Sample records for western continental margin

  1. 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)

  2. Sediment erosion thresholds and characteristics of resuspended aggregates on the western European continental margin

    Microsoft Academic Search

    Laurenz Thomsen; Giselher Gust

    2000-01-01

    Sediment erosion thresholds and characteristics of resuspended aggregates were experimentally determined on cores from the western European continental margin with a ship-borne erosion chamber augmented by image analysis. Bottom sediments (212–4940m water depth) had a thin surface layer that was resuspended as aggregates (median diameter 125–2403?m) under critical shear velocities (u?c) of 0.4–1.2cm s?1. For the underlying sediments, eroded as

  3. 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).

  4. Late paleozoic orogeny in the northwestern Gondwana continental margin, western Argentina and Chile

    NASA Astrophysics Data System (ADS)

    Bonorino, G. González

    During Early Carboniferous-Early Permian time, the segment of the northwestern Gondwana continental margin (western Argentina and Chile) between present-day latitudes 27°S and 50°S was structurally organized into a marginal orogen and a foreland rising into cratonic highlands. Orogenesis and magmatism were governed by subduction of an ancestral Pacific plate under Gondwana. During the Late Devonian-Early Carboniferous, former slope-rise deposits were thickened to form the orogen and were thrust slightly onto the foreland; parts of the orogen emerged above sea level. Following this early diastrophic phase, tectonic uplift in the orogen was regionally checked by erosion until the late Early Permian when uplift rates overcame erosion. The development of this orogen is peculiar in that throughout much of this time: a) the orogenic bett was partly occupied by marine areas; b) there was no reversal of provenance, so that foreland basins were continuously fed from the interior highlands; and c) the orogenic belt did not advance further onto the foreland. Thus the orogen stabilized in position and relief. Magmatism was intense during the period of orogenic stability and was dominated by plutonism; there are no remnants of a well developed volcanic arc. Reconstructions involving Late Devonian-Early Carboniferous collision of crustal masses against Gondwana and a seaward jump of the subduction trench are not entirely substantiated by available data. An alternative suggests that from the Late Devonian to the Early Permian the trench maintained approximately the same position seaward of present-day coastal exposures.

  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. Inner shelf circulation on the north-western continental margin of the Gulf of Cadiz

    NASA Astrophysics Data System (ADS)

    Relvas, Paulo; Garel, Erwan; Drago, Teresa; Taborda, Rui

    2014-05-01

    It has been recurrently attested that the inner shelf, generally defined as the region where the surface and bottom boundary layers overlap, shows a circulation pattern somewhat independent of the outer shelf. The forcing set that governs the outer shelf dynamics is considerable modified at the inner shelf. Over the outer shelf alongshore flows are geostrophic, as a result of the balance between cross-shore pressure gradients, induced by the wind driven upwelling mechanism, and the Coriolis forces. Contrarily, over the inner shelf alongshore flows may be directly driven by the existence of any alongshore pressure gradient, because Coriolis force cannot balance the pressure gradient force due to the presence of the coastline. This is one reason why warm counter-currents flowing over the inner shelf attached to the coast, inshore of the cold water previously upwelled, are recurrently observed in the Eastern Boundary Upwelling Systems, as revealed by sea surface temperature (SST) satellite imagery. The feature is particularly evident along the western part of the northern continental margin of the Gulf of Cadiz, mainly during the decay of coastal upwelling events. Along this coast the wind driven upwelling prevails roughly from April till October, but it is weak and intermittent. We can define two regimes, upwelling and non-upwelling, that temporally alternate in dominating the alongshore circulation. During non-upwelling conditions, the warm near shore counter-current propagates eastwards, sometimes turning northward at the Cape São Vicente, the western limit of the northern margin of the Gulf of Cadiz. In the present research the flow patterns are studied for two periods, May-Dec 2008 and Aug-Dec 2010. Contemporaneous data of the flow velocity and temperature from a bottom-mounted ADCP deployed on the 25 meters isobaths, along with wind data from the ASCAT scatterometer and from a buoy in the Gulf of Cadiz, remotely sensed SST data and sea level data are analysed. Current reversals associated with relaxation/reversal of upwelling favourable winds and raised water temperatures were identified and the vertical structure of the flow inversion was analysed. Results indicate time lagged temporal agreement between patterns of local wind forcing and current velocities, with propagation of the counter-current during relaxations or reversals of upwelling favourable winds. However, results from a momentum analysis indicate large influence from other factors on this propagation. The magnitude of the external influence calculated by the momentum analysis corresponds to forcing by an alongshore pressure gradient involving sea level differences of around 5 cm over a distance of 100 km in barotropic conditions. This agrees well with earlier sea level measurements, supporting that alongshore pressure gradients are the main factor governing the alongshore circulation along the eastern part of the northern margin of the Gulf of Cadiz.

  7. Three dimensional lithospheric structure of the western continental margin of India constrained from gravity modelling: implication for tectonic evolution

    NASA Astrophysics Data System (ADS)

    Arora, K.; Tiwari, V. M.; Singh, B.; Mishra, D. C.; Grevemeyer, I.

    2012-07-01

    This paper describes a 3-D lithospheric density model of the Western Continental Margin of India (WCMI) based on forward modelling of gravity data derived from satellite altimetry over the ocean and surface measurements on the Indian peninsula. The model covers the north-eastern Arabian Sea and the western part of the Indian Peninsula and incorporates constraints from a wide variety of geophysical and geological information. Salient features of the density model include: (1) the Moho depth varying from 13 km below the oceanic crust to 46 km below the continental interior; (2) the lithosphere-asthenosphere boundary (LAB) located at depths between 70 km in the southwestern corner (under oceanic crust) and about 165 km below the continental region; (3) thickening of the crust under the Chagos-Laccadive and Laxmi Ridges and (4) a revised definition of the continent-ocean boundary. The 3-D density structure of the region enables us to propose an evolutionary model of the WCMI that revisits earlier views of passive rifting. The first stage of continental-scale rifting of Madagascar from India at about 90 Ma is marked by relatively small amounts of magmatism. A second episode of rifting and large-scale magmatism was possibly initiated around 70 Ma with the opening of the Gop Rift. Subsequently at around 68 Ma, the drifting away of the Seychelles and formation of the Laxmi Ridge was a consequence of the down-faulting of the northern margin. During this second episode of rifting, the northern part of the WCMI witnessed massive volcanism attributed to interaction with the Reunion hotspot at around 65 Ma. Subsequent stretching of the transitional crust between about 65 and 62 Ma formed the Laxmi Basin, the southward extension of the failed Gop Rift. As the interaction between plume and lithosphere continued, the Chagos-Laccadive Ridge was emplaced on the edge of the nascent oceanic crust/rifted continental margin in the south as the Indian Plate was moving northwards.

  8. 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.

  9. 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.

  10. Benthic foraminiferal distributions on the Uruguayan continental margin (South-western Atlantic) and controlling environmental factors

    NASA Astrophysics Data System (ADS)

    de Mello, Camila; Burone, Leticia; Ortega, Leonardo; Franco-Fraguas, Paula; Lahuerta, Nuria; Mahiques, Michel; Marin, Yamandu

    2014-12-01

    The data on benthic foraminifera analysed from 110 box-core samples collected on the Uruguayan continental margin (outer shelf and upper and middle slope, between 36.54-34.64°S and 51.66-53.71°W) were used to evaluate the distribution of the benthic foraminiferal fauna and its relationship with selected abiotic parameters. Primary productivity (PP) and the organic flux (Jz) reaching the sea floor were also estimated for comparison with the foraminiferal distributions. The study area was characterised by elevated PP and Jz values, mainly in the southernmost region, which were associated with thermohaline fronts due to the presence of the Subtropical Shelf Front. The dominant identified taxa were Rhumblerella sepetibaensis (this is the first study recording the ecology of this species) and the opportunistic species Epistominella exigua, Bulimina spp. and Reophax fusiformis, which displayed maximal densities at the southernmost stations, concurrent with the highest Jz levels. The dominant species and vertical foraminiferal distributions responded to the different environmental conditions impacting the area (e.g. PP, grain size, nutrient content), which were most likely related to the hydrodynamic conditions. Hydrodynamic conditions cause differences in PP according to the locations of water masses and their fronts at the surface, according to the depth and current intensity; they determined energetic differences across the benthic environment, controlling organic matter sedimentation as well as grain size, which influenced oxygen availability within sediments.

  11. Slab rollback and continental break-up in a convergent setting - seismic structure of passive continental margins in the Western Mediterranean Sea

    NASA Astrophysics Data System (ADS)

    Grevemeyer, I.; Ranero, C. R.; Pesquer, D.; Gallart, J.

    2012-04-01

    The Western Mediterranean Sea is a natural laboratory to study the processes of continental extension and rifting in a convergent setting. Gravitational collapse due to tectonic thickening of continental lithosphere and the rollback of a subducting oceanic slab during the latest phases of consumption of the Tethys ocean have led to rapid Neogene extension in an area characterized by a constant convergence of the African and European Plates since Cretaceous time, rifting Spain/Balearic Islands from Algeria, causing passive continental margins on both sides of the Western Mediterranean Basin. However, little is known about the crustal and upper mantle structure of much of the area, including the Algerian-Balearic Basin and the Spanish/Balearic margin. Here we present results from three onshore/offshore seismic refraction and wide-angle lines surveying the Spanish passive continental margin to the south of the town of Alicante and the southern margin of the Balearic promontory. The data were acquired with the German research vessel METEOR in September of 2006 and most ocean-bottom and land stations provided seismic offsets of 40 to 80 km, including wide-angle reflections from the crust/mantle boundary zone (seismic Moho). Profile P03 surveyed the southeastern margin of Spain near the town of Alicante. Profile P04 and P05 approached the Balearic promontory from the south at Ibiza and Mallorca, respectively. All lines extend roughly 100 km into the Algerian-Balearic basin, yielding for the first time constraints on the nature of the crust forming the seafloor between Spain and Algeria. Crust in the centre of the Algerian-Balearic basin is 5-6 km thick and the seismic velocity structure mimics normal oceanic crust, though velocities in the lower crust tended being 0.2-0.4 km/s lower than in typical Atlantic or Pacific crust. Seismic Moho in the Algerian basin occurs at ~11 km below sea level, reaching >24 km under SE Spain and Mallorca. The continent-ocean transition (COT) seems to be rather sharp and we did not find any evidence for velocities intermediate between lower crustal and upper mantle rocks, representing magmatic under plating or lower crustal intrusions as typical for volcanic margins. Further, we did not find any evidence for exposed and serpentinized mantle in the transition zone. Stretching, however, affected a much broader area at the Ibiza segment, while off Alicante and off Mallorca seismic data support a narrow zone affected by stretching. A profound feature in the data is that the earliest crust formed after break-up is 2-4 km thicker with lower crustal velocities 0.2-0.4 km/s lower than near the centre of the Algerian basin. This feature is similar to crust formed in the southern Lau Basin and is believed to be caused by the entrainment of hydrous melts from the adjacent arc during back-arc spreading.

  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. 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.

  14. Characterization of a stratigraphically constrained gas hydrate system along the western continental margin of Svalbard from ocean bottom seismometer data

    NASA Astrophysics Data System (ADS)

    Chabert, Anne; Minshull, Tim A.; Westbrook, Graham K.; Berndt, Christian; Thatcher, Kate E.; Sarkar, Sudipta

    2011-12-01

    The ongoing warming of bottom water in the Arctic region is anticipated to destabilize some of the gas hydrate present in shallow seafloor sediment, potentially causing the release of methane from dissociating hydrate into the ocean and the atmosphere. Ocean-bottom seismometer (OBS) experiments were conducted along the continental margin of western Svalbard to quantify the amount of methane present as hydrate or gas beneath the seabed. P- and S-wave velocities were modeled for five sites along the continental margin, using ray-trace forward modeling. Two southern sites were located in the vicinity of a 30 km long zone where methane gas bubbles escaping from the seafloor were observed during the cruise. The three remaining sites were located along an E-W orientated line in the north of the margin. At the deepest northern site, Vp anomalies indicate the presence of hydrate in the sediment immediately overlying a zone containing free gas up to 100-m thick. The acoustic impedance contrast between the two zones forms a bottom-simulating reflector (BSR) at approximately 195 m below the seabed. The two other sites within the gas hydrate stability zone (GHSZ) do not show the clear presence of a BSR or of gas hydrate. However, anomalously low Vp, indicating the presence of free gas, was modeled for both sites. The hydrate content was estimated from Vp and Vs, using effective-medium theory. At the deepest northern site, modeling suggests a pore-space hydrate concentration of 7-12%, if hydrate forms as part of a connected framework, and about 22% if it is pore-filling. At the two other northern sites, located between the deepest site and the landward limit of the GHSZ, we suggest that hydrate is present in the sediment as inclusions. Hydrate may be present in small quantities at these two sites (4-5%) of the pore space. The variation in lithology for the three sites indicated by high-resolution seismic profiles may control the distribution, concentration and formation of hydrate and free gas.

  15. 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; Canning, Jason C.

    2014-03-01

    The Messinian Salinity Crisis (MSC) resulted from a significant multi-phase drop and subsequent reflooding of the Mediterranean Sea from 5.96 to 5.33 Ma. Well-developed drainage networks, characterized by step-like profiles and abrasion platforms, are associated to this event. The Ebro Continental Margin (Western Mediterranean) presents an additional complexity since the capture of the drainage of the adjacent subaerial Ebro Basin took place sometime prior to the Messinian stage. Using 3D seismic reflection data, this work provides new insights into the origin of the step-like profile of the Messinian erosional surface (MES) and timing of the capture of the subaerial Ebro Basin. The results obtained indicate 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. The mature development attained by the Messinian Ebro River network during the MSC corroborates that the capture of the Ebro Basin occurred prior to the MSC. The 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. Fluvial deposits are outstandingly preserved on the main valleys of the MES. Therefore, the step-like profile of the MES was not created during Zanclean inundation, but during the latest stages of the main Messinian sea-level fall and lowstand.

  16. 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 Precambrian rocks of the Western Anti-Atlas and Cretaceous to Neogene sedimentary rocks from the Northern Tarfaya-Laâyoune-Dakhla Basin. Thermochronological data and t-T path modelling indicate exhumation in the Western Anti-Atlas between Upper Carboniferous and Lower Cretaceous, whereby 9 km of Precambrian-Palaeozoic overburden has been eroded.

  17. Atlantic Rifts and Continental Margins

    NASA Astrophysics Data System (ADS)

    Mohriak, Webster; Taiwani, Manik

    In compiling this volume, we have aimed to develop and enhance our current understanding of the structural evolution and sedimentation processes along divergent continental margins. To counteract the unfortunate situation of a lack of modern seismic and potential fields data on circum-Atlantic passive margins in the literature, we have linked new data from oil companies with that of research institutions. To update the data offered in most volumes used as reference works for the study of continental margins, now upwards of 20 years old, and to remedy the dispersal of important, more recent contributions in specialized journals, we present a current synthesis of materials in one volume focused on the deeper geology of the sedimentary basins along continental margins.

  18. 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).

  19. Crustal structure and magnetic lineation along two geo-traverses from western continental margin of India to Eastern Somali Basin, NW Indian Ocean

    NASA Astrophysics Data System (ADS)

    Chaubey, A. K.; Anshu, A.; Sreejith, K.; Pandey, A.

    2012-12-01

    Shipborne gravity and magnetic data along two parallel geo-traverses spanning from western continental margin of India to off Seychelles are used to delineate crustal structure and magnetic pattern of major structural features - western continental margin of India, Laxmi Basin, Laxmi Ridge, Arabian Basin, slow spreading Carlsberg Ridge and Eastern Somali Basin. The seismically constrained gravity models along the geo-traverses suggest considerable variation in crustal thickness - about 38 km on continental shelf of western India to about 4 km of the Eastern Somali Basin. The Eastern Somali Basin is characterized by thin oceanic crustal thickness (~3 to 4 km) as compared to its conjugate Arabian Basin where thickness varies from 5 to 6 km. The magnetic anomalies along the geo-traverse reveal three distinct zones: (i) a zone of relative high frequency short wavelength younger anomalies over the axial parts of the Carlsberg Ridge, (ii) a zone of well developed Early Tertiary magnetic anomalies in both the Arabian and Eastern Somali basins, and (iii) relative magnetic quiet zone, between the above two zones, representing a hiatus in spreading. Based on the results, we present a comparative analysis of crustal configuration and magnetic pattern of major structural features of the study area and discuss its tectonic evolution.

  20. 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 controls on Ebro margin growth. Deforestation of the drainage basin more than doubled the normal Holocene sediment supply, and construction of dams then reduced the supply by 95%. This reduction of the past 50 years has caused erosion of the delta and contamination of bottom sediment because normal Holocene sediment discharge is not available to prograde the delta or help dilute pollutants. ?? 1990.

  1. 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.

  2. Erosion of continental margins in the Western Mediterranean due to sea-level stagnancy during the Messinian Salinity Crisis

    NASA Astrophysics Data System (ADS)

    Just, Janna; Hübscher, Christian; Betzler, Christian; Lüdmann, Thomas; Reicherter, Klaus

    2011-02-01

    High-resolution multi-channel seismic data from continental slopes with minor sediment input off southwest Mallorca Island, the Bay of Oran (Algeria) and the Alboran Ridge reveal evidence that the Messinian erosional surface is terraced at an almost constant depth interval between 320 and 380 m below present-day sea level. It is proposed that these several hundred- to 2,000-m-wide terraces were eroded contemporaneously and essentially at the same depth. Present-day differences in these depths result from subsidence or uplift in the individual realms. The terraces are thought to have evolved during one or multiple periods of sea-level stagnancy in the Western Mediterranean Basin. According to several published scenarios, a single or multiple periods of relative sea-level stillstand occurred during the Messinian desiccation event, generally known as the Messinian Salinity Crisis. Some authors suggest that the stagnancy started during the refilling phase of the Mediterranean basins. When the rising sea level reached the height of the Sicily Sill, the water spilled over this swell into the eastern basin. The stagnancy persisted until sea level in the eastern basin caught up with the western Mediterranean water level. Other authors assigned periods of sea-level stagnancy to drawdown phases, when inflowing waters from the Atlantic kept the western sea level constant at the depth of the Sicily Sill. Our findings corroborate all those Messinian sea-level reconstructions, forwarding that a single or multiple sea-level stagnancies at the depth of the Sicily Sill lasted long enough to significantly erode the upper slope. Our data also have implications for the ongoing debate of the palaeo-depth of the Sicily Sill. Since the Mallorcan plateau experienced the least vertical movement, the observed terrace depth of 380 m there is inferred to be close to the Messinian depth of this swell.

  3. Structure And Evolution Of The Southwestern Continental Margin Of India

    NASA Astrophysics Data System (ADS)

    Chaubey, A. K.; Ajay, K. K.; Krishna, K. S.; Rao, D. G.; Sar, D.

    2008-12-01

    The present western continental margin of India has been mainly carved due to (i) breakup of India from Madagascar during mid-Cretaceous, (ii) massive Deccan volcanism at Chron 29R (65.6 - 64.8 Ma) followed by rifting between India and Seychelles during the early-Tertiary. The Deccan volcanic event had profound impact on the Indian mainland and adjoining western continental margin, and concealed the pre-existing geology and structure of the margin. As a result, structural features, nature of the crust and margin classification (volcanic/ non-volcanic/ transform) are not well understood. In this study we focus on the southwestern continental margin of India to determine structural pattern and nature of crust, and to provide an updated tectonic framework of the southwest continental margin of India. In order to achieve the objectives we analyzed bathymetry, magnetic, gravity (ship-borne and satellite) and seismic (reflection and refraction) profiles covering important geological provinces of the southwest continental margin of India, which include (i) narrow shelf, (ii) Kerala-Konkan Basin, (iii) Shelf Margin Basin, (iv) the Prathap Ridge complex, (v) the Laccadive Ridge and, (vi) the deep Arabian Basin beyond the western flank of the Laccadive Ridge. Analyses of the data reveal rotated faulted blocks, half graben structures and magmatism in the Shelf Margin Basin, 15 to 19 km crustal thickness of the Laccadive Ridge, and presence of Seaward Dipping Reflectors (SDRs) beneath sedimentary column along the western flank of the Laccadive Ridge. The Shelf Margin Basin exhibits crustal thinning associated with extension, consistent with a rift-type setting. The results suggest that the Laccadive Ridge, bounded by the fault scarps, comprised of stretched and thinned continental crust loaded with volcanic. The inferred SDRs along western flank of the Laccadive Ridge and significant change in magnetic and gravity anomaly signatures across the margin suggest that continent ocean transition lies immediate west of the Laccadive Ridge. Based on the results of the study, we therefore suggest that the entire western continental margin of India is a volcanic rifted margin, developed during rifting between southwest India and Madagascar in the mid-Cretaceous and later, rifting between northwest India and Seychelles during the early-Tertiary.

  4. High-resolution seismic-reflection interpretations of some sediment deposits, Antarctic continental margin: Focus on the western Ross Sea

    USGS Publications Warehouse

    Karl, Herman A.

    1989-01-01

    High-resolution seismic-reflection data have been used to a varying degree by geoscientists to interpret the history of marine sediment accumulations around Antarctica. Reconnaissance analysis of 1-, 3.5-, and 12-kHz data collected by the U.S. Geological Survey in the western Ross Sea has led to the identification of eight echo-character facies and six microtopographic facies in the sediment deposits that overlie the Ross Sea unconformity. Three depositional facies regions, each characterized by a particular assemblage of echo-character type and microtopographic facies, have been identified on the continental shelf. These suites of acoustic facies are the result of specific depositional processes that control type and accumulation of sediment in a region. Evidence of glacial processes and products is uncommon in regions 1 and 2, but is abundant in region 3. McMurdo Sound, region 1, is characterized by a monospecific set of acoustic facies. This unique assemblage probably represents turbidity current deposition in the western part of the basin. Most of the seafloor in region 2, from about latitude 77??S to 75??S, is deeper than 600 m below sealevel. The microtopographic facies and echo-character facies observed on the lower slopes and basin floor there reflect the thin deposits of pelagic sediments that have accumulated in the low-energy conditions that are typical of deep-water environments. In shallower water near the boundary with region 3, the signature of the acoustic facies is different from that in deeper water and probably indicates higher energy conditions or, perhaps, ice-related processes. Thick deposits of tills emplaced by lodgement during the most recent advance of the West Antarctic Ice Sheet are common from latitude 75??S to the northern boundary of the study area just south of Coulman Island (region 3). The signature of microtopographic facies in this region reflects the relief of the base of the grounded ice sheet prior to decoupling from the seafloor. Current winnowing and scour of shallow parts of the seafloor inhibits sediment deposition and maintains the irregular, hummocky relief that characterizes much of the region. Seafloor relief of this type in other polar areas could indicate the former presence of grounded ice. ?? 1989.

  5. Continental Rifting in the Western Ross Sea

    NASA Astrophysics Data System (ADS)

    Davey, F. J.; Cande, S. C.; Stock, J. M.

    2014-12-01

    The Ross Sea forms the north western end of the West Antarctic Rift system, a major continental rift that lies across the western part of Antarctica, and results from rifting during the break-up of Gondwana starting some 180 m.y. ago. In the Ross Sea region, extension comprised a regional thinning associated with the break-up of New Zealand and Australia from Antarctica, and a more focussed extension during the Cenozoic. The last episode of extension, largely from 46 Ma - 25 Ma, formed the Victoria Land Basin (VLB) in the southwest, the Northern Basin (offset from the VLB) in north western Ross Sea and the Adare Basin in the deep ocean to the north. Marine magnetic anomalies associated with the seafloor spreading that formed the Adare Basin, extend continuously onto the continental shelf of the Northern Basin, suggesting that the basin is underlain by oceanic crust, consistent with high gravity anomalies across the continental shelf edge. No seismic data exist for the deeper crust of Northern Basin. The amplitude and gradient of gravity anomalies across the basin limit the depth, density contrast and thickness of the dense body underlying it and are consistent with oceanic crust with steep margins at a depth of about 8 - 10 km. The VLB in contrast shows an extensional thinning of the continental crust. The three basins thus show seafloor spreading in the north, continental rifting at the continental margin, and continental thinning in the south. The pole of rotation for the extension lies to the south of the VLB so the rate of extension increases to the north. In addition, the azimuth of the axis of spreading changes relative to the extension direction, presumably as it followed pre-existing zones of weakness in the Antarctic lithosphere. This leads to a much larger degree of strike slip motion in the Northern Basin rifting that may be a significant factor in the development there of narrow rifting of the continental lithosphere.

  6. Geological interpretation of a low-backscatter anomaly found on the New Jersey continental margin

    E-print Network

    New Hampshire, University of

    Geological interpretation of a low-backscatter anomaly found on the New Jersey continental margin.J.W. Piper Keywords: U.S. mid-Atlantic continental margin continental slope multibeam backscatter submarine canyons Western Boundary Undercurrent Chesapeake Drift An enigmatic low-backscatter, acoustic anomaly

  7. Denudation history of the continental margin of western peninsular India since the early Mesozoic - reconciling apatite fission-track data with geomorphology

    NASA Astrophysics Data System (ADS)

    Gunnell, Y.; Gallagher, K.; Carter, A.; Widdowson, M.; Hurford, A. J.

    2003-10-01

    A comprehensive apatite fission-track (AFT) study of the passive margin of western peninsular India between 12° and 16°N is used to reconstruct the denudation chronology of the continental hinterland. In common with other rifted margins, the morphology is characterised by a low-lying coastal plateau separated from an elevated inland plateau by an erosionally controlled escarpment (Western Ghats). We modelled the fission track data using the commonly adopted annealing algorithm of Laslett et al. [Chem. Geol. 65 (1987) 1-13]. Using the default parametrisation (i.e. an initial track length of 16.3 ?m), it was found that during the Mesozoic denudation rates remained extremely low, increasing sporadically when erosion peaked at 130 Ma (rifting with Antarctica) and 80 Ma (rifting with Madagascar). Denudation rates rose considerably during the Cenozoic, reaching maxima of ca 120 m/Myr. Such values are, however, considered as major overestimates and the effects of the Seychelles rifting at 65 Ma remain suspiciously unrecorded. We explored the consequences of changing the initial track length in this model to a value of 14.5 ?m. In practice, this reduces the rapid Cenozoic denudation artefact, model peak rates during the Mesozoic are much more variable, and during the Cenozoic reach values an order of magnitude lower than with the original initial track length. The response to the Seychelles rifting event is almost immediate. Just as previous model calibrations in AFT analysis have been relatively empirical, this revised approach does not provide insights into the physical mechanisms of low-temperature annealing. However, it is shown to agree much better with independently established geomorphic, cosmogenic, stratigraphic and tectono-magmatic evidence in this and other stable shield regions in terms of both the timing and the magnitude of geological events, and the geomorphic response of the landscape to them.

  8. A critical review of tectonic processes at continental margin orogens

    Microsoft Academic Search

    Steven H. Edelman

    1991-01-01

    A ``cordilleran-type'' orogen has previously been defined as contractional deformation of a continental margin due to subcontinental subduction. The plate tectonic setting of cordilleran-type orogenesis is fundamentally different from that of ``collisional'' orogenesis. Intracontinental orogens such as the Himalayas and Alps form by collision between a continental margin and a continental margin subduction zone. Orogens at continental margins may form

  9. The basins on the Argentine continental margin

    SciTech Connect

    Urien, C.M. [Buenos Aires Technological Institute Petroleum School, Buenos Aires (Argentina)

    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.

  10. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

    Levander, Alan; Bezada, Maximiliano; Niu, Fenglin; Palomeras, Imma; Thurner, Sally; Humphreys, Eugene; Carbonell, Ramon; Gallart, Josep; Schmitz, Michael; Miller, Meghan

    2015-04-01

    Subduction recycling of oceanic lithosphere, a central theme of plate tectonics, is relatively well understood, whereas recycling continental lithosphere is more difficult to recognize, and appears far more complicated. Delamination and localized convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we describe another 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. Seismic images from recent dense broadband seismograph arrays in northeastern South America (SA) and in the western Mediterranean show higher than expected volumes of positive anomalies identified as the subducted Atlantic slab under northeastern SA, and the Alboran slab beneath the Gibraltar arc region (GA). The positive anomalies lie under and are aligned with the continental margins at depths greater than 200 km. Closer to the surface we find that the continental margin lithospheric mantle is significantly thinner than expected beneath the orogens adjacent to the subduction zones. The thinner than expected lithosphere extends inland as far as the edges of nearby cratonic cores. 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, as around GA, inferred by results from active and passive seismic experiments. Viscous removal of continental margin lithosphere creates lithosphere-asthenosphere boundary (LAB) topography which can give rise to secondary downwellings under the continental interior far inland from the subduction zone: We image one under SA and we infer that one or more have occurred in the past under the western Mediterranean. The process of subduction-driven continental margin lithosphere removal reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these two subduction zones.

  11. The fate of subducted continental margins: Two-stage exhumation of the high-pressure to ultrahigh-pressure Western Gneiss Region,

    E-print Network

    Hacker, Bradley R.

    that ultrahigh-pressure (UHP) to high-pressure (HP) rocks across the Western Gneiss Region ponded at the Moho that large-scale, collisional UHP terranes routinely stall at the continental Moho where diminishing body be formed from UHP terranes that were arrested at the Moho and never underwent their final stage

  12. Paleogeographic settings and tectonic deformations of the Barents Sea continental margin in the Cenozoic

    Microsoft Academic Search

    E. V. Shipilov; V. A. Tyuremnov; V. N. Glaznev; V. A. Golubev

    2006-01-01

    The Barents Sea continental margin (hereafter, Barents margin) differs from other passive margins by the most extensive shelf, the giant thickness of sedimentary rocks in basins and troughs, and its unique tectonic position. The outer, almost rectangular promontory of the Barents margin juts out into its deepwater western and northern framing (Fig. 1), identified as the Norwegian?Greenland and Eurasia basins,

  13. The Continental Margin is a Key Source of Iron to the HNLC North Pacific Ocean

    E-print Network

    The Continental Margin is a Key Source of Iron to the HNLC North Pacific Ocean Phoebe J. Lam1 concentrations in the upper 500m of the Western Subarctic Pacific, an iron-limited High Nutrient Low Chlorophyll a key source of bioavailable Fe to the HNLC North Pacific. Keywords: iron, continental margin, HNLC 1

  14. Continental margin tectonics - Forearc processes

    SciTech Connect

    Lundberg, N.; Reed, D.L. (USAF, Geophysics Laboratory, Hanscom AFB, MA (United States))

    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.

  15. 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.

  16. Seismic modelling of gas hydrate and free gas in sediments, from ocean-bottom seismometer data along the continental margin of Western Svalbard

    NASA Astrophysics Data System (ADS)

    Chabert, A.; Minshull, T. A.; Westbrook, G. K.; Berndt, C.

