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Sample records for active continental rift

  1. Continental Rifts

    Science.gov (United States)

    Rosendahl, B. R.

    Continental Rifts, edited by A. M. Quennell, is a new member of the Benchmark Papers in Geology Series, edited in toto by R. W. Fairbridge. In this series the individual volume editors peruse the literature on a given topic, select a few dozen papers of ostensibly benchmark quality, and then reorder them in some sensible fashion. Some of the original papers are republished intact, but many are chopped into “McNuggets™” of information. Depending upon the volume editor, the chopping process can range from a butchering job to careful and prudent pruning. The collecting, sifting, and reorganizing tasks are, of course, equally editor-sensitive. The end product of this series is something akin to a set of Reader's Digest of Geology.

  2. Hydrothermal Petroleum in Active Continental Rift: Lake Chapala, Western Mexico, Initial Results.

    Science.gov (United States)

    Zarate-del Valle, P. F.; Simoneit, B. R.; Ramirez-Sanchez, H. U.

    2003-12-01

    Lake Chapala in western Mexico is located partially in the Citala Rift, which belongs to the well-known neotectonic Jalisco continental triple junction. The region is characterized by active volcanism (Ceboruco, Volcan de Fuego), tectonic (1995 earthquake, M=8, 40-50 mm to SW) and hydrothermal (San Juan Cosala & Villa Corona spas and La Calera sinter deposit) activities. Hydrothermal petroleum has been described in active continental rift (East African Rift) and marine spreading zones (Guaymas Basin, Gulf of California). In 1868 the Mexican local press reported that manifestations of bitumen were appearing in front of the Columba Cap on the mid south shore of Lake Chapala. This bitumen is linked to the lake bottom and when the water level decreases sufficiently it is possible to access these tar bodies as islands. Because of these manifestations the Mexican oil company (PEMEX) drilled an exploration well (2,348m) at Tizapan El Alto without success. Hydrothermal activity is evident in the tar island zone as three in-shore thermal springs (26.8 m depth, 48.5° C, pH 7.8 and oriented N-S). The preliminary analyses by GC-MS of the tar from these islands indicate hydrothermal petroleum derived from lake sedimentary organic matter, generated at low temperatures (150° -200° C). The tars contain no n-alkanes, no PAH or other aromatics, but a major UCM of branched and cyclic hydrocarbons and mature biomarkers derived from lacustrine biota. The biomarkers consist of mainly 17α (H),21β (H)-hopanes ranging from C27 to C34 (no C28), gammacerane, tricyclic terpanes (C20-C26), carotane and its cracking products, and drimanes (C14-C16). The biomarker composition indicates an organic matter source from bacteria and algae, typical of lacustrine ecosystems. 14C dating of samples from two tar islands yielded ages exceeding 40 kyrs, i.e., old carbon from hydrothermal/tectonic remobilization of bitumen from deeper horizons to the surface. The occurrence of hydrothermal petroleum in

  3. Off-axis volcano-tectonic activity during continental rifting: Insights from the transversal Goba-Bonga lineament, Main Ethiopian Rift (East Africa)

    Science.gov (United States)

    Corti, Giacomo; Sani, Federico; Agostini, Samuele; Philippon, Melody; Sokoutis, Dimitrios; Willingshofer, Ernst

    2018-03-01

    The Main Ethiopian Rift, East Africa, is characterized by the presence of major, enigmatic structures which strike approximately orthogonal to the trend of the rift valley. These structures are marked by important deformation and magmatic activity in an off-axis position in the plateaus surrounding the rift. In this study, we present new structural data based on a remote and field analysis, complemented with analogue modelling experiments, and new geochemical analysis of volcanic rocks sampled in different portions of one of these transversal structures: the Goba-Bonga volcano-tectonic lineament (GBVL). This integrated analysis shows that the GBVL is associated with roughly E-W-trending prominent volcano-tectonic activity affecting the western plateau. Within the rift floor, the approximately E-W alignment of Awasa and Corbetti calderas likely represent expressions of the GBVL. Conversely, no tectonic or volcanic features of similar (E-W) orientation have been recognized on the eastern plateau. Analogue modelling suggests that the volcano-tectonic features of the GBVL have probably been controlled by the presence of a roughly E-W striking pre-existing discontinuity beneath the western plateau, which did not extend beneath the eastern plateau. Geochemical analysis supports this interpretation and indicates that, although magmas have the same sub-lithospheric mantle source, limited differences in magma evolution displayed by products found along the GBVL may be ascribed to the different tectonic framework to the west, to the east, and in the axial zone of the rift. These results support the importance of the heterogeneous nature of the lithosphere and the spatial variations of its structure in controlling the architecture of continental rifts and the distribution of the related volcano-tectonic activity.

  4. Comparative Riftology: Insights into the Evolution of Passive Continental Margins and Continental Rifts from the Failed Midcontinent Rift (MCR)

    Science.gov (United States)

    Elling, R. P.; Stein, C. A.; Stein, S.; Kley, J.; Keller, G. R.; Wysession, M. E.

    2017-12-01

    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 evolutionary paths. Of particular interest is the evolution of volcanic passive margins, which are characterized by seaward dipping reflectors, volcanic rocks yielding magnetic anomalies landward of the oldest spreading anomalies, and are underlain by high-velocity lower crustal bodies. How and when these features form remains unclear. Insights are given by the Midcontinent Rift (MCR), which began to form during the 1.1 Ga rifting of Amazonia from Laurentia, but failed when 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 and highly magnetized volcanic section during rifting. Surface exposures and reflection seismic data 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 magnetic anomaly. Because the rift formed as a series of alternating half-grabens, structural asymmetries between conjugate margins would have naturally occurred had it gone to completion. Hence the MCR implies that many passive margin features form prior to seafloor spreading. Massive inversion of the MCR long after it failed has provided a much clearer picture of its structure compared to failed rifts with lesser degrees of inversion. Seismic imaging as well as gravity and magnetic modeling provide important insight into the effects of inversion on failed rifts. The MCR provides an end member for the evolution of actively extending rifts, characterized by upwelling mantle and negative gravity anomalies, to failed

  5. Root zone of a continental rift

    DEFF Research Database (Denmark)

    Kirsch, Moritz; Svenningsen, Olaf

    2016-01-01

    melt are considered to account for the compositional range exhibited by the KIC igneous rocks. U/Pb SIMS geochronological data from zircon rims yield an emplacement age of 578 ± 9 Ma. The KIC is thus younger and more depleted than coeval mafic rocks found in the Seve Nappe, and is interpreted...... to represent a high-level magma plumbing system in a late-stage continental rift. The composition and volume of rift-related igneous rocks in the Seve Nappes are inconsistent with a mantle plume origin, but are thought to record progressive lithospheric thinning and increasing involvement of an asthenospheric......Mafic magmatic rocks formed between ca. 615 and 560 Ma along the Neoproterozoic margins of Baltica and Laurentia are classically attributed to continental rifting heralding the opening of the Iapetus Ocean. We report new data for the Kebnekaise Intrusive Complex (KIC) exposed in the Seve Nappes...

  6. Commercial helium reserves, continental rifting and volcanism

    Science.gov (United States)

    Ballentine, C. J.; Barry, P. H.; Hillegonds, D.; Fontijn, K.; Bluett, J.; Abraham-James, T.; Danabalan, D.; Gluyas, J.; Brennwald, M. S.; Pluess, B.; Seneshens, D.; Sherwood Lollar, B.

    2017-12-01

    Helium has many industrial applications, but notably provides the unique cooling medium for superconducting magnets in medical MRI scanners and high energy beam lines. In 2013 the global supply chainfailed to meet demand causing significant concern - the `Liquid Helium Crisis' [1]. The 2017 closure of Quatar borders, a major helium supplier, is likely to further disrupt helium supply, and accentuates the urgent need to diversify supply. Helium is found in very few natural gas reservoirs that have focused 4He produced by the dispersed decay (a-particle) of U and Th in the crust. We show here, using the example of the Rukwa section of the Tanzanian East African Rift, how continental rifting and local volcanism provides the combination of processes required to generate helium reserves. The ancient continental crust provides the source of 4He. Rifting and associated magmatism provides the tectonic and thermal mechanism to mobilise deep fluid circulation, focusing flow to the near surface along major basement faults. Helium-rich springs in the Tanzanian Great Rift Valley were first identified in the 1950's[2]. The isotopic compositions and major element chemistry of the gases from springs and seeps are consistent with their release from the crystalline basement during rifting [3]. Within the Rukwa Rift Valley, helium seeps occur in the vicinity of trapping structures that have the potential to store significant reserves of helium [3]. Soil gas surveys over 6 prospective trapping structures (1m depth, n=1486) show helium anomalies in 5 out of the 6 at levels similar to those observed over a known helium-rich gas reservoir at 1200m depth (7% He - Harley Dome, Utah). Detailed macroseep gas compositions collected over two days (n=17) at one site allows us to distinguish shallow gas contributions and shows the deep gas to contain between 8-10% helium, significantly increasing resource estimates based on uncorrected values (1.8-4.2%)[2,3]. The remainder of the deep gas is

  7. Regional magnetic anomaly constraints on continental rifting

    Science.gov (United States)

    Vonfrese, R. R. B.; Hinze, W. J.; Olivier, R.; Bentley, C. R.

    1985-01-01

    Radially polarized MAGSAT anomalies of North and South America, Europe, Africa, India, Australia and Antarctica demonstrate remarkably detailed correlation of regional magnetic lithospheric sources across rifted margins when plotted on a reconstruction of Pangea. These major magnetic features apparently preserve their integrity until a superimposed metamorphoric event alters the magnitude and pattern of the anomalies. The longevity of continental scale magnetic anomalies contrasts markedly with that of regional gravity anomalies which tend to reflect predominantly isostatic adjustments associated with neo-tectonism. First observed as a result of NASA's magnetic satellite programs, these anomalies provide new and fundamental constraints on the geologic evolution and dynamics of the continents and oceans. Accordingly, satellite magnetic observations provide a further tool for investigating continental drift to compliment other lines of evidence in paleoclimatology, paleontology, paleomagnetism, and studies of the radiometric ages and geometric fit of the continents.

  8. The Porcupine Basin: from rifting to continental breakup

    Science.gov (United States)

    Reston, Timothy; Gaw, Viola; Klaeschen, Dirk; McDermott, Ken

    2015-04-01

    Southwest of Ireland, the Porcupine Basin is characterized by axial stretching factors that increase southward to values greater than six and typical of rifted margins. As such, the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup, and in particular the processes of mantle serpentinisation, and the onset of detachment faulting. We have processed through to prestack depth migration a series of E-W profiles crossing the basin at different axial stretching factors and linked by a N-S profile running close to the rift axis. Our results constrain the structure of the basin and have implications for the evolution of rifted margins. In the north at a latitude of 52.25N, no clear detachment is imaged, although faults do appear to cut down into the mantle, so that serpentinisation may have started. Further south (51.75N), a bright reflection (here named P) cuts down to the west from the base of the sedimentary section, is overlain by small fault blocks and appears to represent a detachment fault. P may in part follow the top of partially serpentinized mantle: this interpretation is consistent with gravity modelling, with numerical models of crustal embrittlement and mantle serpentinization during extension and with wide-angle data (see posters of Prada and of Watremez). Furthermore, P closely resembles the S reflection west of Iberia, where such serpentinites are well documented. P develops where the crust was thinned to less than 3 km during rifting, again similar to S. Although overall the basin remains symmetrical, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension became asymmetric. Analysis of the depth sections suggests that the detachment may have been active as a rolling hinge rooting at low-angle beneath the Porcupine Bank, consistent with the presence of a footwall of serpentinites. This requires very weak

  9. Normal-Faulting in Madagascar: Another Round of Continental Rifting?

    Science.gov (United States)

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

    2017-12-01

    Analyses of seismicity and seismic structure within Madagascar suggest the current occurrence of crustal extension, which may be related to continental rifting associated with a diffuse boundary between the Somalia and Lwandle tectonic plates. Madagascar has participated in two major rifting events as part of the break-up of Gondwana: the break-away of Greater India (Madagascar, India, the Seychelles) away from mainland Africa during the Jurassic and the break-away of India from Madagascar during the Cretaceous. Seismic activity and the structures obtained from it, using data from the 2-year (2011-2013) MACOMO project, suggest that this break-up may not be finished, and that continental rifts may be developing again. There are fairly high levels of intraplate seismicity within Madagascar: over 800 events located during the 22 months of the deployment. For comparison, a 2-year deployment of seismometers within the upper Midwest of the U.S. yielded just 12 intraplate earthquakes. While the Madagascar seismicity occurs across the island, it is strongly concentrated in the central region, where Cenozoic volcanism has occurred through the Holocene, and earthquakes align along N-S-trending lineations associated with N-S-trending pull-apart graben structures. The thickness of the crust is still >40 km in this region, but it is underlain by a large low-velocity structure within the lithosphere and asthenosphere that is observed in our studies of surface-wave, body-wave, and Pn-phase tomography. Normal faulting is not observed everywhere on the island, however; seismicity in the north is largely strike-slip, and seismicity in the south appears to be largely reverse faulting. Several studies have suggested that the diffuse boundary between the Somalia and Lwandle plates runs roughly E-W across Madagascar. Extensional faulting seems to predominate only within central Madagascar, likely associated with the current volcanic activity, which also appears to be associated with the

  10. Long wavelength magnetic anomalies over continental rifts in cratonic region

    Science.gov (United States)

    Friedman, S. A.; Persaud, P.; Ferre, E. C.; Martín-Hernández, F.; Feinberg, J. M.

    2017-12-01

    New collections of unaltered mantle xenoliths shed light on potential upper mantle contributions to long wavelength magnetic anomalies (LWMA) in continental rifts in cratonic / shield areas. The new material originates from the East African Rift (Tanzania), the Rio Grande Rift (U.S.A.), the Rhine Rift (Germany), and the West Antarctic Rift (Antarctica). The xenoliths sample the uppermost ( 0.2 or Fe geotherms (>60ºC/km) that are characteristic of rifted regions preclude any contribution to LWMA at depths >10 km. Hence, only upper basalts and hypovolcanic mafic sills would constitute potential magnetic sources. In contrast, the margins of these rifted regions consist of refractory cratonic domains, often characterized by oxidized sublithospheric mantle that host significant concentrations of primary magnetite. The higher NRMs of these peridotites (up to 15 A/m, Qn > 2.5) combined with much lower geotherms (as low as 15ºC/km) allows for a 5 to 10 km layer of uppermost mantle to potentially contribute to LWMA. Assuming that Qn values in rift margins are also gradient across the rift would primarily reflect thermal equilibration over time.

  11. Thermomechanical Controls on the Success and Failure of Continental Rift Systems

    Science.gov (United States)

    Brune, S.

    2017-12-01

    Studies of long-term continental rift evolution are often biased towards rifts that succeed in breaking the continent like the North Atlantic, South China Sea, or South Atlantic rifts. However there are many prominent rift systems on Earth where activity stopped before the formation of a new ocean basin such as the North Sea, the West and Central African Rifts, or the West Antarctic Rift System. The factors controlling the success and failure of rifts can be divided in two groups: (1) Intrinsic processes - for instance frictional weakening, lithospheric thinning, shear heating or the strain-dependent growth of rift strength by replacing weak crust with strong mantle. (2) External processes - such as a change of plate divergence rate, the waning of a far-field driving force, or the arrival of a mantle plume. Here I use numerical and analytical modeling to investigate the role of these processes for the success and failure of rift systems. These models show that a change of plate divergence rate under constant force extension is controlled by the non-linearity of lithospheric materials. For successful rifts, a strong increase in divergence velocity can be expected to take place within few million years, a prediction that agrees with independent plate tectonic reconstructions of major Mesozoic and Cenozoic ocean-forming rift systems. Another model prediction is that oblique rifting is mechanically favored over orthogonal rifting, which means that simultaneous deformation within neighboring rift systems of different obliquity and otherwise identical properties will lead to success and failure of the more and less oblique rift, respectively. This can be exemplified by the Cretaceous activity within the Equatorial Atlantic and the West African Rifts that lead to the formation of a highly oblique oceanic spreading center and the failure of the West African Rift System. While in nature the circumstances of rift success or failure may be manifold, simplified numerical and

  12. Geodetic constraints on continental rifting along the Red Sea

    Science.gov (United States)

    Reilinger, R.; McClusky, S.; Arrajehi, A.; Mahmoud, S.; Rayan, A.; Ghebreab, W.; Ogubazghi, G.; Al-Aydrus, A.

    2006-12-01

    We are using the Global Positioning System (GPS) to monitor and quantify patterns and rates of tectonic and magmatic deformation associated with active rifting of the continental lithosphere and the transition to sea floor spreading in the Red Sea. Broad-scale motions of the Nubian and Arabian plates indicate coherent plate motion with internal deformation below the current resolution of our measurements (~ 1-2 mm/yr). The GPS-determined Euler vector for Arabia-Nubia is indistinguishable from the geologic Euler vector determined from marine magnetic anomalies, and Arabia-Eurasia relative motion from GPS is equal within uncertainties to relative motion determined from plate reconstructions, suggesting that Arabia plate motion has remained constant (±10%) during at least the past ~10 Ma. The approximate agreement between broad-scale GPS rates of extension (i.e., determined from relative plate motions) and those determined from magnetic anomalies along the Red Sea rift implies that spreading in the central Red Sea is primarily confined to the central rift (±10-20%). Extension appears to be more broadly distributed in the N Red Sea and Gulf of Suez where comparisons with geologic data also indicate a relatively recent (between 500 and 125 kyr BP) change in the motion of the Sinai block that is distinct from both Nubia and Arabia. In the southern Red Sea, GPS results are beginning to define the motion of the "Danakil micro-plate". We investigate and report on a model involving CCW rotation of the Danakil micro-plate relative to Nubia and magmatic inflation below the Afar Triple Junction that is consistent with available geodetic constraints. Running the model back in time suggests that the Danakil micro-plate has been an integral part of rifting/triple junction processes throughout the history of separation of the Arabian and Nubian plates. On the scale of Nubia-Arabia-Eurasia plate interactions, we show that new area formed at spreading centers roughly equals that

  13. Generation of continental rifts, basins, and swells by lithosphere instabilities

    Science.gov (United States)

    Fourel, Loïc.; Milelli, Laura; Jaupart, Claude; Limare, Angela

    2013-06-01

    Continents may be affected simultaneously by rifting, uplift, volcanic activity, and basin formation in several different locations, suggesting a common driving mechanism that is intrinsic to continents. We describe a new type of convective instability at the base of the lithosphere that leads to a remarkable spatial pattern at the scale of an entire continent. We carried out fluid mechanics laboratory experiments on buoyant blocks of finite size that became unstable due to cooling from above. Dynamical behavior depends on three dimensionless numbers, a Rayleigh number for the unstable block, a buoyancy number that scales the intrinsic density contrast to the thermal one, and the aspect ratio of the block. Within the block, instability develops in two different ways in an outer annulus and in an interior region. In the outer annulus, upwellings and downwellings take the form of periodically spaced radial spokes. The interior region hosts the more familiar convective pattern of polygonal cells. In geological conditions, such instabilities should manifest themselves as linear rifts striking at a right angle to the continent-ocean boundary and an array of domal uplifts, volcanic swells, and basins in the continental interior. Simple scaling laws for the dimensions and spacings of the convective structures are derived. For the subcontinental lithospheric mantle, these dimensions take values in the 500-1000 km range, close to geological examples. The large intrinsic buoyancy of Archean lithospheric roots prevents this type of instability, which explains why the widespread volcanic activity that currently affects Western Africa is confined to post-Archean domains.

  14. 3D numerical simulations of multiphase continental rifting

    Science.gov (United States)

    Naliboff, J.; Glerum, A.; Brune, S.

    2017-12-01

    Observations of rifted margin architecture suggest continental breakup occurs through multiple phases of extension with distinct styles of deformation. The initial rifting stages are often characterized by slow extension rates and distributed normal faulting in the upper crust decoupled from deformation in the lower crust and mantle lithosphere. Further rifting marks a transition to higher extension rates and coupling between the crust and mantle lithosphere, with deformation typically focused along large-scale detachment faults. Significantly, recent detailed reconstructions and high-resolution 2D numerical simulations suggest that rather than remaining focused on a single long-lived detachment fault, deformation in this phase may progress toward lithospheric breakup through a complex process of fault interaction and development. The numerical simulations also suggest that an initial phase of distributed normal faulting can play a key role in the development of these complex fault networks and the resulting finite deformation patterns. Motivated by these findings, we will present 3D numerical simulations of continental rifting that examine the role of temporal increases in extension velocity on rifted margin structure. The numerical simulations are developed with the massively parallel finite-element code ASPECT. While originally designed to model mantle convection using advanced solvers and adaptive mesh refinement techniques, ASPECT has been extended to model visco-plastic deformation that combines a Drucker Prager yield criterion with non-linear dislocation and diffusion creep. To promote deformation localization, the internal friction angle and cohesion weaken as a function of accumulated plastic strain. Rather than prescribing a single zone of weakness to initiate deformation, an initial random perturbation of the plastic strain field combined with rapid strain weakening produces distributed normal faulting at relatively slow rates of extension in both 2D and

  15. From continental to oceanic rifting in the Gulf of California

    Science.gov (United States)

    Ferrari, Luca; Bonini, Marco; Martín, Arturo

    2017-11-01

    The continental margin of northwestern Mexico is the youngest example of the transition from a convergent plate boundary to an oblique divergent margin that formed the Gulf of California rift. Subduction of the Farallon oceanic plate during the Cenozoic progressively brought the East Pacific Rise (EPR) toward the North America trench. In this process increasingly younger and buoyant oceanic lithosphere entered the subduction zone until subduction ended just before most of the EPR could collide with the North America continental lithosphere. The EPR segments bounding the unsubducted parts of the Farallón plate remnants (Guadalupe and Magdalena microplates) also ceased spreading (Lonsdale, 1991) and a belt of the North American plate (California and Baja California Peninsula) became coupled with the Pacific Plate and started moving northwestward forming the modern Gulf of California oblique rift (Nicholson et al., 1994; Bohannon and Parsons, 1995). The timing of the change from plate convergence to oblique divergence off western Mexico has been constrained at the middle Miocene (15-12.5 Ma) by ocean floor morphology and magnetic anomalies as well as plate tectonic reconstructions (Atwater and Severinghaus, 1989; Stock and Hodges, 1989; Lonsdale, 1991), although the onset of transtensional deformation and the amount of right lateral displacement within the Gulf region are still being studied (Oskin et al., 2001; Fletcher et al., 2007; Bennett and Oskin, 2014). Other aspects of the formation of the Gulf of California remain not well understood. At present the Gulf of California straddles the transition from continental transtension in the north to oceanic spreading in the south. Seismic reflection-refraction data indicate asymmetric continent-ocean transition across conjugate margins of rift segments (González-Fernández et al., 2005; Lizarralde et al., 2007; Miller and Lizarralde, 2013; Martín-Barajas et al., 2013). The asymmetry may be related to crustal

  16. Evidence for cross rift structural controls on deformation and seismicity at a continental rift caldera

    Science.gov (United States)

    Lloyd, Ryan; Biggs, Juliet; Wilks, Matthew; Nowacki, Andy; Kendall, J.-Michael; Ayele, Atalay; Lewi, Elias; Eysteinsson, Hjálmar

    2018-04-01

    In continental rifts structural heterogeneities, such as pre-existing faults and foliations, are thought to influence shallow crustal processes, particularly the formation of rift faults, magma reservoirs and surface volcanism. We focus on the Corbetti caldera, in the southern central Main Ethiopian Rift. We measure the surface deformation between 22nd June 2007 and 25th March 2009 using ALOS and ENVISAT SAR interferograms and observe a semi-circular pattern of deformation bounded by a sharp linear feature cross-cutting the caldera, coincident with the caldera long axis. The signal reverses in sign but is not seasonal: from June to December 2007 the region south of this structure moves upwards 3 cm relative to the north, while from December 2007 until November 2008 it subsides by 2 cm. Comparison of data taken from two different satellite look directions show that the displacement is primarily vertical. We discuss potential mechanisms and conclude that this deformation is associated with pressure changes within a shallow (statistically consistent with this fault structure, indicating that the fault has also controlled the migration of magma from a reservoir to the surface over tens of thousands of years. Spatial patterns of seismicity are consistent with a cross-rift structure that extents outside the caldera and to a depth of ∼30 km, and patterns of seismic anisotropy suggests stress partitioning occurs across the structure. We discuss the possible nature of this structure, and conclude that it is most likely associated with the Goba-Bonga lineament, which cross-cuts and pre-dates the current rift. Our observations show that pre-rift structures play an important role in magma transport and shallow hydrothermal processes, and therefore they should not be neglected when discussing these processes.

  17. Implementing real-time GNSS monitoring to investigate continental rift initiation processes

    Science.gov (United States)

    Jones, J. R.; Stamps, D. S.; Wauthier, C.; Daniels, M. D.; Saria, E.; Ji, K. H.; Mencin, D.; Ntambila, D.

    2017-12-01

    Continental rift initiation remains an elusive, yet fundamental, process in the context of plate tectonic theory. Our early work in the Natron Rift, Tanzania, the Earth's archetype continental rift initiation setting, indicates feedback between volcanic deformation and fault slip play a key role in the rift initiation process. We found evidence that fault slip on the Natron border fault during active volcanism at Ol Doniyo Lengai in 2008 required only 0.01 MPa of Coulomb stress change. This previous study was limited by GPS constraints 18 km from the volcano, rather than immediately adjacent on the rift shoulder. We hypothesize that fault slip adjacent to the volcano creeps, and without the need for active eruption. We also hypothesize silent slip events may occur over time-scales less than 1 day. To test our hypotheses we designed a GNSS network with 4 sites on the flanks of Ol Doinyo Lengai and 1 site on the adjacent Natron border fault with the capability to calculate 1 second, 3-5 cm precision positions. Data is transmitted to UNAVCO in real-time with remote satellite internet, which we automatically import to the EarthCube building block CHORDS (Cloud Hosted Real-time Data Services for the Geosciences) using our newly developed method. We use CHORDS to monitor and evaluate the health of our network while visualizing the GNSS data in real-time. In addition to our import method we have also developed user-friendly capabilities to export GNSS positions (longitude, latitude, height) with CHORDS assuming the data are available at UNAVCO in NMEA standardized format through the Networked Transport of RTCM via Internet Protocol (NTRIP). The ability to access the GNSS data that continuously monitors volcanic deformation, tectonics, and their interactions on and around Ol Doinyo Lengai is a crucial component in our investigation of continental rift initiation in the Natron Rift, Tanzania. Our new user-friendly methods developed to access and post-process real-time GNSS

  18. Contrasted continental rifting via plume-craton interaction: Applications to Central East African Rift

    Directory of Open Access Journals (Sweden)

    Alexander Koptev

    2016-03-01

    Full Text Available The East African Rift system (EARS provides a unique system with the juxtaposition of two contrasting yet simultaneously formed rift branches, the eastern, magma-rich, and the western, magma-poor, on either sides of the old thick Tanzanian craton embedded in a younger lithosphere. Data on the pre-rift, syn-rift and post-rift far-field volcanic and tectonic activity show that the EARS formed in the context of the interaction between a deep mantle plume and a horizontally and vertically heterogeneous lithosphere under far-field tectonic extension. We bring quantitative insights into this evolution by implementing high-resolution 3D thermo-mechanical numerical deformation models of a lithosphere of realistic rheology. The models focus on the central part of the EARS. We explore scenarios of plume-lithosphere interaction with plumes of various size and initial position rising beneath a tectonically pre-stretched lithosphere. We test the impact of the inherited rheological discontinuities (suture zones along the craton borders, of the rheological structure, of lithosphere plate thickness variations, and of physical and mechanical contrasts between the craton and the embedding lithosphere. Our experiments indicate that the ascending plume material is deflected by the cratonic keel and preferentially channeled along one of its sides, leading to the formation of a large rift zone along the eastern side of the craton, with significant magmatic activity and substantial melt amount derived from the mantle plume material. We show that the observed asymmetry of the central EARS, with coeval amagmatic (western and magmatic (eastern branches, can be explained by the splitting of warm material rising from a broad plume head whose initial position is slightly shifted to the eastern side of the craton. In that case, neither a mechanical weakness of the contact between the craton and the embedding lithosphere nor the presence of second plume are required to

  19. Inward migration of faulting during continental rifting: Effects of pre-existing lithospheric structure and extension rate

    NARCIS (Netherlands)

    Corti, G.; Ranalli, G.; Agostini, A.; Sokoutis, D.

    Lithospheric-scale analogue models are used to analyse the parameters controlling the typical evolution of deformation during continental narrow rifting, characterized by early activation of large boundary faults and basin subsidence, followed by localization of tectonic activity in internal faults

  20. Thermo-rheological aspects of crustal evolution during continental breakup and melt intrusion : The Main Ethiopian Rift, East Africa

    NARCIS (Netherlands)

    Lavecchia, Alessio; Beekman, Fred; Clark, Stuart R.; Cloetingh, Sierd A P L

    2016-01-01

    The Cenozoic-Quaternary Main Ethiopian Rift (MER) is characterized by extended magmatic activity. Although magmatism has been recognized as a key element in the process of continental breakup, the interaction between melts and intruded lithosphere is still poorly understood. We have performed a 2D

  1. Magma-poor vs. magma-rich continental rifting and breakup in the Labrador Sea

    Science.gov (United States)

    Gouiza, M.; Paton, D.

    2017-12-01

    Magma-poor and magma-rich rifted margins show distinct structural and stratigraphic geometries during the rift to breakup period. In magma-poor margins, crustal stretching is accommodated mainly by brittle faulting and the formation of wide rift basins shaped by numerous graben and half-graben structures. Continental breakup and oceanic crust accretion are often preceded by a localised phase of (hyper-) extension where the upper mantle is embrittled, serpentinized, and exhumed to the surface. In magma-rich margins, the rift basin is narrow and extension is accompanied by a large magmatic supply. Continental breakup and oceanic crust accretion is preceded by the emplacement of a thick volcanic crust juxtaposing and underplating a moderately thinned continental crust. Both magma-poor and magma-rich rifting occur in response to lithospheric extension but the driving forces and processes are believed to be different. In the former extension is assumed to be driven by plate boundary forces, while in the latter extension is supposed to be controlled by sublithospheric mantle dynamics. However, this view fails in explaining observations from many Atlantic conjugate margins where magma-poor and magma-rich segments alternate in a relatively abrupt fashion. This is the case of the Labrador margin where the northern segment shows major magmatic supply during most of the syn-rift phase which culminate in the emplacement of a thick volcanic crust in the transitional domain along with high density bodies underplating the thinned continental crust; while the southern segment is characterized mainly by brittle extension, mantle seprentinization and exhumation prior to continental breakup. In this work, we use seismic and potential field data to describe the crustal and structural architectures of the Labrador margin, and investigate the tectonic and mechanical processes of rifting that may have controlled the magmatic supply in the different segments of the margin.

  2. Contrasted continental rifting via plume-craton interaction : Applications to Central East African Rift

    NARCIS (Netherlands)

    Koptev, Alexander; Burov, Evgueni; Calais, Eric; Leroy, Sylvie; Gerya, Taras; Guillou-Frottier, Laurent; Cloetingh, Sierd

    The East African Rift system (EARS) provides a unique system with the juxtaposition of two contrasting yet simultaneously formed rift branches, the eastern, magma-rich, and the western, magma-poor, on either sides of the old thick Tanzanian craton embedded in a younger lithosphere. Data on the

  3. Morphotectonics of a high plateau on the northwestern flank of the Continental Rift of southeastern Brazil

    Science.gov (United States)

    Modenesi-Gauttieri, May Christine; Takashi Hiruma, Silvio; Riccomini, Claudio

    2002-03-01

    Integration of landform and structural analysis allowed the identification of Late Pleistocene-Holocene pulses of tectonic activity in the Campos do Jordão Plateau with ages and regimes similar to the ones from the continental rift. Fault reactivation along Precambrian shear zones give rise to a series of conspicuous morphotectonic features, determine the formation of stream piracy phenomena, and divide the plateau into smaller blocks. Recognition of these tectonic pulses as well as of their effects in landform development—particularly clear on the Campos de São Francisco at the highest area of the SE edge of the plateau—show that besides the climate-related Quaternary environmental changes significant neotectonic instability should be considered in the geomorphic evolution of the Campos do Jordão Plateau.

  4. Kinematic and thermal evolution of the Moroccan rifted continental margin: Doukkala-High Atlas Transect

    NARCIS (Netherlands)

    Gouiza, M.; Bertotti, G.V.; Hafid, M.; Cloetingh, S.A.P.L.

    2010-01-01

    The Atlantic passive margin of Morocco developed during Mesozoic times in association with the opening of the Central Atlantic and the Alpine Tethys. Extensional basins formed along the future continental margin and in the Atlas rift system. In Alpine times, this system was inverted to form the High

  5. Continental rift architecture and patterns of magma migration: a dynamic analysis based on centrifuge models.

    NARCIS (Netherlands)

    Corti, G.; Bonini, M.; Sokoutis, D.; Innocenti, F.; Manetti, P.; Cloetingh, S.A.P.L.; Mulugeta, G.

    2004-01-01

    Small-scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement. Experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at Moho depths. Results suggest that

  6. Orogenic inheritance and continental breakup: Wilson Cycle-control on rift and passive margin evolution

    Science.gov (United States)

    Schiffer, C.; Petersen, K. D.

    2016-12-01

    Rifts often develop along suture zones between previously collided continents, as part of the Wilson cycle. The North Atlantic is such an example, formed where Pangaea broke apart along Caledonian and Variscan sutures. Dipping upper mantle structures in E. Greenland and Scotland, have been interpreted as fossil subduction zones and the seismic signature indicates the presence of eclogite and serpentinite. We speculate that this orogenic material may impose a rheological control upon post-orogenic extension and we use thermo-mechanical modelling to explore such effects. Our model includes the following features: 1) Crustal thickness anomalies, 2) Eclogitised mafic crust emplaced in the mantle lithosphere, and 3) Hydrated mantle peridotite (serpentinite) formed in a pre-rift subduction setting. Our models indicate that the inherited structures control the location and the structural and magmatic evolution of the rift. Rifting of thin initial crust allows for relatively large amounts of serpentinite to be preserved within the uppermost mantle. This facilitates rapid continental breakup and serpentinite exhumation. Magmatism does not occur before continental breakup. Rifts in thicker crust preserve little or no serpentinite and thinning is more focused in the mantle lithosphere, rather than in the crust. Continental breakup is therefore preceded by magmatism. This implies that pre-rift orogenic properties may determine whether magma-poor or magma-rich conjugate margins are formed. Our models show that inherited orogenic eclogite and serpentinite are deformed and partially emplaced either as dipping structures within the lithospheric mantle or at the base of the thinned continental crust. The former is consistent with dipping sub-Moho reflectors often observed in passive margins. The latter provides an alternative interpretation of `lower crustal bodies' which are often regarded as igneous bodies. An additional implication of our models is that serpentinite, often

  7. Tectonic inheritance and continental rift architecture: Numerical and analogue models of the East African Rift System.

    NARCIS (Netherlands)

    Corti, G.; van Wijk, J.W.; Cloetingh, S.A.P.L.; Morley, C.

    2007-01-01

    The western branch of the East African Rift is composed of an arcuate succession of elongate asymmetric basins, which differ in terms of interaction geometry, fault architecture and kinematics, and patterns of uplift/subsidence and erosion/sedimentation. The basins are located within Proterozoic

  8. Active Magmatic Underplating in Western Eger Rift, Central Europe

    Science.gov (United States)

    Hrubcová, Pavla; Geissler, Wolfram H.; Bräuer, Karin; Vavryčuk, Václav; Tomek, Čestmír.; Kämpf, Horst

    2017-12-01

    The Eger Rift is an active element of the European Cenozoic Rift System associated with intense Cenozoic intraplate alkaline volcanism and system of sedimentary basins. The intracontinental Cheb Basin at its western part displays geodynamic activity with fluid emanations, persistent seismicity, Cenozoic volcanism, and neotectonic crustal movements at the intersections of major intraplate faults. In this paper, we study detailed geometry of the crust/mantle boundary and its possible origin in the western Eger Rift. We review existing seismic and seismological studies, provide new interpretation of the reflection profile 9HR, and supplement it by new results from local seismicity. We identify significant lateral variations of the high-velocity lower crust and relate them to the distribution and chemical status of mantle-derived fluids and to xenolith studies from corresponding depths. New interpretation based on combined seismic and isotope study points to a local-scale magmatic emplacement at the base of the continental crust within a new rift environment. This concept of magmatic underplating is supported by detecting two types of the lower crust: a high-velocity lower crust with pronounced reflectivity and a high-velocity reflection-free lower crust. The character of the underplated material enables to differentiate timing and tectonic setting of two episodes with different times of origin of underplating events. The lower crust with high reflectivity evidences magmatic underplating west of the Eger Rift of the Late Variscan age. The reflection-free lower crust together with a strong reflector at its top at depths of 28-30 km forms a magma body indicating magmatic underplating of the late Cenozoic (middle and upper Miocene) to recent. Spatial and temporal relations to recent geodynamic processes suggest active magmatic underplating in the intracontinental setting.

  9. Gravity study of the Central African Rift system: a model of continental disruption 2. The Darfur domal uplift and associated Cainozoic volcanism

    Science.gov (United States)

    Bermingham, P. M.; Fairhead, J. D.; Stuart, G. W.

    1983-05-01

    Gravity studies of the Darfur uplift, Western Sudan, show it to be associated with a circular negative Bouguer anomaly, 50 mGal in amplitude and 700 km across. A three-dimensional model interpretation of the Darfur anomaly, using constraints deduced from geophysical studies of similar but more evolved Kenya and Ethiopia domes, suggests either a low-density laccolithic body at mid-lithospheric depth (~ 60 km) or a thinned lithosphere with emplacement at high level of low-density asthenospheric material. The regional setting of the Darfur uplift is described in terms of it being an integral part of the Central African Rift System which is shown to be broadly equivalent to the early to middle Miocene stage in the development of the Afro-Arabian Rift System. Comparisons between these rift systems suggest that extensional tectonics and passive rifting, resulting in the subsiding sedimentary rift basins associated with the Ngaoundere, Abu Gabra, Red Sea and Gulf of Aden rifts, are more typical of the early stage development of passive continental margins than the active domal uplift and development of rifted features associated with the Darfur, Kenya and Ethiopia domes.

  10. Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode.

    Science.gov (United States)

    Wright, Tim J; Ebinger, Cindy; Biggs, Juliet; Ayele, Atalay; Yirgu, Gezahegn; Keir, Derek; Stork, Anna

    2006-07-20

    Seafloor spreading centres show a regular along-axis segmentation thought to be produced by a segmented magma supply in the passively upwelling mantle. On the other hand, continental rifts are segmented by large offset normal faults, and many lack magmatism. It is unclear how, when and where the ubiquitous segmented melt zones are emplaced during the continental rupture process. Between 14 September and 4 October 2005, 163 earthquakes (magnitudes greater than 3.9) and a volcanic eruption occurred within the approximately 60-km-long Dabbahu magmatic segment of the Afar rift, a nascent seafloor spreading centre in stretched continental lithosphere. Here we present a three-dimensional deformation field for the Dabbahu rifting episode derived from satellite radar data, which shows that the entire segment ruptured, making it the largest to have occurred on land in the era of satellite geodesy. Simple elastic modelling shows that the magmatic segment opened by up to 8 m, yet seismic rupture can account for only 8 per cent of the observed deformation. Magma was injected along a dyke between depths of 2 and 9 km, corresponding to a total intrusion volume of approximately 2.5 km3. Much of the magma appears to have originated from shallow chambers beneath Dabbahu and Gabho volcanoes at the northern end of the segment, where an explosive fissural eruption occurred on 26 September 2005. Although comparable in magnitude to the ten year (1975-84) Krafla events in Iceland, seismic data suggest that most of the Dabbahu dyke intrusion occurred in less than a week. Thus, magma intrusion via dyking, rather than segmented normal faulting, maintains and probably initiated the along-axis segmentation along this sector of the Nubia-Arabia plate boundary.

  11. The origin and evolution of silicic magmas during continental rifting: new constraints from trace elements and oxygen isotopes from Ethiopian volcanoes

    Science.gov (United States)

    Hutchison, W.; Boyce, A.; Mather, T. A.; Pyle, D. M.; Yirgu, G.; Gleeson, M. L.

    2017-12-01

    The petrologic diversity of rift magmas is generated by two key processes: interaction with the crust via partial melting or assimilation; and closed-system fractional crystallization of the parental magma. It is not yet known whether these two petrogenetic processes vary spatially between different rift settings, and whether there are any significant secular variations during rift evolution. The Ethiopian Rift is the ideal setting to test these hypotheses because it captures the transition from continental rifting to sea-floor spreading and has witnessed the eruption of large volumes of mafic and silicic volcanic rocks since 30 Ma. We use new oxygen isotope (δ18O) and trace element data to fingerprint fractional crystallisation and partial crustal melting processes in Ethiopia and evaluate spatial variations between three active rift segments. δ18O measurements are used to examine partial crustal melting processes. We find that most δ18O data from basalts to rhyolites fall within the bounds of modelled fractional crystallization trajectories (i.e., 5.5-6.5 ‰). Few samples deviate from this trend, emphasising that fractional crystallization is the dominant petrogenetic processes and that little fusible Precambrian crustal material (δ18O of 7-18 ‰) remain to be assimilated beneath the magmatic segments. Trace element systematics (e.g., Ba, Sr, Rb, Th and Zr) further underscore the dominant role of fractional crystallization but also reveal important variations in the degree of melt evolution between the volcanic systems. We find that the most evolved silicic magmas, i.e., those with greatest peralkalinity (molar Na2O+K2O>Al2O3), are promoted in regions of lowest magma flux off-axis and along rift. Our findings provide new information on the nature of the crust beneath Ethiopia's active magmatic segments and also have relevance for understanding ancient rift zones and the geotectonic settings that promote genesis of economically-valuable mineral deposits.

  12. Plate Speed-up and Deceleration during Continental Rifting: Insights from Global 2D Mantle Convection Models.

    Science.gov (United States)

    Brune, S.; Ulvrova, M.; Williams, S.

    2017-12-01

    The surface of the Earth is divided into a jigsaw of tectonic plates, some carrrying continents that disperse and aggregate through time, forming transient supercontinents like Pangea and Rodinia. Here, we study continental rifting using large-scale numerical simulations with self-consistent evolution of plate boundaries, where continental break-up emerges spontaneously due to slab pull, basal drag and trench suction forces.We use the StagYY convection code employing a visco-plastic rheology in a spherical annulus geometry. We consider an incompressible mantle under the Boussinesq approximation that is basally and internally heated.We show that continental separation follows a characteristic evolution with three distinctive phases: (1) A pre-rift phase that typically lasts for several hundreds of millions of years with tectonic quiescence in the suture and extensional stresses that are slowly building up. (2) A rift phase that further divides into a slow rift period of several tens of millions of years where stresses continuously increase followed by a rift acceleration period featuring an abrupt stress drop within several millions of years. The speed-up takes place before lithospheric break-up and therefore affects the structural architecture of the rifted margins. (3) The drifting phase with initially high divergence rates persists over tens of millions of years until the system adjust to new conditions and the spreading typically slows down.By illustrating the geodynamic connection between subduction dynamics and rift evolution, our results allow new interpretations of plate tectonic reconstructions. Rift acceleration within the second phase of rifting is compensated by enhanced convergence rates at subduction zones. This model outcome predicts enhanced subduction velocities, e.g. between North America and the Farallon plate during Central Atlantic rifting 200 My ago, or closure of potential back-arc basins such as in the proto-Andean ranges of South America

  13. Cambrian–early Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: Continental break-up and rifting of the northern Gondwana margin

    Directory of Open Access Journals (Sweden)

    André Pouclet

    2017-01-01

    Full Text Available The Cambrian–lower Ordovician volcanic units of the South Armorican and Occitan domains are analysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle Cambrian–Furongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases: (1 basal Cambrian rhyolites, (2 upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3 upper lower–middle Cambrian continental tholeiites, (4 Tremadocian rhyolites, and (5 upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic continental rift is termed the “Tremadocian Tectonic Belt” underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South Armorican–Medio-European Ocean as a part of the Palaeotethys Ocean.

  14. Numerical model of the transition from continental rifting to oceanization: the case study of the Ligure-Piemontese ocean.

    Science.gov (United States)

    Roda, M.; Marotta, A. M.; Conte, K.; Spalla, M. I.

    2015-12-01

    The transition from continental rifting to oceanization has been investigated by mean of a 2D thermo-mechanical numerical model in which the formation of oceanic crust by mantle serpentinization, due to the hydration of the uprising peridotite, as been implemented. Model predictions have been compared with natural data related to the Permian-Triassic thinning affecting the continental lithosphere of the Alpine domain, in order to identify which portions of the present Alpine-Apennine system, preserving the imprints of Permian-Triassic high temperature (HT) metamorphism, is compatible, in terms of lithostratigraphy and tectono-metamorphic evolution, with a lithospheric extension preceding the opening of the Ligure-Piemontese oceanic basin. At this purpose age, petrological and structural data from the Alpine and Apennine ophiolite complexes are compared with model predictions from the oceanization stage. Our comparative analysis supports the thesis that the lithospheric extension preceding the opening of the Alpine Tethys did not start on a stable continental lithosphere, but developed by recycling part of the old Variscan collisional suture. The HT Permian-Triassic metamorphic re-equilibration overprints an inherited tectonic and metamorphic setting consequent to the Variscan subduction and collision, making the Alps a key case history to explore mechanisms responsible for the re-activation of orogenic scars.

  15. Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation

    Science.gov (United States)

    Rupke, L.; Schmid, D. W.; Perez-Gussinye, M.; Hartz, E. H.

    2013-12-01

    We explore the conditions under which mantle serpentinization may take place during continental rifting with 2D thermotectonostratigraphic basin models. The basic concept follows the idea that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The new model tracks the rheological evolution of the continental crust and allows for kinetically controlled mantle serpentinization processes. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Sedimentation shifts this critical stretching factor to higher values as both deeper burial and the low thermal conductivity of sediments lead to higher crustal temperatures. Serpentinization reactions are therefore only likely in settings with low sedimentation rates and high stretching factors. In addition, we find that the rate of sediment supply has first order controls on the rheology of the lower crust, which may control the overall margin geometry. We further test these concepts in ideas in a case study for the Norwegian margin. In particular, we evaluate whether the inner lower crustal bodies (LCB) imaged beneath the More and Voring margin could be serpentinized mantle. For this purpose we reconstruct multiple 2D transects through a 3D data set. This reconstruction of the Norwegian margin shows that serpentinization reactions are indeed possible and likely during the Jurassic rift phase. Predicted present-day thicknesses and locations of partially serpentinized mantle rocks fit well to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may, in fact, be partially serpentinized mantle.

  16. The evolution of magma during continental rifting: New constraints from the isotopic and trace element signatures of silicic magmas from Ethiopian volcanoes

    Science.gov (United States)

    Hutchison, William; Mather, Tamsin A.; Pyle, David M.; Boyce, Adrian J.; Gleeson, Matthew L. M.; Yirgu, Gezahegn; Blundy, Jon D.; Ferguson, David J.; Vye-Brown, Charlotte; Millar, Ian L.; Sims, Kenneth W. W.; Finch, Adrian A.

    2018-05-01

    Magma plays a vital role in the break-up of continental lithosphere. However, significant uncertainty remains about how magma-crust interactions and melt evolution vary during the development of a rift system. Ethiopia captures the transition from continental rifting to incipient sea-floor spreading and has witnessed the eruption of large volumes of silicic volcanic rocks across the region over ∼45 Ma. The petrogenesis of these silicic rocks sheds light on the role of magmatism in rift development, by providing information on crustal interactions, melt fluxes and magmatic differentiation. We report new trace element and Sr-Nd-O isotopic data for volcanic rocks, glasses and minerals along and across active segments of the Main Ethiopian (MER) and Afar Rifts. Most δ18 O data for mineral and glass separates from these active rift zones fall within the bounds of modelled fractional crystallization trajectories from basaltic parent magmas (i.e., 5.5-6.5‰) with scant evidence for assimilation of Pan-African Precambrian crustal material (δ18 O of 7-18‰). Radiogenic isotopes (εNd = 0.92- 6.52; 87Sr/86Sr = 0.7037-0.7072) and incompatible trace element ratios (Rb/Nb productivity or where crustal structure inhibits magma ascent). This has important implications for understanding the geotectonic settings that promote extreme melt evolution and, potentially, genesis of economically-valuable mineral deposits in ancient rift-settings. The limited isotopic evidence for assimilation of Pan-African crustal material in Ethiopia suggests that the pre-rift crust beneath the magmatic segments has been substantially modified by rift-related magmatism over the past ∼45 Ma; consistent with geophysical observations. We argue that considerable volumes of crystal cumulate are stored beneath silicic volcanic systems (>100 km3), and estimate that crystal cumulates fill at least 16-30% of the volume generated by crustal extension under the axial volcanoes of the MER and Manda Hararo

  17. Relief evolution of the Continental Rift of Southeast Brazil revealed by in situ-produced 10Be concentrations in river-borne sediments

    Science.gov (United States)

    Salgado, André Augusto Rodrigues; Rezende, Eric de Andrade; Bourlès, Didier; Braucher, Régis; da Silva, Juliana Rodrigues; Garcia, Ricardo Alexandrino

    2016-04-01

    This study aims to quantify the denudation dynamics of the Brazilian passive margin along a segment of the Continental Rift of Southeast Brazil. The denudation rates of 30 basins that drain both horsts of the continental rift, including the mountain ranges of the Serra do Mar (seaside horst); and the Serra da Mantiqueira (continental horst); were derived from 10Be concentrations measured in sand-sized river sediment. The mean denudation rate ranges from 9.2 m Ma-1 on the plateau of the Serra do Mar to 37.1 m Ma-1 along the oceanic escarpment of the Serra do Mar. The seaward-facing scarps of both mountain ranges exhibit mean denudation rates that are approximately 1.5 times those of the inland-facing scarps. The escarpments of the horst nearer to the ocean (Serra do Mar) exhibit higher denudation rates (mean 30.2 m Ma-1) than the escarpments of the continental horst (Serra da Mantiqueira) (mean 16.5 m Ma-1). The parameters that impact these denudation rates include the catchment relief, the slope gradient, the rock and the climate. The incongruent combination of a mountainous landscape and moderate to low 10Be-based denudation rates averaging at ∼20 m Ma-1 suggests a reduction in intraplate tectonic activity beginning in the Middle Quaternary or earlier.

  18. Rifting-to-drifting transition of the South China Sea: Moho reflection characteristics in continental-ocean transition zone

    Science.gov (United States)

    Wen, Y.; Li, C.

    2017-12-01

    Dispute remains on the process of continental rifting to subsequent seafloor spreading in the South China Sea (SCS). Several crust-scale multi-channel seismic reflection profiles acquired in the continent-ocean transition zone (COT) of the SCS provide a detailed overview of Moho and deep crustal reflectors and give key information on rifting-to-drifting transition of the area. Moho has strong but discontinuous seismic reflection in COT. These discontinuities are mainly located in the landward side of continent-ocean boundary (COB), and may own to upwelling of lower crustal materials during initial continental extension, leading to numerous volcanic edifices and volcanic ridges. The continental crust in COT shows discontinuous Moho reflections at 11-8.5 s in two-way travel time (twtt), and thins from 18-20.5 km under the uppermost slope to 6-7 km under the lower slope, assuming an average crustal velocity of 6.0 km/s. The oceanic crust has Moho reflections of moderate to high continuity mostly at 1.8-2.2 s twtt below the top of the igneous basement, which means that the crustal thickness excluding sediment layer in COT is 5.4-6.6 km. Subhorizontal Moho reflections are often abruptly interrupted by large seaward dipping normal faults in southern COT but are more continuous compared with the fluctuant and very discontinuous Moho reflections in northern COT. The thickness of thinned continental crust (4.2-4.8 km) is smaller than that of oceanic crust (5.4-6.0 km) near southern COB, indicating that the continental crust has experienced a long period of rifting before seafloor spreading started. The smaller width of northern COT (0-40 km) than in southern COT (0-60 km), and thinner continental crust in southern COT, all indicate that the continental margin rifting and extension was asymmetric. The COT width in the SCS is narrower than that found in other magma-poor continental margins, indicating a swift transition from the final stage of rifting to the inception of

  19. Fault-magma interactions during early continental rifting: Seismicity of the Magadi-Natron-Manyara basins, Africa

    Science.gov (United States)

    Weinstein, A.; Oliva, S. J.; Ebinger, C. J.; Roecker, S.; Tiberi, C.; Aman, M.; Lambert, C.; Witkin, E.; Albaric, J.; Gautier, S.; Peyrat, S.; Muirhead, J. D.; Muzuka, A. N. N.; Mulibo, G.; Kianji, G.; Ferdinand-Wambura, R.; Msabi, M.; Rodzianko, A.; Hadfield, R.; Illsley-Kemp, F.; Fischer, T. P.

    2017-10-01

    Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2-D studies. We analyze seismicity data from a 13 month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses and a new 3-D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6-10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of 0.87 ± 0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along-axis stress rotation of ˜60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ˜N90°E north and south of an active volcanic chain consistent with geodetic data, and ˜N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early-stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.

  20. Fault-Magma Interactions during Early Continental Rifting: Seismicity of the Magadi-Natron-Manyara basins, Africa

    Science.gov (United States)

    Weinstein, A.; Oliva, S. J.; Ebinger, C.; Aman, M.; Lambert, C.; Roecker, S. W.; Tiberi, C.; Muirhead, J.

    2017-12-01

    Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2D studies. We analyze seismicity data from a 13-month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses and a new 3D velocity model reveal lower crustal earthquakes along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6-10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of 0.87 ± 0.03. Focal mechanisms for 66 earthquakes, and a longer time period of relocated earthquakes from global arrays reveal an along-axis stress rotation of 50 o ( N150 oE) in the magmatically active zone. Using Kostrov summation of local and teleseismic mechanisms, we find opening directions of N122ºE and N92ºE north and south of the magmatically active zone. The stress rotation facilitates strain transfer from border fault systems, the locus of early stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Earthquakes are largely driven by stress state around inflating magma bodies, and more dike intrusions with surface faulting, eruptions, and earthquakes are expected.

  1. ACADEMICIAN N.A. LOGATCHEV AND HIS SCIENTIFIC SCHOOL: CONTRUBITION TO STUDIES OF THE CENOZOIC CONTINENTAL RIFTING

    Directory of Open Access Journals (Sweden)

    Sergey V. Rasskazov

    2010-01-01

    Full Text Available N.A. Florensov and N.A. Logatchev pioneered development of fundamental concepts of the structure and evolution of the Baikal system of rift basins. At the turn to the 21st century, in view of the wide availability of scientific research data on the Cenozoic continental rift zones located in Eurasia, Africa and North America, and taking into account the application of new research methods and options to process and analyze huge amounts of geological and geophysical data, a priority was comprehensive modeling of rifting from its origin to the current period of time. This scientific challenge was addressed by the research team under the leadership of N.A. Logachev.

  2. Evolution of basin architecture in an incipient continental rift: the Cenozoic Most Basin, Eger Graben (Central Europe)

    Czech Academy of Sciences Publication Activity Database

    Rajchl, M.; Uličný, David; Grygar, R.; Mach, K.

    2009-01-01

    Roč. 21, č. 3 (2009), s. 269-294 ISSN 0950-091X R&D Projects: GA AV ČR IAA3012705; GA ČR GA205/01/0629; GA ČR(CZ) GA205/06/1823 Institutional research plan: CEZ:AV0Z30120515 Keywords : Cenozoic Most Basin * continental rift * Eger Graben Subject RIV: DB - Geology ; Mineralogy Impact factor: 2.161, year: 2009

  3. Tectonic characteristics and structural styles of a continental rifted basin: Revelation from deep seismic reflection profiles

    Directory of Open Access Journals (Sweden)

    Yuan Li

    2016-09-01

    Full Text Available The Fushan Depression is a half-graben rifted sub-basin located in the southeast of the Beibuwan Basin, South China Sea. The Paleogene Liushagang sequence is the main hydrocarbon-bearing stratigraphic unit in the sub-basin. Using three-dimensional (3-D seismic data and logging data over the sub-basin, we analyzed structural styles and sedimentary characteristics of the Liushagang sequence. Five types of structural styles were defined: ancient horst, traditional slope, flexure slope-break, faulted slope-break and multiple-stage faults slope, and interpretations for positions, background and development formations of each structural style were discussed. Structural framework across the sub-basin reveals that the most remarkable tectonic setting is represented by the central transfer zone (CTZ which divides the sub-basin into two independent depressions, and two kinds of sequence architectures are summarized: (i the western multi-stage faults slope; (ii the eastern flexure slope break belt. Combined with regional stress field of the Fushan Depression, we got plane combinations of the faults, and finally built up plan distribution maps of structural system for main sequence. Also, we discussed the controlling factors mainly focused on subsidence history and background tectonic activities such as volcanic activity and earthquakes. The analysis of structural styles and tectonic evolution provides strong theoretical support for future prospecting in the Fushan sub-basin and other similar rifted basins of the Beibuwan Basin in South China Sea.

  4. Factors influencing seismic wave attenuation in the lithosphere in continental rift zones

    Directory of Open Access Journals (Sweden)

    А. А. Dobrynina

    2017-01-01

    Full Text Available Attenuation of seismic waves in the crust and the upper mantle has been studied in three global rift systems: the Baikal rift system (Eurasia, the North Tanzanian divergence zone (Africa and the Basin and Range Province (North America. Using the records of direct and coda waves of regional earthquakes, the single scattering theory [Aki, Chouet, 1975], the hybrid model from [Zeng, 1991] and the approach described in [Wennerberg, 1993], we estimated the seismic quality factor (QC, frequency parameter (n, attenuation coefficient (δ, and total attenuation (QT. In addition, we evaluated the contributions of two components into total attenuation: intrinsic attenuation (Qi, and scattering attenuation (Qsc. Values of QC are strongly dependent on the frequency within the range of 0.2–16 Hz, as well as on the length of the coda processing window. The observed increase of QC with larger lengths of the coda processing window can be interpreted as a decrease in attenuation with increasing depth. Having compared the depth variations in the attenuation coefficient (δ and the frequency (n with the velocity structures of the studied regions, we conclude that seismic wave attenuation changes at the velocity boundaries in the medium. Moreover, the comparison results show that the estimated variations in the attenuation parameters with increasing depth are considerably dependent on utilized velocity models of the medium. Lateral variations in attenuation of seismic waves correlate with the geological and geophysical characteristics of the regions, and attenuation is primarily dependent on the regional seismic activity and regional heat flow. The geological inhomogeneities of the medium and the age of crust consolidation are secondary factors. Our estimations of intrinsic attenuation (Qi and scattering attenuation (Qsc show that in all the three studied regions, intrinsic attenuation is the major contributor to total attenuation. Our study shows that the

  5. The composition and structure of volcanic rifted continental margins in the North Atlantic: Further insight from shear waves

    Science.gov (United States)

    Eccles, Jennifer D.; White, Robert S.; Christie, Philip A. F.

    2011-07-01

    Imaging challenges caused by highly attenuative flood basalt sequences have resulted in the understanding of volcanic rifted continental margins lagging behind that of non-volcanic rifted and convergent margins. Massive volcanism occurred during break-up at 70% of the passive margins bordering the Atlantic Ocean, the causes and dynamics of which are still debated. This paper shows results from traveltime tomography of compressional and converted shear wave arrivals recorded on 170 four-component ocean bottom seismometers along two North Atlantic continental margin profiles. This traveltime tomography was performed using two different approaches. The first, a flexible layer-based parameterisation, enables the quality control of traveltime picks and investigation of the crustal structure. The second, with a regularised grid-based parameterisation, requires correction of converted shear wave traveltimes to effective symmetric raypaths and allows exploration of the model space via Monte Carlo analyses. The velocity models indicate high lower-crustal velocities and sharp transitions in both velocity and Vp/Vs ratios across the continent-ocean transition. The velocities are consistent with established mixing trends between felsic continental crust and high magnesium mafic rock on both margins. Interpretation of the high quality seismic reflection profile on the Faroes margin confirms that this mixing is through crustal intrusion. Converted shear wave data also provide constraints on the sub-basalt lithology on the Faroes margin, which is interpreted as a pre-break-up Mesozoic to Paleocene sedimentary system intruded by sills.

  6. Continental rupture and the creation of new crust in the Salton Trough rift, Southern California and northern Mexico: Results from the Salton Seismic Imaging Project

    Science.gov (United States)

    Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Kell, Annie; Driscoll, Neal W.; Kent, Graham M.; Harding, Alistair J.; Rymer, Michael J.; González-Fernández, Antonio; Lázaro-Mancilla, Octavio

    2016-10-01

    A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched preexisting crust or higher-grade metamorphosed sediment. The lower crust below 12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 km depth as it does to the south, and a weak reflection suggests Moho at 28 km depth. Structure in adjacent Mexico has slower midcrustal velocity, and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

  7. Continental rupture and the creation of new crust in the Salton Trough rift, southern California and northern Mexico: Results from the Salton Seismic Imaging Project

    Science.gov (United States)

    Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Kell, Annie; Driscoll, Neal W.; Kent, Graham M.; Rymer, Michael J.; Gonzalez-Fernandez, Antonio; Aburto-Oropeza, Octavio

    2016-01-01

    A refraction and wide-angle reflection seismic profile along the axis of the Salton Trough, California and Mexico, was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting. From the northern Salton Sea to the southern Imperial Valley, the crust is 17-18 km thick and approximately one-dimensional. The transition at depth from Colorado River sediment to underlying crystalline rock is gradual and is not a depositional surface. The crystalline rock from ~3 to ~8 km depth is interpreted as sediment metamorphosed by high heat flow. Deeper felsic crystalline rock could be stretched pre-existing crust or higher grade metamorphosed sediment. The lower crust below ~12 km depth is interpreted to be gabbro emplaced by rift-related magmatic intrusion by underplating. Low upper-mantle velocity indicates high temperature and partial melting. Under the Coachella Valley, sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement. Mafic rock does not exist at 12-18 depth as it does to the south, and a weak reflection suggests Moho at ~28 km depth. Structure in adjacent Mexico has slower mid-crustal velocity and rocks with mantle velocity must be much deeper than in the Imperial Valley. Slower velocity and thicker crust in the Coachella and Mexicali valleys define the rift zone between them to be >100 km wide in the direction of plate motion. North American lithosphere in the central Salton Trough has been rifted apart and is being replaced by new crust created by magmatism, sedimentation, and metamorphism.

  8. Opening of the Central Atlantic Ocean: Implications for Geometric Rifting and Asymmetric Initial Seafloor Spreading after Continental Breakup

    Science.gov (United States)

    Klingelhoefer, F.; Biari, Y.; Sahabi, M.; Funck, T.; Benabdellouahed, M.; Schnabel, M.; Reichert, C. J.; Gutscher, M. A.; Bronner, A.; Austin, J. A., Jr.

    2017-12-01

    The structure of conjugate passive margins provides information about rifting styles, the initial phases of the opening of an ocean and the formation of its associated sedimentary basins. The study of the deep structure of conjugate passive continental margins combined with precise plate kinematic reconstructions can provide constraints on the mechanisms of rifting and formation of initial oceanic crust. In this study the Central Atlantic conjugate margins are compared, based on compilation of wide-angle seismic profiles from the NW-Africa Nova Scotian and US passive margins. Plate cinematic reconstructions were used to place the profiles in the position at opening and at the M25 magnetic anomaly. The patterns of volcanism, crustal thickness, geometry, and seismic velocities in the transition zone. suggest symmetric rifting followed by asymmetric oceanic crustal accretion. Conjugate profiles in the southern Central Atlantic image differences in the continental crustal thickness. While profiles on the eastern US margin are characterized by thick layers of magmatic underplating, no such underplate was imaged along the NW-African continental margin. It has been proposed that these volcanic products form part of the CAMP (Central Atlantic Magmatic Province). In the north, two wide-angle seismic profiles acquired in exactly conjugate positions show that the crustal geometry of the unthinned continental crust and the necking zone are nearly symmetric. A region including seismic velocities too high to be explained by either continental or oceanic crust is imaged along the Nova Scotia margin off Eastern Canada, corresponding on the African side to an oceanic crust with slightly elevated velocities. These might result from asymmetric spreading creating seafloor by faulting the existing lithosphere on the Canadian side and the emplacement of magmatic oceanic crust including pockets of serpentinite on the Moroccan margin. A slightly elevated crustal thickness along the

  9. Lateral variations in foreland flexure of a rifted continental margin: The Aquitaine Basin (SW France)

    Science.gov (United States)

    Angrand, P.; Ford, M.; Watts, A. B.

    2017-12-01

    We study the effects of the inherited Aptian to Cenomanian rift on crustal rheology and evolution of the Late Cretaceous to Neogene flexural Aquitaine foreland basin, northern Pyrenees. We use surface and subsurface geological data to define the crustal geometry and the post-rift thermal subsidence, and Bouguer gravity anomalies and flexural modeling to study the lateral variation of the elastic thickness, flexure of the European plate and controlling loads. The Aquitaine foreland can be divided along-strike into three sectors. The eastern foreland is un-rifted and is associated with a simple flexural subsidence. The central sector is affected by crustal stretching and the observed foreland base is modeled by combining topographic and buried loads, with post-rift thermal subsidence. In the western sector the foreland basin geometry is mainly controlled by post-rift thermal subsidence. These three sectors are separated by major lineaments, which affect both crustal and foreland geometry. These lineaments seem to be part of a larger structural pattern that includes the Toulouse and Pamplona Faults. The European foreland shows lateral variations in flexural behavior: the relative role of surface and sub-surface (i.e., buried) loading varies along-strike and the elastic thickness values decrease from the north-east to the south-west where the plate is the most stretched. We suggest that foreland basins are influenced by the thermal state of the underlying lithosphere if it was initiated soon after rifting and that thermal cooling can contribute significantly to subsidence.

  10. Groundwater fluoride enrichment in an active rift setting: Central Kenya Rift case study

    Energy Technology Data Exchange (ETDEWEB)

    Olaka, Lydia A., E-mail: lydiaolaka@gmail.com [Department of Geology, University of Nairobi, P.O Box 30197, Nairobi (Kenya); Wilke, Franziska D.H. [Geoforschungs Zentrum, Telegrafenberg, 14473 Potsdam (Germany); Olago, Daniel O.; Odada, Eric O. [Department of Geology, University of Nairobi, P.O Box 30197, Nairobi (Kenya); Mulch, Andreas [Senckenberg Biodiversity and Climate Research Centre, Senckenberganlage 25, 60325 Frankfurt (Germany); Institut für Geowissenschaften, Goethe Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt (Germany); Musolff, Andreas [UFZ-Helmholtz-Centre for Environmental Research, Department of Hydrogeology, Permoserstr. 15, 04318 Leipzig (Germany)

    2016-03-01

    Groundwater is used extensively in the Central Kenya Rift for domestic and agricultural demands. In these active rift settings groundwater can exhibit high fluoride levels. In order to address water security and reduce human exposure to high fluoride in drinking water, knowledge of the source and geochemical processes of enrichment are required. A study was therefore carried out within the Naivasha catchment (Kenya) to understand the genesis, enrichment and seasonal variations of fluoride in the groundwater. Rocks, rain, surface and groundwater sources were sampled for hydrogeochemical and isotopic investigations, the data was statistically and geospatially analyzed. Water sources have variable fluoride concentrations between 0.02–75 mg/L. 73% exceed the health limit (1.5 mg/L) in both dry and wet seasons. F{sup −} concentrations in rivers are lower (0.2–9.2 mg/L) than groundwater (0.09 to 43.6 mg/L) while saline lake waters have the highest concentrations (0.27–75 mg/L). The higher values are confined to elevations below 2000 masl. Oxygen (δ{sup 18}O) and hydrogen (δD) isotopic values range from − 6.2 to + 5.8‰ and − 31.3 to + 33.3‰, respectively, they are also highly variable in the rift floor where they attain maximum values. Fluoride base levels in the precursor vitreous volcanic rocks are higher (between 3750–6000 ppm) in minerals such as cordierite and muscovite while secondary minerals like illite and kaolinite have lower remnant fluoride (< 1000 ppm). Thus, geochemical F{sup −} enrichment in regional groundwater is mainly due to a) rock alteration, i.e. through long residence times and natural discharge and/or enhanced leakages of deep seated geothermal water reservoirs, b) secondary concentration fortification of natural reservoirs through evaporation, through reduced recharge and/or enhanced abstraction and c) through additional enrichment of fluoride after volcanic emissions. The findings are useful to help improve water management

  11. CENOZOIC CONTINENTAL RIFTING SYMPOSIUM DEDICATED TO THE MEMORY OF ACADEMICIAN N.A. LOGATCHEV, IRKUTSK, RUSSIA, JUNE 7–11, 2010

    Directory of Open Access Journals (Sweden)

    Eugene V. Sklyarov

    2010-01-01

    Full Text Available The information on the «Cenozoic Continental Rifting» Symposium dedicated to the memory of Academician N.A. Logachev is presented. It was held on June 7–11, 2010 at the Institute of the Earth’s Crust, Irkutsk. The scope of conference is presented.

  12. Hydrogeological Modelling of the Geothermal Waters of Alaşehir in the Continental Rift Zone of the Gediz, Western Anatolia, Turkey

    Science.gov (United States)

    Ӧzgür, Nevzat; Bostancı, Yesim; Anilır Yürük, Ezgi

    2017-12-01

    In western Anatolia, Turkey, the continental rift zones of the Büyük Menderes, Küçük Menderes and Gediz were formed by extensional tectonic features striking E-W generally and representing a great number of active geothermal systems, epithermal mineralizations and volcanic rocks from Middle Miocene to recent. The geothermal waters are associated with the faults which strike preferentially NW-SE and NE-SW and locate diagonal to general strike of the rift zones of the Menderes Massif. These NW-SE and NE-SW striking faults were probably generated by compressional tectonic regimes which leads to the deformation of uplift between two extensional rift zones in the Menderes Massif. The one of these rift zones is Gediz which is distinguished by a great number of geothermal waters such as Alaşehir, Kurşunlu, Çamurlu, Pamukkale and Urganlı. The geothermal waters of Alaşehir form the biggest potential in the rift zone of Gediz with a capacity of about 100 to 200 MWe. Geologically, the gneisses from the basement rocks in the study area which are overlain by an Paleozoic to Mesozoic intercalation of mica schists, quartzites and marbles, a Miocene intercalation of conglomerates, sandstones and clay stones and Plio-Quaternary intercalation of conglomerates, sandstones and clay stones discordantly. In the study area, Paleozoic to Mesozoic quartzites and marbles form the reservoir rocks hydrogeologically. The geothermal waters anions with Na+K>Ca>Mg dominant cations and HCO3>Cl> dominant anions are of Na-HCO3 type and can be considered as partial equilibrated waters. According to the results of geochemical thermometers, the reservoir temperatures area of about 185°C in accordance with measured reservoir temperatures. Stabile isotopes of δ18O versus δ2H of geothermal waters of Alaşehir deviate from the meteoric water line showing an intensive water-rock interaction under high temperature conditions. These data are well correlated with the results of the

  13. 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

  14. Seismicity of the Earth 1900-2013 East African Rift

    Science.gov (United States)

    Hayes, Gavin P.; Jones, Eric S.; Stadler, Timothy J.; Barnhart, William D.; McNamara, Daniel E.; Benz, Harley M.; Furlong, Kevin P.; Villaseñor, Antonio; Hayes, Gavin P.; Jones, Eric S.; Stadler, Timothy J.; Barnhart, William D.; McNamara, Daniel E.; Benz, Harley M.; Furlong, Kevin P.; Villaseñor, Antonio

    2014-01-01

    The East African Rift system (EARS) is a 3,000-km-long Cenozoic age continental rift extending from the Afar triple junction, between the horn of Africa and the Middle East, to western Mozambique. Sectors of active extension occur from the Indian Ocean, west to Botswana and the Democratic Republic of the Congo (DRC). It is the only rift system in the world that is active on a continent-wide scale, providing geologists with a view of how continental rifts develop over time into oceanic spreading centers like the Mid-Atlantic Ridge.

