins that form on these margins as a test of the concepts presented

  18. Evidence for mechanical coupling and strong Indian lower crust beneath southern Tibet.

    Science.gov (United States)

    Copley, Alex; Avouac, Jean-Philippe; Wernicke, Brian P

    2011-04-07

    How surface deformation within mountain ranges relates to tectonic processes at depth is not well understood. The upper crust of the Tibetan Plateau is generally thought to be poorly coupled to the underthrusting Indian crust because of an intervening low-viscosity channel. Here, however, we show that the contrast in tectonic regime between primarily strike-slip faulting in northern Tibet and dominantly normal faulting in southern Tibet requires mechanical coupling between the upper crust of southern Tibet and the underthrusting Indian crust. Such coupling is inconsistent with the presence of active 'channel flow' beneath southern Tibet, and suggests that the Indian crust retains its strength as it underthrusts the plateau. These results shed new light on the debates regarding the mechanical properties of the continental lithosphere, and the deformation of Tibet.

  19. On the differences in continental rifting at the Earth, Mars and Venus

    Science.gov (United States)

    Nikishin, A. M.; Milanovsky, E. E.

    1985-01-01

    During the process of continental rifting on Earth, the lower ductile crust stretches, forming a neck, while the upper brittle crust is broken in blocks by faults, and the blocks sink down the thinned lower crust; if the stretching continues, the neck may break and a newly originated oceanic crust is formed at this place. The rift system structure depends on the depth of the boundary surface between the brittle crust and the ductile crust, the litospheric thickness, the tension value, etc.. The rigid brittle rifting when narrow necks form in the lower crust is characteristic of the contemporary Earth; on Mars the brittle rifting with large subsidence was characteristic of the Tharsis upland formation epoch. The ductile rifting is typical of the Venus. The differences in rheologic features of the lithospheres of different planets causes the variation in types of rifting.

  20. 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.

  1. Modes of continental extension in a crustal wedge

    Science.gov (United States)

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

    2015-07-01

    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.

  2. Continental Divide Trail

    Data.gov (United States)

    Earth Data Analysis Center, University of New Mexico — This shapefile was created to show the proximity of the Continental Divide to the Continental Divide National Scenic Trail in New Mexico. This work was done as part...

  3. Bathymetric highs in the mid-slope region of the western continental margin of India - Structure and mode of origin

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, D.G.; Paropkari, A.L.; Krishna, K.S.; Chaubey, A.K.; Ajay, K.K.; Kodagali, V.N.

    brought into focus the presence of mantle based volcanic plugs. Their geomorphological and geophysical expressions and structural pattern and associated deep-seated faults led to suggest continental crust. These features are inferred as rifted fragments...

  4. Deformation and seismicity associated with continental rift zones propagating toward continental margins

    Science.gov (United States)

    Lyakhovsky, V.; Segev, A.; Schattner, U.; Weinberger, R.

    2012-01-01

    We study the propagation of a continental rift and its interaction with a continental margin utilizing a 3-D lithospheric model with a seismogenic crust governed by a damage rheology. A long-standing problem in rift-mechanics, known as thetectonic force paradox, is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism. Our modeling results demonstrate that under moderate rift-driving tectonic forces the rift propagation is feasible even in the absence of magmatism. This is due to gradual weakening and "long-term memory" of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks. We show that the style, rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces, but also on the rate of healing. Accounting for the memory effect provides a feasible solution for thetectonic force paradox. Our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin. Thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin. In such a case, a new fault system is created perpendicular to the direction of the rift propagation. These results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern.

  5. BRYANSK-KURSK-VORONEZH INTRA-CONTINENTAL COLLISIONAL OROGEN (EAST EUROPEAN CRATON

    Directory of Open Access Journals (Sweden)

    M. V. Mints

    2015-09-01

    Full Text Available The article presents new data on the deep crustal structure, origin and evolution of the Bryansk-Kursk-Voronezh orogen in the south-eastern segment of the East European craton; it is composed of the Paleoproterozoic formations and areas of reworked Archean crust. The purpose of this paper is the development and improvement of ideas on intra-continental orogens. The deep structure of the orogen is presented by the 3D model based on results of geological mapping of the Precambrian basement and interpretations of geophysical fields and seismic images of the crust along geotraverse 1-EB and profile DOBRE. It is established that the orogen originated with riftogenic extension of the crust at 2.6–2.5 Ga, that was repeated at 2.2–2.1 Ga, and formation of wide depressions that were efficiently filled in with volcanosedimentary layers including BIF, which accumulation was followed by high-temperature up to granulite facies metamorphism. Suprasubduction magmatism took place at 2.10–2.08 Ga and resulted in formation of the Lipetsk-Losevka volcano-plutonic complex. The active margin was completely formed at about 2.05 Ga. The short duration of subduction-related magmatic activity and the lack of relics of the oceanic lithosphere suggest short-term and spatially limited developing of the oceanic structure. The tectonothermal activity of collisional and postcollision stages was expressed in emplacement of alkaline ultramafic (2.1–2.0 Ga and gabbro-syenite (1.8–1.7 Ga complexes. It is difficult or impossible to explain specific features of the structure and evolution of the orogen in framework of the model of the Cordilleras type  accretionary orogen. Mafic-ultramafic magmatism and indications of intensive heating of the crust suggest a special role of plume type processes that provided for influx of mantle heat and juvenile mantle derived matter. 

  6. Potentially exploitable supercritical geothermal resources in the ductile crust

    Science.gov (United States)

    Watanabe, Noriaki; Numakura, Tatsuya; Sakaguchi, Kiyotoshi; Saishu, Hanae; Okamoto, Atsushi; Ingebritsen, Steven E.; Tsuchiya, Noriyoshi

    2017-01-01

    The hypothesis that the brittle–ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability >10−16 m2) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability–depth relations proposed for the continental crust show no drastic permeability reduction at the BDT. Here we present experimental results suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in rocks with much lower BDT temperatures, such as the granitic rocks that comprise the bulk of the continental crust. We find that permeability behaviour for fractured granite samples at 350–500 °C under effective confining stress is characterized by a transition from a weakly stress-dependent and reversible behaviour to a strongly stress-dependent and irreversible behaviour at a specific, temperature-dependent effective confining stress level. This transition is induced by onset of plastic normal deformation of the fracture surface (elastic–plastic transition) and, importantly, causes no ‘jump’ in the permeability. Empirical equations for this permeability behaviour suggest that potentially exploitable resources exceeding 450 °C may form at depths of 2–6 km even in the nominally ductile crust.

  7. Potentially exploitable supercritical geothermal resources in the ductile crust

    Science.gov (United States)

    Watanabe, Noriaki; Numakura, Tatsuya; Sakaguchi, Kiyotoshi; Saishu, Hanae; Okamoto, Atsushi; Ingebritsen, Steven E.; Tsuchiya, Noriyoshi

    2017-01-01

    The hypothesis that the brittle-ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability >10-16 m2) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability-depth relations proposed for the continental crust show no drastic permeability reduction at the BDT. Here we present experimental results suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in rocks with much lower BDT temperatures, such as the granitic rocks that comprise the bulk of the continental crust. We find that permeability behaviour for fractured granite samples at 350-500 °C under effective confining stress is characterized by a transition from a weakly stress-dependent and reversible behaviour to a strongly stress-dependent and irreversible behaviour at a specific, temperature-dependent effective confining stress level. This transition is induced by onset of plastic normal deformation of the fracture surface (elastic-plastic transition) and, importantly, causes no `jump' in the permeability. Empirical equations for this permeability behaviour suggest that potentially exploitable resources exceeding 450 °C may form at depths of 2-6 km even in the nominally ductile crust.

  8. Creep behavior of microbiotic crust

    Institute of Scientific and Technical Information of China (English)

    2009-01-01

    The creep behavior of microbiotic crust at room temperature was revealed by the creep bending tests of cantilever beam under constant-load conditions.The variation in the deflection with time can be depicted well by a standard creep curve.Creep rupture is a fundamental failure mechanism of microbiotic crust due to creep.A simple theory was then applied to describe this new me-chanical behavior.The existence of creep phenomenon brings into question the validity of widely used methods for measuring the strength of microbiotic crust.

  9. Layered basic complex in oceanic crust, romanche fracture, equatorial atlantic ocean.

    Science.gov (United States)

    Melson, W G; Thompson, G

    1970-05-15

    A layered, basic igneous intrusion, analogous in mineralogy and texture to certain large, continental layered complexes, is exposed in the Romanche Fracture, equatorial Atlantic Ocean. Crustal intrusion of large masses of basic magmas with their subsequent gravity differentiation is probably one of a number of major processes involved in the formation of new oceanic crust during sea-floor spreading.

  10. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: from subduction magmatism to cratonization

    Energy Technology Data Exchange (ETDEWEB)

    Johnson, Simon P.; Korhonen, Fawna; Kirkland, Christopher; Cliff, John B.; Belousova, Elena; Sheppard, Stephen

    2017-01-03

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.

  11. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    Science.gov (United States)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

  12. Moho and magmatic underplating in continental lithosphere

    DEFF Research Database (Denmark)

    Thybo, Hans; Artemieva, Irina M.

    2013-01-01

    fractionation during cooling and solidification in the lower crust. Separation of the low density material from the high-density residue may be a main process of formation of continental crust with its characteristic low average density, also during the early evolution of the Earth. Despite the assumed...... importance of underplating processes and associated fractionation, the available geophysical images of underplated material remain relatively sparse and confined to specific tectonic environments. Direct ponding of magma at the Moho is only observed in very few locations, probably because magma usually...... interacts with the surrounding crustal rocks which leads to smearing of geophysical signals from the underplated material. In terms of processes, there is no direct discriminator between the traditional concept of underplated material and lower crustal magmatic intrusions in the form of batholiths and sill...

  13. Crust Formation in Aluminum Cells

    Science.gov (United States)

    Oedegard, R.; Roenning, S.; Rolseth, S.; Thonstad, J.

    1985-11-01

    This paper examines the catalytic effects offlourides on the ϒ→α-Al2O3 phase transformation by heat treating commercial alumina samples with 2wt% additions of different flouride compounds. The various additives were ranked according to their effect on transformation temperature. Experiments were conducted to explain the high temperature coherence of crusts. The findings indicate that an alumina network is formed during ϒ→α phase transformation, which reinforces the crust on top of the cryolite bath.

  14. Finding of ancient materials in Cathaysia and implication for the formation of Precambrian crust

    Institute of Scientific and Technical Information of China (English)

    YU JinHai; Y S O'REILLY; WANG LiJuan; W L GRIFFIN; JIANG ShaoYong; WANG RuCheng; XU XiSheng

    2007-01-01

    U-Pb dating for fifty-six detrital zircons from a paragneiss in Nanxiong area, northern Guangdong Province, indicates that the latest Neoproterozoic sediments in Cathaysia hinterland are composed of numerous Grenvillian and Neoarchaean clastic materials, as well as some Mesoproterozoic detritus. Minor Paleoarchaean (3.76 Ga) and Mesoarchaean (3.0-3.2 Ga) zircons, which are the oldest zircons in South China, also are firstly found in the sediments, suggesting that the Cathaysia Block may contain very old materials. The Hf isotope compositions of thirty-seven zircons reveal that these clastic materials have different origins. Minor zircons crystallized from magma generated from relatively juvenile crust, while the parental magma of most zircons was derived from ancient crust. Integration of U-Pb dating and Hf isotope analysis of these zircons suggests that the generation of juvenile crust in the Cathaysia block mainly occurred at 2.5-2.6 Ga. Mesoarchaean (3.0-3.3 Ga), late Paleoproterozoic (~1.8 Ga) and Paleoarchaean (~3.7 Ga) may also be important episodes of crustal growth. Grenvillian magmatism is extremely intense,but it mainly involved recycling of ancient crustal components with little formation of juvenile crust. The marked presence of ~2.1 Ga Hf model ages and the absence of the zircons with crystallization ages at ~2.1 Ga suggest that the parental magma of many zircons was probably derived from the mixed source consisting of Neoarchaean and late Paleoproterozoic materials.

  15. Continental emergence in the Late Archean reconciles early and late continental growth models

    Science.gov (United States)

    Flament, Nicolas; Coltice, Nicolas; Rey, Patrice

    2014-05-01

    The analysis of ancient sediments (Rare Earth Element composition of black shales, isotopic strontium composition of marine carbonates, isotopic oxygen composition of zircons) suggests that continental growth culminated around the Archean-Proterozoic transition. In stark contrast, the geochemical analysis of ancient basalts suggests that depletion of the mantle occurred in the Hadean and Eoarchean. This paradox may be solved if continents were extracted from the mantle early in Earth's history, but remained mostly below sea level throughout the Archean. We present a model to estimate the area of emerged land and associated isotopic strontium composition of the mantle and oceans as a function of the coupled evolution of mantle temperature, continental growth and distribution of surface elevations (hypsometry). For constant continental hypsometry and four distinct continental growth models, we show that sea level was between 500 and 2000 m higher in the Archean than at present, resulting in isotopic composition of the mantle and oceans, we show that a reduced area of emerged continental crust can explain why the geochemical fingerprint of continents extracted early in Earth's history was not recorded at the surface of the Earth until the late Archean.

  16. What Is Juvenile Arthritis?

    Science.gov (United States)

    ... Analgesics for Osteoarthritis (Report from AHRQ) Joint Replacement Surgery: Health Information Basics for You and Your Family NIH Pediatric Rheumatology Clinic Health Information Juvenile Arthritis Find a Clinical Trial Journal Articles Juvenile Arthritis PDF Version Size: 123 KB ...

  17. Juvenile Delinquency: An Introduction

    Science.gov (United States)

    Smith, Carolyn A.

    2008-01-01

    Juvenile Delinquency is a term which is often inaccurately used. This article clarifies definitions, looks at prevalence, and explores the relationship between juvenile delinquency and mental health. Throughout, differences between males and females are explored. (Contains 1 table.)

  18. Parenting and juvenile delinquency

    NARCIS (Netherlands)

    Hoeve, Machteld

    2008-01-01

    Juvenile delinquency is a noteworthy problem. This thesis addressed the association between parenting and juvenile delinquency by analyzing the concepts of parenting adopted in family research in relation to criminological concepts and measures of delinquent behavior. Four studies were conducted.

  19. Seismic structure and composition of the crust beneath the southern Scandes, Norway

    DEFF Research Database (Denmark)

    Stratford, Wanda Rose; Thybo, Hans

    2011-01-01

    New results on P and S-wave seismic velocity structure in southern Norway indicate that the crust has an average Poisson's ratio of 0.25, is predominantly of felsic-intermediate composition and lacks a significant mafic lower crust. A crustal scale refraction seismic study (Magnus-Rex — Mantle...... investigations of Norwegian uplift structure, refraction experiment) acquired data along three 300 to 400 km long active source seismic profiles across the Southwest Scandinavian Domain in southern Norway, the youngest section of the Fennoscandian shield. Moho depths in the Domain are 36–40 km, thinning towards...... the continental shelf and Oslo Graben. The high Vp lower crust beneath the Southwest Scandinavian Domain (Vp > 7 km/s) is around 4 km thick. Crustal structure in the adjacent Svecofennian Domain differs significantly; Moho depths reach ~ 50 km and an up to 24 km thick high Vp lower crust is present. Strong P...

  20. Juvenile Arrests, 2007. Juvenile Justice Bulletin

    Science.gov (United States)

    Puzzanchera, Charles

    2009-01-01

    This Bulletin summarizes 2007 juvenile crime and arrest data reported by local law enforcement agencies across the country and cited in the FBI report, "Crime in the United States 2007." The Bulletin describes the extent and nature of juvenile crime that comes to the attention of the justice system. It serves as a baseline for comparison for…

  1. Juvenile polyposis syndrome

    NARCIS (Netherlands)

    L.A.A. Brosens; D. Langeveld; W.A. van Hattem; F.M. Giardiello; G.J.A. Offerhaus

    2011-01-01

    Juvenile polyposis syndrome is a rare autosomal dominant syndrome characterized by multiple distinct juvenile polyps in the gastrointestinal tract and an increased risk of colorectal cancer. The cumulative life-time risk of colorectal cancer is 39% and the relative risk is 34. Juvenile polyps have a

  2. Spatial dynamics of juvenile anchovy in the Bay of Biscay

    KAUST Repository

    Boyra, Guillermo

    2016-07-08

    In autumn 2009, the implementation of two successive acoustic surveys targeting juvenile anchovy (Engraulis encrasicolus) in the Bay of Biscay allowed us to monitor the changes in the spatial distribution and aggregation patterns of juveniles of this species during 45 days under fairly stable meteorological conditions. Juvenile anchovy changed its biological condition and behavior in a different manner in two distinct areas. In the Spanish sector, the juveniles migrated 20 nautical miles (n.mi.) towards the coast, but they remained on the shelf and near the surface during the whole surveyed period. As the advance towards the shelf break progressed, their area of distribution decreased, their density increased and the juveniles spread in fewer but heavier shoals. In the French sector, the juveniles also migrated from slope waters towards the coast at a similar velocity, but they crossed the shelf break into the continental shelf, where they increased their mean depth significantly until gradually adopting the typical nyctemeral migrations of adult anchovy. The mean length of the juveniles that adopted the nyctemeral migrations was significantly higher than that of the juveniles remaining at the surface, suggesting that body size is relevant to accomplish this change. Besides, the stronger temperature gradients between the shelf and oceanic waters in the Spanish sector, favored by a narrow shelf, may have acted as a barrier influencing the distinct observed spatial patterns in the two areas. © 2016 John Wiley & Sons Ltd

  3. Helium isotopes in ferromanganese crusts from the central Pacific Ocean

    Science.gov (United States)

    Basu, S.; Stuart, F.M.; Klemm, V.; Korschinek, G.; Knie, K.; Hein, J.R.

    2006-01-01

    Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and diffusion. High 4He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4He. 3He/4He of VA13/2 samples varies from 18.5 to 1852 Ra due almost entirely to variation in the extraterrestrial He contribution. The highest 3He/4He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping efficiency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3He concentration of the crusts rules out whole, undegassed 4–40 μm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles.

  4. Evolution of Oxidative Continental Weathering

    Science.gov (United States)

    Konhauser, Kurt; Lalonde, Stefan

    2014-05-01

    The Great Oxidation Event (GOE) is currently viewed as a protracted process during which atmospheric oxygen levels increased above 10-5 times the present atmospheric level. This value is based on the loss of sulphur isotope mass independent fractionation (S-MIF) from the rock record, beginning at 2.45 Ga and disappearing by 2.32 Ga. However, a number of recent papers have pushed back the timing for oxidative continental weathering, and by extension, the onset of atmospheric oxygenation several hundreds of million years earlier despite the presence of S-MIF (e.g., Crowe et al., 2013). This apparent discrepancy can, in part, be resolved by the suggestion that recycling of older sedimentary sulphur bearing S-MIF might have led to this signal's persistence in the rock record for some time after atmospheric oxygenation (Reinhard et al., 2013). Here we suggest another possibility, that the earliest oxidative weathering reactions occurred in environments at profound redox disequilibrium with the atmosphere, such as biological soil crusts, riverbed and estuarine sediments, and lacustrine microbial mats. We calculate that the rate of O2 production via oxygenic photosynthesis in these terrestrial microbial ecosystems provides largely sufficient oxidizing potential to mobilise sulphate and a number of redox-sensitive trace metals from land to the oceans while the atmosphere itself remained anoxic with its attendant S-MIF signature. These findings reconcile geochemical signatures in the rock record for the earliest oxidative continental weathering with the history of atmospheric sulphur chemistry, and demonstrate the plausible antiquity of a terrestrial biosphere populated by cyanobacteria. Crowe, S.A., Dossing, L.N., Beukes, N.J., Bau, M., Kruger, S.J., Frei, R. & Canfield, D.E. Atmospheric oxygenation three billion years ago. Nature 501, 535-539 (2013). Reinhard, C.T., Planavsky, N.J. & Lyons, T.W. Long-term sedimentary recycling of rare sulphur isotope anomalies. Nature 497

  5. Juveniles on trial.

    Science.gov (United States)

    Quinn, Kathleen M

    2002-10-01

    This article describes common forensic evaluations requested of juvenile court mental health evaluators. There has been a legal shift toward criminalization of juvenile court, with a greater emphasis on rights, abandonment of the rehabilitative model, and greater movement of adolescents into the adult criminal court. A resulting shift has been the redefinition of juvenile court forensic evaluations toward the specificity of adult forensic work. The challenge for evaluators is to refine their knowledge of the forensic standards and bring knowledge of development, assessment, and diagnosis in juveniles and interview techniques appropriate to juveniles to improve the evaluation and forensic reports.

  6. Modelling of temperatures in continental convergence zones

    Energy Technology Data Exchange (ETDEWEB)

    Toksoz, M.N.; Bird, P.

    1977-08-03

    The thermal histories of continent-continent convergence zones are modelled by a finite-difference technique in an attempt to explain geologic observations of heating and melting in such zones. The suture zone between two converging continents divides a passively heated overriding plate from a quiet continental margin which is suddenly deformed in the collision. Both regions may be metamorphosed and intruded. On the continental-shelf side where mountains are formed by underthrusting within the crust, it was found that adiabatic and radioactive heating are negligible during the orogeny. Shear-strain heating may raise the fault zones to about 500/sup 0/C. At higher temperatures, dislocation creep of crustal rocks would be expected from laboratory results. Even high crustal radioactivity will not produce melting in less than 40 m.y. Thus any plutons in this zone (the granites of the Zagros, Urals, and Himalayas) probably result indirectly by melting of crust that is heated by deep asthenospheric intrusions, which may reach the crust at the time of detachment of the oceanic slab, combined with the effects of friction and water along the subduction plane. Across the suture, the thermal history begins before the collision during the oceanic subduction phase. The sinking slab creates asthenospheric circulations, which warm the passive plate from below and intrude it in an Andean-type arc along the suture (Zagros and Himalayan region). If total subduction exceeds about 3000 km the slow warming has time to weaken the plate and extensive crustal shortening may follow the collision. Crustal shortening and thickening is accompanied by differentiation and volcanism (Tibetan and Grenville orogenies). Thermal modelling of Tibet shows that volcanism cannot be produced in the available time by crustal thickening alone, but requires the initial warming phase as well.

  7. Statistics of Magnetar Crusts Magnetoemission

    Science.gov (United States)

    Kondratyev, V. N.; Korovina, Yu. V.

    2016-05-01

    Soft repeating gamma-ray (SGR) bursts are considered as magnetoemission of crusts of magnetars (ultranamagnetized neutron stars). It is shown that all the SGR burst observations can be described and systematized within randomly jumping interacting moments model including quantum fluctuations and internuclear magnetic interaction in an inhomogeneous crusty nuclear matter.

  8. Statistics of Magnetar Crusts Magnetoemission

    Directory of Open Access Journals (Sweden)

    Kondratyev V. N.

    2016-01-01

    Full Text Available Soft repeating gamma-ray (SGR bursts are considered as magnetoemission of crusts of magnetars (ultranamagnetized neutron stars. It is shown that all the SGR burst observations can be described and systematized within randomly jumping interacting moments model including quantum fluctuations and internuclear magnetic interaction in an inhomogeneous crusty nuclear matter.

  9. The contribution of amphibole from deep arc crust to the silicate Earth's Nb budget

    Science.gov (United States)

    Tiepolo, Massimo; Vannucci, Riccardo

    2014-11-01

    The continental crust (CC) and the depleted mantle (DM) are generally assumed to be complementary reservoirs within the Earth. However, the mixture between CC and upper mantle does not generate the Nb/Ta and Nb/La ratios of chondrites. A reservoir with superchondritic ratios for Nb/Ta and Nb/La is thus required in the Earth's system. The occurrence of a hidden amphibole reservoir in the lower arc crust has been recently proposed. This, coupled with the capability of calcic amphibole to give rise to a superchondritic Nb/Ta and Nb/La reservoir, led us to determine to what extent amphibole-rich ultramafic rocks can account for the Nb (and Nb/Ta, Nb/La as well) imbalance on Earth. We have considered lower crust mafic and ultramafic amphibole-rich intrusive rocks from collisional settings worldwide. Because CC is considered to have primarily formed in collisional setting these rocks are important for its genetic model. We modeled Nb, Ta and La contents of the hidden Nb reservoir by mass balance calculations between continental crust, depleted mantle and primitive mantle. Modeling shows that amphibole-rich mafic lower crust can solve the so-called Nb paradox if large volumes of materials are supposed to be returned into the mantle during the Earth's history. A possible mechanism is recycling, particularly in Precambrian times, of eclogites that underwent pre-eclogitic melting in the amphibolite facies field and then recrystallized under eclogite-facies conditions.

