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Sample records for african continental margins

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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).

  7. 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.

  8. 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…

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  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. The Continental Margins Program in Georgia

    USGS Publications Warehouse

    Cocker, M.D.; Shapiro, E.A.

    1999-01-01

    From 1984 to 1993, the Georgia Geologic Survey (GGS) participated in the Minerals Management Service-funded Continental Margins Program. Geological and geophysical data acquisition focused on offshore stratigraphic framework studies, phosphate-bearing Miocene-age strata, distribution of heavy minerals, near-surface alternative sources of groundwater, and development of a PC-based Coastal Geographic Information System (GIS). Seven GGS publications document results of those investigations. In addition to those publications, direct benefits of the GGS's participation include an impetus to the GGS's investigations of economic minerals on the Georgia coast, establishment of a GIS that includes computer hardware and software, and seeds for additional investigations through the information and training acquired as a result of the Continental Margins Program. These addtional investigations are quite varied in scope, and many were made possible because of GIS expertise gained as a result of the Continental Margins Program. Future investigations will also reap the benefits of the Continental Margins Program.From 1984 to 1993, the Georgia Geologic Survey (GGS) participated in the Minerals Management Service-funded Continental Margins Program. Geological and geophysical data acquisition focused on offshore stratigraphic framework studies, phosphate-bearing Miocene-age strata, distribution of heavy minerals, near-surface alternative sources of groundwater, and development of a PC-based Coastal Geographic Information System (GIS). Seven GGS publications document results of those investigations. In addition to those publications, direct benefits of the GGS's participation include an impetus to the GGS's investigations of economic minerals on the Georgia coast, establishment of a GIS that includes computer hardware and software, and seeds for additional investigations through the information and training acquired as a result of the Continental Margins Program. These additional

  15. 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

  16. 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.

  17. Gas hydrates of outer continental margins

    SciTech Connect

    Kvenvolden, K.A. )

    1990-05-01

    Gas hydrates are crystalline substances in which a rigid framework of water molecules traps molecules of gas, mainly methane. Gas-hydrate deposits are common in continental margin sediment in all major oceans at water depths greater than about 300 m. Thirty-three localities with evidence for gas-hydrate occurrence have been described worldwide. The presence of these gas hydrates has been inferred mainly from anomalous lacoustic reflectors seen on marine seismic records. Naturally occurring marine gas hydrates have been sampled and analyzed at about tensites in several regions including continental slope and rise sediment of the eastern Pacific Ocean and the Gulf of Mexico. Except for some Gulf of Mexico gas hydrate occurrences, the analyzed gas hydrates are composed almost exclusively of microbial methane. Evidence for the microbial origin of methane in gas hydrates includes (1) the inverse relation between methane occurence and sulfate concentration in the sediment, (2) the subparallel depth trends in carbon isotopic compositions of methane and bicarbonate in the interstitial water, and (3) the general range of {sup 13}C depletion ({delta}{sub PDB}{sup 13}C = {minus}90 to {minus}60 {per thousand}) in the methane. Analyses of gas hydrates from the Peruvian outer continental margin in particular illustrate this evidence for microbially generated methane. The total amount of methane in gas hydrates of continental margins is not known, but estimates of about 10{sup 16} m{sup 3} seem reasonable. Although this amount of methane is large, it is not yet clear whether methane hydrates of outer continental margins will ever be a significant energy resource; however, these gas hydrates will probably constitute a drilling hazard when outer continental margins are explored in the future.

  18. 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.

  19. 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.

  20. 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.

  1. 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.

  2. 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

  3. Reconstructing Rodinia by Fitting Neoproterozoic Continental Margins

    USGS Publications Warehouse

    Stewart, John H.

    2009-01-01

    Reconstructions of Phanerozoic tectonic plates can be closely constrained by lithologic correlations across conjugate margins by paleontologic information, by correlation of orogenic belts, by paleomagnetic location of continents, and by ocean floor magmatic stripes. In contrast, Proterozoic reconstructions are hindered by the lack of some of these tools or the lack of their precision. To overcome some of these difficulties, this report focuses on a different method of reconstruction, namely the use of the shape of continents to assemble the supercontinent of Rodinia, much like a jigsaw puzzle. Compared to the vast amount of information available for Phanerozoic systems, such a limited approach for Proterozoic rocks, may seem suspect. However, using the assembly of the southern continents (South America, Africa, India, Arabia, Antarctica, and Australia) as an example, a very tight fit of the continents is apparent and illustrates the power of the jigsaw puzzle method. This report focuses on Neoproterozoic rocks, which are shown on two new detailed geologic maps that constitute the backbone of the study. The report also describes the Neoproterozoic, but younger or older rocks are not discussed or not discussed in detail. The Neoproterozoic continents and continental margins are identified based on the distribution of continental-margin sedimentary and magmatic rocks that define the break-up margins of Rodinia. These Neoproterozoic continental exposures, as well as critical Neo- and Meso-Neoproterozoic tectonic features shown on the two new map compilations, are used to reconstruct the Mesoproterozoic supercontinent of Rodinia. This approach differs from the common approach of using fold belts to define structural features deemed important in the Rodinian reconstruction. Fold belts are difficult to date, and many are significantly younger than the time frame considered here (1,200 to 850 Ma). Identifying Neoproterozoic continental margins, which are primarily

  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. 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

  6. On the Evolution of Glaciated Continental Margins

    NASA Astrophysics Data System (ADS)

    Sverre Laberg, Jan; Rydningen, Tom Arne; Safronova, Polina A.; Forwick, Matthias

    2016-04-01

    Glaciated continental margins, continental margins where a grounded ice sheet repeatedly has been at or near the shelf break, are found at both northern and southern high-latitudes. Their evolution are in several aspects different from their low-latitude counterparts where eustatic sea-level variations possess a fundamental control on their evolution and where fluvial systems provide the main sediment input. From studies of the Norwegian - Barents Sea - Svalbard and NE Greenland continental margins we propose the following factors as the main control on the evolution of glaciated continental margins: 1) Pre-glacial relief controlling the accommodation space, 2) Ice sheet glaciology including the location of fast-flowing ice streams where source area morphology exerts a fundamental control, 3) Composition of the glacigenic sediments where the clay content in previous studies have been found to be important, and 4) Sea-level controlled both by eustacy and isostacy. From three case studies, 1) the western Barents Sea, 2) part of the North Norwegian (Troms), and 3) the Mid-Norwegian margin, the influence on these factors for the sea-floor morphology, sedimentary processes of the continental slope - deep sea and continental margin architecture are discussed. The pre-glacial relief of the mid-Norwegian and Troms margins relates to the onset of rifting and plate break-up from the early Cenozoic while for the SW Barents Sea, plate shear was followed by rifting. A wide zone of extended continental crust occurs offshore mid-Norway while this zone is much narrower offshore Troms leading to a more pronounced pre-glacial relief. Regarding sediment delivery and ice sheet glaciology the western Barents Sea exemplifies very high sediment input corresponding to an estimated average erosion of the source area of ~0.4 mm/yr (SW Barents Sea), much of which is related to subglacial erosion of Mesozoic - Cenozoic sedimentary rocks from large paleo-ice streams. The mid-Norwegian margin

  7. 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

  8. 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.

  9. Systematic mapping of the Spanish continental margin

    NASA Astrophysics Data System (ADS)

    Acosta, Juan; Muñoz, Araceli; Uchupi, Elazar

    2012-07-01

    For economic, environmental, recreational, military, and political reasons it is critical for coastal states to have up-to-date information on their marine margins. Spain began to acquire such data 17 years ago. From 1995 to the present, the Spanish Oceanographic Institute (IEO), a research organization of the state, has carried out a systematic geological and geophysical study of the Spanish margins. Among these projects are (1) the hydrographic and oceanographic study of the Spanish Exclusive Economic Zone (EEZ) that was implemented by the Navy Hydrographic Institute (IHM); (2) the Espace Project, a study of the Spanish continental shelf; and (3) the Capesme Project, which created fisheries maps of the Mediterranean Sea. The latter two projects were carried out in collaboration with the Secretariat General of the Sea (SGM).

  10. Investigating Continental Margins: An Activity to Help Students Better Understand the Continental Margins of North America

    ERIC Educational Resources Information Center

    Poli, Maria-Serena; Capodivacca, Marco

    2011-01-01

    Continental margins are an important part of the ocean floor. They separate the land above sea level from the deep ocean basins below and occupy about 11% of Earth's surface. They are also economically important, as they harbor both mineral resources and some of the most valuable fisheries in the world. In this article students investigate North…

  11. 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.

  12. 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.

  13. Understanding Continental Margin Biodiversity: A New Imperative

    NASA Astrophysics Data System (ADS)

    Levin, Lisa A.; Sibuet, Myriam

    2012-01-01

    Until recently, the deep continental margins (200-4,000 m) were perceived as monotonous mud slopes of limited ecological or environmental concern. Progress in seafloor mapping and direct observation now reveals unexpected heterogeneity, with a mosaic of habitats and ecosystems linked to geomorphological, geochemical, and hydrographic features that influence biotic diversity. Interactions among water masses, terrestrial inputs, sediment diagenesis, and tectonic activity create a multitude of ecological settings supporting distinct communities that populate canyons and seamounts, high-stress oxygen minimum zones, and methane seeps, as well as vast reefs of cold corals and sponges. This high regional biodiversity is fundamental to the production of valuable fisheries, energy, and mineral resources, and performs critical ecological services (nutrient cycling, carbon sequestration, nursery and habitat support). It is under significant threat from climate change and human resource extraction activities. Serious actions are required to preserve the functions and services provided by the deep-sea settings we are just now getting to know.

  14. Understanding continental margin biodiversity: a new imperative.

    PubMed

    Levin, Lisa A; Sibuet, Myriam

    2012-01-01

    Until recently, the deep continental margins (200-4,000 m) were perceived as monotonous mud slopes of limited ecological or environmental concern. Progress in seafloor mapping and direct observation now reveals unexpected heterogeneity, with a mosaic of habitats and ecosystems linked to geomorphological, geochemical, and hydrographic features that influence biotic diversity. Interactions among water masses, terrestrial inputs, sediment diagenesis, and tectonic activity create a multitude of ecological settings supporting distinct communities that populate canyons and seamounts, high-stress oxygen minimum zones, and methane seeps, as well as vast reefs of cold corals and sponges. This high regional biodiversity is fundamental to the production of valuable fisheries, energy, and mineral resources, and performs critical ecological services (nutrient cycling, carbon sequestration, nursery and habitat support). It is under significant threat from climate change and human resource extraction activities. Serious actions are required to preserve the functions and services provided by the deep-sea settings we are just now getting to know.

  15. Remobilization of barium in continental margin sediments

    NASA Astrophysics Data System (ADS)

    McManus, James; Berelson, William M.; Klinkhammer, Gary P.; Kilgore, Tammy E.; Hammond, Douglas E.

    1994-11-01

    The rate of Ba release from California continental margin sediments has been measured, using an in situ benthic flux chamber, and the range of values (25-50 nmol cm -2 y -1) is larger than any previously published benthic flux estimate for this element. The magnitude of the Ba flux suggests that a significant fraction of the Ba raining from the euphotic zone is recycled at the seafloor. Ba:Si regeneration ratios from these margin sediments increase with depth, demonstrating that Ba is decoupled from Si during the earliest stages of diagenesis. On the other hand, Ba regeneration rates and CaCO 3 dissolution rates covary; the coupling between these two constituents is supported by the observation that the Ba: CaCO 3 dissolution flux ratio (1.7 ± 0.4 × 10 -3) is independent of bottomwater depth—even in sediments underlying the oxygen minimum zone along the continental margin. Furthermore, this flux ratio is consistent with both the water column Ba:alkalinity ratio for the world's ocean, as well as the Ba:CaCO 3 ratio in sediment-trap solid phases from the Equatorial Pacific (1.1-2.2 × 10 -3). However, the constancy of the Ba:alkalinity ratio over geologic time remains in question, because the mechanism that controls this relationship remains a mystery. Our flux measurements suggest that diagenesis does not significantly influence the Ba:Ca ratio in the upper 0.5 mm of Pacific sediments, thereby supporting the idea of using the Ba concentration in surface-dwelling benthic forams as a proxy for deep-water chemical conditions ( LEA and BOYLE, 1989, 1990). On the other hand, we predict that if a foraminifer lives 0.5 mm or more below this interface, then diagenetic effects could influence the Ba:Ca ratio that foram species would record. The carrier phase of the particulate Ba reactive during early diagenesis does not appear to be organic matter, oxyhydroxides, or calcium carbonate, but rather a mineral phase related to marine barite or perhaps celestite.

  16. Structure of the North American Atlantic Continental Margin.

    ERIC Educational Resources Information Center

    Klitgord, K. K.; Schlee, J. S.

    1986-01-01

    Offers explanations on the origin of the North American Atlantic continental margin. Provides an analysis and illustrations of structural and strategraphic elements of cross sections of the Atlantic continental margin. Also explains the operations and applications of seismic-relection profiles in studying ocean areas. (ML)

  17. 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.

  18. 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.

  19. 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

  20. 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)

  1. Understanding continental margin biodiversity: a new imperative.

    PubMed

    Levin, Lisa A; Sibuet, Myriam

    2012-01-01

    Until recently, the deep continental margins (200-4,000 m) were perceived as monotonous mud slopes of limited ecological or environmental concern. Progress in seafloor mapping and direct observation now reveals unexpected heterogeneity, with a mosaic of habitats and ecosystems linked to geomorphological, geochemical, and hydrographic features that influence biotic diversity. Interactions among water masses, terrestrial inputs, sediment diagenesis, and tectonic activity create a multitude of ecological settings supporting distinct communities that populate canyons and seamounts, high-stress oxygen minimum zones, and methane seeps, as well as vast reefs of cold corals and sponges. This high regional biodiversity is fundamental to the production of valuable fisheries, energy, and mineral resources, and performs critical ecological services (nutrient cycling, carbon sequestration, nursery and habitat support). It is under significant threat from climate change and human resource extraction activities. Serious actions are required to preserve the functions and services provided by the deep-sea settings we are just now getting to know. PMID:22457970

  2. 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

  3. Organic geochemistry of outer continental margins and deep ocean sediments

    SciTech Connect

    Whelan, J.K.

    1990-01-01

    This report discusses the activities and progress made in the study of continental margins and deep ocean sediments. Topics discussed are the calibration of thermal maturation markers, hydrous pyrolysis studies and sample collection. (KS)

  4. Contemporary phosphorites on the continental margin of peru.

    PubMed

    Veeh, H H; Burnett, W C; Soutar, A

    1973-08-31

    Phosphorite nodules occurring along the biologically productive continental margin of Peru have been dated by uranium-series methods. The radiometric ages range from late Pleistocene to Recent, indicating that phosphorities are currently forming in this area.

  5. Continental margin of Western europe: slope progradation and erosion.

    PubMed

    Curray, J R; Moore, D G; Belderson, R H; Stride, A H

    1966-10-14

    Reflection profiling of the continental margin off western Europe shows seaward-dipping continental-slope deposits that have been dissected by submarine canyons west of the English Channel. These records refute previous interpretation of structural benches of older, nearly horizontal strata outcropping on the slope face.

  6. Continental margin of Western europe: slope progradation and erosion.

    PubMed

    Curray, J R; Moore, D G; Belderson, R H; Stride, A H

    1966-10-14

    Reflection profiling of the continental margin off western Europe shows seaward-dipping continental-slope deposits that have been dissected by submarine canyons west of the English Channel. These records refute previous interpretation of structural benches of older, nearly horizontal strata outcropping on the slope face. PMID:17810307

  7. 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

  8. Late tertiary structure and stratigraphy of north Sinai continental margin

    SciTech Connect

    Ben-Avraham, Z.; Mart, Y.

    1981-06-01

    New seismic data provide information on the structural development and late Tertiary stratigraphy of the continental slope and rise off northern Sinai. The upper continental slope is characterized by a marginal plateau composed of a series of platforms or steps. The lower continental slope is smooth, except for a low ridge paralleling the western part. Numerous diapiric structures along the continental margin north of the Sinai Peninsula emerge from an evaporitic layer of late Tertiary age. The diapirs usually are aligned along west-northwest-trending faults. A salt ridge 90 km long produces a submarine ridge in the lower continental rise. Two main fault trends have been mapped. In the western part of the continental margin they trend west-northwest and, in the eastern part, northeast. These trends parallel the continental slopes of northern Sinai and southern Israel, respectively. The structural grain of the Sinai margin appears to be controlled by two main tectonic elements: (1) rejuvenated basement faults of the continent-ocean transition zone; and (2) salt diapirism due to loading of Messinian evaporites with Nile-derived upper Tertiary clastics.

  9. Comparative biogeochemistry-ecosystem-human interactions on dynamic continental margins

    USGS Publications Warehouse

    Levin, Lisa A.; Liu, Kon-Kee; Emeis, Kay-Christian; Breitburg, Denise L.; Cloern, James; Deutsch, Curtis; Giani, Michele; Goffart, Anne; Hofmann, Eileen E.; Lachkar, Zouhair; Limburg, Karin; Liu, Su-Mei; Montes, Enrique; Naqvi, Wajih; Ragueneau, Olivier; Rabouille, Christophe; Sarkar, Santosh Kumar; Swaney, Dennis P.; Wassman, Paul; Wishner, Karen F.

    2014-01-01

    The ocean’s continental margins face strong and rapid change, forced by a combination of direct human activity, anthropogenic CO2-induced climate change, and natural variability. Stimulated by discussions in Goa, India at the IMBER IMBIZO III, we (1) provide an overview of the drivers of biogeochemical variation and change on margins, (2) compare temporal trends in hydrographic and biogeochemical data across different margins (3) review ecosystem responses to these changes, (4) highlight the importance of margin time series for detecting and attributing change and (5) examine societal responses to changing margin biogeochemistry and ecosystems. We synthesize information over a wide range of margin settings in order to identify the commonalities and distinctions among continental margin ecosystems. Key drivers of biogeochemical variation include long-term climate cycles, CO2-induced warming, acidification, and deoxygenation, as well as sea level rise, eutrophication, hydrologic and water cycle alteration, changing land use, fishing, and species invasion. Ecosystem responses are complex and impact major margin services including primary production, fisheries production, nutrient cycling, shoreline protection, chemical buffering, and biodiversity. Despite regional differences, the societal consequences of these changes are unarguably large and mandate coherent actions to reduce, mitigate and adapt to multiple stressors on continental margins.

  10. Comparative biogeochemistry-ecosystem-human interactions on dynamic continental margins

    NASA Astrophysics Data System (ADS)

    Levin, Lisa A.; Liu, Kon-Kee; Emeis, Kay-Christian; Breitburg, Denise L.; Cloern, James; Deutsch, Curtis; Giani, Michele; Goffart, Anne; Hofmann, Eileen E.; Lachkar, Zouhair; Limburg, Karin; Liu, Su-Mei; Montes, Enrique; Naqvi, Wajih; Ragueneau, Olivier; Rabouille, Christophe; Sarkar, Santosh Kumar; Swaney, Dennis P.; Wassman, Paul; Wishner, Karen F.

    2015-01-01

    The oceans' continental margins face strong and rapid change, forced by a combination of direct human activity, anthropogenic CO2-induced climate change, and natural variability. Stimulated by discussions in Goa, India at the IMBER IMBIZO III, we (1) provide an overview of the drivers of biogeochemical variation and change on margins, (2) compare temporal trends in hydrographic and biogeochemical data across different margins, (3) review ecosystem responses to these changes, (4) highlight the importance of margin time series for detecting and attributing change and (5) examine societal responses to changing margin biogeochemistry and ecosystems. We synthesize information over a wide range of margin settings in order to identify the commonalities and distinctions among continental margin ecosystems. Key drivers of biogeochemical variation include long-term climate cycles, CO2-induced warming, acidification, and deoxygenation, as well as sea level rise, eutrophication, hydrologic and water cycle alteration, changing land use, fishing, and species invasion. Ecosystem responses are complex and impact major margin services. These include primary production, fisheries production, nutrient cycling, shoreline protection, chemical buffering, and biodiversity. Despite regional differences, the societal consequences of these changes are unarguably large and mandate coherent actions to reduce, mitigate and adapt to multiple stressors on continental margins.

  11. Subduction-driven recycling of continental margin lithosphere.

    PubMed

    Levander, A; Bezada, M J; Niu, F; Humphreys, E D; Palomeras, I; Thurner, S M; Masy, J; Schmitz, M; Gallart, J; Carbonell, R; Miller, M S

    2014-11-13

    Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones

  12. Subduction-driven recycling of continental margin lithosphere.

    PubMed

    Levander, A; Bezada, M J; Niu, F; Humphreys, E D; Palomeras, I; Thurner, S M; Masy, J; Schmitz, M; Gallart, J; Carbonell, R; Miller, M S

    2014-11-13

    Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones.

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

    NASA Astrophysics Data System (ADS)

    Basile, Christophe

    2010-05-01

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

  14. 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

  15. Structure of the North American Atlantic Continental Margin.

    USGS Publications Warehouse

    Schlee, J.S.; Klitgord, K.K.

    1986-01-01

    Off E N America, where the structure of the continental margin is essentially constructional, seismic profiles have approximated geologic cross sections up to 10-15km below the sea floor and revealed major structural and stratigraphic features that have regional hydrocarbon potential. These features include a) a block-faulted basement hinge zone; b) a deep, broad, rifted basement filled with clastic sediment and salt; and c) a buried paleoshelf-edge complex that has many forms. The mapping of seismostratigraphic units over the continental shelf, slope, and rise has shown that the margin's developmental state included infilling of a rifted margin, buildup of a carbonate platform, and construction of an onlapping continental-rise wedge that was accompanied by erosion of the slope. -from Authors

  16. Late Cenozoic Underthrusting of the Continental Margin off Northernmost California.

    PubMed

    Silver, E A

    1969-12-01

    The presence of magnetic anomaly 3, age 5 million years, beneath the continental slope off northernmost California, is evidence for underthrusting of the continental margin during the late Cenozoic. Folded and faulted strata near the base of the slope attest to deformation of the eastern edge of the turbidite sedimzents in the Gorda Basin; the deformation observed is exactly that expected from underthrusting. The relative motions of three crustal plates also suggest underthrusting, possibly with a major component of right-lateral slip.

  17. Closing the North American Carbon Budget: Continental Margin Fluxes Matter!

    NASA Astrophysics Data System (ADS)

    Najjar, R.; Benway, H. M.; Siedlecki, S. A.; Boyer, E. W.; Cai, W. J.; Coble, P. G.; Cross, J. N.; Friedrichs, M. A.; Goni, M. A.; Griffith, P. C.; Herrmann, M.; Lohrenz, S. E.; Mathis, J. T.; McKinley, G. A.; Pilskaln, C. H.; Smith, R. A.; Alin, S. R.

    2015-12-01

    Despite their relatively small surface area, continental margins are regions of intense carbon and nutrient processing, export and exchange, and thus have a significant impact on global biogeochemical cycles. In response to recommendations for regional synthesis and carbon budget estimation for North America put forth in the North American Continental Margins workshop report (Hales et al., 2008), the Ocean Carbon and Biogeochemistry (OCB) Program and North American Carbon Program (NACP) began coordinating a series of collaborative, interdisciplinary Coastal CARbon Synthesis (CCARS) research activities in five coastal regions of North America (Atlantic Coast, Pacific Coast, Gulf of Mexico, Arctic, Laurentian Great Lakes) to improve quantitative assessments of the North American carbon budget. CCARS workshops and collaborative research activities have resulted in the development of regional coastal carbon budgets based on recent literature- and model-based estimates of major carbon fluxes with estimated uncertainties. Numerous peer-reviewed papers and presentations by involved researchers have highlighted these findings and provided more in-depth analyses of processes underlying key carbon fluxes in continental margin systems. As a culminating outcome of these synthesis efforts, a comprehensive science plan highlights key knowledge gaps identified during this synthesis and provides explicit guidance on future research and observing priorities in continental margin systems to help inform future agency investments in continental margins research. This presentation will provide an overview of regional and flux-based (terrestrial inputs, biological transformations, sedimentary processes, atmospheric exchanges, lateral carbon transport) synthesis findings and key recommendations in the science plan, as well as a set of overarching priorities and recommendations on observations and modeling approaches for continental margin systems.

  18. The dynamics of continental extension and divergent margin formation

    SciTech Connect

    Sawyer, D.S. )

    1990-05-01

    Continental breakup is a highly variable process. Differences occur in the relative timing and extent of volcanism, uplift, and graben formation as well as in the mode and amount of continental extension before breakup. The authors propose a model that reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends. Continental lithosphere is viewed as a composite material composed of two strong layers, one in the upper mantle and one in the middle crust. Finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust. This leads to early melt segregation and volcanism, margin uplift during the late stages of the extension process, and relatively narrow symmetrical extended margins. In contrast, failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle. This produces initial graben formation, cooling in the lower crust and upper mantle, and broad asymmetrical extended margins. Volcanism only occurs late in the process. Failure at laterally offset weaknesses within both strong layers, perhaps the most common case, leads to a deformation pattern dominated by simple shear. Thus, differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup. They find that certain features of the evolution of the US Atlantic margin, particularly the formation of the hinge zone and the distribution and timing of extension may be better explained using these models.

  19. Geomorphic characterization of the U.S. Atlantic continental margin

    USGS Publications Warehouse

    Brothers, Daniel S.; ten Brink, Uri S.; Andrews, Brian D.; Chaytor, Jason D.

    2013-01-01

    The increasing volume of multibeam bathymetry data collected along continental margins is providing new opportunities to study the feedbacks between sedimentary and oceanographic processes and seafloor morphology. Attempts to develop simple guidelines that describe the relationships between form and process often overlook the importance of inherited physiography in slope depositional systems. Here, we use multibeam bathymetry data and seismic reflection profiles spanning the U.S. Atlantic outer continental shelf, slope and rise from Cape Hatteras to New England to quantify the broad-scale, across-margin morphological variation. Morphometric analyses suggest the margin can be divided into four basic categories that roughly align with Quaternary sedimentary provinces. Within each category, Quaternary sedimentary processes exerted heavy modification of submarine canyons, landslide complexes and the broad-scale morphology of the continental rise, but they appear to have preserved much of the pre-Quaternary, across-margin shape of the continental slope. Without detailed constraints on the substrate structure, first-order morphological categorization the U.S. Atlantic margin does not provide a reliable framework for predicting relationships between form and process.

  20. On isostasy at Atlantic-type continental margins

    NASA Technical Reports Server (NTRS)

    Karner, G. D.; Watts, A. B.

    1982-01-01

    The concept of isostasy describes the manner in which topographic features on the earth's surface are compensated at depth. The present investigation is concerned with the isostatic mechanism at Atlantic-type continental margins. Particular attention is given to the question whether the flexure model of isostasy, which has successfully been used at other geological features in oceans, is applicable at margins. Cross-spectral techniques are used to analyze the relationship between free air gravity and topography at Atlantic-type continental margins. The relatively old eastern North America is found to be associated with the highest value of the effective elastic thickness in the range 10-20 km, while the relatively young Coral Sea/Lord Howe rise is associated with the lowest value of less than 5 km. The differences in estimates of effective elastic thickness between margins can be explained by a simple model in which the flexural strength of the basement increases with age.

  1. Upper mantle viscosity and dynamic subsidence of curved continental margins.

    PubMed

    Sacek, Victor; Ussami, Naomi

    2013-01-01

    Continental rifting does not always follow a straight line. Nevertheless, little attention has been given to the influence of rifting curvature in the evolution of extended margins. Here, using a three-dimensional model to simulate mantle dynamics, we demonstrate that the curvature of rifting along a margin also controls post-rift basin subsidence. Our results indicate that a concave-oceanward margin subsides faster than a convex margin does during the post-rift phase. This dynamic subsidence of curved margins is a result of lateral thermal conduction and mantle convection. Furthermore, the differential subsidence is strongly dependent on the viscosity structure. As a natural example, we analyse the post-rift stratigraphic evolution of the Santos Basin, southeastern Brazil. The differential dynamic subsidence of this margin is only possible if the viscosity of the upper mantle is >2-3 × 10(19) Pa s.

  2. Canada basin: age and history of its continental margin

    SciTech Connect

    Sweeney, J.F.

    1985-02-01

    Presently available age controls suggest that the Canada basin formed during the Cretaceous Period between about 131 and 79 Ma. The opening process began with continental breakup that may have involved all parts of the North American polar margin at about the same time. The opening was completed by the formation of oceanic crust during the extended Cretaceous interval of normal geomagnetic polarity. Features characteristics of continental breakup, insofar as they are known, show systematic regional differences. From Brock to Axel Heiberg Island, continental breakup was associated with an extended (100 + Ma) stratigraphic hiatus and, northeastward from Ellef Ringnes Island, with extensive tholeiitic igneous activity. From Banks Island to northeastern Alaska, the breakup interval was abbreviated (20-30 Ma), and sparse igneous activity occurred. These differences can be produced by changes in the rate and/or amount of crustal stretching during margin formation and would imply relatively faster or more stretching northeast of Brock island. A continental margin of fixed age, exhibiting the indicated pattern of crustal stretching, could be produced along the trailing edge of a rotating block (Arctic Alaska terrane AA) with its pivot near the Mackenzie delta. When the rotation is restored, however, geological discrepancies are evident between Devonian and older rocks across the conjugate margins, suggesting an earlier history of drifting for the AA. Early Paleozoic correlations appear improved if the AA is placed, polar margin to polar margin, against northern Ellesmere Island and Greenland, where in the middle Paleozoic, it was sheared sinistrally along the Canadian margin to its pre-rotated position opposite Banks Island.

  3. The Continental Margins of the Western North Atlantic.

    ERIC Educational Resources Information Center

    Schlee, John S.; And Others

    1979-01-01

    Presents an interpretation of geological and geophysical data, which provides a summary of the structural and sedimentary history of the United States Atlantic Margin. The importance of an understanding of the development of the outer continental shelf to future hydrocarbon exploration is detailed. (BT)

  4. Neogene sedimentation on the outer continental margin, southern Bering Sea

    USGS Publications Warehouse

    Vallier, T.L.; Underwood, M.B.; Gardner, J.V.; Barron, J.A.

    1980-01-01

    Neogene sedimentary rocks and sediments from sites on the outer continental margin in the southern Bering Sea and on the Alaska Peninsula are dominated by volcanic components that probably were eroded from an emergent Aleutian Ridge. A mainland continental source is subordinate. Most sediment in the marine environment was transported to the depositional sites by longshore currents, debris flows, and turbidity currents during times when sea level was near the outermost continental shelf. Fluctuations of sea level are ascribed both to worldwide glacio-eustatic effects and to regional vertical tectonics. Large drainage systems, such as the Yukon and Kuskokwim Rivers, had little direct influence on sedimentation along the continental slope and Unmak Plateau in the southern Bering Sea. Sediments from those drainage systems probably were transported to the floor of the Aleutian Basin, to the numerous shelf basins that underlie the outer continental shelf, and to the Arctic Ocean after passing through the Bering Strait. Environments of deposition at the sites along the outer continental margin have not changed significantly since the middle Miocene. The site on the Alaska Peninsula, however, is now emergent following shallow-marine and transitional sedimentation during the Neogene. ?? 1980.

  5. Ecological theory and continental margins: where shallow meets deep.

    PubMed

    Levin, Lisa A; Dayton, Paul K

    2009-11-01

    Continental margins, where land becomes ocean and plunges to the deep sea, provide valuable food and energy resources, and perform essential functions such as carbon burial and nutrient cycling. They exhibit remarkably high species and habitat diversity, but this is threatened by our increasing reliance on the resources that margins provide, and by warming, expanding hypoxia and acidification associated with climate change. Continental margin ecosystems, with environments, constituents and processes that differ from those in shallow water, demand a new focus, in which ecological theory and experimental methods are brought to bear on management and conservation practices. Concepts of disturbance, diversity-function relationships, top-down versus bottom-up control, facilitation and meta-dynamics offer a framework for studying fundamental processes and understanding future change.

  6. 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.

  7. 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.

  8. A contourite depositional system along the Uruguayan continental margin

    NASA Astrophysics Data System (ADS)

    Hernández Molina, F. J.; Soto, M.; Piola, A. R.; Tomasini, J.; Preu, B.; Thompson, P.; Badalini, G.; Creaser, A.; Violante, R.; Morales, E.; Paterlini, M.; de Santa Ana, H.

    2015-12-01

    For the first time, a multidisciplinary approach for evaluating the influence of bottom currents in the Uruguayan continental margin is presented. Bathymetric data and multichannel 2D and 3D seismic reflection profiles were used to construct a morphosedimentary map to interpret and decode sedimentary and oceanographic processes along the Uruguayan continental margin. Based on these results a significant contourite depositional system on the margin is described, which contains a spectacular array of large erosive, depositional (drifts) and mixed (terrace) features, which have been generated primarily by water masses of Antarctic and subantarctic origin. From the Eocene-Oligocene boundary up to present time, the long-term influence of water masses from higher southern latitudes, in combination with down-slope sedimentary processes have strongly controlled the overall margin morphology. Most of the features described here, were formed during the middle/late Miocene epoch due to paleoceanographic shifts that include the arrival of Antarctic Intermediate Water (AAIW) along the margin, which in combination with deeper Antarctic Bottom Water (AABW) are fundamental in the margin evolution. In combination with Quaternary climatic and eustatic changes in sea level, fluctuations of the Brazil-Malvinas Confluence influenced subsequently glacial and interglacial stages that appear in sedimentary features defined here. These paleoceanographic changes controlled the sedimentary stacking pattern and the locations of high amplitude reflections (HAR) along the contourite terraces, which could be associated to sandier deposits. Fundamental understanding of the above described margin morphologies and the development of associated bedforms in deep marine environments are essential to fully leverage their conceptual implications for hydrocarbon exploration efforts. Futhermore, a more detailed understanding of the margin and its ancient to modern day current dynamics will improve

  9. Holocene subsurface temperature variability in the eastern Antarctic continental margin

    NASA Astrophysics Data System (ADS)

    Kim, Jung-Hyun; Crosta, Xavier; Willmott, Veronica; Renssen, Hans; Bonnin, Jérôme; Helmke, Peer; Schouten, Stefan; Sinninghe Damsté, Jaap S.

    2012-03-01

    We reconstructed subsurface (˜45-200 m water depth) temperature variability in the eastern Antarctic continental margin during the late Holocene, using an archaeal lipid-based temperature proxy (TEX86L). Our results reveal that subsurface temperature changes were probably positively coupled to the variability of warmer, nutrient-rich Modified Circumpolar Deep Water (MCDW, deep water of the Antarctic circumpolar current) intrusion onto the continental shelf. The TEX86L record, in combination with previously published climatic records, indicates that this coupling was probably related to the thermohaline circulation, seasonal variability in sea ice extent, sea temperature, and wind associated with high frequency climate dynamics at low-latitudes such as internal El Niño Southern Oscillation (ENSO). This in turn suggests a linkage between centennial ENSO-like variability at low-latitudes and intrusion variability of MCDW into the eastern Antarctic continental shelf, which might have further impact on ice sheet evolution.

  10. The Antarctic continental margin: Geology and geophysics of offshore Wilkes land

    SciTech Connect

    Eittreim, S.L.; Hampton, M.A.

    1987-01-01

    This book contains 14 chapters. Some of the chapter titles are: An Interpretation of the Multichannel Seismic Reflection Profiles across the Continental Margin of the Dumont D'Urville Sea, off Wilkes Land, East Antarctica; Hydrocarbon Geochemistry of Sediments Offshore from Antarctica: Wilkes Land Continental margin; and the Conjugate Continental margins of Antarctica and Australia.

  11. 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.

  12. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-01-01

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

  14. A transform continental margin rich in hydrocarbons, Gulf of California

    SciTech Connect

    Lonsdale, P.

    1985-07-01

    Conventional and near-bottom geophysical surveys mapped shallow deformational structures and sediment accumulation patterns along part of the actively shearing continental margin of Sonora, including the intersection with a Guaymas Basin spreading center. The principal strike-slip faults occupy a zone 1-2 km (0.6-1.2 mi) wide along the lower slope, with a ridge of tightly folded sediments raised by uplift of sheared basement at the abrupt boundary between continental and oceanic crust. This transform ridge and a continental rise on its seaward slope grows as the fault zone ages away from the spreading-center intersection shoaling at a net rate of 500 m/m.y. (1,600 ft/ m.y.) despite erosion of its crest. A clathrate horizon in the upper 80 m (260 ft) of sediment is inferred from bottomsimulating reflectors in the rise and lower slope, hydrocarbon seeps occur at 1,600 m (1 mi) below sea level along the crest of the transform ridge, and patchy gas accumulation and seepage are recognized on a young marginal plateau.

  15. The Aravalli sequence of Rajasthan, India: A Precambrian continental margin?

    NASA Technical Reports Server (NTRS)

    Macdougall, J. D.; Willis, R.; Lugmair, G. W.; Roy, A. B.; Gopalan, K.

    1985-01-01

    The extent to which plate tectonics in its present form operated during the Precambrian is unknown, but is a subject of considerable current interest. A remarkable succession of Precambrian rocks in Rajasthan, Northwestern India, which may help to shed more light on this question are discussed. Data indicates that the Aravalli sequence has a number of characteristics generally ascribed to active continental margins. Although much more work is required to bear this out, the evidence suggests that the processes operating in such an environment in the early Proterozoic or late Archean were not vastly different from today.

  16. Paleogene continental margin truncation in southwestern Mexico: Geochronological evidence

    NASA Astrophysics Data System (ADS)

    Schaaf, Peter; MoráN-Zenteno, Dante; HernáNdez-Bernal, Maria Del Sol; SolíS-Pichardo, Gabriela; Tolson, Gustavo; KöHler, Hermann

    1995-12-01

    The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW-SE decreasing granitoid intrusion age trend (˜1.2 cm/yr in the 100 Ma-40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW-SSE trending Cretaceous-Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America-Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (˜7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco-Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40-25 Ma, with a velocity of 6.5-4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America-Farallon-Caribbean triple junction. Geological

  17. An Assessment of Global Organic Carbon Flux Along Continental Margins

    NASA Technical Reports Server (NTRS)

    Thunell, Robert

    2004-01-01

    This project was designed to use real-time and historical SeaWiFS and AVHRR data, and real-time MODIS data in order to estimate the global vertical carbon flux along continental margins. This required construction of an empirical model relating surface ocean color and physical variables like temperature and wind to vertical settling flux at sites co-located with sediment trap observations (Santa Barbara Basin, Cariaco Basin, Gulf of California, Hawaii, and Bermuda, etc), and application of the model to imagery in order to obtain spatially-weighted estimates.

  18. Chronobiology of deep-water decapod crustaceans on continental margins.

    PubMed

    Aguzzi, Jacopo; Company, Joan B

    2010-01-01

    Species have evolved biological rhythms in behaviour and physiology with a 24-h periodicity in order to increase their fitness, anticipating the onset of unfavourable habitat conditions. In marine organisms inhabiting deep-water continental margins (i.e. the submerged outer edges of continents), day-night activity rhythms are often referred to in three ways: vertical water column migrations (i.e. pelagic), horizontal displacements within benthic boundary layer of the continental margin, along bathymetric gradients (i.e. nektobenthic), and endobenthic movements (i.e. rhythmic emergence from the substrate). Many studies have been conducted on crustacean decapods that migrate vertically in the water column, but much less information is available for other endobenthic and nektobenthic species. Also, the types of displacement and major life habits of most marine species are still largely unknown, especially in deep-water continental margins, where steep clines in habitat factors (i.e. light intensity and its spectral quality, sediment characteristics, and hydrography) take place. This is the result of technical difficulties in performing temporally scheduled sampling and laboratory testing on living specimens. According to this scenario, there are several major issues that still need extensive research in deep-water crustacean decapods. First, the regulation of their behaviour and physiology by a biological clock is almost unknown compared to data for coastal species that are easily accessible to direct observation and sampling. Second, biological rhythms may change at different life stages (i.e. size-related variations) or at different moments of the reproductive cycle (e.g. at egg-bearing) based on different intra- and interspecific interactions. Third, there is still a major lack of knowledge on the links that exist among the observed bathymetric distributions of species and selected autoecological traits that are controlled by their biological clock, such as the

  19. Carbonate comparison of west Florida continental margin with margins of eastern United States

    SciTech Connect

    Doyle, L.J.

    1986-05-01

    Temperate carbonate margins may have as many similarities to clastic margins as to other carbonate systems. An example is the west Florida continental margin north of Florida Bay, a vast area of more than 150,000 km/sup 2/. The facies of this area differ from those of other Holocene carbonates, such as the Bahama Banks, the Great Barrier Reef, and the Caribbean and Pacific bioherms. The west Florida margin is analogous to the predominantly clastic southeastern US in both physiology and sedimentary processes. The shelf facies is a veneer of carbonate sand, primarily molluscan shell fragments, with low sedimentation rates. It is similar to the southeastern US sand veneer with the clastic component removed. Like the US system, the west Florida shelf has a ridge and swale topography replete with sedimentary structures, such as sand waves, with a series of drainage systems incised into its surface at lower stands of sea level. On the outer edge, it is commonly bounded by outcrops with considerable positive relief. The upper slope of the west Florida margin is a calcilutite, a Holocene chalk deposit accumulating at rates of tens of centimeters/1000 years, comparable to the clastic lutite depositional rates of the eastern US continental slope, and two orders of magnitude higher than deep-sea oozes of similar composition. These relatively high rates are probably caused by fines pumped from and across the coarser shelf-sand sheets in both systems.

  20. 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

  1. Buried Mesozoic rift basins of Moroccan Atlantic continental margin

    SciTech Connect

    Mohamed, N.; Jabour, H.; El Mostaine, M.

    1995-08-01

    The Atlantic continental margin is the largest frontier area for oil and gas exploration in Morocco. Most of the activity has been concentrated where Upper Jurassic carbonate rocks have been the drilling objectives, with only one significant but non commercial oil discovery. Recent exploration activities have focused on early Mesozoic Rift basins buried beneath the post-rift sediments of the Middle Atlantic coastal plain. Many of these basins are of interest because they contain fine-grained lacustrine rocks that have sufficient organic richness to be classified as efficient oil prone source rock. Location of inferred rift basins beneath the Atlantic coastal plain were determined by analysis of drilled-hole data in combination with gravity anomaly and aeromagnetic maps. These rift basins are characterized by several half graben filled by synrift sediments of Triassic age probably deposited in lacustrine environment. Coeval rift basins are known to be present in the U.S. Atlantic continental margin. Basin modeling suggested that many of the less deeply bored rift basins beneath the coastal plain are still within the oil window and present the most attractive exploration targets in the area.

  2. Antarctic glacial history from numerical models and continental margin sediments

    USGS Publications Warehouse

    Barker, P.F.; Barrett, P.J.; Cooper, A. K.; Huybrechts, P.

    1999-01-01

    The climate record of glacially transported sediments in prograded wedges around the Antarctic outer continental shelf, and their derivatives in continental rise drifts, may be combined to produce an Antarctic ice sheet history, using numerical models of ice sheet response to temperature and sea-level change. Examination of published models suggests several preliminary conclusions about ice sheet history. The ice sheet's present high sensitivity to sea-level change at short (orbital) periods was developed gradually as its size increased, replacing a declining sensitivity to temperature. Models suggest that the ice sheet grew abruptly to 40% (or possibly more) of its present size at the Eocene-Oligocene boundary, mainly as a result of its own temperature sensitivity. A large but more gradual middle Miocene change was externally driven, probably by development of the Antarctic Circumpolar Current (ACC) and Polar Front, provided that a few million years' delay can be explained. The Oligocene ice sheet varied considerably in size and areal extent, but the late Miocene ice sheet was more stable, though significantly warmer than today's. This difference probably relates to the confining effect of the Antarctic continental margin. Present-day numerical models of ice sheet development are sufficient to guide current sampling plans, but sea-ice formation, polar wander, basal topography and ice streaming can be identified as factors meriting additional modelling effort in the future.

  3. 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).

  4. The character of the glaciated Mid-Norwegian continental margin

    NASA Astrophysics Data System (ADS)

    Oline Hjelstuen, Berit; Haflidason, Haflidi; Petter Sejrup, Hans

    2010-05-01

    During Pleistocene the development of the NW European continental margin was strongly controlled by the variability in ocean circulation, glaciations and sea-level changes. Repeated occurrence of shelf edge glaciations, from Ireland to Svalbard, started at Marine Isotope Stage 12 (c. 0.5 Ma). During these periods, fast moving ice streams also crossed the Mid-Norwegian continental shelf on a number of locations, and a thick prograding wedge accumulated on the continental slope. During shelf edge glaciations and in early deglaciation phases high sedimentation rates (>2000 cm/ka) existed, and glacigenic debris flows and melt water plumes were deposited. Within these depositional environments we identify three slide events. These slides have affected an area between 2900 and 12000 km2 and involved 580-2400 km3 of sediments, noting that the slide debrites left by the failure events reach a maximum thickness of c. 150 m. The failures have occurred within an area dominated by gradients less than 1 degree, and observation of long run-out distances indicate that hydroplaning was important during slide development. Gas hydrate bearing sediments are identified on the mid-Norwegian continental margin, but appears to be absent in the slide scars. Thus, dissociation of gas hydrates may have promoted conditions for the failures to occur. Within the region of gas hydrate bearing Pleistocene sediments the Nyegga Pockmark Field is observed. This field contains more than 200 pockmarks and is located at a water depth of 600-800 m. The pockmarks identified are up to 15 m deep, between 30 m and 600 m across and reach a maximum area of c. 315 000 m2. The pockmarks are sediment-empty features and are restricted to a <16.2 cal ka BP old sandy mud unit. It seems that the Nyegga Pockmark Field does not show any strong relationship neither to seabed features, sub-seabed structures nor the glacial sedimentary setting. Thus, this implies a more complex development history for the Nyegga

  5. Lower-crustal intrusion on the North Atlantic continental margin.

    PubMed

    White, R S; Smith, L K; Roberts, A W; Christie, P A F; Kusznir, N J; Roberts, A M; Healy, D; Spitzer, R; Chappell, A; Eccles, J D; Fletcher, R; Hurst, N; Lunnon, Z; Parkin, C J; Tymms, V J

    2008-03-27

    When continents break apart, the rifting is sometimes accompanied by the production of large volumes of molten rock. The total melt volume, however, is uncertain, because only part of it has erupted at the surface. Furthermore, the cause of the magmatism is still disputed-specifically, whether or not it is due to increased mantle temperatures. We recorded deep-penetration normal-incidence and wide-angle seismic profiles across the Faroe and Hatton Bank volcanic margins in the northeast Atlantic. Here we show that near the Faroe Islands, for every 1 km along strike, 360-400 km(3) of basalt is extruded, while 540-600 km(3) is intruded into the continent-ocean transition. We find that lower-crustal intrusions are focused mainly into a narrow zone approximately 50 km wide on the transition, although extruded basalts flow more than 100 km from the rift. Seismic profiles show that the melt is intruded into the lower crust as sills, which cross-cut the continental fabric, rather than as an 'underplate' of 100 per cent melt, as has often been assumed. Evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures, with no requirement for either significant active small-scale mantle convection under the rift or the presence of fertile mantle at the time of continental break-up, as has previously been suggested for the North Atlantic Ocean.

  6. Lower-crustal intrusion on the North Atlantic continental margin.

    PubMed

    White, R S; Smith, L K; Roberts, A W; Christie, P A F; Kusznir, N J; Roberts, A M; Healy, D; Spitzer, R; Chappell, A; Eccles, J D; Fletcher, R; Hurst, N; Lunnon, Z; Parkin, C J; Tymms, V J

    2008-03-27

    When continents break apart, the rifting is sometimes accompanied by the production of large volumes of molten rock. The total melt volume, however, is uncertain, because only part of it has erupted at the surface. Furthermore, the cause of the magmatism is still disputed-specifically, whether or not it is due to increased mantle temperatures. We recorded deep-penetration normal-incidence and wide-angle seismic profiles across the Faroe and Hatton Bank volcanic margins in the northeast Atlantic. Here we show that near the Faroe Islands, for every 1 km along strike, 360-400 km(3) of basalt is extruded, while 540-600 km(3) is intruded into the continent-ocean transition. We find that lower-crustal intrusions are focused mainly into a narrow zone approximately 50 km wide on the transition, although extruded basalts flow more than 100 km from the rift. Seismic profiles show that the melt is intruded into the lower crust as sills, which cross-cut the continental fabric, rather than as an 'underplate' of 100 per cent melt, as has often been assumed. Evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures, with no requirement for either significant active small-scale mantle convection under the rift or the presence of fertile mantle at the time of continental break-up, as has previously been suggested for the North Atlantic Ocean. PMID:18368115

  7. Some depositional patterns at continental margin of southeastern Mediterranean Sea

    SciTech Connect

    Mart, Y.; Gai, Y.B.

    1982-04-01

    The upper Miocene to Holocene sedimentary strata in the continental margin of the southeastern Mediterranean Sea depict two depositional regimes. The upper Miocene sequence is predominantly evaporitic and forms the southeastern portion of the upper Miocene evaporites present throughout the Mediterranean region. The Pliocene-Quaternary sequence is predominantly detrital and its major source of sediments has been the Nile River. Interpretation of data derived from several multichannel seismic profiles suggested facial variations in the upper Miocene and the Pliocene-Pleistocene formations. Two depositional facies of the upper Miocene evaporites, indicating basinal and shelf depositional environments, were found. Statistical analyses show correlations of the thickness of the evaporites with their interval seismic velocity, their depth, and the present bathymetry, indicating the autochthonous characteristics of the sequence. The basinal and the shelf depositional facies are separated by a transition zone that trends NNE-SSW and is associated with faulting. It is suggested that this zone, commonly known as The Pelusium Line, was the shelf-edge zone during the late Miocene. Facial analysis of the data pertaining to the Pliocene-Pleistocene sequence depicts its allochthonous characteristics. Statistical negative correlation was calculated between the distance from the continental shelf and the thickness of this sequence, indicating its detrital origin. Variations in thicknesses of both formations compared with the bathymetric depths suggest a post-Miocene subsidence of the southeast Mediterranean basin. 6 figures, 4 tables.

  8. North Atlantic Margins: Case studies of Magmatic Continental Breakup

    NASA Astrophysics Data System (ADS)

    Eccles, J. D.; White, R. S.; Christie, P. A. F.

    2012-04-01

    Continental breakup between Europe and Greenland was accompanied by the rapid eruption of the > 1 million cubic kilometres of extruded basalts forming North Atlantic Igneous Province. With episodes of extension in the region dating back to the Devonian, rifting finally proceeded to full breakup and oceanic spreading in the Paleocene. Flood basalt units flowed up to 150 km over pre-existing sedimentary basins, discrete volcanic centres formed and intrusion into the thinned continental crust occurred. Marine seismic investigations utilising industry-leading seismic reflection imaging technologies and large deployments of ocean bottom seismometers across the Faroes and Hatton Bank margins have been used to better resolve margin structure and composition, improving our understanding of breakup processes. Seismic reflection imaging reveals sub-aerial and submarine seaward-dipping reflector sequences tracking the interplay of uplift (transient and permanent), crustal loading through extrusion and ongoing extension. Lower crustal reflectors, cross-cutting the continental fabric and interpreted as intrusions, are observed within the narrow continent-ocean transition. P-wave tomography of wide-angle reflections and refractions, recorded to offsets of up to ~200 km, reveals unusually thick oceanic crust with lower crustal velocities in excess of those expected for MORB compositions. High P-wave velocities are attributed to magnesium-rich compositions which, combined with the large oceanic crustal thickness, would be consistent with an elevated mantle temperature (~150°C higher than 'normal') at the time of breakup. Vp/Vs ratios derived from tomography of converted shear wave phases also support high magnesium melt composition. P-wave velocities and Vp/Vs ratios across the continent-ocean transition show a mixing trend between magnesium-rich gabbroic compositions (100% for oceanic crust) and compositions consistent with the Lewisian gneiss basement or Early Proterozoic

  9. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

    The study region encompasses a set of three basins located at Northeast Brazilian continental margin: Pernambuco (south sector), Paraíba and Natal platform (north sector). These basins were formed during the last stage of separation between South America and African plates during Cretaceous. The continental breakup in these regions occurred probably during the Middle-Upper Albian (~102 m.y). The adjacent basement rocks belong to Borborema Province (BP), which was formed due a complex superposition between Pre-Cambrian orogenic cycles. The structural framework of BP is dominated by large shear zones that divided this province in three main tectonic domains: South, Central and North. The Pernambuco Basin is located in the South Domain and the Paraíba and Natal platform basins are related to the Central Domain. The tectonic and magmatic evolution of the Pernambuco Basin was influenced by oblique rifting (~ 35° to rift axis) and a thermal anomaly probably caused by the Santa Helena hotspot. The north sector represents a continental shelf characterized by basement high with a narrow platform and an abrupt shelf break on transition to the abyssal plain. The continental platform break of this sector was parallel to the rift axis. In this way, we present a regional structural interpretation of these sectors of Brazilian rifted margin based on interpretation and 2D forward modeling of potential field and 2D seismic data. The magnetic maps (Reduction to magnetic pole and Analytic signal) revealed the influence of an alternating pattern of large narrow magnetic and non-magnetic lineaments, oriented NE-SW, E-W and NW-SE. In the Pernambuco Basin these lineaments (NE-SW and E-W) are related to shear zones in the hyperextended basement which is interpreted as a continuation of the granitic-gneissic and metasedimentary rocks of the South Domain of BP. The Paraíba and Natal platform basins show a slight change in the orientation of structures trending E-W (shear zones in

  11. 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.

  12. The development of the continental margin of eastern North America-conjugate continental margin to West Africa

    USGS Publications Warehouse

    Dillon, William P.; Schlee, J.S.; Klitgord, Kim D.

    1988-01-01

    The continental margin of eastern North America was initiated when West Africa and North America were rifted apart in Triassic-Early Jurassic time. Cooling of the crust and its thinning by rifting and extension caused subsidence. Variation in amounts of subsidence led to formation of five basins. These are listed from south to north. (1) The Blake Plateau Basin, the southernmost, is the widest basin and the one in which the rift-stage basement took longest to form. Carbonate platform deposition was active and persisted until the end of Early Cretaceous. In Late Cretaceous, deposition slowed while subsidence persisted, so a deep water platform was formed. Since the Paleocene the region has undergone erosion. (2) The Carolina Trough is narrow and has relatively thin basement, on the basis of gravity modeling. The two basins with thin basement, the Carolina Trough and Scotian Basin, also show many salt diapirs indicating considerable deposition of salt during their early evolution. In the Carolina Trough, subsidence of a large block of strata above the flowing salt has resulted in a major, active normal fault on the landward side of the basin. (3) The Baltimore Canyon Trough has an extremely thick sedimentary section; synrift and postrift sediments exceed 18 km in thickness. A Jurassic reef is well developed on the basin's seaward side, but post-Jurassic deposition was mainly non-carbonate. In general the conversion from carbonate to terrigenous deposition, characteristics of North American Basins, occurred progressively earlier toward the north. (4) The Georges Bank Basin has a complicated deep structure of sub-basins filled with thick synrift deposits. This may have resulted from some shearing that occurred at this offset of the continental margin. Postrift sediments apparently are thin compared to other basins-only about 8 km. (5) The Scotian Basin, off Canada, contains Jurassic carbonate rocks, sandstone, shale and coal covered by deltaic deposits and Upper

  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. Rapid long-wavelength uplift of the Angolan continental margin

    NASA Astrophysics Data System (ADS)

    Kahle, R. L.; Walker, R. T.; Telfer, M.; Buta-Neto, A.; Dee, M.; Shaw-Kahle, B.; Schwenninger, J. L.; Sloan, A.; Tunguno, C.

    2015-12-01

    Studies of offshore sedimentation have identified hiatuses interpreted as periods of uplift along the coast of Angola, SW Africa. However, these studies do not have sufficient temporal or spatial resolution to determine the rate and extent of these uplift events. The presence of a complicated uplift history on a 'stable' passive continental margin is surprising, and many different mechanisms have been proposed to explain it.We quantify the late Quaternary rate and wavelength of uplift relative to sea-level from suites of coastal terraces along the coastline of Angola. Automated extraction and correlation of terrace remnants from digital topography uncovers a symmetrical uplift, with diameter ~1000 km, and centred near Benguela at latitude 13°S. We perform OSL and radiocarbon dating on sediments overlying a coastal terrace at 30 m at Benguela and show that it formed ~40-45 ka. At this time the sea surface was ~50 m lower than today, implying an uplift rate of ~2.3 mm/yr. This is an order of magnitude higher than previously obtained uplift rates (which are averaged over a longer period).The ~1000 km wavelength and observed rapid uplift rates strongly suggest that the uplift is the surface expression of dynamic mantle processes. This rapid uplift rate cannot have persisted continuously over million year timescales, and we therefore suggest that the topography may be modulated by relatively short-lived pulses of uplift, possibly driven by the pulsing of an underlying plume.

  15. Recent carbonate slope development on southwest Florida continental margin

    SciTech Connect

    Brooks, G.R.; Holmes, C.W.

    1987-05-01

    The southwest Florida continental slope bordering the Florida Strait contains a thick sequence of seaward-prograding sediments. Sediments consist principally of a mixture of shallow water and pelagic carbonate sands and muds deposited rapidly on the upper slope. Sedimentary patterns are interpreted to be a function of high-frequency sea level fluctuations. Most vigorous off-shelf transport and highest sedimentation rates (exceeding 2.5 m/1000 years) occur during early transgressions and late regressions when water depths on the shelf are shallow. During sea level highstands, off-shelf transport is less vigorous and sedimentation rates decrease. During sea level lowstands, no off-shelf transport takes place and erosion of the previously deposited sequence occurs as a result of an increase in erosional capacity of the Florida Current. The presence of at least nine such sequences, all with similar characteristics, indicates that these processes have been occurring since at least the late Pleistocene in response to high-frequency glacial fluctuations. The location of the southwest Florida slope between the rimmed Bahama platform and the nonrimmed remainder of the west Florida margin, as well as similarities with ancient carbonate slope deposits formed during periods when shelf-edge reef-forming organisms were lacking, suggest that depositional patterns on the southwest Florida slope may be indicative of a transition between rimmed and nonrimmed carbonate platform environments. The southwest Florida slope may provide a valuable modern analog for identifying similar transitional environments in the geologic record.