    2009-12-01

    Over the next decades, the shallow parts of continental margins in the Arctic are likely to experience warming of bottom-water. It is, therefore, important to evaluate how methane hydrate beneath the seabed in these margins will react to future increases in bottom-water temperature and whether release of methane from hydrate will have an impact on climate. As part of the International Polar Year initiative, a multidisciplinary marine expedition was carried out in August-September 2008 along the continental margin west of Svalbard in the Arctic Ocean. One of its objectives was to determine the extent of the gas hydrate stability zone (GHSZ) along the continental slope and to quantify the amount of methane present as hydrate or gas beneath the seabed, using seismic techniques. Thirteen ocean-bottom seismometers (OBS) were deployed at 5 representative sites along and across the continental margin. High frequency airguns (GI guns) were fired at 5-s intervals and the data were recorded at a high sampling rate (1 kHz) in the OBS. The records show clear P-wave reflections at short offsets, as well as refracted arrivals at larger offsets, from depths up to 2 km below the seabed. The sub-seabed variation of P-wave velocity was modelled for three sites located above and below the upper limit of GHSZ, using ray-traced forward modelling. The velocity model for the deepest site (~1250 m deep) below the upper limit of the GHSZ shows a zone about 120 metres below the seabed with a greater velocity (1.8 km/s) than expected for terrigenous sediment. This high velocity zone lies above a lower velocity zone (1.55 km/s) and the acoustic contrast between the two zones forms a bottom simulator reflector (BSR) at approximately 170 m below the seabed. The BSR marks the boundary between sediments containing gas hydrate above and free gas below. The velocity model from the shallow site (~480 m deep), below the upper limit of the GHSZ, indicates the presence of a low velocity zone (1.60 km/s) about 160 m below the seabed, which is interpreted as a free-gas reservoir. However, no clear bottom simulator reflector (BSR) has been recognised at this site. The velocity model for the third site (~350 m deep), above the upper limit of the GHSZ indicates high velocity, interpreted as glacial over-compacted sediment, with no clear evidence for free gas. Our preliminary results indicate the presence of gas hydrates at the deepest site of the experiment, but have so far failed to produce unequivocal evidence for gas hydrate at the shallower site where hydrate would be most sensitive to bottom water changes. However, the fact that the presence of free gas can be inferred at two different sites nearly 100 km apart, suggests that free gas is widespread along the Svalbard continental margin and that hydrate may exist close to the upper limit of the GHSZ. Ongoing modelling of the P- and S-wave velocities should provide a better understanding of the sub-seabed distribution of the seismic properties from which the amount of hydrate present in the sediment can be estimated.

  17. 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 emplacement of magmatic intrusions in both the deep basin and the continental margin. Based on the crustal structure, we propose that the eastern Algerian basin opened during the southeastward migration of the European forearc before the collision, along a NW-SE elongated spreading centre that ran perpendicular to the subduction trend. Such an atypical geometry is explained by the diverging directions of the subduction rollback during the backarc opening: eastward for the Corsica-Sardinia block, and southward for the Kabylian blocks. This geometry of the forearc can be interpreted as the surface expression of a slab tear at depth, which is responsible for atypical magmatism in the overlying backarc oceanic basin.

  18. Crustal Structure and Miocene Geodynamic Evolution of the Easternmost Algerian Back-Arc Basin and Continental Margin (western Mediterranean Sea) from Wide-Angle and Multichannel Seismics

    NASA Astrophysics Data System (ADS)

    Bouyahiaoui, B.; Sage, F.; Abtout, A.; Klingelhoefer, F.; Yelles-Chaouche, A.; Schenini, L.; Philippe, S.; Marok, A.; Deverchere, J.; Arab, M.; Galve, A.; Collot, J. Y.

    2014-12-01

    We use new wide-angle seismic and multichannel seismic reflection data (SPIRAL cruise, 2009) and additional geophysical data to study the crustal structure of the eastern Algerian back-arc basin that was born during the Miocene Tethys subduction rollback, before the collision of the European forearc (Kabylian blocks) with the northern African continent ~16-18 Ma ago. In the deep basin, the P-wave velocity model images a thin, 5.5-km-thick oceanic crust with velocity ranging between 4.8 km/s and 7.1 km/s. It is composed of two layers, with a velocity-gradient higher in the upper layer than in the lower one. S-wave modeling indicates a Poisson ratio of 0.28 in the lower crust, supporting a dominant gabbroic composition. Below the continental edge, we define two segments: (1) West of 7°45'E, a typical continental crust with P-wave velocities between 5.2 km/s and 7.0 km/s depicts a gradual seaward thinning of ~15 km over an ~35-km distance characterizing a stretched margin resulting from back-arc extension; (2) East of 7°45'E, the continental crust is a few kilometers thinner, arguing for a variable crustal thickness along the forearc before collision. Based on the deep basin crustal structure and magnetic anomalies, we propose that the eastern Algerian basin opened during the southeastward migration of the European forearc along a NW-SE elongated spreading center that ran perpendicular to the subduction trend, which is an unusual configuration for back-arc opening. Such an atypical geometry of the accretion and the oceanic crustal structure may result from a slab tear at depth related to diverging directions of the subduction rollback (and hence the forearc blocks) during back-arc opening: eastward for the Corsica-Sardinia block, and southward for the Kabylian blocks. The variable thickness of the continental crust along the margin may result from the crustal stretching along the forearc that accommodated the opening of the oceanic domain north of it.

  19. Morphology and time\\/depth distribution of Uvigerina peregrina: continental slope, Eastern Margin, United States

    Microsoft Academic Search

    W. L. Balsam; A. C. Gary; D. F. Williams

    1986-01-01

    Uvigerina is one of the most important genera of benthic foraminifera for biostratigraphic and paleoenvironmental investigations of continental margin sediments from bathyal water depths. The authors present morphological and distributional evidence for distinct subpopulations within Uvigerina peregrina from the continental slope of the eastern US. A detailed record of U. peregrina distribution in the western North Atlantic over water depths

  20. Southern African continental margin: Dynamic processes of a transform margin

    NASA Astrophysics Data System (ADS)

    Parsiegla, N.; Stankiewicz, J.; Gohl, K.; Ryberg, T.; Uenzelmann-Neben, G.

    2009-03-01

    Dynamic processes at sheared margins associated with the formation of sedimentary basins and marginal ridges are poorly understood. The southern African margin provides an excellent opportunity to investigate the deep crustal structure of a transform margin and to characterize processes acting at these margins by studying the Agulhas-Falkland Fracture Zone, the Outeniqua Basin, and the Diaz Marginal Ridge. To do this, we present the results of the combined seismic land-sea experiments of the Agulhas-Karoo Geoscience Transect. Detailed velocity-depth models show crustal thicknesses varying from ˜42 km beneath the Cape Fold Belt to ˜28 km beneath the shelf. The Agulhas-Falkland Fracture Zone is embedded in a 50 km wide transitional zone between continental and oceanic crust. The oceanic crust farther south exhibits relatively low average crustal velocities (˜6.0 km/s), which can possibly be attributed to transform-ridge intersection processes and the thermal effects of the adjacent continental crust during its formation. Crustal stretching factors derived from the velocity-depth models imply that extension in the Outeniqua Basin acted on regional as well as more local scales. We highlight evidence for two episodes of crustal stretching. The first, with a stretching factor ? of 1.6, is interpreted to have influenced the entire Outeniqua Basin. The stresses possibly originated from the beginning breakup between Africa and Antarctica (˜169-155 Ma). The second episode can be associated with a transtensional component of the shear motion along the Agulhas-Falkland Transform from ˜136 Ma. This episode caused additional crustal stretching with ? = 1.3 and is established to only have affected the southern parts of the basin. Crustal velocities directly beneath the Outeniqua Basin are consistent with the interpretation of Cape Supergroup rocks underlying most parts of the basin and the Diaz Marginal Ridge. We propose that the formation of this ridge can be either attributed to a transpressional episode along the Agulhas-Falkland Transform or, more likely, to thermal uplift accompanying the passage of a spreading ridge to the south.

  1. 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…

  2. Evidence for transform margin evolution from the Ivory Coast-Ghana continental margin

    Microsoft Academic Search

    Jean Mascle; Emmanuel Blarez

    1987-01-01

    Results are presented from a recent study (Blarez and Mascle, 1986) of the northern Gulf of Guinea margins, particularly off the eastern Ivory Coast and Ghana, where the continental margin is one of the best-preserved examples of an extinct transform margin. The observations support a four-stage model for transform margin evolution. Tectonically active transform contacts, first between normal continental crusts

  3. Southern African continental margin: Dynamic processes of a transform margin

    Microsoft Academic Search

    N. Parsiegla; J. Stankiewicz; K. Gohl; T. Ryberg; G. Uenzelmann-Neben

    2009-01-01

    Dynamic processes at sheared margins associated with the formation of sedimentary basins and marginal ridges are poorly understood. The southern African margin provides an excellent opportunity to investigate the deep crustal structure of a transform margin and to characterize processes acting at these margins by studying the Agulhas-Falkland Fracture Zone, the Outeniqua Basin, and the Diaz Marginal Ridge. To do

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

    Microsoft Academic Search

    Christophe Basile

    2010-01-01

    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

  5. Continental transformrift interaction adjacent to a continental margin: The Levant case study

    E-print Network

    Lyakhovsky, Vladimir

    2013 Accepted 5 August 2014 Available online 16 September 2014 Keywords: Dead Sea Transform Continental of plate-bounding faults such as the Dead Sea Transform (DST) are strongly relat- ed to interactionsContinental transform­rift interaction adjacent to a continental margin: The Levant case study Amit

  6. Marginal Basins of the Western Pacific: An Overview (Invited)

    NASA Astrophysics Data System (ADS)

    Lewis, S. D.

    2010-12-01

    Marginal ocean basins of the Western Pacific fall into several distinct categories based on their mechanisms of formation: 1) Back-arc basins that likely formed associated with island arc rifting and seafloor spreading, such as the Mariana Trough and the Shikoku-Parece Vela Basin. 2) Ocean basins that formed through processes related to continental rifting, such as the South China Sea and the Sea of Japan, and 3) marginal basins of ambiguous or uncertain origin such as the West Philippine Basin and the Celebes Sea, that may have originated by back-arc spreading, by entrapment of a fragment of an older ocean basin, or by rifting from the Southeast Asian continental margin. The origin of the largest of the western Pacific marginal basins, the West Philippine Basin, has been suggested to be either by back-arc rifting or through entrapment from a major ocean basin. Based on the geological/tectonic characteristics of ocean basin margins, the compositions and ages of oceanic basement, and the stratigraphic and paleomagnetic data from the marginal basins, only the Japan Sea, the South China Sea, the Andaman Sea, and perhaps the Celebes Sea are likely candidates for being formed in relation to the uplift off the Tibetan Plateau. While the age of formation of the West Philippine Basin (55-39 Ma) is roughly synchronous with Tibetan Plateau uplift, the lack of continental crustal blocks or continentally-derived sediment argues against an Asian continental margin origin for the West Philippine Basin. In addition, the evidence for active plate boundaries in the Philippines and along the western side of the West Philippine Basin at this time suggests that it was isolated from Asian deformation perhaps related to Tibetan Plateau uplift. The Celebes Sea is of similar age to the West Philippine Basin, and paleomagnetic data from basement rocks indicates that has moved less than 150 relative to Southeast Asia since it was formed. This evidence, coupled with N-MORB samples recovered from Celebes Sea basement, suggests either an origin as a trapped garment of a larger ocean basin or a basin rifted from the Southeast Asian margin. Those Southeast Asian marginal basins most likely related to the uplift of the Tibetan Plateau are the South China Sea and the Japan Sea. Both basins originated well after the beginning off Tibetan Plateau uplift (18-14 Ma for the Japan Sea and 32-16 Ma for the South China Sea), their proximity to the Asian continental margin, the presence of rifted continental crustal blocks on their margins, and, in the case of the South China Sea, the relationships between continental margin transform faulting and seafloor spreading along the southwest sub-basin, suggests a kinematic relationship between basin formation and continental crustal deformation related to the uplift of the Tibetan Plateau.

  7. Geotechnical characterization of sediments from Hydrate Ridge, Cascadia Continental Margin

    E-print Network

    Tan, Brian B. (Brian Bautista), 1979-

    2004-01-01

    Eight whole core sediment samples were obtained from ODP Site 1244, Hydrate Ridge, Cascadia Continental Margin with the goal of understanding the stress history, consolidation behavior and strength characteristics of the ...

  8. Subduction-Driven Recycling of Continental Margin Lithosphere

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

    While subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, recycling continental lithosphere appears far more complicated and is 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 describe another 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. Seismic images from recent dense broadband arrays on opposite sides of the Atlantic show higher than expected volumes of positive anomalies identified as the subducted Atlantic (ATL) slab under northeastern South America (SA), and the Alboran slab beneath the Gibraltar arc region (GA). The positive anomalies lie under and are aligned with the continental margins at depths greater than 200 km. Closer to the surface we find that the continental margin lithospheric mantle is significantly thinner than expected beneath the orogens adjacent to the subduction zones. Thinner than expected lithosphere extends inland as far as the edges of nearby cratonic cores. 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, as around GA, inferred by results from active and passive seismic experiments. Secondary downwellings develop under the continental interior inland from the subduction zone: We image one under SA and one or more in the past were likely under GA. The process of subduction-driven continental margin lithosphere removal reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these two subduction zones.

  9. Neogene deposition, provenance, and exhumation along a tectonically active, glaciated continental margin, Yakataga and Redwood Formations, southern Alaska syntaxis

    Microsoft Academic Search

    John W Witmer

    2009-01-01

    The southern Alaskan syntaxis is characterized by high coastal topography, extremely high rates of erosion and exhumation, and tidewater glaciation. The eastern margin of the syntaxis is defined by strike-slip tectonics and a wide continental shelf with a distinct shelf-slope break. The western margin of the syntaxis is defined by convergence and an onshore thrust belt that trends offshore into

  10. Electrical resistivity structure under the western Cosmonauts Sea at the continental margin of East Antarctica inferred via a marine magnetotelluric experiment

    NASA Astrophysics Data System (ADS)

    Matsuno, Tetsuo; Nogi, Yoshifumi; Seama, Nobukazu

    2015-06-01

    The western Cosmonauts Sea, off the coast of East Antarctica, was a site of rifting of the Gondwana supercontinent and subsequent early seafloor spreading. To improve our understanding of the breakup of Gondwana, we conducted a marine magnetotelluric experiment to determine the electrical resistivity structure within the uppermost several hundred kilometers beneath the western Cosmonauts Sea. Magnetotelluric response functions at two sites, obtained after considering possible influences of non-plane magnetic field sources, suggest that these responses include distortions by topographic variations and conductive anomalies around the observation sites. Three-dimensional forward modeling confirmed that these distortions due to topographic variations and a thin (?2-km thick) conductive layer immediately under the sites (mostly sediments) are severe. Furthermore, three-dimensional forward modeling to investigate the resistivity structure at deeper depths revealed an upper resistive layer (?300 ?-m), with a thickness of <100 km, and an underlying conductive half-space (?10 ?-m). The upper resistive layer and the underlying conductive structure most likely represent dry and water/melt-rich oceanic upper mantle, respectively. The upper resistive layer may be thinner than anticipated under the old seafloor of the study area (likely >90 Ma), and may suggest a conductive anomaly in the upper mantle produced by mantle convection and/or upwelling.

  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. PMID:25391963

  12. 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 to be emphasized. There is not only one type of transform margins, but as for divergent margins huge changes from one margin to another in both structure and evolution. Multiple types have to be evidenced together with the various parameters that should control the variability. As for divergent margins, special attention should be paid to conjugated transform margins as a tool to assess symmetrical / asymmetrical processes in the oceanic opening. Attention should also be focused on the three-dimensional structure of the intersections between transform and divergent margins, such as the one where the giant oil field Jubilee was recently discovered. There is almost no 3D data available in these area, and their structures still have to be described. An other key point to develop is the mechanical behavior of the lithosphere in and in the vicinity of transform margins. The classical behaviors (isostasy, elastic flexure) have be tested extensively. The localization of the deformation by the transform fault, and the coupling of continental and oceanic lithosphere across the transform fault have to be adressed to understand the evolution of these margins. Again as for divergent margins, new concepts are needed to explain the variations in the post-rift and post-transform subsidence, that can not always be explained by classical subsidence models. But the most remarkable advance in our understanding of transform margins may be related to the study of interactions between the lithosphere and adjacent envelops : deep interactions with the mantle, as underplating, tectonic erosion, or possible lateral crustal flow ; surficial interactions between structural evolution, erosion and sedimentation processes in transform margins may affect the topography and bathymetry, thus the oceanic circulation with possible effects on regional and global climate.

  13. 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

  14. Understanding the thermal evolution of deep-water continental margins

    Microsoft Academic Search

    Nicky White; Mark Thompson; Tony Barwise

    2003-01-01

    Areas of exploration for new hydrocarbons are changing as the hydrocarbon industry seeks new resources for economic and political reasons. Attention has turned from easily accessible onshore regions such as the Middle East to offshore continental shelves. Over the past ten years, there has been a marked shift towards deep-water continental margins (500-2,500 m below sea level). In these more

  15. Investigating the Asymmetry of Northern North Atlantic Volcanic Continental Margins

    NASA Astrophysics Data System (ADS)

    Smith, L. K.; White, R. S.

    2005-12-01

    The Hatton Bank continental margin is a typical example of the volcanic margins present in the northern North Atlantic where voluminous magmatism occurred at the time of continental break-up. The upper crust exhibits characteristically large volumes of extruded lava imaged as seaward-dipping reflectors, which have in the past proved problematic for seismic imaging of the deeper crustal structure. The integrated Seismic Imaging and Modelling of Margins (iSIMM) project recorded profiles in 2002 designed to map specifically the poorly constrained lower crustal structure in this region. 29 four-component ocean-bottom seismometers (OBS) were deployed along a strike line over the region of thickest extrusive and intruded igneous material; 53 OBS were deployed through the mid-point of the strike line, along a dip line extending from the stretched continental crust of the Hatton Basin into the fully oceanic crust of the Iceland Basin. We present a new seismic velocity model for the Hatton Bank volcanic continental margin. Joint wide-angle refraction and reflection tomography was used to determine the seismic velocity structure and depth to Moho across the continent-ocean transition (COT) in both the dip and strike directions. The lower crust beneath the margin exhibits elevated crustal velocities in the range of 7.0-7.4 km/s, which represent new igneous material added to the lower crust in this region at the time of continental break-up. The iSIMM survey is located close to the site of a previous survey carried out in 1986. A comparison of the 1986 results and the results from the 1996 SIGMA survey carried out on the conjugate southeast Greenland margin show a marked asymmetry in crustal structure: the Greenland margin appears to have a COT stretched over ~ 150 km compared to the narrower COT of the Hatton Bank margin, which extends for only ~ 50 km. The new iSIMM survey results provide a refined estimate of crustal structure of the Hatton Bank margin and improved constraints on the intrusive and extrusive igneous components produced during continental break-up in the early Tertiary. Using the new results from the Hatton Bank margin we present an updated comparison of the conjugate margins and investigate the degree of asymmetry in this region of the northern North Atlantic.

  16. Fluxes of dissolved organic carbon from California continental margin sediments

    Microsoft Academic Search

    DAVID J. BURDIGE; W ILLIAM M. BERELSON; KENNETH H. COALE; James McManus; KENNETH S. JOHNSON

    1999-01-01

    Fluxes of dissolved organic carbon (DOC) from marine sediments represent a poorly constrained component of the oceanic carbon cycle that may affect the concentration and composition of DOC in the ocean. Here we report the first in situ measurements of DOC fluxes from continental margin sediments (water depths ranging from 95 to 3,700 m), and compare these fluxes with measured

  17. 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.

  18. 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. PMID:17759946

  19. A deep seismic transect from Hovgård Ridge to northwestern Svalbard across the continental-ocean transition: A sheared margin study

    Microsoft Academic Search

    O. Ritzmann; W. Jokat; W. Czuba; A. Guterch; R. Mjelde; Y. Nishimura

    2004-01-01

    New seismic refraction data were collected across the western Svalbard continental margin off Kongsfjorden (NyÅlesund) during the cruise leg ARK15\\/2 of RV Polarstern. The use of onshore and offshore seismic receivers and a dense air-gun shot pattern provide a detailed view of the velocity structure of Svalbard's continental interior, the continent-ocean transition, and oceanic crust related to the northern Knipovich

  20. 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.

  1. Evolution of an Early Proterozoic Continental Margin: The Coronation Geosyncline and Associated Aulacogens of the Northwestern Canadian Shield

    Microsoft Academic Search

    P. Hoffman

    1973-01-01

    The Coronation geosyncline developed in the early Proterozoic along the western margin of a continental platform (the Slave Province) of Archaean rocks older than 2300 Ma, and culminated between 1725 and 1855 Ma ago with the emplacement of a pair of batholiths (the Bear Province). The evolution of the geosyncline has a strong family resemblance to Phanerozoic geosynclines believed to

  2. Molybdenum and uranium geochemistry in continental margin sediments: Paleoproxy potential

    NASA Astrophysics Data System (ADS)

    McManus, James; Berelson, William M.; Severmann, Silke; Poulson, Rebecca L.; Hammond, Douglas E.; Klinkhammer, Gary P.; Holm, Chris

    2006-09-01

    We measured solid-phase Mo and U concentrations in marine sediments from the California, Mexico, Peru, and Chile margins to ascertain the behavior of Mo and U during early diagenesis in continental margin settings. At sites along the California, Mexico, and Peru margins where there are estimates of mass accumulation rates, authigenic U accumulation rates range from ˜0-50 nmol m -2 day -1. At the California and Mexico margin sites Mo accumulation rates range from 0 to 134 nmol m -2 day -1 whereas at the Peru margin site rates may be as high as 550 nmol m -2 day -1. We observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon (C org). In the case of Mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial. For most of the data presented in this manuscript, continental margin sediments have a Mo:C org accumulation rate ratio of ˜20 ?mol mol -1. This value is significantly lower, however, than those reported for anoxic basins ˜100 ?mol mol -1, but is consistent with reported Mo:C org rain ratios from Mexico margin sediment traps. In contrast to Mo, there appears to be a single U:C org burial ratio of ˜5 ?mol mol -1, which includes a range of environments from anoxic basins and open ocean sites. We interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings. However, we note that any relationship between metal accumulation and organic carbon could be indirect. In the particular case of Mo, based on what is known about Mo geochemistry from the literature, it is possible that Mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study, and that the observed association between Mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial. Using the observed relationships between Mo and C org burial as well as constraints from the Mo isotope budget we estimate that continental margin sediments are an important sink for Mo. The magnitude of this sink (˜0.4 × 10 8 mol y -1, or larger) may be as much as one quarter of the oceanic removal term, and is likely to be larger than the modern anoxic basin sink.

  3. How a Curvilinear Continental Margin Influences Its Subsidence History

    NASA Astrophysics Data System (ADS)

    Sacek, V.; Ussami, N.

    2012-12-01

    Current one-dimensional (1D) and two-dimensional (2D) thermo-mechanical models successfully explain the first-order vertical motions of sedimentary basins created by lithospheric extension. However, the modeling of second-order effects such as extra-subsidence, non-monotonic-subsidence or protracted-subsidence still remains controversial. One aspect that has not been fully considered in the current models is that the rifting direction leading to the continental break-up does not always follow a straight line, which demands a three-dimensional (3D) approach. The aim of this work is to demonstrate the importance of using a 3D model that takes into account the curvature of rifting along the margin and theoretically predicts some of the second-order subsidence observations. Our results indicate that concave oceanward margins tend to subside faster than convex ones. This differential subsidence of the margin is a result of the combined effect of lateral thermal conduction, small-scale mantle (or edge driven) convection and the curvature of the rifting. We have used the finite element code CITCOM (Moresi & Gurnis, 1996; Zhong et al., 2000) to construct 3D numerical models of the mantle convection and its effect on the surface evolution. We observed that the differential subsidence along a curved margin is dependent on the viscosity structure of the mantle: for an asthenospheric viscosity of 5×1020 Pa.s the differential subsidence can reach more than 700 m assuming a sediment filled basin; however, for low asthenospheric viscosity (<2×1019 Pa.s), the pattern of faster subsidence of the concave segment of the margin is not observed and the basin subsidence presents no clear correlation with its geometry. As an application of this 3D conceptual model for curved margin, we analysed the stratigraphic evolution of the Santos Basin, offshore Southeastern Brazil, and we propose that the variations in the subsidence history along the margin can be explained by its 3D geometry and the dynamical evolution of the mantle. We conclude that the incorporation of the third dimension in the study of the subsidence history of divergent margins may also provide information on the physical properties of the mantle. Keywords: 3D numerical model; Basin subsidence; Continental margin. References: L. Moresi, M. Gurnis, Constraints on the lateral strength of slabs from three-dimensional dynamic flow models, Earth Planet. Sci. Lett. 138 (1996) 15-28. S. Zhong, M.T. Zuber, L. Moresi, M. Gurnis, The role of temperature-dependent viscosity and surface plates in spherical shell models of mantle convection, J. Geophys. Res. 105 (2000) 11063-11082.

  4. 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).

  5. 180 Ma continental drainage divide in northeastern Australia: Role of passive margin tectonics

    Microsoft Academic Search

    Jonathan Nott; Stephen Horton

    2000-01-01

    The stratigraphy and sedimentology of Jurassic to Tertiary sediments within the Laura and Carpentaria basins in northeastern Australia show that the continental drainage divide here predates the onset of continental margin formation and has remained stationary since the Middle Jurassic. This result is contrary to models of the evolution of highlands adjacent to passive continental margins that state that divide

  6. Rifted continental margins: The case for depth-dependent extension

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Beaumont, Christopher

    2014-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Huismans, Ritske S.; Beaumont, Christopher

    2015-04-01

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

  8. Summer surface circulation along the Gascoyne continental shelf, Western Australia

    Microsoft Academic Search

    Mun Woo; Charitha Pattiaratchi; William Schroeder

    2006-01-01

    The Gascoyne continental shelf is located along the north-central coastline of Western Australia between latitudes 21° and 28°S. This paper presents CTD and ADCP data collected in November 2000 together with concurrent wind and satellite imagery, to provide a description of the summer surface circulation pattern along the Gascoyne continental shelf and slope. It is shown that the region comprises

  9. 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 diel rhythm of behaviour. Species evolved in a photically variable environment where intra- and inter-specific interactions change along with the community structure over 24 h. Accordingly, the regulation of their biology through a biological clock may be the major evolutionary constraint that is responsible for their reported bathymetric distributions. In this review, our aim is to propose a series of innovative guidelines for a discussion of the modulation of behavioural rhythms of adult decapod crustaceans, focusing on the deep waters of the continental margin areas of the Mediterranean as a paradigm for other marine zones of the world. PMID:20959158

  10. 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.

  11. The Mesozoic south Atlantic Ocean and evolution of its continental margins

    Microsoft Academic Search

    Philip D. Rabinowitz; John LaBrecque

    1979-01-01

    Gravity and magnetic anomalies bordering the continental margins of the southern South Atlantic Ocean are compared, in detail, on conjugate sides of the ridge crest, and a model for the boundary between oceanic and continental basement is given. The area of study includes the predominantly sheared margins of the Agulhas-Falkland fracture zone and the rifted margins of Argentina and southern

  12. The Conjugate Volcanic Continental Margins of the South Atlantic

    NASA Astrophysics Data System (ADS)

    Neben, S.; Franke, D.; Schreckenberger, B.; Temmler, T.

    2005-12-01

    In November-December 2004 the German Federal Institute for Geosciences and Natural Resources (BGR) undertook a marine geophysical cruise along the South Atlantic continental margin off Uruguay and Southern Argentina. In total more than 3,700 km of combined multi-channel reflection seismic (MCS), magnetic and gravity and an additional of 1,300 km with only magnetic and gravity were acquired. In combination with the extensive MCS (>24,000 km) data set from the Argentine and South African margins we are able to interpret conjugate traverses over almost exact conjugate parts of the South Atlantic. Continental break-up and initial sea-floor spreading were accompanied by large-scale magmatism and volcanism as manifested in the seismic data by a huge wedge of seaward dipping reflectors (SDRS). The interpretation of eight refraction/wide-angle seismic traverses show the SDRS are underlain by a distinct and wide spread high-velocity (values 7.1-7.3 km/s) lower crustal body. From the reflection seismic data we tentatively suggest that the (volcanic) features (associated with break-up volcanism?) are mirror images on both margins. The emplacement of the deeply buried, 60-120km wide SDRS was probably episodic as documented by at least three superimposed SDRS units. A similar distinct partitioning of the SDRS into three to four individual wedges is present on the South African margin. From the data a detailed correlation of timing of these individual pulses is intended. Furthermore, it was possible to estimate the amount of extruded material during the phases of emplacement. The seismic data from the Argentine margin document the presence of a narrow, north-east striking half-graben system beneath the upper continental slope. This buried half-graben system developed during the final stage of rifting in the Early Cretaceous, whereas the east-west trending Colorado-Basin probably formed earlier. Further to the south the new data from the Falkland Plateau show widespread small-scale half grabens.

  13. 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.

  14. From collision to extension: The roots of the southeastern continental margin of Brazil

    NASA Astrophysics Data System (ADS)

    Heilbron, Monica; Mohriak, Webster U.; Valeriano, Cláudio M.; Milani, Edison J.; Almeida, Julio; Tupinambá, Miguel

    The South Atlantic Meso-Cenozoic continental margins are located in regions characterized by a long-lived history of Proterozoic extension, structural inversion and compressional remobilization of basement and supracrustal rocks. The roots of the present-day southeastern Brazilian continental margin (e.g. Santos and Campos basins) are associated with terranes directly affected by the Brasiliano orogenic collage. This event was responsible for the Ribeira fold belt, which is characterized by compressional, metamorphic and magmatic episodes from Late Precambrian to the earliest Paleozoic. The initial phases of subsidence of the intracratonic Paraná basin, located west of the Ribeira fold belt, correspond to early Paleozoic siliciclastic rocks deposited in depocenters that were probably controlled by Brasiliano fabrics. The basin-forming stress fields may be related to the lithospheric convergence between Panthalassa oceanic crust and cratonic blocks of western Gondwana. The last phase of subsidence in the Paraná basin is marked by Late Jurassic/Early Cretaceous tholeiitic continental flood basalts. These basalts heralded the breakup of Gondwana. They were also deposited on the Precambrian basement offshore, and are believed to be part of the rift succession. The breakup of western Gondwana and the onset of a new phase of plate divergence in the South Atlantic were marked by thick wedges of seaward-dipping reflectors near the incipient oceanic-ridge spreading center. Subsequently, a few episodes of intraplate tectonic and magmatic activity are also possibly related to compressional stresses resulting from subduction in the Andean margin and ridge push in the mid-Atlantic spreading ridge.