  10. Juvenile polyposis syndrome

    Institute of Scientific and Technical Information of China (English)

    Lodewijk AA Brosens; Danielle Langeveld; W Arnout van Hattem; Francis M Giardiello; G Johan A Offerhaus

    2011-01-01

    Juvenile polyposis syndrome is a rare autosomal dominant syndrome characterized by multiple distinct juvenile polyps in the gastrointestinal tract and an increased risk of colorectal cancer.The cumulative life-time risk of colorectal cancer is 39% and the relative risk is 34.Juvenile polyps have a distinctive histology characterized by an abundance of edematous lamina propria with inflammatory cells and cystically dilated glands lined by cuboidal to columnar epithelium with reactive changes.Clinically, juvenile polyposis syndrome is defined by the presence of 5 or more juvenile polyps in the colorectum,juvenile polyps throughout the gastrointestinal tract or any number of juvenile polyps and a positive family history of juvenile polyposis.In about 50%-60% of patients diagnosed with juvenile polyposis syndrome a germline mutation in the SMAD4 or BMPR1A gene is found.Both genes play a role in the BMP/TGF-beta signalling pathway.It has been suggested that cancer in juvenile polyposis may develop through the so-alled "landscaper mechanism" where an abnormal stromal environment leads to neoplastic transformation of the adjacent epithelium and in the end invasive carcinoma.Recognition of this rare disorder is important for patients and their families with regard to treatment,follow-up and screening of at risk individuals.Each clinician confronted with the diagnosis of a juvenile polyp should therefore consider the possibility of juvenile polyposis syndrome.In addition, juvenile polyposis syndrome provides a unique model to study colorectal cancer pathogenesis in general and gives insight in the molecular genetic basis of cancer. This review discusses clinical manifestations, genetics, pathogenesis and management of juvenile polyposis syndrome.

  11. Fluid activity during exhumation of deep-subducted continental plate

    Institute of Scientific and Technical Information of China (English)

    ZHENG Yongfei

    2004-01-01

    It is well known that a great deal of fluid was released during subduction of oceanic crust, resulting in arc magmatism, quartz veining and metamorphic mineralization of syn-subduction. In contrast, the process of continental subduction is characterized by the relative lack of fluid and thus no arc magmatism has been found so far. During exhumation of deep-subducted continental crust, nevertheless,significant amounts of aqueous fluid became available from the decomposition of hydrous minerals, the decrepitation of primary fluid inclusions, and the exsolution of structural hydroxyls. This kind of metamorphic fluid has recently attracted widespread interests and thus been one of the most important targets in deciphering the geological processes concerning metamorphism, magmatism and mineralization in collisional orogens. A large number of studies involving stable isotopes, fluid inclusions and petrological phase relationships have been accomplished in past a few years with respect to the mobility and amount of metamorphic fluid in UHP metamorphic rocks from the Dabie-Sulu orogenic belt.The results demonstrate that the fluid activity during the exhumation of deep-subducted continental crust has the following effects: (1) amphibolite-facies retrogression due to pervasive fluid flow; (2) formation of HP quartz veins within eclogites due to channelized fluid flow; and (3) partial melting of overlying crustal rocks due to focused fluid flow, producing syn-exhumation magmatism within the orogenic belt.In particular, the aqueous fluid released by decompression exsolution of hydroxyl from UHP minerals is characterized by low salinity and is capable of resulting in pervasive and channellized flow. Therefore, the intensive study of fluid activity during exhumation of UHP metamorphic rocks can not only provide insight into geodynamic processes that occurred in continental collisional belts, but also shed light on understanding of crust-mantle recycling and relevant magmatism in

  12. Slab detachment during continental collision: Influence of crustal rheology and interaction with lithospheric delamination

    Science.gov (United States)

    Duretz, T.; Gerya, T. V.

    2013-08-01

    Collision between continents can lead to the subduction of continental material. If the crust remains coupled to the downgoing slab, a large buoyancy force is generated. This force slows down convergence and promotes slab detachment. If the crust resists to subduction, it may decouple from the downgoing slab and be subjected to buoyant extrusion. We employ two-dimensional thermo-mechanical modelling to study the importance of crustal rheology on the evolution of subduction-collision systems. We propose simple quantifications of the mechanical decoupling between lithospheric levels (σ*) and the potential for buoyant extrusion of the crust (ξ*). The modelling results indicate that a variable crustal rheological structure results in slab detachment, delamination, or the combination of both mechanisms. A strong crust provides coupling at the Moho (low σ*) and remains coherent during subduction (low ξ). It promotes deep subduction of the crust (180 km) and slab detachment. Exhumation occurs in coherent manners via eduction and thrusting. Slab detachment triggers the development of topography (> 4.5 km) close to the suture. A contrasting style of collision occurs using a weak crustal rheology. Mechanical decoupling at the Moho (high σ*) promotes the extrusion of the crust (high ξ), disabling slab detachment. Ongoing shortening leads to buckling of the crust and development of topography on the lower plate. Collisions involving rheologically layered crust allow decoupling at mid-crustal depths. This structure favours both the extrusion of upper crust and the subduction of the lower crust. Such collisions are successively affected by delamination and slab detachment. Topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment. Our results suggest that the occurrence of both delamination (Apennines) and slab detachment (Himalayas) in orogens may indicate differences in the initial crustal structure of

  13. Fluid migration in continental subduction: The Northern Apennines case study

    Science.gov (United States)

    Agostinetti, Nicola Piana; Bianchi, Irene; Amato, Alessandro; Chiarabba, Claudio

    2011-02-01

    Subduction zones are the place in the world where fluids are transported from the foredeep to the mantle and back-to-the-surface in the back-arc. The subduction of an oceanic plate implies the transportation of the oceanic crust to depth and its methamorphization. Oceanic sediments release water in the (relatively) shallower part of the subduction zone, while dehydration of the subducted basaltic crust allows fluid circulation at larger depths. While the water budget in oceanic subduction has been deeply investigated, less attention has been given to the fluids implied in the subduction of a continental margin (i.e. in continental subduction). In this study, we use teleseismic receiver function (RF) analysis to image the process of water migration at depth, from the subducting plate to the mantle wedge, under the Northern Apennines (NAP, Italy). Harmonic decomposition of the RF data-set is used to constrain both isotropic and anisotropic structures. Isotropic structures highlight the subduction of the Adriatic lower crust under the NAP orogens, from 35-40 km to 65 km depth, as a dipping low S-velocity layer. Anisotropic structures indicate the presence of a broad anisotropic zone (anisotropy as high as 7%). This zone develops in the subducted Adriatic lower crust and mantle wedge, between 45 and 65 km depth, directly beneath the orogens and the more recent back-arc extensional basin. The anisotropy is related to the metamorphism of the Adriatic lower crust (gabbro to blueschists) and its consequent eclogitization (blueschists to eclogite). The second metamorphic phase releases water directly in the mantle wedge, hydrating the back-arc upper mantle. The fluid migration process imaged in this study below the northern Apennines could be a proxy for understanding other regions of ongoing continental subduction.

  14. Secular geodynamic changes at the base of the crust recorded by anorthosites

    Science.gov (United States)

    Bybee, G. M.; Parman, S. W.

    2016-12-01

    Proterozoic anorthosites (PA) were formed by fractionation of large basaltic magma bodies as they impinged on the continental crust. Despite their complex magmatic origins, the composition of plagioclases in PA are notably constant, most lying between An 40 and 60. PA first appear in the geological record at 2.7 Ga and disappear at 0.6 Ga. The constancy of mineral composition and texture over 2 Ga is remarkable. But what caused PA to begin to form at 2.7 Ga? And what ended their formation at 0.6 Ga? PA are produced by a distinct set of conditions: 1) voluminous batholiths of high-Al basaltic magma (±104 km3for the largest batholiths), 2) highly effective separation of plagioclase from melt and mafic crystals, 3) crystallization at lower crustal conditions (>30km depth) and 4) fairly anhydrous conditions. The lack of PA prior to 2.7 Ga and after 0.6 Ga implies these conditions did not exist on the Earth at those times, suggesting fundamental secular change. Observations of both the Earth's interior (zircon age distributions, age of cratonic mantle) and atmosphere/hydrosphere (S-MIF in sediments, d13C in organic carbon) suggest 2.7 Ga was a unique period in Earth's history. The appearance of PA at this time implies that whatever was driving these effects involved not just the Earth's surface (e.g. area of exposed continent, continental geometry) as has been postulated, but also secular changes in conditions at the base of the crust. One possibility is that the continental crust thickened or stiffened dramatically at this time, producing conditions necessary for PA production (i.e. high pressure at the crust/mantle boundary) not present in pre-2.7 Ga crust. Longer-lived subduction events could also promote extensive basaltic magma ponding at the base of the crust. At 0.6 Ga, the crust may have become too thick or stiff to allow passage of large, viscous mushes. Thus PA could record the thermo-tectonic evolution of the continental crust. Alternatively, the high

  15. Fluid Release and the Deformation of Subducting Crust

    Science.gov (United States)

    Maunder, Benjamin; van Hunen, Jeroen; Magni, Valentina; Bouilhol, Pierre

    2014-05-01

    It is known that slab dehydration is crucial in subduction dynamics and for the formation of arc-magmatism. Previous studies of this process have constrained this intake and subsequent release of fluids into the mantle wedge by considering the stability hydrous phases within the slab. Other, more dynamical effects of this hydration state and partial melting have also been suggested, such as the possibility of "cold plumes", crustal delamination, and subduction channel return flow. These processes have been inferred to play a role in the generation of continental crust over time through accumulation and melting beneath the overriding plate. Water content and melt fraction have a strong control on the rheology of the system. Therefore we investigate the effect of these parameters on the dynamics of a subducting slab, with the aim to establish the physical bounds on the delamination process. To do this we use a coupled geodynamical-petrological model that tracks dehydration and melting reactions in order to factor in the rheological effect of metamorphism and magmatism on slab and mantle wedge dynamics. We focus primarily on the strength of the subducting crust and the possibility of delamination. We then extend this investigation by considering whether early earth crust formation could have been the result of such a processes by looking at a hypothetical Archean setting.

  16. Neodymium and strontium isotope evidence for crustal contamination of continental volcanics.

    Science.gov (United States)

    Carter, S R; Evensen, N M; Hamilton, P J; O'nions, R K

    1978-11-17

    Combined neodymium and strontium isotope studies on Tertiary volcanics from northwest Scotland indicate that their parental mantle isotopic compositions have been substantially modified in many instances by contamination with the Precambrian continental crust through which they were erupted. The occurrence of samarium-neodymium and rubidium-strontium "pseudoisochrons" of different ages in these contaminated continental volcanics indicates that they are artifacts of the contamination processes and have no temporal significance with respect to mantle fractionation events.

  17. Evolution of magma-poor continental margins from rifting to seafloor spreading.

    Science.gov (United States)

    Whitmarsh, R B; Manatschal, G; Minshull, T A

    2001-09-13

    The rifting of continents involves faulting (tectonism) and magmatism, which reflect the strain-rate and temperature dependent processes of solid-state deformation and decompression melting within the Earth. Most models of this rifting have treated tectonism and magmatism separately, and few numerical simulations have attempted to include continental break-up and melting, let alone describe how continental rifting evolves into seafloor spreading. Models of this evolution conventionally juxtapose continental and oceanic crust. Here we present observations that support the existence of a zone of exhumed continental mantle, several tens of kilometres wide, between oceanic and continental crust on continental margins where magma-poor rifting has taken place. We present geophysical and geological observations from the west Iberia margin, and geological mapping of margins of the former Tethys ocean now exposed in the Alps. We use these complementary findings to propose a conceptual model that focuses on the final stage of continental extension and break-up, and the creation of a zone of exhumed continental mantle that evolves oceanward into seafloor spreading. We conclude that the evolving stress and thermal fields are constrained by a rising and narrowing ridge of asthenospheric mantle, and that magmatism and rates of extension systematically increase oceanward.

  18. The Myanmar continental shelf

    Digital Repository Service at National Institute of Oceanography (India)

    Ramaswamy, V.; Rao, P.S.

    conditions, and neotectonic activity. The most prominent bathymetric feature on the Ayeyarwady continental shelf is the 120 km-wide Martaban Depression, at the centre of which is located the Martaban Canyon. Most of the suspended sediment discharge...

  19. Continental Scientific Drilling Program.

    Science.gov (United States)

    1979-01-01

    Resource Priorities in Continental Drilling J. J. Hemley 12 Aspects of Continental Crustal Structure and Growth Scott Smithson 13 Deep-Drilling Targets...Time of Workshop Allen F. Agnew Library of Congress Clarence R. Allen California Institute of Technology Orson L. Anderson University of California at...Skinner Yale University Norman H. Sleep Northwestern University Laurence L. Sloss Northwestern University Scott B. Smithson University of Wyoming

  20. Record of Permian-Early Triassic continental arc magmatism in the western margin of the Jiamusi Block, NE China: petrogenesis and implications for Paleo-Pacific subduction

    Science.gov (United States)

    Yang, Hao; Ge, Wenchun; Dong, Yu; Bi, Junhui; Wang, Zhihui; Ji, Zheng; Yang, H.; Ge, W. C.; Dong, Y.; Bi, J. H.; Wang, Z. H.; Ji, Z.

    2016-09-01

    In this paper, we report zircon U-Pb ages, Hf isotopes and whole-rock geochemical data for the Permian to Early Triassic granitoids from the western margin of the Jiamusi Block (WJB), NE China. The intermediate to felsic (SiO2 = 59.67-74.04 wt%) granitoids belong to calc-alkaline series and are characterized by enrichments in light rare earth elements and large ion lithophile elements with pronounced negative Nb, Ta and Ti anomalies, revealing typical continental magmatic arc geochemical signatures. The zircon U-Pb determinations on the granodiorite, monzogranite, syenogranite and quartz diorite samples yielded ages between ca. 275-245 Ma, which, together with the published coeval intrusive rocks, indicates that Permian to Early Triassic continental arc magmatism occurred extensively in the WJB. The low and mainly negative zircon ɛ Hf(t) values between -7.6 and +1.6 and the zircon Hf model ages of 1.2-1.8 Ga, which are significantly older than their crystallization ages, suggest that they were mainly derived from reworking of ancient crustal materials with a limited input of juvenile components. The geochemical systematics and petrogenetic considerations indicate that the studied granitoids were generated from a zone of melting, assimilation, storage, and homogenization, i.e., a MASHed zone at the base of Paleo- to Mesoproterozoic continental crust, where large portions of igneous rocks and minor clay-poor sediments involved in the source region. In combination with regional geological data, we argue that the Jiamusi Block was unlikely the rifted segment of the Songliao Block and two possible geodynamical models were proposed to interpret the formation of the ca. 275-245 Ma granitoids in the WJB. In the context of Permian global plate reconstruction, we suggest that Paleo-Pacific plate subduction was initiated in the Permian to Early Triassic beneath the Jiamusi Block, and even whole eastern NE China.

  1. The Seismic Structure of the Crust of Madagascar

    Science.gov (United States)

    Wysession, M. E.; Andriampenomanana Ny Ony, F. S. T.; Tsiriandrimanana, R.; Pratt, M. J.; Aleqabi, G. I.; Wiens, D. A.; Nyblade, A.; Shore, P.; Rambolamanana, G.; Tilmann, F. J.

    2015-12-01

    The structure of Madagascar's crust is determined using both body wave receiver functions as well as an analysis of surface waves using ambient-noise and two-plane-wave earthquake surface waves analyses. The primary data used are from the 2011-2013 MACOMO (Madagascar, the Comoros, and Mozambique) broadband seismic array from the PASSCAL program of IRIS (Incorporated Research Institutions for Seismology), funded by the NSF. Additional data came from the RHUM-RUM project (led by G. Barruol and K. Sigloch), the Madagascar Seismic Profile (led by F. Tilmann), and the GSN. The crustal structure of Madagascar, which had previously only been inferred from a gravity survey assuming isostasy, shows a strong correlation with its tectonic history. Crustal thicknesses are greatest, reaching 45 km, along the spine of Madagascar's mountains, which run north-south across the island. Crustal thicknesses thin to the east and west, which are both regions of tectonic separation, however, with very different results. Extensive crustal thinning occurred along the western coasts of Madagascar when the island rifted away from mainland Africa beginning 160 Ma ago. The crust is as thin as 20 km here, but the thickness of basin sediments is as great as 9 km, with the crystalline basement continental crust thinning to 12 km at its thinnest. Along the east coast, the crustal thickness is within the 33-38 km range, but it is thickest in the two places where mesoarchaean crust was rifted off from the Indian subcontinent when it broke away from Madagascar. Surface wave studies show that velocities beneath Madagascar are generally slow, when compared to global models such as AK135. This appears to be due to the occurrence of Cenozoic intraplate volcanism in three regions of Madagascar (north, central, and southwest), each of which has strong underlying seismic low-velocity anomalies in the lithospheric mantle and asthenosphere.

  2. U-Pb isotopic geochemistry of the post-collisional mafic-ultramafic rocks from the Dabie Mountains--Crust-mantle interaction and LOMU component

    Institute of Scientific and Technical Information of China (English)

    黄方; 李曙光; 周红英; 李惠民

    2003-01-01

    The U-Pb isotope geochemical study of the pyroxenite-gabbro intrusion in the Dabie Mountains shows that the post-collisional mafic-ultramafic rocks of the Dabie Mountains are characterized by relative high Pb contents, low U contents and low U/Pb ratios. These characters may be results of interaction between lithosphere or depleted asthenospheric mantle (DMM) and lower crust, but have nothing to do with mantle plume and subducted continental crust. It was first observed that some samples with lower 206Pb/204Pb and higher 207Pb/204Pb ratios show typical characters of the LOMU component. The Pb, Sr, and Nd isotopic tracing shows that three components are needed in the source of the Zhujiapu pyroxenite-gabbro intrusion. They could be old enriched sub-continental lithospheric mantle (LOMU component), lower crust and depleted asthenospheric mantle. The crust-mantle interaction process producing primitive magma of post-collisional mafic-ultramafic rocks in the Dabie Mountains could be described by a lithospheric delamination and magma underplating model. After continent-continent collision, delamination of the thickened lithosphere induced the upwelling of depleted asthenospheric mantle, which caused partial melting of asthenospheric mantle and residual sub-continental lithospheric mantle. The basaltic magma produced in this process underplated in the boundary between the crust and mantle and interacted with lower crust resulting in the geochemical characters of both enriched lithospheric mantle and lower crust.

  3. A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt

    Institute of Scientific and Technical Information of China (English)

    ZHENG YongFei

    2008-01-01

    The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonics theory in the twenty-first century. The babie-Sulu orogenic belt in China crops out the largest lithotectonic unit containing ultrahigh-pressure metamorphic rocks in the world. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in the Dabie-Sulu rocks. By taking these rocks as the natural laboratory, earth scientists have made seminal contributions to understanding of ultrahigh-pressure metamorphism and continental collision. This paper outlines twelve aspects of outstanding progress, including spatial distribution of the UHP metamorphic rocks, timing of the UHP metamorphism, time-scale of the UHP metamorphism, the protolith nature of deeply subducted continental crust, subduction erosion and crustal detachment during continental collision, the possible depths of continental sub-duction, fluid activity in the continental deep-subduction zone, partial melting during continental colli-sion, element mobility in continental deep-subduction zone, recycling of subducted continental crust, geodynamic mechanism of postcollisional magmatism, and lithospheric architecture of collision oro-gen. Some intriguing questions and directions are also proposed for future studies.

  4. Constraining timescales of focused magmatic accretion and extension in the Afar crust using lava geochronology.

    Science.gov (United States)

    Ferguson, David J; Calvert, Andrew T; Pyle, David M; Blundy, Jon D; Yirgu, Gezahegn; Wright, Tim J

    2013-01-01

    As continental rift zones mature the tectonic and volcanic processes associated with crustal extension become confined to narrow magmatic rift zones, reminiscent of oceanic spreading ridges. The formation of these rift zones and the development of ocean-ridge type topography is a significant milestone in rift evolution as it signifies the localization of crustal extension and rift-related volcanism. Here we show that lavas, which erupted since ~200 ka along part of the on-land Red Sea rift system in Afar, Ethiopia, have a consistent age-progression from the rift axis outwards, indicating that axial dyke intrusion has been the primary mechanism of segment growth and that focused magmatic accretion and extension in the crust have remained stable here over this period. Our results suggest that as this rift segment has formed, in thinned and intruded continental crust, the time-averaged surface opening rate has closely approximated the total extension rate between Africa and Arabia.

  5. Styles of continental subduction and collision and their effect on formation and exhumation of UHP rocks

    Science.gov (United States)

    Warren, C. J.; Beaumont, C.; Jamieson, R. A.

    2006-12-01

    Ultra-high pressure (UHP) metamorphic rocks are exposed in many Phanerozoic mountain belts. It is now widely accepted that they represent continental margin rocks that were subducted to ≥100km during the transition from oceanic subduction to continental collision. A range of subduction-collision mechanisms have been proposed to account for UHP formation and exhumation. A series of upper-mantle-scale geodynamic models is used to test the sensitivity of a subset of these mechanisms to variations in lithosphere density, radioactive heat production, and crustal strength. The subducting crust includes an oceanic domain, a continental margin domain with variable width, thickness, strength, and heat production, and a strong internal continental domain. The models involve dynamic subduction, constant convergence velocity, density changes that accompany the main phase changes during burial, and surface erosion in the collision zone. Results show that continental subduction without retro-continent deformation is favored by relatively dense mantle lithosphere. Subduction of the retro-continent (forearc subduction) is associated with shallow-dipping, low- density mantle lithosphere. Back-thrusting of retro-lithosphere is favored by relatively weak retro-continental crust. UHP material can be exhumed in all 3 model styles, depending on the evolving geometry of the subduction-accretion zone and the degree of decoupling between subducting upper and lower continental crust in this zone. Weaker and/or hotter margins decouple and accrete, and their subsequent exhumation is driven by bulk density contrast (buoyancy) and/or forced expulsion in response to advection of strong continental material into the subduction zone (plunger effect). Stronger and/or colder continental margin material is subducted without decoupling and is then transported laterally, underplating the retro-mantle lithosphere. Model results are compatible with observed geometries and PTt paths from some UHP

  6. Modeling Continental Growth and Mantle Hydration in Earth's Evolution and the Impact of Life

    Science.gov (United States)

    Höning, Dennis; Spohn, Tilman

    2016-04-01

    The evolution of planets with plate tectonics is significantly affected by several intertwined feedback cycles. On Earth, interactions between atmosphere, hydrosphere, biosphere, crust, and interior determine its present day state. We here focus on the feedback cycles including the evolutions of mantle water budget and continental crust, and investigate possible effects of the Earth's biosphere. The first feedback loop includes cycling of water into the mantle at subduction zones and outgassing at volcanic chains and mid-ocean ridges. Water is known to reduce the viscosity of mantle rock, and therefore the speed of mantle convection and plate subduction will increase with the water concentration, eventually enhancing the rates of mantle water regassing and outgassing. A second feedback loop includes the production and erosion of continental crust. Continents are formed above subduction zones, whose total length is determined by the total size of the continents. Furthermore, the total surface area of continental crust determines the amount of eroded sediments per unit time. Subducted sediments affect processes in subduction zones, eventually enhancing the production rate of new continental crust. Both feedback loops affect each other: As a wet mantle increases the speed of subduction, continental production also speeds up. On the other hand, the total length of subduction zones and the rate at which sediments are subducted (both being functions of continental coverage) affect the rate of mantle water regassing. We here present a model that includes both cycles and show how the system develops stable and unstable fixed points in a plane defined by mantle water concentration and surface of continents. We couple these feedback cycles to a parameterized thermal evolution model that reproduces present day observations. We show how Earth has been affected by these feedback cycles during its evolution, and argue that Earth's present day state regarding its mantle water

  7. Identification of radiogenic heat source distribution in the crust: A variational approach

    Indian Academy of Sciences (India)

    R N Singh; Ajay Manglik

    2000-04-01

    Radiogenic heat sources present in the continental crust contribute significantly to the total surface heat flow and temperature distribution in the crust. Various modelsforthe depth distribution of radiogenic sources have been proposed. Among these modelsthe exponential model has been shown to be an optimal, smooth model through the variational approach applied to the heat conduction equation. In the present work, a two-layered model of the crust is considered and heat transport by advection is included in the upper layer. The heat transport in the lower layer is by conduction only. Application of variational principle determines the nature of the radiogenic source distribution in both the layers. The resultsthus obtained indicate a radiogenic source distribution which is more complex than a simple exponential model.

  8. Juvenile giant fibroadenoma

    Directory of Open Access Journals (Sweden)

    Vipul Yagnik

    2011-07-01

    Full Text Available Fibroadenomas are benign solid tumor associated with aberration of normal lobular development. Juvenile giant fibroadenoma is usually single and >5 cm in size /or >500 gms in weight. Important differential diagnoses are: phyllodes tumor and juvenile gigantomastia. Simple excision is the treatment of choice.

  9. Renewing Juvenile Justice

    Science.gov (United States)

    Macallair, Daniel; Males, Mike; Enty, Dinky Manek; Vinakor, Natasha

    2011-01-01

    The Center on Juvenile and Criminal Justice (CJCJ) was commissioned by Sierra Health Foundation to critically examine California's juvenile justice system and consider the potential role of foundations in promoting systemic reform. The information gathered by CJCJ researchers for this report suggests that foundations can perform a key leadership…

  10. Philanthropist in Juvenile Reformatory

    Institute of Scientific and Technical Information of China (English)

    HAN NIU

    2007-01-01

    @@ On the afternoon of February 1, 2007, Chen Guangbiao, a noted philanthropist, found himself in the Jiangsu Provincial Juvenile Reformatory in Jurong City for a ceremony to donate two buses, 100 computers, and 100 desks and 100 chairs for the juvenile offenders to use in their study.