  16. Climatic controls on arid continental basin margin systems

    NASA Astrophysics Data System (ADS)

    Gough, Amy; Clarke, Stuart; Richards, Philip; Milodowski, Antoni

    2016-04-01

    Alluvial fans are both dominant and long-lived within continental basin margin systems. As a result, they commonly interact with a variety of depositional systems that exist at different times in the distal extent of the basin as the basin evolves. The deposits of the distal basin often cycle between those with the potential to act as good aquifers and those with the potential to act as good aquitards. The interactions between the distal deposits and the basin margin fans can have a significant impact upon basin-scale fluid flow. The fans themselves are commonly considered as relatively homogeneous, but their sedimentology is controlled by a variety of factors, including: 1) differing depositional mechanisms; 2) localised autocyclic controls; 3) geometrical and temporal interactions with deposits of the basin centre; and, 4) long-term allocyclic climatic variations. This work examines the basin margin systems of the Cutler Group sediments of the Paradox Basin, western U.S.A and presents generalised facies models for the Cutler Group alluvial fans as well as for the zone of interaction between these fans and the contemporaneous environments in the basin centre, at a variety of scales. Small-scale controls on deposition include climate, tectonics, base level and sediment supply. It has been ascertained that long-term climatic alterations were the main control on these depositional systems. Models have been constructed to highlight how both long-term and short-term alterations in the climatic regime can affect the sedimentation in the basin. These models can be applied to better understand similar, but poorly exposed, alluvial fan deposits. The alluvial fans of the Brockram Facies, northern England form part of a once-proposed site for low-level nuclear waste decommissioning. As such, it is important to understand the sedimentology, three-dimensional geometry, and the proposed connectivity of the deposits from the perspective of basin-scale fluid flow. The developed

  17. 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

  18. Long-term topographic feedbacks along glaciated continental margins

    NASA Astrophysics Data System (ADS)

    Egholm, David L.; Jansen, John; Knudsen, Mads F.; Pedersen, Vivi K.; Brædstrup, Christian F.; Ugelvig, Sofie V.; Andersen, Jane L.; Skov, Daniel

    2016-04-01

    Deep troughs carved by glaciers during Quaternary cold intervals dominate high-latitude mountain ranges in Greenland, Norway, Canada, New Zealand, Chile and Antarctica (Sugden & John, 1976). In addition, areas of relatively low relief are typically found at high elevations between the troughs. These high areas are often draped by block-fields, indicating that slow weathering processes dominated their evolution, at least throughout the most recent series of interglacial periods (Ballantyne, 2010). Ice sheets must have repeatedly covered the high areas during glacial maxima, but the ice did not erode the underlying bedrock; most likely because the ice was predominantly frozen to its bed (Kleman & Stroeven, 1997). The geomorphic contrast between the troughs and the high-elevation, low-relief areas shows that factors controlling glacial erosion can fluctuate dramatically across short distances, and that landscapes in cold regions concurrently support some of the fastest and slowest surface-erosion processes on Earth (Sugden & John, 1976). However, the strong contrast in efficiency of subglacial erosion has likely evolved in response to landscape evolution. The gradual emergence of glacial troughs over time establishes a feedback between ice flux and subglacial erosion, leading to focused ice drainage and accelerating erosion in the troughs and stalling erosion in the high-elevation areas between the troughs (Kessler et al. 2008). This distinct pattern of selective erosion along cold-region continental margins during the Late Quaternary may therefore reflect the product of topographical feedbacks that focus glacial erosion over time. We used computational landscape evolution models to study these topographic feedbacks, with the objective to evaluate landscape evolution in the high areas prior to the development of deep glacial troughs. Our experiments suggest that early overriding ice-sheets most likely eroded the highs and smoothed the regional topography before

  19. Petroleum possibilities in continental margin off central Chile

    SciTech Connect

    Gonzalez, E.

    1986-07-01

    The continental margin off central Chile, from Valparaiso to Valdivia, encompassing an area of 100,000 km/sup 2/, has been the target of exploratory activity by Empresa Nacional del Petroleo since 1970. Exploratory drilling began in 1972. By August 1984, total exploratory efforts had resulted in drilling 14 offshore wells and acquiring 12,130 km of seismic reflection lines. A biogenic gas accumulation was discovered in the F well. Because these attempts to find oil were unsuccessful and because drilling costs have escalated, exploratory activities have been curtailed. Forearc basins off central Chile are characterized by low geothermal gradient and a sedimentary filling of Cretaceous and Tertiary strata. Tertiary sequences are characterized by low organic carbon content, immature humic-type organic matter, and a biogenic gas potential. Cretaceous sequences are characterized by higher organic carbon content, good reservoir rocks, and fair to good source rocks. The organic matter is sapropelic, with vitrinite and liptinites, and is favorable for oil and gas generation. Seismic and well data suggest that Mesozoic and Cenozoic sedimentary rock sequences filling the basins (more than 4000 m thick at the shelf edge) extend 40-70 km beyond the present shelf edge. Mesozoic rocks deposited on the slope may generate petroleum and gas that could migrate upslope and accumulate in traps associated with the faulted basement highs and graben-type depressions existing at the shelf edge. This geologic setting favors the development of large petroleum accumulations along the shelf edge and graben on the sedimentary basins off central Chile.

  20. Transition From Rift to Drift at Obliquely Divergent Continental Rifts: the Paired Rio Muni (W Africa) and NE Brazilian Margins

    NASA Astrophysics Data System (ADS)

    Turner, J. P.; Green, P. F.; Wilson, P. G.; Westbrook, G. K.; Lawrence, S.

    2005-12-01

    We develop a synoptic model for the breakup and drift of a major sheared continental margin system: the Rio Muni basin and its NE Brazilian counterpart. It relates the evolution of crustal structure determined from seismic interpretation (including gravity-modelling of the deep-imaging PROBE dataset) to the cooling history of this margin system yielded from thermal history data (mainly apatite fission track analysis - AFTAr - and vitrinite reflectance data). Shear margins initiate as a leaky transform fracture system accommodating the oblique (i.e. non-orthogonal) divergence of opposing rifted continental margins. As such, the transition from continental breakup (i.e. rupture) to continental drift (i.e. ocean opening) at shear margins exhibits significant differences from that of the much better understood normally divergent rifts, where the spreading vector is normal to the strike of the opposing rift margins. For example, unlike at normally divergent rifts, continental breakup and drift are separate episodes in the early evolution of shear margins. In Rio Muni-NE Brazil, they are recorded by separate breakup and drift unconformities spanning a 15-20Ma. interval, the time taken for the ocean ridge to traverse the length of the margin before a continuous arm of oceanic crust separated Rio Muni from its Brazilian counterpart. In the Rio Muni basin, the c.70km-wide Ascension Fracture Zone (AFZ) exhibits oblique-slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole-lithosphere failure and continental breakup 117Ma. Gravity-modelling of PROBE seismic profiles reveals a land-locked precursor oceanic basin that preceded full ocean opening and which subsequently was stranded on the African margin. Its existence supports the idea of multiple rift suture lines accommodating episodic breakup and it emphasizes the significance of separate breakup and drift episodes at shear margins

  1. A review of the geology and petroleum possibilities of the continental margins of India

    SciTech Connect

    Sahay, B.

    1984-05-01

    Continental margins which flank western and eastern limits of India are broadly divided into West and East Coast. The width of the Western Continental Shelf varies from 300 km near Bombay to approximately 80 kms near Porbandar, narrowing further towards south to 60 kms near Cochin. The Eastern Continental Shelf which extends from Cape Comorin to Calcutta, has the width varying from 2.5 km, about 150 kms south of Madras to 210 kms upto Ganges river delta, near Calcutta.

  2. The role of rifting in the evolution of the continental margin of Eastern Asia: Geophysical evidence

    NASA Astrophysics Data System (ADS)

    Rodnikov, A. G.; Rodnikova, R. D.; Zorina, Yu. G.

    1992-08-01

    The role of rift processes is analysed in the structural evolution of the continental margins of Eastern Asia including the Indo-China Peninsula and North China plain. Paleoreconstructions were made for the Indo-China Peninsula to characterize individual stages of rifting covering the Late Cretaceous-Eocene, Oligocene-Middle Miocene and Late Pliocene-Early Quaternary epochs. The rifting of continental margins occurred synchronously with spreading processes in marginal seas, whereas the formation of rift structures in the North China plain was concurrent with the formation of a deep-water basin of the Philippine Sea. The development of asthenospheric diapire led to crustal extension and was responsible for the formation of rift structures in marginal seas and continental margins.

  3. Continental Margins of the Arctic Ocean: Implications for Law of the Sea

    NASA Astrophysics Data System (ADS)

    Mosher, David

    2016-04-01

    A coastal State must define the outer edge of its continental margin in order to be entitled to extend the outer limits of its continental shelf beyond 200 M, according to article 76 of the UN Convention on the Law of the Sea. The article prescribes the methods with which to make this definition and includes such metrics as water depth, seafloor gradient and thickness of sediment. Note the distinction between the "outer edge of the continental margin", which is the extent of the margin after application of the formula of article 76, and the "outer limit of the continental shelf", which is the limit after constraint criteria of article 76 are applied. For a relatively small ocean basin, the Arctic Ocean reveals a plethora of continental margin types reflecting both its complex tectonic origins and its diverse sedimentation history. These factors play important roles in determining the extended continental shelves of Arctic coastal States. This study highlights the critical factors that might determine the outer edge of continental margins in the Arctic Ocean as prescribed by article 76. Norway is the only Arctic coastal State that has had recommendations rendered by the Commission on the Limits of the Continental Shelf (CLCS). Russia and Denmark (Greenland) have made submissions to the CLCS to support their extended continental shelves in the Arctic and are awaiting recommendations. Canada has yet to make its submission and the US has not yet ratified the Convention. The various criteria that each coastal State has utilized or potentially can utilize to determine the outer edge of the continental margin are considered. Important criteria in the Arctic include, 1) morphological continuity of undersea features, such as the various ridges and spurs, with the landmass, 2) the tectonic origins and geologic affinities with the adjacent land masses of the margins and various ridges, 3) sedimentary processes, particularly along continental slopes, and 4) thickness and

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

    PubMed

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

    2001-09-13

    The rifting of continents involves faulting (tectonism) and magmatism, which reflect the strain-rate and temperature dependent processes of solid-state deformation and decompression melting within the Earth. Most models of this rifting have treated tectonism and magmatism separately, and few numerical simulations have attempted to include continental break-up and melting, let alone describe how continental rifting evolves into seafloor spreading. Models of this evolution conventionally juxtapose continental and oceanic crust. Here we present observations that support the existence of a zone of exhumed continental mantle, several tens of kilometres wide, between oceanic and continental crust on continental margins where magma-poor rifting has taken place. We present geophysical and geological observations from the west Iberia margin, and geological mapping of margins of the former Tethys ocean now exposed in the Alps. We use these complementary findings to propose a conceptual model that focuses on the final stage of continental extension and break-up, and the creation of a zone of exhumed continental mantle that evolves oceanward into seafloor spreading. We conclude that the evolving stress and thermal fields are constrained by a rising and narrowing ridge of asthenospheric mantle, and that magmatism and rates of extension systematically increase oceanward. PMID:11557977

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

    PubMed

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

    2001-09-13

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

  6. 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.

  7. Seismic-reflection signature of cretaceous continental breakup on the wilkes land margin, antarctica.

    PubMed

    Eittreim, S L; Hampton, M A; Childs, J R

    1985-09-13

    The passive (rifted) continental margin of Wilkes Land, Antarctica, is characterized on seismic reflection records by (i) in the south, a block-faulted sequence of highly stratified continental beds overlain by two distinct unconformities; (ii) a transitional, greatly thinned continental crust overlain by material interpreted to be flood basalt; and (iii) in the north, oceanic crust with a boundary ridge at its edge. The Mohorovicić discontinuity can be followed across the continent-ocean boundary and shows a progressive thinning of continental crust to a minimum of 2.5 kilometers at its northern edge.

  8. 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.

  9. 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).

  10. The Cryogenian intra-continental rifting of Rodinia: Evidence from the Laurentian margin in eastern North America

    NASA Astrophysics Data System (ADS)

    McClellan, Elizabeth; Gazel, Esteban

    2014-10-01

    The geologic history of the eastern North American (Laurentian) margin encompasses two complete Wilson cycles that brought about the assembly and subsequent disaggregation of two supercontinents, Rodinia and Pangea. In the southern and central Appalachian region, basement rocks were affected by two episodes of crustal extension separated by > 100 m.y.; a Cryogenian phase spanning the interval 765-700 Ma and an Ediacaran event at ~ 565 Ma. During the Cryogenian phase, the Mesoproterozoic continental crust was intruded by numerous A-type felsic plutons and extensional mafic dikes. At ~ 760-750 Ma a bimodal volcanic sequence erupted onto the uplifted and eroded basement. This sequence, known as the Mount Rogers Formation (MRF), comprises a bimodal basalt-rhyolite lower section and an upper section of dominantly peralkaline rhyolitic sheets. Here, we provide new geochemical evidence from the well-preserved volcanic rocks of the Cryogenian lower MRF, with the goal of elucidating the process that induced the initial stage of the break-up of Rodinia and how this affected the evolution of the eastern Laurentian margin. The geochemical compositions of the Cryogenian lavas are remarkably similar to modern continental intra-plate settings (e.g., East African Rift, Yellowstone-Snake River Plain). Geochemical, geophysical and tectonic evidence suggests that the common denominator controlling the melting processes in these settings is deep mantle plume activity. Thus, evidence from the MRF suggests that the initial phase of extension of the Laurentian margin at ~ 760-750 Ma was possibly triggered by mantle plume activity. It is possible that lithospheric weakness caused by a mantle plume that impacted Rodinia triggered the regional extension and produced the intra-continental rifting that preceded the breakup of the Laurentian margin.

  11. 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.

  12. Manganese and copper fluxes from continental margin sediments

    SciTech Connect

    Heggie, D.; Klinkhammer, G.; Cullen, D.

    1987-05-01

    Total dissolvable Cu and Mn have been measured in sea water collected from the continental shelf of the eastern Bering Sea. Copper concentrations of <3 nmole kg/sup -1/ were measured over the shelf break but concentrations increased to >4 nmole kg/sup -1/ inshore of a hydrographic front over the 100 m isobath. Manganese concentrations also were low over the shelf break, <10 nmole kg/sup -1/, and increased systematically to concentrations >10 nmole kg/sup -1/ inshore of the hydrographic front. Depth distributions of Mn at all continental shelf stations showed gradients into the sediments, with concentrations typically >20 nmole kg/sup -1/ in a bottom layer extending about 30 m off the bottom. Benthic Cu and Mn fluxes are indicated by cross-shelf pore water profiles that show interfacial concentrations more than an order of magnitude greater than in bottom water. These data and the results of a model of metal transport across the shelf suggest that Cu and Mn fluxes, estimated at 2 and 18 nmole cm/sup -2/y/sup -1/, respectively, from continental shelf sediments may be one source of these metals to the deep sea.

  13. Geologic development and characteristics of continental margins, Gulf of Mexico

    SciTech Connect

    Coleman, J.M.; Prior, D.B.; Roberts, H.H.

    1986-09-01

    The continental slope of the Gulf basin covers more than 500,000 km/sup 2/ and consists of smooth and gently sloping surfaces, prominent escarpments, knolls, intraslope basins, and submarine canyons and channels. It is an area of extremely diverse topographic and sedimentologic conditions. The slope extends from the shelf break, roughly at the 200-m isobath, to the upper limit of the continental rise at a depth of 2800 m. The most complex province in the basin, and the one of most interest to the petroleum industry, is the Texas-Louisiana slope, occupying 120,000 km/sup 2/ and in which bottom slopes range from less than 1/sup 0/ to greater than 20/sup 0/ around the knolls and basins. The near-surface geology and topography of the slope is a function of the interplay between episodes of rapid shelf-edge and slope progradation and contemporaneous modification of the depositional sequence by diapirism. Development of discrete depocenters throughout the Neogene results in rapid shelf-edge progradation, often exceeding 15-20 km/m.y. This rapid progradation of the shelf edge leads to development of thick wedges of sediment accumulation on the continental slope. Slope oversteepening, high pore pressures in rapidly deposited soft sediments, and changes in eustatic sea level cause subaqueous slope instabilities such as landslides and debris flows. Large-scale features such as shelf-edge separation scars and landslide-related canyons often result from such processes.

  14. The influence of oceanic fracture zones on the segmentation of continental margins and the evolution of intra-continental rift systems: Case studies from the Atlantic

    NASA Astrophysics Data System (ADS)

    Masterton, S.; Fairhead, J. D.; Green, C. M.

    2012-12-01

    fracture zones do not extend up to the offshore Angolan and conjugate Brazilian margins, we conclude that small offset transform faulting did not influence the evolution of the continental margin as has been previously suggested. On a regional scale, the evolution of the Africa-wide Mesozoic rift system is intimately linked to global plate tectonics and to changes in plate interactions. On a basinal scale, changes in the orientation of the dominant stress field resulting from plate reorganisation have had a clear impact on the deformation history and fault geometries of rift basins. We demonstrate this relationship by correlating the timing of changes in South Atlantic fracture zone geometries and African margin unconformities with major unconformities that are observed in a unified stratigraphy chart for the West and Central African Rift System. We propose a controlling mechanism in which changes in plate stress control the effective elastic strength of a plate, resulting in a focused change in isostatic response over continental margins.

  15. 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

  16. 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

  17. Manganese flux from continental margin sediments in a transect through the oxygen minimum.

    PubMed

    Johnson, K S; Berelson, W M; Coale, K H; Coley, T L; Elrod, V A; Fairey, W R; Iams, H D; Kilgore, T E; Nowicki, J L

    1992-08-28

    The flux of manganese from continental margin sediments to the ocean was measured with a free-vehicle, benthic flux chamber in a transect across the continental shelf and upper slope of the California margin. The highest fluxes were observed on the shallow continental shelf. Manganese flux decreased linearly with bottom water oxygen concentration, and the lowest fluxes occurred in the oxygen minimum zone (at a depth of 600 to 1000 meters). Although the flux of manganese from continental shelf sediments can account for the elevated concentrations observed in shallow, coastal waters, the flux from sediments that intersect the oxygen minimum cannot produce the subsurface concentration maximum of dissolved manganese that is observed in the Pacific Ocean.

  18. Structure and petroleum potential of the Yakutat segment of the northern Gulf of Alaska continental margin

    SciTech Connect

    Bruns, T.R.

    1983-01-01

    This report discusses the structure, geologic history, and petroleum potential of the Yakutat segment, the part of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska. As part of a program of geological and geophysical investigations of the continental margin in the northern Gulf of Alaska, the US Geological Survey collected multichannel seismic reflection data along about 2000 km of tracklines in the study area during 1975, 1977, and 1978. In addition, dredge samples from the continental slope were acquired during the 1977, 1978, and 1979 field seasons. The first part of this paper presents an interpretation of the seismic reflection and refraction data, including structure contour maps, isopach maps, and interpreted seismic sections; the second part is a discussion of the implications for petroleum potential. The primary area of interest is the continental shelf and slope, but some data from strata at the base of the slope are also included.

  19. Manganese flux from continental margin sediments in a transect through the oxygen minimum

    SciTech Connect

    Johnson, K.S. Monterey Bay Aquarium Research Inst., Pacific Grove, CA ); Berelson, W.M.; Iams, H.D.; Kilgore, T.E. ); Coale, K.H.; Coley, T.L.; Elrod, V.A.; Fairey, W.R.; Nowicki, J.L. )

    1992-08-28

    The flux of manganese from continental margin sediments to the ocean was measured with a free-vehicle, benthic flux chamber in a transect across the continental shelf and upper slope of the California margin. The highest fluxes were observed on the shallow continental shelf. Manganese flux decreased linearly with bottom water oxygen concentration, and the lowest flux occurred in the oxygen minimum zone (at a depth of 600 to 1,000 meters). Although the flux of manganese from continental shelf sediments can account for the elevated concentrations observed in shallow, coastal waters, the flux from sediments that intersect the oxygen minimum cannot produce the subsurface concentration maximum of dissolved manganese that is observed in the Pacific Ocean.

  20. Preface - 'Biogeochemistry-ecosystem interaction on changing continental margins in the Anthropocene'

    NASA Astrophysics Data System (ADS)

    Liu, K.-K.; Emeis, Kay-Christian; Levin, Lisa A.; Naqvi, Wajih; Roman, Michael

    2015-01-01

    This special issue is a product of Workshop 1 of IMBIZO III held in Goa, India in January 2013 (Bundy et al., 2013). This IMBIZO (a Zulu word for gathering) has been organized by IMBER (Integrated Marine Biogeochemistry and Ecosystem Research) biannually since 2008. It employs a format of three concurrent but interacting workshops designed to synthesize information on topical research areas in marine science. Workshop 1 addressed the issue, "Biogeochemistry-ecosystem interaction in changing continental margins," which belongs to the purview of the Continental Margins Working Group (CMWG), co-sponsored by IMBER and LOICZ (Land-Ocean Interaction in the Coastal Zone). As a way to explore the emerging issues that concern the CMWG, the workshop had attracted 25 talks and 18 posters that explored the following topics: Human impacts on continental margins

  1. Geodynamic settings of microcontinents, non-volcanic islands and submerged continental marginal plateau formation

    NASA Astrophysics Data System (ADS)

    Dubinin, Evgeny; Grokholsky, Andrey; Makushkina, Anna

    2016-04-01

    Complex process of continental lithosphere breakup is often accompanied by full or semi isolation of small continental blocks from the parent continent such as microcontinents or submerged marginal plateaus. We present different types of continental blocks formed in various geodynamic settings. The process depends on thermo-mechanical properties of rifting. 1) The continental blocks fully isolated from the parent continent. This kind of blocks exist in submerged form (Elan Bank, the Jan-Mayen Ridge, Zenith Plateau, Gulden Draak Knoll, Batavia Knoll) and in non-submerged form in case of large block size. Most of listed submerged blocks are formed in proximity of hot-spot or plume. 2) The continental blocks semi-isolated from the parent continent. Exmouth Plateau, Vøring, Agulhas, Naturaliste are submerged continental plateaus of the indicated category; Sri Lanka, Tasmania, Socotra are islands adjacent to continent here. Nowadays illustration of this setting is the Sinai block located between the two continental rifts. 3) The submerged linear continental blocks formed by the continental rifting along margin (the Lomonosov Ridge). Suggested evolution of this paragraph is the rift propagation along existing transtensional (or another type) transform fault. Future example of this type might be the California Peninsula block, detached from the North American plate by the rifting within San-Andreas fault. 4) The submerged continental blocks formed by extensional processes as the result of asthenosphere flow and shear deformations. Examples are submerged blocks in the central and southern Scotia Sea (Terror Bank, Protector Basin, Discovery Bank, Bruce Bank etc.). 5) The continental blocks formed in the transform fault systems originated in setting of contradict rifts propagation in presence of structure barriers, rifts are shifted by several hundreds kilometers from each other. Examples of this geodynamic setting are Equatorial Atlantic at the initial development stage

  2. Tectonic framework of the Northern California continental margin

    USGS Publications Warehouse

    Clarke, S.H.

    1992-01-01

    The northern coast of California is one of the most seismically active regions in the continental United States. This activity is largely due to tectonic forces resulting from differing relative motions between three extensive lithospheric plates that meet in this region. These crustal plates are bounded by long fault systems-the Cascadia subduction zone, the San Andreas fault system, and the Mendocino fault- that accommodate these differences in plate motion and that are capable of periodically producing damaging earthquakes. Historic earthquake locations are concentrated in the victinity of the tectonically unstable intersection of these tthree plates and their bounding fault systems. 

  3. Illustrations of the importance of mass wasting in the evolution of continental margins

    SciTech Connect

    Pratson, L.; Ryan, W. ); Twichell, D. )

    1990-05-01

    Side-looking sonar imagery and swath bathymetry from a variety of contemporary continental slopes all display erosional scars and debris aprons, illustrating the importance of mass wasting in the evolution of continental margins. The continental slopes examined include slopes fed directly from the fronts of ice sheets, slopes adjacent to continental shelves that were the sites of glacial outwash, slopes supplied exclusively by fluvial drainage, slopes at carbonate platforms, and slopes on accretionary prisms. Examples are drawn from the Atlantic Ocean, the Gulf of Mexico, and the Mediterranean Sea in both passive and active continental margin settings. The sonar imagery and bathymetry used in this study indicate that continental slopes in different tectonic and climatic environments show similar forms of mass wasting. However, in some cases the dominant mode of erosion and/or the overall degree of mass wasting appears to be distinct to particular sedimentary environments. Timing of both recent and older exhumed erosional surfaces identified in the imagery and in seismic reflection profiles is obtained by ground truth observations using submersibles, towed camera sleds, drilling, and coring. These observations suggest that eustatic fluctuations common to all the margins examined do not explain the range in magnitude and areal density of the observed mass wasting. More localized factors such as lithology, diagenesis, pore fluid conditions, sediment supply rates, and seismic ground motion appear to have a major influence in the evolution of erosional scars and their corresponding unconformities.

  4. 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

  5. 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.

  6. Evidence of a dense water vein along the Libyan continental margin

    NASA Astrophysics Data System (ADS)

    Gasparini, G. P.; Bonanno, A.; Zgozi, S.; Basilone, G.; Borghini, M.; Buscaino, G.; Cuttitta, A.; Essarbout, N.; Mazzola, S.; Patti, B.; Ramadan, A. B.; Schroeder, K.; Bahri, T.; Massa, F.

    2008-02-01

    For the first time it was possible to investigate a still poorly known region of the eastern Mediterranean Sea, the Libyan continental margin. An oceanographic cruise, performed during summer 2006, revealed an important and novel feature: a dense vein flowing along the continental slope. The paper describes the vein evolution with some insights on its dynamic and furnishes an estimate of its transport, which results to be comparable with the Adriatic Deep Water production rate. The cascading into a steep canyon which incises the continental shelf suggests that the vein may play an important role in ventilating the deep layers of the Ionian Sea.

  7. 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.

  8. 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.

  9. Internal tidal mixing as a control on continental margin ecosystems

    NASA Astrophysics Data System (ADS)

    Sharples, Jonathan; Moore, C. Mark; Hickman, Anna E.; Holligan, Patrick M.; Tweddle, Jacqueline F.; Palmer, Matthew R.; Simpson, John H.

    2009-12-01

    We show that a breaking internal tide at a shelf edge is a fundamental control on the structural and functional properties of ecosystems. Contrasts in vertical mixing of nitrate between the shelf and the open ocean correspond with horizontal and vertical changes in phytoplankton communities, with largest cells found in surface waters at the shelf edge. Intense fishing activity is commonly seen at continental shelf edges, targeting spawning fish stocks. We suggest that the internal tide, a globally ubiquitous physical process at steep shelf edge bathymetry, supports shelf edge fisheries by providing large-celled phytoplankton for first-feeding fish larvae. The repeatability of the internal tide removes fish from the need to time spawning with a spring bloom. Also, with large phytoplankton cells dominating particulate organic carbon export, the internal tides could be an important influence on spatial and temporal variability in patterns of global carbon sequestration in deep water and sediments.

  10. Convergent tectonics and coastal upwelling: a history of the Peru continental margin ( Pacific).

    USGS Publications Warehouse

    Von Huene, R.; Suess, E.; Emeis, K.C.

    1987-01-01

    Late in 1986, scientists on the ODP drillship JOIDES Resolution confirmed that the upper slope of the Peruvian margin consists of continental crust whereas the lower slope comprises an accretionary complex. An intricate history of horizontal and vertical movements can be detected, and the locations of ancient centers of upwelling appear to have varied, partly due to tectonic movements of the margin. In this review of Leg 112, the 3 scientific leaders on this cruise discuss their results. -from Journal Editor

  11. Fluid seepage at the continental margin offshore Costa Rica and southern Nicaragua

    NASA Astrophysics Data System (ADS)

    Sahling, Heiko; Masson, Douglas G.; Ranero, CéSar R.; Hühnerbach, Veit; Weinrebe, Wilhelm; Klaucke, Ingo; Bürk, Dietmar; Brückmann, Warner; Suess, Erwin

    2008-05-01

    A systematic search for methane-rich fluid seeps at the seafloor was conducted at the Pacific continental margin offshore southern Nicaragua and northern central Costa Rica, a convergent margin characterized by subduction erosion. More than 100 fluid seeps were discovered using a combination of multibeam bathymetry, side-scan sonar imagery, TV-sled observations, and sampling. This corresponds, on average, to a seep site every 4 km along the continental slope. In the northwestern part of the study area, subduction of oceanic crust formed at the East Pacific Rise is characterized by pervasive bending-induced faulting of the oceanic plate and a relatively uniform morphology of the overriding continental margin. Seepage at this part of the margin typically occurs at approximately cone-shaped mounds 50 - 100 m high and up to 1 km wide at the base. Over 60 such mounds were identified on the 240 km long margin segment. Some normal faults also host localized seepage. In contrast, in the southeast, the 220 km long margin segment overriding the oceanic crust formed at the Cocos-Nazca Spreading Centre has a comparatively more irregular morphology caused mainly by the subduction of ridges and seamounts sitting on the oceanic plate. Over 40 seeps were located on this part of the margin. This margin segment with irregular morphology exhibits diverse seep structures. Seeps are related to landslide scars, seamount-subduction related fractures, mounds, and faults. Several backscatter anomalies in side-scan images are without apparent relief and are probably related to carbonate precipitation. Detected fluid seeps are not evenly distributed across the margin but occur in a roughly margin parallel band centered 28 ± 7 km landward of the trench. This distribution suggests that seeps are possibly fed to fluids rising from the plate boundary along deep-penetrating faults through the upper plate.

  12. Organic geochemistry of continental margin and deep ocean sediments

    SciTech Connect

    Whelan, J.K.; Hunt, J.M.; Eglinton, T.; Dickinson, P.; Johnson, C.; Buxton, L.; Tarafa, M.E.

    1990-08-01

    The objective of this research continues to be the understanding of the complex processes of fossil fuel formation and migration. DOE funded research to date has focused on case histories'' of down-hole well profiles of light hydrocarbons, pyrograms, pyrolysis-GC and -GCMS parameters, and biomarker data from wells in the Louisiana and Texas Gulf Coasts the Alaskan North Slope. In the case of the Alaskan North Slope, geological data and one-dimensional maturation modeling have been integrated in order to better constrain possible source rocks, timing, and migration routes for oil and gas generation and expulsion processes.This period, biomarker analyses and organic petrographic analyses were completed for the Ikpikpuk well. In the case of the Gulf Coast, we have obtained a one-dimensional maturation model of the Cost B-1 well in E. Cameron field of the Louisiana Gulf Coast. The completed E. Cameron data set adds to the enigma of the Gulf Coast oils found on the continental shelf of Louisiana. If significant quantities of the oil are coming from relatively organic lean Tertiary rocks, then non-conventional'' expulsion and migration mechanisms, such as gas dissolved in oil must be invoked to explain the Gulf Coast oils reservoired on the Louisiana continental shelf. We are designing and starting to assemble a hydrous pyrolysis apparatus to follow, the laboratory, rates of generation and expulsion of sediment gases. Initiation of some new research to examine {delta}{sup 13}C of individual compounds from pyrolysis is also described. We are beginning to examine both the laboratory and field data from the Gulf Coast in the context of a Global Basin Research Network (GBRN). The purpose is to better understand subsurface fluid flow processes over geologic time in sedimentary basins and their relation to resource accumulation (i.e., petroleum and metal ores). 58 refs.

  13. Extension on rifted continental margins: Observations vs. models.

    NASA Astrophysics Data System (ADS)

    Skogseid, Jakob

    2014-05-01

    Mapping the signature of extensional deformation on rifted margins is often hampered by thick sedimentary or volcanic successions, or because salt tectonics makes sub-salt seismic imaging challenging. Over the past 20 years the literature is witnessing that lack of mapable faults have resulted in a variety of numerical models based on the assumption that the upper crust takes little or no extensional thinning, while the observed reduction of crustal thickness is taken up in the middle and lower crust, as well as in the mantle. In this presentation two case studies are used to highlight the difference that 3D seismic data may have on our understanding. The small patches of 3D resolution data allow us to get a glance of the 'real' signature of extensional faulting, which by analogy can be extrapolate from one margin segment to the next. In the South Atlantic salt tectonics represents a major problem for sub-salt imaging. The conjugate margins of Brazil and Angola are, however, characterized by pronounced crustal thinning as documented by crustal scale 2D reflection and refraction data. Off Angola the 3D 'reality' demonstrates that upper crustal extension by faulting is comparable to the full crustal, as well as lithospheric thinning as derived from refraction data and basin subsidence analysis. The mapped faults are listric low angle faults that seem to detach at mid crustal levels. 2D seismic has in the past been interpreted to indicate that almost no extensional faulting can be mapped towards the base of the so-called 'sag basin'. The whole concept of the 'sag basin', often ascribed to as crustal thinning without upper crustal deformation, is in fact related to this 'lack of observation', and furthermore, have caused the making of different types of dynamic models attempting to account for this. In the NE Atlantic significant Paleocene extensional faulting is locally seen adjacent to the 50 to more than 200 km wide volcanic cover on each side of the breakup axis

  14. 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

  15. The geodynamic province of transitional crust adjacent to magma-poor continental margins

    NASA Astrophysics Data System (ADS)

    Sibuet, J.; Tucholke, B. E.

    2011-12-01

    Two types of 'transitional crust' have been documented along magma-poor rifted margins. One consists of apparently sub-continental mantle that has been exhumed and serpentinized in a regime of brittle deformation during late stages of rifting. A second is highly thinned continental crust, which in some cases is known to have been supported near sea level until very late in the rift history and thus is interpreted to reflect depth-dependent extension. In both cases it is typically assumed that formation of oceanic crust occurs shortly after the breakup of brittle continental crust and thus that the transitional crust has relatively limited width. We here examine two representative cases of transitional crust, one in the Newfoundland-Iberia rift (exhumed mantle) and one off the Angola-Gabon margin (highly thinned continental crust). Considering the geological and geophysical evidence, we propose that depth-dependent extension (riftward flow of weak lower/middle continental crust and/or upper mantle) may be a common phenomenon on magma-poor margins and that this can result in a much broader zone of transitional crust than has hitherto been assumed. Transitional crust in this extended zone may consist of sub-continental mantle, lower to middle continental crust, or some combination thereof, depending on the strength profile of the pre-rift continental lithosphere. Transitional crust ceases to be emplaced (i.e., final 'breakup' occurs) only when emplacement of heat and melt from the rising asthenosphere becomes dominant over lateral flow of the weak lower lithosphere. This model implies a two-stage breakup: first the rupture of the brittle upper crust and second, the eventual emplacement of oceanic crust. Well-defined magnetic anomalies can form in transitional crust consisting of highly serpentinized, exhumed mantle, and they therefore are not diagnostic of oceanic crust. Where present in transitional crust, these anomalies can be helpful in interpreting the rifting

  16. 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.

  17. Block glides offshore Newport Beach, Southern California continental margin

    SciTech Connect

    Greene, H.G.; Clarke, S.H. Jr.; Kennedy, M.P.

    1988-01-01

    The continental slope offshore Newport Beach, California, is characterized by a relatively gentle (approximately 1/sup 0/) grade and is dissected by numerous channels and canyons, of which the most conspicuous is Newport Canyon. An unusual series of block-glide landslides have developed on this lope adjacent to many of these channels. Locally, secondary channels that develop along pull-apart fractures between the slide blocks may service as conduits for downslope sediment movement. A detailed seismic-reflection survey of the area shows that the slope is underlain by soft water-saturated unstable sediment of Quaternary age. The block-glides lie wholly within this sediment; displaced blocks appear to have moved only a short distance downslope and are preserved as intact masses that exhibit downward increasing internal deformation. This deformation reaches a maximum near the front of the displaced mass and in basal beds nearest the slip surface. The morphology of the blocks and their intervening channellike erosional scarps is similar to that of glacial blocks and their associated bergschrunds. The formation of new scarps and the widening of channels formed as pull-aparts by the ongoing process of block movement may contribute to headward erosion and widening of Newport Canyon and its tributaries. Slope failure might be greatly enhanced by strong ground motion associated with nearby earthquakes. The authors suspect that renewed movement occurs on these blocks during major seismic events on the nearby Newport-Inglewood fault (e.g., 1933 M/sub L/ 6.3 event).

  18. Deep-sea Lebensspuren of the Australian continental margins

    NASA Astrophysics Data System (ADS)

    Przeslawski, Rachel; Dundas, Kate; Radke, Lynda; Anderson, Tara J.

    Much of the deep sea comprises soft-sediment habitats dominated by comparatively low abundances of species-rich macrofauna and meiofauna. Although often not observed, these animals bioturbate the sediment during feeding and burrowing, leaving signs of their activities called Lebensspuren ('life traces'). In this study, we use still images to quantify Lebensspuren from the eastern (1921 images, 13 stations, 1300-2200 m depth) and western (1008 images, 11 stations, 1500-4400 m depth) Australian margins using a univariate measure of trace richness and a multivariate measure of Lebensspuren assemblages. A total of 46 Lebensspuren types were identified, including those matching named trace fossils and modern Lebensspuren found elsewhere in the world. Most traces could be associated with waste, crawling, dwellings, organism tests, feeding, or resting, but the origin of 15% of trace types remains unknown. Assemblages were significantly different between the two regions and depth profiles, with five Lebensspuren types accounting for over 95% of the differentiation (ovoid pinnate trace, crater row, spider trace, matchstick trace, mesh trace). Lebensspuren richness showed no strong relationships with depth, total organic carbon, or mud, although there was a positive correlation to chlorin index (i.e., organic freshness) in the eastern margin, with richness increasing with organic freshness. Lebensspuren richness was not related to epifauna either, indicating that epifauna may not be the primary source of Lebensspuren. Despite the abundance and distinctiveness of several traces both in the current and previous studies (e.g., ovoid pinnate, mesh, spider), their origin and distribution remains a mystery. We discuss this and several other considerations in the identification and quantification of Lebensspuren. This study represents the first comprehensive catalogue of deep-sea Lebensspuren in Australian waters and highlights the potential of Lebensspuren as valuable and often

  19. New Insight Into The Crustal Structure of The Continental Margin Off NW Sabah/borneo

    NASA Astrophysics Data System (ADS)

    Barckhausen, U.; Franke, D.; Behain, D.; Meyer, H.

    The continental margin offshore NW Sabah/Borneo (Malaysia) has been investigated with reflection and refraction seismics, magnetics, and gravity during the recent cruise BGR01-POPSCOMS. A total of 4000 km of geophysical profiles has been acquired, thereof 2900 km with reflection seismics. Like in major parts of the South China Sea, the area seaward of the Sabah Trough consists of extended continental lithosphere. We found evidence that the continental crust also underlies the continental slope land- ward of the Trough, a fact that raises many questions about the tectonic history and development of this margin. The characteristic pattern of rotated fault blocks and half grabens and the carbon- ates which are observed all over the Dangerous Grounds can be traced a long way landward of the Sabah Trough beneath the sedimentary succession of the upper plate. The magnetic anomalies which are dominated by the magnetic signatures of relatively young volcanic features also continue under the continental slope. The sedimentary rocks of the upper plate, in contrast, seem to generate hardly any magnetic anoma- lies. We suspect that the volcanic activity coincided with the collision of Borneo and the Dangerous Grounds in middle or late Miocene time. The emplacement of an al- lochtonous terrane on top of the extended continental lithosphere could be explained by overthrusting as a result of the collision or it could be related to gravity sliding following a broad uplift of NW Borneo at the same time.

  20. The continental margin is a key source of iron to the HNLC North Pacific Ocean

    SciTech Connect

    Lam, P.J.; Bishop, J.K.B

    2008-01-15

    Here we show that labile particulate iron and manganese concentrations in the upper 500m of the Western Subarctic Pacific, an iron-limited High Nutrient Low Chlorophyll (HNLC) region, have prominent subsurface maxima between 100-200 m, reaching 3 nM and 600 pM, respectively. The subsurface concentration maxima in particulate Fe are characterized by a more reduced oxidation state, suggesting a source from primary volcagenic minerals such as from the Kuril/Kamchatka margin. The systematics of these profiles suggest a consistently strong lateral advection of labile Mn and Fe from redox-mobilized labile sources at the continental shelf supplemented by a more variable source of Fe from the upper continental slope. This subsurface supply of iron from the continental margin is shallow enough to be accessible to the surface through winter upwelling and vertical mixing, and is likely a key source of bioavailable Fe to the HNLC North Pacific.

  1. 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.

  2. Evolution of the continental margin of southern Spain and the Alboran Sea

    USGS Publications Warehouse

    Dillon, William P.; Robb, James M.; Greene, H. Gary; Lucena, Juan Carlos

    1980-01-01

    Seismic reflection profiles and magnetic intensity measurements were collected across the southern continental margin of Spain and the Alboran basin between Spain and Africa. Correlation of the distinct seismic stratigraphy observed in the profiles to stratigraphic information obtained from cores at Deep Sea Drilling Project site 121 allows effective dating of tectonic events. The Alboran Sea basin occupies a zone of motion between the African and Iberian lithospheric plates that probably began to form by extension in late Miocene time (Tortonian). At the end of Miocene time (end of Messinian) profiles show that an angular unconformity was cut, and then the strata were block faulted before subsequent deposition. The erosion of the unconformity probably resulted from lowering of Mediterranean sea level by evaporation when the previous channel between the Mediterranean and Atlantic was closed. Continued extension probably caused the block faulting and, eventually the opening of the present channel to the Atlantic through the Strait of Gibraltar and the reflooding of the Mediterranean. Minor tectonic movements at the end of Calabrian time (early Pleistocene) apparently resulted in minor faulting, extensive transgression in southeastern Spain, and major changes in the sedimentary environment of the Alboran basin. Active faulting observed at five locations on seismic profiles seems to form a NNE zone of transcurrent movement across the Alboran Sea. This inferred fault trend is coincident with some bathymetric, magnetic and seismicity trends and colinear with active faults that have been mapped on-shore in Morocco and Spain. The faults were probably caused by stresses related to plate movements, and their direction was modified by inherited fractures in the lithosphere that floors the Alboran Sea.

  3. Molecular diversity of sulfate-reducing bacteria from two different continental margin habitats.

    PubMed

    Liu, Xueduan; Bagwell, Christopher E; Wu, Liyou; Devol, Allan H; Zhou, Jizhong

    2003-10-01

    This study examined the natural diversity and distributions of sulfate-reducing bacteria along a natural carbon gradient extending down the shelf-slope transition zone of the eastern Pacific continental margin. Dissimilatory (bi)sulfite reductase gene sequences (dsrAB) were PCR amplified and cloned from five different sampling sites, each at a discrete depth, from two different margin systems, one off the Pacific coast of Mexico and another off the coast of Washington State. A total of 1,762 clones were recovered and evaluated by restriction fragment length polymorphism (RFLP) analysis. The majority of the gene sequences recovered showed site and depth restricted distributions; however, a limited number of gene sequences were widely distributed within and between the margin systems. Cluster analysis identified 175 unique RFLP patterns, and nucleotide sequences were determined for corresponding clones. Several different continental margin DsrA sequences clustered with those from formally characterized taxa belonging to the delta subdivision of the class Proteobacteria (Desulfobulbus propionicus, Desulfosarcina variabilis) and the Bacillus-Clostridium (Desulfotomaculum putei) divisions, although the majority of the recovered sequences were phylogenetically divergent relative to all of the other DsrA sequences available for comparison. This study revealed extensive new genetic diversity among sulfate-reducing bacteria in continental margin sedimentary habitats, which appears to be tightly coupled to slope depth, specifically carbon bioavailability.

  4. Wintertime pytoplankton bloom in the Subarctic Pacific supportedby continental margin iron

    SciTech Connect

    Lam, Phoebe J.; Bishop, James K.B.; Henning, Cara C.; Marcus,Matthew A.; Waychunas, Glenn A.; Fung, Inez

    2004-06-08

    Heightened biological activity was observed in February 1996in the high-nutrient low-chlorophyll (HNLC) subarctic North PacificOcean, a region that is thought to beiron-limited. Here we provideevidence supporting the hypothesis that Ocean Station Papa (OSP) in thesubarctic Pacific received a lateral supply of particulate iron from thecontinental margin off the Aleutian Islands in the winter, coincidentwith the observed biological bloom. Synchrotron X-ray analysis was usedto describe the physical form, chemistry, and depth distributions of ironin size fractionated particulate matter samples. The analysis revealsthat discrete micron-sized iron-rich hotspots are ubiquitous in the upper200m at OSP, more than 900km from the closest coast. The specifics of thechemistry and depth profiles of the Fe hot spots trace them to thecontinental margins. We thus hypothesize that iron hotspots are a markerfor the delivery of iron from the continental margin. We confirm thedelivery of continental margin iron to the open ocean using an oceangeneral circulation model with an iron-like tracer source at thecontinental margin. We suggest that iron from the continental marginstimulated a wintertime phytoplankton bloom, partially relieving the HNLCcondition.

  5. Erosion and tectonics at the margins of continental plateaus

    NASA Technical Reports Server (NTRS)

    Masek, Jeffrey G.; Isacks, Bryan L.; Gubbels, Timothy L.; Fielding, Eric J.

    1994-01-01

    We hypothesize that the steep frontal slope and high peaks of the Beni region and Himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid Pilcomayo region reflects a tectonic landform only slightly modified by erosion. We propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope, eroding back the plateau while enhancing or maintaining the steep long-wavelength slope. A numerical model coupling orographic precipitation, erosion, and tectonic uplift demonstrates the plausibility of this hypothesis. The erosional efflux in both the Beni and Nepal Himalaya have been considerable, and simple mass balance calculations for the Himalaya suggest that during the Neogene, the erosional mass efflux has generally outpaced the tectonic mass influx. This contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period, and/or increased late Cenozoic erosion rates.

  6. Conditions of formation for carbonaceous silicites of the continental margins

    SciTech Connect

    Bazhenova, O.K.

    1986-06-01

    Carbonaceous silicites occur in virtually all systems in Phanerozoic folded regions. They are of practical interest as concentrators of silver, molybdenum, vanadium, and nickel and as source and occasionally reservoir beds for petroleum. Some small oil pools occur in them in basins in Japan (Niigata and Akita), California, and East Sakhalin. Recently, interest has increased because a major pool was discovered in silicites of the Monterey formation: Point Arguello Hueso in the offshore part of the Santa Maria basin. Here the authors consider carbonaceous silicates in the western part of the Pacific active margin, which include Silurian and Devonian phthanites in the Mongolia-Okhotsk belt, and Triassic and Jurassic phthanites in the Sikhote-Alin area, although these rocks are of fairly local occurrence in the section. The authors have examined silicites in Kamchatka, Sakhalin, and Chukotka: diatomites, tuff-diatomites, and opokas, together with their recrystallized analogs. They occur in the Paleogene, but they are most abundant in the Miocene and Pliocene, as well as in the Jurassic, Cretaceous, and Eocene, particularly in the Miocene of California and Japan. 16 references.

  7. Sequence stratigraphy and continental margin development of the northwestern shelf of the South China Sea

    SciTech Connect

    Chen, P.P.H. ); Zhi Yuong Chen; Qi Min Zhang )

    1993-05-01

    Based on the sequence stratigraphic analyses of the Tertiary in the northwestern continental margin of the South China Sea, this paper summarizes the regional tectonics and their influences on the basin developments and discusses the systems tract distributions through the continental margin growth. The study area is underlain by two Tertiary basins separated by a major basin boundary fault across which two distinctly different basin structural styles developed. The Qiongdongnan basin shows characteristic rift structures and is closely related to the opening of the South China Sea. The Yinggehai basin is filled with thick Neogene monoclines overlying basement without apparent rift structures. The Yinggehai's genesis has been linked to the tectonism along the Red River suture zone. The margin development is characterized by depositional environments of sediment influx constantly exceeding accommodation space. Under these environments, the highstand deltas frequently developed near shelf edges, where vertically stacked shallow water sands of both highstand deltas and lowstand wedges/deltas could produce favorable exploration targets. High rates of sedimentation in the study area allow the identification of fourth-order cyclicity in the Quaternary. Regional sequence stratigraphic correlations suggest that many sequence boundaries appear to be synchronous, but not all boundaries exist in all basins along the eastern and southern continental margins of mainland China. The margin evolved through the repeated development of lowstand fans, lowstand wedges, transgressive systems tracts, and highstand deltas. Prior to the Pliocene, the shelf was sourced both from Vietnam and Hainan Island and, as a result, two shelf margins developed. The southwestern margin stopped growing in the late Miocene, and the northern margin continued to prograde with possible additional sediment inputs through the Red River system in the Quaternary. 29 refs., 22 figs.

  8. Trophic state of sediments from two deep continental margins off Iberia: a biomimetic approach

    NASA Astrophysics Data System (ADS)

    Dell'Anno, A.; Pusceddu, A.; Corinaldesi, C.; Canals, M.; Heussner, S.; Thomsen, L.; Danovaro, R.

    2012-12-01

    The trophic state of benthic deep-sea ecosystems can greatly influence key ecological processes (e.g. biomass production and nutrient cycling). Thus, assessing the trophic state of the sediment at different spatial and temporal scales is crucial for a better understanding of deep-sea ecosystem functioning. Here, using a biomimetic approach based on enzymatic digestion of protein and carbohydrate pools, we assess the bioavailability of organic detritus and its nutritional value in the uppermost layer of deep-sea sediments from open slopes and canyons of the Catalan (NW Mediterranean) and Portuguese (NE Atlantic) continental margins, offshore east and west Iberia, respectively. Patterns of sediment trophic state were analyzed in relation to increasing water depth, including repeated samplings over a 3 yr period in the Catalan margin. Bioavailable organic matter and its nutritional value were significantly higher in the Portuguese margin than in the Catalan margin, thus reflecting differences in primary productivity of surface waters reported for the two regions. Similarly, sediments of the Catalan margin were characterized by significantly higher food quantity and quality in spring, when higher primary production processes occur in surface waters, than in summer and autumn. In both continental margins, bioavailable organic C concentrations did not vary or increase with increasing water depth. Differences in the benthic trophic state of canyons against open slopes were more evident in the Portuguese than in the Catalan margin. Overall our findings indicate that deep-sea sediments are characterized by relatively high amounts of bioavailable organic matter. We suggest that the interactions between biological-related processes in surface waters and particle transport and deposition dynamics can play a crucial role in shaping the quantity and distribution of bioavailable organic detritus and its nutritional value along deep continental margins.

  9. Trophic state of benthic deep-sea ecosystems from two different continental margins off Iberia

    NASA Astrophysics Data System (ADS)

    Dell'Anno, A.; Pusceddu, A.; Corinaldesi, C.; Canals, M.; Heussner, S.; Thomsen, L.; Danovaro, R.

    2013-05-01

    The bioavailability of organic matter in benthic deep-sea ecosystems, commonly used to define their trophic state, can greatly influence key ecological processes such as biomass production and nutrient cycling. Here, we assess the trophic state of deep-sea sediments from open slopes and canyons of the Catalan (NW Mediterranean) and Portuguese (NE Atlantic) continental margins, offshore east and west Iberia, respectively, by using a biomimetic approach based on enzymatic digestion of protein and carbohydrate pools. Patterns of sediment trophic state were analyzed in relation to increasing water depth, including repeated samplings over a 3 yr period in the Catalan margin. Two out of the three sampling periods occurred a few months after dense shelf water cascading events. The benthic deep-sea ecosystems investigated in this study were characterized by high amounts of bioavailable organic matter when compared to other deep-sea sediments. Bioavailable organic matter and its nutritional value were significantly higher in the Portuguese margin than in the Catalan margin, thus reflecting differences in primary productivity of surface waters reported for the two regions. Similarly, sediments of the Catalan margin were characterized by significantly higher food quantity and quality in spring, when the phytoplankton bloom occurs in surface waters, than in summer and autumn. Differences in the benthic trophic state of canyons against open slopes were more evident in the Portuguese than in the Catalan margin. In both continental margins, bioavailable organic C concentrations did not vary or increase with increasing water depth. Overall, our findings suggest that the intensity of primary production processes along with the lateral transfer of organic particles, even amplified by episodic events, can have a role in controlling the quantity and distribution of bioavailable organic detritus and its nutritional value along these continental margin ecosystems.

  10. Evolution of continental slope gullies on the northern california margin

    USGS Publications Warehouse

    Spinelli, G.A.; Field, M.E.

    2001-01-01

    A series of subparallel, downslope-trending gullies on the northern California continental slope is revealed on high-resolution seismic reflection profiles imaging the uppermost 50 m of sediment. The gullies are typically 100 m wide and have 1 to 3 m of relief. They extend for 10 to 15 km down the slope and merge into larger channels that feed the Trinity Canyon. In the lower half of the 50 m stratigraphic section, the gullies increase in both relief and number up section, to maxima at a surface 5 to 10 m below the last glacial maximum lowstand surface. Gully relief increased as interfluves aggraded more rapidly than thalwegs. Erosion is not evident in the gully bottoms, therefore gully growth was probably due to reduced sediment deposition within the gullies relative to that on interfluves. As the gullies increased in relief, their heads extended upslope toward the shelfbreak. At all times, a minimum of 10 km of non-gullied upper slope and shelf stretched between the heads of the gullies and the paleo-shoreline; the gullies did not connect with a subaerial drainage network at any time. Gully growth occurred when the gully heads were in relatively shallow water (??? 200 m paleo-water depth) and were closest to potential sediment sources. We suggest that prior to the last glacial maximum, the Mad River, then within 10 km of the gully heads, supplied sediment to the upper slope, which fed downslope-eroding sediment flows. These flows removed sediment from nearly parallel gullies at a rate slightly slower than sediment accumulation from the Eel River, 40 km to the south. The process or processes responsible for gully growth and maintenance prior to the last glacial maximum effectively ceased following the lowstand, when sea level rose and gully heads lay in deeper water (??? 300 m water depth), farther from potential sediment sources. During sea-level highstand, the Mad River is separated from the gully heads by a shelf 30 km wide and no longer feeds sediment flows

  11. New Insight Into the Crustal Structure of the Continental Margin offshore NW Sabah/Borneo

    NASA Astrophysics Data System (ADS)

    Barckhausen, U.; Franke, D.; Behain, D.; Meyer, H.

    2002-12-01

    The continental margin offshore NW Sabah/Borneo (Malaysia) has been investigated with reflection and refraction seismics, magnetics, and gravity during the recent cruise BGR01-POPSCOMS. A total of 4000 km of geophysical profiles has been acquired, thereof 2900 km with reflection seismics. The focus of investigations was on the deep water areas. The margin looks like a typical accretionary margin and was presumably formed during the subduction of a proto South China Sea. Presently, no horizontal movements between the two plates are being observed. Like in major parts of the South China Sea, the area seaward of the Sabah Trough consists of extended continental lithosphere which is characterised by a pattern of rotated fault blocks and half grabens and a carbonate platform of Early Oligocene to Early Miocene age. We found evidence that the continental crust also underlies the Sabah Trough and the adjacent continental slope, a fact that raises many questions about the tectonic history and development of this margin. The tectonic pattern of the Dangerous Grounds' extended continental crust can be traced a long way landward of the Sabah Trough beneath the sedimentary succession of the upper plate. The magnetic anomalies which are dominated by the magnetic signatures of relatively young volcanic features also continue under the continental slope. The sedimentary rocks of the upper plate, in contrast, seem to generate hardly any magnetic anomalies. Based on the new data we propose the following scenario for the development of the NW Sabah continental margin: Seafloor spreading in the present South China Sea started at about 30 Ma in the Late Oligocene. The spreading process separated the Dangerous Grounds area from the SE Asian continent and ceased in late Early Miocene when the oceanic crust of the proto South China Sea was fully subducted in eastward direction along the Borneo-Palawan Trough. During Lower and/or Middle Miocene, Borneo rotated counterclockwise and was

  12. The Case for a Thermal Origin of Magmatism on the North Atlantic Continental Margin

    NASA Astrophysics Data System (ADS)

    White, R.

    2008-12-01

    The cause of the magmatism on 'volcanic' continental margins is still disputed, specifically as to whether it is due to increased mantle temperatures. New normal incidence and wide-angle seismic profiles across the Faroe and Hatton Bank volcanic margins in the NE Atlantic enable us to constrain the seismic velocities and volumes of both the extruded and intruded melt. Near the Faroe Islands, for every 1 km along strike, 360-- 400 cubic kilometers of basalt was extruded, while 540--600 cubic kilometers was intruded into the continent- ocean transition (COT). Lower-crustal intrusions are focused mainly into a narrow zone about 50 km wide on the COT, whereas extruded basalts flow more than 100 km from the rift. Deep-penetration seismic profiles show that melt is intruded into the lower crust as sills which cross-cut the continental fabric, rather than as 'underplate' of 100 per cent melt as has previously often been assumed. This means that measured lower-crustal velocities represent a mixture of continental crust and new igneous rock. Tomographic inversion of wide-angle traveltimes from 85 ocean bottom seismometers constrain average lower-crustal seismic velocities as 6.9-7.3 km/s under the COT, intermediate between the velocities of the continental crust and fully igneous oceanic crust on either side. By comparison with theoretical curves of igneous thickness versus seismic velocity (H- Vp), our observations are consistent with the dominant control on the melt production being elevated mantle temperatures, with no requirement for either significant active small-scale mantle convection under the rift or of the presence of fertile mantle at the time of continental breakup as suggested for the North Atlantic by other authors. The mantle temperature anomaly was c. 130-150°C above normal at the time of continental breakup, decreasing steadily by about 75°C over the first 10 Ma of seafloor spreading. Comparison with the conjugate Greenland margins reveal a similar

  13. Anthropogenic impacts on continental margins: New frontiers and engagement arena for global sustainability research and action

    NASA Astrophysics Data System (ADS)

    Liu, K. K.; Glavovic, B.; Limburg, K.; Emeis, K. C.; Thomas, H.; Kremer, H.; Avril, B.; Zhang, J.; Mulholland, M. R.; Glaser, M.; Swaney, D. P.

    2014-12-01

    There is an urgent need to design and implement transformative governance strategies that safeguard Earth's life-support systems essential for long-term human well-being. From a series of meetings of the Continental Margins Working Group co-sponsored by IMBER and LOICZ of IGBP, we conclude that the greatest urgency exists at the ocean-land interface - the continental margins or the Margin - which extends from coastlands over continental shelves and slopes bordering the deep ocean. The Margin is enduring quadruple squeeze from (i) Population growth and rising demands for resources; (ii) Ecosystem degradation and loss; (iii) Rising CO2, climate change and alteration of marine biogeochemistry and ecosystems; and (iv) Rapid and irreversible changes in social-ecological systems. Some areas of the Margin that are subject to the greatest pressures (e.g. the Arctic) are also those for which knowledge of fundamental processes remains most limited. Aside from improving our basic understanding of the nature and variability of the Margin, priority issues include: (i) investment reform to prevent lethal but profitable activities; (ii) risk reduction; and (iii) jurisdiction, equity and fiscal responsibility. However, governance deficits or mismatches are particularly pronounced at the ocean-edge of the Margin and the prevailing Law of the Sea is incapable of resolving these challenges. The "gold rush" of accelerating demands for space and resources, and variability in how this domain is regulated, move the Margin to the forefront of global sustainability research and action. We outline a research strategy in 3 engagement arenas: (a) knowledge and understanding of dynamic Margin processes; (b) development, innovation and risk at the Margin; and (c) governance for sustainability on the Margin. The goals are (1) to better understand Margin social-ecological systems, including their physical and biogeochemical components; (2) to develop practical guidance for sustainable development

  14. 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

  15. Accretion, subduction, and underplating along the southern Alaska continental margin

    SciTech Connect

    Plafker, G.; Ambos, E.L.; Fuis, G.S.; Mooney, W.D.; Nokleberg, W.J.; Campbell, D.L.

    1985-01-01

    In 1984-1985 the Trans Alaska Crustal Transect (TACT) program completed geologic, seismic refraction, gravity, and magnetic studies along a 350-km-long corridor that extends northward from the Gulf of Alaska coast near Cordova to the Denali fault at the Richardson Highway. From south to north, this segment of the transect traverses: 1) part of the Prince William terrance (PWT), composed of an accreted Paleocene and Eocene deep-sea fan complex, oceanic volcanic rocks, and pelagic sediments; 2) the Chugach terrane (CGT) composed of a) accreted Late Cretaceous flysch and oceanic basaltic rocks, b) accreted and subducted (.) Late Jurassic to Early Cretaceous sheared melange, and c) subducted Early (.) Jurassic or older blueschist/greenschist; and 3) Wrangellia-Peninsular terranes (WRT/PET) consisting primarily of late Paleozoic intraoceanic andesitic arc rocks with associated mafic and ultramafic plutonic rocks, an overlying distinctive Triassic sedimentary and volcanic sequence, and superposed intrusive and extrusive magmatic rocks of the Jurassic Talkeetna arc. At the southern margin of both the CGT and WRT/PET, shallow high-velocity zones characterized by positive gravity and magnetic anomalies reflect uplift of mafic and ultramafic basement along these thrusts. The Contact and Border Ranges fault systems appear to merge into a subhorizontal low-velocity zone of uncertain origin that underlies the CGT and southern WRT/PET at 5-9 km depth. A few kilometers beneath the shallow low-velocity zone in a 30-km-thick stack of eight northward-dipping layers of alternating high and low velocity, interpreted as subducted and underplated mantle and oceanic crust rocks. Distribution of earthquake hypocenters suggests that active subduction involves at least the lowest two and possibly the lower four layers.

