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Sample records for african craton wac

  1. Paleogeographic Evolution of the Late Neoproterozoic and Early Phanerozoic with New Paleomagnetic Constraints from West African Craton

    NASA Astrophysics Data System (ADS)

    Robert, B.; Besse, J.; Blein, O.; Greff-Lefftz, M.; Baudin, T.; Fernando, L.; Meslouh, S.; Belbadaoui, M.

    2015-12-01

    The paleogeographic evolution of the late Neoproterozoic and early Phanerozoic is dominated by the dispersion of Rodinia and the assembly of Gondwana. The timing of these two episodes is still highly debated, partly due to the low number of good quality paleomagnetic data. In order to better constrain the paleogeography for this epoch, we bring new paleomagnetic data on volcanic series from the West African Craton (WAC), which is a key block to understand the evolution of these two supercontinents. We have sampled well dated pyroclastic and lava flows from the groups of Ouarzazate (upper Ediacaran) and Taroudant (lower Cambrian) in the Anti-Atlas (Morocco). 500 samples from 105 sites were thermally demagnetized in laboratory. Our results highlight two major groups of directions, mainly carried by minerals of the titano-hematite family. Magnetite may also contribute sometimes to the magnetization. The first group displays a single polarity direction, with a shallow inclination and a south-east declination. This direction close to the expected direction derived from the Permo-Carboniferous segment of the Gondwana apparent polar wander path (APWP) is due to a remagnetization acquired during the Kiaman reversed polarity superchron (320-262Ma). The second group, observed in the Ouarzazate and Taroudant groups, consists of a dual polarity high inclination direction and may represent the characteristic magnetization. On the basis of geologic and paleomagnetic data from literature, we constructed an APWP for both WAC and Amazonia between 615 and 530Ma, assuming these two blocks were already accreted. We found a paleomagnetic solution in which Laurentia and WAC-Amazonia remained attached from ~615Ma up to the late Ediacaran, Laurentia remaining at low latitude during this period. Around ~550Ma, WAC-Amazonia separated from Laurentia and finally collided with the other Gondwanan blocks during the lower Cambrian, marking the final accretion of Gondwana.

  2. Evolution of a Paleoproterozoic “weak type” orogeny in the West African Craton (Ivory Coast)

    NASA Astrophysics Data System (ADS)

    Vidal, M.; Gumiaux, C.; Cagnard, F.; Pouclet, A.; Ouattara, G.; Pichon, M.

    2009-11-01

    The Paleoproterozoic domain of the Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of long-lived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of "domes and basins" geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of long-lived Paleoproterozoic "weak type" orogen.

  3. African hot spot volcanism: small-scale convection in the upper mantle beneath cratons.

    PubMed

    King, S D; Ritsema, J

    2000-11-10

    Numerical models demonstrate that small-scale convection develops in the upper mantle beneath the transition of thick cratonic lithosphere and thin oceanic lithosphere. These models explain the location and geochemical characteristics of intraplate volcanos on the African and South American plates. They also explain the presence of relatively high seismic shear wave velocities (cold downwellings) in the mantle transition zone beneath the western margin of African cratons and the eastern margin of South American cratons. Small-scale, edge-driven convection is an alternative to plumes for explaining intraplate African and South American hot spot volcanism, and small-scale convection is consistent with mantle downwellings beneath the African and South American lithosphere. PMID:11073447

  4. Lower Cambrian-Ediacaran Paleogeography and True Polar Wander with New Paleomagnetic Constraints from West African Craton

    NASA Astrophysics Data System (ADS)

    Robert, B.; Besse, J.; Blein, O.; Greff-Lefftz, M.; Baudin, T.; Fernando, L.; Meslouh, S.; Belbadaoui, M.

    2014-12-01

    Paleomagnetic data from Laurentia and Baltica continents suggest fast large oscillations of the virtual geomagnetic poles (VGP) from high to low latitudes during the Ediacaran (635-542 Ma). These data are interpreted in the literature either as oscillations of the Earth magnetic dipole between polar and equatorial positions, or as True Polar Wander (TPW), implying a very fast tumbling of continents and perhaps, of whole Earth. In this study, we try to test these hypotheses by bringing new paleomagnetic data on volcanic series from another continent, the West African Craton (WAC). We have sampled well dated pyroclastic and lava flows from the Ouarzazate and Taroudant groups in the Anti-Atlas, (Morocco). 480 samples from 105 sites were thermally demagnetized in our laboratory. Our preliminary results highlight two major groups of directions, mainly carried by hematite, magnetite also contributing sometimes to the magnetization. The first group consists of a dual polarity high inclination direction that may represent the original magnetization. The observed paleolatitude is compatible with that predicted by the lower Cambrian-Ediacaran apparent polar wander path (APWP) of Gondwana, assuming that the WAC was already accreted to Gondwana at this age. Nevertheless, a complete agreement between our pole and the APWP needs a local rotation of around 80° on a vertical axis. The second group displays a single polarity direction, with a shallow inclination and a south-east declination. This direction is close to the expected direction derived from the Permo-Carboniferous segment of the Gondwana APWP, and may represent a remagnetization acquired during the Kiaman reversed polarity superchron. Our preliminary paleomagnetic results thus display large changes in the VGP position, as also evidenced by others on Baltica and Laurentia. However, their interpretation does not favor TPW episodes or equatorial Earth magnetic dipole during the lower Cambrian-Ediacaran periods, but

  5. Paleoproterozoic Paleomagnetic Data from South American and African Cratons: An Unorthodox Configuration of Atlantica?

    NASA Astrophysics Data System (ADS)

    Rapalini, A. E.; Sanchez Bettucci, L.; Badgen, E.; Vásquez, C. A.

    2013-05-01

    Several South American and African cratonic blocks share similar geologic evolutions in mid- Paleoproterozoic times. The Rio de la Plata, Congo-Sao Francisco, Guyana (Proto-Amazonia) and West-Africa cratons, plus other minor blocks, display a very significant crustal forming phase from around 2.2 to 2.0 Ga. This includes juvenile basic to ultrabasic as well as bimodal metavolcanic rocks intercalated with metasediments, highly deformed and metamorphosed granitoids and gneisses and 2.1-1.9 Ga undeformed granitoids that indicate crustal stabilization. These similarities have been interpreted by several authors as evidence of continental assembly that led to formation of the Atlantica "supercontinent". According to this model, Atlantica was formed by the above mentioned cratons in the late Rhyacian and/or early Orosinian with each block occupying a position virtually identical to the one they presented within Gondwana in the Phanerozoic. Testing the existence and configuration of Atlantica is very important for any model of global paleogeographic evolution as well as for the geologic evolution of large areas of South America and Africa. In the last decade, paleomagnetic data from well dated rocks in the interval 2.1-1.95 Ga have become available from the Rio de la Plata, Sao Francisco, Guyana and West Africa cratons, allowing testing the Atlantica hypothesis. Coeval paleomagnetic poles from these blocks cannot be reconciled in the traditional Atlantica configuration suggesting that it is not a valid paleogeographic hypothesis. However, a radically different configuration permits a conciliation of the paleomagnetic data with a single land-mass encompassing the four blocks. According to the available data, the cratons underwent a rapid displacement from polar to mid-latitudes during and immediately after amalgamation between around 2.1 and 2.0 Ga. Data also suggest that West Africa might have been the last block to be accreted.

  6. Tectonic evolution of the Oudalan-Gorouol greenstone belt in NE Burkina Faso and Niger, West African craton.

    NASA Astrophysics Data System (ADS)

    Tshibubudze, Asinne; Hein, Kim A. A.

    2010-05-01

    The Oudalan-Gorouol Greenstone Belt (OGGB) forms part of the Palaeoproterozoic as the Baoulé-Mossi domain of the West African Craton (WAC) and hosts gold deposits at Essakane, Gossey, Korizena, and Falagountou in NE Burkina Faso, and Kossa goldfield in Niger. The Birimian supracrustal sequences in the OGGB are dominated by meta-volcanoclastic greywacke intercalated meta-conglomerate, siltstone and shale, carbonate (dolomite) and volcanic units pillow basalts). The belt is surrounded by plutonic rocks including granite, TTG suite granitoids and granite gneiss. The sequences where subjected to two phases of deformation, and several phases of contact metamorphosed to hornblende-hornfels facies during emplacement of pyroxenite-gabbro-norite, granodiorite-tonalite and gabbro dykes and porphyritic sills. The OGGB is bounded and/or crosscut by several major NNE to NE-trending shear zones including the steeply east-dipping Markoye Shear Zone (western margin of the OGGB), Tin Takanet-Bellekcire Shear Zone, Dori Shear Zone, Kargouna Shear Zone, Takabougou Shear Zone, and Bom Kodjelé Shear Zone (transects the centre of the OGGB). The structures were readily identified using LANDSAT, Aster, aeromagnetic and RTP magnetic data, with follow-up strategic mapping, highlighting the value of interpreting geophysical and remotely sensed data in regional mapping in Burkina Faso and Niger. Structural studies completed in 2007 adjacent to the Essakane gold mine indicated that the NE-trending, first-order crustal-scale Markoye Shear Zone (MSZ) has undergone at least two phases of reactivation concomitant to two phases of regional deformation (Tshibubudze et al., 2009). The first phase of deformation, D1, resulted in the formation of NNW-NW trending folds and thrusts during dextral-reverse displacement on the MSZ. The deformation predates the Eburnean Orogeny is termed the Tangaean Event (meaning low hills in the Moré language of Burkina Faso) and is tentatively dated at ca. 2170

  7. Mat-related sedimentary structures in Neoproterozoic peritidal passive margin deposits of the West African Craton (Anti-Atlas, Morocco)

    NASA Astrophysics Data System (ADS)

    Bouougri, E.; Porada, H.

    2002-11-01

    Proterozoic inliers in the central Anti-Atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the Neoproterozoic continental margin of the West African Craton (WAC). The low-grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes. The well-preserved structures include wrinkle structures, erosion marks, microbial sand chips, spindle-shaped and subcircular microbial shrinkage cracks, and possibly gas domes and cabbage-head structures. Thin sections exhibit mat fragments and dispersed grains of hematite/limonite after pyrite in fine-grained quartzitic storm deposits. Post-storm layers frequently consist of matrix-supported sand-sized to silt-sized grains and are overlain by argillaceous veneers including isolated silt-sized grains and black carbonaceous laminae. The muddy veneers are considered to represent compacted stacks of microbial mats (biolaminites), which colonized and biostabilized storm and post-storm layers, and thus prevented them from eroding. In the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions, it may be expected that Proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally, e.g. behind protective barriers, in the subtidal zone and beyond. Mat-related structures, however, need specific conditions for their formation and preservation: Wrinkle structures, erosion marks, and microbial sand chips require tractional currents and soon deposition and burial, respectively. Such conditions are preferably met in intertidal and supratidal zones. Spindle-shaped and subcircular cracks require mat shrinkage due to either desiccation or "syneresis". Crack propagation implies progressive shrinkage, while superposition of crack generations indicates repeated alternation

  8. Trace element enrichment in off-craton peridotites: comparison of off-craton Proterozoic and Pan-African mantle beneath southern Africa

    NASA Astrophysics Data System (ADS)

    le Roex, Anton; Class, Cornelia

    2014-05-01

    Trace element abundances in constituent clinopyroxene, garnet and phlogopite from off-craton mantle beneath the Proterozoic Namaqua-Natal belt (South Africa) and in clinopyroxene and amphibole from the Pan African Damara belt (Namibia) have been determined by laser ablation ICP-MS. Chondrite normalised rare earth element abundances in clinopyroxenes show a wide range of patterns from LREE depleted, through sinusoidal patterns to strongly LREE enriched. In garnets, the patterns range from strongly LREE depleted with flat HREE, through sinusoidal patterns to 'humped' sinusoidal patterns. Numerical modelling shows that the range of REE enrichment patterns can be generated via reactive porous flow mechanisms involving an alkaline silicate melt and where, in the Proterozoic mantle, there is a high fluid/solid ratio (i.e. all clinopyroxene and garnet are products of modal metasomatism and/or near fully equilibrated with a metasomatic fluid). In contrast, clinopyroxene in the Pan African mantle shows evidence for cryptic metasomatism, where enrichment of pre-existing clinopyroxene by alkaline silicate melts has occurred at the distal regions of an advancing metasomatic front. Extreme fluid/melt compositions are not required to account for the enrichment patterns Numerical modelling of primitive mantle normalised trace element abundances in clinopyroxenes show that the commonly observed strong relative depletions in Nb-Ta and Rb-Ba present in clinopyroxenes can be accounted for by crystallisation of accessory phlogopite (in Proterozoic mantle) and amphibole (in Pan African mantle) from the metasomatising melt/fluid. In contrast, strong relative depletion in Zr-Hf in some clinopyroxenes in both Proterozoic and Pan African mantle regions requires either metasomatic crystallisation of zircon, or involvement of a carbonatitic melt as metasomatic agent. Trace element enrichment patterns seen in clinopyroxene and garnet in the Proterozoic off-craton regions of southern Africa

  9. Upper mantle structure beneath southern African cratons from seismic finite-frequency P- and S-body wave tomography

    NASA Astrophysics Data System (ADS)

    Youssof, M.; Thybo, H.; Artemieva, I. M.; Levander, A.

    2015-06-01

    We present a 3D high-resolution seismic model of the southern African cratonic region from teleseismic tomographic inversion of the P- and S-body wave dataset recorded by the Southern African Seismic Experiment (SASE). Utilizing 3D sensitivity kernels, we invert traveltime residuals of teleseismic body waves to calculate velocity anomalies in the upper mantle down to a 700 km depth with respect to the ak135 reference model. Various resolution tests allow evaluation of the extent of smearing effects and help defining the optimum inversion parameters (i.e., damping and smoothness) for regularizing the inversion calculations. The fast lithospheric keels of the Kaapvaal and Zimbabwe cratons reach depths of 300-350 km and 200-250 km, respectively. The paleo-orogenic Limpopo Belt is represented by negative velocity perturbations down to a depth of ˜ 250 km, implying the presence of chemically fertile material with anomalously low wave speeds. The Bushveld Complex has low velocity down to ˜ 150 km, which is attributed to chemical modification of the cratonic mantle. In the present model, the finite-frequency sensitivity kernels allow to resolve relatively small-scale anomalies, such as the Colesberg Magnetic Lineament in the suture zone between the eastern and western blocks of the Kaapvaal Craton, and a small northern block of the Kaapvaal Craton, located between the Limpopo Belt and the Bushveld Complex.

  10. Tectonic evolution of the Oudalan-Gorouol greenstone belt in NE Burkina Faso and Niger, West African craton.

    NASA Astrophysics Data System (ADS)

    Tshibubudze, Asinne; Hein, Kim A. A.

    2010-05-01

    The Oudalan-Gorouol Greenstone Belt (OGGB) forms part of the Palaeoproterozoic as the Baoulé-Mossi domain of the West African Craton (WAC) and hosts gold deposits at Essakane, Gossey, Korizena, and Falagountou in NE Burkina Faso, and Kossa goldfield in Niger. The Birimian supracrustal sequences in the OGGB are dominated by meta-volcanoclastic greywacke intercalated meta-conglomerate, siltstone and shale, carbonate (dolomite) and volcanic units pillow basalts). The belt is surrounded by plutonic rocks including granite, TTG suite granitoids and granite gneiss. The sequences where subjected to two phases of deformation, and several phases of contact metamorphosed to hornblende-hornfels facies during emplacement of pyroxenite-gabbro-norite, granodiorite-tonalite and gabbro dykes and porphyritic sills. The OGGB is bounded and/or crosscut by several major NNE to NE-trending shear zones including the steeply east-dipping Markoye Shear Zone (western margin of the OGGB), Tin Takanet-Bellekcire Shear Zone, Dori Shear Zone, Kargouna Shear Zone, Takabougou Shear Zone, and Bom Kodjelé Shear Zone (transects the centre of the OGGB). The structures were readily identified using LANDSAT, Aster, aeromagnetic and RTP magnetic data, with follow-up strategic mapping, highlighting the value of interpreting geophysical and remotely sensed data in regional mapping in Burkina Faso and Niger. Structural studies completed in 2007 adjacent to the Essakane gold mine indicated that the NE-trending, first-order crustal-scale Markoye Shear Zone (MSZ) has undergone at least two phases of reactivation concomitant to two phases of regional deformation (Tshibubudze et al., 2009). The first phase of deformation, D1, resulted in the formation of NNW-NW trending folds and thrusts during dextral-reverse displacement on the MSZ. The deformation predates the Eburnean Orogeny is termed the Tangaean Event (meaning low hills in the Moré language of Burkina Faso) and is tentatively dated at ca. 2170

  11. Southern African perspectives on the long-term morpho-tectonic evolution of cratonic interiors

    NASA Astrophysics Data System (ADS)

    Kounov, Alexandre; Viola, Giulio; Dunkl, István; Frimmel, Hartwig E.

    2013-08-01

    We propose a refined conceptual model for the Paleo- and Mesozoic morpho-tectonic evolution of the southern African cratonic interior. Constraints are derived from new zircon and apatite fission-track and (U-Th-[Sm])/He dates (ZFT, AFT, ZHe and AHe) of rocks from the Augrabies Falls and Fish River Canyon regions in South Africa and southern Namibia, respectively. The combined ZFT and ZHe thermochronological results suggest a smooth and simple tectonic evolution, wherein the study area cooled monotonically as one coherent block from the Early Silurian to the Mid Triassic in response to very low denudation rates of less than 5 m/myr. Some of the new zircon ages may indicate a discrete and short-lived period of enhanced cooling interrupting this monotonic cooling during the Mid Devonian-Early Carboniferous. We tentatively correlate this episode to the events that caused the regional hiatus that separates the Cape Supergroup from the overlying Karoo Supergroup. Apatite fission-track and (U-Th-[Sm])/He data joint modeling reveals a period of accelerated regional cooling through 120 to 40 °C between 100 and 65 Ma ago. We interpret the latter as most probably due to regional uplift in combination with high river gradients and enhanced erosion rates in the Orange and Fish River basins, which, during the Cretaceous, were probably part of the greater Kalahari River catchment area. Based on the apatite results, a denudation rate of ca. 25 m/myr was calculated for the Late Cretaceous. At that time the area was probably characterized by an elevated average altitude and low relief, as indicated by the AFT and AHe age patterns.

  12. Tinkering toward WAC Utopia.

    ERIC Educational Resources Information Center

    Cohen, Samuel

    2002-01-01

    Notes that Writing Across the Curriculum (WAC) is growing at a perceived time of crisis in education and perceived strengthening of the forces of globalization. Suggests WAC should tinker with its existing techniques to promote critical thinking in even the most basic exercises. Concludes that in doing so, it can help students develop into…

  13. Analogies Between the East African Rift Around the Tanzania Craton and the Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Montesi, L. G.

    2013-12-01

    Continental rifts and oceanic spreading centers both accommodate plate divergence but their morphologies are often quite different. Yet, ultraslow spreading centers, especially the Southwest Indian ridge at the 9 to 16°E area (SWIR), present good analogies for the East African Rift (EAR), including localized volcanism, avolcanic segments, and a continuous but not straight rift axis. The archetypal oceanic spreading center features transform offsets. Volcanism is continuous along the ridge axis and is most vigorous at the center of spreading segments. By contrast, continental rifts do not feature transform offsets. The orientation of the rift can change along strike. Several rift segments are purely tectonic, with relatively isolated volcanic centers. The EAR around the Tanzania Craton clearly shows this kind of morphology. Ultraslow spreading centers share many of these features. The SWIR, in particular, displays dramatic changes in orientation, with volcanism localized at the junction between segments of different obliquity. Melt production and transport are controlled by the effective spreading rate, a combination of plate divergence velocity and rift obliquity. Ultraslow spreading center all have an effective spreading rate less than 13 mm/yr. At that speed the thickness of the thermal boundary layer is similar to the depth from which magma can be effectively extracted, opening the possibility for long-distance transport of magma along axis without extraction. Volcanic centers correspond to the location where the magma transport system first encounters a tectonically damaged zone that enables extraction to the surface. The effective velocity of the EAR in the Kenya dome is less than 4mm/yr firmly on par with ultraslow ridges. In fact, to generate magma by mantle upwelling at such a slow opening rate requires a higher mantle temperature or fertility than in the oceanic domain. Both opening rate and effective velocity increase northward along the Eastern branch

  14. Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example

    NASA Astrophysics Data System (ADS)

    Beauvais, Anicet; Chardon, Dominique

    2013-05-01

    long-term erosion of tropical shields is crucial to constraining the role of lateritic regolith covers as prominent sinks and sources of CO2 and sediments in the context of long-term Cenozoic climate change. It is also a key to understanding long-term landform evolution processes operating over most of the continental surface and their control onto the sediment routing system. We study the surface evolution of West Africa over three erosion periods (~ 45-24, ~ 24-11 and ~ 11-0 Ma) recorded by relicts of three subcontinental-scale lateritic paleolandsurfaces whose age is bracketed by 39Ar/40Ar dating of lateritic K-Mn oxides. Denudation depths and rates compiled from 380 field stations show that despite heterogeneities confined to early-inherited reliefs, the subregion underwent low and homogeneous denudation (~ 2-20 m Ma-1) over most of its surface whatever the considered time interval. This homogeneity is further documented by a worldwide compilation of cratonic denudation rates, over long-term, intermediate and modern Cenozoic time scales (100-107 yr). These results allow defining a steady state cratonic denudation regime that is weathering-limited, i.e., controlled by the thickness of the (lateritic) regolith available for stripping. Steady state cratonic denudation regimes are enabled by maintained compartmentalization of the base levels between river knick points controlled by relief inheritance. Under such regimes, lowering of base levels and their fossilization are primarily imposed by long-term eustatic sea level fall and climate rather than by epeirogeny. The expression of steady state cratonic denudation regimes in clastic sedimentary fluxes remains to be investigated.

  15. Seismic structure of the crust and lithospheric mantle of the southern African cratonic region

    NASA Astrophysics Data System (ADS)

    Youssof, M.; Thybo, H.; Artemieva, I. M.; Levander, A.

    2013-12-01

    We present a new seismic model for the structure of the crust and lithospheric mantle in southern Africa constrained by a joint study of seismic receiver functions and finite-frequency tomography, using the high-quality data from the South Africa Seismic Experiment (SASE). A) The crust has a highly heterogeneous structure with short wavelength variations in (i) thickness, (ii) composition (reflected in Vp/Vs-ratio calculated for all SASE stations), and (iii) Moho sharpness (which is quantified and mapped for the entire region) (Youssof et al., Tectonophysics, in review). By mapping these three parameters, we distinguish ~20 crustal blocks that do not everywhere coincide with surface tectonic features. Our RFs also demonstrate strong azimuthal anisotropy in the crust, with a typical crustal contribution to the total S-wave splitting of at least 30%. Spatial correlation of the S-wave polarization directions of crustal and mantle anisotropy indicates (i) the presence of three distinct Archean lithospheric terranes and (ii) coupling between the crust and lithospheric mantle in most of the study area, with a strong decoupling in western Kaapvaal where the crustal anisotropy is strongest. The similarity of anisotropy directions in the crust and mantle beneath much of the Kaapvaal craton indicates that (a) the seismic anisotropy originates at the time of cratonization and (b) the observed correspondence between the present direction of absolute plate motion (APM) and lithosphere anisotropy is coincidental. B) A new 3D high-resolution seismic model of the lithospheric mantle has been determined from finite frequency tomographic inversions of teleseismic P- and S- body wave data. The two velocity models are very similar in structure, but differ in the relative P- and S-wave velocity anomalies. We find that: 1) the fast lithospheric keels extends very deep, perhaps to depths of 300-350 km and 250 km beneath the Kaapvaal and Zimbabwe cratons, respectively, and 2) the Archean

  16. North African petroleum geology: regional structure and stratigraphic overview of a hydrocarbon-rich cratonic area

    SciTech Connect

    O'Connor, T.E.; Kanes, W.H.

    1985-02-01

    North Africa, including Sinai, contains some of the most important hydrocarbon-producing basins in the world. The North African Symposium is devoted to examining the exploration potential of the North African margin in light of the most recent and promising exploration discoveries. The geologic variety of the region is extraordinary and can challenge any exploration philosophy. Of primary interest are the Sirte basin of Libya, which has produced several billion barrels of oil, and the Gulf of Suez, a narrow, evaporite-capped trough with five fields that will produce more than 5 billion bbl. Both are extensional basins with minimal lateral movement and with good source rocks in direct proximity to reservoirs. Structural models of these basins give firm leads for future exploration. More difficult to evaluate are the Tethyan realm basins of the northern Sinai, and the Western Desert of Egypt, the Cyrenaican Platform of Libya, and the Tunisia-Sicily shelf area, where there are only limited subsurface data. These basins are extensional in origin also, but have been influenced by lateral tectonics. Favorable reservoirs exist, but source rocks have been a problem locally. Structural models with strong stratigraphic response offer several favorable play concepts. The Paleozoic Ghadames basin in Libya, Tunisia, and Algeria has the least complex structural history, and production appears to be limited to small structures. A series of stratigraphic models indicates additional areas with exploration potential. The Paleozoic megabasin of Morocco, with its downfaulted Triassic grabens, remains an untested but attractive area.

  17. Nature and evolution of Neoproterozoic ocean-continent transition: Evidence from the passive margin of the West African craton in NE Mali

    NASA Astrophysics Data System (ADS)

    Renaud, Caby

    2014-03-01

    The Timétrine massif exposed west of the Pan-African suture zone in northeastern Mali belongs to the passive margin of the West African craton facing to the east intra-oceanic arc assemblages and 730 Ma old pre-collisional calc-alkaline plutons. The Timétrine lithologic succession includes from the base to the top Mesoproterozoic cratonic to passive margin formations overlain by deep-sea Fe-Mg schists. Submarine metabasalts and two ultramafic massifs of serpentinized mantle peridotites are inserted as olistoliths towards the top whereas turbidites of continental origin represent the younger unit. Field and petrological data have revealed a distinct metasedimentary sequence attached to the serpentinized peridotites. It essentially consists of impure carbonates, Fe jaspers and polymictic breccias containing altered blocks of mantle peridotites, most rocks being enriched in detrital chromite. This association is interpreted as reworked chemical and detrital sediments derived from the alteration of mafic-ultramafic rocks. It is argued that mantle exhumation above sea floor took place during the Neoproterozoic rifting and crustal thinning period under possible tropical conditions, as suggested by the large volume of silicified serpentinites. In spite of greenschist facies metamorphic overprint characterized by widespread Fe-rich blue amphiboles that are not diagnostic of high-pressure conditions, it is possible to reconstruct a former ocean-continent transition similar to that evidenced for the Mesozoic period, followed by the deposition of syn-to post rift terrigeneous turbidites roughly coeval with ocean spreading some time before 800 Ma. It is concluded that the serpentinite massifs were tectonically emplaced first in an extensional setting, then incorporated within deep-sea sediments as olistoliths and finally transported westward during late Neoproterozoic collisional tectonics onto the West African craton.

  18. The basement of the Punta del Este Terrane (Uruguay): an African Mesoproterozoic fragment at the eastern border of the South American Río de La Plata craton

    NASA Astrophysics Data System (ADS)

    Basei, Miguel A. S.; Peel, Elena; Sánchez Bettucci, Leda; Preciozzi, Fernando; Nutman, Allen P.

    2011-04-01

    The Punta del Este Terrane (eastern Uruguay) lies in a complex Neoproterozoic (Brasiliano/Pan-African) orogenic zone considered to contain a suture between South American terranes to the west of Major Gercino-Sierra Ballena Suture Zone and eastern African affinities terranes. Zircon cores from Punta del Este Terrane basement orthogneisses have U-Pb ages of ca. 1,000 Ma, which indicate an lineage with the Namaqua Belt in Southwestern Africa. U-Pb zircon ages also provide the following information on the Punta del Este terrane: the orthogneisses containing the ca. 1,000 Ma inheritance formed at ca. 750 Ma; in contrast to the related terranes now in Africa, reworking of the Punta del Este Terrane during Brasiliano/Pan-African orogenesis was very intense, reaching granulite facies at ca. 640 Ma. The termination of the Brasiliano/Pan-African orogeny is marked by formation of acid volcanic and volcanoclastic rocks at ca. 570 Ma (Sierra de Aguirre Formation), formation of late sedimentary basins (San Carlos Formation) and then intrusion at ca. 535 Ma of post-tectonic granitoids (Santa Teresa and José Ignacio batholiths). The Punta del Este Terrane and unrelated western terranes represented by the Dom Feliciano Belt and the Río de La Plata Craton were in their present positions by ca. 535 Ma.

  19. Metamorphic record and Thermo-mechanical modelling of lower crust exhumation during the Palaeoproterozoic Eburnean orogeny, West African Craton

    NASA Astrophysics Data System (ADS)

    Gerbault, Muriel; Ganne, Jerome; Block, Sylvain

    2014-05-01

    A thermo-mechanical interpretation of the metamorphic evolution of moderate- to high-pressure volcano-sedimentary rocks (6-8 to >10 Kb) in the Birimian Province (2.2-2.0 Ga) of the West African Craton is explored in terms of burial and exhumation processes. Metamorphic data collected in Burkina Faso, southwest Ghana and eastern Senegal suggest that during the Eburnean orogeny (~2.1 Ga),this Palaeoproterozoic Birimian crust was dominated by moderate apparent geothermal gradients of 20-30°C/km (M2a), that produced greenschist- to amphibolite-facies metamorphic assemblages associated with regional shortening and granitoid intrusions. The M2a gradient is superimposed on a colder thermal regime (M1 : <10-15 °C/km) that produced high-P greenschist- to blueschist-facies metamorphic assemblages, and which most likely recorded the earlier formation of the protolith. The geodynamical origin of M1 is not directly addressed here. Thermo-mechanical two-dimensional numerical models were built in order to test whether late-stage compressional tectonics could generate the exhumation of meta-sediments, collected in Ca0-poor granitoids and which record elevated metamorphic pressures (P> 6-8 Kb). The poor data quality provide limited constraints on the appropriate initial setup conditions, and a number of tests have led us to conceptualize the spatial distribution of a hypothezised succession of volcanic island arcs emplaced on top of CaO rich TTG (Tonalite- Trondjhemite-Granodiorite suites) basement, tectonically paired with sedimentary basins. We postulated therefore the preexistence of wide (about 250 km) and thick flexural sedimentary basins (depth 15 km) in an orogenic mafic crust (about 20 km thick), underplated by a more felsic and lighter layer representing a TTG melange. The numerical results show that a mechanism of burial, heating and exhumation of meta-sediments can occur by simultaneous folding and gravitational instabilities within the broad extent of the basin

  20. Petrochemical and petrophysical characterization of the lower crust and the Moho beneath the West African Craton, based on Xenoliths from Kimberlites

    NASA Technical Reports Server (NTRS)

    Haggerty, Stephen E.; Toft, Paul B.

    1988-01-01

    Additional evidence to the composition of the lower crust and uppermost mantle was presented in the form of xenolith data. Xenoliths from the 2.7-Ga West African Craton indicate that the Moho beneath this shield is a chemically and physically gradational boundary, with intercalations of garnet granulite and garnet eclogite. Inclusions in diamonds indicate a depleted upper mantle source, and zenolith barometry and thermometry data suggest a high mantle geotherm with a kink near the Moho. Metallic iron in the xenoliths indicates that the uppermost mantle has a significant magnetization, and that the depth to the Curie isotherm, which is usually considered to be at or above the Moho, may be deeper than the Moho.

  1. Geochronology of the Birim Supergroup of the West African craton in the Wa-Bolé region of west-central Ghana: Implications for the stratigraphic framework

    NASA Astrophysics Data System (ADS)

    de Kock, G. S.; Armstrong, R. A.; Siegfried, H. P.; Thomas, E.

    2011-01-01

    The Birim rocks of the West African craton comprise belts of greenschist- to amphibolite-grade gneiss and schist, and subparallel basins of greenschist-grade phyllite of volcaniclastic and epiclastic origin, which were intruded by igneous rocks. The granitoids intruded between 2213 and 2060 Ma and overlap with the volcaniclastic units dated between 2211 and 2064 Ma. The simultaneous occurrence of the magmatic events and irregular distribution of the rock ages hamper the formulation of a stratigraphic succession. SHRIMP spot analyses were done on older cores, crystals and rims from 23 rocks from the Bolé-Wa region in west-central Ghana. The crystallization ages range from 2195 to 2118 Ma, the inherited ages from 2876 to 2130 Ma, and metamorphic ages from 2114 to 2090 Ma. Aided by metamorphic, structural and chemical studies an older geotectonic cycle (2195-2150 Ma), containing the Dole and Guropie Suite and Bolé Group, was established. These units were subjected to several orthogonal and shear deformation events. These events were followed by the contemporaneous Sawla calc-alkaline monzonitic plutonism (2132-2126 Ma) and deposition of the epiclastic Maluwe Group (2137-2125 Ma) of calc-alkaline felsic to tholeiitic volcanic origin. Deformation of the basin beds was succeeded by the intrusion of the Tanina Suite granitoids of 2122-2120 Ma, which, themselves, were deformed prior to 2119 Ma. At 2118 Ma syenite and gabbro intruded along conjugate extension fractures. The gabbro and syenite of the Wakawaka Suite were only affected by three events of brittle strike-slip faulting. The first had significant displacement along NNE- to NE-directed shear zones, while the latter only formed conjugate joint systems with limited transport. Palaeo- to Neoarchaean cores, the oldest yet reported in the Baoulé Mossi domain, are restricted to the gneissic Dole Suite biotite granites. The presence of Dole-, Guropie-, Sawla-, and Tanina-aged older cores and grains in younger rocks

  2. Lithostratigraphic framework and correlation of the Neoproterozoic northern West African Craton passive margin sequence (Siroua-Zenaga-Bouazzer Elgraara Inliers, Central Anti-Atlas, Morocco): an integrated approach

    NASA Astrophysics Data System (ADS)

    Bouougri, El Hafid; Saquaque, Ali

    2004-06-01

    The Neoproterozoic volcano-sedimentary cover (˜0.8 Ga) deposited on the northern passive margin of the West African craton occurs in the central Anti-Atlas inliers and is a ˜2 km thick succession formally termed the Tizi n-Taghatine Group. The group comprises two sedimentary packages (lower and upper) separated by a middle volcanic unit. An integrated approach of lithostratigraphy, sedimentology, sequence stratigraphy and paleogeographic setting have been combined to provide well constrained correlation and lithostratigraphic organization. The Tizi n-Taghatine Group is formally subdivided into 12 Formations, 10 of which are defined in the lower sedimentary package while the middle volcanic unit and the upper sedimentary package constitute the two uppermost Formations. The lithostratigraphic organization and subdivision of the Tizi n-Taghatine Group constitutes a stratigraphic record of major paleogeographic settings and large cyclic changes in depositional system due to relative sea-level variations. The integrated approach of combination and comparison of various data provides significant information on the nature, origin and time significance of the formal unit boundaries. This leads to proposing a correlative scheme for the Tizi n-Taghatine Group across the Neoproterozoic central Anti-Atlas suture zone.

  3. Neotectonic faults and stress field in the East African Rift System around the Tanzanian Craton - A contribution to the seismotectonic map of Africa

    NASA Astrophysics Data System (ADS)

    Delvaux, Damien; Macheyeki, Athanas Simon; Fernandes, Rui-Manuel; Ayele, Atalay; Meghraoui, Mustapha

    2015-04-01

    As a contribution to the UNESCO-IUGS IGCP 601 project "Seismotectonics and seismic hazards in Africa" and in preparation of the Seismotectonic Map of Africa, we compiled the neotectonic faults related to the East African Rift System around the Tanzanian craton. The initial aim was to identify and map the potentially active faults. Faults are usually defined as active when they show seismogenic displacement during the last 10,000 to 100,000 years, generally on the basis of paleoseismic investigation. In East Africa, however, very few faults have been studied by paleoseismic techniques and even fewer have known historical seismic activation. To address this issue, we mapped faults that show morphological indications of displacement. We used the SRTM DTM (90 and 30 m when available to us), with artificial shading as basis for identify neotectonic faults, in combination with existing data from geological maps, publications and reports, complemented by our own field observations. Thermal springs often occur along tectonically active faults. We use them to distinguish present-day faulting from other mapped faults as they are in most cases structurally controlled. In parallel, we used also the available focal mechanisms and geological fault-slip data to constrain the stress second-order stress field (at the scale of rift segments) and locally also the third-order stress field (at the local scale). All these elements are combined and compared with existing kinematic models for the East African Rift based on earthquake slip vectors, GPS measurements and geologic indicators. The comparison evidences some local discrepancies between the stress field and the direction of opening, probably due to the interactions between different rift segments, as in the Rukwa rift, Mbeya southern junction between the eastern and western rift branches, and in the Manyara-Natron area.

  4. Support for a Uniformitarian Model of Continental Mantle Lithosphere Formation from the "Near-Cratonic" Composition of Proterozoic Southern African Mantle Lithosphere

    NASA Astrophysics Data System (ADS)

    Janney, P. E.

    2014-12-01

    The transition at the end of the Archean between the generation of cratonic and mobile belt continental lithosphere is regarded as a first-order change in the mode of generation of continental lithosphere. It is widely debated whether this transition represented a fundamental change in the process by which the lithospheric mantle was generated (i.e., as melting residues of deep-seated mantle upwellings to residues of relatively shallow mantle melting at subduction zones), or whether it primarily reflected a more gradual change in the conditions (i.e., temperatures, depths and degrees of melting) of lithosphere generation in a suprasubduction zone setting. The marked contrast, in many cases, between the major element compositions of peridotite xenoliths from Archean cratons and those from adjacent post-Archean mobile belts has accentuated the significance of this transition. Peridotite xenoliths from the post-Archean mobile belt terranes surrounding the Kaapvaal craton in southern Africa are clearly Proterozoic in age from Re-Os isotope constraints, but they are unusual in that they share several key similarities in composition and mineralogy with Archean Kaapvaal peridotites (e.g., low bulk-rock Al2O3, relatively low modal olivine and high modal orthopyroxene). Although they lack the low FeO and high olivine Mg# values of the most extreme Kaapvaal samples, they show a very large degree of overlap (extending to olivine Mg# values of greater than 93 for example). These similarities support a common mode of origin for cratonic and post-cratonic lithosphere in southern Africa (although varying somewhat in the degrees and depths of melt extraction) and a similar history of post-formation modification. A comparison of the conditions of melt extraction for cratonic and post-cratonic lithosphere inferred from compatible and mildly incompatible trace elements will be presented.

  5. Going It Alone: Supporting Writing across the Curriculum (WAC) When There Is No WAC Program.

    ERIC Educational Resources Information Center

    Gale, Charlotte

    This paper explains why a comprehensive Writing across the Curriculum (WAC) program at the author/educator's university, the University of the Sciences in Philadelphia (USP), would encounter strong opposition. The paper first points out that the author, as the director of the University's Writing Center, has tried for 7 years to make a WAC program…

  6. WAC: Closing Doors or Opening Doors for Second Language Writers?

    ERIC Educational Resources Information Center

    Cox, Michelle

    2011-01-01

    Written by a WAC program director and second language writing studies scholar, this article raises questions about how second language writers are faring in WAC programs and the extent to which the fields of second language writing and WAC are informed by each other's scholarship. In this article, Cox draws from her review of 26 journal articles…

  7. Évidences paléomagnétiques et paléontologiques en faveur d'une position antipodale du craton Ouest-Africain et de la Chine du Nord : conséquences paléogéographiquesPalaeomagnetic and palaeontologic evidence for an antipodal position of the West African Craton and of northern China in the Rodinia puzzle: palaeogeographic consequences

    NASA Astrophysics Data System (ADS)

    Lefort, Jean-Pierre; Aı̈fa, Tahar; Bourrouilh, Robert

    2004-02-01

    Reappraisal of the palaeomagnetic data previously published about the West African Craton and their comparison with the most recent information obtained on doleritic dykes and stromatolite-bearing formations allow us to propose, after a critical examination of the old geochronological data, a new path for the West African Craton between 2200 and 700 Ma. This path has been compared with that of eastern China (which can be simply divided into a northern and a southern China block) between 1400 and 700 Ma. We can observe that these two blocks show a common path before their accretion to the Rodinia super-continent. These two blocks, which were located close the equator and at the outboard of Rodinia at about 1000 Ma, were also characterized by identical stromatolitic assemblages. This result shows that West Africa and Eastern China represented the antipodal continental margins of Rodinia. One can also deduce from our data that the accretion of Rodinia mainly resulted from north-south-oriented displacements. To cite this article: J.-P. Lefort et al., C. R. Geoscience 336 (2004).

  8. Photometric normalization of LROC WAC images

    NASA Astrophysics Data System (ADS)

    Sato, H.; Denevi, B.; Robinson, M. S.; Hapke, B. W.; McEwen, A. S.; LROC Science Team

    2010-12-01

    The Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) acquires near global coverage on a monthly basis. The WAC is a push frame sensor with a 90° field of view (FOV) in BW mode and 60° FOV in 7-color mode (320 nm to 689 nm). WAC images are acquired during each orbit in 10° latitude segments with cross track coverage of ~50 km. Before mosaicking, WAC images are radiometrically calibrated to remove instrumental artifacts and to convert at sensor radiance to I/F. Images are also photometrically normalized to common viewing and illumination angles (30° phase), a challenge due to the wide angle nature of the WAC where large differences in phase angle are observed in a single image line (±30°). During a single month the equatorial incidence angle drifts about 28° and over the course of ~1 year the lighting completes a 360° cycle. The light scattering properties of the lunar surface depend on incidence(i), emission(e), and phase(p) angles as well as soil properties such as single-scattering albedo and roughness that vary with terrain type and state of maturity [1]. We first tested a Lommel-Seeliger Correction (LSC) [cos(i)/(cos(i) + cos(e))] [2] with a phase function defined by an exponential decay plus 4th order polynomial term [3] which did not provide an adequate solution. Next we employed a LSC with an exponential 2nd order decay phase correction that was an improvement, but still exhibited unacceptable frame-to-frame residuals. In both cases we fitted the LSC I/F vs. phase angle to derive the phase corrections. To date, the best results are with a lunar-lambert function [4] with exponential 2nd order decay phase correction (LLEXP2) [(A1exp(B1p)+A2exp(B2p)+A3) * cos(i)/(cos(e) + cos(i)) + B3cos(i)]. We derived the parameters for the LLEXP2 from repeat imaging of a small region and then corrected that region with excellent results. When this correction was applied to the whole Moon the results were less than optimal - no surprise given the

  9. Photometric normalization of LROC WAC images

    NASA Astrophysics Data System (ADS)

    Sato, H.; Boyd, A. K.; Denevi, B. W.; Robinson, M. S.; Hapke, B. W.; McEwen, A. S.; Humm, D. C.; LROC Science Operations Team

    2011-12-01

    Monthly global Lunar Reconnaissance Orbiter (LRO) Wide Angle Camera (WAC) observations of the Moon, acquired with varying emission and incidence angles, enable the precise derivation of spatially resolved Hapke photometric parameters [1]. The WAC global mosaics are stacked in a time series to enable phase curve fitting using a tile-by-tile method, with a wide range of phase angle in each tile. Tile-by-tile calculations provide photometric parameters for each tile (currently 1° by 1° from 80° to -80° latitude), resulting in resolved near-global photometric parameter maps (w, xi, Bco, and hc)[2]. Incidence, emission, and phase angles are computed using a new WAC stereometric digital terrain model (100 m/pixel)[3]. In the tile-by-tile method, we obtain low phase observations only near the equator resulting in an inability to accurately estimate Bco at higher latitudes (Hapke parameter controls the y-intercept of phase curve). We interpolated Bco poleward of +/- 5° latitudes, using a negative correlation between w and Bco observed in the equator +/- 5° latitudes. This interpolation method can decrease one free parameter, resulting in short calculation time and much less failed tiles. The normalized image using this parameter map shows almost no tile boundary, which shows that the tile-by-tile method works well. After the tile-by-tile method was applied with twenty months of data over a full range of beta angle, we observed an I/F offset with time. This offset results in reflectance differences at month-to-month boundaries in a global mosaic constructed from sequences acquired over different months (constructed to minimize incidence angle differences as a function of latitude). After significant tests of any possible parameters controlling the residual I/F value, incidence angle was revealed to be a dominant factor, indicating that the photometry model may not completely accounting for incidence angle. The main function dealing with incidence angle is the Lommel

  10. Origin of cratonic basins

    NASA Astrophysics Data System (ADS)

    Dev. Klein, George; Hsui, Albert T.

    1987-12-01

    Tectonic subsidence curves show that the Illinois, Michigan, and Williston basins formed by initial fault-controlled mechanical subsidence during rifting and by subsequent thermal subsidence. Thermal subsidence began around 525 Ma in the Illinois Basin, 520 460 Ma in the Michigan Basin, and 530 500 Ma in the Williston Basin. In the Illinois Basin, a second subsidence episode (middle Mississippian through Early Permian) was caused by flexural foreland subsidence in response to the Alleghanian-Hercynian orogeny. Resurgent Permian rifting in the Illinois Basin is inferred because of intrusion of well-dated Permian alnoites; such intrusive rocks are normally associated with rifting processes. The process of formation of these cratonic basins remains controversial. Past workers have suggested mantle phase changes at the base of the crust, mechanical subsidence in response to isostatically uncompensated excess mass following igneous intrusions, intrusion of mantle plumes into the crust, or regional thermal metamorphic events as causes of basin initiation. Cratonic basins of North America, Europe, Africa, and South America share common ages of formation (around 550 to 500 Ma), histories of sediment accumulation, temporal volume changes of sediment fills, and common dates of interregional unconformities. Their common date of formation suggests initiation of cratonic basins in response to breakup of a late Precambrian super-continent. This supercontinent acted as a heat lens that caused partial melting of the lower crust and upper mantle followed by emplacement of anorogenic granites during extensional tectonics in response to supercontinent breakup. Intrusion of anorogenic granites and other partially melted intrusive rocks weakened continental lithosphere, thus providing a zone of localized regional stretching and permitting formation of cratonic basins almost simultaneously over sites of intrusion of these anorogenic granites and other partially melted intrusive rocks.

  11. Origin of cratonic basins

    SciTech Connect

    de V. Klein, G.; Hsui, A.T.

    1987-12-01

    Tectonic subsidence curves show that the Illinois, Michigan, and Williston basins formed by initial fault-controlled mechanical subsidence during rifting and by subsequent thermal subsidence. Thermal subsidence began around 525 Ma in the Illinois Basin, 520-460 Ma in the Michigan Basin, and 530-500 Ma in the Williston Basin. In the Illinois Basin, a second subsidence episode (middle Mississippian through Early Permian) was caused by flexural foreland subsidence in response to the Alleghanian-Hercynian orogeny. Past workers have suggested mantle phase changes at the base of the crust, mechanical subsidence in response to isostatically uncompensated excess mass following igneous intrusions, intrusion of mantle plumes into the crust, or regional thermal metamorphic events as causes of basin initiation. Cratonic basins of North America, Europe, Africa, and South America share common ages of formation, histories of sediment accumulation, temporal volume changes of sediment fills, and common dates of interregional unconformities. Their common date of formation suggests initiation of cratonic basins in response to breakup of a late Precambrian supercontinent. This supercontinent acted as a heat lens that caused partial melting of the lower crust and upper mantle followed by emplacement of anorogenic granites during extensional tectonics in response to supercontinent breakup. Intrusion of anorogenic granites and other partially melted intrusive rocks weakened continental lithosphere, thus providing a zone of localized regional stretching and permitting formation of cratonic basins almost simultaneously over sites of intrusion of these anorogenic granites and other partially melted intrusive rocks.

  12. Geological evolution of the Antongil Craton, NE Madagascar

    USGS Publications Warehouse

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

    2010-01-01

    interpreted as being the only manifestation of the Pan-African orogeny seen in the craton, which led to the assembly of the tectonic blocks that comprise the island. ?? 2010 NERC.

  13. Craton stability and continental lithosphere dynamics during plume-plate interaction

    NASA Astrophysics Data System (ADS)

    Wang, H.; Van Hunen, J.; Pearson, D.

    2013-12-01

    the craton itself. Given the considerable debate on the uplift history of southern African plateau (Nyblade and Sleep, 2003), our numerical models that encompass lithospheric heterogeneity within cratons could help to achieve a better understanding of this issue.

  14. WAC for the New Millennium: Strategies for Continuing Writing-Across-the-Curriculum Programs.

    ERIC Educational Resources Information Center

    McLeod, Susan H., Ed.; Miraglia, Eric, Ed.; Soven, Margot, Ed.; Thaiss, Christopher, Ed.

    Celebrating the achievements of Writing Across the Curriculum (WAC) by highlighting the promise of its future, this book presents 12 essays that describe how WAC programs have adapted and continue to adapt to meet new challenges. Essays in the book explain strategies for continuing WAC programs in an atmosphere of change; explore new avenues of…

  15. How to make a craton

    NASA Astrophysics Data System (ADS)

    Lee, C.; Chin, E. J.; Erdman, M.; Gaschnig, R. M.; Lederer, G. W.; Savage, P. S.; Zhong, S.; Zincone, S.

    2013-12-01

    Most Archean cratons are underlain by long-lived 200-300 km thick thermal boundary layers, significantly thicker than oceanic boundary layers, which eventually subduct. The longevity of cratons is perplexing because cold thermal boundary layers should be gravitationally unstable or should thermally erode with time. However, it is agreed that thermal contraction of the cratonic root is compensated by intrinsic compositional buoyancy due to extreme melt depletion. This melt depletion is also thought to have dehydrated the peridotitic residue, strengthening the cratonic mantle, making it resistant to thermo-mechanical erosion. Exactly how cratonic mantle arrives at this chemically buoyant and dehydrated state is unknown. Possible scenarios include formation by melting within a large plume head, accretion of oceanic lithosphere, and accretion of sub-arc mantle. The high degrees of melting would seem to imply formation in hot plume heads, but low Al and heavy rare earth element contents suggest formation in the spinel stability field, implying formation at shallower depths than their current equilibration pressures. We present a new thermobarometer designed to estimate the average melting pressures and temperatures of residual peridotites using whole rock major element compositions. We find that the average melting pressures and temperatures of cratonic peridotites range between 3-4 GPa and 1600 °C. If cratonic peridotites melted via adiabatic decompression, these average pressures represent maximum bounds on the final pressures of melt extraction. Currently, cratonic peridotites derive from 4-7 GPa, implying that the building blocks of peridotites experienced an increase of 1-3 GPa, equivalent to 30-90 km of overburden. Our results thus imply that cratonic mantle most likely formed by tectonic thickening of oceanic or arc lithospheres. But because both arc and oceanic lithospheres might be expected to be wet due to hydrous flux melting and serpentinization

  16. Limiting depth of magnetization in cratonic lithosphere

    NASA Technical Reports Server (NTRS)

    Toft, Paul B.; Haggerty, Stephen E.

    1988-01-01

    Values of magnetic susceptibility and natural remanent magnetization (NRM) of clino-pyroxene-garnet-plagioclase granulite facies lower crustal xenoliths from a kimberlite in west Africa are correlated to bulk geochemistry and specific gravity. Thermomagnetic and alternating-field demagnetization analyses identify magnetite (Mt) and native iron as the dominant magnetic phases (totaling not more than 0.1 vol pct of the rocks) along with subsidiary sulfides. Oxidation states of the granulites are not greater than MW, observed Mt occurs as rims on coarse (about 1 micron) Fe particles, and inferred single domain-pseudosingle domain Mt may be a result of oxidation of fine-grained Fe. The deepest limit of lithospheric ferromagnetism is 95 km, but a limit of 70 km is most reasonable for the West African Craton and for modeling Magsat anomalies over exposed Precambrian shields.

  17. Separating Siamese Twins: Can We Extricate WAC from Writing Centers?

    ERIC Educational Resources Information Center

    Blumner, Jacob S.

    This paper addresses the culture of writing in higher education from a multicultural perspective of those within the "monolith." The paper first notes that writing programs, more specifically writing across the curriculum (WAC), and writing centers work in similar ways by benefiting each other and sharing the broad mission of improving students'…

  18. WAC Revisited: You Get What You Pay for

    ERIC Educational Resources Information Center

    Perelman, Les

    2011-01-01

    In 1982, the author wrote an essay for the second issue of "The Writing Instructor," "Approaches to Comprehensive Writing: Integrating Writing into the College Curriculum," reviewing the early stages of the modern Writing Across the Curriculum (WAC)/Writing in the Disciplines (WID) movement. In this article, the author revisits his essay and…

  19. The State of WAC/WID in 2010: Methods and Results of the U.S. Survey of the International WAC/WID Mapping Project

    ERIC Educational Resources Information Center

    Thaiss, Chris; Porter, Tara

    2010-01-01

    As writing across the curriculum (WAC) has matured and diversified as a concept and as an organizational structure in U.S. higher education, there has arisen a need for accurate, up-to-date information on the presence and characteristics of WAC and writing-in-the-disciplines (WID) programs. Following on the only previous nationwide survey of…

  20. Opposition effect of the Moon from LROC WAC data

    NASA Astrophysics Data System (ADS)

    Velikodsky, Yu. I.; Korokhin, V. V.; Shkuratov, Yu. G.; Kaydash, V. G.; Videen, Gorden

    2016-09-01

    LROC WAC images acquired in 5 bands of the visible spectral range were used to study the opposition effect for two mare and two highland regions near the lunar equator. Opposition phase curves were extracted from the images containing the opposition by separating the phase-curve effect from the albedo pattern by comparing WAC images at different phase angles (from 0° to 30°). Akimov's photometric function and the NASA Digital Terrain Model GLD100 were used in the processing. It was found that phase-curve slopes at small phase angles directly correlate with albedo, while at larger phase angles, they are anti-correlated. We suggest a parameter to characterize the coherent-backscattering component of the lunar opposition surge, which is defined as the maximum phase angle for which the opposition-surge slope increases with growing albedo. The width of the coherent-backscattering opposition effect varies from approximately 1.2° for highlands in red light to 3.9° for maria in blue light. The parameter depends on albedo, which is in agreement with the coherent-backscattering theory. The maximum amplitude of the coherent opposition effect is estimated to be near 8%. Maps of albedo and phase-curve slope at phase angles larger than those, at which the coherent-backscattering occurs, were built for the areas under study. Absolute calibration of WAC images was compared with Earth-based observations: the WAC-determined albedo is very close to the mean lunar albedo calculated using available Earth-based observations.

  1. Widespread refertilization of cratonic and circum-cratonic lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Tang, Yan-Jie; Zhang, Hong-Fu; Ying, Ji-Feng; Su, Ben-Xun

    2013-03-01

    Studies of mantle xenoliths have confirmed that Archean subcontinental lithospheric mantle (SCLM) is highly depleted in basaltic components (such as Al, Ca and Na) due to high-degree extraction of mafic and ultramafic melts and thus is refractory and buoyant, which made it chronically stable as tectonically independent units. However, increasing studies show that ancient SCLM can be refertilized by episodic rejuvenation events like infiltration of upwelling fertile material. The North China Craton is one of the most typical cases for relatively complete destruction of its Archean keel since the eruption of Paleozoic kimberlites, as is evidenced by a dramatic change in the compositions of mantle xenoliths sampled by Paleozoic to Cenozoic magmas, reflecting significant lithospheric thinning and the change in the character of the SCLM. The compositional change has been interpreted as the result of refertilization of Archean SCLM via multiple-stage peridotite-melt reactions, suggested by linear correlations between MgO and indices of fertility, covariations of Al2O3 with CaO, La/Yb, 87Sr/86Sr, 143Nd/144Nd, 187Os/188Os and Re-depletion ages (TRD), high Re abundances, scatter in Re-Os isotopic plot, variable in situ TRD ages of sulfides, and correlation between TRD ages and olivine Fo of peridotite xenoliths in Paleozoic kimberlites and Cenozoic basalts on the craton. By integrating major and trace element, Sr, Nd and Os isotopic compositions of peridotite xenoliths and orogenic massif peridotites from the continents of Europe, Asia, America, Africa and Australia, together with previous studies of petrology and geochemistry of global peridotites, we suggest that (1) refertilization of cratonic and circum-cratonic lithospheric mantle is widespread; (2) Archean SCLM worldwide has experienced a multi-stage history of melt depletion and refertilization since segregation from the convecting mantle; (3) cratonic SCLM may be more susceptible to compositional change caused by

  2. Three-Dimensional Forward Modeling of Magnetotelluric Data Over Cratonic Lithosphere and Attendent Geological Structures: Case Study of the Zimbabwe Craton

    NASA Astrophysics Data System (ADS)

    Miensopust, M. P.; Jones, A. G.; Farquharson, C. G.; MT Team

    2007-12-01

    The Southern African MagnetoTelluric Experiment (SAMTEX) is covering a huge area - containing parts of South Africa, Namibia and nearly the whole of Botswana - with acquisition of magnetotelluric (MT) data. The project's aim is to gain more information on the lithospheric geometries of the geological structures of this region which contains some of the oldest lithospheric pieces on Earth - the cratons (e.g. Kaapvaal and Zimbabwe cratons). Since the standard modeling of MT data is only in two-dimensions and the recently developed three-dimensional inversion programs require a lot of computation time and high speed computers with large memories, 3D forward modeling is a good compromise on the way to full 3D interpretation of MT data. Forward modeling of subsurface structures similar to the cratonic areas of study will give some indication of how the MT responses should look like and if they change significantly when the cratons have a different shape or extent or resistivity. The determined synthetic data then can be compared with the real data collected in the SAMTEX project. The area for the case study is the western edge of the Zimbabwe craton in eastern Botswana. The craton is surrounded by the Damara Mobile Belt and the Magondi Mobile Belt to the north and west and the Limpopo belt to the south. The giant northern Botswana dyke swarm is cross cutting the craton in about WNW to ESE direction. Based on this geological information, a 3D model was created to calculate synthetic MT responses using the forward modeling routine implemented in the 3D inversion program MT3Dinv (developed by the Geophysical Inversion Facility, University of British Columbia). We will show the results of this modeling exercise and compare them with the observations.

  3. On the Origin of Cratonic Sag Basins: Did They Sag?

    NASA Astrophysics Data System (ADS)

    Morgan, Jason P.

    2015-04-01

    Cratonic sag basins are regions of long-lived, extremely slow (~20-30 m/Myr) shallow water and terrestrial sediment accumulation that have no striking signs of tectonic activity (cf. Allen and Armitage, 2012). In their evolution, hundreds of Myr-long periods of slow sediment accumulation are separated by unconformities. The mechanisms for their formation resist geodynamic characterization by other common hypotheses for basin subsidence because of their extremely slow subsidence and lack of evident tectonic activity. I propose their dynamics are better understood within the geodynamic context of continental cratons that ride over a ~250km-deep sub-asthenospheric mantle with lateral temperature variations between a few wide and persistent 1000s-km broad ~1400C 'superplume' upwelling mantle structures (e.g. currently beneath S. African Atlantic and French Polynesia) and prevalent surrounding ~1150C average temperature sub-asthenospheric mantle. When continents pass over typical mantle plumes, buoyant plume material tends to drain beneath the continent along junctions between cratons where the lithosphere is relatively thin, keeping the lithosphere over regions where plume material drains hotter than the average temperature of ~250km-deep mantle. (e.g., the Cameroon Line.) Regions where melting of plume material occurs during decompression associated with either plume ascent or lateral drainage beneath continents are associated with the addition of a buoyant rind of more depleted mantle to the continent. In addition, regions where plume material can pond in a relatively thin sub-lithospheric 'anti-basin' beneath a continent, or that stay stationary for long times over super plumes will heat to a lithospheric basal temperature of ~1400C instead of ~1150C, with ~700m of associated uplift. (e.g., Southern Africa). In this scenario (cf. Yamamoto, Morgan, and Morgan in "Plumes, Plates, and Paradigms"), it is the relative plume-passage-induced uplift of arches between

  4. Report on Analyses of WAC Samples of Evaporator Overheads - 2004

    SciTech Connect

    Oji, L

    2005-03-18

    In November and December of 2004, the Tank Farm submitted annual samples from 2F, 2H and 3H Evaporator Overhead streams for characterization to verify compliance with the new Effluent Treatment Facility (ETF) Waste Acceptance Criteria (WAC) and to look for organic species. With the exception of slightly high ammonia in the 2F evaporator overheads and high radiation control guide number for the 3H and 2F evaporator overhead samples, all the overheads samples were found to be in compliance with the Effluent Treatment Facility WAC. The ammonium concentration in the 2F-evaporator overhead, at 33 mg/L, was above the ETF waste water collection tank (WWCT) limits of 28 mg/L. The RCG Number for the 3H and 2F evaporator samples at, respectively, 1.38E-02 and 8.24E-03 were higher than the WWCT limit of 7.69E-03. The analytical detection limits for americium-241 and radium-226 in the evaporator samples were not consistently met because of low WWCT detection limits and insufficient evaporator samples.

  5. Notes from the Margins: WAC, WID, and the Politics of Place(ment)

    ERIC Educational Resources Information Center

    Cook, Paul G.

    2014-01-01

    This institutional autoethnography (IAE) explores the political and pedagogical dynamics of WPA and WAC/WID work within an exceedingly small, resolutely single-sex, and assuredly rural liberal arts campus ecology. Working within a theoretical framework informed by WAC/WID's historical commitment to increasing literacy in students from diverse…

  6. Implementation of Writing across the Curriculum (WAC) Learning Approaches in Social Work and Sociology Gerontology Courses

    ERIC Educational Resources Information Center

    Kolb, Patricia

    2013-01-01

    This article describes the goals and methods of the international Writing Across the Curriculum (WAC) movement in higher education, and WAC-enriched learning approaches that the author used in teaching a social work gerontology practice course and a sociological theories of aging course. The author's in-class, low-stakes, nongraded writing…

  7. Metasomatism and the Weakening of Cratons: A Mechanism to Rift Cratons

    NASA Astrophysics Data System (ADS)

    Wenker, Stefanie; Beaumont, Christopher

    2016-04-01

    The preservation of cratons is a demonstration of their strength and resistance to deformation. However, several cratons are rifting now (e.g. Tanzania and North China Craton) or have rifted in the past (e.g. North Atlantic Craton). To explain this paradox, we suggest that widespread metasomatism of the originally cold depleted dehydrated craton mantle lithosphere root can act as a potential weakening mechanism. This process, particularly melt metasomatism, increases root density through a melt-peridotite reaction, and reduces root viscosity by increasing the temperature and rehydrating the cratonic mantle lithosphere. Using 2D numerical models, we model silicate-melt metasomatism and rehydration of cold cratonic mantle lithosphere that is positioned beside standard Phanerozoic lithosphere. The models are designed to investigate when a craton is sufficiently weakened to undergo rifting and is no longer protected by the initially weaker adjacent standard Phanerozoic lithosphere. Melt is added to specified layers in the cratonic mantle lithosphere at a uniform volumetric rate determined by the duration of metasomatism (3 Myr, 10 Myr or 30 Myr), until a total of ~30% by volume of melt has been added. During melt addition heat and mass are properly conserved and the density and volume increase by the respective amounts required by the reaction with the peridotite. No extensional boundary conditions are applied to the models during the metasomatism process. As expected, significant refertilization leads to removal and thinning of progressively more gravitationally unstable cratonic mantle lithosphere. We show that the duration of metasomatism dictates the final temperature in the cratonic upper mantle lithosphere. Consequently, when extensional boundary conditions are applied in our rifting tests in most cases the Phanerozoic lithosphere rifts. The craton rifts only in the models with the hottest cratonic upper mantle lithosphere. Our results indicate rifting of cratons

  8. Origin and evolution of the Amazonian craton

    NASA Technical Reports Server (NTRS)

    Gibbs, A. K.; Wirth, K. R.

    1986-01-01

    The Amazonian craton appears to be formed and modifed by processes much like those of the better-known Precambrian cratons, but the major events did not always follow conventional sequences nor did they occur synchronously with those of other cratons. Much of the craton's Archean style continental crust formation, recorded in granite-greenstone and high-grade terranes, occurred in the Early Proterozoic: a period of relative quiescence in many other Precambrian regions. The common Archean to Proterozoic transition in geological style did not occur here, but an analogous change from abundant marine volcanism to dominantly continental sedimentary and eruptive styles occurred later. Amazonian geology is summarized, explaining the evolution of the craton.

  9. Tectonics of the junction region between the East European craton and West Arctic platform

    NASA Astrophysics Data System (ADS)

    Baluev, A. S.; Morozov, Y. A.; Terekhov, E. N.; Bayanova, T. B.; Tyupanov, S. N.

    2016-09-01

    The region of the junction and interaction between the East European Craton (EEC) and the West Arctic Craton (WAC) is regarded as a complexly built zone or assembly of both the volumetric and dividing linear tectonic elements: the Trollfjord-Rybachi-Kanin (TRK) Lineament, the pericratonic subsidence zone of the EEC, the Karpinskii Lineament, the Murmansk Block of the Fennoscandian (Baltic) Shield, and the Kolmozero-Voronya Zone, which are briefly characterized in this paper. Evidences of thrusting have been established not only in the TRK Suture Zone and on the Rybachi Peninsula, which represent a fragment of the Timanides fold-thrust belt, but also to the southwest, in the Upper Riphean and Vendian terrigenous sequences making up the Sredni Peninsula and related to the pericratonic trough of the VEC. Two phases of fold-thrust deformations with elements of left-lateral strike-slip offset pertaining to the activity and evolution of the lineament suture dividing the Sredni and Rybachi peninsulas have been recorded. The variously oriented fault-fold systems within this fault zone are evidence for multistage deformation and can be explained by an at least twostage change in the kinematics that control displacement along the fault. The disintegrated granitic massifs of the Archean crystalline basement tectonically squeezed out in the upper crust as protrusions are localized within TRK Fault Zone. Plagiogranitic bodies, which underwent superposed fault-fold deformations of both kinematic stages, are an evidence of the vigorous tectonic event that predated folding and two-stage strike-slip displacement along the TRK Fault—by thrusting of Riphean sequences from north to south toward the Archean craton. The nappe-thrust regional structure was formed at this stage; elements of it have been recognized in the Sredni, Rybachi, and Kanin peninsulas. The main stages of tectonic evolution in the junction zone between the EEC and the WAP have been revealed and substantiated.

  10. Ambient Noise Imaging of Craton Modification: Examples from the Kaapvaal craton

    NASA Astrophysics Data System (ADS)

    Porter, R. C.; Shirey, S. B.; Fouch, M. J.; James, D. E.

    2013-12-01

    Mapping of crustal and upper-mantle seismic velocities can give important insight into the geologic evolution of cratonic regions. In order to better understand the formation and evolution of the Kaapvaal craton and surrounding regions, we use ambient noise tomography to produce a new crustal and upper-mantle 3D shear-velocity model for southeastern Africa using data collected during the (1995-1999) Kaapvaal Seismic Experiment. Ambient noise tomography enables imaging of shear-velocity variations at smaller scales (10s of km) and shallower depths (< 10 km) than previously determined using natural-source body- and surface-wave techniques within the region. The goal of this work is to image geological features of the Kaapvaal-Zimbabwe craton such as kimberlite pipe density, flood basalt feeder dike systems, granite-greenstone belts, layered mafic intrusions and craton assembly/collision patterns at mid- lower crustal levels where information is typically lacking. Our results provide important constraints on the evolution of the Kaapvaal craton, including the collision with the Zimbabwe craton and the formation of the Bushveld Complex and Witwatersrand basin, and the impact of these events on lithospheric structure. Preliminary images exhibit key first-order features of the craton. At depth ranges of 4-12 km, reduced shear velocities correlate with the Bushveld-Malopo Farms complexes. At a depth interval of 10-18 km, lower velocities are associated with the Witwatersrand sedimentary basin, and higher velocities may be attributed to unroofed mid-crustal rocks of high metamorphic grade formed during the Kaapvaal-Zimbabwe craton collision. At depths of 36-46 km, velocity patterns reflect Moho topography, with thinner crust associated with the undisturbed regions of the Kaapvaal and Zimbabwe cratons and thicker crust associated with the Limpopo Belt and Namaqua-Natal Mobile Belt. At greater depths, higher upper-mantle velocities are observed in the southern Kaapvaal

  11. Cratonic lithosphere: an electrifying view (Invited)

    NASA Astrophysics Data System (ADS)

    Jones, A. G.

    2013-12-01

    Deep-probing electromagnetic studies reveal the vertical and lateral electrical conductivity structure of cratonic lithosphere. At lithospheric temperatures and for silicate mantle minerals, semi-conduction is from small polaron hopping and, in the presence of water, proton conduction, both of which is thermally-driven and can be described by an Arrhenius equation. There is little compositional sensitivity, save for the far higher water contents prevalent in pyroxenes (typically 200-400 wt ppm) compared to olivine (typically 10-100 wt ppm), with the latter exhibiting a depth dependence and the former none. Seismological methods are sensitive to temperature and composition, and virtually insensitive to low amounts of water. Taken together, the two are highly complementary. Seismological and magnetotelluric studies across Southern Africa can be employed together to constraint temperature and water content, where there is a significant difference at 100 km depth between the Kaapvaal Craton compared to the Angola (Congo) Craton compared to the Zimbabwe Craton. The Congo Craton is driest but hottest, the Kaapvaal Craton exhibits laterally-varying water content and is coldest, and the Zimbabwe Craton is the wettest and intermediate in temperature. Such thermal and water content variation results in lateral rheological variation, with the wettest and warmest being more deformable. For the Canadian Shield, there is again significant lateral variation at 100 km, with the Superior Province being mostly dry, and the Slave Craton and Rae/Hearne Provinces being far wetter. By 200 km however, all of the Canadian Shield is dry, consistent with the xenolith-based observations from the Kaapvaal Craton of far higher water contents in olivine at 100 km (50-100 wt ppm) decreasing with depth to around 10 wt ppm at 200 km. The lithosphere beneath the Kimberley area of the Kaapvaal Craton has been forward modeled in a manner that is self-consistent not only with the seismological and

  12. LAB and other lithospheric discontinuities below Cratons

    NASA Astrophysics Data System (ADS)

    Sodoudi, Forough

    2013-04-01

    Cratons are extremely stable continental areas of the Earth's crust, which have been formed and remained largely unchanged since Precambrian. However, their formation and how they survived destruction over billions of years remains a subject of debate. Seismic properties of the cratonic lithosphere reflect its composition and physical state and obtain basic constraints on processes of the formation and evolution of continents. Insight on these issues may be gained by determining the depth and the nature of the Lithosphere-Asthenosphere Boundary (LAB), which is a necessary element of the plate tectonic theory. However, It has proved quite "elusive" beneath the oldest continental areas. What is missing to date is a consensus on the feature that would correspond to the LAB and whether such a feature exists everywhere beneath cratons. The relatively recently developed S receiver function technique employing S-to-P conversions appears promising for detecting the LAB with a sufficiently high resolution and density. A growing number of regional observations obtained from S receiver function studies has detected discontinuities characterized by a significant negative velocity contrast in the upper mantle. However, challenges still remain in detecting the S-to-P conversions from the LAB beneath the Precambrian cratons. Some recent SRF studies observed a deep (> 160 km) negative velocity contrast beneath cratons and interpreted it as the LAB. For example, a deep LAB at about 250 km was reported beneath the Kalahari craton by different authors. Similar results were also obtained beneath some parts of the Canadian shield, East European Craton, Australia, the Arabian Shield and Tanzania craton. In contrast, other SRF studies found no evidence for negative discontinuities at these depths in the North American craton, in Kalahari craton or in Australia. Instead they revealed a very sharp negative velocity gradient at much shallower depth (60-150 km), leading some authors to infer

  13. 3-d numerical modeling for the interaction of mantle plumes with cratonic keel

    NASA Astrophysics Data System (ADS)

    Lin, S.; Kuo, B.

    2003-04-01

    The magmatism of the Ethiopian and east African plateaus is one of the largest active continental igneous provinces on Earth. Great volumes of volcanic deposits have been thought to be originated from the upwelling mantle or plumes under the African continent [e.g., Ebinger et al., 1989]. Major, trace element and radiogenic isotope ratios (Sr, Nd and Pb) and the dating data [George et al., 1998] suggest that there are at least two mantle plumes, i.e., the latter Afar plume and earlier Kenya plume, beneath the East African rift system. It was proposed [e.g., Rogers et al., 2000] that the northeastward plate motion over the Kenya plume produced the magmatism from southern Ethiopia to northern Tanzania since about 45 Ma, and that the Afar plume later generated the magma in the Ethiopia Plateau. Meanwhile, it has been found that the Tanzania Craton in central Africa has survived the thermal erosion of the mantle plumes and the extensional tectonics in this region [e.g., Ritsema and van Heijst, 2000]. Here we investigate how the plume material changes its directions when it meets the tectonically stable cratonic keel using 3-D numerical experiments. The stronger temperature dependence of viscosity as well as the hotter plumes can at times provide higher buoyancy flux and determine how far the plume material can reach. In the meantime, the cratonic keel can divert the plume material and induce the edge-driven convection. Numerical models have been designed to address the double-plume hypothesis, in which the plumes were initiated at different periods of time and interacted with the cratonic keel on a moving plate. The numerical models and a comparison between the models and geological constraints will be presented.

  14. Water in the Cratonic Mantle Lithosphere

    NASA Technical Reports Server (NTRS)

    Peslier, A. H.

    2016-01-01

    The fact that Archean and Proterozoic cratons are underlain by the thickest (>200 km) lithosphere on Earth has always puzzled scientists because the dynamic convection of the surrounding asthenosphere would be expected to delaminate and erode these mantle lithospheric "keels" over time. Although density and temperature of the cratonic lithosphere certainly play a role in its strength and longevity, the role of water has only been recently addressed with data on actual mantle samples. Water in mantle lithologies (primarily peridotites and pyroxenites) is mainly stored in nominally anhydrous minerals (olivine, pyroxene, garnet) where it is incorporated as hydrogen bonded to structural oxygen in lattice defects. The property of hydrolytic weakening of olivine [4] has generated the hypothesis that olivine, the main mineral of the upper mantle, may be dehydrated in cratonic mantle lithospheres, contributing to its strength. This presentation will review the distribution of water concentrations in four cratonic lithospheres. The distribution of water contents in olivine from peridotite xenoliths found in kimberlites is different in each craton (Figure 1). The range of water contents of olivine, pyroxene and garnet at each xenolith location appears linked to local metasomatic events, some of which occurred later then the Archean and Proterozoic when these peridotites initially formed via melting. Although the low olivine water contents (<10 ppm wt H2O) at > 6 GPa at the base of the Kaapvaal cratonic lithosphere may contribute to its strength, and prevent its delamination, the wide range of those from Siberian xenoliths is not compatible with providing a high enough viscosity contrast with the asthenophere. The water content in olivine inclusions from Siberian diamonds, on the other hand, have systematically low water contents (<20 ppm wt H2O). The xenoliths may represent a biased sample of the cratonic lithosphere with an over-­abundance of metasomatized peridotites with

  15. Asymmetry and polarity of the South Atlantic conjugated margins related to the presence of cratons: a numerical study

    NASA Astrophysics Data System (ADS)

    Andrés-Martínez, Miguel; Pérez-Gussinyé, Marta; de Monserrat Navarro, Albert; Morgan, Jason P.

    2015-04-01

    Tectonic asymmetry of conjugated passive margins, where one margin is much narrower than the conjugate one, is commonly observed at many passive margins world-wide. Conjugate margin asymmetry has been suggested to be a consequence of lateral changes in rheology, composition, temperature gradient or geometries of the crust and lithosphere. Here we use the South Atlantic margins (from Camamu/Gabon to North Santos/South Kwanza) as a natural laboratory to understand conjugate margin asymmetry. Along this margin sector the polarity of the asymmetry changes. To the North, the Brazilian margin developed in the strong Sao Francisco craton, and this constitutes the narrow side of the conjugate pair. To the South, the Brazilian margin developed in the Ribeira fold belt, and the margin is wide. The opposite is true for the African side. We have thus numerically analysed how the relative distance between the initial location of extension and the craton influences the symmetry/asymmetry and polarity of the conjugate margin system. Our numerical model is 2D visco-elasto-plastic and has a free surface, strain weakening and shear heating. The initial set-up includes a cratonic domain, a mobile belt and a transition area between both. We have run tests with different rheologies, thickness of the lithosphere, and weak seeds at different distances from the craton. Results show asymmetric conjugated margins, where the narrower margin is generally the closest to the craton. Our models also allow us to study how the polarity is controlled by the distance between the initial weakness and the craton, and help to understand how the presence of cratonic domains affects the final architecture of the conjugated margins.

  16. Implementation of Writing Across the Curriculum (WAC) learning approaches in social work and sociology gerontology courses.

    PubMed

    Kolb, Patricia

    2013-01-01

    This article describes the goals and methods of the international Writing Across the Curriculum (WAC) movement in higher education, and WAC-enriched learning approaches that the author used in teaching a social work gerontology practice course and a sociological theories of aging course. The author's in-class, low-stakes, nongraded writing assignments facilitated students' development of knowledge about gerontological practice and sociological theories, as well as analytical thinking. The assignments are influenced by WAC's perspective that when students write their reactions to information, their understanding and retention of information improves; that writing can facilitate the application of new content to students' own lives and interests; and that increased frequency of writing increases writing comfort and maintenance and can result in the improvement of writing skills. The students' reactions to the assignments have been very positive.

  17. RESULTS FOR THE FOURTH QUARTER 2010 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M

    2011-02-22

    This report details the chemical and radionuclide contaminant results for the characterization of the 2010 Fourth Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Liquid Waste Operations (LWO) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: (1) The concentrations of the reported chemical and radioactive contaminants were less than their respective WAC targets or limits unless noted in this section. (2) The reported detection limits for {sup 94}Nb, {sup 247}Cm and {sup 249}Cf are above the requested limits from Reference 2. However, they are below the limits established in Reference 3. (3) There is an estimated concentration of trimethylbenzene (2.25 mg/L). This is not a WAC analyte, but it is the first time this organic compound has been detected in a quarterly WAC sample from Tank 50. (4) The reported detection limit for Norpar 13 is greater than the limit from Table 4 and Attachment 8.2 of the WAC. (5) The reported detection limit for Isopar L is greater than the limit from Table 3 of the WAC. (6) Isopar L and Norpar 13 have limited solubility in aqueous solutions making it difficult to obtain consistent and reliable sub-samples. The values reported in this memo are the concentrations in the sub-sample as detected by the GC/MS; however, the results may not accurately represent the concentrations of the analytes in Tank 50.

  18. Mesozoic Thermal Evolution of the Kaapvaal Craton Mantle Root.

    NASA Astrophysics Data System (ADS)

    Bell, D. R.