  15. Isotopic characterisation of the sub-continental lithospheric mantle beneath Zealandia, a rifted fragment of Gondwana

    DEFF Research Database (Denmark)

    Waight, Tod Earle; Scott, James M.; van der Meer, Quinten Har Adriaan

    2013-01-01

    The greater New Zealand region, known as Zealandia, represents an amalgamation of crustal fragments accreted to the paleo-Pacific Gondwana margin and which underwent significant thinning during the subsequent split from Australia and Antarctica in the mid-Cretaceous following opening of the Tasma...... Sea and the Southern Ocean. We present Sr, Nd and Pb isotopes and laser ablation trace element data for a comprehensive suite of clinopyroxene separates from spinel peridotite xenoliths (lherzolite to harzburgite) from the sub-continental lithospheric mantle across southern New Zealand...... composition, age or geographical separation. These isotopic compositions indicate that the sub-continental lithospheric mantle under southern New Zealand has a regionally distinct and pervasive FOZO to HIMU – like signature. The isotopic signatures are also similar to those of the alkaline magmas...... that transported the xenoliths and suggest that most of the HIMU signature observed in the volcanics could be derived from a major source component in the sub-continental lithospheric mantle. Trace element abundances in clinopyroxene are highly heterogeneous and vary from LREE-enriched, relatively flat and MORB...

  16. Barite-forming environments along a rifted continental margin, Southern California Borderland

    Science.gov (United States)

    Hein, James R.; Zierenberg, Robert A.; Maynard, J. Barry; Hannington, Mark D.

    2007-01-01

    The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. δ34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have δ34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/86Sr ratios for the barites vary over a narrow range of 0.70830–0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time

  17. Physics-based and statistical earthquake forecasting in a continental rift zone: the case study of Corinth Gulf (Greece)

    Science.gov (United States)

    Segou, Margarita

    2016-01-01

    I perform a retrospective forecast experiment in the most rapid extensive continental rift worldwide, the western Corinth Gulf (wCG, Greece), aiming to predict shallow seismicity (depth statistics, four physics-based (CRS) models, combining static stress change estimations and the rate-and-state laboratory law and one hybrid model. For the latter models, I incorporate the stress changes imparted from 31 earthquakes with magnitude M ≥ 4.5 at the extended area of wCG. Special attention is given on the 3-D representation of active faults, acting as potential receiver planes for the estimation of static stress changes. I use reference seismicity between 1990 and 1995, corresponding to the learning phase of physics-based models, and I evaluate the forecasts for six months following the 1995 M = 6.4 Aigio earthquake using log-likelihood performance metrics. For the ETAS realizations, I use seismic events with magnitude M ≥ 2.5 within daily update intervals to enhance their predictive power. For assessing the role of background seismicity, I implement a stochastic reconstruction (aka declustering) aiming to answer whether M > 4.5 earthquakes correspond to spontaneous events and identify, if possible, different triggering characteristics between aftershock sequences and swarm-type seismicity periods. I find that: (1) ETAS models outperform CRS models in most time intervals achieving very low rejection ratio RN = 6 per cent, when I test their efficiency to forecast the total number of events inside the study area, (2) the best rejection ratio for CRS models reaches RN = 17 per cent, when I use varying target depths and receiver plane geometry, (3) 75 per cent of the 1995 Aigio aftershocks that occurred within the first month can be explained by static stress changes, (4) highly variable performance on behalf of both statistical and physical models is suggested by large confidence intervals of information gain per earthquake and (5) generic ETAS models can adequately

  18. Tectono-stratigraphy of the Lower Cretaceous Syn-rift Succession in Bongor Basin, Chad: Insights into Structural Controls on Sedimentary Infill of a Continental Rift

    Science.gov (United States)

    Chen, C.; Ji, Y.; Wei, X.; An, F.; Li, D.; Zhu, R.

    2017-12-01

    In a rift basin, the dispersal and deposition of sediments is significantly influenced by the paleo-topography, which is highly controlled by the evolution and interaction of normal faults in different scales. To figure out the impact of faults evolution and topographic elements towards sedimentary fillings, we investigated the Lower Cretaceous syn-rift package in Bongor Basin, south of Chad Republic. Constrained with 2D and 3D seismic data, core data and logging information, a sequence stratigraphy architecture and a variety of depositional systems are recognized, including fan delta, braided delta, sub-lacustrine fan and lacustrine system. We also studied the spatial distribution and temporal evolution of clastic depositional systems of the syn-rift complex, and valuable insights into structural controls of sequence architectures and depositional systems are provided. During the evolution of rift basin, marginal structures such as relay ramps and strike-slipping boundary transfer fault are major elements that influence the main sediments influx points. Release faults in the hanging-wall could form a differential evolution pattern for accommodation, and effect the deposition systems in the early stage of rift evolution. Oblique crossing-faults, minor faults that develop on the erosional uplift in the interior foot-wall, would cut the uplifts and provide faulted-through paths for the over-filled sediments in the accommodation space, making it possible to develop sedimentary systems towards the center of basin during the early stage of rift evolution, although the origins of such minor faults still need further discussion. The results of this research indicate that different types of fault interactions have a fundamental control on patterns of sediment dispersal during early stage of rift basins.

  19. Role of magmatism in continental lithosphere extension: an introduction to tectnophysics special issue

    Energy Technology Data Exchange (ETDEWEB)

    Van Wijk, Jolante W [Los Alamos National Laboratory

    2008-01-01

    The dynamics and evolution of rifts and continental rifted margins have been the subject of intense study and debate for many years and still remain the focus of active investigation. The 2006 AGU Fall Meeting session 'Extensional Processes Leading to the Formation of Basins and Rifted Margins, From Volcanic to Magma-Limited' included several contributions that illustrated recent advances in our understanding of rifting processes, from the early stages of extension to breakup and incipient seafloor spreading. Following this session, we aimed to assemble a multi-disciplinary collection of papers focussing on the architecture, formation and evolution of continental rift zones and rifted margins. This Tectonophysics Special Issue 'Role of magmatism in continental lithosphere extension' comprises 14 papers that present some of the recent insights on rift and rifted margins dynamics, emphasising the role of magmatism in extensional processes. The purpose of this contribution is to introduce these papers.

  20. Continental rift formation and transition to ocean sea floor spreading : A case study of the Afar triple junction

    NARCIS (Netherlands)

    Lavecchia, A.L.

    2017-01-01

    Lithosphere extension, thinning and breakup are fundamental processes in geodynamics. During rift development, both the lithosphere and the mantle are involved in a coupled system, in which the main mechanisms and the forces associated with them often vary during the rift evolution. Furthermore, the

  1. Structure and Geochemistry of the Continental-Oceanic Crust Boundary of the Red Sea and the Rifted Margin of Western Arabia

    Science.gov (United States)

    Dilek, Y.; Furnes, H.; Schoenberg, R.

    2009-12-01

    The continental-oceanic crust boundary and an incipient oceanic crust of the Red Sea opening are exposed within the Arabian plate along a narrow zone of the Tihama Asir coastal plain in SW Saudi Arabia. Dike swarms, layered gabbros, granophyres and basalts of the 22 Ma Tihama Asir (TA) continental margin ophiolite represent products of magmatic differentiation formed during the initial stages of rifting between the African and Arabian plates. Nearly 4-km-wide zone of NW-trending sheeted dikes are the first products of mafic magmatism associated with incipient oceanic crust formation following the initial continental breakup. Gabbro intrusions are composed of cpx-ol-gabbro, cpx-gabbro, and norite/troctolite, and are crosscut by fine-grained basaltic dikes. Granophyre bodies intrude the sheeted dike swarms and are locally intrusive into the gabbros. Regional Bouger gravity anomalies suggest that the Miocene mafic crust represented by the TA complex extends westward beneath the coastal plain sedimentary rocks and the main trough of the Red Sea. The TA complex marks an incipient Red Sea oceanic crust that was accreted to the NE side of the newly formed continental rift in the earliest stages of seafloor spreading. Its basaltic to trachyandesitic lavas and dikes straddle the subalkaline-mildly alkaline boundary. Incompatible trace element relationships (e.g. Zr-Ti, Zr-P) indicate two distinct populations. The REE concentrations show an overall enrichment compared to N-MORB; light REEs are enriched over the heavy ones ((La/Yb)n > 1), pointing to an E-MORB influence. Nd-isotope data show ɛNd values ranging from +4 to +8, supporting an E-MORB melt source. The relatively large variations in ɛNd values also suggest various degrees of involvement of continental crust during ascent and emplacement, or by mixing of another mantle source.

  2. Linking the tectonic evolution with fluid history in magma-poor rifted margins: tracking mantle- and continental crust-related fluids

    Science.gov (United States)

    Pinto, V. H. G.; Manatschal, G.; Karpoff, A. M.

    2014-12-01

    The thinning of the crust and the exhumation of subcontinental mantle is accompanied by a series of extensional detachment faults. Exhumation of mantle and crustal rocks is intimately related to percolation of fluids along detachment faults leading to changes in mineralogy and chemistry of the mantle, crustal and sedimentary rocks. Field observation, analytical methods, refraction/reflection and well-core data, allowed us to investigate the role of fluids in the Iberian margin and former Alpine Tethys distal margins and the Pyrenees rifted system. In the continental crust, fluid-rock interaction leads to saussuritization that produces Si and Ca enriched fluids found in forms of veins along the fault zone. In the zone of exhumed mantle, large amounts of water are absorbed in the first 5-6 km of serpentinized mantle, which has the counter-effect of depleting the mantle of elements (e.g., Si, Ca, Mg, Fe, Mn, Ni and Cr) forming mantle-related fluids. Using Cr-Ni-V and Fe-Mn as tracers, we show that in the distal margin, mantle-related fluids used detachment faults as pathways and interacted with the overlying crust, the sedimentary basin and the seawater, while further inward parts of the margin, continental crust-related fluids enriched in Si and Ca impregnated the fault zone and may have affected the sedimentary basin. The overall observations and results enable us to show when, where and how these interactions occurred during the formation of the rifted margin. In a first stage, continental crust-related fluids dominated the rifted systems. During the second stage, mantle-related fluids affected the overlying syn-tectonic sediments through direct migration along detachment faults at the future distal margin. In a third stage, these fluids reached the seafloor, "polluted" the seawater and were absorbed by post-tectonic sediments. We conclude that a significant amount of serpentinization occurred underneath the thinned continental crust, that the mantle-related fluids

  3. Subsurface images of the Eastern Rift, Africa, from the joint inversion of body waves, surface waves and gravity: investigating the role of fluids in early-stage continental rifting

    Science.gov (United States)

    Roecker, S.; Ebinger, C.; Tiberi, C.; Mulibo, G.; Ferdinand-Wambura, R.; Mtelela, K.; Kianji, G.; Muzuka, A.; Gautier, S.; Albaric, J.; Peyrat, S.

    2017-08-01

    The Eastern Rift System (ERS) of northern Tanzania and southern Kenya, where a cratonic lithosphere is in the early stages of rifting, offers an ideal venue for investigating the roles of magma and other fluids in such an environment. To illuminate these roles, we jointly invert arrival times of locally recorded P and S body waves, phase delays of ambient noise generated Rayleigh waves and Bouguer anomalies from gravity observations to generate a 3-D image of P and S wave speeds in the upper 25 km of the crust. While joint inversion of gravity and arrival times requires a relationship between density and wave speeds, the improvement in resolution obtained by the combination of these disparate data sets serves to further constrain models, and reduce uncertainties. The most significant features in the 3-D model are (1) P and S wave speeds that are 10-15 per cent lower beneath the rift zone than in the surrounding regions, (2) a relatively high wave speed tabular feature located along the western edge of the Natron and Manyara rifts, and (3) low (∼1.71) values of Vp/Vs throughout the upper crust, with the lowest ratios along the boundaries of the rift zones. The low P and S wave speeds at mid-crustal levels beneath the rift valley are an expected consequence of active volcanism, and the tabular, high-wave speed feature is interpreted to be an uplifted footwall at the western edge of the rift. Given the high levels of CO2 outgassing observed at the surface along border fault zones, and the sensitivity of Vp/Vs to pore-fluid compressibility, we infer that the low Vp/Vs values in and around the rift zone are caused by the volcanic plumbing in the upper crust being suffused by a gaseous CO2 froth on top of a deeper, crystalline mush. The repository for molten rock is likely located in the lower crust and upper mantle, where the Vp/Vs ratios are significantly higher.

  4. ALVIN investigation of an active propagating rift system, Galapagos 95.5° W

    Science.gov (United States)

    Hey, R.N.; Sinton, J.M.; Kleinrock, M.C.; Yonover, R.N.; MacDonald, K.C.; Miller, S.P.; Searle, R.C.; Christie, D.M.; Atwater, T.M.; Sleep, Norman H.; Johnson, H. Paul; Neal, C.A.

    1992-01-01

    ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5° W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.−1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.

  5. Microstructural and seismic properties of the upper mantle underneath a rifted continental terrane (Baja California): An example of sub-crustal mechanical asthenosphere?

    NARCIS (Netherlands)

    Palasse, L.N.; Vissers, R.L.M.; Paulssen, H.; Basu, A.R.; Drury, M.R.

    2012-01-01

    The Gulf of California rift is a young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. The xenolith bearing lavas of the San Quintin volcanic area provide lower crust and upper mantle samples from

  6. Seismic velocity structure of the crust in NW Namibia: Impact of rifting and mantle plume activity

    Science.gov (United States)

    Bauer, K.; Heit, B.; Muksin, U.; Yuan, X.

    2017-12-01

    The continental crust in northwestern Namibiamainly was formed during to the Neoproterozoic assembly of Gondwana. The collision of old African and South American cratonic coressuch as the Congo, Kalahari and Rio de la Plata cratons led tothe development of the Pan-African Damara orogen. The fold systemconsists of an intracratonic branch in northern central Namibia (named Damara Belt), and two coast-parallel branches, the Kaoko Belt in northern Namibia and the Gariep Belt in the border region between Namibia and theRepublic of South Africa. During the Early Cretaceous opening of the South Atlantic ocean, the crust in NW Namibia was prominently affected by the Tristan da Cunha mantle plume, as evidenced by the emplacement of the Etendeka continental flood basalts.A local earthquake tomography was carried out in NW Namibia to investigateif and to what degree the deeper continental crust was modified by the magmaticactivity during rifting and the impingement of the Tristan da Cunhamantle plume. We analyzed data from 28 onshore stations of the temporaryWALPASS seismic network. Stations were covering the continental marginaround the landfall of the Walvis Ridge, parts of the Kaoko Belt and Damara Belt,and marginally the southwestern edges of the Congo craton.First arrivals of P and S waves were identified and travel times werepicked manually. 1D inversion was carried out with VELEST to derivestarting models and the initial seismicity distribution, and SIMUL2000was used for the subsequent 3D tomographic inversion. The resultingseismicity distribution mainly follows the structures of the Pan-Africanorogenic belts. The majority of events was localized in the upper crust,but additional seismicity was also found in the deeper crust.An anomaly of increased P velocities is revealed in the deep crust under the Etendekaflood basalt province. Increased P velocities can be explained by mafic and ultra-maficmaterial which intruded in the lower crust. The anomaly appears to be rather

  7. Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu-Bonin-Mariana Arc: Syntectonic granitic crust formation during back-arc rifting

    Science.gov (United States)

    Tani, Kenichiro; Dunkley, Daniel J.; Chang, Qing; Nichols, Alexander R. L.; Shukuno, Hiroshi; Hirahara, Yuka; Ishizuka, Osamu; Arima, Makoto; Tatsumi, Yoshiyuki

    2015-08-01

    A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan-West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu-Bonin-Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.

  8. Mastritherium (Artiodactyla, Anthracotheriidae) from Wadi Sabya, southwestern Saudi Arabia; an earliest Miocene age for continental rift-valley volcanic deposits of the Red Sea margin

    Science.gov (United States)

    Madden, Gary T.; Schmidt, Dwight Lyman; Whitmore, Frank C.

    1983-01-01

    A lower jaw fragment with its last molar (M/3) from the Baid formation in Wadi Sabya, southwestern Saudi Arabia, represents the first recorded occurrence in the Arabian Peninsula of an anthracotheriid artiodactyl (hippo-like, even-toed ungulate). This fossil is identified as a primitive species of Masritherium, a North and East African genus restricted, previously to the later early Miocene. This identification indicates that the age of the Baid formation, long problematical, is early Miocene and, moreover, shows that the age of the fossil site is earliest Miocene (from 25 to 21Ma). The Wadi Sabya anthracothere is the first species of fossil mammal recorded from western Saudi Arabia, and more important, it indicates an early Miocene age for the volcanic deposits of a continental rift-valley that preceded the initial sea-floor spreading of the Red Sea.

  9. Fault kinematics and tectonic stress in the seismically active Manyara Dodoma Rift segment in Central Tanzania Implications for the East African Rift

    Science.gov (United States)

    Macheyeki, Athanas S.; Delvaux, Damien; De Batist, Marc; Mruma, Abdulkarim

    2008-07-01

    The Eastern Branch of the East African Rift System is well known in Ethiopia (Main Ethiopian Rift) and Kenya (Kenya or Gregory Rift) and is usually considered to fade away southwards in the North Tanzanian Divergence, where it splits into the Eyasi, Manyara and Pangani segments. Further towards the south, rift structures are more weakly expressed and this area has not attracted much attention since the mapping and exploratory works of the 1950s. In November 4, 2002, an earthquake of magnitude Mb = 5.5 struck Dodoma, the capital city of Tanzania. Analysis of modern digital relief, seismological and geological data reveals that ongoing tectonic deformation is presently affecting a broad N-S trending belt, extending southward from the North Tanzanian Divergence to the region of Dodoma, forming the proposed "Manyara-Dodoma Rift segment". North of Arusha-Ngorongoro line, the rift is confined to a narrow belt (Natron graben in Tanzania) and south of it, it broadens into a wide deformation zone which includes both the Eyasi and Manyara grabens. The two-stage rifting model proposed for Kenya and North Tanzania also applies to the Manyara-Dodoma Rift segment. In a first stage, large, well-expressed topographic and volcanogenic structures were initiated in the Natron, Eyasi and Manyara grabens during the Late Miocene to Pliocene. From the Middle Pleistocene onwards, deformations related to the second rifting stage propagated southwards to the Dodoma region. These young structures have still limited morphological expressions compared to the structures formed during the first stage. However, they appear to be tectonically active as shown by the high concentration of moderate earthquakes into earthquake swarms, the distribution of He-bearing thermal springs, the morphological freshness of the fault scarps, and the presence of open surface fractures. Fault kinematic and paleostress analysis of geological fault data in basement rocks along the active fault lines show that recent

  10. Physical Processes Contributing To Small-scale Vertical Movements During Changing Inplane Stresses In Rift Basins and At Passive Continental Margins

    Science.gov (United States)

    Paulsen, G. E.; Nielsen, S. B.; Hansen, D. L.

    The vertical movements during a regional stress reversal in a rifted basin or on a passive continental margin are examined using a numerical 2D thermo-mechanical finite element model with a visco-elastic-plastic rheology. Three different physical mechanisms are recognized in small-scale vertical movements at small inplane force variations: elastic dilatation, elastic flexure, and permanent deformation. Their rela- tive importance depend on the applied force, the duration of the force, and the thermal structure of the lithosphere. Elastic material dilatation occurs whenever the stress state changes. A reversal from extension to compression therefore immediately leads to elastic dilatation, and re- sults in an overall subsidence of the entire profile. Simultaneously with dilatation the lithosphere reacts with flexure. The significance of the flexural component strongly depends on the thermal structure of the lithosphere. The polarity and amplitude of the flexure depends on the initial (before compression) loading of the lithosphere. Gener- ally, the flexural effects lead to subsidence of the overdeep in the landward part of the basin and a small amount of uplift at the basin flanks. The amplitudes of the flexural response are small and comparable with the amplitudes of the elastic dilatation. With continuing compression permanent deformation and lithospheric thickening becomes increasingly important. Ultimately, the thickened part of the lithosphere stands out as an inverted zone. The amount of permanent deformation is directly connected with the size and duration of the applied force, but even a relatively small force leads to inversion tectonics in the landward part of the basin. The conclusions are: 1) small stress induced vertical movements in rift basins and at passive continental margins are the result of a complex interaction of at least three different processes, 2) the total sediment loaded amplitudes resulting from these pro- cesses are small (2-300 m) for

  11. Receiver function and magnetotelluric analysis to understand the first stage of a continental lithospheric break-up : case of the North Tanzanian Rift

    Science.gov (United States)

    Plasman, M.; Tiberi, C.; Tarits, P.; Hautot, S.; Gautier, S.; Ebinger, C. J.; Mulibo, G. D.; Khalfan, M.

    2015-12-01

    First stage of continental break-up, though intensively studied, is yet poorly understood. This is partly because actual rifting areas are either too mature (more than 10 My) or not easily accessible (thick sediment cover or under water). The North Tanzania part of the East African Rift is the place of a lithosphere's early break-up (less than 5My) in response to a combination of regional pulling forces and mantle upwelling. Deformation there results from complex interactions between magmatic intrusions, faulting, asthenospheric dynamics and far field stresses. CoLiBrEA (ANR) and CRAFTI (NSF) are two multidisciplinary projects which collaboratively focus on this area to understand the interactions between faults and magma, the role of inherited structures and rheological heterogeneities of the lithosphere. For that purpose, we deployed 38 broadband seismic stations in the Natron and Ngorongoro areas from January 2013 to December 2014 and carried out a 120 km East-West magnetotelluric (MT) profile to image the crustal and mantle structures. The 3D resistivity model, obtained from the inversion of the MT data along the profile, shows an highly heterogeneous crust with three-dimensional structures over a more homogeneous upper mantle. The first inversion result from the receiver function (RF) by the Zhu and Kanamori's inversion method show a thick crust (~35 km) with important variations (maximum 15km) especially in the Ngorongoro area, and an average Vp/Vs ratio of 1.75. We then completed this study by combining the MT data and the RF at the 11 sites of the EW profile. For each site, we built a 1D velocity model (Vs and VpVs) obtained by combining the Sambridge forward solution with a non linear descent research algorithm and constrained by the resistivity structure. The inversion shows an heterogeneous crust obviously dominated by the Moho interface at different depths, with low velocity layers mainly corresponding to low resistivity features.

  12. Images of the East Africa Rift System from the Joint Inversion of Body Waves, Surface Waves, and Gravity: Investigating the Role of Magma in Early-Stage Continental Rifting

    Science.gov (United States)

    Roecker, S. W.; Ebinger, C. J.; Tiberi, C.; Mulibo, G. D.; Ferdinand-Wambura, R.; Muzuka, A.; Khalfan, M.; Kianji, G.; Gautier, S.; Albaric, J.; Peyrat, S.

    2015-12-01

    With several rift segments at different stages of the rifting cycle, and the last orogenic episode more than 500 Mya, the young (Ngorongoro caldera appears to be physically cut off from the magma beneath the main part of the rift zone by a relatively thin (< 10 km) wide zone of higher shear wave speeds that lies along the western edge of the fault-bounded rift. The narrow ridge of higher velocity lower crustal material may be a consequence of flexural uplift of the rift flank in response to stretching of strong, cratonic lithosphere.

  13. Variability of the geothermal gradient across two differently aged magma-rich continental rifted margins of the Atlantic Ocean

    OpenAIRE

    Gholamrezaie, Ershad; Scheck-Wenderoth, Magdalena (Dr.); Sippel, Judith (Dr.); Strecker, Manfred R. (Prof. Dr.)

    2018-01-01

    Abstract. The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D models differentiate various sedimentary, crustal, and mantle units and i...

  14. Heat flow in the rifted continental margin of the South China Sea near Taiwan and its tectonic implications

    Science.gov (United States)

    Liao, Wei-Zhi; Lin, Andrew T.; Liu, Char-Shine; Oung, Jung-Nan; Wang, Yunshuen

    2014-10-01

    Temperature measurements carried out on 9 hydrocarbon exploration boreholes together with Bottom Simulating Reflectors (BSRs) from reflection seismic images are used in this study to derive geothermal gradients and heat flows in the northern margin of the South China Sea near Taiwan. The method of Horner plot is applied to obtain true formation temperatures from measured borehole temperatures, which are disturbed by drilling processes. Sub-seafloor depths of BSRs are used to calculate sub-bottom temperatures using theoretical pressure/temperature phase boundary that marks the base of gas hydrate stability zone. Our results show that the geothermal gradients and heat flows in the study area range from 28 to 128 °C/km and 40 to 159 mW/m2, respectively. There is a marked difference in geothermal gradients and heat flow beneath the shelf and slope regions. It is cooler beneath the shelf with an average geothermal gradient of 34.5 °C/km, and 62.7 mW/m2 heat flow. The continental slope shows a higher average geothermal gradient of 56.4 °C/km, and 70.9 mW/m2 heat flow. Lower heat flow on the shelf is most likely caused by thicker sediments that have accumulated there compared to the sediment thickness beneath the slope. In addition, the continental crust is highly extended beneath the continental slope, yielding higher heat flow in this region. A half graben exists beneath the continental slope with a north-dipping graben-bounding fault. A high heat-flow anomaly coincides at the location of this graben-bounding fault at the Jiulong Ridge, indicating vigorous vertical fluid convection which may take place along this fault.

  15. Morphostructural evidence for Recent/active extension in Central Tanzania beyond the southern termination of the Kenya Rift.

    Science.gov (United States)

    Le Gall, B.; Rolet, J.; Gernigon, L.; Ebinger, C.; Gloaguen, R.

    2003-04-01

    The southern tip zone of the Kenya Rift on the eastern branch of the East African System is usually thought to occur in the so-called North Tanzanian Divergence. In this region, the narrow (50 km-wide) axial graben of southern Kenya splays southwards, via a major EW-trending volcanic lineament, into a 200 km-wide broad rifted zone with three separate arms of normal faulting and tilted fault blocks (Eyasi, Manyara and Pangani arms from W to E). Remote sensing analysis from Central Tanzania demonstrates that rift morphology exists over an area lying 400 km beyond the southern termination of the Kenya Rift. The most prominent rift structures are observed in the Kilombero region and consist of a 100 km-wide range of uplifted basement blocks fringed to the west by an E-facing half-graben inferred to reach depths of 6-8 km from aeromagnetic dataset. Physiographic features (fault scarps), and river drainage anomalies suggest that the present-day rift pattern in the Kilombero extensional province principally results from Recent/Neogene deformation. That assumption is also supported by the seismogenic character of a number of faults. The Kilombero half-graben is superimposed upon an earlier rift system, Karoo in age, which is totally overprinted and is only evidenced from its sedimentary infill. On the other hand, the nature and thickness of the inferred Neogene synrift section is still unknown. The Kilombero rifted zone is assumed to connect northwards into the central rift arm (Manyara) of the South Kenya Rift via a seismically active transverse fault zone that follows ductile fabrics within the Mozambican crystalline basement. The proposed rift model implies that incipient rifting propagates hroughout the cold and strong crust/lithosphere of Central Tanzania along Proterozoic (N140=B0E) basement weakness zones and earlier Karoo (NS)rift structures. A second belt of Recent-active linked fault/basins also extends further East from the Pangani rift arm to the offshore

  16. The East African rift system

    Science.gov (United States)

    Chorowicz, Jean

    2005-10-01

    This overview paper considers the East African rift system (EARS) as an intra-continental ridge system, comprising an axial rift. It describes the structural organization in three branches, the overall morphology, lithospheric cross-sections, the morphotectonics, the main tectonic features—with emphasis on the tension fractures—and volcanism in its relationships with the tectonics. The most characteristic features in the EARS are narrow elongate zones of thinned continental lithosphere related to asthenospheric intrusions in the upper mantle. This hidden part of the rift structure is expressed on the surface by thermal uplift of the rift shoulders. The graben valleys and basins are organized over a major failure in the lithospheric mantle, and in the crust comprise a major border fault, linked in depth to a low angle detachment fault, inducing asymmetric roll-over pattern, eventually accompanied by smaller normal faulting and tilted blocks. Considering the kinematics, divergent movements caused the continent to split along lines of preexisting lithospheric weaknesses marked by ancient tectonic patterns that focus the extensional strain. The hypothesis favored here is SE-ward relative divergent drifting of a not yet well individualized Somalian plate, a model in agreement with the existence of NW-striking transform and transfer zones. The East African rift system comprises a unique succession of graben basins linked and segmented by intracontinental transform, transfer and accommodation zones. In an attempt to make a point on the rift system evolution through time and space, it is clear that the role of plume impacts is determinant. The main phenomenon is formation of domes related to plume effect, weakening the lithosphere and, long after, failure inducing focused upper mantle thinning, asthenospheric intrusion and related thermal uplift of shoulders. The plume that had formed first at around 30 Ma was not in the Afar but likely in Lake Tana region (Ethiopia

  17. Morpho-structural evolution of a volcanic island developed inside an active oceanic rift: S. Miguel Island (Terceira Rift, Azores)

    Science.gov (United States)

    Sibrant, A. L. R.; Hildenbrand, A.; Marques, F. O.; Weiss, B.; Boulesteix, T.; Hübscher, C.; Lüdmann, T.; Costa, A. C. G.; Catalão, J. C.

    2015-08-01

    The evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass-wasting processes. More specifically, volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation. The island of S. Miguel (Azores) has developed during the last 1 Myr inside the active Terceira Rift, a major tectonic structure materializing the present boundary between the Eurasian and Nubian lithospheric plates. In this work, we depict the evolution of the island, based on high-resolution DEM data, stratigraphic and structural analyses, high-precision K-Ar dating on separated mineral phases, and offshore data (bathymetry and seismic profiles). The new results indicate that: (1) the oldest volcanic complex (Nordeste), composing the easternmost part of the island, was dominantly active between ca. 850 and 750 ka, and was subsequently affected by a major south-directed flank collapse. (2) Between at least 500 ka and 250 ka, the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main E-W collapse scar. (3) Since 250 kyr, the western part of this succession (Furnas area) was affected by multiple vertical collapses; associated plinian eruptions produced large pyroclastic deposits, here dated at ca. 60 ka and less than 25 ka. (4) During the same period, the eastern part of the landslide scar was enlarged by retrogressive erosion, producing the large Povoação valley, which was gradually filled by sediments and young volcanic products. (5) The Fogo volcano, in the middle of S. Miguel, is here dated between ca. 270 and 17 ka, and was affected by, at least, one southwards flank collapse. (6) The Sete Cidades volcano, in the western end of the island, is here dated between ca. 91 and 13 ka, and experienced mutliple caldera collapses; a landslide to the North is also suspected from the presence of a

  18. Acid-activated structural reorganization of the Rift Valley fever virus Gc fusion protein

    NARCIS (Netherlands)

    Boer, de S.M.; Kortekaas, J.A.; Spel, L.; Rottier, P.J.M.; Moormann, R.J.M.; Bosch, B.J.

    2012-01-01

    Entry of the enveloped Rift Valley fever virus (RVFV) into its host cell is mediated by the viral glycoproteins Gn and Gc. We investigated the RVFV entry process and its pH-dependent activation mechanism in particular using our recently developed nonspreading RVFV particle system. Entry of the virus

  19. Magmatism and deformation during continental breakup

    Science.gov (United States)

    Keir, Derek

    2013-04-01

    The rifting of continents and the transition to seafloor spreading is characterised by extensional faulting and thinning of the lithosphere, and is sometimes accompanied by voluminous intrusive and extrusive magmatism. In order to understand how these processes develop over time to break continents apart, we have traditionally relied on interpreting the geological record at the numerous fully developed, ancient rifted margins around the world. In these settings, however, it is difficult to discriminate between different mechanisms of extension and magmatism because the continent-ocean transition is typically buried beneath thick layers of volcanic and sedimentary rocks, and the tectonic and volcanic activity that characterised breakup has long-since ceased. Ongoing continental breakup in the African and Arabian rift systems offers a unique opportunity to address these problems because it exposes several sectors of tectonically active rift sector development spanning the transition from embryonic continental rifting in the south to incipient seafloor spreading in the north. Here I synthesise exciting, multidisciplinary observational and modelling studies using geophysical, geodetic, petrological and numerical techniques that uniquely constrain the distribution, time-scales, and interactions between extension and magmatism during the progressive breakup of the African Plate. This new research has identified the previously unrecognised role of rapid and episodic dike emplacement in accommodating a large proportion of extension during continental rifting. We are now beginning to realise that changes in the dominant mechanism for strain over time (faulting, stretching and magma intrusion) impact dramatically on magmatism and rift morphology. The challenge now is to take what we're learned from East Africa and apply it to the rifted margins whose geological record documents breakup during entire Wilson Cycles.

  20. 3D Numerical Rift Modeling with Application to the East African Rift System

    Science.gov (United States)

    Glerum, A.; Brune, S.; Naliboff, J.

    2017-12-01

    As key components of plate tectonics, continental rifting and the formation of passive margins have been extensively studied with both analogue models and numerical techniques. Only recently however, technical advances have enabled numerical investigations into rift evolution in three dimensions, as is actually required for including those processes that cause rift-parallel variability, such as structural inheritance and oblique extension (Brune 2016). We use the massively parallel finite element code ASPECT (Kronbichler et al. 2012; Heister et al. 2017) to investigate rift evolution. ASPECT's adaptive mesh refinement enables us to focus resolution on the regions of interest (i.e. the rift center), while leaving other areas such as the asthenospheric mantle at coarse resolution, leading to kilometer-scale local mesh resolution in 3D. Furthermore, we implemented plastic and viscous strain weakening of the nonlinear viscoplastic rheology required to develop asymmetric rift geometries (e.g. Huismans and Beaumont 2003). Additionally created plugins to ASPECT allow us to specify initial temperature and composition conditions based on geophysical data (e.g. LITHO1.0, Pasyanos et al. 2014) or to prescribe more general along-strike variation in the initial strain seeding the rift. Employing the above functionality, we construct regional models of the East African Rift System (EARS), the world's largest currently active rift. As the EARS is characterized by both orthogonal and oblique rift sections, multi-phase extension histories as well as magmatic and a-magmatic branches (e.g. Chorowicz 2005; Ebinger and Scholz 2011), it constitutes an extensive natural laboratory for our research into the 3D nature of continental rifting. References:Brune, S. (2016), in Plate boundaries and natural hazards, AGU Geophysical Monograph 219, J. C. Duarte and W. P. Schellart (Eds.). Chorowicz, J. (2005). J. Afr. Earth Sci., 43, 379-410. Ebinger, C. and Scholz, C. A. (2011), in Tectonics of

  1. Initiation of Extension in South China Continental Margin during the Active-Passive Margin Transition: Thermochronological and Kinematic Constraints

    Science.gov (United States)

    Zuo, X.; Chan, L. S.

    2015-12-01

    The South China continental margin is characterized by a widespread magmatic belt, prominent NE-striking faults and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from active to passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. Our zircon fission-track data in this region show two episodes of exhumation: The first episode, occurring during 170-120Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115-70Ma, including the Yunkai Terrane and the Nanling Range. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the fact that exhumation of the granite-dominant Nanling Range occurred earlier than that of the gneiss-dominant Yunkai Terrane. In addition to the difference in rock types, the heat from Jurassic-Early Cretaceous magmatism in Nanling may have softened the upper crust, causing the area to exhume more readily than Yunkai. Numerical modeling results also indicate that (1) high lithospheric geothermal gradient, high slab dip angle and low convergence velocity favor the reversal of crustal stress state from compression to extension in the upper continental plate; (2) late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension imply that the Late Cretaceous-early Paleogene red-bed basins in South China could have formed during the late stage of the subduction, accounting for the occurrence of volcanic events in some sedimentary basins. We propose that the rifting started as early as Late Cretaceous, probably before the cessation of subduction process.

  2. Boundary separating the seismically active reelfoot rift from the sparsely seismic Rough Creek graben, Kentucky and Illinois

    Science.gov (United States)

    Wheeler, R.L.

    1997-01-01

    The Reelfoot rift is the most active of six Iapetan rifts and grabens in central and eastern North America. In contrast, the Rough Creek graben is one of the least active, being seismically indistinguishable from the central craton of North America. Yet the rift and graben adjoin. Hazard assessment in the rift and graben would be aided by identification of a boundary between them. Changes in the strikes of single large faults, the location of a Cambrian transfer zone, and the geographic extent of alkaline igneous rocks provide three independent estimates of the location of a structural boundary between the rift and the graben. The boundary trends north-northwest through the northeastern part of the Fluorspar Area Fault Complex of Kentucky and Illinois, and has no obvious surface expression. The boundary involves the largest faults, which are the most likely to penetrate to hypocentral depths, and the boundary coincides with the geographic change from abundant seismicity in the rift to sparse seismicity in the graben. Because the structural boundary was defined by geologic variables that are expected to be causally associated with seismicity, it may continue to bound the Reelfoot rift seismicity in the future.

  3. The geology and geophysics of the Oslo rift

    Science.gov (United States)

    Ruder, M. E.

    1981-01-01

    The regional geology and geophysical characteristics of the Oslo graben are reviewed. The graben is part of a Permian age failed continental rift. Alkali olivine, tholefitic, and monzonitic intrusives as well as basaltic lavas outline the extent of the graben. Geophysical evidence indicates that rifting activity covered a much greater area in Skagerrak Sea as well as the Paleozoic time, possibly including the northern Skagerrak Sea as well as the Oslo graben itself. Much of the surficial geologic characteristics in the southern part of the rift have since been eroded or covered by sedimentation. Geophysical data reveal a gravity maximum along the strike of the Oslo graben, local emplacements of magnetic material throughout the Skagerrak and the graben, and a slight mantle upward beneath the rift zone. Petrologic and geophysical maps which depict regional structure are included in the text. An extensive bibliography of pertinent literature published in English between 1960 and 1980 is also provided.

  4. CONTINENTAL RIFTING, ACCOMPANYING PROCESSES” SYMPOSIUM DEDICATED TO THE MEMORY OF N.A. LOGACHEV AND E.E. MILANOVSKY, ‘ACADEMICIANS OF RIFTING’, 20–23 AUGUST 2013, IRKUTSK, RUSSIA

    Directory of Open Access Journals (Sweden)

    S. V. Rasskazov

    2013-01-01

    Full Text Available  The article provides information about “Continental rifting, accompanying processes” Symposium dedicated to the memory of N.A. Logachev and E.E. Milanovsky, ‘Academicians of Rifting’. The symposium took place on 20–23 August 2013. It was initiated and arranged by the Institute of the Earth's Crust SB RAS and the Geological Faculty of the Irkutsk State University (Irkutsk with participation of the Geological Faculty of Moscow State University. Topics of the symposium are reviewed.  

  5. Contribution of slab melting to magmatism at the active rifts zone in the middle of the Izu-Bonin arc

    Science.gov (United States)

    Hirai, Y.; Okamura, S.; Sakamoto, I.; Shinjo, R.; Wada, K.; Yoshida, T.

    2016-12-01

    The active rifts zone lies just behind the Quaternary volcanic front in the middle of the Izu-Bonin arc. Volcanism at the active rifts zone has been active since ca. 2 Ma, and late Quaternary basaltic lavas (< 0.1 Ma) and hydrothermal activity occur along the central axis of the rifts (Taylor, 1992; Ishizuka et al., 2003). In this paper we present new Sr, Nd, and Hf isotope and trace element data for the basalts erupted in the active rifts zone, including the Aogashima, Myojin and Sumisu rifts. Two geochemical groups can be identified within the active rift basalts: High-Zr basalts (HZB) and Low-Zr basalts (LZB). In the case of the Sumisu rift, the HZB exhibits higher in K2O, Na2O, Y, Zr and Ni, and also has higher Ce/Yb and Zr/Y, lower Ba/Th than the LZB. Depletion of Zr-Hf in the N-MORB spidergram characterizes the LZB from the Aogashima, Myojin and Sumisu rifts. The 176Hf/177Hf ratios are slightly lower in the HZB than in the LZB, decoupling of 176Hf/177Hf ratios and 143Nd/144Nd ratios. Estimated primary magma compositions suggest that primary magma segregation for the HZB occurred at depths less than 70 km ( 2 GPa), whereas the LZB more than 70 km (2 3 GPa). ODP Leg126 site 788, 790, and 791 reached the basaltic basement of the Sumisu rift (Gill et al., 1992). The geochemical data and stratigraphic relations of the basement indicate that the HZB is younger than the LZB. Geochemical modelling demonstrates that slab-derived melt mixed with mantle wedge produces the observed isotopic and trace elemental characteristics. The LZB volcanism at the early stage of the back-arc rifting is best explained by a partial melting of subducted slab saturated with trace quantities of zircon under low-temperature conditions in the mantle wedge. On the other hand, the HZB requires a partial melt of subducted slab accompanied by full dissolution of zircon under high-temperature conditions in the mantle wedge, which could have been caused by hot asthenospheric injection during the

  6. Hydrothermal bitumen generated from sedimentary organic matter of rift lakes - Lake Chapala, Citala Rift, western Mexico

    Energy Technology Data Exchange (ETDEWEB)

    Zarate del Valle, Pedro F. [Departamento de Quimica, Universidad de Guadalajara - CUCEI, Ap. Postal 4-021, Guadalajara, Jalisco CP 44410 (Mexico); Simoneit, Bernd R.T. [Environmental and Petroleum Geochemistry Group, College of Oceanic and Atmospheric Sciences, Oregon State University, Building 104, Corvallis, OR 97331-5503 (United States)]. E-mail: simoneit@coas.oregonstate.edu

    2005-12-15

    Lake Chapala is in the Citala Rift of western Mexico, which in association with the Tepic-Zacoalco and Colima Rifts, form the well-known neotectonic Jalisco continental triple junction. The rifts are characterized by evidence for both paleo- and active hydrothermal activity. At the south shore of the lake, near the Los Gorgos sublacustrine hydrothermal field, there are two tar emanations that appear as small islands composed of solid, viscous and black bitumen. Aliquots of tar were analyzed by GC-MS and the mixtures are comprised of geologically mature biomarkers and an UCM. PAH and n-alkanes are not detectable. The biomarkers consist mainly of hopanes, gammacerane, tricyclic terpanes, carotane and its cracking products, steranes, and drimanes. The biomarker composition and bulk C isotope composition ({delta} {sup 13}C = -21.4%) indicate an organic matter source from bacteria and algae, typical of lacustrine ecosystems. The overall composition of these tars indicates that they are hydrothermal petroleum formed from lacustrine organic matter in the deeper sediments of Lake Chapala exceeding 40 ka ({sup 14}C) in age and then forced to the lakebed by tectonic activity. The absence of alkanes and the presence of an UCM with mature biomarkers are consistent with rapid hydrothermal oil generation and expulsion at temperatures of 200-250 deg. C. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be considered in future energy resource exploration in such regions.

  7. The Volcanic Myths of the Red Sea - Temporal Relationship Between Magmatism and Rifting

    Science.gov (United States)

    Stockli, D. F.; Bosworth, W.

    2017-12-01

    The Cenozoic Red Sea is one of the premier examples of continental rifting and active break-up. It has been cited as an example for both prototypical volcanic, pure shear rift systems with limited crustal stretching as well as magma-poor simple-shear rifting and highly asymmetric rift margins characterized by low-angle normal faults. In light of voluminous Oligocene continental flood basalts in the Afar/Ethiopian region, the Red Sea has often been viewed as a typical volcanic rift, despite evidence for asymmetric extension and hyperextended crust (Zabargad Island). An in-depth analysis of the timing, spatial distribution, and nature of Red Sea volcanism and its relationship to late Cenozoic extensional faulting should shed light on some of the misconceptions. The Eocene appearance of the East African super-plume was not accompanied by any recognized significant extensional faulting or rift-basin formation. The first phase of volcanism more closely associated with the Red Sea occurred in northern Ethiopia and western Yemen at 31-30 Ma and was synchronous with the onset of continental extension in the Gulf of Aden. Early Oligocene volcanism has also been documented in southern and central Saudi Arabia and southern Sudan. However, this voluminous Oligocene volcanism entirely predates Red Sea extensional faulting and rift formation. Marking the onset of Red Sea rifting, widespread, spatially synchronous intrusion of basaltic dikes occurred at 24-21 Ma along the entire Red Sea-Gulf of Suez rift and continuing into northern Egypt. While the initiation of lithospheric extension in the central and northern and central Red Sea and Gulf of Suez was accompanied by only sparse basaltic volcanism and possible underplating, the main phase of rifting in the Miocene Red Sea/Gulf of Suez completely lacks any significant rift-related volcanism, suggesting plate-boundary forces probably drove overall separation of Arabia from Africa. During progressive rifting, there is also no

  8. Melt Origin Across a Rifted Continental Margin: A Case for Subduction-related Metasomatic Agents in the Lithospheric Source of Alkaline Basalt, Northwest Ross Sea, Antarctica

    Science.gov (United States)

    Panter, K. S.; Castillo, P.; Krans, S. R.; Deering, C. D.; McIntosh, W. C.; Valley, J. W.; Kitajima, K.; Kyle, P. R.; Hart, S. R.; Blusztajn, J.

    2017-12-01

    Alkaline magmatism within the West Antarctic rift system in the NW Ross Sea (NWRS) includes a chain of shield volcanoes extending 260 km along the coast, numerous seamounts located on the continental shelf and hundreds more within the oceanic Adare Basin. Dating and geochemistry confirm that the seamounts are Pliocene‒Pleistocene in age and petrogenetically akin to the mostly Miocene volcanism on the continent as well as to a much broader region of alkaline volcanism that altogether encompasses areas of West Antarctica, Zealandia and Australia. All of these regions were contiguous prior to Gondwana breakup at 100 Ma, suggesting that the magmatism is interrelated. Mafic alkaline magmas (> 6 wt.% MgO) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in Si-undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations, LREE/HREE and Nb/Y ratios. Radiogenic isotopes also vary with Nd and Pb ratios increasing and Sr ratios decreasing ocean-ward. The variations are not explained by crustal contamination or by changes in degree of mantle partial melting but are likely a function of the thickness and age of mantle lithosphere. The isotopic signature of the most Si-undersaturated and incompatible element enriched basalts best represent the composition of the sub-lithospheric source with low 87Sr/86Sr (≤ 0.7030) and δ18Oolivine (≤ 5.0 ‰), high 143Nd/144Nd ( 0.5130) and 206Pb/204Pb (≥ 20) ratios. The isotopic `endmember' is derived from recycled material and was transferred to the lithospheric mantle by small degree melts to form amphibole-rich metasomes. Later melting of the metasomes produced silica-undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continent-ward. Ancient or more recent ( 550‒100 Ma) subduction along the margin of Gondwana supplied the recycled

  9. Mid-Continent Rift: Rift, LIP, or Both?

    Science.gov (United States)

    Stein, C. A.; Stein, S. A.; Kley, J.; Hindle, D.; Keller, G. R., Jr.

    2014-12-01

    North America's Midcontinent Rift (MCR) is traditionally considered to have formed by midplate extension and volcanism ~1.1 Ga that ended due to compression from the Grenville orogeny, the ~1.3 - ~0.98 Ga assembly of Amazonia (Precambrian northeast South America), Laurentia (Precambrian North America), and other continents into the supercontinent of Rodinia. We find that a more plausible scenario is that it formed as part of the rifting of Amazonia from Laurentia and became inactive once seafloor spreading was established. The MCR has aspects both of a continental rift - a segmented linear depression filled with sedimentary and igneous rocks - and a large igneous province (LIP). Comparison of areas and volumes for a range of continental LIPS shows that the MCR volcanic rocks are significantly thicker than the others. The MCR flood basalts have steeper dips and thicker overlying sediments than other continental flood basalts, and were deposited in a subsiding basin after most extension ended, indicating that they are better viewed as post-rift than syn-rift rocks. Hence we view the MCR as a LIP deposited in crust weakened by rifting, and thus first a rift and then a LIP.

  10. Active Magmatic Underplating in Western Eger Rift, Central Europe

    Czech Academy of Sciences Publication Activity Database

    Hrubcová, Pavla; Geissler, W.H.; Bräuer, K.; Vavryčuk, Václav; Tomek, Č.; Kämpf, H.

    2017-01-01

    Roč. 36, č. 12 (2017), s. 2846-2862 ISSN 0278-7407 R&D Projects: GA ČR GA17-19297S; GA ČR GC16-19751J Institutional support: RVO:67985530 Keywords : active intraplate magmatic underplating * mantle-derived fluids * high-velocity lower crust * reflection-free magma body Subject RIV: DC - Siesmology, Volcanology, Earth Structure OBOR OECD: Volcanology Impact factor: 3.784, year: 2016

  11. Petrogenesis and tectonic setting of an basalt-Trachyte-Rhyolite suite in the Spilli area (south of Siahkal, north of Iran: evidences of continental rift-related bimodal magmatism in Alborz

    Directory of Open Access Journals (Sweden)

    Shahrooz Haghnazar

    2016-09-01

    Full Text Available The spilli volcanic rocks suite consisting of Basalt- Trachyte- Rhyolite with upper Cretaceous, outcrop in the northern part of Alborz and south of Siahkal area (east of the Guilan province. Based on geochemical data, the studied suite attributed to transitional to alkali series. Negative correlation of Al2O3, CaO, P2O5 and positive correlation of Rb and Th versus SiO2 reveal the occurrence of fractional crystallization process. Also, the negative correlation of Sr versus Y, Sr/Zr versus Sr and CaO/Al2O3 versus SiO2 show that fractionation of plagioclase has played an important role in petrogenesis of the spilli Suite. The hypotheses is supported by the negative anomalies of Eu, Ba and Sr. The overall geochemical evidences indicate that the basic rocks belong to intra-continental rift zone whereas the felsic rocks are classified as A1 type derived from parent basaltic magmas via fractional crystallization in an anorogenic setting. The studied magmatism share many similarities with bimodal magmatism in continental rift zones.

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

    Science.gov (United States)

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

    2009-12-01

    Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has carried out seismic experiments using a multichannel reflection system and ocean bottom seismographs (OBSs) in the Izu-Ogasawara (Bonin)-Mariana (IBM) arc region since 2002 to understand growth process of continental crust. The source was an airgun array with a total capacity of 12,000 cubic inches and the OBSs as the receiver were deployed with an interval of 5 km for all seismic refraction experiments. As the results, we obtained crustal structures across the whole IBM arc with an interval of 50 km and detected the structural characteristics showing the crustal growth process. The IBM arc is one of typical oceanic island arc, which crustal growth started from subduction of an oceanic crust beneath the other oceanic crust. The arc crust has developed through repeatedly magmatic accretion from subduction slab and backarc opening. The volcanism has activated in Eocene, Oligocene, Miocene and Quaternary (e.g., Taylor, 1992), however, these detailed locations of past volcanic arc has been remained as one of unknown issues. In addition, a role of crustal rifting for the crustal growth has also been still unknown issue yet. Our seismic structures show three rows of past volcanic arc crusts except current arc. A rear arc and a forearc side have one and two, respectively. The first one, which was already reported by Kodaira et al. (2008), distributes in northern side from 27 N of the rear arc region. The second one, which develops in the forearc region next to the recent volcanic front, distributes in whole of the Izu-Ogasawara arc having crustal variation along arc direction. Ones of them sometimes have thicker crust than that beneath current volcanic front and no clear topographic high. Last one in the forearc connects to the Ogasawara Ridge. However, thickest crust is not always located beneath these volcanic arcs. The initial rifting region like the northern end of the Mariana Trough and the Sumisu

  13. Surface morphology of active normal faults in hard rock: Implications for the mechanics of the Asal Rift, Djibouti

    Science.gov (United States)

    Pinzuti, Paul; Mignan, Arnaud; King, Geoffrey C. P.

    2010-10-01

    Tectonic-stretching models have been previously proposed to explain the process of continental break-up through the example of the Asal Rift, Djibouti, one of the few places where the early stages of seafloor spreading can be observed. In these models, deformation is distributed starting at the base of a shallow seismogenic zone, in which sub-vertical normal faults are responsible for subsidence whereas cracks accommodate extension. Alternative models suggest that extension results from localised magma intrusion, with normal faults accommodating extension and subsidence only above the maximum reach of the magma column. In these magmatic rifting models, or so-called magmatic intrusion models, normal faults have dips of 45-55° and root into dikes. Vertical profiles of normal fault scarps from levelling campaign in the Asal Rift, where normal faults seem sub-vertical at surface level, have been analysed to discuss the creation and evolution of normal faults in massive fractured rocks (basalt lava flows), using mechanical and kinematics concepts. We show that the studied normal fault planes actually have an average dip ranging between 45° and 65° and are characterised by an irregular stepped form. We suggest that these normal fault scarps correspond to sub-vertical en echelon structures, and that, at greater depth, these scarps combine and give birth to dipping normal faults. The results of our analysis are compatible with the magmatic intrusion models instead of tectonic-stretching models. The geometry of faulting between the Fieale volcano and Lake Asal in the Asal Rift can be simply related to the depth of diking, which in turn can be related to magma supply. This new view supports the magmatic intrusion model of early stages of continental breaking.

  14. Tectonics and stratigraphy of the East Brazil Rift system: an overview

    Science.gov (United States)

    Hung Kiang Chang; Kowsmann, Renato Oscar; Figueiredo, Antonio Manuel Ferreira; Bender, AndréAdriano

    1992-10-01

    The East Brazilian Rift system (Ebris) constitutes the northern segment of the South Atlantic rift system which developed during the Mesozoic breakup of South America and Africa. Following crustal separation in the Late Aptian, it evolved into a passive continental margin. Along the continental margin six basins are recognized, while three onshore basins form part of an aborted rift. Three continental syn-rift stratigraphic sequences are recognized, spanning Jurassic to Barremian times. The Jurassic (Syn-rift I) and Neocomian (Syn-rift II) phases were most active in the interior rift basins. During the Barremian (Syn-rift III), rift subsidence rates were twice as large as during the Neocomian (Syn-rift II), both in the interior rift and in the marginal rift segments, indicating that rift axis did not migrate from the interior to the marginal setting. Rift magmatism was centered on the southern EBRIS and peaked between 130 and 120 Ma during syn-rift phase II. Rift phase III was followed by a transitional marine, evaporitic megasequence of Aptian age, which directly overlies the rift unconformity and a marine drift megasequence which spans Albian to Recent times. During the Late Cretaceous, sedimentation rates responded to first-order eustatic sea-level fluctuations. Tertiary accelerated sedimentation rates can be related to local clastic supply which filled in spaces inherited from previous starved conditions. Between 60 and 40 Ma, post-rift magmatism, centered on the Abrolhos and Royal Charlotte banks, is probably related to development of a hot spot associated with the Vitória-Trindade Seamount Chain. Although crossing three distinct Precambrian tectono-thermal provinces, ranging from Archean through Late Proterozoic, rift structures follow a general NE trend, subparallel to the principal basement fabric. A NW-SE oriented stress field appears to be compatible with both Neocomian and Barremian phases of crustal extension. Profiles transverse to the rift axis

  15. Imaging an off-axis volcanic field in the Main Ethiopian Rift using 3-D magnetotellurics

    Science.gov (United States)

    Huebert, J.; Whaler, K. A.; Fisseha, S.; Hogg, C.

    2017-12-01

    In active continental rifts, asthenospheric upwelling and crustal thinning result in the ascent of melt through the crust to the surface. In the Main Ethiopian Rift (MER), most volcanic activity is located in magmatic segments in the rift centre, but there are areas of significant off-axis magmatism as well. The Butajira volcanic field is part of the Silti Debre Zeyt Fault (SDZF) zone in the western Main Ethiopian Rift. It is characterized by densely clustered volcanic vents (mostly scoria cones) and by limited seismic activity, which is mainly located along the big border faults that form the edge of a steep escarpment. Seismic P-Wave tomography reveals a crustal low velocity anomaly in this area. We present newly collected Magnetotelluric (MT) data to image the electrical conductivity structure of the area. We deployed 12 LMT instruments and 27 broadband stations in the western flank of the rift to further investigate the along-rift and depth extent of a highly conductive region under the SDZF which was previously identified by MT data collected on the central volcano Aluto and along a cross-rift transverse. This large conductor was interpreted as potential pathways for magma and fluid in the crust. MT Stations were positioned in five NW-SE running 50 km long profiles, covering overall 100km along the rift and providing good coverage for a 3-D inversion of the data to image this enigmatic area of the MER.

  16. Estimation of age of Dali-Ganis rifting and associated volcanic activity, Venus

    Science.gov (United States)

    Basilevsky, A. T.

    1993-01-01

    This paper deals with the estimation of age for the Dali and Ganis Chasma rift zones and their associated volcanism based on photogeologic analysis of stratigraphic relations of rift-associated features with impact craters which have associated features indicative of their age. The features are radar-dark and parabolic, and they are believed to be mantles of debris derived from fallout of the craters' ejecta. They are thought to be among the youngest features on the Venusian surface, so their 'parent' craters must also be very young, evidently among the youngest 10 percent of Venus' crater population. Dali Chasma and Ganis Chasma are a part of a system of rift zones contained within eastern Aphrodite and Atla Regio which is a significant component of Venus tectonics. The rifts of this system are fracture belts which dissect typical Venusian plains with rare islands of tessera terrain. The rift zone system consists of several segments following each other (Diane, Dali, Ganis) and forming the major rift zone line, about 10,000 km long, which has junctions with several other rift zones, including Parga Chasma Rift. The junctions are usually locations of rift-associated volcanism in the form of volcanic edifices (Maat and Ozza Montes) or plain-forming flows flooding some areas within the rift zones and the adjacent plains.

  17. A new perspective on evolution of the Baikal Rift

    Directory of Open Access Journals (Sweden)

    Victor D. Mats

    2011-07-01

    The three-stage model of the rift history does not rule out the previous division into two major stages but rather extends its limits back into time as far as the Maastrichtian. Our model is consistent with geological, stratigraphic, structural, and geophysical data and provides further insights into the understanding of rifting in the Baikal region in particular and continental rifting in general.

  18. Quaternary volcano-tectonic activity in the Soddo region, western margin of the Southern Main Ethiopian Rift

    NARCIS (Netherlands)

    Corti, G.; Sani, F.; Philippon, M.; Sokoutis, D.; Willingshofer, E.; Molin, P.

    We present an analysis of the distribution, timing, and characteristics of the volcano-tectonic activity on the western margin of the Southern Main Ethiopian Rift in the Soddo area (latitudes between ~7°10'N and ~6°30'N). The margin is characterized by the presence of numerous normal faults, with

  19. Seismic imaging of the geodynamic activity at the western Eger rift in central Europe

    Czech Academy of Sciences Publication Activity Database

    Mullick, N.; Buske, S.; Hrubcová, Pavla; Růžek, Bohuslav; Shapiro, S.; Wigger, P.; Fischer, T.

    647-648, 19 April (2015), s. 105-111 ISSN 0040-1951 R&D Projects: GA ČR GA13-08971S Institutional support: RVO:67985530 Keywords : European Cenozoic Rift System * Eger Rift * West Bohemian Massif Subject RIV: DC - Siesmology, Volcanology, Earth Structure Impact factor: 2.650, year: 2015

  20. Is the Proterozoic Ladoga Rift (SE Baltic Shield) a rift?

    DEFF Research Database (Denmark)

    Artemieva, Irina; Shulgin, Alexey

    2015-01-01

    , and geophysical characteristics typical of continental rifts in general and demonstrate that, except for magmatic and, perhaps, some gravity signature, the Lake Ladoga region lacks any other rift features. We also compare the geophysical data from the Lake Ladoga region with similar in age Midcontinent and Valday...... interpreted as an intracratonic Ladoga rift (graben). We question the validity of this geodynamic interpretation by analyzing regional geophysical data (crustal structure, heat flow, Bouguer gravity anomalies, magnetic anomalies, and mantle Vs velocities). We provide a complete list of tectonic, magmatic...... rifts, and provide alternative explanations for Mesoproterozoic geodynamic evolution of the southern Baltic Shield. We propose that Mesoproterozoic mafic intrusions in southern Fennoscandia may be associated with a complex deformation pattern during reconfiguration of (a part of) Nuna (Columbia...

  1. How does continental lithosphere break-apart? A 3D seismic view on the transition from magma-poor rifted margin to magmatic oceanic lithosphere

    Science.gov (United States)

    Emmanuel, M.; Lescanne, M.; Picazo, S.; Tomasi, S.

    2017-12-01

    In the last decade, high-quality seismic data and drilling results drastically challenged our ideas about how continents break apart. New models address their observed variability and are presently redefining basics of rifting as well as exploration potential along deepwater rifted margins. Seafloor spreading is even more constrained by decades of scientific exploration along Mid Oceanic Ridges. By contrast, the transition between rifting and drifting remains a debated subject. This lithospheric breakup "event" is geologically recorded along Ocean-Continent Transitions (OCT) at the most distal part of margins before indubitable oceanic crust. Often lying along ultra-deepwater margin domains and buried beneath a thick sedimentary pile, high-quality images of these domains are rare but mandatory to get strong insights on the processes responsible for lithospheric break up and what are the consequences for the overlying basins. We intend to answer these questions by studying a world-class 3D seismic survey in a segment of a rifted margin exposed in the Atlantic. Through these data, we can show in details the OCT architecture between a magma-poor hyper-extended margin (with exhumed mantle) and a classical layered oceanic crust. It is characterized by 1- the development of out-of-sequence detachment systems with a landward-dipping geometry and 2- the increasing magmatic additions oceanwards (intrusives and extrusives). Geometry of these faults suggests that they may be decoupled at a mantle brittle-ductile interface what may be an indicator on thermicity. Furthermore, magmatism increases as deformation migrates to the future first indubitable oceanic crust what controls a progressive magmatic crustal thickening below, above and across a tapering rest of margin. As the magmatic budget increases oceanwards, full-rate divergence is less and less accommodated by faulting. Magmatic-sedimentary architectures of OCT is therefore changing from supra-detachment to magmatic

  2. Concentration of strain in a marginal rift zone of the Japan backarc during post-rift compression

    Science.gov (United States)

    Sato, H.; Ishiyama, T.; Kato, N.; Abe, S.; Shiraishi, K.; Inaba, M.; Kurashimo, E.; Iwasaki, T.; Van Horne, A.; No, T.; Sato, T.; Kodaira, S.; Matsubara, M.; Takeda, T.; Abe, S.; Kodaira, C.

    2015-12-01

    Late Cenozoic deformation zones in Japan may be divided into two types: (1) arc-arc collision zones like those of Izu and the Hokkaido axial zone, and (2) reactivated back-arc marginal rift (BMR) systems. A BMR develops during a secondary rifting event that follows the opening of a back-arc basin. It forms close to the volcanic front and distant from the spreading center of the basin. In Japan, a BMR system developed along the Sea of Japan coast following the opening of the Japan Sea. The BMR appears to be the weakest, most deformable part of the arc back-arc system. When active rifting in the marginal basins ended, thermal subsidence, and then mechanical subsidence related to the onset of a compressional stress regime, allowed deposition of up to 5 km of post-rift, deep-marine to fluvial sedimentation. Continued compression produced fault-related folds in the post-rift sediments, in thin-skin style deformation. Shortening reached a maximum in the BMR system compared to other parts of the back-arc, suggesting that it is the weakest part of the entire system. We examined the structure of the BMR system using active source seismic investigation and earthquake tomography. The velocity structure beneath the marginal rift basin shows higher P-wave velocity in the upper mantle/lower crust which suggests significant mafic intrusion and thinning of the upper continental crust. The syn-rift mafic intrusive forms a convex shape, and the boundary between the pre-rift crust and the mafic intrusive dips outward. In the post-rift compressional stress regime, the boundary of the mafic body reactivated as a reverse fault, forming a large-scale wedge thrust and causing further subsidence of the rift basin. The driver of the intense shortening event along the Sea of Japan coast in SW Japan was the arrival of a buoyant young (15 Ma) Shikoku basin at the Nankai Trough. Subduction stalled and the backarc was compressed. As the buoyant basin cooled, subduction resumed, and the rate of

  3. Imaging rifting at the lithospheric scale in the northern East African Rift using S-to-P receiver functions

    Science.gov (United States)

    Lavayssiere, A.; Rychert, C.; Harmon, N.; Keir, D.; Hammond, J. O. S.; Kendall, J. M.; Leroy, S. D.; Doubre, C.

    2017-12-01

    The lithosphere is modified during rifting by a combination of mechanical stretching, heating and potentially partial melt. We image the crust and upper mantle discontinuity structure beneath the northern East African Rift System (EARS), a unique tectonically active continental rift exposing along strike the transition from continental rifting in the Main Ethiopian rift (MER) to incipient seafloor spreading in Afar and the Red Sea. S-to-P receiver functions from 182 stations across the northern EARS were generated from 3688 high quality waveforms using a multitaper technique and then migrated to depth using a regional velocity model. Waveform modelling of data stacked in large conversion point bins confirms the depth and strength of imaged discontinuities. We image the Moho at 29.6±4.7 km depth beneath the Ethiopian plateaux with a variability in depth that is possibly due to lower crustal intrusions. The crust is 27.3±3.9 km thick in the MER and thinner in northern Afar, 17.5±0.7 km. The model requires a 3±1.2% reduction in shear velocity with increasing depth at 68.5±1.5 km beneath the Ethiopian plateaux, consistent with the lithosphere-asthenosphere boundary (LAB). We do not resolve a LAB beneath Afar and the MER. This is likely associated with partial melt near the base of the lithosphere, reducing the velocity contrast between the melt-intruded lithosphere and the partially molten asthenosphere. We identify a 4.5±0.7% increase in velocity with depth at 91±3 km beneath the MER. This change in velocity is consistent with the onset of melting found by previous receiver functions and petrology studies. Our results provide independent constraints on the depth of melt production in the asthenosphere and suggest melt percolation through the base of the lithosphere beneath the northernmost East African rift.

  4. Time evolution of a rifted continental arc: Integrated ID-TIMS and LA-ICPMS study of magmatic zircons from the Eastern Srednogorie, Bulgaria

    Science.gov (United States)

    Georgiev, S.; von Quadt, A.; Heinrich, C. A.; Peytcheva, I.; Marchev, P.

    2012-12-01

    Eastern Srednogorie in Bulgaria is the widest segment of an extensive magmatic arc that formed by convergence of Africa and Europe during Mesozoic to Tertiary times. Northward subduction of the Tethys Ocean beneath Europe in the Late Cretaceous gave rise to a broad range of basaltic to more evolved magmas with locally associated Cu-Au mineralization along this arc. We used U-Pb geochronology of single zircons to constrain the temporal evolution of the Upper Cretaceous magmatism and the age of basement rocks through which the magmas were emplaced in this arc segment. High precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) was combined with laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) for spatial resolution within single zircon grains. Three tectono-magmatic regions are distinguished from north to south within Eastern Srednogorie: East Balkan, Yambol-Burgas and Strandzha. Late Cretaceous magmatic activity started at ~ 92 Ma in the northernmost East Balkan region, based on stratigraphic evidence and limited geochronology, with the emplacement of minor shallow intrusions and volcanic rocks onto pre-Cretaceous basement. In the southernmost Strandzha region, magmatism was initiated at ~ 86 Ma with emplacement of gabbroic to dioritic intrusions and related dikes into metamorphic basement rocks that have previously been overprinted by Jurassic-Lower Cretaceous metamorphism. The Yambol-Burgas region is an extensional basin between the East Balkan and the Strandzha regions, which broadens and deepens toward the Black Sea further east and is filled with a thick pile of marine sediments and submarine extrusive volcanic rocks accompanied by coeval intrusions. This dominantly mafic magmatism in the intermediate Yambol-Burgas region commenced at ~ 81 Ma and produced large volumes of potassium-rich magma until ~ 78 Ma. These shoshonitic to ultrapotassic basaltic to intermediate magmas formed by differentiation of ankaramitic (high

  5. Two-Dimensional Numerical Modeling of Intracontinental Extension: A Case Study Of the Baikal Rift Formation

    DEFF Research Database (Denmark)

    Yang, H.; Chemia, Zurab; Artemieva, Irina

    The Baikal Rift zone (BRZ) is a narrow ( 10 km) active intra-continental basin, located at the boundary between the Amurian and Eurasian Plates. Although the BRZ is one of the major tectonically active rift zones in the world andit has been a subject of numerous geological...... on topography,basin depth, the structure of the crust, lithosphere thickness, and the location of major tectonic faults. Our goal is to determine the physical models that reproduce reasonably well the ob-served deformation patterns of the BRZ.We perform a systematic analysis of the pa-rameter space in order...

  6. The temporal and spatial distribution of upper crustal faulting and magmatism in the south Lake Turkana rift, East Africa

    Science.gov (United States)

    Muirhead, J.; Scholz, C. A.

    2017-12-01

    During continental breakup extension is accommodated in the upper crust largely through dike intrusion and normal faulting. The Eastern branch of the East African Rift arguably represents the premier example of active continental breakup in the presence magma. Constraining how faulting is distributed in both time and space in these regions is challenging, yet can elucidate how extensional strain localizes within basins as rifting progresses to sea-floor spreading. Studies of active rifts, such as the Turkana Rift, reveal important links between faulting and active magmatic processes. We utilized over 1100 km of high-resolution Compressed High Intensity Radar Pulse (CHIRP) 2D seismic reflection data, integrated with a suite of radiocarbon-dated sediment cores (3 in total), to constrain a 17,000 year history of fault activity in south Lake Turkana. Here, a set of N-S-striking intra-rift faults exhibit time-averaged slip-rates as high as 1.6 mm/yr, with the highest slip-rates occurring along faults within 3 km of the rift axis. Results show that strain has localized into a zone of intra-rift faults along the rift axis, forming an approximately 20 km-wide graben in central parts of the basin. Subsurface structural mapping and fault throw profile analyses reveal increasing basin subsidence and fault-related strain as this faulted graben approaches a volcanic island in the center of the basin (South Island). The long-axis of this island trends north-south, and it contains a number of elongate cones that support recent emplacement of N-S-striking dike intrusions, which parallel recently active intra-rift faults. Overall, these observations suggest strain localization into intra-rift faults in the rift center is likely a product of both volcanic loading and the mechanical and thermal effects of diking along the rift axis. These results support the establishment of magmatic segmentation in southern Lake Turkana, and highlight the importance of magmatism for focusing upper

  7. Integrated Remote Sensing and Geophysical Investigations of the Geodynamic Activities at Lake Magadi, Southern Kenyan Rift

    Directory of Open Access Journals (Sweden)

    Akinola Adesuji Komolafe

    2012-01-01

    Full Text Available The tectonic lineaments and thermal structure of Lake Magadi, southern Kenyan rift system, were investigated using ASTER data and geophysical methods. Five N-S faults close to known hot springs were identified for geoelectric ground investigation. Aeromagnetic data were employed to further probe faults at greater depths and determine the Curie-point depth. Results indicate a funnel-shaped fluid-filled (mostly saline hydrothermal zone with relatively low resistivity values of less than 1 Ω-m, separated by resistive structures to the west and east, to a depth of 75 m along the resistivity profiles. There was evidence of saline hydrothermal fluid flow toward the surface through the fault splays. The observed faults extend from the surface to a depth of 7.5 km and are probably the ones that bound the graben laterally. They serve as major conduits for the upward heat flux in the study area. The aeromagnetics spectral analysis also revealed heat source emplacement at a depth of about 12 km. The relative shallowness implies a high geothermal gradient evidenced in the surface manifestations of hot springs along the lake margins. Correlation of the heat source with the hypocenters showed that the seismogenetic zone exists directly above the magmatic intrusion, forming the commencement of geodynamic activities.

  8. The role of signal transducer and activator of transcription 3 in Rift Valley fever virus infection

    International Nuclear Information System (INIS)

    Pinkham, Chelsea; An, Soyeon; Lundberg, Lindsay; Bansal, Neha; Benedict, Ashwini; Narayanan, Aarthi; Kehn-Hall, Kylene

    2016-01-01

    Rift Valley fever (RVF) is a zoonotic disease that can cause severe illness in humans and livestock, triggering spontaneous abortion in almost 100% of pregnant ruminants. In this study, we demonstrate that signal transducer and activator of transcription 3 (STAT3) is phosphorylated on its conserved tyrosine residue (Y705) following RVFV infection. This phosphorylation was dependent on a major virulence factor, the viral nonstructural protein NSs. Loss of STAT3 had little effect on viral replication, but rather resulted in cells being more susceptible to RVFV-induced cell death. Phosphorylated STAT3 translocated to the nucleus, coinciding with inhibition of fos, jun, and nr4a2 gene expression, and the presence of STAT3 and NSs at the nr4a2 promoter. NSs was found predominantly in the cytoplasm of STAT3 null cells, indicating that STAT3 influences NSs nuclear localization. Collectively, these data demonstrate that STAT3 functions in a pro-survival capacity through modulation of NSs localization. - Highlights: • STAT3 is phosphorylated on tyrosine residue 705 following RVFV infection. • Phosphorylation of STAT3 was dependent on the viral protein NSs. • STAT3 -/- MEFs were more susceptible to RVFV-induced cell death. • Loss of STAT3 led to an increase in pro-apoptotic gene expression. • STAT3 functions in a pro-survival capacity by modulation of NSs localization.

  9. p53 Activation following Rift Valley fever virus infection contributes to cell death and viral production.