  16. 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

  17. 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.

  18. Structure of continental margin off Mid-Atlantic states (Baltimore Canyon Trough)

    USGS Publications Warehouse

    Schlee, John Stevens; Behrendt, John Charles; Mattick, Robert E.; Taylor, P.T.

    1975-01-01

    Increasing interest in the Atlantic continental margin as a future petroleum province has resulted in several recent papers (Emmerich, 1974; Burk and Drake, 1974) that attempt to summarize the structure and stratigraphic framework of this area. Most papers tend to portray the margin as a wedge of Mesozoic and Cenozoic sediment that thins at the edge of the shelf over a "basement ridge" and then thickens again under the continental rise. Off the northeastern United States, the sediment wedge under the shelf attains a thickness of 8-11 km in the Georges Bank basin (Schultz and Glover, 1974; Mattick and others, 1974; Sheridan, 1974b; Behrendt and others, 1974) and 12 km in thickness in the Baltimore Canyon trough off the middle Atlantic states of Delaware, Maryland, Virginia and New Jersey (fig. 1). Seaward of the continental shelf and its sediment prism, Emery and Uchupi (1972, figs. 133-135) infer slump deposits (eroded in some areas) covering a buried ridge thought to extend from the Laurentian Channel to Cape Hatteras, where it splits in two. The lower slope and continental rise are inferred by Drake and later investigators to be a thick prism of deep sea sediment (turbidites, hemipelagic clays, slump deposits) overlying oceanic basement in a welt that parallels the continental edge and reaches a maximum thickness of 6 km (Emery and Uchupi, 1972, fig. 188).

  19. 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.

  20. Paleoceanographic model of neogene phosphorite deposition, u.s. Atlantic continental margin.

    PubMed

    Riggs, S R

    1984-01-13

    The Neogene stratigraphic section of the southeastern U.S. continental shelf-coastal plain system is characterized by (i) a series of major regional phosphogenic episodes; (ii) a strong spatial relationship between the structural or topographic framework and phosphate deposition; and (iii) distinct cyclical and regional patterns of deposition of the terrigenous, carbonate, and phosphate lithofacies. The complex depositional patterns are explained by a paleoceanographic model based upon the interaction of glacial eustatic sea-level fluctuations, associated changes in climate, and the dynamics of the Gulf Stream in response to the bathymetric configurations of the continental margin during the past 20 million years.

  1. Arctic and Antarctic submarine gullies—A comparison of high latitude continental margins

    NASA Astrophysics Data System (ADS)

    Gales, J. A.; Forwick, M.; Laberg, J. S.; Vorren, T. O.; Larter, R. D.; Graham, A. G. C.; Baeten, N. J.; Amundsen, H. B.

    2013-11-01

    Submarine gullies are common features of high latitude continental slopes and, over the last decade, have been shown to play a key role in continental margin evolution, submarine erosion, downslope sediment transport, slope deposits, and the architecture of petroleum reservoirs. However, the processes that form these gullies, the timescales over which they develop, and the environmental controls influencing their morphology remain poorly constrained. We present the first systematic and comparative analysis between Arctic and Antarctic gullies with the aim of identifying differences in slope character, from which we infer differences in processes operating in these environments. Quantitative analysis of multibeam echosounder data along 2441 km of the continental shelf and upper slope and morphometric signatures of over 1450 gullies show that six geomorphically distinct gully types exist on high latitude continental margins. We identify distinct differences between Arctic and Antarctic gully morphologies. In the Arctic data sets, deep relief (> 30 m gully incision depth at 50 m below the shelf edge) and shelf-incising gullies are lacking. These differences have implications for the timescales over which the gullies were formed and for the magnitude of the flows that formed them. We consider two hypotheses for these differences: (1) some Antarctic gullies developed through several glacial cycles; and (2) larger Antarctic gullies were formed since the Last Glacial Maximum as a result of erosive flows (i.e., sediment-laden subglacial meltwater) being more abundant on parts of the Antarctic margin over longer timescales. A second difference is that unique gully signatures are observed on Arctic and on Antarctic margins. Environmental controls, such as the oceanographic regime and geotechnical differences, may lead to particular styles of gully erosion observed on Arctic and Antarctic margins.

  2. Supercritical Submarine Channel Morphodynamics from Integrated Investigation of the Western North American Continental Margin

    NASA Astrophysics Data System (ADS)

    Covault, J. A.; Fildani, A.; Hubbard, S. M.; Hughes Clarke, J. E.; Kostic, S.; Paull, C. K.; Sylvester, Z.

    2015-12-01

    Submarine channels are conduits through which turbidity currents and related mass movements transport sediment into the deep sea, thereby playing important roles in the development of continental margins and biogeochemical cycles. To gain a better understanding of submarine channel morphodynamic evolution we explore a variety of channel systems from the western North American continental margin with varying sinuosity and levee geometry, terraces, channel cut-offs, and sediment waves in incipient channels, along thalwegs of well-developed channels, and on levees. Repeat bathymetric surveys of submarine channels in fjords of British Columbia and the Monterey canyon underscore the transience of fine-scale detail in channelized geomorphology, and multi-phase bed reworking, local deposition, and bypass of turbidity currents. Numerical modeling is combined with interpretations of channel geomorphology and strata in the Monterey and San Mateo canyon-channel systems to demonstrate that some of the sediment waves are likely to be cyclic steps. Submarine cyclic steps are long-wave, upstream-migrating bedforms in which each bedform in the series is bounded by a hydraulic jump in an overriding turbidity current, which is Froude-supercritical over the lee side of the bedform and Froude-subcritical over the stoss side. Submarine turbidity currents are susceptible to supercritical flow because of the reduced gravitational acceleration of dilute suspensions. Higher submarine slopes common to the North American continental margin also promote supercritical flow, which might not be as common across lower slopes of large passive margins such as the Amazon, Indus, and Bengal submarine fans. We posit that cyclic steps are a common morphodynamic expression in many continental margins. Continued integration of high-resolution data, such as repeat geophysical surveys, acoustic doppler current profiler measurements, and turbidite outcrops, which provide insights into the longer

  3. Anomalous Subsidence at the Ocean Continent Transition of the Gulf of Aden Rifted Continental Margin

    NASA Astrophysics Data System (ADS)

    Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie

    2013-04-01

    It has been proposed that some rifted continental margins have anomalous subsidence and that at break-up they were elevated at shallower bathymetries than the isostatic response predicted by classical rift models (McKenzie, 1978). The existence of anomalous syn- or early-post break-up subsidence of this form would have important implications for our understanding of the geodynamics of continental break-up and sea-floor spreading initiation. We have investigated subsidence of the young rifted continental margin of the eastern Gulf of Aden, focussing on the western Oman margin (break-up age 17.6 Ma). Lucazeau et al. (2008) have found that the observed bathymetry here is approximately 1 km shallower than the predicted bathymetry. In order to examine the proposition of an anomalous early post break-up subsidence history of the Omani Gulf of Aden rifted continental margin, we have determined the subsidence of the oldest oceanic crust adjacent to the continent-ocean boundary (COB) using residual depth anomaly (RDA) analysis corrected for sediment loading and oceanic crustal thickness variation. RDAs corrected for sediment loading using flexural backstripping and decompaction have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous subsidence of the Gulf of Aden rifted continental margin. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions of Crosby and McKenzie (2009). Non-zero RDAs at the Omani Gulf of Aden rifted continental margin can be the result of non standard oceanic crustal thickness or the effect of mantle dynamic topography or a non-classical rift and break-up model. Oceanic crustal basement thicknesses from gravity inversion together with Airy isostasy have been used to predict a "synthetic" gravity RDA, in order to determine the RDA contribution from non-standard oceanic crustal thickness. Gravity inversion, used to determine crustal basement thickness

  4. Ophiolites and Continental Margins of the Mesozoic Western U.S. Cordillera

    NASA Astrophysics Data System (ADS)

    Dilek, Y.

    2001-12-01

    The Mesozoic tectonic history of the western U.S. Cordillera records evidence for multiple episodes of accretionary and collisional orogenic events and orogen-parallel strike-slip faulting. Paleozoic-Jurassic volcanic arc complexes and subduction zone assemblages extending from Mexico to Canada represent an East-Pacific magmatic arc system and an accretionary-type orogen evolved along the North American continental margin. Discontinuous exposures of Paleozoic upper mantle rocks and ophiolitic units structurally beneath this magmatic arc system are remnants of the Panthalassan oceanic lithosphere, which was consumed beneath the North American continent. Pieces of this subducted Panthalassan oceanic lithosphere that underwent high-P metamorphism are locally exposed in the Sierra Nevada foothills (e.g. Feather River Peridotite) indicating that they were subsequently (during the Jurassic) educted in an oblique convergent zone along the continental margin. This west-facing continental margin arc evolved in a broad graben system during much of the Jurassic as a result of extension in the upper plate, keeping pace with slab rollback of the east-dipping subduction zone. Lower to Middle Jurassic volcanoplutonic complexes underlain by an Upper Paleozoic-Lower Mesozoic polygenetic ophiolitic basement currently extend from Baja California-western Mexico through the Sierra-Klamath terranes to Stikinia-Intermontane Superterranes in Canada and represent an archipelago of an east-facing ensimatic arc terrane that developed west and outboard of the North American continental margin arc. The Smartville, Great Valley, and Coast Range ophiolites (S-GV-CR) in northern California are part of this ensimatic terrane and represent the island arc, arc basement, and back-arc tectonic settings, respectively. The oceanic Josephine-Rogue-Chetco-Rattlesnake-Hayfork tectonostratigraphic units in the Klamath Mountains constitute a west-facing island arc system in this ensimatic terrane as a

  5. Tectonics of the West Iberia continental margin from seismic reflection data

    NASA Astrophysics Data System (ADS)

    Henning, Alison Teagan

    Continental rifting is a fundamental component of the plate tectonic cycle. The West Iberia passive margin is a classic example of a nonvolcanic rifted margin. The West Iberia margin contains an enigmatic north-south ridge of serpentinized peridotite located within the ocean-continent transition. Interpretation of multichannel seismic data and tectonic subsidence analyses suggests that the ridge is located within a broad zone of exhumed mantle that has been serpentinized. This implies that seafloor spreading does not immediately follow continental breakup. Where the peridotite ridge is well-developed, it parallels a deeply-penetrating, west-dipping normal fault. Hydrodynamic circulation drove seawater down this fault close to the beginning of seafloor spreading and caused a concentration of serpentinization at its base. This water-driven process of formation accounts for the variability of the ridge along strike. Prestack depth migration of a 340 km long seismic reflection profile across the margin served as the basis for stratigraphic interpretation. The proximal margin displays horsts and grabens, with 1--2 km thick synrift deposits from Early Cretaceous (Valanginian) rifting. The deep water margin displays rotated blocks with distinct internal seismic patterns. These blocks formed during the final Cretaceous rifting event that led to continental breakup, and contain earlier Valanginian synrift deposits as part of the rotated blocks, thus providing evidence of two separate rifting stages along this profile. A large mantle-penetrating fault located seaward of the last rotated block of continental crust provides evidence for a third and final Cretaceous rifting event that may have been coeval with early seafloor spreading. Two independent estimates of crustal thickness along this line indicate stretching factors of 50% on the proximal margin (corresponding to a continental crustal thickness of ˜16 km), increasing to 100% in the deep water. Plate tectonics is one

  6. Paleogeography and evolution of the Ordovician/Silurian (Whiterockian-Llandoverian) continental margin in central Nevada

    SciTech Connect

    Britt, L.W. )

    1991-02-01

    In central Nevada, stratigraphic successions of Whiterockian-Llandoverian lithofacies, transitional with autochthonous platform/shelf carbonates to the east, occur in isolated windows in outer slope to basinal lithotopes of the Roberts Mountains allochthon. Petrologic, chronostratigraphic and lithostratigraphic, and paleontologic comparison of those successions with platform/shelf facies to the east is integral for reconstruction of Ordovician-Silurian platform margin paleogeography and pre-Antler genesis of the western North American continental margin. Numerous facies changes and/or stratigraphic omissions in central Nevada can be related to sea level fluctuation and aggradation/progradation of the carbonate platform to the east, and not to a postulated, offshore geanticline (i.e., the Toiyabe Ridge). Stratigraphic omission of the Eureka Quartzite above Pogonip equivalents in transitional successions of the Toquima Range and the presence of correlative quartzite in outer slope/basinal parautochthonous facies of the Toiyabe Range suggest development of a possible bypass-margin during the Middle Ordovician. Deposition of Late Ordovician platform margin dolostones (Ely Springs Dolostone) and upper ramp limestones (Hanson Creek Formation and Martin Ridge strata) followed Late Ordovician transgression that drowned the margin and reestablished the carbonate factory. Glacioeustatic drawdown of Late Ordovician-earliest Silurian seas due to the Gondwanan glacial fluctuation can be recognized in strata along the platform margin and upper ramp. Rapid, Early Silurian transgression produced dark-gray carbonates and may have induced marginal flexure and regional, massive slope failure in central Nevada, generating stratigraphic hiatuses west of the platform margin.

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    The architecture of magma-poor continental margins is remarkably variable. The width of highly thinned continental crust (with a thickness < 10 km) varies from 70 km off Iberia, and 200 km offshore Angola, to over 300 km in the Antarctic Enderby Basin. The respective conjugate margin, however, is restricted to few tens of kilometres resulting in large scale crustal asymmetry. Growing evidence from rifted continental margins in the North and South Atlantic, as well as from the East Australia/Lord Howe Rise margin pair supports the idea that rifts with a very wide margin and a narrow conjugate are rather the rule than the exception. In this study, we use numerical thermo-mechanical models to investigate the dynamics of rifting. Our simulations apply an elasto-visco-plastic rheology formulation that relies on laboratory-derived flow laws for crustal and mantle rock. The models are constrained by geophysical and geological observations like limited melt generation, cold initial geotherms, and mafic lower crustal rheology. We show that small-scale lateral rift migration simultaneously explains the observed margin asymmetry and the presence of highly thinned continental crust. Rift migration results from two fundamental processes: (1) Strain hardening in the rift centre due to cooling of upwelling mantle material; (2) Formation of a low viscosity exhumation channel adjacent to the rift centre that is generated by heat transfer from the upwelling mantle and enhanced by viscous strain softening. Rift migration takes place in a steady-state manner and is accomplished by oceanward-younging sequential faults within the upper crust and balanced through lower crustal flow. We demonstrate that the rate of extension has paramount control on margin width. Since higher velocities lead to elevated heat flow within the rift and hence to hot and weak lower crust, a larger low-viscosity exhumation channel is generated that facilitates rift migration leading to wider margins. The South

  8. How widely is the Andean type of continental margin represented in the Archean

    NASA Technical Reports Server (NTRS)

    Burke, Kevin

    1988-01-01

    Application of the principle of uniformitarianism to the Archean was discussed in a search for evidence of Archean-type continental margins in Archean rocks. The author cautioned that Archean rocks represent only 2 percent of the current exposure of the continents, half of which is in the North American Superior Province. Care must be taken in interpreting the global tectonic significance of relatively small exposures of Archean rocks, such as South India. Andean margins were characterized by their elongate shape, magmatic associations, and isotopic signatures. Although the compositional evidence alone will always be ambiguous, it was suggested that supporting structural evidence may aid in the identification of Archean Andean margins. Andean margin remains have been recognized in the Superior Province of Canada by these criteria, and the author suggested that the Closepet granite of South India may represent another example.

  9. Variations in heat flow across the Goban Spur and Galicia Bank continental margins

    NASA Astrophysics Data System (ADS)

    Louden, K. E.; Sibuet, J.-C.; Foucher, J.-P.

    1991-09-01

    We present the results of 44 new heat flow stations which were taken in 1984 and 1989 in profiles across the Goban Spur and Galicia Bank continental margins (NW Atlantic Ocean). Simple extensional models indicate that the heat flow across both these Early Cretaceous rifted margins should increase from values of 45-50 mW/m2 over oceanic crust to 65-80 mW/m2 on the continents. The rate of this increase should help to constrain the mechanism (simple versus pure shear) by which the upper, more radiogenic continental crust is thinned. Measurements across Goban Spur increase from values of 40-45 mW/m2 over oceanic crust to 50-55 mW/m2 near the continental shelf. They follow the predicted trend for pure-shear rifting, but only if the value of upper crustal radiogenic heating is low (1-2 μW/m3). Otherwise, they would require the upper crust to thin more rapidly than the total crustal thickness, as with a lower plate, simple-shear margin. Measurements across Galicia Bank show a very different pattern, with similar values over oceanic crust but much lower values (30-35 mW/m2) nearer land. This is difficult to reconcile with any simple, single rifting event but is more compatible with an origin as a pure-shear or lower plate rather than upper plate margin. We also note that oceanic values of heat flow require asthenospheric temperatures 100°C lower than normal for both margins. This indicates that the triple junction in existance between these margin segments during the breakup of Iberia, Europe, and North America was not the site of a major mantle plume.

  10. Morphology of late Quaternary submarine landslides along the U.S. Atlantic continental margin

    USGS Publications Warehouse

    Twichell, D.C.; Chaytor, J.D.; ten Brink, U.S.; Buczkowski, B.

    2009-01-01

    The nearly complete coverage of the U.S. Atlantic continental slope and rise by multibeam bathymetry and backscatter imagery provides an opportunity to reevaluate the distribution of submarine landslides along the margin and reassess the controls on their formation. Landslides can be divided into two categories based on their source areas: those sourced in submarine canyons and those sourced on the open continental slope and rise. Landslide distribution is in part controlled by the Quaternary history of the margin. They cover 33% of the continental slope and rise of the glacially influenced New England margin, 16% of the sea floor offshore of the fluvially dominated Middle Atlantic margin, and 13% of the sea floor south of Cape Hatteras. The headwall scarps of open-slope sourced landslides occur mostly on the lower slope and upper rise while they occur mostly on the upper slope in the canyon-sourced ones. The deposits from both landslide categories are generally thin (mostly 20-40??m thick) and comprised primarily of Quaternary material, but the volumes of the open-slope sourced landslide deposits can be larger (1-392??km3) than the canyon-sourced ones (1-10??km3). The largest failures are located seaward of shelf-edge deltas along the southern New England margin and near salt domes that breach the sea floor south of Cape Hatteras. The spatial distribution of landslides indicates that earthquakes associated with rebound of the glaciated part of the margin or earthquakes associated with salt domes were probably the primary triggering mechanism although other processes may have pre-conditioned sediments for failure. The largest failures and those that have the potential to generate the largest tsunamis are the open-slope sourced landslides.

  11. Deep crustal structure and continent-ocean boundary along the Galicia continental margin (NW Iberia)

    NASA Astrophysics Data System (ADS)

    Druet, María; Muñoz-Martín, Alfonso; Carbó, Andrés; Acosta, Juan; Granja Bruña, José Luis; Llanes, Pilar; Vázquez, Juan-Tomás; Ercilla, Gemma

    2016-04-01

    The Galicia continental margin is a magma-poor rifted margin with an extremely complex structure. Its formation involves several rifting episodes during the Mesozoic in the vicinity of a ridge triple junction, which produces a change in the orientation of the main structures. In addition, there is an overimposed Cenozoic partial tectonic inversion along its northern border. Although this continental margin has been widely studied since the 70's, most studies have focused on its western part in the transition to the Iberia Abyssal Plain, and there is a significant lack of information on the north and northwestern flanks of this margin. This fact, along with its great structural complexity, has resulted in the absence of a previous comprehensive regional geodynamic model integrating all the processes observed. In the present study we integrate a large volume of new geophysical data (gravity, swath bathymetry and 2D multichannel reflection seismic). Data come from the systematic mapping of the Spanish EEZ project which provides a dense grid of gravity data and full seafloor coverage with swath bathymetry, and from the ERGAP project which provides serially-arranged 2D seismic reflection profiles across the NW Iberia margin. The combined interpretation and modelling of this new information has arisen significant constraints on the origin, the deep crustal structure and the physiographic complexity of the margin, as well as on the characterization of the along- and across-strike variation of the ocean-continent transition along NW Iberia margin. The analysis of this information leads us to propose a conceptual model for the initiation of the tectonic inversion of a magma-poor rifted margin. Finally, a framework for the geodynamic evolution of the Galicia margin has been constructed, involving three main stages: A) an early stage from the end of rifting and oceanic drift in the Bay of Biscay (Santonian); B) an intermediate stage with the beginning of tectonic inversion in

  12. Northwest margin of California continental borderland: marine geology and tectonic evolution.

    USGS Publications Warehouse

    Crouch, J.K.

    1981-01-01

    The NW margin of the California continental border land consists of 2 NW-trending pre-Neogene lithologic belts blanketed by Miocene and younger strata. These represent facies corresponding to the subduction complex and forearc-basin deposits of a late Mesozoic and Paleogene continental-margin arc-trench system. The outer belt, which forms the acoustic basement is characterized by: 1) moderately high compressional velocities; 2) discordant and discontinuous seismic reflectors; and 3) nonfossiliferous and diverse rock types. Landward, the inner Great Valley sequence belt is characterized by: 1) intermediate compressional velocities; 2) concordant and relatively continuous reflectors, and 3) a thick turbidite sequence. Well-developed wrench-fault structures in overlying Miocene strata record a transition from Paleogene subduction to Neogene transform faulting. The timing of volcanism and uplift, and of the younger wrenching event, closely matches the plate-tectonic model of Atwater and Molnar.-from Author

  13. Neogene rotations and quasicontinuous deformation of the Pacific Northwest continental margin

    SciTech Connect

    England, P. ); Wells, R.E. )

    1991-10-01

    Paleomagnetically determined rotations about vertical axes of 15 to 12 Ma flows of the Miocene Columbia River Basalt Group of Oregon and Washington decrease smoothly with distance from the plate margin, consistent with a simple physical model for continental deformation that assumes the lithosphere behaves as a thin layer of fluid. The average rate of northward translation of the continental margin since 15 Ma calculated from the rotations, using this model, is about 15 mm/year, which suggests that much of the tangential motion between the Juan de Fuca and North American plates since middle Miocene time has been taken up by deformation of North America. The fluid-like character of the large-scale deformation implies that the brittle upper crust follows the motions of the deeper parts of the lithosphere.

  14. Surface current patterns suggested by suspended sediment distribution over the outer continental margin, Bering Sea

    USGS Publications Warehouse

    Karl, Herman A.; Carlson, P.R.

    1987-01-01

    Samples of total suspended matter (TSM) were collected at the surface over the northern outer continental margin of the Bering Sea during the summers of 1980 and 1981. Volume concentrations of surface TSM averaged 0.6 and 1.1 mg l-1 for 1980 and 1981, respectively. Organic matter, largely plankton, made up about 65% of the near-surface TSM for both years. Distributions of TSM suggested that shelf circulation patterns were characterized either by meso- and large- scale eddies or by cross-shelf components of flow superimposed on a general northwesterly net drift. These patterns may be caused by large submarine canyons which dominate the physiography of this part of the Bering Sea continental margin. ?? 1987.

  15. Cenozoic tectonic subsidence in the Southern Continental Margin, South China Sea

    NASA Astrophysics Data System (ADS)

    Fang, Penggao; Ding, Weiwei; Fang, Yinxia; Zhao, Zhongxian; Feng, Zhibing

    2016-10-01

    We analyzed two recently acquired multichannel seismic profiles across the Dangerous Grounds and the Reed Bank area in the South China Sea. Reconstruction of the tectonic subsidence shows that the southern continental margin can be divided into three stages with variable subsidence rate. A delay of tectonic subsidence existed in both areas after a break-up, which was likely related to the major mantle convection during seafloor spreading, that was triggered by the secondary mantle convection below the continental margin, in addition to the variation in lithospheric thickness. Meanwhile, the stage with delayed subsidence rate differed along strikes. In the Reed Bank area, this stage is between 32-23.8 Ma, while in the Dangerous Grounds, it was much later (between 19-15.5 Ma). We believe the propagated rifting in the South China Sea dominated the changes of this delayed subsidence rate stage.

  16. The U.S. Atlantic continental margin: the best-known gas hydrate locality: Chapter 13

    USGS Publications Warehouse

    Dillon, William P.; Max, Michael D.; Max, M.D.

    2003-01-01

    One of the few attempts to date to map gas hydrate over a large area has been made on the Atlantic continental margin of the United States (Dillon et al., 1993, 1994, 1995). This work has resulted in the production of an extensive data base of seismic reflection lines including both single and multichannel lines, and complete GLORIA sidescan sonar coverage. This work was part of the assessment of the U.S. EEZ and was carried out by the U.S. Geological Survey. Earlier efforts were made by Tucholke et al. (1977) and Shipley, et al. (1979). Research along the U.S. SE continental margin of the U.S. is continuing.

  17. SEISMIC AND GEOCHEMICAL EVIDENCE FOR SHALLOW GAS IN SEDIMENT ON NAVARIN CONTINENTAL MARGIN, BERING SEA.

    USGS Publications Warehouse

    Carlson, Paul R.; Golan-Bac, Margaret; Karl, Herman A.; Kvenvolden, Keith A.

    1985-01-01

    Marine sesmic studies coupled with geochemical investigations demonstrate tha hydrocarbon gases are ubiquitous in the near-surface sediment of the Navarin continental margin in the northern Bering Sea. Three types of acoustic anomalies appear to be related to the presence of gas in the sediment. These anomalies are most prevalent in the northern half of the Navarin basin. Acoustic anomalies attributed to gas hydrates and to diagenetic boundaries are present on seismic records of the lower slope between Navarinsky and Zhemchug Canyons. Hydrocarbon gases, methane through butanes, are common in the surface sediment of the Navarin continental margin. The source of methane is mainly biogenic, but the hydrocarbon gas compositions in 17 of 141 cores suggest the presence of thermogenic gas. No direct correlation could be found between acoustic anomalies and gas concentrations in the sediment. Refs.

  18. Influence of submarine morphology on bottom water flow across the western Ross Sea continental margin

    USGS Publications Warehouse

    Davey, F.J.; Jacobs, S.S.

    2007-01-01

    Multibeam sonar bathymetry documents a lack of significant channels crossing outer continental shelf and slope of the western Ross Sea. This indicates that movement of bottom water across the shelf break into the deep ocean in this area is mainly by laminar or sheet flow. Subtle, ~20 m deep and up to 1000 m wide channels extend down the continental slope, into tributary drainage patterns on the upper rise, and then major erosional submarine canyons. These down-slope channels may have been formed by episodic pulses of rapid down slope water flow, some recorded on bottom current meters, or by sub-ice melt water erosion from an icesheet grounded at the margin. Narrow, mostly linear furrows on the continental shelf thought to be caused by iceberg scouring are randomly oriented, have widths generally less than 400 m and depths less than 30m, and extend to water depths in excess of 600 m.

  19. 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.

  20. Climate modulated erosion and sediment flux control offshore crustal structure at South China Sea continental margin

    NASA Astrophysics Data System (ADS)

    Clift, P. D.; Brune, S.; Quinteros, J.

    2015-12-01

    Rifted continental lithosphere subsides as a consequence of combined crustal thinning and mantle lithosphere cooling yet basins on some continental margins experience anomalous subsidence events that postdate active extension. Deep basins on the northern margin of the South China Sea, notably the Baiyun Sag, show basement subsidence accelerating after ~21 Ma, postdating extension by several million years. Similar subsidence events are seen after 5 Ma in the Song Hong Basin and after 11 Ma in the Qiongdongnan Basin. We combine geophysical observations and numerical forward modeling to show that loading of the offshore basins by increased sediment flux caused by faster onshore erosion following Early Miocene monsoon intensification is a viable trigger for ductile flow after the cessation of active extension. Loading works in conjunction with onshore uplift to drive flow of the lower crust away from the rift axis. As well as sediment supply rates distribution patterns and drainage capture can be significant in controlling crustal flow and thinning. This illustrates that offshore basin dynamics at continental margins with weak crust can be controlled by onshore surface processes in a newly recognized form of climate-tectonic coupling.

  1. 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).

  2. The dynamics of continental breakup-related magmatism on the Norwegian volcanic margin

    NASA Astrophysics Data System (ADS)

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

    2007-12-01

    The Vøring margin off mid-Norway was initiated during the earliest Eocene (~54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, an ocean bottom seismometer survey was acquired on the Norwegian margin to constrain continental breakup and early seafloor spreading processes. The profile P-wave model described here crosses the northern part of the Vøring Plateau. Maximum igneous crustal thickness was found to be 18 km, decreasing to ~6.5 km over ~6 M.y. after continental breakup. Both the volume and the duration of excess magmatism after breakup is about twice of what is observed off the Møre Margin south of the Jan Mayen Fracture Zone, which offsets the margin segments by ~170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~100 km to the Lofoten margin, but without a margin offset. There is a strong correlation between magma productivity and early plate spreading rate, which are highest just after breakup, falling with time. This is seen both at the Møre and the Vøring margin segments, suggesting a common cause. A model for the breakup- related magmatism should be able to (1) explain this correlation, (2) the magma production peak at breakup, and (3) the magmatic segmentation. Proposed end-member hypotheses are elevated upper-mantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection fluxing mantle rocks through the melt zone. Both the average P-wave velocity and the major-element data at the Vøring margin indicate a low degree of melting consistent with convection. However, small scale convection does not easily explain the issues listed above. An elaboration of the mantle plume model by N. Sleep, in which buoyant plume material fills the rift-topography at the base of the lithosphere, can explain these: When the continents break apart, the buoyant plume-material flows up into the rift zone, causing excess magmatism by both elevated

  3. The dynamics of continental breakup-related magmatism on the Norwegian volcanic margin

    NASA Astrophysics Data System (ADS)

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

    2004-12-01

    The Vøring margin off mid-Norway was initiated during the earliest Eocene (~54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, an ocean bottom seismometer survey was acquired on the Norwegian margin to constrain continental breakup and early seafloor spreading processes. The profile P-wave model described here crosses the northern part of the Vøring Plateau. Maximum igneous crustal thickness was found to be 18 km, decreasing to ~6.5 km over ~6 M.y. after continental breakup. Both the volume and the duration of excess magmatism after breakup is about twice of what is observed off the Møre Margin south of the Jan Mayen Fracture Zone, which offsets the margin segments by ~170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~100 km to the Lofoten margin, but without a margin offset. There is a strong correlation between magma productivity and early plate spreading rate, which are highest just after breakup, falling with time. This is seen both at the Møre and the Vøring margin segments, suggesting a common cause. A model for the breakup- related magmatism should be able to (1) explain this correlation, (2) the magma production peak at breakup, and (3) the magmatic segmentation. Proposed end-member hypotheses are elevated upper-mantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection fluxing mantle rocks through the melt zone. Both the average P-wave velocity and the major-element data at the Vøring margin indicate a low degree of melting consistent with convection. However, small scale convection does not easily explain the issues listed above. An elaboration of the mantle plume model by N. Sleep, in which buoyant plume material fills the rift-topography at the base of the lithosphere, can explain these: When the continents break apart, the buoyant plume-material flows up into the rift zone, causing excess magmatism by both elevated

  4. 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

  5. Constraining lithosphere deformation modes during continental breakup for the Iberia-Newfoundland conjugate rifted margins

    NASA Astrophysics Data System (ADS)

    Jeanniot, Ludovic; Kusznir, Nick; Mohn, Geoffroy; Manatschal, Gianreto; Cowie, Leanne

    2016-06-01

    A kinematic model of lithosphere and asthenosphere deformation has been used to investigate lithosphere stretching and thinning modes during continental rifting leading to breakup and seafloor spreading. The model has been applied to two conjugate profiles across the Iberia-Newfoundland rifted margins and quantitatively calibrated using observed present-day water loaded subsidence and crustal thickness, together with observed mantle exhumation, subsidence and melting generation histories. The kinematic model uses an evolving prescribed flow-field to deform the lithosphere and asthenosphere leading to lithospheric breakup from which continental crustal thinning, lithosphere thermal evolution, decompression melt initiation and subsidence are predicted. We explore the sensitivity of model predictions to extension rate history, deformation migration and buoyancy induced upwelling. The best fit calibrated models of lithosphere deformation evolution for the Iberia-Newfoundland conjugate margins require; (1) an initial broad region of lithosphere deformation with passive upwelling, (2) lateral migration of deformation, (3) an increase in extension rate with time, (4) focussing of the deformation and (5) buoyancy induced upwelling. The model prediction of exhumed mantle at the Iberia-Newfoundland margins, as observed, requires a critical threshold of melting to be exceeded before melt extraction. The preferred calibrated models predict faster extension rates and earlier continental crustal separation and mantle exhumation for the Iberia Abyssal Plain-Flemish Pass conjugate margin profile than for the Galicia Bank-Flemish Cap profile to the north. The predicted N-S differences in the deformation evolution give insights into the 3D evolution of Iberia-Newfoundland margin crustal separation.

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  7. Ice Sheet History from Antarctic Continental Margin Sediments: The ANTOSTRAT Approach

    USGS Publications Warehouse

    Barker, P.F.; Barrett, P.J.; Camerlenghi, A.; Cooper, A. K.; Davey, F.J.; Domack, E.W.; Escutia, C.; Kristoffersen, Y.; O'Brien, P.E.

    1998-01-01

    The Antarctic Ice Sheet is today an important part of the global climate engine, and probably has been so for most of its long existence. However, the details of its history are poorly known, despite the measurement and use, over two decades, of low-latitude proxies of ice sheet volume. An additional way of determining ice sheet history is now available, based on understanding terrigenous sediment transport and deposition under a glacial regime. It requires direct sampling of the prograded wedge of glacial sediments deposited at the Antarctic continental margin (and of derived sediments on the continental rise) at a small number of key sites, and combines the resulting data using numerical models of ice sheet development. The new phase of sampling is embodied mainly in a suite of proposals to the Ocean Drilling Program, generated by separate regional proponent groups co-ordinated through ANTOSTRAT (the Antarctic Offshore Acoustic Stratigraphy initiative). The first set of margin sites has now been drilled as ODP Leg 178 to the Antarctic Peninsula margin, and a first, short season of inshore drilling at Cape Roberts, Ross Sea, has been completed. Leg 178 and Cape Roberts drilling results are described briefly here, together with an outline of key elements of the overall strategy for determining glacial history, and of the potential contributions of drilling other Antarctic margins investigated by ANTOSTRAT. ODP Leg 178 also recovered continuous ultra-high-resolution Holocene biogenic sections at two sites within a protected, glacially-overdeepened basin (Palmer Deep) on the inner continental shelf of the Antarctic Peninsula. These and similar sites from around the Antarctic margin are a valuable resource when linked with ice cores and equivalent sections at lower latitude sites for studies of decadal and millenial-scale climate variation.

  8. Submarine canyon and slope processes of the U.S. Atlantic continental margin

    USGS Publications Warehouse

    McGregor, B.A.

    1983-01-01

    Two regions on the U.S. Atlantic continental margin were surveyed using single-channel, seismic-reflection profiling techniques: the Mid-Atlantic Continental Slope and Rise seaward of New Jersey in the vicinity of Baltimore Canyon and the Continental Slope and upper Rise just north of Cape Hatteras. Submarine canyons are the dominant morphologic feature in both areas. The Continental Slope in the Baltimore Canyon area has a general sea-floor gradient of 3?-4? and a width of approximately 40 km, whereas the study area north of Cape Hatteras has a general sea-floor gradient of approximately 9? and a width of 20 km. The dominant slope process differs in each area. In the Baltimore Canyon area, subbottom reflectors suggest that sediment deposition with progradation of the slope is related to canyon processes. In the study area north of Cape Hatteras, the canyons appear erosional and mass wasting is the dominant erosional process. Dominant slope processes appear to be correlated with the width and sea-floor gradient of the Continental Slope. Although the absolute age of the canyons is difficult to determine without rotary-drill cores for stratigraphic control, Baltimore Canyon is suggested to be older than the shelf-indenting canyon just north of Cape Hatteras. An anomalously large ridge flanking Baltimore Canyon on the upper rise appears to be related to canyon depositional and erosional processes.

  9. Rift migration explains continental margin asymmetry and crustal hyper-extension.

    PubMed

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

    2014-06-06

    When continents break apart, continental crust and lithosphere are thinned until break-up is achieved and an oceanic basin is formed. The most remarkable and least understood structures associated with this process are up to 200 km wide areas of hyper-extended continental crust, which are partitioned between conjugate margins with pronounced asymmetry. Here we show, using high-resolution thermo-mechanical modelling, that hyper-extended crust and margin asymmetry are produced by steady state rift migration. We demonstrate that rift migration is accomplished by sequential, oceanward-younging, upper crustal faults, and is balanced through lower crustal flow. Constraining our model with a new South Atlantic plate reconstruction, we demonstrate that larger extension velocities may account for southward increasing width and asymmetry of these conjugate magma-poor margins. Our model challenges conventional ideas of rifted margin evolution, as it implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other.

  10. Rift migration explains continental margin asymmetry and crustal hyper-extension

    PubMed Central

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

    2014-01-01

    When continents break apart, continental crust and lithosphere are thinned until break-up is achieved and an oceanic basin is formed. The most remarkable and least understood structures associated with this process are up to 200 km wide areas of hyper-extended continental crust, which are partitioned between conjugate margins with pronounced asymmetry. Here we show, using high-resolution thermo-mechanical modelling, that hyper-extended crust and margin asymmetry are produced by steady state rift migration. We demonstrate that rift migration is accomplished by sequential, oceanward-younging, upper crustal faults, and is balanced through lower crustal flow. Constraining our model with a new South Atlantic plate reconstruction, we demonstrate that larger extension velocities may account for southward increasing width and asymmetry of these conjugate magma-poor margins. Our model challenges conventional ideas of rifted margin evolution, as it implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other. PMID:24905463

  11. Scheme of 3 interfaces with local isostatic compensation on the Argentine continental margin

    NASA Astrophysics Data System (ADS)

    Pedraza De Marchi, A. C.; Ghidella, M. E.; Tocho, C.

    2013-05-01

    The segment of Argentine continental margin located between 39°S and the Malvinas platform (~49°S) is of passive type and volcanic characteristics revealed by seaward-dipping seismic reflectors sequences (SDRs). The free air gravity edge-effect associated with passive continental margins is one of the most distinctive characteristics of gravity in marine regions. This effect is in large part due to the transition between continental and oceanic crusts, because of their different thicknesses. In this presentation we investigate the Airy type isostatic compensation scheme by using three interfaces in a forward calculation with different approximations of Parker's expression to obtain the isostatic anomaly. After that we perform the inversion of the anomaly thus obtained in order to find the Moho's deflection necessary to compensate it (or minimize it) by using the same scheme of interfaces and the iterative Parker-Oldenburg method (Oldenburg, D., 1974) with more terms in the inversion. The crust-mantle interface (Moho) thus calculated represents a more realistic surface than the one calculated using one term in the inversion and the surface estimated with topographic data and sediment thickness. Even considering that the experiment constitutes a schematic assumption just to test the numerical methods involved, we find that in the comparison with the only available digitized refraction profile, the inverted Moho interface reproduces fairly well the Moho that the seismic profile yields, for the case of the iterative method. This suggests that the inverse calculation with the iterative method is sensible to the presence of the SDRS, at least for this sole profile. Keywords: isostatic anomaly, Moho, passive continental margins Oldenburg, D., 1974. The inversion and interpretation of gravity anomalíes, Geophysics, vol. 39, no. 4, p. 526-536.

  12. 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

  13. Dating and Recognition of Major Glaciations Events - Evidence from Unglaciated Continental Margins

    NASA Astrophysics Data System (ADS)

    Pierantoni Gamboa, Luiz Antonio; Scarparo, Armando; Severiano de Vasconcelos, Claudemir; Trindade Rodrigues de Freitas, João.

    2010-05-01

    Dating the multiple glacial episodes on Antarctic interior basins and margins can be very difficult as erosional events can be profound, and useful fossils for dating are missing in most of the depositional sequences. However, some episodes can be recognized in certain basins located along the paths of the currents generated or intensified by major climatic changes, as observed in southern Brazilian continental margin. At least two major pulses indicative of intense transport of sediments can be recognized in southern Brazilian continental margin sequences during the Oligo-Miocene. Interlayed in finer sediments, a widespread occurrence of sand waves and submarine dunes of Late-Miocene age has been mapped. This coarse grained stratigraphic interval was identified using 3D seismic surveys and dated using the muddy intervals above and below, sampled in several exploratory wells. The flow along the slope was towards the north. Another interval affected by intense bottom currents was identified in Oligocene-age rocks. An erosive event occurred along the entire Brazilian margin, just after the deposition of anomalous Braarudosphaera-chalks (Braarudosphaera-rich layers that record recurrent specific coccolithophorid blooms) at 28.5 Ma. Palaeoceanographic evidence suggests that both events were triggered by glacial events in Antarctica. An intensification of bottom currents marked by a major regional unconformity eroded at about 28.5 Ma and intense reworking at the Late Miocene can be correlated with major glacial events recorded in the Antarctic continental sequences. Detailed studies using available seismic and drilled material could recognize events of smaller magnitude.

  14. Evolution of Northeast Atlantic Magmatic Continental Margins from an Ethiopian-Afar Perspective

    NASA Astrophysics Data System (ADS)

    England, R. W.; Cornwell, D. G.; Ramsden, A. M.

    2014-12-01

    One of the major problems interpreting the evolution of magmatic continental margins 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 constraints we have examined the Ethiopian - Afar rift system to try to understand the rifting process. The Main Ethiopian rift contains an embryonic magmatic passive margin dominated by faulting at the margins of the rift and en-echelon magmatic zones at the centre. Further north toward Afar the rift becomes in-filled with extensive lava flows fed from fissure systems in the widening rift zone. This rift system provides, along its length, a series of 'snapshots' into the possible tectonic evolution of a magmatic continental margin. Deep seismic profiles crossing the NE Atlantic margins reveal ocean dipping reflector sequences (ODRS) overlying extended crust and lower crustal sill complexes of intruded igneous rock, which extend back beneath the continental margin. The ODRS 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 from fissures which are now observed as the ODRS. Also in the seismic profiles we identify volcanic constructs on the ODRS which we interpret as the equivalent of the present day fissure eruptions seen in Afar. The ocean ward dip on the ODRS is

  15. Tectonic elements of the continental margin of East Antarctica, 38-164ºE

    USGS Publications Warehouse

    O'Brien, P.E.; Stagg, H.M.J.

    2007-01-01

    The East Antarctic continental margin from 38–164ºE is divided into western and eastern provinces that developed during the separation of India from Australia–Antarctica (Early Cretaceous) and Australia from Antarctica (Late Cretaceous). In the overlap between these provinces the geology is complex and bears the imprint of both extension/spreading episodes, with an overprinting of volcanism. The main rift-bounding faults appear to approximately coincide with the outer edge of the continental shelf. Inboard of these faults, the sedimentary cover thins above shallowing basement towards the coast where crystalline basement generally crops out. The continental slope and the landward flanks of the ocean basins, are blanketed by up to 9–10 km of mainly post-rift sediments in margin-parallel basins, except in the Bruce Rise area. Beneath this blanket, extensive rift basins are identified off Enderby and Wilkes Land/Terre Adélie; however, their extent and detailed structures are difficult to determine.

  16. 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

  17. 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

  18. Cenozoic tectonic jumping and implications for hydrocarbon accumulation in basins in the East Asia Continental Margin

    NASA Astrophysics Data System (ADS)

    Suo, Yanhui; Li, Sanzhong; Yu, Shan; Somerville, Ian D.; Liu, Xin; Zhao, Shujuan; Dai, Liming

    2014-07-01

    Tectonic migration is a common geological process of basin formation and evolution. However, little is known about tectonic migration in the western Pacific margins. This paper focuses on the representative Cenozoic basins of East China and its surrounding seas in the western Pacific domain to discuss the phenomenon of tectonic jumping in Cenozoic basins, based on structural data from the Bohai Bay Basin, the South Yellow Sea Basin, the East China Sea Shelf Basin, and the South China Sea Continental Shelf Basin. The western Pacific active continental margin is the eastern margin of a global convergent system involving the Eurasian Plate, the Pacific Plate, and the Indian Plate. Under the combined effects of the India-Eurasia collision and retrogressive or roll-back subduction of the Pacific Plate, the western Pacific active continental margin had a wide basin-arc-trench system which migrated or ‘jumped’ eastward and further oceanward. This migration and jumping is characterized by progressive eastward younging of faulting, sedimentation, and subsidence within the basins. Owing to the tectonic migration, the geological conditions associated with hydrocarbon and gashydrate accumulation in the Cenozoic basins of East China and its adjacent seas also become progressively younger from west to east, showing eastward younging in the generation time of reservoirs, seals, traps, accumulations and preservation of hydrocarbon and gashydrate. Such a spatio-temporal distribution of Cenozoic hydrocarbon and gashydrate is significant for the oil, gas and gashydrate exploration in the East Asian Continental Margin. Finally, this study discusses the mechanism of Cenozoic intrabasinal and interbasinal tectonic migration in terms of interplate, intraplate and underplating processes. The migration or jumping regimes of three separate or interrelated events: (1) tectonism-magmatism, (2) basin formation, and (3) hydrocarbon-gashydrate accumulation are the combined effects of the

  19. Ocean Drilling Program Leg 178 (Antarctic Peninsula): Sedimentology of glacially influenced continental margin topsets and foresets

    USGS Publications Warehouse

    Eyles, N.; Daniels, J.; Osterman, L.E.; Januszczak, N.

    2001-01-01

    Ocean Drilling Program Leg 178 (February-April 1998) drilled two sites (Sites 1097 and 1103) on the outer Antarctic Peninsula Pacific continental shelf. Recovered strata are no older than late Miocene or early Pliocene (<4.6 Ma). Recovery at shallow depths in loosely consolidated and iceberg-turbated bouldery sediment was poor but improved with increasing depth and consolidation to allow description of lithofacies and biofacies and interpretation of depositional environment. Site 1097 lies on the outer shelf within Marguerite Trough which is a major outlet for ice expanding seaward from the Antarctic Peninsula and reached a maximum depth drilled of 436.6 m below the sea floor (mbsf). Seismic stratigraphic data show flat-lying upper strata resting on strata that dip gently seaward. Uppermost strata, to a depth of 150 mbsf, were poorly recovered, but data suggest they consist of diamictites containing reworked and abraded marine microfauna. This interval is interpreted as having been deposited largely as till produced by subglacial cannibalization of marine sediments (deformation till) recording ice sheet expansion across the shelf. Underlying gently dipping strata show massive, stratified and graded diamictite facies with common bioturbation and slump stuctures that are interbedded with laminated and massive mudstones with dropstones. The succession contains a well-preserved in situ marine microfauna typical of open marine and proglacial marine environments. The lower gently dipping succession at Site 1097 is interpreted as a complex of sediment gravity flows formed of poorly sorted glacial debris. Site 1103 was drilled in that part of the continental margin that shows uppermost flat-lying continental shelf topsets overlying steeper dipping slope foresets seaward of a structural mid-shelf high. Drilling reached a depth of 363 mbsf with good recovery in steeply dipping continental slope foreset strata. Foreset strata are dominated by massive and chaotically

  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. Basement Tectonics 8: Characterization and comparison of ancient and mesozoic continental margins. Proceedings

    SciTech Connect

    Bartholomew, M.J.; Hyndman, D.W.; Mogk, D.W.; Mason, R.

    1988-12-31

    The International Conference on Basement Tectonics was held in Butte, Montana, August 8--12,1988. Historically, basement tectonics conferences have focused on such topics as reactivation of faults, the influence of basement faults on metallogeny and hydrocarbon accumulation, and the use of geophysical and remote sensing techniques to interpret subsurface and surface geology. The 8th Conference diverged from past conferences in that a unifying theme was selected. Because ancient major terrane or cratonic boundaries are often postulated to be fault zones which are subsequently reactivated, the conference was organized to examine all aspects of ancient continental margins and terrane boundaries and to compare younger (Mesozoic) ones, about which more is known, with older (Paleozoic and Precambrian) ones. Moreover, because the 8th Conference was held in the northwestern United States, a greater emphasis was placed on the Mesozoic margin of western North America and the North American shield. The seven oral sessions and four poster sessions all dealt with aspects of the conference theme: characterization and comparison of ancient continental margins. The papers will be indexed individually.

  2. Basement Tectonics 8: Characterization and comparison of ancient and mesozoic continental margins

    SciTech Connect

    Bartholomew, M.J. . Inst. of Earth Sciences and Resources); Hyndman, D.W. . Dept. of Geology); Mogk, D.W. . Dept. of Earth Sciences); Mason, R. . Dept. of Geological Sciences)

    1988-01-01

    The International Conference on Basement Tectonics was held in Butte, Montana, August 8--12,1988. Historically, basement tectonics conferences have focused on such topics as reactivation of faults, the influence of basement faults on metallogeny and hydrocarbon accumulation, and the use of geophysical and remote sensing techniques to interpret subsurface and surface geology. The 8th Conference diverged from past conferences in that a unifying theme was selected. Because ancient major terrane or cratonic boundaries are often postulated to be fault zones which are subsequently reactivated, the conference was organized to examine all aspects of ancient continental margins and terrane boundaries and to compare younger (Mesozoic) ones, about which more is known, with older (Paleozoic and Precambrian) ones. Moreover, because the 8th Conference was held in the northwestern United States, a greater emphasis was placed on the Mesozoic margin of western North America and the North American shield. The seven oral sessions and four poster sessions all dealt with aspects of the conference theme: characterization and comparison of ancient continental margins. The papers will be indexed individually.

  3. Continental Margins and the Law of the Sea - an `Arranged Marriage' with Huge Research Potential

    NASA Astrophysics Data System (ADS)

    Parson, L.

    2005-12-01

    The United Nations Convention on the Law of the Sea (UNCLOS) requires coastal states intending to secure sovereignty over continental shelf territory extending beyond 200 nautical miles to submit geological/geophysical data, along with their analysis and synthesis of the relevant continental margin in support of their claim. These submissions are scrutinised and assessed by a UN Commission of experts who decide if the claim is justified, and thereby ultimately allowing the exploitation of non-living resources into this extended maritime space. The amount of data required to support the case will vary from margin to margin, depending on the local geological evolution, but typically will involve the running of new, dedicated marine surveys, mostly bathymetric and seismic. Key geological/geophysical issues revolve around proof of `naturalness' of the prolongation of land mass (cue - wide-angle seismics, deep drilling and sampling programmes) and shelf and slope morphology and sediment section thickness (cue - swath bathymetry and multichannel seismics programmes). These surveys, probably primarily funded by government agencies anxious not to lose out on the `land grab', will generate datasets which will inevitably boost not only the research effort leading to increased understanding of margin evolution in academic terms, but also contribute to wider applied aspects of the work such as those leading to refinement of deepwater hydrocarbon resource potential. It is conservatively estimated that in the region of fifty coastal states world-wide have a significant potential for claiming continental shelf beyond 200 nautical miles, and that the total area available as extended shelf could easily exceed 7 million square kilometres. However, while for the vast majority of these states a UNCLOS deadline of 2009 exists for submitting a claim - to date only four have done so (Russia, Brazil, Australia and Ireland). It is therefore predictable, if not inevitable, that within the

  4. 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.

  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. Shelf basin exchange along the Siberian continental margin: Modification of Atlantic Water and Lower Halocline Water

    NASA Astrophysics Data System (ADS)

    Bauch, Dorothea; Cherniavskaia, Ekaterina; Timokhov, Leonid

    2016-09-01

    Salinity and stable oxygen isotope (δ18O) evidence shows a modification of Atlantic Water in the Arctic Ocean by a mixture of sea-ice meltwater and meteoric waters along the Barents Sea continental margin. On average no further influence of meteoric waters is detectable within the core of the Atlantic Water east of the Kara Sea as indicated by constant δ18O, while salinity further decreases along the Siberian continental slope. Lower Halocline Waters (LHW) may be divided into different types by Principal Component Analysis. All LHW types show the addition of river water and an influence of sea-ice formation to a varying extent. The geographical distribution of LHW types suggest that the high salinity type of LHW forms in the Barents and Kara seas, while other LHW types are formed either in the northwestern Laptev Sea or from southeastern Kara Sea waters that enter the northwestern Laptev Sea through Vilkitsky Strait. No further modification of LHW is seen in the eastern Laptev Sea but the distribution of LHW-types suggest a bifurcation of LHW at this location, possibly with one branch continuing along the continental margin and a second branch along the Lomonosov Ridge. We see no pronounced distinction between onshore and offshore LHW types, as the LHW components that are found within the halocline over the basin also show a narrow bottom-bound distribution at the continental slope that is consistent with a shelf boundary current as well as a jet of water entering the western Laptev Sea from the Kara Sea through Vilkitsky Strait.

  7. Tertiary evolution and petroleum potential of Oregon-Washington continental margin

    SciTech Connect

    Snavely, P.D. Jr.

    1986-07-01

    The Oregon-Washington continental margin was the site of a deep marginal basin in which more than 7000 m of Tertiary sedimentary and volcanic rocks accumulated. Oceanic basalts of Paleocene to early Eocene age form the basin floor and are interpreted to represent eruptions in an elongate trough formed by rifting of the continental margin. Middle Eocene turbidite sandstone overlapped both the oceanic crust and the pre-Tertiary rocks of the Klamath Mountains, thus indicating that suturing of the Coast Range-Olympic terrane to North America was about 50 Ma. Oblique convergence between the Farallon and North American plates occurred during most of the middle Eocene to middle Miocene. Sedimentation, punctuated by episodes of volcanism, was essentially continuous in a forearc basin whose axis lay along the present inner continental shelf. The oblique interaction between the plates was interrupted by two periods of more head-on convergence during the middle late Eocene and late middle Miocene. Thick accretionary melange wedges of Eocene and of late Oligocene to late middle Miocene ages were formed during these strongly compressive episodes. Geochemical analyses indicate that the accretionary melange wedges, which crop out along the west side of the Olympic Peninsula and beneath the adjacent shelf, have the highest potential for oil and gas generation. They are the source rocks for numerous gas seeps and oil and gas shows in exploratory wells, and for the 12,000 bbl of 38.9 /sup 0/ API paraffin-based oil produced from a well drilled on the southwest Washington coast. Potential exploration targets exist where the Eocene and Oligocene-Miocene melanges are underplated to a position beneath the lower Eocene oceanic basalt. Hydrocarbons generated in the melanges could migrate upward into structures in strata that overlie the basalt in the upper plate.

  8. Molybdenum isotope signatures from the Yangtze block continental margin and its indication to organic burial rate

    NASA Astrophysics Data System (ADS)

    Zhou, L.; Zhou, H. B.; Huang, J. H.

    2007-12-01

    The paper presents the molybdenum isotope data, along with the trace element content, to investigate the geochemical behavior of authigenic Mo during long-term burial in sediments in continental margin settings of the Yangtze block, as well as their indication to the burial of original organic carbon. The burial rate of original organic carbon were estimated on the basis of the amount of sedimentary sulfur (TS content), whilst the carbon loss by aerobic degradation was estimated according to calculated Mn contents. On these points, the original organic carbon flux was calculated, exhibiting a large range of variation (2.54-15.82 mmol/m2/day). The strong correlation between sedimentary Mo isotope values and organic carbon burial rates previously proposed on the basis of the investigations on modern ocean sediments was also used here to estimate the organic carbon burial rate. The data gained through this model showed that organic carbon burial rates have large variations, ranging from 0.43- 2.87mmol/m2/day. Although the two sets of data gained through different geochemical records in the Yangtze block show a deviation of one order of magnitude, they do display a strong correlation. It is thus tempting to speculate that the Mo isotope signature of sediments may serve as a tracer for the accumulation rate of original organic carbon in the continental margin sediments. Keywords: Molybdenum isotopes; organic carbon burial rate; ancient continental margin setting ACKNOWLEDGMENTS We thank Professor Xie Shucheng for his constructive review comments. This research is co-supported by the Program for Changjiang Scholars and Innovative Research Team in University (grants IRT0441), the SinoPec project (grant no. G0800-06-ZS-319) and the National Nature Science Foundation of China (grants 40673020).

  9. Stratigraphic evolution of Mesozoic continental margin and oceanic sequences northwest Australia and north Himalayas

    SciTech Connect

    Gradstein, F.M. ); Von Rad, U. )

    1990-05-01

    The authors are investigating continental margin to ocean sequences of the incipient Indian Ocean as it replaced central Tethys. Objectives of this study are the dynamic relation between sedimentation, tectonics, and paleogeography. Principal basins formation along the northern edge of eastern Gondwana started in the Late Permian to the Triassic. By the Late Triassic-Early Jurassic, platform carbonates with thin, organic-rich lagoonal shales were laid down in a subtropical climate. This unit, which harbors some of the oldest known nannofossils, shows repeated shallowing-upward sequences. Subsequent southward drift of the Gondwana margin during the Middle Jurassic increased siliciclastic input in Nepal, when widespread sediment starvation or erosion during local uplift took place off parts of northwest Australia. A middle Callovian-early Oxfordian hiatus in Nepal is submarine and appears global in extent. The overlying 250-m-thick organic-rich black shales, correlative to the Oxford/Kimmeridge clays of circum-Atlantic petroleum basins, may be traced along the northern Himalayan Range, and probably represent an extensive continental slope deposit formed under an oxygen minimum layer in southern Tethys. The deposit's diverse foraminiferal microfauna was previously only known from boreal Laurasia. The Callovian breakup unconformity, off northwest Australia, precedes onset of sea-floor spreading at least 15-25 Ma. Sea-floor spreading, leading to the present Indian Ocean started in the Argo Abyssal Plain around 140 Ma, at the end of the Jurassic, was about 15 m.y. later than previously postulated. Australia and Greater India separated as early as the Late Valanginian, about 130 Ma. Mafic volcaniclastics in Nepalese deltaic sediments probably testify to concurrent continental margin volcanic activity, which may be a precursor to the slightly younger Rajmahal traps in eastern India.

  10. Seafloor morphology of the Montenegro/N. Albania Continental Margin (Adriatic Sea-Central Mediterranean)

    NASA Astrophysics Data System (ADS)

    Del Bianco, Fabrizio; Gasperini, Luca; Giglio, Federico; Bortoluzzi, Giovanni; Kljajic, Zoran; Ravaioli, Mariangela

    2014-12-01

    High-resolution multibeam morpho-bathymetric maps and a dense grid of seismic reflection profiles show relict and palimpsest geomorphologic features along the Montenegro/Northern Albanian Continental Margin. This sector of the Eastern Adriatic shelf, at the external front of the Dinarides Chain, is characterized by highly variable seafloor patterns and depositional styles, and shows a peculiar alternation of large-scale troughs and ridges, probably caused by tectonic compressive deformations. These tectonically controlled morphologies are overprinted by the result of sedimentary processes, such as progradation at river outflows, erosion, and reworking of sediments by longshore currents, as well as gravity-driven process caused by sediment loading and seismic shaking. Physiographic domains along this shelf-slope margin include (i) an inner and an outer shelf, separated by two major topographic highs, the Kotor and the Bar ridges; (ii) a drowned lobate delta formed during the last phase of sea level fall, likely fed by the Buna/Bojana drainage basin; and (iii) a continental slope affected by gravity-driven faulting and mass-wasting processes. Seafloor reflectivity maps, ground-truthed by grain-size analysis of bottom sediments, reveal that fine-grained deposits accumulate in the inner shelf, while other sectors appear starved. The effects of the last sea-level rise is testified by the presence of seabed forms diagnostic of erosion or depositional processes, such us large dunes, sediment ridges and sediment waves, which were studied to infer the effect of bottom currents under the present-day oceanographic regime and in the recent past. This paper presents a first description of geomorphologic features observed along the Montenegro/Northern Albanian Continental Margin, in the context of Late Quaternary sea-level changes.