    2002-12-01

    Recent thermobarometry1 and accessory-mineral thermochronology2 studies on mantle and lower crustal xenoliths proposed a thermal pulse that affected the southern African mantle lithosphere during the Mesozoic and may relate to supercontinent breakup and associated mantle upwelling. Effects are concentrated on the margins of the Archean craton and in the surrounding Proterozoic terrains, but are not obvious within the core of the craton. In order to examine more closely the evidence for a thermal effect within the craton itself, we performed thermobarometry studies of a large suite of peridotite xenoliths from the 114 Ma Newlands orangeite (group II kimberlite). A variety of mineral thermobarometers were applied to analyses of coexisting minerals separated from fifty xenoliths. The resultant P-T arrays lie sub-parallel to those defined by xenoliths from the 84 Ma Kimberley group I kimberlites, some 50 km to the SE, but at temperatures from 60 to 120 C lower (depending on thermobarometer) at a given depth. This result, which is consistent with seismic images of a 250 - 300 km thick mantle root, confirms previous reports3,4 of unusually low temperatures, and inferences from garnet xenocryst studies5 that proposed a change from <37 to 40 mW/m2 model geotherms over this 30 Ma time interval. The corresponding depth difference for Kimberley and Newlands isotherms ranges from 12 - 20 km and exceeds expectations for mid-Cretaceous uplift5. It thus appears that the mantle root of the Kaapvaal craton has been heated, at least in the local regions sampled by Group I kimberlites, and that both the inflected and un-inflected limbs of Mesozoic group-I kimberlite-hosted geotherms on the Kaapvaal Craton may represent a perturbed thermal state of limited lateral extent. Petrologic studies on Kimberley xenoliths have documented the common presence of metasomatic diopside, phlogopite, and amphibole that are not common in the Newlands peridotites. Modal, trace element and isotope data

  19. RESULTS FOR THE SECOND QUARTER 2011 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Eibling, R.

    2011-08-25

    The Saltstone Facility is designed and permitted to immobilize and dispose of low-level radioactive and hazardous liquid waste (salt solution) remaining from the processing of radioactive material at the Savannah River Site. Low-level waste (LLW) streams from the Effluent Treatment Project (ETP), H-Canyon, the DDA (Deliquification, Dissolution, and Adjustment) process, and the decontaminated salt solution product from the Actinide Removal Process/Modular Caustic Side Solvent Extraction (CSSX) Unit (ARP/MCU) process are stored in Tank 50 until the LLW can be transferred to the Saltstone Facility for treatment and disposal. The LLW must meet the specified waste acceptance criteria (WAC) before it is processed into saltstone. The specific chemical and radionuclide contaminants and their respective WAC limits are listed in the current Saltstone WAC. Waste Solidification Engineering (WSE) requested that Savannah River National Laboratory (SRNL) perform quarterly analysis on saltstone samples. The concentrations of chemical and radionuclide contaminants are measured to ensure the saltstone produced during each quarter is in compliance with the current WAC. This report documents the concentrations of chemical and radionuclide contaminants for the 2011 Second Quarter samples collected from Tank 50 on April 4, 2011 and discusses those results in further detail than the previously issued results report. The following conclusions are drawn from the analytical results provided in this report: (1) The concentrations of the reported chemical and radioactive contaminants were less than their respective WAC targets or limits unless noted in this section. (2) The reported detection limit for {sup 59}Ni is above the requested limit from Reference 2 but below the established limit in Reference 3. (3) The reported detection limit for {sup 94}Nb is above the requested limit from Reference 2; however, it is below the established limits in Reference 3. (4) The reported concentration of

  20. Lessons for WAC/WID from Language Learning Research: Multicompetence, Register Acquisition, and the College Writing Student

    ERIC Educational Resources Information Center

    Hall, Jonathan; Navarro, Nela

    2011-01-01

    This article is a collaboration between WAC/WID and second language acquisition (SLA) specialists. It examines alternate disciplinary notions of the place of writing among other skills and adapts concepts from SLA theory and pedagogy with the goal of providing new interdisciplinary options for WAC/WID research and classroom practice.

  1. Stalled Orogen Linked to East Antarctic Craton Assembly

    NASA Astrophysics Data System (ADS)

    Martos, Y. M.; Ferraccioli, F.; Finn, C.; Bell, R. E.; Jordan, T. A.; Damaske, D.

    2015-12-01

    The interior of East Antarctica is often regarded as a coherent Archean craton surrounded by Paleo to Neoproterozoic orogenic belts. Here we use recent aerogeophysical, satellite magnetic, satellite gravity and passive seismic results in central East Antarctica to challenge this view. Firstly, anomalously thick crust (compared to most other cratons) has been imaged in East Antarctica by both passive seismic and gravity modelling with values up to 60 km (Ferraccioli et al., 2011, Nature; An et al., 2015, JGR). The thick crust underlies both the Gamburtsev Subglacial Mountains and an elevated region between Lake Vostok and Dronning Maud Land, referred to as the East Antarctic Mountain Ranges plateau (An et al., 2015). Second, satellite magnetic data reveal that the Gamburtev Province lies in between the Ruker/Princess Elizabeth Land, Vostok, Nimrod/South Pole and Recovery provinces. The Nimrod/ South Pole province is a Paleo to Meso Proterozoic igneous province formed along the edges of the Archean/Paleoproterozoic Mawson continent (e.g. Goodge and Fanning, 2010 JGR). Our aerogeophysical and sediment provenance data interpretations suggest that the Gamburtsev Province represents a distinct Grenvillian-age orogenic belt. A stalled orogen with thick crust (i.e. an orogen where widespread orogenic collapse and root delamination has not occurred)- is preserved in the interior of East Antarctica resembling e.g. the Paleoproterozoic Trans Hudson Orogen and segments of Grenvillian orogens in Laurentia. The stalled orogen may relate to widespread accretionary and collisional events within Rodinia. However, passive seismic interpretations (An et al., 2015) favour linking crustal thickening to the Pan-African age assembly of Greater India, East Antarctica and Australia within Gondwana (e.g. Aitken et al., 2014 GRL). Further aerogeophysical observations over Princess Elizabeth Land are timely to enable more robust correlations with geological observations and to help dating the

  2. Cyclic Cratonic Carbonates and Phanerozoic Calcite Seas.

    ERIC Educational Resources Information Center

    Wilkinson, Bruce H.

    1982-01-01

    Discusses causes of cyclicity in cratonic carbonate sequences and evidence for and potential significance of postulated primary calcite sediment components in past Paleozoic seas, outlining problems, focusing on models explaining existing data, and identifying background. Future sedimentary geologists will need to address these and related areas…

  3. The Tachakoucht-Iriri-Tourtit arc complex (Moroccan Anti-Atlas): Neoproterozoic records of polyphased subduction-accretion dynamics during the Pan-African orogeny

    NASA Astrophysics Data System (ADS)

    Triantafyllou, Antoine; Berger, Julien; Baele, Jean-Marc; Diot, Hervé; Ennih, Nasser; Plissart, Gaëlle; Monnier, Christophe; Watlet, Arnaud; Bruguier, Olivier; Spagna, Paul; Vandycke, Sara

    2016-05-01

    We report new mapping, tectonic, metamorphic and U-Pb zircon dating data on the polyphased Tachakoucht-Iriri and Tourtit arc-related units within the Moroccan Pan-African belt (Sirwa window, Anti-Atlas). The studied area contains four different sub-units, from south to north: (1) the Tachakoucht gneisses intruded to its northern part by (2) Iriri intrusions. To the north, the Tachakoucht-Iriri massif is thrusted by (3) the south-verging 760 Ma Khzama ophiolitic sequence intruded by (4) the Tourtit meta-granitic complex. The Tachakoucht gneiss represents former andesitic to dacitic porphyritic rocks crystallized around 740-720 Ma in an intra-oceanic arc setting (IOAS). Subsequently, it has been buried and metamorphosed to 700 °C, 8 kbar in response to early accretion of the arc onto the West African Craton (WAC). This tectono-metamorphic event also led to the dismembering and stacking of back-arc ophiolite onto the arc unit. Subsequently, the Iriri intrusions, a suite of hydrous mafic dykes (hornblende gabbro and fine-grained basalt) and ultramafic (hornblendite) plutons showing subduction zone affinities, intruded the Tachakoucht gneiss under P-T conditions of 750-800 °C and 2-5 kbar. Emplacement of Iriri intrusions led locally to pronounced partial melting of the Tachakoucht gneiss and to the production of leucogranitic melts. These melts crop out into the Iriri-Tachakoucht gneiss contacts as leucogneissic bands (former leucosomes, dated at 651 ± 5 Ma) but also intruded the Khzama ophiolite to form the Tourtit granite (dated at 651 ± 3 Ma). These ages (651-641 Ma) also constrain the timing of Iriri intrusion emplacement. The entire complex has been overprinted by a second deformation event under greenschist to amphibolite facies conditions marked by transposition of primary structures and a development of mylonitic shear zones. These results and those published on the Bou Azzer window show that two phases of subduction-related magmatism occurred in the Anti

  4. Beyond the L2 Metaphor: Towards a Mutually Transformative Model of ESL/WAC Collaboration.

    ERIC Educational Resources Information Center

    Matsuda, Paul Kei; Jablonski, Jeffrey

    A metaphor sometimes used in teaching discipline-based academic writing is that the experience of learning to write in various academic contexts is like learning a new language (L2). This approach is critically examined here, and its implications for "writing across the curriculum" (WAC) programs at the college level are discussed. It is argued…

  5. Building on Common Ground: Overcoming Resistance to WAC in the Technical College.

    ERIC Educational Resources Information Center

    Weiser, Michael S.

    Writing-Across-the-Curriculum (WAC) program administrators who wish to work effectively and amicably with faculty in the two-year technical college would do well to remember two principles: (1) work first with the type of writing that already exists in the curriculum; and (2) try to speak a language to the faculty outside the field of composition…

  6. WAC at Century's End: Haunted by the Ghost of Fred Newton Scott.

    ERIC Educational Resources Information Center

    McLeod, Susan H.

    1997-01-01

    Discusses two danger signs for writing across the curriculum (WAC) program survival, dealing with their cross-curricular structure, and their heavy dependence on just one person for their health and continuation. Outlines two cases from the history of writing instruction at the University of Michigan providing a precautionary tale for both kinds…

  7. Peer Response to Low Stakes Writing in a WAC Literature Classroom.

    ERIC Educational Resources Information Center

    Sargent, M. Elizabeth

    1997-01-01

    College courses which use writing across the curriculum (WAC) principles encourage students to write about assigned readings and to respond to each other's writings, resulting in better learning of course concepts and the experience of participating in a discipline's ongoing conversation. Guidelines for setting up peer response groups in both…

  8. RESULTS FOR THE FOURTH QUARTER 2011 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Bannochie, C.

    2012-01-31

    The Saltstone Facility is designed and permitted to immobilize and dispose of low-level radioactive and hazardous liquid waste (salt solution) remaining from the processing of radioactive material at the Savannah River Site. Low-level waste (LLW) streams from the Effluent Treatment Project (ETP), H-Canyon, and the decontaminated salt solution product from the Actinide Removal Process/Modular Caustic Side Solvent Extraction (CSSX) Unit (ARP/MCU) process are stored in Tank 50 until the LLW can be transferred to the Saltstone Facility for treatment and disposal. The LLW must meet the specified waste acceptance criteria (WAC) before it is processed into saltstone. The specific chemical and radionuclide contaminants and their respective WAC limits are in the current Saltstone WAC. Waste Solidification Engineering (WSE) requested that Savannah River National Laboratory (SRNL) perform quarterly analysis on saltstone samples. The concentrations of chemical and radionuclide contaminants are measured to ensure the saltstone produced during each quarter is in compliance with the current WAC. This report documents the concentrations of chemical and radionuclide contaminants for the 2011 Fourth Quarter samples collected from Tank 50 on October 12, 2011 and discusses those results in further detail than the previously issued results report.

  9. RESULTS FOR THE THIRD QUARTER 2011 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M.

    2011-10-20

    The Saltstone Facility is designed and permitted to immobilize and dispose of low-level radioactive and hazardous liquid waste (salt solution) remaining from the processing of radioactive material at the Savannah River Site. Low-level waste (LLW) streams from the Effluent Treatment Project (ETP), H-Canyon, and the decontaminated salt solution product from the Actinide Removal Process/Modular Caustic Side Solvent Extraction (CSSX) Unit (ARP/MCU) process are stored in Tank 50 until the LLW can be transferred to the Saltstone Facility for treatment and disposal. The LLW must meet the specified waste acceptance criteria (WAC) before it is processed into saltstone. The specific chemical and radionuclide contaminants and their respective WAC limits are in the current Saltstone WAC. Waste Solidification Engineering (WSE) requested that Savannah River National Laboratory (SRNL) perform quarterly analysis on saltstone samples. The concentrations of chemical and radionuclide contaminants are measured to ensure the saltstone produced during each quarter is in compliance with the current WAC. This report documents the concentrations of chemical and radionuclide contaminants for the 2011 Third Quarter samples collected from Tank 50 on July 7, 2011 and discusses those results in further detail than the previously issued results report.

  10. Results For The First Quarter 2012 Tank 50 WAC Slurry Sample: Chemical And Radionuclide Contaminant Results

    SciTech Connect

    Bannochie, C. J.

    2012-07-16

    This report details the chemical and radionuclide contaminant results for the characterization of the 2012 First Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Waste Solidification Engineering (WSE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this memorandum: The concentrations of the reported chemical and radioactive contaminants were less than their respective WAC targets or limits unless noted; The reported detection limit for {sup 94}Nb is above the requested limit but below the estimated limit; {sup 247}Cm and {sup 249}Cf are above the requested limits. However, they are below the limits established; The reported detection limit for Norpar 13 is greater than the limit from the WAC; The reported detection limit for Isopar L is greater than the limit from WAC; Isopar L and Norpar 13 have limited solubility in aqueous solutions making it difficult to obtain consistent and reliable sub-samples; The values reported in this report are the concentrations in the sub-sample as detected by the instrument, however, the results may not accurately represent the concentrations of the analytes in Tank 50; The low insoluble solids content increases the measurement uncertainty for insoluble species.

  11. Reinventing WAC (Again): The First-Year Seminar and Academic Literacy

    ERIC Educational Resources Information Center

    Brent, Doug

    2005-01-01

    Academically oriented first-year seminars can be good venues for teaching many of the concepts important to WAC programs, including extended engagement with a research topic and situated writing. A qualitative study of a first-year seminar program at the University of Calgary highlights faculty members' and students' responses.

  12. Is Nubia plate rigid? A geodetic study of the relative motion of different cratonic areas within Africa.

    NASA Astrophysics Data System (ADS)

    Njoroge, M. W.; Malservisi, R.; Hugentobler, U.; Mokhtari, M.; Voytenko, D.

    2014-12-01

    Plate rigidity is one of the main paradigms of plate tectonics and a fundamental assumption in the definition of a global reference frame as ITRF. Although still far for optimal, the increased GPS instrumentation of the African region can allow us to understand how rigid one of the major plate can be. The presence of diffused band of seismicity, the Cameroon volcanic line, Pan African Kalahari orogenic belt and East Africa Rift suggest the possibility of relative motion among the different regions within the Nubia. The study focuses on the rigidity of Nubia plate. We divide the plate into three regions: Western (West Africa craton plus Nigeria), Central (approximately the region of the Congo craton) and Southern (Kalahari craton plus South Africa) and we utilize Euler Vector formulation to study internal rigidity and eventual relative motion. Developing five different reference frames with different combinations of the 3 regions, we try to understand the presence of the relative motion between the 3 cratons thus the stability of the Nubia plate as a whole. All available GPS stations from the regions are used separately or combined in creation of the reference frames. We utilize continuous stations with at least 2.5 years of data between 1994 and 2014. Given the small relative velocity, it is important to eliminate eventual biases in the analysis and to have a good estimation in the uncertainties of the observed velocities. For this reason we perform our analysis using both Bernese and Gipsy-oasis codes to generate time series for each station. Velocities and relative uncertainties are analyzed using the Allan variance of rate technique, taking in account for colored noise. An analysis of the color of the noise as function of latitude and climatic region is also performed to each time series. Preliminary results indicate a slight counter clockwise motion of West Africa craton with respect to South Africa Kalahari, and South Africa Kalahari-Congo Cratons. In addition

  13. RESULTS FOR THE FIRST QUARTER 2009 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M.; Diprete, C.; Bibler, N.

    2009-10-06

    This report details the chemical and radionuclide contaminant results for the characterization of the 2009 First Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Liquid Waste Operations (LWO) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: (1) The concentrations of the chemical and radioactive contaminants were all less than their respective WAC Targets or Limits except for Am-242m. (2) The radionuclide Am-242m was not detected; however, its detection limit is above the WAC Target given in Attachment 8.4. The higher detection limit was expected based on current analytical capabilities as stated in the Task Technical and Quality Assurance Plan (TTQAP). (3) The reported detection limit of isopropanol was lower than its WAC Limit for accident analysis but higher than its WAC concentration given in Table 4 for vault flammability. The higher detection limit was expected based on current analytical capabilities and is documented in the Task Technical and Quality Assurance Plan (TTQAP). (4) The reported detection limit for Isopar L is lower than its WAC limit for accident analysis in Appendix 8.1 but higher than its WAC concentration given in Table 3 in reference to vault flammability. The higher detection limit was expected based on current analytical capabilities as stated in the Task Technical and Quality Assurance Plan (TTQAP). (5) Isopar L and Norpar 13 have limited solubility in aqueous solutions making it difficult to obtain consistent and reliable sub-samples. The values reported in this memo are the concentrations in the sub-sample as detected by the GC/MS; however, the results may not accurately

  14. The Río de la Plata craton and the assembly of SW Gondwana

    NASA Astrophysics Data System (ADS)

    Rapela, C. W.; Pankhurst, R. J.; Casquet, C.; Fanning, C. M.; Baldo, E. G.; González-Casado, J. M.; Galindo, C.; Dahlquist, J.

    2007-07-01

    origin within Gondwana-forming blocks, for which the closest identifiable sources are 'Brazilian' and 'African' (Namaqua-Natal). Consequently, the preferred model for the Pampean orogeny is that the Río de la Plata craton reached its present position by large-scale dextral strike-slip movement against fore-arc sedimentary sequences that had developed on the southern and western margins of the Kalahari craton during the Early Cambrian. In the final stage the displaced sedimentary sequences outboard of the RPC collided with the Mesoproterozoic Western Sierras Pampeanas terrane, which was at the time attached to the large Amazonia craton and other smaller continental blocks, such as Arequipa-Antofalla and Río Apa. Protracted relative displacement of the RPC after the Pampean Orogeny led to its final position.

  15. RESULTS FOR THE SECOND QUARTER 2010 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M.; Bibler, N.

    2010-08-04

    This report details the chemical and radionuclide contaminant results for the characterization of the 2010 Second Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC).1 Information from this characterization will be used by Liquid Waste Operations (LWO) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: (1) The concentrations of the reported chemical and radioactive contaminants were less than their respective WAC targets or limits unless noted in this section. (2) The reported detection limits for {sup 94}Nb and {sup 144}Ce are above both the established and requested limits from References 4 and 6. (3) The reported detection limits for {sup 247}Cm and {sup 249}Cf are above the requested limits from Reference 4. However, they are below the limits established in Reference 6. (4) The reported detection limit for Isopar L is greater than the limit from Table 3 of the WAC. (5) A measurable concentration of Norpar 13 is present in the sample. The reported concentration is greater than the requested limit from Table 4 and Attachment 8.2 of the WAC. (6) Isopar L and Norpar 13 have limited solubility in aqueous solutions making it difficult to obtain consistent and reliable sub-samples. The values reported in this memo are the concentrations in the sub-sample as detected by the GC/MS; however, the results may not accurately represent the concentrations of the analytes in Tank 50. (7) The detection limit for isopropanol has been lowered from 0.5 mg/L to 0.25 mg/L{sup 7}. This revised limit now satisfies the limit in Table 4 of the WAC.

  16. RESULTS FOR THE THIRD QUARTER 2010 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M.; Bibler, N.

    2010-12-09

    This report details the chemical and radionuclide contaminant results for the characterization of the 2010 Third Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Liquid Waste Operations (LWO) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: (i) The concentrations of the reported chemical and radioactive contaminants were less than their respective WAC targets or limits unless noted in this section. (ii) The reported detection limits for {sup 94}Nb, {sup 247}Cm and {sup 249}Cf are above the requested limits from Reference 4. However, they are below the limits established in Reference 3. (iii) The reported detection limit for {sup 242m}Am is greater than the requested limit from Attachment 8.4 of the WAC. (iv) The reported detection limit for Isopar L is greater than the limit from Table 3 of the WAC. (v) The reported concentration of Isopropanol is greater than the limit from Table 4 of the WAC. (vi) Isopar L and Norpar 13 have limited solubility in aqueous solutions making it difficult to obtain consistent and reliable sub-samples. The values reported in this memo are the concentrations in the sub-sample as detected by the GC/MS; however, the results may not accurately represent the concentrations of the analytes in Tank 50.

  17. Seismic reflection profiling of Neoarchean cratons

    NASA Astrophysics Data System (ADS)

    van der Velden, Arie Jan

    Deep seismic reflection data from the Superior and Slave cratons in Canada and the Yilgarn craton in Australia are processed with similar processing flows and display parameters to facilitate comparative analysis. The profiles are characterized by subhorizontal to shallowly dipping reflection fabrics in the crust and upper mantle. These reflection fabrics are interpreted as ˜2.8-2.6 Ga ductile structural fabrics associated with peak orogenesis that led to cratonization. A re-evaluation of the seismic data has led to alternative interpretations compared to those published previously. In western Ontario, at Red Lake, divergent reflection patterns are interpreted as products of mainly collisional tectonics rather than extensional tectonics, and at Pickle Lake, mantle reflections connect to a mapped suture and strikeslip fault system. In western Quebec, steep structures are interpreted on the north flank of the Opatica domain. In the Kalgoorlie area of western Australia, subhorizontal upper crustal reflections are interpreted as pre-deformational layers within anticlines. A new tectonic model is presented for the western Slave Province in which divergent reflections at Yellowknife are interpreted to be associated with convergence between the Snare arc and the central Slave basement complex. Reflections that project from the reflection Moho into the upper mantle are observed on all profiles and are interpreted as relict subduction zones and/or major terrane-bounding structures. Listric mid-crustal reflections resembling roofing shingles are interpreted as products of underthrusting and subcretion. Strike-slip faults are manifested as near-vertical zones of reflection truncations. Greenstone belts are often poorly reflective. These reflection patterns are consistent with tectonic models in which greenstone belts form adjacent to protocratons and are thickened by protocontinent-dipping subduction, tectonic underplating, formation of nappes, and thrust-and-fold structures

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

    NASA Astrophysics Data System (ADS)

    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.

  19. Cold cratonic roots and thermal blankets: How continents affect mantle convection

    USGS Publications Warehouse

    Trubitsyn, V.P.; Mooney, W.D.; Abbott, D.H.

    2003-01-01

    Two-dimensional convection models with moving continents show that continents profoundly affect the pattern of mantle convection. If the continents are wider than the wavelength of the convection cells (???3000 km, the thickness of the mantle), they cause neighboring deep mantle thermal upwellings to coalesce into a single focused upwelling. This focused upwelling zone will have a potential temperature anomaly of about 200??C, much higher than the 100??C temperature anomaly of upwelling zones generated beneath typical oceanic lithosphere. Extensive high-temperature melts (including flood basalts and late potassic granites) will be produced, and the excess temperature anomaly will induce continental uplift (as revealed in sea level changes) and the eventual breakup of the supercontinent. The mantle thermal anomaly will persist for several hundred million years after such a breakup. In contrast, small continental blocks (<1000 km diameter) do not induce focused mantle upwelling zones. Instead, small continental blocks are dragged to mantle downwelling zones, where they spend most of their time, and will migrate laterally with the downwelling. As a result of sitting over relatively cold mantle (downwellings), small continental blocks are favored to keep their cratonic roots. This may explain the long-term survival of small cratonic blocks (e.g., the Yilgarn and Pilbara cratons of western Australia, and the West African craton). The optimum size for long-term stability of a continental block is <3000 km. These results show that continents profoundly affect the pattern of mantle convection. These effects are illustrated in terms of the timing and history of supercontinent breakup, the production of high-temperature melts, and sea level changes. Such two-dimensional calculations can be further refined and tested by three-dimensional numerical simulations of mantle convection with moving continental and oceanic plates.

  20. Structural framework across the Bastar craton - the Eastern Ghats Granulite Belt interface: Implications for making of eastern Gondwana

    NASA Astrophysics Data System (ADS)

    Patole, Vishal; Nasipuri, Pritam

    2015-04-01

    existence of a single landmass since the time of Grenvillian orogeny (1000 Ma) and it is suggested that the same configuration was preserved up to the formation of East Gondwana. The EGGB orogenic belt is considered as intercontinental high grade domain with no evidence for the closing of Mesoproterozoic oceans and the younger Pan-African metamorphic events are the results of intercontinental reactivation of old crustal weakness during the Gondwana assembly in the late Neoproterozoic-Paleozoic. However, recent palaeomagnetic data from India and Australia indicate that India was at much higher palaeolatitude compared to the Australia-Antarctica block, and the accretion of Eastern Ghats Granulite Belt over Bastar craton should have occurred during the Pan-African orogeny. The presence of NNW-SSW trending melt bands and the increase in the intensity of melting and tightness of folding near the contact indicate that Eastern Ghats Granulite belt collided with the Bastar craton during the Paleozoic.

  1. How craton margins are preserved: Insights from geodynamic models

    NASA Astrophysics Data System (ADS)

    Currie, Claire A.; van Wijk, Jolante

    2016-10-01

    Lateral variations in lithosphere thickness are observed in many continental regions, especially at the boundary between the ancient cratonic core and the adjacent more juvenile lithosphere. In some places, such as the North America craton margin in western Canada and the Sorgenfrei-Tornquist Zone in northern Europe, the transition in lithosphere thickness has a steep gradient (>45°) and it appears to be a long-lived feature (at least 50 Ma). We use thermal-mechanical numerical models to address the dynamics of lithospheric thickness changes on timescales of 100 Ma. Models start with the juxtaposition of 60 km thick lithosphere ("mobile belt") and 160 km thick lithosphere ("craton"). In the reference model, all mantle materials have a damp olivine rheology and a density comparable to primitive mantle. With this configuration, edge-driven mantle convection occurs at the craton boundary, resulting in a lateral smoothing of the thickness transition. The density and rheology of the craton mantle lithosphere are then varied to approximate changes in composition and water content. For all densities, a steep transition is maintained only if the craton strength is 5-50 times stronger than the reference damp olivine. If dry olivine is an upper limit on strength, only cratonic mantle with moderate compositional buoyancy (20-40 kg/m3 less dense than primitive mantle) remains stable. At higher densities, the thick lithosphere is eroded through downwellings, and the craton margin migrates inboard. Conversely, a compositionally buoyant craton destabilises through lateral spreading below the mobile belt.

  2. RESULTS FOR THE THIRD QUARTER 2009 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M.; Diprete, C.; Bibler, N.

    2009-11-13

    This report details the chemical and radionuclide contaminant results for the characterization of the 2009 Third Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Liquid Waste Operations (LWO) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. Recently, a review of the radionuclide inventory in Saltstone Vaults 1 and 4 identified several additional radionuclides, not currently in the WAC, which require quantification ({sup 40}K, {sup 108m}Ag, {sup 133}Ba, {sup 207}Bi, {sup 227}Ac, {sup 228}Ra, {sup 228}Th, {sup 231}Pa, {sup 247}Cm, {sup 249}Cf, {sup 251}Cf). In addition, several of the radionuclides previously reported with minimum detection limits below the requirements listed in the WAC required analysis with reduced detection limits to support future inventory reporting requirements ({sup 22}Na, {sup 26}Al, {sup 59}Ni, {sup 94}Nb, {sup 106}Ru, {sup 144}Ce, {sup 152}Eu, {sup 155}Eu, {sup 226}Ra). This added scope was formally requested in a revision to the standing Technical Task Request for CY2009 Saltstone support and is further discussed in several supporting documents. The following conclusions are drawn from the analytical results provided in this report: (1) The concentrations of the reported chemical and radioactive contaminants are less than their respective WAC targets or limits unless noted in this section. (2) The reported detection limits for {sup 59}Ni, {sup 94}Nb, {sup 247}Cm, and {sup 249}Cf are above the limits requested by LWO; however, they are below the achievable limits established by Analytical Development (AD). (3) The reported detection limit of isopropanol is lower than its WAC Limit for accident analysis in Appendix 8.1, but higher than its WAC concentration given in

  3. RESULTS FOR THE FOURTH QUARTER TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M.; Bibler, N.

    2010-01-27

    The Saltstone Facility is designed and permitted to immobilize and dispose of low-level radioactive and hazardous liquid waste (salt solution) remaining from the processing of radioactive material at the Savannah River Site. Low-level waste (LLW) streams from the Effluent Treatment Project (ETP), H-Canyon, the DDA (Deliquification, Dissolution, and Adjustment) process, and the decontaminated salt solution product from the Actinide Removal Process/Modular Caustic Side Solvent Extraction (CSSX) Unit (ARP/MCU) process are stored in Tank 50 until the LLW can be transferred to the Saltstone Facility for treatment and disposal. The LLW must meet the specified waste acceptance criteria (WAC) before it is processed into saltstone. The specific chemical and radionuclide contaminants and their respective WAC limits are listed in the current Saltstone WAC. SRS Liquid Waste Operations (LWO) requested that Savannah River National Laboratory (SRNL) perform quarterly analysis on saltstone samples. The concentrations of chemical and radionuclide contaminants are measured to ensure the saltstone produced during each quarter is in compliance with the current WAC. This report documents the concentrations of chemical and radionuclide contaminants for the 2009 Fourth Quarter samples collected from Tank 50 on October 2, 2009 and discusses those results in further detail than the previously issued results report. This report details the chemical and radionuclide contaminant results for the characterization of the 2009 Fourth Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Liquid Waste Operations (LWO) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results

  4. REPORT ON THE ANALYSIS OF WAC SAMPLES FROM EVAPORATOR OVERHEADS FOR 2011

    SciTech Connect

    Washington, A.

    2012-03-29

    This report tabulates the chemical analysis of the 3H, 2H and 2F evaporator overhead samples including the inorganic, organic, and radionuclide species according the ETP WAC (rev.4). In addition, the physical properties including pH, total suspended solids, and average particle size are listed for each sample. The report identifies all sample receipt dates, preparation methods, and analyses completed to accumulate these values. All values were found to be within the ETP WAC specifications for WWCT except for the {sup 137}Cs concentration for the 2F sample which was slightly above the limit ({approx}5%). However, tank farm personnel sample each tank overhead for beta/gamma prior to transfer using a pulse height analyzer and these field readings were all well within limits. Additional actions will be taken to help understand the reason in the discrepancy of the field measurement versus laboratory analysis.

  5. Episodic and multistaged gravitational instability of cratonic lithosphere and its implications for reactivation of the North China Craton

    NASA Astrophysics Data System (ADS)

    Wang, Yongming; Huang, Jinshui; Zhong, Shijie

    2015-03-01

    Archean cratons are the most stable tectonic units and their lithospheric mantle is chemically depleted and buoyant relative to the underlying mantle. The chemical depletion leads to high viscosity that maintains the long-term stability of cratons. However, the eastern part of the North China Craton (˜1200 km in horizontal length scale) had been extensively reactivated and modified over a time scale of ˜100 Myr in the Mesozoic and Cenozoic. While the causes for the weakening of the North China Craton, a necessary condition for its reactivation, are still in debate, we investigate gravitational instability of compositionally buoyant lithosphere, by computing 2-D thermochemical convection models with different buoyancy number, lithospheric viscosity, and rheology. We find that the gravitational instability of cratonic lithosphere can happen over a larger range of buoyancy numbers with non-Newtonian rheology, but lithospheric instability with Newtonian rheology only happens with relatively small buoyancy numbers. For cratonic lithosphere with non-Newtonian rheology and relatively weak temperature-dependent viscosity, the instability starts in the cold, shallow part of the lithosphere and has small horizontal length scale (<300 km), leading to efficient thermal and chemical mixing with the underlying mantle. For cratonic lithosphere such as the eastern North China Craton, the instability process is episodic and consists of multiple instability events that may last for ˜100 Myr. The instability process revealed from our study explains the observations of episodic magmatism/volcanism events, geochemical mixing, and time scales associated with the reactivation of the North China Craton.

  6. RESULTS FOR THE FIRST QUARTER 2010 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M.; Bibler, N.

    2010-05-05

    This report details the chemical and radionuclide contaminant results for the characterization of the 2010 First Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Liquid Waste Operations (LWO) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: (1) The concentrations of the reported chemical and radioactive contaminants were less than their respective WAC targets or limits unless noted in this section. (2) The reported detection limit for Isopar L is greater than the limit from Table 3 of the WAC. (3) The reported detection limits for {sup 59}Ni and {sup 94}Nb are above the requested limits from Reference 4. However, they are each below the limits established in Reference 6. (4) The reported detection limit for isopropanol is greater than the requested limit from Table 4 of the WAC. (5) The reported detection limits for 247Cm and 249Cf are above the requested limits from Reference 4. However, they are below the limits established in Reference 6. (6) Isopar L and Norpar 13 have limited solubility in aqueous solutions making it difficult to obtain consistent and reliable sub-samples. The values reported in this memo are the concentrations in the sub-sample as detected by the GC/MS; however, the results may not accurately represent the concentrations of the analytes in Tank 50.

  7. RESULTS FOR THE FIRST QUARTER 2012 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Bannochie, C.

    2012-06-06

    This report details the chemical and radionuclide contaminant results for the characterization of the 2012 First Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Waste Solidification Engineering (WSE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this memorandum: (1) The concentrations of the reported chemical and radioactive contaminants were less than their respective WAC targets or limits unless noted in this section; (2) The reported detection limit for {sup 94}Nb is above the requested limit from Reference 2 but below the estimated limit in Reference 3; (3) {sup 247}Cm and {sup 249}Cf are above the requested limits from Reference 2. however, they are below the limits established in Reference 3; (4) The reported detection limit for Norpar 13 is greater than the limit from Table 4 and Attachment 8.2 of the WAC; (5) The reported detection limit for Isopar L is greater than the limit from Table 3 of the WAC; (6) Isopar L and Norpar 13 have limited solubility in aqueous solutions making it difficult to obtain consistent and reliable sub-samples, the values reported in this report are the concentrations in the sub-sample as detected by the instrument; however, the results may not accurately represent the concentrations of the analytes in Tank 50; and (7) The low insoluble solids content increases the measurement uncertainty for insoluble species.

  8. Activation of ULK Kinase and Autophagy by GABARAP Trafficking from the Centrosome Is Regulated by WAC and GM130.

    PubMed

    Joachim, Justin; Jefferies, Harold B J; Razi, Minoo; Frith, David; Snijders, Ambrosius P; Chakravarty, Probir; Judith, Delphine; Tooze, Sharon A

    2015-12-17

    Starvation-induced autophagy requires activation of the ULK complex at the phagophore. Two Golgi proteins, WAC and GM130, regulate autophagy, however their mechanism of regulation is unknown. In search of novel interaction partners of WAC, we found that GM130 directly interacts with WAC, and this interaction is required for autophagy. WAC is bound to the Golgi by GM130. WAC and GM130 interact with the Atg8 homolog GABARAP and regulate its subcellular localization. GABARAP is on the pericentriolar matrix, and this dynamic pool contributes to autophagosome formation. Tethering of GABARAP to the Golgi by GM130 inhibits autophagy, demonstrating an unexpected role for a golgin. WAC suppresses GM130 binding to GABARAP, regulating starvation-induced centrosomal GABARAP delivery to the phagophore. GABARAP, unlipidated and lipidated, but not LC3B, GABARAPL1, and GATE-16, specifically promotes ULK kinase activation dependent on the ULK1 LIR motif, elucidating a unique non-hierarchical role for GABARAP in starvation-induced activation of autophagy. PMID:26687599

  9. Results for the Fourth Quarter 2012 Tank 50 WAC Slurry Sample: Chemical and Radionuclide Contaminants

    SciTech Connect

    Bannochie, Christopher J.

    2014-09-30

    This report details the chemical and radionuclide contaminant results for the characterization of the 2012 Fourth Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC).1 Information from this characterization will be used by Waste Solidification Engineering (WSE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: The concentration of the reported chemical and radioactive contaminants were less than their respective WAC Limits and Targets, unless noted in this section. Norpar 13 and Isopar L have higher detection limits5 compared with the Saltstone WAC1. The data provided in this report is based upon the concentrations in the sub-sample, and due to the limited solubility in aqueous solution, may not represent the concentrations of the analytes in Tank 50. Diisooctyl adipate (or diisooctyl hexanedioate) was measured at 1.30E+00 mg/L in one of two replicate measurements conducted on an at-depth sample.a The organic analysis of the at-depth sample was conducted at the request of SRR.4 This analyte was below the detection limit in the surface sample. The low insoluble solids content increases the measurement uncertainty for insoluble species.

  10. Results for the Third Quarter 2013 Tank 50 WAC Slurry Sample: Chemical and Radionuclide Contaminants

    SciTech Connect

    Bannochie, Christopher J.

    2014-09-30

    This report details the chemical and radionuclide contaminant results for the characterization of the 2013 Third Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC) in effect at that time.1 Information from this characterization will be used by DWPF & Saltstone Facility Engineering (DSFE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: SRR WAC targets or limits were met for all analyzed chemical and radioactive contaminates unless noted in this section. 59Ni, 94Nb, 247Cm, 249Cf, and 251Cf are above the requested SRR target concentrations.2 However, they are below the detection limits established by SRNL.3 Norpar 13 and Isopar L have higher detection limits4 compared with the Saltstone WAC.1 The data provided in this report is based upon the concentrations in the sub-sample, and due to the limited solubility in aqueous solution, may not represent the concentrations of the analytes in Tank 50. Finally, the low insoluble solids content increases the measurement uncertainty for insoluble species.

  11. Results For The First Quarter 2013 Tank 50 WAC Slurry Sample: Chemical And Radionuclide Contaminants

    SciTech Connect

    Bannochie, C. J.

    2013-05-14

    This report details the chemical and radionuclide contaminant results for the characterization of the 2013 First Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Waste Solidification Engineering (WSE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: SRR WAC targets or limits were met for all analyzed chemical and radioactive contaminates unless noted in this section; {sup 59}Ni, {sup 94}Nb, {sup 247}Cm, {sup 249}Cf, and {sup 251}Cf are above the requested SRR target concentrations. However, they are below the detection limits established by SRNL; Norpar 13 and Isopar L have higher detection limits compared with the Saltstone WAC. The data provided in this report is based upon the concentrations in the sub-sample, and due to the limited solubility in aqueous solution, may not represent the concentrations of the analytes in Tank 50; and, The low insoluble solids content increases the measurement uncertainty for insoluble species.

  12. Geology of interior cratonic sag basins

    SciTech Connect

    Leighton, M.W.; Eidel, J.J.; Kolata, D.R.; Oltz, D.F. )

    1990-05-01

    Interior cratonic sag basins are thick accumulations of sediment, generally more or less oval in shape, located entirely in the interiors of continental masses. Some are single-cycle basins and others are characterized by repeated sag cycles or are complex polyhistory basins. Many appear to have developed over ancient rift systems. Interior cratonic sag basins are typified by a dominance of flexural over fault-controlled subsidence, and a low ratio of sediment volume to surface area of the basin. The Baltic, Carpentaria, Illinois, Michigan, Parana, Paris, and Williston basins are examples of interior cratonic sag basins. Tectonics played a dominant role in controlling the shapes and the geometries of the juxtaposed packets of sedimentary sequences. While the mechanics of tectonic control are not clear, evidence suggests that the movements are apparently related to convergence of lithospheric plates and collision and breakup of continents. Whatever the cause, tectonic movements controlled the freeboard of continents, altering base level and initiating new tectono-sedimentologic regimes. Sag basins situated in low latitudes during their development commonly were sites of thick carbonates (e.g., Illinois, Michigan, Williston, and Paris basins). In contrast, siliciclastic sedimentation characterized basins that formed in higher latitudes (e.g., Parana and Carpentaria basins). Highly productive sag basins are characterized by widespread, mature, organic-rich source rocks, large structures, and good seals. Nonproductive basins have one or more of the following characteristics: immature source rocks, leaky plumbing, freshwater flushing, and/or complex geology due to numerous intrusions that inhibit mapping of plays.

  13. The deep water cycle and origin of cratonic flood basalts: two examples from the Siberian craton

    NASA Astrophysics Data System (ADS)

    Ivanov, A. V.

    2014-12-01

    Cratonic flood basalt volcanism is the most puzzling phenomenon compared to all other types of intraplate volcanism. Cratons are thick and cold; the two parameters which suppress melting of either sublithospheric or lithospheric mantle in dry conditions. Fusible eclogites at hot plume geotherm start to melt in sublithospheric depth (~ 230 km), but geochemical arguments require that, in addition to eclogitic component, significant volume of flood basalts are from peridotitic mantle. Dry peridotitic mantle cannot be melted unless the lithospheric thickness reduced to about 60 km. That is why modern plume models incorporate lithospheric delamination and/or initially thinned lithosphere to explain cratonic flood basalts. However, if lithosphere remained thick, which was the case for the Siberian craton by the time of formation of its Devonian (Vilyui) and Permo-Triassic (Siberian) flood basalt provinces, then plume model is unable to explain the flood basalt volcanism. If mantle is wet, the peridotitic solidus lowered such as it can start to melt at sublithospheric depth (for example, 2 wt. % H2O-bearing peridotite starts to melt at ~320 km depth even at a normal mantle geotherm). In this presentation I will show that fluxing of mantle via the deep water cycle process may explain the Siberian craton flood basalts and many other continental flood basalts. According to the deep water cycle model, water is carried to the mantle transition zone by fast subducting slabs (may be in form of solid ice VII), then water is released from the slabs due to warming to the ambient mantle temperature, then localized hydration creates buoyant wet diapirs (or melt-bearing diapirs), the diapirs raise up to the sublithosheric depth were melt accumulates for the following tectonically triggered flood basalt eruptions.

  14. RESULTS FOR THE FIRST QUARTER 2011 TANK 50 WAC SLURRY SAMPLE: CHEMICAL AND RADIONUCLIDE CONTAMINANT RESULTS

    SciTech Connect

    Reigel, M.

    2011-06-15

    This report details the chemical and radionuclide contaminant results for the characterization of the 2011 First Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Waste Solidification Engineering (WSE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: (1) The concentrations of the reported chemical and radioactive contaminants were less than their respective WAC targets or limits unless noted in this section; (2) The reported detection limit for {sup 59}Ni is above both the requested limits from Reference 2 and the established limits in Reference 3; (3) The reported detection limit for {sup 94}Nb is above the requested limit from Reference 2; however, it is below the established limits in Reference 3. This is a change from previously reported results; (4) The reported concentration of {sup 242m}Am is above the target in Listed in Attachment 8.4 of the Saltstone WAC. This is a change from the previously reported results; (5) {sup 247}Cm and {sup 249}Cf are above the requested limits from Reference 2. However, they are below the limits established in Reference 3; (6) The reported detection limit for Norpar 13 is greater than the limit from Table 4 and Attachment 8.2 of the WAC; (7) The reported detection limit for Isopar L is greater than the limit from Table 3 of the WAC; and (8) Isopar L and Norpar 13 have limited solubility in aqueous solutions making it difficult to obtain consistent and reliable sub-samples. The values reported in this memo are the concentrations in the sub-sample as detected by the instrument; however, the results may not accurately represent the concentrations of the analytes in Tank

  15. Thermotectonic Evolution of the Archean Yilgarn Craton, Western Australia: Insights into Gondwana Amalgmation and Breakup

    NASA Astrophysics Data System (ADS)

    Lu, S.; Phillips, D.; Kohn, B. P.; Gleadow, A. J. W.; Matchan, E.; Li, G.

    2015-12-01

    The post-cratonisation history of the Yilgarn craton is somewhat fragmentary due to the paucity or absence of a stratigraphic record. Previously reported Rb-Sr biotite cooling ages suggest that the western craton margin was subjected to late 'Pan-African' tectonism (~400-600 Ma) and E-W compression, resulting in tectonic sedimentary loading. We report 40Ar/39Ar results on muscovite and single biotite grains from well-documented sample sites broadly comparable to those sampled for Rb-Sr biotite analysis. Muscovites record consistent ages between ~2400 and 2200 Ma. Coexisting single-grain biotite results, however, reveal significant age variations, showing similar ages of ~2500 Ma across much of the craton but decreasing abruptly to ~620-1000 Ma over a ~50-100 km wide belt at the western margin. Petrographic and chemical studies indicate that biotites are distinctly different in origin, i.e. magmatic to the east versus hydrothermal to the west. The more scattered biotite ages along the western margin are therefore unlikely to represent cooling ages, but rather indicate probable fluid-induced partial or complete biotite recrystallization at ~620 Ma. This age is in agreement with palaeomagnetic reconstructions suggesting oblique collision between Great India and Australian during Gondwana amalgamation. Complementary zircon He data from low radiation damage grains ([eU] <900 ppm), yield consistent ages 256 ±13 Ma, whereas those with [eU] values of 900-2000 ppm are negatively correlated with younger ages (~ 40-200 Ma). Coexisting apatite fission track ages are similar in range to zircon He ages, whereas apatite He ages are younger (227 ±18 Ma). These data confirm that much of the Yilgarn craton experienced extremely slow cooling (<0.1 ºC/Myr) from the Late Archean to Permian times. However, during the Late Permian to Triassic, the craton apparently experienced accelerated cooling (>4 ºC/Myr), possibly related to earliest rifting between Great India and Western

  16. Disentangling Middle Paleozoic sea level and tectonic events in cratonic margins and cratonic basins of North America

    NASA Astrophysics Data System (ADS)

    Bond, Gerard C.; Kominz, Michelle A.

    1991-04-01

    The cratonic margins and basins of North America contain evidence of distinct changes in relative sea level, one of the most intriguing of which occurred in middle Paleozoic time. The change in relative sea level began in Frasnian time (Late Devonian) and continued through Visean time (Middle Mississippian) in the Cordilleran miogeocline, in the Southern Oklahoma Aulacogen, in the Appalachian miogeocline and in the Michigan, Illinois, and Williston basins. The synchroneity and wide geographic distribution of this event are striking and would seem to argue for an eustatic mechanism. An estimate of the middle Paleozoic sea level rise relative to the stable craton in Iowa suggests that while a large sea level rise occurred, it is smaller than the magnitude of subsidence in the cratonic basins and margins. Flexural foreland basin models do not appear to account for the all of the events in the cratonic margins, and thermal subsidence mechanisms do not seem appropriate for the subsidence in the cratonic basins. The middle Paleozoic stratigraphic record from the North American craton and its margins, therefore, poses a basic problem of identifying a mechanism for producing a large-amplitude rise in sea level relative to the stable craton at the same time as a synchronous onset of tectonic subsidence in widespread basinal and marginal settings of diverse tectonic origin. One plausible mechanism for the tectonic subsidence in the basins and margins is a pulse of intraplate compressive stress. The origin of the large sea level rise relative to the stable craton could reflect an unusually large eustatic sea level change, but we cannot eliminate the possibility of a small component of subsidence or change in dynamic topography of the North American craton. The synchroneity of the sea level rise relative to the craton with the subsidence of basins and margins may be fortuitous, but it is also predicted by recent mantle convection models for the early stages of accretion of

  17. RESULTS FOR THE FOURTH QUARTER 2013 TANK 50 WAC SLURRY SAMPLE CHEMICAL AND RADIONUCLIDE CONTAMINANTS

    SciTech Connect

    Bannochie, C.

    2014-04-01

    This report details the chemical and radionuclide contaminant results for the characterization of the 2013 Fourth Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC) in effect at that time. Information from this characterization will be used by DWPF & Saltstone Facility Engineering (DSFE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: SRR WAC targets or limits were met for all analyzed chemical and radioactive contaminants unless noted in this section. {sup 59}Ni, {sup 94}Nb, {sup 247}Cm, {sup 249}Cf, and {sup 251}Cf are above the requested SRR target concentrations. However, they are below the detection limits established by SRNL. Norpar 13 and Isopar L have higher detection limits compared with the Saltstone WAC. The data provided in this report is based upon the concentrations in the sub-sample, and due to the limited solubility of these materials in aqueous solution, may not represent the concentrations of the analytes in Tank 50. The low insoluble solids content increases the measurement uncertainty for insoluble species. The semivolatile organic analysis (SVOA) method employed in the measurement of Norpar 13 and tributyl phosphate (TBP) has resulted in the erroneous reporting of a variety of small chain alcohols, including 4-methyl-3-hexanol and 5-methyl-3-hexanol, in previous quarterly sample reports. It has now been determined that these alcohols are an artifact of the sample preparation. Further work is being conducted in SRNL to delineate the conditions that produce these alcohols, and these findings will be reported separately.

  18. Results For The Fourth Quarter 2012 Tank 50 WAC Slurry Sample: Chemical And Radionuclide Contaminants

    SciTech Connect

    Bannochie, C. J.

    2013-02-05

    This report details the chemical and radionuclide contaminant results for the characterization of the 2012 Fourth Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC).Information from this characterization will be used by Waste Solidification Engineering (WSE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System. The following conclusions are drawn from the analytical results provided in this report: The concentration of the reported chemical and radioactive contaminants were less than their respective WAC Limits and Targets, unless noted in this section; Norpar 13 and Isopar L have higher detection limits compared with the Saltstone WAC. The data provided in this report is based upon the concentrations in the sub-sample, and due to the limited solubility in aqueous solution, may not represent the concentrations of the analytes in Tank 50; Diisooctyl adipate (or diisooctyl hexanedioate) and 5-methyl-3-hexanol, plasticizers, were measured at 1.30E+00 mg/L and 3.00E+00 mg/L, respectively, in one of two replicate measurements conducted on an at-depth sample. The organic analysis of the at-depth sample was conducted at the request of SRR. These analytes were below the detection limits for the surface sample; and, The low insoluble solids content increases the measurement uncertainty for insoluble species.

  19. Results For The Fourth Quarter 2014 Tank 50 WAC Slurry Sample: Chemical And Radionuclide Contaminants

    SciTech Connect

    Crawford, C.

    2015-09-30

    This report details the chemical and radionuclide contaminant results for the characterization of the Calendar Year (CY) 2014 Fourth Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC) in effect at that time. Information from this characterization will be used by DWPF & Saltstone Facility Engineering (DSFE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System.

  20. Results for the Third Quarter 2012 Tank 50 WAC Slurry Sample: Chemical and Radionuclide Contaminants

    SciTech Connect

    Bannochie, C. J.

    2012-10-26

    This report details the chemical and radionuclide contaminant results for the characterization of the 2012 Third Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC). Information from this characterization will be used by Waste Solidification Engineering (WSE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System.

  1. Results For The Third Quarter 2013 Tank 50 WAC Slurry Sample

    SciTech Connect

    Bannochie, Christopher J.

    2013-11-26

    This report details the chemical and radionuclide contaminant results for the characterization of the 2013 Third Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC) in effect at that time. Information from this characterization will be used by DWPF & Saltstone Facility Engineering (DSFE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System.

  2. Results for the second quarter 2014 tank 50 WAC slurry sample chemical and radionuclide contaminants

    SciTech Connect

    Bannochie, C.

    2014-09-04

    This report details the chemical and radionuclide contaminant results for the characterization of the 2014 Second Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC) in effect at that time. Information from this characterization will be used by DWPF & Saltstone Facility Engineering (DSFE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System.

  3. Results for the Third Quarter 2014 Tank 50 WAC slurry sample: Chemical and radionuclide contaminants

    SciTech Connect

    Crawford, Charles L.

    2015-01-08

    This report details the chemical and radionuclide contaminant results for the characterization of the 2014 Third Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC) in effect at that time.1 Information from this characterization will be used by DWPF & Saltstone Facility Engineering (DSFE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System.

  4. Results For The Second Quarter 2013 Tank 50 WAC Slurry Sample: Chemical And Radionuclide Contaminants

    SciTech Connect

    Bannochie, Christopher J.

    2013-07-31

    This report details the chemical and radionuclide contaminant results for the characterization of the 2013 Second Quarter sampling of Tank 50 for the Saltstone Waste Acceptance Criteria (WAC) in effect at that time. Information from this characterization will be used by Saltstone Facility Engineering (SFE) to support the transfer of low-level aqueous waste from Tank 50 to the Salt Feed Tank in the Saltstone Facility in Z-Area, where the waste will be immobilized. This information is also used to update the Tank 50 Waste Characterization System.

  5. Late Precambrian aulacogens of the North China craton

    NASA Technical Reports Server (NTRS)

    Qian, X.

    1985-01-01

    According to tectonic styles, the Precambrian evolution history of the North China craton may be subdivided into four stages: (1) Archean consolidation in 3.5 to 2.5 Ga, (2) Early Proterozoic rifting in 2.5 to 1.8 Ga, (3) Late Precambrian aulacogen in 1.8 to 0.8 Ga and (4) Platform regime after 0.8 Ga. In the Late Precambrian aulacogen stage of the North China craton there were two main aulacogens, Yanliao and Zhongtiao (Y and Z), developed in Middle Proterozoic time with an age 1.8 to 1.0 Ga. Their NE trend caused them to meet together in the central part of the craton and build up one great Y-Z aulacogen throughout the craton.

  6. Patterns and origin of igneous activity around the Tanzanian craton

    NASA Astrophysics Data System (ADS)

    Foley, S. F.; Link, K.; Tiberindwa, J. V.; Barifaijo, E.

    2012-01-01

    Tertiary and later igneous activity is common on and around the Tanzanian craton, with primitive magma compositions ranging from kimberlites and varieties of picrites through nephelinites, basanites and alkali basalts. This review focuses on elucidating the conditions of origin of the melts, addressing the question of the state and involvement of the Tanzanian cratonic lithosphere in magma genesis. The Tanzanian craton is anomalous with a surface elevation of >1100 m reflecting buoyancy supported by a subcratonic plume whose effects are seen in the volcanics of both western and eastern rift branches. Magmatism on the craton and at its edge has high K/Na and primitive melts show fractionation dominated by olivine. Slightly further from the craton pyroxene fractionation dominates and K/Na ratios in the magmas are lower. Off-craton melts are nephelinites, basanites and alkali basalts with low K/Na. Potassium enrichment in the melts correlates with the occurrence of phlogopite in mantle-derived xenoliths, and also with carbonate in the magmas. This is attributed to melting at >140 km depths of mixed source regions containing phlogopite pyroxenite and peridotite, whereby the carbonate is derived from oxidation of diamonds concentrated near the base of the cratonic lithosphere. Mixed source regions are required by arrays of radiogenic isotopes such as Os and Sr in the volcanic rocks. The temporal progression of lamproites to phlogopite + carbonate-rich rocks to melilitites, nephelinites and alkali basalts seen during the erosion of the North Atlantic craton are seen around the Tanzanian craton as the coeval occurrence kimberlites, kamafugites and related rocks, nephelinites and alkali basalts showing spatial instead of temporal variation. This is due to the different stages of development of rifting around the craton: in northwestern Uganda and northern Tanzania, K-rich volcanism occurs at the craton edge, whereas nephelinites, basanites and alkali basalts occur where

  7. The Thermal Structure and Strength of Cratons and their Margins

    NASA Astrophysics Data System (ADS)

    Jaupart, C. P.; Mareschal, J. C.

    2015-12-01

    The large cratons of today are made of younger terranes that wrap around older cores. Deformation due to accretion did not proceed in homogeneous fashion and was concentrated in the younger belts. This is illustrated clearly in the Archean Superior Province, Canada. In this area, one observes little imbrication of accreted crust and craton core, in contrast to the laterally extensive thrusting that has affected the younger terranes to the South. The boundary between the craton core and accreted belts is a nearly vertical interface delineated by steeply dipping electrical and seismic anomalies extending to the base of the lithosphere. These steeply dipping structures have been interpreted as relicts of the subduction that drove accretion. By contrast, the sub-crustal subduction remnant that is imaged beneath younger terranes to the south shows up as a moderately dipping (≈30°) structure. These observations suggest a stiff craton surrounded by weaker belts. This strength contrast may have affected later events, such as the Keweenawan rifing, which propagated northward through the accreted terranes but stopped short of impinging the craton core. In the Superior Province, crustal heat production is much higher in the accreted terranes than in the craton core, implying higher temperatures and lower mechanical strength. Such a remarkable dichotomy also exists in South Africa, where the Limpopo and Namaqua belts are characterized by higher heat flux and crustal heat production than the adjacent Archean Kaapvaal and Zimbabwe cratons. The generality of this cannot be assessed on the basis of heat flow and heat production data which are scarce in most other cratons. These cratons, however, are characterized by post-orogenic high temperature metamorphism which is best explained by high crustal heat production. This is true, for example, for the Jimperding metamorphic belt at the edge of the Yilgarn craton, Western Australia. Thus, cratons appear to be surrounded, and

  8. Construction and destruction of some North American cratons

    NASA Astrophysics Data System (ADS)

    Snyder, D. B.; Humphreys, G.

    2015-12-01

    Construction histories of Archean cratons remain poorly understood; their destruction is even less clear because of, by definition, its rarity. By assembling geophysical and geochemical data in 3-D lithosphere models, a clearer understanding of the geometry of major structures within the Rae, Slave and Wyoming cratons of central North America is now possible. Little evidence exists of subducted slabs similar to modern oceanic lithosphere in these construction histories whereas underthrusting and wedging of proto-continental lithosphere is inferred from multiple dipping discontinuities. Archean continental building blocks may resemble the modern lithosphere of Ontong-Java-Hikurangi oceanic plateau. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can be correlated to surface rocks. The 3.6-2.6 Ga Rae, Slave and Wyoming cratons comprise smaller continental terranes that 'cratonized' during a granitic bloom at 2.61-2.55 ga. Cratonization probably represents the final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho atop depleted lithospheric mantle. Peak thermo-tectonic events at 1.86-1.7 Ga broadly metasomatized, mineralized and recrystallized mantle and lower crustal rocks, apparently making mantle peridotite more 'fertile' and conductive by introducing or concentrating sulfides or graphite throughout the lithosphere at 80-120 km depths. This metasomatism may have also weakened the lithosphere or made it more susceptible to tectonic or chemical erosion. The arrival of the subducted Shatsky Rise conjugate at the Wyoming craton at 65-75 Ma appears to have eroded and displaced the thus weakened base of the craton below 140-160 km. This replaced old refertilized continental mantle with new depleted oceanic mantle. Is this the same craton?

  9. Plume-cratonic lithosphere interaction recorded by water and other trace elements in peridotite xenoliths from the Labait volcano, Tanzania

    NASA Astrophysics Data System (ADS)

    Hui, Hejiu; Peslier, Anne H.; Rudnick, Roberta L.; Simonetti, Antonio; Neal, Clive R.

    2015-06-01

    Water and other trace element concentrations in olivine (1-39 ppm H2O), orthopyroxene (10-150 ppm H2O), and clinopyroxene (16-340 ppm H2O) of mantle xenoliths from the Labait volcano, located on the edge of the Tanzanian craton along the eastern branch of the East African Rift, record melting and subsequent refertilization by plume magmas in a stratified lithosphere. These water contents are at the lower end of the range observed in other cratonic mantle lithospheres. Despite correlations between water content and indices of melting in orthopyroxene from the shallow peridotites, and in both olivine and orthopyroxene from the deep peridotites, water concentrations are too high for the peridotites to be simple residues. Instead, the Labait water contents are best explained as reflecting interaction between residual peridotite with a melt having relatively low water content (<1 wt.% H2O). Plume-derived melts are the likely source of water and other trace element enrichments in the Labait peridotites. Only garnet may have undergone addition of water from the host magma as evidenced by water content increasing toward the kelyphite rim in one otherwise homogeneous garnet. Based on modeling of the diffusion profile, magma ascent occurred at 4-28 m/s. In summary, plume-craton interaction appears to result in only moderate water enrichment of the lithosphere.

  10. Water in the Cratonic Mantle: Insights from FTIR Data on Lac De Gras Xenoliths (Slave Craton, Canada)

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Brandon, Alan D.; Schaffer, Lillian Aurora; O'Reilly, Suzanne Yvette; Griffin, William L.; Morris, Richard V.; Graff, Trevor G.; Agresti, David G.

    2014-01-01

    The mantle lithosphere beneath the cratonic part of continents is the deepest (> 200 km) and oldest (>2-3 Ga) on Earth, remaining a conundrum as to how these cratonic roots could have resisted delamination by asthenospheric convection over time. Water, or trace H incorporated in mineral defects, could be a key player in the evolution of continental lithosphere because it influences melting and rheology of the mantle. Mantle xenoliths from the Lac de Gras kimberlite in the Slave craton were analyzed by FTIR. The cratonic mantle beneath Lac de Gras is stratified with shallow (<145 km) oxidized ultradepleted peridotites and pyroxenites with evidence for carbonatitic metasomatism, underlain by reduced and less depleted peridotites metasomatized by kimberlite melts. Peridotites analyzed so far have H O contents in ppm weight of 7-100 in their olivines, 58 to 255 in their orthopyroxenes (opx), 11 to 84 in their garnet, and 139 in one clinopyroxene. A pyroxenite contains 58 ppm H2O in opx and 5 ppm H2O in its olivine and garnet. Olivine and garnet from the deep peridotites have a range of water contents extending to higher values than those from the shallow ones. The FTIR spectra of olivines from the shallow samples have more prominent Group II OH bands compared to the olivines from the deep samples, consistent with a more oxidized mantle environment. The range of olivine water content is similar to that observed in Kaapvaal craton peridotites at the same depths (129-184 km) but does not extend to as high values as those from Udachnaya (Siberian craton). The Slave, Kaapvaal and Siberian cratons will be compared in terms of water content distribution, controls and role in cratonic root longevity.

  11. Magnetotelluric deep soundings, gravity and geoid in the south São Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating

    NASA Astrophysics Data System (ADS)

    Pinto, Luis Gustavo Rodrigues; de Pádua, Marcelo Banik; Ussami, Naomi; Vitorello, Ícaro; Padilha, Antonio Lopes; Braitenberg, Carla

    2010-09-01

    positive geoid anomaly (+ 10 m) and pre-tholeiites (age > 138 Ma), carbonatites and kimberlites along the west African continental margin (Angola and Namibia) reinforces the presumed age of the São Francisco-Congo craton rejuvenation to be prior to its fragmentation in the Lower Cretaceous.

  12. A Precambrian cratonic block in the west-central Chihuahua - The Sierra del Nido cratonic block

    SciTech Connect

    Goodell, P.C. . Dept. of Geological)

    1993-02-01

    Precambrian rocks in west-central Chihuahua have been recognized by Denison (1969) and Mauger et al. (1983), on the basis of radiometric dating. The rocks are rhyolite clasts, and an allucthonous block, respectively, however their source direction and vergence can be measured. They point back to and are on the edge of a large, uniform, negative Bouguer gravity anomaly, having values greater than 200 milligals. The isotopic geochemical character of several Tertiary felsic fields within this anomalous are has been determined, and initial strontium isotopic ratios are all greater than 0.7055. Outside the anomalous area these ratios are lower, and Basin and Range extension tectonism is more evident. It is proposed that a Precambrian cratonic block, the Sierra del Nido, is present in the crust in west-central Chihuahua. It is reasonable to propose that it was decreted from North America during a Precambrian extensional (1.1. By ) event, from somewhere along the Arizona Transition Zone-Texas Linament region. The Sierra del Nido Block is separated form the ATZ-TL by a region of disrupted craton and extended crust, the Basin and Range Province. Implications of the pressure of the Sierra del Nido Block on other regional tectonic events will be discussed.

  13. Archaean Crustal Growth, Proterozoic Terrane Amalgamation and the Pan-African Orogeny, as Recorded in the NE African Sedimentary Record.

    NASA Astrophysics Data System (ADS)

    Najman, Y.; Fielding, L.; Millar, I.; Butterworth, P.; Andò, S.; Padoan, M.; Barfod, D. N.; Kneller, B. C.

    2015-12-01

    The cratons of Central Africa are formed of various blocks of Archaean and Palaeoproterozoic crust, flanked or truncated by Palaeoproterozoic to Mesoproterozoic orogenic belts. The geology of east Africa has largely been shaped by the events of the Pan-African Orogeny when east and west Gondwana collided to form 'Greater Gondwana' at the end of the Neoproterozoic. The Pan-African orogeny in NE Africa involved the collision of Archaean cratons and the Saharan Metacraton with the Arabian Nubian Shield, a terrane comprising Neoproterozoic juvenile oceanic island arcs. Phanerozoic cover sedimentary rocks, eroded from the Pan-African orogenies, blanket much of NE Africa. Detrital data from these Phanerozoic cover sedimentary rocks, and modern rivers draining both the cover the basement, provide a wealth of information on basement evolution, of particular relevance for regions where the basement itself is poorly exposed due to ancient or modern sedimentary cover. From samples collected in Uganda, Ethiopia, Sudan and Egypt, we provide combined U-Pb and Hf-isotope zircon, U-Pb rutile and Ar-Ar mica datasets, heavy mineral analyses, and bulk trace element data, from Archaean basement, Phanerozoic cover and modern river sediment from the Nile and its tributaries to document the evolution of the North African crust. The data document early crust-forming events in the Congo Craton and Sahara Metacraton, phased development of the Arabian Nubian Shield culminating in the Neoproterozoic assembly of Gondwana during the Pan African Orogeny, and the orogen's subsequent erosion, with deposition of voluminous Phanerozoic cover.

  14. Photometric parameter maps of the Moon derived from LROC WAC images

    NASA Astrophysics Data System (ADS)

    Sato, H.; Robinson, M. S.; Hapke, B. W.; Denevi, B. W.; Boyd, A. K.

    2013-12-01

    Spatially resolved photometric parameter maps were computed from 21 months of Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) images. Due to a 60° field-of-view (FOV), the WAC achieves nearly global coverage of the Moon each month with more than 50% overlap from orbit-to-orbit. From the repeat observations at various viewing and illumination geometries, we calculated Hapke bidirectional reflectance model parameters [1] for 1°x1° "tiles" from 70°N to 70°S and 0°E to 360°E. About 66,000 WAC images acquired from February 2010 to October 2011 were converted from DN to radiance factor (I/F) though radiometric calibration, partitioned into gridded tiles, and stacked in a time series (tile-by-tile method [2]). Lighting geometries (phase, incidence, emission) were computed using the WAC digital terrain model (100 m/pixel) [3]. The Hapke parameters were obtained by model fitting against I/F within each tile. Among the 9 parameters of the Hapke model, we calculated 3 free parameters (w, b, and hs) by setting constant values for 4 parameters (Bco=0, hc=1, θ, φ=0) and interpolating 2 parameters (c, Bso). In this simplification, we ignored the Coherent Backscatter Opposition Effect (CBOE) to avoid competing CBOE and Shadow Hiding Opposition Effect (SHOE). We also assumed that surface regolith porosity is uniform across the Moon. The roughness parameter (θ) was set to an averaged value from the equator (× 3°N). The Henyey-Greenstein double lobe function (H-G2) parameter (c) was given by the 'hockey stick' relation [4] (negative correlation) between b and c based on laboratory measurements. The amplitude of SHOE (Bso) was given by the correlation between w and Bso at the equator (× 3°N). Single scattering albedo (w) is strongly correlated to the photometrically normalized I/F, as expected. The c shows an inverse trend relative to b due to the 'hockey stick' relation. The parameter c is typically low for the maria (0.08×0.06) relative to the

  15. Towards an Integrated Seismic Characterization of the Slave Craton

    NASA Astrophysics Data System (ADS)

    Rondenay, S.; Snyder, D. B.; Chen, C.; Straub, K. M.; Bank, C.; Bostock, M. G.

    2005-12-01

    Archean cratons form the core of the majority of Earth's continents and offer a unique window into the evolution of continents and plate tectonics over geological time. The dynamics that led to the evolution and stabilization of cratons over one billion years ago, however, remain poorly understood. The Archean Slave province, located in the NW Canadian Shield, is an ideal site to study the formation of cratons due to its high degree of preservation and petrological evidence that its lithosphere possesses a distinct stratification resulting from cratonic assembly. The last decade has witnessed an explosion of seismological work in the region, with more than 45 broadband seismic stations deployed over variable lengths of time by the University of British Columbia, the Geological Survey of Canada, the POLARIS consortium and MIT. These data have been subjected to a wide array of seismic analyses: body- and surface-wave tomographic inversions were applied to the complete dataset, whereas receiver functions and shear-wave splitting were applied to subsets of the data. When considered together, these results yield an unprecedented seismic characterization of the Slave province. The Slave's lithosphere is, on average, 200 km thick and displays seismic velocities that are ~2-3% faster than surrounding Proterozoic orogens and ~2% faster than average cratonic values. At smaller scales, a low velocity anomaly centered to the south of the Lac de Gras kimberlite field is observed between 50-300 km depth. The anomaly has a radius of ~100 km, it exhibits a 2.8% slowness contrast with respect to the surrounding mantle, and may represent post-stabilization alteration of the cratonic lithosphere by processes responsible for kimberlite magmatism. Coherent results from shear-wave splitting and surface-wave analyses show evidence for two layers of anisotropy beneath the Slave craton: one in the uppermost lithosphere that may be associated with crustal structure from the last episode of

  16. Heat flux and topography constraints on thermochemical structure below North China Craton regions and implications for evolution of cratonic lithosphere

    NASA Astrophysics Data System (ADS)

    Wang, Yongming; Huang, Jinshui; Zhong, Shijie; Chen, Jiaming

    2016-04-01

    The eastern North China Craton (NCC) has undergone extensive reactivation during the Mesozoic and Cenozoic, while the western NCC has remained stable throughout its geological history. Geophysical and geochemical observations, including heat flux, surface topography, crustal and lithospheric thicknesses, and volcanism, show significant contrast between the eastern and western NCC. These observations provide constraints on thermochemical structure and reactivation process of the eastern NCC, thus helping understand the dynamic evolution of cratonic lithosphere. In this study, we determined the residual topography for the NCC region by removing crustal contribution to the topography. We found that the residual topography of the eastern NCC region is generally 0.3-0.9 km higher than the western NCC. We computed a large number of two-dimension thermochemical convection models for gravitational instability of cratonic lithosphere and quantified surface heat flux and topography contrasts between stable and destabilized parts of cratonic lithosphere. These models consider different chemical buoyancy (i.e., buoyancy number B) and viscosity for the cratonic lithosphere. Our models suggest that to explain the difference in heat flux (25-30 mW/m2) and residual topography (0.3-0.9 km) between the eastern and western NCC regions, the buoyancy number B is required to be ~0.3-0.4. This range of B implies that as much as 50% of the original cratonic lithospheric material remains in the present-day eastern NCC lithosphere and its underlying shallow mantle and that the new lithosphere in the eastern NCC may be a mixture of the relics of old craton materials and the normal mantle.

  17. Precambrian Cratons and Fold-Belts in Brazil: Overview

    NASA Astrophysics Data System (ADS)

    Fuck, R.

    2008-05-01

    The main Precambrian terrains recognized in Brazil comprise the Amazonian, São Francisco and Rio de la Plata cratons, surrounded by Neoproterozoic Brasiliano fold belts, making up the Borborema, Mantiqueira and Tocantins provinces. The Amazonian craton comprises an Archean core, surrounded by Paleoproterozoic terrains (Maroni-Itacaiunas, Ventuari-Tapajós, Rio Negro-Juruena), which southwestwards give way to the Mesoproterozoic Rondoniano-San Ignacio and Sunsas belts, the latter thought to be related to the Grenville belt of North America. The São Francisco craton comprises several Archean blocks (Gavião, Serrinha, Jequié) amalgamated by the Paleoproterozoic high-grade Itabuna-Salvador-Curaçá orogen. The Rio de la Plata craton, largely covered by Phanerozoic strata, is made of Paleoproterozoic basement gneiss and several Paleoproterozoic greenstone belts. Other cratonic blocks are hidden below large Phanerozoic basins, like the Paranapanema and Parnaíba blocks below the Paraná and Parnaíba basins, respectively. Several smaller Archean/Paleoproterozoic blocks appear within the Brasiliano provinces: some were strongly reworked during the Neoproterozoic orogenic events (São José do Campestre, Pernambuco-Alagoas, Goiás, Guanhães, Juiz de Fora, Curitiba), others were only marginally affected (São Luiz, Rio Apa, Luís Alves). The Brasiliano provinces are the result of Neoproterozoic to early Paleozoic orogenic events within the framework of West Gondwana amalgamation. The Mantiqueira Province extends from eastern Brazil to southern Uruguay and includes the Araçuaí, Ribeira and Dom Feliciano fold belts, bordering the São Francisco, Paranapanema and Rio de la Plata cratons and surrounding the Luís Alves craton. The Tocantins province in central Brazil includes the Araguaia, Paraguay and Brasília fold belts, the former bordering the Amazonian craton, the second bordering both the southern Amazonian craton and the Rio Apa block, and the last established on

  18. The 3-dimensional construction of the Rae craton, central Canada

    NASA Astrophysics Data System (ADS)

    Snyder, David B.; Craven, James A.; Pilkington, Mark; Hillier, Michael J.

    2015-10-01

    Reconstruction of the 3-dimensional tectonic assembly of early continents, first as Archean cratons and then Proterozoic shields, remains poorly understood. In this paper, all readily available geophysical and geochemical data are assembled in a 3-D model with the most accurate bedrock geology in order to understand better the geometry of major structures within the Rae craton of central Canada. Analysis of geophysical observations of gravity and seismic wave speed variations revealed several lithospheric-scale discontinuities in physical properties. Where these discontinuities project upward to correlate with mapped upper crustal geological structures, the discontinuities can be interpreted as shear zones. Radiometric dating of xenoliths provides estimates of rock types and ages at depth beneath sparse kimberlite occurrences. These ages can also be correlated to surface rocks. The 3.6-2.6 Ga Rae craton comprises at least three smaller continental terranes, which "cratonized" during a granitic bloom. Cratonization probably represents final differentiation of early crust into a relatively homogeneous, uniformly thin (35-42 km), tonalite-trondhjemite-granodiorite crust with pyroxenite layers near the Moho. The peak thermotectonic event at 1.86-1.7 Ga was associated with the Hudsonian orogeny that assembled several cratons and lesser continental blocks into the Canadian Shield using a number of southeast-dipping megathrusts. This orogeny metasomatized, mineralized, and recrystallized mantle and lower crustal rocks, apparently making them more conductive by introducing or concentrating sulfides or graphite. Little evidence exists of thin slabs similar to modern oceanic lithosphere in this Precambrian construction history whereas underthrusting and wedging of continental lithosphere is inferred from multiple dipping discontinuities.

  19. Archean crustal evolution of the northern North China Craton

    NASA Technical Reports Server (NTRS)

    Qian, Xianglin; Chen, Yaping; Liu, Jinzhong

    1988-01-01

    The Archean granultie facies rocks of the North China (Sino-Korean) Craton mostly occur inside the northern boundary forming a unique and spectacular granulite belt trending roughly E-W from eastern Hebei, North China in the east to Mt. Daqinchan, western Inner Mongolia in the west, ranging about 1,000 km long. Over the years in the middle portion of this Archean high-grade metamorphic belt a stratigraphic unconformity between the khondalite rock assemblage and the medium in composition granulite assemblage in Datong-Xinghe area is determined. The geological structural properties of the North China Craton are discussed.

  20. The Role of Water in the Stability of Cratonic Keels

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Woodland, Alan B.; Bell, David R.; Lazarov, Marina

    2011-01-01

    Cratons are typically underlain by large, deep, and old lithospheric keels (to greater than 200 km depth, greater than 2.5 Ga old) projecting into the asthenosphere (e.g., Jordan, 1978; Richardson et al., 1984). This has mystified Earth scientists as the dynamic and relatively hot asthenosphere should have eroded away these keels over time (e.g., Sleep, 2003; O'Neill et al., 2008; Karato, 2010). Three key factors have been invoked to explain cratonic root survival: 1) Low density makes the cratonic mantle buoyant (e.g., Poudjom Djomani et al., 2001). 2) Low temperatures (e.g., Pollack, 1986; Boyd, 1987), and 3) low water contents (e.g., Pollack, 1986), would make cratonic roots mechanically strong. Here we address the mechanism of the longevity of continental mantle lithosphere by focusing on the water parameter. Although nominally anhydrous , olivine, pyroxene and garnet can accommodate trace amounts of water in the form of H bonded to structural O in mineral defects (e.g., Bell and Rossman, 1992). Olivine softens by orders of magnitude if water (1-1000 ppm H2O) is added to its structure (e.g., Mackwell et al., 1985). Our recent work has placed constraints on the distribution of water measured in peridotite minerals in the cratonic root beneath the Kaapvaal in southern Africa (Peslier et al., 2010). At P greater than 5 GPa, the water contents of pyroxene remain relatively constant while those of olivine systematically decrease from 50 to less than 10 ppm H2O at 6.4 GPa. We hypothesized that at P greater than 6.4 GPa, i.e. at the bottom of the cratonic lithosphere, olivines are essentially dry (greater than 10 ppm H2O). As olivine likely controls the rheology of the mantle, we calculated that the dry olivines could be responsible for a contrast in viscosity between cratonic lithosphere and surrounding asthenosphere large enough to explain the resistance of cratonic root to asthenospheric delamination.

  1. Interaction between an incipient rift and a cratonic lithosphere : The North Tanzania Rift seen from some seismic tools

    NASA Astrophysics Data System (ADS)

    Gautier, Stéphanie; Plasman, Matthieu; Tiberi, Christel; Lopez, Marie; Peyrat, Sophie; Perrot, Julie; Albaric, Julie; Déverchère, Jacques; Deschamps, Anne; Ebinger, Cindy; Roecker, Steven; Mulibo, Gabriel; Wambura, Richard Ferdinand; Muzuka, Alfred; Msabi, Michael; Gama, Remigius

    2016-04-01

    The North Tanzania part of the East African Rift is the place of an incipient break up of the lithosphere. This continental rifting happens on the edge of the Tanzanian craton, and their interaction leads to major changes in the surface deformation. The evolution of the rift and its morphology is strongly linked to the inherited structures, particularly the Proterozoic belts and the craton itself. It is thus of prime interest to image the structure of the craton edges to fully understand its impact on the localisation of the current deformation at the surface. Since 2007 different multidisciplinary projects have taken place in this area to address this question. We present here a work based on a collaborative work between French, American and Tanzanian institutes that started in 2013. About 35 seismological stations were installed for 2 years in the Natron lake region, and 10 short period instruments were added for 9 months in the Manyara area to record local and telesismic events. We have analysed more than a hundred teleseismic events to compute the receiver function, and we finally obtain a Moho map of the region as well as azimuthal distribution of converted phases. The stations located on the edge of the rift and near the craton present a continuous evolution of their crustal pattern in the RF signal. Especially, we identify a clear phase at about 7s for those stations that corresponds to an interface separating a normal upper mantle from a very slow mantle at about 70 km depth. We first model those receiver functions to perfectly fit the signal, and more precisely the transverse component, which shows a strong and coherent pattern. Second, the local seismic network we have installed for 9 months in Manyara region advantageously completed the 2007 SEISMOTANZ network. In this part of the rift the seismicity is deep (20-30 km) and clustered without any magmatism record at the surface, opposite to Natron area. We could then relocalize the deep seismicity observed

  2. The story of a craton from heart to margins: illuminating cratonic lithosphere with Rayleigh wave phase velocities in Eastern Canada

    NASA Astrophysics Data System (ADS)

    Petrescu, L.; Darbyshire, F. A.; Gilligan, A.; Bastow, I. D.; Totten, E. J.

    2015-12-01

    Cratons are Precambrian continental nuclei that are geologically distinct from modern continental regions and are typically underlain by seismically fast lithospheric roots (keels) to at least 200 km depth. Both plate and non-plate tectonic origin theories such as stacking of subducted slabs or multiple mantle plume underplating have been proposed to explain keel growth.Eastern Canada is an ideal continental region to investigate cratonization processes and the onset of plate tectonics. It comprises part of the largest Archean craton in the world, the Superior Province, flanked by a ~1.1 Ga Himalayan-scale orogenic belt, the Grenville Province, and the 500-300 Ma old Appalachian orogenic province, following the same general SW-NE axial trend. The region is also cross-cut by the Great Meteor Hotspot track, providing an excellent opportunity to study the interaction of hotspot tectonism with progressively younger lithospheric domains.We investigate the lithospheric structure of Precambrian Eastern Canada using teleseismic earthquake data recorded at permanent and temporary networks. We measure interstation dispersion curves of Rayleigh wave phase velocities between ~15 and 220 s, and compare the results to standard continental and cratonic reference models. We combine the dispersion curves via a tomographic inversion which solves for isotropic phase velocity heterogeneity and azimuthal anisotropy across the region at a range of periods. The phase velocity maps indicate variations in lithospheric properties from the heart of the Superior craton to the SE Canadian coast.The new regional-scale models will help to understand the processes that generated, stabilized and reworked the cratonic roots through their billion-year tectonic history. We investigate how surface tectonic boundaries relate to deeper lithospheric structural changes, and consider the effects of the multiple Wilson cycles that affected Laurentia.