    Directory of Open Access Journals (Sweden)

    Dana Austin

    Full Text Available Rift Valley fever virus (RVFV is an emerging viral zoonosis that is responsible for devastating outbreaks among livestock and is capable of causing potentially fatal disease in humans. Studies have shown that upon infection, certain viruses have the capability of utilizing particular cellular signaling pathways to propagate viral infection. Activation of p53 is important for the DNA damage signaling cascade, initiation of apoptosis, cell cycle arrest and transcriptional regulation of multiple genes. The current study focuses on the role of p53 signaling in RVFV infection and viral replication. These results show an up-regulation of p53 phosphorylation at several serine sites after RVFV MP-12 infection that is highly dependent on the viral protein NSs. qRT-PCR data showed a transcriptional up-regulation of several p53 targeted genes involved in cell cycle and apoptosis regulation following RVFV infection. Cell viability assays demonstrate that loss of p53 results in less RVFV induced cell death. Furthermore, decreased viral titers in p53 null cells indicate that RVFV utilizes p53 to enhance viral production. Collectively, these experiments indicate that the p53 signaling pathway is utilized during RVFV infection to induce cell death and increase viral production.

  10. The role of signal transducer and activator of transcription 3 in Rift Valley fever virus infection

    Energy Technology Data Exchange (ETDEWEB)

    Pinkham, Chelsea; An, Soyeon; Lundberg, Lindsay; Bansal, Neha; Benedict, Ashwini; Narayanan, Aarthi; Kehn-Hall, Kylene, E-mail: kkehnhal@gmu.edu

    2016-09-15

    Rift Valley fever (RVF) is a zoonotic disease that can cause severe illness in humans and livestock, triggering spontaneous abortion in almost 100% of pregnant ruminants. In this study, we demonstrate that signal transducer and activator of transcription 3 (STAT3) is phosphorylated on its conserved tyrosine residue (Y705) following RVFV infection. This phosphorylation was dependent on a major virulence factor, the viral nonstructural protein NSs. Loss of STAT3 had little effect on viral replication, but rather resulted in cells being more susceptible to RVFV-induced cell death. Phosphorylated STAT3 translocated to the nucleus, coinciding with inhibition of fos, jun, and nr4a2 gene expression, and the presence of STAT3 and NSs at the nr4a2 promoter. NSs was found predominantly in the cytoplasm of STAT3 null cells, indicating that STAT3 influences NSs nuclear localization. Collectively, these data demonstrate that STAT3 functions in a pro-survival capacity through modulation of NSs localization. - Highlights: • STAT3 is phosphorylated on tyrosine residue 705 following RVFV infection. • Phosphorylation of STAT3 was dependent on the viral protein NSs. • STAT3 -/- MEFs were more susceptible to RVFV-induced cell death. • Loss of STAT3 led to an increase in pro-apoptotic gene expression. • STAT3 functions in a pro-survival capacity by modulation of NSs localization.

  11. The MOZART Project - MOZAmbique Rift Tomography

    Science.gov (United States)

    Fonseca, J. F.; Chamussa, J. R.; Domingues, A.; Helffrich, G. R.; Fishwick, S.; Ferreira, A. M.; Custodio, S.; Brisbourne, A. M.; Grobbelaar, M.

    2012-12-01

    Project MOZART (MOZAmbique Rift Tomography) is an ongoing joint effort of Portuguese, Mozambican and British research groups to investigate the geological structure and current tectonic activity of the southernmost tip of the East African Rift System (EARS) through the deployment of a network of 30 broad band seismic stations in Central and Southern Mozambique. In contrast with other stretches of the EARS to the North and with the Kapvaal craton to the West and South, the lithosphere of Mozambique was not previously studied with a dense seismographic deployment on account of past political instability, and many questions remain unanswered with respect to the location and characteristics of the EARS to the south of Tanzania. In recent years, space geodesy revealed the existence of three microplates in and off Mozambique - Victoria, Rovuma, Lwandle - whose borders provide a connection of the EARS to the South West Indian Ridge as required by plate tectonics. However, the picture is still coarse concerning the location of the rift structures. The 2006 M7 Machaze earthquake in Central Mozambique highlighted the current tectonic activity of the region and added a further clue to the location of the continental rift, prompting the MOZART deployment. Besides helping unravel the current tectonics, the project is expected to shed light on the poorly known Mesoproterozoic structure described by Arthur Holmes in 1951 as the Mozambique Belt, and on the mechanisms of transition from stable craton to rifted continental crust, through the development of a tomographic model for the lithosphere. The MOZART network is distributed South of the Zambezi river at average inter-station spaces of the order of 100 km and includes four stations across the border in South Africa. Data exchange was agreed with AfricaArray. The deployment proceeded in two phases in March 2011, and November and December 2011. Decommissioning is foreseen for August 2013. We report preliminary results for this

  12. Geological constraints on continental arc activity since 720 Ma: implications for the link between long-term climate variability and episodicity of continental arcs

    Science.gov (United States)

    Cao, W.; Lee, C. T.

    2016-12-01

    Continental arc volcanoes have been suggested to release more CO2 than island arc volcanoes due to decarbonation of wallrock carbonates in the continental upper plate through which the magmas traverse (Lee et al., 2013). Continental arcs may thus play an important role in long-term climate. To test this hypothesis, we compiled geological maps to reconstruct the surface distribution of granitoid plutons and the lengths of ancient continental arcs. These results were then compiled into a GIS framework and incorporated into GPlates plate reconstructions. Our results show an episodic nature of global continental arc activity since 720 Ma. The lengths of continental arcs were at minimums during most of the Cryogenian ( 720-670 Ma), the middle Paleozoic ( 460-300 Ma) and the Cenozoic ( 50-0 Ma). Arc lengths were highest during the Ediacaran ( 640-570 Ma), the early Paleozoic ( 550-430 Ma) and the entire Mesozoic with peaks in the Early Triassic ( 250-240 Ma), Late Jurassic-Early Cretaceous ( 160-130 Ma), and Late Cretaceous ( 90-65 Ma). The extensive continental arcs in the Ediacaran and early Paleozoic reflect the Pan-African events and circum-Gondwana subduction during the assembly of the Gondwana supercontinent. The Early Triassic peak is coincident with the final closure of the paleo-Asian oceans and the onset of circum-Pacific subduction associated with the assembly of the Pangea supercontinent. The Jurassic-Cretaceous peaks reflect the extensive continental arcs established in the western Pacific, North and South American Cordillera, coincident with the initial dispersal of the Pangea. Continental arcs are favored during the final assembly and the early-stage dispersal of a supercontinent. Our compilation shows a temporal match between continental arc activity and long-term climate at least since 720 Ma. For example, continental arc activity was reduced during the Cryogenian icehouse event, and enhanced during the Early Paleozoic and Jurassic-Cretaceous greenhouse

  13. On causal links between flood basalts and continental breakup

    Science.gov (United States)

    Courtillot, V.; Jaupart, C.; Manighetti, I.; Tapponnier, P.; Besse, J.

    1999-03-01

    Temporal coincidence between continental flood basalts and breakup has been noted for almost three decades. Eight major continental flood basalts have been produced over the last 300 Ma. The most recent, the Ethiopian traps, erupted in about 1 Myr at 30 Ma. Rifting in the Red Sea and Gulf of Aden, and possibly East African rift started at about the same time. A second trap-like episode occurred around 2 Ma and formation of true oceanic crust is due in the next few Myr. We find similar relationships for the 60 Ma Greenland traps and opening of the North Atlantic, 65 Ma Deccan traps and opening of the NW Indian Ocean, 132 Ma Parana traps and South Atlantic, 184 Ma Karoo traps and SW Indian Ocean, and 200 Ma Central Atlantic Margin flood basalts and opening of the Central Atlantic Ocean. The 250 Ma Siberian and 258 Ma Emeishan traps seem to correlate with major, if aborted, phases of rifting. Rifting asymmetry, apparent triple junctions and rift propagation (towards the flood basalt area) are common features that may, together with the relative timings of flood basalt, seaward dipping reflector and oceanic crust production, depend on a number of plume- and lithosphere- related factors. We propose a mixed scenario of `active/passive' rifting to account for these observations. In all cases, an active component (a plume and resulting flood basalt) is a pre-requisite for the breakup of a major oceanic basin. But rifting must be allowed by plate-boundary forces and is influenced by pre-existing heterogeneities in lithospheric structure. The best example is the Atlantic Ocean, whose large-scale geometry with three large basins was imposed by the impact points of three mantle plumes.

  14. Fault Growth and Propagation and its Effect on Surficial Processes within the Incipient Okavango Rift Zone, Northwest Botswana, Africa (Invited)

    Science.gov (United States)

    Atekwana, E. A.

    2010-12-01

    The Okavango Rift Zone (ORZ) is suggested to be a zone of incipient continental rifting occuring at the distal end of the southwestern branch of the East African Rift System (EARS), therefore providing a unique opportunity to investigate neotectonic processes during the early stages of rifting. We used geophysical (aeromagnetic, magnetotelluric), Shuttle Radar Tomography Mission, Digital Elevation Model (SRTM-DEM), and sedimentological data to characterize the growth and propagation of faults associated with continental extension in the ORZ, and to elucidate the interplay between neotectonics and surficial processes. The results suggest that: (1) fault growth occurs by along axis linkage of fault segments, (2) an immature border fault is developing through the process of “Fault Piracy” by fault-linkages between major fault systems, (3) significant discrepancies exits between the height of fault scarps and the throws across the faults compared to their lengths in the basement, (4) utilization of preexisting zones of weakness allowed the development of very long faults (> 25-100 km) at a very early stage of continental rifting, explaining the apparent paradox between the fault length versus throw for this young rift, (5) active faults are characterized by conductive anomalies resulting from fluids, whereas, inactive faults show no conductivity anomaly; and 6) sedimentlogical data reveal a major perturbation in lake sedimentation between 41 ka and 27 ka. The sedimentation perturbation is attributed to faulting associated with the rifting and may have resulted in the alteration of hydrology forming the modern day Okavango delta. We infer that this time period may represent the age of the latest rift reactivation and fault growth and propagation within the ORZ.

  15. Comparison of hydrothermal activity between the Adriatic and the Red Sea rift margins

    Science.gov (United States)

    Ball, Philip; Incerpi, Nicolò; Birkle, Peter; Lacsamana, Elizabeth; Manatschal, Gianreto; Agar, Susan; Zhang, Shuo; Borsato, Ron

    2017-04-01

    Detailed field studies, and access to high-quality seismic reflection and refraction data have led to an improved understanding of the architecture and evolution of magma poor and magma rich margins. Associated with the spatial-temporal evolution of the rift, it is evident that there are evolving, extensive, fluid-rock interactions due to the infiltration of fluids within the sediment, basement and lithospheric mantle. Key questions therefore arise: What are the different fluid-rock reactions that can be typed to different geodynamic stages of the rift evolution? What are their compositions and how do they interact with their environment (basement, sediments, evaporites, hydrosphere, and magmatism)? What are the implications for the evolution of the margin rheology, thermal structure, depositional environments/organic matter maturity, and reservoir quality? The Adriatic paleo-rifted margin is preserved in both SE Switzerland and northern Italy. The field exposures provide a unique opportunity to study the fluid flow history of a hyperextended magma poor extensional margin. Analysis of breccias, cement veins and replacement minerals reveal that the margin records a complex, long-lasting history of dolomitization, calcification and silicification during the Jurassic rifting. The Red Sea by contrast is a young rifted margin. It differs from the paleo-Adriatic margin by several characteristics: volcanism is more evident, and syn-tectonic sediments, including evaporites (halite and anhydrite) are thicker. Several core and fluid samples are available from both onshore and offshore wells, which reveal rift-related hydrothermal alteration. In addition, we find evidence for the presence of an extreme dynamic hydraulic system with infiltration of surface water into sub-salt units during Late Pleistocene. In this study we present results from petrographic and geochemical analysis of basement and sedimentary rocks from Adriatic field-derived samples and core/subsurface fluid

  16. Episodic nature of continental arc activity since 750 Ma: A global compilation

    Science.gov (United States)

    Cao, Wenrong; Lee, Cin-Ty A.; Lackey, Jade Star

    2017-03-01

    Continental arcs have been recently hypothesized to outflux large amounts of CO2 compared to island arcs so that global flare-ups in continental arc magmatism might drive long-term greenhouse events. Quantitative testing of this hypothesis, however, has been limited by the lack of detailed studies on the spatial distribution of continental arcs through time. Here, we compile a worldwide database of geological maps and associated literature to delineate the surface exposure of granitoid plutons, allowing reconstruction of how the surface area addition rate of granitoids and the length of continental arcs have varied since 750 Ma. These results were integrated into an ArcGIS framework and plate reconstruction models. We find that the spatial extent of continental arcs is episodic with time and broadly matches the detrital zircon age record. Most vigorous arc magmatism occurred during the 670-480 Ma and the 250-50 Ma when major greenhouse events are recognized. Low continental arc activity characterized most of the Cryogenian, middle-late Paleozoic, and Cenozoic when climate was cold. Our results indicate that plate tectonics is not steady, with fluctuations in the nature of subduction zones possibly related in time to the assembly and dispersal of continents. Our results corroborate the hypothesis that variations in continental arc activity may play a first order role in driving long-term climate change. The dataset presented here provides a quantitative basis for upscaling continental arc processes to explore their effects on mountain building, climate, and crustal growth on a global scale.

  17. A temperature-limited assessment of the risk of Rift Valley fever transmission and establishment in the continental United States of America

    Directory of Open Access Journals (Sweden)

    Sarah K. Konrad

    2012-05-01

    Full Text Available The rapid spread of West Nile virus across North America after its introduction in 1999 highlights the potential for foreign arboviruses to become established in the United States of America. Of particular concern is Rift Valley fever virus (RVFV, which has been responsible for multiple African epidemics resulting in death of both humans and livestock, as well as major economic disruption due to livestock loss and trade restrictions. Modern globalization, travel, and commerce allow viruses to easily jump from one continent to another; and it is likely only a matter of time before RVFV reaches North American shores. We used a degree-day model in combination with livestock population data and a pathways analysis to identify regions and times where RVFV is most likely to enter and become established in the United States of America. Transmission risk of the disease varies across the country from 325 annual risk days in parts of Florida to zero risk days in the far North and in high mountain regions. Areas of particular concern are where there are a high number of possible transmission days, a large livestock population, and proximity to likely locations for the disease to enter the country via mosquito vector or human host. These areas should be monitored closely during transmission “risk seasons” so that if the virus does enter the country and begins to become established, it can be quickly controlled and eliminated before spreading further. Areas most at risk include the Baltimore and New York City metro areas as well as much of the region between these urban centers; most of Texas, especially around Houston; Florida; Atlanta; southwest Nebraska; southern California and Arizona; and the central valley of California.

  18. 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 geothermal gradients (up to 60°C/km) known in the region, and higher temperature closer to the pre-rift 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.

  19. The Role of Rift Obliquity in Formation of the Gulf of California

    Science.gov (United States)

    Bennett, Scott Edmund Kelsey

    The Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to large-offset normal faults in adjacent pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic basins. The Gulf of California rift has accommodated oblique divergence of the Pacific and North America tectonic plates in northwestern Mexico since Miocene time. Due to its infancy, the rifted margins of the Gulf of California preserve a rare onshore record of early continental break-up processes from which to investigate the role of rift obliquity in strain localization. Using new high-precision paleomagnetic vectors from tectonically stable sites in north-central Baja California, I compile a paleomagnetic transect of Miocene ignimbrites across northern Baja California and Sonora that reveals the timing and distribution of dextral shear associated with inception of this oblique rift. I integrate detailed geologic mapping, basin analysis, and geochronology of pre-rift and syn-rift volcanic units to determine the timing of fault activity on Isla Tiburon, a proximal onshore exposure of the rifted North America margin, adjacent to the axis of the Gulf of California. The onset of strike-slip faulting on Isla Tiburon, ca. 8 - 7 Ma, was synchronous with the onset of transform faulting along a significant length of the nascent plate boundary within the rift. This tectonic transition coincides with a clockwise azimuthal shift in Pacific-North America relative motion that increased rift obliquity. I constrain the earliest marine conditions on southwest Isla Tiburon to ca. 6.4 - 6.0 Ma, coincident with a regional latest Miocene marine incursion in the northern proto-Gulf of California. This event likely flooded a narrow, incipient topographic depression along a ˜650 km-long portion of the latest Miocene plate boundary and corresponds in time and space with formation of a newly

  20. Extension parallel to the rift zone during segmented fault growth: application to the evolution of the NE Atlantic

    Directory of Open Access Journals (Sweden)

    A. Bubeck

    2017-11-01

    Full Text Available The mechanical interaction of propagating normal faults is known to influence the linkage geometry of first-order faults, and the development of second-order faults and fractures, which transfer displacement within relay zones. Here we use natural examples of growth faults from two active volcanic rift zones (Koa`e, island of Hawai`i, and Krafla, northern Iceland to illustrate the importance of horizontal-plane extension (heave gradients, and associated vertical axis rotations, in evolving continental rift systems. Second-order extension and extensional-shear faults within the relay zones variably resolve components of regional extension, and components of extension and/or shortening parallel to the rift zone, to accommodate the inherently three-dimensional (3-D strains associated with relay zone development and rotation. Such a configuration involves volume increase, which is accommodated at the surface by open fractures; in the subsurface this may be accommodated by veins or dikes oriented obliquely and normal to the rift axis. To consider the scalability of the effects of relay zone rotations, we compare the geometry and kinematics of fault and fracture sets in the Koa`e and Krafla rift zones with data from exhumed contemporaneous fault and dike systems developed within a > 5×104 km2 relay system that developed during formation of the NE Atlantic margins. Based on the findings presented here we propose a new conceptual model for the evolution of segmented continental rift basins on the NE Atlantic margins.

  1. Variability of the geothermal gradient across two differently aged magma-rich continental rifted margins of the Atlantic Ocean: the Southwest African and the Norwegian margins

    Directory of Open Access Journals (Sweden)

    E. Gholamrezaie

    2018-02-01

    Full Text Available The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D models differentiate various sedimentary, crustal, and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature–depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition. According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3-D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal lithosphere–asthenosphere boundary (LAB depth influence the shallow thermal field pattern. The results indicate an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated in the present day.

  2. Variability of the geothermal gradient across two differently aged magma-rich continental rifted margins of the Atlantic Ocean: the Southwest African and the Norwegian margins

    Science.gov (United States)

    Gholamrezaie, Ershad; Scheck-Wenderoth, Magdalena; Sippel, Judith; Strecker, Manfred R.

    2018-02-01

    The aim of this study is to investigate the shallow thermal field differences for two differently aged passive continental margins by analyzing regional variations in geothermal gradient and exploring the controlling factors for these variations. Hence, we analyzed two previously published 3-D conductive and lithospheric-scale thermal models of the Southwest African and the Norwegian passive margins. These 3-D models differentiate various sedimentary, crustal, and mantle units and integrate different geophysical data such as seismic observations and the gravity field. We extracted the temperature-depth distributions in 1 km intervals down to 6 km below the upper thermal boundary condition. The geothermal gradient was then calculated for these intervals between the upper thermal boundary condition and the respective depth levels (1, 2, 3, 4, 5, and 6 km below the upper thermal boundary condition). According to our results, the geothermal gradient decreases with increasing depth and shows varying lateral trends and values for these two different margins. We compare the 3-D geological structural models and the geothermal gradient variations for both thermal models and show how radiogenic heat production, sediment insulating effect, and thermal lithosphere-asthenosphere boundary (LAB) depth influence the shallow thermal field pattern. The results indicate an ongoing process of oceanic mantle cooling at the young Norwegian margin compared with the old SW African passive margin that seems to be thermally equilibrated in the present day.

  3. FATORES CONTROLADORES DA EVOLUÇÃO DO RELEVO NO FLANCO NNW DO RIFT CONTINENTAL DO SUDESTE DO BRASIL: UMA ANÁLISE BASEADA NA MENSURAÇÃO DOS PROCESSOS DENUDACIONAIS DE LONGO-TERMO

    Directory of Open Access Journals (Sweden)

    Éric Andrade Rezende

    2013-03-01

    Full Text Available O presente trabalho tem como objetivo estudar os fatores controladores da evolução do relevo de um trecho do interflúvio entre a bacia interiorana do rio Grande, um dos formadores do rio Paraná, e a bacia do rio Paraíba do Sul. Este divisor hidrográfico está contido em uma das porções mais elevadas da Serra da Mantiqueira e coincide com o flanco NNW do Rift Continental do Sudeste do Brasil. Para este estudo foram selecionadas nove sub-bacias localizadas no extremo sul de Minas Gerais, entre os municípios de Itamonte e Bocaina de Minas. Essas sub-bacias tiveram suas taxas denudacionais estimadas pela mensuração da produção do isótopo cosmogênico 10Be em sedimentos fluviais. Os resultados mostram que a taxa média de denudação das sub-bacias do rio Paraíba do Sul, voltadas para o gráben (17,39 m/Ma, é superior a das sub-bacias dos rios Grande e Aiuruoca, voltadas para o interior continental (12,24 m/Ma. Entre os fatores controladores das taxas de denudação, se destaca a boa correlação existente entre as taxas e dois parâmetros morfométricos: amplitude de relevo e declividade média das sub-bacias. A influência da litologia também se mostra importante, já que a resistência dos granitos frente aos processos erosivos é um dos fatores que condiciona taxas denudacionais bastante baixas na área. Os resultados indicam que o granito Maromba é a unidade litológica mais resistente entre aquelas presentes nas sub-bacias amostradas. As baixas taxas mensuradas, referentes às últimas dezenas de milhares de anos, contrastam com o relevo rejuvenescido e com as elevadas altitudes desse setor da Mantiqueira, onde há um reconhecido papel da neotectônica na morfogênese. Deste modo, é provável que tenha havido uma atenuação da atividade tectônica no Quaternário Superior, com a consequente estabilização dos níveis de base e diminuição dos processos denudacionais.

  4. Polyphase Rifting and Breakup of the Central Mozambique Margin

    Science.gov (United States)

    Senkans, Andrew; Leroy, Sylvie; d'Acremont, Elia; Castilla, Raymi

    2017-04-01

    The breakup of the Gondwana supercontinent resulted in the formation of the Central Mozambique passive margin as Africa and Antarctica were separated during the mid-Jurassic period. The identification of magnetic anomalies in the Mozambique Basin and Riiser Larsen Sea means that post-oceanisation plate kinematics are well-constrained. Unresolved questions remain, however, regarding the initial fit, continental breakup process, and the first relative movements of Africa and Antarctica. This study uses high quality multi-channel seismic reflection profiles in an effort to identify the major crustal domains in the Angoche and Beira regions of the Central Mozambique margin. This work is part of the integrated pluri-disciplinary PAMELA project*. Our results show that the Central Mozambique passive margin is characterised by intense but localised magmatic activity, evidenced by the existence of seaward dipping reflectors (SDR) in the Angoche region, as well as magmatic sills and volcanoclastic material which mark the Beira High. The Angoche region is defined by a faulted upper-continental crust, with the possible exhumation of lower crustal material forming an extended ocean-continent transition (OCT). The profiles studied across the Beira high reveal an offshore continental fragment, which is overlain by a pre-rift sedimentary unit likely to belong to the Karoo Group. Faulting of the crust and overlying sedimentary unit reveals that the Beira High has recorded several phases of deformation. The combination of our seismic interpretation with existing geophysical and geological results have allowed us to propose a breakup model which supports the idea that the Central Mozambique margin was affected by polyphase rifting. The analysis of both along-dip and along-strike profiles shows that the Beira High initially experienced extension in a direction approximately parallel to the Mozambique coastline onshore of the Beira High. Our results suggest that the Beira High results

  5. Preferential rifting of continents - A source of displaced terranes

    Science.gov (United States)

    Vink, G. E.; Morgan, W. J.; Zhao, W.-L.

    1984-01-01

    Lithospheric rifting, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential rifting of continents, the total strength of different lithospheres is compared by integrating the limits of lithospheric stress with depth. Comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of three. Also, a thickened crust can halve the total strength of normal continental lithosphere. Because the weakest area acts as a stress guide, any rifting close to an ocean-continent boundary would prefer a continental pathway. This results in the formation of small continental fragments or microplates that, once accreted back to a continent during subduction, are seen as displaced terranes. In addition, the large crustal thicknesses associated with suture zones would make such areas likely locations for future rifting episodes. This results in the tendency of new oceans to open along the suture where a former ocean had closed.

  6. Gravity anomalies, crustal structure and rift tectonics at the Konkan ...

    Indian Academy of Sciences (India)

    trolled by the mode of extension and thinning of continental ... facilitates to evaluate the mechanism of rifting, thermal as ..... estimated as the median depth between the back- stripped .... and gravity modeling with an application to the Gulf of.

  7. On risk assessment in the petroleum activities on the Norwegian and UK continental shelves

    International Nuclear Information System (INIS)

    Avena, Terje; Pitblado, Robin

    1998-01-01

    This article discusses some key principles and tools of safety management, with focus on risk analyses, risk interpretation, risk acceptance criteria and risk communication, as well as emergency preparedness. The starting point for the discussion is the experience from the activity on the Norwegian and UK continental shelves, and the implementation of new safety regulations in particular

  8. Atlantic update, July 1986--June 1990: Outer Continental Shelf oil and gas activities

    Energy Technology Data Exchange (ETDEWEB)

    Karpas, R.M.; Gould, G.J.

    1990-10-01

    This report describes outer continental shelf oil and gas activities in the Atlantic Region. This edition of the Atlantic Update includes an overview of the Mid-Atlantic Planning Area and a summary of the Manteo Prospect off-shore North Carolina. 6 figs., 8 tabs.

  9. Review of the Cambrian volcanic activity in Morocco: geochemical fingerprints and geotectonic implications for the rifting of West Gondwana

    Science.gov (United States)

    Pouclet, André; El Hadi, Hassan; Álvaro, J. Javier; Bardintzeff, Jacques-Marie; Benharref, Mohammed; Fekkak, Abdelilah

    2018-03-01

    Volcanic activities related to the opening of a Cambrian rift in Morocco were widespread from the Fortunian to the Cambrian Epoch 3. Numerous data are available from northwestern volcanic sites, particularly in the western High Atlas, but they are scarce from the southeastern sites. New data are documented here from the volcanic formations exposed in the Jbel Tazoult n'Ouzina of the Tafilalt Province, eastern Anti-Atlas and dated to Cambrian Epoch 2-3. The Cambrian volcanic activities recorded in the High Atlas, Anti-Atlas, and Coastal Meseta are synthesized to refine their stratigraphic setting and to characterize their magmatic affinities and fingerprints. Six volcanic pulses are determined as tholeiitic, transitional, and alkaline suites. The tholeiitic and transitional magmas originated from primitive mantle and E-MORB-type sources with a spinel- and garnet-bearing lherzolite composition. Some of them were modified by assimilation-fractional crystallisation processes during crust-mantle interactions. The alkaline magmas fit with an OIB-type and a garnet-bearing lherzolite source. The palaeogeographic distribution of the magmatic suites was controlled by the lithospheric thinning of the Cambrian Atlas Rift and lithospheric constraints of the Pan-African metacraton and West African craton.

  10. Continentward-dipping detachment fault system and asymmetric rift structure of the Baiyun Sag, northern South China Sea

    Science.gov (United States)

    Zhou, Zhichao; Mei, Lianfu; Liu, Jun; Zheng, Jinyun; Chen, Liang; Hao, Shihao

    2018-02-01

    The rift architecture and deep crustal structure of the distal margin at the mid-northern margin of the South China Sea have been previously investigated by using deep seismic reflection profiles. However, one fundamental recurring problem in the debate is the extensional fault system and rift structure of the hyperextended rift basins (Baiyun Sag and Liwan Sag) within the distal margin because of the limited amount of seismic data. Based on new 3D seismic survey data and 2D seismic reflection profiles, we observe an array of fault blocks in the Baiyun Sag, which were tilted towards the ocean by extensional faulting. The extensional faults consistently dip towards the continent. Beneath the tilted fault blocks and extensional faults, a low-angle, high-amplitude and continuous reflection has been interpreted as the master detachment surface that controls the extension process. During rifting, the continentward-dipping normal faults evolved in a sequence from south to north, generating the asymmetric rift structure of the Baiyun Sag. The Baiyun Sag is separated from the oceanic domain by a series of structural highs that were uplifted by magmatic activity in response to the continental breakup at 33 Ma and a ridge jump to the south at 26-24 Ma. Therefore, we propose that magmatism played a significant role in the continental extension and final breakup in the South China Sea.

  11. Pre-existing oblique transfer zones and transfer/transform relationships in continental margins: New insights from the southeastern Gulf of Aden, Socotra Island, Yemen

    Science.gov (United States)

    Bellahsen, N.; Leroy, S.; Autin, J.; Razin, P.; d'Acremont, E.; Sloan, H.; Pik, R.; Ahmed, A.; Khanbari, K.

    2013-11-01

    Transfer zones are ubiquitous features in continental rifts and margins, as are transform faults in oceanic lithosphere. Here, we present a structural study of the Hadibo Transfer Zone (HTZ), located in Socotra Island (Yemen) in the southeastern Gulf of Aden. There, we interpret this continental transfer fault zone to represent a reactivated pre-existing structure. Its trend is oblique to the direction of divergence and it has been active from the early up to the latest stages of rifting. One of the main oceanic fracture zones (FZ), the Hadibo-Sharbithat FZ, is aligned with and appears to be an extension of the HTZ and is probably genetically linked to it. Comparing this setting with observations from other Afro-Arabian rifts as well as with passive margins worldwide, it appears that many continental transfer zones are reactivated pre-existing structures, oblique to divergence. We therefore establish a classification system for oceanic FZ based upon their relationship with syn-rift structures. Type 1 FZ form at syn-rift structures and are late syn-rift to early syn-OCT. Type 2 FZ form during the OCT formation and Type 3 FZ form within the oceanic domain, after the oceanic spreading onset. The latter are controlled by far-field forces, magmatic processes, spreading rates, and oceanic crust rheology.

  12. Continentward-Dipping Normal Faults, Boudinage and Ductile Shear at Rifted Passive Margins

    Science.gov (United States)

    Clerc, C. N.; Ringenbach, J. C.; Jolivet, L.; Ballard, J. F.

    2017-12-01

    Deep structures resulting from the rifting of the continental crust are now well imaged by seismic profiles. We present a series of recent industrial profiles that allow the identification of various rift-related geological processes such as crustal boudinage, ductile shear of the base of the crust and low-angle detachment faulting. Along both magma-rich and magma-poor rifted margins, we observe clear indications of ductile deformation of the deep continental crust. Large-scale shallow dipping shear zones are identified with a top-to-the-continent sense of shear. This sense of shear is consistent with the activity of the Continentward-Dipping Normal Faults (CDNF) that accommodate the extension in the upper crust. This pattern is responsible for an oceanward migration of the deformation and of the associated syn-tectonic deposits (sediments and/or volcanics). We discuss the origin of the Continentward-Dipping Normal Faults (CDNF) and investigate their implications and the effect of sediment thermal blanketing on crustal rheology. In some cases, low-angle shear zones define an anastomosed pattern that delineates boudin-like structures that seem to control the position and dip of upper crustal normal faults. We present some of the most striking examples from several locations (Uruguay, West Africa, South China Sea…), and discuss their rifting histories that differ from the classical models of oceanward-dipping normal faults.

  13. Lithology and temperature: How key mantle variables control rift volcanism

    Science.gov (United States)

    Shorttle, O.; Hoggard, M.; Matthews, S.; Maclennan, J.

    2015-12-01

    Continental rifting is often associated with extensive magmatic activity, emplacing millions of cubic kilometres of basalt and triggering environmental change. The lasting geological record of this volcanic catastrophism are the large igneous provinces found at the margins of many continents and abrupt extinctions in the fossil record, most strikingly that found at the Permo-Triassic boundary. Rather than being considered purely a passive plate tectonic phenomenon, these episodes are frequently explained by the involvement of mantle plumes, upwellings of mantle rock made buoyant by their high temperatures. However, there has been debate over the relative role of the mantle's temperature and composition in generating the large volumes of magma involved in rift and intra-plate volcanism, and even when the mantle is inferred to be hot, this has been variously attributed to mantle plumes or continental insulation effects. To help resolve these uncertainties we have combined geochemical, geophysical and modelling results in a two stage approach: Firstly, we have investigated how mantle composition and temperature contribute to melting beneath Iceland, the present day manifestation of the mantle plume implicated in the 54Ma break up of the North Atlantic. By considering both the igneous crustal production on Iceland and the chemistry of its basalts we have been able to place stringent constraints on the viable temperature and lithology of the Icelandic mantle. Although a >100°C excess temperature is required to generate Iceland's thick igneous crust, geochemistry also indicates that pyroxenite comprises 10% of its source. Therefore, the dynamics of rifting on Iceland are modulated both by thermal and compositional mantle anomalies. Secondly, we have performed a global assessment of the mantle's post break-up thermal history to determine the amplitude and longevity of continental insulation in driving excess volcanism. Using seismically constrained igneous crustal

  14. 3D Numerical Model of Continental Breakup via Plume Lithosphere Interaction Near Cratonic Blocks: Implications for the Tanzanian Craton

    Science.gov (United States)

    Koptev, A.; Calais, E.; Burov, E. B.; Leroy, S. D.; Gerya, T.

    2014-12-01

    Although many continental rift basins and their successfully rifted counterparts at passive continental margins are magmatic, some are not. This dichotomy prompted end-member views of the mechanism driving continental rifting, deep-seated and mantle plume-driven for some, owing to shallow lithospheric stretching for others. In that regard, the East African Rift (EAR), the 3000 km-long divergent boundary between the Nubian and Somalian plates, provides a unique setting with the juxtaposition of the eastern, magma-rich, and western, magma-poor, branches on either sides of the 250-km thick Tanzanian craton. Here we implement high-resolution rheologically realistic 3D numerical model of plume-lithosphere interactions in extensional far-field settings to explain this contrasted behaviour in a unified framework starting from simple, symmetrical initial conditions with an isolated mantle plume rising beneath a craton in an east-west tensional far field stress. The upwelling mantle plume is deflected by the cratonic keel and preferentially channelled along one of its sides. This leads to the coeval development of a magma-rich branch above the plume head and a magma-poor one along the opposite side of the craton, the formation of a rotating microplate between the two rift branches, and the feeding of melt to both branches form a single mantle source. The model bears strong similarities with the evolution of the eastern and western branches of the central EAR and the geodetically observed rotation of the Victoria microplate. This result reconciles the passive (plume-activated) versus active (far-field tectonic stresses) rift models as our experiments shows both processes in action and demonstrate the possibility of developing both magmatic and amagmatic rifts in identical geotectonic environments.

  15. Seismic evidence for arc segmentation, active magmatic intrusions and syn-rift fault system in the northern Ryukyu volcanic arc

    Science.gov (United States)

    Arai, Ryuta; Kodaira, Shuichi; Takahashi, Tsutomu; Miura, Seiichi; Kaneda, Yoshiyuki

    2018-04-01

    Tectonic and volcanic structures of the northern Ryukyu arc are investigated on the basis of multichannel seismic (MCS) reflection data. The study area forms an active volcanic front in parallel to the non-volcanic island chain in the eastern margin of the Eurasian plate and has been undergoing regional extension on its back-arc side. We carried out a MCS reflection experiment along two across-arc lines, and one of the profiles was laid out across the Tokara Channel, a linear bathymetric depression which demarcates the northern and central Ryukyu arcs. The reflection image reveals that beneath this topographic valley there exists a 3-km-deep sedimentary basin atop the arc crust, suggesting that the arc segment boundary was formed by rapid and focused subsidence of the arc crust driven by the arc-parallel extension. Around the volcanic front, magmatic conduits represented by tubular transparent bodies in the reflection images are well developed within the shallow sediments and some of them are accompanied by small fragments of dipping seismic reflectors indicating intruded sills at their bottoms. The spatial distribution of the conduits may suggest that the arc volcanism has multiple active outlets on the seafloor which bifurcate at crustal depths and/or that the location of the volcanic front has been migrating trenchward over time. Further distant from the volcanic front toward the back-arc (> 30 km away), these volcanic features vanish, and alternatively wide rift basins become predominant where rapid transitions from normal-fault-dominant regions to strike-slip-fault-dominant regions occur. This spatial variation in faulting patterns indicates complex stress regimes associated with arc/back-arc rifting in the northern Okinawa Trough.[Figure not available: see fulltext.

  16. Initiation of extension in South China continental margin during the active-passive margin transition: kinematic and thermochronological constraints

    Science.gov (United States)

    ZUO, Xuran; CHAN, Lung

    2015-04-01

    The southern South China Block is characterized by a widespread magmatic belt, prominent NE-striking fault zones and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from an active to a passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in this area. In this study, we used zircon fission-track dating (ZFT) and numerical modeling to examine the timing and kinematics of the active-passive margin transition. Our ZFT results on granitic plutons in the SW Cathaysia Block show two episodes of exhumation of the granitic plutons. The first episode, occurring during 170 Ma - 120 Ma, affected local parts of the Nanling Range. The second episode, a more regional exhumation event, occurred during 115 Ma - 70 Ma. Numerical geodynamic modeling was conducted to simulate the subduction between the paleo-Pacific plate and the South China Block. The modeling results could explain the observation based on ZFT data that exhumation of the granite-dominant Nanling Range occurred at an earlier time than the gneiss-dominant Yunkai Terrane. In addition to the difference in geology between Yunkai and Nanling, the heating from Jurassic-Early Cretaceous magmatism in the Nanling Range may have softened the upper crust, causing the area to exhume more readily. Numerical modeling results also indicate that (1) high slab dip angle, high geothermal gradient of lithosphere and low convergence velocity favor the subduction process and the reversal of crustal stress state from compression to extension in the upper plate; (2) the late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction. The inversion of stress regime in the continental crust from compression to crustal extension has shed light on the geological condition producing the red bed basins during Late Cretaceous

  17. Seismic cycle and seismic risk of an active faults network: the Corinth rift case (Greece)

    International Nuclear Information System (INIS)

    Boiselet, Aurelien

    2014-01-01

    The Corinth rift (Greece) is one of the regions with the highest strain rates (16 mm/y extension rate) in the Euro-Mediterranean area and as such it has long been identified as a site of major importance for earthquake studies in Europe (20 years of research by the Corinth Rift Laboratory and 4 years of in-depth studies by the ANR-SISCOR project). This enhanced knowledge, acquired in particular, in the western part of the Gulf of Corinth (CRL region), an area about 50 by 40 km 2 , between the city of Patras to the west and the city of Aigion to the east, provides an excellent opportunity to compare fault-based (FB) and classical seismo-tectonic (ST) approaches currently used in seismic hazard assessment studies. An homogeneous earthquake catalogue was thus constructed for the purpose of this study along with a comprehensive database of all relevant geological, geodetic and geophysical information available in the literature and recently collected within the ANR-SISCOR project. The homogenized Mw earthquake catalogue is composed of data from the National Observatory of Athens and from the university of Thessaloniki as well as data acquired through historical and instrumental work performed within the ANR-SISCOR group for the CRL region. A frequency magnitude analysis confirms that seismicity rates are governed by Gutenberg-Richter (GR) statistic for 1.2 =6 earthquakes were computed for the region of study. Time dependent models (Brownian Passage time and Weibull probability distributions) were also explored. The probability (normalized by area) of a M≥6.0 earthquake is found to be greater in the CRL region compared to the eastern part of the Corinth rift. Probability estimates corresponding to the 16. and 84. percentile are also provided, as a means of representing the range of uncertainties in the results. Probability estimates based on the ST-approach are then compared to those based on the FB approach approach. In general ST tends to overestimate probabilities

  18. Seismic imaging of the metamorphism of young sediment into new crystalline crust in the actively rifting Imperial Valley, California

    Science.gov (United States)

    Han, Liang; Hole, John; Stock, Joann; Fuis, Gary S.; Williams, Colin F.; Delph, Jonathan; Davenport, Kathy; Livers, Amanda

    2016-01-01

    Plate-boundary rifting between transform faults is opening the Imperial Valley of southern California and the rift is rapidly filling with sediment from the Colorado River. Three 65–90 km long seismic refraction profiles across and along the valley, acquired as part of the 2011 Salton Seismic Imaging Project, were analyzed to constrain upper crustal structure and the transition from sediment to underlying crystalline rock. Both first arrival travel-time tomography and frequency-domain full-waveform inversion were applied to provide P-wave velocity models down to ∼7 km depth. The valley margins are fault-bounded, beyond which thinner sediment has been deposited on preexisting crystalline rocks. Within the central basin, seismic velocity increases continuously from ∼1.8 km/s sediment at the surface to >6 km/s crystalline rock with no sharp discontinuity. Borehole data show young sediment is progressively metamorphosed into crystalline rock. The seismic velocity gradient with depth decreases approximately at the 4 km/s contour, which coincides with changes in the porosity and density gradient in borehole core samples. This change occurs at ∼3 km depth in most of the valley, but at only ∼1.5 km depth in the Salton Sea geothermal field. We interpret progressive metamorphism caused by high heat flow to be creating new crystalline crust throughout the valley at a rate comparable to the ≥2 km/Myr sedimentation rate. The newly formed crystalline crust extends to at least 7–8 km depth, and it is shallower and faster where heat flow is higher. Most of the active seismicity occurs within this new crust.

  19. Seismic imaging of the metamorphism of young sediment into new crystalline crust in the actively rifting Imperial Valley, California

    Science.gov (United States)

    Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Williams, Colin F.; Delph, Jonathan R.; Davenport, Kathy K.; Livers, Amanda J.

    2016-11-01

    Plate-boundary rifting between transform faults is opening the Imperial Valley of southern California and the rift is rapidly filling with sediment from the Colorado River. Three 65-90 km long seismic refraction profiles across and along the valley, acquired as part of the 2011 Salton Seismic Imaging Project, were analyzed to constrain upper crustal structure and the transition from sediment to underlying crystalline rock. Both first arrival travel-time tomography and frequency-domain full-waveform inversion were applied to provide P-wave velocity models down to ˜7 km depth. The valley margins are fault-bounded, beyond which thinner sediment has been deposited on preexisting crystalline rocks. Within the central basin, seismic velocity increases continuously from ˜1.8 km/s sediment at the surface to >6 km/s crystalline rock with no sharp discontinuity. Borehole data show young sediment is progressively metamorphosed into crystalline rock. The seismic velocity gradient with depth decreases approximately at the 4 km/s contour, which coincides with changes in the porosity and density gradient in borehole core samples. This change occurs at ˜3 km depth in most of the valley, but at only ˜1.5 km depth in the Salton Sea geothermal field. We interpret progressive metamorphism caused by high heat flow to be creating new crystalline crust throughout the valley at a rate comparable to the ≥2 km/Myr sedimentation rate. The newly formed crystalline crust extends to at least 7-8 km depth, and it is shallower and faster where heat flow is higher. Most of the active seismicity occurs within this new crust.

  20. Colorado Basin Structure and Rifting, Argentine passive margin

    Science.gov (United States)

    Autin, Julia; Scheck-Wenderoth, Magdalena; Loegering, Markus; Anka, Zahie; Vallejo, Eduardo; Rodriguez, Jorge; Marchal, Denis; Reichert, Christian; di Primio, Rolando

    2010-05-01

    partly supports this hypothesis and shows two main directions of faulting: margin-parallel faults (~N30°) and rift-parallel faults (~N125°). A specific distribution of the two fault sets is observed: margin-parallel faults are restrained to the most distal part of the margin. Starting with a 3D structural model of the basin fill based on seismic and well data the deeper structure of the crust beneath the Colorado Basin can be evaluate using isostatic and thermal modelling. Franke, D., et al. (2002), Deep Crustal Structure Of The Argentine Continental Margin From Seismic Wide-Angle And Multichannel Reflection Seismic Data, paper presented at AAPG Hedberg Conference "Hydrocarbon Habitat of Volcanic Rifted Passive Margins", Stavanger, Norway Franke, D., et al. (2006), Crustal structure across the Colorado Basin, offshore Argentina Geophysical Journal International 165, 850-864. Gladczenko, T. P., et al. (1997), South Atlantic volcanic margins Journal of the Geological Society, London 154, 465-470. Hinz, K., et al. (1999), The Argentine continental margin north of 48°S: sedimentary successions, volcanic activity during breakup Marine and Petroleum Geology 16(1-25). Hirsch, K. K., et al. (2009), Tectonic subsidence history and thermal evolution of the Orange Basin, Marine and Petroleum Geology, in press, doi:10.1016/j.marpetgeo.2009.1006.1009

  1. Phanerozoic Rifting Phases And Mineral Deposits

    Science.gov (United States)

    Hassaan, Mahmoud

    2016-04-01

    connected with NW,WNW and N-S faults genetically related to volcano-hydrothermal activity associated the Red Sea rifting. At Sherm EL-Sheikh hydrothermal manganese deposit occurs in Oligocene clastics within fault zone. Four iron-manganese-barite mineralization in Esh-Elmellaha plateau are controlled by faults trending NW,NE and nearly E-W intersecting Miocene carbonate rocks. Barite exists disseminated in the ores and as a vein in NW fault. In Shalatee - Halaib district 24 manganese deposits and barite veins with sulphide patches occur within Miocene carbonates distributed along two NW fault planes,trending 240°and 310° and occur in granite and basalt . Uranium -lead-zinc sulfide mineralization occur in Late Proterozoic granite, Late Cretaceous sandstones, and chiefly in Miocene clastic-carbonate-evaporate rocks. The occurrences of uranium- lead-zinc and iron-manganese-barite mineralization have the characteristic features of hypogene cavity filling and replacement deposits correlated with Miocene- Recent Aden volcanic rocks rifting. In western Saudi Arabia barite-lead-zinc mineralization occurs at Lat. 25° 45' and 25° 50'N hosted by Tertiary sediments in limestone nearby basaltic flows and NE-SW fault system. The mineralized hot brines in the Red Sea deeps considered by the author a part of this province. The author considers the constant rifting phases of Pangea and then progressive fragmentation of Western Gondwana during the Late Carboniferous-Lias, Late Jurassic-Early Aptian, Late Aptian - Albian and Late Eocene-Early Miocene and Oligocene-Miocene, responsible for formation of the mineral deposits constituting the M provinces. During these events, rifting, magmatism and hydrothermal activities took place in different peri-continental margins.

  2. Tectonostratigraphy of the Passive Continental Margin Offshore Indus Pakistan

    Science.gov (United States)

    Aslam, K.; Khan, M.; Liu, Y.; Farid, A.

    2017-12-01

    The tectonic evolution and structural complexities are poorly understood in the passive continental margin of the Offshore Indus of Pakistan. In the present study, an attempt has been made to interpret the structural trends and seismic stratigraphic framework in relation to the tectonics of the region. Seismic reflection data revealed tectonically controlled, distinct episodes of normal faulting representing rifting at different ages and transpression in the Late Eocene time. This transpression has resulted in the reactivation of the Pre-Cambrian basement structures. The movement of these basement structures has considerably affected the younger sedimentary succession resulting in push up structures resembling anticlines. The structural growth of the push-up structures was computed. The most remarkable tectonic setting in the region is represented by the normal faulting and by the basement uplift which divides the rifting and transpression stages. Ten mappable seismic sequences have been identified on the seismic records. A Jurassic aged paleo-shelf has also been identified on all regional seismic profiles which is indicative of Indian-African Plates separation during the Jurassic time. Furthermore, the backstripping technique was applied which has been proved to be a powerful technique to quantify subsidence/uplift history of rift-type passive continental margins. The back strip curves suggest that transition from an extensional rifted margin to transpression occurred during Eocene time (50-30 Ma). The backstripping curves show uplift had happened in the area. We infer that the uplift has occurred due to the movement of basement structures by the transpression movements of Arabian and Indian Plates. The present study suggests that the structural styles and stratigraphy of the Offshore Indus Pakistan were significantly affected by the tectonic activities during the separation of Gondwanaland in the Mesozoic and northward movement of the Indian Plate, post-rifting

  3. Characterising East Antarctic Lithosphere and its Rift Systems using Gravity Inversion

    Science.gov (United States)

    Vaughan, Alan P. M.; Kusznir, Nick J.; Ferraccioli, Fausto; Leat, Phil T.; Jordan, Tom A. R. M.; Purucker, Michael E.; Golynsky, A. V. Sasha; Rogozhina, Irina

    2013-04-01

    Since the International Geophysical Year (1957), a view has prevailed that East Antarctica has a relatively homogeneous lithospheric structure, consisting of a craton-like mosaic of Precambrian terranes, stable since the Pan-African orogeny ~500 million years ago (e.g. Ferracioli et al. 2011). Recent recognition of a continental-scale rift system cutting the East Antarctic interior has crystallised an alternative view of much more recent geological activity with important implications. The newly defined East Antarctic Rift System (EARS) (Ferraccioli et al. 2011) appears to extend from at least the South Pole to the continental margin at the Lambert Rift, a distance of 2500 km. This is comparable in scale to the well-studied East African rift system. New analysis of RadarSat data by Golynsky & Golynsky (2009) indicates that further rift zones may form widely distributed extension zones within the continent. A pilot study (Vaughan et al. 2012), using a newly developed gravity inversion technique (Chappell & Kusznir 2008) with existing public domain satellite data, shows distinct crustal thickness provinces with overall high average thickness separated by thinner, possibly rifted, crust. Understanding the nature of crustal thickness in East Antarctica is critical because: 1) this is poorly known along the ocean-continent transition, but is necessary to improve the plate reconstruction fit between Antarctica, Australia and India in Gondwana, which will also better define how and when these continents separated; 2) lateral variation in crustal thickness can be used to test supercontinent reconstructions and assess the effects of crystalline basement architecture and mechanical properties on rifting; 3) rift zone trajectories through East Antarctica will define the geometry of zones of crustal and lithospheric thinning at plate-scale; 4) it is not clear why or when the crust of East Antarctica became so thick and elevated, but knowing this can be used to test models of

  4. Rotation, narrowing and preferential reactivation of brittle structures during oblique rifting

    Science.gov (United States)

    Huismans, R. S.; Duclaux, G.; May, D.

    2017-12-01

    Occurrence of multiple faults populations with contrasting orientations in oblique continental rifts and passive margins has long sparked debate about relative timing of deformation events and tectonic interpretations. Here, we use high-resolution three-dimensional thermo-mechanical numerical modeling to characterize the evolution of the structural style associated with moderately oblique rifting in the continental lithosphere. Automatic analysis of the distribution of active extensional shears at the surface of the model demonstrates a characteristic deformation sequence. We show that upon localization, Phase 1 wide oblique en-échelon grabens develop, limited by extensional shears oriented orthogonal to σ3. Subsequent widening of the grabens is accompanied by a progressive rotation of the Phase 1 extensional shears that become sub-orthogonal the plate motion direction. Phase 2 is marked by narrowing of active deformation resulting from thinning of the continental lithosphere and development of a second-generation of extensional shears. During Phase 2 deformation localizes both on plate motion direction-orthogonal structures that reactivate rotated Phase 1 shears, and on new oblique structures orthogonal to σ3. Finally, Phase 3 consists in the oblique rupture of the continental lithosphere and produces an oceanic domain where oblique ridge segments are linked with highly oblique accommodation zones. We conclude that while new structures form normal to σ3 in an oblique rift, progressive rotation and long-term reactivation of Phase 1 structures promotes orthorhombic fault systems, critical to accommodate upper crustal extension and control oblique passive margin architecture. The distribution, orientation, and evolution of frictional-plastic structures observed in our models is remarkably similar to documented fault populations in the Gulf of Aden conjugate passive margins, which developed in moderately oblique extensional settings.

  5. Rift Valley fever dynamics in Senegal: a project for pro-active adaptation and improvement of livestock raising management

    Directory of Open Access Journals (Sweden)

    Murielle Lafaye

    2013-11-01

    Full Text Available The multi-disciplinary French project “Adaptation à la Fièvre de la Vallée du Rift” (AdaptFVR has concluded a 10-year constructive interaction between many scientists/partners involved with the Rift Valley fever (RVF dynamics in Senegal. The three targeted objectives reached were (i to produce - in near real-time - validated risk maps for parked livestock exposed to RVF mosquitoes/vectors bites; (ii to assess the impacts on RVF vectors from climate variability at different time-scales including climate change; and (iii to isolate processes improving local livestock management and animal health. Based on these results, concrete, pro-active adaptive actions were taken on site, which led to the establishment of a RVF early warning system (RVFews. Bulletins were released in a timely fashion during the project, tested and validated in close collaboration with the local populations, i.e. the primary users. Among the strategic, adaptive methods developed, conducted and evaluated in terms of cost/benefit analyses are the larvicide campaigns and the coupled bio-mathematical (hydrological and entomological model technologies, which are being transferred to the staff of the “Centre de Suivi Ecologique” (CSE in Dakar during 2013. Based on the results from the AdaptFVR project, other projects with similar conceptual and modelling approaches are currently being implemented, e.g. for urban and rural malaria and dengue in the French Antilles.

  6. Fractional activation of accumulation-mode particles in warm continental stratiform clouds

    International Nuclear Information System (INIS)

    Gillani, N.V.; Daum, P.H.; Schwartz, S.E.; Leaitch, W.R.; Strapp, J.W.; Isaac, G.A.

    1991-07-01

    The degree of activation of accumulation-mode particles (AMP) in clouds has been studied using continuous (1 second average) aircraft measurements of the number concentrations of cloud droplets (N cd , 2 to 35 μm diameter) and of unactivated AMP (N amp , 0.17 to 2.07 μm diameter) in cloud interstitial air. The magnitude and spatial variation of the activated fraction (F) of all measured particles (defined as F triple-bond N cd /N tot , where N tot = N cd + N amp ) are investigated, based on measurements made during ten aircraft flights in non-precipitating warm continental stratiform clouds near Syracuse NY in the fall of 1984. Based on instantaneous observations throughout the clouds, the spatial distribution of F was found to be quite nonuniform. In general, F was low in cloud edges and where total particle loading was high and/or cloud convective activity was low. In the interior of clouds, the value of F exceeded 0.9 for 36% of the data, but was below 0.6 for 28%. Factors influencing F the most were the total particle loading (N tot ) and the thermal stability of the cloud layer. The dependence of F on N tot in cloud interior was characterized by two distinct regimes. For N tot -3 , F was generally close to unity and relatively insensitive to N tot . For N tot > 800 cm -3 , F tended to decrease with increasing N tot . This decrease was greatest in a stable stratus deck embedded in a warm moist airmass. The results suggest that, in warm continental stratiform clouds, the process of particle activation becomes nonlinear and self-limiting at high particle loading. The degree of this nonlinearity depends on cloud convective activity (thermal instability)

  7. Polyphased rifting to post-breakup evolution of the Coral Sea region, Papua New Guinea

    Science.gov (United States)

    Bulois, Cédric; Pubellier, Manuel; Chamot-Rooke, Nicolas; Delescluse, Matthias

    2016-04-01

    The Coral Sea Basin, offshore Papua New Guinea, is generally described as a rift propagator that opened through the Australian craton during the Late Cretaceous. Rifting was later followed by spreading activity during Palaeocene to lowermost Eocene times and basin inversion during the Cenozoic. Herein, we specifically describe the extensional structures and show that the area has actually a much longer history that dates back from the Late Palaeozoic. A special focus is made on the northern margin of the Coral Sea Basin along which subsurface and HD topographic data were recently acquired. Extension took place discontinuously from the Late Palaeozoic to the Lower Cenozoic through several rift megacycles that include extensional pulses and relaxation episodes. The first rift megacycle (R1), poorly documented, occurred during the Triassic along an old Permo-Triassic, NS-trending structural fabric. Evidence of Permo-Triassic features is principally observed in the western part of the Coral Sea near the Tasman Line, a major lithospheric discontinuity that marks the eastern limit of the underlying Australian craton in Papua New Guinea. This early Triassic framework was reactivated during a Jurassic rifting stage (R2), resulting in small (~10/20km) tilted basins bounded by major NS, NE-SW and EW normal faults. Extension formed a large basin, floored by oceanic crust that might have connected with the Tethys Ocean. The Owen Stanley Oceanic Basin containing deep-marine sediments now obducted in the Ocean Stanley Thrust Belt are likely to represent this oceanic terrane. Both R1 and R2 megacycles shaped the geometry of the Jurassic Australian margin. A third Cretaceous extensional megacycle (R3) only reactivated the largest faults, cutting through the midst of this early stretched continental margin. It formed wider, poorly tilted basins and terminated with the onset of the Coral Sea seafloor spreading from Danian to Ypresian times (61.8 to 53.4 Myr). Then, the overall

  8. Magmatic architecture within a rift segment: Articulate axial magma storage at Erta Ale volcano, Ethiopia

    Science.gov (United States)

    Xu, Wenbin; Rivalta, Eleonora; Li, Xing

    2017-10-01

    Understanding the magmatic systems beneath rift volcanoes provides insights into the deeper processes associated with rift architecture and development. At the slow spreading Erta Ale segment (Afar, Ethiopia) transition from continental rifting to seafloor spreading is ongoing on land. A lava lake has been documented since the twentieth century at the summit of the Erta Ale volcano and acts as an indicator of the pressure of its magma reservoir. However, the structure of the plumbing system of the volcano feeding such persistent active lava lake and the mechanisms controlling the architecture of magma storage remain unclear. Here, we combine high-resolution satellite optical imagery and radar interferometry (InSAR) to infer the shape, location and orientation of the conduits feeding the 2017 Erta Ale eruption. We show that the lava lake was rooted in a vertical dike-shaped reservoir that had been inflating prior to the eruption. The magma was subsequently transferred into a shallower feeder dike. We also find a shallow, horizontal magma lens elongated along axis inflating beneath the volcano during the later period of the eruption. Edifice stress modeling suggests the hydraulically connected system of horizontal and vertical thin magmatic bodies able to open and close are arranged spatially according to stresses induced by loading and unloading due to topographic changes. Our combined approach may provide new constraints on the organization of magma plumbing systems beneath volcanoes in continental and marine settings.

  9. Recent and Hazardous Volcanic Activity Along the NW Rift Zone of Piton De La Fournaise Volcano, La Réunion Island

    Science.gov (United States)

    Walther, G.; Frese, I.; Di Muro, A.; Kueppers, U.; Michon, L.; Metrich, N.

    2014-12-01

    Shield volcanoes are a common feature of basaltic volcanism. Their volcanic activity is often confined to a summit crater area and rift systems, both characterized by constructive (scoria and cinder cones; lava flows) and destructive (pit craters; caldera collapse) phenomena. Piton de la Fournaise (PdF) shield volcano (La Réunion Island, Indian Ocean) is an ideal place to study these differences in eruptive behaviour. Besides the frequent eruptions in the central Enclos Fouqué caldera, hundreds of eruptive vents opened along three main rift zones cutting the edifice during the last 50 kyrs. Two short rift zones are characterized by weak seismicity and lateral magma transport at shallow depth (above sea level). Here we focus on the third and largest rift zone (15km wide, 20 km long), which extends in a north-westerly direction between PdF and nearby Piton des Neiges volcanic complex. It is typified by deep seismicity (up to 30 km), emitting mostly primitive magmas, testifying of high fluid pressures (up to 5 kbar) and large-volume eruptions. We present new field data (including stratigraphic logs, a geological map of the area, C-14 dating and geochemical analyses of the eruption products) on one of the youngest (~6kyrs) and largest lava field (Trous Blancs eruption). It extends for 24km from a height of 1800 m asl, passing Le Tampon and Saint Pierre cities, until reaching the coast. The source area of this huge lava flow has been identified in an alignment of four previously unidentified pit craters. The eruption initiated with intense fountaining activity, producing a m-thick bed of loose black scoria, which becomes densely welded in its upper part; followed by an alternation of volume rich lava effusions and strombolian activity, resulting in the emplacement of meter-thick, massive units of olivine-basalt alternating with coarse scoria beds in the proximal area. Activity ended with the emplacement of a dm-thick bed of glassy, dense scoria and a stratified lithic

  10. Controls of inherited lithospheric heterogeneity on rift linkage: Numerical and analog models of interaction between the Kenyan and Ethiopian rifts across the Turkana depression

    Science.gov (United States)

    Brune, Sascha; Corti, Giacomo; Ranalli, Giorgio

    2017-09-01

    Inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts. The Turkana depression in the East African Rift connects the Main Ethiopian Rift to the north with the Kenya rift in the south. This region is characterized by a NW-SE trending band of thinned crust inherited from a Mesozoic rifting event, which is cutting the present-day N-S rift trend at high angle. In striking contrast to the narrow rifts in Ethiopia and Kenya, extension in the Turkana region is accommodated in subparallel deformation domains that are laterally distributed over several hundred kilometers. We present both analog experiments and numerical models that reproduce the along-axis transition from narrow rifting in Ethiopia and Kenya to a distributed deformation within the Turkana depression. Similarly to natural observations, our models show that the Ethiopian and Kenyan rifts bend away from each other within the Turkana region, thus forming a right-lateral step over and avoiding a direct link to form a continuous N-S depression. The models reveal five potential types of rift linkage across the preexisting basin: three types where rifts bend away from the inherited structure connecting via a (1) wide or (2) narrow rift or by (3) forming a rotating microplate, (4) a type where rifts bend towards it, and (5) straight rift linkage. The fact that linkage type 1 is realized in the Turkana region provides new insights on the rheological configuration of the Mesozoic rift system at the onset of the recent rift episode.

  11. The effects of thick sediment upon continental breakup: seismic imaging and thermal modeling of the Salton Trough, southern California

    Science.gov (United States)

    Han, L.; Hole, J. A.; Lowell, R. P.; Stock, J. M.; Fuis, G. S.; Driscoll, N. W.; Kell, A. M.; Kent, G. M.; Harding, A. J.; Gonzalez-Fernandez, A.; Lázaro-Mancilla, O.

    2015-12-01

    Continental rifting ultimately creates a deep accommodation space for sediment. When a major river flows into a late-stage rift, thick deltaic sediment can change the thermal regime and alter the mechanisms of extension and continental breakup. The Salton Trough, the northernmost rift segment of the Gulf of California plate boundary, has experienced the same extension as the rest of the Gulf, but is filled to sea level by sediment from the Colorado River. Unlike the southern Gulf, seafloor spreading has not initiated. Instead, seismicity, high heat flow, and minor volcanoes attest to ongoing rifting of thin, transitional crust. Recently acquired controlled-source seismic refraction and wide-angle reflection data in the Salton Trough provide constraints upon crustal architecture and active rift processes. The crust in the central Salton Trough is only 17-18 km thick, with a strongly layered but relatively one-dimensional structure for ~100 km in the direction of plate motion. The upper crust includes 2-4 km of Colorado River sediment. Crystalline rock below the sediment is interpreted to be similar sediment metamorphosed by the high heat flow and geothermal activity. Meta-sediment extends to at least 9 km depth. A 4-5 km thick layer in the middle crust is either additional meta-sediment or stretched pre-existing continental crust. The lowermost 4-5 km of the crust is rift-related mafic magmatic intrusion or underplating from partial melting in the hot upper mantle. North American lithosphere in the Salton Trough has been almost or completely rifted apart. The gap has been filled by ~100 km of new transitional crust created by magmatism from below and sedimentation from above. These processes create strong lithologic, thermal, and rheologic layering. While heat flow in the rift is very high, rapid sedimentation cools the upper crust as compared to a linear geotherm. Brittle extension occurs within new meta-sedimentary rock. The lower crust, in comparison, is

  12. Asthenospheric flow and origin of volcanism in the Baikal rift area

    NARCIS (Netherlands)

    Lebedev, S.; Meier, T.; Hilst, R.D. van der

    2006-01-01

    The origin of low-volume, hotspot-like volcanism often observed in continental rift areas is debated, as is the nature of the flow in the mantle beneath. In this paper we assemble seismic constraints on the mantle flow below the Baikal Rift Zone. We combine new evidence from upper-mantle

  13. Predicting Rift Valley Fever Inter-epidemic Activities and Outbreak Patterns: Insights from a Stochastic Host-Vector Model.

    Directory of Open Access Journals (Sweden)

    Sansao A Pedro

    2016-12-01

    Full Text Available Rift Valley fever (RVF outbreaks are recurrent, occurring at irregular intervals of up to 15 years at least in East Africa. Between outbreaks disease inter-epidemic activities exist and occur at low levels and are maintained by female Aedes mcintoshi mosquitoes which transmit the virus to their eggs leading to disease persistence during unfavourable seasons. Here we formulate and analyse a full stochastic host-vector model with two routes of transmission: vertical and horizontal. By applying branching process theory we establish novel relationships between the basic reproduction number, R0, vertical transmission and the invasion and extinction probabilities. Optimum climatic conditions and presence of mosquitoes have not fully explained the irregular oscillatory behaviour of RVF outbreaks. Using our model without seasonality and applying van Kampen system-size expansion techniques, we provide an analytical expression for the spectrum of stochastic fluctuations, revealing how outbreaks multi-year periodicity varies with the vertical transmission. Our theory predicts complex fluctuations with a dominant period of 1 to 10 years which essentially depends on the efficiency of vertical transmission. Our predictions are then compared to temporal patterns of disease outbreaks in Tanzania, Kenya and South Africa. Our analyses show that interaction between nonlinearity, stochasticity and vertical transmission provides a simple but plausible explanation for the irregular oscillatory nature of RVF outbreaks. Therefore, we argue that while rainfall might be the major determinant for the onset and switch-off of an outbreak, the occurrence of a particular outbreak is also a result of a build up phenomena that is correlated to vertical transmission efficiency.

  14. Assessment and recommendations for two sites with active and potential aquaculture production in Rift Valley and Coast Provinces, Kenya

    Science.gov (United States)

    Kenya has a long history of local fish consumption. The population in the Lake Victoria area (Rift Valley Province) Northwest of Nairobi and coastal communities (Coast Province) have historically included fish in their diet. Migration from villages to urban areas and increasing commerce has created ...

  15. The Example of Eastern Africa: the dynamic of Rift Valley fever and tools for monitoring virus activity

    Science.gov (United States)

    Rift Valley fever is a mosquito-borne viral zoonosis that primarily affects animals but also has the capacity to infect humans. Outbreaks of this disease in eastern Africa are closely associated with periods of heavy rainfall and forecasting models and early warning systems have been developed to en...

  16. Extensional Fault Evolution and its Flexural Isostatic Response During Iberia-Newfoundland Rifted Margin Formation

    Science.gov (United States)

    Gómez-Romeu, J.; Kusznir, N.; Manatschal, G.; Roberts, A.

    2017-12-01

    During the formation of magma-poor rifted margins, upper lithosphere thinning and stretching is achieved by extensional faulting, however, there is still debate and uncertainty how faults evolve during rifting leading to breakup. Seismic data provides an image of the present-day structural and stratigraphic configuration and thus initial fault geometry is unknown. To understand the geometric evolution of extensional faults at rifted margins it is extremely important to also consider the flexural response of the lithosphere produced by fault displacement resulting in footwall uplift and hangingwall subsidence. We investigate how the flexural isostatic response to extensional faulting controls the structural development of rifted margins. To achieve our aim, we use a kinematic forward model (RIFTER) which incorporates the flexural isostatic response to extensional faulting, crustal thinning, lithosphere thermal loads, sedimentation and erosion. Inputs for RIFTER are derived from seismic reflection interpretation and outputs of RIFTER are the prediction of the structural and stratigraphic consequences of recursive sequential faulting and sedimentation. Using RIFTER we model the simultaneous tectonic development of the Iberia-Newfoundland conjugate rifted margins along the ISE01-SCREECH1 and TGS/LG12-SCREECH2 seismic lines. We quantitatively test and calibrate the model against observed target data restored to breakup time. Two quantitative methods are used to obtain this target data: (i) gravity anomaly inversion which predicts Moho depth and continental lithosphere thinning and (ii) reverse post-rift subsidence modelling to give water and Moho depths at breakup time. We show that extensional faulting occurs on steep ( 60°) normal faults in both proximal and distal parts of rifted margins. Extensional faults together with their flexural isostatic response produce not only sub-horizontal exhumed footwall surfaces (i.e. the rolling hinge model) and highly rotated (60

  17. The influence of inherited structures on magmatic and amagmatic processes in the East African Rift.

    Science.gov (United States)

    Biggs, J.; Lloyd, R.; Hodge, M.; Robertson, E.; Wilks, M.; Fagereng, A.; Kendall, J. M.; Mdala, H. S.; Lewi, E.; Ayele, A.

    2017-12-01

    The idea that crustal heterogeneities, particularly inherited structures, influence the initiation and evolution of continental rifts is not new, but now modern techniques allow us to explore these controls from a fresh perspective, over a range of lengthscales, timescales and depths. In amagmatic rifts, I will demonstrate that deep fault structure is controlled by the stress orientation during the earliest phase of rifting, while the surface expression exploits near-surface weaknesses. I will show that pre-existing structures control the storage and orientation of deeper magma reservoirs in magmatic rifts, while the tectonic stress regime controls intra-rift faulting and shallow magmatism and stresses related to surface loading and cycles of inflation and deflation dominate at volcanic edifices. Finally, I will show how cross-rift structures influence short-term processes such as deformation and seismicity. I will illustrate the talk throughout using examples from along the East African Rift, including Malawi, Tanzania, Kenya and Ethiopia.

  18. Possible Different Rifting Mechanisms Between South and North Part of the Fenhe-Weihe Rift Zone Revealed by Shear Velocity Structures

    Science.gov (United States)

    Ai, S.; Zheng, Y.

    2017-12-01

    As an active intraplate continental rift, FWR plays an important role in accommodating the trans-tension in the Trans North China Craton (TNCO). Velocity field derived from GPS measurements reveals that the northern part of FWR is still under extension in N105°E direction at a rate of 4±2 mm/yr [Shen et al., 2000]. Actually, the FWR has been the most seismically active region in NCC. Bouguer gravity profile and seismic sounding lines [Xu and Ma, 1992] revealed a 2-3 km uplift of Moho depth beneath Taiyuan basin and 5-6 km beneath the Southwestern rift zone, those geophysical observations give clues to the un-evenly upwelling of the asthenosphere beneath the rift system and the different rifting process of the FWR. Therefore, studying the extension process of FWR is meaningful to understanding the NCC geodynamics associated with rifting tectonism. Using vertical continuous waveforms recorded during 2014 from CEarray, we construct a reliable and detailed 3-D crustal and uppermost mantle S-wave velocity structure of FWR, using a Bayesian Monte-Carlo method to jointly interpret teleseismic P-wave receiver functions and Rayleigh wave dispersions [Shen et al., 2013]. In the upmost crust, FWR appear as awful low velocity anomaly zone (LVZ), while the Taihang and Lvliang mountain ranges are imaged as strong high velocity anomaly zones(HVZ). In the middle crust, the low velocity zones still keep their LVZ features Additionally, nearly the whole FWR appears as a linearly LVZ line separating Taihang Uplift and Lvliang Uplift, except beneath Shilingguan and Linshi blocks that separate the Xinxian, Taiyuan and Linfen Basins, consisting with the high seismicity there. The velocity of the lower crust beneath Taiyuan and Weihe Basin are relatively higher than the rest rift regions, we interpret them as the limited mafic underplating beneath the TNCO. From the lower crust to upper mantle, the Datong volcanic zone display robust low velocity features, though the lowest velocity

  19. 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

  20. Structure and tectonics of western continental margin of India: Implication for geologic hazards

    Digital Repository Service at National Institute of Oceanography (India)

    Chaubey, A.K.; Ajay, K.K.

    characteristics of Western Continental Margin of India (WCMI) are closely related to the tectonic history of the Indian subcontinent, its break up during continental rifting, magmatic and sedimentary history, northward movement of India and finally collision... Continental Flood Basalt (DCFB) province on the western and central Indian (Duncan. 1990) as well as continental flood basalt on the Praslin Island in the Seychelles microcontinent (Devey and Stephens, 1991). The DCFB is the largest known continental flood...

  1. Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: A hot rift to drift transition?

    Science.gov (United States)

    Beltrando, Marco; Zibra, Ivan; Montanini, Alessandra; Tribuzio, Riccardo

    2013-05-01

    Rift-related thinning of continental basement along distal margins is likely achieved through the combined activity of ductile shear zones and brittle faults. While extensional detachments responsible for the latest stages of exhumation are being increasingly recognized, rift-related shear zones have never been sampled in ODP sites and have only rarely been identified in fossil distal margins preserved in orogenic belts. Here we report evidence of the Jurassic multi-stage crustal thinning preserved in the Santa Lucia nappe (Alpine Corsica), where amphibolite facies shearing persisted into the rift to drift transition. In this nappe, Lower Permian meta-gabbros to meta-gabbro-norites of the Mafic Complex are separated from Lower Permian granitoids of the Diorite-Granite Complex by a 100-250 m wide shear zone. Fine-grained syn-kinematic andesine + Mg-hornblende assemblages in meta-tonalites of the Diorite-Granite Complex indicate shearing at T = 710 ± 40 °C at P Lucia basement. These results imply that middle to lower crustal rocks can be cooled and exhumed rapidly in the last stages of rifting, when significant crustal thinning is accommodated in less than 5 Myr through the consecutive activity of extensional shear zones and detachment faults. High thermal gradients may delay the switch from ductile shear zone- to detachment-dominated crustal thinning, thus preventing the exhumation of middle and lower crustal rocks until the final stages of rifting.