  11. Tectonic significance of Synrift sediment packages across the Congo continental margin

    SciTech Connect

    McGinnis, J.P.; Karner, G.D.; Driscoll, N.W. ); Brumbaugh, W.D. ); Cameron, N. )

    1993-09-01

    The tectonic and stratigraphic development of the Congo continental margin reflects the timing, magnitude, and distribution of lithospheric extension responsible for its formation. Details of the lithospheric extension process are recorded in the stratigraphic successions preserved along and across the margin. By using the stratal relationships (e.g., onlap, downlap, and truncation) and lithofacies determined from seismic reflection and exploratory well data as input into our basin-modeling strategy, we have developed an integrated approach to determine the relationship between the timing, magnitude, and distribution of lithospheric extension across the margin. Two hinge zones, an eastern and Atlantic hinge formed along the Congo margin in response to discrete extensional events occurring from the Berriasian to the Aptian. The eastern hinge zone demarcates the eastern limit of the broadly distributed Berriasian extension. This extension resulted in the formation of deep anoxic, lacustrine systems. In contrast, the Atlantic hinge, located [approximately]90 km west of the eastern hinge, marks the eastern limit of a second phase of extension, which began in the Hauterivian. Consequent footwall uplift and rotation exposed the earlier synrift and prerift stratigraphy to at least wave base causing varying amounts of erosional truncation across the Atlantic hinge zone along much of the Gabon, Congo, and Angola margins. The absence of the Melania Formation across the Congo margin implies that uplift of the Atlantic hinge was relatively minor compared to that across the Angola and Gabon margins. In addition, material eroded from the adjacent and topographically higher hinge zones may in part account for the thick wedge of sediment deposited seaward of the Congo Atlantic hinge. A third phase of extension reactivated both the eastern and Atlantic hinge zones and was responsible for creating the accommodation space for Marnes Noires source rock deposition.

  12. 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.

  13. The benthic manganese cycle along the Oregon-California continental margin

    NASA Astrophysics Data System (ADS)

    McManus, J.; Berelson, W.; Severmann, S.; Roy, M.; Chase, Z.; Muratli, J. M.; Hastings, R. H.; Goni, M. A.; Mix, A. C.

    2010-12-01

    High rates of sedimentary organic carbon degradation within continental margin sediments combined with abundant supply of reactive Mn-oxides from continental weathering result in a sedimentary environment where Mn is readily solubilized through microbial or abiotic Mn-oxide reduction. Here we examine the consequences of the interplay among the delivery of terrigenous material, high organic carbon oxidation rates, and Mn reduction within the sediments along the Oregon-California continental margin. In areas of high terrigenous sediment discharge, the benthic Mn efflux is enhanced over typical continental margin rates—even at sites with comparable organic carbon oxidation rates. Our preliminary flux estimates suggest that the highest benthic Mn effluxes may exceed ~50 µmol m-2 day-1 off the Eel River; prior work and our new results show that open margin (i.e., non-shelf) environments typically have Mn efflux rates < 10 µmol m-2 day-1. A survey of surface sediments from the Umpqua River discharge area supports the notion that shallow (~ < 100 m) shelf sediments may be a net source of Mn to the water column. These sediments, which are bathed intermittently by hypoxic waters, have Mn to Al ratios as low as ~ 2 x 10-3 g g-1; for comparison, Umpqua river sediments have Mn to Al values of ~6 - 7 x 10-3 g g-1 for the 20 - 63 micron size fraction (VanLaningham, 2007). Mn to Al ratios of ~ 8 - 11 x 10-3 g g-1 (implying possible Mn enrichment relative to river sediments) occur between ~125 and 150 m water depth. Ratios decrease again at greater depths to values of ~7 x 10-3 g g-1, close to that of river sediment. Collectively our data support the idea of an efflux of Mn from shelf sediments. Although the benthic incubation chamber data all indicate a Mn efflux from the sediments, the sedimentary Mn distributions indicate the possibility of re-deposition at depths of ~125 - 150 m. Further work will need to evaluate diagenetic processes and provenance effects on the sediment

  14. Origins and fates of DOM along the New England continental margin. Techical progress report, Year 1

    SciTech Connect

    Fry, B.; Hopkinson, C.; Altabet, M.

    1993-06-01

    We have focused on methods development and completed an initial survey of Dissolved Organic Matter (DOM) concentration and isotopic composition during the 1994 spring bloom on Georges Bank. The methods development phase assures that high quality measurements will be made in years 2 and 3, and emphasizes developing two independent methods for DON determination. The survey work on Georges Bank will be extended in years 2 and 3 to determine seasonal and spatial changes in DOM concentrations and isotopic compositions along the eastern US continental margin, from Georges Bank to Cape Hatteras.

  15. Origins and fates of DOM along the New England continental margin

    SciTech Connect

    Fry, B.; Hopkinson, C. . Ecosystems Center); Altabet, M. )

    1993-01-01

    We have focused on methods development and completed an initial survey of Dissolved Organic Matter (DOM) concentration and isotopic composition during the 1994 spring bloom on Georges Bank. The methods development phase assures that high quality measurements will be made in years 2 and 3, and emphasizes developing two independent methods for DON determination. The survey work on Georges Bank will be extended in years 2 and 3 to determine seasonal and spatial changes in DOM concentrations and isotopic compositions along the eastern US continental margin, from Georges Bank to Cape Hatteras.

  16. The present day formation of apatite in Mexican continental margin sediments

    NASA Astrophysics Data System (ADS)

    Jahnke, Richard A.; Emerson, Steven R.; Roe, Kevin K.; Burnett, William C.

    1983-02-01

    Results of pore water and sediment analyses from the western Mexican continental margin strongly suggest the present day formation of apatite. The interstitial water phosphate and fluoride profiles indicate chemical removal at a depth which corresponds to a large maximum in the phosphorus content of the sediments. Apatite is identified within this maximum via X-ray diffraction but is elsewhere undetectable in the core. Radioisotopic thorium, uranium, and radium data support the conclusion that this deposit is modern. The present day depositional environment is consistent with those reported by other workers for phosphorite formation with the exception that pore water magnesium is not depleted below its seawater value.

  17. Observations of intermediate nepheloid layers on the northern California continental margin

    NASA Astrophysics Data System (ADS)

    McPhee-Shaw, E. E.; Sternberg, R. W.; Mullenbach, B.; Ogston, A. S.

    2004-04-01

    Conductivity-temperature-depth and transmissometer surveys were undertaken to investigate the characteristics and seasonal nature of intermediate nepheloid layers (INLs) over the outer shelf and upper slope of the northern California margin, near Eureka, CA. Observed INLs could generally be grouped into one of two categories: INLs that formed and spread seaward from the continental shelf, and INLs generated at continental slope depths greater than 150 m. Shelf INLs, forming between 70 and 150-m depth on the outer shelf and extending seaward 15-20 km past the shelf break, were prevalent during winter and early spring. Continental slope INLs were found during all seasons of the year. Their suspended particulate matter concentration was lower than that of the shelf INLs, and their horizontal extent was 3-7 km seaward from the slope. Mooring data from 450-m depth show strong bottom-trapped semidiurnal internal tidal energy and asymmetry between upslope and downslope semidiurnal motions, indicative of bore-like behavior. These analyses suggest that critical reflection of internal tides is a common feature of the outer continental slope in this region. The location of slope-depth INLs was often observed to coincide with regions where the topographic slope angle was critical for internal tide reflection. In addition, seasonal variability in the span of critical topography over the slope matches seasonal variability in the density of observed INLs over the continental slope. Slope-depth INLs appear to be associated with energetic tidal motions and interaction between internal tides and bathymetry, whereas shelf-depth INLs are likely formed by resuspension events on the outer shelf and subsequent transport offshore by mesoscale circulation processes.

  18. The gravity field of the U.S. Atlantic continental margin

    USGS Publications Warehouse

    Grow, A.J.; Bowin, C.O.; Hutchinson, D.O.

    1979-01-01

    Approximately 39,000 km of marine gravity data collected during 1975 and 1976 have been integrated with U.S. Navy and other available data over the U.S. Atlantic continental margin between Florida and Maine to obtain a 10 mgal contour free-air gravity anomaly map. A maximum typically ranging from 0 to +70 mgal occurs along the edge of the shelf and Blake Plateau, while a minimum typically ranging from -20 to -80 mgal occurs along the base of the continental slope, except for a -140 mgal minimum at the base of the Blake Escarpment. Although the maximum and minimum free-air gravity values are strongly influenced by continental slope topography and by the abrupt change in crustal thickness across the margin, the peaks and troughs in the anomalies terminate abruptly at discrete transverse zones along the margin. These zones appear to mark major NW-SE fractures in the subsided continental margin and adjacent deep ocean basin, which separate the margin into a series of segmented basins and platforms. Rapid differential subsidence of crustal blocks on either side of these fractures during the early stages after separation of North America and Africa (Jurassic and Early Cretaceous) is inferred to be the cause of most of the gravity transitions along the length of margin. The major transverse zones are southeast of Charleston, east of Cape Hatteras, near Norfolk Canyon, off Delaware Bay, just south of Hudson Canyon and south of Cape Cod. Local Airy isostatic anomaly profiles (two-dimensional, without sediment corrections) were computed along eight multichannel seismic profiles. The isostatic anomaly values over major basins beneath the shelf and rise are generally between -10 and -30 mgal while those over the platform areas are typically 0 to +20 mgal. While a few isostatic anomaly profiles show local 10-20 mgal increases seaward of the East Coast Magnetic Anomaly (ECMA: inferred to mark the ocean-continent boundary), the lack of a consistent correlation indicates that the

  19. 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

  20. Variability of subseafloor viral abundance at the geographically and geologically distinct continental margins.

    PubMed

    Yanagawa, Katsunori; Morono, Yuki; Yoshida-Takashima, Yukari; Eitoku, Masamitsu; Sunamura, Michinari; Inagaki, Fumio; Imachi, Hiroyuki; Takai, Ken; Nunoura, Takuro

    2014-04-01

    We studied the relationship between viral particle and microbial cell abundances in marine subsurface sediments from three geographically distinct locations in the continental margins (offshore of the Shimokita Peninsula of Japan, the Cascadia Margin off Oregon, and the Gulf of Mexico) and found depth variations in viral abundances among these sites. Viruses in sediments obtained offshore of the Shimokita and in the Cascadia Margin generally decreased with increasing depth, whereas those in sediments from the Gulf of Mexico were relatively constant throughout the investigated depths. In addition, the abundance ratios of viruses to microbial cells notably varied among the sites, ranging between 10(-3) and 10(1) . The subseafloor viral abundance offshore of the Shimokita showed a positive relationship with the microbial cell abundance and the sediment porosity. In contrast, no statistically significant relationship was observed in the Cascadia Margin and the Gulf of Mexico sites, presumably due to the long-term preservation of viruses from enzymatic degradation within the low-porosity sediments. Our observations indicate that viral abundance in the marine subsurface sedimentary environment is regulated not only by in situ production but also by the balance of preservation and decay, which is associated with the regional sedimentation processes in the geological settings.

  1. Preliminary digital geologic map of the Penokean (early Proterozoic) continental margin in northern Michigan and Wisconsin

    USGS Publications Warehouse

    Cannon, W.F.; Ottke, Doug

    1999-01-01

    The data on this CD consist of geographic information system (GIS) coverages and tabular data on the geology of Early Proterozoic and Archean rocks in part of the Early Proterozoic Penokean orogeny. The map emphasizes metasedimentary and metavolcanic rocks that were deposited along the southern margin of the Superior craton and were later deformed during continental collision at about 1850 Ma. The area includes the famous iron ranges of the south shore region of the Lake Superior district. Base maps, both as digital raster graphics (DRG) and digital line graphs (DLG) are also provided for the convenience of users. The map has been compiled from many individual studies, mostly by USGS researchers, completed during the past 50 years, including many detailed (1:24,000 scale) geologic maps. Data was compiled at 1:100,000 scale and preserves most of the details of source materials. This product is a preliminary release of the geologic map data bases during ongoing studies of the geology and metallogeny of the Penokean continental margin. Files are provided in three formats: Federal Spatial Data Transfer format (SDTS), Arc export format (.e00) files, and Arc coverages. All files can be accessed directly from the CD-ROM using either ARC/INFO 7.1.2 or later or Arc View 3.0 or later software. ESRI's Arc Explorer, a free GIS data viewer available at the web site: http://www.esri.com/software/arcexplorer/index.html also provides display and querying capability for these files.

  2. Macrofaunal Biodiversity Response to Natural Gradients of Multiple Stressors on Continental Margins

    NASA Astrophysics Data System (ADS)

    Sperling, E. A.; Frieder, C.; Levin, L. A.

    2015-12-01

    Sharp increases in atmospheric CO2 are resulting in ocean warming, acidification and deoxygenation. Rates of change are unprecedented, raising questions about whether (and how) communities will adapt and if responses will reflect synergistic interactions among multiple stressors. Changes to benthic biodiversity on continental margins have important implications for carbon cycle processes and other ecosystem services. One strategy to interpret adaptation potential and predict future faunal change is to examine ecological shifts along natural gradients in the modern ocean. Here, we assess the explanatory power of major climate stressors for macrofaunal diversity and evenness along continental margins using variance partitioning techniques. Sharp drops in diversity are seen as O2 levels decline through the 0.5 - 0.15 ml/l (~22 - 6 μM; ~21 - 5 matm) range, and as temperature increases through the 7-10°C range. pCO2 shows a strong effect in the Arabian Sea but very little effect in the Eastern Pacific Ocean. In contrast, very little variation in evenness is explained by these three global change variables. The identification of sharp thresholds in ecological response are used here to predict seafloor areas most at risk to future marine global change, although the existence of clear regional differences cautions against applying global thresholds.

  3. First evidence for the presence of iron oxidizing zetaproteobacteria at the Levantine continental margins.

    PubMed

    Rubin-Blum, Maxim; Antler, Gilad; Tsadok, Rami; Shemesh, Eli; Austin, James A; Coleman, Dwight F; Goodman-Tchernov, Beverly N; Ben-Avraham, Zvi; Tchernov, Dan

    2014-01-01

    During the 2010-2011 E/V Nautilus exploration of the Levantine basin's sediments at the depth of 300-1300 m, densely patched orange-yellow flocculent mats were observed at various locations along the continental margin of Israel. Cores from the mat and the control locations were collected by remotely operated vehicle system (ROV) operated by the E/V Nautilus team. Microscopic observation and phylogenetic analysis of microbial 16S and 23S rRNA gene sequences indicated the presence of zetaproteobacterial stalk forming Mariprofundus spp.-like prokaryotes in the mats. Bacterial tag-encoded FLX amplicon pyrosequencing determined that zetaproteobacterial populations were a dominant fraction of microbial community in the biofilm. We show for the first time that zetaproteobacterial may thrive at the continental margins, regardless of crustal iron supply, indicating significant fluxes of ferrous iron to the sediment-water interface. In light of this discovery, we discuss the potential bioavailability of sediment-water interface iron for organisms in the overlying water column. PMID:24614177

  4. Fission track analysis, rift shoulder uplift, and tectonic modeling of the Norwegian Continental Margin

    SciTech Connect

    Andriessen, P.; Van Der Beek, P.; Cloetingh, S.; Rohrman, M. )

    1993-09-01

    Apatite fission track analysis from southern Norway and Sweden, across the Permian Carboniferous Oslo rift, are presented and discussed in relation to different rifting scenarios. Vertical and horizontal apatite fission tack profiles in middle and southern Norway unravel the post-Carboniferous history of the Fennoscandian shield. Fission track apatite ages range from 240 Ma in the south to 160 Ma in the north, and according to spontaneous fission track length measurements, they must be interpreted as mixed ages, indicating minor amounts of Paleozoic-Mesozoic sedimentary cover. Apatite fission track length and age modeling suggest rapid cooling and uplift in the Tertiary for the southernmost part of Norway, suggesting a differential uplift of the basement. the obtained data are important for the reconstruction of burial and thermal histories of Cenozoic sedimentary basins of the Norwegian continental margin in the northern North Sea, where diverse rifting events, intraplate stress regimes, and inversion tectonics are involved. Fission track analysis puts constraints on tectonic modeling of uplift of rift flanks and the Norwegian continental margin and yields information for these assessment of hydrocarbon potentials of the sedimentary basins.

  5. First evidence for the presence of iron oxidizing zetaproteobacteria at the Levantine continental margins.

    PubMed

    Rubin-Blum, Maxim; Antler, Gilad; Tsadok, Rami; Shemesh, Eli; Austin, James A; Coleman, Dwight F; Goodman-Tchernov, Beverly N; Ben-Avraham, Zvi; Tchernov, Dan

    2014-01-01

    During the 2010-2011 E/V Nautilus exploration of the Levantine basin's sediments at the depth of 300-1300 m, densely patched orange-yellow flocculent mats were observed at various locations along the continental margin of Israel. Cores from the mat and the control locations were collected by remotely operated vehicle system (ROV) operated by the E/V Nautilus team. Microscopic observation and phylogenetic analysis of microbial 16S and 23S rRNA gene sequences indicated the presence of zetaproteobacterial stalk forming Mariprofundus spp.-like prokaryotes in the mats. Bacterial tag-encoded FLX amplicon pyrosequencing determined that zetaproteobacterial populations were a dominant fraction of microbial community in the biofilm. We show for the first time that zetaproteobacterial may thrive at the continental margins, regardless of crustal iron supply, indicating significant fluxes of ferrous iron to the sediment-water interface. In light of this discovery, we discuss the potential bioavailability of sediment-water interface iron for organisms in the overlying water column.

  6. Physical resuspension and vertical mixing of sediments on a high energy continental margin (Sydney, Australia).

    PubMed

    Matthai, C; Birch, G F; Jenkinson, A; Heijnis, H

    2001-01-01

    Four sediment cores from the continental margin adjacent to Sydney were analyzed for 210Pb, 137Cs, trace metals (Ag, Cd, Co, Cu, Mn, Ni, Pb, Zn), iron, dry bulk density, mud and moisture content. The concentrations of trace metals in the total sediment are low at all sites, although slightly elevated concentrations of Ag, Cu, Pb and Zn are present in the fine fraction of sediment (< 62.5 microns) near a major ocean outfall. Concentrations of trace metals in the fine fraction of sediment are similar in the upper 10-15 cm, indicating strong vertical mixing of the sediments, whereas an upward coarsening grain size in the upper 1-3 cm of sediment supports physical resuspension during storms. Sediment accumulation rates on the middle shelf adjacent to Sydney were estimated from downcore profiles of 210Pb and 137Cs and range between 0.2 and 0.4 cm yr-1. Although the mass fluxes of Cu, Pb and Zn within a distance of 2 km from the outfall (up to 36.1, 30.8 and 86.2 micrograms cm-2 yr-1, respectively) are greater than 20 km north of the outfall (< 23.5 micrograms cm-2 yr-1), the low concentrations of trace metals in sediments near the outfall support an efficient dispersal of anthropogenic contaminants on this continental margin.

  7. Sinking jelly-carbon unveils potential environmental variability along a continental margin.

    PubMed

    Lebrato, Mario; Molinero, Juan-Carlos; Cartes, Joan E; Lloris, Domingo; Mélin, Frédéric; Beni-Casadella, Laia

    2013-01-01

    Particulate matter export fuels benthic ecosystems in continental margins and the deep sea, removing carbon from the upper ocean. Gelatinous zooplankton biomass provides a fast carbon vector that has been poorly studied. Observational data of a large-scale benthic trawling survey from 1994 to 2005 provided a unique opportunity to quantify jelly-carbon along an entire continental margin in the Mediterranean Sea and to assess potential links with biological and physical variables. Biomass depositions were sampled in shelves, slopes and canyons with peaks above 1000 carcasses per trawl, translating to standing stock values between 0.3 and 1.4 mg C m(2) after trawling and integrating between 30,000 and 175,000 m(2) of seabed. The benthopelagic jelly-carbon spatial distribution from the shelf to the canyons may be explained by atmospheric forcing related with NAO events and dense shelf water cascading, which are both known from the open Mediterranean. Over the decadal scale, we show that the jelly-carbon depositions temporal variability paralleled hydroclimate modifications, and that the enhanced jelly-carbon deposits are connected to a temperature-driven system where chlorophyll plays a minor role. Our results highlight the importance of gelatinous groups as indicators of large-scale ecosystem change, where jelly-carbon depositions play an important role in carbon and energy transport to benthic systems.

  8. Seismic stratigraphy of Long Island platform, United States Atlantic Continental Margin

    SciTech Connect

    Jowett, R.A.; Hutchinson, D.R.

    1987-09-01

    Approximately 2000 km of single- and multichannel seismic reflection profiles collected over the Long Island platform on the US Atlantic continental margin show that the basement beneath the platform was rifted prior to the separation of Africa from North America and that it subsided after the separation. Postrift sediment thicknesses range from less than 1 km in the northwest part of the platform to several kilometers in the southeast, near the Atlantis and Nantucket rift basins. Flanking the platform are the Georges Bank basin to the east and Baltimore Canyon Trough to the south, where sedimentary rocks are 10-15 km thick. Nine major unconformities have been delineated in analysis of the seismic profiles. The most conspicuous unconformities are correlated with the end of rifting and the upper surfaces of the Bathonian, Tithonian, Albian, Turonian-Coniacian, Maestrichtian, upper Eocene, mid-Oligocene, and mid-Miocene sections. Ages are determined by tracing reflectors and unconformities to the COST (Continental Offshore Stratigraphic Test), AMCOR (Atlantic Margin Coring Project), and coastal wells. Several of these unconformities coincide with pronounced fluctuations in the Vail curve of relative sea level.

  9. Biodiversity response to natural gradients of multiple stressors on continental margins.

    PubMed

    Sperling, Erik A; Frieder, Christina A; Levin, Lisa A

    2016-04-27

    Sharp increases in atmospheric CO2 are resulting in ocean warming, acidification and deoxygenation that threaten marine organisms on continental margins and their ecological functions and resulting ecosystem services. The relative influence of these stressors on biodiversity remains unclear, as well as the threshold levels for change and when secondary stressors become important. One strategy to interpret adaptation potential and predict future faunal change is to examine ecological shifts along natural gradients in the modern ocean. Here, we assess the explanatory power of temperature, oxygen and the carbonate system for macrofaunal diversity and evenness along continental upwelling margins using variance partitioning techniques. Oxygen levels have the strongest explanatory capacity for variation in species diversity. Sharp drops in diversity are seen as O2 levels decline through the 0.5-0.15 ml l(-1) (approx. 22-6 µM; approx. 21-5 matm) range, and as temperature increases through the 7-10°C range. pCO2 is the best explanatory variable in the Arabian Sea, but explains little of the variance in diversity in the eastern Pacific Ocean. By contrast, very little variation in evenness is explained by these three global change variables. The identification of sharp thresholds in ecological response are used here to predict areas of the seafloor where diversity is most at risk to future marine global change, noting that the existence of clear regional differences cautions against applying global thresholds. PMID:27122565

  10. Sinking jelly-carbon unveils potential environmental variability along a continental margin.

    PubMed

    Lebrato, Mario; Molinero, Juan-Carlos; Cartes, Joan E; Lloris, Domingo; Mélin, Frédéric; Beni-Casadella, Laia

    2013-01-01

    Particulate matter export fuels benthic ecosystems in continental margins and the deep sea, removing carbon from the upper ocean. Gelatinous zooplankton biomass provides a fast carbon vector that has been poorly studied. Observational data of a large-scale benthic trawling survey from 1994 to 2005 provided a unique opportunity to quantify jelly-carbon along an entire continental margin in the Mediterranean Sea and to assess potential links with biological and physical variables. Biomass depositions were sampled in shelves, slopes and canyons with peaks above 1000 carcasses per trawl, translating to standing stock values between 0.3 and 1.4 mg C m(2) after trawling and integrating between 30,000 and 175,000 m(2) of seabed. The benthopelagic jelly-carbon spatial distribution from the shelf to the canyons may be explained by atmospheric forcing related with NAO events and dense shelf water cascading, which are both known from the open Mediterranean. Over the decadal scale, we show that the jelly-carbon depositions temporal variability paralleled hydroclimate modifications, and that the enhanced jelly-carbon deposits are connected to a temperature-driven system where chlorophyll plays a minor role. Our results highlight the importance of gelatinous groups as indicators of large-scale ecosystem change, where jelly-carbon depositions play an important role in carbon and energy transport to benthic systems. PMID:24367499

  11. Middle Jurassic to early Cretaceous igneous rocks along eastern North American continental margin

    SciTech Connect

    Jansa, L.F.; Pe-Piper, G.

    1988-03-01

    Late Middle Jurassic and Early Cretaceous mafic dikes, sills, flows, and local volcaniclastic sediments are intercalated within continental shelf sediments from the Baltimore Canyon Trough northward to the Grand Banks of Newfoundland. The igneous rocks on the eastern North American margin are mainly alkali basalts of intraplate affinity. The late Middle Jurassic igneous activity was of short duration, at about 140 Ma, and was restricted to Georges Bank where it led to construction of several volcanic cones. The main period of igneous activity was concentrated at about 120 Ma in the Aptian/Berremian. The activity consists of dike swarms in Baltimore Canyon, occasional dikes on the Scotian Shelf, and the growth of stratovolcanoes on the Scotian Shelf and Grand Banks. Younger dikes (approx. 95 Ma) also are present on the Grand Banks. With regard to oil exploration on the continental margin, care must be taken to properly identify igneous and volcaniclastic rocks on mechanical logs, drill cuttings, and cores. Reflection seismic profiles can be used to map the areal extent of sills, flows, and low-angle dikes, which commonly show distinctive seismic responses. However, steeply dipping dikes generally produce little, if any, seismic response. Isotopic-age determinations of igneous rocks, combined with biostratigraphic-age determinations of adjacent strata, are invaluable for stratigraphic correlation, establishing chronology of seismic sequences, and analysis of basin sedimentation and tectonic history. 9 figures, 2 tables.

  12. Sinking Jelly-Carbon Unveils Potential Environmental Variability along a Continental Margin

    PubMed Central

    Lebrato, Mario; Molinero, Juan-Carlos; Cartes, Joan E.; Lloris, Domingo; Mélin, Frédéric; Beni-Casadella, Laia

    2013-01-01

    Particulate matter export fuels benthic ecosystems in continental margins and the deep sea, removing carbon from the upper ocean. Gelatinous zooplankton biomass provides a fast carbon vector that has been poorly studied. Observational data of a large-scale benthic trawling survey from 1994 to 2005 provided a unique opportunity to quantify jelly-carbon along an entire continental margin in the Mediterranean Sea and to assess potential links with biological and physical variables. Biomass depositions were sampled in shelves, slopes and canyons with peaks above 1000 carcasses per trawl, translating to standing stock values between 0.3 and 1.4 mg C m2 after trawling and integrating between 30,000 and 175,000 m2 of seabed. The benthopelagic jelly-carbon spatial distribution from the shelf to the canyons may be explained by atmospheric forcing related with NAO events and dense shelf water cascading, which are both known from the open Mediterranean. Over the decadal scale, we show that the jelly-carbon depositions temporal variability paralleled hydroclimate modifications, and that the enhanced jelly-carbon deposits are connected to a temperature-driven system where chlorophyll plays a minor role. Our results highlight the importance of gelatinous groups as indicators of large-scale ecosystem change, where jelly-carbon depositions play an important role in carbon and energy transport to benthic systems. PMID:24367499

  13. Submarine erosion and karstification on the west Florida Continental margin: disparate environments yield similar features

    SciTech Connect

    Doyle, L.J.; Brooks, G.; Herbert, J.H.

    1985-01-01

    Thousands of kilometers of high resolution seismic profiles from the carbonate West Florida continental margin reveal two large bands of solution features. One band is found on the inner portion of the shelf and includes a variety of buried and filled karst features, the most spectacular of which are large solution valleys, paleodrainage extensions of the extent Tampa Bay and Charlotte Harbor estuaries. These features were probably formed subaerially during lower stands of sea level. This band of karst dies out in a line at mid-shelf between 75 m and 100 m water depth, marking a low stand of sea level. A second band of solution features is found on the upper slope at water depths of between 500 and 800 m. It is partially exposed in outcrop. Although many of the features are similar in appearance to those of the karst inner shelf band, their origin in submarine. They could have formed from dissolution by groundwater percolating down from the Florida mainland, from submarine erosion by the Loop Current which sweeps this portion of the slope or by a combination of the above processes. Although karstification is considered a continental process, extensive solution feature scan also form in the marine environment. Subsequently, these can be raised above sea level and be modified by continental processes, making it difficult to distinguish marine or partially marine solution features form those of traditional subaerial origin.

  14. Boninitic magmatism in a continental margin setting, Yukon- Tanana terrane, southeastern Yukon, Canada

    NASA Astrophysics Data System (ADS)

    Piercey, Stephen J.; Murphy, Donald C.; Mortensen, James K.; Paradis, Suzanne

    2001-08-01

    Mid-Paleozoic mafic rocks in the Finlayson Lake region of the Yukon-Tanana terrane, southeastern Yukon, Canada, have the diagnostic geochemical signatures of boninites: high MgO, Cr, Ni, and Co contents, intermediate SiO2 contents, high Mg#'s (MgO/ (MgO+FeO*), Al2O3/TiO2, and Zr(Hf)/middle rare earth element (REE) ratios; low TiO2, REE, and high-field-strength element contents; and U-shaped primitive mantle normalized trace element patterns. However, unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms, those from the Finlayson Lake region are part of a mid-Paleozoic continental margin arc-backarc magmatic system. We propose a model in which the boninitic rocks from the Finlayson Lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust. In the oceanic segment, upwelling asthenosphere induced melting of a subducted-slab metasomatized refractory mantle source to form boninitic magmatism. In the continental sector, upwelling asthenospheric mantle, and/or the melts derived thereof, induced crustal melting, which explains the large volume of temporally equivalent felsic volcanic and intrusive rocks.

  15. Continental margin volcanism at sites of spreading ridge subduction: Examples from southern Alaska and western California

    NASA Astrophysics Data System (ADS)

    Cole, Ronald B.; Stewart, Brian W.

    2009-01-01

    Episodes of spreading ridge subduction occurred during late Oligocene to early Miocene time along western California and during late Paleocene to early Eocene time along southern Alaska. In each case, ridge subduction and subsequent slab window formation has had a profound influence on continental margin magmatism. Foremost, in each setting there was a hiatus in arc magmatism following ridge subduction, followed by the onset of volcanism in zones of local extension within arc-front, forearc, and accretionary prism settings. These near-trench volcanic rocks are distinctive from adjacent arc rocks and represent unique episodes of continental margin magmatism. The western California volcanic rocks were erupted into several local extensional basins and form discrete volcanic centers within each basin. The southern Alaska volcanic rocks, which form the Caribou Creek volcanic field, were erupted in a broad zone of extension that trended orthogonally to the continental margin. Basalts from the western California volcanic centers and the Caribou Creek volcanic field are tholeiitic and have depleted Nd and Sr isotope compositions with ɛ Nd( t) as high as + 9.3 and + 10.9 and 87Sr/ 86Sr( t) as low as 0.70258 and 0.70278, respectively. These basalts are unique because they are the most geochemically depleted basalts yet documented along the continental margin of the northern Cordillera. The basalts of each group also have high Ti contents (TiO 2 above 1.5%) and low ratios of fluid-mobile and other large ion lithophile elements compared to high field strength elements. For example, Ba/Ta ratios among the basalts range from about 40 to 600, with most samples below 270, while arc basalts typically have Ba/Ta ratios greater than 450. The basalts also have Th/Yb and Ta/Yb ratios in the range of mid-ocean-ridge basalts and do not exhibit the typical enrichment in Th/Yb that characterizes arc basalts. The basaltic andesites through dacites and rhyolites have more enriched isotope

  16. The Hamburg klippe: Record of the destruction of the proto-North American continental margin

    SciTech Connect

    Lash, G.G. . Dept. of Geosciences)

    1993-03-01

    The Hamburg klippe, a Taconic allochthon in the central Appalachian orogen of southeastern Pennsylvania, has long been in the focus of debate. Although initially interpreted by some to be either autochthonous or simply a facies of the parautochthonous Martinsburg Formation, essentially all workers now agree that rocks of the Hamburg klippe comprise a far-travelled sequence of deep-marine deposits thrust onto the proto-North American platform in Ordovician time. The klippe is composed of two tectonic slices that contain rocks that formed in very different tectonic environments. The structurally lowest slice, the Greenwich slice, is interpreted to be an ancient subduction complex that formed in Cambrian-Ordovician time. This slice consists chiefly of turbidite sandstone and hemipelagic mudstone of Middle Ordovician age (85%) underlain by relatively thin sequences of red and light-green pelagic mudstone, deep-water limestone and chert of Early to Middle Ordovician age (14%). The vertical stratigraphy of the deposits of the Greenwich slice is interpreted to reflect migration of a site on oceanic lithosphere from an abyssal plain setting toward and ultimately into a trench southeast of the proto-North American continental margin by Middle Ordovician time. The tectonically highest slice, the Richmond slice, is composed predominantly of Middle Ordovician carbonate turbidites and black shale interpreted to have accumulated on the subsiding proto-North American continental margin as it approached the trench to the southeast. Eventually the Richmond slice was thrust to the northwest over the subduction complex which was, by this time, on the continental platform. Similar rock types in other allochthons in the Appalachian orogen may reflect an analogous mode of origin.

  17. Cenozoic prograding sequences of the Antarctic continental margin - What balance between structural and eustatic control

    SciTech Connect

    Cooper, A.K. ); Barrett, P. ); Hinz, K. ); Stagg, H. ); Traube, V. )

    1990-05-01

    Multichannel seismic reflection profiles across the Antarctic continental margin commonly reveal prograding sedimentary sequences that are bounded by unconformities. These sequences are as much as 5 km thick and, where sampled, are composed entirely of late Eocene( )-early Oligocene and younger glacial rocks. On nonpolar margins, prograding sequences generally are attributed to relative changes in sea level, sediment supply, and tectonism. Around Antarctica, ice sheets have also been important in controlling the geometry and location of prograding sequences. The Antarctic sequences may provide a proximal record of major Cenozoic ice volume changes and related sea level changes not obtainable from low-latitude continental shelves. Presently, the Antarctic record is poorly known because of limited core data. Two categories of prograding (P) and aggrading (A) sigmoidal sequences are observed around Antarctica: (1) P sequences that build principally outward (common) and (2) AP sequences that build largely upward and outward (less common). P sequences may result principally from grounded ice sheets, and AP sequences from open-marine basinal processes. Major rift embayments of Antarctica (e.g., eastern Ross Sea eastern Weddell Sea Lambert graben Wilkes basin) are also pathways for major ice movement. In general, most areas with P sequences lie within or adjacent to Mesozoic or older rift embayment, whereas the primary area with AP sequences (eastern Ross Sea) lies within a likely Cenozoic rift embayment. The Pacific side of the Antarctic Peninsula where Cenozoic ice sheets and Cenozoic tectonism have been active, is also marked by a P sequence. Scientific drilling on the Antarctic continental shelf has recovered openwater glacial deposits (Ross Sea) as well as glacial diamicts that were deposited beneath and in front of grounded glacier ice (Ross Sea and Prydz Bay).

  18. Distribution of deep-water corals along the North American continental margins: Relationships with environmental factors

    NASA Astrophysics Data System (ADS)

    Bryan, Tanya L.; Metaxas, Anna

    2006-12-01

    Despite the increasing attention to assemblages of deep-water corals in the past decade, much of this research has been focused on documenting and enumerating associated fauna. However, an understanding of the distribution of most species of coral and the ecological processes associated with these assemblages is still lacking. In this study, we qualitatively and quantitatively described the habitats of two families of deep-water corals in relation to six oceanographic factors (depth, slope, temperature, current, chlorophyll a concentration and substrate) on the Pacific and Atlantic Continental Margins of North America (PCM and ACM study areas, respectively). This study focused primarily on the distributions of Primnoidae and Paragorgiidae because of the large number of documented occurrences. For each environmental factor, deep-water coral locations were compared to the surrounding environment using χ2 tests. On both continental margins, coral locations were found to be not randomly distributed within the study areas, but were within specific ranges for most environmental factors. In the PCM study area, Paragorgiidae and Primnoidae locations were found in areas with slopes ranging from 0° to 10.0°, temperature from -2.0 to 11.0 °C and currents from 0 to 143 cm s -1. In the ACM study area, Paragorgiidae and Primnoidae locations were found in areas with slopes ranging from 0° to 1.4°, temperature ranging from 0 to 11.0 °C and currents ranging from 0 to 207 cm s -1. Although the patterns in habitat characteristics were similar, differences existed between families with respect to particular environmental factors. In both study areas, most environmental parameters in locations where corals occurred were significantly different from the average values of these parameters as determined with χ2 tests ( p<0.05) except for substrate in Paragorgiidae locations and depth in Primnoidae locations on the PCM. This is the first study to show coral distributional patterns

  19. Solonker ophiolite in Inner Mongolia, China: A late Permian continental margin-type ophiolite

    NASA Astrophysics Data System (ADS)

    Luo, Zhi-wen; Xu, Bei; Shi, Guan-zhong; Zhao, Pan; Faure, M.; Chen, Yan

    2016-09-01

    The Solonker ophiolite is exposed along the border between Mongolia and China within the Solonker zone, the southeastern Central Asian Orogenic Belt (CAOB), and it is composed dominantly of serpentinized peridotite with subordinate gabbro, basaltic lava, radiolarian-bearing siliceous rocks, and minor plagiogranite. Meanwhile, layered mafic-ultramafic cumulates are not ubiquitous. In this study, zircon grains from two gabbros and a plagiogranite yield 206Pb/238U ages of 259 ± 6 Ma, 257 ± 3 Ma and 263 ± 1 Ma. These data were interpreted to represent the formation age of the Solonker ophiolite. The studied gabbros and basalts have a tholeiitic composition, showing a MORB affinity. They are also characterized by enrichment of Pb and depletion of Nb relative to La and Th. Furthermore, the studied gabbros contain inherited zircon grains and display a large range of zircon Hf isotopes (εHf(t) = - 5.27 to + 10.19). These features imply that crustal contamination played an important role in the generation of these mafic rocks. Major elements derived from the radiolarian-bearing siliceous rocks suggest a continental margin setting. This is confirmed by rock association. Terrigenous rocks (sandstones and siltstones) interstratified with siliceous rocks. U-Pb dating of detrital zircon grains in sandstones from both the northern and southern sides of the Solonker ophiolite belt, along with published data, reveals that the Late Carboniferous-Early Permian strata in fault contact with the Solonker ophiolite was deposited above Early Paleozoic orogens. The lines of petrological, geochemical, geochronological, and isotopic evidence led us to propose that the Solonker ophiolite is a Late Permian continental margin-type body formed during the early stages of opening of an ocean basin, following rifting and break-up of the Early Paleozoic orogens. Accordingly, the Permian Solonker zone is characterized by an intra-continental extensional setting.

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

    USGS Publications Warehouse

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

    1991-01-01

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

  1. Transition from magma dominant to magma poor rifting along the Nova Scotia Continental Margin

    NASA Astrophysics Data System (ADS)

    Lau, K. H.; Louden, K. E.; Nedimović, M. R.; Whitehead, M.; Farkas, A.; Watremez, L.; Dehler, S. A.

    2011-12-01

    Passive margins have been characterized as magma-dominant (volcanic) or magma-poor (non-volcanic). However, the conditions under which margins might switch states are not well understood as they typically have been studied as end member examples in isolation to each other. The Nova Scotia (NS) continental margin, however, offers an opportunity to study the nature of such a transition between the magma-dominant US East Coast margin to the south and the magma-poor Newfoundland margin to the north within a single rift segment. This transition is evidenced by a clear along-strike reduction in features characteristic of syn-rift volcanism from south-to-north along the NS margin, such as the weakening of the East Coast Magnetic Anomaly (ECMA) and the coincident disappearance of seaward dipping reflector sequences (SDRS) on multichannel seismic (MCS) reflection profiles. Results from recent industry MCS profiles along and across the margin suggest a potentially narrow magma-dominant to magma-poor along-strike transition between the southern and the central NS margin. Such a transition is broadly consistent with results of several widely-spaced, across-strike ocean bottom seismometer (OBS) wide-angle profiles. In the southern region, the crustal structure exhibits a narrow (~120-km wide) ocean-continent transition (OCT) with a high velocity (7.2 km/s) lower crust, interpreted as a gabbro-rich underplated melt, beneath the SDRS and the ECMA, similar to crustal models across the US East Coast. In contrast, profiles across the central and northern margin contain a much wider OCT (150-200-km wide) underlain by a low velocity mantle layer (7.3-7.9 km/s), interpreted as partially serpentinized olivine, which is similar to the magma-poor Newfoundland margin to the north. However, the central-to-northern OBS profiles also exhibit significant variations within the OCT and the along-strike continuity of these OCT structures is not yet clear. In November 2010, we acquired, in the

  2. Continental Margin of Kamchatka Peninsula, Russia: the Mode and Nature of Crustal Growth in the Accretionary Orogen

    NASA Astrophysics Data System (ADS)

    Konstantinovskaya, E. A.; Bindeman, I. N.

    2001-12-01

    Tectonic accretion of island arc terranes is the process widely developed in Pacific Rim in the present and in the past. The mode and nature of crustal growth of continental margins during arc accretion are various and essentially determined by deformation of the margin. The Cenozoic Kamchatka orogen formed by the accretion of two island arc terranes: Achaivayam-Valaginskaya arc (A-V, Eocene) (2) and Kronotskaya arc (terminal Miocene) to the continental margin of Asia. During the Early Eocene, the southern segment of the A-V arc collided with the Sredinny metamorphic massif, which was the frontal part of the Asian continental margin (3). New results from SHRIMP dating of zircons (1) from metamorphic rocks of Sredinny massif (Kolpakovskaya series) show that the massif contains an abundance of Archean, Proterozoic and Phanerozoic detrital zircon cores, and ubiquitous 77 Ma rims. The youngest ages are from four 47-53 Ma unzoned zircon cores, with dull cathodoluminescence, and irregular morphology. We regard the 47-53 Ma episode of zircon growth in the Sredinny massif as evidence for superimposed metamorphism induced by continental margin subduction at the beginning of its collision with the A-V arc in the early Eocene. Physical modeling experiments of arc-continent collision suggest that deformation at continental margin is controlled by strength of the subducting crust. Failure, accretion and erosion-activated extrusion/exhumation of the subducted crust occur in the continental margin in the case when the margin is weakened by pre-existing faulting, extension, or heating. At the beginning of the continental margin subduction, crust of the margin fails along the continent-vergent thrust. The subducted crustal slice is, then, completely scraped from the mantle base and accreted to the fore-arc block. Subsequent thrusting and thickening of the subducting crust within the continental margin lead to formation of the accretionary orogen composed of crustal slices in front

  3. Anomalous Subsidence at Rifted Continental Margins: Distinguishing Mantle Dynamic Topography from Anomalous Oceanic Crustal Thickness

    NASA Astrophysics Data System (ADS)

    Cowie, L.; Kusznir, N. J.

    2012-12-01

    It has been proposed that some continental rifted margins have anomalous subsidence histories and that at breakup they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn or post breakup subsidence of this form would have important implications for our understanding of the geodynamics of continental breakup and rifted continental margin formation, margin subsidence history and the evolution of syn and post breakup depositional systems. We have investigated three rifted continental margins; the Gulf of Aden, Galicia Bank and the Gulf of Lions, to determine whether the oceanic crust in the ocean-continent transition of these margins has present day anomalous subsidence and if so, whether it is caused by mantle dynamic topography or anomalous oceanic crustal thickness. Residual depth anomalies (RDA) corrected for sediment loading, using flexural backstripping and decompaction, have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous oceanic bathymetry and subsidence at these margins. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions from Crosby & McKenzie (2009). Non-zero sediment corrected RDAs may result from anomalous oceanic crustal thickness with respect to the global average, or from mantle dynamic uplift. Positive RDAs may result from thicker than average oceanic crust or mantle dynamic uplift; negative RDAs may result from thinner than average oceanic crust or mantle dynamic subsidence. Gravity inversion incorporating a lithosphere thermal gravity anomaly correction and sediment thickness from 2D seismic data has been used to determine Moho depth and oceanic crustal basement thickness. The reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The gravity inversion crustal basement thicknesses

  4. Lithoprobe east: marine deep seismic reflection results across the Appalachians and the rifted continental margin northeast of Newfoundland

    SciTech Connect

    Keen, C.E.; Stockmal, G.S.; O'Brien, S.J.; Quinlan, G.

    1985-01-01

    Marine deep seismic reflection data have been collected across the Appalachian Orogen and the rifted continental margin northeast of Newfoundland. Results across the Appalachian Orogen show the extent of the ancient Grenvillian passive margin beneath the terranes to the east, the nature of terrane boundaries at depth, and the relationships between surface geological features and the seismically defined crustal geometry. The results across the rifted margin define a decollement zone below the faulted continental basement. These basement fault blocks do not exhibit a listric geometry. The continental crust thins beneath the sedimentary basins occupying the rifted margin, although there does not appear to be a simple relationship between crustal thickness and basin subsidence. The ocean-continent transition is marked by a landward dip of the oceanic crust which disappears below the thinned continental crust near the transition. The ocean-continent boundary is not a vertical boundary between crustal types, rather there appears to be continuity of oceanic-type crust below the continent, perhaps forming a high velocity lower crustal layer across the continental margin.

  5. A newly discovered Pliocene volcanic field on the western Sardinia continental margin (western Mediterranean)

    NASA Astrophysics Data System (ADS)

    Conforti, Alessandro; Budillon, Francesca; Tonielli, Renato; De Falco, Giovanni

    2016-02-01

    A previously unknown submerged volcanic field offshore western Sardinia (western Mediterranean Sea), has been identified based on swath bathymetric data collected in 2009, 2010 and 2013, and high-resolution seismic profiles collected in 2011 and 2013. About 40 conical-shaped volcanic edifices (maximum width of about 1600 m and maximum height of about 180 m) and several lava outcrops (up to 1,200 m wide) were recognized at 20 to 150 m water depth over an area of 800 km2. The volcanic edifices are mainly eruptive monogenic vents, mostly isolated with a rather distinct shape, or grouped to form a coalescent volcanic body in which single elements are often still recognizable. High-resolution seismics enabled identifying relationships between the volcanic bodies and continental margin successions. The edifices overlie a major erosional surface related to the margin exposure following the Messinian salinity crisis, and are overlain by or interbedded with an early Pliocene marine unit. This seismo-stratigraphic pattern dates the volcanic activity to the early Pliocene, in agreement with the radiometric age of the Catalano island lavas (4.7 Ma) reported in earlier studies. The morphometry of the volcanic bodies suggests that cone erosion was higher at shallow water depths. Indeed, most of the shallow edifices are strongly eroded and flattened at 125 to 130 m water depth, plausibly explained by recurrent sub-aerial exposure during Pleistocene sea-level lowstands, whereas cones in deeper water are much better preserved. Volcanic vents and lava deposits, hereafter named the Catalano volcanic field (CVF), are emplaced along lineaments corresponding to the main directions of the normal fault system, which lowered the Sinis Basin and the western Sardinia continental margin. The CVF represents a volumetrically relevant phase of the late Miocene - Quaternary anorogenic volcanic cycle of Sardinia, which is related to the first stage of the extensional tectonics affecting the island

  6. New Low-Temperature Thermochronology Reveals Contrasting Modes of Continental Extension Across the Sonoran Rifted Margin

    NASA Astrophysics Data System (ADS)

    Kohn, B. P.; Fletcher, J. M.; Gleadow, A. J.; Calmus, T.; Nourse, J. A.

    2003-12-01

    The Sonoran rifted margin extends 250 km from the western flanks of the Sierra Madre Occidental to the Gulf of California and contains a classic Basin and Range morphology that indicates "broad-rift" mode of continental extension. However, new low-temperature thermochronology reveals that the Sonoran rifted margin is also internally composed of at least two temporally and spatially distinct belts that display other distinct styles of extension. Mountain ranges that lie within a narrow belt (20 km wide) along the coast of the Gulf of California between Puerto Libertad and Bahia Kino yield highly discordant apatite fission track (AFT) ages that range from 5 to 54 Ma and likely reflect the strong tilting of these tectonic blocks. The widespread occurrence of AFT ages between 5 and 7 Ma, which are typically found in the deepest crustal levels of the tilt blocks, and the presence of Quaternary scarps indicate that extension in the coastal region largely occurred from late Miocene to recent times. We infer that this belt is dominated by a "narrow-rift" mode of extension where deformation has been focused to produce the Gulf depression. Well inland from the coast (175 km east) is a belt of metamorphic core complexes that extends more than 200 km from Magdalena to Mazatan and typically yields older and more concordant AFT ages from 14 to 23 Ma. However, the presence of ages as young as 8 to 11 Ma indicate that the "metamorphic-core-complex" mode of extension in this belt likely overlapped in time with the "narrow-rift" mode of extension in the Gulf of California. We conclude that the juxtaposition of major deformation belts each with different modes of continental extension reflects the diverse processes that have affected the Sonoran margin through time.

  7. Cenozoic vertical motions of the western continental margin of Peninsular India

    NASA Astrophysics Data System (ADS)

    Richards, Fred; Hoggard, Mark; White, Nicky

    2016-04-01

    Despite the cessation of rifting at ˜65 Ma and its remoteness from active convergence, the topography of Peninsular India is dominated by a dramatic, high-elevation escarpment along its western margin: the Western Ghats (˜1 - 1.5 km amsl). Inland of the escarpment, South Indian topography exhibits a long-wavelength (>1000 km), low-angle (˜0.1°) eastward tilt down to the Krishna-Godavari and Cauvery deltas on the eastern continental margin. Offshore, oceanic residual depth measurements show an identical long-wavelength asymmetry from highs of +1 km in the Arabian Sea to lows of -1.2 km in the Bay of Bengal. Strong evidence from margin stratigraphy, dated palaeosurfaces, thermochronology, cosmogenic nuclides and marine terraces combine to suggest that, following a period of relative quiescence from 50 Ma - 25 Ma, the present-day topography evolved in response to Neogene uplift and erosion along the western Indian margin. By jointly inverting 530 longitudinal river profiles for uplift rate and calibrating our inversions against these geological constraints, we successfully place this Cenozoic landscape evolution into a more complete spatio-temporal framework. The results demonstrate slow growth of the eastward tilt from 50 Ma - 25 Ma (≤0.02 mm a‑1), preceding a phase of increasingly rapid development - initiating in the south - from 25 Ma onwards (≤0.2 mm a‑1). The onset of rapid uplift pre-dates the initial intensification of the Indian monsoon by >15 Ma, suggesting that rock uplift and not climate change is primarily responsible for the modern-day relief of the peninsula. Previous studies have aimed to explain this topographic evolution by invoking flexural isostatic mechanisms involving denudation, sediment loading and/or underplating. However, seismological constraints show that South Indian topography deviates significantly from crustal isostatic expectations, while the 9.8‑2.2+3.8 km effective elastic thickness of the region generates ˜125 km

  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.

    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

  9. 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

  10. Geology and petroleum potential of Shumagin continental margin, western Gulf of Alaska

    SciTech Connect

    Bruns, T.R.; Von Huene, R.; Culotta, R.D.; Lewis, S.D.; Ladd, J.W.

    1986-07-01

    Interpretations of multichannel seismic reflection data indicate that the Shumagin continental margin seaward of the Border Ranges fault is underlain by two major seismic sequences, separated by an erosional unconformity beneath the shelf and by the time-correlative conformity seaward. Rocks above the unconformity are late Miocene and younger. Rocks below the unconformity can be as young as middle Miocene beneath the outer shelf and slope, seaward of a paleoshelf break. However, beneath the shelf they are primarily Late Cretaceous turbidites of the Shumagin Formation and Paleocene granodiorite. Late Miocene and younger structures of the Shumagin margin include Shumagin, Sanak, and Unimak basins and Unimak Ridge, a midslope structural high. Strata in Sanak and Unimak basins were deposited on a subsiding outer shelf and slope, and trapped behind Unimak Ridge and its now-buried structural continuation. Sanak and Unimak basins are in part bounded by northwest-trending extensional faults that parallel both the early Tertiary Beringian margin and a transverse tectonic boundary that segments the fore-arc. These faults may have developed during collapse and extension along the southeastward continuation of the old Beringian margin, analogous to the processes that created the Bering Shelf basins. The most promising areas of the Shumagin margin for petroleum potential are Sanak, and Unimak basins, which contain strata 8 and 4.5 km thick, respectively, and beneath the outer shelf and slope. Paleogene source rocks like those on the adjacent Alaska Peninsula may be preserved offshore, seaward of the inferred paleoshelf break. Reservoir rocks might have formed from granitic-rich erosional products derived during Oligocene and Miocene erosion of the shelf plutons.

  11. The role of continental margins in the final stages of arc formation: Constraints from teleseismic tomography of the Gibraltar and Calabrian Arc (Western Mediterranean)

    NASA Astrophysics Data System (ADS)

    Argnani, Andrea; Cimini, Giovanni Battista; Frugoni, Francesco; Monna, Stephen; Montuori, Caterina

    2016-05-01

    The deep seismicity and lateral distribution of seismic velocity in the Central Western Mediterranean, point to the existence under the Alboran and Tyrrhenian Seas of two lithospheric slabs reaching the mantle transition zone. Gibraltar and Calabrian narrow arcs correspond to the slabs. Similarities in the tectonic and mantle structure of the two areas have been explained by a common subduction and roll-back mechanism, in which the two arcs are symmetrical end members. We present a new 3-D tomographic model at mantle scale for the Calabrian Arc and compare it with a recently published model for the Gibraltar Arc by Monna et al. (2013a). The two models, calculated with inversion of teleseismic phase arrivals, have a scale and parametrization that allow for a direct comparison. The inclusion in both inversions of ocean bottom seismometer broadband data improves the resolution of the areas underlying the seafloor networks. This additional information is used to resolve the deep structure and constrain the reconstruction of the Central Western Mediterranean geodynamic evolution. The Gibraltar tomography model suggests that the slab is separated from the Atlantic oceanic domain by a portion of African continental margin, whereas the Calabrian model displays a continuous oceanic slab that is connected, via a narrow passage (~ 350 km), to the Ionian basin oceanic domain. Starting from the comparison of the two models we propose the following interpretation: within the Mediterranean geodynamic regime (dominated by slab rollback) the geometry of the African continental margin, located on the lower plate, represents a critical control on the evolution of subduction. As buoyant continental lithosphere entered the subduction zones, slab pull caused tears in the subducted lithosphere. This tectonic response, which occurred in the final stages of arc evolution and was strongly controlled by the paleogeography of the subducted plates, explains the observed differences between the

  12. Pyrophaeophorbide- a as a tracer of suspended particulate organic matter from the NE Pacific continental margin

    NASA Astrophysics Data System (ADS)

    Bianchi, Thomas S.; Bauer, James E.; Druffel, Ellen R. M.; Lambert, Corey D.

    Pyrophaeophorbide- a, a degradation product of chlorophyll- a, is predominantly formed by grazing processes in sediments as well as in the water column. Water column profiles of pyrophaeophorbide- a/suspended particulate organic carbon (SPOC) concentrations, at an abyssal site in the northeast (NE) Pacific (Sta M, 34°50'N, 123°00'W; 4100 m water depth), show low concentrations (0.01-0.1 ng/μg SPOC) at surface and mesopelagic depths, and increasing concentrations with closer proximity to the sea floor (0.05-0.6 ng/μg SPOC). However, in June 1992, the deep maximum of pyrophaeophorbide- a/SPOC in the water column of Sta M extended higher into the water column, as much as 1600 m above the bottom (mab) (2500 m water depth); in other seasons they only extended up to 650 mab (3450 m water depth). Previous studies have demonstrated lateral transport of particulate matter from the continental shelf to the deep ocean off the coast of northern California. Recent work suggests that the benthic boundary layer (BBL) extends to 50 mab, based on sediment trap and transmissometry measurements (Smith, K.L., Kaukmann, R.S., Baldwin, R.J., 1994. Coupling of near-bottom pelagic and benthic processes at abyssal depths. Limnology and Oceanography 39, 1101-1118.), and that lateral transport is significant only during summer, which is consistent with our observations. A partial vertical profile of pyrophaeophorbide- a/SPOC from the north central (NC) Pacific provides some evidence that the deep maximum may be absent due to the distance of this site from the continental margin. Thus, the observed deep maximum of pyrophaeophorbide- a/SPOC at Sta M is likely due mainly to lateral transport from the continental slope rather than to local vertical resuspension in the BBL exclusively. Pyrophaeophorbide- a concentrations in SPOC at Sta M were negatively correlated with Δ 14C values of SPOC (SPOC samples from Druffel, E.R.M., Bauer, J.E., Williams, P.M., Griffin, S.A. and Wolgast, D., 1996

  13. 7Be as a tracer of flood sedimentation on the northern California continental margin

    USGS Publications Warehouse

    Sommerfield, C. K.; Nittrouer, C. A.; Alexander, C. R.