  3. Physico-chemical constraints on cratonic lithosphere discontinuities

    NASA Astrophysics Data System (ADS)

    Aulbach, Sonja; Rondenay, Stéphane; Huismans, Ritske

    2014-05-01

    The origins of the mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere boundary (LAB) have received much attention over the recent years. Peculiarities of cratonic lithosphere construction - compositional and rheological stratification due to thickening in collisional settings or by plume subcretion, multiple metasomatic overprints due to longevity - offer a variety of possibilities for the generation of discontinuities. Interconnected small degrees of conductive partial melt (carbonate-rich melts, such as carbonatites and kimberlites, or highly alkaline melts) at the cratonic LAB, which produce seismic discontinuities, may be generated in the presence of volatiles. These depress the peridotite solidus sufficiently to intersect the mantle adiabat at depths near the cratonic LAB at ~160-220 km, i.e. above the depth of metal saturation where carbonatite becomes unstable. The absence of agreement between the different seismic and magnetotelluric estimates for the depth of the LAB beneath Kaapvaal may be due to impingement of a plume, leading to a pervasively, but heterogeneously metasomatised ('asthenospherised') hot and deep root. Such a root and hot sublithosphere may yield conflicting seismic-thermal-geochemical depths for the LAB. The question arises whether the chemical boundary layer should be defined as above or below the asthenospherised part of the SCLM, which has preserved isotopic, compositional (non-primitive olivine forsterite content) and physical evidence (e.g. from teleseismic tomography and receiver functions) for a cratonic heritage and which therefore is still distinguishable from the asthenospheric mantle. If cratonic lithosphere overlies anomalously hot mantle for extended periods of time, the LAB may be significantly thinned, aided by penetration of relatively high-degree Fe-rich partial melts, as has occurred beneath the Tanzanian craton. Xenoliths from the deep Slave craton show little evidence for 'asthenospherisation'. Its root

  4. Lunar textural analysis based on WAC-derived kilometer-scale roughness and entropy maps

    NASA Astrophysics Data System (ADS)

    Li, Bo; Wang, XueQiang; Zhang, Jiang; Chen, Jian; Ling, Zongcheng

    2016-06-01

    In general, textures are thought to be some complicated repeated patterns formed by elements, or primitives which are sorted in certain rules. Lunar surfaces record the interactions between its outside environment and itself, thus, based on high-resolution DEM model or image data, there are some topographic features which have different roughness and entropy values or signatures on lunar surfaces. Textures of lunar surfaces can help us to concentrate on typical topographic and photometric variations and reveal the relationships between obvious features (craters, impact basins, sinuous rilles (SRs) and ridges) with resurfacing processes on the Moon. In this paper, the term surface roughness is an expression of the variability of a topographic or photometric surface at kilometer scale, and the term entropy can characterize the variability inherent in a geological and topographic unit and evaluate the uncertainty of predictions made by a given geological process. We use the statistical moments of gray-level histograms in different-sized neighborhoods (e.g., 3, 5, 10, 20, 40 and 80 pixels) to compute the kilometer-scale roughness and entropy values, using the mosaic image from 70°N to 70°S obtained by Lunar Reconnaissance Orbiter (LRO) Wide Angle Camera (WAC). Large roughness and entropy signatures were only found in the larger scale maps, while the smallest 3-pixel scale map had more disorderly and unsystematic textures. According to the entropy values in 10-pixel scale entropy map, we made a frequency curve and categorized lunar surfaces into three types, shadow effects, maria and highlands. A 2D scatter plot of entropy versus roughness values was produced and we found that there were two point clusters corresponding to the highlands and maria, respectively. In the last, we compared the topographic and photometric signatures derived from Lunar Orbiter Laser Altimeter (LOLA) data and WAC mosaic image. On the lunar surfaces, the ridges have obvious multilevel

  5. Recycling lower continental crust in the North China craton.

    PubMed

    Gao, Shan; Rudnick, Roberta L; Yuan, Hong-Ling; Liu, Xiao-Ming; Liu, Yong-Sheng; Xu, Wen-Liang; Ling, Wen-Li; Ayers, John; Wang, Xuan-Che; Wang, Qing-Hai

    2004-12-16

    Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton. PMID:15602559

  6. Recycling lower continental crust in the North China craton.

    PubMed

    Gao, Shan; Rudnick, Roberta L; Yuan, Hong-Ling; Liu, Xiao-Ming; Liu, Yong-Sheng; Xu, Wen-Liang; Ling, Wen-Li; Ayers, John; Wang, Xuan-Che; Wang, Qing-Hai

    2004-12-16

    Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earth's continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

  7. Longevity and rheology of cratons: key constraints from surface topography

    NASA Astrophysics Data System (ADS)

    Francois, T.; Burov, E. B.; Meyer, B.; Agard, P.

    2011-12-01

    Archean cratons are stable remnants of Earth's early continental lithosphere. Their structure, composition and survival over geological time spans make them ones of the most unique and enigmatic features of the Earth's surface. It has become evident from both geophysical and petrological studies that cratons exhibit deep lithospheric roots, which remained stable ever since their formation in the early Archean. The question of how some of the cratons survived destruction over timescales of billions of years remains a subject of vigorous debate. In order to understand what controls the long-term stability of the cratons, we investigated the impact of the thermo-rheological structure of the lithosphere on the evolution of both surface topography and cratonic roots using fully coupled thermo-mechanical numerical models (600*3000 km, free upper surface topography, layered lithospheric structure). Our model has a particular focus on the Canadian Shield, where considerable structural and thermal data are available from both geological and geophysical studies. In particular, we compare the implications of the "Cratonic" "Jelly-Sandwich" rheology (JS; strong dry olivine mantle, strong crust, cold geotherm with Moho temperature of 400°C, thermal lithosphere thickness of 250 km) with those of the "Crème Brûlée" rheology (CB; strong crust, weak wet olivine mantle, Moho temperature of 600°C, thermal lithosphere thickness of 150 km) (Figure 1). Our experiments show that, in the case of a laterally homogeneous lithosphere and in the absence of tectonic shortening or extension (blocked borders), both JS and CB rheologies may account for the stability of the shield and its surface topography. In this case continental lithosphere remains stable over large time spans, even for the weakest wet olivine mantle (but for "cold" thermal gradients). Nevertheless, for a laterally heterogeneous crust, as is the case for the Canadian Shield and most cratons, dry olivine mantle JS rheology

  8. Early Mesozoic tectonic settings of the northern North China craton

    NASA Astrophysics Data System (ADS)

    Meng, Qing-Ren; Wei, Hong-Hong; Wu, Guo-Li; Duan, Liang

    2014-01-01

    This paper aims to reconstruct early Mesozoic tectonic settings of the northern North China craton based on investigations of Triassic-Middle Jurassic successions and age determinations of some lithostratigraphic units and dikes cutting tilted early Mesozoic strata. It used to be regarded that the northern North China craton experienced fold-thrust deformations in the Triassic to Early Jurassic based on the occurrences of unconformity beneath Triassic/Lower Jurassic units, synorogenic conglomerate, and Late Triassic thrusting. We revisited early Mesozoic stratigraphy of the Xiabancheng basin in the middle Yanshan belt to restore tectonic environments during that period of time. Detrital zircon data reveal that the Liujiagou and Ermaying Formations are actually of Middle and Late Triassic ages, respectively. Field observations show that the Ermaying and Xingshikou Formations are conformable, making up a sequence from fluvial to alluvial-fan facies. It is also noticeable that Lower-Middle Jurassic Xingshikou through Xiahuayuan succession is typified by fining- and deepening-upward depositional trend and contains abundant volcaniclastic rocks, indicative of continued subsidence of the Xiabancheng basin in a volcanic setting. All the results cast doubts on the long-held view that the first phase of shortening in the northern North China craton happened in the Late Triassic or Early Jurassic. The angular unconformity beneath Upper Jurassic Tiaojishan volcanics registered shortening in the northern North China craton, which plausibly took place around 170 Ma on account of zircon U-Pb ages of two dikes penetrating deformed Middle Triassic-Lower Jurassic strata and ages of Tiaojishan volcanics above the unconformity. Given the widespread occurrence of magmatism, rift basins and metamorphic core complexes during the Triassic and Early Jurassic, it follows that the northern North China craton was more likely under an extensional setting during the early Mesozoic.

  9. Temperature-dependent transient creep and dynamics of cratonic lithosphere

    NASA Astrophysics Data System (ADS)

    Birger, Boris I.

    2013-11-01

    Large-scale mantle convection forms the upper boundary layer (lithosphere) where the vertical temperature drop is about 1300 K. Theoretical rheology and laboratory experiments with rock samples show that transient creep occurs while creep strains are sufficiently small. The transient creep is described by the temperature-dependent Andrade rheological model. Since plate tectonics allows only small deformations in lithospheric plates, creep of the lithosphere plates is transient whereas steady-state creep, described by non-Newtonian power-law rheological model, takes place in the underlying mantle. The solution of stability problem shows that the lithosphere is stable but small-scale convective oscillations are attenuated very weakly in regions of thickened lithosphere beneath continental cratons (subcratonic roots) where the thickness of the lithosphere is about 200 km. These oscillations create small-scale convective cells (the horizontal dimensions of the cells are of the order of the subcratonic lithosphere thickness). Direction of motion within the cells periodically changes (the period of convective oscillations is of the order of 3 × 108 yr). In this study, the oscillations of cratonic lithosphere caused by initial relief perturbation are considered. This relief perturbation is assumed to be created by overthrusting in orogenic belts surrounding cratons. The perturbation of the Earth's surface relief leads to a fast isothermal process of isostatic recovery. In the presence of vertical temperature gradient, vertical displacements, associated with the recovery process in the lithosphere interior, instantly produce the initial temperature perturbations exciting thermoconvective oscillations in the cratonic lithosphere. These small-amplitude convective oscillations cause oscillatory crustal movements which form sedimentary basins on cratons.

  10. The structure of the Amazonian craton: Available geophysical evidence

    NASA Astrophysics Data System (ADS)

    Rosa, João Willy Corrêa; Rosa, José Wilson Corrêa; Fuck, Reinhardt A.

    2016-10-01

    The Amazonian craton, which covers a large area of South America, and is thought to have been stable since the end of the Mesoproterozoic, has recently benefited from a series of regional geophysical surveys. The Amazonian craton comprises the northern Guyana shield and the southern Central Brazil shield. It has become the main subject of seismological studies aiming to determine crustal thickness. Moho thickness maps that cover a large part of the South American continent summarize these studies. Receiver function studies, aided by surface wave dispersion tomography, were also useful tools applied in the region over the past decade. These have been improved by the addition of temporary and permanent regional seismological arrays and stations. An interesting NNW-SSE Moho depth anomaly, pointing to crustal thickening of up to 60 km in the central Guyana shield and a 50 km thick anomaly of the southern Central Brazil shield were recently identified. Areas with crustal thickening correspond to Paleoproterozoic magmatic arcs. The upper mantle seismic anisotropy in part of the region has been determined from SKS splitting studies. The currently available seismic anisotropy information shows that the orientation of the determined anisotropic axis is related to the frozen in anisotropy hypothesis for the Amazonian craton. The orientation of the anisotropic axis shows no relation to the current South American plate motion in the Amazonian craton. Most recently, detailed information for the two shields has benefited from a series of high-resolution, regional aerogeophysical surveys, made available by CPRM, the Brazilian Geological Survey. In addition to the mentioned contribution from seismology for imaging deeper crustal structures, regional gravity surveys have been expanded, adding to previous Bouguer anomaly maps, and deep drilling information from early exploration efforts have been compiled for the Amazon basin, which covers the Amazonian craton separating the Guyana

  11. Flat Slab Subduction, Trench Suction, and Craton Destruction: Comparison of the North China, Wyoming, and Brazilian Cratons

    NASA Astrophysics Data System (ADS)

    Kusky, Timothy; Windley, Brian; Wang, Lu; Wang, Zhensheng; Li, Xiaoyong; Zhu, Peimin

    2015-04-01

    We define and test a unifying plate tectonic driving mechanism that explains the characteristics of the destruction of cratonic lithospheric roots. We document and model the relationships between flat slab subduction, trench suction, and craton destruction, using examples from the North China and Wyoming cratons, each of which locally lost approximately 100 km of their lithospheric roots in the Cretaceous and which show spatio-temporal relationships with episodes of flat slab subduction in the mantle transition zone associated with deep mantle hydration, coupled with slab rollback and concomitant influx of fertile mantle material to accommodate the space created by slab rollback. A similar process has more recently operated along the western side of the Brazilian craton where it is thrust beneath the thickened crust of the Andes in an area of trench rollback. The importance of the mutual interaction between these processes for destruction of cratonic roots may be greater than currently perceived. Together with the other processes of subduction erosion and arc subduction, larger amounts of continental lithosphere may have been subducted or otherwise returned to the sub-lithospheric mantle than previously appreciated. When oceanic lithosphere subducts, it hydrates the upper mantle beneath an arc from well-known dehydration reactions. However, some hydrous phases (e.g., Phase A, Phase E, and -γt and β-phase olivine) are stable to much greater depths and dehydrate even when a slab is in the mantle transition zone. It is estimated that 40% of the water subducted in hydrated oceanic crust, mantle, sediments, and subducted continental material reaches the mantle transition zone between 410 and 660 km. For instance lawsonite may contain up to 11% water, and is stable up to 11 GPa or about 300 km and serpentinites can contain up to 13% water and are stable up to 7 GPa, and after conversion to denser hydrous phases such as β-phase olivine they can be stable up to 50 GPa

  12. Accretion and reworking beneath the North China Craton

    NASA Astrophysics Data System (ADS)

    Zheng, J. P.; Griffin, W. L.; Ma, Q.; O'Reilly, S. Y.; Xiong, Q.; Tang, H. Y.; Zhao, J. H.; Yu, C. M.; Su, Y. P.

    2012-09-01

    How has the Earth's continental lithosphere evolved? Most of our knowledge is derived from surface exposures, but xenoliths carried in volcanic rocks can be an important source of information. The North China Craton (NCC) is one of the oldest in the world and Phanerozoic volcanic rocks with abundant xenoliths are widespread, making it an ideal area to study the formation and evolution of continents. New analyses of U-Pb ages and Hf isotopes in zircon were obtained for lower crustal xenoliths from four localities including the Paleozoic Yingxian lamproites, and the basalts of Pingquan (Paleocene), Hebi and Nushan (Neogene). Published ages and compositions of lower crustal and upper mantle xenoliths from the NCC are synthesized to constrain the accretion and reworking processes that have affected the deep lithosphere beneath the craton. The peridotite bodies within the Dabie-Sulu ultrahigh-pressure (UHP) belt, along the southern edge of the NCC, are compared with the xenolith peridotites to constrain early Mesozoic dynamics. The oldest components of the NCC may be ~ 4.0 Ga old. The craton experienced complex accretion and reworking processes in its deep lithosphere, accompanied by the formation (or aggregation) and differentiation of the ancient continental nucleus. The small size of the NCC, compared with many other cratons worldwide, made it more susceptible to the effects of marginal subduction and collision with surrounding blocks. The subcontinental lithosphere mantle (SCLM) was generally coupled with the lower crust through the Paleozoic, while decoupling occurred in late Mesozoic-Cenozoic time, except locally (such as the Neoarchen lower crust and SCLM in Hebi), suggesting strong interactions between the asthenosphere and the lithosphere (both upper mantle and lower crust) in Phanerozoic time. In the lower crust, the ancient components of the craton were re-worked in Paleoarchean (3.80-3.65 Ga) time. The craton also experienced two important accretionary

  13. Contrasting crustal evolution processes in the Dharwar craton

    NASA Astrophysics Data System (ADS)

    Lancaster, Penelope J.; Dey, Sukanta; Storey, Craig D.; Mitra, Anirban; Bhunia, Rakesh K.

    2015-04-01

    Archaean cratons provide a critical window into early Earth dynamics, preserving a record of crustal evolution processes that include the start of 'modern' plate tectonics and the development of supercontinents. However, these same processes can also destroy or rework substantial volumes of crust, and the oldest extant Archaean terrane is only ~3.9 Ga. Where Archaean rocks are exposed, bulk techniques such as Pb and Nd isotopes can provide considerable information about continental formation, but these may have been altered by later metamorphic events. Another method is to examine sedimentary units, which can preserve fragments of the crust that are no longer exposed at the Earth's surface. Resistant detrital minerals such as zircon have particular use in these studies, as they incorporate a range of isotopic and geochemical tracers and can survive multiple crystallisation and/or sedimentary events. In this manner, a more complete record of a craton's evolution may be obtained, with the benefit of contributions from contrasting isotopic systems. The Dharwar craton of southern India is one such Archaean block, comprising >2.7 Ga trondjhemite-tonalite-granodiorite (TTG) gneisses, volcano-sedimentary belts (>3.0 and 2.9-2.6 Ga) and 2.7-2.5 Ga calc-alkaline to potassic granitoids. These rocks preserve evidence for several cycles of supracrustal formation, deformation, metamorphism and granitic activity during the Precambrian. New in situ U-Pb-Hf analyses of detrital zircons from across the craton indicate significant juvenile crustal extraction events at ~3.3 and 2.7 Ga, and continuous extraction from 3.7-3.3 Ga. Reworking in the older western block at ~3.0 Ga marks the onset of cratonisation, most likely due to 'modern' plate tectonic processes, while reworking in both the western and younger eastern block at 2.55-2.50 Ga indicates accretion of the two terranes and final cratonisation much later than in most other Archaean terranes (~2.7 Ga). Different patterns of

  14. Paleomagnetism of the Wyoming Craton: A Pre-Laurentian Puzzle

    NASA Astrophysics Data System (ADS)

    Kilian, T.; Chamberlain, K.; Mitchell, R. N.; Evans, D. A.; Bleeker, W.; Lecheminant, A. N.

    2010-12-01

    The Archean Wyoming craton is mostly buried beneath Phanerozoic sediments in the Rocky Mountains of the west central United States. Exposures of the craton are entirely in thrust-bounded Laramide uplifts and contain numerous swarms of Neoarchean-Proterozoic mafic dikes. U-Pb ages from these dikes include ~2685 Ma from a dike in the Owl Creek Mountains (Frost et al., 2006) as well as another in the Bald Mountain region of the Bighorn Mountains (this study), ~2170 Ma from the Wind River Mountain quartz diorite (Harlan et al., 2003), ~2110 Ma from a dike in the Granite Mountains (Bowers and Chamberlain, 2006), ~2010 Ma from a Kennedy dike in the Laramie Range (Cox et al., 2000), and ~780 Ma for dikes in the Beartooth and Teton Mountains (Harlan et al., 1997). These possible age ranges of magmatic events will allow a detailed comparison with other cratons, especially Superior and Slave. Prior to the assembly of Laurentia, Wyoming may have been connected with Slave in supercraton Sclavia (Bleeker, 2003; Frost et al., 2007), or alternatively, Wyoming may have been attached to the present southern margin of Superior in the supercraton Superia, as judged by similarities of the thrice-glaciated Huronian and Snowy Pass sedimentary successions (Roscoe and Card, 1993). Paleomagnetic results will be presented from over 150 dikes in the Wyoming craton. All dikes were from the basement uplifts of the Beartooth Mountains, Bighorn Mountains, Owl Creek Mountains, Granite Mountains, Ferris Mountains and Laramie Range. Dikes range in widths from 1 to >100 meters, and trends vary across all orientations. Stable remanence is observed in majority of sites with at least 8 different directions from the various uplifts. Structural corrections are applied when necessary to restore shallowly dipping Cambrian strata to horizontal. The paleomagnetic study is being integrated with precise U-Pb geochronology of dikes that bear stable remanence directions. Results will eventually allow a

  15. On craton thinning/destruction: Insight from 2D thermal-mechanical numerical modeling

    NASA Astrophysics Data System (ADS)

    Liao, J.

    2014-12-01

    Although most cratons maintain stable, some exceptions are present, such as the North China craton, North Atlantic craton, and Wyoming craton, which have experienced dramatic lithospheric deformation/thinning. Mechanisms triggering cratonic thinning remains enigmatic [Lee et al., 2011]. Using a 2D thermo-mechanical coupled numerical model [Gerya and Yuen, 2007], we investigate two possible mechanisms: (1) stratification of cratonic lithospheric mantle, and (2) rheological weakening due to hydration.Lithospheric mantle stratification is a common feature in cratonic areas which has been demonstrated by geophysical and geochemical studies [Thybo and Perchuc, 1997; Griffin et al., 2004; Romanowicz, 2009; Rychert and Shearer, 2009; Yuan and Romanowicz, 2010]. The influence of lithospheric mantle stratification during craton evolution remains poorly understood. A rheologically weak layer representing hydrated and/or metasomatized composition is implemented in the lithospheric mantle. Our results show that the weak mantle layer changes the dynamics of lithospheric extension by enhancing the deformation of the overlying mantle and crust and inhibiting deformation of the underlying mantle [Liao et al., 2013; Liao and Gerya, 2014]. Modeling results are compared with North China and North Atlantic cratons. Our work indicates that although the presence of a weak layer may not be sufficient to initiate craton deformation, it enhances deformation by lowering the required extensional plate boundary force. Rheological weakening due to hydration is a possible mechanism triggering/enhancing craton deformation, especially for cratons jaxtaposing with a subduction, since water can release from a subducting slab. We investigate the influence of wet mantle flow laws [Hirth and Kohlstedt, 2003], in which a water parameter (i.e. constant water content) is involved. Our results show that wet dislocation alone does not accelerate cratonic deformation significantly. However, if wet diffusion

  16. Asymmetry of Non-Volcanic Passive Margins Induced by the Proximity of a Craton

    NASA Astrophysics Data System (ADS)

    Andres-Martinez, M.; Perez-Gussinye, M.; Morgan, J. P.; Araujo, M. N.

    2015-12-01

    Symmetry of conjugated rifted margins is controlled by the rheology of the crust and the mantle, extension velocities and heterogeneities in the lithosphere. However, there is a lack of knowledge on how the feedbacks between these initial conditions influence the final architecture of passive margins and the polarity of the asymmetry. Here we focus on cratons as stiff heterogeneities which potentially induce asymmetry. For simplicity, we choose to address only non-volcanic rifted margins developed next to cratons, such as the Brazil-Congo and Australia-Antarctica margin pairs. In the South Atlantic case, where cratons are closer to the margins (north of Sao Francisco craton and north and south of Congo craton) the margins are narrow, while wide margins develop far away from cratons. Extreme asymmetry occurs where rifting takes place close to a craton in one margin (narrow) and a fold belt in the conjugate (wide). The same is observed for the Australia-Antarctic pair in the sector of Recherche basin, where the Australian margin is narrow next to the Yilgarn craton and widens towards the east as it lays further from the craton. We use numerical models in order to study how cratons induce asymmetry of conjugated rifted margins and affect the polarity of the asymmetry. We ran experiments with different lower crustal rheologies for a fold belt lithosphere in order to understand which rheologies 'naturally' result in asymmetric margins. We also ran experiments where a cratonic lithosphere is placed next to a fold belt lithosphere, and where rifting is initiated by a weak seed in the fold belt at different distances from the craton. We found that where some fold belt experiments result in symmetric margins, their equivalent experiments with craton result in asymmetric margins. Furthermore, strong- and intermediate-rheology experiments with cratons showcase narrow margins in the craton side and wide margins on the fold belt side. We also observe that the distance from the

  17. Small-scale upper mantle extension beneath a destroyed craton

    NASA Astrophysics Data System (ADS)

    Zhao, L.; Zheng, T.; Chen, L.; Ai, Y.; He, Y.; Xu, X.

    2014-12-01

    The North China Craton (NCC), as an unusual craton with part of its thick lithosphere destructed, records the geodynamic processes associated with the convergence of Eurasia and the Pacific and Philippine plates lasting from the Mesozoic to the Cenozoic. How the cratonic lithosphere deformed in response to the extensional tectonics caused by the oceanic plate subduction, however, remains debated. In order to investigate the mantle deformation of the NCC, we present new shear wave splitting measurements and updated tomographic models beneath a 900-km long profile across the north NCC. Compared to our other observations in the NCC, this profile is shorter but also crosses a region that experienced strong lithospheric destruction, therefore provides a good opportunity to improve our understanding of upper mantle deformation during the craton destruction. The upper mantle deformation is studied using SKS data from 60 broadband stations with average spacing of 15 km. For the data from events occurring at distances of 85º-115º, fast polarization directions and delay times (fδt) are retrieved by a routine method, while for the events at distances < 85º, waveform modeling are applied to obtain (fδt) after separating the effects of S and SKS. The measured splitting parameters show small-scale variations from east to west: the major fast directions, trending NE-SW or NW-SE in contrast, distribute intermittently along the profile. We plot the splitting parameters overlapping on the geological map and the tomography image for a depth range of 120-300 km. Comparison shows good consistency of the splitting pattern and structural features both at shallow and deep depths: NW-SE trending fast directions are observed at stations located within the basins or extensional zones like metamorphic core complexes, with the fast direction parallel to the extensional or stretching directions; the fast directions and the shear-wave velocity anomalies within the upper mantle

  18. An evaluation of the zircon method of isotopic dating in the Southern Arabian Craton

    USGS Publications Warehouse

    Cooper, J.A.; Stacey, J.S.; Stoeser, D.G.; Fleck, R.J.

    1979-01-01

    A zircon study has been made on eleven samples of igneous rocks from the Saudi Arabian Craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time. Tonalite, granodiorite, and crosscutting leucoadamellite bodies in the southern part of the An Nimas Bathylith yield ages in the time range 820-760 Ma. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas Bathylith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could be more episodic than previously assumed.

  19. Volatile organic compound production and consumption by microbial plankton communities on the NOAA WACS cruise

    NASA Astrophysics Data System (ADS)

    Giovannoni, S. J.; Halsey, K.; Thrash, J. C.; De Gouw, J. A.; Graus, M.

    2013-12-01

    Information about biological sources and sinks of volatile organic compounds (VOCs) in the ocean surface could result in a better understanding of the underlying causes of variation in air/sea VOC fluxes, and potentially could alter predictions about the impact of climate change on ocean surface ecology and air/sea interactions. The goal of this work was to measure rates of biological production, oxidation and assimilation of volatile organic compounds (VOCs) by microbial plankton communities along the WACS cruise transect from Boston to Bermuda in August 2013. Tangential flow filtration was used to concentrate microbial plankton communities for incubation in environmentally controlled dynamic stripping chambers under simulated ocean surface layer conditions. Gas streams exiting the chambers were monitored in real time with proton transfer reaction mass spectrometry (PTR-MS). In separate experiments aliquots of plankton suspensions were incubated with 14C-methanol, 14C-TMAO, and 14C-pyruvate, and the assimilation of 14C into biomass and the production of 14C-CO2 were measured. Results showed that the highly productive George's Bank plankton community has a high capacity for methanol and trimethylamine oxide (TMAO) oxidation. Biomass was estimated by counting cells. The rate of incorporation of methanol-carbon into biomass was only 21% of the rate of methanol oxidation to CO2. Similar results were observed for TMAO. These experiments also allowed estimates of kinetic constants for both compounds. The half-saturation constants (Ks) for methanol oxidation were similar in natural populations collected at George's Bank and the Sargasso Sea (12.8 and 9.9 μM, respectively). Interestingly, the Ks values for TMAO oxidation were an order of magnitude lower than for methanol in plankton communities sampled from both sites (0.5 and 0.3 μM, respectively). These results provide additional evidence that microbial plankton have a high capacity for oxidation of these low

  20. Origin of Aristarchus Olivine Deposits Based on M3, WAC, and Diviner Analyses

    NASA Astrophysics Data System (ADS)

    Wiseman, S. M.; Mustard, J. F.; Donaldson Hanna, K. L.; Isaacson, P.; Jolliff, B. L.; Besse, S.; Staid, M.; Pieters, C. M.

    2011-12-01

    The Aristarchus region contains geologically diverse deposits and the Aristarchus impact crater, located on the SE margin of the plateau near the contact between plateau materials and western Procellarum basalts, has exposed materials with variable compositions. Of particular interest is the origin of olivine-bearing deposits that occur on the SE portion of the crater rim and ejecta in association with impact melt [1]. NW portions of the rim and ejecta expose plateau materials and are spectrally dominated by pyroxene in the VNIR. Spectra of the NW rim and ejecta are consistent with a noritic composition and with the inferred origin of the plateau as uplifted upper crust [2,3,4]. Therefore, it is unlikely that the olivine- bearing materials, which exhibit a strong 1 micron olivine absorption and only minor pyroxene contributions, are derived from plateau materials similar to those exposed in the NW portion of the crater. Potential sources of the olivine-bearing material excavated by the impact include western Procellarum basalts or buried material associated with the Marius Hills volcanic complex. Alternatively, the olivine-bearing deposits could be derived from a shallow pluton that is not represented by other surface exposures or could have formed as re-crystallized impact melt. Both the western Procellarum basalts [5,6] and some units associated with the Marius Hills [7] are olivine-bearing. In order to differentiate between these hypotheses, we are integrating spectral data in the UV/VIS (LRO WAC), VIS/NIR (Chandrayaan-1 M3), and TIR (LRO Diviner) to further characterize the assemblages of minerals that occur in association with the olivine-bearing deposits in Aristarchus crater, western Procellarum, and units within the Marius Hills volcanic complex. [1] Mustard et al., 2011, JGR 116. [2] McEwen et al., 1994, Science 266. [3] Lucey et al., 1986, LPSC 16. [4] Chevrel et al., 2009, Icarus 199. [5] Staid and Pieters 2001, JGR. [6] Staid et al., 2011, JGR 116. [7

  1. [Professor Wacław Kuśnierczyk (1908-1997)--Pro Memoria in the century of birthday].

    PubMed

    Brozek, Krzysztof; Kozakiewicz, Jacek; Kierzek, Andrzej

    2009-01-01

    Wacław Kuśnierczyk was born in 1908 in Sniatyń. He received the degree in medicine at Jan Kazimierz University in Lwów in 1932. He did his PhD degree under Professor Zaleski supervision in 1938 at Jan Kazimierz University. At that time he concentrated his scientific activity on research on tuberculosis. In 1953 he obtained the title of second degree specialist in ear, nose and throat diseases. He became a chief of Otolaryngology at Urban Hospital No 4 in Katowice in 1960. Since then this eminent physician was working on tumours located in upper respiratory tract and the possibility of its endoscopic diagnosis at Silesian Academy of Medicine in Katowice. As one of the first he pointed out the negative influence of smoking cigarettes on cancer of larynx. It was Wacław Kuśnierczyk who implemented new priorities for integrated programs in patient care, research, education and cancer prevention. He has published widely in peer reviewed journals and has edited or contributed to many books. He has given many major lectures and is the recipient of numerous prestigious awards for his scientific accomplishments. The achievement of Professor Kuśnierczyk were the valuable source of information for the physicians. In 1997, on the 31st of January he died in Katowice.

  2. Teleseismic receiver functions modeling of the eastern Indian craton

    NASA Astrophysics Data System (ADS)

    Mandal, Prantik; Biswas, Koushik

    2016-09-01

    We estimate receiver functions (RFs) through the time-domain deconvolution using three-component broadband data of 100 teleseismic events (30° ⩽ ∧ ⩽ 90°) from 15 seismographs in the eastern Indian craton. Estimated radial RFs show a positive phase at 4.6-5.8 s delay time corresponding to the crustal thicknesses of 37-46 km. Through the differential evolution (DE) waveform inversion modeling of radial receiver functions, we delineate the crustal structure at 15 broadband stations. On an average, the Archean Singhbhum Odisha Craton (SOC) is characterized by a thick crust of 43 ± 3 km in comparison to a relatively thin crust of 41 ± 1 km underlying the Proterozoic Chotanagpur Granite Gneissic terrain (CGGT). While, a thin crust of 38 ± 1 km characterizes the younger Eastern Ghats Mobile Belt (EGMB). The main results of our modeling reveal a 46 km thick Archean crust underlying the Singhbhum granite (SG) of 3.6 Ga, which is characterized by a 3 km crustal thickening probably resulted from the Archean subduction process. Our modeling also detects a 2-3 km crustal thinning with the thinnest crust of 37 km below the region near South Singhbhum Shear Zone, which could be attributed to the 1.6 Ga plume activity associated with Dalma volcanic. Our modeling also led to the delineation of a crustal thinning of 2-3 km underlying the region in EGMB, which was influenced by a much younger (∼117 Ma) Rajmahal magmatism associated with the Gondwana break-up episode. However, our study could not detect any age-dependent variation of crustal thicknesses in the eastern Indian craton. The main result of our modeling suggests a two-phase crustal evolution process for the SOC viz. older E-W crustal thickening due to E-W plate compression and later crustal thinning episodes associated with the Dalma volcanism in the north and the Rajmahal volcanism in the South.

  3. Latest Jurassic-early Cretaceous regressive facies, northeast Africa craton

    SciTech Connect

    van Houten, F.B.

    1980-06-01

    Nonmarine to paralic detrital deposits accumulated in six large basins between Algeria and the Arabo-Nubian shield during major regression in latest Jurassic and Early Cretaceous time. The Ghadames Sirte (north-central Libya), and Northern (Egypt) basins lay along the cratonic margin of northeastern Africa. The Murzuk, Kufra, and Southern (Egypt) basins lay in the south within the craton. Data for reconstructing distribution, facies, and thickness of relevant sequences are adequate for the three northern basins only. High detrital influx near the end of Jurassic time and in mid-Cretaceous time produced regressive nubian facies composed largely of low-sinuosity stream and fahdelta deposits. In the west and southwest the Ghadames, Murzuk, and Kufra basins were filled with a few hundred meters of detritus after long-continued earlier Mesozoic aggradation. In northern Egypt the regressive sequence succeeded earlier Mesozoic marine sedimentation; in the Sirte and Southern basins correlative deposits accumulated on Precambrian and Variscan terranes after earlier Mesozoic uplift and erosion. Waning of detrital influx into southern Tunisia and adjacent Libya in the west and into Israel in the east initiated an Albian to early Cenomanian transgression of Tethys. By late Cenomanian time it had flooded the entire cratonic margin, and spread southward into the Murzuk and Southern basins, as well as onto the Arabo-Nubian shield. Latest Jurassic-earliest Cretaceous, mid-Cretaceous, and Late Cretaceous transgressions across northeastern Africa recorded in these sequences may reflect worldwide eustatic sea-level rises. In contrast, renewed large supply of detritus during each regression and a comparable subsidence history of intracratonic and marginal basins imply regional tectonic control. 6 figures.

  4. Remobilization in the cratonic lithosphere recorded in polycrystalline diamond

    PubMed

    Jacob; Viljoen; Grassineau; Jagoutz

    2000-08-18

    Polycrystalline diamonds (framesites) from the Venetia kimberlite in South Africa contain silicate minerals whose isotopic and trace element characteristics document remobilization of older carbon and silicate components to form the framesites shortly before kimberlite eruption. Chemical variations within the garnets correlate with carbon isotopes in the diamonds, indicating contemporaneous formation. Trace element, radiogenic, and stable isotope variations can be explained by the interaction of eclogites with a carbonatitic melt, derived by remobilization of material that had been stored for a considerable time in the lithosphere. These results indicate more recent formation of diamonds from older materials within the cratonic lithosphere.

  5. Remobilization in the cratonic lithosphere recorded in polycrystalline diamond

    PubMed

    Jacob; Viljoen; Grassineau; Jagoutz

    2000-08-18

    Polycrystalline diamonds (framesites) from the Venetia kimberlite in South Africa contain silicate minerals whose isotopic and trace element characteristics document remobilization of older carbon and silicate components to form the framesites shortly before kimberlite eruption. Chemical variations within the garnets correlate with carbon isotopes in the diamonds, indicating contemporaneous formation. Trace element, radiogenic, and stable isotope variations can be explained by the interaction of eclogites with a carbonatitic melt, derived by remobilization of material that had been stored for a considerable time in the lithosphere. These results indicate more recent formation of diamonds from older materials within the cratonic lithosphere. PMID:10947983

  6. Heat flow anomaly in Lesotho - implications for the southern boundary of the Kaapvaal craton

    SciTech Connect

    Jones, M.Q.W. )

    1992-10-01

    Heat flow studies in southern Africa reveal a pattern of low heat flow in Archean cratons compared with Proterozoic mobile belts that provides grounds for modeling in which cratons have lower mantle heat flux and greater lithospheric thickness. Measurements at nine locations in a 90-km-long north-south traverse show an increase of heat flow from typically cratonic values of about 45 mW/sq m in the north to about 80 mW/sq m where the craton abuts on the Natal belt in the south. The change occurs within 30 km, suggesting that the anomaly largely reflects an increase in radioactive heat from the upper crust in the orogenic front. The change in mantle heat flux across the boundary is probably considerably smaller. These and other results suggest that the surficial craton boundary is further north than usually depicted. 20 refs.

  7. Numerical modeling of continental rifting: Implications for the East African Rift system

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    The East African Rift system (EARS) provides a unique system with juxtaposition of two contrasting yet simultaneously formed rift branches, the eastern, magma-rich, and the western, magma-poor, on either side of the old thick Tanzanian craton embedded into younger lithosphere. Here we take advantage of the improvements in our understanding of deep structures, geological evolution and recent kinematics, together with new cutting edge numerical modeling techniques to design a three-dimensional ultra-high resolution viscous plastic thermo-mechanical numerical model that accounts for thermo-rheological structure of the lithosphere and hence captures the essential geophysical features of the central EARS. Based on our experiments, we show that in case of the mantle plume seeded slightly to the northeast of the craton center, the ascending plume material is deflected by the cratonic keel and preferentially channeled along the eastern side of the craton, leading to formation of a large rift zone characterized by important magmatic activity with substantial amounts of melts derived from mantle plume material. This model is in good agreement with the observations in the EARS, as it reproduces the magmatic eastern branch and at the same time, anticlockwise rotation of the craton. However, this experiment does not reproduce the observed strain localization along the western margin of the cratonic bloc. To explain the formation of contrasting magmatic and amagmatic rift branches initiating simultaneously on either side of a non-deforming block as observed in the central EARS, we experimentally explored several scenarios of which three can be retained as specifically pertaining to the EARS: (1) The most trivial first scenario assumes rheologically weak vertical interface simulating the suture zone observed in the geological structure along the western border of the craton; (2) The second scenario involves a second smaller plume initially shifted in SW direction; (3) Finally, a

  8. Crustal evolution and the eclogite to granulite phase transition in xenoliths from the West African Craton

    NASA Technical Reports Server (NTRS)

    Haggerty, S. E.; Hills, D. V.; Toft, P. B.

    1988-01-01

    A suite of eclogite and granulite facies xenoliths from kimberlite pipes in the Archean Man Shield of West Africa is described. The xenoliths include lithologies ranging in composition from komatiite to anorthosite and appear to be geochemically, petrologically, and geophysically related. The suite may represent fractionation of felsic material separated from ancient mantle and added to early Archean crust. The samples can be used to define a xenolith geotherm, which may represent an ancient episode of high heat flow. The samples also imply that the crust-mantle boundary is a gradational and possibly interlayered geochemical, mineralogical, and seismic transition. It is speculated that the depleted subcontinental mantle required by diamond bearing coalescence of smaller depletion cells formed by extraction of ancient crustal components. These depleted zones are surrounded by fertile asthenospheric mantle, which may have given rise to later flood basalts such as the Karroo and Parana Provinces.

  9. 3-D Structure of the Slave and Rae Cratons Provides Clues to Their Construction

    NASA Astrophysics Data System (ADS)

    Snyder, D. B.

    2013-12-01

    Deep geologic structures within cratons that make up continental cores were long neglected. Recently acquired geophysical data from large observational arrays and geochemical data resulting from exploration for diamond has now made possible co-registration of large-scale (400-km depth), truly 3-dimensional data sets. P-waves, surface waves and magnetotelluric observations provide 3-D wavespeed and conductivity models. Multi-azimuthal receiver functions map seismic discontinuity surfaces in 3-D. Xenolith suites erupted in kimberlites provide rock samples at key lithospheric depths, albeit at sparsely distributed locations. These multi-disciplinary models are becoming available for several key cratons worldwide; here the deep structure of the Slave and Rae cratons of the Canadian Shield is described. Lithospheric layers with tapered, wedge-shaped margins are common. Slave craton layers are sub-horizontal and indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. The central Rae craton has predominantly dipping discontinuities that indicate construction at 1.9 Ga by thrusting similar to that observed in crustal ';thick-skinned' fold-and-thrust belts. 3-D mapping of conductivity and metasomatism, the latter via mineral recrystallization and resetting of isotopic ages, overprints primary structures in both cratons. Distribution of more conductivitve mantle suggests that assumed causative pervasive metasomatism occurs at 100-200 km depths with ';chimneys' reaching to shallower depths, typically in locations where kimberlites or mineralization has occurred.

  10. Diamond genesis, seismic structure, and evolution of the Kaapvaal-Zimbabwe craton.

    PubMed

    Shirey, Steven B; Harris, Jeffrey W; Richardson, Stephen H; Fouch, Matthew J; James, David E; Cartigny, Pierre; Deines, Peter; Viljoen, Fanus

    2002-09-01

    The lithospheric mantle beneath the Kaapvaal-Zimbabwe craton of southern Africa shows variations in seismic P-wave velocity at depths within the diamond stability field that correlate with differences in the composition of diamonds and their syngenetic inclusions. Middle Archean mantle depletion events initiated craton keel formation and early harzburgitic diamond formation. Late Archean accretionary events involving an oceanic lithosphere component stabilized the craton and contributed a younger Archean generation of eclogitic diamonds. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the Archean diamond suite. PMID:12215642

  11. A Sharp Edge of the Cratonic Lithosphere of North America

    NASA Astrophysics Data System (ADS)

    Harper, T. B.; Skryzalin, P. A.; Menke, W. H.; Levin, V. L.; Darbyshire, F. A.

    2015-12-01

    Using teleseismic travel time delays, we develop a tomographic model of the lithosphere beneath northeastern North America, from the shore of James Bay in Quebec to the Atlantic coast of New England and to a depth of 300 km. Three major terranes lie within this cratonic margin: the 2.7 Ga Superior province, the 1 Ga Grenville orogenic belt and the 0.3-0.4 Ga Appalachian terranes, which are bounded by the Grenville Front (GF) and Appalachian Front (AF), respectively. Additionally, the 0.8 Ga Avalon terrain was accreted to coastal New England by strike-skip faulting during the Appalachian orogeny. Our tomographic model uses earthquake seismograms recorded by permanent US and Canadian stations, the Transportable Array and the temporary QMIII deployment. All data were corrected for instrument response and record sections were examined visually to identify gross errors in response and timing. Differential arrival times of P and PKP waves were determined by cross-correlation and have a maximum amplitude of about ±1 second. In our model, lithospheric boundaries do not correlate well with geological boundaries, nor do they strike parallel to them. The seismically-fast (by 5% relative to AK135) cratonic lithosphere of North America is much thicker than that of the younger terranes, extending to 200 km or more depth but with a sharp east-dipping eastern edge located (at Moho depths) 100-250 km northwest of the GF. The lithosphere beneath the Grenville and Appalachian terranes, which were affected by subduction during the Grenville and Appalachian orogenies, is slower (by 4%). A sliver of seismically-fast lithosphere, extending to ~150 km depth, occurs along the Atlantic coast and is interpreted as the Avalonian lithosphere.

  12. Plume-induced dynamic instabilities near cratonic blocks: Implications for P-T-t paths and metallogeny

    NASA Astrophysics Data System (ADS)

    Guillou-Frottier, L.; Burov, E.; Cloetingh, S.; Le Goff, E.; Deschamps, Y.; Huet, B.; Bouchot, V.

    2012-06-01

    Plume head-lithosphere interactions around cratonic blocks result in thermo-mechanical disturbances that lead to heating and burial phases of crustal rocks. We present results from numerical models of plume head-cratonic blocks interactions where a free upper surface condition and realistic rheologies are accounted for. These models include distinct cratonic blocks embedded within a continental lithosphere and separated by several hundreds of kilometers. Surface topography, thermal field and effective viscosity values are tracked for 20 Myr of interactions. The modeled dynamic interaction of a plume head around cratonic blocks results in two main types of instabilities, each of them resulting in a distinct P-T-t path. The "slab-like" instability, focused on cratonic edges when plume head is away from the craton center, shows a near-isothermal burial phase, while the "drip-like" instability occurring above plume head material results in a near-isobaric heating phase. Consequently, both clockwise and counterclockwise P-T-t paths can be expected around cratons, as actually observed around the Tanzanian craton and other cratonic areas. Metallogenic data from gemstone-bearing rocks in south-east Africa and data from ultrahigh temperature and ultrahigh pressure metamorphism are compatible with our model. It appears that vertical mantle dynamics around cratons may also explain thermobarometric signatures that are often attributed to horizontal tectonics.

  13. On the relations between cratonic lithosphere thickness, plate motions, and basal drag

    USGS Publications Warehouse

    Artemieva, I.M.; Mooney, W.D.

    2002-01-01

    An overview of seismic, thermal, and petrological evidence on the structure of Precambrian lithosphere suggests that its local maximum thickness is highly variable (140-350 km), with a bimodal distribution for Archean cratons (200-220 km and 300-350 km). We discuss the origin of such large differences in lithospheric thickness, and propose that the lithospheric base can have large depth variations over short distances. The topography of Bryce Canyon (western USA) is proposed as an inverted analog of the base of the lithosphere. The horizontal and vertical dimensions of Archean cratons are strongly correlated: larger cratons have thicker lithosphere. Analysis of the bimodal distribution of lithospheric thickness in Archean cratons shows that the "critical" surface area for cratons to have thick (>300 km) keels is >6-8 ?? 106 km2 . Extrapolation of the linear trend between Archean lithospheric thickness and cratonic area to zero area yields a thickness of 180 km. This implies that the reworking of Archean crust should be accompanied by thinning and reworking of the entire lithospheric column to a thickness of 180 km in accord with thickness estimates for Proterozoic lithosphere. Likewise, extrapolation of the same trend to the size equal to the total area of all Archean cratons implies that the lithospheric thickness of a hypothesized early Archean supercontinent could have been 350-450 km decreasing to 280-400 km for Gondwanaland. We evaluate the basal drag model as a possible mechanism that may thin the cratonic lithosphere. Inverse correlations are found between lithospheric thickness and (a) fractional subduction length and (b) the effective ridge length. In agreement with theoretical predictions, lithospheric thickness of Archean keels is proportional to the square root of the ratio of the craton length (along the direction of plate motion) to the plate velocity. Large cratons with thick keels and low plate velocities are less eroded by basal drag than small

  14. Two modes of extension in the North China Craton: Insights from numerical modeling

    NASA Astrophysics Data System (ADS)

    Lin, F.; Liu, M.

    2013-12-01

    The North China Craton was formed around 1.8 Ga by collision and amalgamation of the Eastern and Western blocks. It remained tectonically stable since then until Mesozoic, when its eastern part experienced wide-distributed extension and volcanism. This distributed extension waned down in the early Cenozoic, and was replaced by localized extension (rifting) in the western part of the North China Craton. We have developed a series of viscoplastic finite element models to investigate the lithospheric conditions associated with these changing modes of extension in the North China Craton. Our results show that the wide-distributed extension in North China requires a thin and hot lithosphere, which is probably the result of delamination or thermal erosion of the lithospheric root under the eastern part of the North China Craton. The localized rifting during the late Cenozoic in the western part of the North China Craton indicates a relatively cold and thick lithosphere. Furthermore, preexisting lithospheric weakening was needed to explain the formation of the Late Cenozoic rift zones. These preexisting weakening zones may be inherited from the Paleoprotozoic collision that formed the North China Craton.

  15. Intra-cratonic melting as a result of delamination of mantle lithosphere - insight from numerical modelling

    NASA Astrophysics Data System (ADS)

    Gorczyk, W.; Vogt, K.; Gerya, T.; Hobbs, B. E.

    2012-12-01

    It is becoming increasingly apparent that intense deformation, metamorphism and metasomatism occur within continental cratonic blocks far removed form subducting margins Such changes may occur intra-cratonically arising from lithospheric thickening and the development of gravitational instabilities, but mostly occur at the boundary of cratonic blocks. The contact of two cratons is characterized by rheological lateral variations within mantle-lithosphere and overlying crust. Tectonic stresses acting on craton/craton boundaries may lead to thinning or thickening due to delamination of the mantle lithosphere. This is reflected in tectonic deformation, topography evolution, melting and crustal metamorphism. To understand the controls on these processes a number of 2D, coupled petrological thermo-mechanical numerical experiments has been performed to test the response of a laterally weakened zone to a compressional regime. The results indicate that the presence of water-bearing minerals in the lithosphere and lower crust is essential to initiate melting, which in the later stages may expand to dry melting of crust and mantle. In the case of anhydrous crust and lithosphere, no melting occurs. Thus a variety of instabilities, melting behaviour and topographic responses occurs at the base of the lithosphere as well as intensive faulting and buckling in the crust dependent on the strength and "water" content of the lithosphere.

  16. Dating Kimberlite Eruption and Erosion Phases Using Perovskite, Zircon, and Apatite (U-Th)/He Geochronology to Link Cratonic Lithosphere Evolution and Surface Processes

    NASA Astrophysics Data System (ADS)

    Stanley, J. R.; Flowers, R. M.

    2015-12-01

    In many cases it is difficult to evaluate the synchronicity and thus potential connections between disparate geologic events, such as the links between processes in the mantle lithosphere and at the surface. Developing new geochronologic tools and strategies for integrating existing chronologic data with other information is essential for addressing these problems. Here we use (U-Th)/He dating of multiple kimberlitic minerals to date kimberlite eruption and cratonic erosion phases. This approach permits us to more directly assess the link between unroofing and thermomodification of the lithosphere by tying our results to information obtained from mantle-derived clasts in the same pipes. Kimberlites are rich sources of information about the composition of the cratonic lithosphere and its evolution over time. Their xenoliths and xenocrysts can preserve a snapshot of the entire lithosphere and its sedimentary cover at the time of eruption. Accurate geochronology of these eruptions is crucial for interpreting spatiotemporal trends, but kimberlites can be difficult to date using standard techniques. Here we show that the mid-temperature thermochonometers of the zircon and perovskite (U-Th)/He (ZHe, PHe) systems can be viable tools for dating kimberlite eruption. When combined with the low temperature sensitivity of (U-Th)/He in apatite (AHe), the (U-Th)/He system can be used to date both the emplacement and the erosional cooling history of kimberlites. The southern African shield is an ideal location to test the utility of this approach because the region was repeatedly intruded by kimberlites in the Cretaceous, with two major pulses at ~200-110 Ma and ~100-80 Ma. These kimberlites contain a well-studied suite of mantle xenoliths and xenocrysts that document lithospheric heating and metasomatism over this interval. Our ZHe and PHe dates overlap with published eruption ages and add new ages for undated pipes. Our AHe dates constrain the spatial patterns of Cretaceous

  17. Mantle Discontinuities and the Origins of the U.S. Cratonic Lithosphere

    NASA Astrophysics Data System (ADS)

    Fischer, K. M.; Hopper, E.

    2014-12-01

    The goals of this work are to probe how mantle lithosphere discontinuity structure varies beneath the cratonic terranes of the northern U.S. and to relate this structure to the processes that created and modified the cratonic mantle. Our region samples the Archean Wyoming, Medicine Hat and Superior cratons, and the Proterozoic terranes that lie between them. We imaged the mantle using Sp phases recorded by permanent and temporary seismic networks, including EarthScope's Transportable Array. Sp receiver functions for individual waveforms were obtained by extended time multi-taper deconvolution, and migrated into a 3D volume using common conversion point stacking, a spline representation of phase Fresnel zones, and 3D models for crust and mantle structure. The stack was bootstrapped. In the cratonic mantle, we observe multiple mid-lithospheric discontinuities (MLDs) that are characterized by three types of structures: a relatively continuous negative discontinuity (velocity decrease with depth) that lies in the 65-100 km depth range; deeper negative MLDs (80-145 km) that are more discontinuous and intermittent; and occasional positive MLDs at the greatest depths (>125 km). In contrast to the tectonically active western U.S., beneath cratonic regions we typically do not observe a strong negative discontinuty at the base of the tomographically-defined lithosphere, indicating that the transition to asthenospheric properties is gradual. The MLDs indicate strong layering in the cratonic mantle lithosphere. In multiple cases, one negative discontinuity dips below another, consistent with a slab of lithosphere imbricated beneath pre-existing cratonic mantle. One of the clearest examples is a north-dipping phase at depths of 80-130 km beneath the Cheyenne Belt, the suture between the Wyoming Craton and the accreted Proterozoic terranes to its south. In Sept. 2013, an unusual earthquake occurred within the high velocity mantle of the Wyoming craton at ~76 km, a depth that

  18. Late Cretaceous to Present evolution of the NW Africa peri-cratonic in the Africa-Eurasia plate convergence context

    NASA Astrophysics Data System (ADS)

    Ghorbal, B.; Bertotti, G.; Andriessen, P. A. M.

    2009-04-01

    Africa-Eurasia plate convergence is the main mechanism to explain topographic evolution and patterns of Tertiary vertical motions recorded around the entire Mediterranean and even further east. However, most of the studies are concentrated on the Eurasian side of the Mediterranean Realm. Along the NW Africa pericratonic zone (western Mediterranean side) extending longitudinally from the Anti-Atlas to the Rif Mountains, the highest topography is observed in the High Atlas intracontinental belt and in the Pan-African Anti-Atlas belt, and not in the youngest belt, the Rif. The combination of AFT and (U-Th)/He low-thermal dating, performed on pre-Cenozoic basement rocks along the Moroccan pericratonic transect (500km) yield ages ranging respectively between 90-9Ma and 65-7Ma, documenting vertical motions of subsidence and exhumation in between Late Cretaceous and Present. Time-Temperature models show that vertical movements are spatially zoned through Morocco, with the highest amplitude of vertical movements in the High Atlas (>4-5km) and more modest amounts in the Anti-Atlas and the Western Meseta (<2.5km) from Late Cretaceous onward. Precious information provided by the AFT and AHe ages indicates that the entire NW African peri-cratonic zone including the Western Meseta and the Anti-Atlas in addition to the Atlas and the Rif systems experienced Tertiary deformation. Two stages of folding are distinguished on the basis of low-thermal dating results along the pericratonic transect. The first is a lithospherical folding of ~500km in the Late Cretaceous (confirming that this process is a primary response to recently induced compressional stress fields) and the second is a crustal folding of ~100-150km wavelength in the Late Eocene that occurred independently to the mantle, requiring therefore the existence of a decoupling in between the base of the crust and the high mantle.

  19. Spatial distribution of eclogite in the Slave cratonic mantle: The role of subduction

    NASA Astrophysics Data System (ADS)

    Kopylova, Maya G.; Beausoleil, Yvette; Goncharov, Alexey; Burgess, Jennifer; Strand, Pamela

    2016-03-01

    We reconstructed the spatial distribution of eclogites in the cratonic mantle based on thermobarometry for ~ 240 xenoliths in 4 kimberlite pipes from different parts of the Slave craton (Canada). The accuracy of depth estimates is ensured by the use of a recently calibrated thermometer, projection of temperatures onto well-constrained local peridotitic geotherms, petrological screening for unrealistic temperature estimates, and internal consistency of all data. The depth estimates are based on new data on mineral chemistry and petrography of 148 eclogite xenoliths from the Jericho and Muskox kimberlites of the northern Slave craton and previously reported analyses of 95 eclogites from Diavik and Ekati kimberlites (Central Slave). The majority of Northern Slave eclogites of the crustal, subduction origin occurs at 110-170 km, shallower than in the majority of the Central Slave crustal eclogites (120-210 km). The identical geochronological history of these eclogite populations and the absence of steep suture boundaries between the central and northern Slave craton suggest the lateral continuity of the mantle layer relatively rich in eclogites. We explain the distribution of eclogites by partial preservation of an imbricated and plastically dispersed oceanic slab formed by easterly dipping Proterozoic subduction. The depths of eclogite localization do not correlate with geophysically mapped discontinuities. The base of the depleted lithosphere of the Slave craton constrained by thermobarometry of peridotite xenoliths coincides with the base of the thickened lithospheric slab, which supports contribution of the recycled oceanic lithosphere to formation of the cratonic root. Its architecture may have been protected by circum-cratonic subduction and shielding of the shallow Archean lithosphere from the destructive asthenospheric metasomatism.

  20. African Aesthetics

    ERIC Educational Resources Information Center

    Abiodun, Rowland

    2001-01-01

    No single traditional discipline can adequately supply answers to the many unresolved questions in African art history. Because of the aesthetic, cultural, historical, and, not infrequently, political biases, already built into the conception and development of Western art history, the discipline of art history as defined and practiced in the West…

  1. "African Connection."

    ERIC Educational Resources Information Center

    Adelman, Cathy; And Others

    This interdisciplinary unit provides students in grades kindergarten through seventh grade an opportunity to understand diversity through a study of Africa as a diverse continent. The project is designed to provide all elementary students with cultural enrichment by exposing them to African music, art, storytelling, and movement. This project can…

  2. The Preservation of Meso- Archean Refractory Lithospheric Mantle Underneath the Eastern Margin of the Tanzania Craton

    NASA Astrophysics Data System (ADS)

    Shu, Q.; Liu, J.; Pearson, G. D.; Gibson, S. A.

    2014-12-01

    Numerous studies on the petrology and geochemistry of peridotite xenoliths from the Tanzanian Craton and its rifted margins have investigated the origin, chemical change and thermal state of the cratonic roots from its core area (Nzega and Mwadui), its Northern (Marsabit) and Eastern margin Labait and Lashaine area (e.g. Dawson, 1964; Henjes-Kunst and Altherr, 1991; Lee & Rudnick, 1999; Chesley et al., 1999; Gibson et al., 2013). These studies suggest that the Tanzanian cratonic mantle formed via high degrees of melt extraction in the Archean (oldest Re-depletion age TRD = 3.4 Ga, Burton et al., 2000) and sev­eral episodes of refertilization. In order to gain further temporal and chemical understanding on the effects of tectonic processes on cratonic roots, we carried out a Re-Os isotopic study on peridotites (n = 11) from Lashaine, which will be followed by Lu-Hf, Sm-Nd and Sr isotope investigations of the constituent minerals of the same samples. The preliminary whole-rock Os isotope data from Lashaine peridotites show a large range of 187Os/188Os (0.1061 - 0.1261), with TRD ages from Meso-Archean to very young (3.1 Ga to 0.3 Ga). There is a negative correlation between TRD and bulk alumina contents. One sample with the lowest Al2O3 yields the oldest age of 3.1 Ga. Five samples range from 2.5 to 2.8 Ga, three give ages close to 2 Ga, and one sample with a high Al2O3 has a TRD at 0.3 Ga. The positive Al2O3-187Os/188Os correlation trend passes above the PM composition may reflect ancient metasomatism by high Re/Os melts or recent metasomatism by very radiogenic Os plume-derived melts. These processes could be related to the evolution of the peripheral Proterozoic mobile belts, or Cenozoic rifting on the Eastern margin. Collectively, our new Os isotope data demonstrate that Meso-Archean (at least 3.1 Ga old) mantle portions are still retained underneath the rifted Eastern margin of the Craton. This is in line with previous results indicating that Archean cratonic

  3. High Water Contents in the Siberian Cratonic Mantle: An FTIR Study of Udachnaya Peridotite Xenoliths

    NASA Technical Reports Server (NTRS)

    Doucet, Luc S.; Peslier, Anne H.; Ionov, Dimitri A.; Brandon, Alan D.; Golovin, Alexander V.; Ashchepkov, Igor V.

    2013-01-01

    Water is believed to be a key factor controlling the long-term stability of cratonic lithosphere, but mechanisms responsible for the water content distribution in the mantle remain poorly constrained. Water contents were obtained by FTIR in olivine, pyroxene and garnet for 20 well-characterized peridotite xenoliths from the Udachnaya kimberlite (central Siberian craton) and equilibrated at 2-7 GPa. Water contents in minerals do not appear to be related to interaction with the host kimberlite. Diffusion modeling indicates that the core of olivines preserved their original water contents. The Udachnaya peridotites show a broad range of water contents in olivine (6.5 +/- 1.1 to 323 +- 65 ppm H2O (2 sigma)), and garnet (0 - 23 +/- 6 ppm H2O). The water contents of olivine and garnet are positively correlated with modal clinopyroxene, garnet and FeO in olivine. Water-rich garnets are also rich in middle rare earth elements. This is interpreted as the result of interaction between residual peridotites and water rich-melts, consistent with modal and cryptic metasomatism evidenced in the Siberian cratonic mantle. The most water-rich Udachnaya minerals contain 2 to 3 times more water than those from the Kaapvaal craton, the only craton with an intact mantle root for which water data is available. The highest water contents in olivine and orthopyroxene in this study (>= 300 ppm) are found at the bottom of the lithosphere (> 6.5 GPa). This is in contrast with the Kaapvaal craton where the olivines of peridotites equilibrated at > 6.4 GPa have < 1 ppm H2O. The latter "dry" olivine may make the base of the Kaapvaal cratonic root strong and thus protects it from erosion by the convective mantle The calculated viscosity for water-rich Udachnaya peridotites at > 6 GPa is lower or similar (8.4× 10(exp 16) to 8.0× 10(exp 18) Pa./s) to that of the asthenosphere (<= 3.7x10(exp 18) Pa./s ). Such lithologies would not be able to resist delamination by the convecting asthenosphere

  4. Supercontinents, True Polar Wander, and Paleogeography of the Slave Craton

    NASA Astrophysics Data System (ADS)

    Mitchell, Ross Nelson

    evidence in support of a long-lived, Ediacaran-aged hotspot. The consistency of paleocurrent directions derived from the Great Slave Supergroup argues against significant regional vertical-axis rotations and for large and rapid TPW to explain discordant paleomagnetic directions observed within section. The last frontier for paleomagnetic constraints on supercontinents, TPW, and the antiquity of plate tectonics is earliest Proterozoic time. Laurentia, one of Earth's oldest continents that formed at the core supercontinent Nuna, contains several cratons that have adequate paleomagnetic and geochronologic data with which to test for evidence of early TPW and relative plate motion, the hallmark of tectonics. Although past comparisons have been made between the Slave and Superior cratons at each "bookend" of Laurentia, new paleomagnetic data supported by baked contact tests allow for conclusive early Proterozoic reconstructions. Similar to periods following Proterozoic supercontinents Nuna and Rodinia, early Proterozoic time is characterized by large TPW oscillations and large-scale plate reorganizations prior to amalgamation, possibly indicating the presence of `Kenorland', an Archean supercontinent.

  5. Layered structure of the lithospheric mantle changes dynamics of craton extension

    NASA Astrophysics Data System (ADS)

    Liao, J.; Gerya, T.; Wang, Q.

    2013-11-01

    Although presence of weak layers due to hydration and/or metasomatism in the lithospheric mantle of cratons has been detected by both geophysical and geochemical studies, its influence on craton evolution remains elusive. Using a 2‒D thermomechanical viscoelastoplastic numerical model, we studied the craton extension of a heterogeneous lithospheric mantle with a rheologically weak layer. Our results demonstrate that the effect of the weak mantle layer is twofold: (1) enhances deformation of the overlying lithosphere and (2) inhibits deformation of the underlying lithospheric mantle. Depending on the weak‒layer depth, the Moho temperature and extension rate, three extension patterns are found (1) localized mantle necking with exposed weak layer, (2) widespread mantle necking with exposed weak layer, and (3) widespread mantle necking without exposed weak layer. The presence of the weak mantle layer reduces long‒term acting boundary forces required to sustain extensional deformation of the lithosphere.

  6. East asian gold: Deciphering the anomaly of phanerozoic gold in precambrian cratons

    USGS Publications Warehouse

    Goldfarb, R.J.; Hart, C.; Davis, G.; Groves, D.

    2007-01-01

    Early Cretaceous orogenic gold deposits in eastern Asia are globally unique in that large Phanerozoic lode gold deposits occur in Archean-Paleoproterozoic cratons. In the northern Pacific region, ca. 125 Ma orogenic gold deposits in the North China, Yangzte, and Siberian craton margins, as well as in young terranes in California, may ultimately relate to the giant Cretaceous mantle plume in the southern Pacific basin and the relatively rapid tectonic consequences along both continental margins from resulting Pacific plate reconfigurations. In eastern Asia, such consequences include reactivation of and fluid flow along major fault systems, with fluid focusing into simultaneously forming, isolated core complexes of uncertain genesis. Deposition of gold ores in previously devolatilized high-grade Precambrian metamorphic rocks requires an exotic source of ore fluid, most likely subducted Mesozoic oceanic crust and/or overlying sediment. An implication is that Phanerozoic metamorphic core complexes in other destabilized craton margins could host large gold resources. ?? 2007 by Economic Geology.

  7. Passive margin asymmetry and its polarity in the presence of a craton

    NASA Astrophysics Data System (ADS)

    Andres-Martinez, Miguel; Perez-Gussinye, Marta; Neto-Araujo, Mario; Morgan, Jason

    2016-04-01

    When continental lithosphere is extended to break-up it forms two conjugate passive margins. In many instances these margins are asymmetric: while one is wide and extensively faulted, the conjugate thins more abruptly and exhibits little faulting. Recent observational studies have suggested that this asymmetry results from the formation of an oceanward-younging sequential normal fault array on the future wide margin. Numerical models have shown that fault sequentiality arises as a result of asymmetric uplift of the hot mantle towards the hanging wall of the active fault, which weakens this area and promotes the formation of a new oceanward fault. In numerical models the polarity of the asymmetry is random. It results from spontaneous preferential localization of strain in a given fault, a process reinforced by strain weakening effects. Slight changes in the experiments initial grid result in an opposite polarity of the asymmetry. However, along a long stretch of the South Atlantic margins, from the Camamu-Gabon to the North Santos-South Kwanza conjugates, the polarity is not random and is very well correlated with the distance of the rift to nearby cratons. Here, we use numerical experiments to show that the presence of a thick cratonic root inhibits asthenospheric flow from underneath the craton towards the adjacent fold belt, while flow from underneath the fold belt towards the craton is favoured. This enhances and promotes sequential faulting towards the craton and results in a wide faulted margin located in the fold belt and a narrow conjugate margin in the craton side, thereby determining the polarity of the asymmetry, as observed in nature.

  8. Obesity and African Americans

    MedlinePlus

    ... Data > Minority Population Profiles > Black/African American > Obesity Obesity and African Americans African American women have the ... ss6304.pdf [PDF | 3.38MB] HEALTH IMPACT OF OBESITY More than 80 percent of people with type ...

  9. Heat Flow, Lower Crustal Thermochronology, and Transient Geotherms in the Mesozoic Southern African Continental Lithosphere

    NASA Astrophysics Data System (ADS)

    Schmitz, M. D.; Bell, D. R.; Bowring, S. A.

    2002-12-01

    There is a well-established debate regarding the role of higher mantle heat flux for accommodating the elevated average surface heat flow in the Proterozoic orogenic belts relative to the Archean cratonic regions of southern Africa1,2. Advocates of steeper off-craton lithospheric mantle thermal gradients commonly support their arguments with thermobarometric data indicating elevated temperatures at a given depth in off-craton versus cratonic mantle xenolith suites3,4, and together such inferences have cemented a paradigm of differential lithospheric thickness between Proterozoic orogenic belts (thin) and Archean cratons (thick). However, this inherently steady-state interpretation of lithospheric thermomechanical structure is at odds with emerging data pointing toward transient thermal perturbations and irreversible chemical modifications to the southern African lithosphere during the Late Mesozoic5,6. This contribution seeks to illustrate how an essentially "cratonic" thermal state in the Proterozoic lithospheric mantle of southern Africa remains compatible with nominally elevated surface heat flow in the same regions. Model geotherms have been constructed utilizing published surface heat flow, heat production and thermal conductivity measurements1, and seismically-derived crustal thickness estimates7. New heat-producing element concentration data for kimberlite-borne high-pressure granulite xenoliths from the Proterozoic domains constrain lower crustal heat production to be in the range of 0.15 to 0.2 μW/m3. A family of geologically realistic crustal heat production models accommodates surface heat flow >50 mW/m2, while maintaining moderate basal mantle heat flux <15 mW/m2, with associated lithospheric mantle geotherms and thickness indistinguishable from those of cratonic mantle. This family of models is further consistent with constraints on pre-Mesozoic lower crustal paleotemperatures of <450°C imposed by rutile U-Pb thermochronology in the craton

  10. Seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of Kalahari Craton

    NASA Astrophysics Data System (ADS)

    Sodoudi, F.; Yuan, X.; Kind, R.; Lebedev, S.; Tilmann, F. J.

    2013-12-01

    S receiver functions obtained from the data of 97 seismic stations present evidence for the existence of a layered and thick lithosphere beneath the Kalahari Craton. We identified three negative discontinuities within the lithosphere of the Archean cratons and Proterozoic mobile belts of southern Africa. We also employed a novel combination of SRFs and surface-wave analysis to constrain the anisotropic properties of the lithosphere and its internal layering. Our results show that frozen-in anisotropy and compositional changes can generate sharp Mid-Lithospheric Discontinuities (MLD) at depths of 85 and 150-200 km, respectively. We found that a 50 km thick anisotropic layer containing 3% S wave anisotropy and with a fast-velocity axis different from that in the layer beneath can account for the first MLD at about 85 km depth. This depth is largely consistent with that of 8° discontinuity suggested as a global characteristic of cratonic lithosphere. Significant correlation between the depths of an apparent boundary separating the depleted and metasomatic refertilized lithosphere, as inferred from chemical tomography, and those of our second MLD (at 150-200 km depth) led us to characterize this negative discontinuity as a compositional boundary, most likely due to the modification of the cratonic mantle lithosphere by magma infiltration. We detected this MLD at a depth of about 150 km beneath the Zimbabwe Craton and Limpopo belt with a steep deepening to about 200 km underneath the Kaapvaal Craton and its passive margin. The deepening of this boundary is spatially correlated with the surficial expression of the ancient Thabazimbi-Murchison Lineament (TML). This may imply that the translithospheric TML isolates the lithospheric block of the relatively younger Limpopo terrane from that of the ancient Kaapvaal terrane. Finally, the largest velocity contrast (3.6-4.7%) is observed at a boundary located at depths of 260-280 km beneath the Archean domains and the older

  11. An Integrated Crustal Analysis of Craton-Terrane Transition

    NASA Astrophysics Data System (ADS)

    Contenti, S. M.; Gu, Y. J.; Shen, L.

    2012-12-01

    velocities along the Thorsby/Rimbey domains. The proximity of these two domains to the STZ may shed new light on the spatial variability of subduction during the Proterozoic era. The combination of crustal depth, velocity, Vp/Vs ratios and anisotropy enables a careful re-examination of the existing tectonic model regarding the domain formation and reworking near the western edge of the North American Craton.

  12. The Hales discontinuity and upper mantle anisotropy beneath cratons

    NASA Astrophysics Data System (ADS)

    Musacchio, G.; White, D. J.; Thomson, C. J.

    2003-04-01

    Seismic velocity discontinuities are commonly found within the upper 100 km of the mantle lithosphere, with great variability in their depth, lateral extent, and the polarity of velocity jump. Among the more commonly observed is the Hales discontinuity, identified in a variety of tectonic environments, and commonly associated with a high-velocity, highly reflective and sometimes anisotropic layer. In the Archean Western Superior Province of the Canadian Shield, long range R/WAR profiling gives a high-resolution estimate of the mantle V_P in the shallow upper mantle, providing a more certain determination of the nature of the Hales dicontinuity. Ray-based travel-time inversion of the data, have shown that Vp in the uppermost mantle is 8.0-8.3 km/s. A 15-20 km thick layer (layer-H) with >6% seismic anisotropy (N-S V_P of 8.3 km/s and E-W V_P of 8.8 km/s) dips northward at ˜10^o from a minimum depth of 48-50 km. The attitude of layer-H is consistent with the general tectonic strike; its depth range (50-75 km) falls within that of the Hales discontinuity. If a link between the Hales discontinuity and layer-H can be drawn, observations strengthen the objection that the estimated velocity contrast (0.2 to 0.4 km/s depending on the direction of wave propagation) is relatively high if layer-H represents a phase transition, and thus (re)opens the debate on the nature of shallow upper-mantle boundaries beneath continents. The high V_P and intermediate anisotropy of upper-mantle layer-H requires a harzburgite peridotitic composition with the a-axis of olivine aligned E-W. Layer-H might have emplaced during accretion (2.7 Ga Kenoran orogeny) of the North American proto-craton and be relic oceanic lithosphere. The Hales discontinuity might be an expression of continents accretion and map relic slabs in the shallow upper mantle.

  13. Petrology, 40Ar/39Ar age, Sr-Nd isotope systematics, and geodynamic significance of an ultrapotassic (lamproitic) dyke with affinities to kamafugite from the easternmost margin of the Bastar Craton, India

    NASA Astrophysics Data System (ADS)

    Rao, N. V. Chalapathi; Atiullah; Burgess, R.; Nanda, Purnendu; Choudhary, A. K.; Sahoo, Samarendra; Lehmann, B.; Chahong, Ngazipmi

    2016-04-01

    We report the mineralogy, bulk-rock geochemistry, 40Ar/39Ar (whole-rock) age and radiogenic (Sr and Nd) isotope composition of an ultrapotassic dyke from Sakri (Nuapada lamproite field) located at the tectonic contact between the easternmost margin of the Bastar craton and Eastern Ghats Mobile Belt, India. The Sakri dyke has a mineralogy which strongly resembles a lamproite sensu stricto (viz.,Ti-rich phlogopite, Na-poor diopside, Fe-rich sanidine, ulvospinel trend and Sr-rich apatite). However, its bulk-rock major element geochemical characteristics (viz., extreme silica-undersaturated nature) resemble sensu lato kamafugite from Toro Ankole, Uganda, East African Rift, and Alto Paranaiba Province, Brazil. The Sakri dyke also displays certain compositional peculiarities (viz., high degree of evolution of mica composition from phlogopite to biotite, elevated titanium and aluminum in clinopyroxene and significantly lower bulk Mg#) when compared to the ultrapotassic rocks from various Indian cratons. 40Ar/39Ar dating gave a plateau age of 1045 ± 9 Ma which is broadly similar to that of other Mesoproterozoic (i) lamproites from the Bastar and Bundelkhand cratons, and (ii) kimberlites from the Eastern Dharwar craton. Initial bulk-rock Sr (0.705865-0.709024) and Nd (0.511063-0.511154) isotopic ratios reveal involvement of an `enriched' source region with long-term incompatible element enrichment and a depleted mantle (TDM) Nd model age of 2.56 Ga straddling the Archaean-Proterozoic chronostratigraphic boundary. The bulk-rock incompatible trace element ratios (Ta/Yb, Th/Yb, Rb/Ba and Ce/Y) of the Sakri ultrapotassic dyke negate any significant influence of crustal contamination. Small-degree melting (1 to 1.5 %) of a mixed garnet-facies and spinel-facies phlogopite lherzolite can account for its observed REE concentrations. Whereas the emplacement of the Sakri ultrapotassic dyke is related to the amalgamation of the supercontinent of Rodinia, its overlapping geochemical

  14. Late-Archaean Potassic Granite from the Bundelkhand Craton, Central India

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    Late-Archaean granitoids, show wide range of compositional variation: (i) TTG like granitoids with strongly fractionated REE patterns, which can be both Na-rich and K-Mg-rich (Sanukitoids) (ii) K-rich, Mg-poor biotite granites with less fractionated REE patterns and showing negative Eu-anomalies (type area, the Closepet Granite, Eastern Dharwar Craton, India). Amongst them Late-Archaean Sanukitoid or K-rich Closepet-type granitoids are most widely reported from the Archaean Cratons world-wide: Superior Province, Canada, Pilbara Craton, Yilgarn Craton, Antarctica, Limpopo Belt, Dharwar Craton. Several models proposed so far for the origin of these granitoids mostly include partial melting of hydrated basalts, reaction of slab melts with mantle wedge peridotites, re-melting of an enriched mantle and then mixing of the resulting melt with the anatectic melt generated during the melting of continental crust in subduction zone settings. The Closepet-type potassic biotite-rich granites were mostly produced by re-melting of TTG-like continental basements most likely in a subduction zone setting. Most of the proposed models suggest such partial melting to have taken place in garnet-stability field and some in orthopyroxene-stability field. In this study we report late-Archaean (~2.61-2.5 Ga) potassic granite from the Bundelkhand Craton in central India. The Late-Archaean granitoids recorded from the craton are intrusive into the high-grade supracrustal rocks of the craton. They are classified as coarse grained grey, pink porphyritic granite, medium granied pink granite, granite porphyry and fine-grained pink granite. The supracrustal rocks of the craton have been metamorphosed at ~2.78 Ga under high-pressure conditions (~17-18 kbar)- medium temperature (600ºC) in a subduction zone setting. The intrusions of the granitoids at ~2.6-2.5 Ga mark the stability of the craton. The pink-porphyritic granite studied here preserves plagioclase-potash feldspar

  15. SRNL PHASE 1 ASSESSMENT OF THE WAC/DQO AND UNIT OPERATIONS FOR THE WTP WASTE QUALIFICATION PROGRAM

    SciTech Connect

    Peeler, D.; Adamson, D.; Bannochie, C.; Cozzi, A.; Eibling, R.; Hay, M.; Hansen, E.; Herman, D.; Martino, C.; Nash, C.; Pennebaker, F.; Poirier, M.; Reboul, S.; Stone, M.; Taylor-Pashow, K.; White, T.; Wilmarth, B.

    2012-05-16

    The Hanford Tank Waste Treatment and Immobilization Plant (WTP) is currently transitioning its emphasis from a design and construction phase toward start-up and commissioning. With this transition, the WTP Project has initiated more detailed assessments of the requirements related to actual processing of the Hanford Site tank waste. One particular area of interest is the waste qualification program to be implemented to support the WTP. Given the successful implementation of similar waste qualification efforts at the Savannah River Site (SRS), based on critical technical support and guidance from the Savannah River National Laboratory (SRNL), WTP requested the utilization of subject matter experts from SRNL to support a technology exchange to perform a review of the WTP waste qualification program, discuss the general qualification approach at SRS, and to identify critical lessons learned through the support of DWPF's sludge batch qualification efforts. As part of Phase 1, SRNL subject matter experts in critical technical and/or process areas reviewed specific WTP waste qualification information. The Phase 1 review was a collaborative, interactive, and iterative process between the two organizations. WTP provided specific analytical procedures, descriptions of equipment, and general documentation as baseline review material. SRNL subject matter experts reviewed the information and, as appropriate, requested follow-up information or clarification to specific areas of interest. This process resulted in multiple teleconferences with key technical contacts from both organizations resolving technical issues that lead to the results presented in this report. This report provides the results of SRNL's Phase 1 review of the WAC-DQO waste acceptance criteria and processability parameters, and the specific unit operations which are required to support WTP waste qualification efforts. The review resulted in SRNL providing concurrence, alternative methods, or gap identification

  16. Gondwanan basement terranes of the Variscan-Appalachian orogen: Baltican, Saharan and West African hafnium isotopic fingerprints in Avalonia, Iberia and the Armorican Terranes

    NASA Astrophysics Data System (ADS)

    Henderson, Bonnie J.; Collins, William Joseph; Murphy, James Brendan; Gutierrez-Alonso, Gabriel; Hand, Martin

    2016-06-01

    Iberia, Avalonia and the "Armorican" terranes form key constituents of the Variscan-Appalachian orogen, but their Neoproterozoic origins along the northern Gondwanan margin continue to be strongly debated. Here, we present a new detrital zircon U-Pb-Hf dataset from Neoproterozoic-Silurian sedimentary sequences in NW Iberia and Avalonia, in conjunction with the comprehensive existing datasets from potential source cratons, to demonstrate that the provenance of each terrane is relatively simple and can be traced back to three major cratons. The enigmatic Tonian-Stenian detrital zircons in autochthonous Iberian rocks were derived from the Saharan metacraton in the latest Neoproterozoic-early Cambrian. Avalonia is commonly considered to have been derived from the Amazonian margin of Gondwana, but the hafnium isotopic characteristics of the detrital zircon grains in early Neoproterozoic rocks bear much stronger similarities to Baltica. The hafnium isotopic array also suggests the early Avalonian oceanic arc was built on a sliver of "Grenvillian-type crust" (~ 2.0-1.0 Ga) possibly of Baltican affinity at ~ 800 Ma, prior to accretion with a continental margin at ~ 640 Ma. The Upper Allochthon of Iberia is frequently linked to the West African Craton in the late Neoproterozoic-early Cambrian, however the hafnium isotopic array presented here does not support this connection; rather it is more similar to the hafnium array from Avalonia. The Armorican terranes have strong detrital zircon isotopic links to the West African Craton during the late Neoproterozoic-Cambrian.