  11. Parenting and juvenile delinquency

    NARCIS (Netherlands)

    Hoeve, Machteld

    2008-01-01

    Juvenile delinquency is a noteworthy problem. This thesis addressed the association between parenting and juvenile delinquency by analyzing the concepts of parenting adopted in family research in relation to criminological concepts and measures of delinquent behavior. Four studies were conducted. Th

  12. Juvenile Confinement in Context

    Science.gov (United States)

    Mendel, Richard A.

    2012-01-01

    For more than a century, the predominant strategy for the treatment and punishment of serious and sometimes not-so-serious juvenile offenders in the United States has been placement into large juvenile corrections institutions, alternatively known as training schools, reformatories, or youth corrections centers. America's heavy reliance on…

  13. Parenting and juvenile delinquency

    NARCIS (Netherlands)

    Hoeve, Machteld

    2008-01-01

    Juvenile delinquency is a noteworthy problem. This thesis addressed the association between parenting and juvenile delinquency by analyzing the concepts of parenting adopted in family research in relation to criminological concepts and measures of delinquent behavior. Four studies were conducted. Th

  14. Origin of Permian andesites from Xi Ujimqin, the Hinggan Mountains: Contributions of lower crust North China Carton

    Science.gov (United States)

    Gao, X.

    2014-12-01

    Andesite magmas at convergent margins are enriched in silica compared to magmas erupting at mid-ocean ridges and intra-plate volcanoes. Determining the cause (s) of silica enrichment is fundamental for models of continental crust formation, arc growth rates and across-arc mass balances (Plank and Langmuir, 1993; Rudnick, 1995; White et al., 2006).The Xi-Ujimqin is located the eastern segment of the Central Asian Orogenic Belt , the CAOB is composed of mainly subduction-accretion complexes, intruded by vast plutons of mainly magmatic arc origin and covered in places by their volcanic derivatives (Sengor et al. 1993; Sengor and Natalin 1996). As the most important site for Phanerozoic crustal growth in the world, the subduction-accretion complexes added ~ 5.3 million km2of material to Asia, half of which may be of juvenile origin (Sengor et al. 1993).The andesitic lavas of Daotenuoer Fms. at Xi-Ujimqin have different trace and Sr-Nd-Pb isotopic characteristic from them, these rocks span a SiO2 range of 56.83 ~ 59.17% and MgO of range of 1.21~2.91%, characterized by obviously LREE/HREE fractionation (Ce/YbCN = 11.6 ~ 16.8), strong LILE enrichment and variable Nb-Ta and Sr depletion. They have more radiogenic Sr and less radiogenic Nd and Pb isotopic composition (87Sr/86Sr(i) = 0.7063 ~ 0.7066, eNd(t) = -10.5 ~ -6.3,TDM = 1.22 ~ 1.51Ga, 206Pb / 204Pb(i) = 17.29 ~ 17.33, D7/4 = 3.7 ~ 6.9, D 8/4 = 94 ~ 104) than the contemporaneous intermediate-felsic volcanic lavas. Zircon U-Pb dating results give an emplacement age of ~253 Ma for the these rocks by LA-ICP-MS, corresponding to the Late Permian. The features of major, trace and Sr-Nd-Pb isotope of These rocks imply its sources are of attributes of lower crust North China Carton (NCC) during Paleozoic subduction and collision related to the closure of the Chinese segment of the Paleo-Asian Ocean between the NCC and south Mongolia Block (Sengor et al., 1993).

  15. One-Dimensional Velocity Structure of the Crust in Fujian, Southeast China

    Directory of Open Access Journals (Sweden)

    Hui-Teng Cai

    2015-01-01

    Full Text Available 12095 P-wave phase data were selected in this study from four NW-oriented and four NE-oriented explosion sounding survey lines conducted along the coast and midland of Southeast China during the years 2010 and 2012. The 1-D crust P-wave velocity model was obtained in the continental margin of Southeast China (Fujian Province using the travel time residual as the threshold and performing linear iterative inversion. This crust model includes 5 layers with the velocities being 5.04, 5.44, 6.06, 6.16, 6.39 km s-1, respectively, with the bottom depths being 0.23, 2.82, 6.44, 18.81, 30.42 km, respectively, and the uppermost mantle velocity being 8.08 km s-1. Compared with previous work the four P-wave phase data could effectively reflect the shallow and deep crust characteristics. The joint inversion method involves both the velocity and depth. The results in this paper could therefore be more reasonable and applicable than previous findings, with fairly good control in both the shallow and deep crusts. These findings have practical significance for compiling the earthquake travel time table and precisely locating earthquakes in this area. This work also provides an accurate preliminary model for subsequent 2-D and 3-D velocity structure inversions in the Southeast China (Fujian Province continental margin.

  16. Continental margin radiography from a potential field and sediment thickness standpoint: the Iberian Atlantic Margin

    Energy Technology Data Exchange (ETDEWEB)

    Catalan, M.; Martos, Y. M.; Martin-Davila, J.; Munoz-Martin, A.; Carbo, A.; Druet, M.

    2015-07-01

    This study reviews the state of knowledge in the Iberian Atlantic margin. In order to do this, the margin has been divided into three provinces: the Galicia margin, the southern Iberian abyssal plain, and the Tagus abyssal plain. We have used potential field and sediment thickness data. This has allowed us to study the crust, setting limits for the continental crust domain, and the amplitude of the so-called ocean-continent transition, whose end marks the beginning of the oceanic crust. The study shows the continental crust in the Galician margin to be the widest, about 210 km in length, whilst the ocean-continent transition varies slightly in this province: between 65 km wide in the south and 56 km wide in the north. This result shows up some differences with the hypothesis of other authors. The situation in the southern Iberian abyssal plain is nearly the opposite. Its continental crust extends approximately 60 km, whilst the ocean-continent transition zone is 185 km long. The Tagus abyssal plain study shows a faster morphological evolution than the others, according with the amount of crustal thinning β, the ocean-continent transition domain spanning 100 km. These results support a transitional intermediate character for almost the whole Tagus plain, in contrary to what other authors have stated. (Author)

  17. Juvenile mammary papillomatosis; Papilomatosis juvenil mamaria

    Energy Technology Data Exchange (ETDEWEB)

    Alvarez, M.; Jimenez, A. V. [Hospital Reina Sofia. Cordoba (Spain)

    2001-07-01

    Juvenile mammary papillomatosis is a benign proliferative disease of young patients, generally under 30 years of age. The most frequent clinical presentation is the existence of an elastic and mobile lymph node of the breast. Anatomopathologically, it is characterized because it presents ductal epithelial hyperplasia, sometimes with marked atypia, and there are numerous cysts having different sizes among the findings. It has been associated with an increase in the incidence of breast cancer, both in the patient herself as well as her family. We review the literature on the subject and present the mammographic and ultrasonographic findings of a 22 year old woman diagnosed of juvenile mammary papillomatosis. (Author) 12 refs.

  18. Modes of continental extension in a lithospheric wedge

    Science.gov (United States)

    Wu, G.; Lavier, L. L.; Choi, E.

    2014-12-01

    We studied extension of a lithospheric wedge as an approximation to an orogenic belt or a continental margin. We ran a series of numerical models to quantify the effects of the strength of the lower crust and a mid-crustal shear zone (MCSZ) on the extension processes. When the MCSZ is present, we found that the regional lower crustal flow plays a critical role in controlling the modes of extension. The compensation is long-wavelength when the lower crust flows from the highest to the lowest elevation in order to compensate upper crustal thinning. In response to this motion, the mantle flows towards the highest elevation in order to balance for the lower crust leaving the area under the highest topography. For weak (wet quartz regime with partial melting) or intermediate (wet quartz regime), or strong (dry quartz regime) lower crust, we recognized three predominantly decoupled modes of extension characterized by 1) significant lower crustal exhumation exemplified as a large massif, 2) formation of core complexes and detachment faults, and 3) distributive domino faulting, respectively. Without the MCSZ, however, the lower crustal flow is essentially subdued with predominantly coupled extension. For weak or intermediate, or strong lower crust, we recognized three coupled modes characterized by 1) localized generally symmetric crustal exhumation, 2) distributed grabens and narrow rifts, and 3) wide continental margins, respectively. The MCSZ controls the degree of decoupling of the lower crustal flow such that a frictionally stronger MCSZ does not change the behaviors of the models but results in a more distributed extension. Due to the long-wavelength compensation, subhorizontal Moho is achieved where intensive extension occurred for all the decoupled models with a MCSZ. Natural counterparts for each mode may be easily identified, for instance, in the Basin and Range or the Aegean.

  19. Underplating of basaltic magmas and crustal growth in a continental arc: Evidence from Late Mesozoic intermediate-felsic intrusive rocks in southern Qiangtang, central Tibet

    Science.gov (United States)

    Hao, Lu-Lu; Wang, Qiang; Wyman, Derek A.; Ou, Quan; Dan, Wei; Jiang, Zi-Qi; Wu, Fu-Yuan; Yang, Jin-Hui; Long, Xiao-Ping; Li, Jie

    2016-02-01

    account ophiolites in the Bangong-Nujiang Suture and Late Mesozoic magmatic rocks in the southern Qiangtang sub-block, we suggest that this area was located in a continental arc setting. Moreover, from the Late Jurassic to Early Cretaceous, the ancient lower crust in the southern Qiangtang sub-block was gradually replaced by mantle-derived juvenile materials. The crustal evolution indicates that, in a continental arc, basaltic magma underplating plays a key role in vertical crustal growth.

  20. Comparative riftology: insights from crustal structure into the evolution of continental rifts and passive continental margins

    Science.gov (United States)

    Kley, Jonas; Stein, Carol; Stein, Seth; Keller, Randy; Wysession, Michael; Frederiksen, Andrew

    2017-04-01

    Continental rifts evolve to seafloor spreading and are preserved in passive margins, or fail and remain as fossil features in continents. Rifts at different stages give insight into these different evolutionary paths. Of particular interest is how volcanic passive margins evolve. These features are characterized by sequences of volcanic rocks yielding magnetic anomalies landward of and sometimes larger than the oldest spreading anomalies. Seaward-dipping reflectors (SDR) occur in stretched continental crust landward of the oldest oceanic crust and are underplated by high-velocity lower crustal bodies. How and when these features form remains unclear. Insights are given by the Midcontinent Rift (MCR), formed by 1.1 Ga rifting of Amazonia from Laurentia, that failed once seafloor spreading was established elsewhere. MCR volcanics are much thicker than other continental flood basalts, due to deposition in a narrow rift rather than a broad region, giving a rift's geometry but a LIP's magma volume. The MCR provides a snapshot of the deposition of a thick highly magnetized volcanic section during rifting. Surface exposures and seismic-reflection data in and near Lake Superior show a rift basin filled by inward-dipping flood basalt layers. Had the rift evolved to seafloor spreading, the basin would have split into two sets of volcanics with opposite-facing SDRs, each with a strong magnetic anomaly. Because the rift formed as a series of alternating half-grabens, structural asymmetries between conjugate margins can naturally occur. Hence the MCR shows that many features form prior to breakup. Because the MCR was massively inverted by regional compression long after it failed and was uplifted, its structure is better known than failed rifts that incurred lesser degrees of inversion. It provides an end member for the evolution of actively extending rifts, characterized by upwelling mantle and negative gravity anomalies, in contrast to failed and inverted rifts without

  1. Mesoproterozoic continental arc magmatism and crustal growth in the eastern Central Tianshan Arc Terrane of the southern Central Asian Orogenic Belt: Geochronological and geochemical evidence

    Science.gov (United States)

    He, Zhen-Yu; Klemd, Reiner; Zhang, Ze-Ming; Zong, Ke-Qing; Sun, Li-Xin; Tian, Zuo-Lin; Huang, Bo-Tao

    2015-11-01

    Numerous microcontinents are known to occur in the Central Asian Orogenic Belt (CAOB), one of the largest accretionary orogens and the most significant area of Paleozoic crustal growth in the world. The evolution of the Precambrian crust in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB. Here, we present systematic zircon U-Pb dating and Hf isotope studies of Mesoproterozoic gneissic granitoids from the eastern Central Tianshan Arc Terrane (CTA) of the southern CAOB. The investigated intermediate to felsic (SiO2 = 60.48-78.92 wt.%) granitoids belong to the calcic- to calc-alkaline series and usually have pronounced negative Nb, Ta and Ti anomalies, relative enrichments of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) while heavy rare earth elements (HREEs) and high field strength elements (HFSEs) are depleted, revealing typical active continental margin magmatic arc geochemical characteristics. These spatially-distant rocks show consistent zircon U-Pb crystallization ages from ca. 1.45 to 1.40 Ga and thus constitute a previously unknown Mesoproterozoic continental magmatic arc covering hundreds of kilometers in the eastern segment of the CTA. Furthermore the high and mainly positive zircon εHf(t) values between - 1.0 and + 8.6 and the zircon Hf model ages of 1.95 to 1.55 Ga, which are slightly older than their crystallization ages, suggest that they were mainly derived from rapid reworking of juvenile material with a limited input of an ancient crustal component. Therefore, the formation of these granitoids defines an extensive Mesoproterozoic intermediate to felsic, subduction-related intrusive magmatic arc activity that was active from at least 1.45 to 1.40 Ga, involving significant juvenile continental growth in the eastern segment of the CTA. Furthermore the zircon U-Pb and Hf isotopic data challenge the common belief that the CTA was part of the Tarim Craton during Paleo

  2. Crust and subduction zone structure of Southwestern Mexico

    Science.gov (United States)

    Suhardja, Sandy Kurniawan; Grand, Stephen P.; Wilson, David; Guzman-Speziale, Marco; Gomez-Gonzalez, Juan Martin; Dominguez-Reyes, Tonatiuh; Ni, James

    2015-02-01

    Southwestern Mexico is a region of complex active tectonics with subduction of the young Rivera and Cocos plates to the south and widespread magmatism and rifting in the continental interior. Here we use receiver function analysis on data recorded by a 50 station temporary deployment of seismometers known as the MARS (MApping the Rivera Subduction zone) array to investigate crustal structure as well as the nature of the subduction interface near the coast. The array was deployed in the Mexican states of Jalisco, Colima, and Michoacan. Crustal thickness varies from 20 km near the coast to 42 km in the continental interior. The Rivera plate has steeper dip than the Cocos plate and is also deeper along the coast than previous estimates have shown. Inland, there is not a correlation between the thickness of the crust and topography indicating that the high topography in northern Jalisco and Michoacan is likely supported by buoyant mantle. High crustal Vp/Vs ratios (greater than 1.82) are found beneath the trenchward edge of magmatism including below the Central Jalisco Volcanic Lineament and the Michoacan-Guanajuato Volcanic Field implying a new arc is forming closer to the trench than the Trans Mexican Volcanic Belt. Elsewhere in the region, crustal Vp/Vs ratios are normal. The subducting Rivera and Cocos plates are marked by a dipping shear wave low-velocity layer. We estimate the thickness of the low-velocity layer to be 3 to 4 km with an unusually high Vp/Vs ratio of 2.0 to 2.1 and a drop in S velocity of 25%. We postulate that the low-velocity zone is the upper oceanic crust with high pore pressures. The low-velocity zone ends from 45 to 50 km depth and likely marks the basalt to eclogite transition.

  3. Pulsar glitches: The crust is not enough

    CERN Document Server

    Andersson, N; Ho, W C G; Espinoza, C M

    2012-01-01

    Pulsar glitches are traditionally viewed as a manifestation of vortex dynamics associated with a neutron superfluid reservoir confined to the inner crust of the star. In this Letter we show that the non-dissipative entrainment coupling between the neutron superfluid and the nuclear lattice leads to a less mobile crust superfluid, effectively reducing the moment of inertia associated with the angular momentum reservoir. Combining the latest observational data for prolific glitching pulsars with theoretical results for the crust entrainment we find that the required superfluid reservoir exceeds that available in the crust. This challenges our understanding of the glitch phenomenon, and we discuss possible resolutions to the problem.

  4. The Deep Crust Magmatic Refinery, Part 2 : The Magmatic Output of Numerical Models.

    Science.gov (United States)

    Bouilhol, P.; Riel, N., Jr.; Van Hunen, J.

    2016-12-01

    Metamorphic and magmatic processes occurring in the deep crust ultimately control the chemical and physical characteristic of the continental crust. A complex interplay between magma intrusion, crystallization, and reaction with the pre-existing crust provide a wide range of differentiated magma and cumulates (and / or restites) that will feed the upper crustal levels with evolved melt while constructing the lower crust. With growing evidence from field and experimental studies, it becomes clearer that crystallization and melting processes are non-exclusive but should be considered together. Incoming H2O bearing mantle melts will start to fractionate to a certain extent, forming cumulates but also releasing heat and H2O to the intruded host-rock allowing it to melt in saturated conditions. The end-result of such dynamic system is a function of the amount and composition of melt input, and extent of reaction with the host which is itself dependent on the migration mode of the melts. To better constrain lower crust processes, we have built up a numerical model [see Riel et al. associated abstract for methods] to explore different parameters, unravelling the complex interplay between melt percolation / crystallization and degassing / re-melting in a so called "hot zone" model. We simulated the intrusion of water bearing mantle melts at the base of an amphibolitized lower crust during a magmatic event that lasts 5 Ma. We varied several parameters such as Moho depth and melt rock ratio to better constrain what controls the final melt / lower crust composition.. We show the evolution of the chemical characteristics of the melt that escape the system during this magmatic event, as well as the resulting lower crust characteristics. We illustrate how the evolution of melt major elements composition reflects the progressive replacement of the crust towards compositions that are dominated by the mantle melt input. The resulting magmas cover a wide range of composition from

  5. Modification of Thickened Orogenic Crust by a Subducting Ridge: Disruption of the Andean Lower Crust of Southern Peru by the Subducting Aseismic Nazca Ridge

    Science.gov (United States)

    Bishop, B.; Beck, S. L.; Zandt, G.; Wagner, L. S.; Long, M. D.; Tavera, H.

    2015-12-01

    The subduction of oceanic plateaus or aseismic ridges represent significant perturbations to the long term development of subduction systems and associated orogenies, the consequences of which are variable and determined by the physical characteristics of both the overriding and subducting plates. Flat subduction of the ~18 km oceanic crust of the aseismic Nazca Ridge under the 50 km to 65 km continental crust of the Peruvian Andes provides an opportunity to investigate these consequences. Through analysis of 2233 teleseismic P-wave receiver functions from 55 broadband seismometers deployed in southern Peru for the PULSE, CAUGHT and PeruSE seismic experiments we have identified the South American continental Moho and subducted Nazca oceanic Moho to a higher degree of detail than previously possible in the region.We find that the continental Moho beneath the Western and Eastern Cordilleras of the Peruvian Andes is at a depth >60 km to the north and south of the subducted Nazca Ridge but at 500 km from the trench.

  6. Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth

    Science.gov (United States)

    Chowdhury, Priyadarshi; Gerya, Taras; Chakraborty, Sumit

    2017-09-01

    The rock record and geochemical evidence indicate that continental recycling has been occurring since the early history of the Earth. The stabilization of felsic continents in place of Earth's early mafic crust about 3.0 to 2.0 billion years ago, perhaps due to the initiation of plate tectonics, implies widespread destruction of mafic crust during this time interval. However, the physical mechanisms of such intense recycling on a hotter, (late) Archaean and presumably plate-tectonic Earth remain largely unknown. Here we use thermomechanical modelling to show that extensive recycling via lower crustal peeling-off (delamination but not eclogitic dripping) during continent-continent convergence was near ubiquitous during the late Archaean to early Proterozoic. We propose that such destruction of the early mafic crust, together with felsic magmatism, may have caused both the emergence of silicic continents and their subsequent isostatic rise, possibly above the sea level. Such changes in the continental character have been proposed to influence the Great Oxidation Event and, therefore, peeling-off plate tectonics could be the geodynamic trigger for this event. A transition to the slab break-off controlled syn-orogenic recycling occurred as the Earth aged and cooled, leading to reduced recycling and enhanced preservation of the continental crust of present-day composition.

  7. Petrology and tectonics of Phanerozoic continent formation: From island arcs to accretion and continental arc magmatism

    Science.gov (United States)

    Lee, C.-T.A.; Morton, D.M.; Kistler, R.W.; Baird, A.K.

    2007-01-01

    Mesozoic continental arcs in the North American Cordillera were examined here to establish a baseline model for Phanerozoic continent formation. We combine new trace-element data on lower crustal xenoliths from the Mesozoic Sierra Nevada Batholith with an extensive grid-based geochemical map of the Peninsular Ranges Batholith, the southern equivalent of the Sierras. Collectively, these observations give a three-dimensional view of the crust, which permits the petrogenesis and tectonics of Phanerozoic crust formation to be linked in space and time. Subduction of the Farallon plate beneath North America during the Triassic to early Cretaceous was characterized by trench retreat and slab rollback because old and cold oceanic lithosphere was being subducted. This generated an extensional subduction zone, which created fringing island arcs just off the Paleozoic continental margin. However, as the age of the Farallon plate at the time of subduction decreased, the extensional environment waned, allowing the fringing island arc to accrete onto the continental margin. With continued subduction, a continental arc was born and a progressively more compressional environment developed as the age of subducting slab continued to young. Refinement into a felsic crust occurred after accretion, that is, during the continental arc stage, wherein a thickened crustal and lithospheric column permitted a longer differentiation column. New basaltic arc magmas underplate and intrude the accreted terrane, suture, and former continental margin. Interaction of these basaltic magmas with pre-existing crust and lithospheric mantle created garnet pyroxenitic mafic cumulates by fractional crystallization at depth as well as gabbroic and garnet pyroxenitic restites at shallower levels by melting of pre-existing lower crust. The complementary felsic plutons formed by these deep-seated differentiation processes rose into the upper crust, stitching together the accreted terrane, suture and former

  8. Basins in ARC-continental collisions

    Science.gov (United States)

    Draut, Amy E.; Clift, Peter D.; Busby, Cathy; Azor, Antonio

    2012-01-01

    Arc-continent collisions occur commonly in the plate-tectonic cycle and result in rapidly formed and rapidly collapsing orogens, often spanning just 5-15 My. Growth of continental masses through arc-continent collision is widely thought to be a major process governing the structural and geochemical evolution of the continental crust over geologic time. Collisions of intra-oceanic arcs with passive continental margins (a situation in which the arc, on the upper plate, faces the continent) involve a substantially different geometry than collisions of intra-oceanic arcs with active continental margins (a situation requiring more than one convergence zone and in which the arc, on the lower plate, backs into the continent), with variable preservation potential for basins in each case. Substantial differences also occur between trench and forearc evolution in tectonically erosive versus tectonically accreting margins, both before and after collision. We examine the evolution of trenches, trench-slope basins, forearc basins, intra-arc basins, and backarc basins during arc-continent collision. The preservation potential of trench-slope basins is low; in collision they are rapidly uplifted and eroded, and at erosive margins they are progressively destroyed by subduction erosion. Post-collisional preservation of trench sediment and trench-slope basins is biased toward margins that were tectonically accreting for a substantial length of time before collision. Forearc basins in erosive margins are usually floored by strong lithosphere and may survive collision with a passive margin, sometimes continuing sedimentation throughout collision and orogeny. The low flexural rigidity of intra-arc basins makes them deep and, if preserved, potentially long records of arc and collisional tectonism. Backarc basins, in contrast, are typically subducted and their sediment either lost or preserved only as fragments in melange sequences. A substantial proportion of the sediment derived from

  9. Cryopyrin-Associated Autoinflammatory Syndromes (CAPS) - Juvenile

    Science.gov (United States)

    ... Cryopyrin-Associated Autoinflammatory Syndrome (CAPS) (Juvenile) Dermatomyositis (Juvenile) Familial Mediterranean Fever (Juvenile) Fibromyalgia Giant Cell Arteritis Glucocorticoid-induced Osteoperosis ...

  10. Juvenil idiopatisk arthritis

    DEFF Research Database (Denmark)

    Herlin, Troels

    2002-01-01

    The new classification of juvenile idiopathic arthritis (JIA) is described in this review. Clinical characteristics divide JIA in to subtypes: systemic, oligoarticular (persistent and extended type), RF-positive and--negative polyarticular, enthesitis-related arthritis and psoriatic arthritis...

  11. Juvenile Rockfish Recruitment Cruise

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — In 1983, the groundfish analysis project began a series of yearly cruises designed to assess the annual abundance of juvenile rockfish along the central California...

  12. Juvenile Idiopathic Arthritis

    Science.gov (United States)

    ... Physical Therapy Regular Exercise en español Artritis idiopática juvenil It may begin with a swollen knuckle, a ... may suddenly appear and disappear, developing in one area and then another. High fevers that tend to ...