    1999-01-01

    Sediment inventories of the cosmogenic radionuclide 7Be (t1/2=53 d) were measured on the Eel River shelf and slope (northern California continental margin) to investigate sedimentation processes associated with coastal river flooding. Seabed coring shortly after major riverflow events in 1995 and 1997 documented a shelf-wide flood deposit, and subsequent radionuclide studies determined 7Be to be a powerful tracer of fine-grained river sediment. In addition, distinctive signatures of 234Th and 210Pb were observed in oceanic flood deposits and provided additional information regarding depositional processes. During the 1995–1997 monitoring period, 7Be was present (2–35 dpm cm-2) in shelf and slope sediments only after periods of high rainfall and river runoff during the winter months. It is suggested that fluvial input was the primary source of 7Be in shelf sediments after the floods. 7Be sediment inventories and sediment-trap fluxes determined after the 1997 flood revealed that fine-grained fluvial sediments were rapidly (within one month) broadcast over the continental margin, to the 500 m isobath. Dispersal was apparently facilitated by energetic storm waves, which resuspended and redistributed some fraction of the suspended load residing on the shelf prior to accretion as flood deposits. These observations illustrate that floods are an important sedimentary process for modern environments of the Eel shelf and slope, and perhaps for other fluviomarine sedimentary systems of the northern California continental margin. Ratios of the 210Pb sediment-accumulation rate (100 yr average) to the 7Be deposition rate (1–2 month average) for shelf sites illustrate the episodic nature of shelf sedimentation, and suggest that a minimum of 3–30 depositional events complete the most recent stratigraphic record. This observation is consistent with the magnetude and frequency of fluvial sediment input, as Eel River floods with return periods of 3–33 yr (3% of the time of

  14. Mesozoic architecture of a tract of the European-Iberian continental margin: Insights from preserved submarine palaeotopography in the Longobucco Basin (Calabria, Southern Italy)

    NASA Astrophysics Data System (ADS)

    Santantonio, Massimo; Fabbi, Simone; Aldega, Luca

    2016-01-01

    The sedimentary successions exposed in northeast Calabria document the Jurassic-Early Cretaceous tectonic-sedimentary evolution of a former segment of the European-Iberian continental margin. They are juxtaposed today to units representing the deformation of the African and Adriatic plates margins as a product of Apenninic crustal shortening. A complex pattern of unconformities reveals a multi-stage tectonic evolution during the Early Jurassic, which affected the facies and geometries of siliciclastic and carbonate successions deposited in syn- and post-rift environments ranging from fluvial to deep marine. Late Sinemurian/Early Pliensbachian normal faulting resulted in exposure of the Hercynian basement at the sea-floor, which was onlapped by marine basin-fill units. Shallow-water carbonate aprons and reefs developed in response to the production of new accommodation space, fringing the newborn islands which represent structural highs made of Paleozoic crystalline and metamorphic rock. Their drowning and fragmentation in the Toarcian led to the development of thin caps of Rosso Ammonitico facies. Coeval to these deposits, a thick (> 1 km) hemipelagic/siliciclastic succession was sedimented in neighboring hanging wall basins, which would ultimately merge with the structural high successions. Footwall blocks of the Early Jurassic rift, made of Paleozoic basement and basin-margin border faults with their onlapping basin-fill formations, are found today at the hanging wall of Miocene thrusts, overlying younger (Middle/Late Jurassic to Late Paleogene) folded basinal sediments. This paper makes use of selected case examples to describe the richly diverse set of features, ranging from paleontology to sedimentology, to structural geology, which are associated with the field identification of basin-margin unconformities. Our data provide key constraints for restoring the pre-orogenic architecture of a continental margin facing a branch of the Liguria-Piedmont ocean in the

  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

    Starting with the break up of Gondwana, the northwest Indian Ocean and its continental margins in Madagascar, East Africa and western India formed by divergence of the African and Indian plates and were shaped by a complicated sequence of plate boundary relocations, ridge propagation events, and the independent movement of the Seychelles microplate. As a result, attempts to reconcile the different plate-tectonic components and processes into a coherent kinematic model have so far been unsatisfactory. A new high-resolution plate kinematic model has been produced in an attempt to solve these problems, using seafloor spreading data and rotation parameters generated by a mixture of visual fitting of magnetic isochron data and iterative joint inversion of magnetic isochron and fracture zone data. Using plate motion vectors and plate boundary geometries derived from this model, the first-order regional stress pattern was modelled for distinct phases of margin formation. The stress pattern is correlated with the tectono-stratigraphic history of related sedimentary basins. The plate kinematic model identifies three phases of spreading, from the Jurassic to the Paleogene, which resulted in the formation of three main oceanic basins. Prior to these phases, intracontinental 'Karoo' rifting episodes in the late Carboniferous to late Triassic had failed to break up Gondwana, but initiated the formation of sedimentary basins along the East African and West Madagascan margins. At the start of the first phase of spreading (183 to 133 Ma) predominantly NW - SE extension caused continental rifting that separated Madagascar/India/Antarctica from Africa. Maximum horizontal stresses trended perpendicular to the local plate-kinematic vector, and parallel to the rift axes. During and after continental break-up and subsequent spreading, the regional stress regime changed drastically. The extensional stress regime became restricted to the active spreading ridges that in turn adopted trends

  16. A geophysical overview of the southern continental margin of North America in the Late Precambrian/Cambrian

    SciTech Connect

    Keller, G.R. . Dept. of Geological Sciences)

    1993-02-01

    Recent geophysical studies have shed considerable light on the nature and extent of the southern Continental margin of North America which formed during the Late Precambrian/Cambrian. To the east between Arkansas and Alabama, the PASSCAL/Ouachita seismic experiment and older results in Mississippi indicate that this margin is largely preserved beneath allochthonous rocks emplaced during the Ouachita orogeny. Here the margin is fairly abrupt suggesting transtension was important in this origin. The Wiggins and Sabine blocks appear to be continental fragments which may have formed along this margin. In Texas, the margin extends around the Llano uplift in a sinuous fashion. In this area, Mesozoic extension clouds the picture considerably. However, structural complexities along this portion of he margin can be inferred form gravity data. In West Texas, the margin bends westward and then southward. A very deep oil exploration test which was recently drilled in this area provides valuable constraints for geophysical models of this portion of the margin. Recent results in Mexico allows the authors to trace this margin further south into the state of Chihuahua than previously possible.

  17. Chirostylidae of Australia's western continental margin (Crustacea : Decapoda: Anomura), with the description of five new species.

    PubMed

    Mccallum, Anna W; Poore, Gary C B

    2013-01-01

    Five new species from the squat lobster family Chirostylidae are described from the continental margin of western Australia: Uroptychus albus sp. nov., Uroptychus bardi sp. nov., Uroptychus jawi sp. nov., Uroptychus taylorae sp. nov., and Uroptychus worrorra sp. nov. New records of Indo-West Pacific species for Australia are: Gastroptychus brachyterus Baba, 2005, Gastroptychus investigatoris Alcock, 1899, Uroptychodes grandirostris (Yokoya, 1933), Uroptychodes inortenseni (Van Dam, 1939), Uroptychus scandens Benedict, 1902, Uroptychus ciliatus (Van Dam, 1933) and Uroptychus vandamae Baba, 1988. New distributional records are given for species previously recorded from Australia: Uroptychus flindersi Ahyong & Poore, 2004, Uroptychus hesperius Ahyong & Poore, 2004, Uroptychusjoloensis Van Dam, 1939, Uroptychus nigricapillis Alcock, 1901, and Uroptychus spinirostris (Ahyong & Poore, 2004). These new records expand the number of chirostylid species in Australia from 34 to 46. Keys to Australian species of the genera Gastroptychus, Uroptychodes and Uroptychus are provided. PMID:26266295

  18. Carbonate dissolution and sedimentation on the mid-atlantic continental margin.

    PubMed

    Balsam, W L

    1982-09-01

    The calcium carbonate content was determined for core tops from two transects on the upper slope to lower rise on the mid-Atlantic continental margin. Carbonate content in the sediment increases from approximately 5 percent (by weight) on the upper slope to more than 30 percent on the upper rise. A zone of low-carbonate content extends from 3000 to 4400 meters. Below 4400 meters, the percent carbonate increases. An examination of dissolution indices in these core tops indicates that the low-carbonate zone is associated with intense dissolution. Below 4400 meters, dissolution decreases and carbonate is well preserved. The decrease in dissolution occurs where the high-velocity core of the Western Boundary Undercurrent is first encountered.

  19. Chirostylidae of Australia's western continental margin (Crustacea : Decapoda: Anomura), with the description of five new species.

    PubMed

    Mccallum, Anna W; Poore, Gary C B

    2013-01-01

    Five new species from the squat lobster family Chirostylidae are described from the continental margin of western Australia: Uroptychus albus sp. nov., Uroptychus bardi sp. nov., Uroptychus jawi sp. nov., Uroptychus taylorae sp. nov., and Uroptychus worrorra sp. nov. New records of Indo-West Pacific species for Australia are: Gastroptychus brachyterus Baba, 2005, Gastroptychus investigatoris Alcock, 1899, Uroptychodes grandirostris (Yokoya, 1933), Uroptychodes inortenseni (Van Dam, 1939), Uroptychus scandens Benedict, 1902, Uroptychus ciliatus (Van Dam, 1933) and Uroptychus vandamae Baba, 1988. New distributional records are given for species previously recorded from Australia: Uroptychus flindersi Ahyong & Poore, 2004, Uroptychus hesperius Ahyong & Poore, 2004, Uroptychusjoloensis Van Dam, 1939, Uroptychus nigricapillis Alcock, 1901, and Uroptychus spinirostris (Ahyong & Poore, 2004). These new records expand the number of chirostylid species in Australia from 34 to 46. Keys to Australian species of the genera Gastroptychus, Uroptychodes and Uroptychus are provided.

  20. Antiquity of the continental slope along the middle-atlantic margin of the United States.

    PubMed

    Prior, D B; Coleman, J M; Doyle, E H

    1984-03-01

    A detailed high-resolution geophysical study of part of the continental slope along the mid-Atlantic margin of the United States indicates that it is an ancient, relict landscape largely unmodified by modern slope processes. The slope morphology is heavily influenced by bedrock outcrops, including joints and bedding planes, rather than by any single degradational process. A pelagic drape averaging 3 to 5 meters in thickness blankets the slope. Carbon-14 dates from eight drop cores show that the drape was deposited in late Pleistocene and Holocene times. The Holocene part of the drape, comprising the uppermost 1 meter, was deposited at a continuous rate of 10 centimeters per 1000 years. Most features on the slope predate the drape cover.

  1. Slope Instability and Gas Hydrates in the Hudson Canyon Region, U.S. Atlantic Continental Margin

    NASA Astrophysics Data System (ADS)

    Rona, P. A.; Robb, J. M.; Butman, B.; Scranton, M. I.; Kingman, K. E.; Tucholke, B. E.; Twichell, D.

    2004-12-01

    The continental slope and the upper rise centered on Hudson Canyon offshore New York and New Jersey lie within a major gas-hydrate province. This region exhibits evidence of gravitational mass movements and possible methane expulsion, as inferred from our bathymetric and water-column surveys conducted in 2002 with support from NOAA/OE, and prior data. The bathymetric data cover our study area (200 km by 110 km; 37\\deg40'N to 39\\deg50'N, 70\\deg00'W to 72\\deg30'W) from the inner edge of the continental slope (depth 200 m) seaward to the middle rise (c.3500 m). The world's largest hub of submarine telecommunications cables partially passes through this area. Evidence of gravitational mass movements and of probable gas release is extensive. Examples of the former include: (1) blocks of landward-dipping strata up to 2-km wide and 150-m high that lie at the base of the continental slope (water depth 2100-2200 m) seaward of an over-pressured zone beneath the continental slope (639 mbsf in ODP Hole 1073A; water depth 650 m; Dugan and Flemings, 2000); (2) boulders of Eocene chalk that litter the lower slope and upper rise; (3) a semicircular, tabular glide block, about 20 km in diameter, which thickens to about 150 m at its seaward margin; the block is centered at 39\\deg23.5'N, 71\\deg10.0'W between 2450 and 2600 m depth on the upper rise, about 15 km downslope from a congruent scarp at 2200 m on the lower slope; (4) apparent penecontemporaneous faulting and gliding in strata inclined sub-parallel to the seafloor along the upper rise; 5) apparent clogging of Hudson Canyon with hummocky sediment at a right-angle turn of the axis (depth 3368 m; 38\\deg39.6'N, 71\\deg01.8'W); 6) changes in stratification from the upper to middle rise; uneven layering beneath the upper rise (seafloor mean inclination 0.75\\deg down to 2700 m) is inferred to reflect disturbance by gravitational mass movements; even layering parallel to the seafloor beneath the middle rise (inclination

  2. 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

  3. Authigenic carbonate formation at hydrocarbon seeps in continental margin sediments: A comparative study

    USGS Publications Warehouse

    Naehr, T.H.; Eichhubl, P.; Orphan, V.J.; Hovland, M.; Paull, C.K.; Ussler, W.; Lorenson, T.D.; Greene, H. Gary

    2007-01-01

    Authigenic carbonates from five continental margin locations, the Eel River Basin, Monterey Bay, Santa Barbara Basin, the Sea of Okhotsk, and the North Sea, exhibit a wide range of mineralogical and stable isotopic compositions. These precipitates include aragonite, low- and high-Mg calcite, and dolomite. The carbon isotopic composition of carbonates varies widely, ranging from -60??? to +26???, indicating complex carbon sources that include 13C-depleted microbial and thermogenic methane and residual, 13C-enriched, bicarbonate. A similarly large variability of ??18O values (-5.5??? to +8.9???) demonstrates the geochemical complexity of these sites, with some samples pointing toward an 18O-enriched oxygen source possibly related to advection of 18O-enriched formation water or to the decomposition of gas hydrate. Samples depleted in 18O are consistent with formation deeper in the sediment or mixing of pore fluids with meteoric water during carbonate precipitation. A wide range of isotopic and mineralogical variation in authigenic carbonate composition within individual study areas but common trends across multiple geographic areas suggest that these parameters alone are not indicative for certain tectonic or geochemical settings. Rather, the observed variations probably reflect local controls on the flux of carbon and other reduced ions, such as faults, fluid conduits, the presence or absence of gas hydrate in the sediment, and the temporal evolution of the local carbon reservoir. Areas with seafloor carbonates that indicate formation at greater depth below the sediment-water interface must have undergone uplift and erosion in the past or are still being uplifted. Consequently, the occurrence of carbonate slabs on the seafloor in areas of active hydrocarbon seepage is commonly an indicator of exhumation following carbonate precipitation in the shallow subsurface. Therefore, careful petrographic and geochemical analyses are critical components necessary for the

  4. Authigenic apatite formation and burial in sediments from non-upwelling, continental margin environments

    SciTech Connect

    Ruttenberg, K.C.; Berner, R.A. )

    1993-03-01

    Evidence for precipitation of authigenic carbonate fluorapatite (CFA) in Long Island Sound and Mississippi Delta sediments suggests that formation of CFA is not restricted to environments of active coastal upwelling. The authors present porewater data suggestive of CFA formation in both these areas. Application of a sequential leaching procedure, designed specifically to separate authigenic carbonate fluorapatite from other phosphorus-containing phases, including detrital apatite of igneous or metamorphic origin, provides strong supporting evidence for authigenic apatite formation in these sediments. The size of the authigenic apatite reservoir increases with depth, indicating continued formation of CFA during early diagenesis. This depth increase is mirrored by a decrease in solid-phase organic P at both sites, suggesting that CFA is forming at the expense of organic P. Mass balance considerations, application of diagenetic models to intersitital water nutrient data, and the saturation state of the interstitial water are consistent with this interpretation. Diagenetic redistribution of phosphorus among the different solid-phase reservoirs is observed at both sites, and results in near perfect retention of P by these sediments over the depth intervals sampled. Formation of CFA in continental margins which do not conform to the classically defined regions of phosphorite formation renders CFA a quantitatively more important sink than has previously been recognized. Including this reservoir as a newly identified sink for reactive P in the ocean, the residence time of P in the modern ocean must be revised downward. The implication for ancient oceans of CFA formation in continental margin sediments other than phosphorites is that phosphorite formation may be less a representation of episodicity in removal of reactive P from the oceans than of localized concentration of CFA in phosphatic sediments by secondary physical processes. 90 refs., 5 figs., 2 tabs.

  5. Earth-System Scales of Biodiversity Variability in Shallow Continental Margin Seafloor Ecosystems

    NASA Astrophysics Data System (ADS)

    Moffitt, S. E.; White, S. M.; Hill, T. M.; Kennett, J.

    2015-12-01

    High-resolution paleoceanographic sedimentary sequences allow for the description of ecosystem sensitivity to earth-system scales of climate and oceanographic change. Such archives from Santa Barbara Basin, California record the ecological consequences to seafloor ecosystems of climate-forced shifts in the California Current Oxygen Minimum Zone (OMZ). Here we use core MV0508-20JPC dated to 735,000±5,000 years ago (Marine Isotope Stage 18) as a "floating window" of millennial-scale ecological variability. For this investigation, previously published archives of planktonic δ18O (Globigerina bulloides) record stadial and interstadial oscillations in surface ocean temperature. Core MV0508-20JPC is an intermittently laminated archive, strongly influenced by the California Current OMZ, with continuously preserved benthic foraminifera and discontinuously preserved micro-invertebrates, including ophiuroids, echinoderms, ostracods, gastropods, bivalves and scaphopods. Multivariate statistical approaches, such as ordinations and cluster analyses, describe climate-driven changes in both foraminiferal and micro-invertebrate assemblages. Statistical ordinations illustrate that the shallow continental margin seafloor underwent predictable phase-shifts in oxygenation and biodiversity across stadial and interstadial events. A narrow suite of severely hypoxic taxa characterized foraminiferal communities from laminated intervals, including Bolivina tumida, Globobulimina spp., and Nonionella stella. Foraminiferal communities from bioturbated intervals are diverse and >60% similar to each other, and they are associated with echinoderm, ostracod and mollusc fossils. As with climate shifts in the latest Quaternary, there is a sensitive benthic ecosystem response in mid-Pleistocene continental margins to climatically related changes in OMZ strength.

  6. Rifted Structure of the Vietnam Continental Margin Near the South China Sea Spreading Center

    NASA Astrophysics Data System (ADS)

    Reid, I. D.; Fyhn, M. B.; Boldreel, L. O.; Nielsen, L. H.; Duc, N. A.; Huyen, N. T.; Thang, L. D.

    2007-12-01

    The extinct spreading center of the South China Sea intersects the continental margin off Vietnam, providing an excellent opportunity to study the interaction of these two features. As part of a collaborative project between the Geological Survey of Denmark and Greenland, the University of Copenhagen and the Vietnam Petroleum Institute, the crustal structure of this area has been investigated by the use of seismic reflection profiles, to provide control on the sedimentary and basement structure, combined with modelling of gravity data from global satellite altimetry, to constrain the crustal thickness. A complex pattern of rifting is seen, which may be ascribed to the complex stress fields of the propagating rift axis, together with an apparent progression in structure. In the more oceanic area, the rifting is relatively sharp, with fairly rapid crustal thnning of about 10 km. Towards the continent, in the region of the tip of the rift axis, the crustal thinning is less, around 5-7 km, and takes place over a greater distance. In the absence of data on the deep crustal structure it is not possible to determine the absolute crustal thickness with certainty, but the gravity modelling suggests that the pre-existing crust was no more than 20 km thick, having been thinned in earlier stages of formation of the South China Sea. A preliminary analysis of the isostatic balance along the various transects was inconclusive but suggests that the sedimentary sequences are largely isostatically compensated, rather than being supported by lithospheric rigidity. Detailed modelling of the rifting and subsidence may provide further insight into the processes that occur when an oceanic spreading center intersects and propagates into a continental margin.

  7. 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.

  8. Geophysical investigations of the southern continental margin of Australia and the conjugate sector of East Antarctica

    NASA Astrophysics Data System (ADS)

    Kong, M.

    1980-08-01

    The magnetic quiet zones on the rifted continental margins of the Southeast Indian Ocean are associated with a characteristic pattern of geophysical anomalies. Each quiet zone is bounded on its seaward side by the oldest identified marine magnetic lineation, for the most part anomaly 22. Trend and extent of anomaly 22 were used to determine a new pole of rotation, located at 1.5 degrees N. lat., 37.0 degrees E. long., for the fit of Australia against Antarctica. This reconstruction is considerably less tight than previously proposed ones, due to the inclusion of an intracratonic rift zone, and avoids any overlap of continental material; it also satisfies the existing geological constraints governing the positioning of Australia against Antarctica. The geometry of the Australian-Antarctic rift in conjuction with the evidence for a continential depositional environment of the rift-valley sedimentary sequence necessitates the existence of a structural barrier near the western extremity of the rift. Such a barrier is postulated to have extended from the Naturaliste Block of southwestern Australia to the Bruce Spur off Knox Coast, Antarctica.

  9. Distribution and sources of organic matter in surface sediments of the eastern continental margin of India

    NASA Astrophysics Data System (ADS)

    Krishna, M. S.; Naidu, S. A.; Subbaiah, Ch. V.; Sarma, V. V. S. S.; Reddy, N. P. C.

    2013-12-01

    sources and distribution of organic matter (OM) in surface sediments of the eastern continental margin of India, including the region influenced by river discharge, were investigated using content, molar C:N ratios and stable isotopes of carbon and nitrogen. Despite relatively high water column integrated chlorophyll-a concentrations were found in the continental shelf than the slope; however, the lower sediment organic carbon (SOC) was found in the former than the latter region suggesting that in situ production did not play significant role on preservation of SOC in the coastal Bay of Bengal. The broad range of δ13C of SOC (-23.2 to -16.7‰) suggests that OM is a broad mixture of terrestrial and marine OM. Relative contributions from terrestrial C3 and C4 plants and marine sources are quantified as 34%, 23%, and 43%, respectively, indicating that dominant source of allochthonous OM (~57%) in the coastal Bay of Bengal. Relatively higher contribution of OM from C4 plants was found in the sediments at off river Krishna indicating that this region received detritus of agricultural crops such as jowar, bajra, and sugar cane, which are dominant in its drainage basin, during SW monsoon. This study revealed that relatively high OM preserved in the slope than shelf region along the coastal Bay of Bengal and the composition of OM is primarily controlled by the type of agricultural crops and vegetation in the drainage basin of the river.

  10. Late Cretaceous - Cenozoic development of outer continental margin, southwestern Nova Scotia

    SciTech Connect

    Swift, S.A.

    1987-06-01

    The growth pattern for the outer continental margin of Nova Scotia during the Late Cretaceous and Cenozoic was studied using seismic stratigraphy and well data. Sediment accumulation was broadly controlled by temporal changes in relative sea level, but significant spatial and temporal changes in accumulation patterns were caused by changes in sediment supply rate, morphology, erosion by abyssal currents, and salt tectonics. A Jurassic-Early Cretaceous carbonate platform remained exposed until the Late Cretaceous and controlled the location and steepness of the paleoslope until the late Miocene. Local erosion of the outer shelf and slope in the late Paleocene-early Eocene produced chalky fans on the upper rise. The relationship between erosion of the shelf in the late Eocene and early Oligocene, and abyssal current erosion of the upper rise in the Oligocene, is unclear. Seaward extensions of Tertiary shelf-edge canyons are poorly defined except for the Eocene fans. In the Miocene, abyssal currents eroded a bench on the upper continental rise. Subsequently, sediments lapped onto and buried the paleoslope. The lower rise above horizon A/sup u/ (Oligocene) is composed of fans and olistostromes shed from halokinetic uplift of the upper rise. Current eroded unconformities are common in the rise sequence, but the only current deposit is a Pliocene interval (< 300 m) restricted to the lowermost rise. Pleistocene turbidity currents eroded the present canyon morphology. 15 figures, 2 tables.

  11. Petroleum geology of the mid-Atlantic continental margin, offshore Virginia

    USGS Publications Warehouse

    Bayer, K.C.; Milici, R.C.

    1989-01-01

    The Baltimore Canyon Trough, a major sedimentary basin on the Atlantic continental shelf, contains up to 18 km of Mesozoic and Cenozoic strata. The basin has been studied extensively by multichannel common depth point (CDP) seismic reflection profiles and has been tested by drilling for hydrocarbon resources in several places. The Mesozoic and Cenozoic strata contained in the basin were deposited in littoral to bathyal depositional settings and contain immature to marginally mature oil-prone and gas-prone kerogen. The more deeply buried strata of Early Mesozoic age are more likely to be thermally mature than are the younger strata with respect to hydrocarbon generation, but contain terrestrially derived coaly organic matter that would be prone to yield gas, rather than oil. An analysis of available CDP seismic reflection data has indicated that there are several potential hydrocarbon plays in the area offshore of Virginia. These include: (1) Lower Mesozoic synrift basins that appear similar to those exposed in the Appalachian Piedmont, (2) a stratigraphic updip pinchout of strata of Early Mesozoic age in the offshore region near the coast, (3) a deeply buried paleoshelf edge, where seismic reflectors dip sharply seaward; and (4) a Cretaceous/Jurassic shelf edge beneath the present continental rise. Of these, the synrift basins and Cretaceous/Jurassic shelf edge are considered to be the best targets for exploration. ?? 1989.

  12. The Argentine Continental Margin: Rift Basins, Crustal Architecture, and Sedimentation Regime

    NASA Astrophysics Data System (ADS)

    Neben, S.; Franke, D.; Hinz, K.; Meyer, H.; Roeser, H. A.; Schreckenberger, B.; Nevistic, V. A.; Mancilla, O.

    2001-12-01

    With the objective to contribute to a better understanding of the processes of rifting and magmatism/volcanism during the Early Cretaceous breakup of Gondwana, the Federal Institute for Geosciences and Natural Resources (BGR) acquired 12,000 km of MCS data and two wide-angle reflection/refraction lines on the Argentine continental margin between 38° S and 44° S in 1998/1999. The new seismic data document the presence of a narrow, north-east striking half-graben system beneath the present upper slope. This buried half-graben system developed during the final stage of rifting in Early Cretaceous time, whereas the east-west trending Colorado-Basin probably formed earlier. However, this wide sedimentary basin was superimposed by the rifting process that led to the opening of the South Atlantic. Continental breakup and initial sea-floor spreading were accompanied by large-scale magmatism and volcanism manifested in the seismic data by a huge wedge of seaward dipping reflectors (SDRS), and a distinct high-velocity (average value 7.3 km/s) lower crustal body beneath the SDRS. Emplacement of the deeply buried, 60-120km wide SDRS was episodic documented by at least three superimposed SDRS units. The drift deposits, locally more than 6,000m thick, and subdivided by five seismic unconformities, were mapped along the shelf, slope, and deep sea.

  13. 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

  14. Pleistocene marine ice sheets and ice shelves at the East Siberian continental margin

    NASA Astrophysics Data System (ADS)

    Niessen, Frank; Kuk Hong, Jong; Hegewald, Anne; Matthiessen, Jens; Stein, Rüdiger; Kim, Sookwan; Jensen, Laura; Jokat, Wilfried; Nam, Seung Il

    2014-05-01

    RV "Polarstern" cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA03B (2012) investigated an area in the Arctic Ocean located between the Chukchi Borderland and the East Siberian Sea (between 165°W and 170°E). Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present evidence that the western Arctic Ocean had a glaciated continental margin during several glacial periods of the Pleistocene (Niessen et al. 2013). At the southern end of the Mendeleev Ridge and on the Chukchi and East Siberian continental slopes ice sheets and ice shelves grounded in up to 1200 m present water depth. We found mega-scale glacial lineations (MSGL) associated with deposition of glaciogenic wedges and debris-flow deposits indicative of sub-glacial erosion and deposition close to the former grounding lines. Glacially lineated areas are associated with large-scale erosion, capped with diamicton and draped by, in places, several metres of pelagic sediments. On the Arlis Plateau, a detailed bathymetric map exhibits several generations of MSGL, which we interpret as relicts of different Pleistocene glaciations. Traces of former grounding line positions suggest that an ice shelf of approximately 900 m in thickness has spread across the Southern Mendeleev Ridge in a north-easterly direction. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. A preliminary age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3 and clearly predates the Last Glacial Maximum. The oldest and deepest event predates MIS 6. The youngest grounding event on the Arlis Plateau is tentatively dated to have occurred during MIS 4. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for albedo, ocean-atmosphere heat exchange

  15. Subsidence and eustasy at the continental margin of eastern North America

    NASA Technical Reports Server (NTRS)

    Watts, A. B.; Steckler, M. S.

    1979-01-01

    Biostratigraphic data from the COST B-2 well off New York and four deep commercial wells off Nova Scotia have been used to remove the effect of sediment loading at the Atlantic-type continental margin off the East Coast of North America. The resulting subsidence contains terms due to both 'tectonic' and 'eustatic' effects. By assuming the tectonic subsidence is thermal in origin these effects can be separated. The 'eustatic' effects have been isolated by least squares fitting an exponential curve to the subsidence data. The resulting sea-level curve shows a maximum rise in sea level during the Late Cretaceous era which probably does not exceed 150 m. The tectonic subsidence has been interpreted in terms of a simple thermal model for the cooling lithosphere. Based on this model the thermal thickness of the lithosphere and the total amount of crustal thinning are estimated. These estimates which are consistent with surface ship gravity and GEOS-3 altimeter measurements are used to define the structural elements which control the tectonic evolution of the margin.

  16. Cretaceous source rock characterization of the Atlantic Continental margin of Morocco

    SciTech Connect

    Jabour, H. )

    1993-02-01

    Characterization of the petroleum potential for the Atlantic margin of Morocco has been based primarily on limited, antiently acquired organic geochemical data. These indicate the area of drilling behind the paleoshelf edge to be only fair in organic carbon and C15+ extract values with predominantly terrestrial kerogen types. Recently acquired geochemical data obtained from relatively recent drilling both behind and beyond the paleoshelf edge indicate 4 depositional facies containing hydrogen rich amorphous kerogen assemblages. These are: (1) Lower to Mid Jurassic inner shelf facies probably deposited in algal rich lagoon-like, (2) Lower Cretaceous non marine coaly facies probably deposited in algal rich swamplike environments, (3) Middle Cretaceous facies characterized by restrited anoxic environment with sediments rich in marine kerogen types deposited under sluggish wather circulation, (4) Upper Cretaceous to Tertiary outer-shelf to Upper slope facies probably deposited under algal-rich upwelling systems. Of these, the Cretaceous facies is the most widespread and represents the best source rock potential characteristics. Correlation of these facies to recently acquired good quality seismic packages allows for extrapolation of probable organic facies distribution throughout the continental margin. This should enhance the hydrocarbon potential of the Mesozoic and Cenozoic sediments both landward and seaward of the paleoshelf edge and thus permits refinement of strategies for hydrocarbon exploration in the area.

  17. Maturation of Tertiary sediments in the Asian Continental Margins: A basis for hydrocarbon generation studies

    SciTech Connect

    Miki, Takashi )

    1994-07-01

    In the marginal areas of the Asian continent, the Paleogene and Miocene coal-bearing formations are sporadically distributed. In some areas, particularly in the sea regions, their equivalents are possibly explored for oil and gas. The basins mainly formed as tectonic depressions, and are filled with fluvial to marine clastic rocks. The formations show marked lateral variation in thickness, lithology, and sediment characteristics, which are related to the geotectonic settings of the basins at active plate margins. Remarkable accumulation of overburden and high paleogeothermal conditions, which are marked in northern Kyushu, Japan, and Thailand, influenced diagenesis. Organic and inorganic maturation studies in northern Kyushu reveal a progress of diagenesis from the inland of Kyushu toward the sea region essentially controlled by additional heat supply from the sea region during and after sedimentation. The sediments on the land surface are chiefly overmatured, and/or contain minor amounts of organic carbon. High paleogeothermal influence on Tertiary maturation is clear also in northern Thailand. The high paleotemperature conditions in these areas may be related to tectonic interaction between the oceanic and continental plates.

  18. Physiography of the Monterey Bay National Marine Sanctuary and implications about continental margin development

    USGS Publications Warehouse

    Greene, H.D.; Maher, N.M.; Paull, C.K.

    2002-01-01

    Combined EM-300 multibeam bathymetric data and satellite photography reveal the physiography of the continental margin between 35°50′ and 37°03′N and from the shoreline west of 122°40′ and 122°37′W, which includes Monterey Bay, in a previously unprecedented detail. Patterns in these images clearly reveal the processes that are actively influencing the current geomorphology of the Monterey Bay region, including the Monterey Bay National Marine Sanctuary (MBNMS). Our data indicates that seafloor physiography within the MBNMS results from plate margin tectonic deformation, including uplift and erosion along structural lineaments, and from fluid flow. Mass wasting is the dominant process active within the Ascension–Monterey and Sur–Partington submarine canyon systems and along the lower slopes. Meanders, slump dams, and constricted channels within the submarine canyons, especially within Monterey Canyon, slow and interrupt down-canyon sediment transport. We have identified for the first time thin sediment flows, rotational slumps, rills, depressions that may be associated with pipes, and other fluid-induced features we call ‘scallops’ off the Ascension slope, and suggest that fluid flow has sculptured the seafloor morphologies here. These unusual seafloor morphologies are similar to morphologies found in terrestrial areas modified by ground-water flow.

  19. Ten years of studies on Maryland's inner Continental Margin and coastal bays

    USGS Publications Warehouse

    Kerhin, R.T.; Conkwright, R.; Wells, D.

    1999-01-01

    During the past ten years of the Association of American State Geologists-Mineral Management Service Continental Margins Program, the Maryland Geological Survey investigated the sedimentological, paleontological, stratigraphical and geophysical character of Maryland's inner continental shelf. Based on seismic records and sedimentological analyses completed during the first four years, a late Quaternary stratigraphic model was developed. Five distinct stratigraphic units were identified and described on the Maryland inner shelf. These units represent late Pleistocene interglacial deposits, the oldest of which corresponds to pre-Illinoian (oxygen-isotope stages 7 and/or 9) transgressive shelf sands. Overlying the Q1 unit, the Q2 unit is a 6-meter thick mud sequence of oxygen-isotope stage 5 (128-75 ka) age. Units Q3 and Q4 representing fluvial and leading edge estuarine deposits (oxygen-isotope stages 4, 3 and 2) filled numerous paleochannels that were incised into units Q2 and Q1. Modern trailing-edge transgressive shelf shoals (Unit Q5) discontinuously cap the sequence. The 5th and 6th years studies reported on the economic minerals of surficial and cored sediments. Vibracores collected off the Maryland's shelf during previous studies were analyzed for mineral types and abundances, weight percent of general size fractions, and heavy mineral (HM) content. Mineralogic maturity indices were compiled to correlate the THM and economic heavy minerals (EHM) abundances with position offshore, sediment type, and the indices themselves. For the 7th year, the Maryland Geological Survey re-examined geophysical records and lithological data originally collected by the Army Corps of Engineers to locate and assess beach fill borrow areas for the Ocean City Beach Replenishment Project. Data from 163 vibracores and over 300 kilometers of high-resolution seismic profile records collected off Ocean City, Maryland, supported the stratigraphic model developed by MGS during the first

  20. A model of the methane cycle, permafrost, and hydrology of the Siberian continental margin

    NASA Astrophysics Data System (ADS)

    Archer, D.

    2014-06-01

    A two-dimensional model of a passive continental margin was adapted to the simulation of the methane cycle on Siberian continental shelf and slope, attempting to account for the impacts of glacial/interglacial cycles in sea level, alternately exposing the continental shelf to freezing conditions with deep permafrost formation during glacial times, and immersion in the ocean in interglacial times. The model is used to gauge the impact of the glacial cycles, and potential anthropogenic warming in the deep future, on the atmospheric methane emission flux, and the sensitivities of that flux to processes such as permafrost formation and terrestrial organic carbon (Yedoma) deposition. Hydrological forcing drives a freshening and ventilation of pore waters in areas exposed to the atmosphere, which is not quickly reversed by invasion of seawater upon submergence, since there is no analogous saltwater pump. This hydrological pump changes the salinity enough to affect the stability of permafrost and methane hydrates on the shelf. Permafrost formation inhibits bubble transport through the sediment column, by construction in the model. The impact of permafrost on the methane budget is to replace the bubble flux by offshore groundwater flow containing dissolved methane, rather than accumulating methane for catastrophic release when the permafrost seal fails during warming. By far the largest impact of the glacial/interglacial cycles on the atmospheric methane flux is attenuation by dissolution of bubbles in the ocean when sea level is high. Methane emissions are highest during the regression (soil freezing) part of the cycle, rather than during transgression (thawing). The model-predicted methane flux to the atmosphere in response to a warming climate is small, relative to the global methane production rate, because of the ongoing flooding of the continental shelf. A slight increase due to warming could be completely counteracted by sea level rise on geologic time scales

  1. A model of the methane cycle, permafrost, and hydrology of the Siberian continental margin

    DOE PAGES

    Archer, D.

    2014-06-03

    A two-dimensional model of a passive continental margin was adapted to the simulation of the methane cycle on Siberian continental shelf and slope, attempting to account for the impacts of glacial/interglacial cycles in sea level, alternately exposing the continental shelf to freezing conditions with deep permafrost formation during glacial times, and immersion in the ocean in interglacial times. The model is used to gauge the impact of the glacial cycles, and potential anthropogenic warming in the deep future, on the atmospheric methane emission flux, and the sensitivities of that flux to processes such as permafrost formation and terrestrial organic carbonmore » (Yedoma) deposition. Hydrological forcing drives a freshening and ventilation of pore waters in areas exposed to the atmosphere, which is not quickly reversed by invasion of seawater upon submergence, since there is no analogous saltwater pump. This hydrological pump changes the salinity enough to affect the stability of permafrost and methane hydrates on the shelf. Permafrost formation inhibits bubble transport through the sediment column, by construction in the model. The impact of permafrost on the methane budget is to replace the bubble flux by offshore groundwater flow containing dissolved methane, rather than accumulating methane for catastrophic release when the permafrost seal fails during warming. By far the largest impact of the glacial/interglacial cycles on the atmospheric methane flux is attenuation by dissolution of bubbles in the ocean when sea level is high. Methane emissions are highest during the regression (soil freezing) part of the cycle, rather than during transgression (thawing). The model-predicted methane flux to the atmosphere in response to a warming climate is small, relative to the global methane production rate, because of the ongoing flooding of the continental shelf. A slight increase due to warming could be completely counteracted by sea level rise on geologic time

  2. Geology of the continental margin beneath Santa Monica Bay, Southern California, from seismic-reflection data

    USGS Publications Warehouse

    Fisher, M.A.; Normark, W.R.; Bohannon, R.G.; Sliter, R.W.; Calvert, A.J.

    2003-01-01

    We interpret seismic-reflection data, which were collected in Santa Monica Bay using a 70-in3 generator-injector air gun, to show the geologic structure of the continental shelf and slope and of the deep-water, Santa Monica and San Pedro Basins. The goal of this research is to investigate the earthquake hazard posed to urban areas by offshore faults. These data reveal that northwest of the Palos Verdes Peninsula, the Palos Verdes Fault neither offsets the seafloor nor cuts through an undeformed sediment apron that postdates the last sea level rise. Other evidence indicates that this fault extends northwest beneath the shelf in the deep subsurface. However, other major faults in the study area, such as the Dume and San Pedro Basin Faults, were active recently, as indicated by an arched seafloor and offset shallow sediment. Rocks under the lower continental slope are deformed to differing degrees on opposite sides of Santa Monica Canyon. Northwest of this canyon, the continental slope is underlain by a little-deformed sediment apron; the main structures that deform this apron are two lower-slope anticlines that extend toward Point Dume and are cored by faults showing reverse or thrust separation. Southeast of Santa Monica Canyon, lower-slope rocks are deformed by a complex arrangement of strike-slip, normal, and reverse faults. The San Pedro Escarpment rises abruptly along the southeast side of Santa Monica Canyon. Reverse faults and folds underpinning this escarpment steepen progressively southeastward. Locally they form flower structures and cut downward into basement rocks. These faults merge downward with the San Pedro Basin fault zone, which is nearly vertical and strike slip. The escarpment and its attendant structures diverge from this strike-slip fault zone and extend for 60 km along the margin, separating the continental shelf from the deep-water basins. The deep-water Santa Monica Basin has large extent but is filled with only a thin (less than 1.5-km

  3. Continental margin deformation along the Andean subduction zone: Thermo-mechanical models

    NASA Astrophysics Data System (ADS)

    Gerbault, Muriel; Cembrano, J.; Mpodozis, C.; Farias, M.; Pardo, M.

    2009-12-01

    The Chilean Andes extend north-south for about 3000 km over the subducting Nazca plate, and show evidence of local rheological controls on first-order tectonic features. Here, rheological parameters are tested with numerical models of a subduction driven by slab-pull and upper plate velocities, and which calculate the development of stress and strain over a typical period of 4 Myr. The models test the effects of subduction interface strength, arc and fore-arc crust rheology, and arc temperature, on the development of superficial near-surface faulting as well as viscous shear zones in the mantle. Deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities. Upper plate shortening and trench advance are both correlated, and favored, to a first-order by upper plate weakness, and to a second-order by interface strength. In cases of a strong interface, a weak fore-arc crust is dragged downward by “tectonic erosion”, a scenario for which indications are found along the northern Chilean margin. In contrast for a resistant fore-arc, the slab-pull force transmits to the surface and produces topographic subsidence. This process may explain present-day subsidence of the Salar de Atacama basin and/or the persistence of a Central Depression. Specific conditions for northern Chile produce a shear zone that propagates from the subduction zone in the mantle, through the Altiplano lower crust into the Sub-Andean crust, as proposed by previous studies. Models with a weak interface in turn, allow buoyant subducted material to rise into the continental arc. In case of cessation of the slab-pull, this buoyant material may rise enough to change the stress state in the continental crust, and lead to back-arc opening. In a case of young and hydrated oceanic plate forced by the slab-pull to subduct under a resistant continent, this plate is deviated and indented by the continental mantle, and stretches horizontally

  4. Distribution, abundance and trail characteristics of acorn worms at Australian continental margins

    NASA Astrophysics Data System (ADS)

    Anderson, T. J.; Przeslawski, R.; Tran, M.

    2011-04-01

    Acorn worms (Enteropneusta), which were previously thought to be a missing link in understanding the evolution of chordates, are an unusual and potentially important component of many deep-sea benthic environments, particularly for nutrient cycling. Very little is known about their distribution, abundance, or behaviour in deep-sea environments around the world, and almost nothing is known about their distribution within Australian waters. In this study, we take advantage of two large-scale deep-sea mapping surveys along the eastern (northern Lord Howe Rise) and western continental margins of Australia to quantify the distribution, abundance and trail-forming behaviour of this highly unusual taxon. This is the first study to quantify the abundance and trail behaviour of acorn worms within Australian waters and provides the first evidence of strong depth-related distributions. Acorn worm densities and trail activity were concentrated between transect-averaged depths of 1600 and 3000 m in both eastern and western continental margins. The shallow limit of their depth distribution was 1600 m. The deeper limit was less well-defined, as individuals were found in small numbers below 3000 down to 4225 m. This distributional pattern may reflect a preference for these depths, possibly due to higher availability of nutrients, rather than a physiological constraint to greater depths. Sediment characteristics alone were poor predictors of acorn worm densities and trail activity. High densities of acorn worms and trails were associated with sandy-mud sediments, but similar sediment characteristics in either shallower or deeper areas did not support similar densities of acorn worms or trails. Trail shapes varied between eastern and western margins, with proportionally more meandering trails recorded in the east, while spiral and meandering trails were both common in the west. Trail shape varied by depth, with spiral-shaped trails dominant in areas of high acorn worm densities

  5. Development of The Argentine Continental Margin: Rift Basins and Volcanic Episodes

    NASA Astrophysics Data System (ADS)

    Neben, S.; Franke, D.; Hinz, K.; Meyer, H.; Roeser, H.; Schreckenberger, B.

    With the objective to contribute to a better understanding of the processes of rifting and magmatism/volcanism during the Early Cretaceous break-up of Gondwana, the Federal Institute for Geosciences and Natural Resources (BGR) acquired 12,000 km of MCS data and two wide-angle reflection/refraction lines on the Argentine continen- tal margin between 38S and 44S in 1998/1999. Together with already existing data sets, we know have 22,000 km of MCS data available. The seismic data document the presence of a narrow, north-west striking half-graben system beneath the present upper slope. This buried half-graben system developed during the final stage of rifting in Early Cretaceous time, whereas the east-west trending Colorado-Basin probably formed earlier. However, this wide sedimentary basin was superimposed and reacti- vated by the rifting process that led to the opening of the South Atlantic. Continental break-up and initial sea-floor spreading were accompanied by large-scale magmatism and volcanism. This is manifested in the seismic data by a huge wedge of seaward dipping reflectors (SDRS), and a distinct high-velocity (average value 7.3 km/s) lower crustal body beneath the SDRS. Extrapolating DSDP/ODP-drilling results from other volcanic margins, especially in the North Atlantic, we infer that the SDRS consist of tholeitic basalts. The emplacement of the deeply buried, 60-120km wide SDRS was episodic documented by at least three superimposed SDRS units. From our dense grid of new magnetic data we infer further that the emplacement of the volcanic units was diachronous (older units in the south and younger units in the north). The presence of the volcanic edifices at the continental margin of Argentine is related to the activity of the Tristan da Cunha hot spot which led to the emplacement of the Parana-Etendeka Flood basalts on South America and South Africa and to the Rio-Grande-Rise/Walvis- Ridge complex. The drift deposits, locally more than 6,000m thick, and

  6. Cenozoic ice sheet history from East Antarctic Wilkes Land continental margin sediments

    USGS Publications Warehouse

    Escutia, C.; De Santis, L.; Donda, F.; Dunbar, R.B.; Cooper, A. K.; Brancolini, Giuliano; Eittreim, S.L.

    2005-01-01

    The long-term history of glaciation along the East Antarctic Wilkes Land margin, from the time of the first arrival of the ice sheet to the margin, through the significant periods of Cenozoic climate change is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice sheet to the Wilkes Land margin resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma during the early Oligocene cooling climate trend. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low-angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across unconformity WL-U8 is interpreted to represent the transition, at the end of the middle Miocene "climatic optimum" (14-10 Ma), from a subpolar regime with dynamic ice sheets (i.e., ice sheets come and go) to a regime with persistent but oscillatory ice sheets. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, which we infer to be of middle Miocene age. During deposition of the two uppermost sequences, WL-S8 and WL-S9, there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin (14-10 Ma). Sequence WL-S9 forms under a polar

  7. Organic matter diagenesis and hydrocarbon generation on outer Continental Margin of northwestern Australia

    SciTech Connect

    Meyers, P.A.; Snowdon, L.R.; Heggie, D.; Bent, A.

    1989-03-01

    Organic geochemical analyses of sediments and rocks obtained from drill sites on the Exmouth and Wombat Plateaus and the Argo Abyssal Plain on the northwestern margin of Australia were done onboard the JOIDES Resolution during Ocean Drilling Program Legs 122 and 123. These analyses provide information about the sources of organic matter to these offshore locations from Triassic to Holocene times and also indicate the degree of postdepositional diagenesis and maturation the organic matter has experienced. Because this margin has interest to petroleum explorationists, these data have practical as well as fundamental significance. Triassic claystones (equivalent to the onshore Mungeroo Formation) from the Wombat Plateau contain up to several percent of land-derived organic carbon. Neocomian siltstones and claystones (equivalent to the Barrow Group and Muderong Shale) from the Exmouth Plateau hold similar organic matter but at lower concentrations. Younger sediments are generally very lean in organic matter. Gas chromatographic analysis of extractable hydrocarbons shows a large and often dominant contribution of continental components, notably n-alkanes with a strong odd/even ratio and tricyclic diterpanes. Both Rock-Eval and hydrocarbon results agree in indicating low to moderate levels of thermal maturity. Locations on the Exmouth Plateau typically contain large amounts of thermogenic gaseous hydrocarbons dominated by methane. Concentrations peak in Senonian chalk sequences. In Neocomian siltstones and claystones, methane-ethane ratios diminish as concentrations decrease. The source of these hydrocarbons is likely to be the Triassic coals and coaly material below the Dingo claystone, which was not drilled during these legs but has been characterized from industry wells on this passive margin.

  8. 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.

  9. The Davie Ridge: a Marginal Transform Ridge not Formed During Continental Breakup

    NASA Astrophysics Data System (ADS)

    Phethean, J. J. J.; Van Hunen, J.; McCaffrey, K. J. W.; Davies, R. J.

    2014-12-01

    The breakup of Gondwana translated Madagascar southwards relative to Africa along the Davie Fracture Zone (DFZ). This fracture zone now forms the Transform Passive Continental Margin (TPCM) from Kenya to Mozambique. The Davie Ridge (DR), a transform marginal ridge, has formed along the DFZ between 5 and 2°S and 22 and 11°S, but with little expression in-between. It has been proposed that this marginal ridge was formed by the thermal effects of a passing Mid Ocean Ridge (MOR) during the separation of Gondwana. Plate kinematic reconstructions, however, constrained by ocean magnetic anomalies, show that the MOR only passed between the north and south expressions of the DR. Therefore the positive linear gravity anomalies of the DR cannot be attributed to the effects of a passing MOR, and some other mechanism must be found to explain their formation. Interpretation of seismic reflection profiles along the DR shows that the gravity highs occur adjacent to large basin structures. In the north this correlates with a basin-bounding basement high of ~Albian age, and in the south with the rift flank uplifts of the currently active Quirimbas graben. This suggests that the northern and southern DR segments are instead shoulder uplifts resulting from two separate extensional episodes during different stress regimes. These are the Cretaceous NE-SW extension during the breakup of the south Atlantic, and the E-W extension of the Neogene-recent Afar-East Africa rift system, respectfully. The lack of deformation and DR formation along the region of the TPCM passed by the MOR suggests it has been coupled by thermal effects and/or the injection of magma.

  10. Geoacoustic characteristics at the DH-2 long-core sediments in the Korean continental margin of the East Sea

    NASA Astrophysics Data System (ADS)

    Ryang, Woo Hun; Kim, Seong Pil; Hahn, Jooyoung

    2015-04-01

    A long core of 27.2 m was acquired at the DH-2 site (37°34.355'N and 129°19.516'E) in the Korean continental margin of the western East Sea. The core site is located near the Donghae City and the water depth is 316.6 m deep. The long-core sediment was recovered using the Portable Remotely Operated Drill (PROD), a fully contained drilling system, remotely operated at the seafloor. The recovered core sediments were analyzed for physical, sedimentological, and geoacoustic properties mostly at 10~30 cm intervals. Based on the long-core data with subbottom and air-gun profiles at the DH-2 core site, geoacoustic characteristics of the deeper sedimentary successions were firstly investigated in the Korean continental margin of the western East Sea. The geoacoustic measurements comprise 86 P-wave velocities and 76 attenuation values. These geoacoustic characteristics of the DH-2 long core probably contribute for reconstruction of geoacoustic models reflecting vertical and lateral variability of acoustic properties in the Korean continental margin of the western East Sea. Keywords: long core, geoacoustic, East Sea, continental margin, P-wave speed Acknowledgements: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0025733) and by the Agency of Defense Development (UD140003DD).

  11. 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.

  12. Lithospheric Shear Velocity Models Beneath Continental Margins in Antarctica Inferred From Genetic Algorithm Inversion for Teleseismic Receiver Functions

    NASA Astrophysics Data System (ADS)

    Kanao, M.; Shibutani, T.

    2005-12-01

    Seismic shear velocity models of the crust and the uppermost mantle were studied by teleseismic receiver function analyses beneath the permanent stations of the Federation of Digital Seismographic Networks (FDSN) at Antarctic continental margins. In order to eliminate the starting model dependency, a non-linear Genetic Algorithm (GA) was introduced in the time domain inversion of the receiver functions. A plenty of velocity models with an acceptable fit to the receiver function waveforms were generated during the inversion, and a stable model was produced by employing a weighted average of the best 1,000 models encountered in the development of the GA. The shear velocity model beneath the MAW (67.6S, 62.9E) has a sharp Moho boundary at 44 km depth that might have involved in a reworked metamorphic event of adjacent Archaean Napier Complex. A fairly sharp Moho was identified about 28 km depth beneath DRV (66.7S, 140.0E), with a middle grade variation of the crustal velocities that might have been caused by the Early Proterozoic metamorphism. A similar sharp Moho has been found at 40 km beneath SYO (69.0S, 39.6E). Thus Moho depth is consistent with that from refraction / wide-angle reflection surveys around the station. Fairly complicated velocity variations within the crust may have a relationship with lithology of granulite facies metamorphic rocks in the shallow crust associated with Pan-African events. Broadening low velocity zones about 30 km depths with transitional crust-mantle boundary at VNDA (77.5S, 161.9E), might be caused by the rift system besides the Trans Antarctic Mountains. As for the Antarctic Peninsular, very broad Moho was found around 36 km depths around PMSA (64.8S, 64.0W). The evidence of velocity variations within the crust reflects the tectonic histories of each terrain where these permanent stations are located.

  13. Circum-Pacific accretion of oceanic terranes to continental blocks: accretion of the Early Permian Dun Mountain ophiolite to the E Gondwana continental margin, South Island, New Zealand

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair

    2016-04-01

    Accretionary orogens, in part, grow as a result of the accretion of oceanic terranes to pre-existing continental blocks, as in the circum-Pacific and central Asian regions. However, the accretionary processes involved remain poorly understood. Here, we consider settings in which oceanic crust formed in a supra-subduction zone setting and later accreted to continental terranes (some, themselves of accretionary origin). Good examples include some Late Cretaceous ophiolites in SE Turkey, the Jurassic Coast Range ophiolite, W USA and the Early Permian Dun Mountain ophiolite of South Island, New Zealand. In the last two cases, the ophiolites are depositionally overlain by coarse clastic sedimentary rocks (e.g. Permian Upukerora Formation of South Island, NZ) that then pass upwards into very thick continental margin fore-arc basin sequences (Great Valley sequence, California; Matai sequence, South Island, NZ). Field observations, together with petrographical and geochemical studies in South Island, NZ, summarised here, provide evidence of terrane accretion processes. In a proposed tectonic model, the Early Permian Dun Mountain ophiolite was created by supra-subduction zone spreading above a W-dipping subduction zone (comparable to the present-day Izu-Bonin arc and fore arc, W Pacific). The SSZ oceanic crust in the New Zealand example is inferred to have included an intra-oceanic magmatic arc, which is no longer exposed (other than within a melange unit in Southland), but which is documented by petrographic and geochemical evidence. An additional subduction zone is likely to have dipped westwards beneath the E Gondwana margin during the Permian. As a result, relatively buoyant Early Permian supra-subduction zone oceanic crust was able to dock with the E Gondwana continental margin, terminating intra-oceanic subduction (although the exact timing is debatable). The amalgamation ('soft collision') was accompanied by crustal extension of the newly accreted oceanic slab, and

  14. Structural Precursors to Continental Break-Up; the Faroe Islands, NE Atlantic Margin

    NASA Astrophysics Data System (ADS)

    Walker, R. J.; Holdsworth, R. E.; Imber, J.; Ellis, D.

    2008-12-01

    During the Palaeogene the NE Atlantic margin was subjected to a series of extension events immediately prior to and during continental break-up (at ca. 54 Ma). In the Faroes region of the margin, palaeostress analyses on faults exposed on the Faroe Islands indicate that the extension vector rotated in an anticlockwise sense from NE-SW to NW-SE. Remnants of the Faroe Islands Basalt Group (FIBG) exposed on the islands were emplaced at or around sea-level, to a total stratigraphic thickness in excess of 6.6 km, requiring a comparable magnitude of subsidence; to date, the structures preserved on the Faroe Islands have been inferred as being concurrent with subsidence. However, no onshore studies have accounted for the uplift events that must have occurred to bring the Faroe Islands to their current elevation (the highest peak at 882 m a.s.l.). The purpose of this study is to constrain the relative timings and kinematics of structures exposed on the Faroe Islands in order to investigate the regional tectonic deformation regime during continental break-up and sea-floor spreading; processes that have not hitherto been resolved using geophysical techniques. For the first time, we provide structural evidence that suggests uplift was accommodated by reactivation of pre-existing structures in the period immediately following emplacement of the FIBG. Structures on the islands provide clear evidence for a 3-phase tectonic evolution: (1a) anticlockwise rotation from E-W to NE-SW extension, facilitated first on N-S (dip-slip) faults, followed by NW-SE (dip-slip) faults. NE- SW extension (1b) continued with emplacement of a NW-SE- and NNE-SSW-oriented dyke swarm. Event-1 began prior to the deposition of the coal-bearing Prestfjall Formation, and was sustained through to emplacement of the Enni Formation, resulting in notable thickness variations across the Judd, Brynhild and Westray fault-zones. Further anticlockwise rotation of the extension vector led to (2a) the emplacement of

  15. Geometry of the Iapetus Baltoscandian continental margin; evidence for basement highs from the external imbricate zone.

    NASA Astrophysics Data System (ADS)

    Rice, A. Hugh N.

    2015-04-01

    The geometry of the Iapetus Baltoscandian continental margin prior to Scandinavian Caledonian collision is important, since only with a detailed initial input can synthetic palaeogeographic and deformation models be correctly applied. The Scandes comprise ~SE-directed nappes pierced by tectonic windows exposing basement with condensed, post-Gaskiers-glaciation (582-580Ma) cover sequences. Here, evidence, largely from the Lower Allochthon (external imbricate zone), for major displacement of these basement rocks ('Window Allochthon'), is summarized; palaeogeographically they formed a topographic-high along the Baltoscandian continental margin. In the Oslo Graben and East Finnmark areas (southernmost/northernmost Scandinavia), the transition from (par)-autochthon to allochthon is preserved (Osen-Roa Nappe Complex/Gaissa Thrust Belt; ORNC/GTB). These areas give reliable templates for other parts of the orogen, where the orogen leading edge has been extensively eroded. In the ORNC and GTB, bulk shortening was ~50%, with values rising towards the hinterland; metamorphic grades also increase towards the hinterland. Balanced-sections restore the trailing-edges of the ORNC and GTB to Norwegian coastal areas. In Finnmark, restoration places pre-Marinoan (pre~650 Ma) GTB anchizone-grade rocks above epizone-grade post-Gaskiers rocks lying unconformably on basement in the Komagfjord tectonic window. In southern Norway, restored pre-Gaskiers ORNC rocks overlie Cambro-Ordovician sediments unconformable on basement in the Atnsjøen/Spekedalen windows and WGR. Caledonian Middle Allochthon deformation in Finnmark was SE-directed and in the GTB E- to ESE-directed. In the Komagfjord window basement, Caledonian imbrication was SE-directed, but the overlying basal Middle Allochthon mylonites have an out-of-sequence E-ESE overprint. Thus the Komagfjord basement/cover lies structurally between the Middle and Lower Allochthons. In the Atnsjøen/Spekedalen windows, SE-directed Caledonian

  16. 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…

  17. Germanium-silicon fractionation in a river-influenced continental margin: The Northern Gulf of Mexico

    NASA Astrophysics Data System (ADS)

    Baronas, J. Jotautas; Hammond, Douglas E.; Berelson, William M.; McManus, James; Severmann, Silke

    2016-04-01

    In this study we have sampled the water column and sediments of the Gulf of Mexico to investigate the effects of high riverine terrigenous load and sediment redox conditions on the cycling of Ge and Si. Water column Ge/Si ratios across the Gulf of Mexico continental shelf range from 1.9 to 25 μmol/mol, which is elevated compared to the global ocean value of 0.7 μmol/mol. The Ge enrichment in the Gulf of Mexico seawater is primarily due to anthropogenic contamination of the Mississippi river, which is the main Ge and Si source to the area, and to a smaller extent due to discrimination against Ge during biogenic silica (bSi) production (Ge/Si = 1.2-1.8 μmol/mol), especially by radiolarians and siliceous sponges (Ge/Si = 0.6-1.1 μmol/mol). Most sediment pore waters (Ge/Si = 0.3-4.5 μmol/mol) and sediment incubation experiments (benthic flux Ge/Si = 0.9-1.2 μmol/mol) indicate precipitation of authigenic phases that sequester Ge from pore waters (non-opal sink). This process appears to be independent of oxidation-reduction reactions and suggests that authigenic aluminosilicate formation (reverse weathering) may be the dominant Ge sink in marine sediments. Compilation of previously published data shows that in continental margins, non-opal Ge burial flux is controlled by bSi supply, while in open ocean sediments it is 10-100 times lower and most likely limited by the supply of lithogenic material. We provide a measurement-based estimate of the global non-opal Ge burial flux as 4-32 Mmol yr-1, encompassing the 2-16 Mmol yr-1 needed to keep the global marine Ge cycle at steady state.

  18. 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

  19. Three-Dimensional Numerical Modeling of Crustal Growth at Active Continental Margins

    NASA Astrophysics Data System (ADS)

    Zhu, G.; Gerya, T.; Tackley, P. J.

    2011-12-01

    Active margins are important sites of new continental crust formation by magmatic processes related to the subduction of oceanic plates. We investigate these phenomena using a three-dimensional coupled petrological-geochemical-thermomechanical numerical model, which combines a finite-difference flow solver with a non-diffusive marker-in-cell technique for advection (I3ELVIS code, Gerya and Yuen, PEPI,2007). The model includes mantle flow associated with the subducting plate, water release from the slab, fluid propagation that triggers partial melting at the slab surface, melt extraction and the resulting volcanic crustal growth at the surface. The model also accounts for variations in physical properties (mainly density and viscosity) of both fluids and rocks as a function of local conditions in temperature, pressure, deformation, nature of the rocks, and chemical exchanges. Our results show different patterns of crustal growth and surface topography, which are comparable to nature, during subduction at active continental margins. Often, two trench-parallel lines of magmatic activity, which reflect two maxima of melt production atop the slab, are formed on the surface. The melt extraction rate controls the patterns of new crust at different ages. Moving free water reflects the path of fluids, and the velocity of free water shows the trend of two parallel lines of magmatic activity. The formation of new crust in particular time intervals is distributed in finger-like shapes, corresponding to finger-like and ridge-like cold plumes developed atop the subducting slabs (Zhu et al., G-cubed,2009; PEPI,2011). Most of the new crust is basaltic, formed from peridotitic mantle. Granitic crust extracted from melted sediment and upper crust forms in a line closer to the trench, and its distribution reflects the finger-like cold plumes. Dacitic crust extracted from the melted lower crust forms in a line farther away from the trench, and its distribution is anticorrelated with

  20. Methane Derived Authigenic Carbonates from the Upper Continental Margin of the Bay of Biscay (France)

    NASA Astrophysics Data System (ADS)

    Pierre, C.; Blanc-Valleron, M. M.; Dupré, S.

