Science.gov

Sample records for african passive margin

  1. Geometry and structural evolution of Lorbeus diapir, northwestern Tunisia: polyphase diapirism of the North African inverted passive margin

    NASA Astrophysics Data System (ADS)

    Masrouhi, Amara; Bellier, Olivier; Koyi, Hemin

    2014-04-01

    Detailed geologic mapping, structural analysis, field cross-sections, new dating based on planktonic foraminifera, in addition to gravity signature of Lorbeus diapir, are used to characterize polyphase salt diapirism. This study highlights the role of inherited faulting, which controls and influences the piercement efficiency and the style and geometry of the diapir; and also the localization of evaporite early ascent displaying diapiric growth during extension. Salt was extruded along the graben axis developed within extensional regional early Cretaceous tectonic associated with the North African passive margin evolution. Geologic data highlight reactive diapirism during Albian time (most extreme extension period) and passive diapirism during the late Cretaceous post-rift stage. Northeastern Maghreb salt province gives evidences that contractional deformations are not associated with significant diapirism. During shortening, the initial major graben deforms as complex anticlines where diapirs are squeezed and pinched from their feeding.

  2. Volcanic passive margins

    NASA Astrophysics Data System (ADS)

    Geoffroy, Laurent

    2005-12-01

    Compared to non-volcanic ones, volcanic passive margins mark continental break-up over a hotter mantle, probably subject to small-scale convection. They present distinctive genetic and structural features. High-rate extension of the lithosphere is associated with catastrophic mantle melting responsible for the accretion of a thick igneous crust. Distinctive structural features of volcanic margins are syn-magmatic and continentward-dipping crustal faults accommodating the seaward flexure of the igneous crust. Volcanic margins present along-axis a magmatic and tectonic segmentation with wavelength similar to adjacent slow-spreading ridges. Their 3D organisation suggests a connection between loci of mantle melting at depths and zones of strain concentration within the lithosphere. Break-up would start and propagate from localized thermally-softened lithospheric zones. These 'soft points' could be localized over small-scale convection cells found at the bottom of the lithosphere, where adiabatic mantle melting would specifically occur. The particular structure of the brittle crust at volcanic passive margins could be interpreted by active and sudden oceanward flow of both the unstable hot mantle and the ductile part of the lithosphere during the break-up stage. To cite this article: L. Geoffroy, C. R. Geoscience 337 (2005).

  3. Thermal history from both sides of the South Atlantic passive margin - A comparison: Argentinean pampa vs. South African escarpement.

    NASA Astrophysics Data System (ADS)

    Kollenz, Sebastian; Glasmacher, Ulrich A.

    2014-05-01

    The eastern Argentina South Atlantic passive continental margin is distinguished by a very flat topography. Out of the so called Pampean flat two mountain ranges are arising. These mountain ranges, the Sierras Australes and the Sierras Septentrionales, are located in the State of Buenos Aires south of the capital Buenos Aires. In existing literature the Sierras Australes are correlated with the South African cape fold belt (Torsvik 2009; Lopez Gamundi & Rossello 1998). Existing thermochronological data shows different post-breakup cooling histories for both areas and different AFT-ages. Published thermochronological ages (e.g. Raab et al. 2002, 2005, Gallagher et al et al. 1998)from the south African escarpement vary around 150 and 100 Ma (Gallagher et al. 1998). Only some spots in the eastern part of South Africa towards the pacific margin show older ages of 250 Ma and older than 350 Ma (Gallagher et al. 1998). New thermochronological data (AHe, AFT and ZHe) from the Sierras Australes indicate a different cooling history by revealing a range of varying ages due to younger tectonic activity. By comparing the data sets from both areas it is getting clear that the post-rift evolution of both continents is differing very strong. Gallagher, K., Brown, R. and Johnson, C. 1998. Fission track analysis and its application to geological problems. Annual review of Earth and Planetary Science, 26, 519-572. Lopez Gamundi, O.R., Rossello, E.A. (1998): Basin fill evolution and paleotectonic patterns along the Samfrau geosyncline: the Sauce Grande basin-Ventana foldbelt (Argentina) and Karoo basin-Cape foldbelt (South Africa) revisited. Geol Rundsch 86 :819-834. Raab, M.J., Brown, R.W., Gallagher, K., Carter, A. and Webber, K. 2002. late Cretaceous reactivation of major crustal shear zones in northern Namibia: constraints from apatite fission track analysis. Tectonophysics. 349, 75-92. Raab, M.J., Brown, R.W., Gallagher, K., Webber, K. and Gleadow, A.J.W. 2005. denudational and

  4. Submarine allochthonous salt sheets: Gravity-driven deformation of North African Cretaceous passive margin in Tunisia - Bled Dogra case study and nearby salt structures

    NASA Astrophysics Data System (ADS)

    Masrouhi, Amara; Bellier, Olivier; Ben Youssef, Mohamed; Koyi, Hemin

    2014-09-01

    We used structural, stratigraphic and sedimentologic data, together with a comparison of nearby structures and a Bouguer gravity map, to evaluate the evolution of the Bled Dogra salt structure (northern Tunisia) during the Cretaceous. Triassic salt sheets are recognized in the northwestern region of the Tunisian Atlas. These salt sheets are the result of Cretaceous thick and/or thin-skinned extension along the south Tethyan margin. The Bled Dogra salt structure is one of these submarine allochthonous salt sheets, which was emplaced during the Early Cretaceous. The geologic framework, during this period, produces conditions for a predominantly gravity-driven deformation: extension has produced space for the salt to rise; vigorous differential sedimentation created differential loading that resulted in the emplacement and extrusion of a large volume of Triassic salt and formation of large submarine salt sheets. Geologic field data suggest an interlayered Triassic salt sheet within Albian sequences. Salt was extruded at the sea floor during the Early-Middle Albian and was initially buried by Middle-Late Albian strata. The Coniacian corresponds to a second transgressive cover onto the salt sheet after the gliding of the first salt cover (Late Albian-Turonian). In addition, this northwest Tunisian area exposes evidences for salt flow and abundant slump features at the base of a northward facing submarine slope, which was probably dominant from the Early Cretaceous to Santonian. Two gravity deformation processes are recognized: gravity gliding and gravity spreading. Acting concurrently, these two processes appear indistinguishable in this geologic context. Like the present-day salt-involved passive margins - such as the northern Gulf of Mexico, the Atlantic margin of Morocco, the Brazilian Santos basin, the Angola margin, Cadiz in western Iberia, and the Red Sea - the North African Cretaceous passive margin in Tunisia provides evidences that deformation in a passive-margin

  5. 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.

  6. 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

  7. Plio-Quaternary paleostresses in the Atlantic passive margin of the Moroccan Meseta: Influence of the Central Rif escape tectonics related to Eurasian-African plate convergence

    NASA Astrophysics Data System (ADS)

    Chabli, Ahmed; Chalouan, Ahmed; Akil, Mostapha; Galindo-Zaldívar, Jesús; Ruano, Patricia; Sanz de Galdeano, Carlos; López-Garrido, Angel Carlos; Marín-Lechado, Carlos; Pedrera, Antonio

    2014-07-01

    The Atlantic Moroccan Meseta margin is affected by far field recent tectonic stresses. The basement belongs to the variscan orogen and was deformed by hercynian folding and metamorphism followed by a post-Permian erosional stage, producing the flat paleorelief of the region. Tabular Mesozoic and Mio-Plio-Quaternary deposits locally cover the Meseta, which has undergone recent uplift, while north of Rabat the subsidence continues in the Gharb basin, constituting the foreland basin of the Rif Cordillera. The Plio-Quaternary sedimentary cover of the Moroccan Meseta, mainly formed by aeolian and marine terraces deposits, is affected by brittle deformations (joints and small-scale faults) that evidence that this region - considered up to date as stable - is affected by the far field stresses. Striated faults are recognized in the oldest Plio-Quaternary deposits and show strike-slip and normal kinematics, while joints affect up to the most recent sediments. Paleostress may be sorted into extensional, only affecting Rabat sector, and three main compressive groups deforming whole the region: (1) ENE-WSW to ESE-WNW compression; (2) NNW-SSE to NE-SW compression and (3) NNE-SSW compression. These stresses can be attributed mainly to the NW-SE oriented Eurasian-African plate convergence in the western Mediterranean and the escape toward the SW of the Rif Cordillera. Local paleostress deviations may be related to basement fault reactivation. These new results reveal the tectonic instability during Plio-Quaternary of the Moroccan Meseta margin in contrast to the standard passive margins, generally considered stable.

  8. 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.

  9. Offshore Benin, a classic passive margin

    SciTech Connect

    Mathalone, J.M.P. )

    1991-03-01

    Offshore Benin comprises a narrow east-west continental shelf, some 30 km wide. A sharp shelf break running parallel to the coast borders the shelf, whereupon water depths rapidly increase to over 7000 ft. The area lies within the Dahomey Embayment, one of a series of Cretaceous and younger basins lining the coast of Africa that owe their inception to the Late Mesozoic break-up of the Gondwanaland Continent. The basin extends some 100 km inland, but sedimentary section is thin onshore compared to a maximum of 20,000 ft of sediment offshore. Initial sedimentation in this basin was of Neocomian alluvial and lacustrine clastics. These were deposited in east-west-trending narrow half-grabens associated with the initial break up of the South American and African continents. They are covered unconformably by more extensive Albian and Cenomanian transgressive clastics and shallow marine Turonian sandstones which are the main reservoir at Seme, Benin's only oilfield. The Senonian section offshore comprises passive margin deep sea clastic sediments prograding southwards. Very large proximal deep sea channels up to 2500 ft thick are developed in this interval. These channels are associated with excellent petroleum source rocks, averaging 4-5% oil-prone organic carbon, and form the main exploration target in the area when configured in a trap morphology. Seismic data quality is excellent in the region allowing detailed examination of the relationships between the rifted section and later units. In addition, these data illustrate clearly both internal and external morphology of the Senonian proximal deep sea channels.

  10. Marginality Theory and the African American Student.

    ERIC Educational Resources Information Center

    Grant, G. Kathleen; Breese, Jeffrey R.

    1997-01-01

    Reports on a study of African-American college students at a state university in the Midwest. The study examined the effects of marginality on their college experience and performance. Identifies six reactions to marginality and provides case study examples of each. Includes extensive references and verbatim comments from the students. (MJP)

  11. Earthquakes at North Atlantic passive margins

    SciTech Connect

    Gregersen, S. ); Basham, P.W. )

    1989-01-01

    The main focus of this volume is the earthquakes that occur at and near the continental margins on both sides of the North Atlantic. The book, which contains the proceedings of the NATO workshop on Causes and Effects of Earthquakes at Passive Margins and in Areas of Postglacial Rebound on Both Sides of the North Atlantic, draws together the fields of geophysics, geology and geodesy to address the stress and strain in the Earth's crust. The resulting earthquakes produced on ancient geological fault zones and the associated seismic hazards these pose to man are also addressed. Postglacial rebound in North America and Fennoscandia is a minor source of earthquakes today, during the interglacial period, but evidence is presented to suggest that the ice sheets suppressed earthquake strain while they were in place, and released this strain as a pulse of significant earthquakes after the ice melted about 9000 years ago.

  12. Focused fluid flow in passive continental margins.

    PubMed

    Berndt, Christian

    2005-12-15

    Passive continental margins such as the Atlantic seaboard of Europe are important for society as they contain large energy resources, and they sustain ecosystems that are the basis for the commercial fish stock. The margin sediments are very dynamic environments. Fluids are expelled from compacting sediments, bottom water temperature changes cause gas hydrate systems to change their locations and occasionally large magmatic intrusions boil the pore water within the sedimentary basins, which is then expelled to the surface. The fluids that seep through the seabed at the tops of focused fluid flow systems have a crucial role for seabed ecology, and study of such fluid flow systems can also help in predicting the distribution of hydrocarbons in the subsurface and deciphering the climate record. Therefore, the study of focused fluid flow will become one of the most important fields in marine geology in the future.

  13. Orogenic structural inheritance and rifted passive margin formation

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    Structural inheritance is related to mechanical weaknesses in the lithosphere due to previous tectonic events, e.g. rifting, subduction and collision. The North and South Atlantic rifted passive margins that formed during the breakup of Western Gondwana, are parallel to the older Caledonide and the Brasiliano-Pan-African orogenic belts. In the South Atlantic, 'old' mantle lithospheric fabric resulting from crystallographic preferred orientation of olivine is suggested to play a role during rifted margin formation (Tommasi and Vauchez, 2001). Magnetometric and gravimetric mapping of onshore structures in the Camamu and Almada basins suggest that extensional faults are controlled by two different directions of inherited older Brasiliano structures in the upper lithosphere (Ferreira et al., 2009). In the South Atlantic Campos Basin, 3D seismic data indicate that inherited basement structures provide a first order control on basin structure (Fetter, 2009). Here we investigate the role of structural inheritance on the formation of rifted passive margins with high-resolution 2D thermo-mechanical numerical experiments. The numerical domain is 1200 km long and 600 km deep and represents the lithosphere and the sublithospheric mantle. Model experiments were carried out by creating self-consistent orogenic inheritance where a first phase of orogen formation is followed by extension. We focus in particular on the role of varying amount of orogenic shortening, crustal rheology, contrasting styles of orogen formation on rifted margin style, and the time delay between orogeny and subsequent rifted passive formation. Model results are compared to contrasting structural styles of rifted passive margin formation as observed in the South Atlantic. Ferreira, T.S., Caixeta, J.M., Lima, F.D., 2009. Basement control in Camamu and Almada rift basins. Boletim de Geociências da Petrobrás 17, 69-88. Fetter, M., 2009. The role of basement tectonic reactivation on the structural evolution

  14. Passive margin formation, Timor Sea, Australia

    SciTech Connect

    Hillis, R.R. )

    1990-06-01

    Recent ODP data show that sea-floor spreading began in the Argo Abyssal Plain in the earliest Cretaceous, and not the Callovian-Oxfordian as had previously been believed. These data are now consistent with the Callovian-Valanginian rifting observed on seismic records over the adjacent continental shelf (Vulcan subbasin, western Timor Sea). Tectonic subsidence plots have been constructed for well, extrapolated well, and significant off-well (seismically based) locations in the Vulcan subbasin and adjacent highs. The fully corrected plots show relatively little tectonic subsidence during the Callovian-Valanginian rift phase, even in the depocenter of the Swan Graben, where the Callovian-Valanginian interval reaches its maximum thickness. This is atypical for a passive margin basin. Assuming an extensional origin for the margin, the absence of tectonic subsidence is considered to indicate that continental rifting in the area was wet (accompanied by major volcanic activity). Recent studies have shown that extensive volcanism may occur where rift zones cut through regions of anomalously hot mantle (100-200{degree}C above normal). The addition to the crust of igneous material, the density of which has been modified by adiabatic decompression, inhibits syn-rift subsidence. A wet rifting model also has implications for the origin of the nearby marginal plateaux such as the Scott Plateau. Their relatively thick crust and lack of subsidence may be due to igneous underplating associated with wet rifting. As such the plateaux may be regarded as transitional between oceanic and continental crust. The post-Valanginian Cretaceous subsidence of the Vulcan subbasin and adjacent areas is consistent with typical post-rift thermal subsidence, the predicted exponentially decaying subsidence history for a wet rift being indistinguishable from that of a dry rift.

  15. Colorado Basin Structure and Rifting, Argentine passive margin

    NASA Astrophysics Data System (ADS)

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

    2010-05-01

    The Argentine margin presents a strong segmentation with considerable strike-slip movements along the fracture zones. We focus on the volcanic segment (between the Salado and Colorado transfer zones), which is characterized by seaward dipping reflectors (SDR) all along the ocean-continent transition [e.g. Franke et al., 2006; Gladczenko et al., 1997; Hinz et al., 1999]. The segment is structured by E-W trending basins, which differs from the South African margin basins and cannot be explained by classical models of rifting. Thus the study of the relationship between the basins and the Argentine margin itself will allow the understanding of their contemporary development. Moreover the comparison of the conjugate margins suggests a particular evolution of rifting and break-up. We firstly focus on the Colorado Basin, which is thought to be the conjugate of the well studied Orange Basin [Hirsch et al., 2009] at the South African margin [e.g. Franke et al., 2006]. This work presents results of a combined approach using seismic interpretation and structural, isostatic and thermal modelling highlighting the structure of the crust. The seismic interpretation shows two rift-related discordances: one intra syn-rift and the break-up unconformity. The overlying sediments of the sag phase are less deformed (no sedimentary wedges) and accumulated before the generation of oceanic crust. The axis of the Colorado Basin trends E-W in the western part, where the deepest pre-rift series are preserved. In contrast, the basin axis turns to a NW-SE direction in its eastern part, where mainly post-rift sediments accumulated. The most distal part reaches the margin slope and opens into the oceanic basin. The general basin direction is almost orthogonal to the present-day margin trend. The most frequent hypothesis explaining this geometry is that the Colorado Basin is an aborted rift resulting from a previous RRR triple junction [e.g. Franke et al., 2002]. The structural interpretation

  16. Mesozoic evolution of northeast African shelf margin, Libya and Egypt

    SciTech Connect

    Aadland, R.K.; Schamel, S.

    1989-03-01

    The present tectonic features of the northeast African shelf margin between the Nile delta and the Gulf of Sirte are products of (1) precursory late Paleozoic basement arches, (2) early Mesozoic rifting and plate separation, and (3) Late Cretaceous structural inversion. The 250 km-wide and highly differentiated Mesozoic passive margin in the Western Desert region of Egypt is developed above a broad northwest-trending Late Carboniferous basement arch. In northeastern Libya, in contrast, the passive margin is restricted to just the northernmost Cyrenaica platform, where subsidence was extremely rapid in the Jurassic and Early Cretaceous. The boundary between the Western Desert basin and the Cyrenaica platform is controlled by the western flank of the basement arch. In the middle Cretaceous (100-90 Ma), subsidence accelerated over large areas of the Western desert, further enhancing a pattern of east-west-trending subbasins. This phase of rapid subsidence was abruptly ended about 80 Ma by the onset of structural inversion that uplifted the northern Cyrenaica shelf margin and further differentiated the Western Desert subbasin along a northeasterly trend.

  17. Lithosphere structure and subsidence evolution of the conjugate S-African and Argentine margins

    NASA Astrophysics Data System (ADS)

    Dressel, Ingo; Scheck-Wenderoth, Magdalena; Cacace, Mauro; Götze, Hans-Jürgen; Franke, Dieter

    2016-04-01

    The bathymetric evolution of the South Atlantic passive continental margins is a matter of debate. Though it is commonly accepted that passive margins experience thermal subsidence as a result of lithospheric cooling as well as load induced subsidence in response to sediment deposition it is disputed if the South Atlantic passive margins were affected by additional processes affecting the subsidence history after continental breakup. We present a subsidence analysis along the SW African margin and offshore Argentina and restore paleobathymetries to assess the subsidence evolution of the margin. These results are discussed with respect to mechanisms behind margin evolution. Therefore, we use available information about the lithosphere-scale present-day structural configuration of these margins as a starting point for the subsidence analysis. A multi 1D backward modelling method is applied to separate individual subsidence components such as the thermal- as well as the load induced subsidence and to restore paleobathymetries for the conjugate margins. The comparison of the restored paleobathymetries shows that the conjugate margins evolve differently: Continuous subsidence is obtained offshore Argentina whereas the subsidence history of the SW African margin is interrupted by phases of uplift. This differing results for both margins correlate also with different structural configurations of the subcrustal mantle. In the light of these results we discuss possible implications for uplift mechanisms.

  18. Plate tectonic evolution of circum-Antarctic passive margins

    SciTech Connect

    Scotese, C.R.; Lawver, L.A.; Sclater, J.G.; Mayes, C.L.; Norton, I.; Royer, J.

    1987-05-01

    Passive margins that formed during the Late Jurassic and Cretaceous account for approximately 80% of the 15,000-km circumference of Antarctica. There are no passive margins younger than Late Cretaceous. Approximately 28% of these margins are Late Jurassic in age, 24% are Early Cretaceous in age, and the remaining 48% formed during the Late Cretaceous. The tectonic style of the rifting events that formed these margins varies considerably along the perimeter of Antarctica. In several areas the initiation of sea-floor spreading was preceded by a long period of extension and predrift stretching (Wilkes Land). Along other portions of the margin, rifting proceeded rapidly with little evidence for a lengthy phase of pre-drift extension (Queen Maud Land). Though extension is the dominant tectonic style, there is evidence for large-scale strike-slip movement associated with the early phases of continental breakup along the coasts of Crown Princess Martha Land and Victoria Land. Except for a short segment of the margin between the West Antarctic peninsula and Marie Byrdland, the Antarctic passive margins have not been affected by subsequent subduction-related compressive deformation. This presentation will review the plate tectonic evolution of the Circum-Antarctic passive margins during five time intervals: Early Jurassic, Late Jurassic, Early Cretaceous, mid-Cretaceous, and latest Cretaceous. A map illustrating the relative amounts of extension along the margin of Antarctica will be presented, and a computer animation illustrating the breakup of Gondwana from an Antarctic perspective will be shown.

  19. Sedimentary loading, lithospheric flexure and subduction initiation at passive margins

    SciTech Connect

    Erickson, S.G. . Dept. of Earth Sciences)

    1992-01-01

    Recent theoretical models have demonstrated the difficulty of subduction initiation at passive margins, whether subduction is assumed to initiate by overcoming the shear resistance on a thrust fault through the lithosphere or by failure of the entire lithosphere in bending due to sedimentary loading. A mechanism for subduction initiation at passive margins that overcomes these difficulties incorporates the increased subsidence of a marginal basin during decoupling of a previously locked margin. A passive margin may decouple by reactivation of rift-related faults in a local extensional or strike-slip setting. Flexure of marginal basins by sedimentary loading is modeled here by the bending of infinite and semi-infinite elastic plates under a triangular load. The geometry of a mature marginal basin fits the deflection produced by loading of an infinite plate in which the flexural rigidity of continental lithosphere is larger than that of oceanic lithosphere. Decoupling of such a locked passive margin by fault reactivation may cause the lithospheric bending behavior of the margin to change from that of an infinite plate to that of a semi-infinite plate, with a resultant increase in deflection of the marginal basin. The increase in deflection depends on the flexural rigidities of continental and oceanic lithosphere. For flexural rigidities of 10[sup 30]-10[sup 31] dyn-cm (elastic lithosphere thicknesses 24--51 km), the difference in deflections between infinite and semi-infinite plates is 15--17 km, so that decoupling sinks the top of the oceanic lithosphere to depths of ca 35 km. Additional sedimentation within the basin and phase changes within the oceanic crust may further increase this deflection. Subduction may initiate if the top of the oceanic lithosphere sinks to the base of the adjacent elastic lithosphere.

  20. Post-Rift Compressional Deformation on the Passive Margin of a young Mediterranean Backarc Basin (Eastern Sardinian Margin, Tyrrhenian Sea)

    NASA Astrophysics Data System (ADS)

    Chanier, F.; Gaullier, V.; Maillard, A.; Thinon, I.; Sage, F.; Lymer, G.; Vendeville, B.; Giresse, P.; Bassetti, M. A.; Lofi, J.

    2014-12-01

    Compressional deformation has been reported on many passive margins, mostly attributed to thin-skinned tectonics in response to gravity gliding or spreading from viscous layers (overpressured shales, salt décollement). However some of the reported structures are obviously related to regional stress and also affect the basement, not only the upper sedimentary cover. Such deformation has been documented and discussed in the last decade mainly from the northern Atlantic margins (Doré et al., 2008 ; Pereira et al., 2011, & ref. herein). The compressional structures on passive margins have been notably considered as linked to tectonomagmatic and active asthenospheric upwelling, post-breakup compression and compactional stresses. The western margin of the Tyrrhenian Sea (Central Mediterranean) is a passive margin formed during the late Miocene opening of a back-arc basin in relation with the roll-back and retreat of the Ionian subducting lithosphere (African Plate). From our new data, we can show evidence for compressional features that developed in the Pliocene, shortly after the main rifting period on the western Tyrrhenian Sea (Middle to Late Miocene) and beginning of oceanic spreading (Earliest Pliocene). We could describe such structures across the inner margin onshore, from field analysis, as well as offshore, from newly acquired seismic data (METYSS 1 & 3; Gaullier et al. 2014). The characters and distribution of such compressional deformation, occurring very shortly after the onset of oceanic spreading in the deep basin (earliest Pliocene), allow us to discuss the possible interactions between breakup processes and inversion episodes on passive margins. Doré A.G., Lundin E.R., Kusznir N.J., & Pascal C., 2008. Potential mechanisms for the genesis of Cenozoic domal structures on the NE Atlantic margin: Pros and cons and some new ideas. Geol. Soc. London Spec. Pub., 306, 1-26. Gaullier V., Chanier F., et al., 2014. Salt tectonics and crustal tectonics along the

  1. How applicable are current models of volcanic passive margin formation?

    NASA Astrophysics Data System (ADS)

    Paton, Douglas; Bellingham, Paul; Pindell, Jim

    2015-04-01

    The Argentinian Passive Margin provides an exceptional example of a volcanic passive margin that has been a focus, along with much of the South Atlantic, for a number of studies investigating volcanic margin evolution. In this study we use recently acquired, well imaged, seismic reflection data to constrain the margin architecture. These new data provide significant improvements in imaging throughout the oceanic and continental lithosphere that enables us to interpret lower and mid crustal reflectivity, the continental and oceanic moho, seaward dipping reflections and oceanic crust domains. Despite this high quality imaging uncertainty still remains in both the interpretation of the data as well as the geophysical properties of the margin, including the extent of lower crustal magmatic bodies, the geometry of break-up volcanics and Seaward Dipping Reflection emplacement. Constraining these interpretations have a fundamental control in our understanding of the processes involved in continental rifting and break up. Interpretation of previous data, as well as existing models of the margin, do not account for such uncertainty in the interpretations. In this study we present multiple seismic-structural interpretations for data that are geometrically valid. We then use a number of techniques, including kinematic restorations, gravity modelling, backstripping and subsidence analysis to test the validity of each of the models. By addressing the uncertainty inherent in any sub-surface data we can better constrain the suite of likely scenarios. This enables us to challenge the current models of lithospheric stretching in volcanic passive margins.

  2. Cretaceous to Eocene passive margin sedimentation in Northeastern Venezuela

    SciTech Connect

    Erikson, J.P. )

    1993-02-01

    Twenty two palinspastic paleogeographic maps are presented for the Cretaceous to Eocene strata of the Serrania del Interior of northeastern Venezuela. The mapped lithologies, environmental conditions, and evolving depositional systems record [approximately]90 m.y. of dominantly marine sedimentation on the only observable Mesozoic passive margin in the Western Hemisphere. The depositional systems of the passive margin are heterogeneous at lateral (i.e., along-margin) length scales greater than [approximately]40 km. The primary lateral heterogeneity is caused by a major Lower Cretaceous deltaic system that emanated southwest of the Serrania del Interior. All important intervals, such as the laterally variable Aptian-Albian El Cantil platform limestone and the hydrocarbon source rocks of the Upper Cretaceous Querecual and San Antonio formations, are related to probable causal mechanisms and environmental conditions. Stratigraphic events have been interpreted as of either local or regional extent; based on a combination of outcrop sedimentologic analyses and regional depositional systems interpretation. The 3-dimensional distribution of depositional systems and systems tracts reveals 4-6 regional sequence boundaries separated by 4-20 m.y. Subsidence analyses support the facies interpretation of a passive margin by showing continuous, thermally dominated subsidence during the Cretaceous to Eocene interval. Subsidence and accumulation rates increased and facies changed significantly in the Oligocene, indicating the end of passive margin sedimentation and the initiation of foredeep subsidence and accumulation associated with overthrusting the eastward-advancing Caribbean Plate.

  3. Factors controlling structural style and magmatism in passive margins

    NASA Astrophysics Data System (ADS)

    Lu, Gang; Huismans, Ritske S.

    2015-04-01

    Comparing volcanic and non-volcanic passive margins, the distinct variability in geometry and subsidence history implies that the thermo-mechanical conditions vary at the time of rifting. Volcanic rifted margins (such as in the North Atlantic) show large magmatic activity and shallow water condition at the rift-drift transition, implying high geothermal gradients. For non-volcanic rifted margins where the initial thermal condition is potentially colder, it may develop in two end-member styles (Type I and Type II). Type-I margin with limited magmatism can be observed at Iberia-Newfoundland conjugate margins where the continental crustal thins across a narrow region and large tracts of continental mantle lithosphere are exposed at the seafloor. Type-II margin as observed in the ultra-wide central South Atlantic margins, in contrast, has normal magmatic activity and has a strongly thinned continental crust that span very wide regions (>250 km) below which the continental mantle lithosphere was removed. Here we perform thermo-mechanical finite element numerical experiments to investigate factors that are potentially important for the formation of volcanic and non-volcanic passive margins. Forward numerical models are used to predict the structural styles and characteristic magmatism associated with each of these end members. A number of parameters including different rheological stratifications and thermal gradients are tested and factors that control the degree of magmatism and structural style during rifting are focused.

  4. The Algerian Margin: an Example of a Reactivation in Compression of a Complex Cenozoic Passive Margin

    NASA Astrophysics Data System (ADS)

    Domzig, A.; Deverchere, J.; Yelles, K.; Govers, R.; Wortel, R.; Petit, C.; Cataneo, A.; Kherroubi, A.; Teams, M.

    2007-12-01

    The Western Mediterranean underwent a complex Cenozoic history involving subduction of the Tethys Ocean as well as subduction roll-back and associated opening of back-arc basins. During the Oligo-Miocene, the subduction roll-back to the south led to the collision of the Kabylies into the African plate, but subduction continued towards west, causing the Alboran slab to migrate towards the Gibraltar Arc. Northern Africa is at the southern border of this system and is therefore a major study area in the context of slow convergent plates to study the reactivation in compression of a Cenozoic passive margin but also the records of past geodynamic processes. This work aims to characterize the multi-scale structure of the offshore Algerian margin, based on the MARADJA'03 and MARADJA2/SAMRA'05 cruises data (multibeam bathymetry, seismic-reflection, side-scan sonar, backscattering, CHIRP, gravimetry). Tectonic (geomorphology, folds, faults) records reveal large recent and active structures as well as the geological inheritance of the margin. In western Algeria, slab roll-back is likely to have been accompanied by lithospheric tearing (STEP fault) as it has been modelled at a regional scale (Govers and Wortel, 2005): we provide first evidence for the presence of such structure(s) offshore Algeria. The geodynamical conditions have now changed, and we are facing new types of structures. Two main tectonic styles are identified: reverse to the centre and east; and strike-slip to the west. In Central Algeria, the compressional structures are active blind thrusts (Plio-Quaternary) verging to the north (opposite to pre-existing features) expressed as asymmetrical folds, sub-perpendicular to the convergence direction and often en echelon. These faults may all trigger M=6-7.5 earthquakes (e.g. Khair al Din fault near Algiers). Among them, the fault associated with the 2003 Boumerdes event (Mw=6.8) would continue to the surface by flats and ramps creating piggy-back basins or

  5. Sequence stratigraphy on an early Cretaceous passive margin, Exmouth Plateau

    SciTech Connect

    Boyd, R.; Gorur, N.; Ito, M.; O'Brien, D.; Wilkens, R.; Tang, C.

    1989-03-01

    Permian-Jurassic rifting of northwestern Australia resulted in the development of a passive continental margin flanking the northeastern Indian Ocean. On this margin the relatively thin synrift to postrift sedimentary sequence of southern Exmouth Plateau was drilled during ODP Leg 122. A sequence-stratigraphy analysis of the complete Mesozoic-Cenozoic sedimentary succession at Sites 762 and 763 was derived from a synthesis of seismic stratigraphy, wireline logs, lithostratigraphy, biostratigraphy, and magnetostratigraphy. Results indicate that during breakup, the southern Exmouth Plateau was a transform margin with an extensional component. Between the Tithonian and Valanginian, a thick clastic wedge prograded from the transform margin south of Site 763 northwestward toward Site 762 and onto subsiding continental crust. Southern clastic supply decreased into the Aptian-Cenomanian, and cyclic deposition of deep-water mudstones continued during subsidence of the earlier shelf margin wedge. Between the Albian and Cenomanian, deposition gradually became dominated by pelagic carbonates. Two regional unconformities mark the Cenomanian/Turonian and Cretaceous/Tertiary boundaries. Each was an erosional event, succeeded by renewed pelagic carbonate deposition that began in the distal northern basin and onlapped progressively toward the topographic high, which persisted into the Tertiary along the southern margin. The entire Jurassic to Holocene record at the southern Exmouth Plateau ODP sites is less than 1500 m thick and represents a classic rift to mature ocean passive-margin succession.

  6. Geology of New England passive margin

    SciTech Connect

    Austin, J.A. Jr.; Uchupi, E.; Shaughnessy, D.R. III; Ballard, R.D.

    1980-01-01

    The New England continental margin began to develop in the Middle Triassic, when rifting of Precambrian/Paleozoic terrane produced a complex arrangement of horsts and grabens. During the Late Triassic-Early Jurassic, these grabens were filled with terrigenous clastics, volcanics, and evaporites. When plate separation took place and seafloor spreading began approximately 195 to 190 m.y.B.P., the newly formed continental edge was uplifted and eroded, truncating preexisting rift structures. As North America began to drift away from Africa, subsidence occurred along a series of normal faults now beneath the outer continental shelf. This hinge zone may represent the boundary between continental crust and a transitional zone of continental and oceanic crustal fragments. Atop the faulted and subsiding crustal platform, thick sediments were deposited. The lower part of the drift sequence is an evaporite-carbonate unit of Early-Middle Jurassic age, and the upper part is a clastic wedge of Middle Jurassic to Cenozoic age. More than 80% of these sediments are Jurassic. Their total thickness may be as much as 13 km beneath the southeastern part of Georges Bank.

  7. Multiple uplift phases inferred from the Southwest African Atlantic margin

    NASA Astrophysics Data System (ADS)

    Scheck-Wenderoth, Magdalena; Cacace, Mauro; Dressel, Ingo

    2015-04-01

    The South Atlantic basins offshore Namibia and South Africa stored more than 10 km thick sedimentary successions that are separated by major unconformities into several sequences. These sedimentary units rest on a thinned continental crust of a magmatic passive margin. Using a 3D forward modelling approach considering flexural compensation of a rheologically differentiated lithosphere in response to sedimentary loading after stretching on one hand and the thermal feed-back between cooling of the stretched lithosphere and insulating sediments on the other hand we derive quantitative estimates on how vertical movements have influenced the margin after stretching. The approach combines the consideration of observations on sediment configuration as well as on crustal thickness (ß-factor) with the process of lithosphere thinning and subsequent thermal re-equilibration. These estimates are conservative estimates as they are based on the preserved sediments only whereas eroded sediments are not considered. Nevertheless, the approach considers thermo-mechanical coupling in 3D and both initial conditions as well as sedimentary history are constrained by observations. Specific effects include the delayed thermal re-equilibration of the thinned lithosphere due to deposition of insulating sediments and the related thermal feedback on lithosphere rheology and therefore on the flexural response to sediment loading. Our results indicate that in addition to predominantly continuous subsidence also phases of uplift have affected the southwestern African margin during the syn-rift and post-rift evolution. The spatio-temporal variation of vertical movements is controlled by the amount of initial thinning of the lithosphere, the variation of rheological characteristics (lithology and temperature) but also by the distribution of sediment supply (loading and thermal insulation).

  8. Volcanic passive margins: another way to break up continents

    PubMed Central

    Geoffroy, L.; Burov, E. B.; Werner, P.

    2015-01-01

    Two major types of passive margins are recognized, i.e. volcanic and non-volcanic, without proposing distinctive mechanisms for their formation. Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere. In contrast with non-volcanic margins, continentward-dipping detachment faults accommodate crustal necking at both conjugate volcanic margins. These faults root on a two-layer deformed ductile crust that appears to be partly of igneous nature. This lower crust is exhumed up to the bottom of the syn-extension extrusives at the outer parts of the margin. Our numerical modelling suggests that strengthening of deep continental crust during early magmatic stages provokes a divergent flow of the ductile lithosphere away from a central continental block, which becomes thinner with time due to the flow-induced mechanical erosion acting at its base. Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain. Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle. PMID:26442807

  9. Volcanic passive margins: another way to break up continents.

    PubMed

    Geoffroy, L; Burov, E B; Werner, P

    2015-10-07

    Two major types of passive margins are recognized, i.e. volcanic and non-volcanic, without proposing distinctive mechanisms for their formation. Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere. In contrast with non-volcanic margins, continentward-dipping detachment faults accommodate crustal necking at both conjugate volcanic margins. These faults root on a two-layer deformed ductile crust that appears to be partly of igneous nature. This lower crust is exhumed up to the bottom of the syn-extension extrusives at the outer parts of the margin. Our numerical modelling suggests that strengthening of deep continental crust during early magmatic stages provokes a divergent flow of the ductile lithosphere away from a central continental block, which becomes thinner with time due to the flow-induced mechanical erosion acting at its base. Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain. Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle.

  10. Volcanic passive margins: another way to break up continents.

    PubMed

    Geoffroy, L; Burov, E B; Werner, P

    2015-01-01

    Two major types of passive margins are recognized, i.e. volcanic and non-volcanic, without proposing distinctive mechanisms for their formation. Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere. In contrast with non-volcanic margins, continentward-dipping detachment faults accommodate crustal necking at both conjugate volcanic margins. These faults root on a two-layer deformed ductile crust that appears to be partly of igneous nature. This lower crust is exhumed up to the bottom of the syn-extension extrusives at the outer parts of the margin. Our numerical modelling suggests that strengthening of deep continental crust during early magmatic stages provokes a divergent flow of the ductile lithosphere away from a central continental block, which becomes thinner with time due to the flow-induced mechanical erosion acting at its base. Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain. Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle. PMID:26442807

  11. The convective mantle and the not so passive margins

    NASA Astrophysics Data System (ADS)

    Moucha, Robert; Forte, Alessandro; Glisovic, Petar; Rowley, David; Mitrovica, Jerry; Simmons, Nathan; Grand, Stephen

    2010-05-01

    Numerous studies have shown that dynamic topography driven by convective mantle flow has profound implications for the inference and interpretation of long term sea level variations obtained from either backstripping analysis of bore-hole data and/or seismic sequence analysis at sites within so-called passive continental margins (e.g. Moucha et al., 2008; Spasojević et al., 2008; Conrad et al., 2009). In this presentation, we will explore the geodynamic implications of convective flow on the stability of passive margins throughout the late Cenozoic by carrying out backward mantle flow simulations starting with present-day heterogeneity derived from a high resolution joint seismic-geodynamic tomography model (Simmons et al., 2009) that yields excellent fits to present day surface observables (e.g. dynamic topography and the geoid). Uncertainties in our reconstruction of margin topography that originate from the numerical method of backward convection to the staring models of mantle heterogeneity and rheology are fully investigated and compared with the geological record from the northeastern US margin and the Angolan margin of Africa.

  12. Crustal thinning and tectonic geomorphology: redefining the passive margin

    NASA Astrophysics Data System (ADS)

    Redfield, T.; Osmundsen, P. T.

    2012-04-01

    We describe Scandinavia's passive margin in terms of a hyper-extended distal margin, a variably tapered proximal margin that includes the outer onshore areas, and an upwarped, unstretched, continent-sloping hinterland that terminates against the "undeformed" cratonic interior. Two benchmark locations, defined as the taper break (TB) and the Hinterland Break in Slope (HBSL), occur at the inner boundary of the distal margin and at the transition from the continent-sloping hinterland and craton, respectively. The elevation of the seaward-facing escarpment is directly scaled to the distance between the taper break and the Hinterland Break in Slope. Scaling relationships between the taper of the crystalline crust in the direction of the distal margin and the length/dip of the hinterland backslope follow directly. The shape factors of major catchments are directly scaled to the taper of the proximal margin and drainage azimuths are parallel to the mean transport lineation recorded from a distinct population of range-bounding normal faults. Topographic expressions of the footwalls and offsets in apatite fission-track age-patterns indicate that fault movement controlled topography, locally and regionally inboard of sharp crustal tapers long after the main phase of crustal thinning. We extend our definition of the passive margin to other post-breakup margins. One particularly fine example is SE Brasil. New data (Zalan et al., 2011) suggest the direct correlation of SE Brasil's Taper Break with its escarpment elevation in a manner consistent with our Scandinavian and global observations. The Taper Hypothesis appears to hold across old and young, glaciated, and unglaciated margins. Following the stretching, thinning, and exhumation phase, an "accommodation phase" is warranted. During accommodation, the initially elevated escarpments can be eroded to very low base levels and subsequently undergo inboard rejuvenation by footwall uplift, in response to tensile stresses

  13. Mesozoic Source-to-Sink of the African margin of the Equatorial Atlantic

    NASA Astrophysics Data System (ADS)

    ye, jing; Chardon, Dominique; rouby, delphine; Guillocheau, François; Robin, Cecile; Loparev, Artiom; Huyghe, damien; Dall'Asta, Massimo; Brown, Roderick; wildman, mark; webster, david

    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. We produced paleogeographic maps at the scale of West Africa spanning the continental domain and offshore basins since 200 Ma. Mapping spatial and temporal distribution of domains either in erosion (sources) or in accumulation (sinks) document the impact of the successive rifting of Central and Equatorial Atlantic on the physiography of the area. We use low temperature thermochronology dating along three transects perpendicular to the margin (Guinea, Ivory Coast and Benin) to determine periods and domains of denudation in that framework. We compare these data to the Mesozoic accumulation histories in passive margin basins and discuss their stratigraphic expression according to the type of margin segment they are preserved in. Syn-rift architectures (Early Cretaceous) are largely impacted by transform faults that define sub-basins with contrasted width of crustal necking zone (narrower in transform segments than in oblique/normal segments). During the Late Cretaceous post-rift, sedimentary wedges record a transgression along the all margin. Proximal parts of the sedimentary wedge are preserved in basins developing on segments with wide crustal necking zone while they were eroded away in basins developing on narrow segments. As a difference, the Cenozoic wedge is everywhere preserved across the whole width of the margin.

  14. Lithosphere extension and magmatism at volcanic passive margins

    NASA Astrophysics Data System (ADS)

    Geoffroy, Laurent; Gernigon, Laurent; Werner, Philippe

    2014-05-01

    We present onshore and offshore evidences suggesting that volcanic passive margins are distinct in origin and evolution from non-volcanic hyper-extended margins. Consecutively, they should not be integrated in a single evolutionary process and do not necessarily represent the ultimate stage of an hyper-extension with or without mantle exhumation. Volcanic passive margins usually form in mobile areas between cratonic areas which may have been submitted to long-term periods of divergence and convergence or strike-slip tectonics. In the NE-Atlantic, for example, a complete illustration of a Wilson cycle is illustrated between Greenland and Baltica cratonic areas. From the Devonian to the end of the Jurassic, the Caledonian orogenic crust has suffered from a number of wrench and extensional tectonic stretching episodes. The late-Jurassic/Early Cretaceous extension was severe, leading to extreme crustal thinning (e.g. Rockall Through, Vøring Basin, Lofoten Basin) and was followed by a long-term regional thermal subsidence of the NE-Atlantic lithosphere. Meanwhile, pre-thinning lithospheric thickness was restored progressively during ~80 Myr, in spite of some tectonic reactivation occurring in Late Cretaceous (e.g. Outer Vøring Basin) resulting in little coeval stretching and thinning. During the Paleocene (or even earlier, especially in the Rockall area) a regional mantle melting event occurred. The mantle melted in specific locations but led ultimately to a large igneous province formation during the onset of breakup. The NE-Atlantic continental crust was at this time extremely heterogeneous due to its tectonic inheritance but we think that generally the lithosphere was much thicker than during the Jurassic-Cretaceous event, and thus much stronger. Although we must consider the existence of some extension during the latest Cretaceous and Paleocene, the main stretching and thinning event leading to volcanic passive margins formations and successful break-up occurred

  15. Late Mesozoic North African continental margin: Sedimentary sequences and subsidence history

    SciTech Connect

    Kuhnt, W.; Obert, D.

    1988-08-01

    Cretaceous facies types and subsidence history have been studied along two well outcropping and almost complete transversals through the Tellian units of the Mesozoic North African margin, the Western Rif (Morocco), and the Babors (Algeria). Sedimentologic observations and characteristic foraminiferal assemblages enabled estimates for Late Cretaceous paleobathymetries. Both palinspastic reconstruction and sedimentologic and biofacies analyses led to the following results. (1) The morphology and evolution of the Cretaceous North African margin, which in general represents a classic passive continental margin, were complicated by various factors such as Late Cretaceous compressional and lateral movements, the onset of (tectonically controlled ) diapirism, and the existence of intramarginal highs and basins. (2) The Cretaceous subsidence history of both areas can be divided into four stages which are accompanied by characteristic sedimentary formations: (I) distension and subsidence of the margin (Early Cretaceous); (II) a first compressional phase with uplift and slight metamorphism in the Albian/early Cenomanian which affected mainly the northerly paleogeographic zones, accompanied by first diapiric movements and resedimentation of Triassic saliferous material; (III) a Late Cretaceous stage of subsidence (Cenomanian-Santonian); and (IV) a second compressional phase starting with the Campanian and reflected by the formation of sedimentary klippes and olistostromes. (3) As a general trend, sedimentary basins deepened from south to north during Campanian/Maastrichtian time, giving rise to a characteristic succession of bathymetric zones which have been observed on both transversals.

  16. Postrift history of the eastern central Atlantic passive margin: Insights from the Saharan region of South Morocco

    NASA Astrophysics Data System (ADS)

    Leprêtre, Rémi; Missenard, Yves; Barbarand, Jocelyn; Gautheron, Cécile; Saddiqi, Omar; Pinna-Jamme, Rosella

    2015-06-01

    The passive margin of South Morocco is a low-elevated passive margin. It constitutes one of the oldest margins of the Atlantic Ocean, with an Early Jurassic breakup, and little geological data are available concerning its postrift reactivation so far. We investigated the postrift thermal history of the onshore part of the margin with low-temperature thermochronology on apatite crystals. Fission track and (U-Th-Sm)/He ages we obtained are significantly younger than the breakup (~190 Ma). Fission track ages range from 107 ± 8 to 175 ± 16 Ma, with mean track lengths from 10.7 ± 0.3 to 12.5 ± 0.2 µm. (U-Th-Sm)/He ages range from 14 ± 1 to 185 ± 15 Ma. Using inverse modeling of low-temperature thermochronological data, we demonstrate that the South Moroccan continental margin underwent a complex postrift history with at least two burial and exhumation phases. The first exhumation event occurred during Late Jurassic/Early Cretaceous, and we attribute this to mantle dynamics rather than to intrinsic rifting-related processes such as flexural rebound. The second event, from Late Cretaceous to early Paleogene, might record the onset of Africa/Europe convergence. We show a remarkably common behavior of the whole Moroccan passive margin during its early postrift evolution. The present-day differences result from a segmentation of the margin domains due to the Africa/Europe convergence. Finally we propose that varying retained strengths during rifting and also the specific crustal/lithospheric geometry of stretching explain the difference between the topographical expressions on the continental African margin compared to its American counterpart.

  17. Data based 3D modelling of the southwest African continental margin

    NASA Astrophysics Data System (ADS)

    Freymark, J.; Sippel, J.; Scheck-Wenderoth, M.; Götze, H.-J.; Reichert, C.

    2012-04-01

    The volcanic passive continental margin of southwest Africa was formed in consequence of rifting and continental break-up of Gondwana in the Late Mesozoic. Our study focusses on an area extending from the Walvis Ridge in the north to the Agulhas Falkland Fracture Zone in the south including some important petroliferous sedimentary basins such as the Walvis Basin, the Luderitz Basin, and the Orange Basin. Due to decades of industrial exploration and scientific research, some of these areas reveal a large pool of structural and geophysical data. Thus, much is known about the individual tectonic and depositional histories of several subdomains of the area. The goal of our study is to understand the margin in its entirety. We present a 3D model of the present-day configuration of the southwest African continental margin. This model integrates well information, seismic reflection and refraction data, a previously published 3D structural model (Maystrenko et al., 2011), as well as freely available global data sets on the crustal structure (e.g. crust2.0 of Bassin, Laske & Masters, 2000). To extrapolate local information on crustal thickness (respectively the depth of the Moho) across the whole margin, we perform 3D gravity modelling using the software IGMAS+ (Götze & Schmidt, 2010; Schmidt et al., 2011). As parts of the first results, we show margin-wide depth and thickness distributions of a Palaeozoic to Cenozoic sedimentary layer and a Paleoproterozoic to Mesozoic crystalline crustal layer.

  18. Turbiditic systems on passive margins: fifteen years of fruitful industry-academic exchanges.

    NASA Astrophysics Data System (ADS)

    Guillocheau, F.

    2012-04-01

    During the last fifteen years, with the oil discovery in deep offshore plays, new tools have been developed that deeply modified our knowledge on sedimentary gravity processes on passive margins: geometry, physical processes, but also the importance of the topography and the quantification of the stratigraphic parameters of control. The major breakthrough was of course the extensive 3D seismic data available around most of the world margins with a focus on gravity-tectonics dominated margins. The first major progress was the characterization of the sinuous channels infilling, their diversity and different models for their origin. This also was a better knowledge of the different types of slopes (graded vs. above-graded) and the extension of the concept of accommodation to deep-water environments (ponded, healed-slope, incised submarine valley and slope accommodation). The second step was the understanding of the synsedimentary deformations for the location and the growth of turbiditic systems on margins dominated by gravity tectonics, with the importance of the sedimentary flux and its variation through time and space. The third step is now the integration of the sedimentary system, from the upstream erosional catchment to the abyssal plain (source to sink approach), with the question of the sediment routing system. During the last 100 Ma, continents experienced major changes of both topography and climate. In the case of Africa, those are (1) the growth of the plateaus (and mainly the South African one) around 90-80 Ma (Late Cretaceous) and 40-20 Ma (Late Eocene-Early Miocene) and (2) a climate evolution from hot humid (50-40 Ma) to hot dry conditions since 20-15 Ma. This evolution changed the topography, the processes of erosion and the volume and nature (weathered vs. non weathered rocks) materials. Those are primary processes for controlling the deposition of turbiditic systems, and then to predict the location of sands. This will be discussed along the

  19. Modelling the role of magmatic intrusions in the post-breakup thermal evolution of Volcanic Passive Margins

    NASA Astrophysics Data System (ADS)

    Peace, Alexander; McCaffrey, Ken; Imber, Jonny; van Hunen, Jeroen; Hobbs, Richard; Gerdes, Keith

    2013-04-01

    Passive margins are produced by continental breakup and subsequent seafloor spreading, leaving a transition from continental to oceanic crust. Magmatism is associated with many passive margins and produces diagnostic criteria that include 1) abundant breakup related magmatism resulting in a thick igneous crust, 2) a high velocity zone in the lower crust and 3) seaward dipping reflectors (SDRs) in seismic studies. These Volcanic Passive Margins (VPMs) represent around 75% of the Atlantic passive margins, but beyond this high level description, these magma-rich settings remain poorly understood and present numerous challenges to petroleum exploration. In VPMs the extent to which the volume, timing, location and emplacement history of magma has played a role in controlling heat flow and thermal evolution during margin development remains poorly constrained. Reasons for this include; 1) paucity of direct heat flow and thermal gradient measurements at adequate depth ranges across the margins, 2) poor onshore exposure 3) highly eroded flood basalts and 4) poor seismic imaging beneath thick offshore basalt sequences. As a result, accurately modelling the thermal history of the basins located on VPMs is challenging, despite the obvious importance for determining the maturation history of potential source rocks in these settings. Magmatism appears to have affected the thermal history of the Vøring Basin on the Norwegian VPM, in contrast the effects on the Faeroe-Shetland Basin was minimal. The more localised effects in the Faeroe-Shetland Basin compared to Vøring Basin may be explained by the fact that the main reservoir sandstones appear to be synchronous with thermal uplift along the basin margin and pulsed volcanism, indicating that the bulk of the magmatism occurred at the basin extremities in the Faeroe-Shetland Basin, where its effect on source maturation was lessened. Our hypothesis is that source maturation occurs as a result of regional temperature and pressure

  20. Coupling Thermo-Mecanical Simulation and Stratigraphic Modelling: Impact of Lithosphere Deformation on Stratigraphic Architecture of Passive Margin Basins

    NASA Astrophysics Data System (ADS)

    Rouby, D.; Huismans, R. S.; Braun, J.

    2013-12-01

    The aim of this study is to revise the view of the long-term stratigraphic trends of passive margins to include the impact of the coupling between the lithosphere deformation and the surface processes. However, modeling coupling lithosphere deformation and surface processes usually address large-scale deformation processes, i.e. they cannot resolve the stratigraphic trend of the simulated basins. On the other hand, models dedicated to stratigraphic simulation do not include these feedbacks of erosion/sedimentation on deformation processes. The recent development of a numerical modeling tool, coupling the thermal and flexural evolution of the lithosphere and including the (un)loading effects of surface processes in 3D (Flex3D; J. Braun), allows us to propose a new procedure to investigate, in 3D, the evolution of passive margins, from the scale of the lithosphere to the detailed stratigraphic architecture, including syn- and post-rift phases and onshore and offshore domains. To do this, we first simulate the syn-rift phase of lithosphere stretching by thermo-mechanical modeling (Sopal, R. Huismans). We use the resulting lithosphere geometry as input of the 3D flexural modeling to simulate the post-rift evolution of the margin. We then use the resulting accumulation and subsidence histories as input of the stratigraphic simulation (Dionisos, D. Granjeon) to model the detailed stratigraphic architecture of the basin. Using this procedure, we evaluate the signature of various boundary conditions (lithosphere geometries and thermal states, stretching distributions, surface processes efficiencies and drainage organization) in the uplift/subsidence and denudation histories as well as the stratigraphic architecture of the associated sedimentary basins. We apply the procedure to the case study of passives margins surrounding the West African craton, for which we have compiled data constraining the denudation and accumulation history, and the long term stratigraphic

  1. Numerical modeling of spatially-variable precipitation and passive margin escarpment evolution

    NASA Astrophysics Data System (ADS)

    Colberg, J. S.; Anders, A. M.

    2014-02-01

    Passive margin escarpments are long-lasting landforms which can remain steep and high over tens of millions of years. Precipitation variability has seldom been considered as a contributor to passive margin escarpment evolution. However, large and distinct variability in precipitation rates with elevation is observed in mountainous landscapes. We assess the potential impacts of spatially variable precipitation on passive margin escarpment evolution with a numerical model that couples a simple elevation-dependent precipitation rule with the CASCADE surface processes model and a simple flexural uplift model. Modeled topography is more sensitive to the elevation of the precipitation maximum than to the effective elastic thickness of the lithosphere. If precipitation is maximized at the escarpment crest, the escarpment decreases in elevation, slope and proximity to the coast more rapidly than if precipitation is focused at low elevations. Low elevation precipitation maxima contribute to increasing escarpment slope over time and preservation of a high-elevation escarpment. Using Tropical Rainfall Measuring Mission (TRMM) precipitation radar data, we establish precipitation/elevation relationships at high-elevation passive margins across the tropics and document the occurrence of patterns similar to those included in our model. Despite uncertainty as to the robustness of precipitation patterns and the simplified tectonic and geomorphic framework of our model we conclude that spatial variability in precipitation is a potentially significant contributor to passive margin escarpment evolution.

  2. Geochemical discrimination of siliciclastic sediments from active and passive margin settings

    NASA Astrophysics Data System (ADS)

    Verma, Surendra P.; Armstrong-Altrin, John S.

    2016-03-01

    Discrimination of active and passive margins is important from both academic and economic aspects. This can only be successfully achieved, however, if there are major compositional differences among sediments derived from different continental margins. A worldwide database of active and passive margin settings was established from published major and trace element geochemical data of Neogene to Quaternary siliciclastic sediments. These data were used to evaluate the performance of existing discrimination diagrams, which were shown to work unsatisfactorily with success values of mostly between 0% and 30%. Because these diagrams were not based on a statistically coherent methodology, we proposed two new discriminant functions from linear discriminant analysis of multinormally distributed isometric log-transformed ratios of major and combined major and trace elements. These new diagrams showed very high percent success values of about 87%-97% and 84%-86% for the active and passive margins, respectively, for the original database. Excellent performance of the multidimensional diagrams and related discriminant functions was confirmed from 11 test studies involving Quaternary to Holocene siliciclastic sediments from known tectonic margins. The expected result of an active or passive margin was obtained, with most samples plotting correctly in the respective field.

  3. Comparison of submarine gully morphologies in passive and active margin settings

    NASA Astrophysics Data System (ADS)

    Jackson, C.; Shumaker, L.; Johnstone, S.; Graham, S. A.

    2015-12-01

    Passive and active tectonic margins have inherently different hypsometry, due to local patterns of deformation and subsequent impacts on the style of sedimentation. One way we can analyze and compare the two settings is through observation of submarine gullies, which are small channel features that form along the continental slope as it descends to the ocean floor. By documenting the geometries of gullies that have formed on passive margins and gullies that have formed on active margins, we attempt to distinguish differences in gully morphologies in these two settings. We manually mapped over 600 gullies and interfluves from shaded relief and contour maps generated from bathymetric data across the globe, including the coast of California, the Beaufort Sea, and the Black Sea. We extrapolated and plotted elevation profiles of the gullies along their downslope distance, and compared a range of gully properties, such as length, spacing, and slope, to look at the correlations among those elements of gullies and their tectonic setting. We find that gullies forming on active margins show the greatest variability in their slopes, exhibiting both the steepest and the shallowest slopes of the dataset. The slopes of the passive margin gullies fall within the range of the active margin gully slopes, but interestingly, we note patterns in the ranges of gully steepness at different localities. These results differ from our our anticipation that active margin gullies are steeper than passive margin gullies, but suggest that gullies in all settings display a variety of morphologies. Additional mapping of active margin gullies will better determine if there are morphological differences between the two settings.

  4. Transition from a passive continental margin to an active margin documented by time-facies profiles and geohistory diagrams

    SciTech Connect

    Kenter, J.A.M.; Reymer, J.J.G.; van der Straaten, H.C.

    1988-08-01

    The Upper Cretaceous to Neogene sediments in the northern part of the external zone of the Betic Cordilleras (southeast Spain) reflect the evolution of a passive continental margin into an active margin. Time-facies profiles and geohistory diagrams were constructed to identify and date tectonic events and sea level changes in the sedimentary record. During Late Cretaceous to middle Eocene time, parallel-trending shallow marine facies belts at the edge of a slowly subsiding basin evidence a passive continental margin setting. The period from middle Eocene to early Miocene is rather poorly documented. After initial shallowing the whole area emerged and continental conditions prevailed from the late Eocene to early Miocene. The subareal exposure may have been caused by the compound effect of the worldwide Oligocene sea level drop and overall tectonic uplift. Miocene sediments reflect the compressional tectonic regime of an active margin. The parallel facies belts were disrupted and the area was segmented into several tectonic blocks, each displaying an individual sedimentary record due to differential vertical movement. During middle to late Miocene time a major compressional phase generated northeast-trending folds and activated strike-slip and thrust faults. This tectonic phase led to a highly differentiated topography, resulting in the development of local depocenters - each with a unique tectonic and depositional record.

  5. Mesozoic evolution of the northeast African shelf margin, Libya and Egypt

    SciTech Connect

    Aadland, R.K.; Schamel, S.

    1988-08-01

    The present tectonic features of the northeast African shelf margin between the Nile delta and the Gulf of Sirte are products of (1) precursory late Paleozoic basement arches, (2) early Mesozoic rifting and plate separation, and (3) Late Cretaceous structural inversion. Isopach and structural maps, cross sections, and sediment accumulation (geohistory) curves constructed from 89 wells in the Western Desert and 27 wells in northeastern Libya depict the structural and stratigraphic development of the northeast African shelf margin.

  6. Continental environment variations (climate, erosion) recorded by Marine quaternary sediments of the northwestern and eastern African margins

    NASA Astrophysics Data System (ADS)

    Faugères, J. C.; Pujol, Cl.

    Samples collected from 4 sites on the northwestern and eastern African margins were used to test the reliability of marine sedimentary record of continental environmental variations, during the last Glacial and Interglacial climatic cycle. On the northwestern margin which is passive and stable (between Cape Verde and Cape Blanc), climatic variations are marked by parameters such as sedimentary facies, sedimentary dynamics, sedimentation rates or faunal assemblages. These parameters are controlled by climatic changes that modify continental environments (erosion conditions, rate of terrigenous supplies) and marine environments as well (sea-level, currents and biogenic sediment productivity). On the opposite, in the Gulf of Aden, 3 sites show the extent to which tectonics may affect the record of environment modifications due to climatic changes. In the East of the Gulf, on the Sukra margin that is passive but with young and still active structures, the continental slope is uneven with tectonic basins acting as sediment traps. Here, several parameters like sedimentation rates become unreliable for they no longer reflect the importance of terrigenous inputs nor that of primary productivity. Further to the West, the deep narrow trough of Alula Fartak and the epicontinental domain belonging to the Assal rift (Ghubbet el Kharab), are part of highly active tectonic and volcanic margins. Continental environment variations cease to be recorded through sedimentological parameters which are closely related to morpho-structural and volcanic factors.

  7. Hydrocarbon traps within passive-margin evolution of Louisiana

    SciTech Connect

    Lavoie, D. ); Lowrie, A.

    1993-09-01

    The evolutionary dynamics of the Louisiana continental margin as applied to the Neogene to present are sufficiently well understood that we present a preliminary model. The external components influencing the geologic evolution are sediment input (amount, type, and transport mechanisms) and sea level oscillations (periodicity and range). The internal dynamics are subsidence (rate, total amount, and location), salt tectonics (type and rate of motion), and sediment deposition (amount, type and mechanisms). The model presented is restricted geographically to the offshore region, from the shelf to the Sigsbee Escarpment, and temporally during the Neogene, the past 20 m.y. The notion that tectonic periodicity controls the evolutionary dynamics is integral to the model. The general loci of maximal deposition and tectonics are dictated by Milankovitch fourth-order cycles ranging from 1 x 10[sup 4] to 1 x 10 [sup 5] yr. superimposed on third-order cycles of up to 1 to 2 x 10[sup 6]yr. This model suggests a highly energetic phase in overall continental margin evolution during which the Sigsbee salt wedge migrated past an arbitrary fixed reference point, changing the physiography from lower slope to shelf. The energetic phase, which lasts between 2 and 4 m.y., separated two much longer phases are the drift phase, characterized by sedimentation along lower continental rises and abyssal plains, and a depositional phase, generally minor, and erosion along the shelf, coastal plain, and interior basins. This latter phase is characterized by regional subsidence and [open quotes]catch-up[close quotes] deposition as equilibrium along the continent is maintained. We also discuss hydrocarbon traps and their ephemeral nature with the overall continental margin.

  8. Tectonic heritage and intra-crustal decoupling: consequences for inversion of a passive margin

    NASA Astrophysics Data System (ADS)

    Burov, E. B.; Duretz, T.; Bellahsen, N.; Le Pourhiet, L.

    2012-12-01

    We investigate the consequences of tectonic heritage and rheological structure on the style and evolution of collision process resulting from inversion of rifted margins. Recent studies of structure and evolution of passive margins have led to significant reconsideration of conceptual models of margin evolution, specifically regarding the concepts of proximal inversion during the post-rift collision. For example, for a number of margins, it has been shown that the crust very locally thins in the vicinity of the continent-ocean transition, from about 30 km to a few km in thickness, along a very short distance. This observation has been interpreted as the result of large crustal detachments that result in thinning of the medium and lower crusts, or as the result of a particular sequence of activity of steep faults. This localization of deformation occurs after a period of distributed rifting during which forms the proximal margin, and follows a phase of exhumation of the mantle to the surface. It seems clear that the burial (during the collision) of such complex margins must necessarily differ from that of margins which structure would be more continuous. We implement a parametric thermo-mechanical numerical study of the role of geometry and of the inherited structure of the inverted margins in the subduction-collision transition. In the experiments, we first form a margin by applying passive extension to continental lithosphere of different structure. After the margin is formed, we apply compression allowing for different periods of relaxation. The experiments demonstrate strong dependence of the developing collision style on the initial thermo-rheological structure and geometry of the margin and on the delay between the extension and compression phase. The resulting collision modes vary from subduction of the continental margin to pure shear thickening and, in some cases, to obduction of the oceanized lithosphere. Our experiments also treated as a particular case

  9. Tectonic heritage and intra-crustal decoupling: consequences for inversion of a passive margin

    NASA Astrophysics Data System (ADS)

    Burov, Evgueni; Le Pourhiet, Laetitia; Mezri, Leila; Duretz, Thibault; Bellahsen, Nicolas

    2013-04-01

    We investigate the consequences of tectonic heritage and rheological structure on the style and evolution of collision process resulting from inversion of rifted margins. Recent studies of structure and evolution of passive margins have led to significant reconsideration of conceptual models of margin evolution, specifically regarding the concepts of proximal inversion during the post-rift collision. For example, for a number of margins, it has been shown that the crust very locally thins in the vicinity of the continent-ocean transition, from about 30 km to a few km in thickness, along a very short distance. This observation has been interpreted as the result of large crustal detachments that result in thinning of the medium and lower crusts, or as the result of a particular sequence of activity of steep faults. This localization of deformation occurs after a period of distributed rifting during which forms the proximal margin, and follows a phase of exhumation of the mantle to the surface. It seems clear that the burial (during the collision) of such complex margins must necessarily differ from that of margins which structure would be more continuous. We implement a parametric thermo-mechanical numerical study of the role of geometry and of the inherited structure of the inverted margins in the subduction-collision transition. In the experiments, we first form a margin by applying passive extension to continental lithosphere of different structure. After the margin is formed, we apply compression allowing for different periods of relaxation. The experiments demonstrate strong dependence of the developing collision style on the initial thermo-rheological structure and geometry of the margin and on the delay between the extension and compression phase. The resulting collision modes vary from subduction of the continental margin to pure shear thickening and, in some cases, to obduction of the oceanized lithosphere. Our experiments also treated as a particular case

  10. Stratigraphic signature of lithospheric deformation style in post-rift passive margin basins

    NASA Astrophysics Data System (ADS)

    Rouby, Delphine; Huismans, Ritske; Robin, Cecile; Braun, Jean; Granjeon, Didier

    2016-04-01

    We revise commonly accepted models explaining long-term stratigraphic trends along Atlantic-type passive margins by including the impact of complex lithosphere deformation at depth and it's coupling with surface processes. To achieve this, we simulated the evolution of a passive margin basin using a cascade of three modeling tools: a thermo-mechanical model of the syn-rift stretching of the lithosphere, a flexural and thermal model of the post-rift stage that includes coupling with surface processes and, finally, a stratigraphic model of the associated sedimentary basin architecture. We compare two necking styles that lead to different margin geometries: wide and narrow margins that form by heterogeneous stretching. Wide margins, forming thinner and wider sedimentary wedges, show significantly larger aggradation component and longer preservation duration, in more continental/proximal depositional facies. Narrow margins are characterized by enhanced erosion and by-pass during transgression. Through a parametric analysis we constrain the relative contribution of lithosphere deformation and surface processes on the stratigraphic trends and show that both may contribute equally to the stratigraphic architecture. For example, enhanced erosion in narrow margins impacts the volume of sediments delivered to the basin, which, in turn, significantly increases the subsidence. Our simulations also underline the importance of the assumed sediment transport length, which controls whether the main depocentres remain in the necking zone or reach the more distal parts of the margin.

  11. ODP Leg 107 results from continental margin east of Sardinia (Mediterranean Sea): a transect across a very young passive margin

    SciTech Connect

    Kastens, K.A.; Mascle, J.; Auroux, C.; Bonatti, E.; Broglia, C.; Channell, J.; Curzi, P.; Emeis, K.; Glacon, G.; Hasegawa, S.; Hieke, W.

    1987-05-01

    A 200-km wide zone east of Sardinia, characterized by thin continental crust with tilted, listric(.)-fault-bounded blocks, has been interpreted as a passive continental margin formed during back-arc opening of the Tyrrhenian Sea. Leg 107 of the Ocean Drilling Project drilled a transect of four sites across this margin plus three sites in the basaltic basin. Site 654, closest to Sardinia, recovered a transgressive sequence attributed to basin subsidence: coarse-grained, iron-oxide rich, subaerial conglomerates underlie oyster-bearing sands followed upsection by open-water Tortonian marine marls. The synrift sequence, as inferred from seismic reflection profiles, correlates with sediments of Tortonian to Messinian age. Farther east the synrift sediments are younger: site 652, near the continental/oceanic transition, recovered an inferred synrift sequence of Messinian to early Pliocene age. The pan-Mediterranean Messinian desiccation event is represented at the western two sites (654 and 653) by a basinal facies including laminated gypsum, whereas at the eastern two sites the Messinian facies are terrestrial (lacustrine at 652 and subaerial at 656). They therefore infer that subsidence was more advanced at the western sites than at the eastern sites as of 5 Ma. Leg 107 results suggest that subsidence and stretching were diachronous across the passive margin, beginning and ending several million years earlier in the west than in the east. This asynchroneity may result from the inherent asymmetry of back-arc basin opening, or it may be a common characteristic of passive margins which has been revealed by the unusually precise time resolution of this data set.

  12. Late Permian to mid-Cretaceous carbonate platform along the passive margin of the southeastern Mediterranean

    SciTech Connect

    Derin, B.; Garfunkel, Z.

    1988-08-01

    Starting from the Late Permian and throughout most of the Mesozoic, up to 5 km of shallow-water carbonates formed in a subsiding belt along the northern fringe of the Arabo-African continent, passing inland into thinner and clastic-rich sections. This sedimentation pattern was established in the Late Permian and evolved in several distinct stages that depended on global oscillations of sea level and local tectonic events. In Middle Triassic to early Liassic times, several pulses of faulting and magmatism, interpreted as rifting, occurred along the subsiding belt of carbonate deposition and produced a passive continental margin. Tectonic activity ended by the Roarcian, and since then a rather uniform shallow-water carbonate shelf formed. It was delimited by shoals of high-energy sediments and fringing reefs, inland of which low-energy carbonate muds and sometimes dolomite accumulated. By the Late Jurassic a more than 1.5 km-high continental slope developed, separating the shallow-water domain from a starved deep-water basin. In latest Jurassic and earliest Cretaceous time the activity of an intraplate hot spot caused extensive magmatism, uplifting, and erosion. The eroded material formed a thick sediment body in front of the continental slope. Since late Valanginian time, renewed regional subsidence and a rise in sea level led to a new phase of carbonate deposition over a wide shallow-water shelf. This was fringed by an accentuated continental slope covered by a basinward-thinning wedge-shaped apron of calciclastic sediments. This region was terminated by Senonian and later deformation related to plate collision in the Alpine orogenic belt.

  13. Late Cenozoic flexural deformation of the middle U.S. Atlantic passive margin

    NASA Technical Reports Server (NTRS)

    Pazzaglia, Frank J.; Gardner, Thomas, W.