  13. Juvenile Sex Offenders.

    Science.gov (United States)

    Ryan, Eileen P; Otonichar, Joseph M

    2016-07-01

    Sexual offending by juveniles accounts for a sizable percentage of sexual offenses, especially against young children. In this article, recent research on female juvenile sex offenders (JSOs), risk factors for offending in juveniles, treatment, and the ways in which these youth may differ from general delinquents will be reviewed. Most JSOs do not go on to develop paraphilic disorders or to commit sex offenses during adulthood, and as a group, they are more similar to nonsexual offending juvenile delinquents than to adult sex offenders. Recent research has elucidated some differences between youth who commit sex offenses and general delinquents in the areas of atypical sexual interests, the use of pornography, and early sexual victimization during childhood.

  14. Temporal evolution of continental lithospheric strength in actively deforming regions

    Science.gov (United States)

    Thatcher, W.; Pollitz, F.F.

    2008-01-01

    It has been agreed for nearly a century that a strong, load-bearing outer layer of earth is required to support mountain ranges, transmit stresses to deform active regions and store elastic strain to generate earthquakes. However the dept and extent of this strong layer remain controversial. Here we use a variety of observations to infer the distribution of lithospheric strength in the active western United States from seismic to steady-state time scales. We use evidence from post-seismic transient and earthquake cycle deformation reservoir loading glacio-isostatic adjustment, and lithosphere isostatic adjustment to large surface and subsurface loads. The nearly perfectly elastic behavior of Earth's crust and mantle at the time scale of seismic wave propagation evolves to that of a strong, elastic crust and weak, ductile upper mantle lithosphere at both earthquake cycle (EC, ???10?? to 103 yr) and glacio-isostatic adjustment (GIA, ???103 to 104 yr) time scales. Topography and gravity field correlations indicate that lithosphere isostatic adjustment (LIA) on ???106-107 yr time scales occurs with most lithospheric stress supported by an upper crust overlying a much weaker ductile subtrate. These comparisons suggest that the upper mantle lithosphere is weaker than the crust at all time scales longer than seismic. In contrast, the lower crust has a chameleon-like behavior, strong at EC and GIA time scales and weak for LIA and steady-state deformation processes. The lower crust might even take on a third identity in regions of rapid crustal extension or continental collision, where anomalously high temperatures may lead to large-scale ductile flow in a lower crustal layer that is locally weaker than the upper mantle. Modeling of lithospheric processes in active regions thus cannot use a one-size-fits-all prescription of rheological layering (relation between applied stress and deformation as a function of depth) but must be tailored to the time scale and tectonic

  15. Initiation of continental accretion: metamorphic conditions

    Science.gov (United States)

    Clement, Conand; Frederic, Mouthereau; Gianreto, Manatschal; Adbeltif, Lahfid

    2017-04-01

    The physical processes involved at the beginning of the continental collision are largely unknown because they are transient and therefore hardly identifiable from the rock record. Despite the importance of key parameters for understanding mountain building processes, especially the formation of deep mountain roots and their impacts on earthquakes nucleation, rock/fluid transfers and oil/gas resources in the continental crust, observations from the earliest collision stages remain fragmentary. Here, we focus on the example of Taiwan, a young and active mountain belt where the transition from oceanic subduction, accretion of the first continental margin to mature collision can be followed in space and time. We present preliminary results and provide key questions regarding the reconstruction of time-pressure-temperature paths of rocks & fluids to allow discriminating between rift-related thermal/rheological inheritance and burial/heating phases during convergence. Previous studies have focused on peak temperatures analyzed by Raman Spectrometry of Carbonaceous Matter from the deeper structural layers exposed in the Central Range of Taiwan. In the pre-rift sediments, these studies reported a positive gradient from West to Est, and values from units. Cross sections and maps with high resolution peak temperatures are in process as well as pressure estimations to determine how the sediments were metamorphosed. In addition to this work, we report a few inherited temperatures in the 390-570 °C range, indicating recycling of organic matter from metasediments that recorded HT events, likely originated from higher grade metamorphic units of mainland China, which have been eroded and deposited in the post-rift sediments.

  16. CSDP: Seismology of continental thermal regime

    Energy Technology Data Exchange (ETDEWEB)

    Aki, K.

    1989-04-01

    This is a progress report for the past one year of research (year 2 of 5-year project) under the project titled CSDP: Seismology of Continental Thermal Regime'', in which we proposed to develop seismological interpretation theory and methods applicable to complex structures encountered in continental geothermal areas and apply them to several candidate sites for the Continental Scientific Drilling Project. During the past year, two Ph.D. thesis works were completed under the present project. One is a USC thesis on seismic wave propagation in anisotropic media with application to defining fractures in the earth. The other is a MIT thesis on seismic Q and velocity structure for the magma-hydrothermal system of the Valles Caldera, New Mexico. The P.I. co-organized the first International Workshop on Volcanic Seismology at Capri, Italy in October 1988, and presented the keynote paper on the state-of-art of volcanic seismology''. We presented another paper at the workshop on Assorted Seismic Signals from Kilauea Volcano, Hawaii. Another international meeting, namely, the Chapman Conference on seismic anisotropy in the earth's crust at Berkeley, California in May 1988, was co-organized by the co-P.I. (P.C.L), and we presented our work on seismic waves in heterogeneous and anisotropic media. Adding the publications and presentations made in the past year to the list for the preceding year, the following table lists 21 papers published, submitted or presented in the past two years of the present project. 65 refs., 334 figs., 1 tab.

  17. Juvenile Spondyloarthritis Treatment Recommendations

    OpenAIRE

    Tse, Shirley; Burgos-Vargas, Ruben; Colbert, Robert A

    2012-01-01

    No specific recommendations for the treatment of juvenile spondyloarthritis have been established. Important differences exist in how spondyloarthritis begins and progresses in children and adults, supporting the need for pediatric-specific recommendations. Recently published recommendations for the treatment of juvenile arthritis consider children with sacroiliitis in a separate group, and allow for more accelerated institution of a TNF inhibitor depending on disease activity and prognostic ...

  18. Parenting and juvenile delinquency

    OpenAIRE

    Hoeve, Machteld

    2008-01-01

    Juvenile delinquency is a noteworthy problem. This thesis addressed the association between parenting and juvenile delinquency by analyzing the concepts of parenting adopted in family research in relation to criminological concepts and measures of delinquent behavior. Four studies were conducted. The first study addressed a meta-analysis on parenting characteristics and styles in relation to delinquency. In this meta-analysis, previous manuscripts were systematically analyzed, computing mean ...

  19. Geodynamic investigation of the processes that control Lu-Hf isotopic differences between different mantle domains and the crust

    Science.gov (United States)

    Jones, Rosie; van Keken, Peter; Hauri, Erik; Vervoort, Jeff; Ballentine, Chris J.

    2016-04-01

    The chemical and isotopic composition of both the Earth's mantle and the continental crust are greatly influenced by subduction zone processes, such as the formation of continental crust through arc volcanism and the recycling of surface material into the deep mantle. Here we use a combined geodynamical-geochemical approach to investigate the long term role of subduction on the Lu-Hf isotopic evolution of the mantle and the continental crust. We apply the geodynamic model developed by Brandenburg et al., 2008. This model satisfies the geophysical constraints of oceanic heat flow and average plate velocities, as well as geochemical observations such as 40Ar in the atmosphere, and reproduces the geochemical distributions observed in multiple isotope systems which define the HIMU, MORB and EM1 mantle endmembers. We extend this application to investigate the detail of terrestrial Lu-Hf isotope distribution and evolution, and specifically to investigate the role of sediment recycling in the generation of EM2 mantle compositions. The model has been updated to produce higher resolution results and to include a self-consistent reorganisation of the plates with regions of up-/down-wellings. The model assumes that subduction is initiated at 4.5 Ga and that a transition from 'dry' to 'wet' subduction occurred at 2.5 Ga. The modelling suggests that the epsilon Hf evolution of the upper mantle can be generated through the extraction and recycling of the oceanic crust, and that the formation of continental crust plays a lesser role. Our future intention is to utilise the model presented here to investigate the differences observed in the noble gas compositions (e.g., 40Ar/36Ar, 3He/4He) of MORB and OIB. Brandenburg, J.P., Hauri, E.H., van Keken, P.E., Ballentine, C.J., 2008. Earth and Planetary Science Letters 276, 1-13.

  20. Continental growth and mantle hydration as intertwined feedback cycles in the thermal evolution of Earth

    Science.gov (United States)

    Höning, Dennis; Spohn, Tilman

    2016-06-01

    A model of Earth's continental coverage and mantle water budget is discussed along with its thermal evolution. The model links a thermal evolution model based on parameterized mantle convection with a model of a generic subduction zone that includes the oceanic crust and a sedimentary layer as carriers of water. Part of the subducted water is used to produce continental crust while the remainder is subducted into the mantle. The total length of the subduction zones is calculated from the total surface area of continental crust assuming randomly distributed continents. The mantle viscosity is dependent of temperature and the water concentration. Sediments are generated by continental crust erosion, and water outgassing at mid-oceanic ridges closes the water cycle. We discuss the strongly coupled, non-linear model using a phase plane defined by the continental coverage and mantle water concentration. Fixed points are found in the phase plane at which the rates of change of both variables are zero. These fixed points evolve with time, but in many cases, three fixed points emerge of which two are stable and an intermediate point is unstable with respect to continental coverage. With initial conditions from a Monte-Carlo scheme we calculate evolution paths in the phase plane and find a large spread of final states that all have a mostly balanced water budget. The present day observed 40% continental surface coverage is found near the unstable fixed point. Our evolution model suggests that Earth's continental coverage formed early and has been stable for at least 1.5 Gyr. The effect of mantle water regassing (and mantle viscosity depending on water concentration) is found to lower the present day mantle temperature by about 120 K, but the present day mantle viscosity is affected little. The water cycle thus complements the well-known thermostat effect of viscosity and mantle temperature. Our results further suggest that the biosphere could impact the feedback cycles by

  1. Molecular mobility in crispy bread crust

    NARCIS (Netherlands)

    Nieuwenhuijzen, van N.H.

    2008-01-01

    The aim of the PhD study on molecular mobility was to analyse the molecular grounds for the deterioration of crispy/crunchy characteristics of cellular solid foods. A fresh baguette for example has a crispy crust and a moist and soft interior. Moisture migrates from crumb to crust. Already at a wate

  2. Water Uptake Mechanism in Crispy Bread Crust

    NARCIS (Netherlands)

    Nieuwenhuijzen, van N.H.; Meinders, M.B.J.; Tromp, R.H.; Hamer, R.J.; Vliet, van T.

    2008-01-01

    Crispness is an important quality characteristic of dry solid food products such as crispy rolls. Its retention is directly related to the kinetics of water uptake by the crust. In this study, a method for the evaluation of the water sorption kinetics in bread crust is proposed. Two different sorpti

  3. THERMAL CONTINENTALISM IN EUROPE

    Directory of Open Access Journals (Sweden)

    APOSTOL L.

    2015-03-01

    Full Text Available In the context of current climate changes, this article aims to highlight the continental characteristics of Europe’s climate (including a temporal evolution, regarding the multiannual thermal averages. For this purpose, 78 meteorological stations have been selected, placed approximately on two pairs of transects on West-East and South-North directions. The data were extracted from www.giss.nasa.gov (NASA Goddard Institute for Space Studies, statistically processed (Open Office and mapped (www.saga-gis.org. For the lapse of time 1961-2010, the analysis of multiannual temperature averages has shown the following: if the multiannual average temperature is strongly influenced by latitude, its deviations are more dependent on longitude; the multiannual average thermal amplitude, as well as the Gorczynski continentality index, are strongly related to longitude; their temporal evolution has shown a significant decrease in the Eastern half of the continent and an increase (although less significant in Western Europe.

  4. The role of amphibole in the evolution of arc magmas and crust: the case from the Jurassic Bonanza arc section, Vancouver Island, Canada

    Science.gov (United States)

    Larocque, Jeff; Canil, Dante

    2010-04-01

    The Jurassic Bonanza arc, on Vancouver Island, British Columbia, represents an exhumed island arc crustal section of broadly diorite composition. We studied bodies of mafic and ultramafic cumulates within deeper levels of the arc to constrain the conditions and fractionation pathways leading from high-Mg basalt to andesite and dacite. Major element trends coupled with textural information show the intercumulus crystallization of amphibole, as large oikocrysts enclosing olivine in primitive cumulates controls the compositions of liquids until the onset of plagioclase crystallization. This process is cryptic, occurring only in the plutonic section, and explains the paucity of amphibole in mafic arc volcanics and the change in the Dy/Yb ratios in many arc suites with differentiation. The correlation of octahedral Al in hornblende with pressure in liquidus experiments on high-Mg basalts is applied as an empirical barometer to hornblendes from the Bonanza arc. It shows that crystallization took place at 470-880 MPa in H2O-saturated primitive basaltic magmas. There are no magmatic equivalents to bulk continental crust in the Bonanza arc; no amount of delamination of ultramafic cumulates will shift the bulk arc composition to the high-Mg# andesite composition of bulk continental crust. Garnet removal from wet magmas appears to be the key factor in producing continental crust, requiring high pressures and thick crust. Because oceanic island arcs are built on thinner crust, the long-term process generating the bulk continental crust is the accretion of island arcs to continental margins with attendant tectonic thickening.

  5. Continental Rifts and Resources

    Science.gov (United States)

    Stein, Holly J.

    2017-04-01

    Nearly all resource-forming systems involve upward mobility of fluids and melts. In fact, one of the most effective means of chemically transforming the earth's crust can be readily observed in the rift environment. Imposition of rifting is based on deeper stresses that play out in the crust. At its most fundamental level, rifting transfers heat and fluids to the crust. Heat delivered by fluids aids both in transport of metal and maturation of hydrocarbons. The oxidizing capacity of fluids on their arrival in the deep crust, whether derived from old slabs, depleted upper mantle and/or deeper, more primitive mantle, is a fundamental part of the resource-forming equation. Oxidizing fluids transport some metals and breakdown kerogen, the precursor for oil. Reducing fluids transport a different array of metals. The tendency is to study the resource, not the precursor or the non-economic footprint. In doing so, we lose the opportunity to discover the involvement and significance of initiating processes; for example, externally derived fluids may produce widespread alteration in host rocks, a process that commonly precedes resource deposition. It is these processes that are ultimately the transferable knowledge for successful mineral and hydrocarbon exploration. Further limiting our understanding of process is the tendency to study large, highly complex, and economically successful ore-forming or petroleum systems. In order to understand their construction, however, it is necessary to put equal time toward understanding non-economic systems. It is the non-economic systems that often clearly preserve key processes. The large resource-forming systems are almost always characterized by multiple episodes of hydrothermal overprints, making it difficult if not impossible to clearly discern individual events. Understanding what geologic and geochemical features blocked or arrested the pathway to economic success or, even worse, caused loss of a resource, are critical to

  6. Vocational Teachers' Role in Serving Juvenile Offenders.

    Science.gov (United States)

    Meers, Gary D.

    1983-01-01

    Educators need to understand the juvenile justice system to understand what juvenile offenders go through while completing their sentences. This article reviews cases and juvenile charge classifications, and presents a model for alternative sentencing options for juveniles. (JOW)

  7. U-series Disequilibria in Continental Arcs: NE Japan Case

    Science.gov (United States)

    Tepley, F. J.; Gill, J. B.; Williams, R. W.

    2005-12-01

    Basalts and andesites from continental arcs typically are close to equilibrium between (238U) and (230Th), and have subdued excesses of 226Ra compared to oceanic arcs (Turner et al., 2003). There is ambiguity whether these and other geochemical features derive from subducted sediment, subcontinental lithosphere, or the crust. We report new 238U-234U-230Th-226Ra data for 20 historical to Holocene samples from Asama in the south of NE Japan to Tarumai in the north. Most straddle the equiline with 4.

  8. The effect of energy feedbacks on continental strength.

    Science.gov (United States)

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

    2006-07-06

    The classical strength profile of continents is derived from a quasi-static view of their rheological response to stress--one that does not consider dynamic interactions between brittle and ductile layers. Such interactions result in complexities of failure in the brittle-ductile transition and the need to couple energy to understand strain localization. Here we investigate continental deformation by solving the fully coupled energy, momentum and continuum equations. We show that this approach produces unexpected feedback processes, leading to a significantly weaker dynamic strength evolution. In our model, stress localization focused on the brittle-ductile transition leads to the spontaneous development of mid-crustal detachment faults immediately above the strongest crustal layer. We also find that an additional decoupling layer forms between the lower crust and mantle. Our results explain the development of decoupling layers that are observed to accommodate hundreds of kilometres of horizontal motions during continental deformation.

  9. Vertical stretching and crustal thickening at Nanga Parbat, Pakistan Himalaya: A model for distributed continental deformation during mountain building

    Science.gov (United States)

    Butler, R. W. H.; Casey, M.; Lloyd, G. E.; Bond, C. E.; McDade, P.; Shipton, Z.; Jones, R.

    2002-08-01

    The localization of strain in the continental crust during compressional tectonics is examined using the active structures at the Nanga Parbat massif, an exhumed tract of Indian continental crust in the Pakistan Himalaya. This large-scale (~40 km wavelength) structure is considered to involve the whole crust. Thrusting at the modern surface places gneisses of the Indian continental crust onto Holocene deposits. At the Raikhot transect, the thrust zone carries a relatively narrow (2 km wide) shear zone within which minor structures are asymmetric and the deformation apparently noncoaxial. However, modeling of foliation and augen preferred orientation/ellipticity suggests that the bulk deformation is a combination of relatively small simple shear strains (γ = 1) with larger stretching strains. Heterogeneous stretching within the shear zone was accommodated by localized shearing on metabasic layers so that strain is partitioned. Outside this shear zone on the transect there is penetrative deformation throughout the Nanga Parbat massif. This broadly distributed deformation shows no asymmetry or evidence of rotation. Rather this deformation is better described as near pure-shear subvertical stretching. Augen ellipticities suggest subvertical stretches of greater than 200%. Consideration of plausible changes in crustal thickness during the amplification of the Nanga Parbat structure suggests the magnitude of vertical stretch decays with depth. Presumably these strains in the deep crust are more distributed but weaker than in the exposed middle crustal sections, assuming conservation of horizontal shortening displacement with depth. These studies suggest that penetrative vertical stretching through dominantly pure shear deformation is an effective mechanism for thickening the continental crust and that models which assume that simple shear zones penetrate the whole crust need not be of ubiquitous applicability.

  10. Trunk asymmetry in juveniles

    Directory of Open Access Journals (Sweden)

    Triantafyllopoulos Georgios

    2008-09-01

    Full Text Available Abstract Background Trunk asymmetry (TA is a common phenomenon in children, but its incidence in juveniles is not known. The present cross sectional study reports TA in normal juveniles and provides data which describe the evolution of TA from early childhood to adolescence. Materials and methods The scoliometer readings in both standing and sitting forward bending position (FBP of 3301 children, (1645 boys, and 1656 girls aged from 3 to 9 years old were studied. TA was quantified by measuring angle of trunk rotation (ATR and children were categorized as symmetric (ATR = 0°, mild asymmetric (ATR 1° – 6° and severely asymmetric (ATR ≥ 7°. The difference of TA between standing and sitting FBP as well as differences between boys and girls in frequency of TA were also calculated. The scoliometer readings were analyzed by age to reveal at which age the juvenile pattern of TA changes into the adolescent one. Results 74.2% of boys and 77% of girls were symmetric (ATR = 0° in the thoracic region in standing FBP, while 82.7% of boys and 84.1% of girls were symmetric in the thoracic region in sitting FBP. Juvenile girls are more symmetric than boys but severe TA was found almost the same between the two genders. A significant reduction in the frequency of mild TA from standing into sitting FBP, in all the examined regions in both boys and girls was found, but in severe TA this reduction is very small. Analysing scoliometer readings by age it appears that significant TA changes take place between 8–9 years of age for boys and between 6–7 and 8–9 years for girls. TA in boys is changing into the adolescent pattern at a later age than in girls. Conclusion Juveniles were found more symmetric than adolescents, who were studied previously in a different study. Furthermore, juvenile girls were found more symmetric than boys. Juvenile TA pattern seems to be in accordance with the higher incidence of juvenile idiopathic scoliosis in boys. Furthermore

  11. Gravity and structure of the continental margins of southwestern Mexico and northwestern Guatemala

    Science.gov (United States)

    Couch, Richard; Woodcock, Stephen

    1981-03-01

    Geophysical measurements over the eastern end of the Tehuantepec Ridge and adjacent continental margins of southern Mexico and northern Guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces. A positive free-air gravity anomaly which extends northwestward along the outer continental shelf of Guatemala curves abruptly landward in the Gulf of Tehuantepec. The positive shelf anomaly is on trend with the positive anomaly of the Nicoya Peninsula, Costa Rica, and suggests that rocks genetically related to Cretaceous rocks of the Nicoya Complex extend northwestward along the continental shelf to the Gulf of Tehuantepec. A crustal and subcrustal cross section of the continental margin of Guatemala, constrained by gravity, magnetic, and seismic refraction data, indicates that the rock strata causing the outer shelf gravity high dip landward, consistent with imbricate thrusting of the oceanic crust beneath and into the continental margin. A model crustal cross section of the continental margin of southern Mexico, north of the Tehuantepec Ridge, shows a markedly different margin structure with a relatively small amount of continental accretion and a continental crustal block extending to within approximately 25 km of the trench axis.

  12. Juvenile Incarceration and Health.

    Science.gov (United States)

    Barnert, Elizabeth S; Perry, Raymond; Morris, Robert E

    2016-03-01

    Addressing the health status and needs of incarcerated youth represents an issue at the nexus of juvenile justice reform and health care reform. Incarcerated youth face disproportionately higher morbidity and higher mortality compared to the general adolescent population. Dental health, reproductive health, and mental health needs are particularly high, likely as a result of lower access to care, engagement in high-risk behaviors, and underlying health disparities. Violence exposure and injury also contribute to the health disparities seen in this population. Further, juvenile incarceration itself is an important determinant of health. Juvenile incarceration likely correlates with worse health and social functioning across the life course. Correctional health care facilities allow time for providers to address the unmet physical and mental health needs seen in this population. Yet substantial challenges to care delivery in detention facilities exist and quality of care in detention facilities varies widely. Community-based pediatricians can serve a vital role in ensuring continuity of care in the postdetention period and linking youth to services that can potentially prevent juvenile offending. Pediatricians who succeed in understanding and addressing the underlying social contexts of their patients' lives can have tremendous impact in improving the life trajectories of these vulnerable youth. Opportunities exist in clinical care, research, medical education, policy, and advocacy for pediatricians to lead change and improve the health status of youth involved in the juvenile justice system.

  13. On the Evolution of Glaciated Continental Margins

    Science.gov (United States)

    Sverre Laberg, Jan; Rydningen, Tom Arne; Safronova, Polina A.; Forwick, Matthias

    2016-04-01

    Glaciated continental margins, continental margins where a grounded ice sheet repeatedly has been at or near the shelf break, are found at both northern and southern high-latitudes. Their evolution are in several aspects different from their low-latitude counterparts where eustatic sea-level variations possess a fundamental control on their evolution and where fluvial systems provide the main sediment input. From studies of the Norwegian - Barents Sea - Svalbard and NE Greenland continental margins we propose the following factors as the main control on the evolution of glaciated continental margins: 1) Pre-glacial relief controlling the accommodation space, 2) Ice sheet glaciology including the location of fast-flowing ice streams where source area morphology exerts a fundamental control, 3) Composition of the glacigenic sediments where the clay content in previous studies have been found to be important, and 4) Sea-level controlled both by eustacy and isostacy. From three case studies, 1) the western Barents Sea, 2) part of the North Norwegian (Troms), and 3) the Mid-Norwegian margin, the influence on these factors for the sea-floor morphology, sedimentary processes of the continental slope - deep sea and continental margin architecture are discussed. The pre-glacial relief of the mid-Norwegian and Troms margins relates to the onset of rifting and plate break-up from the early Cenozoic while for the SW Barents Sea, plate shear was followed by rifting. A wide zone of extended continental crust occurs offshore mid-Norway while this zone is much narrower offshore Troms leading to a more pronounced pre-glacial relief. Regarding sediment delivery and ice sheet glaciology the western Barents Sea exemplifies very high sediment input corresponding to an estimated average erosion of the source area of ~0.4 mm/yr (SW Barents Sea), much of which is related to subglacial erosion of Mesozoic - Cenozoic sedimentary rocks from large paleo-ice streams. The mid-Norwegian margin

  14. Microphytic crusts: 'topsoil' of the desert

    Science.gov (United States)

    Belnap, Jayne

    1990-01-01

    Deserts throughout the world are the home of microphytic, or cryptogamic, crusts. These crusts are dominated by cyanobacteria, previously called blue-green algae, and also include lichens, mosses, green algae, microfungi and bacteria. They are critical components of desert ecosystems, significantly modifying the surfaces on which they occur. In the cold deserts of the Colorado Plateau (including parts of Utah, Arizona, Colorado, and New Mexico), these crusts are extraordinarily well-developed, and may represent 70-80% of the living ground cover.