  17. The nature of cratonic lithosphere: Combining constraints from seismology, mineral physics, and petrology

    NASA Astrophysics Data System (ADS)

    Dalton, Colleen; Faul, Ulrich; Hirsch, Aaron

    2014-05-01

    In recent years, the prevailing notion of Precambrian continental lithosphere as a thick boundary layer (~200-300 km) with a very depleted composition and temperature structure controlled by steady-state conductive cooling has been challenged by several lines of seismological evidence. One, profiles of shear velocity with depth beneath cratons exhibit lower wave speed at shallow depths and higher wave speed at greater depths than can be explained by temperature alone. These profiles are also characterized by positive or flat velocity gradients with depth in the uppermost mantle and anomalously high attenuation, both of which are difficult to reconcile with the low temperatures and large thermal gradient expected in the thermal boundary layer. Two, body-wave receiver-function studies have detected a mid-lithospheric discontinuity that requires a large and abrupt velocity decrease with depth in cratonic regions that cannot be achieved by thermal gradients alone. We have used a forward-modeling approach to identify the suite of shear-velocity profiles that are consistent with phase-velocity observations made for Rayleigh waves traversing cratons in North America, Africa, and Australia. We have also calculated the range of lithospheric temperatures and compositions that are consistent with the elastic and anelastic seismological models, using laboratory measurements on the sensitivity of velocity and attenuation to temperature, major-element composition, and mineralogy. Finally, we consider the implications of the models for the long-term stability of cratons.

  18. Lithospheric architecture of the Slave craton, northwest Canada, as determined from an interdisciplinary 3-D model

    NASA Astrophysics Data System (ADS)

    Snyder, D. B.; Hillier, M. J.; Kjarsgaard, B. A.; de Kemp, E. A.; Craven, J. A.

    2014-05-01

    geologic structures characteristic of mantle lithosphere within cratons found in continent interiors are interpreted using geo-registered diverse data sets from the Slave craton of northwest Canada. We developed and applied a new method for mapping seismic discontinuities in three dimensions using multiyear observations at sparse, individual broadband receivers. New, fully 3-D conductivity models used all available magnetotelluric data. Discontinuity surfaces and conductivity models were geo-registered with previously published P-wave and surface-wave velocity models to confirm first-order structures such as a midlithosphere discontinuity. Our 3-D model to 400 km depth was calibrated by "drill hole" observations derived from xenolith suites extracted from kimberlites. A number of new structural discontinuities emerge from direct comparison of coregistered data sets and models. Importantly, we distinguish primary mantle layers from secondary features related to younger metasomatism. Subhorizontal Slave craton layers with tapered, wedge-shaped margins indicate construction of the craton core at 2.7 Ga by underthrusting and flat stacking of lithosphere. Mapping of conductivity and metasomatism in 3-D, the latter inferred via mineral recrystallization and resetting of isotopic ages in xenoliths, indicates overprinting of the primary layered structures. The observed distribution of relatively conductive mantle at 100-200 km depths is consistent with pervasive metasomatism; vertical "chimneys" reaching to crustal depths in locations where kimberlites erupted or where Au mineralization is known.

  19. A comparison of real-time radiography results and visual characterization results with emphasis on WIPP WAC and TRAMPAC compliance issues

    SciTech Connect

    Hailey, S.M.

    1994-11-01

    Visual characterization provides a means of confirming the real-time radiography (RTR) certification process and process knowledge. RTR and visual characterization have been conducted on thirty-three drums containing transuranic (TRU) waste in support of the Waste Isolation Pilot Plant (WIPP) Experimental Test Program (WETP) at the Idaho National Engineering Laboratory. Argonne National Laboratory - West (ANL-W) detected a small can of liquid in one of these drums during the visual examination, resulting in a WIPP Waste Acceptance Criteria (WIPP-WAC) miscertification. The remaining thirty-two drums were certified correctly by the RTR system at the Stored Waste Examination Pilot Plant (SWEPP) for WIPP-WAC and TRUPACT-II Authorized Methods for Payload Control (TRAMPAC) requirements. TRAMPAC contains restrictions on the weights of specific materials allowed in the waste, based on the shipping category. Items on the restricted list for a given shipping category are allowed in quantities less than 1 percent of the weight of the waste. RTR can estimate the weights of certain broad categories in homogeneous waste forms, however, the capability to estimate weights at the 1 percent level is not presently realistic. Process knowledge forms the basis of conformance to these weight requirements. Visual characterization suggests process knowledge is not completely adequate at this level.

  20. Africans in America.

    ERIC Educational Resources Information Center

    Hart, Ayanna; Spangler, Earl

    This book introduces African-American history and culture to children. The first Africans in America came from many different regions and cultures, but became united in this country by being black, African, and slaves. Once in America, Africans began a long struggle for freedom which still continues. Slavery, the Civil War, emancipation, and the…

  1. Therapy with African Families.

    ERIC Educational Resources Information Center

    Nwadiora, Emeka

    1996-01-01

    Informs helping professionals about the unique history and challenges of African families to guide them toward providing ethnically sensitive psychological services to African immigrant families in need. African families undergo great stress when faced with the alienation of being Black and African in a Euro-American culture. (SLD)

  2. African Outreach Workshop 1974.

    ERIC Educational Resources Information Center

    Schmidt, Nancy J.

    This report discusses the 1974 African Outreach Workshop planned and coordinated by the African Studies Program at the University of Illinois at Urbana-Champaign. Its major aim was to assist teachers in developing curriculum units on African using materials available in their local community. A second aim was for the African Studies Program to…

  3. Multiscale finite-frequency Rayleigh wave tomography of the Kaapvaal craton

    NASA Astrophysics Data System (ADS)

    Chevrot, S.; Zhao, L.

    2007-04-01

    We have measured phase delays of fundamental-mode Rayleigh waves for 12 events recorded by the Southern Africa Seismic Experiment at frequencies between 0.005 and 0.035 Hz. A novel multiscale finite-frequency tomographic method based on wavelet decomposition of 3-D sensitivity kernels for the phase of Rayleigh waves is used to map the shear velocities in the upper mantle beneath southern Africa. The kernels are computed by summing coupled normal modes over a very fine grid surrounding the seismic array. To estimate and minimize the biases in the model resulting from structures outside the tomographic grid, a jackknife inversion method is implemented. The contribution of heterogeneities outside the target volume is significant, but produces artefacts in the tomographic model that are easily identified and discarded before interpretation. With structures on length scales as short as 100 km retrieved beneath the array, the deep structure of the Kaapvaal craton is revealed with unprecedented detail. Outside the array, the corresponding resolution is 200 km. High velocity cratonic roots are confined to the Archean craton, and extend to depths of at least 250 km. Confirming earlier surface structural studies, we recognize two distinct units in the Kaapvaal craton. The eastern Witwatersrand block and the western Kimberley block are separated by a major near-vertical translithospheric boundary which coincides with the Colesberg Lineament. Lower than average velocities south and east of the Kaapvaal craton reveal extensive metasomatism and heating of the lithosphere, probably related to the Karoo magmatic event and to the opening of the South Atlantic Ocean.

  4. Neoproterozoic-Early Paleozoic rifting of the craton margin in eastern Kentucky: Evidence from subsidence analysis

    SciTech Connect

    Goodman, P.T. . Dept. of Geological Sciences); Walker, D. )

    1992-01-01

    Analysis of subsidence along the craton margin in eastern Kentucky indicates a Neoproterozoic to Early through Middle Cambrian rifting event developing on a subsiding passive margin of the Laurentian craton to the Iapetus Ocean. Subsidence associated with rifting is confined to the Rome Trough; an internally broken half-graben within the Laurentian craton; the trough trends sub-parallel to the Appalachian orogenic belt. In cross section the through as an abrupt faulted margin on the carton side and a tapering, gentle extension toward the orogenic belt. The stratigraphic sequence within the Rome Trough and toward the orogen consists of Neoproterozoic or early Cambrian basal sands overlying Grenville basement, and succeeded by silts, shales and discontinuous carbonates of the Rome Fm. that are overlain by shales and carbonates of the Conesauga Fm. Stratigraphic relationships suggest that an out-of-sequence, inboard rift developed along the Laurentian margin adjacent to a drift-phase continental shelf represented by strata of the Blue Ridge and Valley and Ridge. Analysis of the subsidence history of this region reveals trends which support the notion that the subsidence history of this area cannot be accounted for by typical passive-margin development. The subsidence history of the area within the Rome Trough presents a pattern of high thermal subsidence and produces beta values greater than in areas nearer the craton margin. These data indicate that an inboard locus of anomalous crustal extension occurred in the area of the Rome Trough while the remainder of the cratonal margin underwent drift-phase subsidence, and that the timing and magnitude of this event is related to the development of the Iapetan margin.

  5. Constraints on Shear Velocity in the Cratonic Upper Mantle From Rayleigh Wave Phase Velocity

    NASA Astrophysics Data System (ADS)

    Hirsch, A. C.; Dalton, C. A.

    2014-12-01

    In recent years, the prevailing notion of Precambrian continental lithosphere as a thick boundary layer (200-300 km), defined by a depleted composition and a steady-state conductively cooled temperature structure, has been challenged by several lines of seismological evidence. One, profiles of shear velocity with depth beneath cratons exhibit lower wave speed at shallow depths and higher wave speed at greater depths than can be explained by temperature alone. These profiles are also characterized by positive or flat velocity gradients with depth and anomalously high attenuation in the uppermost mantle, both of which are difficult to reconcile with the low temperatures and large thermal gradient expected with a thermal boundary layer. Two, body-wave receiver-function studies have detected a mid-lithospheric discontinuity that requires a large and abrupt velocity decrease with depth in cratonic regions that cannot be achieved by thermal gradients alone. Here, we used forward-modeling to identify the suite of shear-velocity profiles that are consistent with phase-velocity observations made for Rayleigh waves that primarily traversed cratons in North America, South America, Africa, and Australia. We considered two approaches; with the first, depth profiles of shear velocity were predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the crust and upper mantle. With the second approach, depth profiles of shear velocity were randomly generated. In both cases, Rayleigh wave phase velocity was calculated from the Earth models and compared to the observed values. We show that it is very difficult to match the observations with an Earth model containing a low-velocity zone in the upper mantle; instead, the best-fit models contain a flat or positive velocity gradient with depth. We explore the implications of this result for the thermal and

  6. Ambient noise tomography of the Cameroon Volcanic Line and Northern Congo craton: new constraints on the structure of the lithosphere

    NASA Astrophysics Data System (ADS)

    Guidarelli, M.; Aoudia, A.

    2016-03-01

    We investigate the lithospheric structure of Cameroon inverting Rayleigh waves obtained from the cross-correlation of ambient seismic noise. We correlate seismic records between 32 broad-band stations and we obtain good quality Rayleigh waves for 310 interstation paths. We measure group velocity dispersion curves from the reconstructed Rayleigh waves in the period range 10-35 s and we invert the group velocities for tomographic images. After the tomography the group velocities are then inverted, together with longer period group velocity measurements from existing literature, to compute a 3-D S-wave velocity model of the Cameroon lithosphere down to 100 km depth. Our results provide an unprecedented mapping of the physical properties of the different crustal units and their correlations with surface geology, as well as with mantle lithospheric variations. The Cameroon Volcanic Line (CVL) appears as a segmented feature exhibiting different physical properties along strike. The active Mt Cameroon volcano is underlain by very low velocities, unlike the other segments of the CVL. The along-strike variations in crustal structure suggest that lateral heterogeneities in lithospheric thickness and physical properties have influenced the location and distribution of magmatism. The crust beneath the Central African Shear Zone exhibits a sizeable low velocity anomaly. The lithosphere beneath Cameroon is characterised by a heterogeneous crust with a relatively constant thickness and a low velocity uppermost mantle at the edge of the Congo Craton. Our results favour processes combining small-scale upwelling at the edge of a thick lithosphere and reactivation of Precambrian basement structures to explain the distribution of Holocene-Recent magmatism and plateau uplift. Our results also indicate that Mt Cameroon and surroundings areas are the most at risk zones for magmatic activity during this stage of CVL development.

  7. Multi Scale Imaging of Seismic Structure beneath the Western Branch of the East-African Rift

    NASA Astrophysics Data System (ADS)

    Jakovlev, A.; Rumpker, G.; Koulakov, I.

    2010-12-01

    In this study, we investigate the crustal and upper mantle velocity structure beneath the East African Rift System (EARS) as a whole and beneath the Rwenzori Mountains in western Uganda in particular. The most interesting features here is the mountain range of approximately 50 km-wide, with a length of about 150 km, which is situated within the western branch of the East African rift zone and reaches the altitudes of more than 5000 m. The joint tomographic inversion was used to investigate the velocity structure beneath the Rwenzoris on a local scale down to the depth of about 75 - 80 km. Arrival times from 2053 local and 284 teleseismic earthquakes recorded by a temporary network equipped 35 short period and broad-band stations were used as an input for the joint tomographic inversion. The network is covering an area of 140×90 km2. To estimate resolution ability and stability of the obtained results we performed many different tests. The tests show that the best horizontal resolution is achieved in the northern part of the study area, where the density of the ray coverage is highest. In shallow depths where local earthquake rays dominate, the vertical and horizontal resolution is relatvely high, while in deeper sections covered with teleseismic rays, the anomalies seem to be strongly smeared and can be interpreted only on a qualitative level. Velocity structure in the upper crust agrees with the distribution of the main geological units, such as sedimentary basins, igneous outcrops, thermal fields etc. For the uppermost mantle, our results reveal an inclined boundary between the high-velocity Tanzania craton and low-velocity patterns beneath the rift. The same position of the contact between craton and rift zone was obtained in the regional tomographic study of the EARS as a whole. This regional study was based on the ISC data, which contains information about arrival times of signals from earthquakes at more than 7000 stations of the global seismological network

  8. Neoproterozoic geodynamic evolution of SW-Gondwana: a southern African perspective

    NASA Astrophysics Data System (ADS)

    Frimmel, H. E.; Basei, M. S.; Gaucher, C.

    2011-04-01

    Our current understanding of the tectonic history of the principal Pan-African orogenic belts in southwestern Africa, reaching from the West Congo Belt in the north to the Lufilian/Zambezi, Kaoko, Damara, Gariep and finally the Saldania Belt in the south, is briefly summarized. On that basis, possible links with tectono-stratigraphic units and major structures on the eastern side of the Río de la Plata Craton are suggested, and a revised geodynamic model for the amalgamation of SW-Gondwana is proposed. The Río de la Plata and Kalahari Cratons are considered to have become juxtaposed already by the end of the Mesoproterozoic. Early Neoproterozoic rifting led to the fragmentation of the northwestern (in today's coordinates) Kalahari Craton and the splitting off of several small cratonic blocks. The largest of these ex-Kalahari cratonic fragments is probably the Angola Block. Smaller fragments include the Luis Alves and Curitiba microplates in eastern Brazil, several basement inliers within the Damara Belt, and an elongate fragment off the western margin, named Arachania. The main suture between the Kalahari and the Congo-São Francisco Cratons is suspected to be hidden beneath younger cover between the West Congo Belt and the Lufilian/Zambezi Belts and probably continues westwards via the Cabo Frío Terrane into the Goiás magmatic arc along the Brasilia Belt. Many of the rift grabens that separated the various former Kalahari cratonic fragments did not evolve into oceanic basins, such as the Northern Nosib Rift in the Damara Belt and the Gariep rift basin. Following latest Cryogenian/early Ediacaran closure of the Brazilides Ocean between the Río de la Plata Craton and the westernmost fragment of the Kalahari Craton, the latter, Arachania, became the locus of a more than 1,000-km-long continental magmatic arc, the Cuchilla Dionisio-Pelotas Arc. A correspondingly long back-arc basin (Marmora Basin) on the eastern flank of that arc is recognized, remnants of which

  9. Mid-lithospheric discontinuity and its roles in the dynamic evolution of the craton-example from the North China Craton

    NASA Astrophysics Data System (ADS)

    Chen, Ling; Wei, Zigen; Jiang, Mingming; Ling, Yuan

    2016-04-01

    Mid-lithospheric discontinuity and its roles in the dynamic evolution of the craton - example from the North China Craton Ling Chen1,2, Zigen Wei3, Mingming Jiang1, Yuan Ling1 1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing100101, China 3. State Key Laboratory of Geodesy and Earth's Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China Detailed knowledge of lithospheric structure is essential for understanding the long-term evolution and dynamics of continents. We present an integrated lithospheric structural image along an E-W profile across the North China Craton (NCC) derived from the teleseismic data recorded at two dense seismic arrays in combination with other geophysical and geological observations. Our S- and P-receiver function images show substantial undulations of the lithosphere-asthenosphere boundary (LAB), from 60-100 km in the eastern NCC to ~160-200 km in the central-western NCC, and <150-km in the Qilian orogenic belt further to the west, accompanying marked lithospheric structural variations. This agrees with previous studies that suggest the occurrence of fundamental destruction in the eastern NCC but localized lithospheric thinning and modifications in the central-western NCC. A negative velocity discontinuity is identified at the depth of ~80-100 km within the thick lithosphere of the central-western NCC, spatially coincident with the top interface of a relatively low velocity layer in the overall high velocity mantle root imaged by surface wave tomography. Detailed data analyses show that this mid- or intra-lithospheric discontinuity has considerably larger S-to-P and P-to-S conversion amplitudes than the LAB below, which provides observational constraints to further decipher the origin of the discontinuity. Our imaging results corroborate

  10. Changes in the mantle transition zone from central Europe to the East European Craton as seen by receiver functions

    NASA Astrophysics Data System (ADS)

    Knapmeyer-Endrun, B.; Krüger, F.; Passeq Working Group

    2012-04-01

    The depth to which lithospheric roots of cratons influence the surrounding mantle has important consequences for our understanding of the thermal structure of the mantle and its geodynamics. Still, this information is often not well constrained: previous studies of depth variations in mantle transition zone (MTZ) discontinuities beneath cratons led to variable results, depending on the craton under investigation. Here, we present evidence that the influence of the East European Craton extends into the MTZ. We study teleseismic P-receiver functions recorded at more than 400 stations across central and eastern Europe, from the Netherlands to Estonia. Using P-to-S converted waves, receiver functions yield relative travel times to the 410 km and 660 km discontinuities and are especially useful in mapping variations in MTZ thickness. The main data source in our study is the multinational PASSEQ experiment, which achieved the densest coverage with seismic stations yet in Poland and Lithuania by using about 200 temporary stations installed from 2006 to 2008. These recordings are supplemented by national and regional networks and broad-band data from older temporary deployments. The large data set allows us to image the MTZ across the Trans-European Suture Zone (TESZ), the western boundary of the East European Craton, in high resolution. We observe significantly advanced onset times, by as much as 2 s compared to standard Earth models and up to 3 s compared to the Central European Platform, for conversions from both MTZ discontinuities for locations within the craton. This effect is expected and due to the higher velocities in the mantle above the MTZ, e.g. the cratonic root. It gets even more pronounced when the opposing effect of variable crustal thickness (50 km and more within the craton in contrast to 30 to 35 km in central Europe) is considered. Additionally, the differential travel time between the two discontinuities is increased by 1 s beneath the craton, most

  11. Diamonds and the african lithosphere.

    PubMed

    Boyd, F R; Gurney, J J

    1986-04-25

    Data and inferences drawn from studies of diamond inclusions, xenocrysts, and xenoliths in the kimberlites of southern Africa are combined to characterize the structure of that portion of the Kaapvaal craton that lies within the mantle. The craton has a root composed in large part of peridotites that are strongly depleted in basaltic components. The asthenosphere boundary shelves from depths of 170 to 190 kilometers beneath the craton to approximately 140 kilometers beneath the mobile belts bordering the craton on the south and west. The root formed earlier than 3 billion years ago, and at that time ambient temperatures in it were 900 degrees to 1200 degrees C; these temperatures are near those estimated from data for xenoliths erupted in the Late Cretaceous or from present-day heat-flow measurements. Many of the diamonds in southern Africa are believed to have crystallized in this root in Archean time and were xenocrysts in the kimberlites that brought them to the surface.

  12. Variations of the lithospheric strength across the edges of the North American craton and their relation to intraplate earthquakes

    NASA Astrophysics Data System (ADS)

    Tesauro, Magdala; Kaban, Mikhail; Mooney, Walter; Cloetingh, Sierd

    2016-04-01

    Seismic tomography models revealed a pronounced velocity contrast between the cratonic and off-cratonic regions of North America. However, the location of the transition between the fast/slow velocities characterizing the Archean-Proterozoic and Phanerozoic regions, respectively, is still under debate. In order to understand the structure of the edges of North American (NA) cratons, we analyze the results of two recent thermal and strength models of the NA continent, obtained using seismic and gravity data (Kaban et al., 2014; Tesauro et al., 2014; 2015). We could observe that in the peripheral parts of the cratons, as the Proterozoic Canadian Platform, the Grenville, and the western part of the Yavapai-Mazatzal province, the integrated strength for one model is 10 times larger than the other one, due to a temperature difference of >200°C in the uppermost mantle. The differences in the effective elastic thickness (Te) between the two models are less pronounced. In both models, Proterozoic regions reactivated by Meso-Cenozoic tectonics (e.g., Rocky Mountains and the Mississippi Embayment), are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer. Furthermore, intraplate earthquakes are distributed along the edges of the cratons, demonstrating that tectonic stress accumulates there, while the cores of the cratons remain undeformed. In both models, intraplate seismicity occurs in weak lithosphere or in the regions characterized by pronounced contrasts of strength and Te.

  13. Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

    NASA Astrophysics Data System (ADS)

    Pilet, Sebastien; Guex, Jean; Muntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Schaltegger, Urs

    2016-04-01

    The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here, we present a synthesis of stratigraphic constraints on the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries combined with geochronological data in order to establish the sequence of events that initiate two of the major mass extinctions recorded in Earth's history. This synthesis demonstrates that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. The initial regressive events recorded at T-J and Pl-To boundaries seem difficult to reconcile either with large initial CO2 degassing associated with plume activity or by volatile-release (CO2, CH4, Cl2) from deep sedimentary reservoirs during contact metamorphism associated to dykes and sills intrusion because massive CO2 degassing is expected to produce super greenhouse conditions. We evaluate, here, an alternative suggesting that the initial cooling could be due to gas release during the initial thermal erosion of the cratonic lithosphere due to emplacement of the CAMP and Karoo-Ferrar volcanic provinces. Petrological constraints on primary magmas indicate that the mantle is hotter and melts more extensively to produce LIP lavas than for current oceanic islands basalts. However, available data suggest that the Karoo and CAMP areas were underlain by thick lithosphere (>200 km) prior to continental break up. The presence of thick lithosphere excludes significant melting of the asthenospheric mantle without initial stage of thermal erosion of the cratonic lithosphere. This initial step of thermal erosion / thermal heating of the cratonic lithosphere is critical to understand the volatile budget associated with LIPs while

  14. Mantle density beneath the Siberian craton based on free board constrains

    NASA Astrophysics Data System (ADS)

    Cherepanova, Yulia; Artemieva, Irina

    2014-05-01

    We present the mantle density model of the Archean-Proterozoic Siberian Craton (SC). The density model is constrained by free-board (buoyancy) modeling (Lachenbruch and Morgan, 1990). The approach assumes isostatic compensation of the region, and is justified by the near-zero free-air gravity for most of the region, except for the flanking orogenic belts with high topography. Despite a relatively uniform topography of the SC (ca. 400 m for most of the region and reaching 700 m in the shields), the craton has a strongly heterogeneous crustal structure with large regional variations in Moho and average crustal Vp (Cherepanova et al., 2013) which reflects its complex tectonic evolution. Formed by amalgamation of several Archean terranes, the craton has been significantly affected by Proterozoic collisional and extensional events, the late- Proterozoic rifting at its margins, the Devonian rifting of the Vilyui rift, several pulses of kimberlite magmatism, and the Permo-Triassic trap basalt magmatism. The strong lateral and vertical heterogeneity of the lithospheric mantle has been documented so far in the studied of the mantle xenoliths from kimberlite pipes and in a limited number of geophysical studies. Here we extend geophysical analysis of mantle compositional heterogeneity by evaluating mantle density structure and interpreting its regional variations in terms of mantle mg#. We link regional large-amplitude variations in mantle depletion to the tectonic evolution of the craton and compare these results with geophysical models and petrologic data. We speculate on the origin of compositional heterogeneity of the lithospheric mantle, which is in overall agreement with results of a joint analysis of seismic and thermal data (Artemieva, 2009) and mantle xenolith studies which provide information on metasomatic enrichment of the depleted lithospheric mantle by the tectonic events. The results indicate the heterogeneous structure of mantle density, with the average

  15. Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions

    NASA Astrophysics Data System (ADS)

    Miller, Meghan S.; Eaton, David W.

    2010-09-01

    Delineating mantle interfaces can provide important clues for understanding the formation of continents. We use S-wave receiver functions to investigate lithospheric structure along a transect extending from Vancouver Island to Baffin Island. Observed Sp converted waves allow for interpretation of boundaries in the depth range expected for tectonic plates. Receiver functions show a distinct negative amplitude feature, interpreted as the lithosphere-asthenosphere boundary, at shallow depths beneath British Columbia (˜85km), deepening abruptly at the eastern edge of the Cordillera to ˜260km beneath the Canadian Shield. Dipping mid-lithospheric discontinuities extend beneath several giant ca. 1.8 Ga epicontinental magmatic arcs, consistent with formation of cratonic lithosphere by arc accretion. This model provides a plausible explanation for global mid-lithospheric discontinuities within cratons and aids in understanding their formation.

  16. Olivine water contents in the continental lithosphere and the longevity of cratons.

    PubMed

    Peslier, Anne H; Woodland, Alan B; Bell, David R; Lazarov, Marina

    2010-09-01

    Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet. PMID:20811455

  17. Olivine water contents in the continental lithosphere and the longevity of cratons.

    PubMed

    Peslier, Anne H; Woodland, Alan B; Bell, David R; Lazarov, Marina

    2010-09-01

    Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet.

  18. Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction.

    PubMed

    Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

    2016-01-01

    The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere. PMID:27009463

  19. Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

    NASA Astrophysics Data System (ADS)

    Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

    2016-03-01

    The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.

  20. Regional geophysics and the basement of cratonic basins: a comparative study with the Michigan basin

    SciTech Connect

    Hinze, W.J.; Lidiak, E.G.

    1986-08-01

    The basement of the Michigan basin consists of four major provinces - the complex metasedimentary, metavolcanic, and igneous rocks of the Penokean orogenic assemblage in the north, the felsic anorogenic igneous rocks to the south, the highly metamorphosed schists, gneisses, and related igneous intrusions of the Grenville province in the east, and a middle Proterozoic rift zone, which transects the basin from the north to the southeast margin. Sparse basement drill holes and characteristic geophysical patterns support this interpretation. The direct geologic information on the basement of other cratonic basins is not as well known. However, regional geophysical surveys and sparse, poorly distributed basement drill holes provide information on the complex character and structural relationships of the basement of other basins. Like the Michigan basin, many cratonic basins (e.g., Illinois, Williston, and Paris basins) are underlain by dense and commonly more magnetic rocks than adjacent areas. As in the Michigan basin, these rocks are interpreted to have a profound effect on the origin and tectonic development of the basins. Geologic and geophysical evidence indicates that many of these dense basement rocks originated in rifts that formed hundreds of millions of years prior to basin development. A comparison of the basement in cratonic basins provides important constraints on the origin and tectonic development of the Michigan basin.

  1. How Did the North China Craton Move from Nuna to Rodinia

    NASA Astrophysics Data System (ADS)

    Zhang, S.

    2015-12-01

    Several models that include the paleogeographic positions of the North China Craton (NCC) in Precambrian supercontinents Nuna and Rodinia were tested in this study using Proterozoic global paleomagnetic database. During ca. 1.8 - 1.4 Ga, the NCC can be placed far from Laurentia and Baltica. In Nuna, the present southern margin of the NCC faced an open ocean, whilst its present northern margin was likely connected to Indian and Australian cratons. In Rodinia, however, during ca. 1.1 - 0.85 Ga, the paleomagnetic data indicate a proximate connection between the NCC and the arctic side of Laurentia. These two models, if both are right, suggest that the NCC may have moved a long distance since the departure from Nuna to the joining of Rodinia. Moreover, paleomagnetic data and our reconstructions require many cratons to drift away from their connections in Nuna after ca. 1.35 Ga. These reconstructions strongly challenges the currently influential speculation of limited breakup of Nuna, which is mainly based on the observation of low abundance of passive margins that were formed during the boring billion years (ca. 1.75 - 0.75 Ga).

  2. Lithospheric Structure of the Northeastern North China Craton Imaged by S Receiver Functions

    NASA Astrophysics Data System (ADS)

    Wang, Xingchen; Ding, Zhifeng; Zhu, Lupei

    2016-08-01

    Lithosphere thickness variation is important for understanding the significant tectonic reactivation of the North China Craton (NCC) in the Mesozoic and Cenozoic time. Here, we determined the lithospheric structure in the northeastern NCC using S receiver functions from 305 teleseismic events recorded by 223 seismic stations. The Moho and lithosphere-asthenosphere boundary (LAB) are imaged clearly beneath the region. The Moho depth decreases from ~45 km beneath the western NCC to ~25 km beneath the eastern NCC. We found that the lithospheric thickness varies from 60 to 80 km beneath the Trans-North China Orogen (TNCO) and eastern NCC with no significant change of the LAB depth. The lithosphere thickness beneath the northwestern Ordos plateau is 100-130 km. In addition, there is a mid-lithosphere discontinuity at a depth of 80 km beneath the plateau that is connected to the base of thinned lithosphere in TNCO and eastern NCC. We suggest that the mid-lithosphere discontinuity represents a mechanically weak zone in the original cratonic lithosphere of the NCC. The material in the lower lithosphere of the craton, when warmed and hydrated by water released from the subducting slab of Western Pacific, became weak due to decrease in viscosity and/or partial melting and was subsequently removed through small-scale mantle convections.

  3. Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction.

    PubMed

    Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

    2016-03-24

    The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere.

  4. Geotectonic setting and metallogeny of the northern São Francisco craton, Bahia, Brazil

    NASA Astrophysics Data System (ADS)

    Teixeira, João Batista Guimarães; da Silva, Maria da Glória; Misi, Aroldo; Cruz, Simone Cerqueira Pereira; da Silva Sá, José Haroldo

    2010-11-01

    This paper aims at establishing a tectonic and temporal framework to characterize the metallogenic processes that contributed to the origin of the mineral provinces in the northern São Francisco Craton. Many Archean mineralizations (eg. massive sulfide zinc, lead, zinc and copper, besides magnesite-talc, iron-titanium-vanadium, iron, chromite and manganese) were generated before the assembly of the Craton. Deposits of chromite, nickel, gold and emerald were produced during the Paleoproterozoic orogenic cycle, when the Craton was amalgamated into the Atlantica paleocontinent. An extension event is recorded in the Neoproterozoic, during the breakup of Rodinia, associated with deposits of phosphorite and uranium. Kimberlite diamond and gold mineralization were generated during the Brasiliano orogenic cycle, coeval with the amalgamation of West Gondwana. A long-lasting and rather uniform crustal stress is recorded in the area during the Cambrian period. Resetting of the isotopic and magnetic systems that affected the Neoproterozoic sediments of the Irecê Basin at about 520 Ma was attributed to the regional-scale fluid migration and mineralization in the aftermath of the Brasiliano orogenic cycle.

  5. Reactivation and mantle dynamics of North China Craton: insight from P-wave anisotropy tomography

    NASA Astrophysics Data System (ADS)

    Tian, You; Zhao, Dapeng

    2013-12-01

    We determined the first 3-D P-wave anisotropic tomography beneath the North China Craton (NCC) using a large number of high-quality arrival-time data from local earthquakes and teleseismic events, which reveals depth-dependent azimuthal anisotropy in the crust and upper mantle down to 600 km depth. In the NCC western block, the fast velocity direction (FVD) varies from east-west in the southern part to northeast-southwest in the northern part, which may reflect either the interaction between the Yangtze block and NCC or fossil lithospheric fabrics in the craton. Under the NCC eastern block, a uniform northwest-southeast FVD is revealed in the lower part of the upper mantle (300-410 km depths) and the mantle transition zone (410-660 km depths), which may reflect horizontal and upwelling flows in the big mantle wedge (BMW) above the stagnant Pacific slab in the mantle transition zone. The NCC central block exhibits a northeast-southwest FVD, consistent with the surface tectonic orientation there, suggesting that the cold and thick (>300 km) cratonic root of the NCC western block may obstruct the northwest-southeast trending mantle flow induced by the Pacific Plate subduction, resulting in a northeast-southwest trending mantle flow under the central block. Our present results indicate that the corner flow in the BMW associated with the deep subduction of the Pacific Plate is the main cause of NCC reactivation and mantle dynamics under East China.

  6. Thermal erosion of cratonic lithosphere as a potential trigger for mass-extinction

    PubMed Central

    Guex, Jean; Pilet, Sebastien; Müntener, Othmar; Bartolini, Annachiara; Spangenberg, Jorge; Schoene, Blair; Sell, Bryan; Schaltegger, Urs

    2016-01-01

    The temporal coincidence between large igneous provinces (LIPs) and mass extinctions has led many to pose a causal relationship between the two. However, there is still no consensus on a mechanistic model that explains how magmatism leads to the turnover of terrestrial and marine plants, invertebrates and vertebrates. Here we present a synthesis of ammonite biostratigraphy, isotopic data and high precision U-Pb zircon dates from the Triassic-Jurassic (T-J) and Pliensbachian-Toarcian (Pl-To) boundaries demonstrating that these biotic crises are both associated with rapid change from an initial cool period to greenhouse conditions. We explain these transitions as a result of changing gas species emitted during the progressive thermal erosion of cratonic lithosphere by plume activity or internal heating of the lithosphere. Our petrological model for LIP magmatism argues that initial gas emission was dominated by sulfur liberated from sulfide-bearing cratonic lithosphere before CO2 became the dominant gas. This model offers an explanation of why LIPs erupted through oceanic lithosphere are not associated with climatic and biotic crises comparable to LIPs emitted through cratonic lithosphere. PMID:27009463

  7. Ambient noise tomography of the East African Rift in Mozambique

    NASA Astrophysics Data System (ADS)

    Domingues, Ana; Silveira, Graça; Ferreira, Ana M. G.; Chang, Sung-Joon; Custódio, Susana; Fonseca, João F. B. D.

    2016-03-01

    Seismic ambient noise tomography is applied to central and southern Mozambique, located in the tip of the East African Rift (EAR). The deployment of MOZART seismic network, with a total of 30 broad-band stations continuously recording for 26 months, allowed us to carry out the first tomographic study of the crust under this region, which until now remained largely unexplored at this scale. From cross-correlations extracted from coherent noise we obtained Rayleigh wave group velocity dispersion curves for the period range 5-40 s. These dispersion relations were inverted to produce group velocity maps, and 1-D shear wave velocity profiles at selected points. High group velocities are observed at all periods on the eastern edge of the Kaapvaal and Zimbabwe cratons, in agreement with the findings of previous studies. Further east, a pronounced slow anomaly is observed in central and southern Mozambique, where the rifting between southern Africa and Antarctica created a passive margin in the Mesozoic, and further rifting is currently happening as a result of the southward propagation of the EAR. In this study, we also addressed the question concerning the nature of the crust (continental versus oceanic) in the Mozambique Coastal Plains (MCP), still in debate. Our data do not support previous suggestions that the MCP are floored by oceanic crust since a shallow Moho could not be detected, and we discuss an alternative explanation for its ocean-like magnetic signature. Our velocity maps suggest that the crystalline basement of the Zimbabwe craton may extend further east well into Mozambique underneath the sediment cover, contrary to what is usually assumed, while further south the Kaapval craton passes into slow rifted crust at the Lebombo monocline as expected. The sharp passage from fast crust to slow crust on the northern part of the study area coincides with the seismically active NNE-SSW Urema rift, while further south the Mazenga graben adopts an N-S direction

  8. Structural and geochronological constraints on the Pan-African tectonic evolution of the northern Damara Belt, Namibia

    NASA Astrophysics Data System (ADS)

    Lehmann, Jérémie; Saalmann, Kerstin; Naydenov, Kalin V.; Milani, Lorenzo; Belyanin, George A.; Zwingmann, Horst; Charlesworth, Guy; Kinnaird, Judith A.

    2016-01-01

    The Pan-African Orogen formed by convergence of numerous continental blocks during the Neoproterozoic to early Cambrian. This convergence eventually led to amalgamation of Gondwana, a supercontinent crosscut by a network of highly oblique linear orogenic belts that locally intersect each other, as in NW Namibia, where the NNW trending Kaoko Belt joins the NE trending Damara Belt. The northern Damara Belt has preserved well three regional Pan-African tectonic events due to the dominance of weak Neoproterozoic marine sediments (Damara Supergroup) that have been affected by low-grade metamorphism. A newly discovered early N-S horizontal contraction, dated by 40Ar/39Ar at ~590 Ma, is tentatively linked to convergence between the Congo and Kalahari cratons. This was superseded by collision between the Congo and Rio de la Plata cratons between 580 and 530 Ma that thickened and exhumed the orogenic crust of the Kaoko Belt and produce upper crustal N-S oriented folds of earlier fold trains and associated axial planar schistosities in the northern Damara Belt. A switch from E-W to NW-SE horizontal shortening occurred at ~530 Ma as a result of collision with the Kalahari Craton, triggering extensive syn-orogenic magmatism in the entire Damara Belt. During this last event, southward indentation and underthrusting of the Congo Craton promontory below the Neoproterozoic cover sequences produced a deformation front in the northern Damara Belt. Our results show that highly oblique convergent processes competed over a period of ~120 Ma to build Gondwana in Namibia during the late Neoproterozoic to early Cambrian.

  9. The African Connection

    ERIC Educational Resources Information Center

    Oguntoyinbo, Lekan

    2012-01-01

    From student and faculty exchanges to joint research projects, U.S. universities maintain a broad spectrum of collaborative relationships with African universities. It's unclear how many U.S. colleges and universities have partnerships with African universities. The African Studies Association, an organization of scholars, doesn't keep that kind…

  10. Linguistic Imperialism: African Perspectives.

    ERIC Educational Resources Information Center

    Phillipson, Robert

    1996-01-01

    Responds to an article on aspects of African language policy and discusses the following issues: multilingualism and monolingualism, proposed changes in language policy from the Organization for African Unity and South African initiatives, the language of literature, bilingual education, and whose interests English-language teaching is serving.…

  11. Seismic Tomography of the Arctic: Continental Cratons, Ancient Orogens, Oceanic Lithosphere and Convecting Mantle Beneath (Invited)

    NASA Astrophysics Data System (ADS)

    Lebedev, S.; Schaeffer, A. J.

    2013-12-01

    Lateral variations in seismic velocities in the upper mantle, mapped by seismic tomography, reflect primarily the variations in the temperature of the rock at depth. Seismic tomography thus reveals lateral changes in the temperature and thickness of the lithosphere; it maps deep boundaries between tectonic blocks with different properties and with different age of the lithosphere. Our new global, shear-wave tomographic model of the upper mantle and the crust is constrained by an unprecedentedly large number of broadband waveform fits (nearly one million seismograms, with both surface and S waves included) and provides improved resolution of the lithosphere across the whole of the Arctic region, compared to other available models. The most prominent high-velocity anomalies, seen down to 150-200 km depths, indicate the cold, thick, stable mantle lithosphere beneath Precambrian cratons. The northern boundaries of the Canadian Shield's and Greenland's cratonic lithosphere closely follow the coastlines, with the Greenland and North American cratons clearly separated from each other. In Eurasia, in contrast, cratonic lithosphere extends hundreds of kilometres north of the coast of the continent, beneath the Barents and eastern Kara Seas. The boundaries of the Archean cratons mapped by tomography indicate the likely offshore extensions of major Phanerozoic sutures in northern Eurasia. The old oceanic lithosphere of the Canada Basin is much colder and thicker than the younger lithosphere beneath the adjacent Amundsen Basin, north of the Gakkel Ridge. Beneath the slow-spreading Gakkel Ridge, we detect the expected low-velocity anomaly associated with partial melting in the uppermost mantle; the anomaly is weaker, however, than beneath faster-spreading ridges globally. South of the ridge, the Nansen Basin shows higher seismic velocities in the upper mantle beneath it, compared to the Amundsen Basin. At 150-250 km depth, most of the oceanic portions of the central Arctic (the

  12. Melt-Peridotite Reaction Mechanism of North China Craton Destruction: Experimental Constrains

    NASA Astrophysics Data System (ADS)

    Wang, C.; Jin, Z.; Gao, S.; Zhang, J.; Zheng, S.

    2009-12-01

    Although the geoscientsts in the world have reached an agreement about the Mesozoic thinning of North China Craton, the dynamics of this process is still under debate, especially the thinning mechanism. There are two popular thinning mechanisms were proposed for the North China Craton, one is the delamination model and the other is thermo-mechanical erosion model. The delamination model has aroused great interests since Gao et al. use it to explain the formation of Xilonggou high Mg# andesite from Liaoxi region of Liaoning Province. These Lavas, which possessed an adakitic signature with high Mg#, were believed to origin by the interaction between silica rich and yttrium delepted melt, which are formed by partile melting of low crust eclogite, and asthenospheric peridotite. The discovery of zoned mantle olivine and pyroxene xenocrysts, especially zoned mantle peridotitic xenoliths in Mesozoic basaltic rocks from eastern North China, indicates that peridotite-melt reaction was widespread in Mesozoic lithospheric mantle beneath the southeastern North China craton. Such interaction could be an important mechanism for the compositional transformation and rapid enrichment of the lithospheric mantle. We conducted a series of experiments at a pressure of 2.0 GPa and temperatures of 1250-1400 °C using Bixiling eclogite and Damaping peridotite as starting materials to study the mechanism of melt-peridotite reaction. Our results show that olivine and orthopyroxene were dissolved in the reacting melts between eclogite and peridotite, resulting in clinopyroxene precipitation along the interface of melts and lherzolite. The experimental run products, possessing a lherzolite-pyroxenite-garnet pyroxenite sequence, are identical to the composite mantle xenoliths in the Neogene Hannuoba basalt of the North China craton. The later is considered as strong evidence of the maffic lower continental crust recycling into the mantle during the Mesozoic Era. The reacting melts have a high

  13. Aeromagnetic interpretation in the south-central Zimbabwe Craton: (reappraisal of) crustal structure and tectonic implications

    NASA Astrophysics Data System (ADS)

    Ranganai, Rubeni T.; Whaler, Kathryn A.; Ebinger, Cynthia J.

    2015-12-01

    Regional aeromagnetic data from the south-central Zimbabwe Craton have been digitally processed and enhanced for geological and structural mapping and tectonic interpretation integrated with gravity data, to constrain previous interpretations based on tentative geologic maps and provide new information to link these structural features to known tectonic events. The derived maps show excellent correlation between magnetic anomalies and the known geology, and extend lithological and structural mapping to the shallow/near subsurface. In particular, they reveal the presence of discrete crustal domains and several previously unrecognised dykes, faults, and ultramafic intrusions, as well as extensions to others. Five regional structural directions (ENE, NNE, NNW, NW, and WNW) are identified and associated with trends of geological units and cross-cutting structures. The magnetic lineament patterns cut across the >2.7 Ga greenstone belts, which are shown by gravity data to be restricted to the uppermost 10 km of the crust. Therefore, the greenstone belts were an integral part of the lithosphere before much of the upper crustal (brittle) deformation occurred. Significantly, the observed magnetic trends have representatives craton-wide, implying that our interpretation and inferences can be applied to the rest of the craton with confidence. Geological-tectonic correlation suggests that the interpreted regional trends are mainly 2.5 Ga (Great Dyke age) and younger, and relate to tectonic events including the reactivation of the Limpopo Belt at 2.0 Ga and the major regional igneous/dyking events at 1.8-2.0 Ga (Mashonaland), 1.1 Ga (Umkondo), and 180 Ma (Karoo). Thus, their origin is here inferred to be inter- and intra-cratonic collisions and block movements involving the Zimbabwe and Kaapvaal Cratons and the Limpopo Belt, and later lithospheric heating and extension associated with the break-up of Gondwana. The movements produced structures, or reactivated older fractures

  14. Visible and infrared properties of unaltered to weathered rocks from Precambrian granite-greenstone terrains of the West African Craton

    NASA Astrophysics Data System (ADS)

    Metelka, Václav; Baratoux, Lenka; Jessell, Mark W.; Naba, Séta

    2015-12-01

    In situ and laboratory 0.35 μm-2.5 μm spectra of rocks from a Paleoproterozoic granite-greenstone terrain along with its Neoproterozoic sedimentary cover and derived regolith materials were examined in western Burkina Faso. The reflectance spectra show the influence of typical arid to semi-arid weathering with the formation of desert varnish, iron films, and dust coatings. Fe and Mg-OH absorption features related to chlorite, amphibole, pyroxene, epidote, and biotite are observable in the mafic and intermediate meta-volcanic rocks as well as in the granodiorites and tonalites. Al-OH absorption caused by kaolinite, smectite, illite/muscovite are typical for meta-volcano-sedimentary schists, Tarkwaian-type detrital meta-sediments, sandstones of the Taoudeni basin, all of the weathered surfaces and regolith materials. Ferric and ferrous iron absorptions related to both primary rock-forming minerals and secondary weathering minerals (goethite, hematite) were observed in most of the sampled materials. The results show that although weathering alters the spectral signature of the fresh rock, indicative absorption features located in the short wave infrared region remain detectable. In addition, spectra of soils partially reflect the mineral composition of the weathered rock surfaces. The analysis of the hyperspectral data shows the potential of differentiating between the sampled surfaces. The library presents a primary database for the geological and regolith analysis of remote sensing data in West Africa.

  15. Influence of cratonic lithosphere on slab geometry and mantle flow: insights from 3D time-dependent modelling.

    NASA Astrophysics Data System (ADS)

    Taramón, Jorge M.; Rodríguez-González, Juan; Negredo, Ana M.

    2014-05-01

    Recent studies show a clear correlation between the occurrence of flat subduction and the proximity of areas of high elastic/thermal thickness in the overriding plate. A plausible explanation is that cold overriding plates lead to colder mantle wedge, increasing the hydrodynamic suction and decreasing the slab dip. In particular, recent numerical modeling has shown that the presence of cratonic lithosphere in the overriding plate has a significant effect on subducting slabs. In this study we quantify the influence of cratonic areas in the overriding plate on subduction dynamics. We present 3D thermo-mechanical and time-dependent numerical models of buoyancy-driven subduction processes. A non-Newtonian pseudo-plastic rheology is assumed. Different simulations have been performed to quantify the effect of different factors, such as the craton width, thermal thickness and distante to the trench. Modelling results indicate that presence of cratonic lithosphere in the overriding plate produces strong along-trench variations of the slab geometry. These variations are maintained and propagated at great depths as the slab sinks deeper into the mantle. Significant trench-parallel flow in the mantle wedge is generated by time-dependent changes in slab dip. For cases of reduced slab pull, the slab and the base of the craton become coupled, which causes a dramatic reduction of subduction velocity and the formation of a slab gap. The presence of cratons may have an important role on subduction episodicity and provide a new mechanism to explain slab gaps in areas where cratons have been located close to trenches, as is the case of South America and the Cenozoic subduction of North America. We further emphasize that the lithospheric structure of the overriding plate should be taken into account in analysis and modelling studies of subduction zones.

  16. The building and stabilization of an Archean Craton in the Superior Province, Canada, from a heat flow perspective

    NASA Astrophysics Data System (ADS)

    Jaupart, C.; Mareschal, J.-C.; Bouquerel, H.; Phaneuf, C.

    2014-12-01

    How continental lithosphere responds to tectonic stresses and mantle convective processes is determined in large part by its mechanical strength and temperature distribution, which depend on crustal heat production. In order to establish reliable crustal and thermal models for the Superior Craton, Canadian Shield, new measurements of heat flux and heat production in 28 deep boreholes at 16 sites are combined with a larger set of older data. The Superior Province was assembled by the docking of volcanic/plutonic and metasedimentary terranes and continental fragments to the southern margin of an older core around 2.7 Ga. The average heat flux is much lower in the craton core than in the accreted terranes, 31 versus 43 mW m-2. The major accreted volcanic/plutonic belts share the same heat production characteristics, testifying to the remarkable uniformity of crust-building mechanisms. The marked difference between the crusts of the core and the accreted belts supports the operation of two different crust-forming processes. The crust of the craton core has an enriched upper layer, in contrast to that of the younger belts which lack marked internal differentiation. At the end of amalgamation, the lithosphere of the craton core was colder and mechanically stronger than the lithosphere beneath newly accreted material. Surrounding the craton core with weaker belts may have ensured its stability against tectonic and mantle convection perturbations. This large strength contrast accounts for the lack of lithospheric imbrication at the edge of the craton core as well as for the different characteristics of seismic anisotropy in the lithospheres of the craton core and the younger terranes.

  17. The East African Orogen: Accretion versus Collision

    NASA Astrophysics Data System (ADS)

    Kröner, A.; Muhongo, S.; Sommer, H.; Vogt, M.

    2003-04-01

    The East African Orogen is an extensive Neoproterozoic (Pan-African) orogenic belt extending from Arabia to Mozambique and containing elements of both accretion and collision tectonics. The predominantly upper crustal northern part (Arabian-Nubian Shield, ANS) consists of Neoproterozoic juvenile arc assemblages that accreted onto the African continent along ophiolite-decorated sutures. In contrast, the tectonic evolution of the predominantly middle to lower crustal southern part (Mozambique belt, MB) is still poorly understood, and simple continental collision models as previously applied are not compatible with new isotopic and petrological data. Published Nd isotopic systematics and our new zircon ages demonstrate that large parts of the high-grade MB in Tanzania consists of late Archaean to Palaeoproterozoic granitoid gneisses which may either constitute an extension of the Tanzania craton to the E, reworked during the Pan-African orogeny, or these rocks may constitute a separate terrane, or terranes, tectonically interdigitated with Neoproterozoic gneisses, similar to the situation in Madagascar. Small-scale tectonic interlayering of >1800 Ma and 650-800 Ma gneisses have been documented at several localities, and the amount of pre-Neoproterozoic crust in the MB appears to be ˜70% or more. Relatively small volumes of ˜1000-1100 Ma granitoids have so far only been recorded in southern Tanzania, and their significance in the orogenic evolution and their relation to more extensive rocks of this age in northern Mozambique are not known. High-grade metamorphism in the MB of Tanzania led to granulite and charnockite formation and occurred at 620-640 Ma, slightly earlier than in Mozambique (˜615 Ma) but significantly earlier than in Malawi (˜550-580 Ma) and in Madagascar (˜550-560 Ma). The significance of these age differences is not understood. Petrological data suggest both clockwise and anti-clockwise PT path for the metamorphic assemblages, suggesting that

  18. Lithospheric structure of the southern African subcontinent from surface wave tomography

    NASA Astrophysics Data System (ADS)

    Raveloson, A.; Nyblade, A.; Mulibo, G.; Mangongolo, A.; Tugume, F.

    2012-12-01

    In this study the lithospheric structure of the southern African subcontinent is examined using a new 3D shear wave velocity model. The lithospheric structure of Africa consists of several Archean cratons and Proterozoic mobile belts. Many intracratonic and rift basin are found within both the Archean and Protozoic terrains. We investigate the 3D shear wave velocity structure of the lithosphere by tomographically modeling Rayleigh wave dispersion curves. Rayleigh waves group velocities from 10 to 125 s period were determined by using multiple-filter and phase-matched techniques. We used seismic events with a magnitude greater than 4.5 and depth shallower than 100 km recorded from 1990 to 2011 on many stations belonging to temporary or permanent networks. We used events and stations within the African plate in an effort to minimize the contributions from outside structure. We constructed the 3D S wave model in two steps. In the first step we measured group velocities of fundamental mode Rayleigh waves. In the second stage we extracted a dispersion curve from the group velocity maps and inverted them to obtain 1D shear wave velocity models. The 1D models are then combined at a regular spatial interval to create a 3D shear wave velocity model. New features revealed in our model include a region of lower wave speeds beneath the cuvette central separating several Archean blocks of the Congo craton and a region of fast lithosphere in northern Mozambique associated with the Ruvuma microplate.

  19. Superplume Under the Siberian Craton: Evidence from Metasomatic Signatures in Kimberlitic Xenoliths

    NASA Astrophysics Data System (ADS)

    Pokhilenko, N.; Taylor, L. A.; Agashev, A. M.; Pokhilenko, L.; Baziotis, I. P.; Carmody, L.; Smith, G. B.

    2012-12-01

    A superplume is proposed to have existed under the Siberian Craton from the mid-Paleozoic to the late Mesozoic. The most dramatic result of this was the eruption of 17 million km3 of the Siberian trap magma at 250 ±2 Ma. However, the effects of this plume were recorded earlier in Devonian (~360 Ma) kimberlitic peridotites, as minor "basaltic" metasomatic overprinting. Indeed, peridotites from later kimberlites (~145 Ma) show evidence for similar, yet extensive, metasomatism, thereby bracketing the major plume climax with possible effects both prior and after this extensive activity. Indeed, a comparative analysis of petrological, mineralogical, and geochemical results from these early and late, Devonian versus Jurassic, peridotites have shown that the thickness of lithosphere of North-Eastern part of Siberian Platform was around 230 km at Middle Paleozoic time and decreased to 140-150 km by Late Mesozoic time. The Devonian Udachnaya kimberlite (central craton), one of the largest diamondiferous pipes in the world, provides a suite of garnet peridotites (>90%), many of which possess a 'sheared' texture. These peridotites possess evidence for a late-stage cryptic metasomatism, particularly in the garnets, exemplified by a notable increase in CaO, with a near constant Cr2O3 content, as the garnet compositions move from the harzburgitic field into that of lherzolites. Conversely, the xenolith suite from the Mesozoic Obnazhennaya kimberlite (north-east craton) is comprised of more pyroxenite xenoliths, with rare peridotite lithologies. The metasomatic over-print on these xenoliths has produced a more fertile chemistry, with the garnets becoming less Cr-rich, following a lherzolitic trend. These two trends in the garnets suggest that a long-lasting metasomatic ultra-event, the same metasomatic processes (and possibly similar sources), operated during both time periods. Changes from harzburgitic in the older Udachnaya peridotite rims to lherzolitic core-rim compositions

  20. Surface-wave Tomography of East African Rift System using Ambient Seismic Noise

    NASA Astrophysics Data System (ADS)

    Kim, S.; Kang, T.; Baag, C.; Nyblade, A. A.

    2008-12-01

    The surface-wave tomography technique for the ambient seismic noise is applied to the east African rift system to investigate shallow crustal structures of the region. Even if the technique has been widely used in many regions to investigate crustal structure in the world, there have been difficulties in application of the technique to the east African region because of unstable data conditions of PASSCAL experiments. A meticulous check of record by record enables us of applying the technique to understand the tectonic environment of the region. The long-period data of one month showing good quality in cross-correlation results are used in this study. They are from the 1994-95 Tanzania Passive-Source Seismic Experiment for the Tanzania craton and its surrounding rift zone, and from the 2000-02 Ethiopia/Kenya Broadband Seismic Experiment and the adjacent permanent stations of the African Array for the Ethiopia rift. The Rayleigh- and Love-wave group-speed maps were inverted using LSQR algorithm for several period bands (5 - 50 s). The preliminary group-speed distribution maps yield results roughly consistent with regional geology. The tomographic images of the Tanzania region show a strong high velocity anomaly at the location corresponding to the Tanzania craton and low velocity anomalies at the surrounding rift regions. For the Ethiopia regions, the features of low velocity anomalies roughly agree with the Tertiary volcanic regions. Combining the Tanzania and Ethiopia broadband arrays, the outline of the east African rift system can be identified as the low velocity anomalies in the surface-wave tomographic results. The structural variation with depth and the feature of the regional shear-wave anisotropy of crust will be explored by converting group- speed dispersion curves into shear-wave velocity structure.

  1. Craton vs. rift uppermost mantle contributions to magnetic anomalies in the United States interior

    NASA Astrophysics Data System (ADS)

    Friedman, S. A.; Feinberg, J. M.; Ferré, E. C.; Demory, F.; Martín-Hernández, F.; Conder, J. A.; Rochette, P.

    2014-06-01

    The interpretation of satellite magnetic information (Magsat, Oersted, CHAMP, Swarm) requires the understanding of the mineralogy of crustal and mantle sources. Also, spectral analysis of magnetic data over forearcs and cratons calls for upper mantle contribution. The prospect of such a contribution contradicts the view that the mantle is too hot and its magnetism is too weak to influence magnetic anomalies. Here we examine the rock magnetic properties of fresh mantle xenoliths from four settings across the United States: phlogopite-spinel dunites from the Bearpaw Mountains, Montana, and lherzolites/harzburgites from San Carlos, Arizona; Kilbourne Hole, New Mexico; and Knippa, Texas. Paleomagnetic results show single-component natural remanent magnetizations (NRMs), which, combined with optical and secondary electron microscopy support the lack of post-eruption alteration and absence of host-rock contamination. The NRM carriers include magnetite at Bearpaw Mountain and San Carlos, and pyrrhotite at Kilbourne Hole and Knippa. These four areas show continental crust of distinct thicknesses and various geotherms. The potential mantle contribution to magnetic anomalies is forward modeled using crustal thickness, current geotherm and average magnetic properties of xenoliths. The San Carlos and Kilbourne Hole mantle, situated near the Rio Grande Rift is too hot and its magnetism is too weak to contribute to anomalies. The sulfide-dominated assemblage at Knippa does not support magnetization at mantle depths. In contrast, the Bearpaw Mountains combine a relatively cold geotherm (craton) and abundance of magnetite formed at mantle depth. This cratonic mantle, metasomatized by fluids from the Farallon plate, may contribute to long wavelength magnetic anomalies.

  2. Local Re-Cratonization of the Wyoming Province and the Uplift of the Black Hills

    NASA Astrophysics Data System (ADS)

    Bezada, M.; Humphreys, E.; Schmandt, B.

    2015-12-01

    The Archean Wyoming Craton is a primary building block of the core of the North American continent. It resisted tectonic deformation for over a billion years, but during the Laramide orogeny significant crustal shortening was accommodated by basement-involved thrusting. Xenoliths suggest that ~50 km of cratonic mantle were removed from the Wyoming Province by basal erosion during this time. This orogenic event resulted in the formation of several mountain ranges in Wyoming which are part of the Rocky Mountains. The eastern limit of the Rocky Mountains in Wyoming is typically defined by the Big Horn and Laramie ranges. The Black Hills represent a topographic anomaly having a similar trend to the Big Horn and the Laramie Mts, but lying ~200 km NE of the Rocky Mountain Front (RMF); while the intervening region is relatively undeformed. Subduction of the conjugate to the Shatsky Rise is the leading hypothesis to explain the flattening of the Farallon slab that lead to the Laramide orogeny. The leading edge of the Shatsky conjugate would have been the conjugate to the Tamu Massif, the largest known single volcano on the planet. The reconstructed path of the Tamu conjugate plausibly places it beneath the region between the RMF and the Black Hills, where a high seismic velocity anomaly is observed by body wave tomography to depths exceeding 200 km. We propose that subcretion of the highly depleted mantle lithosphere of the Tamu cojugate re-cratonized a region of the Wyoming Province forming a distinct rigid block. We further suggest that the new block successfully resisted tectonic deformation and transferred the stresses of the Laramide orogeny northeastward to generate the basement-cored uplift of the Black Hills.

  3. "13 BB star" - broadband seismic array at the edge of East European Craton in Poland

    NASA Astrophysics Data System (ADS)

    Polkowski, Marcin; Grad, Marek; Wilde-Piórko, Monika; Suchcicki, Jerzy; Arant, Tadeusz

    2014-05-01

    "13 BB star" is a dense array of 13 Reftek 151-120 broadband seismometers located in unpopulated forests in northern Poland - just on the edge of East European Craton. The shape of array was designed as a "star" with one central station and 12 station located on two circles (radius of about 30 and 60 km) around it. This geometry allows us to study seismic waves (in particular surface waves) incoming from all azimuths. Our "13 BB star" array stations are self powered (solar panels and batteries), equipped with local storage system and on-line transmission of seismic and auxiliary data using 2G and 3G cellular network. We also developed on-line application for monitoring array status (transmission, connections, power, temperature, etc.) and on-line data visualization. Main advantages of our array are: 1) 120s broadband seismometers recording with 100Hz sampling, 2) density of array - distances between neighboring stations does not exceed 30 km, 3) station placement in underground wells, 4) stations located in forests far (usually 2-3 km) from industries, roads, villages and other human activity. "13 BB star" started operating in July 2013 and since that time recorded several local, regional, and teleseismic events proving good array functionality. The scientific aim of the "13 BB star" project is development of a quantitative model of the lithosphere-asthenosphere-structure in the marginal zone of the East European Craton in northern Poland. New acquired data will be analyzed using integrated seismic methods, which will yield images of lithosphere-asthenosphere system hitherto unknown for this area. The knowledge of detailed structure of the cratonic lithosphere-asthenosphere system is crucial for the better understanding of the regional, as well as global mantle dynamics and evolution of the Earth's interior. This work was supported by NCN-grant DEC 2011/02/A/ST10/00284.

  4. Cretaceous Apparent Polar Wander Relative to the Major Cratons and Displacement Estimates of Baja British Columbia

    NASA Astrophysics Data System (ADS)

    Enkin, R. J.

    2004-12-01

    When paleogeographic interpretations derived from independent observations conflict, the methods and results from each discipline come under careful scrutiny, as illustrated by the Baja British Columbia controversy. Cretaceous paleomagnetic data from a large region of the Canadian Cordillera render paleopoles which are far-sided with respect to cratonic North American poles, suggesting this region, designated Baja British Columbia, translated northward during Late Cretaceous - Paleogene time. Criticism of this interpretation based on other geological reasoning prompted me to perform new reviews of Cretaceous to Eocene paleomagnetic results from the Cordillera and from the major cratons of the globe. The global review follows the method of Besse and Courtillot (1991; 2002). One difference between our methods is that I compiled paleomagnetic results from highly studied rock units to single results to balance data weightings spatially and temporally, thus reducing the number of individual results. For the period 160 to 40 Ma, 51 poles were included compared to 92 poles by Besse and Courtillot (2002). Differences between apparent polar wander paths in their and my analyses are never significant at 95% confidence, however mean pole positions differ by up to 500 km, which is important for paleogeographic analysis. The global distribution of sampling localities and the tight clustering of the paleomagnetic poles after plate reconstruction provide invaluable confirmation of plate tectonically derived Euler rotations, the reliability of paleomagnetic remanence directions, and the geocentric dipole geometry of the geomagnetic field. My Cordilleran review shows that paleolatitudes derived from plutons and remagnetized rocks are significantly more scattered than those derived from bedded rocks. Using bedded rocks only, the paleomagnetic record shows that Baja British Columbia sat 2100 ± 500 km south of its present position with respect to cratonic North America during the

  5. Sediment budget of cratons: insights from West Africa over the Cenozoic

    NASA Astrophysics Data System (ADS)

    Jean-Louis, G.; Chardon, D.; Rouby, D.; Beauvais, A.

    2015-12-01

    The sediment load of rivers constitutes the material that builds the stratigraphic successions found from continental margins to the deep ocean. Measure of this flux is relevant to understanding continental controls on denudation, riverine transport and basin filling. An increasing number of sediment yield measures is available but whether these modern values can be extrapolated at geological timescales for large watersheds is still questioned. One reason is the lack of long-term data. Here, we present a sediment budget for Sub-Saharan West Africa over the Cenozoic to compare with the modern rates. The denudation of this cratonic area is constrained using three regional lateritic paleo-landsurfaces that formed during periods of enhanced weathering since the Paleocene-Eocene greenhouse peak. The 3D interpolation of these surfaces allowed building three successive denudation maps for the 45-24, 24-11 and 11-0 Ma intervals together with reconstructions of the paleo-drainage. The regional distribution of erosion suggests the influence of lithospheric deformation, concentrated around a southern marginal upwarp and eastern hotspot swells. The export of large-scale drainages was calculated by converting denudated volumes into sediment fluxes using the porosity and density of lateritic regolith. Exported volumes calculated for the Niger watershed fall within the same range as the Cenozoic clastic accumulations of the Niger delta. Comparisons also show that modern fluxes can be an order of magnitude above the long-term fluxes for moderately large watersheds but that modern and long-term yields are similar for the largest watersheds (e.g. Niger, Volta, Senegal). These results suggest that the export of very large cratonic watersheds is independent of the measurement timescale and that their modern yields can be extrapolated at long-timescale. Finally, it allows assessing the relative contribution of cratons, i.e. non-active orogenic areas, to the global sediment budgets at

  6. Cambrian-Ordovician craton margin section, southern Great Basin: A sequence stratigraphic perspective

    SciTech Connect

    Cooper, J.D. ); Edwards, J.C. )

    1991-02-01

    The Upper Cambrian through Upper Ordovician cratonal to miogeoclinal transition section in southern Nevada and eastern California accumulated on a gentle homoclinal ramp and includes a variety of peritidal and subtidal carbonate facies. This section consists of a westward-thickening wedge assigned to the Nopah and Mountain Springs formations and can be related to one type 1 and least four type 2 sequences. The basal part of the section is the Dunderberg Shale Member of the Nopah Formation, which is composed of meter-scale cyclical shale-carbonate bundles. Carbonate interbeds signify a variety of peritidal to deep subtidal paleoenvironments that comprise shelf margin and transgressive systems tracts. The progradation of a thrombolitic bank complex in the overlying upper Nopah may reflect emergence on the craton, evidenced by the Sauk II-Sauk III cratonic disconformity. The overlying A' member of the Mountain Springs Formation rests in sharp and, in places, slightly discordant contact with the top of the Nopah. This contact is interpreted as a marine flooding surface and type 2 sequence boundary. Member A consists of Lower Ordovician transgressive and highstand systems tracts comprised of oolitic shoal and back-shoal to intertidal carbonates. The type 1 unconformity that separates member A from overlying B' member is the top of the first-order Sauk Sequence. The B' member is a thin stratal interval of late Middle to Late Ordovician age and consists of dark, burrow-mottled skeletal wackestone and mudstone that is part of a transgressive systems tract. It is bounded above by another type 1 unconformity and therefore represents the first-order Tippecanoe sequence of Sloss.

  7. Bouguer images of the North American craton and its structural evolution

    NASA Technical Reports Server (NTRS)

    Arvidson, R. E.; Bowring, S.; Eddy, M.; Guinness, E.; Leff, C.; Bindschadler, D.

    1984-01-01

    Digital image processing techniques have been used to generate Bouguer images of the North American craton that diplay more of the granularity inherent in the data as compared with existing contour maps. A dominant NW-SE linear trend of highs and lows can be seen extending from South Dakota, through Nebraska, and into Missouri. The structural trend cuts across the major Precambrian boundary in Missouri, separating younger granites and rhyolites from older sheared granites and gneisses. This trend is probably related to features created during an early and perhaps initial episode of crustal assembly by collisional processes. The younger granitic materials are probably a thin cover over an older crust.

  8. Unusual lithospheric structure and evolutionary pattern of the cratonic segments of the South Indian shield

    NASA Astrophysics Data System (ADS)

    Agrawal, P. K.; Pandey, O. P.

    2004-02-01

    The southern Indian shield, characterised by several prominent geological and geophysical features, can be divided into three distinct tectonic segments: Western Dharwar craton (WDC), Eastern Dharwar craton (EDC) and Southern Granulite terrain (SGT). With the exception of WDC, the entire crust beneath EDC and SGT has been remobilized several times since their formation during the mid- to late Archeans (3.0-2.5 Ga). In order to understand the evolutionary history of these segments, a multiparametric geological and geophysical study has been made which indicates that the south Indian shield, characterized by a reduced heat flow of 23-38 mW/m2 has a much thinner (88-163 km) lithosphere compared to ˜200-450 km found in other global shields. In the EDC-SGT terrain, high velocity upper crust is underlain by considerably low mantle velocity with a thick high conductive/low velocity zone sandwiched at mid crustal level. Our study reveals that the entire EDC region is underlain by granulite facies rocks with a density of about 2.85 to 3.16 g/cm3 at a shallow depth of about 8 km in the southern part and at even shallower depth of about 1 to 2 km below the Hyderabad granitic region in the north. Cratonic mantle lithosphere beneath EDC may contain a highly conductive, anisotropic and hydrous metasomatic zone between the depth of 90 and 105 km where estimated temperatures are in the range of 850-975°C. It is likely that before the early Proterozoic, the entire south Indian shield was a coherent crustal block which subsequently got segmented due to persistent plume-induced episodic thermal reactivations during the last 2.7 Ga. These reactivations led to self destruction of cratonic roots giving rise to negative buoyancy at deeper levels which may have been responsible for crustal remobilisations, followed by regional uplifting and erosion of once substantially thick greenstone belts. Consequently, the crustal column beneath the EDC has become highly evolved and now corresponds

  9. The ambient noise and earthquake surface wave tomography of the North China Craton

    NASA Astrophysics Data System (ADS)

    Pan, J.; Obrebski, M. J.; Wu, Q.; Li, Y.

    2010-12-01

    The North China Craton (NCC) is unique for its unusual Phanerozoic tectonic activity. The NCC was internally tectonically stable until Jurassic when its southern margin collided with the Yangzte Craton. Subsequently, the eastern and central part of the NCC underwent distinctive evolutions during the Late Mesozoic and Cenozoic. In contrast to the Erdos block located in the western part of NCC and that seems to have preserved the typical features of a stable craton, the eastern NCC has experienced significant lithospheric thinning as evidenced by widespread magmatism activity and normal faulting, among other manifestations. The eastern part of the NCC is also one of the most seismically active intracontinental regions in the world. Here we focus on the region that comprises the North China Basin and the Yanshan-Taihang Mountains, two major tectonic units located to the east and in the center of the NCC, respectively. We combine ambient noise data and ballistic surface wave data recorded by the dense temporary seismic array deployed in the North China to obtain phase velocity maps at periods ranging from 5s to 60s. 1587 and 3667 ray paths were obtained from earthquake surface waves and ambient noise correlations, respectively. The phase velocity distribution was reconstructed with grid size 0.25x0.25 degrees and 0.5x0.5 degrees from ambient noise tomography and earthquake surface wave tomography. For periods shorter than 15s, the phase velocity variations are well correlated with the principal geological units in the NCC, with low-speed anomalies corresponding to the major sedimentary basins and high-speed anomalies coinciding with the main mountain ranges. Within the period range from 20s to 30s, the phase velocity variations seem to be related to the local variations of the crustal thickness. For the periods above 30s, the strength of the phase velocity variations decreases with increasing periods, which may imply that the uppermost mantle is much more homogeneous

  10. Relative sea-level changes during Middle Ordovician through Mississippian deposition in the Iowa area, North American craton

    USGS Publications Warehouse

    Witzke, B.J.; Bunker, B.J.; ,

    1996-01-01

    The Tippecanoe and Kaskaskia cratonic megasequences in the Iowa area are subdivided into a succession of third-order transgressive-regressive (T-R) depositional cycles of ~1 to 3 m.y. duration. Cratonic deposition in the Iowa area is categorized into two broad-scale facies groupings, each dominated by shallowing-upward patterns: 1) an inner shelf with shallow subtidal to peritidal facies, and 2) a middle shelf dominated by subtidal facies. Relative changes in sea level, as documented in the cycles, are considered primarily to reflect eustatic patterns, but local variations in subsidence history and sedimentation rates complicate interpretations of the eustatic signal. Minor modification of certain sequence stratigraphic paradigms that were developed originally for continental margin settings is suggested for application to the Iowa cratonic cycles.

  11. Complex evolution of the lower crust beneath the southeastern North China Craton: The Junan xenoliths and xenocrysts: Comment

    NASA Astrophysics Data System (ADS)

    Yuan, Ya-Juan; Xia, Bin

    2015-10-01

    The Junan granulite xenoliths and xenocrysts have a distinct overlap in U-Pb ages and Lu-Hf isotopic compositions with the Precambrian basement of the Yangtze craton. We thus believe that the Junan granulite cannot be derived from the North China lower crust. Moreover, broad deep seismic and magnetotelluric probing across the Sulu UHP terrane indicates the former is well coupled with the lower crust of the Yangtze craton. Therefore, we suggest that the Junan granulites are most likely to have been derived from the Yangtze Precambrian basement and their zircon U-Pb-Lu-Hf isotope systematics are more likely to indicate the complex evolution of the Precambrian lower crust along the northern margin of the Yangtze craton.

  12. The Río de la Plata Craton: a review of units, boundaries, ages and isotopic signature

    NASA Astrophysics Data System (ADS)

    Oyhantçabal, Pedro; Siegesmund, Siegfried; Wemmer, Klaus

    2011-04-01

    A review of the lithostratigraphic units in the Río de la Plata Craton and of new and previously published geochronological, isotopic and geophysical data is presented. Sm-Nd TDM model ages between 2.6 and 2.2 Ga characterize the Piedra Alta Terrane of this craton. Crystallization ages between 2.2 and 2.1 Ga for the metamorphic protoliths and 2.1-2.0 Ga for the post-orogenic granitoids indicate juvenile crust, followed by a short period of crustal recycling. Cratonization of this terrane occurred during the late Paleoproterozoic. Younger overprinting is not observed, suggesting it had a thick and strong lithosphere in the Neoproterozoic. A similar scenario is indicated for the Tandilia Belt of Argentina. Sm-Nd TDM model ages for the Nico Pérez Terrane show two main events of crustal growth (3.0-2.6 and 2.3-1.6 Ga). The crystallization ages on zircon ranges between 3.1 and 0.57 Ga, which is evidence for long-lived crustal reworking. The age for cratonization is still uncertain. In the Taquarembó Block, which is considered the prolongation of the Nico Pérez Terrane in southern Brazil, a similar scenario can be observed. These differences together with contrasting geophysical signatures support the redefinition of the Río de la Plata Craton comprising only the Piedra Alta Terrane and the Tandilia Belt. The Sarandí del Yí Shear Zone is regarded as the eastern margin of this Craton.

  13. The African superswell

    NASA Technical Reports Server (NTRS)

    Nyblade, Andrew A.; Robinson, Scott W.

    1994-01-01

    Maps of residual bathymetry in the ocean basins around the African continent reveal a broad bathymetric swell in the southeastern Atlantic Ocean with an amplitude of about 500 m. We propose that this region of anomalously shallow bathymetry, together with the contiguous eastern and southern African plateaus, form a superswell which we refer to as the African superswell. The origin of the African superswell is uncertain. However, rifting and volcanism in eastern Africa, as well as heat flow measurements in southern Africa and the southeastern Atlantic Ocean, suggest that the superswell may be attributed, at least in part, to heating of the lithosphere.

  14. Zircon from East Antarctica: evidence for Archean intracrustal recycling in the Kaapvaal-Grunehogna Craton from O and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Marschall, H. R.; Hawkesworth, C. J.; Storey, C.; Leat, P. T.; Dhuime, B.

    2010-12-01

    The Grunehogna Craton (GC, East Antarctica) is interpreted as part of the Archean Kaapvaal Craton of southern Africa prior to Gondwana breakup. The basement of the GC is only exposed within a small area comprising the dominantly leucocratic Annandagstoppane (ADT) granite. The granite (and hence the craton) has been dated previously only by Rb-Sr and Pb-Pb mica and whole-rock methods. Here, the crystallisation age of the granite was determined to 3,067 ± 8 Ma by U-Pb dating of zircon. This age is coeval with granitoids and volcanics in the Swaziland and Witwatersrand blocks of the Kaapvaal Craton. Inherited grains in the ADT granite were discovered with ages of up to 3,433 ±7 Ma, and are the first evidence of Palaeoarchean basement in Dronning-Maud Land. The age spectrum of the inherited grains reflects well-known tectono-magmatic events in the Kaapvaal Craton and form important pieces of evidence for the connection of the GC to the Kaapvaal Craton for at least three billion years and probably longer. Whole-rock chemistry and zircon O isotopes demonstrate a supracrustal sedimentary source for the granite, and Hf model ages show that at least two or three different crustal sources were contributing to the magma with model ages of ~3.50, ~3.75 and possibly ~3.90 Ga, respectively. 3.1 Ga granites covering ~60 % of the outcrop area of the Kaapvaal-Grunehogna Craton played a major role in the mechanical stabilisation of the continental crust during the establishment of the craton in the Mesoarchean. Combined zircon Hf-O isotope data and the lack of juvenile additions to the crust in the Mesoarchean strongly suggest that crustal melting and granite formation was caused by the deep burial of clastic sediments and subsequent incubational heating of the crust. Intracrustal recycling of this type may be an important process during cratonisation and the long-term stabilisation of continental crust.

  15. The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana

    NASA Astrophysics Data System (ADS)

    Hofmann, Mandy; Linnemann, Ulf; Hoffmann, Karl-Heinz; Gerdes, Axel; Eckelmann, Katja; Gärtner, Andreas

    2014-07-01

    This paper presents combined U/Pb, Th/U and Hf isotope analyses on detrital and magmatic zircon grains together with whole-rock geochemical analyses of two basement and eight sedimentary rock samples from the Namuskluft and the Dreigratberg in southern Namibia (Gariep Belt). The sedimentary sections evolved during the Cryogenian on the SW part of the Kalahari Craton and where therefore deposited in an active rift setting during the break-up of Rodinia. Due to insufficient palaeomagnetic data, the position of the Kalahari Craton within Rodinia is still under discussion. There are possibilities to locate Kalahari along the western side of Australia/Mawsonland (Pisarevski et al. in Proterozoic East Gondwana: supercontinent assembly and break-up, Geological Society, London, 2003; Evans in Ancient Orogens and modern analogues. Geological Society, London, 2009; and others) or together with the Congo-Sao Francisco and Rio de la Plata Cratons (Li et al. in Prec Res 45: 203-2014, 2008; Frimmel et al. in Int J Earth Sci (Geol Rundsch) 100: 323-354, 2011; and others). It is sill unclear which craton rifted away from the Kalahari Craton during the Cryogenian. Although Middle to Upper Cryogenian magmatic activity is known for the SE Kalahari Craton (our working area) (Richtersveld Suite, Rosh Pinah Fm), all the presented samples show no U/Pb zircon ages younger than ca. 1.0 Ga and non-older than 2.06 Ga. The obtained U/Pb ages fit very well to the exposed basement of the Kalahari Craton (1.0-1.4 Ga Namaqua Province, 1.7-2.0 Ga Vioolsdrif Granite Suite and Orange River Group) and allow no correlation with a foreign craton such as the Rio de la Plata or Australia/Mawsonland. Lu-Hf isotopic signatures of detrital zircon point to the recycling of mainly Palaeoproterozoic and to a smaller amount of Archean crust in the source areas. ɛHf( t) signatures range between -24 and +14.8, which relate to TDM model ages between 1.05 and 3.1 Ga. Only few detrital zircon grains derived from

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

    PubMed

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

    2003-06-19

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

  17. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton.

    PubMed

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan

    2014-01-01

    Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ(7)Li in Archean lithospheric mantle. The δ(7)Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from -27 to +21, in marked deviation from the δ(7)Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1-2 ppm). The Li abundances and δ(7)Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ(7)Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust. PMID:24589693

  18. The character of the Moho and lower crust within Archean cratons and the tectonic implications

    NASA Astrophysics Data System (ADS)

    Abbott, Dallas H.; Mooney, Walter D.; VanTongeren, Jill A.

    2013-12-01

    Undisturbed mid Archean crust (stabilized by 3.0-2.9 Ga) has several characteristics that distinguish it from post Archean crust. Undisturbed mid-Archean crust has a low proportion of internal seismic boundaries (as evidenced by converted phases in seismic receiver functions), lacks high seismic velocities in the lower crust and has a sharp, flat Moho. Most of the seismic data on mid-Archean crust comes from the undisturbed portions of the Kaapvaal and Zimbabwe (Tokwe segment) cratons. Around 67-74% of younger Archean crust (stabilized by 2.8-2.5 Ga) has a sharp, flat Moho. Much of the crust with a sharp, flat Moho also lacks strong internal seismic boundaries, but there is not a one to one correspondence. In cases where its age is known, basaltic lower crust in Archean terranes is often but not always the result of post Archean underplating. Undisturbed mid-Archean cratons are also characterized by lower crustal thicknesses (Archean median range = 32-39 km vs. post-Archean average = 41 km) and lower crustal seismic velocities. These observations are shown to be distinct from those observed in any modern-day tectonic environment. The data presented here are most consistent with a model in which Archean crust undergoes delamination of dense lithologies at the garnet-in isograd resulting in a flat, sharp Moho reflector and a thinner and more felsic-intermediate crust. We discuss the implications of this model for several outstanding paradoxes of Archean geology.