    1994-01-01

    Despite the century-long recognition of regional epeirogeny along the middle Atlantic passive margin, relatively few studies have focused on understanding postrift uplift mechanisms. Here, we demonstrate that epeirogenic uplift of the central Appalachian Piedmont and subsidence of the Salisbury Embayment represent first-order, flexural isostatic processes driven by continental denudation and offshore deposition. Our results show that regional epeirogenic processes, present on all Atlantic-type passive margins, are best resolved by specific stratigraphic and geomorphic relationships, rather than topography. A simple one-dimensional geodynamic model, constrained by well-dated Baltimore Canyon trough, Coastal Plain, and lower Susquehanna River (piedmont) stratigraphy, simulates flexural deforamtion of the U.S. Atlantic margin. The model represents the passive margin lithosphree as a uniformly thick elastic plate, without horizontal compressive stresses, that deforms flexurally under the stress of strike-averaged, vertically applied line loads. Model results illustrate a complex interaction among margin stratigraphy and geomorphology, the isostatic repsonse to denudational and depositional processes, and the modulating influence of exogenic forces such as eustasy. The current elevation, with respect to modern sea level, of fluvial terraces and correlateive Coastal Plain deposits or unconformities is successfully predicted through the synthesis of paleotopography, eustatic change, and margin flexure. Results suggest that the middle U.S. Atlantic margin landward of East Coast Magnetic Anomaly is underlain by lithoshpere with an average elastic thickness of 40 km (flexural rigidity, D = 4 X 10(exp 23) N m), the margin experience an average, long-term denudation rate of approximately 10m/m.y., and the Piedmont has been flexurally upwaped between 35 and 130 meters in the last 15 m.y. Long term isostatic continental uplift resulting rom denudation and basin subsidence

  14. Late Cenozoic flexural deformation of the middle U.S. Atlantic passive margin

    NASA Astrophysics Data System (ADS)

    Pazzaglia, Frank J.; Gardner, Thomas W.

    1994-06-01

    Despite the century-long recognition of regional epeirogeny along the middle Atlantic passive margin, relatively few studies have focused on understanding postrift uplift mechanisms. Here, we demonstrate that epeirogenic uplift of the central Appalachian Piedmont and subsidence of the Salisbury Embayment represent first-order, flexural isostatic processes driven by continental denudation and offshore deposition. Our results show that regional epeirogenic processes, present on all Atlantic-type passive margins, are best resolved by specific stratigraphic and geomorphic relationships, rather than topography. A simple one-dimensional geodynamic model, constrained by well-dated Baltimore Canyon trough, Coastal Plain, and lower Susquehanna River (Piedmont) stratigraphy, simulates flexural deformation of the U.S. Atlantic margin. The model represents the passive margin lithosphere as a uniformly thick elastic plate, without horizontal compressive stresses, that deforms flexurally under the stress of strike-averaged, vertically applied line loads. Model results illustrate a complex interaction among margin stratigraphy and geomorphology, the isostatic response to denudational and depositional processes, and the modulating influence of exogenic forces such as eustasy. The current elevation, with respect to modern sea level, of fluvial terraces and correlative Coastal Plain deposits or unconformities is successfully predicted through the synthesis of paleotopography, eustatic change, and margin flexure. Results suggest that the middle U.S. Atlantic margin landward of East Coast Magnetic Anomaly is underlain by lithosphere with an average elastic thickness of 40 km (flexural rigidity, D = 4 × 1023 N m), the margin experiences an average, long-term denudation rate of approximately 10 m/m.y., and the Piedmont has been flexurally upwarped between 35 and 130 meters in the last 15 m.y. Long-term isostatic continental uplift resulting from denudation and basin subsidence

  15. Eastern Venezuela Basin's Post-Jurassic evolution as a passive transform margin basin

    SciTech Connect

    George, R.P. Jr. ); Sams, R.H. )

    1993-02-01

    Passive transform margins are segments of rifted continental margins bounded by transform faults that are active during rifting and that become inactive during drifting. Examples include the northern coast of Brazil and its matching margin along the Liberia-Nigeria coast. We propose that the northern margin of the Eastern Venezuela Basin was dominantly a passive transform margin during the Cretaceous and early Paleogene, rather than a purely passive margin. Published microplate reconstructions of the southern Caribbean show Jurassic separation of the Bahamas platform from northern South America along a northwest-trending transform fault postulated to lie just northeast of Trinidad and the Guianas. We conjecture that the [open quotes]Deflexion de Barcelona[close quotes] (a northwest-trending zone of strike slip faults along the southwestern edge of the Serrania del Interior) is controlled by a basement geofracture that is the onshore expression of Jurassic transform fault southwest of and subparallel to the southwestern Bahamas transform. Implications of this conjecture for the Eastern Venezuela Basin include: (1) absence of McKenzie-type regional crustal stretching, Mesozoic thermal anomaly, and Mesozoic thermal-tectonic subsidence; (2) abrupt rather than gradual seaward changes in crustal thickness; (3)abrupt lateral changes in thickness and facies of Mesozoic sediments, as in the Piaui-Ceara basins of northern Brazil; (4) tendency for structural styles developed during Neogene compression to include more strike-slip faults and en enchelon fold sets (because of reactivation of Mesozoic transforms) than would be expected by structural inversion of a purely passive margin.

  16. Cretaceous sequence stratigraphy of the Northern South American Passive Margin: Implications for tectonic evolution

    SciTech Connect

    Kauffman, E.G.; Villamil, T.; Johnson, C.C. )

    1993-02-01

    The passive margin of northern South America, from Colombia to northeastern Venezuela, was relatively stable through the Cretaceous and only broadly affected by the entry of the Caribbean Plate into the Protocaribbean Basin. This region offers a unique opportunity to test the relative effects of global sealevel change, autocyclic sedimentologic processed, and regional tectonics in shaping the stratigraphic record of Cretaceous passive margins. High-resolution stratigraphic studies of Colombia and Venezuela have established a precise system of regional chronology and correlation with resolution <1 Ma (50-500 ka for the middle Cretaceous). This allows precise separation of allocyclic and autocyclic controls on facies development. This new chronology integrates assemblage zone biostratigraphy with event/cycle chronostratigraphy. Newly measured Cretaceous sections in Venezuela and throughout Colombia are calibrated to this new chronology, and sequence stratigraphic units independently defined to the third-order of resolution. Graphic correlation of all sections is used to identify sequences with regional stratigraphic expression, and those which correlate to sequence stratigraphic standards of North America, Europe and the global cycles of Hag et al. (1988). 50-60 percent of the stratigraphic sequences across the South American passive margin correlate to other continents and to the global sequence stratigraphic standard, reflecting strong eustatic influence on Cretaceous sedimentation across northern South America. The remaining sequences in this region reflect tectonic modification of the passive margin and autocyclic sedimentary processes.

  17. Origin of the Blue Ridge escarpment along the passive margin of Eastern North America

    USGS Publications Warehouse

    Spotila, J.A.; Bank, G.C.; Reiners, P.W.; Naeser, C.W.; Naeser, N.D.; Henika, B.S.

    2004-01-01

    The Blue Ridge escarpment is a rugged landform situated within the ancient Appalachian orogen. While similar in some respects to the great escarpments along other passive margins, which have evolved by erosion following rifting, its youthful topographic expression has inspired proposals of Cenozoic tectonic rejuvenation in eastern North America. To better understand the post-orogenic and post-rift geomorphic evolution of passive margins, we have examined the origin of this landform using low-temperature thermochronometry and manipulation of topographic indices. Apatite (U-Th)/He and fission-track analyses along transects across the escarpment reveal a younging trend towards the coast. This pattern is consistent with other great escarpments and fits with an interpretation of having evolved by prolonged erosion, without the requirement of tectonic rejuvenation. Measured ages are also comparable specifically to those measured along other great escarpments that are as much as 100 Myr younger. This suggests that erosional mechanisms that maintain rugged escarpments in the early post-rift stages may remain active on ancient passive margins for prolonged periods. The precise erosional evolution of the escarpment is less clear, however, and several end-member models can explain the data. Our preferred model, which fits with all data, involves a significant degree of erosional escarpment retreat in the Cenozoic. Although this suggests that early onset of topographic stability is not required of passive margin evolution, more data are required to better constrain the details of the escarpment's development. ?? 2003 Blackwell Publishing Ltd.

  18. Geological Studies in eastern Venezuela and Trinidad from Cretaceous passive margin to Neogene transpressional thrust belt

    SciTech Connect

    Algar, S.T.; Erikson, J.E.; Pindell, J.L. )

    1991-03-01

    Sedimentological and structural analyses of Trinidad's Northern and Central ranges and Venezuela's Serrania del Interior have led to new interpretations of northeastern. South America's tectonic evolution within the Southern Caribbean Plate Boundary Zone. Medial ( ) Jurassic through early Cenozoic passive margin sediments make up the majority of these areas and were deposited somewhat to the WNW (between 80 and 130 km for Northern Range of Trinidad) of their present positions prior to structural shortening. Neogene southeastward displacement of Jurassic-Cretaceous passive slope and rise sediments (Northern Range) drove propagation of thrusts southward into the Serrania and Central ranges. Displacements were driven by migration of the Caribbean Plate relative to South America. Thus, the Serrania and Central ranges are the western hemisphere's only exposed Mesozoic-Cenozoic passive Atlantic margin stratigraphic section. As such, they provide a Cretaceous-Paleogene record of passive margin sedimentation at a thermally subsiding margin where the complicating effects of tectonism are absent. This makes these sections especially suited for studies of eustatic sea level behavior. Preliminary assessments are shown which suggest that sea level changes for Cretaceous to Paleogene time are not as pronounced as the frequent large and rapid sea level falls and rises that are promoted by some.

  19. Isostatic and dynamic support of high topography on a North Atlantic passive margin

    NASA Astrophysics Data System (ADS)

    Pedersen, Vivi K.; Huismans, Ritske S.; Moucha, Robert

    2016-07-01

    Substantial controversy surrounds the origin of high topography along passive continental margins. Here we focus on the well-documented elevated passive margin in southwestern Scandinavia, and quantify the relative contributions of crustal isostasy and dynamic topography in controlling the present topography. We find that majority of the topography is compensated by the crustal structure, suggesting a topographic age that is in accord with the 400 Myr old Caledonian orogenesis. In addition, we propose that dynamic uplift of ∼300 m has rejuvenated existing topography locally in the coastal region over the last 10 Myr. Such uplift, combined with a general sea level fall, can help explain a variety of observations that have traditionally been interpreted in favor of a peneplain uplift model. We conclude that high topography along the Scandinavian margin cannot represent remnants of a peneplain uplifted within the last 20 Myr. The topography must have been high since the Caledonian orogeny.

  20. Numerical experiments on the influence of melt and serpentinization on passive margin structure

    NASA Astrophysics Data System (ADS)

    Tetreault, Joya; Buiter, Susanne

    2014-05-01

    Passive margins are often classified as magma-rich or magma-poor, with distinctly different crustal architectures. For example, end-member magma-rich margins have thick sequences of seaward-dipping reflectors, short necking zones, and thick oceanic crusts, whereas many magma-poor margins have wide necking zones, hyper-extended crust, and exhumed serpentinized mantle. Melt and magmatic processes can strongly affect the mantle and crust during various stages of extension. At late stages of extension, serpentinization of upper mantle rocks will also affect crustal strength. We aim to study the influence of melt and serpentinization on structures developed during passive margin formation. Melt and serpentinization are two processes that can alter crust and mantle rheologic properties during different stages of extension. Introducing melt into a rift system will alter the thermal field, rheology, and density of crust and lithosphere. The presence of large amounts of melt (7-8%) in upper mantle rocks will significantly lower the viscosity. In addition, depleted mantle rocks can have significant loss of water that would result in raising the viscosity by about a factor of 100. Intrusion and underplating of magma to the lower crust can cause metamorphism and thus density and rheological changes of the surrounding crust. Furthermore, analogue experiments have shown that magmatic underplating will induce strain localization in the crust during extension. In regions such as the magma-poor margins of the North Atlantic, the serpentinization of mantle peridotites after sufficient thinning of continental crust can lead to strain localization that will subsequently affect the margin architecture. Upper mantle rocks become serpentinized at temperatures lower than 400 degrees when seawater infiltrates. The lower frictional properties that serpentinized peridotites have at these temperatures work to localize strain and allow detachment faults to form. We use 2D numerical experiments

  1. Linking margin morphology to sedimentary processes along the US East Coast passive continental margin

    NASA Astrophysics Data System (ADS)

    Brothers, D. S.; ten Brink, U. S.; Andrews, B.; Twichell, D.

    2010-12-01

    The morphology of the US East Coast continental slope and rise has a surprising amount of along-margin variation. Multibeam bathymetry datasets that cover the slope and rise from Cape Hatteras to Georges Bank provide a unique opportunity to analyze both first-order and higher-order morphologies, including submarine canyons, landslides, slumps and sedimentary bedforms. Using the morphological characterization coupled with seismic and core data, we hope to better understand how ancient and modern sedimentary processes control the shape of the margin. As a first step, the margin bathymetry was subdivided into 20 shelf-perpendicular regions from which several statistical parameters were analyzed. Within each region, the slope gradient was computed separately for down-slope and across-slope aspect directions. Distribution curves in each region for down- and across-slope gradients and seafloor roughness as functions of depth were grouped according to their statistical similarities. Four basic groups emerge and each approximately corresponds to known regions of Quaternary glacial, fluvial, current-controlled and gravity-driven sedimentary transport. In the second part of the study, published lithologic and chronostratigraphic frameworks of this margin were used to examine the relationship between seafloor morphology and the underlying geology. Along the upper continental rise, thick Quaternary deposits appear to have a strong influence on the short- and long-wavelength variation in rise topography, revealing a complex interplay between down-slope and along-slope sediment transport. Despite the close correlation between continental slope morphology and Quaternary environmental conditions, initial results suggest that the underlying, older, stratigraphy also plays a primary role. Along the continental slope, Quaternary processes appear to control the relief of slope-confined canyons and other short-wavelength (<5 km) topography, but the first order morphology of the slope

  2. New Insights into Passive Margin Development from a Global Deep Seismic Reflection Dataset

    NASA Astrophysics Data System (ADS)

    Bellingham, Paul; Pindell, James; Graham, Rod; Horn, Brian

    2014-05-01

    The kinematic and dynamic evolution of the world's passive margins is still poorly understood. Yet the need to replace reserves, a high oil price and advances in drilling technology have pushed the international oil and gas industry to explore in the deep and ultra-deep waters of the continental margins. To support this exploration and help understand these margins, ION-GXT has acquired, processed and interpreted BasinSPAN surveys across many of the world's passive margins. Observations from these data lead us to consider the modes of subsidence and uplift at both volcanic and non-volcanic margins. At non-volcanic margins, it appears that frequently much of the subsidence post-dates major rifting and is not thermal in origin. Rather the subsidence is associated with extensional displacement on a major fault or shear zone running at least as deep as the continental Moho. We believe that the subsidence is structural and is probably associated with the pinching out (boudinage) of the Lower Crust so that the Upper crust effectively collapses onto the mantle. Eventually this will lead to the exhumation of the sub-continental mantle at the sea bed. Volcanic margins present more complex challenges both in terms of imaging and interpretation. The addition of volcanic and plutonic material into the system and dynamic effects all impact subsidence and uplift. However, we will show some fundamental observations regarding the kinematic development of volcanic margins and especially SDRs which demonstate that the process of collapse and the development of shear zones within and below the crust are also in existence at this type of margin. A model is presented of 'magma welds' whereby packages of SDRs collapse onto an emerging sub-crustal shear zone and it is this collapse which creates the commonly observed SDR geometry. Examples will be shown from East India, Newfoundland, Brazil, Argentina and the Gulf of Mexico.

  3. 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

  4. Geodynamics of passive margins: insights from the DFG Schwerpunktprogramm SAMPLE for the South Atlantic and beyond

    NASA Astrophysics Data System (ADS)

    Bunge, Hans-Peter

    2016-04-01

    The DFG Priority Program SAMPLE (South Atlantic Margin Processes and Links with onshore Evolution: http://www.sample-spp.de/), which is to be completed 2016, has studied the evolution of the South Atlantic from its Cretaceous inception to the present day. The program has an explicit interdisciplinary focus, drawing on constraints from deep Earth geophysics, lithosphere and basin dynamics, petrology, landscape evolution and geodesy, thus linking processes that are commonly studied in isolation. Starting from the premise that passive margins are first-order geo-archives, the program has placed the South Atlantic opening history into an observational and theoretical context that considers seismic imaging, plate motion histories, uplift and subsidence events, magmatic and surface evolution, together with models of mantle convection and lithosphere dynamics. A primary lesson is that passive margins are active, displaying a range of vertical motion (i.e. dynamic topography) events, apparently correlated with plate motion changes, that do not conform to traditional rifting models of passive margins. I will summarize some observational results of the program, and place them into a geodynamic context.

  5. Mountain building along a passive margin: Late Neogene tectonism in southeastern Victoria, Australia

    NASA Astrophysics Data System (ADS)

    Webb, John A.; Gardner, Thomas W.; Kapostasy, Dan; Bremar, Kathy A.; Fabel, Derek

    2011-01-01

    The Hoddle Ranges (maximum elevation of ~ 750 m above sea level) lie along the southeastern Australian passive margin. Detailed geological/geomorphological studies of the southern margin of the ranges, focusing on a fault block of Oligocene-Miocene sedimentary rocks, have constrained the landscape evolution. In the mid-Cretaceous, this area changed from a subduction zone accumulating volcanogenic sediments to a passive, low-relief margin, which was crossed by north-flowing rivers. In the Paleocene, the eruption of basaltic volcanics formed a low divide which diverted these rivers towards the northeast, so that sediments deposited on either side of the divide contain different heavy mineral assemblages. In the middle Late Miocene-Early Pliocene the area was subjected to a period of relatively rapid mountain building, the Kosciuszko Uplift, as broadly NW-SE oriented compression created the Hoddle Ranges at an uplift rate of ~ 0.15 mm/a. Uplift was not uniform; a small southern block was uplifted only ~ 200 m, and its surface has acted as a local base level for the Agnes River which flows across it, with a major knickpoint on the southern side. The Southeastern Highlands to the north were uplifted by up to 700-1000 m during the Kosciuszko Uplift, similar to the maximum increase in elevation of the Hoddle Ranges (~ 600 m). The Kosciuszko Uplift tectonism occurred at rates greater than typical of passive margins, and belies Australia's reputation as a tectonically stable continent.

  6. Space-for-time substitution and the evolution of submarine canyons in a passive, progradational margin.

    NASA Astrophysics Data System (ADS)

    Micallef, Aaron; Ribó, Marta; Canals, Miquel; Puig, Pere; Lastras, Galderic; Tubau, Xavier

    2013-04-01

    40% of submarine canyons worldwide are located in passive margins, where they constitute preferential conduits of sediment and biodiversity hotspots. Recent studies have presented evidence that submarine canyons incising passive, progradational margins can co-evolve with the adjacent continental slope during long-term margin construction. The stages of submarine canyon initiation and their development into a mature canyon-channel system are still poorly constrained, however, which is problematic when attempting to reconstruct the development of passive continental margins. In this study we analyse multibeam echosounder and seismic reflection data from the southern Ebro margin (western Mediterranean Sea) to document the stages through which a first-order gully develops into a mature, shelf-breaching canyon and, finally, into a canyon-channel system. This morphological evolution allows the application of a space-for-time substitution approach. Initial gully growth on the continental slope takes place via incision and downslope elongation, with limited upslope head retreat. Gravity flows are the main driver of canyon evolution, whereas slope failures are the main agent of erosion; they control the extent of valley widening, promote tributary development, and their influence becomes more significant with time. Breaching of the continental shelf by a canyon results in higher water/sediment loads that enhance canyon development, particularly in the upper reaches. Connection of the canyon head with a paleo-river changes evolution dynamics significantly, promoting development of a channel and formation of depositional landforms. Morphometric analyses demonstrate that canyons develop into geometrically self-similar systems that approach steady-state and higher drainage efficiency. Canyon activity in the southern Ebro margin is pulsating and enhanced during sea level lowstands. Rapid sedimentation by extension of the palaeo-Millars River into the outermost shelf and upper

  7. Elevated Passive Continental Margins may form much Later than the time of Rifting

    NASA Astrophysics Data System (ADS)

    Chalmers, J. A.; Japsen, P.; Green, P. F.; Bonow, J.; Lidmar-Bergstrom, K.

    2004-12-01

    Many current models of the development of elevated passive continental margins assume that they are either the remains of foot-wall uplift at the time of rifting or due to underplating by magma from a plume or other mantle source. We have studied the rift and post-rift history of such a passive margin in West and South Greenland and have concluded that the present-day elevations developed 25-60 million years after cessation of rifting and local volcanism, suggesting that additional factors need to be considered when modelling such margins. The morphology of West Greenland is similar to that of other elevated passive margins ion many parts of the world. There are high-level, large-scale, quasi-planar landscapes (planation surfaces) at altitudes of 1-2 km cut by deeply incised valleys. The gradient from the highest ground to the coast is much steeper than that away from the coast. We combined analysis of the morphology of the landscape with studies of fission tracks and the preserved stratigraphic record both on- and off-shore. Rifting and the commencement of sea-floor spreading in the Early Paleogene was accompanied by voluminous high-temperature volcanism. Kilometer-scale uplift at the time of rifting was followed shortly afterwards by kilometer-scale subsidence and possibly by transgression of marine sediments across the rift margin. The present elevated margin formed during three episodes of uplift during the Neogene, 25-60 million years after the cessation of rifting and local volcanism. The quasi-planar planation surfaces presently at 1-2 km altitude are the end-products of denudation to near sea-level in the mid- and late Cenozoic and these surfaces were uplifted to their present altitudes during the Neogene events. Rivers then incised the summit surface to form valleys that were further enlarged and deepened by glaciers. Similar elevated margins exist all around the northern North Atlantic and in many other parts of the world; eastern North America, on both

  8. Elevated Passive Continental Margins may form much Later than the time of Rifting

    NASA Astrophysics Data System (ADS)

    Chalmers, J. A.; Japsen, P.; Green, P. F.; Bonow, J.; Lidmar-Bergstrom, K.

    2007-12-01

    Many current models of the development of elevated passive continental margins assume that they are either the remains of foot-wall uplift at the time of rifting or due to underplating by magma from a plume or other mantle source. We have studied the rift and post-rift history of such a passive margin in West and South Greenland and have concluded that the present-day elevations developed 25-60 million years after cessation of rifting and local volcanism, suggesting that additional factors need to be considered when modelling such margins. The morphology of West Greenland is similar to that of other elevated passive margins ion many parts of the world. There are high-level, large-scale, quasi-planar landscapes (planation surfaces) at altitudes of 1-2 km cut by deeply incised valleys. The gradient from the highest ground to the coast is much steeper than that away from the coast. We combined analysis of the morphology of the landscape with studies of fission tracks and the preserved stratigraphic record both on- and off-shore. Rifting and the commencement of sea-floor spreading in the Early Paleogene was accompanied by voluminous high-temperature volcanism. Kilometer-scale uplift at the time of rifting was followed shortly afterwards by kilometer-scale subsidence and possibly by transgression of marine sediments across the rift margin. The present elevated margin formed during three episodes of uplift during the Neogene, 25-60 million years after the cessation of rifting and local volcanism. The quasi-planar planation surfaces presently at 1-2 km altitude are the end-products of denudation to near sea-level in the mid- and late Cenozoic and these surfaces were uplifted to their present altitudes during the Neogene events. Rivers then incised the summit surface to form valleys that were further enlarged and deepened by glaciers. Similar elevated margins exist all around the northern North Atlantic and in many other parts of the world; eastern North America, on both

  9. The Eastern Sardinian Margin (Tyrrhenian Sea, Western Mediterranean) : a key area to study the rifting and post-breakup evolution of a back-arc passive continental margin

    NASA Astrophysics Data System (ADS)

    Gaullier, Virginie; Chanier, Frank; Vendeville, Bruno; Maillard, Agnès; Thinon, Isabelle; Graveleau, Fabien; Lofi, Johanna; Sage, Françoise

    2016-04-01

    The Eastern Sardinian passive continental margin formed during the opening of the Tyrrhenian Sea, which is a back-arc basin created by continental rifting and oceanic spreading related to the eastward migrating Apennine subduction system (middle Miocene to Pliocene). Up to now, rifting in this key area was considered to be pro parte coeval with the Messinian Salinity Crisis (MSC, 5.96-5.32 Ma). We use the MSC seismic markers and the deformation of viscous salt and its brittle overburden as proxies to better delineate the timing of rifting and post-rift reactivation, and especially to quantify vertical and horizontal movements. On this young, highly-segmented margin, the Messinian Erosion Surface and the Upper and Mobile Units are systematically associated, respectively, to basement highs and deeper basins, showing that a rifted deep-sea domain already existed by Messinian times, therefore a major pre-MSC rifting episode occurred across the entire domain. Data show that there are no signs of Messinian syn-rift sediments, hence no evidence for rifting after Late Tortonian times. Moreover, because salt tectonics creates fan-shaped geometries in sediments, syn-rift deposits have to be carefully re-examined to distinguish the effects of crustal tectonics (rifting) and salt tectonics. We also precise that rifting is clearly diachronous from the upper margin (East-Sardinia Basin) to the lower margin (Cornaglia Terrace) with two unconformities, attributed respectively to the necking and to the lithospheric breakup unconformities. The onshore part of the upper margin has been recently investigated in order to characterize the large crustal faults affecting the Mesozoic series (geometry, kinematics and chronology) and to decipher the role of the structural inheritance and of the early rifting. Seaward, we also try to constrain the architecture and timing of the continent-ocean transition, between the hyper-extended continental crust and the first oceanic crust. Widespread

  10. Preliminary assessment of a Cretaceous-Paleogene Atlantic passive margin, Serrania del Interior and Central Ranges, Venezuela/Trinidad

    SciTech Connect

    Pindell, J.L.; Drake, C.L. ); Pitman, W.C. )

    1991-03-01

    For several decades, Cretaceous arc collision was assumed along northern Venezuela based on isotopic ages of metamorphic minerals. From subsidence histories in Venezuelan/Trinidadian basins, however, it is now clear that the Cretaceous metamorphic rocks were emplaced southeastward as allochthons above an autochthonous suite of rocks in the Cenozoic, and that the pre-Cenozoic autochthonous rocks represent a Mesozoic passive margin. The passive margin rocks have been metamorphosed separately during overthrusting by the allochthons in central Venezuela, but they are uplifted but not significantly metamorphosed in Eastern Venezuela and Trinidad. There, in the Serrania del Interior and Central Ranges of Venezuela/Trinidad, Mesozoic-Paleogene passive margin sequences were uplifted in Neogene time, when the Caribbean Plate arrived from the west and transpressionally inverted the passive margin. Thus, this portion of South America's Atlantic margin subsided thermally without tectonism from Jurassic to Eocene time, and these sections comprise the only Mesozoic-Cenozoic truly passive Atlantic margin in the Western Hemisphere that is now exposed for direct study. Direct assessments of sedimentological, depositional and faunal features indicative of, and changes in, water depth for Cretaceous and Paleogene time may be made here relative to a thermally subsiding passive margin without the complications of tectonism. Work is underway, and preliminary assessments presented here suggest that sea level changes of Cretaceous-Paleogene time are not as pronounced as the frequent large and rapid sea level falls and rises that are promoted by some.

  11. African-American Women in the Professoriate: Addressing Social Exclusion and Scholarly Marginalization through Mentoring

    ERIC Educational Resources Information Center

    Lloyd-Jones, Brenda

    2014-01-01

    African-American women and other underrepresented faculty members often report experiences of social exclusion and scholarly marginalization in mainstream institutions of higher education. This lack of inclusion challenges their retention and hinders them from becoming productive members of the professoriate, positioning them at a disadvantage for…

  12. Magmatism at passive margins: Effect of depth-dependent rifting and depleted continental lithospheric counterflow

    NASA Astrophysics Data System (ADS)

    Lu, Gang; Huismans, Ritske

    2016-04-01

    Rifted continental margins may have a variety of structural and magmatic styles, resulting in narrow or wide, magma-dominated or magma-poor conjugate margins. Some magma-poor margins differ from the classical uniform extension (McKenzie) model in that continental crust breaks up significantly earlier or later than continental mantle lithosphere and establishment of mature mid-ocean ridge is significantly delayed. The best-known examples are observed at: 1) the Iberia-Newfoundland conjugate margins (Type I) with a narrow transition between oceanic and continental crust; and 2) ultra-wide central South Atlantic margins (Type II) where the continental crust spans wide regions while the mantle lithosphere beneath has been removed. These margins are explained by depth-dependent extension. In this study, we perform 2D thermo-mechanical finite element numerical experiments to investigate magmatism at passive margins with depth-dependent extension. A melting prediction model is coupled with the thermo-mechanical model, in which temperature, density and viscosity feedbacks are considered. For the standard models, the crust is either strong and coupled (Type I-A models), or weak and decoupled (Type II-A models) with mantle lithosphere. In addition, models with a buoyant, depleted (cratonic) lower mantle lithosphere (referred as C models) are also investigated. We illustrate that Type I-A/C models develop Type I narrow margins, whereas Type II-A/C models develop Type II wide margins. In the C models, the buoyant lower mantle lithosphere flows laterally towards the ridge (i.e. the counterflow), resulting in the exhumation (in Type I-C models) or underplating (in Type II-C models) of the continental mantle lithosphere. Magmatic productivity is strongly prohibited when counterflow is developed. We argue that Type I-A and I-C models are comparable with the Aden Gulf rifted margins and the Iberia-Newfoundland conjugate margins, respectively. The Type II-A/C models are consistent

  13. Stages in evolution of Paleozoic carbonate platform and basin margin types - western United States passive Continental Margin

    SciTech Connect

    Cook, H.E.; Taylor, M.E.

    1987-05-01

    Late Precambrian rifting along the western edge of North America established a passive continental margin that became the site of 5000 m of platform and basin carbonate sediments over a 150-m.y. interval (Cambrian-Devonian). This megaplatform evolved through several stages: (1) Cambrian-Silurian, distally steepened nonrimmed ramp with base-of-slope fan (Hales Limestone) to homoclinal ramp (Hanson Creek Formation); to (2) Silurian-Devonian, rimmed platform (Lone Mountain Dolomite) having low-angle depositional slopes and slope aprons (Roberts Mountains Formation) and basinal debris sheets (Tor Limestone); to (3) Devonian, rimmed platforms having high-angle bypass slopes, slides, and base-of-slope aprons (McColley Canyon Formation and Devils Gate Limestone). The position of the rifted continental margin controlled the overall trend of the platform-slope break. Postrift subsidence with superimposed eustatic sea level changes allowed the platform to accumulate 5000 m of sediment. The stratigraphic progression from nonrimmed ramps in the Cambrian to rimmed platforms with high-angle bypass slopes in the Devonian was a function of both the gradual steepening of the slope, as the platform margin built up and prograded seaward, and the evolution of reef and bank-building organisms through time. Evolution of adjacent basinal carbonates was strongly influenced by slope declivity and relative sea level changes. As slope declivity increased through time, sedimentary processes on the slope changed from small-scale sediment gravity flows that accreted on the slope (ex: Roberts Mountains Formation slope apron) to large-scale sediment gravity flows that deposited debris in base-of-slope settings (ex: Devils Gate Limestone base-of-slope apron).

  14. Changes in plate motion and vertical movements along passive continental margins

    NASA Astrophysics Data System (ADS)

    Japsen, P.; Cobbold, P. R.; Chalmers, J. A.; Green, P. F.; Bonow, J. M.

    2012-04-01

    The origin of the forces that produce elevated, passive continental margins (EPCMs) has been a hot topic in geoscience for many years. Studies of individual margins have led to models, which explain high elevations by invoking specific conditions for each margin in question. We have studied the uplift history of several margins and have found some striking coincidences between episodes of uplift and changes in plate motion. In the Campanian, Eocene and Miocene, pronounced events of uplift and erosion affected not only SE Brazil (Cobbold et al., 2001), but also NE Brazil and SW Africa (Japsen et al., 2012a). The uplift phases in Brazil also coincided with three main phases of Andean orogeny (Cobbold et al., 2001, 2007). These phases, Peruvian (90-75 Ma), Incaic (50-40 Ma), and Quechuan (25-0 Ma), were also periods of relatively rapid convergence at the Andean margin of South America (Pardo-Casas and Molnar, 1987). Because Campanian uplift in Brazil coincides, not only with rapid convergence at the Andean margin of South America, but also with a decline in Atlantic spreading rate, we suggest that all these uplift events have a common cause, which is lateral resistance to plate motion (Japsen et al., 2012a). Because the uplift phases in South America and Africa are common to the margins of two diverging plates, we also suggest that the driving forces can transmit across the spreading axis, probably at great depth, e.g. in the asthenosphere (Japsen et al., 2012a). Similarly, a phase of uplift and erosion at the Eocene-Oligocene transition (c. 35 Ma), which affected margins around the North Atlantic, correlates with a major plate reorganization there (Japsen et al., 2012b). Passive continental margins clearly formed as a result of extension. Despite this, the World Stress Map shows that, where data exist, all EPCMs are today under compression. We maintain that folds, reverse faults, reactivated normal faults and strike-slip faults that are typical of EPCMs are a result

  15. Stratigraphical links between Miocene Alpine Foreland basin and Gulf of Lion Passive Margin during lowstands

    NASA Astrophysics Data System (ADS)

    Rubino, Jean-Loup; Gorini, Christian; Leroux, Estelle; Aslanian, Daniel; Rabineau, Marina; Parize, Olivier; Besson, David

    2015-04-01

    Miocene peri-alpine foreland basin is connected toward the south with the Gulf of Lion passive margin and is predominantly filled by marine shallow water molassic deposits ranging from lower Miocene to Pliocene in age. Nine to ten depositional sequences are recorded and partly preserved in this basin and can be traced into the post rift part of the Gulf of Lion. One of the most surprising feature of the stratigraphic infill is the total lack of lowstand deposits within the foreland basin ; All superimposed sequences only includes transgressive and highstand System Tracts separated by erosional sequence boundaries and the development of incised valley networks filled by tidal deposits during transgression; Besson et al. 2005. It means that the entire foreland basin in SE France is exposed during lowstand periods without any preservation of fluvial deposits. By place few forced regression wedges are preserved at the transition between the foreland and the passive margin, close to the present day coastline. To date no real lowstand wedges have never been reported in the offshore of the Gulf of Lion. A reinterpretation of the best old vintage 2D dip seismic profiles along the passive margin validates the idea that the foreland basin is entirely exposed as well as the proximal part of the passive margin; first because some incised valleys can be occasionally picked on the shelf and second mainly because well defined superimposed or juxtaposed prograding lowstand wedges with nicely defined clinoforms onlapping the sequence boundaries can be recognized on the distal part of the shelf from the Burdigalian to the Messinian. Their ages being constrains by the Calmar well calibration. Unfortunately, they can't be continuously mapped all along the shelf break because of the strong erosion related to the Messinian Unconformity and the associated huge sea level fall.So we have to explain why during the lowstands, exceptionally long fluvial valley networks (more than 300km) can

  16. Morphotectonic evolution of passive margins undergoing active surface processes: large-scale experiments using numerical models.

    NASA Astrophysics Data System (ADS)

    Beucher, Romain; Huismans, Ritske S.

    2016-04-01

    Extension of the continental lithosphere can lead to the formation of a wide range of rifted margins styles with contrasting tectonic and geomorphological characteristics. It is now understood that many of these characteristics depend on the manner extension is distributed depending on (among others factors) rheology, structural inheritance, thermal structure and surface processes. The relative importance and the possible interactions of these controlling factors is still largely unknown. Here we investigate the feedbacks between tectonics and the transfers of material at the surface resulting from erosion, transport, and sedimentation. We use large-scale (1200 x 600 km) and high-resolution (~1km) numerical experiments coupling a 2D upper-mantle-scale thermo-mechanical model with a plan-form 2D surface processes model (SPM). We test the sensitivity of the coupled models to varying crust-lithosphere rheology and erosional efficiency ranging from no-erosion to very efficient erosion. We discuss how fast, when and how the topography of the continents evolves and how it can be compared to actual passive margins escarpment morphologies. We show that although tectonics is the main factor controlling the rift geometry, transfers of masses at the surface affect the timing of faulting and the initiation of sea-floor spreading. We discuss how such models may help to understand the evolution of high-elevated passive margins around the world.

  17. Isostatic and dynamic support of high topography on a North Atlantic passive margin

    NASA Astrophysics Data System (ADS)

    Kathrine Pedersen, Vivi; Huismans, Ritske S.; Moucha, Robert

    2016-04-01

    Substantial controversy surrounds the origin and recent evolution of high topography along passive continental margins in the North Atlantic, with suggested age of formation ranging from early Paleozoic Caledonian orogenesis to Neogene uplift of a Mesozoic peneplain. Here we focus on the well-documented high passive margin in southwestern Scandinavia, and quantify the relative contributions of crustal isostasy and dynamic topography in controlling the present topography. We find that most topography is compensated by the crustal structure, suggesting a topographic age related to ~400 Myr old Caledonian orogenesis. In addition, we infer that dynamic uplift (~300 m) has rejuvenated existing topography locally in the coastal region within the last ~10 Myr due to mantle convection. Such uplift has, in combination with a general eustatic sea-level fall and concurrent erosion-driven isostatic rock-column uplift, the potential to increase erosion of coastal-near regions and explain observations that have traditionally been interpreted in favor of the peneplain uplift model. We conclude that high topography along the Scandinavian margin cannot represent remnants of a peneplain uplifted within the last ~20 Myr. Topography must have been high since the Caledonian orogeny.

  18. 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.

  19. Evolution of North Atlantic Passive Margins Controlled by the Iceland Mantle Plume

    NASA Astrophysics Data System (ADS)

    Parnell-Turner, R. E.; White, N. J.; Henstock, T.; Murton, B. J.; Jones, S. M.

    2015-12-01

    Evolution of North Atlantic passive margins has been profoundly influenced by the Iceland mantle plume over the past 60 Ma. Residual depth anomalies of oceanic lithosphere, long wavelength gravity anomalies and seismic tomographic models show that upwelling mantle material extends from Baffin Bay to Western Norway. At fringing passive margins such as Northwest Scotland, there is evidence for present-day dynamic support of the crust. The Iceland plume is bisected by the Reykjanes Ridge ridge, which acts as a tape-recorder of the temporal variability of the plume. We present regional seismic reflection profiles that traverse the oceanic basin between northwest Europe and Greenland. A diachronous pattern of V-shaped ridges and troughs are imaged beneath marine sediments, revealing a complete record of transient periodicity that can be traced continuously back to ~55 Myrs. This periodicity increases from ~3 to ~8 Ma with clear evidence for minor, but systematic, asymmetric crustal accretion. V-shaped ridges grow with time and reflect small (5-30°C) changes in mantle temperature, consistent with episodic generation of hot solitary waves triggered by growth of thermal boundary layer instabilities within the mantle. Our continuous record of convective activity suggests that the otherwise uniform thermal subsidence of sedimentary basins, which fringe the North Atlantic Ocean, has been punctuated by periods of variable dynamic topography. This record can explain a set of diverse observations from the geologic record. Paleogene unconformities in the Faroe-Shetland Basin, the punctuated deposition of contourite drifts and variations in deep-water current strength can all be explained by transient mantle plume behavior. These signals of convective activity should lead to improved insights into the fluid dynamics of the mantle, and into the evolution of volcanic passive margins.

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

    NASA Astrophysics Data System (ADS)

    Zuo, X.; Chan, L. S.

    2015-12-01

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

  1. Rapid late pleistocene incision of Atlantic passive-margin river gorges

    USGS Publications Warehouse

    Reusser, L.J.; Bierman, P.R.; Pavich, M.J.; Zen, E.-A.; Larsen, J.; Finkel, R.

    2004-01-01

    The direct and secondary effects of rapidly changing climate caused large rivers draining the Atlantic passive margin to incise quickly into bedrock beginning about 35,000 years ago. Measured in samples from bedrock fluvial terraces, 10-beryllium shows that both the Susquehanna and Potomac Rivers incised 10- to 20-meter-deep gorges along steep, convex lower reaches during the last glacial cycle. This short-lived pulse of unusually rapid downcutting ended by 13,000 to 14,000 years ago. The timing and rate of downcutting are similar on the glaciated Susquehanna and unglaciated Potomac Rivers, indicating that regional changes, not simply glacial melt-water, initiated incision.

  2. Variscan to Neogene thermal and exhumation history at the Moroccan passive continental margin assessed by low temperature thermochronology

    NASA Astrophysics Data System (ADS)

    Sehrt, M.; Glasmacher, U. A.; Stockli, D. F.; Kluth, O.; Jabour, H.

    2012-04-01

    In North Africa, a large amount of Mesozoic terrigenous sedimentary rocks are deposited in most of the basins along the continental margin indicating a major episode of erosion occurred during the rift and early post-rift period in the Central Atlantic. In the Tarfaya-Dakhla Basin, Morocco the sedimentary cover reaches thicknesses of up to 9000 m. The presence of high surface elevations in the Anti-Atlas mountain belt (2500 m) indicates a potential source area for the surrounding basins. The NE-SW oriented Anti-Atlas of Morocco is located at the northwestern fringe of the West African Craton and south of the High Atlas and represents the Phanerozoic foreland of the Late Paleozoic North African Variscides and the Cenozoic Atlas Belt. Variscan deformation affected most of Morocco. Paleozoic basins were folded and thrusted, with the major collision dated as late Devonian to Late Carboniferous. Zircon fission-track ages of 287 (±23) to 331 (±24) Ma confirmed the main exhumation referred to the Variscan folding, followed by rapid exhumation and the post-folding erosion. Currently, phases of uplift and exhumation in the Anti-Atlas during the Central Atlantic rifting and places where the associated erosion products are deposited are poorly constrained and there is little quantitative data available at present. The objective of the study is to determine the thermal and exhumation history of the Anti-Atlas and the connected Tarfaya-Dakhla Basin at the Moroccan passive continental margin. Besides zircon fission-track dating, apatite and zircon (U-Th-Sm)/He and apatite fission-track analyses and furthermore 2-D modelling with 'HeFTy' software has been carried out at Precambrian rocks of the Western Anti-Atlas and Cretaceous to Neogene sedimentary rocks from the Northern Tarfaya-Dakhla Basin. The apatite fission-track ages of 120 (±13) to 189 (±14) Ma in the Anti-Atlas and 176 (±20) to 216 (±18) Ma in the Tarfaya Basin indicate very obvious a Central Atlantic opening

  3. 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.

  4. DHOFAR Seismic Experiment: First results to understand the breakup processes in a passive margin context

    NASA Astrophysics Data System (ADS)

    Tiberi, C.; Leroy, S.; D'Acremont, E.; Pointu, A.; Ebinger, C.; Brisbourne, A.; Denton, P.; Al-Lazki, A.; Al-Azri, H.; Bin Monshir Bahlaf, S.; Brunet, C.; Famin, V.; Labrousse, L.

    2004-12-01

    The process of strain localisation preceding the onset of seafloor spreading is still poorly understood, though extensively studied. The reason is the differences in lithospheric properties, proximity to hot spot(s) and melt generation and extraction that lead to a variety of structural styles with major differences. The eastern Gulf of Aden represents a natural laboratory to study passive continental margins for many reasons: post-rift sedimentary strata are relatively thin, both onshore and nearshore structures are well-exposed, and conjugate margins can be precisely reconstructed. A first cruise (ENCENS-SHEBA), in 2000, has established the structural and geophysical framework using bathymetric swath mapping and underway geophysics. Later on, the Dhofar seismic experiment consisted in the deployment of a network of 11 broadband seismic stations from March 2003 to March 2004 on the northern margin, in the Dhofar area, southern Oman. This experiment was dedicated to the detailed study of the crust and upper mantle beneath the northern passive margin. CMG40TD 3-components seismometers from SEIS-UK network were used. We have recorded hundreds of teleseismic events with a good azimuthal coverage. Three main studies are then attempted. First, a teleseismic image of the first 200 km depth will be established using the P- and PKP-phases. This will allow a 3D representation of crustal and upper mantle velocity structures. We present here the preliminary results from the study of the traveltime residuals. The image resolution shall be enhanced by combining gravity data. Second, a receiver function analysis will locally determine the depth of the main interfaces (eg, Moho boundary). Finally, events recorded within the 1000-6000 km distance range will improve the regional S-wave velocity structure in this area and will help to locate the main regional wide structures related to this extended area.

  5. Comparison of marine gas hydrates in sediments of an active and passive continental margin

    USGS Publications Warehouse

    Kvenvolden, K.A.

    1985-01-01

    Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3191 m water depth on a spit-like extension of the continental rise on a passive margin in the Atlantic Ocean. Site 568, at 2031 m water depth, is in upper slope sediment of an active accretionary margin in the Pacific Ocean. Both sites are characterized by high rates of sedimentation, and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that transgress sedimentary structures and parallel the seafloor, suggested the presence of gas hydrates at both sites, and, during coring, small samples of gas hydrate were recovered at subbottom depths of 238m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates wer methane, ethane, and CO2. Residual methane in sediments at both sites usually exceeded 10 mll-1 of wet sediment. Carbon isotopic compositions of methane, CO2, and ??CO2 followed parallel trends with depth, suggesting that methane formed mainly as a result of biological reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. These geochemical characteristics define some of the conditions associated with the occurrence of gas hydrates formed by in situ processes in continental margin sediments. ?? 1984.

  6. Equating the basal unconformity with the palaeoplain: a model for passive margins

    NASA Astrophysics Data System (ADS)

    Ollier, C. D.; Pain, C. F.

    1997-05-01

    The palaeoplain is the terrestrial continental surface, little changed from the landsurface that existed before continental breakup. The basal unconformity occurs offshore, separating older continental rocks from younger post-rift sediments. This paper considers the palaeoplain and the basal unconformity to be the same surface. It presents a hypothesis for continental passive margins that integrates the geomorphology near the continental boundary with the offshore geomorphology. By recognising the identity of the palaeoplain with the basal unconformity the hypothesis links the terrestrial story to the offshore story. Many details that were separated in earlier models come together in a single model. Continental rifting starts with a rift valley stage. At this stage the pre-breakup landsurface, the palaeoplain at the edge of the continent, is downwarped. On the seaward side post-rift sediments accumulate on the submerged palaeoplain, which thus becomes the basal unconformity. This model explains the parallel structures of continental divide, present coast line, and continental edge. It also explains the creation of the basal unconformity in a short time, and the comparative volumes of onshore erosion and offshore sedimentation. Onshore, the model explains the modifications of drainage and the great escarpments that are characteristic of passive margins. It also explains oldlands observed near the coast.

  7. Upper Jurassic and Lower Cretaceous facies relationships in a passive margin basin, western North Atlantic

    SciTech Connect

    Prather, B.E.

    1988-02-01

    Correlation of facies from hydrocarbon-bearing continental and transitional marine sandstones to time-equivalent high-energy shelf-margin carbonates provide insight into hydrocarbon habitats of the Baltimore Canyon basin. These facies occur within a thick (> 10,000 ft) prograded wedge of shelf sediments in this passive margin basin. Wells drilled to test structural closures in shallow-water (< 600 ft) areas of Baltimore Canyon penetrate clastic facies which are time-equivalent to the downdip carbonate facies tested in deep-water wells. Numerous hydrocarbon shows, including a noncommercial gas and gas-condensate accumulation, occur with sandstone units that were deposited in prograding continental/fluvial and transitional marine environments located updip of the Oxfordian/Kimmeridgian carbonate shelf edge. The continental and transitional facies are overlain by a fine-grained deltaic complex which forms a regionally extensive top seal unit. The deltaic complex was deposited during aggradation of the Kimmeridgian through Berriasian shelf-margin carbonates penetrated by the deep-water wells. Deep-water wells (> 5000 ft) drilled off the continental shelf edge to test large structural closures along the downdip termination of the Upper Jurassic/Lower Cretaceous carbonate shelf edge encountered no significant hydrocarbon shows. Reservoir rocks in these wells consist of (1) oolite grainstone which was deposited within a shoal-water complex located at the Aptian shelf edge, and (2) coral-stromatoporoid grainstone and boundstone which formed an aggraded shelf-margin complex located at the Kimmeridgian through Berriasian shelf edge. Structural closures with reservoir and top seals are present in both updip and downdip trends. The absence of hydrocarbon shows in downdip carbonate reservoirs suggests a lack of source rocks available to charge objectives at the shelf margin.

  8. Geological Consequences of Unequal Loading of Sedimentary Units, at Passive, Transform, and Convergent Margins

    NASA Astrophysics Data System (ADS)

    Moore, C.; Dugan, B.; Flemings, P.; Iturrino, G.; Sawyer, D.; Behrmann, J.; John, C.

    2005-12-01

    An investigation of the effects of unequal loading of permeable sedimentary units and potential lateral flow was a primary objective of IODP Exp. 308 in the Gulf of Mexico. In addition to occurrence at passive margins, the geological consequences of unequal loading of aquifers is prominent at transform and at convergent margins. The development of a pull-apart basin along the San Andreas Transform Fault system resulted in a large difference in sedimentary loading in the upper Miocene Santa Margarita Sandstone near Santa Cruz CA. The overlying diatomaceous mudstone shows a variation in thickness from about 3 km at the basin center to several hundred meters at the basin margin. At the basin margin the underlying sandstone injects the overlying mudstone as dikes and sills and also flowed onto the Miocene seafloor as a sand volcano. In the Late Cretaceous-Paleocene Great Valley Forearc Basin Sequence, similar sandstone dikes and sills occur at the basin margin where the overlying sedimentary cover is minimized. However the lateral variation in thickness of the overlying mudstone sequence (1100m to ~ 800 m) is less dramatic than in the San Andreas Fault system example. The abundance of sills in both the Great Valley and San Andreas Fault system examples unequivocally indicate that the fluid pressures reached lithostatic values. In both examples, hydrocarbons reduced the density of fluids and assisted in reaching fluid pressures equal to the overburden. In the Ursa Basin Exp. 308 measured fluid pressures of ~ 0.6 of effective vertical stress in the muds overlying the unequally loaded permeable sandy "Blue Unit". Drilling and preliminary investigation of the seismic and borehole imaging data indicate no evidence of dikes or sills emanating from the Blue Unit. Therefore, the fluid pressure conditions in the Blue Unit and the overlying muds apparently define a lower limit for the formation of clastic intrusions.

  9. Volcanic Versus Non-Volcanic Passive Margins: Two Different Ways to Break-up Continents

    NASA Astrophysics Data System (ADS)

    Geoffroy, L.; Burov, E. B.; Werner, P.; Unternehr, P.

    2014-12-01

    Volcanic passive margins (VPMs) are distinctive features of Larges Igneous Provinces. They characterize continental breakup associated with the extrusion and intrusion of large volumes of magma, predominantly mafic. In Large Igneous Provinces, regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere, suggesting that mantle melting is a cause of continental break-up, not a consequence. Early melt covers as volcanic traps large cratonic or/and cratonic-edge continental areas. Crustal dilatation through dyking in the upper crust and magma underplating at Moho level is thought to occur massively during this early stage. Lithosphere extension leading to break-up and VPMs development is coeval with a 3D focusing of mantle melting, giving rise to VPMs. From a combination of deep seismic reflection profiles and onshore observations, we show that the mechanism of continental breakup at volcanic passive margins is very different from the one generally proposed for non-magmatic systems. Crustal extension and coeval extrusion of thick wedges of seaward-dipping basalts are accommodated by continentward-dipping detachment-faults at both conjugate margins. Those faults root on a deformed ductile crust whose composition seems partly magmatic. Our numerical modeling show that hardening of deep continental crust during the early magmatic stages provokes a divergent flow of the ductile lithosphere (mantle and lower crust) away from a central continental block which thins through advection with time. Magma-assisted crustal-scale faults dipping continentward root over this flowing material, isolating micro-continents which may be lost in the future oceanic domain. The structure and tectonic evolution of volcanic passive margins cannot therefore be compared to non-volcanic ones, where major detachment faults dip oceanward during the necking-stage and where mantle is finally exhumed during the mechanical breakup. Confusions may exist where ancient hyper

  10. Meso-Cenozoic Source-to-Sink analysis of the African margin of the Equatorial Atlantic

    NASA Astrophysics Data System (ADS)

    Chardon, Dominique; Rouby, Delphine; Huyghe, Damien; Ye, Jing; Guillocheau, François; Robin, Cécile; Dall'Asta, Massimo; Brown, Roderick; Webster, David

    2015-04-01

    The Transform Source to Sink Project (TS2P) objective is to link the evolution of the offshore sedimentary basins of the African margin of the Equatorial Atlantic and their source areas on the West African Craton. The margin consists in alternating transform and oblique margin portions from Guinea, in the West, to Nigeria, in the East. Such a longitudinal structural variability is associated with variation in the margin width, continental geology and relief, drainage networks and subsidence/accumulation patterns that we analyzed using offshore seismic data and onshore geology and geomorphology. We compare syn- to post rift offshore geometry and long-term stratigraphic history of each of the margin segments. Transform faults appear to play a major role in shaping Early Cretaceous syn-rift basin architectures. Immediate post-rift Late Cretaceous sedimentary wedges record a transgression and are affected by the reactivation of some of transform faults. We produced A new type of inland paleogeographic maps for key periods since the end of the Triassic, allowing delineation of intracratonic basins having accumulated material issued from erosion of the marginal upwarps that have grown since break-up along the margin. We use offshore and onshore basin analysis to estimate sediment accumulation and integrate it in a source-to-sink analysis where Mesozoic onshore denudation will be estimated by low-temperature thermochronology. Cenozoic erosion and drainage history of the continental domain have been reconstructed from the spatial analysis of dated and regionally correlated geomorphic markers. The stationary drainage configuration of the onshore domain since 30 Ma offers the opportunity to correlate the detailed onshore morphoclimatic record based on the sequence of lateritic paleolandsurfaces to offshore stratigraphy, eustasy and global climatic proxies since the Oligocene. Within this framework, we simulate quantitative solute / solid erosional fluxes based on the

  11. Passive Participle Marking by African American English-Speaking Children Reared in Poverty

    ERIC Educational Resources Information Center

    Pruitt, Sonja L.; Oetting, Janna B.; Hegarty, Michael

    2011-01-01

    Purpose: In this study, the authors examined the linguistic profile of African American English (AAE)-speaking children reared in poverty by focusing on their marking of passive participles and by comparing the results with the authors' previous study of homophonous forms of past tense (S. Pruitt & J. Oetting, 2009). Method: The data were from 45…

  12. Salt tectonics and gravity driven deformation: Structural guidelines for exploration in passive margin

    SciTech Connect

    Mauduit, T.; Gwenael G.; Brun, J.P.

    1995-08-01

    The West African Margin, (Gulf of Guinea) presents spectacular examples of gravity driven deformation above a salt decollement (i.e. growth faulting, rafts, diapirs and contractional structures) which have been documented by numerous Oil and Gas investigations. Seismic data demonstrate that the variation of deformation styles in space and time appear to be function of: regional geometry of the margin (i.e. value of basal slope and presence/absence of residual reliefs below the salt layers) and, mode, rate and repartition of sedimentation. The role and effects of the above parameters were analyzed using laboratory modeling investigation based on basic structural patterns identified through seismic data. Models are built with sand and silicone putty, that respectively represent the frictional behavior of upper Cretaceous-Cenozoic cover and the viscous behavior of the upper Aptian salt. They are scaled to fit observed natural configurations. Results are compared with examples from the Gulf of Guinea on the basis of seismic data. This approach allowed to better understand the evolution of the margin and therefore the reservoir distributions and traps geometries.

  13. Development of passive volcanic margins of the Central Atlantic and initial opening of ocean

    NASA Astrophysics Data System (ADS)

    Melankholina, E. N.; Sushchevskaya, N. M.

    2015-01-01

    Geological and geophysical data on the Central Atlantic are discussed in order to elucidate the tectonic setting of the initial magmatic activity, rifting, and breakup resulting in the origination of Mesozoic ocean. The structural, magmatic, and historical aspects of the problem are considered. It has been established that the initial dispersed rifting and low-capacity magmatism at proximal margins was followed by the migration of the process toward the central part of region with the formation of distal zones and the development of vigorous magmmatism, further breakup of the lithosphere and ocean opening. Magmatism, its sources, and the features of newly formed magmatic crust at both the rifting and breakup stages of margin development are discussed and compared with subsequent spreading magmatism. Sr, Nd, and Pb isotopic compositions show that the magmatic evolution of the Central Atlantic proximal margins bears the features of two enriched components, one of which is related to the EM-1 source, developing only at the North American margin. Another enriched component typical of the province as a whole is related to the EM-2 source. To a lesser extent, this component is expressed in igneous rocks of Guyana, which also bear the signature of the MORB-type depleted source typical of spreading tholeiites in the Atlantic Ocean. Similar conditions are assumed for subsequent magmatism at the distal margins and for the early spreading basalts in the adjacent Atlantic belt, which also contain a small admixture of enriched material. A comparison of the magmatism at the margins of Central and North Atlantic reveals their specificity distinctly expressed in isotopic compositions of igneous rocks. In contrast to the typical region of the North Atlantic, the immediate melting of the enriched lithospheric source without the participation of plume-related melts is reconstructed for the proximal margins of the Central Atlantic. At the same time, decompression and melting in the

  14. Ductile deformation of the continental crust below volcanic and non-volcanic passive margins: insight from high quality industrial seismic profiles

    NASA Astrophysics Data System (ADS)

    Clerc, Camille; Jolivet, Laurent; Ringenbach, Jean-Claude; Ballard, Jean-François

    2015-04-01

    High quality industrial seismic profiles have now been acquired along most of the world's passive margins. Stunningly increasing resolution over the past decades leads to unravel unexpected structures and to see real images of models drawn from the integration of field data. Some profiles show clear indications of ductile deformation of the deep continental crust, more or less localized along large-scale shallow-dipping shear zones. Maximums of deformation are suggested at the very base of the continental crust, and the Moho appears to be strongly sheared. These shear zones show a top-to-the-continent sense of shear consistent with the activity of counter-regional (continentward) normal faults observed in the upper crust. This pattern is responsible for a migration of the deformation and associated sedimentation or volcanic activity toward the ocean. We present some of the most striking examples and discuss their implications for the time-temperature-subsidence history of the margins. The distal domain of the non-magmatic margins is generally represented with an important sag basin (i.e. West African margins). This kind of sag basin is usually described as a vertically subsiding basin without differential tilting and resting on a highly thinned, little faulted continental crust. In contrast, we present new interpretations of seismic profiles across the West African margins showing evidences of intense syn-sedimentary tectonic activity within the Sag-basin. Sequences of low-angle normal faults horizontalizing in a hyper-stretched and ductile continental crust control a migration of the depot-center toward the ocean, in response to the horizontal extraction of the base of the continental crust and upper mantle. Finally, the hyper-thinned continental crust has undergone a ductile stretching under a cover of early syn-rift sediments, which implies a probable high thermal regime during rifting.

  15. Plate Kinematic model of the NW Indian Ocean and derived regional stress history of the East African Margin

    NASA Astrophysics Data System (ADS)

    Tuck-Martin, Amy; Adam, Jürgen; Eagles, Graeme

    2015-04-01

    normal to the plate divergence vector. Away from the active ridges, compressional horizontal stresses caused by ridge-push forces were transmitted through the subsiding oceanic lithosphere, with an SH max orientation parallel to plate divergence vectors. These changes are documented by the lower Bajocian continental breakup unconformity, which can be traced throughout East African basins. At 133 Ma, the plate boundary moved from north to south of Madagascar, incorporating it into the African plate and initiating its separation from Antarctica. The orientation of the plate divergence vector however did not change markedly. The second phase (89 - 61 Ma) led to the separation of India from Madagascar, initiating a new and dramatic change in stress orientation from N-S to ENE-WSW. This led to renewed tectonic activity in the sedimentary basins of western Madagascar. In the third phase (61 Ma to present) asymmetric spreading of the Carlsberg Ridge separated India from the Seychelles and the Mascarene Plateau via the southward propagation of the Carlsberg Ridge to form the Central Indian Ridge. The anti-clockwise rotation of the independent Seychelles microplate between chrons 28n (64.13 Ma) and 26n (58.38 Ma) and the opening of the short-lived Laxmi Basin (67 Ma to abandonment within chron 28n (64.13 - 63.10 Ma)) have been further constrained by the new plate kinematic model. Along the East African margin, SH max remained in a NE - SW orientation and the sedimentary basins experienced continued thick, deep water sediment deposition. Contemporaneously, in the sedimentary basins along East African passive margin, ridge-push related maximum horizontal stresses became progressively outweighed by local gravity-driven NE-SW maximum horizontal stresses trending parallel to the margin. These stress regimes are caused by sediment loading and extensional collapse of thick sediment wedges, predominantly controlled by margin geometry. Our study successfully integrates an interpretation

  16. Cambrian-lower Middle Ordovician passive carbonate margin, southern Appalachians: Chapter 14

    USGS Publications Warehouse

    Read, J. Fred; Repetski, John E.

    2012-01-01

    The southern Appalachian part of the Cambrian–Ordovician passive margin succession of the great American carbonate bank extends from the Lower Cambrian to the lower Middle Ordovician, is as much as 3.5 km (2.2 mi) thick, and has long-term subsidence rates exceeding 5 cm (2 in.)/k.y. Subsiding depocenters separated by arches controlled sediment thickness. The succession consists of five supersequences, each of which contains several third-order sequences, and numerous meter-scale parasequences. Siliciclastic-prone supersequence 1 (Lower Cambrian Chilhowee Group fluvial rift clastics grading up into shelf siliciclastics) underlies the passive margin carbonates. Supersequence 2 consists of the Lower Cambrian Shady Dolomite–Rome-Waynesboro Formations. This is a shallowing-upward ramp succession of thinly bedded to nodular lime mudstones up into carbonate mud-mound facies, overlain by lowstand quartzose carbonates, and then a rimmed shelf succession capped by highly cyclic regressive carbonates and red beds (Rome-Waynesboro Formations). Foreslope facies include megabreccias, grainstone, and thin-bedded carbonate turbidites and deep-water rhythmites. Supersequence 3 rests on a major unconformity and consists of a Middle Cambrian differentiated rimmed shelf carbonate with highly cyclic facies (Elbrook Formation) extending in from the rim and passing via an oolitic ramp into a large structurally controlled intrashelf basin (Conasauga Shale). Filling of the intrashelf basin caused widespread deposition of thin quartz sandstones at the base of supersequence 4, overlain by widespread cyclic carbonates (Upper Cambrian lower Knox Group Copper Ridge Dolomite in the south; Conococheague Formation in the north). Supersequence 5 (Lower Ordovician upper Knox in the south; Lower to Middle Ordovician Beekmantown Group in the north) has a basal quartz sandstone-prone unit, overlain by cyclic ramp carbonates, that grade downdip into thrombolite grainstone and then storm

  17. From rifting to passive margin: the examples of the Red Sea, Central Atlantic and Alpine Tethys

    NASA Astrophysics Data System (ADS)

    Favre, P.; Stampfli, G. M.

    1992-12-01

    Evolution of the Red Sea/Gulf of Suez and the Central Atlantic rift systems shows that an initial, transtensive rifting phase, affecting a broad area around the future zone of crustal separation, was followed by a pre-oceanic rifting phase during which extensional strain was concentrated on the axial rift zone. This caused lateral graben systems to become inactive and they evolved into rift-rim basins. The transtensive phase of diffuse crustal extension is recognized in many intra-continental rifts. If controlling stress systems relax, these rifts abort and develop into palaeorifts. If controlling stress systems persist, transtensive rift systems can enter the pre-oceanic rifting stage, during which the rift zone narrows and becomes asymmetric as a consequence of simple-shear deformation at shallow crustal levels and pure shear deformation at lower crustal and mantle-lithospheric levels. Preceding crustal separation, extensional denudation of the lithospheric mantle is possible. Progressive lithospheric attenuation entails updoming of the asthenosphere and thermal doming of the rift shoulders. Their uplift provides a major clastic source for the rift basins and the lateral rift-rim basins. Their stratigraphic record provides a sensitive tool for dating the rift shoulder uplift. Asymmetric rifting leads to the formation of asymmetric continental margins, corresponding in a simple-shear model to an upper plate and a conjugate lower plate margin, as seen in the Central Atlantic passive margins of the United States and Morocco. This rifting model can be successfully applied to the analysis of the Alpine Tethys palaeo-margins (such as Rif and the Western Alps).

  18. Joint analysis of seismic, gravity, magnetism and seismological data for passive margin structure imaging

    NASA Astrophysics Data System (ADS)

    D'Acremont, E.; Leroy, S.; Tiberi, C.; Pointu, A.; Ebinger, C.

    2004-12-01

    The eastern Gulf of Aden is a key place for investigating seafloor spreading processes and strain localisation, given its thin post-rift sedimentary strata, the good exposure of onshore and nearshore rift structures, the lack of salt deformation structures and its large distance away from the Afar plume. Further, exploratory well data exist for stratigraphic ties, and the two conjugate passive margins can be reconstructed within lateral errors smaller than 10 km. First, the 2000 ENCENS-SHEBA cruise has revealed the structural and geophysical framework using bathymetric swath mapping and underway geophysics (Leroy et al. 2004; d'Acremont et al. submitted). Second, the Dhofar Seismic network in 2004, on the onshore Northern margin (11 BB stations for receiver function and tomography studies) has improved our understanding of the rifting and the oceanic spreading processes in this area, as well as the transitional phase between them. A smaller deformation wavelength prevails on the northern margin, which is also steeper and narrower than the southern one. The southern-rifted domain is about twice as large as the northern one, while the crust is thinner in the northern margin. Besides the influence of rifting obliquity, this asymmetry of the structural pattern could be a consequence of inherited basins and faults associated with the Jurassic rifting episode that affected the southern domain. The transition between the thinned continental crust and the onset of oceanic seafloor spreading is characterized by an ocean-continent transition (OCT). Although its precise nature remains unknown, two possible origins can be proposed with respect to our data; either an exhumed mantle, or an ultra-thinned continental crust intruded by partial melt products from the underlying mantle. Between the Alula-Fartak and Socotra transform faults, the non-volcanic margins and the OCT are segmented by two transfer fault zones trending N027°E. These zones define three N110°E trending

  19. Changes in biological productivity along the northwest African margin over the past 20,000 years

    NASA Astrophysics Data System (ADS)

    Bradtmiller, Louisa I.; McGee, David; Awalt, Mitchell; Evers, Joseph; Yerxa, Haley; Kinsley, Christopher W.; deMenocal, Peter B.

    2016-01-01

    The intertropical convergence zone and the African monsoon system are highly sensitive to climate forcing at orbital and millennial timescales. Both systems influence the strength and direction of the trade winds along northwest Africa and thus directly impact coastal upwelling. Sediment cores from the northwest African margin record upwelling-related changes in biological productivity connected to changes in regional and hemispheric climate. We present records of 230Th-normalized biogenic opal and Corg fluxes using a meridional transect of four cores from 19°N-31°N along the northwest African margin to examine changes in paleoproductivity since the last glacial maximum. We find large changes in biogenic fluxes synchronous with changes in eolian fluxes calculated using end-member modeling, suggesting that paleoproductivity and dust fluxes were strongly coupled, likely linked by changes in wind strength. Opal and Corg fluxes increase at all sites during Heinrich Stadial 1 and the Younger Dryas, consistent with an overall intensification of the trade winds, and changes in the meridional flux gradient indicate a southward wind shift at these times. Biogenic fluxes were lowest, and the meridional flux gradients were weakest during the African Humid Period when the monsoon was invigorated due to precessional changes, with greater rainfall and weaker trade winds over northwest Africa. These results expand the spatial coverage of previous paleoproxy studies showing similar changes, and they provide support for modeling studies showing changes in wind strength and direction consistent with increased upwelling during abrupt coolings and decreased upwelling during the African Humid Period.

  20. The three scales of submarine groundwater flow and discharge across passive continental margins

    USGS Publications Warehouse

    Bratton, J.F.

    2010-01-01

    Increased study of submarine groundwater systems in recent years has provided a wealth of new data and techniques, but some ambiguity has been introduced by insufficient distinguishing of the relevant spatial scales of the phenomena studied. Submarine groundwater flow and discharge on passive continental margins can be most productively studied and discussed by distinct consideration of the following three spatial scales: (1) the nearshore scale, spanning approximately 0-10 m offshore and including the unconfined surficial aquifer; (2) the embayment scale, spanning approximately 10 m to as much as 10 km offshore and including the first confined submarine aquifer and its terminus; and (3) the shelf scale, spanning the width and thickness of the aquifers of the entire continental shelf, from the base of the first confined aquifer downward to the basement, and including influences of geothermal convection and glacioeustatic change in sea level. ?? 2010 by The University of Chicago. All rights reserved.

  1. Subsidence, extension and thermal history of the West African margin in Senegal

    NASA Astrophysics Data System (ADS)

    Brun, Marie Véronique Latil; Lucazeau, Francis

    1988-10-01

    The subsidence of the Atlantic margin in Senegal clearly shows two rapid stages related to the formation of (1) the Central Atlantic during the early Jurassic (around 200 Ma), and (2) the Equatorial Atlantic during the Cretaceous (100 Ma). A simple model of extension is used to interpret the subsidence history and to derive the thermal evolution of this basin. The present-day gravity, bathymetry, bottom hole temperatures (BHT) in oil exploration boreholes and heat flow density are used to control the validity of the model. Two cross sections from the outcropping basement to oceanic crust are used, one in Casamance and the other one at the south to latitude of Dakar. The model can fully explain the first-order subsidence history as well as the present-day observations, and therefore can provide valuable information about the thermal evolution of sediments and about the structure of the continental crust along the margin. Comparisons with the opposite margin in North America (Blake Plateau and Carolina trough) indicate a rather different evolution (the North American margin did not undergo the second stage of rifting) and a different crustal structure (crustal thinning is less important on the African margin).

  2. Propagation of syn-sedimentary faults within the passive margin Otway Basin, Australia

    NASA Astrophysics Data System (ADS)

    Tanner, David; Ziesch, Jennifer; Krawczyk, Charlotte

    2016-04-01

    Faults are often interpreted in 3-D seismics, but they are more rarely analysed in terms of their kinematics and growth. This is nevertheless important to understand the true structural development of a tectonic structure. We attempt such an analysis here and show how neighbouring faults grew in very different fashions. We interpreted a 3-D reflection seismic cube (32.3 km × 14.35 km × 4100 ms TWT) from the onshore Otway Basin, which is part of the passive margin that developed from the Late Jurassic onwards in response to the breakup of the southern Australian margin. Over 2.2 km thick syn-rift Late Cretaceous to recent sediments were deposited, which we identified between a dense pattern of SW-dipping growth faults. Analysis of thickness maps shows faulting was constantly active during sediment deposition, and yet the faults were in retreat until ca. 50 Ma, when nearly all died out completely. No seismically-visible post-rift faulting took place. Using juxtaposition maps of the faults we observe two very different behaviours of the faults' tip line propagation and isolines of fault slip: while all fault strike lengths decrease stratigraphically upwards, the isolines and tip lines are either symmetrical or strongly asymmetrical (but always in a dextral sense). We interpret the latter as oblique dextral propagation of the faults. The distribution of the oblique dextral and dip-slip faults suggests strain partitioning took place on a kilometre scale.

  3. Middle Cambrian to Late Ordovician evolution of the Appalachian margin: Foundering of a passive margin to form a subduction zone and volcanic arc

    SciTech Connect

    Washington, P.A. , Southern Pines, NC )

    1994-03-01

    From late Middle Cambrian to early Late Ordovician time, the Appalachian passive margin experienced a series of orogenic events culminating in the Taconic orogeny. Most of these events are generally viewed as enigmatic and isolated, but they can be viewed as a coherent tectonic sequence of events. The early stages involved broad uplifts and localized extension, especially of internal shelf and adjacent continental interiors. Later stages involved increased subsidence rates of the outer shelf, resulting in retreat of the outer margin of the carbonate platform.The beginning of volcanic activity coincides with, or immediately follows, the rapid subsidence. Onset of compressional orogenesis is often temporally separated from the initial rapid subsidence. These events can be integrated into a tectonic model in which the passive margin is converted into an active Andean margin. Early uplift and extension events represented the surface expression of the beginning of deep-seated downward mantle convection. Subsequent rapid subsidence events represented the mechanical failure of the lithosphere as the convection reaches maturity. Failure of the lithosphere resulted in a subduction zone that quickly created arc volcanism. The compressive Taconic orogenesis occurred when the arc was thrust back onto the shelf margin as the subduction zone migrated continentward in response to progressively channeled convective flow.