  15. Biogenic crust dynamics on sand dunes

    CERN Document Server

    Kinast, Shai; Yizhaq, Hezi; Ashkenazy, Yosef

    2012-01-01

    Sand dunes are often covered by vegetation and biogenic crusts. Despite their significant role in dune stabilization, biogenic crusts have rarely been considered in studies of dune dynamics. Using a simple model, we study the existence and stability ranges of different dune-cover states along gradients of rainfall and wind power. Two ranges of alternative stable states are identified: fixed crusted dunes and fixed vegetated dunes at low wind power, and fixed vegetated dunes and active dunes at high wind power. These results suggest a cross-over between two different forms of desertification.

  16. Reconstructing Rodinia by Fitting Neoproterozoic Continental Margins

    Science.gov (United States)

    Stewart, John H.

    2009-01-01

    extensional in origin, supports recognition of the Neoproterozoic fragmentation pattern of Rodinia and outlines the major continental masses that, prior to the breakup, formed the supercontinent. Using this pattern, Rodinia can be assembled by fitting the pieces together. Evidence for Neoproterozoic margins is fragmentary. The most apparent margins are marked by miogeoclinal deposits (passive-margin deposits). The margins can also be outlined by the distribution of continental-margin magmatic-arc rocks, by juvenile ocean-floor rocks, or by the presence of continent-ward extending aulacogens. Most of the continental margins described here are Neoproterozoic, and some had an older history suggesting that they were major, long-lived lithospheric flaws. In particular, the western margin of North America appears to have existed for at least 1,470 Ma and to have been reactivated many times in the Neoproterozoic and Phanerozoic. The inheritance of trends from the Mesoproterozoic by the Neoproterozoic is particularly evident along the eastern United States, where a similarity of Mesoproterozoic (Grenville) and Neoproterozoic trends, as well as Paleozoic or Mesozoic trends, is evident. The model of Rodinia presented here is based on both geologic and paleomagnetic information. Geologic evidence is based on the distribution and shape of Neoproterozoic continents and on assembling these continents so as to match the shape, history, and scale of adjoining margins. The proposed model places the Laurasian continents?Baltica, Greenland, and Laurentia?west of the South American continents (Amazonia, Rio de La Plata, and Sa? Francisco). This assembly is indicated by conjugate pairs of Grenville-age rocks on the east side of Laurentia and on the west side of South America. In the model, predominantly late Neoproterozoic magmatic-arc rocks follow the trend of the Grenville rocks. The boundary between South America and Africa is interpreted as the site of a Wilson cycle

  17. Extending juvenility in grasses

    Energy Technology Data Exchange (ETDEWEB)

    Kaeppler, Shawn; de Leon Gatti, Natalia; Foerster, Jillian

    2017-04-11

    The present invention relates to compositions and methods for modulating the juvenile to adult developmental growth transition in plants, such as grasses (e.g. maize). In particular, the invention provides methods for enhancing agronomic properties in plants by modulating expression of GRMZM2G362718, GRMZM2G096016, or homologs thereof. Modulation of expression of one or more additional genes which affect juvenile to adult developmental growth transition such as Glossy15 or Cg1, in conjunction with such modulation of expression is also contemplated. Nucleic acid constructs for down-regulation of GRMZM2G362718 and/or GRMZM2G096016 are also contemplated, as are transgenic plants and products produced there from, that demonstrate altered, such as extended juvenile growth, and display associated phenotypes such as enhanced yield, improved digestibility, and increased disease resistance. Plants described herein may be used, for example, as improved forage or feed crops or in biofuel production.

  18. Continental scientific drilling program. [Los Alamos, New Mexico, July 17-21, 1978

    Energy Technology Data Exchange (ETDEWEB)

    1979-01-01

    The dynamics, structure, evolution, and genesis of the continents offer a major scientific challenge. At the same time, society faces many problems that require information about the continental crust for solution. A workshop on continental drilling for scientific purposes addressed the questions of how to maximize the scientific value of current and planned efforts of federal agencies and industry and how to supplement these efforts with holes drilled solely for scientific purpoes. Four panels addressed the scientific and associated societal problems relating to basement structures and deep continental basins, thermal regimes, mineral resources, and earthuqakes. These panels identified the main problems in each area that could be solved by information obtained from drill holes. A fifth panel considered needs for technological developments. The importance of a communications and coordinating mechanism to maximize the scientific results was noted. To this end, a Continental Scientific Drilling Program is outlined, including two advisory and guiding committees, one concerned with scientific objectives, the other with operations. (RWR)

  19. DERMATOMIOSITIS JUVENIL Y EMBARAZO

    OpenAIRE

    Evans M,Gregorio; Poulsen R,Ronald; Blanco R,Romiely; Luna V,Viviana

    2002-01-01

    La dermatomiositis juvenil es un desorden inflamatorio crónico multisistémico del tejido conectivo. Tiene una incidencia de 2-3/100.000/año. Con la disminución en la mortalidad experimentada en los últimos decenios, la atención está cifrada en la morbilidad a largo plazo y en las alteraciones funcionales. Con un tratamiento agresivo los niños con dermatomiositis juvenil generalmente tienen un futuro promisorio, sin incapacidad o con incapacidad mínima. La mortalidad actualmente se estima cerc...

  20. Juvenile idiopatiske inflammatoriske myopatier

    Directory of Open Access Journals (Sweden)

    Helga Sanner

    2009-10-01

    Full Text Available Juvenile idiopathic inflammatory myopathies (JIIM is a group of rare autoimmune systemic diseases in children and adolescents, characterized by chronic skeletal muscle inflammation. Unlike in adults, dermatomyositis (JDM is by far the most common of the idiopathic inflammatory myopathies in children and adolescents. The hallmark of JDM is calcinosis, lipodystrophy and vasculitis, findings that differs the juvenile form of dermatomyosits from the adult form. JDM is still diagnosed and classified by Bohan and Peter’s criteria from 1975. There are limited data on long time outcome of this disease

  1. Temperature distribution in magnetized neutron star crusts

    CERN Document Server

    Geppert, U; Page, D

    2004-01-01

    We investigate the influence of different magnetic field configurations on the temperature distribution in neutron star crusts. We consider axisymmetric dipolar fields which are either restricted to the stellar crust, ``crustal fields'', or allowed to penetrate the core, ``core fields''. By integrating the two-dimensional heat transport equation in the crust, taking into account the classical (Larmor) anisotropy of the heat conductivity, we obtain the crustal temperature distribution, assuming an isothermal core. Including quantum magnetic field effects in the envelope as a boundary condition, we deduce the corresponding surface temperature distributions. We find that core fields result in practically isothermal crusts unless the surface field strength is well above $10^{15}$ G while for crustal fields with surface strength above a few times $10^{12}$ G significant deviations from isothermality occur at core temperatures inferior or equal to $10^8$ K. At the stellar surface, the cold equatorial region produce...

  2. Juvenile cellulitis in a puppy – case reportsCelulite juvenil canina – relato de casos

    Directory of Open Access Journals (Sweden)

    Carlos Eduardo Fonseca-Alves

    2012-08-01

    Full Text Available Juvenile cellulitis or juvenile sterile granulomatous lymphadenitis is a rare disorder that affects puppies between three weeks to six months years old. Clinical signs include alopecia, edema, papules, pustules and crusts especially on eyelids. Definitive diagnosis requires cytological and histological evaluation and early and aggressive therapy is recommended, once scars after recovery can be severe .The choice treatment is the high dose of corticosteroids use such as prednisone. Three animals of canine species were attended at the Veterinary Hospital Clinical Small Animal Service presenting different clinical signs. Hemogram, skin lesions and submandibular lymph nodes cytological examination was collected and analyzed. The treatment was instituted, using cephalexin (22mg/kg, twice daily up to control of secondary bacterial infection, and prednisone (2mg/kg, once a day until clinical resolution. Complete cure was obtained at the end of treatment. The aim of this work is to report three clinical cases of juvenile cellulitis in dogs. A celulite juvenil ou linfadenite granulomatosa estéril juvenil, é um distúrbio raro que acomete filhotes com idade entre três semanas a seis meses. Os sinais clínicos incluem alopecia, edema, pápulas, pústulas e crostas especialmente em pálpebras. O diagnóstico definitivo requer exames citológico e histopatológico e a terapia precoce e agressiva é preconizada, pois as cicatrizes após recuperação podem ser graves. O tratamento de escolha consiste no uso de altas doses dos glicocorticoides, como prednisona. Foram atendidos no Hospital Veterinário, três animais da espécie canina, apresentando sinais clínicos variados. Foram realizados hemograma, exame citológico das lesões de pele e dos linfonodos submandibulares. O tratamento foi instituído, utilizando-se cefalexina na dose de 22 mg/kg, duas vezes ao dia, até controle de infecção bacteriana secundária e prednisona na dose de 2mg/kg, uma vez

  3. Neutron Star Crust and Molecular Dynamics Simulation

    CERN Document Server

    Horowitz, C J; Schneider, A; Berry, D K

    2011-01-01

    In this book chapter we review plasma crystals in the laboratory, in the interior of white dwarf stars, and in the crust of neutron stars. We describe a molecular dynamics formalism and show results for many neutron star crust properties including phase separation upon freezing, diffusion, breaking strain, shear viscosity and dynamics response of nuclear pasta. We end with a summary and discuss open questions and challenges for the future.

  4. The boring billion? – Lid tectonics, continental growth and environmental change associated with the Columbia supercontinent

    Directory of Open Access Journals (Sweden)

    Nick M.W. Roberts

    2013-11-01

    Full Text Available The evolution of Earth's biosphere, atmosphere and hydrosphere is tied to the formation of continental crust and its subsequent movements on tectonic plates. The supercontinent cycle posits that the continental crust is periodically amalgamated into a single landmass, subsequently breaking up and dispersing into various continental fragments. Columbia is possibly the first true supercontinent, it amalgamated during the 2.0–1.7 Ga period, and collisional orogenesis resulting from its formation peaked at 1.95–1.85 Ga. Geological and palaeomagnetic evidence indicate that Columbia remained as a quasi-integral continental lid until at least 1.3 Ga. Numerous break-up attempts are evidenced by dyke swarms with a large temporal and spatial range; however, palaeomagnetic and geologic evidence suggest these attempts remained unsuccessful. Rather than dispersing into continental fragments, the Columbia supercontinent underwent only minor modifications to form the next supercontinent (Rodinia at 1.1–0.9 Ga; these included the transformation of external accretionary belts into the internal Grenville and equivalent collisional belts. Although Columbia provides evidence for a form of ‘lid tectonics’, modern style plate tectonics occurred on its periphery in the form of accretionary orogens. The detrital zircon and preserved geological record are compatible with an increase in the volume of continental crust during Columbia's lifespan; this is a consequence of the continuous accretionary processes along its margins. The quiescence in plate tectonic movements during Columbia's lifespan is correlative with a long period of stability in Earth's atmospheric and oceanic chemistry. Increased variability starting at 1.3 Ga in the environmental record coincides with the transformation of Columbia to Rodinia; thus, the link between plate tectonics and environmental change is strengthened with this interpretation of supercontinent history.

  5. 100 Years of Studies of the Crust and Upper Mantle in Croatia

    Science.gov (United States)

    Herak, D.; Herak, M.; Tkalcic, H.

    2009-12-01

    The study of properties of the earth’s crust and upper mantle started in Croatia one century ago, by the seminal paper in which A. Mohorovicic proved the existence of the crust-mantle boundary. Most of Mohorovicic’s work which followed was dedicated to improving travel-time curves of crustal phases. During the 1980-es most of the research—mostly based on interpretation of several deep seismic profiles running across the country—was directed towards determination of the Moho-topography. The interest for the study of elastic properties of the crust was revived in the 1990-es, when several papers appeared which dealt with determination of velocities and attenuation (coda-Q) in the greater circum-Adriatic region. More recently, a large database of Pg-phase arrival times was used to assess the azimuthal anisotropy of the P-wave velocity within the crust in the NW Croatia and in parts of the External Dinarides. It was found that the direction of the fast velocities closely correspond to the direction of predominant current tectonic stress field. Current research based on the data obtained within a large international seismic experiment (ALP2002) lead to new constraints on the crustal structure in northern and western Croatia. Lithospheric structure beneath coastal and continental Croatia is being studied also by broadband teleseismic waveform modeling using receiver functions. The results indicate that the Mohorovicic discontinuity in Dalmatia may lie considerably deeper than presented on recent maps of the Moho topography in Europe.

  6. In situ Detection of Microbial Life in the Deep Biosphere in Igneous Ocean Crust.

    Science.gov (United States)

    Salas, Everett C; Bhartia, Rohit; Anderson, Louise; Hug, William F; Reid, Ray D; Iturrino, Gerardo; Edwards, Katrina J

    2015-01-01

    The deep biosphere is a major frontier to science. Recent studies have shown the presence and activity of cells in deep marine sediments and in the continental deep biosphere. Volcanic lavas in the deep ocean subsurface, through which substantial fluid flow occurs, present another potentially massive deep biosphere. We present results from the deployment of a novel in situ logging tool designed to detect microbial life harbored in a deep, native, borehole environment within igneous oceanic crust, using deep ultraviolet native fluorescence spectroscopy. Results demonstrate the predominance of microbial-like signatures within the borehole environment, with densities in the range of 10(5) cells/mL. Based on transport and flux models, we estimate that such a concentration of microbial cells could not be supported by transport through the crust, suggesting in situ growth of these communities.

  7. Titanium isotopic evidence for felsic crust and plate tectonics 3.5 billion years ago.

    Science.gov (United States)

    Greber, Nicolas D; Dauphas, Nicolas; Bekker, Andrey; Ptáček, Matouš P; Bindeman, Ilya N; Hofmann, Axel

    2017-09-22

    Earth exhibits a dichotomy in elevation and chemical composition between the continents and ocean floor. Reconstructing when this dichotomy arose is important for understanding when plate tectonics started and how the supply of nutrients to the oceans changed through time. We measured the titanium isotopic composition of shales to constrain the chemical composition of the continental crust exposed to weathering and found that shales of all ages have a uniform isotopic composition. This can only be explained if the emerged crust was predominantly felsic (silica-rich) since 3.5 billion years ago, requiring an early initiation of plate tectonics. We also observed a change in the abundance of biologically important nutrients phosphorus and nickel across the Archean-Proterozoic boundary, which might have helped trigger the rise in atmospheric oxygen. Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

  8. In-situ detection of microbial life in the deep biosphere in igneous ocean crust

    Directory of Open Access Journals (Sweden)

    Everett Cosio Salas

    2015-11-01

    Full Text Available The deep biosphere is a major frontier to science. Recent studies have shown the presence and activity of cells in deep marine sediments and in the continental deep biosphere. Volcanic lavas in the deep ocean subsurface, through which substantial fluid flow occurs, present another potentially massive deep biosphere. We present results from the deployment of a novel in-situ logging tool designed to detect microbial life harbored in a deep, native, borehole environment within igneous oceanic crust, using deep ultraviolet native fluorescence spectroscopy. Results demonstrate the predominance of microbial-like signatures within the borehole environment, with densities in the range of 105 cells/mL. Based on transport and flux models, we estimate that such a concentration of microbial cells could not be supported by transport through the crust, suggesting in situ growth of these communities.

  9. The amount of recycled crust in sources of mantle-derived melts.

    Science.gov (United States)

    Sobolev, Alexander V; Hofmann, Albrecht W; Kuzmin, Dmitry V; Yaxley, Gregory M; Arndt, Nicholas T; Chung, Sun-Lin; Danyushevsky, Leonid V; Elliott, Tim; Frey, Frederick A; Garcia, Michael O; Gurenko, Andrey A; Kamenetsky, Vadim S; Kerr, Andrew C; Krivolutskaya, Nadezhda A; Matvienkov, Vladimir V; Nikogosian, Igor K; Rocholl, Alexander; Sigurdsson, Ingvar A; Sushchevskaya, Nadezhda M; Teklay, Mengist

    2007-04-20

    Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

  10. Juvenile Battens Disease.

    Science.gov (United States)

    Gayton, Romayne

    1987-01-01

    Ten children diagnosed with juvenile Battens disease were tested over a three-year period in general intelligence, memory, listening and speech, motor skills, and general learning. Results showed that the patients followed a predetermined pattern but that the time span for development of memory, communication, and behavior problems varied greatly.…

  11. Juvenile Victimization and Delinquency.

    Science.gov (United States)

    Esbensen, Finn-Aage; Huizinga, David

    1991-01-01

    Demographic characteristics of juvenile victims of crime and a potential relationship between victimization and self-reported delinquency are examined for 877 adolescents from a large midwestern city. Lifetime victimization rates (LVRs) are higher for those involved in delinquency, and LVRs rise with age and higher levels of delinquent behavior.…

  12. Juvenile Battens Disease.

    Science.gov (United States)

    Gayton, Romayne

    1987-01-01

    Ten children diagnosed with juvenile Battens disease were tested over a three-year period in general intelligence, memory, listening and speech, motor skills, and general learning. Results showed that the patients followed a predetermined pattern but that the time span for development of memory, communication, and behavior problems varied greatly.…

  13. Juvenile idiopathic arthritis

    NARCIS (Netherlands)

    Prakken, Berent; Albani, Salvatore; Martini, Alberto

    2011-01-01

    Juvenile idiopathic arthritis is a heterogeneous group of diseases characterised by arthritis of unknown origin with onset before age of 16 years. Pivotal studies in the past 5 years have led to substantial progress in various areas, ranging from disease classification to new treatments. Gene expres

  14. Properties of middle-late Proterozoic volcanic rocks in South Qinling and the Precambrian continental break-up

    Institute of Scientific and Technical Information of China (English)

    夏林圻; 夏祖春; 徐学义

    1996-01-01

    In South Qinling, the volcanic series of the middle-late Proterozoic Yunxi Group, Yaolinghe Group, Xi.xiang Group and Bikou Group have characteristics of the continental rift volcanic rocks or continental flood basalts and are formed in continental intraplate tensional setting. The enrichment of incompatible elements, high εNd values and low-medium 87Sr/86Sr initial ratios of these volcanic rocks indicate that they were derived from asthenospheric plume. Under the action of the intense pull-apart in lithosphere, the mantle plume upwelled, quickly decompressed and melted, and finally produced magma. This tensional process made the continental crust break and eventually led to an oceanic basin in late Proterozoic. The middle-late Proterozoic volcanism is a precursor of Precarabrian continental break-up in the South Qinling.

  15. Magma-compensated crustal thinning in continental rift zones.

    Science.gov (United States)

    Thybo, H; Nielsen, C A

    2009-02-12

    Continental rift zones are long, narrow tectonic depressions in the Earth's surface where the entire lithosphere has been modified in extension. Rifting can eventually lead to rupture of the continental lithosphere and creation of new oceanic lithosphere or, alternatively, lead to formation of wide sedimentary basins around failed rift zones. Conventional models of rift zones include three characteristic features: surface manifestation as an elongated topographic trough, Moho shallowing due to crustal thinning, and reduced seismic velocity in the uppermost mantle due to decompression melting or heating from the Earth's interior. Here we demonstrate that only the surface manifestation is observed at the Baikal rift zone, whereas the crustal and mantle characteristics can be ruled out by a new seismic profile across southern Lake Baikal in Siberia. Instead we observe a localized zone in the lower crust which has exceptionally high seismic velocity and is highly reflective. We suggest that the expected Moho uplift was compensated by magmatic intrusion into the lower crust, producing the observed high-velocity zone. This finding demonstrates a previously unknown role for magmatism in rifting processes with significant implications for estimation of stretching factors and modelling of sedimentary basins around failed rift structures.

  16. Southeast continental shelf studies

    Energy Technology Data Exchange (ETDEWEB)

    Menzel, D.W.

    1979-02-12

    Research efforts on the southeast continental shelf currently describe the manner in which fluctuations in Gulf Stream motion influence biological and chemical processes. Current meter arrays are maintained in the Georgia Bight and in Onslow Bay to describe general circulation patterns and to identify forcing functions. biological studies describe processes affecting temporal and spatial variations on the shelf and have attempted to track the biological history of intruded Gulf Stream water masses. Chemical studies examine the influence of both physical and biological variables on the distribution and fate of trace elements. The current state of knowledge is reviewed, the hypotheses developed and are described, a rationale for testing these hypotheses is given. 1 figure, 1 table.

  17. NSF Continental Lithosphere Program

    Science.gov (United States)

    Mayhew, Michael; MacGregor, Ian

    For several months the Continental Lithosphere Program (CL) of the National Science Foundation has been subject to a major review. The process was stimulated by a series of budget setbacks over the past few years. Although Presidential budget requests have been very favorable for the Division of Earth Sciences (EAR), and there has been strong support within the National Science Foundation and Congress, actual appropriations by Congress have been disappointing.In each year the final allocation to EAR has been affected by external factors beyond the control of the Foundation. In the four fiscal years from 1986 through 1989 the factors include reductions tied to the Gramm-Rudman deficit reduction measures, congressional reaction to the October 1987 stock market crash, and two years of protection for the Ocean Sciences part of the NSF budget that was paid for from the budgets of the Atmospheric and Earth Sciences divisions.

  18. Lead isotopic evidence for evolutionary changes in magma-crust interaction, Central Andes, southern Peru

    Energy Technology Data Exchange (ETDEWEB)

    Barreiro, B.A. (California Univ., Santa Barbara (USA). Dept. of Geological Sciences); Clark, A.H. (Queen' s Univ., Kingston, Ontario (Canada). Dept. of Geological Sciences)

    1984-07-01

    Lead isotopic measurements were made on Andean igneous rocks of Jurassic to Recent age in Moquegua and Tacna Departments, southernmost Peru, to clarify the petrogenesis of the rocks and, in particular, to investigate the effect of crustal thickness on rock composition. This location in the Cordillera Occidental is ideal for such a study because the ca. 2 Ga Precambrian basement rocks (Arequipa massif) have a distinct Pb isotopic signature which is an excellent tracer of crustal interaction, and because geomorphological research has shown that the continental crust was here thickened drastically in the later Tertiary.

  19. Los crustáceos decápodos de aguas profundas del mar ecuatoriano.

    OpenAIRE

    Cornejo Antepara, Matilde Elvira

    2010-01-01

    Se analizaron 63 muestras de crustáceos decápodos, colectados entre las latitudes 1º18’N y 3º12’S, en el talud continental, entre 500 a 1500 metros de profundidad. Identificándose 44 especies ubicadas así: del Suborden Dendrobranchiata, Superfamilia Penaeoidea 4, del Suborden Pleocyemata están los infraórdenes: Caridea con 18 especies, Astacidea 1, Palinura 3, Anomura 11 y de Brachyura 7. Se elaboró una clave para las familias encontradas; así como, para las especies identificadas. El an...

  20. Late Mesozoic crust-mantle interaction and lower crust components in South China: A geochemical study of mafic granulite xenoliths from Cenozoic basalts

    Institute of Scientific and Technical Information of China (English)

    YU; Jinhai; (于津海); XU; Xisheng; (徐夕生); ZHOU; Xinmin; (周新民)

    2003-01-01

    Mafic granulite xenoliths collected from Cenozoic basalts in SE China can be classified as magmatic granulite and cumulate granulite. Magmatic granulites are characterized by highly concentrated Al2O3, K2O, P2O5, Ba, Sr, Pb and REE, and low contents of Nb, Zr, Hf and Th, and have an incompatible element abundance pattern similar to that of continental arc basalts. Cumulate granulites aredepleted in K2O, P2O5, Rb, Cs and Ba. These granulite xenoliths were the products of crystallization and recrystallization of the basaltic magma underplating into crust-mantle boundary in Late Mesozoic. Sr and Nd isotopic compositions and variation trend of these mafic rocks are the result of crust-mantle mixing and controlled by assimilation and fractional crystallization process (AFC). However,trace element and major element variations were mainly controlled by fractionalcrystallization. The granulites are similar in geochemistry to surface Late Mesozoic gabbro and basalt in the study area, suggesting a close petrogenetic link between them. Late Mesozoic basaltic magma activities are the most important cause for the formation of extensive contemporaneous granite and rhyolite in the study area. This study and previous data indicate that the lower crust beneath South China is composed of a variety of Paleo- to Meso-proterozoic metamorphic rocks and Late Mesozoic mafic granulites.

  1. Inherited structure and coupled crust-mantle lithosphere evolution: Numerical models of Central Australia

    Science.gov (United States)

    Heron, Philip J.; Pysklywec, Russell N.

    2016-05-01

    Continents have a rich tectonic history that have left lasting crustal impressions. In analyzing Central Australian intraplate orogenesis, complex continental features make it difficult to identify the controls of inherited structure. Here the tectonics of two types of inherited structures (e.g., a thermally enhanced or a rheologically strengthened region) are compared in numerical simulations of continental compression with and without "glacial buzzsaw" erosion. We find that although both inherited structures produce deformation in the upper crust that is confined to areas where material contrasts, patterns of deformation in the deep lithosphere differ significantly. Furthermore, our models infer that glacial buzzsaw erosion has little impact at depth. This tectonic isolation of the mantle lithosphere from glacial processes may further assist in the identification of a controlling inherited structure in intraplate orogenesis. Our models are interpreted in the context of Central Australian tectonics (specifically the Petermann and Alice Springs orogenies).