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

    PubMed

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

    2003-06-19

    Komatiites are ultramafic volcanic rocks containing more than 18 per cent MgO (ref. 1) that erupted mainly in the Archaean era (more than 2.5 gigayears ago). Although such compositions occur in later periods of Earth history (for example, the Cretaceous komatiites of Gorgona Island), the more recent examples tend to have lower MgO content than their Archaean equivalents. Komatiites are also characterized by their low incompatible-element content, which is most consistent with their generation by high degrees of partial melting (30-50 per cent). Current models for komatiite genesis include the melting of rock at great depth in plumes of hot, diapirically rising mantle or the melting of relatively shallow mantle rocks at less extreme, but still high, temperatures caused by fluxing with water. Here we report a suite of ultramafic lava flows from the Commondale greenstone belt, in the southern part of the Kaapvaal Craton, which represents a previously unrecognized type of komatiite with exceptionally high forsterite content of its igneous olivines, low TiO(2)/Al(2)O(3) ratio, high silica content, extreme depletion in rare-earth elements and low Re/Os ratio. We suggest a model for their formation in which a garnet-enriched residue left by earlier cratonic volcanism was melted by hydration from a subducting slab. PMID:12815428

  20. The Late Triassic and Late Jurassic stress fields and tectonic transmission of North China craton

    NASA Astrophysics Data System (ADS)

    Hou, Guiting; Wang, Yanxin; Hari, K. R.

    2010-09-01

    The transmission of the tectonic regime from the Paleo-Asian Ocean to the Pacific Ocean during Mesozoic era was reconstructed using the modeling of Late Triassic (T 3) and Late Jurassic (J 3) stress fields employing two dimensional linear finite element models (2-D FEM). The model at T 3 proposes that Qinling-Dabie-Sulu orogens coevally collided and the model J 3 proposes that Subei block continued to collide with the North China block along the Sulu orogen while the collision of the Qinling-Dabie orogen was terminated. The stress fields at T 3 and J 3 during the two episodes were calculated based on mechanical conditions under different deviatoric stresses acting along the boundaries of the North China craton by elastic finite modeling. The transmission between two episodes of stress fields resulted from Qinling-Dabie-Sulu collision between North China and South China in the Late Triassic period, and from continued collision between the Subei block and North China by the NW-trending movement of Izanagi plate during Late Jurassic. The results from modeling of the Mesozoic stress fields of the North China suggest that late Jurassic was the key transmission period of the tectonic regime of the North China block when large scale thrusting triggered the subsequent destruction of the North China craton.

  1. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton

    PubMed Central

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M.; Xiao, Yan

    2014-01-01

    Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ7Li in Archean lithospheric mantle. The δ7Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from −27 to +21, in marked deviation from the δ7Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1–2 ppm). The Li abundances and δ7Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ7Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ7Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust. PMID:24589693

  2. Abnormal lithium isotope composition from the ancient lithospheric mantle beneath the North China Craton.

    PubMed

    Tang, Yan-Jie; Zhang, Hong-Fu; Deloule, Etienne; Su, Ben-Xun; Ying, Ji-Feng; Santosh, M; Xiao, Yan

    2014-03-04

    Lithium elemental and isotopic compositions of olivines in peridotite xenoliths from Hebi in the North China Craton provide direct evidence for the highly variable δ(7)Li in Archean lithospheric mantle. The δ(7)Li in the cores of olivines from the Hebi high-Mg# peridotites (Fo > 91) show extreme variation from -27 to +21, in marked deviation from the δ(7)Li range of fresh MORB (+1.6 to +5.6) although the Li abundances of the olivines are within the range of normal mantle (1-2 ppm). The Li abundances and δ(7)Li characteristics of the Hebi olivines could not have been produced by recent diffusive-driven isotopic fractionation of Li and therefore the δ(7)Li in the cores of these olivines record the isotopic signature of the subcontinental lithospheric mantle. Our data demonstrate that abnormal δ(7)Li may be preserved in the ancient lithospheric mantle as observed in our study from the central North China Craton, which suggest that the subcontinental lithospheric mantle has experienced modification of fluid/melt derived from recycled oceanic crust.

  3. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution

    NASA Astrophysics Data System (ADS)

    Fritz, H.; Abdelsalam, M.; Ali, K. A.; Bingen, B.; Collins, A. S.; Fowler, A. R.; Ghebreab, W.; Hauzenberger, C. A.; Johnson, P. R.; Kusky, T. M.; Macey, P.; Muhongo, S.; Stern, R. J.; Viola, G.

    2013-10-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world´s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara-Congo-Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian-Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite-Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650-620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo-Tanzania-Bangweulu Cratons and the Zimbabwe-Kalahari Craton. They closed during the ∼600-500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600-550 Ma extension is recorded in the Arabian-Nubian Shield and the Eastern Granulite-Cabo Delgado Nappe Complex. Later ∼550-480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings resulted in the evolution of

  4. Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution☆

    PubMed Central

    Fritz, H.; Abdelsalam, M.; Ali, K.A.; Bingen, B.; Collins, A.S.; Fowler, A.R.; Ghebreab, W.; Hauzenberger, C.A.; Johnson, P.R.; Kusky, T.M.; Macey, P.; Muhongo, S.; Stern, R.J.; Viola, G.

    2013-01-01

    The East African Orogen, extending from southern Israel, Sinai and Jordan in the north to Mozambique and Madagascar in the south, is the world́s largest Neoproterozoic to Cambrian orogenic complex. It comprises a collage of individual oceanic domains and continental fragments between the Archean Sahara–Congo–Kalahari Cratons in the west and Neoproterozoic India in the east. Orogen consolidation was achieved during distinct phases of orogeny between ∼850 and 550 Ma. The northern part of the orogen, the Arabian–Nubian Shield, is predominantly juvenile Neoproterozoic crust that formed in and adjacent to the Mozambique Ocean. The ocean closed during a protracted period of island-arc and microcontinent accretion between ∼850 and 620 Ma. To the south of the Arabian Nubian Shield, the Eastern Granulite–Cabo Delgado Nappe Complex of southern Kenya, Tanzania and Mozambique was an extended crust that formed adjacent to theMozambique Ocean and experienced a ∼650–620 Ma granulite-facies metamorphism. Completion of the nappe assembly around 620 Ma is defined as the East African Orogeny and was related to closure of the Mozambique Ocean. Oceans persisted after 620 Ma between East Antarctica, India, southern parts of the Congo–Tanzania–Bangweulu Cratons and the Zimbabwe–Kalahari Craton. They closed during the ∼600–500 Ma Kuungan or Malagasy Orogeny, a tectonothermal event that affected large portions of southern Tanzania, Zambia, Malawi, Mozambique, Madagascar and Antarctica. The East African and Kuungan Orogenies were followed by phases of post-orogenic extension. Early ∼600–550 Ma extension is recorded in the Arabian–Nubian Shield and the Eastern Granulite–Cabo Delgado Nappe Complex. Later ∼550–480 Ma extension affected Mozambique and southern Madagascar. Both extension phases, although diachronous,are interpreted as the result of lithospheric delamination. Along the strike of the East African Orogen, different geodynamic settings

  5. 16 Extraordinary African Americans.

    ERIC Educational Resources Information Center

    Lobb, Nancy

    This collection for children tells the stories of 16 African Americans who helped make America what it is today. African Americans can take pride in the heritage of these contributors to society. Biographies are given for the following: (1) Sojourner Truth, preacher and abolitionist; (2) Frederick Douglass, abolitionist; (3) Harriet Tubman, leader…

  6. African Studies Computer Resources.

    ERIC Educational Resources Information Center

    Kuntz, Patricia S.

    African studies computer resources that are readily available in the United States with linkages to Africa are described, highlighting those most directly corresponding to African content. Africanists can use the following four fundamental computer systems: (1) Internet/Bitnet; (2) Fidonet; (3) Usenet; and (4) dial-up bulletin board services. The…

  7. Understanding African American Males

    ERIC Educational Resources Information Center

    Bell, Edward Earl

    2010-01-01

    The purpose of this study was to assess the socialization skills, self-esteem, and academic readiness of African American males in a school environment. Discussions with students and the School Perceptions Questionnaire provided data for this investigation. The intended targets for this investigation were African American students; however, there…

  8. Africans Away from Home.

    ERIC Educational Resources Information Center

    Clarke, John Henrik

    Africans who were brought across the Atlantic as slaves never fully adjusted to slavery or accepted its inevitability. Resistance began on board the slave ships, where many jumped overboard or committed suicide. African slaves in South America led the first revolts against tyranny in the New World. The first slave revolt in the Caribbean occurred…

  9. Keeping African Masks Real

    ERIC Educational Resources Information Center

    Waddington, Susan

    2012-01-01

    Art is a good place to learn about our multicultural planet, and African masks are prized throughout the world as powerfully expressive artistic images. Unfortunately, multicultural education, especially for young children, can perpetuate stereotypes. Masks taken out of context lose their meaning and the term "African masks" suggests that there is…

  10. Educating African American Males

    ERIC Educational Resources Information Center

    Bell, Edward E.

    2010-01-01

    Background: Schools across America spend money, invest in programs, and sponsor workshops, offer teacher incentives, raise accountability standards, and even evoke the name of Obama in efforts to raise the academic achievement of African American males. Incarceration and college retention rates point to a dismal plight for many African American…

  11. A Pan-African thermal event in southern India

    NASA Astrophysics Data System (ADS)

    Miller, Jonathan S.; Santosh, M.; Pressley, Rachel A.; Clements, Alina S.; Rogers, John J. W.

    UPb zircon data from five igneous suites confirm previous studies that demonstrated widespread Pan-African magmatism in the Granulite Terrain of southern India. Ages determined here are ˜560 Ma for the Peralimala Granite and ˜555 Ma for the Kalpatta Granite, both north of the Palghat-Cauvery lineament, and ˜585 Ma for a charnockite in the Cardamom massif south of the lineament. Zircon from a pegmatite in the Kerala khondalite belt at Melankode yields an age of 512 Ma. Resetting of zircons in the 2500-Ma Arsikere Granite of the western Dharwar craton probably occurred at ˜450 Ma. These ages and the concentration of Pan-African granitic magmatism around the Indian portion of a broad region of granulite-facies metamorphism in East Gondwana demostrates generation of a restricted area of high temperature either above a rising plume or a zone of rifting. Mantle-derived fluids continued to move upward through the crust of southern India for at least 100 m.y. after the peak of magmatism, and the entire region was still cooling at 400 Ma.

  12. African horse sickness and African carnivores.

    PubMed

    Alexander, K A; Kat, P W; House, J; House, C; O'Brien, S J; Laurenson, M K; McNutt, J W; Osburn, B I

    1995-11-01

    African horse sickness (AHS) is a disease that affects equids, and is principally transmitted by Culicoides spp. that are biological vectors of AHS viruses (AHSV). The repeated spread of AHSV from sub-Saharan Africa to the Middle East, northern Africa and the Iberian peninsula indicate that a better understanding of AHS epizootiology is needed. African horse sickness has long been known to infect and cause mortality among domestic dogs that ingest virus contaminated meat, but it is uncertain what role carnivores play in transmission of the virus. We present evidence of widespread natural AHS infection among a diversity of African carnivore species. We hypothesize that such infection resulted from ingestion of meat and organs from AHS-infected prey species. The effect of AHS on the carnivores is unknown, as is their role in the maintenance cycle of the disease.

  13. Diabetes in African Americans

    PubMed Central

    Marshall, M

    2005-01-01

    African Americans have a high risk for type 2 diabetes. Genetic traits, the prevalence of obesity, and insulin resistance all contribute to the risk of diabetes in the African American community. African Americans have a high rate of diabetic complications, because of poor glycaemic control and racial disparities in health care in the USA. African Americans with diabetes may have an atypical presentation that simulates type 1 diabetes, but then their subsequent clinical course is typical of type 2 diabetes. Culturally sensitive strategies, structured disease management protocols, and the assistance of nurses, diabetic educators, and other health care professionals are effective in improving the outcome of diabetes in the African American community. PMID:16344294

  14. African bees to control African elephants

    NASA Astrophysics Data System (ADS)

    Vollrath, Fritz; Douglas-Hamilton, Iain

    2002-11-01

    Numbers of elephants have declined in Africa and Asia over the past 30 years while numbers of humans have increased, both substantially. Friction between these two keystone species is reaching levels which are worryingly high from an ecological as well as a political viewpoint. Ways and means must be found to keep the two apart, at least in areas sensitive to each species' survival. The aggressive African bee might be one such method. Here we demonstrate that African bees deter elephants from damaging the vegetation and trees which house their hives. We argue that bees can be employed profitably to protect not only selected trees, but also selected areas, from elephant damage.

  15. A Sm-Nd and Pb isotope study of Archaean greenstone belts in the southern Kaapvaal Craton, South Africa

    NASA Technical Reports Server (NTRS)

    Wilson, A. H.; Carlson, R. W.

    1989-01-01

    An Sm-Nd and Pb study on a wide variety of lithologies in Archaean greenstone belt fragments in the southern Kaapvaal Craton reveals a complex petrogenetic history. The fragments are important because they represent a 350 km transect through the craton south of Barberton to its southern margin. The Commondale greenstone belt yields a precise Sm-Nd age of 3334 + or - 18 Ma on an exceptionally well preserved peridotite suite of komatiitic affinity. The wide range of Sm/Nd from 0.6 to 1.0 is attributed to the unusual occurrence of orthopyroxene in the spinifex-bearing rocks. A considerably younger age of about 3.2 Ga is suggested for the Nondweni greenstone belt close to the southern margin of the craton on the basis of separate Sm-Nd isochrons on individual lithologies ranging from komatiite, through komatiitic basalt and basalt to felsic volcanic rocks. On the basis of the present study the greenstone belts appear to have been emplaced at progressively younger ages toward the southern margin of the craton.

  16. Petrology and U-Pb geochronology of lower crustal xenoliths and the development of a craton, Slave Province, Canada

    NASA Astrophysics Data System (ADS)

    Davis, W. J.; Canil, D.; MacKenzie, J. M.; Carbno, G. B.

    2003-12-01

    Lower crustal xenoliths recovered from Eocene to Cambrian kimberlites in the central and southern Slave craton are dominated by mafic granulites (garnet, clinopyroxene, plagioclase±orthopyroxene), with subordinate metatonalite and peraluminous felsic granulites. Geothermobarometry indicates metamorphic conditions of 650-800 °C at pressures of 0.9-1.1 GPa. The metamorphic conditions are consistent with temperatures expected for the lower crust of high-temperature low-pressure (HT-LP) metamorphic belts characteristic of Neoarchean metamorphism in the Slave craton. U-Pb geochronology of zircon, rutile and titanite demonstrate a complex history in the lower crust. Mesoarchean protoliths occur beneath the central Slave supporting models of an east-dipping boundary between Mesoarchean crust in the western and Neoarchean crust in the eastern Slave. At least, two episodes of igneous and metamorphic zircon growth occurred in the interval 2.64-2.58 Ga that correlate with the age of plutonism and metamorphism in the upper crust, indicating magmatic addition to the lower crust and metamorphic reworking during this period. In addition, discrete periods of younger zircon growth at ca. 2.56-2.55 and 2.51 Ga occurred 20-70 my after the cessation of ca. 2.60-2.58 Ga regional HT-LP metamorphism and granitic magmatism in the upper crust. This pattern of younger metamorphic events in the deep crust is characteristic of the Slave as well as other Archean cratons (e.g., Superior). The high temperature of the lower crust immediately following amalgamation of the craton, coupled with evidence for continued metamorphic zircon growth for >70 my after 'stabilization' of the upper crust, is difficult to reconcile with a thick (200 km), cool lithospheric mantle root beneath the craton prior to this event. We suggest that thick tectosphere developed synchronously or after these events, most likely by imbrication of mantle beneath the craton at or after ca. 2.6 Ga. The minimum age for

  17. Moho depth and crustal structure of the Siberian Craton and the West Siberian Basin: An appraisal of existing seismic data

    NASA Astrophysics Data System (ADS)

    Cherepanova, Yulia; Artemieva, Irina M.; Thybo, Hans

    2013-04-01

    We present a digital model of the crustal structure of the Archean-Proterozoic Siberian craton and the Paleozoic-Mesozoic West Siberian basin, based on seismic profiles published since 1960 in international and Russian journals, books, theses and reports. Data quality was assessed and quantitatively assigned to each profile based on acquisition and interpretation method and completeness of the model. The database represents major improvement in coverage and resolution with a nominal sample interval of 50 km before interpolation onto a uniform grid. It includes depth to Moho, thickness and average P-wave velocity of five crustal layers (sediments, and upper, middle, lower, and lowermost crust) and Pn velocity. Results are presented in maps and cross-sections, which demonstrate strong crustal heterogeneity. Crustal structure shows weak correlation with tectono-thermal age and strong correlation with tectonic setting. Sedimentary thickness varies from 0-3 km in stable craton to 10-20 km in extended regions. Typical Moho depths are 44-46 km in stable Archean crust, 40-42 km in Proterozoic craton and Neoproterozoic/Paleozoic orogens, 35-38 km in extended cratonic crust, and 38-40 km in the West Siberian basin. Average crustal velocity is ~6.2-6.4 km/s, ranging from <5.8 km/s in deep sedimentary basins to ~6.6 km/s around the up-to 54 km thick Anabar shield crust. The cratonic crust generally consists of three layers and has no high-velocity lowermost crust (Vp~7.4 km/s), which is observed only in magmatic areas. Upper mantle Pn velocities are generally ~8.2 km/s in the craton and West Siberian Basin, lower in Baikalian and Caledonian areas, higher in the Tunguska and Viluy basins, and abnormally high (8.6-8.9 km/s) around kimberlite fields. We provide an extensive summary of the tectonic and geodynamic evolution of the region and discuss the origin of crustal heterogeneity and processes of crustal evolution in Precambrian cratons and major Phanerozoic basins and rift

  18. Influence of pH and sodium chloride on selected functional and physical properties of African breadfruit (Treculia africana Decne) kernel flour.

    PubMed

    Badifu, G I; Akubor, P I

    2001-01-01

    African breadfruit seeds were cleaned, parboiled (98 degrees C) for 15 min, drained and dehulled. The kernels were sun-dried, then milled and sieved into flour. Nitrogen solubility (NS), water absorption capacity (WAC), emulsion activity (EA), viscosity, foaming and emulsifying properties of the flour were determined as functions of pH and NaCl concentration. The NS was pH dependent with a minimum at pH 4 and maximum at pH 10. The flour also exhibited minimum and maximum foam capacity at pH 4 and 10, respectively. The minimum emulsion activity was at pH 4, a value which increased from 7-16% at pH 12. The addition of NaCl at concentrations of 0.2 to 0.4 M improved WAC, NS, foaming and emulsion properties of the flour. Sodium chloride enhanced NS of the flour at pH 4-6 and EA at pH 2-8. The EA at pH 2, 10 and 12 correlated negatively (r = -0.30) with NaCl concentration levels; however, the correlation was not significant (p > 0.05). The flour dispersions had lower viscosities at acid pH and in the presence of NaCl than at neutral and alkaline pH values. Results indicated the flour could be used in food product supplementation.

  19. The record of the Amazon craton in two supercontinents: Paleomagnetic and geological constraints for Mesoproterozoic to Paleozoic times

    NASA Astrophysics Data System (ADS)

    Tohver, E.

    2008-05-01

    The Amazon craton plays a fundamental role in the evolution of two supercontinents, the late Mesoproterozoic Rodinia, the break-up of which led to thethe late Neoproterozoic-early Paleozoic formation of Gondwana. A recent review of the paleomagnetic database for South America and Africa highlights the different role of the principal elements of western Gondwana elements Amazonia, conjoined with West Africa, versus the more centrally- located pieces of Gondwana; -Congo-São Francisco, Kalahari, the Rio de Plata, and the accreted terranes of the Arabia-Nubia shield. Whereas the Amazon-West Africa conjoined craton appears to have been alongside Laurentia within the framework of Rodinia, the other "central Gondwana" cratons were not part of Rodinia. New paleomagnetic data from the SW Amazon craton demonstrate the transpressive evolution of the Grenvillian collision, which resulted in thousands of kilometers of along-strike between the Amazon and Laurentia cratons. Portions of Amazonian crust stranded within the North American craton, notably the Blue Ridge province of the southern Appalachians, is evidence for this long-lived motion. An extensive review of recent thermochronological data from the North American Grenville Province and new data from the SW Amazon belts of "Grenvillian" age reveals the effects of differential post-orogenic exhumation. Restoration of this exhumation gives us a crustal-scale cross-section of the synorogenic structure, marked by thrust-related imbrication on the North American side, and large-scale, strike-slip faults on the Amazon side. It is this asymmetric structure that accounts for the differences in tectonic style between the two cratons. The timing of the break-up of Rodinia is still mostly unconstrained by geochronological data from rift-related sediments from the Amazon side. The Paraguai belt that marks the SE margin of the Amazon craton is a curved, fold-and-thrust belt that affected the late Neoproterozoic-Cambrian sediments that

  20. Eastern Edge of the Laurentian Cratonic Lithosphere Beneath Southern Quebec from Teleseismic P Waves

    NASA Astrophysics Data System (ADS)

    Menke, W. H.; Neitz, T.; Levin, V. L.; Darbyshire, F. A.; Bastow, I. D.

    2014-12-01

    The eastern margin of Laurentia was deformed by the late-Proterozoic Grenville orogeny, which metamorphosed the original Archean-age cratonic crust and added extensive intrusive rocks. The Laurentian crust clearly extends as far east as the Laurentian Highlands, just north of the St. Laurence River, where Grenville-deformed rocks outcrop. Small outliers of Grenville-deformed rocks amongst west-thrust Paleozoic sediments, 20-50 km east of the Appalachian Front in southern Quebec, suggest that the Laurentian crust extends beneath the shallow thrust sheets of this region, too. On the other hand, Laurentia does not extend east of the Norumbega fault in coastal Maine, for the crust there is derived from the Avalon micro-continent. We search for the eastern edge of Laurentia within this ~250 km wide interval using relative arrival times of teleseismic P waves which ascend sub-vertically through the lithosphere beneath the region. These times are expected to be most sensitive to upper mantle compressional velocity and so to be able to discriminate the cratonic lithosphere on the basis of its faster than average speed. We use signal-correlation techniques to measured delay times for all broadband seismic stations in the region, including the QMIII array, which is especially designed to have high station density near the Appalachian Front. As expected, we observe central Quebec to have anomalously early times and coastal Maine to have anomalously late times, by as much as ±1s, when compared to the predictions of the global AK135 traveltime model. The boundary between the two arrival time regimes is sharp and is collinear with the Appalachian Front, to within the ± 25 km spatial resolution of our study. We hypothesize that it represents the eastern edge of the Laurentian cratonic lithosphere. Tomographic inversion of the data indicates a 0.2 km/s (2.4%) drop in compressional velocity of the shallow (90 km deep) mantle from west to east across the boundary. This is a strong

  1. Exploiting radiation damage control on apatite (U Th)/He dates in cratonic regions

    NASA Astrophysics Data System (ADS)

    Flowers, Rebecca M.

    2009-01-01

    Apatites from four pairs of samples of Precambrian basement from the western Canadian shield were analyzed by (U-Th)/He thermochronometry to test for the influence of radiation damage on apatite (U-Th)/He dates in this cratonic region. Recent studies have demonstrated that the accumulation of radiation damage increases the apatite He retentivity, so that apatites with a span of effective U concentrations, eU, that experienced the same thermal history may be characterized by a range of closure temperatures. In this investigation, each sample pair consisted of a mafic dike cross-cutting felsic gneisses from a single outcrop or nearby outcrops that contained apatites with a span of eU. The apatites yielded (U-Th)/He dates from 846 to 123 Ma, and were positively correlated with eU within each sample pair. These results can be explained using a model that tracks the evolution of He mobility in response to the accumulation of radiation damage. When coupled with regional geological constraints, the data appear to require partial to complete He loss due to burial and reheating in Phanerozoic time. New apatite fission- track dates and length data were obtained for five of these samples. The apatite fission- track dates are Proterozoic regardless of apatite eU. Thermal history simulations indicate that the apatite fission-track data are compatible with the (U-Th)/He results, although the thermal histories are not identical in detail and the fission-track results alone do not require Phanerozoic heating. Together the data are consistent with burial of this region by ≥ 1 km of Phanerozoic strata that were subsequently denuded, thus pointing toward significant Phanerozoic deposition in the North American cratonic interior hundreds of kilometers east of where previously documented. The results suggest that exploiting radiation damage control on apatite (U-Th)/He dates through investigation of surface sample apatites with a span of closure temperatures can impose tighter

  2. Paleogene post-collisional lamprophyres in western Yunnan, western Yangtze Craton: Mantle source and tectonic implications

    NASA Astrophysics Data System (ADS)

    Lu, Yong-Jun; Campbell McCuaig, T.; Li, Zheng-Xiang; Jourdan, Fred; Hart, Craig J. R.; Hou, Zeng-Qian; Tang, Suo-Han

    2015-09-01

    A suite of lamprophyres, spatially associated with mafic lavas and potassic felsic intrusive rocks, was emplaced between 36.5 ± 0.2 and 33.7 ± 0.5 Ma (based on phlogopite 40Ar/39Ar dating) on the eastern side of the Ailao Shan-Red River shear zone in the western Yangtze Craton. These shoshonitic and ultrapotassic intrusive rocks post-date the ~ 60-55 Ma collisional event between the Indian and the Asian continents. They are characterized by: (1) enrichment in large-ion lithophile elements and light rare-earth elements with (La/Sm)n = 3.15-7.15; (2) strong positive Pb spikes; (3) depletion in high-field-strength elements (e.g. Nb/La = 0.08-0.98); (4) high initial 87Sr/86Sr (0.706-0.709) with negative εNd(t) values of - 10.5 to - 0.9; (5) old Nd model ages of 1542-945 Ma; and (6) radiogenic (207Pb/204Pb)i of 15.57-15.70 and (208Pb/204Pb)i (38.70-39.06). These features suggest that the mantle source was metasomatized by Proterozoic subduction beneath the Yangtze Craton. The lamprophyres have similar trace element patterns, and Sr-Nd-Pb isotope compositions, as coeval mafic lava, indicating a common source of metasomatized veined continental lithospheric mantle (CLM). Lower degree partial melting of metasomatic veins likely generated the lamprophyres, whereas the coeval mafic lava was likely derived from melting of phlogopite harzburgite. The lamprophyres and mafic lava have similar Sr-Nd isotope systematics as CLM-derived Neoproterozoic mafic rocks and Late Permian Emeishan low-Ti basalt in the region, indicating that they share the same Proterozoic source. We envisage that mantle plumes thermally eroded the Proterozoic metasomatized CLM beneath the western part of the Yangtze Craton during 825-750 Ma and 260-250 Ma, although residual metasomatized domains remained before being tapped by delamination after the India-Asia continental collision during the Paleogene period.

  3. Shear wave anisotropy beneath the Cascadia subduction zone and western North American craton

    NASA Astrophysics Data System (ADS)

    Currie, Claire A.; Cassidy, John F.; Hyndman, Roy D.; Bostock, Michael G.

    2004-04-01

    We have examined shear wave splitting of SKS phases at 26 permanent broadband stations in western North America to constrain regional trends in anisotropy at the Cascadia subduction zone (CSZ) and adjacent regions. At forearc stations above the Juan de Fuca Plate, the fast directions are approximately parallel to the direction of absolute plate motion of the main Juan de Fuca Plate (~N70°E). Delay times of 1.0 to 1.5 s indicate a mantle source for the anisotropy, most likely strain-induced lattice-preferred orientation of anisotropic mantle minerals. The anisotropy may be related to present-day subduction-induced deformation of the mantle beneath the subducting plate. The delay times show an increase with distance from the deformation front (trench), which may be indicative of 3-5 per cent anisotropy within the forearc mantle wedge, with a fast direction parallel to the subduction direction. Above the Explorer Plate at the northern end of the CSZ, the fast directions are N25°E. This may reflect either the more northerly subduction direction of that plate, or a transition from subduction-related deformation to along-margin flow parallel to the transcurrent Queen Charlotte Fault to the north. At four stations in the central backarc of the CSZ, fast directions are parallel to the Juan de Fuca-North America convergence direction, consistent with mantle deformation due to subduction-induced mantle wedge flow, as well as deformation of the uppermost backarc mantle associated with motion of the overriding plate. No clear splitting was observed at the two most northern backarc stations, indicating either little horizontal anisotropy or highly complex anisotropy beneath these stations, possibly associated with complex mantle flow around the northern edge of the subducted plate. The hot, thin backarc lithosphere of the Cascadia subduction zone extends to the Rocky Mountain Trench, the western boundary of the cold, stable North America craton. At two stations on the North

  4. Cover sequences at the northern margin of the Antongil Craton, NE Madagascar

    USGS Publications Warehouse

    Bauer, W.; Walsh, G.J.; De Waele, B.; Thomas, Ronald J.; Horstwood, M.S.A.; Bracciali, L.; Schofield, D.I.; Wollenberg, U.; Lidke, D.J.; Rasaona, I.T.; Rabarimanana, M.H.

    2011-01-01

    The island of Madagascar is a collage of Precambrian, generally high-grade metamorphic basement domains, that are locally overlain by unmetamorphosed sedimentary rocks and poorly understood low-grade metasediments. In the Antalaha area of NE Madagascar, two distinct cover sequences rest on high-grade metamorphic and igneous basement rocks of the Archaean Antongil craton and the Neoproterozoic Bemarivo belt. The older of these two cover sequences, the Andrarona Group, consists of low-grade metasedimentary rocks. The younger sequence, the newly defined Ampohafana Formation, consists of unmetamorphosed sedimentary rocks. The Andrarona Group rests on Neoarchaean granites and monzogranites of the Antongil craton and consists of a basal metagreywacke, thick quartzites and an upper sequence of sericite-chlorite meta-mudstones, meta-sandstones and a volcaniclastic meta-sandstone. The depositional age of the volcaniclastic meta-sandstone is constrained in age by U–Pb laser-ablation ICP-MS analyses of euhedral zircons to 1875 ± 8 Ma (2σ). Detrital zircons of Archaean and Palaeoproterozoic age represent an input from the Antongil craton and a newly defined Palaeoproterozoic igneous unit, the Masindray tonalite, which underlies the Andrarona Group, and yielded a U–Pb zircon age of 2355 ± 11 Ma (2σ), thus constraining the maximum age of deposition of the basal part of the Andrarona Group. The Andrarona Group shows a low-grade metamorphic overprint in the area near Antalaha; illite crystallinity values scatter around 0.17°Δ2Θ CuKα, which is within the epizone. The Ampohafana Formation consists of undeformed, polymict conglomerate, cross-bedded sandstone, and red mudstone. An illite crystallinity value of >0.25°Δ2Θ CuKα obtained from the rocks is typical of the diagenetic zone. Occurrences of rhyodacite pebbles in the Ampohafana Formation and the intrusion of a basaltic dyke suggest a deposition in a WSW-ENE-trending graben system during the opening of the Indian

  5. Diamonds and the african lithosphere.

    PubMed

    Boyd, F R; Gurney, J J

    1986-04-25

    Data and inferences drawn from studies of diamond inclusions, xenocrysts, and xenoliths in the kimberlites of southern Africa are combined to characterize the structure of that portion of the Kaapvaal craton that lies within the mantle. The craton has a root composed in large part of peridotites that are strongly depleted in basaltic components. The asthenosphere boundary shelves from depths of 170 to 190 kilometers beneath the craton to approximately 140 kilometers beneath the mobile belts bordering the craton on the south and west. The root formed earlier than 3 billion years ago, and at that time ambient temperatures in it were 900 degrees to 1200 degrees C; these temperatures are near those estimated from data for xenoliths erupted in the Late Cretaceous or from present-day heat-flow measurements. Many of the diamonds in southern Africa are believed to have crystallized in this root in Archean time and were xenocrysts in the kimberlites that brought them to the surface. PMID:17743571

  6. The Precambrian of Transangaria, Yenisei Ridge (Siberia): Neoproterozoic microcontinent, Grenville-age orogen, or reworked margin of the Siberian craton?

    NASA Astrophysics Data System (ADS)

    Kuzmichev, Alexander B.; Sklyarov, Eugene V.

    2016-01-01

    The Yenisei Ridge was traditionally perceived as an uplifted segment of the western Siberian craton affected by Neoproterozoic collision events. However, the suggestions for Archaean or Palaeoproterozoic ('Siberian') basement in Transangaria have not been confirmed by reliable geochronological data. A new view regards most of the Ridge, namely, its Transangarian segment, to be an exotic Neoproterozoic terrane that collided with Siberia in the late Neoproterozoic. This paper presents new U-Pb SHRIMP zircon ages demonstrating that Archaean rocks (2611 ± 12 Ma) actually exist in this territory. We also provide a review of published U-Pb zircon ages for igneous and metamorphic rocks of Transangaria together with our new age data. This geochronological dataset clarifies the geology of the Yenisei Ridge and leads to new conclusions, as follows. (1) It is likely that Transangaria was originally underlain by an Archaean-Palaeoproterozoic basement, similar to that of the Siberian craton. (2) Geochronological data do not confirm the idea of widespread "Greenvillian age" granitoides in Transangaria. (3) The Neoproterozoic evolution of the Yenisei Ridge segment of the Siberian craton margin includes the following events. (i) Collision of an unidentified terrane with the western margin (in recent coordinates) of the Siberian craton during 900-855 Ma. The colliding terrane is no longer present in the current structure. (ii) Dextral shearing during 830-800 Ma may have been caused by counter-clockwise rotation of the Siberian craton. (iii) Extensional conditions prevailed during 800-700 Ma. The Isakovka oceanic basin formed at this time interval. (iv) Thrusting of the Isakovka island arc and accretionary prism onto the Siberian margin occurred during the late Neoproterozoic (650-630 Ma) and caused high-pressure metamorphism.

  7. Sone megafan: A non-Himalayan megafan of craton origin on the southern margin of the middle Ganga Basin, India

    NASA Astrophysics Data System (ADS)

    Sahu, Sudarsan; Saha, Dipankar; Dayal, Shankar

    2015-12-01

    Researchers in the last few decades have focused on the fluvial megafans at the base of Himalayan foothills in the Ganga Basin. No major effort has so far been made to delineate any such large depositional environment at the base of the northern Indian peninsular craton at the distal parts of the basin. In this work, for the first time, we document a megafan, unusually created by the cratonic Sone River in the marginal plains south of the Ganga. The geomorphology of the megafan surface, distribution of palaeochannels, sedimentology, and areal extent of the megafan are described. The study also reconstructs the tectonic and palaeoclimatic conditions, which might have helped in sediment supply from the cratonic catchment areas of the Sone. We suggest tectonic control over the Sone River channel dynamics and development of the megafan. The oval- and fan-shaped, craton-derived sediment body covers an area of around 12,000 km2 in the marginal plains and about 9000 km2 in the central alluvium north of the Ganga River. The megafan is around 190 km long and ~ 240 km wide. The mean slope of the megafan surface is ~ 0.03°. An onlap of Himalayan sediments has occurred over the megafan at its toe parts caused by southward encroachment of the Ganga during the Holocene. The megafan sediments comprise brownish yellow fine to coarse sand and gravels with an admixture of carbonaceous nodules (kankars). The thickness of the megafan sediment, which directly overlies the Precambrian basement, varies from only a few meters in the south near the peninsular craton to ~ 1000 m at the north in the central alluvial plain. The granular zones form prolific aquifer systems in an otherwise poor hydrogeological setting of the marginal plains.

  8. Cenozoic Source-to-Sink of the African margin of the Equatorial Atlantic

    NASA Astrophysics Data System (ADS)

    Rouby, Delphine; Chardon, Dominique; Huyghe, Damien; Guillocheau, François; Robin, Cecile; Loparev, Artiom; Ye, Jing; Dall'Asta, Massimo; Grimaud, Jean-Louis

    2016-04-01

    The objective of the Transform Source to Sink Project (TS2P) is to link the dynamics of the erosion of the West African Craton to the offshore sedimentary basins of the African margin of the Equatorial Atlantic at geological time scales. This margin, alternating transform and oblique segments from Guinea to Nigeria, shows a strong structural variability in the margin width, continental geology and relief, drainage networks and subsidence/accumulation patterns. We analyzed this system combining onshore geology and geomorphology as well as offshore sub-surface data. Mapping and regional correlation of dated lateritic paleo-landscape remnants allows us to reconstruct two physiographic configurations of West Africa during the Cenozoic. We corrected those reconstitutions from flexural isostasy related to the subsequent erosion. These geometries show that the present-day drainage organization stabilized by at least 29 Myrs ago (probably by 34 Myr) revealing the antiquity of the Senegambia, Niger and Volta catchments toward the Atlantic as well as of the marginal upwarp currently forming a continental divide. The drainage rearrangement that lead to this drainage organization was primarily enhanced by the topographic growth of the Hoggar swell and caused a major stratigraphic turnover along the Equatorial margin of West Africa. Elevation differences between paleo-landscape remnants give access to the spatial and temporal distribution of denudation for 3 time-increments since 45 Myrs. From this, we estimate the volumes of sediments and associated lithologies exported by the West African Craton toward different segments of the margin, taking into account the type of eroded bedrock and the successive drainage reorganizations. We compare these data to Cenozoic accumulation histories in the basins and discuss their stratigraphic expression according to the type of margin segment they are preserved in.

  9. A major Archean, gold- and crust-forming event in the Kaapvaal craton, South Africa.

    PubMed

    Kirk, Jason; Ruiz, Joaquin; Chesley, John; Walshe, John; England, Gavin

    2002-09-13

    The 2.89- to 2.76-gigayear-old conglomerates of the Central Rand Group of South Africa host an immense concentration of gold. The gold and rounded pyrites from the conglomerates yield a rhenium-osmium isochron age of 3.03 +/- 0.02 gigayears and an initial 187Os/188Os ratio of 0.1079 +/- 0.0001. This age is older than that of the conglomerates. Thus, the gold is detrital and was not deposited by later hydrothermal fluids. This Middle Archean gold mineralization event corresponds to a period of rapid crustal growth in which much of the Kaapvaal craton was formed and is evidence for a significant noble metal flux from the mantle. PMID:12228713

  10. The Montesbelos mass-flow (southern Amazonian craton, Brazil): a Paleoproterozoic volcanic debris avalanche deposit?

    NASA Astrophysics Data System (ADS)

    Roverato, M.

    2016-07-01

    The present contribution documents the extremely well-preserved Paleoproterozoic architecture of the Montesbelos breccia (named here for the first time), which is interpreted as a rare example of a subaerial paleoproterozoic (>1.85 Ga) granular-dominated mass-flow deposit, few of which are recorded in the literature. Montesbelos deposit is part of the andesitic Sobreiro Formation located in the São Felix do Xingu region, southern Amazonian craton, northern Brazil. The large volume, high variability of textural features, presence of broken clasts, angular low sphericity fragments, mono- to heterolithic character, and the size of the outcrops point to a volcanic debris avalanche flow. Fluviatile sandy material and debris flows are associated with the deposit as a result of post-depositional reworking processes.

  11. East-China Geochemistry Database (ECGD):A New Networking Database for North China Craton

    NASA Astrophysics Data System (ADS)

    Wang, X.; Ma, W.

    2010-12-01

    North China Craton is one of the best natural laboratories that research some Earth Dynamic questions[1]. Scientists made much progress in research on this area, and got vast geochemistry data, which are essential for answering many fundamental questions about the age, composition, structure, and evolution of the East China area. But the geochemical data have long been accessible only through the scientific literature and theses where they have been widely dispersed, making it difficult for the broad Geosciences community to find, access and efficiently use the full range of available data[2]. How to effectively store, manage, share and reuse the existing geochemical data in the North China Craton area? East-China Geochemistry Database(ECGD) is a networking geochemical scientific database system that has been designed based on WebGIS and relational database for the structured storage and retrieval of geochemical data and geological map information. It is integrated the functions of data retrieval, spatial visualization and online analysis. ECGD focus on three areas: 1.Storage and retrieval of geochemical data and geological map information. Research on the characters of geochemical data, including its composing and connecting of each other, we designed a relational database, which based on geochemical relational data model, to store a variety of geological sample information such as sampling locality, age, sample characteristics, reference, major elements, rare earth elements, trace elements and isotope system et al. And a web-based user-friendly interface is provided for constructing queries. 2.Data view. ECGD is committed to online data visualization by different ways, especially to view data in digital map with dynamic way. Because ECGD was integrated WebGIS technology, the query results can be mapped on digital map, which can be zoomed, translation and dot selection. Besides of view and output query results data by html, txt or xls formats, researchers also can

  12. A major Archean, gold- and crust-forming event in the Kaapvaal craton, South Africa.

    PubMed

    Kirk, Jason; Ruiz, Joaquin; Chesley, John; Walshe, John; England, Gavin

    2002-09-13

    The 2.89- to 2.76-gigayear-old conglomerates of the Central Rand Group of South Africa host an immense concentration of gold. The gold and rounded pyrites from the conglomerates yield a rhenium-osmium isochron age of 3.03 +/- 0.02 gigayears and an initial 187Os/188Os ratio of 0.1079 +/- 0.0001. This age is older than that of the conglomerates. Thus, the gold is detrital and was not deposited by later hydrothermal fluids. This Middle Archean gold mineralization event corresponds to a period of rapid crustal growth in which much of the Kaapvaal craton was formed and is evidence for a significant noble metal flux from the mantle.

  13. Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

    NASA Astrophysics Data System (ADS)

    Baptiste, V.; Tommasi, A.

    2014-01-01

    We calculated the seismic properties of 47 mantle xenoliths from 9 kimberlitic pipes in the Kaapvaal craton based on their modal composition, the crystal-preferred orientations (CPO) of olivine, ortho- and clinopyroxene, and garnet, the Fe content of olivine, and the pressures and temperatures at which the rocks were equilibrated. These data allow constraining the variation of seismic anisotropy and velocities within the cratonic mantle. The fastest P and S2 wave propagation directions and the polarization of fast split shear waves (S1) are always subparallel to olivine [100] axes of maximum concentration, which marks the lineation (fossil flow direction). Seismic anisotropy is higher for high olivine contents and stronger CPO. Maximum P wave azimuthal anisotropy (AVp) ranges between 2.5 and 10.2% and the maximum S wave polarization anisotropy (AVs), between 2.7 and 8%. Changes in olivine CPO symmetry result in minor variations in the seismic anisotropy patterns, mainly in the apparent isotropy directions for shear wave splitting. Seismic properties averaged over 20 km-thick depth sections are, therefore, very homogeneous. Based on these data, we predict the anisotropy that would be measured by SKS, Rayleigh (SV) and Love (SH) waves for five endmember orientations of the foliation and lineation. Comparison to seismic anisotropy data from the Kaapvaal shows that the coherent fast directions, but low delay times imaged by SKS studies, and the low azimuthal anisotropy with with the horizontally polarized S waves (SH) faster than the vertically polarized S wave (SV) measured using surface waves are best explained by homogeneously dipping (45°) foliations and lineations in the cratonic mantle lithosphere. Laterally or vertically varying foliation and lineation orientations with a dominantly NW-SE trend might also explain the low measured anisotropies, but this model should also result in backazimuthal variability of the SKS splitting data, not reported in the

  14. Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

    NASA Astrophysics Data System (ADS)

    Virginie, Baptiste; Andrea, Tommasi

    2014-05-01

    We calculated the seismic properties of 47 mantle xenoliths from 9 kimberlitic pipes in the Kaapvaal craton based on their modal composition, the crystal preferred orientations (CPO) of olivine, ortho- and clinopyroxene, and garnet, the Fe content of olivine, and the pressures and temperatures at which the rocks were equilibrated. These data allow constraining the variation of seismic anisotropy and velocities within the cratonic mantle. The fastest P and S2 waves propagation direction and the polarization of fast split shear wave (S1) are always subparallel to olivine [100] axes maximum concentration, which marks the lineation (fossil flow direction). Seismic anisotropy is higher for high olivine contents and stronger CPO. Maximum P-wave azimuthal anisotropy (AVp) ranges between 2.5 and 10.2% and the maximum S-wave polarization anisotropy (AVs), between 2.7 and 8%. Changes in olivine CPO symmetry result in minor variations in the seismic anisotropy patterns, mainly in the apparent isotropy directions for shear wave splitting. Seismic properties averaged over 20 km thick depth sections are, therefore, very homogeneous. Based on these data, we predict the anisotropy that would be measured by SKS, Rayleigh (SV) and Love (SH) waves for 5 end-member orientations of the foliation and lineation. Comparison to seismic anisotropy data in the Kaapvaal shows that the coherent fast directions, but low delay times imaged by SKS studies and the low azimuthal anisotropy with SH faster than SV measured using surface waves are best explained by a homogeneously dipping (45°) foliation and lineation in the cratonic mantle lithosphere. Laterally or vertically varying foliation and lineation orientations with a dominantly NW-SE trend might also explain the low measured anisotropies, but this model should also result in backazimuthal variability of the SKS splitting data, not reported in the seismological data. The strong compositional heterogeneity of the Kaapvaal peridotite

  15. Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology

    USGS Publications Warehouse

    Fanning, C.M.; Flint, R.B.; Parker, A.J.; Ludwig, K. R.; Blissett, A.H.

    1988-01-01

    Through the application of both conventional U-Pb zircon analyses and small-sample U-Pb isotopic analyses, the nature and timing of tectonic events leading to the formation of the Gawler Craton have been defined more precisely. Constraints on deposition of Early Proterozoic iron formation-bearing sediments have been narrowed down to the period 1960-1847 Ma. Deformed acid volcanics, including the economically important Moonta Porphyry, have zircon ages of ??? 1790 and 1740 Ma. The voluminous acid Gawler Range Volcanics and correlatives to the east were erupted over a short interval at 1592 ?? 2 Ma, and were intruded by anorogenic granites at ??? 1575 Ma. Small-sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses, generally yielding more-concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight-line discordia. ?? 1988.

  16. Wisconsin gravity minimum: Solution of a geologic and geophysical puzzle and implications for cratonic evolution

    SciTech Connect

    Allen, D.J.; Hinze, W.J. )

    1992-06-01

    An intense Bouguer gravity anomaly minimum extending across much of Wisconsin cannot be explained by the surface Phanerozoic sedimentary strata, the basement Precambrian geology, or the topography of the region. The most intense ({minus}100 mgal) part of the minimum coincides with the 1.47 Ga anorogenic granitic Wolf River batholith of northeastern Wisconsin. In southern Wisconsin, however, the densities of the Precambrian basement rocks, which are older than the batholith, provide no clue to the origin of the anomaly. The gradients of the minimum indicate that the source of the anomaly is in the upper crust. Furthermore, nearby deep seismic reflection data indicate that lower crustal structures do not significantly contribute to the gravity minimum. Thus, the minimum is appropriately interpreted as originating from the low-density Wolf River batholith that crops out only in northeastern Wisconsin but is buried beneath a veneer of older rocks in the southern and central parts of the state. Gravity modeling suggests that the batholith is at least 10 km thick and encompasses an area of {approximately}50,000 km{sup 2}. This interpretation provides an important clue to the origin of similar negative gravity anomalies of the Phanerozoic strata-covered craton. Also, the presence of this massive granitic body appears to have influenced the evolution of the craton - e.g., by controlling the location of the 1.1 Ga Midcontinent rift system and the Paleozoic Wisconsin arch. The fact that the Wolf River batholith is mostly buried suggests that central Wisconsin has been tectonically stable for the past 1.47 b.y. and that the Precambrian basement has been minimally eroded.

  17. (142)Nd evidence for an enriched Hadean reservoir in cratonic roots.

    PubMed

    Upadhyay, Dewashish; Scherer, Erik E; Mezger, Klaus

    2009-06-25

    The isotope (146)Sm undergoes alpha-decay to (142)Nd, with a half-life of 103 million years. Measurable variations in the (142)Nd/(144)Nd values of rocks resulting from Sm-Nd fractionation could therefore only have been produced within about 400 million years of the Solar System's formation (that is, when (146)Sm was extant). The (142)Nd/(144)Nd compositions of terrestrial rocks are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High (142)Nd/(144)Nd values measured in some Archaean rocks from Greenland hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low (142)Nd/(144)Nd values in 1.48-gigayear-(Gyr)-old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India. These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low (142)Nd/(144)Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are 'hidden' in Hadean crust, the D'' layer of the lowermost mantle or, as we propose here, also within the roots of old cratons. PMID:19553997

  18. (142)Nd evidence for an enriched Hadean reservoir in cratonic roots.

    PubMed

    Upadhyay, Dewashish; Scherer, Erik E; Mezger, Klaus

    2009-06-25

    The isotope (146)Sm undergoes alpha-decay to (142)Nd, with a half-life of 103 million years. Measurable variations in the (142)Nd/(144)Nd values of rocks resulting from Sm-Nd fractionation could therefore only have been produced within about 400 million years of the Solar System's formation (that is, when (146)Sm was extant). The (142)Nd/(144)Nd compositions of terrestrial rocks are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High (142)Nd/(144)Nd values measured in some Archaean rocks from Greenland hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low (142)Nd/(144)Nd values in 1.48-gigayear-(Gyr)-old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India. These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low (142)Nd/(144)Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are 'hidden' in Hadean crust, the D'' layer of the lowermost mantle or, as we propose here, also within the roots of old cratons.

  19. Unusual lithospheric structure beneath the Hyderabad granitic region, eastern Dharwar craton, south India

    NASA Astrophysics Data System (ADS)

    Pandey, O. P.; Agrawal, P. K.; Chetty, T. R. K.

    2002-03-01

    Using multiparametric geological and geophysical data, the evolutionary history of the lithosphere beneath the Late Archean—early Proterozoic Hyderabad granitic region (HGR) of the eastern Dharwar craton of south Indian shield has been attempted. Our study reveals that the entire granitic region and the surroundings (˜200 km×200 km) is being neotectonically uplifted possibly due to a major intrusive body situated at sub-crustal depth, leading to changes in river courses and also erosion of several kilometers (˜10 km) from its upper crustal column. Beneath this region, the Moho is elevated (32-33 km), the reduced heat flow is of the order of 28 mW/m 2 and the asthenosphere is located at shallow depths of about 124 km, i.e. far less than 200-400 km depth found in Precambrian shield areas of the globe. The cratonic mantle lithosphere beneath HGR contains a highly conductive (15 Ωm) hydrous and seismically anisotropic metasomatic zone between depth of 90 and 100 km, where the estimated temperatures could be in the range of 900-1000 °C. The surface granitic layer, containing unusually high radioactivity (5.25 μW/m 3), appears very thin probably a kilometer or so, beneath which the entire crustal column seems to be differentiated and made up of low radioactive granulite facies (?) rocks of acid to intermediate composition. Persistent episodic thermal reactivations during the last 2.6 gega years and continuous uplifting seems to have played a major role towards making its crustal column unusual.

  20. Is the Paleoproterozoic Jiao-Liao-Ji Belt (North China Craton) a rift?

    NASA Astrophysics Data System (ADS)

    Li, Zhuang; Chen, Bin; Wei, Chunjing

    2016-04-01

    As a typical example of the Paleoproterozoic crust in the Eastern Block of the North China Craton, the Paleoproterozoic Jiao-Liao-Ji Belt consists principally of the Liaohe Group (and its equivalents), Liaoji granites and mafic intrusions. Previous studies indicate that the evolution of the Jiao-Liao-Ji Belt has been mainly attributed to the opening and closing of an intracontinental rift along the eastern continental margin of the North China Craton. Here we synthesize the Paleoproterozoic magmatism, sedimentation, metamorphism and metallogeny against the rift model and propose a process of arc-continent collision between the northern Longgang and the southern Nangrim Blocks. This conclusion is consistent with the observations, including that (a) the 2.0- to 2.2-Ga magmatism shows a typical subalkaline series, rather than a bimodal distribution, since the mafic rocks mostly have arc affinities and the acidic-intermediate rocks belong to the calc-alkaline series; (b) the main source of the 1.9- to 2.0-Ga sedimentary rocks is the Paleoproterozoic arc materials, indicating a fore-arc or back-arc basin setting; (c) a couple of big borate deposits occur in the boron-rich volcanic rocks that were formed in convergent continental margins; (d) the North and South Liaohe Groups show different rock associations and metamorphic histories (P-T paths); and (e) the Nangrim and Longgang Blocks vary in lithological units, geochronology and metamorphic features. Thus, an arc-continent collision tectonic scenario for the Paleoproterozoic Jiao-Liao-Ji Belt is involved: (a) a southward subduction in the period 2.0-2.2 Ga; (b) sedimentation during the period 1.9-2.0 Ga; (c) arc-continent collision at ca. 1.9 Ga; and (d) post-collisional extension at 1.82-1.87 Ga, marking the end of the Paleoproterozoic tectonothermal event.

  1. Low water contents in diamond mineral inclusions: Proto-genetic origin in a dry cratonic lithosphere

    NASA Astrophysics Data System (ADS)

    Taylor, Lawrence A.; Logvinova, Alla M.; Howarth, Geoffrey H.; Liu, Yang; Peslier, Anne H.; Rossman, George R.; Guan, Yunbin; Chen, Yang; Sobolev, Nikolay V.

    2016-01-01

    The mantle is the major reservoir of Earth's water, hosted within Nominally Anhydrous Minerals (NAMs) (e.g., Bell and Rossman, 1992; Peslier et al., 2010; Peslier, 2010; Nestola and Smyth, 2015), in the form of hydrogen bonded to the silicate's structural oxygen. From whence cometh this water? Is the water in these minerals representative of the Earth's primitive upper mantle or did it come from melting events linked to crustal formation or to more recent metasomatic/re-fertilization events? During diamond formation, NAMs are encapsulated at hundreds of kilometers depth within the mantle, thereby possibly shielding and preserving their pristine water contents from re-equilibrating with fluids and melts percolating through the lithospheric mantle. Here we show that the NAMs included in diamonds from six locales on the Siberian Craton contain measurable and variable H2O concentrations from 2 to 34 parts per million by weight (ppmw) in olivine, 7 to 276 ppmw in clinopyroxene, and 11-17 ppmw in garnets. Our results suggest that if the inclusions were in equilibrium with the diamond-forming fluid, the water fugacity would have been unrealistically low. Instead, we consider the H2O contents of the inclusions, shielded by diamonds, as pristine representatives of the residual mantle prior to encapsulation, and indicative of a protogenetic origin for the inclusions. Hydrogen diffusion in the diamond does not appear to have modified these values significantly. The H2O contents of NAMs in mantle xenoliths may represent some later metasomatic event(s), and are not always representative of most of the continental lithospheric mantle. Results from the present study also support the conclusions of Peslier et al. (2010) and Novella et al. (2015) that the dry nature of the SCLM of a craton may provide stabilization of its thickened continental roots.

  2. Astronomy for African development

    NASA Astrophysics Data System (ADS)

    Govender, Kevindran

    2011-06-01

    In recent years there have been a number of efforts across Africa to develop the field of astronomy as well as to reap benefit from astronomy for African people. This presentation will discuss the case of the SALT (Southern African Large Telescope) Collateral Benefits Programme (SCBP) which was set up to ensure societal benefit from astronomy. With African society as the target, the SCBP has embarked on various projects from school level education to public understanding of science to socio-economic development, the latter mainly being felt in the rural communities surrounding the South African Astronomical Observatory (home to SALT). A development plan for ``Astronomy in Africa'' will also be discussed. This plan has been drawn up with input from all over Africa and themed ``Astronomy for Education''. The Africa case stands as a good example for the IYA cornerstone project ``Developing Astronomy Globally'' which focuses on developing regions.

  3. African American Health

    MedlinePlus

    ... specific health concerns. Differences in the health of groups can result from Genetics Environmental factors Access to care Cultural factors On this page, you'll find links to health issues that affect African Americans.

  4. African American Suicide

    MedlinePlus

    ... accounted for 83.8% of Caucasian elderly suicides. • Firearms were the predominant method of suicide among African ... per 100,000 annually. Source: Centers for Disease Control and Prevention. National Vital Statistics System. Mortality Data. ...

  5. AfricaArray seismological studies of the structure and evolution of the African continent

    NASA Astrophysics Data System (ADS)

    Durrheim, Raymond; Nyblade, Andrew; Brandt, Martin; Tugume, Fred; Mulibo, Gabriel; Kgaswane, Eldridge; Mangongolo, Azangi; Manzi, Musa; El Tahir, Nada; Loots, Letticia

    2014-05-01

    The AfricaArray programme was launched in 2005 to conduct research that promotes development in Africa by building human and infrastructural capacity in support of the mineral exploration, mining, geohazard and environmental sectors. The AfricaArray "backbone" network now consists of 51 geophysical observatories in 20 sub-Saharan countries. Most stations are equipped with broadband seismometers, while 25 stations have continuous GPS sensors and 22 stations have meteorological packs. In addition, several temporary seismic arrays have been deployed to investigate the seismotectonics of the East African Rift System, the extent of the Congo craton, and the rifting of Mozambique and Madagascar. In this paper we will present results pertinent to large-scale crustal and mantle geodynamic processes that have been obtained by AfricaArray researchers. Brandt and Mulibo elucidated the relationship between the African Superplume, Superswell and the East African Rift System by studying the seismic velocity structure of the mantle. Kgaswane jointly inverted P-wave receiver functions (PRFs) and surface waves, and found that the Kalahari Craton lower crust is largely mafic, except for a few terrains such as the Kimberley. Kgaswane also produced evidence that supports a link between the eastern and western lobes of the Bushveld Complex. Mangongolo used surface wave tomography to define the south-western boundary of the Congo Craton. El Tahir used PRFs to investigate the crustal structure of the Khartoum Basin, while Tugume determined the Moho depths and Poisson's ratios of the Precambrian crust in East Africa. Manzi reprocessed 3D reflection seismic data covering part of the Witwatersrand goldfields using seismic attribute analysis methods, and has provided new constraints on the evolution of the Basin during the Neoarchean. Loots interpreted a 105 km 2D seismic reflection profile immediately to the north of the Cape Fold Belt, imaging the Karoo and Cape Supergroup rocks and the

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

    NASA Technical Reports Server (NTRS)

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

    1988-01-01

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

  7. 2.6-2.7 Ga continental crust growth in Yangtze craton, South China

    NASA Astrophysics Data System (ADS)

    Chen, K.; Gao, S.; Wu, Y.

    2013-12-01

    A combined study of zircon U-Pb and Lu-Hf isotopes and whole rock major and trace elements and Sr-Nd isotopes has been conducted for 10 granitic and tonalitic-trondhjemitic-granodioritic (TTG) gneisses from the Kongling terrain, the only known Archean microcontinent in the Yangtze craton, South China. The results reveal a significant magmatic event at ~2.6-2.7 Ga (Fig. 1), in addition to the previously reported ~2.9 Ga and ~3.2-3.3 Ga magmatism (Zhang et al., 2006; Jiao et al., 2009; Gao et al., 2011). The ~2.6-2.7 Ga rocks show relatively high REE (530-1074 ppm), apparently negative Eu anomaly (Eu/Eu* = 0.22-0.35), low #Mg (19.51-22.63) and low LaN/YbN (10.3-24.2). Besides, they have high K-feldspar proportion and relatively evaluated (K2O+Na2O)/CaO, TFeO/MgO, Zr, Nb, Ce and Y contents. Their 10000 × Ga/Al ratios range between 3.00 and 3.54. All these features suggest that the protoliths of these gneisses are A-type granites. Most of the ~2.6-2.7 Ga zircon grains have ɛHf(t) values >0 (up to 7.93, close to the depleted mantle value). This clearly indicates a considerably higher proportion of new crustal components in the ~2.6-2.7 Ga granitoids compared to the ~3.2-3.3 Ga and ~2.9 Ga TTGs. Our results support the conclusion of worldwide studies of igneous and detrital zircons that age peaks at 2.65-2.76 Ga represent increases in the volume of juvenile continental crust. The present study also confirms the existence of the two older magmatic events in the Kongling terrain. Both whole rock ɛNd(t) values (-3.74 to 1.59) and the zircon ɛHf(t) values (-11.18 to 3.55) for the ~2.9 Ga TTG and the Hf isotopes of ~3.2-3.3 Ga igneous zircons (-7.37 to 3.12) are chondritic or subchondritic, suggesting that they were mainly generated by reworking of older rocks with a small amount of new crustal additions. References Gao, S., Yang, J., Zhou, L., Li, M., Hu, Z.C., Guo, J.L., Yuan, H.L., Gong, H.J., Xiao, G.Q., Wei, J.Q., 2011. Age and growth of the Archean Kongling terrain

  8. The timing of gold mineralization across the eastern Yilgarn craton using U-Pb geochronology of hydrothermal phosphate minerals

    NASA Astrophysics Data System (ADS)

    Vielreicher, Noreen; Groves, David; McNaughton, Neal; Fletcher, Ian

    2015-04-01

    The highly mineralized Eastern Goldfields of the eastern Yilgarn craton is an amalgamation of dominantly Neoarchaean granitoid-greenstone terranes and domains that record a history of early rifting, followed by westward directed collision with initial arc formation, collision and clastic basin formation, and final accretion to the western Yilgarn proto-craton between 2.66 and 2.60 billion years ago. The gold deposits that define this region as a world-class gold province are the product of orogenic processes that operated during accretion late in the tectonic history, after initial compressional deformation (D1-D2) and the majority of granitoid magmatism. Minor gold was also deposited throughout the entire tectonic history in magmatic-hydrothermal-related systems. However, such mineralization (mostly < 0.3 g/t gold) is nowhere economic unless it overprints, or is overprinted by, much higher-grade orogenic gold lodes. Robust SHRIMP U-Pb geochronology of gold-related hydrothermal xenotime and monazite supports structural studies that gold mineralization occurred during late transpressional events (D3-D4), shortly before cratonization. However, westward migration of collision and accretion produced a complementary diachroneity in the timing of gold mineralization of 5 to 20 m.y. between c. 2.65 Ma in the east (including Laverton District, Kurnalpi Terrane) to c. 2.63 Ma in the west (including Kalgoorlie Terrane) across the eastern part of the craton. The robust geochronology refutes previous suggestions that significant gold mineralization events extended from DE to D4 in the evolution of the orogen and that the Kalgoorlie gold deposits formed over a period of 45 m.y. The crustal continuum model is applicable within terranes where orogenic gold depositional events were penecontemporaneous, but must be modified to account for diachroneity of orogenic events and gold mineralization across the Eastern Goldfields.

  9. Deformation of the Continental Lithosphere at the Margins of the North American Craton: Constraints from Seismic Anisotropy

    NASA Astrophysics Data System (ADS)

    Long, M. D.; Benoit, M. H.; Ford, H. A.; Wirth, E. A.; Aragon, J. C.; Abrahams, L.; McNamara, J.; Jackson, K.

    2015-12-01

    Earth's continents exhibit striking properties, including relatively thick and low-density crust and a strong, thick, long-lived mantle lithosphere. Major questions related to the formation, stability, evolution, and dynamics of cratonic lithosphere remain unanswered. One promising avenue for understanding the stability of cratonic lithosphere through geologic time is to understand how their margins are deformed via tectonic processes such as orogenesis and rifting. Here we present results of several recent and ongoing studies which aim to constrain past lithospheric deformation along the eastern margin of the North American craton. Each of these studies focuses on constraining seismic anisotropy, or the directional dependence of seismic wavespeeds, in the lithospheric upper mantle. Because there is a causative link between upper mantle deformation and the resulting seismic anisotropy, studies of anisotropic structure in the upper mantle beneath continental interiors can shed light on the deformation processes associated with past tectonic events. The recent explosion in the availability of seismic data in the eastern United States, largely due to the EarthScope initiative, has enabled detailed studies of lithospheric deformation using anisotropic receiver function (RF) analysis and SKS splitting analysis. A comparison of lithospheric structure inferred from RFs for stations located to the east of the Grenville deformation front with those located within the cratonic interior argues for extensive deformation of the lithosphere during the formation and/or breakup of Rodinia. The pattern of fast SKS splitting directions measured at USArray Transportable Array (TA) stations shows clear evidence for a specific lithospheric anisotropy signature at stations beneath the Appalachian Mountains, indicating strong, coherent lithospheric deformation associated with Appalachian orogenesis. The Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment, a linear array

  10. Re-Os isotopic composition of peridotitic sulphide inclusions in diamonds from Ellendale, Australia: Age constraints on Kimberley cratonic lithosphere

    NASA Astrophysics Data System (ADS)

    Smit, K. V.; Shirey, S. B.; Richardson, S. H.; le Roex, A. P.; Gurney, J. J.

    2010-06-01

    Sulphide-bearing diamonds recovered from the ˜20 Ma Ellendale 4 and 9 lamproite pipes in north-western Australia were investigated to determine the nitrogen aggregation state of the diamonds and Re-Os isotope geochemistry of the sulphide inclusions. The majority of diamond studies have been based on diamonds formed in the sub-continental lithospheric mantle (SCLM) below stable cratons, whereas the Ellendale lamproites intrude the King Leopold Orogen, south of the Kimberley craton. The sulphide inclusions consist of pyrrhotite-pentlandite-chalcopyrite assemblages, and can be divided into peridotitic and eclogitic parageneses on the basis of their Ni and Os contents. A lherzolitic paragenesis for the high-Ni sulphide inclusions is suggested from their Re and Os concentrations. Regression analysis of the Re-Os isotope data for the lherzolitic sulphides yields an age of 1426 ± 130 Ma, with an initial 187Os/ 188Os ratio of 0.1042 ± 0.0034. The upper limit of the uncertainty on the 187Os/ 188Os initial ratio gives a Re depletion age of 2.96 Ga, indicating the presence of SCLM beneath Ellendale since at least the Mesoarchaean, with the lherzolitic diamond-forming event much younger and unrelated to the craton keel stabilisation. The nitrogen aggregation state of the diamonds and calculated mantle residence temperatures suggest an origin and storage of the Ellendale diamonds in a stable cratonic SCLM, consistent with the King Leopold Orogen being cratonised by about 1.8 Ga. The diamonds do not show evidence for pervasive deformation or platelet degradation, which suggests that the diamonds had a relatively undisturbed 1.4 billion year mantle storage history.

  11. Precambrian Processes, the Trans-Hudson Orogen, and Cratonic Keels: Insights From Teleseismic Tomography in Northern Hudson Bay, Canada

    NASA Astrophysics Data System (ADS)

    Liddell, M. V.; Bastow, I. D.; Gilligan, A.; Kendall, J. M.; Darbyshire, F. A.

    2015-12-01

    Earth conditions in the Precambrian, and their effect upon the formation of cratons and orogenies from that era, are not fully understood. For example, the precise onset of modern plate tectonics remains ambiguous; it has been hypothesised to have begun anywhere from ~4.1Ga (Hopkins, 2008) to ~1Ga (Stern, 2005). Also, the exceptional depth to which fast wave-speed and geoid anomalies extend beneath some cratons points to the existence of thick "cratonic keels", the origin of which remains unexplained. To improve our understanding of the early Earth processes, geological evidence preserved within ancient plates that have remained largely unchanged since the Precambrian can be used. The rocks of northern Hudson Bay include Archean domains, the Paleoproterozoic Trans-Hudson Orogen (THO), and lie atop one of the largest cratonic keels on Earth (Bastow et al., 2013), making this region an ideal laboratory for study of Precambrian processes. Here, we use seismological data recorded at Canadian POLARIS and Hudson Bay Lithospheric Experiment (HuBLE) stations to perform a relative arrival-time study of northern Hudson Bay region and the THO. Waveforms are aligned using the adaptive stacking routine of Rawlinson et al. (2004), and inversions are produced using the Fast Marching Tomography (FMTOMO) inversion code of Rawlinson et al. (2006). Our inversions provide an improved velocity model of the lithosphere and upper mantle of northern Canada, suggesting updated boundaries between lithospheric blocks at mantle depths and constituting new body-wave constraints on their structure. The results are used to address a number of outstanding questions regarding the processes that formed the THO and the Laurentian Keel of North America.

  12. He-Ar isotope geochemistry of iron and gold deposits reveals heterogeneous lithospheric destruction in the North China Craton

    NASA Astrophysics Data System (ADS)

    Shen, Junfeng; Li, Shengrong; Santosh, M.; Meng, Kai; Dong, Guochen; Wang, Yanjuan; Yin, Na; Ma, Guanggang; Yu, Hongjun

    2013-12-01

    The North China Craton (NCC) provides a classic example for extensive destruction of the cratonic lithosphere. The Mesozoic magmatism which contributed to the decratonization of the NCC was also accompanied by the formation of a variety of mineral deposits. In order to gain further insights into the cratonic destruction process, typical iron and gold deposits are investigated here. Helium-argon isotopic data on pyrite, from typical skarn iron deposits of the Beiminghe and Fushan in the Han-Xing district of the central NCC, and the Linglong and Canzhuang gold deposits in the Jiaodong district in the eastern NCC, are presented in this paper. The 3He/4He, 40Ar/36Ar and 40Ar/4He ratios show generally uniform patterns within the individual deposits and reveal a complex evolutionary history of the ore-forming fluids with varying degree of crust-mantle interaction. The ore-forming fluids associated with the gold mineralization at the Jiaodong mine have higher content of fluids of mantle origin with mantle helium ranging from 1.24% to 18.02% (average 6.73%; N = 18). In contrast, the ore-forming fluids related to the iron ore deposits contain less mantle contribution with mantle helium ranging from 0.12% to 4.96% (average 1.29%; N = 10). Our results suggest complex and heterogeneous crust-mantle processes associated with the magmatism and metallogeny, where the lithosphere of the eastern NCC was subjected to more extensive thinning and destruction as compared with that in the western part, consistent with the observations from geophysical studies in the region. Our study demonstrates that fluids associated with the Mesozoic metallogenic processes in the NCC provide useful insights into the geodynamics of destruction and refertilization of the cratonic lithosphere.

  13. Grenville tectonic events and evolution of the Yenisei Ridge at the western margin of the Siberian Craton

    NASA Astrophysics Data System (ADS)

    Likhanov, I. I.; Nozhkin, A. D.; Reverdatto, V. V.; Kozlov, P. S.

    2014-09-01

    Geological, petrologic, geochemical, and isotopic geochronological evidence for Grenville events at the western margin of the Siberian Craton are considered. These events were related to assembly of the Rodinia supercontinent. Multiple manifestations of riftogenic and within-plate magmatism at the final stage of orogenic evolution gave rise to breakdown of Rodinia and the formation of the Paleoasian ocean. The results allowed us to develop a new concept on the Precambrian geological evolution of the Yenisei Ridge and the processes that created its tectonic structure. The chronological sequence of events in the history of the Transangarian Yenisei Ridge is based on geological evidence and isotopic dating of Precambrian complexes variable in geodynamic nature. Four tectonic stages dated at 1.4-1.1, 1.1-0.9, 0.90-0.85, and 0.8-0.6 Ga were controlled by collision and extension recognized from large regional linear crustal structural elements. The evolution of the Transangarian Yenisei Ridge, which lasted for ˜650 Ma, corresponds in duration to supercontinental cycles that begin from rifting and breakdown of the predated supercontinent and was completed by orogeny and the formation of a new supercontinent. The regional geodynamic history correlates with the synchronous sequence and similar style of tectonothermal events at the periphery of the large Precambrian Laurentia and Baltica cratons. This is evidenced by paleocontinental reconstructions, which confirm close spatiotemporal links of Siberia with cratons in the northern Atlantic 1400-600 Ma ago and indicate incorporation of the Siberian Craton into the ancient Nuna and Rodinia supercontinents.

  14. Interaction of the Siberian craton and Central Asian Orogenic Belt (CAOB) recorded by detrital zircons from Transbaikalia

    NASA Astrophysics Data System (ADS)

    Powerman, V.; Shatsillo, A.; Chumakov, N.; Kapitonov, I.; Hourigan, J. K.

    2015-12-01

    The goal of this study is to pinpoint the beginning of interaction of two gigantic crustal structures: the Siberian Craton and the Central Asian Orogenic Belt (CAOB). We hypothesize that the beginning of convergence should be recorded in the Neoproterozoic passive margin strata of Siberian Craton by the first appearance of extraregional Neoproterozoic zircons. In order to test this hypothesis, we have acquired U-Pb zircon age distributions from twelve Neoproterozoic clastic rocks from the Baikal-Patom margin of Siberia and one sample from the volcaniclastic Padrinsky Group that was deposited atop accreted CAOB crust. Stratigraphically lower strata from the Siberian margin yield Archean - Paleoproterozoic detrital zircon ages, which are similar to, and probably derived from the Siberian Precambrian craton. A few extra-regional Mesoproterozoic grains are also present. The provenance shift happens in the upper portion of the section and is marked by a strong influx of extra-regional Neoproterozoic sediments. The youngest grains of 610 Ma constrain the sedimentation age and confine the timing of interaction between CAOB and Siberia in this region. Neoproterozoic zircons also dominate the overlying sedimentary unit, suggesting the continuance of the convergence. The coeval volcanoclastic unit on the CAOB side has a similar U-Pb detrital age distribution, strengthening the provenance link. Analysis of the local tectonics suggests that the beginning of accretion might have started even before the first appearance of Neoproterozoic zircon: during the development of a regional unconformity, capped by 635 Ma (?) "Snowball Earth" tillites of Dzhemkukan Fm. The absence of Neoproterozoic zircons in Dzhemkukan Fm. is probably explained by a thin-skinned tectonics that did not result in massive orogenesis . Our data are in good correlation with other Neoproterozoic sedimentary basins of southern Siberian Craton, including Cisbaikalia and Bodaibo Synclinorium.

  15. African-Americans and Alzheimer's

    MedlinePlus

    ... Share Plus on Google Plus African-Americans and Alzheimer's alz.org | IHaveAlz Introduction 10 Warning Signs Brain ... African-Americans are at a higher risk for Alzheimer's disease. Many Americans dismiss the warning signs of ...

  16. Biomarkers in Archaean banded iron formations : examples from Pilbara and Dhawar Craton

    NASA Astrophysics Data System (ADS)

    Orberger, B.; Pinti, D. L.; Cloquet, C.; Hashizume, K.; Soyama, H.; Jayananda, M.; Wirth, R.; Gallien, J. P.; Massault, M.; Rouchon, V.

    The origin of Archeaen banded iron formations (BIF) and the role of biosphere in Fe precipitation is still highly debated. In order to elucidate these processes, detailed mineralogical and textural analyses combined with δ 15 N, δ 56 Fe and δ 13 C data were obtained on Fe-oxide bands from Marble Bar chert Unit (MB, 3.46 Ga, Pilbara craton, W. Australia) and a BIF from the Bababudan Group (BG, 2.7-2.9 Ga, Dhawar Craton, Southern India). Both samples are composed of alternating quartz and Feoxide bands with wavy micro-textures. CI-normalized REE patterns show that MB reflects hydrothermal fluid/basalt interactions, while BG precipitated from a hydrothermal fluid/seawater mixture. In MB, nano-cristalline hematite replaced magnetite, Mgcalcite and Fe-sulfides producing a matlike surface, preserving nanometric N-bearing amorphous carbon nodules. Measured C/N ratios (2.3 to 52) are typical of Precambrian organic matter. The δ 56 Fe of -0.40±0.02% suggests MOR-hydrothermal fluids as a Fe-source, while a δ 15 N of +7.4±0.4% is compatible with nitrification- denitrification processes and δ 13 C of -19.9±0.1% support an organic origin. BG is composed of intergrown magnetite and hematite. Disseminated grunerite and magnetite grew during low T metamorphism. Fe-oxide spherules compose vermicular- filaments that nucleated perpendicular to quartz surfaces. Fe-oxide spherule bunches are perfectly preserved in the silica bands forming micrometric mats, which contain heterogeneously distributed N (˜0.09at.%) and C (0.51 at.%, C/N=5.73). Bulk δ 13 C of -15.35%±0.10 points to an organic origin for C. The ?56 Fe in Fe and Si layers (0.75% to 2.16%) is compatible with a chemical precipitation for BIF. A negative correlation between ?56 Fe and the Th/U ratio suggests that Fe isotopic variations are related to fluid circulation and re-precipitation of Fe-oxides. High ?15 N, on one Feoxide layer, of +21.8±0.7%, corresponds to that observed for Archeaen BIFs and may be related

  17. Synsedimentary deformation and the paleoseismic record in Marinoan cap carbonate of the southern Amazon Craton, Brazil

    NASA Astrophysics Data System (ADS)

    Soares, Joelson Lima; Nogueira, Afonso César Rodrigues; Domingos, Fábio; Riccomini, Claudio

    2013-12-01

    Event Layers in Neoproterozoic cap carbonates of Brazil's southwestern Amazon Craton record post-Marinoan synsedimentary seismicity. The 35 m-thick cap carbonates overlie glaciogenic sediments related to the Marinoan glaciation (635 Ma) and are comprised of two units: the lower cap consists of dolomite (˜15 m thick) and the upper cap is limestone (˜25 m thick). The cap dolomite includes pinkish crystalline dolostone with even parallel lamination, stratiform stromatolites, eventual tube structures and megaripple bedded peloidal dolostone interpreted as shallow (euphotic) platform deposits. The cap limestone onlaps the cap dolomite and consists of red marl, gray to black bituminous lime mudstone, bituminous shale with abundant calcite crystal fans (pseudomorphs after aragonite) and even parallel lamination interpreted as moderately deep to deep platform deposits. Five successive events of synsedimentary deformation were recognized in the cap carbonates exposed at Mirassol d'Oeste and Tangará da Serra, in Central Brazil: Event 1 - large to small-scale load cast structures in the contact between dolostones and glaciogenic sediments; Event 2 - stromatolitic lamination truncated by tube structures; Event 3 - vertical to subvertical fractures and faults, and large-scale synclines and anticlines with chevron folds; Event 4 - conglomerate and breccia filling neptunian dykes limited by undeformed beds; and Event 5 - slump and sliding deposits found only in the upper part of the cap limestone. Event 1 was produced by hydroplastic dynamics likely induced by isostatic rebound during ice cap melting in the final stages of the Marinoan glaciation. Events 2 and 5 are autocyclic in nature, and related to depositional processes. Event 2 is linked to fluid and methane escape from organic degradation of microbial mats and domes that formed tubestones; Event 5 is associated to collapse and sliding/slumping in the platform and slope. The reliable orientations of synsedimentary faults

  18. Paleoproterozoic volcanism in the southern Amazon Craton (Brazil): insight into its origin and deposit textures

    NASA Astrophysics Data System (ADS)

    Roverato, Matteo; Juliani, Caetano

    2014-05-01

    The Brazilian Amazon craton hosts a primitive volcanic activity that took place in a region completely stable since 1.87 Ga. The current geotectonic context is very different from what caused the huge volcanism that we are presenting in this work. Volcanic rocks in several portions of the Amazon craton were grouped in the proterozoic Uatumã supergroup, a well-preserved magmatic region that covers an area with more than 1,200,000 km2. In this work one specific region is considered, the southwestern Tapajos Gold province (TGP) that is part of the Tapajós-Parina tectonic province (Tassinari and Macambri, 1999). TGP consists of metamorphic, igneous and sedimentary sequences resulted from a ca. 2.10-1.87 Ga ocean-continent orogeny. High-K andesites to felsic volcanic sequences and plutonic bodies, andesitic/rhyolitic epiclastic volcanic rocks and A-type granitic intrusions form part of this volcanism/plutonism. In this work we focus particularly our attention on welded, reomorphic and lava-like rhyolitic ignimbrites and co-ignimbrite brecchas. Fiamme texture of different welding intensity, stretched obsidian fragments, "glassy folds", relict pumices, lithics, rotated crystals of feldspars, bipiramidal quarz, and devetrification spherulites are the common features represented by our samples. Microscopical images are provided to characterize the deposits analyzed during this preliminary research. The lack of continuum outcrops in the field made more difficult the stratigraphic reconstruction, but the superb preservation of the deposits, apparently without any metamorphic evidences (not even low-grade), permits a clearly description of the textures and a differentiation between deposits. A detailed exploration of this ancient andesitic and rhyolitic volcanic activity could contribute greatly to the knowledge of the Amazon territory and in particular for the recognition of the various units that form the supergroup Uatumã, especially in relation to different eruptive

  19. Archaean triple junction as a plumbing system: an example from the eastern Pilbara craton (Western Australia)

    NASA Astrophysics Data System (ADS)

    Thébaud, N.; Rey, P.; Philippot, P.