  4. The reactivation of the SW Iberian passive margin: a brief review

    NASA Astrophysics Data System (ADS)

    Duarte, Joao; Rosas, Filipe; Terrinha, Pedro; Schellart, Wouter; Almeida, Pedro; Gutscher, Marc-André; Riel, Nicolas; Ribeiro, António

    2016-04-01

    On the morning of the 1st of November of 1755 a major earthquake struck offshore the Southwest Iberian margin. This was the strongest earthquake ever felt in Western Europe. The shake, fire and tsunami devastated Lisbon, was felt as far as Finland and had a profound impact on the thinkers of that time, in particular on the Enlightenment philosophers such as Voltaire, Rousseau and Kant. The Great Lisbon Earthquake is considered by many as the event that marks the birth of modern geosciences; and made of this region one of the most well studied areas in the world. After the 1755 earthquake, Kant and others authors wrote several treaties dealing with the causes and dynamics of earthquakes and tsunamis and were close to identify some key elements of what we now call plate tectonics. More than two hundred years later, in the year of 1969, the region was struck by another major earthquake. This was precisely during the period in which the theory of plate tectonics was being built. Geoscientists like Fukao (1973), Purdy (1975) and Mackenzie (1977) immediately focused their attention in the area. They suggested that these events were related with "transient" subduction of Africa below Iberia, along the East-West Azores-Gibraltar plate boundary. Several years later, Ribeiro (1989) suggested that instead of Africa being subducted below Iberia, it was the West Iberian passive margin that was being reactivated, a process that may, in time, lead to the formation of a new subduction zone. In the turning of the millennium, a subducting slab was imaged bellow the Gibraltar Straits, a remanent of the Western Mediterranean arc system that according to Gutscher et al. (2002) was related with ongoing subduction. Recently, it was proposed that a causal link between the Gibraltar subduction system and the reactivation of the SW Iberian margin might exist. In addition, the large-scale Africa-Eurasia convergence is inducing compressive stresses along the West Iberian margin. The margin

  5. Crustal architecture and deep structure of the Namibian passive continental margin around Walvis Ridge from wide-angle seismic data

    NASA Astrophysics Data System (ADS)

    Behrmann, Jan H.; Planert, Lars; Jokat, Wilfried; Ryberg, Trond; Bialas, Jörg; Jegen, Marion

    2013-04-01

    The opening of the South Atlantic ocean basin was accompanied by voluminous magmatism on the conjugate continental margins of Africa and South America, including the formation of the Parana and Entendeka large igneous provinces (LIP), the build-up of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDR), as well as the formation of paired hotspot tracks on the rifted African and South American plates, the Walvis Ridge and the Rio Grande Rise. The area is considered as type example for hotspot or plume-related continental break-up. However, SDR, and LIP-related features on land are concentrated south of the hotspot tracks. The segmentation of the margins offers a prime opportunity to study the magmatic signal in space and time, and investigate the interrelation with rift-related deformation. A globally significant question we address here is whether magmatism drives continental break-up, or whether even rifting accompanied by abundant magmatism is in response to crustal and lithospheric stretching governed by large-scale plate kinematics. In 2010/11, an amphibious set of wide-angle seismic data was acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The experiments were designed to provide crustal velocity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, constrain the velocity signature of SDR sequences, as well as the extent of magmatic addition to the lower crust near the continent-ocean transition. Sediment cover down to the igneous basement was additionally constrained by reflection seismic data. Here, we present tomographic analysis of the seismic data of one long NNW oriented profile parallel to the continental margin across Walvis Ridge, and a second amphibious profile from the Angola Basin across Walvis Ridge and into the continental interior, crossing the area of the Etendeka

  6. Comparison of Sedimentary Processes on Adjacent Passive and Active Continental Margins Offshore of Southwest Taiwan Based on Echo Character Studies

    NASA Astrophysics Data System (ADS)

    Liu, C.; Chiu, J.

    2008-12-01

    Echo character recorded on Chirp sub-bottom sonar data from offshore area of southwest Taiwan were analyzed to examine and compare the sedimentary processes of adjacent passive and active continental margin settings. Seafloor echoes in the study area are classified into four types: (1) distinct echoes, (2) indistinct echoes, (3) hyperbolic echoes, and (4) irregular echoes. Based on the mapped distribution of the echo types, the sedimentary processes offshore of southwest Taiwan are different in the two tectonic settings. On the passive South China Sea margin, slope failure is the main process on the upper continental slope, whereas turbidite deposits accumulate in the lower continental slope. In contrast, the submarine Taiwan orogenic wedge is characterized by fill-and-spill processes in the intraslope basins of the upper slope, and mass-transport deposits are observed in the canyons and on the lower Kaoping slope. This difference is largely caused by the huge influx of terrigenous sediments into the submarine Taiwan orogenic wedge province compared to the passive South China Sea continental margin. In the passive South China Sea margin, loading and movement of the Taiwan orogenic wedge has had significant effect on the seafloor morphology, and triggered retrogressive failures. Gas hydrate dissociation may have enhanced the slope failure processes at some locations.

  7. Uplift along passive continental margins, changes in plate motion and mantle convection

    NASA Astrophysics Data System (ADS)

    Japsen, Peter; Green, Paul F.; Chalmers, James A.; Bonow, Johan M.

    2014-05-01

    The origin of the forces that produce elevated, passive continental margins (EPCMs) is a hot topic in geoscience. It is, however, a new aspect in the debate that episodes of uplift coincide with changes in plate motion. This has been revealed, primarily, by studies of the burial, uplift and exhumation history of EPCMs based on integration on stratigraphic landscape analysis, low-temperature thermochronology and evidence from the geological record (Green et al., 2013). In the Campanian, Eocene and Miocene, uplift and erosion affected the margins of Brazil and Africa (Japsen et al., 2012b). The uplift phases in Brazil coincided with main phases of Andean orogeny which were periods of relatively rapid convergence at the Andean margin of South America (Cobbold et al., 2001). Because Campanian uplift in Brazil coincides, not only with rapid convergence at the Andean margin of South America, but also with a decline in Atlantic spreading rate, Japsen et al. (2012b) suggested that all these uplift events have a common cause, which is lateral resistance to plate motion. Because the uplift phases are common to margins of diverging plates, it was also suggested that the driving forces can transmit across the spreading axis; probably at great depth, e.g. in the asthenosphere. Late Eocene, Late Miocene and Pliocene uplift and erosion shaped the elevated margin of southern East Greenland (Bonow et al., in review; Japsen et al., in review). These regional uplift phases are synchronous with phases in West Greenland, overlap in time with similar events in North America and Europe and also correlate with changes in plate motion. The much higher elevation of East Greenland compared to West Greenland suggests dynamic support in the east from the Iceland plume. Japsen et al. (2012a) pointed out that EPCMs are typically located above thick crust/lithosphere that is closely juxtaposed to thinner crust/lithosphere. The presence of mountains along the Atlantic margin of Brazil and in East

  8. Deep-sea environment and biodiversity of the West African Equatorial margin

    NASA Astrophysics Data System (ADS)

    Sibuet, Myriam; Vangriesheim, Annick

    2009-12-01

    The long-term BIOZAIRE multidisciplinary deep-sea environmental program on the West Equatorial African margin organized in partnership between Ifremer and TOTAL aimed at characterizing the benthic community structure in relation with physical and chemical processes in a region of oil and gas interest. The morphology of the deep Congo submarine channel and the sedimentological structures of the deep-sea fan were established during the geological ZAIANGO project and helped to select study sites ranging from 350 to 4800 m water depth inside or near the channel and away from its influence. Ifremer conducted eight deep-sea cruises on board research vessels between 2000 and 2005. Standardized methods of sampling together with new technologies such as the ROV Victor 6000 and its associated instrumentation were used to investigate this poorly known continental margin. In addition to the study of sedimentary environments more or less influenced by turbidity events, the discovery of one of the largest cold seeps near the Congo channel and deep coral reefs extends our knowledge of the different habitats of this margin. This paper presents the background, objectives and major results of the BIOZAIRE Program. It highlights the work achieved in the 16 papers in this special issue. This synthesis paper describes the knowledge acquired at a regional and local scale of the Equatorial East Atlantic margin, and tackles new interdisciplinary questions to be answered in the various domains of physics, chemistry, taxonomy and ecology to better understand the deep-sea environment in the Gulf of Guinea.

  9. Locating hyperextended passive margins based on plate reconstructions and limits of oceanic crust derived from potential fields data.

    NASA Astrophysics Data System (ADS)

    Norton, I.; Lawver, L.; Gahagan, L.

    2008-12-01

    Recent advances in understanding of passive margins have emphasized that there is a spectrum of margin styles, ranging from volcanic to hyperextended. All extensional margins will eventually develop sea floor spreading if continental separation continues long enough; the differing margin styles reflect local response of the lithosphere and asthenosphere to ongoing extension. Hyperextended margins can be viewed as extensional systems where continental separation has progressed to a point where there is no more continental crust left, but the asthenosphere has not reached the melting conditions necessary for creation of oceanic crust. The result is that the lithosphere starts to delaminate and mantle is exhumed. The trend then is to form hyperextended margins with exhumation where there is a large amount of continental separation before sea floor spreading begins. One documented area is the Iberia-Newfoundland system, where plate reconstructions suggest that separation between Iberia and Newfoundland was more than 500 km before onset of sea floor spreading. In the South Atlantic, seismic data suggests that mantle exhumation did occur in some areas of the salt basins. Plate reconstructions here suggest more than 300 km of movement between South America and Africa before sea floor spreading was able to start. To try and predict other passive margins where exhumed mantle may underlie significant portions of the margins we have compared extension amounts calculated from plate reconstructions to widths of the margins. This requires mapping of the limits of oceanic crust in areas with inadequate seismic or difficult seismic imaging of deep crustal structure. A regional tool for mapping this boundary is the Bouguer gravity anomaly, with the horizontal gradient of the Bouguer anomaly being a refinement of the method. Studies of the Gulf of Mexico, Gulf of Aden and the Australia - Antarctica conjugate margins are presented here. These studies show that it is possible to

  10. Mineral, Virginia earthquake illustrates seismicity of a passive-aggressive margin

    NASA Astrophysics Data System (ADS)

    Stein, S. A.; Pazzaglia, F. J.; Meltzer, A.; Berti, C.; Wolin, E.; Kafka, A. L.

    2011-12-01

    The August 2011 M5.8 Virginia earthquake illustrated again that "passive" continental margins, at which the continent and neighboring seafloor are part of the same plate, are often seismically active. This phenomenon occurs worldwide, with the east coast of North America a prime example. Examples from North to South include the 1933 M 7.3 Baffin Bay, 1929 M 7.2 Grand Banks of Newfoundland, 1755 M 6 Cape Ann, Massachusetts, and 1886 M 7 Charleston earthquakes. The mechanics of these earthquakes remains unclear. Their overall alignment along the margin suggests that they reflect reactivation of generally margin-parallel faults remaining from continental convergence and later rifting by the modern stress field. This view accords with the occurrence of the Virginia earthquake by reverse faulting on a margin-parallel NE-SW striking fault. However, it occurred on the northern edge of the central Virginia seismic zone, a seismic trend normal to the fault plane, margin, and associated structures, that has no clear geologic expression. Hence it is unclear why this and similar seismic zones have the geometry they do. Although it is tempting to correlate these zones with extensions of Atlantic fracture zones, this correlation has little explanatory power given the large number of such zones. It is similarly unclear whether these zones and the intervening seismic gaps reflect areas that are relatively more active over time, or are instead the present loci of activity that migrates. It is also possible that the presently-active zones reflect long-lived aftershocks of large prehistoric earthquakes. The forces driving the seismicity are also unclear. In general, seismic moment release decreases southward along the margin, consistent with the variation in vertical motion rates observed by GPS, suggesting that glacial-isostatic adjustment (GIA) provides some of the stresses involved. However, in the mid-Atlantic region - south of the area of significant GIA - deformed stratigraphic

  11. Effect of obliquity and structural inheritance on the dynamics of passive margins

    NASA Astrophysics Data System (ADS)

    Le Pourhiet, Laetitia; Mezri, Leila; May, Dave; Leroy, Sylvie

    2015-04-01

    In order to better understand the formation of oblique margins, we have made use of top of the art 3D thermo-mechanical numerical models. The model set up is based on the well-known offset weak notch model. We impose two offset weak-zones which consist of damaged material, in order to ensure the nucleation of two offset continental rift segments and test how the spacing of the notch influence the structures. This set up is well establish and understood for two layers mechanical models resembling sand box. Contrarily to previously published studies, we have ensured that the model are large enough (600 km by 1200 km by 200 km) in order to capture the length scale of oblique passive margin. We have made use of massively parallel architecture of pTatin 3D and its efficient non linear solver in order to achieve a 2km cube resolution on the whole mesh and also to run the models for sufficient number of time step to capture the rifting and the effect of the onset of spreading in both the normal and oblique segments. In order to study the impact of tectonic inheritance on the dynamic of the oblique margins, two different type of crust have been tested. One regular crust which is homogeneous in composition and one post-orogenic crust, in which the lower crust is made of less mafic material, in order to simulate the presence of upper crustal material underplated during a previous orogenic event. This post orogenic crust was previously showed to amplified strain localisation on strike slip segment in 3D and to favour the occurrence of asymmetric detachment structure in extension. In 2D models, the necking of the mantle is forced to occur in similar direction as the necking of the crust. In 3D, the decoupling between the upper crust and the upper mantle leads to an obliquity between the different level of necking and results without any change in the other all kinematics in the boundary in three main faulting direction with time which reflects jumps of the strength of the

  12. Fluid transport processes in the passive margins of the Eastern Mediterranean

    NASA Astrophysics Data System (ADS)

    Bertoni, Claudia; Foschi, Martino; Cartwright, Joe; Levell, Bruce

    2015-04-01

    We analyse and produce a synoptic model of the different styles of fluid transport occurring in the various passive margin settings in the Eastern Mediterranean. The common tectonic-stratigraphic setting is dominated, from the Mesozoic, by the interaction of the Tethyan platforms with Cenozoic to recent, mainly clastic, deposits interacting with the ubiquitous thick late Miocene (Messinian) evaporitic sediments. This created different specific modes of fluid-lithology coupling behaviours, and generated an extraordinary suite of seismically resolvable fluid flow phenomena, including mud volcanoes, pockmarks, dissolution/collapse structures, chimneys and pipes. We integrate this evidence with the analysis of the regional pressure/temperature gradient, and with published hydrocarbon generation models, to propose a regional synthesis of all fluid transport processes in a specific basinal context. We place the fluid flow evidence observed in the Eastern Mediterranean in the framework of the three main fluid flow settings, which are typically defined in sedimentary basins, in terms of depth: 1) A thermobaric fluid regime, where fluid transport is limited and convection can be the dominant transport mechanism, 2) A thermogenic regime, where fluids supplied by hydrocarbon generation can migrate by hydraulic fracturing and advection (along open faults/conduits), by matrix flow and in the longer term, by diffusion processes, 3) A shallow compactional regime, where the fluids are generated by sediment dewatering and shallow diagenesis, and the main transport mechanism is characterised by vertical fluid flow, either through advection and hydrofracturing along faults, or matrix flow. In the Eastern Mediterranean passive margins, this depth-related subdivision needs to be modified in order to accommodate the influence of the laterally and vertically extensive evaporitic series, which acts as a regional aquitard/aquiclude to water or a seal to hydrocarbon flow. The presence of

  13. Sp receiver function imaging of a passive margin: Transect across Texas's Gulf Coastal Plain

    NASA Astrophysics Data System (ADS)

    Ainsworth, Ryan; Pulliam, Jay; Gurrola, Harold; Evanzia, Dominic

    2014-09-01

    The Gulf Coast of Texas has been the subject of intensive geological and geophysical investigation in pursuit of hydrocarbons but studies that penetrate beyond the upper crust are limited to a few refraction profiles and regional surface wave investigations. The passing of EarthScope's Transportable Array has facilitated regional investigations of the lithosphere but its 70-km station spacing does not allow many important tectonic features to be imaged. A broadband seismic transect across the Texas Gulf Coastal Plain was therefore performed in order to image deep structure beneath this passive margin and the transition to the neighboring craton. A 2D Sp receiver function common conversion point (CCP) stacked image produced for this transect reveals several discontinuities in the sub-crustal lithosphere. The region nearest the shoreline is underlain by an anomalous ∼18 km thick low velocity layer that produces a strong negative pulse in the Sp receiver functions. The drop in velocity is too large to be due to any reasonable change in Fe or Mg content but could be produced by partial melt or mantle hydration. It is unlikely that partial melt would still be found in a 160-180-year-old passive margin, such as the Gulf Coast, but hydration, possibly introduced by a through-going Balcones fault system, and resulting serpentinization could produce the observed anomaly. An event with negative polarity appears at a depth of ∼110 km, which we interpret to be the lithosphere-asthenosphere boundary (LAB). Thermal variations alone would not produce a sufficiently sharp discontinuity to be imaged by Sp converted phases. Recent shear-wave splitting studies revealed unusually large delay times in this region, along with fast polarization directions that differ from measurements on the Laurentian craton. Large delay times may imply significant flow, which could also produce frictional heating, due to shearing, and partial melt, which would steepen the velocity gradients. An

  14. The role of small-scale convection on the formation of volcanic passive margins

    NASA Astrophysics Data System (ADS)

    Van Hunen, J.; Phethean, J. J. J.

    2014-12-01

    Several models have been presented in the literature to explain volcanic passive margins (VPMs), including variation in rifting speed or history, enhanced melting from mantle plumes, and enhanced flow through the melting zone by small-scale convection (SSC) driven by lithospheric detachments. Understanding the mechanism is important to constrain the paleo-heat flow and petroleum potential of VPM. Using 2D and 3D numerical models, we investigate the influence of SSC on the rate of crust production during continental rifting. Conceptually, SSC results in up/downwellings with a typical spacing of a few-100 km, and may lead to enhanced decompression melting. Subsequent mantle depletion changes buoyancy (from latent heat consumption and compositional changes), and affects mantle dynamics under the MOR and potentially any further melting. Decompression melting leads to a colder, thermally denser residue (from consumption of latent heat of melting), but also a compositionally more buoyant one. A parameter sensitivity study of the effects of mantle viscosity, spreading rate, mantle temperature, and a range material parameters indicates that competition between thermal and compositional buoyancy determines the mantle dynamics. For mantle viscosities ηm > ~1022 Pa s, no SSC occurs, and a uniform 7-8 km-thick oceanic crust forms. For ηm < ~1021 Pa s, SSC is vigorous and can form VPMs with > 10-20 km crust. If thermal density effects dominate, a vigorous (inverted) convection may drive significant decompression melting, and create VPMs. Such dynamics could also explain the continent-dipping normal faults that are commonly observed at VPMs. After the initial rifting phase, the crustal thickness reduces significantly, but not always to a uniformly thick 7-8 km, as would be appropriate for mature oceanic basins. Transverse convection rolls may result in margin-parallel crustal thickness variation, possibly related to observations such as the East-Coast Magnetic Anomaly.

  15. Inherited segmentation of the Iberian-African margins and tectonic reconstruction of a diffuse plate boundary.

    NASA Astrophysics Data System (ADS)

    Fernàndez, Manel; Torne, Montserrat; Vergés, Jaume; Casciello, Emilio

    2016-04-01

    Diffuse plate-boundary regions are characterized by non-well defined contacts between tectonic plates thus making difficult their reconstruction through time. The Western Mediterranean is one of these regions, where the convergence between the African and Iberian plates since Late Cretaceous resulted in the Betic-Rif arcuate orogen, the Gulf of Cadiz imbricate wedge, and the Alboran back-arc basin. Whereas the Iberia-Africa plate boundary is well defined west to the Gorringe Bank and along the Gloria Fault, it becomes much more diffuse eastwards with seismicity spreading over both the south-Iberian and north-African margins. Gravity data, when filtered for short wavelengths, show conspicuous positive Bouguer anomalies associated with the Gorringe Bank, the Gulf of Cadiz High and the Ronda/Beni-Bousera peridotitic massifs reflecting an inherited Jurassic margin segmentation. The subsequent Alpine convergence between Africa and Iberia reactivated these domains, producing crustal-scale thrusting in the Atlantic segments and eventually subduction in the proto-Mediterranean segments. The Jurassic segmentation of the Iberia-Africa margins substantiates the double-polarity subduction model proposed for the region characterized by a change from SE-dipping polarity in the Gorringe, Gulf of Cadiz and Betic-Rif domains, to NW-dipping polarity in the proto-Algerian domain. Therefore, the Algerian and Tyrrhenian basins in the east and the Alboran basin in the west are the result of SSE-E and NW-W retreating slabs of oceanic and/or hyper-extended Tethyan domains, respectively.

  16. Imaging the lithosphere of rifted passive margins using waveform tomography: North Atlantic, South Atlantic and beyond

    NASA Astrophysics Data System (ADS)

    Lebedev, Sergei; Schaeffer, Andrew; Celli, Nicolas Luca

    2016-04-01

    Lateral variations in seismic velocities in the upper mantle reflect variations in the temperature of the rocks at depth. Seismic tomography thus provides a proxy for lateral changes in the temperature and thickness of the lithosphere. It can map the deep boundaries between tectonic blocks with different properties and age of the lithosphere. Our 3D tomographic models of the upper mantle and the crust at the Atlantic and global scales are constrained by an unprecedentedly large global dataset of broadband waveform fits (over one million seismograms) and provide improved resolution of the lithosphere, 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, including those in North America, Greenland, northern and eastern Europe, Africa and South America. The dominant, large-scale, low-velocity feature is the global system of mid-ocean ridges, with broader low-velocity regions near hotspots, including Iceland. Currently active continental rifts show highly variable expression in the upper mantle, from pronounced low velocities to weak anomalies; this correlates with the amount of magmatism within the rift zone. Rifted passive margins have typically undergone cooling since the rifting and show more subtle variations in their seismic-velocity structure. Their thermal structure and evolution, however, are also shaped by 3D geodynamic processes since their formation, including cooling by the adjacent cratonic blocks inland and heating by warm oceanic asthenosphere.

  17. Post-rift uplift, paleorelief and sedimentary fluxes: the case example of the African margin of the South Atlantic

    NASA Astrophysics Data System (ADS)

    Guillocheau, F.; Dauteuil, O.

    2012-04-01

    volume of eroded sediments. This can explain abnormal stratigraphic response along the African South Atlantic passive margins, such as thin clayey basin floor fans at time of uplift and erosion of weathering profiles. Keywords: Africa, Cenozoic, Siliciclastic sediment fluxes, Tectonics, Climate

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  19. Tectonic denudation of upper mantle along passive margins: a model based on drilling (ODP Leg 103) and diving (Galinaute cruise) results, western Galicia Margin, Spain

    SciTech Connect

    Boillot, G.; Winterer, E.L.; Recq, M.; Girardeau, J.; Kornprobst, J.; Loreau, J.P.; Malod, J.; Mougenot, D.

    1987-05-01

    During ODP Leg 103 (April-June 1985) and the Galinaute cruise (June-July 1986), serpentinized peridotite (clinopyroxene-spinel harzburgite) was recovered within the basement approximately at the boundary between the North Atlantic ocean crust to the west and the thinned continental crust of the Galicia passive margin (Spain) to the east. The exposure of mantle-derived peridotite on the sea floor occurred at the end of the period of rifting, roughly 110 Ma. Ductile shear zones observed in the peridotite are consistent with movements along a deep, low-angle normal fault rooted within the upper mantle and dipping eastward beneath the Galicia margin. To explain the tectonic denudation of the mantle at the ocean-continent boundary, they use a nonuniform stretching model for the lithosphere, set up from Wernicke's model.

  20. Dating fluid flow in developing passive margins using low-temperature thermochronology

    NASA Astrophysics Data System (ADS)

    Gleadow, A. J.; Seiler, C.; Kohn, B. P.

    2012-12-01

    Despite the importance of fluid flow for mass flux and remobilisation in the Earth's crust, the age of past fluid flow events is often difficult to determine, particularly in the low-temperature environment of the shallow crust. This is partly because mineral phases precipitated by low-temperature fluids are either lacking or not very easy to date. Low-temperature thermochronometers such as apatite fission track (AFT) and (U-Th)/He (AHe) systems are, in theory, ideally suited to investigate the temperature interval of hydrothermal fluids near the Earth's surface and could be used to date fluid flow in the shallow crust. In passive margins, however, rift-related faulting, exhumation and post-breakup erosion often result in a much stronger regional cooling signal that relates to tectonic events rather than fluid flow. Moreover, the response of low-temperature thermochronometers to transient and potentially short-lived thermal events associated with hydrothermal fluids has not been studied systematically and is poorly known. In this study, we report AFT and AHe results from two young, regionally important faults that were active at different stages of passive margin evolution in the Gulf of California rift system. In the first case, we investigate the thermal history of the Libertad fault in central Baja California, which represents the breakaway fault for Late Miocene to recent rifting. Regional background AFT and AHe ages range between ~60-35Ma, they predate rifting and are likely associated with steady erosional unroofing of the basement. In contrast, a closely spaced 3D grid of samples from the Libertad escarpment records a distinct Late Miocene thermal event at ~9-8Ma that can be traced several kilometres along the base and a few hundred metres up the escarpment face. In the second case, we collected a 2D grid of samples orthogonal to the Ballenas transform, a transform fault located ~3-5km offshore the coast of central Baja California that is part of the current

  1. A passive margin-type submarine fan complex, Permian Ecca Group, South Africa

    SciTech Connect

    Wickens, H.D. ); Bouma, A.H. )

    1991-03-01

    A submarine fan complex, comprising five arenaceous fan systems separated by basinal shale units, occurs in the southwestern part of the intracratonic Karoo basin in South Africa. Although basin development is related to a subduction zone bordering the palaeo-Pacific ocean to the south of Gondwanaland and the evolution of the Cape Fold Belt, the entire Lower Permian Ecca Group basin-fill succession reflects depositional characteristics of a passive-margin setting. The submarine fan complex, 250 m thick, originated from sediments supplied by Mississippi-type deltas dominating the Ecca coastline. The fine grain-size and low sand/shale ratio of the submarine fan and deltaic deposits reflect the maturity of the ancient river systems. Outcrops of the fan complex are well exposed and cover an area of 650 km{sup 2}. The strata are not affected by folding, and deep erosion allows three-dimensional viewing of mid-fan to outer-fan deposits. Features of interest include stacked lobe deposits displayed along 2.5 km of a 60 m high cliff section, and a transverse cliff section through channel-fill deposits 500 m wide. Paleocurrent directions reveal that each sequence had its own main source area located to the northwest and south of its present geographic location. The cyclic nature of the fan complex is attributed to relative sea-level changes; deposition took place on the basin floor in water depths that do not exceed 500 m. Shoaling of the basin to wave base depths is reflected in the pro-delta and delta front deposits overlying the uppermost fan sequence. Major factors in controlling direction of fan progradation were delta switching and basin floor topography.

  2. Satellite and aircraft passive microwave observations during the Marginal Ice Zone Experiment in 1984

    NASA Astrophysics Data System (ADS)

    Gloersen, Per; Campbell, William J.

    1988-06-01

    During the Marginal Ice Zone Experiment in the Fram Strait in June-July 1984, a number of aircraft with microwave sensors and the scanning multichannel microwave radiometer (SMMR) on board the Nimbus 7 satellite were used to acquire large-scale and mesoscale ice-ocean observations in conjunction with local surface measurements made by experimenters based on helicopter-equipped ice-strengthened vessels. An analysis of the data acquired during six flights of one such aircraft, the NASA CV-990 airborne laboratory, is discussed in this paper. Included in the instrument complement of the CV-990 were two passive microwave imagers operating at wavelengths of 0.33 and 1.55 cm and the airborne multichannel microwave radiometer (AMMR) operating at wavelengths of 0.81, 1.4, and 1.7 cm for both horizontal and vertical polarizations. Total and multiyear sea ice concentrations calculated from the AMMR data were found to agree with similar calculations using SMMR data. This is the first check of the performance of the SMMR Team ice algorithm for near-melting point conditions. The temperature dependence of the multiyear sea ice concentration determination near the melting point was found to be the same for both airborne and spacecraft instrument data and to be correlated with presence or absence of clouds. Finally, it was found that a spectral gradient ratio using the data from both the 0.33- and 1.55-cm radiometers provides more reliable distinctions between low total ice concentrations and open water storm effects near the ice edge than does either singly.

  3. Thermal history and evolution of the South Atlantic passive continental margin in northern Namibia

    NASA Astrophysics Data System (ADS)

    Menges, Daniel; Karl, Markus; Glasmacher, Ulrich Anton

    2013-04-01

    From Permo-Carboniferous to Mid Jurassic northern Namibia was affected by deep erosion of the Damara Orogen, Permo-Triassic collisional processes along the southern margin of Gondwana and eastern margin of Africa (Coward and Daly 1984, Daly et al. 1991), and the deposition of the Nama Group sediments and the Karoo megasequence. The lithostratigraphic units consist of Proterozoic and Cambrian metamorphosed rocks with ages of 534 (7) Ma to 481 (25) Ma (Miller 1983, Haack 1983), as well as Mesozoic sedimentary and igneous rocks. The Early Jurassic Karoo flood basalt lavas erupted rapidly at 183 (1) Ma (Duncan et al. 1997). The Early Cretaceous Paraná-Etendeka flood basalts (132 (1) Ma) and mafic dike swarms mark the rift stage of the opening of the South Atlantic (Renne et al. 1992, Milner et al. 1995, Stewart et al. 1996, Turner et al. 1996). The "passive" continental margin in northern Namibia is a perfect location to quantify exhumation and uplift rates, model the long-term landscape evolution and provide information on the influence of mantle processes on a longer time scale. The poster will provide first information on the long-term landscape evolution and thermochronological data. References Coward, M. P. and Daly, M. C., 1984. Crustal lineaments and shear zones in Africa: Their relationships to plate movements, Precambrian Research 24: 27-45. Duncan, R., Hooper, P., Rehacek, J., March, J. and Duncan, A. (1997). The timing and duration of the Karoo igneous event, southern Gondwana, Journal of Geophysical Research 102: 18127-18138. Haack, U., 1983. Reconstruction of the cooling history of the Damara Orogen by correlation of radiometric ages with geography and altitude, in H. Martin and F. W. Eder (eds), Intracontinental fold belts, Springer Verlag, Berlin, pp. 837-884. Miller, R. M., 1983. Evolution of the Damara Orogen, Vol. 11, Geological Society, South Africa Spec. Pub.. Milner, S. C., le Roex, A. P. and O'Connor, J. M., 1995. Age of Mesozoic igneous rocks in

  4. From northern Gondwana passive margin to arc dismantling: a geochemical discrimination of Ordovician volcanisms (Sardinia, Italy)

    NASA Astrophysics Data System (ADS)

    Gaggero, L.; Oggiano, G.; Buzzi, L.; Funedda, A.

    2009-04-01

    In Sardinia, one of the southernmost remain of the European Variscan belt, a crustal section through northern Gondwanan paleodomains is largely preserved. It bears significant evidence of igneous activity, recently detailed in field relationships and radiometric dating (Oggiano et al., submitted). A Cambro - Ordovician (491.7 ± 3.5 Ma ÷ 479.9 ± 2.1 Ma, LA-ICP-MS U-Pb zircon age) bimodal volcanic suite occurs with continuity in external and inner Variscan nappes of Sardinia below the so-called Sardic unconformity. The igneous suite represents an intraplate volcanic activity developed through subsequent episodes: i) an intermediate explosive and effusive volcanism, i.e. pyroclastic fall deposits and lava flows, embedded into epicontinental clastic sediments, culminating in silicic ignimbrite eruptions, and ii) mafic effusives. Geochemical data document a transitional, within-plate signature, e.g. the average Th/Ta (4.5) and La/Nb (2.7) overlap the upper continental crust values. The volcanites are characterized by slight fractionation of LREEs, nearly flat HREE abundance. The negative Eu anomaly increases towards evolved compositions. Some prominent HREE depletion (GdCN/YbCN = 13.8), and the high Nb/Y suggest a garnet-bearing source. The high 87Sr radiogenic content (87Sr/86Sr 490 Ma = 0.71169) and the epsilon Nd 490 Ma value of -6.54 for one dacite sample, imply a time integrated LREE-enriched source with a high Rb/Sr, such as a metasedimentary source. The stratigraphy of the succession and the geochemical composition of igneous members suggest a volcanic passive margin along the northern Gondwana at the early Ordovician. The bimodal Mid-Ordovician arc volcanism (465.4 ± 1.4 Ma, U-Pb zircon age; Oggiano et al., submitted) is developed in the external nappes (e.g. in Sarrabus and Sarcidano) and in the foreland occurs as clasts at the base of the Hirnantian succession (Leone et al. 1991). The Mid Ordovician sub-alkalic volcanic suite has reliable stratigraphic and

  5. Transcurrent reactivation of Australia's western passive margin: An example of intraplate deformation from the central Indo-Australian plate

    NASA Astrophysics Data System (ADS)

    Hengesh, J. V.; Whitney, B. B.

    2016-05-01

    Australia's northwestern passive margin intersects the eastern termination of the Java trench segment of the Sunda arc subduction zone and the western termination of Timor trough along the Banda arc tectonic collision zone. Differential relative motion between the Sunda arc subduction zone and the Banda arc collision zone has reactivated the former rifted margin of northwestern Australia evidenced by Pliocene to Quaternary age deformation along a 1400 km long offshore fault system. The fault system has higher rates of seismicity than the adjacent nonextended crustal terranes, has produced the largest historical earthquake in Australia (1941 ML 7.3 Meeberrie event), and is dominated by focal mechanism solutions consistent with dextral motion along northeast trending fault planes. The faults crosscut late Miocene unconformities that are eroded across middle Miocene inversion structures suggesting multiple phases of Neogene and younger fault reactivation. Onset of deformation is consistent with the timing of the collision of the Scott Plateau part of the passive continental margin with the former Banda trench between 3.0 Ma and present. The range of estimated maximum horizontal slip rates across the zone is ~1.4 to 2.6 mm yr-1, at the threshold of geodetically detectable motion, yet significant with respect to an intraplate tectonic setting. The folding and faulting along this part of the continental margin provides an example of intraplate deformation resulting from kinematic transitions along a distant plate boundary and demonstrates the presence of a youthful evolving intraplate fault system within the Indo-Australian plate.

  6. Space-for-time substitution and the evolution of a submarine canyon-channel system in a passive progradational margin

    NASA Astrophysics Data System (ADS)

    Micallef, Aaron; Ribó, Marta; Canals, Miquel; Puig, Pere; Lastras, Galderic; Tubau, Xavier

    2014-09-01

    Space-for-time substitution is a concept that has been widely applied, but not thoroughly tested, in some fields of geomorphology. The objective of this study is to test whether the concept of space-for-time substitution is valid in reconstructing the evolution of a submarine canyon-channel system in a passive progradational margin. We use multibeam echosounder data and in situ measurements from the south Ebro Margin to analyse the morphology and morphometry of a sequence of submarine valleys ordered in terms of increasing valley thalweg length. The morphological model of submarine valley evolution that we can propose from this analysis is very similar to established models in the literature, which leads us to conclude that time can be substituted by space when reconstructing the evolution of submarine canyon and channel systems in the south Ebro Margin. By extracting morphometric information from the application of the space-for-time substitution model to our data, we identify a series of morphological patterns as a submarine canyon evolves in a passive progradational margin. These include the geometric similarity of canyon planform shape, an increase in canyon draining efficiency and in the influence of flank slope failures, and an evolution towards equilibrium between canyon form and imposed water and sediment load without net erosion or deposition taking place. We also observe that canyon elongation is higher downslope and that the canyon undergoes an early stage of rapid incision similar to the process of "erosion narrowing" reported in terrestrial rivers. We demonstrate that the conclusions of our study are not limited to submarine valleys in the south Ebro Margin but are applicable to other margins around the world.

  7. The Tethys Himalayan passive margin from Late Triassic to Early Cretaceous (South Tibet)

    NASA Astrophysics Data System (ADS)

    Jadoul, Flavio; Berra, Fabrizio; Garzanti, Eduardo

    1998-04-01

    The Mesozoic succession of South Tibet, both in lithologies and stratigraphic thicknesses, compares much more closely with that of central Nepal than has been reported in the literature. Facies distribution, from relatively proximal environments in the south to more distal settings in the north, reflects the paleogeography of the Tethys Himalaya passive margin, representing the southern edge of Neotethys. As in central Nepal and NW Himalaya, accumulation rates increased greatly in the latest Triassic, when very thick shelfal siliciclastics and locally volcaniclastics (Tarap Formation) were followed by coastal sandstones (Zhamure Formation). Contrary to reports in the literature, this latter unit reaches well into the Liassic at least in proximal southern sectors. The base of the overlying Kioto Group, yielding rich faunal associations characterized by benthic foraminifers and Lithiotis, similar to those of western Tethys carbonate platforms, is dated as middle Liassic (most probably late Pliensbachian). The monotonous growth of carbonate ramps during the middle Liassic to early Dogger was interrupted by two siliciclastic episodes related to major paleogeographic changes in the Toarcian (middle part of the Kioto Group) and in the Bajocian-Bathonian (Laptal Formation). An early Callovian flooding event (Ferruginous Oolite Formation)—widespread all along the Tethys Himalaya—was locally followed by deposition of a markedly lenticular pelagic unit, previously reported only from the Thakkhola Graben (Dangar Formation). The overlying Spiti Shale is reduced to only 20 m in southern sectors, where it is not younger than the Oxfordian, whereas in northern sectors the unit is several tens of m thick and mainly ascribed to the Tithonian. Volcaniclastic deposition (Wölong Formation)—reported herein for the first time—probably began as early as the Kimmeridgian/early Tithonian and lasted until Aptian times, when it was replaced by sedimentation of relatively deep

  8. Deciphering the influence of the thermal processes on the early passive margins formation

    NASA Astrophysics Data System (ADS)

    Bousquet, Romain; Nalpas, Thierry; Ballard, Jean-François; Ringenbach, Jean-Claude; Chelalou, Roman; Clerc, Camille

    2015-04-01

    Many large-scale dynamic processes, from continental rifting to plate subduction, are intimately linked to metamorphic reactions. This close relation between geodynamic processes and metamorphic reactions is, in spite of appearances, yet poorly understood. For example, during extension processes, rocks will be exposed to important temperature, pressures and stress changes. Meanwhile less attention has been paid to other important aspects of the metamorphic processes. When reacting rocks expand and contract, density and volume changes will set up in the surrounding material. While several tectonic models are proposed to explain the formation of extensive basins and passive margins ( simple shear detachment mantle exhumation .... ) a single thermal model (McKenzie , 1978), as a dogma, is used to understanding and modeling the formation and evolution of sedimentary basins . This model is based on the assumption that the extension is only by pure shear and it is instantaneous. Under this approach, the sedimentary deposits occur in two stages. i) A short step , 1 to 10 Ma , controlled by tectonics. ii) A longer step , at least 50 Ma as a result of the thermal evolution of the lithosphere.
However, most stratigraphic data indicate that less thermal model can account for documented vertical movements. The study of the thermal evolution , coupled with other tectonic models , and its consequences have never been studied in detail , although the differences may be significant and it is clear that the petrological changes associated with changes in temperature conditions , influence changes reliefs.
In addition, it seems that the relationship between basin formation and thermal evolution is not always the same:
- Sometimes the temperature rise above 50 to 100 Ma tectonic extension. In the Alps, a significant rise in geothermal gradient Permo -Triassic followed by a "cold" extension , leading to the opening of the Ligurian- Piedmont ocean, from the Middle Jurassic .

  9. Paleobathymetry from 3-D flexural backstripping: Implementation and application to NW Australia and Liberia passive margins

    NASA Astrophysics Data System (ADS)

    Lovely, Peter; Chauvin, Benjamin; Brennan, Patrick; Laroche, Matt

    2015-04-01

    eustatic sea level, allows for emergent topography, and overcomes potential pitfalls associated with the analytical solution for a "filled" basin. We review the numerical implementation of flexural backstripping, and discuss implications, as well as limitations, of paleobathymetric maps for source rock preservation and reservoir presence in two diverse passive margin settings: offshore Liberia and the Northwest Shelf of Australia.

  10. Treatment of Passive Component Reliability in Risk-Informed Safety Margin Characterization FY 2010 Report

    SciTech Connect

    Robert W Youngblood

    2010-09-01

    The Risk-Informed Safety Margin Characterization (RISMC) pathway is a set of activities defined under the U.S. Department of Energy (DOE) Light Water Reactor Sustainability Program. The overarching objective of RISMC is to support plant life-extension decision-making by providing a state-of-knowledge characterization of safety margins in key systems, structures, and components (SSCs). A technical challenge at the core of this effort is to establish the conceptual and technical feasibility of analyzing safety margin in a risk-informed way, which, unlike conventionally defined deterministic margin analysis, is founded on probabilistic characterizations of SSC performance.

  11. Cenozoic denudation rates of the West African marginal upwarp recorded by lateritic paleotopographies

    NASA Astrophysics Data System (ADS)

    Beauvais, Anicet; Chardon, Dominique

    2013-04-01

    Quantifying 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, particularly passive margins, 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 3 sub-continental scale lateritic paleolandsurfaces whose age is bracketed by 39Ar/40Ar dating of lateritic K-Mn oxides [1]. Denudation depths and rates compiled from 380 field stations show that despite heterogeneities confined to early-inherited reliefs, the sub-region 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 results suggest that Cenozoic post-rift vertical mobility of marginal upwarps in the tropical belt was unable to modify slow, weathering-controlled, steady state denudation regimes. The potentially complex expression of steady-state cratonic denudation regimes in clastic sedimentary fluxes remains to be investigated. [1] Beauvais et al., Journal of

  12. The Neotethyan Sanandaj-Sirjan zone of Iran as an archetype for passive margin-arc transitions

    NASA Astrophysics Data System (ADS)

    Hassanzadeh, Jamshid; Wernicke, Brian P.

    2016-03-01

    The Sanandaj-Sirjan zone of Iran is a northwest trending orogenic belt immediately north of the Zagros suture, which represents the former position of the Neotethys Ocean. The zone contains the most extensive, best preserved record of key events in the formation and evolution of the Neotethys, from its birth in Late Paleozoic time through its demise during the mid-Tertiary collision of Arabia with Eurasia. The record includes rifting of continental fragments off of the northern margin of Gondwanaland, formation of facing passive continental margins, initiation of subduction along the northern margin, and progressive development of a continental magmatic arc. The latter two of these events are critical phases of the Wilson Cycle that, elsewhere in the world, are poorly preserved in the geologic record because of superimposed events. Our new synthesis reaffirms the similarity between this zone and various terranes to the north in Central Iran. Late Paleozoic rifting, preserved as A-type granites and accelerated subsidence, was followed by a phase of pronounced subsidence and shallow marine sedimentation in Permian through Triassic time, marking the formation and evolution of passive margins on both sides of the suture. Subduction and arc magmatism began in latest Triassic/Early Jurassic time, culminating at ~170 Ma. The extinction of arc magmatism in this zone, and its shift northeastward to form the subparallel Urumieh-Dokhtar arc, occurred diachronously along strike, in Late Cretaceous or Paleogene time. Post-Cretaceous uplift transformed the zone from a primarily marine borderland into a marine archipelago that persisted until mid-Tertiary time.

  13. Flow of material under compression in weak lower continental crust can cause post-rift uplift of passive continental margins

    NASA Astrophysics Data System (ADS)

    Chalmers, James

    2014-05-01

    There are mountain ranges up to more than 2 km high along many passive continental margins (e.g. Norway, eastern Australia, eastern Brazil, SE and SW Africa, east and west Greenland etc.), dubbed Elevated Passive Continental Margins (EPCMs). EPCMs contain several features in common and observations indicate that uplift of these margins took place after continental break-up. There are many explanations for their formation but none that satisfy all the observations. Lack of a geodynamical mechanism has meant that there has been difficulty in getting the community to accept the observational evidence. Formation of a passive continental margin must take place under conditions of tension. After rifting ceases, however, the margin can come under compression from forces originating elsewhere on or below its plate, e.g. orogeny elsewhere in the plate or sub-lithospheric drag. The World Stress Map (www.world-stress-mp.org) shows that, where data exists, all EPCMs are currently under compression. Under sufficient compression, crust and/or lithosphere can fold, and Cloetingh & Burov (2010) showed that many continental areas may have folded in this way. The wavelengths of folding observed by Cloetingh & Burov (2010) imply that the lower crust is likely to be of intermediate composition; granitic lower crust would fold with a shorter wavelength and basic lower crust would mean that the whole lithosphere would have to fold as a unit resulting in a much longer wavelength. Continental crust more than 20 km thick would be separated from the mantle by a weak layer. However, crust less thick than that would contain no weak layers would become effectively annealed to the underlying strong mantle. Under sufficient horizontal compression stress, material can flow in the lower weak layer towards a continental margin from the continental side. The annealed extended crust and mantle under the rift means, however, that flow cannot continue towards the ocean. Mid- and lower crustal material

  14. Optical dating of aeolian dynamism on the West African Sahelian margin

    NASA Astrophysics Data System (ADS)

    Stokes, S.; Bailey, R. M.; Fedoroff, N.; O'Marah, K. E.

    2004-04-01

    The Sahelian Margin of West Africa is widely recognised as an area of recent environmental catastrophe and human suffering arising from food shortage and land degradation associated with prolonged drought. The propensity of this region to suffer drought has been related, using environmental data collected during the period of instrumental records, to a combination of low mean annual rainfall levels and a high degree of rainfall variability which relates to sea surface temperature anomalies in the adjacent tropical Atlantic Ocean. Despite the significant environmental and human consequences of such droughts, there is a paucity of long-term environmental data for the West African Sahel. Aeolian dune reactivations in this area are a potentially highly useful environmental archive of past periods of extended drought conditions, which may have resulted in localised or widespread dune reactivation. Here we describe the initial results from an ongoing programme of research, which seeks to develop a detailed record of past dune reactivations in Mali. We find evidence for repeated Holocene dune reactivation events and a significant number of reactivations, which commenced at the time of onset of the last major drought cycle in the early 1970s. We obtain ages as young as 20-30 years for some significant dune units (thickness up to 1 m) and describe the results of experiments which test the performance of our dating exercise. We specifically test for the significance of preheat temperature on single aliquot regeneration (SAR) equivalent dose determinations and recycling ratios; neither are found vary significantly as a function of preheating. Optical dating of sand sized quartz could provide a useful tool for palaeogeographical mapping of ancient and historical dune reactivations in this region and elsewhere.

  15. Magma-poor and magma-rich segments along the hyperextended, pre-Caledonian passive margin of Baltica

    NASA Astrophysics Data System (ADS)

    Andersen, Torgeir B.; Alsaif, Manar; Corfu, Fernando; Jakob, Johannes; Planke, Sverre; Tegner, Christian

    2015-04-01

    The Scandinavian Caledonides constitute a more than 1850 km long 'Himalayan-type' orogen, formed by collision between Baltica-Avalonia and Laurentia. Subduction-related magmatism in the Iapetus ended at ~430 Ma and continental convergence continued for ~30 Myr until ~400 Ma. The collision produced a thick orogenic wedge comprising the stacked remnants of the rifted to hyperextended passive Baltican margin (Andersen et al. 2012), as well as suspect, composite and outboard terranes, which were successively emplaced as large-scale nappe complexes onto Baltica during the Scandian collision (see Corfu et al. 2014 for a recent review). Large parts (~800 km) of the mountain-belt in central Scandinavia, particularly in the Särv and Seve Nappes and their counterparts in Troms, are characterised by spectacular dyke complexes emplaced into continental sediments (e.g. Svenningsen 2001, Hollocher et al. 2007). These constitute a magma-rich segment formed along the margin of Baltica or within hyperextended continental slivers outboard of Baltica. The intensity of the pre-Caledonian magmatism is comparable to that of the present NE-Atlantic and other volcanic passive margins. The volumes and available U-Pb ages of 610-597 Ma (Baird et al. 2014 and refs therein) suggest that the magmatism was short lived, intense and therefore compatible with a large igneous province (LIP). By analogy with present-day margins this LIP may have been associated with continental break-up and onset of sea-floor spreading. The remnants of the passive margin both north and south of the magma-rich segment have different architectures, and are almost devoid of rift/drift related magmatic rocks. Instead, these magma-poor segments are dominated by heterogeneous sediment-filled basins characterised by the abundant presence of solitary bodies of variably altered mantle peridotites, also commonly present as detrital serpentinites. These basins are interpreted to have formed by hyperextension. We suggest that

  16. A Geomorphological Analysis of the Cenozoic Rejuvenation of the Southwestern Norwegian 'Passive' Margin

    NASA Astrophysics Data System (ADS)

    McDermott, Jeni; Redfield, Tim; Terje Osmundsen, Per; Arnhold, Chad; Conrad, Dan

    2015-04-01

    Although the Norwegian and Greenland rifted margins underwent Early Paleocene breakup, the southwestern Norwegian continental margin exhibits 2 to 3 km-high, sharply asymmetric seaward-facing escarpments and a 250+ km long topographic displacement gradient, a morphology not consistent with simple margin evolution models that predict subsidence and cooling as the dominate processes in tectonically-quiescent regions. Such atypical margins present a paradox: How is high, rugged topography along rifted margins maintained for tens to hundreds of millions of years after the cessation of extension? Recent work indicates the offshore crustal thinning gradient, a measure of the length from the continental escarpment to the location of the maximum crustal thickness, may play a controlling role: where the gradient is sharp the topography is most elevated; where gentle, the escarpments are lower. Although controversy remains, it is generally accepted, based on offshore geophysical data and onshore geomorphology, thermochronology, and structural geology, that the southwestern Norwegian escarpment has undergone topographic rejuvenation during the Cenozoic. Although several mechanistic models invoking various contributions of active tectonism have been proposed, from remnant topography recently carved by extensive glaciation to active uplift along large-scale onshore margin-parallel faults, the rejuvenating mechanism has not been resolved. Non-glacial components of rock column uplift may possibly be occurring today: tectonic control of major drainage patterns has been proposed and recent work in the Møre-Trøndelag Fault Complex provides compelling evidence for discrete fault-bound tectonic blocks with unique exhumation histories. We are seeking to constrain the primary mode of Cenozoic deformation along the western Norwegian continental rifted margin by utilizing a tiered approach with distinct but complementary techniques encompassing tectonic geomorphology, structural geology

  17. Comparative development of the Western United States and southern Kazakhstan, Soviet Union - Two early Paleozoic carbonate passive margins

    SciTech Connect

    Cooke, H.E. ); Taylor, M.E. ); Zhemchuzhnikov, S.V.; Apollonov, M.K.; Ergaliev, G. Kh.; Sargaskaev, Z.S. ); Dubinina, S.V. )

    1991-02-01

    Early Paleozoic passive continental margins of the Western united States and southern Kazakhstan evolved at low latitudes on rifted Precambrian continental crust adjacent to the proto-Pacific Ocean. In the Western United States, early Paleozoic carbonate submarine fans and slides formed on continental slopes in central Nevada. Coeval shoal-water carbonate sediments occurred to the east, in Utah, where they interfingered with siliciclastic sediments and onlapped the craton. In contrast, early Paleozoic carbonate sediments of the Malyi Karatau, southern Kazakhstan, were deposited on isolated microcontinental blocks that developed during Late Proterozoic rifting of the continental crust. Comparison of stratigraphic sections from Nevada and Malyi Karatau indicate a similar upward-shallowing and seaward-prograding evolution. The Hot Creek Range section in Nevada consists of the Upper Cambrian Swarbrick Formation and Tybo Shale, and Upper Cambrian and lowest Ordovician Hales Limestone. These depositional facies include basin plain (about 500 m), carbonate submarine fan and slides (200 m), upperslope (150 m), and platform margin (150 m). The Kyrshabakty and Batyrbay sections in the Malyi Karatau consist of Cambrian and lowest Ordovician rocks of the Shabakty Suite. Stratigraphic sections in both the Western United States and Malyi Karatau record three coeval episodes of sea level lowstands. These lowstands, which the authors interpret to be eustatic, are recognized by times of seaward collapse of large segments of the platform margins and deeper water slopes and by solution breccias and faunal discontinuities in shoal-water platform-interior sites.

  18. The Golden Lady: The Storied Life of a Multilingual Teacher and Author of Supplemental Reading Materials in a Marginalized South African Language

    ERIC Educational Resources Information Center

    Horan, Deborah A.; Sailors, Misty; Martinez, Miriam; Skerrett, Allison; Makalela, Leketi

    2012-01-01

    Personal narratives can be powerful venues for understanding human experiences. In this paper, we tell the story of Lutanyani, a Black South African multilingual teacher and author of supplemental reading materials in a marginalized South African language. Through various word images, we convey the role of language, in particular written language,…

  19. Long-term landscape evolution of the South Atlantic passive continental margin along the Kaoko- and Damara Belts, NW-Namibia

    NASA Astrophysics Data System (ADS)

    Menges, Daniel; Glasmacher, Ulrich Anton; Hackspacher, Peter; Schneider, Gabriele; Salomon, Eric

    2015-04-01

    The Kaoko Belt in northwestern Namibia originates in the collision of the Rio de la Plata and Kongo Craton during the Pan-African Orogeny in the Neoproterozoic (1) and represents the northern arm of the Damara Orogen. NW-Namibias continental crust mainly consists of the NE-SW striking intracontinental branch of the Pan-African Damara mobile belt, which separates the Congo from the Kalahari craton. The Damara Orogen is divided into several tectonostratigraphic zones that are bounded by steeply dipping, ductile shear zones. These regional lineaments can be traced at least 150 km offshore (2). The lithostratigraphic units consist of Proterozoic and Cambrian metamorphosed rocks (534 (7) Ma - 481 (25) Ma (3) as well as Mesozoic sedimentary and igneous rocks. From Permo-Carboniferous to Mid Jurassic northern Namibia was affected by deep erosion of the Damara Orogen, Permo-Triassic collisional processes along the southern margin of Gondwana and eastern margin of Africa (4), and the deposition of the Nama Group sediments and the Karoo megasequence (5). Between the Otjihorongo and the Omaruru Lineament-Waterberg Thrust early Mesozoic tectonic activity is recorded by coarse clastic sediments deposited within NE trending half-graben structures. The Early Jurassic Karoo flood basalt lavas erupted rapidly at 183±1 Ma (6). The Early Cretaceous Paraná-Etendeka flood basalts (132±1 Ma) and mafic dike swarms mark the rift stage of the opening of the South Atlantic (7). Early Cretaceous alkaline intrusions (137-124 Ma) occur preferentially along Mesozoic half-graben structures and are called the Damaraland Igneous Province (8). Late Cretaceous alkaline intrusions and kimberlite pipes occur in northern Namibia. Post Early Paleocene siliciclastic sedimentation in Namibia was largely restricted to a 150 km wide zone (9) and is represented by the Tsondab Sandstone Formation (~ 300 m thickness). The oldest part has an age of early Paleocene and the upper part span from middle Miocene

  20. Evidence of multiple stretching and episodic subsidence of a passive continental margin: Indian examples

    SciTech Connect

    Agrawal, A. )

    1990-05-01

    The western continental margin of India, south of the Narmada lineament underwent two major phases of stretching and rapid tectonic subsidence, as seen in more than 30 deep offshore wells. The initial phase of unloaded basement subsidence took place in the Paleocene, and was probably related to K/T rifting of India along this margin. At this time, the continental crust underwent some stretching leading to the formation of several north-south-trending depressions, namely the Vijayadurg, Surat-Panna, the Kori-Comorin, and on-land Cambay graben, which provided a site for the rapid accumulation of hydrocarbon source rocks in the depressions. The second phase of rapid basement subsidence along this margin occurred in the Early Miocene Further stretching of the already blockfaulted crust caused the complete development of the westernmost Kori-Comorin depression but the effect was less significant on the main shelf. The possibility of a heating event associated with the stretching, inducing maturation of hydrocarbons in the area, is under investigation. Similar early Miocene tectonic subsidence also is seen along the peripheries of the Arabian Sea and the Bay of Bengal. This phase of tectonic movements was probably related to the locking of the Owen fracture zone, the 20-Ma plate reorganization in the Indian Ocean and the renewed uplift of the Himalayas.

  1. Moving beyond the Margins: An Exploration of Low Performing African American Male College Students

    ERIC Educational Resources Information Center

    Jackson, Ronald C.

    2011-01-01

    Data have shown that African American male college students are being outperformed. Compared to all other populations by ethnicity and gender, African American males most often fare the worst in terms of persistence, performance, and completion. The impetus of this study was to explore the motivation of those that have low academic performance and…

  2. Link between Mid-Ocean Ridge kinematics and uplift of passive continental margins

    NASA Astrophysics Data System (ADS)

    Døssing, A.; Japsen, P.; Nielsen, T.; Thybo, H.; Dahl-Jensen, T.

    2012-04-01

    Tectonic models predict post-rift subsidence of rift margins after initial flexural rebound and transgression of a sedimentary wedge over the subsiding mar¬gin as the lithosphere cools with time. However, studies of North Atlantic rifted margins show that thermal subsidence following breakup at the Paleocene-Eocene transition was interrupted by significant uplift movements. These vertical movements represent a long-standing enigma and they have been linked to sea-level fluctuations, climate deterioration and tectonics but as yet they remain unexplained. Here we combine regional Multi-Channel Seismic reflection data across the NE Greenland Shelf, the Greenland Fracture Zone (GFZ) and continental East Greenland Ridge (EGR) in the northern NE Atlantic and stratigraphic data from a drill core. We show that a mid-Miocene change from down-faulting to uplift along the GFZ-EGR correlates with significant uplift of the NE Greenland margin. This tectonic change is associated with a regional unconformity that marks the first occurrences of mass-¬wasted deposits in the deep sea off the NE Greenland Shelf and the development of prograd¬ing mega-sequences and angular truncation of hemipelagic sediments below the unconformity, respectively, on the outer and inner NE Greenland Shelf. We attribute the tectonic changes at the GFZ to the development of a modern, continuous spreading system along the Mohns-Knipovich Ridge segments that led to an opening of the Fram Strait corridor, to large-scale changes in ocean circulation and climate and possibly to medium-scale (20-30 m) sea-level fluctuations. While these consequences of the tectonic changes may have affected the amplitude of uplift in NE Greenland, they cannot explain the uplift at the GFZ-EGR in deep sea. We therefore find that plate-tectonic changes produced the driving force for the mid-Miocene uplift in NE Greenland.

  3. Control of hyper-extended passive margin architecture on subduction initiation with application to the Alps and present-day North Atlantic ocean

    NASA Astrophysics Data System (ADS)

    Candioti, Lorenzo; Bauville, Arthur; Picazo, Suzanne; Mohn, Geoffroy; Kaus, Boris

    2016-04-01

    Hyper-extended magma-poor margins are characterized by extremely thinned crust and partially serpentinized mantle exhumation. As this can act as a zone of weakness during a subsequent compression event, a hyper-extended margin can thus potentially facilitate subduction initiation. Hyper-extended margins are also found today as passive margins fringing the Atlantic and North Atlantic ocean, e.g. Iberia and New Foundland margins [1] and Porcupine, Rockwall and Hatton basins. It has been proposed in the literature that hyper-extension in the Alpine Tethys does not exceed ~600 km in width [2]. The geodynamical evolution of the Alpine and Atlantic passive margins are distinct: no subduction is yet initiated in the North Atlantic, whereas the Alpine Tethys basin has undergone subduction. Here, we investigate the control of the presence of a hyper-extended margin on subduction initiation. We perform high resolution 2D simulations considering realistic rheologies and temperature profiles for these locations. We systematically vary the length and thickness of the hyper-extended crust and serpentinized mantle, to better understand the conditions for subduction initiation. References: [1] G. Manatschal. New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps. Int J Earth Sci (Geol Rundsch) (2004); 432-466. [2] G. Mohn, G. Manatschal, M. Beltrando, I. Haupert. The role of rift-inherited hyper-extension in alpine-type orogens. Terra Nova (2014); 347-353.

  4. Comparative analysis of post-breakup basin evolution along the South-American and South-African margins, southern Atlantic

    NASA Astrophysics Data System (ADS)

    Strozyk, F.; Back, S.; Kukla, P. A.

    2012-04-01

    Recently, considerable attempts have been made to compare the sedimentary basin evolution and the associated tectonic framework on both sides of the South-Atlantic. However, yet there are still unresolved questions concerning the tectono-sedimentary styles of margin basin evolution that markedly differ from north to south. Amongst the most striking observations is that multiple phases of uplift and subsidence are recorded after the break-up of the southern South Atlantic margin segment on both sides of the Florianopolis-Walvis Ridge volcanic complex, features that are regarded as atypical when compared to published examples of other post-breakup margin successions. Adding to the heterogeneity of the system, the northernmost segment of the South Atlantic rift and salt basins is also characterized by a pronounced asymmetry, with the Brazilian margin now comprising narrower and deeper rift basins with less salt than the Congo-Gabon conjugate margin. This project deals with a large-scale comparison of this very different post-breakup tectono-stratigraphic development of the southern and northern South American and African continental margins that both record thick post-rift sedimentary successions. To gain detail of the basin margin evolution, we focus on a regional comparison between the post-breakup records archived in the large offshore southern Brazil basins (Pelotas, Santos, Campos) and the post-breakup continental margin successions of offshore Namibia (e.g. Orange Basin) and southern Angola (e.g. Kwanza Basin). A tectonic-stratigraphic comparison of representative geological transects provides a comprehensive basin-to-basin documentation of key factors influencing margin development which include the subsidence development through time, the sediment (in-)flux and storage patterns and the respective type of basin fill (e.g. salt vs. non-salt systems; carbonate-rich vs. clastics-dominated systems). Data from the salt-prone areas offshore South America and southern

  5. A Broadband Investigation of the Texas/Gulf of Mexico Passive Margin

    NASA Astrophysics Data System (ADS)

    Evanzia, D.; Ainsworth, R.; Pratt, K. W.; Pulliam, J.; Gurrola, H.

    2012-12-01

    The lithosphere of central and east Texas underwent two cycles of continental rifting and orogeny from the formation of Laurentia and assembly through the breakup of Pangea. The craton itself, exposed in the Llano uplift of central Texas, formed ~1.4 Ga as part of the great expanse of Mesoproterozoic crust that makes up southern Laurentia. Some of this crust was deformed during the Grenville orogeny ~1.1 Ga. Southern Laurentia was subsequently stable until rifting began in Cambrian time (~530 Ma). Suturing of Gondwana to Laurentia (310-290 Ma) during the assembly of Pangea formed the Ouachita orogen in west Texas. Sometime before 200 Ma rifting was initiated, opening the Gulf of Mexico (GoM). In north and east Texas the Ouachita front lies north of GoM rifting but, according to deep seismic data, Ouachita structures appear to coincide with GoM rifting in south and central Texas. This suggests that rifting in that region occurred along structures that were weakened previously by Ouachita deformation and reactivated during the Jurassic opening of the GoM. It is not clear whether the process that created the Gulf of Mexico and led to the formation of Texas' Gulf Coast Plain (GCP) is best described as "active" or "passive" rifting. A recent study interpreted the GCP to be a volcanic rifted margin—an active rifting process—using available gravity, magnetic, drilling and geological data, but older studies describe the opening of the GoM as a passive event. In the coastal plain, a large magnetic anomaly suggests that the crust here was modified by volcanism. Seismic data are sparse and of limited quality in the Gulf Coast region so we conducted a 2.5-year broadband seismograph transect across the GCP in an effort to clarify its structure and origin. In all, twenty-three broadband seismographs were deployed in a line from Matagorda Island, in the Gulf of Mexico, to Johnson City, TX, on the uplifted Llano Plateau from July 2010 to December 2012. These seismographs have

  6. A Geo-traverse at a Passive Continental Margin: the Tagus Abyssal Plain, West Iberia

    NASA Astrophysics Data System (ADS)

    Afilhado, A.; Matias, L.; Mendes-Victor, L.

    2006-12-01

    The crustal and lithospheric mantle structure was investigated along a 370km profile at the south segment of the west Iberian margin, from 12.9W to 8.7W, at approximately 38N. The profile crosses from inland unthinned continental to oceanic crust, IAM5. Both MCS data and wide-angle, WA, data were considered. WA data set includes 6 OBSs and 2 inland seismic stations. Free air and total field magnetic anomalies profiles were extracted from available grided data. WA data cinematic and dynamic modeling provided a 2D velocity model that proved to be consistent with the free air anomaly profile. 2.5D generalized inversion on pseudo- susceptibility was performed. A Bouguer anomaly grid was calculated and a similar procedure was performed for the undulation of the geoid grid. First and second derivatives grids of this surface and its upward continued surfaces were calculated to locate lineated lows and highs. MCS and WA data sets indicate four main crustal domains. East of 9.4W the complete crustal section of slightly thinned continental crust is present. From 9.4W to 9.7W crustal thinning is abrupt and the lower continental crust pinches out. From 9.7W to 10.5W the transitional crust has a complex structure that varies both horizontally and vertically. Within the eastern most transitional domain the exhumed mid continental crust thins to zero; small scale heterogeneities exist at its lower interface; the under-laying lower crust nature is unknown, correlating both to exhumed continental mantle and to continental gabbros. From 10.2W to 10.5W the transitional crust is very heterogeneous, having higher than usual density at its upper levels; the absence of Moho reflections suggests that exhumed and intruded mantle might be present at the lower transitional crust. West of 10.5W thin oceanic crust is found, probably generated in a slow spreading environment; around 12W the data suggests a very thin or even absent layer 3. Deep reflectors imaged within the oceanic domain and

  7. Ductile extensional shear zones in the lower crust of a passive margin

    NASA Astrophysics Data System (ADS)

    Clerc, Camille; Jolivet, Laurent; Ringenbach, Jean-Claude

    2015-12-01

    We describe and interpret an unpublished set of ION Geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the Uruguayan volcanic rifted margin. We distinguish two main groups of reflectors in the lowermost continental crust. A first group, at depths ranging from 32 km below the continent to 16 km in the continent-ocean transition, comprises reflectors continuous over tens of kilometers, peculiarly visible near the mantle-crust boundary. A second group of reflectors dipping toward the ESE (oceanward) is widely distributed in the lower crust. These reflectors are slightly curved and tend to merge and become sub-parallel with the first group of reflectors. Together they draw the pattern of thick shallow-dipping top-to-the continent shear zones affecting the lower continental crust. Such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn-tectonic volcanic formations (SDR). A major portion of the continental crust behaved in a ductile manner and recorded a component of top-to-the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle.

  8. Low temperature thermochronology and topographic evolution of the South Atlantic passive continental margin in the region in eastern Argentina

    NASA Astrophysics Data System (ADS)

    Pfister, Sabrina; Kollenz, Sebastian; Glasmacher, Ulrich A.

    2014-05-01

    To understand the evolution of the passive continental margin in Argentina low temperature thermochronology is an appropriate method, which will lead to new conclusions in this area. The Tandilia System, also called Sierras Septentrionales, is located south of the Río de la Plato Craton in eastern Argentina in the state of Buenos Aires. North of the hills Salado basin is located whereas the Claromecó basin is situated south of the mountain range. In contrary to most basins along the southamerican passive continental margin the Tandilia-System and the neighbouring basins trend perpendicular to the coast line. The topography is fairly flat with altitudes of. The igneous-metamorphic basement is pre-proterozoic in age and build up of mainly granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks and granitoid plutons it is overlain by a series of Neoproterozoic to early Paleozoic sediments (Cingolani, 2010), like siliciclastics, dolostones, shales and limestones (Demoulin et al., 2005). The aim of the study is to quantify the long-term landscape evolution of the passive continental margin in eastern Argentina in terms of thermal history, exhumation and tectonic activities. For that purpose, samples were taken from the Sierra Septentrionales and analyzed with the apatite fission-track method. Further 2-D thermokinematic modeling was conducted with the computer code HeFTy (Ketcham, 2005; Ketcham 2007; Ketcham et al., 2009). The results indicate apatite fission track ages between 101.6 (9.4) to 228.9 (22.3) Ma, what means all measured ages are younger as their formation age. That shows all samples have been reset. Six samples accomplished enough confined tracks and were used to test geological t-T models against the AFT data set. These models give a more detailed insight on the cooling history and tectonic activities in the research area. References: Cingolani C. A. (2010): The Tandilia System of Argentina as a southern extension of the Río de la

  9. Incipient oceanic accretion at the foot of the Gondwanian passive margin : the Neyriz ophiolites (Iran)

    NASA Astrophysics Data System (ADS)

    Jannessary, M. R.; Whitechurch, H.; Ricou, L. E.; Muntener, O.; Ildefonse, B.; Manatschal, G.

    2003-04-01

    directly on the harzburgites. The structures and petrology of the Neyriz ophiolite reflect likely the initiation of a proto-ridge intruding an old oceanic lithosphere at the distal part of the pre-existing Triassic continental margin. Thus, Neyriz and Oman ophiolites that are issued from the same Cretaceous accretion system in the Tethys are different. The former attest from the incipience of an ultra low spreading ridge, the later from steady state processes at a fast to ultra fast spreading ridge.

  10. Remnants of a hyperextended passive margin in a Caledonian mélange unit below the Jotun nappe, B\\overdalen, Central-south Norway

    NASA Astrophysics Data System (ADS)

    Alsaif, Manar; Jakob, Johannes; Andersen, Torgeir; Corfu, Fernando

    2015-04-01

    The Scandinavian Caledonides have been long studied, yet their ever unfolding complexity renders them far from being fully understood. It has been recognized that the Caledonian Allochthons have neither a linear nor straightforward along-strike relationship (Corfu et al. 2014). A mélange unit has been recently identified as a separate tectonic unit (Andersen et al. 2012). This unit is structurally positioned below crystalline nappes previously assigned to the Middle Allochthon. The mélange comprises meta-sediments and minor meta-basalt/gabbro, but most intriguingly, numerous solitary meta-peridotites. These occur as 'Alpine type' meta-peridotites, serpentinites, soapstones and detrital serpentinites. We present results of a field study of the mélange in the B\\overdalen area, structurally below the Jotun nappe, and suggest that this provides further evidence that the regional mélange unit was formed in a hyperextended passive margin. The meta-peridotites represent exhumed serpentinized mantle and are intimately associated with meta-sediments. The sediments are garnetiferous chlorite-muscovite schists, graphitic schists, phyllites, amphibolites, meta-sandstones as well as quartzite-pebble dominated conglomerates. It is suggested that this highly heterogeneous unit formed during the early stages of rifting and hyperextension along the Baltican passive margin. Characteristics of the detrital peridotites suggests that serpentinite-talc protrusions may have formed islands. The processes involved are observed on modern margins where the best-studied example is the Iberia-Newfoundland passive margin. Work in present-day margins (mostly seismic reflection data) elucidate the large-scale structure of hyperextended margins, while studies of ancient exposed examples in mountain belts provide insight into the lithology, geochemistry and details of these margins. The widespread distribution of hyperextended margins in modern margins and the increasing number of recognizable

  11. Long-term landscape evolution, cooling and exhumation history at the Moroccan passive continental margin, Western Anti-Atlas

    NASA Astrophysics Data System (ADS)

    Sehrt, Manuel; Glasmacher, Ulrich A.