  2. Geometry of the neoproterozoic and paleozoic rift margin of western Laurentia: Implications for mineral deposit settings

    Science.gov (United States)

    Lund, K.

    2008-01-01

    The U.S. and Canadian Cordilleran miogeocline evolved during several phases of Cryogenian-Devonian intracontinental rifting that formed the western mangin of Laurentia. Recent field and dating studies across central Idaho and northern Nevada result in identification of two segments of the rift margin. Resulting interpretations of rift geometry in the northern U.S. Cordillera are compatible with interpretations of northwest- striking asymmetric extensional segments subdivided by northeast-striking transform and transfer segments. The new interpretation permits integration of miogeoclinal segments along the length of the western North American Cordillera. For the U.S. Cordillera, miogeoclinal segments include the St. Mary-Moyie transform, eastern Washington- eastern Idaho upper-plate margin, Snake River transfer, Nevada-Utah lower-plate margin, and Mina transfer. The rift is orthogonal to most older basement domains, but the location of the transform-transfer zones suggests control of them by basement domain boundaries. The zigzag geometry of reentrants and promontories along the rift is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains. Likewise, transform transfer zones localized subsequent transcurrent structures and igneous activity. Sediment-hosted mineral deposits trace the same zigzag geometry along the margin. Sedimentary exhalative (sedex) Zn-Pb-Ag ??Au and barite mineral deposits formed in continental-slope rocks during the Late Devonian-Mississippian and to a lesser degree, during the Cambrian-Early Ordovician. Such deposits formed during episodes of renewed extension along miogeoclinal segments. Carbonate-hosted Mississippi Valley- type (MVT) Zn-Pb deposits formed in structurally reactivated continental shelf rocks during the Late Devonian-Mississippian and Mesozoic due to reactivation of preexisting structures. The distribution and abundance of sedex and MVT deposits are controlled by the

  3. Direct impacts of outer continental shelf activities on wetland loss in the central Gulf of Mexico

    International Nuclear Information System (INIS)

    Baumann, R.H.; Turner, R.E.

    1990-01-01

    The direct impacts of outer continental shelf (OCS) development on recent wetland loss in the northern Gulf of Mexico were quantified using aerial imagery, field surveys, and literature review. The total direct impacts accounted for an estimated 25.6 percent of total net wetland loss within the Louisiana portion of the study area from 1955/56 to 1978. Of the total direct impacts of 73,905 ha, OCS-related activities accounted for 11,589-13,631 ha of the wetland loss during the same time interval. Although this is a substantial areal loss, it represents only 4.0-4.7 percent of the total Louisiana wetland loss from 1955/56 to 1978, and 15.7-18.4 percent of direct impacts. Direct impacts from OCS pipelines averages 2.49 ha/km, lower than published guidelines, and totaled 12,012 ha. Lowest impacts are for backfilled pipelines in the Chenier Plain of western Louisiana and for small young pipeline canals does not appear to be an important factor for total new wetland loss in the coastal zone because few pipelines are open to navigation and, for the examples found, the impact width was not significantly different than for open pipelines closed to navigation. Navigation channels account for a minimum of 16,902 ha of habitat change. Direct impacts per unit length of navigation channel average 20 times greater than pipelines

  4. Large-scale variation in lithospheric structure along and across the Kenya rift

    Science.gov (United States)

    Prodehl, C.; Mechie, J.; Kaminski, W.; Fuchs, K.; Grosse, C.; Hoffmann, H.; Stangl, R.; Stellrecht, R.; Khan, M.A.; Maguire, Peter K.H.; Kirk, W.; Keller, Gordon R.; Githui, A.; Baker, M.; Mooney, W.; Criley, E.; Luetgert, J.; Jacob, B.; Thybo, H.; Demartin, M.; Scarascia, S.; Hirn, A.; Bowman, J.R.; Nyambok, I.; Gaciri, S.; Patel, J.; Dindi, E.; Griffiths, D.H.; King, R.F.; Mussett, A.E.; Braile, L.W.; Thompson, G.; Olsen, K.; Harder, S.; Vees, R.; Gajewski, D.; Schulte, A.; Obel, J.; Mwango, F.; Mukinya, J.; Riaroh, D.

    1991-01-01

    The Kenya rift is one of the classic examples of a continental rift zone: models for its evolution range from extension of the lithosphere by pure shear1, through extension by simple shear2, to diapiric upwelling of an asthenolith3. Following a pilot study in 19854, the present work involved the shooting of three seismic refraction and wide-angle reflection profiles along the axis, across the margins, and on the northeastern flank of the rift (Fig. 1). These lines were intended to reconcile the different crustal thickness estimates for the northern and southern parts of the rift4-6 and to reveal the structure across the rift, including that beneath the flanks. The data, presented here, reveal significant lateral variations in structure both along and across the rift. The crust thins along the rift axis from 35 km in the south to 20 km in the north; there are abrupt changes in Mono depth and uppermost-mantle seismic velocity across the rift margins, and crustal thickening across the boundary between the Archaean craton and PanAfrican orogenic belt immediately west of the rift. These results suggest that thickened crust may have controlled the rift's location, that there is a decrease in extension from north to south, and that the upper mantle immediately beneath the rift may contain reservoirs of magma generated at greater depth.

  5. Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes

    Science.gov (United States)

    L., Passarelli; E., Rivalta; A., Shuler

    2014-01-01

    As continental rifts evolve towards mid-ocean ridges, strain is accommodated by repeated episodes of faulting and magmatism. Discrete rifting episodes have been observed along two subaerial divergent plate boundaries, the Krafla segment of the Northern Volcanic Rift Zone in Iceland and the Manda-Hararo segment of the Red Sea Rift in Ethiopia. In both cases, the initial and largest dike intrusion was followed by a series of smaller intrusions. By performing a statistical analysis of these rifting episodes, we demonstrate that dike intrusions obey scaling relationships similar to earthquakes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law. Due to the effects of magma supply, the timing of secondary dike intrusions differs from that of the aftershocks. This work provides evidence of self-similarity in the rifting process. PMID:24469260

  6. The influence of tectonic inheritance on crustal extension style following failed subduction of continental crust: applications to metamorphic core complexes in Papua New Guinea

    Science.gov (United States)

    Biemiller, J.; Ellis, S. M.; Little, T.; Mizera, M.; Wallace, L. M.; Lavier, L.

    2017-12-01

    The structural, mechanical and geometric evolution of rifted continental crust depends on the lithospheric conditions in the region prior to the onset of extension. In areas where tectonic activity preceded rift initiation, structural and physical properties of the previous tectonic regime may be inherited by the rift and influence its development. Many continental rifts form and exhume metamorphic core complexes (MCCs), coherent exposures of deep crustal rocks which typically surface as arched or domed structures. MCCs are exhumed in regions where the faulted upper crust is displaced laterally from upwelling ductile material along a weak detachment fault. Some MCCs form during extensional inversion of a subduction thrust following failed subduction of continental crust, but the degree to which lithospheric conditions inherited from the preceding subduction phase control the extensional style in these systems remains unclear. For example, the Dayman Dome in Southeastern Papua New Guinea exposes prehnite-pumpellyite to greenschist facies rocks in a smooth 3 km-high dome exhumed with at least 24 km of slip along one main detachment normal fault, the Mai'iu Fault, which dips 21° at the surface. The extension driving this exhumation is associated with the cessation of northward subduction of Australian continental crust beneath the oceanic lithosphere of the Woodlark Plate. We use geodynamic models to explore the effect of pre-existing crustal structures inherited from the preceding subduction phase on the style of rifting. We show that different geometries and strengths of inherited subduction shear zones predict three distinct modes of subsequent rift development: 1) symmetric rifting by newly formed high-angle normal faults; 2) asymmetric rifting along a weak low-angle detachment fault extending from the surface to the brittle-ductile transition; and 3) extension along a rolling-hinge structure which exhumes deep crustal rocks in coherent rounded exposures. We

  7. Tectonic-magmatic interplay during the early stages of oceanic rifting: temporal constraints from cosmogenic 3He dating in the Dabbahu rift segment, Afar

    Science.gov (United States)

    Williams, A.; Pik, R.; Burnard, P.; Medynski, S.; Yirgu, G.

    2009-12-01

    The Afar Rift in Ethiopia is one of the only subaerial locations in the world where the transition from continental break-up to oceanic-spreading can be observed. Extension and volcanism in the Afar is concentrated in tectono-magmatic segments (TMS), similar in size and morphology to those that characterize mid-ocean ridge systems. However, unlike their submarine equivalents, the Afar TMS contain large silicic central volcanoes, implying that magma differentiation plays an important role in the early evolution of the oceanic rifts. The Dabbahu TMS at the south of the western Afar rift system has recently been the site of significant activity. A massive seismic event in late 2005, triggered by dyke injection, heralded the onset of new rifting period. Volcanism associated with the periods of magma-driven extension has been both silicic (explosive) and basaltic (fissural). The most recent activity in the Afar thus testifies to the close interplay of tectonics and magmatism in rifting environments. In an effort to decipher the long-term structural and volcanic evolution of Dabbahu TMS, we combine cosmogenic 3He dating with geological interpretation of ASTER images and major and trace element analyses of the main volcanic units present. The cosmogenic dating method has advantages over other geochronological tools in that we can target both volcanic and tectonic surfaces of a few Kyr to several Myr age. At Baddi Volcano, an off-axis stratovolcano located west of the Dabbahu rift-axis, basaltic lava flows overlie an acidic base, previously dated at 290 ka using the K-Ar technique (Lahitte et al., 2003). Following preliminary sampling in 2007, we determined cosmogenic 3He ages of 57 ka and 45 ka for two basaltic flows on the flanks of Baddi. We now investigate whether this presumed replenishment of the Baddi magma chamber represents a replenishment of the entire sub-rift plumbing system, and how this in turn relates to the onset and maintenance of surface deformation

  8. Seismically active fracture zones in the continental wedge above the Andean subduction zone in the Arica Elbow region

    Czech Academy of Sciences Publication Activity Database

    Vaněk, Jiří; Hanuš, Václav; Slancová, Alice; Špičák, Aleš

    2007-01-01

    Roč. 9, č. 1-4 (2007), s. 39-57 ISSN 0163-3171 R&D Projects: GA ČR GA205/95/0264; GA AV ČR IAA3012805 Grant - others:UNESCO(FR) IGCP project No. 345 Institutional research plan: CEZ:AV0Z30120515 Source of funding: V - iné verejné zdroje Keywords : continental lithosphere * Wadati-Benioff zone * seismically active zones Subject RIV: DC - Siesmology, Volcanology, Earth Structure

  9. Factors controlling the mode of rift interaction in brittle-ductile coupled systems: A 3D numerical study

    NARCIS (Netherlands)

    Allken, V.; Huismans, R.S.; Thieulot, C.

    2012-01-01

    The way individual faults and rift segments link up is a fundamental aspect of lithosphere extension and continental break-up. Little is known however about the factors that control the selection of the different modes of rift interaction observed in nature. Here we use state-of-the-art large

  10. Effects of energy-related activities on the Atlantic Continental Shelf

    Energy Technology Data Exchange (ETDEWEB)

    Manowitz, B [ed.

    1975-01-01

    Sixteen papers were presented and are announced separately. Coastal waters, continental shelf geology and aquatic ecosystems are studied for modelling basic data for assessment of possible environmental impacts from offshore energy development. Sediment transport and wave phenomena are modelled for understanding water pollution transport and diffusion. (PCS)

  11. Kanda fault: A major seismogenic element west of the Rukwa Rift (Tanzania, East Africa)

    Science.gov (United States)

    Vittori, Eutizio; Delvaux, Damien; Kervyn, François

    1997-09-01

    The NW-SE trending Rukwa Rift, part of the East African Rift System, links the approximately N-S oriented Tanganyika and Nyassa (Malawi) depressions. The rift has a complex half-graben structure, generally interpreted as the result of normal and strike-slip faulting. Morphological and structural data (e.g. fault scarps, faceted spurs, tilting of Quaternary continental deposits, volcanism, seismicity) indicate Late Quaternary activity within the rift. In 1910 an earthquake of M = 7.4 (historically the largest felt in Africa) struck the Rukwa region. The epicentre was located near the Kanda fault, which affects the Ufipa plateau, separating the Rukwa depression from the south-Tanganyika basin. The geomorphic expression of the Kanda fault is a prominent fresh-looking scarp more than 180 km long, from Tunduma to north of Sumbawanga, that strikes roughly NW-SE, and dips constantly northeast. No evidence for horizontal slip was observed. Generally, the active faulting affects a very narrow zone, and is only locally distributed over several subparallel scarps. The height of the scarp progressively decreases towards the northwest, from about 40-50 m to a few metres north of Sumbawanga. Faulted lacustrine deposits exposed in a road cut near Kaengesa were dated as 8340 ± 700 and 13 600 ± 1240 radiocarbon years. These low-energy deposits now hang more than 15 m above the present-day valley floor, suggesting rapid uplift during the Holocene. Due to its high rate of activity in very recent times, the Kanda Fault could have produced the 1910 earthquake. Detailed paleoseismological studies are used to characterize its recent history. In addition, the seismic hazard posed by this fault, which crosses the fast growing town of Sumbawanga, must be seriously considered in urban planning.

  12. Syn-rift unconformities punctuating the lower-middle Cambrian transition in the Atlas Rift, Morocco

    Science.gov (United States)

    Álvaro, J. Javier; Ezzouhairi, Hassan; Clausen, Sébastien; Ribeiro, M. Luisa; Solá, Rita

    2015-04-01

    The Cambrian Tamdroust and Bab n'Ali Volcanic Complexes represent two magmatic episodes developed in the latest Ediacaran-Cambrian Atlas Rift of Morocco. Their rifting pulses were accompanied by accumulation of volcanosedimentary edifices (dominated by effusive lava flows in the former and explosive acidic aprons in the latter) associated with active tilting and uplift. Sealing of their peneplaned horst-and-graben palaeotopographies led to the onset of distinct onlapping geometries and angular discordances capping eroded basements ranging from the Ediacaran Ouarzazate Supergroup to the Cambrian Asrir Formation. Previous interpretations of these discordances as pull-apart or compressive events are revised here and reinterpreted in an extensional (rifting) context associated with active volcanism. The record of erosive unconformities, stratigraphic gaps, condensed beds and onlapping patterns across the traditional "lower-middle Cambrian" (or Cambrian Series 2-3) transition of the Atlas Rift must be taken into consideration for global chronostratigraphic correlation based on their trilobite content.

  13. Study of the magmatism related to the rifting of the central and southern Atlantic: 40Ar/39Ar geochronology and geochemistry of Jurassic intrusives of Guinea and French Guyana/Surinam, and Cretaceous intrusives of Brazil

    International Nuclear Information System (INIS)

    Deckart, K.

    1996-01-01

    The initial stage of continental rifting in the Central and South Atlantic has been accompanied by tholeiitic magmatism, which is mainly represented by sills, dykes, layered intrusions and lava flows. During the rifting progression, the syn-rift stage in the South Atlantic has been accompanied by abundant alkaline magmatism. A geochronological and geochemical study has been performed on these formations with the aim to contribute to the understanding of the early continental rifting processes and their evolution. 40 Ar/ 39 Ar analyses have been done on tholeiitic intrusives of Guinea and French Guyana/Surinam, tholeiitic dykes, associated with the Parana volcanism (Brazil), and alkaline dykes in the region of Rio de Janeiro (Brazil). The geochemical and isotopic study has been focused on the tholeiitic intrusions from Guinea and French Guyana/Surinam. These three arms may represent the three branches of a triple junction which was active between 134 to 129 Ma, and which was at the origin of at least the northern Parana traps. Even if the principal magmatic activity can be related to the thermal anomaly due to the Tristan da Cunha hotspot, which favours an active rifting, the tectonic system of the triple junction is not compatible in time and space with this hotspot and therefore with this geodynamic model. It is possible that the Parana traps (133-130 Ma) are only partly contemporaneous and therefore, they might be not related to the same mode of geodynamic initiation. Biotites from the alkaline magmatics of the dyke swarm (NE-SW) near Rio de Janeiro display plateau ages between 82 and 70 Ma; this intense alkaline magmatism was related to vertical movements characterising the syn-rift stage not only in SE-Brazil but also in equatorial Africa. (author)

  14. Syn-rift unconformities punctuating the lower-middle Cambrian transition in the Atlas Rift, Morocco

    OpenAIRE

    Álvaro, J. Javier; Ezzouhairi, Hassan; Clausen, Sébastien; Ribeiro, Maria Luísa; Solá, Ana Rita

    2015-01-01

    The Cambrian Tamdroust and Bab n’Ali Volcanic Complexes represent two magmatic episodes developed in the latest Ediacaran–Cambrian Atlas Rift of Morocco. Their rifting pulses were accompanied by accumulation of volcanosedimentary edifices (dominated by effusive lava flows in the former and explosive acidic aprons in the latter) associated with active tilting and uplift. Sealing of their peneplaned horst-and-graben palaeotopographies led to the onset of distinct onlapping geometrie...

  15. Active Deformation of Malawi Rift's North Basin Hinge Zone Modulated by Reactivation of Preexisting Precambrian Shear Zone Fabric

    Science.gov (United States)

    Kolawole, F.; Atekwana, E. A.; Laó-Dávila, D. A.; Abdelsalam, M. G.; Chindandali, P. R.; Salima, J.; Kalindekafe, L.

    2018-03-01

    We integrated temporal aeromagnetic data and recent earthquake data to address the long-standing question on the role of preexisting Precambrian structures in modulating strain accommodation and subsequent ruptures leading to seismic events within the East African Rift System. We used aeromagnetic data to elucidate the relationship between the locations of the 2009 Mw 6.0 Karonga, Malawi, earthquake surface ruptures and buried basement faults along the hinge zone of the half-graben comprising the North Basin of the Malawi Rift. Through the application of derivative filters and depth-to-magnetic-source modeling, we identified and constrained the trend of the Precambrian metamorphic fabrics and correlated them to the three-dimensional structure of buried basement faults. Our results reveal an unprecedented detail of the basement fabric dominated by high-frequency WNW to NW trending magnetic lineaments associated with the Precambrian Mughese Shear Zone fabric. The high-frequency magnetic lineaments are superimposed by lower frequency NNW trending magnetic lineaments associated with possible Cenozoic faults. Surface ruptures associated with the 2009 Mw 6.0 Karonga earthquake swarm aligned with one of the NNW-trending magnetic lineaments defining a normal fault that is characterized by right-stepping segments along its northern half and coalesced segments on its southern half. Fault geometries, regional kinematics, and spatial distribution of seismicity suggest that seismogenic faults reactivated the basement fabric found along the half-graben hinge zone. We suggest that focusing of strain accommodation and seismicity along the half-graben hinge zone is facilitated and modulated by the presence of the basement fabric.

  16. Prediction, assessment of the Rift Valley fever activity in East and Southern Africa 2006-2008 and possible vector control strategies.

    Science.gov (United States)

    Anyamba, Assaf; Linthicum, Kenneth J; Small, Jennifer; Britch, Seth C; Pak, Edwin; de La Rocque, Stephane; Formenty, Pierre; Hightower, Allen W; Breiman, Robert F; Chretien, Jean-Paul; Tucker, Compton J; Schnabel, David; Sang, Rosemary; Haagsma, Karl; Latham, Mark; Lewandowski, Henry B; Magdi, Salih Osman; Mohamed, Mohamed Ally; Nguku, Patrick M; Reynes, Jean-Marc; Swanepoel, Robert

    2010-08-01

    Historical outbreaks of Rift Valley fever (RVF) since the early 1950s have been associated with cyclical patterns of the El Niño/Southern Oscillation (ENSO) phenomenon, which results in elevated and widespread rainfall over the RVF endemic areas of Africa. Using satellite measurements of global and regional elevated sea surface temperatures, elevated rainfall, and satellite derived-normalized difference vegetation index data, we predicted with lead times of 2-4 months areas where outbreaks of RVF in humans and animals were expected and occurred in the Horn of Africa, Sudan, and Southern Africa at different time periods from September 2006 to March 2008. Predictions were confirmed by entomological field investigations of virus activity and by reported cases of RVF in human and livestock populations. This represents the first series of prospective predictions of RVF outbreaks and provides a baseline for improved early warning, control, response planning, and mitigation into the future.

  17. DEFORMATION WAVES AS A TRIGGER MECHANISM OF SEISMIC ACTIVITY IN SEISMIC ZONES OF THE CONTINENTAL LITHOSPHERE

    Directory of Open Access Journals (Sweden)

    S. I. Sherman

    2013-01-01

    fault zones, a brief description of the method for assessment of spatial and temporal regularities in locations of earthquake epicentres in zones of dynamic influence of faults is provided. The method can be applied to estimate a dominating direction of movement of the epicentres, which corresponds to the phase velocity of the deformation wave disturbing meta-stability of the fault-block medium, leading to displacement of neighbouring blocks and thus causing a seismic event (Fig. 14. By integration of vectors of migration of epicentres at active faults, it is possible to demonstrate a pattern of vectors of movements of the deformation waves in the seismic zones of the continental lithosphere (Fig. 18.Regional and trans-regional deformation waves are analyzed. For seismic zones of Central Asia, vectors of deformation waves are established, a scheme showing regional orientations of the waves is developed, and main wave parameters (length and time period are estimated (Fig. 19. Three depth levels of deformation waves are distinguished: the whole lithosphere, the upper brittle part of the lithosphere, and the top part of the brittle layer (Fig. 20.It is concluded that the leading factor of gradual accumulation of earthquake foci, which takes place regularly in space and time in seismic zones, are deformation waves that influence the geophysical medium. This understanding of the fundamental basis of seismic process needs to be more thoroughly justified with application of modern concepts, its revised phenomenological concept and development of a model representing a seismic zones as a geologically and geophysically independent structure of the lithosphere, which has its specific properties, based on which testing of the lithosphere becomes possible for purposes of potential earthquake prediction.

  18. Atlantic continental margin of the United States

    Science.gov (United States)

    Grow, John A.; Sheridan, Robert E.; Palmer, A.R.

    1982-01-01

    The objective of this Decade of North American Geology (D-NAG) volume will be to focus on the Mesozoic and Cenozoic evolution of the U.S. Atlantic continental margin, including the onshore coastal plain, related onshore Triassic-Jurassic rift grabens, and the offshore basins and platforms. Following multiple compressional tectonic episodes between Africa and North America during the Paleozoic Era that formed the Appalachian Mountains, the Mesozoic and Cenozoic Eras were dominated by tensional tectonic processes that separated Africa and North America. Extensional rifting during Triassic and Early Jurassic times resulted in numerous tensional grabens both onshore and offshore, which filled with nonmarine continental red beds, lacustrine deposits, and volcanic flows and debris. The final stage of this breakup between Africa and North America occurred beneath the present outer continental shelf and continental slope during Early or Middle Jurassic time when sea-floor spreading began to form new oceanic crust and lithosophere between the two continents as they drifted apart. Postrift subsidence of the marginal basins continued in response to cooling of the lithosphere and sedimentary loading.Geophysical surveys and oil-exploration drilling along the U.S. Atlantic continental margin during the past 5 years are beginning to answer many questions concerning its deep structure and stratigraphy and how it evolved during the rifting and early sea-floor-spreading stages of the separation of this region from Africa. Earlier geophysical studies of the U.S. continental margin used marine refraction and submarine gravity measurements. Single-channel seismic-reflection, marine magnetic, aeromagnetic, and continuous gravity measurements became available during the 1960s.

  19. The effect of tectonic evolution on lacustrine syn-rift sediment patters in Qikou Sag, Bohaiwan Basin, eastern China

    Science.gov (United States)

    Liao, Y.; Wang, H.; Xu, W.

    2013-12-01

    Normal fault arrays and associated relay ramps between two overlapping en-echelon normal faults are well known to control the deposition and distribution of sediments in alluvial, fluvial and deltaic systems in rift settings. The influence of transfer zones or relay ramps on sediment routes and dispersal patterns in subaqueous (deeper marine/lacustrine), however, is barely studied and hence less clear. Previous experimental studies indicate that subaqueous relay ramps may act as sediment transportation pathways if certain conditions are available. In this study, we integrate detailed structural and stratigraphic analysis with three-dimensional seismic data and limited well log data from the Qikou Sag to examine the tectonic evolution and the syn-rift sediment patterns response to fault growth and linkage in an active rift setting. Qikou Sag is located at the center of Huanghua Depression, Bohaiwan Basin of eastern China. Structurally, it is a typical continental rift basin characterized by a linked system of two NEE-SWW-striking half-grabens and one E-W-striking graben. Qikou sag is filled with Eocene-Oligocene syn-rift sediments and Miocene to Quaternary post-rift sediments. The Eocene-Oligocene rifting stage can be divided into early rifting period (43-36.5 Ma, the third member and second member of Shahejie Formation, Es3 and Es2), stable rifting period (36.5-29Ma, the first member of Shaehejie Formation, Es1) and fault-depressed diversionary period (29-24.6Ma, the Dongying Formation, Ed). This study focus on the early syn-rift, the third and second member of Shehejie Formation, which is mostly dark-grey mudstone interbedded with fine to coarse-grained sandstone deposited by large-scale turbidity currents in deep-lake. In particular, we use a combination of thickness variability and facies distributions, onlap patterns within a high-resolution sequence stratigraphic framework, integrated with structural geometry, fault activity and subsidence history analysis to

  20. SYN-RIFT SANDSTONЕS: THE FEATURES OF BULK CHEMICAL COMPOSITIONS, AND POSITIONS ON PALEOGEODYNAMIC DISCRIMINANT DIAGRAMS

    Directory of Open Access Journals (Sweden)

    A. V. Maslov

    2018-01-01

    Full Text Available From the early 1980s, the data on the bulk chemical composition of sandstones and mudstones are actively involved for interpretation of the paleogeodynamic settings for sedimentary sequences. Discriminant diagrams such as K2O/Na2O–SiO2/Al2O3 [Maynard et al., 1982], (Fe2O3*+MgO–K2O/Na2O and others [Bhatia, 1983], SiO2–K2O/Na2O [Roser, Korsch, 1986], (K2O+Na2O–SiO2/20–(TiO2+Fe2O3+MgO [Kroonenberg, 1994] etc., are now widely used in regional investigations to classify terrigenous rocks from several paleogeodynamic settings (passive and active continental margins, oceanic and continental volcanic arcs etc. with a certain ‘percentage of consistency’. The first diagrams DF1–DF2 for syn-rift compositions were published in the early 2010s [Verma, Armstrong-Altrin, 2013]. This article analyzes the bulk chemical compositions of syn-rift sandstones from intracratonic rifts and rifts formed during the break-up of the Columbia and Gondwana supercontinents, rifts within volcanic arcs and related to the collapse of collision orogens (for example, Permian sandstones of the Malužiná formation, Western Carpathians, Slovakia. Our database includes the Neoproterozoic Uinta Mountain Group (USA, the Cretaceous Omdurman formation of the Khartoum Basin (Sudan, the siliciclastic deposits of the Kalahari Basin (East African rift zone, the sandstones of the Vindhyan Supergroup (India, the Neoproterozoic Ui Group of the Uchur-Maya region (Southeast Siberia, the Meso-Neoproterozoic Banxi Group (Southern China, the Mesoproterozoic Belt-Purcell Supergroup (USA, the Oronto and Bayfield Groups of the Midcontinent (USA, as well as the sandstones of the Upper Precambrian Ai and Mashak formations, and the metasedimentary rocks of the Arsha Group (Southern Urals. The article examines: (1 the position of the syn-rift sandstone compositions (fields on the log(SiO2/Al2O3–log(Na2O/K2O classification diagram and the F1–F2 diagram, which gives the possible

  1. Geomorphic and Structural Evidence for Rolling Hinge Style Deformation in the Footwall of an Active Low Angle Normal Fault, Mai'iu Fault, Woodlark Rift, SE Papua New Guinea

    Science.gov (United States)

    Mizera, M.; Little, T.; Norton, K. P.; Webber, S.; Ellis, S. M.; Oesterle, J.

    2016-12-01

    While shown to operate in oceanic crust, rolling hinge style deformation remains a debated process in metamorpic core complexes (MCCs) in the continents. The model predicts that unloading and isostatic uplift during slip causes a progressive back-tilting in the upper crust of a normal fault that is more steeply dipping at depth. The Mai'iu Fault in the Woodlark Rift, SE Papua New Guinea, is one of the best-exposed and fastest slipping (probably >7 mm/yr) active low-angle normal faults (LANFs) on Earth. We analysed structural field data from this fault's exhumed slip surface and footwall, together with geomorphic data interpreted from aerial photographs and GeoSAR-derived digital elevation models (gridded at 5-30 m spacing), to evaluate deformational processes affecting the rapidly exhuming, domal-shaped detachment fault. The exhumed fault surface emerges from the ground at the rangefront near sea level with a northward dip of 21°. Up-dip, it is well-preserved, smooth and corrugated, with some fault remnants extending at least 29 km in the slip direction. The surface flattens over the crest of the dome, beyond where it dips S at up to 15°. Windgaps perched on the crestal main divide of the dome, indicate both up-dip tectonic advection and progressive back-tilting of the exhuming fault surface. We infer that slip on a serial array of m-to-km scale up-to-the-north, steeply S-dipping ( 75°) antithetic-sense normal faults accommodated some of the exhumation-related, inelastic bending of the footwall. These geomorphically well expressed faults strike parallel to the main Mai'iu fault at 110.9±5°, have a mean cross-strike spacing of 1520 m, and slip with a consistent up-to-the-north sense of throw ranging from <5 m to 120 m. Apparently the Mai'iu Fault was able to continue slipping despite having to negotiate this added fault-roughness. We interpret the antithetic faulting to result from bending stresses, and to provide the first clear examples of rolling hinge

  2. Continental tectonics and continental kinetics

    International Nuclear Information System (INIS)

    Allegre, C.J.; Jaupart, C.; Paris-7 Univ., 75

    1985-01-01

    We present a model of continental growth which combines the results of geochemical studies and tectonic ideas about the evolution of continents through geological time. The process of continental growth is mainly controlled by surface phenomena. Continental material is extracted from the mantle along subduction zones at the periphery of oceans, and is destroyed in collision zones where it is remobilized and made available for subduction. We derive an equation for S, the portion of the Earth's surface occupied by continents, which reads as follows: dS/dt=a . √(1-S)-b . S. Coefficients a and b depend on the geometry of plates, on their number and on their velocities. We assume that they decrease exponentially with time with the same time-scale α. This model satisfies both geochemical and tectonic constraints, and allows the integration of several current observations in a single framework. (orig.)

  3. Petrological Constraints on Melt Generation Beneath the Asal Rift (Djibouti)

    Science.gov (United States)

    Pinzuti, P.; Humler, E.; Manighetti, I.; Gaudemer, Y.; Bézos, A.

    2010-12-01

    The temporal evolution of the mantle melting processes in the Asal Rift is evaluated from the chemical composition of 95 lava flows sampled along 10 km of the rift axis and 8 km off-axis (that is for the last 650 ky). The major element composition and the trace element ratios of aphyric basalts across the Asal Rift show a symmetric pattern relative to the rift axis and preserved a clear signal of mantle melting depth variations. FeO, Fe8.0, Sm/YbN and Zr/Y increase, whereas SiO2 and Lu/HfN decrease from the rift axis to the rift shoulders. These variations are qualitatively consistent with a shallower melting beneath the rift axis than off-axis and the data show that the melting regime is inconsistent with a passive upwelling model. In order to quantify the depth range and extent of melting, we invert Na8.0 and Fe8.0 contents of basalts based on a pure active upwelling model. Beneath the rift axis, melting paths are shallow, from 60 to 30 km. These melting paths are consistent with adiabatic melting in normal-temperature asthenosphere, beneath an extensively thinned mantle lithosphere. In contrast, melting on the rift shoulders occurred beneath a thick mantle lithosphere and required mantle solidus temperature 180°C hotter than normal (melting paths from 110 to 75 km). The calculated rate of lithospheric thinning is high (6.0 cm yr-1) and could explain the survival of a metastable garnet within the mantle at depth shallower than 90 km beneath the modern Asal Rift.

  4. How sedimentation affects rift segment interaction during oblique extension: a 4D analogue modelling study

    Science.gov (United States)

    Zwaan, Frank; Schreurs, Guido; Adam, Jürgen

    2017-04-01

    prevents the viscous sand/silicone layer from isostatic rising as observed in the models without sedimentation. In models with syn-rift sedimentation, the rift wedge sinks into the viscous seed below. Analysis of surface structures indicates that sedimentation also results in differences in internal fault structures within the rift. In order to validate our first experimental results, we will (re)run various models in a X-Ray CT-scanner to reveal their internal structure in detail. Combined with high-resolution surface scanning of the model topography, a thorough 4D surface and internal evolution assessment will be possible, including tomographic strain analysis using Digital Volume Correlation techniques (DVC). REFERENCES Bialas and Buck, 2009, How sediment promotes narrow rifting: Application to the Gulf of California. Tectonics 28, TC4014 Brune, Popov and Sobolev, 2012, Modeling suggests that oblique extension facilitates rifting and continental break-up. Journal of Geophysical Research 117, B08402 Zwaan, Naliboff, Schreurs and Buiter, 2016, Insights into the effects of oblique extension on continental rift interaction from 3D analogue and numerical models. Tectonophysics 693, Part B, 239-260

  5. Along-Axis Structure and Crustal Construction Processes of Spreading Segments in Iceland: Implications for Magmatic Rifts

    Science.gov (United States)

    Siler, D. L.; Karson, J. A.

    2017-10-01

    Magmatic rift systems are composed of discrete spreading segments defined by morphologic, structural, and volcanic features that vary systematically along strike. In Iceland, structural features mapped in the glaciated and exhumed Miocene age upper crust correlate with analogous features in the seismically and volcanically active neovolcanic zone. Integrating information from both the active rift zones and ancient crust provides a three-dimensional perspective of crustal structure and the volcanic and tectonic processes that construct crust along spreading segments. Crustal exposures in the Skagi region of northern Iceland reveal significant along-strike variations in geologic structure. The upper crust at exhumed magmatic centers (segment centers) is characterized by a variety of intrusive rocks, high-temperature hydrothermal alteration, and geologic evidence for kilometer-scale subsidence. In contrast, the upper crust along segment limbs, which extend along strike from magmatic centers, is characterized by thick sections of gently dipping lava flows, cut by varying proportions of subvertical dikes. This structure implies relatively minor upper crustal subsidence and lateral dike intrusion. The differing modes of subsidence beneath segment centers and segment limbs require along-axis mass redistribution in the underlying upper, middle, and lower crust during crustal construction. This along-axis material transport is accomplished through lateral dike intrusion in the upper crust and by along-axis flow of magmatic to high-temperature solid-state gabbroic material in the middle and lower crust. These processes, inferred from outcrop evidence in Skagi, are consistent with processes inferred to be important during active rifting in Iceland and at analogous magmatic oceanic and continental rifts.

  6. Surface analogue outcrops of deep fractured basement reservoirs in extensional geological settings. Examples within active rift system (Uganda) and proximal passive margin (Morocco).

    Science.gov (United States)

    Walter, Bastien; Géraud, Yves; Diraison, Marc

    2014-05-01

    structures). Two field cases, located in Morocco and Uganda, allow us to investigate basement complexes at different stages of an extension process and give us analog geological data of similar fractured basement reservoirs. Border faults and associated fracture networks of an active rifting system propagated in Proterozoic basement rocks are analyzed in the Albertine rift system in Uganda. Brittle structures developed along a proximal passive margin of the Atlantic domain are analyzed in Proterozoic basements rocks in Western Anti-Atlas in Morocco.

  7. Transition From a Magmatic to a Tectonic Rift System : Seismotectonics of the Eyasi- Manyara Region, Northern Tanzania, East Africa

    Science.gov (United States)

    Albaric, J.; Perrot, J.; Deschamps, A.; Deverchere, J.; Wambura, R. F.; Tiberi, C.; Petit, C.; Le Gall, B.; Sue, C.

    2008-12-01

    How a rift system propagates and breaks throughout a cold and thick continental crust remains poorly known. Only few places allow to address the question. In the East African Rift System (EARS), the eastern magma- rich branch abruptly splits into two amagmatic arms (the Eyasi and Manyara faulted systems), south of a E-W volcanic chain (the Ngorongoro-Kilimanjaro transverse volcanic belt), as crossing the Archaean Tanzanian craton margin. We present the first detailed seismotectonic picture of the Eyasi-Manyara rifts where a network of ~25 seismometers was settled from June to November 2007 (SEISMO-TANZ'07 seismological experiment). From the seismicity recorded by the network, we identify active faults and discuss the stress field framework obtained from the inversion of focal mechanisms. We use the determined depth of earthquakes (1) to discuss the crustal structure of the transition zone from a magma-rich to a magma-starved section of the EARS and (2) to further emphasize the rheological control on depth distributions in the EARS (Albaric et al., Tectonophysics, 2008). The stress and strain directions deduced from our work are also used to question recently published kinematics and conceptual models of the EARS (Calais et al., Geol. Soc. London, 2006 ; Le Gall et al., Tectonophysics, 2008).

  8. The Relationships of Subparallel Synthetic Faults and Pre-existing Structures in the Central Malawi Rift

    Science.gov (United States)

    Johnson, S.; Mendez, K.; Beresh, S. C. M.; Mynatt, W. G.; Elifritz, E. A.; Laó-Dávila, D. A.; Atekwana, E. A.; Abdelsalam, M. G.; Chindandali, P. R. N.; Chisenga, C.; Gondwe, S.; Mkumbura, M.; Kalaguluka, D.; Kalindekafe, L.; Salima, J.

    2017-12-01

    The objective of our research is to explore the evolution of synthetic fault systems in continental rifts. It has been suggested that during the rifting process border faults may become locked and strain is then accommodated within the hanging wall. The Malawi Rift provides an opportunity to study the evolution of these faults within a young (8 Ma), active and magma-poor continental rift. Two faults in central Malawi may show the transference of strain into the hanging wall. These faults are the older Chirobwe-Ntcheu with a length of 115 km and a scarp height of 300-1000 m and the younger Bilila-Mtakataka with a length of 130 km and a scarp height of 4-320 m. We used high-resolution aeromagnetic data and 30m resolution Shuttle Radar Topography Mission (SRTM) digital elevation models (DEM) to provide a 3D spatial characterization of the fault system. Additionally 10cm resolution DEMs were created using unmanned aerial system (UAS) derived aerial photography and Structure from Motion to document the regional Precambrian foliation and joint patterns. Moreover, displacement profiles where extracted from the SRTM-DEM data to compare the segmentation and linkage of the outer and inner faults. Our preliminary results show that the strike of each fault is approximately NW-SE which follows the strike of the Precambrian fabric. The magnetic fabric has a strike of NW-SE in the south changing to NE-SW in the north suggesting that the faults are controlled in part by an inherited Precambrian fabric. The displacement profile of the inner Bilila-Mtakataka fault is asymmetric and displays five fault segments supporting the interpretation that this is a relatively young fault. The expected results of this study are information about segmentation and displacement of each fault and their relationship to one another. The results from the aeromagnetic data utilizing Source Parameter Imaging to produce an approximate depth to basement which will support the displacement profiles derived

  9. Using of Remote Sensing Techniques for Monitoring the Earthquakes Activities Along the Northern Part of the Syrian Rift System (LEFT-LATERAL),SYRIA

    Science.gov (United States)

    Dalati, Moutaz

    Earthquake mitigation can be achieved with a better knowledge of a region's infra-and substructures. High resolution Remote Sensing data can play a significant role to implement Geological mapping and it is essential to learn about the tectonic setting of a region. It is an effective method to identify active faults from different sources of Remote Sensing and compare the capability of some satellite sensors in active faults survey. In this paper, it was discussed a few digital image processing approaches to be used for enhancement and feature extraction related to faults. Those methods include band ratio, filtering and texture statistics . The experimental results show that multi-spectral images have great potentials in large scale active faults investigation. It has also got satisfied results when deal with invisible faults. Active Faults have distinct features in satellite images. Usually, there are obvious straight lines, circular structures and other distinct patterns along the faults locations. Remotely Sensed imagery Landsat ETM and SPOT XS /PAN are often used in active faults mapping. Moderate and high resolution satellite images are the best choice, because in low resolution images, the faults features may not be visible in most cases. The area under study is located Northwest of Syria that is part of one of the very active deformation belt on the Earth today. This area and the western part of Syria are located along the great rift system (Left-Lateral or African- Syrian Rift System). Those areas are tectonically active and caused a lot of seismically events. The AL-Ghab graben complex is situated within this wide area of Cenozoic deformation. The system formed, initially, as a result of the break up of the Arabian plate from the African plate. This action indicates that these sites are active and in a continual movement. In addition to that, the statistic analysis of Thematic Mapper data and the features from a digital elevation model ( DEM )produced from

  10. Tectonic history and the biogeography of the freshwater fishes from the coastal drainages of eastern Brazil: an example of faunal evolution associated with a divergent continental margin

    Directory of Open Access Journals (Sweden)

    Alexandre Cunha Ribeiro

    Full Text Available The eastern Brazilian coastal drainages are of great biogeographical significance, because of their highly endemic fish faunas. Phylogenetic patterns suggest a close biotic relationship between the rivers that flow into the Atlantic and those on the adjacent upland crystalline shield. However, little has been said on the dynamics of the geological processes causally related to the cladogenetic events between these areas. Distributional and phylogenetic patterns suggest a close association with the geological history of the passive continental margin of South America, from the Cretaceous to the present day. In this area megadome uplifts, rifting, vertical movements between rifted blocks and the erosive retreat of the South American eastern continental margin are hypothesized as the main geological forces controlling the distribution of freshwater fishes. The tectonic activity associated with the break-up of Gondwana and separation of South America and Africa formed six megadomes that control most of the current courses of the main crystalline shield river basins. Except for basins located at the edges of such megadomes, these river systems developed long, circuitous routes over the ancient Brazilian crystalline shield before emptying into the recently opened Atlantic Ocean. Initial cladogenetic events between upland crystalline drainages and Atlantic tributaries were probably associated with vicariant processes, and some ancient basal sister-groups of widespread inclusive taxa are found in these coastal hydrographic systems. Later, generalized erosive denudation resulted in an isostatic adjustment of the eastern margin of the platform. These, along with reactivations of ancient rifts led to vertical movements between rifted blocks and gave rise, in southeastern Brazil, to taphrogenic (rift related basins. These basins, such as the Taubaté, São Paulo, Curitiba and Volta Redonda basins, among others, captured adjacent upland drainages and fauna

  11. Crustal structure variations along the NW-African continental margin: A comparison of new and existing models from wide-angle and reflection seismic data

    Science.gov (United States)

    Klingelhoefer, Frauke; Biari, Youssef; Sahabi, Mohamed; Aslanian, Daniel; Schnabel, Michael; Matias, Luis; Benabdellouahed, Massinissa; Funck, Thomas; Gutscher, Marc-André; Reichert, Christian; Austin, James A.

    2016-04-01

    Deep seismic data represent a key to understand the geometry and mechanism of continental rifting. The passive continental margin of NW-Africa is one of the oldest on earth, formed during the Upper Triassic-Lower Liassic rifting of the central Atlantic Ocean over 200 Ma. We present new and existing wide-angle and reflection seismic data from four study regions along the margin located in the south offshore DAKHLA, on the central continental margin offshore Safi, in the northern Moroccan salt basin, and in the Gulf of Cadiz. The thickness of unthinned continental crust decreases from 36 km in the North to about 27 km in the South. Crustal thinning takes place over a region of 150 km in the north and only 70 km in the south. The North Moroccan Basin is underlain by highly thinned continental crust of only 6-8 km thickness. The ocean-continent transition zone shows a variable width between 40 and 70 km and is characterized by seismic velocities in between those of typical oceanic and thinned continental crust. The neighbouring oceanic crust is characterized by a thickness of 7-8 km along the complete margin. Relatively high velocities of up to 7.5 km/s have been imaged between magnetic anomalies S1 and M25, and are probably related to changes in the spreading velocities at the time of the Kimmeridgian/Tithonian plate reorganization. Volcanic activity seems to be mostly confined to the region next to the Canary Islands, and is thus not related to the initial opening of the ocean, which was associated to only weak volcanism. Comparison with the conjugate margin off Nova Scotia shows comparable continental crustal structures, but 2-3 km thinner oceanic crust on the American side than on the African margin.

  12. Determination of in-situ rock stresses related to petroleum activities on the Norwegian Continental Shelf

    Energy Technology Data Exchange (ETDEWEB)

    Fejerskov, Morten

    1996-12-31

    In-situ rock stresses have proved to be important for exploration and production of hydrocarbons. This thesis uses various stress determination techniques to characterize the in-situ stress field on the Norwegian Continental Shelf. Leak off tests and borehole breakouts have been used to determine the in-situ stress in deep well bores. Overcoring and earthquake focal mechanisms data have also been compiled and evaluated together with stress information from petroleum wells. A new test procedure is suggested to improve the quality of leak off tests that emphasises the importance of accurate pressure determination, longer test periods and digital data sampling. Four-arm calliper logs are used to identify borehole breakouts on the Tampen Spur and Horda Platform. However, it proved difficult to distinguish borehole breakouts from other drilling processes since the elongations of the borehole consistently appeared in the direction of hole azimuth; drilling-induced key seats are here observed in vertical wells. This behaviour is discussed but not fully understood. This reduces the number of reliable stress data and their quality enormously and even casts some doubt about the quality of previous breakout studies from the North Sea. A highly compressive horizontal stress field seems to be present onshore and offshore Norway. Different stress determination techniques yield very consistent stress orientation, regional and internal variation. A 1. order stress direction is identified, where the maximum horizontal stress direction is rotating from N-S in the Barents Sea to NW-SE in the Norwegian Sea and WNW-ESE in the northern North Sea. At Tampen Spur, a dominant WNW-ESE maximum horizontal stress direction, normal to the major tectonic structures is identified as well as a minimum stress close to the vertical stress, the latter indicating high horizontal stresses. 250 refs., 91 figs., 14 tabs.

  13. Continental Mathematics League.

    Science.gov (United States)

    Quartararo, Joseph

    2002-01-01

    This article describes the activities of the Continental Mathematics League, which offers a series of meets for children in grades 3 though 9. In addition, a Calculus League and a Computer Contest are offered. The league allows schools to participate by mail so that rural schools can participate. (CR)

  14. Incorporating Cutting Edge Scientific Results from the Margins-Geoprisms Program into the Undergraduate Curriculum, Rupturing Continental Lithosphere Part II: Introducing Euler Poles Using Baja-North America Relative Plate Motion Across the Gulf of California

    Science.gov (United States)

    Loveless, J. P.; Bennett, S. E. K.; Cashman, S. M.; Dorsey, R. J.; Goodliffe, A. M.; Lamb, M. A.

    2014-12-01

    The NSF-MARGINS Program funded a decade of research on continental margin processes. The NSF-GeoPRISMS Mini-lesson Project, funded by NSF-TUES, is designed to integrate the significant findings from the MARGINS program into open-source college-level curriculum. The Gulf of California (GOC) served as the focus site for the Rupturing Continental Lithosphere (RCL) initiative, which addressed several scientific questions: What forces drive rift initiation, localization, propagation and evolution? How does deformation vary in time and space, and why? How does crust evolve, physically and chemically, as rifting proceeds to sea-floor spreading? What is the role of sedimentation and magmatism in continental extension? We developed two weeks of curriculum, including lectures, labs, and in-class activities that can be used as a whole or individually. This component of the curriculum introduces students to the Euler pole description of relative plate motion (RPM) by examining the tectonic interactions of the Baja California microplate and North American plate. The plate boundary varies in rift obliquity along strike, from highly oblique and strike-slip dominated in the south to slightly less oblique and with a larger extensional component in the north. This Google Earth-based exercise provides students with a visualization of RPM using small circle contours of the local direction and magnitude of Baja-North America movement on a spherical Earth. Students use RPM to calculate the fault slip rates on transform, normal, and oblique-slip faults and examine how the varying faulting styles combine to accommodate RPM. MARGINS results are integrated via comparison of rift obliquity with the structural style of rift-related faults around the GOC. We find this exercise to fit naturally into courses about plate tectonics, geophysics, and especially structural geology, given the similarity between Euler pole rotations and stereonet-based rotations of structural data.

  15. The rifted margin of Saudi Arabia

    Science.gov (United States)

    McClain, J. S.; Orcutt, J. A.

    The structure of rifted continental margins has always been of great scientific interest, and now, with dwindling economic oil deposits, these complex geological features assume practical importance as well. The ocean-continent transition is, by definition, laterally heterogeneous and likely to be extremely complicated. The southernmost shotpoints (4, 5, and 6) in the U.S. Geological Survey seismic refraction profile in the Kingdom of Saudi Arabia lie within a transition region and thus provide a testing ground for methods that treat wave propagation in laterally heterogeneous media. This portion of the profile runs from the Farasan Islands in the Red Sea across the coast line and the Hijaz-Asir escarpment into the Hijaz-Asir tectonic province. Because the southernmost shotpoint is within the margin of the Saudi sub-continent, the full transition region is not sampled. Furthermore, such an experiment is precluded by the narrowness of the purely oceanic portion of the Red Sea.

  16. Orogenic structural inheritance and rifted passive margin formation

    Science.gov (United States)

    Salazar Mora, Claudio A.; Huismans, Ritske S.

    2016-04-01

    of Campos Basin, offshore Brazil: Evidence from 3D seismic analysis and section restoration. Marine and Petroleum Geology 26, 873-886. Tommasi, A., Vauchez, A., 2001. Continental rifting parallel to ancient collisional belts: An effect of the mechanical anisotropy of the lithospheric mantle. Earth and Planetary Science Letters 185, 199-210.

  17. Continental rift-setting and evolution of Neoproterozoic Sindreth ...

    Indian Academy of Sciences (India)

    Stefan Schöbel

    2017-09-05

    Sep 5, 2017 ... evolution; a period of global scale plate break-up .... Provenance analysis of LCU and UCU. The left column shows thin ... analysis allows the determination of principal .... ity, partly even in the sub-meter scale, between.

  18. The evolution of rifting process in the tectonic history of the Earth

    Science.gov (United States)

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

    1985-01-01

    The continental rifting is the response of the lithosphere to the oriented tension. The distribution of viscosity in the lithosphere plays an essential role during all stages of the rifting. The viscosity is a function of the temperature, the lithostatic pressure, the rock composition, the deformation rate and other factors. The temperature is the most important factor. The vertical section of continental lithosphere of the rift zone may be divided into the following layers: the upper crust, in which brittle deformation prevails; the medialcrust, in which the role of plastic deformation increases; the lower crust, in which plastic deformation prevails; and the uppermost plastic part of the mantle overlapping asthenosphere. The depth of the boundaries in the crust layers are mainly controlled by the temperature.

  19. Crustal structure and rift tectonics across the Cauvery–Palar basin ...

    Indian Academy of Sciences (India)

    The Cauvery–Palar basin is a major peri-cratonic rift basin located along the Eastern Continental. Margin of India ..... density P(k) of a magnetized body having infinite extensions in the .... aly data must be brought down to the sea level through ...

  20. Rift Valley Fever.

    Science.gov (United States)

    Hartman, Amy

    2017-06-01

    Rift Valley fever (RVF) is a severe veterinary disease of livestock that also causes moderate to severe illness in people. The life cycle of RVF is complex and involves mosquitoes, livestock, people, and the environment. RVF virus is transmitted from either mosquitoes or farm animals to humans, but is generally not transmitted from person to person. People can develop different diseases after infection, including febrile illness, ocular disease, hemorrhagic fever, or encephalitis. There is a significant risk for emergence of RVF into new locations, which would affect human health and livestock industries. Copyright © 2017 Elsevier Inc. All rights reserved.

  1. Comparative study of the base activity levels of the Spanish continental nuclear power stations

    International Nuclear Information System (INIS)

    Gregorides de los Santos y Perez-Minguez, A.M.

    1980-01-01

    In this work the pre-operational activity levels of the sites of some Spanish nuclear power stations are discussed. These power stations are located near the Tagus and Ebro river basins. A report of the communities investigated is made. The total alpha and beta activity as well as the long half life activity has been researched there. The importance of this question is obvious for it gives a regional average of the base activity in the basins of these two big Spanish rivers [fr

  2. Geometry of the Arabia-Somalia Plate Boundary into Afar: Preliminary Results from the Seismic Profile Across the Asal Rift (Djibouti)

    Science.gov (United States)

    Vergne, J.; Doubre, C.; Mohamed, K.; Tiberi, C.; Leroy, S.; Maggi, A.

    2010-12-01

    In the Afar Depression, the Asal-Ghoubbet Rift in Djibouti is a young segment on land at the propagating tip of the Aden Ridge. This segment represents an ideal laboratory to observe the mechanisms of extension and the structural evolutions involved, from the continental break-up to the first stage of oceanic spreading. However, we lack first order information about the crustal and upper mantle structure in this region, which for example prevent detailed numerical modeling of the deformations observed at the surface from GPS or InSAR. Moreover the current permanent network is not well suited to precisely constrain the ratio of seismic/aseismic deformation and to characterize the active deformation and the rifting dynamics. Since November 2009 we have maintained a temporary network of 25 seismic stations deployed along a 150 km-long profile. Because we expect rapid variations of the lithospheric structure across the 10 km-wide central part of the rift, we gradually decreased the inter-stations spacing to less than 1 km in the middle section of the profile. In order to obtain a continuous image of the plate boundary, from the topographic surface to the upper mantle, several techniques and methods will be applied: P and S wave receiver functions, tomographies based on body waves, surface waves and seismic noise correlation, anisotropy, and finally a gravity-seismic joint inversion. We present some preliminary results deduced from the receiver functions applied to the data acquired during the first months of the experiment. We migrate several sets of receiver functions computed in various frequency bands to resolve both mantle interfaces and fine scale structures within the thin crust in the center of the rift. These first images confirm a rapid variation of the Moho depth on both sides of the rift and a very complex lithospheric structure in the central section with several low velocity zones within the top 50km that might correspond to magma lenses.

  3. The lithosphere of the East African Rift and Plateau (Afar-Ethiopia-Turkana) : insights from Integrated 3-D density modelling

    OpenAIRE

    Woldetinsae, Girma

    2005-01-01

    The area encompassing the Eastern branch of the East African Rift System (EARS: Afar-Ethiopia-Turkana) and associated plateaux is an ideal region to investigate extension and magmatism associated with rupturing continental lithosphere. Ethiopia covers an important part of the EARS. It contains the major section of the ca. 5000 km Afro-Arabian rift and includes the transition between the Arabo-Nubian-Shield and the Mozambique Belt. A compilation of over 45000 onshore and offshore gravity stati...

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

    Directory of Open Access Journals (Sweden)

    Moikwathai Moidaki

    2013-02-01

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

  5. Fire activity and hydrological dynamics in the past 5700 years reconstructed from Sphagnum peatlands along the oceanic-continental climatic gradient in northern Poland

    Science.gov (United States)

    Marcisz, Katarzyna; Gałka, Mariusz; Pietrala, Patryk; Miotk-Szpiganowicz, Grażyna; Obremska, Milena; Tobolski, Kazimierz; Lamentowicz, Mariusz

    2017-12-01

    Fire is a critical component of many ecosystems and, as predicted by various climate models, fire activity may increase significantly in the following years due to climate change. Therefore, knowledge about the past fire activity of various ecosystems is highly important for future nature conservation purposes. We present results of high-resolution investigation of fire activity and hydrological changes in northern Poland. We analyzed microscopic charcoal from three Sphagnum-dominated peatlands located on the south of Baltic, on the oceanic-continental (west-east) climatic gradient, and reconstructed the history of fire in the last 5700 years. We hypothesize that air circulation patterns are highly important for local fire activity, and that fire activity is more intensive in peatlands influenced by continental air masses. We have found out that forest fires have been occurring regularly since the past millennia and were linked to climatic conditions. We show that fire activity (related to climate and fuel availability) was significantly higher in sites dominated by continental climate (northeastern Poland) than in the site located under oceanic conditions (northwestern Poland)-microscopic charcoal influx was 13.3 times higher in the eastern study site of the gradient, compared to the western study site. Recorded fire activity patterns were different between the sites in a long timescale. Moreover, most of the recorded charcoal peaks occurred during high water tables. Rising human pressure has caused droughts and water table instability, and substantial increase in fire activity in the last 400 years.

  6. High content image-based screening of a protease inhibitor library reveals compounds broadly active against Rift Valley fever virus and other highly pathogenic RNA viruses.

    Directory of Open Access Journals (Sweden)

    Rajini Mudhasani

    2014-08-01

    Full Text Available High content image-based screening was developed as an approach to test a protease inhibitor small molecule library for antiviral activity against Rift Valley fever virus (RVFV and to determine their mechanism of action. RVFV is the causative agent of severe disease of humans and animals throughout Africa and the Arabian Peninsula. Of the 849 compounds screened, 34 compounds exhibited ≥ 50% inhibition against RVFV. All of the hit compounds could be classified into 4 distinct groups based on their unique chemical backbone. Some of the compounds also showed broad antiviral activity against several highly pathogenic RNA viruses including Ebola, Marburg, Venezuela equine encephalitis, and Lassa viruses. Four hit compounds (C795-0925, D011-2120, F694-1532 and G202-0362, which were most active against RVFV and showed broad-spectrum antiviral activity, were selected for further evaluation for their cytotoxicity, dose response profile, and mode of action using classical virological methods and high-content imaging analysis. Time-of-addition assays in RVFV infections suggested that D011-2120 and G202-0362 targeted virus egress, while C795-0925 and F694-1532 inhibited virus replication. We showed that D011-2120 exhibited its antiviral effects by blocking microtubule polymerization, thereby disrupting the Golgi complex and inhibiting viral trafficking to the plasma membrane during virus egress. While G202-0362 also affected virus egress, it appears to do so by a different mechanism, namely by blocking virus budding from the trans Golgi. F694-1532 inhibited viral replication, but also appeared to inhibit overall cellular gene expression. However, G202-0362 and C795-0925 did not alter any of the morphological features that we examined and thus may prove to be good candidates for antiviral drug development. Overall this work demonstrates that high-content image analysis can be used to screen chemical libraries for new antivirals and to determine their

  7. Tectonic-Volcanic Interplay in the Dabbahu Segment of the Afar Rift from Cosmogenic 3He Constraints

    Science.gov (United States)

    Williams, A.; Pik, R.; Burnard, P.; Lahitte, P.; Yirgu, G.; Adem, M.

    2008-12-01

    The Afar Rift in Ethiopia is one of the only subaerial locations in the world where the transition from continental break-up to oceanic-spreading can be observed. Extension and volcanism in the Afar is concentrated in tectono-magmatic segments (TMS), similar in size and morphology to those that characterise spreading ridges. The Dabbahu TMS is the southernmost of the western Afar and has recently been the site of significant activity. A massive seismic event in late 2005, triggered by the injection of an 8-m wide dyke, heralded the onset of a new rifting period in the Dabbahu TMS. Volcanic activity associated with the periods of magma-driven extension, which have recurred at 4-8 mth intervals, has been both silicic (explosive) and basaltic (fissural). The most recent activity in the Afar thus testifies to the close interplay of tectonics and magmatism in rifting environments. In an effort to decipher the long-term structural and volcanic evolution of Dabbahu TMS we have employed the cosmogenic nuclide dating technique to provide chronological data for the segment. This method has advantages over other geochronological tools in that we can target both volcanic and tectonic surfaces of a few Kyr to several Myr age. Baddi Volcano, located off-axis on the western margin of the TMS, is a bimodal central stratovolcano typical of the Afar TMS. Late-stage basaltic lava flows cap an acidic base, which has been dated at 290 ± 4 ka using the K-Ar technique (Lahitte et al., 2003). Following preliminary sampling in 2007, we have determined a cosmogenic 3He age of 53.4 ± 3.7 ka from multiple samples from one of the basaltic flows on the NW flank of Baddi. These data show a significant time gap (240 Kyr) between the final phase of acidic volcanism and the onset of basaltic activity at the central volcanoes, presumably related to the rate of magma chamber replenishment. To test whether the spectacular shift to basaltic activity at 53 ka represents replenishment of the entire sub-rift

  8. 77 FR 5830 - Commercial Wind Leasing and Site Assessment Activities on the Atlantic Outer Continental Shelf...

    Science.gov (United States)

    2012-02-06

    ... Areas (WEAs), and utilizes coordinated environmental studies, large-scale planning processes, and... proposal will be given full environmental review at that time. BOEM is seeking public input regarding the... characterization activities (i.e., geological and geophysical surveys and core samples), a lessee must submit the...

  9. 78 FR 27427 - Outer Continental Shelf (OCS) Geological and Geophysical Exploration Activities in the Gulf of...

    Science.gov (United States)

    2013-05-10

    ... timeframe. The activities considered within this PEIS are associated with GOM OCS oil and gas exploration... and NMFS as the PEIS is developed. Background: A variety of G&G techniques are used to characterize... surveys are conducted to: (1) Obtain data for hydrocarbon exploration and production; (2) aid in siting...

  10. Hydrothermal fluid flow within a tectonically active rift-ridge transform junction: Tjörnes Fracture Zone, Iceland

    Science.gov (United States)

    Lupi, M.; Geiger, S.; Graham, C. M.

    2010-05-01

    We investigate the regional fluid flow dynamics in a highly faulted transform area, the Tjörnes Fracture Zone in northern Iceland which is characterized by steep geothermal gradients, hydrothermal activity, and strong seismicity. We simulate fluid flow within the Tjörnes Fracture Zone using a high-resolution model that was based on the available geological and geophysical data and has the aim to represent the complex geological structures and the thermodynamical processes that drive the regional fluid flow in a physically realistic way. Our results show that convective heat flow and mixing of cold and saline seawater with deep hydrothermal fluids controls the large-scale fluid flow. The distribution of faults has a strong influence on the local hydrodynamics by focusing flow around clusters of faults. This explains the nature of isolated upflow zones of hot hydrothermal fluids which are observed in the Tjörnes Fracture Zone. An important emergent characteristic of the regional fluid flow in the Tjörnes Fracture Zone are two separate flow systems: one in the sedimentary basins, comprising more vigorous convection, and one in the crystalline basement, which is dominated by conduction. These two flow systems yield fundamental insight into the connection between regional hydrothermal fluid flow and seismicity because they form the basis of a toggle switch mechanism that is thought to have caused the hydrogeochemical anomalies recorded at Húsavik before and after the 5.8 M earthquake in September 2002.

  11. Late Ordovician (post-Sardic) rifting branches in the North Gondwanan Montagne Noire and Mouthoumet massifs of southern France

    DEFF Research Database (Denmark)

    Javier Álvaro, J.; Colmenar Lallena, Jorge; Monceret, Eric

    2016-01-01

    , and the subsequent Middle-Ordovician stratigraphic gap is related to the Sardic phase. Upper Ordovician sedimentation started in the rifting branches of Cabrières and Mouthoumet with deposition of basaltic lava flows and lahar deposits (Roque de Bandies and Villerouge formations) of continental tholeiite signature...... (CT), indicative of continental fracturing.The infill of both rifting branches followed with the onset of (1) Katian (Ka1-Ka2) conglomerates and sandstones (Glauzy and Gascagne formations), which have yielded a new brachiopod assemblage representative of the Svobodaina havliceki Community; (2) Katian...

  12. Macrophysical properties of continental cumulus clouds from active and passive remote sensing

    Energy Technology Data Exchange (ETDEWEB)

    Kassianov, Evgueni I.; Riley, Erin A.; Kleiss, Jessica; Long, Charles N.; Riihimaki, Laura D.; Flynn, Donna M.; Flynn, Connor J M.; Berg, Larry K.

    2017-10-06

    Cloud amount is an essential and extensively used macrophysical parameter of cumulus clouds. It is commonly defined as a cloud fraction (CF) from zenith-pointing ground-based active and passive remote sensing. However, conventional retrievals of CF from the remote sensing data with very narrow field-of-view (FOV) may not be representative of the surrounding area. Here we assess its representativeness using an integrated dataset collected at the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program's Southern Great Plains (SGP) site in Oklahoma, USA. For our assessment with focus on selected days with single-layer cumulus clouds (2005-2016), we include the narrow-FOV ARM Active Remotely Sensed Clouds Locations (ARSCL) and large-FOV Total Sky Imager (TSI) cloud products, the 915-MHz Radar Wind Profiler (RWP) measurements of wind speed and direction, and also high-resolution satellite images from Landsat and the Moderate Resolution Imaging Spectroradiometer (MODIS). We demonstrate that a root-mean-square difference (RMSD) between the 15-min averaged ARSCL cloud fraction (CF) and the 15-min averaged TSI fractional sky cover (FSC) is large (up to 0.3). We also discuss how the horizontal distribution of clouds can modify the obtained large RMSD using a new uniformity metric. The latter utilizes the spatial distribution of the FSC over the 100° FOV TSI images obtained with high temporal resolution (30 sec sampling). We demonstrate that cases with more uniform spatial distribution of FSC show better agreement between the narrow-FOV CF and large-FOV FSC, reducing the RMSD by up to a factor of 2.

  13. Continental divide

    International Nuclear Information System (INIS)

    Quinn, F.J.

    1991-01-01

    The historical precedents to the idea of continent-wide diversion of water in North America are reviewed, starting from early perceptions of continental drainage and the era of canal building that reached its peak in the mid-1800s. The attitude that natural landscapes can be rearranged to suit human needs has persisted from that era with the proposal for continent-wide water diversion megaprojects, many involving the movement of water from Canada to the southwestern USA. Over 50 water diversions exist in Canada, with a total diverted flow of 4,400 m 3 /s. The density of interconnected and almost-connected lakes and rivers has favored such diversions. Of these diversions, 95% of their storage capacity and 96% of their flow is for hydroelectric power generation. The number of diversions in the USA is similar but water volumes are only a sixth of those in Canada, and the water is mainly used for irrigation or water supply. Experience in both countries shows that diversions are contained by political boundaries. No large-scale diversion of fresh water across the international boundary has received any government support, and no significant change in this policy is anticipated. In the water-short areas of the USA, conservation and reallocation of water resources are receiving priority. 19 refs., 2 figs., 2 tabs

  14. Analogue modelling of continental extension: a review focused on the relations between the patterns of deformation and the presence of magma.

    NARCIS (Netherlands)

    Corti, G.; Bonini, M.; Conticelli, S.; Innocenti, F.; Manetti, P.; Sokoutis, D.

    2003-01-01

    Continental extension may occur in two main different modes, narrow and wide rifting, which mainly differ in the width of the deformed region. A third mechanism, the core complex, has been considered either a distinct mode of extension or a local anomaly within wide rifts. In terms of causative

  15. East Antarctic rifting triggers uplift of the Gamburtsev Mountains

    Science.gov (United States)

    Ferraccioli, F.; Finn, Carol A.; Jordan, Tom A.; Bell, Robin E.; Anderson, Lester M.; Damaske, Detlef

    2011-01-01

    The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains’ origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere.

  16. A Middle-Upper Miocene fluvial-lacustrine rift sequence in the Song Ba Rift, Vietnam

    DEFF Research Database (Denmark)

    Lars H., Nielsen; Henrik I., Petersen; Nguyen D., Dau

    2007-01-01

    The small Neogene Krong Pa graben is situated within the continental Song Ba Rift, which is bounded by strike-slip faults that were reactivated as extensional faults in Middle Miocene time. The 500 m thick graben-fill shows an overall depositional development reflecting the structural evolution...... subsidence rate and possibly a higher influx of water from the axial river systems the general water level in the graben rose and deep lakes formed. High organic preservation in the lakes prompted the formation of two excellent oil-prone lacustrine source-rock units. In the late phase of the graben...... as carrier beds, whereas the braided fluvial sandstones and conglomerates along the graben margins may form reservoirs. The Krong Pa graben thus contains oil-prone lacustrine source rocks, effective conduits for generated hydrocarbons and reservoir sandstones side-sealed by the graben faults toward...

  17. Diverse Eruptions at Approximately 2,200 Years B.P. on the Great Rift, Idaho: Inferences for Magma Dynamics Along Volcanic Rift Zones

    Science.gov (United States)

    Hughes, S. S.; Nawotniak, S. E. Kobs; Borg, C.; Mallonee, H. C.; Purcell, S.; Neish, C.; Garry, W. B.; Haberle, C. W.; Lim, D. S. S.; Heldmann, J. L.

    2016-01-01

    Compositionally and morphologically diverse lava flows erupted on the Great Rift of Idaho approximately 2.2 ka (kilo-annum, 1000 years ago) during a volcanic "flare-up" of activity following an approximately 2 ky (kiloyear, 1000 years) hiatus in eruptions. Volcanism at Craters of the Moon (COTM), Wapi and Kings Bowl lava fields around this time included primitive and evolved compositions, separated over 75 kilometers along the approximately 85 kilometers-long rift, with striking variability in lava flow emplacement mechanisms and surface morphologies. Although the temporal associations may be coincidental, the system provides a planetary analog to better understand magma dynamics along rift systems, including that associated with lunar floor-fractured craters. This study aims to help bridge the knowledge gap between ancient rift volcanism evident on the Moon and other terrestrial planets, and active rift volcanism, e.g., at Hawai'i and Iceland.

  18. Rift Structure in Eastern Papua New Guinea From the Joint Inversion of Receiver Functions and Seismic Noise

    Science.gov (United States)

    Abers, G. A.; Obrebski, M. J.; Jin, G.; Eilon, Z.

    2014-12-01

    The recent CDPapua seismic array in the active D'Entrecasteaux-Woodlark Rift provides insights into how continental crust accommodates large extension. Here, >100 km of extension has occurred in the last 4-6 Ma, exhuming rocks from 100 km depth. To better understand the modes of deformation of the crust, we analyze shear wave velocity (Vs) distribution for a set of temporary land and ocean bottom broadband stations. We resolve the depth of the main velocity contrasts using receiver function (RF) analysis, alleviating the intrinsic trade-off between depth and velocity intrinsic by joint inversion with dispersion constraints (10 - 100 s) from earthquake surface waves and ambient noise. A transdimensional Bayesian scheme explores the model space (Vs in each layer, number of interfaces and their respective depths), minimizing the number of layers required to fit the observations given their noise level. Preliminary results suggest that the Moho is sharp in most places, with a depth of 28-38 km and 20-27 km below the Papuan Peninsula and the highly-extended D'Entracasteaux Islands, respectively. The mid-lower crust of these regions appears to be similar and consistent with felsic compositions, 3.25≤Vs≤3.5 km/s, and may represent the Owen-Stanley Metamorphic Belt or underlying continental rocks. A fast layer (3.75≤Vs≤4 km/s) is observed below the Papuan Peninsula in the 20-30 km depth range and may indicate more mafic lower crust. In contrast, faster velocities between 10 and 20km depth are modeled below the Goodenough Basin (3.75≤Vs≤4 km/s) and the Trobriand Basin (3.5≤Vs≤3.75 km/s) where rocks of the Papuan Ultramafic Belt have been suggested, although these results partly depend upon complicated signals from ocean-bottom seismometers. Well-located seismicity shows that active fault systems generally follow the boundaries between regions of different crustal velocity structure. Overall these results confirm a continental velocity structure for the

  19. Seismicity and lithospheric structure of Central Mozambique: implications for the southward propagation of the East African Rift System

    Science.gov (United States)

    Fonseca, J. F. B. D.; Domingues, A.