  2. Early Cretaceous subduction of continental crust at the Diego de Almagro archipelago, southern Chile

    Institute of Scientific and Technical Information of China (English)

    FranciscoHervé; C.MarkFanning

    2003-01-01

    In the Diego de Almagro archipelago of southern Chile, a quartz rich mica schist and a mylonitized granite contain Late Jurassic zircons of ca 166 and 170 Ma respectively. These rocks were metamorphosed during the Cretaceous in a subduction zone environment, which developed blueschist assemblages in metabasalts interleaved with the mica schist. The dated rocks were probably part of the acid large igneous province developed in southwestern Gondwanaland during the extensional phase which preceded the dismembering of the supercontinent. They constitute evidence that tectonic erosion of the margin occurred, as these siliceous igneous rocks, formed in the South American upper plate, were transported in the subduction zone to some 20 km depth prior to their exhumation. These rocks are in tectonic contact through the Seno Arcabuz shear zone, with late Permian turbidites of the Duque de York complex, which did not undergo blueschist metamorphism.

  3. Three-dimensional thermal structure of the Chinese continental crust and upper mantle

    Institute of Scientific and Technical Information of China (English)

    2007-01-01

    We invert S-wave velocities for the 3D upper-mantle temperatures, in which the position with a temperature crossing the 1300℃ adiabat is corresponding to the top of the seismic low velocity zone. The temperatures down to the depth of 80 km are then calculated by solving steady-state thermal conduction equation with the constraints of the inverted upper-mantle temperatures and the surface temperatures, and then surface heat flows are calculated from the crustal temperatures. The misfit between the calculated and observed surface heat flow is smaller than 20% for most regions. The result shows that, at a depth of 25 km, the crustal temperature of eastern China (500―600℃) is higher than that of western China (<500℃). At a depth of 100 km, temperatures beneath eastern and southeastern China are higher than the adiabatic temperature of 1300℃, while that beneath west China is lower. The Tarim craton and the Sichuan basin show generally low temperature. At a depth of 150 km, temperatures beneath south China, eastern Yangtze craton, North China craton and around the Qiangtang terrane are higher than the adiabatic temperature of 1300℃, but is the lowest beneath the Sichuan basin and the regions near the Indian-Eurasian collision zone. At a depth of 200 km, very low temperature occurs beneath the Qinghai-Tibet Plateau and the south to the Tarim craton.

  4. A metagenomic window into carbon metabolism at 3 km depth in Precambrian continental crust.

    Science.gov (United States)

    Magnabosco, Cara; Ryan, Kathleen; Lau, Maggie C Y; Kuloyo, Olukayode; Sherwood Lollar, Barbara; Kieft, Thomas L; van Heerden, Esta; Onstott, Tullis C

    2016-03-01

    Subsurface microbial communities comprise a significant fraction of the global prokaryotic biomass; however, the carbon metabolisms that support the deep biosphere have been relatively unexplored. In order to determine the predominant carbon metabolisms within a 3-km deep fracture fluid system accessed via the Tau Tona gold mine (Witwatersrand Basin, South Africa), metagenomic and thermodynamic analyses were combined. Within our system of study, the energy-conserving reductive acetyl-CoA (Wood-Ljungdahl) pathway was found to be the most abundant carbon fixation pathway identified in the metagenome. Carbon monoxide dehydrogenase genes that have the potential to participate in (1) both autotrophic and heterotrophic metabolisms through the reversible oxidization of CO and subsequent transfer of electrons for sulfate reduction, (2) direct utilization of H2 and (3) methanogenesis were identified. The most abundant members of the metagenome belonged to Euryarchaeota (22%) and Firmicutes (57%)-by far, the highest relative abundance of Euryarchaeota yet reported from deep fracture fluids in South Africa and one of only five Firmicutes-dominated deep fracture fluids identified in the region. Importantly, by combining the metagenomics data and thermodynamic modeling of this study with previously published isotopic and community composition data from the South African subsurface, we are able to demonstrate that Firmicutes-dominated communities are associated with a particular hydrogeologic environment, specifically the older, more saline and more reducing waters.

  5. Integrated geologic and geophysical studies of North American continental intraplate seismicity

    Science.gov (United States)

    Van Lanen, X.; Mooney, W.D.

    2007-01-01

    The origin of earthquakes within stable continental regions has been the subject of debate over the past thirty years. Here, we examine the correlation of North American stable continental region earthquakes using five geologic and geophysical data sets: (1) a newly compiled age-province map; (2) Bouguer gravity data; (3) aeromagnetic anomalies; (4) the tectonic stress field; and (5) crustal structure as revealed by deep seismic-reflection profiles. We find that: (1) Archean-age (3.8-2.5 Ga) North American crust is essentially aseismic, whereas post-Archean (less than 2.5 Ga) crust shows no clear correlation of crustal age and earthquake frequency or moment release; (2) seismicity is correlated with continental paleorifts; and (3) seismicity is correlated with the NE-SW structural grain of the crust of eastern North America, which in turn reflects the opening and closing of the proto- and modern Atlantic Ocean. This structural grain can be discerned as clear NE-SW lineaments in the Bouguer gravity and aeromagnetic anomaly maps. Stable continental region seismicity either: (1) follows the NE-SW lineaments; (2) is aligned at right angles to these lineaments; or (3) forms clusters at what have been termed stress concentrators (e.g., igneous intrusions and intersecting faults). Seismicity levels are very low to the west of the Grenville Front (i.e., in the Archean Superior craton). The correlation of seismicity with NE-SW-oriented lineaments implies that some stable continental region seismicity is related to the accretion and rifting processes that have formed the North American continental crust during the past 2 b.y. We further evaluate this hypothesis by correlating stable continental region seismicity with recently obtained deep seismic-reflection images of the Appalachian and Grenville crust of southern Canada. These images show numerous faults that penetrate deep (40 km) into the crust. An analysis of hypocentral depths for stable continental region earthquakes

  6. Osmium isotope and highly siderophile element systematics of the lunar crust

    Science.gov (United States)

    Day, James M. D.; Walker, Richard J.; James, Odette B.; Puchtel, Igor S.

    2010-01-01

    Coupled 187Os/ 188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 ± 0.3 pg g - 1 Os, 1.5 ± 0.6 pg g - 1 Ir, 6.8 ± 2.7 pg g - 1 Ru, 16 ± 15 pg g - 1 Pt, 33 ± 30 pg g - 1 Pd and 0.29 ± 0.10 pg g - 1 Re (˜ 0.00002 × CI) and Re/Os ratios that were modestly elevated ( 187Re/ 188Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (˜ 0.00007 × CI) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle-crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments. If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust-mantle concentration ratios ( D-values) must be ≤ 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust-mantle HSE partitioning for the Earth and Moon are

  7. Osmium isotope and highly siderophile element systematics of the lunar crust

    Science.gov (United States)

    Day, J.M.D.; Walker, R.J.; James, O.B.; Puchtel, I.S.

    2010-01-01

    Coupled 187Os/188Os and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundance data are reported for pristine lunar crustal rocks 60025, 62255, 65315 (ferroan anorthosites, FAN) and 76535, 78235, 77215 and a norite clast in 15455 (magnesian-suite rocks, MGS). Osmium isotopes permit more refined discrimination than previously possible of samples that have been contaminated by meteoritic additions and the new results show that some rocks, previously identified as pristine, contain meteorite-derived HSE. Low HSE abundances in FAN and MGS rocks are consistent with derivation from a strongly HSE-depleted lunar mantle. At the time of formation, the lunar floatation crust, represented by FAN, had 1.4 ?? 0.3 pg g- 1 Os, 1.5 ?? 0.6 pg g- 1 Ir, 6.8 ?? 2.7 pg g- 1 Ru, 16 ?? 15 pg g- 1 Pt, 33 ?? 30 pg g- 1 Pd and 0.29 ?? 0.10 pg g- 1 Re (??? 0.00002 ?? CI) and Re/Os ratios that were modestly elevated (187Re/188Os = 0.6 to 1.7) relative to CI chondrites. MGS samples are, on average, characterised by more elevated HSE abundances (??? 0.00007 ?? CI) compared with FAN. This either reflects contrasting mantle-source HSE characteristics of FAN and MGS rocks, or different mantle-crust HSE fractionation behaviour during production of these lithologies. Previous studies of lunar impact-melt rocks have identified possible elevated Ru and Pd in lunar crustal target rocks. The new results provide no supporting evidence for such enrichments. If maximum estimates for HSE in the lunar mantle are compared with FAN and MGS averages, crust-mantle concentration ratios (D-values) must be ??? 0.3. Such D-values are broadly similar to those estimated for partitioning between the terrestrial crust and upper mantle, with the notable exception of Re. Given the presumably completely different mode of origin for the primary lunar floatation crust and tertiary terrestrial continental crust, the potential similarities in crust-mantle HSE partitioning for the Earth and Moon are somewhat

  8. An updated global earthquake catalogue for stable continental regions: Reassessing the correlation with ancient rifts

    Science.gov (United States)

    Schulte, S.M.; Mooney, W.D.

    2005-01-01

    We present an updated global earthquake catalogue for stable continental regions (SCRs; i.e. intraplate earthquakes) that is available on the Internet. Our database contains information on location, magnitude, seismic moment and focal mechanisms for over 1300 M (moment magnitude) ??? 4.5 historic and instrumentally recorded crustal events. Using this updated earthquake database in combination with a recently published global catalogue of rifts, we assess the correlation of intraplate seismicity with ancient rifts on a global scale. Each tectonic event is put into one of five categories based on location: (i) interior rifts/taphrogens, (ii) rifted continental margins, (iii) non-rifted crust, (iv) possible interior rifts and (v) possible rifted margins. We find that approximately 27 per cent of all events are classified as interior rifts (i), 25 per cent are rifted continental margins (ii), 36 per cent are within non-rifted crust (iii) and 12 per cent (iv and v) remain uncertain. Thus, over half (52 per cent) of all events are associated with rifted crust, although within the continental interiors (i.e. away from continental margins), non-rifted crust has experienced more earthquakes than interior rifts. No major change in distribution is found if only large (M ??? 6.0) earthquakes are considered. The largest events (M ??? 7.0) however, have occurred predominantly within rifts (50 per cent) and continental margins (43 per cent). Intraplate seismicity is not distributed evenly. Instead several zones of concentrated seismicity seem to exist. This is especially true for interior rifts/taphrogens, where a total of only 12 regions are responsible for 74 per cent of all events and as much as 98 per cent of all seismic moment released in that category. Of the four rifts/taphrogens that have experienced the largest earthquakes, seismicity within the Kutch rift, India, and the East China rift system, may be controlled by diffuse plate boundary deformation more than by the

  9. Investigating Compositional Links Between Arc Magmas And The Subducted Altered Oceanic Crust

    Science.gov (United States)

    Straub, S. M.

    2015-12-01

    Arc magmatism is causally related to the recycling of materials from the subducting plate. Numerous studies showed that the recycled material flux is dominated by recycled continental crust (oceanic sediment, eroded crust) and altered oceanic igneous crust (AOC). The crustal component is highly enriched, and thus its signal in arc magmas can readily be distinguished from mantle wedge contributions. In contrast, the impact of the AOC flux is much more difficult to detect, since the AOC isotopically resembles the mantle. Mass balance studies of arc input and output suggest that the recycled flux from the thick (6000 meter on average) AOC may buffer the flux of the recycled continental crust to the point of concealment in arc settings where the latter is volumetrically minor. In particular, highly fluid- mobile elements Sr and Pb in arc magmas are strongly influenced by the AOC, implying that the arc chemistry may allow for inferring the Sr and Pb isotopic composition of the subducted AOC. This hypothesis is being tested by a compilation of published data of high-quality trace element and isotope compositions from global arcs. In agreement with previous studies, our results confirm that the Sr-rich fluids released from the AOC control the arc Sr isotopes, whereby the slightly elevated 87Sr/86Sr (up to 0.705) of many arcs may principally reflect the similarly elevated Sr isotope ratios of the AOC rather than a recycled crustal component. In contrast, the arc Pb isotope ratios are influenced by both the AOC and the recycled crustal component which create the typical binary mixing arrays. These arrays should then point to the Pb isotope composition of the AOC and the recycled crust, respectively. However, as the proportions of these end members may strongly vary in arc magmas, the exact 206Pb/204Pb of the subducted AOC in a given setting is challenging. Remarkably, the Pb isotope systematics from well-constrained western Aleutian (minimal sediment subduction) and central

  10. Anomalous heat flow belt along the continental margin of Brazil

    Science.gov (United States)

    Hamza, Valiya M.; Vieira, Fabio P.; Silva, Raquel T. A.

    2017-06-01

    A comprehensive analysis of thermal gradient and heat flow data was carried out for sedimentary basins situated in the continental margin of Brazil (CMB). The results point to the existence of a narrow belt within CMB, where temperature gradients are higher than 30 °C/km and the heat flow is in excess of 70 mW/m2. This anomalous geothermal belt is confined between zones of relatively low to normal heat flow in the adjacent continental and oceanic regions. The width of the belt is somewhat variable, but most of it falls within the range of 100-300 km. The spatial extent is relatively large in the southern (in the basins of Pelotas, Santos and Campos) and northern (in the basins of Potiguar and Ceará) parts, when compared with those in the central parts (in the basins of South Bahia, Sergipe and Alagoas). The characteristics of heat flow anomalies appear to be compatible with those produced by thermal sources at depths in the lower crust. Hence, magma emplacement at the transition zone between lower crust and upper mantle is considered the likely mechanism producing such anomalies. Seismicity within the belt is relatively weak, with focal depths less than 10 km for most of the events. Such observations imply that "tectonic bonding" between continental and oceanic segments, at the transition zone of CMB, is relatively weak. Hence, it is proposed that passive margins like CMB be considered as constituting a type of plate boundary that is aseismic at sub-crustal levels, but allows for escape of significant amounts of earth's internal heat at shallow depths.

  11. Late Onset Juvenile Xanthogranuloma

    Directory of Open Access Journals (Sweden)

    Punithwavathy K

    1999-01-01

    Full Text Available A 19 year old female was seen with multiple skin coloured and hyperpigmented macules, discrete as well as grouped papules and nodules of varying sizes distributed over the face, neck, extensor and flexor aspects of both upper and lower extremities including joints. The trunk was spared. Some of the lesions showed features of spontaneous regression. Investigations confirmed the diagnosis of juvenile xanthogranuloma. Lesions regressed satisfactorily with liquid nitrogen cryotherapy.

  12. Juvenile Incarceration and Health

    OpenAIRE

    Barnert, ES; R Perry; Morris, RE

    2015-01-01

    © 2015. Addressing the health status and needs of incarcerated youth represents an issue at the nexus of juvenile justice reform and health care reform. Incarcerated youth face disproportionately higher morbidity and higher mortality compared to the general adolescent population. Dental health, reproductive health, and mental health needs are particularly high, likely as a result of lower access to care, engagement in high-risk behaviors, and underlying health disparities. Violence exposure a...

  13. Crust formation in drying colloidal suspensions

    KAUST Repository

    Style, R. W.

    2010-06-30

    During the drying of colloidal suspensions, the desiccation process causes the suspension near the air interface to consolidate into a connected porous matrix or crust. Fluid transport in the porous medium is governed by Darcy\\'s law and the equations of poroelasticity, while the equations of colloid physics govern processes in the suspension. We derive new equations describing this process, including unique boundary conditions coupling the two regions, yielding a moving-boundary model of the concentration and stress profiles during drying. A solution is found for the steady-state growth of a nedimensional crust during constant evaporation rate from the surface. The solution is used to demonstrate the importance of the system boundary conditions on stress profiles and diffusivity in a drying crust. © 2011 The Royal Society.

  14. Juvenile Ultracool Dwarfs

    CERN Document Server

    Rice, Emily L; Cruz, Kelle; Barman, Travis; Looper, Dagny; Malo, Lison; Mamajek, Eric E; Metchev, Stanimir; Shkolnik, Evgenya L

    2011-01-01

    Juvenile ultracool dwarfs are late spectral type objects (later than ~M6) with ages between 10 Myr and several 100 Myr. Their age-related properties lie intermediate between very low mass objects in nearby star-forming regions (ages 1-5 Myr) and field stars and brown dwarfs that are members of the disk population (ages 1-5 Gyr). Kinematic associations of nearby young stars with ages from ~10-100 Myr provide sources for juvenile ultracool dwarfs. The lowest mass confirmed members of these groups are late-M dwarfs. Several apparently young L dwarfs and a few T dwarfs are known, but they have not been kinematically associated with any groups. Normalizing the field IMF to the high mass population of these groups suggests that more low mass (mainly late-M and possibly L dwarf) members have yet to be found. The lowest mass members of these groups, along with low mass companions to known young stars, provide benchmark objects with which spectroscopic age indicators for juvenile ultracool dwarfs can be calibrated and...

  15. The Early Evolution of Mars' Crust

    Science.gov (United States)

    Samuel, H.; Baratoux, D.; Kurita, K.

    2014-12-01

    The Mars crustal density and thickness have been recently re-evaluated using petrological constraints from remote sensing, in-situ data, and SNC meteorites. This work indicates that the present-day Martian crust is denser and thicker than previously proposed if essentially basaltic in composition. As a consequence, the average crustal thickness would be commensurable with the depth of the basalt/eclogite transition, re-opening the question of crustal recycling on Early Mars and more generally throughout all its history. We have therefore investigated the conditions under which a thick ancient crust with an eclogitic root could survive through the history of Mars using numerical modelling. Delamination may occur if the combination of poorly constrained physical parameters induces the presence of gravitationally unstable layers and favors a rheological decoupling. To study the conditions and the time scales for the occurrence of crustal delamination on Mars, we investigated the influence of critical parameters for a plausible range of values corresponding to the Martian mantle. For each case we follow the dynamic evolution over geological times of a three-layer system (i.e., crust-mantle with a distinction between low pressure, buoyant basaltic crust and higher pressure, denser eclogitic material). We systematically varied four governing parameters within plausible ranges: (1) the basalt-eclogite transition depth, (2) the density difference between the mantle and the basaltic crust, (3) the density difference between the eclogitic crust and the lithosphere & mantle, (4) the viscous rheology. These experiments allow determining the average Martian crustal thickness at early and late evolutionary stages.

  16. Three-dimensional density model of the Nazca plate and the Andean continental margin

    Science.gov (United States)

    Tassara, AndréS.; GöTze, Hans-Jürgen; Schmidt, Sabine; Hackney, Ron

    2006-09-01

    We forward modeled the Bouguer anomaly in a region encompassing the Pacific Ocean (85°W) and the Andean margin (60°W) between northern Peru (5°S) and Patagonia (45°S). The three-dimensional density model that reproduces the gravity field is a continental-scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic Nazca plate, subducted slab and continental margin with a minimum number of bodies. We predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressure-temperature conditions appropriate for the Andean setting. A database of independent geophysical information constrains the geometry of the top of the subducted slab, locally the Moho of the oceanic and continental crusts and, indirectly, the lithosphere-asthenosphere boundary underneath the continental plate. Other boundaries, notably the intracrustal density discontinuity separating upper from lower crust below the continent, were not constrained and their geometry is the result of fitting the observed and calculated Bouguer anomaly during forward modeling. This contribution presents the model to the Andean geoscientific community and contains some tools, like a sensitivity analysis, that helps potential users of the model to interpret its results. We describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena (e.g., modification of oceanic plate structure by hot spots, shape of the subducted slab, thermal structure of the continental lithosphere, compensation mechanism and formation of orogenic relieve, causes of Andean segmentation).

  17. Hafnium isotope stratigraphy of ferromanganese crusts

    Science.gov (United States)

    Lee, D.-C.; Halliday, A.N.; Hein, J.R.; Burton, K.W.; Christensen, J.N.; Gunther, D.

    1999-01-01

    A Cenozoic record of hafnium isotopic compositions of central Pacific deep water has been obtained from two ferromanganese crusts. The crusts are separated by more than 3000 kilometers but display similar secular variations. Significant fluctuations in hafnium isotopic composition occurred in the Eocene and Oligocene, possibly related to direct advection from the Indian and Atlantic oceans. Hafnium isotopic compositions have remained approximately uniform for the past 20 million years, probably reflecting increased isolation of the central Pacific. The mechanisms responsible for the increase in 87Sr/86Sr in seawater through the Cenozoic apparently had no effect on central Pacific deep-water hafnium.

  18. Reconstruction of the East Africa and Antarctica continental margins

    Science.gov (United States)

    Nguyen, Luan C.; Hall, Stuart A.; Bird, Dale E.; Ball, Philip J.

    2016-06-01

    The Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana and Pangea. Previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent-ocean-boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate Antarctic margin near the Riiser-Larsen Sea. Satellite-derived gravity data have been used to trace the orientations and landward limits of fracture zones. A 3-D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Crustal thicknesses together with fracture zone terminations reveal COBs that are significantly closer to the African and Antarctic coasts than previously recognized. Correlation of fracture zone azimuths and identified COBs suggests Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. An areal-balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins. Both margins reveal a trend of increasing extension from east to west. Our results suggest Africa underwent extension of 60-120 km, while Antarctic crust was stretched by 105-180 km. Various models tested to determine the direction of extension during rifting suggest that Antarctica moved away from Africa in a WNW-ESE direction during the period between 184 and 171 Ma prior to the onset of seafloor spreading.

  19. Genetics Home Reference: juvenile idiopathic arthritis

    Science.gov (United States)

    ... Home Health Conditions juvenile idiopathic arthritis juvenile idiopathic arthritis Printable PDF Open All Close All Enable Javascript ... view the expand/collapse boxes. Description Juvenile idiopathic arthritis refers to a group of conditions involving joint ...

  20. Genetics Home Reference: juvenile primary osteoporosis

    Science.gov (United States)

    ... Home Health Conditions juvenile primary osteoporosis juvenile primary osteoporosis Enable Javascript to view the expand/collapse boxes. ... PDF Open All Close All Description Juvenile primary osteoporosis is a skeletal disorder characterized by thinning of ...

  1. Genetics Home Reference: juvenile myoclonic epilepsy

    Science.gov (United States)

    ... Home Health Conditions juvenile myoclonic epilepsy juvenile myoclonic epilepsy Enable Javascript to view the expand/collapse boxes. ... PDF Open All Close All Description Juvenile myoclonic epilepsy is a condition characterized by recurrent seizures (epilepsy). ...

  2. Continental United States Hurricane Strikes

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The Continental U.S. Hurricane Strikes Poster is our most popular poster which is updated annually. The poster includes all hurricanes that affected the U.S. since...

  3. Petrogenesis and its significance to continental dynamics of the Neogene high-potassium calc-alkaline volcanic rock association from north Qiangtang, Tibetan Plateau

    Institute of Scientific and Technical Information of China (English)

    赖绍聪; 刘池阳; S.Y.O’Reilly

    2001-01-01

    Detailed studies indicate that the main rock type of the Neogene high-potassium calc-alkaline volcanic rock association from north Qiangtang is andesite, dacite and rhyolite. They belong to typical crust-generation magmatic system and originate from the special thickened crust of the Tibetan Plateau by dehydration melting. This group of rocks exhibits LREE enrichment but no remarkable Eu anomaly that shows their source region should be a thickened deep crust consisting of eclogitic mass group, implying that the crust had been thickened and an eclogitic deep crust had been formed during the Neogene period in Qiangtang area. This understanding is important and significant to making further discussion on the uplift mechanism and continental dynamics of the Tibetan Plateau.