    2014-05-01

    The ENE-trending Anti-Atlas of Morocco is located at the northwestern fringe of the West African Craton and south of the High Atlas and represents the Phanerozoic foreland of the Late Palaeozoic North African Variscides and the Cenozoic Atlas Belt. The Anti-Atlas mountain belt extends from the Atlantic Ocean over 500 km into the Moroccan interior and shows a rugged topography with elevations of about 2700 m. The exhumation of the Precambrian basement and the deformation and erosion of the Palaeozoic cover is mainly related to the Variscan orogeny in the Upper Carboniferous-Lower Permian. Subsequently, exhumation of the inliers occurred in the Triassic-Jurassic, as the Anti-Atlas formed the shoulder of the Atlantic rift and finally in the Upper Eocene-Pleistocene, contemporaneously with the uplift of the Atlas belt. In Morocco, a large amount of Mesozoic terrigenous sedimentary rocks are deposited in most of the basins along the continental margin indicating a major episode of erosion during the rift and early post-rift period in the Central Atlantic. In the Tarfaya-Laâyoune-Dakhla Basin, south of the Anti-Atlas, the sedimentary cover reach a thickness of up to 12 km. The presence of high surface elevations in the Anti-Atlas mountain belt indicates a potential source area for the surrounding basins. Currently, phases of exhumation in the Anti-Atlas during the Central Atlantic rifting and places where the associated erosion products are deposited are poorly constrained and there is little quantitative data available at present. The present study was focused on the thermal and exhumation history of the Western Anti-Atlas, the burial and inversion history of the Tarfaya-Laâyoune-Dakhla Basin and on provenance analysis of the Meso-Cenozoic sedimentary rocks in the basin. In order to characterize the t-T history, apatite and zircon fission-track dating, apatite and zircon (U-Th-Sm)/He dating and furthermore 2-D modelling with 'HeFTy' software has been carried out at

  12. From Cadomian arc to Ordovician passive margin: geochemical records preserved in metasedimentary successions of the Orlica-Śnieżnik Dome in SW Poland

    NASA Astrophysics Data System (ADS)

    Szczepański, Jacek; Ilnicki, Sławomir

    2014-04-01

    The chemical composition of metamorphosed siliciclastic rocks in the Orlica-Śnieżnik Dome (Bohemian Massif) identifies the main sources for the Neoproterozoic [the Młynowiec Formation (MF)], Early Cambrian [the Stronie Formation (SF)] and Late Cambrian/Early Ordovician [the Goszów quartzites (GQ)] sediments. The MF developed from erosion of a Cadomian magmatic arc along the northern Gondwana margin. The variegated SF, with supra-subduction affinities, shows chemical characteristics pointing to erosion of the freshly exhumed Cadomian orogen and detritus deposition in the back-arc basin. The very different chemical features of the GQ indicate deposition in a basin sited on a passive continental margin. The explanation proposed for the observed changes in chemical composition involves three main stages: (1) The pre ~540 Ma evolution of an active continental margin and related back-arc basin ceased with the collision and accretion of the magmatic arc to the Gondwana margin; (2) Early Cambrian rift to drift transition (540-500 Ma) and development of a depositional basin filled with detritus derived from remnants of the magmatic arc; (3) Peri-Gondwana break-up leading to the formation of shallow-water passive margin depositional basins filled with quartz-rich detritus resembling Early Ordovician Armorican quartzites known from other parts of the Variscan Belt.

  13. The habitat of petroleum in the Brazilian marginal and west African basins: A biological marker investigation

    SciTech Connect

    Mello, M.R.; Soldan, A.L. ); Maxwell, J.R. ); Figueira, J. )

    1990-05-01

    A geochemical and biological marker investigation of a variety of oils from offshore Brazil and west Africa, ranging in age from Lower Cretaceous to Tertiary, has been done, with the following aims: (1) assessing the depositional environment of source rocks, (2) correlating the reservoired oils, (3) comparing the Brazilian oils with their west African counterparts. The approach was based in stable isotope data; bulk, elemental, and hydrous pyrolysis results; and molecular studies involving quantitative geological marker investigations of alkanes using GC-MS and GC-MS-MS. The results reveal similarities between groups of oils from each side of the Atlantic and suggest an origin from source rocks deposited in five types of depositional environment: lacustrine fresh water, lacustrine saline water, marine evaporitic/carbonate, restricted marine anoxic, and marine deltaic. In west Africa, the Upper Cretaceous marine anoxic succession (Cenomanian-Santonian) appears to be a major oil producer, but in Brazil it is generally immature. The Brazilian offshore oils have arisen mainly from the pre-salt sequence, whereas the African oils show a balance between origins from the pre-salt and marine sequences. The integration of the geochemical and geological data indicate that new frontiers of hydrocarbon exploration in the west African basins must consider the Tertiary reservoirs in the offshore area of Niger Delta, the reservoirs of the rift sequences in the shallow-water areas of south Gabon, Congo, and Cuanza basins, and the reservoirs from the drift sequences (post-salt) in the deep-water areas of Gabon, Congo Cabinda, and Cuanza basins.

  14. Principles of Geological Mapping of Marine Sediments (with Special Reference to the African Continental Margin). Unesco Reports in Marine Science No. 37.

    ERIC Educational Resources Information Center

    Lisitzin, Alexandre P.

    Designed to serve as a complement to the Unesco Technical Papers in Marine Science, this report concentrates on theoretical and practical problems of geological mapping of the sea floor. An introduction is given to geological mapping procedures at continental margins as well as some practical recommendations taking as an example the African region…

  15. Thermal history, exhumation and long-term landscape evolution of the South Atlantic passive continental margin, Kaoko Belt, NW Namibia

    NASA Astrophysics Data System (ADS)

    Menges, Daniel; Glasmacher, Ulrich A.; Hackspacher, Peter C.; Schneider, Gabriele; Zentner, Henning; Karl, Markus

    2014-05-01

    After the Damara Orogeny at the end of the Neoproterozoic the Kaoko Belt in northwestern Namibia was affected by deep erosion of the Damara Sequence, followed by the depositon of the Karoo Supergroup from Permo-Carboniferous to Early Cretaceous. The lithostratigraphic units consist of Late Proterozoic to Cambrian metamorphosed rocks and intrusive complexes of the Damara Group, with ages of 534 (7) Ma to 481 (25) Ma (Miller 1983), that are unconformably overlain by terrestrial deposits of the Karoo Supergroup (Stollhofen 1999), comprising two flood basalt events: the Karoo flood basalts, at 183 (1) Ma (Duncan et al. 1997), and the Early Cretaceous Paraná-Etendeka flood basalts, at 132 (1) Ma (Renne et al. 1996). The latter marking the rift stage of the opening of the South Atlantic. The "passive" continental margin along the Kaoko Belt in northern Namibia is a perfect location to quantify exhumation and uplift rates, model the long-term landscape evolution and provide information about the major processes controlling the landscape evolution in this region. The poster/talk will present thermochronological data, t-T-models and exhumation rates for the Kaoko belt, NW Namibia. References Miller, R. M., 1983. Evolution of the Damara Orogen, Vol. 11, Geol. Soc., South Africa Spec. Pub.. Renne, P.R., Glen, J.M., Milner, S.C., Duncan, A.R., 1996. Age of Etendeka flood volcanism and associated intrusions in southwestern Africa, Geology 24 (7): 659- 662. Duncan, R., Hooper, P., Rehacek, J., March, J. and Duncan, A., 1997. The timing and duration of the Karoo igneous event, southern Gondwana, J. Geophys. Res. 102: 18127-18138. Stollhofen, H., 1999. Karoo Synrift-Sedimentation und ihre tektonische Kontrolle am entstehenden Kontinentalrand Namibias, Z.dt.geol.Ges. 149: 519-632.

  16. Can rifting evolution and passive margins architecture be driven by relative rheological heterogeneities? Insight from analogue modelling focused on South Atlantic margins.

    NASA Astrophysics Data System (ADS)

    Cappelletti, Alessio; Nestola, Yago; Tsikalas, Filippos; Salvi, Francesca; Argnani, Andrea; Cavozzi, Crisitan; Meda, Marco

    2016-04-01

    Crustal transect joined with lithospherical-scale analogue experiments are used to unreveal the evolution of the Central Segment of the South Atlantic margin. Specifically we analized the Santos and Campos basins along the Brazilian margin, where crustal inhomogeneities affects both rifting evolution and structural architecture of the conjugate margins. The results show that heterogeneities located within the lower crust can have a remarkable impact on the along-margin segmentation promoting focused and deeper basins related to a relatively "weak" rheology, and articulated basins with horsts and grabens in response to a relative "strong" rheology on the equivalent parts of the conjugate pairs. At the early-stage of rift evolution the deformation is concentrated at the proximal margin. At this stage, if a weak lower crust rheology heterogeneity exists, a main deep listric half-graben fault and associated thick and wedge shaped syn-rift basin sequences are developed; on the contrary, a strong lower crust rheology produce a more planar, rotated, domino-type faulted basins with thinner sequences directly controlled by the individual fault-blocks. At the late-stage rift evolution, once the effects of the initial crustal rheology inhomogeneities are reduced due to the lithosperic thinning process, the outer margin records a late syn-rift sequence which shows comparable thicknesses for both cases of lower crust rheologies. This tectono-stratigraphic evolution of the rifting process gives rise to along-margin alterations in symmetry versus asymmetry of the width and structural architecture. The presented models show that the tectono-stratigraphic evolution of rifting process can produces along margin switching of width and structural architecture. The change in architecture is due to the relative rheological contrast with respect to the surrounding in the lower crust. This produces a different, "relative", behavior for the lower crust if next to "weak" or to "strong

  17. Climatic changes along the northwestern African Continental Margin over the last 30 kyrs

    NASA Astrophysics Data System (ADS)

    Ternois, Yann; Sicre, Marie-Alexandrine; Paterne, Martine

    2000-01-01

    Two sediment cores were investigated to study the regional climatic variability of the NW African upwelling over the late Quaternary. Biomarker data and Sea Surface Temperatures (SSTs) predicted from alkenones at 25°N and 21°N exhibited distinct features. The amplitude of the last deglaciation was estimated to 4.5°C at 25°N and 2-2.5°C at 21°N. At 25°N, terrestrial and marine glacial inputs were higher than Holocene ones as a result of a strengthening of the trade winds and intensification of oceanic production. In contrast, at 21°N, warmer SSTs and lower organic carbon and alkenone productions during the last glacial suggest a regression of the upwelling and therefore a change of wind regime. Low glacial n-alkanols are consistent with the migration of the vegetation belt during the maximum of African aridity, while their decrease towards the core-top may be indicative of anthropogenic disturbances.

  18. The northern slope of South China Sea: an ideal site for studying passive margin extension and breakup

    NASA Astrophysics Data System (ADS)

    Zhou, D.; Sun, Z.; Pang, X.; Wu, X.; Xu, H.; Qiu, N.

    2011-12-01

    sags. Its complex shape differs from that of any other sag in the northern SCS, also differs from that of the lower slope of SE Atlantic margin and the Gulf of Mexico where thrust belts developed by gravitational sliding. Multi-staged magmatic activities have contributed to but could not fully explain the structural complexities of the LWS. Perhaps basement structures have played an important role as the sag might be developed upon the relict Mesozoic West Pacific subduction system. In addition, two horizons of deep-seated waving reflectors are identified beneath the LWS, which are suspected to be respectively a detachment surface and the intra-crustal shear zones related to lower-crust flow. A good understanding of these features may help answering the fundamental question on what controls the style, magnitude, and segmentation of passive margin extension and breakup, what is the mechanism, and what differences between marginal sea and open oceans in their evolution and dynamics. Preliminary attempt has been made taking into account basement structure, kinematics of bounding blocks, lithospheric rheology, lower crustal flow, as well as enhanced sediment supply by monsoon strengthening. This study is funded by CNSF40976033.

  19. Long-term landscape evolution of the South Atlantic passive continental margin along the Kaoko- and Damara Belts, NW-Namibia

    NASA Astrophysics Data System (ADS)

    Menges, Daniel; Glasmacher, Ulrich Anton; Hackspacher, Peter; Schneider, Gabriele; Salomon, Eric

    2015-04-01

    The Kaoko Belt in northwestern Namibia originates in the collision of the Rio de la Plata and Kongo Craton during the Pan-African Orogeny in the Neoproterozoic (1) and represents the northern arm of the Damara Orogen. NW-Namibias continental crust mainly consists of the NE-SW striking intracontinental branch of the Pan-African Damara mobile belt, which separates the Congo from the Kalahari craton. The Damara Orogen is divided into several tectonostratigraphic zones that are bounded by steeply dipping, ductile shear zones. These regional lineaments can be traced at least 150 km offshore (2). The lithostratigraphic units consist of Proterozoic and Cambrian metamorphosed rocks (534 (7) Ma - 481 (25) Ma (3) as well as Mesozoic sedimentary and igneous rocks. From Permo-Carboniferous to Mid Jurassic northern Namibia was affected by deep erosion of the Damara Orogen, Permo-Triassic collisional processes along the southern margin of Gondwana and eastern margin of Africa (4), and the deposition of the Nama Group sediments and the Karoo megasequence (5). Between the Otjihorongo and the Omaruru Lineament-Waterberg Thrust early Mesozoic tectonic activity is recorded by coarse clastic sediments deposited within NE trending half-graben structures. The Early Jurassic Karoo flood basalt lavas erupted rapidly at 183±1 Ma (6). The Early Cretaceous Paraná-Etendeka flood basalts (132±1 Ma) and mafic dike swarms mark the rift stage of the opening of the South Atlantic (7). Early Cretaceous alkaline intrusions (137-124 Ma) occur preferentially along Mesozoic half-graben structures and are called the Damaraland Igneous Province (8). Late Cretaceous alkaline intrusions and kimberlite pipes occur in northern Namibia. Post Early Paleocene siliciclastic sedimentation in Namibia was largely restricted to a 150 km wide zone (9) and is represented by the Tsondab Sandstone Formation (~ 300 m thickness). The oldest part has an age of early Paleocene and the upper part span from middle Miocene

  20. X-ray synchrotron diffraction study of natural gas hydrates from African margin

    NASA Astrophysics Data System (ADS)

    Bourry, Christophe; Charlou, Jean-Luc; Donval, Jean-Pierre; Brunelli, Michela; Focsa, Cristian; Chazallon, Bertrand

    2007-11-01

    Natural gas hydrates recovered from the Congo-Angola basin and Nigerian margins are analyzed by synchrotron X-ray powder diffraction. Biogenic methane is the most abundant gas trapped in the samples and others minor components (CO2, H2S) are co-clathrated in a type I cubic lattice structure. The refinement for the type I structure gives lattice parameters of a = 11.8646 (39) Å and a = 11.8619 (23) Å for specimens from Congo-Angola and Nigerian margins respectively at 90 K. These values, intermediate between the lattice constant of less pure methane specimens and pure artificial methane hydrates, indicate that lattice constants can be affected by the presence of encaged CO2, H2S and other gas molecules, even in small amounts. Thermal expansion is also presented for Congo-Angola hydrate in the temperature range 90-200 K. The coefficients are comparable with values reported for synthetic hydrates at low temperature and tend to approach thermal expansion of ice at higher temperature.

  1. 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

  2. Biogeography, biodiversity and fluid dependence of deep-sea cold-seep communities at active and passive margins

    NASA Astrophysics Data System (ADS)

    Sibuet, Myriam; Olu, Karine

    1998-01-01

    To date, several cold-seep areas which fuel chemosynthesis-based benthic communities have been explored, mainly by deployment of manned submersibles. They are located in the Atlantic and in the Eastern and Western Pacific oceans and in the Mediterranean Sea, in depths ranging between 400 and 6000 m in different geological contexts in passive and active margins. Our study is based on a review of the existent literature on 24 deep cold seeps. The geographic distribution of seeps, the variations of origin and composition of fluids, and rates of fluid flow are presented as they are important factors which explain the spatial heterogeneity and the biomass of biological communities. Methane-rich fluid of thermogenic and/or biogenic origin is the principal source of energy for high-productive communities; however, production of sulphide by sulphate reduction in the sediment also has a major role. The dominant seep species are large bivalves belonging to the families Vesicomyidae or Mytilidae. Other symbiont-containing species occur belonging to Solemyidae, Thyasiridae, Lucinidae bivalves, Pogonophora worms, Cladorhizidae and Hymedesmiidae sponges. Most of the symbiont-containing cold-seep species are new to science. Different symbiont-containing species rely on sulphide or methane oxidation, or both, via chemoautotrophic endosymbiotic bacteria. A total of 211 species, from which 64 are symbiont-containing species, have been inventoried. Patterns in biodiversity and biogeography are proposed. A large majority of the species are endemic to a seep area and the symbiont-containing species are mainly endemic to the cold-seep ecosystem. A comparison of species found in other deep chemosynthesis-based ecosystems, hydrothermal vents, whale carcass and shipwreck reduced habitats, reveals from the existing data, that only 13 species, of which five are symbiont-containing species occur, at both seeps and hydrothermal vents. The species richness of cold-seep communities decreases

  3. Cenozoic Tectonic Activity of the "Passive" North America Margin: Evidence for Cenozoic Activity on Mesozoic or Paleozoic Faults

    NASA Astrophysics Data System (ADS)

    Nedorub, O. I.; Knapp, C. C.

    2012-12-01

    The tectonic history of the Eastern North American Margin (ENAM) incorporates two cycles of continental assembly, multiple pulses of orogeny, rifting, and post-rift geodynamic evolution. This is reflected in the heterogeneous lithosphere of the ENAM which contains fault structures originated in Paleozoic to Mesozoic eras. The South Georgia Rift basin is probably the largest Mesozoic graben within its boundaries that is associated with the breakup of Pangea. It is composed of smaller sub-basins which appear to be bounded by high-angle normal faults, some of which may have been inverted in late Cretaceous and Cenozoic eras. Paleozoic structures may have been reactivated in Cenozoic time as well. The ENAM is characterized by N-NE maximum horizontal compressive stress direction. This maximum compressional stress field is sub-parallel to the strike of the Atlantic Coast province fault systems. Camden, Augusta, Allendale, and Pen Branch faults are four of the many such reactivated faults along the southern part of ENAM. These faults are now buried under the 0-400 m of loosely consolidated Cretaceous and Cenozoic age sediments and thus are either only partially mapped or currently not recognized. Some of the objectives of this study are to map the subsurface expression and geometry of these faults and to investigate the post Cretaceous deformation and possible causes of fault reactivation on a passive margin. This study employs an integrated geophysical approach to investigate the upper 200 m of identified locations of the above mentioned faults. 2-D high-resolution shallow seismic reflection and refraction methods, gravity surveys, GPR, 2-D electrical resistivity and well data are used for analyses and interpretation. Preliminary results suggest that Camden fault shows signs of Cenozoic reactivation through an approximately 30 m offset NW side up mainly along a steeply dipping fault zone in the basal contact of Coastal Plain sediments with the Carolina Piedmont. Drill

  4. The Lamu Basin deepwater fold-and-thrust belt: An example of a margin-scale, gravity-driven thrust belt along the continental passive margin of East Africa

    NASA Astrophysics Data System (ADS)

    Cruciani, Francesco; Barchi, Massimiliano R.

    2016-03-01

    In recent decades, advances in seismic processing and acquisition of new data sets have revealed the presence of many deepwater fold-and-thrust belts (DW-FTBs), often developing along continental passive margins. These kinds of tectonic features have been intensively studied, due to their substantial interest. This work presents a regional-scale study of the poorly explored Lamu Basin DW-FTB, a margin-scale, gravity-driven system extending for more than 450 km along the continental passive margin of Kenya and southern Somalia (East Africa). A 2-D seismic data set was analyzed, consisting of both recently acquired high-quality data and old reprocessed seismic profiles, for the first detailed structural and stratigraphic interpretation of this DW-FTB. The system originated over an Early to mid-Cretaceous shale detachment due to a mainly gravity-spreading mechanism. Analysis of synkinematic strata indicates that the DW-FTB was active from the Late Cretaceous to the Early Miocene, but almost all of the deformation occurred before the Late Paleocene. The fold-and-thrust system displays a marked N-S variation in width, the northern portion being more than 150 km wide and the southern portion only a few dozen kilometers wide; this along-strike variation is thought to be related to the complex tectonosedimentary evolution of the continental margin at the Somalia-Kenya boundary, also reflected in the present-day bathymetry. Locally, a series of volcanic edifices stopped the basinward propagation of the DW-FTB. A landward change in the dominant structural style, from asymmetric imbricate thrust sheets to pseudo-symmetric detachment folds, is generally observed, related to the landward thickening of the detached shales.

  5. Beyond Passivity: Constructions of Femininities in a Single-Sex South African School

    ERIC Educational Resources Information Center

    Bhana, Deevia; Pillay, Nalini

    2011-01-01

    In the context of the calamitous effects of gender violence on the experience of schooling for South African girls, single-sex schools have been advanced as a strategy to protect girls from violence. In this paper, the experiences of a selected group of girls in a single-sex school in Durban, South Africa are illustrated to provide a counter…

  6. The great West African Tertiary coastal uplift: Fact or fiction? A perspective from the Angolan divergent margin

    NASA Astrophysics Data System (ADS)

    Jackson, M. P. A.; Hudec, M. R.; Hegarty, K. A.

    2005-12-01

    We explore exhumation in the coastal Kwanza Basin by combining analyses of Tertiary hiatuses and apatite fission tracks. Planktonic biozones show five major hiatuses in the Oligo-Miocene and Plio-Pleistocene. Between gaps, Oligo-Miocene strata accumulated under marine conditions. A marine setting refutes the idea of a massively raised coastal plateau in the mid-Tertiary. Marine conditions continued until ˜5 Ma. Fission track data suggest three thermal events: ˜150 Ma, during rifting and volcanism; ˜100-70 Ma, during shortening and volcanism; and ˜20-10 Ma, during exhumation. Tertiary uplift was spatially highly variable. For the Kwanza Basin, we infer that Tertiary uplift on the West African margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas. Massive uplift (2000-4000 m) of the Precambrian craton had little structural effect in the outer basin. Instead, minor uplifts on the shelf drove late Tertiary deformation on the slope.

  7. Bacterial diversity and biogeochemistry of different chemosynthetic habitats of the REGAB cold seep (West African margin, 3160 m water depth)

    NASA Astrophysics Data System (ADS)

    Pop Ristova, P.; Wenzhöfer, F.; Ramette, A.; Zabel, M.; Fischer, D.; Kasten, S.; Boetius, A.

    2012-07-01

    The giant pockmark REGAB (West African margin, 3160 m water depth) is an active methane-emitting cold seep ecosystem, where the energy derived from microbially mediated oxidation of methane supports high biomass and diversity of chemosynthetic communities. Bare sediments interspersed with heterogeneous chemosynthetic assemblages of mytilid mussels, vesicomyid clams and siboglinid tubeworms form a complex seep ecosystem. To better understand if benthic bacterial communities reflect the patchy distribution of chemosynthetic fauna, all major chemosynthetic habitats at REGAB were investigated using an interdisciplinary approach combining porewater geochemistry, in situ quantification of fluxes and consumption of methane, as well bacterial community fingerprinting. This study revealed that sediments populated by different fauna assemblages show distinct biogeochemical activities and are associated with distinct sediment bacterial communities. The methane consumption and methane effluxes ranged over one to two orders of magnitude across habitats, and reached highest values at the mussel habitat, which hosted a different bacterial community compared to the other habitats. Clam assemblages had a profound impact on the sediment geochemistry, but less so on the bacterial community structure. Moreover, all clam assemblages at REGAB were restricted to sediments characterized by complete methane consumption in the seafloor, and intermediate biogeochemical activity. Overall, variations in the sediment geochemistry were reflected in the distribution of both fauna and microbial communities; and were mostly determined by methane flux.

  8. Bacterial diversity and biogeochemistry of different chemosynthetic habitats of the REGAB cold seep (West African margin, 3160 m water depth)

    NASA Astrophysics Data System (ADS)

    Pop Ristova, P.; Wenzhöfer, F.; Ramette, A.; Zabel, M.; Fischer, D.; Kasten, S.; Boetius, A.

    2012-12-01

    The giant pockmark REGAB (West African margin, 3160 m water depth) is an active methane-emitting cold seep ecosystem, where the energy derived from microbially mediated oxidation of methane supports high biomass and diversity of chemosynthetic communities. Bare sediments interspersed with heterogeneous chemosynthetic assemblages of mytilid mussels, vesicomyid clams and siboglinid tubeworms form a complex seep ecosystem. To better understand if benthic bacterial communities reflect the patchy distribution of chemosynthetic fauna, all major chemosynthetic habitats at REGAB were investigated using an interdisciplinary approach combining pore water geochemistry, in situ quantification of fluxes and consumption of methane, as well as bacterial community fingerprinting. This study revealed that sediments populated by different fauna assemblages show distinct biogeochemical activities and are associated with distinct sediment bacterial communities. The methane consumption rates and methane effluxes ranged over one to two orders of magnitude across habitats, and reached highest values at the mussel habitat, which hosted a different bacterial community compared to the other habitats. Clam assemblages had a profound impact on the sediment geochemistry, but less so on the bacterial community structure. Moreover, all clam assemblages at REGAB were restricted to sediments characterized by complete methane consumption in the seafloor, and intermediate biogeochemical activity. Overall, variations in the sediment geochemistry were reflected in the distribution of both fauna and microbial communities; and were mostly determined by methane flux.

  9. Basin Dynamics and Sedimentary Infilling of Miocene Sandstone Reservoir Systems In Eastern Tunisian African Margin

    NASA Astrophysics Data System (ADS)

    Bédir, Mourad; Khomsi, Sami

    2015-04-01

    Most of hydrocarbon accumulations and aquifers within the Cap Bon, Gulf of Hammamet and Sahel basins in eastern tunisian foreland are reservoired within the Upper Miocene Birsa and Saouaf sandstones and shales Formations. In the gulf of Hammamet, these sandstones constitutes oil and gas fields and are exploited on anticline highs and described as varying from shoreface to shallow marine and typically exhibit excellent reservoir quality of 30% to 35% porosity and good permeability from 500 to 1100 md. In addition, the fracturing of faults enhanced the reservoir quality potential. In contrary, the same hydrocarbon reservoirs are important hydrogeologic ones in the Cap Bon and Sahel basins with huge amount of hundred millions of cubic meters of water only partially exploited. Integrated wire line logging correlations, seismic sequence stratigraphic, tectonics and outcrop geologic analogue studies had permitted to highlight the basin structuring and sedimentary environments of sequence deposits infilling of the reservoir distribution between high platforms to subsiding graben and syncline basins bounded by deep-seated transtensive and transpressive flower faults. Seven third order sequence deposits limited by downlap prograding and onlap/toplap aggrading/retrograding system tracts extend along the eastern margin around the three basins by facies and thickness variances. System tracts exhibit around high horst and graben a channelized and levee infillings extending from 100 meters to more than a kilometer of width. They present a stacked single story and multistory channels types showing space lateral and vertical migrations along NE-SW, E-W and N-S directions. Paleogeographic depositional reservoir fair maps distribution highlight deltaic horst domain with floodplain and incised valley of fluvial amalgamed and braided sandstones distributary channels that occupy the high folded horsts. Whereas folded horst-graben and syncline borders domain of Shelf prodelta are

  10. Long-term subsidence, cooling, and exhumation history along the South Atlantic passive continental margin in NW-Namibia

    NASA Astrophysics Data System (ADS)

    Menges, Daniel; Glasmacher, Ulrich Anton; Salomon, Eric; Hackspacher, Peter Christian; Schneider, Gabi

    2016-04-01

    In northwest Namibia the Kaoko Belt is one of the most important Precambrian crustal segments that have stored the subsidence, cooling, and exhumation history of Namibia since the Neoproterozoic. ZFT-ages are processed to give new insights on this early evolution. Paleozoic to Mesozoic sedimentary rocks of the Karoo Supergroup and the Lower Cretaceous volcanic rocks of the Etendeka sequence overlay the Proterozoic metamorphic and intrusive rocks (1). New apatite fission-track (AFT) ages range from 390.9 (17.9) Ma to 80.8 (6.0) Ma. Along the coast apatites of Proterozoic rock samples reveal the youngest ages. Further inland the ages increase significantly. In addition, rapid change of AFT-ages occurs on both sides of major thrust and shear zones. Using the oldest thermochronological data the revealed t-T paths indicate a long era of exhumation, starting at the end of the Pan-African Orogeny in the Neoproterozoic and continuing into the Permo-Carboniferous. The subsequent sedimentation of the Karoo Supergroup initiates a new era of subsidence until the end of Triassic (2). The subsequent period of denudation ends abruptly with the rapid deposition of the Etendeka basalts in the Early Cretaceous (3). The maximum thickness of the Etendeka volcanic suite has been estimated, using the apatite fission-track data, to about 3.2 (1.2) km. With the ongoing opening of the South Atlantic and the formation of the continental margin the Kaoko Belt went through a rapid cooling event starting ~ 130 Ma and ending ~ 80 Ma, at a mean rate of 0.034 km/Ma for the western, and 0.018 km/Ma for the northern and eastern Kaoko Belt. This cooling event was accompanied by a reactivation of major fault zones, like the Purros Mylonite Zone (4). Thereafter, stable conditions were established, with denudation rates generally lower than 0.010 km/Ma, until the Neogene, where a second cooling event led to increased exhumation rates around 0.042 km/Ma. The total amount of denudation in the last 130 Ma

  11. Heterogeneous Cenozoic cooling of central Britain: insights into the complex evolution of the North Atlantic passive margin

    NASA Astrophysics Data System (ADS)

    Łuszczak, Katarzyna; Persano, Cristina; Stuart, Finlay

    2015-04-01

    The western flank of the North Atlantic passive margin has experienced multiple episodes of rock uplift and denudation during the Cenozoic that have been locally variable in scale. Two regional scale exhumation events have been identified: early Palaeogene and Neogene [see 1 for review]. The former has been identified both onshore and offshore and it appears to be temporally coincident with basaltic magmatism related to the arrival of the proto-Iceland mantle plume beneath thinned continental lithosphere, which may have cause long wavelength, low amplitude dynamic uplift. Quantifying the amount of early Palaeogene exhumation using mineral thermochronometers may be complicated by elevated heat flow. The magnitude and timing of exhumation during the Neogene is even less clear, as is the driving mechanism. Quantifying the amount of early Palaeogene exhumation, determining the precise timing as well as the amount of uplift and erosion in the Neogene, require detailed application of low temperature thermochronometers. Here we present the first multiple low temperature thermochronometer study from S Scotland, N England and N Wales. New apatite fission track (AFT) data are integrated with apatite and zircon (U-Th-Sm)/He (AHe and ZHe, respectively) ages to establish regional rock cooling history from 200°C to 30°C. To precisely constrain the early Palaeogene cooling history, and to better define the possible Neogene cooling event, >20 single grain AHe ages have been produced on key samples and modelled using the newly codified HelFrag technique. The new AFT and AHe ages confirm earlier studies that show the Lake District and North Pennines experienced rapid cooling from >120°C in the Palaeogene. The amount of cooling/exhumation gradually decreases northwards into S Scotland and southwards in N Wales; there is no evidence for the rapid Palaeogene event in areas ~70 km from the Lake District centre. Inverse modelling of the AHe and AFT data suggest that the rapid cooling

  12. Crustal structure of the Southeast Georgia embayment-Carolina trough: Preliminary results of a composite seismic image of a continental suture ( ) and a volcanic passive margin

    SciTech Connect

    Austin, J.A. Jr.; Stoffa, P.L.; Phillips, J.D. ); Oh, Jinyong ); Sawyer, D.S. ); Purdy, G.M.; Reiter, E. ); Makris, J. )

    1990-10-01

    New deep-penetration multichannel seismic reflection data, combined with refraction results and magnetics modeling, support a hypothesis that the Carolina trough is a Mesozoic volcanic passive margin exhibiting a seaward-dipping wedge and associated underplating. The structure of Carolina platform continental crust is consistent with the late Paleozoic continental collision that produced the Appalachians, but imbrication has had no obvious effect on shallower structures produced by Mesozoic extension and volcanism. The origin of prominent magnetic anomalies crossing the Southeast Georgia embayment can be explained by processes attending Mesozoic separation of Africa and North America, and is not related to a Paleozoic continental suture, as previously postulated.

  13. Rates, causes, and dynamic of long-term landscape evolution of the South Atlantic "passive continental margin", Brazil and Namibia, as revealed by thermo-kinematic numerical modeling.

    NASA Astrophysics Data System (ADS)

    Christian, Stippich; Anton, Glasmacher Ulrich; Peter, Christian, Hackspacher

    2014-05-01

    The aim of the research is to quantify the long-term landscape evolution of the South Atlantic passive continental margin (SAPCM) in SE-Brazil and NW-Namibia. Excellent onshore outcrop conditions and complete rift to post-rift archives between Sao Paulo and Porto Alegre and in the transition from Namibia to Angola (onshore Walvis ridge) allow a high precision quantification of exhumation, and uplift rates, influencing physical parameters, long-term acting forces, and process-response systems. Research will integrate the published and partly published thermochronological data from Brazil and Namibia, and test lately published new concepts on causes of long-term landscape evolution at rifted margins. The climate-continental margin-mantle coupled process-response system is caused by the interaction between endogenous and exogenous forces, which are related to the mantle-process driven rift - drift - passive continental margin evolution of the South Atlantic, and the climate change since the Early/Late Cretaceous climate maximum. Special emphasis will be given to the influence of long-living transform faults such as the Florianopolis Fracture Zone (FFZ) on the long-term topography evolution of the SAPCM's. A long-term landscape evolution model with process rates will be achieved by thermo-kinematic 3-D modeling (software code PECUBE and FastCape). Testing model solutions obtained for a multidimensional parameter space against the real thermochronological and geomorphological data set, the most likely combinations of parameter rates, and values can be constrained. The data and models will allow separating the exogenous and endogenous forces and their process rates.

  14. Deep crustal structure of the North-West African margin from combined wide-angle and reflection seismic data (MIRROR seismic survey)

    NASA Astrophysics Data System (ADS)

    Biari, Y.; Klingelhoefer, F.; Sahabi, M.; Aslanian, D.; Schnurle, P.; Berglar, K.; Moulin, M.; Mehdi, K.; Graindorge, D.; Evain, M.; Benabdellouahed, M.; Reichert, C.

    2015-08-01

    The structure of the Moroccan and Nova Scotia conjugate rifted margins is of key importance for understanding the Mesozoic break-up and evolution of the northern central Atlantic Ocean basin. Seven combined multichannel reflection (MCS) and wide-angle seismic (OBS) data profiles were acquired along the Atlantic Moroccan margin between the latitudes of 31.5° and 33° N during the MIRROR seismic survey in 2011, in order to image the transition from continental to oceanic crust, to study the variation in crustal structure, and to characterize the crust under the West African Coast Magnetic Anomaly (WACMA). The data were modeled using a forward modeling approach. The final models image crustal thinning from 36 km thickness below the continent to approximately 8 km in the oceanic domain. A 100 km wide zone characterized by rough basement topography and high seismic velocities up to 7.4 km/s in the lower crust is observed westward of the West African Coast Magnetic Anomaly. No basin underlain by continental crust has been imaged in this region, as has been identified north of our study area. Comparison to the conjugate Nova Scotian margin shows a similar continental crustal thickness and layer geometry, and the existence of exhumed and serpentinized upper mantle material on the Canadian side only. The oceanic crustal thickness is lower on the Canadian margin.

  15. Tectonic development of passive continental margins of the southern and central Red Sea with a comparison to Wilkes Land, Antarctica

    USGS Publications Warehouse

    Bohannon, R.G.; Eittreim, S.L.

    1991-01-01

    The continental margins of the southern and central Red Sea and most of Wilkes Land, Antarctica have bulk crustal configurations and detailed structures that are best explained by a prolonged history of magmatic expansion that followed a brief, but intense period of mechanical extension. Extension on the Red Sea margins was spatially confined to a rift that was 20-30 km in width. The rifting phase along the Arabian margin of the central and southern Red Sea occurred 25-32 Ma ago, primarily by detachment faulting at upper crustal levels and ductile uniform stretching at depth. Rifting was followed by an early magmatic phase during which the margin was invaded by dikes and plutons, primarily of gabbro and diorite, at 20-24 Ma, after the crust was mechanically thinned from 40 km to ??? 20 km. We infer continued spreading after that in which broad shelves were formed by a process of magmatic expansion, because the offshore crust is only 8-15 km thick, including sediment, and seismic reflection data do not depict horst and graben or half graben structures from which mechanical extension might be inferred. The Wilkes Land margin is similar to the Arabian example. The margin is about 150 km in width, the amount of upper crustal extension is too low to explain the change in sub-sediment crustal thickness from ??? 35 km on the mainland to < 10 km beneath the margin and reflectors in the deepest seismic sequence are nearly flat lying. Our model requires large volumes of melt in the early stages of continental rifting. The voluminous melt might be partly a product of nearby hot spots, such as Afar and partly the result of an initial period of partial fusion in the deep continental lithosphere under lower temperatures than ordinarily required by dry solidus conditions. ?? 1991.

  16. Comparative analysis of the Late Cretaceous to Recent post-breakup basin evolution of the South-American and South-African margin of the southern Atlantic

    NASA Astrophysics Data System (ADS)

    Kukla, Peter; Back, Stefan

    2010-05-01

    Recently, considerable attempts have been made to compare the sedimentary basin evolution and the associated tectonic framework on both sides of the South-Atlantic (e.g. Mohriak et al., 2008, and references therein). Yet there are still unresolved questions. Amongst the most striking observations is that multiple phases of volcanism, uplift and subsidence are recorded after the break-up of the southern South Atlantic margin segment on both sides of the Florianopolis - Walvis Ridge volcanic complex, features that are regarded as atypical when compared to published examples of other post-breakup continental margin successions. However, the tectono-sedimentary and magmatic styles markedly differ from south to north across the volcanic complex. In seismic reflection data, voluminous extrusives are manifested by the occurrence of large wedges of seaward dipping reflector sequences south of the volcanic complex, whilst large volumes of Cretaceous mafic alkaline rocks only occur north of the Florianopolis - Walvis Ridge complex. It can be expected that these differences are of a broad importance for the understanding of both break-up and post-breakup processes. This presentation focuses on a comparison of the post-breakup stratigraphic development of the South American and South African continental margins that both record thick post-rift sedimentary successions. Basins along the southern African margin are much narrower in comparison to their South American counterparts, constituting a pronounced margin asymmetry across the Atlantic. Adding to the heterogeneity of the system, the northernmost segment of the South Atlantic rift and salt basins is also characterized by a pronounced asymmetry, with the Brazilian margin now comprising narrower and deeper rift basins with less salt than the Congo-Gabon conjugate margin. In general, it seems that in the salt-prone areas both offshore South America and southern Africa, salt-related tectonics are amongst the key parameters

  17. Re-exploration of cratonic basins using passive-margin sequence-stratigraphic concepts: examples from upper Paleozoic rocks, eastern margin, Midland basin

    SciTech Connect

    Brown, L.F. Jr.

    1989-03-01

    Use of 5000 well logs and extensive outcrop information with a 22,000-mi/sup 2/ test region on the eastern margin of the Midland basin permitted delineation of 16 probably third-order type 1 depositional sequences. Sandstone-isolith maps of siliciclastic highstand and lowstand systems tracts show that most structural traps produce from highstand fluvial-deltaic reservoirs, but most stratigraphic traps discovered to date occur within lowstand depositional systems, principally incised valley fills and basin-floor fans. Hydrocarbons are rarely trapped in retrogradational (transgressive) systems tracts. Maps of lowstand tracts refocus attention on reservoirs that can be predicted to exist basinward of preexisting shelf edges. A basinward shift of exploration emphasis from incised valley-fill reservoirs to other lowstand elements - such as basin-floor fans, canyon and leveed-channel fills, and lowstand progradational deltaic wedges - could lead to plays where lenticular reservoir sandstones and marine-condensed source and seal shales exhibit the optimum conditions for pinch-out traps.

  18. Source to sink study of non-cylindrical rifted passive margins: the case of the Gulf of Guinea

    NASA Astrophysics Data System (ADS)

    Chardon, Dominique; Rouby, Delphine; Robin, Cécile; Calves, Gérome; Grimaud, Jean-louis; Guillocheau, François; Beauvais, Anicet; Braun, Jean

    2013-04-01

    The aim of our project is to analyze quantitatively the post-rift evolution of a transform margin in order to determine how the spatially complex rifting processes that produced a tridimensional stretching of the lithosphere might impact the post rift evolution of the margin and the associated sedimentary systems. More specifically, we investigate its impact on vertical motion (uplift/subsidence), sediment transfer (erosion/accumulation) and stratigraphic architecture of sedimentary basins. We also intend to characterize the stratigraphic signature of independent geodynamic processes potentially affecting the margin during the post rift phase such as mantle dynamic, change in climate and erosion processes (chemical vs mechanical erosion). In this framework, the Atlantic margin from the Senegal to the Niger Delta is an ideal case study for which we compiled a unique dataset constraining over the Cenozoic: (i) the paleodrainage evolution and the denudation history for the whole area contributing to the sedimentary basins, and (ii) the accumulation history of the latter. From the reconstruction of the 3D geometry of paleo-alteration land surfaces, we show a complete reorganization of the drainage between 45 and 25 Myr. It resulted from the capture by the Niger of a formerly endoreic drainage isolated from the margin by a marginal bulge, as well as, by the incision and downwarp of this bulge by coastal drainage such as the Volta River. This relief had therefore a major impact on the export of sediment to the basins during the Cenozoic and both geomorphologic study and numerical modeling of the 3D flexure of this margin suggest it might be inherited from the rifting phase. Also, we compiled 13 geological sections along the margin to evaluate the accumulation histories of 3 domains: the Senegal basin, the Niger Delta and the Northern Margin of the Gulf of Guinea. All basins showed an acceleration in accumulation rates between 45 and 25 Myr. The 3D numerical modeling of

  19. Geochemical and tectonic relationships in the east Indonesian arc-continent collision region: Implications for the subduction of the Australian passive margin

    NASA Astrophysics Data System (ADS)

    van Bergen, M. J.; Vroon, P. Z.; Hoogewerff, J. A.

    1993-07-01

    Van Bergen, M.J., Vroon, P.Z. and Hoogewerff, J.A., 1993. Geochemical and tectonic relationships in the east Indonesian arc-continent collision region: implications for the subduction of the Australian passive margin. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near The Earth's Surface. Tectonophysics, 223: 97-116. Variations in the isotopic signatures of volcanics along the East Sunda Banda Arc reflect changes in the nature and amount of sedimentary material supplied by the northeast Indian Ocean floor and the adjacent Australian passive continental margin, which form the two major domains of the Indian Ocean plate that approach the arc system. A compilation of isotopic data for 200-500-km-long arc sectors shows that the trend in magmatic signatures follows distinct subduction/collision stages reached by the corresponding oceanic and continental-margin sections entering the trench system. Maximum amounts of magma source contamination are inferred for volcanics near an extinct sector north of Timor, where the Australian continent started to collide with the arc first. Pb-Nd isotopic source mixing models point to contamination by sediments with variations in composition, similar to observed along-arc changes in sediments entering the trench. The results indicate an increasing contribution of subducted continental material in the direction of the collision region. Mass-balance calculations, considering the magmatic output and minimum input of subducted continental material required to generate the composition of the volcanic arc in the collision region, are difficult to reconcile with subduction of ocean-floor sediments alone. Thicknesses of sediments presently covering oceanic crust near the margin are close to calculated thicknesses of the sediments fluxed into the trench and magmatically returned to the arc crust, but cannot account for the additional volumes of material accreted on

  20. Apatite fission track dating and long-term landscape evolution of the South Atlantic passive continental margin in the region of the Sierras Septentrionales in eastern Argentina

    NASA Astrophysics Data System (ADS)

    Pfister, S.; Glasmacher, P. A.; Kollenz, S.

    2013-12-01

    To understand the evolution of the passive continental margin in Argentina apatite fission track dating is an appropriate method, which will lead to new conclusions in this area. The Tandilia System, also called Sierras Septentrionales, is located south of the Río de la Plato Craton in eastern Argentina in the state of Buenos Aires. North of the hills Salado basin is orientated whereas the Claromeó basin is located south of the mountain range. In contrary to most basins along the southamerican passive continental margin the Tandilia-System and the neighbouring basins trend perpendicular to the coast line. The topography ranges between 50 and 250m within the study area and is therefore fairly flat. The igneous-metamorphic basement is pre-proterozoic in age build up of mainly granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks and granitoid plutons and is overlain by a series of Neoproterozoic to early Paleozoic sediments (Cingolani, 2010). The aim of the study is to evaluate the long-term landscape evolution of the passive continental margin in eastern Argentina in terms of thermal history and exhumation. For that purpose samples were taken from the Sierra Septentrionales basement analyzed for the apatite-FT method. The results so far indicate apatite fission track ages between 146.2 (10.1) Ma and 200.4 (12.7) Ma, which shows all samples have been reseted. Still ongoing length measurements will lead to 2D thermo kinematic Hefty (Ketcham, 2005; Ketcham et al., 2009; Ketcham, 2007) models. This will leads to further more insights on the cooling history and tectonic activities in the research area. References: Cingolani C. A. (2010): The Tandilia System of Argentina as a southern extension of the Río de la Plata craton: an overview. Int. J. Earth Sci. (Geol. Rundsch.) (2011) 100:221-242, doi 10.1007/s00531-010-0611-5. Ketcham, R. A. (2005): Forward and inverse modeling of low-temperature thermochronometry data, in Low

  1. Lower crustal high-velocity bodies along North Atlantic passive margins, and their link to Caledonian suture zone eclogites and Early Cenozoic magmatism

    NASA Astrophysics Data System (ADS)

    Mjelde, Rolf; Kvarven, Trond; Faleide, Jan Inge; Thybo, Hans

    2016-02-01

    In this study we use crustal-scale Ocean Bottom Seismic models to infer the presence of two types of lower crustal bodies at North Atlantic passive margins; Type I, primarily interpreted as Early Eocene magmatic intrusions, and Type II, interpreted as Caledonian eclogites. We discuss how these eclogites might be related to the main Caledonian Suture Zone and other tectonic features in a conjugate North Atlantic setting. Based on the first-order approximation that P-wave velocities can be related to rock strength, the narrower continental margin at the southern (Møre) transect may be explained by stronger lower crust there, compared with the northern (Vøring) transect. This difference in strength, possibly resulting in a steeper dip in the subducting Baltica Plate south of the proto-Jan Mayen Lineament, may explain the asymmetry in extensional style observed across this lineament. Our interpretation locates the main suture off mid-Norway close to the Møre Trøndelag Fault Zone on the Møre Margin, along the western boundary of the Trøndelag Platform on the Vøring Margin, and further northwards beneath the Lofoten Ridge. The Lower Crustal Body Type I is about 60% thicker on the Greenland side, for both transects, and its thickness along the northern transect is more than twice that of the southern transect. These differences are consistent with sub-lithospheric interaction between the Icelandic hotspot and the continental rift/oceanic accretion system around the time of continental break-up.

  2. Kinematics of a growth fault/raft system on the West African margin using 3-D restoration

    NASA Astrophysics Data System (ADS)

    Rouby, Delphine; Raillard, Stéphane; Guillocheau, François; Bouroullec, Renaud; Nalpas, Thierry

    2002-04-01

    The ability to quantify the movement history associated with growth structures is crucial in the understanding of fundamental processes such as the growth of folds or faults in 3-D. In this paper, we present an application of an original approach to restore in 3-D a listric growth fault system resulting from gravity-induced extension located on the West African margin. Our goal is to establish the 3-D structural framework and kinematics of the study area. We construct a 3-D geometrical model of the fault system (from 3-D seismic data), then restore six stratigraphic surfaces and reconstruct the 3-D geometry of the system at six incremental steps of its history. The evolution of the growth fault/raft system corresponds to the progressive separation of two rafts by regional extension, resulting in the development of an intervening basin located between them that evolved in three main stages: (1) the rise of an evaporite wall, (2) the development of a symmetric basin as the elevation of the diapir is reduced and buried, and (3) the development of asymmetric basins related to two systems of listric faults (the main fault F1 and the graben located between the rollovers and the lower raft). Important features of the growth fault/raft system could only be observed in 3-D and with increments of deformation restored. The rollover anticline (associated with the listric fault F1) is composed of two sub-units separated by an E-W oriented transverse graben indicating that the displacement field was divergent in map view. The rollover units are located within the overlap area of two fault systems and displays a 'mock-turtle' anticline structure. The seaward translation of the lower raft is associated with two successive vertical axis rotations in the opposite sense (clockwise then counter-clockwise by about 10°). This results from the fact that the two main fault systems developed successively. Fault system F1 formed during the Upper Albian, and the graben during the Cenomanian

  3. Revealing the long-term landscape evolution of the South Atlantic passive continental margin, Brazil and Namibia, by thermokinematic numerical modeling using the software code Pecube.

    NASA Astrophysics Data System (ADS)

    Stippich, Christian; Glasmacher, Ulrich Anton; Hackspacher, Peter

    2015-04-01

    The aim of the research is to quantify the long-term landscape evolution of the South Atlantic passive continental margin (SAPCM) in SE-Brazil and NW-Namibia. Excellent onshore outcrop conditions and complete rift to post-rift archives between Sao Paulo and Porto Alegre and in the transition from Namibia to Angola (onshore Walvis ridge) allow a high precision quantification of exhumation, and uplift rates, influencing physical parameters, long-term acting forces, and process-response systems. Research will integrate the published and partly published thermochronological data from Brazil and Namibia, and test lately published new concepts on causes of long-term landscape evolution at rifted margins. The climate-continental margin-mantle coupled process-response system is caused by the interaction between endogenous and exogenous forces, which are related to the mantle-process driven rift - drift - passive continental margin evolution of the South Atlantic, and the climate change since the Early/Late Cretaceous climate maximum. Special emphasis will be given to the influence of long-living transform faults such as the Florianopolis Fracture Zone (FFZ) on the long-term topography evolution of the SAPCM's. A long-term landscape evolution model with process rates will be achieved by thermo-kinematic 3-D modeling (software code PECUBE1,2 and FastScape3). Testing model solutions obtained for a multidimensional parameter space against the real thermochronological and geomorphological data set, the most likely combinations of parameter rates, and values can be constrained. The data and models will allow separating the exogenous and endogenous forces and their process rates. References 1. Braun, J., 2003. Pecube: A new finite element code to solve the 3D heat transport equation including the effects of a time-varying, finite amplitude surface topography. Computers and Geosciences, v.29, pp.787-794. 2. Braun, J., van der Beek, P., Valla, P., Robert, X., Herman, F., Goltzbacj

  4. Regional seismic stratigraphy and controls on the Quaternary evolution of the Cape Hatteras region of the Atlantic passive margin, USA

    USGS Publications Warehouse

    Mallinson, D.J.; Culver, S.J.; Riggs, S.R.; Thieler, E.R.; Foster, D.; Wehmiller, J.; Farrell, K.M.; Pierson, J.

    2010-01-01

    Seismic and core data, combined with amino acid racemization and strontium-isotope age data, enable the definition of the Quaternary stratigraphic framework and recognition of geologic controls on the development of the modern coastal system of North Carolina, U.S.A. Seven regionally continuous high amplitude reflections are defined which bound six seismic stratigraphic units consisting of multiple regionally discontinuous depositional sequences and parasequence sets, and enable an understanding of the evolution of this margin. Data reveal the progressive eastward progradation and aggradation of the Quaternary shelf. The early Pleistocene inner shelf occurs at a depth of ca. 20-40 m beneath the western part of the modern estuarine system (Pamlico Sound). A mid- to outer shelf lowstand terrace (also early Pleistocene) with shelf sand ridge deposits comprising parasequence sets within a transgressive systems tract, occurs at a deeper level (ca. 45-70 m) beneath the modern barrier island system (the Outer Banks) and northern Pamlico Sound. Seismic and foraminiferal paleoenvironmental data from cores indicate the occurrence of lowstand strandplain shoreline deposits on the early to middle Pleistocene shelf. Middle to late Pleistocene deposits occur above a prominent unconformity and marine flooding surface that truncates underlying units, and contain numerous filled fluvial valleys that are incised into the early and middle Pleistocene deposits. The stratigraphic framework suggests margin progradation and aggradation modified by an increase in the magnitude of sea-level fluctuations during the middle to late Pleistocene, expressed as falling stage, lowstand, transgressive and highstand systems tracts. Thick stratigraphic sequences occur within the middle Pleistocene section, suggesting the occurrence of high capacity fluvial point sources debouching into the area from the west and north. Furthermore, the antecedent topography plays a significant role in the evolution

  5. Oceanic Intraplate Magmatism Off The Vøring Volcanic Passive Margin, Norway - Constraints On Origin From Seismic Velocity Structure

    NASA Astrophysics Data System (ADS)

    Breivik, A. J.; Faleide, J. I.; Mjelde, R.

    2009-12-01

    Early Eocene continental breakup in the NE Atlantic was associated with voluminous extrusive and intrusive magmatism, and initial seafloor spreading produced anomalously thick oceanic crust. During 2003, a wide-angle seismic data set was collected on the Norwegian Margin under the Euromargins 2003 program. One target was a bathymetric high of unclear nature on oceanic crust off the Vøring Plateau. Velocity modeling of the data shows a 14-15 km thick crust supporting the high, and both velocity structure and dimension are similar to the thick igneous crust created at the Vøring margin during breakup. By analyzing stratigraphic disturbances of sedimentary strata on multichannel seismic reflection data around the high, we argued in a recent G3 paper that the bathymetric relief of this Mid- to Late Eocene oceanic crust was established much later, probably Late Miocene. We estimated that ~57500 cubic-km of igneous material was added to the crust at that time, forcing up the high by dominantly lower-crustal growth. This explanation has, however, not been universally accepted, and it has been argued that the crustal thickness may be primary, tied to the strong magmatism following crustal breakup, and later uplift caused by movement on East Jan Mayen Fracture Zone bordering the high to the Southwest. We have looked closer at the velocity structure of the igneous crust of the high in an attempt to constrain the melting mode of the mantle. In recent years several groups have compared igneous crustal thickness to mean seismic velocity (H-Vp) at the northeast Atlantic margins, and always found a positive correlation. This correlation indicates that the breakup magmatism was caused by elevated mantle temperature, cooling down as seafloor spreading progressed. We obtain the same result for the margin part of our profile, but the H-Vp diagram for the bathymetric high itself shows a low correlation. We conclude that melting was quite uniform and the melting degree moderate here

  6. Geophysical Investigations of Crustal Structure of Cenozoic Rifting Basin in Passive Continental Margin: The Pearl River Mouth Basin, South China Sea

    NASA Astrophysics Data System (ADS)

    Qiu, N.

    2015-12-01

    The Pearl River Mouth Basin (PRMB) initiated in the Cenozoic with rifting, and became a part of the South China Sea (SCS) rifted passive continental margin. Decades of industrial exploration in this proliferous region have produced lots of geological and geophysical data. In order to get the first order crustal scale structure, we integrate well data, multi channel seismic reflection, and the observed gravity field for a joint inversion. The Cenozoic sediment of PRMB comprises of several stratigraphic sequences, including the terrestrial facies, the marine facies and the transitional facies. The sedimentary model takes into account of two main parts that refer to the Paleogene to Neogene unit and the Neogene to Quaternary unit, which were respectively formed during the intercontinental rifting stage and the passive continental margin post-rifting stage. By integrating long cable seismic profiles, interval velocity and performing gravity modelling, we have modelled the sub-sedimentary basement. There are some high-density bodies in the lower part of crust (ρ> 2.8 g/cm3), most of which were probably made up by emplacement from the upper mantle into the lower crust. The crystalline continental crust spans from unstretched domains (as thick as about 25 km) near the continental shelf to extremely thinned domains (of less than 6 km thickness) in the sag center. The presented crust-scale structural model shows that the crystalline crust of the Liwan Sag (LWS) and Baiyun sag (BYS) are thinner than other parts of PRMB, especially, the crystalline crust thickness in BYS is even less than 6 km. we could preliminary infer that the crystalline crust may be more easily stretched and be thinned by the existence of hot and soft substances at the lower crust.

  7. Linking lithology and land use to sources of dissolved and particulate organic matter in headwaters of a temperate, passive-margin river system

    NASA Astrophysics Data System (ADS)

    Longworth, B. E.; Petsch, S. T.; Raymond, P. A.; Bauer, J. E.

    2007-09-01

    A number of rivers have been found to transport highly aged organic matter [OM]; however, the sources of this aged material remain a matter of debate. One potential source may be erosion and weathering of headwater lithologies rich in ancient sedimentary OM. In this study, waters, suspended particulates, streambed sediments, rocks and soils from fourteen small headwater watersheds of a mid-size, temperate, passive margin river were sampled and characterized by Δ 14C, δ 13C, and POC/TPN ratios to identify sources of particulate and dissolved OM delivered to the river mainstem. These headwater sites encompass a range in lithology (OM-rich shales, OM-lean carbonate/mudstone facies, and OM-free crystalline rocks) and land use types (forested and agricultural), and allow investigation of the influence of agriculture and bedrock types on stream OM characteristics. Streams draining large areas of both agricultural land use and OM-rich lithology contain particulate OM [POM] that is more 14C-depleted than streams draining forested, shale-free watersheds. However, this is not sufficient to account for the significantly lower Δ 14C-POC measured in the river mainstem. Dissolved OM [DOM] Δ 14C are in all cases enriched compared to POM from the same stream, but are otherwise highly variable and unrelated to either land use or lithology. POC/TPN ratios were likewise highly variable. POC and DOC δ 13C signatures were similar across all watersheds. Based on isotope mass balance, 14C-free fossil OM sources contribute 0-12% of total stream POM. Although these results do not unequivocally separate the influences of land use and lithology, watershed coverage by shale and agriculture are both important controls on stream Δ 14C-POC. Thus export of aged, particle-associated OM may be a feature of river systems along both passive and active continental margins.

  8. Deep structure of the Texas Gulf passive margin and its Ouachita-Precambrian basement: Results of the COCORP San Marcos arch survey

    SciTech Connect

    Culotta, R.; Latham, T.; Oliver, J.; Brown, L.; Kaufman, S. ); Sydow, M. )

    1992-02-01

    This COCORP deep seismic survey provides a comprehensive image of the southeast-Texas part of the Gulf passive margin and its accreted Ouachita arc foundation. Beneath the updip limit of the Cenozoic sediment wedge, a prominent antiformal structure is imaged within the interior zone of the buried late Paleozoic Ouachita orogen. The structure appears to involve Precambrian Grenville basement. The crest of the antiform is coincident with the Cretaceous-Tertiary Luling-Mexia-Talco fault zone. Some of these faults dip to the northwest, counter to the general regional pattern of down-to-the-basin faulting, and appear to sole into the top of the antiform, suggesting that the Ouachita structure has been reactivated as a hingeline to the subsiding passive margin. The antiform may be tied via this fault system and the Ouachita gravity gradient to the similar Devils River, Waco, and Benton uplifts, interpreted as Precambrian basement-cored massifs. Above the Paleozoic sequence, a possible rift-related graben is imaged near the updip limit of Jurassic salt. Paleoshelf edges of the major Tertiary depositional sequences are marked by expanded sections disrupted by growth faults and shale diapirs. Within the Wilcox Formation, the transect crosses the mouth of the 900-m-deep Yoakum Canyon, a principal pathway of sediment delivery from the Laramide belt to the Gulf. Beneath the Wilcox, the Comanchean (Lower Cretaceous) shelf edge, capped by the Stuart City reef, is imaged as a pronounced topographic break onlapped by several moundy sediment packages. Because this segment of the line parallels strike, the topographic break may be interpreted as a 2,000-m-deep embayment in the Cretaceous shelf-edge, and possibly a major submarine canyon older and deeper than the Yoakum Canyon.

  9. Detrital zircon U-Pb analysis of the Neoproterozoic, Timanian, passive-margin successions in North Norway and significance for Arctic reconstructions

    NASA Astrophysics Data System (ADS)

    Zhang, W.; Roberts, D.; Pease, V.

    2013-12-01

    Sediment provenance investigations which combine paleocurrent data with isotopic analysis (e.g. U-Pb, Lu-Hf), geochemical studies, and/or heavy-mineral analysis, can help to refine paleogeographic reconstructions by, for example, mapping depositional systems, constraining lateral displacements in orogens, characterizing crust which is no longer exposed, stratigraphic correlation, etc. Hence, sediment provenance analysis can provide insights into circum-Arctic tectonic evolution. Sediment recycling, however, can complicate, obscure, and even mislead in testing regional tectonic scenarios. Consequently, it is important to distinguish between first- and second-cycle detritus, as well as to fully characterize all detrital sources including sedimentary ones. The Neoproterozoic, passive-margin successions of the Timanian margin, Finnmark, northern Norway, include the 9 km-thick, deep-marine to deltaic, Barents Sea Group and, to the south, a fluvial to shallow-marine, platformal domain. A study aiming to assess the primary detrital signature of these successions is ongoing. Many detrital investigations in other Arctic terranes claim to recognize the Timanian ';fingerprint' (c. 610-560 Ma zircon from subduction-related granitoids generated during Timanian orogenesis), yet to what extent the passive-margin sediments are recycled or recognized in younger sediments has not been addressed. Here, we present further results of this U-Pb detrital zircon investigation, extending the study into two of the Caledonian nappes farther west. Our results show the following main features of the detrital zircon populations: 1. Styret Formation, Løkviksfjellet Group, Varanger Peninsula (Sample STY1): a multimodal spread extends from c.2.0-1.1 Ga with four peaks of concordant grains at around 1.20-1.15 Ga, 1.55-1.45 Ga, 1.65-1.60 Ga, and 1.85-1.70 Ga, and subsidiary peaks at 2.90-2.50 Ga. 2. Ifjord Formation, Laksefjord Nappe Complex (Sample IFJ1): one group dominates the probability plot

  10. Passive margin uplift around the North Atlantic region and its role in Northern Hemisphere late Cenozoic glaciation

    NASA Astrophysics Data System (ADS)

    Eyles, Nicholas

    1996-02-01

    Tectonic-climatic models of late Cenozoic global cooling emphasize the importance of middle latitude uplifts (e.g., Tibetan Plateau and the American west) but ignore widespread tectonic events on the margins of the North Atlantic Ocean. Pleistocene glaciations, after 2.5 Ma, are characterized by circum North Atlantic continental ice sheets that formed by the coalescence of perennial snow fields on extensive plateau surfaces in eastern Canada, northwest Britain, and Scandinavia. Plateaus record Cenozoic uplift of peneplains in response to semisynchronous magmatic underplating and thermal buoyancy of rifted continental margins. High-standing plateaus are very sensitive to small reductions in summer temperature. As late Cenozoic climate cooling proceeded, driven by uplift in regions external to the North Atlantic region, elevated plateaus became sites for extensive snow fields and ultimately ice sheets. Circum-Atlantic uplift took place in the key latitudinal belt that is most sensitive to orbitally forced changes in solar irradiation; this, together with albedo effects from large snow fields, could have amplified the relatively weak Milankovitch signal.

  11. The northern Egyptian continental margin

    NASA Astrophysics Data System (ADS)

    Badawy, Ahmed; Mohamed, Gad; Omar, Khaled; Farid, Walid

    2015-01-01

    Africa displays a variety of continental margin structures, tectonics and sedimentary records. The northern Egyptian continental margin represents the NE portion of the North African passive continental margin. Economically, this region is of great importance as a very rich and productive hydrocarbon zone in Egypt. Moreover, it is characterized by remarkable tectonic setting accompanied by active tectonic processes from the old Tethys to recent Mediterranean. In this article, seismicity of the northern Egyptian continental margin has been re-evaluated for more than 100-years and the source parameters of three recent earthquakes (October 2012, January 2013 and July 2013) have been estimated. Moment tensor inversions of 19th October 2012 and 17th January 2013 earthquakes reveal normal faulting mechanism with strike-slip component having seismic moment of 3.5E16 N m and 4.3E15 N m respectively. The operation of the Egyptian National Seismic Network (ENSN) since the end of 1997 has significantly enhanced the old picture of earthquake activity across northern Egyptian continental margin whereas; the record-ability (annual rate) has changed from 2-events/year to 54-event/year before and after ENSN respectively. The spatial distribution of earthquakes foci indicated that the activity tends to cluster at three zones: Mediterranean Ridge (MR), Nile Cone (NC) and Eratosthenes Seamount (ERS). However, two seismic gaps are reported along Levant Basin (LEV) and Herodotus Basin (HER).

  12. Alpine geodynamic evolution of passive and active continental margin sequences in the Tauern Window (eastern Alps, Austria, Italy): a review

    NASA Astrophysics Data System (ADS)

    Kurz, W.; Neubauer, F.; Genser, J.; Dachs, E.

    The Penninic oceanic sequence of the Glockner nappe and the foot-wall Penninic continental margin sequences exposed within the Tauern Window (eastern Alps) have been investigated in detail. Field data as well as structural and petrological data have been combined with data from the literature in order to constrain the geodynamic evolution of these units. Volcanic and sedimentary sequences document the evolution from a stable continent that was formed subsequent to the Variscan orogeny, to its disintegration associated with subsidence and rifting in the Triassic and Jurassic, the formation of the Glockner oceanic basin and its consumption during the Upper Cretaceous and the Paleogene. These units are incorporated into a nappe stack that was formed during the collision between a Penninic Zentralgneis block in the north and a southern Austroalpine block. The Venediger nappe and the Storz nappe are characterized by metamorphic Jurassic shelf deposits (Hochstegen group) and Cretaceous flysch sediments (Kaserer and Murtörl groups), the Eclogite Zone and the Rote Wand-Modereck nappe comprise Permian to Triassic clastic sequences (Wustkogel quartzite) and remnants of platform carbonates (Seidlwinkl group) as well as Jurassic volcanoclastic material and rift sediments (Brennkogel facies), covered by Cretaceous flyschoid sequences. Nappe stacking was contemporaneous to and postdated subduction-related (high-pressure) eclogite and blueschist facies metamorphism. Emplacement of the eclogite-bearing units of the Eclogite zone and the Glockner nappe onto Penninic continental units (Zentralgneis block) occurred subsequent to eclogite facies metamorphism. The Eclogite zone, a former extended continental margin, was subsequently overridden by a pile of basement-cover nappes (Rote Wand-Modereck nappe) along a ductile out-of-sequence thrust. Low-angle normal faults that have developed during the Jurassic extensional phase might have been inverted during nappe emplacement.

  13. The role of Variscan to pre-Jurassic active extension in controlling the architecture of the rifted passive margin of Adria: the example of the Canavese Zone (Western Southern Alps, Italy)

    NASA Astrophysics Data System (ADS)

    Succo, Andrea; De Caroli, Sara; Centelli, Arianna; Barbero, Edoardo; Balestro, Gianni; Festa, Andrea

    2016-04-01

    The Canavese Zone, in the Italian Western Southern Alps, represents the remnant of the Jurassic syn-rift stretching, thinning and dismemberment of the distal passive margin of Adria during the opening of the Penninic Ocean (i.e., Northern Alpine Tethys). Our findings, based on detailed geological mapping, structural analysis and stratigraphic and petrographic observations, document however that the inferred hyper-extensional dismemberment of this distal part of the passive margin of Adria, up to seafloor spreading, was favored by the inherited Variscan geometry and crustal architecture of the rifted margin, and by the subsequent Alpine-related strike-slip deformation. The new field data document, in fact, that the limited vertical displacement of syn-extensional (syn-rift) Jurassic faults was ineffective in producing and justifying the crustal thinning observed in the Canavese Zone. The deformation and thinning of the continental basement of Adria are constrained to the late Variscan time by the unconformable overlying of Late Permian deposits. Late Cretaceous-Early Paleocene and Late Cenozoic strike-slip faulting (i.e., Alpine and Insubric tectonic stages) reactivated previously formed faults, leading to the formation of a complex tectonic jigsaw which only partially coincides with the direct product of the Jurassic syn-rift dismemberment of the distal part of the passive margin of Adria. Our new findings document that this dismemberment of the rifted continental margin of Adria did not simply result from the syn-rift Jurassic extension, but was strongly favored by the inheritance of older (Variscan and post-Variscan) tectonic stages, which controlled earlier lithospheric weakness. The formation of rifted continental margins by extension of continental lithosphere leading to seafloor spreading is a complex and still poorly understood component of the plate tectonic cycle. Geological mapping of rifted continental margins may thus provide significant information to

  14. Siliciclastic Progradation Within a Neogene Carbonate Passive Margin - Northern Carnarvon Basin of the Northwest Shelf of Australia

    NASA Astrophysics Data System (ADS)

    Sanchez, C. M.; Fulthorpe, C. S.; Austin, J. A.

    2008-12-01

    Interpretations of extensive industry subsurface data (3D and 2D seismic data, wireline logs and completion reports) and results from previous studies suggest that relative sea level changes and ocean currents controlled observed variations in carbonate vs. siliciclastic sediment type and stratal architecture in the Northern Carnarvon Basin, Northwest Shelf of Australia, during the Neogene. The basin has been a site of predominantly carbonate sedimentation since the late Paleogene. However, significant (at least 30-70 km) siliciclastic progradation on top of this carbonate shelf started in the late middle Miocene. Carbonate- dominated sedimentation resumed in the late Miocene/Pliocene. Siliciclastic-rich seismic packages show dip- oriented progradation northwestward, as well as strike-oriented progradation northeastward. Clinoform heights, up to approx. 140 m, and a dip-elongated lobe morphology suggest deposition by a long-lived (approx. 6 My) deltaic system which is no longer active on this margin. Consistent northeastward switching of these interpreted delta lobes and their progradation over the preexisting carbonate shelf sediments suggest wave-dominated conditions, with strong northeastward long-shore currents, for this delta system during a time of relative base level fall. During this siliciclastic progradation event, carbonate sedimentation continued in the updrift direction southwest of the delta, but carbonates were not accumulated downdrift or northeast, where progressively younger deltaic siliciclastics filled available accommodation space. By the beginning of the Pliocene, siliciclastic sedimentation had retreated towards the Australian coast, allowing widespread carbonate production to resume; interpreted shallow-water carbonate platforms then developed over subtle topographic highs created by the underlying deltaic lobes.

  15. Post-rift uplift and focused fluid flow in the passive margin of northern South China Sea

    NASA Astrophysics Data System (ADS)

    Wu, Shiguo; Gao, Jinwei; Zhao, Shujuan; Lüdmann, Thomas; Chen, Duanxin; Spence, George

    2014-03-01

    An important post-rifting tectonic event with extensive uplift and fluid flow took place at the northern margin of the South China Sea (SCS) during the late Miocene. Based on an analysis of two- and three-dimensional seismic data plus drilling well data, the tectonic event mainly affected the Dongsha Rise and the adjacent depressions. Tectonic deformation likely occurred in the Dongsha Rise, the eastern part of the Panyu Swell and several depressions of the Zhu I Sag. The post-rifting deformation in the study area was characterized by faulting, erosion, igneous activity and hydrothermal fluid flow. The tectonic movement caused abundant secondary NW and WNW tensional and transtensional faults, with lengths of 1 to 10 km. It also created a clear angular unconformity which marks the time of the tectonic movement. Late Neogene igneous rocks are found in the strata in the Dongsha Rise only close to the continent-ocean transition zone. Geothermal activity occurred in the Liuhua carbonate platform and adjacent depressions. Deformation is more intense in the eastern area than in the western area. According to sequence stratigraphy, we infer that the tectonic event mainly occurred in the late period of Late Miocene and ended at the Mio/Pliocene boundary (5.5 Ma). The dynamic mechanism for tectonic movement could be associated with subduction of the South China Sea slab beneath the Philippine Sea Plate at the Manila trench. When subduction of lower density crust within the continental-ocean transition zone was initiated, large resistance stresses may have led to lithosphere bending and crustal uplift in the Dongsha area.

  16. Long-term landscape evolution of the South Atlantic "passive" continental margin in Eastern Argentina using apatite fission-track thermochronology

    NASA Astrophysics Data System (ADS)

    Pfister, Sabrina; Kollenz, Sebastian; Glasmacher, Ulrich A.