  15. Macrobenthic community structure over the continental margin of Crete (South Aegean Sea, NE Mediterranean)

    NASA Astrophysics Data System (ADS)

    Tselepides, Anastasios; Papadopoulou, Konstantia-N.; Podaras, Dimitris; Plaiti, Wanda; Koutsoubas, Drosos

    2000-08-01

    Macrobenthic faunal composition, abundance, biomass and diversity together with a suite of sedimentary environmental parameters were investigated on a seasonal basis in order to determine factors regulating faunal distribution over the oligotrophic continental margin of the island of Crete (South Aegean Sea, North Eastern Mediterranean). Macrofaunal species composition was similar to that of the western Mediterranean and the neighboring Atlantic having several common dominant species. Mean benthic biomass, abundance and diversity decreased with depth, with a major transition zone occurring at 540 m, beyond which values declined sharply. At comparable depths biomass and abundance values were considerably lower to those found in the Atlantic, high-lighting the extreme oligotrophy of the area. The continental margin of Crete was characterised by a high diversity upper continental shelf environment (dominated by surface deposit feeding polychaetes) and a very low diversity slope and deep-basin environment (dominated by carnivorous and filter feeding polychaetes). Classification and ordination analyses revealed the existence of four principle clusters divided by a faunal boundary between 200 and 540 m, as well as beyond 940 m depth. Significant correlations between macrofauna and sediment parameters led to the conclusion that besides depth, food availability (as manifested by the concentration of chloroplastic pigments) is the principle regulating factor in the system. Such being the case, the prevailing hydrographic features that structure the pelagic food web and are directly responsible for the propagation of organic matter to the benthos also affect its community structure.

  16. Exploring the Continental Margin of Israel: “Telepresence” at Work

    NASA Astrophysics Data System (ADS)

    Coleman, Dwight F.; Austin, James A., Jr.; Ben-Avraham, Zvi; Ballard, Robert D.

    2011-03-01

    A multidisciplinary team of American and Israeli scientists conducted ocean exploration with a “telepresence” component offshore Israel in September 2010 on board the new E/V Nautilus, which is a reincarnation of the former East German R/V Alexander von Humboldt. This was the first comprehensive geological and biological exploration of the Israel continental margin using deep submergence vehicle systems. Diverse seafloor environments in water depths between 500 and 1300 meters were sampled and imaged using two remotely operated vehicle (ROV) systems, Hercules and Argus. The ROV dives within three areas (Figure 1) investigated high-priority acoustic targets representing geological, biological, or archaeological features as identified by the onboard scientific team. During the dives, biological and geological samples and more than 100 kilometers of high-resolution side-scan sonar data were collected.

  17. Wintertime phytoplankton bloom in the subarctic Pacific supported by continental margin iron

    E-print Network

    Bishop, James K.B.

    Wintertime phytoplankton bloom in the subarctic Pacific supported by continental margin iron Phoebe-chlorophyll (HNLC) subarctic North Pacific Ocean, a region that is thought to be iron-limited. Here we provide a lateral supply of particulate iron from the continental margin off the Aleutian Islands in the winter

  18. 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 metamorphic basement of the Faroes and Hatton Bank areas respectively. Sedimentary units forming a low velocity zone beneath the flood basalts across the Faroe Ridge and into the Faroe-Shetland Trough are hypothesised to represent Paleocene sedimentary rock emplaced as transient thermal uplift across the nascent rift zone led to increased weathering and clastic sediment transport from Greenland.

  19. The Chukchi Borderland: a Sediment-starved Rifted Continental Margin

    NASA Astrophysics Data System (ADS)

    Hutchinson, D. R.; Houseknecht, D.; Mosher, D. C.; Hart, P. E.; Jackson, H. R.; Lebedeva-Ivanova, N. N.; Shimeld, J.; Chian, D.

    2013-12-01

    The origin and geologic structure of the Chukchi Borderland region, approximately 650 by 400 km in size, has been the subject of speculation since the earliest ice island research groups discovered its existence more than 60 years ago. Multichannel seismic reflection and refraction data acquired between 2007 and 2011, together with legacy seismic data show fragments of high-standing basement (continental) horsts. The structure is draped with less than a kilometer of sediment. Between the high-standing blocks are deep grabens with locally tilted but mostly flat-lying deposits generally only 1-2 km thick. Northwind Escarpment, along the eastern boundary of the Borderland, is a 600-km-long fault adjacent to the deeply subsided and hyper-extended crust of the Canada Basin to the east. The long, linear, sub-parallel orientation of the major structures (including Northwind Escarpment) is consistent with transtensional deformation of the Borderland. The general paucity of thick sediments indicates a sediment-starved environment. Both the North Chukchi Basin on the west and an unnamed deeply buried valley east on the Beaufort margin provide sediment-routing conduits through which sediment by-passed the Borderland throughout much of the Cretaceous history of the growing Brooks Range to the south. Canada Basin deposits also show strata thicken towards the southwest, suggesting sediment influx via the deeply buried valley on the Beaufort margin. On the northeastern side of the Canada Basin, the region is underlain by horst and graben structures with orientations similar to the Chukchi Borderland, but the intervening valleys are filled with as much as two km of sediment and the entire feature is buried beneath another 2 km of post-rift sediment. The similarity of structural styles on both sides of the Canada Basin suggests that this style of transtensional rifting could have been widespread during the early extension of this part of the Arctic and perhaps the Chukchi Borderland and parts of the Canadian Arctic Archipelago margin were conjugates prior to rifting. Seismic data also show that volcanism associated with the High-Arctic Large Igneous Province to the north has intruded or flowed over the northern parts of the Borderland. The Chukchi Borderland, because of its lack of sedimentary cover, offers a unique window into the early rifting history of the Canada Basin and the transition from rifted to hyper-extended continental crust.

  20. Alpine inversion of the North African margin and delamination of its continental lithosphere

    NASA Astrophysics Data System (ADS)

    Roure, FrançOis; Casero, Piero; Addoum, Belkacem

    2012-06-01

    This paper aims at summarizing the current extent and architecture of the former Mesozoic passive margin of North Africa from North Algeria in the west up to the Ionian-Calabrian arc and adjacent Mediterranean Ridge in the east. Despite that most paleogeographic models consider that the Eastern Mediterranean Basin as a whole is still underlain by remnants of the Permo-Triassic or a younger Cretaceous Tethyan-Mesogean ocean, the strong similarities documented here in structural styles and timing of inversion between the Saharan Atlas, Sicilian Channel and the Ionian abyssal plain evidence that this portion of the Eastern Mediterranean Basin still belongs to the distal portion of the North African continental margin. A rim of Tethyan ophiolitic units can be also traced more or less continuously from Turkey and Cyprus in the east, in onshore Crete, in the Pindos in Greece and Mirdita in Albania, as well as in the Western Alps, Corsica and the Southern Apennines in the west, supporting the hypothesis that both the Apulia/Adriatic domain and the Eastern Mediterranean Basin still belong to the former southern continental margin of the Tethys. Because there is no clear evidence of crustal-scale fault offsetting the Moho, but more likely a continuous yet folded Moho extending between the foreland and the hinterland beneath the Mediterranean arcs, we propose here a new model of delamination of the continental lithosphere for the Apennines and the Aegean arcs. In this model, only the mantle lithosphere of Apulia and the Eastern Mediterranean is still locally subducted and recycled in the asthenosphere, most if not all the northern portion of the African crust and coeval Moho being currently decoupled from its former, currently delaminated and subducted mantle lithosphere.

  1. The development of the continental margin of eastern North America—conjugate continental margin to West Africa

    NASA Astrophysics Data System (ADS)

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

    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 Cretaceous deeper water chalk and shale.

  2. 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.

  3. 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.

  4. The challenge of computer graphics in continental western Europe

    Microsoft Academic Search

    RICHARD A. GUEDJ

    1974-01-01

    Computer graphics applications, software, and hard-ware research, are examined and evaluated in Continental Western Europe. Growth has been slow. The reasons are the high cost of hardware and the complexity of the required software. The past reveals a rather dim picture. Mechanical engineering applications have shown some promising success. Electrical engineering applications have been effective mainly for PCB and IC

  5. The challenge of computer graphics in Continental Western Europe

    Microsoft Academic Search

    R. A. Guedj

    1974-01-01

    Computer graphics applications, software, and hardware research, are examined and evaluated in Continental Western Europe. Growth has been slow. The reasons are the high cost of hardware and the complexity of the required software. The past reveals a rather dim picture. Mechanical engineering applications have shown some promising success. Electrical engineering applications have been effective mainly for PCB and IC

  6. Abbot Ice Shelf, the Amundsen Sea Continental Margin and the Southern Boundary of the Bellingshausen Plate Seaward of West Antarctica

    NASA Astrophysics Data System (ADS)

    Cochran, J. R.; Tinto, K. J.; Bell, R. E.

    2014-12-01

    The Abbot Ice Shelf extends 450 km along the coast of West Antarctica between 103°W and 89°W and straddles the boundary between the Bellingshausen Sea continental margin, which overlies a former subduction zone, and Amundsen Sea rifted continental margin. Inversion of NASA Operation IceBridge airborne gravity data for sub-ice bathymetry shows that the western part of the ice shelf, as well as Cosgrove Ice Shelf to the south, are underlain by a series of east-west trending rift basins. The eastern boundary of the rifted terrain coincides with the eastern boundary of rifting between Antarctica and Zealandia and the rifts formed during the early stages of this rifting. Extension in these rifts is minor as rifting quickly jumped north of Thurston Island. The southern boundary of the Cosgrove Rift is aligned with the southern boundary of a sedimentary basin under the Amundsen Embayment continental shelf to the west, also formed by Antarctica-Zealandia rifting. The shelf basin has an extension factor, ?, of 1.5 - 1.7 with 80 -100 km of extension occurring in an area now ~250 km wide. Following this extension early in the rifting process, rifting centered to the north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf has been tectonically quiescent and has primarily been shaped though subsidence, sedimentation and the passage of the West Antarctic Ice Sheet back and forth across it. The former Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to its incorporation into the Antarctic Plate at ~62 Ma. During the latter part of its existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence between the Bellingshausen and Antarctic plates east of 102°W. Seismic reflection and gravity data show that this convergence is expressed by an area of intensely deformed sediments beneath the continental slope from 102°W to 95°W and by incipient subduction beneath the Bellingshausen Gravity Anomaly on the western edge of a salient of the Antarctic plate near 94°W. West of 102°W, relative motion was extensional and occurred in a diffuse zone occupied by the Marie Byrd Seamounts that are dated at 65-56 Ma and extend 800 km along the continental margin near the base of the continental rise.

  7. Eocene-Pliocene planktonic foraminifera biostratigraphy from the continental margin of the southwest Caribbean

    E-print Network

    Bermingham, Eldredge

    in the integration of other events derived from benthonic foraminifera, nannoplankton, pollenEocene-Pliocene planktonic foraminifera biostratigraphy from the continental margin@eafit.edu.co ABSTRACT: Biostratigraphy in northern Colombia has traditionally been done using foraminifera. Quantitative

  8. Reconstructing the Last Pleistocene (Late Devensian) Glaciation on the Continental Margin of Northwest Britain 

    E-print Network

    Davison, Stephen

    The continental margin in the area west of Shetland was subjected to repeated and extensive ice sheet advances during the Late Pleistocene. Seabed imagery, seismic survey and borehole core data show the Late Devensian ...

  9. The evolution of lithospheric deformation and crustal structure from continental margins to oceanic spreading centers

    E-print Network

    Behn, Mark Dietrich, 1974-

    2002-01-01

    This thesis investigates the evolution of lithospheric deformation and crustal structure from continental margins to mid-ocean ridges. The first part (Ch. 2) examines the style of segmentation along the U.S. East Coast ...

  10. Factors controlling the crustal density structure underneath active continental margins with implications for their evolution

    Microsoft Academic Search

    Andrés Tassara

    2006-01-01

    The design and interpretation of gravity-based Earth models require sufficient knowledge of the effect exerted on continental crustal density by chemical composition, pressure-temperature (PT) conditions, and water content. This need has motivated the development of a petrophysical modeling for 55 major element analyses compiled to characterize the geochemical differentiation trend of active continental margins. Equilibrium mineral assemblages and densities were

  11. Hydrology, morphology and sedimentology of the Campos continental margin, offshore Brazil

    Microsoft Academic Search

    A. R. Viana; J. C. Faugeres; R. O. Kowsmann; J. A. M. Lima; L. F. G. Caddah; J. G. Rizzo

    1998-01-01

    Slope sand deposits have accumulated from at least the Neogene to the Present on the southeastern Brazilian continental margin (Campos Basin area). This region shows sand accumulations concentrated on the upper portion and on the base of the continental slope with a middle to lower slope bypass zone. A synthesis of preliminary results, supported by recent cores, high-resolution geophysical surveys,

  12. Gravity and structure of the continental margins of southwestern Mexico and northwestern Guatemala

    Microsoft Academic Search

    Richard Couch; Stephen Woodcock

    1981-01-01

    Geophysical measurements over the eastern end of the Tehuantepec Ridge and adjacent continental margins of southern Mexico and northern Guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces. A positive free-air gravity anomaly whch extends northwestward along the outer continental shelf of Guatemala curves abruptly landward in the Gulf

  13. 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.

  14. Subducted continental margin imaged i n the Carpathians of Czechoslovakia

    Microsoft Academic Search

    Cestmir Tomek; Jeremy Hall

    1993-01-01

    Deep seismic-reflection data across the frontal part of the Carpathian arc image subducted European continental crust beneath the thick accretionary wedge of the Neogene west Carpathian arc. The upper continental crust of the lower plate is depressed with high curvature to a depth off about 25 km below the arc, but the corresponding Moho appears to be nearly horizontal. Flow

  15. Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction

    Microsoft Academic Search

    A. R. Chappell; N. J. Kusznir

    2008-01-01

    This paper describes a method for determining Moho depth, lithosphere thinning factor (gamma = 1 - 1\\/beta) and the location of the ocean-continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. The lateral density changes caused by the elevated geotherm in thinned continental

  16. Crustal structure of the northern Nova Scotia rifted continental margin (eastern Canada)

    Microsoft Academic Search

    Thomas Funck; H. Ruth Jackson; Keith E. Louden; Sonya A. Dehler; Yue Wu

    2004-01-01

    The Nova Scotia continental margin off eastern Canada marks a transition from a volcanic to a nonvolcanic style of rifting. The northern (nonvolcanic) segment of the margin was studied by a 490-km-long refraction seismic line with dense air gun shots, coincident with previous deep reflection profiles. A P wave velocity model was developed from forward and inverse modeling of the

  17. Crustal structure of the Newfoundland rifted continental margin from constrained 3-D gravity inversion

    Microsoft Academic Search

    J. Kim Welford; Jeremy Hall

    2007-01-01

    The rifting history of the Atlantic continental margin of Newfoundland is very complex and so far has been investigated at the crustal scale primarily with the use of 2-D seismic surveys. While informative, the results generated from these surveys cannot easily be interpreted in a regional sense due to their sparse sampling of the margin. A 3-D gravity inversion of

  18. 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

  19. Seismic-reflection signature of cretaceous continental breakup on the Wilkes Land margin, Antarctica

    USGS Publications Warehouse

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

    1985-01-01

    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 Mohorovic??ic?? 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.

  20. Morphology and time/depth distribution of Uvigerina peregrina: continental slope, Eastern Margin, United States

    SciTech Connect

    Balsam, W.L.; Gary, A.C.; Williams, D.F.

    1986-05-01

    Uvigerina is one of the most important genera of benthic foraminifera for biostratigraphic and paleoenvironmental investigations of continental margin sediments from bathyal water depths. The authors present morphological and distributional evidence for distinct subpopulations within Uvigerina peregrina from the continental slope of the eastern US. A detailed record of U. peregrina distribution in the western North Atlantic over water depths from 700 to 4400 m was determined for the last 25 k.y. Modern U. peregrina is found in core tops from water depths of 700-3000 m, reaching its acme between 1300 and 1800 m. During the glacial maximum (18 k.y.B.P.), this species occupied depths from at least 2800 to 4350 m. Beginning 16 k.y.B.P., Uvigerina disappeared from both the shallow and deep parts of its range, until the last remnants of this glacial-age population disappeared 7.6 k.y.B.P. from a depth of 3600 m. Fourier shape analysis shows that the modern and glacial-age U. peregrina are morphologically distinct. These observations indicate the following: (1) modern Uvigerina has occupied depths below 1800 m only during the last 4 k.y.; (2) the lack of continuity between modern and glacial-age specimens suggests that separate subpopulations or morphotypes of this species existed; (3) water mass properties in the western North Atlantic are a major control on the distribution of Uvigerina; (4) paleobathymetric reconstructions using Uvigerina should consider what morphotypes are the basis for the depth zonation.

  1. Structural highs on the western continental slope of India: Implications for regional tectonics

    NASA Astrophysics Data System (ADS)

    Mukhopadhyay, Ranadhir; Rajesh, M.; De, Sutirtha; Chakraborty, Bishwajit; Jauhari, Pratima

    2008-04-01

    Integrated geological and geophysical explorations over the last two decades along the central western continental margin of India reveal, from east to west, a terrace-like flat shelf, an advancing shelf break, a NW-SE-oriented wide shelf margin basin, the Upper Slope Ridge, the Prathap Ridge and the Laccadive Ridge. The basin on the continental slope extends between the shelf break and the eastern slope of the Laccadive Ridge, and is divided longitudinally into a relatively narrow eastern and a wide western basin by the prominent Upper Slope Ridge. Detailed multibeam swath mapping now over an area of little more than 20,300 km 2 within the shelf margin basin located on the upper continental slope now reveals the occurrence of several isolated structural highs of variable height. Four of these highs are located over the Upper Slope Ridge and a similar number on the Prathap Ridge. Of these, four highs have summit height > 1000 m, two have heights between 500 and 999 m, while the remaining two have heights less than 499 m. The Digital Terrain Model developed from the processed multibeam swath bathymetry data offers a three-dimensional perspective of these structural highs. The tectonic understanding from multibeam bathymetry findings and satellite gravity data, supported by other geophysical information, suggest that these structural highs are likely to be the peaks of the Upper Slope Ridge and the Prathap Ridge. We discuss the possible origin and mode of emplacement of these structural highs (and ridges) in the light of regional tectonics and suggest that both the Upper Slope and Prathap ridges may have been formed during the separation of India from Madagascar during the middle Cretaceous.

  2. Atmospheric methane emissions along the western Svalbard margin

    NASA Astrophysics Data System (ADS)

    Pohlman, J.; Greinert, J.; Silyakova, A.; Casso, M.; Ruppel, C. D.; Mienert, J.; Lund Myhre, C.; Bunz, S.

    2014-12-01

    Documented warming of intermediate waters by ~1oC over the past 30 years along the western Svalbard margin has been suggested as a driver of climate-change induced dissociation of marine methane hydrate. However, recent evidence suggests methane release from gas hydrate has been occurring for thousands of years near the upper limit of methane hydrate stability and that seasonal changes in bottom water temperature may be more important than longer-term warming of intermediate waters. Nevertheless, this area has been and remains an active area for researching the physical and climate controls of methane release from the seafloor, yet the amount of methane reaching the atmosphere (the ultimate climate driver) in this region is largely unknown. As part of the MOCA project led by the Norwegian Institute for Air Research (NILU), water column and atmospheric marine boundary layer methane data were collected in June 2014 aboard the R/V Helmer Hanssenduring a collaboration among CAGE at University of Troms?, NILU, GEOMAR, and the USGS. The results provide a continuous record of surface methane concentration and carbon isotope data from continental slope sites near temperature-sensitive hydrate-bearing seeps along the shelf-break and upper slope, the deep-water pockmarked gas-venting Vestnesa Ridge and a shallow water seep area within the Forlandet moraine complex at the shelf. Surface water methane and associated data used to calculate sea-air fluxes were obtained with the cavity ring-down spectrometer-based USGS Gas Analysis System (USGS-GAS). Only the shallow seep site (~90 m water depth) had appreciable methane in surface waters. We conducted an exhaustive survey of this site, mapping the full extent of the surface methane plume. To provide three-dimensional constraints, we acquired 65 vertical dissolved methane profiles to delineate the vertical and horizontal extent of the subsurface methane plume. Using these data, we assess how effectively shallow arctic seeps transmit methane to the atmosphere by determining what fraction of methane in the geochemical plume is emitted to the atmosphere. We also compare the methane mass flux from the seep site to an adjacent section of the Prins Karls Forland coastal margin to constrain the relative importance of different types of high-latitude seafloor methane emissions.

  3. Rift basins in western margin of India and their hydrocarbon prospects with special reference to Kutch basin

    SciTech Connect

    Biswas, S.K.

    1982-10-01

    The western continental margin of India can be classed as a divergent or passive margin. The western continental shelf is an extensive carbonate bank (Bombay offshore basin) passing into clastic sediments on the north and south. Three craton-margin embayed basins-Kutch, Cambay, and Narmada- in the northern part of the shelf, are filled predominantly with clastic sediments. These basins occupy grabens bounded by faults diverging seaward. The grabens were formed by three rift systems along major Precambrian tectonic trends. The rifting developed sequentially from north to south around the Saurashtra horst. Kutch basin was formed in the Early Jurassic, followed by Cambay basin in Early Cretaceous time, and the Narmada in the Late Cretaceous. It appears that these rifting events occurred at successive stages during the northward migration of the Indian plate after its break from Gondwanaland in Late Triassic or Early Jurassic. It is inferred that these rift basins opened up successively as a result of the counterclockwise drift of the Indian craton. Bombay offshore and Cambay are two major oil-producing basins in the western margin. These basins are characterized by high geothermal gradients attributed to the shallowness of the mantle in this region. Oil has not been found in KUtch basin, which is mainly an onshore Mesozoic basin. The basin basin depocenter shifted offshore at the northwestern part of the continental shelf where the shelf is wide.

  4. The structure of the Alpine distal margin: insights from the proximal margin shortening kinematics during collision, Western Alps

    NASA Astrophysics Data System (ADS)

    Bellahsen, N.; Mouthereau, F.; Lacombe, O.; Jolivet, L.

    2011-12-01

    In mountain belts, along strike variations of inherited passive margin structure can affect the collision dynamics. Particularly, the structure the distal part can potentially drive the collision evolution and thus the proximal part inversion. In most orogens however (as in the Alps for example), the initial structure of the subducted distal margin is largely hidden by subduction- and exhumation-related deformations and metamorphism. In this contribution, the structure of the European distal margin in Western Alps is discussed in the light of the collision kinematics. More precisely, we aim at constraining the structure of the "distal" Dauphinois/Helvetic that might be the western lateral termination of the Valais ocean in Switzerland separating Europe and the Brianconnais block. The Dauphinois/Helvetic zone deformation is characterized and described using several balanced cross-sections. The timing, amount of shortening and exhumation of various External Crystalline Massifs from France to Switzerland (Oisans, Mont Blanc, Aar) are compared. In the Aar and Mont Blanc massifs, the averaged amount of shortening and exhumation during Oligo-Miocene times is around three times higher than in the Oisans massif. The P,T conditions are of higher grade in the Aar and Mont Blanc and the foreland basin (the Oligo-miocene molasse) is more developed. Finally, deformation mechanisms and geometries are significantly different, from brittle-ductile to ductile, from south to north, respectively. We tentatively propose that these differences can be due to an evolution from hyper-extended crust with mantle exhumation in the Northeast (Aar) to an "aborted rift" configuration in the Southwest (Oisans). The presence of a (light) continental crust in the Southwest may have prevented a strong tectonic burying of the proximal margin and promoted its early shortening.

  5. Climate changes control offshore crustal structure at South China Sea continental margin

    NASA Astrophysics Data System (ADS)

    Clift, Peter D.; Brune, Sascha; Quinteros, Javier

    2015-06-01

    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. 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. 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.

  6. On the use of rayleigh wave group velocities for the analysis of continental margins

    Microsoft Academic Search

    S. A. P. L. Cloetingh; G. Nolet; R. Wortel

    1979-01-01

    Rayleigh wave group velocities provide a low-cost means for a quick assessment of averaged local properties of the Earth's crust in continental margin regions of the Atlantic type. Sufficiently accurate measurements (with a standard error of 0.3 km\\/s or less) of group velocities in continental shelf areas at periods between 5 and 30 seconds provide important information about structural parameters.

  7. 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 mantle exhumation for the Iberia Abyssal Plain - Flemish Pass conjugate margin profile than for the Galicia Bank - Flemish Cap profile. The predicted N-S differences in deformation mode evolution give insights into the 3D evolution of Iberia-Newfound margin breakup.

  8. 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

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

    E-print Network

    Levin, Lisa

    and energy resources, and perform essential functions such as carbon burial and nutrient cycling differentiation [8] and some of the planet's highest species diversity [9]. The margins also provide essential realized and potential energy resources in the form of oil, gas and methane hydrates, as well as forming

  10. Lomonosov Ridge---A double-sided continental margin

    Microsoft Academic Search

    W. Jokat; G. Uenzelmann-Neben; Y. Kristoffersen; T. M. Rasmussen

    1992-01-01

    The first two traverses of marine multichannel seismic data across the Lomonosov Ridge (central Arctic), by the German research icebreaker Polarstern and the Swedish icebreaker Oden, demonstrate a prograded margin toward the Amerasian side and fault-bounded half grabens toward the Eurasian side of the ridge. Nearly 450 m of undisturbed flat-lying strata have been deposited on top of the peneplaned

  11. Allochthonous deep-water basin deposits of the western US: Implications for Paleozoic paleogeography and plate margin tectonics

    SciTech Connect

    Miller, E.L. (Stanford Univ., CA (United States). Geology Dept.)

    1993-04-01

    The stratigraphy and sedimentology of the lower Paleozoic Roberts Mts. and upper Paleozoic Golconda allochthons can be used to reconstruct their general paleogeographic setting in the Paleozoic. Basalt pillow lavas and radiolarian chert, were once considered straightforward evidence that the allochthons represented imbricated ocean crust formed at sites far removed from continental influences. Better stratigraphic definition, provenance studies and geochemistry of lavas now indicate that clastic components were derived from the continental shelf or interior and basalts in the Roberts Mountains allochthon were erupted in an intraplate setting through thinned continental crust (Madrid, 1987). Both in the earliest Mississippian and in the Late Permian, the Antler Basin (Roberts Mts.) and the Havallah Basin (Golconda) received proximal detritus from island arc sources to the west, immediately prior to closure of the basins by thrust-faulting. These data suggest that both systems of basins formed as marginal basins by rifting on the continental shelf (Antler Basin) and along the continental margin (Havallah Basin) and were flanked to the west by active island arcs at least during part of their history. As such, their stratigraphy provides a great deal of insight regarding tectonism along the western plate margin of North America during the Paleozoic.

  12. A new reconstruction of the Paleozoic continental margin of southwestern North America: Implications for the nature and timing of continental truncation and the possible role of the Mojave-Sonora megashear

    USGS Publications Warehouse

    Stevens, C.H.; Stone, P.; Miller, J.S.

    2005-01-01

    Data bearing on interpretations of the Paleozoic and Mesozoic paleogeography of southwestern North America are important for testing the hypothesis that the Paleozoic miogeocline in this region has been tectonically truncated, and if so, for ascertaining the time of the event and the possible role of the Mojave-Sonora megashear. Here, we present an analysis of existing and new data permitting reconstruction of the Paleozoic continental margin of southwestern North America. Significant new and recent information incorporated into this reconstruction includes (1) spatial distribution of Middle to Upper Devonian continental-margin facies belts, (2) positions of other paleogeographically significant sedimentary boundaries on the Paleozoic continental shelf, (3) distribution of Upper Permian through Upper Triassic plutonic rocks, and (4) evidence that the southern Sierra Nevada and western Mojave Desert are underlain by continental crust. After restoring the geology of western Nevada and California along known and inferred strike-slip faults, we find that the Devonian facies belts and pre-Pennsylvanian sedimentary boundaries define an arcuate, generally south-trending continental margin that appears to be truncated on the southwest. A Pennsylvanian basin, a Permian coral belt, and a belt of Upper Permian to Upper Triassic plutons stretching from Sonora, Mexico, into westernmost central Nevada, cut across the older facies belts, suggesting that truncation of the continental margin occurred in the Pennsylvanian. We postulate that the main truncating structure was a left-lateral transform fault zone that extended from the Mojave-Sonora megashear in northwestern Mexico to the Foothills Suture in California. The Caborca block of northwestern Mexico, where Devonian facies belts and pre-Pennsylvanian sedimentary boundaries like those in California have been identified, is interpreted to represent a missing fragment of the continental margin that underwent ???400 km of left-lateral displacement along this fault zone. If this model is correct, the Mojave-Sonora megashear played a direct role in the Pennsylvanian truncation of the continental margin, and any younger displacement on this fault has been relatively small. ?? 2005 Geological Society of America.

  13. Analysis of shallow gas and fluid migration within the Plio-Pleistocene sedimentary succession of the SW Barents Sea continental margin using 3D seismic data

    Microsoft Academic Search

    Karin Andreassen; Espen Glad Nilssen; Christian M. Ødegaard

    2007-01-01

    Three-dimensional (3D) seismic data acquired for hydrocarbon exploration reveal that gas accumulations are common within the 2 3 km thick Plio-Pleistocene stratigraphic column of the south-western Barents Sea continental margin. The 3D seismic data have relatively low-frequency content (<40 Hz) but, due to dense spatial sampling, long source-receiver offsets, 3D migration and advanced interpretation techniques, they provide surprisingly detailed images

  14. Stratigraphic Record from the Antarctic Continental Margin and the Nature of Cenozoic Climate and Ice Volume Changes

    Microsoft Academic Search

    L. R. Bartek

    2001-01-01

    Ocean Drilling Program core and seismic stratigraphic analyses indicate that large ice sheets were present in Antarctica at least by early Paleogene time and expanded out over large areas of the continental margin by the Eocene. Seismic stratigraphic analysis shows that seismic facies similar to those produced in the Pleistocene are present throughout the Cenozoic continental margin strata in the

  15. Interactions of 3D mantle flow and continental lithosphere near passive margins R.J. Farrington a,

    E-print Network

    Sandiford, Mike

    northward drift commenced (Tikku and Cande, 1999). The continental insulation mechanism has also beenInteractions of 3D mantle flow and continental lithosphere near passive margins R.J. Farrington a and thick continental lithosphere. The resultant flow in the upper mantle is driven by a combination

  16. Continental margin evolution of the northern Arabian platform in Syria

    SciTech Connect

    Best, J.A.; Barazangi, M. (Cornell Univ., Ithaca, NY (United States)); Al-Saad, D.; Sawaf, T.; Gebran, A. (Syrian Petroleum Company, Damascus (Syrian Arab Republic))

    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.