    2003-04-01

    It is largely accepted that the development of Archaean ore deposits is related to the mobilisation of fluids through the crust, their channelling along shear zones and concentration in domains bounded by zones of low permeability. In many Archaean cratons a number of modern studies argue that the regional structure, together with the characteristics of metamorphism and plutonism are compatible with the gravity-driven exhumation of granitic domes and sagduction of greenstone basins. Structural mapping in the Pilbara confirms that strain, in Archaean domes and basins, displays a large but coherent spatial variability. Planar fabric develops in domal position, planar-linear fabric develops at the contact between two granitic domes, and intense linear and vertical fabric develops at foliation triple junction where three or more granitic domes interact. We believe this network is the backbone element of an Archaean plumbing system that has channeled and concentrated ore bearing fluids. The Warawoonna syncline in the East Pilbara craton (Western Australia) offers a horizontal section through one spectacular example of triple junction. On the regional scale we have recognized a main drain along a shear zone characterized by a hydraulic breccia. A detailed sampling has been conducted across this breccia and bulk stable isotope (d18O) analyses conducted on mafic, ultramafic and felsic rocks show significant variations in its vicinity that is most likely the signature of fluid rock interaction. At the outcrop scale, the Warawoonna syncline is pervasively intruded by a few quartz veins generations one of which is oriented at high angle to the direction of maximum finite stretching. Fluid inclusions study (micro-thermometry, RAMAN spectroscopy) has been conducted on quartz veins. These preliminary analyses show primary and secondary populations of fluid inclusions with contrasted composition of the trapped phases (e.g. H2O-CO2, H2O, H2O-CO2-CH4…). This is consistent with a

  20. Metasomatic Control of Water in Garnet and Pyroxene from Kaapvaal Craton Mantle Xenoliths

    NASA Technical Reports Server (NTRS)

    Peslier, Anne H.; Woodland, Alan B.; Bell, David R.; Lazarov, Marina; Lapen, Thomas J.

    2012-01-01

    Fourier transform infrared spectrometry (FTIR) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) were used to determine water, rare earth (REE), lithophile (LILE), and high field strength (HFSE) element contents in garnet and pyroxene from mantle xenoliths, Kaapvaal craton, southern Africa. Water enters these nominally anhydrous minerals as protons bonded to structural oxygen in lattice defects. Pyroxene water contents (150-400 ppm in clinopyroxene; 40-250 ppm in orthopyroxene) correlate with their Al, Fe, Ca and Na and are homogeneous within a mineral grains and a xenolith. Garnets from Jagersfontein are chemically zoned for Cr, Ca, Ti and water contents. Garnets contain 0 to 20 ppm H2 Despite the fast diffusion rate of H in mantle m inerals, the observations above indicate that the water contents of mantle xenolith minerals were not disturbed during kimberlite entrainment and that the measured water data represent mantle values. Trace elements in all minerals show various degrees of light REE and LILE enrichments indicative of minimal to strong metasomatism. Water contents of peridotite minerals from the Kaapvaal lithosphere are not related to the degree of depletion of the peridotites. Instead, metasomatism exerts a clear control on the amount of water of mantle minerals. Xenoliths from each location record specific types of metasomatism with different outcomes for the water contents of mantle minerals. At pressures . 5.5 GPa, highly alkaline melts metasomatized Liqhobong and Kimberley peridotites, and increased the water contents of their olivine, pyroxenes and garnet. At higher pressures, the circulation of ultramafic melts reacting with peridotite resulted in co-variation of Ca, Ti and water at the edge of garnets at Jagersfontein, overall decreasing their water content, and lowered the water content of olivines at Finsch Mine. The calculated water content of these melts varies depending on whether the water content of the peridotite

  1. Magnesium isotopic heterogeneity across the cratonic lithosphere in eastern China and its origins

    NASA Astrophysics Data System (ADS)

    Wang, Ze-Zhou; Liu, Sheng-Ao; Ke, Shan; Liu, Yi-Can; Li, Shu-Guang

    2016-10-01

    Available data in the literature have demonstrated a broad magnesium (Mg) isotope range for mantle and lower continental crustal rocks, implying an isotopically heterogeneous continental lithosphere, but its origin has not been thoroughly understood. Here, to investigate the primary cause of lithospheric Mg isotopic heterogeneity, we report major-trace elements, Sr and Mg isotope data for thirty deep-seated mafic xenoliths, which sampled different lithospheric depths in the southeastern North China Craton (NCC). The xenoliths are classified into three types based upon petrology and mineralogy, sampling from middle continental crust (Group I), lower continental crust (Group II) and lithospheric mantle (Group III), respectively. The Group I xenoliths have mantle-like to slightly high δ26Mg values (- 0.32 ‰ to + 0.01 ‰), whereas some of the Group II xenoliths have very low δ26Mg values (- 0.93 ‰ to - 0.07 ‰), reflecting substantial reaction with intracrustal carbonate-derived fluids. Combined with data in the literature, the results suggest that the Mg isotopic composition of the lower continental crust is much more heterogeneous and lighter on average relative to the middle continental crust. Except for one sample, the Group III xenoliths have extremely low δ26Mg values (- 1.23 ‰ to - 0.73 ‰), the lightest among values already reported for mantle-derived rocks including peridotites and basalts. They also have highly variable 87Sr/86Sr ratios, of 0.70387 to 0.71675. The covariation of Mg and Sr isotopes in Group III xenoliths can be explained by Mg and Sr isotopic exchange reactions during mantle metasomatism, implying that the sub-continental mantle has been significantly modified by fluids derived from recycled carbonate-pelite bearing oceanic crust. Together with the metasomatism age of ∼400 Ma obtained for one Group III xenolith, the results provide new evidence for the presence of extremely low-δ26Mg rocks in the lithosphere and indicate ancient

  2. Characterizing azimuthal anisotropy at the mid-lithospheric discontinuity in the Superior and Wyoming Cratons

    NASA Astrophysics Data System (ADS)

    Ford, H. A.; Long, M. D.; Wirth, E. A.

    2015-12-01

    The recent observation of a ubiquitous mid-lithospheric discontinuity (MLD) within tectonically stable Proterozoic- to Archean-aged lithosphere has led to questions about the formation of the MLD and the continental lithosphere. The MLD has been typically imaged as a drop in velocity from Sp receiver functions, but establishing a single mechanism for a global negative velocity gradient within stable continental lithosphere has proven difficult. One proposed mechanism, azimuthal anisotropy, has been suggested however, it is unclear how such anisotropy could produce a consistently negative phase in Sp receiver functions. More fundamentally, it is unclear how lithospheres with different tectonic histories would all have a boundary in anisotropy located at similar depths. To better understand the relationship between the MLD, anisotropy and tectonic history we have calculated radial and transverse component Ps receiver functions for seven permanent US Array stations within the Superior and Wyoming Cratons. The two cratons are similar in age and collided during the Trans-Hudson orogen at ~1.8 Ga, forming the core of present day Laurentia. The receiver functions were generated using a multi-taper cross correlation technique and events were limited to distances of 30 to 100°, and magnitudes greater than 5.8. Seismograms were inspected for data quality; receiver function results were binned as functions of back azimuth and epicentral distance. At stations DGMT, EGMT, LAO, and LKWY, a large amplitude negative phase is observed 8-10 seconds after the direct P arrival on the radial component. This arrival corresponds to a depth of approximately 80 to 100 km and is interpreted to be the MLD. Preliminary analysis of the transverse component finds significant energy within the same time window as the radial component MLD with changes in polarity observed in some instances. Further work is needed before the character of these polarity changes, and their associated anisotropy

  3. The record of time in cratonic interior strata: Does exceptionally slow subsidence necessarily result in exceptionally poor stratigraphic completeness?

    USGS Publications Warehouse

    Runkel, Anthony C.; Miller, J.F.; McKay, R.M.; Palmer, A.R.; Taylor, John F.

    2008-01-01

    A newly constructed a high-resolution chronostratigraphic and lithostratigraphic framework for the Upper Cambrian and Lower Ordovician Sauk Sequence in the cratonic interior of North America provides insight into the long-standing question of how time is recorded in sedimentary packages deposited in shallow epeiric seas across regions with exceptionally slow subsidence. It reveals that time is recorded in these strata in a manner fundamentally similar to the way it is in a number of nearshore marine-dominated sedimentary packages that were deposited under conditions of markedly higher subsidence rates. The principal consequence of slow subsidence in the cratonic interior appears largely to be a pronounced shingling of chronostratigraphic units perpendicular to depositional strike. An evaluation of relative stratigraphic completeness of the Upper Cambrian and Lower Ordovician of this region suggests that a number of routine interpretations and assumptions must be re-evaluated. Our results are inconsistent with the common interpretation that: (1) cratonic interior sedimentary packages are exceptionally stratigraphically incomplete; and (2) that conditions of very slow subsidence and a bathymetrically shallow shelf by themselves preclude deposition of a relatively complete record of time. In refuting these conventional assumptions, our conclusions have implications for a variety of approaches that require a fundamental understanding of the stratigraphic record of time, such as efforts to construct eustatic sea level curves and evaluations of the fossil record of evolution.

  4. Riphean rifting and major Palaeoproterozoic crustal boundaries in the basement of the East European Craton: geology and geophysics

    NASA Astrophysics Data System (ADS)

    Bogdanova, S. V.; Pashkevich, I. K.; Gorbatschev, R.; Orlyuk, M. I.

    1996-12-01

    The East European Craton, which occupies the northeastern half of the European continent, is characterized by the presence of giant, even transcratonic systems of Riphean (Meso- to Neoproterozoic) rifts (aulacogens). The largest of these Riphean rift systems tend to follow the course of Palaeoproterozoic sutures and junction zones that subdivide the craton into three crustal segments with autonomous development histories. These segments, named Fennoscandia, Volgo-Uralia and Sarmatia, are marked by distinct T-MAGSAT anomalies. Genetically, the intersegment junction zones represent different types of collisional and accretional interaction between the component crustal segments of the craton. In accordance with this, the Riphean rift systems vary in width, the presence or absence of Moho uplifts, and other properties like, for instance, heat flow values. However, all of them follow belts of weakly magnetized crust studded with local magnetic maxima due to the presence of mafic rocks associated with the rifting. The rift system-intersegment junction zone relationships, in consequence, similarly represent a range of variation and cannot be referred to one single model.

  5. English as an African Language.

    ERIC Educational Resources Information Center

    Desai, Gaurav

    1993-01-01

    Discusses the role of the English language in postcolonial African literature, focusing on the politics of language, "Africanized" English, and the social languages used in Chinua Achebe's novels and concludes that English today is as much an African language as a British or American one. (Contains 37 references.) (MDM)

  6. The Struggles over African Languages

    ERIC Educational Resources Information Center

    Maseko, Pam; Vale, Peter

    2016-01-01

    In this interview, African Language expert Pam Maseko speaks of her own background and her first encounter with culture outside of her mother tongue, isiXhosa. A statistical breakdown of South African languages is provided as background. She discusses Western (originally missionary) codification of African languages and suggests that this approach…

  7. The Hlagothi Complex: The identification of fragments from a Mesoarchaean large igneous province on the Kaapvaal Craton

    NASA Astrophysics Data System (ADS)

    Gumsley, A. P.; de Kock, M. O.; Rajesh, H. M.; Knoper, M. W.; Söderlund, U.; Ernst, R. E.

    2013-08-01

    In this paper, we present geochronological, geochemical and palaeomagnetic results from the Hlagothi Complex and a NW-trending dolerite dyke swarm on the southeastern region of the Kaapvaal Craton in northern KwaZulu-Natal, South Africa. The Hlagothi Complex consists of layered sills of meta-peridotite, pyroxenite and gabbro intruding into the Pongola Supergroup. U-Pb baddeleyite ages on the Hlagothi Complex and a NW-trending dyke of 2866 ± 2 Ma and 2874 ± 2 Ma, respectively, reveal a ca. 2.87 Ga magmatic event on the southeastern Kaapvaal Craton. The geochemical signature of the Hlagothi Complex recognises two discrete groupings, with a magmatic source that is chemically distinct from those of the older rift-related Nsuze and Dominion groups. Additional units on the Kaapvaal Craton can be linked with this new ‘Hlagothi' event based on spatial and temporal association, and geochemistry: 1) the Thole Complex, 2) parts of the Usushwana Complex, and 3) flood basalts within the Mozaan Group and Central Rand Group. The association between all these units suggests a previously unrecognised large igneous province in the southeastern Kaapvaal Craton. Our palaeomagnetic data identifies a possible primary magnetisation within the least-altered lithologies of the Hlagothi Complex (with a virtual geographic pole at 23.4°N, 53.4°E, dp = 8.2° and dm = 11.8°). The bulk of samples however, displayed two episodes of remagnetisation. These are likely to be associated with 2.85 to 2.75 Ga aged granitoids across the southeastern Kaapvaal Craton, and tectonic activity in the nearby Meso- to Neoproterozoic Namaqua-Natal mobile belt. A short-lived (≤ 8 Ma) mantle plume is proposed to have caused the ca. 2.87 Ga magmatism, and also may well have controlled sedimentation within the Pongola-Witwatersrand basin. Volcanism during uplift would have been fed through a series of feeder dykes and sills, of which the Hlagothi Complex and NW-trending dykes are part of.

  8. Deposition and diagenesis of a cratonic Silurian platform reef, Pipe Creek Jr. , Indiana

    SciTech Connect

    Simo, A.; Lehmann, P.

    1988-02-01

    Petrographic and geochemical characteristics of the Pipe Creek Jr. paragenesis record the stratigraphic and burial evolution of the cratonic Silurian platform of Indiana during Late Silurian to Pennsylvanian. A variety of several diagenetic fluids acting over geological time affected the reef. The paragenetic sequence is as follows: (1) precipitation of turbid, fibrous, blotchy cathodoluminescent (CL) cement; (2) dolomitization of mud-rich facies; (3) precipitation of clear, zoned CL equant calcite cements; (4) fracturing and karst formation, partially filled by geopetal silt and sandstone; (5) precipitation of clear, dull CL, ferroan to nonferroan equant calcite cement, ferroan dolomite overgrowth and equant dolomite cement in moldic porosity, caves and fractures; (6) microdissolution and hydrocarbon emplacement; and (7) stylolitization. The New Albany Shale was both the hydrocarbon source and top seal to the fossil Pipe Creek Jr. oil field with original oil in place estimated at 11 million bbl. The level of organic metamorphism of the New Albany Shale, the oil residue, and the two-phase fluid inclusions in the burial cements suggest that sediments accumulated on the platform throughout Mississippian time.

  9. Deposition and diagenesis of a cratonic Silurian platform reef, Pipe Creek Jr. , Indiana

    SciTech Connect

    Simo, A.; Lehmann, P.

    1988-01-01

    Petrographic and geochemical characteristics of the Pipe Creek Jr. paragenesis record the stratigraphic and burial evolution of the cratonic Silurian platform of Indiana during Late Silurian to Pennsylvanian. A variety of several diagenetic fluids acting over geological time affected the reef. The paragenetic sequence is as follows: (1) precipitation of turbid, fibrous, blotchy cathodoluminescent (CL) cement; (2) dolomitization of mud-rich facies; (3) precipitation of clear, zoned CL equant calcite cements; (4) fracturing and karst formation, partially filled by geopetal silt and sandstone; (5) precipitation of clear, dull CL, ferroan to nonferroan equant calcite cement, ferroan dolomite overgrowth and equant dolomite cement in moldic porosity, caves and fractures; (6) microdissolution and hydrocarbon emplacement; and (7) stylolitization. Carbonate grew and fibrous cements precipitated in an open marine environment. During Late Silurian an increasingly restricted environment stopped reef growth and dolomite replaced mud-rich faces. The reefs were then subaerially exposed and two meteoric cement sequences, non-luminescent to bright luminescent, precipitated prior to Mid-Devonian fracture-controlled karsting. Caves and fractures crosscut former cement stages and were filled by sandstones. Later, the platform was buried by the late Mid-Devonian organic-rich New Albany Shale, and clear, dull CL calcite cement and ferroan dolomite precipitated. Hydrocarbon migration postdates all cements and created minor moldic porosity and predates stylolitization.

  10. Upper Devonian-Tournaisian facies and oil resources of the Russian craton's eastern margin

    SciTech Connect

    Ulmishek, G.F.

    1987-01-01

    Upper Devonian-Tournaisian facies on the Russian craton's eastern margin indicate deposition in two distinct paleomorphic environments: bathimetrically expressed basins and shallow platforms. Shallow-water carbonate sedimentation persisted on the platforms, and black, thin-bedded, organic-rich shales and limestones of the Domanik facies were deposited in stagnant basins that stretched more than 2000 km from the Arctic Ocean to the Caspian Sea. Intermittently introduced clastic material and detrital carbonates formed progradational shelves. Barrier reefs along platform edges, atolls and reef mounds on basin margins, and smaller patch reefs on platforms were abundant. The basins were finally filled with Tournaisian (in the Timan-Pechora province) or basal Visean (in the Volga-Ural province) clastics. The North Caspian deep-water basin survived until the end of Early Permian time when it was filled with a thick salt formation. The organic-rich Domanik facies is the major source rock in the Volga-Ural, Timam-Pechora, and North Caspian petroleum provinces. About one-third of oil reserves occurs predominantly in structural traps in Middle Devonian-lower Frasnian clastics that directly underlie Domanik rocks. Most of the remaining two-thirds is found in reefs, especially in drape structures over the reefs in Tournaisian carbonates and basal Visean clastics. 61 refs., 16 figs.

  11. Using Ocean Tidal Load Response to Explore the Elastic Structure of the Amazonian Craton

    NASA Astrophysics Data System (ADS)

    Martens, H. R.; Simons, M.; Rivera, L. A.; Owen, S. E.; Ito, T.

    2014-12-01

    We investigate ocean tidal load response in South America using observations of GPS displacements from Brazil, Argentina, and Uruguay. Spatial variations in the tidal loading response allow us to constrain absolute ranges of density and the two elastic moduli through the regional crust and upper mantle. We process 30-second GPS data using the GIPSY-OASIS II software to obtain position estimates every 5-minutes, with special attention paid to removing tropospheric delay effects. We then extract tidal loading response signals from multiple years of processed GPS time series using generalized harmonic analysis techniques, whereby satellite modulation corrections and the astronomical argument are updated at each epoch. To compare with our observations, we construct a range of forward models by convolving modern ocean tidal loading models (e.g., FES2012, TPX08-Atlas) with Greens functions for Earth structure. The development of our own load Love number and Greens function computation code provides us with the facility to explore a wide range of 1D, layered elastic Earth models. Finally, we convert our forward modeling methods into a Bayesian inversion framework to explore the range of density and elastic structural models for the Amazonian Craton that are consistent with our observations.

  12. Paleoproterozoic felsic volcanism of the Tapajós Mineral Province, Southern Amazon Craton, Brazil

    NASA Astrophysics Data System (ADS)

    Roverato, M.; Giordano, D.; Echeverri-Misas, C. M.; Juliani, C.

    2016-01-01

    Amazonian rocks record one amongst the most complete and best-preserved Paleoproterozoic magmatic episodes on Earth. The present contribution documents the extremely well preserved paleoproterozoic architecture of a series of felsic rocks found in the Tapajós Mineral Province (TMP), located in the western part of Pará State, southern Amazon Craton, north of Brazil. These rocks are the first to be investigated to comprehend, based on their textural evidences, their emplacement mechanisms. Textural characterization allowed to identify three main facies with, as following reported, 1) chaotic ("Breccia") group, 2) eutaxitic ("Eutax") group and 3) parataxitic ("Paratax") group vitrophyric textures. Given the superb preservation of our samples, the investigated rocks are grouped, according to their grade of welding, into a wide variety of lithofacies from very low-grade to high-grade and rheomorphic ignimbrites. In the "Paratax group" strong similarities with banding in lava flows are observed. Based on the presented data we discuss the effusive or explosive origin of the observed flow mechanisms.

  13. Depth-dependent extension, two-stage breakup and cratonic underplating at rifted margins.

    PubMed

    Huismans, Ritske; Beaumont, Christopher

    2011-05-01

    Uniform lithospheric extension predicts basic properties of non-volcanic rifted margins but fails to explain other important characteristics. Significant discrepancies are observed at 'type I' margins (such as the Iberia-Newfoundland conjugates), where large tracts of continental mantle lithosphere are exposed at the sea floor, and 'type II' margins (such as some ultrawide central South Atlantic margins), where thin continental crust spans wide regions below which continental lower crust and mantle lithosphere have apparently been removed. Neither corresponds to uniform extension. Instead, either crust or mantle lithosphere has been preferentially removed. Using dynamical models, we demonstrate that these margins are opposite end members: in type I, depth-dependent extension results in crustal-necking breakup before mantle-lithosphere breakup and in type II, the converse is true. These two-layer, two-stage breakup behaviours explain the discrepancies and have implications for the styles of the associated sedimentary basins. Laterally flowing lower-mantle cratonic lithosphere may underplate some type II margins, thereby contributing to their anomalous characteristics.

  14. Density heterogeneity of the cratonic mantle and dynamic topography in southern Africa

    NASA Astrophysics Data System (ADS)

    Artemieva, Irina; Vinnik, Lev

    2016-04-01

    An unusually high topography in southern Africa may be caused by the dynamic support of the mantle below the lithosphere base and/or by a low density (high depletion) of the cratonic lithospheric mantle. We use free-board constraints to examine the relative contributions of the both factors to surface topography and present the model of density structure of the lithospheric mantle in southern Africa. The results indicate that 0.5-1.0 km of topography requires contribution from the sublithospheric mantle because it cannot be explained by the lithosphere structure within the petrologically permitted range of mantle densities. We propose that this additional topography may be associated with the low-density region below the depth of isostatic compensation (LAB). A likely candidate is the low velocity layer between the lithospheric base and the mantle transition zone, where a temperature anomaly of 100-200 deg may produce the required extra contribution to regional topographic uplift. The calculated lithospheric mantle density values are in an overall agreement with xenolith-based data for lithospheric terranes of different ages and show an overall trend in mantle density increase from Archean to younger lithospheric terranes. A significant anomaly in mantle depletion beneath the Limpopo belt and the Bushveld Complex may result from regional melt-metasomatism. Density anomalies in the lithospheric mantle show an overall inverse correlation with seismic Vp, Vs velocities at 100-150 km depth; however, density-velocity relationship is strongly non-unique. Manuscripts in revision, Gondwana Research (2016)

  15. Revisiting the stratigraphy of the Mesoproterozoic Chhattisgarh Supergroup, Bastar craton, India based on subsurface lithoinformation

    NASA Astrophysics Data System (ADS)

    Mukherjee, Arunangshu; Ray, Ranjan Kumar; Tewari, Dinesh; Ingle, Vijay Kumar; Sahoo, Bikram Kumar; Y Khan, M. W.

    2014-04-01

    In the last 10 years, several teams of geologists from different institutions in India and abroad have vigorously investigated the Chhattisgarh basin (Bastar craton, India). Based on the new results and the lithologs of more than 350 water wells, resistivity and gamma-ray logs, and extensive geological traverses, we present a revised geological map, relevant cross sections, a new comprehensive stratigraphic column and a discussion of the new findings. Major outcomes of this revision are: (1) confirming the existence of two sub-basins (Hirri and Baradwar) and two depocentres; (2) establishing the age of the basin to be essentially Mesoproterozoic; (3) discarding the `unclassified Pandaria Formation' and classifying the package of Pandaria rock units into Chandi, Tarenga, Hirri and Maniari formations in the Hirri sub-basin; (4) accepting the `group' status of the Singhora Group and the newly proposed Kharsiya Groups in the Baradwar sub-basin; (5) establishing an intrabasinal correlation of formations; (6) reappraising the thicknesses of the different formations; and (7) finding that the geometry of the basin is `bowl-shaped', which is compatible with a sag model for the origin and evolution of the basin.

  16. Very early Archean crustal-accretion complexes preserved in the North Atlantic craton

    SciTech Connect

    Nutman, A.P. ); Collerson, K.D. )

    1991-08-01

    The North Atlantic craton contains very early Archean supracrustal rocks, orthogneisses, and massive ultramafic rocks. Most units of supracrustal rocks are dominated by mafic volcanic rocks, layered gabbros, and banded iron formations, bust some also contain abundant felsic volcanic-sedimentary rocks, quartzites, and marbles. Some quartzites contain detrital zircons derived from rocks identical in age to felsic volcanic-sedimentary rocks in these sequences (ca. 3800 Ma) and also from older (ca. 3850 Ma) sources. The presence of the ca. 3850 Ma detrital zircons suggests that the supracrustal units containing them were deposited on, or close to, ca. 3850 Ma sialic crust. The massive ultramafic rocks have chemical affinities to upper mantle rocks. The voluminous suites of tonalitic gneisses are dominated by 3700-3730 Ma bodies that intrude the supracrustal sequences, but they also locally contain components with ages between 3820 and 3920 Ma. The diverse supracrustal units, upper mantle rocks, and {ge} 3820 Ma components in the gneisses were tectonically interleaved in very early Archean convergent plate boundaries, giving rise to accretion complexes. In the period 3700-3730 Ma, voluminous tonalitic magmas produced by partial melting of predominantly mafic rocks in the base of the accretion complexes were emplaced at higher levels, forming juvenile continental crust and leaving behind a refractory lower crustal to upper mantle substrate.

  17. Geochemistry of the Madawara Igneous Complex, Bundelkhand Craton, Central India: Implications for PGE Metallogeny

    NASA Astrophysics Data System (ADS)

    Manavalan, Satyanarayanan; Singh, Surya Prakash; Balaram, Vysetti; Niranjan, Mohanty

    2015-12-01

    The southern part of the Bundelkhand craton contains a series of a E-W trending mafic and ultramafic rocks, about 40 km in length and 2-4 km wide, that occur as intrusions within the Bundelkhand Gneissic Complex (BnGC). They are confined between the Madawara- Karitoran and Sonrai-Girar shear zones. Dunite, harzburgite, lherzolite and websterite are the commonly occurring ultramafic rocks that have high MgO, Ni, Cr, PGE and low Al2O3, CaO, K2O, TiO2 and V contents, and shows peridotitic affinity. A distinct trend of crystallization from peridotite to komatiitic basalt has been inferred from geochemical plots, which also indicates the occurrence of at least two varieties among the ultramafic suite of the Madawara ultramafic complex, namely, Group I comprising dunite, spinel peridotite, harzburgite and lherzolite, and Group II consisting of pyroxenite, websterite and olivine websterite. In several places, the rocks of Group II have an intrusive relationship with Group I, and are relatively enriched in total platinum group elements (PGE ~ 300 ppb). The discrimination diagrams suggest that the PGE are enriched in low sulphur-fugacity source magma at moderate to deeper depths by high degree of partial melting of the mantle.

  18. Palaeointensity determinations on rocks from Palaeoproterozoic dykes from the Kaapvaal Craton (South Africa)

    NASA Astrophysics Data System (ADS)

    Shcherbakova, V. V.; Shcherbakov, V. P.; Zhidkov, G. V.; Lubnina, N. V.

    2014-06-01

    Palaeointensity study of the Proterozoic-Archean volcanic rocks from the Kaapvaal Craton South Africa) are reported. Palaeomagnetic study of this collection was performed earlier by Olsson et al. Electron microscope observations, thermomagnetic and hysteresis measurements indicate the presence of single-domain and pseudo-single-domain (SD-PSD) magnetite grains as the main magnetic mineral. The samples demonstrated a very good stability to heating, the electron micrograph observations revealed magnetite-ilmenite exsolution structure. Palaeointensity determinations were obtained by Coe-modified Thellier procedure. A total 58 samples from 14 sites were studied but only seven samples from one site NL28 of the Early Proterozoic age of 1.9 Ga passed palaeointensity selection criteria. Reliable palaeointensity determinations were obtained by both Thellier and Wilson methods on 18 cubes (subsamples) from site NL28 yielding rather low mean virtual dipole moment (VDM) = (2.82 ± 0.12) ×1022 Am2 which is in agreement with the suggestion of existence of the `Proterozoic dipole low period'.

  19. The Genetic Structure and History of Africans and African Americans

    PubMed Central

    Tishkoff, Sarah A.; Reed, Floyd A.; Friedlaender, Françoise R.; Ehret, Christopher; Ranciaro, Alessia; Froment, Alain; Hirbo, Jibril B.; Awomoyi, Agnes A.; Bodo, Jean-Marie; Doumbo, Ogobara; Ibrahim, Muntaser; Juma, Abdalla T.; Kotze, Maritha J.; Lema, Godfrey; Moore, Jason H.; Mortensen, Holly; Nyambo, Thomas B.; Omar, Sabah A.; Powell, Kweli; Pretorius, Gideon S.; Smith, Michael W.; Thera, Mahamadou A.; Wambebe, Charles; Weber, James L.; Williams, Scott M.

    2010-01-01

    Africa is the source of all modern humans, but characterization of genetic variation and of relationships among populations across the continent has been enigmatic. We studied 121 African populations, four African American populations, and 60 non-African populations for patterns of variation at 1327 nuclear microsatellite and insertion/deletion markers. We identified 14 ancestral population clusters in Africa that correlate with self-described ethnicity and shared cultural and/or linguistic properties. We observed high levels of mixed ancestry in most populations, reflecting historical migration events across the continent. Our data also provide evidence for shared ancestry among geographically diverse hunter-gatherer populations (Khoesan speakers and Pygmies). The ancestry of African Americans is predominantly from Niger-Kordofanian (~71%), European (~13%), and other African (~8%) populations, although admixture levels varied considerably among individuals. This study helps tease apart the complex evolutionary history of Africans and African Americans, aiding both anthropological and genetic epidemiologic studies. PMID:19407144

  20. Application of multi-dimensional discrimination diagrams and probability calculations to Paleoproterozoic acid rocks from Brazilian cratons and provinces to infer tectonic settings

    NASA Astrophysics Data System (ADS)

    Verma, Sanjeet K.; Oliveira, Elson P.

    2013-08-01

    In present work, we applied two sets of new multi-dimensional geochemical diagrams (Verma et al., 2013) obtained from linear discriminant analysis (LDA) of natural logarithm-transformed ratios of major elements and immobile major and trace elements in acid magmas to decipher plate tectonic settings and corresponding probability estimates for Paleoproterozoic rocks from Amazonian craton, São Francisco craton, São Luís craton, and Borborema province of Brazil. The robustness of LDA minimizes the effects of petrogenetic processes and maximizes the separation among the different tectonic groups. The probability based boundaries further provide a better objective statistical method in comparison to the commonly used subjective method of determining the boundaries by eye judgment. The use of readjusted major element data to 100% on an anhydrous basis from SINCLAS computer program, also helps to minimize the effects of post-emplacement compositional changes and analytical errors on these tectonic discrimination diagrams. Fifteen case studies of acid suites highlighted the application of these diagrams and probability calculations. The first case study on Jamon and Musa granites, Carajás area (Central Amazonian Province, Amazonian craton) shows a collision setting (previously thought anorogenic). A collision setting was clearly inferred for Bom Jardim granite, Xingú area (Central Amazonian Province, Amazonian craton) The third case study on Older São Jorge, Younger São Jorge and Maloquinha granites Tapajós area (Ventuari-Tapajós Province, Amazonian craton) indicated a within-plate setting (previously transitional between volcanic arc and within-plate). We also recognized a within-plate setting for the next three case studies on Aripuanã and Teles Pires granites (SW Amazonian craton), and Pitinga area granites (Mapuera Suite, NW Amazonian craton), which were all previously suggested to have been emplaced in post-collision to within-plate settings. The seventh case

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

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

  2. From Archaean oceans to Late Proterozoic cratons: The origin, evolution and preservation of the continental material on earth

    NASA Astrophysics Data System (ADS)

    Celal Şengör, A. M.

    2013-04-01

    The earth's outermost rocky shell consists of two major components: continents and oceans. Oceans are continuously generated along spreading centres and also continuously destroyed by subduction along deep-sea trenches. Because of their ephemeral nature they have an average age of 0.1 Ga. The oceanic crust has an average density of about 2.9 g/cc. It covers nearly 60% of the planet's surface, but makes up only 20% of its crust and a vanishingly small 0.00099% of its total mass. No part of the oceanic crust has any permanence: even its slightly more buoyant parts (oceanic plateaux) are ultimately subducted unless protected within a continental embayment (e.g. the Pre-Caspian Depression). The only exception to this rule may be the Tarim Block, which may be a trapped oceanic plateau of Proterozoic age now functioning as a craton. Whether the Black and the South Caspian oceans will have a similar destiny cannot now be told, although the latter is now being subducted along the Apsheron sill. Continents by contrast seem permanent parts of the crust: Their average density is 2.85g/cc and mean age is 2.0 Ga and in places the oldest rocks are dated to be older than 3.8 Ga. Some individual zircons recovered from continents in a few places are older than 4 Ga; some have yielded evidence for wet mantle melting and reworked continental crust as far back as 4,370 million years ago. In fact, there is now 176Hf/177Hf evidence consistent with the view that a volume of continental crust close to the present one may have formed by 4.4. to 4.5. Ga, but then entirely mixed back into the mantle by the beginning of the Archaean, both by a permobile subduction regime and heavy meteorite bombardment-controlled mixing! The present rock record on continents began to be kept since about 3.8 Ga ago, i.e., since the end of the heavy meteorite bombardment. The preserved Archaean tectonic record shows a great resemblance to the Turkic-type orogens of the Phanerozoic and probably formed in a

  3. On the nature and origin of highly-refractory Archean lithosphere: Petrological and geophysical constraints from the Tanzanian craton

    NASA Astrophysics Data System (ADS)

    Gibson, S. A.; McMahon, S. C.; Day, J. A.; Dawson, J. B.

    2012-12-01

    The nature and timescales of garnet formation are important to understanding how subcontinental lithospheric mantle (SCLM) has evolved since the Archean, and also to mantle dynamics, because the presence of garnet greatly influences the density of the lower lithosphere and hence the long-term stability of thick (150 to 220 km) subcratonic lithosphere. Nevertheless, the widespread occurrence of garnet in the SCLM remains one of the 'holy grails' of mantle petrology. Garnets found in mantle xenoliths from the eastern margin of the Tanzanian Craton (Lashaine) have diverse compositions and provide major constraints on how the underlying deep (120 to 160 km) mantle evolved during the last 3 billion years. Certain harzburgite members of the xenolith suite contain the first reported occurrence of pyrope garnets with rare-earth element patterns similar to hypothetical garnets proposed to have formed in the Earth's SCLM during the Archean, prior to metasomatism [Stachel et al., 2004]. These rare ultradepleted low-Cr garnets occur in low temperature (~1050 oC) xenoliths derived from depths of ~120 km and coexist in chemical and textural equilibrium with highly-refractory olivine (Fo95.4) and orthopyroxene (Mg#=96.4). These phases are all more magnesian than generally encountered in global mantle harzburgites and diamond inclusions. The ultradepleted garnets form interconnecting networks around grains of orthopyroxene which give the rocks a banded appearance: we propose that the increase in pressure associated with cratonization may have caused isochemical exsolution of ultradepleted garnet from orthopyroxene. These unique garnets have not previously been identified in global suites of mantle xenoliths or diamond inclusions. We believe they are rare because their low concentrations of trace elements make them readily susceptible to geochemical overprinting. This highly-refractory low-density peridotite may be common in the 'shallow' SCLM but not normally brought to the

  4. High-resolution imaging of the Kaapvaal Craton using P and S wave receiver functions and P- and S- finite frequency tomography

    NASA Astrophysics Data System (ADS)

    Youssof, M.; Thybo, H.; Levander, A.; Artemieva, I. M.

    2012-12-01

    We are investigating the Kaapvaal craton with Ps- and Sp-wave receiver functions and P- and S- finite-frequency body wave tomography. We have produced 3D common conversion point (CCP) stacks of Ps and Sp receiver functions image volumes for comparison to 3D body wave finite-frequency travel-time inversions. These two methods together provide high vertical and lateral resolution of the upper mantle beneath the Kaapvaal craton. For a conversion point d, we have calculated the ray path of converted phases PdS and Sdp and their arrival times relative to P and S, respectively, by ray tracing the 1D AK135 velocity model. About ~1000 individual S-wave receiver functions and ~7000 P-wave receiver functions have been converted to depth and laterally migrated to their conversion point in 3D using the 1D reference velocity model, with signals stacked for signal enhancement. Converted wave imaging and traveltime tomography complement one another: The tomograms provide the laterally variable velocity structures needed for image focusing. The converted wave images provide higher resolution images of both lateral and vertical impedance changes, removing some of the smearing inherent in tomographic methods. In our particular case of investigating the Kaapvaal craton, the seismic imaging has confirmed the existence of a boundary beneath the Kaapvaal at 350 km, which we speculate is an unusually deep Lithosphere-Asthenosphere Boundary (LAB). The topography of the LAB varies considerably from shallow depths (150 km) beneath the non-Archean regions to the deepest point (350 km) beneath the center of the Kaapvaal craton. In the receiver functions the LAB appears as a strong negative amplitude (in comparison to the Moho) beneath the Kaapvaal craton including under its edges, but it is a much weaker signal under the surrounding mobile belts. Our new model of Kaapvaal craton enhances the evidence for the existence of extremely deep cratonic keel, which reach 350 km with clear layered

  5. The Other African Americans.

    ERIC Educational Resources Information Center

    Matory, J. Lorand

    Black North America is ethnically and culturally diverse. It contains many groups who do not call themselves or have not always called themselves "Negro,""Black,""African-American," and so forth, such as Louisiana Creoles of color and many of the Indian tribes east of the Mississippi. There are also numerous North American ethnic groups of African…

  6. African Oral Tradition Literacy.

    ERIC Educational Resources Information Center

    Green, Doris

    1985-01-01

    Presents the basic principles of two systems for notating African music and dance: Labanotation (created to record and analyze movements) and Greenotation (created to notate musical instruments of Africa and to parallel Labanotation whereby both music and dance are incorporated into one integrated score). (KH)

  7. Elective: African Literature.

    ERIC Educational Resources Information Center

    Jenkins, Kenneth V.

    The make-up of a course in African literature for high school students is discussed. It is pointed out that the course can be constructed on already familiar lines. High school students will be able to describe clearly, for example, the relationship between environment and character or the dilemma of characters caught between traditional values…

  8. Long lasting epeirogenic uplift from mantle plumes and the origin of the Southern African Plateau

    NASA Astrophysics Data System (ADS)

    Nyblade, Andrew A.; Sleep, Norman H.

    2003-12-01

    We investigate under what conditions the present-day long wavelength anomalous elevation (˜500 m) of the southern African Plateau can be attributed to Mesozoic plume events. The anomalous, long wavelength topography of the southern African Plateau cannot be easily explained by recent heating of the upper mantle, as opposed to other areas of the African Superswell, and geomorphological studies provide few hard constraints on the timing of the uplift. A Mesozoic origin for the plateau uplift is attractive in that southern Africa experienced several large magmatic events in the Mesozoic. We formulate numerical and scaling models to investigate if plume material ponded beneath cratonic lithosphere in the Mesozoic could produce 500 m of present-day elevation. We find that starting plume heads are ineffective at producing much long lasting uplift because the material spreads into a thin layer, tens of km thick. The ponded plume material also suppresses secondary convection by having a stable boundary at its base. Over time, heat continues to flow out of the top of the lithosphere, and a net imbalance of heat flow in the lithosphere develops, resulting in subsidence. Even after the plume material has cooled to the temperature of the mantle adiabat, secondary convection supplies less heat to the lithosphere than is lost upward by conduction. The cratonic lithosphere returns to its original thermal structure on a timescale of less than 200 m.y. Even two mantle starting plume heads, one at the time of Karoo volcanism (ca.183 Ma) and the other at the time of kimberlite eruption (ca. 80-90 Ma) in our models, cannot produce much (<200 m) of the present-day anomalous elevation. However, significant uplift can be generated by plume tails, provided they linger beneath the lithosphere for ˜25-30 m.y., and if the uplift effects of Mesozoic plume heads and tails are considered together, then it is possible to account for ˜500 m of present-day elevation. Consequently, a Mesozoic

  9. High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture

    USGS Publications Warehouse

    Runkel, Anthony C.; Miller, J.F.; McKay, R.M.; Palmer, A.R.; Taylor, John F.

    2007-01-01

    Well-known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental-scale tectonism, oceanographic dynamics of epeiric seas, and eustasy. We used a multi-disciplinary approach to construct a high-resolution stratigraphic framework for lower Paleozoic strata in the cratonic interior of North America. Within this framework, these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins, settings markedly different from the cratonic interior. Parasequences, parasequence stacking patterns, systems tracts, maximum flooding intervals, and sequence-bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification. The similarity of cratonic interior and foreland basin successions in size, geometry, constituent facies, and local stacking patterns of nearshore parasequences is especially striking. This similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same, despite marked differences in tectonism, physiography, and bathymetry between the two settings. Our results do not support the widespread perception that Paleozoic cratonic interior successions are so anomalous in stratal geometries, and constitute such a poor record of time, that they are poorly suited for modern sequence stratigraphic analyses. The particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another. Thus, Paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a

  10. The Neoproterozoic Trans-Saharan/Trans-Brasiliano shear zones: Suggested Tibetan Analogs

    NASA Astrophysics Data System (ADS)

    Attoh, K.; Brown, L. D.

    2008-05-01

    The Trans-Saharan Borborema (TSB) belt is a product of the assembly of Gondwana, a supercontinent that formed from cratonic fragments derived from Rodinia and other vagrant lithospheric blocks. Recent reconstructions show the West African craton (WAC) and Congo- San-Francisco craton juxtaposed by the closure of the Brasiliano (Pharuside, Adamastor) ocean during early stages of the Pan-African orogenic cycle in northwest Gondwana. The Dahomeyide and Pharuside segments of the resulting orogenic belt preserve well- organized lithotectonic units on the eastern margin of the WAC. The foreland units consist of craton-verging nappe stacks formed from the deformed margin of the WAC and its cover rocks. The near-hinter land is underlain by granitoid gneisses postulated to represent ca 600 Ma juvenile crust, exposed in the Accra-Benin plain. Further east from the suture zone is the Nigerian province, which includes rocks that were extensively reworked apparently during the Pan-African. The Borborema province of northeastern Brasil is the correlative/ extension of Nigerian. It is underlain by rocks intensively reworked during the Brasiliano (Pan-African) orogeny and juxtaposed along a series of shear zones. A distinct feature of the TBS are these extensive shear zones, many of which are typified by dextral wrench shear. In West Africa the prominent examples extend from the Sahara to the coastline and include the Hoggar, which splays into the Kandi fault, which itself has numerous splays in Benin, Togo and southeastern Ghana. In Brazil, nearly all reconstructions show that the continuation of the Kandi Fault is the Sobral fault which is inferred to be the northern segment of the Trans-Brasiliano lineament (TBL). If correct, the TBL and its TBS extensions constitute a 4000 km long dextral shear zone, perhaps the longest coherent shear zone on earth. We suggest that the geometry of these shear zones and associated Pan African sutures have instructive analogs in the Tibet

  11. Derivation of detrital rutile in the Yaoundé region from the Neoproterozoic Pan-African belt in southern Cameroon (Central Africa)

    NASA Astrophysics Data System (ADS)

    Stendal, Henrik; Toteu, Sadrack Félix; Frei, Robert; Penaye, Joseph; Njel, Urbain Olivier; Bassahak, Jean; Nni, Jean; Kankeu, Boniface; Ngako, Vincent; Hell, Joseph Victor

    2006-04-01

    Rutile, as an important component in alluvial or eluvial heavy mineral deposits, is known in southern Cameroon. These deposits are underlain by the Neoproterozoic low- to high-grade Yaoundé Group. Geochemical, thermometric, fluid inclusion and Pb isotopic studies of the rutile from alluvial and eluvial concentrates and from medium-grade micaschist from the nearby Yaoundé region permit the following conclusions: (1) alluvial and eluvial rutile of the Yaoundé region are derived from the degradation of metapelites, metamafic rocks and pegmatites of the nearby Yaoundé Group; (2) rutile in the Yaoundé Group formed during the Pan-African metamorphism, or was inherited as detrital rutile from a ˜900 Ma source. The study also shows that the rutile can be used to trace the history of the Pan-African belt north of the Congo craton.

  12. LA-SF-ICP-MS zircon U-Pb geochronology of granitic rocks from the central Bundelkhand greenstone complex, Bundelkhand craton, India

    NASA Astrophysics Data System (ADS)

    Verma, Sanjeet K.; Verma, Surendra P.; Oliveira, Elson P.; Singh, Vinod K.; Moreno, Juan A.

    2016-03-01

    The central Bundelkhand greenstone complex in Bundelkhand craton, northern India is one of the well exposed Archaean supracrustal amphibolite, banded iron formation (BIF) and felsic volcanic rocks (FV) and associated with grey and pink porphyritic granite, tonalite-trondhjemite-granodiorite (TTG). Here we present high precision zircon U-Pb geochronological data for the pinkish porphyritic granites and TTG. The zircons from the grey-pinkish porphyritic granite show three different concordia ages of 2531 ± 21 Ma, 2516 ± 38 Ma, and 2514 ± 13 Ma, which are interpreted as the best estimate of the magmatic crystallization age for the studied granites. We also report the concordia age of 2669 ± 7.4 Ma for a trondhjemite gneiss sample, which is so far the youngest U-Pb geochronological data for a TTG rock suite in the Bundelkhand craton. This TTG formation at 2669 Ma is also more similar to Precambrian basement TTG gneisses of the Aravalli Craton of north western India and suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Aravalli craton of the North-western India.

  13. Dynamics of mantle rock metasomatic transformation in permeable lithospheric zones beneath Siberian craton

    NASA Astrophysics Data System (ADS)

    Sharapov, Victor; Sorokin, Konstantin; Perepechko, Yury

    2015-04-01

    The numerical descriptions of hydrodynamic model of two - velocity heat and mass transfer in permeable zones above the asthenospheric lenses was formulated and solved basing on the study the composition of inclusions in minerals of low crust ultra metamorphic rocks and lithospheric mantle metasomatites and estimation of thermodynamic conditions of the processes. Experimental study of influence of the simulated hot reduced gas flows on the minerals of low crust and mantle xenolith of the Siberian craton platform (SP) give the basic information for this processes. In detail: 1. Thermobarometric study of composition of inclusions in granulite and lithospheric mantle rocks beneath the diamondiferous cratons allowed to estimate the gas phase compositions during the metamorphism and metasomatism as well as products of their re equilibration during decompression. 2. Results of the pilot study of the influence of hot gas impact flows on minerals of mantle xenoliths are taken into account. This allowed to reproduce the elements and heterophase kinetics of interactions within a temperature range of about 300 to 1300o on relative to the interactions between the solid, liquid and gas phases. 3. Correct mathematical two-velocities model of fluid dynamics for compressible multiphase fluid -rock systems. 4. Numerical schemes are simulated and solved for the problems of quantitative description of 2D dynamics behavior of P and T within the permeable zone above the asthenospheric lens. 5. Quantitative description of heterophase non isothermal fluid-rock interaction within the framework of the approximation was obtained on the basis of the parallel solutions of the exchange between the ideal gas flow and solid phase according to the model of multi-reservoir reactors based on minimization of the Gibbs potential. Qualitatively the results of numerical simulation are as follows: 1) appearance in permeable zones of the any composition fluid flows from the upper mantle magma chambers

  14. Numerical simulation of the thinning of North China Craton by thermal convection

    NASA Astrophysics Data System (ADS)

    Qiao, Y.; Shi, Y.

    2011-12-01

    Several mechanisms have been proposed for North China Craton lithospheric thinning.This research based on a two-dimensional model,small-scale convection in the upper mantle-lithosphere system with depth and temperature dependent viscosity by using numerical technique to examine the mechanism of thinning by thermal convection. We use the finite difference method and MIC(marks in cell) technique to solve the equations of continuity, conservation of momentum and conservation of energy.The infinite-Prandtl-number approximation is made and the inertial term in the momentum equation is neglected. Fluid flow is assumed to be incompressible and the Boussinesq approximation is made.We do the benchmark work to test our code. For the temperature-dependent viscosity (case 2a of Blankenbach et al. [1989]),these quantities are within 1%. The error shows our code is suit for this problem.Using a 2-D upper-mantle model 700 KM deep and 700 KM wide and model resolution are 71*71 nodes.The boundary conditions are constrained in normal direction and free-slip along all boundaries,and thermal insulation condition in two sides and temperatures are assigned for top and bottom boundaries. We assume the initial temperature on the top is 273K, 1673K at the bottom, and 1573K at the bottom of the lithosphere (200KM depth) to describe the thick, cold and stable North China Craton.Then,we calculate thermal evolutions in the upper mantle if temperature at its bottom is raised due to convections or plumes in the lower mantle. Temperature at the bottom is assumed as 1773,1873,1973,and 2073K for different models to study their effects on lithospheric thinning process.The results as followed:1)we confirmed that lithosphere thinning can occur when the bottom temperature uprising intensify the mantle convection.2)we discuss two different viscosity η0=1.0e22 and η0=1.0e23. when η0=1.0e22, the lithospheric can thin from 200 to 100KM in 14.55Ma,to 103.75 KM in 14.95Ma, to 118.75KM in 15Ma, to 126.25KM

  15. Sulfide Mineralization In The Marble Bar Greenstone Belt Around Mount Edger Batholith, Pilbara Craton, Western Australia

    NASA Astrophysics Data System (ADS)

    Kitazono, S.; Nedachi, M.; Taguchi, S.

    2004-12-01

    Pilbara Craton is one of the most important regions in the world to understand the evolution of early Earth, because the geological history is well preserved through the metamorphism of low grade. Pilbara Craton, Western Australia, consists of two different tectonic components formed 3.6 and 2.8 Ga; an older Archean granite occupying the east Pilbara and greenstone belt. In the east Pilbara, the most conspicuous structures are broad domal granitoid complex separated by narrow synformal greenstone belts, and a model of continuous lithostratigraphy in the greenstones in which the dominant structures were produced by multi-stage granitoid diapirism. The Marble Bar greenstone belt is distributed around the Mount Edgar of granitoid pluton, and numerous hydrothermal gold veins are distributed in the greenstone near the boundary of pluton. Also base-metal veins and volcanogenic sedimentary type deposits are located in the same area. In this study, we examined the hydrothermal mineralization observed in the core samples of the Marble Bar greenstone belt, drilled at the Salgash area by the Archean Biosphere Drilling Project (ABDP). The Salgash drill hole is composed of tuff breccias with numerous fragments of black shale of 100 m in thickness, alternation of sandstone and shale of 40 m in thickness, basaltic lava and tuff of 30 m in thickness, and shale and sandstone of 110m in thickness with some sills of basalt and ultramafic rock. The rocks had been metamorphosed, and the grade is near the boundary between green schist and amphibolite facies. Low REE content (43 to 88 ppm), low La/Yb ratio (6.3 to 14.3), and high Eu/Eu* ratio (0.9 to 1.3) of the volcanic rocks are ordinal as the basaltic rock in Archean greenstone belts. On the other hand, these rocks show extremely high values of Cr (1500ppm), Ni (700ppm), Co (70 ppm), and Zn (600 ppm). The C isotopic ratios of carbonate in the volcanic rocks are around -3.8 permil. The clastic sediment sandstone and black shale show

  16. Palaeointensity and palaeodirection determinations of Paleoproterozoic dykes in the Kaapvaal Craton (South Africa)

    NASA Astrophysics Data System (ADS)

    Shcherbakova, V. V.; Lubnina, N. V.; Shcherbakov, V. P.; Zhidkov, G. V.

    2012-04-01

    A combined palaeodirectional and palaeointensity study of a representative collection from the Bushveld Igneous Complex from 27 dolerite dykes from the 2.9, 2.7, and 1.8 Ga age swarms radiating SE, E and NE, respectively [Olsson et al., 2010] was carried out. Conventional progressive thermal or AF demagnetization was applied to all specimens. The palaeomagnetic directions have been calculated after thermal demagnetization. The ChRMs were isolated over the temperature interval 440-590 C and their intensities amount to 95% of total NRMs. Paleopole calculate from the primary high-temperature component, separated in the 2.9 Ga SE-dykes, is close to the paleopoles, obtained by Wingate (1998) and Strik et al. (2007) for 2.78 Ga volcanics. The paleopole calculated for the 2.7 Ga age E-trending dykes of the eastern region does not correspond to any of the previously obtained Archean-Paleoproterozoic paleopoles for the Kaapvaal Craton. The paleopole calculated for some NE-trending dykes of the Black Ridge swarm in the NE region is close to the 1.87 Ga pole of the Kaapvaal Craton obtained by Hanson et al. (2004). Palaeointensity determinations were carried out on rocks from ten dykes of different ages using Thellier-Coe method with the "check-points" procedure on specimens of 1 cm in edge length cut from either drilled cores or hand samples. Rock magnetic measurements were made on sister specimens. Curie temperatures and the thermal stability of magnetic minerals were estimated from thermomagnetic heating-cooling cycles to incrementally higher temperatures Ti with a Curie balance in an external magnetic field H = 0.45 T. To assess the magnetic hardness and mineralogy of samples, measurements of magnetic susceptibility and hysteresis loop parameters were performed. The domain structure (DS) was estimated also from the thermomagnetic criterion by evaluating the tails of pTRMs. Wilson's method of palaeointensity determination based on comparison of thermodemagnetization curves

  17. The Fazenda Largo off-craton kimberlites of Piauí State, Brazil

    NASA Astrophysics Data System (ADS)

    Kaminsky, Felix V.; Sablukov, Sergei M.; Sablukova, Ludmila I.; Zakharchenko, Olga D.

    2009-10-01

    In the late 1990s, the Fazenda Largo kimberlite cluster was discovered in the Piauí State of Brazil. As with earlier known kimberlites in this area - Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos - this cluster is located within the Palaeozoic Parnaiba Sedimentary Basin that separates the São Francisco and the Amazonian Precambrian cratons. Locations of kimberlites are controlled by the 'Transbrasiliano Lineament'. The Fazenda Largo kimberlites are intensely weathered, almost completely altered rocks with a fine-grained clastic structure, and contain variable amounts of terrigene admixture (quartz sand). These rocks represent near-surface volcano-sedimentary deposits of the crater parts of kimberlite pipes. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to average kimberlite. The composition of the deep-seated material in the Fazenda Largo kimberlites is quite diverse: among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and large xenoliths of pyrope dunite. High-pressure minerals are predominantly of the ultramafic suite, Cr-association minerals (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Kimberlites from all four pipes contain rare grains of G10 pyrope of the diamond association, but chromian spinel of the diamond association was not encountered. By their tectonic position, by geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic.

  18. Cratons formation by global plume-lid tectonics in the early Earth

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Fischer, Ria; Sizova, Elena

    2016-04-01

    Modern geodynamics and continental growth are critically driven by subduction and plate tectonics, however how this tectonic regime started and what geodynamic regime was before remains controversial. Based on 2D and 3D magmatic-thermo-mechanical numerical experiments we suggest that a distinct Venus-like plume-lid tectonics regime operated on Earth before plate tectonics, which was associated with widespread tectono-magmatic heat and mass exchange between the crust and the mantle. This regime was characterized by the presence of weak internally deformable highly heterogeneous lithosphere with low topography, massive juvenile crust production from mantle derived melts, mantle-flows-driven crustal deformation, magma-assisted crustal convection and widespread development of lithospheric delamination and eclogitic drips. Both proto-continental and proto-oceanic domains were formed in this regime by a combination of eclogitic drips and ultra-slow spreading. Plume-lid tectonics also resulted in growth of hot internally convecting moderately-depleted chemically buoyant eclogite-rich proto-continental mantle layer. Later, this layer could be rapidly cooled by internal convection and consolidated to form eclogite-rich sub-continental lithospheric mantle (SCLM) domains. Numerical models also show feasibility of short-lived deep subduction of ultra-depleted eclogite-poor proto-oceanic lithosphere formed by ultra-slow spreading. Subsequent rising and accretion of this highly buoyant lithosphere to the bottom of unrelated heterogeneous crustal terrains may offer another way of Archean cratonization associated with eclogite-poor SCLM formation. Numerical models also suggest that plume-induced subduction may likely played a crucial role for making transition from global plume-lid tectonics to global plate tectonics.

  19. Neoproterozoic extension in the greater dharwar craton: A reevaluation of the "betsimisaraka suture" in madagascar

    USGS Publications Warehouse

    Tucker, R.D.; Roig, J.-Y.; Delor, C.; Amlin, Y.; Goncalves, P.; Rabarimanana, M.H.; Ralison, A.V.; Belcher, R.W.

    2011-01-01

    The Precambrian shield of Madagascar is reevaluated with recently compiled geological data and new U-Pb sensitive high-resolution ion microprobe (SHRIMP) geochronology. Two Archean domains are recognized: the eastern Antongil-Masora domain and the central Antananarivo domain, the latter with distinctive belts of metamafic gneiss and schist (Tsaratanana Complex). In the eastern domain, the period of early crust formation is extended to the Paleo-Mesoarchean (3.32-3.15 Ga) and a supracrustal sequence (Fenerivo Group), deposited at 3.18 Ga and metamorphosed at 2.55 Ga, is identified. In the central domain, a Neoarchean period of high-grade metamorphism and anatexis that affected both felsic (Betsiboka Suite) and mafic gneisses (Tsaratanana Complex) is documented. We propose, therefore, that the Antananarivo domain was amalgamated within the Greater Dharwar Craton (India + Madagascar) by a Neoarchean accretion event (2.55-2.48 Ga), involving emplacement of juvenile igneous rocks, high-grade metamorphism, and the juxtaposition of disparate belts of mafic gneiss and schist (metagreenstones). The concept of the "Betsimisaraka suture" is dispelled and the zone is redefined as a domain of Neoproterozoic metasedimentary (Manampotsy Group) and metaigneous rocks (Itsindro-Imorona Suite) formed during a period of continental extension and intrusive igneous activity between 840 and 760 Ma. Younger orogenic convergence (560-520 Ma) resulted in east-directed overthrusting throughout south Madagascar and steepening with local inversion of the domain in central Madagascar. Along part of its length, the Manampotsy Group covers the boundary between the eastern and central Archean domains and is overprinted by the Angavo-Ifanadiana high-strain zone that served as a zone of crustal weakness throughout Cretaceous to Recent times.

  20. Ultrahigh-temperature metamorphism under isobaric heating: New evidence from the North China Craton

    NASA Astrophysics Data System (ADS)

    Yang, Qiong-Yan; Santosh, M.; Tsunogae, Toshiaki

    2014-12-01

    The Khondalite Belt within Inner Mongolia Suture Zone (IMSZ) in the North China Craton (NCC) preserves evidence for extreme crustal metamorphism under ultra-high temperature (UHT) conditions at ca. 1.92 Ga, associated with the subduction-collision tectonics between the Yinshan and Ordos Blocks. Here we report a new locality in Hongsigou where cordierite- and spinel-bearing granulites record UHT metamorphism. The prograde, peak, and retrograde mineral assemblages in these pelitic granulites have been identified based on petrography and mineral chemistry as: Bt1 + Grt1 + Sil1 + Kfs1 + Pl1 + Ilm + Qtz1, Grt1 + Sil2 + Kfs2 + Pl2 + Spl + Ilm + Qtz2 + Liq, and Crd + Grt2 + Sil3 + Kfs2 + Pl2 + Ilm + Qtz2 respectively. The peak metamorphic conditions of the pelitic granulite were estimated as 930-1050 °C and 6.5-7.5 kbar based on pseudosection analysis in the system NCKFMASHTO, suggesting extreme thermal metamorphism. We report LA-ICPMS zircon U-Pb data from the granulite which show weighted mean 207Pb/206Pb age of 1881 ± 6.6 Ma, marking the timing of UHT metamorphism. Lu-Hf analyses of the zircons show εHf(t) values within a restricted range of -4.2 to 0.3 and together with Hf model ages, a Paleoproterozoic arc magmatic source is inferred for the detrital zircons. The estimated P-T path for the UHT granulite suggests isobaric heating followed by cooling and decompression along a clockwise trajectory, different from the anti-clockwise P-T paths defined in earlier studies for the 1.92 Ga UHT rocks from the IMSZ. The younger age and the isobaric heating trajectory suggest that the Hongsigou UHT rocks are related to heat input from underplated mafic magmas following continental collision.

  1. Chemical transfers along slowly eroding catenas developed on granitic cratons in southern Africa

    USGS Publications Warehouse

    Khomo, Lesego; Bern, Carleton R.; Hartshorn, Anthony S.; Rogers, Kevin H.; Chadwick, Oliver A.

    2013-01-01

    A catena is a series of distinct but co-evolving soils arrayed along a slope. On low-slope, slowly eroding catenas the redistribution of mass occurs predominantly as plasma, the dissolved and suspended constituents in soil water. We applied mass balance methods to track how redistribution via plasma contributed to physical and geochemical differentiation of nine slowly eroding (~ 5 mm ky− 1) granitic catenas. The catenas were arrayed in a 3 × 3 climate by relief matrix and located in Kruger National Park, South Africa. Most of the catenas contained at least one illuviated soil profile that had undergone more volumetric expansion and less mass loss, and these soils were located in the lower halves of the slopes. By comparison, the majority of slope positions were eluviated. Soils from the wetter climates (550 and 730 mm precipitation yr− 1) generally had undergone greater collapse and lost more mass, while soils in the drier climate (470 mm yr− 1) had undergone expansion and lost less mass. Effects of differences in catena relief were less clear. Within each climate zone, soil horizon mass loss and strain were correlated, as were losses of most major elements, illustrating the predominant influence of primary mineral weathering. Nevertheless, mass loss and volumetric collapse did not become extreme because of the skeleton of resistant primary mineral grains inherited from the granite. Colloidal clay redistribution, as traced by the ratio of Ti to Zr in soil, suggested clay losses via suspension from catena eluvial zones. Thus illuviation of colloidal clays into downslope soils may be crucial to catena development by restricting subsurface flow there. Our analysis provides quantitative support for the conceptual understanding of catenas in cratonic landscapes and provides an endmember reference point in understanding the development of slowly eroding soil landscapes.

  2. Origin of the Adventure Subglacial Trench linked to Cenozoic extension in the East Antarctic Craton

    NASA Astrophysics Data System (ADS)

    Cianfarra, P.; Salvini, F.

    2016-02-01

    The Antarctic plate occupies a unique geodynamic setting being mostly surrounded by divergent or transform margins. Major intracontinental basins and highlands characterize its bedrock, buried under the 34 Ma East Antarctic Ice Sheet (EAIS). Their formation atop of the cratonic lithosphere in the interior of East Antarctica remains a major open question. Post-Mesozoic intraplate extensional tectonic activity has been proposed for their development and is supported by this work. Here we focus on the Adventure Subglacial Trench (AST) whose origin is poorly constrained and controversial, as currently available geophysical models suggest either extensional or compressional tectonic origin. The AST is an over 250-km-long, 60-km-wide subglacial trough, elongated in the NNW-SSE direction adjacent to the westernmost flank of the Wilkes Subglacial Basin, and is parallel to regional scale alignments of magnetic and gravimetric anomalies. Geophysical campaigns allowed better definition of the AST physiography showing its typical half-graben geometry. The rounded morphology of the western flank of the AST was simulated through tectonic numerical modelling. We consider the subglacial landscape to primarily reflect a preserved relict of the tectonic processes affecting the interior of East Antarctica in the Cenozoic, due to the negligible erosion/deposition capability of the EAIS. The bedrock morphology was replicated through the activity of the listric Adventure Fault, characterized by a basal detachment at the base of the crust (34 km) and a vertical displacement of 2.5 km. This length suggests its regional/crustal importance. The predicted displacement is interpreted either as a newly formed fault or as the partial reactivation of a weaker zone along a major Precambrian crustal-scale tectonic boundary. The extensional regime in the AST is part of a more extensive 800-km long intraplate corridor characterized by nearly along-strike extension in Cenozoic times with a left

  3. Terranes in the foreland of the East European Craton imaged by the CELEBRATION 2000 experiment

    NASA Astrophysics Data System (ADS)

    Celebration 2000 Working Group; Malinowski, M.

    2003-04-01

    The results of the CELEBRATION 2000 seismic experiment help us to resolve the question of the western extent of the East European Craton (EEC) crust in SE Poland, as well as to delineate main paleozoic terranes in this region. Profiles CEL01 and CEL02 cross each other in the Holy Cross Mts. area and end in the EEC. Profile CEL03 runs through the TESZ along the border of the EEC. 2-D P-wave velocity models along those lines were obtained by 2-D ray tracing technique using package SEIS83 (Červeny and Pšencík, 1983). The model along CEL02 profile shows great contrast in the crustal structure between Upper Silesian and Malopolska terranes, separated by the Krakow - Lubliniec Zone. The Malopolska Block is characterised by Vp values similar to the rest of the TESZ (profiles CEL03, TTZ, and lines from POLONAISE'97 experiment): 5.4-5.8 km/s down to 20 km depth. We have found an unusually high velocity body (HVB) (6.8 - 7.2 km/s at 15-17 km depth) beneath CEL01 and CEL02 profiles in the Lublin Trough region, which coincides with positive gravity anomalies. Further towards the West, such velocities have been imaged on CEL03 profile - in the axis of the TESZ we have modelled a thin and elongated HVB, which might be a result of the magmatic activity along the EEC border and probably is an westward extension of the HVB from CEL01 and CEL02 lines. The Moho depth varies from 45 km in the EEC part to 35-32 km in the Malopolska Block and the Upper Silesian Block. It comes from our models that EEC lower crust seems to extend as far as Holy Cross Mts. Fault, however this view is still debated.

  4. Geological Setting of Diamond Drilling for the Archean Biosphere Drilling Project, Pilbara Craton, Western Australia

    NASA Astrophysics Data System (ADS)

    Hickman, A.

    2004-12-01

    The Archean Biosphere Drilling Project (ABDP) is a collaborative international research project conducting systematic (bio)geochemical investigations to improve our understanding of the biosphere of the early Earth. The Pilbara Craton of Western Australia, which includes exceptionally well preserved 3.52 to 2.70 Ga sedimentary sequences, was selected for an innovative sampling program commencing in 2003. To avoid near-surface alteration and contamination effects, sampling was by diamond drilling to depths of between 150 and 300 m, and was located at sites where the target lithologies were least deformed and had lowest metamorphic grade (below 300°C). The first of five successful drilling sites (Jasper Deposit) targeted red, white and black chert in the 3.46 Ga Marble Bar Chert Member. This chert marks the top of a thick mafic-felsic volcanic cycle, the third of four such cycles formed by mantle plumes between 3.52 and 3.43 Ga. The geological setting was a volcanic plateau founded on 3.72 to 3.60 Ga sialic crust (isotopic evidence). The second hole (Salgash) was sited on the basal section of the fourth cycle, and sampled sulfidic (Cu-Zn-Fe), carbon-rich shale and sandstone units separated by flows of peridotite. The third hole (Eastern Creek) was sited on the margin of a moderately deep-water rift basin, the 2.95 to 2.91 Ga Mosquito Creek Basin. This is dominated by turbidites, but the sandstones and carbon-rich shales intersected at the drilling site were deposited in shallower water. The fourth and fifth holes, located 300 km apart, sampled 2.77 to 2.76 Ga continental formations of the Fortescue Group; both holes included black shales.

  5. The seismotectonics of Southeastern Tanzania: Implications for the propagation of the eastern branch of the East African Rift

    NASA Astrophysics Data System (ADS)

    Mulibo, Gabriel D.; Nyblade, Andrew A.

    2016-04-01

    Seismicity patterns and focal mechanisms in southeastern Tanzania, determined from data recorded on temporary and permanent AfricaArray seismic stations, have been used to investigate the propagation direction of the Eastern branch of the East African Rift System southward from the Northern Tanzania Divergence Zone (NTDZ). Within the NTDZ, the rift zone is defined by three segments, the Eyasi segment to the west, the Manyara segment in the middle, and the Pangani segment to the east. Results show that most of the seismicity (~ 75%) extends to the south of the Manyara segment along the eastern margin of the Tanzania Craton, and at ~ 6-7° S latitude trends to the SE along the northern boundary of the Ruvuma microplate, connecting with a N-S zone of seismicity offshore southern Tanzania and Mozambique. A lesser amount of seismicity (~ 25%) is found extending from the SE corner of the Tanzania Craton at ~ 6-7° S latitude southwards towards Lake Nyasa. This finding supports a model of rift propagation via the Manyara segment to the southeast of the Tanzania Craton along the northern boundary of the Ruvuma microplate. However, given the limited duration of the seismic recordings used in this study, the possibility of another zone of extension developing to the south towards Lake Nyasa (Malawi) cannot be ruled out. Focal mechanisms along the boundary between the Victoria and the Ruvuma microplates and offshore southeastern Tanzania show a combination of normal and strike slip faulting indicating mainly extension with some sinistral motion, consistent with the mapped geologic faults and a clockwise rotation of the Ruvuma microplate.

  6. Cratonic roots under North America are shifted by basal drag: new evidence from gravity and geodynamic modeling

    NASA Astrophysics Data System (ADS)

    Kaban, M. K.; Petrunin, A.; Mooney, W. D.

    2013-12-01

    The impact of basal drag on the long-lived cratonic roots has been debated since the discovering of plate tectonics. Previously, evidence for a shifted mantle structure under North America was postulated from a comparison of the surface expression of the Great Meteor hotspot track versus its location at 200 km depth as inferred from seismic tomography (Eaton and Frederiksen, 2007). We present new results that are based on the integrative modeling of gravity and seismic data. The starting point is the residual gravity anomaly and residual topography, which are computed by removing of the crustal effect and of the effect of temperature variations in the upper mantle from the observed fields (Mooney and Kaban, 2010). After the temperature correction both residual fields chiefly reflect compositional density heterogeneity of the upper mantle. The residual gravity and topography are jointly inverted to determine the 3D density structure of the upper mantle. The inversion technique accounts for the fact that although these parameters are controlled by the same factors, the effect depends on depth and wavelength. Therefore, we can resolve the vertical distribution of density more reliable than by interpreting only one parameter. We found a strong negative anomaly under the North American craton, as expected for a depleted mantle. However, starting from a depth of about 200 km the depleted root is shifted west-southwest. The maximal shift reaches about 1000 km at a depth of 300 km. The direction agrees with the North American plate movement and with the anisotropy pattern in the upper mantle (e.g. Bokelmann, 2002). The results of the gravity modeling are confirmed by geodynamic modeling. The mantle flow is estimated from the density and temperature distribution derived from seismic tomography models. A 3D viscosity model is supplemented with weak boundaries based on an integrated model of plate boundary deformations. The calculated plate velocities are in a good agreement

  7. Crustal and uppermost mantle structure of the eastern margin of the Yilgarn Craton (Australia) from passive seismic data

    NASA Astrophysics Data System (ADS)

    Sippl, Christian; Tkalčić, Hrvoje; Kennett, Brian; Spaggiari, Catherine; Gessner, Klaus

    2016-04-01

    The Yilgarn Craton in Western Australia is one of the largest units of Archean lithosphere on earth. Along its southern and southeastern margin, it is bounded by the Albany-Fraser Orogen (AFO), a Paleo- to Mesoproterozoic extensioal-accretionary orogen. In this contribution, we investigate the crustal and upper mantle structure of the AFO and adjacent regions using passive seismic data collected during the recent ALFREX experiment. Since the entire region has not been significantly reactivated since the Mesoproterozoic, the old signature of craton edge modification should have been well preserved until today. From November 2013 until January 2016, we operated a temporary passive seismic network consisting of 70 stations in the eastern Albany-Fraser Orogen. The array had an average station spacing of about 40 km and was designed to fill the gap between recently acquired active seismic profiles. We present results from the analysis of P receiver functions and ambient noise tomography using the ALFREX data. Receiver functions were used to derive a Moho depth map via H-K stacking, for direct imaging (common conversion point stacking) as well as joint inversion with surface wave dispersion data to derive 1D S-velocity profiles beneath the stations. The obtained receiver functions show a marked change of character from west to east across the array. Whereas they feature clear and sharp Moho phases for stations on the Yilgarn Craton, significantly more crustal complexity and fainter Moho phases are seen throughout the AFO. Crustal thickness increases from 36-39 km for the Yilgarn Craton to values between 42 and 48 km across the AFO, decreasing to around 40 km in the east. Ambient noise cross-correlations were used to derive maps of phase and group velocities of Rayleigh waves at periods between 1 and 30 seconds. A three-dimensional model of S wavespeeds throughout the area was then computed by pixelwise inversion of dispersion curves. Obtained S wavespeeds are generally

  8. 2480 Ma mafic magmatism in the northern Black Hills, South Dakota: A new link connecting the Wyoming and Superior cratons

    USGS Publications Warehouse

    Dahl, P.S.; Hamilton, M.A.; Wooden, J.L.; Foland, K.A.; Frei, R.; McCombs, J.A.; Holm, D.K.

    2006-01-01

    The Laramide Black Hills uplift of southwest South Dakota exposes a Precambrian crystalline core of ???2560-2600 Ma basement granitoids nonconformably overlain by two Paleoproterozoic intracratonic rift successions. In the northern Black Hills, a 1 km thick, layered sill (the Blue Draw metagabbro) that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift-basin sedimentation. Ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207Pb/206Pb upper-intercept age of 2480 ?? 6 Ma (all age errors ??2??), comparable to two-point 207Pb/206Pb errorchron ages obtained by Pb stepwise leaching of the same titanites. Nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 ?? 16 to 2284 ?? 20 Ma (i.e., differentially reset along U-Pb concordia), and hornblende from an associated metadiorite yields a partially reset date with oldest apparent-age increments ranging between 2076 ?? 16 and 2010 ?? 8 Ma. We interpret these data as indicating that an episode of gabbroic magmatism occurred at 2480 Ma, in response to earlier rifting of the eastern edge of the Wyoming craton. Layered mafic intrusions of similar thickness and identical age occur along a rifted belt in the southern Superior craton (Sudbury region, Ontario). Moreover, these mafic intrusions are spatially aligned using previous supercontinent restorations of the Wyoming and Superior cratons (Kenorland-Superia configurations). This new "piercing point" augments one previously inferred by spatial-temporal correlation of the Paleoproterozoic Huronian (southern Ontario) and Snowy Pass (southeastern Wyoming) supergroups. We propose that layered mafic intrusions extending from Nemo, South Dakota, to Sudbury, Ontario, delineate an axial rift zone along which Wyoming began to separate from Superior during initial fragmentation of the Neoarchean

  9. New Precambrian paleomagnetic and geochronological results from the North China Craton and their implications for supercontinent Nuna reconstruction

    NASA Astrophysics Data System (ADS)

    Zhang, S.

    2013-05-01

    A recent update of available global paleomagnetic data demonstrate that a coherent Nuna formed by ca. 1.75 Ga, and lasted till at least ca. 1.4 Ga (Zhang et al, 2012). This paleomagnetism-based reconstruction of Nuna is also in agreement with those proposed, geologically based models, including the SAMBA connection between Baltica, Amazonia and Western Africa (Johansson, 2009), the Nuna core connection between Laurentia, Baltica and Siberia (Evans and Mitchell, 2011), the proto-SWEAT connection between Laurentia, Mawson block and Australian blocks (Payne et al., 2009) and the NCB-India connection (Zhao et al., 2011). The North China Craton (NCC) is positioned by superimposing its four high quality poles between 1.78 Ga and ~1.44 Ga atop coeval poles from the combined cratonic assemblage, including Laurentia, Baltica, Siberia and Australian blocks. In this reconstruction, the present south margin of the NCB represented an active continental margin in Nuna and likely faced an open ocean, whilst its north margin was connected to a large landmass. Recently, Su et al (2012) reported a precise age of 1611±8 Ma (zircon U-Pb LA-MC- ICPMS) obtained from a tuff bed in the Luoyukou Formation in western Henan Province. This age suggested that two high quality paleomagnetic poles from the Baicaoping and Yunmengshan formations in this region, which were considered as of 1.2-1.3 Ga, should be of ~1.61 - 1.78 Ga. Although with poor age constraint, the two poles also support the NCC's position in the Nuna. In addition, new paleomagnetic results have been obtained from the well dated ~1.33 Ga mafic sills in the northern NCC (Zhang et al, 2011; Chen et al, 2013). However, because there is no coeval pole from other major cratons, whether the NCC had still connected with them is not conclusive.

  10. Phanerozoic orogeny triggers reactivation and exhumation in the northern part of the Archean-Paleoproterozoic North China Craton

    NASA Astrophysics Data System (ADS)

    Zhang, Hong-Fu; Zou, Dong-Ya; Santosh, M.; Zhu, Bin

    2016-09-01

    Cratons and orogenic belts are integral components of ancient continents. In some cases, stable continents can be reactivated and even largely destroyed by younger events. The mechanisms of reactivation or destruction of ancient cratons remain equivocal. Here we compile zircon U-Pb data from the metamorphic rocks of the Hongqiyingzi Group in the northern part of the North China Craton (NCC) to evaluate this problem. The results reveal episodic tectonothermal events related to three major periods of Latest Neoarchean-Earliest Paleoproterozoic (2.6-2.4 Ga), Paleoproterozoic (1.85-1.95 Ma) and Phanerozoic (480-220 Ma, with peak at 360-320 Ma). The Neoarchean-Paleoproterozoic crust as represented by the Hongqiyingzi Group was subducted to eclogite facies at ca. 1.95 Ga, followed by the final collision between the Eastern and Western Blocks at ca. 1.85 Ga to produce the Trans-North China Orogen (TNCO). The ubiquitous presence of Phanerozoic concordant zircons in the Hongqiyingzi Group suggests the episodic exhumation of the TNCO in the northern segment during the Phanerozoic. The 460-420 Ma concordant zircons record the onset of exhumation, triggered by the Ordovician-Silurian southward subduction of the Paleoasian Ocean. Widespread occurrence of 360-320 Ma metamorphic and magmatic zircons in the Hongqiyingzi Group demonstrates that the Carboniferous-Permian was an important period of exhumation for the TNCO, possibly related to back-arc extension. A few Permian-Triassic concordant zircons record late Permian accretion and Triassic post-accretional uplifting. Our study demonstrates that the young orogeny during the formation of the Central Asian Orogenic Belt resulted in extensive reactivation of not only the Archean continental crust but also the Paleoproterozoic orogen in the northern NCC.

  11. Contrasting Triassic ferroan granitoids from northwestern Liaoning, North China: Magmatic monitor of Mesozoic decratonization and a craton-orogen boundary

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaohui; Yuan, Lingling; Xue, Fuhong; Zhang, Yanbin

    2012-07-01

    The diversity exhibited by ferroan granitoids testifies to multiple distinct crustal and mantle processes generally within extensional environments and thus holds the key to monitoring important geodynamic and crustal evolutionary processes and calibrating terrane tectonic affinity. This study documents three Middle to Late Triassic ferroan granitoid suites from northwestern Liaoning, North China, and proposes distinct petrogenetic scenarios for each. The Middle Triassic (ca. 238 Ma) Ping'andi (PAD) granites are mainly calc-alkali and peraluminous. Their radiogenic whole-rock Nd and zircon Hf isotopic signatures argue for an origin consistent with partial melting of a juvenile quartzofeldspathic crust formed by prior mantle-derived magmatic underplating and their differentiations. By contrast, the Late Triassic (ca. 220 Ma) Dashaoleng (DSL) and Sijiazi (SJZ) suites show an evolved character from alkali-calcic to alkali and from metaluminous to peraluminous. Their variably evolved whole-rock Sr-Nd and mixed zircon Hf isotopic compositions suggest that both suites were formed by variable mixing between depleted mantle-derived mafic magma and old crust-derived felsic magma, with distinctively higher input from juvenile components in the DSL suite. These contrasting ferroan granitoid suites not only provide a spatial marker for monitoring juxtaposition of the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB) along a lithospheric-scale boundary fault in the region, but also present a temporal snapshot that records a southwardly-progressing crustal growth scenario possibly in response to lithospheric dripping within a post-orogenic extensional regime. The cratonic scale synthesis further indicates that diachronous decratonization pattern of the NCC might record episodic response of the craton to evolved plate tectonic processes with two contrasting Phanerozoic orogenic systems.

  12. Evolution of the Kaapvaal Craton as viewed from geochemical and SmNd isotopic analyses of intracratonic pelites

    NASA Astrophysics Data System (ADS)

    Jahn, Bor-Ming; Condie, Kent C.