    2015-04-01

    To understand the evolution of the "passive" continental margin in Argentina low temperature thermochronology is an appropriate method, which might lead to new insights in this area. The Tandilia System, also called Sierras Septentrionales, is located south of the Río de la Plato Craton in eastern Argentina in the state of Buenos Aires. North of the hills the Salado basin is located whereas the Claromecó basin is situated south of the mountain range. In contrary to most basins along the South American "passive" continental margin, the Tandilia-System and the neighbouring basins trend perpendicular to the coast line. The topography is fairly flat with altitudes up to 350 m. The igneous-metamorphic basement is pre-Proterozoic in age and build up of mainly granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks and granitoid plutons. It is overlain by a series of Neoproterozoic to early Paleozoic sedimentary rocks (Cingolani 2011), like siliciclastic rocks, dolostones, shales and limestones (Demoulin 2005). The aim of the study is to quantify the long-term landscape evolution of the "passive" continental margin in eastern Argentina in terms of thermal, exhumation and tectonic evolution. For that purpose, samples were taken from the basement of the Sierra Septentrionales and analyzed with the apatite fission-track method. Further 2-D thermokinematic modeling was conducted with the computer code HeFTy (Ketcham 2005; Ketcham 2007; Ketcham et al. 2009). Because there are different hypotheses in literature regarding the geological evolution of this area two different models were generated, one after Demoulin et al. (2005) and another after Zalba et al.(2007). All samples were taken from the Neoproterozoic igneous-metamorphic basement. Apatite fission-track ages range from 101.6 (9.4) to 228.9 (22.3) Ma, and, therefore, are younger than their formation age, indicating all samples have been thermally reset. Six samples accomplished enough confined

  17. Glacially-influenced late Pleistocene stratigraphy of a passive margin: New Jersey's Record of the North American ice sheet

    USGS Publications Warehouse

    Carey, J.S.; Sheridan, R.E.; Ashley, G.M.; Uptegrove, J.

    2005-01-01

    Glacial isostasy and the sediment supply changes associated with the waxing and waning of ice sheets have dramatic effects on the stratigraphy of adjacent continental shelves. In ancient stratigraphic records, the glacial influences on such deposits could be difficult to recognize because of the removal of coeval terrestrial glacial deposits by erosion. This study illustrates the effects of the Laurentide Ice Sheet on a basin near its maximum limit, the New Jersey continental shelf. Analysis of 1600 km of Geopulse???, Uniboom???, Minisparker??? and airgun profiles reveals four depositional sequences that have a maximum thickness of ???75 m near the shelf edge. Sequences I and IV correspond to the major glacial-interglacial sea level changes at Marine Isotope Chron (MIC) 6/5e and 2/1, whereas sequences II and III reflect smaller-scale sea-level fluctuations during chrons 4/3c and 3b/3a, respectively. Sequences I and IV are characterized by relatively thick low stand to early transgressive deposits near the shelf edge formed during times of increased sediment supply, but are thin and discontinuous across much of the shelf. Reflection horizons in these units deepen northward in the northern half of the study area due to collapse of a peripheral bulge that formed at the margin of the Laurentide Ice Sheet. The Hudson River moved from a more southerly drainage pattern to the modern Hudson Shelf Valley position, possibly under the influence of the advancing peripheral bulge. Sequences II and III are largely preserved within a broad mid-shelf swale likely created by the migration of an ancestral Hudson River, and their thickness implies much higher sedimentation rates during chrons 4 and 3 than seen today. If the terrestrial glacial record was eroded, the increased rates of sedimentation during the Pleistocene, dominance of sediments derived from northern New England, and northward tilting of strata could be interpreted as a result of uplift of a northern source area. The

  18. Changes in opal fluxes along the northwest African margin during the last glacial period; linking high and low latitude patterns of productivity

    NASA Astrophysics Data System (ADS)

    Bradtmiller, L. I.; Galgay, M.; McGee, D.; Kinsley, C. W.; Anderson, R. F.

    2014-12-01

    Recent studies have proposed competing hypotheses to explain increased opal fluxes in high and low latitudes during the most recent deglaciation. Anderson et al. (2009) rely on increased wind-driven upwelling in the Southern Ocean to explain the increased availability of Si in both the Southern Ocean and tropical thermoclines, leading to increased opal fluxes in both regions coincident with the deglacial rise in CO2. Meckler et al. (2013) suggest that a decrease in the presence of North Atlantic intermediate water (GNAIW) during the deglaciation allowed Si-rich southern-sourced waters to fill the tropical Atlantic leading to increased opal burial. We attempt to distinguish between these two mechanisms by reconstructing opal fluxes and fluxes of windblown dust over the past ~65ka at four sites along the northwest African margin. The records include the deglaciation, including Heinrich Event 1 (H1) and the Younger Dryas (YD), as well as several earlier Heinrich events. We find that opal and dust fluxes increase simultaneously during the deglaciation, and more highly resolved cores record H1 and the YD as distinct peaks within the deglaciation. Furthermore, opal and dust fluxes scale approximately linearly with one another during these events. We observe opal peaks associated with most Heinrich Events through H6. Finally, we observe a strong similarity between patterns of opal flux in the Southern Ocean and along the African Margin. This suggests that the pattern of diatom productivity and opal flux along the African Margin reflects a combination of changes in wind strength due to shifting temperature gradients, and changes in the export of silica-rich water from the Southern Ocean, both as a result of the global scale climate changes associated with Heinrich Events. Anderson, R. F., S. Ali, L. I. Bradtmiller, S. H. H. Nielsen, M. Q. Fleisher, B. E. Anderson and L. H. Burckle. Wind-Driven Upwelling in the Southern Ocean and the Deglacial Rise in Atmospheric CO2

  19. Transition from island-arc to passive setting on the continental margin of Gondwana: U Pb zircon dating of Neoproterozoic metaconglomerates from the SE margin of the Teplá Barrandian Unit, Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Sláma, Jiří; Dunkley, Daniel J.; Kachlík, Václav; Kusiak, Monika A.

    2008-12-01

    yielded a mean U-Pb age of 610 ± 17 Ma arguing for active arc magmatism during the latest Neoproterozoic. The overall age distribution of detrital zircons and the absence of Grenvillian zircon ages favour a location of the Teplá-Barrandian Unit close to the West African craton at the Neoproterozoic/Paleozoic boundary. The rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying Cambrian shallow-water clastics alternating with predominantly felsic calc-alkaline to alkaline volcanics. During the Cambrian oblique plate convergence, the active continental margin was transformed into belts of horsts and narrow pull-apart or small rift basins. After the Ordovician break-off of Armorican Terrane Assemblage blocks from Gondwana, the extensional regime in the Teplá-Barrandian Unit continued until the beginning of Variscan orogeny in the Middle Devonian.

  20. Case Studies of Massive Gravity Slides Imaged in 3D Seismic Volumes: Passive Margin and Basinal Settings (West Africa and Northwest Europe)

    NASA Astrophysics Data System (ADS)

    Benjamin, U. K.; Le, A. N.; Oluboyo, A. P.; Irving, D. H.; Huuse, M.

    2010-12-01

    Regionally extensive scour surfaces on continental margins and in epeiric basins develop due to two major processes: submarine landslides, mainly affecting slopes in excess of 1 degree and subglacial scour beneath ice streams draining alpine or continental ice sheets. Both types of surfaces can extend for tens to hundreds of kilometres in the dip and strike directions, with tens of metres relief across tens of kilometres long scours and ridges. Correct interpretation within a sediment system tract enlightens models of synchronous glacial, marine and climatic conditions. We present four megaslides imaged in, and interpreted from 3D seismic volumes acquired in diverse settings: 1: On the passive margin shelf of Cameroon, large-scale, downslope-oriented striations extend 40 km from the upper slope to the deep basin area. Individual striations are 20-50 m wide, 8 km long, 5-10 ms TWT deep, and broadly arcuate. The striations present in two sets and mark the base of a chaotic-to-discontinuous, high-amplitude package with chaotic, low amplitude reflections in the unstriated area. The facies is interpreted as a zone of repeated gravity sliding during the early Pleistocene in response to tectono-climatic forcing. 2: A basal surface from the Moray Firth, UK Central North Sea, tentatively dated as Paleocene in age. It is 20 km in width, over 40 km long and displays parallel grooves measuring typically 100-1000 m wide, 10-20 km long, of sinuous form along the section imaged in the dataset and with evidence of post-slump failure along the margins of the slide zone. It is interpreted as scour caused by a single sliding event. 3: An areally extensive scoured surface measuring 45x45 km that is part of a larger Pliocene system on the Angolan margin. The basal shear surface is marked by divergent scours. The geometry and spatial location of this flow are confined by listric faults and salt-cored folds with compressional thrusts within the distal toe of deposits on the flanks of

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  2. Sea-level Controls on the Sediment Architecture of the US New Jersey Passive Margin During Oligocene and Miocene Times: IODP Expedition 313 Preliminary Results

    NASA Astrophysics Data System (ADS)

    Proust, J.; Mountain, G.; Ando, H.; Browning, J. V.; Hesselbo, S. P.; Hodgson, D. M.; Rabineau, M.; Sugarman, P.; Scienceparty, E.

    2010-12-01

    In May-July 2009, IODP Expedition 313 used an ECORD "mission-specific" jack-up platform 45-67 km off the coast of New Jersey, in 35 m of water, to core and log Upper Paleogene and Neogene sequences. The goal was to estimate the amplitudes, rates and mechanisms of sea-level change and to evaluate sequence stratigraphic facies models that predict depositional environments, sediment compositions, and stratal geometries in response to sea-level change. Three holes, as deep as 757mbsf, were drilled through mid-shelf clinoform deposits which complement the coastal plain (ODP Legs 150X, 174AX) and slope (Leg 150) core datasets, building up a 200 km-long “New Jersey transect” across the US Atlantic passive margin. The cores reveal a silt-rich supply systems notably depleted in clays and a marked difference in facies between top-set beds landward of clinoform rollovers and toe-set beds seaward of clinoform toes. The top-set facies are well sorted silts and sands deposited in offshore to shoreface, mixed wave to river-dominated shelf environments. Toe-set silts and silty clays were deposited below wave base typically interbedded with poorly-sorted silts and sands deposited by down-slope gravity transport processes such as sandy debris flows and turbidity currents during periods of clinoform slope/rollover degradation. The open shelf experienced frequent cycles of dysoxia. In situ and reworked glauconite is a common component of top-set and toe-set strata that also show sharp changes in pore water salinity. Chronostratigraphic control is provided by Sr-isotopic ages measured on mollusc shells and foraminifer tests, biostratigraphic zonation of multiple fossil groups (foraminifers, dinocysts nannofossils and diatoms), magnetic reversal chronology and pollen markers. A full set of wireline logs allows more than 16 surfaces and/or seismic sequence bounding unconformities mapped around the regional seismic grid to be tied confidently to the cores. These surfaces bound a

  3. Genesis of the Doğankuzu and Mortaş Bauxite deposits, Taurides, Turkey: separation of Al, Fe, and Mn and implications for passive margin metallogeny

    USGS Publications Warehouse

    Öztürk, Hüseyin; Hein, James R.; Hanilçi, Nurullah

    2002-01-01

    The Taurides region of Turkey is host to a number of important bauxite, Al-rich laterite, and Mn deposits. The most important bauxite deposits, Doğankuzu and Mortaş, are karst-related, unconformity-type deposits in Upper Cretaceous limestone. The bottom contact of the bauxite ore is undulatory, and bauxite fills depressions and sinkholes in the footwall limestone, whereas its top surface is concordant with the hanging-wall limestone. The thickness of the bauxite varies from 1 to 40 m and consists of böhmite, hematite, pyrite, marcasite, anatase, diaspore, gypsum, kaolinite, and smectite. The strata-bound, sulfide- and sulfate-bearing, low-grade lower part of the bauxite ore bed contains pyrite pseudomorphs after hematite and is deep red in outcrop owing to supergene oxidation. The lower part of the bauxite body contains local intercalations of calcareous conglomerate that formed in fault-controlled depressions and sinkholes. Bauxite ore is overlain by fine-grained Fe sulfide-bearing and calcareous claystone and argillaceous limestone, which are in turn overlain by massive, compact limestone of Santonian age. That 50-m-thick limestone is in turn overlain by well-bedded bioclastic limestone of Campanian or Maastrichtian age, rich with rudist fossils. Fracture fillings in the bauxite orebody are up to 1 m thick and consist of bluish-gray-green pyrite and marcasite (20%) with böhmite, diaspore, and anatase. These sulfide veins crosscut and offset the strata-bound sulfide zones. Sulfur for the sulfides was derived from the bacterial reduction of seawater sulfate, and Fe was derived from alteration of oxides in the bauxite. Iron sulfides do not occur within either the immediately underlying or overlying limestone. The platform limestone and shale that host the bauxite deposits formed at a passive margin of the Tethys Ocean. Extensive vegetation developed on land as the result of a humid climate, thereby creating thick and acidic soils and enhancing the transport of

  4. Anatomy of extremely thin marine sequences landward of a passive-margin hinge zone: Neogene Calvert Cliffs succession, Maryland, U.S.A.

    SciTech Connect

    Kidwell, S.M.

    1997-03-01

    Detailed examination of Neogene strata in cliffs 25--35 m high along the western shore of Chesapeake Bay, Maryland, reveals the complexity of the surviving record of siliciclastic sequences {approximately}150 km inland of the structural hinge zone of the Atlantic passive margin. Previous study of the lower to middle Miocene Calvert (Plum Point Member) and Choptank Formations documented a series of third-order sequences 7--10 m thick in which lowstand deposits are entirely lacking, transgressive tracts comprise a mosaic of condensed bioclastic facies, and regressive (highstand) tracts are present but partially truncated by the next sequence boundary; smaller-scale (fourth-order) cyclic units could not be resolved. Together, these sequences constitute the transgressive and early highstand tracts of a larger (second-order Miocene) composite sequence. The present paper documents stratigraphic relations higher in the Calvert Cliffs succession, including the upper Miocene St. Marys Formation, which represents late highstand marine deposits of the Miocene second-order sequence, and younger Neogene fluvial and tidal-inlet deposits representing incised-valley deposits of the succeeding second-order cycle. The St. Marys Formation consists of a series of tabular units 2--5 m thick, each with an exclusively transgressive array of facies and bounded by stranding surfaces of abrupt shallowing. These units, which are opposite to the flooding-surface-bounded regressive facies arrays of model parasequences, are best characterized as shaved sequences in which only the transgressive tract survives, and are stacked into larger transgressive, highstand, and forced-regression sets.

  5. Reconstruction of pre-rift Pyrenean relief in the Oligo-Quitanian Camargue Basin (Gulf of Lion passive margin, SE France): Implications on thermal history of basins

    SciTech Connect

    Benedicto, A.; Labaume, P.; Seranne, M.

    1995-08-01

    Fault reconstruction techniques commonly assume horizontal pre-rift level datum to calculate fault geometry from hanging-wall geometry or viceversa. Example from Camargue basin shows that neglecting pre-rift relief may lead to important errors in calculating the fault and hanging-wall geometries, and the total extension. These errors have direct implications on reconstruction of the thermal history of basins. The Camargue basin results front NW-SE extension and rifting of the Gulf of Lion passive margin. More than 4000m of Oligo-Aquitanian syn-rift series unconformably overlie a crust previously thickened during Pyrenean orogeny. The half-graben basin is controlled by the SE-dipping listric Nimes basement fault which generated a typical roll-over. As both fault and hanging-wall geometries are constrained, the pre-rift surface topography can be restored, using three reconstruction techniques. Either the constant-bed-length and constant-heave techniques produce a depression in the axis of the basin and a relief (1500m and 12(X)m respectively) atop the roll-over. The simple-shear (a=60{degrees}) technique generates a 1500m topography atop the roll-over, more coherent with regional data. Testing the hypothesis of a pre-rift horizontal datum leads to a roll-over 1400m too deep. Pre-rift surface elevation corresponds to the residual topography herited from the Pyrenean orogeny. Consequently, there has been some 1000m subsidence more than predicted by the syn-rift sedimentary record.

  6. New (U-Th)/He titanite data from a complex orogen-passive margin system: A case study from northern Mozambique

    NASA Astrophysics Data System (ADS)

    Bauer, Friederike U.; Jacobs, Joachim; Emmel, Benjamin U.; van Soest, Matthijs C.

    2016-08-01

    New titanite (U-Th)/He (He) data on basement rocks from NE Mozambique are presented. The objective was to test the applicability of titanite He thermochronology in an orogen-passive margin setting and to better constrain the exhumation history across the Lurio Belt, a major structural discontinuity in Mozambique. Therefore, samples from existing geochronological and thermochronological studies were dated using titanite He thermochronology. Resulting titanite He data (from abraded crystals) provide average cooling ages from 178 ± 15 to 383 ± 23 Ma. The data fit well into the age pattern obtained from previous thermochronological studies in NE Mozambique, revealing differential exhumation across the Lurio Belt. The basement to the north experienced earlier cooling than that to the south, while overall youngest titanite He ages are from the Lurio Belt, indicating reactivation linked to the post-collisional extension and break-up of Gondwana. Thermal history modelling revealed two possibilities, able to account for the different cooling histories of NE Mozambique since initial Gondwana break-up in Permian times: One involves a transient sedimentary overburden that buried and (re)heated the southern basement, with subsequent basin inversion at ˜250 Ma in response to rift shoulder uplift. The second model implies delayed cooling of the southern basement, possibly due to delamination of the crustal root shortly after Gondwana formation. The formerly upwelling asthenosphere and the subsequently formed sag basin might have caused a prolonged thermal effect. Titanite He ages and thermal histories point to rift shoulder uplift of the southern part and increased thermal activity within the reactivated Lurio Belt, signifying first rifting activities as precursor of Gondwana break-up.

  7. New (U-Th)/He titanite data from a complex orogen-passive margin system: A case study from northern Mozambique

    NASA Astrophysics Data System (ADS)

    Bauer, Friederike U.; Jacobs, Joachim; Emmel, Benjamin U.; van Soest, Matthijs C.

    2016-08-01

    New titanite (U-Th)/He (He) data on basement rocks from NE Mozambique are presented. The objective was to test the applicability of titanite He thermochronology in an orogen-passive margin setting and to better constrain the exhumation history across the Lurio Belt, a major structural discontinuity in Mozambique. Therefore, samples from existing geochronological and thermochronological studies were dated using titanite He thermochronology. Resulting titanite He data (from abraded crystals) provide average cooling ages from 178 ± 15 to 383 ± 23 Ma. The data fit well into the age pattern obtained from previous thermochronological studies in NE Mozambique, revealing differential exhumation across the Lurio Belt. The basement to the north experienced earlier cooling than that to the south, while overall youngest titanite He ages are from the Lurio Belt, indicating reactivation linked to the post-collisional extension and break-up of Gondwana. Thermal history modelling revealed two possibilities, able to account for the different cooling histories of NE Mozambique since initial Gondwana break-up in Permian times: One involves a transient sedimentary overburden that buried and (re)heated the southern basement, with subsequent basin inversion at ∼250 Ma in response to rift shoulder uplift. The second model implies delayed cooling of the southern basement, possibly due to delamination of the crustal root shortly after Gondwana formation. The formerly upwelling asthenosphere and the subsequently formed sag basin might have caused a prolonged thermal effect. Titanite He ages and thermal histories point to rift shoulder uplift of the southern part and increased thermal activity within the reactivated Lurio Belt, signifying first rifting activities as precursor of Gondwana break-up.

  8. Examining the Impact of Historical/Developmental, Sociodemographic, and Psychological Factors on Passive Suicide among African-American Men

    ERIC Educational Resources Information Center

    Tucker, Tameka M.

    2009-01-01

    Nationally published reports on death rates for substance abuse (drug-alcohol related), violence (homicide), and risky sexual behaviors (HIV/AIDS) among African-American men are deeply concerning. The goal of this study was to examine the relationship between historical/developmental factors (masculine identity, racial identity, racism),…

  9. Quaternary sedimentary processes on the northwestern African continental margin - An integrated study using side-scan sonar, high-resolution profiling, and core data

    SciTech Connect

    Masson, D.G.; Huggett, Q.J.; Weaver, P.P.E. ); Kidd, R.B. ); Gardner, J.V. )

    1991-08-01

    Side-scan sonar data, cores, and high-resolution profiles have been used to produce an integrated model of sedimentation for the continental margin west of the Canary Islands. Long-range side-scan sonar (GLORIA) data and a grid of 3.5-kHz profiles, covering some 200,000 km{sup 2} allow a regional appraisal of sedimentation. More detailed studies of selected areas have been undertaken using a new 30 kHz deep-towed side-scan sonar (TOBI) developed by the U.K. Institute of Oceanographic Sciences. Sediment cores have been used both to calibrate acoustic facies identified on sonographs and for detailed stratigraphic studies. The most recent significant sedimentation event in the area is to Saharan Sediment Slide, which carried material from the upper continental slope off West Africa to the edge of the Madeira Abyssal Plain, a distance of some 1000 km. The authors data shows the downslope evolution of the debris flow. Near the Canaries, it is a 20-m-thick deposit rafting coherent blocks of more than 1 km diameter; side-scan records show a strong flow-parallel fabric on a scale of tens of meters. On the lower slope, the debris flow thins to a few meters, the flow fabric disappears, and the rafted blocks decrease to meters in diameter. Side-scan data from the lower slope show that the Saharan Slide buries an older landscape of turbidity current channels, typically 1 km wide and 50 m deep. Evidence from the Madeiran Abyssal Plain indicates a history of large but infrequent turbidity currents, the emplacement of which is related to the effects of sea level changes on the northwest African margin.

  10. Geochemistry and geochronology of the Mkhondo suite, Swaziland: evidence for passive-margin deposition and granulite facies metamorphism in the Late Archean of Southern Africa

    NASA Astrophysics Data System (ADS)

    Condie, K. C.; Kröner, A.; Milisenda, C. C.

    1996-05-01

    The Archean Mkhondo suite in southern Swaziland is a multiply deformed succession of metasediments intruded with amphibolite dykes and sills and granitoid gneisses. Mineral and textural relationships indicate an early period of granulite facies metamorphism, followed later by amphibolite facies metamorphism. Geothermobarometry indicates maximum temperatures of 700-900°C and burial depths of 25-3 km. Paragneisses and biotite quartzites have LREE enriched patterns with small negative Eu anomalies, whereas white quartzites show variable REE patterns and low REE concentrations. BIF has slight LREE enrichment and Eu anomalies. Amphibolites have moderate LREE enrichment and depletions in TaNb and P. Unlike many Archean granitoids, the Mkhondo granitoid gneisses are high in K and other LILE, have large negative Eu anomalies and are not depleted in HREE. SHRIMP {U}/{Pb} isotopic analyses of detrital zircons from a biotite quartzite define a source age of ˜3600-3460 Ma. A deformed granitoid in tectonic contact with the Mkhondo suite yields a zircon evaporation {207Pb}/{206Pb} mean age of 3192±5 Ma, which is interpreted as the age of emplacement. A zircon evaporation age of a granitic melt patch in paragneiss, as well as whole-rock and garnet SmNd isotopic ages, suggest that the peak of high-grade metamorphism in the Mkhondo suite occurred at about 2750 Ma. This is the first evidence for Late Archean high-grade metamorphism in the southeastern Kaapvaal craton. The age data of this study restrict deposition of the Mkhondo suite to between ˜3.2 and ˜2.75 Ga. Mkhondo paragneisses are interpreted as shales with biotite quartzites as iron- and quartz-rich detrital sediments. Geochemical mixing calculations indicate that the sediment sources were composed of basalt (±komatiite), TTG and Eu-depleted granitoids. The Mkhondo assemblage may have been deposited along a passive continental margin or in a continental interior basin. The presence of minor BIF with positive Eu

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

    NASA Astrophysics Data System (ADS)

    Walter, Bastien; Géraud, Yves; Diraison, Marc

    2014-05-01

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

  12. Pre-collisional geodynamic context of the southern margin of the Pan-African fold belt in Cameroon

    NASA Astrophysics Data System (ADS)

    Nkoumbou, C.; Barbey, P.; Yonta-Ngouné, C.; Paquette, J. L.; Villiéras, F.

    2014-11-01

    We reassess the geodynamic context close to the Congo craton during the pre-collisional period of the Pan-African orogeny from whole-rock major and trace element compositions and isotopic data obtained in the westward extension of the Yaounde series (Boumnyebel area, Cameroon). The series consists of metasediments (micaschists, minor calc-silicate rocks and marbles) and meta-igneous rocks (hornblende gneisses, amphibolites, metagabbros, pyroxenites and talcschists) recrystallized under high-pressure conditions. Chemically, the micaschists correspond to shales and greywackes similar to the Yaounde high-grade gneisses. 87Sr/86Sr initial ratios (0.7084-0.7134), moderately negative εNd(620 Ma) values (-5.75 to -7.81), Nd model ages (1.66 < TDM < 1.74 Ga) and radiometric ages point to the conclusion that the Yaounde basin was filled with siliciclastic sediments derived from both reworked older continental crust (Palaeoproterozoic to Archaean in age) and Neoproterozoic juvenile volcanogenic material. This occurred in the same time span (625-1100 Ma) as the deposition of the Lower Dja, Yokadouma, Nola and Mintom series (Tonian-Cryogenian). Dolomitic marble associated with mafic/ultramafic rocks and characterized by high Cr (854-1371 ppm) and Ni (517-875 ppm) contents, are considered to result from chemical precipitation in relation with submarine magmatic activity. Talcschists (orthopyroxenitic to harzburgitic in composition) show primitive-mantle-normalized multi-element patterns with significant negative Nb-Ta anomalies, and slopes similar to that of average metasomatically altered lithospheric mantle. These rocks could be mantle slices involved in the collision tectonics. Amphibolites show the compositions of island-arc basalts with systematic negative Nb-Ta anomalies, 87Sr/86Sr initial ratios mostly <0.7047 and positive εNd(620 Ma) values (+1.41 to +6.58). They are considered to be the expression of incipient oceanisation to the north of the Congo craton during the

  13. Geology and metallogeny of the Ar Rayn terrane, eastern Arabian shield: Evolution of a Neoproterozoic continental-margin arc during assembly of Gondwana within the East African orogen

    USGS Publications Warehouse

    Doebrich, J.L.; Al-Jehani, A. M.; Siddiqui, A.A.; Hayes, T.S.; Wooden, J.L.; Johnson, P.R.

    2007-01-01

    characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile, with opposite subduction polarity. The Ar Rayn terrane represents a continental margin arc that lay above a west-dipping subduction zone along a continental block represented by the Afif composite terrane. The concentration of epithermal, porphyry Cu and IOCG mineral systems, of central arc affiliation, along the AAF suggests that the AAF is not an ophiolitic suture zone, but originated as a major intra-arc fault that localized magmatism and mineralization. West-directed oblique subduction and ultimate collision with a land mass from the east (East Gondwana?) resulted in major transcurrent displacement along the AAF, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the AAF against a back-arc basin assemblage represented by the Abt schist of the Ad Dawadimi terrane. Our findings indicate that arc formation and accretionary processes in the Arabian shield were still ongoing into the latest Neoproterozoic (Ediacaran), to about 620-600 Ma, and lead us to conclude that evolution of the Ar Rayn terrane (arc formation, accretion, syn- to postorogenic plutonism) defines a final stage of assembly of the Gondwana supercontinent along the northeastern margin of the East African orogen. ?? 2007 Elsevier B.V. All rights reserved.

  14. Underthrusting of passive margin strata into deep crustal hot zones associated with Cretaceous arc magmatism in North America: links and timescales of magmatic vs. tectonic thickening

    NASA Astrophysics Data System (ADS)

    Chin, E. J.; Lee, C.; Tollstrup, D. L.; Xie, L.; Wimpenny, J.; Yin, Q.

    2011-12-01

    The North American Cordillera experienced lithospheric thickening during the Cretaceous as a result of subduction-induced magmatism and tectonic shortening. Several studies suggest correlations between increased plate convergence rates and crustal underthrusting with apparent magmatic flux and evolved isotopic excursions, yet questions still remain regarding causality between tectonic and magmatic thickening. Here, we use lower crustal garnet-bearing metaquartzite (80% SiO2) xenoliths hosted in late Miocene basalts in the central Sierra Nevada Batholith, California to constrain the P-T-t (pressure-temperature-time) history of crustal thickening. The xenoliths are equigranular in texture and are comprised of >50% quartz, ~10% metamorphic garnet, <40% plagioclase, and trace rutile, kyanite, and biotite. High quartz mode, abundant well-rounded detrital zircons, and oriented graphite laths demonstrating sedimentary or metamorphic layering point to a supracrustal sedimentary protolith. However, final equilibration temperatures using titanium-in-quartz thermometry are 700 - 800 °C, and final equilibration pressures using the GASP barometer yield 0.9 - 1.3 GPa, indicating the metaquartzites equilibrated within a hot lower crust (18 - 45 km). Low whole-rock REE totals, lack of whole-rock HREE enrichment relative to LREE and MREE, and absence of positive Eu anomalies suggest that significant melting in the garnet or plagioclase fields did not occur. The whole-rock trace element geochemistry is also consistent with an initially garnet-free protolith. Simultaneous LA-ICP-MS measurements of U-Pb and Hf isotopes in detrital zircons show that all zircons have discordant U-Pb with variable upper intercept ages (1.7, 2.7, 3.3 Ga; consistent with Hf model ages), but common lower intercept ages (100 Ma). The above indicate that protoliths of the metaquartzites were North American Proterozoic to Paleozoic passive margin sediments which were simultaneously emplaced into the lower

  15. Petrophysical models of high velocity lower crust on the South Atlantic rifted margins: whence the asymmetry?

    NASA Astrophysics Data System (ADS)

    Trumbull, Robert B.; Franke, Dieter; Bauer, Klaus; Sobolev, Stephan V.

    2015-04-01

    Lower crustal bodies with high seismic velocity (Vp > 7km/s) underlie seaward-dipping reflector wedges on both margins of the South Atlantic, as on many other volcanic rifted margins worldwide. A comprehensive geophysical study of the South Atlantic margins by Becker et al. (Solid Earth, 5: 1011-1026, 2014) showed a strong asymmetry in the development of high-velocity lower crust (HVLC), with about 4 times larger volumes of HVLC on the African margin. That study also found interesting variations in the vertical position of HVLC relative to seaward-dipping reflectors which question a simple intrusive vs. extrusive relationship between these lower- and upper crustal features. The asymmetry of HVLC volumes on the conjugate margins is paradoxically exactly the opposite to that of surface lavas in the Paraná-Etendeka flood basalt province, which are much more voluminous on the South American margin. This contribution highlights the asymmetric features of magma distribution on the South Atlantic margins and explores their geodynamic significance. Petrophysical models of the HVLC are presented in the context of mantle melt generation, based on thickness-velocity (H-Vp) relations. These suggest that the greater volumes and average Vp values of HVLC on the African margin are due to active upwelling and high temperature, whereas passive upwelling under a thick lithospheric lid suppressed magma generation on the South American margin. The contrast in mantle upwelling rate and lithospheric thickness on the two margins predictably causes differential uplift, and this may help explain the greater accomodation space for surface lavas on the South American side although melt generation was strongest under the African margin.

  16. Deep continental margin reflectors

    USGS Publications Warehouse

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

    1985-01-01

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

  17. 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.

  18. Salt tectonics and thermal imprint along an inverted passive margin: the Montcaou anticline, Chaînons Béarnais, North Pyrenean Zone

    NASA Astrophysics Data System (ADS)

    Menant, Armel; Aubourg, Charles; Cuyala, Jean-Baptiste; Hoareau, Guilhem; Callot, Jean-Paul; Péré, Eve; Labaume, Pierre; Ducoux, Maxime

    2016-04-01

    Resulting from the late Cretaceous-Tertiary Iberia-Eurasia convergence, the building of the Pyrenean belt followed a pre-orogenic period of rifting where the Eurasian margin was extremely stretched. The geometry and the evolution of this paleo-margin, now constituting the North Pyrenean Zone, remain however controversial. Although localized high-temperature deformation and isolated peridotite bodies have been related to crustal thinning, processes controlling the distribution of these hot paleo-temperatures and mantle outcrops are still unknown. In this study we investigate the possible role of salt tectonics, recognized in the Aquitanian basin and the Pyrenean foreland, on the development of such thermal anomalies and the exhumation of peridotites bodies. We thus performed a detailed structural and thermal characterization of the region of the Montcaou anticline (Chaînon Béarnais, North Pyrenean Zone) where salt structures have been already described. We propose balanced geological cross-sections along this anticline displaying a peridotite body in its core, embedded in Triassic evaporitic deposits. In addition, to assess the thermal imprint occurring in this area, we measured a wide set of paleo-temperature proxies, using RAMAN spectrometry on carbonaceous material. Intensively folded Jurassic and lower Cretaceous sedimentary formations (with evidences of overturned sedimentary sections), erosional unconformities and strong thickness variations in Urgonian limestones associated to the Montcaou anticline suggest a salt ridge or diapir growth since upper Aptian times. Superimposition of Pyrenees-related compressional deformation then allowed salt structure tilting and propagation of top-to-the-north thrust faults. In this region, the distribution of thermal anomalies (up to 420 °C), as well as occurrences of high-temperature scapolite minerals, seems correlated with these salt structures. Indeed, high thermal conductivity of salt material could enhance the

  19. Timing of magmatism following initial convergence at a passive margin, southwestern U.S. Cordillera, and ages of lower crustal magma sources

    USGS Publications Warehouse

    Barth, A.P.; Wooden, J.L.

    2006-01-01

    Initiation of the Cordilleran magmatic arc in the southwestern United States is marked by intrusion of granitic plutons, predominantly composed of alkali-calcic Fe- and Sr-enriched quartz monzodiorite and monzonite, that intruded Paleoproterozoic basement and its Paleozoic cratonal-miogeoclinal cover. Three intrusive suites, recognized on the basis of differences in high field strength element and large ion lithophile element abundances, contain texturally complex but chronologically distinctive zircons. These zircons record heterogeneous but geochemically discrete mafic crustal magma sources, discrete Permo-Triassic intrusion ages, and a prolonged postemplacement thermal history within the long-lived Cordilleran arc, leading to episodic loss of radiogenic Pb. Distinctive lower crustal magma sources reflect lateral heterogeneity within the composite lithosphere of the Proterozoic craton. Limited interaction between derived magmas and middle and upper crustal rocks probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin magmatic arc. ?? 2006 by The University of Chicago. All rights reserved.

  20. Long-term evolution of the western South Atlantic passive continental margin in a key area of SE Brazil revealed by thermokinematic numerical modeling using the software code Pecube

    NASA Astrophysics Data System (ADS)

    Stippich, Christian; Krob, Florian; Glasmacher, Ulrich A.; Hackspacher, Peter C.

    2016-04-01

    The aim of the research is to quantify the long-term evolution of the western South Atlantic passive continental margin (SAPCM) in SE-Brazil. Excellent onshore outcrop conditions and extensive pre-rift to post-rift archives between São Paulo and Laguna allow a high precision quantification of exhumation, and rock uplift rates, influencing physical parameters, long-term acting forces, and process-response systems. Research will integrate published1 and partly published thermochronological data from Brazil, and test lately published new concepts on causes of long-term landscape and lithospheric evolution in southern Brazil. Six distinct lithospheric blocks (Laguna, Florianópolis, Curitiba, Ilha Comprida, Peruibe and Santos), which are separated by fracture zones1 are characterized by individual thermochronological age spectra. Furthermore, the thermal evolution derived by numerical modeling indicates variable post-rift exhumation histories of these blocks. In this context, we will provide information on the causes for the complex exhumation history of the Florianópolis, and adjacent blocks. The climate-continental margin-mantle coupled process-response system is caused by the interaction between endogenous and exogenous forces, which are related to the mantle-process driven rift - drift - passive continental margin evolution of the South Atlantic, and the climate change since the Early/Late Cretaceous climate maximum. Special emphasis will be given to the influence of long-living transform faults such as the Florianopolis Fracture Zone (FFZ) on the long-term topography evolution of the SAPCM's. A long-term landscape evolution model with process rates will be achieved by thermo-kinematic 3-D modeling (software code PECUBE2,3 and FastScape4). Testing model solutions obtained for a multidimensional parameter space against the real thermochronological and geomorphological data set, the most likely combinations of parameter rates, and values can be constrained. The

  1. Against Marginalization and Criminal Reading Curriculum Standards for African American Adolescents in Low-level Tracks: A Retrospective of Baldwin's Essay.

    ERIC Educational Resources Information Center

    Tatum, Alfred W.

    2000-01-01

    Invokes James Baldwin's 1963 essay, "A Talk to Teachers," as a brilliant statement of the challenge facing teachers of African American students. Argues that there must be a thrust toward comprehensive reading instruction that encompasses explicit strategy instruction and authentic opportunities to read culturally relevant materials. (SR)

  2. Evolution of the South Atlantic passive continental margin and lithosphere dynamic movement in Southern Brazil derived from zircon and apatite (U-Th-Sm)/He and fission-track data

    NASA Astrophysics Data System (ADS)

    Krob, Florian; Stippich, Christian; Glasmacher, Ulrich A.; Hackspacher, Peter C.

    2016-04-01

    Passive continental margins are important geoarchives related to mantle dynamics, the breakup of continents, lithospheric dynamics, and other processes. The main concern yields the quantifying long-term lithospheric evolution of the continental margin between São Paulo and Laguna in southeastern Brazil since the Neoproterozoic. We put special emphasis on the reactivation of old fracture zones running into the continent and their constrains on the landscape evolution. In this contribution, we represent already consisting thermochronological data attained by fission-track and (U-Th-Sm)/He analysis on apatites and zircons. The zircon fission-track ages range between 108.4 (15.0) and 539.9 (68.4) Ma, the zircon (U-Th-Sm)/He ages between 72.9 (5.8) and 427.6 (1.8) Ma whereas the apatite fission-track ages range between 40.0 (5.3) and 134.7 (8.0) Ma, and the apatite (U-Th-Sm)/He ages between 32.1 (1.52) and 92.0 (1.86) Ma. These thermochronological ages from metamorphic, sedimentary and intrusive rocks show six distinct blocks (Laguna, Florianópolis, Curitiba, Ilha Comprida, Peruibe and Santos) with different evolution cut by old fracture zones. Furthermore, models of time-temperature evolution illustrate the differences in Pre- to post-rift exhumation histories of these blocks. The presented data will provide an insight into the complex exhumation history of the continental margin based on the existing literature data on the evolution of the Paraná basin in Brazil and the latest thermochronological data. We used the geological model of the Paraná basin supersequences (Rio Ivaí, Paraná, Gondwana I-III and Bauru) to remodel the subsidence and exhumation history of our consisting thermochronological sample data. First indications include a fast exhumation during the early Paleozoic, a slow shallow (northern blocks) to fast and deep (Laguna block) subduction from middle Paleozoic to Mesozoic time and a extremely fast exhumation during the opening of the South Atlantic

  3. Influence of lower crustal rheology on onset and distribution of melting and serpentinisation during rifting: comparison with the Brazilian/African conjugate margins

    NASA Astrophysics Data System (ADS)

    Perez-Gussinye, M.; Araujo, M. N.; Romeiro, M. T.; Martinez, M. A.; Morgan, J. P.; Ros, E.

    2014-12-01

    The onset and distribution of melting and serpentinisation during rifting determine the continent-ocean transition width and composition and have been shown to depend on extension velocity. Conductive cooling during slow rifting favors serpentinisation and inhibits melting (Perez-Gussinye et al., 2006). Here we use numerical modeling to show that, additionally, lower crustal rheology, which also controls margin symmetry and width (Brune et al. 2014), strongly influences the onset and distribution of melting and serpentinisation. We find that strong lower crust rheologies effectively couple deformation in upper crust and mantle and lead to rapid crustal break-up through crust-cutting faults (see Brune et al., 2014), allowing serpentinisation to start relatively early and producing narrow, symmetric margins. Coupling of lithospheric layers leads to rapid asthenospheric uplift and the onset of melting at a relatively early stage during extension. For slow velocities, serpentinisation starts before melting, and the little magma produced probably ponds under the serpentinite layer exhumed after crustal break-up, generating a wide continent-ocean transition. For the same extension velocities, relatively weak lower crust shows a long initial phase of distributed faulting, with moderate lithospheric thinning, followed by a long phase of sequential, oceanward younging faults, producing wider, asymmetric margins. Serpentinisation is insignificant because lower crustal flow towards the tip of the active fault inhibits the formation of crust cutting faults. Asthenospheric upwelling is less pronounced, and the onset and amount of melting is delayed with respect to the stronger lower crust case. When crustal break-up occurs magma rises to form oceanic crust and hence a narrow continent-ocean transition. Along Brazil and Africa the margin's symmetry, width and continent-ocean transition type change as the onshore terranes in which they developed go from cratons to mobile belts

  4. Detrital zircon U-Pb geochronology and whole-rock Nd-isotope constraints on sediment provenance in the Neoproterozoic Sergipano orogen, Brazil: From early passive margins to late foreland basins

    NASA Astrophysics Data System (ADS)

    Oliveira, E. P.; McNaughton, N. J.; Windley, B. F.; Carvalho, M. J.; Nascimento, R. S.

    2015-11-01

    SHRIMP U-Pb detrital zircon geochronology and depleted-mantle Nd-model ages of clastic rocks were combined to understand the sediment provenance in the Neoproterozoic Sergipano Belt. The Sergipano is the main orogenic belt between the Borborema province and the São Francisco Craton, eastern South America; it is divisible into several lithostratigraphic domains from North to South: Canindé, Poço Redondo-Marancó, Macururé, Vaza Barris, and Estância. Nd model ages (TDM) and detrital zircon U-Pb SHRIMP geochronology indicate that the protoliths of clastic metasedimentary rocks from the Marancó and Macururé domains were mostly derived from eroded late Mesoproterozoic to early Neoproterozoic rocks (1000-900 Ma), whereas detritus of similar rocks from the Canindé domain came from a younger source (ca. 700 Ma and 1000 Ma). Samples from the Vaza Barris domain show the greatest scatter of both TDM and zircon ages amongst all domains, but with important contributions from Proterozoic sources (690-1050 Ma and ca. 2100 Ma) and less from Archaean sources. The Estância domain samples have zircon population peaks at 570 Ma, 600 Ma, and 920-980 Ma, with a few older grains; one diamictite contains only ca. 2150 Ma zircon grains. Our preliminary results support a model in which sediments of the Marancó and Macururé domains were deposited on a continental margin of the ancient Borborema plate before its collision with the São Francisco Craton; the Canindé domain is likely to be an aborted Neoproterozoic rift assemblage within the southern part of the Borborema plate (Pernambuco-Alagoas massif). The basal units of the Vaza Barris and Estância domains have clast sources from the São Francisco Craton and are best interpreted as passive margin sediments. However, the uppermost units of the Estância and Vaza Barris domains come from foreland basins formed during collision of Borborema plate with the São Francisco Craton.

  5. East Africa continental margins

    SciTech Connect

    Bosellini, A.

    1986-01-01

    New well data from Somalia, together with the history of sea-floor spreading in the Indian Ocean derived from magnetic anomalies, show that the East African margins from latitude 15/sup 0/S into the Gulf of Aden comprise four distinct segments that formed successively by the southward drift of Madagascar from Somalia during the Middle to Late Jurassic and Early Cretaceous, by the northeastward drift of India along the Owen Transform during the Late Cretaceous and Paleocene, and by the opening of the Gulf of Aden during the Neogene.

  6. Austrian phase on the northern African margin inferred from sequence stratigraphy and sedimentary records in southern Tunisia (Chotts and Djeffara areas)

    NASA Astrophysics Data System (ADS)

    Lazzez, Marzouk; Zouaghi, Taher; Ben Youssef, Mohamed

    2008-08-01

    A multidisciplinary study concerning Aptian and Albian deposits is reported from petroleum wells and the exposed section. The biostratigraphic and sedimentological analysis defined four sedimentary units. Well-logging signals' analysis allows us to refine the record resolution on Aptian series and reveals, in the Djeffara field, a transgressive system tract (TST) and a highstand system tract (HST). Exceptionally, the first sequence (S1) in the Mareth 1 well and the fifth sequence in the two wells Mareth 1 and Gourine 1 reveal the lower-stand system tract (LST). The unconformities characterized by the absence of Upper Aptian (Clansayesian) and Lower to Middle Albian deposits signed by a significant gamma-ray reduction. The Middle and Upper Albian is represented by only one deposit sequence (S6) in Mareth 1. Towards the south, in the Gourine well, two deposit sequences were identified (S6 and S7); to specify the Aptian and Albian evolution of the deposit sequences, a tentative correlation has been established between the Chotts and Djeffara areas. This correlation allows us to characterize the sedimentary unconformities related to the tectonics and eustatic events. The Chotts and the Djeffara deposition areas were developed, characterized by an irregular subsidence and separated by the Tebaga Medenine high area. The Aptian-Albian subsidence platform of southern Tunisia may be considered as a block diagram of environmental deposit with regressive and transgressive trends, showing the impact of tectonic deformations on the palaeogeographic evolution of southeastern Tunisia during the Austrian phase. This study also must be replaced within regional structural patterns that may explain both the sequential and sedimentological evolution of the area. Deformations regionally identified are integrated in the more general context of both Tethyan and Atlantic areas related to the drift of the African platform.

  7. Enriched asthenosphere melting beneath the nascent North African margin: trace element and Nd isotope evidence in middle-late Triassic alkali basalts from central Sicily (Italy)

    NASA Astrophysics Data System (ADS)

    Cirrincione, Rosolino; Fiannacca, Patrizia; Lustrino, Michele; Romano, Vanessa; Tranchina, Annunziata; Villa, Igor M.

    2016-03-01

    During the dismembering of the Pangea supercontinent, middle-late Triassic sub-volcanic alkaline rocks were emplaced in central Sicily. These rocks have an alkali basaltic composition and show OIB-like incompatible element patterns in primitive mantle-normalized diagrams (e.g., enrichments in HFSE and LREE coupled with high HFSE/LILE ratios), as well as slightly positive \\varepsilon_{Nd} values. Only subtle effects of crustal contamination at shallow depths emerge from geochemical data. These characteristics are very different compared with the Permian calcalkaline magmas from elsewhere in SW Europe still carrying the geochemical signature of modifications related to the Variscan orogeny. The mineralogical, geochemical and isotopic compositions of the investigated samples from central Sicily are also different from the coeval shoshonitic volcano-plutonic formations of Southern Alps (Dolomites). The incompatible element composition and Nd isotopic ratios are consistent with low-degree partial melting of a moderately depleted asthenospheric mantle source, with a negligible involvement of the thinned continental crust. The studied alkaline basalts represent the only known evidence of a segment of the Triassic rift system associated with early Pangea breakup in central Sicily. The close similarity of the central Sicily Triassic alkali basalts with coeval basalts emplaced along former orogenic sutures across the peri-Mediterranean area suggests a common origin related, at least partly, to asthenospheric passive upwelling following the tectonic collapse of the Variscan Belt. These rocks provide new constraints on the spatial-temporal distribution, magma source evolution and geodynamic meaning of the widespread Permo-Triassic basic magmatism developed after the end of the Variscan Orogeny in southwestern Europe.

  8. The Role of Plumes in Breakup Processes - Traces Found in the Deep Crustal Structure at the Intersection of Walvis Ridge with the African Continental Margin

    NASA Astrophysics Data System (ADS)

    Fromm, T.; Jokat, W.; Behrmann, J. H.; Ryberg, T.; Weber, M. H.

    2014-12-01

    Large igneous provinces (LIP) are often found in close temporal and spatial proximity with continental breakups, supporting the model, that an arriving mantle plume produces large amounts of melt and has a massive influence on the breakup process. The South Atlantic is a classical example, with flood basalts on both adjacent continents and a paired age progressing ridge system connecting them with the current hotspot location at Tristan da Cunha. To estimate the influence of the plume on the preexisting continental crust, a large-scale geophysical experiment was conducted in 2011 at the intersection of Walvis Ridge with the African continent. We present four P-wave velocity models derived from seismic refraction data. One extends 430 km along the ridge crest and continues onshore to a total length of 730 km, while the other three crossing the ridge perpendicular: one (480 km long) far offshore in the oceanic regime, one (600 km) close to shelf break and the last one (400 km) onshore. Crustal velocities beneath Walvis Ridge range between 5.5 km/s and 7.0 km/s, which are typical velocities for oceanic crust. The crustal thickness, however, is approximately three times larger than of normal oceanic crust: 17 km in the western part increasing to 22 km towards the continent. The continent ocean transition is characterized by 30 km thick crust with a high velocity body (HVB) in the lower crust and seismic velocities up to 7.5 km/s. The western extend of the HVB is to a similar distance from shore as for HVBs observed south of Walvis Ridge. In contrast, the eastern boundary lies well within the continental domain, at the 40 km thick crust of the Kaoko fold belt. Here, the variation of seismic velocities indicates that hot material intruded the continental crust during the initial rifting stage. However, beyond this relatively sharp boundary (40 km wide), the remaining continental crust seems unaffected by intrusions and the root of the Kaoko belt is no eroded. The cross

  9. Post-orogenic evolution of the Sierras Septentrionales and the Sierras Australes and links to the evolution of the eastern Argentina South Atlantic passive continental margin constrained by low temperature thermochronometry and 2D thermokinematic modeling

    NASA Astrophysics Data System (ADS)

    Kollenz, Sebastian; Glasmacher, Ulrich Anton; Rossello, Eduardo A.

    2013-04-01

    The eastern Argentina South Atlantic passive continental margin is distinguished by a very flat topography. Out of the so called Pampean flat two mountain ranges are arising. These mountain ranges, the Sierras Australes and the Sierras Septentrionales, are located in the State of Buenos Aires south of the capital Buenos Aires. North of the Sierras Septentrionales the Salado basin is located. The Sierras Septentrionales and the Sierras Australes are also divided by a smaller intracratonic basin. Further in the South the Colorado basin is located. The Sierras Australes is a variscian fold belt originated by strong phases of metamorphosis, but till now it is unclear by how many tectonic phases the area was influenced (Tomezzoli & Vilas, 1999). It consists of Proterozoic to Paleozoic rocks. The Sierras Septentrionales consists mainly of Precambrian crystalline rocks. The Precambrian sequences are overlain by younger Sediments (Cingolani, 2010). The aim is to understand the long-term landscape evolution of the area by quantifiying erosion- and exhumation-rates and by dating ancient rock-uplift-events. Another goal is to find out how the opening of the south atlantic took effect on this region. To fulfill this goal, thermochronological techniques, such as fission-track dating and (U-Th-Sm)/He dating has been applied to samples from the region. Because there was no low-temperature thermochronology done in this area, both techniques were applied on apatites and zircons. Furthermore, numerical modeling of the cooling history has provided the data base for the quantification of the exhumation rates. The first data-set shows clusters of different ages which can be linked to tectonic activities during late Paleozoic times. Also the thermokinematic modeling is leading to new insights of the evolution of both mountain ranges. References: Renata Nela Tomezzoli and Juan Francisco Vilas (1999): Palaeomagnetic constraints on the age of deformation of the Sierras Australes thrust and

  10. 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,…

  11. 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.

  12. Marginality principle

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Soil is a fragile resource supplying many goods and services. Given the diversity of soil across the world and within a landscape, there are many different capacities among soils to provide the basic soil functions. Marginality of soils is a difficult process to define because the metrics to define ...

  13. Geochemical study (major, trace elements and Pb-Sr-Nd isotopes) of mantle material obducted onto the North African margin (Edough Massif, North Eastern Algeria): Tethys fragments or lost remnants of the Liguro-Provençal basin?

    NASA Astrophysics Data System (ADS)

    Bosch, Delphine; Hammor, Dalila; Mechati, Mehdi; Fernandez, Laure; Bruguier, Olivier; Caby, Renaud; Verdoux, Patrick

    2014-06-01

    The Maghrebides, Betics and some parts of the Calabrian, NE Sicilian and Tuscan allochtons constitute dismembered fragments of the Alpine belt that resulted from the Cenozoic collision between Africa and Eurasia and the opening of the Western Mediterranean basin. Mineral and whole-rock geochemical analyses have been performed on three distinct outcrops of mantle material from the Edough Massif of NE Algeria, namely the Bou Maiza (BM) gabbros, the La Voile Noire (LVN) amphibolites and the Sidi Mohamed (SM) peridotites. In all samples, Sr isotopes are largely affected by seawater alteration (87Sr/86Sract. > 0.70384 and up to 0.70888) and cannot be used to evaluate the nature of the source reservoirs. SM peridotites display variable depleted mantle Nd isotopic signatures (εNdact. from + 7.0 to + 12.2) and geochemical features suggesting no significant chemical depletion as a result of partial melting and melt extraction (Mg# < 90; slightly LREE-depleted patterns with La/YbN = 0.33-0.39). These rocks are interpreted as parts of the subcontinental lithospheric mantle incorporated into the crustal units of the Edough Massif during the early stages of opening of the Algerian basin. BM gabbros and LVN amphibolites show geochemical signatures indicating derivation from a common depleted mantle reservoir (εNd > + 7.9) and are likely cogenetic, but without filiation with the SM peridotites. Pb isotopes indicate a contribution of sediments in the source reservoir, which is attributed to contamination solely by hydrous fluids released from a sedimentary component. This observation, together with a LILE-enrichment, suggests a back-arc basin environment. These results indicate that BM and LVN units were obducted onto the North African margin and subsequently fragmented, probably during doming and exhumation of the lower continental crust of the Edough massif. Doming resulted in opposite movements of the overlying oceanic units, southward for the BM units and northward for LVN

  14. Geodynamic models of convergent margin tectonics: transition from rifted margin to overthrust belt and consequences for foreland-basin development

    SciTech Connect

    Stockmal, G.S.; Beaumont, C.; Boutilier, R.

    1986-02-01

    A quantitative geodynamic model for overthrusting of a passive continental margin during attempted continental subduction demonstrates the mechanical and thermal coupling between overthrust loads, the lithosphere, and the associated foreland basin. The model treated the lithosphere as a two-dimensional nonuniform elastic plate whose strength is controlled thermally. The thermal and flexural evolution of a margin is followed from initial rifting and passive-margin development, through overthrusting and foreland-basin deposition, to postdeformational erosion.

  15. East African and Kuunga Orogenies in Tanzania - South Kenya

    NASA Astrophysics Data System (ADS)

    Fritz, H.; Hauzenberger, C. A.; Tenczer, V.

    2012-04-01

    Tanzania and southern Kenya hold a key position for reconstructing Gondwana consolidation because here different orogen belts with different tectonic styles interfere. The older, ca. 650-620 Ma East African Orogeny resulted from the amalgamation of arc terranes in the northern Arabian-Nubian Shield (ANS) and continental collision between East African pieces and parts of the Azania terrane in the south (Collins and Pisarevsky, 2005). The change form arc suturing to continental collision settings is found in southern Kenya where southernmost arcs of the ANS conjoin with thickened continental margin suites of the Eastern Granulite Belt. The younger ca. 570-530 Ma Kuunga orogeny heads from the Damara - Zambesi - Irumide Belts (De Waele et al., 2006) over Tanzania - Mozambique to southern India and clashes with the East African orogen in southern-central Tanzania. Two transitional orogen settings may be defined, (1) that between island arcs and inverted passive continental margin within the East African Orogen and, (2) that between N-S trending East African and W-E trending Kuungan orogenies. The Neoproterozoic island arc suites of SE-Kenya are exposed as a narrow stripe between western Azania and the Eastern Granulite belt. This suture is a steep, NNW stretched belt that aligns roughly with the prominent southern ANS shear zones that converge at the southern tip of the ANS (Athi and Aswa shear zones). Oblique convergence resulted in low-vorticity sinstral shear during early phases of deformation. Syn-magmatic and syn-tectonic textures are compatible with deformation at granulite metamorphic conditions and rocks exhumed quickly during ongoing transcurrent motion. The belt is typified as wrench tectonic belt with horizontal northwards flow of rocks within deeper portions of an island arc. The adjacent Eastern Granulite Nappe experienced westward directed, subhorizontal, low-vorticity, high temperature flow at partly extreme metamorphic conditions (900°C, 1.2 to 1.4 GPa

  16. Lithospheric thickness jumps at the S-Atlantic continental margins from satellite gravity data and modelled isostatic anomalies

    NASA Astrophysics Data System (ADS)

    Shahraki, Meysam; Schmeling, Harro; Haas, Peter

    2016-04-01

    Isostatic equilibrium is a good approximation for passive continental margins. In these regions, geoid anomalies are proportional to the local dipole moment of density-depth distributions, which can be used to constrain the thickness of lithospheric jumps and corresponding tectonic stress. We analysed satellite derived geoid data and, after filtering, extracted typical averaged profiles across the Western and Eastern passive margins of the South Atlantic. They show geoid jumps of 8.1 m and 7.0 m for the Argentinian and African sides, respectively. Together with topography data and reasonable assumptions about densities these jumps are interpreted as isostatic geoid anomalies and yield best-fitting crustal and lithospheric thicknesses. They reveal a small asymmetry between the African and S-American crusts and lithospheres by a few kilometers. On both sides, the continental lithosphere is about 15 - 30km thicker than the oceanic lithosphere. To keep such geoid jumps stable over O(100Ma) fully dynamic models show that lithospheric viscosities must be of the order of 1e23 Pa s.

  17. GROWTH OF THE GREAT ESCARPMENT ACROSS THE INDIAN MARGIN OF SOUTH AFRICA: a couple stratigraphic-geomorphologic study

    NASA Astrophysics Data System (ADS)

    Baby, Guillaume; Guillocheau, François; Robin, Cécile; Dall'Asta, Massimo

    2015-04-01

    The South African Plateau is formed by marginal bulges clustered around an intracontinental basin (the Kalahari Basin) with a mean elevation between 1000 and 1400 m. On seaward side, marginal bulges form major escarpments that can reach an elevation up to 3500 m in the Drakensberg area, boundering the high elevation continent from a dissected coastal region. The factors controlling escarpment evolution of those high-elevation passive margins are highly debated. On the one hand, geomorphic studies interpret escarpments in term of pulses of uplift and scarp retreat (King, The Natal Monocline, 1982; Partridge & Maud, S.Afr.J.Geol., 1987). On the other hand, thermochronological data and numerical models of escarpment erosion (Gallagher & Brown, Phil.Trans.R.Soc.Lon., 1999; Van der Beek et al., J.Geophys.Res., 2002) suggest that escarpments predate the breakup with a minimal escarpment retreat during post-rift margin evolution. To answer this question, we studied the Indian margin of South Africa (from Bushveld area to Port-Elizabeth) using sequence stratigraphy analysis of industrial seismic lines and wells. This study is coupled with an analysis of the adjacent landforms, constrained by dated sediments and weathering deposits. The first outcomes of our study are: 1. A first uplift during Late Cenomanian (95-90 Ma) created an initial escarpment along the Indian coast. 2. A second uplift occurred during the latest Cretaceous to earliest Cenozoïc with a sequential tilting and truncations of the inner part of the margin followed by the incision of pediments on the seaward side of the initial escarpment, 3. A third uplift that occurred during Late Eocene - Early Oligocene and Miocene with the incision of two new generations of pediments. These preliminary results suggest that the "Great Escarpment" along the Indian coast of South Africa results from the stepping of at least four generations of pediments which record the polyphasic uplift history of the South African

  18. The deep structure of the South Atlantic rifted margins and the implications of the magmatic processes for the break-up

    NASA Astrophysics Data System (ADS)

    Becker, Katharina; Dieter, Franke; Trumbull, Robert; Schnabel, Michael; Heyde, Ingo; Schreckenberger, Bernd; Koopmann, Hannes; Bauer, Klaus; Jokat, Wilfried; Krawczyk, Charlotte

    2014-05-01

    The high velocity lower crust HVLC (Vp > 7km/s) together with seaward dipping reflectors (SDRs) and continental flood basalts are specific characteristics of volcanic rifted margins. The nature and origin of the HVLC is still under discussion. Here we provide a comprehensive study of the deep crustal structure of the South Atlantic rifted margins in which we focus on variations in the distribution and size of HVLC bodies along and across the margins. Two new and five existing refraction lines complemented by gravity models cover the area between the Rio Grande Rise - Walvis Ridge to the Falkland Agulhas Fracture Zone. Three seismic lines on the South American margin outline the change from a non-magmatic margin (lacking seaward dipping reflectors) in the south to a well-developed volcanic rifted margin off Uruguay in the north. While the HVLC exhibit a consistent increase in the cross-sectional area along both margins from South to North, we observe a major asymmetry across the margins. The African margin has about two-three times thicker and four times more voluminous HVLC than the South American margin. Importantly, the erupted lavas in the Etendeka-Paraná Provinces show the opposite asymmetry. Also the spatial position of the HVLC with regard to the inner SDRs varies consistently along both margins. Close to the Falkland Agulhas Fracture zone a small body of HVLC is not accompanied by seaward dipping reflectors. In the central segment, HVLC is centered under the SDRs inner wedge but in the north, HVLC also extends further seawards. These observations question a simple extrusive/intrusive relationship between SDRs and HVLC, and they imply differences in the timing of the HVLC formation during the rifting and break-up process. We conclude that the HVLC is predominantly a magmatic feature related mantle melting during break-up. Melt generation models suggest that the greater thickness of HVLC on the African margin is due to active upwelling combined with elevated

  19. 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.

  20. Late differentiation of proximal and distal margins in the Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Bache, F.; Leroy, S.; D'Acremont, E.; Autin, J.; Watremez, L.; Rouzo, S.

    2009-04-01

    , F. Klingelhoefer, C. Labails, L. Matias, H. Nouzé, and M. Rabineau, 2008, Brazilian and African Passive Margins of the Central Segment of the South Atlantic Ocean: Kinematic constraints: Tectonophysics, v. doi: 10.1016/j.tecto.2008.12.016. Autin, J., 2008, Déchirure continentale et segmentation du Golfe d'Aden oriental en contexte de rifting oblique: Ph. D. thesis, Université Pierre et Marie Curie, Paris VI, 310 p. Bache, F., 2008, Evolution Oligo-Miocène des marges du micro océan Liguro Provençal.: Ph. D. thesis, Université de Bretagne Occidentale/CNRS/IFREMER. http://www.ifremer.fr/docelec/notice/2008/notice4768-EN.htm, Brest, 328 p. Boillot, G., J. Girardeau, and J. Kornprobst, 1988, The rifting of the Galicia margin: crustal thinning and emplacement of mantle rocks on the seafloor (ODP Leg 103). In Boillot, G., Winterer, E.L., et al., Proc. ODP, Sci. Results, v. 103, College Station, TX (Ocean Drilling Program), p. 741-756. d'Acremont, E., S. Leroy, M. O. Beslier, N. bellahsen, M. Fournier, C. Robin, M. Maia, and P. Gente, 2005, Structure and evolution of the eastern Gulf of Aden conjugate margins from seismic reflection data: Geophys. J. Int., v. 160, p. 869-890. d'Acremont, E., S. Leroy, M. Maia, P. Patriat, M. O. Beslier, N. Bellahsen, M. Fournier, and P. Gente, 2006, Structure and evolution of the eastern Gulf of Aden: insights from magnetic and gravity data (Encens-Sheba MD117 cruise): Geophys. J. Int., v. 165, p. 786-803. Dupré, S., G. Bertotti, and S. Cloetingh, 2007, Tectonic history along the South Gabon Basin: Anomalous early post-rift subsidence: Mar. Pet. Geol., v. 24, p. 151-172. Labails, C., 2007, La marge sud-marocaine et les premières phases d'ouverture de l'océan Atlantique Central: Ph. D. thesis, Université de Bretagne Occidentale, Brest. Leroy, S., P. Gente, M. Fournier, E. d'Acremont, P. Patriat, M. O. Beslier, N. Bellahsen, M. Maia, A. Blais, J. Perrot, A. Al-Kathiri, S. Merkouriev, J. M. Fleury, P. Y. Ruellan, C. Lepvrier, and P

  1. An Analysis of Wilson Cycle Plate Margins

    NASA Astrophysics Data System (ADS)

    Buiter, S.; Torsvik, T. H.

    2012-12-01

    The Wilson Cycle theory that oceans close and open along the same suture is a powerful concept in analyses of ancient plate tectonics. It implies that collision zones are structures that are able to localize extensional deformation for long times after the collision has waned. However, some sutures are seemingly never reactivated and already Tuzo Wilson recognized that Atlantic break-up did not follow the precise line of previous junction. We have reviewed margin pairs around the Atlantic and Indian Oceans with the aim to evaluate the extent to which oceanic opening used former sutures, summarize delay times between collision and break-up, and analyze the role of mantle plumes in continental break-up. We aid our analyses with plate tectonic reconstructions using GPlates (www.gplates.org). Although at first sight opening of the North Atlantic Ocean largely seems to follow the Iapetus and Rheic sutures, a closer look reveals deviations. For example, Atlantic opening did not utilize the Iapetus suture in Great Britain and rather than opening along the younger Rheic suture north of Florida, break-up occurred along the older Pan-African structures south of Florida. We find that today's oceanic Charlie Gibbs Fracture Zone, between Ireland and Newfoundland, is aligned with the Iapetus suture. We speculate therefore that in this region the Iapetus suture was reactivated as a transform fault. As others before us, we find no correlation of suture and break-up age. Often continental break-up occurs some hundreds of Myrs after collision, but it may also take over 1000 Myr, as for example for Australia - Antarctica and Congo - São Francisco. This places serious constraints on potential collision zone weakening mechanisms. Several studies have pointed to a link between continental break-up and large-scale mantle upwellings. It is, however, much debated whether plumes use existing rifts as a pathway, or whether plumes play an active role in causing rifting. We find a positive

  2. Deep Stucture of the Northwestern Atlantic Moroccan Margin Studied by OBS and Deep Multichannel Seismic Reflection.

    NASA Astrophysics Data System (ADS)

    MALOD, J. A.; Réhault, J.; Sahabi, M.; Géli, L.; Matias, L.; Diaz, J.; Zitellini, N.

    2001-12-01

    The Northwestern Atlantic Moroccan margin, a conjugate of the New Scotland margin, is one of the oldest passive margin of the world. Continental break up occurred at early Liassic time and the deep margin is characterized by a large salt basin. A good knowledge of this basin is of major interest to improve the initial reconstruction between Africa, North America and Iberia (Eurasia). It is also a good opportunity to study a mature passive margin and model its structure and evolution.Moreover, there is a need to assess the geological hazards linked to the neotectonic activity within the Africa-Eurasia plate boundary. These topics have been adressed during the SISMAR cruise carried out from April 9th to May 4th 2001.During this cruise, 3667 km of multichannel seismic reflection (360 channels, 4500 m long streamer, 4800 ci array of air guns) were recorded together with refraction records by means of 48 OBH/OBS drops. Simultaneously, some of the marine profiles have been extended onshore with 16 portable seismic land stations. We present the initial results of this study. Off El Jadida, the Moho and structures within the thinned continental crust are well imaged on both the reflection and refraction records. In the northern area, off Casablanca, we follow the deepening of the moroccan margin beneath the up to 9 sec (twtt) allochtonous series forming a prism at the front the Rif-Betic chain. Sismar cruise has been also the opportunity to record long seismic profiles making the junction between the Portuguese margin and the Moroccan one, and crossing the Iberian-African plate boundary. This allows to observe the continuity of the sedimentary sequence after the end of the large inter-plate motion in Early Cretaceous. In addition to the authors, SISMAR Group includes: AMRHAR Mostafa, BERMUDEZ VASQUEZ Antoni, CAMURRI Francesca, CONTRUCCI Isabelle, CORELA Carlos, DIAZ Jordi, DORVAL Philippe, EL ARCHI Abdelkrim, EL ATTARI Ahmed, GONZALEZ Raquel, HARMEGNIES Francois, JAFFAL

  3. Passive Accelerometer

    NASA Technical Reports Server (NTRS)

    Naumann, Robert J.; Baugher, Charles; Alexander, Iwan

    1992-01-01

    Motion of ball in liquid indicates acceleration. Passive accelerometer measures small accelerations along cylindrical axis. Principle of operation based on Stokes' law. Provides accurate measurements of small quasi-steady accelerations. Additional advantage, automatically integrates out unwanted higher-frequency components of acceleration.

  4. Petroleum geology of Cote d`Ivoir (Abidjan margin)

    SciTech Connect

    Reymond, A.

    1995-08-01

    The Cote d`Ivoire sedimentary basin is part of a typical transform passive margin developed along the West African coast from Liberia to Ghana. It straddles the coastline and the sedimentary section expands dramatically South of the East-West trending Lagune Fault, with up to 10,000 metres of sediments from Aptian to Present in age. Albo-Aptian rift series, mainly continental clastics without evaporites, have accumulated progressively in a tilted semi-graben. The drift stage marks a widespread marine sedimentation organized in progradational sequences which blanket the Albian block-fault topography of the continental break-up. Reservoirs are mainly clastics and present in the section from Middle Albian to Maastrichtian. Sand bodies are associated with identified submarine fans, infill structures or channelized units deposited in a shelf or outer-shelf environment. Trapping opportunities are due to block-faulting in the rift section, or gravity tectonics in the drift section, often combined with sand pinch-outs to constitute mixed structural-stratigraphic traps. Thick top-seal units formed by marine shales are widespread. Source-rocks have been shown to belong mainly to the rift series and they have been characterized in terms of geochemistry and maturation timing. An efficient simulation model has been used to recontruct the expulsion, migration and trapping of hydrocarbons along a selected North-South cross-section and to better define the Petroleum Systems.

  5. [Passive smoking].

    PubMed

    Grandjean, E; Weber, A; Fischer, T

    1979-03-01

    Passive smoking is the involuntary inspiration of smoky indoor air. Based on the information available today, it may be assumed that passive smoking normally is no health hazard as far as the classical smoker's diseases (lung cancer, myocardial infarct, etc.) are concerned. Nevertheless, it is probable that irritations caused by tobacco smoke have an unfavorable influence on the health of small children and that of already sick persons. The main problem of passive smoking is annoyance due to odor and irritations of eyes and respiratory organs. Our investigations in a climatic chamber with healthy subjects show that air pollution caused by tobacco smoke as indicated by 5 ppm CO leads to marked eye irritations--objectively as well as subjectively--in 15 to 20% of the subjects. This corresponds to smoking 10 cigarettes per hour in a small room with an air ventilation rate of four times per hour. If air pollution caused by tobacco smoke lies below the level of 2 ppm CO, irritations and annoyance for healthy persons are regarded as low and tolerable. This corresponds to about four cigarettes per hour under the same circumstances.