  17. Distribution and morphology of large submarine sediment slides and slumps on Atlantic continental margins

    Microsoft Academic Search

    Robert W. Embley; Robert D. Jacobi

    1977-01-01

    Numerous large sediment slides and slumps have been discovered and surveyed on the continental margins of Northwest Africa, Southwest Africa, Brazil (Amazon Cone), the Mediterranean, the Gulf of Mexico, and North America over the past 10 years. The mass movements are of two primary types: (1) translational slides, and (2) rotational slumps. Translational slides are characterized by a slide scar

  18. Submarine mass movements on continental margins HOMA J. LEE*, JACQUES LOCAT, PRISCILLA DESGAGNS, JEFFREY D. PARSONS,

    E-print Network

    Lin, Andrew Tien-Shun

    Submarine mass movements on continental margins HOMA J. LEE*, JACQUES LOCAT, PRISCILLA DESGAGNÉS **Institut de Ciències del Mar, Barcelona 08003, Spain ABSTRACT Submarine landslides can be important currents. Recent submarine land- slide research has: shown that landslides and sediment waves may generate

  19. Elevated, passive continental margins: Long-term highs or Neogene uplifts? New evidence from West Greenland

    Microsoft Academic Search

    Peter Japsen; Johan M. Bonow; Paul F. Green; James A. Chalmers; Karna Lidmar-Bergström

    2006-01-01

    It is commonly assumed that elevated, passive continental margins have remained uplifted since the time of rifting. In many areas, e.g. Scandinavia, the timing and extent of uplift movements are difficult to determine because the uplifted area consists almost exclusively of ancient metamorphic rocks. However, the preserved Mesozoic–Cenozoic sedimentary and volcanic record of West Greenland makes this a key area

  20. Gas hydrate stability and the assessment of heat flow through continental margins

    Microsoft Academic Search

    Ingo Grevemeyer; Heinrich Villinger

    2001-01-01

    SUMMARY A prominent feature across some continental margins is a bottom-simulating reflector (BSR). This seismic reflection generally coincides with the depth predicted for the base of the gas hydrate stability field. Because the occurrence of gas hydrates is controlled by temperature and pressure conditions, it has been suggested that BSRs mark an isotherm and they have therefore been used to

  1. Escape of methane gas from the seabed along the West Spitsbergen continental margin

    E-print Network

    Rohling, Eelco

    Escape of methane gas from the seabed along the West Spitsbergen continental margin Graham K August 2009. [1] More than 250 plumes of gas bubbles have been discovered emanating from the seabed the seabed by reducing the extent of the GHSZ, causing the liberation of methane from decomposing hydrate

  2. Glacial to Holocene fluctuations in hydrography and productivity along the southwestern continental margin of India

    Microsoft Academic Search

    M. Thamban; V. Purnachandra Rao; R. R. Schneider; P. M. Grootes

    2001-01-01

    Oxygen isotope and sedimentological records of a sediment core from the southwestern continental margin of India are used to reconstruct the fluctuations in sea surface hydrography and productivity during the last deglaciation. The ?18O records of Globigerinoides ruber and Globigerinoides sacculifer exhibit high amplitude oscillations during the early deglaciation and Holocene. Significant variations in ?18O during the Holocene and a

  3. The western margin of Mexico is ideally suited for testing two opposing models for

    E-print Network

    Busby, Cathy

    controlled crustal growth by continental margin rifting and addition of new igneous and volcaniclastic to be part of a single arc. However, we divide it into four distinc- tive tectonostratigraphic assemblages of a subduction zone produce numerous arc-related basins, some rifted off the continental margin and others formed

  4. 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.

  5. Do fracture zones define continental margin segmentation? — Evidence from the French Guiana margin

    Microsoft Academic Search

    C. J. Greenroyd; C. Peirce; M. Rodger; A. B. Watts; R. W. Hobbs

    2008-01-01

    Plate reconstructions suggest that the French Guiana margin in the west equatorial Atlantic is a highly segmented margin with both rift- and transform-style features. We describe here the results of modelling coincident multi-channel and wide-angle seismic, gravity and magnetic data acquired along two transects of this margin. The resulting models not only highlight the degree of structural segmentation but also

  6. Temporal and spatial patterns of Cenozoic and Late Mesozoic erosion and deposition along the western margin of southern Africa

    SciTech Connect

    Brown, R.W.; Gleadow, A.J.W. (Latrobe Univ., Bundoora (Australia)); Rust, D.J.; Summerfield, M.A. (Univ. of Edinburgh (Scotland))

    1990-05-01

    Compared with subsidence history and eustatic sea level change, sediment supply has been a neglected component of studies of passive margin stratigraphy. The spatial and temporal pattern of sediment supply to continental margin, however, is a critical factor in determining the architecture of offshore sedimentary sequences. Sediment routing across passive margins is controlled primarily by their tectonic development and the consequent morphological evolution of the subaerial part of the margin. By combining offshore sediment volume and sedimentation rate data based on isopach maps and borehole records with apatite fission-track analysis and denudational modeling onland, the depositional history of the western margin of southern Africa has been related to its geomorphic response to continental rifting. The sediment volume data indicate a declining rate of sedimentation after rifting in the Early Cretaceous despite a probable enlargement of the sediment source area through time. Similarly, apatite fission-track ages and confined track length distributions indicate an Early Cretaceous episode of relatively high erosion rates which affected areas both inland and oceanward of the major topographic discontinuity along the margin represented by the Great Escarpment. Late Cenozoic rates of erosion and sediment supply have been low, although much of the sediment source area is still at a significant elevation. Although aridity may have contributed to this reduction in sediment supply, the morphological response to the tectonic evolution of the margin has also been crucial.

  7. 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.

  8. 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.

  9. The continent-ocean transition of the rifted South China continental margin

    NASA Astrophysics Data System (ADS)

    Cameselle, Alejandra L.; Ranero, César R.; Franke, Dieter; Barckhausen, Udo

    2014-05-01

    The continent to ocean transition (COT) architecture of rifted margins represents a key aspect in the study of the variability of different rifting systems and thus, to understand lithospheric extension and final break-up processes. We used 2250 km of reprocessed multichannel seismic data along 4 regional lines and magnetic data acquired across the NW South China continental margin to investigate a previously poorly defined COT. The along-strike structure of the NW subbasin of the South China Sea presents different amounts of extension allowing the study of conjugate pairs of continental margins and their COT in a relative small region. The time-migrated seismic sections allow us to interpreted clear continental and oceanic domains from differences in internal reflectivity, faulting style, fault-block geometry, the seismic character of the top of the basement, the geometry of sediment deposits, and Moho reflections. The continental domain is characterized by arrays of normal faults and associated tilted blocks overlaid by syn-rift sedimentary units. The Moho is imaged as sub-horizontal reflections that define a fairly continuous boundary typically at 8-10 s TWT. Estimation of the thickness of the continental crust using 6 km/s average velocity indicates a ~22 km-thick continental crust under the uppermost slope thinning abruptly to ~9-6 km under the lower slope. The oceanic crust has a comparatively highly reflective top of basement, little-faulting, not discernible syn-tectonic strata, and fairly constant thickness (4-8 km) over tens of km distance defined by usually clear Moho reflections. The COT can be very well defined based on MSC images and occurs across a ~5-10 km narrow zone. Rifting in the NW subbasin resulted in asymmetric conjugate margins. Arrays of tilted fault blocks covered by abundant syn-rift sediment are displayed across the northwestern South China continental margin, whereas the conjugate Macclesfield Bank margin shows abrupt thinning and little faulting. Seismic profiles also show a clear change in the tectonic structure of the margin from NE to SW. On the two NE-most lines, the abrupt crustal thinning occurs over a 20-40 km wide area resulting in final breakup. To the SW, the area of stretched continental crust extends over a comparatively broader ~100-110 km segment of tilted fault-blocks. We interpret that the 3D structural variability and the narrow COT is related to the lateral NE to SW propagation of a spreading center. The early spreading center propagation in the NE suddenly stopped continental stretching during ongoing rifting, causing an abrupt break-up and a narrow COT. Later arrival of spreading center to the SW resulted in a comparatively broader segment of highly stretched continental crust. We suggest that the final structure of the northwest South China continental margin have been governed by the 3D interaction between rifting and oceanic spreading center propagation to a degree larger than by the local lithospheric structure during rifting.

  10. 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.

  11. 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.

  12. 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).

  13. 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.

  14. 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.

  15. 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 down the gullies, which fill with sediment from the distal Eel River. Approximately one-half of the subsurface gullies have no expression on the seafloor, because they have completely filled with sediment following the last glacial maximum lowstand of sea level. Copyright ?? 2001, SEPM (Society for Sedimentary Geology).

  16. 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 thickens towards the East forming a giant buried drift and also towards the West building a unit of plastered drifts below the Piedra Buena Terrace. Here, the maximum thickness of this unit is ~1.4 s (TWT). In contrast to this the sediments of late Miocene to recent age are very thin or completely eroded over the Piedra Buena terrace but form drifts at the Valentin Feilberg terrace that can be further divided into subunits whose reflections have stratified facies with good lateral continuity. Mounded drift structures on the western and eastern edges of the terrace are bounding an onlap fill structure possibly associated with bottom currents of reduced activity. With an assumed age of ~15 Ma for reflector AR5 the average sedimentation rate since the middle Miocene is estimated to be > 10 cm/ka and thus would make a drill site on the terrace suitable for high resolution palaeoclimate studies.

  17. 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.

  18. Synthesis of the crustal structure of the transform continental margin off Ghana, northern Gulf of Guinea

    Microsoft Academic Search

    R. A. Edwards; R. B. Whitmarsh; R. A. Scrutton

    1997-01-01

    Results of a detailed geophysical transect across the transform continental margin off Ghana, at the eastern end of the Romanche\\u000a Fracture Zone in the Equatorial Atlantic, are presented. Seismic refraction, single-channel seismic reflection, gravity, and\\u000a magnetic data were collected, and seismic, gravity, and magnetic models along the transect are shown. The 6- to 11-km-wide\\u000a ocean–continent transition (OCT) is characterized by

  19. The Pindos Fold-and-thrust belt (Greece): inversion kinematics of a passive continental margin

    Microsoft Academic Search

    Konstantinos Skourlis; Theodor Doutsos

    2003-01-01

    Continuous exposure in the Pindos mountain chain (Greece) and the detailed stratigraphic measurements in the area enable us to construct eight balanced cross sections across the Pindos Fold-and-thrust belt (PFTB) and to approach quantitatively some parameters which controlled foreland evolution. The 160-km-wide passive continental margin of the Apulian continent in Greece was progressively shortened from east to west at rates

  20. Chapter 5 Cenozoic Climate History from Seismic Reflection and Drilling Studies on the Antarctic Continental Margin

    Microsoft Academic Search

    Alan K. Cooper; Giuliano Brancolini; Carlota Escutia; Yngve Kristoffersen; Rob Larter; German Leitchenkov; Phillip O'Brien; Wilfried Jokat

    2008-01-01

    Seismic stratigraphic studies and scientific drilling of the Antarctic continental margin have yielded clues to the evolution of Cenozoic climates, depositional paleoenvironments and paleoceanographic conditions. This paper draws on studies of the former Antarctic Offshore Stratigraphy Project and others to review the geomorphic and lithostratigraphic offshore features that give insights into the long-duration (m.y.) and short-term (k.y.) changes that document

  1. Revisiting Submarine Mass Movements Along The U.S. Atlantic Continental Margin: Implications For Tsunami Hazards

    Microsoft Academic Search

    J. D. CHAYTOR; D. Twichell; U. Brink; B. Buczkowski; B. Andrews

    Interest in the generation of tsunamis by submarine mass movements has warranted a reassessment of their distribution and\\u000a the nature of submarine landslides offshore of the eastern U.S. The recent acquisition and analysis of multibeam bathymetric\\u000a data over most of this continental slope and rise provides clearer view into the extent and style of mass movements on this\\u000a margin. Debris

  2. Benthonic foraminiferal distributions and quantitative transfer functions for the northwest European continental margin

    Microsoft Academic Search

    H. P. Sejrup; H. J. B. Birks; D. Klitgaard Kristensen; H. Madsen

    2004-01-01

    A database of benthonic foraminiferal data from 298 sediment surface-samples from the northwest European (Ireland to Svalbard) and Iceland margin has been compiled. Samples deeper than 500 m on the continental slope and shallower than 30 m in coastal areas are not included. Bottom-water temperatures at the sites range between ?1 and 12.5 °C and salinity between 33.5 and 35.5‰.

  3. Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

    Microsoft Academic Search

    Taras Gerya; Bernhard Stöckhert

    2006-01-01

    The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each

  4. Late Quaternary sedimentation on the Portuguese continental margin: climate-related processes and products

    Microsoft Academic Search

    J. H. Baas; J. Mienert; F. Abrantes; M. A. Prins

    1997-01-01

    The late Quaternary sedimentary history of the continental margin off Portugal was reconstructed from sediment gravity cores. Hemipelagic sedimentation (lithofacies A) was dominant during glacial times. It was interrupted periodically by deposition of shelf- and upper-slope-derived silty and sandy terrigenous material by dilute turbidity currents (lithofacies B and C), ice-rafted debris during distinct periods of breakdown of North Atlantic ice

  5. High-Resolution Holocene Records of Paleoceanographic and Paleoclimatic Variability from the Southern Alaskan Continental Margin

    Microsoft Academic Search

    B. P. Finney; J. M. Jaeger; A. C. Mix; E. A. Cowan; S. S. Gulick; L. A. Mayer; N. G. Pisias; R. D. Powell; F. Prahl; J. S. Stoner

    2004-01-01

    We are investigating sediments from the fjords and continental margin of southern Alaska to develop high-resolution climatic and oceanographic records for the Late Quaternary. Our goal is to better understand linkages between climatic, terrestrial and oceanic systems in this tectonically active and biologically productive region. A field program was conducted aboard the R\\/V Maurice Ewing in August\\/September 2004 utilizing geophysical

  6. Denudation of the Cte d'Ivoire-Ghana transform continental margin from apatite fission tracks *Florence Bigot-Cormier, **Christophe Basile, ***Grard Poupeau,

    E-print Network

    Denudation of the Côte d'Ivoire-Ghana transform continental margin from apatite fission tracks Apatite fission track analysis of samples from the shoulder (marginal ridge) of the Côte d'Ivoire-Ghana transform continental margin reveal a cooling of the margin between 85 and 65 Ma for the central and eastern

  7. Sediment storage and stability along the western Tibetan plateau margin

    NASA Astrophysics Data System (ADS)

    Blöthe, J. H.; Munack, H.; Korup, O.

    2012-04-01

    The Indus is one of the major rivers draining the western Tibetan plateau. Many studies have stressed rapid incision and uplift along the river's course through the western Himalayan syntaxis, whereas process rates in the upper reaches near the western Tibetan plateau margin have yet to be quantified. Moreover, little is known about the volumetric amount of sediment that is stored along the low-gradient plateau margin, let alone the potential rates at which this sediment may be released by glacial and fluvial processes. We start filling this knowledge gap by offering a first-order regional quantification of intramontane sediment storage. We compare different geospatial algorithms for objectively delineating sediment storage contained in large valley fills from digital topographic data, and estimate the stored total sediment using a probabilistic volume-area scaling approach. Before applying this scaling to real topography, we conducted a geometrical scaling for different shape factors to quantitatively constrain prediction errors associated with bedrock geometry. Finally, we applied the volume-area scaling to using 90-m SRTM sample data representing different valley types and lithologies in the Ladakh and Zanskar Ranges drained by the upper Indus River. Our estimates show that >40 km3 of sediment are stored in the 15,000 km2 Zanskar catchment, which is mostly an arid bedrock landscape with mean elevations of ~3500 m. Storage potential on hillslopes is limited such that most material is perched along deeply incised reaches (~80%) or infilling low-gradient headwaters (~20%), where the otherwise steep and rugged drainage network of the Zanskar grades into the gently sloping low-relief topography that characterizes the Tibetan Plateau. Sediment storage covers between 3 and 8 % of the total catchment areas of dissected basins. This is consistent with storage estimates from other mountain belts with grossly differing climatic and lithological conditions. However, the fraction of sediment storage may be as high as 25% in low-relief high-elevation basins on the plateau. Depending on published estimates of regional rates of denudation and exhumation, we infer average sediment residence times of ~25 to >260 kyr in this region along the western Tibetan Plateau margin. This estimate is consistent with the preservation and landform ages of some of the oldest glacigenic deposits in the Himalaya-Tibet orogen, and points to the importance of sediment flux and storage in preserving bedrock topography, while providing spatially distributed reservoirs for highly episodic sediment transport events.

  8. Plume-lithosphere interactions near a passive continental margin: a thermo-mechanical modelling study

    NASA Astrophysics Data System (ADS)

    François, Thomas; Cloetingh, Sierd; Burov, Evgueni; Matenco, Liviu

    2015-04-01

    Plume head-lithosphere (PLI) interactions have important consequences both for tectonic and mineralogical evolution of the lithosphere and are often considered to be an important factor of continental break-up. Nevertheless, the interaction between plume and post break-up tectonics (i.e. evolution of passive margins) remain unclear. The passive margins represent important geometrical, thermal and rheological barriers that interact with the plume head material during its emplacement below the lithosphere. For example on the Scandinavia's North Atlantic passive margin the large Cenozoic uplift comprised uplift of basin margins as well as accelerated subsidence of basin centres adjacent to the uplifted landmasses while the compressional reactivation coincides with the postulated Iceland-plume events associated with massive magma emplacement. The goal of this study is to understand the role of the Iceland plume in the Cenozoic evolution of the Scandinavia's North Atlantic passive margin. To investigate the interactions between the plume and passive margin we use fully coupled thermo-mechanical 2D numerical code (Flamar v12). The model area is 700 km deep and 1500 km wide comprising rheologically realistic lithosphere and the entire upper mantle Our models have free upper surface boundary, surface erosion, account for the rheological stratification (upper crust, lower crust, lithospheric mantle and asthenosphere), brittle-elastic-ductile rheology, metamorphic phase changes (density and physical properties) and for the specific crustal and thermal structure of the Scandinavia's North Atlantic passive margin. We have tested several parameters including the lateral position of the plume, the rate of extension and the thermo-rheological profile of the continental lithosphere.

  9. 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.

  10. Mesozoic sedimentary and magmatic evolution of the Arabian continental margin, northern Syria: evidence from the Baer–Bassit Melange 

    E-print Network

    Al-Riyami, Khalil; Robertson, Alastair H F

    2002-01-01

    One of the few detailed records of Mesozoic deep-water sedimentation and volcanism preserved along the tectonically emplaced Arabian continental margin is from the Baer–Bassit region of northern Syria. South-Tethyan units ...

  11. Submarine slope failures along the convergent continental margin of the Middle America Trench

    NASA Astrophysics Data System (ADS)

    Harders, Rieka; Ranero, CéSar R.; Weinrebe, Wilhelm; Behrmann, Jan H.

    2011-06-01

    We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. We have used multibeam bathymetry along ˜1300 km of the Middle America Trench of the Central America Subduction Zone and deep-towed side-scan sonar data. We found abundant evidence of large-scale slope failures that were mostly previously unmapped. The features are classified into a variety of slope failure types, creating an inventory of 147 slope failure structures. Their type distribution and abundance define a segmentation of the continental slope in six sectors. The segmentation in slope stability processes does not appear to be related to slope preconditioning due to changes in physical properties of sediment, presence/absence of gas hydrates, or apparent changes in the hydrogeological system. The segmentation appears to be better explained by changes in slope preconditioning due to variations in tectonic processes. The region is an optimal setting to study how tectonic processes related to variations in intensity of subduction erosion and changes in relief of the underthrusting plate affect mass wasting processes of the continental slope. The largest slope failures occur offshore Costa Rica. There, subducting ridges and seamounts produce failures with up to hundreds of meters high headwalls, with detachment planes that penetrate deep into the continental margin, in some cases reaching the plate boundary. Offshore northern Costa Rica a smooth oceanic seafloor underthrusts the least disturbed continental slope. Offshore Nicaragua, the ocean plate is ornamented with smaller seamounts and horst and graben topography of variable intensity. Here mass wasting structures are numerous and comparatively smaller, but when combined, they affect a large part of the margin segment. Farther north, offshore El Salvador and Guatemala the downgoing plate has no large seamounts but well-defined horst and graben topography. Off El Salvador slope failure is least developed and mainly occurs in the uppermost continental slope at canyon walls. Off Guatemala mass wasting is abundant and possibly related to normal faulting across the slope. Collapse in the wake of subducting ocean plate topography is a likely failure trigger of slumps. Rapid oversteepening above subducting relief may trigger translational slides in the middle Nicaraguan upper Costa Rican slope. Earthquake shaking may be a trigger, but we interpret that slope failure rate is lower than recurrence time of large earthquakes in the region. Generally, our analysis indicates that the importance of mass wasting processes in the evolution of margins dominated by subduction erosion and its role in sediment dynamics may have been previously underestimated.

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

    PubMed

    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

  13. Approximation of surface wave mode conversion at a passive continental margin by a mode-matching technique

    Microsoft Academic Search

    T. Meier; P. G. Malischewsky

    2000-01-01

    At a continental margin sharp structural changes cause strong lateral heterogeneityin S-wave velocity, P-wave velocity and density. Therefore, mode conversion of surface wave modes is expected. Passive continental margins may be modelled by vertical discontinuities. Mode conversion is then expressed in terms of reflection and transmission coefficients. Long-period seismograms are calculated by mode summation over incident, reflected and transmitted modes.

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

    Microsoft Academic Search

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

    2011-01-01

    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

  15. Ophiolitic basement to the Great Valley forearc basin, California, from seismic and gravity data: Implications for crustal growth at the North American continental margin

    USGS Publications Warehouse

    Godfrey, N.J.; Beaudoin, B.C.; Klemperer, S.L.; Levander, A.; Luetgert, J.; Meltzer, A.; Mooney, W.; Trehu, A.

    1997-01-01

    The nature of the Great Valley basement, whether oceanic or continental, has long been a source of controversy. A velocity model (derived from a 200-km-long east-west reflection-refraction profile collected south of the Mendocino triple junction, northern California, in 1993), further constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn is underlain by a westward extension of lower-density continental crust (Sierran affinity material). We used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model. The crustal section of Great Valley ophiolite is 7-8 km thick, and the Great Valley ophiolite relict oceanic Moho is at 11-16 km depth. The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western margin of the Great Valley, where the 5-7-km-thick Great Valley ophiolite mantle section dips west into the present-day mantle. There are 16-18 km of lower-density Sierran affinity material beneath the Great Valley ophiolite mantle section, such that a second, deeper, "present-day" continental Moho is at about 34 km depth. At mid-crustal depths, the boundary between the eastern extent of the Great Valley ophiolite and the western extent of Sierran affinity material is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin. We suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

  16. Reply to `Oligocene to Holocene sediment drifts and bottom-currents on the slope of Gabon continental margin (West Africa).

    E-print Network

    Demouchy, Sylvie

    Discussion Reply to `Oligocene to Holocene sediment drifts and bottom- currents on the slope of Gabon continental margin (West Africa). Consequences for sedimentation and southeast Atlantic upwelling for the sedimentary structures occurring on the upper continental slope of southern Gabon. We welcome the opportunity

  17. Seismic investigation of the continental margin off- and onshore Valparaiso, Chile

    NASA Astrophysics Data System (ADS)

    Flueh, E. R.; Vidal, N.; Ranero, C. R.; Hojka, A.; von Huene, R.; Bialas, J.; Hinz, K.; Cordoba, D.; Dañobeitia, J. J.; Zelt, C.

    1998-03-01

    At the latitude of Valparaiso, Chile, a fundamental change in the configuration of the Benioff zone, volcanic activity, and the structure of the continental margin occurs opposite the subducting Juan Fernandez Ridge. Three legs of the German {R}/{V}Sonne (cruises SO101, SO103 and SO104) surveyed the continental margin and oceanic plate offshore Valparaiso, aiming at studying the crustal structure and investigating possible causes for the change in slab configuration. Sonne cruise SO101 investigated the tectonic setting with swath-mapping bathymetry, magnetics and high-resolution seismics. Following these investigations cruise SO103 collected land-sea wide-angle seismic data, and coincident deep seismic reflection data were acquired during cruise SO104. Coincident near-vertical and wide-angle seismic measurements were made along two profiles. Profile 1, located at the south of the study area, away from the influence of the subducting ridge, crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed. Profile 2, located in the north, runs from the Juan Fernandez Ridge to the Chilean coast. The crustal velocity models obtained for the two profiles show that the continental crust extends to the middle-lower slope boundary, which is also reflected in morphology. In addition, they show that the crustal structure of the oceanic plate is rather similar, but the plate seems to be slightly more inclined along the northern profile (13° versus 10° in the south). The two profiles are only about 70 km apart but their structures differ significantly. No straightforward correlation exists between the two profiles that can be attributed to ridge collision. The data support that the 1985 central Chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate.

  18. Continental break-up history of a deep magma-poor margin based on seismic reflection data (northeastern Gulf of Aden margin, offshore Oman)

    NASA Astrophysics Data System (ADS)

    Autin, Julia; Leroy, Sylvie; Beslier, Marie-Odile; d'Acremont, Elia; Razin, Philippe; Ribodetti, Alessandra; Bellahsen, Nicolas; Robin, Cécile; Al Toubi, Khalfan

    2010-02-01

    Rifting between Arabia and Somalia started around 35 Ma followed by spreading at 17.6 Ma in the eastern part of the Gulf of Aden. The first-order segment between Alula-Fartak and Socotra-Hadbeen fracture zones is divided into three second-order segments with different structure and morphology. Seismic reflection data were collected during the Encens Cruise in 2006 on the northeastern margin. In this study, we present the results of Pre-Stack Depth Migration of the multichannel seismic data from the western segment, which allows us to propose a tectono-stratigraphic model of the evolution of this segment of the margin from rifting to the present day. The chronological interpretation of the sedimentary sequences is mapped out within relation to the onshore observations and existing dating. After a major development of syn-rift grabens and horsts, the deformation localized where the crust is the thinnest. This deformation occurred in the distal margin graben (DIM) at the northern boundary of the ocean-continent transition (OCT) represented by the OCT ridge. At the onset of the OCT formation differential uplift induced a submarine landslide on top of the deepest tilted block and the crustal deformation was restricted to the southern part of the DIM graben, where the continental break-up finally occurred. Initial seafloor spreading was followed by post-rift magmatic events (flows, sills and volcano-sedimentary wedge), whose timing is constrained by the analysis of the sedimentary cover of the OCT ridge, correlated with onshore stratigraphy. The OCT ridge may represent exhumed serpentinized mantle intruded by post-rift magmatic material, which modified the OCT after its emplacement.

  19. 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 predominantly the result of post-eruption differential subsidence, as opposed to syn-eruption extension. The timing of intrusion of the lower crustal sill complexes remains unclear but they are most likely to have been emplaced as the supply of magma increased, which implies they are a late stage addition.

  20. 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; Behrmann, Jan-Hinrich; Papenberg, Cord; Geersen, Jacob; Ridente, Domenico; Latino Chiocci, Francesco; Urlaub, Morelia; Bialas, Jörg; Micallef, Aaron

    2015-04-01

    Mount Etna is the largest active volcano in Europe. Its volcano edifice is located on top of continental crust close to the Ionian shore in east Sicily. Instability of the eastern flank of the volcano edifice is well documented onshore. The continental margin is supposed to deform as well. Little, however, is known about the offshore extension of the eastern volcano flank and its adjacent continental margin, which is 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 and processed a new marine high-resolution seismic and hydro-acoustic dataset. The data provide new detailed insights into the heterogeneous geology and tectonics of shallow continental margin structures offshore Mt Etna. In a similiar manner as observed onshore, the submarine realm is characterized by different blocks, which are controlled by local- and regional tectonics. We image a compressional regime at the toe of the continental margin, which is bound to an asymmetric basin system confining the eastward movement of the flank. In addition, we constrain the proposed southern boundary of the moving flank, which is identified as a right lateral oblique fault movement north of Catania Canyon. From our findings, we consider a major coupled volcano edifice instability and continental margin gravitational collapse and spreading to be present at Mt Etna, as we see a clear link between on- and offshore tectonic structures across the entire eastern flank. The new findings will help to evaluate hazards and risks accompanied by Mt Etna's slope- and continental margin instability and will be used as a base for future investigations in this region.

  1. Belt-parallel mantle flow beneath a halted continental collision: The Western Alps Guilhem Barruol a,

    E-print Network

    Demouchy, Sylvie

    Belt-parallel mantle flow beneath a halted continental collision: The Western Alps Guilhem Barruol: seismic anisotropy SKS splitting lithosphere upper mantle Western Alps Constraining mantle deformation belts, is a particularly important objective of "mantle tectonics" that may bring a depth extent

  2. Variability of fluvial sediment supply to the Laptev Sea continental margin during Late Weichselian to Holocene times: implications from clay-mineral records

    NASA Astrophysics Data System (ADS)

    Müller, Claudia; Stein, Ruediger

    2000-08-01

    Three sediment cores from the Laptev Sea continental margin were investigated for their clay mineralogy by X-ray diffraction to study the fluvial sediment supply since the late Weichselian. In the study area, the clay-mineral composition of surface sediments is characterized by distinct regional variations. The source area for smectite in the eastern Eurasian Basin is the Putoran Plateau drained by the Khatanga and Yenisei rivers. Currents caused by river discharge and the inflow of Atlantic water masses along the Eurasian continental margin are responsible for sediment distribution. In the sediment cores, smectite and illite contents show an opposite trend which mainly results from variable smectite supply. During MIS 2 the amount of smectite on the Laptev Sea continental margin never exceeds 10 rel.%. Probably, reduced river discharge and the lowered sea level during MIS 2 caused a decreased sediment supply to the Laptev Sea. Additionally, the Putoran Plateau was covered by an ice sheet during the Late Weichselian preventing the erosion of smectite-rich soils. In contrast, maximum smectite contents (up to 30 rel.%) in Holocene sediments result from increased sediment input by the Khatanga River and from the Kara Sea through the Vilkitsky Strait and via St. Anna Trough into the western Laptev Sea.