    2017-12-01

    South of latitude 5ºS, there is scarce support for a single preferred location of continental rifting in SE Africa. Building on the complexity already displayed further north around the Victoria microplate, the structures associated with rifting activity are now distributed over three branches: one directed towards the SW through Zambia and into the Okawango rift in Botswana; one running offshore along the Mozambique Channel; and a central rift system through lake Malawi and Central Mozambique. Our investigation focuses on this central branch, whose tectonic relevance was highlighted by the M7 Machaze earthquake in 2006. Through the temporary deployment of 30 broadband stations in central Mozambique we were able to document that the Shire and Urema grabens linking the Malawi rift to the Machaze epicentral area are seismically active, correlating with a 300 km long narrow band of seismicity reaching the lower crust. No significant seismicity was recorded along the Mazenga graben, south of the Machaze epicentral area. A tomographic model derived from ambient noise analysis showed a strong correlation between the seismicity and a sharp NNE-SSW boundary between the fast crust of the Zimbabwe and Kaapvaal cratons and slower crust underneath the Mozambique Coastal Plains. The seismicity shuts down were this trend rotates to a more N-S direction as the Lebombo monocline is approached. 20th Century seismicity of SE Africa shows a clear cluster in time, with five M>6 earthquakes concentrated in the 1950's, distributed along the edges of the Zimbabwe craton and spanning distances of 600 km. Spatial correlation with such range is hard to reconcile with stress transmission in the crust and may point to the interaction of the cratonic root with asthenospheric flow. Under this light, the M6.5 Central Botswana earthquake of April 2017 and the M7 Machaze earthquake of 2006, both located in the vicinity of the borders of the Kaapvaal craton, may bear a similar correlation. The

  20. NW Africa post-rift tectonics: fieldwork constraints from an "unfitting" anticline in west Morocco

    Science.gov (United States)

    Fernández-Blanco, David; Gouiza, Mohamed

    2015-04-01

    The evolution of the Moroccan Atlantic rifted margin is marked by a period of abnormal and excessive early post-rift subsidence during the Late Jurassic-Early Cretaceous affecting the proximal coastal basins, the continental shelf and the distal deep basins, which acted coevally to km-scale uplift and erosion of large domains to the east. The tectonics of the uplift event are still unclear, as it took place 30 to 50 Myr after lithospheric breakup between Morocco and Nova Scotia and prior to the Atlas/Alpine contraction, which gave rise to the Atlas and the Rif mountain belts. The Essaouira-Haha basin, located on the coastal plain of the Atlantic rifted margin of Morocco, and bounded by two uplifted Paleozoic basement highs (i.e. the Massif Ancien of Marrakech, to the east, and the Jebilet, to the northeast), is an ideal location to investigate the tectonic processes that might have triggered these vertical movements. Although most of the deformation observed in the basin is classically attributed to Upper Cretaceous halokinesis and Neogene Atlas contraction, recent works have shown the existence of contractional structures. We carry out a structural analysis of the Jbel Amsittene Anticline, located in the middle of the Essaouira-Haha basin to investigate the tectonics of its formation and its relationship with the above-mentioned exhumation. We show structural field data along several cross-sections transecting the anticline, and characterize a salt-cored fault propagation fold verging north, with a Triassic salt acting as a detachment plane. Regional kinematic indicators and structures show overall NNW-SSE to NNE-SSW shortening and active tectonics during the postrift phase, as indicated by syn-tectonic wedges seen for the Late Jurassic to Early Cretaceous period. These facts discard the "salt-drives-tectonics" theory to let "tectonic-drives-salt" one to rise, and point to factors other than small-cell mantle convection acting during the evolution of the Moroccan

  1. Besshi-type mineral systems in the Palaeoproterozoic Bryah Rift-Basin, Capricorn Orogen, Western Australia: Implications for tectonic setting and geodynamic evolution

    Directory of Open Access Journals (Sweden)

    Franco Pirajno

    2016-05-01

    Full Text Available In this contribution we use VMS mineral systems in the Bryah rift-basin to constrain the tectonic setting of the widespread mafic and ultramafic magmatism that characterises the rift-basin in question. Two distinct, but temporally closely associated, lithostratigraphic sequences, Narracoota and Karalundi Formations, are discussed. The Karalundi Formation is the main host of VMS mineral systems in the region. The Karalundi Formation consists of turbiditic and immature clastic sediments, which are locally intercalated with basaltic hyaloclastites, dolerites and banded jaspilites. We propose that the basaltic hyaloclastites, dolerites and clastics and jaspilites rocks, form a distinct unit of the Karalundi Formation, named Noonyereena Member. The VMS mineral systems occur near the north-east trending Jenkin Fault and comprise the giant and world-class DeGrussa and the Red Bore deposits. The nature of these deposits and their intimate association with terrigenous clastic rocks and dominantly marine mafic volcanic and subvolcanic rocks, as well as the common development of peperitic margins, are considered indicative of a Besshi-type environment, similar to that of present-day Gulf of California. Our Re-Os age data from a primary pyrite yielded a mean model age of 2012 ± 48 Ma, which coincides (within error with recent published Re-Os data (Hawke et al., 2015 and confirms the timing of the proposed geodynamic evolution. We propose a geodynamic model that attempts to explain the presence of the Narracoota and Karalundi Formations as the result of mantle plume activity, which began with early uplift of continental crust with intraplate volcanism, followed by early stages of rifting with the deposition of the Karalundi Formation (and Noonyereena Member, which led to the formation of Besshi-type VMS deposits. With on-going mantle plume activity and early stages of continental separation, an oceanic plateau was formed and is now represented by mafic

  2. Constraints Imposed by Rift Inheritance on the Compressional Reactivation of a Hyperextended Margin: Mapping Rift Domains in the North Iberian Margin and in the Cantabrian Mountains

    Science.gov (United States)

    Cadenas, P.; Fernández-Viejo, G.; Pulgar, J. A.; Tugend, J.; Manatschal, G.; Minshull, T. A.

    2018-03-01

    The Alpine Pyrenean-Cantabrian orogen developed along the plate boundary between Iberia and Europe, involving the inversion of Mesozoic hyperextended basins along the southern Biscay margin. Thus, this margin represents a natural laboratory to analyze the control of structural rift inheritance on the compressional reactivation of a continental margin. With the aim to identify former rift domains and investigate their role during the subsequent compression, we performed a structural analysis of the central and western North Iberian margin, based on the interpretation of seismic reflection profiles and local constraints from drill-hole data. Seismic interpretations and published seismic velocity models enabled the development of crustal thickness maps that helped to constrain further the offshore and onshore segmentation. Based on all these constraints, we present a rift domain map across the central and western North Iberian margin, as far as the adjacent western Cantabrian Mountains. Furthermore, we provide a first-order description of the margin segmentation resulting from its polyphase tectonic evolution. The most striking result is the presence of a hyperthinned domain (e.g., Asturian Basin) along the central continental platform that is bounded to the north by the Le Danois High, interpreted as a rift-related continental block separating two distinctive hyperextended domains. From the analysis of the rift domain map and the distribution of reactivation structures, we conclude that the landward limit of the necking domain and the hyperextended domains, respectively, guide and localize the compressional overprint. The Le Danois block acted as a local buttress, conditioning the inversion of the Asturian Basin.

  3. Late Jurassic-Early Cretaceous continental convergence and intracontinental orogenesis in East Asia: A synthesis of the Yanshan Revolution

    Science.gov (United States)

    Dong, Shuwen; Zhang, Yueqiao; Zhang, Fuqin; Cui, Jianjun; Chen, Xuanhua; Zhang, Shuanhong; Miao, Laicheng; Li, Jianhua; Shi, Wei; Li, Zhenhong; Huang, Shiqi; Li, Hailong

    2015-12-01

    The basic tectonic framework of continental East Asia was produced by a series of nearly contemporaneous orogenic events in the late Middle Jurassic to Early Cretaceous. Commonly, the Late Mesozoic orogenic processes were characterized by continent-continent collision, large-scale thrusting, strike-slip faulting and intense crustal shortening, crustal thickening, regional anatexis and metamorphism, followed by large-scale lithospheric extension, rifting and magmatism. To better understand the geological processes, this paper reviews and synthesizes existing multi-disciplinary geologic data related to sedimentation, tectonics, magmatism, metamorphism and geochemistry, and proposes a two-stage tectono-thermal evolutionary history of East Asia during the late Middle Jurassic to Early Cretaceous (ca. 170-120 Ma). In the first stage, three orogenic belts along the continental margins were formed coevally at ca. 170-135 Ma, i.e., the north Mongol-Okhotsk orogen, the east paleo-Pacific coastal orogen, and the west Bangong-Nujiang orogen. Tectonism related to the coastal orogen caused extensive intracontinental folding and thrusting that resulted in a depositional hiatus in the Late Jurassic, as well as crustal anatexis that generated syn-kinematic granites, adakites and migmatites. The lithosphere of the East Asian continent was thickened, reaching a maximum during the latest Jurassic or the earliest Cretaceous. In the second stage (ca. 135-120 Ma), delamination of the thickened lithosphere resulted in a remarkable (>120 km) lithospheric thinning and the development of mantle-derived magmatism, mineralization, metamorphic core complexes and rift basins. The Middle Jurassic-Early Cretaceous subduction of oceanic plates (paleo-Pacific, meso-Tethys, and Mongol-Okhotsk) and continent-continent collision (e.g. Lhasa and Qiangtang) along the East Asian continental margins produced broad coastal and intracontinental orogens. These significant tectonic activities, marked by

  4. Kinematics of the entire East African Rift from GPS velocities

    Science.gov (United States)

    Floyd, M.; King, R. W.

    2017-12-01

    Through a collaborative effort of the GeoPRISMS East Africa Rift GPS Working Group, we have collected and collated all of the publicly available continuous and survey-mode data for the entire rift system between 1994 and 2017 and processed these data as part of a larger velocity solution for Africa, Arabia and western Eurasia. We present here our velocity solution encompassing the major bounding plates and intervening terranes along the East African Rift from the Red Sea to the Malawi Rift and adjacent regions for GPS sites with data spans of at least 2.4 years, and north and east velocity uncertainties less than 1.5 mm/yr. To obtain realistic uncertainties for the velocity estimates, we attempted at each stage of the analysis to account for the character of the noise: During phase processing, we used an elevation-dependent weighting based on the phase residuals for each station; we then examined each position time series, removing outliers and reweighting appropriately to account for the white noise component of the errors; and e accounted for temporal correlations by estimating an equivalent random-walk magnitude for each continuous site and applying the median value (0.5 mm/√yr) to all survey-mode sites. We rigorously estimate relative rotation rates of Nubia, by choosing subset of well-determined sites such that the effective weights of western, northeastern and southern Africa were roughly equivalent, and Somalia, for which the estimate is dominated by three sites (MALI, RCMN, SEY1) whose uncertainties are a factor of 2-3 smaller than those of the other sites. For both plates, the weighted root-mean-square of the velocity residuals is 0.5 mm/yr. Our unified velocity solution provides a geodetic framework and constraints on the continental-scale kinematics of surface motions as well as more local effects both within and outside of the rift structures. Specific focus areas with denser coverage than previous fields include the Danakil block, the Afar Rift, the

  5. Post-rift deformation of the Red Sea Arabian margin

    Science.gov (United States)

    Zanoni, Davide; Schettino, Antonio; Pierantoni, Pietro Paolo; Rasul, Najeeb

    2017-04-01

    Starting from the Oligocene, the Red Sea rift nucleated within the composite Neoproterozoic Arabian-Nubian shield. After about 30 Ma-long history of continental lithosphere thinning and magmatism, the first pulse of oceanic spreading occurred at around 4.6 Ma at the triple junction of Africa, Arabia, and Danakil plate boundaries and propagated southward separating Danakil and Arabia plates. Ocean floor spreading between Arabia and Africa started later, at about 3 Ma and propagated northward (Schettino et al., 2016). Nowadays the northern part of the Red Sea is characterised by isolated oceanic deeps or a thinned continental lithosphere. Here we investigate the deformation of thinned continental margins that develops as a consequence of the continental lithosphere break-up induced by the progressive oceanisation. This deformation consists of a system of transcurrent and reverse faults that accommodate the anelastic relaxation of the extended margins. Inversion and shortening tectonics along the rifted margins as a consequence of the formation of a new segment of ocean ridge was already documented in the Atlantic margin of North America (e.g. Schlische et al. 2003). We present preliminary structural data obtained along the north-central portion of the Arabian rifted margin of the Red Sea. We explored NE-SW trending lineaments within the Arabian margin that are the inland continuation of transform boundaries between segments of the oceanic ridge. We found brittle fault zones whose kinematics is consistent with a post-rift inversion. Along the southernmost transcurrent fault (Ad Damm fault) of the central portion of the Red Sea we found evidence of dextral movement. Along the northernmost transcurrent fault, which intersects the Harrat Lunayyir, structures indicate dextral movement. At the inland termination of this fault the evidence of dextral movement are weaker and NW-SE trending reverse faults outcrop. Between these two faults we found other dextral transcurrent

  6. Some aspects of the role of rift inheritance on Alpine-type orogens

    Science.gov (United States)

    Tugend, Julie; Manatschal, Gianreto; Mohn, Geoffroy; Chevrot, Sébastien

    2017-04-01

    Processes commonly recognized as fundamental for the formation of collisional orogens include oceanic subduction, arc-continent and continent-continent collision. As collisional belts result from the closure of oceanic basins and subsequent inversion of former rifted margins, their formation and evolution may also in theory be closely interlinked with the initial architecture of the former rifted margins. This assumption is indeed more likely to be applicable in the case of Alpine-type orogens, mainly controlled by mechanical processes and mostly devoid of arc-related magmatism. More and more studies from present-day magma-poor rifted margins illustrate the complex evolution of hyperextended domains (i.e. severely thinned continental crust (images across the Pyrenees (PYROPE) and the Alps (CIFALPS) reveal a surprisingly comparable present-day overall crustal and lithospheric structure. Based on the comparison between the two orogens we discuss: (1) the nature and depth of decoupling levels inherited from hyperextension; (2) the implications for restorations and interpretations of orogenic roots (former hyperextended domains vs. lower crust only); and (3) the nature and major role of buttresses in controlling the final stage of collisional processes. Eventually, we discuss the variability of the role of rift-inheritance in building Alpine-type orogens. The Pyrenees seem to represent one extreme, where rift-inheritance is important at different stages of collisional processes. In contrast, in the Alps the role of rift-inheritance is subtler, likely because of its more complex and polyphase compressional deformation history.

  7. Dykes and structures of the NE rift of Tenerife, Canary Islands: a record of stabilisation and destabilisation of ocean island rift zones

    Science.gov (United States)

    Delcamp, A.; Troll, V. R.; van Wyk de Vries, B.; Carracedo, J. C.; Petronis, M. S.; Pérez-Torrado, F. J.; Deegan, F. M.

    2012-07-01

    Many oceanic island rift zones are associated with lateral sector collapses, and several models have been proposed to explain this link. The North-East Rift Zone (NERZ) of Tenerife Island, Spain offers an opportunity to explore this relationship, as three successive collapses are located on both sides of the rift. We have carried out a systematic and detailed mapping campaign on the rift zone, including analysis of about 400 dykes. We recorded dyke morphology, thickness, composition, internal textural features and orientation to provide a catalogue of the characteristics of rift zone dykes. Dykes were intruded along the rift, but also radiate from several nodes along the rift and form en échelon sets along the walls of collapse scars. A striking characteristic of the dykes along the collapse scars is that they dip away from rift or embayment axes and are oblique to the collapse walls. This dyke pattern is consistent with the lateral spreading of the sectors long before the collapse events. The slump sides would create the necessary strike-slip movement to promote en échelon dyke patterns. The spreading flank would probably involve a basal decollement. Lateral flank spreading could have been generated by the intense intrusive activity along the rift but sectorial spreading in turn focused intrusive activity and allowed the development of deep intra-volcanic intrusive complexes. With continued magma supply, spreading caused temporary stabilisation of the rift by reducing slopes and relaxing stress. However, as magmatic intrusion persisted, a critical point was reached, beyond which further intrusion led to large-scale flank failure and sector collapse. During the early stages of growth, the rift could have been influenced by regional stress/strain fields and by pre-existing oceanic structures, but its later and mature development probably depended largely on the local volcanic and magmatic stress/strain fields that are effectively controlled by the rift zone growth

  8. Rift Valley fever outbreak, Mauritania, 1998: seroepidemiologic, virologic, entomologic, and zoologic investigations.

    Science.gov (United States)

    Nabeth, P; Kane, Y; Abdalahi, M O; Diallo, M; Ndiaye, K; Ba, K; Schneegans, F; Sall, A A; Mathiot, C

    2001-01-01

    A Rift Valley fever outbreak occurred in Mauritania in 1998. Seroepidemiologic and virologic investigation showed active circulation of the Rift Valley fever virus, with 13 strains isolated, and 16% (range 1.5%-38%) immunoglobulin (Ig) M-positivity in sera from 90 humans and 343 animals (sheep, goats, camels, cattle, and donkeys). One human case was fatal.

  9. Microstructural evolution and seismic anisotropy of upper mantle rocks in rift zones. Geologica Ultraiectina (300)

    NARCIS (Netherlands)

    Palasse, L.N.

    2008-01-01

    This thesis investigates field-scale fragments of subcontinental upper mantle rocks from the ancient Mesozoic North Pyrenean rift and Plio-Pleistocene xenoliths from the active Baja California rift, in order to constrain the deformation history of the uppermost mantle. The main focus of the study is

  10. Rhyolites associated to Ethiopian CFB: Clues for initial rifting at the Afar plume axis

    Science.gov (United States)

    Natali, Claudio; Beccaluva, Luigi; Bianchini, Gianluca; Siena, Franca

    2011-12-01

    A comprehensive tectono-magmatic model based on new geochemical and field data is discussed in order to highlight the significance of the high-TiO 2 bimodal picrite basalt/rhyolite association in the north-eastern sector of the Ethiopian Plateau, which is considered to be the axial zone of the 30 Ma Continental Flood Basalt activity related to the Afar plume (Beccaluva et al., 2009). In this area the volcanic sequence consists of approximately 1700 m of high TiO 2 (4-6.5%) picrite basalts, covered by rhyolitic ignimbrites and lavas, with an average thickness of 300 m, which discontinuously extend over an area of nearly 13,500 km 2 (ca. 3600 km 3). Petrogenetic modelling, using rock and mineral chemical data and phase equilibria calculations by PELE and MELTS, indicates that: 1) picrite basalts could generate rhyolitic, sometimes peralkaline, residual melts with persistently high titanium contents (TiO 2 0.4-1.1%; Fluorine 0.2-0.3%; H 2O 2-3%; density ca. 2.4) corresponding to liquid fractions 9-16%; 2) closed system fractional crystallisation processes developed at 0.1-0.3 GPa pressure and 1390-750 °C temperature ranges, under QFM fO 2 conditions; 3) the highest crystallisation rate - involving 10-13% of Fe-Ti oxide removal - in the temperature range 1070-950 °C, represents a transitory (short-lived) fractionation stage, which results in the absence of erupted silica intermediate products (Daly gap). The eruption of low aspect ratio fluorine-rich rhyolitic ignimbrites and lavas capping the basic volcanics implies a rapid change from open- to closed-system tectono-magmatic conditions, which favoured the trapping of parental picrite basalts and their fractionation in upwardly zoned magma chambers. This evolution resulted from the onset of continental rifting, which was accompanied by normal faulting and block tilting, and the formation of shallow - N-S elongated - fissural chambers parallel to the future Afar Escarpment. The eruption of large volumes of rhyolitic

  11. Rift architecture and evolution: The Sirt Basin, Libya: The influence of basement fabrics and oblique tectonics

    Science.gov (United States)

    Abdunaser, K. M.; McCaffrey, K. J. W.

    2014-12-01

    The Cretaceous-Tertiary northwest-trending Sirt Basin system, Libya, is a rift/sag basin formed on Pan-African to Paleozoic-aged basement of North Africa. In this study, we investigate the rift-basin architecture and tectonic framework of the western Sirt Basin. Using remote sensed data, supported by borehole data from about 300 deep wells and surface geologic maps, we constructed geological cross sections and surface geology maps. Indication of the relative timing of structures and movement along faults has been determined where possible. Direction statistics for all the interpreted linear features acquired in the study area were calculated and given as a total distribution and then the totals are broken down by the major basin elements of the area. Hundreds of lineaments were recognized. Their lengths, range between a hundred meters up to hundreds of kilometers and the longest of the dominant trends are between N35W-N55W and between N55E-N65E which coincides with Sirt Basin structures. The produced rose diagrams reveal that the majority of the surface linear features in the region have four preferred orientations: N40-50W in the Zallah Trough, N45-55W in the Dur al Abd Trough, N35-55W in the Az Zahrah-Al Hufrah Platform, and in contrast in the Waddan Uplift a N55-65E trend. We recognize six lithostratigraphic sequences (phases) in the area's stratigraphic framework. A Pre-graben (Pre-rift) initiation stage involved the Pre-Cretaceous sediments formed before the main Sirt Basin subsidence. Then followed a Cretaceous to Eocene graben-fill stage that can divided into four structurally-active and structurally-inactive periods, and finally a terminal continental siliciclastics-rich package representing the post-rift stage of the development in post-Eocene time. In general five major fault systems dissect and divide the study area into geomorphological elevated blocks and depressions. Most of the oil fields present in the study area are associated with structural hinge

  12. The regional structure of the Red Sea Rift revised

    Science.gov (United States)

    Augustin, Nico; van der Zwan, Froukje M.; Devey, Colin W.; Brandsdóttir, Bryndís

    2017-04-01

    The Red Sea Rift has, for decades, been considered a text book example of how young ocean basins form and mature. Nevertheless, most studies of submarine processes in the Red Sea were previously based on sparse data (mostly obtained between the late 1960's and 1980's) collected at very low resolution. This low resolution, combined with large gaps between individual datasets, required large interpolations when developing geological models. Thus, these models generally considered the Red Sea Rift a special case of young ocean basement formation, dividing it from North to South into three zones: a continental thinning zone, a "transition zone" and a fully developed spreading zone. All these zones are imagined, in most of the models, to be separated by large transform faults, potentially starting and ending on the African and Arabian continental shields. However, no consensus between models e.g. about the locations (or even the existence) of major faults, the nature of the transition zone or the extent of oceanic crust in the Red Sea Rift has been reached. Recently, high resolution bathymetry revealed detailed seafloor morphology as never seen before from the Red Sea, very comparable to other (ultra)slow spreading mid-ocean ridges such as the Gakkel Ridge, the Mid-Atlantic Ridge and SW-Indian Ridge, changing the overall picture of the Red Sea significantly. New discoveries about the extent, movement and physical properties of submarine salt deposits led to the Red Sea Rift being linked to the young Aptian-age South Atlantic. Extensive crosscutting transform faults are not evident in the modern bathymetry data, neither in teleseismic nor vertical gravity gradient data and comparisons to Gakkel Ridge and the SW-Indian Ridge suggest that the Red Sea is much simpler in terms of structural geology than was previously thought. Complicated tectonic models do not appear necessary and there appears to be large areas of oceanic crust under the Red Sea salt blankets. Based on

  13. Is the Gop rift oceanic? A reevaluation of the Seychelles-India conjugate margins

    Science.gov (United States)

    Guan, Huixin; Werner, Philippe; Geoffroy, Laurent

    2016-04-01

    Recent studies reevaluated the timing and evolution of the breakup process between the Seychelles continental ridge and India, and the relationship between this evolution and mantle melting associated with the Deccan Igneous Province1,2,3. Those studies, mainly based on gravity and seismic refraction surveys, point that the oceanic domain located between the Seychelles and the Laxmi Ridge (here designed as the Carlsberg Basin) is the youngest oceanic domain between India and the Seychelles. To the East of the Laxmi Ridge, the aborted Gop Rift is considered as an older highly magmatic extensional continental system with magmatism, breakup and oceanic spreading being coeval with or even predating the emplacement of the major pulse of the Deccan trapps. This interpretation on the oceanic nature of the Gop Rift conflicts with other extensive surveys based on magnetic and seismic reflection data4 which suggest that the Gop Rift is an extended syn-magmatic continental domain. In our work based (a) on the existing data, (b) on new deep-seismic reflection surveys (already published by Misra5) down to the Moho and underlying mantle and (c) on new concepts on the geometry of volcanic passive margins, we propose a distinct interpretation of the Seychelles-India system. As proposed by former authors6,7, the Indian margin suffered some continental stretching and thinning before the onset of the Deccan traps during the Mesozoic. Thus continental crust thickness cannot be used easily as a proxy of syn-magmatic stretching-thinning processes or even to infer the presence or not of oceanic-type crust based, solely, on crustal thickness. However, some remarkable features appear on some of the deep penetration seismic lines we studied. We illustrate that the whole Seychelles/India system, before the opening of the present-day "Carlsberg Basin" may simply be regarded as a pair of sub-symmetric conjugate volcanic passive margins (VPMs) with inner and outer SDR wedges dipping towards the

  14. Passive recording of an active transform, an example from the Levant continental margin and the Dead Sea Fault

    Science.gov (United States)

    Lang, Guy; Lazar, Michael; Schattner, Uri

    2017-04-01

    Transform faults accommodate lateral motion between two adjacent plates. Records of plate motion and consequent boundary development on land is, at times, scarce and limited to structures along the fault axis. Investigation of a passive continental margin adjacent to the plate boundary might broaden the scope and provide estimates for its structural development. To examine this hypothesis, we analyzed depth and time migrated 3D seismic data together with four boreholes located along the southern Levant continental margin, ca. 100 Km from the continental Dead Sea fault (DSF). The analysis focus on the Plio-Pleistocene sequence, a key period in the development of the DSF. It includes formation of structural maps, stacking pattern investigation and calculation of sedimentation rates based on decompacted 3D depth data. These, in turn, enabled the reconstruction of margin development. This includes Messinian-earliest Zanclean NNE-SSW sinistral strike-slip faulting followed by Zanclean-Late Gelasian syn-depositional folding striking in the same direction. Abrupt change is marked by the Top Gelasian surface that shows indications of regional mass slumping. Successive Mid-Late Pleistocene progradation marks a basinward shift of the depocenter. Progradation controls margin sedimentation rates during the mid-late Pleistocene. These were found to increase throughout the whole Plio-Pleistocene, in contrast to reported sediment discharge from the Nile, which was shown to decrease after the Gelasian. Correlations to onshore findings, suggest that the continental margin records strain localization on the DSF during the Pliocene-Gelasian. This trend peaked at 1.8 Ma when short wavelength strain ceased along the margin, and differential subsidence commenced basinwards. This is attributed to consequent deepening of the DSF plate boundary.

  15. Rift systems of the Russian Eastern Arctic shelf and Arctic deep water basins: link between geological history and geodynamics

    Directory of Open Access Journals (Sweden)

    A. M. Nikishin

    2017-01-01

    Full Text Available In our study, we have developed a new tectonic scheme of the Arctic Ocean, which is based mainly on seismic profiles obtained in the Arctic-2011, Arctic-2012 and Arctic-2014 Projects implemented in Russia. Having interpreted many seismic profiles, we propose a new seismic stratigraphy of the Arctic Ocean. Our main conclusions are drawn from the interpretation of the seismic profiles and the analysis of the regional geological data. The results of our study show that rift systems within the Laptev, the East Siberian and the Chukchi Seas were formed not earlier than Aptian. The geological structure of the Eurasian, Podvodnikov, Toll and Makarov Basins is described in this paper. Having synthesized all the available data on the study area, we propose the following model of the geological history of the Arctic Ocean: 1. The Canada Basin formed till the Aptian (probably, during Hauterivian-Barremian time. 2. During the Aptian-Albian, large-scale tectonic and magmatic events took place, including plume magmatism in the area of the De Long Islands, Mendeleev Ridge and other regions. Continental rifting started after the completion of the Verkhoyansk-Chukotka orogenу, and rifting occurred on the shelf of the Laptev, East Siberian, North Chukchi and South Chukchi basins, and the Chukchi Plateau; simultaneously, continental rifting started in the Podvodnikov and Toll basins. 3. Perhaps the Late Cretaceous rifting continued in the Podvodnikov and Toll basins. 4. At the end of the Late Cretaceous and Paleocene, the Makarov basin was formed by rifting, although local spreading of oceanic crust during its formation cannot be excluded. 5. The Eurasian Basin started to open in the Early Eocene. We, of course, accept that our model of the geological history of the Arctic Ocean, being preliminary and debatable, may need further refining. In this paper, we have shown a link between the continental rift systems on the shelf and the formation history of the Arctic

  16. Fylla Bank: structure and evolution of a normal-to-shear rifted margin in the northern Labrador Sea

    DEFF Research Database (Denmark)

    Døssing, Arne

    2011-01-01

    ‐strike discontinuities in oceanic crust in the Labrador Sea to define margin segmentation in southern West Greenland, including the borders of Fylla Bank. A structural‐kinematic model presented here thus suggests that the Cretaceous–Cenozoic poly‐phase rifting to some extent was controlled by pre‐existing crustal fabric......, the Bank may be compared with the Demerara Plateau, part of the French Guinea‐Northeast Brazil continental margin. Seismic reflection interpretations presented in this study show that Fylla Bank is situated above an extensive basin complex, herein referred to as Fylla Structural Complex, which contains...... an up to 5‐km‐thick Cretaceous–Cenozoic sedimentary succession above an inferred pre‐Cretaceous basement. Seismic mapping of basement structures show that the complex is dominated by NNW‐/NW‐striking rift basins in its southern part and NNE‐striking rift basins in its northern part. The rift basins...

  17. The Age of Rift-Related Basalts in East Antarctica

    Science.gov (United States)

    Leitchenkov, G. L.; Belyatsky, B. V.; Kaminsky, V. D.

    2018-01-01

    The Lambert Rift, which is a large intracontinental rift zone in East Antarctica, developed over a long period of geological time, beginning from the Late Paleozoic, and its evolution was accompanied by magmatic activity. The latest manifestation of magmatism is eruption of alkaline olivine-leucite basalts on the western side of the Lambert Rift; Rb-Sr dating referred its time to the Middle Eocene, although its genesis remained vague. In order to solve this problem, we found geochronometer minerals in basaltic samples and 68 apatite grains appeared to be suitable for analysis. Their ages and ages of host basalts, determined by the U-Pb local method on the SIMS SHRIMP-II, were significantly different (323 ± 31 Ma) from those assumed earlier. This age corresponds to the earliest stage of crustal extension in East Antarctica and to most of Gondwana. The new data crucially change the ideas about the evolution of Lambert Rift and demonstrate the ambiguity of K-Ar dates of the alkali effusive formed under long-term rifting.

  18. State-of-stress in magmatic rift zones: Predicting the role of surface and subsurface topography

    Science.gov (United States)

    Oliva, S. J. C.; Ebinger, C.; Rivalta, E.; Williams, C. A.

    2017-12-01

    Continental rift zones are segmented along their length by large fault systems that form in response to extensional stresses. Volcanoes and crustal magma chambers cause fundamental changes to the density structure, load the plates, and alter the state-of-stress within the crust, which then dictates fracture orientation. In this study, we develop geodynamic models scaled to a structure, petrologic and thermodynamic studies constrain material densities, and seismicity and structural analyses constrain active and time-averaged kinematics. This area is an ideal test area because a 60º stress rotation is observed in time-averaged fault and magma intrusion, and in local seismicity, and because this was the site of a large volume dike intrusion and seismic sequence in 2007. We use physics-based 2D and 3D models (analytical and finite elements) constrained by data from active rift zones to quantify the effects of loading on state-of-stress. By modeling varying geometric arrangements, and density contrasts of topographic and subsurface loads, and with reasonable regional extensional forces, the resulting state-of-stress reveals the favored orientation for new intrusions. Although our models are generalized, they allow us to evaluate whether a magmatic system (surface and subsurface) can explain the observed stress rotation, and enable new intrusions, new faults, or fault reactivation with orientations oblique to the main border faults. Our results will improve our understanding of the different factors at play in these extensional regimes, as well as contribute to a better assessment of the hazards in the area.

  19. Geological evolution of the Boset-Bericha Volcanic Complex, Main Ethiopian Rift: 40Ar/39Ar evidence for episodic Pleistocene to Holocene volcanism

    Science.gov (United States)

    Siegburg, Melanie; Gernon, Thomas M.; Bull, Jonathan M.; Keir, Derek; Barfod, Dan N.; Taylor, Rex N.; Abebe, Bekele; Ayele, Atalay

    2018-02-01

    episodic activity within the BBVC and at several rift segments in the MER is observed, and facilitates continental rifting.

  20. 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...

  1. Keweenaw hot spot: Geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System

    Science.gov (United States)

    Hutchinson, D.R.; White, R.S.; Cannon, W.F.; Schulz, K.J.

    1990-01-01

    The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which we call the Keweenaw hot spot. Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection profiles constrain end-member models of melt thickness and stretching factors, which yield an inferred mantle potential temperature of 1500°–1570°C during rifting. Combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased, and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material. The isotopic signature of Keweenawan volcanic rocks, presented in a companion paper by Nicholson and Shirey (this issue), is consistent with our model of passive rifting above an asthenospheric mantle plume.

  2. Reassessment of source parameters for three major earthquakes in the East African rift system from historical seismograms and bulletins

    Directory of Open Access Journals (Sweden)

    O. Kulhánek

    2000-06-01

    Full Text Available Source parameters for three majo earthquakes in the East African rift are re-computed from historical seismograms and bulletins. The main shock and the largest foreshock of the August 25, 1906 earthquake sequence in the main Ethiopian rift are re-located on the eastern shoulder of the rift segment.The magnitude of the main shock is estimated to be 6.5 (Mw from spectral analysis. The December 13, 1910 earthquake in the Rukwa rift (Western Tanzania indicated a significant strike-slip component from teleseismcs body-waveform inversion for fault mechanism and seismic moment. The January 6, 1928 earthquake in the Gregory rift (Kenya showed a multiple rupture process and unusually long duration for a size of 6.6(Mw. The May 20, 1990 earthquake in Southern Sudan, mentioned merely for the sake of comparison, is the largest of all instrumentally recorded events in the East African rift system. Despite the fact that the mode of deformation in the continental rift is predominantly of extensional nature, the three largest earthquakes known to occur in the circum-Tanzanian craton have shallow focal depths and significant strike-slip component in their fault mechanisms. This and similar works will enrich the database for seismic hazard assessment in East Africa.

  3. Geodynamic modelling of the rift-drift transition: Application to the Red Sea

    Science.gov (United States)

    Fierro, E.; Schettino, A.; Capitanio, F. A.; Ranalli, G.

    2017-12-01

    The onset of oceanic accretion after a rifting phase is generally accompanied by an initial fast pulse of spreading in the case of volcanic margins, such that the effective spreading rate exceeds the relative far-field velocity between the two plates for a short time interval. This pulse has been attributed to edge-driven convention (EDC), although our numerical modelling shows that the shear stress at the base of the lithosphere cannot exceed 1 MPa. In general, we have developed a 2D numerical model of the mantle instabilities during the rifting phase, in order to determine the geodynamic conditions at the rift-drift transition. The model was tested using Underworld II software, variable rheological parameters, and temperature and stress-dependent viscosity. Our results show an increase of strain rates at the top of the lithosphere with the lithosphere thickness as well as with the initial width of the margin up to 300 km. Beyond this value, the influence of the initial rift width can be neglected. An interesting outcome of the numerical model is the existence of an axial zone characterized by higher strain rates, which is flanked by two low-strain stripes. As a consequence, the model suggests the existence of an area of syn-rift compression within the rift valley. Regarding the post-rift phase, we propose that at the onset of a seafloor spreading, a phase of transient creep allows the release of the strain energy accumulated in the mantle lithosphere during the rifting phase, through anelastic relaxation. Then, the conjugated margins would be subject to post-rift contraction and eventually to tectonic inversion of the rift structures. To explore the tenability of this model, we introduce an anelastic component in the lithosphere rheology, assuming both the classical linear Kelvin-Voigt rheology and a non-linear Kelvin model. The non-linear model predicts viable relaxation times ( 1-2Myrs) to explain the post-rift tectonic inversion observed along the Arabian

  4. Receiver functions analysis in Northern Tanzania to understand the earliest stage of rifting

    Science.gov (United States)

    Tiberi, C.; Albaric, J.; Deschamps, A.; Deverchere, J.; Ebinger, C. J.; Ferdinand, R. W.; Gautier, S.; Lambert, C.; Msabi, M.; Mtelela, K.; Muzuka, A.; Perrot, J.; Rasendra, N.; Roecker, S. W.; Rodzianko, A.; Witkin, E.

    2013-12-01

    The East African Rift (EAR) is the site of stretching and breakup of the lithosphere in response to a combination of regional pulling forces and mantle upwellings. Deformation results from complex interactions between magmatic intrusions, faulting, asthenospheric dynamism and far field stresses. It thus involves both deep processes and local inherited fabrics. In the frame of two international projects CRAFTI (NSF) and CoLiBrEA (ANR), we gather our skills to lead a multidisciplinary project in order to characterize the factors involved in continental rifting. We target the first 5 My of a magmatic rift initiating in thick (>150 km) continental lithosphere, where we can directly image and detect fault and magma interactions, the role of inherited and rheological heterogeneities of the lithosphere on rift localisation. We deployed 35 broadband seismic stations in Natron and Ngorongoro areas in January 2013 to characterize crustal and mantle structures of the rift. The stations were equipped by 3 component sensors and Reftek Recorders to continuously record teleseisms as well as local seismicity. We present here a receiver function analyse on the teleseismic events recorded during the first 6 months of the experiment. Both P- and S-waves receiver functions were proceeded to document the modification of the crust and the mantle due to plate stretching and magmatic processes. The Vp/Vs ratio informs on the state of the crust, which is affected by magmatic and fluids intrusions at different depths. The S-wave receiver function gives insight into the lithosphere state and the nature of the mantle beneath the rift (archean or plume affected).

  5. Geophysical and geochemical models of the Earth's shields and rift zones

    International Nuclear Information System (INIS)

    Chung, D.H.

    1977-01-01

    This report summarizes a collection of, synthesis of, and speculation on the geophysical and geochemical models of the earth's stable shields and rift zones. Two basic crustal types, continental and oceanic, and two basic mantle types, stable and unstable, are described. It is pointed out that both the crust and upper mantle play a strongly interactive role with surface geological phenomena ranging from the occurrence of mountains, ocean trenches, oceanic and continental rifts to geographic distributions of earthquakes, faults, and volcanoes. On the composition of the mantle, there is little doubt regarding the view that olivine constitutes a major fraction of the mineralogy of the earth's upper mantle. Studies are suggested to simulate the elasticity and composition of the earth's lower crust and upper mantle

  6. Crustal structure variations along the NW-African continental margin: a comparison of new and existing models from wide angle and reflection seismic data

    Science.gov (United States)

    Biari, Y.; Klingelhoefer, F.; Sahabi, M.; Aslanian, D.; Philippe, S.; Louden, K. E.; Berglar, K.; Moulin, M.; Mehdi, K.; Graindorge, D.; Evain, M.; Benabellouahed, M.; Reichert, C. J.

    2014-12-01

    Deep seismic data represent a key to understand the geometry and mechanism of continental rifting. The passive continental margin of NW-Africa is one of the oldest on earth, formed during the Upper Triassic-Lower Liassic rifting of the central Atlantic Ocean over 200 Ma. We present new and existing wide-angle and reflection seismic data from three study regions along the margin located in the North Moroccan salt basin, on the central continental margin offshore Safi and in the south, offshore Dakhla. In each of the study areas several combined wide-angle and reflection seismic profiles perpendicular and parallel to the margin have been acquired and forward modelled using comparable methods. The thickness of unthinned continental crust decreases from 36 km in the North to about 27 km in the South. In the North Moroccan Basin continental crust thins from originally 36 km to about 8 km in a 150 km wide zone. The basin itself is underlain by highly thinned continental crust. Offshore safi thinning of the continental crust is confined to a 130 km wide zone with no neighboring sedimentary basin underlain by continental crust. In both areas the zone of crustal thinning is characterised by the presence of large blocks and abundant salt diapirs. In the south crustal thinning is more rapid in a zone of 90 km and asymmetric with the upper crust thinning more closely to the continent than the lower crust, probably due to depth-dependent stretching and the presence of the precambrian Reguibat Ridge on land. Oceanic crust is characterised by a thickness of 7-8 km along the complete margin. Relatively high velocities of up to 7.5 km/s have been imaged between magnetic anomalies S1 and M25, and are probably related to changes in the spreading velocities at the time of the Kimmeridgian/Tithonian plate reorganisation. Volcanic activity seems to be confined to the region next to the Canary Islands, and is thus not related to the initial opening of the oceanic, which was related to no

  7. Detection of Rift Valley Fever Virus Interepidemic Activity in Some Hotspot Areas of Kenya by Sentinel Animal Surveillance, 2009–2012

    Directory of Open Access Journals (Sweden)

    Jacqueline Kasiiti Lichoti

    2014-01-01

    Full Text Available Rift Valley fever virus causes an important zoonotic disease of humans and small ruminants in Eastern Africa and is spread primarily by a mosquito vector. In this region, it occurs as epizootics that typically occur at 5–15-year intervals associated with unusual rainfall events. It has hitherto been known that the virus is maintained between outbreaks in dormant eggs of the mosquito vector and this has formed the basis of understanding of the epidemiology and control strategies of the disease. We show here that seroconversion and sporadic acute disease do occur during the interepidemic periods (IEPs in the absence of reported cases in livestock or humans. The finding indicates that previously undetected low-level virus transmission during the IEPs does occur and that epizootics may also be due to periodic expansion of mosquito vectors in the presence of both circulating virus and naïve animals.

  8. The role of magmatic loads and rift jumps in generating seaward dipping reflectors on volcanic rifted margins

    Science.gov (United States)

    Buck, W. Roger

    2017-05-01

    The largest volcanic constructs on Earth are the seismically imaged seaward dipping reflector (SDR) units found offshore of many rifted continental margins, including most that border the Atlantic Ocean. Whether their formation requires large magnitude (i.e. 10 s of km) of normal fault slip or results from the deflection of the lithosphere by the weight of volcanic flows is controversial. Though there is evidence for faulting associated with some SDRs, this paper considers the range of structures that can be produced by magmatic and volcanic loading alone. To do this an idealized mechanical model for the construction of rift-related volcanic flow structures is developed. Dikes open as plates move away from the center of a model rift and volcanic flows fill the depression produced by the load caused by dike solidification. The thin elastic plate flexure approximation allows a closed form description of the shape of both the contacts between flows and between the flows and underlying dikes. The model depends on two independent parameters: the flexure parameter, α, and the maximum isostatically supported extrusive layer thickness, w0. For reasonable values of these parameters the model reproduces the observed down-dip thickening of flows and the range of reflector dip angles. A numerical scheme using the analytic results allows simulation of the effect of temporal changes in the locus of magmatic spreading as well as changes in the amount of volcanic infill. Either jumps in the location of the center of diking or periods with no volcanism result in separate units or "packages" of model SDRs, in which the flow-dike contact dips landward, consistent with observations previously attributed only to listric normal fault offset. When jumps in the spreading center are small (i.e. less than α) they result in thicker, narrower volcanic units on one side of a rift compared to those on the other side. This is similar to the asymmetric distributions of volcanic packages seen

  9. Devonian magmatism in the Timan Range, Arctic Russia - subduction, post-orogenic extension, or rifting?

    Science.gov (United States)

    Pease, V.; Scarrow, J. H.; Silva, I. G. Nobre; Cambeses, A.

    2016-11-01

    Devonian mafic magmatism of the northern East European Craton (EEC) has been variously linked to Uralian subduction, post-orogenic extension associated with Caledonian collision, and rifting. New elemental and isotopic analyses of Devonian basalts from the Timan Range and Kanin Peninsula, Russia, in the northern EEC constrain magma genesis, mantle source(s) and the tectonic process(es) associated with this Devonian volcanism to a rift-related context. Two compositional groups of low-K2O tholeiitic basalts are recognized. On the basis of Th concentrations, LREE concentrations, and (LREE/HREE)N, the data suggest two distinct magma batches. Incompatible trace elements ratios (e.g., Th/Yb, Nb/Th, Nb/La) together with Nd and Pb isotopes indicate involvement of an NMORB to EMORB 'transitional' mantle component mixed with variable amounts of a continental component. The magmas were derived from a source that developed high (U,Th)/Pb, U/Th and Sm/Nd over time. The geochemistry of Timan-Kanin basalts supports the hypothesis that the genesis of Devonian basaltic magmatism in the region resulted from local melting of transitional mantle and lower crust during rifting of a mainly non-volcanic continental rifted margin.

  10. Spatial and Temporal Strain Distribution Along the Central Red Sea Rift - A Study of the Hamd-Jizil Basin in Saudi Arabia

    Science.gov (United States)

    Szymanski, E.; Stockli, D.; Johnson, P.; Kattan, F. H.; Al Shamari, A.

    2006-12-01

    Numerous models exploring the rupturing modes and mechanisms of continental lithosphere are based on geological evidence from the Red Sea/Gulf of Suez rift system. Individually, the Red Sea basin is the prototype for many models of orthogonal continental rifting. Despite being a classic example of continental extension, many temporal and spatial strain distribution aspects, as well as the dynamic evolution of the rift architecture of the Red Sea, remain poorly constrained. Critical data come mostly from the Gulf of Suez and the Egyptian and Yemeni margins of the Red Sea; the rift flanks in Sudan and Saudi Arabia have remained largely unstudied, leaving a large information gap along the central portions of the rift system. Improving continental lithosphere rupture models requires an absolute understanding of the timing and magnitude of strain partitioning along the full rift flank. This study focuses on the development of extensional structures, syn- extensional sedimentary deposits, and rift-related Tertiary basaltic volcanism along the central flank of the rift system in Saudi Arabia. Geo- and thermochronometric techniques are used to elucidate the evolution of inboard and outboard strain markers manifested by structurally-controlled extensional basins that parallel the trend of the main Red Sea rift. Constraints on the dynamics of rift flank deformation are achieved through the collection of thermochronometric transects that traverse both the entire Arabian shield and individual normal faults that bound inland basins. Preliminary results show inland basins as asymmetric half-grabens filled by tilted Cenozoic sedimentary strata and separated by exhumed basement fault blocks. The most prominent extensional basin is the NW-trending Hamd-Jizil basin, located north of Madinah, measuring ~200 km along strike and up to 20 km in width. The Hamd-Jizil basin is structurally characterized by two half-grabens exposing a series of syn-rift siliciclastic sedimentary sections

  11. Wilson cycle passive margins: Control of orogenic inheritance on continental breakup

    DEFF Research Database (Denmark)

    Petersen, Kenni D.; Schiffer, Christian

    2016-01-01

    Abstract Rifts and passive margins often develop along old suture zones where colliding continents merged during earlier phases of the Wilson cycle. For example, the North Atlantic formed after continental break-up along sutures formed during the Caledonian and Variscan orogenies. Even though suc...

  12. Thermal evolution of a hyperextended rift basin, Mauléon Basin, western Pyrenees

    Science.gov (United States)

    Hart, Nicole R.; Stockli, Daniel F.; Lavier, Luc L.; Hayman, Nicholas W.

    2017-06-01

    Onshore and offshore geological and geophysical observations and numerical modeling have greatly improved the conceptual understanding of magma-poor rifted margins. However, critical questions remain concerning the thermal evolution of the prerift to synrift phases of thinning ending with the formation of hyperextended crust and mantle exhumation. In the western Pyrenees, the Mauléon Basin preserves the structural and stratigraphic record of Cretaceous extension, exhumation, and sedimentation of the proximal-to-distal margin development. Pyrenean shortening uplifted basement and overlying sedimentary basins without pervasive shortening or reheating, making the Mauléon Basin an ideal locality to study the temporal and thermal evolution of magma-poor hyperextended rift systems through coupling bedrock and detrital zircon (U-Th)/He thermochronometric data from transects characterizing different structural rifting domains. These new data indicate that the basin was heated during early rifting to >180°C with geothermal gradients of 80-100°C/km. The proximal margin recorded rift-related exhumation/cooling at circa 98 Ma, whereas the distal margin remained >180°C until the onset of Paleocene Pyrenean shortening. Lithospheric-scale numerical modeling shows that high geothermal gradients, >80°C/km, and synrift sediments >180°C, can be reached early in rift evolution via heat advection by lithospheric depth-dependent thinning and blanketing caused by the lower thermal conductivity of synrift sediments. Mauléon Basin thermochronometric data and numerical modeling illustrate that reheating of basement and synrift strata might play an important role and should be considered in the future development of conceptual and numerical models for hyperextended magma-poor continental rifted margins.

  13. The real right of the state and municipal taxation on activities performed on the Continental Shelf; O real direito de tributacao estadual e municipal sobre atividades executadas na Plataforma Continental

    Energy Technology Data Exchange (ETDEWEB)

    Belo, Heraldo da Costa [PETROBRAS S.A., Rio de Janeiro, RJ (Brazil); Belo, Rodrigo Marcellino da Costa

    2008-07-01

    and services that are carried out in the continental Brazilian platform. The continental platform does not constitute territory of the States and Local authority, but yes territory in the Union, so the standard itself of regency limits the rights of power, jurisdiction and legislation only the Union in exclusion to too many federate beings. So, notice, in regard to the beginning of the legality and to the value of the legal security, the necessity of resolving this dilemma, since to a businessman who it is responsibility to business activity cannot be subdued to size been of uncertainty that, like natural reflex, matters in increase of costs of production that they will be gone over again to the final product, raising the price whose products the production of the oil producing industry is an input, or even his products themselves been intended for the final consumption. (author)

  14. Sediment-infill volcanic breccia from the Neoarchean Shimoga greenstone terrane, western Dharwar Craton: Implications on pyroclastic volcanism and sedimentation in an active continental margin

    Science.gov (United States)

    Manikyamba, C.; Saha, Abhishek; Ganguly, Sohini; Santosh, M.; Lingadevaru, M.; Rajanikanta Singh, M.; Subba Rao, D. V.

    2014-12-01

    We report sediment-infill volcanic breccia from the Neoarchean Shimoga greenstone belt of western Dharwar Craton which is associated with rhyolites, chlorite schists and pyroclastic rocks. The pyroclastic rocks of Yalavadahalli area of Shimoga greenstone belt host volcanogenic Pb-Cu-Zn mineralization. The sediment-infill volcanic breccia is clast-supported and comprises angular to sub-angular felsic volcanic clasts embedded in a dolomitic matrix that infilled the spaces in between the framework of volcanic clasts. The volcanic clasts are essentially composed of alkali feldspar and quartz with accessory biotite and opaques. These clasts have geochemical characteristics consistent with that of the associated potassic rhyolites from Daginkatte Formation. The rare earth elements (REE) and high field strength element (HFSE) compositions of the sediment-infill volcanic breccia and associated mafic and felsic volcanic rocks suggest an active continental margin setting for their generation. Origin, transport and deposition of these rhyolitic clasts and their aggregation with infiltrated carbonate sediments may be attributed to pyroclastic volcanism, short distance transportation of felsic volcanic clasts and their deposition in a shallow marine shelf in an active continental margin tectonic setting where the rhyolitic clasts were cemented by carbonate material. This unique rock type, marked by close association of pyroclastic volcanic rocks and shallow marine shelf sediments, suggest shorter distance between the ridge and shelf in the Neoarchean plate tectonic scenario.

  15. 16 Years, 16 Cruises, 1.6 Billion Soundings: a Compilation of High-Resolution Multibeam Bathymetry of the Active Plate Boundary Along the Chilean Continental Margin

    Science.gov (United States)

    Weinrebe, W.; Flueh, E. R.; Hasert, M.; Behrmann, J. H.; Voelker, D.; Geersen, J.; Ranero, C. R.; Diaz-Naveas, J. L.

    2011-12-01

    Chile, a country stranding the active plate boundary between the South-American and the Nazca Plate is afflicted by recurrent earthquakes and hazardous volcanic eruptions. The strongest earthquake ever recorded occurred here, and volcanic hazards are frequent. Consequently, this area has been studied by geoscientists for many years to improve the understanding of subduction zone processes. Swath bathymetry mapping of the ocean floor has proven to bear a large potential for the interpretation of subduction-related processes, such as tectonic deformation of the marine forearc, release and migration of fluids as well as earthquake-triggered mass wasting. Multibeam bathymetry data of 16 major cruises of German, British, and Chilean research vessels recorded between 1995 and December 2010, in total more than 10,000 data files comprising about 1.6 billion soundings, have now been carefully reprocessed, compiled and merged into a unifying set of high-resolution bathymetric maps of the Chilean continental margin from latitude 40°S to 20°S. The imprint of subsurface processes on the surface morphology is well displayed in the case of the Chilean continental margin. The 3,500 km long Chilean convergent margin is not uniform, as various segments with different tectonic characteristics can be distinguished. Major factors that control margin morphology and thus the style of subduction are (1) relief and structure of the incoming oceanic plate, (2) supply of trench sediment, (3) turbidite transport within the trench, and (4) the input of terrigeneous sediments down the continental slope. A major segment boundary occurs at latitude 32°-33° S where the hotspot-related volcanic chain of Juan Fernandez is presently subducting. South of the area of ridge subduction the trench is filled with turbidites, and accretionary ridges develop across the base of the slope along most of the segment, whereas north of this boundary the turbiditic infill is reduced and subduction erosion is

  16. 3D Thermo-Mechanical Models of Plume-Lithosphere Interactions: Implications for the Kenya rift

    Science.gov (United States)

    Scheck-Wenderoth, M.; Koptev, A.; Sippel, J.

    2017-12-01

    We present three-dimensional (3D) thermo-mechanical models aiming to explore the interaction of an active mantle plume with heterogeneous pre-stressed lithosphere in the Kenya rift region. As shown by the recent data-driven 3D gravity and thermal modeling (Sippel et al., 2017), the integrated strength of the lithosphere for the region of Kenya and northern Tanzania appears to be strongly controlled by the complex inherited crustal structure, which may have been decisive for the onset, localization and propagation of rifting. In order to test this hypothesis, we have performed a series of ultra-high resolution 3D numerical experiments that include a coupled mantle/lithosphere system in a dynamically and rheologically consistent framework. In contrast to our previous studies assuming a simple and quasi-symmetrical initial condition (Koptev et al., 2015, 2016, 2017), the complex 3D distribution of rock physical properties inferred from geological and geophysical observations (Sippel et al., 2017) has been incorporated into the model setup that comprises a stratified three-layer continental lithosphere composed of an upper and lower crust and lithospheric mantle overlaying the upper mantle. Following the evidence of the presence of a broad low-velocity seismic anomaly under the central parts of the East African Rift system (e.g. Nyblade et al, 2000; Chang et al., 2015), a 200-km radius mantle plume has been seeded at the bottom of a 635 km-depth model box representing a thermal anomaly of 300°C temperature excess. In all model runs, results show that the spatial distribution of surface deformation is indeed strongly controlled by crustal structure: within the southern part of the model box, a localized narrow zone stretched in NS direction (i.e. perpendicularly to applied far-field extension) is aligned along a structural boundary within the lower crust, whereas in the northern part of the model domain, deformation is more diffused and its eastern limit coincides with

  17. The NE Rift of Tenerife: towards a model on the origin and evolution of ocean island rifts; La dorsal NE de Tenerife: hacia un modelo del origen y evolucion de los rifts de islas oceanicas

    Energy Technology Data Exchange (ETDEWEB)

    Carracedo, J. C.; Guillou, H.; Rodriguez Badiola, E.; Perez-Torrado, F. J.; Rodriguez Gonzalez, A.; Peris, R.; Troll, V.; Wiesmaier, S.; Delcamp, A.; Fernandez-Turiel, J. L.

    2009-07-01

    The NE Rift of Tenerife is an excellent example of a persistent, recurrent rift, providing important evidence of the origin and dynamics of these major volcanic features. The rift developed in three successive, intense and relatively short eruptive stages (a few hundred ka), separated by longer periods of quiescence or reduced activity: A Miocene stage (7266 {+-}156 ka), apparently extending the central Miocene shield of Tenerife towards the Anaga massif; an Upper Pliocene stage (2710{+-} 58 ka) and the latest stage, with the main eruptive phase in the Pleistocene. Detailed geological (GIS) mapping, geomagnetic reversal mapping and stratigraphic correlation, and radioisotopic (K/Ar) dating of volcanic formations allowed the reconstruction of the latest period of rift activity. In the early phases of this stage the majority of the eruptions grouped tightly along the axis of the rift and show reverse polarity (corresponding to the Matuyama chron). Dykes are of normal and reverse polarities. In the final phase of activity, eruptions are more disperse and lavas and dykes are consistently of normal polarity (Brunhes chron). Volcanic units of normal polarity crossed by dykes of normal and reverse polarities yield ages apparently compatible with normal subchrons (M-B Precursor and Jaramillo) in the Upper Matuyama chron. Three lateral collapses successively mass-wasted the rift: The Micheque collapse, completely concealed by subsequent nested volcanism, and the Guimar and La Orotava collapses, that are only partially filled. Time occurrence of collapses in the NE rift apparently coincides with glacial stages, suggesting that giant landslides may be finally triggered by sea level chan-ges during glaciations. Pre-collapse and nested volcanism is predominantly basaltic, except in the Micheque collapse, where magmas evolved towards intermediate and felsic (trachytic) compositions. Rifts in the Canary Islands are long-lasting, recurrent features, probably related to primordial

  18. Early evolution of the southern margin of the Neuquén Basin, Argentina: Tectono-stratigraphic implications for rift evolution and exploration of hydrocarbon plays

    Science.gov (United States)

    D'Elia, Leandro; Bilmes, Andrés; Franzese, Juan R.; Veiga, Gonzalo D.; Hernández, Mariano; Muravchik, Martín

    2015-12-01

    Long-lived rift basins are characterized by a complex structural and tectonic evolution. They present significant lateral and vertical stratigraphic variations that determine diverse basin-patterns at different timing, scale and location. These issues cause difficulties to establish facies models, correlations and stratal stacking patterns of the fault-related stratigraphy, specially when exploration of hydrocarbon plays proceeds on the subsurface of a basin. The present case study corresponds to the rift-successions of the Neuquén Basin. This basin formed in response to continental extension that took place at the western margin of Gondwana during the Late Triassic-Early Jurassic. A tectono-stratigraphic analysis of the initial successions of the southern part of the Neuquén Basin was carried out. Three syn-rift sequences were determined. These syn-rift sequences were located in different extensional depocentres during the rifting phases. The specific periods of rifting show distinctly different structural and stratigraphic styles: from non-volcanic to volcanic successions and/or from continental to marine sedimentation. The results were compared with surface and subsurface interpretations performed for other depocentres of the basin, devising an integrated rifting scheme for the whole basin. The more accepted tectono-stratigraphic scheme that assumes the deposits of the first marine transgression (Cuyo Cycle) as indicative of the onset of a post-rift phase is reconsidered. In the southern part of the basin, the marine deposits (lower Cuyo Cycle) were integrated into the syn-rift phase, implying the existence of different tectonic signatures for Cuyo Cycle along the basin. The rift climax becomes younger from north to south along the basin. The post-rift initiation followed the diachronic ending of the main syn-rift phase throughout the Neuquén Basin. Thus, initiation of the post-rift stage started in the north and proceeded towards the south, constituting a

  19. Stress and slip partitioning during oblique rifting: comparison between data from the Main Ethiopian Rift and laboratory experiments

    Science.gov (United States)

    Corti, G.; Philippon, M.; Sani, F.; Keir, D.

    2012-04-01

    Oblique rifting in the central and northern Main Ethiopian Rift (MER) has resulted in a complex structural pattern characterized by two differently oriented fault systems: a set of NE-SW-trending boundary faults and a system of roughly NNE-SSW-oriented fault swarms affecting the rift floor (Wonji faults). Boundary faults formed oblique to the regional extension vector, likely as a result of the oblique reactivation of a pre-existing deep-seated rheological anisotropy, whereas internal Wonji faults developed sub-orthogonal to the stretching direction. Previous works have successfully reconciled this rift architecture and fault distribution with the long-term plate kinematics; however, at a more local scale, fault-slip and earthquake data reveal significant variations in the orientation the minimum principal stress and related fault-slip direction across the rift valley. Whereas fault measurements indicate a roughly N95°E extension on the axial Wonji faults, a N105°E to N110°E directed minimum principal stress is observed along boundary faults. Both fault-slip data and analysis of seismicity indicate a roughly pure dip-slip motion on the boundary faults, despite their orientation (oblique to the regional extension vector) should result in an oblique displacement. To shed light on the process driving the variability of data derived from fault-slip (and seismicity) analysis we present crustal-scale analogue models of oblique rifting, deformed in a large-capacity centrifuge by using materials and boundary conditions described in several previous modeling works. As in these previous works, the experiments show the diachronous activation of two fault systems, boundary and internal, whose pattern strikingly resemble that observed in previous lithospheric-scale modeling, as well as that described in the MER. Internal faults arrange in two different, en-echelon segments connected by a transfer zone where strike-slip displacement dominates. Whereas internal faults develop

  20. Deeply concealed half-graben at the SW margin of the East European Craton (SE Poland — Evidence for Neoproterozoic rifting prior to the break-up of Rodinia

    Directory of Open Access Journals (Sweden)

    P. Krzywiec

    2018-01-01

    Full Text Available Baltica was one of continents formed as a result of Rodinia break-up 850–550 Ma. It was separated from Amazonia(? by the Tornquist Ocean, the opening of which was preceded by Neoproterozoic extension in a network of continental rifts. Some of these rifts were subsequently aborted whereas the Tornquist Rift gave rise to splitting of Rodinia and formation of the Tornquist Ocean. The results of 1-D subsidence analysis at the fossil passive margin of Baltica provided insight in the timing and kinematics of continental rifting that led to break-up of Rodinia. Rifting was associated with Neoproterozoic syn-rift subsidence accompanied by deposition of continental coarse-grained sediments and emplacement of continental basalts. Transition from a syn-rift to post-rift phase in the latest Ediacaran to earliest early Cambrian was concomitant with deposition of continental conglomerates and arkoses, laterally passing into mudstones. An extensional scenario of the break-up of Rodinia along the Tornquist Rift is based on the character of tectonic subsidence curves, evolution of syn-rift and post-rift depocenters in time, as well as geochemistry and geochronology of the syn-rift volcanics. It is additionally reinforced by the high-quality deep seismic reflection data from SE Poland, located above the SW edge of the East European Craton. The seismic data allowed for identification of a deeply buried (11–18 km, well-preserved extensional half-graben, developed in the Palaeoproterozoic crystalline basement and filled with a Neoproterozoic syn-rift volcano-sedimentary succession. The results of depth-to-basement study based on integration of seismic and gravity data show the distribution of local NE–SW elongated Neoproterozoic depocenters within the SW slope of the East European Craton. Furthermore, they document the rapid south-eastwards thickness increase of the Neoproterozoic succession towards the NW–SE oriented craton margin. This provides evidence

  1. Compressional reactivation of hyperextended domains on a rifted margin: a requirement for a reappraisal of traditional restoration procedures?

    Science.gov (United States)

    Cadenas Martínez, P.; Fernandez Viejo, G.; Pulgar, J. A.

    2017-12-01

    The North Iberian margin is an inverted hyperextended rifted margin that preserves the initial stages of compressional reactivation. Rift inheritance conditioned in a determinant way the contractional reactivation. The underthrusting of the hyperextended distal domains beneath the platform and the formation of an accretionary wedge at the toe of the slope focused most of the compression. The underthrusting gave place to the formation of a crustal root and the uplifting of the Cantabrian Mountains onshore. Meanwhile, the main rift basins within the continental platform were slightly inverted. Plate kinematic reconstructions and palinspatic restorations have provided different shortening values. Thereby, the amount of shortening linked with the Cenozoic compression is still unclear and a matter of debate on this area.In this work, we present a full cross-section at the central part of the North Iberian margin developed from the restoration of a high quality depth migrated seismic profile running from the continental platform to the Biscay abyssal plain. A shortening calculation gives an estimate of about 1 km within the Asturian Basin, in the continental platform, while in the accretionary wedge at the bottom of the slope, shortening values ranges between 12 km and 15 km. The limited values estimated within the Asturian Basin support the mild inversion observed within this basin, which preserves most of the extensional imprint. Within the abyssal plain, shortening values differ from previous estimations and cannot account for a high amount of compression in the upper crust. Deformation of the hyperextended crust and the exhumed mantle domains inherited from the rifting processes would have accommodated most of the compression. Restoration of these domains seems to be the key to decipher the structure and the tectonic evolution of the reactivated rifted margin but cannot be solved accurately using traditional restoration methods. This leads to a reappraisal of the

  2. Probable existence of a Gondwana transcontinental rift system in western India: Implications in hydrocarbon exploration in Kutch and Saurashtra offshore: A GIS-based approach

    Science.gov (United States)

    Mazumder, S.; Tep, Blecy; Pangtey, K. K. S.; Das, K. K.; Mitra, D. S.

    2017-08-01

    The Gondwanaland assembly rifted dominantly during Late Carboniferous-Early Permian forming several intracratonic rift basins. These rifts were subsequently filled with a thick sequence of continental clastic sediments with minor marine intercalations in early phase. In western part of India, these sediments are recorded in enclaves of Bikaner-Nagaur and Jaisalmer basins in Rajasthan. Facies correlatives of these sediments are observed in a number of basins that were earlier thought to be associated with the western part of India. The present work is a GIS based approach to reconnect those basins to their position during rifting and reconstruct the tectono-sedimentary environment at that time range. The study indicates a rift system spanning from Arabian plate in the north and extending to southern part of Africa that passes through Indus basin, western part of India and Madagascar, and existed from Late Carboniferous to Early Jurassic. Extensions related to the opening of Neo-Tethys led to the formation of a number of cross trends in the rift systems that acted as barriers to marine transgressions from the north as well as disrupted the earlier continuous longitudinal drainage systems. The axis of this rift system is envisaged to pass through present day offshore Kutch and Saurashtra and implies a thick deposit of Late Carboniferous to Early Jurassic sediments in these areas. Based on analogy with other basins associated with this rift system, these sediments may be targeted for hydrocarbon exploration.

  3. The major tectonic boundaries of the Northern Red Sea rift, Egypt derived from geophysical data analysis

    Science.gov (United States)

    Saleh, Salah; Pamukçu, Oya; Brimich, Ladislav

    2017-09-01

    In the present study, we have attempted to map the plate boundary between Arabia and Africa at the Northern Red Sea rift region including the Suez rift, Gulf of Aqaba-Dead Sea transform and southeastern Mediterranean region by using gravity data analysis. In the boundary analysis method which was used; low-pass filtered gravity anomalies of the Northern Red Sea rift region were computed. Different crustal types and thicknesses, sediment thicknesses and different heat flow anomalies were evaluated. According to the results, there are six subzones (crustal blocks) separated from each other by tectonic plate boundaries and/or lineaments. It seems that these tectonic boundaries reveal complex structural lineaments, which are mostly influenced by a predominant set of NNW-SSE to NW-SE trending lineaments bordering the Red Sea and Suez rift regions. On the other side, the E-W and N-S to NNE-SSW trended lineaments bordering the South-eastern Mediterranean, Northern Sinai and Aqaba-Dead Sea transform regions, respectively. The analysis of the low pass filtered Bouguer anomaly maps reveals that the positive regional anomaly over both the Red Sea rift and South-eastern Mediterranean basin subzones are considered to be caused by the high density of the oceanic crust and/or the anomalous upper mantle structures beneath these regions whereas, the broad medium anomalies along the western half of Central Sinai with the Suez rift and the Eastern Desert subzones are attributed to low-density sediments of the Suez rift and/or the thick upper continental crustal thickness below these zones. There are observable negative anomalies over the Northern Arabia subzone, particularly in the areas covered by Cenozoic volcanics. These negative anomalies may be attributed to both the low densities of the surface volcanics and/or to a very thick upper continental crust. On the contrary, the negative anomaly which belongs to the Gulf of Aqaba-Dead Sea transform zone is due to crustal thickening

  4. The earthquakes of stable continental regions. Volume 2: Appendices A to E. Final report

    International Nuclear Information System (INIS)

    Johnston, A.C.; Kanter, L.R.; Coppersmith, K.J.; Cornell, C.A.

    1994-12-01

    The objectives of the study were to develop a comprehensive database of earthquakes in stable continental regions (SCRs) and to statistically examine use of the database for the assessment of large earthquake potential. We identified nine major and several minor SCRs worldwide and compiled a database of geologic characteristics of tectonic domains within each SCR. We examined all available earthquake data from SCRs, from historical accounts of events with no instrumental ground-motion data to present-day instrumentally recorded events. In all, 1,385 events were analyzed. Using moment magnitude 4.5 as the lower bound threshold for inclusion in the database, 870 were assigned to an SCR, 124 were found to be transitional to an SCR, and 391 were examined, but rejected. We then performed a seismotectonic analysis to determine what distinguishes seismic activity in SCRs from other types of crust, such as active plate margins or active continental regions. General observations are: (1) SCRs comprise nearly two-thirds of all continental crust of which 25% is considered to be extended (i.e., rifted); (2) the majority of seismic energy release and the largest earthquakes in SCRs have occurred in extended crust; and (3) active plate margins release seismic energy at a rate per unit area approximately 7,000 times the average for non-extended SCRs. Finally, results of a statistical examination of distributions of historical maximum earthquakes between different crustal domain types indicated that additional information is needed in order to adequately constrain estimates of maximum earthquakes for any given region. Thus, a Bayesian approach was developed in which statistical constraints from the database were used to develop a prior distribution, which may then be combined with source-specific information to constrain maximum magnitude assessments for use in probabilistic seismic hazard analyses

  5. Numerical modelling of edge-driven convection during rift-to-drift transition: application to the Red Sea

    Science.gov (United States)

    Fierro, Elisa; Capitanio, Fabio A.; Schettino, Antonio; Morena Salerno, V.

    2017-04-01

    We use numerical modeling to investigate the coupling of mantle instabilities and surface tectonics along lithospheric steps developing during rifting. We address whether edge driven convection (EDC) beneath rifted continental margins and shear flow during rift-drift transition can play a role in the observed post-rift compressive tectonic evolution of the divergent continental margins along the Red Sea. We run a series of 2D simulations to examine the relationship between the maximum compression and key geometrical parameters of the step beneath continental margins, such as the step height due to lithosphere thickness variation and the width of the margins, and test the effect of rheology varying temperature- and stress-dependent viscosity in the lithosphere and asthenosphere. The development of instabilities is initially illustrated as a function of these parameters, to show the controls on the lithosphere strain distribution and magnitude. We then address the transient evolution of the instabilities to characterize their duration. In an additional suite of models, we address the development of EDC during plate motions, thus accounting for the mantle shearing due to spreading. Our results show an increase of strain with the step height as well as with the margin width up to 200 km. After this value the influence of ridge margin can be neglected. Strain rates are, then, quantified for a range of laboratory-constrained constitutive laws for mantle and lithosphere forming minerals. These models propose a viable mechanism to explain the post-rift tectonic inversion observed along the Arabian continental margin and the episodic ultra-fast sea floor spreading in the central Red Sea, where the role of EDC has been invoked.

  6. Post-rift magmatism in the Pearl River Mouth Basin, northern South China Sea

    Science.gov (United States)

    Xu, H.; Zhao, F.; Xia, S.; Sun, J.; Fan, C.

    2017-12-01

    Multi-beam, 2D seismic reflection and borehole data reveal that post-rift magmatism are widespread in the northern margin of South China Sea. A large-scale volcanic complex was identified at water depths of 500 to 3000 m, covering an area of ca. 8000 km2. This volcanic complex includes seamounts, igneous sills, dykes and intruded volcanic bodies. Combining data from exploration wells BY7-1 and BY2 with published seismic stratigraphic data, we can highlight multiple extrusive events from the Early Oligocene to Early Miocene, reflecting progressive continental breakup in the South China Sea. Most intruded magma through the continental crust also uplifted sediments up to the T6 unconformity. Given the evidence in this work that Early Miocene magmatic bodies were developed above or along faults, we suggest that post-rift magmatism in the northern margin of the South China Sea was largely controlled by the faults. Reactivation events in the faults are suggested to have generated preferential vertical pathways for the ascent of magma within a context of progressive continental breakup and thinned continental crust, as the South China Sea was being formed.

  7. Continental transform margins : state of art and future milestones

    Science.gov (United States)

    Basile, Christophe

    2010-05-01

    Transform faults were defined 45 years ago as ‘a new class of fault' (Wilson, 1965), and transform margins were consequently individualized as a new class of continental margins. While transform margins represent 20 to 25 % of the total length of continent-ocean transitions, they were poorly studied, especially when compared with the amount of data, interpretations, models and conceptual progress accumulated on divergent or convergent continental margins. The best studied examples of transform margins are located in the northern part of Norway, south of South Africa, in the gulf of California and on both sides of the Equatorial Atlantic. Here is located the Côte d'Ivoire - Ghana margin, where the more complete data set was acquired, based on numerous geological and geophysical cruises, including ODP Leg 159. The first models that encompassed the structure and evolution of transform margins were mainly driven by plate kinematic reconstructions, and evidenced the diachronic end of tectonic activity and the non-cylindrical character of these margins, with a decreasing strike-slip deformation from the convex to the concave divergent-transform intersections. Further thermo-mechanical models were more specifically designed to explain the vertical displacements along transform margins, and especially the occurrence of high-standing marginal ridges. These thermo-mechanical models involved either heat transfer from oceanic to continental lithospheres across the transform faults or tectonically- or gravity-driven mass transfer in the upper crust. These models were far from fully fit observations, and were frequently dedicated to specific example, and not easily generalizable. Future work on transform continental margins may be expected to fill some scientific gaps, and the definition of working directions can benefit from the studies dedicated to other types of margins. At regional scale the structural and sedimentological variability of transform continental margins has

  8. Deep continental margin reflectors

    Science.gov (United States)

    Ewing, J.; Heirtzler, J.; Purdy, M.; Klitgord, Kim D.

    1985-01-01

    In contrast to the rarity of such observations a decade ago, seismic reflecting and refracting horizons are now being observed to Moho depths under continental shelves in a number of places. These observations provide knowledge of the entire crustal thickness from the shoreline to the oceanic crust on passive margins and supplement Consortium for Continental Reflection Profiling (COCORP)-type measurements on land.