  6. Fault Orientations at Obliquely Rifted Margins: Where? When? Why?

    NASA Astrophysics Data System (ADS)

    Brune, Sascha

    2015-04-01

    Present-day knowledge of rifted margin formation is largely based on 2D seismic lines, 2D conceptual models, and corroborated by 2D numerical experiments. However, the 2D assumption that the extension direction is perpendicular to the rift trend is often invalid. In fact, worldwide more than 75% of all rifted margin segments have been formed under significant obliquity exceeding 20° (angle measured between extension direction and rift trend normal): During formation of the Atlantic Ocean, oblique rifting dominated at the sheared margins of South Africa and Patagonia, the Equatorial Atlantic margins, separation of Greenland and North America, and it played a major role in the protracted rift history of the North East Atlantic. Outside the Atlantic Ocean, oblique rifting occurred during the split between East and West Gondwana, the separation of India and Australia, India and Madagascar, Australia and Antarctica, as well as Arabia and Africa. It is presently observed in the Gulf of California, the Aegean and in the East African Rift. Despite its significance, the degree to which oblique lithospheric extension affects first-order rift and passive margin properties like surface stress pattern, fault azimuths, and basin geometry, is still not entirely clear. This contribution provides insight in crustal stress patterns and fault orientations by applying a 3D numerical rift model to oblique extensional settings. The presented forward experiments cover the whole spectrum of oblique extension (i.e. rift-orthogonal extension, low obliquity, high obliquity, strike-slip deformation) from initial deformation to breakup. They are conducted using an elasto-visco-plastic finite element model and involve crustal and mantle layers accounting for self-consistent necking of the lithosphere. Results are thoroughly compared to previous analogue experiments, which yields many similarities but also distinct differences for late rift stages and for high obliquity. Even though the model

  7. Continental Margins: Linking Ecosystems

    NASA Astrophysics Data System (ADS)

    Kelly-Gerreyn, Boris; Rabalais, Nancy; Middelburg, Jack; Roy, Sylvie; Liu, Kon-Kee; Thomas, Helmuth; Zhang, Jing

    2008-02-01

    Impacts of Global, Local and Human Forcings on Biogeochemical Cycles and Ecosystems, IMBER/LOICZ Continental Margins Open Science Conference; Shanghai, China, 17-21 September 2007; More than 100 scientists from 25 countries came together to address global, regional, local, and human pressures interactively affecting continental margin biogeochemical cycles, marine food webs, and society. Continental margins cover only 12% of the global ocean area yet account for more than 30% of global oceanic primary production. In addition, continental margins are the most intensely used regions of the world's ocean for natural commodities, including productive fisheries and mineral and petroleum resources. The land adjacent to continental margins hosts about 50% of the world's population, which will bear many direct impacts of global change on coastal margins. Understanding both natural and human-influenced alterations of biogeochemical cycles and ecosystems on continental margins and the processes (including feedbacks) that threaten sustainability of these systems is therefore of global interest.

  8. The basins on the Argentine continental margin

    SciTech Connect

    Urien, C.M.

    1996-08-01

    After the stabilization of the central Gondwana Craton, orogenic belts were accreted, as a result of convergence events and an extensive passive margin developed in southwestern Gondwana. Thermal subsidence in Parana, Karoo-Ventania basins and the Late Paleozoic-Early Mesozoic rifts, were modified by the Gondwana breakup and the South Atlantic opening. Early Paleozoic marine transgressions deposited the Table Mountain Group in Ventania. In southwestern Patagonia foreland clastics were deposited. Magmatic arcs and marine units indicate a tectonic trough was formed, alternating with continental sequences, over Late Paleozoic metamorphics and intrusives, resulting from plastered terrains along the Gondwana margin. In Patagonia, Permo-Carboniferous continental and glacio marine clastics infill the basins, while in Ventania, paralic sequences, grade from neritic to continental to the northeast, extending beneath the continental margin. The Triassic-Jurassic rift basins progressed onto regional widespread acid lavas and were infilled by lagoonal organic-rich sequences. Early drift phase built basins transverse to the margin, with fluvio-lacustrine sequences: Salado, Colorado, Valdes-Rawson, San Julian and North Malvinas intracratonic basins, which underwent transtensional faulting. Post-Oxfordian to Neocomian brackish sequences, onlapped the conjugate basins during the margin`s drift, with petroleum systems, as in Austral and Malvinas. In the Valanginian, basic extrusions commenced to form on the continental border, heralding the oceanic phase. Due to thermal subsidence, offlaping sediments prograded onto the remaining half-grabens. Several petroleum systems, proven and hypothetical, are identified in this region.

  9. [Marginalization and health. Introduction].

    PubMed

    Yunes, J

    1992-06-01

    The relationship between marginalization and health is clear. In Mexico, for example, life expectancy is 53 years for the poorest population sectors and 20 years more for the wealthiest. Infant mortality in poor Colombian families is twice that of wealthier families, and one-third of developing countries the rural population is only half as likely as the urban to have access to health services. Women in the Southern hemisphere are 12 times likelier than those in the Northern to die of maternal causes. The most important step in arriving at a solution to the inequity may be to analyze in depth the relationship between marginality and health. Marginality may be defined as the lack of participation of individuals or groups in certain key phases of societal life, such as production, consumption, or political decision making. Marginality came to be viewed as a social problem only with recognition of the rights of all individuals to participate in available social goods. Marginality is always relative, and marginal groups exist because central groups determine the criteria for inclusion in the marginal and central groups. Marginality thus always refers to a concrete society at a specific historical moment. Marginal groups may be of various types. At present, marginal groups include women, rural populations, people with AIDS or mental illness or certain other health conditions, refugees, ethnic or religious groups, homosexuals, and the poor, who are the largest group of marginal persons in the world. Even in developed countries, 100-200 million persons live below the poverty line. Latin America is struggling to emerge from its marginal status in the world. The economic crisis of the 1980s increased poverty in the region, and 40% are not considered impoverished. Latin America is a clear example of the relationship between marginality and health. Its epidemiologic profile is intimately related to nutrition, availability of potable water, housing, and environmental

  10. [Marginalization and health. Introduction].

    PubMed

    Yunes, J

    1992-06-01

    The relationship between marginalization and health is clear. In Mexico, for example, life expectancy is 53 years for the poorest population sectors and 20 years more for the wealthiest. Infant mortality in poor Colombian families is twice that of wealthier families, and one-third of developing countries the rural population is only half as likely as the urban to have access to health services. Women in the Southern hemisphere are 12 times likelier than those in the Northern to die of maternal causes. The most important step in arriving at a solution to the inequity may be to analyze in depth the relationship between marginality and health. Marginality may be defined as the lack of participation of individuals or groups in certain key phases of societal life, such as production, consumption, or political decision making. Marginality came to be viewed as a social problem only with recognition of the rights of all individuals to participate in available social goods. Marginality is always relative, and marginal groups exist because central groups determine the criteria for inclusion in the marginal and central groups. Marginality thus always refers to a concrete society at a specific historical moment. Marginal groups may be of various types. At present, marginal groups include women, rural populations, people with AIDS or mental illness or certain other health conditions, refugees, ethnic or religious groups, homosexuals, and the poor, who are the largest group of marginal persons in the world. Even in developed countries, 100-200 million persons live below the poverty line. Latin America is struggling to emerge from its marginal status in the world. The economic crisis of the 1980s increased poverty in the region, and 40% are not considered impoverished. Latin America is a clear example of the relationship between marginality and health. Its epidemiologic profile is intimately related to nutrition, availability of potable water, housing, and environmental

  11. Neotectonics in the northern equatorial Brazilian margin

    NASA Astrophysics Data System (ADS)

    Rossetti, Dilce F.; Souza, Lena S. B.; Prado, Renato; Elis, Vagner R.

    2012-08-01

    An increasing volume of publications has addressed the role of tectonics in inland areas of northern Brazil during the Neogene and Quaternary, despite its location in a passive margin. Hence, northern South America plate in this time interval might have not been as passive as usually regarded. This proposal needs further support, particularly including field data. In this work, we applied an integrated approach to reveal tectonic structures in Miocene and late Quaternary strata in a coastal area of the Amazonas lowland. The investigation, undertaken in Marajó Island, mouth of the Amazonas River, consisted of shallow sub-surface geophysical data including vertical electric sounding and ground penetrating radar. These methods were combined with morphostructural analysis and sedimentological/stratigraphic data from shallow cores and a few outcrops. The results revealed two stratigraphic units, a lower one with Miocene age, and an upper one of Late Pleistocene-Holocene age. An abundance of faults and folds were recorded in the Miocene deposits and, to a minor extent, in overlying Late Pleistocene-Holocene strata. In addition to characterize these structures, we discuss their origin, considering three potential mechanisms: Andean tectonics, gravity tectonics related to sediment loading in the Amazon Fan, and rifting at the continental margin. Amongst these hypotheses, the most likely is that the faults and folds recorded in Marajó Island reflect tectonics associated with the history of continental rifting that gave rise to the South Atlantic Ocean. This study supports sediment deposition influenced by transpression and transtension associated with strike-slip divergence along the northern Equatorial Brazilian margin in the Miocene and Late Pleistocene-Holocene. This work records tectonic evidence only for the uppermost few ten of meters of this sedimentary succession. However, available geological data indicate a thickness of up to 6 km, which is remarkably thick for

  12. "Teaching while Black": Narratives of African American Student Affairs Faculty

    ERIC Educational Resources Information Center

    Patton, Lori D.; Catching, Christopher

    2009-01-01

    African American faculty have historically been underrepresented within predominantly white institutions (PWIs) and deal with academic isolation, marginalization of their scholarship, and racial hostility. Little is known about the experiences of African American faculty who teach in student affairs graduate programs. The purpose of this study was…

  13. Abrupt plate accelerations shape rifted continental margins

    NASA Astrophysics Data System (ADS)

    Brune, Sascha; Williams, Simon E.; Butterworth, Nathaniel P.; Müller, R. Dietmar

    2016-08-01

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth’s major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength–velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.

  14. Abrupt plate accelerations shape rifted continental margins.

    PubMed

    Brune, Sascha; Williams, Simon E; Butterworth, Nathaniel P; Müller, R Dietmar

    2016-08-11

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength--velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.

  15. Abrupt plate accelerations shape rifted continental margins.

    PubMed

    Brune, Sascha; Williams, Simon E; Butterworth, Nathaniel P; Müller, R Dietmar

    2016-08-11

    Rifted margins are formed by persistent stretching of continental lithosphere until breakup is achieved. It is well known that strain-rate-dependent processes control rift evolution, yet quantified extension histories of Earth's major passive margins have become available only recently. Here we investigate rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions. We find that rifted margins feature an initial, slow rift phase (less than ten millimetres per year, full rate) and that an abrupt increase of plate divergence introduces a fast rift phase. Plate acceleration takes place before continental rupture and considerable margin area is created during each phase. We reproduce the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions. The extension models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength--velocity feedback, which can be robustly inferred for diverse lithosphere configurations and rheologies. Our results explain differences between proximal and distal margin areas and demonstrate that abrupt plate acceleration during continental rifting is controlled by the nonlinear decay of the resistive rift strength force. This mechanism provides an explanation for several previously unexplained rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time. PMID:27437571

  16. The Zambezi sedimentary system (coastal plain - deep sea fan): a record of the vertical movements of the Mozambican margin since Cretaceous times.

    NASA Astrophysics Data System (ADS)

    Ponte, Jean Pierre; Robin, Cecile; Guillocheau, Francois; Baby, Guillaume; Dall'Asta, Massimo; Popescu, Speranta; Suc, Jean Pierre; Droz, Laurence; Rabineau, Marina; Moulin, Maryline

    2016-04-01

    The Mozambique margin is an oblique to transform margin which feeds one of the largest African turbiditic system, the Zambezi deep-sea fan (1800 km length and 400 km wide; Droz and Mougenot., AAPG Bull., 1987). The Zambezi sedimentary system is characterized by (1) a changing catchment area through time with evidences of river captures (Thomas and Shaw, J. Afr. Earth. Sci, 1988) and (2) a delta, storing more than 12 km of sediment, with no gravitary tectonics. The aim of this study is to carry out a source to sink study along the Zambezi sedimentary system and to analyse the margin evolution (vertical movements, climate change) since Early Cretaceous times. The used data are seismic lines (industrial and academic) and petroleum wells (with access to the cuttings). Our first objective was to perform a new biochronostratigraphic framework based on nannofossils, foraminifers, pollen and spores on the cuttings of three industrial wells. The second target was to recognize the different steps of the growth of the Zambezi sedimentary systems. Four main phases were identified: • Late Jurassic (?) - early Late Cretaceous: from Neocomian to Aptian times, the high of the clinoforms is getting higher, with the first occurrence of contouritic ridges during Aptian times. • Late Cretaceous - Early Paleocene: a major drop of relative sea-level occurred as a consequence of the South African Plateau uplift. The occurrence of two depocenters suggests siliciclastic supplies from the Bushveld and from the North Mozambique domain. • Early Paleocene - Eocene: growth of carbonate platforms and large contouritic ridges. • Oligocene - Present-day: birth of the modern Zambezi Delta, with quite low siliciclastic supply during Oligocene times, increasing during Miocene times. As previously expected (Droz and Mougenot) some sediments of the so-called Zambezi fans are coming from a feeder located east of the Davie Ridge. This study was founded by TOTAL and IFREMER in the frame of the

  17. Transform continental margins - part 1: Concepts and models

    NASA Astrophysics Data System (ADS)

    Basile, Christophe

    2015-10-01

    This paper reviews the geodynamic concepts and models related to transform continental margins, and their implications on the structure of these margins. Simple kinematic models of transform faulting associated with continental rifting and oceanic accretion allow to define three successive stages of evolution, including intra-continental transform faulting, active transform margin, and passive transform margin. Each part of the transform margin experiences these three stages, but the evolution is diachronous along the margin. Both the duration of each stage and the cumulated strike-slip deformation increase from one extremity of the margin (inner corner) to the other (outer corner). Initiation of transform faulting is related to the obliquity between the trend of the lithospheric deformed zone and the relative displacement of the lithospheric plates involved in divergence. In this oblique setting, alternating transform and divergent plate boundaries correspond to spatial partitioning of the deformation. Both obliquity and the timing of partitioning influence the shape of transform margins. Oblique margin can be defined when oblique rifting is followed by oblique oceanic accretion. In this case, no transform margin should exist in the prolongation of the oceanic fracture zones. Vertical displacements along transform margins were mainly studied to explain the formation of marginal ridges. Numerous models were proposed, one of the most used is being based on thermal exchanges between the oceanic and the continental lithospheres across the transform fault. But this model is compatible neither with numerical computation including flexural behavior of the lithosphere nor with timing of vertical displacements and the lack of heating related to the passing of the oceanic accretion axis as recorded by the Côte d'Ivoire-Ghana marginal ridge. Enhanced models are still needed. They should better take into account the erosion on the continental slope, and the level of coupling

  18. Fundamental studies of passivity and passivity breakdown

    SciTech Connect

    Macdonald, D.D.; Urquidi-Macdonald, M.; Song, H.; Biaggio-Rocha, S.; Searson, P.

    1991-11-01

    This report summarizes the findings of our fundamental research program on passivity and passivity breakdown. During the past three and one half years in this program (including the three year incrementally-funded grant prior to the present grant), we developed and experimentally tested various physical models for the growth and breakdown of passive films on metal surfaces. These models belong to a general class termed point defects models'' (PDMs), in which the growth and breakdown of passive films are described in terms of the movement of anion and cation vacancies.

  19. Ocean margins workshop

    SciTech Connect

    1990-12-31

    The Department of Energy (DOE) is announcing the refocusing of its marine research program to emphasize the study of ocean margins and their role in modulating, controlling, and driving Global Change phenomena. This is a proposal to conduct a workshop that will establish priorities and an implementation plan for a new research initiative by the Department of Energy on the ocean margins. The workshop will be attended by about 70 scientists who specialize in ocean margin research. The workshop will be held in the Norfolk, Virginia area in late June 1990.

  20. The geodynamics of the Levant margin

    NASA Astrophysics Data System (ADS)

    Ben-Avraham, Z.

    2006-12-01

    The Levant continental margin, offshore Israel, Lebanon and Syria, is usually defined as a passive margin that was formed through rifting processes. During the formation two major continental fragments are assumed to separate from the northern edge of the Afro-Arabian plate to form the Levant basin: the Tauride and Eratosthenes blocks. Today an oceanic crust and, in places, a very thin continental crust are present between the Levant margin and Eratosthenes seamount. The margin can be divided into two distinct provinces that are separated by the Carmel Structure, which extends from seawards to the northwest across the continental shelf and slope. The preservation of segmentation, both in the shallow and in the deep structure, insinuates that the two segments were formed through different continental breakup processes, which continue to dictate the style of sediment accumulation. The nature and development of the continental margin offshore Israel were the subject of numerous studies, which suggest that the southern Levant segment (south of the Carmel Structure) was formed through continental rifting processes. In contrast, the northern segment, from the Carmel structure northwards and offshore southern Lebanon, was hardly studied before. Recent studies however indicate that the northern segment shows a strong similarity to classical transform margins in the world. In view of the new classification of the northern Levant margin a modified scenario is suggested for: (a) the initial stages in which the Levant margin was formed; and (b) the present day structural differences between the two segments of the margin. At present, the northern Levant continental margin is being reactivated by transpressional faulting of the marine continuation of the Carmel fault which bends northward at the base of the continental slope due to the rheological discontinuity in this region. This fault system coincides with the sharp continental-oceanic crustal transition, and acts as an

  1. Vaccination of mice with a modified Vaccinia Ankara (MVA) virus expressing the African horse sickness virus (AHSV) capsid protein VP2 induces virus neutralising antibodies that confer protection against AHSV upon passive immunisation.

    PubMed

    Calvo-Pinilla, Eva; de la Poza, Francisco; Gubbins, Simon; Mertens, Peter Paul Clement; Ortego, Javier; Castillo-Olivares, Javier

    2014-02-13

    In previous studies we showed that a recombinant Modified Vaccinia Ankara (MVA) virus expressing the protein VP2 of AHSV serotype 4 (MVA-VP2) induced virus neutralising antibodies in horses and protected interferon alpha receptor gene knock-out mice (IFNAR-/-) against challenge. We continued these studies and determined, in the IFNAR-/- mouse model, whether the antibody responses induced by MVA-VP2 vaccination play a key role in protection against AHSV. Thus, groups of mice were vaccinated with wild type MVA (MVA-wt) or MVA-VP2 and the antisera from these mice were used in a passive immunisation experiment. Donor antisera from (a) MVA-wt; (b) MVA-VP2 vaccinated; or (c) MVA-VP2 vaccinated and AHSV infected mice, were transferred to AHSV non-immune recipient mice. The recipients were challenged with virulent AHSV together with MVA-VP2 vaccinated and MVA-wt vaccinated control animals and the levels of protection against AHSV-4 were compared between all these groups. The results showed that following AHSV challenge, mice that were passively immunised with MVA-VP2 vaccinated antisera were highly protected against AHSV disease and had lower levels of viraemia than recipients of MVA-wt antisera. Our study indicates that MVA-VP2 vaccination induces a highly protective humoral immune response against AHSV.

  2. Fundamental studies on passivity and passivity breakdown

    SciTech Connect

    Macdonald, D.D.; Urquidi-Macdonald, M.

    1993-06-01

    Using photoelectrochemical impedance and admittance spectroscopies, a fundamental and quantitative understanding of the mechanisms for the growth and breakdown of passive films on metal and alloy surfaces in contact with aqueous environments is being developed. A point defect model has been extended to explain the breakdown of passive films, leading to pitting and crack growth and thus development of damage due to localized corrosion.

  3. Tropospheric Passive Remote Sensing

    NASA Technical Reports Server (NTRS)

    Keafer, L. S., Jr. (Editor)

    1982-01-01

    The long term role of airborne/spaceborne passive remote sensing systems for tropospheric air quality research and the identification of technology advances required to improve the performance of passive remote sensing systems were discussed.

  4. Polar continental margins: Studies off East Greenland

    NASA Astrophysics Data System (ADS)

    Mienert, J.; Thiede, J.; Kenyon, N. H.; Hollender, F.-J.

    The passive continental margin off east Greenland has been shaped by tectonic and sedimentary processes, and typical physiographic patterns have evolved over the past few million years under the influence of the late Cenozoic Northern Hemisphere glaciations. The Greenland ice shield has been particularly affected.GLORIA (Geological Long Range Inclined Asdic), the Institute of Oceanographic Sciences' (IOS) long-range, side-scan sonar, was used on a 1992 RV Livonia cruise to map large-scale changes in sedimentary patterns along the east Greenland continental margin. The overall objective of this research program was to determine the variety of large-scale seafloor processes to improve our understanding of the interaction between ice sheets, current regimes, and sedimentary processes. In cooperation with IOS and the RV Livonia, a high-quality set of seafloor data has been produced. GLORIA'S first survey of east Greenland's continental margin covered several 1000- × 50-km-wide swaths (Figure 1) and yielded an impressive sidescan sonar image of the complete Greenland Basin and margin (about 250,000 km2). A mosaic of the data was made at a scale of 1:375,000. The base map was prepared with a polar stereographic projection having a standard parallel of 71°.

  5. Interlanguage Passive Construction

    ERIC Educational Resources Information Center

    Simargool, Nirada

    2008-01-01

    Because the appearance of the passive construction varies cross linguistically, differences exist in the interlanguage (IL) passives attempted by learners of English. One such difference is the widely studied IL pseudo passive, as in "*new cars must keep inside" produced by Chinese speakers. The belief that this is a reflection of L1 language…

  6. 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…

  7. "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…

  8. [Fundamental studies of passivity and passivity breakdown

    SciTech Connect

    Macdonald, D.D.

    1993-07-01

    We developed and experimentally tested physical models for growth and breakdown of passive films on metal surfaces. These models are ``point defect models,`` in which the growth and breakdown are described in terms of movement of anion and cation vacancies. The work during the past 5 years resulted in: theory of growth and breakdown of passive films, theory of corrosion-resistant alloys, electronic structure of passive films, and estimation of damage functions for energy systems. Proposals are give for the five ongoing tasks. 10 figs.

  9. Flexural behaviour of the north Algerian margin and tectonic implications

    NASA Astrophysics Data System (ADS)

    Hamai, Lamine; Petit, Carole; Abtout, Abdeslem; Yelles-Chaouche, Abdelkarim; Déverchère, Jacques

    2015-06-01

    The Algerian margin formed through back-arc opening of the Algerian basin (Mediterranean Sea) resulting from the roll-back of the Tethyan slab. Recent geophysical data acquired along the Algerian margin showed evidence of active or recent compressive deformation in the basin due to the ongoing Africa-Eurasia convergence. Published data from four wide-angle seismic profiles have allowed imaging the deep structure of the Algerian margin and its adjacent basins. In this study, we converted these velocity models into density models, then into isostatic anomalies. This allowed us to image an isostatic disequilibrium (relative to a local isostasy model) reaching a maximum amplitude at the margin toe. Converting isostatic anomalies into Moho depth variations shows that the Moho extracted from wide-angle seismic data is deeper than the one predicted by a local isostasy model in the oceanic domain, and shallower than it in the continental domain. These anomalies can be interpreted by opposite flexures of two plates separated by a plate boundary located close to the margin toe. We use a finite element model to simulate the lithospheric flexure. The amplitude of the equivalent vertical Moho deflection is larger in the central part of the study area (6-7 km) than on the easternmost and westernmost profiles (3 km). The effective elastic thickness used to best match the computed deflection is always extremely low (always less than 10 km) and probably reflects the relatively low strength of the lithosphere close to the plate boundary. Comparison with other wide-angle seismic profiles across an active and a passive margin show that the North Algerian margin displays isostatic anomalies close to that of an active margin. Finally, plate flexure is highest at the southern tip of the ocean-continent transition, possibly indicating that a former passive margin detachment is reactivated as a crustal scale reverse fault pre-dating a future subduction.

  10. North Sinai-Levant rift-transform continental margin

    SciTech Connect

    Ressetar, R.; Schamel, S.; Travis, C.J.

    1985-01-01

    The passive continental margin of northern Egypt and the Levant coast formed during the Early mesozoic as the relatively small Anatolia plate broke away from northern Africa. The oceanic basin of the eastern Mediterranean and the unusual right-angle bend in the North Sinai-Levant shelf margin are both products of plate separation along a rift-transform fracture system, the south arm of Tethys. The north-south trending Levant transform margin is considerably narrower than the east-west trending rift margin of northern Egypt. Both exhibit similar facies and depositional histories through the mid-Tertiary. Analysis of subsurface data and published reports of the regional stratigraphy point to a three-stage tectonic evolution of this passive margin. The Triassic through mid-Cretaceous was marked by crustal breakup followed by rapid rotational subsidence of the shelf margins about hinge lines located just south and east of the present shorelines. Reef carbonates localized on the shelf edge separated a deep marine basin to the north from a deltaic-shallow marine platform to the south and east. In the Late Cretaceous-Early Tertiary, inversion of earlier formed half-grabens produced broad anticlinal upwarps of the Syrian Arc on the shelf margin that locally influenced facies patterns. The episode of inversion corresponds with the onset of northward subduction of the Africa plate beneath southern Asia. Beginning in the Oligocene and continuing to the present, there has been renewed subsidence of the North Sinai shelf margin beneath thick, outward building clastic wedges. The source of this large volume of sediment is the updomed and erosionally stripped margins of the Suez-Red Sea Rift and the redirected Nile River.

  11. Continental margin sedimentation: from sediment transport to sequence stratigraphy

    USGS Publications Warehouse

    Nittrouer, Charles A.; Austin, James A.; Field, Michael E.; Kravitz, Joseph H.; Syvitski, James P.M.; Wiberg, Patricia L.; Nittrouer, Charles A.; Austin, James A.; Field, Michael E.; Kravitz, Joseph H.; Syvitski, James P. M.; Wiberg, Patricia L.

    2007-01-01

    This volume on continental margin sedimentation brings together an expert editorial and contributor team to create a state-of-the-art resource. Taking a global perspective, the book spans a range of timescales and content, ranging from how oceans transport particles, to how thick rock sequences are formed on continental margins. - Summarizes and integrates our understanding of sedimentary processes and strata associated with fluvial dispersal systems on continental shelves and slopes - Explores timescales ranging from particle transport at one extreme, to deep burial at the other - Insights are presented for margins in general, and with focus on a tectonically active margin (northern California) and a passive margin (New Jersey), enabling detailed examination of the intricate relationships between a wide suite of sedimentary processes and their preserved stratigraphy - Includes observational studies which document the processes and strata found on particular margins, in addition to numerical models and laboratory experimentation, which provide a quantitative basis for extrapolation in time and space of insights about continental-margin sedimentation - Provides a research resource for scientists studying modern and ancient margins, and an educational text for advanced students in sedimentology and stratigraphy

  12. Power margin improvement for OFDMA-PON using hierarchical modulation.

    PubMed

    Cao, Pan; Hu, Xiaofeng; Zhuang, Zhiming; Zhang, Liang; Chang, Qingjiang; Yang, Qi; Hu, Rong; Su, Yikai

    2013-04-01

    We propose and experimentally demonstrate a hierarchical modulation scheme to improve power margin for orthogonal frequency division multiple access-passive optical networks (OFDMA-PONs). In a PON system, under the same launched optical power, optical network units (ONUs) have different power margins due to unequal distribution fiber lengths. The power margin of the PON system is determined by the ONU with the lowest power margin. In our proposed scheme, ONUs with long and short distribution fibers are grouped together, and downstream signals for the paired ONUs are mapped onto the same OFDM subcarriers using hierarchical modulation. In a pair of ONUs, part of the power margin of the ONU with short distribution fiber is re-allocated to the ONU with long distribution fiber. Therefore, the power margin of the ONU with the longest distribution fiber can be increased, leading to the power margin improvement of the PON system. Experimental results show that the hierarchical modulation scheme improves the power margin by 2.7 dB for an OFDMA-PON system, which can be used to support more users or extend transmission distance.

  13. Reconstructing Rodinia by Fitting Neoproterozoic Continental Margins

    USGS Publications Warehouse

    Stewart, John H.

    2009-01-01

    extensional in origin, supports recognition of the Neoproterozoic fragmentation pattern of Rodinia and outlines the major continental masses that, prior to the breakup, formed the supercontinent. Using this pattern, Rodinia can be assembled by fitting the pieces together. Evidence for Neoproterozoic margins is fragmentary. The most apparent margins are marked by miogeoclinal deposits (passive-margin deposits). The margins can also be outlined by the distribution of continental-margin magmatic-arc rocks, by juvenile ocean-floor rocks, or by the presence of continent-ward extending aulacogens. Most of the continental margins described here are Neoproterozoic, and some had an older history suggesting that they were major, long-lived lithospheric flaws. In particular, the western margin of North America appears to have existed for at least 1,470 Ma and to have been reactivated many times in the Neoproterozoic and Phanerozoic. The inheritance of trends from the Mesoproterozoic by the Neoproterozoic is particularly evident along the eastern United States, where a similarity of Mesoproterozoic (Grenville) and Neoproterozoic trends, as well as Paleozoic or Mesozoic trends, is evident. The model of Rodinia presented here is based on both geologic and paleomagnetic information. Geologic evidence is based on the distribution and shape of Neoproterozoic continents and on assembling these continents so as to match the shape, history, and scale of adjoining margins. The proposed model places the Laurasian continents?Baltica, Greenland, and Laurentia?west of the South American continents (Amazonia, Rio de La Plata, and Sa? Francisco). This assembly is indicated by conjugate pairs of Grenville-age rocks on the east side of Laurentia and on the west side of South America. In the model, predominantly late Neoproterozoic magmatic-arc rocks follow the trend of the Grenville rocks. The boundary between South America and Africa is interpreted as the site of a Wilson cycle

  14. Predicting service life margins

    NASA Technical Reports Server (NTRS)

    Egan, G. F.

    1971-01-01

    Margins are developed for equipment susceptible to malfunction due to excessive time or operation cycles, and for identifying limited life equipment so monitoring and replacing is accomplished before hardware failure. Method applies to hardware where design service is established and where reasonable expected usage prediction is made.

  15. Marginalization and School Nursing

    ERIC Educational Resources Information Center

    Smith, Julia Ann

    2004-01-01

    The concept of marginalization was first analyzed by nursing researchers Hall, Stevens, and Meleis. Although nursing literature frequently refers to this concept when addressing "at risk" groups such as the homeless, gays and lesbians, and those infected with HIV/AIDS, the concept can also be applied to nursing. Analysis of current school nursing…

  16. Phanerozoic polycyclic evolution of the southwestern Angola margin: New insights for apatite fission track and (U-Th)/He methodologies

    NASA Astrophysics Data System (ADS)

    Venancio da Silva, Bruno; Hackspacher, Peter; Carina Siqueira Ribeiro, Marli; Glasmacher, Ulrich Anton

    2016-04-01

    , respectively (8,9). Our preliminary data suggest a polycyclic evolution of the southewestern Angola margin and support the importance of the Cenozoic event in the area which has been widely reported along the Angolan margin (2,4,10,11) but has not been evident in other regions of southern Africa where it has been documented mean Cretaceous events (12,13,14,15). Differences in magnitude of Late Cretaceous events between southern Angola and northern Namibia (16,17) suggest a likely basement control linked to different tectonic-denudation episodes, with the Neoproterozoic shear zones absorbing more deformation than the Congo craton during the shortening events of the margin during Late Cretaceous times. Acknowledgments: Capes /AULP 2012 (Proc. 28/13). Professor Antonio Olimpio Gonçalves, FCT/Univ. Agostinho Neto, Angola References 1. Giresse, P., Hoang, C. T., & Kouyoumontzakis, G., 1984. Analysis of vertical movements deduced from a geochronological study of marine Pleistocene deposits, southern coast of Angola. Journal of African Earth Sciences (1983), 2(2), 177-187. 2. Guiraud, M., Buta-Neto, A., & Quesne, D., 2010. Segmentation and differential post-rift uplift at the Angola margin as recorded by the transform-rifted Benguela and oblique-to-orthogonal-rifted Kwanza basins. Marine and Petroleum Geology, 27(5), 1040-1068. 3 Hudec, M. R., & Jackson, M. P., 2002. Structural segmentation, inversion, and salt tectonics on a passive margin: Evolution of the Inner Kwanza Basin, Angola. Geological Society of America Bulletin, 114(10), 1222-1244. 4. Jackson, M. P. A., Hudec, M. R., & Hegarty, K. A., 2005. The great West African Tertiary coastal uplift: Fact or fiction? A perspective from the Angolan divergent margin. Tectonics, 24(6). 5. Donelick, R. A., O'Sullivan, P. B., & Ketcham, R. A., 2005. Apatite fission-track analysis. Reviews in Mineralogy and Geochemistry, 58(1), 49-94. 6. Ketcham, R. A., 2003. Observations on the relationship between crystallographic orientation and

  17. The Ebro margin study, northwestern Mediterranean Sea - an introduction

    USGS Publications Warehouse

    Maldonado, A.; Hans, Nelson C.

    1990-01-01

    The Ebro continental margin from the coast to the deep sea off northeastern Spain was selected for a multidisciplinary project because of the abundant Ebro River sediment supply, Pliocene and Quaternary progradation, and margin development in a restricted basin where a variety of controlling factors could be evaluated. The nature of this young passive margin for the last 5 m.y. was investigated with particular emphasis on marine circulation, sediment dynamics, sediment geochemistry, depositional facies, seismic stratigraphy, geotechnical properties, geological hazards and human influences. These studies show the importance of marine circulation, variation in sediment supply, sea-level oscillation and tectonic setting for the understanding of modern and ancient margin depositional processes and growth patterns. ?? 1990.

  18. The Brazilian continental margin

    NASA Astrophysics Data System (ADS)

    Martins, L. R.; Coutinho, P. N.

    1981-04-01

    The Brazilian continental margin, with its interesting morphology, structure and sediments, has become better known only during the last two decades. Six physiographical provinces can be recognized at the continental margin and the adjacent coast: (1) Cabo Orange-Parnaiba delta; (2) Parnaiba delta-Cabo Sa˜o Roque; (3) Cabo Sa˜o Roque-Belmonte; (4) Belmonte-Cabo Frio; (5) Cabo Frio-Cabo Santa Marta; and (6) Cabo Santa Marta-Chui. The shelf is rather wide near the Amazon Mouth, becoming narrower eastwards, continuing very narrow along the northeastern and eastern coast, and becoming wider again in the south towards the Plate River. Prominent morphological features along the margin are the Amazon cone, the marginal plateaus off northeastern Brazil, the Sa˜o Francisco cone and canyon, the Abrolhos Bank, and the deep-sea plateaus of Pernambuco and Sa˜o Paulo. On the shelf proper a number of relief elements exist, such as sand waves east of the Amazon, submarine terraces at various places, and irregularities of structural origin. The shelf break is rather smooth in the far north and south, more abrupt in the remainder. Surface sediments of the Brazilian shelf show five distinct facies types: littoral quartz sands, mud, transition sand-mud, coralline algae, and biodetrital. The terrigenous elastic fractions dominate off the Amazon and in southern Brazil; between these areas they occupy a very narrow strip near the coast. The carbonate facies, predominantly composed of calcareous algae, is abundant between the Parnaiba delta and Cabo Frio; to the south this facies is more biodetrital and restricted to the outer shelf. Economically important on the Brazilian continental margin besides oil, are sands and gravels, carbonate deposits, evaporites and some subsurface coal. Other possible mineral resources could be phosphate, heavy minerals and clays for ceramics.

  19. 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 ...

  20. 'Marginal' BY Draconis stars

    NASA Technical Reports Server (NTRS)

    Bopp, Bernard W.

    1987-01-01

    Spectroscopic observations of 52 dK-dM stars, obtained at 640-665 nm (with spectral resolution 70-90 pm) using CCD detectors on the coude-feed telescope at KPNO since 1982, are reported. Data for four stars found to have diluted absorption or weak emission above continuum at H-alpha are presented in tables and spectra and discussed in detail. These objects (Gliese numbers 256, 425A, 900, and 907.1) are shown to be 'marginal' BY Dra stars, single objects of age 2.5-3 Gyr with activity and rotational velocity (3-5 km/s) between those of normal dM stars and those of true BY Dra stars. An explanation based on evolution from the BY Dra stage through marginal BY Dra to inactive dM is proposed.

  1. [Marginality and infant mortality].

    PubMed

    Jimenez Ornelas, R

    1988-01-01

    This study is concerned with differentials in infant and child mortality among low-income urban groups in Mexico. Mortality differentials within and among marginal socioeconomic groups in suburbs of Mexico City and Leon are analyzed and compared using data collected in interviews in 1980 and 1983. The results indicate that the health benefits associated with modernization, such as improved sanitation, can sometimes be offset by their negative impact on mortality, such as industrial accidents and environmental pollution.

  2. Chapter 34: Geology and petroleum potential of the rifted margins of the Canada Basin

    USGS Publications Warehouse

    Houseknecht, D.W.; Bird, K.J.

    2011-01-01

    Three sides of the Canada Basin are bordered by high-standing, conjugate rift shoulders of the Chukchi Borderland, Alaska and Canada. The Alaska and Canada margins are mantled with thick, growth-faulted sediment prisms, and the Chukchi Borderland contains only a thin veneer of sediment. The rift-margin strata of Alaska and Canada reflect the tectonics and sediment dispersal systems of adjacent continental regions whereas the Chukchi Borderland was tectonically isolated from these sediment dispersal systems. Along the eastern Alaska-southern Canada margin, termed herein the 'Canning-Mackenzie deformed margin', the rifted margin is deformed by ongoing Brooks Range tectonism. Additional contractional structures occur in a gravity fold belt that may be present along the entire Alaska and Canada margins of the Canada Basin. Source-rock data inboard of the rift shoulders and regional palaeogeographic reconstructions suggest three potential source-rock intervals: Lower Cretaceous (Hauterivian-Albian), Upper Cretaceous (mostly Turonian) and Lower Palaeogene. Burial history modelling indicates favourable timing for generation from all three intervals beneath the Alaska and Canada passive margins, and an active petroleum system has been documented in the Canning-Mackenzie deformed margin. Assessment of undiscovered petroleum resources indicates the greatest potential in the Canning-Mackenzie deformed margin and significant potential in the Canada and Alaska passive margins. ?? 2011 The Geological Society of London.

  3. 17 CFR 242.103 - Nasdaq passive market making.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... 17 Commodity and Securities Exchanges 3 2010-04-01 2010-04-01 false Nasdaq passive market making. 242.103 Section 242.103 Commodity and Securities Exchanges SECURITIES AND EXCHANGE COMMISSION (CONTINUED) REGULATIONS M, SHO, ATS, AC, AND NMS AND CUSTOMER MARGIN REQUIREMENTS FOR SECURITY...

  4. Passive magnetic bearing configurations

    DOEpatents

    Post, Richard F.

    2011-01-25

    A journal bearing provides vertical and radial stability to a rotor of a passive magnetic bearing system when the rotor is not rotating and when it is rotating. In the passive magnetic bearing system, the rotor has a vertical axis of rotation. Without the journal bearing, the rotor is vertically and radially unstable when stationary, and is vertically stable and radially unstable when rotating.

  5. Overcoming Passive Behavior.

    ERIC Educational Resources Information Center

    Kay, Marilyn

    1986-01-01

    Passivity in learning disabled children is identified as either inborn or as "learned helplessness," and the role of the teacher in overcoming passivity is noted. Teachers can help students understand themselves, become active agents in learning, and use self monitoring devices. (CL)

  6. Passive solar heating

    NASA Astrophysics Data System (ADS)

    Claridge, David E.; Mowris, Robert J.

    1985-11-01

    Buildings have been designed to use solar gains for winter heating for several millenia, but the quantitative basis for passive solar design has only been developed in the last decade. A simplified lumped capacitance model is used to provide insight into the physics of passive building behavior. Three passive design methods are described: the Solar Load Ratio (SLR) method based on correlations to simulation results; the Gordon/Zarmi closed form analytical mode;; and the ``unutilizability'' model of Monsen and Klein. Model predictions are compared with measured results; agreement is good if measured building characteristics are used. Numerous passive houses use less than 2 Btu/ft2-DD for auxiliary heating and consensus is developing that modest levels of passive glazing combined with superinsulation techniques can provide the best feature of both approaches.

  7. Passive solar construction handbook

    SciTech Connect

    Levy, E.; Evans, D.; Gardstein, C.

    1981-08-01

    Many of the basic elements of passive solar design are reviewed. The unique design constraints presented in passive homes are introduced and many of the salient issues influencing design decisions are described briefly. Passive solar construction is described for each passive system type: direct gain, thermal storage wall, attached sunspace, thermal storage roof, and convective loop. For each system type, important design and construction issues are discussed and case studies illustrating designed and built examples of the system type are presented. Construction details are given and construction and thermal performance information is given for the materials used in collector components, storage components, and control components. Included are glazing materials, framing systems, caulking and sealants, concrete masonry, concrete, brick, shading, reflectors, and insulators. The Load Collector Ratio method for estimating passive system performance is appended, and other analysis methods are briefly summarized. (LEW)

  8. New evidence for active deformation off the Annaba region (easternmost Algerian margin): Estimates of Quaternary shortening and slip rates from folding analyses

    NASA Astrophysics Data System (ADS)

    Kherroubi, A.; Deverchere, J.; Yelles, A.; Mercier de Lepinay, B.; Domzig, A.; Graindorge, D.; Bracene, R.; Cattaneo, A.; Gaullier, V.

    2007-12-01

    From the first marine investigations off the region of Annaba (easternmost part of Algeria) made after the MARADJA2 survey in November 2005, a set of large active faults and folds was discovered near the foot of the margin. This active thrust system resumes a previous passive margin and creates growth strata deposition on the backlimbs of large Quaternary folds, resulting in the development of perched basins at the foot of the margin since less than 1 Ma. The system forms a set of overlapping fault segments verging towards the Algerian basin. From the seismic line analysis (high-resolution and Chirp data), and using sedimentation rates obtained from coring and extrapolated, a shortening rate of about 0.9 mm/yr is estimated. If we consider a single fault dipping at about 45° (hypothesized from the Boumerdès 2003 rupture zone), a fault slip rate of about 1.3 mm/yr can be postulated. Considering recent kinematic studies, this fault-related fold system could accommodate a significant part of the present-day submarine shortening rate between the European and African plates, predicted by Stich et al. (2006) to be 1.5 mm/yr from GPS modelling. This faulting is apparently active at a faster rate than the similar faulting observed at the foot of the margin NW of Algiers. This system of faults off Annaba represents a major threat for the region. Indeed, the location of this system changes drastically the seismic hazard assessment of the region, since no large historical earthquake is reported there since 3 centuries at least.

  9. Interaction of tectonic and depositional processes that control the evolution of the Iberian Gulf of Cadiz margin

    USGS Publications Warehouse

    Maldonado, A.; Nelson, C.H.

    1999-01-01

    This study provides an integrated view of the growth patterns and factors that controlled the evolution of the Gulf of Cadiz continental margin based on studies of the tectonic, sedimentologic and oceanographic history of the area. Seven sedimentary regimes are identified, but there are more extensive descriptions of the late Cenozoic regimes because of the larger data base. The regimes of the Mesozoic passive margin include carbonate platforms, which become mixed calcareous-terrigenous deposits during the Late Cretaceous-early Tertiary. The Oligocene and Early Miocene terrigenous regimes developed, in contrast, over the active and transcurrent margins near the African-Iberian plate boundary. The top of the Gulf of Cadiz olistostrome, emplaced in the Late Miocene, is used as a key horizon to define the 'post-orogenic' depositional regimes. The Late Miocene progradational margin regime is characterized by a large terrigenous sediment supply to the margin and coincides with the closing of the Miocene Atlantic-Mediterranean gateways. The terrigenous drift depositional regime of the Early Pliocene resulted from the occurrence of high eustatic sea level and the characteristics of the Mediterranean outflow currents that developed after the opening of the Strait of Gibraltar. The Late Pliocene and Quaternary regimes are dominated by sequences of deposits related to cycles of high and low sea levels. Deposition of shelf-margin deltas and slope wedges correlate with regressive and low sea level regimes caused by eustasy and subsidence. During the highstand regimes of the Holocene, inner shelf prograding deltas and deep-water sediment drifts were developed under the influence of the Atlantic inflow and Mediterranean outflow currents, respectively. A modern human cultural regime began 2000 years ago with the Roman occupation of Iberia; human cultural effects on sedimentary regimes may have equalled natural factors such as climate change. Interplay of tectonic and

  10. Amphetamine margin in sports

    SciTech Connect

    Laties, V.G.; Weiss, B.

    1981-10-01

    The amphetamines can enhance athletic performance. That much seem clear from the literature, some of which is reviewed here. Increases in endurance have been demonstrated in both humans and rats. Smith and Beecher, 20 years ago, showed improvement of running, swimming, and weight throwing in highly trained athletes. Laboratory analogs of such performances have also been used and similar enhancement demonstrated. The amount of change induced by the amphetamines is usually small, of the order of a few percent. Nevertheless, since a fraction of a percent improvement can make the difference between fame and oblivion, the margin conferred by these drugs can be quite important.

  11. The decade of the African child.

    PubMed

    Schuftan, C

    1995-11-01

    The Organization of African States and UNICEF in 1992 jointly proposed that 1994-2003 be designated the Decade of the African Child. Although Africa would like to solve its children's problems on its own, especially in health and nutrition, it cannot do so alone. This paper identifies nine challenges and windows of opportunity on which consensus exists in Africa, and where donors can collaborate with moral, technical, and financial support to improve every African child's quality of life. Sections discuss the empowerment factor; health and nutrition policies; breaking out of poverty; women and child care; the right to know; linking people, primary health care, and nutrition; the need for early warning systems; restructuring the economy; and the changing face of the 1990s. The author stresses that neither governments, nongovernmental organizations, nor donors can afford to be passive observers of the pressing problems of the 1990s. Concerted efforts are needed now more than ever to fight important problems head-on.

  12. Evolution of Northeast Atlantic magmatic continental margins

    NASA Astrophysics Data System (ADS)

    England, Richard; Cornwell, David; Ramsden, Alice

    2014-05-01

    One of the major problems interpreting the evolution of magmatic continental margins such as those which dominate the Irish, UK and Norwegian margins of the NE Atlantic is that the structure which should record the pre-magmatic evolution of the rift and which potentially influences the character of the rifting process is partially or completely obscured by thick basalt lava flows and sills. A limited number of deep reflection seismic profiles acquired with tuned seismic sources have penetrated the basalts and provide an image of the pre-magmatic structure, otherwise the principle data are lower resolution wide-angle/refraction profiles and potential field models which have greater uncertainties associated with them. In order to sidestep the imaging contraints we have examined the Ethiopian ñ Afar rift system to try to understand the rifting process. This magmatic rift system provides, along its length, a series of ësnapshotsí into the possible tectonic evolution of a magmatic continental margin which are associated with different amounts of extension. The Main Ethiopian rift contains an embryonic magmatic passive margin dominated by faulting at the margins of the rift and en-echlon magmatic zones at the centre. Further north toward Afar the rift becomes infilled with extensive lava flows fed from fissure systems in the widening rift zone. Deep seismic profiles crossing the NE Atlantic margins reveal ocean dipping reflector sequences (ODRS) of basaltic lavas overlying extended crust and lower crustal sill complexes of intruded igneous rock, often referred to as underplate, which extend back beneath the continental margin. The ODRS show a variety of morphologies and settings but frequently occur in fault bounded rift structures along the margins. We suggest, by analogy to the observations that can be made in the Ethiopia Afar rift that these fault bounded basins largely form at the embryonic rift stage and are then partially or completely filled with lavas fed

  13. The Cadiz margin study off Spain: An introduction

    USGS Publications Warehouse

    Nelson, C.H.; Maldonado, A.

    1999-01-01

    The Cadiz continental margin of the northeastern Gulf of Cadiz off Spain was selected for a multidisciplinary project because of the interplay of complex tectonic history between the Iberian and African plates, sediment supply from multiple sources, and unique Mediterranean Gateway inflow and outflow currents. The nature of this complex margin, particularly during the last 5 million years, was investigated with emphasis on tectonic history, stratigraphic sequences, marine circulation, contourite depositional facies, geotechnical properties, geologic hazards, and human influences such as dispersal of river contaminants. This study provides an integrated view of the tectonic, sediment supply and oceanographic factors that control depositional processes and growth patterns of the Cadiz and similar modem and ancient continental margins.

  14. Post Rift Evolution of the Indian Margin of Southern Africa

    NASA Astrophysics Data System (ADS)

    Baby, Guillaume; Guillocheau, François; Robin, Cécile; Dall'asta, Massimo

    2016-04-01

    The objective of this study is to discuss the evolution of the South African Plateau along the Indian margin of Southern Africa. Since the classical works of A. du Toit and L.C. King and the improvement of thermochronological methods and numerical models, the question of the uplift of South African Plateau was highly debated with numerous scenarios: early Cretaceous at time of rifting (Van der Beek et al., J.Geophys.Res., 2002), late Cretaceous (Braun et al., Solid Earth, 2014), late Cenozoic (Burke & Gunnell, Geol.Soc.of America, 2008). Limited attention has been paid on the constraints provided by the offshore stratigraphic record of the surrounding margins. The objective of our study is to integrate onshore and offshore data (seismic profiles and industrial wells) to (1) analyse the infill of the whole margin (21°S to 31°S) from its hinterland to the distal deep water basin, (2) to constrain and quantify the vertical movements. We discuss the impact on accommodation and sediments partitioning, and their significance on South African Plateau uplift history. 1. Sedimentary basins of the Indian margin of Southern Africa are related to the break-up of Gondwana during late Jurassic, resulting in rifts and flexural basins. First marine incursions started during early Cretaceous times (oldest marine outcropping sediments are of Barremian age ~128 Ma). The region developed as a normal continental shelf at the Aptian-Albian transition (~113 Ma). 2. The Cretaceous geological history of the basins is characterized by differential uplift and subsidence of the basement, controlled by structures inherited from break up. As example, major early Cretaceous depocenters of the margin are located on the north of Save-Limpopo uplift (Forster, Paleogography, Paleoclimatology, Paleoecology, 1975) showing an eastward drainage pattern, maybe related to a proto Limpopo drainage. Those observations suggest that the escarpment bordering the Bushveld depression is an old relief inherited

  15. Tectonostratigraphic evolution of Cenozoic marginal basin and continental margin successions in the Bone Mountains, Southwest Sulawesi, Indonesia

    NASA Astrophysics Data System (ADS)

    van Leeuwen, Theo M.; Susanto, Eko S.; Maryanto, Sigit; Hadiwisastra, Sapri; Sudijono; Muhardjo; Prihardjo

    2010-06-01

    The Bone Mountains, located in Southwest Sulawesi along the SE margin of Sundaland, are composed of Oligocene to possibly lower Miocene marginal basin successions (Bone Group) that are juxtaposed against continental margin assemblages of Eocene-Miocene age (Salokalupang Group). Three distinct units make up the latter: (i) Middle-Upper Eocene volcaniclastic sediments with volcanic and limestone intercalations in the upper part (Matajang Formation), reflecting a period of arc volcanism and carbonate development along the Sundaland margin; (ii) a well-bedded series of Oligocene calc-arenites (Karopa Formation), deposited in a passive margin environment following cessation of volcanic activity, and (iii) a series of Lower-Middle Miocene sedimentary rocks, in part turbiditic, which interfinger in the upper part with volcaniclastic and volcanic rocks of potassic affinity (Baco Formation), formed in an extensional regime without subduction. The Bone Group consists of MORB-like volcanics, showing weak to moderate subduction signatures (Kalamiseng Formation), and a series of interbedded hemipelagic mudstones and volcanics (Deko Formation). The Deko volcanics are in part subduction-related and in part formed from melting of a basaltic precursor in the overriding crust. We postulate that the Bone Group rocks formed in a transtensional marginal basin bordered by a transform passive margin to the west (Sundaland) and by a newly initiated westerly-dipping subduction zone on its eastern side. Around 14-13 Ma an extensional tectonic event began in SW Sulawesi, characterized by widespread block-faulting and the onset of potassic volcanism. It reached its peak about 1 Ma year later with the juxtaposition of the Bone Group against the Salokalupang Group along a major strike-slip fault (Walanae Fault Zone). The latter group was sliced up in variously-sized fragments, tilted and locally folded. Potassic volcanism continued up to the end of the Pliocene, and locally into the Quaternary.

  16. 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…

  17. 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)

  18. 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…

  19. Evolution of Devonian carbonate-shelf margin, Nevada

    USGS Publications Warehouse

    Morrow, J.R.; Sandberg, C.A.

    2008-01-01

    The north-trending, 550-km-long Nevada segment of the Devonian carbonate-shelf margin, which fringed western North America, evidences the complex interaction of paleotectonics, eustasy, biotic changes, and bolide impact-related influences. Margin reconstruction is complicated by mid-Paleozoic to Paleogene compressional tectonics and younger extensional and strike-slip faulting. Reports published during the past three decades identify 12 important events that influenced development of shelf-margin settings; in chronological order, these are: (1) Early Devonian inheritance of Silurian stable shelf inargin, (2) formation of Early to early Middle 'Devonian shelf-margin basins, (3) propradation of later Middle Devonian shelf margin, (4) late Middle Devonian Taghanic ondap and continuing long-term Frasnian transgression, (5) initiation of latest Middle Devonian to early Frasnian proto-Antler orogenic forebulge, (6) mid-Frasnian Alamo Impact, (7) accelerated development of proto-Antler forebulge and backbulge Pilot basin, (8) global late Frasnian sentichatovae sea-level rise, (9) end-Frasnian sea-level fluctuations and ensuing mass extinction, (10) long-term Famennian regression and continept-wide erosion, (11) late Famennian emergence: of Ahtler orogenic highlands, and (12) end-Devonian eustatic sea-level fall. Although of considerable value for understanding facies relationships and geometries, existing standard carbonate platform-margin models developed for passive settings else-where do not adequately describe the diverse depositional and, structural settings along the Nevada Devonian platform margin. Recent structural and geochemical studies suggest that the Early to Middle Devonian-shelf-margin basins may have been fault-bound and controlled by inherited Precambrian structure. Subsequently, the migrating latest Middle to Late Devonian Antler orogenic forebulge exerted a dominant control on shelf-margin position, morphology, and sedimentation. ??Geological Society of

  20. Cenozoic evolution of the Antarctic Peninsula continental margin

    SciTech Connect

    Anderson, J.B. )

    1990-05-01

    Cenozoic evolution of the Antarctic Peninsula continental margin has involved a series of ridge (Aluk Ridge)-trench collisions between the Pacific and Antarctic plates. Subduction occurred episodically between segments of the Pacific plate that are bounded by major fracture zones. The age of ridge-trench collisions decreases from south to north along the margin. The very northern part of the margin, between the Hero and Shackleton fracture zones, has the last surviving Aluk-Antarctic spreading ridge segments and the only remaining trench topography. The sedimentary cover on the northern margin is relatively thin generally less than 1.5 km, thus providing a unique setting in which to examine margin evolution using high resolution seismic methods. Over 5,000 km of high resolution (water gun) seismic profiles were acquired from the Antarctic Peninsula margin during four cruises to the region. The margin is divided into discrete fracture-zone-bounded segments; each segment displays different styles of development. Highly tectonized active margin sequences have been buried beneath a seaward-thickening sediment wedge that represents the passive stage of margin development Ice caps, which have existed in the Antarctic Peninsula region since at least the late Oligocene, have advanced onto the continental shelf on numerous occasions, eroding hundreds of meters into the shelf and depositing a thick sequence of deposits characterized by till tongues and glacial troughs. Glacial erosion has been the main factor responsible for overdeepening of the shelf; isostasy is of secondary importance. As the shelf was lowered by glacial erosion, it was able to accommodate thicker and more unstable marine ice sheets. The shelf also became a vast reservoir for cold, saline shelf water, one of the key ingredients of Antarctic bottom water.

  1. South Atlantic Margin Processes and Links with Onshore Evolution: Overview of the German Priority Program SAMPLE (Invited)

    NASA Astrophysics Data System (ADS)

    Trumbull, R. B.

    2013-12-01

    Since 2009 the SAMPLE program (www.spp-sample.de) provides a platform for research into the causes and effects of continental breakup and the evolution of passive margins. SAMPLE encompasses 28 projects from 13 German institutions and many international partnerships. The 6-year program will run through 2015. At the core of the program are observational studies that are interlinked by modelling projects examining the interplay of deep mantle dynamics, lithospheric stress fields, pre-rift fabric and melt-weaking on localizing rifting. Geophysics teams collect and integrate existing data from wide-angle seismic profiles, reprocessed multichannel seismics, as well as gravity, magnetics and heat-flow studies to construct self-consistent lithospheric-scale 3-D models along the conjugate margins. Key interests are variations in margin architecture, distribution of magmatic features and the evolution of sedimentary basins (subsidence and thermal histories). An exciting new contribution of SAMPLE geophysics is a linked set of seismic, seismologic and magnetotelluric experiments along the Walvis Ridge, including onshore NW Namibia and the Tristan da Cunha hotspot. In the deep mantle, we examine evidence from global seismic tomography for dramatic low seismic-velocity regions near the core-mantle boundary beneath southern Africa and their implications for dynamics in the deep Earth and the thermo-chemical nature of plumes. Petrologic studies focus on near-primary mantle melts represented by Mg-rich mafic dikes. Projects address the origin of magmas and crust-mantle interaction, and the environmental impact of mega-scale volcanism during breakup. Thermobarometry results from the African margin reveal a N-to-S decrease in mantle potential temperatures from 1520°C (N) to 1380° (S), which supports a thermal plume origin for excessive melt production in the north. Thermochronology data from both conjugate margins reveal complex and puzzling patterns in the denudation history

  2. Race on the Superhighway: How E-Mail Affects African American Student Writers.

    ERIC Educational Resources Information Center

    Redd, Teresa M.; Massey, Victoria W.

    1997-01-01

    Examines three claims about -mail and its implications for African-American students: e-mail (1) blends elements of oral and written language; (2) fosters a sense of community; and (3) leads to the enfranchisement of marginalized writers. Explores these claims through an extended e-mail exchange between African-American students at Howard…

  3. African American Women Principals: Heeding the Call to Serve as Conduits for Transforming Urban School Communities

    ERIC Educational Resources Information Center

    Newcomb, Whitney Sherman; Niemeyer, Arielle

    2015-01-01

    African American women leaders are often found in urban schools that have been exhausted of resources and lack support. However, due to their disproportionate representation in urban schools, African American women principals have become adept at uniting and engaging stakeholders in marginalized school settings into action. The intent for this…

  4. Superstar or Scholar? African American Male Youths' Perceptions of Opportunity in a Time of Change

    ERIC Educational Resources Information Center

    Conchas, Gilberto Q.; Lin, Alex R.; Oseguera, Leticia; Drake, Sean J.

    2015-01-01

    Through a Multiple Marginality Framework, this exploratory case study highlights how African American male youth in an urban high school setting perceive the opportunity structure during the historic election of the first African American President. Youth optimism generated by Obama's election gives students a sense of hope despite the persistent…

  5. Hood River Passive House

    SciTech Connect

    Hales, D.

    2013-03-01

    The Hood River Passive Project was developed by Root Design Build of Hood River Oregon using the Passive House Planning Package (PHPP) to meet all of the requirements for certification under the European Passive House standards. The Passive House design approach has been gaining momentum among residential designers for custom homes and BEopt modeling indicates that these designs may actually exceed the goal of the U.S. Department of Energy's (DOE) Building America program to reduce home energy use by 30%-50% (compared to 2009 energy codes for new homes). This report documents the short term test results of the Shift House and compares the results of PHPP and BEopt modeling of the project.

  6. Uplift, exhumation and erosion along the Angolan continental margin: an integrated approach

    NASA Astrophysics Data System (ADS)

    Gröger, Heike R.; Machado, Vladimir; Di Pinto, Giuseppe

    2013-04-01

    The topographical development along the SW African margin is not exclusively rift-related. In addition to the onset of rifting in the Early Cretaceous, additional Late Cretaceous and Cenozoic events of uplift, exhumation and erosion are discussed. Thermochronology has proven to be a valuable tool to constrain phases of exhumation in passive continental margins. For South Africa and Namibia a large number of thermochronological data are available. Angola on the other hand is still scarcely investigated. This study is based on thermochronological data from onshore Angola, integrated with quantitative morphotectonic analysis and the on- and offshore stratigraphic record. In South Africa and Namibia published thermochronological data document pronounced Early and Late Cretaceous cooling events, which can be related to 2.5-3.5 km of removed section during the Cretaceous. An additional 1-2 km of removed section are estimated during the Cenozoic. In Angola predominantly Permo-Triassic apatite fission track ages indicate significantly less Cretaceous to Cenozoic erosion (< 2.5 km). The apatite fission track data do not provide high resolution constraints on the syn-post rift topographical development along the Angolan margin. However, thermal modelling points to a pronounced Miocene final cooling event. River bed topography upstream the Angolan escarpment is in equilibrium, while the escarpment itself forms a major knick zone. Downstream the main knick point towards the coast, river long-profiles are characterised by convex reaches which are the evidence of an immature, non steady-state topography. Estimation from knick point migration reveals about 1 km uplift within the Cenozoic (< 57 Ma). Published basin reconstructions offshore South Africa and Namibia confirm the general picture of pronounced Cretaceous erosion in the offshore sedimentary record of the basins south of the Walvis ridge (Walvis, Lüderitz and Orange basin). More pronounced Cenozoic erosion in Angola

  7. Young Urban African American Adolescents' Experience of Discretionary Time Activities

    ERIC Educational Resources Information Center

    Bohnert, Amy M.; Richards, Maryse H.; Kolmodin, Karen E.; Lakin, Brittany L.

    2008-01-01

    This cross-sectional study examined the daily discretionary time experiences of 246 (107 boys, 139 girls) fifth through eighth grade urban African American adolescents using the Experience Sampling Method. Relations between the types of activities (i.e., active structured, active unstructured, passive unstructured) engaged in during discretionary…

  8. Geological history and petroleum resources of the continental margins in the central sector of Tethys

    SciTech Connect

    Geodekyan, A.A.; Zabanbark, A.; Konyukov, A.I.

    1993-01-01

    The history of the closure of Tethys explains the distribution and nature of occurrence of petroleum. The enormous resources known in basins of the former passive Gondwanan margin, including those of the Persian Gulf, are mostly in carbonate reservoirs. In contrast, the resources in basins of the former active Eurasian margin, from Spain to Iran, are very much smaller. 4 refs., 3 figs., 6 tabs.

  9. Ivory Coast-Ghana margin: model of a transform margin

    SciTech Connect

    Mascle, J.; Blarez, E.

    1987-05-01

    The authors present a marine study of the eastern Ivory Coast-Ghana continental margins which they consider one of the most spectacular extinct transform margins. This margin has been created during Early-Lower Cretaceous time and has not been submitted to any major geodynamic reactivation since its fabric. Based on this example, they propose to consider during the evolution of the transform margin four main and successive stages. Shearing contact is first active between two probably thick continental crusts and then between progressively thinning continental crusts. This leads to the creation of specific geological structures such as pull-apart graben, elongated fault lineaments, major fault scarps, shear folds, and marginal ridges. After the final continental breakup, a hot center (the mid-oceanic ridge axis) is progressively drifting along the newly created margin. The contact between two lithospheres of different nature should necessarily induce, by thermal exchanges, vertical crustal readjustments. Finally, the transform margin remains directly adjacent to a hot but cooling oceanic lithosphere; its subsidence behavior should then progressively be comparable to the thermal subsidence of classic rifted margins.

  10. Reconstruction of the East Africa and Antarctica continental margins

    NASA Astrophysics Data System (ADS)

    Nguyen, Luan C.; Hall, Stuart A.; Bird, Dale E.; Ball, Philip J.

    2016-06-01

    The Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana and Pangea. Previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent-ocean-boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate Antarctic margin near the Riiser-Larsen Sea. Satellite-derived gravity data have been used to trace the orientations and landward limits of fracture zones. A 3-D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Crustal thicknesses together with fracture zone terminations reveal COBs that are significantly closer to the African and Antarctic coasts than previously recognized. Correlation of fracture zone azimuths and identified COBs suggests Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. An areal-balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins. Both margins reveal a trend of increasing extension from east to west. Our results suggest Africa underwent extension of 60-120 km, while Antarctic crust was stretched by 105-180 km. Various models tested to determine the direction of extension during rifting suggest that Antarctica moved away from Africa in a WNW-ESE direction during the period between 184 and 171 Ma prior to the onset of seafloor spreading.

  11. Vortex formation analysis of a piston-cylinder apparatus with passively varying output inspired by jellyfish

    NASA Astrophysics Data System (ADS)

    Villanueva, Alex; Priya, Shashank

    2012-11-01

    The flow analysis of a robotic jellyfish (Robojelly) has led to the observation of an increase in performance due to passive flexible margin. Flexible margin are common on animals using an oscillating mode of propulsion. The understanding of flexible margins is therefore important for a better understanding of animal propulsion and bio-inspired propulsion. This work focuses on analyzing the effects of stiffness and geometry of flexible margins. A piston-cylinder apparatus was used with flexible margin at the output to test the different flexible margin configurations. These results characterize the effects of the different flexible margin parameters on vortex circulation and size. Office of Naval Research through contract number N00014-08-1-0654.

  12. Adolescent Substance Use: The Role of Demographic Marginalization and Socioemotional Distress

    ERIC Educational Resources Information Center

    Benner, Aprile D.; Wang, Yijie

    2015-01-01

    We investigated the links between racial/ethnic marginalization (i.e., having few same-race/ethnic peers at school) and adolescents' socioemotional distress and subsequent initiation of substance use (alcohol and marijuana) and substance use levels. Data from 7,731 adolescents (52% female; 55% White, 21% African American, 16% Latino, 8% Asian…

  13. Salt diapirs bordering the continental margin of northern kenya and southern somalia.

    PubMed

    Rabinowitz, P D; Coffin, M F; Falvey, D

    1982-02-01

    The presence of newly discovered diapirs of presumed salt origin is documented for the continental margin of northeastern Kenya and southeastern Somalia. These structures are probably a manifestation of a significant thickness of Lower Jurassic evaporites deposited during the rift and early-drift stages of the separation of Madagascar from the African continent.

  14. Salt diapirs bordering the continental margin of northern kenya and southern somalia.

    PubMed

    Rabinowitz, P D; Coffin, M F; Falvey, D

    1982-02-01

    The presence of newly discovered diapirs of presumed salt origin is documented for the continental margin of northeastern Kenya and southeastern Somalia. These structures are probably a manifestation of a significant thickness of Lower Jurassic evaporites deposited during the rift and early-drift stages of the separation of Madagascar from the African continent. PMID:17842401

  15. Crustal-scale architecture and segmentation of the Argentine margin and its conjugate off South Africa

    NASA Astrophysics Data System (ADS)

    Blaich, Olav A.; Faleide, Jan Inge; Tsikalas, Filippos; Franke, Dieter; León, Enric

    2009-07-01

    Integration of regional seismic reflection and refraction profiles and potential field data across the Argentine margin and its conjugate off South Africa, complemented by crustal-scale gravity modelling, is used to reveal and illustrate the whole-crust architecture, onshore-offshore crustal structure correlations, the character of the continent-ocean boundary/transition and the relationship of crustal structure to regional variation of potential field anomalies. The study reveals, within these two provinces, distinct along-margin structural and magmatic changes that are spatially related to a number of conjugate transfer systems governing the margin segmentation and evolution, clearly implying structural inheritance. In particular, the Colorado transfer system on the Argentina margin, marks a distinct along-margin boundary in the distribution and volume of breakup-related magmatism. Similarly, the Hope transfer system on the conjugate South Africa margin also marks a distinct along-margin transition from a zone of relative magnetic quiescence to a zone of prominent magnetic anomalies. Furthermore, the study indicates that the `G-magnetic anomaly' along the South Africa margin probably defines the eastern limit of the continent-ocean transition (COT) rather than a discrete continent-ocean boundary (COB). Potential field plate reconstructions of the South Atlantic suggest conjugate margin asymmetry, characterized by a rather broad Argentine margin conjugate to a narrow South Africa margin. In detail, the Argentine margin is characterized by a sharp and relatively constant COT, whereas the COT along the conjugate South Africa margin is considerably wider. An along-strike tectonomagmatic asymmetry variation is also observed and is expressed by the northward increase in width of the COT on the South African margin. The study clearly shows that integration of regional seismic reflection and refraction profiles, potential field data and gravity modelling provide a

  16. Marginal deformation of crustal plates as key to crustal motion, crustal spirals, and the driving force

    SciTech Connect

    Wood, B.G.M.

    1986-07-01

    Present plate tectonic models concentrate on compressive- and extensive-type plate margins, often incorporating shear margins as a subtype of compressive margins. However, if a single moving plate is considered, it becomes apparent that the leading edge is a compressive margin, the trailing edge is an extensive margin, and the lateral edges are shear margins. Conversely, if a plate's margin can be recognized by identifying areas of subduction (compression), rifting (extension), and strike slip and buckle folding (shearing), then not only can a plate be identified but its motion can also be inferred. The Pacific plate provides an excellent example. It is bounded by subduction trenches along its west-northwest margin, extension rifts along its east-southeast margin, and shear and buckle-fold complexes along its south-southwest and north-northeast margins. A west-northwest motion is inferred. As other major plates are examined, two striking features are revealed. A west-northwest to northwest motion is consistently identified, and the plates line up end to end forming a northwest-spiraling segmented band encircling the globe. The lateral margins of this band form the well-known Tethyan shear system. The plates comprising this band are of varying sizes and composition, and the extent of deformation along a plate margin is directly related to plate size. The Pacific and Eurasian plates dominate in size and marginal deformation. The tail of this north-spiraling ribbon of crustal plates is deformed in the Southern Hemisphere, most notably in the area of the African, Indian, and Australian plates. Each southern plate has a strong north component of motion as well as a counterclockwise spiraling action. The plates appear to have rotated in response to drag along the southern margin of the Pacific and Eurasian plates.

  17. African American Homeschooling and the Quest for a Quality Education

    ERIC Educational Resources Information Center

    Mazama, Ama; Lundy, Garvey

    2015-01-01

    Academic interest in homeschooling has increased over the last decade, as what was once perceived as a marginal development, has, in fact, turned into a significant and growing phenomenon. There has been, in recent years, a noticeable surge in African American involvement in the homeschooling movement as well. However, there continues to be a…

  18. Exploring Artistic Practice in Global Communities of the African Diaspora

    ERIC Educational Resources Information Center

    Ellis, Auburn E.

    2014-01-01

    In 2012 an African Centered single case study was conducted in the United States. The problem is as follows: K-12 practitioners in urban areas are faced with unique circumstances while serving marginalized students in urban areas. As a response to this issue, the purpose of this study was to identify and describe curricula used in three African…

  19. Deep Crustal Structure of S-Atlantic Margins

    NASA Astrophysics Data System (ADS)

    Becker, K.; Schnabel, M.; Franke, D.; Heyde, I.; Schreckenberger, B.; Koopmann, H.; Krawczyk, C. M.; Trumbull, R. B.

    2013-12-01

    We investigate the crustal structure along the southern South Atlantic margins with a focus on the high velocity lower crustal bodies (HVLC). This is a distinct zone at the base of the crust, where seismic P-wave velocities exceed 7.0 km/s and locally reach values up to 7.7 km/s. The study is based on a selected set of refraction seismic lines on conjugate margin segments of Uruguay-Argentina and Namibia-South Africa, acquired during marine geophysical cruises in 2004 and 1998. We performed new P-wave tomography complemented with gravity modeling along two crustal transects, and combine these with previous seismic and gravity models. The results are used to examine the interplay of rifting and magmatism during the evolution of the South Atlantic, what activated the spreading phase and how this is reflected in the distribution of high velocity lower crust. On all sections we observe HVLC, even on a magma poor southernmost section at the western margin. The HVLC varies strongly in shape and size along the margin. From South to North the area of the HVLC on 2D velocity sections increases on both margins. However, the HVLC bodies along the South American margin are much smaller than on the South African margin, possibly indicating asymmetric break up. A striking feature is the distinct seaward shift of the HVLC relative to the seaward dipping reflectors (SDRs). While in the south, the HVLC is situated below the SDRs, towards the north the HVLC formed seaward of the SDRs. From this seaward migration we infer that the formation of HVLC in the magma-rich northern sections may have formed at least partly after rifting and break up.