  4. Extended parental care in crustaceans: an update Cuidado parental extendido en crustáceos: conocimiento actual

    Directory of Open Access Journals (Sweden)

    MARTIN THIEL

    2003-06-01

    increase in dispersal distances of crustaceans with XPC via rafting, surpassing that of crustaceans with pelagic larvae. Since crustaceans with XPC may be particularly susceptible to changing environmental conditions, especially in the terrestrial environment where populations are often small and locally restricted, conservation of biodiversity should focus on these (and other invertebrate species with XPCMuchas especies de crustáceos presentan cuidado parental extendido (XPC, donde individuos juveniles completamente desarrollados son cuidados por los padres. En la presente contribución se revisa el conocimiento actual del XPC, sus consecuencias y además se identifican tópicos importantes para ser investigados en el futuro. Los crustáceos que presentan XPC pueden ser encontrados en ambientes marinos, límnicos y terrestres, pero el XPC es más conspicuo y aparentemente más común en el ambiente terrestre. En todas las especies analizadas, las hembras desarrollan las principales tareas relacionadas con el XPC. Crustáceos que portan sus juveniles en alguna estructura del cuerpo (i.e. marsupio durante el XPC liberan a estos tempranamente, mientras que las especies que habitan refugios pueden albergar a sus crías incluso cuando éstas alcanzan estadios tardíos de desarrollo, o bien, la adultez. Además de proveer un microhabitat adecuado y seguro para las crías, las madres comparten el alimento con, limpian y/o defienden activamente a sus juveniles. Entre los beneficios más importantes del XPC figura el incremento de las tasas de crecimiento y sobrevivencia de los juveniles. El XPC puede producir conflictos entre las crías o entre los padres y sus crías, especialmente durante la etapa tardía del XPC, cuando algunos recursos (alimento y espacio pueden llegar a ser aún más limitantes. De manera similar, debido a la proximidad y frecuente interacción entre padres y crías, epibiontes (e.g., parásitos pueden ser transferidos desde los padres hacia los juveniles

  5. Lithospheric Thermal Isostasy of North Continental Margin of the South China Sea

    Institute of Scientific and Technical Information of China (English)

    Chen Shi; Zhang Jian; Sun Yujun; Shi Yaolin

    2009-01-01

    Accompanied with rifting and detaching of the north continental margin of the South China Sea,the ernst and the lithosphere become thinner away from the continental margin resulting from the tectonic activities,such as tensile deformation,thermal uplift,and cooling subsidence,etc..Integrated with thermal,gravimetric,and isostatic analysis techniques,based on the seismic interpretation of the deep penetration seismic soundings across the northern margin of the South China Sea,we reconstructed the lithospheric thermal structure and derived the variation of the crust boundary in the east and west parts of the seismic profde by using gravity anomaly data.We mainly studied the thermal isostasy problems using the bathymetry of the profiles and calculated the crust thinning effect due to the thermal variety in the rifting process.The results Indicate that the thermal isostasy may reach 2.5 kin,and the compositional variations in the ilthospheric density and thickness may produce a variation of 4.0 kin.Therefore,the compositional isostatic correction is very important to recover the relationship between surface heat flow and topography.Moreover,because of the high heat flow characteristic of the continental margin,building the model of lithospheric geotherm in this region is of great importan for studying the Cenozoic tectonic thermal evolution of the north passive continental margin of the South China Sea.

  6. Seismological constraints on the crustal structures generated by continental rejuvenation in northeastern China.

    Science.gov (United States)

    Zheng, Tian-Yu; He, Yu-Mei; Yang, Jin-Hui; Zhao, Liang

    2015-10-07

    Crustal rejuvenation is a key process that has shaped the characteristics of current continental structures and components in tectonic active continental regions. Geological and geochemical observations have provided insights into crustal rejuvenation, although the crustal structural fabrics have not been well constrained. Here, we present a seismic image across the North China Craton (NCC) and Central Asian Orogenic Belt (CAOB) using a velocity structure imaging technique for receiver functions from a dense array. The crustal evolution of the eastern NCC was delineated during the Mesozoic by a dominant low seismic wave velocity with velocity inversion, a relatively shallow Moho discontinuity, and a Moho offset beneath the Tanlu Fault Zone. The imaged structures and geochemical evidence, including changes in the components and ages of continental crusts and significant continental crustal growth during the Mesozoic, provide insight into the rejuvenation processes of the evolving crust in the eastern NCC caused by structural, magmatic and metamorphic processes in an extensional setting. The fossil structural fabric of the convergent boundary in the eastern CAOB indicates that the back-arc action of the Paleo-Pacific Plate subduction did not reach the hinterland of Asia.

  7. Pulsar Glitches: The Crust may be Enough

    CERN Document Server

    Piekarewicz, J; Horowitz, C J

    2014-01-01

    Pulsar glitches-the sudden spin-up in the rotational frequency of a neutron star-suggest the existence of an angular-momentum reservoir confined to the inner crust of the neutron star. Large and regular glitches observed in the Vela pulsar have originally constrained the fraction of the stellar moment of inertia that must reside in the solid crust to about 1.4%. However, crustal entrainment-which until very recently has been ignored-suggests that in order to account for the Vela glitches, the fraction of the moment of inertia residing in the crust must increase to about 7%. This indicates that the required angular momentum reservoir may exceed that which is available in the crust. We explore the possibility that uncertainties in the equation of state provide enough flexibility for the construction of models that predict a large crustal thickness and consequently a large crustal moment of inertia. Given that analytic results suggest that the crustal moment of inertia is sensitive to the transition pressure at ...

  8. Dew formation and activity of biological crusts

    NARCIS (Netherlands)

    Veste, M.; Heusinkveld, B.G.; Berkowicz, S.M.; Breckle, S.W.; Littmann, T.; Jacobs, A.F.G.

    2008-01-01

    Biological soil crusts are prominent in many drylands and can be found in diverse parts of the globe including the Atacama desert, Chile, the Namib desert, Namibia, the Succulent-Karoo desert, South Africa, and the Negev desert, Israel. Because precipitation can be negligible in deserts ¿ the

  9. Norwegian crusted scabies: an unusual case presentation.

    Science.gov (United States)

    Maghrabi, Michael M; Lum, Shireen; Joba, Ameha T; Meier, Molly J; Holmbeck, Ryan J; Kennedy, Kate

    2014-01-01

    Scabies is a contagious condition that is transmitted through direct contact with an infected person and has been frequently associated with institutional and healthcare-facility outbreaks. The subtype Norwegian crusted scabies can masquerade as other dermatologic diseases owing to the heavy plaque formation. Successful treatment has been documented in published reports, including oral ivermectin and topical permethrin. Few case studies documenting the treatment of Norwegian crusted scabies have reported the use of surgical debridement as an aid to topical and/or oral treatment when severe plaque formation has been noted. A nursing home patient was admitted to the hospital for severe plaque formation of both feet. A superficial biopsy was negative for both fungus and scabies because of the severity of the plaque formation on both feet. The patient underwent a surgical, diagnostic biopsy of both feet, leading to the diagnosis of Norwegian crusted scabies. A second surgical debridement was then performed to remove the extensive plaque formation and aid the oral ivermectin and topical permethrin treatment. The patient subsequently made a full recovery and was discharged back to the nursing home. At 2 and 6 months after treatment, the patient remained free of scabies infestation, and the surgical wound had healed uneventfully. The present case presentation has demonstrated that surgical debridement can be complementary to the standard topical and oral medications in the treatment of those with Norwegian crusted scabies infestation.

  10. Juvenile Justice Bulletin: Aftercare Services. Juvenile Justice Practices Series.

    Science.gov (United States)

    Gies, Steve V.

    This bulletin examines aftercare services that provide youth with comprehensive health, education, family, and vocational services upon their release from the juvenile justice system. Aftercare can be defined as reintegrative services that prepare out-of-home placed juveniles for reentry into the community by reestablishing the necessary…

  11. Oceanization starts from below during continental rupturing in the northern Red Sea

    Science.gov (United States)

    Cai, Y.; Ligi, M.; Bonatti, E.; Bosworth, W.; Cipriani, A.; Palmiotto, C.; Rasul, N. M.; Ronca, S.; Sanfilippo, A.; Seyler, M.; Nomani, S.; AlQutub, A. S.

    2015-12-01

    The role of magmatism in continental rupturing and in the birth of a new ocean is not well understood. Continental rupture can take place with intense and voluminous volcanism, as in the Southern Red Sea or in a relatively amagmatic mode, as in the Northern Red Sea. Mantle upwelling and melting may be affected by the south to north decreasing opening rate of the Red Sea and by the influence of the Afar plume, also decreasing from south to north. The tholeiitic basalts of the Red Sea spreading system contrast with the extensive Cenozoic basaltic lava fields of the western part of the Arabian peninsula that form one of the largest alkali basalt provinces in the world. In order to establish possible relationship between the Red Sea rift evolution and the western Saudi Arabia intraplate alkali volcanism, field work was carried out on Lunayyir, Ishara, al Kura and Khaybar volcanic fields. Collected samples cover a wide range of chemical diversity (from olivine basalt to trachyte) and span over a 20 Ma interval. We attempt a comparison of the geochemistry of igneous rocks from western Arabia dykes and volcanic fields with those from the Red Sea axis and from the islands of Zabargad and Brothers in the northern Red Sea, that represent basaltic melts injected into the thinned continental crust before continental rupturing and initiation of seafloor spreading. Gabbros drilled in the western Red Sea and exposed on the Brothers islands suggest that continental break up in the northern Red Sea, a relatively non-volcanic rift, is preceded by intrusion of oceanic-type basaltic melts that crystallize at progressively shallower crustal depths as rifting progresses towards continental break-up. A seismic reflection profile running across the central part of the southern Thetis basin shows a ~5 km wide reflector that marks the roof of a magma chamber located ~3.5 km below seafloor. The presence of a few kilometers deep subrift magma chamber soon after the initiation of oceanic

  12. Earth's crust model of the South-Okhotsk Basin by wide-angle OBS data

    Science.gov (United States)

    Kashubin, Sergey N.; Petrov, Oleg V.; Rybalka, Alexander V.; Milshtein, Evgenia D.; Shokalsky, Sergey P.; Verba, Mark L.; Petrov, Evgeniy O.

    2017-07-01

    -waves velocity models of the sedimentary strata and the upper consolidated crust. Velocity values in the upper consolidated crust beneath the South-Okhotsk Basin (Vp = 5.50-5.80 km/s, Vp/Vs = 1.74-1.76) allow interpretation of this 2.5-3.5-km-thick layer to be consistent with a felsic (granodioritic) crust. These results suggest that the Earth's crust in this region can be considered continental in nature, rather than previously accepted oceanic crust. Even though, the crust is thinned and stretched at this location.

  13. MULTI-ARC BASIN SYSTEM OF THE KUNLUN OROGENIC BELT AND PAN-CATHYSIAN CONTINENTAL ACCRETION

    Institute of Scientific and Technical Information of China (English)

    YIN Fuguang; PAN Guitang; LI Xingzhen

    2003-01-01

    After Rodinia supercontinent was disintegrated in Late Proterozoic, an ocean, namely, Tethys Ocean, occurred between Gondwana continental group and Pan-Cathaysian continental group from Late Proterozoic to Mesozoic. From Early Paleozoic to Mesozoic, Tethys Ocean was subducted toward Pan-Cathaysian block group, which results in backarc expansion, arc-land collision and forearc accretion. When the backarc basin expands and reaches the small oceanic basin, ophiolite melange will be generated. As accretion had already occurred in the south of the continental margin in the earlier stage, the succeeding backarc expansion and the frontal arc position were migrated toward south correspondingly. Therefore, multiple ophiolite belts and magmatic rock belts occurred, and show a trend of decreasing age from north toward south. As the continental margin was split and migrated toward south and reached a high latitude position, i.e., with the shortening and subduction of oceanic crust, the sedimentary bodies at high latitude was accreted continuously toward low latitude area together with the formation of oceanic island, mixing of cold-type and warm-type organism was generated. Moreover,blocks split and separated from Pan-Cathaysian or Gondwana continental group cannot traverse the oceanic median ridge and joins with another continental block. As a result, the Kunlun belt on the SW margin of the Pan-Cathaysian land was resulted from the multi-arc orogenesis such as the backarc seabed expansion, arc-arc collision, arc-land collision oceanic bed, and the continuous southward accretion process.

  14. Miastenia gravis juvenil Juvenile myasthenia gravis

    Directory of Open Access Journals (Sweden)

    Oscar Papazian

    2009-01-01

    Full Text Available La miastenia gravis juvenil (MGJ es un trastorno crónico auto inmune en el cual existen anticuerpos séricos que al unirse a los receptores de acetilcolin nicotínicos de la membrana muscular de la placa motora alteran la transmisión neuromuscular. El resultado es fatiga muscular precoz con progresión a la parálisis durante estados de contracción muscular iterativos (movimientos o sostenidos (posturas y más raramente parálisis permanente durante el reposo. Los músculos inervados por los nervios craneales, especialmente los extraoculares y elevadores de los párpados, tienen más tendencia a la debilidad muscular persistente que los inervados por otros pares craneales y las extremidades. Las formas clínicas de presentación son generalizadas, oculares y respiratorias. El diagnóstico se sospecha mediante la anamnesia, la fatiga anormal se comprueba mediante el examen físico y la estimulación eléctrica iterativa del nervio que inerva al músculo afectado pero no paralizado. Se corrobora mediante la administración de inhibidores de la acetilcolin esterasa (IACE que al aumentar la cantidad de acetilcolin en la hendidura sináptica, corrigen la fatiga o la debilidad muscular transitoriamente. Se hace el diagnóstico de certeza mediante la demostración sérica de anticuerpos contra los receptores de acetilcolin (ACRA. El tratamiento es a largo plazo sintomático con IACE y etiopatogénico con inmunosupresores, plasmaféresis, gamma globulina endovenosa y timectomía. El curso es crónico. La remisión espontánea o después de tratamiento sintomático o etiopatogénico ocurre entre 1-10 años respectivamente. La mortalidad es prácticamente nula aun durantes las crisis miastenias gracias a la educación de padres, pacientes y público en general sobre el tema, al desarrollo del sistema de respuesta rápida de auxilio domiciliario y las unidades de cuidados intensivos y el empleo de la ventilación asistida profiláctica, plasmaféresis y

  15. Juvenile sterile granulomatous dermatitis (puppy strangle in Pekingese and German shepherd puppies

    Directory of Open Access Journals (Sweden)

    Mohammad Abbaszadeh Hasiri

    2015-07-01

    Full Text Available Juvenile sterile granulomatous dermatitis is an uncommon granulomatous and pustular disorder of the face, pinnae, and submandibular lymph nodes of puppies. A 10-week-old male Pekingese and a 8-week-old female German shepherd presented with submandibular lymphadenomegaly, skin lesions on muzzle and periocular area (Papules, crusts and pustules. The case did not respond to antibiotic therapy. Results of a hemogram, biochemical panel, and urinalysis were normal. Due to skin scraping, cytology examination (impression smear, fungal and bacterial culture and response to therapy puppy strangle (juvenile cellulitis was diagnosed. The puppies made a full recovery on glucocorticoid therapy. The present case report describes the first report of juvenile sterile granulomatous dermatitis in Iran.

  16. Juvenile Dermatomyositis in Pregnancy

    Directory of Open Access Journals (Sweden)

    Anthony Emeka Madu

    2013-01-01

    Full Text Available Juvenile dermatomyositis has variable clinical presentations both in and outside of pregnancy. A literature review indicated that optimal maternal and fetal outcomes can be anticipated when the pregnancy is undertaken while the disease is in remission. Poorer outcomes are associated with flare-up of the disease in early pregnancy compared with exacerbation in the second or third trimester, when fetal prognosis is usually good. We present a case of JDM in pregnancy with disease exacerbation late in pregnancy and review of the relevant literature.

  17. Juvenile dermatomyositis in pregnancy.

    Science.gov (United States)

    Madu, Anthony Emeka; Omih, Edwin; Baguley, Elaine; Lindow, Stephen W

    2013-01-01

    Juvenile dermatomyositis has variable clinical presentations both in and outside of pregnancy. A literature review indicated that optimal maternal and fetal outcomes can be anticipated when the pregnancy is undertaken while the disease is in remission. Poorer outcomes are associated with flare-up of the disease in early pregnancy compared with exacerbation in the second or third trimester, when fetal prognosis is usually good. We present a case of JDM in pregnancy with disease exacerbation late in pregnancy and review of the relevant literature.

  18. Juvenile hyaline fibromatosis.

    Science.gov (United States)

    Larralde, M; Santos-Muñoz, A; Calb, I; Magariños, C

    2001-01-01

    Juvenile hyaline fibromatosis (JHF) is a rare autosomal recessive disease with onset in infancy or early childhood. It is characterized by papulonodular skin lesions, soft tissue masses, gingival hypertrophy, and flexion contractures of the large joints. The light and electron microscopic features are very distinctive. Here we report an 8-month-old boy with characteristic stiffness of the knees and elbows and pink confluent papules on the paranasal folds, and periauricular and perianal regions. He also had hard nodules all over the scalp and around the mouth, and severe gingival hypertrophy. Histologic and ultrastructural features were typical of JHF. Clinical features, pathology, and physiology are discussed.

  19. New insights into continental rifting from a damage rheology modeling

    Science.gov (United States)

    Lyakhovsky, Vladimir; Segev, Amit; Weinberger, Ram; Schattner, Uri

    2010-05-01

    Previous studies have discussed how tectonic processes could produce relative tension to initiate and propagate rift zones and estimated the magnitude of the rift-driving forces. Both analytic and semi-analytic models as well as numerical simulations assume that the tectonic force required to initiate rifting is available. However, Buck (2004, 2006) estimated the minimum tectonic force to allow passive rifting and concluded that the available forces are probably not large enough for rifting of thick and strong lithosphere in the absence of basaltic magmatism (the "Tectonic Force" Paradox). The integral of the yielding stress needed for rifting over the thickness of the normal or thicker continental lithosphere are well above the available tectonic forces and tectonic rifting cannot happen (Buck, 2006). This conclusion is based on the assumption that the tectonic stress has to overcome simultaneously the yielding stress over the whole lithosphere thickness and ignore gradual weakening of the brittle rocks under long-term loading. In this study we demonstrate that the rifting process under moderate tectonic stretching is feasible due to gradual weakening and "long-term memory" of the heavily fractured brittle rocks, which makes it significantly weaker than the surrounding intact rock. This process provides a possible solution for the tectonic force paradox. We address these questions utilizing 3-D lithosphere-scale numerical simulations of the plate motion and faulting process base on the damage mechanics. The 3-D modeled volume consists of three main lithospheric layers: an upper layer of weak sediments, middle layer of crystalline crust and lower layer of the lithosphere mantle. Results of the modeling demonstrate gradual formation of the rift zone in the continental lithosphere with the flat layered structure. Successive formation of the rift system and associated seismicity pattern strongly depend not only on the applied tectonic force, but also on the healing

  20. Juvenile Justice in Rural America.

    Science.gov (United States)

    Jankovic, Joanne, Ed.; And Others

    Producing a much-needed organized body of literature about rural juvenile justice, 14 papers (largely from the 1979 National Symposium on Rural Justice) are organized to identify current issues, identify forces causing changes in current systems, review programs responding to rural juvenile justice problems, and provide planning models to aid…

  1. Sedimentological context of the continental sabkhas of Abu Dhabi

    Science.gov (United States)

    Lokier, Stephen; Paul, Andreas; Bixiao, Xin

    2017-04-01

    For more than half a century, the coastal sabkhas of Abu Dhabi have been the focus of intensive research focusing on deposition, early diagenesis and the role of microbial communities. Given all of this activity, it is somewhat surprising that their continental counterparts have been largely neglected with only a brief mention in larger-scale regional studies. This study redresses this imbalance by documenting the sedimentological, mineralogical and early diagenetic characteristics of continental sabkhas that are hosted in the Rub al Khali desert of the United Arab Emirates. During reconnaissance surveys it has been established that organic-rich microbial mats and evaporite minerals, both similar to those observed in the coastal sabkha, also occur in these continental sabkha settings. Satellite imagery was utilised to identify potential field locations for surface and shallow sub surface investigation; subsequent field reconnaissance established the validity of sites in terms of anthropogenic disruption and accessibility. At each site, surface features were described in detail, particularly with reference to any microbial communities or evaporite crusts; sample pits were dug in order to document sub-surface facies geometries and to recover both sediment and pore water samples for subsequent analysis. In each pit, a range of environmental parameters was measured over a prolonged period, including surface and sub-surface temperatures, ground water salinity and dissolved oxygen. Sediment samples were subjected to a range of analyses in order to establish and quantify primary sediment composition and any early diagenetic mineral phases. The results of this study are used to build an atlas of sedimentary structures and textures that are associated with continental sabkha settings. These observations allow us to establish the defining sedimentological and early diagenetic characteristics that can be employed to identify similar depositional environments in ancient

  2. A mechanism to thin the continental lithosphere at magma-poor margins.

    Science.gov (United States)

    Lavier, Luc L; Manatschal, Gianreto

    2006-03-16

    Where continental plates break apart, slip along multiple normal faults provides the required space for the Earth's crust to thin and subside. After initial rifting, however, the displacement on normal faults observed at the sea floor seems not to match the inferred extension. Here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults. Our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust (< 10 km) by exhumation of mid-crustal and mantle material. This transitional crust is capped by sub-horizontal detachment surfaces over distances exceeding 100 km with little visible deformation. Our rift model is based on numerical experiments constrained by geological and geophysical observations from the Alpine Tethys and Iberia/Newfoundland margins. Furthermore, we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust. The presence of such lower crustal gabbros is well constrained for the Alpine Tethys system. Initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution. This is achieved in our models by the formation of weak ductile shear zones.

  3. Cadomian (˜560 Ma) crust buried beneath the northern Arabian Peninsula: Mineral, chemical, geochronological, and isotopic constraints from NE Jordan xenoliths

    Science.gov (United States)

    Stern, Robert J.; Ali, Kamal A.; Ren, Minghua; Jarrar, Ghaleb H.; Romer, Rolf L.; Leybourne, Matthew I.; Whitehouse, Martin J.; Ibrahim, Khalil M.

    2016-02-01

    In order to better understand the nature and formation of the lower continental crust beneath northern Arabia, we studied lower crustal xenoliths brought up by Neogene basalts in NE Jordan. Most of these xenoliths are comprised of primary phases plagioclase + two-pyroxenes with magnetite and ilmenite. Most clinopyroxene are augite whereas orthopyroxene mostly are hypersthene (Mg# = 50-80). Plagioclase feldspar is dominantly andesine-labradorite; pyrope-rich garnet and Fe-rich olivine (Fo75 to Fo62) are rare. These xenoliths represent cumulates formed from intermediate magmas that pooled in the lower crust. Many xenoliths also contain small, fine-grained K-rich zones interpreted as melt pockets reflecting late magmatic infiltration of the lower crust. The xenoliths display a wide range in major element compositions (37-51 wt.% SiO2, 4-15 wt.% MgO and 0.1-6.3 wt.% TiO2), enrichment in Ba, K, Sr, Pb and Eu, and some trace element ratios atypical of bulk continental crust (e.g., K/Rb = 1265 ± 565, K/U = 63 000 ± 60 080 and Th/U = 0.96 ± 0.56); these extreme ratios reflect widespread K-metasomatism associated with melt pockets. The magmas from which these cumulates formed may have been generated at a reararc convergent margin setting. Four U-Pb zircon populations yield indistinguishable ages of 554 ± 4 Ma; 559 ± 5 Ma; 559 ± 6 Ma, and 563 ± 5 Ma. Initial 87Sr/86Sr values (0.70260-0.70352) and positive εNd(560) (with the exception of a single, more radiogenic sample (+9.6), range = + 1.3 to +4.8) indicate that the lower crust sampled by the xenoliths originated in the asthenospheric mantle, with little or no interaction with older crust, although Pb isotopic compositions allow for some interaction with older or subducted crustal materials. We interpret the geochemistry and mineralogy of these xenoliths to indicate that the lower crust beneath NE Jordan is mafic and comprised of plagioclase-rich 2-pyroxene igneous rocks. The lower crust of this area formed by

  4. Juvenile arthritis and uveitis.

    Science.gov (United States)

    Kanski, J J

    1990-01-01

    The association between juvenile arthritis and uveitis is reviewed. Some children with the HLA-B27 related spondyloarthropathies develop anterior uveitis. About 20% of patients with juvenile rheumatoid arthritis (JRA) who are negative for IgM rheumatoid factor develop a frequently bilateral, nongranulomatous chronic anterior uveitis. Risk factors for uveitis in JRA patients are: female gender, pauciarticular onset of arthritis, presence of circulating antinuclear antibodies, and the antigens HLA-DW5 and HLA-DPw2. Uveitis is rare after seven years or more have elapsed from the onset of arthritis. The visual prognosis in patients with uveitis is good in 25% and fair in 50%. The remaining 25% develop visual impairment from complicated cataract and/or secondary inflammatory glaucoma. The potential benefit of cytotoxic agents in the treatment of intractable uveitis is outweighed by the risk of serious side effects. The management of secondary inflammatory glaucoma is unsatisfactory, but the results of treatment of complicated cataracts by lensectomy-vitrectomy are good.

  5. Aggressive juvenile mandibular fibromatosis.

    Science.gov (United States)

    Ivanov, Georgi P; Atanasov, Dimitar T; Anavi, Beniamin L

    2013-01-01

    Aggressive juvenile fibromatosis of the jawbones is a rare tumor presenting as infiltrative mass with unpredictable evolution. We report herein a 17-year-old student with a 6-month history of radiologically proven resorption of a part of the mandible, lingual displacement of tooth 34 and malocclusion. Alveolar ridge resorption and three dark-brown foci in the bone were seen after the tooth was extracted. Histological study showed the tumor tissue to have a bundle-like structure; immunohistochemically it was positive for vimentin, smooth muscle actin, beta-catenin, Ki-67 (5%), and negative for desmin and cytokeratin 34bE12. The golden standard in the diagnostics of desmoid fibromatoses is the nuclear or membrane expression of beta-catenin, which is found in 90% of the cases. Differential diagnosis include mandibular fibroma, well-differentiated fibrosarcoma, fibrosing histiocytoma, and infiltration from adjacent soft-tissue tumor. Aggressive juvenile fibromatosis should be managed by radical excision. Local recurrences are not rare, but metastases do not develop. In rare cases this type of fibromatosis has been known to regress spontaneously. Aggressive fibromatosis is a diagnostic challenge, since it remains in the grey zone between benign and malignant lesions of the oral cavity.