    1995-06-01

    Precambrian cratonic pelites from the Kaapvaal Craton in southern Africa have similar REE patterns with relative LREE enrichment and absence of significant depletion in HREEs. They have a narrow range of 147Sm/144Nd ratios with a mean value of 0.118, which is identical to the mean value of ≈450 worldwide fine-grained samples of all ages obtained by isotopic dilution analyses. This value is probably the best estimate for the upper continental crust. The Kaapvaal pelites also have distinct Cr/Th ratios, but overlap in Eu/Eu∗ ratios, suggesting that variable provenance and sedimentary recycling were important both during and after the Archean. Because the light REE budget is controlled chiefly by granitoids, which mask contributions of mafic-ultramafic components, the relatively uniform Sm/Nd ratios in sediments do not indicate a near-constant composition for the upper continental crust. Most Kaapvaal pelites have negative ɛNd( T) values, indicating important contributions of older crustal sources. Overall, there is a slight decrease of ɛNd( T) values with decreasing age, but no clear distinction is apparent at the A/P boundary at 2.5 Ga. Almost all of Kaapvaal pelites have TDM ages greater than their depositional ages but younger than 3.6 Ga, suggesting an absence of rocks older than 3.6 Ga in the Kaapvaal Craton. The debate on growth or no-growth of continents depends much on the choice of parameters in model calculations. The crucial parameters include sediment flux in subduction zones and delaminated lower crust, and the Sm/Nd ratio of continental crust. Unfortunately, the available data are ambiguous in modelling studies. Neodymium isotopic data and Sm/Nd ratios cannot be taken as a robust argument against the no-continental-growth model advocated by R. L. Armstrong (1991).

  13. Understanding traditional African healing

    PubMed Central

    MOKGOBI, M.G.

    2015-01-01

    Traditional African healing has been in existence for many centuries yet many people still seem not to understand how it relates to God and religion/spirituality. Some people seem to believe that traditional healers worship the ancestors and not God. It is therefore the aim of this paper to clarify this relationship by discussing a chain of communication between the worshipers and the Almighty God. Other aspects of traditional healing namely types of traditional healers, training of traditional healers as well as the role of traditional healers in their communities are discussed. In conclusion, the services of traditional healers go far beyond the uses of herbs for physical illnesses. Traditional healers serve many roles which include but not limited to custodians of the traditional African religion and customs, educators about culture, counselors, social workers and psychologists. PMID:26594664

  14. Assessment of Undiscovered Petroleum Resources of the North and East Margins of the Siberian Craton, Russian Federation

    USGS Publications Warehouse

    Klett, T.R.

    2008-01-01

    Four geologic provinces located along the north and east margins of the Siberian craton were assessed for undiscovered crude oil, natural gas, and natural gas liquids/condensates resources as part of the U.S. Geological Survey's (USGS) Circum-Arctic Oil and Gas Resource Appraisal. Using a geology-based methodology, the USGS estimated the mean undiscovered, conventional petroleum resources in these provinces to be approximately 28 billion barrels of oil equivalent, including approximately 8 billion barrels of crude oil, 106 trillion cubic feet of natural gas, and 3 billion barrels of natural gas liquids.

  15. Distinct Thermal and Metasomatic Characteristics of Mantle Lithosphere Beneath Two Proterozoic Terranes Bordering the Kaapvaal Craton of Southern Africa

    NASA Astrophysics Data System (ADS)

    Janney, P. E.; Shiimi, E. T.

    2015-12-01

    There is a first order contrast in compositional and thermal properties between cold and infertile Archean cratonic mantle and younger, warmer and more fertile Proterozoic lithosphere, but it has also become apparent that coherent thermal and compositional differences exist between adjacent Proterozoic terranes, even in regions that have been stable for over 1 Ga. We report new thermobarometry and in-situ trace element data for garnet peridotite xenoliths from several late Cretaceous (100-70 Ma) kimberlite localities in the western Namaqua-Natal Belt (NNB) and Rehoboth Province (RP), which bound the Archean Kaapvaal craton to the west and south, respectively. The localities include some for which no data have been reported previously. Re-depletion model ages from Os isotopes indicate that the lithosphere beneath the NNB and RP is mainly Early Proterozoic (Pearson et al., Chem. Geol., 2004; Janney et al. J. Petrol., 2010) and there is no evidence from xenolith modal proportions for significant differences in average fertility between lithospheric terranes. Equilibration pressures for garnet peridotites from both terranes fall in a similar range (2 to 5 GPa). However, peridotites from the RP typically have P and T values that fall on or very close to the Kaapvaal cratonic geotherm (apart from a group of peridotites from the Gibeon kimberlites with pressures > 4 GPa that follow an adiabatic gradient; e.g. Franz et al., J. Geol., 1996) whereas peridotites from the western NNB have temperatures roughly 100°C warmer than the cratonic geotherm over the whole depth range. Peridotites from the 140 Ma Melton Wold kimberlite, also in the western NNB, lack these warmer temperatures and suggest that warming was contemporaneous with Late Cretaceous kimberlite magmatism. Metasomatic enrichment in incompatible elements (consistent with interaction with kimberlitic melts) is more pronounced in NNB as compared to RP peridotites. The association of higher temperatures with a greater

  16. The Western Edge of Cratonic North America and Topography of the Northern U.S. Rocky Mountains

    NASA Astrophysics Data System (ADS)

    Foster, D. A.; Russo, R. M.; van der Lee, S.; Mueller, P. A.

    2009-12-01

    We used seismic structure of the upper mantle determined via waveform inversions of surface and regional shear waves (Beadle and van der Lee, 2007) to examine the 3-D geometry of the base of North American lithosphere at the junction between thick, stable cratonic eastern North America and the thinner, recently tectonized western part of the continent. This boundary has been affected by long-term subduction beneath North America. Variability in convergence rates and directions, and especially in slab dip, have been postulated as important controls on the configuration of the transition from thick to thin lithosphere, and on the distribution and degree of crustal deformation and volcanism in the western U.S. We show that the lithospheric thickness transition at depths of 70-130 km - defined as contours of zero shear velocity anomaly - correlates strongly with the high topography of Laramide uplifts in the northern Rockies, which lie west of this seismically defined craton edge. The transition from thick to thin lithosphere also includes an embayment symmetrically centered on the Yellowstone hotspot, offset cratonward from the surface position of the hotspot by ca. 140-180 km at depths of 130-150 km. We interpret this structure as a reduction of cratonic seismic velocities reflecting the thermal halo around the hotspot, and perhaps associated with the separation of the lower lithosphere. The steep velocity gradient (boundary) east of the hotspot occurs along the Big Horn Mountains, and distributed mountain ranges of southwestern Montana. The steep transition between thin and thick lithosphere turns sharply west along the northern margin of the Helena thrust salient-Lewis and Clark fault zone, where it may reflect the edge of the Archean Medicine Hat Block and/or the northern termination of the influence shallow Farallon slab subduction the during Laramide time. Laramide-style basement uplifts are absent north of this zone and the eastern front ranges of the Rockies

  17. The Majorqaq Belt: A record of Neoarchaean orogenesis during final assembly of the North Atlantic Craton, southern West Greenland

    NASA Astrophysics Data System (ADS)

    Dyck, Brendan; Reno, Barry L.; Kokfelt, Thomas F.

    2015-04-01

    Over the past 4.5 billion years (Ga) the Earth has undergone a secular change in tectonic regimes. Archaean tectonics was a markedly different process than observed today, involving a hotter planet with a thinner lithosphere, thicker oceanic crust, and smaller proto-continents. The transition to a more modern type of plate tectonics required the initiation of subduction of oceanic lithosphere. The onset of subduction in the Meso- to Neoarchaean (ca. 3.2 to 2.5 Ga) is shown by geochemical signatures indicative of crustal recycling and petrologic evidence of tectonically driven thickening of the continental crust. This transition is recorded in the rock record of the North Atlantic Craton, which preserves a history of crust formation through to terrane accretion and the collision of crustal blocks. Here we present petrologic, chronologic, chemical and geophysical evidence from the Majorqaq Belt, a ~ 100 km wide east-west striking zone, defined by a high gravity anomaly, voluminous metasedimentary units, olivine-rich ultramafic enclaves, and anomalously high K2O, Ni, and MgO stream-sediment chemistry. We use a 207Pb/206Pb zircon LA-ICP-MS age of 2.557 ± 0.005 Ga (2σ) from an upper-amphibolite facies metapelitic migmatite to propose Neoarchaean suturing of the Maniitsoq crustal block to the North Atlantic Craton. Ti-in-zircon temperatures and zircon-garnet REE distribution coefficients constrain zircon crystallisation to 815-729 °C, which is at, or near the solidus during leucosome crystallisation. Our data reveal a clockwise P-T path with a high-pressure assemblage of garnet-kyanite-plagioclase-rutile-biotite-quartz which grew at an apparent thermal gradient of ~ 660 °C/GPa during subduction-driven compression of surface-derived sediments to a minimum-peak-pressure of ~ 1.15 GPa at ~ 760 °C followed by the growth of gedrite-cordierite-sillimanite-ilmenite at minimum-peak-temperature conditions of ~ 810 °C at ~ 0.94 GPa. We propose the evolution of the Majorqaq

  18. Seismic imaging of crust beneath the Dharwar Craton, India, from ambient noise and teleseismic receiver function modelling

    NASA Astrophysics Data System (ADS)

    Borah, Kajaljyoti; Rai, S. S.; Prakasam, K. S.; Gupta, Sandeep; Priestley, Keith; Gaur, V. K.

    2014-05-01

    We use cross-correlation of continuous 18 months (2009 February to 2010 August) ambient noise data recorded over 35 broad-band seismographs in the Archean Dharwar Craton and the adjoining granulite terrain to generate Rayleigh-wave group velocity maps in the period 5-28 s. This is supplemented with longer period data (40-70 s) from earthquake source. Combined group velocity measurement was inverted jointly with the teleseismic receiver functions obtained at 50 stations (includes 15 stations operated during 1998-2002) to produce shear velocity image of the crust. The velocity image reveals thinner crust (34-38 km) in the late Archean (˜2.7 Ga) Eastern Dharwar Craton (EDC), while all other terrains (mid-Archean and Proterozoic) have crustal thickness from 40 to over 50 km. The mid-Archean (3.36 Ga) greenstone belt of the Western Dharwar Craton (WDC) has the thickest crust (˜50 km). The average crustal Vs beneath the EDC is ˜3.70-3.78 km s-1 as compared to 3.80-3.95 km s-1 beneath the WDC. We observe significant lateral variation in the thickness of lower crust (Vs ˜ 3.8-4.2 km s-1): ˜10-15 km in the EDC compared to ˜20-30 km in the WDC. The lowermost part of the crust (Vs ≥ 4.0 km s-1) is thin (<5 km) beneath the EDC in contrast to more thickness (10-27 km) beneath the WDC. Our analysis suggests intermediate composition for the crust beneath the EDC similar to those for other cratons. In contrast, the mid-Archean exposed WDC crust has more mafic composition and exceptional thickness-a scenario at variance with the global observations. We interpret this thick mafic crust to represent undeformed geological segment of 3.36 Ga. The EDC with a nearly flat Moho, felsic to intermediate composition of crust and thin basal layer may represent a reworked terrain during the late-Archean.

  19. Petrogenesis and Tectonic Implications of Paleoproterozoic Metapelitic Rocks in the Archean Kongling Complex from the Northern Yangtze Craton, South China

    NASA Astrophysics Data System (ADS)

    Li, Y.; Zheng, J.; Wang, W.; Xiong, Q.

    2015-12-01

    The Archean Kongling Complex in the northern Yangtze Craton is an ideal target to investigate the Precambrian accretion and evolution of continental crust in South China. This study aims to unravel the crustal evolution and tectonic setting of the Yangtze Craton during the Paleoproterozoic time, using integrated studies of petrography, zircon U-Pb and Hf isotopes and whole-rock geochemistry of Paleoproterozoic metapelitic rocks in the Kongling Complex. These rocks contain garnet, sillimanite, biotite, plagioclase, minor graphite and ilmenite. Zircons from the samples show nebulous sector-zoning and rim-core structure, suggesting both metamorphic origin and detrital origin with metamorphic overprints. The metamorphic zircons and metamorphic overprints have concordant 207Pb/206Pb age at ~2.0 Ga, while detrital grains yield three distinct concordant-age populations of >2.5 Ga, 2.4-2.2 Ga and 2.2-2.1 Ga. The age patterns indicate that the depositional age of the metasedimentary rocks was 2.1-2.0 Ga. Those 2.2-2.1 Ga detrital zircons with variable ɛHf(t) values (-7.28 to 2.97) suggest the addition of juvenile materials from depleted mantle to the crust during 2.2-2.1 Ga. The 2.4-2.2 Ga zircons have Hf model ages (TDM2) of ~2.6-3.5 Ga and >2.5 Ga zircons have TDM2 ages varying from 2.9 Ga to 3.3 Ga. The new data suggest that the Kongling Complex was originally a Paleoarchean (old up to 3.5 Ga) continental nucleus, which experienced multiple episodes of growth and reworking events at 3.3-3.2 Ga, 2.9 Ga, 2.7-2.6 Ga, 2.4-2.2 Ga and 2.2-2.1 Ga. In combination with available data, the new results in this study suggest a continent-arc-continent evolution model to explain the tectonic evolution of the Yangtze Craton during the Paleoproterozoic time: the western margin of Yangtze Craton was originally an individual continent, which underwent a reworking event during 2.4-2.2 Ga and a crust growth event caused by continent-arc collision during 2.2-2.1 Ga; it subsequently collided

  20. Brittle Deformation in the Ordos Basin in response to the Mesozoic destruction of the North China Craton

    NASA Astrophysics Data System (ADS)

    Wang, Q.; Jiang, L.

    2012-12-01

    Craton is continental block that has been tectonically stable since at least Proterozoic. Some cratons, however, become unstable for some geodynamic reasons. The North China Craton (NCC) is an example. Structure geological, geochemical, and geophysical works have revealed that the NCC was destructed in Cretaceous and that lithosphere thickness beneath the eastern NCC were thinned by 120 km. The present study will focus on deformation of the western NCC, and to understand the effect of the Mesozoic destruction of the North China Craton (NCC). Structural partitioning of the Ordos Basin, which is located in the western NCC, from the eastern NCC occurred during the Mesozoic. Unlike the eastern NCC where many Cretaceous metamorphic core complexes developed, sedimentary cover of the NCC remains nearly horizontal and deformation is manifested by joint. We visited 216 sites of outcrops and got 1928 joints measurements, among which 270 from Jurassic sandstones, 1378 from the Upper Triassic sandstones, 124 from the Middle and Lower Triassic sandstones, and 156 from Paleozoic sandstones. In the interior of the Ordos Basin, joints developed quite well in the Triassic strata, while joints in the Jurassic stata developed weakly and no joint in the Cretaceous strata. The Mesozoic stratigraphic thickness are: 1000 meters for the Lower Triassic, the Middle Triassic sandstone with thickness of 800 meters, 3000 meters for the Upper Triassic, 4000 meters for the Jurassic, and 1100 meters for the Lower Cretaceous. The vertical difference in joint development might be related to the burying depth of the strata: the higher the strata, the smaller the lithostatic stress, and then the weaker the joint. Joints in all stratigraphic levels showed a similar strain direction with the sigma 1 (the maximum pressure stress) vertical and the sigma 3 (the minimum pressure stress) horizontal and running N-S. The unconformity below the Cretaceous further indicates that joints in Jurassic and Triassic

  1. Crustal structure of the Archaean granite-greenstone terrane in the northern portion of the Kaapvaal Craton

    NASA Technical Reports Server (NTRS)

    Debeer, J. H.; Stettler, E. H.; Barton, J. M., Jr.; Vanreenen, D. D.; Bearncombe, J. R.

    1986-01-01

    Recent investigations of the electrical resistivity, gravity and aeromagnetic signatures of the various granite-greenstone units in the northern portion of the Kaapvaal craton have revealed three features of significance: (1) the Archean greenstone belts are shallow features, rarely exceeding 5 km in depth; (2) the high resistivity upper crustal layer typical of the lower grade granite-greenstone terranes is absent in the granulite facies terrane; and (3) the aeromagnetic lineation patterns allow the granite-greenstone terrane to be subdivided into geologically recognizable tectono-metamorphic domains on the basis of lineation frequency and direction. A discussion follows.

  2. Electrical resistivity structure of the upper mantle beneath Northeastern China: Implications for rheology and the mechanism of craton destruction

    NASA Astrophysics Data System (ADS)

    Dong, Zeyi; Tang, Ji; Unsworth, Martyn; Chen, Xiaobin

    2015-03-01

    The North China Craton (NCC) and Central Asian Orogen Belt (CAOB) in Northeastern China experienced a range of tectonic events during the Phanerozoic, dominated by lithospheric thinning of the eastern NCC in the late Mesozoic and Cenozoic. In order to better understand the tectonic evolution of the NCC and the CAOB, new broadband and long period magnetotelluric data were collected along a north-west to south-east trending profile that extended from the CAOB across the Yanshan Belt, the Tanlu Fault Zone to the Liaodong Peninsula. A two-dimensional (2-D) resistivity model was derived from inversion of the transverse electric mode, transverse magnetic mode and vertical magnetic field data. In the crust of the CAOB, the resistivity model shows a northwest dipping low resistivity zone beneath the Solonker suture that is identified as the suture zone formed by the collision between the Siberian and North China cratons. The upper mantle of the CAOB is characterized by moderate resistivity values (300-1000 Ω m) that are best explained by the presence of hydrogen dissolved in olivine. The water concentration of the CAOB mantle is comparable to values reported for the asthenosphere and cratons that have been significantly hydrated. The NCC upper mantle is generally lower in resistivity than the CAOB upper mantle, and a zone of lower resistivity is observed in the upper mantle at the southeast end of the profile beneath the NCC (<100 Ω m) which requires around 1% partial melt to account for the observed resistivity. Superimposed on this southeast decrease in upper mantle resistivity, three low resistivity zones were imaged: (1) below the Xilamulun fault, (2) close to the North-South Gravity Lineament, and (3) between the northern Yanshan Belt and Tanlu Fault Zone. The low resistivities can be explained as regions of partial melts or fluids, perhaps caused by asthenospheric upwelling. Together with seismic imaging results and geochemical data, the resistivity model shows

  3. Paleogeography of the Amazon craton at 1.2 Ga: early Grenvillian collision with the Llano segment of Laurentia

    NASA Astrophysics Data System (ADS)

    Tohver, Eric; van der Pluijm, B. A.; Van der Voo, R.; Rizzotto, G.; Scandolara, J. E.

    2002-05-01

    A paleomagnetic, geochronologic and petrographic study was undertaken on the flat-lying gabbros and basalts of the Nova Floresta Formation of Rondônia state, western Brazil in order to constrain the Mesoproterozoic paleogeography of the Amazon craton. Measurement of the anisotropy of magnetic susceptibility on the gabbroic samples reveals a flat-lying foliation with a radiating pattern of lineations, supporting the field evidence that the gabbros are part of a large, undeformed sill. Petrographic observations of oxides in the gabbros reveals two populations of magnetite grains produced during the original cooling of the sill: large, oxyexsolved titanomagnetite grains and fine-grained magnetite in igneous reaction rims. New 40Ar/ 39Ar age dating of biotite and plagioclase yield ages of ˜1.2 Ga, which represent the rapid cooling following emplacement of the mafic magma. Whole rock dating of basalt samples yields total gas ages of 1062±3 Ma, similar to the ˜1.0 Ga K/Ar ages reported by previous workers. However, the strong compositional dependence of the age spectrum renders this younger whole rock age unreliable except as a minimum constraint. A single magnetic component is found in the basalts, indistinguishable from the characteristic remanence found in the gabbros that is oriented WNW and steeply upward. This magnetization is considered to be primary and was acquired during the cooling of the sill and associated lavas. A paleomagnetic pole calculated from the Nova Floresta Formation ( n=16 sites, Plat.=24.6°N, Plong.=164.6°E, A95=5.5°, Q=5), the first reported pole for the Amazon craton for the 1200-600 Ma Rodinia time period, constrains the paleogeographic position of Amazonia at ˜1.2 Ga. Juxtaposition of the western Amazon craton with the Llano segment of the Laurentia's Grenville margin causes the NF pole to lie on the 1.2 Ga portion of the combined APWP for Laurentia and Greenland, which indicates that a collision with the Amazon craton could have

  4. Human African trypanosomiasis.

    PubMed

    Lejon, Veerle; Bentivoglio, Marina; Franco, José Ramon

    2013-01-01

    Human African trypanosomiasis or sleeping sickness is a neglected tropical disease that affects populations in sub-Saharan Africa. The disease is caused by infection with the gambiense and rhodesiense subspecies of the extracellular parasite Trypanosoma brucei, and is transmitted to humans by bites of infected tsetse flies. The disease evolves in two stages, the hemolymphatic and meningoencephalitic stages, the latter being defined by central nervous system infection after trypanosomal traversal of the blood-brain barrier. African trypanosomiasis, which leads to severe neuroinflammation, is fatal without treatment, but the available drugs are toxic and complicated to administer. The choice of medication is determined by the infecting parasite subspecies and disease stage. Clinical features include a constellation of nonspecific symptoms and signs with evolving neurological and psychiatric alterations and characteristic sleep-wake disturbances. Because of the clinical profile variability and insidiously progressive central nervous system involvement, disease staging is currently based on cerebrospinal fluid examination, which is usually performed after the finding of trypanosomes in blood or other body fluids. No vaccine being available, control of human African trypanosomiasis relies on diagnosis and treatment of infected patients, assisted by vector control. Better diagnostic tools and safer, easy to use drugs are needed to facilitate elimination of the disease.

  5. Diversity among African Pygmies

    PubMed Central

    Ramírez Rozzi, Fernando V.; Sardi, Marina L.

    2010-01-01

    Although dissimilarities in cranial and post-cranial morphology among African pygmies groups have been recognized, comparative studies on skull morphology usually pull all pygmies together assuming that morphological characters are similar among them and different with respect to other populations. The main aim of this study is to compare cranial morphology between African pygmies and non-pygmies populations from Equatorial Africa derived from both the Eastern and the Western regions in order to test if the greatest morphological difference is obtained in the comparison between pygmies and non-pygmies. Thirty three-dimensional (3D) landmarks registered with Microscribe in four cranial samples (Western and Eastern pygmies and non-pygmies) were obtained. Multivariate analysis (generalized Procrustes analysis, Mahalanobis distances, multivariate regression) and complementary dimensions of size were evaluated with ANOVA and post hoc LSD. Results suggest that important cranial shape differentiation does occur between pygmies and non-pygmies but also between Eastern and Western populations and that size changes and allometries do not affect similarly Eastern and Western pygmies. Therefore, our findings raise serious doubt about the fact to consider African pygmies as a homogenous group in studies on skull morphology. Differences in cranial morphology among pygmies would suggest differentiation after divergence. Although not directly related to skull differentiation, the diversity among pygmies would probably suggest that the process responsible for reduced stature occurred after the split of the ancestors of modern Eastern and Western pygmies. PMID:21049030

  6. African horse sickness.

    PubMed

    Zientara, S; Weyer, C T; Lecollinet, S

    2015-08-01

    African horse sickness (AHS) is a devastating disease of equids caused by an arthropod-borne virus belonging to the Reoviridae family, genus Orbivirus. It is considered a major health threat for horses in endemic areas in sub-Saharan Africa. African horse sickness virus (AHSV) repeatedly caused large epizootics in the Mediterranean region (North Africa and southern Europe in particular) as a result of trade in infected equids. The unexpected emergence of a closely related virus, the bluetongue virus, in northern Europe in 2006 has raised fears about AHSV introduction into Europe, and more specifically into AHSV-free regions that have reported the presence of AHSV vectors, e.g. Culicoides midges. North African and European countries should be prepared to face AHSV incursions in the future, especially since two AHSV serotypes (serotypes 2 and 7) have recently spread northwards to western (e.g. Senegal, Nigeria, Gambia) and eastern Africa (Ethiopia), where historically only serotype 9 had been isolated. The authors review key elements of AHS epidemiology, surveillance and prophylaxis. PMID:26601437

  7. Is the Ventersdorp rift system of southern Africa related to a continental collision between the Kaapvaal and Zimbabwe Cratons at 2.64 Ga AGO?

    NASA Technical Reports Server (NTRS)

    Burke, K.; Kidd, W. S. F.; Kusky, T.

    1985-01-01

    Rocks of the Ventersdorp Supergroup were deposited in a system of northeast trending grabens on the Kaapvaal Craton approximately 2.64 Ga ago contemporary with a continental collision between the Kaapvaal and Zimbabwe Cratons. It is suggested that it was this collision that initiated the Ventersdorp rifting. Individual grabens strike at high angles toward the continental collision zone now exposed in the Limpopo Province where late orogenic left-lateral strike-slip faulting and anatectic granites are recognized. The Ventersdorp rift province is related to extension in the Kaapvaal Craton associated with the collision, and some analogy is seen with such rifts as the Shansi and Baikal Systems associated with the current India-Asia continental collision.

  8. East African Rift Valley, Kenya

    NASA Technical Reports Server (NTRS)

    1990-01-01

    This rare, cloud free view of the East African Rift Valley, Kenya (1.5N, 35.5E) shows a clear view of the Turkwell River Valley, an offshoot of the African REift System. The East African Rift is part of a vast plate fracture which extends from southern Turkey, through the Red Sea, East Africa and into Mozambique. Dark green patches of forests are seen along the rift margin and tea plantations occupy the cooler higher ground.

  9. First precise U-Pb baddeleyite ages of 1500 Ma mafic dykes from the São Francisco Craton, Brazil, and tectonic implications

    NASA Astrophysics Data System (ADS)

    Silveira, E. M.; Söderlund, U.; Oliveira, E. P.; Ernst, R. E.; Leal, A. B. Menezes

    2013-08-01

    The São Francisco Craton (SFC) is an Archaean-Proterozoic craton that hosts a significant number of mafic intrusions. Previous attempts to date the igneous emplacement of these rocks are limited to a few poor precision Rb-Sr and Sm-Nd isochrons. Here, dykes from the Curaçá belt, in the northeastern portion of the craton, as well as from the Chapada Diamantina region in the central part, were dated using U-Pb on baddeleyite (ID-TIMS). These dykes yielded similar baddeleyite U-Pb ages of 1506.7 ± 6.9 Ma (Curaçá) and 1501.0 ± 9.1 Ma (Chapada), with converging trends indicating a possible magmatic centre located near the western margin of the São Francisco Craton. The two dyke swarms are subalkaline, and have light rare earth enriched chondrite-normalized patterns but the Curaçá dykes have lower Mg-number and higher abundances of most incompatible trace elements than the Chapada dykes. Geochemical modelling and trace element ratios suggest that the two dyke swarms had different petrogenesis, with the Chapada dykes being derived from more depleted mantle sources and the Curaçá dykes from enriched sources, however the mantle sources of both dyke swarms appear to have had contributions from an enriched plume. Current geochronological and palaeomagnetic data suggest a coherent São Francisco-Congo block from at least the late Mesoproterozoic until the opening of the South Atlantic (ca. 130 Ma ago). The discovery of ca. 1500 Ma intrusions in the São Francisco Craton is a major step forward to a more complete barcode record for the SFC-Congo Craton, which can be used in future palaeo-continental reconstructions.

  10. Cordilleran cratonal-miogeoclinal hinge zone in the Mojave Desert and southern Great Basin: What is it How can it be recognized Where is it

    SciTech Connect

    Cooper, J.D. . Dept. of Geological Sciences); Fedo, C.M. . Dept. of Geological Sciences)

    1993-04-01

    Commonly referred to as hingelines, ancient cratonal-miogeoclinal transitions are best defined by rift-related structures because during the rift-to-drift history of continental margins, hingelines exert their most profound influences as active tectonic and physiographic features. Where rift-related structures are not preserved, stratigraphic development, as the most direct response to early continental margin tectonic activity, is the more sensitive indicator of paleotectonic setting. Accordingly, it is the initial, Neoproterozoic-Early Cambrian basement-cover deposits of the evolving southwestern US Cordilleran passive margin that provide the clear signature of cratonal-miogeoclinal affinity and hinge definition. At a facies scale, the similarities between cratonal and miogeoclinal strata are compelling; thus the major difference between the craton and miogeocline expressed not by lithofacies, but by stratigraphic unit representation and thickness. Throughout the eastern Mojave Desert and southern Great Basin regions, three distinct expressions of Upper Proterozoic-Lower Cambrian stratigraphic unit representation and thickness relate to hinge definition and location. Basal cratonal stratigraphic sections begin with middle member Wood Canyon Formation that rests nonconformably on Proterozoic basement. Basal miogeoclinal stratigraphic sections are typified by a kilometers-thick sub-Wood Canyon Formation interval ([+-]Pahrump Group; Noonday Dolomite, Johnnie Formation, Stirling Quartzite) and the presence of lower member Wood Canyon Formation. A third, transitional stratigraphic succession begins with a comparatively thin, poorly developed sub-Wood Canyon Formation interval and contains no lower member Wood Canyon Formation. This transitional stratigraphic style represents a craton-margin setting and is crucial for constraining a hinge.

  11. The African Plate: A history of oceanic crust accretion and subduction since the Jurassic

    NASA Astrophysics Data System (ADS)

    Gaina, C.; Torsvik, T. H.; Labails, C.; van Hinsbergen, D.; Werner, S.; Medvedev, S.

    2012-04-01

    Initially part of Gondwana and Pangea, and now surrounded almost entirely by spreading centres, the African plate moved relatively slowly for the last 200 million years. Yet both Africa's cratons and passive margins were affected by tectonic stresses developed at distant plate boundaries. Moreover, the African plate was partly underlain by hot mantle (at least for the last 300 Ma) - either a series of hotspots or a superswell, or both - that contributed to episodic volcanism, basin-swell topography, and consequent sediment deposition, erosion, and structural deformation. A systematic study of the African plate boundaries since the opening of surrounding oceanic basins is presently lacking. This is mainly because geophysical data are sparse and there are still controversies regarding the ages of oceanic crust. The publication of individual geophysical datasets and more recently, global Digital Map of Magnetic Anomalies (WDMAM, EMAG2) prompted us to systematically reconstruct the ages and extent of oceanic crust around Africa for the last 200 Ma. Location of Continent Ocean Boundary/Continent Ocean Transition and older oceanic crust (Jurassic and Cretaceous) are updates in the light of gravity, magnetic and seismic data and models of passive margin formation. Reconstructed NeoTethys oceanic crust is based on a new model of microcontinent and intr-oceanic subduction zone evolution in this area.The new set of oceanic palaeo-age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and will be used as input for quantifying the paleo-ridge push and slab pull that contributed to the African plate palaeo-stresses and had the potential to influence the formation of sedimentary basins.

  12. Detrital zircon geochronology by LA-ICP-MS of the Neoarchean Manjeri Formation in the Archean Zimbabwe craton- the disappearance of Eoarchean crust by 2.7 Ga?

    NASA Astrophysics Data System (ADS)

    Sawada, Hikaru; Maruyama, Shigenori; Sakata, Shuhei; Hirata, Takafumi

    2016-01-01

    For the ca. 2.7 Ga (Neoarchean) clastic rocks in the Belingwe greenstone belt (Manjeri Formation), U-Pb age of detrital zircon grains were analyzed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The Manjeri Formation, composed of sandstone, quartzite, and limestone with stromatolites, was deposited in a shallow marine setting along the ancient passive continental margin of the Zimbabwe craton. Nearly 100 detrital zircon grains, for each from two sandstone samples in central Zimbabwe, were separated for U-Pb dating. Age spectra of detrital zircon grains of two analyzed sandstones of the Manjeri Formation show more or less the same multiple age clusters: i.e., ca. 2.9 Ga, ca. 3.15 Ga, ca. 3.3 Ga, and ca. 3.5 Ga. These age spectra reflect nature of the provenance of proto-Zimbabwe, which was considerably different from that of the modern Zimbabwe craton. The age clusters of ca. 2.9 Ga, ca. 3.3 Ga, and ca. 3.5 Ga are correlated with those of extant basement rocks of the present Zimbabwe craton, whereas the ca. 3.15 Ga cluster has no corresponding unit within the craton, except for detrital zircons from the 2.65 Ga Shamvaian Group in a neighboring area of the Zimbabwe craton. The extremely old (3.85 Ga; Eoarchean) detrital zircon grains, previously reported from the ca. 2.9 Ga and ca. 3.4 Ga sandstones elsewhere in the craton, were not detected at all in the present two analyzed samples. As no Eoarchean (>3.8 Ga) basement rocks remain in the Zimbabwe craton at present either, the present study confirmed that the Eoarchean crustal rocks once occurred in the proto-Zimbabwe craton but they had been removed secondarily from the provenance of Neoarchean and younger basins prior to 2.7 Ga. Possible geologic processes for such disappearance of older crusts may include the physiological separation by continental rifting, subduction erosion, and/or other crustal recycling processes.

  13. A Teacher's Guide to African Narratives. Studies in African Literature.

    ERIC Educational Resources Information Center

    O'Brien, Sara Talis

    This guide is designed to help secondary school teachers include African literature in their classes. It furnishes English and social studies teachers with a foundation for teaching African literature by offering critical commentary on the texts themselves. A synthesis of anthropological and historical material is presented to help both teachers…

  14. Paleoproterozoic andesitic volcanism in the southern Amazonian craton (northern Brazil); lithofacies analysis and geodynamic setting

    NASA Astrophysics Data System (ADS)

    Roverato, Matteo; Juliani, Caetano; Capra, Lucia; Dias Fernandes, Carlos Marcelo

    2016-04-01

    Precambrian volcanism played an important role in geological evolution and formation of new crust. Most of the literature on Precambrian volcanic rocks describes settings belonging to subaqueous volcanic systems. This is likely because subaerial volcanic rocks in Proterozoic and Archean volcano-sedimentary succession are poorly preserved due to erosive/weathering processes. The late Paleoproterozoic Sobreiro Formation (SF) here described, seems to be one of the rare exceptions to the rule and deserves particular attention. SF represents the subaerial expression of an andesitic magmatism that, linked with the upper felsic Santa Rosa F., composes the Uatumã Group. Uatumã Group is an extensive magmatic event located in the Xingú region, southwestern of Pará state, Amazonian Craton (northern Brazil). The Sobreiro volcanism is thought to be related to an ocean-continent convergent margin. It is characterized by ~1880 Ma well-preserved calc-alkaline basaltic/andesitic to andesitic lava flows, pyroclastic rocks and associated reworked successions. The superb preservation of its rock-textures allowed us to describe in detail a large variety of volcaniclastic deposits. We divided them into primary and secondary, depending if they result from a direct volcanic activity (pyroclastic) or reworked processes. Our study reinforces the importance of ancient volcanic arcs and rocks contribution to the terrestrial volcaniclastic sedimentation and evolution of plate tectonics. The volcanic activity that produced pyroclastic rocks influenced the amount of detritus shed into sedimentary basins and played a major role in the control of sedimentary dispersal patterns. This study aims to provide, for the first time, an analysis of the physical volcanic processes for the subaerial SF, based in field observation, lithofacies analysis, thin section petrography and less geochemical data. The modern volcanological approach here used can serve as a model about the evolution of Precambrian

  15. Paleomagnetism of the Mississippian HP pipe and the western margin of the North American craton

    NASA Astrophysics Data System (ADS)

    Symons, D. T. A.; Lewchuk, M. T.

    The HP pipe is a small oval (80m×40m) kimberlitic diatreme near Golden, B.C., in the fold-and-thrust belt of the Rocky Mountains. The pipe intrudes gently-dipping (˜10°S) Late Cambrian to Middle Ordovician limestones with nearly vertical contacts. It is comprised of carbonate breccia fragments and pyroxenite nodules in a light-green serpentine matrix and gives a Rb-Sr age of 348±7 Ma. A later phase of dark-green dikes cut the breccia and adjacent limestones. AF and thermal step demagnetization of 168 specimens from 42 oriented blocks was done using an automated cryogenic magnetometer in a shielded room for remanence measurement. The diatreme breccia and dikes retain a stable A remanence component after thermal cleaning above ˜400°C and AF cleaning above ˜20 mT. Their combined mean direction of 129.8°, -44.3° (α95 = 6.5°) after tilt correction gives a concordant Mississippian pole position of 138.5°E, 44.5°N (Dp = 5.1°, Dm = 8.2°). The limestones provide a positive contact test by giving a stable B component direction of 114.4°, -22.7° (α95 = 12.4°) with a concordant Late Cambrian to Middle Ordovician pole of 141.0°E, 24.3°N (δp = 7.0°, δm = 13.3°). These concordant poles indicate that the fold-and-thrust belt is part of the North American craton. Thus the suture with allochthonous terranes to the west must be located at or west of the Rocky Mountain Trench. Both the diatreme and limestones retain a low-coercivity and unblocking-temperature C component, isolated by vector subtraction without tilt correction, that records a minor overprint from the Laramide Orogeny combined with Pleistocene viscous remanence.

  16. Mantle dynamics of the Paleoproterozoic North China Craton: A perspective based on seismic tomography

    NASA Astrophysics Data System (ADS)

    Santosh, M.; Zhao, Dapeng; Kusky, Timothy

    2010-01-01

    We investigate the mantle dynamics beneath the North China Craton (NCC) and surrounding regions based on a synthesis of recent P-wave mantle tomographic data down to depths of 600-800 km and their correlation with the surface geological features, with particular reference to the Paleoproterozoic tectonic events associated with the incorporation of the NCC within the Columbia supercontinent amalgam. From the tomographic images, we identify a hot corridor in the mantle transition zone beneath the central region of the Western Block of the NCC sandwiched between two cold corridors. This scenario is similar to the donut-shaped high-velocity anomaly surrounding a region of low-velocity anomaly in the lowermost mantle under the Pacific and suggests that the cold regions might represent slab graveyards which provide the fuel for the plumes rising from the center. A tomographic transect along the collisional suture of the NCC with the Columbia supercontinent, covering the Yinshan-Ordos Blocks in the Western Block through the Central Orogenic Belt and into the Eastern Block of the NCC reveals a ca. 250 km thick lithospheric keel below the Ordos Block defined by a prominent high-velocity anomaly. We identify slab break-off and asthenospheric upwelling in this region and suggest that this process probably initiated the thermal and material erosion of the tectosphere beneath the Eastern Block from the Paleoproterozoic, which was further intensified during the Mesozoic when a substantial part of the sub-continental mantle lithosphere was lost. We visualize heat input from asthenosphere and interaction between asthenosphere and overlying carbonated tectosphere releasing CO 2-rich fluids for the preservation of ultra-high temperature (ca. 1000 °C) metamorphic rocks enriched in CO 2 as well as high-pressure mafic granulites as a paired suite in this region. We also identify a hot swell of the asthenosphere rooted to more than 200 km depth and reaching up to the shallow mantle in

  17. Multidisciplinary Study of the Precambrian Biosphere and Surficial Oxygenation, Kaapvaal Craton, South Africa: The Agouron Cores

    NASA Astrophysics Data System (ADS)

    Kirschvink, J. L.; Beukes, N. J.; Evans, D. A.; Grotzinger, J. P.; Knoll, A. H.; Sumner, D. Y.

    2004-12-01

    The Campbellrand-Kuruman carbonate-iron formation stratigraphic succession, which drapes the Kaapvaal craton of South Africa, provides a unique opportunity to study the latest Archean/Earliest Proterozoic time interval in a multidisciplinary fashion, for four principal reasons: 1) The >1 km-thick succession of carbonates, cherts, shales, and associated iron formations is a storehouse of various geochemical and paleoclimatic proxy records, 2) the carbonate platform has never been significantly buried and contains abundant limestone, thus offering strong potential for preservation of organic biomarkers, 3) the occurrence of early chert and abundant early sea-floor carbonate crusts provide good potential for the preservation of microfossils and magnetofossils, and 4) much of the stratigraphic succession has not been significantly deformed and we have estabilshed a chronostratigraphic framework in which shallow water facies can be traced down the ancient paleoslope into facies deposited at water depths > 250 meters within a sequence stratigraphic context, supplemented with correlation of three impact spherule layers. The geologic framework provided by this sequence of rock offers an unparalleled opportunity to study the structure and composition of the Archean ocean and to merge this information with co-existing paleontological and geochemical records. With support from the Agouron Institute, two separate cores, each ~ 1.5 km in length, were drilled through the margin of the carbonate platform, spaced so as to intercept the transitional facies at two paleodepths. The holes were deviated slightly from vertical so that a ball-mark system could be used to obtain absolute orientation. To enhance the utility for paleomagnetic investigations, core barrels and bits were demagnetized routinely with a portable mu-metal shielded coil assembly to reduce remagnetization problems, and all core slicing was done with non-magnetic blades. To minimize contamination problems for

  18. The Relationship Between Lithospheric Thickness and Tectonic Subsidence in Mildly- Extended Intra-Cratonic Basins

    NASA Astrophysics Data System (ADS)

    Crosby, A.; White, N.; Fishwick, S.

    2008-12-01

    In extensional sedimentary basins, the duration of post-rift subsidence depends on the thermal time constant and hence on the thickness of the lithosphere. It is well known that this thickness varies by at least a factor of two over the continents, and that many intra-cratonic basins have continued to accumulate accommodation space for longer than expected given a standard thickness of 125 km. In this study, we make use of recent advances in mapping the thickness of the lithosphere using surface wave tomography, and a global database of backstripped well-logs, to assess the applicability of the classic pure shear model to the stretching and subsidence of regions where the lithosphere is unusually thick. We start by using the known density structure of oceanic lithosphere, and independent seismic observations of crustal structure and lithosphere thickness, to isostatically quantify the average density depletion of the mantle. We find our observations are consistent with geochemical constraints. Using backstripped wells from basins where there has been only one obvious period of extension, and an adapted stretching model which incorporates temperature and composition-dependent thermal diffusivity and expansivity and the advection of depleted mantle, we then invert observations of tectonic subsidence for both thinning factors and lithospheric thickness, and compare our results with seismic observations at the present day. The subsidence of some basins, such as the Michigan Basin, is consistent with the thicker lithospheric template predicted using seismology. In other cases, such as the West Siberian Basin, the lithospheric thickness estimated from seismic tomography and subsidence analysis disagree. We also find strong local gradients in apparent lithospheric thickness, such as between the Michigan and Illinois Basins. The Congo Basin in central Africa can be explained fairly well by simple extension of locally thick lithosphere. However, the pronounced negative

  19. Ferric iron contents of clinopyroxene from cratonic mantle and partitioning behaviour with garnet

    NASA Astrophysics Data System (ADS)

    Woodland, A. B.

    2009-11-01

    Together with garnet and spinel, clinopyroxene is known to be an important carrier of Fe 3+ in mantle-derived peridotites. The systematics of Fe 3+ incorporation in clinopyroxene and the Fe 3+-partitioning behaviour between clinopyroxene and garnet have been investigated from a large suite of garnet peridotite xenoliths (> 50) mostly originating from the upper mantle beneath the Kaapvaal craton (Southern Africa). A correlation between Na and Fe 3+ is apparent that is not observed for clinopyroxene from spinel peridotites. This indicates a change in how Fe 3+ substitutes into clinopyroxene when going from the spinel to the garnet peridotite facies in the upper mantle. A general decrease in Fe 3+/∑Fe with increasing temperature is also observable, however, when considered in terms of Fe 3+ cations per formula unit this negative correlation with temperature disappears. This partial decoupling Fe 3+/∑Fe values from the actual Fe 3+ contents can be understood in terms of two factors: 1) Fe 3+ incorporation is controlled by a number of parameters, including ƒO 2, the degree of depletion of a given sample, as well as the composition of the clinopyroxene itself, and 2) Fe 3+/∑Fe in clinopyroxene can also be directly influenced by changes in Fe 2+ content, which is known to be temperature dependent in the presence of opx and garnet. The partitioning of Fe 3+ between clinopyroxene and garnet can be considered in terms of two different Fe 3+-Al exchange equilibria involving Na-bearing components in clinopyroxene and either Ca-bearing or Fe 2+-bearing components in the coexisting garnet. Results from the natural samples suggest a temperature dependence, with Fe 3+ being progressively partitioned into garnet with increasing temperature. However, the magnitude of this effect remains open due to uncertainties in available thermodynamic data. Analysis of the two exchange equilibria point to an error in the tabulated standard enthalpy of formation of NaFe 3+Si 2O 6 aegerine

  20. Geophysical Character and Geochemical Evolution of the Mesoproterozoic Figueira Branca Intrusive Suite, SW Amazon Craton (Brazil)

    NASA Astrophysics Data System (ADS)

    Louro, Vinicius; Cawood, Peter; Mantovani, Marta

    2016-04-01

    The Jauru Terrain hosts the Figueira Branca Intrusive Suite (FBS) in the SW of the Amazon Craton (Brazil). The FBS is a series of 1425 Ma layered mafic intrusions, previously interpreted as anorogenic. The FBS area is located in foreland to the Santa Helena orogen, formed by the subduction of the Rio Alegre Terrain under the Jauru Terrain. Potential field methods (magnetic and gravity), gamma-ray spectrometry, geochemical and isotope data were used to characterize and to model the extent of FBS magmatism, the distribution of faults and shear zones in the area, to evaluate affinities of the magmatic activity, and the relation between the FBS and the Santa Helena orogen. The geophysical methods identified three anomalies corresponding with FBS outcrops. A fourth anomaly with significantly higher amplitude was observed to the north of the three anomalies. From south to north, the anomalies were named Indiavaí, Azteca, Figueira Branca and Jauru. These anomalies were modeled and indicated a northwest-southeast trend, parallel to regional shear zones. The gamma-ray data enabled the collection of 50 samples from the FBS rocks, the Alto Jauru group that hosts the FBS, from nearby intrusive suites, and the Rio Alegre Terrain. The 30 freshest samples were analyzed by X-ray fluorescence for oxides and some trace elements, 20 by ICP-MS for Rare-Earth Elements and 10 for Nd-Sr isotope analyses. The FBS samples were gabbros and gabbro-norites with Nb/Yb-Th/Yb and TiO2/Yb-Nb/Yb ratios indicating varying degrees of crustal interaction. The TiO2/Yb-Nb/Yb data suggested a subduction related component and the ɛNd-ɛSr indicated a juvenile source. Samples from coeval adjacent intermediate magma suites displayed similar characteristics, which suggest derivation from a bimodal source probably related with the subduction of the Rio Alegre Terrain. We interpreted the tectonic setting of the FBS as a result of a roll-back of the subducted slab, which resulted in rejuvenation of the

  1. Differential cooling across the accreted terrain-North American cratonic boundary in Idaho-Oregon

    NASA Astrophysics Data System (ADS)

    Fayon, A. K.; Tikoff, B.; Kahn, M.; Byerly, A.

    2014-12-01

    The Cretaceous-age margin of western North America, as exposed along the Idaho-Oregon border, juxtaposes accreted terrains to the west and cratonic North America to the east. The Idaho batholith intrudes exclusively east of this boundary. New (U-Th)/He zircon ages from samples collected across the boundary between 44 and 45 °N latitude illustrate the variability in cooling below 200 °C and further document the abrupt nature of this margin. Three samples collected from the accreted terrains near the Idaho border yield the oldest ages: 102.3 ± 1.6 Ma (± 1σ; Sparta Butte, OR); 88.6 ± 1.3 Ma (Snake River at Pittsburg Landing, ID); and 83.5 ± 1.4 Ma (above Pittsburg Landing, ID). These ages bracket the inferred timing of deformation within the western Idaho shear zone (WISZ), which currently demarcates this boundary. Immediately to the east, within the WISZ, samples yield ages of 76.2 ± 1.4 to 63.2 ± 1.0 Ma and record steady cooling following deformation. East of the WISZ, within the Atlanta lobe of the Idaho batholith, cooling ages vary from 65.0 ± 1.0 to 20.2 ± 0.3 Ma. The overall pattern of cooling ages is older on the margins of the Atlanta lobe and younger in the center closer to the Sawtooth Valley. Higher density sampling in the Sawtooth and White Cloud Mountains near Stanley, ID reveal internal young Miocene deformation associated with extension. Ages within the Sawtooth Mountains range from 34.2 ± 0.5 Ma (Atlanta lobe) to 20.2 ± 0.3 Ma (Sawtooth granite), whereas two samples of the Atlanta lobe from the White Cloud Mountains to the east yield ages of 68.5 ± 1.0 Ma and 64.2 ± 1.0 Ma. The cooling age pattern likely reflects exhumation of the Sawtooth Mountains along an E-dipping, low-angle normal fault during the Oligocene-Miocene. Overall, the pattern of cooling ages clearly demonstrates distinct domains of thermal stability - notably accreted terrains, WISZ, and the Atlanta lobe - with respect to cooling below 200 °C.

  2. Tectono-metamorphic evolution of the Paleoproterozoic ultra-high temperatures Khondalite Belt, North China Craton.

    NASA Astrophysics Data System (ADS)

    Lobjoie, Cyril; Trap, Pierre; Lin, Wei; Goncalves, Philippe; Marquer, Didier

    2016-04-01

    In the North China Craton, the Khondalite belt is a famous Paleoproterozoic domain where ultra-high temperatures (UHT) metamorphism was extensively documented over an area of 1000 square kilometers. Numerous petrological analyses argue for P-T conditions around 0.6-0.8GPa for temperature above 900°C for peak metamorphism. Unfortunately, the scarcity of available structural data prevents any discussion about thermo-mechanical behavior of the orogenic crust suffering high thermal regime. In this contribution, we present a detail structural analysis of the Khondalite belt that allowed to distinguish two main deformation events, named D1 and D2. The deformation D1 led to the formation of the S1 foliation that dips weakly toward the South-East. S1 holds a N70°E trending mineral and stretching L1 lineation that is sub-horizontal or plunges weakly to the East. The D1 fabrics is reworked by the dextral transpressional D2 deformation responsible for the development of km-scale S2-C2-C'2 system. The N30°E trending S2 foliation is sub-vertical to highly dipping toward the East. Kilometer-scale C2 and C'2 shear zones are sub-vertical and trend N70°E and N90-100°E, respectively. Petrological study and phase diagram modeling suggest that both D1 and D2 developed at UHT conditions. Garnet and spinel-bearing migmatites recording D1 fabric yield 0.7GPa for ca. 950-1015°C P-T conditions. Within D2 shear zones, numerous granitoids and mafic bodies are injected. Mafic intrusions are responsible for UHT contact metamorphism that can occur at low pressure as recorded in an olivine-bearing migmatite. This may suggest that the D2 S-C-C' system form an interconnected network of kilometer scale shear zones that act as pathways for percolation of mafic magmas from the mantle up to the base of the upper crust. Our results allow to discuss the role of localized heat advection along crustal-scale shear zones as a possible mechanism responsible for UHT metamorphism at regional scale, with

  3. Nature and evolution of the lower crust in the eastern North China craton: A review

    NASA Astrophysics Data System (ADS)

    Jiang, Neng; Guo, Jinghui; Chang, Guohu

    In this paper, published data for granulite terrain rocks exposed at the surface, lower crustal xenoliths, and Mesozoic intermediate-felsic igneous rocks from the eastern North China craton (NCC) are integrated to constrain the nature and evolution of the lower crust in this area. U-Pb zircon dating shows that the protolith ages for most of the granulite terrain rocks are 2500 to 2600 Ma and that many of them experienced 1800-1900 Ma metamorphism. Lower crustal xenoliths entrained in volcanic rocks with ages varying from ~ 460 to ~ 10 Ma suggest that the lower crust is dominated by Neoarchean rocks, although there may be minor rocks with ages of Meso- to Paleoarchean (> 3000 Ma), ~ 45 Ma and possibly ~ 1900 Ma locally. The Mesozoic intrusive rocks, although varying from diorite to granite and spanning from Triassic to Cretaceous, contain ~ 2500 Ma inherited zircons and have magmatic zircons with Hf crust model ages (TDMHf, C) ages of 2500-2700 Ma and whole-rock Sr-Nd isotopic compositions falling within the field of the granulite terrain rocks, pointing to their derivation by the melting of Neoarchean lower crust. The combined data for the granulite terrain rocks, lower crustal xenoliths and Mesozoic intermediate-felsic igneous rocks indicate that the present lower crust is dominated by rocks with Neoarchean ages and is intermediate to mafic in composition (i.e., SiO2 < 62%). The (87Sr/86Sr)i, ɛNd (t) and ɛHf (t) of the lower crust at 130 Ma are considered to be 0.705 to 0.716, - 10 to - 28 and - 13 to - 28, respectively. The ɛNd (t) range is very different from that proposed previously (- 32 to - 44). The large range of ɛHf (t) for the lower crust implies that significant ɛHf (t) variations for magmatic zircons from the Mesozoic intermediate-felsic igneous rocks do not necessarily reflect mixing of mantle- and crustal-magmas as commonly thought, instead they may reflect heterogeneity in the ancient lower crust. Given that the voluminous Mesozoic intermediate

  4. Multifractal spatial organisation in hydrothermal gold systems of the Archaean Yilgarn craton, Western Australia

    NASA Astrophysics Data System (ADS)

    Munro, Mark; Ord, Alison; Hobbs, Bruce

    2015-04-01

    A range of factors controls the location of hydrothermal alteration and gold mineralisation in the Earth's crust. These include the broad-scale lithospheric architecture, availability of fluid sources, fluid composition and pH, pressure-temperature conditions, microscopic to macroscopic structural development, the distribution of primary lithologies, and the extent of fluid-rock interactions. Consequently, the spatial distribution of alteration and mineralization in hydrothermal systems is complex and often considered highly irregular. However, despite this, do they organize themselves in a configuration that can be documented and quantified? Wavelets, mathematical functions representing wave-like oscillations, are commonly used in digital signals analysis. Wavelet-based multifractal analysis involves incrementally scanning a wavelet across the dataset multiple times (varying its scale) and recording its degree of fit to the signal at each interval. This approach (the wavelet transform modulus maxima method) highlights patterns of self-similarity present in the dataset and addresses the range of scales over which these patterns replicate themselves (expressed by their range in 'fractal dimension'). Focusing on seven gold ore bodies in the Archaean Yilgarn craton of Western Australia, this study investigates whether different aspects of hydrothermal gold systems evolve to organize themselves spatially as multifractals. Four ore bodies were selected from the Sunrise Dam deposit (situated in the Laverton tectonic zone of the Kurnalpi terrane) in addition to the Imperial, Majestic and Salt Creek gold prospects, situated in the Yindarlgooda dome of the Mount Monger goldfield (approximately 40km due east of Kalgoorlie). The Vogue, GQ, Cosmo East and Astro ore bodies at Sunrise Dam were chosen because they exhibit different structural geometries and relationships between gold and associated host-rock alteration styles. Wavelet-based analysis was conducted on 0.5m and 1m

  5. Mesoarchean sanukitoid rocks of the Rio Maria Granite-Greenstone Terrane, Amazonian craton, Brazil

    NASA Astrophysics Data System (ADS)

    de Oliveira, Marcelo Augusto; Dall'Agnol, Roberto; Althoff, Fernando Jacques; da Silva Leite, Albano Antonio

    2009-02-01

    The Archean sanukitoid Rio Maria Granodiorite yielded zircon ages of ˜2.87 Ga and is exposed in large domains of the Rio Maria Granite-Greenstone Terrane, southeastern Amazonian craton. It is intrusive in the greenstone belts of the Andorinhas Supergroup, in the Arco Verde Tonalite and Caracol Tonalitic Complex (older TTGs). Archean potassic leucogranites, younger TTGs and the Paleoproterozoic granites of Jamon Suite are intrusive in the Rio Maria Granodiorite. The more abundant rocks of the Rio Maria Granodiorite have granodioritic composition and display medium to coarse even-grained textures. These rocks show generally a gray color with greenish shades due to strongly saussuritized plagioclase, and weak WNW-ESE striking foliation. The significant geochemical contrasts between the occurrences of Rio Maria Granodiorite in different areas suggest that this unit corresponds in fact to a granodioritic suite of rocks derived from similar but distinct magmas. Mingling processes involving the Rio Maria Granodiorite and similar mafic to intermediate magmas are able to explain the constant occurrence of mafic enclaves in the granodiorite. The associated intermediate rocks occur mainly near Bannach, where mostly quartz diorite and quartz monzodiorite are exposed. The dominant rocks are mesocratic, dark-green rocks, with fine to coarse even-grained texture. The Rio Maria Granodiorite and associated intermediate rocks show similar textural and mineralogical aspects. They follow the calc-alkaline series trend in some diagrams. However, they have high-Mg#, Cr, and Ni conjugate with high contents of large ion lithophile elements (LILEs), typical of sanukitoids series. The patterns of rare earth elements of different rocks are similar, with pronounced enrichment in light rare earth elements (LREEs) and strong to moderate fractionation of heavy rare earth elements (HREEs). Field aspects and petrographic and geochemical characteristics denote that the granodiorites and

  6. Three-dimensional field perspective on deformation, flow, and growth of the lower continental crust (Dharwar craton, India)

    NASA Astrophysics Data System (ADS)

    Chardon, Dominique; Jayananda, Mudlappa

    2008-02-01

    The study of fabric development and juvenile batholith emplacement across the tilted crustal section of the Eastern Dharwar craton shows that horizontal, constrictional deformation affected large volumes of the midcrust and lower crust at the time of regional partial melting and magmatic accretion. Constriction is achieved by a combination of coeval shallow and steep planar fabrics sharing a common horizontal elongation direction, two sets of conjugate strike-slip shears, and extensional shear zones. The Eastern Dharwar craton illustrates an end-member deformation mode by which a particularly weakened lithosphere responds to shortening by developing distributed, horizontal plane strain on a crustal scale, resulting from the combination of crustal shortening and lateral gravity-driven flow. Thinning accompanying constrictional deformation is interpreted as compensating for juvenile magmatic accretion and thickening of greenstone belts and as acting to maintain a stable crustal thickness. Such a midcrustal to lower crustal deformation process may provide a resolution of the batholithic room problem in a softened crust submitted to lateral shortening and may explain nearly isobaric retrograde pressure-temperature-time paths of high temperature-low pressure high-grade terrains.

  7. Paleoproterozoic high-pressure metamorphism in the northern North China Craton and implications for the Nuna supercontinent

    PubMed Central

    Wan, Bo; Windley, Brian F.; Xiao, Wenjiao; Feng, Jianyun; Zhang, Ji'en

    2015-01-01

    The connection between the North China Craton (NCC) and contiguous cratons is important for the configuration of the Nuna supercontinent. Here we document a new Paleoproterozoic high-pressure (HP) complex dominated by garnet websterite on the northern margin of the NCC. The peak metamorphism of the garnet websterite was after ∼1.90 Ga when it was subducted to eclogite facies at ∼2.4 GPa, then exhumed back to granulite facies at ∼0.9 GPa before ∼1.82 Ga. The rock associations with their structural relationships and geochemical affinities are comparable to those of supra-subduction zone ophiolites, and supported by subduction-related signatures of gabbros and basalts. We propose that a ∼1.90 Ga oceanic fragment was subducted and exhumed into an accretionary complex along the northern margin of the NCC. Presence of the coeval Sharyzhalgai complex with comparable HP garnet websterites in the southern Siberian active margin favours juxtaposition against the NCC in the Paleoproterozoic. PMID:26388458

  8. Paleoproterozoic high-pressure metamorphism in the northern North China Craton and implications for the Nuna supercontinent.

    PubMed

    Wan, Bo; Windley, Brian F; Xiao, Wenjiao; Feng, Jianyun; Zhang, Ji'en

    2015-09-21

    The connection between the North China Craton (NCC) and contiguous cratons is important for the configuration of the Nuna supercontinent. Here we document a new Paleoproterozoic high-pressure (HP) complex dominated by garnet websterite on the northern margin of the NCC. The peak metamorphism of the garnet websterite was after ∼1.90 Ga when it was subducted to eclogite facies at ∼2.4 GPa, then exhumed back to granulite facies at ∼0.9 GPa before ∼1.82 Ga. The rock associations with their structural relationships and geochemical affinities are comparable to those of supra-subduction zone ophiolites, and supported by subduction-related signatures of gabbros and basalts. We propose that a ∼1.90 Ga oceanic fragment was subducted and exhumed into an accretionary complex along the northern margin of the NCC. Presence of the coeval Sharyzhalgai complex with comparable HP garnet websterites in the southern Siberian active margin favours juxtaposition against the NCC in the Paleoproterozoic.

  9. Antiquity of the Rı´o de la Plata craton in Tandilia, southern Buenos Aires province, Argentina

    NASA Astrophysics Data System (ADS)

    Pankhurst, R. J.; Ramos, A.; Linares, E.

    2003-05-01

    Rb-Sr and Sm-Nd whole-rock data for granitoids and orthogneisses from the western part of the Sierras Septentrionales of the southern Buenos Aires province yield an errorchron of 2009±71 Ma (initial 87Sr/ 86Sr=0.7041, MSWD=69) and an isochron of 2140±88 Ma (initial 143Nd/ 144Nd=0.50977), respectively. As in previous investigations, the Rb-Sr data are clearly disturbed, but the Sm-Nd isochron may record the age of emplacement of igneous precursors. These results reaffirm that this region is the southern extension of the crystalline basement of the Rı´o de la Plata craton. The Sm-Nd age, though not very precise, is slightly older than previously demonstrated but consistent with most recent U-Pb studies of the craton exposed in Uruguay and Brazil. Crust-derived Sm-Nd model ages averaging 2620±80 Ma indicate that, though the principal rock-forming events were Paleoproterozoic, a Late Archaean prehistory is possible. However, the data place strict constraints on the nature and intensity of post-2000 Ma activity in this area, which seems to be confined to tholeiitic dyke emplacement and hydrothermal reactivation.

  10. Paleoproterozoic high-pressure metamorphism in the northern North China Craton and implications for the Nuna supercontinent.

    PubMed

    Wan, Bo; Windley, Brian F; Xiao, Wenjiao; Feng, Jianyun; Zhang, Ji'en

    2015-01-01

    The connection between the North China Craton (NCC) and contiguous cratons is important for the configuration of the Nuna supercontinent. Here we document a new Paleoproterozoic high-pressure (HP) complex dominated by garnet websterite on the northern margin of the NCC. The peak metamorphism of the garnet websterite was after ∼1.90 Ga when it was subducted to eclogite facies at ∼2.4 GPa, then exhumed back to granulite facies at ∼0.9 GPa before ∼1.82 Ga. The rock associations with their structural relationships and geochemical affinities are comparable to those of supra-subduction zone ophiolites, and supported by subduction-related signatures of gabbros and basalts. We propose that a ∼1.90 Ga oceanic fragment was subducted and exhumed into an accretionary complex along the northern margin of the NCC. Presence of the coeval Sharyzhalgai complex with comparable HP garnet websterites in the southern Siberian active margin favours juxtaposition against the NCC in the Paleoproterozoic. PMID:26388458

  11. A precisely dated Proterozoic palaeomagnetic pole from the North China craton, and its relevance to palaeocontinental reconstruction

    NASA Astrophysics Data System (ADS)

    Halls, Henry C.; Li, Jianghai; Davis, Don; Hou, Guiting; Zhang, Baoxing; Qian, Xianglin

    2000-10-01

    A palaeomagnetic pole position, derived from a precisely dated primary remanence, with minimal uncertainties due to secular variation and structural correction, has been obtained for China's largest dyke swarm, which trends for about 1000km in a NNW direction across the North China craton. Positive palaeomagnetic contact tests on two dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. The age of one of these dykes, based on U-Pb dating of primary zircon, is 1769.1+/-2.5Ma. The mean palaeomagnetic direction for 19 dykes, after structural correction, is D=36°, I=-5°, k=63, α95=4°, yielding a palaeomagnetic pole at Plat=36°N, Plong=247°E, dp=2°, dm=4° and a palaeolatitude of 2.6°S. Comparison of this pole position with others of similar age from the Canadian Shield allows a continental reconstruction that is compatible with a more or less unchanged configuration of Laurentia, Siberia and the North China craton since about 1800Ma

  12. Transition from a localized to wide deformation along Eastern branch of Central East African Rift: Insights from 3D numerical models

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

    The Central East African Rift (CEAR) bifurcates in two branches (eastern, magma-rich and western, magma-poor) surrounding strong Tanzanian craton. Intensive magmatism and continental flood basalts are largely present in many of the eastern rift segments, but other segments, first of all the western branch, exhibit very small volcanic activity. The Eastern rift is characterized by southward progression of the onset of volcanism, the extensional features and topographic expression of the rift vary significantly north-southward: in northern Kenya the deformation is very wide (some 150-250 km in E-W direction), to the south the rift narrows to 60-70 km, yet further to the south the deformation widens again in the so-called Tanzania divergence zone. Widening of the Eastern branch within its southern part is associated with the impingement of the southward-propagating rift on the strong Masai block situated to east of the Tanzanian craton. To understand the mechanisms behind this complex deformation distribution, we implemented a 3Dl ultra-high resolution visco-plastic thermo-mechanical numerical model accounting for thermo-rheological structure of the lithosphere and hence captures essential features of the CEAR. The preferred model has a plume seeded slightly to the northeast of the craton center, consistent with seismic tomography, and produces surface strain distribution that is in good agreement with observed variation of deformation zone width along eastern side of Tanzanian craton: localized above bulk of mantle material deflected by cratonic keel narrow high strain zone (Kenia Rift) is replaced by wide distributed deformations within areas situated to north (northern Kenya, Turkana Rift) and to south (Tanzania divergence, Masai block) of it. These results demonstrate significant differences in the impact of the rheological profile on rifting style in case of dominant active rifting compared to dominant passive rifting. Narrow rifting, conventionally attributed to

  13. African American Administrators and Staff

    ERIC Educational Resources Information Center

    Wright, Dianne; Taylor, Janice D.; Burrell, Charlotte; Stewart, Gregory

    2006-01-01

    This article explores the issues of African American participation in the administrative ranks of the academy. The authors find that African Americans tend to hold positions that are marginal in academic organizations, lacking power and influence, and that not much has changed over recent decades. Forces influencing this condition are explored,…

  14. African-American Children's Literature.

    ERIC Educational Resources Information Center

    Kline, Lucinda

    This paper examines the history of African American children's literature, the present-day status of it, and ventures predictions about its future. The paper also considers the historic and social factors of the debate about whether an author who is not African American can write a book that will/should be accepted in this category of children's…

  15. African-American Sacred Music.

    ERIC Educational Resources Information Center

    Bailey, A. Peter

    1991-01-01

    The history of African-American sacred music is traced from the time of slavery to the present interest in gospel music. The religious music of African Americans is geared toward liberation themes. It is important that this music does not dilute its power through cross-over with other music forms. (SLD)

  16. Africanization in the United States

    PubMed Central

    Pinto, M. Alice; Rubink, William L.; Patton, John C.; Coulson, Robert N.; Johnston, J. Spencer

    2005-01-01

    The expansion of Africanized honeybees from South America to the southwestern United States in <50 years is considered one of the most spectacular biological invasions yet documented. In the American tropics, it has been shown that during their expansion Africanized honeybees have low levels of introgressed alleles from resident European populations. In the United States, it has been speculated, but not shown, that Africanized honeybees would hybridize extensively with European honeybees. Here we report a continuous 11-year study investigating temporal changes in the genetic structure of a feral population from the southern United States undergoing Africanization. Our microsatellite data showed that (1) the process of Africanization involved both maternal and paternal bidirectional gene flow between European and Africanized honeybees and (2) the panmitic European population was replaced by panmitic mixtures of A. m. scutellata and European genes within 5 years after Africanization. The post-Africanization gene pool (1998–2001) was composed of a diverse array of recombinant classes with a substantial European genetic contribution (mean 25–37%). Therefore, the resulting feral honeybee population of south Texas was best viewed as a hybrid swarm. PMID:15937139

  17. Tectonics of the North African Variscides (Morocco, western Algeria): an outline

    NASA Astrophysics Data System (ADS)

    Hoepffner, Christian; Houari, Mohamed Rachid; Bouabdelli, Mohamed

    2006-01-01

    The Palaeozoic terranes that crop out north of the South Atlasic Line constitute the Variscan Belt of North Africa. Subdivision of the belt into five structural zones separated by major shear zones results from a polyphase evolution including very localised Prevariscan events (450-430 Ma), which correspond to the Caledonian cycle, and Variscan events involving three main stages: Eovariscan (370-360 Ma), Mesovariscan (330-320 Ma), and Neovariscan (300-290 Ma), followed by Tardi-Variscan events during Early Permian-Triassic times. During the Variscan evolution, the geodynamic context is transtensive-transpressive, and controls the opening and closure of Devono-Carboniferous basins. The Variscan deformations are accommodated by folding and combination of thrusting and strike-slip faulting associated with a low-grade to medium-grade metamorphic evolution, characteristic of the external zones of the orogens. The lack of oceanic crust, and thus of suture zone in the Palaeozoic series suggests that the North African Variscan Belt is an intracontinental part of the Variscan orogen, which evolved near the margin of the West African Craton. The links with the other Periatlantic Palaeozoic segments still remain a subject of discussion. To cite this article: C. Hoepffner et al., C. R. Geoscience 338 (2006).

  18. The southern and central parts of the "Souttoufide" belt, Northwest Africa

    NASA Astrophysics Data System (ADS)

    Villeneuve, Michel; Gärtner, Andreas; Youbi, Nasrrddine; El Archi, Abdelkrim; Vernhet, Elodie; Rjimati, Ech-Cherki; Linnemann, Ulf; Bellon, Hervé; Gerdes, Axel; Guillou, Omar; Corsini, Michel; Paquette, Jean-Louis

    2015-12-01

    The Souttoufide belt is situated between the High-Atlas in the north, the Mauritanides in the south and the West African Craton as well as the Tindouf basin in the east. Thus, this belt is a part of the West African fold belt system which surrounds the West African Craton. Outcrops occur only in three areas, while the rest of the Souttoufide belt is covered by Meso- to Cenozoic sediments. These areas are, from the south to the north, the Adrar Souttouf Massif, the Smara Zemmour area, and the western part of the Anti-Atlas. Because of the continuing processing and numerous publications, the Anti-Atlas cannot be dealt with in detail. Fundamental geological work in both of the other areas was mainly done during the second third of the 20th century, followed by several decades of inactivity. Numerous studies were done in the Adrar Souttouf massif during the last decade, while the Smara-Zemmour area still remains poorly investigated. Most of them were focused on geochronological analyses and geochemistry. Accordingly, the obtained data led to a new interpretation. The Adrar Souttouf is interpreted as a Pan-African belt exhibiting a Neoproterozoic island arc reworked by the Variscan collision between the North American Craton (NAC) and the West African Craton (WAC). The Smara-Zemmour area is considered as the external part of this Variscan belt thrusted onto the Palaeozoic Tindouf basin. The western Anti-Atlas as considered as a Pan-African active margin reworked during the Variscan orogen. Together, these three areas, lead to the hypothesis of a Late Pan-African belt capped by the Early and Middle Palaeozoic covers and then affected by the Middle Carboniferous tectonic event (330 Ma) related to the Pangaean assembly. Correlations with adjacent belts (Mauritanides and Anti-Atlas) are considered.

  19. The history of African trypanosomiasis

    PubMed Central

    Steverding, Dietmar

    2008-01-01

    The prehistory of African trypanosomiasis indicates that the disease may have been an important selective factor in the evolution of hominids. Ancient history and medieval history reveal that African trypanosomiasis affected the lives of people living in sub-Saharan African at all times. Modern history of African trypanosomiasis revolves around the identification of the causative agents and the mode of transmission of the infection, and the development of drugs for treatment and methods for control of the disease. From the recent history of sleeping sickness we can learn that the disease can be controlled but probably not be eradicated. Current history of human African trypanosomiasis has shown that the production of anti-sleeping sickness drugs is not always guaranteed, and therefore, new, better and cheaper drugs are urgently required. PMID:18275594

  20. Cancer statistics for African Americans.

    PubMed

    Ghafoor, Asma; Jemal, Ahmedin; Cokkinides, Vilma; Cardinez, Cheryll; Murray, Taylor; Samuels, Alicia; Thun, Michael J

    2002-01-01

    The American Cancer Society provides estimates on the number of new cancer cases and deaths, and compiles health statistics on African Americans in a biennial publication, Cancer Facts and Figures for African Americans. The compiled statistics include cancer incidence, mortality, survival, and lifestyle behaviors using the most recent data on incidence and survival from the National Cancer Institute's (NCI) Surveillance, Epidemiology, and End Results (SEER) program, mortality data from the National Center for Health Statistics (NCHS), and behavioral information from the Behavior Risk Factor Surveillance System (BRFSS), Youth Risk Behavior Surveillance System (YRBSS), and National Health Interview Survey (NHIS). It is estimated that 132,700 new cases of cancer and 63,100 deaths will occur among African Americans in the year 2003. Although African Americans have experienced higher incidence and mortality rates of cancer than whites for many years, incidence rates have declined by 2.7 percent per year in African-American males since 1992, while stabilizing in African-American females. During the same period, death rates declined by 2.1 percent and 0.4 percent per year among African-American males and females, respectively. The decrease in both incidence and death rates from cancer among African-American males was the largest of any racial or ethnic group. Nonetheless, African Americans still carry the highest cancer burden among US racial and ethnic groups. Most cancers detectable by screening are diagnosed at a later stage and survival rates are lower within each stage of disease in African Americans than in whites. The extent to which these disparities reflect unequal access to health care versus other factors is an active area of research.

  1. Bioenergy and African transformation.

    PubMed

    Lynd, Lee R; Sow, Mariam; Chimphango, Annie Fa; Cortez, Luis Ab; Brito Cruz, Carlos H; Elmissiry, Mosad; Laser, Mark; Mayaki, Ibrahim A; Moraes, Marcia Afd; Nogueira, Luiz Ah; Wolfaardt, Gideon M; Woods, Jeremy; van Zyl, Willem H

    2015-01-01

    Among the world's continents, Africa has the highest incidence of food insecurity and poverty and the highest rates of population growth. Yet Africa also has the most arable land, the lowest crop yields, and by far the most plentiful land resources relative to energy demand. It is thus of interest to examine the potential of expanded modern bioenergy production in Africa. Here we consider bioenergy as an enabler for development, and provide an overview of modern bioenergy technologies with a comment on application in an Africa context. Experience with bioenergy in Africa offers evidence of social benefits and also some important lessons. In Brazil, social development, agricultural development and food security, and bioenergy development have been synergistic rather than antagonistic. Realizing similar success in African countries will require clear vision, good governance, and adaptation of technologies, knowledge, and business models to myriad local circumstances. Strategies for integrated production of food crops, livestock, and bioenergy are potentially attractive and offer an alternative to an agricultural model featuring specialized land use. If done thoughtfully, there is considerable evidence that food security and economic development in Africa can be addressed more effectively with modern bioenergy than without it. Modern bioenergy can be an agent of African transformation, with potential social benefits accruing to multiple sectors and extending well beyond energy supply per se. Potential negative impacts also cut across sectors. Thus, institutionally inclusive multi-sector legislative structures will be more effective at maximizing the social benefits of bioenergy compared to institutionally exclusive, single-sector structures. PMID:25709714

  2. African oil plays

    SciTech Connect

    Clifford, A.J. )

    1989-09-01

    The vast continent of Africa hosts over eight sedimentary basins, covering approximately half its total area. Of these basins, only 82% have entered a mature exploration phase, 9% have had little or no exploration at all. Since oil was first discovered in Africa during the mid-1950s, old play concepts continue to bear fruit, for example in Egypt and Nigeria, while new play concepts promise to become more important, such as in Algeria, Angola, Chad, Egypt, Gabon, and Sudan. The most exciting developments of recent years in African oil exploration are: (1) the Gamba/Dentale play, onshore Gabon; (2) the Pinda play, offshore Angola; (3) the Lucula/Toca play, offshore Cabinda; (4) the Metlaoui play, offshore Libya/Tunisia; (5) the mid-Cretaceous sand play, Chad/Sudan; and (6) the TAG-I/F6 play, onshore Algeria. Examples of these plays are illustrated along with some of the more traditional oil plays. Where are the future oil plays likely to develop No doubt, the Saharan basins of Algeria and Libya will feature strongly, also the presalt of Equatorial West Africa, the Central African Rift System and, more speculatively, offshore Ethiopia and Namibia, and onshore Madagascar, Mozambique, and Tanzania.

  3. Bioenergy and African transformation.

    PubMed

    Lynd, Lee R; Sow, Mariam; Chimphango, Annie Fa; Cortez, Luis Ab; Brito Cruz, Carlos H; Elmissiry, Mosad; Laser, Mark; Mayaki, Ibrahim A; Moraes, Marcia Afd; Nogueira, Luiz Ah; Wolfaardt, Gideon M; Woods, Jeremy; van Zyl, Willem H

    2015-01-01

    Among the world's continents, Africa has the highest incidence of food insecurity and poverty and the highest rates of population growth. Yet Africa also has the most arable land, the lowest crop yields, and by far the most plentiful land resources relative to energy demand. It is thus of interest to examine the potential of expanded modern bioenergy production in Africa. Here we consider bioenergy as an enabler for development, and provide an overview of modern bioenergy technologies with a comment on application in an Africa context. Experience with bioenergy in Africa offers evidence of social benefits and also some important lessons. In Brazil, social development, agricultural development and food security, and bioenergy development have been synergistic rather than antagonistic. Realizing similar success in African countries will require clear vision, good governance, and adaptation of technologies, knowledge, and business models to myriad local circumstances. Strategies for integrated production of food crops, livestock, and bioenergy are potentially attractive and offer an alternative to an agricultural model featuring specialized land use. If done thoughtfully, there is considerable evidence that food security and economic development in Africa can be addressed more effectively with modern bioenergy than without it. Modern bioenergy can be an agent of African transformation, with potential social benefits accruing to multiple sectors and extending well beyond energy supply per se. Potential negative impacts also cut across sectors. Thus, institutionally inclusive multi-sector legislative structures will be more effective at maximizing the social benefits of bioenergy compared to institutionally exclusive, single-sector structures.

  4. Timing the structural events in the Palaeoproterozoic Bolé-Nangodi belt terrane and adjacent Maluwe basin, West African craton, in central-west Ghana

    NASA Astrophysics Data System (ADS)

    de Kock, G. S.; Théveniaut, H.; Botha, P. M. W.; Gyapong, W.

    2012-04-01

    The Maluwe basin, north-adjacent to the Sunyani basin, is the northernmost of the northeast-trending Eburnean volcaniclastic depositories in Ghana. These basins are separated from one another by remnants of Eoeburnean crust, all formed during the evolution of an arc-backarc basins complex in a Palaeoproterozoic intraoceanic environment. The Bolé-Nangodi belt terrane to the northwest, of mostly Eoeburnean crust is fault bounded with the Maluwe basin along the northeast-trending Bolé-Navrongo fault zone. The stratigraphic sequence, which was the key to unravelling the structural evolution of the study area, was established by means of field observations aided by precision SHRIMP geochronology. The quartzitic, pelitic, quartzofeldspathic and granitic gneisses of the Eoeburnean crust (>2150 Ma) experienced complex metamorphic mineral growth and migmatitization, mostly under static crustal conditions and were subjected to several deformation episodes. The foliated mafic and metasedimentary enclaves within the Ifanteyire granite establish deformation to have taken place prior to ˜2195 Ma, while the tectonically emplaced Kuri amphibolites within the 2187-Ma gneissic Gondo granite indicate a stage of rifting followed by collision. Deformation of granite dykes in the Gondo granites at ˜2150 Ma concluded the development of the Eoeburnean orogenic cycle (DEE). The Sawla Suite, contemporaneous with the deposition of the Maluwe Group, intruded the tectonic exhumed Bolé-Nangodi terrane during extension between ˜2137 and 2125 Ma. The rifting separated the Abulembire fragment from the Bolé-Nangodi terrane. During subsequent northwestward subduction of young back-arc basin oceanic crust the volcaniclastic strata of the Maluwe Group and Sawla granitoids were deformed (DE1) under chlorite/sericite greenschist-grade conditions. The NE-trending folds had subhorizontal axes and subvertical axial planes. Simultaneous to the DE1 orogenesis the molasses of the Banda Group was deposited concordantly on the submerged Sunyani strata after a hiatus of 20 million years. After cessation of the NW-SE-directed compression the early Tanina Suite intruded as batholiths, dykes and sheets and produced garnet, staurolite, sillimanite and kyanite in their thermal aureoles. Docking of the Sunyani basin produced the DE2 thrust related folding and stacking in the deformed and granitoid invaded Maluwe basin as well as the single stage sin- and anticlinoria in the Sunyani and Banda Groups. In the Maluwe basin the Abulembire fragment acted as a resistor and the approaching front rotated anticlockwise and clockwise around the barrier to form west- and north-directed piggy-back thrust-stacking and deformation of the Tanina Suite granitoids. Due to the low metamorphic conditions the DE2 fabric is limited to crenulation cleavages in the more psammitic and pelitic units. The fold axes are double plunging (N-S and E-W) up to 60° with the axial planar fabric subvertical. Post-D2 tectonic relaxation has allowed the emplacement of the last Tanina Suite calc-alkaline melts and was succeeded by N-S extension fracturing (DE3) along which mantle derived Wakawaka gabbroids and syenite intruded. The DE1 folding occurred between 2125 and 2122 Ma and DE2 before 2119 Ma. The tectonic relaxation occurred at 2118 Ma. Around 2100 Ma, NE-SW directed strike-slip shearing (DE4), fractured the Bolé-Nangodi terrane and enhanced the basin-belt boundary. Along the boundary, the displacement was dextral along vertical faults but, southward, it became more east-over-west thrust related. Associated tension gashes are filled with vein quartz and pegmatite and typical of the brittle sector of the crust. Tectonism in this part of the intraoceanic accretionary arc back-arc complex was concluded by limited, right-lateral strike-slip (DE5) movement which formed some breccias.