    2014-12-01

    Extensive seafloor carbonate pavements are present at water depth from 140 to 180 meters on the upper continental margin of the Bay of Biscay, 50 to 60 km away from the present-day coastline. They form at the seafloor meter-high sub-circular reliefs with a diameter from 10 m to 100 m that are surrounded by light brown silto-sandy unconsolidated sediments. All these structures are associated with active methane seeps that cover an area of 80km from N to S and up to 8km from W to E. These carbonates were sampled during the two cruises GAZCOGNE 1 (july-august 2013) and GAZCOGNE 2 (september 2013). The carbonate crusts are porous sandstones, dark brown to black by impregnation with Fe-Mn oxides/hydroxides. Subseafloor concretions are homogenous light to medium grey fine-grained sandstones. The bulk carbonate content varies in the range 36-42 weight %. The carbonate mineralogy is dominated by aragonite that cements the detrital grains whereas calcite comes from the biogenic carbonates. Dolomite occurs in significant amount in a few samples. Circular cavities of 5 to 10 µm of diameter in the carbonate cement represent traces of gas bubbles; smaller holes in the aragonite crystals are due to carbonate dissolution by CO2 issued from aerobic oxidation of methane. The oxygen isotopic compositions of the bulk carbonate (+1.7 to +4.5‰) and aragonite cement (-0.2 to +1.4‰) are lower than the values in equilibrium with the present-day temperature and salinity conditions. This indicates that the carbonate precipitated in mixtures of seawater and continental water, i.e. in a context of submarine groundwater discharge. The carbon isotopic compositions of the bulk carbonate (-51.9 to -38.2‰) and aragonite cement (-49.9 to -29.3‰) demonstrate that most carbon derived from methane oxidized as bicarbonate during microbial anaerobic oxidation of methane. The GAZCOGNE study is co-funded by TOTAL and IFREMER as part of the PAMELA (Passive Margin Exploration Laboratories

  1. Dynamic sedimentation of Paleoproterozoic continental margin iron formation, Labrador Trough, Canada: Paleoenvironments and sequence stratigraphy

    NASA Astrophysics Data System (ADS)

    Pufahl, P. K.; Anderson, S. L.; Hiatt, E. E.

    2014-07-01

    The Paleoproterozoic Sokoman Formation (ca. 1.88 Ga) of the Labrador Trough, eastern Canada, is a ca. 100-m-thick succession of interbedded iron formation and fine-grained, terrigenous clastic sedimentary rocks. Detailed examination of drill cores and outcrops indicates a dynamic paleoshelf where an oxygen-stratified water column, coastal upwelling of hydrothermally derived Fe and Si, as well as tide- and storm-generated currents controlled lithofacies character. Vertical and lateral facies stacking patterns record deposition through two relative sea-level cycles that produced seven distinct lithofacies comprising two unconformity-bounded sequences. Sequence 1 reflects deposition of hematitic peritidal iron formation as deep as the upper shoreface. Sequence 2 is truncated by later erosion and encompasses the change to deeper-water accumulation of magnetite and Fe silicate-rich iron formation. The character and lateral distribution of redox-sensitive facies indicate that iron formation accumulation was controlled as much by shelf hydraulics as oxygen levels. The development of a suboxic surface ocean is interpreted to reflect photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that flourished in nutrient-rich, upwelled waters offshore. Deposition of other continental margin iron formations also occurred on Paleoproterozoic shelves that were favorably positioned for coastal upwelling. Variability between iron formations reflects intrinsic factors such as shelf profile, fluvial contribution, eolian input, evaporation rates, and coastal current systems, which influenced upwelling dynamics and the delivery of Fe, Si, and nutrients. Aridity onshore was a primary depositional control since it governed the transport and type of diluting terrigenous clastics as well as evaporative precipitation along the coastline. As in the Phanerozoic, unconformities, and transgressive and maximum flooding surfaces frame iron

  2. Key controls of surface carbonate system dynamics around the northwest European continental margin

    NASA Astrophysics Data System (ADS)

    Jiang, Z.; Hydes, D. J.; Hartman, S. E.; Hartman, M. C.

    2011-12-01

    Monthly sampling coupled to continuous underway observation from a ship-of-opportunity (Pride of Bilbao) provides new insights into the relative importance of processes controlling the seasonal to inter-annual variability of the carbonate system around the northwest European continental margin. Total alkalinity (TA) and total dissolved inorganic carbon (DIC) were determined alongside measurements of nutrients and continuous records of temperature, salinity, chlorophyll-fluorescence, and dissolved oxygen (DO). The northwest European continental margin is temperate latitude system with a strong seasonal cycle in biological productivity determined by light, nutrient supply, and stratification. Here we contrast findings in two areas: the shallow non stratified English Channel (depth ~50 m) and seasonally stratified oligotrophic waters of the central Bay of Biscay (depth >3000 m). In the Bay of Biscay, the seasonal variations of the carbonate system, nutrient, and DO were mainly controlled by the winter mixing and spring phytoplankton bloom. DIC and nutrients in the Bay increased from autumn and reached the annual maxima in later winter, they then decreased significantly during the spring bloom corresponding to the biological uptake. DIC fell during the spring bloom with a near Redfield ratio in relation to the nutrient uptake. In contrast, post bloom in summer, a continued decrease in DIC in the absence of measurable nitrate was possibly related to the nutrient supply from the turbulent mixing. pCO2 and pH showed a double peak in the annual cycles modulated by temperature which counterbalanced the influence of winter mixing and biological production. The inter-annual biogeochemical variability was closely related to the changes in winter mixed layer depth and the phytoplankton biomass. The Bay of Biscay acted as a sink for atmospheric CO2 in all seasons, with higher air-to-sea CO2 fluxes observed in cold winter and the productive spring season. In the more dynamic

  3. Gas emissions at the continental margin west off Svalbard: mapping, sampling, and quantification

    NASA Astrophysics Data System (ADS)

    Sahling, H.; Römer, M.; Pape, T.; Bergès, B.; dos Santos Fereirra, C.; Boelmann, J.; Geprägs, P.; Tomczyk, M.; Nowald, N.; Dimmler, W.; Schroedter, L.; Glockzin, M.; Bohrmann, G.

    2014-05-01

    We mapped, sampled, and quantified gas emissions at the continental margin west of Svalbard during R/V Heincke cruise He-387 in late summer 2012. Hydroacoustic mapping revealed that gas emissions were not limited to a zone just above 396 m below sea level (m b.s.l.). Flares from this depth gained significant attention in the scientific community in recent years because they may be caused by bottom water-warming induced hydrate dissolution in the course of global warming and/or by recurring seasonal hydrate formation and decay. We found that gas emissions occurred widespread between about 80 and 415 m b.s.l. which indicates that hydrate dissolution might only be one of several triggers for active hydrocarbon seepage in that area. Gas emissions were remarkably intensive at the main ridge of the forlandet moraine complex in 80 to 90 m water depths, and may be related to thawing permafrost. Focused seafloor investigations were performed with the remotely operated vehicle (ROV) "Cherokee". Geochemical analyses of gas bubbles sampled at about 240 m b.s.l. as well as at the 396 m gas emission sites revealed that the vent gas is primarily composed of methane (> 99.70%) of microbial origin (average δ13C = -55.7‰ V-PDB). Estimates of the regional gas bubble flux from the seafloor to the water column in the area of possible hydrate decomposition were achieved by combining flare mapping using multibeam and single beam echosounder data, bubble stream mapping using a ROV-mounted horizontally-looking sonar, and quantification of individual bubble streams using ROV imagery and bubble counting. We estimated that about 53 × 106 mol methane were annually emitted at the two areas and allow a large range of uncertainty due to our method (9 to 118 × 106 mol yr-1). These amounts, first, show that gas emissions at the continental margin west of Svalbard were in the same order of magnitude as bubble emissions at other geological settings, and second, may be used to calibrate models

  4. Gas emissions at the continental margin west of Svalbard: mapping, sampling, and quantification

    NASA Astrophysics Data System (ADS)

    Sahling, H.; Römer, M.; Pape, T.; Bergès, B.; dos Santos Fereirra, C.; Boelmann, J.; Geprägs, P.; Tomczyk, M.; Nowald, N.; Dimmler, W.; Schroedter, L.; Glockzin, M.; Bohrmann, G.

    2014-11-01

    We mapped, sampled, and quantified gas emissions at the continental margin west of Svalbard during R/V Heincke cruise He-387 in late summer 2012. Hydroacoustic mapping revealed that gas emissions were not limited to a zone just above 396 m water depth. Flares from this depth have gained significant attention in the scientific community in recent years because they may be caused by bottom-water warming-induced hydrate dissolution in the course of global warming and/or by recurring seasonal hydrate formation and decay. We found that gas emissions occurred widespread between about 80 and 415 m water depth, which indicates that hydrate dissolution might only be one of several triggers for active hydrocarbon seepage in that area. Gas emissions were remarkably intensive at the main ridge of the Forlandet moraine complex in 80 to 90 m water depths, and may be related to thawing permafrost. Focused seafloor investigations were performed with the remotely operated vehicle (ROV) "Cherokee". Geochemical analyses of gas bubbles sampled at about 240 m water depth as well as at the 396 m gas emission sites revealed that the vent gas is primarily composed of methane (> 99.70%) of microbial origin (average δ13C = -55.7‰ V-PDB). Estimates of the regional gas bubble flux from the seafloor to the water column in the area of possible hydrate decomposition were achieved by combining flare mapping using multibeam and single-beam echosounder data, bubble stream mapping using a ROV-mounted horizontally looking sonar, and quantification of individual bubble streams using ROV imagery and bubble counting. We estimated that about 53 × 106 mol methane were annually emitted at the two areas and allow for a large range of uncertainty due to our method (9 to 118 × 106 mol yr-1). First, these amounts show that gas emissions at the continental margin west of Svalbard were on the same order of magnitude as bubble emissions at other geological settings; second, they may be used to calibrate

  5. Assessing the importance of tropical cyclones on continental margin sedimentation in the Mississippi delta region

    NASA Astrophysics Data System (ADS)

    Dail, Michael. B.; Reide Corbett, D.; Walsh, J. P.

    2007-08-01

    Recent research on the Mississippi margin indicates notable seasonal variation in seabed dynamics. During years with minimal tropical-system activity, sediments initially deposited from late spring to early fall are remobilized by wind-driven currents and wave energy during extra-tropical weather systems in the winter. This research reveals the profound significance of tropical cyclones on Louisiana Shelf sedimentation. The amount of material delivered to and advected across the shelf by recent tropical cyclones is considerably larger than that related to winter storm systems. In Fall 2004, the river-dominated shelf of Louisiana was impacted by three tropical systems in less than a month, including Hurricane Ivan. Ivan, with maximum sustained winds in excess of 74 m s -1 (144 knots) and a minimum measured central pressure of 910 mbar, was the eighth most intense Atlantic hurricane on record at the time. In order to assess the impact these tropical systems had on the continental margin west of the Mississippi delta, seabed samples were collected from box cores in October 2004 and analyzed for particle-reactive radionuclides 234Th, 7Be, and 210Pb. Radiochemical data and observations from X-radiographs indicate event-driven sediment deposits ranged from 4 to 30 cm on the shelf and 2-6 cm in the Mississippi Canyon. These deposits exhibit distinct radiochemical signatures and differ visually and texturally from the underlying sediment. The well-developed physical stratification and graded nature of the deposits observed in core X-radiographs suggests that the sediment could have been deposited from sediment-gravity flows. Inventories of 7Be and 7Be/ 234Th xs ratios reveal this series of cyclones transported considerably more material to the outer shelf and slope than periods of minimal tropical-system activity. When compared to seasonal depositional rates created by winter storms, tropical-cyclone-related event deposits on the middle and outer shelf are up to an order

  6. Wide Angle Converted Shear Wave Analysis of North Atlantic Volcanic Rifted Continental Margins

    NASA Astrophysics Data System (ADS)

    Eccles, J. D.; White, R. S.; Christie, P. A.

    2007-12-01

    High-quality, wide-angle, ocean bottom seismometer (OBS) data have been acquired with a low frequency (9 Hz) seismic source across the Faroes and Hatton Bank volcanic rifted continental margins in the North Atlantic. In these regions thick Tertiary flood basalt sequences provide a challenge to deep seismic imaging. S-wave arrivals, which are dominantly converted from P- to S-waves at the sediment-top basalt interface, were recorded at 170 4-component OBS locations. Variation in the conversion efficiency was observed along the profiles. Tomographic inversion of over 70,000 converted S-wave crustal diving waves and Moho reflections was performed to produce S-wave velocity models and hence, when combined with pre-existing P-wave velocity models, a measure of the Vp/Vs ratio structure of the crust. Resolution testing shows the structure of the oceanic crust and continent-ocean transition is generally well resolved on both profiles. Lateral and vertical changes in Vp/Vs resolves changing crustal composition within, and between, oceanic and continental crust, including regions in the lower crust at the continent-ocean transition with high P-wave velocities of up to 7.5 km/s and low Vp/Vs ratios of ~ 1.75 associated with intense high-temperature intrusion at the time of break-up. Vp/Vs ratios of 1.75-1.80 at the base of the thickened oceanic crust are also lower than generally reported in normal oceanic crust. The P-wave travel-time tomography revealed a low velocity zone (LVZ) beneath the basalt on the Faroes margin and additional constraint on the Vp/Vs of the LVZ beneath the Fugloy Ridge has been gained by analysing the relative travel-time delays between basalt and basement refractions for P- and S-waves. This approach is less subject to the velocity-depth ambiguity associated with velocity inversions than is the determination of P- or S- wave velocity alone. Comparison of the calculated Vp/Vs ratio and P-wave velocity with measurements from relevant lithologies

  7. Numerical Modelling of the Transition from Continental Rifting to Mantle Exhumation at the West Iberia Margin.

    NASA Astrophysics Data System (ADS)

    Reston, T. J.; Perez-Gussinye, M.; Phipps Morgan, J.

    2003-12-01

    The continental margin of West Iberia lacks significant synrift magmatism and exhibits a zone up to 100 km wide thought to consist mainly of serpentinized and exhumed mantle between the thinned continental and the oceanic crust. However, the existence of linear magnetic anomalies pose an ambiguity regarding the exact amount of synrift magmatism produced during mantle exhumation at the surface. We investigate how the thinned continental crust gives way to a broad zone of exhumed and serpentinized mantle with little synrift magmatism. For this we use a finite element code that includes brittle and ductile deformation in both crust and mantle, production of serpentine and melt. Serpentinisation is only allowed to occur when the entire crust has become brittle so that large amounts of water can reach the mantle through brittle faults. The increase in temperature due to the exothermic nature of serpentinisation and the decrease in the coefficient of friction where serpentinisation occurs is also taken into account. Melt production includes the effect of increased depletion in mantle temperatures. In a first approximation, melt is assumed to migrate instantaneously upwards and accumulate at crustal levels. We present tests with a range of extension velocities and asthenospheric temperatures. Preliminary model runs shows how the entire crust becomes brittle after it is has reached a thickness of less than 10 km. For slow rifting velocities (< 5 mm/yr), serpentinisation occurs prior to melting, whereas for faster rifting velocities the opposite is true. In all models, crustal separation and the exposure of mantle at the continent-ocean transition (COT) occurs after the entire crust has become brittle. The relative amount of serpentinite and melt in the COT depends on the rifting velocity, with slower velocities promoting the production of more serpentinite than melt. However, for a normal mantle temperature (1300 C), even for the slow extension rate of 5 mm/yr, 3-4 km of

  8. Geophysical Investigation of Continental Margin Development, and Early Spreading History of the South Atlantic South of the Walvis Ridge/Rio Grande Rise.

    NASA Astrophysics Data System (ADS)

    Dragoi, D.; Hall, S.; Bird, D.

    2006-12-01

    Crustal models of the transition from continental (>30 km) to normal oceanic crust between 22°S and 32°S on the African side and from 28°S and 43°S on the S. American side have been constructed from extensive gravity and magnetic anomaly data together with more limited seismic reflection and refraction data. Offshore gravity highs related to major crustal thinning can be followed along each margin. A smaller, more seaward gravity high observed on the African side coincides with previously mapped magnetic anomaly M4 and appears to delineate the landward limit of normal oceanic crust. The transition zone width is relatively uniform ~320±30 km on the African margin but increases southward from ~300 km to >400 km on the S. American side. The zone of magmatic underplating beneath the thinned crust has a uniform width of ~200 km on the African side but is somewhat narrower (~130 km) and also decreases southwards on the S. American side. Seafloor spreading magnetic anomalies C31 to C34, and M0 to M4 have been identified on both sides. Distinctive C34 anomalies can be clearly correlated except where masked by large amplitude (~1000 nT) anomalies produced by seafloor topography of the Walvis Ridge. C34 spreading rates are slightly asymmetrical with 42 mm/yr on the S. American side compared with 38 mm/yr on the African side. M0 to M4 anomalies are more difficult to identify and reliably correlate over each margin. M0-M4 spreading appears to be asymmetric with more rapid spreading on the S. American side. The C34-M0 distance is noticeably larger on the S. American side suggesting that asymmetrical spreading may continue into the Cretaceous Quiet Zone. A more probable explanation for the asymmetry is that one or more ridge jumps occurred between 84 and 120 Ma. Residualized free air satellite gravity data have been used to delineate fracture zones (FZs) associated with the early opening. More than 10 flow lines determined from these FZs intersect the mapped C34, M0 and M4

  9. Turbidite paleoseismology along the active continental margin of Chile - Feasible or not?

    NASA Astrophysics Data System (ADS)

    Bernhardt, Anne; Melnick, Daniel; Hebbeln, Dierk; Lückge, Andreas; Strecker, Manfred R.

    2015-07-01

    Much progress has been made in estimating recurrence intervals of great and giant subduction earthquakes using terrestrial, lacustrine, and marine paleoseismic archives. Recent detailed records suggest these earthquakes may have variable recurrence periods and magnitudes forming supercycles. Understanding seismic supercycles requires long paleoseismic archives that record timing and magnitude of such events. Turbidite paleoseismic archives may potentially extend past earthquake records to the Pleistocene and can thus complement commonly shorter-term terrestrial archives. However, in order to unambiguously establish recurring seismicity as a trigger mechanism for turbidity currents, synchronous deposition of turbidites in widely spaced, isolated depocenters has to be ascertained. Furthermore, characteristics that predispose a seismically active continental margin to turbidite paleoseismology and the correct sample site selection have to be taken into account. Here we analyze 8 marine sediment cores along 950 km of the Chile margin to test for the feasibility of compiling detailed and continuous paleoseismic records based on turbidites. Our results suggest that the deposition of areally widespread, synchronous turbidites triggered by seismicity is largely controlled by sediment supply and, hence, the climatic and geomorphic conditions of the adjacent subaerial setting. The feasibility of compiling a turbidite paleoseismic record depends on the delicate balance between sufficient sediment supply providing material to fail frequently during seismic shaking and sufficiently low sedimentation rates to allow for coeval accumulation of planktonic foraminifera for high-resolution radiocarbon dating. We conclude that offshore northern central Chile (29-32.5°S) Holocene turbidite paleoseismology is not feasible, because sediment supply from the semi-arid mainland is low and almost no Holocene turbidity-current deposits are found in the cores. In contrast, in the humid region

  10. Shaping of the Southern Adriatic Continental Margin Through Widespread Mass Wasting and Bottom Currents

    NASA Astrophysics Data System (ADS)

    Trincardi, F.; Cattaneo, A.; Correggiari, A.; Verdicchio, G.; Tobi, T.

    2003-12-01

    New TOBI seafloor images and VHR Chirp sonar profiles reveal widespread collapsing of the South Adriatic continental slope including: multiple overlapping slide scars affecting more than 100 km of Pleistocene regressive shelf-margin deposits below the shelf edge, extensive blocky slides on the lower slope (block sizes up to 200 x 500 m), and a basin wide acoustically-transparent deposit up to 40 m thick, buried under a late-Pleistocene-Holocene mud section, in water depths greater than 1000 m. Lateral variations in internal geometry and seafloor morphology likely reflects along margin differences in sediment composition and thickness of the mass-transport deposits and variable run outs (up to 40 km). Preliminary correlation to published cores in the area indicates that the main mass transport deposit reached the basin floor during the Last Glacial Maximum, but younger failures of smaller size may have affected the slope in more recent times. The new data show that the entire slope area is swept by bottom currents generating furrowed areas up to several tens of km2 in extent, moats on the downdrift side of seafloor irregularities (slide blocks) and preferential sediment deposition on the inferred updrift side, and growth of upslope-migrating sediment waves. In particular, two fields of sediment waves can be mapped in water depths of 400-600 and 1000 m. The former may reflect the activity of the Levantine Intermediate Waters, while the latter may be linked to the downslope flow of the Northern Adriatic Dense Waters. Future work will focus on the definition of intervals of growth vs quiescence of the sediment waves, their possible relation to paleoceanographic reconfigurations, and their interplay with seafloor irregularities dictated by slope instability. A smaller-scale possibly comparable interaction of seafloor morphology generated by sediment deformation and differential deposition of muddy deposits on the shelf emphasizes the importance of bottom

  11. Shelfal sediment transport by undercurrents forces turbidity current activity during high sea level, Chile continental margin

    NASA Astrophysics Data System (ADS)

    Bernhardt, Anne; Hebbeln, Dierk; Regenberg, Marcus; Lückge, Andreas; Strecker, Manfred. R.

    2016-04-01

    Understanding the links between terrigenous sediment supply and marine transport and depositional processes along tectonically active margins is essential to decipher turbidite successions as potential archives of climatic and seismic forcings and to comprehend timing and quantity of marine clastic deposition. Sequence stratigraphic models predict coarse-grained terrigenous sediment delivery to deep-marine sites mainly during sea-level fall and lowstand. Marine clastic deposition during periods of transgression and highstand has been attributed to the continued geomorphic connectivity between terrestrial sediment sources and marine sinks (e.g., rivers connected to submarine canyons) often facilitated by narrow shelves, high sediment supply causing delta migration to the shelf edge, and/or abrupt increases in sediment supply due to climatic variability or catastrophic events. To decipher the controls on Holocene highstand turbidite deposition, we analyzed twelve sediment cores of spatially disparate, coeval Holocene turbidite systems along the Chile margin (29-40°S) with changing climatic and geomorphic characteristics but uniform changes of sea level. Intraslope basins in north-central Chile (29-33°S) offshore a narrow to absent shelf record a shut-off of turbidite activity during the Holocene. In contrast, core sites in south-central Chile (36-40°S) offshore a wide continental shelf have repeatedly experienced turbidite deposition during sea-level highstand conditions, even though most of the depocenters are not connected via canyons to sediment sources. The interplay of stable high sediment supply related to strong onshore precipitation in combination with a wide shelf, over which undercurrents move sediment towards the shelf edge, appears to control Holocene turbidite sedimentation and sediment export to the deep sea.

  12. Natural constraints on exploring Antarctica's continental margin, existing geophysical and geological data basis, and proposed drilling program

    SciTech Connect

    Anderson, J.B.

    1987-05-01

    There have been a number of multichannel seismic reflection and seismic refraction surveys of the Antarctic continental shelf. While glacial erosion has left acoustic basement exposed on portions of the inner shelf, thick sedimentary sequences occur on the passive margin of east Antarctica. The thickness and age of these strata vary due to different breakup histories of the margin. Several sedimentary basins have been identified. Most are rift basins formed during the early stages of Antarctica's separation from other Gondwana continents and plateaus. The west Antarctic continental shelf is extensive, being approximately twice the size of the Gulf of Mexico shelf. It has been poorly surveyed to date, owing mainly to its perennial sea ice cover. Gradual subduction of the spreading center from south to north along the margin resulted in old active margin sequences being buried beneath passive margin sequences. The latter should increase in thickness from north to south along the margin although no data bear this out. Hydrocarbon potential on the northern portion of the west Antarctic margin is considered low due to a probable lack of reservoir rocks. Establishment of ice sheets on Antarctica caused destruction of land vegetation and greatly restricted siliciclastic sand-producing environments. So only sedimentary basins which contain pre-early Miocene deposits have good hydrocarbon prospectivity. The Antarctic continental shelf is the deepest in the world, averaging 500 m and in places being more than a kilometer deep. The shelf has been left rugged by glacial erosion and is therefore prone to sediment mass movement. Widespread sediment gravity flow deposits attest to this. The shelf is covered with sea ice most of the year and in a few areas throughout the year. Icebergs, drift freely in the deep waters of the shelf; drift speeds of 1 to 2.5 km/year are not uncommon.

  13. 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.

  14. A small number of candidate gene SNPs reveal continental ancestry in African Americans.

    PubMed

    Kodaman, Nuri; Aldrich, Melinda C; Smith, Jeffrey R; Signorello, Lisa B; Bradley, Kevin; Breyer, Joan; Cohen, Sarah S; Long, Jirong; Cai, Qiuyin; Giles, Justin; Bush, William S; Blot, William J; Matthews, Charles E; Williams, Scott M

    2013-01-01

    Using genetic data from an obesity candidate gene study of self-reported African Americans and European Americans, we investigated the number of Ancestry Informative Markers (AIMs) and candidate gene SNPs necessary to infer continental ancestry. Proportions of African and European ancestry were assessed with STRUCTURE (K = 2), using 276 AIMs. These reference values were compared to estimates derived using 120, 60, 30, and 15 SNP subsets randomly chosen from the 276 AIMs and from 1144 SNPs in 44 candidate genes. All subsets generated estimates of ancestry consistent with the reference estimates, with mean correlations greater than 0.99 for all subsets of AIMs, and mean correlations of 0.99 ± 0.003; 0.98 ± 0.01; 0.93 ± 0.03; and 0.81 ± 0.11 for subsets of 120, 60, 30, and 15 candidate gene SNPs, respectively. Among African Americans, the median absolute difference from reference African ancestry values ranged from 0.01 to 0.03 for the four AIMs subsets and from 0.03 to 0.09 for the four candidate gene SNP subsets. Furthermore, YRI/CEU Fst values provided a metric to predict the performance of candidate gene SNPs. Our results demonstrate that a small number of SNPs randomly selected from candidate genes can be used to estimate admixture proportions in African Americans reliably.

  15. Recurrent Pleistocene sub-marine slide events on the south Vøring Plateau, mid-Norwegian continental margin

    NASA Astrophysics Data System (ADS)

    Hjelstuen, B.; Skaug, M.; Haflidason, H.

    2009-12-01

    Throughout the Pleistocene the sediment input and depositional environments on the continental slope off Norway have been strongly controlled by variability in the ocean circulation, glaciations and sea-level changes. Repeated occurrence of shelf edge glaciations along the whole NW European margin, from Ireland to Svalbard, started at Marine Isotope Stage 12 (c. 0.5 Ma). During these periods, fast moving ice streams crossed the continental shelf on a number of locations, and large submarine fans and prograding wedges accumulated on the continental slope. During glacial maximums and in the early phases of the deglaciations high sedimentation rates, >2000 cm/ka, characterised the Norwegian continental margin. Within these depositional environments more than 30 large-scale mass failures have been identified. Here, we report on three slide events on the south Vøring Plateau, on the mid-Norwegian margin. These slides have affected an area between 2900 and 12000 km2 and involved 580-2400 km3 of sediments, noting that the slide debrites left by the failure events reach a maximum thickness of c. 150 m. For comparison the Holocene Storegga Slide, considered the largest exposed sub-marine slide in the world today, mobilized c. 2500-3500 km3 of masses and affected an area of c. 95 000 km2. The Vøring Plateau sub-marine slides are most likely younger than 0.5 Ma, thus occurring in a time period of repeated ice sheet growth to the shelf edge and high accumulation rates. The failures have occurred within an area dominated by gradients less than 1 degree, and observation of long run-out distances indicate that hydroplaning was important during slide development. Our multichannel seismic profiles further show that gas hydrate bearing sediments are observed on the mid-Norwegian continental margin. Thus, dissociation of gas hydrates and high sedimentation rates may have promoted conditions for failures to occur.

  16. Occurrence and formation of dolomite in organic-rich continental margin sediments

    SciTech Connect

    Baker, P.A.; Burns, S.J.

    1985-11-01

    Dolomite is presently forming at rates less than 500 m/m.y. in continental margin marine sediments having organic carbon contents greater than 0.5 wt. %. At higher sedimentation rates, the dolomite content of the sediments is greatly diluted. At lower organic carbon content, the pore-water chemistry precludes dolomite formation. The reactants for dolomite formation have the following sources: magnesium supplied from overlying seawater, calcium supplied by the dissolution of calcium carbonate, and carbon supplied from carbonate dissolution and organic carbon oxidation. The rate of dolomite formation appears to be limited by the rate of calcium carbonate dissolution. This rate can be quantitatively calculated from the magnesium diffusional and advective fluxes. Conversely, in ancient sediments the depth and timing of dolomite formation can be calculated from the amount of dolomite and the sedimentation rate. In dolomite-rich sediments these calculations demonstrate that most of the dolomite formed within a few tens of meters of the sea floor within the zone of microbial sulfate reduction. Such dolomites have negative values of delta/sup 13/C. In dolomite-poor sediments dolomite may form at greater burial depth, below the zone of sulfate reduction, resulting in positive values of delta/sup 13/C. The distribution coefficient of strontium in dolomite has been determined to have an average value of 0.06. Using this value, predicted strontium contents for submarine dolomites range from 150 ppm in dolomite-rich sections to 1,290 ppm in dolomite-poor sections.

  17. Formation of modern and Paleozoic stratiform barite at cold methane seeps on continental margins

    USGS Publications Warehouse

    Torres, M.E.; Bohrmann, G.; Dube, T.E.; Poole, F.G.

    2003-01-01

    Stratiform (bedded) Paleozoic barite occurs as large conformable beds within organic- and chert-rich sediments; the beds lack major sulfide minerals and are the largest and most economically significant barite deposits in the geologic record. Existing models for the origin of bedded barite fail to explain all their characteristics: the deposits display properties consistent with an exhalative origin involving fluid ascent to the seafloor, but they lack appreciable polymetallic sulfide minerals and the corresponding strontium isotopic composition to support a hydrothermal vent source. A new mechanism of barite formation, along structurally controlled sites of cold fluid seepage in continental margins, involves barite remobilization in organic-rich, highly reducing sediments, transport of barium-rich fluids, and barite precipitation at cold methane seeps. The lithologic and depositional framework of Paleozoic and cold seep barite, as well as morphological, textural, and chemical characteristics of the deposits, and associations with chemosymbiotic fauna, all support a cold seep origin for stratiform Paleozoic barite. This understanding is highly relevant to paleoceanographic and paleotectonic studies, as well as to economic geology.

  18. Late Cretaceous - early Tertiary dextral transpression in north Sinai: Reactivation of the Tethyan Continental Margin

    SciTech Connect

    Moustafa, A.R.; Khalil, M.H. )

    1988-08-01

    Detailed photogeologic study and field checks indicate the North Sinai folds are associated with northwest-dipping upthrusts, especially on their southeastern steeply dipping flanks. These northeast-southwest-plunging folds include both large folded ranges (tens of kilometers long, e.g., Gebels Yelleq, El Maghara, and El Halal) and smaller folds (2-10 km long). The smaller folds have right-stepping en echelon arrangement and define six east-northeast elongated belts which were probably formed by right-lateral wrenching in Late Cretaceous-early Tertiary time. These belts are called the G, El Amrar belt, the G. El Mistan belt, the G. Um Latiya belt, the G. Falig belt, the El Giddi Pass-G. El Minsherah-G. El Burqa belt, and the Mitla Pass-G. Kherim-G. Araif El Naq belt. The existence of northwest-dipping upthrusts within and between these en echelon fold belts probably indicates the wrenching was convergent. The en echelon fold belts are proposed to overlie pre-existing deep-seated faults which could have been formed by the Late Triassic-Liassic rifting of north Africa-Arabia to form the southern passive continental margin of the Tethys sea. Mesozoic rocks thicken across these faults. Late Cretaceous-early Tertiary reactivation of these faults by dextral transpression probably resulted from the oblique movement between Africa and Eurasia to close the Tethys sea.

  19. A mechanism to thin the continental lithosphere at magma-poor margins.

    PubMed

    Lavier, Luc L; Manatschal, Gianreto

    2006-03-16

    Where continental plates break apart, slip along multiple normal faults provides the required space for the Earth's crust to thin and subside. After initial rifting, however, the displacement on normal faults observed at the sea floor seems not to match the inferred extension. Here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults. Our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust (< 10 km) by exhumation of mid-crustal and mantle material. This transitional crust is capped by sub-horizontal detachment surfaces over distances exceeding 100 km with little visible deformation. Our rift model is based on numerical experiments constrained by geological and geophysical observations from the Alpine Tethys and Iberia/Newfoundland margins. Furthermore, we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust. The presence of such lower crustal gabbros is well constrained for the Alpine Tethys system. Initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution. This is achieved in our models by the formation of weak ductile shear zones.

  20. Revisiting submarine mass movements along the U.S. Atlantic Continental Margin: implications for tsunami hazards

    USGS Publications Warehouse

    Chaytor, J.D.; Twichell, D.C.; ten Brink, U.S.; Buczkowski, B.J.; Andrews, B.D.

    2007-01-01

    Interest in the generation of tsunamis by submarine mass movements has warranted a reassessment of their distribution and the nature of submarine landslides offshore of the eastern U.S. The recent acquisition and analysis of multibeam bathymetric data over most of this continental slope and rise provides clearer view into the extent and style of mass movements on this margin. Debris flows appear to be the dominant type of mass movement, although some translational slides have also been identified. Areas affected by mass movements range in size from less than 9 km2 to greater than 15,200 km2 and reach measured thicknesses of up to 70 m. Failures are seen to originate on either the open-slope or in submarine canyons. Slope-sourced failures are larger than canyonsourced failures, suggesting they have a higher potential for tsunami generation although the volume of material displaced during individual failure events still needs to be refined. The slope-sourced failures are most common offshore of the northern, glaciated part of the coast, but others are found downslope of shelf-edge deltas and near salt diapirs, suggesting that several geological conditions control their distribution.

  1. Silicon uptake by sponges: a twist to understanding nutrient cycling on continental margins

    PubMed Central

    Maldonado, Manuel; Navarro, Laura; Grasa, Ana; Gonzalez, Alicia; Vaquerizo, Isabel

    2011-01-01

    About 75% of extant sponge species use dissolved silicon (DSi) to build a siliceous skeleton. We show that silicon (Si) uptake by sublittoral Axinella demosponges follows an enzymatic kinetics. Interestingly, maximum uptake efficiency occurs at experimental DSi concentrations two orders of magnitude higher than those in the sponge habitats, being unachievable in coastal waters of modern oceans. Such uptake performance appears to be rooted in a former condition suitable to operate at the seemingly high DSi values characterizing the pre-Tertiary (>65 mya) habitats where this sponge lineage diversified. Persistence of ancestral uptake systems causes sponges to be outcompeted by the more efficient uptake of diatoms at the low ambient DSi levels characterizing Recent oceans. Yet, we show that sublittoral sponges consume substantial coastal DSi (0.01–0.90 mmol Si m−2 day−1) at the expenses of the primary-production circuit. Neglect of that consumption hampers accurate understanding of Si cycling on continental margins. PMID:22355549

  2. Styles of neotectonic fault reactivation within a formerly extended continental margin, North West Shelf, Australia

    NASA Astrophysics Data System (ADS)

    Whitney, Beau B.; Hengesh, James V.; Gillam, Dan

    2016-08-01

    We have investigated the locations and patterns of neotectonic deformation in the Carnarvon basin along the Mesozoic rifted margin of Western Australia to evaluate the characteristics of post-Neogene tectonic reactivation. Geological, geophysical, geotechnical, and bathymetric data demonstrate that preferentially oriented rift-era structures have been reactivated under the current neotectonic stress regime. The most recent pulse of neotectonic reactivation initiated during the Plio-Pleistocene (4.0 to 1.8 million years ago) and is ongoing. Reactivated structures in the region demonstrate a variety of styles of deformation consistent with dextral-transpression. Structural styles include both positive and negative flower structures, restraining and releasing bends, and hourglass structures. Barrow Island lies within a broad kinematic restraining bend that appears to warp the MIS 5e marine terrace on the island. Fold reconstructions of Neogene strata on the Cape Range and Barrow anticlines yield uplift rates consistent with uplift rates determined from folded late Pleistocene units in the Cape region. Although tectonic rates are low compared to interplate settings, evidence for active tectonic deformation precludes this part of the Australian plate from being classified as a Stable Continental Region.

  3. Morphology and sedimentation along the Kongsfjord Channel System, Svalbard continental margin, European Arctic

    NASA Astrophysics Data System (ADS)

    Forwick, M.; Vorren, T. O.; Hass, C. H.; Laberg, J.; Vanneste, M.

    2011-12-01

    The continental margin off northwestern Svalbard (between 79° and 80° N) has been affected by repeated mass wasting, as well as from northward-flowing contourite currents. One on the most striking morphological features is the approximately 120 km long, SE-NW oriented Kongsfjord Channel System (KCS) that can be traced from the shelf break at 250-300 m water depth to a maximum water depth of about 4000 m. Several channels/gullies starting at the shelf break merge into the main channel at around 1400 m. The main channel is at maximum 500 m wide and 80 m deep. Repeated interaction of the channel with contourite deposition has occurred. This includes relocation of the channel's axis related to the northward migration of the contourites, as well as renewed incision of contourites by activitiy within the channel. Two sediment cores were retrieved from either side of the main channel at 1430 m water depth, and two from about 1560 m water depth, respectively. The stratigraphy of these cores can easily be correlated to dated cores from the west Spitsbergen continental margin (Jessen et al., 2010). Preliminary results reveal that the sedimentation rates (SRs) at the shallower locations are generally higher than at the deeper stations during the past c. 30,000 years. At the sites from 1430 m water depth, the SRs were rather uniform during the past c. 10,000 years. However, prior to c. 10,000 cal. years BP, the SRs were significantly higher at the location of the core that was retrieved northeast of the channel. We assume that this is related to enhanced down-slope sediment transport within the channel and deposition to the right (northeast) of the transport direction - due to the Coriolis force and/or the northward-flowing contourite currents. The sources for sediment transport within the channel are most probably 1) enhanced sediment supply from the Kongsfjorden ice stream during the last glacial, and 2) winnowing of fine-grained material in the upper parts of the KCS and

  4. Rift to drift transition in Siberian Arctic and its impact on continental margin architecture

    NASA Astrophysics Data System (ADS)

    Drachev, S. S.

    2003-04-01

    The East Siberian Arctic Continental Margin (ESAM) represents a rare case of rifting to spreading transition. Present-day geodynamics of this plate tectonic interplay is characterized by a very slow plate divergence in the Laptev Sea as this regions is located just landward of the slowest spreading center worldwide (the Gakkel Ridge), close to the pole of North American/Eurasian plate rotation. However the existing geological and geophysical data, mainly seismic reflection and potential field data, allow conclusion that this situation has been far different in the past. Just after its formation at the end of Late Cretaceous through a series of plate convergence and folding episodes the crust of the ESAM has been strongly modified by an intense rifting. The earliest rift episode took place eastward of the present Laptev Sea, in the East Siberian Sea and probably Chukchi seas, where presently abandoned rifts are stretched landward along the principal weakened zones in the ESAM basement. This rifting might have been related to a spreading episode in the Amerasia Basin and perhaps was triggered by a mantle plume ca. 120 mln. yr. ago (De Long and Franz Joseph Land basalts). Outer parts of the ERAM might have also been rifted away to create marginal blocks, as the Arlis and Chukchi plateau. Second rift event was clearly related to the opening of the Eurasia Basin, preceding it and remaining active through the Cenozoic. The rift to drift transition has been taking place in a huge, “dry” and still active Laptev Rift System, which is a landward projection of the Gakkel Ridge spreading axis. This extension had a major effect on the western ERAM causing strong normal faulting and crustal thinning, up to 70% in some places. However, total crustal extension in the Laptev Rift System is considerably smaller than a value of total opening of the Eurasia Basin, so the spreading is not completely accommodated by the rifting. It may be speculated that a major portion of this

  5. Integrated seismic, geophysical and geological interpretation of Meso-Atlantic Gulf of Guinea continental margin evolution, and hydrocarbon potential of the Cotonou (Dahomey or Benin) basin

    SciTech Connect

    Babalola, O.O.

    1990-01-01

    The assembled aeromagnetic, reflection-seismic, well-log, and gravity data, eliminate the large, problematic gaps in published geophysical data over the shallow-marine and coastal onshore. Data interpretation reveal discordant fracture zones beneath the Niger Delta region, indicating the Gulf of Guinea basins originated as a series of pull-apart basins, that favorable maturation estimates, migration pathways to good source-rocks, and trapping stratigraphic and structural configurations exist for the accumulation of hydrocarbons in several parts of the basin. Depth-to-basement data from exploratory wells in the basins were evaluated with thermo-mechanical subsidence models, to make geodynamic estimates of lithospheric extension. Seismic stratigraphic and structural analysis illustrate tectonic control of clastic and carbonate sedimentation, and the interplay of basinal subsidence with eustatic sea-level changes. The results support a hypothesis that during the breakup of Africa and South America, the Gulf of Guinea cul-de-sac consisted of several microplates, generated from brittle deformation of continental crust in response to mantle convection stresses from below, as well as torsional stresses from the northward of the South Atlantic rip into the Brasilo-West African region. Relative motion between five of these plates is invoked as the evolutionary model, accounting for the observed tectonic physiography as well as the extensional and compressional features of the Cotonou basin and the peri-Niger Delta region. The generation of short-lived continental microplates is also advanced as a model for breakup of large continental plates, as sea-floor spreading is established along nascent continental margins.

  6. Increased continental-margin slumping frequency during sea-level lowstands above gas hydrate-bearing sediments

    SciTech Connect

    Paull, C.K.; Buelow, W.J.; Ussler, W. III; Borowski, W.S.

    1996-02-01

    We present {sup 14}C data on sediment samples from cores of the upper 7 m of the sediment column overlying a major continental-rise gas hydrate field on the southern Carolina Rise and inner Blake Ridge offshore the southeastern United States. The data show that glacial-age sediments are underrepresented in the cores. The observation is consistent with a previously predicted association between sea-level lowstands and increased frequency of sea-floor slumping on continental margins containing gas hydrates. 26 refs., 3 figs.

  7. Distributions of dissolved organic and inorganic carbon and radiocarbon in the eastern North Pacific continental margin

    NASA Astrophysics Data System (ADS)

    Bauer, James E.; Druffel, Ellen R. M.; Wolgast, David M.; Griffin, Sheila; Masiello, Caroline A.

    Temporal variations in the natural radiocarbon ( 14C) signatures of dissolved organic and inorganic carbon (DOC and DIC, respectively) in seawater have been studied previously (Druffel, E.R.M., Bauer, J.E., Williams, P.M., Griffin, S., Wolgast, D.M., 1996. Seasonal variability of radiocarbon in particulate organic carbon in the northeast Pacific. J. Geophys. Res. 101, 20 543-20 552; Bauer, J.E., Druffel, E.R.M., Williams, P.M., Wolgast, D.M., Griffin, S., 1998. Temporal variability in dissolved organic carbon and radiocarbon in the eastern North Pacific Ocean. J. Geophys. Res. 103, 2867-2882) at a long-term time-series station (Sta. M: 32°N, 123W) in the eastern North Pacific located at the eastern edge of the North Pacific abyssal plain. In June 1995 a transect was made from Sta. M inshore to approximately 500 m depth in order to evaluate the distributions of 14C in DOC and DIC from the abyssal plain to the upper continental slope. Concentrations and Δ 14C values of DOC in mixed layer waters (25 and 85 m) decreased toward the upper slope. In deeper waters, concentrations and Δ 14C values were in general similar at all three sites. Differences in DOC concentrations and Δ 14C-DOC between Sta. M and the rise and upper slope sites were explained in part by the mixing of DOC and Δ 14C along constant density ( σt) surfaces. However, specific deviations from conservative behavior due to mixing were observed for Δ 14C-DOC at mesopelagic (˜700 m) and near-bottom (˜3600- 3900 m) depths of the continental rise. Comparable findings are reported for DIC, where σt-normalized concentrations and Δ 14C values in Sta. M, rise and upper slope waters were similar, with the exception of slight increases in concentrations and Δ 14C values in near-bottom waters of the rise. These observations indicate that both DOC and DIC in continental rise and slope surface waters of the eastern North Pacific Ocean margin are comprised of a component of actively upwelled material derived

  8. Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin

    USGS Publications Warehouse

    Holbrook, W.S.; Brocher, T.M.; ten Brink, U.S.; Hole, J.A.

    1996-01-01

    Wide-angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Francisco Bay Area Block and across the offshore continental margin. San Francisco Bay is underlain by a veneer (<300 m) of sediments, beneath which P wave velocities increase rapidly from 5.2 km/s to 6.0 km/s at 7 km depth, consistent with rocks of the Franciscan subduction assemblage. The base of the Franciscan at-15-18 km depth is marked by a strong wide-angle reflector, beneath which lies an 8- to 10-km-thick lower crust with an average velocity of 6.75??0.15 km/s. The lower crust of the Bay Area Block may be oceanic in origin, but its structure and reflectivity indicate that it has been modified by shearing and/or magmatic intrusion. Wide-angle reflections define two layers within the lower crust, with velocities of 6.4-6.6 km/s and 6.9-7.3 km/s. Prominent subhorizontal reflectivity observed at near-vertical incidence resides principally in the lowermost layer, the top of which corresponds to the "6-s reflector" of Brocher et al. [1994]. Rheological modeling suggests that the lower crust beneath the 6-s reflector is the weakest part of the lithosphere; the horizontal shear zone suggested by Furlong et al. [1989] to link the San Andreas and Hayward/Calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust. A transect across the continental margin from the paleotrench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seismic velocity and wide-angle reflectivity suggests that crustal composition changes across major transcurrent fault systems. Pacific oceanic crust extends 40-50 km landward of the paleotrench but, contrary to prior models, probably does not continue beneath the Salinian Block, a Cretaceous arc complex that lies west of the San Andreas fault in the Bay Area. The thickness (10 km) and high

  9. 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

  10. Palaeoceanographic significance of sedimentary features at the Argentine continental margin revealed by multichannel seismic reflection data

    NASA Astrophysics Data System (ADS)

    Gruetzner, Jens; Uenzelmann-Neben, Gabriele; Franke, Dieter

    2010-05-01

    The thermohaline circulation in the Argentine Basin today is characterized by the interaction of northward flowing Antarctic water masses (Antarctic Intermediate Water, AAIW; Circumpolar Deep Water, CDW; Antarctic Bottom Water, AABW) and southward flowing North Atlantic Deep Water (NADW). The transfer of heat and energy via both AABW and NADW constitutes an important component in maintaining the global conveyor belt. We aim at a better understanding of both paths and intensity of this current system in the past by investigating an extensive (> 11000 km) set of high quality seismic reflection profiles from the Argentine continental margin. The profiles show a significant contourite system containing both erosive and depositional features that formed through the evolution of water masses and their modifications (path, physical and chemical properties) due to plate tectonic events such as the opening of the Drake Passage or the extensive emplacement of volcanic flows at the Rio Grande Rise. Overall the depositional features indicate that along slope (contour current) transport dominates over down slope (turbiditic) processes at the southern Argentine margin south of 45° S. Further to the North down slope transport was more extensive as indicated by the presence of submarine canyons crossing the slope down to a depth of ~3500 m. Here we present preliminary results from the southern part of the continental margin (42°-50° S) where we focus on a set of ~50 km wide terraces on the slope and rise separated by contouritic channels. The terraces developed over time in alternating constructional (depositional) and erosive phases. An initial age frame was developed by mapping regional reflectors and seismic units known from previous studies. The sedimentary layer between regional reflectors AR 4 and AR 5 spanning roughly the time interval from the Eocene/Oligocene boundary to the early middle Miocene is thin (0.1 - 0.4 s TWT) below the Valentine Feilberg Terrace but

  11. Measurement of sediment and crustal thickness corrected RDA for 2D profiles at rifted continental margins: Applications to the Iberian, Gulf of Aden and S Angolan margins

    NASA Astrophysics Data System (ADS)

    Cowie, Leanne; Kusznir, Nick

    2014-05-01

    Subsidence analysis of sedimentary basins and rifted continental margins requires a correction for the anomalous uplift or subsidence arising from mantle dynamic topography. Whilst different global model predictions of mantle dynamic topography may give a broadly similar pattern at long wavelengths, they differ substantially in the predicted amplitude and at shorter wavelengths. As a consequence the accuracy of predicted mantle dynamic topography is not sufficiently good to provide corrections for subsidence analysis. Measurements of present day anomalous subsidence, which we attribute to mantle dynamic topography, have been made for three rifted continental margins; offshore Iberia, the Gulf of Aden and southern Angola. We determine residual depth anomaly (RDA), corrected for sediment loading and crustal thickness variation for 2D profiles running from unequivocal oceanic crust across the continental ocean boundary onto thinned continental crust. Residual depth anomalies (RDA), corrected for sediment loading using flexural backstripping and decompaction, have been calculated by comparing observed and age predicted oceanic bathymetries at these margins. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions from Crosby & McKenzie (2009). Non-zero sediment corrected RDAs may result from anomalous oceanic crustal thickness with respect to the global average or from anomalous uplift or subsidence. Gravity anomaly inversion incorporating a lithosphere thermal gravity anomaly correction and sediment thickness from 2D seismic reflection data has been used to determine Moho depth, calibrated using seismic refraction, and oceanic crustal basement thickness. Crustal basement thicknesses derived from gravity inversion together with Airy isostasy have been used to correct for variations of crustal thickness from a standard oceanic thickness of 7km. The 2D profiles of RDA corrected for both sediment loading and non-standard crustal

  12. ROV study of a giant pockmark on the Gabon continental margin

    NASA Astrophysics Data System (ADS)

    Ondréas, H.; Olu, K.; Fouquet, Y.; Charlou, J. L.; Gay, A.; Dennielou, B.; Donval, J. P.; Fifis, A.; Nadalig, T.; Cochonat, P.; Cauquil, E.; Bourillet, J. F.; Moigne, M. Le; Sibuet, M.

    2005-11-01

    A giant, 800-m wide pockmark, called Regab, was discovered along the Equatorial African margin at 3160-m water depth and was explored by remote operated vehicle (ROV) as part of the Zaiango (1998-2000) and Biozaire (2001-2003) projects carried out conjointly by TOTAL and a number of French research institutes. A microbathymetric map obtained using the ROV sensors shows that the pockmark actually consists of a cluster of smaller pockmarks aligned N70 along a 15-m deep depression. Methane was recorded all over the pockmark, the highest values along the axis of the depression where massive carbonate crusts and dense seep communities were also found. Several faunal species belong to the Vesicomyidae and Mytilidae bivalve families, as well as to Siboglinidae (Vestimentifera) tubeworms. Preliminary analyses confirm their association with symbiotic bacteria, thus documenting their dependence on fluid seeps. The pockmark appears to be related to an infilled channel, visible on the seismic data 300 m below the seafloor, which may act as a reservoir for biogenic fluids supplied to the trap from the surrounding sediments.

  13. Geohistory analysis of the Santa Maria basin, California, and its relationship to tectonic evolution of the continental margin

    SciTech Connect

    McCrory, P.A.; Arends, R.G. ); Ingle, J.C. Jr. ); Isaacs, C.M.; Stanley, R.G. ); Thornton, M.L.C. )

    1991-02-01

    The Santa Maria basin of central California is a geologically complex area located along the tectonically active California continental margin. The record of Cenozoic tectonism preserved in Santa Maria strata provides an opportunity to compare the evolution of the region with plate tectonic models for Cenozoic interactions along the margin. Geohistory analysis of Neogene Santa Maria basin strata provides important constraints for hypotheses of the tectonic evolution of the central California margin during its transition from a convergent to a transform plate boundary. Preliminary analyses suggest that the tectonic evolution of the Santa Maria area was dominated by coupling between adjacent oceanic plates and the continental margin. This coupling is reflected in the timing of major hiatuses within the basin sedimentary sequence and margin subsidence and uplift which occurred during periods of tectonic plate adjustment. Stratigraphic evidence indicates that the Santa Maria basin originated on the continental shelf in early Miocene time. A component of margin subsidence is postulated to have been caused by cessation of spreading on adjacent offshore microplates approximately 19-18 ma. A sharp reduction in rate of tectonic subsidence in middle Miocene time, observed in the Santa Maria basin both onshore and offshore, was coeval with rotation of crustal blocks as major shearing shifts shoreward. Tectonic uplift of two eastern sites, offshore Point Arguello and near Point Sal, in the late Miocene may have been related to a change to transpressional motion between the Pacific and North American plates, as well as to rotation of the western Transverse Ranges in a restraining geometry.

  14. 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

  15. Gulf of Aden: Structure and evolution of a young ocean basin and continental margin

    SciTech Connect

    Cochran, J.R.

    1981-01-10

    New marine geophysical data are used to describe the structure and history of the Gulf of Aden. Magnetic anomaly data shows seafloor spreading magnetic anomalies of Sheba Ridge from the axial anomaly to anomaly 5 (10 m.y. B.P.) between the Owen fracture zone and 45 /sup 0/E and to anomaly 2' (3 m.y. B.P.) or anomaly 3 (4 m.y. B.P.) west of 45 /sup 0/E. The data does not support the two episodes of seafloor spreading recently proposed. Landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies. The magnetic quiet zone boundary is also a structural boundary effectively marking the edge of Sheba Ridge, with deeper basement lacking a significant topographic gradient found on the landward side. A magnetic quiet zone is found not only where Sheba Ridge splits continental lithosphere but also on East Sheba Ridge where the ridge splits the old oceanic lithosphre of the Owen and Somali basins. There the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the Owen fracture zone. The magnetic quiet zone boundary is not an isochron in either the Gulf of Aden or the Red Sea, suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading. The offset on the Dead Sea Rift is used to estimate that from 80 to 160 km of opening, amounting to between 65% and 200% extension of the initial rift valley, occurred in the Gulf of Aden and Red Sea prior to the establishment of a mid-ocean ridge. It is suggested that the development of a new ocean basin occurs in two stages. The first involves diffuse extension over an area perhaps 10 km wide in a rift valley environment without an organized spreading center. This is followed by concentration of the extension at a single axis and the beginning of true seafloor spreading.

  16. Spatial Extent of Wave-Supported Fluid Mud on the Waipaoa Continental Margin

    NASA Astrophysics Data System (ADS)

    Hale, R. P.; Ogston, A. S.; Walsh, J. P.; Orpin, A. R.

    2013-12-01

    Data from acoustic and optical sensors provide a powerful tool to connect near-bed water-column processes with the deposits they generate. Ideally, the product of water-column and seabed interactions can then be applied more broadly to understand systems as a whole, in both space and time. Recent observational research has allowed for an improved understanding of shelf sediment-transport dynamics in many coastal systems, including the dynamic Waipaoa Sedimentary System (WSS), on the east coast of the north island of New Zealand. This narrow shelf (~20 km) on an active continental margin is subject to strong environmental forcings in the form of high waves (>5 m), strong currents (>50 cm/s), and frequent floods of the Waipaoa River, which delivers an average of 15 MT of sediment to Poverty Bay and the coastal environment each year. A year-long study of the WSS during 2010-2011 combined observational data from instrumented tripods at three locations on the continental shelf, with repeat sediment cores collected in four-month intervals, to identify and assess the mechanisms of cross- and off-shelf sediment transport. Observational data identified that cross-shelf sediment transport is stochastic, typically driven by high-wave events, with 40% of the net annual cross-shelf flux for one tripod location occurring during a single wave-supported fluid mud (WSFM) in July 2010. Fortunately, this event was recorded in the instrument data, and the resulting deposit was plainly visible in x-radiograph images. This particular WSFM was observed in x-radiographs collected as deep as ~50 m, and as far as ~28 km from the mouth of the Waipaoa River, and is more prevalent on the northern portion of the shelf. A critical water depth is not the only criteria for WSFM deposition, as some shallower areas on the southern shelf, which were subject to high bed stress, show no evidence of WSFM in this event, while cores collected in deeper areas (e.g. lower bed stress) on the northern shelf

  17. From continental priorities to local conservation: a multi-level analysis for African tortoises.

    PubMed

    Bombi, Pierluigi; D'Amen, Manuela; Luiselli, Luca

    2013-01-01

    Terrestrial tortoises are the most endangered group of vertebrates but they are still largely ignored for defining global conservation priorities. In this paper, we explored within a hierarchical framework the potential contribution of prioritization studies at the continental scale to the planning of local initiatives for the conservation of African tortoises at the regional level. First, we modeled the distribution of all the African tortoise species, we calculated three indicators of conservation priority (i.e., species richness, conservation value, and complementarity), and we carried out a gap analysis at continental scale. Second, we focused on the most important region for tortoise conservation and performed the same analyses at higher resolution. Finally, we compared the results from the two scales for understanding the degree to which they are complementary. Southern Africa emerged from the continental analysis as the most important region for tortoises. Within this area, the high-resolution analysis pointed out specific core sites for conservation. The relative degree of species protection was assessed similarly at the two different resolutions. Two species appeared particularly vulnerable at both scales. Priority indices calculated at high resolution were correlated to the values calculated for the corresponding cells at low resolution but the congruence was stronger for species richness. Our results suggest to integrate the calculation of conservation value and complementarity into a hierarchical framework driven by species richness. The advantages of large scale planning include its broad perspective on complementarity and the capability to identify regions with greatest conservation potential. In this light, continental analyses allow targeting fine scale studies toward regions with maximum priority. The regional analyses at fine scale allow planning conservation measure at a resolution similar to that required for the practical implementation

  18. Sediment accumulation on the Southern California Bight continental margin during the twentieth century

    USGS Publications Warehouse

    Alexander, C.R.; Lee, H.J.

    2009-01-01

    Sediment discharged into the portion of the Southern California Bight extending from Santa Barbara to Dana Point enters a complex system of semi-isolated coastal cells, narrow continental shelves, submarine canyons, and offshore basins. On both the Santa Monica and San Pedro margins, 210Pb accumulation rates decrease in an offshore direction (from ??0.5 g cm-2yr-1 to 0.02 g cm-2yr -1), in concert with a fining in sediment grain size (from 4.5?? to 8.5??), suggesting that offshore transport of wave-resuspended material occurs as relatively dilute nepheloid layers and that hemiplegic sedimentation dominates the supply of sediment to the outer shelf, slope, and basins. Together, these areas are effectively sequestering up to 100% of the annual fluvial input. In contrast to the Santa Monica margin, which does not display evidence of mass wasting as an important process of sediment delivery and redistribution, the San Pedro margin does provide numerous examples of failures and mass wasting, suggesting that intraslope sediment redistribution may play a more important role there. Basin deposits in both areas exhibit evidence of turbidites tentatively associated with both major floods and earthquakes, sourced from either the Redondo Canyon (San Pedro Basin) or Dume Canyon (Santa Monica Basin). On the Palos Verdes shelf, sediment-accumulation rates decrease along and across the shelf away from the White's Point outfall, which has been a major source of contaminants to the shelf deposits. Accumulation rates prior to the construction of the outfall were ??0.2 g cm-2yr-1 and increased 1.5-3.7 times during peak discharges from the outfall in 1971. The distal rate of accumulation has decreased by ??50%, from 0.63 g cm -2yr-1 during the period 1971-1992 to 0.29 g cm -2yr-1 during the period 1992-2003. The proximal rate of accumulation, however, has only decreased ??10%, from 0.83 g cm -2yr-1 during the period 1971-1992 to 0.73 g cm -2yr-1 during the period 1992-2003. Effluent

  19. Estimated post-Messinian sediment supply and sedimentation rates on the Ebro continental margin, Spain

    USGS Publications Warehouse

    Nelson, C.H.