  9. An Epidemiological Model of Rift Valley Fever with Spatial Dynamics

    Directory of Open Access Journals (Sweden)

    Tianchan Niu

    2012-01-01

    Full Text Available As a category A agent in the Center for Disease Control bioterrorism list, Rift Valley fever (RVF is considered a major threat to the United States (USA. Should the pathogen be intentionally or unintentionally introduced to the continental USA, there is tremendous potential for economic damages due to loss of livestock, trade restrictions, and subsequent food supply chain disruptions. We have incorporated the effects of space into a mathematical model of RVF in order to study the dynamics of the pathogen spread as affected by the movement of humans, livestock, and mosquitoes. The model accounts for the horizontal transmission of Rift Valley fever virus (RVFV between two mosquito and one livestock species, and mother-to-offspring transmission of virus in one of the mosquito species. Space effects are introduced by dividing geographic space into smaller patches and considering the patch-to-patch movement of species. For each patch, a system of ordinary differential equations models fractions of populations susceptible to, incubating, infectious with, or immune to RVFV. The main contribution of this work is a methodology for analyzing the likelihood of pathogen establishment should an introduction occur into an area devoid of RVF. Examples are provided for general and specific cases to illustrate the methodology.

  10. Which Fault Orientations Occur during Oblique Rifting? Combining Analog and Numerical 3d Models with Observations from the Gulf of Aden

    Science.gov (United States)

    Autin, J.; Brune, S.

    2013-12-01

    Oblique rift systems like the Gulf of Aden are intrinsically three-dimensional. In order to understand the evolution of these systems, one has to decode the fundamental mechanical similarities of oblique rifts. One way to accomplish this, is to strip away the complexity that is generated by inherited fault structures. In doing so, we assume a laterally homogeneous segment of Earth's lithosphere and ask how many different fault populations are generated during oblique extension inbetween initial deformation and final break-up. We combine results of an analog and a numerical model that feature a 3D segment of a layered lithosphere. In both cases, rift evolution is recorded quantitatively in terms of crustal fault geometries. For the numerical model, we adopt a novel post-processing method that allows to infer small-scale crustal fault orientation from the surface stress tensor. Both models involve an angle of 40 degrees between the rift normal and the extensional direction which allows comparison to the Gulf of Aden rift system. The resulting spatio-temporal fault pattern of our models shows three normal fault orientations: rift-parallel, extension-orthogonal, and intermediate, i.e. with a direction inbetween the two previous orientations. The rift evolution involves three distinct phases: (i) During the initial rift phase, wide-spread faulting with intermediate orientation occurs. (ii) Advanced lithospheric necking enables rift-parallel normal faulting at the rift flanks, while strike-slip faulting in the central part of the rift system indicates strain partitioning. (iii) During continental break-up, displacement-orthogonal as well as intermediate faults occur. We compare our results to the structural evolution of the Eastern Gulf of Aden. External parts of the rift exhibit intermediate and displacement-orthogonal faults while rift-parallel faults are present at the rift borders. The ocean-continent transition mainly features intermediate and displacement

  11. Crustal and mantle structure and anisotropy beneath the incipient segments of the East African Rift System: Preliminary results from the ongoing SAFARI

    Science.gov (United States)

    Yu, Y.; Reed, C. A.; Gao, S. S.; Liu, K. H.; Massinque, B.; Mdala, H. S.; moidaki, M.; Mutamina, D. M.; Atekwana, E. A.; Ingate, S. F.; Reusch, A.; Barstow, N.

    2013-12-01

    Despite the vast wealth of research conducted toward understanding processes associated with continental rifting, the extent of our knowledge is derived primarily from studies focused on mature rift systems, such as the well-developed portions of the East African Rift System (EARS) north of Lake Malawi. To explore the dynamics of early rift evolution, the SAFARI (Seismic Arrays for African Rift Initiation) team deployed 50 PASSCAL broadband seismic stations across the Malawi, Luangwa, and Okavango rifts of the EARS during the summer of 2012. The cumulative length of the profiles is about 2500 km and the planned recording duration is 2 years. Here we present the preliminary results of systematic analyses of data obtained from the first year of acquisition for all 50 stations. A total of 446 high-quality shear-wave splitting measurements using PKS, SKKS, and SKS phases from 84 teleseismic events were used to constrain fast polarization directions and splitting times throughout the region. The Malawi and Okavango rifts are characterized by mostly NE trending fast directions with a mean splitting time of about 1 s. The fast directions on the west side of the Luangwa Rift Zone are parallel to the rift valley, and those on the east side are more N-S oriented. Stacking of approximately 1900 radial receiver functions reveals significant spatial variations of both crustal thickness and the ratio of crustal P and S wave velocities, as well as the thickness of the mantle transition zone. Stations situated within the Malawi rift demonstrate a southward increase in observed crustal thickness, which is consistent with the hypothesis that the Malawi rift originated at the northern end of the rift system and propagated southward. Both the Okavango and Luangwa rifts are associated with thinned crust and increased Vp/Vs, although additional data is required at some stations to enhance the reliability of the observations. Teleseismic P-wave travel-time residuals show a delay of about

  12. Constraining the dynamic response of subcontinental lithospheric mantle to rifting using Re-Os model ages in the Western Ross Sea, Antarctica

    Science.gov (United States)

    Doherty, C.; Class, C.; Goldstein, S. L.; Shirey, S. B.; Martin, A. P.; Cooper, A. F.; Berg, J. H.; Gamble, J. A.

    2012-12-01

    lithosphere extends beneath the TAM. With further analyses we hope to determine if there is lateral flow of cratonic lithosphere into the rift. Huismans, R., Beaumount, C., 2011. Depth-dependent extension, two stage breakup and cratonic underplating at rifted margins. Nature 473, 74-78. Reisberg, L.C., Lorand, J.P., 1995. Longevity of sub-continental mantle lithosphere from osmium isotope systematics in orogenic peridotite massifs. Nature 376, 159-162. Rudnick, R.L., Walker, R.J., 2009. Interpreting ages from Re-Os isotopes in peridotites. Lithos 1125, 1083-1095.

  13. Contributions to knowledge of the continental margin of Uruguay. Uruguayan continental margin: Physiographic and seismic analysis

    International Nuclear Information System (INIS)

    Preciozzi, F

    2014-01-01

    This work is about the kind of continental margins such as a )Atlantic type passive margins which can be hard or soft b) An active or Pacific margins that because of the very frequent earthquakes develop a morphology dominated by tectonic processes. The Uruguayan continental margin belongs to a soft Atlantic margin

  14. Asymmetric rifting, breakup and magmatism across conjugate margin pairs: insights from Newfoundland to Ireland

    Science.gov (United States)

    Peace, Alexander L.; Welford, J. Kim; Foulger, Gillian R.; McCaffrey, Ken J. W.

    2017-04-01

    Continental extension, subsequent rifting and eventual breakup result in the development of passive margins with transitional crust between extended continental crust and newly created oceanic crust. Globally, passive margins are typically classified as either magma-rich or magma-poor. Despite this simple classification, magma-poor margins like the West Orphan Basin, offshore Newfoundland, do exhibit some evidence of localized magmatism, as magmatism to some extent invariably accompanies all continental breakup. For example, on the Newfoundland margin, a small volcanic province has been interpreted near the termination of the Charlie Gibbs Fracture Zone, whereas on the conjugate Irish margin within the Rockall Basin, magmatism appears to be more widespread and has been documented both in the north and in the south. The broader region over which volcanism has been identified on the Irish margin is suggestive of magmatic asymmetry across this conjugate margin pair and this may have direct implications for the mechanisms governing the nature of rifting and breakup. Possible causes of the magmatic asymmetry include asymmetric rifting (simple shear), post-breakup thermal anomalies in the mantle, or pre-existing compositional zones in the crust that predispose one of the margins to more melting than its conjugate. A greater understanding of the mechanisms leading to conjugate margin asymmetry will enhance our fundamental understanding of rifting processes and will also reduce hydrocarbon exploration risk by better characterizing the structural and thermal evolution of hydrocarbon bearing basins on magma-poor margins where evidence of localized magmatism exists. Here, the latest results of a conjugate margin study of the Newfoundland-Ireland pair utilizing seismic interpretation integrated with other geological and geophysical datasets are presented. Our analysis has begun to reveal the nature and timing of rift-related magmatism and the degree to which magmatic asymmetry

  15. Holocene environmental change in the Albertine Rift : sediment-based evidence from Virunga volcanoes

    OpenAIRE

    McGlynn, Gayle

    2012-01-01

    The Albertine Rift is one of the most important conservation areas in tropical Africa, with exceptionally high levels of biodiversity (including rare and endemic montane flora and fauna) and high human population densities. Environmental changes in the Albertine Rift during the Holocene have been influenced both by long-term climate change and by human activity, although establishing clear cause-effect relationships is often problematic - particularly as this area is known to have had a long ...

  16. Extension style in the Orphan Basin during the Mesozoic North Atlantic rifting

    Science.gov (United States)

    Gouiza, Mohamed; Hall, Jeremy

    2013-04-01

    The Orphan Basin, lying along the Newfoundland passive continental margin, has formed in Mesozoic time during the opening of the North Atlantic Ocean and the breakup of Iberia/Eurasia from North America. Regional deep seismic reflection profiles across the basin indicate that the Neoproterozoic basement has been affected by repeated extensional episodes between the Late Triassic/Jurassic and the Early Cretaceous. Deformation initiated in the eastern part of the Orphan basin in the Jurassic and migrated toward the west in the Early Cretaceous, resulting in numerous rift structures filled with Jurassic-Lower Cretaceous syn-rift successions and sealed by thick Upper Cretaceous-Cenozoic post-rift sediments. The seismic data show an extremely attenuated crust underneath the eastern and western part of the deep basin, forming two sub-basins associated with the development of rifting. The two sub-basins are separated by a wide structural high with a relatively thick crust and are bounded to the west by the continental shelf domain. Restoration of the Orphan Basin along a 2D crustal section (520 km long), yields a total amount of stretching of about 144 km, while the total crustal thinning indicates an extension of around 250 km, assuming mass conservation along the section and an initial crustal thickness of 28 km. Brittle deformation accommodated by normal faults is documented in the seismic profiles and affected essentially the present-day upper portion of the crust, and represents only 60% of the total extension which thinned the Orphan crust. The remaining crustal thinning must involve other deformation processes which are not (easily) recognizable in the seismic data. We propose two models that could explain discrepancies between brittle deformation and total crustal thinning during lithospheric extension. The first model assumes the reactivation of pre-rift inherited structures, which act as crustal-scale detachments during the early stages of rifting. The second

  17. The role of long-term strain history on the generation and amplification of inherited heterogeneities in continental lithosphere extensional settings

    Science.gov (United States)

    Morena Salerno, V.; Capitanio, Fabio A.

    2017-04-01

    The Earth's lithosphere is characters by various types of heterogeneities, at different scales and located at variable depth. They can be represented at crustal level by remnants of earlier tectonics evolution, such as previous orogenetic structures, remains of passive margins and magmatic bodies intrusion, or at deeper level by mantle anisotropies. These heterogeneities can severely affect the stress and strain localization in subsequent continental lithospheric extension and rift basins evolution, hence contributing to the formation of diverse and complex rift basin types and architectures. In order to explain the difference in rift basin and passive margin types, their subsidence patterns and melt production, previous studies have exanimated the role of initial heterogeneities, rheological layering, geothermal gradients, and extension rates during a single rifting event. However, this approach does not consider the previous strain history of many basins that are characterized by multiple rifting events. In this study we use numerical models of a pristine lithosphere undergoing two rifting events separated by cooling, to show the effect of early events on later evolution. The strain histories are controlled by the variation of velocity of boundary displacement during two rifting events. We use both fast and slow first rifting events, followed by a cooling period, producing diverse mechanical heterogeneities at Moho level that represent inherited initial conditions for the second rifting event. These inherited heterogeneities range from several small perturbations distributed along the numerical domain at the end of the slowest first rifting event, to a single large perturbation at the end of first fastest rifting event. In the second rifting event, the inherited heterogeneities are amplified at different degree and time, depending on the velocity of boundary displacement used. To highlight the role of previous strain history, we parametrize the inherited

  18. A joint inversion for shear velocity and anisotropy: the Woodlark Rift, Papua New Guinea

    Science.gov (United States)

    Eilon, Zachary; Abers, Geoffrey A.; Gaherty, James B.

    2016-08-01

    Trade-offs between velocity and anisotropy heterogeneity complicate the interpretation of differential traveltime data and have the potential to bias isotropic tomographic models. By constructing a simple parametrisation to describe an elastic tensor with hexagonal symmetry, we find analytic solutions to the Christoffel equations in terms of fast and slow horizontal velocities that allow us to simultaneously invert differential traveltime data and splitting data from teleseismic S arrivals to recover 3-D velocity and anisotropy structure. This technique provides a constraint on the depth-extent of shallow anisotropy, otherwise absent from interpretations based on SKS splitting alone. This approach is well suited to the young Woodlark Rift, where previous studies have found strong velocity variation and substantial SKS splitting in a continental rift with relatively simple geometry. This study images a low-velocity rift axis with ≤4 per cent spreading-parallel anisotropy at 50-100 km depth that separates regions of pre-existing lithospheric fabric, indicating the synchronous development of extensional crystallographic preferred orientation and lithospheric thinning. A high-velocity slab fragment north of the rift axis is associated with strike-parallel anisotropic fast axes, similar to that seen in the shallow mantle of some subduction zones. In addition to the insights provided by the anisotropy structure, the improvement in fit to the differential traveltime data demonstrates the merit to a joint inversion that accounts for anisotropy.

  19. Fluids circulation during the Miocene rifting of the Penedès half-graben, NE Iberian Peninsula

    Science.gov (United States)

    Baqués, Vinyet; Travé, Anna; Cantarero, Irene

    2013-04-01

    The Penedès half-graben, located in the north-western part of the Mediterranean, is a NE-SW oriented basin generated during the Miocene rifting. This graben is bounded to the northwest by the SE-dipping Vallès-Penedès fault, which places the Mesozoic rocks in contact with the Miocene basin-fill. The basin is filled with an up to 4 km thick succession of sediments divided into three lithostratigraphic units. From base to top: (1) a lower continental complex, (2) a continental to marine complex and (3) an upper continental complex. These units are covered by Pliocene deposits which onlap a Messinian regional erosive surface. The structural features within the Penedès half-graben allow defining three deformational phases during the Miocene rifting. The first, during the syn-rift, two successive stages of NE-SW normal faults were formed. The second, during the early post-rift, one stage of NE-SW normal faults and one minor compression phase with a dextral directional developed. The third, during the late post-rift, two successive stages of N-S trending extensional fractures (faults and joints) and one minor compression with a sinistral component developed. The fractures related to the syn-rift stage acted as conduits for meteoric fluids both, in the phreatic and in the vadose zone. During the early post-rift, Fe2+- rich fluids precipitated oxides along the NE-SW fault planes. The dextral directional faults served as conduits for meteoric fluids which reequilibrated totally the marine Miocene host rocks under the phreatic environment. The late post-rift stage was characterized by marine fluids upflowing through the N-S fractures, probably derived from the Miocene marine interval, which mixed with meteoric fluids producing dolomitization. The second set of N-S fractures served as conduits for meteoric fluids characterised by δ13C-depleted soil-derived CO2 attributed to precipitation in the vadose zone. The change from phreatic to vadose meteoric environment and the

  20. A Surface Wave's View of the Mid-Continent Rift

    Science.gov (United States)

    Foster, A. E.; Darbyshire, F. A.; Schaeffer, A. J.

    2017-12-01

    The presence of the Mid-Continent Rift (MCR), a 1.1Ga failed rift in central North America, raises many questions. We address the following: what lasting effects has it had on the continental lithosphere? Though many studies have looked at the area with a variety of data types, the combination of USArray Transportable Array stations to the south, permanent and temporary Canadian stations to the north, and SPREE stations in strategic locations crossing the rift provide a new opportunity for a regional surface-wave study. We select 80 stations with roughly 200 km spacing, resulting in dense path coverage of a broad area centered on the MCR. We use teleseismic data for all earthquakes from January 2005-August 2016 with a magnitude greater than 6.0, amounting to over 1200 events, and we make Rayleigh wave two-station dispersion measurements for all station pairs with suitable event-station geometry. We invert these measurements for anisotropic phase-velocity maps at periods of 20-200 s, yielding information not only on the wave speed but also the current fabric of the lithosphere, a complicated record of strain from formation, through modification from orogeny, attempted rifting, and hotspot interaction, to present day plate motion. We observe a clear signature of the MCR at short (20-25 s) periods, with the slowest phase-velocity anomaly in the region aligning with the strongest gravity anomaly. At increasing periods, and thus greater depths, this slowest anomaly shifts to beneath the center of Lake Superior (30-40 s). Eventually, it appears to merge with a slow anomaly to the north associated with the Nipigon Embayment, and contrasts sharply with an adjacent fast anomaly in the western Superior Province. In our preliminary anisotropy results, we observe weak anisotropy at the latitude of the MCR and to the south, whereas to the north of the MCR we find strong anisotropy. This is similar to the spatial variations in magnitude of delay times from shear-wave splitting

  1. Cross-continental comparison of the association between the physical environment and active transportation in children: a systematic review.

    Science.gov (United States)

    D'Haese, Sara; Vanwolleghem, Griet; Hinckson, Erica; De Bourdeaudhuij, Ilse; Deforche, Benedicte; Van Dyck, Delfien; Cardon, Greet

    2015-11-26

    The purpose of this systematic review was to determine the relationship between a wide range of physical environmental characteristics and different contexts of active transportation in 6- to 12-year-old children across different continents. A systematic search was conducted in six databases (Pubmed, Web of Science, Cinahl, SportDiscus, TRIS and Cochrane) resulting in 65 papers, eligible for inclusion. The investigated physical environmental variables were grouped into six categories: walkability, accessibility, walk/cycle facilities, aesthetics, safety, recreation facilities. The majority of the studies were conducted in North America (n = 35), Europe (n = 17) and Australia (n = 11). Active transportation to school (walking or cycling) was positively associated with walkability. Walking to school was positively associated with walkability, density and accessibility. Evidence for a possible association was found for traffic safety and all forms of active transportation to school. No convincing evidence was found for associations between the physical environment and active transportation during leisure. General safety and traffic safety were associated with active transportation to school in North America and Australia but not associated with active transportation to school in Europe. The physical environment was mainly associated with active transportation to school. Continent specific associations were found, indicating that safety measures were most important in relation to active commuting to school in North America and Australia. There is a need for longitudinal studies and studies conducted in Asia, Africa and South-America and studies focusing specifically on active transportation during leisure.

  2. Thermochronological evidence for polyphase post-rift reactivation in SE Brazil

    Science.gov (United States)

    Cogné, N.; Gallagher, K.; Cobbold, P. R.; Riccomini, C.

    2012-04-01

    The continental margin of SE Brazil shows good evidence for tectonic activity well after the break-up of Western Gondwana (see Cobbold et al., 2001 for a review). Additionally, SE Brazil ranks as an HEPM (high elevation passive margin), summits reaching 2800 m. To constrain the onshore evolution of the margin, especially during the Tertiary, we did a new thermochronological and structural study. After an initial regional study, during which we found additional evidence for a major phase of exhumation during the Late Cretaceous to Early Tertiary (Cogné et al., 2011), we focussed on a region that was clearly subject to Tertiary tectonics. This region includes the Tertiary Taubaté basin and the adjacent Serra do Mar and Serra da Mantiqueira. We used two thermochronolgical methods on the same samples, apatite fission tracks (AFT) and U-Th/He on apatite (AHe). AFT ages range from 129.3±4.3 Ma to 60.7±1.9 Ma with mean track lengths (MTL) from 14.31±0.24 μm to 11.41±0.23 μm, whereas AHe ages range from 519.6±16.6 to 10.1±0.1 Ma. A subset of AHe ages, selected on the basis of data consistency and geological arguments, has a smaller range (122.4±2.5 to 45.1±1.5 Ma). We have combined inverse and forward modelling to assess the range of acceptable thermal histories. Results of inverse modelling confirm our earlier study by showing a Late Cretaceous phase of cooling. Around the onshore Taubaté Basin, for a limited number of samples, another period of cooling occurred during the Early Tertiary, around the time when the basin formed. The inferred thermal histories for most of the samples also imply a later reheating, followed by a Neogene cooling. According to forward modelling, the evidence for reheating seems to be robust around the margins of the Taubaté Basin, but elsewhere the data cannot really discriminate between this and a less complex thermal history. However forward modelling and geologically independent information support the conclusion that the whole

  3. Mantle convection patterns reveal the enigma of the Red Sea rifting

    Science.gov (United States)

    Petrunin, Alexey; Kaban, Mikhail; El Khrepy, Sami; Al-Arifi, Nassir

    2017-04-01

    Initiation and further development of the Red Sea rift (RSR) is usually associated with the Afar plume at the Oligocene-Miocene separating the Arabian plate from the rest of the continent. Usually, the RSR is divided into three parts with different geological, tectonic and geophysical characteristics, but the nature of this partitioning is still debatable. To understand origin and driving forces responsible for the tectonic partitioning of the RSR, we have developed a global mantle convection model based on the refined density model and viscosity distribution derived from tectonic, rheological and seismic data. The global density model of the upper mantle is refined for the Middle East based on the high-resolution 3D model (Kaban et al., 2016). This model based on a joint inversion of the residual gravity and residual topography provides much better constraints on the 3D density structure compared to the global model based on seismic tomography. The refined density model and the viscosity distribution based on a homologous temperature approach provide an initial setup for further numerical calculations. The present-day snapshot of the mantle convection is calculated by using the code ProSpher 3D that allows for strong lateral variations of viscosity (Petrunin et al., 2013). The setup includes weak plate boundaries, while the measured GPS velocities are used to constrain the solution. The resulting mantle flow patterns show clear distinctions among the mantle flow patterns below the three parts of the RSR. According to the modeling results, tectonics of the southern part of the Red Sea is mainly determined by the Afar plume and the Ethiopian rift opening. It is characterized by a divergent mantle flow, which is connected to the East African Rift activity. The rising mantle flow is traced down to the transition zone and continues in the lower mantle for a few thousand kilometers south-west of Afar. The hot mantle anomaly below the central part of the RSR can be

  4. Eradicating tsetse from the Southern Rift Valley of Ethiopia

    International Nuclear Information System (INIS)

    2003-01-01

    Farming activities in Ethiopia, as in much of sub-Saharan Africa, are restricted by the presence of tsetse flies (Glossina spp.). These carry the livestock and human disease, trypanosomosis, which severely affects agricultural production and human well-being. In collaboration with the Ethiopian authorities, the International Atomic Energy Agency is sponsoring a Sterile Insect Technique (SIT) programme to eradicate tsetse from the Southern Rift Valley of Ethiopia. (IAEA)

  5. Basement control in the development of the early cretaceous West and Central African rift system

    Science.gov (United States)

    Maurin, Jean-Christophe; Guiraud, René

    1993-12-01

    The structural framework of the Precambrian basement of the West and Central African Rift System (WCARS) is described in order to examine the role of ancient structures in the development of this Early Cretaceous rift system. Basement structures are represented in the region by large Pan-African mobile belts (built at ca. 600 Ma) surrounding the > 2 Ga West African, Congo and Sao Francisco cratons. Except for the small Gao trough (eastern Mali) located near the contact nappe of the Pan-African Iforas suture zone along the edge of the West African craton, the entire WCARS is located within the internal domains of the Pan-African mobile belts. Within these domains, two main structural features occur as the main basement control of the WCARS: (1) an extensive network of near vertical shear zones which trend north-south through the Congo, Brazil, Nigeria, Niger and Algeria, and roughly east-west through northeastern Brazil and Central Africa. The shear zones correspond to intra-continental strike-slip faults which accompanied the oblique collision between the West African, Congo, and Sao Francisco cratons during the Late Proterozoic; (2) a steep metamorphic NW-SE-trending belt which corresponds to a pre-Pan-African (ca. 730 Ma) ophiolitic suture zone along the eastern edge of the Trans-Saharian mobile belt. The post-Pan-African magmatic and tectonic evolution of the basement is also described in order to examine the state of the lithosphere prior to the break-up which occurred in the earliest Cretaceous. After the Pan-African thermo-tectonic event, the basement of the WCARS experienced a long period of intra-plate magmatic activity. This widespread magmatism in part relates to the activity of intra-plate hotspots which have controlled relative uplift, subsidence and occasionally block faulting. During the Paleozoic and the early Mesozoic, this tectonic activity was restricted to west of the Hoggar, west of Aïr and northern Cameroon. During the Late Jurassic

  6. Initiation and evolution of the Oligo-Miocene rift basins of southwestern Europe: Contribution of deep seismic reflection profiling

    Science.gov (United States)

    Bois, C.

    1993-11-01

    Southwestern European Oligo-Miocene rift basins have recently been investigated by deep seismic reflection profiling. The study of these data, together with other geophysical and geological data, shows that the rifts, which run from the Rhinegraben to the western Mediterranean, do not form a single clearcut system. The N-trending rifts (Rhinegraben, Bresse and Limagne) were developed on a cold and rigid lithosphere affected by the Alpine collision. The NE-trending rifts (southeastern France, Gulf of Lions and Valencia Trough) were formed slightly later in a backarc basin associated with an active segment of the European-Iberian plate that was heated, affected by widespread calcalkaline volcanism and probably weakened. All the southwestern European rifts and basins together may, however, be related to a common heritage represented by the boundary between the European-Iberian and African-Apulian plates that was created in the Jurassic with the initiation of the Tethys Ocean. The present features of the southwestern European Oligo-Miocène rift basins may be explained by a combination of three geodynamic mechanisms: mechanical stretching of the lithosphere, active mantle uplifting, and subordinate lithospheric flexuring. All the rifts were probably initiated by passive stretching. A systematic discrepancy between stretching derived from fault analysis and attenuation of the crust has been observed in all the rifts. This suggests that these rifts were subsequently reworked by one or several active mantle upwelling events associated with late shoulder uplift, asthenosphere upwelling and anomalous P-wave velocities in the lowermost crust and the uppermost mantle. Crustal attenuation may have been achieved by mantle intrusion, metamorphism of the deep crust and/or its delamination. Some of the rifts were affected by lithospheric flexuring. Combinations, in various proportions, of a small number of geodynamic mechanisms probably controlled many basins in the world. This

  7. Deeply concealed half-graben at the SW margin of the East European Craton (SE Poland) — Evidence for Neoproterozoic rifting prior to the break-up of Rodinia

    OpenAIRE

    P. Krzywiec; P. Poprawa; M. Mikołajczak; S. Mazur; M. Malinowski

    2018-01-01

    Baltica was one of continents formed as a result of Rodinia break-up 850–550 Ma. It was separated from Amazonia(?) by the Tornquist Ocean, the opening of which was preceded by Neoproterozoic extension in a network of continental rifts. Some of these rifts were subsequently aborted whereas the Tornquist Rift gave rise to splitting of Rodinia and formation of the Tornquist Ocean. The results of 1-D subsidence analysis at the fossil passive margin of Baltica provided insight in the timing and ki...

  8. Interactions between magma and the lithospheric mantle during Cenozoic rifting in Central Europe

    Science.gov (United States)

    Meyer, Romain; Elkins-Tanton, Linda T.

    2010-05-01

    During the Cenozoic, extensive intraplate volcanic activity occurred throughout Central Europe. Volcanic eruptions extend over France (the Massif Central), central Germany (Eifel, Vogelsberg, Rhön; Heldburg), the Czech Republic (the Eger graben) and SW Poland (Lower Silesia), a region ~1,200 km wide. The origin of this predominantly alkaline intraplate magmatism is often genetically linked to one or several mantle plumes, but there is no convincing evidence for this. We have measured Pb isotope ratios, together with major and trace elements, in a representative set of mafic to felsic igneous rocks from the intra-plate Cenozoic Rhön Mts. and the Heldburg dike swarm in order to gain insight into the melting source and petrogenetic history of these melts. Three different mafic rock types (tholeiitic basalt, alkali basalt, basanite) were distinguished based on petrography and geochemistry within the investigated areas. Except for the lherzolite-bearing phonolite from the Veste Heldburg all other evolved magmas are trachytes. REE geochemistry and calculated partial melting modeling experiments for the three mafic magma types point to different degrees of partial melting in a garnet-bearing mantle source. In addition a new version of the ternary Th-Hf-Ta diagram is presented in this study as a useful petrological tool. This diagram is not only able to define potentially involved melting source end-members (e.g. asthenosphere, sub-continental lithospheric mantle and continental crust) but also interactions between these members are illustrated. An advantage of this diagram compared to partial melting degree sensitive multi-element diagrams is that a ternary diagram is a closed system. An earlier version of this diagram has been recently used to establish the nature and extent of crust mantle melt interaction of volcanic rifted margins magmas (Meyer et al. 2009). The Th-Hf-Ta geochemistry of the investigated magmas is similar to spinel and garnet xenoliths from different

  9. The influence of tectonic and volcanic processes on the morphology of the Iberian continental margins; Influencia de los procesos tectonicos y volcanicos en la morfologia de los margenes continentales ibericos

    Energy Technology Data Exchange (ETDEWEB)

    Maestro, A.; Bohoyo, F.; Lopez-Martinez, J.; Acosta, J.; Gomez-Ballesteros, M.; Llaave, E.; Munoz, A.; Terrinha, P. G.; Dominguez, M.; Fernandez-Saez, F.

    2015-07-01

    The Iberian continental margins are mainly passive margins. Nevertheless, the northern sector of the margin was active during some stages of its geological evolution. The southern sector is considered as a transformed margin, which defines the boundary between the Iberian and African plates. This margin was also an active margin in the past. The different types, origins and intensities of the endogenic processes that have affected he Iberian continental margins have led to the development of various tectonic and volcanic morphologies. The North Atlantic rifting allowed the development of large marginal platforms in the Cantabrian and Galician margins the North-Atlantic Ocean spreading. The reactivation of Variscan faults during the Mesozoic and Cenozoic controlled the strike of some of the largest canyons in the Iberian margins. The Gulf of Cadiz margin is characterized by the development of morphologies related to salt tectonic, fluid seepage, thrust fronts and strike-slip fault lineaments hundreds of kilometres long. The Alboran basin and the Betic margin show morphologies connected with the Miocene rift phase, which generated volcanic edifices and various structural reliefs, and with the subsequent compressive phase, when folds and strike-slip, reverse faults, diapirs and mud volcanoes were developed. Finally, the Catalan-Valencian margin and the Balearic promontory are characterized by the presence of horst and graben structures related to the development of the Valencia trough during the Paleogene. The morphological features of endogenic origin have largely controlled the location and extent of the sedimentary processes and morphological products along the Iberian margins. (Author)

  10. Strike-slip tectonics during rift linkage

    Science.gov (United States)

    Pagli, C.; Yun, S. H.; Ebinger, C.; Keir, D.; Wang, H.

    2017-12-01

    The kinematics of triple junction linkage and the initiation of transforms in magmatic rifts remain debated. Strain patterns from the Afar triple junction provide tests of current models of how rifts grow to link in area of incipient oceanic spreading. Here we present a combined analysis of seismicity, InSAR and GPS derived strain rate maps to reveal that the plate boundary deformation in Afar is accommodated primarily by extensional tectonics in the Red Sea and Gulf of Aden rifts, and does not require large rotations about vertical axes (bookshelf faulting). Additionally, models of stress changes and seismicity induced by recent dykes in one sector of the Afar triple junction provide poor fit to the observed strike-slip earthquakes. Instead we explain these patterns as rift-perpendicular shearing at the tips of spreading rifts where extensional strains terminate against less stretched lithosphere. Our results demonstrate that rift-perpendicular strike-slip faulting between rift segments achieves plate boundary linkage during incipient seafloor spreading.

  11. Kinematics of the Ethiopian Rift and Absolute motion of Africa and Somalia Plates

    Science.gov (United States)

    Muluneh, A. A.; Cuffaro, M.; Doglioni, C.

    2013-12-01

    faster SW motion of Africa with respect to Somalia plate is due to a possibly lower viscosity in the top asthenosphere (Low-Velocity Zone) beneath Africa. These findings have significant implications for the evolution of continental rifting in transtensional settings and provide evidence for the kinematics and tectonics of the Ethiopian rift in the context of the Africa-Somalia plate interaction in the mantle reference frame.

  12. Evidences of late quaternary neotectonic activity and sea-level changes along the western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, V.P.; Veerayya, M.; Thamban, M.; Wagle, B.G.

    The offshore data on sea-level changes along the western margin of India have been reviewed and evidences of Late Quaternary neotectonic activity and subsidence are documented, based on the diagenetic textures of limestones from deeper submarine...

  13. High-resolution seismic survey for the characterization of planned PIER-ICDP fluid-monitoring sites in the Eger Rift zone

    Science.gov (United States)

    Simon, H.; Buske, S.

    2017-12-01

    The Eger Rift zone (Czech Republic) is a intra-continental non-volcanic region and is characterized by outstanding geodynamic activities, which result in earthquake swarms and significant CO2 emanations. Because fluid-induced stress can trigger earthquake swarms, both natural phenomena are probably related to each other. The epicentres of the earthquake swarms cluster at the northern edge of the Cheb Basin. Although the location of the cluster coincides with the major Mariánské-Lázně Fault Zone (MLFZ) the strike of the focal plane indicates another fault zone, the N-S trending Počátky-Plesná Zone (PPZ). Isotopic analysis of the CO2-rich fluids revealed a significant portion of upper mantle derived components, hence a magmatic fluid source in the upper mantle was postulated. Because of these phenomena, the Eger Rift area is a unique site for interdisciplinary drilling programs to study the fluid-earthquake interaction. The ICDP project PIER (Probing of Intra-continental magmatic activity: drilling the Eger Rift) will set up an observatory, consisting of five monitoring boreholes. In preparation for the drilling, the goal of the seismic survey is the characterization of the projected fluid-monitoring drill site at the CO2 degassing mofette field near Hartoušov. This will be achieved by a 6 km long profile with dense source and receiver spacing. The W-E trending profile will cross the proposed drill site and the surface traces of MLFZ and PPZ. The outcome of the seismic survey will be a high-resolution structural image of potential reflectors related to these fault zones. This will be achieved by the application of advanced pre-stack depth migration methods and a detailed P-wave velocity distribution of the area obtained from first arrival tomography. During interpretation of the seismic data, a geoelectrical resistivity model, acquired along the same profile line, will provide important constraints, especially with respect to fluid pathways.

  14. Sulfate reduction and methane oxidation activity below the sulfate-methane transition zone in Alaskan Beaufort Sea continental margin sediments: Implications for deep sulfur cycling

    Science.gov (United States)

    Treude, Tina; Krause, Stefan; Maltby, Johanna; Dale, Andrew W.; Coffin, Richard; Hamdan, Leila J.

    2014-11-01

    Two ∼6 m long sediment cores were collected along the ∼300 m isobath on the Alaskan Beaufort Sea continental margin. Both cores showed distinct sulfate-methane transition zones (SMTZ) at 105 and 120 cm below seafloor (cmbsf). Sulfate was not completely depleted below the SMTZ but remained between 30 and 500 μM. Sulfate reduction and anaerobic oxidation of methane (AOM) determined by radiotracer incubations were active throughout the methanogenic zone. Although a mass balance could not explain the source of sulfate below the SMTZ, geochemical profiles and correlation network analyses of biotic and abiotic data suggest a cryptic sulfur cycle involving iron, manganese and barite. Inhibition experiments with molybdate and 2-bromoethanesulfonate (BES) indicated decoupling of sulfate reduction and AOM and competition between sulfate reducers and methanogens for substrates. While correlation network analyses predicted coupling of AOM to iron reduction, the addition of manganese or iron did not stimulate AOM. Since none of the classical archaeal anaerobic methanotrophs (ANME) were abundant, the involvement of unknown or unconventional phylotypes in AOM is conceivable. The resistance of AOM activity to inhibitors implies deviation from conventional enzymatic pathways. This work suggests that the classical redox cascade of electron acceptor utilization based on Gibbs energy yields does not always hold in diffusion-dominated systems, and instead biotic processes may be more strongly coupled to mineralogy.

  15. Active convergence between the Lesser and Greater Caucasus in Georgia: Constraints on the tectonic evolution of the Lesser-Greater Caucasus continental collision

    Science.gov (United States)

    Sokhadze, G.; Floyd, M.; Godoladze, T.; King, R.; Cowgill, E. S.; Javakhishvili, Z.; Hahubia, G.; Reilinger, R.

    2018-01-01

    We present and interpret newly determined site motions derived from GPS observations made from 2008 through 2016 in the Republic of Georgia, which constrain the rate and locus of active shortening in the Lesser-Greater Caucasus continental collision zone. Observation sites are located along two ∼160 km-long profiles crossing the Lesser-Greater Caucasus boundary zone: one crossing the Rioni Basin in western Georgia and the other crossing further east near the longitude of Tbilisi. Convergence across the Rioni Basin Profile occurs along the southern margin of the Greater Caucasus, near the surface trace of the north-dipping Main Caucasus Thrust Fault (MCTF) system, and is consistent with strain accumulation on the fault that generated the 1991 MW6.9 Racha earthquake. In contrast, convergence along the Tbilisi Profile occurs near Tbilisi and the northern boundary of the Lesser Caucasus (near the south-dipping Lesser Caucasus Thrust Fault), approximately 50-70 km south of the MCTF, which is inactive within the resolution of geodetic observations (< ± 0.5 mm/yr) at the location of the Tbilisi Profile. We suggest that the southward offset of convergence along strike of the range is related to the incipient collision of the Lesser-Greater Caucasus, and closing of the intervening Kura Basin, which is most advanced along this segment of the collision zone. The identification of active shortening near Tbilisi requires a reevaluation of seismic hazards in this area.

  16. ODP Leg 210 Drills the Newfoundland Margin in the Newfoundland-Iberia Non-Volcanic Rift

    Science.gov (United States)

    Tucholke, B. E.; Sibuet, J.

    2003-12-01

    The final leg of the Ocean Drilling Project (Leg 210, July-September 2003) was devoted to studying the history of rifting and post-rift sedimentation in the Newfoundland-Iberia rift. For the first time, drilling was conducted in the Newfoundland Basin along a transect conjugate to previous drill sites on the Iberia margin (Legs 149 and 173) to obtain data on a complete `non-volcanic' rift system. The prime site during this leg (Site 1276) was drilled in the transition zone between known continental crust and known oceanic crust at chrons M3 and younger. Extensive geophysical work and deep-sea drilling have shown that this transition-zone crust on the conjugate Iberia margin is exhumed continental mantle that is strongly serpentinized in its upper part. Transition-zone crust on the Newfoundland side, however, is typically a kilometer or more shallower and has much smoother topography, and seismic refraction data suggest that the crust may be thin (about 4 km) oceanic crust. A major goal of Site 1276 was to investigate these differences by sampling basement and a strong, basinwide reflection (U) overlying basement. Site 1276 was cored from 800 to 1737 m below seafloor with excellent recovery (avg. 85%), bottoming in two alkaline diabase sills >10 m thick that are estimated to be 100-200 meters above basement. The sills have sedimentary contacts that show extensive hydrothermal metamorphism. Associated sediment structural features indicate that the sills were intruded at shallow levels within highly porous sediments. The upper sill likely is at the level of the U reflection, which correlates with lower Albian - uppermost Aptian(?) fine- to coarse-grained gravity-flow deposits. Overlying lower Albian to lower Oligocene sediments record paleoceanographic conditions similar to those on the Iberia margin and in the main North Atlantic basin, including deposition of `black shales'; however, they show an extensive component of gravity-flow deposits throughout.

  17. Aeromagnetic and gravity investigations of the Coastal Area and Continental Shelf of Liberia, West Africa, and their relation to continental drift

    Science.gov (United States)

    Behrendt, John C.; Wotorson, Cletus S.

    1970-01-01

    An aeromagnetic survey has shown the existence of several basins in which magnetic basement depths are greater than 5 km on the continental shelf off Liberia. Magnetic diabase of 176 to 192 m.y. (Jurassic) in age intruding the Paleozoic (?) rocks and overlain by younger rocks onshore requires the distinction between “magnetic basement” and “basement.” Several lines of evidence suggest that the Paleozoic(?) rocks are less than 1 km thick; this implies that the diabase does not introduce a large error in depth-to-basement estimates. The dikes or their extrusive equivalents are traceable, on the basis of the magnetic data, beneath the younger sedimentary rock in the basins to the edge of the continental slope. The magnetic data also delineate a second zone of diabase dikes 90 km inland, parallel to the coast, which cross the entire country. The intrusion of the younger dikes probably coincides with rifting at the beginning of the separation of Africa and South America, and the associated magnetic anomaly zones appear to be parallel with and continuous into the anomaly bands in the Atlantic. A major northeast-trending break in the magnetic fabric intersects the coast near 9° W. and is associated with Eburnean age rocks (about 2000 m.y.) to the southeast as contrasted with Liberian-age rocks (about 2700 m.y.) to the northwest. Change in magnetic fabric direction inland from northeast to northwest in the coastal area allows recognition of a boundary between the Liberian-age rocks inland and Pan-African-age (about 550 m.y.) rocks in the coastal area northwest of about 9° 20'W. Sets of north-northwest-and west-northwest—trending faults of 1 to 2 km vertical displacement cut the Cretaceous sedimentary rocks onshore and can be traced into the offshore basins. Vertical displacements of several kilometers in the magnetic basement underlying the continental shelf suggest a pattern of block faulting all along the coast and continental shelf. Negative Bouguer

  18. COBBOOM: The Continental Breakup and Birth of Oceans Mission

    Directory of Open Access Journals (Sweden)

    Joann M. Stock

    2007-09-01

    Full Text Available The rupture of continents and creation of new oceans is a fundamental yet primitively understood aspect of the plate tectonic cycle. Building upon past achievements by ocean drilling and geophysical and geologic studies, we propose “The Continental Breakup and Birth of Oceans Mission (COBBOOM” as the next major phase of discovery, for which sampling by drilling will be essential.In September 2006, fifty-one scientists from six continents gathered in Pontresina, Switzerland to discuss current knowledge of continental breakup and sedimentary basin formation and how the Integrated Ocean Drilling Program (IODP can deepen that knowledge Coffin et al., 2006. Workshop participants discussed a global array of rifted margins (Fig. 1, formulated the critical problems to beaddressed by future drilling and related investigations, and identified key rift systems poised for IODP investigations. 

  19. Transtensional Rifting in the Late Proto-Gulf of California Near Bahía Kino, Sonora, México

    Science.gov (United States)

    Bennett, S. E.; Oskin, M. E.; Dorsey, R. J.

    2009-12-01

    We investigate the role of obliquity in continental rupture from the example of the Gulf of California rift. Focused transtensional strain adjacent to strike-slip faults, ubiquitous in oblique rifts, may act as a catalyst for lithospheric rupture. To test this hypothesis we completed detailed structural mapping, fault kinematic analysis, basin analysis, and paleomagnetism of pre- and syn-rift volcanic and sedimentary rocks exposed in coastal Sonora, near Bahía Kino, México. This area is host to the NW-striking, dextral Sacrificio and Bahía Kino faults onshore that are likely linked to the offshore De Mar transform fault that accommodated Gulf opening. Three fault-bounded sedimentary basins formed unconformably above the 12.50 ± 0.08 Ma Tuff of San Felipe. The 6.53 ± 0.18 Ma Tuff of Cerro Tordillo and the 6.39 ± 0.02 Ma Tuffs of Mesa Cuadrada are interbedded in the lower part of the non-marine basin fill. In one of these basins, we used these tuff markers to calibrate a sedimentation rate of 1.2 ± 0.2 mm/yr and a tilting rate of 0.12 ± 0.02 °/kyr. These rapid rates suggest transtensional strain and related basin subsidence initiated ca. 6.6 Ma, near the end of proto-Gulf time. Paleomagnetism of the Tuff of San Felipe and Tuffs of Mesa Cuadrada in coastal Sonora show variable amounts of clockwise vertical-axis rotation when compared to paleomagnetic reference sites in Baja California. Fault blocks in the central and southern parts of the study area are rotated counter-clockwise 15° to clockwise 35°. Strike-slip faults in this area accommodate up to 10 km of slip. In contrast, ~53° of clockwise rotation occurred in the northern part of the study area, where strike-slip faults are absent. In this northern area, transtensional deformation occurred primarily by block rotation and ~6 km of normal slip on the low-angle (5-15°) Punta Chueca fault. After correcting for variable amounts of rotation, fault blocks display a consistent tilt down to the ENE. Pre-rift

  20. Post-Pan-African tectonic evolution of South Malawi in relation to the Karroo and recent East African rift systems

    Science.gov (United States)

    Castaing, C.

    1991-05-01

    Structural studies conducted in the Lengwe and Mwabvi Karroo basins and in the basement in South Malawi, using regional maps and published data extended to cover Southeast Africa, serve to propose a series of geodynamic reconstructions which reveal the persistence of an extensional tectonic regime, the minimum stress σ3 of which has varied through time. The period of Karroo rifting and the tholeiitic and alkaline magmatism which terminated it, were controlled by NW-SE extension, which resulted in the creation of roughly NE-SW troughs articulated by the Tanganyika-Malawi and Zambesi pre-transform systems. These were NW-SE sinistral-slip systems with directions of movement dipping slightly to the Southeast, which enabled the Mwanza fault to play an important role in the evolution of the Karroo basins of the Shire Valley. The Cretaceous was a transition period between the Karroo rifting and the formation of the Recent East African Rift System. Extension was NE-SW, with some evidence for a local compressional episode in the Lengwe basin. Beginning in the Cenozoic, the extension once more became NW-SE and controlled the evolution in transtension of the Recent East African Rift System. This history highlights the major role of transverse faults systems dominated by strike-slip motion in the evolution and perpetuation of the continental rift systems. These faults are of a greater geological persistence than the normal faults bounding the grabens, especially when they are located on major basement anisotropies.

  1. Measurement of inter- and intra-annual variability of landscape fire activity at a continental scale: The Australian case

    Science.gov (United States)

    Grant J. Williamson; Lynda D. Prior; Matt Jolly; Mark A. Cochrane; Brett P. Murphy; David M. J. S. Bowman

    2016-01-01

    Climate dynamics at diurnal, seasonal and inter-annual scales shape global fire activity, although difficulties of assembling reliable fire and meteorological data with sufficient spatio-temporal resolution have frustrated quantification of this variability. Using Australia as a case study, we combine data from 4760 meteorological stations with 12 years of satellite-...

  2. Origin of three-armed rifts in volcanic islands: the case of El Hierro (Canary Islands)

    Science.gov (United States)

    Galindo Jiménez, Inés; Becerril Carretero, Laura; Martí Molist, Joan; Gudmundsson, Agust

    2015-04-01

    Rifts zones in volcanic oceanic islands are common structures that have been explained through several theories/models. However, despite all these models it is as yet unclear whether it is the intense intrusive activity or the sector collapses that actually control the structural evolution and geometry of oceanic-island rift zones. Here we provide a new hypothesis to explain the origin and characteristics of the feeding system of oceanic-island rift zones based on the analysis of more than 1700 surface, subsurface (water galleries), and submarine structural data from El Hierro (Canary Islands). El Hierro's geological structure is primarily controlled by a three-armed rift-zone, the arms striking NE, WSW and S. Between the rift axes there are three valleys formed during huge landslides: El Golfo, El Julan, and Las Playas. Our results show: (1) a predominant NE-SW strike of structural elements, which coincides with the main regional trend of the Canary Archipelago as a whole; (2) a clear radial strike distribution of structural elements for the whole volcanic edifice (including submarine flanks) with respect to the centre of the island; (3) that the rift zones are mainly subaerial structures and do not propagate through the submarine edifice; (4) that it is only in the NE rift that structures have a general strike similar to that of the rift as a whole, and; (5) that in the W and S rifts there is not clear main direction, showing the structural elements in the W rift a fan distribution coinciding with the general radial pattern in the island as a whole. Based on these data, we suggest that the radial-striking structures reflect comparatively uniform stress fields that operated during the constructive episodes, mainly conditioned by the combination of overburden pressure, gravitational spreading, and magma-induced stresses. By contrast, in the shallower parts of the edifice, that is, the NE-SW, N-S and WNW-ESE-striking structures, reflect local stress fields related

  3. Focused seismicity triggered by flank instability on Kīlauea's Southwest Rift Zone

    Science.gov (United States)

    Judson, Josiah; Thelen, Weston A.; Greenfield, Tim; White, Robert S.

    2018-03-01

    Swarms of earthquakes at the head of the Southwest Rift Zone on Kīlauea Volcano, Hawai´i, reveal an interaction of normal and strike-slip faulting associated with movement of Kīlauea's south flank. A relocated subset of earthquakes between January 2012 and August 2014 are highly focused in space and time at depths that are coincident with the south caldera magma reservoir beneath the southern margin of Kīlauea Caldera. Newly calculated focal mechanisms are dominantly dextral shear with a north-south preferred fault orientation. Two earthquakes within this focused area of seismicity have normal faulting mechanisms, indicating two mechanisms of failure in very close proximity (10's of meters to 100 m). We suggest a model where opening along the Southwest Rift Zone caused by seaward motion of the south flank permits injection of magma and subsequent freezing of a plug, which then fails in a right-lateral strike-slip sense, consistent with the direction of movement of the south flank. The seismicity is concentrated in an area where a constriction occurs between a normal fault and the deeper magma transport system into the Southwest Rift Zone. Although in many ways the Southwest Rift Zone appears analogous to the more active East Rift Zone, the localization of the largest seismicity (>M2.5) within the swarms to a small volume necessitates a different model than has been proposed to explain the lineament outlined by earthquakes along the East Rift Zone.

  4. Mid–Late Neoproterozoic rift-related volcanic rocks in China: Geological records of rifting and break-up of Rodinia

    Directory of Open Access Journals (Sweden)

    Linqi Xia

    2012-07-01

    Full Text Available Early Cambrian and Mid–Late Neoproterozoic volcanic rocks in China are widespread on several Precambrian continental blocks, which had aggregated to form part of the Rodinia supercontinent by ca. 900 Ma. On the basis of petrogeochemical data, the basic lavas can be classified into two major magma types: HT (Ti/Y > 500 and LT (Ti/Y  0.85 and HT2 (Nb/La ≤ 0.85, and LT1 (Nb/La > 0.85 and LT2 (Nb/La ≤ 0.85 subtypes, respectively. The geochemical variation of the HT2 and LT2 lavas can be accounted for by lithospheric contamination of asthenosphere- (or plume- derived magmas, whereas the parental magmas of the HT1 and LT1 lavas did not undergo, during their ascent, pronounced lithospheric contamination. These volcanics exhibit at least three characteristics: (1 most have a compositional bimodality; (2 they were formed in an intracontinental rift setting; and (3 they are genetically linked with mantle plumes or a mantle surperplume. This rift-related volcanism at end of the Mid–Neoproterozoic and Early Cambrian coincided temporally with the separation between Australia–East Antarctica, South China and Laurentia and between Australia and Tarim, respectively. The Mid–Late Neoproterozoic volcanism in China is the geologic record of the rifting and break-up of the supercontinent Rodinia.

  5. Twenty-five years of geodetic measurements along the Tadjoura-Asal rift system, Djibouti, East Africa

    Science.gov (United States)

    Vigny, Christophe; de Chabalier, Jean-Bernard; Ruegg, Jean-Claude; Huchon, Philippe; Feigl, Kurt L.; Cattin, Rodolphe; Asfaw, Laike; Kanbari, Khaled

    2007-06-01

    Since most of Tadjoura-Asal rift system sits on dry land in the Afar depression near the triple junction between the Arabia, Somalia, and Nubia plates, it is an ideal natural laboratory for studying rifting processes. We analyze these processes in light of a time series of geodetic measurements from 1978 through 2003. The surveys used triangulation (1973), trilateration (1973, 1979, and 1981-1986), leveling (1973, 1979, 1984-1985, and 2000), and the Global Positioning System (GPS, in 1991, 1993, 1995, 1997, 1999, 2001, and 2003). A network of about 30 GPS sites covers the Republic of Djibouti. Additional points were also measured in Yemen and Ethiopia. Stations lying in the Danakil block have almost the same velocity as Arabian plate, indicating that opening near the southern tip of the Red Sea is almost totally accommodated in the Afar depression. Inside Djibouti, the Asal-Ghoubbet rift system accommodates 16 ± 1 mm/yr of opening perpendicular to the rift axis and exhibits a pronounced asymmetry with essentially null deformation on its southwestern side and significant deformation on its northeastern side. This rate, slightly higher than the large-scale Arabia-Somalia motion (13 ± 1 mm/yr), suggests transient variations associated with relaxation processes following the Asal-Ghoubbet seismovolcanic sequence of 1978. Inside the rift, the deformation pattern exhibits a clear two-dimensional pattern. Along the rift axis, the rate decreases to the northwest, suggesting propagation in the same direction. Perpendicular to the rift axis, the focus of the opening is clearly shifted to the northeast, relative to the topographic rift axis, in the "Petit Rift," a rift-in-rift structure, containing most of the active faults and the seismicity. Vertical motions, measured by differential leveling, show the same asymmetric pattern with a bulge of the northeastern shoulder. Although the inner floor of the rift is subsiding with respect to the shoulders, all sites within the

  6. Transient deformation in the Asal-Ghoubbet Rift (Djibouti) since the 1978 diking event: Is deformation controlled by magma supply rates?

    Science.gov (United States)

    Smittarello, D.; Grandin, R.; de Chabalier, J. B.; Doubre, C.; Deprez, A.; Masson, F.; Socquet, A.; Ibrahim Ahmed, S.

    2016-12-01

    Within the Afar Depression, the Asal-Ghoubbet Rift (AG Rift)in Djibouti lies in the subaerial continuation of the Aden ridge system. This segment constitutes a natural laboratory to study rifting processes and mechanisms involved in continental breakup and oceanic spreading. In November 1978, an exceptional rifting event occurred in the AG Rift. Regularly upgraded and improved geodetic technology has been used to monitor this event and the postdiking deformation. In light of recent results obtained for the Manda Hararo-Dabbahu rifting event (2005-2010), we propose that the horizontal and vertical geodetic data can be modeled with a double source, involving a dike-like inflation component aligned along the rift axis and a spherical pressure source located at midsegment below the Fieale caldera. By revisiting the codiking data, we propose that the reservoir below Fieale could have fed, at least partially, the 1978 injection and the contemporaneous Ardoukoba eruption and potentially induced local subsidence due to magma draining out of the central reservoir. As an alternative to previously proposed viscoelastic relaxation models, we reinterpret postdiking observations using a purely elastic rheology. We determine the relative contribution of a midsegment reservoir inflation and a dike-like opening component, together with their respective time evolutions. Our results suggest that interactions between steadily accumulating tectonic strain and temporal variations in melt supply to the shallow magma plumbing system below the AG Rift may entirely explain the geodetic observations and that viscoelastic deformation processes played a minor role in the 30 years following the 1978 rifting event.

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

    Science.gov (United States)

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

    2014-12-01

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

  8. Records of continental slope sediment flow morphodynamic responses to gradient and active faulting from integrated AUV and ROV data, offshore Palos Verdes, southern California Borderland

    Science.gov (United States)

    Maier, Katherine L.; Brothers, Daniel; Paull, Charles K.; McGann, Mary; Caress, David W.; Conrad, James E.

    2016-01-01

    Variations in seabed gradient are widely acknowledged to influence deep-water deposition, but are often difficult to measure in sufficient detail from both modern and ancient examples. On the continental slope offshore Los Angeles, California, autonomous underwater vehicle, remotely operated vehicle, and shipboard methods were used to collect a dense grid of high-resolution multibeam bathymetry, chirp sub-bottom profiles, and targeted sediment core samples that demonstrate the influence of seafloor gradient on sediment accumulation, depositional environment, grain size of deposits, and seafloor morphology. In this setting, restraining and releasing bends along the active right-lateral Palos Verdes Fault create and maintain variations in seafloor gradient. Holocene down-slope flows appear to have been generated by slope failure, primarily on the uppermost slope (~ 100–200 m water depth). Turbidity currents created a low relief (water depositional systems. These results help to bridge gaps in scale between existing deep-sea and experimental datasets and may provide constraints for future numerical modeling studies.

  9. Impacts of Sea-Level Rise and Human Activity on a Tropical Continental Shelf, RN State, NE Brazil

    Science.gov (United States)

    Vital, H.; Barros Pereira, T. R.; Lira, H. F.; Tabosa, W. F.; Eichler, P.; Stattegger, K.; Sen Gupta, B. K.; Gomes, M. P.; Nogueira, M. L. D. S.; Pierri, G. C. S.

    2014-12-01

    The northeastern Brazilian, tropical coast-shelf system along the Atlantic Ocean is a sediment-starved zone, because of low relief, small drainage basins, and a semiarid climate. This work presents the major results of a study of environmental changes, particularly those related to Holocene sea-level rise, affecting the coast and shallow waters of Rio Grande do Norte (RN) State, NE Brazil. The methods included bottom-sediment characterization, bioindicator tracking, and integrated shallow-water geophysical investigation. This coastline is marked by active sea cliffs carved into tablelands alternating with reef- or dune-barrier sections, beach rocks and lagoons, whereas the shelf is a narrow, very shallow, and highly energetic system. Overall, the area is under the natural influence of tides (with a semidiurnal mesotidal regime) and the anthropogenic influence of salt exploration, oil industry, shrimp farms, tourism, and wind-farms. Sedimentation during the Holocene has been controlled mainly by sea-level variation, longshore currents, and the advance and westward propagation of active dunes along the coast. As in other areas around the world, growing numbers of permanent and seasonal residents choose to live at or near the ocean. Coastal erosion is a cause for concern along many Brazilian beaches, and several erosion hot spots are already recognized in RN State. Curves of Holocene relative sea-level variation were established for RN State, but the absence of long-term oceanographic observations in the last centuries or that of detailed altimetry maps hinders the evaluation of different risk scenarios at the local level. Nevertheless, impacts of the current sea-level rise and human activity can be observed along the RN coastal-shelf system. Particular aspects of the study, such as oil-spill monitoring, coastal-water sewage contamination, and coastal erosion, will be highlighted.

  10. Extensional fault geometry and its flexural isostatic response during the formation of the Iberia - Newfoundland conjugate rifted margins

    Science.gov (United States)

    Gómez-Romeu, Júlia; Kusznir, Nick; Manatschal, Gianreto; Roberts, Alan

    2017-04-01

    Despite magma-poor rifted margins having been extensively studied for the last 20 years, the evolution of extensional fault geometry and the flexural isostatic response to faulting remain still debated topics. We investigate how the flexural isostatic response to faulting controls the structural development of the distal part of rifted margins in the hyper-extended domain and the resulting sedimentary record. In particular we address an important question concerning the geometry and evolution of extensional faults within distal hyper-extended continental crust; are the seismically observed extensional fault blocks in this region allochthons from the upper plate or are they autochthons of the lower plate? In order to achieve our aim we focus on the west Iberian rifted continental margin along the TGS and LG12 seismic profiles. Our strategy is to use a kinematic forward model (RIFTER) to model the tectonic and stratigraphic development of the west Iberia margin along TGS-LG12 and quantitatively test and calibrate the model against breakup paleo-bathymetry, crustal basement thickness and well data. RIFTER incorporates the flexural isostatic response to extensional faulting, crustal thinning, lithosphere thermal loads, sedimentation and erosion. The model predicts the structural and stratigraphic consequences of recursive sequential faulting and sedimentation. The target data used to constrain model predictions consists of two components: (i) gravity anomaly inversion is used to determine Moho depth, crustal basement thickness and continental lithosphere thinning and (ii) reverse post-rift subsidence modelling consisting of flexural backstripping, decompaction and reverse post-rift thermal subsidence modelling is used to give paleo-bathymetry at breakup time. We show that successful modelling of the structural and stratigraphic development of the TGS-LG12 Iberian margin transect also requires the simultaneous modelling of the Newfoundland conjugate margin, which we

  11. Atividade anti-helmíntica de plantas nativas do continente americano: uma revisão Anthelmintic activity of native American plants: a review

    Directory of Open Access Journals (Sweden)

    R.G. Sousa

    2013-01-01

    Full Text Available O presente trabalho faz uma revisão de plantas nativas do continente americano com atividade anti-helmíntica com a finalidade de subsidiar pesquisas e o desenvolvimento de novos medicamentos na área de produtos naturais. Na literatura, foram citadas 22 famílias de plantas, tais como: Moraceae, Asteraceae e Cucurbitaceae, e estudadas cerca de 35 espécies, como a Ficus sp, Cucurbita máxima, e Ruta graveolens, usadas por sua alegada atividade anti-helmíntica. Destas espécies, 26 foram ativas contra alguns helmintos: Strongyloides stercolaris, Ancylostomidae, Trichostrongylus sp, Capillaria sp, Ascaris lumbricoides, Taenia saginata, Haemonchus contortus, e Hymenolepsis diminuta. Foi verificado maior número de estudos com parasitas de animais, possivelmente devido ao desenvolvimento de resistência dos helmintos aos tratamentos convencionais disponíveis.This paper is a review of native American plants showing anthelmintic activity in order to give support to research and development of new drugs in the field of natural products. In the literature, 22 plant families were cited, such as: Moraceae, Asteraceae and Cucurbitaceae, and around 35 species were studied, including Ficus sp, Cucurbita maxima and Ruta graveolens, used for their putative anthelmintic activity. From these species, 26 were active against some helminthes: Strongyloides stercolaris, Ancylostomidae, Trichostrongylus sp, Capillaria sp, Ascaris lumbricoides, Taenia saginata, Haemonchus contortus and Hymenolepsis diminuta. Data indicated a larger number of studies with animal parasites, probably due to the resistance of these helminthes to the conventional treatments available.

  12. Deepening, and repairing, the metabolic rift.

    Science.gov (United States)

    Schneider, Mindi; McMichael, Philip

    2010-01-01

    This paper critically assesses the metabolic rift as a social, ecological, and historical concept describing the disruption of natural cycles and processes and ruptures in material human-nature relations under capitalism. As a social concept, the metabolic rift presumes that metabolism is understood in relation to the labour process. This conception, however, privileges the organisation of labour to the exclusion of the practice of labour, which we argue challenges its utility for analysing contemporary socio-environmental crises. As an ecological concept, the metabolic rift is based on outmoded understandings of (agro) ecosystems and inadequately describes relations and interactions between labour and ecological processes. Historically, the metabolic rift is integral to debates about the definitions and relations of capitalism, industrialism, and modernity as historical concepts. At the same time, it gives rise to an epistemic rift, insofar as the separation of the natural and social worlds comes to be expressed in social thought and critical theory, which have one-sidedly focused on the social. We argue that a reunification of the social and the ecological, in historical practice and in historical thought, is the key to repairing the metabolic rift, both conceptually and practically. The food sovereignty movement in this respect is exemplary.

  13. Impact of rheological layering on rift asymmetry

    Science.gov (United States)

    Jaquet, Yoann; Schmalholz, Stefan M.; Duretz, Thibault

    2015-04-01

    Although numerous models of rift formation have been proposed, what triggers asymmetry of rifted margins remains unclear. Parametrized material softening is often employed to induce asymmetric fault patterns in numerical models. Here, we use thermo-mechanical finite element models that allow softening via thermal weakening. We investigate the importance of lithosphere rheology and mechanical layering on rift morphology. The numerical code is based on the MILAMIN solver and uses the Triangle mesh generator. Our model configuration consists of a visco-elasto-platic layered lithosphere comprising either (1) only one brittle-ductile transition (in the mantle) or (2) three brittle-ductile transitions (one in the upper crust, one in the lower crust and one in the mantle). We perform then two sets of simulations characterized by low and high extensional strain rates (5*10-15 s-1, 2*10-14 s-1). The results show that the extension of a lithosphere comprising only one brittle-ductile transition produces a symmetric 'neck' type rift. The upper and lower crusts are thinned until the lithospheric mantle is exhumed to the seafloor. A lithosphere containing three brittle-ductile transitions favors strain localization. Shear zones at different horizontal locations and generated in the brittle levels of the lithosphere get connected by the weak ductile layers. The results suggest that rheological layering of the lithosphere can be a reason for the generation of asymmetric rifting and subsequent rift morphology.

  14. Surface deformation in volcanic rift zones

    Science.gov (United States)

    Pollard, D.D.; Delaney, P.T.; Duffield, W.A.; Endo, E.T.; Okamura, A.T.

    1983-01-01

    The principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes, some of which feed fissure eruptions. Elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression. Concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift. If eruption occurs the characteristic ridge-trough-ridge displacement profile changes to a single ridge, centered at the fissure, and the erupted lava alters the local topography. A well-developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events. To investigate this process we compute the elastic displacements and stresses in a homogeneous, two-dimensional half-space driven by a pressurized crack that may breach the surface. A derivative graphical method permits one to estimate the three geometric parameters of the dike (height, inclination, and depth-to-center) and the mechanical parameter (driving pressure/rock stiffness) from a smoothly varying displacement profile. Direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation, notably normal faulting, creates discontinuities in the profile. Geological structures (open cracks, normal faults, buckles, and thrust faults) form because of stresses induced by dike emplacement and fissure eruption. Theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation. ?? 1983.

  15. Kinematic evolution of the southwestern Arabian continental margin: implications for the origin of the Red Sea

    Science.gov (United States)

    Voggenreiter, W.; Hötzl, H.

    The tectonic and magnetic evolution of the Jizan coastal plain (Tihama Asir) in southwest Arabia was dominated by SW-NE lithospheric extension related to the development of the Red Sea Rift. A well-exposed, isotopically-dated succession of magmatic rocks (Jizan Group volcanics, Tihama Asir Magmatic Complex) allows a kinematic analysis for this part of the Arabian Red Sea margin. A mafic dyke swarm and several generations of roughly NW-trending normal faults characterized the continental rift stage from Oligocene to early Miocene time. Major uplift of the Arabian graben shoulder probably began about 14 Ma ago. By this time, extension and magmatism ceased in the Jizan area and were followed by an approximately 10 Ma interval of tectonic and magmatic quiescence. A second phase of extension began in the Pliocene and facilitated a vast outpouring of alkaliolivine basalts on the coastal plain. The geometry of faulting in the Jizan area supports a Wernicke-type simple-shear mechanism of continental rifting for the southern Arabian continental margin of the Red Sea.

  16. Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

    Science.gov (United States)

    Plasman, M.; Tiberi, C.; Ebinger, C.; Gautier, S.; Albaric, J.; Peyrat, S.; Déverchère, J.; Le Gall, B.; Tarits, P.; Roecker, S.; Wambura, F.; Muzuka, A.; Mulibo, G.; Mtelela, K.; Msabi, M.; Kianji, G.; Hautot, S.; Perrot, J.; Gama, R.

    2017-07-01

    Rifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15-18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE-SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8-9 per cent. We relate this interface to melt ponding

  17. Outer Continental Shelf Lands Act

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — This data represents geographic terms used within the Outer Continental Shelf Lands Act (OCSLA or Act). The Act defines the United States outer continental shelf...

  18. Crustal structure of an exhumed IntraCONtinental Sag (ICONS): the Mekele Basin in Northern Ethiopia.

    Science.gov (United States)

    Alemu, T. B.; Abdelsalam, M. G.

    2017-12-01

    The Mekele Sedimentary Basin (MSB) in Ethiopia is a Paleozoic-Mesozoic IntraCONtinental Sag (ICONS) exposed due to Cenozoic domal and rift flank uplift associated with the Afar mantle plume and Afar Depression (AD). ICONS are formed over stable lithosphere, and in contrast to rift and foreland basins, show circular-elliptical shape in map view, saucer shaped in cross section, and concentric gravity minima. Surface geological features of the MSB have been shown to exhibit geologic characteristics similar to those of other ICONS. We used the World Gravity Map (WGM 2012) data to investigate subsurface-crustal structure of the MSB. We also used 2D power spectrum analysis and inversion of the gravity field to estimate the Moho depth. Our results show the Bouguer anomalies of the WGM 2012 ranges between 130 mGal and - 110 mGal with the highest values within the AD. Despite the effect of the AD on the gravity anomalies, the MSB is characterized by the presence of gravity low anomaly that reaches in places -110 mGal, especially in its western part. The Moho depth estimates, from both spectral analysis and inversion of the gravity data, is between 36 and 40 km depth over most of the western and southern margins of the MSB. However, as the AD is approached, in the eastern margins of the MSB, crustal thickness estimates are highly affected by the anomalously thin and magmatic segment of the AD, and the Moho depth range between 30 and 25 km. Our results are consistent with that of seismic studies in areas far from the MSB, but within the Northwestern Ethiopian Plateau where the MSB is located. Those studies have reported an abrupt decrease in Moho depth from 40 km beneath the Northwestern plateau, to 20 km in the adjacent AD. Though the MSB is small (100 kmX100 km) compared to other ICONS, and affected by the neighboring AD, it is characterized by elliptical gravity minima and a relatively thicker crust that gradually thickens away from the rift. In addition, seismic imaging

  19. Early Jurassic Volcanism in the South Lhasa Terrane, Southern Tibet: Record of Back-arc Extension in the Active Continental Margin

    Science.gov (United States)

    Wei, Y.; Zhao, Z.; Zhu, D. C.; Wang, Z.; Liu, D.; Mo, X.

    2015-12-01

    Indus-Yarlung Zangbo Suture Zone (IYZSZ) represents the Mesozoic remnants of the Neo-Tethyan Ocean lithosphere after its northward subduction beneath the Lhasa Terrane. The evolution of the Neo-Tethyan Ocean prior to India-Asia collision remains unclear. To explore this period of history, we investigate zircon U-Pb geochronology, geochemistry and Nd-Hf isotopes of the Early Jurassic bimodal-like volcanic sequence around Dagze area, south Tibet. The volcanic sequence comprises calc-alkaline basalts to rhyolites whereas intermediate components are volumetrically restricted. Zircons from a basaltic andesite yielded crystallization age of 178Ma whereas those from 5 silicic rocks were dated at 183-174Ma, which suggest that both the basaltic and the silicic rocks are coeval. The basaltic rocks are enriched in LREE and LILE, and depleted in HFSE, with Epsilon Nd(t) of 1.6-4.0 and zircon Epsilon Hf(t) of 0.7-11.8, which implies that they were derived from a heterogenetic mantle source metasomatized by subduction components. Trace element geochemistry shows that the basaltic rocks are compositionally transitional from normal mid-ocean ridge basalts (N-MORB) to island arc basalts (IAB, e.g. Zedong arc basalts of ~160-155Ma in the south margin of Lhasa Terrane), with the signature of immature back-arc basin basalts. The silicic rocks display similar Nd-Hf isotopic features of the Gangdese batholith with Epsilon Nd(t) of 0.9-3.4 and zircon Epsilon Hf(t) of 2.4-17.7, indicating that they were possibly generated by anatexis of basaltic juvenile lower crust, instead of derived from the basaltic magma. These results support an Early to Middle Jurassic (183-155Ma) model that the back-arc extension tectonic setting were existing in the active continental margin in the south Lhasa Terrane.

  20. Study of southern CHAONAN sag lower continental slope basin deposition character in Northern South China Sea

    Science.gov (United States)

    Tang, Y.