  20. Phanerozoic polycyclic evolution of the southwestern Angola margin: New insights for apatite fission track and (U-Th)/He methodologies

    NASA Astrophysics Data System (ADS)

    Venancio da Silva, Bruno; Hackspacher, Peter; Carina Siqueira Ribeiro, Marli; Glasmacher, Ulrich Anton

    2016-04-01

    , respectively (8,9). Our preliminary data suggest a polycyclic evolution of the southewestern Angola margin and support the importance of the Cenozoic event in the area which has been widely reported along the Angolan margin (2,4,10,11) but has not been evident in other regions of southern Africa where it has been documented mean Cretaceous events (12,13,14,15). Differences in magnitude of Late Cretaceous events between southern Angola and northern Namibia (16,17) suggest a likely basement control linked to different tectonic-denudation episodes, with the Neoproterozoic shear zones absorbing more deformation than the Congo craton during the shortening events of the margin during Late Cretaceous times. Acknowledgments: Capes /AULP 2012 (Proc. 28/13). Professor Antonio Olimpio Gonçalves, FCT/Univ. Agostinho Neto, Angola References 1. Giresse, P., Hoang, C. T., & Kouyoumontzakis, G., 1984. Analysis of vertical movements deduced from a geochronological study of marine Pleistocene deposits, southern coast of Angola. Journal of African Earth Sciences (1983), 2(2), 177-187. 2. Guiraud, M., Buta-Neto, A., & Quesne, D., 2010. Segmentation and differential post-rift uplift at the Angola margin as recorded by the transform-rifted Benguela and oblique-to-orthogonal-rifted Kwanza basins. Marine and Petroleum Geology, 27(5), 1040-1068. 3 Hudec, M. R., & Jackson, M. P., 2002. Structural segmentation, inversion, and salt tectonics on a passive margin: Evolution of the Inner Kwanza Basin, Angola. Geological Society of America Bulletin, 114(10), 1222-1244. 4. Jackson, M. P. A., Hudec, M. R., & Hegarty, K. A., 2005. The great West African Tertiary coastal uplift: Fact or fiction? A perspective from the Angolan divergent margin. Tectonics, 24(6). 5. Donelick, R. A., O'Sullivan, P. B., & Ketcham, R. A., 2005. Apatite fission-track analysis. Reviews in Mineralogy and Geochemistry, 58(1), 49-94. 6. Ketcham, R. A., 2003. Observations on the relationship between crystallographic orientation and

  1. Wireless passive radiation sensor

    SciTech Connect

    Pfeifer, Kent B; Rumpf, Arthur N; Yelton, William G; Limmer, Steven J

    2013-12-03

    A novel measurement technique is employed using surface acoustic wave (SAW) devices, passive RF, and radiation-sensitive films to provide a wireless passive radiation sensor that requires no batteries, outside wiring, or regular maintenance. The sensor is small (<1 cm.sup.2), physically robust, and will operate unattended for decades. In addition, the sensor can be insensitive to measurement position and read distance due to a novel self-referencing technique eliminating the need to measure absolute responses that are dependent on RF transmitter location and power.

  2. 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…

  3. 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.…

  4. Flexural rigidity and loading mechanisms across the Irish Atlantic margin.

    NASA Astrophysics Data System (ADS)

    Daly, E.; Brown, C.; Stark, C.; Tiberi, C.; Ebinger, C.

    2003-04-01

    Lithospheric rheology is known to exert a significant control on tectonic style, yet it has not been assessed across the Irish Atlantic margin. The effective elastic thickness Te of the lithosphere provides a good measure of the lateral variability in rheology without requiring detailed information on lithospheric composition, strain rate, etc. Through the analysis of the isostatic mechanism, its spatial variability is examined across the Irish Atlantic margin. A new mathematical technique employing Multi-Resolution Analysis (wavelets) provides a significant improvement in the spatial resolution of Te variations compared with standard spectral analysis techniques for computing coherence. Variations in the distribution of loading mechanisms have been shown to have profound effects on estimated Te values and the rheology of the continental lithosphere. In the Rockall Basin a 3d seismically constrained gravity inversion showed the presence of surface and subsurface loading mechanisms. However the surface load due to sedimentation dominates. The load ratio f generated as part of the wavelet coherence method corroborates with the results of the gravity inversion, thus lending further evidence to the geological meaning and relevance of f, and therefore the need to incorporate f in a spectral based analysis of the effective elastic thickness of the lithosphere. Overall Te is low across the margin (5-16 km) following a NE-SW Caledonian trend. The weakest lithosphere is in the southern Rockall Basin, Porcupine Bank and Porcupine Basin and the strongest lithosphere is along the Hatton-Rockall region. The correlation between the Te variation and the Euler depth solutions and analytic signal values along the eastern Rockall Bank indicate that there is clear geological and rheological divide between the Rockall-Hatton region to the NW and the Rockall Basin to the SE. On a global scale the estimated Te values are consistent with the results from other passive margins. The reason

  5. Method of passivating semiconductor surfaces

    DOEpatents

    Wanlass, Mark W.

    1990-01-01

    A method of passivating Group III-V or II-VI semiconductor compound surfaces. The method includes selecting a passivating material having a lattice constant substantially mismatched to the lattice constant of the semiconductor compound. The passivating material is then grown as an ultrathin layer of passivating material on the surface of the Group III-V or II-VI semiconductor compound. The passivating material is grown to a thickness sufficient to maintain a coherent interface between the ultrathin passivating material and the semiconductor compound. In addition, a device formed from such method is also disclosed.

  6. Method of passivating semiconductor surfaces

    DOEpatents

    Wanlass, M.W.

    1990-06-19

    A method is described for passivating Group III-V or II-VI semiconductor compound surfaces. The method includes selecting a passivating material having a lattice constant substantially mismatched to the lattice constant of the semiconductor compound. The passivating material is then grown as an ultrathin layer of passivating material on the surface of the Group III-V or II-VI semiconductor compound. The passivating material is grown to a thickness sufficient to maintain a coherent interface between the ultrathin passivating material and the semiconductor compound. In addition, a device formed from such method is also disclosed.

  7. Effects of a flexible margin on Robojelly vortex structures

    NASA Astrophysics Data System (ADS)

    Villanueva, Alex; Stewart, Kelley; Vlachos, Pavlos; Priya, Shashank

    2011-11-01

    An Unmanned Underwater Vehicle (UUV) inspired by jellyfish morphology and propulsion mechanism, termed ``Robojelly,'' was used to analyze the effects of the flexible margin on jellyfish propulsion. The natural animal has a bell section which deforms at a different phase then the rest of the bell. This lagging section, referred to as flexible margin or flap, is delimited by the bell margin and an inflexion point. The flap was replicated on the robotic vehicle by a flexible passive material to conduct a systematic parametric study. In a preliminary experiment, Robojelly was tested without a flap and with a flap. This revealed a thrust increase over an order of magnitude. We hypothesize that the length of this passive flap affects the vortex ring circulation strength of the jellyfish which can lead to higher efficiency and thrust. Velocity field measurements were performed using planar Time Resolved Digital Particle Image Velocimetry (TRDPIV) to analyze the change in vortex structures as a function of flap length. The robot input parameters stayed constant over the different configurations tested thus maintaining a near constant power consumption. Results clearly demonstrate that the flap plays an important role in the propulsion mechanism of Robojelly and provides an anatomical understanding of natural jellyfish.

  8. Hood River Passive House

    SciTech Connect

    Hales, David

    2014-01-01

    The Hood River Passive Project was developed by Root Design Build of Hood River Oregon using the Passive House Planning Package (PHPP) to meet all of the requirements for certification under the European Passive House standards. The Passive House design approach has been gaining momentum among residential designers for custom homes and BEopt modeling indicates that these designs may actually exceed the goal of the U.S. Department of Energy's (DOE) Building America program to "reduce home energy use by 30%-50% (compared to 2009 energy codes for new homes). This report documents the short term test results of the Shift House and compares the results of PHPP and BEopt modeling of the project. The design includes high R-Value assemblies, extremely tight construction, high performance doors and windows, solar thermal DHW, heat recovery ventilation, moveable external shutters and a high performance ductless mini-split heat pump. Cost analysis indicates that many of the measures implemented in this project did not meet the BA standard for cost neutrality. The ductless mini-split heat pump, lighting and advanced air leakage control were the most cost effective measures. The future challenge will be to value engineer the performance levels indicated here in modeling using production based practices at a significantly lower cost.

  9. Hood River Passive House

    SciTech Connect

    Hales, D.

    2014-01-01

    The Hood River Passive Project was developed by Root Design Build of Hood River Oregon using the Passive House Planning Package (PHPP) to meet all of the requirements for certification under the European Passive House standards. The Passive House design approach has been gaining momentum among residential designers for custom homes and BEopt modeling indicates that these designs may actually exceed the goal of the U.S. Department of Energy's (DOE) Building America program to reduce home energy use by 30%-50% (compared to 2009 energy codes for new homes). This report documents the short term test results of the Shift House and compares the results of PHPP and BEopt modeling of the project. The design includes high R-Value assemblies, extremely tight construction, high performance doors and windows, solar thermal DHW, heat recovery ventilation, moveable external shutters and a high performance ductless mini-split heat pump. Cost analysis indicates that many of the measures implemented in this project did not meet the BA standard for cost neutrality. The ductless mini-split heat pump, lighting and advanced air leakage control were the most cost effective measures. The future challenge will be to value engineer the performance levels indicated here in modeling using production based practices at a significantly lower cost.

  10. Manufacturing Marginality among Women and Latinos in Neoliberal America

    PubMed Central

    Massey, Douglas S.

    2014-01-01

    Intersectionality is the study of how categorical distinctions made on the basis of race, class, and gender interact to generate inequality, and this concept has become a primary lens by which scholars have come to model social stratification in the United States. In addition to the historically powerful interaction between race and class, gender interactions have become increasingly powerful in exacerbating class inequalities while the growing exclusion of foreigners on the basis of legal status has progressively marginalized Latinos in U.S. society. As a result, poor whites and immigrant-origin Latinos have increasingly joined African Americans at the bottom of American society to form a new, expanded underclass. PMID:25309007

  11. Manufacturing Marginality among Women and Latinos in Neoliberal America.

    PubMed

    Massey, Douglas S

    2014-01-01

    Intersectionality is the study of how categorical distinctions made on the basis of race, class, and gender interact to generate inequality, and this concept has become a primary lens by which scholars have come to model social stratification in the United States. In addition to the historically powerful interaction between race and class, gender interactions have become increasingly powerful in exacerbating class inequalities while the growing exclusion of foreigners on the basis of legal status has progressively marginalized Latinos in U.S. society. As a result, poor whites and immigrant-origin Latinos have increasingly joined African Americans at the bottom of American society to form a new, expanded underclass. PMID:25309007

  12. Robust Stabilization of a Class of passive Nonlinear Systems

    NASA Technical Reports Server (NTRS)

    Joshi, Suresh M.; Kelkar, Atul G.

    1996-01-01

    The problem of feedback stabilization is considered for a class of nonlinear, finite dimensional, time invariant passive systems that are affine in control. Using extensions of the Kalman-Yakubovch lemma, it is shown that such systems can be stabilized by a class of finite demensional, linear, time-invariant controllers which are strictly positive real in the weak or marginal sense. The stability holds regardless of model uncertainties, and is therefore, robust.

  13. Marginal seas—Terminological crisis

    NASA Astrophysics Data System (ADS)

    Mazarovich, A. O.

    2011-07-01

    The terms marginal sea, peripheral sea, and backarc sea are widely used in the contemporary Russian geological literature as synonyms but do not have, in my opinion, unequivocal treatment. The application of the term marginal sea is briefly discussed. The seas of the Pacific transitional zone are reviewed. It is proposed to define a marginal sea as a marine basin a few thousand kilometers in extent and connected with the open ocean. Domains underlain by crust of the continental and oceanic types must coexist therein. The domains with oceanic crust are expressed in the topography as deepwater basins (one or several), where fragments of continental crust may also occur. A marginal sea must be bounded by at least one island arc.

  14. Northeast Atlantic Igneous Province volcanic margin development

    NASA Astrophysics Data System (ADS)

    Mjelde, R.; Breivik, A. J.; Faleide, J. I.

    2009-04-01

    Early Eocene continental breakup in the NE Atlantic Volcanic Province (NAIP) was associated with voluminous extrusive and intrusive magmatism, and initial seafloor spreading produced anomalously thick oceanic crust. Recent publications based on crustal-scale wide-angle seismic data show that there is a positive correlation between igneous crustal thickness (H) and average P-wave velocity (Vp) on all investigated margins in the NAIP. Vp can be used as a proxy for crustal composition, which can be related to the mode of mantle melting. A positive H-Vp correlation indicates that excessive mantle melting the first few million years after breakup was driven by an initial increased temperature that cools off as seafloor spreading develops, consistent with a mantle plume model. Variations in mantle composition can explain excess magmatism, but will generate a negative H-Vp correlation. Active mantle convection may increase the flux of mantle rocks through the melting zone above the rate of passive corner flow, which can also produce excessive magmatism. This would produce little H-Vp correlation, and place the curve lower than the passive flow melting curve in the diagram. We have compiled earlier published results with our own analyses of published and unpublished data from different groups to look for systematic variations in the mantle melting mode along the NAIP margins. Earlier studies (Holbrook et al., 2002, White et al, 2008) on the southeast Greenland conjugate system, indicate that the thick igneous crust of the southern NAIP (SE Greenland ? Hatton Bank) was dominated by increased mantle temperature only, while magmatism closer to the southern side of and including the Greenland-Iceland-Færøy Ridge (GIFR) was created by combined temperature increase and active mantle convection. Recent publications (Breivik et al., 2008, White et al, 2008) north of the GIFR for the Norway Basin segment, indicate temperature dominated magmatism between the Jan Mayen Fracture

  15. Reconstruction of the East Africa and Antarctica continental margins

    NASA Astrophysics Data System (ADS)

    Nguyen, L. C.; Hall, S. A.; Ball, P.; Bird, D. E.

    2015-12-01

    The Early Jurassic separation of Antarctica from Africa plays an important role in our understanding of the dispersal of Gondwana. Previously proposed reconstruction models often contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent-ocean boundary (COB) and determining the amount and distribution of extended continental crust. This study focuses on the evolution of the African margin adjacent to the Mozambique Basin and the conjugate margin of Antarctica near the Riiser Larsen Sea. New satellite-derived gravity data have been used to trace the orientations and landward limits of fracture zones in the study area. A 3-D gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust. Information on crustal thickness along with the identification of fracture zones reveal the COBs that are located significantly closer to the coasts of Africa and Antarctica than previously recognized. Correlation of both fracture zone azimuths and the identified COBs over the conjugate margins suggest Antarctica began drifting away from Africa at approximately 171 Ma in a roughly SSE direction. Of several scenarios examined, the Beira High is most likely oceanic and may be a conjugate feature of the southern Astrid Ridge. An areal-balancing method that involves restoring the crust to a uniform pre-rift thickness has been used to perform the non-rigid reconstruction for both non-volcanic and volcanic margin with magmatic underplating. Based on the results, Africa underwent extension of 65-105 km while Antarctic crust was stretched by 90-190 km. Both margins reveal a trend of increasing extension from east to west. Various models tested to determine the direction of extension during rifting suggest that Antarctica underwent a counter-clockwise rotation with respect to Africa between 186-171 Ma prior to the onset of seafloor spreading.

  16. Steel Industry Marginal Opportunity Analysis

    SciTech Connect

    none,

    2005-09-01

    The Steel Industry Marginal Opportunity Analysis (PDF 347 KB) identifies opportunities for developing advanced technologies and estimates both the necessary funding and the potential payoff. This analysis determines what portion of the energy bandwidth can be captured through the adoption of state-of-the-art technology and practices. R&D opportunities for addressing the remainder of the bandwidth are characterized and plotted on a marginal opportunity curve.

  17. African American Female Professors' Strategies for Successful Attainment of Tenure and Promotion at Predominately White Institutions: It Can Happen

    ERIC Educational Resources Information Center

    Jones, Brandolyn; Hwang, Eunjin; Bustamante, Rebecca M.

    2015-01-01

    In their pursuit of tenure and promotion, African American female faculty members continue to prevail over workplace adversities such as ridicule, marginalization, alienation, isolation, and lack of information. In this descriptive phenomenological study, the lived experiences of five African American female professors who successfully navigated…

  18. Norwegian remote sensing experiment in a marginal ice zone

    USGS Publications Warehouse

    Farrelly, B.; Johannessen, J.A.; Svendsen, E.; Kloster, K.; Horjen, I.; Matzler, C.; Crawford, J.; Harrington, R.; Jones, L.; Swift, C.; Delnore, V.E.; Cavalieri, D.; Gloersen, P.; Hsiao, S.V.; Shemdin, O.H.; Thompson, T.W.; Ramseier, R.O.; Johannessen, O.M.; Campbell, W.J.

    1983-01-01

    The Norwegian Remote Sensing Experiment in the marginal ice zone north of Svalbard took place in fall 1979. Coordinated passive and active microwave measurements were obtained from shipborne, airborne, and satellite instruments together with in situ observations. The obtained spectra of emissivity (frequency range, 5 to 100 gigahertz) should improve identification of ice types and estimates of ice concentration. Mesoscale features along the ice edge were revealed by a 1.215-gigahertz synthetic aperture radar. Ice edge location by the Nimbus 7 scanning multichannel microwave radiometer was shown to be accurate to within 10 kilometers.

  19. Passivated niobium cavities

    SciTech Connect

    Myneni, Ganapati Rao; Hjorvarsson, Bjorgvin; Ciovati, Gianluigi

    2006-12-19

    A niobium cavity exhibiting high quality factors at high gradients is provided by treating a niobium cavity through a process comprising: 1) removing surface oxides by plasma etching or a similar process; 2) removing hydrogen or other gases absorbed in the bulk niobium by high temperature treatment of the cavity under ultra high vacuum to achieve hydrogen outgassing; and 3) assuring the long term chemical stability of the niobium cavity by applying a passivating layer of a superconducting material having a superconducting transition temperature higher than niobium thereby reducing losses from electron (cooper pair) scattering in the near surface region of the interior of the niobium cavity. According to a preferred embodiment, the passivating layer comprises niobium nitride (NbN) applied by reactive sputtering.

  20. Passive broadband acoustic thermometry

    NASA Astrophysics Data System (ADS)

    Anosov, A. A.; Belyaev, R. V.; Klin'shov, V. V.; Mansfel'd, A. D.; Subochev, P. V.

    2016-04-01

    The 1D internal (core) temperature profiles for the model object (plasticine) and the human hand are reconstructed using the passive acoustothermometric broadband probing data. Thermal acoustic radiation is detected by a broadband (0.8-3.5 MHz) acoustic radiometer. The temperature distribution is reconstructed using a priori information corresponding to the experimental conditions. The temperature distribution for the heated model object is assumed to be monotonic. For the hand, we assume that the temperature distribution satisfies the heat-conduction equation taking into account the blood flow. The average error of reconstruction determined for plasticine from the results of independent temperature measurements is 0.6 K for a measuring time of 25 s. The reconstructed value of the core temperature of the hand (36°C) generally corresponds to physiological data. The obtained results make it possible to use passive broadband acoustic probing for measuring the core temperatures in medical procedures associated with heating of human organism tissues.

  1. Passive fetal monitoring sensor

    NASA Technical Reports Server (NTRS)

    Zuckerwar, Allan J. (Inventor); Hall, Earl T. (Inventor); Baker, Donald A. (Inventor); Bryant, Timothy D. (Inventor)

    1992-01-01

    An ambulatory, passive sensor for use in a fetal monitoring system is discussed. The invention is comprised of a piezoelectric polymer film, combined with a metallic mounting plate fastened to a belt, and electrically connected to a signal processing unit by means of a shielded cable. The purpose of the sensor is to receive pressure pulses emitted by a fetus inside an expectant mother. Additionally, the monitor will filter out pressure pulses arising from other sources, such as the maternal heart.

  2. Passively actuated valve

    DOEpatents

    Modro, S. Michael; Ougouag, Abderrafi M.

    2005-09-20

    A passively actuated valve for isolating a high pressure zone from a low pressure zone and discontinuing the isolation when the pressure in the high pressure zone drops below a preset threshold. If the pressure in the high pressure zone drops below the preset threshold, the valve opens and allows flow from the high pressure zone to the low pressure zone. The valve remains open allowing pressure equalization and back-flow should a pressure inversion between the two pressure zone occur.

  3. Tectonic evolution of Brazilian equatorial continental margin basins

    SciTech Connect

    Azevedo, R.P. )

    1993-02-01

    The structural style and stratigraphic relationships of sedimentary basins along the Brazilian Equatorial Atlantic Continental Margin were used to construct an empirical tectonic model for the development of ancient transform margins. The model is constrained by detailed structural and subsidence analyses of several basins along the margin. The structural framework of the basins was defined at shallow and deep levels by the integration of many geophysical and geological data sets. The Barreirinhas and Para-Maranhao Basins were divided in three tectonic domains: the Tutoia, Caete, and Tromai subbasins. The Caete area is characterized by northwest-southeast striking and northeast-dipping normal faults. A pure shear mechanism of basin formation is suggested for its development. The structure of the Tutoia and Tromai subbasins are more complex and indicative of a major strike-slip component with dextral sense of displacement, during early stages of basin evolution. These two later subbasins were developed on a lithosphere characterized by an abrupt transition (<50 km wide) from an unstretched continent to an oceanic lithosphere. The subsidence history of these basins do not comply with the classical models developed for passive margins or continental rifting. The thermo-mechanical model proposed for the Brazilian equatorial margin includes heterogeneous stretching combined with shearing at the plate margin. The tectonic history comprises: (1) Triassic-Jurassic limited extension associated with the Central Atlantic evolution; (2) Neocomian intraplate deformation consisting of strike-slip reactivation of preexisting shear zones; (3) Aptian-Cenomanian two-phase period of dextral shearing; and (4) Late Cretaceous-Cenozoic sea-floor spreading.

  4. Geology and petroleum potential of Adelie Coast margin, east Antarctica

    SciTech Connect

    Wanneson, J.

    1987-05-01

    The few rock outcrops on Adelie Coast-Wilkes Land consist mainly of Precambrian plutonic rocks and metasediments. On the continental margin, several multichannel seismic surveys, including the 1982 IFP survey, reveal the presence of a thick sedimentary basin, especially beneath the outer continental shelf and upper slope, where it may exceed 6000 m. Thin basin results from the creation and evolution of a continental margin, initiated some 100 Ma from the separation of Australia and Antarctica. Beneath the outer shelf, which is 400-500 m deep, the sedimentary series consist of four units separated by three major unconformities: (1) a predrift unit including a Precambrian basement, possible Paleozoic and early Mesozoic sediments, and a Mesozoic syn-rift sequence; (2) an upper Eocene to Oligocene unit in a shallow marine environment; and (3) a Neogene glacial prograding unit. The predrift and early postrift units are considered to be a promising target with reference to other passive margins, although no major hydrocarbon accumulation has been discovered as yet on the Australian conjugate margin.

  5. Early Miocene sequence development across the New Jersey margin

    USGS Publications Warehouse

    Monteverde, D.H.; Mountain, Gregory S.; Miller, K.G.

    2008-01-01

    Sequence stratigraphy provides an understanding of the interplay between eustasy, sediment supply and accommodation in the sedimentary construction of passive margins. We used this approach to follow the early to middle Miocene growth of the New Jersey margin and analyse the connection between relative changes of sea level and variable sediment supply. Eleven candidate sequence boundaries were traced in high-resolution multi-channel seismic profiles across the inner margin and matched to geophysical log signatures and lithologic changes in ODP Leg 150X onshore coreholes. Chronologies at these drill sites were then used to assign ages to the intervening seismic sequences. We conclude that the regional and global correlation of early Miocene sequences suggests a dominant role of global sea-level change but margin progradation was controlled by localized sediment contribution and that local conditions played a large role in sequence formation and preservation. Lowstand deposits were regionally restricted and their locations point to both single and multiple sediment sources. The distribution of highstand deposits, by contrast, documents redistribution by along shelf currents. We find no evidence that sea level fell below the elevation of the clinoform rollover, and the existence of extensive lowstand deposits seaward of this inflection point indicates efficient cross-shelf sediment transport mechanisms despite the apparent lack of well-developed fluvial drainage. ?? 2008 The Authors. Journal compilation ?? 2008 Blackwell Publishing.

  6. Formation of Australian continental margin highlands driven by plate-mantle interaction

    NASA Astrophysics Data System (ADS)

    Müller, R. Dietmar; Flament, Nicolas; Matthews, Kara J.; Williams, Simon E.; Gurnis, Michael

    2016-05-01

    Passive margin highlands occur on most continents on Earth and play a critical role in the cycle of weathering, erosion, and atmospheric circulation. Yet, in contrast to the well-developed understanding of collisional mountain belts, such as the Alps and Himalayas, the origin of less elevated (1-2 km) passive margin highlands is still unknown. The eastern Australian highlands are a prime example of these plateaus, but compared to others they have a well-documented episodic uplift history spanning 120 million years. We use a series of mantle convection models to show that the time-dependent interaction of plate motion with mantle downwellings and upwellings accounts for the broad pattern of margin uplift phases. Initial dynamic uplift of 400-600 m from 120-80 Ma was driven by the eastward motion of eastern Australia's margin away from the sinking eastern Gondwana slab, followed by tectonic quiescence to about 60 Ma in the south (Snowy Mountains). Renewed uplift of ∼700 m in the Snowy Mountains is propelled by the gradual motion of the margin over the edge of the large Pacific mantle upwelling. In contrast the northernmost portion of the highlands records continuous uplift from 120 Ma to present-day totalling about 800 m. The northern highlands experienced a continuous history of dynamic uplift, first due to the end of subduction to the east of Australia, then due to moving over a large passive mantle upwelling. In contrast, the southern highlands started interacting with the edge of the large Pacific mantle upwelling ∼ 40- 50 million years later, resulting in a two-phase uplift history. Our results are in agreement with published uplift models derived from river profiles and the Cretaceous sediment influx into the Ceduna sub-basin offshore southeast Australia, reflecting the fundamental link between dynamic uplift, fluvial erosion and depositional pulses in basins distal to passive margin highlands.

  7. Mesozoic and Cenozoic evolution of the SW Iberian margin

    NASA Astrophysics Data System (ADS)

    Ramos, Adrià; Fernández, Oscar; Terrinha, Pedro; Muñoz, Josep Anton; Arnaiz, Álvaro

    2016-04-01

    The SW Iberian margin lies at the eastern termination of the Azores-Gibraltar Fracture Zone (AGFZ), the diffuse transform plate boundary between Africa and Iberia (Sartori et al., 1994). It comprises the Gulf of Cadiz and the Algarve Basin, which were developed under two main different regional stages of deformation. During the Mesozoic, the SW Iberian margin evolution since the Late Triassic was dominated by the Pangea break-up and the Central Atlantic opening up to Early Jurssic, followed by the westernmost Tethyan opening up to Mid/Late Jurassic, and the North Atlantic rifting from Late Jurassic to Early Cretaceous (e.g., Schettino and Turco, 2010). This phase of extension led to the formation of E-W to NE-SW trending, basement-involved extensional faults, the triggering of salt tectonics and the uplifting of basement highs (e.g., Guadalquivir Bank). This extensional phase was responsible not only for the sedimentary depocenter distribution, but also for the crustal configuration of this passive margin, extending from continental crust in the proximal part, to oceanic crust in the distal and deepest portion of the margin. Since the Late Cretaceous, the margin was inverted due to the N-S convergence between Africa and Iberia, being still undergoing collision given the dominance of reverse fault earthquake mechanisms (e.g., Zitellini et al., 2009). The shortening in the margin is mainly accommodated by the north-dipping foliation of the basin, expressed by south-directed blind thrusts affecting the present-day bathymetry, re-activating the basement highs and the salt tectonics, and controlling the Cenozoic depocenters. The emplacement of the Betics to the east led to the westward emplacement of the gravitational unit partially overlying the sedimentary basins, corresponding to the Allochthonous Unit of the Gulf of Cadiz (AUGC). Our observations of the margin configuration have been based on the interpretation of 2D and 3D seismic reflection surveys throughout the

  8. Deep seismic studies of conjugate profiles from the Nova Scotia - Moroccan and the Liguro-Provencal margin pairs

    NASA Astrophysics Data System (ADS)

    Klingelhoefer, F.; Biari, Y.; Sahabi, M.; Aslanian, D.; Philippe, S.; Schnabel, M.; Moulin, M.; Louden, K. E.; Funck, T.; Reichert, C. J.

    2014-12-01

    The structure of conjugate passive margins provides information about rifting styles, opening of an ocean and formation of it's associated sedimentary basins. In order to distinguish between tectonic inheritance and structures directly related to rifting of passive margins conjugate profiles have to be acquired on margins on diverse locations and different ages. In this study we use new and existing reflection and wide-angle seismic data from two margin pairs, the 200 Ma year old Nova-Scotia - Morocco margin pair and the only 20 Ma Gulf of Lions - Sardinia margin pair. On both margin pairs wide-angle seismic data combined with reflection seismic data were acquired on conjugate profiles on sea and extended on land. Forward modelling of the deep crustal structure along the four transects indicates that a high velocity zone (HVZ) (> 7.2 km/s) is present at the base of the lower crust on all four margins along the ocean-continental transition zone (OCT). This may represent either exhumed upper mantle material or injection of upper mantle material into proto-oceanic crust at the onset of sea-floor spreading. However the width of the HVZ might strongly differ between conjugates, which may be the result of tectonic inheritance, for example the presence of ancient subduction zones or orogens. Both margin pairs show a similar unthinned continental crustal thickness. Crustal thinning and upper-to-lower crustal thickness vary between margin pairs, but remain nearly symmetric on conjugate profiles and might therefore depend on the structure and mechanical properties of the original continental crust. For the Mediterranean margin pair, the oceanic crust is similar on both sides, with a thickness of only 4-5 km. For the Atlantic margin pair, oceanic crustal thickness is higher on the Moroccan Margin, a fact that can be explained by either asymmetric spreading or by the volcanic underplating, possibly originating from the Canary Hot Spot.

  9. Atlantic marginal basins of Africa

    SciTech Connect

    Moore, G.T.

    1988-02-01

    The over 10,000-km long Atlantic margin of Africa is divisible into thirty basins or segments of the margin that collectively contain over 18.6 x 10/sup 6/ km/sup 3/ of syn-breakup and post-breakup sediments. Twenty of these basins contain a sufficiently thick volume of sediments to be considered prospects. These basins lie, at least partially, within the 200 m isobath. The distribution of source rocks is broad enough to give potential to each of these basins. The sedimentation patterns, tectonics, and timing of events differ from basin to basin and are related directly to the margin's complex history. Two spreading modes exist: rift and transform. Rifting dates from Late Triassic-Early Jurassic in the northwest to Early Cretaceous south of the Niger Delta. A complex transform fault system separated these two margins. Deep-water communication between the two basins became established in the middle Cretaceous. This Mesozoic-Cenozoic cycle of rifting and seafloor spreading has segmented the margin and where observable, basins tend to be bounded by these segments.

  10. 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.

  11. Dimensionality influence on passive scalar transport

    NASA Astrophysics Data System (ADS)

    Iovieno, M.; Ducasse, L.; Tordella, D.

    2011-12-01

    We numerically investigate the advection of a passive scalar through an interface placed inside a decaying shearless turbulent mixing layer. We consider the system in both two and three dimensions. The dimensionality produces a different time scaling of the diffusion, which is faster in the two-dimensional case. Two intermittent fronts are generated at the margins of the mixing layer. During the decay these fronts present a sort of propagation in both the direction of the scalar flow and the opposite direction. In two dimensions, the propagation of the fronts exhibits a significant asymmetry with respect to the initial position of the interface and is deeper for the front merged in the high energy side of the mixing. In three dimensions, the two fronts remain nearly symmetrically placed. Results concerning the scalar spectra exponents are also presented.

  12. Widespread methane leakage from the sea floor on the northern US Atlantic margin

    USGS Publications Warehouse

    Skarke, Adam; Ruppel, Carolyn; Kodis, Mali'o; Brothers, Daniel S.; Lobecker, Elizabeth A.

    2014-01-01

    Methane emissions from the sea floor affect methane inputs into the atmosphere, ocean acidification and de-oxygenation, the distribution of chemosynthetic communities and energy resources. Global methane flux from seabed cold seeps has only been estimated for continental shelves, at 8 to 65 Tg CH4 yr−1, yet other parts of marine continental margins are also emitting methane. The US Atlantic margin has not been considered an area of widespread seepage, with only three methane seeps recognized seaward of the shelf break. However, massive upper-slope seepage related to gas hydrate degradation has been predicted for the southern part of this margin, even though this process has previously only been recognized in the Arctic. Here we use multibeam water-column backscatter data that cover 94,000 km2 of sea floor to identify about 570 gas plumes at water depths between 50 and 1,700 m between Cape Hatteras and Georges Bank on the northern US Atlantic passive margin. About 440 seeps originate at water depths that bracket the updip limit for methane hydrate stability. Contemporary upper-slope seepage there may be triggered by ongoing warming of intermediate waters, but authigenic carbonates observed imply that emissions have continued for more than 1,000 years at some seeps. Extrapolating the upper-slope seep density on this margin to the global passive margin system, we suggest that tens of thousands of seeps could be discoverable.

  13. New Mesozoic-Cenozoic palaeotectonic maps used to shed light on Tethyan geological development (Eastern Mediterranean, Taurides and Arabian margin)

    NASA Astrophysics Data System (ADS)

    Robertson, A. H. F.; Parlak, O.; Ustaömer, T.

    2012-04-01

    The main objective here is to present and discuss a series of new palaeotectonic maps for the region that includes the easternmost Mediterranean, the Arabian margin and the Taurides, and which assimilate much recently published information. Critical to this is a review of the Taurides which tests alternative reconstructions that involve either the creation of a series of rifts within a large continental area and/or an array of or microcontinents separated by Mesozoic ocean basins. The proposed reconstructions envisage a long-lived Palaeozoic-Early Cenozoic Tethys bordering Eurasia (Rheic and Palaeotethyan oceans), northward subduction of these oceans beneath Eurasia and the rifting of continental fragments from Gondwana (e.g. during Ordovician and Triassic). Consideration of field relations indicates that the various platform units in southern Turkey (e.g. Bey Dağları; Malatya-Keban; Kirşehir) do not restore as a single large Tauride continent. Instead continental fragments rifted from Gondwana during the Triassic to open several Mesozoic oceanic basins, notably the large Southern Neotethyan ocean, the Berit ocean (new name) and the Inner Tauride ocean, while the İzmir-Arkara-Erzincan ocean developed adjacent to Eurasia. In general, Mid-Permian to Mid-Triassic pulsed rifting culminated in Late Triassic-Early Jurassic spreading. After Early-Mid Jurassic passive subsidence, Late Jurassic-Early Cretaceous was characterised by alkaline, within-plate magmatism related to plume activity or renewed rifting. Late Cretaceous ophiolites formed above subduction zones in several oceanic basins. During latest the Cretaceous ophiolites were emplaced southwards onto the Tauride and Arabian platforms. The Southern Neotethys sutured with the Arabian margin during the Early-Middle Miocene, while ocean crust remained in the Eastern Mediterranean further west. The leading edge of the North African continental margin, the Eratosthenes High, collided with a subduction trench south

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

    NASA Astrophysics Data System (ADS)

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

    1983-05-01

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

  15. Fundamental studies of passivity and passivity breakdown. Final report

    SciTech Connect

    Macdonald, D.D.; Urquidi-Macdonald, M.; Song, H.; Biaggio-Rocha, S.; Searson, P.

    1991-11-01

    This report summarizes the findings of our fundamental research program on passivity and passivity breakdown. During the past three and one half years in this program (including the three year incrementally-funded grant prior to the present grant), we developed and experimentally tested various physical models for the growth and breakdown of passive films on metal surfaces. These models belong to a general class termed ``point defects models`` (PDMs), in which the growth and breakdown of passive films are described in terms of the movement of anion and cation vacancies.

  16. 12 CFR 220.4 - Margin account.

    Code of Federal Regulations, 2011 CFR

    2011-01-01

    ... 12 Banks and Banking 3 2011-01-01 2011-01-01 false Margin account. 220.4 Section 220.4 Banks and... BROKERS AND DEALERS (REGULATION T) § 220.4 Margin account. (a) Margin transactions. (1) All transactions not specifically authorized for inclusion in another account shall be recorded in the margin...

  17. 12 CFR 220.4 - Margin account.

    Code of Federal Regulations, 2012 CFR

    2012-01-01

    ... 12 Banks and Banking 3 2012-01-01 2012-01-01 false Margin account. 220.4 Section 220.4 Banks and... BROKERS AND DEALERS (REGULATION T) § 220.4 Margin account. (a) Margin transactions. (1) All transactions not specifically authorized for inclusion in another account shall be recorded in the margin...

  18. Post-breakup Basin Evolution along the South-Atlantic Margins

    NASA Astrophysics Data System (ADS)

    Strozyk, Frank; Back, Stefan; Kukla, Peter

    2014-05-01

    The post-breakup tectono-stratigraphic evolution of large offshore basins along the South American and African continental margins record strongly varying post-rift sedimentary successions. The northernmost segment of the South Atlantic rift and salt basins is characterized by a pronounced asymmetry, with the Brazilian margin comprising narrower and deeper rift basins with less salt in comparison to the Congo-Gabon conjugate margin. Another important observation is that multiple phases of uplift and subsidence are recorded after the break-up of the southern South Atlantic on both sides of the Florianopolis-Walvis Ridge volcanic complex, features that are regarded as atypical when compared to published examples of other post-breakup margin successions. A regional comparison based on tectonic-stratigraphic analysis of selected seismic transects between the large basins offshore southern Brazil (Espirito Santo Basin, Campos Basin, Santos Basin, Pelotas Basin) and southwest Africa (Lower Congo Basin, Kwanza Basin, Namibe Basin, Walvis Basin) provides a comprehensive basin-to-basin documentation of the key geological parameters controlling ocean and continental margin development. This comparison includes the margin configuration, subsidence development through time, sediment influx and storage patterns, type of basin fill (e.g. salt vs. non-salt systems; carbonate-rich vs. clastics-dominated systems) and finally major tectonic and magmatic events. Data from the salt basins indicate that salt-related tectonic deformation is amongst the prime controls for the non-uniform post-rift margin development. The diversity in the stratigraphic architecture of the conjugate margins offshore southern Brazil, Namibia and Angola reflects variations in the interplay of a number of controlling factors, of which the most important are (a) the structural configuration of each margin segment at the time of break-up, (b) the post break-up subsidence history of the respective margin segment

  19. 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…

  20. 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…

  1. 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…

  2. 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…

  3. 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…

  4. 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…

  5. Changes in North African dust deposition: 35 ka through the Last Glacial Maximum

    NASA Astrophysics Data System (ADS)

    Kinsley, C. W.; McGee, D.; Winckler, G.; deMenocal, P. B.; Stuut, J. W.; Bradtmiller, L. I.

    2013-12-01

    Past changes in atmospheric circulation and aridity in the North African region can be explored by examining continuous records of reconstructed eolian dust accumulation in West African margin sediments. Recent high-resolution reconstructions of dust deposition by McGee et al. (2013) from a meridional transect of cores stretching from 27°N to 19°N along the northwest African margin indicate dramatic changes in North African dust emissions over the last 20 ka. Times of high dust emissions were documented during Heinrich Stadial 1 and the Younger Dryas, and lower dust emissions during the African Humid Period. Here we present a continuation of these records, combining grain size endmember modeling with 230Th-normalized fluxes in these cores to document spatial and temporal changes in dust loads and grain size distributions within the North African dust plume from 20 to ~35 ka. Our results provide quantitative estimates of the magnitude of dust flux changes associated with previous Heinrich Stadials, and lend insight to the nature of the North African dust plume through the entirety of the Last Glacial Maximum. References: McGee, D., deMenocal, P.B., Winckler, G., Stuut, J.B.W., Bradtmiller, L.I., 2013. The magnitude, timing and abruptness of changes in North African dust deposition over the last 20,000 yr. Earth And Planetary Science Letters 371-372, 163-176.

  6. 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.

  7. Passive bistatic radar analysis

    NASA Astrophysics Data System (ADS)

    O'Hagan, Daniel W.; Kuschel, H.; Schiller, Joachim

    2009-06-01

    Passive Bistatic Radar (PBR) research is at its zenith with several notable PBR systems currently operational, or available for deployment. Such PBRs include the Manastash Ridge Radar (MRR) developed for and by academia; Silent Sentry developed as a commercial concern by Lockheed Martin; and Homeland Alerter (HA100) also a commercial system developed by Thales. However at present, despite the existence of numerous PBR prototypes, take up of commercial passive radar technology remains slow. This is due in part to technology immaturity, in part to politics, and particularly due to the fact that monostatic radars perform so well. If PBRs are to enjoy longevity as a viable technology then it is imperative that they address certain niche application areas, with the aforementioned MRR being one prime example of this. The focus of this paper will be an analysis of a PBR system that utilised FM radio signals of opportunity to detect aircraft targets with an RCS generally not lower than 20 m2. The paper will demonstrate the theoretical detection coverage of an FM based PBR operating in a severe interference environment.

  8. Fly ash carbon passivation

    DOEpatents

    La Count, Robert B; Baltrus, John P; Kern, Douglas G

    2013-05-14

    A thermal method to passivate the carbon and/or other components in fly ash significantly decreases adsorption. The passivated carbon remains in the fly ash. Heating the fly ash to about 500 and 800 degrees C. under inert gas conditions sharply decreases the amount of surfactant adsorbed by the fly ash recovered after thermal treatment despite the fact that the carbon content remains in the fly ash. Using oxygen and inert gas mixtures, the present invention shows that a thermal treatment to about 500 degrees C. also sharply decreases the surfactant adsorption of the recovered fly ash even though most of the carbon remains intact. Also, thermal treatment to about 800 degrees C. under these same oxidative conditions shows a sharp decrease in surfactant adsorption of the recovered fly ash due to the fact that the carbon has been removed. This experiment simulates the various "carbon burnout" methods and is not a claim in this method. The present invention provides a thermal method of deactivating high carbon fly ash toward adsorption of AEAs while retaining the fly ash carbon. The fly ash can be used, for example, as a partial Portland cement replacement in air-entrained concrete, in conductive and other concretes, and for other applications.

  9. Passive damping technology demonstration

    NASA Astrophysics Data System (ADS)

    Holman, Robert E.; Spencer, Susan M.; Austin, Eric M.; Johnson, Conor D.

    1995-05-01

    A Hughes Space Company study was undertaken to (1) acquire the analytical capability to design effective passive damping treatments and to predict the damped dynamic performance with reasonable accuracy; (2) demonstrate reasonable test and analysis agreement for both baseline and damped baseline hardware; and (3) achieve a 75% reduction in peak transmissibility and 50% reduction in rms random vibration response. Hughes Space Company teamed with CSA Engineering to learn how to apply passive damping technology to their products successfully in a cost-effective manner. Existing hardware was selected for the demonstration because (1) previous designs were lightly damped and had difficulty in vibration test; (2) multiple damping concepts could be investigated; (3) the finite element model, hardware, and test fixture would be available; and (4) damping devices could be easily implemented. Bracket, strut, and sandwich panel damping treatments that met the performance goals were developed by analysis. The baseline, baseline with damped bracket, and baseline with damped strut designs were built and tested. The test results were in reasonable agreement with the analytical predictions and demonstrated that the desired reduction in dynamic response could be achieved. Having successfully demonstrated this approach, it can now be used with confidence for future designs as a means for reducing weight and enhancing reliability.

  10. Plasma preparation and storage for African elephants (Loxodonta africana).

    PubMed

    Knauf, Sascha; Blad-Stahl, Julia; Lawrenz, Arne; Schuerer, Ulrich; Wehrend, Axel

    2009-03-01

    The use of plasma as a life-saving tool for neonatal African elephants (Loxodonta africana) that failed passive transfer of immunoglobulins is proposed. The methodology of blood sampling, plasma extraction, and plasma storage is described. Values for cellular component sedimentation and biochemical parameters of extracted plasma that was collected from 2 female elephants is presented. The proposal for a central plasma bank for elephants in European zoos is suggested. PMID:19368242

  11. Stability margins for Hurwitz polynomials

    NASA Technical Reports Server (NTRS)

    Chapellat, Herve; Bhattacharyya, S. P.; Keel, L. H.

    1988-01-01

    The authors treat the robust stability issue using the characteristic polynomial, for two different cases: first in coefficient space with respect to perturbations in the coefficient of the characteristic polynomial; and then for a control system containing perturbed parameters in the transfer function description of the plant. In coefficient space, a simple expression is first given for the l-(squared) stability margin for both the monic and nonmonic cases. Following this, a method is given to find the l(infinity) margin, and the method is extended to reveal much larger stability regions. In parameter space the authors consider all single-input (multi-output) or single-output (multi-input) systems with a fixed controller and a plant described by a set of transfer functions which are ratios of polynomials with variable coefficients. A procedure is presented to calculate the radius of the largest stability ball in the space of these variable parameters. The calculation serves as a stability margin for the control system. The formulas that result are quasi-closed-form expressions for the stability margin and are computationally efficient.

  12. 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

  13. 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

  14. Passive-solar construction handbook

    SciTech Connect

    Levy, E.; Evans, D.; Gardstein, C.

    1981-02-01

    Many of the basic elements of passive solar design are reviewed. Passive solar construction is covered according to system type, each system type discussion including a general discussion of the important design and construction issues which apply to the particular system and case studies illustrating designed and built examples of the system type. The three basic types of passive solar systems discussed are direct gain, thermal storage wall, and attached sunspace. Thermal performance and construction information is presented for typical materials used in passive solar collector components, storage components, and control components. Appended are an overview of analysis methods and a technique for estimating performance. (LEW)

  15. 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.

  16. Visualizing MARGINS Data in Undergraduate Courses using GeoMapApp (Invited)

    NASA Astrophysics Data System (ADS)

    Wetzel, L. R.

    2009-12-01

    Undergraduates can explore continental margins and plate boundaries through two exercises, Profiling Earth's Surface using GeoMapApp and What Kind of Continental Margin am I? Active or Passive? Both activities introduce students to GeoMapApp, an easy-to-use mapping program focused on marine geology and geophysics developed at Lamont-Doherty Earth Observatory. In the Profiling Earth's Surface exercise, students relate large-scale continental and oceanic landforms to lithospheric plates, the underlying asthenosphere, earthquakes, and volcanoes. After using GeoMapApp to create a profile showing elevation, students add additional geologic features by hand. In the What Kind of Continental Margin am I? activity, students use GeoMapApp to investigate earthquake data to locate subducting slabs; examine topographic data to determine volcanic arc locations relative to trenches; and integrate earthquake, volcano, and bathymetric data to distinguish between passive and active margins. A webinar recorded in the summer of 2009, Teaching with MARGINS Data and GeoMapApp, illustrates how to use this and two other related activities. Students in introductory-level courses are likely to take about an hour to complete each exercise. To use GeoMapApp, students must download the freely available software to their Mac or PC (www.geomapapp.org) and maintain an internet connection to access base maps and datasets. On-line tutorials describing specific GeoMapApp functions are available to assist both students and faculty. Both activities were created in direct response to the NSF-funded Mini-Lessons initiative to develop undergraduate teaching modules to utilize MARGINS resources, including scientific data and visualization tools such as GeoMapApp. The exercises and the webinar are part of the MARGINS Data in the Classroom collection available at the on-line Science Education Resource Center at Carleton College (serc.carleton.edu/margins).

  17. 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.

  18. Integrated geophysical study of Newfoundland continental margin (east coast Canada)

    SciTech Connect

    Enachescu, M.E.

    1987-05-01

    A synergetic approach is used to delineate the tectono-structural framework and establish a model of the geologic evolution of the Newfoundland continental margin. Over 100,000 mi of regional and detailed reflection seismic, various potential field maps and profiles, and selected well information constitute the data base of this study. More than two decades of exploratory effort was recently rewarded by significant oil discoveries contained in the Upper Jurassic-Early Cretaceous sands of the Jeanne d'Arc basin. Although a part of the East Coast North America chain (Florida to Baffin Bay) of passive margin basins, the Jeanne d'Arc basin had a unique intracratonic setting during its development. This prolific hydrocarbon-bearing basin is only one of the structural provinces identified on the Newfoundland Shelf. Other adjacent basins, sediment-covered ridges, platforms, and basement horsts were identified through integration of all available geophysical and geological data. All of the structural provinces defined in this paper are part of an aborted rift system which initially developed in the Late Triassic and was first activated at the end of the Jurassic and then again in Aptian time. Thermal subsidence prevailed during the Late Cretaceous and Tertiary. The extensional history of the Newfoundland passive margin was additionally complicated by intensive salt tectonics. A large 2-D and 3-D seismic base of more than 100,000 mi shows numerous and complex hydrocarbon traps are present, but up to now only the extensional sedimentary cycle has been found to be productive.

  19. A review of Wilson Cycle plate margins: What is the role of mantle plumes in continental break-up along former sutures?

    NASA Astrophysics Data System (ADS)

    Buiter, Susanne; Torsvik, Trond

    2013-04-01

    It was Tuzo Wilson (1966) who recognised that the different faunal distributions on both sides of the present-day North Atlantic Ocean required the existence of an earlier proto-Atlantic Ocean. The observation that the present-day Atlantic Ocean mainly opened along a former suture was a crucial step in the formulation of the Wilson Cycle theory. The theory implies that collision zones are structures that are able to localize extensional deformation for long times after the collision has waned. We review margin pairs around the Atlantic and Indian Oceans with the aim to evaluate the extent to which oceanic opening used former sutures and to analyse the role of mantle plumes in continental break-up. We aid our analyses with plate tectonic reconstructions using GPlates (www.gplates.org). Already Wilson recognized that Atlantic break-up did not always follow the precise line of previous junction. For example, Atlantic opening did not utilize the Iapetus suture in Great Britain and rather than opening along the younger Rheic suture north of Florida, break-up occurred along the older Pan-African structures south of Florida. As others before us, we find no correlation of suture and break-up age. Often continental break-up occurs some hundreds of Myrs after collision, but it may also take more than a Gyr, as for example for Australia-Antarctica and Congo-São Francisco. This places serious constraints on potential collision zone weakening mechanisms. Several studies have pointed to a link between continental break-up and large-scale mantle upwellings. It is, however, much debated whether plumes use existing rifts as a pathway, or whether plumes play an active role in causing rifting. It is also important to realise that in several cases break-up cannot be related to plume activity. Examples are the Iberia-Newfoundland, Equatorial Atlantic Ocean, and Australia-Antarctica plate margins. For margins that are associated with large igneous provinces (LIPs), we find a positive

  20. Segmented African Lithosphere Beneath Anatolia Imaged by Teleseismic P-Wave Tomography

    NASA Astrophysics Data System (ADS)

    Biryol, Cemal; Zandt, George; Beck, Susan; Ozacar, Atilla

    2010-05-01

    Anatolia, a part of the Alpine-Himalayan orogenic belt, is shaped by a variety of complex tectonic processes that define the major tectonic provinces across which different deformation regimes exist. Collision related plateau formation dominates the present lithospheric deformation to the east and slab roll-back related back-arc extension takes place in the west. The two zones are connected at the northern part of the region by strike-slip faulting along the right-lateral North Anatolian Fault Zone. Recent seismological studies show that the Eastern Anatolian Plateau (EAP) is supported by hot asthenosphereric material that was emplaced beneath the plateau following the detachment of subducted Arabian lithosphere. The westward continuation of the deeper structure of Anatolia was previously less well constrained due to the lack of geophysical observations. In order to study the deeper lithosphere and mantle structure beneath Anatolia, we used teleseismic P-wave tomography and data from several temporary and permanent seismic networks deployed in the region. A major part of the data comes from the North Anatolian Fault passive seismic experiment (NAF) that consists of 39 broadband seismic stations operated at the north central part of Anatolia between 2005 and 2008. We also used data collected from permanent seismic stations of the National Earthquake Monitoring Center (NEMC) and stations from the Eastern Turkey Seismic Experiment (ETSE). Approximately 34,000 P-wave travel time residuals, measured in multiple frequency bands, are inverted using approximate finite-frequency sensitivity kernels. Our tomograms reveal a fast anomaly that corresponds to the subducted portion of the African lithosphere along the Cyprean Arc. This fast anomaly dips northward beneath central Anatolia with an angle of approximately 45 degrees. However, the anomaly disappears rather sharply to the east beneath the western margin of the EAP and to the west beneath the Isparta Angle. The western

  1. Adaptive passive fathometer processing.

    PubMed

    Siderius, Martin; Song, Heechun; Gerstoft, Peter; Hodgkiss, William S; Hursky, Paul; Harrison, Chris

    2010-04-01

    Recently, a technique has been developed to image seabed layers using the ocean ambient noise field as the sound source. This so called passive fathometer technique exploits the naturally occurring acoustic sounds generated on the sea-surface, primarily from breaking waves. The method is based on the cross-correlation of noise from the ocean surface with its echo from the seabed, which recovers travel times to significant seabed reflectors. To limit averaging time and make this practical, beamforming is used with a vertical array of hydrophones to reduce interference from horizontally propagating noise. The initial development used conventional beamforming, but significant improvements have been realized using adaptive techniques. In this paper, adaptive methods for this process are described and applied to several data sets to demonstrate improvements possible as compared to conventional processing.

  2. Passive containment cooling system

    DOEpatents

    Billig, Paul F.; Cooke, Franklin E.; Fitch, James R.

    1994-01-01

    A passive containment cooling system includes a containment vessel surrounding a reactor pressure vessel and defining a drywell therein containing a non-condensable gas. An enclosed wetwell pool is disposed inside the containment vessel, and a gravity driven cooling system (GDCS) pool is disposed above the wetwell pool in the containment vessel and is vented to the drywell. An isolation pool is disposed above the GDCS pool and includes an isolation condenser therein. The condenser has an inlet line disposed in flow communication with the drywell for receiving the non-condensable gas along with any steam released therein following a loss-of-coolant accident (LOCA). The condenser also has an outlet line disposed in flow communication with the drywell for returning to the drywell both liquid condensate produced upon cooling of the steam and the non-condensable gas for reducing pressure within the containment vessel following the LOCA.

  3. Passive Ball Capture Joint

    NASA Technical Reports Server (NTRS)

    Cloyd, Richard A. (Inventor); Bryan, Thomas C. (Inventor)

    2003-01-01

    A passive ball capture joint has a sleeve with a plurality of bores distributed about a circumference thereof and formed therethrough at an acute angle relative to the sleeve's longitudinal axis. A spring-loaded retainer is slidingly fitted in each bore and is biased such that, if allowed, will extend at least partially into the sleeve to retain a ball therein. A ring, rotatably mounted about the bores, has an interior wall defining a plurality of shaped races that bear against the spring-loaded retainers. A mechanized rotational force producer is coupled to the ring. The ring can be rotated from a first position (that presses the retainers into the sleeve to lock the ball in place) to a second position (that allows the retainers to springback out of the sleeve to release the ball).

  4. Passive containment cooling system

    DOEpatents

    Billig, P.F.; Cooke, F.E.; Fitch, J.R.

    1994-01-25

    A passive containment cooling system includes a containment vessel surrounding a reactor pressure vessel and defining a drywell therein containing a non-condensable gas. An enclosed wetwell pool is disposed inside the containment vessel, and a gravity driven cooling system (GDCS) pool is disposed above the wetwell pool in the containment vessel and is vented to the drywell. An isolation pool is disposed above the GDCS pool and includes an isolation condenser therein. The condenser has an inlet line disposed in flow communication with the drywell for receiving the non-condensable gas along with any steam released therein following a loss-of-coolant accident (LOCA). The condenser also has an outlet line disposed in flow communication with the drywell for returning to the drywell both liquid condensate produced upon cooling of the steam and the non-condensable gas for reducing pressure within the containment vessel following the LOCA. 1 figure.

  5. Passive focus sensor

    NASA Astrophysics Data System (ADS)

    Engelhardt, Kai; Knop, Karl

    1995-05-01

    A focus-sensor module that could take the place of the visual-image control for professional large-format cameras was fabricated. In addition, a passive focus-sensing method was shown to work at arbitrary locations and orientations in the recording plane of large-format professional cameras. A focus resolution of better than 0.1 mm and a range of measurement of +/- 5 mm at the image side were obtained at a minimum level of illuminance and with an aperture f/5.6 of the imaging lens. In the current method, three out of four images that arose from various sections of the camera's objective lens were applied for triangulation. The demonstrated approach was based on a linear photodiode array and employed one-dimensional image information for focus sensing.

  6. Passive magnetic bearing system

    SciTech Connect

    Post, Richard F.

    2014-09-02

    An axial stabilizer for the rotor of a magnetic bearing provides external control of stiffness through switching in external inductances. External control also allows the stabilizer to become a part of a passive/active magnetic bearing system that requires no external source of power and no position sensor. Stabilizers for displacements transverse to the axis of rotation are provided that require only a single cylindrical Halbach array in its operation, and thus are especially suited for use in high rotation speed applications, such as flywheel energy storage systems. The elimination of the need of an inner cylindrical array solves the difficult mechanical problem of supplying support against centrifugal forces for the magnets of that array. Compensation is provided for the temperature variation of the strength of the magnetic fields of the permanent magnets in the levitating magnet arrays.

  7. Commentary on "Capturing the Evasive Passive"

    ERIC Educational Resources Information Center

    Lillo-Martin, Diane; Snyder, William

    2009-01-01

    Passives has been the focus of much research in language acquisition since the 1970s. It has been clear from this research that young children seldom produce passives spontaneously, particularly "long" or "full" passives with a by-phrase; and they usually perform poorly on experimental tests of the comprehension of passives, especially passives of…

  8. Temperature initiated passive cooling system

    DOEpatents

    Forsberg, C.W.

    1994-11-01

    A passive cooling system for cooling an enclosure only when the enclosure temperature exceeds a maximum standby temperature comprises a passive heat transfer loop containing heat transfer fluid having a particular thermodynamic critical point temperature just above the maximum standby temperature. An upper portion of the heat transfer loop is insulated to prevent two phase operation below the maximum standby temperature. 1 fig.

  9. Temperature initiated passive cooling system

    DOEpatents

    Forsberg, Charles W.

    1994-01-01

    A passive cooling system for cooling an enclosure only when the enclosure temperature exceeds a maximum standby temperature comprises a passive heat transfer loop containing heat transfer fluid having a particular thermodynamic critical point temperature just above the maximum standby temperature. An upper portion of the heat transfer loop is insulated to prevent two phase operation below the maximum standby temperature.

  10. Paleogeodynamic evolution of the Northern South America margin through 13 maps from Maastrichtian to present

    SciTech Connect

    Stephan, J.F. )

    1993-02-01

    The paleogeodynamic history of the Northern South America margin (NSAM) for the last 75 Ma is depicted through 13 maps. Five major episodes can be distinguished: In Maastrichtian and Paleocene times, the NSAM is still a passive margin including, from west to east, the northeast-trending Tinaco-Caucagua Promontory (TCP) and the Coast Range Realm (CRR); From Lower to Upper Eocene the Villa de Cura-Tobago Cretaceous Arc obliquely collides with the margin, generating a northeast-trending foreland flysch basin (i.e., Matatere, Guarico, and Rio Guache flysch). By the end of Eocene, the TCP and CRR have been imbricated under the arc and thrusted southeastward, together with the flysch nappes, onto the upper margin. The allochthon front is stabilized roughly along an Acarigua-Caracas line; Oligocene and lowermost Miocene times correspond to a drastic geometric and kinematic reorganization probably related to a strong slow-down of the Caribbean plate movement. Subsidence and transtension are dominant; From late Lower Miocene to early Upper Miocene, the remnant central and eastern passive margin is tectonized due to the fast eastward transpressive shift of the Caribbean plate; In late Upper Miocene times, a second geodynamic reorganization occurs which gives rise to the present-day pattern where transpression is mostly active in Trinidad, Falcon, and the Merida Andes.

  11. New Insights into SouthWest Africa Margin Evolution; Integrating Reconstructions and Restorations

    NASA Astrophysics Data System (ADS)

    Paton, Douglas; Markwick, Paul; Hodgson, Neil; Rowlands, Holly; Thompson, Phil

    2015-04-01

    Over the last few years there has been a significant increase in the quality and availability of passive margin scale transect derived from new geophysical techniques. Coupled with plate reconstructions this has unquestionably led to a paradigm shift in our understanding of the architecture of conjugate passive continental margins and the transition from continental to oceanic lithosphere. These sections, however, still commonly only consider architecture of the margin by placing conjugate sections together in their pre-break up position without considering realistic architecture of the margin at the time of deposition. In this study we use plate reconstructions to consider location of sections at a variety of time steps. We then apply stratigraphic and structural techniques to determine the geometry of the depositional sequence to predict the architecture and water depth of the margin at the time of deposition. Our study focuses on the south-eastern Atlantic and we use these techniques to understand key time intervals, including the geometry at the end of the rift phase, the emplacement of seaward dipping reflections, the Barremian sag phase and early Cretaceous deltaic sequences. This provides us with new insights into the Southern Atlantic basin evolution as well as providing better constraints for lithospheric processes and palaeogeographic reconstructions during these intervals that are fundamental to the hydrocarbon prospectivity of the region.

  12. Structural design/margin assessment

    NASA Technical Reports Server (NTRS)

    Ryan, R. S.

    1993-01-01

    Determining structural design inputs and the structural margins following design completion is one of the major activities in space exploration. The end result is a statement of these margins as stability, safety factors on ultimate and yield stresses, fracture limits (fracture control), fatigue lifetime, reuse criteria, operational criteria and procedures, stability factors, deflections, clearance, handling criteria, etc. The process is normally called a load cycle and is time consuming, very complex, and involves much more than structures. The key to successful structural design is the proper implementation of the process. It depends on many factors: leadership and management of the process, adequate analysis and testing tools, data basing, communications, people skills, and training. This process and the various factors involved are discussed.

  13. Preservation of FFTF Data Related to Passive Safety Testing

    SciTech Connect

    Wootan, David W.; Butner, R. Scott; Omberg, Ronald P.; Makenas, Bruce J.; Nielsen, Deborah L.

    2010-10-01

    experience to a large-size LMR and obtain data for validating design analysis computer codes, and 3) to develop and test passive safety enhancements that might be used for future LMRs. These tests were designed to provide data sufficient to allow separation of fuel temperature effects from structural temperature effects. The data developed through this testing program were used to verify the predictive capability of passive safety analysis methods as well as provide a data base for calibrating design tools such as the SASSYS/SAS4A codes. These tests were instrumental in improving understanding of reactivity feedback mechanisms in LMRs and demonstrating passive safety margins available in an LMR. Knowledge preservation at the FFTF is focused on the areas of design, construction, startup, and operation of the reactor. This information may be of potential use for international exchanges with other LMR programs around the world. This information provides the basis for creating benchmarks for validating and testing large scale computer programs. All information preserved to date is now being stored and categorized consistent with the IAEA international standardized taxonomy. The test results information exists in several different formats depending upon the final stage of the test evaluation. Over 100 documents relevant to passive safety testing have been identified and are being recovered, scanned, and catalogued. Attempts to recover plant data tapes are also in progress. Documents related to passive safety testing are now being categorized consistent with internationally agreed upon IAEA standards. Documents are being converted to electronic format compatible with a general search engine being developed by INL. The data from the FFTF passive safety tests provides experimental verification of structural reactivity effects that should be very useful to innovative designers seeking to optimize passive safety in the design of new LMRs.

  14. [Marginality, ethnic groups and health].

    PubMed

    Corretger, J M; Fortuny, C; Botet, F; Valls, O

    1992-06-01

    Main marginated ethnic groups in Span are to be found among gypsies and 3rd world immigrants. The first group include about 250,000 persons and the second group more tan half a million people. Their origins and their being past of the less fortunate social layers made them a group of health risk. Pediatric pathologies are those favored by socio-economic shortcomings as well as hygienic-sanitary deficiencies. Imported pediatric pathologies have a small incident.

  15. Fluid-escape structures and slope instabilities along the French Guiana margin

    NASA Astrophysics Data System (ADS)

    Loncke, L.; Gaullier, V.; Basile, C.; Maillard, A.; Patriat, M.; Vendeville, B. C.; Folens, L.; Roest, W.

    2009-04-01

    Many of the world's passive margins are shear margins. Those margins present a very steep ocean-continent boundary which is expressed by high surface-slope gradients. In this type of margins, complex rift structures including wrench and strike-slip faults affect the continental crust. These rift basins usually trap organic matter, hence kerogen that later become hydrocarbons. Along the Guiana margin, late cretaceous black Shales provide additional organic matter. The French Guiana margin has been recently surveyed (GUYAPLAC cruise, 2003) allowing the discovery of a giant pockmark field likely caused by active degassing of deep reservoirs and expressed at the surface through giant elongated pock-marks. These pockmarks are systematically associated with massive slope instabilities and underlying polygonal fault network. Major seaward collapses seem to have affected the margin, and the breakup unconformity is tilted seaward. We believe that fluid overpressure above Cretaceous under-consolidated organic rocks may have destabilized part of the sedimentary cover, allowing massive escape of fluids toward the surface, as is suggested further North by geotechnical analyses made after leg ODP 207 (O'Regan & Moran, 2007). A compactional origin of fluids is also possible. In any case, the specific structural pattern of the Guiana transform margin, with a seaward tilted geometry and no marginal ridge, may favour particularly active fluid releases and slope instabilities (favoured by the decrease in sediment's strength related to high pore-fluid pressure). As suggested by O'Regan & Moran (2007) it is possible also that fluid migration occurs along the break-up unconformity, which crops out along the very steep continental slope. If this is correct, a great part of fossil hydrocarbon resources may escape to the surface along of the Guiana and Surinam continental margins. References: O'Regan M. & K. Moran, 2007. Compressibility, permeability and stress history of sediments from

  16. Structural framework, stratigraphy, and evolution of Brazilian marginal basins

    SciTech Connect

    Ojeda, H.A.O.

    1982-06-01

    The structural framework of the Brazilian continental margin is basically composed of eight structural types: antithetic tilted step-fault blocks, synthetic untilted step-fault blocks, structural inversion axes, hinges with compensation grabens, homoclinal structures, growth faults with rollovers, diapirs, and igneous structures. The antithetic tilted and synthetic untilted step-fault blocks are considered as synchronous, complementary structural systems, separated by an inversion axis. Two evaporitic cycles (Paripueira and Ibura) were differentiated in the Sergipe-Alagoas type basin and tentatively correlated to the evaporitic section of other Brazilian marginal basis. Four phases are considered in the evolution of the Brazilian marginal basins: pre-rift, rift, transitional, and drift. During the pre-rift phase (Late Jurassic-Early Cretaceous), continental sediments were deposited in peripheral intracratonic basins. In the rift phase (Early Cretaceous), the breakup of the continental crust of the Gondwana continent gave rise to a central graben and rift valleys where lacustrine sediments were deposited. The transitional phase (Aptian) developed under relative tectonic stability, when evaporitic and clastic lacustrine sequences were being deposited. In the drift phase (Albian to Holocene), a regionl homoclinal structure developed, consisting of two distinct sedimentary sequences, a lower clastic-carbonate and an upper clastic. From the Albian to the Holocene Epoch, structures associated to plastic displacement of salt or shale developed in many Brazilian marginal basins. Two phases of major igneous activity occurred: one in the Early Cretaceous associated with the rift phase of the Gondwana continent, and the other in the Tertiary during the migration phase of the South American and African plates.

  17. Tectonic structure and evolution of the Atlantic continental margin

    SciTech Connect

    Klitgord, K.D.; Schouten, H.; Hutchinson, D.R.

    1985-01-01

    The Atlantic continental margin developed across the boundary between continental and oceanic crust as rifting and then sea-floor spreading broke apart and separated the North American and African plates, forming the Atlantic Ocean Basin. Continental rifting began in Late Triassic with reactivation of Paleozoic thrust faults as normal faults and with extension across a broad zone of subparallel rift basins. Extension became localized in Early to Middle Jurassic along the zone that now underlies the large marginal basins, and other rift zones, such as the Newark, Hartford, and Fundy basins, were abandoned. Rifting and crustal stretching between the two continents gave way to sea-floor spreading Middle Jurassic and the formation of oceanic crust. This tectonic evolution resulted in formation of distinctive structural features. The marginal basins are underlain by a thinner crust and contain a variety of fault-controlled structures, including half-grabens, seaward- and landward-tilted blocks, faults that die out within the crust, and faults that penetrate the entire crust. This variable structure probably resulted from the late Triassic-Early Jurassic pattern of normal, listric, and antithetic faults that evolved from the Paleozoic thrust fault geometry. The boundary between marginal basins and oceanic crust is marked approximately by the East Coast Magnetic Anomaly (ECMA). A major basement fault is located in the Baltimore Canyon trough at the landward edge of the ECMA and a zone of seaward dipping reflectors is found just seaward of the ECMA off Georges Bank. The fracture zone pattern in Mesozoic oceanic crust can be traced landward to the ECMA.

  18. Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

    NASA Astrophysics Data System (ADS)

    Rooney, Tyrone O.; Mohr, Paul; Dosso, Laure; Hall, Chris

    2013-02-01

    The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ˜31 Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin

  19. The sedimentary supply of African sedimentary basins over the last 250 Ma

    NASA Astrophysics Data System (ADS)

    Rouby, Delphine; Guillocheau, Francois; Robin, Cecile; Helm, Catherine

    2010-05-01

    The African continent is bordered by passive margins and bears intracontinental basins preserving the terrigeneous sediment resulting from its erosion, and as such, recording the dynamics of its relief variation. Our objective is to bring new constraints on the uplift and erosion of the African continent over the last 250 Ma from the perspective of the stratigraphic architecture of its sedimentary basins. The novel aspect of our approach is to integrate the evolution of both the domains in erosion and in sedimentation (i.e. from the drainage divide of the domain in erosion down to the most distal deposits over the oceanic crust), and to review published data to quantify the terrigeneous supply eroded in the drainage area and preserved in the basins. One objective is to evaluate the conditions under which this simple approach, based on already published data, can be used to infer continental relief variations, the sedimentary archives of the domain in erosion being by definition scarce and denudation evaluation by thermochronology usually relying on hypotheses on past heat flows. We quantify the siliciclastic sedimentary volumes preserved in African basins correcting from porosity and in-situ (e.g. carbonate) production, with a particular attention to the determination of uncertainties resulting from parameters such as: velocity laws used to depth conversion of TWT data, biostratigraphic used for calibration in absolute ages, lithology assumed for porosity removal. We use two approaches with complementary spatial and temporal resolutions. (1) When data are available (e.g along the South African and Namibian Atlantic margins), we determine the long-term signal of sedimentary supply (x10 Ma) from 3D mass balance calculations comparing sedimentary volumes deduced from offshore isopach maps on one hand and erosion volumes deduced from the present day geometry of geomorphic markers and thermochronology data on the other hand. We show that our approach provide a good

  20. 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.

  1. 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.

  2. 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. ...

  3. Temporal and spatial patterns of Cenozoic and Late Mesozoic erosion and deposition along the western margin of southern Africa

    SciTech Connect

    Brown, R.W.; Gleadow, A.J.W. ); Rust, D.J.; Summerfield, M.A. )

    1990-05-01

    Compared with subsidence history and eustatic sea level change, sediment supply has been a neglected component of studies of passive margin stratigraphy. The spatial and temporal pattern of sediment supply to continental margin, however, is a critical factor in determining the architecture of offshore sedimentary sequences. Sediment routing across passive margins is controlled primarily by their tectonic development and the consequent morphological evolution of the subaerial part of the margin. By combining offshore sediment volume and sedimentation rate data based on isopach maps and borehole records with apatite fission-track analysis and denudational modeling onland, the depositional history of the western margin of southern Africa has been related to its geomorphic response to continental rifting. The sediment volume data indicate a declining rate of sedimentation after rifting in the Early Cretaceous despite a probable enlargement of the sediment source area through time. Similarly, apatite fission-track ages and confined track length distributions indicate an Early Cretaceous episode of relatively high erosion rates which affected areas both inland and oceanward of the major topographic discontinuity along the margin represented by the Great Escarpment. Late Cenozoic rates of erosion and sediment supply have been low, although much of the sediment source area is still at a significant elevation. Although aridity may have contributed to this reduction in sediment supply, the morphological response to the tectonic evolution of the margin has also been crucial.