  5. Paleomagnetism of the Marble Bar Chert Member, Western Australia: Implications for apparent polar wander path for Pilbara craton during Archean time

    NASA Astrophysics Data System (ADS)

    Suganuma, Yusuke; Hamano, Yozo; Niitsuma, Sachiko; Hoashi, Masamichi; Hisamitsu, Toshio; Niitsuma, Nobuaki; Kodama, Kazuto; Nedachi, Munetomo

    2006-12-01

    The Archean Biosphere Drilling Project (ABDP) drilled a continuous 270 m long oriented core from the Towers Formation, which includes the Marble Bar Chert Member (3456.1-3476.0 Ma) in the Pilbara craton, northwestern Australia. A paleomagnetic study of 261 discrete specimens, collected from a 158.5 to 182.0 m section of the Marble Bar Chert Member, revealed two distinct magnetic components (LT and MT). The MT component yields seven different mean paleomagnetic directions clustered as MB1 to MB7. These, together with the published paleomagnetic poles of early Archean rocks from the Pilbara craton, draw a continuous paleomagnetic pole path, which likely to be regarded as the early to late Archean apparent polar wander path (APWP) for the Pilbara craton. The APWP implies that the Pilbara craton underwent a latitudinal drift of about 21° during the interval when the magnetization of the Marble Bar Chert Member was acquired. The estimated speed of the lateral drift is 12-112 cm/yr (120-1120 km/Myr), which is large compared with current plate motion velocities, suggesting that continents might have moved during the Archean faster than in the Phanerozoic.

  6. Proterozoic evolution of the western margin of the Wyoming craton: Implications for the tectonic and magmatic evolution of the northern Rocky Mountains

    USGS Publications Warehouse

    Foster, D.A.; Mueller, P.A.; Mogk, D.W.; Wooden, J.L.; Vogl, J.J.

    2006-01-01

    Defining the extent and age of basement provinces west of the exposed western margin of the Archean Wyoming craton has been elusive because of thick sedimentary cover and voluminous Cretaceous-Tertiary magmatism. U-Pb zircon geochronological data from small exposures of pre-Belt supergroup basement along the western side of the Wyoming craton, in southwestern Montana, reveal crystallization ages ranging from ???2.4 to ???1.8 Ga. Rock-forming events in the area as young as ???1.6 Ga are also indicated by isotopic (Nd, Pb, Sr) signatures and xenocrystic zircon populations in Cretaceous-Eocene granitoids. Most of this lithosphere is primitive, gives ages ???1.7-1.86 Ga, and occurs in a zone that extends west to the Neoproterozoic rifted margin of Laurentia. These data suggest that the basement west of the exposed Archean Wyoming craton contains accreted juvenile Paleoproterozoic arc-like terranes, along with a possible mafic underplate of similar age. This area is largely under the Mesoproterozoic Belt basin and intruded by the Idaho batholith. We refer to this Paleoproterozoic crust herein as the Selway terrane. The Selway terrane has been more easily reactivated and much more fertile for magma production and mineralization than the thick lithosphere of the Wyoming craton, and is of prime importance for evaluating Neoproterozoic continental reconstructions. ?? 2006 NRC Canada.

  7. Chapter 27: Geology and petroleum potential of the north and east margins of the Siberian Craton, north of the Arctic Circle

    USGS Publications Warehouse

    Klett, T.R.; Wandrey, C.J.; Pitman, J.K.

    2011-01-01

    The Siberian Craton consists of crystalline rocks and superimposed Precambrian sedimentary rocks deposited in rift basins. Palaeozoic rocks, mainly carbonates, were deposited along the margins of the craton to form an outwardly younger concentric pattern that underlies an outward-thickening Mesozoic sedimentary section. The north and east margins of the Siberian Craton subsequently became foreland basins created by compressional deformation during collision with other tectonic plates. The Tunguska Basin developed as a Palaeozoic rift/sag basin over Proterozoic rifts. The geological provinces along the north and east margins of the Siberian Craton are immature with respect to exploration, so exploration-history analysis alone cannot be used for assessing undiscovered petroleum resources. Therefore, other areas from around the world having greater petroleum exploration maturity and similar geological characteristics, and which have been previously assessed, were used as analogues to aid in this assessment. The analogues included those of foreland basins and rift/sag basins that were later subjected to compression. The US Geological Survey estimated the mean undiscovered, technically recoverable conventional petroleum resources to be approximately 28 billion barrels of oil equivalent, including approximately 8 billion barrels of crude oil, 103 trillion cubic feet of natural gas and 3 billion barrels of natural gas liquids. ?? 2011 The Geological Society of London.

  8. Juvenile accretion (2360-2330 Ma) in the São Francisco Craton, and implications for the Columbia supercontinent: evidence from U-Pb zircon ages, Sr-Nd-Hf and geochemical constraints

    NASA Astrophysics Data System (ADS)

    Teixeira, W.; Ávila, C.

    2012-12-01

    The Mineiro and the Itabuna-Salvador-Curaçá belts are segments of an Early Proterozoic orogen, in the São Francisco/West Congo-North Gabon craton. The latter segment includes island-arc rocks with preserved portions of the accretionary prism and back-arc basins, developed between 2.4 and 2.0 Ga. The Mineiro belt evolved marginally to the Minas passive margin basin (<2.55 to 2.35 Ga). It contains mainly granitoid rocks with ages between 2.25-2.20 Ga and 2.12-2.08 Ga, along with coeval back arc sequences. The overall framework includes regional metamorphism and related faults and shear zones across both belts. Similar tectonic features are portrayed by the West Central African belt (of Eburnean age) by considering the early contiguous African counterpart. We present an integrated geochronologic and geochemical study for the Resende Costa orthogneiss (Mineiro belt): the gneissic rocks are slightly metaluminous to peraluminous, subalkaline, show varied SiO2 (69 to 73wt.%) contents, and low K2O and high- Na2O +CaO ones. Chemically, they are compatible with high Al2O3 trondhjemites. They also show weak positive Eu/Eu* anomalies, low Rb (24 to 70ppm), Ba (500 to 1000ppm), Th (2.1 to 8.5ppm) contents, very high Sr/Y ratios (75 to 158) and variable LREE and low HREE patterns (Yb < 1.23 ppm). The Resende Costa pluton yields two U-Pb (LA-ICPMS) zircon crystallization ages (2358±10 Ma and 2356±12 Ma), while the zircon rims yield 2133±32 Ma, interpreted as the age of metamorphism. The Sm/Nd TDM whole rock model ages are between 2.35-2.50 Ga, whereas the ɛNd(t) values range from +1.2 to +3.0, ɛSr(t) from +10 to -6, and ɛHf(t) in zircon between -3 to +6. The nearby Ramos gneissic pluton gives U-Pb zircon age of 2331±17 Ma, TDM age of 2.4 Ga, ɛNd(t) +2.2, ɛHf(t) (-9/+9) and ɛSr(t) +40 values. The overall signature implies to short crustal residence for the protholiths with minor contamination during the petrogenesis. Published data reveal that the nearby Lagoa Dourada

  9. Zircon from Mesoproterozoic sediments sheds light on the subduction-collision history at the eastern active continental margin of the Archaean Kalahari-Grunehogna Craton

    NASA Astrophysics Data System (ADS)

    Marschall, H.; Hawkesworth, C. J.; Leat, P. T.; Dhuime, B.; Storey, C.

    2013-12-01

    The Grunehogna Craton (East Antarctica) was a part of the Archean Kalahari Craton of southern Africa prior to Gondwana breakup. Granite from the basement of the craton has been dated by U-Pb zircon dating to 3,067 Ma with inherited grains showing ages of up to 3,433 Ma [1]. At the eastern margin of the craton, the Ahlmannryggen nunataks comprise an ~2000 m thick pile of clastic and volcanic sediments of the Ritscherflya Supergroup. These were sourced from eroding a proximal active continental arc as demonstrated through the age distribution and internal zoning of detrital zircon [2]. Detrital zircon grains from the Ritscherflya Supergroup show an age distribution with a dominant age peak at ~1,130 Ma, i.e., close to the sedimentation age. Older age peaks include those at 1370 Ma, 1725 Ma, 1880 Ma, 2050 Ma, and 2700 Ma. Palaeo- and Mesoarchaean zircon grains (2800-3445 Ma) were also discovered, corresponding to the age of the Kalahari-Grunehogna Craton basement. Most significantly we found a number of inherited Archaean cores in ~1130 Ma zircons. They demonstrate that the volcanic arc was indeed located on Archaean continental crust, rather than in Mesoproterozoic, intra-oceanic island arcs. The age spectrum of the zircons bears strong evidence for (i) derivation of the entire Ritscherflya sediment sequence from an active continental convergent margin; (ii) a cratonic provenance of part of the sediments from population peaks coinciding with major tectono-thermal events in the Kalahari Craton; (iii) at least some of the active volcanism being located on cratonic basement rather than a juvenile island arc. Detrital zircons in the ~1130 Ma age group show several distinct populations in their Hf isotopic compositions. The dominant group shows negative ɛHf values of -11.5 corresponding to a model age (TDM) of ~2700 Ma (average crustal 176Lu/177Hf = 0.015). A smaller group shows ɛHf values of +2 to +6, which may represent mantle-derived subduction-zone volcanism at

  10. Successfully Educating Our African-American Students

    ERIC Educational Resources Information Center

    Moncree-Moffett, Kareem

    2013-01-01

    The purpose of this empirical study was to explore the lived experiences of African American retired female teachers who have prior experience with educating urban African American students in public schools. Also explored are the experiences of active African American female teachers of urban African American students and comparisons are…

  11. Traces of the crustal units and the upper-mantle structure in the southwestern part of the East European Craton

    NASA Astrophysics Data System (ADS)

    Janutyte, I.; Kozlovskaya, E.; Majdanski, M.; Voss, P. H.; Budraitis, M.; Passeqworking Group

    2014-08-01

    The presented study is a part of the passive seismic experiment PASSEQ 2006-2008, which took place around the Trans-European Suture Zone (TESZ) from May 2006 to June 2008. The data set of 4195 manually picked arrivals of teleseismic P waves of 101 earthquakes (EQs) recorded in the seismic stations deployed to the east of the TESZ was inverted using the non-linear teleseismic tomography algorithm TELINV. Two 3-D crustal models were used to estimate the crustal travel time (TT) corrections. As a result, we obtain a model of P-wave velocity variations in the upper mantle beneath the TESZ and the East European Craton (EEC). In the study area beneath the craton, we observe up to 3% higher and beneath the TESZ about 2-3% lower seismic velocities compared to the IASP91 velocity model. We find the seismic lithosphere-asthenosphere boundary (LAB) beneath the TESZ at a depth of about 180 km, while we observe no seismic LAB beneath the EEC. The inversion results obtained with the real and the synthetic data sets indicate a ramp shape of the LAB in the northern TESZ, where we observe values of seismic velocities close to those of the craton down to about 150 km. The lithosphere thickness in the EEC increases going from the TESZ to the NE from about 180 km beneath Poland to 300 km or more beneath Lithuania. Moreover, in western Lithuania we find an indication of an upper-mantle dome. In our results, the crustal units are not well resolved. There are no clear indications of the features in the upper mantle which could be related to the crustal units in the study area. On the other hand, at a depth of 120-150 km we indicate a trace of a boundary of proposed palaeosubduction zone between the East Lithuanian Domain (EL) and the West Lithuanian Granulite Domain (WLG). Also, in our results, we may have identified two anorogenic granitoid plutons.

  12. Aeromagnetic signatures reveal a back-arc basin imposed upon the inherited rifted margin of the East Antarctic craton

    NASA Astrophysics Data System (ADS)

    Armadillo, E.; Ferraccioli, F.; Jordan, T. A.; Bozzo, E.

    2009-12-01

    The Wilkes Subglacial Basin (WSB) represents a largely unexplored, approximately 1400 km-long and up to 600 km-wide subglacial depression, buried beneath the over 3 km-thick East Antarctic Ice Sheet. During the 2005-06 austral summer an extensive aerogeophysical survey was flown to investigate the WSB adjacent to northern Victoria Land (NVL), and included the acquisition of new airborne radar, aeromagnetic and aerogravity data. Several contrasting models for the origin of the basin have been previously proposed, and are based primarily on relatively sparse gravity data. These range from Cenozoic flexure, to distributed crustal extension of unknown age (possibly Mesozoic to Cenozoic), and even compression along the margin of craton. Our recent aeromagnetic data reveal that the basin is structurally controlled and has a tectonic origin, at least adjacent to NVL. The eastern margin of the basin is imposed upon an Early Paleozoic thrust fault belt, which can be traced under the ice using aeromagnetic signatures from exposures in Oates Land and the Ross Sea coast. Aeromagnetic patterns reveal that the western margin of the basin is imposed upon a Proterozoic-age shear zone mapped in the Mertz Glacier, and that is interpreted from geological studies to represent the continuation of a coeval shear zone in Australia. The broad aeromagnetic and satellite magnetic low over the WSB contrasts with the high over the un-reworked Proterozoic craton to the west of the basin, and is interpreted to reflect Neoproterozoic-age sediments deposited along the rifted margin of the craton. Magnetic intrusions within the WSB are interpreted as back-arc plutons that formed later in response to Cambrian-Ordovician age subduction along the paleo-Pacific margin of Gondwana. The aeromagnetic interpretation for a former broad back-arc basin in the WSB is supported by the occurrence of low-grade metasedimentary rocks of back-arc affinity in Oates Land, and also by the similarity in long

  13. Lithospheric structure, composition, and thermal regime of the East European Craton: Implications for the subsidence of the Russian platform

    USGS Publications Warehouse

    Artemieva, I.M.

    2003-01-01

    A new mechanism for Paleozoic subsidence of the Russian, or East European, platform is suggested, since a model of lithosphere tilting during the Uralian subduction does not explain the post-Uralian sedimentation record. Alternatively, I propose that the Proterozoic and Paleozoic rifting (when a platform-scale Central Russia rift system and a set of Paleozoic rifts were formed) modified the structure and composition of cratonic lithosphere, and these tectono-magmatic events are responsible for the post-Uralian subsidence of the Russian platform. To support this hypothesis, (a) the thermal regime and the thickness of the lithosphere are analyzed, and (b) lithospheric density variations of non-thermal origin are calculated from free-board constraints. The results indicate that Proterozoic and Paleozoic rifting had different effects on the lithospheric structure and composition. (1) Proterozoic rifting is not reflected in the present thermal regime and did not cause significant lithosphere thinning (most of the Russian platform has lithospheric thickness of 150-180 km and the lithosphere of the NE Baltic Shield is 250-300 km thick). Paleozoic rifting resulted in pronounced lithospheric thinning (to 120-140 km) in the southern parts of the Russian platform. (2) Lithospheric density anomalies suggest that Proterozoic-Paleozoic rifting played an important role in the platform subsidence. The lithospheric mantle of the Archean-early Proterozoic part of the Baltic Shield is ??? 1.4 ?? 0.2% less dense than the typical Phanerozoic upper mantle. However, the density deficit in the subcrustal lithosphere of most of the Russian platform is only about (0.4-0.8) ?? 0.2% and decreases southwards to ???0%. Increased densities (likely associated with low depletion values) in the Russian platform suggest strong metasomatism of the cratonic lithosphere during rifting events, which led to its subsidence. It is proposed that only the lower part of the cratonic lithosphere was

  14. Anisotropic Lithospheric layering in the North American craton, revealed by Bayesian inversion of short and long period data

    NASA Astrophysics Data System (ADS)

    Roy, Corinna; Calo, Marco; Bodin, Thomas; Romanowicz, Barbara

    2016-04-01

    Competing hypotheses for the formation and evolution of continents are highly under debate, including the theory of underplating by hot plumes or accretion by shallow subduction in continental or arc settings. In order to support these hypotheses, documenting structural layering in the cratonic lithosphere becomes especially important. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at 100-140 km depths, which is too shallow to be consistent with the lithosphere-asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. This leads to the conclusion that 1) the cratonic lithosphere may be thinner than expected, contradicting tomographic and other geophysical or geochemical inferences, or 2) that the receiver function studies detect a mid-lithospheric discontinuity rather than the LAB. On the other hand, several recent studies documented significant changes in the direction of azimuthal anisotropy with depth that suggest layering in the anisotropic structure of the stable part of the North American continent. In particular, Yuan and Romanowicz (2010) combined long period surface wave and overtone data with core refracted shear wave (SKS) splitting measurements in a joint tomographic inversion. A question that arises is whether the anisotropic layering observed coincides with that obtained from receiver function studies. To address this question, we use a trans-dimensional Markov-chain Monte Carlo (MCMC) algorithm to generate probabilistic 1D radially and azimuthal anisotropic shear wave velocity profiles for selected stations in North America. In the algorithm we jointly invert short period (Ps Receiver Functions, surface wave dispersion for Love and Rayleigh waves) and long period data (SKS waveforms). By including three different data types, which sample different volumes of the Earth and have different sensitivities to 
structure, we overcome the problem of

  15. The Presence of a Stable Block bounded by Active Zones (Mobile Belts) in the southwestern North American Proterozoic craton

    NASA Astrophysics Data System (ADS)

    Goodell, P.; Martinez P, C.; Mahar, M. A.

    2014-12-01

    Bouguer gravity data, initial Sr isotope values, zircon U-Pb, and multiple occurrences of felsic Proterozoic rocks, have revealed an elevated, less deformed, felsic cratonic block in the northern Mexico. The block is situated in western Chihuahua and is bounded by active zones or mobile belts on three sides, and is here referred to as the Western Chihuahua Cratonic Block (WCCB). Bouguer gravity data clearly indicate a region of a highly negative anomaly (< -200 mgal) in contrast to adjoining areas. The region is large and the anomaly is relatively smooth over broad areas; the WCCB appears as a smaller version of the Colorado Plateau. The block is characterized by high initial Sr isotope ratios (<0.706). Several occurrences of Proterozoic rocks are located within or next to the WCCB, and they reveal the character of the Bouguer anomaly. On the east, at Los Filtros, Proterozoic rocks crop out in a basement cored uplift interpreted to having been derived from the WCCB during the Ouachita orogeny. At Sierra La Mojina boulders of 1.1 Ga granites are found in Permian conglomerates. And at Basasiachic, xenoliths of 1.1 Ga granites are present in ash flow tuffs. Establishment of the Precambrian character of the WCCB is of importance, and these multiple occurrences are evidence. Prior studies of the Sierra Madre Occidental suggest that the region was uplifted because of a vast Cenozoic batholith presumed to lie under the SLIP (Silicic Large Igneous Province), the Upper Volcanic Series. The present study challenges that conclusion and maintains the SMO is underlain by Proterozoic silicic crust. The geology of age dated samples supports this. The WCCB is surrounded on three sides by Active Zones or Mobile Belts, which have been active extensional and translational zones periodically over a long period of time. On the east are the Paleozoic Pedrogosa Basin, Mesozoic Chihuahua Trough and Cenozoic Rio Grande Rift, the first two of which also continue around the northern border

  16. Geochemical Evolution of Cratonic Lithospheric Mantle: A 3.6 Ga Story of Persistence and Transformation (Invited)

    NASA Astrophysics Data System (ADS)

    O'Reilly, S. Y.; Griffin, W. L.; Pearson, N. J.

    2013-12-01

    Over the last decade, our view of the Earth has changed dramatically with the rapid development of planetary- and lithosphere-scale seismic tomography at increasingly higher resolution, and other global datasets such as gravity, magnetic and magnetotellurics. We now have access to geochemical (including isotopic) data for a large proportion of the elements in the periodic table, at very low levels in bulk rocks and in tiny volumes of tiny volumes. These advances, along with imaging of microstructures, enormous databanks for tectonic syntheses, and robust geodynamic modelling, have generated new concepts in understanding lithosphere evolution, structure and processes. The 4-D Lithosphere Mapping methodology introduced the integration of geophysical and geochemical datasets to construct geologically realistic sections of rock types and major boundaries in the deep lithosphere, in different timeslices. Correlation of petrological and geophysical data allows broader extrapolation of Lithosphere Mapping beyond xenolith sampling sites. The lithosphere-asthenosphere boundary (LAB) is a first-order Earth discontinuity: it may shallow due to extension and metasomatism, or deepen due to vertical accretion. Its (paleo-)location can be traced geochemically (a proxy for paleogeophysics), and cratonic LAB is commonly marked by significant concentrations of eclogites. Convergent geochronology datasets of Hf isotopic model ages for zircons and Re-Os model ages for mantle sulfides and PGMs, reinforced by other geochemical and tectonic criteria, indicate that over 70% of the SCLM and its overlying crust (now mostly lower crust) formed at about 3.5 Ga, probably in a series of global overturn events that marked a change in Earth's fundamental geodynamic behaviour. This primitive SCLM, the roots of the Archean cratons up to 300km deep, was geochemically highly depleted, and subsequently played a major role in crustal metallogeny for many ore types. Firstly, the high degree of buoyancy

  17. The Beiminghe skarn iron deposit, eastern China: Geochronology, isotope geochemistry and implications for the destruction of the North China Craton

    NASA Astrophysics Data System (ADS)

    Shen, Jun-Feng; Santosh, M.; Li, Sheng-Rong; Zhang, Hua-Feng; Yin, Na; Dong, Guo-Cheng; Wang, Yan-Juan; Ma, Guang-Gang; Yu, Hong-Jun

    2013-01-01

    The Beiminghe (BMH) iron ore in the southern part of the Taihang Mountain (TM), Hebei province, is one of the largest skarn iron deposits in China. Here we report phlogopite 40Ar-39Ar and zircon U-Pb age data, as well as sulfur, lead, and He-Ar isotope geochemistry of pyrite from the ores and skarnitized rocks in the deposit in an attempt to constrain the timing and mechanism of formation of the mineralization. The phlogopite 40Ar-39Ar and LA-ICP-MS zircon U-Pb data show markedly consistent ages constraining the timing of ore formation as 136-137 Ma. The presence of several inherited zircons with late Archean or Paleoproterozoic ages indicates the participation of the basement rocks during the ore-forming process. The δ34S values of pyrite from the ores range from 12.2 to 16.5‰, with 206Pb/204Pb = 17.84-18.79, 207Pb/204Pb = 15.46-15.62, and 208Pb/204Pb = 37.93-39.75, suggesting that continental crust is the major contributor. This is further confirmed by the He-Ar isotope data (3He/4He = 0.0648-0.1886 Ra, mean 0.1237Ra; 40Ar/36Ar = 311.7-22909.4; and 40Ar*/4He = 0.036-0.421). The Mesozoic magmatism and metallogeny in the BMH correlate well with the peak event of lithospheric thinning and destruction of the North China Craton during this process, the early Precambrian lower crustal rocks in the region were re-melted through underplating of mantle magmas, leading to the formation of the Beiminghe monzodioritic pluton. Minor mantle input occurred during the evolution of the monzodiorite magma, which scavenged the ore-forming materials from the lower crust. Interaction of the magmas and fluids with the surrounding rocks resulted in the formation of the Beiminghe skarn iron deposits. The magmatism and metallogeny in the Taihang Mountain are signatures of the extensive craton destruction and lithospheric thinning in the eastern part of the North China Craton during Mesozoic, probably associated with Pacific slab subduction.

  18. EarthScope in Midcontinent North America: Investigating the Architecture and Tectonic History of Cratonic-Platform Lithosphere

    NASA Astrophysics Data System (ADS)

    Marshak, S.; Larson, T.; Hamburger, M. W.; Pavlis, G. L.; Gilbert, H. J.; Parke, M.

    2010-12-01

    The transportable array of EarthScope will sweep across the Midcontinent of North America during 2011 and 2012. The central portion of this swath, between latitudes 36°N and 38°N, covers a "type example" of cratonic-platform lithosphere, where a veneer of Paleozoic sedimentary strata overlies Precambrian crystalline basement. In anticipating this scientific opportunity, we have compiled a unique suite of geologic, geophysical, subsurface, and topographic data sets for this area. The maps emphasize that, in spite of low topographic relief, the region has large subsurface structural relief. Specifically, its western portion includes a large intracratonic uplift (the Ozark Plateau), whereas its central portion includes a major intracratonic basin (the Illinois Basin). The elevation difference between the Cambrian-Precambrian unconformity at the crest of the Ozark Plateau and the same horizon at the base of the Illinois Basin (< 100 km to the east) is over 7.5 km. The region also includes the northern end of the Mississippi embayment (an anomalous depression), three major Proterozoic lithosphere accretionary boundaries (borders of the Yavapai, Mazatzal, and Grenville belts), one of the world's largest anorogenic igneous provinces (the Eastern Granite-Rhyolite Province), pronounced gravity and magnetic anomalies, and numerous fault-and-fold zones. Many of the zones remain active, both within and outside the notorious New Madrid seismic zone, making the central Midcontinent one of the most seismically active examples of cratonic platform lithosphere anywhere. As part of the USArray deployment in this region, a number of research groups (some of whom met at an EarthScope Workshop held in Urbana) have proposed dense, Flex-Array networks that would densify the sparser Transportable Array network. We propose an experiment that would span the Ozark Dome and the Illinois Basin, the Rough Creek Graben and other fault zones including the Wabash Valley seismic zone. This

  19. Paleoproterozoic felsic magmatism of the Karelian Craton: petrogeochemistry, isotope geochemistry, and genesis

    NASA Astrophysics Data System (ADS)

    Bogina, M. M.; Zlobin, V. L.

    2012-04-01

    Intense rift magmatism at the Karelian craton at the Archean-Proterozoic boundary (2.5-2.4 Ga) was mainly represented by mafic rocks, which compose mafic-ultramafic layered intrusions and basaltic lava fields. Felsic rocks of this age are of limited abundance, but provide insight into the nature of magmatism in an extensional regime associated with incipient rifting. These felsic rocks are represented by metavolcanic rocks of dacitic to rhyolitic composition, which are spatially related with mafic volcanics, their plutonic analogues, as well as fields of "remobilized granites" among the Late Archean granitoids. Their U-Pb ages vary from 2434+8 Ma and 2361 + 15 Ma, with the younger ages of the latters. The Paleoproterozoic felsic voclanics and granitoids are ascribed to the calc-alkaline to subalkaline series with total alkalis from 4.1 to 7.3 wt % and the K predominance over Na. Their distinctive feature is elevated TiO2 content (up to 1.19 wt%), which is not typical of siliceous rocks and expressed in the presence of numerous rutile needles in quartz. Practically all rocks have high Fe mole fraction. In terms of alumina saturation index (ASI), the volcanics show wide variations from metaluminous to highly peraluminous rocks (from 0.9 to 1.6), whereas granites are metaluminous rocks (ASI = 0.9-1.0). The studied rocks show fractionated REE patterns with wide LREE variations ((La/Yb)N=2.09-17.08; (La/Sm)N=1.65-4.4) at weak HREE variations (Gd/Yb)N=0.66-2.09), which is typical of the rocks formed in an intracontinental setting. In the petrotectonic diagram, the granitoids fall in the field of A-type anorogenic granites, which is consistent with rifting setting of their formation. In terms of Y-Nb-Ga relations, they correspond to the rocks derived by crustal melting. Sm-Nd isotopic studies revealed that the volcanics are characterized by negative eNd from -3.6 to -2.4, while the granites have more radiogenic composition from -1.7 to -2.0. The model ages vary from 2770

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  1. Geology, Mineralogy and Geochemistry of the Mount Deans Pegmatite Field, Eastern Yilgarn Craton/Australia

    NASA Astrophysics Data System (ADS)

    Dittrich, Thomas; Seifert, Thomas; Schulz, Bernhard

    2014-05-01

    Lithium-Cesium-Tantal (LCT) pegmatites are an important source for the rare metals Li, Cs and Ta, commodities that are now consumed in a rapidly increasing amount in high technology applications. Despite that LCT pegmatites are characteristic features for the Archaean geology of Western Australia, only the Greenbushes, Cattlin Creek and Wodgina deposits are currently exploited for Li and Ta. Therefore, Western Australia still possesses a great potential for the identification of additional resources for Li, Ta and possibly also Cs. The present study presents an overview of the geology, mineralogy and geochemistry of the Mount Deans pegmatite field, located c. 13 km S of Norseman, in the Eastern Goldfields Terrane of the Yilgarn Craton. The Mount Deans pegmatite field is Meso- to Neo-Archaean in age and hosted in the N-S trending Dundas Hill greenstone belt. The pegmatite field covers an area of 6 km in N-S and 4 km in E-W extension and comprises several dozens of individual pegmatite sheets and lenses. Structurally the pegmatite bodies are subdivided into two distinct types. Type I occurs predominantly in the southern part, is gently dipping (5-10°) to various directions and has variable thicknesses (3-25 m). Type II occurs in the northern part of the pegmatite field, dips steeply (50-90°) with a general N-S striking and has only a limited thickness (10 cm to 5 m). A clear distinction can also be made through their internal structure and mineralogy. Type I pegmatites exhibit a distinct structural and mineralogical zoning, whereas type II pegmatites are unzoned. Also albite, zinnwaldite, lepidolite and quartz form the bulk of the pegmatite; lepidolite is considerably more common in type II. Based on its peraluminous and strongly calc-alkaline character, as well as its enrichment in rare elements (i.e., Li, Rb, Cs, Ta, Nb, F), the pegmatites at Mount Deans are interpreted as LCT-pegmatites. However, despite the occurrence of rare element minerals like cassiterite

  2. New data for Eclogites and mantle xenocrysts and megacrysts from kimberlites of Dharwar craton , southern India.

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor; Ravi, Subramanian; Shankar Nayak, Shiva; Kaminsky, , Felix; Naflos, Theodoros; Vladykin, Nikolai

    2013-04-01

    Eclogitic mantle xenoliths from Proterozoic (1100) Kalyandurg kimberlite field (KL-4 pipe) Dharwar craton, India as well as the xenocrysts from pipes Wajrakarur kimberlite field pipes and others were analyzed by EPMA and LAM ICP MS methods. The eclogites (often with kyanite) (Patel et al., 2006) are composed mainly from garnet and Cpx, intergrain material is mainly represented by the carbonates and Ca- silicates. Garnets reveal Hi- CaO content to (10-12%) and Cpx are omphacites very low in FeO (1-3%) high Al2O3 (8-14% ) and Na2O (2-6 %) differing from the studied samples (Patel et al., 2009). The typical grosspidites (with kyanites coesite, K-Cpx and sanidine) have irregular compositions of minerals and fingerprint structures probably related to the crystallization from fluid. The compositions of the Cpx from Wajrakarur and other pipes reveal Hi - Cr2O3 (5%) content often higher then FeO and Na2O (4%). Garnets are in Lherzolite field in CaO - Cr2O3 (to 12%) diagram. Ilmenites with TiO2 variations (58-42%) show two trends of Cr2O3 enrichments accompanied by the general NiO and V2O5 decrease. Trace element s for eclogitic Cpx reveal high La/Ybn ratios, Eu peaks and flattened HREE. Garnet REE are not equilibrated and highly inclined. The TRE spiderdiagrams show depletion in HFSE (Ta>Nb), the most depleted show Y through for most depleted varieties. Garnets reveal U peak but low Sr CPx peaks in both Ba and Sr. The REE patterns Cpx xenocrysts from Wajrakarur are very similar in shape with varying incompatible part. They are showing high La/Ybn by the order of 2 and small humps in Ce-Pr. Spidergrams show small depletion in Zr- Hf and U and all incompatible elements and through in Pb . The REE of ilmenite xenocrysts show two models: high La/Ybn by 2 orders or nearly flattened patterns. Chromites show depletion in La-Pr elements. Ilmenite's TRE spiderrams show peaks in Nb-Ta and Pb and Zr- Hf . PTXFO2 diagrams for SCLM beneath the Wajrakarur and nearby fields show rather

  3. Structure and evolution of the lithospheric mantle beneath Siberian craton, thermobarometric study

    NASA Astrophysics Data System (ADS)

    Ashchepkov, Igor V.; Pokhilenko, Nikolai P.; Vladykin, Nikolai V.; Logvinova, Alla M.; Afanasiev, Valentin P.; Pokhilenko, Lyudmila N.; Kuligin, Sergei S.; Malygina, Elena V.; Alymova, Natalia A.; Kostrovitsky, Sergey I.; Rotman, Anatolii Y.; Mityukhin, Sergey I.; Karpenko, Mikhail A.; Stegnitsky, Yuri B.; Khemelnikova, Olga S.

    2010-04-01

    Monomineral thermobarometry (MTB) data derived from EMP analyses for heavy monomineral separates from > 20 Yakutian kimberlite pipes were used to compile a SSE-NNW traverse of the mantle beneath the Siberian craton. Orthopyroxene (Opx) MBT for the mantle section beneath Udachnaya gives three PT paths: a low temperature (LT) conductive branch (Boyd et al., 1997), estimated with thermometers of Krogh (1988) or/and O'Neill and Wood (1979), and two other HT paths. The three paths correspond to different values of Fe# in Ol (0.075, 0.085 and 0.11). They are reproduced by the modified MTB equations for clinopyroxene, garnet, chromite and ilmenites (Ashchepkov et al., 2008a), a mono-version of the O'Neill and Wood (1979) thermometer with corrections to Cr and Ca/Mg ratios which mark conductive geotherm. PT estimates for garnets and pyroxenes reflect mantle layering, whereas those for ilmenite reflect varying conditions of polybaric mantle protokimberlite systems and metasomatism. MTB for xenoliths from the Udachnaya, Mir, Dalnyaya, and Komsomolskaya kimberlites show the colder branches if PT path using heavy mineral analyses them according to PT for xenoliths according to TB (O'Neill and Wood, 1979; Finnerty and Boyd, 1987; McGregor, 1974; Brey and Kohler, 1990) producing smoother geotherms. MTB gives a wider range PT points reflecting heating near magmatic channels. Regularities of mantle sections and layering in the Daldyn field are recognized on the PT and P- X diagrams. The lower part of the mantle sections were heated by protokimberlite melts which created the megacrystalline associations. PT values of sub-calcic garnets correlate with those of picroilmenites. Mantle columns beneath the large pipes reveal stepped 7-12 layering for Udachnaya pipe units correlating with the peaks of Re/Os ages (Griffin et al., 2002b) is marked by periodic increase in Fe# in minerals. Pyroxenites and 'hot' eclogites (Pokhilenko et al., 1999) are found in layers at ˜ ˜ 40, 50-65, and

  4. Negro, Black, Black African, African Caribbean, African American or what? Labelling African origin populations in the health arena in the 21st century

    PubMed Central

    Agyemang, C.; Bhopal, R.; Bruijnzeels, M.

    2005-01-01

    Broad terms such as Black, African, or Black African are entrenched in scientific writings although there is considerable diversity within African descent populations and such terms may be both offensive and inaccurate. This paper outlines the heterogeneity within African populations, and discusses the strengths and limitations of the term Black and related labels from epidemiological and public health perspectives in Europe and the USA. This paper calls for debate on appropriate terminologies for African descent populations and concludes with the proposals that (1) describing the population under consideration is of paramount importance (2) the word African origin or simply African is an appropriate and necessary prefix for an ethnic label, for example, African Caribbean or African Kenyan or African Surinamese (3) documents should define the ethnic labels (4) the label Black should be phased out except when used in political contexts. PMID:16286485

  5. Negro, Black, Black African, African Caribbean, African American or what? Labelling African origin populations in the health arena in the 21st century.

    PubMed

    Agyemang, Charles; Bhopal, Raj; Bruijnzeels, Marc

    2005-12-01

    Broad terms such as Black, African, or Black African are entrenched in scientific writings although there is considerable diversity within African descent populations and such terms may be both offensive and inaccurate. This paper outlines the heterogeneity within African populations, and discusses the strengths and limitations of the term Black and related labels from epidemiological and public health perspectives in Europe and the USA. This paper calls for debate on appropriate terminologies for African descent populations and concludes with the proposals that (1) describing the population under consideration is of paramount importance (2) the word African origin or simply African is an appropriate and necessary prefix for an ethnic label, for example, African Caribbean or African Kenyan or African Surinamese (3) documents should define the ethnic labels (4) the label Black should be phased out except when used in political contexts. PMID:16286485

  6. African and African Caribbean users' perceptions of inpatient services.

    PubMed

    Secker, J; Harding, C

    2002-04-01

    It has been suggested that well-documented differences in African and African Caribbean people's contact with mental health services may stem from the organization, processes and practices of services themselves. This article presents the findings of a qualitative study which explored the inpatient experiences of a sample of African and African Caribbean people. Although some positive experiences were described, in the main, participants' accounts revolved around a sense of loss of control and around experiences of overt and implicit racism. Underpinning these experiences were relationships with staff that were perceived to be unhelpful. On the basis of both the positive and negative experiences described, we draw conclusions about the changes required to ensure that inpatient services more effectively meet the needs of this group.

  7. The African Millennium Villages

    PubMed Central

    Sanchez, Pedro; Palm, Cheryl; Sachs, Jeffrey; Denning, Glenn; Flor, Rafael; Harawa, Rebbie; Jama, Bashir; Kiflemariam, Tsegazeab; Konecky, Bronwen; Kozar, Raffaela; Lelerai, Eliud; Malik, Alia; Modi, Vijay; Mutuo, Patrick; Niang, Amadou; Okoth, Herine; Place, Frank; Sachs, Sonia Ehrlich; Said, Amir; Siriri, David; Teklehaimanot, Awash; Wang, Karen; Wangila, Justine; Zamba, Colleen

    2007-01-01

    We describe the concept, strategy, and initial results of the Millennium Villages Project and implications regarding sustainability and scalability. Our underlying hypothesis is that the interacting crises of agriculture, health, and infrastructure in rural Africa can be overcome through targeted public-sector investments to raise rural productivity and, thereby, to increased private-sector saving and investments. This is carried out by empowering impoverished communities with science-based interventions. Seventy-eight Millennium Villages have been initiated in 12 sites in 10 African countries, each representing a major agroecological zone. In early results, the research villages in Kenya, Ethiopia, and Malawi have reduced malaria prevalence, met caloric requirements, generated crop surpluses, enabled school feeding programs, and provided cash earnings for farm families. PMID:17942701

  8. COCORP profiles from the Montana plains: The Archean cratonic crust and a lower crustal anomaly beneath the Williston basin

    SciTech Connect

    Latham, T.S. ); Best, J.; Chaimov, T.; Oliver, J.; Brown, L.; Kaufman, S. )

    1988-12-01

    New COCORP deep seismic reflection profiles from the Montana plains between the Rocky Mountains and the Williston basin image the crystalline continental basement of the Archean Wyoming cratonic province on a regional scale. The crust is, in general, reflective throughout its entire thickness. West of the Williston basin, the crust-mantle boundary is at the base of the reflective zone and is not marked by the presence of any distinctive reflections. The lowermost crust beneath the Williston basin is, in contrast, characterized by a prominent, laterally extensive zone of relatively high-amplitude reflections. If, as the spatial correlation suggest, the anomalously reflective lower crustal zone is causally related to the subsidence of the basin, then the data place constraints in addition to those of the sedimentary record on physical models for the evolution of the Williston basin.

  9. Polyphase neotectonic movements in the Gavilgarh Fault Zone, central Indian craton: evidences from geomorpho-tectonic analysis

    NASA Astrophysics Data System (ADS)

    Bhattacharjee, Dipanjan; Chattopadhyay, Anupam; Jain, Vikrant

    2014-05-01

    The central part of Indian craton is believed to be a stable continental region with low strain build-up and long earthquake recurrence periods. It comprises two major Archean cratonic fragments (i.e. the Bundelkhand and the Bastar Cratons) and a Proterozoic mobile belt called Central Indian Tectonic Zone (CITZ), along which the cratonic fragments were amalgamated in the Proterozoic. Gavilgarh Fault Zone (GFZ) is an important component of CITZ and is represented by a >250 km long, ENE-WSW trending fault line which demarcates the southern boundary of the Satpura mountains. Although the eastern part of the lineament shows evidences of polyphase tectonic movements in the Meso-Neoproterozic (Chattopadhyay and Khasdeo, 2011), there is no focussed analysis of neotectonic activity in this fault zone although a number of earthquakes have been recorded within the CITZ in last 100 years or so. The present study comprises structural mapping and geomorphological analysis of a 200 km long stretch of the GFZ lineament. GFZ shows evidences of reverse fault-slip movements that possibly resulted in an uplift of the northern side, as deeper level rocks (e.g. Paleozoic Gondwana sandstones) are juxtaposed against the overlying Deccan Trap basalts of Mesozoic age along the fault line. Crushing of basalts along the lineament, asymmetric folds within Gondwana sandstone, inclination of Anisotropic Magnetic Susceptibility (AMS) axes etc. provide evidences for fault-drag folding related to the post-Deccan reverse faulting. Drainages crosscutting the lineament adjusted with the tectonic uplift either by incising their own sediments and bed rock or by increasing their sinuosity, only in the northern side, as seen in the satellite images. Hypsometric Integral values suggest that the immature/in-equilibrated drainage basins were restricted in the north while mature/equilibrated basins developed in the south of the lineament. Longitudinal profiles and S-L Index of the river profiles, prepared

  10. Tectonic evolution of greenstone-Gneiss association in Dharwar Craton, South India: Problems and perspectives for future research

    NASA Technical Reports Server (NTRS)

    Rao, Y. J. B.

    1986-01-01

    The two fold stratigraphic subdivision of the Archean-Proterozoic greenstone-gneiss association of Dharwar craton into an older Sargur group (older than 2.9 Ga.) and a younger Dharwar Supergroup serves as an a priori stratigraphic model. The concordant greenstone (schist)-gneiss (Peninsular gneiss) relationships, ambiguities in stratigraphic correlations of the schist belts assigned to Sargur group and difficulties in deciphering the older gneiss units can be best appreciated if the Sargur group be regarded as a trimodal association of: (1) ultrabasic-mafic metavolcanics (including komatiites), (2) clastic and nonclastic metasediments and paragneisses and (3) mainly tonalite/trondhemite gneisses and migmatites of diverse ages which could be as old as c. 3.4 ga. or even older. The extensive occurrence of this greenstone-gneiss complex is evident from recent mapping in many areas of central and southern Karnataka State.

  11. U-Pb ID-TIMS zircon ages of TTG gneisses of the Aravalli Craton of India

    NASA Astrophysics Data System (ADS)

    Chauhan, Hiredya; Saikia, Ashima; Kaulina, Tatiana; Bayanova, Tamara; Ahmad, Talat

    2015-04-01

    The crystalline basement of the Aravalli Craton is a heterogeneous assemblage dominated by granitic gneisses and granites with sporadic occurrences of amphibolites and dismembered sedimentary enclaves (Upadhyaya et al., 1992). This assemblage is known to have experienced multiple deformation and metamorphic events followed by emplacement of voluminous granites and basaltic dykes. Based on Sm-Nd whole rock data on the basement Mewar orthogneisses of Jhamarkotra region (Gopalan et al., 1990) and Pb/Pb ages of zircon from Gingla Granites which intrudes the basement (Wiedenbeck et al., 1996), it has been inferred that the whole magmatic episode leading to the formation of the basement spanned from 3300 to 2400 Ma and that the Aravalli cratonic block had broadly stabilized by 2500 Ma on which the younger Aravalli and Delhi Supergroup unconformably deposited. However, no comprehensive age data on the basement gneisses from the study area spanning the entire magmatic episode is available. This work attempts to provide a time frame work for evolution of the basement gneisses of the Aravalli Craton. We present here U-Pb zircon ages from the Precambrian basement TTG gneisses of the Aravalli Craton of north western India. Pb and U were measured on multicollector Finnigan-MAT 262 mass spectrometer. The temperatures of measurements were 1300°C for Pb and 1500°C for U. Pb isotope ratios were corrected for mass fractionation with a factor of 0.10% per amu, based on repeat analyses of the standard NBS SRM 982. The U analyses were corrected for mass fractionation with a factor of 0.003% per amu, based on repeat analyses of the NBS U 500 standard. Reproducibility of the U-Pb ratios was determined from the repeated analysis of standard zircon IGFM-87 (Ukraine) and taken as 0.5% for 207Pb/235U and 206Pb/238U ratios, respectively, at 95% confidence level. All calculations were done using the programs PBDAT and ISOPLOT (Ludwig 1991, 2008). Four zircon fractions corresponding to four

  12. The Archean Dongwanzi ophiolite complex, North China craton: 2.505-billion-year-old oceanic crust and mantle.

    PubMed

    Kusky, T M; Li, J H; Tucker, R D

    2001-05-11

    We report a thick, laterally extensive 2505 +/- 2.2-million-year-old (uranium-lead ratio in zircon) Archean ophiolite complex in the North China craton. Basal harzburgite tectonite is overlain by cumulate ultramafic rocks, a mafic-ultramafic transition zone of interlayered gabbro and ultramafic cumulates, compositionally layered olivine-gabbro and pyroxenite, and isotropic gabbro. A sheeted dike complex is rooted in the gabbro and overlain by a mixed dike-pillow lava section, chert, and banded iron formation. The documentation of a complete Archean ophiolite implies that mechanisms of oceanic crustal accretion similar to those of today were in operation by 2.5 billion years ago at divergent plate margins and that the temperature of the early mantle was not extremely elevated, as compared to the present-day temperature. Plate tectonic processes similar to those of the present must also have emplaced the ophiolite in a convergent margin setting.

  13. The Archean Dongwanzi ophiolite complex, North China craton: 2.505-billion-year-old oceanic crust and mantle.

    PubMed

    Kusky, T M; Li, J H; Tucker, R D

    2001-05-11

    We report a thick, laterally extensive 2505 +/- 2.2-million-year-old (uranium-lead ratio in zircon) Archean ophiolite complex in the North China craton. Basal harzburgite tectonite is overlain by cumulate ultramafic rocks, a mafic-ultramafic transition zone of interlayered gabbro and ultramafic cumulates, compositionally layered olivine-gabbro and pyroxenite, and isotropic gabbro. A sheeted dike complex is rooted in the gabbro and overlain by a mixed dike-pillow lava section, chert, and banded iron formation. The documentation of a complete Archean ophiolite implies that mechanisms of oceanic crustal accretion similar to those of today were in operation by 2.5 billion years ago at divergent plate margins and that the temperature of the early mantle was not extremely elevated, as compared to the present-day temperature. Plate tectonic processes similar to those of the present must also have emplaced the ophiolite in a convergent margin setting. PMID:11349144

  14. Complex evolution of the lower crust beneath the southeastern North China Craton: The Junan xenoliths and xenocrysts: Reply

    NASA Astrophysics Data System (ADS)

    Tang, Huayun; Zheng, Jianping; Griffin, William L.; O‧Reilly, Suzanne Y.; Yu, Chunmei; Pearson, Norman J.; Ping, Xianquan; Xia, Bing; Yang, Huaben

    2015-10-01

    In our paper, we suggested that the Junan granulite xenoliths and xenocrysts record evolution of the Precambrian lower crust beneath the southeastern North China Craton (NCC). Yuan and Xia (2015) disagree with us. However, they have not fully considered the evolutional histories of the NCC lithosphere, and geochemical and isotopic compositions of the Junan xenoliths. We also contend that they have misinterpreted the available geophysical data. Synthesizing the geochronological characteristics of the NCC lower crust, nature of the Junan granulite xenoliths, and reinterpretation of the resistivity profile, we again emphasize that the Junan granulite xenoliths are tectonically affiliated to the NCC lower crust, and the Junan zircon data could reflect the complex evolution of the lower crust beneath the southeastern NCC.

  15. Central African Republic.

    PubMed

    1989-11-01

    The Central African Republic contains 242,000 square miles, which rolling terrain almost 2000 feet above sea level. The climate is tropical, and it has a population of 2.8 million people with a 2.5% growth rate. There are more than 80 ethnic groups including Baya 34%, Banda 28%, Sara 10%, Mandja 9%, Mboum 9%, and M'Baka 7%. The religions are traditional African 35%, protestant 25%, Roman Catholic 25%, and Muslim 15%, and the languages are French and Sangho. The infant mortality rate is 143/1000, with expectancy at 49 years and a 40% literacy rate. The work force of 1 million is 70% agricultural, industry 6% and commerce and service 6% and government 3%. The government consists of a president assisted by cabinet ministers and a single party. Natural resources include diamonds, uranium, timber, gold, and oil, and major industries are beverages, textiles, and soap. Agricultural products feature coffee, cotton, peanuts, tobacco, food crops and livestock. Most of the population live in rural areas and most of the 80 ethnic groups have their own language. This is one of the world's least developed countries, with a per capita income of $375/year. The main problems with development are the poor transportation infrastructure, and the weak internal and international marketing systems. The US and various international organizations have aided in agriculture development, health programs, and family planning. US investment is mainly in diamond and gold mining, and although oil drilling has been successful it is not economically feasible at current prices.

  16. Superposed folding and associated fracturing influence hypogene karst development in Neoproterozoic carbonates, São Francisco Craton, Brazil

    NASA Astrophysics Data System (ADS)

    Ennes-Silva, Renata A.; Bezerra, Francisco H. R.; Nogueira, Francisco C. C.; Balsamo, Fabrizio; Klimchouk, Alexander; Cazarin, Caroline L.; Auler, Augusto S.

    2016-01-01

    Porosity and permeability along fractured zones in carbonates could be significantly enhanced by ascending fluid flow, resulting in hypogene karst development. This work presents a detailed structural analysis of the longest cave system in South America to investigate the relationship between patterns of karst conduits and regional deformation. Our study area encompasses the Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves, which are ca. 107 km and 34 km long, respectively. This cave system occurs in Neoproterozoic carbonates of the Salitre Formation in the northern part of the São Francisco Craton, Brazil. The fold belts that are around and at the craton edges were deformed in a compressive setting during the Brasiliano orogeny between 750 and 540 Ma. Based on the integrated analysis of the folds and brittle deformation in the caves and in outcrops of the surrounding region, we show the following: (1) The caves occur in a tectonic transpressive corridor along a regional thrust belt; (2) major cave passages, at the middle storey of the system, considering both length and frequency, developed laterally along mainly (a) NE-SW to E-W and (b) N to S oriented anticline hinges; (3) conduits were formed by dissolutional enlargement of subvertical joints, which present a high concentration along anticline hinges due to folding of competent grainstone layers; (4) the first folding event F1 was previously documented in the region and corresponds with NW-SE- to N-S-trending compression, whereas the second event F2, documented for the first time in the present study, is related to E-W compression; and (5) both folding events occurred during the Brasiliano orogeny. We conclude that fluid flow and related dissolution pathways have a close relationship with regional deformation events, thus enhancing our ability to predict karst patterns in layered carbonates.

  17. Rift propagation at craton margin.: Distribution of faulting and volcanism in the North Tanzanian Divergence (East Africa) during Neogene times

    NASA Astrophysics Data System (ADS)

    Le Gall, B.; Nonnotte, P.; Rolet, J.; Benoit, M.; Guillou, H.; Mousseau-Nonnotte, M.; Albaric, J.; Deverchère, J.

    2008-02-01

    A revised kinematic model is proposed for the Neogene tectono-magmatic development of the North Tanzanian Divergence where the axial valley in S Kenya splits southwards into a wide diverging pattern of block faulting in association with the disappearance of volcanism. Propagation of rifting along the S Kenya proto-rift during the last 8 Ma is first assumed to have operated by linkage of discrete magmatic cells as far S as the Ngorongoro-Kilimanjaro transverse volcanic belt that follows the margin of cratonic blocks in N Tanzania. Strain is believed to have nucleated throughout the thermally-weakened lithosphere in the transverse volcanic belt that might have later linked the S Kenya and N Tanzania rift segments with marked structural changes along-strike. The North Tanzanian Divergence is now regarded as a two-armed rift pattern involving: (1) a wide domain of tilted fault blocks to the W (Mbulu) that encompasses the Eyasi and Manyara fault systems, in direct continuation with the Natron northern trough. The reactivation of basement fabrics in the cold and intact Precambrian lithosphere in the Mbulu domain resulted in an oblique rift pattern that contrasts with the orthogonal extension that prevailed in the Magadi-Natron trough above a more attenuated lithosphere. (2) To the E, the Pangani horst-like range is thought to be a younger (< 1 Ma) structure that formed in response to the relocation of extension S of the Kilimanjaro magmatic center. A significant contrast in the mechanical behaviour of the stretched lithosphere in the North Tanzanian diverging rift is assumed to have occurred on both sides of the Masai cratonic block with a mid-crustal decoupling level to the W where asymmetrical fault-basin patterns are dominant (Magadi-Natron and Mbulu), whereas a component of dynamical uplift is suspected to have caused the topographic elevation of the Pangani range in relation with possible far-travelled mantle melts produced at depth further N.

  18. High-quality heat flow determination from the crystalline basement of the south-east margin of North China Craton

    NASA Astrophysics Data System (ADS)

    Jiang, Guangzheng; Tang, Xiaoyin; Rao, Song; Gao, Peng; Zhang, Linyou; Zhao, Ping; Hu, Shengbiao

    2016-03-01

    Very few of heat flow data have come from the crystalline basement in the North China Craton but rather from boreholes in the sedimentary cover of oil-gas basins. Explorations for hot dry rock (HDR) geothermal resources and porphyry gold deposits in eastern China offer now valuable opportunities to study the terrestrial heat flow in the crystalline basement. In this study, we obtained continuous temperature logs from two boreholes (the LZ borehole with a depth of 3471 m and the DR borehole with a depth of 2179 m) located in the south-east margin of the North China Craton. The boreholes have experienced long shut-in times (442 days and 261 days for the LZ borehole and DR borehole, respectively); thus, it can be expected that the temperature conditions have re-equilibrated after drilling and drill-mud circulation. Rock thermal conductivity and radiogenic heat production were measured for 68 crystalline rock samples from these two boreholes. The measured heat-flow density was determined to be 71.8 ± 2.3 mW m-2 (for the LZ borehole) and 91.5 ± 1.2 mW m-2 (for the DR borehole). The heat flow for the LZ borehole is close to the value of 75 mW m-2 determined in the Chinese Continental Scientific Drilling main hole (CCSD MH), both being in the Sulu-Dabie orogenic belt and thus able to verify each other. The value for the DR borehole is higher than the above two values, which supports former high heat-flow values determined in the Bohai Bay Basin.

  19. Magnetic mineralogy of pyroxenite xenoliths from Hannuoba basalts, northern North China Craton: Implications for magnetism in the continental lower crust

    NASA Astrophysics Data System (ADS)

    Li, Zhiyong; Zheng, Jianping; Zeng, Qingli; Liu, Qingsheng; Griffin, W. L.

    2014-02-01

    Studies of the petrology, mineral chemistry, and rock magnetic properties of nine pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These pyroxenites are weakly magnetic with low average susceptibility (χ) and saturation isothermal remanent magnetization (Mrs) of 39.59 × 10-8 m3 kg-1 and 12.05 × 10-3 Am2 kg-1, respectively. The magnetic minerals are mainly magnetite, pyrrhotite, and Fe-rich spinel, which significantly contribute to χ and natural remanent magnetization. Magnetite occurs as interstitial microcrystals together with zeolite aggregates, indicating a secondary origin in a supergene environment. In contrast, pyrrhotite and Fe-rich spinel were formed prior to the xenoliths' ascent to the surface, as evidenced by their dominant occurrence as tiny inclusions and thin exsolution lamellae in pyroxene. The Fe-rich spinel has ~ 50% mole fraction of Fe3O4 and corresponds to the strongest magnetization, and its coexistence with Mg-rich spinel implies a reheating event due to the underplating of basaltic magma. Besides, armalcolite and ilmenite were found in the reaction rims between xenoliths and the basalt, but they contribute little to the whole rock magnetization. However, these pyroxenite xenoliths would be nonmagnetic at in situ depths, as well as peridotite and mafic granulite xenoliths derived from the crust-mantle transition zone (~ 32-42 km). Therefore, we suggest the limiting depth of magnetization at the boundary between weakly magnetic deep-seated (lower crust and upper mantle) xenoliths and strongly magnetic Archean granulite facies rocks (~ 32 km) in Hannuoba, northern North China Craton.

  20. A modern analogue for tectonic, eustatic, and climatic processes in cratonic basins: Gulf of Carpentaria, northern Australia

    USGS Publications Warehouse

    Edgar, N. Terence; Cecil, C. Blaine; Mattick, R.E.; de Deckker, Patrick; Djajadihardja, Yusuf S.

    2003-01-01

    The Gulf of Carpentaria is a tropical, silled epicontinental sea and may be a modern analogue for ancient cratonic basins. For the purpose of this study, the Gulf of Carpentaria is compared to Pennsylvanian cratonic basins of the United States. During the Pennsylvanian, the North American continent moved from the Southern Hemisphere, through the Equator, into the Northern Hemisphere. Today, the Gulf of Carpentaria–New Guinea region is a few degrees south of the Equator and is moving towards it. During the Pennsylvanian, the world was subjected to major glaciations and associated sea-level changes. The island of New Guinea and the Gulf of Carpentaria have undergone similar processes during the Quaternary. A reconnaissance seismic survey of the gulf conducted by the USGS and the Australian National University (ANU), combined with oil-exploration well data, provided the first step in a systematic evaluation of a modern tropical epicontinental system. During the Cenozoic, the region was dominated by terrestrial sedimentation in a temperate climate. At the same time, carbonates were being deposited on the northern shelf edge of the Australian Plate. During the Miocene, carbonate deposition expanded southward into the gulf region. Then in the Late Miocene, carbonate sedimentation was replaced by terrigenous clastics derived from the developing Central Range of the island of New Guinea, which developed a wetter climate while moving northwards into the tropics. At least 14 basin-wide transgressive–regressive cycles are identified by channels that were eroded under subaerial conditions since about the Miocene. Comparison of the modern Gulf of Carpentaria sequences with those of the Pennsylvanian reveals many similarities.

  1. The Ufa indenter: stratigraphic and geophysic evidences for an actual indentation of the Southern Urals by the East European craton

    NASA Astrophysics Data System (ADS)

    Lefort, Jean-Pierre; Danukalova, Guzel

    2014-07-01

    Study of the altitudes of the lowest part of the Upper Cretaceous-Eocene and Aktschagylian-Quaternary stratigraphic ensembles known on the western slope of the Southern Urals evidences the existence of an East-West elongated dome which follows the N53° latitude. This ridge is superimposed at depth with the remnants of the Sernovodsk-Abdulino Aulacogen and with the Belaya tear fault, which support the existence of a recent rejuvenation of these old structures. North of these disruptions the Southern Urals display a clear bent towards the East. Detailed microstructural studies show that this curvature is associated with a typical stress pattern which suggests the existence of an indentation of the fold belt by the East European craton. The hypothesis of an Ufa indenter is not supported by an equivalent East-West deep fault north of the bend. However, a long N100° magnetic anomaly, interpreted as a shear zone, suggests that the indenter is a reality. Quaternary uplift and crustal thickening at its front as well as seismological data support our interpretation. It is not stressed that the curvature of the Urals observed at 56° latitude results solely from this recent indentation. It is only assumed that the actual indentation is rejuvenating a former unevenness which existed before in the East European craton. Study of the inner part of the indenter shows that this type of structure is not necessarily rigid and undeformed. Some of the structures described on the URSEIS deep seismic line could be much younger than previously expected.

  2. Study on the genesis of Yishui banded iron formation (BIF) in the North China Craton: geochemical characteristics and tectonic environment

    NASA Astrophysics Data System (ADS)

    Moon, I.; Lee, I.; Yang, X.

    2015-12-01

    The Yishui BIFs are located in the Taishan Group, Shandong province of Eastern Block of North China Craton. The iron ore samples were collected from the mine pits. Major elements were analyzed by X-ray Fluoresence Spectromemter (XRF). Trace elements and REY (REE + Y) were analyzed by Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Petrological, mineralogical and geochemical analyses of iron ores and their wall rock (amphibolite) were conducted to trace the genesis of Yishui BIF. Iron ores of Yishui BIF are mainly composed of SiO2 and Fe2O3T (SiO2+ Fe2O3T= 85.8 to 98.8 wt%) and consistent with major mineral components which are quartz and iron oxide such as magnetite and hematite. Low contents of TiO2 (0.01 to 0.09 wt%) , Al2O3 (0.42 to 1.18 wt%) and HFSE indicate no or little effect of detrital contamination. Iron ores have positive La, Eu, Gd, Er and Y anomalies with enriched HREE in PAAS normalized REY graph. The REY patterns of iron ores were used as a fingerprint to trace the source of iron and silica. Distinctive positive Eu anomalies (Eu/Eu*= 2.44-4.19), Y anomalies (Y/Y*=0.97 - 4.19), slightly negative Ce anomalies (Ce/Ce*= 0.87-0.97) and enriched HREE ((La/Yb)SN= 0.17-0.32) indicate that mixture of seawater and high-temperature hydrothermal fluid (>250 ◦C). Depositional environment in North China Craton implies that Yishui BIFs were formed at Neoarchean and associated arc-related tectonic setting. All these data suggest that Yishui BIFs belong to typical Algoma-type BIF.

  3. Early African Hominids: Pedagogic Patterns.

    ERIC Educational Resources Information Center

    Newman, James L.

    1984-01-01

    By studying early African hominids, students can learn about the interactive testing and creative aspects of scientific thinking and sharpen their geographical skills. It is impossible to study this topic without giving prominence to space and time. (RM)

  4. Structural analysis of the Itapucumí Group in the Vallemí region, northern Paraguay: Evidence of a new Brasiliano/Pan-African mobile belt

    NASA Astrophysics Data System (ADS)

    Campanha, Ginaldo Ademar da Cruz; Warren, Lucas; Boggiani, Paulo César; Grohmann, Carlos Henrique; Cáceres, Alberto Arias

    The Neoproterozoic (Ediacaran) Itapucumí Group in northern Paraguay is composed of carbonate and siliciclastic rocks, including ooid grainstones, marls, shales and sandstones, containing Cloudina fossils in the eastern region. It is almost undeformed over the Rio Apa Cratonic Block but shows a strong deformational pattern at its western edge. A detailed structural analysis of the Itapucumí Group was conducted in the Vallemí Mine, along with a regional survey in other outcrops downstream in the Paraguay River and in the San Alfredo, Cerro Paiva and Sargento José E. López regions. In the main Vallemí quarry, the structural style is characterized by an axial-plane slaty cleavage in open to isoclinal folds, sometimes overturned, associated with N-S trending thrust faults and shear zones of E-vergence and with a low-grade chlorite zone metamorphism. The structural data presented here are compatible with the hypothesis of a newly recognized mobile belt on the western side of the Rio Apa Cratonic Block, with opposite vergence to that of the Paraguay Mobile Belt in Brazil. Both belts are related to the Late Brasiliano/Pan-African tectonic cycle with a Lower Cambrian deformation and metamorphism age. The deformation could be due to the late collision of the Amazonian Craton with the remainder of Western Gondwana or to the western active plate boundary related to the Pampean Belt. The structural and lithologic differences between the western Itapucumí Group in the Vallemí and Paraguay River region and the eastern region, near San Alfredo and Cerro Paiva, suggest that this group could be divided into two lithostratigraphic units, but more stratigraphic and geochronological analyses are required to confirm this possibility.

  5. Hepatitis C in African Americans.

    PubMed

    Saab, Sammy; Jackson, Christian; Nieto, Jose; Francois, Fritz

    2014-10-01

    The care of hepatitis C virus (HCV) in African Americans represents an opportunity to address a major health disparity in medicine. In all facets of HCV infection, African Americans are inexplicably affected, including in the prevalence of the virus, which is higher among them compared with most of the racial and ethnic groups. Ironically, although fibrosis rates may be slow, hepatocellular carcinoma and mortality rates appear to be higher among African Americans. Sustained viral response (SVR) rates have historically significantly trailed behind Caucasians. The reasons for this gap in SVR are related to both viral and host factors. Moreover, low enrollment rates in clinical trials hamper the study of the efficacy of anti-viral therapy. Nevertheless, the gap in SVR between African Americans and Caucasians may be narrowing with the use of direct-acting agents. Gastroenterologists, hepatologists, primary care physicians, and other health-care providers need to address modifiable risk factors that affect the natural history, as well as treatment outcomes, for HCV among African Americans. Efforts need to be made to improve awareness among health-care providers to address the differences in screening and referral patterns for African Americans.

  6. Provenance and reconnaissance study of detrital zircons of the Palaeozoic Cape Supergroup in South Africa: revealing the interaction of the Kalahari and Río de la Plata cratons

    NASA Astrophysics Data System (ADS)

    Fourie, Pieter H.; Zimmermann, Udo; Beukes, Nicolas J.; Naidoo, Thanusha; Kobayashi, Katsuro; Kosler, Jan; Nakamura, Eizo; Tait, Jenny; Theron, Johannes N.

    2011-04-01

    In order to facilitate the understanding of the geological evolution of the Kalahari Craton and its relation to South America, the provenance of the first large-scale cratonic cover sequence of the craton, namely the Ordovician to Carboniferous Cape Supergroup was studied through geochemical analyses of the siliciclastics, and age determinations of detrital zircon. The Cape Supergroup comprises mainly quartz-arenites and a Hirnantian tillite in the basal Table Mountain Group, subgreywackes and mudrocks in the overlying Bokkeveld Group, while siltstones, interbedded shales and quartz-arenites are typical for the Witteberg Group at the top of the Cape Supergroup. Palaeocurrent analyses indicate transport of sediment mainly from northerly directions, off the interior of the Kalahari Craton with subordinate transport from a westerly source in the southwestern part of the basin near Cape Town. Geochemical provenance data suggest mainly sources from passive to active continental margin settings. The reconnaissance study of detrital zircons reveals a major contribution of Mesoproterozoic sources throughout the basin, reflecting the dominance of the Namaqua-Natal Metamorphic Belt, situated immediately north of the preserved strata of Cape Supergroup, as a source with Archaean-aged zircons being extremely rare. We interpret the Namaqua-Natal Metamorphic Belt to have been a large morphological divide at the time of deposition of the Cape Supergroup that prevented input of detrital zircons from the interior early Archaean Kaapvaal cratonic block of the Kalahari Craton. Neoproterozoic and Cambrian zircons are abundant and reflect the basement geology of the outcrops of Cape strata. Exposures close to Cape Town must have received sediment from a cratonic fragment that was situated off the Kalahari Craton to the west and that has subsequently drifted away. This cratonic fragment predominantly supplied Meso- to Neoproterozoic, and Cambrian-aged zircon grains in addition to minor

  7. Post-collisional Pan-African granitoids and rare metal pegmatites in western Nigeria: Age, petrogenesis, and the 'pegmatite conundrum'

    NASA Astrophysics Data System (ADS)

    Goodenough, K. M.; Lusty, P. A. J.; Roberts, N. M. W.; Key, R. M.; Garba, A.

    2014-07-01

    The Minna area of western Nigeria lies within a Pan-African orogenic belt that extends along the margin of the West African Craton, from Algeria southwards through Nigeria, Benin and Ghana, and into the Borborema Province of Brazil. This belt is characterised by voluminous post-collisional granitoid plutons that are well exposed around the city of Minna. In this paper we present new information about their age and petrogenesis. The Pan-African plutons around Minna can be divided into two main groups: a group of largely peraluminous biotite-muscovite granites that show varying levels of deformation in late Pan-African shear zones; and a younger group of relatively undeformed, predominantly metaluminous hornblende granitoids. Pegmatites, including both barren and rare-metal types, occur at the margins of some of the plutons. New U-Pb zircon dating presented here, in combination with published data, indicates an early phase of magmatism at c. 790-760 Ma in the Minna area. This magmatism could be related either to continental rifting, or to subduction around the margins of an existing continent. The peraluminous biotite-muscovite granites were intruded at c. 650-600 Ma during regional shearing in the orogenic belt, and are likely to have formed largely by crustal melting. Subsequent emplacement of metaluminous granitoids at c. 590 Ma indicates the onset of post-orogenic extension in this area, with a contribution from mantle-derived magmas. The rare-metal pegmatites represent the youngest intrusions in this area and thus are likely to have formed in a separate magmatic episode, post-dating granite intrusion.

  8. The African Pediatric Fellowship Program: Training in Africa for Africans.

    PubMed

    Wilmshurst, Jo M; Morrow, Brenda; du Preez, Avril; Githanga, David; Kennedy, Neil; Zar, Heather J

    2016-01-01

    Africa has a significant burden of childhood disease, with relatively few skilled health care professionals. The African Paediatric Fellowship Programme was developed by the Department of Pediatrics and Child Health at the University of Cape Town to provide relevant training for African child health professionals, by Africans, within Africa. Trainees identified by partner academic institutions spend 6 months to 2 years training in the Department of Pediatrics and allied disciplines. They then return to their home institution to build practice, training, research, and advocacy. From 2008 to 2015, 73 physicians have completed or are completing training in general pediatrics or a pediatric subspecialty. At 1 year posttraining, 98% to 100% are practicing back in their home institution. The impact of the returning fellows is evident from their practice interventions, research collaborations, and positions as stakeholders who can change health care policies. Thirty-three centers in 13 African countries are partners with the program, and the program template is now followed by other partner sites in Africa. Increasing and retaining the skills pool of African child health specialists is building a network of motivated, highly skilled clinicians who are equipped to advance child health in Africa. PMID:26659458

  9. The African Pediatric Fellowship Program: Training in Africa for Africans.

    PubMed

    Wilmshurst, Jo M; Morrow, Brenda; du Preez, Avril; Githanga, David; Kennedy, Neil; Zar, Heather J

    2016-01-01

    Africa has a significant burden of childhood disease, with relatively few skilled health care professionals. The African Paediatric Fellowship Programme was developed by the Department of Pediatrics and Child Health at the University of Cape Town to provide relevant training for African child health professionals, by Africans, within Africa. Trainees identified by partner academic institutions spend 6 months to 2 years training in the Department of Pediatrics and allied disciplines. They then return to their home institution to build practice, training, research, and advocacy. From 2008 to 2015, 73 physicians have completed or are completing training in general pediatrics or a pediatric subspecialty. At 1 year posttraining, 98% to 100% are practicing back in their home institution. The impact of the returning fellows is evident from their practice interventions, research collaborations, and positions as stakeholders who can change health care policies. Thirty-three centers in 13 African countries are partners with the program, and the program template is now followed by other partner sites in Africa. Increasing and retaining the skills pool of African child health specialists is building a network of motivated, highly skilled clinicians who are equipped to advance child health in Africa.

  10. East African Rift

    NASA Technical Reports Server (NTRS)

    2008-01-01

    Places where the earth's crust has formed deep fissures and the plates have begun to move apart develop rift structures in which elongate blocks have subsided relative to the blocks on either side. The East African Rift is a world-famous example of such rifting. It is characterized by 1) topographic deep valleys in the rift zone, 2) sheer escarpments along the faulted walls of the rift zone, 3) a chain of lakes within the rift, most of the lakes highly saline due to evaporation in the hot temperatures characteristic of climates near the equator, 4) voluminous amounts of volcanic rocks that have flowed from faults along the sides of the rift, and 5) volcanic cones where magma flow was most intense. This example in Kenya displays most of these features near Lake Begoria.

    The image was acquired December 18, 2002, covers an area of 40.5 x 32 km, and is located at 0.1 degrees north latitude, 36.1 degrees east longitude.

    The U.S. science team is located at NASA's Jet Propulsion Laboratory, Pasadena, Calif. The Terra mission is part of NASA's Science Mission Directorate.

  11. An African ethic for nursing?

    PubMed

    Haegert, S

    2000-11-01

    This article derives from a doctoral thesis in which a particular discourse was used as a 'paradigm case'. From this discourse an ethic set within a South African culture arose. Using many cultural 'voices' to aid the understanding of this narrative, the ethic shows that one can build on both a 'justice' and a 'care' ethic. With further development based on African culture one can take the ethic of care deeper and reveal 'layers of understanding'. Care, together with compassion, forms the foundation of morality. Nursing ethics has followed particular western moral philosophers. Often nursing ethics has been taught along the lines of Kohlberg's theory of morality, with its emphasis on rules, rights, duties and general obligations. These principles were universalistic, masculine and noncontextual. However, there is a new ethical movement among Thomist philosophers along the lines to be expounded in this article. Nurses such as Benner, Bevis, Dunlop, Fry and Gadow--to name but a few--have welcomed the concept of an 'ethic of care'. Gilligan's work gave a feminist view and situated ethics in the everyday aspects of responsiveness, responsibility, context and concern. Shutte's search for a 'philosophy for Africa' has resulted in finding similarities in Setiloane and in Senghor with those of Thomist philosophers. Using this African philosophy and a research participant's narrative, an African ethic evolves out of the African proverb: 'A person is a person through other persons', or its alternative rendering: 'I am because we are: we are because I am.' This hermeneutic narrative reveals 'the way affect imbues activity with ethical meaning' within the context of a black nursing sister in a rural South African hospital. It expands upon the above proverb and incorporates the South African constitutional idea of 'Ubuntu' (compassion and justice or humanness).

  12. The Kimberlites and related rocks of the Kuruman Kimberlite Province, Kaapvaal Craton, South Africa

    NASA Astrophysics Data System (ADS)

    Donnelly, Cara L.; Griffin, William L.; O'Reilly, Suzanne Y.; Pearson, Norman J.; Shee, Simon R.

    2011-03-01

    The Kuruman Kimberlite Province is comprised of 16 small pipes and dikes and contains some of the oldest known kimberlites (>1.6 Ga). In this study, 12 intrusions are subdivided into three groups with distinct petrology, age, and geochemical and isotopic compositions: (1) kimberlites with groundmass perovskites defining a Pb-Pb isochron age of 1787 ± 69 Ma, (2) orangeite with a U-Pb perovskite age of 124 ± 16 Ma, and (3) ultramafic lamprophyres (aillikite and mela-aillikite) with a zircon U-Pb age of 1642 ± 46 Ma. The magma type varies across the Province, with kimberlites in the east, lamprophyres in the west and orangeite and ultramafic lamprophyres to the south. Differences in the age and petrogenesis of the X007 orangeite and Clarksdale and Aalwynkop aillikites suggest that these intrusions are probably unrelated to the Kuruman Province. Kimberlite and orangeite whole-rock major and trace element compositions are similar to other South African localities. Compositionally, the aillikites typically lie off kimberlite and orangeite trends. Groundmass mineral chemistry of the kimberlites has some features more typical of orangeites. Kimberlite whole-rock Sr and Nd isotopes show zoning across the Province. When the kimberlites erupted at ~1.8 Ga, they sampled a core volume (ca 50 km across) of relatively depleted SCLM that was partially surrounded by a rim of more metasomatized mantle. This zonation may have been related to the development of the adjacent Kheis Belt (oldest rocks ~2.0 Ga), as weaker zones surrounding the more resistant core section of SCLM were more extensively metasomatized.

  13. Terminal suturing of Gondwana along the southern margin of South China Craton: Evidence from detrital zircon U-Pb ages and Hf isotopes in Cambrian and Ordovician strata, Hainan Island

    NASA Astrophysics Data System (ADS)

    Xu, Yajun; Cawood, Peter A.; Du, Yuansheng; Zhong, Zengqiu; Hughes, Nigel C.

    2014-12-01

    Hainan Island, located near the southern end of mainland South China, consists of the Qiongzhong Block to the north and the Sanya Block to the south. In the Cambrian, these blocks were separated by an intervening ocean. U-Pb ages and Hf isotope compositions of detrital zircons from the Cambrian succession in the Sanya Block suggest that the unit contains detritus derived from late Paleoproterozoic and Mesoproterozoic units along the western margin of the West Australia Craton (e.g., Northampton Complex) or the Albany-Fraser-Wilkes orogen, which separates the West Australia and Mawson cratons. Thus, in the Cambrian the Sanya Block was not part of the South China Craton but rather part of the West Australian Craton and its environs. In contrast, overlying Late Ordovician strata display evidence for input of detritus from the Qiongzhong Block, which constituted part of the southeastern convergent plate margin of the South China Craton in the early Paleozoic. The evolving provenance record of the Cambrian and Ordovician strata suggests that the juxtaposition of South China and West Australian cratons occurred during the early to mid-Ordovician. The event was linked with the northern continuation of Kuungan Orogeny, with South China providing a record of final assembly of Gondwana.

  14. Comparative tectonic and dynamic analysis of cratons, orogens, basins, and metallogeny: A special volume to honor the career of Brian F. Windley

    NASA Astrophysics Data System (ADS)

    Kusky, Tim; Xiao, Wenjiao

    2015-11-01

    Cratons, orogens, and basins of the world each show a distinctive pattern of evolution and metallogeny, and relationships to supercontinent cycles. Some aspects of these histories have remained similar through time, yet others have changed with Earth's changing biota, heat production and flow, and atmospheric composition and temperature. To understand the similarities and differences between these cratons, orogens, and basins through time, we need systematic comparative tectonic analyses between these different elements from similar and widely different ages. One of the pioneers in comparative tectonic analysis is Brian F. Windley, whose research has spanned all of these topics, and more, for more than five decades. This special issue is a tribute to his career.

  15. Roberts Victor Eclogites: The MacGregor Legacy of Archean Oceanic Lithosphere Subduction and its Role in Craton Formation

    NASA Astrophysics Data System (ADS)

    Shirey, S. B.; Schmitz, M. D.; Wiechert, U.; Carlson, R. W.

    2005-12-01

    Eclogite xenoliths from the 125 Ma old, Group II, Roberts Victor kimberlite have long been of interest (MacGregor et al, 1968) because of their diversity, abundance, large size, occurrence with peridotite and their high carbon/diamond content. Coesite, corundum, kyanite, Ca-, Mg-, and Fe- rich eclogites are available but those classified as Group I, Group II (as defined by MacGregor, 1970) or diamondiferous were selected with the goal to better understand eclogite petrogenesis, Kaapvaal cratonic keel evolution, diamond formation, and eclogite metasomatism. Recent laser fluorination oxygen isotope data (δ18O) on gt (GI = 5.8 to 6.9; GII = 2.1 to 5.1) match earlier data (Garlick et al, 1971; MacGregor and Manton, 1986), while ion-probe trace element contents of gt (e.g. chondrite normalized Ce G1 = 0.2 to 0.5; GII = 0.002 to 0.07) and cpx (G1 = 7 to 20; GII = 0.2 to 2) and whole-rock Re-Os (G1 Re = 0.19 to 3.41 ppb; GII Re = 0.006 to 0.38 ppb) highlight even more distinct differences between Groups I and II. These differences must be a pre-metamorphic signature of their original protoliths and not just due to pressure differences or partial melting during emplacement. Using ophiolites and composites of oceanic crust as a guide (e.g. MacGregor and Manton, 1986), Group I eclogites could represent the volcanic rocks of Layer 2 of Archean oceanic crust whereas Group II might represent the cumulate, intrusive rocks of Layer 3. Group II eclogites have positive Eu anomalies and lower REE and Re contents which support this idea. The Re-Os systematics of the oceanic lithosphere is poorly known, especially in the Archean, but Roberts Victor eclogite Re-Os and trace element abundances and major element compositions suggest a basaltic komatiitic protolith as might typify slightly hotter ocean ridges in the Archean. A U-Pb age of 3.061±0.006 Ga on zircon grains separated from a Group I Roberts Victor eclogite and a same-age but scattered whole-rock Re-Os isotope array

  16. The Diamondiferous Lithospheric Mantle Underlying the Eastern Superior Craton: Evidence From Mantle Xenoliths From the Renard Kimberlites, Quebec

    NASA Astrophysics Data System (ADS)

    Hunt, L.; Stachel, T.; Armstrong, J. P.; Simonetti, A.

    2009-05-01

    The Renard kimberlite cluster consists of nine pipes located within a 2km2 area in the northern Otish Mountains of Quebec. The pipes are named Renards 1 to 10, with subsequent investigation revealing Renards 5 and 6 to join at depth (now Renard 65). The pipes are located within the eastern portion of the Superior