  3. Comparison of ship and satellite bio-optical measurements on the continental margin of the NE Gulf of Mexico

    Microsoft Academic Search

    Chuanmin Hu; Frank E. Muller-Karger; Douglas C. Biggs; Kendall L. Carder; Bisman Nababan; Denis Nadeau; Joe Vanderbloemen

    2003-01-01

    Surface flow-through data were collected three times per year from November 1997 to August 1999 from the continental margin of the NE Gulf of Mexico on hydrographic cruises that surveyed 11 cross-margin transects from the 10 m to the 1000 m isobath. These data, calibrated using standard water sampling and filtration methods, show seasonal and spatial patterns in riverine outflow

  4. The Crustal Structure of the Northern South China Sea continental margin revealed by Multi-Channel Seismic Reflection and Ocean Bottom Seismometer Observations

    NASA Astrophysics Data System (ADS)

    Wang, Hao; Chao Huang, I.; Shine Liu, Char; Chang, Emmy T. Y.

    2015-04-01

    The South China Sea (SCS) is an ideal place to examine the nature of continental rifting, break-up, and the onset of seafloor spreading. Being mostly inactive today, the SCS basin is measured to get spreading in the early Oligocene and to generate a series of syn-rift structures on the margins, therefore, to know the crustal structures of continental margin can help us to understand the evolution of the SCS. In this study, we use multi-channel seismic (MCS) reflection data to reveal the upper crustal structures and ocean bottom seismometer (OBS) data to probe the lower crustal structures. Accompanying the MCS experiments with active sources, 39 OBS stations were deployed along 2 NW-SE trending profiles in the northern SCS. The eastern profile is located southeast of Dongsha atoll, while the western profile extends from the Zhu II depression to the NW sub-basin of SCS. For construction crustal velocity models, we extract shallow velocity structure from the MCS profile data, then we conduct travel-time tomographic inversion on OBS data to derive 2D velocity models. Finally, forward modeling using RAYINVR is subsequently applied to refine the velocity models. Both MCS profiles show that the basement has been offset by normal faults and thick sediments are deposited in the grabens. Many volcanic bodies are observed in the eastern profile, but few appear in the western profile. The OBS velocity model of the eastern profile reveals that the crustal thickness decreases gradually toward the oceanic basin. However the continental crust thins abruptly from continental slope toward the NW sub-basin. A high velocity layer (>7km/s) in the lower crust can be identified in the eastern profile, but not in the western profile. This high velocity layer has been interpreted to be underplating material, however, another possibility is that it might be serpentinized upper mantle, which frequently found in necking zones and COTs zones.

  5. Continental Margin Tectonics Along the Convergent Plate Boundary of Central Chile

    NASA Astrophysics Data System (ADS)

    Weinrebe, W.; Ranero, C. R.; Diaz, J.; Reichert, C.; Vera, E. E.

    2003-12-01

    Multibeam bathymetry along central Chile provides a detailed map of recent tectonic deformation of the margin and incoming oceanic plate from about 28? S to 36? S. The data were collected during R/V SONNE cruises 101, 102, 104 and 161 and a cruise with R/V Vidal Gormaz. Individual pings were edited and cleaned and the different surveys have been merged after depth calculations using a different measured velocity function for each of them. The oceanic Nazca plate is covered by about 100 m of pelagic sediment and the morphology of the igneous basement is displayed well in the bathymetric maps. The oceanic plate topography changes markedly along the subduction zone and exerts a first order control in the distribution of trench sediment infill and in the tectonic style of deformation of the margin. A major boundary occurs at latitude 32?-33? S where the hotspot volcanic chain of Juan Fernadez is currently subducting. The chain subducts oblique to the margin strike and thus the tectonic boundary has been migrating along the subduction zone through time. South of the area of ridge subduction the trench is filled with turbidites and a 20-40 km wide accretionary prism occurs at the front of the continental slope. The upper slope has a smooth morphology indicative of a quiet tectonic domain. At the current area of ridge subduction and north of it (28?-33?S) the trench has a reduced turbiditic infill. The trench infill seems to be at minimum at 31-32S and slightly larger to the north as the trench axis becomes deeper. Here, a small ridge at the slope toe may indicate that reduced accretion is active. The continental slope is deeper and more rugged that to the south displaying a series of small midslope basins. Here, the continental slope morphotectonic structure is the product of tectonic erosion due to the passage of the volcanic ridge.

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

    SciTech Connect

    Gradstein, F.M. (Bedford Institute of Oceanography, Dartmouth, Nova Scotia (Canada)); Von Rad, U. (Bundesanstalt fuer Geowissenschaften und Rohstoffe, Hannover (West Germany))

    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.

  7. Crustal structure of the northern Nova Scotia rifted continental margin (eastern Canada)

    NASA Astrophysics Data System (ADS)

    Funck, Thomas; Jackson, H. Ruth; Louden, Keith E.; Dehler, Sonya A.; Wu, Yue

    2004-09-01

    The Nova Scotia continental margin off eastern Canada marks a transition from a volcanic to a nonvolcanic style of rifting. The northern (nonvolcanic) segment of the margin was studied by a 490-km-long refraction seismic line with dense air gun shots, coincident with previous deep reflection profiles. A P wave velocity model was developed from forward and inverse modeling of the wide-angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles. The continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5.7-6.9 km/s. Crustal thinning down to 3 km occurs in a 180-km-wide zone and the sediment cover in this area is up to 15 km thick. Farther seaward, a 150-km-wide transition zone is observed with a 5-km-thick lower layer (7.2-7.6 km/s) interpreted as partially serpentinized mantle. At the landward end, this layer is overlain by highly altered continental crust (5.4 km/s) extending up to the seaward limit of the Jurassic salt province. Farther seaward, the upper layer is interpreted as exhumed and highly serpentinized mantle (5.1 km/s) separated from the lower layer by subhorizontal reflectivity, which probably represents a serpentinization front. Oceanic crustal thickness is 4 km with layer 2 velocities of 4.6-5.0 km/s. Layer 3 velocities of 6.4-6.55 km/s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile. This reduced magma production might be related to the proximity of the Newfoundland transform margin.

  8. 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.

  9. Analysis of shallow gas and fluid migration within the Plio-Pleistocene sedimentary succession of the SW Barents Sea continental margin using 3D seismic data

    Microsoft Academic Search

    Karin Andreassen; Espen Glad Nilssen; Christian M. Ødegaard

    2007-01-01

    Three-dimensional (3D) seismic data acquired for hydrocarbon exploration reveal that gas accumulations are common within the\\u000a 2–3 km thick Plio-Pleistocene stratigraphic column of the south-western Barents Sea continental margin. The 3D seismic data\\u000a have relatively low-frequency content (<40 Hz) but, due to dense spatial sampling, long source-receiver offsets, 3D migration\\u000a and advanced interpretation techniques, they provide surprisingly detailed images of inferred gas

  10. Si-WEBS, a European network for the study of Si fluxes on continental margins

    NASA Astrophysics Data System (ADS)

    Ragueneau, O.; Si-Webs Team

    2003-04-01

    Diatoms play an essential role in the export of carbon (C) towards both higher trophic levels and the deep ocean. They have a crucial need for silicon (Si) to build their frustule, but this element has clearly been neglected in studies of carbon and nutrient (N, P) fluxes in continental margins. Over the last 20 years however, coastal ecosystems of temperate regions became particularly sensitive to declining Si:N and Si:P nutrient ratios. Such declines have been related to increased eutrophication and the build-up of dams in river systems. As a result of these anthropogenic perturbations, many ecosystems have switched from nitrate limitation to silicic acid (DSi) limitation, with important consequences for phytoplankton dynamics (from diatoms to less desirable species) and cascading effects on pelagic and benthic food webs. Short-term consequences of Si availability on the shelf mostly affect the resource whereas long-term consequences may affect carbon dioxide (CO2) sequestration on the shelf and the auxiliary biological pump. Continental margins also play a filtering role so that changes in Si delivery to the hydrosphere and/or retention along the Land-Ocean-Continuum (LOC) may have a long-term impact on the oceanic C cycle. Here, we suggest an approach to improve our understanding of (1) the role of Si in the functioning of coastal ecosystems and (2) Si delivery to the open ocean at global scale. This approach implies (1) extending the LOICZ budgeting approach to the element Si to derive worldwide Si budgets on continental margins; (2) improving our knowledge of the processes that control Si transformations along the LOC. The EU-SiWEBS Research Training Network (2002-2006) will work in this last direction, by (a) improving the parameterization of the Si cycle in three river, coastal zone and open ocean models, (b) building quantitative modeling tools to describe Si transformations along the land-ocean continuum, and (c) using these tools to evaluate the ecological, biogeochemical and socio-economical consequences of natural and anthropogenic perturbations of the Si cycle. Although centered on coastal zone processes, SiWEBS will clearly build links towards the two ends of continental margins, by linking the terrestrial and aquatic Si cycles and by providing a means of quantifying temporal variations in Si river inputs to the global ocean.

  11. 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

    The Atacama region in northern Chile hosts the driest desert on Earth and is the world's premier iodine production province. The origin of iodine enrichment in Atacama is controversial and fundamentally different processes have been invoked over the years that involve marine, eolian and more recently deep sedimentary fluid and groundwater sources. As a result of the very limited geochemical iodine data in Atacama and the western South American margin, the origin of iodine enrichment in this region still remains elusive. In this study, we present a comprehensive survey of iodine concentrations and isotopic ratios (129I/I) of different reservoirs in the Atacama Desert of northern Chile, including nitrate soils, supergene copper deposits, marine sedimentary rocks, geothermal fluids, groundwater and meteoric water. Nitrate soils along the eastern slope of the Coastal Cordillera are found to have mean iodine concentrations of at least three orders of magnitude higher than the mean crustal abundances of ?0.12 ppm, with a mean concentration of ?700 ppm. Soils above giant copper deposits in the Central Depression are also highly enriched in iodine (100's of ppm range), and Cu-iodide and iodate minerals occur in the supergene enrichment zones of some of these deposits. Further east in the Precordillera, Jurassic sedimentary shales and limestones show above-background iodine concentrations, the latter averaging ?50 ppm in the southern portion of the study area. The highest iodine concentrations in fluids were measured in groundwater below nitrate soils in the Coastal Range (?3.5-10 ppm) and in geothermal waters (1-3 ppm) along the volcanic arc. Although highly variable, the iodine isotopic ratios (129I/I) of Jurassic marine sedimentary rocks (?300-600 × 10-15), nitrate soils (?150-1500 × 10-15) and waters (?215 × 10-15) are consistently low (<1500 × 10-15), indicating that recent anthropogenic additions are almost negligible in most surficial and deeper 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.

  12. 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. PMID:24308555

  13. 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.

  14. Assessment of OSCAT winds for coastal circulation on the north western continental shelf of India

    NASA Astrophysics Data System (ADS)

    Salim, M.; Nagendra, K.; Bansal, S.; Nayak, R. K.; Rao, M. S.; Sasmal, S. K.; Dutt, C. B. S.; Rao, K. H.; Dadhwal, V. K.

    2014-11-01

    Winds and tides are the major driving forces of the circulation in the coastal and marginal seas. Data Interpolating Variation Analysis (DIVA) method is used to generate spatial and time series data of sea surface winds for the period 2010-2013 at daily time scale from the OSCAT observations. Validity and consistency of the data were examined against the in situ observations and ECMWF re-analysis at different time scales. Amplitude of semi-annual cycle of OSCAT winds in the coastal domain is 30 % larger than the ECMWF winds while the amplitude of annual cycle of OSCAT winds is 20 % smaller than the ECMWF winds. On the open oceans, intensity of respective semi-annual cycles are mostly similar while annual cycle of OSCAT wind is 20 % smaller than the ECMWF winds. Wind driven currents over the western continental shelf of India were simulated by forcing OSCAT and ECMWF winds to a coastal circulation model. It is observed that the mean seasonal circulations from both the simulations are identical spatial pattern however the magnitude of simulated currents based on OSCAT winds are much stronger than ECMWF wind forcing. These currents used in a lagrangian tracer transport code to model the oil-spill events occurred in this region. It revealed that OSCAT based ocean currents has performed better in simulating the trajectory than the ECMWF wind driven currents.

  15. Evolution of the Southwest Indian continental divergent margin: Constraints from 40Ar-39Ar dating of lateritic paleolandsurfaces

    NASA Astrophysics Data System (ADS)

    Bonnet, Nicolas; Beauvais, Anicet; Chardon, Dominique; Arnaud, Nicolas

    2015-04-01

    The western continental passive margin of Peninsular India is marked by the Western Ghats escarpment, which separates a coastal lowland from an East-dipping highland plateau and is carved both into the 63-Ma old Deccan traps and their Archean basement. Previous studies suggested establishment of the escarpment by differential erosion across an elevated rift shoulder, and thermochronologic models predicted escarpment formation from higher denudation in the coastal lowland than on the plateau until ~ 50 Ma. We provided complementary time constraints on the evolution of the passive margin by 40Ar-39Ar dating of supergene K-Mn oxides (cryptomelane) sampled in lateritic formations exposed on paleosurfaces, which are preserved as relicts on both sides of the escarpment. Three main lateritic paleosurfaces were identified in the highland at altitude ranges of 1200-1000 m (S1), 1000-900 m (S2) and 850-600 m (S3), and a lower paleosurface in the lowland at 150-50 m (S4). All the 40Ar-39Ar ages obtained on either side of the escarpment document major weathering periods for each paleosurface: 53 to 45 Ma (S1-S4) synchronously with the bauxitic weathering, 40 to 32 Ma (S2), 30 to 23 Ma (S3), and 24 to 19 Ma (S4). These ages indicate that most of the incision and dissection of plateau landsurfaces S1, S2, and S3 must therefore have taken place after 45, 32 and 23 Ma respectively, while the coastal lowland surface S4 was incised after 19 Ma. Preservation of laterites as old as 47 Ma in the coastal lowland implies that the escarpment already existed in the Mid-Eocene while intense bauxitic weathering was taking place on both sides of the escarpment. The ages obtained in the lowland are also indicative of limited erosion (~ 4 m Ma-1) at the foot of the escarpment since 45 Ma, and particularly low incipient incision of the lowland (~ 5 m Ma-1) since 19 Ma. Ages obtained on the highland plateau indicate further Neogene denudation inland but at less than 15 m Ma-1 since 45 Ma, and incision lower than 6 m Ma-1 since 23 Ma. Limited erosion in the coastal lowland contrasts with Late Neogene increase in clastic fluxes on the offshore margin, which may therefore be attributed to erosion of the shelf edge or material imported from the Indus fan. Our results attest to the antiquity of the first-order relief and topography of the high-elevation margin of Southwest India and to a divergent erosion pattern on either side of its escarpment since at least 47 Ma.

  16. 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.

  17. The impact of ocean deoxygenation on iron release from continental margin sediments

    NASA Astrophysics Data System (ADS)

    Scholz, Florian; McManus, James; Mix, Alan C.; Hensen, Christian; Schneider, Ralph R.

    2014-06-01

    In the oceans' high-nitrate-low-chlorophyll regions, such as the Peru/Humboldt Current system and the adjacent eastern equatorial Pacific, primary productivity is limited by the micronutrient iron. Within the Peruvian upwelling area, bioavailable iron is released from the reducing continental margin sediments. The magnitude of this seafloor source could change with fluctuations in the extension or intensity of the oxygen minimum zones. Here we show that measurements of molybdenum, uranium and iron concentrations can be used as a proxy for sedimentary iron release, and use this proxy to assess iron release from the sea floor beneath the Peru upwelling system during the past 140,000 years. We observe a coupling between levels of denitrification, as indicated by nitrogen isotopes, trace metal proxies for oxygenation, and sedimentary iron concentrations. Specifically, periods with poor upper ocean oxygenation are characterized by more efficient iron retention in the sediment and a diminished iron supply to the water column. We attribute efficient iron retention under more reducing conditions to widespread sulphidic conditions in the surface sediment and concomitant precipitation of iron sulphides. We argue that iron release from continental margin sediments is most effective in a narrow redox window where neither oxygen nor sulphide is present. We therefore suggest that future deoxygenation in the Peru upwelling area would be unlikely to result in increased iron availability, whereas in weaker oxygen minimum zones partial deoxygenation may enhance the iron supply.

  18. Hydrology, morphology and sedimentology of the Campos continental margin, offshore Brazil

    NASA Astrophysics Data System (ADS)

    Viana, A. R.; Faugeres, J. C.; Kowsmann, R. O.; Lima, J. A. M.; Caddah, L. F. G.; Rizzo, J. G.

    1998-01-01

    Slope sand deposits have accumulated from at least the Neogene to the Present on the southeastern Brazilian continental margin (Campos Basin area). This region shows sand accumulations concentrated on the upper portion and on the base of the continental slope with a middle to lower slope bypass zone. A synthesis of preliminary results, supported by recent cores, high-resolution geophysical surveys, geotechnical investigations and environmental research, is presented and permits a prelitrunary analysis of the sedimentological mechanisms operational in this area. These point toward a temporal and spatial multiscale set of phenomena responsible for sand deposits. At any sea-level stand these deposits are dependent on: (1) a suitable sediment source; (2) offshelf transport mechanisms; (3) a morphostructural and hydrodynamic context responsible for the deposition of these sands in the upper portion of continental slopes. The proposed scenario of depositional processes concerns: (1) a set of hydrological processes such as surface currents and counter-currents, waves, tides and eddies with sufficient energy to form submarine sand dune fields at the outer shelf; (2) the offshelf export of this sediment under a combined action of spillover, internal waves, eddies 'seafloor polishing effect' and gravity processes (turbidity currents); and (3) the slope sand deposits and their distribution controlled by the action of contour currents, mass movements and the morphological context, such as canyons, gullies or scarps.

  19. 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.

  20. New geophysical data from the southern Levantine continental margin and outer Nile cone - the GEMME project

    NASA Astrophysics Data System (ADS)

    Huebscher, C.; Ben-Avraham, Z.; Dehghani, A.; Gohl, K.

    2003-04-01

    In order to investigate the tectonic and sedimentary setting of the southern Levantine continental margin as well as the Late Quaternary paleoceanography of the outer Nile Cone a geophysical and geological survey - the GEMME project - was carried out in the eastern Mediterranean between February and March 2002. The German research vessel METEOR operated for 5 weeks in the territorial waters of Israel and Egypt. The experiments included refraction and reflection seismics, gravity and magnetics, swath sounding and subbottom profiling, and sediment sampling with gravity and multi-corer. The high-resolution stratigraphy of the Post-Messinian sediment prism reflects the interplay between sediment input, transport mechanisms, uplift and subsidence, halokinetics, and changes of sea level and climate. We found a wide abundance of gassy clastic sediments above the basal Pliocene unconformity and relation between chemoherms, faulting, and gas/fluid migration. The dominant disturbances along the continental slope off Israel are most likely triggered by salt (gravity) tectonic. The position of the landward termination of the Messinian evaporites is located beneath the present shelf in the north but beneath the lower continental slope of bottomset in the south. The salt tectonic is most likely responsible for the along-strike variation of the margin morphology. The Pelusium line reflects the salt tectonic and is presumably not a basement tectonic feature. We found evidence of active tectonic off Haifa Bay in the prolongation of the Carmel rift and it related branches. Salt tectonic features are correlated with channel levee complexes evolved on the outer Nile Cone, which is different to other submarine fans without a mobile layer beneath where the location of the complexes is an autocyclic process.

  1. Late Cenozoic structural and tectonic development of the western margin of the central Andean Plateau in southwest Peru

    NASA Astrophysics Data System (ADS)

    Schildgen, Taylor F.; Hodges, Kip V.; Whipple, Kelin X.; Pringle, Malcolm S.; van Soest, Matthijs; Cornell, Katrina

    2009-08-01

    Structural and thermochronologic studies of the western margin of the central Andean Plateau show changing styles of deformation through time that give insights into tectonic evolution. In southwest Peru, uplift of the plateau proceeded in several distinct phases. First, NW striking, NE dipping reverse faults accommodated uplift prior to ˜14-16 Ma. Subsequent uplift of the plateau relative to the piedmont (between the plateau and the Pacific Ocean) occurred between ˜14 and 2.2 Ma and was accommodated by NW striking, SW dipping normal faults and subparallel monoclinal folds. The youngest phase of uplift affected the piedmont region and the plateau margin as a coherent block. Although the uplift magnitude associated with phase 1 is unknown, phases 2 and 3 resulted in at least 2.4-3.0 km of uplift. Up to 1 km of this may have occurred during phase 3. Geodynamic processes occurring in both the continental interior and the subduction zone likely contributed to uplift.

  2. Progressive increase in number and volume of ice-marginal lakes on the western margin of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Carrivick, Jonathan; Quincey, Duncan

    2015-04-01

    The evolution in number, area and volume of ice-marginal lakes in western Greenland is very poorly documented or understood. It is important to understand ice-marginal lake evolution because they provide an element of meltwater retention, affect ice-margin character and behaviour, and potentially glacier dynamics. This study uses repeat satellite imagery acquired between 1987 and 2010 to reveal a net 44 % (± 6.5 %) increase in the number of lakes, a net 20 % (± 6.5 %) expansion in total lake surface area and an increase of 12% (± 3.3 %) in the estimated volume of meltwater retained along a 1300 km length of the ice margin in western Greenland. Whilst ~ 12 % (± 1.6 %) of the ice margin holds lakes at any one time there is considerable complexity in lake evolution; many lakes have coalesced, drained partially or fully, or become detached from the ice margin. The total lake volume equates to 144 % of the annual runoff combined from Gothab and Jakobshavn hydrological catchments. The rate of increase in meltwater retention between 1987 and 2010 was similar to the rate of increase in ice sheet surface runoff over the same time period. If the study region is representative of the whole Greenland ice sheet margin then as a first-order estimate ~5 % of the increased runoff over the last 25 years has been intercepted enroute to the oceans by the increased ice-marginal lake capacity. Interactions between these ice-marginal lakes, the western Greenland ice sheet and climate should be determined to provide insight into future land-terminating ice-marginal conditions, runoff retention and meltwater and sediment fluxes to the oceans.

  3. 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.; Nedimovi?, 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 OCTOPUS survey, wide-angle seismic data along a 240-km-long margin parallel profile extending from the central to the northern margin segments along an existing industry MCS profile (Ion/GX Technology NovaSPAN 5100). Twenty OBSs at 10-km spacing were analysed. A preliminary p-wave velocity model along the profile indicates that the cross-strike structures are continuous within the OCT. However, a substantial anisotropy in velocity (~8% lower parallel to the margin) is observed within the OCT. This result is consistent with an interpretation of partially serpentinized mantle that flowed perpendicular to the margin during its extension. In addition, along strike variations are also observed along the profile, which suggest a higher degree of volcanism and a thinner layer of serpentinized mantle to the southwest. These results provide a framework for future studies to the southwest to further investigate the transition to a magma-dominant regime towards the US East Coast.

  4. Seismicity and Attenuation of the Continental Margin Off New York and Southern/ New England from OBS Data

    NASA Astrophysics Data System (ADS)

    Ten Brink, U. S.; Flores, C. H.; McGuire, J. J.; Collins, J. A.

    2014-12-01

    Earthquake data recorded during two ocean-bottom-seismometer (OBS) deployments on the continental slope off New York and Southern New England are used to evaluate micro-seismicity and attenuation of the continental margin. The deployments were partly motivated by the detection by land seismometers of several ML2.5-3.8 continental margin earthquakes in this region during the previous four years. The data come from a five-month-long cluster deployment (1 km radius) of eleven short-period OBS in July 2012 at a water depth of ~835 m and a subsequent nine-month deployment of a network of five short-period OBS spaced 50 km apart at water depths between 900-2400 m. The land-based seismic network did not identify seismic activity on the margin during the two deployments. The OBS network located only one earthquake of ML~1 near the shelf edge, suggesting that seismic activity of the margin is probably successfully monitored by land seismometers. A successful land-based detection of earthquakes on the margin may indicate that low seismic attenuation extends from the Eastern U.S. to the continental shelf and slope. To better quantify the attenuation of the margin we derive the peak spectral attenuation of 6-8 ML2.8-4.1 earthquakes recorded by the OBS during the two deployments. These earthquakes were located on land close to shore and offshore between New Jersey and Nova Scotia, and propagation paths are almost entirely within the continental margin. Onshore earthquakes of ML ?2.6 were not detected by the OBS. Information about the seismicity and attenuation of the margin will be used to evaluate the probability of earthquake-generated landslides.

  5. 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 together with Airy isostasy have been used to predict a "synthetic" gravity derived RDA. Sediment corrected RDA for oceanic crust in the Gulf of Aden are positive (+750m) indicating anomalous uplift with respect to normal subsidence. Gravity inversion predicts normal thickness oceanic crust and a zero "synthetic" gravity derived RDA in the oceanic domain. The difference between the positive sediment corrected RDA and the zero "synthetic" gravity derived RDA, implies that the anomalous subsidence reported in the Gulf of Aden is the result of mantle dynamic uplift. For the oceanic crust outboard of Galicia Bank both the sediment corrected RDA and the "synthetic" gravity derived RDA are negative (-800m) and of similar magnitude, indicating anomalous subsidence, which is the result of anomalously thin oceanic crust, not mantle dynamic topography. We conclude that there is negligible mantle dynamic topography influencing the Galicia Bank region. In the Gulf of Lions, gravity inversion predicts thinner than average oceanic crust. Both sediment corrected RDA (-1km) and "synthetic" gravity derived RDA (-500m) are negative. The more negative sediment corrected RDA compared with the "synthetic" gravity derived RDA implies that the anomalous subsidence in the Gulf of Lions is the result of mantle dynamic subsidence as well as thinner than average oceanic crust.

  6. Modern dolomite deposition in continental, saline lakes, western Victoria, Australia

    Microsoft Academic Search

    P. de Deckker; William M. Last

    1988-01-01

    Microcrystalline dolomite forms a major constituent of Holocene sediments of numerous continental, saline playa lakes in southeastern Australia. The lake waters are highly supersaturated with respect to dolomite as well as other Mg carbonates, but undersaturated or near saturation with respect to calcite and aragonite. The dolomite shows no replacement textures and most likely formed by direct precipitation. Conditions in

  7. Siberian Arctic Continental Margin: Constraints and Uncertainties of Plate Tectonic Models

    NASA Astrophysics Data System (ADS)

    Drachev, S. S.

    2004-12-01

    Siberian Arctic Continental Margin (SACM) reveals a complicated tectonic history resulted from three major events: (1) Mesozoic collisions of various allochtonous blocks with Paleo-Siberian continental margin, (2) Opening of Canada Basin, and (3) Opening of Eurasia Basin. Despite considerable progress was achieved in the past 15 years owing to CDP seismic reflection surveys and satellite observations, some major points of SACM's structure and history are still poorly understood. According to the most accepted model, opening of the Canada Basin led to separation and counterclockwise rotation of North Alaskan-Chukchi Microplate until it collided with Siberian/Omolon margin along South Anyui Suture. However, the time and geometry of the opening are not properly constrained yet. Uniform rotation of North Alaskan-Chukchi Microplate by 66 deg. causes a significant overlap in the East Siberian Sea that cannot be explained by later extension of the SACM. Accepted age of the basin opening is 130-80 Ma, however, geological data show that South Anyui Suture was already completely closed by Aptian. In contrast, Cretaceous flood basalts suggest even later opening of the Canada Basin, which may have begun around 125 Ma. Chukchi Borderland, when remained at its present position, prevents closure of the Amerasia Basin. We suggest it was conjugated to what now the northeastern margin of East Siberian Sea is. Then it was detached from the SACM and moved to the present-day position during spreading episodes within the Arctic basins. However, the time of this event is unconstrained yet. Late Cretaceous-Cenozoic extension of the SACM was related to opening of the Eurasia Basin, and, probably, North Atlantic and Labrador Sea. It led to significant modification of SACM's initial architecture and created several profound rift systems. Extension of northern Laptev Shelf totals at least 190 km, which is about 47 % of the total divergence within adjacent Eurasia Basin. The northern East Siberian and Chukchi seas and Chukchi Borderland are the best candidates to account for some 200 km of the "missing extension". Using all available data we have revised structure and geological history of the SACM and speculated its relationships to the Canadian Arctic Margin in a "pre-Canada Basin" Arctic.

  8. Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

    NASA Astrophysics Data System (ADS)

    Gerya, Taras; Stöckhert, Bernhard

    2006-04-01

    The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5 3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10 20 Myr (after 400 600 km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc: (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.

  9. Neotectonic evolution of the Brazilian northeastern continental margin based on sedimentary facies and ichnology

    NASA Astrophysics Data System (ADS)

    Gandini, Rosana; Rossetti, Dilce de Fátima; Netto, Renata Guimarães; Bezerra, Francisco Hilário Rego; Góes, Ana Maria

    2014-09-01

    Quaternary post-Barreiras sediments are widespread along Brazil's passive margin. These deposits are well exposed in the onshore Paraíba Basin, which is one of the rift basins formed during the Pangean continental breakup. In this area, the post-Barreiras sediments consist of sandstones with abundant soft-sediment deformation structures related to seismicity contemporaneous with deposition. The trace fossils Thalassinoides and Psilonichnus are found up to 38 m above modern sea level in sandstones dated between 60.0 (± 1.4) and 15.1 (± 1.8) ka. The integration of ichnological and sedimentary facies suggests nearshore paleoenvironments. Such deposits could not be related to eustatic sea-level rise, as this time coincides with the last glaciation. Hence, an uplift of 0.63 mm/yr, or 1.97 mm/yr if sea level was 80 m lower in the last glaciation, would have been required to ascend the post-Barreiras sediments several meters above the present-day sea level during the last 60 ka. This would suggest that the post-rift stage of the South American eastern passive margin may have experienced tectonic reactivation more intense than generally recognized. Although more complete data are still needed, the information presented herein may play an important role in studies aiming to decipher the Quaternary evolution of this passive margin.

  10. Neogene changes in Southern Ocean sedimentation based on mass accumulation rates at four continental margins

    NASA Astrophysics Data System (ADS)

    Handwerger, David A.; Jarrard, Richard D.

    2003-12-01

    High-resolution mass accumulation rates (MAR) were determined from lithologic logs based on downhole log and continuous core data for six sites at four continental margins around the Southern Ocean. Total MAR was calculated at Ocean Drilling Program (ODP) Site 1095 on the margin of the Antarctic Peninsula and ODP Site 1165 on the margin of Prydz Bay, Antarctica. Carbonate and noncarbonate (terrigenous) MAR were calculated for ODP Sites 1123 and 1124 east of New Zealand and Sites 1168 and 1172 west and southeast of Tasmania. Shifts in carbonate MAR are seen around Tasmania and New Zealand at 23 and 14 Ma, suggesting changes in deep water circulation and surface carbonate productivity at these times. Carbonate MAR dropped at Sites 1124 and 1168 at 23 Ma and increased at 14 Ma at Sites 1123, 1168, and 1172. The overall character of total MAR at Antarctic Sites 1095 and 1165 is a continual and relatively constant decrease in total MAR during the Neogene, with periods of stepwise decrease. One such decrease is seen at 14 Ma at Site 1165, coincident with increased carbonate MAR in New Zealand and Tasmania (Sites 1123, 1124, 1168 and 1172) and likely related to mid-Miocene expansion of the East Antarctic Ice Sheet. Another step drop in total MAR is seen at ˜9 Ma at Site 1165, possibly reflecting diversion of sediment to a newly formed Prydz Channel fan.