    2009-12-01

    Northern South China Sea Margin locates in Eurasian plate,Indian-Australia plate,Pacific Plates.The South China Sea had underwent a complicated tectonic evolution in Cenozoic.During rifting,the continental shelf and slope forms a series of Cenozoic sedimentary basins,including Qiongdongnan basin,Pearl River Mouth basin,Taixinan basin.These basins fill in thick Cenozoic fluviolacustrine facies,transitional facies,marine facies,abyssal facies sediment,recording the evolution history of South China Sea Margin rifting and ocean basin extending.The studies of tectonics and deposition of depression in the Southern Chaonan Sag of lower continental slope in the Norther South China Sea were dealt with,based on the sequence stratigraphy and depositional facies interpretation of seismic profiles acquired by cruises of“China and Germany Joint Study on Marine Geosciences in the South China Sea”and“The formation,evolution and key issues of important resources in China marginal sea",and combining with ODP 1148 cole and LW33-1-1 well.The free-air gravity anomaly of the break up of the continental and ocean appears comparatively low negative anomaly traps which extended in EW,it is the reflection of passive margin gravitational effect.Bouguer gravity anomaly is comparatively low which is gradient zone extended NE-SW.Magnetic anomaly lies in Magnetic Quiet Zone at the Northern Continental Margin of the South China Sea.The Cenozoic sediments of lower continental slope in Southern Chaonan Sag can be divided into five stratum interface:SB5.5,SB10.5,SB16.5,SB23.8 and Hg,their ages are of Pliocene-Quaternary,late Miocene,middle Miocene,early Miocene,paleogene.The tectonic evolution of low continental slope depressions can be divided into rifting,rifting-depression transitional and depression stages,while their depositional environments change from river to shallow marine and abyssa1,which results in different topography in different stages.The topographic evolvement in the study

  1. The role of mechanical heterogeneities during continental breakup: a 3D lithospheric-scale modelling approach

    Science.gov (United States)

    Duclaux, Guillaume; Huismans, Ritske S.; May, Dave

    2015-04-01

    How and why do continents break? More than two decades of analogue and 2D plane-strain numerical experiments have shown that despite the origin of the forces driving extension, the geometry of continental rifts falls into three categories - or modes: narrow rift, wide rift, or core complex. The mode of extension itself is strongly influenced by the rheology (and rheological behaviour) of the modelled layered system. In every model, an initial thermal or mechanical heterogeneity, such as a weak seed or a notch, is imposed to help localise the deformation and avoid uniform stretching of the lithosphere by pure shear. While it is widely accepted that structural inheritance is a key parameter for controlling rift localisation - as implied by the Wilson Cycle - modelling the effect of lithospheric heterogeneities on the long-term tectonic evolution of an extending plate in full 3D remains challenging. Recent progress in finite-element methods applied to computational tectonics along with the improved accessibility to high performance computers, now enable to switch from plane strain thermo-mechanical experiments to full 3D high-resolution experiments. Here we investigate the role of mechanical heterogeneities on rift opening, linkage and propagation during extension of a layered lithospheric systems with pTatin3d, a geodynamics modeling package utilising the material-point-method for tracking material composition, combined with a multigrid finite-element method to solve heterogeneous, incompressible visco-plastic Stokes problems. The initial model setup consists in a box of 1200 km horizontally by 250 km deep. It includes a 35 km layer of continental crust, underlaid by 85 km of sub-continental lithospheric mantle, and an asthenospheric mantle. Crust and mantle have visco-plastic rheologies with a pressure dependent yielding, which includes strain weakening, and a temperature, stress, strain-rate-dependent viscosity based on wet quartzite rheology for the crust, and wet

  2. Rifting and Subsidence in the Gulf of Mexico: Implications for Syn-rift, Sag, and Salt Sections, and Subsequent Paleogeography

    Science.gov (United States)

    Pindell, J. L.; Graham, R.; Horn, B.

    2013-05-01

    Thick (up to 5 km), rapid (depression where basement had already subsided tectonically, and thus could receive up to 5 km of salt, roughly the isostatic maximum on exhumed mantle, hyper-thinned continent, or new ocean crust. ION-GXT and other seismic data along W Florida and NW Yucatán show that (1) mother salt was only 1 km thick in these areas, (2) that these areas were depositionally connected to areas of thicker deposition, and (3) the top of all salt was at global sea level, and hence the sub-salt unconformity along Florida and Yucatán was only 1 km deep by end of salt deposition. These observations fit the air-filled chasm hypothesis; however, two further observations make that mechanism highly improbable: (1) basinward limits of sub-salt unconformities along Florida/Yucatán are deeper than top of adjacent ocean crust emplaced at ~2.7 km subsea (shown by backstripping), and (2) deepest abyssal sediments over ocean crust onlap the top of distal salt, demonstrating that the salt itself was rapidly drowned after deposition. Study of global ION datasets demonstrates the process of "rapid outer marginal collapse" at most margins, which we believe is achieved by low-angle detachment on deep, landward-dipping, Moho-equivalent surfaces such that outer rifted margins are hanging walls of crustal scale half-grabens over mantle. The tectonic accommodation space produced (up to 3 km, < 3 Ma) can be filled by ~5 km of sag/salt sequences with little apparent hanging wall rifting. When salt (or other) deposition lags behind, or ends during, outer marginal collapse, deep-water settings result. We suggest that this newly identified, "outer marginal detachment phase", normally separates the traditional "rift" from "drift" stages during continental margin creation. Importantly, this 2-3 km of subsidence presently is neither treated as tectonic nor as thermal in traditional subsidence analysis; thus, Beta estimates may be excessive at many outer margins. Outer marginal

  3. Rayleigh Wave Phase Velocities Beneath the Central and Southern East African Rift System

    Science.gov (United States)

    Adams, A. N.; Miller, J. C.

    2017-12-01

    This study uses the Automated Generalized Seismological Data Function (AGSDF) method to develop a model of Rayleigh wave phase velocities in the central and southern portions of the East African Rift System (EARS). These phase velocity models at periods of 20-100s lend insight into the lithospheric structures associated with surficial rifting and volcanism, as well as basement structures that pre-date and affect the course of rifting. A large dataset of >700 earthquakes is used, comprised of Mw=6.0+ events that occurred between the years 1995 and 2016. These events were recorded by a composite array of 176 stations from twelve non-contemporaneous seismic networks, each with a distinctive array geometry and station spacing. Several first-order features are resolved in this phase velocity model, confirming findings from previous studies. (1) Low velocities are observed in isolated regions along the Western Rift Branch and across the Eastern Rift Branch, corresponding to areas of active volcanism. (2) Two linear low velocity zones are imaged trending southeast and southwest from the Eastern Rift Branch in Tanzania, corresponding with areas of seismic activity and indicating possible incipient rifting. (3) High velocity regions are observed beneath both the Tanzania Craton and the Bangweulu Block. Furthermore, this model indicates several new findings. (1) High velocities beneath the Bangweulu Block extend to longer periods than those found beneath the Tanzania Craton, perhaps indicating that rifting processes have not altered the Bangweulu Block as extensively as the Tanzania Craton. (2) At long periods, the fast velocities beneath the Bangweulu Block extend eastwards beyond the surficial boundaries, to and possibly across the Malawi Rift. This may suggest the presence of older, thick blocks of lithosphere in regions where they are not exposed at the surface. (3) Finally, while the findings of this study correspond well with previous studies in regions of overlapping

  4. Trace element and Sr-Nd-Pb isotope geochemistry of Rungwe Volcanic Province, Tanzania: Implications for a superplume source for East Africa Rift magmatism

    Directory of Open Access Journals (Sweden)

    Paterno R Castillo

    2014-09-01

    Full Text Available The recently discovered high, plume-like 3He/4He ratios at Rungwe Volcanic Province (RVP in southern Tanzania, similar to those at the Main Ethiopian Rift in Ethiopia, strongly suggest that magmatism associated with continental rifting along the entire East African Rift System (EARS has a deep mantle contribution (Hilton et al., 2011. New trace element and Sr-Nd-Pb isotopic data for high 3He/4He lavas and tephras from RVP can be explained by binary mixing relationships involving Early Proterozoic (+/- Archaean lithospheric mantle, present beneath the southern EARS, and a volatile-rich carbonatitic plume with a limited range of compositions and best represented by recent Nyiragongo lavas from the Virunga Volcanic Province also in the Western Rift. Other lavas from the Western Rift and from the southern Kenya Rift can also be explained through mixing between the same endmember components. In contrast, lavas from the northern Kenya and Main Ethiopian rifts can be explained through variable mixing between the same mantle plume material and the Middle to Late Proterozoic lithospheric mantle, present beneath the northern EARS. Thus, we propose that the bulk of EARS magmatism is sourced from mixing among three endmember sources: Early Proterozoic (+/- Archaean lithospheric mantle, Middle to Late Proterozoic lithospheric mantle and a volatile-rich carbonatitic plume with a limited range of compositions. We propose further that the African Superplume, a large, seismically anomalous feature originating in the lower mantle beneath southern Africa, influences magmatism throughout eastern Africa with magmatism at RVP and Main Ethiopian Rift representing two different heads of a single mantle plume source. This is consistent with a single mantle plume origin of the coupled He-Ne isotopic signatures of mantle-derived xenoliths and/or lavas from all segments of the EARS (Halldorsson et al., 2014.

  5. Neoproterozoic rift basins and their control on the development of hydrocarbon source rocks in the Tarim Basin, NW China

    Science.gov (United States)

    Zhu, Guang-You; Ren, Rong; Chen, Fei-Ran; Li, Ting-Ting; Chen, Yong-Quan

    2017-12-01

    The Proterozoic is demonstrated to be an important period for global petroleum systems. Few exploration breakthroughs, however, have been obtained on the system in the Tarim Basin, NW China. Outcrop, drilling, and seismic data are integrated in this paper to focus on the Neoproterozoic rift basins and related hydrocarbon source rocks in the Tarim Basin. The basin consists of Cryogenian to Ediacaran rifts showing a distribution of N-S differentiation. Compared to the Cryogenian basins, those of the Ediacaran are characterized by deposits in small thickness and wide distribution. Thus, the rifts have a typical dual structure, namely the Cryogenian rifting and Ediacaran depression phases that reveal distinct structural and sedimentary characteristics. The Cryogenian rifting basins are dominated by a series of grabens or half grabens, which have a wedge-shaped rapid filling structure. The basins evolved into Ediacaran depression when the rifting and magmatic activities diminished, and extensive overlapping sedimentation occurred. The distributions of the source rocks are controlled by the Neoproterozoic rifts as follows. The present outcrops lie mostly at the margins of the Cryogenian rifting basins where the rapid deposition dominates and the argillaceous rocks have low total organic carbon (TOC) contents; however, the source rocks with high TOC contents should develop in the center of the basins. The Ediacaran source rocks formed in deep water environment of the stable depressions evolving from the previous rifting basins, and are thus more widespread in the Tarim Basin. The confirmation of the Cryogenian to Ediacaran source rocks would open up a new field for the deep hydrocarbon exploration in the Tarim Basin.

  6. Young rift kinematics in the Tadjoura rift, western Gulf of Aden, Republic of Djibouti

    Science.gov (United States)

    Daoud, Mohamed A.; Le Gall, Bernard; Maury, René C.; Rolet, JoëL.; Huchon, Philippe; Guillou, Hervé

    2011-02-01

    The Tadjoura rift forms the westernmost edge of the westerly propagating Sheba ridge, between Arabia and Somalia, as it enters into the Afar depression. From structural and remote sensing data sets, the Tadjoura rift is interpreted as an asymmetrical south facing half-graben, about 40 km wide, dominated by a large boundary fault zone to the north. It is partially filled up by the 1-3 Myr old Gulf Basalts which onlapped the older Somali Basalts along its shallower southern flexural margin. The major and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basaltic types, namely the Gulf, Somali, Goumarre, and Hayyabley Basalts. These results, together with radiometric age data, lead us to propose a revised volcano-stratigraphic sketch of the two exposed Tadjoura rift margins and to discriminate and date several distinct fault networks of this oblique rift. Morphological and statistical analyses of onshore extensional fault populations show marked changes in structural styles along-strike, in a direction parallel to the rift axis. These major fault disturbances are assigned to the arrest of axial fault tip propagation against preexisting discontinuities in the NS-oriented Arta transverse zone. According to our model, the sinistral jump of rifting into the Asal-Ghoubbet rift segment results from structural inheritance, in contrast with the en échelon or transform mechanism of propagation that prevailed along the entire length of the Gulf of Aden extensional system.

  7. The lakes of the Jordan Rift Valley

    International Nuclear Information System (INIS)

    Gat, J.R.

    2001-01-01

    This paper presents a summary of the proceedings of a workshop on the Lakes of the Jordan Rift Valley that was held in conjunction with the CRP on The Use of Isotope Techniques in Lake Dynamics Investigations. The paper presents a review of the geological, hydrogeological and physical limnological setting of the lakes in the Jordan Rift Valley, Lake Hula, Lake Kinneret and the Dead Sea. This is complemented by a description of the isotope hydrology of the system that includes the use of a wide range of isotopes: oxygen-18, deuterium, tritium, carbon-14, carbon-13, chlorine isotopes, boron-11 and helium-3/4. Environmental isotope aspects of the salt balances of the lakes, their palaeolimnology and biogeochemical tracers are also presented. The scope of application of isotopic tracers is very broad and provides a clear insight into many aspects of the physical, chemical and biological limnology of the Rift Valley Lakes. (author)

  8. Thermal springs, fumaroles and gas vents of continental Yemen: Their relation with active tectonics, regional hydrology and the country's geothermal potential

    International Nuclear Information System (INIS)

    Minissale, Angelo; Mattash, Mohamed A.; Vaselli, Orlando; Tassi, Franco; Al-Ganad, Ismail N.; Selmo, Enrico; Shawki, Nasr M.; Tedesco, Dario; Poreda, Robert; Ad-Dukhain, Abdassalam M.; Hazzae, Mohammad K.

    2007-01-01

    Most thermal springs of continental Yemen (about 65 emergences at 48 sampling sites) and a couple of fumaroles and boiling water pools have been sampled and analyzed for chemical and isotopic composition in the liquid phase and the associated free-gas phase. Whatever the emergence, all the water discharges have an isotopic signature of meteoric origin. Springs seeping out from high altitudes in the central volcanic plateau show a prevalent Na-HCO 3 -composition, clearly affected by an anomalous flux of deep CO 2 deriving from active hydrothermal systems located in the Jurassic Amran Group limestone sequence and/or the Cretaceous Tawilah Group, likely underlying the 2000-3000 m thick volcanic suite. At lower elevations, CO 2 also affects the composition of some springs emerging at the borders of the central volcanic plateau. Although mixing to a limited extent with organic CO 2 infiltrating together with the meteoric recharge waters cannot be ruled out, all the CO 2 -rich gas samples have a δ 13 C-CO 2 signature that falls in the range of mantle CO 2 (-3 13 C 3 He/ 4 He (1 a 2 -rich springs and also some mixed N 2 -CO 2 gas vents in the far east Hadramaut region support the presence of mantle magmas and related hydrothermal systems residing at the crust level in several areas of Yemen. This well agrees with the presence of Quaternary basaltic magmatic activity along the Gulf of Aden, as well as inside the central Yemen volcanic plateau. Presently, the thermal springs of Yemen are prevalently used for spas and/or bathing. Nevertheless, liquid- and gas-geothermometry and geological considerations suggest that there are at least three areas (Al Lisi, Al Makhaya and Damt) inside the Yemen volcanic plateau (around Dhamar) that may be promising prospects for the future development of geothermal energy in Yemen. Alternatively, they could be used as a source of energy for small-to-medium scale agriculture and/or industrial purposes. Moreover, most of the thermal water

  9. Thermal springs, fumaroles and gas vents of continental Yemen: Their relation with active tectonics, regional hydrology and the country's geothermal potential

    Energy Technology Data Exchange (ETDEWEB)

    Minissale, Angelo [CNR - Italian Council for Research, Institute of Geosciences and Earth Resources of Florence, Via La Pira 4, 50121 Florence (Italy)]. E-mail: minissa@igg.cnr.it; Mattash, Mohamed A. [Ministry of Oil and Mineral Resources, Geological Survey and Minerals Resources Board, P.O. Box 297, Sana' a (Yemen); Vaselli, Orlando [Department of Earth Sciences, Via La Pira 4, 50121 Firenze (Italy); CNR - Italian Council for Research, Institute of Geosciences and Earth Resources of Florence, Via La Pira 4, 50121 Firenze (Italy); Tassi, Franco [Department of Earth Sciences, Via La Pira 4, 50121 Firenze (Italy); Al-Ganad, Ismail N. [Ministry of Oil and Mineral Resources, Geological Survey and Minerals Resources Board, P.O. Box 297, Sana' a (Yemen); Selmo, Enrico [Department of Earth Sciences, Parco Area delle Scienze 157A, 43100 Parma (Italy); Shawki, Nasr M. [Department of Geology, University of Ta' iz, P.O. Box 5679, Ta' iz (Yemen); Tedesco, Dario [Department of Environmental Sciences, Second University of Naples, Via Vivaldi 43, 81100 Caserta (Italy); Poreda, Robert [Department of Earth and Environmental Sciences, 227 Hutchinson Hall, Rochester, NY 14627 (United States); Ad-Dukhain, Abdassalam M. [Ministry of Oil and Mineral Resources, Geological Survey and Minerals Resources Board, P.O. Box 297, Sana' a (Yemen); Hazzae, Mohammad K. [Ministry of Oil and Mineral Resources, Geological Survey and Minerals Resources Board, P.O. Box 297, Sana' a (Yemen)

    2007-04-15

    Most thermal springs of continental Yemen (about 65 emergences at 48 sampling sites) and a couple of fumaroles and boiling water pools have been sampled and analyzed for chemical and isotopic composition in the liquid phase and the associated free-gas phase. Whatever the emergence, all the water discharges have an isotopic signature of meteoric origin. Springs seeping out from high altitudes in the central volcanic plateau show a prevalent Na-HCO{sub 3}-composition, clearly affected by an anomalous flux of deep CO{sub 2} deriving from active hydrothermal systems located in the Jurassic Amran Group limestone sequence and/or the Cretaceous Tawilah Group, likely underlying the 2000-3000 m thick volcanic suite. At lower elevations, CO{sub 2} also affects the composition of some springs emerging at the borders of the central volcanic plateau. Although mixing to a limited extent with organic CO{sub 2} infiltrating together with the meteoric recharge waters cannot be ruled out, all the CO{sub 2}-rich gas samples have a {delta} {sup 13}C-CO{sub 2} signature that falls in the range of mantle CO{sub 2} (-3 < {delta} {sup 13}C < -7 per mille V-PDB). The relatively high {sup 3}He/{sup 4}He (1 < R/R {sub a} < 3.2) ratios measured in all the CO{sub 2}-rich springs and also some mixed N{sub 2}-CO{sub 2} gas vents in the far east Hadramaut region support the presence of mantle magmas and related hydrothermal systems residing at the crust level in several areas of Yemen. This well agrees with the presence of Quaternary basaltic magmatic activity along the Gulf of Aden, as well as inside the central Yemen volcanic plateau. Presently, the thermal springs of Yemen are prevalently used for spas and/or bathing. Nevertheless, liquid- and gas-geothermometry and geological considerations suggest that there are at least three areas (Al Lisi, Al Makhaya and Damt) inside the Yemen volcanic plateau (around Dhamar) that may be promising prospects for the future development of geothermal energy

  10. The Myanmar continental shelf

    Digital Repository Service at National Institute of Oceanography (India)

    Ramaswamy, V.; Rao, P.S.

    reveal a minimum of 18 m thick strata of modern muds (Fig. 2g). At the outer boundary of the Gulf of Myanmar Continental Shelf 8 Martaban (15oN Latitude), brown muds overlie coarse sands indicating that modern deltaic sediments... on the Myeik Bank (Rodolfo, 1969a). Modern sediments on the Ayeyarwady shelf General composition, Texture and Grain-size: The distribution and sediment texture on the Ayeyarwady shelf shows fine-grained sediments comprising silty-clay and clayey...

  11. Tectonic inheritance in the development of the Kivu - north Tanganyika rift segment of the East African Rift System: role of pre-existing structures of Precambrian to early Palaeozoic origin.

    Science.gov (United States)

    Delvaux, Damien; Fiama Bondo, Silvanos; Ganza Bamulezi, Gloire

    2017-04-01

    The present architecture of the junction between the Kivu rift basin and the north Tanganyika rift basin is that of a typical accommodation zone trough the Ruzizi depression. However, this structure appeared only late in the development of the Western branch of the East African Rift System and is the result of a strong control by pre-existing structures of Precambrian to early Palaeozoic origin. In the frame of a seismic hazard assessment of the Kivu rift region, we (Delvaux et al., 2016) constructed homogeneous geological, structural and neotectonic maps cross the five countries of this region, mapped the pre-rift, early rift and Late Quaternary faults and compiled the existing knowledge on thermal springs (assumed to be diagnostic of current tectonic activity along faults). We also produced also a new catalogue of historical and instrumental seismicity and defined the seismotectonic characteristics (stress field, depth of faulting) using published focal mechanism data. Rifting in this region started at about 11 Ma by initial doming and extensive fissural basaltic volcanism along normal faults sub-parallel to the axis of the future rift valley, as a consequence of the divergence between the Nubia and the Victoria plate. In a later stage, starting around 8-7 Ma, extension localized along a series of major border faults individualizing the subsiding tectonic basins from the uplifting rift shoulders, while lava evolved towards alkali basaltic composition until 2.6 Ma. During this stage, initial Kivu rift valley was extending linearly in a SSW direction, much further than its the actual termination at Bukavu, into the Mwenga-Kamituga graben, up to Namoya. The SW extremity of this graben was linked via a long oblique transfer zone to the central part of Lake Tanganyika, itself reactivating an older ductile-brittle shear zone. In the late Quaternary-early Holocene, volcanism migrated towards the center of the basin, with the development of the Virunga volcanic massif

  12. Oppositely directed pairs of propagating rifts in back-arc basins: Double saloon door seafloor spreading during subduction rollback

    Science.gov (United States)

    Martin, A. K.

    2006-06-01

    When a continent breaks up into two plates, which then separate from each other about a rotation pole, it can be shown that if initial movement is taken up by lithospheric extension, asthenospheric breakthrough and oceanic accretion propagate toward the pole of rotation. Such a propagating rift model is then applied to an embryonic centrally located rift which evolves into two rifts propagating in opposite directions. The resultant rhombic shape of the modeled basin, initially underlain entirely by thinned continental crust, is very similar to the Oligocene to Burdigalian back-arc evolution of the Valencia Trough and the Liguro-Provencal Basin in the western Mediterranean. Existing well and seismic stratigraphic data confirm that a rift did initiate in the Gulf of Lion and propagated southwest into the Valencia Trough. Similarly, seismic refraction, gravity, and heat flow data demonstrate that maximum extension within the Valencia Trough/Liguro-Provencal Basin occurred in an axial position close to the North Balearic Fracture Zone. The same model of oppositely propagating rifts, when applied to the Burdigalian/Langhian episode of back-arc oceanic accretion within the Liguro-Provencal and Algerian basins, predicts a number of features which are borne out by existing geological and geophysical, particularly magnetic data. These include the orientation of subparallel magnetic anomalies, presumed to be seafloor spreading isochrons, in both basins; concave-to-the-west fracture zones southwest of the North Balearic Fracture Zone, and concave-to-the-east fracture zones to its northeast; a spherical triangular area of NW oriented seafloor spreading isochrons southwest of Sardinia; the greater NW extension of the central (youngest?) magnetic anomaly within this triangular area, in agreement with the model-predicted northwestward propagation of a rift in this zone; successively more central (younger) magnetic anomalies abutting thinned continental crust nearer to the pole of

  13. Geodetic measurements and models of rifting in Northern Iceland for 1993-1998 (Invited)

    Science.gov (United States)

    Ali, T.; Feigl, K.; Thurber, C. H.; Masterlark, T.; Carr, B.; Sigmundsson, F.

    2010-12-01

    Rifting occurs as episodes of active deformation in individual rift segments of the Northern Volcanic Zone (NVZ) in Iceland. Here we simulate deformation around the Krafla central volcano and rift system in NVZ in order to explain InSAR data acquired between 1993 and 1998. The General Inversion for Phase Technique (GIPhT) is used to model the InSAR phase data directly, without unwrapping [Feigl and Thurber, Geophys. J. Int., 2009]. Using a parallel simulated annealing algorithm, GIPhT minimizes the non-linear cost function that quantifies the misfit between observed and modeled values of the phase. We test the hypothesis that the observed deformation can be explained by a combination of at least three processes including: (i) secular plate spreading, (ii) post rifting relaxation following the Krafla rifting episode (1975-1984), and (iii) deflation of a shallow magma chamber beneath the central volcano. The calibration parameters include material properties of upper/lower crust and mantle as well as flux rates for the elements of the plumbing system. The best fitting Maxwell model favors a stronger lower crust (~1.0E+20 Pa.s) and a mantle viscosity of ~1.0E+18 Pa.s as well as a shallow deflating magma chamber. The deformation appears to be linear in time over the observed interval.

  14. Contrasting basin architecture and rifting style of the Vøring Basin, offshore mid-Norway and the Faroe-Shetland Basin, offshore United Kingdom

    Science.gov (United States)

    Schöpfer, Kateřina; Hinsch, Ralph

    2017-04-01

    The Vøring and the Faroe-Shetland basins are offshore deep sedimentary basins which are situated on the outer continental margin of the northeast Atlantic Ocean. Both basins are underlain by thinned continental crust whose structure is still debated. In particular the nature of the lower continental crust and the origin of high velocity bodies located at the base of the lower crust are a subject of discussion in recent literature. Regional interpretation of 2D and 3D seismic reflection data, combined with well data, suggest that both basins share several common features: (i) Pre-Cretaceous faults that are distributed across the entire basin width. (ii) Geometries of pre-Jurassic strata reflecting at least two extensional phases. (iii) Three common rift phases, Late Jurassic, Campanian-Maastrichtian and Palaeocene. (iv) Large pre-Cretaceous fault blocks that are buried by several kilometres of Cretaceous and Cenozoic strata. (iii). (v) Latest Cretaceous/Palaeocene inversion. (vi) Occurrence of partial mantle serpentinization during Early Cretaceous times, as proposed by other studies, seems improbable. The detailed analysis of the data, however, revealed significant differences between the two basins: (i) The Faroe-Shetland Basin was a fault-controlled basin during the Late Jurassic but also the Late Cretaceous extensional phase. In contrast, the Vøring Basin is dominated by the late Jurassic rifting and subsequent thermal subsidence. It exhibits only minor Late Cretaceous faults that are localised above intra-basinal and marginal highs. In addition, the Cretaceous strata in the Vøring Basin are folded. (ii) In the Vøring Basin, the locus of Late Cretaceous rifting shifted westwards, affecting mainly the western basin margin, whereas in the Faroe-Shetland Basin Late Cretaceous rifting was localised in the same area as the Late Jurassic phase, hence masking the original Jurassic geometries. (iii) Devono-Carboniferous and Aptian/Albian to Cenomanian rift phases

  15. Testing Predictions of Continental Insulation using Oceanic Crustal Thicknesses

    Science.gov (United States)

    Hoggard, Mark; Shorttle, Oliver; White, Nicky

    2016-04-01

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

  16. Modeling the Sedimentary Infill of Lakes in the East African Rift: A Case Study of Multiple versus Single Rift Basin Segments

    Science.gov (United States)

    Zhang, C.; Scholz, C. A.

    2016-12-01

    The sedimentary basins in the East African Rift are considered excellent modern examples for investigating sedimentary infilling and evolution of extensional systems. Some lakes in the western branch of the rift have formed within single-segment systems, and include Lake Albert and Lake Edward. The largest and oldest lakes developed within multi-segment systems, and these include Lake Tanganyika and Lake Malawi. This research aims to explore processes of erosion and sedimentary infilling of the catchment area in single-segment rift (SSR) and multi-segment rift (MSR) systems. We consider different conditions of regional precipitation and evaporation, and assess the resulting facies architecture through forward modeling, using state-of-the-art commercial basin modeling software. Dionisos is a three-dimensional numerical stratigraphic forward modeling software program, which simulates basin-scale sediment transport based on empirical water- and gravity-driven diffusion equations. It was classically used to quantify the sedimentary architecture and basin infilling of both marine siliciclastic and carbonate environments. However, we apply this approach to continental rift basin environments. In this research, two scenarios are developed, one for a MSR and the other for a SSR. The modeled systems simulate the ratio of drainage area and lake surface area observed in modern Lake Tanganyika and Lake Albert, which are examples of MSRs and SSRs, respectively. The main parameters, such as maximum subsidence rate, water- and gravity-driven diffusion coefficients, rainfall, and evaporation, are approximated using these real-world examples. The results of 5 million year model runs with 50,000 year time steps show that MSRs are characterized by a deep water lake with relatively modest sediment accumulation, while the SSRs are characterized by a nearly overfilled lake with shallow water depths and thick sediment accumulation. The preliminary modeling results conform to the features

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

    Science.gov (United States)

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

    2013-12-01

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

  18. The Thermal Evolution of the Southeast Baffin Island Continental Margin: An Integrated Apatite Fission Track and Apatite (U-Th)/He Study

    Science.gov (United States)

    Jess, S.; Stephenson, R.; Brown, R. W.

    2017-12-01

    The elevated continental margins of the North Atlantic continue to be a focus of considerable geological and geomorphological debate, as the timing of major tectonic events and the age of topographic relief remain controversial. The West Greenland margin, on the eastern flank of Baffin Bay, is believed by some authors to have experienced tectonic rejuvenation and uplift during the Neogene. However, the opposing flank, Baffin Island, is considered to have experienced a protracted erosional regime with little tectonic activity since the Cretaceous. This work examines the thermal evolution of the Cumberland Peninsula, SE Baffin Island, using published apatite fission track (AFT) data with the addition of 103 apatite (U-Th)/He (AHe) ages. This expansion of available thermochronological data introduces a higher resolution of thermal modelling, whilst the application of the newly developed `Broken Crystals' technique provides a greater number of thermal constraints for an area dominated by AHe age dispersion. Results of joint thermal modelling of the AFT and AHe data exhibit two significant periods of cooling across the Cumberland Peninsula: Devonian/Carboniferous to the Triassic and Late Cretaceous to present. The earliest phase of cooling is interpreted as the result of major fluvial systems present throughout the Paleozoic that flowed across the Canadian Shield to basins in the north and south. The later stage of cooling is believed to result from rift controlled fluvial systems that flowed into Baffin Bay during the Mesozoic and Cenozoic during the early stages and culmination of rifting along the Labrador-Baffin margins. Glaciation in the Late Cenozoic has likely overprinted these later river systems creating a complex fjordal distribution that has shaped the modern elevated topography. This work demonstrates how surface processes, and not tectonism, can explain the formation of elevated continental margins and that recent methodological developments in the field of

  19. Geodetic Measurements and Numerical Models of Rifting in Northern Iceland for 1993-1999

    Science.gov (United States)

    Ali, T.; Feigl, K.; Masterlark, T.; Carr, B. B.; Sigmundsson, F.; Thurber, C. H.

    2009-12-01

    Rifting occurs as episodes of active deformation in individual rift segments of the Northern Volcanic Zone (NVZ) of Iceland. To measure the deformation, we use interferometric analysis of synthetic aperture radar (InSAR) data acquired between 1993 and 1999. Preliminary results suggest that a complex interplay of multiple inflating and deflating sources at depth is required to account for the observed deformation. In an effort to integrate heterogeneous constraining information (kinematic plate spreading, seismic tomography and anisotropy, and thermal and rheologic structures), we develop finite element models that simulate the underlying sources and processes associated with rifting events to quantitatively understand the magmatic plumbing system beneath Krafla central volcano and rift segment, the site of the most recent rifting episode in the NVZ. Calibration parameters include the positions, geometries, and flux rates for elements of the plumbing system, as well as material properties. The General Inversion for Phase Technique (GIPhT) [Feigl and Thurber, Geophys. J. Int., 2009] is used to model the InSAR phase data directly, without unwrapping parameters. It operates on wrapped phase values ranging from -1/2 to +1/2 cycles. By defining a cost function that quantifies the misfit between observed and modeled values in terms of wrapped phase, GIPhT can estimate parameters in a geophysical model by minimizing the cost function. Since this approach can handle noisy, wrapped phase data, it avoids the pitfalls of phase-unwrapping approaches. Consequently, GIPhT allows the analysis, interpretation and modeling of more interferometric pairs than approaches that require unwrapping. GIPhT also allows statistical testing of hypotheses because the wrapped phase residuals follow a Von Mises distribution. As a result, the model parameters estimated by GIPhT include formal uncertainties. We test the hypothesis that deformation in the rift zone occurred at a constant (secular

  20. Introduction in New perspectives on Rio Grande rift basins: from tectonics to groundwater

    Science.gov (United States)

    Hudson, Mark R.; Grauch, V.J.S.

    2013-01-01

    Basins of the Rio Grande rift have long been studied both for their record of rift development and for their potential as host of natural resources. Early workers described the basin geomorphology and the character of infilling sediments (e.g. Siebenthal, 1910; Bryan, 1938; Speigel and Baldwin, 1963), and subsequent research compilations provided general stratigraphic and tectonic overviews of rift basins and described their geophysical characteristics within the crust (Hawley, 1978; Riecker, 1979; Baldridge et al., 1984; Keller, 1986). Subsurface knowledge gained from hydrocarbon exploration activities coupled with detailed surface studies of basins and their flanking uplifts were presented in Geological Society of America (GSA) Special Paper 291, edited by Keller and Cather (1994a).

  1. The Mesozoic rift basins of eastern North America: Potential reservoir or Explorationist's folly

    Energy Technology Data Exchange (ETDEWEB)

    Pyron, A.

    1991-08-01

    Mesozoic rift basins are found on the East Coast of North America from Georgia to Nova Scotia. The basins formed as a result of extensional activity associated with the breakup of Pangaea. The internal geometry of the basins includes a depositional sequence ranging from coarse fanglomerates to fine-grained siltstones and argillites. Since these Mesozoic rift basins were first studied, they have not been considered to be likely spots for hydrocarbon accumulations. Recently, geologists have reconsidered these Mesozoic basins and have developed a more synergistic approach that suggests that many of these rift basins might be suitable targets for exploration. By analogy, these Mesozoic basins are correlative to similar basins in northwestern Africa, where significant reserved of oil and natural gas have been developed. The similarity between the productive basins in northwestern Africa and the Mesozoic basins of North America and their proximity to major markets provides sufficient rationale to further investigate these basins.

  2. Lithological Influences on Occurrence of High-Fluoride Waters in The Central Kenya Rift

    Science.gov (United States)

    Olaka, L. A.; Musolff, A.; Mulch, A.; Olago, D.; Odada, E. O.

    2013-12-01

    Within the East African rift, groundwater recharge results from the complex interplay of geology, land cover, geomorphology, climate and on going volcano-tectonic processes across a broad range of spatial and temporal scales. The interrelationships between these factors create complex patterns of water availability, reliability and quality. The hydrochemical evolution of the waters is further complex due to the different climatic regimes and geothermal processes going on in this area. High fluoridic waters within the rift have been reported by few studies, while dental fluorosis is high among the inhabitants of the rift. The natural sources of fluoride in waters can be from weathering of fluorine bearing minerals in rocks, volcanic or fumarolic activities. Fluoride concentration in water depends on a number of factors including pH, temperature, time of water-rock formation contact and geochemical processes. Knowledge of the sources and dispersion of fluoride in both surface and groundwaters within the central Kenya rift and seasonal variations between wet and dry seasons is still poor. The Central Kenya rift is marked by active tectonics, volcanic activity and fumarolic activity, the rocks are majorly volcanics: rhyolites, tuffs, basalts, phonolites, ashes and agglomerates some are highly fractured. Major NW-SE faults bound the rift escarpment while the rift floor is marked by N-S striking faults We combine petrographic, hydrochemistry and structural information to determine the sources and enrichment pathways of high fluoridic waters within the Naivasha catchment. A total of 120 water samples for both the dry season (January-February2012) and after wet season (June-July 2013) from springs, rivers, lakes, hand dug wells, fumaroles and boreholes within the Naivasha catchment are collected and analysed for fluoride, physicochemical parameters and stable isotopes (δ2 H, δ18 O) in order to determine the origin and evolution of the waters. Additionally, 30 soil and

  3. Rifte Guaritas basin compartmentation in Camaqua

    International Nuclear Information System (INIS)

    Preissler, A; Rolim, S; Philipp, R.

    2010-01-01

    The study contributes to the knowledge of the tectonic evolution of the Guaritas rift basin in Camaqua. Were used aero magnetic geophysical data for modeling the geometry and the depth of the structures and geological units. The research was supported in processing and interpretation of Aster images (EOS-Terra), which were extracted from geophysical models and digital image

  4. Rift Valley Fever, Mayotte, 2007–2008

    Science.gov (United States)

    Giry, Claude; Gabrie, Philippe; Tarantola, Arnaud; Pettinelli, François; Collet, Louis; D’Ortenzio, Eric; Renault, Philippe; Pierre, Vincent

    2009-01-01

    After the 2006–2007 epidemic wave of Rift Valley fever (RVF) in East Africa and its circulation in the Comoros, laboratory case-finding of RVF was conducted in Mayotte from September 2007 through May 2008. Ten recent human RVF cases were detected, which confirms the indigenous transmission of RFV virus in Mayotte. PMID:19331733

  5. Molecular Rift: Virtual Reality for Drug Designers.

    Science.gov (United States)

    Norrby, Magnus; Grebner, Christoph; Eriksson, Joakim; Boström, Jonas

    2015-11-23

    Recent advances in interaction design have created new ways to use computers. One example is the ability to create enhanced 3D environments that simulate physical presence in the real world--a virtual reality. This is relevant to drug discovery since molecular models are frequently used to obtain deeper understandings of, say, ligand-protein complexes. We have developed a tool (Molecular Rift), which creates a virtual reality environment steered with hand movements. Oculus Rift, a head-mounted display, is used to create the virtual settings. The program is controlled by gesture-recognition, using the gaming sensor MS Kinect v2, eliminating the need for standard input devices. The Open Babel toolkit was integrated to provide access to powerful cheminformatics functions. Molecular Rift was developed with a focus on usability, including iterative test-group evaluations. We conclude with reflections on virtual reality's future capabilities in chemistry and education. Molecular Rift is open source and can be downloaded from GitHub.

  6. Coulomb Stress Change and Seismic Hazard of Rift Zones in Southern Tibet after the 2015 Mw7.8 Nepal Earthquake and Its Mw7.3 Aftershock

    Science.gov (United States)

    Dai, Z.; Zha, X.; Lu, Z.

    2015-12-01

    In southern Tibet (30~34N, 80~95E), many north-trending rifts, such as Yadong-Gulu and Lunggar rifts, are characterized by internally drained graben or half-graben basins bounded by active normal faults. Some developed rifts have become a portion of important transportation lines in Tibet, China. Since 1976, eighty-seven >Mw5.0 earthquakes have happened in the rift regions, and fifty-five events have normal faulting focal mechanisms according to the GCMT catalog. These rifts and normal faults are associated with both the EW-trending extension of the southern Tibet and the convergence between Indian and Tibet. The 2015 Mw7.8 Nepal great earthquake and its Mw7.3 aftershock occurred at the main Himalayan Thrust zone and caused tremendous damages in Kathmandu region. Those earthquakes will lead to significant viscoelastic deformation and stress changes in the southern Tibet in the future. To evaluate the seismic hazard in the active rift regions in southern Tibet, we modeled the slip distribution of the 2015 Nepal great earthquakes using the InSAR displacement field from the ALOS-2 satellite SAR data, and calculated the Coulomb failure stress (CFS) on these active normal faults in the rift zones. Because the estimated CFS depends on the geometrical parameters of receiver faults, it is necessary to get the accurate fault parameters in the rift zones. Some historical earthquakes have been studied using the field data, teleseismic data and InSAR observations, but results are in not agreement with each other. In this study, we revaluated the geometrical parameters of seismogenic faults occurred in the rift zones using some high-quality coseismic InSAR observations and teleseismic body-wave data. Finally, we will evaluate the seismic hazard in the rift zones according to the value of the estimated CFS and aftershock distribution.

  7. The distribution and hydrogeological controls of fluoride in the groundwater of central Ethiopian rift and adjacent highlands

    Science.gov (United States)

    Ayenew, Tenalem

    2008-05-01

    Occurrence of fluoride (F) in groundwater has drawn worldwide attention, since it has considerable impact on human health. In Ethiopia high concentrations of F in groundwaters used for community water supply have resulted in extensive dental and skeletal fluorosis. As a part of a broader study, the distribution of F in groundwater has been investigated, and compared with bedrock geology and pertinent hydrochemical variables. The result indicates extreme spatial variations. High F concentration is often associated with active and sub-active regional thermal fields and acidic volcanics within high temperature rift floor. Variations in F can also be related to changes in calcium concentration resulting from dissolution of calcium minerals and mixing with waters of different chemical composition originated from variable hydrogeological environment across the rift valley. The concentration of F dramatically declines from the rift towards the highlands with the exception of scattered points associated with thermal springs confined in local volcanic centers. There are also interactions of F-rich alkaline lakes and the surrounding groundwater. Meteoric waters recharging volcanic aquifers become enriched with respect to F along the groundwater flow path from highland recharge areas to rift discharge areas. Locally wells drilled along large rift faults acting as conduits of fresh highland waters show relatively lower F. These areas are likely to be possible sources of better quality waters within the rift. The result of this study has important implications on site selection for water well drilling.

  8. Late Cenozoic cooling history of the central Menderes Massif and the contribution of erosion to rock exhumation during active continental extension

    Science.gov (United States)

    Nilius, Nils-Peter; Wölfler, Andreas; Heineke, Caroline; Glotzbach, Christoph; Hetzel, Ralf; Hampel, Andrea; Akal, Cüneyt; Dunkl, István

    2017-04-01

    erosion rates from catchments in the exposed footwall of the Büyük Menderes detachment indicates that erosion has contributed 10-40% to the exhumation of metamorphic rocks beneath the detachment. Our finding underlines that the contribution of erosion to rock exhumation cannot be neglected in regions of active continental extension. References Buscher, J.T., Hampel, A., Hetzel, R., Dunkl, I, Glotzbach, C., Struffert, A., Akal, C., Rätz, M. 2013. Quantifying rates of detachment faulting and erosion in the central Menderes Massif (western Turkey) by thermochronology and cosmogenic 10Be. J. Geol. Soc. London. 170, 669-683. Gessner, K., Ring, U., Johnson, C., Hetzel, R., Passchier, C.W., Güngör, T., 2001. An active bivergent rolling-hinge detachment system: Central Menderes metamorphic core complex in western Turkey. Geology 29, 611-614. Gessner, K., Gallardo, L.A., Markwitz, V., Ring, U., Thomson, S.N., 2013. What caused the denudation of the Menderes Massif: Review of crustal evolution, lithosphere structure, and dynamic topography in southwest Turkey. Gondwana Research 24, 243-274. Wölfler, A., Glotzbach, C., Heineke, C., Nilius, N.P., Hetzel, R., Hampel, A., Akal, C., Dunkl, I., Christl, M. (manuscript in revision for Tectonophysics). Late Cenozoic cooling history of the central Menderes Massif: timing and slip rate of the Büyük Menderes detachment and the relative contribution of normal faulting and erosion to rock exhumation.

  9. Volcanism in the Sumisu Rift. Pt. 2

    International Nuclear Information System (INIS)

    Hochstaedter, A.G.; Gill, J.B.; Morris, J.D.

    1990-01-01

    A bimodal suite of volcanic rocks collected from the Sumisu Rift by ALVIN provide present day example of the first magmatic products of arc rifting during the initiation of back-arc spreading. The trace element and isotopic composition of these rocks, which are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, differ from those of arc rocks and N-MORB in their relative incorporation of both subduction-related and non-subduction-related components. Subduction-related components, i.e., those that distinguish volcanic arc basalts from N-MORB, are less pronounced in rift lavas than in arc lavas. Alkali and alkaline earth to high field strength element and REE ratios as well as 87 Sr/ 86 Sr are intermediate between those of N-MORB and Izu arc lavas and indicate that Sumisu Rift basalts are similar to BABB erupted in other, more mature back-arc basins. These results show that back-arc basins may begin their magmatic evolution with BABB rather than more arc-like lavas. Evidence of non-subduction related components remains after the effects of subduction related components are removed or accounted for. Compared to the arc, higher HFSE and REE concentrations, contrasting REE patterns, and ≤ε Nd in the rift reflect derivation of rift lavas from more enriched components. Although SR basalt resembles E-MORB in many trace element ratios, it is referred to as BABB because low concentrations of Nb are similar to those in volcanic arcs and H 2 O/REE and H 2 O/K 2 O exceed those of E-MORB. Differences in HREE pattern and ε Nd require that the E-MORB characteristics result from source heterogeneities and not lower degrees of melting. Enriched mantle beneath the rift may reflect enriched blobs entrained in a more depleted matrix, or injection of new, more enriched mantle. High 208 Pb/ 204 Pb and moderate 207 Pb/ 204 Pb ratios with respect to Pacific MORB also reflect ancient mantle enrichment. (orig.)

  10. The continental waters pollution

    International Nuclear Information System (INIS)

    Marsily, G. de

    1996-01-01

    This work deals with the continental water pollution. The sewage affect considerably the quality of some rivers water and of some basins. Moreover, a slow and general damage of natural waters has been established. The direct effects on men and on the natural medium (climatic change, aquatic ecosystems, water cycle) are given as well as the protection means (waste processing, the water-bearing bed and underground water protection, the aquatic ecosystems protection and planning) used and future to abate the water pollution. (O.L.). 17 refs., 6 tabs

  11. The Sidi Ifni transect across the rifted margin of Morocco (Central Atlantic): Vertical movements constrained by low-temperature thermochronology

    Science.gov (United States)

    Charton, Rémi; Bertotti, Giovanni; Arantegui, Angel; Bulot, Luc

    2018-05-01

    The occurrence of km-scale exhumations during syn- and post-rift stages has been documented along Atlantic continental margins, which are also characterised by basins undergoing substantial subsidence. The relationship between the exhuming and subsiding domains is poorly understood. In this study, we reconstruct the evolution of a 50 km long transect across the Moroccan rifted margin from the western Anti-Atlas to the Atlantic basin offshore the city of Sidi Ifni. Low-temperature thermochronology data from the Sidi Ifni area document a ca. 8 km exhumation between the Permian and the Early/Middle Jurassic. The related erosion fed sediments to the subsiding Mesozoic basin to the NW. Basement rocks along the transect were subsequently buried by 1-2 km between the Late Jurassic and the Early Cretaceous. From late Early/Late Cretaceous onwards, rocks present along the transect were exhumed to their present-day position.

  12. Rift magmatism on the Eurasia basin margin: U–Pb baddeleyite ages of alkaline dyke swarms in North Greenland

    DEFF Research Database (Denmark)

    Thórarinsson, Sigurjón B.; Söderlund, Ulf; Døssing, Arne

    2015-01-01

    The opening of the Arctic Ocean involved multiple stages of continental rifting and intrusion of extensive dyke swarms. To trace tectonomagmatic processes of the High Arctic, we present the first U–Pb ages for alkaline dyke swarms of North Greenland. Concordia ages of 80.8 ± 0.6 and 82.1 ± 1.5 Ma...... indicate that north–south and east–west dykes are coeval. The north–south dykes reflect initial east–west rifting that led to break-up along the Gakkel Ridge and formation of the Eurasia Basin. The east–west dykes reflect local variations in the stress field associated with reactivated Palaeozoic faults...

  13. Rifting Thick Lithosphere - Canning Basin, Western Australia

    Science.gov (United States)

    Czarnota, Karol; White, Nicky

    2016-04-01

    The subsidence histories and architecture of most, but not all, rift basins are elegantly explained by extension of ~120 km thick lithosphere followed by thermal re-thickening of the lithospheric mantle to its pre-rift thickness. Although this well-established model underpins most basin analysis, it is unclear whether the model explains the subsidence of rift basins developed over substantially thick lithosphere (as imaged by seismic tomography beneath substantial portions of the continents). The Canning Basin of Western Australia is an example where a rift basin putatively overlies lithosphere ≥180 km thick, imaged using shear wave tomography. Subsidence modelling in this study shows that the entire subsidence history of the account for the observed subsidence, at standard crustal densities, the lithospheric mantle is required to be depleted in density by 50-70 kg m-3, which is in line with estimates derived from modelling rare-earth element concentrations of the ~20 Ma lamproites and global isostatic considerations. Together, these results suggest that thick lithosphere thinned to > 120 km is thermally stable and is not accompanied by post-rift thermal subsidence driven by thermal re-thickening of the lithospheric mantle. Our results show that variations in lithospheric thickness place a fundamental control on basin architecture. The discrepancy between estimates of lithospheric thickness derived from subsidence data for the western Canning Basin and those derived from shear wave tomography suggests that the latter technique currently is limited in its ability to resolve lithospheric thickness variations at horizontal half-wavelength scales of <300 km.

  14. The Effects of Rapid Sedimentation upon Continental Breakup: Kinematic and Thermal Modeling of the Salton Trough, Southern California, Based upon Recent Seismic Images

    Science.gov (United States)

    Han, L.; Hole, J. A.; Lowell, R. P.; Stock, J. M.; Fuis, G. S.

    2016-12-01

    The Salton Seismic Imaging Project (SSIP) illuminated crustal and upper mantle structure of the Salton Trough, the northern-most rift segment of the Gulf of California plate boundary. The crust is 17-18 km thick and homogeneous for 100 km in the plate motion direction. New crust is being created by distributed rift magmatism, Colorado River sedimentation, and metamorphism of the sediment. A 5 km thick pre-existing crustal layer may still exist. The crust has not broken apart to enable initiation of seafloor spreading. A one-dimensional time-dependent kinematic and thermal model was developed to simulate these observations. We assume that all crustal layers are stretched uniformly during extension. Distributed mafic magmatism and sedimentation are added simultaneously to compensate for the crustal thinning. The ratio of magmatism to sedimentation is constrained by the seismic observations. Heat is transported by thermal conduction and by advection due to stretching of the crust. A constant temperature boundary at the Moho is used to represent partial melting in the upper mantle. Assuming a constant plate motion rate, the zone of active rifting extends linearly with time. The crustal thickness and internal structure also evolve with time. The model constraints are the observed seismic structure and heat flow. The model rapidly reaches quasi-steady state, and could continue for many millions of years. The observed seismic structure and heat flow are reproduced after 3 Myr. The yield strength profile calculated from lithology and model temperature indicates that ductile deformation in the middle and lower crust dominates the crustal rheology. Rapid sedimentation delays crustal breakup and the initiation of seafloor spreading by maintaining the thickness of the crust and keeping it predominantly ductile. This process probably occurs wherever a large river flows into an active rift driven by far-field extension. It may have built passive margins in many locations

  15. The modest seismicity of the northern Red Sea rift

    Science.gov (United States)

    Mitchell, Neil C.; Stewart, Ian C. F.

    2018-05-01

    Inferring tectonic movements from earthquakes (`seismotectonics') relies on earthquakes faithfully recording tectonic motions. In the northern half of the Red Sea, however, events of magnitude 5.0 and above are almost entirely absent from global catalogues, even though GPS and other plate motion data suggest that the basin is actively rifting at ˜10 mm yr-1. Seismic moments computed here from event magnitudes contributed to the International Seismology Centre (ISC) suggest that the moment release rate is more than an order of magnitude smaller than for the southern Red Sea and for the Southwest Indian Ridge (SWIR), which is spreading at a comparable rate to the central Red Sea and is more remote from recording stations. A smaller moment release rate in the northern Red Sea might be anticipated from its smaller spreading rate, but seismic coupling coefficients, which account for spreading rate variations, are also one order of magnitude smaller than for the other two areas. We explore potential explanations for this apparently reduced seismicity. The northern Red Sea is almost continuously covered with thick evaporites and overlying Plio-Pleistocene sediments. These deposits may have reduced the thickness of the seismogenic layer, for example, by elevating lithosphere temperatures by a thermal blanketing effect or by leading to excess pore fluid pressures that reduce effective stress. The presence of subdued seismicity here implies that tectonic movements can in places be poorly recorded by earthquake data and requires that alternative data be sought when investigating the active tectonics of sedimented rifts in particular.

  16. Lithosphere/asthenosphere interaction during continental breakup: preliminary isotopic date on the passive Galicia margin (North-Atlantic)

    International Nuclear Information System (INIS)

    Charpentier, S.; Kornprobst, J.; Chazot, G.; Cornen, G.

    1998-01-01

    The Galicia Margin ultramafic ridge has been cross-cut by diorites, pyroxenites and gabbros before the end of the rifting stage, and then by dolerites, after the continental break-full; it has been further overlaid by basaltic lava flows. The younger the rocks, the higher the initial ξ Nd (2.2-8.8). This evolution would be the result of the contamination of liquids extracted from the asthenosphere, by the enriched (ξ Ndi =4.0) and partially melted previous continental lithosphere. Time-decreasing contamination is related to progressive lithospheric thinning from the end to the beginning of oceanic spreading. (authors)

  17. Origin and structural evolution of the Cenozoic Rift System of Southeastern Brazil; Origem e evolucao estrutural do Sistema de Riftes Cenozoicos do Sudeste do Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Zalan, Pedro Victor [PETROBRAS S.A., Salvador, BA (Brazil). E e P. Gerencia de Gestao de Projetos Exploratorios], E-mail: zalan@petrobras.com.br; Oliveira, Joao Alberto Bach de

    2005-05-15

    The southeastern region of Brazil did not constitute a typical passive margin as one would expect from the premises of Plate Tectonics. After 25 m.y. that rifting (134-114 Ma) ceased an uplift of epeirogenic nature of the continental crust started in response to the drifting of the South American Plate over a thermal anomaly (Trindade hot spot). This Late Cretaceous (89-65 Ma) uplift was accompanied by intense alkaline (over non-extended crust) and basaltic (over thinned crust) magmatism. A marked absence of tectonism, however, also characterized this event. The resulting highlands extended over 300 000 km{sup 2} (Cretaceous Serra do Mar), and they were the main source area for coniacian-maastrichtian sediments of the Santos, Campos and Parana Basins. By the end of the rising (exactly at the K/T boundary) a widespread erosional surface had developed (Japi Surface) and leveled the top of the highlands at around 2 000 m (in relation to present-day sea level). This mega-plateau was adjacent to the subsiding Santos and Campos Basins and created an isostatically unstable situation. Gravitational collapse began around 7 m.y. after K/T, towards the depocenters of the basins. From Late Paleocene to Early Miocene (58-20 Ma) the continental crust broke and collapsed into a series of grabens, thus forming corridors (rifts) parallel to the current coastline. The ancient eastern edge of the Cretaceous Serra do Mar coincided with the current cretaceous hinge line of the Santos and Campos Basins. The topographic remnants of the mega-plateau nowadays form the highest parts of the Mantiqueira and Serra do Mar Ranges, modified by elastic rebound and tilting of the fault blocks. (author)

  18. Constraining the Thermal History of the Midcontinent Rift System with Clumped Isotopes and Organic Thermal Maturity Indices

    Science.gov (United States)

    Gallagher, T. M.; Sheldon, N. D.; Mauk, J. L.; Gueneli, N.; Brocks, J. J.

    2015-12-01

    The Mesoproterozoic (~1.1 Ga) North American Midcontinent Rift System (MRS) has been of widespread interest to researchers studying its economic mineral deposits, continental rifting processes, and the evolution of early terrestrial life and environments. For their age, the MRS rocks are well preserved and have not been deeply buried, yet a thorough understanding of the regional thermal history is necessary to constrain the processes that emplaced the mineral deposits and how post-burial alteration may have affected various paleo-records. To understand the thermal history of the MRS better, this study presents carbonate clumped isotope (Δ47) temperatures from deposits on the north and south sides of the rift. Due to the age of these deposits and known post-depositional processes, uncertainties exist about whether the clumped isotope signature has been reset. To test this, three generations of calcite were analyzed from the Nonesuch Fm. from the White Pine mine in Michigan including: sedimentary limestone beds, early diagenetic carbonate nodules, and hydrothermal calcite veins associated with the emplacement of copper mineralization. Clumped isotope temperatures from the White Pine mine range from 84 to 131°C, with a hydrothermal vein producing the hottest temperature. The clumped isotope temperature range for samples throughout the rift expands to 41-134°C. The hottest temperatures are associated with areas of known copper mineralization, whereas the coolest temperatures are found on the northern arm of the rift in Minnesota, far from known basin-bounding faults. Our hottest temperatures are broadly consistent with preexisting maximum thermal temperature estimates based on clay mineralogy, fluid inclusions, and organic geochemistry data. Clumped isotope results will also be compared to new hydrocarbon maturity data from the Nonesuch Fm., which suggest that bitumen maturities consistently fall within the early oil window across Michigan and Wisconsin.

  19. Intraplate seismicity along the Gedi Fault in Kachchh rift basin of western India

    Science.gov (United States)

    Joshi, Vishwa; Rastogi, B. K.; Kumar, Santosh

    2017-11-01

    The Kachchh rift basin is located on the western continental margin of India and has a history of experiencing large to moderate intraplate earthquakes with M ≥ 5. During the past two centuries, two large earthquakes of Mw 7.8 (1819) and Mw 7.7 (2001) have occurred in the Kachchh region, the latter with an epicenter near Bhuj. The aftershock activity of the 2001 Bhuj earthquake is still ongoing with migration of seismicity. Initially, epicenters migrated towards the east and northeast within the Kachchh region but, since 2007, it has also migrated to the south. The triggered faults are mostly within 100 km and some up to 200 km distance from the epicentral area of the mainshock. Most of these faults are trending in E-W direction, and some are transverse. It was noticed that some faults generate earthquakes down to the Moho depth whereas some faults show earthquake activity within the upper crustal volume. The Gedi Fault, situated about 50 km northeast of the 2001 mainshock epicenter, triggered the largest earthquake of Mw 5.6 in 2006. We have carried out detailed seismological studies to evaluate the seismic potential of the Gedi Fault. We have relocated 331 earthquakes by HypoDD to improve upon location errors. Further, the relocated events are used to estimate the b value, p value, and fractal correlation dimension Dc of the fault zone. The present study indicates that all the events along the Gedi Fault are shallow in nature, with focal depths less than 20 km. The estimated b value shows that the Gedi aftershock sequence could be classified as Mogi's type 2 sequence, and the p value suggests a relatively slow decay of aftershocks. The fault plane solutions of some selected events of Mw > 3.5 are examined, and activeness of the Gedi Fault is assessed from the results of active fault studies as well as GPS and InSAR results. All these results are critically examined to evaluate the material properties and seismic potential of the Gedi Fault that may be useful

  20. Neotectonism - An offshore evidence from eastern continental shelf off Visakhapatnam

    Digital Repository Service at National Institute of Oceanography (India)

    Subrahmanyam, A.S.; Venkateswarlu, K.; Murthy, K.S.R.; Rao, M.M.M.; Rao, K.M.; Raju, Y.S.N.

    tremor provide evidence of Neo-tectonic activity in this regio. The epicentral region falls in a shallow marine environment ideal for generating a geophysical database for stable continental region earthquakes....

  1. Historical volcanism and the state of stress in the East African Rift System

    Directory of Open Access Journals (Sweden)

    Geoffrey Wadge

    2016-09-01

    Full Text Available Crustal extension at the East African Rift System (EARS should, as a tectonic ideal, involve a stress field in which the direction of minimum horizontal stress is perpendicular to the rift. A volcano in such a setting should produce dykes and fissures parallel to the rift. How closely do the volcanoes of the EARS follow this? We answer this question by studying the 21 volcanoes that have erupted historically (since about 1800 and find that 7 match the (approximate geometrical ideal. At the other 14 volcanoes the orientation of the eruptive fissures/dykes and/or the axes of the host rift segments are oblique to the ideal values. To explain the eruptions at these volcanoes we invoke local (non-plate tectonic variations of the stress field caused by: crustal heterogeneities and anisotropies (dominated by NW structures in the Protoerozoic basement, transfer zone tectonics at the ends of offset rift segments, gravitational loading by the volcanic edifice (typically those with 1-2 km relief and magmatic pressure in central reservoirs. We find that the more oblique volcanoes tend to have large edifices, large eruptive volumes and evolved and mixed magmas capable of explosive behaviour. Nine of the volcanoes have calderas of varying ellipticity, 6 of which are large, reservoir-collapse types mainly elongated across rift (e.g. Kone and 3 are smaller, elongated parallel to the rift and contain active lava lakes (e.g. Erta Ale, suggesting different mechanisms of formation and stress fields. Nyamuragira is the only EARS volcano with enough sufficiently well-documented eruptions to infer its long-term dynamic behaviour. Eruptions within 7 km of the volcano are of relatively short duration (<100 days, but eruptions with more distal fissures tend to have greater obliquity and longer durations, indicating a changing stress field away from the volcano. There were major changes in long-term magma extrusion rates in 1977 (and perhaps in 2002 due to major along-rift

  2. 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 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...... and evolution of Precambrian lithosphere: A global study. Journal of Geophysical Research 106, 16387–16414.] show strong correlation with tectono-thermal ages and with regional variations in lithospheric thickness constrained by surface heat flow data and seismic velocities. In agreement with xenolith data...

  3. Origin of the Eastern Mediterranean: Neo-Tethys Rifting Along a Cryptic Cadomian Suture with Afro-Arabia

    Science.gov (United States)

    Avigad, D.; Abbo, A.; Gerdes, A.

    2016-12-01

    The East Mediterranean is a land-locked basin, a remnant of Neo-Tethys. It was formed in the Permo-Triassic as a result of the drift of the Tauride block from the Afro-Arabian margin of Gondwana. Herein we show that rather than being a genuine Afro-Arabia crustal fragment, the Tauride block is underlain by a Late Neoproterozoic Cadomian basement, which differs significantly from the Neoproterozoic "Pan-African" basement of NE Africa from which it was detached. Resembling other Cadomian terranes of Western Europe, the Tauride basement is chiefly a greywacke succession deposited in a mid to late Ediacaran back-arc basin formed on the periphery of Afro-Arabia, above the southward subducting proto-Tethys. The back-arc region was deformed and metamorphosed to various degrees and intruded by latest Ediacaran-Cambrian granites and volcanics during the Cadomian orogeny. Unlike the protracted (ca .300 m.y.) Neoproterozoic crustal evolution recorded in Afro-Arabia, the Cadomian basement of the Taurides evolved briefly, over ca. 50 m.y. We show that the entire cycle of sedimentation, metamorphism and magmatism in the Tauribe basement took place in the late Ediacaran-Cambrian and lagged after Neoproterozoic Pan-African orogeny and igneous activity in Afro-Arabia. The Cadomian orogeny had accreted the Taurides, and adjoining peri-Gandwana Cadomian terranes, with an already-consolidated Afro-Arabian continent. Permo-Triassic rifting of the East Mediterranean occurred close to the transition between these two domains. Rifting has thus been inherited from, and superimposed on late Ediacaran structures formed in front of the current Afro-Arabia margin of Gondwana during Cadomian orogeny. The boundary between the Cadomian edifice and the Pan-African crust of Afro-Arabia appears to lie nowadays on the southern margin of the Mediterranean, extending from Morocco in the west to Arabia in the east. Hence, the continental margin of the East Mediterranean, including in the Levant basin

  4. Cs-137 geochronology, epithermal neutron activation analysis, and principal component analysis of heavy metals pollution of the Black Sea anoxic continental shelf sediments

    Science.gov (United States)

    Duliu, O. G.; Cristache, C.; Oaie, G.; Culicov, O. A.; Frontasyeva, M. V.

    2009-04-01

    Anthropogenic Cs-137 Gamma-ray Spectroscopy assay (GrSA) performed at the National Institute of Research and Development for Physics and Nuclear Engineering - Bucharest (Romania) in correlation with Epithermal Neutrons Activation Analysis (ENAA) performed at the Joint Institute of Nuclear Researches - Dubna (Russia) were used to investigate a 50 cm core containing unconsolidated sediments collected at a depth of 600 m off Romanian town of Constantza, located in the anoxic zone of the Black Sea Continental Shelf. A digital radiography showed the presence of about 265 distinct laminae, 1 to 3 mm thick, a fact attesting a stationary sedimentary process, completely free of bioturbation. After being radiographed, the core was sliced into 45 segments whose thickness gradually increased from 0.5 to 5 cm, such that the minimum thickness corresponded to the upper part of the core. From each segment two aliquots of about 0.5 g and 50 g were extracted for subsequent ENAA and Cs-137 GrSA. The Cs-137 vertical profile evidenced two maxima, one of them was very sharp and localized at a depth of 1 cm and the other very broad, almost undistinguished at about 8 cm depth, the first one being attributed to 1986 Chernobyl accident. Based on these date, we have estimated a sedimentation ratio of about 0.5 mm/year, value taken as reference for further assessment of recent pollution history. By means of ENAA we have determined the vertical content of five presumed pollutants, e.i. Zn, As, Br, Sn and Sb and of Sc, as natural, nonpolluting element. In the first case, all five elements presented a more or less similar vertical profile consisting of an almost exponential decrease for the first 10 cm below sediment surface followed by a plateau until the core base, i.e. 50 cm below surface, dependency better described by the equation: c(z) = c0 [1+k exp (-z/Z)] (1) where: where c(z) represents the concentration vertical profile; z represents depth (in absolute value); c0 represents the plateau

  5. Oblique rift opening revealed by reoccurring magma injection in central Iceland

    KAUST Repository

    Ruch, Joel; Wang, Teng; Xu, Wenbin; Hensch, Martin; Jonsson, Sigurjon

    2016-01-01

    -field deformation has rarely been captured, hindering progress in understanding rifting mechanisms and evolution. Here we show new evidence of oblique rift opening during a rifting event influenced by pre-existing fractures and two centuries of extension deficit

  6. Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia

    KAUST Repository

    Pallister, John S.; McCausland, Wendy A.; Jonsson, Sigurjon; Lu, Zhong; Zahran, Hani M.; El-Hadidy, Salah Y.; Aburukbah, Abdallah; Stewart, Ian C F; Lundgren, Paul R.; White, Randal A.; Moufti, Mohammed Rashad H

    2010-01-01

    The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8 km long with 91 cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10 km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees. © 2010 Macmillan Publishers Limited. All rights reserved.

  7. Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia

    KAUST Repository

    Pallister, John S.

    2010-09-26

    The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8 km long with 91 cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10 km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees. © 2010 Macmillan Publishers Limited. All rights reserved.

  8. Broad accommodation of rift-related extension recorded by dyke intrusion in Saudi Arabia

    Science.gov (United States)

    Pallister, J.S.; McCausland, W.A.; Jonsson, Sigurjon; Lu, Z.; Zahran, H.M.; El, Hadidy S.; Aburukbah, A.; Stewart, I.C.F.; Lundgren, P.R.; White, R.A.; Moufti, M.R.H.

    2010-01-01

    The extensive harrat lava province of Arabia formed during the past 30 million years in response to Red Sea rifting and mantle upwelling. The area was regarded as seismically quiet, but between April and June 2009 a swarm of more than 30,000 earthquakes struck one of the lava fields in the province, Harrat Lunayyir, northwest Saudi Arabia. Concerned that larger damaging earthquakes might occur, the Saudi Arabian government evacuated 40,000 people from the region. Here we use geologic, geodetic and seismic data to show that the earthquake swarm resulted from magmatic dyke intrusion. We document a surface fault rupture that is 8 km long with 91 cm of offset. Surface deformation is best modelled by the shallow intrusion of a north-west trending dyke that is about 10 km long. Seismic waves generated during the earthquakes exhibit overlapping very low- and high-frequency components. We interpret the low frequencies to represent intrusion of magma and the high frequencies to represent fracturing of the crystalline basement rocks. Rather than extension being accommodated entirely by the central Red Sea rift axis, we suggest that the broad deformation observed in Harrat Lunayyir indicates that rift margins can remain as active sites of extension throughout rifting. Our analyses allowed us to forecast the likelihood of a future eruption or large earthquake in the region and informed the decisions made by the Saudi Arabian government to return the evacuees.

  9. 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.

  10. Local stresses, dyke arrest and surface deformation in volcanic edificesand rift zones

    Directory of Open Access Journals (Sweden)

    L. S. Brenner

    2004-06-01

    Full Text Available Field studies indicate that nearly all eruptions in volcanic edifices and rift zones are supplied with magma through fractures (dykes that are opened by magmatic overpressure. While (inferred dyke injections are frequent during unrest periods, volcanic eruptions are, in comparison, infrequent, suggesting that most dykes become arrested at certain depths in the crust, in agreement with field studies. The frequency of dyke arrest can be partly explained by the numerical models presented here which indicate that volcanic edifices and rift zones consisting of rocks of contrasting mechanical properties, such as soft pyroclastic layers and stiff lava flows, commonly develop local stress fields that encourage dyke arrest. During unrest, surface deformation studies are routinely used to infer the geometries of arrested dykes, and some models (using homogeneous, isotropic half-spaces infer large grabens to be induced by such dykes. Our results, however, show that the dyke-tip tensile stresses are normally much greater than the induced surface stresses, making it difficult to explain how a dyke can induce surface stresses in excess of the tensile (or shear strength while the same strength is not exceeded at the (arrested dyke tip. Also, arrested dyke tips in eroded or active rift zones are normally not associated with dyke-induced grabens or normal faults, and some dykes arrested within a few metres of the surface do not generate faults or grabens. The numerical models show that abrupt changes in Young's moduli(stiffnesses, layers with relatively high dyke-normal compressive stresses (stress barriers, and weak horizontal contacts may make the dyke-induced surface tensile stresses too small for significant fault or graben formation to occur in rift zones or volcanic edifices. Also, these small surface stresses may have no simple relation to the dyke geometry or the depth to its tip. Thus, for a layered crust with weak contacts, straightforward

  11. Creating global comparative analyses of tectonic rifts, monogenetic volcanism and inverted relief

    Science.gov (United States)

    van Wyk de Vries, Benjamin

    2016-04-01

    I have been all around the world, and to other planets and have travelled from the present to the Archaean and back to seek out the most significant tectonic rifts, monogenetic volcanoes and examples of inverted relief. I have done this to provide a broad foundation of the comparative analysis for the Chaîne des Puys - Limagne fault nomination to UNESCO world Heritage. This would have been an impossible task, if not for the cooperation of the scientific community and for Google Earth, Google Maps and academic search engines. In preparing global comparisons of geological features, these quite recently developed tools provide a powerful way to find and describe geological features. The ability to do scientific crowd sourcing, rapidly discussing with colleagues about features, allows large numbers of areas to be checked and the open GIS tools (such as Google Earth) allow a standardised description. Search engines also allow the literature on areas to be checked and compared. I will present a comparative study of rifts of the world, monogenetic volcanic field and inverted relief, integrated to analyse the full geological system represented by the Chaîne des Puys - Limagne fault. The analysis confirms that the site is an exceptional example of the first steps of continental drift in a mountain rift setting, and that this is necessarily seen through the combined landscape of tectonic, volcanic and geomorphic features. The analysis goes further to deepen the understanding of geological systems and stresses the need for more study on geological heritage using such a global and broad systems approach.

  12. Present-day Opening of the Natron Rift: Tectonic and Magmatic Processes at Work

    Science.gov (United States)

    Calais, E.; Dalaison, M.; Saria, E.; Doubre, C.; Masson, F.

    2017-12-01

    The young Natron basin (system, is an important locale to study the initial stage of continental rifting. It was the locus of a rarely observed tectono-magmatic event in July 2007, with slow slip on an intra-basin normal fault followed by a 10 km-long dike intrusion underneath the Gelai shield volcano. Here we report on a series of GPS observations over a 20-site network spanning the basin, measured repeatedly since 2013. We observe a long wavelength ( 200 km wide) extension with a horizontal rate of about 2 mm/yr, consistent with recentlty published regional kinematic models, and a velocity gradient centered on the west-bounding fault of the Natron basin. Initial models show that the data is best fit by a normal fault dipping 60 degrees to the east and slipping at a rate of 6 mm/yr. Superimposed on this long wavelength extension, we observe a smaller scale ( 30 km wide) extensional signal in the middle of the basin, roughly coincident with the location of the Gelai volcano, which was the locale of the 2007 seismic-magmatic crisis. We investigate the relative importance of tectonic faulting, post-diking relaxation following the 2007 intrusion (as observed for instance in Afar or Iceland after similar events), and melt recharge of the intra-basin magmatic system in present-day extension across this young segment of the East African Rift.

  13. Boron Isotopic Composition of Metasomatized Mantle Xenoliths from the Western Rift, East Africa

    Science.gov (United States)

    Hudgins, T.; Nelson, W. R.

    2017-12-01

    The Western Branch of the East African Rift System is known to have a thick lithosphere and sparse, alkaline volcanism associated with a metasomatized mantle source. Recent work investigating the relationship between Western Branch metasomatized mantle xenoliths and associated lavas has suggested that these metasomes are a significant factor in the evolution of the rift. Hydrous/carbonated fluids or silicate melts are potent metasomatic agents, however gaining insight into the source of a metasomatic agent proves challenging. Here we investigate the potential metasomatic fluid sources using B isotope analysis of mineral separates from Western Branch xenoliths. Preliminary SIMS analyses of phlogopite from Katwe Kikorongo and Bufumbira have and average B isotopic composition of -28.2‰ ± 5.1 and -16.4‰ ± 3.6, respectively. These values are are dissimilar to MORB (-7.5‰ ± 0.7; Marschall and Monteleone, 2015), primitive mantle (-10‰ ± 2; Chaussidon and Marty, 1995), and bulk continental crust (-9.1‰ ± 2.4; Marschall et al., 2017) and display significant heterogeneity across a relatively short ( 150km) portion of the Western Branch. Though displaying large variability, these B isotopic compositions are indicative of a metasomatic agent with a more negative B isotopic composition than MORB, PM, or BCC. These results are consistent with fluids that released from a subducting slab and may be related to 700 Ma Pan-African subduction.

  14. 3D crustal model of the US and Canada East Coast rifted margin

    Science.gov (United States)

    Dowla, N.; Bird, D. E.; Murphy, M. A.

    2017-12-01

    We integrate seismic reflection and refraction data with gravity and magnetic data to generate a continent-scale 3D crustal model of the US and Canada East Coast, extending north from the Straits of Florida to Newfoundland, and east from the Appalachian Mountains to the Central Atlantic Ocean. The model includes five layers separated by four horizons: sea surface, topography, crystalline basement, and Moho. We tested magnetic depth-to-source techniques to improve the basement morphology, from published sources, beneath the continental Triassic rift basins and outboard to the Jurassic ocean floor. A laterally varying density grid was then produced for the resultant sedimentary rock layer thickness based on an exponential decay function that approximates sedimentary compaction. Using constant density values for the remaining layers, we calculated an isostatically compensated Moho. The following structural inversion results of the Moho, controlled by seismic refraction depths, advances our understanding of rift-to-drift crustal geometries, and provides a regional context for additional studies.