  4. African-American lesbian identity management and identity development in the context of family and community.

    PubMed

    Miller, Shannon J

    2011-01-01

    Don't Ask, Don't Tell is gaining attention in family studies literature as a cultural specific context to understand lesbian, gay, and bisexual visibility in African-American families and communities. This policy suggests that sexual minorities are accepted within African-American families and communities as long as they do not label themselves or acknowledge publicly that they engage in same-sex relationships. The narratives of two African-American lesbians (aged 26 and 27 years) are chronicled in the present study to reveal their lesbian identity development, lesbian identity management, and how they defined and navigated Don't Ask, Don't Tell. They encountered challenges and successes in a quest to find communities that would embrace and affirm their multiple marginalized identities. Their stories are offered as a point of entry to further inquiry concerning African-American lesbian visibility and identity proclamation within African-American families and communities.

  5. Passive blast pressure sensor

    DOEpatents

    King, Michael J.; Sanchez, Roberto J.; Moss, William C.

    2013-03-19

    A passive blast pressure sensor for detecting blast overpressures of at least a predetermined minimum threshold pressure. The blast pressure sensor includes a piston-cylinder arrangement with one end of the piston having a detection surface exposed to a blast event monitored medium through one end of the cylinder and the other end of the piston having a striker surface positioned to impact a contact stress sensitive film that is positioned against a strike surface of a rigid body, such as a backing plate. The contact stress sensitive film is of a type which changes color in response to at least a predetermined minimum contact stress which is defined as a product of the predetermined minimum threshold pressure and an amplification factor of the piston. In this manner, a color change in the film arising from impact of the piston accelerated by a blast event provides visual indication that a blast overpressure encountered from the blast event was not less than the predetermined minimum threshold pressure.

  6. New passive helicopter detector

    SciTech Connect

    Elliott, G.R.

    1985-01-01

    Sandia has developed a new helicopter detector. The device relies on the correlation between the acoustic wave from the helicopter and the resulting coupled seismic wave. A significant feature of this approach is that the detector is completely passive; there is no radio frequency radiation. Intended for deployment as a perimeter sensor around a site, the unit offers a low nuisance/false alarm rate and a high probability of detection for a wide range of helicopters. Reliable detection occurs when the target is at high altitude and also very near the earth's surface. Detection ranges start at one kilometer for the small, four-place, civilian helicopter and approach five kilometers for heavier, military types. The system has two parts: a transducer package containing a microphone and a geophone and a digital processor. Development is underway for a model which will be AC powered and well suited to permanent facilities. A prototype unit using a lightweight, battery powered processor is being constructed for rapid-deployment applications. 6 figs.

  7. Passive-solar greenhouse

    SciTech Connect

    Not Available

    1982-01-01

    Our project objective was to design, construct, and operate a commercialized (16' x 50') passive, solar greenhouse. The structure was originally intended as a vegetable forcing facility to produce vegetable crops in the off-season. Building and size constraints and economic considerations convinced us to use the greenhouse for producing bedding plants and vegetable starts in the spring, high value vegetables (tomatoes, cucumbers) in the fall and forced bulbs in the winter. This crop sequence allows us to use the greenhouse all year without additional heat as the crops are adopted to the temperature regime of the greenhouse during each particular season. In our first season, the greenhouse performed beautifully. The lowest temperature recorded was 38/sup 0/F after 4 cold, cloudy days in February. The production of bedding plants has allowed us to diversify our products and the early transplants we produced were a great asset to our vegetable farming operation. Although construction cost (4.57 sq. ft.) is higher than that of a conventional polyethylene-covered, quonset-type greenhouse (approx. $1.92 sq. ft.), our annual operating cost is cheaper than that of a conventional greenhouse (0.49 cents sq. ft. versus 0.67 cents sq. ft.) due to a longer usable lifetime of the structure and the elimination of heating costs. Our structure has been toured by interested individuals, school and farm groups. We plan to publicize the structure and its advantages by promoting more visits to the site.

  8. Passive Acoustic Vessel Localization

    NASA Astrophysics Data System (ADS)

    Suwal, Pasang Sherpa

    This thesis investigates the development of a low-cost passive acoustic system for localizing moving vessels to monitor areas where human activities such as fishing, snorkeling and poaching are restricted. The system uses several off-the-shelf sensors with unsynchronized clocks where the Time Difference of Arrival (TDOA) or time delay is extracted by cross-correlation of the signal between paired sensors. The cross-correlation function uses phase correlation or Phase Transform (PHAT) which whitens the cross-spectrum in order to de-emphasize dominant frequency components. Using the locations of pairs of sensors as foci, hyperbolic equations can be defined using the time delay between them. With three or more sensors, multiple hyperbolic functions can be calculated which intersect at a unique point: the boat's location. It is also found that increasing separation distances between sensors decreased the correlation between the signals. However larger separation distances have better localization capability than with small distances. Experimental results from the Columbia and Willamette Rivers are presented to demonstrate performance.

  9. Passive Vaporizing Heat Sink

    NASA Technical Reports Server (NTRS)

    Knowles, TImothy R.; Ashford, Victor A.; Carpenter, Michael G.; Bier, Thomas M.

    2011-01-01

    A passive vaporizing heat sink has been developed as a relatively lightweight, compact alternative to related prior heat sinks based, variously, on evaporation of sprayed liquids or on sublimation of solids. This heat sink is designed for short-term dissipation of a large amount of heat and was originally intended for use in regulating the temperature of spacecraft equipment during launch or re-entry. It could also be useful in a terrestrial setting in which there is a requirement for a lightweight, compact means of short-term cooling. This heat sink includes a hermetic package closed with a pressure-relief valve and containing an expendable and rechargeable coolant liquid (e.g., water) and a conductive carbon-fiber wick. The vapor of the liquid escapes when the temperature exceeds the boiling point corresponding to the vapor pressure determined by the setting of the pressure-relief valve. The great advantage of this heat sink over a melting-paraffin or similar phase-change heat sink of equal capacity is that by virtue of the =10x greater latent heat of vaporization, a coolant-liquid volume equal to =1/10 of the paraffin volume can suffice.

  10. Constraining Lithosphere Deformation Modes during Continental Breakup for the Iberia-Newfoundland Conjugate Margins

    NASA Astrophysics Data System (ADS)

    Jeanniot, L.; Kusznir, N. J.; Mohn, G.; Manatschal, G.

    2014-12-01

    How the lithosphere and asthenosphere deforms during continental rifting leading to breakup and sea-floor spreading initiation is poorly understood. Observations at present-day and fossil analogue rifted margins show a complex OCT architecture which cannot be explained by a single simplistic lithosphere deformation modes. This OCT complexity includes hyper-extended continental crust and lithosphere, detachments faults, exhumed mantle, continental slivers and scattered embryonic oceanic crust. We use a coupled kinematic-dynamic model of lithosphere and asthenosphere deformation to determine the sequence of lithosphere deformation modes leading to continental breakup for Iberia-Newfoundland conjugate margin profiles. We quantitatively calibrate the models using observed present-day water loaded subsidence and crustal thickness, together with subsidence history and the age of melt generation. Flow fields, representing a sequence of lithosphere deformation modes, are generated by a 2D finite element viscous flow model (FE-Margin), and used to advect lithosphere and asthenosphere temperature and material. FE-Margin is kinematically driven by divergent deformation in the upper 15-20 km of the lithosphere inducing passive upwelling below. Buoyancy enhanced upwelling (Braun et al. 2000) is also kinematically included. Melt generation by decompressional melting is predicted using the methodology of Katz et al., 2003. The extension magnitudes used in the lithosphere deformation models are taken from Sutra et al (2013). The best fit calibrated models of lithosphere deformation evolution for the Iberia-Newfoundland conjugate margins require (i) an initial broad region of lithosphere deformation and passive upwelling, (ii) lateral migration of deformation, (iii) an increase in extension rate with time, (iv) focussing of deformation and (v) buoyancy induced upwelling. The preferred calibrated models predict faster extension rates and earlier continental crustal rupture and

  11. Continental margin evolution of the northern Arabian platform in Syria

    SciTech Connect

    Best, J.A.; Barazangi, M. ); Al-Saad, D.; Sawaf, T.; Gebran, A. )

    1993-02-01

    Synthesis of available geological and geophysical data in the Syrian Arab Republic permits a descriptive account of the pre-Cenozoic geologic history of the northern Arabian platform. The northern Arabian platform appears to be a composite plate similar up to that interpreted in the rocks of the Arabian shield. The structural and stratigraphic relationships of the Paleozoic and Mesozoic sedimentary sections in Syria record the transformation of an eastward-facing Gondwana passive margin in the early Paleozoic into a westward-facing Levantine margin in the Mesozoic, at which time the northern platform was closely associated with the creation of the eastern Mediterranean basin. Timing of the margin transformation is inferred from the orientation and thickness variations of Lower Triassic rocks, but the transformation may have initiated as early as the Permian. The diversity and timing of geological features in Syria suggest that the northern Arabian platform did not behave as a rigid plate throughout its geological history. The present-day Palmyride mountain belt, located within the northern Arabian platform in Syria and initiated in the early Mesozoic as a northeast-trending rift nearly perpendicular to the Levantine margin, subsequently was inverted in the Cenozoic by transpression. The location of the rift may be associated with the reactivation of a zone of crustal weakness, i.e., a Proterozoic suture zone previously proposed from modeling of Bouguer gravity data. Thus, the northern and southern parts of the Arabian platform are similar in their respective geologic histories during the Proterozoic and Paleozoic; however, the northern Arabian platform was greatly affected by Mesozoic rifting and the creation of the eastern Mediterranean basin during the Mesozoic. 13 figs.

  12. Silenced, Silence, Silent: Motherhood in the Margins

    ERIC Educational Resources Information Center

    Carpenter, Lorelei; Austin, Helena

    2007-01-01

    This project explores the experiences of women who mother children with ADHD. The authors use the metaphor of the text and the margin. The text is the "motherhood myth" that describes a particular sort of "good" mothering. The margin is the space beyond that text. This marginal space is inhabited by some or all of the mothers they spoke with, some…

  13. 12 CFR 220.4 - Margin account.

    Code of Federal Regulations, 2013 CFR

    2013-01-01

    ... 12 Banks and Banking 3 2013-01-01 2013-01-01 false Margin account. 220.4 Section 220.4 Banks and...) CREDIT BY BROKERS AND DEALERS (REGULATION T) § 220.4 Margin account. (a) Margin transactions. (1) All transactions not specifically authorized for inclusion in another account shall be recorded in the...

  14. 12 CFR 220.4 - Margin account.

    Code of Federal Regulations, 2014 CFR

    2014-01-01

    ... 12 Banks and Banking 3 2014-01-01 2014-01-01 false Margin account. 220.4 Section 220.4 Banks and...) CREDIT BY BROKERS AND DEALERS (REGULATION T) § 220.4 Margin account. (a) Margin transactions. (1) All transactions not specifically authorized for inclusion in another account shall be recorded in the...

  15. Diminishing Marginal Utility in Economics Textbooks

    ERIC Educational Resources Information Center

    Dittmer, Timothy

    2005-01-01

    Many introductory microeconomics textbook authors derive the law of demand from the assumption of diminishing marginal utility. Authors of intermediate and graduate textbooks derive demand from diminishing marginal rate of substitution and ordinal preferences. These approaches are not interchangeable; diminishing marginal utility for all goods is…

  16. Will Passive Protection Save Congo Forests?

    PubMed

    Galford, Gillian L; Soares-Filho, Britaldo S; Sonter, Laura J; Laporte, Nadine

    2015-01-01

    Central Africa's tropical forests are among the world's largest carbon reserves. Historically, they have experienced low rates of deforestation. Pressures to clear land are increasing due to development of infrastructure and livelihoods, foreign investment in agriculture, and shifting land use management, particularly in the Democratic Republic of Congo (DRC). The DRC contains the greatest area of intact African forests. These store approximately 22 billion tons of carbon in aboveground live biomass, yet only 10% are protected. Can the status quo of passive protection - forest management that is low or nonexistent - ensure the preservation of this forest and its carbon? We have developed the SimCongo model to simulate changes in land cover and land use based on theorized policy scenarios from 2010 to 2050. Three scenarios were examined: the first (Historical Trends) assumes passive forest protection; the next (Conservation) posits active protection of forests and activation of the national REDD+ action plan, and the last (Agricultural Development) assumes increased agricultural activities in forested land with concomitant increased deforestation. SimCongo is a cellular automata model based on Bayesian statistical methods tailored for the DRC, built with the Dinamica-EGO platform. The model is parameterized and validated with deforestation observations from the past and runs the scenarios from 2010 through 2050 with a yearly time step. We estimate the Historical Trends trajectory will result in average emissions of 139 million t CO2 year-1 by the 2040s, a 15% increase over current emissions. The Conservation scenario would result in 58% less clearing than Historical Trends and would conserve carbon-dense forest and woodland savanna areas. The Agricultural Development scenario leads to emissions of 212 million t CO2 year-1 by the 2040s. These scenarios are heuristic examples of policy's influence on forest conservation and carbon storage. Our results suggest that 1

  17. Will Passive Protection Save Congo Forests?

    PubMed

    Galford, Gillian L; Soares-Filho, Britaldo S; Sonter, Laura J; Laporte, Nadine

    2015-01-01

    Central Africa's tropical forests are among the world's largest carbon reserves. Historically, they have experienced low rates of deforestation. Pressures to clear land are increasing due to development of infrastructure and livelihoods, foreign investment in agriculture, and shifting land use management, particularly in the Democratic Republic of Congo (DRC). The DRC contains the greatest area of intact African forests. These store approximately 22 billion tons of carbon in aboveground live biomass, yet only 10% are protected. Can the status quo of passive protection - forest management that is low or nonexistent - ensure the preservation of this forest and its carbon? We have developed the SimCongo model to simulate changes in land cover and land use based on theorized policy scenarios from 2010 to 2050. Three scenarios were examined: the first (Historical Trends) assumes passive forest protection; the next (Conservation) posits active protection of forests and activation of the national REDD+ action plan, and the last (Agricultural Development) assumes increased agricultural activities in forested land with concomitant increased deforestation. SimCongo is a cellular automata model based on Bayesian statistical methods tailored for the DRC, built with the Dinamica-EGO platform. The model is parameterized and validated with deforestation observations from the past and runs the scenarios from 2010 through 2050 with a yearly time step. We estimate the Historical Trends trajectory will result in average emissions of 139 million t CO2 year-1 by the 2040s, a 15% increase over current emissions. The Conservation scenario would result in 58% less clearing than Historical Trends and would conserve carbon-dense forest and woodland savanna areas. The Agricultural Development scenario leads to emissions of 212 million t CO2 year-1 by the 2040s. These scenarios are heuristic examples of policy's influence on forest conservation and carbon storage. Our results suggest that 1

  18. Will Passive Protection Save Congo Forests?

    PubMed Central

    Galford, Gillian L.; Soares-Filho, Britaldo S.; Sonter, Laura J.; Laporte, Nadine

    2015-01-01

    Central Africa’s tropical forests are among the world’s largest carbon reserves. Historically, they have experienced low rates of deforestation. Pressures to clear land are increasing due to development of infrastructure and livelihoods, foreign investment in agriculture, and shifting land use management, particularly in the Democratic Republic of Congo (DRC). The DRC contains the greatest area of intact African forests. These store approximately 22 billion tons of carbon in aboveground live biomass, yet only 10% are protected. Can the status quo of passive protection — forest management that is low or nonexistent — ensure the preservation of this forest and its carbon? We have developed the SimCongo model to simulate changes in land cover and land use based on theorized policy scenarios from 2010 to 2050. Three scenarios were examined: the first (Historical Trends) assumes passive forest protection; the next (Conservation) posits active protection of forests and activation of the national REDD+ action plan, and the last (Agricultural Development) assumes increased agricultural activities in forested land with concomitant increased deforestation. SimCongo is a cellular automata model based on Bayesian statistical methods tailored for the DRC, built with the Dinamica-EGO platform. The model is parameterized and validated with deforestation observations from the past and runs the scenarios from 2010 through 2050 with a yearly time step. We estimate the Historical Trends trajectory will result in average emissions of 139 million t CO2 year-1 by the 2040s, a 15% increase over current emissions. The Conservation scenario would result in 58% less clearing than Historical Trends and would conserve carbon-dense forest and woodland savanna areas. The Agricultural Development scenario leads to emissions of 212 million t CO2 year-1 by the 2040s. These scenarios are heuristic examples of policy’s influence on forest conservation and carbon storage. Our results

  19. Determination of pyrotechnic functional margin

    NASA Technical Reports Server (NTRS)

    Bement, Laurence J.; Schimmel, Morry L.

    1993-01-01

    Following the failure of a previously qualified pyrotechnically actuated pin puller design, an investigation led to a redesign and requalification. The emphasis of the second qualification was placed on determining the functional margin of the pin puller by comparing the energy deliverable by the pyrotechnic cartridge to the energy required to accomplish the function. Also determined were the effects of functional variables. This paper describes the failure investigation, the test methods employed and the results of the evaluation, and provides a recommended approach to assure the successful functioning of pyrotechnic devices.

  20. Three-dimensional marginal separation

    NASA Technical Reports Server (NTRS)

    Duck, Peter W.

    1988-01-01

    The three dimensional marginal separation of a boundary layer along a line of symmetry is considered. The key equation governing the displacement function is derived, and found to be a nonlinear integral equation in two space variables. This is solved iteratively using a pseudo-spectral approach, based partly in double Fourier space, and partly in physical space. Qualitatively, the results are similar to previously reported two dimensional results (which are also computed to test the accuracy of the numerical scheme); however quantitatively the three dimensional results are much different.

  1. Continental margin tectonics - Forearc processes

    SciTech Connect

    Lundberg, N.; Reed, D.L. )

    1991-01-01

    Recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of U.S. research from the period 1987-1990. Topics addressed include the geometry of accretionary prisms (Coulomb wedge taper and vertical motion in response to tectonic processes), offscraping vs underplating or subduction, the response to oblique convergence, fluids in forearc settings, the thermal framework and the effects of fluid advection, and serpentinite seamounts. Also included is a comprehensive bibliography for the period.

  2. Managing margins through physician engagement.

    PubMed

    Sears, Nicholas J

    2012-07-01

    Hospitals should take the following steps as they seek to engage physicians in an enterprisewide effort to effectively manage margins: Consider physicians' daily professional practice requirements and demands for time in balancing patient care and administrative duties. Share detailed transactional supply data with physicians to give them a behind-the-scenes look at the cost of products used for procedures. Institute physician-led management and monitoring of protocol compliance and shifts in utilization to promote clinical support for change. Select a physician champion to provide the framework for managing initiatives with targeted, efficient communication. PMID:22788036

  3. 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

  4. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

    Levander, Alan; Bezada, Maximiliano; Niu, Fenglin; Palomeras, Imma; Humphreys, Eugene; Carbonell, Ramon; Gallart, Josep; Schmitz, Michael; Miller, Meghan

    2016-04-01

    Subduction recycling of oceanic lithosphere, a central theme of plate tectonics, is relatively well understood. Recycling continental lithosphere is more difficult to recognize, can take a number of different forms, and appears to require an external trigger for initiation. Delamination and localized convective downwelling are two processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. We describe a related process that can lead to the loss of continental lithosphere adjacent to a subduction zone: Subducting oceanic plates can entrain and recycle lithospheric mantle from an adjacent continent and disrupt the continental lithosphere far inland from the subduction zone. Body wave tomograms from dense broadband seismograph arrays in northeastern South America (SA) and the western Mediterranean show larger than expected volumes of positive velocity anomalies which we identify as the subducted Atlantic slab under northeastern SA, and the Alboran slab beneath the Gibraltar arc (GA). The positive anomalies lie under and are aligned with the continental margins at sublithospheric depths. The continental margins along which the subduction zones have traversed, i.e. the northeastern SA plate boundary and east of GA, have significantly thinner lithosphere than expected. The thinner than expected lithosphere extends inland as far as the edges of nearby cratons as determined from receiver function images and surface wave tomography. These observations suggest that subducting oceanic plates viscously entrain and remove continental mantle lithosphere from beneath adjacent continental margins, modulating the surface tectonics and pre-conditioning the margins for further deformation. The latter can include delamination of the entire lithospheric mantle and include the lower crust, as around GA, inferred by results from active and passive seismic experiments. Viscous removal of continental margin lithosphere creates LAB topography leading

  5. The Northwestern Atlantic Moroccan Margin From Deep Multichannel Seismic Reflection

    NASA Astrophysics Data System (ADS)

    Malod, J. A.; Réhault, J. P.; Sahabi, M.; Géli, L.; Matias, L.; Zitellini, N.; Sismar Group

    The NW Atlantic Moroccan margin, a conjugate of the Nova Scotia margin, is one of the oldest passive margins of the world. Continental break up occurred in the early Jurassic and the deep margin is characterized by a large salt basin. The SISMAR cruise (9 April to 4 May 2001) acquired 3667 km of 360 channel seismic reflection profiles. In addition, refraction data were recorded by means of 48 OBH/OBS deployments. Simultaneously, some of the marine profiles were extended onshore with 16 portable seismic land stations. WNW-ESE profiles 4 and 5 off El Jadida show a good section of the margin. The crustal thinning in this region is fairly abrupt. These profiles image the crust above a strong seismic reflector at about 12 s.twt., interpreted as the Moho. The crust exhibits several different characteristics from the continent towards the ocean.: - highly diffractive with a thickness larger than 25 km beneath the shelf. - stratified at a deep level and topped by few "tilted blocks" with a diffractive acoustic facies and for which 2 hypotheses are proposed: either continental crust tilted during the rifting or large landslides of crustal and sedimentary material slid down later. Liassic evapor- ites are present but seem less thick than to the south. - layered with seaward dipping reflectors: this type of crust correlates with the magnetic anomaly S1 and corresponds to the continent-ocean transition. - diffractive with an oceanic character. Oceanwards, the crust becomes more typically oceanic, but shows internal reflectors that may be re- lated to compressional reactivation during the Tertiary attested by large scale inverted basins. Our results allow us to discuss the nature and location of the continent-ocean transition at a regional scale and the rifting to spreading evolution of the very ma- ture continental margin off El Jadida. This provide us with some constraints for the initial reconstruction between Africa, North America and Iberia. Moreover, these re- sults help

  6. Orion Passive Thermal: Control Overview

    NASA Technical Reports Server (NTRS)

    Alvarez-Hermandez, Angel; Miller, Stephen W.

    2009-01-01

    A general overview of the NASA Orion Passive Thermal Control System (PTCS) is presented. The topics include: 1) Orion in CxP Hierarchy; 2) General Orion Description/Orientation; and 3) Orion PTCS Overview.

  7. [Passive smoking. Effects on health].

    PubMed

    Trédaniel, J; Zalcman, G; Boffetta, P; Hirsch, A

    1993-05-15

    Passive smoking--also called involuntary or environmental smoking--is the exposure of non-smokers to the tobacco smoke released by smokers. The physico-chemical composition of tobacco smoke, and notably its contents in toxic and carcinogenic substances, is the same in the secondary stream between puffs as in the primary stream released by the smoker. The pathogenic effects of passive smoking are increasingly well known and accepted. A high incidence of respiratory tract infections and of chronic respiratory and asthmatic symptoms is observed in children. In adults, passive smoking seems to be one of the main risk factors for cardiovascular diseases. Its repercussions on the respiratory tracts is difficult to evaluate, but there are marked by an increase of respiratory symptoms and perhaps of chronic obstructive lung diseases. Finally, it is now recognized that passive smoking is a major risk factor for primary lung cancer in non-smokers exposed to tobacco smoke. PMID:8235360

  8. Passivating metals on cracking catalysts

    SciTech Connect

    Mckay, D.L.

    1980-01-15

    Metals such as nickel, vanadium and iron contaminating a cracking catalyst are passivated by contacting the cracking catalyst under elevated temperature conditions with antimony selenide, antimony sulfide, antimony sulfate, bismuth selenide, bismuth sulfide, or bismuth phosphate.

  9. Microwave and physical properties of sea ice in the winter marginal ice zone

    NASA Technical Reports Server (NTRS)

    Tucker, W. B., III; Perovich, D. K.; Gow, A. J.; Grenfell, T. C.; Onstott, R. G.

    1991-01-01

    Surface-based active and passive microwave measurements were made in conjunction with ice property measurements for several distinct ice types in the Fram Strait during March and April 1987. Synthesis aperture radar imagery downlinked from an aircraft was used to select study sites. The surface-based radar scattering cross section and emissivity spectra generally support previously inferred qualitative relationships between ice types, exhibiting expected separation between young, first-year and multiyear ice. Gradient ratios, calculated for both active and passive data, appear to allow clear separation of ice types when used jointly. Surface flooding of multiyear floes, resulting from excessive loading and perhaps wave action, causes both active and passive signatures to resemble those of first-year ice. This effect could possibly cause estimates of ice type percentages in the marginal ice zone to be in error when derived from aircraft- or satellite-born sensors.

  10. Dynamics of the continental margins

    SciTech Connect

    Not Available

    1990-11-01

    On 18--20 June 1990, over 70 oceanographers conducting research in the ocean margins of North America attended a workshop in Virginia Beach, Virginia. The purpose of the workshop was to provide the Department of Energy with recommendations for future research on the exchange of energy-related materials between the coastal and interior ocean and the relationship between the ocean margins and global change. The workshop was designed to optimize the interaction of scientists from specific research disciplines (biology, chemistry, physics and geology) as they developed hypotheses, research questions and topics and implementation plans. The participants were given few restraints on the research they proposed other than realistic time and monetary limits. The interdisciplinary structure of the meeting promoted lively discussion and creative research plans. The meeting was divided into four working groups based on lateral, vertical, air/sea and sediment/water processes. Working papers were prepared and distributed before the meeting. During the meeting the groups revised the papers and added recommendations that appear in this report, which was reviewed by an Executive Committee.

  11. Westward intensification in marginal seas

    NASA Astrophysics Data System (ADS)

    Chen, Gengxin; Xue, Huijie

    2014-03-01

    An idealized model was used to examine why the strong western boundary current (WBC) is observed in the South China Sea (SCS) but not in the Gulf of Mexico (GOM) and Japan/East Sea (JES). Results suggested that the stronger WBC in the SCS is mainly attributed to the direct contribution of the inflow and the strong monsoon. Although the Gulf Stream transports a large amount of water into the GOM, the passage in the southeast corner guides the inflow out of the gulf and inhibits the inflow from intensifying the WBC. Meanwhile, the wind stress in the GOM is weakest among the three marginal seas. The meridional ocean ridge and the particular layout of the continental slope of JES prevent the whole basin from participating in the westward intensification. Besides, the throughflow has adverse effects on the formulation of WBC in JES. The variation of Coriolis parameter with latitude leads to the westward intensification in marginal seas. However, a strong WBC cannot be observed in the absence of reasonable collocation of wind, inflow, and topography.

  12. 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 ...

  13. Observational constraints and models for conjugate North Atlantic volcanic rifted margins formation

    NASA Astrophysics Data System (ADS)

    Huismans, R. S.; Faleide, J. I.; Planke, S.

    2009-12-01

    The amount of breakup-related magmatism in the northeast Atlantic cannot be explained by passive decompression melting of sub-lithospheric mantle with a normal potential temperature. Three competing end-member hypotheses are proposed for the formation of this excess magmatic activity: 1) excess magmatism results from elevated mantle potential temperatures associated with mantle plume processes, 2) rifting induced small scale convection at the base of the lithosphere enhances the flux of material through the melt window during rifting and mid-oceanic ridge spreading, and 3) mantle heterogeneities with lower melt temperatures of the melt source may produce larger than expected magmatic productivity. We review observations and present forward numerical models of passive margin formation including melt production. Key characteristics that require explanation include: 1) igneous crustal thickness versus mean igneous seismic velocity relationships for magmatic underplated bodies on the Norwegian-Greenland conjugate margins that indicate a cooling trend and seismic velocities that point to a moderate or non-existent mantle thermal anomaly during and after breakup; 2) rapid along margin variations in the observed igneous crustal thickness that suggest a local lithospheric control on melt productivity; 3) significant asymmetries in melt production existing between the Norwegian-Greenland conjugate margins; 4) post rift magmatic activity 30 my after breakup with igneous seismic velocity indicating normal mantle temperature; 5) anomalous low magmatic productivity in the oceanic Norway basin following moderate excess productivity that suggests anomalous low mantle temperatures following breakup, 6) anomalous mantle lithosphere thinning in the distal margin in the late syn-rift, 7) uplift and erosion of highly thinned crustal blocks in the distal margin. Forward numerical models of passive margin formation and melt productivity suggest the following key controls: 1) rifting

  14. Postsecondary Educators' Cultural and Institutional Awareness of Issues Faced by African American Students with Disabilities

    ERIC Educational Resources Information Center

    Becton, Alicia B.; Foster, Amanda L.; Chen, Roy K.

    2016-01-01

    Being a part of an ethnic minority group and a student with a disability (SWD) often presents as a barrier to college retention and graduation rates among members of this marginalized group. Purpose: To examine educators' awareness of racial and institutional influences that impact African American SWD. Method: Data for this study were gathered…

  15. JBHE Readers Select the Most Important African Americans of the Twentieth Century.

    ERIC Educational Resources Information Center

    Journal of Blacks in Higher Education, 1999

    1999-01-01

    Presents the results of a survey of readers' opinions about African Americans who made the greatest contributions to American society during the 20th century. Martin Luther King, Jr., received the most votes by a large margin, followed by Thurgood Marshall, W.E.B. Du Bois, and Malcolm X. Discusses survey results by various categories. (SM)

  16. Gender in the Early Years: Boys and Girls in an African Working Class Primary School

    ERIC Educational Resources Information Center

    Bhana, Deevia; Nzimakwe, Thokozani; Nzimakwe, Phumzile

    2011-01-01

    Understanding the ways in which young boys and girls give meaning to gender and sexuality is vital, and is especially significant in the light of South Africa's commitment to gender equality. Yet the, gendered cultures of young children in the early years of South African primary schools remains a, marginal concern in debate, research and…

  17. Passive Wake Vortex Control

    SciTech Connect

    Ortega, J M

    2001-10-18

    works by placing shape memory alloy (SMA) control surfaces on the submarine's diving planes and periodically oscillating them. The modulated control vortices generated by these surfaces interact with the tip vortices on the diving planes, causing an instability to rapidly occur. Though several numerical simulations have been presented, experimental verification does not appear to be available in the open literature. The authors address this problem through a concept called passive wake vortex control (PWVC), which has been demonstrated to rapidly break apart a trailing vortex wake and render it incoherent. PWVC functions by introducing unequal strength, counter-rotating control vortices next to the tip vortices. The presence of these control vortices destabilizes the vortex wake and produces a rapidly growing wake instability.

  18. 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)

  19. 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…

  20. Tracking small mountainous river derived terrestrial organic carbon across the active margin marine environment

    NASA Astrophysics Data System (ADS)

    Childress, L. B.; Blair, N. E.; Orpin, A. R.

    2015-12-01

    Active margins are particularly efficient in the burial of organic carbon due to the close proximity of highland sources to marine sediment sinks and high sediment transport rates. Compared with passive margins, active margins are dominated by small mountainous river systems, and play a unique role in marine and global carbon cycles. Small mountainous rivers drain only approximately 20% of land, but deliver approximately 40% of the fluvial sediment to the global ocean. Unlike large passive margin systems where riverine organic carbon is efficiently incinerated on continental shelves, small mountainous river dominated systems are highly effective in the burial and preservation of organic carbon due to the rapid and episodic delivery of organic carbon sourced from vegetation, soil, and rock. To investigate the erosion, transport, and burial of organic carbon in active margin small mountainous river systems we use the Waipaoa River, New Zealand. The Waipaoa River, and adjacent marine depositional environment, is a system of interest due to a large sediment yield (6800 tons km-2 yr-1) and extensive characterization. Previous studies have considered the biogeochemistry of the watershed and tracked the transport of terrestrially derived sediment and organics to the continental shelf and slope by biogeochemical proxies including stable carbon isotopes, lignin phenols, n-alkanes, and n-fatty acids. In this work we expand the spatial extent of investigation to include deep sea sediments of the Hikurangi Trough. Located in approximately 3000 m water depth 120 km from the mouth of the Waipaoa River, the Hikurangi Trough is the southern extension of the Tonga-Kermadec-Hikurangi subduction system. Piston core sediments collected by the National Institute of Water and Atmospheric Research (NIWA, NZ) in the Hikurangi Trough indicate the presence of terrestrially derived material (lignin phenols), and suggest a continuum of deposition, resuspension, and transport across the margin

  1. Joint geophysical and petrological models for the lithosphere structure of the Antarctic Peninsula continental margin

    NASA Astrophysics Data System (ADS)

    Yegorova, Tamara; Bakhmutov, Vladimir; Janik, Tomasz; Grad, Marek

    2011-01-01

    The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.

  2. Deep seismic reflection study of a passive margin, southeatern Gulf of Guinea

    SciTech Connect

    Rosendahl, B.R.; Groschel-Becker, H.; Meyers, J.; Kaczmarick, K. )

    1991-04-01

    A large grid of deep-imaging, marine seismic reflection data has been acquired in the Gulf of Guinea. The data show that the architecture of old Atlantic igneous crust and upper mantle is highly variable, particularly if reflection Moho is taken to be the base of the crust. Most abrupt changes in oceanic basement thickness and depth to Moho can be correlated with fracture-zone crossings, but significant variations can occur between fracture zones and along flow lines, especially near the ocean-continent transition. Reflection Moho is usually continuous from ocean to continent and does not display any systematic changes in character, continuity, or reflection time even beneath the innermost shelf areas. There are several varieties of intracrustal reflectors, including those that mark different levels within the oceanic gabbroic complex and events that diagonally link the top of oceanic seismic layer 2 and Moho. Different types of sub-Moho dipping reflections also are observed. Some are associated with fracture zones, some originate within continental crust and dip toward the ocean, dissecting Moho without offsetting it, and still others originate at the oceanic Moho and dip toward the continent. The transition from oceanic to continental crust is generally quite sharp north of lat 1{degree}S, but the exact nature of the transition ranges from rift-block geology to abrupt juxtapositions of oceanic and continental crustal rocks. South of about lat 1{degree}S, the transition to continental crust is more gradual, involving a progressive thickening of oceanic crust toward land. This difference may relate to the occurrence of much more oblique initial rifting north of 1{degree}S.

  3. Passive margins: U.S. Geological Survey Line 19 across the Georges Bank basin

    USGS Publications Warehouse

    Klitgord, Kim D.; Schlee, John S.; Grow, John A.

    1987-01-01

    Georges Bank is a shallow part of the Atlantic continental shelf southeast of New England (Emery and Uchupi, 1972, 1984). This bank, however, is merely the upper surface of several sedimentary basins overlying a block-faulted basement of igneous and metamorphic crystalline rock. Sedimentary rock forms a seaward-thickening cover that has accumulated in one main depocenter and several ancillary depressions, adjacent to shallow basement platforms of paleozoic and older crystalline rock. Georges Bank basin contains a thickness of sedimentary rock greater than 10 km, whereas the basement platforms that flank the basin are areas of thin sediment accumulation (less than 5 km).

  4. The Nature of the Passive Margin in the Area of the Canary Islands, Central Atlantic Ocean

    NASA Astrophysics Data System (ADS)

    Neumann, E.; Vannucci, R.; Tiepolo, M.; Griffin, W. L.; Pearson, N. J.

    2004-05-01

    The Canary Islands (CI) form a roughly E-W trending island chain normal to the coast of Africa. The base of the continental slope lies only 30-40 km east of the easternmost island, Fuerteventura. The oceanic lithosphere beneath CI formed about 180-150 Ma ago, during the earliest stages of opening of the central Atlantic Ocean (e.g. Roest et al., 1992). The CI hotspot has been active during the last 60-70 million years (Geldmacher et al., 2001). Detailed studies of mantle and crustal xenoliths from different islands, including point analyses of trace elements and 87Sr/86Sr isotopic ratios in minerals, show that the lithosphere beneath CI consists of highly depleted oceanic crust and upper mantle that have been metasomatized to different degrees during the CI intraplate event. The original composition of the upper mantle is best preserved in the REE concentrations in ol and opx porphyroclasts and in cpx neoblasts in the most refractory spinel harzburgite xenoliths. These are strongly depleted in MREE relative to HREE. LAM-MC-ICPMS Sr isotope analyses of cpx in these xenoliths give 87Sr/86Sr ratios of 0.7027-0.7028. This is within the range of N-MORB and significantly below the range of CI basaltic rocks (=0.7030). Modeling based on major and trace elements in the most refractory sp harzburgites suggests that the lithospheric mantle beneath CI represents the residue after about 25 percent depletion relative to the Primordial Mantle. Such a high degree of partial melting results in complete exhaustion of cpx. In these rocks cpx appears mainly as small neoblasts along the boundaries of opx porphyroclasts, and is believed to be the results of exsolution of opx, followed by recrystallization. Cpx in more highly metasomatized peridotites give 87Sr/86Sr ratios in the range 0.7029-0.7033, and have flat to LREE-enriched REE patterns. Two types of gabbroic xenoliths have been retrieved. One type, dominated by augite+plag+hornblende+mt+-ol, is interpreted as cumulates formed from alkali basaltic Canarian magmas. The other type (opx-gabbros) is mildly deformed and consists of ol+cpx+plag+-opx. Some of these show evidence of reactions with enriched magmas. Cpx and opx in the most pristine opx-gabbros have strongly depleted REE patterns. Estimates indicate formation of the opx-gabbros from N-MORB parent melts with (La/Sm)N=0.16 and (Sm/Yb)N=0.52; that is among the lowest ones recorded for MORB magmas. We conclude that in the area of CI, the oldest oceanic lithosphere was highly depleted. There is no evidence that transitional melts were involved in the formation of the oldest oceanic crust in this part of the Atlantic Ocean. Furthermore, there is no evidence that continental lithosphere is present anywhere beneath the Canary Islands. The continent-ocean transition in the area of the Canary Islands thus appears to be quite sharp, located just east of the easternmost CI, and to be quite different from the 80-130 km wide continent-ocean transition zone found further north in the Iberia Abyssal Plain (e.g. Whitmarsh and Sawyer, 1996). Geldmacher et al., 2001. J. Volc. Geotherm. Res. 111, 55-87 Roest et al., 1992. Marine Geophys. Res. 14, 1-24 Whitmarsh and Sawyer, 1996. Proc. ODP Scientific Res. 149, 713-733

  5. Mesozoic carbonate-siliciclastic platform to basin systems of a South Tethyan margin (Egypt, East Mediterranean)

    NASA Astrophysics Data System (ADS)

    Tassy, Aurélie; Crouzy, Emmanuel; Gorini, Christian; Rubino, Jean-Loup

    2015-04-01

    The Mesozoïc Egyptian margin is the south margin of a remnant of the Neo-Tethys Ocean, at the African northern plate boundary. East Mediterranean basin developed during the late Triassic-Early Jurassic rifting with a NW-SE opening direction (Frizon de Lamotte et al., 2011). During Mesozoïc, Egypt margin was a transform margin with a NW-SE orientation of transform faults. In the Eastern Mediterranean basin, Mesozoïc margins are characterized by mixed carbonate-siliciclastics platforms where subsidence and eustacy are the main parameters controlling the facies distribution and geometries of the platform-to-basin transition. Geometries and facies on the platform-slope-basin system, today well constrained on the Levant area, where still poorly known on the Egyptian margin. Geometries and stratigraphic architecture of the Egyptian margin are revealed, thanks to a regional seismic and well data-base provided by an industrial-academic group (GRI, Total). The objective is to understand the sismostratigraphic architecture of the platform-slope-basin system in a key area from Western Desert to Nile delta and Levant margin. Mapping of the top Jurassic and top Cretaceous show seismic geomorphology of the margin, with the cartography of the hinge line from Western Desert to Sinaï. During the Jurassic, carbonate platform show a prograding profile and a distally thickening of the external platform, non-abrupt slope profiles, and palaeovalleys incisions. Since the Cretaceous, the aggrading and retrograding mixed carbonate-siliciclastic platform show an alternation of steep NW-SE oblique segments and distally steepened segments. These structures of the platform edge are strongly controlled by the inherited tethyan transform directions. Along the hinge line, embayments are interpreted as megaslides. The basin infilling is characterised by an alternation of chaotic seismic facies and high amplitude reflectors onlaping the paleoslopes. MTC deposits can mobilize thick sedimentary

  6. Great earthquakes along the Western United States continental margin: implications for hazards, stratigraphy and turbidite lithology

    NASA Astrophysics Data System (ADS)

    Nelson, C. H.; Gutiérrez Pastor, J.; Goldfinger, C.; Escutia, C.

    2012-11-01

    We summarize the importance of great earthquakes (Mw ≳ 8) for hazards, stratigraphy of basin floors, and turbidite lithology along the active tectonic continental margins of the Cascadia subduction zone and the northern San Andreas Transform Fault by utilizing studies of swath bathymetry visual core descriptions, grain size analysis, X-ray radiographs and physical properties. Recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California margins are analyzed using two methods: (1) radiometric dating (14C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The H method provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that, on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great earthquakes are approximately 300 yr for the Cascadia subduction zone and 130 yr for the northern San Andreas Fault, which indicates both fault systems are in (Cascadia) or very close (San Andreas) to the early window for another great earthquake. On active tectonic margins with great earthquakes, the volumes of mass transport deposits (MTDs) are limited on basin floors along the margins. The maximum run-out distances of MTD sheets across abyssal-basin floors along active margins are an order of magnitude less (~100 km) than on passive margins (~1000 km). The great earthquakes along the Cascadia and northern California margins cause seismic strengthening of the sediment, which

  7. 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

  8. Post-breakup Basin Evolution along the South-Atlantic Margins in Brazil and Angola/Namibia

    NASA Astrophysics Data System (ADS)

    Kukla, P. A.; Strozyk, F.; Back, S.

    2013-12-01

    The post-breakup tectono-stratigraphic evolution of large offshore basins along the South American and African continental margins record strongly varying post-rift sedimentary successions. The northernmost segment of the South Atlantic rift and salt basins is characterized by a pronounced asymmetry, with the Brazilian margin comprising narrower and deeper rift basins with less salt in comparison to the Congo-Gabon conjugate margin. Another important observation is that multiple phases of uplift and subsidence are recorded after the break-up of the southern South Atlantic on both sides of the Florianopolis-Walvis Ridge volcanic complex, features that are regarded as atypical when compared to published examples of other post-breakup margin successions. A regional comparison based on tectonic-stratigraphic analysis of selected seismic transects between the large basins offshore southern Brazil (Espirito Santo Basin, Campos Basin, Santos Basin, Pelotas Basin) and southwest Africa (Lower Congo Basin, Kwanza Basin, Namibe Basin, Walvis Basin) provides a comprehensive basin-to-basin documentation of the key geological parameters controlling ocean and continental margin development. This comparison includes the margin configuration, subsidence development through time, sediment influx and storage patterns, type of basin fill (e.g. salt vs. non-salt systems; carbonate-rich vs. clastics-dominated systems) and finally major tectonic and magmatic events. Data from the salt basins indicate that salt-related tectonic deformation is amongst the prime controls for the non-uniform post-rift margin development. The diversity in the stratigraphic architecture of the conjugate margins offshore southern Brazil, Namibia and Angola reflects variations in the interplay of a number of controlling factors, of which the most important are (a) the structural configuration of each margin segment at the time of break-up, (b) the post break-up subsidence history of the respective margin segment

  9. Margins for cervical and vulvar cancer.

    PubMed

    Khanna, Namita; Rauh, Lisa A; Lachiewicz, Mark P; Horowitz, Ira R

    2016-03-01

    Surgery is the primary treatment for vulvar cancer as well as early-stage carcinoma of the cervix. This article reviews the significance of margin status after surgery on overall survival, need for further surgical intervention, and role for possible adjuvant therapy. It summarizes the abundant literature on margin status in vulvar cancer and highlights the need for further investigation on the prognostic significance of margins in cervical cancer. In addition, it reviews other important operative considerations.

  10. Formation of the volcanic rifted margin off Argentina/Uruguay, South Atlantic

    NASA Astrophysics Data System (ADS)

    Franke, D.; Reichert, C.; Ladage, S.; Schnabel, M.; Schreckenberger, B.; Neben, S.; Hinz, K.

    2009-04-01

    The Federal Institute for Geosciences and Natural Resources (BGR), Germany has investigated the passive continental margins offshore Argentina and Uruguay since the early 90ies. Numerous marine geophysical surveys have meanwhile established a databasis of more than 25.000 km of regional multi-channel reflection seismic lines, accompanied with magnetic and gravity profiles. These data document that the Early Cretaceous South Atlantic continental break-up and initial sea-floor spreading were accompanied by large-scale, transient volcanism emplacing voluminous extrusives, manifested in the seismic data by huge wedges of seaward dipping reflectors (SDRs). These deeply buried and 60-120 km wide SDRs were emplaced episodically as suggested by at least three superimposed SDRS units. Distinct along-margin variations in the architecture, volume, and width of the SDRs wedges correlate with large scale margin segmentation. We identify at least four domains bounded by the Falkland Fracture Zone/Falkland Transfer, the Colorado Transfer, the Ventana Transfer and the Salado Transfer. The individual transfer zones may have acted as barriers for propagating rifts during the SDR emplacement phase, selectively directing rift segments in left stepping patterns along the western South Atlantic margin. The rift segments are offset systematically in a left stepping pattern along the western South Atlantic margin. Albeit we found extensive variations in the architecture, style and extent of the seaward dipping reflector sequences a general trend is that the largest volumes are emplaced close to the proposed transfer zones and the width of the SDRs wedges decreases northward within the individual margin segments. The different volcano-tectonic architectures of the margin segments and the distribution of the extruded magmas indicates that the emplacement of the volcanic material was controlled by the tectonic setting and the pre-rift lithosphere configuration within individual margin

  11. Some thoughts on future directions in margin research

    SciTech Connect

    Mutter, J.C. )

    1990-05-01

    Looking through the abstracts of presentations for this symposium, the authors see a science in transition. The quarter century since plate tectonic theory provided geoscience with a guiding framework in which to carry forward research into global tectonic phenomenon has witnessed a flourishing of research into the structure and evolution of continental margins. Margin evolution can now be placed firmly within this theoretical construct, thereby providing one of the key elaborations of the original theory that has given it its strength and value. What, then, is the next stage Are they in a period of normal science characterized by the detailed verification of an established paradigm, or can they hope for a really new direction Two approaches seem to present themselves. One is to focus attention on anomalies-things that don't quite fit or are not very easily covered by the present paradigm. Volcanic passive margins don't fit very well within the simple kinematic statement of plate tectonics, nor do eustasy and its effect on sedimentary sequences. While study of these square pegs in the round hole of plate theory may not bring them to a new paradigm, they might well provide significant modifications to the current wisdom. Perhaps a more fruitful approach is to propose that they are at a point in the development of other science at which the era of verification of the plate tectonic paradigm is sufficiently mature (perhaps even at its end) that the proper direction now is to establish a rigorous physical basis for the phenomena that are kinematically described by plate theory. The objective should be to created a higher order theory and thereby illuminate these known phenomena. Numerical and analytical modeling of deformational processes, magma generation, and migration are making leading contributions to this endeavor.

  12. Offshore investigations on Wilkes land-Victoria land margin, Antarctica

    SciTech Connect

    Eittreim, S.L.

    1984-04-01

    In January 1984, the US Geological Survey research vessel S. P. Lee carried out investigations of the Antarctic continental margin in the Wilkes Land Victoria Land areas, using 24-channel and high-resolution seismic, sonobuoy refraction, gravity, magnetic, and bottom-sampling methods. This investigation augmented previous surveys of the Dumont d'Urville area by the French Petroleum Institute and explored new areas west and east to the boundary between the onshore Wilkes basin and the Victoria Land highlands. These surveys defined sediment thickness distribution and seismic stratigraphy in this frontier area. The tectonic style of the boundary between the East Antarctic craton and the younger crust of West Antarctica in the Ross Sea is revealed by one multichannel seismic line across this important boundary. The initial breakup of Antarctical from Australia occurred as a slowly spreading phase during the middle Cretaceous. According to Deep Sea Drilling Project results on the Tasman Rise, conditions of restricted circulation existed in the growing basin between the continents before the late Eocene. After the late Eocene, the major oceanic circulation pattern was established. Before that time, conditions were favorable for preservation of organic-carbon deposits on the sea floor. Among the questions to be addressed with this data are the following. How do apparent subsidence rates of this passive margin compare with others around the world. Does the onshore subglacial Wilkes basins to the Otway and Ceduna basins of Australia exists. What is the effect of the ice cap on the stratigraphy of this margin. Do the two major Tertiary ice advances have conspicuous seismic-stratigraphic signatures.

  13. On the Marginal Stability of Glassy Systems

    NASA Astrophysics Data System (ADS)

    Yan, Le; Baity-Jesi, Marco; Müller, Markus; Wyart, Matthieu

    2015-03-01

    In various glassy systems that are out of equilibrium, like spin glasses and granular packings, the dynamics appears to be critical: avalanches involving almost the whole system could happen. A recent conceptual breakthrough argues that such glassy systems sample the ensemble of marginal stable states, which inevitably results into critical dynamics. However, it is unclear how the marginal stability is dynamically guaranteed. We investigate this marginal stability assumption by studying specifically the critical athermal dynamics of the Sherrington-Kirkpatrick model. We discuss how a pseudo-gap in the density distribution of local fields characterizing the marginal stability arises dynamically.

  14. [Resection margins in conservative breast cancer surgery].

    PubMed

    Medina Fernández, Francisco Javier; Ayllón Terán, María Dolores; Lombardo Galera, María Sagrario; Rioja Torres, Pilar; Bascuñana Estudillo, Guillermo; Rufián Peña, Sebastián

    2013-01-01

    Conservative breast cancer surgery is facing a new problem: the potential tumour involvement of resection margins. This eventuality has been closely and negatively associated with disease-free survival. Various factors may influence the likelihood of margins being affected, mostly related to the characteristics of the tumour, patient or surgical technique. In the last decade, many studies have attempted to find predictive factors for margin involvement. However, it is currently the new techniques used in the study of margins and tumour localisation that are significantly reducing reoperations in conservative breast cancer surgery.

  15. Assessment of seismic margin calculation methods

    SciTech Connect

    Kennedy, R.P.; Murray, R.C.; Ravindra, M.K.; Reed, J.W.; Stevenson, J.D.

    1989-03-01

    Seismic margin review of nuclear power plants requires that the High Confidence of Low Probability of Failure (HCLPF) capacity be calculated for certain components. The candidate methods for calculating the HCLPF capacity as recommended by the Expert Panel on Quantification of Seismic Margins are the Conservative Deterministic Failure Margin (CDFM) method and the Fragility Analysis (FA) method. The present study evaluated these two methods using some representative components in order to provide further guidance in conducting seismic margin reviews. It is concluded that either of the two methods could be used for calculating HCLPF capacities. 21 refs., 9 figs., 6 tabs.

  16. 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…

  17. 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)

  18. 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…

  19. Passivated ambipolar black phosphorus transistors.

    PubMed

    Yue, Dewu; Lee, Daeyeong; Jang, Young Dae; Choi, Min Sup; Nam, Hye Jin; Jung, Duk-Young; Yoo, Won Jong

    2016-07-01

    We report the first air-passivated ambipolar BP transistor formed by applying benzyl viologen, which serves as a surface charge transfer donor for BP flakes. The passivated BP devices exhibit excellent stability under both an ambient atmosphere and vacuum; their transistor performance is maintained semi-permanently. Unlike their intrinsic p-type properties, passivated BP devices present advantageous ambipolar properties with much higher electron mobility up to ∼83 cm(2) V(-1) s(-1) from 2-terminal measurement at 300 K, compared to other reported studies on n-type BP transistors. On the basis of the n-type doping effect that originated from benzyl viologen, we also systematically investigated the BP thickness dependence of our devices on electrical properties, in which we found the best electron transport performance to be attained when an ∼10 nm thick BP flake was used. PMID:27283027

  20. Pulmonary effects of passive smoking: the Indian experience

    PubMed Central

    Gupta, D; Aggarwal, AN; Jindal, SK

    2003-01-01

    There are only a few studies done on pulmonary effects of passive smoking from India, which are summarized in this paper. Several vernacular tobacco products are used in India, bidis (beedis) being the commonest form of these. Bidis contain a higher concentration of nicotine and other tobacco alkaloids compared to the standard cigarettes (e.g., the sum of total nicotine and minor tobacco alkaloids was 37.5 mg in bidi compared to 14–16 mg in Indian or American cigarettes in one study). A large study performed on 9090 adolescent school children demonstrated environmental tobacco smoke (ETS) exposure to be associated with an increased risk of asthma. The odds ratio for being asthmatic in ETS-exposed as compared to ETS-unexposed children was 1.78 (95% CI: 1.33–2.31). Nearly one third of the children in this study reported non-specific respiratory symptoms and the ETS exposure was found to be positively associated with the prevalence of each symptom. Passive smoking was also shown to increase morbidity and to worsen the control of asthma among adults. Another study demonstrated exposure to ETS was a significant trigger for acute exacerbation of asthma. Increased bronchial hyper-responsiveness was also demonstrated among the healthy nonsmoking adult women exposed to ETS. Passive smoking leads to subtle changes in airflow mechanics. In a study among 50 healthy nonsmoking women passively exposed to tobacco smoke and matched for age with 50 unexposed women, forced expiratory volume in first second (FEV1) and peak expiratory flow (PEF) were marginally lower among the passive smokers (mean difference 0.13 L and 0.20 L-1, respectively), but maximal mid expiratory flow (FEF25–75%), airway resistance (Raw) and specific conductance (sGaw) were significantly impaired. An association between passive smoking and lung cancer has also been described. In a study conducted in association with the International Agency for Research on Cancer, the exposure to ETS during childhood was

  1. Pulmonary effects of passive smoking: the Indian experience

    PubMed Central

    Gupta, D; Aggarwal, AN; Jindal, SK

    2003-01-01

    There are only a few studies done on pulmonary effects of passive smoking from India, which are summarized in this paper. Several vernacular tobacco products are used in India, bidis (beedis) being the commonest form of these. Bidis contain a higher concentration of nicotine and other tobacco alkaloids compared to the standard cigarettes (e.g., the sum of total nicotine and minor tobacco alkaloids was 37.5 mg in bidi compared to 14–16 mg in Indian or American cigarettes in one study). A large study performed on 9090 adolescent school children demonstrated environmental tobacco smoke (ETS) exposure to be associated with an increased risk of asthma. The odds ratio for being asthmatic in ETS-exposed as compared to ETS-unexposed children was 1.78 (95% CI: 1.33–2.31). Nearly one third of the children in this study reported non-specific respiratory symptoms and the ETS exposure was found to be positively associated with the prevalence of each symptom. Passive smoking was also shown to increase morbidity and to worsen the control of asthma among adults. Another study demonstrated exposure to ETS was a significant trigger for acute exacerbation of asthma. Increased bronchial hyper-responsiveness was also demonstrated among the healthy nonsmoking adult women exposed to ETS. Passive smoking leads to subtle changes in airflow mechanics. In a study among 50 healthy nonsmoking women passively exposed to tobacco smoke and matched for age with 50 unexposed women, forced expiratory volume in first second (FEV1) and peak expiratory flow (PEF) were marginally lower among the passive smokers (mean difference 0.13 L and 0.20 L-1, respectively), but maximal mid expiratory flow (FEF25–75%), airway resistance (Raw) and specific conductance (sGaw) were significantly impaired. An association between passive smoking and lung cancer has also been described. In a study conducted in association with the International Agency for Research on Cancer, the exposure to ETS during childhood was

  2. Constraining lithosphere deformation mode evolution for the Iberia-Newfoundland rifted margins

    NASA Astrophysics Data System (ADS)

    Jeanniot, Ludovic; Kusznir, Nick; Mohn, Geoffroy; Manatschal, Gianreto

    2015-04-01

    The deformation of lithosphere and asthenosphere and its evolution during continental rifting leading to breakup and seafloor spreading initiation is poorly understood. The resulting margin architecture and OCT structure is complex and diverse, and observations at magma poor margins includes hyper-extended continental crust and lithosphere, detachments faults, exhumed mantle, continental slivers and scattered embryonic oceanic crust. A coupled kinematic-dynamic model of lithosphere and asthenosphere deformation has been used to investigate the sequence of lithosphere deformation modes for 2 conjugate margin profiles for the Iberia-Newfoundland rifted margins. We use the observed water-loaded subsidence and crustal thickness, together with subsidence history and the age of melt generation, to test and constrain lithosphere and asthenosphere deformation models. A sequence of lithosphere deformation modes is represented by a succession of flow-fields, which are generated by a 2D finite element viscous flow model (FE-Margin), and is used to advect lithosphere and asthenosphere temperature and material. FE-Margin is kinematically driven by divergent deformation in the upper 15-20 km of the lithosphere inducing passive upwelling below. Buoyancy enhanced upwelling (e.g. Braun et al. 2000) is also kinematically included. The methodology of Katz et al., 2003 is used to predict melt generation by decompressional melting. The magnitude of extension used in the modelling is consistent with that proposed by Sutra et al (2013). The best fit calibrated models of lithosphere deformation evolution for the Iberia-Newfoundland conjugate margins require (i) an initial broad region of lithosphere deformation and passive upwelling, (ii) lateral migration of deformation, (iii) an increase in extension rate with time, (iv) focussing of deformation and (v) buoyancy induced upwelling. The preferred calibrated models predict faster extension rates and earlier continental crustal rupture and

  3. Passivation of high temperature superconductors

    NASA Technical Reports Server (NTRS)

    Vasquez, Richard P. (Inventor)

    1991-01-01

    The surface of high temperature superconductors such as YBa2Cu3O(7-x) are passivated by reacting the native Y, Ba and Cu metal ions with an anion such as sulfate or oxalate to form a surface film that is impervious to water and has a solubility in water of no more than 10(exp -3) M. The passivating treatment is preferably conducted by immersing the surface in dilute aqueous acid solution since more soluble species dissolve into the solution. The treatment does not degrade the superconducting properties of the bulk material.

  4. The anodic passivation of lithium

    SciTech Connect

    James, S.D.

    1983-10-01

    The anodic passivation of Li has been characterized at room temperature in a variety of electrolytes (propylene carbonate, thionyl chloride, sulfur dioxide), as a function of convection and current density and in the presence of water and other impurities. In thionyl chloride the effect of salt concentration (0.5-4.5M, LiA1C1/sub 4/) and acidity (0.5-3M, A1C1/sub 3/) has been studied. The evidence accumulated suggests that anodic passivation is caused by anodic enrichment and eventual precipitation of electrolyte salt in superficial anolyte.

  5. Crustal Structure of the Gulf of Aden Continental Margins, from Afar to Oman, by Ambient Noise Seismic Tomography

    NASA Astrophysics Data System (ADS)

    Korostelev, F.; Weemstra, C.; Boschi, L.; Leroy, S. D.; Ren, Y.; Stuart, G. W.; Keir, D.; Rolandone, F.; Ahmed, A.; Al Ganad, I.; Khanbari, K. M.; Doubre, C.; Hammond, J. O. S.; Kendall, J. M.

    2014-12-01

    Continental rupture processes under mantle plume influence are still poorly known although extensively studied. The Gulf of Aden presents volcanic margins to the west, where they are influenced by the Afar hotspot, and non volcanic margins east of longitude 46° E. We imaged the crustal structure of the Gulf of Aden continental margins from Afar to Oman to evaluate the role of the Afar plume on the evolution of the passive margin and its extent towards the East. We use Ambient Noise Seismic Tomography to better understand the architecture and processes along the Gulf of Aden. This recent method, developed in the last decade, allows us to study the seismic signal propagating between two seismic stations. Ambient Noise Seismic Tomography is thus free from artifacts related to the distribution of earthquakes. We collected continuous records from about 200 permanent or temporary stations since 1999 to compute Rayleigh phase velocity maps over the Gulf of Aden.

  6. Storm tracks near marginal stability

    NASA Astrophysics Data System (ADS)

    Ambaum, Maarten; Novak, Lenka

    2015-04-01

    The variance of atmospheric storm tracks is characterised by intermittent bursts of activity interspersed with relatively quiescent periods. Most of the poleward heat transport by storm tracks is due to a limited number of strong heat flux events, which occur in a quasi-periodic fashion. This behaviour is in contradiction with the usual conceptual model of the storm tracks, which relies on high growth rate background flows which then spawn weather systems that grow in an exponential or non-normal fashion. Here we present a different conceptual model of the atmospheric storm tracks which is built on the observation that, when including diabatic and other dissipative effects, the storm track region is in fact most of the time marginally stable. The ensuing model is a nonlinear oscillator, very similar to Volterra-Lotka predator-prey models. We demonstrate the extensions of this model to a stochastically driven nonlinear oscillator. The model produces quasi-periodic behaviour dominated by intermittent heat flux events. Perhaps most surprisingly, we will show strong evidence from re-analysis data for our conceptual model: the re-analysis data produces a phase-space plot that is very similar indeed to the phase-space plot for our nonlinear oscillator model.

  7. 17 CFR 242.403 - Required margin.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... 17 Commodity and Securities Exchanges 3 2010-04-01 2010-04-01 false Required margin. 242.403 Section 242.403 Commodity and Securities Exchanges SECURITIES AND EXCHANGE COMMISSION (CONTINUED) REGULATIONS M, SHO, ATS, AC, AND NMS AND CUSTOMER MARGIN REQUIREMENTS FOR SECURITY FUTURES Customer...

  8. Marginal Utility and Convex Indifference Curves.

    ERIC Educational Resources Information Center

    Jackson, A.A.

    1981-01-01

    Reviews discussion of the relationship between marginal utility and indifference curves which has been presented in recent issues of "Economics." Concludes that indifference analysis does not embody the assumptions of marginal utility theory and that there is no simple relationship between these concepts that does not entail unacceptable…

  9. 17 CFR 31.18 - Margin calls.

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ....18 Commodity and Securities Exchanges COMMODITY FUTURES TRADING COMMISSION LEVERAGE TRANSACTIONS § 31.18 Margin calls. (a) No leverage transaction merchant shall liquidate a leverage contract because of a margin deficiency without effecting personal contact with the leverage customer. If a...

  10. Dependency and Marginality in Kingston, Jamaica.

    ERIC Educational Resources Information Center

    Clarke, Colin G.

    1983-01-01

    Kingston, capital of Jamaica, has been molded by three institutions: colonialism, the sugar plantation, and slavery. It has an enormous marginal population living in permanent poverty and not absorbable into the labor force. This marginality, fundamentally related to dependent capitalism, sustains itself by keeping wages low. (CS)

  11. [Immigration and political marginality in Argentina].

    PubMed

    Forte, R

    1996-01-01

    The author analyzes immigration in Argentina, with a focus on the extent to which the political system has contributed to the social and economic marginalization of migrants since the mid-nineteenth century. Sociocultural characteristics of migrants are examined. Patterns of land distribution and ownership are discussed, and the impact on movements of migrants within the country and on urban marginality is investigated.

  12. Marginal Teachers: Their Perceptions of Improvement.

    ERIC Educational Resources Information Center

    Devlin-Scherer, Roberta

    For over a decade, parents whose children are in the public education system have expressed concerns regarding the quality of teaching in their schools; yet little energy and study have been devoted to the topic of incompetent or marginal teacher improvement. To describe how marginal teachers perceived their improvement to be related to a staff…

  13. Tumor margin detection using optical biopsy techniques

    NASA Astrophysics Data System (ADS)

    Zhou, Yan; Liu, Cheng-hui; Li, Jiyou; Li, Zhongwu; Zhou, Lixin; Chen, Ke; Pu, Yang; He, Yong; Zhu, Ke; Li, Qingbo; Alfano, Robert R.

    2014-03-01

    The aim of this study is to use the Resonance Raman (RR) and fluorescence spectroscopic technique for tumor margin detection with high accuracy based on native molecular fingerprints of breast and gastrointestinal (GI) tissues. This tumor margins detection method utilizes advantages of RR spectroscopic technique in situ and in real-time to diagnose tumor changes providing powerful tools for clinical guiding intraoperative margin assessments and postoperative treatments. The tumor margin detection procedures by RR spectroscopy were taken by scanning lesion from center or around tumor region in ex-vivo to find the changes in cancerous tissues with the rim of normal tissues using the native molecular fingerprints. The specimens used to analyze tumor margins include breast and GI carcinoma and normal tissues. The sharp margin of the tumor was found by the changes of RR spectral peaks within 2 mm distance. The result was verified using fluorescence spectra with 300 nm, 320 nm and 340 nm excitation, in a typical specimen of gastric cancerous tissue within a positive margin in comparison with normal gastric tissues. This study demonstrates the potential of RR and fluorescence spectroscopy as new approaches with labeling free to determine the intraoperative margin assessment.

  14. Sedimentary basins on the connugate margins of South America and Africa

    SciTech Connect

    Moore, G.T. )

    1990-05-01

    An Early Cretaceous spreading system formed the South Atlantic by separating South America from Africa along two subparallel major transform fault systems. The distribution of major sedimentary depocenters is controlled by the complex interplay of two factors: the late Mesozoic-Cenozoic cycle of sea-floor spreading and the legacy of a Precambrian collage of ancient cores that comprised western Gondwana. Three spreading modes created this configuration: rift, transform, and subduction. Each produces a different geometry and tectonic framework for the accumulation of sediment. Rifted margins (60%) contain basins that are elongate, form with their depocenter axes inboard of the ocean-continent transition, and rest on a tectonically complex, foundered basement. Transform margins have abrupt ocean-continent transitions. Such margins (30%) may be sediment starved or contain a thick sedimentary section controlled by the volcanic ridges of transform faults. Off Tierra del Fuego, Burdwood Bank is bounded on the north by a fossil (aseismic) subduction zone. The associated basin is an elongate, deformed accretionary prism of sediments on a gently dipping, faulted oceanic plate. The South Atlantic margins are divisible into 68 basins or segments that collectively contain over 33 {times} 106 km{sup 3} of syn- and postbreakup sediments. The South American margin contains 22 {times} 10{sup 6} km{sup 3} in 46 basins, and the African margin, 11 {times} 10{sup 6} km{sup 3} in 22 basins. Over 65% of the basins have a sediment column greater than 5 km with some depocenters that locally exceed 10 km. The source rock quality and character vary along both margins. The top of the oil generation window averages about 3.3 km; however, due to differing thermal histories, individual basins can depart significantly from this average.

  15. Deep structure of the Argentine margin inferred from 3D gravity and temperature modelling, Colorado Basin

    NASA Astrophysics Data System (ADS)

    Autin, J.; Scheck-Wenderoth, M.; Götze, H.-J.; Reichert, C.; Marchal, D.

    2016-04-01

    Following previous work on the Colorado Basin using a 3D crustal structural model, we now investigate the presence of lower crustal bodies at the base of the crust using 3D lithospheric gravity modelling and calculations of the conductive thermal field. Our first study highlighted two fault directions and depocentres associated with thinned crust (NW-SE in the West and NE-SW at the distal margin). Fault relative chronology argues for two periods of extension: (1) NW-SE faulting and thinning in the western Colorado Basin and (2) NE-SW faulting and thinning related to the continental breakup and formation of the NE-SW-striking volcanic margins of the Atlantic Ocean. In this study, the geometry of modelled high-density Lower Crustal Bodies (LCBs) enables the reproduction of the gravimetric field as well as of the temperature measured in wells down to 4500 m. The modelled LCBs correlate with geological observations: (1) NW-SE LCBs below the deepest depocentres in the West, (2) NE-SW LCBs below the distal margin faults and the seaward dipping reflectors. Thus the proposed poly-phased evolution of the margin could as well correspond to two emplacement phases of the LCBs. The calculated conductive thermal field fits the measured temperatures best if the thermal properties (thermal conductivity and radiogenic heat production) assigned to the LCBs correspond to either high-grade metamorphic rocks or to mafic magmatic intrusions. To explain the possible lithology of the LCBs, we propose that the two successive phases of extension are accompanied by magma supply, emplaced (1) in the thinnest crust below the older NW-SE depocentres, then (2) along the NE-SW continentward boundary of the distal margin and below the volcanic seaward dipping reflectors. The South African conjugate margin records only the second NE-SW event and we discuss hypotheses which could explain these differences between the conjugate margins.

  16. The Passive Aggressive Conflict Cycle

    ERIC Educational Resources Information Center

    Whitson, Signe

    2013-01-01

    Understanding the Passive Aggressive Conflict Cycle (PACC) helps observers to be able to look beyond behavior and better understand what is occurring beneath the surface. This article presents a real-life example of a seemingly minor conflict between a teacher and child that elicited an apparent major overreaction by the adult. Also provided is a…

  17. Orion Passive Thermal Control Overview

    NASA Technical Reports Server (NTRS)

    Miller, Stephen W.

    2007-01-01

    An viewgraph presentation of Orion's passive thermal control system is shown. The topics include: 1) Orion in CxP Hierarchy; 2) General Orion Description/Orientation; 3) Module Descriptions and Images; 4) Orion PTCS Overview; 5) Requirements/Interfaces; 6) Design Reference Missions; 7) Natural Environments; 8) Thermal Models; 9) Challenges/Issues; and 10) Testing

  18. [Passive smoking--active killer].

    PubMed

    Palavra, Irena Rojnić; Franelić, Iva Pejnović; Milanović, Sanja Musić; Puljić, Kresimir

    2013-01-01

    Although still not perceived in this way, passive smoking is a public health issue of great importance. World Health Organization estimates that as a result of passive exposure to tobacco smoke each year 600,000 people die, of which 165,000 children. There are 33% of men, 35% of women and 40% of children who do not smoke, but are exposed to second hand smoke, and still only 11% of the world population is protected by adequate smoke-free legislation. Scientific literature provides evidence that passive exposure to tobacco smoke can result in numerous adverse health effects: asthma and allergies, respiratory infections and (middle) ear infections, cancers of various localization, accelerated atherosclerosis and cardiovascular diseases, retardation of growth and development in children, and in pregnancy it can lead to congenital anomalies and premature birth as well as lower body weight and length of the child. Certainly, the scariest consequence of all is sudden infant death syndrome, also called "death in the crib". Smoke-free policies have proven their effectiveness, but while implementing the laws, it is necessary to raise public awareness of the hazards of, both active and passive, exposure to tobacco smoke. PMID:24490334

  19. Monitored passive-solar buildings

    NASA Astrophysics Data System (ADS)

    Jones, R. W.

    1982-06-01

    Selected performance results from six monitored passive and hybrid solar heated buildings are presented. These employ: a two story trombe wall; a thermosyphoning solar air heater with rock bin storage; a greenhouse; a composite concrete and water trombe wall; two story sunspace; and, for a mobile/modular home, direct gain and roof pond.

  20. Distinguishing Terrestrial Organic Carbon in Marginal Sediments of East China Sea and Northern South China Sea

    NASA Astrophysics Data System (ADS)

    Kandasamy, Selvaraj; Lin, Baozhi; Wang, Huawei; Liu, Qianqian; Liu, Zhifei; Lou, Jiann-Yuh; Chen, Chen-Tung Arthur; Mayer, Lawrence M.

    2016-04-01

    Knowledge about the sources, transport pathways and behavior of terrestrial organic carbon in continental margins adjoining to large rivers has improved in recent decades, but uncertainties and complications st