  11. 7Be as a tracer of flood sedimentation on the northern California continental margin

    NASA Astrophysics Data System (ADS)

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

    1999-03-01

    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 record) have supplied >80% of the total 85 yr suspended load. Based on radionuclide and hydrologic data, it can be concluded that a small number of flood depositional events have had a disproportionate impact on the sedimentary record of the Eel shelf.

  12. 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 record) have supplied >80% of the total 85 yr suspended load. Based on radionuclide and hydrologic data, it can be concluded that a small number of flood depositional events have had a disproportionate impact on the sedimentary record of the Eel shelf.

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

    SciTech Connect

    Keller, G.R. (Univ. of Texas, El Paso, TX (United States). 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.

  14. The continental Etirol-Levaz slice (Western Alps, Italy): Tectonometamorphic evolution of an extensional allochthon

    NASA Astrophysics Data System (ADS)

    Ewerling, Kathrin; Obermüller, Gerrit; Kirst, Frederik; Froitzheim, Nikolaus; Nagel, Thorsten; Sandmann, Sascha

    2013-04-01

    The Etirol-Levaz slice (ELS) in the western Valtournenche of Italy is a continental fragment trapped between two oceanic units, the eclogite-facies Zermatt-Saas Zone in the footwall and the greenschist-facies Combin Zone in the hanging wall. It has been interpreted as an extensional allochthon derived from the Adriatic continental margin and stranded inside the Piemont-Ligurian oceanic domain during Jurassic rifting (Dal Piaz et al., 2001; Beltrando et al., 2010). The slice consists of Variscan high-grade gneisses, micaschists and metabasics overprinted under eclogite-facies conditions during Early Tertiary Alpine subduction. Eclogites generally consist of garnet + omphacite ± epidote ± amphibole ± phengite ± quartz. We investigate their metamorphic history using equilibrium phase diagrams, mineral compositions, and textural relations between prograde, peak, and retrograde phases. In sample FD328, garnets have compositions of Alm52-61 Grs18-41 Prp5-22 Sps0.5-2 and typical growth zoning. Some garnet grains are brittlely fractured, strongly corroded and overgrown by epidote. Amphibole occurs as a major phase in the matrix and shows a progressive evolution from glaucophane in the core to pargasitic hornblende towards the rim. Sample FD329 with a particular Ca-rich bulk composition (18.3 wt% Ca) displays two distinct garnet generations. Perfectly euhedral cores show compositions of Grs42-45 Alm47-51 Prp3-6 Sps2-7 and typical prograde growth zoning. These cores are overgrown by irregularly shaped rims characterised by an initial rise in Mn and the Fe-Mg ratio. Omphacite in this sample with jadeite-contents of 19-28 mol% apparently has been fractured and annealed by jadeite-poor (7-12 mol%) omphacite suggesting brittle behaviour at eclogite-facies conditions or two high-pressure stages with lower metamorphic conditions in between. We discuss whether the ELS experienced the same monocyclic metamorphic history as the Zermatt-Saas Zone or not. Some of our observations suggest that the ELS experienced two independent stages of high-pressure metamorphism during the Alpine orogeny, e.g. as proposed by Rubatto et al. (2011) for the Sesia Nappe. A lower-pressure stage in between might have been associated with brittle fracturing of high-pressure phases like garnet, glaucophane, and omphacite while the second generations of these minerals might indicate a new stage of increasing pressures and/or temperatures. References Beltrando, M., Rubatto, D. & Manatschal, G. (2010): From passive margins to orogens: The link between ocean-continent transition zones and (ultra)high-pressure metamorphism. Geology, 6, 559-562. Dal Piaz, G.V., Cortiana, G., Del Moro, A., Martin, S., Pennacchioni, G. & Tartarotti, P. (2001): Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt-Saas ophiolite, western Alps. Int. J. Earth Sci., 90, 668-684. Rubatto, D., Regis, D., Hermann, J., Boston, K., Engi, M., Beltrando, M. & McAlpine, S.R.B. (2011): Yo-yo subduction recorded by accessory minerals in the Italian Western Alps. Nature Geoscience, 4, 338-342.

  15. Identification and inversion of converted shear waves: case studies from the European North Atlantic continental margins

    NASA Astrophysics Data System (ADS)

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

    2009-10-01

    Wide-angle shear wave arrivals, converted from compressional to shear waves at crustal interfaces, enable crustal Vp/Vs ratios to be determined which provide valuable constraint on geological interpretations. Analysis of the converted shear wave phases represents the next logical step in characterizing the crustal structure and composition following multichannel seismic structural imaging and tomographic inversion of the wide-angle compressional wave phases. In this offshore study across two passive margins extending from stretched continental to fully oceanic crust, the high-data density (2-10 km ocean bottom seismometer, OBS, spacing) and a consistent, efficient conversion interface produced shear wave data sets suitable for traveltime inversion. The shear waves were recorded by three orthogonal geophones in each OBS. Arrival phases, visible to 180 km offset, were identified using their arrival times, moveout velocities and particle motions. Across the North Atlantic volcanic rifted continental margins studied, breakup was accompanied by the eruption of large volumes of basalts of the North Atlantic Igneous Province. The interface between post-volcanic sediments and the top of the basalts provides the dominant conversion boundary across the oceanic crust and the continent-ocean transition. However, the shear wave data quality was significantly diminished at the continental ends of the profiles where the thick basalt flows and hence this conversion interface feathers out and crustal attenuation increases. Initial modelling of the converted shear wave phases was carried out using a layer-based approach with arrivals converted on the way up used to constrain the Vp/Vs ratio of the post-volcanic sedimentary sequence beneath each OBS. To produce a model with continuous crustal S-wave velocities, the compressional wave velocities beneath the sediment-top basalt interface were transformed into starting shear wave velocities using a constant value of Vp/Vs and the inversion carried out by specifying the appropriate ray path. Once the data set had been fully interpreted, correction of the traveltimes to effective symmetric ray paths enabled us to apply a regularized grid inversion. Such inversions are less subjective than the layer-based approach and yield more robust minimum structure results with quantifiable errors, except in the vicinity of a known subbasalt low-velocity zone encountered on the Faroes margin. Monte Carlo analyses were performed for this approach; the average model from multiple inversions using randomized starting models and traveltimes shows the structure required by the traveltimes and the model standard deviation gives an estimate of uncertainty. Model and inversion parametrizations were fully tested and optimum parameters chosen for compressional and shear wave inversions. This allows, after appropriate model smoothing, an estimate to be made of the spatial variation of the Vp/Vs ratio within the crust. There are marked gradients in Vp, Vs and Vp/Vs ratio across the continent-ocean transition, which may result from intrusion of high magnesium mafic igneous material into the crystalline continental crust. The Vp/Vs ratio, used in conjunction with Vp, also provides constraints on the subbasalt lithologies forming the low-velocity zone. We conclude from such an analysis that this zone is unlikely to be composed entirely of igneous hyaloclastite material; some proportion of clastic sedimentary rocks is likely to be present. The Vp/Vs and Vp properties of the units underlying the low-velocity zone are inconsistent with crystalline continental basement and this unit is likely to represent a sill-intruded Mesozoic sedimentary sequence from a pre-breakup sedimentary basin.

  16. 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 signi?cant uplift movements. These vertical movements represent a long-standing enigma and they have been linked to sea-level ?uctuations, climate deterioration and tectonics but as yet they remain unexplained. Here we combine regional Multi-Channel Seismic re?ection 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 signi?cant 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 ?uctuations. 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.

  17. Quaternary mass wasting on the western Black Sea margin, offshore of Amasra

    NASA Astrophysics Data System (ADS)

    Dondurur, Derman; Küçük, H. Mert; Çifçi, Günay

    2013-04-01

    In recent years, the western Black Sea margin has become well-studied due to its potential for petroleum plays in relatively deeper waters. In 2010, multi-channel seismic, multibeam bathymetry and Chirp high resolution seismic data were collected in order to define the existing geohazards along the margin, to identify the seabed morphology and to determine mass movement types and their run-out distances. Seismic data indicate that the western Black Sea margin is an unstable region with sediment erosion. Particularly, an unstable area offshore of Amasra in the NW consisting of four slides and four buried debris lobes is named the Amasra mass failure zone. Different types of sliding with varying sizes and different mechanisms are observed. These include sliding in the steep slope zones where block-type sliding occurs, smaller-scale slides on the canyon walls, and relatively larger slides in the Amasra mass failure zone. Block-type sliding is observed on the upper continental slope to the south as well as on the canyon walls. They are formed along the rotational faults and occur due to the gravitational loading on the steep slope zones possibly triggered by local seismic activity. In addition, seven large debris lobes identified in the northern toe of the slope buried in the Quaternary sediments triggered by excess pore pressures due to high sediment input and submarine fluid flow. We suggest that earthquake activity may be an important agent for all kind of mass movements in the area. In addition, we propose that the slides in the Amasra mass failure zone are triggered by excess pore pressures in shallow sediments due to the submarine fluid flow possibly produced from gas hydrate dissociation. Warmer Mediterranean seawater input during the rapid transgression period after the Last Glacial Maximum in the Black Sea together with the rapid sedimentation resulted in destabilization of gas hydrates, which caused excess pore pressures in shallow sediments leading to massive sediment failures. Small-scale normal faults around the scarps may be a secondary factor promoting the failures providing the suitable pathways for the fluid flow as well as the suitable weak surfaces for the sliding.

  18. Dynamic support by the Icelandic plume and vertical tectonics of the northeast Atlantic continental margins

    NASA Astrophysics Data System (ADS)

    Clift, Peter D.; Turner, Jonathan

    1995-12-01

    Late Paleocene-early Eocene continental rifting in the northeast Atlantic differs significantly from earlier episodes of margin formation in the Central Atlantic. At a nonvolcanic margin, rifting occurs over a wide area, with little associated magmatism. Postrift subsidence decreases in a predictable, exponential pattern with time. In contrast, subsidence analysis of Ocean Drilling Program and Deep Sea Drilling Project drill sites from the Vøring Plateau, Hatton Bank, and East Greenland Margin show that in these areas the continent-ocean transition (COT) is very sharp, with ß increasing over a horizontal distance of 30-50 km from values of 1.1-1.50 on the continent side to ? at the COT. Drilling penetrated the thick seaward dipping basaltic sequences that typify the East Greenland Margin at Site 917. Fluvial sandstones underlying the basalts show that the area was subaerially exposed prior to continental breakup, but the amount and timing of any uplift are presently unconstrained. Sediment backstripping techniques allow a comparison between the reconstructed and predicted subsidence histories and thus an estimate of the thermal anomaly through time. Anomalous slow subsidence in the early postrift period at 63°N offshore East Greenland is attributed to support by the Icelandic plume. Dynamic support is weaker on the Hatton Bank and on the Vøring Plateau. Variations in the strength and duration of the support suggest that the plume was a large, 1000-km-radius structure that lay under the Greenland craton at the time of breakup. Current data suggest that it may have crossed the East Greenland coast at 40 Ma. Discrepancies between uniform stretching models and the reconstructed subsidence for sites on the East Greenland shelf and the Vøring Plateau allow the amount of igneous underplating at the time of breakup to be estimated. Calculations suggest a maximum of around 8.8 km of gabbroic underplating occurred at the time of breakup under the East Greenland shelf and 2.4 km under the Vøring Plateau, although both these bodies thin rapidly away from the COT.

  19. 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., III; 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 correct interpretation of processes related to hydrocarbon seepage in continental margin environments and elsewhere. ?? 2007 Elsevier Ltd. All rights reserved.

  20. Built-up of the continental margin offshore Central Mozambique from marine geophysical investigations

    NASA Astrophysics Data System (ADS)

    Heyde, I.; Block, M.; Ehrhardt, A.; Reichert, C. J.; Schreckenberger, B.

    2009-12-01

    In September/October 2007, along with institutes from Germany, France and Portugal BGR conducted the cruise MoBaMaSis (Mozambique Basin Marine Seismic Survey) using RV MARION DUFRESNE. The goal of the marine geophysical measurements offshore central Mozambique was the investigation of the continental margin in terms of its structure and formation history with special focus on the opening history of Eastern Gondwana and the hydrocarbon potential. A total of four long transects (450 to 225 km long) and a number of connection lines were acquired from the shelf and the slope into the deep Mozambique Basin. The data comprises multichannel seismic reflection (MCS), magnetic, gravimetric and swath bathymetry. On the eastern two transects two on-/offshore seismic refraction studies were carried out. Apart from results of the MCS and the magnetic work, in particular the results of the gravity data are presented. A 3D density model was developed. In the Mozambique Basin a large thick sedimentary succession of up to 8 km thickness from Jurassic to present is observed. Two deep reaching wells supported, at least in part, the identification of stratigraphy. Faint indications for SDR sequences related to volcanic flows are found in the northern part of the study area. In the south, the Beira High represents a prominent structure. The basement high with sediments of considerable reduced thickness is characterized by a distinct gravity minimum. A possible explanation is that the high is formed by a continental fragment. In addition, no clear magnetic chrons are identifiable. Thus, stretched continental crust is assumed underlying this part of the Mozambique Basin.

  1. 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 ?13C 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.

  2. Average QLg, QSn, and observation of Lg blockage in the Continental Margin of Nova Scotia

    NASA Astrophysics Data System (ADS)

    Mousavi, S. Mostafa; Cramer, Chris H.; Langston, Charles A.

    2014-10-01

    The term "Lg blockage" refers to the sudden disappearance of the Lg phase along a particular propagation path which is commonly seen at continental-oceanic transition zones. In this paper we present observational evidence of Lg blockage across the continental margin of Nova Scotia in eastern Canada. Regional Lg and Sn spectra from 91 events with epicentral distances between 100 and 1200 km and magnitudes between 2.5 and 4.7 are inverted simultaneously for the source spectrum, site amplification, and average attenuation. The vertical displacement spectra were estimated between 0.9 and 10.75 Hz. The assumptions include a fixed frequency-independent geometric spreading rate for Lg and a frequency-dependent spreading model for the Sn. Estimates for the apparent regional attenuations are QLg (f) = 615(±25) f0.35(±0.04) and QSn (f) = 404(±23) f0.45(±0.03). Results from this study provide an accurate parameterization of observed amplitude spectra and are valuable for representing wave propagation in the region. Based on the observation of a strong trade-off between Sn and Lg amplitudes which have different attenuation characteristics, we conclude any attenuation study based on measuring amplitude of a package of several different phases, without taking into consideration the propagation characteristics of individual waveforms at the region of study, may bias the estimation of average regional Q.

  3. 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 and West Greenland, close to where continental crust starts to thin towards oceanic crust, illustrates the common association between EPCMs and the edges of cratons. These observations indicate that the elevation of EPCMs may be due to processes operating where there is a rapid change in crustal/lithosphere thickness. Vertical motion of EPCMs may thus be related to lithosphere-scale folding caused by compressive stresses at the edge of a craton (e.g. Cloetingh et al., 2008). The compression may be derived either from orogenies elsewhere on a plate or from differential drag at the base of the lithosphere by horizontal asthenospheric flow (Green et al., 2013). Bonow, Japsen, Nielsen. Global Planet. Change in review. Cloetingh, Beekman, Ziegler, van Wees, Sokoutis, 2008. Geol. Soc. Spec. Publ. (London) 306. Cobbold, Meisling, Mount, 2001. AAPG Bull. 85. Green, Lidmar-Bergström, Japsen, Bonow, Chalmers, 2013. GEUS Bull. 2013/30. Japsen, Chalmers, Green, Bonow 2012a, Global Planet. Change 90-91. Japsen, Bonow, Green, Cobbold, Chiossi, Lilletveit, Magnavita, Pedreira, 2012b. GSA Bull. 124. Japsen, Green, Bonow, Nielsen. Global Planet. Change in review.

  4. The Mesozoic Continental Magmatism in Brazil: its Role in the Western Gondwana Evolution from Integrated Paleomagnetic and Geochemical Data

    NASA Astrophysics Data System (ADS)

    Ernesto, M.; Marques, L. S.

    2011-12-01

    Most of the Paleozoic era in the South American platform represents a period of tectonic quiescence during which large sedimentary basins evolved. Subsequently an intense magmatic activity took place preceding the disclosure of the Gondwana from Pangea, and later the disruption of the western Gondwana blocks (South America and Africa separation). In Brazil Early Jurassic (~220-180 Ma) tholeiitic basalts erupted mostly in the northern area (Amazonas and Parnaíba basins), whereas the Early Cretaceous (~140-120 Ma) is best represented by the huge magmatism of the Serra Geral Formation (Paraná basin, southeastern Brazil). An intense associated intrusive activity in the form of dykes and sills of both ages is widespread all over the country but tends to concentrate towards the continental margins. The integration of paleomagnetic and geochemical data on the Brazilian Mesozoic magmatism put some constraints on the timing, duration and the mantle sources involved in the generation of the magma products related to the different magmatic events.

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

    NASA Astrophysics Data System (ADS)

    ZUO, Xuran; CHAN, Lung

    2015-04-01

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

  6. Numerical Modeling of Salt Tectonics on Passive Continental Margins: Preliminary Assessment of the Effects of Sediment Loading,

    E-print Network

    Beaumont, Christopher

    Numerical Modeling of Salt Tectonics on Passive Continental Margins: Preliminary Assessment Sciences The University of Leeds LS2 9JT Leeds United Kingdom Abstract Salt tectonics in passive model of frictional-plastic sedimentary overburden overlying a linear viscous salt layer. We present

  7. Surficial clay mineral distribution on the southwestern continental margin of India: evidence of input from the Bay of Bengal

    Microsoft Academic Search

    Onkar S. Chauhan; A. R. Gujar

    1996-01-01

    Analyses of spatial distribution of clay minerals, sediment texture, and > 63 ?m 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

  8. Surficial clay mineral distribution on the southwestern continental margin of India: evidence of input from the Bay of Bengal

    Microsoft Academic Search

    Onkar S. Chauhan; A. R. Gujar

    1996-01-01

    Analyses of spatial distribution of clay minerals, sediment texture, and > 63 mum 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

  9. 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.

  10. Phanerozoic history of Western Australia related to continental drift

    Microsoft Academic Search

    J. J. Veevers

    1971-01-01

    From north to south, the sedimentary basins of Western Australia change from broad platforms of wholly marine strata that span the entire Phanerozoic (Bonaparte Gulf and Canning Basins) through the intermediate Carnarvon Basin to rifts of nonmarine Permian and Mesozoic strata (Perth Basin). These contrasts in age, facies, and structure reflect different positions of the basins in Gondwanaland: the Bonaparte

  11. 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.

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

    SciTech Connect

    Jabour, H. (ONAREP, Rabat (Morocco))

    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.

  13. 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.

  14. New Data From the Southern Levantine Continental Margin and outer Nile Cone - the GEMME Project

    NASA Astrophysics Data System (ADS)

    Huebscher, C. P.; Ben-Avraham, Z.; Dehghani, A.; Gohl, K.; Paetzold, J.

    2002-12-01

    In order to investigate the tectonic and sedimentary setting of the southern Levantine continental margin as well as the Late Quaternary paleoceanography of the outer Nile Cone a geophysical and geological survey - the GEMME project - was carried out in the eastern Mediterranean between February and March 2002. The German research vessel METEOR operated for 5 weeks in the territorial waters of Israel and Egypt. The experiments included refraction and reflection seismics, gravity and magnetics, swath sounding and subbottom profiling, and sediment sampling with gravity and multi-corer. We found a strong correlation between salt tectonic features and the position of channel levee complexes evolved on the outer Nile Cone, which is different to other submarine fans without a mobile layer beneath where the location of the complexes is an autocyclic process. The high-resolution stratigraphy of the Post-Messinian sediment prism reflects the interplay between sediment input, transport mechanisms, uplift and subsidence, halokinetics, and changes of sea level and climate. We found a wide abundance of gassy clastic sediments above the basal Pliocene unconformity and relation between chemoherms, faulting, and gas/fluid migration. The dominant disturbances along the continental slope off Israel are most likely triggered by salt (gravity) tectonic. The position of the landward termination of the Messinian evaporites is located beneath the present shelf in the north but beneath the lower continental slope of bottomset in the south. The salt tectonic is most likely responsible for the along-strike variation of the margin morphology. The Pelusium line reflects the salt tectonic and is presumably not a basement tectonic feature. We found evidence of active tectonic off Haifa Bay in the prolongation of the Carmel rift and it related branches. The paleoclimate history of the Nile deposits will be studied from the extensive sediment core collection. Four different sediment profiles were covered, i.e. three core transects representing the three different provinces of the Nile fan and one core transect across the continental margin of southern Israel. High resolution dating by AMS 14C and high resolution logger methods will reveal detailed sedimentological and geochemical chronologies in proximal and distal provinces of the marine Nile fan. High resolution geochemical and stable isotope chronologies will reveal climatic and oceanographic changes in the southeastern Mediterranean under the impact of the Nile. Special emphasis will be given to reconstruct the late glacial and Holocene climatic record and to compare it with the terrestrial archives of African and Middle East climate change. It will be also possible to reconstruct changes in sedimentation in post-Aswan dam times. To better understand land-ocean interactions it will be needed to link our high resolution marine sediment records with terrestrial paleoclimate archives. We also expect an improved understanding of the impact of short term global climatic changes on the deposition of Nile sediments and the climate of the southeastern Mediterranean. The analysis of the data will be carried out within a science network, that developed during the initial phase of the EUROCORES/EUROMARGIN program of the ESF. Institutions form Germany, Egypt, England, France, Israel, Netherlands, and Palestine are involved.

  15. A nitrogen budget for the continental margin of the Peruvian oxygen minimum zone

    NASA Astrophysics Data System (ADS)

    Dengler, Marcus; Bryant, Lee; Sommer, Stefan; Bourbonnais, Annie; Dullo, Christian; Dale, Andy

    2015-04-01

    In oxic environments, nitrogen (N) is frequently a limiting nutrient for primary production and hence a controlling element in marine ecosystems. The fixed form of N, i.e., bioavailable N for primary production, is primarily in the oxidized form of nitrate (NO3-). However, in the sub-oxic environments of oxygen minimum zones (OMZs), N-species are biochemically converted to biogenic N2 gas which is then released, or lost, to the atmosphere. N-cycling under sub-oxic conditions thus diminishes the oceanic pool of bioavailable N. It has been suggested that although OMZs constitute only about 1% of global ocean volume, they account for about 20-40% of global oceanic N loss. However, to date these estimates are subject to largely uncertainties. Here, we quantify the rate of N-cycling and the associated N-loss by evaluating all terms of a benthic-pelagic nutrient transport budget at the continental margin off Peru using observations from an extensive measurement program conducted along the continental slope and shelf region at 12°S. The data set was collected during austral summer in 2013 and consists of nutrient, microstructure and CTD/O2 profiles as well as shipboard velocity data from two research cruises, a glider swarm experiment and current time series from a moored array. To constrain the benthic contribution to the nutrient budget, benthic nutrient fluxes were measured in benthic chambers using Biogeochemical Observatory (BIGO) landers. Detailed budget determinations were performed on the upper continental slope and shelf break as well as at the shelf. Both regions were anoxic but different with regard to nutrient distribution as well as benthic nutrient release rates. Three major conclusions can be inferred from the study: (1) Unexpectedly, the results showed that diapycnal nutrient fluxes, driven by turbulent mixing caused by the breaking of non-linear internal waves, was one to two orders of magnitude larger than advective and lateral-diffusive fluxes. (2) The relative contribution of benthic nutrient fluxes to nutrient cycling was between 30% and 50%. (3) Nitrogen conversion rates on the shelf (50m-100m water depth) were an order of magnitude larger that at the continental slope (200m-300m water depth). The strong differences in the magnitude of the nutrient cycling rates most likely originate from the presence of sulfidic bottom waters that were observed on the shelf.

  16. 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) section of what are probably post-Miocene rocks and sediment. Extrapolating ages obtained from Ocean Drilling Program site 1015 indicates that this sedimentary cover is Quaternary, possibly no older than 600 ka. Folds and faults along the base of the San Pedro Escarpment began to form during 8-13 ka ago. Refraction-velocity data show that high-velocity rocks, probably the Catalina Schist or Miocene volcanic rocks, underlie the sedimentary section. The San Pedro Basin developed along a strike-slip fault, widens to the southeast, and is deformed by faults having apparent reverse separation and by folds near Redondo Canyon and the Palos Verdes Peninsula.

  17. The Wide Bay Canyon system: A case study of canyon morphology on the east Australian continental margin

    NASA Astrophysics Data System (ADS)

    Yu, P. W.; Hubble, T.; Airey, D.; Gallagher, S. J.; Clarke, S. L.

    2014-12-01

    A voyage was conducted aboard the RV Southern Surveyor in early 2013 to investigate the east Australian continental margin. From the continental slope of the Wide Bay region offshore Fraser Island, Queensland, Australia, remote sensing data and sediment samples were collected. Bathymetric data reveals that the continental slope of the region presents a mature canyon system. Eight dredge samples were recovered from the walls of Wide Bay Canyon and the adjacent, relatively intact continental slope along the entire length of the slope, from the start of the shelf break to the toe, in water depths ranging from 1100-2500 m. For these samples, sediment composition, biostratigraphic age, and bulk mineralogy data are reported. These slope-forming sediments are primarily comprised of calcareous sandy-silts. Occasional terrestrial plant fossils and minerals can be found in a mostly marine-fossiliferous composition, suggesting minor but significant riverine and aeolian input. Biostratigraphic dates extracted from the foraminiferal contents of these samples indicate that the intra-canyon and slope material was deposited between Middle Miocene to Pliocene, implying that the incision of this section of the margin and formation of the erosional features took place no earlier than the Pliocene. In conjunction with bathymetric data of the local continental slope, the depositional origins of this section of the east Australian continental margin, and the timing of major morphological events such as slope failure and canyon incision can be interpreted. The Wide Bay Canyon system can serve as a representative case study of local canyon formation, allowing a better understanding of the past or ongoing processes that are shaping the margin and giving way to similar morphologies.

  18. 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.

  19. A volcanic province near the western termination of the Charlie-Gibbs Fracture Zone at the rifted margin, offshore northeast Newfoundland

    NASA Astrophysics Data System (ADS)

    Keen, C. E.; Dafoe, L. T.; Dickie, K.

    2014-06-01

    A mid-Cretaceous to Late Cretaceous volcanic province, named here the Charlie-Gibbs Volcanic Province, is described near the western termination of the Charlie-Gibbs Fracture Zone, against the rifted continental margin northeast of Newfoundland. We used seismic data to map 14 volcanic seamounts, now buried below younger sediments. They rise 0.7 to 2 s two-way time (twt) above the surrounding basement level and are about 8-30 km wide. Some are conical while others are more flat-topped. Underlying igneous units resembling flows and sills are also observed. Based on magnetic modeling of the large positive magnetic anomalies associated with the seamounts, the total thickness of igneous rocks can locally reach about 8 km. This magmatism occurred in the vicinity of the Charlie-Gibbs Fracture Zone and extends about 150 km to the north along the rifted continental margin. The volcanic province also forms the northern boundary of the Jurassic-Early Cretaceous Orphan Basin, along a major transform margin there. Truncation of rift-related structures which extend to deep crustal levels is observed at the transform, along trends similar to those of prerift Appalachian terrane boundaries on the adjacent shelf. This suggests the existence of a preexisting weak zone in the continental lithosphere within which a complex strike-slip fault system developed and may have controlled the location of final continental breakup between the Rockall and North American plates in the Late Cretaceous.

  20. Using crustal thickness and subsidence history on the Iberia-Newfoundland margins to constrain lithosphere deformation modes during continental breakup

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex lithosphere deformation history during continental breakup and seafloor spreading initiation leading to complex OCT architecture with hyper-extended continental crust and lithosphere, exhumed mantle and scattered embryonic oceanic crust and continental slivers. Initiation of seafloor spreading requires both the rupture of the continental crust and lithospheric mantle, and the onset of decompressional melting. Their relative timing controls when mantle exhumation may occur; the presence or absence of exhumed mantle provides useful information on the timing of these events and constraints on lithosphere deformation modes. A single lithosphere deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere deformation events for two profiles across the present-day conjugate Iberia-Newfoundland margins, using forward modelling of continental breakup and seafloor spreading initiation calibrated against observations of crustal basement thickness and subsidence. Flow fields, representing a sequence of lithosphere deformation modes, are generated by a 2D finite element viscous flow model (FeMargin), and used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the upper 15-20 km of the lithosphere inducing passive upwelling beneath that layer; extensional faulting and magmatic intrusions deform the topmost upper lithosphere, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). Buoyancy enhanced upwelling, as predicted by Braun et al. (2000) is also kinematically included in the lithosphere deformation model. Melt generation by decompressional melting is predicted using the parameterization and methodology of Katz et al. (2003). The distribution of lithosphere deformation, the contribution of buoyancy driven upwelling and their spatial and temporal evolution including lateral migration are determined by using a series of numerical experiments, tested and calibrated against observations of crustal thicknesses and water-loaded subsidence. Pure-shear widths exert a strong control on the timing of crustal rupture and melt initiation; to satisfy OCT architecture, subsidence and mantle exhumation, we need to focus the deformation from a broad to a narrow region. The lateral migration of the deformation flow axis has an important control on the rupture of continental crust and lithosphere, melt initiation, their relative timing, the resulting OCT architecture and conjugate margin asymmetry. The numerical models are used to predict margin isostatic response and subsidence history.

  1. Explaining darker deep convective clouds over the western Pacific in comparison to tropical continental regions

    NASA Astrophysics Data System (ADS)

    Sohn, B.; Choi, M.

    2013-12-01

    A recent study, based on analysis of Moderate Resolution Imaging Spectroradiometer (MODIS) solar channel measurements, reports that deep convective clouds (DCCs) over the western Pacific are found to be darker than DCCs over continental tropical regions such as Africa and South America. This phenomenon is quite interesting although an immediate attempt to explain the reason may be the different cloud microphysics within the deep convective clouds between land and ocean. However, in fact, attempt has not made what causes such difference. In this study, aiming at understanding why DCCs over the western Pacific show generally lower reflectivity in comparison to tropical African and South American regions, regional differences in optical properties of DCCs are examined using Cloud Profile Radar (CPR) onboard CloudSat and Cloud Aerosol Lidar Infrared Pathfinder Satellite Observation (CALIPSO) measurements. Analysis of four January months of 2007-2010, demonstrates that there are distinct difference in ice water path (IWP) between the western Pacific and other two continental regions. Also found is small but meaningful differences in the effective radius. Results lead to a conjecture that smaller IWP over the western Pacific is the main cause inducing smaller reflectance. Radiative transfer experiments investigating what optical parameters of DCCs are responsible for the darker clouds over the western Pacific make it clear that smaller ice water content available in the western Pacific is the main cause.