  15. Petroleum resources assessment on the continental shelf basin of the south sea

    Energy Technology Data Exchange (ETDEWEB)

    Cheong, Tae Jin; Kwak, Young Hun; Oh, Jae Ho; Bong, Pil Youn; Son, Jin Dam; Lee, Ho Young; Ryu, Byung Jae; Son, Byoung Kook; Hwang, In Geul; Kwon, Young In; Lee, Young Ju; Kim, Hak Ju; Park, Kwan Soon; Park, Keun Pil; Sunwoo, Don [Korea Institute of Geology Mining and Materials, Taejon (Korea, Republic of)

    1996-12-01

    On the basis of diverse microfossils such as foraminifers, ostracods, micromulluscs, fossil spores and pollen and calcareous nannofossils derived from 14 drill holes, the sediments are divided into two part, the upper marine part and the lower nonmarine one. Marine part is subdivided into 4 foraminiferal zones and 3 nannofossil zones of Plio-Pleistocene age. In the lower part ranging from Oligocene to Late Miocene 4 palynomoph assemblages are established, which reflect climatic changes fluctuating between subtropical and cool temperate. Some fine sediments occurring in the South Sea continental shelf are rich in organic matter to be hydrocarbon source rock. The organic matter is mainly compared to type 3. However, lower part of the Geobuk-1 and Okdom-1 shows more oil prone geochemical characteristics than other wells. The kerosene is mixture type 1 and type 3 organic matter. The main oil generation zone located between 2,500 m and 3,000 m and gas generation zone from 3,500 m to 4,000 m approximately. Hydrocarbon accumulation could be expected in the trap formed in the period earlier than 10 Ma. as the hydrocarbon started to be expelled at 10 Ma. according to the modeling. Approximately 13,000 Line-km of multichannel seismic data integrated with 14 wells and gravity and magnetic data were analyzed to investigate the structural and stratigraphic evolution of southern part of offshore Korea. The northeast-southwest trending Taiwan-Sinzi Uplift Belt separates the area into two regions with different tectonic features, northwestern and southwestern regions. Tectonics of the rifting phase in both regions have been established on the basis of structural and stratigraphic analyses of depositional sequences and their seismic expressions. In the northwestern region, the large rifting activity appears to be occurred during Oligocene and pre-Oligocene, while in the southeastern region it occurred from late Miocene to early Pliocene. The potential hydrocarbon traps associated

  16. 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...

  17. 40Ar/39Ar dating and geochemistry of tholeiitic magmatism related to the early opening of the Central Atlantic rift

    International Nuclear Information System (INIS)

    Sebai, A.; Feraud, G.; Bertrand, H.; Hanes, J.

    1991-01-01

    Tholeiitic effusive and intrusive magnetism from Iberia, Morocco, Algeria and Mali, realted to the early opening of the Central Atlantic rift, was dated by the 40 Ar/ 39 Ar step-heating method. Four plateau ags, rangin from 203.7±2.7 to 197.1±1.8 Ma, were obtained on plagioclase from dykes from theTaoudenni area (Mali) and two lava flows from Morocco. The Messejana dyke (Iberia), which previously yielded discrepant conventional K-Ar dates, did not furnish any 40 Ar/ 39 Ar plateau dates. However, there is a clear inverse relationship between apparent age and K/Ca atio for gas fractions from a plagioclase separate (proportional to the alteration degree) which, combined with dates obtained on amphibole, biotite and pyroxene, allows us to determine an age of around 200 Ma for this body. These data, and those obtained on the Foum Zguid (Morocco) and the Ksi Ksou (Algeria) dykes, give evidence of a brief magmatic event (between 206 and 195 Ma ago) which affected a large area ca. 2500 km long. Trace-element modelling shows that most of these formations originated from a homogeneous, enriched, source material. Such a brief magmatic episode related to the opening of a continental rift is in agreement with findings in other magmatic provinces (e.g. the Deccan traps and the Red Sea rift, precisely dated by the 40 Ar/ 39 Ar method as well). (orig.)

  18. Aulacogens, the Donets Basin (Eastern Ukraine, Southwestern Russia, and the new classification of rifts: Towards a proper terminology

    Directory of Open Access Journals (Sweden)

    Ruban Dmitry A.

    2012-01-01

    Full Text Available Some intra-cratonic basins are traditionally called “aulacogens”. This term has persisted in the geoscience literature since its invention by Soviet geologists in the mid-20th century before the triumph of the plate tectonics, but its meaning has evolved. Attempts to change its meaning from descriptive to genetic have led to a broad spectrum of opinions on the definition of aulacogens. Some specialists related them to continental rifts, while others have restricted aulacogens to the only particular rift systems or peculiar stages in the evolution of young cratons. The Donets Basin is a typical aulacogen stretching across the southern margin of the East European Craton. A brief review of present knowledge of this basin shows that its nature is rather incompatible with the present understanding of aulacogens. Instead, the new classification of rifts offers a more precise terminology for its exact characteristics. It is suggested that the term “aulacogen” should only be restricted to those basins for which it has been applied historically.

  19. A bottom-driven mechanism for distributed faulting: Insights from the Gulf of California Rift

    Science.gov (United States)

    Persaud, P.; Tan, E.; Choi, E.; Contreras, J.; Lavier, L. L.

    2017-12-01

    The Gulf of California is a young oblique rift that displays a variation in rifting style along strike. Despite the rapid localization of strain in the Gulf at 6 Ma, the northern rift segment has the characteristics of a wide rift, with broadly distributed extensional strain and small gradients in topography and crustal thinning. Observations of active faulting in the continent-ocean transition of the Northern Gulf show multiple oblique-slip faults distributed in a 200 x 70 km2area developed some time after a westward relocation of the plate boundary at 2 Ma. In contrast, north and south of this broad pull-apart structure, major transform faults accommodate Pacific-North America plate motion. Here we propose that the mechanism for distributed brittle deformation results from the boundary conditions present in the Northern Gulf, where basal shear is distributed between the Cerro Prieto strike-slip fault (southernmost fault of the San Andreas fault system) and the Ballenas Transform fault. We hypothesize that in oblique-extensional settings whether deformation is partitioned in a few dip-slip and strike-slip faults, or in numerous oblique-slip faults may depend on (1) bottom-driven, distributed extension and shear deformation of the lower crust or upper mantle, and (2) the rift obliquity. To test this idea, we explore the effects of bottom-driven shear on the deformation of a brittle elastic-plastic layer with pseudo-three dimensional numerical models that include side forces. Strain localization results when the basal shear is a step-function while oblique-slip on numerous faults dominates when basal shear is distributed. We further investigate how the style of faulting varies with obliquity and demonstrate that the style of faulting observed in the Northern Gulf of California is reproduced in models with an obliquity of 0.7 and distributed basal shear boundary conditions, consistent with the interpreted obliquity and boundary conditions of the study area. Our

  20. 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

  1. The stress shadow induced by the 1975-1984 Krafla rifting episode

    KAUST Repository

    Maccaferri, F.

    2013-03-01

    It has been posited that the 1975–1984 Krafla rifting episode in northern Iceland was responsible for a significant drop in the rate of earthquakes along the Húsavík-Flatey Fault (HFF), a transform fault that had previously been the source of several magnitude 6–7 earthquakes. This compelling case of the existence of a stress shadow has never been studied in detail, and the implications of such a stress shadow remain an open question. According to rate-state models, intense stress shadows cause tens of years of low seismicity rate followed by a faster recovery phase of rate increase. Here, we compare the long-term predictions from a Coulomb stress model of the rifting episode with seismological observations from the SIL catalog (1995–2011) in northern Iceland. In the analyzed time frame, we find that the rift-induced stress shadow coincides with the eastern half of the fault where the observed seismicity rates are found to be significantly lower than expected, given the historical earthquake activity there. We also find that the seismicity rates on the central part of the HFF increased significantly in the last 17 years, with the seismicity progressively recovering from west to east. Our observations confirm that rate-state theory successfully describes the long-term seismic rate variation during the reloading phase of a fault invested by a negative Coulomb stress. Coincident with this recovery, we find that the b-value of the frequency-magnitude distribution changed significantly over time. We conclude that the rift-induced stress shadow not only decreased the seismic rate on the eastern part of the HFF but also temporarily modified how the system releases seismic energy, with more large magnitude events in proportion to small ones. This behavior is currently being overturned, as rift-induced locking is now being compensated by tectonic forcing.

  2. The stress shadow induced by the 1975-1984 Krafla rifting episode

    Science.gov (United States)

    Maccaferri, F.; Rivalta, E.; Passarelli, L.; Jónsson, S.

    2013-03-01

    It has been posited that the 1975-1984 Krafla rifting episode in northern Iceland was responsible for a significant drop in the rate of earthquakes along the Húsavík-Flatey Fault (HFF), a transform fault that had previously been the source of several magnitude 6-7 earthquakes. This compelling case of the existence of a stress shadow has never been studied in detail, and the implications of such a stress shadow remain an open question. According to rate-state models, intense stress shadows cause tens of years of low seismicity rate followed by a faster recovery phase of rate increase. Here, we compare the long-term predictions from a Coulomb stress model of the rifting episode with seismological observations from the SIL catalog (1995-2011) in northern Iceland. In the analyzed time frame, we find that the rift-induced stress shadow coincides with the eastern half of the fault where the observed seismicity rates are found to be significantly lower than expected, given the historical earthquake activity there. We also find that the seismicity rates on the central part of the HFF increased significantly in the last 17 years, with the seismicity progressively recovering from west to east. Our observations confirm that rate-state theory successfully describes the long-term seismic rate variation during the reloading phase of a fault invested by a negative Coulomb stress. Coincident with this recovery, we find that the b-value of the frequency-magnitude distribution changed significantly over time. We conclude that the rift-induced stress shadow not only decreased the seismic rate on the eastern part of the HFF but also temporarily modified how the system releases seismic energy, with more large magnitude events in proportion to small ones. This behavior is currently being overturned, as rift-induced locking is now being compensated by tectonic forcing.

  3. Esophageal cancer in north rift valley of western Kenya | Wakhisi ...

    African Journals Online (AJOL)

    Esophageal cancer in north rift valley of western Kenya. ... Our finding also contrast with an earlier reported study that indicated that Rift Valley is a low prevalence area for this type of cancer. The mean age ... This may lead to identification of molecular biomarkers to be used in future for the early detection of this neoplasm.

  4. Rift Valley fever potential mosquito vectors and their infection status ...

    African Journals Online (AJOL)

    Background: Rift Valley fever (RVF) is a mosquito-borne viral zoonotic disease. Rift Valley fever virus (RVFV) has been isolated from more than 40 species of mosquitoes from eight genera. This study was conducted to determine the abundance of potential mosquito vectors and their RVFV infection status in Ngorongoro ...

  5. Salt Lakes of the African Rift System: A Valuable Research ...

    African Journals Online (AJOL)

    Salt Lakes of the African Rift System: A Valuable Research Opportunity for Insight into Nature's Concenrtated Multi-Electrolyte Science. JYN Philip, DMS Mosha. Abstract. The Tanzanian rift system salt lakes present significant cultural, ecological, recreational and economical values. Beyond the wealth of minerals, resources ...

  6. Breaking Ice 2: A rift system on the Ross Ice Shelf as an analog for tidal tectonics on icy moons

    Science.gov (United States)

    Brunt, K. M.; Hurford, T., Jr.; Schmerr, N. C.; Sauber, J. M.; MacAyeal, D. R.

    2016-12-01

    Ice shelves are the floating regions of the polar ice sheets. Outside of the influence of the narrow region of their grounding zone, they are fully hydrostatic and strongly influenced by the ocean tides. Recent observational and modeling studies have assessed the effect of tides on ice shelves, including: the tidal influence on the ice-shelf surface height, which changes by as much as 6 to 7 m on the southern extreme of the Ronne-Filchner Ice Shelf; the tidal modulation of the ice-shelf horizontal flow velocities, which changes the mean ice-flow rate by as much as two fold on the Ross Ice Shelf; and the tidal contribution to fracture and rift propagation, which eventually leads to iceberg calving. Here, we present the analysis of 16 days of continuous GPS data from a rift system near the front of the Ross Ice Shelf. While the GPS sites were installed for a different scientific investigation, and not optimized to assess tidal rifting mechanics, they provide a first-order sense of the tidal evolution of the rift system. These analyses can be used as a terrestrial analog for tidal activity on icy satellites, such as Europa and Enceladus, moons of Jupiter and Saturn, respectively. Using remote sensing and modeling of the Ross Ice Shelf rift system, we can investigate the geological processes observed on icy satellites and advance modeling efforts of their tidal-tectonic evolution.

  7. Más allá del valle del Rift: la evidencia arqueológica del Plio-Pleistoceno fuera de África Oriental

    Directory of Open Access Journals (Sweden)

    Ignacio de la TORRE SÁINZ

    2009-12-01

    Full Text Available RESUMEN: La mayor parte de los trabajos sobre la arqueología del Plio-Pleistoceno en Africa se centran en la evidencia del valle del Rift. Allí existen extensas columnas crono-estratigráficas que permiten contextualizar de forma fiable los conjuntos arqueo-paleontológicos y reconstruir así la secuencia evolutiva de nuestro género. Se suele obviar de esa forma la evidencia del resto de África, donde existen grandes áreas sedimentarias muy poco conocidas desde un punto de vista arqueológico. En este trabajo se reflexionará sobre el registro arqueológico plio-pleistocénico en las zonas alejadas del valle del Rift, y se tratará de reconstruir cómo fue la primera colonización del continente africano.ABSTRACT: Most of the works about the African Plio-Pleistocene are focused on the Rift Valley evidence. In this area there are long chrono-stratigraphical columns that build reliable contexts for the archaeo-paleontological sites, providing the reconstruction of the evolutionary sequence of our Genus. Evidence on the rest of Africa, where there are poor-known sedimentary areas from an archaeological point of view, is usually dismissed. In this work we will reflect on the Plio-Pleistocene record beyond the Rift Valley, trying to reconstruct the first settlement of the African continent.

  8. Peridotites and mafic igneous rocks at the foot of the Galicia Margin: an oceanic or continental lithosphere? A discussion

    Energy Technology Data Exchange (ETDEWEB)

    Korprobst, J.; Chazot, G.

    2016-10-01

    An ultramafic/mafic complex is exposed on the sea floor at the foot of the Galicia Margin (Spain and Portugal). It comprises various types of peridotites and pyroxenites, as well as amphibole-diorites, gabbros, dolerites and basalts. For chronological and structural reasons (gabbros were emplaced within peridotites before the continental break-up) this unit cannot be assigned to the Atlantic oceanic crust. The compilation of all available petrological and geochemical data suggests that peridotites are derived from the sub-continental lithospheric mantle, deeply transformed during Cretaceous rifting. Thus, websterite dykes extracted from the depleted MORB mantle reservoir (DMM), were emplaced early within the lithospheric harzburgites; subsequent boudinage and tectonic dispersion of these dykes in the peridotites, during deformation stages at the beginning of rifting, resulted in the formation of fertile but isotopically depleted lherzolites. Sterile but isotopically enriched websterites, would represent melting residues in the peridotites, after significant partial melting and melt extraction related to the thermal erosion of the lithosphere. The latter melts are probably the source of brown amphibole metasomatic crystallization in some peridotites, as well as of the emplacement of amphibole-diorite dykes. Melts directly extracted from the asthenosphere were emplaced as gabbro within the sub-continental mantle. Mixing these DMM melts together with the enriched melts extracted from the lithosphere, provided the intermediate isotopic melt-compositions - in between the DMM and Oceanic Islands Basalts reservoir - observed for the dolerites and basalts, none of which are characterized by a genuine N-MORB signature. An enriched lithospheric mantle, present prior to rifting of the Galicia margin, is in good agreement with data from the Messejana dyke (Portugal) and more generally, with those of all continental tholeiites of the Central Atlantic Magmatic Province (CAMP

  9. Magmatic dyking and recharge in the Asal Rift, Republic of Djibouti

    Science.gov (United States)

    Peltzer, G.; Harrington, J.; Doubre, C.; Tomic, J.

    2012-12-01

    The Asal Rift, Republic of Djibouti, has been the locus of a major magmatic event in 1978 and seems to have maintained a sustained activity in the three decade following the event. We compare the dyking event of 1978 with the magmatic activity occurring in the rift during the 1997-2008 time period. We use historical air photos and satellite images to quantify the horizontal opening on the major faults activated in 1978. These observations are combined with ground based geodetic data acquired between 1973 and 1979 across the rift to constrain a kinematic model of the 1978 rifting event, including bordering faults and mid-crustal dykes under the Asal Rift and the Ghoubbet Gulf. The model indicates that extension was concentrated between the surface and a depth of 3 km in the crust, resulting in the opening of faults, dykes and fissures between the two main faults, E and gamma, and that the structure located under the Asal Rift, below 3 km, deflated. These results suggest that, during the 1978 event, magmatic fluids transferred from a mid-crustal reservoir to the shallow structures, injecting dykes and filling faults and fissures, and reaching the surface in the Ardoukoba fissural eruption. Surface deformation observed by InSAR during the 1997-2008 decade reveals a slow, yet sustained inflation and extension across the Asal Rift combined with continuous subsidence of the rift inner floor. Modeling shows that these observations cannot be explained by visco-elastic relaxation, a process, which mostly vanishes 20 to 30 years after the 1978 event. However, the InSAR observations over this decade are well explained by a kinematic model in which an inflating body is present at mid-crustal depth, approximately under the Fieale caldera, and shallow faults accommodate both horizontal opening and down-dip slip. The total geometric moment rate, or inflation rate, due to the opening of the mid-crustal structure and the deeper parts of the opening faults is 3 106 m3yr. Such a

  10. Two-dimensional, average velocity field across the Asal Rift, Djibouti from 1997-2008 RADARSAT data

    Science.gov (United States)

    Tomic, J.; Doubre, C.; Peltzer, G.

    2009-12-01

    Located at the western end of the Aden ridge, the Asal Rift is the first emerged section of the ridge propagating into Afar, a region of intense volcanic and tectonic activity. We construct a two-dimensional surface velocity map of the 200x400 km2 region covering the rift using the 1997-2008 archive of InSAR data acquired from ascending and descending passes of the RADARSAT satellite. The large phase signal due to turbulent troposphere conditions over the Afar region is mostly removed from the 11-year average line of sight (LOS) velocity maps, revealing a clear deformation signal across the rift. We combine the ascending and descending pass LOS velocity fields with the Arabia-Somalia pole of rotation adjusted to regional GPS velocities (Vigny et al., 2007) to compute the fields of the vertical and horizontal, GPS-parallel components of the velocity over the rift. The vertical velocity field shows a ~40 km wide zone of doming centered over the Fieale caldera associated with shoulder uplift and subsidence of the rift inner floor. Differential movement between shoulders and floor is accommodated by creep at 6 mm/yr on Fault γ and 2.7 mm/yr on Fault E. The horizontal field shows that the two shoulders open at a rate of ~15 mm/yr, while the horizontal velocity decreases away from the rift to the plate motion rate of ~11 mm/yr. Part of the opening is concentrated on faults γ (5 mm/yr) and E (4 mm/yr) and about 4 mm/yr is distributed between Fault E and Fault H in the southern part of the rift. The observed velocity field along a 60 km-long profile across the eastern part of the rift can be explained with a 2D mechanical model involving a 5-9 km-deep, vertical dyke expanding horizontally at a rate of 5 cm/yr, a 2 km-wide, 7 km-deep sill expanding vertically at 1cm/yr, and down-dip and opening of faults γ and E. Results from 3D rift models describing along-strike velocity decrease away from the Goubbet Gulf and the effects of a pressurized magma chamber will be

  11. 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

  12. Major and micro seismo-volcanic crises in the Asal Rift, Djibouti

    Science.gov (United States)

    Peltzer, G.; Doubre, C.; Tomic, J.

    2009-05-01

    The Asal-Ghoubbet Rift is located on the eastern branch of the Afar triple junction between the Arabia, Somalia, and Nubia tectonic plates. The last major seismo-volcanic crisis on this segment occurred in November 1978, involving two earthquakes of mb=5+, a basaltic fissure eruption, the development of many open fissures across the rift and up to 80 cm of vertical slip on the bordering faults. Geodetic leveling revealed ~2 m of horizontal opening of the rift accompanied by ~70 cm of subsidence of the inner-floor, consistent with models of the elastic deformation produced by the injection of magma in a system of two dykes. InSAR data acquired at 24-day intervals during the last 12 years by the Canadian Radarsat satellite over the Asal Rift show that the two main faults activated in 1978 continue to slip with periods of steady creep at rates of 0.3-1.3 mm/yr, interrupted by sudden slip events of a few millimeters, in 2000 and 2003. Slip events are coincident with bursts of micro earthquakes distributed around and over the Fieale volcanic center in the eastern part of the Asal Rift. In both cases (the 1978 crisis and micro-slip events), the observed geodetic moment released by fault slip exceeds by a few orders of magnitude the total seismic moment released by earthquakes over the same period. Aseismic fault slip is likely to be the faults response to a changing stress field associated with a volcanic process and not due to dry friction on faults. Sustained injection of magma (1978 crisis) and/or crustal fluids (micro-slip events) in dykes and fissures is a plausible mechanism to control fluid pressure in the basal parts of faults and trigger aseismic slip. In this respect, the micro-events observed by InSAR during a 12-year period of low activity in the rift and the 1978 seismo-volcanic episode are of same nature.

  13. Continental breakup by oblique extension: the Gulf of California

    Science.gov (United States)

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

    2017-12-01

    We address two aspects of oblique extension: 1) the evolution of pull-apart basins, and how/when they may evolve into seafloor spreading segments; and 2) the formation of microcontinents. The Gulf of California formed by oblique extension. Breakup resulted in oceanic crust generation in the southern and central parts, while in the northern Gulf/Salton Trough a thick layer of (meta-)sediments overlies thinned continental crust. We propose a simple mechanism to explain this N-S variation. We assume that oblique rifting of the proto-Gulf province resulted in pull-apart basins, and use numerical models to show that such pull-apart basins do not develop into seafloor spreading segments when their length-to-width ratios are small, as is the case in the northern Gulf. In the central and southern Gulf the length-to-width ratios were larger, promoting continent rupture. The mechanisms behind this fate of pull-apart basins will be discussed in the presentation. In the southern Gulf, potential field models show that the Tamayo Bank in the southern Gulf is likely a microcontinent, separated from the main continent by the Tamayo trough. The thickness of the ocean crust in the Tamayo trough is anomalously small, suggesting that initial seafloor spreading was magma-starved and unsuccessful, causing the location of rifting and seafloor spreading to jump. As a consequence a sliver of continent broke off, forming the microcontinent. We suggest that worldwide this may be a common process for microcontinent formation.

  14. Basement tectonics and flexural subsidence along western continental margin of India

    Directory of Open Access Journals (Sweden)

    D.K. Pandey

    2017-09-01

    Full Text Available The Paleocene-recent post-rift subsidence history recorded in the Mumbai Offshore Basin off western continental margin of India is examined. Results obtained through 2-D flexural backstripping modelling of new seismic data reveal considerable thermo-tectonic subsidence over last ca. 56 Myr. Reverse post-rift subsidence modelling with variable β stretching factor predicts residual topography of ca. 2000 m to the west of Shelf Margin Basin and fails to restore late Paleocene horizon and the underlying igneous basement to the sea level. This potentially implies that: (1 either the igneous basement formed during the late Cretaceous was emplaced under open marine environs; or (2 a laterally varying cumulative subsidence occurred within Mumbai Offshore Basin (MOB during ca. 68 to ca. 56 Ma. Pre-depositional topographic variations at ca. 56 Ma across the basin could be attributed to the extensional processes such as varied lower crustal underplating along Western Continental Margin of India (WCMI. Investigations about basement tectonics after unroofing of sediments since late Paleocene from this region support a transitional and heavily stretched nature of crust with high to very high β factors. Computations of past sediment accumulation rates show that the basin sedimentation peaked during late Miocene concurrently with uplift of Himalayan–Tibetan Plateau and intensification of Indian monsoon system. Results from basin subsidence modelling presented here may have significant implications for further studies attempting to explore tectono–climatic interactions in Asia.

  15. Monitoring of fumarole discharge during the 1975-1982 rifting in Krafla volcanic center, North Iceland

    Energy Technology Data Exchange (ETDEWEB)

    Oskarsson, N.

    1984-09-01

    Fumarole discharge chemistry in the Krafla geothermal field changed regionally during the 1975-1982 rifting activity. The discharge chemistry previously encountered in the Krafla fumarole grounds was masked by a carbon dioxide-rich gas during the first weeks of rifting. The new discharge composition remained unchanged until 1983 when the first signs of recovery of the previous equilibrium composition appeared at the margins of the area. The outgassing carbon dioxide is released from the deep aquifers beneath the area by the interaction of magmatic gas with the hydrothermal system. In addition to juvenile magmatic carbon the outgassing contains carbon released from the hydrothermal system upon reaction with acid magmatic gases. Increased boiling of the hydrothermal fumaroles was induced by the lowering of the partial pressure of steam due to increased gas content in the fumarole conduits. At the center of rifting activity above the magma chamber the fumarole discharge was temporarily mixed with magmatic gases during local effusive activity. Hydrogen was the dominating magmatic gas in that discharge due to the preferred degassing of hydrogen from magma at shallow levels. These ''hydrogen pulses'' increased in magnitude and duration towards the end of rifting in 1982. The discharge chemistry correlates with the expansion of the magma reservoir of the volcano (regional change of long duration) and local volcanism (short-lived change, hydrogen pulses). The chemical monitoring of fumaroles in Krafla shows that the chemical surveillance of volcanos needs rapid methods for sampling and complete chemical analysis which can be interpreted in terms of reactions and magmatic processes.

  16. The transition from diffuse to focused extension: Modeled evolution of the West Antarctic Rift system

    Science.gov (United States)

    Huerta, Audrey D.; Harry, Dennis L.

    2007-03-01

    Two distinct stages of extension are recognized in the West Antarctic Rift system (WARS). During the first stage, beginning in the Late Cretaceous, extension was broadly distributed throughout much of West Antarctica. A second stage of extension in the late Paleogene was focused primarily in the Victoria Land Basin, near the boundary with the East Antarctic craton. The transition to focused extension was roughly coeval with volcanic activity and strike-slip faulting in the adjacent Transantarctic Mountains. This spatial and temporal correspondence suggests that the transition in extensional style could be the result of a change in plate motions or impingement of a plume. Here we use finite element models to study the processes and conditions responsible for the two-stage evolution of rifting in the WARS. Model results indicate that the transition from a prolonged period of broadly distributed extension to a later period of focused rifting did not require a change in the regional stress regime (changes in plate motion), or deep mantle thermal state (impingement of a plume). Instead, we attribute the transition from diffuse to focused extension to an early stage dominated by the initially weak accreted lithosphere of West Antarctica, and a later stage that concentrated around a secondary weakness located at the boundary between the juvenile West Antarctica lithosphere and Precambrian East Antarctic craton. The modeled transition in extension from the initially weak West Antarctica region to the secondary weakness at the West Antarctic-East Antarctic boundary is precipitated by strengthening of the West Antarctica lithosphere during syn-extensional thinning and cooling. The modeled syn-extensional strengthening of the WARS lithosphere promotes a wide-rift mode of extension between 105 and ˜ 65 Ma. By ˜ 65 Ma most of the extending WARS region becomes stronger than the area immediately adjacent to the East Antarctic craton and extension becomes concentrated near the

  17. Guidebook to Rio Grande rift in New Mexico

    Science.gov (United States)

    Hawley, J.W.

    1978-01-01

    Discusses the details of geologic features along the rift zone. Included are short papers on topics relative to the overall region. These papers and the road logs are of special interest to any one pursuing further study of the rift. This book is a comprehensive guide to the middle and late Cenozoic geology of the Rio Grande region of Colorado and New Mexico. Though initially used on field trips for the International Symposium on Tectonics and Magmatism of the Rio Grande rift, the guidebook will be useful to anyone interested in the Cenozoic history of the 600-mi-long area extending from central Colorado to El Paso, Texas.

  18. Footwall degradation styles and associated sedimentary facies distribution in SE Crete: Insights into tilt-block extensional basins on continental margins

    Science.gov (United States)

    Alves, Tiago M.; Cupkovic, Tomas

    2018-05-01

    Depositional facies resulting from footwall degradation in extensional basins of SE Crete are studied based on detailed geological maps, regional transects, lithological columns and outcrop photos. During an extensional episode affecting Crete in the late Miocene-early Pliocene, depocentres trending N20°E and N70°E were filled with fan deltas, submarine mass-wasting deposits, sandy turbidites and fine-grained hemipelagites sourced from both nearby and distal sediment sources. Deposition of proximal continental and shallow-marine units, and relatively deep (marine) turbidites and mass-transport deposits, occurred within a complex mosaic of tectonically controlled depocentres. The new geological maps and transects in this work reveal that depositional facies in SE Crete were controlled by: a) their relative proximity to active faults and uplifting footwall blocks, b) the relative position (depth and relative height above sea level) of hanging-wall basins, and c) the nature of the basement units eroded from adjacent footwall blocks. Distal sediment sources supplied background siliciclastic sediment ('hemipelagites'), which differ markedly from strata sourced from local footwalls. In parallel, mass-transport of sediment was ubiquitous on tectonically active slopes, and so was the presence of coarse-grained sediment with sizes varying from large blocks > 50 m-wide to heterolithic mass-transport deposits and silty-sandy turbidites. We expect similar tectono-sedimentary settings to have predominated in tectonically active Miocene basins of the eastern Mediterranean, in which hydrocarbon exploration is occurring at present, and on rifted continental margins across the world.

  19. The role of inheritance in structuring hyperextended rift systems

    Science.gov (United States)

    Manatschal, Gianreto; Lavier, Luc; Chenin, Pauline

    2015-04-01

    A long-standing question in Earth Sciences is related to the importance of inheritance in controlling tectonic processes. In contrast to physical processes that are generally applicable, assessing the role of inheritance suffers from two major problems: firstly, it is difficult to appraise without having insights into the history of a geological system; and secondly all inherited features are not reactivated during subsequent deformation phases. Therefore, the aim of our presentation is to give some conceptual framework about how inheritance may control the architecture and evolution of hyperextended rift systems. We use the term inheritance to refer to the difference between an "ideal" layer-cake type lithosphere and a "real" lithosphere containing heterogeneities and we define 3 types of inheritance, namely structural, compositional and thermal inheritance. Moreover, we assume that the evolution of hyperextended rift systems reflects the interplay between their inheritance (innate/"genetic code") and the physical processes at play (acquired/external factors). Thus, by observing the architecture and evolution of hyperextended rift systems and integrating the physical processes, one my get hints on what may have been the original inheritance of a system. Using this approach, we focus on 3 well-studied rift systems that are the Alpine Tethys, Pyrenean-Bay of Biscay and Iberia-Newfoundland rift systems. For the studied examples we can show that: 1) strain localization on a local scale and during early stages of rifting is controlled by inherited structures and weaknesses 2) the architecture of the necking zone seems to be influenced by the distribution and importance of ductile layers during decoupled deformation and is consequently controlled by the thermal structure and/or the inherited composition of the curst 3) the location of breakup in the 3 examples is not significantly controlled by the inherited structures 4) inherited mantle composition and rift

  20. New foci of cutaneous leishmaniasis in central Kenya and the Rift Valley.

    Science.gov (United States)

    Sang, D K; Okelo, G B; Ndegwa, C W; Ashford, R W

    1993-01-01

    Active case detection and investigations of sandfly resting places in suspected transmission sites of cutaneous leishmaniasis in central Kenya and the Rift Valley resulted in the identification of several foci of the disease in Samburu, Isiolo, Laikipia, Nakuru and Nyandarua districts. The foci occurred in areas ranging from semi-arid lowlands at 400 m altitude to highland plateaux at 2500 m, including the floor of the Rift Valley, and were mostly inhabited by recently settled communities, nomads and migrant charcoal burners. Four species of Phlebotomus, 3 of the subgenus Larroussius (P. pedifer, P. aculeatus and P. guggisbergi) and one Paraphlebotomus (P. saevus) were collected from caves, rock crevices and tree hollows found in river valleys and in lava flows.

  1. The seismicity related to the southern part of the Kenya Rift

    Science.gov (United States)

    Hollnack, D.; Stangl, R.

    1998-04-01

    In 1990 the Geology Department of the University of Nairobi started to build up a seismological network for Kenya, which has been operating since 1993. In this paper the actual state of this seismological network is described. Additionally, the first results on the seismic activity in the southern part of Kenya and adjacent areas between October 1993 and August 1996 are presented and are compared with historical data. Out of more than 2000 recorded local earthquakes 435 could be localised within the study area with local magnitudes of up to 5. The distribution of the events shows three areas of prominent seismicity: the Rift Valley between Nakuru and northern Tanzania; the area northeast of Kilimanjaro; and the Nyanza Rift in western Kenya. In a first attempt to assess the seismic hazard for the study area, a seismic energy map for the period of observation is given.

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

    Science.gov (United States)

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

    2016-12-01

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

  3. Cratonic roots and lower crustal seismicity: Investigating the role of deep intrusion in the Western rift, Africa

    Science.gov (United States)

    Drooff, C.; Ebinger, C. J.; Lavayssiere, A.; Keir, D.; Oliva, S. J.; Tepp, G.; Gallacher, R. J.

    2017-12-01

    Improved seismic imaging beneath the African continent reveals lateral variations in lithospheric thickness, and crustal structure, complementing a growing crust and mantle xenolith data base. Border fault systems in the active cratonic rifts of East Africa are characterized by lower crustal seismicity, both in magmatic sectors and weakly magmatic sectors, providing constraints on crustal rheology and, in some areas, magmatic fluid migration. We report new seismicity data from magmatic and weakly magmatic sectors of the East African rift zone, and place the work in the context of independent geophysical and geochemical studies to models for strain localization during early rifting stages. Specifically, multidisciplinary studies in the Magadi Natron rift sectors reveal volumetrically large magmatic CO2 degassing along border faults with seismicity along projections of surface dips to the lower crust. The magmatic CO2 degassing and high Vp/Vs ratios and reflectivity of the lower crust implies that the border fault serves a conduit between the lower crustal underplating and the atmospheric. Crustal xenoliths in the Eastern rift sector indicate a granulitic lower crust, which is relatively weak in the presence of fluids, arguing against a strong lower crust. Within magmatic sectors, seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Within some weakly magmatic sectors, lower crustal earthquakes also occur along projections of border faults to the lower crust (>30 km), and they are prevalent in areas with high Vp/Vs in the lower crust. Within the southern Tanganyika rift, focal mechanisms are predominantly normal with steep nodal planes. Our comparative studies suggest that pervasive metasomatism above a mantle plume, and melt extraction in thin zones between cratonic roots, lead to

  4. Impacts of continental arcs on global carbon cycling and climate

    Science.gov (United States)

    Lee, C. T.; Jiang, H.; Carter, L.; Dasgupta, R.; Cao, W.; Lackey, J. S.; Lenardic, A.; Barnes, J.; McKenzie, R.

    2017-12-01

    greenhouse climates, but after they die magmatically, they remain geomorphically active and become a net CO2 sink, helping to drive climate towards cooler conditions. Tectonic oscillations that drive fluctuations in the activity of continental arcs thus may be responsible for greenhouse-icehouse oscillations in the Phanerozoic.

  5. Relations between tectonics and sedimentation along the Eastern Sardinian margin (Western Tyrrhenian Sea) : from rifting to reactivation

    Science.gov (United States)

    Gaullier, Virginie; Chanier, Frank; Vendeville, Bruno; Lymer, Gaël; Maillard, Agnès; Thinon, Isabelle; Lofi, Johanna; Sage, Françoise; Giresse, Pierre; Bassetti, Maria-Angela

    2014-05-01

    salt and its Late Messinian and Early Pliocene brittle overburden. "METYSAR" fieldwork onshore was conducted in the Orosei region and showed that the main present-day Cedrino river follows the trend of a paleo-valley that cuts through the underlying granitic basement and alterites. These deposits, along with the basement, were likely eroded during Messinian times, then reworked during a marine transgression. Micro-fauna in these fine-grained marine sediments are of Upper Pliocene age. The strata dip by 20° to 30° and trend NNE-SSW, a direction which is sub-parallel to the main tectonic structures involved in the rifting of the margin. The tilted Pliocene strata were overlain by volcanic flows, some dating from Upper Pliocene time. Field mapping has evidenced that there was a paleo-topographic relief, trending NNE-SSW, that controlled the sediment deposition. These results indicate that the post-Messinian tectonic activity, which is also visible offshore, controlled the sedimentary architecture and the paleogeography of this area. Onshore, there are signs of neither Lower-Pliocene marine deposits nor Gilbert deltas. The absence of such sedimentary edifices, which are characteristic of the Pliocene refilling of the Mediterranean basin are clues about significant post-rift vertical movements in the Tyrrhenian sea.

  6. Artificial Synthesis of Conserved Segment S Gene Fragment of Rift ...

    African Journals Online (AJOL)

    Fragment of Rift Valley Fever Virus and Preliminary Study of Its Reverse ... blindness, encephalitis, hemorrhagic hepatitis, and, in serious ... over 30 mosquito species [5] of mosquito. As ..... RVFV, as a mosquito-borne virus for a multitude.

  7. SALT LAKES OF THE AFRICAN RIFT SYSTEM: A VALUABLE ...

    African Journals Online (AJOL)

    dell

    rift lake locations fitting the description. “endorheic” (closed) ... updating, as well as harness the scholarship ... Ionic concentrations are location and season .... Progress and effects of weathering of Lake Natron Basin rock formations; a hill in.

  8. Comparison of sampling techniques for Rift Valley Fever virus ...

    African Journals Online (AJOL)

    time for trapping potential vectors for Rift Valley Fever virus. ..... Krockel, U., Rose, A., Eiras, A.E. & Geier, M. (2006) New tools for surveillance of adult yellow fever ... baited trapping systems for sampling outdoor mosquito populations in ...

  9. Continental breakup of the Central Atlantic and the initiation of the southern Central Atlantic Magmatic Province: revisiting the role of a mantle plume

    Science.gov (United States)

    Rohrman, M.

    2017-12-01

    Central Atlantic breakup is strongly associated with magmatism of the Central Atlantic Magmatic Province (CAMP), although the exact mechanism, as well as the temporal and spatial relations, have so far been poorly constrained. Here, I propose a mantle plume origin for the 200 Ma southern Central Atlantic Province (CAMP), based on an original plume conduit location off southeastern Florida, linking Early Jurassic rift systems: One rift arm is defined by the Takutu rift in present-day Guyana and Brazil, extending all the way past the Demerara Rise. This rift is linking up with a second arm from the Bahamas basin to the Blake Plateau basin. Finally, there is the third, failed rift between the Demerara Rise and the Guinea Plateau. This rift system post-dates earlier Triassic rift systems along the US eastcoast and in the subsurface of Arkansas, Texas, the Gulf of Mexico and northern South America. Chronostratigraphic analysis of outcrop, wells and seismic data near the proposed conduit, suggest initial Rhaetian uplift, followed by dike/sill intrusions feeding flood basalts and the initiation of igneous centers at the triple point. The latter resulted in various subsequent uplift and subsidence events, as a result of volcanic construction and erosion. The load of the volcanic edifice generated a point of weakness, allowing favorable plate stresses to generate rift systems, propagating away from the rift junction and eventually break up Pangea. The breakup is marked by the magmatic breakup (un)conformity on seismic data, separating hotspot/plume sourced Seaward Dipping reflectors (SDRs) within the continental rift system, from early ocean spreading sourced SDRs. As ocean spreading continued, the volcanic construction evolved into a hotspot track, now recognized as the Bahamas island trail. Time progression of this hotspot track resembles the present-day Iceland hotspot track, as suggested by plate reconstructions (Figure 1). Based on melting depth estimates from Sm

  10. Lithosphere destabilization by melt percolation during pre-oceanic rifting: Evidence from Alpine-Apennine ophiolitic peridotites

    Science.gov (United States)

    Piccardo, Giovanni; Ranalli, Giorgio

    2017-04-01

    Orogenic peridotites from Alpine-Apennine ophiolite Massifs (Lanzo, Voltri, External and Internal Ligurides, - NW Italy, and Mt. Maggiore - Corsica) derive from the mantle lithosphere of the Ligurian Tethys. Field/structural and petrologic/geochemical studies provide constraints on the evolution of the lithospheric mantle during pre-oceanic passive rifting of the late Jurassic Ligurian Tethys ocean. Continental rifting by far-field tectonic forces induced extension of the lithosphere by means of km-scale extensional shear zones that developed before infiltration of melts from the asthenosphere (Piccardo and Vissers, 2007). After significant thinning of the lithosphere, the passively upwelling asthenosphere underwent spinel-facies decompression melting along the axial zone of the extensional system. Silica-undersaturated melt fractions percolated through the lithospheric mantle via diffuse/focused porous flow and interacted with the host peridotite through pyroxenes-dissolving/olivine-precipitating melt/rock reactions. Pyroxene dissolution and olivine precipitation modified the composition of the primary silica-undersaturated melts into derivative silica-saturated melts, while the host lithospheric spinel lherzolites were transformed into pyroxene-depleted/olivine-enriched reactive spinel harzburgites and dunites. The derivative liquids interacted through olivine-dissolving/orthopyroxene+plagioclase-crystallizing reactions with the host peridotites that were impregnated and refertilized (Piccardo et al., 2015). The saturated melts stagnated and crystallized in the shallow mantle lithosphere (as testified by diffuse interstitial crystallization of euhedral orthopyroxene and anhedral plagioclase) and locally ponded, forming orthopyroxene-rich/olivine-free gabbro-norite pods (Piccardo and Guarnieri, 2011). Reactive and impregnated peridotites are characterized by high equilibration temperatures (up to 1250 °C) even at low pressure, plagioclase-peridotite facies

  11. Recognized Multiple Rifts of the Neoproterozoic in the Initiation of the Tarim Craton (NW China) and Their Tectonic Implications

    Science.gov (United States)

    He, B.; Jiao, C.; Huang, T.; Zhou, X.; Cai, Z.; Cao, Z.; Jiang, Z.; Cui, J.; Yu, Z.; Chen, W.

    2017-12-01

    The Tarim Basin is the largest, oil-bearing and superimposed basin in the northwest of China. The development and tectonic property of the initial Tarim basin have been acutely disputed and remain enigmatic. Urgently need to reveal the origin and formation dynamics of the Tarim Carton and evaluate the potential of the deep energy resources. However, covered by vast desert and huge-thickness sedimentary strata, suffered by multiple tectonic movements, seismic data with low signal- to- noise ratio in the deep are the critical difficulties. We analyse 4 field outcrops, 18 wells, 27 reprocessed seismic reflection profiles with high SNR across the basin and many ancillary ones and aeromagnetic data. We find about 20 normal fault-controlled rift depressions of the Cryogenian and Ediacaran scattered in the Tarim basin, which developed on the Precambrian metamorphic and crystalline basements and covered by the epeiric sea and basin facies sediments of the Lower Cambrian. The structural styles of the rifts are mainly half grabens, symmetrical troughs and horst-grabens. The regional differences exist obviously in spatial and temporal. The WNW-ESE-trending faults occur in the central part and northern of the basin and the NE, and the NEE-trending faults occur in the southern parts, which response with the anomaly of aeromagnetic. Some main faults of the Ediacaran inherited from the Cryogenian and some occurred newly, the more rifting depressions occurred during the Ediacaran. The extensional NNW-SSE-oriented and NNE-SSW-oriented paleostress field occurred simultaneously during rifting, and accompanied with the clockwise shearing. According to the activities of syn-sedimentary faults, magmatic events and sediments, the tectonic properties of the rifts are different depending on their locations in the Tarim craton. The rifting phases mainly occurred from 780 Ma to 615 Ma. The formation of rifts were associated with the opening of the South Tianshan Ocean and the South Altun

  12. Heat flow in the Sumisu Rift, Izu-Ogasawara (Bonin) arc

    Energy Technology Data Exchange (ETDEWEB)

    Yamazaki, Toshitsugu

    1988-01-27

    Heat flow in the Sumisu rift of the Izu-Ogasawara Arc. was measured. Temperature gradients measured at 11 locations give the heat flow of 38 - 700 mW/m. The wide fluctuation is attributed to the existing hydrothermal system but can barely be explained by the influence of the thermal conductivity which depends upon the surface and basement rock topography. High values are found in the area of upwelling hot water and low values in the downwelling area, proving the hypothesis that the Sumisu Rift is in the early period of back-arc spreading. In the measurement at five locations at dense intervals (0.5 - 1 km) across the geological faults, high heat flow is found near the base of the submarine canyons. The wavelength of local thermal flow variation is several km, that is close to the distance between geological faults. The hydrothermal circulation is caused by the high aquiclude of the active faults and the low viscosity of high temperature interstitial water although there exists in this area an alluvial deposit of 1500 m thick, or the largest in the Sumisu Rift, which has low aquiclude and low heat convection. (5 figs, 1 tab, 23 refs)

  13. Volcanic geology and eruption frequency, lower east rift zone of Kilauea volcano, Hawaii

    Science.gov (United States)

    Moore, Richard B.

    1992-08-01

    Detailed geologic mapping and radiocarbon dating of tholeiitic basalts covering about 275 km2 on the lower east rift zone (LERZ) and adjoining flanks of Kilauea volcano, Hawaii, show that at least 112 separate eruptions have occurred during the past 2360 years. Eruptive products include spatter ramparts and cones, a shield, two extensive lithic-rich tuff deposits, aa and pahoehoe flows, and three littoral cones. Areal coverage, number of eruptions and average dormant interval estimates in years for the five age groups assigned are: (I) historic, i.e. A D 1790 and younger: 25%, 5, 42.75; (II) 200 400 years old: 50%, 15, 14.3: (III) 400 750 years old: 20%, 54, 6.6; (IV) 750 1500 years old: 5%, 37, 20.8; (V) 1500 3000 years old: LERZ during the past 1500 years. Estimated volumes of the exposed products of individual eruptions range from a few tens of cubic meters for older units in small kipukas to as much as 0.4 km3 for the heiheiahulu shield. The average dormant interval has been about 13.6 years during the past 1500 years. The most recent eruption occurred in 1961, and the area may be overdue for its next eruption. However, eruptive activity will not resume on the LERZ until either the dike feeding the current eruption on the middle east rift zone extends farther down rift, or a new dike, unrelated to the current eruption, extends into the LERZ.

  14. Giant seismites and megablock uplift in the East African Rift: evidence for Late Pleistocene large magnitude earthquakes.

    Science.gov (United States)

    Hilbert-Wolf, Hannah Louise; Roberts, Eric M

    2015-01-01

    In lieu of comprehensive instrumental seismic monitoring, short historical records, and limited fault trench investigations for many seismically active areas, the sedimentary record provides important archives of seismicity in the form of preserved horizons of soft-sediment deformation features, termed seismites. Here we report on extensive seismites in the Late Quaternary-Recent (≤ ~ 28,000 years BP) alluvial and lacustrine strata of the Rukwa Rift Basin, a segment of the Western Branch of the East African Rift System. We document examples of the most highly deformed sediments in shallow, subsurface strata close to the regional capital of Mbeya, Tanzania. This includes a remarkable, clastic 'megablock complex' that preserves remobilized sediment below vertically displaced blocks of intact strata (megablocks), some in excess of 20 m-wide. Documentation of these seismites expands the database of seismogenic sedimentary structures, and attests to large magnitude, Late Pleistocene-Recent earthquakes along the Western Branch of the East African Rift System. Understanding how seismicity deforms near-surface sediments is critical for predicting and preparing for modern seismic hazards, especially along the East African Rift and other tectonically active, developing regions.

  15. Thermochronological constraints on the Cambrian to recent geological evolution of the Argentina passive continental margin

    Science.gov (United States)

    Kollenz, Sebastian; Glasmacher, Ulrich A.; Rossello, Eduardo A.; Stockli, Daniel F.; Schad, Sabrina; Pereyra, Ricardo E.

    2017-10-01

    Passive continental margins are geo-archives that store information from the interplay of endogenous and exogenous forces related to continental rifting, post-breakup history, and climate changes. The recent South Atlantic passive continental margins (SAPCMs) in Brazil, Namibia, and South Africa are partly high-elevated margins ( 2000 m a.s.l.), and the recent N-S-trending SAPCM in Argentina and Uruguay is of low elevation. In Argentina, an exception in elevation is arising from the higher topography (> 1000 m a.s.l.) of the two NW-SE-trending mountain ranges Sierras Septentrionales and Sierras Australes. Precambrian metamorphic and intrusive rocks, and siliciclastic rocks of Ordovician to Permian age represent the geological evolution of both areas. The Sierras Australes have been deformed and metamorphosed (incipient - greenschist) during the Gondwanides Orogeny. The low-temperature thermochronological (LTT) data (history of the Gondwanides and the Mesozoic and Cenozoic South Atlantic geological evolution. Upper Carboniferous zircon (U-Th/He)-ages (ZHe) indicate the earliest cooling below 180 °C/1 Ma. Most of the ZHe-ages are of Upper Triassic to Jurassic age. The apatite fission-track ages (AFT) of Sierras Septentrionales and the eastern part of Sierras Australes indicate the South Atlantic rifting and, thereafter. AFT-ages of Middle to Upper Triassic on the western side of the Sierras Australes are in contrast, indicating a Triassic exhumation caused by the eastward thrusting along the Sauce Grande wrench. The corresponding t-T models report a complex subsidence and exhumation history with variable rates since the Ordovician. Based on the LTT-data and the numerical modelling we assume that the NW-SE-trending mountain ranges received their geographic NW-SE orientation during the syn- to post-orogenic history of the Gondwanides.

  16. Monitoring of the Syrian rift valley using radon technique

    International Nuclear Information System (INIS)

    Al-Hilal, M.; Al-Ali, A.; Jubeli, Y.

    1997-02-01

    Groundwater radon data were recorded once every two months from six monitoring sites of the Syrian rift valley during the year 1996. Radon samples were measured from deep artesian wells and from continuously-flowing springs that are distributed along this most active seismic zone in Syria. The available data were integrated with previously measured groundwater radon data from the same stations in order to estimate the range of normal radon fluctuations in the region. The estimation of such range may enable the separation between usual groundwater radon variations from other outliers which may indicate possible tectonic activities or earthquake hazards in the study area. Periodical radon measurements based on two months intervals and long distance between sampling stations does not enable us to trust with high level of confidence the connection between radon values and any possible earth dynamics. Therefore, shorter measuring time with closer monitoring sites are highly recommended to achieve the optimum advantage of such application. (Author). 8 Figs., 2 Tabs., 10 Refs

  17. Pits, rifts and slumps: the summit structure of Piton de la Fournaise

    Science.gov (United States)

    Carter, Adam; van Wyk de Vries, Benjamin; Kelfoun, Karim; Bachèlery, Patrick; Briole, Pierre

    2007-06-01

    A clear model of structures and associated stress fields of a volcano can provide a framework in which to study and monitor activity. We propose a volcano-tectonic model for the dynamics of the summit of Piton de la Fournaise (La Reunion Island, Indian Ocean). The summit contains two main pit crater structures (Dolomieu and Bory), two active rift zones, and a slumping eastern sector, all of which contribute to the actual fracture system. Dolomieu has developed over 100 years by sudden large collapse events and subsequent smaller drops that include terrace formation. Small intra-pit collapse scars and eruptive fissures are located along the southern floor of Dolomieu. The western pit wall of Dolomieu has a superficial inward dipping normal fault boundary connected to a deeper ring fault system. Outside Dolomieu, an oval extension zone containing sub-parallel pit-related fractures extends to a maximum distance of 225 m from the pit. At the summit the main trend for eruptive fissures is N80°, normal to the north south rift zone. The terraced structure of Dolomieu has been reproduced by analogue models with a roof to width ratio of approximately 1, suggesting an original magma chamber depth of about 1 km. Such a chamber may continue to act as a storage location today. The east flank has a convex concave profile and is bounded by strike-slip fractures that define a gravity slump. This zone is bound to the north by strike-slip fractures that may delineate a shear zone. The southern reciprocal shear zone is probably marked by an alignment of large scoria cones and is hidden by recent aa lavas. The slump head intersects Dolomieu pit and may slide on a hydrothermally altered layer known to be located at a depth of around 300 m. Our model has the summit activity controlled by the pit crater collapse structure, not the rifts. The rifts become important on the mid-flanks of the cone, away from pit-related fractures. On the east flank the superficial structures are controlled

  18. Geodynamics of rift-plume interaction in Iceland as constrained by new 40Ar/ 39Ar and in situ U-Pb zircon ages

    Science.gov (United States)

    Martin, E.; Paquette, J. L.; Bosse, V.; Ruffet, G.; Tiepolo, M.; Sigmarsson, O.

    2011-11-01

    The interaction between a rift zone and a mantle plume leads to exceptional situations in Iceland where the island is 1.5 wider than expected, given the North-Atlantic spreading rate. In order to give a better idea of the timeframe of this evolution, we present 32 new 40Ar/ 39Ar and in-situ U-Pb dating on zircon from 16 volcanic systems located from the west to east coasts of Iceland. The North Iceland Rift Zone (NIRZ) was initiated at least 12 Ma ago. Furthermore, during these last 12 Ma, the NIRZ half spreading rate was between 0.7 and 1.2 cm/yr and it propagated to the south at a rate of 1.0-1.2 cm/yr. The excess width of Iceland can thus not be explained by faster spreading rate in the past. Here we discuss a model that explains the ~ 200 km 'excess' of crust, taking into account the eastward relocation of the rift zone and corresponding older crustal capture over the course of Iceland's geological history. The most recent rift relocation is dated at approximately 6 Ma at Snæfellsnes Peninsula in the west, whereas the oldest volcanic systems (15-13 Ma) from the extreme north east of Iceland were most likely generated at the Kolbeinsey ridge north of Iceland rather than in the NIRZ itself. The need for rift relocations and crustal capture to explain the width of Iceland strongly suggests that during rift-plume interaction the mantle plume plays an active role. It forces the active rift zone to be frequently relocated by rift jumps above its center leaving inactive rift zones as older synclines in the geological record. This result in an eastward position of the rift zone in Iceland relative to the North Atlantic ridge, and it can be predicted that in a few tens of millions of years the Mid-Atlantic ridge and the Icelandic plume may become decoupled.

  19. Geomorphic Response to Spatial and Temporal Tectonic uplift on the Kenya Rift of East African Rift System

    Science.gov (United States)

    Xue, L.; Abdelsalam, M. G.

    2017-12-01

    Tectonic uplifts of the shoulders of the East Africa Rift System (EARS) have significant impact on the geological record by reorganizing drainage systems, increasing sediment supply, and changing climate and biogeography. Recent studies in geochronology, geomorphology and geophysics have provided some understanding of the timing of tectonic uplift and its distribution pattern of the (EARS). We do not know how the vertical motion is localized along the rift axis and the relative roles of upwelling of magma and rift extensional processes play in tectonic uplift history. This work presents detailed morphometric study of the fluvial landscape response to the tectonic uplift and climate shifting of the Kenya Rift shoulders in order to reconstruct their incision history, with special attention to timing, location, and intensity of uplift episodes. This work compiles the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) and Sentinel-2A data, summarized previous 39Ar-40Ar and thermochronology data, and calculates long-term incision rate and geomorphic proxies (normalized steepness and chi-integral) along the Kenya Rift. It also models the age of tectonic/climatic events by using knickpoint celerity model and R/SR integrative approach. It found that the maximum long-term incision rates of 300 mm/kyr to be at the central Kenya Rift, possibly related to the mantle-driven process and rapid tectonic uplift. The geomorphic proxies indicate southward decreasing pattern of the short-term incision rate, possibly related to the migration of the mantle plume.

  20. Oblique transfer of extensional strain between basins of the middle Rio Grande rift, New Mexico: Fault kinematic and paleostress constraints

    Science.gov (United States)

    Minor, Scott A.; Hudson, Mark R.; Caine, Jonathan S.; Thompson, Ren A.

    2013-01-01

    bulk sinistral-normal oblique shear along the Santo Domingo rift segment in Pliocene and later time. Regional geologic evidence suggests that the width of active rift faulting became increasingly confined to the Santo Domingo Basin and axial parts of the adjoining basins beginning in the late Miocene. We infer that the Santo Domingo clockwise stress perturbations developed coevally with the oblique rift segment mainly due to mechanical interactions of large faults propagating toward each other from the adjoining basins as the rift narrowed. Our results suggest that negligible bulk strike-slip displacement has been accommodated along the north-trending rift during much of its development, but uncertainties in the maximum ages of fault slip do not allow us to fully evaluate and discriminate between earlier models that invoked northward or southward rotation and translation of the Colorado Plateau during early (Miocene) rifting.

  1. Remote sensing studies and morphotectonic investigations in an arid rift setting, Baja California, Mexico

    Science.gov (United States)

    El-Sobky, Hesham Farouk

    The Gulf of California and its surrounding land areas provide a classic example of recently rifted continental lithosphere. The recent tectonic history of eastern Baja California has been dominated by oblique rifting that began at ˜12 Ma. Thus, extensional tectonics, bedrock lithology, long-term climatic changes, and evolving surface processes have controlled the tectono-geomorphological evolution of the eastern part of the peninsula since that time. In this study, digital elevation data from the Shuttle Radar Topography Mission (SRTM) from Baja California were corrected and enhanced by replacing artifacts with real values that were derived using a series of geostatistical techniques. The next step was to generate accurate thematic geologic maps with high resolution (15-m) for the entire eastern coast of Baja California. The main approach that we used to clearly represent all the lithological units in the investigated area was objectoriented classification based on fuzzy logic theory. The area of study was divided into twenty-two blocks; each was classified independently on the basis of its own defined membership function. Overall accuracies were 89.6%, indicating that this approach was highly recommended over the most conventional classification techniques. The third step of this study was to assess the factors that affected the geomorphologic development along the eastern side of Baja California, where thirty-four drainage basins were extracted from a 15-m-resolution absolute digital elevation model (DEM). Thirty morphometric parameters were extracted; these parameters were then reduced using principal component analysis (PCA). Cluster analysis classification defined four major groups of basins. We extracted stream length-gradient indices, which highlight the differential rock uplift that has occurred along fault escarpments bounding the basins. Also, steepness and concavity indices were extracted for bedrock channels within the thirty-four drainage basins. The

  2. Geology of the Elephanta Island fault zone, western Indian rifted margin, and its significance for understanding the Panvel flexure

    Science.gov (United States)

    Samant, Hrishikesh; Pundalik, Ashwin; D'souza, Joseph; Sheth, Hetu; Lobo, Keegan Carmo; D'souza, Kyle; Patel, Vanit

    2017-02-01

    The Panvel flexure is a 150-km long tectonic structure, comprising prominently seaward-dipping Deccan flood basalts, on the western Indian rifted margin. Given the active tectonic faulting beneath the Panvel flexure zone inferred from microseismicity, better structural understanding of the region is needed. The geology of Elephanta Island in the Mumbai harbour, famous for the ca. mid-6th century A.D. Hindu rock-cut caves in Deccan basalt (a UNESCO World Heritage site) is poorly known. We describe a previously unreported but well-exposed fault zone on Elephanta Island, consisting of two large faults dipping steeply east-southeast and producing easterly downthrows. Well-developed slickensides and structural measurements indicate oblique slip on both faults. The Elephanta Island fault zone may be the northern extension of the Alibag-Uran fault zone previously described. This and two other known regional faults (Nhava-Sheva and Belpada faults) indicate a progressively eastward step-faulted structure of the Panvel flexure, with the important result that the individual movements were not simply downdip but also oblique-slip and locally even rotational (as at Uran). An interesting problem is the normal faulting, block tectonics and rifting of this region of the crust for which seismological data indicate a normal thickness (up to 41.3 km). A model of asymmetric rifting by simple shear may explain this observation and the consistently landward dips of the rifted margin faults.

  3. The Statoil/Hydro fusion in a continental shelf perspective

    International Nuclear Information System (INIS)

    Osmundsen, Petter

    2007-01-01

    The article analysis the consequences of the Statoil/Hydro merger on the development on the Norwegian continental shelf. Various indicators of results of the market power the merger are presented and the effects of reduced competition on the markets are discussed. The advantages of the merger on the activities on the shelf are analyzed. (tk)

  4. New Insight Into the Crustal Structure of the Continental Margin offshore NW Sabah/Borneo

    Science.gov (United States)

    Barckhausen, U.; Franke, D.; Behain, D.; Meyer, H.

    2002-12-01

    thrusted onto the edge of the rifted continental block of the Dangerous Grounds. The subducted oceanic crust of the proto South China Sea must today be located below the Eastern part of Sabah and not along the present NW Sabah Trough.

  5. Contrasting styles of post-caldera volcanism along the Main Ethiopian Rift: Implications for contemporary volcanic hazards

    Science.gov (United States)

    Fontijn, Karen; McNamara, Keri; Zafu Tadesse, Amdemichael; Pyle, David M.; Dessalegn, Firawalin; Hutchison, William; Mather, Tamsin A.; Yirgu, Gezahegn

    2018-05-01

    The Main Ethiopian Rift (MER, 7-9°N) is the type example of a magma-assisted continental rift. The rift axis is populated with regularly spaced silicic caldera complexes and central stratovolcanoes, interspersed with large fields of small mafic scoria cones. The recent (latest Pleistocene to Holocene) history of volcanism in the MER is poorly known, and no eruptions have occurred in the living memory of the local population. Assessment of contemporary volcanic hazards and associated risk is primarily based on the study of the most recent eruptive products, typically those emplaced within the last 10-20 ky. We integrate new and published field observations and geochemical data on tephra deposits from the main Late Quaternary volcanic centres in the central MER to assess contemporary volcanic hazards. Most central volcanoes in the MER host large mid-Pleistocene calderas, with typical diameters of 5-15 km, and associated ignimbrites of trachyte and peralkaline rhyolite composition. In contrast, post-caldera activity at most centres comprises eruptions of peralkaline rhyolitic magmas as obsidian flows, domes and pumice cones. The frequency and magnitude of events varies between individual volcanoes. Some volcanoes have predominantly erupted obsidian lava flows in their most recent post-caldera stage (Fentale), whereas other have had up to 3 moderate-scale (VEI 3-4) explosive eruptions per millennium (Aluto). At some volcanoes we find evidence for multiple large explosive eruptions (Corbetti, Bora-Baricha, Boset-Bericha) which have deposited several centimetres to metres of pumice and ash in currently densely populated regions. This new overview has important implications when assessing the present-day volcanic hazard in this rapidly developing region. Supplementary Table 2 Main Ethiopian Rift outcrop localities with brief description of geology. All coordinates in Latitude - Longitude, WGS84 datum. Sample names (as listed in Supplementary Table 3a) follow outcrop name

  6. Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins

    Directory of Open Access Journals (Sweden)

    Green, Paul F.

    2013-12-01

    Full Text Available The continental margin of West Greenland is similar in many respects to other elevated, passive continental margins (EPCMs around the world. These margins are characterised by extensive regions of low relief at elevations of 1–2 kilometres above sea level sloping gently inland, with a much steeper, oceanward decline, often termed a 'Great Escarpment', terminating at a coastal plain. Recent studies, based on integration of geological, geomorphological and thermochronological evidence, have shown that the high topography of West Greenland was formed by differential uplift and dissection of an Oligo-Miocene peneplain since the late Miocene, many millions of years after continental break-up between Greenland and North America. In contrast, many studies of other EPCMs have proposed a different style of development in which the high plateaux and the steep, oceanward decline are regarded as a direct result of rifting and continental separation. Some studies assume that the elevated regions have remained high since break-up, with the high topography continuously renewed by isostasy. Others identify the elevated plains as remnants of pre-rift landscapes. Key to understanding the development of the West Greenland margin is a new approach to the study of landforms, stratigraphic landscape analysis, in which the low-relief, high-elevation plateaux at EPCMs are interpreted as uplifted peneplains: low-relief surfaces of large extent, cutting across bedrock of different age and resistance, and originally graded to sea level. Identification of different generations of peneplain (re-exposed and epigene from regional mapping, combined with geological constraints and thermochronology, allows definition of the evolution leading to the formation of the modern-day topography. This approach is founded particularly on results from the South Swedish Dome, which document former sea levels as base levels for the formation of peneplains. These results support the view

  7. Sr and Nd isotope geochemistry and tectonics during subduction and rifting in Sierra Santa Ursula, Sonora, Northwestern Mexico

    International Nuclear Information System (INIS)

    Mora-Klepeis, G.

    2000-01-01

    The western margin of North America was affected by a convergent plate boundary from the Cretaceous through the Early Tertiary. Volcanic rocks produced by subduction-related arc magmatism in northwestern Mexico are concentrated in two northwest-trending belts subparallel to the continental margin. One of these is the Sierra Madre Occidental, where mid-Tertiary magmatism consisted mostly of calc-alkaline rhyolitic ignimbrite and minor andesite produced between ∼ 46 and 28 Ma (McDowell et al., 1990). The second (younger) northwest-trending belt is located along the eastern margin of the Baja California Peninsula and in the Gulf of California region of mainland Mexico. This belt is composed mostly of andesite, but includes some basalt and dacite whose ages range from about 24 to 11 Ma (Hausback, 1984). A transition to rifting began after a mid-Tertiary cessation of subduction, eventually creating the Gulf of California extensional province. Four markedly different magma types comprising mainly tholeiitic and alkalic rocks and minor calc-alkaline and peralkaline rocks were erupted throughout the last 13 Ma and record the history of rifting of the Gulf of California (Sawlan, 1991). The aim of the present paper is to distinguish the nature of the 24-8.5 Ma magmatism emplaced on the eastern side of the Gulf of California in the state of Sonora, by the use of stratigraphic, geochemical and isotopic data. Preliminary Sr and Nd results show that three groups of magmas are present in the area suggesting a heterogeneous source. This can be interpreted as the result of magmas being erupted at different stages of subduction and rifting during the tectonic evolution of this part on North America

  8. The Mesozoic-Cenozoic igneous intrusions and related sediment-dominated hydrothermal activities in the South Yellow Sea Basin, the Western Pacific continental margin

    Science.gov (United States)

    Yumao, Pang; Xunhua, Zhang; Guolin, Xiao; Luning, Shang; Xingwei, Guo; Zhenhe, Wen

    2018-04-01

    Various igneous complexes were identified in multi-channel seismic reflection profiles from the South Yellow Sea Basin. It is not rare that magmatic intrusions in sedimentary basins cause strong thermal perturbations and hydrothermal activities. Some intrusion-related hydrothermal vent complexes have been identified and they are considered to originate from the deep sedimentary contact aureole around igneous intrusions and terminate in upper vents structures, and are linked by a vertical conduit system. The upper vent complexes are usually eye-shaped, dome-shaped, fault-related, crater-shaped or pock-shaped in seismic profiles. A schematic model was proposed to illustrate the structures of different types of hydrothermal vent complexes. A conceptual conduit model composed of an upper pipe-like part and a lower branching part was also derived. Hydrothermal vent complexes mainly developed during the Middle-Late Cretaceous, which is coeval with, or shortly after the intrusion. The back-arc basin evolution of the area which is related to the subduction of the Paleo-Pacific plate during the Mesozoic-Cenozoic may be the principal factor for voluminous igneous complexes and vent complexes in this area. It is significant to study the characteristics of igneous complexes and related hydrothermal vent complexes, which will have implications for the future study of this area.

  9. Patterns and variability in geochemical signatures and microbial activity within and between diverse cold seep habitats along the lower continental slope, Northern Gulf of Mexico

    Science.gov (United States)

    Bowles, Marshall; Hunter, Kimberley S.; Samarkin, Vladimir; Joye, Samantha

    2016-07-01

    We collected 69 sediment cores from distinct ecological and geological settings along the deep slope in the Northern Gulf of Mexico to evaluate whether specific geochemical- or habitat-related factors correlated with rates of microbial processes and geochemical signatures. By collecting replicate cores from distinct habitats across multiple sites, we illustrate and quantify the heterogeneity of cold seep geochemistry and microbial activity. These data also document the factors driving unique aspects of the geochemistry of deep slope gas, oil and brine seeps. Surprisingly little variation was observed between replicate (n=2-5) cores within sites for most analytes (except methane), implying that the common practice of collecting one core for geochemical analysis can capture the signature of a habitat in most cases. Depth-integrated concentrations of methane, dissolved inorganic carbon (DIC), and calcium were the predominant geochemical factors that correlated with a site's ecological or geological settings. Pore fluid methane concentration was related to the phosphate and DIC concentration, as well as to rates of sulfate reduction. While distinctions between seep habitats were identified from geochemical signatures, habitat specific geochemistry varied little across sites. The relative concentration of dissolved inorganic nitrogen versus phosphorus suggests that phosphorus availability limits biomass production at cold seeps. Correlations between calcium, chloride, and phosphate concentrations were indicative of brine-associated phosphate transport, suggesting that in addition to the co-migration of methane, dissolved organic carbon, and ammonium with brine, phosphate delivery is also associated with brine advection.

  10. Development of continental margins of the Atlantic Ocean and successive breakup of the Pangaea-3 supercontinent

    Science.gov (United States)

    Melankholina, E. N.; Sushchevskaya, N. M.

    2017-01-01

    Comparative tectonic analysis of passive margins of the Atlantic Ocean has been performed. Tectonotypes of both volcanic and nonvolcanic margins are described, and their comparison with other passive Atlantic margins is given. The structural features of margins, peculiarities of magmatism, its sources and reasons for geochemical enrichment of melts are discussed. The important role of melting of the continental lithosphere in the development of magmatism is demonstrated. Enriched EM I and EM II sources are determined for the lower parts of the volcanic section, and a depleted or poorly enriched source is determined for the upper parts of the volcanic section based on isotope data. The conclusions of the paper relate to tectonic settings of the initial occurrence of magmatism and rifting and breakup during the period of opening of the Mesozoic Ocean. It was found out that breakup and magmatism at proximal margins led only to insignificant structural transformations and reduction of the thickness of the ancient continental crust, while very important magmatic events happened later in the distal zone. New growth of magmatic crust at the stage of continental breakup is determined as a typical feature of distal zones of the margins under study. The relationship of development of margins with the impact of deep plumes as the source of magmatic material or a heat source only is discussed. Progradation of the zone of extension and breakup into the areas of cold lithosphere of the Atlantic and the formation of a single tectonomagmatic system of the ocean are under consideration.

  11. Crustal structure of the rifted volcanic margins and uplifted plateau of Western Yemen from receiver function analysis

    Science.gov (United States)

    Ahmed, Abdulhakim; Tiberi, Christel; Leroy, Sylvie; Stuart, Graham W.; Keir, Derek; Sholan, Jamal; Khanbari, Khaled; Al-Ganad, Ismael; Basuyau, Clémence

    2013-06-01

    We analyse P-wave receiver functions across the western Gulf of Aden and southern Red Sea continental margins in Western Yemen to constrain crustal thickness, internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism, faulting and mechanical crustal thinning during continental breakup. We analyse teleseismic data from 21 stations forming the temporary Young Conjugate Margins Laboratory (YOCMAL) network together with GFZ and Yemeni permanent stations. Analysis of computed receiver functions shows that (1) the thickness of unextended crust on the Yemen plateau is ˜35 km; (2) this thins to ˜22 km in coastal areas and reaches less than 14 km on the Red Sea coast, where presence of a high-velocity lower crust is evident. The average Vp/Vs ratio for the western Yemen Plateau is 1.79, increasing to ˜1.92 near the Red Sea coast and decreasing to 1.68 for those stations located on or near the granitic rocks. Thinning of the crust, and by inference extension, occurs over a ˜130-km-wide transition zone from the Red Sea and Gulf of Aden coasts to the edges of the Yemen plateau. Thinning of continental crust is particularly localized in a <30-km-wide zone near the coastline, spatially co-incident with addition of magmatic underplate to the lower crust, above which on the surface we observe the presence of seaward dipping reflectors (SDRs) and thickened Oligo-Miocene syn-rift basaltic flows. Our results strongly suggest the presence of high-velocity mafic intrusions in the lower crust, which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the SDRs. Our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western Gulf of Aden and southern Red Sea volcanic margins followed by a second phase of extension along the Red Sea margin.

  12. An investigation of the structure beneath Magadi area in southern ...

    African Journals Online (AJOL)

    Magadi area is located in the southern part of the Kenyan rift, an active continental rift that is part of the East African Rift system. Local seismic activity monitored previously around Lake Magadi revealed an earthquake cluster caused by swarm activity in the rift centre at shallow depths, which was probably triggered by ...