    1990-01-01

    Because of the extensive data base of seismic profiles, radiometric ages, and stratigraphic time markers such as the subaerial Messinian surface, sedimentation rates and Ebro River sediment discharge can be estimated for different periods and environments of the Ebro continental margin. New values for sediment discharge (i.e., 6.2 versus previous estimates of 2-3.5 million t/yr) for the Holocene highstand are more reliable but remain minimum estimates because a small proportion of Ebro sediment advected to the Balearic Rise and Abyssal Plain cannot be accounted for, especially during lowstands. The general highstand conditions of the Pliocene, which were similar to those of the Holocene, resulted in a low discharge of Ebro River sediment (ca. 6.5 million t/yr) and an even thickness of sediment across the margin that deposited at rates of about 24-40 cm/ky. In contrast, sediment supply increased two-three times during the Pleistocene, the margin prograded rapidly and deposition occurred at rates of 101-165 cm/ky on the outer shelf and slope, but basin floor rates remained anomalously low (21-26 cm/ky) because sediment was drained and broadly dispersed eastward in Valencia Trough. During the late Pleistocene rise of sea level, the main depocenters progressively shifted shoreward and sedimentation rates greatly decreased from 175 cm/ky on the upper slope during the early transgression to 106 cm/ky on the outer shelf and then to 63 cm/ky on the mid-shelf during the late transgression as the river sediment discharge dropped to half by Holocene time. Maximal sedimentation rates occurred in active depocenters of sediment dispersal such as the Holocene delta (370 cm/ky) or the youngest Pleistocene Oropesa channel-levee complex (705 cm/ky) where deposition rates increased by an order of magnitude or more compared to average Ebro shelf (38 cm/ky) or base-of-slope rates in the Pleistocene (21 cm/ky). The sedimentation rates verify the importance of sea-level control on the

  20. H/V spectral ratios of the continental margin sediments offshore southwestern Taiwan

    NASA Astrophysics Data System (ADS)

    Lin, Jing-Yi; Cheng, Win-Bin; Chin, Shao-Jinn; Hsu, Shu-Kun

    2015-04-01

    For decades, it has been mentioned that submarine slope failures are spatially linked to the presence of gas hydrates/gas-charged sediments. When triggered by earthquakes, over steepen and instable sediments may prompt breakouts of the slopes containing gas hydrates and cause submarine landslides and tsunamis. Widely distributed BSRs have been observed in the area offshore of southwestern Taiwan where the active accretionary complex meets with the passive China continental margin. In the region, large or small scale landslides were also reported based on seismic interpretations. In order to clarify the link between earthquake, landslide and the presence of gas hydrate, we evaluate the response of seafloor sediments in regard to passive dynamic loads. Horizontal-to-vertical (H/V) spectral ratios are used to characterize the local sediment response. Ambient noise as well as distant earthquake is used as generators of the passive dynamic loads. Based on this study, we aim to characterize the site in terms of its physical properties and the local site effect produced by shallow marine sediments. Estimating H/V spectral ratios of data recorded by the short period OBSs (Ocean Bottom Seismometer) deployed in the active and paqssive margin offshore southwestern Taiwan show similar spectral characteristics and provide a general understanding of the preferential vibration modes of sediment systems. The results show that the maximal H/V ratios appeared in the range of 5-10 Hz, where the horizontal amplitudes increased by an order of magnitude relative to the vertical amplitude. The stations located in the northwestern part of study area were characterized by another relatively small peak at proximately 2 Hz, which may indicates the presence of a discontinuity of sediments. For most stations, the H/V ratios estimated based on the earthquakes (i.e. strong input signal) and noise (background, micro-seismic noise) records were characterized by different pattern. No distinct peak

  1. Anaerobic oxidation of methane in the Concepción Methane Seep Area, Chilean continental margin

    NASA Astrophysics Data System (ADS)

    Steeb, P.; Linke, P.; Scholz, F.; Schmidt, M.; Liebetrau, V.; Treude, T.

    2012-04-01

    Within subduction zones of active continental margins, large amounts of methane can be mobilized by dewatering processes and transported to the seafloor along migration pathways. A recently discovered seep area located off Concepción (Chile) at water depth between 600 to 1100 mbsl is characterized by active methane vent sites as well as massive carbonates boulders and plates which probably are related to methane seepage in the past. During the SO210 research expedition "Chiflux" (Sept-Oct 2010), sediment from the Concepción Methane Seep Area (CSMA) at the fore arc of the Chilean margin was sampled to study microbial activity related to methane seepage. We sampled surface sediments (0-30cm) from sulfur bacteria mats, as well as clam, pogonophoran, and tubeworm fields with push cores and a TV-guided multicorer system. Anaerobic oxidation of methane (AOM) and sulfate reduction rates were determined using ex-situ radioisotope tracer techniques. Additionally, porewater chemistry of retrieved cores as well as isotopic composition and age record of surrounding authigenic carbonates were analyzed. The shallowest sulfate-methane-transition zone (SMTZ) was identified at 4 cm sediment depth hinting to locally strong fluid fluxes. However, a lack of Cl- anomalies in porewater profiles indicates a shallow source of these fluids, which is supported by the biogenic origin of the methane (δ13C -70‰ PDB). Sulfide and alkalinity was relatively high (up to 20 mM and 40 mEq, respectively). Rates of AOM and sulfate reduction within this area reached magnitudes typical for seeps with variation between different habitat types, indicating a diverse methane supply, which is affecting the depths of the SMTZ. Rates were highest at sulfur a bacteria mats (20 mmol m-2 d-1) followed by a large field of dead clams, a pogonophoran field, a black sediment spot, and a carbonate rich clam field. Lowest rates (0.2 mmol m-2 d-1) were measured in close vicinity to these hot spots. Abundant massive

  2. Late Miocene sedimentary architecture of the Ebro Continental Margin (Western Mediterranean): Implications to the Messinian Salinity Crisis

    NASA Astrophysics Data System (ADS)

    Cameselle, Alejandra L.; Urgeles, Roger; De Mol, Ben; Camerlenghi, Angelo

    2014-05-01

    The Messinian Salinity Crisis (MSC) resulted from a significant multi-phase drop and subsequent reflooding of the Mediterranean Sea during the Late Miocene. In a relatively short time span (5.96 to 5.33 Ma), partial desiccation of the basin and consequent subaerial exposure of the continental margins resulted in widespread erosion of continental shelves and slopes and regressive erosion along major fluvial valleys. Using 3D seismic reflection data from the Ebro Margin (Western Mediterranean), we provide new insights into the origin of the Messinian Erosional Surface (MES) and timing of the capture of the subaerial Ebro Basin. The observed sedimentary architecture of the Ebro Continental Margin indicates a sedimentary-active continental slope and delta progradation during Middle-Late Miocene, in a normal regressive context associated to a pre-Messinian proto-Ebro River. Configuration of the clinoforms below the MES suggests that deltaic sediments of the Messinian Paleo-Ebro River deposited during the Tortonian and initial Messinian sea-level drawdown. The MES formed at the top of the Tortonian Highstand, where a fluvial network was deeply carved, and in the topset region of the Messinian Falling Stage Systems Tract, where minor erosion occurred. The patterns of Messinian erosion and sedimentation produced a MES with a step-like profile. Significant Miocene progradation and the mature development attained by the Messinian Ebro River network during the MSC indicate that capture of the Ebro Basin occurred prior to the MSC. Fluvial deposits are outstandingly preserved on the main valleys of the MES indicating that re-flooding of the margin was extremely rapid. Therefore, the step-like profile of the MES was created during the latest stages of the main Messinian sea-level drawdown and lowstand.

  3. Quantification of gas bubble emissions from submarine hydrocarbon seeps at the Makran continental margin (offshore Pakistan)

    NASA Astrophysics Data System (ADS)

    RöMer, Miriam; Sahling, Heiko; Pape, Thomas; Bohrmann, Gerhard; Spieß, Volkhard

    2012-10-01

    Evidence for twelve sites with gas bubble emissions causing hydroacoustic anomalies in 18 kHz echosounder records (`flares') was obtained at the convergent Makran continental margin. The hydroacoustic anomalies originating from hydrocarbon seeps at water depths between 575 and 2870 m disappeared after rising up to 2000 m in the water column. Dives with the remotely operated vehicle `Quest 4000 m' revealed that several individual bubble vents contributed to one hydroacoustic anomaly. Analyzed gas samples suggest that bubbles were mainly composed of methane of microbial origin. Bubble size distributions and rise velocities were determined and the volume flux was estimated by counting the emitted bubbles and using their average volume. We found that a low volume flux (Flare 1 at 575 mbsl: 90 ml/min) caused a weak hydroacoustic signal in echograms whereas high volume fluxes (Flare 2 at 1027 mbsl: 1590 ml/min; Flare 5 C at 2870 mbsl: 760 ml/min) caused strong anomalies. The total methane bubble flux in the study area was estimated by multiplying the average methane flux causing a strong hydroacoustic anomaly in the echosounder record with the total number of equivalent anomalies. An order-of-magnitude estimate further considers the temporal variability of some of the flares, assuming a constant flux over time, and allows a large range of uncertainty inherent to the method. Our results on the fate of bubbles and the order-of-magnitude estimate suggest that all of the ˜40 ± 32 × 106 mol methane emitted per year within the gas hydrate stability zone remain in the deep ocean.

  4. Hydrogeological structure of a seafloor hydrothermal system related to backarc rifting in a continental margin setting

    NASA Astrophysics Data System (ADS)

    Ishibashi, Jun-ichiro

    2016-04-01

    Seafloor hydrothermal systems in the Okinawa Trough backarc basin are considered as related to backarc rifting in a continental margin setting. Since the seafloor is dominantly covered with felsic volcaniclastic material and/or terrigenous sediment, hydrothermal circulation is expected to be distributed within sediment layers of significantly high porosity. Deep drilling through an active hydrothermal field at the Iheya North Knoll in the middle Okinawa Trough during IODP Expedition 331 provided a unique opportunity to directly access the subseafloor. While sedimentation along the slopes of the knoll was dominated by volcanic clasts of tubular pumice, intense hydrothermal alteration was recognized in the vicinity of the hydrothermal center even at very shallow depths. Detailed mineralogical and geochemical studies of hydrothermal clay minerals in the altered sediment suggest that the prevalent alteration is attributed to laterally extensive fluid intrusion and occupation within the sediment layer. Onboard measurements of physical properties of the obtained sediment revealed drastic changes of the porosity caused by hydrothermal interactions. While unaltered sediment showed porosity higher than 70%, the porosity drastically decreased in the layer of anhydrite formation. On the other hand, the porosity remained high (~50%) in the layer of only chlorite alteration. Cap rock formation caused by anhydrite precipitation would inhibit the ascent of high temperature fluids to the seafloor. Moreover, an interbedded nature of pelagic mud units and matrix-free pumice deposits may prompt formation of a tightly layered architecture of aquifers and aquicludes. This sediment architecture should be highly conducive to lateral flow pseudo-parallel to the surface topography. Occurrence of sphalerite-rich sulfides was recognized as associated with detrital and altered sediment, suggesting mineralization related to subsurface chemical processes. Moreover, the vertical profiles of

  5. The Mississippian Antler foreland and continental margin in southern Nevada: The Eleana Formation reinterpreted

    SciTech Connect

    Cashman, P.H.; Trexler, J.H. Jr. )

    1991-02-01

    Rocks mapped as the Mississippian Eleana Formation at the type locality on the Nevada Test Site appear to comprise two completely different, but coeval, sedimentary units. In the Eleana Range (Western Eleana Formation), the strata are siliciclastic and carbonate turbidites of Mississippian age. From immediately east of the Eleana Range to Syncline Ridge (Eastern Eleana Formation), the strata are Devonian-Mississippian mudstone and quartzite conformably overlying Devonian limestone and underlying Pennsylvanian limestone. Although the contact between the two sedimentary packages is not exposed, small-scale structures document an east-dipping fault contact and reverse motion. Sandstone petrography and stratigraphic considerations support the age data in identifying two separate Mississippian units. Sandstones from the Western Eleana are chert litharenites with significant amount of feldspar and both volcanic and sedimentary lithic grains. These rocks are interpreted to be a submarine fan deposit; southwest-directed paleocurrent indicators suggest that they were deposited in an elongate trough, filled axially from the northeast. The source of the sediments was the antler allochthon and foreland basin. The authors tentatively correlate this section with the Dale Canyon-Chainman-Diamond Peak section near Eureka, Nevada. Sandstones from the Eastern Eleana are quartz arenites with rare chert and detrital heavy minerals. These strata are tentatively interpreted to be a shallow shelf deposit, with sediments derived from the continent to the east. They tentatively correlate this section with the Guilmette-Pilot-Scotty Wash-Chainman section of eastern Nevada. These sedimentary systems are initially separated an unknown distance across the late Paleozoic continental margin.

  6. The Late Paleozoic evolution of the Gondwanaland continental margin in northern Chile

    NASA Astrophysics Data System (ADS)

    Bell, C. M.

    Tectonic activity on the Gondwanaland continental margin in northern Chile and northwestern Argentina has been continuous from the early Paleozoic to the present. Paleozoic accretion resulted from the buildup of accretionary and magmatic arc complexes, and possibly from the addition of exotic terranes. Paleozoic strata between 25°S and 29°S in northern Chile comprise two north-south elongated strips separated by a 100-km-wide graben infilled with younger rocks. The western strip consists of deep-sea turbidites and basic lavas of the Devonian or Early Carboniferous Las Tórtolas Formation. Subduction of these rocks during Carboniferous times produced the Chañaral mélange in the area south of 26°30'S. The mélange probably resulted from intrastratal movements of partly consolidated strata within an accretionary wedge. Further tectonic deformation of both the turbidites and the mélange was produced by northeast directed subduction. The subduction complex is bounded to the east by the Atacama strike-slip fault system. To the east of the graben are relatively undeformed Early Carboniferous lacustrine sedimentary rocks of the Chinches Formation. These were deposited in a deep, elongated basin, possibly of pull-apart type resulting from strike-slip movement parallel to the coastline. Late Carboniferous to Early Permian magmatic activity superimposed on both these sedimentary successions suggests seaward migration of the subduction zone. The development of the Mesozoic and Cenozoic Andean complex, which overlies the Paleozoic rocks with a marked unconformity, was not accompanied by the accretion of a further subduction complex.

  7. A statistical overview of mass movement characteristics on the North American atlantic outer continental margin

    USGS Publications Warehouse

    Booth, James S.; O'Leary, Dennis W.

    1992-01-01

    An analysis of 179 mass movements on the North American Atlantic continental slope and upper rise shows that slope failures have occurred throughout the geographic extent of the outer margin. Although the slope failures show no striking affinity for a particular depth as an origination level, there is a broad, primary mode centered at about 900 m. The resulting slides terminate at almost all depths and have a primary mode at 1100 m, but the slope/rise boundary (at 2200 m) also is an important mode. Slope failures have occurred at declivities ranging from 1° to 30° (typically, 4°); the resultant mass movement deposits vary in width from 0.2 to 50 km (typically, 1-2 km) and in length from 0.3 to 380 km (typically, 2–4 km), and they have been reported to be as thick as 650 m. On a numeric basis, mass movements are slightly more prevalent on open slopes than in other physiographic settings, and both translational and rotational failure surfaces are common. The typical mass movement is disintegrative in nature. Open slope slides tend to occur at lower slope angles and are larger than canyon slides. Further, large‐scale slides rather than small‐scale slides tend to originate on gentle slopes (≍ 3-4°). Rotational slope failures appear to have a slightly greater chance of occurring in canyons, but there is no analogous bias associated with translational failures. Similarly, disintegrative slides seem more likely to be associated with rotational slope failures than translational ones and are longer than their nondisintegrative counterparts. The occurrence of such a variety of mass movements at low declivities implies that a regional failure mechanism has prevailed. We suggest that earthquakes or, perhaps in some areas, gas hydrates are the most likely cause of the slope failures.

  8. Geoacoustic models of the Donghae-to-Gangneung region in the Korean continental margin of the East Sea

    NASA Astrophysics Data System (ADS)

    Ryang, Woo Hun; Kim, Seong Pil; Hahn, Jooyoung

    2016-04-01

    Geoacoustic model is to provide a model of the real seafloor with measured, extrapolated, and predicted values of geoacoustic environmental parameters. It controls acoustic propagation in underwater acoustics. In the Korean continental margin of the East Sea, this study reconstructed geoacoustic models using geoacoustic and marine geologic data of the Donghae-to-Gangneung region (37.4° to 37.8° in latitude). The models were based on the data of the high-resolution subbottom and air-gun seismic profiles with sediment cores. The Donghae region comprised measured P-wave velocities and attenuations of the cores, whereas the Gangneung region comprised regression values using measured values of the adjacent areas. Geoacoustic data of the cores were extrapolated down to a depth of the geoacoustic models. For actual modeling, the P-wave speed of the models was compensated to in situ depth below the sea floor using the Hamilton method. These geoacoustic models of this region probably contribute for geoacoustic and underwater acoustic modelling reflecting vertical and lateral variability of acoustic properties in the Korean continental margin of the western East Sea. Keywords: geoacoustic model, environmental parameter, East Sea, continental margin Acknowledgements: This research was supported by the research grants from the Agency of Defense Development (UD140003DD and UE140033DD).

  9. Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation—including examples from the Norwegian Sea

    NASA Astrophysics Data System (ADS)

    Rüpke, Lars H.; Schmid, Daniel W.; Perez-Gussinye, Marta; Hartz, Ebbe

    2013-10-01

    The conditions permitting mantle serpentinization during continental rifting are explored within 2-D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature-dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin, we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2-D transects have been reconstructed through the 3-D data set by Scheck-Wenderoth and Maystrenko (2011). We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle.

  10. New insights into the structure of Norwegian continental margins from modern aeromagnetic compilations

    NASA Astrophysics Data System (ADS)

    Ebbing, J.; Olesen, O.; Gernigon, L.; Skilbrei, J. R.

    2007-12-01

    We present the aeromagnetic compilation of the Norwegian mainland and its shelf area and its importance for geological models and tectonic studies. The combined data-set reveal that the bedrock structures are continuous from the Baltic Shield under the Caledonian orogen into the continental shelf and that the late-Caledonian collapse of the Caledonian orogene has influenced the style of extension on the Norwegian shelf. On the margin, modern high-resolution aeromagnetic surveys with small line-spacing, more accurate navigation and sensitive magnetometers have revealed the existence of significant magnetic anomalies arising from sedimentary layers. Sub-cropping Late Paleozoic to Tertiary sedimentary units along the Trøndelag-Nordland coast produce a very distinct anomaly pattern. The asymmetry of the anomalies, with a steep gradient and a negative anomaly to the east and a more gentle gradient to the west, relate the anomalies to a strata gently dipping westward. Susceptibility measurements on core samples, hand specimens and in situ on bedrock exposures are essential for the interpretation of these anomalies. Remapping of the oceanic crust has also improved our under-standing of the Tertiary opening of the North Atlantic as previously interpreted oceanic fracture zones zones do not exist; these were artefacts of poor navigation and wide line spacing of the vintage datasets. Tectonic reconstruction has shown that the opening of the Norwegian-Greenland Sea between the Jan Mayen and Senja fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies and without jumps in spreading axis. Transfer zones have previously been associated with oceanic fracture zones along the Mid-Norwegian and East-Greenland margins. Transfer zones are important entry points for sedimentary drainage systems, a relationship that has also been suggested for the transport of Cretaceous sands to the mid-Norwegian margin. Our new interpretation has consequently

  11. Hanging canyons of Haida Gwaii, British Columbia, Canada: Fault-control on submarine canyon geomorphology along active continental margins

    NASA Astrophysics Data System (ADS)

    Harris, Peter T.; Barrie, J. Vaughn; Conway, Kim W.; Greene, H. Gary

    2014-06-01

    Faulting commonly influences the geomorphology of submarine canyons that occur on active continental margins. Here, we examine the geomorphology of canyons located on the continental margin off Haida Gwaii, British Columbia, that are truncated on the mid-slope (1200-1400 m water depth) by the Queen Charlotte Fault Zone (QCFZ). The QCFZ is an oblique strike-slip fault zone that has rates of lateral motion of around 50-60 mm/yr and a small convergent component equal to about 3 mm/yr. Slow subduction along the Cascadia Subduction Zone has accreted a prism of marine sediment against the lower slope (1500-3500 m water depth), forming the Queen Charlotte Terrace, which blocks the mouths of submarine canyons formed on the upper slope (200-1400 m water depth). Consequently, canyons along this margin are short (4-8 km in length), closely spaced (around 800 m), and terminate uniformly along the 1400 m isobath, coinciding with the primary fault trend of the QCFZ. Vertical displacement along the fault has resulted in hanging canyons occurring locally. The Haida Gwaii canyons are compared and contrasted with the Sur Canyon system, located to the south of Monterey Bay, California, on a transform margin, which is not blocked by any accretionary prism, and where canyons thus extend to 4000 m depth, across the full breadth of the slope.

  12. Crustal rheological strength heterogeneities control the formation of continental plateau margins

    NASA Astrophysics Data System (ADS)

    Liu, Chang; Zhu, Bojing; Yang, Xiaolin

    2015-08-01

    The margins bordering the Tibetan Plateau show two end-member morphologies in topographic gradient: steep margins and low-gradient margins. To investigate the formation mechanism of convergent intracontinental plateau margins, we conduct 2D numerical experiments to simulate crustal deformation process across plateau margins. Our numerical experiments demonstrate that heterogeneities in crustal rheological strength control significantly the formation of plateau margins when subjected to crustal convergence. A very steep margin is the result of crustal convergence between plateau with weak lower crust and foreland basin with strong lower crust. By contrast, a low-gradient margin could result from crustal convergence between plateau and foreland with less strength contrast. This finding suggests that the diversity in topographic gradient along the Tibetan Plateau borders reflects heterogeneities in crustal rheological strength across the plateau margins. Steep gradient at the margins indicate large crustal rheological strength contrasts between the weak ductile lower crust of the Tibetan Plateau and its strong surrounding foreland basins, like the Sichuan Basin, the Tarim Basin and the Qaidam Basin. Beneath these steep margins the horizontal flow of the Tibetan ductile lower crust is inhibited and forced to extrude to support escarpments. Low-gradient at the margins indicate less crustal strength variations between the plateau and outer forelands, like at the northeastern and southeastern margins, where they might be outlets for the weak ductile Tibetan lower crust to flow away from the plateau.

  13. Historical changes in terrestrially derived organic carbon inputs to Louisiana continental margin sediments over the past 150 years

    NASA Astrophysics Data System (ADS)

    Sampere, Troy P.; Bianchi, Thomas S.; Allison, Mead A.

    2011-03-01

    Major rivers (and associated deltaic environments) provide the dominant pathway for the input of terrestrial-derived organic carbon in sediments (TOCT) to the ocean. Natural watershed processes and land-use changes are important in dictating the amount and character of carbon being buried on continental margins. Seven core sites were occupied on the Louisiana continental margin aboard the R/V Pelican in July 2003 along two major sediment transport pathways south and west of the Mississippi River mouth. Lignin profiles in these age-dated cores (210Pb geochronology) indicate artificial reservoir retention as a primary control on organic carbon quantity and quality reaching the margin post-1950, whereas pre-1950 sediments may reflect soil erosion due to land clearing and farming practices. Lignin (Λ8) concentrations (range 0.2 to 1.7) also indicate that TOCT delivery rates/decay processes have probably remained relatively consistent from proximal to distal stations along transects. The down-core profile at the Canyon station seems to be temporally linked and connected to inner shelf deposition, suggestive of rapid cross-shelf transport. Sources of terrestrially derived organic carbon were reflective of mixed angiosperms over the last 150 years in cores west and south of the Mississippi River delta. The lignin-phenol vegetation index (LPVI) (range 130.0 to 510) proved to be a sensitive indicator of source changes in these sediments and eliminated some of the variability compared to C/V (range 0.01 to 0.4) and S/V (range 0.9 to 2.1) ratios. Stochastic events such as hurricanes and large river floods have a measurable, albeit ephemeral, effect on the shelf TOCT record. Burial of TOCT on the river-dominated Louisiana continental margin is largely driven by anthropogenic land-use alterations in the last 150 years. Land-use changes in the Mississippi River basin and river damming have likely affected carbon cycling and TOCT burial on the Louisiana continental margin over a

  14. Continent-ocean transition at the western Barents Sea/Svalbard continental margin

    SciTech Connect

    Eldholm, O.; Faleide, J.I.; Myhre, A.M.

    1987-12-01

    The change in crustal type at the western Barents Sea/Svalbard margin takes place over a narrow zone related to primary rift and shear structures reflecting the stepwise opening of the Greenland Sea. Regionally, the margin is composed of two large shear zones and a central rifted-margin segment. Local transtension and transpression at the plate boundary caused the early Cenozoic tectonism in Svalbard and the western Barents Sea, and might explain the prominent marginal gravity and velocity anomalies.

  15. The Bay of Bengal and the Statement of Understanding Concerning the Establishment of the Outer Edge of the Continental Margin: Regional Implications for Delimiting the Juridical Continental Shelf

    NASA Astrophysics Data System (ADS)

    Mridha, M.; Varma, H.; Macnab, R.

    2005-12-01

    The Bay of Bengal is the site of massive depositions of sediment from the Ganga-Brahmaputra river systems, which discharge an estimated 2300 million tons of material into the Indian Ocean every year. The accumulated material comprises an enormous fan that extends some 4000 km from the Mouths of the Ganges, a delta system which encompasses the entire coast of Bangladesh and a segment of the coast of India. The major tectonic elements of the Bay of Bengal and surrounding areas are: the passive eastern continental margin of India; the 85E Ridge; the Ninetyeast Ridge; the intervening basin buried beneath deep sediment; and the Sunda Arc system with the associated back-arc Andaman Basin. Except for the Nikitin Seamounts which rise above the seabed just south of the Equator, the 85E Ridge is totally covered by thick sediment. The Ninetyeast Ridge, on the other hand, protrudes above the seabed as far north as 10N, where it plunges beneath the thickening sediment and separates the deposits into the Bengal Fan and the smaller Nicobar Fan. The 85E and Ninetyeast Ridges present the most significant relief in the crystalline basement underlying the Bay of Bengal, and should therefore figure substantially in any analysis of sediment thickness pursuant to the delimitation of the outer continental shelf. In this region, the sediment thickness provision of Article 76 has been modified by a Statement of Understanding in Annex II of the Final Act of the Third UN Conference on the Law of the Sea. To avoid a perceived inequity that might arise from the application of the standard one percent sediment thickness formula of Article 76, the Statement introduced a new formula: a qualified State in this region, even if it has a narrow physiographic continental shelf, may establish the outer edge of its continental margin by a line where the thickness of sedimentary rock is not less than one km. This presentation will describe the development of a joint formula line for the States that

  16. Sequence stratigraphy and systems tract analysis of the Neogene-Quaternary continental margin off the Zambezi delta, Mozambique

    SciTech Connect

    Kolla, V. ); Macurda, D.B. Jr. ); Nelson, H.R. Jr. )

    1991-03-01

    During the Neogene and Quaternary, the Zambezi River built a broad delta-platform from 75 to over 100 km into the Indian Ocean. A regional seismic grid shows numerous discontinuities in the delta platform, slope, and deeper basinal areas. Based on downward shifts of reflection terminations and onlaps at or below shelf edge, more than 25 sequences have been identified. Within the gross Neogene package, the basal section is characterized by aggradation, followed by rapid and significant (oblique) progradation, which is then followed by numerous aggradational-progradation and progradation packages in the upper, younger sections. From recognition of aggradation-progradation patterns and from well information, it appears that the first, significant, and rapid progradation occurred since mid-Miocene. The earliest of the Neogene sequence appears thicker towards south and thinner towards north, opposite of the younger sequences. The number of sequences, their modes of stacking, and thickness distributions reflect relative sea-level changes and the points of sediment input as the Zambezi River shifted in position from south to north in time. The Zambezi passive continental margin, located in the Indian Ocean basin, is a stable platform as opposed to the unstable continental margins off the Mississippi, McKenzie, and Niger deltas and is far from the stable margins that were the basis of the Haq et al. cycle-chart (1987). Thus the Zambezi continental margin provides an independent test case for verification of eustatic cycles and for the evaluation of allogenic (eustatic) versus autogenic (subsidence and delta switching) effects on depositional systems and systems tracts.

  17. A time-transgressive Holocene onset from Globorotalia menardii records on Brazilian continental margin sediments

    NASA Astrophysics Data System (ADS)

    Iwai, F. S.; Costa, K. B.; Toledo, F. A. D. L.; Santarosa, A. C. A.; Chiessi, C. M.; Camillo, E., Jr.; Quadros, J. P.

    2014-12-01

    The planktic foraminifer Globorotalia menardii presents a cyclic behavior within Pleistocene glacial cycles on Atlantic; it disappears during glacial periods and returns to this ocean after deglaciations. Therefore, G. menardii has been used to identify limits between those cycles and the last limit is recognized as the Holocene onset. The Holocene onset has been reported before as being more than 4 kyrs later than expected at the equatorial Atlantic Ocean based on a G. menardii record (Broecker & Pena, 2014). In this study, we explore the time-transgressive Holocene onset of G. menardii in the Atlantic from 21 piston cores collected along the Brazilian continental margin, between 7 ˚N and 33 ˚S. Radiocarbon dating was conducted on Globigerinoides ruber on samples prior to and after G. menardii reappearance in the cores. Reservoir-age corrected 14C dates vary between 17 and 6.5 cal kyrs; the older ages are found at ~14 ˚S and younger ages at 6 ˚N and 33 ˚S. From these ages and latitudes, we hypothesize that G. menardii's population has spread at higher rates southward. From the scenario observed on Brazilian coast it is possible to conclude that although ocean circulation has an important role on dispersion of planktonic foraminifera, it may be superimposed by ecological constraints of the species. G. menardii absence during glacials is linked to the Agulhas Leakage activity, which is prevented from getting to the Atlantic due the northern position of the Subtropical Convergence Zone during glacials. On interglacials, warm and saline waters carrying G. menardii are transported into the Subtropical Gyre currents, achieving Brazil's coast through the South Equatorial Current and spreading south and northward through Brazil Current and North Brazil Current, respectively. Nonetheless, from velocity and volume registered for this currents, we would expect a higher G. menardii dispersion rate northward. A faster southward dispersal during the deglaciation suggests

  18. A water column study of methane around gas flares located at the West Spitsbergen continental margin

    NASA Astrophysics Data System (ADS)

    Gentz, Torben; Damm, Ellen; Schneider von Deimling, Jens; Mau, Susan; McGinnis, Daniel Frank; Schlüter, Michael

    2014-01-01

    In the Arctic Seas, the West Spitsbergen continental margin represents a prominent methane seep area. In this area, free gas formation and gas ebullition as a consequence of hydrate dissociation due to global warming are currently under debate. Recent studies revealed shallow gas accumulation and ebullition of methane into the water column at more than 250 sites in an area of 665 km2. We conducted a detailed study of a subregion of this area, which covers an active gas ebullition area of 175 km2 characterized by 10 gas flares reaching from the seafloor at~245 m up to 50 m water depth to identify the fate of the released gas due to dissolution of methane from gas bubbles and subsequent mixing, transport and microbial oxidation. The oceanographic data indicated a salinity-controlled pycnocline situated ~20 m above the seafloor. A high resolution sampling program at the pycnocline at the active gas ebullition flare area revealed that the methane concentration gradient is strongly controlled by the pycnocline. While high methane concentrations of up to 524 nmol L-1 were measured below the pycnocline, low methane concentrations of less than 20 nmol L-1 were observed in the water column above. Variations in the δ13CCH4 values point to a 13C depleted methane source (~-60‰ VPDB) being mainly mixed with a background values of the ambient water (~-37.5‰ VPDB). A gas bubble dissolution model indicates that ~80% of the methane released from gas bubbles into the ambient water takes place below the pycnocline. This dissolved methane will be laterally transported with the current northwards and most likely microbially oxidized in between 50 and 100 days, since microbial CH4 oxidation rates of 0.78 nmol d-1 were measured. Above the pycnocline, methane concentrations decrease to local background concentration of ~10 nmol L-1. Our results suggest that the methane dissolved from gas bubbles is efficiently trapped below the pycnocline and thus limits the methane concentration in

  19. Pathways of carbon oxidation in continental margin sediments off central Chile.

    PubMed

    Thamdrup, B; Canfield, D E

    1996-12-01

    Rates and oxidative pathways of organic carbon mineralization were determined in sediments at six stations on the shelf and slope off Concepcion Bay at 36.5 degrees S. The depth distribution of C oxidation rates was determined to 10 cm from accumulation of dissolved inorganic C in 1-5-d incubations. Pathways of C oxidation were inferred from the depth distributions of the potential oxidants (O2, NO3-, and oxides of Mn and Fe) and from directly determined rates of SO4(2-) reduction. The study area is characterized by intense seasonal upwelling, and during sampling in late summer the bottom water over the shelf was rich in NO3- and depleted of O2. Sediments at the four shelf stations were covered by mats of filamentous bacteria of the genera Thioploca and Beggiatoa. Carbon oxidation rates at these sites were extremely high near the sediment surface (>3 micromol cm-3 d-1) and decreased exponentially with depth. The process was entirely coupled to SO4(2-) reduction. At the two slope stations where bottom-water O2 was > 100 microM, C oxidation rates were 10-fold lower and varied less with depth; C oxidation coupled to the reduction of O2, NO3-, and Mn oxides combined to yield an estimated 15% of the total C oxidation between 0 and 10 cm. Carbon oxidation through Fe reduction contributed a further 12-29% of the depth-integrated rate, while the remainder of C oxidation was through SO4(2-) reduction. The depth distribution of Fe reduction agreed well with the distribution of poorly crystalline Fe oxides, and as this pool decreased with depth, the importance of SO4(2-) reduction increased. The results point to a general importance of Fe reduction in C oxidation in continental margin sediments. At the shelf stations, Fe reduction was mainly coupled to oxidation of reduced S. These sediments were generally H2S-free despite high SO4(2-) reduction rates, and precipitation of Fe sulfides dominated H2S scavenging during the incubations. A large NO3- pool was associated with the

  20. Back-arc rifting at a continental margin: A case study from the Okinawa trough

    NASA Astrophysics Data System (ADS)

    Arai, R.; Kaiho, Y.; Takahashi, T.; Nakanishi, A.; Fujie, G.; Kodaira, S.; Kaneda, Y.

    2014-12-01

    The Okinawa trough, a back-arc basin formed behind the Ryukyu arc-trench system, southwest Japan, represents an active rifting zone associated with extension of the continental lithosphere. The basin is located at the southeastern margin of the Eurasian plate and characterized by axial rift valleys with over 1.0 km depth and ~100 km width. Previous studies suggest that the early rifting phase started late Miocene and crustal extension is currently active at a full rate of 30 to 50 mm/yr. Within the basin, numerous active hydrothermal vents are observed, suggesting that the crustal rifting enhances melt/heat transfer from the deep mantle up to the seafloor. However, internal structure beneath the back-arc basin and its relation to the rifting system are little documented. Complex regional tectonic setting, such as active collision in Taiwan to the west, oblique subduction of the Philippine Sea slab, and changing spreading rate along the rift axis, may also have significant influences on the thermal structure and flow within the mantle wedge, but their relative roles in controlling the rifting mode and magmatic supply are still poorly understood. As a step toward filling this gap in knowledge, we started a new 7-year project that consists of four two-dimensional active-source seismic experiments and extensive passive-source seismic observations along the Ryukyu arc. In 2013, active-source seismic data were collected on the first line that crosses the southernmost part of the Ryukyu arc-trench and Okinawa trough at 124-125°E. For refraction/wide-angle reflection analyses, a total of 60 ocean bottom seismographs were deployed with approximately 6 km spacing on a ~390-km-long profile. On the same line, multichannel seismic (MCS) reflection profiling was also carried out. Seismic velocity models obtained by first arrival tomography show that beneath the volcanic arc a thick layer (~10 km) of the middle crust with Vp = 6.0-6.8 km/s is developed, a typical feature in the

  1. Sources, sinks and long-term cycling of iodine in the hyperarid Atacama continental margin

    NASA Astrophysics Data System (ADS)

    Álvarez, Fernanda; Reich, Martin; Pérez-Fodich, Alida; Snyder, Glen; Muramatsu, Yasuyuki; Vargas, Gabriel; Fehn, Udo

    2015-07-01

    reservoirs. Geochemical mixing models reveal that the measured 129I/I ratios in Atacama are in agreement with multiple sources of iodine that include variable contributions from old organic iodine sources (i.e., marine sedimentary rocks) and younger fluids such as pore waters, geothermal fluids and meteoric waters. Our results show that the large variation observed in the iodine isotopic ratios of different reservoirs (129I/I from 150 to 1580 × 10-15) is indicative of significant mixing and circulation of fluids of meteoric, sedimentary and volcanic origin along the Chilean continental margin in the last 30 million years. We conclude that this protracted and large-scale fluid flow was driven by tectonic uplift and highly influenced by the climatic history of the Atacama Desert. The combination of such factors has played an unforeseen role in transporting and accumulating iodine and other soluble components in the Atacama region, and is evidence that elemental remobilization is a key process in the overall crustal cycle of iodine over scales of millions of years.

  2. Geology and physiography of the continental margin north of Alaska and implications for the origin of the Canada Basin

    USGS Publications Warehouse

    Grantz, Arthur; Eittreim, Stephen L.; Whitney, O.T.

    1979-01-01

    The continental margin north of Alaska is of Atlantic type. It began to form probably in Early Jurassic time but possibly in middle Early Cretaceous time, when the oceanic Canada Basin of the Arctic Ocean is thought to have opened by rifting about a pole of rotation near the Mackenzie Delta. Offsets of the rift along two fracture zones are thought to have divided the Alaskan margin into three sectors of contrasting structure and stratigraphy. In the Barter Island sector on the east and the Chukchi sector on the west the rift was closer to the present northern Alaska mainland than in the Barrow sector, which lies between them. In the Barter Island and Chukchi sectors the continental shelf is underlain by prisms of clastic sedimentary rocks that are inferred to include thick sections of Jurassic and Neocomian (lower Lower Cretaceous) strata of southern provenance. In the intervening Barrow sector the shelf is underlain by relatively thin sections of Jurassic and Neocomian strata derived from northern sources that now lie beneath the outer continental shelf. The rifted continental margin is overlain by a prograded prism of Albian (upper Lower Cretaceous) to Tertiary clastic sedimentary rocks that comprises the continental terrace of the western Beaufort and northern Chukchi Seas. On the south the prism is bounded by Barrow arch, which is a hingeline between the northward-tilted basement surface beneath the continental shelf of the western Beaufort Sea and the southward-tilted Arctic Platform of northern Alaska. The Arctic platform is overlain by shelf clastic and carbonate strata of Mississippian to Cretaceous age, and by Jurassic and Cretaceous clastic strata of the Colville foredeep. Both the Arctic platform and Colville foredeep sequences extend from northern Alaska beneath the northern Chukchi Sea. At Herald fault zone in the central Chukchi Sea they are overthrust by more strongly deformed Cretaceous to Paleozoic sedimentary rocks of Herald arch, which trends

  3. Metallogeny of the northeastern Pacific Rim: an example of the distribution of ore deposits along a growing continental margin

    USGS Publications Warehouse

    Goldfarb, R.J.; Hart, C.J.; Mortensen, J.K.; Weber, Graeme

    1999-01-01

    The distribution of mineral deposits within northwestern North America (Alaska, Yukon, and northern British Columbia) allows for an in-depth examination of the metallogenic patterns of a growing continental margin. A more complete understanding of the tectonic evolution of this part of the Pacific Rim, achieved over the last 15 to 20 years, now allows for the placement of ore systems into a well-defined plate tectonic framework. Ore deposits older than about 185 Ma represent hydrothermal systems that were active in the platform/shelf environment of ancestral North America's miogeocline or hydrothermal systems developed in oceanic arcs and continental fragments more distal to the craton. These include important SEDEX, VMS, and pre-accretionary porphyry deposits. In contrast, most mineral deposits younger than about 185 Ma were formed within the growing Cordilleran orogen, as terranes were accreted to the continental margin during interactions between the North America and Pacific/Farallon/Kula plates. Such syn- to post-accretionary mineralised systems include many large lode gold and porphyry/skarn systems.

  4. Crustal structure of the eastern Algerian continental margin and adjacent deep basin: implications for late Cenozoic geodynamic evolution of the western Mediterranean

    NASA Astrophysics Data System (ADS)

    Bouyahiaoui, B.; Sage, F.; Abtout, A.; Klingelhoefer, F.; Yelles-Chaouche, K.; Schnürle, P.; Marok, A.; Déverchère, J.; Arab, M.; Galve, A.; Collot, J. Y.

    2015-06-01

    We determine the deep structure of the eastern Algerian basin and its southern margin in the Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction in this region. This study is based on new geophysical data collected during the SPIRAL cruise in 2009, which included a wide-angle, 240-km-long, onshore-offshore seismic profile, multichannel seismic reflection lines and gravity and magnetic data, complemented by the available geophysical data for the study area. The analysis and modelling of the wide-angle seismic data including refracted and reflected arrival travel times, and integrated with the multichannel seismic reflection lines, reveal the detailed structure of an ocean-to-continent transition. In the deep basin, there is an ˜5.5-km-thick oceanic crust that is composed of two layers. The upper layer of the crust is defined by a high velocity gradient and P-wave velocities between 4.8 and 6.0 km s-1, from the top to the bottom. The lower crust is defined by a lower velocity gradient and P-wave velocity between 6.0 and 7.1 km s-1. The Poisson ratio in the lower crust deduced from S-wave modelling is 0.28, which indicates that the lower crust is composed mainly of gabbros. Below the continental edge, a typical continental crust with P-wave velocities between 5.2 and 7.0 km s-1, from the top to the bottom, shows a gradual seaward thinning of ˜15 km over an ˜35-km distance. This thinning is regularly distributed between the upper and lower crusts, and it characterizes a rifted margin, which has resulted from backarc extension at the rear of the Kabylian block, here represented by the Edough Massif at the shoreline. Above the continental basement, an ˜2-km-thick, pre-Messinian sediment layer with a complex internal structure is interpreted as allochthonous nappes of flysch backthrusted on the margin during the collision of Kabylia with the African margin. The crustal structure, moreover, provides evidence for Miocene

  5. 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.

  6. Cenozoic prograding sequences of the Antarctic continental margin: a record of glacio-eustatic and tectonic events

    USGS Publications Warehouse

    Cooper, A. K.; Barrett, P.J.; Hinz, K.; Traube, V.; Letichenkov, G.; Stagg, H.M.J.

    1991-01-01

    Sedimentary sections up to 6-14 km thick lie beneath many areas of the Antarctic continental margin. The upper parts of the sections contain up to 6 km of Cenozoic glacial and possibly non-glacial sequences that have prograded the continental shelf up to 85 km. We describe the Cenozoic sequences using two general categories based on their acoustic geometries. Type IA sequences, which account for most prograding of the Antarctic continental shelf, have complex sigmoidal geometries and some acoustic characteristics atypical of low-latitude margins, such as troughs and mounds lying parallel and normal to the shelf edge and high velocities (2.0-2.6 km/s) for flat layers within 150 m of the seafloor. Type IIA sequences, which principally aggrade the paleoshelf, lie beneath type IA sequences and have mostly simple geometries and gently dipping reflections. The prograding sequences are commonly located near the seaward edges of major Mesozoic and older margin structures. Relatively rapid Cenozoic subsidence has occured due to the probable rifting in the Ross Sea, thermal subsidence in the Antarctic Peninsula, and isostatic crustal flexure in Wilkes Land. In Prydz Bay and the Weddell Sea, prograding sequences cover Mesozoic basins that have undergone little apparent Cenozoic tectonism. Grounded ice sheets are viewed by us, and others, as the principal mechanism for depositing the Antarctic prograding sequences. During the initial advance of grounded ice the continental shelf is flexurally overdeepened, the inner shelf is heavily eroded, and gently dipping glacial strata are deposited on the shelf (i.e type IIA sequences). The overdeepened shelf profile is preserved (a) during glacial times, by grounded ice sheets episodically crossing the shelf, eroding sediments from onshore and inner shelf areas, and depositing sediments at the front of the ice sheet as outer shelf topset-banks and continental slope foreset-aprons (i.e. type IA sequences), and (b) during interglacial

  7. 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

  8. Factors controlling late Cenozoic continental margin growth from the Ebro Delta to the western Mediterranean deep sea

    USGS Publications Warehouse

    Nelson, C.H.; Maldonado, A.

    1990-01-01

    The Ebro continental margin sedimentation system originated with a Messinian fluvial system. This system eroded both a major subaerial canyon cutting the margin southeastward from the present Ebro Delta and an axial valley that drained northeastward down Valencia Trough. Post-Messinian submergence of this topography and the Pliocene regime of high sea levels resulted in a marine hemipelagic drape over the margin. Late Pliocene to Pleistocene glacial climatic cycles, drainagebasin deforestation, and sea-level lowstands combined to increase sediment supply, cause the margin to prograde, and create a regime of lowstand sediment-gravity flows in the deeper margin. The depositional patterns of regressive, transgressive and highstand sea-level regimes suggest that location of the sediment source near the present Ebro Delta throughout the late Cenozoic, southward current advection of sediment, and greater subsidence in the southern margin combined to cause generally asymmetric progradation of the margin to the southeast. Thicker, less stable deposits filling the Messinian subaerial canyon underwent multiple retrograde failures, eroded wide gullied canyons and formed unchanneled base-of-slope sediment aprons in the central margin area; other margin areas to the north and south developed a series of channel-levee complexes. On the basin floor, the formation of Valencia Valley over the Messinian subaerial valley and earlier faults led to draining of about 20% of the Ebro Pleistocene sediment from channel-levee complexes through the valley to prograde Valencia Fan as much as 500 km northeast of the margin. Thus, the Ebro margin has two growth directions, mainly southeastward during higher sea levels, and eastward to northeastward during lower sea levels. The northeastward draining of turbidity currents has produced unusually thin and widely dispersed turbidite systems compared to those on ponded basin floors. During the past few centuries, man's impact has exceeded natural

  9. Continental breakup and the dynamics of rifting in back-arc basins: The Gulf of Lion margin

    NASA Astrophysics Data System (ADS)

    Jolivet, Laurent; Gorini, Christian; Smit, Jeroen; Leroy, Sylvie

    2015-04-01

    Deep seismic profiles and subsidence history of the Gulf of Lion margin reveal an intense stretching of the distal margin and strong postrift subsidence, despite weak extension of the onshore and shallow offshore portions of the margin. We revisit this evolution from the geological interpretation of an unpublished multichannel seismic profile and other published geophysical data. We show that an 80 km wide domain of thin lower continental crust, the "Gulf of Lion metamorphic core complex," is present in the ocean-continent transition zone and exhumed mantle makes the transition with oceanic crust. The exhumed lower continental crust is bounded upward and downward by shallow north dipping detachments. The presence of exhumed lower crust in the deep margin explains the discrepancy between the amount of extension deduced from normal faults in the upper crust and total extension. We discuss the mechanism responsible for exhumation and present two scenarios: the first one involving a simple coupling between mantle extension due to slab retreat and crustal extension and the second one involving extraction of the lower crust and mantle from below the margin by the southeastward flow of hot asthenosphere in the back-arc region during slab rollback. In both scenarios, the combination of Eocene crustal thickening related to the Pyrenees, the nearby volcanic arc, and a shallow lithosphere-asthenosphere boundary weakened the upper mantle and lower crust enough to make them flow southeastward. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after breakup.

  10. Stratigraphic and isotopic link between the northern Stikine terrane and an ancient continental margin assemblage, Canadian Cordillera

    SciTech Connect

    Jackson, J.L.; Gehrels, G.E.; Patchett, P.J. ); Mihalynuk, M.G. )

    1991-12-01

    Geologic and isotopic data strongly imply a Late Triassic depositional link between a juvenile volcanic arc (northern Stikine terrane) and an outboard ancient continental margin assemblage (Nisling terrane) in the Canadian Cordillera. Two sandstone samples and a schist clast from a conglomerate layer at the base of the Upper Triassic Stuhini Group (northern Stikine terrane) have Nd-depleted mantle model ages of 1400-1430 and 1600 Ma, respectively; other Stuhini Group rocks have model ages of 390,660 and 690 Ma. Three samples of Nisling terrane schist and gneiss yield Nd model ages of 910, 1770, and 2450 Ma and highly radiogenic {sup 87}Sr/{sup 86}Sr ratios. These isotopic data corroborate stratigraphic evidence that detritus at the base of northern Stikine was shed from the Nisling terrane and strengthen interpretations that these terranes became linked by Late Triassic time. Thus, Upper Triassic strata of the northern Stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment, a rifted fragment of the North American margin, or the in situ North American margin.

  11. Advection of Sea-Ice Meltwater and Halocline Water Along the Siberian Continental Margin

    NASA Astrophysics Data System (ADS)

    Bauch, D.; Torres-Valdes, S.; Polyakov, I.; Chernyavskaya, E.; Novikhin, A.; Dmitrenko, I.; McKay, J. L.; Mix, A. C.

    2014-12-01

    Our study is based on hydrochemical and stable oxygen isotope data at the Laptev Sea continental slope from summers 2005-2009 and reveals a general pattern in water mass distribution and potential shelf-basin exchange. Despite considerable inter-annual variations, a frontal system can be inferred between shelf, continental slope and central Eurasian Basin waters in the upper 100 m of the water column along the continental slope. Net sea-ice melt is consistently found at the continental slope. However, the sea-ice meltwater signal is independent from the local retreat of the sea-ice edge and appears to be advected from upwind locations. In addition to the along-slope frontal system at the continental shelf break, a strong gradient is identified on the Laptev Sea shelf at ~122-126°E with an eastward increase of riverine and sea-ice related brine water contents. These waters cross the shelf break at ~140°E and feed the Low Salinity Halocline Water (LSHW, salinity S<33) in the upper 50 m of the water column. High silicate concentrations in Laptev Sea bottom waters may lead to speculation about a link to the local silicate maximum found within the salinity range of ~33 to 34.5, typical for the Lower Halocline Water (LHW) at the continental slope. However brine signatures and nutrient ratios from the central Laptev Sea differ from those observed at the continental slope. Similar to the advection of the sea-ice melt signal along the Laptev Sea continental slope the nutrient signal at 50-70 m water depth within the LHW might also be fed by advection parallel to the slope. Thus, our analyses suggest that advective processes from upstream locations play a significant role in the meltwater distribution and halocline formation in the northern Laptev Sea. Inter-annual variations within the properties of LHW are further investigated.

  12. Paternal lineages signal distinct genetic contributions from British Loyalists and continental Africans among different Bahamian islands.

    PubMed

    Simms, Tanya M; Martinez, Emanuel; Herrera, Kristian J; Wright, Marisil R; Perez, Omar A; Hernandez, Michelle; Ramirez, Evelyn C; McCartney, Quinn; Herrera, Rene J

    2011-12-01

    Over the past 500 years, the Bahamas has been influenced by a wide array of settlers, some of whom have left marked genetic imprints throughout the archipelago. To assess the extent of each group's genetic contributions, high-resolution Y-chromosome analyses were performed, for the first time, to delineate the patriarchal ancestry of six islands in the Northwest (Abaco and Grand Bahama) and Central (Eleuthera, Exuma, Long Island, and New Providence) Bahamas and their genetic relationships with previously published reference populations. Our results reveal genetic signals emanating primarily from African and European sources, with the predominantly sub-Saharan African and Western European haplogroups E1b1a-M2 and R1b1b1-M269, respectively, accounting for greater than 75% of all Bahamian patrilineages. Surprisingly, we observe notable discrepancies among the six Bahamian populations in their distribution of these lineages, with E1b1a-M2 predominating Y-chromosomes in the collections from Abaco, Exuma, Eleuthera, Grand Bahama, and New Providence, whereas R1b1b1-M269 is found at elevated levels in the Long Island population. Substantial Y-STR haplotype variation within sub-haplogroups E1b1a7a-U174 and E1b1ba8-U175 (greater than any continental African collection) is also noted, possibly indicating genetic influences from a variety of West and Central African groups. Furthermore, differential European genetic contributions in each island (with the exception of Exuma) reflect settlement patterns of the British Loyalists subsequent to the American Revolution.

  13. Geology of the offshore Southeast Georgia Embayment, U.S. Atlantic continental margin, based on multichannel seismic reflection profiles

    USGS Publications Warehouse

    Buffler, Richard T.; Watkins, Joel S.; Dillon, William P.

    1979-01-01

    The sedimentary section is divided into three major seismic intervals. The intervals are separated by unconformities and can be mapped regionally. The oldest interval ranges in age from Early Cretaceous through middle Late Cretaceous, although it may contain Jurassic rocks where it thickens beneath the Blake Plateau. It probably consists of continental to nearshore clastic rocks where it onlaps basement and grades seaward to a restricted carbonate platform facies (dolomite-evaporite). The middle interval (Upper Cretaceous) is characterized by prograding clinoforms interpreted as open marine slope deposits. This interval represents a Late Cretaceous shift of the carbonate shelf margin from the Blake Escarpment shoreward to about its present location, probably due to a combination of co tinued subsidence, an overall Late Cretaceous rise in sea level, and strong currents across the Blake Plateau. The youngest (Cenozoic) interval represents a continued seaward progradation of the continental shelf and slope. Cenozoic sedimentation on the Blake Plateau was much abbreviated owing mainly to strong currents.

  14. Geoacoustic model at the DH-1 long-core site in the Korean continental margin of the East Sea

    NASA Astrophysics Data System (ADS)

    Ryang, Woo Hun; Kim, Seong Pil

    2014-05-01

    A long core of 23.6 m was acquired at the DH-1 site (37°36.651'N and 129°19.709'E) in the Korean continental margin of the western East Sea. The core site is located near the Donghae City and the water depth is 357.8 m deep. The long-core sediment was recovered using the Portable Remotely Operated Drill (PROD), a fully contained drilling system, remotely operated at the seafloor. The recovered core sediments were analyzed for physical, sedimentological, and geoacoustic properties mostly at 10~30 cm intervals. Based on the long-core data with subbottom and air-gun profiles at the DH-1 core site, a geoacoustic model was firstly reconstructed including water mass. The geoacoustic model comprises 7 geoacoustic units of the core sediments, based on the measurements of 125 P-wave velocities and 121 attenuations. The P-wave speed was compensated to in situ depth below the sea floor using the Hamilton method. The geoacoustic model DH-1 probably contributes for reconstruction of geoacoustic models reflecting vertical and lateral variability of acoustic properties in the Korean continental margin of the western East Sea. Keywords: long core, geoacoustic model, East Sea, continental margin, P-wave speed Acknowledgements: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0025733) and by the Ministry of Knowledge Economy through the grant of Marine Geology and Geophysical Mapping Project (GP2010-013).

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  16. Denudation history and landscape evolution of the northern East-Brazilian continental margin from apatite fission-track thermochronology

    NASA Astrophysics Data System (ADS)

    Jelinek, A. R.; Chemale, F.; van der Beek, P. A.; Guadagnin, F.; Cupertino, J. A.; Viana, A.

    2014-10-01

    We reconstruct the history of denudation and landscape evolution of the northern East- Brazilian continental margin using apatite fission-track thermochronology and thermal history modeling. This part of the Brazilian Atlantic margin is morphologically characterized by inland and coastal plateaus surrounding a wide low-lying inland region, the Sertaneja Depression. The apatite fission track ages and mean track lengths vary from 39 ± 4 to 350 ± 57 Ma and from 10.0 ± 0.3 to 14.2 ± 0.2 μm, respectively, implying a protracted history of spatially variable denudation since the Permian at relatively low rates (<50 m My-1). The Sertaneja Depression and inland plateaus record Permian-Early Jurassic (300-180 Ma) denudation that precedes rifting of the margin by > 60 Myrs. In contrast, the coastal regions record up to 2.5 km of Late Jurassic-Early Cretaceous (150-120 Ma) denudation, coeval with rifting of the margin. The samples from elevated coastal regions, the Borborema Plateau and the Mantiqueira Range, record cooling from temperatures above 120 °C since the Late Cretaceous extending to the Cenozoic. We interpret this denudation as related to post-rift uplift of these parts of the margin, possibly resulting from compressional stresses transmitted from the Andes and/or magmatism at that time. Several samples from these areas also record accelerated Neogene (<30 Ma) cooling, which may record landscape response to a change from a tropical to a more erosive semi-arid climate during this time. The inferred denudation history is consistent with the offshore sedimentary record, but not with evolutionary scenarios inferred from the recognition of “planation surfaces” on the margin. The denudation history of the northeastern Brazilian margin implies a control of pre-, syn- and post-rift tectonic and climatic events on landscape evolution.

  17. Low-Temperature Thermochronology Applied to Constrain the Multi-Episodic Thermotectonic Evolution of the Southeastern Continental Margin of Brazil

    NASA Astrophysics Data System (ADS)

    Mendes, L. D.; Heilbron, M. C. P. L.; Hodges, K. V.; Van Soest, M. C.; Silva, L. G. A. E.

    2015-12-01

    Low-temperature thermochronology was applied to constrain the Mesozoic and Cenozoic tectonic evolution of the continental margin of southeast Brazil. Using apatite (U-Th)/He thermochronology (AHe), we acquired data from 107 crystals of basement samples collected from a NW-SE transect in the Mantiqueira Mountains to the Guanabara Graben, as well as from the NE-SE transverse faults. The data range from 43.5 ± 1.9 Ma to 250.1 ± 8.7 Ma (2 σ) for corrected ages. The Neo-Cretaceous, Eo-Cretaceous, and Paleocene are the main recorded AHe ages, in order of importance. The Eo-Cretaceous ages indicate the occurrence of older thermal events related to a pre-rifting phase (~121 Ma). The Neo-Cretaceous ages signify the importance of tectonic and magmatic events, and regional uplifting for the thermal history of the study area, including ages related to the Serra do Mar Mountains uplift (~86 Ma). Paleocene ages seem to be related to the reactivation (~65 Ma), which was responsible for the continental rifts in the southeastern Brazil. Finally, the Eocene ages (49.7 Ma and 43.5 Ma), which are from samples restricted to the Resende Basin border faults, indicate a continental rift reactivation. Time-temperature (t-T) paths obtained from inverse modeling, performed using HeFTy (Ketcham, 2005) with a Radiation Damage Diffusion and Annealing Model (Flowers et al., 2009), suggests rapid cooling episodes for all samples. The main thermal events show a direct correlation with the timing of regional tectonic events: reactivation phases, continental margin uplift, and the sedimentary record. Apatite (U-Th)/He ages increase with distance from the coast and with elevation. However, these patterns are discontinued by samples of younger ages as a result of the reactivation process of pre-existing structures. The total estimated denudation range from 1.2 to 2.8 km. The erosion rates range from 15.2 to 35.3 m/My. Thus, the multi-episodic thermal events, which led to the formation of important

  18. Modeling the conversion of hydroacoustic to seismic energy at island and continental margins: preliminary analysis of Ascension Island data

    SciTech Connect

    Harben, P.; Rodgers, A.