  2. 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 greenschist facies stretching lineations underlie SSE-directed very-low metamorphic grade deformation in the restored ORNC. In both sections, restorations place lower metamorphic grades rocks over higher grades. Thus, stratigraphic, structural and metamorphic evidence indicate that the basement rocks in the windows in southernmost and northernmost Scandinavia are fully allochthonous. Elsewhere, metamorphic data from the Luo-Pakte/Rombak section indicates that the Rombak window basement is allochthonous. Restoration of the Blaik Nappe Complex (LA) in the Central Scandes produces a stratigraphic repetition over cover sediments on basement in the Børgefjellet tectonic window. The basement window rocks are given to be allochthonous. Thus where data is available, an allochthonous model has been applied. An allochthonous origin for the Window Allochthon is expected, since the lower part of the overlying Middle Allochthon is often dominated by basement, sometimes with massively thick debris-flow deposits requiring a proximal topographic-high source area. A simplified palaeogeographic model for the Iapetus Baltoscandian continental margin, applicable with minor variations to all of the orogen, shows a thin-shelf sequence seen in the Autochthon, which extends for an unknown distance under the nappe pile, and in the Lower Allochthon, passing into a deeper water basin (Lower Allochthon). Outboard, lay a topographic basement high, with a thin cover succession (Window Allochthon). The basement dominated parts of the Middle Allochthon which lay west of the restored Window Allochthon should properly be assigned to the latter. These passed into a deeper basin (Middle Allochthon sediments) which merged with Iapetus oceanic crust.

  3. Crustal Configuration of the Terrace off Trivandrum, Southwestern Continental Margin of India

    NASA Astrophysics Data System (ADS)

    Kurian, J.; Vadakkeyakath, Y.; Bhattacharya, G. C.; Sivaramakrishnan, R.; Sk221 Scientific Team*

    2010-12-01

    The Terrace off Trivandrum (TOT) is an anomalous bathymetric protrusion in the southwestern continental margin of India. Recent studies based on paleogeographic reconstructions have suggested that the Terrace off Trivandrum and the bathymetric notch in the Northern Madagascar Ridge are conjugate features related to India-Madagascar separation. While the nature of crust underlying the Northern Madagascar Ridge is ambiguous, no information is available on the nature of the crust in the Terrace off Trivandrum. In this context, the present study was carried out to understand the crustal configuration of TOT based on recently acquired bathymetry, multi-channel seismic reflection, gravity and magnetic data. The seismic reflection data clearly brings out the block-faulted nature of the basement and presence of a nearly N-S trending wide basement high in the central part of the TOT. This region is characterized by the presence of several prominent high amplitude magnetic anomalies as well as several positive residual gravity anomalies superimposed over a broad negative free-air gravity anomaly. At places, the prominent gravity and magnetic anomalies coincide with the bathymetric features suggesting their probable genesis as volcanic emplacements. In the central part of the TOT, the prominent gravity anomalies coincide with the wide basement high. However, no distinctive basement features are conspicuous in the seismic section corresponding to the prominent magnetic anomalies observed at places over this basement high. Integrated modelling of gravity and magnetic data constrained by seismic reflection information suggests that the crustal configuration of the TOT region is comparable to a thinned continental crust intermingled with intruded / extruded volcanics. *SK221 Scientific Team: M.M. Subramaniam, A. Tyagi, P. Vohat, A. Bhattacharya, V.S.K. Rao, P. Shetkar, R.D. Singh, U.K. Singh, K.V. Swamy, S. Upadhyaya

  4. A Holocene paleosecular variation record from the northwestern Barents Sea continental margin

    NASA Astrophysics Data System (ADS)

    Sagnotti, Leonardo; Macrì, Patrizia; Lucchi, Renata; Rebesco, Michele; Camerlenghi, Angelo

    2011-11-01

    A high-resolution paleomagnetic and rock magnetic study has been carried out on sediment cores collected in glaciomarine silty-clay sequences from the continental shelf and slope of the southern Storfjorden trough-mouth fan, on the northwestern Barents Sea continental margin. The Storfjorden sedimentary system was investigated during the SVAIS and EGLACOM cruises, when 10 gravity cores, with a variable length from 1.03 m to 6.41 m, were retrieved. Accelerator mass spectrometry (AMS) 14C analyses on 24 samples indicate that the cores span a time interval that includes the Holocene, the last deglaciation phase and in some cores the last glacial maximum. The sediments carry a well-defined characteristic remanent magnetization and have a valuable potential to reconstruct the paleosecular variation (PSV) of the geomagnetic field, including relative paleointensity (RPI) variations. The paleomagnetic data allow reconstruction of past dynamics and amplitude of the geomagnetic field variations at high northern latitudes (75°-76° N). At the same time, the rock magnetic and paleomagnetic data allow a high-resolution correlation of the sedimentary sequences and a refinement of their preliminary age models. The Holocene PSV and RPI records appear particularly sound, since they are consistent between cores and they can be correlated to the closest regional stacking curves (UK PSV, FENNOSTACK and FENNORPIS) and global geomagnetic model for the last 7 ka (CALS7k.2). The computed amplitude of secular variation is lower than that outlined by some geomagnetic field models, suggesting that it has been almost independent from latitude during the Holocene.

  5. 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 the finger-like plumes. We demonstrate the potential applicability of our model to clustering of arc magmatism in several subduction zones, such as Baja California (Ramos-Velázquez et al., Revista Mexicana de Ciencias Geológicas,2008), North Island of New Zealand (Booden et al., J. Volcanol. Geotherm. Res., 2010), Northeast Japan (Kimura and Yoshida,Journal of Petrology, 2006); Ecuador (Schütte et al., Tectonophysics,2010) and Lesser Antilles (Labanieh et al., EPSL,2010).

  6. 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 ?13C = -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 models predicting hydrate dissolution at present and in the future; and third, they may serve as a baseline (year 2012) estimate of the bubble flux that will potentially increase in the future due to ever-increasing global-warming-induced bottom water warming and hydrate dissociation.

  7. 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 ?13C = -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 predicting hydrate dissolution at present and in the future, third, may serve as baseline (year 2012) estimate of the bubble flux that will potentially increase in future due to ever-increasing global-warming induced bottom water-warming and hydrate dissolution.

  8. Deep Crustal Structure of the Continental Margin off the Explora Escarpment and in the Lazarev Sea, East Antarctica

    NASA Astrophysics Data System (ADS)

    Jokat, Wilfried; Ritzmann, Oliver; Reichert, Christian; Hinz, Karl

    2004-09-01

    This study presents the results of a seismic refraction experiment that was carried out off Dronning Maud Land (East Antarctica) along the Explora Escarpment (14° W 12° W) and close to Astrid Ridge (6°E). Oceanic crust of about 10 km thickness is observed northwest of the Explora Escarpment. Stretched continental crust, observed southeast of the escarpment, is most likely intruded by volcanic material at all crustal levels. Seismic velocities of 7.0 7.4 km/s are modelled for the lower crust. The northern boundary of this high velocity body coincides approximately with the Explora Escarpment. The upper crystalline crust is overlain by a 4-km thick and 70-km wide wedge of volcanic material: the Explora Wedge. Seismic velocities for the oceanic crust north of the Explora Escarpment are in good agreement with global studies. The oceanic crust in the region of the Lazarev Sea is also up to 10-km thick. The lower crystalline crust shows seismic velocities of up to 7.4 km/s. This, together with the larger crustal thickness might point to higher mantle temperatures during the formation of the oceanic crust. The more southerly rifted continental crust is up to 25-km thick, and also has seismic velocities of 7.4 km/s in the lower crystalline crust. This section is interpreted to consist of stretched continental crust, which is heavily intruded by volcanic material up to approximately 8-km depth. Multichannel seismic data indicate that, in this region, two volcanic wedges are present. The wedges are interpreted to have evolved during different time/rift periods. The wedges have a total width of at least 180 km in the Lazarev Sea. Our results support previous findings that the continental margin off Dronning Maud Land between ?2°E and ?13°E had a complex and long-lived rift history. Both continental margins can be classified as rifted volcanic continental margins that were formed during break-up of Gondwana.

  9. North American continental margin records of the Paleocene-Eocene thermal maximum: Implications for global carbon and hydrological cycling

    NASA Astrophysics Data System (ADS)

    John, CéDric M.; Bohaty, Steven M.; Zachos, James C.; Sluijs, Appy; Gibbs, Samantha; Brinkhuis, Henk; Bralower, Timothy J.

    2008-06-01

    The impacts of the Paleocene-Eocene thermal maximum (PETM) (˜55 Ma), one of the most rapid and extreme warming events in Earth history, are well characterized in open marine and terrestrial environments but are less so on continental margins, a major carbon sink. Here, we present stable isotope, carbonate content, organic matter content, and C:N ratio records through the PETM from new outcrop sections in California and from cores previously drilled on the New Jersey margin. Foraminifer ?18O data suggest that midlatitude shelves warmed by a similar magnitude as the open ocean (5°C-8°C), while the carbon isotope excursion (CIE), recorded both in carbonate and organic matter ?13C records, is slightly larger (3.3-4.5‰) than documented in open ocean records. Sediment accumulation rates increase dramatically during the CIE in marked contrast to the open ocean sites. In parallel, mass accumulation rates of both organic and inorganic carbon also increased by an order of magnitude. The estimated total mass of accumulated carbon in excess of pre-CIE rates suggests that continental margins, at least along North America, became carbon sinks during the CIE, mainly because of weathering feedbacks and rising sea level. This result is significant because it implies that the negative feedback role of carbon burial on continental margins was greater than previously recognized.

  10. 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 between 36 and 38°S frequent Holocene turbidite deposition may generally correspond to paleoseismic events. However, high terrigenous sedimentation rates prevent high-resolution radiocarbon dating. The climatic transition region between 32.5 and 36°S appears to be best suited for turbidite paleoseismology.

  11. Neogene Changes in Southern Ocean Sedimentation Based on Mass Accumulation Rates at Four Continental Margins

    NASA Astrophysics Data System (ADS)

    Handwerger, D. A.; Jarrard, R. D.

    2001-12-01

    For six sites at four continental margins around the Southern Ocean, downhole and continuous-core logs were converted to lithologic logs. These logs, along with porosity logs and age data, were used to calculate high-resolution mass accumulation rates (MAR). Total MAR was determined for ODP Site 1095 on the margin of the Antarctic Peninsula and ODP Site 1165 offshore Prydz Bay, Antarctica. Carbonate and non-carbonate (terrigenous) MAR were calculated for ODP Sites 1123 and 1124 east of New Zealand and Sites 1168 and 1172 west and southeast of Tasmania. Changes in carbonate MAR are evident around Tasmania and New Zealand at 23 Ma and 14 Ma, suggesting changes in deep water circulation or surface carbonate productivity at these times, possibly related to changes in Antarctic glaciation. After 23 Ma there is a pronounced drop in carbonate MAR at Sites 1124 and 1168. Carbonate MAR increases at 14 Ma at Sites 1123, 1168 and 1172. The broad pattern of MAR evolution at Antarctic Sites 1095 and 1165 is continual and relatively constant decrease over the past 23 Ma, but with episodes of stepwise decrease. One such decrease is seen at 14 Ma at Site 1165, coincident with increased carbonate MAR in New Zealand and Tasmania and likely related to mid-Miocene expansion of the East Antarctic Ice Sheet. Another sudden drop in total MAR occurs at about 9 Ma at both Antarctic sites. The drop at Site 1165 may reflect diversion of sediment to a newly formed Prydz Channel fan, and the drop at Site 1095 may reflect a provenance shift.

  12. Biogeochemical characteristics of the surface sediments along the western continental shelf of India

    Microsoft Academic Search

    Josia Jacob; K. A. Jayaraj; H. Habeeb Rehman; N. Chandramohanakumar; K. K. Balachandran; T. V. Raveendran; Thresiamma Joseph; Maheswari Nair; C. T. Achuthankutty

    2009-01-01

    Surface sediments from the western continental shelf of India were analysed for total organic carbon (TOC), total nitrogen (TN), total hydrolysable carbohydrates (TCHO) and proteins (PRT) during the late summer monsoon (September–October, 2003) and pre-monsoon (March–April, 2004) seasons. The region experienced entirely different hydrographic characteristics and productivity patterns during the two seasons. Low oxygenated, cold, nutrient rich (DO<180 ?M, SST<28°C, NO3>2 ?M)

  13. 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 thickness provide a measurement of anomalous uplift or subsidence which we attribute to mantle dynamic topography. We compare our sediment and crustal thickness corrected RDA analysis results with published predictions of mantle dynamic topography from global models.

  14. 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.

  15. Fluid flow during early compartmentalisation of rafts: A North Sea analogue for divergent continental margins

    NASA Astrophysics Data System (ADS)

    Alves, Tiago M.; Elliott, Claire

    2014-11-01

    High-quality 3D seismic data tied to eighteen (18) boreholes are used to investigate the styles of faulting and associated fluid flow features in Triassic-early Jurassic rafts of the Broad Fourteens Basin, Southern North Sea. The study area is presented as an analogue for continental margins experiencing early stage gravitational gliding, i.e. prior to complete separation and downslope translation of individual rafts. In such a setting, and for present-day stress conditions, fault slip data indicate that chasms and faults separating rafts in the Broad Fourteens Basin comprise structures subject to dip slip and strike-slip reactivation. Chasms and faults sub-parallel to these latter chasms comprise the most significant bypass areas for fluid sourced from pre-salt strata. Faults sub-parallel to the main chasms show limited propagation into Early Cretaceous and Cenozoic strata draping the rafts, a character further stressed by the depth of occurrence of fluid pipes and dim spots. This is an important observation, and leads us to postulate that faults formed during early stage rafting control fluid flow in regions where gravitational gliding is limited such as West and Equatorial Africa, Southeast Brazil and parts of the Gulf of Mexico.

  16. An Idealized Model of Organic Carbon Dynamics on the Continental Margin of the Eastern United States

    NASA Astrophysics Data System (ADS)

    Siedlecki, S. A.; Archer, D.; Mahadevan, A.

    2003-12-01

    Continental margins play a significant role in the production and burial of organic carbon in the ocean, bur these areas are poorly resolved global circulation models. In this study, a high-resolution three-dimensional, nonhydrostatic model of an idealized eastern coastal United States after Mahadevan and Archer, 1998, was modified to simulate organic carbon production and export off the shelf. The model assumes a periodic north and south boundary, and an offshore boundary at the shelf-break density front determined by bathymetry. The model uses a free surface and a sigma grid in the vertical. The model is initialized with a vertical nutrient profile taken from the open Atlantic Ocean. As the winds are given time to influence the region, upwelling conditions can result in the vertical movement of water. Vertical diffusion also carries nutrients into the euphotic zone. Excess nutrients in the euphotic zone are converted to particles that advect with the flow while sinking with a velocity of 10-5 m/s. Remineralization is treated as a first-order decay. We will vary the alongshore wind stress, shelf width, and vertical diffusivity to determine their respective impacts on organic carbon export. Eventually, we hope to parameterize the impact of coastal circulation on the carbon cycle with global circulation and carbon models. Mahadevan, A., Archer, D., Modeling a Limited Region of the Ocean, Journal of Computational Physics 145, 555-574, 1998.

  17. Evidence for current-controlled sedimentation along the southern Mozambique continental margin since Early Miocene times

    NASA Astrophysics Data System (ADS)

    Preu, Benedict; Spieß, Volkhard; Schwenk, Tilmann; Schneider, Ralph

    2011-12-01

    Major plastered drift sequences were imaged using high-resolution multichannel seismics during R/V Meteor cruises M63/1 and M75/3 south of the Mozambique Channel along the continental margin of Mozambique off the Limpopo River. Detailed seismic-stratigraphic analyses enabled the reconstruction of the onset and development of the modern, discontinuous, eddy-dominated Mozambique Current. Major drift sequences can first be identified during the Early Miocene. Consistent with earlier findings, a progressive northward shift of the depocenter indicates that, on a geological timescale, a steady but variable Mozambique Current existed from this time onward. It can furthermore be shown that, during the Early/Middle Miocene, a coast-parallel current was established off the Limpopo River as part of a lee eddy system driven by the Mozambique Current. Modern sedimentation is controlled by the interplay between slope morphology and the lee eddy system, resulting in upwelling of Antarctic Intermediate Water. Drift accumulations at larger depths are related to the reworking of sediment by deep-reaching eddies that migrate southward, forming the Mozambique Current and eventually merging with the Agulhas Current.

  18. 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. Oblique extension is recorded also by strike- and oblique-slip fault geometry within the AFZ and buckling of Aptian synrift rocks in response to block rotation and local transpression. Principal cooling episodes affecting both margins identified from the AFTAr occurred in the Mid-Cretaceous 110-90Ma., following a period of high heatflow during which the geothermal gradient was more than twice that of the present day 25°C/km gradient, and 45-35Ma. coinciding with a major change in the pole of rotation at Chron 34 some 84Ma. Additional cooling episodes between 85-75Ma. and 10-2Ma. are restricted to the African side. Given that this margin system underwent breakup some 117Ma., it appears that thermal equilibrium was maintained throughout the initial breakup meaning that no significant uplift and erosion is recorded by the thermal history data during this period. Instead, Mid-Cretaceous cooling is interpreted as a response to the influence of i) the c.105Ma. St Helena Plume and ii) transpressional shortening and inversion that accompanied the breakup to drift period.

  19. 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 (Kwanza basin) is corroborated by enhanced Oligocene and Miocene sedimentation offshore. Thus the on- and offshore geological record in Angola appear directly linked. Cenozoic erosion onshore is mirrored by enhanced Oligocene to Miocene sedimentation offshore. The geomorphological information as well as the stratigraphic record are compatible with the Cenozoic cooling and exhumation as suggested by thermal modelling of apatite fission track data. Although direct indicators for Cretaceous cooling and erosion are missing in Angola, minor amounts of Cretaceous erosion may be disguised by the Miocene final event.

  20. New geophysical constraints on the stratigraphy and structure of the southern New England continental margin

    NASA Astrophysics Data System (ADS)

    Flores, C. H.; Brothers, D. S.; ten Brink, U. S.; Chaytor, J. D.

    2011-12-01

    A number of factors influence the evolution of continental slopes including changes in sediment supply, base level and energy flux. Slopes can be categorized as either constructional or destructional depending on the balance between erosion and deposition. The southern New England continental slope represents one of few regions of the U.S. Atlantic margin that maintains constructional form: the slope is ~50 km wide, has convex curvature that gradually increases in gradient from the shelf down to the lower slope, and contains relatively few, widely spaced canyons. Enhanced preservation of slope sedimentary sequences offers a unique opportunity to examine the interplay between sediment delivery, downslope transport and basin physiography. Also, examining these relationships helps understand the geohazards along this section of the U.S. Atlantic margin. We present results from recent seismic reflection surveys conducted along this section of the margin, offshore southern New England and Hudson Canyon in water depths between 500 to 2500 meters. More than 820-km of 2-D seismic reflection data were collected in 2010 and 2011 with a 6 KJ sparker source and a 48-64 channel digital streamer (6.25-m group interval). Seismic horizons are imaged more than 1-sec (two-way travel) below the seafloor, with frequencies between 40-650 Hz, providing up to 2-m vertical resolution. The processed data have been integrated with swath bathymetry and reflection profiles collected by the USGS in the 1970's to examine the relationship between shallow Pleistocene deposits with underlying Tertiary and Cretaceous strata. Despite its constructional form and lack of canyons, the northeastern section of the study region contains pervasive landslides that truncate the upper rise and the slope. We observe a thick, base-of-slope Pleistocene sedimentary wedge that infills steep lower-slope paleo-relief. Based on stratal geometry, the wedge appears to consist of stacked, upslope migrating sediment wave deposits that onlap a regional unconformity. Higher gradient and relief across the lower-slope appears to be limiting upper and middle slope progradation of Pleistocene sequences. Closer to Hudson Canyon, the lower-slope has a relaxed gradient, less relief and the base-of-slope unconformity is more subtle. Pleistocene beds are concordant from the upper slope to the rise. Here, a graded slope profile, a slope in equilibrium, has developed possibly because slope failure of the upper slope appears to be balanced by deposition and aggradation of the lower slope, allowing overall progradation to occur. The southern New England slope also contains evidence for faulting and fluid/gas expulsion extending from irregular, high-amplitude Cretaceous(?) beds through the overlying unconsolidated Pleistocene section. In some instances these features are coincident with large-scale landslide features.

  1. 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 lower-crustal velocity of Pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby Pioneer seamount. The Salinian Block consists of a 15-km-thick layer of velocity 6.0-6.2 km/s overlying a 5-km-thick, high-velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceous arc-derived lower crust, or a magmatically underplated layer. The strong structural variability across the margin attests to the activity of strike-slip faulting prior to and during development of the transcurrent Pacific/North American plate boundary around 29 Ma. Copyright 1996 by the American Geophysical Union.

  2. Petrology and age of volcanic-arc rocks from the continental margin of the Bering Sea: implications for Early Eocene relocation of plate boundaries

    USGS Publications Warehouse

    Davis, A.S.; Pickthorn, L.-B.G.; Vallier, T.L.; Marlow, M. S.

    1989-01-01

    Eocene volcanic flow and dike rocks from the Beringian margin have arc characteristics, implying a convergent history for this region during the early Tertiary. Chemical and mineralogical compositions are similar to those of modern Aleutian-arc lavas. They also resemble volcanic-arc compositions from western mainland Alaska, although greater chemical diversity and a stronger continental influence are observed in the Alaskan mainland rocks. Early Eocene ages of 54.4-50.2 Ma for the Beringian samples are well constrained by conventional K-Ar ages of nine plagioclase separates and by concordant 40Ar/39Ar incremental heating and total-fusion experiments. A concordant U-Pb zircon age of 53 Ma for the quartz-diorite dike is in good agreement with the K-Ar data. Plate motion studies of the North Pacific Ocean indicate more northerly directed subduction prior to the Tertiary and a continuous belt of arc-type volcanism extending from Siberia, along the Beringian margin, into mainland Alaska. Around 56 Ma (chron 25-24), subduction changed to a more westerly direction and subduction-related volcanism ceased for most of mainland Alaska. The increasingly oblique angle of convergence should have ended subduction along the Beringian margin as well. However, consistent ages of 54-50 Ma indicate a final pulse in arc-type magmatism during this period of plate adjustment. -from Authors

  3. HyFlux - Part I: Regional Modeling of Methane Flux From Near-Seafloor Gas Hydrate Deposits on Continental Margins

    Microsoft Academic Search

    I. R. MacDonald; V. Asper; O. P. Garcia; M. Kastner; I. Leifer; T. Naehr; E. Solomon; S. Yvon-Lewis; B. Zimmer

    2008-01-01

    HyFlux - Part I: Regional modeling of methane flux from near-seafloor gas hydrate deposits on continental margins MacDonald, I.R., Asper, V., Garcia, O., Kastner, M., Leifer, I., Naehr, T.H., Solomon, E., Yvon-Lewis, S., and Zimmer, B. The Dept. of Energy National Energy Technology Laboratory (DOE\\/NETL) has recently awarded a project entitled HyFlux: \\

  4. Lithospheric density variations and Moho structure of the Irish Atlantic continental margin from constrained 3-D gravity inversion

    Microsoft Academic Search

    J. Kim Welford; Patrick M. Shannon; Brian M. O'Reilly; Jeremy Hall

    2010-01-01

    A 3-D density anomaly model of the Irish Atlantic continental margin was generated from a regional inversion of the free air gravity data constrained by bathymetric and sediment thickness information. The model results are compared against a recent regional Moho structure compilation from velocity models from crustal-scale wide-angle reflection\\/refraction surveys. Using the inverted model, a regional map of Moho structure

  5. Classification of sea-floor features associated with methane seeps along the Gulf of Cádiz continental margin

    Microsoft Academic Search

    Ricardo León; Luis Somoza; Teresa Medialdea; Adolfo Maestro; Victor Díaz-Del-Río; María del Carmen Fernández-Puga

    2006-01-01

    Based on recently gathered swath-bathymetry, high- to ultra-high-resolution seismic, and underwater camera data, along with dredging and coring samples, this paper examines the relationship between sea-floor features and the nature of hydrocarbon-enriched fluid and gas leaks from degassing of deeply buried sediments along the continental margin of the Gulf of Cádiz (eastern Central Atlantic). A classification into three main groups

  6. Structures of the northeasternmost South China Sea continental margin and ocean basin: geophysical constraints and tectonic implications

    Microsoft Academic Search

    Chun-Feng Li; Zuyi Zhou; Jiabiao Li; Hujun Hao; Jianhua Geng

    2007-01-01

    The northeastern part of the South China Sea is a special region in many aspects of its tectonics. Both recent drilling into\\u000a the Mesozoic and new reflection seismic surveys in the area provide a huge amount of data, fostering new understanding of\\u000a the continental margin basins and regional tectonic evolution. At least four half-grabens are developed within the Northern\\u000a Depression

  7. 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.

  8. Palaeo-ice streams on the west Greenland continental margin during the last glacial cycle

    NASA Astrophysics Data System (ADS)

    O'Cofaigh, Colm; Dowdeswell, Julian; Kilfeather, Aoibheann; Jennings, Anne; Evans, Jeffrey; Noormets, Riko; Walton, Mariah

    2010-05-01

    The Greenland Ice Sheet is currently experiencing short-term mass-balance and dynamic changes at low elevations. These changes may reflect recent climate/ocean warming, or alternatively they may be part of the natural cycle of ice sheet growth and decay. Key to resolving this question is an understanding of long-term changes in Greenland Ice Sheet behaviour during the Late Pleistocene and the Holocene. However, our understanding of the long-term changes in the dynamic behaviour of the ice sheet is still poor, and major outstanding questions remain regarding past ice-sheet extent, and the timing and controls on initial ice retreat in many areas of Greenland. This is particularly the case on the west Greenland continental shelf bordering Baffin Bay. In this region, several major fast flowing outlets, including Jakobshavns Isbrae, currently drain the ice sheet. Marine geophysical and geological data collected in 2009 from the continental shelf and slope on the central west Greenland margin provide a detailed record of the landform and sediment record of these outlets during the last glacial cycle. Multibeam swath bathymetric data show the presence of streamlined bedforms focused along cross-shelf troughs. These streamlined bedforms record the former presence of major fast-flowing ice sheet outlets emanating from Disko Bugt (into which Jakobshavns Isbrae currently drains) and the Umanak fjord system. Geophysical and core evidence indicate that these outlets were grounded as far as the outer shelf/shelf edge. Major submarine fans are present at the mouths of the cross-shelf bathymetric troughs and cores and acoustic records from these fans show that they are composed of a variety of glacially-related mass flow sediments including turbidites and debris flow deposits. The landform-sediment record of these former ice sheet outlets will be discussed as will the results of radiocarbon dating investigations of the timing of ice sheet advance and retreat on the shelf. Dates indicate significantly later deglaciation of the ice sheet from the outer shelf than previous reconstructions have suggested.

  9. 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 did observe WSFM. Interestingly, several cores on the southern shelf do appear to preserve evidence of previous wave-reworking of the seabed. It appears that the presence of a river plume and associated sediment, as well as the direction in which it is advected, are instrumental in WSFM generation.

  10. Lena Trough (Arctic Ocean): Active mantle exhumation on a continental rifted margin

    NASA Astrophysics Data System (ADS)

    Snow, J. E.; Hellebrand, E.; von der Handt, A.; Nauret, F.

    2004-12-01

    Lena Trough is the northern continuation of the Mid-Atlantic Ridge through Fram Strait and into the Arctic Ocean. The rifting of Lena Trough began in the Miocene, and significantly, is the final and the most recent event in the separation of the North American from the Eurasian continent. Lena Trough was mapped in 1999, 2001 and 2004 by PFS Polarstern (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany), revealing sea floor structures that are inconsistent with any normally conceived mid-ocean ridge spreading, and instead indicative of late continental rifting. Lena Trough is shown to be a deep, fault-bounded basin with depths of 3800-4200m, and irregular, steep valley sides that are oblique to the spreading direction. Basement horst structures that outcrop as sigmoidal ridges with steeply dipping sides project out of the valley floor. These basement ridges are roughly parallel along flow lines to the valley walls on either side. Ridge-orthogonal topography is simply absent (ie no segments trending parallel nor fracture zones perpendicular to Gakkel Ridge). Most faults trend approximately SSE-NNW, an obliquity with respect to Gakkel Ridge (SW-NE) of about 55°. The basement ridges are composed nearly entirely of fertile mantle peridotite, as are the valley walls. Only at the northern and southern extremities of Lena Trough do basalts appear at all. The peridotites compositions are consistent with either continental or oceanic (asthenospheric) mantle. They show evidence of low-degree mantle melting, followed by high-level stagnation in a thick lithosphere. This evidence (veining, impregnation) is more evident where little or no basaltic cover is present, while peridotites dredged in the vicinity of basalts tend to be more residual. This may indicate some degree of magmatic focusing in the absence of a basaltic crust per se. Lena Trough contains rare, highly alkaline basalts that are unlike any compositions dredged from mid-ocean ridges. While nearly all alkaline E-MORB have less then 49 wt. % SiO2 and less than 15% Al2O3, the Lena Basalts have nearly 52% SiO2, and 18% Al2O3. This suggests that the melts formed at the quartz eclogite peritectic with residual garnet rather than at the peridotitic peritectic usual for MORB. Their trace element and isotopic characteristics moreover reflect a ubiquitous enriched component found in the Western Gakkel Ridge and the Mohn’s Ridge. We suggest that the Lena Trough has undergone almost no partial melting, and that the rare basalts found there are nearly uniquely the result of melting of early-melting heterogeneities (veins) in the upwelling asthenosphere.

  11. Lena Trough (Arctic Ocean): Active mantle exhumation on a continental rifted margin

    NASA Astrophysics Data System (ADS)

    Snow, J. E.; Hellebrand, E.; von der Handt, A.; Nauret, F.