  6. Intensive Ammonia and Methane Oxidation in Organic Liquid Manure Crusts

    DEFF Research Database (Denmark)

    Nielsen, Daniel Aagren; Nielsen, Lars Peter; Schramm, Andreas;

    of the crusts. PCR targeting the unique methane and ammonia monooxygenases were applied together with FISH to detect the presence of the two bacterial groups. Potential activity was assessed by short term slurry incubations of crust samples while monitoring NO2- production or CH4 consumption. Crusts were...... also CH4 emission mitigation, an organic surface crust can be effective if populations of MOB and AOB are allowed to build up....

  7. The Lord Howe Rise continental ribbon: a fragment of eastern Gondwana that reveals the drivers of continental rifting and plate tectonics

    Science.gov (United States)

    Saito, S.; Hackney, R. I.; Bryan, S. E.; Kimura, J. I.; Müller, D.; Arculus, R. J.; Mortimer, N. N.; Collot, J.; Tamura, Y.; Yamada, Y.

    2016-12-01

    Plate tectonics and resulting changes in crustal architecture profoundly influence global climate, oceanic circulation, and the origin, distribution and sustainability of life. Ribbons of continental crust rifted from continental margins are one product of plate tectonics that can influence the Earth system. Yet we have been unable to fully resolve the tectonic setting and evolution of huge, thinned, submerged, and relatively inaccessible continental ribbons like the Lord Howe Rise (LHR), which formed during Cretaceous fragmentation of eastern Gondwana. Thinned continental ribbons like the LHR are not easily explained or predicted by plate-tectonic theory. However, because Cretaceous rift basins on the LHR preserve the stratigraphy of an un-accreted and intact continental ribbon, they can help to determine whether plate motion is self-organised—passively driven by the pull of negatively-buoyant subducting slabs—or actively driven by convective flow in the mantle. In a self-organising scenario, the LHR formed in response to ocean-ward retreat of the long-lived eastern Gondwana subduction zone and linked upper-plate extension. In the mantle-driven scenario, the LHR resulted from rifting near the eastern edge of Gondwana that was triggered by processes linked to emplacement of a silicic Large Igneous Province. These scenarios can be distinguished using the ribbon's extensional history and the composition and tectonic affinity of igneous rocks within rift basins. However, current knowledge of LHR rift basins is based on widely-distributed marine and satellite geophysical data, limited dredge samples, and sparse shallow drilling (<600 m below-seafloor). This limits our ability to understand the evolution of extended continental ribbons, but a recent deep crustal seismic survey across the LHR and a proposed IODP deep stratigraphic well through a LHR rift basin provide new opportunities to explore the drivers behind rifting, continental ribboning and plate tectonics.

  8. Microbial community structure in three deep-sea carbonate crusts

    NARCIS (Netherlands)

    Heijs, S. K.; Aloisi, G.; Bouloubassi, I.; Pancost, R. D.; Pierre, C.; Damste, J. S. Sinninghe; Gottschal, J. C.; van Elsas, J. D.; Forney, L. J.

    2006-01-01

    Carbonate crusts in marine environments can act as sinks for carbon dioxide. Therefore, understanding carbonate crust formation could be important for understanding global warming. In the present study, the microbial communities of three carbonate crust samples from deep-sea mud volcanoes in the eas

  9. Permeability of crust is key to crispness retention

    NARCIS (Netherlands)

    Hirte, A.; Hamer, R.J.; Meinders, M.B.J.; Primo-Martin, C.

    2010-01-01

    Bread loses crispness rapidly after baking because water originating from the wet crumb accumulates in the dry crust. This water accumulation might be increased by the dense and low permeable character of the bread crust. Our objective was to investigate the influence of permeability of the crust on

  10. Permeability of crust is key to crispness retention

    NARCIS (Netherlands)

    Hirte, A.; Hamer, R.J.; Meinders, M.B.J.; Primo-Martin, C.

    2010-01-01

    Bread loses crispness rapidly after baking because water originating from the wet crumb accumulates in the dry crust. This water accumulation might be increased by the dense and low permeable character of the bread crust. Our objective was to investigate the influence of permeability of the crust on

  11. Bioenergetics of Continental Serpentinites

    Science.gov (United States)

    Cardace, D.; Meyer-Dombard, D. R.

    2011-12-01

    Serpentinization is the aqueous alteration of ultramafic (Fe- and Mg-rich) rocks, resulting in secondary mineral assemblages of serpentine, brucite, iron oxyhydroxides and magnetite, talc, and possibly carbonate and silica-rich veins and other minor phases-all depending on the evolving pressure-temperature-composition of the system. The abiotic evolution of hydrogen and possibly organic compounds via serpentinization (McCollom and Bach, 2009) highlights the relevance of this geologic process to carbon and energy sources for the deep biosphere. Serpentinization may fuel life over long stretches of geologic time, throughout the global seabed and in exposed, faulted peridotite blocks (as at Lost City Hydrothermal Field, Kelley et al., 2005), and in obducted oceanic mantle units in ophiolites (e.g., Tiago et al., 2004). Relatively little work has been published on life in continental serpentinite settings, though they likely host a unique resident microbiota. In this work, we systematically model the serpentinizing fluid as an environmental niche. Reported field data for high and moderate pH serpentinizing fluids were modeled from Cyprus, the Philippines, Oman, Northern California, New Caledonia, Yugoslavia, Portugal, Italy, Newfoundland Canada, New Zealand, and Turkey. Values for Gibbs Energy of reaction (ΔGr), kJ per mole of electrons transferred for a given metabolism, are calculated for each field site. Cases are considered both for (1) modest assumptions of 1 nanomolar hydrogen and 1 micromolar methane, based on unpublished data for a similar northern California field site (Cardace and Hoehler, in prep.) and (2) an upper estimate of 10 nanomolar hydrogen and 500 micromolar methane. We survey the feasibility of microbial metabolisms for key steps in the nitrogen cycle, oxidation of sulfur in pyrite, iron oxidation or reduction reactions, sulfate reduction coupled to hydrogen or methane oxidation, methane oxidation coupled to the reduction of oxygen, and

  12. 76 FR 2919 - Outer Continental Shelf Official Protraction Diagram and Supplemental Official Outer Continental...

    Science.gov (United States)

    2011-01-18

    ... Bureau of Ocean Energy Management, Regulation and Enforcement Outer Continental Shelf Official Protraction Diagram and Supplemental Official Outer Continental Shelf Block Diagrams AGENCY: Bureau of Ocean... American Datum of 1983 (NAD 83) Outer Continental Shelf Official Protraction Diagram and...

  13. Juvenile justice mental health services.

    Science.gov (United States)

    Thomas, Christopher R; Penn, Joseph V

    2002-10-01

    As the second century of partnership begins, child psychiatry and juvenile justice face continuing challenges in meeting the mental health needs of delinquents. The modern juvenile justice system is marked by a significantly higher volume of cases, with increasingly complicated multiproblem youths and families with comorbid medical, psychiatric, substance abuse disorders, multiple family and psychosocial adversities, and shrinking community resources and alternatives to confinement. The family court is faced with shrinking financial resources to support court-ordered placement and treatment programs in efforts to treat and rehabilitate youths. The recognition of high rates of mental disorders for incarcerated youth has prompted several recommendations for improvement and calls for reform [56,57]. In their 2000 annual report, the Coalition for Juvenile Justice advocated increased access to mental health services that provide a continuum of care tailored to the specific problems of incarcerated youth [58]. The specific recommendations of the report for mental health providers include the need for wraparound services, improved planning and coordination between agencies, and further research. The Department of Justice, Office of Juvenile Justice and Delinquency Prevention has set three priorities in dealing with the mental health needs of delinquents: further research on the prevalence of mental illness among juvenile offenders, development of mental health screening assessment protocols, and improved mental health services [59]. Other programs have called for earlier detection and diversion of troubled youth from juvenile justice to mental health systems [31,56]. Most recently, many juvenile and family courts have developed innovative programs to address specific problems such as truancy or substance use and diversionary or alternative sentencing programs to deal with first-time or nonviolent delinquents. All youths who come in contact with the juvenile justice system

  14. ALL-RUSSIA CONFERENCE «FLUID REGIME OF ENDOGENIC PROCESSES IN THE CONTINENTAL LITHOSPHERE», IRKUTSK, RUSSIA, OCTOBER 6–9, 2015

    OpenAIRE

    F. A. Letnikov; Yu.V. Danilova; T. S. Sokolova; A. V. Levin

    2015-01-01

    Chronicle of All-Russian conference “Fluid regime of endogenic processes of continental lithosphere” devoted to 50 anniversary of Laboratory of petrology, geochemistry and ore genesis of the Institute of the Earth’s crust is presented. Description of events accompanying the conference is given and overview of the presented reports is made.

  15. ALL-RUSSIA CONFERENCE «FLUID REGIME OF ENDOGENIC PROCESSES IN THE CONTINENTAL LITHOSPHERE», IRKUTSK, RUSSIA, OCTOBER 6–9, 2015

    Directory of Open Access Journals (Sweden)

    F. A. Letnikov

    2015-12-01

    Full Text Available Chronicle of All-Russian conference “Fluid regime of endogenic processes of continental lithosphere” devoted to 50 anniversary of Laboratory of petrology, geochemistry and ore genesis of the Institute of the Earth’s crust is presented. Description of events accompanying the conference is given and overview of the presented reports is made.

  16. ALL-RUSSIA CONFERENCE «FLUID REGIME OF ENDOGENIC PROCESSES IN THE CONTINENTAL LITHOSPHERE», IRKUTSK, RUSSIA, OCTOBER 6–9, 2015

    OpenAIRE

    F. A. Letnikov; Yu. V. Danilova; T. S. Sokolova; Levin, A V

    2015-01-01

    Chronicle of All-Russian conference “Fluid regime of endogenic processes of continental lithosphere” devoted to 50 anniversary of Laboratory of petrology, geochemistry and ore genesis of the Institute of the Earth’s crust is presented. Description of events accompanying the conference is given and overview of the presented reports is made.

  17. Crusted Scabies in the Burned Patient

    DEFF Research Database (Denmark)

    Berg, Jais Oliver; Alsbjørn, Bjarne

    2011-01-01

    The objectives of this study were 1) to describe a case of crusted scabies (CS) in a burned patient, which was primarily undiagnosed and led to a nosocomial outbreak in the burn unit; 2) to analyze and discuss the difficulties in diagnosing and treating this subset of patients with burn injury......; and 3) to design a treatment strategy for future patients. Case analysis and literature review were performed. The index patient had undiagnosed crusted scabies (sive Scabies norvegica) with the ensuing mite hyperinfestation when admitted to the department with minor acute dermal burns. Conservative...... healing and autograft healing were impaired because of the condition. Successful treatment of the burns was only accomplished secondarily to scabicide treatment. An outbreak of scabies among staff members indirectly led to diagnosis. CS is ubiquitous, and diagnosis may be difficult. This is the first...

  18. Towards a metallurgy of neutron star crusts

    CERN Document Server

    Kobyakov, D

    2013-01-01

    In the standard picture of the crust of a neutron star, matter there is simple: a body-centered-cubic (bcc) lattice of nuclei immersed in an essentially uniform electron gas. We show that at densities above that for neutron drip ($\\sim4\\times10^11$) g cm$^{-3}$ or roughly one thousandth of nuclear matter density, the interstitial neutrons give rise to an attractive interaction between nuclei that renders the lattice unstable. We argue that the likely equilibrium structure is similar to that in displacive ferroelectric materials such as BaTiO$_3$. As a consequence, properties of matter in the inner crust are expected to be much richer than previously appreciated and we mention consequences for observable neutron star properties.

  19. Towards a metallurgy of neutron star crusts.

    Science.gov (United States)

    Kobyakov, D; Pethick, C J

    2014-03-21

    In the standard picture of the crust of a neutron star, matter there is simple: a body-centered-cubic lattice of nuclei immersed in an essentially uniform electron gas. We show that, at densities above that for neutron drip (∼ 4 × 1 0(11)  g cm(-3) or roughly one-thousandth of nuclear matter density), the interstitial neutrons give rise to an attractive interaction between nuclei that renders the lattice unstable. We argue that the likely equilibrium structure is similar to that in displacive ferroelectric materials such as BaTiO3. As a consequence, the properties of matter in the inner crust are expected to be much richer than previously appreciated, and we mention possible consequences for observable neutron star properties.

  20. Three-Dimensional Numerical Modeling of Crustal Growth at Active Continental Margins

    Science.gov (United States)

    Zhu, G.; Gerya, T.; Tackley, P. J.

    2011-12-01

    Active margins are important sites of new continental crust formation by magmatic processes related to the subduction of oceanic plates. We investigate these phenomena using a three-dimensional coupled petrological-geochemical-thermomechanical numerical model, which combines a finite-difference flow solver with a non-diffusive marker-in-cell technique for advection (I3ELVIS code, Gerya and Yuen, PEPI,2007). The model includes mantle flow associated with the subducting plate, water release from the slab, fluid propagation that triggers partial melting at the slab surface, melt extraction and the resulting volcanic crustal growth at the surface. The model also accounts for variations in physical properties (mainly density and viscosity) of both fluids and rocks as a function of local conditions in temperature, pressure, deformation, nature of the rocks, and chemical exchanges. Our results show different patterns of crustal growth and surface topography, which are comparable to nature, during subduction at active continental margins. Often, two trench-parallel lines of magmatic activity, which reflect two maxima of melt production atop the slab, are formed on the surface. The melt extraction rate controls the patterns of new crust at different ages. Moving free water reflects the path of fluids, and the velocity of free water shows the trend of two parallel lines of magmatic activity. The formation of new crust in particular time intervals is distributed in finger-like shapes, corresponding to finger-like and ridge-like cold plumes developed atop the subducting slabs (Zhu et al., G-cubed,2009; PEPI,2011). Most of the new crust is basaltic, formed from peridotitic mantle. Granitic crust extracted from melted sediment and upper crust forms in a line closer to the trench, and its distribution reflects the finger-like cold plumes. Dacitic crust extracted from the melted lower crust forms in a line farther away from the trench, and its distribution is anticorrelated with

  1. Excited nuclei in neutron star crusts

    CERN Document Server

    Takibayev, Nurgali; Nasirova, Diana

    2012-01-01

    The paper considers the chains of successive electron capture reactions by nuclei of the iron group which take place in the crystal structures of neutron star envelopes. It is shown that as a result of such reactions the daughter nuclei in excited states accumulate within certain layers of neutron star crusts. The phonon model of interactions is proposed between the excited nuclei in the crystalline structure, as well as formation of highly excited nuclear states which emit neutrons and higher energy photons.

  2. Lime-Crusted Rammed Earth: Materials Study

    OpenAIRE

    Mileto, Camilla; Vegas López-Manzanares, Fernando; Alejandre, Francisco Javier; Martín, Juan Jesús; Garcia Soriano, Lidia

    2013-01-01

    This study analyses the durability of rammed-earth wall construction techniques. The analysis focuses on three medieval masonry types from the Castle of Villavieja (Castellón, Spain) using two variations of lime-reinforced rammed earth in its walls: lime-crusted rammed earth and brick-reinforced rammed earth. Materials analysis reveals the good properties of the materials used in the outer wall facing despite its age. It also clearly shows how deterioration depends more on the construction t...

  3. Topological characterization of neutron star crusts

    CERN Document Server

    Dorso, C O; López, J A

    2012-01-01

    Neutron star crusts are studied using a classical molecular dynamics model developed for heavy ion reactions. After the model is shown to produce a plethora of the so-called "pasta" shapes, a series of techniques borrowed from nuclear physics, condensed matter physics and topology are used to craft a method that can be used to characterize the shape of the pasta structures in an unequivocal way.

  4. Modelling of sea floor spreading initiation and rifted continental margin formation

    Science.gov (United States)

    Tymms, V. J.; Isimm Team

    2003-04-01

    Recent observations of depth dependent (heterogeneous) stretching where upper crustal extension is much less than that of the lower crust and lithospheric mantle at both non-volcanic and volcanic margins plus the discovery of broad domains of exhumed continental mantle at non-volcanic rifted margins are not predicted by existing quantitative models of rifted margin formation which are usually based on intra-continental rift models subjected to very large stretching factors. New conceptual and quantitative models of rifted margin formation are required. Observations and continuum mechanics suggest that the dominant process responsible for rifted continental margin formation is sea-floor spreading of the young ocean ridge, rather than pre-breakup intra-continental rifting. Simple fluid flow models of ocean ridge processes using analytical iso-viscous corner-flow demonstrate that the divergent motion of the upwelling mantle beneath the ocean ridge, when viewed in the reference frame of the young continental margin, shows oceanward flow of the lower continental crust and lithospheric mantle of the young rifted margin giving rise to depth dependent stretching as observed. Single-phase fluid-models have been developed to model the initiation of sea-floor spreading and the thermal, stretching and thinning evolution of the young rifted continental margin. Finite element fluid-flow modelling incorporating the evolving temperature dependent viscosity field on the fluid flow also show depth dependent stretching of the young continental margin. Two-phase flow models of ocean ridges incorporating the transport of both solid matrix and melt fluid (Spiegelman &Reynolds 1999) predict the divergent motion of the asthenosphere and lithosphere matrix, and the focusing of basaltic melt into the narrow axial zone spreading centre at ocean ridges. We are adapting two-phase flow models for application to the initiation of sea-floor spreading and rifted continental margin formation. i

  5. Miranda Rights: Implications for Juveniles with Disabilities

    Science.gov (United States)

    Katsiyannis, Antonis; Barrett, David E.; Losinski, Mickey L.

    2011-01-01

    Juvenile delinquency in the United States has been a persistent concern for decades. Consequently, because more juveniles have been referred to juvenile court and the arrest rate of preteen offenders has increased to almost three times that of older youth, the persistent and often controversial issue of the capacity of juvenile offenders to waive…

  6. Pyrolysis of waste plastic crusts of televisions.

    Science.gov (United States)

    Liu, Xinmin; Wang, Zhen; Xu, Dongyan; Guo, Qingjie

    2012-09-01

    The disposal of waste plastic crusts of televisions is an issue that is gaining increasing interest around the world. In this investigation, the pyrolysis and catalytic cracking of the waste television crusts mainly composed of acrylonitrile--butadiene-styrene copolymer was studied. Thermogravimetric analysis was used for initial characterization of the pyrolysis of the waste plastic, but most of the investigations were carried out using a 600 mL tubing reactor. Effects of temperature, reaction time and catalyst on the pyrolysis of the waste television crusts were investigated. The results showed that the oil yield increased with increasing temperature or with prolongation of reaction time. With increasing temperature, the generating percentage of gasoline and diesel oil increased, but the heavy oil yield decreased. Zinc oxide, iron oxide and fluid catalytic cracking catalyst (FCC catalyst) were employed to perform a series of experiments. It was demonstrated that the liquid product was markedly improved and the reaction temperature decreased 100 degrees C when FCC was used. The composition ofpyrolysis oils was analysed using gas chromatography-mass spectrometry, and they contained 36.49% styrene, 19.72% benzenebutanenitrile, 12.1% alpha-methylstyrene and 9.69% dimethylbenzene.

  7. Juvenile prison in parallel legislation

    Directory of Open Access Journals (Sweden)

    Lutovac Mitar

    2016-01-01

    Full Text Available The need for punishment of juveniles occurred from the time when there was no clear line separating them from the adult criminal population. At the same time, the evolution of the juvenile punishment is not in itself involve substantial changes to their criminal status. On the contrary, the status of minors in society did not show serious differences regarding the status of young adults, as well as the adult elderly. On the other hand, on the ground of their punishment is recorded deviations that go in the direction of application of mild corporal punishment. Closing the minor was performed in a physically separate parts of the general penal institutions with the use of a lower degree of restrictions while serving juvenile prison. Due to the different treatment of minors during the evolution of their criminal status leads to their different treatment in comparative law. That is why we are witnessing the existence of numerous differences in the juvenile punishment in some countries in the world. On the European continent there is a wide range of different legal solutions when it comes to punishing juveniles. There are considerable differences in the procedure pronouncing juvenile prison and in particular penal treatment of juveniles in penitentiary institutions. For these reasons, the author has decided to show the basic statutory provisions in the part that relates to the issue of punishment of minors in the legislation of individual countries.

  8. Update on juvenile myasthenia gravis.

    Science.gov (United States)

    Liew, Wendy K M; Kang, Peter B

    2013-12-01

    Juvenile myasthenia gravis is a relatively rare autoimmune neuromuscular disorder. The pathophysiology of juvenile myasthenia gravis is similar to that of adult myasthenia gravis, though there remain important differences regarding presentation and therapeutic options. We review the pathophysiology, clinical presentation, and treatment options for juvenile myasthenia gravis. Randomized clinical studies of myasthenia gravis have been carried out primarily in adult populations. As juvenile myasthenia gravis is rare, it has been difficult to collect prospective randomized controlled data to evaluate treatment outcomes and efficacy. A recent retrospective series suggests that, as in adult myasthenia gravis, thymectomy is a viable therapeutic option for selected cases of generalized juvenile myasthenia gravis. This is corroborated by the clinical experience of the authors in a referral center with a cohort of patients affected by juvenile myasthenia gravis over a number of years. Recent studies illustrate that some, but not all, adult research on myasthenia gravis is applicable to children and adolescents with juvenile myasthenia gravis. Adult research can inform pediatric studies, but should not be regarded as a substitute for dedicated research in those populations.

  9. A metasomatic mechanism for the formation of Earth's earliest evolved crust

    Science.gov (United States)

    Baker, Don R.; Sofonio, Kassandra

    2017-04-01

    Following giant impacts the early Hadean Earth was shrouded in a steam atmosphere for durations on the order of 1 Ma. In order to investigate the potential of this atmosphere to fractionate major elements between various silicate reservoirs and influence a planet's geochemical evolution, we performed experiments simulating the interaction of a post-giant-impact steam atmosphere with a bulk silicate Earth (BSE) composition. Our experiments indicate that the composition of the solute in a water-rich atmosphere at 10 MPa and ∼727 °C is remarkably similar to that of Earth's modern continental crust and would constitute up to 10% of the solution mass. This solute composition is similar to solute compositions previously measured at higher pressures, but distinct from those of near-solidus peridotite melts. Mass balance calculations based upon the hypothesis that Earth's initial water concentration was similar to that in CI carbonaceous chondrites, and that degassing and metasomatism produced the BSE, indicate that metasomatism could produce from 10 to 300% of the mass of the modern crust. If instead the amount of metasomatism is estimated by the difference between the water concentration in the BSE and in the depleted upper mantle, then a mass of up to approximately 4% of the current crust could be produced by metasomatism. Using results of earlier research we find that the solute is expected to have a smaller Sm/Nd ratio than the residual BSE, and if the solute was formed early in Earth's history its Nd isotopic signatures would be highly enriched. Although we cannot be certain that the metasomatic process created a significant fraction of Earth's crust in the early Hadean, our research indicates that it has the potential to form crustal nuclei and possibly was responsible for the production of incompatible-element enriched reservoirs in the early Earth, as seen in the isotopic signatures of Archean rocks.

  10. Noble Gas Isotopic Compositions of Cobalt-rich Ferromanganese Crusts from the Western Pacific Ocean and Their Geological Implications

    Institute of Scientific and Technical Information of China (English)

    SUN Xiaoming; XUE Ting; HE Gaowen; YE Xianren; ZHANG Mei; LU Hongfeng; WANG Shengwei

    2007-01-01

    Noble gas isotopic compositions of various layers in three-layered (outer, porous and compact layers) cobalt-rich ferromanganese crusts and their basaltic and phosphorite substrates from the western Pacific Ocean were analyzed by using a high vacuum gas mass spectrum. The analytical results show that the noble gases in the Co-rich crusts have derived mainly from the ambient seawater,extraterrestrial grains such as interplanetary dust particles (IDPs) and wind-borne continental dust grains, and locally formation water in the submarine sediments, but different noble gases have different sources. He in the crusts derives predominantly from the extraterrestrial grains, with a negligible amount of radiogenic He from the eolian dust grains. Ar is sourced mainly from the dissolved air in the seawater and insignificantly from radiogenic Ar in the eolian continental dust grains or the formation water. Xe and Ne derive mainly from the seawater, with minor amounts of extraterrestrial Xe and Ne in the IDPs. Compared with the porous and outer layers, the compact layer has a relatively high 4He content and lower 3He/4He ratios, suggesting that marine phosphatization might have greatly modified the noble gas isotopic compositions of the crusts. Besides, the 3He/4He values of the basaltic substrates of the cobalt-rich crusts are very low and their R/Ra ratios are mostly <0.1 Ra, which are similar to that of phosphorite substrates (0.087 Ra), but much lower than that of fresh submarine MORB (8.75±2.14Ra) or seamount basalts (3-43 Ra), implying that the basaltic substrates have suffered strong water/rock interaction and reacted with radiogenic 4He and P-rich upwelling marine currents during phosphatization. The trace elements released in the basalt/seawater interaction might favor the growth of cobalt-rich crusts. The relatively low 3He/4He values in the seamount basalts may be used as an important exploration criterion for the cobalt-rich ferromanganese crusts.

  11. Juvenile chronic arthritis.