    1999-07-26

    Seismic stations at islands and continental margins will be an essential component of the International Monitoring System (IMS) for event location and identification in support of Comprehensive Nuclear-Test-Ban Treaty (CTBT) monitoring. Particularly important will be the detection and analysis of hydroacoustic-to-seismic converted waves (T-phases) at island or continental margins. Acoustic waves generated by sources in or near the ocean propagate for long distances very efficiently due to the ocean sound speed channel (SOFAR) and low attenuation. When ocean propagating acoustic waves strike an island or continental margin they are converted to seismic (elastic) waves. We are using a finite difference code to model the conversion of hydroacoustic T-waves at an island or continental margin. Although ray-based methods are far more efficient for modeling long-range (> 1000 km) high-frequency hydroacoustic propagation, the finite difference method has the advantage of being able to model both acoustic and elastic wave propagation for a broad range of frequencies. The method allows us to perform simulations of T-phases to relatively high frequencies ({>=}10 Hz). Of particular interest is to identify factors that affect the efficiency of T-phase conversion, such as the topographic slope and roughness at the conversion point and elastic velocity structure within the island or continent. Previous studies have shown that efficient T-phase conversion occurs when the topographic slope at the conversion point is steep (Cansi and Bethoux, 1985; Talandier and Okal, 1998). Another factor impacting T-phase conversion may be the near-shore structure of the sound channel. It is well known that the depth to the sound channel axis decreases in shallow waters. This can weaken the channeled hydroacoustic wave. Elastic velocity structure within the island or continent will impact how the converted seismic wave is refracted to recording stations at the surface and thus impact the T

  19. Seabed fluid expulsion along the upper slope and outer shelf of the U.S. Atlantic continental margin

    USGS Publications Warehouse

    Brothers, D.S.; Ruppel, C.; Kluesner, J.W.; ten Brink, U.S.; Chaytor, J.D.; Hill, J.C.; Andrews, B.D.; Flores, C.

    2014-01-01

    Identifying the spatial distribution of seabed fluid expulsion features is crucial for understanding the substrate plumbing system of any continental margin. A 1100 km stretch of the U.S. Atlantic margin contains more than 5000 pockmarks at water depths of 120 m (shelf edge) to 700 m (upper slope), mostly updip of the contemporary gas hydrate stability zone (GHSZ). Advanced attribute analyses of high-resolution multichannel seismic reflection data reveal gas-charged sediment and probable fluid chimneys beneath pockmark fields. A series of enhanced reflectors, inferred to represent hydrate-bearing sediments, occur within the GHSZ. Differential sediment loading at the shelf edge and warming-induced gas hydrate dissociation along the upper slope are the proposed mechanisms that led to transient changes in substrate pore fluid overpressure, vertical fluid/gas migration, and pockmark formation.

  20. Geochemical zonation and characteristics of cold seeps along the Makran continental margin off Pakistan

    NASA Astrophysics Data System (ADS)

    Fischer, D.; Bohrmann, G.; Zabel, M.; Kasten, S.

    2009-04-01

    Several highly dynamic and spatially extended cold seeps were found and analyzed on the Makran accretionary wedge off Pakistan during R/V Meteor cruise M74-3 in 2007. In water depths of 550m to 2870m along the continental slope nine different gas escape structures were examined some of which are situated within a stable oxygen minimum zone (OMZ) between 150m and 1100m water depth (von Rad et al., 1996, 2000). Echosounder data indicate several gas bubble streams in the water column. The gas seepage presumably originates from squeezing of massive sediment packages being compressed by subduction at the continental margin off Pakistan. Gas- and fluid venting and associated surface-near anaerobic oxidation of methane (AOM) feed several cold seepage systems in the seabed. The seep sites show strong inter- and intraspecific variability of benthic chemosynthetic microhabitats. Singular seeps are often colonized by different chemosynthetic organisms in a concentric fashion. The seep-center, where active bubble ebullition occurs, is often colonized by large hydrogen sulfide-oxidizing bacteria, which are surrounded by a rim inhabited by small chemosynthetic clams and tube worms. These different habitats and the associated sediments show distinct geochemical zonations and gradients. Geochemical analyses of pore water and sediment samples obtained via ROV (push corer) show that concentrations of hydrogen sulfide and alkalinity rapidly increase to >15 mmol/l and >35 mmol/l respectively several cm below the seafloor in the center of the cold seep. In places, sulfate is depleted to concentrations below detection limit at the same depth (ROV push core GeoB 12313-6). Ammonium concentrations in this core on the other hand show a different pattern: In the center of the cold seep, which is colonized by bacterial assemblages, ammonium concentrations fluctuate around 100 µmol/l and peak with 274.4 µmol/l just above the aforementioned sulfide maximum values at 5 cm followed by a rapid

  1. Multichannel seismic depth sections and interval velocities over outer continental shelf and upper continental slope between Cape Hatteras and Cape Cod: rifted margins

    USGS Publications Warehouse

    Grow, John A.; Mattick, Robert E.; Schlee, John S.

    1979-01-01

    Six computer-generated seismic depth sections over the outer continental shelf and upper slope reveal that subhorizontal Lower Cretaceous reflectors continue 20 to 30 km seaward of the present shelf edge. Extensive erosion on the continental slope has occurred primarily during the Tertiary, causing major unconformities and retreat of the shelf edge to its present position. The precise age and number of erosional events is not established, but at least one major erosional event is thought to be Oligocene and related to a marine regression in response to a worldwide eustatic lowering of sea level. Velocities derived from the multichannel data reveal distinctive ranges and lateral trends as functions of sediment age, depth of burial, and distance from the coastline. Seismic units beneath the shelf and slope of inferred Tertiary age range from 1.7 to 2.7 km/sec, increasing with age and depth of burial. Units interpreted as Upper Cretaceous rocks beneath the shelf range from 2.3 to 3.6 km/sec and show a distinct lateral increase across the shelf followed by a decrease beneath the present continental slope. Inferred Lower Cretaceous and Upper Jurassic rocks beneath the shelf increase from 3.7 to 4.8 km/sec from nearshore to offshore and indicate a change in facies from clastic units below the inner shelf to carbonate units beneath the outer shelf and upper continental slope. Both reflection and refraction data suggest that thin, high-velocity limestone units (5.0 km/sec) are present within the Lower Cretaceous and Upper Jurassic units beneath the outermost shelf edge, but that these change lithology or pinch out before reaching the middle shelf. Although lateral changes in velocity across the shelf and local velocity inversions appear, the interval velocities along the length of the margin show excellent continuity between Cape Hatteras and Cape Cod. The high-velocity horizons within the Lower Cretaceous and Upper Jurassic shelf-edge complex indicate the presence of a

  2. Reconstruction of Holocene southern African continental climate using biomarkers from salt pan sediments

    NASA Astrophysics Data System (ADS)

    Belz, Lukas; Schüller, Irka; Wehrmann, Achim; Wilkes, Heinz

    2015-04-01

    The climate system of southern Africa is strongly influenced by large scale atmospheric and marine circulation processes and, therefore, very sensitive to global climate change. Recent publications provided evidence for strong spatial and temporal climate variability in southern Africa over the Holocene. It is of major importance to understand the mechanisms driving the southern African climate system in order to estimate regional implications of current global change. However, proxy datasets from continental geoarchives especially of the semi-arid western Kalahari region are still scarce. A main problem is the absence of conventional continental climatic archives, due to the lack of lacustrine systems. In this study we are exploring the utility of sediments from western Kalahari salt pans, i.e. local depressions which are flooded temporarily during rainfall events. Besides the analyses of basic geochemical bulk parameters including TOC, δ13Corg, TIC, δ13Ccarb, δ18Ocarb, TN, δ15N, the paleo-climatic approach focuses on reconstruction of local vegetation assemblages to identify changes in the ecosystem. This is pursued using plant biomarkers, particularly leaf wax n-alkanes and n-alcohols and their stable carbon and hydrogen isotopic signatures. Preliminary results show prominent shifts in n-alkane distribution and δ13C values of the C33 homologue during late Pleistocene and early Holocene. These shifts correlate to changes of the TOC content. Our data indicate changes in carbon sources which possibly reflect major environmental changes.

  3. Reconstruction of multiple tectonic events in continental margins by integrated tectonostratigraphic and geochronological analysis: the Mesozoic to Paleogene Caribbean-South American interaction in northeastern Colombia

    NASA Astrophysics Data System (ADS)

    Cardona, Agustin; Montes, Camilo; Bayona, German; Valencia, Victor; Ramirez, Diego; Zapata, Sebastian; Lara, Mario; Lopez-Martinez, Margarita; Thomson, Stuart; Weber, Marion

    2013-04-01

    Although the older record and successive tectonic scenarios experienced by a continental margin is commonly fragmentary, integrated field, petrological and geochronological analysis can reconstruct the long term tectonic evolution of continental margins and characterized major controls on the orogenic style. We present new geochronological constraints from igneous and low to very low grade metasedimentary rocks from the Caribbean continental margin of northeastern Colombia (Guajira region) in order to reconstruct the different tectonic events recorded by the margin before, during and following the arc-continent collision with the front of the Caribbean plate. Zircon U-Pb LA-ICP-MS geochronology results from leucogranites associated with garnet amphibolites, tonalites and volcanic rocks that made the continental basement of northeastern Colombia reveals and Early to Middle Mesozoic tectonic activity with peaks at ca. 220-230 Ma and 170-180 Ma. This magmatic record is related to a collisional belt link to the final agglutination of Pangea and was followed by an overimposed far field back-arc setting associated to the subduction of the Pacific (Farrallon) plate under the Pangea supercontinent. Muscovite and biotite Ar-Ar geochronology from basement rocks and low grade Mesozoic metasediments also reveals the existence of Middle Jurassic to Early Cretaceous thermal events link to the final opening of the proto-Caribbean ocean. The South American continental margin was subsequently affected by an arc-continent collisional event with the front of the Caribbean plate. This event is recorded by the growth of a Banda-type collisional melange that mixed South American continental margin sediments with mafic and ultramafic blocks of intra-oceanic arc origin, the formation of a coherent metasedimentary belt also made of South American margin sediments, and the mylonitization of the continental basement. Ar-Ar temporal constraints on the low grade metasedimentary rocks and

  4. Impact of gravity processes on the initial post-rift stages of construction and evolution of a continental margin: Insights from the eastern Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Baurion, Celine; Gorini, Christian; Leroy, Sylvie; Migeon, Sebastien; Lucazeau, Francis; Bache, Francois; Zaragosi, Sebastien; Smit, Jeroen; Al-Toubi, Khalfan; dos Reis, Antonio

    2013-04-01

    The study of the post-rift sediment architecture and continental slope morphology leads to a reconstruction of the initial stages of formation and evolution of gravity-driven processes on the northern margin of the eastern Gulf of Aden. The slope-related features and associated deposits in the deep basin along this young passive margin are investigated through the analysis of a set of seismic-reflection and multibeam bathymetry data. This study demonstrates how preconditioning and triggering factors (tectonics, climate and eustatic variations) can interact and control the margin morphology and post-rift sediment architecture in a source-to-sink perspective. The combined geomorphological and stratigraphic study of this margin allows us to identify three morphological domains inherited from the structural segmentation. The monsoon climate combined with a major eustatic lowstand is proposed as the most likely set of factors preconditioning slope destabilisation on the whole margin. These factors also enhance the effect of the late post-rift uplift of the eastern morphological domain of the studied margin. The formation and distribution of the slope-related features are thus mainly controlled by active faults on the continental slope and the potential effect of bottom currents at the base of the continental slope. The oversteepening of the continental slope in the eastern domain of the studied margin is probably the main triggering factor controlling the generation of failure processes and subsequent canyon formation by upslope erosion. The analysis of canyon location and morphology along the uplifted part of the continental slope reveals the long-term influence of secondary slope-related features, contour currents and turbidite flows on the development of canyons. As a consequence of the late post-rift uplift that only affected the eastern part of the studied margin, huge volumes of sediment were accumulated in mass-transport complexes at the foot of numerous slope

  5. Nutrient distributions, transports, and budgets on the inner margin of a river-dominated continental shelf

    EPA Science Inventory

    Physical and biogeochemical processes determining the distribution and fate of nutrients delivered by the Mississippi and Atchafalaya rivers to the inner (<50 m depth) Louisiana continental shelf (LCS) were examined using a three-dimensional hydrodynamic model of the LCS and obse...

  6. Trace metals and organochlorines in sediments near a major ocean outfall on a high energy continental margin (Sydney, Australia).

    PubMed

    Matthai, C; Birch, G F

    2000-12-01

    Sewage effluent from a large ocean outfall south of Sydney, southeastern Australia, is efficiently dispersed on this high energy continental margin. An enrichment of Ag, Cu, Pb and Zn is only detectable in the fine fraction (<62.5 microm) of sediment. Ag, Co, Cu, Ni, Pb and Zn in the bulk sample correlate strongly with the mud content of surficial sediment, making an identification of the anthropogenic trace metal source difficult using total sediment analyses. The concentrations of HCB and DDE in the total sediment are also slightly elevated near the outfall. In the vicinity of the outfall, the estimated sewage component in the fine fraction of sediment, using Ag, Cu and Zn in a conservative, two-endmember physical mixing model, is <5% and is <0.25% of the total sediment. A greater anthropogenic Pb component in the fine fraction (mean: 24.8%) of surficial sediment compared to Ag, Cu and Zn may suggest a source other than sewage to Sydney continental margin sediments.

  7. Benthic respiration and standing stock on two contrasting continental margins in the western Indian Ocean: the Yemen-Somali upwelling region and the margin off Kenya

    NASA Astrophysics Data System (ADS)

    Duineveld, G. C. A.; De Wilde, P. A. W. J.; Berghuis, E. M.; Kok, A.; Tahey, T.; Kromkamp, J.

    During the Netherlands Indian Ocean Project (NIOP, 1992-1993) sediment community oxygen consumption (SCOC) was measured on two continental margins in the Indian Ocean with different productivity: the productive upwelling region off Yemen-Somalia and the supposedly less productive Kenyan margin, which lacks upwelling. The two margins also differ in terms of river input (Kenya) and the more severe oxygen minimum in the Arabian Sea. Simultaneously with SCOC, distributions of benthic biomass and phytodetritus were studied. Our expectation was that benthic processes in the upwelling margin of the Arabian Sea would be relatively enhanced as a result of the higher productivity. On the Kenyan margin, SCOC (range l-36 mmol m -2 d -1) showed a clear decrease with increasing water depth, and little temporal variation was detected between June and December. Highest SCOC values of this study were recorded at 50 m depth off Kenya, with a maximum of 36 mmol m -2 d -1 in the northernmost part. On the margin off Yemen-Somalia, SCOC was on average lower and showed little downslope variation, 1.8-5.7 mmol m -2 d -1, notably during upwelling, when the zone between 70 and 1700 m was covered with low O 2 water (10-50 μM). After cessation of upwelling, SCOC at 60 m depth off Yemen increased from 5.7 to 17.6 mmol m -2 d -1 concurrently with an increase of the near-bottom O 2 concentration (from 11 to 153 μM), suggesting a close coupling between SCOC and O 2 concentration. This was demonstrated in shipboard cores in which the O 2 concentration in the overlying water was raised after the cores were first incubated under in situ conditions (17 μM O 2). This induced an immediate and pronounced increase of SCOC. Conversely, at deeper stations permanently within the oxygen minimum zone (OMZ), SCOC showed little variation between monsoon periods. Hence, organic carbon degradation in sediments on a large part of the Yemen slope appears hampered by the oxygen deficiency of the overlying water

  8. 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

  9. 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.

  10. Anomalous Subsidence of the Ocean Continent Transition at Rifted Continental Margins: Observations from the Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Cowie, L.; Kusznir, N. J.

    2011-12-01

    It has been proposed that some continental rifted margins have anomalous early subsidence histories and that at break-up they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn- or early post-breakup subsidence, of this form, would have important implications for our understanding of the geodynamics of continental breakup and sea-floor spreading initiation and important consequences for syn- and post-breakup depositional systems. Possible explanations for anomalous subsidence during continental breakup could include transient effects as the continental geotherm evolves towards an oceanic form, or small scale convection. Lucazeau et al. (2008) have reported anomalously high heat-flows in the ocean continent transition (OCT) of the young rifted margin of the Eastern Gulf of Aden which would have implications for its subsidence history. In order to verify (or otherwise) the proposition of an anomalous early post-breakup subsidence history in the Eastern Gulf of Aden, we have determined anomalous oceanic subsidence using residual depth anomaly (RDA) analysis and have compared lithosphere thinning across the OCT measured using subsidence analysis with continental crustal basement thinning from gravity inversion. Both 3D regional and localised 2D analyses have been carried out. The localised studies focus on published seismic reflection lines (Autin et al, 2010; D'Acremont et al, 2005; Fournier et al, 2007; Leroy et al, 2004; Leroy et al, 2010; Lucazeau et al 2008; Lucazeau et al 2010). RDAs have been calculated by comparing observed and predicted oceanic bathymetries. Regional 3D RDAs for the Gulf of Aden, without a sediment correction, show positive RDAs between 3km and 4km at the rifted margins decreasing to 0.5km at the ocean ridge axis. Localised 2D sediment corrected RDA profiles determined within and adjacent to the OCT of the Eastern Gulf of Aden are also

  11. Cannibalization of a continental margin by regional scale mass wasting: an example from the Central Tyrrhenian Sea

    NASA Astrophysics Data System (ADS)

    Chiocci, F. L.; Martorelli, E.; Bosman, A.

    2003-04-01

    Over the last decade instability features, due to different kind of processes (sliding, slumping, debris flow), were recognized to be very common on continental margin, thus highlighting the significant role of mass wasting processes. Despite their relatively frequent occurrence, the complete cannibalization of a whole span of continental margin is quite rare. Pontine Islands continental slope (Central Tyrrhenian Sea) has been investigated by long- range side scan sonar (T.O.B.I.), high-resolution seismics and gravity coring. TOBI data were collected on the whole continental slope, from the shelf break (150 m w.d.) down to the abyssal plain (3600 m w.d.), covering an area of 2.000 km2. Pontine islands slope is located at the apex of the Vavilov Basin, one of the two abyssal plains of the Tyrrhenian back-arc basin. The continental slope is extremely narrow and steep (5-10°, locally up to 30°), with a complex morphology characterized by wide channels and ridges. The complex morphology is mainly due to instability-erosional phenomena that produce the complete cannibalization of the continental margin, so that only 2% of the area is not affected by erosion and covered by hemipelagic sediments. Instability features range in size from hundreds of sq. meters to tens of sq. kilometres. Processes involved are very different and inter-related. They seem to be controlled by seafloor gradient, i.e. slides/slumps are located on the steepest areas (i.e. the upper continental slope and flank of slope ridges), while wide mass flow deposits are located on deeper areas characterized by smoother topography. When gradient is low also exfoliation of stratified deposits occurs both on semicircular scars and on flat surface, producing a step-like morphology. Slides and slumps are both characterized by isolated and coalescing scarps highlighting simple or complex failure. In some areas a retrogressive pattern was recognized; in the upper slope it produced a several hundred of meters

  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. Quantifying Continental Margin Deformation North and South of the Opening of the Gulf of California—Evidence for Subduction Erosion?

    NASA Astrophysics Data System (ADS)

    Peterman, E. M.; Grove, M.; Kimbrough, D. L.

    2010-12-01

    A striking contrast exists in the geology of the continental margin north and south of the opening of the Gulf of California. For 1200km north of the Gulf opening, a reasonably well-preserved forearc region is associated with strongly magnetic 130-100 Ma plutonic rocks, volcanics and ophiolitic basement. This package of rocks is also coupled with a weakly magnetic eastern belt of 100-90 Ma tonalite and trondhjemite. In contrast, the southern continental margin bound by the Acapulco Trench lacks both a forearc basin and the 130-100 Ma plutonic belt. Instead, Late Cretaceous and Early to Middle Cenozoic granitoids crop out both along the coast and in the offshore region east of the Acapulco Trench, extending 1300 km south to 16°N. The geologic contrast between the northern and southern segments has been attributed to Cenozoic subduction erosion of the latter. The transition region between these two regimes occurs ~50 km northwest of the Gulf opening at 23°N along the Pacific coast of Baja California. North of Todos Santos, Late Cretaceous and younger strata depositionally overlie the western margin of the 130-100 Ma plutonic belt. The plutonic rocks are well-expressed in aeromagnetic images, even where buried. South of Todos Santos, 100-90 m.y. granitoids crop out along the coast and underlie the offshore Todos Santos basin; the strongly magnetic basement and undeformed forearc rocks are absent. Near Todos Santos, 94 Ma orthogneiss intercalated with upper amphibolite facies (garnet ± andalusite) Late Triassic(?) wallrocks are structurally juxtaposed above epidote amphibolite calc-schist of the mid-Cretaceous accretionary complex, suggesting significant forearc deformation. Previous 40Ar/39Ar data gathered south of Todos Santos reveal 5-10 m.y. of discordance between hornblende and biotite in the 100-90 m.y. granitoids. The age discordance could be related to either transient heating from 83-68 Ma granitoids located to the east or subduction erosion deformation

  14. The limits of seaward spreading and slope instability at the continental margin offshore Mt Etna, imaged by high-resolution 2D seismic data

    NASA Astrophysics Data System (ADS)

    Gross, Felix; Krastel, Sebastian; Geersen, Jacob; Behrmann, Jan Hinrich; Ridente, Domenico; Chiocci, Francesco Latino; Bialas, Jörg; Papenberg, Cord; Cukur, Deniz; Urlaub, Morelia; Micallef, Aaron

    2016-01-01

    Mount Etna is the largest active volcano in Europe. Instability of its eastern flank is well documented onshore, and continuously monitored by geodetic and InSAR measurements. Little is known, however, about the offshore extension of the eastern volcano flank, defining a serious shortcoming in stability models. In order to better constrain the active tectonics of the continental margin offshore the eastern flank of the volcano, we acquired a new high-resolution 2D reflection seismic dataset. The data provide new insights into the heterogeneous geology and tectonics at the continental margin offshore Mt Etna. The submarine realm is characterized by different blocks, which are controlled by local- and regional tectonics. A compressional regime is found at the toe of the continental margin, which is bound to a complex basin system. Both, the clear link between on- and offshore tectonic structures as well as the compressional regime at the easternmost flank edge, indicate a continental margin gravitational collapse as well as spreading to be present at Mt Etna. Moreover, we find evidence for the offshore southern boundary of the moving flank, which is identified as a right lateral oblique fault north of Catania Canyon. Our findings suggest a coupled volcano edifice/continental margin instability at Mt Etna, demonstrating first order linkage between on- and offshore tectonic processes.

  15. Sedimentology of seismo-turbidites off the Cascadia and northern California active tectonic continental margins, Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Gutierrez Pastor, Julia; Nelson, Hans; Goldfinger, Chris; Escutia, Carlota

    2013-04-01

    Holocene turbidites from turbidite channel systems along the active tectonic continental margins of the Cascadia subduction zone (offshore Vancouver Island to Mendocino Triple Junction) and the northern San Andreas Transform Fault (the Triple Junction to San Francisco Bay), have been analyzed for sedimentologic features related to their seismic origin. Centimeter thick silt/sand beds (turbidite base) capped by mud layers (turbidite tail) and interbedded with hemipelagic silty clay intervals with high biogenic content have been characterized by visual core descriptions, grain-size analysis, X-ray radiographs and physical properties. Along the northern California margin in upstream single tributary canyons and channels, most turbidites are uni-pulsed (classic fining up) whereas downstream below multiple tributary canyon and channel confluences, most deposits are stacked turbidites. Because each set of stacked turbidites has no hemipelagic sediment between each turbidite unit and each unit has a distinct mineralogy from a different tributary canyon, we interpret that a stacked turbidite is deposited by several coeval turbidity currents fed by multiple tributary canyons and channels with synchronous triggering from a single San Andreas Fault earthquake. The Cascadia margin is characterized by individual multi-pulsed turbidites that contain multiple coarse-grained sub-units without hemipelagic sediment between pulses. Because the number and character of multiple coarse-grained pulses for each correlative multi-pulsed turbidite is almost always constant both upstream and downstream in different channel systems for 600 km along the margin,we interpret that the earthquake shaking or aftershock signature is usually preserved, for the much stronger Cascadia (≥9 Mw) compared to weaker California (≥8Mw) earthquakes, which result in upstream uni-pulsed turbidites and downstream stacked turbidites. Consequently, both the strongest (≥9 Mw) great earthquakes and downstream

  16. Reproductive biology and recruitment of the deep-sea fish community from the NW Mediterranean continental margin

    NASA Astrophysics Data System (ADS)

    Fernandez-Arcaya, U.; Rotllant, G.; Ramirez-Llodra, E.; Recasens, L.; Aguzzi, J.; Flexas, M. M.; Sanchez-Vidal, A.; López-Fernández, P.; García, J. A.; Company, J. B.

    2013-11-01

    Temporal patterns in deep-sea fish reproduction are presently unknown for the majority of deep continental margins. A series of seasonal trawling surveys between depths of 300 to 1750 m in the Blanes submarine canyon and its adjacent open slope (NW Mediterranean) were conducted. The bathymetric size distributions and reproductive cycles of the most abundant species along the NW Mediterranean margin were analyzed to assess the occurrence of (i) temporal patterns in reproduction (i.e., spawning season) along a bathymetric gradient and (ii) preferential depth strata for recruitment. The fish assemblages were grouped in relation to their bathymetric distribution: upper slope, middle slope and lower slope species. Middle-slope species (i.e., 800-1350 m) showed short (i.e., highly seasonal) reproductive activity compared to the upper (300-800 m) and lower (1350-1750 m) ones. Our results, together with those previously published for megabenthic crustacean decapods in the area, suggest a cross-phyla depth-related trend of seasonality in reproduction. In the middle and lower slope species, the reproductive activity reached a maximum in the autumn-winter months and decreased in the spring. The observed seasonal spawning patterns appear to be ultimately correlated with changes in the downward transport of organic particles and with seasonal changes in the physicochemical characteristics of the surrounding water masses. The distribution of juveniles was associated with the bathymetric stratum where intermediate nepheloid layers interact with the continental margins, indicating that this stratum acts as a deep-sea fish nursery area.

  17. Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid-Atlantic continental margin

    NASA Astrophysics Data System (ADS)

    Obelcz, Jeffrey; Brothers, Daniel; Chaytor, Jason; Brink, Uri ten; Ross, Steve W.; Brooke, Sandra

    2014-06-01

    Shelf-sourced submarine canyons are common features of continental margins and are fundamental to deep-sea sedimentary systems. Despite their geomorphic and geologic significance, relatively few passive margin shelf-breaching canyons worldwide have been mapped using modern geophysical methods. Between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the US Mid-Atlantic margin: Wilmington, Baltimore, Washington, and Norfolk canyons. More than 5700 km2 of high-resolution multibeam bathymetry and 890 line-km of sub-bottom CHIRP profiles were collected along the outer shelf and uppermost slope (depths of 80-1200 m). The data allowed us to compare and contrast the fine-scale morphology of each canyon system. The canyons have marked differences in the morphology and orientation of canyon heads, steepness and density of sidewall gullies, and the character of the continental shelf surrounding canyon rims. Down-canyon axial profiles for Washington, Baltimore and Wilmington canyons have linear shapes, and each canyon thalweg exhibits morphological evidence for recent, relatively small-scale sediment transport. For example, Washington Canyon displays extremely steep wall gradients and contains ~100 m wide, 5-10 m deep, v-shaped incisions down the canyon axis, suggesting modern or recent sediment transport. In contrast, the convex axial thalweg profile, the absence of thalweg incision, and evidence for sediment infilling at the canyon head, suggest that depositional processes strongly influence Norfolk Canyon during the current sea-level high-stand. The north walls of Wilmington, Washington and Norfolk canyons are steeper than the south walls due to differential erosion, though the underlying cause for this asymmetry is not clear. Furthermore, we speculate that most of the geomorphic features observed within the canyons (e.g., terraces, tributary canyons, gullies, and hanging valleys) were formed during the Pleistocene, and show only

  18. Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid-Atlantic continental margin

    USGS Publications Warehouse

    Obelcz, Jeffrey; Brothers, Daniel S.; Chaytor, Jason D.; ten Brink, Uri S.; Ross, Steve W.; Brooke, Sandra

    2013-01-01

    Shelf-sourced submarine canyons are common features of continental margins and are fundamental to deep-sea sedimentary systems. Despite their geomorphic and geologic significance, relatively few passive margin shelf-breaching canyons worldwide have been mapped using modern geophysical methods. Between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the US Mid-Atlantic margin: Wilmington, Baltimore, Washington, and Norfolk canyons. More than 5700 km2 of high-resolution multibeam bathymetry and 890 line-km of sub-bottom CHIRP profiles were collected along the outer shelf and uppermost slope (depths of 80-1200 m). The data allowed us to compare and contrast the fine-scale morphology of each canyon system. The canyons have marked differences in the morphology and orientation of canyon heads, steepness and density of sidewall gullies, and the character of the continental shelf surrounding canyon rims. Down-canyon axial profiles for Washington, Baltimore and Wilmington canyons have linear shapes, and each canyon thalweg exhibits morphological evidence for recent, relatively small-scale sediment transport. For example, Washington Canyon displays extremely steep wall gradients and contains ~100 m wide, 5–10 m deep, v-shaped incisions down the canyon axis, suggesting modern or recent sediment transport. In contrast, the convex axial thalweg profile, the absence of thalweg incision, and evidence for sediment infilling at the canyon head, suggest that depositional processes strongly influence Norfolk Canyon during the current sea-level high-stand. The north walls of Wilmington, Washington and Norfolk canyons are steeper than the south walls due to differential erosion, though the underlying cause for this asymmetry is not clear. Furthermore, we speculate that most of the geomorphic features observed within the canyons (e.g., terraces, tributary canyons, gullies, and hanging valleys) were formed during the Pleistocene, and show only

  19. Geologic development and characteristics of the continental margins, Gulf of Mexico. Research report, 1983-1986

    SciTech Connect

    Coleman, J.M.; Prior, D.B.; Roberts, H.H.

    1986-01-01

    The continental slope of the Gulf Basin covers more than 500,000 sq km and consists of smooth and gently sloping surfaces, prominent escarpments, knolls, intraslope basins, and submarine canyons and channels. It is an area of extremely diverse topographic and sedimentologic conditions. The slope extends from the shelf break, roughly at the 200 m isobath, to the upper limit of the continental rise, at a depth of 2800 m. The most-complex province in the basin, and the one of most interest to the petroleum industry, is the Texas-Louisiana slope, occupying 120,000 sq km and in which bottom slopes range from < 1 deg to > 20 deg around the knolls and basins. The near-surface geology and topography of the slope are functions of the interplay between episodes of rapid shelf-edge and slope progradation and contemporaneous modification of the depositional sequence by diapirism. Development of discrete depo-centers throughout the Neogene results in rapid shelf-edge progradation, often in excess of 15-20 km/my. This rapid progradation of the shelf edge leads to development of thick wedges of sediment accumulation on the continental slope. Oversteeping, high pore pressures in rapidly deposited soft sediments and changes in eustatic sea level cause subaqueous slope instabilities such as landsliding and debris flows. Large scale features such as shelf edge separation scars and landslide related canyons often results from such processes.

  20. 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.

  1. Cenozoic magmatism in the northern continental margin of the South China Sea: evidence from seismic profiles

    NASA Astrophysics Data System (ADS)

    Zhang, Qiao; Wu, Shiguo; Dong, Dongdong

    2016-06-01

    Igneous rocks in the northern margin of the South China Sea (SCS) have been identified via high resolution multi-channel seismic data in addition to other geophysical and drilling well data. This study identified intrusive and extrusive structures including seamounts and buried volcanoes, and their seismic characteristics. Intrusive features consist of piercement and implicit-piercement type structures, indicating different energy input associated with diapir formation. Extrusive structures are divided into flat-topped and conical-topped seamounts. Three main criteria (the overlying strata, the contact relationship and sills) were used to distinguish between intrusive rocks and buried volcanos. Three criteria are also used to estimate the timing of igneous rock formation: the contact relationship, the overlying sedimentary thickness and seismic reflection characteristics. These criteria are applied to recognize and distinguish between three periods of Cenozoic magmatism in the northern margin of the SCS: before seafloor spreading (Paleocene and Eocene), during seafloor spreading (Early Oligocene-Mid Miocene) and after cessation of seafloor spreading (Mid Miocene-Recent). Among them, greater attention is given to the extensive magmatism since 5.5 Ma, which is present throughout nearly all of the study area, making it a significant event in the SCS. Almost all of the Cenozoic igneous rocks were located below the 1500 m bathymetric contour. In contrast with the wide distribution of igneous rocks in the volcanic rifted margin, igneous rocks in the syn-rift stage of the northern margin of the SCS are extremely sporadic, and they could only be found in the southern Pearl River Mouth basin and NW sub-sea basin. The ocean-continent transition of the northern SCS exhibits high-angle listric faults, concentrated on the seaward side of the magmatic zone, and a sharply decreased crust, with little influence from a mantle plume. These observations provide further evidence to

  2. 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.

  3. Factors controlling early stage salt tectonics at rifted continental margins and their thermal consequences

    NASA Astrophysics Data System (ADS)

    Goteti, Rajesh; Beaumont, Christopher; Ings, Steven J.

    2013-06-01

    We use 2-D thermomechanical models to investigate the early evolution of rifted margin salt tectonics in terms of the competition among margin tilt, salt flow, and sediment aggradation. Model experiments include initial geometry of the rifted margin and autochthonous salt basin, subsequent synrift and thermal subsidence, sediment and water loading, and sediment compaction. We also calculate the thermal evolution of the system to investigate the impact of the high thermal conductivity of the salt (halite). Model Set 1 demonstrates a two-phase response to salt deposition: short-term thermal equilibration between the salt and crust and longer-term relaxation in which the salt basin thermal image penetrates to a depth on the order of its width. Set 2 addresses the competition among margin tilt, salt flow, and sediment aggradation. Set 3 examines other factors, the salt basin width and depth, and the rifted margin width, which potentially affect the system evolution. Set 4 shows that sawtooth subsalt topography, representing faulted basement grabens, does not strongly impede salt flow. The model results are discussed in terms of a ternary diagram with apices representing tilt, salt flow, and sedimentation rates. Characteristic styles include the following: (1) tilt and rapid salt flow draining salt to the distal basin before the sediment aggrades, (2) an equivalent system with faster aggradation that captures draining salt as diapirs between minibasins, and (3) rapid sediment aggradation in which diapir-minibasins systems develop before the salt drains. Thermal consequences of these styles are discussed. A preliminary comparison shows that salt structures resembling these styles occur in the southwest Nova Scotian margin.

  4. Aedes (Stegomyia) aegypti in the continental United States: a vector at the cool margin of its geographic range.

    PubMed

    Eisen, Lars; Moore, Chester G

    2013-05-01

    After more than a half century without recognized local dengue outbreaks in the continental United States, there were recent outbreaks of autochthonous dengue in the southern parts of Texas (2004-2005) and Florida (2009-2011). This dengue reemergence has provoked interest in the extent of the future threat posed by the yellow fever mosquito, Aedes (Stegomyia) aegypti (L.), the primary vector of dengue and yellow fever viruses in urban settings, to human health in the continental United States. Ae. aegypti is an intriguing example of a vector species that not only occurs in the southernmost portions of the eastern United States today but also is incriminated as the likely primary vector in historical outbreaks of yellow fever as far north as New York, Philadelphia, and Boston, from the 1690s to the 1820s. For vector species with geographic ranges limited, in part, by low temperature and cool range margins occurring in the southern part of the continental United States, as is currently the case for Ae. aegypti, it is tempting to speculate that climate warming may result in a northward range expansion (similar to that seen for Ixodes tick vectors of Lyme borreliosis spirochetes in Scandinavia and southern Canada in recent decades). Although there is no doubt that climate conditions directly impact many aspects of the life history of Ae. aegypti, this mosquito also is closely linked to the human environment and directly influenced by the availability of water-holding containers for oviposition and larval development. Competition with other container-inhabiting mosquito species, particularly Aedes (Stegomyia) albopictus (Skuse), also may impact the presence and local abundance of Ae. aegypti. Field-based studies that focus solely on the impact of weather or climate factors on the presence and abundance of Ae. aegypti, including assessments of the potential impact of climate warming on the mosquito's future range and abundance, do not consider the potential confounding

  5. Early Cretaceous rifting and exposure of periodotite on the Galicia continental margin: preliminary results of ocean drilling program Leg 103

    SciTech Connect

    Winterer, E.; Boillot, G.; Meyer, A.; Applegate, J.; Baltuck, M.; Bergen, J.; Comas, M.; Davies, T.; Dunham, K.; Evans, C.; Girardeau, J.

    1985-01-01

    Results of drilling near the ocean-continent boundary on the Galicia margin of Iberia shed new light on the timing of rifting and demonstrate the presence at the foot of the margin of a ridge of foliated, lineated, sheared and serpentinized harzburgite, probably representing oceanic mantle. Fifty km east of the periodotite ridge, on a continental fault block, the stratigraphic section sampled during Leg 103 above Hercynian basement comprises: (1) at least 250m of Upper Jurassic and possibly lowest Cretaceous limestone, dolomite and minor sandstone and claystone deposited in relatively shallow water before rifting began; (2) about 20m of Valanginian calpionellid marlstone, probably deposited in moderate depths at the onset of rifting; (3) from about 500 to 1500m of Valanginian and Hauterivian turbidite sandstone rich in terrestrial plant debris, and Barremian and Aptian( ) claystone and marlstone deposited in deeper water during rifting; and (4) about 700m of sediments deposited after Aptian time, when rifting ceased and oceanic spreading between Iberia and Newfoundland began. The lithology and seismic stratigraphy of the wedges of clastic sediments laid down during rifting show the progressive filling of basins that formed by episodic listric faulting that began very early in the Cretaceous and continued for about 25 my. The Lower Cretaceous turbidite sandstone cored on the Galicia margin correlates with thick Lower Cretaceous turbidites cored off Morocco during DSDP Leg 50, and with Wealden deltaic and fluviatile deposits on both sides of the Atlantic.

  6. 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.

  7. Cenozoic unconformities and depositional supersequences of North Atlantic continental margins: testing the Vail model

    SciTech Connect

    Poag, C.W.; Ward, L.W.

    1987-02-01

    Integrated outcrop, borehole, and seismic reflection stratigraphy from the US and Irish margins of the North Atlantic basin reveals a framework of Cenozoic depositional supersequences and interregional unconformities that resembles the Vail depositional model. Paleobathymetric and paleoceanographic analyses of associated microfossil assemblages indicate a genetic link between the depositional framework and the relative position of sea level.

  8. Cenozoic unconformities and depositional supersequences of North Atlantic continental margins: testing the Vail model

    USGS Publications Warehouse

    Poag, C. Wylie; Ward, Lauck W.

    1987-01-01

    Integrated outcrop, borehole, and seismic reflection stratigraphy from the U.S. and Irish margins of the North Atlantic basin reveals a framework of Cenozoic depositional supersequences and interregional unconformities that resembles the Vail depositional model. Paleo-bathymetric and paleoceanographic analyses of associated microfossil assemblages indicate a genetic link between the depositional framework and the relative position of sea level.

  9. Gravimetric determination of the continental-oceanic boundary of the Argentine continental margin (from 36°S to 50°S)

    NASA Astrophysics Data System (ADS)

    Arecco, María Alejandra; Ruiz, Francisco; Pizarro, Guillermo; Giménez, Mario; Martínez, Patricia; Ramos, Víctor A.

    2016-01-01

    This paper presents the gravimetric analysis together with seismic data as an integral application in order to identify the continental-oceanic crust boundary (COB) of the Argentine continental margin from 36°S to 50°S in a continuous way. The gravimetric and seismic data are made up of large grids of data obtained from satellite altimetry and marine research. The methodology consists of three distinct methods: (i) the application of enhancement techniques to gravimetric anomalies, (ii) the calculation of crustal thinning from 3-D gravity inversion modelling of the crust-mantle discontinuity and (iii) 2-D gravimetric modelling supported by multichannel reflection and refraction seismic profiles. In the first method, the analytic signal, Theta map, and tilt angle and its horizontal derivative were applied. In the second method, crustal thickness was obtained as the difference in the depths of the crystalline basement and the crust-mantle discontinuity; the latter was obtained via gravimetric inversion. Finally, 2-D modelling was performed from free-air anomalies in two representative sections by considering as restriction surfaces those coming from the interpretation of seismic data. The results of the joint application of enhancement techniques and 2-D and 3-D modelling have enabled continuous interpretation of the COB. In this study, the COB was determined continuously from the integration of 2-D profiles of the enhancement techniques, taking account of crustal thickness and performing 2-D gravimetric modelling. The modelling technique was complemented by regional studies integrated with multichannel seismic reflection and seismic refraction lines, resulting in consistent enhancement techniques.

  10. 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

  11. Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing - an overview and synthesis

    NASA Astrophysics Data System (ADS)

    Liu, K.-K.; Kang, C.-K.; Kobari, T.; Liu, H.; Rabouille, C.; Fennel, K.

    2014-07-01

    In this special issue we examine the biogeochemical conditions and marine ecosystems in the major marginal seas of the western North Pacific Ocean, namely, the East China Sea, the Japan/East Sea to its north and the South China Sea to its south. They are all subject to strong climate forcing as well as anthropogenic impacts. On the one hand, continental margins in this region are bordered by the world's most densely populated coastal communities and receive tremendous amounts of land derived materials. On the other hand, the Kuroshio, the strong western boundary current, which is modulated by climate oscillation, exerts strong influences over all three marginal seas. Because these continental margins sustain arguably the most productive marine ecosystems, changes in these stressed ecosystems may threaten the livelihood of a large human population. This special issue reports the latest observations of the biogeochemical conditions and ecosystem functions in the three marginal seas. The studies exemplify many faceted ecosystem functions and biogeochemical expressions, but they reveal only a few long term trends mainly due to lack of long term records. It is critical to develop and sustain time series observations in order to detect biogeochemical changes and ecosystem responses in continental margins and to attribute the causes for better management of the environment and resources in these marginal seas.

  12. Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing - an overview and synthesis

    NASA Astrophysics Data System (ADS)

    Liu, K.-K.; Kang, C.-K.; Kobari, T.; Liu, H.; Rabouille, C.; Fennel, K.

    2014-12-01

    In this special issue we examine the biogeochemical conditions and marine ecosystems in the major marginal seas of the western North Pacific Ocean, namely, the East China Sea, the Japan/East Sea to its north and the South China Sea to its south. They are all subject to strong climate forcing as well as anthropogenic impacts. On the one hand, continental margins in this region are bordered by the world's most densely populated coastal communities and receive tremendous amount of land-derived materials. On the other hand, the Kuroshio, the strong western boundary current of the North Pacific Ocean, which is modulated by climate oscillation, exerts strong influences over all three marginal seas. Because these continental margins sustain arguably some of the most productive marine ecosystems in the world, changes in these stressed ecosystems may threaten the livelihood of a large population of humans. This special issue reports the latest observations of the biogeochemical conditions and ecosystem functions in the three marginal seas. The studies exemplify the many faceted ecosystem functions and biogeochemical expressions, but they reveal only a few long-term trends mainly due to lack of sufficiently long records of well-designed observations. It is critical to develop and sustain time series observations in order to detect biogeochemical changes and ecosystem responses in continental margins and to attribute the causes for better management of the environment and resources in these marginal seas.

  13. Epibenthic megacrustaceans from the continental margin and slope of the Southwestern Gulf of Mexico: factors responsible for variability in species composition and diversity

    NASA Astrophysics Data System (ADS)

    Gaytan-Caballero, A.; Escobar, E.; Villalobos-Hiriart, J. L.

    2007-05-01

    Specimens collected from trawls on board UNAM's R/V Justo Sierra along 4 years has allowed us to describe the community structure of megacrustaceans collected on the continental margin (45 to 156 m) and upper slope (251 to 705m) in the Mexican Ridges and the Campeche Bank of the southern Gulf of Mexico. The species composition, species richness, density and diversity varied among geographic regions and in the depth gradient. A total of 76 species were identified and grouped in 2 orders, 5 infraorders, 37 families and 53 genera. This study extends the known geographic ranges of the species Homolodromia monstrosa and the proposal of Munida constricta and Munidopsis polita. The largest number of species was recorded in the Mexican Ridges (9+3.16) and on the upper continental shelf (10+3.5); lower values were found on the continental margin. The largest densities were recorded on the continental margin in the Mexican Ridges. Megacrustaceans show in general low frequencies and low abundances, characterizing them as rare components of benthic assemblages. In spite of the great similarity among continental shelf and upper continental slope, two subgroups were recognized reflecting specific habitat and time variation

  14. Evolution of the Red Sea Continental Margin from Integrated Analyses of Gravity, Magnetic, and Receiver Function Observations

    NASA Astrophysics Data System (ADS)

    Reed, C. A.; Mohamed, A. A.; Gao, S. S.; Mickus, K. L.; Liu, K. H.; Yu, Y.; Elsheikh, A. A.

    2014-12-01

    The development of evolutionary models and constraints for the extensional mechanisms which govern continental rifting is of fundamental significance toward understanding the breakup of continents and the role of volcanism in achieving successful rifting. To analyze the transitional nature of the Red Sea rift (RSR) passive margins and to quantify the mechanism through which extension has been accommodated, we examined a total of 3531 high-quality radial receiver functions from multiple temporary deployments in Saudi Arabia and the Levant as well as data recently acquired by the Egyptian National Seismic Network. Egypt is characterized by a relatively constant crustal thickness of approximately 37 km, while the southern Arabian Shield is roughly 35 km on average. The crust beneath the Eastern Desert of Egypt is significantly thinned with an average thickness of about 26 km. Observations of Vp/Vs across the Arabian-Nubian Shield indicate highly similar intermediate to mafic compositions, supporting well-accepted theories for juvenile arc accretion of relatively uniform makeup. Thinned crust as far as 130 km inland on the Egyptian margin indicates a highly asymmetric crustal structure across the Red Sea, supporting a model invoking simple shear extensional mechanisms. Joint modeling using satellite gravity and magnetic data with RF Moho depth constraints reveals the presence of high-density high-magnetic susceptibility mafic complexes which we interpret as volcanic margins in the northern RSR at ~25.5°N and the southern RSR at ~19.5°N. We believe the development of the northern RSR margin is accompanied by isolated volcanism associated with slow spreading rates since the Oligocene.

  15. Structure and petroleum potential of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska

    USGS Publications Warehouse

    Bruns, T.R.

    1982-01-01

    Major structural features of the Yakutat segment, the segment of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska, are delineated by multichannel seismic reflection data. A large structural high is centered on Fairweather Ground and lies generally at the edge of the shelf from Cross Sound to west of the Alsek Valley. A basement uplift, the Dangerous River zone, along which the seismic acoustic basement shallows by up to two kilometers, extends north from the western edge of Fairweather Ground towards the mouth of the Dangerous River. The Dangerous River zone separates the Yakutat segment into two distinct subbasins. The eastern subbasin has a maximum sediment thickness of about 4 km, and the axis of the basin is near and parallel to the coast. Strata in this basin are largely of late Cenozoic age (Neogene and Quaternary) and approximately correlate with the onshore Yakataga Formation. The western subbasin has a maximum of at least 9 km of sediment, comprised of a thick (greater than 4.5 km) Paleogene section overlain by late Cenozoic strata. The Paleogene section is truncated along the Dangerous River zone by a combination of erosion, faulting, and onlap onto the acoustic basement. Within the western subbasin, the late Cenozoic basin axis is near and parallel to the coast, but the Paleogene basin axis appears to trend in a northwest direction diagonally across the shelf. Sedimentary strata throughout the Yakutat shelf show regional subsidence and only minor deformation except in the vicinity of the Fairweather Ground structural high, near and along the Dangerous River zone, and at the shoreline near Lituya Bay. Seismic data across the continental slope and adjacent deep ocean show truncation at the continental slope of Paleogene strata, the presence of a thick (to 6 km) undeformed or mildly deformed abyssal sedimentary section at the base of the slope that in part onlaps the slope, and a relatively narrow zone along the slope or at

  16. 3-D view of erosional scars on U. S. Mid-Atlantic continental margin

    SciTech Connect

    Farre, J.A.; Ryan, W.B.

    1985-06-01

    Deep-towed side-scan and bathymetric data have been merged to present a 3-D view of the lower continental slope and upper continental rise offshore Atlantic City, New Jersey. Carteret Canyon narrows and becomes nearly stranded on the lower slope where it leads into one of two steep-walled, flat-floored erosional chutes. The floors of the chutes, cut into semilithified middle Eocene siliceous limestones, are marked by downslope-trending grooves. The grooves are interpreted to be gouge marks formed during rock and sediment slides. On the uppermost rise, beneath the chutes, is a 40-m deep depression. The origin of the depression is believed to be related to material moving downslope and encountering the change in gradient at the slope/rise boundary. Downslope of the depression are channels, trails, and allochthonous blocks. The lack of significant post-early Miocene deposits implies that the lower slope offshore New Jersey has yet to reach a configuration conducive to sediment accumulation. The age of erosion on the lower slope apparently ranges from late Eocene-early Miocene to the recent geologic past.

  17. Data file, Continental Margin Program, Atlantic Coast of the United States: vol. 2 sample collection and analytical data

    USGS Publications Warehouse

    Hathaway, John C.

    1971-01-01

    The purpose of the data file presented below is twofold: the first purpose is to make available in printed form the basic data relating to the samples collected as part of the joint U.S. Geological Survey - Woods Hole Oceanographic Institution program of study of the Atlantic continental margin of the United States; the second purpose is to maintain these data in a form that is easily retrievable by modern computer methods. With the data in such form, repeate manual transcription for statistical or similar mathematical treatment becomes unnecessary. Manual plotting of information or derivatives from the information may also be eliminated. Not only is handling of data by the computer considerably faster than manual techniques, but a fruitful source of errors, transcription mistakes, is eliminated.

  18. High-pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east- central Alaska

    USGS Publications Warehouse

    Dusel-Bacon, C.; Hansen, V.L.; Scala, J.A.

    1995-01-01

    Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska: the northern, structurally higher Taylor Mountain terrane and the southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane. The pressure, temperature, kinematic and age data are interpreted to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. -from Authors

  19. Surficial clay mineral distribution on the southwestern continental margin of India: evidence of input from the Bay of Bengal

    NASA Astrophysics Data System (ADS)

    Chauhan, Onkar S.; Gujar, A. R.

    1996-03-01

    Analyses of spatial distribution of clay minerals, sediment texture, and > 63 μm fractions of the grab samples from the S W continental margin of India exhibit: (i) higher contents of illite and chlorite on the lower slope and (ii) a well-defined no-clay zone on the entire shelf. Kaolinite and smectite are also present in significant quantities on the slope with traces of gibbsite and palygorskite in some samples. The high contents of illite and chlorite (clay minerals which are not abundant in the soils and estuarine sediments of this region) in the southern region of the study area are evidence for sediment contribution from the Bay of Bengal waters (BBW), which enter this region after the SW monsoon. Distribution trends of kaolinite, smectite, gibbsite, and laterite granules on the slope are suggestive of contribution from chemically weathered soils of Peninsular India.

  20. 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.

  1. Ascension Submarine Canyon, California - Evolution of a multi-head canyon system along a strike-slip continental margin

    USGS Publications Warehouse

    Nagel, D.K.; Mullins, H.T.; Greene, H. Gary

    1986-01-01

    Ascension Submarine Canyon, which lies along the strike-slip (transform) dominated continental margin of central California, consists of two discrete northwestern heads and six less well defined southeastern heads. These eight heads coalesce to form a single submarine canyon near the 2700 m isobath. Detailed seismic stratigraphic data correlated with 19 rock dredge hauls from the walls of the canyon system, suggest that at least one of the two northwestern heads was initially eroded during a Pliocene lowstand of sea level ???3.8 m.y. B.P. Paleogeographic reconstructions indicate that at this time, northwestern Ascension Canyon formed the distal channel of nearby Monterey Canyon and has subsequently been offset by right-lateral, strike-slip faulting along the San Gregorio fault zone. Some of the six southwestern heads of Ascension Canyon may also have been initially eroded as the distal portions of Monterey Canyon during late Pliocene-early Pleistocene sea-level lowstands (???2.8 and 1.75 m.y. B.P.) and subsequently truncated and offset to the northwest. There have also been a minimum of two canyon-cutting episodes within the past 750,000 years, after the entire Ascension Canyon system migrated to the northwest past Monterey Canyon. We attribute these late Pleistocene erosional events to relative lowstands of sea level 750,000 and 18,000 yrs B.P. The late Pleistocene and Holocene evolution of the six southeastern heads also appears to have been controlled by structural uplift of the Ascension-Monterey basement high at the southeastern terminus of the Outer Santa Cruz Basin. We believe that uplift of this basement high sufficiently oversteepened submarine slopes to induce gravitational instability and generate mass movements that resulted in the erosion of the canyon heads. Most significantly, though, our results and interpretations support previous proposals that submarine canyons along strike-slip continental margins can originate by tectonic trunction and lateral

  2. Volcano-sedimentary processes operating on a marginal continental arc: the Archean Raquette Lake Formation, Slave Province, Canada

    NASA Astrophysics Data System (ADS)

    Mueller, W. U.; Corcoran, P. L.

    2001-06-01

    The 200-m thick, volcano-sedimentary Raquette Lake Formation, located in the south-central Archean Slave Province, represents a remnant arc segment floored by continental crust. The formation overlies the gneissic Sleepy Dragon Complex unconformably, is laterally interstratified with subaqueous mafic basalts of the Cameron River volcanic belt, and is considered the proximal equivalent of the turbidite-dominated Burwash Formation. A continuum of events associated with volcanism and sedimentation, and controlled by extensional tectonics, is advocated. A complex stratigraphy with three volcanic and three sedimentary lithofacies constitute the volcano-sedimentary succession. The volcanic lithofacies include: (1) a mafic volcanic lithofacies composed of subaqueous pillow-pillow breccia, and subaerial massive to blocky flows, (2) a felsic volcanic lithofacies representing felsic flows that were deposited in a subaerial environment, and (3) a felsic volcanic sandstone lithofacies interpreted as shallow-water, wave- and storm-reworked pyroclastic debris derived from explosive eruptions. The sedimentary lithofacies are represented by: (1) a conglomerate-sandstone lithofacies consistent with unconfined debris flow, hyperconcentrated flood flow and talus scree deposits, as well as minor high-energy stream flow conglomerates that formed coalescing, steep-sloped, coarse-clastic fan deltas, (2) a sandstone lithofacies, interpreted as hyperconcentrated flood flow deposits that accumulated at the subaerial-subaqueous interface, and (3) a mudstone lithofacies consistent with suspension sedimentation in a small restricted lagoon-type setting. The Raquette Lake Formation is interpreted as a fringing continental arc that displays both high-energy clastic sedimentation and contemporaneous effusive and explosive mafic and felsic volcanism. Modern analogues that develop along active plate margins in which continental crust plays a significant role include Japan and the Baja California

  3. Large particle flux of 239+240Pu on the continental margin of the East China Sea.

    PubMed

    Yamada, Masatoshi; Aono, Tatsuo

    2002-03-15

    Settling particles were collected from three locations in the East China Sea continental margin and analyzed for 239+240Pu. Two types of sediment traps were used, cylindrical traps and conical time-series traps. Surface sediment samples collected from five locations were also analyzed for 239+240Pu. Data from cylindrical traps showed there was a clear tendency for total mass fluxes to increase with depth at all three stations, and there was an especially large increase near the bottom. 239+240Pu concentrations in settling particles increased with depth from 1.76 mBq/g at 97-m depth to 3.0 mBq/g at 120-m depth and ranged from approximately 3 to 4 mBq/g at depths greater than 120 m. 239+240Pu concentrations collected in the near-bottom traps were approximately two times higher than those in the underlying surface sediments. Like total mass fluxes there was a clear tendency for 239+240Pu fluxes to increase with depth at every station, and the highest 239+240Pu fluxes were observed near the bottom. 239+240Pu concentrations in the time-series traps had little variation throughout the sampling period, though the total mass fluxes showed a large variation. A high variability of 239+240Pu fluxes occurred in very short period of time (1/2 day). The large fluxes of 239+240Pu might be attributed to episodic lateral transport of particles that flow down the continental slope with the nepheloid layer which was considered to be significant for 239+240Pu transport on the continental slope in the East China Sea.

  4. Two modes of Weddell Sea Bottom Water Production: continental margin gravity currents and open ocean convection, which wins and when?

    NASA Astrophysics Data System (ADS)

    Gordon, Arnold L.

    2014-05-01

    There are 2 processes by which Southern Ocean surface waters may reach into the deep ocean: gravity currents over the continental slope and convection within the open ocean. In February 1977 the Islas Orcadas found clear evidence of the latter process, when it observed the remnants of a convective 'chimney' near Maud Rise, in the Weddell Sea. This observation was key in linking deep ocean convective processes to the "Great Weddell Polynya", a 250,000-km2 area virtually free of sea ice during the winters of 1974-1976. Further research from AWI research vessel Polarstern revealed the vulnerability of central Weddell gyre, particularly in the Maud Rise region, to breakdown of water column stability. Climate forcing related to prolonged period of negative or neutral Southern Annular Mode, as was the situation before the "Great Weddell Polynya", acts to reduce freshwater input to the Weddell Sea and thus serves as a trigger for open ocean convection and Polynya development. Similar condition may be occurred during the1912 Deutschland expedition into the Weddell Sea. We speculate that during glacial times, with sea level 130 m lower and the glacial ice extended to shelf break, with the Southern Annular Mode very much in a prolonged negative mode, open ocean production of Weddell Sea Bottom Water [and perhaps that of the Ross Sea too] was prevalent. The bottom water product during the open ocean convection mode may be expected to be saltier than that produced along the continental margin, which would incorporate glacial melt.

  5. Quantifying the distribution and abundance of rippled scour depressions (RSDs) on the seafloor of California's continental margin using autoclassfication models

    NASA Astrophysics Data System (ADS)

    Davis, A. C.; Mueller, C.; Hallenbeck, T.; Carrillo, J.; Gomez, J.

    2010-12-01

    The California Seafloor Mapping Project (CSMP) is a cooperative initiative creating a comprehensive, high-resolution (2-5m) coastal/marine geologic and habitat base map for all of California’s State waters (Mean high water to three nautical miles). This massive dataset covering > 8500 sq. km of coastal seafloor is enabling researchers to study patterns and distribution of near shore habitats and geomorphology on a scale never before possible. Data from CSMP reveal the presence of rippled scour depressions (RSD) as the most prominent features on the continental shelf. These features are found worldwide and are characterized as depressions (.4m-1m) of coarse grain sediment and long period sand waves surrounded by a fine sediment plateau. While previous studies have described the geomorphologies of RSDs and speculated on their origin, this is the first regional study describing their patterns of abundance and distribution on a scale of 1000s of km. The purpose of this study is to use auto classification methods to quantify the spatial extent and distribution of three benthic habitats (rock, sediment, RSD) within the state waters of California. Using CSMP acoustic backscatter imagery and derived bathymetric products (rugosity, bathymetric position index, and slope), we developed a habitat classification model in ArcGIS to assign benthic habitat into one of these three classes. These results will then be used to quantify and characterize spatial patterns in the distribution and abundance of these habitats along the California continental margin.

  6. 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.

  7. A model of the methane cycle, permafrost, and hydrology of the Siberian continental margin

    DOE PAGES

    Archer, D.

    2015-05-21

    A two-dimensional model of a sediment column, with Darcy fluid flow, biological and thermal methane production, and permafrost and methane hydrate formation, is subjected to glacial–interglacial cycles in sea level, alternately exposing the continental shelf to the cold atmosphere during glacial times and immersing it in the ocean in interglacial times. The glacial cycles are followed by a "long-tail" 100 kyr warming due to fossil fuel combustion. The salinity of the sediment column in the interior of the shelf can be decreased by hydrological forcing to depths well below sea level when the sediment is exposed to the atmosphere. Theremore » is no analogous advective seawater-injecting mechanism upon resubmergence, only slower diffusive mechanisms. This hydrological ratchet is consistent with the existence of freshwater beneath the sea floor on continental shelves around the world, left over from the last glacial period. The salt content of the sediment column affects the relative proportions of the solid and fluid H2O-containing phases, but in the permafrost zone the salinity in the pore fluid brine is a function of temperature only, controlled by equilibrium with ice. Ice can tolerate a higher salinity in the pore fluid than methane hydrate can at low pressure and temperature, excluding methane hydrate from thermodynamic stability in the permafrost zon