    2007-12-01

    Lena Trough is the northern continuation of the Mid-Atlantic Ridge through Fram Strait and into the Arctic Ocean. The rifting of Lena Trough began in the Miocene, and significantly, is the final and the most recent event in the separation of the North American from the Eurasian continent. Lena Trough was mapped in 1999, 2001 and 2004 by PFS Polarstern (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany), revealing sea floor structures that are inconsistent with any normally conceived mid-ocean ridge spreading, and instead indicative of late continental rifting. Lena Trough is shown to be a deep, fault-bounded basin with depths of 3800-4200m, and irregular, steep valley sides that are oblique to the spreading direction. Basement horst structures that outcrop as sigmoidal ridges with steeply dipping sides project out of the valley floor. These basement ridges are roughly parallel along flow lines to the valley walls on either side. Ridge-orthogonal topography is simply absent (ie no segments trending parallel nor fracture zones perpendicular to Gakkel Ridge). Most faults trend approximately SSE-NNW, an obliquity with respect to Gakkel Ridge (SW-NE) of about 55°. The basement ridges are composed nearly entirely of fertile mantle peridotite, as are the valley walls. Only at the northern and southern extremities of Lena Trough do basalts appear at all. The peridotites compositions are consistent with either continental or oceanic (asthenospheric) mantle. They show evidence of low-degree mantle melting, followed by high-level stagnation in a thick lithosphere. This evidence (veining, impregnation) is more evident where little or no basaltic cover is present, while peridotites dredged in the vicinity of basalts tend to be more residual. This may indicate some degree of magmatic focusing in the absence of a basaltic crust per se. Lena Trough contains rare, highly alkaline basalts that are unlike any compositions dredged from mid-ocean ridges. While nearly all alkaline E-MORB have less then 49 wt. % SiO2 and less than 15% Al2O3, the Lena Basalts have nearly 52% SiO2, and 18% Al2O3. This suggests that the melts formed at the quartz eclogite peritectic with residual garnet rather than at the peridotitic peritectic usual for MORB. Their trace element and isotopic characteristics moreover reflect a ubiquitous enriched component found in the Western Gakkel Ridge and the Mohn’s Ridge. We suggest that the Lena Trough has undergone almost no partial melting, and that the rare basalts found there are nearly uniquely the result of melting of early-melting heterogeneities (veins) in the upwelling asthenosphere.

  12. 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 is observed for the H/V pattern calculated during earthquakes. This phenomenon may suggest that no clear sedimentary boundary exist when a stronger motion applies. We found that the resonance frequency for the relative rigid material, such as mud diapir, is relatively higher, about 9 Hz. Moreover, their main resonance frequency is not affected by occurrence of earthquakes. On the general sedimentary layer and marine landslide, the resonance frequency shows relatively low value, about 7~8 Hz. When the site is affected by earthquakes, the main frequency becomes unclear. Finally, when a site is characterized by very thick sedimentary layer, the frequency of about 5 Hz is the lowest observed in the experiment.

  13. High-resolution and Deep Crustal Imaging Across The North Sicily Continental Margin (southern Tyrrhenian Sea)

    NASA Astrophysics Data System (ADS)

    Agate, M.; Bertotti, G.; Catalano, R.; Pepe, F.; Sulli, A.

    Three multichannel seismic reflection profiles across the North Sicily continental mar- gin have been reprocessed and interpreted. Data consist of an unpublished high pene- tration seismic profile (deep crust Italian CROP Project) and a high-resolution seismic line. These lines run in the NNE-SSW direction, from the Sicilian continental shelf to the Tyrrhenian abyssal plain (Marsili area), and are tied by a third, high penetration seismic line MS104 crossing the Sisifo High. The North Sicily continental margin represents the inner sector of the Sicilian-Maghrebian chain that is collapsed as con- sequence of extensional tectonics. The chain is formed by a tectonic wedge (12-15 km thick. It includes basinal Meso-Cenozoic carbonate units overthrusting carbonate platform rock units (Catalano et al., 2000). Presently, main culmination (e.g. Monte Solunto) and a number of tectonic depressions (e.g. Cefalù basin), filled by >1000 m thick Plio-Pleistocene sedimentary wedge, are observed along the investigated tran- sect. Seismic attributes and reflector pattern depicts a complex crustal structure. Be- tween the coast and the M. Solunto high, a transparent to diffractive band (assigned to the upper crust) is recognised above low frequency reflective layers (occurring be- tween 9 and 11 s/TWT) that dips towards the North. Their bottom can be correlated to the seismological (African?) Moho discontinuity which is (26 km deep in the Sicilian shelf (Scarascia et al., 1994). Beneath the Monte Solunto ridge, strongly deformed re- flectors occurring between 8 to 9.5 s/TWT (European lower crust?) overly the African (?) lower crust. The resulting geometry suggests underplating of the African crust respect to the European crust (?). The already deformed crustal edifice is dissected by a number of N-dipping normal faults that open extensional basins and are associ- ated with crustal thinning. The Plio-Pleistocene fill of the Cefalù basin can be subdi- vided into three subunits by well-developed unconformities. The stratal pattern of the lower subunit (Early Pliocene?) points out thrust-top basin. The intermediate subunit (Middle-Late Pliocene?) shows a wide sedimentary lateral accretion with syntectonic growth geometries. Upper Pliocene layers are overlain by well-stratified sediments of supposedly Pleistocene to Recent age, which drape and smooth underlying features (Pepe et al., 2000). Crustal thinning is (2 in the Cefalù basin and reach (3.54 north of Sisifo volcano, where crustal separation occurs and oceanic crust emplaced (Marsili 1 basin). In this area the Moho is located at (8 s/TWT, corresponding to 10-km depth. References Catalano R., Franchino A., Merlini S. e Sulli A., 2000. Mem. Soc. Geol. It., 55, 5-16. Pepe F., Bertotti G., Cella F. Marsella E., 2000. Tectonics, 19, 241-257. Scarascia S., Lozej A. Cassinis R., 1994. Boll. Geof. Teor. Appl., 36 (141-144), 5-19. 2

  14. Constraints of mapped and unfolded slabs on Oligocene to present-day Western Mediterranean plate reconstructions: potential role of north Iberia continental delamination

    NASA Astrophysics Data System (ADS)

    Lee, Yi-Te; Wu, Jonny; Wu, Yi-Min; Suppe, John; Sibuet, Jean-Claude; Chevrot, Sebastien

    2015-04-01

    Seismic tomographic images of subducted lithospheric remnants under the western Mediterranean have provided new constraints for Oligocene to present-day plate reconstructions. In this study, we mapped slabs under the western Mediterranean and Iberia from regional seismic tomography (Chevrot et al., 2014; Bezada et al., 2013) and from MITP08 global tomography (Li et al., 2008). A newly developed method was used to unfold (ie. structurally restore) the mapped slabs to a model spherical Earth surface, minimizing area and shape distortion. Slab constraints were input into plate tectonic reconstructions using Gplates software. Our mapping confirms the existence of western Mediterranean slabs including the Betic-Alboran, Algerian, and Calabrian slabs that were previously identified by Spakman and Wortel (2004). When unfolded these mapped slabs fit together in an Oligocene plate reconstruction, within tomographic resolution limits. Slab stretching was not required. Slab segmentation supports the existence of a North Balearic transform. Here we emphasize the potential importance for western Mediterranean tectonics of antoher slab under Iberia that we call the 'mid-Iberia slab'. This slab was first identified by Sibuet et al. (2004) and interpreted to be a Neotethyan suture. We have mapped this slab in detail from recent regional tomography (Chevrot et al., 2014). Our mapped slab is sub-vertical and strikes E-W under the southern margins of the Duero and Ebro basins. We newly interpret this slab to be delaminated northern Iberian continental lithosphere. We propose that continental delamination occurred during the Oligocene and produced uplifted Iberian Meseta topography, internally-drained basins, and high mean elevations that still persist today. We show how Oligocene northern Iberian continental delamination could have initiated subduction and rollback of the western Mediterranean

  15. Continental growth at convergent margins facing large ocean basins: a case study from Mesozoic convergent-margin basins of

    E-print Network

    Lee, Cin-Ty Aeolus

    convergent-margin basins of Baja California, Mexico Cathy Busby* Department of Geological Sciences-mail address: busby@geol.ucsb.edu (C. Busby). www.elsevier.com/locate/tecto Tectonophysics 392 (2004) 241

  16. 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.

  17. Gas and Fluid Expulsion at the Congo continental margin identfied from seismoacoustic data

    NASA Astrophysics Data System (ADS)

    Spiess, V.; Fekete, N.; Ding, F.; Caparachin, C.; Foucher, J.

    2008-12-01

    During R/V Meteor Cruise M76/3 in June/July 2008, seismic and acoustic methods were applied to study the distribution of seep structures and associated subsurface feeder systems. From the combination of swath bathymetry and backscatter, sediment echosounder, water column imaging and high-resolution multichannel seismics, numerous new seep sites could be identified. From previous studies, a few 'giant' pockmarks had been documented, representing deeply rooted migration zones and a few hundred meters wide and a few meters to more than ten meters deep depressions as the morphological expressions of fluid and gas expulsions. The new studies confirmed a widespread occurrence of such structures for the wider area of the continental margins of Gabon, Congo and Angola in deeper water. Spatial surveys have further shown that seep structures are present on different scales, in particular also with smaller sizes of tens of meters in diameter and a morphology on the meter scale. While these structures seem to be related to relatively shallow gas reservoirs, larger structures reveal roots to gas reservoirs in several hundred meters sub-bottom depth. At some of these locations, gas flares could be identified in the water column of some hundred to over thousand meters height. In comparison of working areas north and south of the Congo Canyon, it became evident that different driving forces and sedimentary and tectonic boundary conditions may be responsible for fluid seepage and its distribution. While in the North a thick sediment cover restricts seepage to selected zones of weakness and higher permeability, salt diapirism in the South is massively fracturing overlying sediments, have created numerous promising morphological features at the seafloor. However, only few active seeps could be found in the area of salt diapirism. Future work will particularly focus on the details of seep systems, the comparison with site-specific information from coring and video surveys and the integrated interpretation of the acoustic and seismic data sets.

  18. Buried Mesozoic rift basins of the U. S. middle Atlantic continental margin

    SciTech Connect

    Benson, R.N. (Univ. of Delaware, Newark (United States))

    1991-08-01

    The Atlantic continental margin is one of the frontier areas for oil and gas exploration in the US. Most the activity has been offshore where Upper Jurassic-Lower Cretaceous siliciclastic and carbonate rocks have been the drilling objectives, with only one significant but noncommercial gas discover. Onshore, recent exploration activities have focused on early Mesozoic rift basins buried beneath the postrift sediments of the middle Atlantic coastal plain. Many of the basins are of interest because they contain fine-grained lacustrine rocks that have sufficient organic richness, if not lost through hydrocarbon generation, to be classified as source beds for oil or gas. Locations of inferred rift basins beneath the middle Atlantic coastal plain were determined by analysis of drill-hole data in combination with gravity anomaly and aeromagnetic maps. Two basins in Delaware and the Queen Anne basin of Maryland are imaged on a regional Vibroseis profile. Areas enclosing inferred rift basins in the offshore region were mapped from interpretation of seismic reflection profiles. Assuming that petroleum source beds are present in the basin (synrift) rocks, hydrocarbon-generation models (Lopatin method) indicate that for a basin just offshore Delaware that is buried by 7 km of postrift sediments, only dry gas would be present in reservoir rocks; for the Norfolk basin of the Virginia coast buried by only 3 km of postrift rocks, the upper few hundred meters of synrift rocks are still within the oil-generation window. The less deeply buried basins beneath the coastal plain likely are still within the oil window.

  19. 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.

  20. Please fill out information in all gray boxes Title: Global and local controls on continental margin depositional cyclicity: Canterbury Basin, eastern

    E-print Network

    Yang, Zong-Liang

    facies, SBs 4-12. Drift, moat. Drift, main body. #12;1 Global and local controls on continental marginPlease fill out information in all gray boxes Title: Global and local controls on continental: (5 or less) Sea level, seismic stratigraphy, sediment drifts, tectonics Area: New Zealand Contact

  1. 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

    The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield. The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar fault zone (AAF), and is nonconformably overlain on the east by Phanerozoic sedimentary rocks. The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions. The layered rocks of the arc, the Al Amar group (>689 Ma to ???625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates, and are divided into an eastern and western sequence. Plutonic rocks of the terrane form three distinct lithogeochemical groups: (1) low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity (632-616 Ma) in the western part of the terrane, (2) high-Al TTG/adakite of arc affinity (689-617 Ma) in the central and eastern part of the terrane, and (3) syn- to post-orogenic alkali granite (607-583 Ma). West-dipping subduction along a trench east of the terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG. The Ar Rayn terrane contains significant resources in epithermal Au-Ag-Zn-Cu-barite, enigmatic stratiform volcanic-hosted Khnaiguiyah-type Zn-Cu-Fe-Mn, and orogenic Au vein deposits, and the potential for significant resources in Fe-oxide Cu-Au (IOCG), and porphyry Cu deposits. Khnaiguiyah-type deposits formed before or during early deformation of the Al Amar group eastern sequence. Epithermal and porphyry deposits formed proximal to volcanic centers in Al Amar group western sequence. IOCG deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane. Orogenic gold veins are largely associated with north-striking faults, particularly in and near the AAF, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes. Geologic, structural, and metallogenic 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.

  2. Revisiting the magnetic anomalies along the West Australian margin identifies a new continental fragment that accreted to Sumatra during the Early Eocene

    NASA Astrophysics Data System (ADS)

    Gibbons, A.; Whittaker, J. M.; Müller, P.

    2010-12-01

    Plate models reconstructing the formation of the West Australian margin differ in their treatment of the section of the Australian margin extending from the Wallaby-Zenith Fracture Zone to the tip of the Exmouth Plateau. Some reconstructions model Greater India as the conjugate, while others do not model any conjugate plate at all. The formation of the passive margin on the Australian plate implies that there must have been a conjugate continental plate that rifted away. Our revised reconstruction that includes all the abyssal plains along the West Australian margin reveals that, apart from Greater India and Argoland, a third continental block (Gascoyneland) must also have rifted from Australia since the Jurassic. From 132 Ma, while initially moving about the same Euler pole as Greater India, it formed the stretched continental crust of the Exmouth Plateau and then the oceanic crust of the Gascoyne and Cuvier abyssal plains. At 115 Ma Gascoyneland began moving in a northerly direction while Greater India continued westward only later moving northward from approximately 95 Ma when it was located entirely west of Gascoyneland. Gascoyneland did not pass west of the Investigator Ridge, a north-south-oriented linear feature at 98°E marking the western limits of the curved fracture zones of the Wharton Basin. Gascoyneland’s change in direction of plate motion would have formed these curved fracture zones and, assuming the N-S orientation of the Investigator Ridge continued into now subducted oceanic crust, would have reached West Sumatra at around 60 Ma. Plate tectonic models indicate that Sumatra was derived from accreted continental fragments originating from Gondwana (Metcalfe, 1996), although the continuity of Triassic sediments in West Sumatra, Sibumasu and East Malaya contradict this (Barber and Crow, 2003). The Woyla Group, consisting of the Sikuleh, Natal and Bengkulu terranes located along the west coast of Sumatra, has been identified as an oceanic arc, which accreted during the Jurassic-Early Cretaceous after the formation of a short-lived, narrow marginal sea (Cameron et al., 1980). The same authors considered the Woyla Group to overly continental crust due to the presence of the Sikuleh granitoid batholith. We propose Gascoyneland is now buried beneath the Woyla Terrane. References Cameron, N.R., Clarke, M.C.G., Aldiss, D.T., Aspden, J.A. and Djunuddin, A. (1980) The geological evolution of northern Sumatra. Indones. Petrol. Assoc., Proceedings 9th Annual Convention, Jakarta, 1980, pp. 149-187. Barber, A.J., and Crow, M.J., 2003, An evaluation of plate tectonic models for the development of Sumatra: Gondwana Research, v. 6, p. 1-28. Metcalfe, I., 1996, Gondwanaland Dispersion, Asian Accretion and Evolution of Eastern Tethys: Australian Journal of Earth Sciences, v. 43, p. 605-623.

  3. 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.

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

    SciTech Connect

    Kolla, V. (Elf Exploration, Inc., Houston, TX (United States)); Macurda, D.B. Jr. (The Energists, Houston, TX (United States)); Nelson, H.R. Jr. (Landmark Graphics Corp., Houston, TX (United States))

    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.

  5. Holocene temperature history at the western Greenland Ice Sheet margin reconstructed from lake sediments

    E-print Network

    Briner, Jason P.

    Holocene temperature history at the western Greenland Ice Sheet margin reconstructed from lake form 15 October 2012 Accepted 19 October 2012 Available online 29 November 2012 Keywords: Greenland t Predicting the response of the Greenland Ice Sheet to future climate change presents a major challenge

  6. Thermal history and evolution of the Rio de Janeiro - Barbacena section of the southeastern Brazilian continental margin

    NASA Astrophysics Data System (ADS)

    Neri Gezatt, Julia; Stephenson, Randell; Macdonald, David

    2015-04-01

    The transect between the Brazilian cities of Rio de Janeiro and Barbacena (22°54'S, 43°12'W and 21°13'S, 43°46'W, respectively) runs through a segment of a complex range of N-NE/S-SW trending basement units of the Ribeira Belt and southern Sao Francisco Craton, intensely reworked during the Brasiliano-Pan-African orogenic cycle. The ortho- and paragneisses in the area have metamorphic ages between 650 and 540 Ma and are intruded by pre-, syn- and post-tectonic granitic bodies. The transect, perpendicular to the strike direction of the continental margin, crosses the Serra do Mar escarpment, where the sample density is higher in order to better constrain occasional significant age changes. For logistical reasons, the 40 samples collected were processed in two separate batches for apatite fission track (AFT) analysis. The first batch comprised 19 samples, from which 15 produced fission track ages. Analyses were carried out at University College London (UCL), following standard procedures. Preliminary results for the study show AFT ages between 85.9±6.3 and 54.1±4.2 Ma, generally with younger ages close to the coast and progressively older ages towards the continental interior. The highest area sampled, around the city of Teresopolis, ranges from 740 to 1216 m above sea level and shows ages between 85.9±6.3 and 71.3±5.3 Ma. There is no evident lithological or structural distribution control. Medium track length values range from 12.57 to 13.89 µm and distributions are unimodal. Thermal history modelling was done using software QTQt. Individual sample model cooling curves can be divided into two groups: a dominant one, showing a single, slower cooling trend, and a second one with a rapid initial cooling curve, which becomes less steep around 65 Ma. In both groups the maximum paleotemperatures are around 110 Ma. The thermal history model for the first batch of samples is compatible with a single cooling event for the area following continental rifting and formation of the Atlantic Ocean. The preliminary results add to the growing thermochronological data base for the southeastern Brazilian continental margin and to deciphering the complex evolution of the region, as well as to the knowledge about the development and evolution of divergent continental margins in general. In a regional setting, AFT ages from this study, though not broadly variant locally, are distinct from basement rock AFT ages for adjacent areas produced by other authors along the southeastern continental margin. Similar ages are found at the southern Bocaina Plateau, for example, where structural control of age distribution is evident. Such regional thermal age difference has been previously attributed to continental scale structural compartmentalization throughout the continental passive margin, related to Late Cretaceous and Cenozoic reactivation of the E-W fracture zones linked to rifting of the South Atlantic. The present AFT results are compatible with Late Cretaceous reactivation but show no relation with younger events.

  7. High-resolution and Deep Crustal Imaging Across The North Sicily Continental Margin (southern Tyrrhenian Sea)

    Microsoft Academic Search

    M. Agate; G. Bertotti; R. Catalano; F. Pepe; A. Sulli

    2002-01-01

    Three multichannel seismic reflection profiles across the North Sicily continental mar- gin have been reprocessed and interpreted. Data consist of an unpublished high pene- tration seismic profile (deep crust Italian CROP Project) and a high-resolution seismic line. These lines run in the NNE-SSW direction, from the Sicilian continental shelf to the Tyrrhenian abyssal plain (Marsili area), and are tied by

  8. Azimuthal seismic anisotropy in a zone of exhumed continental mantle, West Iberia margin

    Microsoft Academic Search

    P. B. Cole; T. A. Minshull; R. B. Whitmarsh

    2002-01-01

    P-wave azimuthal seismic anisotropy of the uppermost mantle has been shown to indicate directions of extension, both recent and fossil, in oceanic and continental settings. We have determined the P-wave anisotropy of a zone of exhumed continental mantle beneath the southern Iberia Abyssal Plain, as a possible indication of the initial direction of separation of Iberia from North America in

  9. 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 times, like today, by little or no clastic sedimentation on the continental shelf other than beneath retreated ice shelves lying far from the continental sheld edge. Ice streams carve broad depressions across the shelf and carry abundant basal sediments directly to the continental shelf edge, thereby creating troughmouth fans and sheet-like prograding sequences (i.e. type IA sequences). Numerous acoustic unconformities and multiple overcompacted layers within the prograding sequences suggest major fluctuations of the Antarctic Ice Sheet. The available drilling and seismic interpretations provide the following history: (1) Cenozoic ice sheets have existed in places near the continental shelf since middle to late Eocene time. (2) A grounded Antarctic ice sheet first expanded to the continental shelf edge, with probable overdeepening of the outer shelf, in late Eucene to early Oligocene time in Prydz Bay, possibly in early Miocene time in the Ross Sea, and at least by middle Miocene time in the Weddell Sea. (3) The relative amounts of shelf prograding and inferred ice-volume variations (and related sea-level changes) have increased since middle to late Miocene time in the eastern Ross Sea, Prydz Bay, and possibly Weddell Sea. Our analysis is preliminary. Further acoustic surveys and scientific drilling are needed to resolve the proximal Antarctic record of glacio-eustatic, climatic, and tectonic events recorded by the prograding sequences. ?? 1991.

  10. Automatic detection of Floating Ice at Antarctic Continental Margin from Remotely Sensed Image with Object-oriented Matching

    NASA Astrophysics Data System (ADS)

    Zhao, Z.

    2011-12-01

    Changes in ice sheet and floating ices around that have great significance for global change research. In the context of global warming, rapidly changing of Antarctic continental margin, caving of ice shelves, movement of iceberg are all closely related to climate change and ocean circulation. Using automatic change detection technology to rapid positioning the melting Region of Polar ice sheet and the location of ice drift would not only strong support for Global Change Research but also lay the foundation for establishing early warning mechanism for melting of the polar ice and Ice displacement. This paper proposed an automatic change detection method using object-based segmentation technology. The process includes three parts: ice extraction using image segmentation, object-baed ice tracking, change detection based on similarity matching. An approach based on similarity matching of eigenvector is proposed in this paper, which used area, perimeter, Hausdorff distance, contour, shape and other information of each ice-object. Different time of LANDSAT ETM+ data, Chinese environment disaster satellite HJ1B date, MODIS 1B date are used to detect changes of Floating ice at Antarctic continental margin respectively. We select different time of ETM+ data(January 7, 2003 and January 16, 2003) with the area around Antarctic continental margin near the Lazarev Bay, which is from 70.27454853 degrees south latitude, longitude 12.38573410 degrees to 71.44474167 degrees south latitude, longitude 10.39252222 degrees,included 11628 sq km of Antarctic continental margin area, as a sample. Then we can obtain the area of floating ices reduced 371km2, and the number of them reduced 402 during the time. In addition, the changes of all the floating ices around the margin region of Antarctic within 1200 km are detected using MODIS 1B data. During the time from January 1, 2008 to January 7, 2008, the floating ice area decreased by 21644732 km2, and the number of them reduced by 83080. The results show that the object-based information extraction algorithm can obtain more precise details of a single object, while the change detection method based on similarity matching can effectively tracking the change of floating ice.

  11. Ancient impact structures on modern continental shelves: The Chesapeake Bay, Montagnais, and Toms Canyon craters, Atlantic margin of North America

    USGS Publications Warehouse

    Poag, C. Wylie; Plescia, J.B.; Molzer, P.C.

    2002-01-01

    Three ancient impact craters (Chesapeake Bay - 35.7 Ma; Toms Canyon - 35.7 Ma; Montagnais - 51 Ma) and one multiring impact basin (Chicxulub - 65 Ma) are currently known to be buried beneath modern continental shelves. All occur on the passive Atlantic margin of North America in regions extensively explored by seismic reflection surveys in the search for oil and gas reserves. We limit our discussion herein to the three youngest structures. These craters were created by submarine impacts, which produced many structural and morphological features similar in construction, composition, and variability to those documented in well-preserved subaerial and planetary impact craters. The subcircular Chesapeake Bay (diameter 85 km) and ovate Montagnais (diameter 45-50 km) structures display outer-rim scarps, annular troughs, peak rings, inner basins, and central peaks similar to those incorporated in the widely cited conceptual model of complex impact craters. These craters differ in several respects from the model, however. For example, the Montagnais crater lacks a raised lip on the outer rim, the Chesapeake Bay crater displays only small remnants of a raised lip, and both craters contain an unusually thick body of impact breccia. The subtriangular Toms Canyon crater (diameter 20-22 km), on the other hand, contains none of the internal features of a complex crater, nor is it typical of a simple crater. It displays a prominent raised lip on the outer rim, but the lip is present only on the western side of the crater. In addition, each of these craters contains some distinct features, which are not present in one or both of the others. For example, the central peak at Montagnais rises well above the elevation of the outer rim, whereas at Chesapeake Bay, the outer rim is higher than the central peak. The floor of the Toms Canyon crater is marked by parallel deep troughs and linear ridges formed of sedimentary rocks, whereas at Chesapeake Bay, the crater floor contains concentric faults and compression ridges formed in rocks of the crystalline basement. The Chesapeake Bay crater is distinguished further by its cluster of at least 23 adjacent secondary craters. The North American tektite strewn field, a widespread deposit of distal ejecta, is thought to be derived from the Chesapeake Bay impact, perhaps with a small contribution from the Toms Canyon impact. No ejecta field is known to be associated with the Montagnais impact. No immediate major extinction event is directly linked to any of these three impacts. There is evidence, however, that the Chesapeake Bay and Toms Canyon impacts helped initiate a long-term pulse of warm global climate, whose eventual dissipation coincided with an early Oligocene mass extinction event, 2 Ma after the impacts.

  12. A seismic study to investigate the prospect of gas hydrate in Mahanadi deep water basin, northeastern continental margin of India

    NASA Astrophysics Data System (ADS)

    Prakash, Anand; Samanta, B. G.; Singh, N. P.

    2010-12-01

    The presence of gas hydrates, one of the new alternative energy resources for the future, along the Indian continental margins has been inferred mainly from bottom simulating reflectors (BSR) and the gas stability zone thickness mapping. Gas hydrate reserves in Krishna Godawari Basin have been established with the help of gas-hydrate related proxies inferred from multidisciplinary investigations. In the present study, an analysis of 3D seismic data of nearly 3,420 km2 area of Mahanadi deep water basin was performed in search of seismic proxies related with the existence of natural gas hydrate in the region. Analysis depicts the presence of BSR-like features over a large areal extent of nearly 250 km2 in the central western part of the basin, which exhibit all characteristics of a classical BSR associated with gas hydrate accumulation in a region. The observed BSR is present in a specific area restricted to a structural low at the Neogene level. The coherency inversion of pre-stack time migration (PSTM) gathers shows definite inversion of interval velocity across the BSR interface which indicates hydrate bearing sediments overlying the free gas bearing sediments. The amplitude versus offset analysis of PSTM gathers shows increase of amplitude with offset, a common trend as observed in BSR associated with gas hydrate accumulation. Results suggest the possibility of gas hydrate accumulation in the central part of the basin specifically in the area of structural low at the Neogene level. These results would serve as preliminary information for selecting prospective gas hydrate accumulation areas for further integrated or individual study from geophysical, geological, geochemical and microbiological perspectives for confirmation of gas hydrate reserves in the area. Further, on the basis of these results it is envisaged that biogenic gas might have been generated in the region which under suitable temperature and pressure conditions might have been transformed into the gas hydrates, and therefore, an integrated study comprising geophysical, geological, geochemical and microbiological data is suggested to establish the gas hydrate reserves in Mahanadi deep water basin.

  13. Seafloor Mapping of the Southeast Iberian Continental Slope and Western Algero-Balearic Abyssal Plain

    NASA Astrophysics Data System (ADS)

    Lastras, G.; Canals, M.; León, C.; Elvira, E.; Pascual, L.; Muñoz, A.; de Cárdenas, E.; Acosta, J.

    2014-12-01

    We present the multibeam bathymetry and derived maps of the southeast Iberian margin from Cabo de Palos to Cabo de Gata, 37º35'N to 35º45'N and 2º10'W to 0º20'E, from the coastline down to the Algero-Balearic abyssal plain at depths exceeding 2,600 m. Data were obtained during different surveys in 2004, 2006 and 2007 on board R/V Vizconde de Eza with a Simrad EM300 multibeam echo-sounder, as part of the CAPESME Project, a collaboration between the Spanish Institute of Oceanography (IEO) and General Secretariat of Fisheries (SGP), aiming at creating maps of the fishing grounds of the Mediterranean continental margins of Spain. The edition of the maps has been carried out within the Complementary Action VALORPLAT (Scientific valorisation of multibeam bathymetry data from the Spanish continental shelf and slope), funded by the Spanish Ministry of Economy and Competitivity. Multibeam bathymetry data from the continental shelf obtained within the ESPACE project, also in a cooperative frame between IEO and SGP, completes the whole picture from the coastline to the deep abyssal plain. The map series is constituted by a general map at 1:400,000 scale and 14 detailed maps at 1:75,000 scale, which include inset maps on slope gradients and seafloor nature (rock or sediment type), the later obtained with rock dredges and Shipeck sediment dredges. Both the detailed maps and the general map are available in paper print, and the whole collection is also distributed in an edited USB. The geological features displayed in the different maps include the continental shelf, with abundant geomorphic features indicative of past sea-level changes, the continental slope carved by a large number of submarine canyons and gullies, including Palos, Tiñoso, Cartagena Este, Cartagena Oeste, Águilas, Almanzora, Alias, Garrucha and Gata submarine canyons, the Mazarrón, Palomares and Al-Mansour escarpments of probable tectonic origin, the Abubácer, Maimonides and Yusuf ridges, the Águilas and Al-Mansour seamounts, and the Algero-Balearic abyssal plain where prominent halokinetic deformation structures have been observed.

  14. Genesis of giant promontories during two-stage continental breakup and implications for post-Rodinia circum-Arctic margins (Invited)

    NASA Astrophysics Data System (ADS)

    Bradley, D. C.

    2013-12-01

    Giant promontories are a seldom-noted feature of the present-day population of passive margins. A number of them formed during the breakup of Pangea: the South Tasman Rise and Naturaliste Plateau off Australia, the Grand Banks and Florida off North America, the Falkland Plateau off South America, and the Horn of Africa. Giant promontories protrude hundreds of kms seaward from a corner of the continent and are not to be confused with the low-amplitude irregularies that occur at intervals along most passive margins. Giant promontories that might have formed during the breakup of the earlier supercontinents, Rodinia and Nuna, have not been recognized. Properties of the modern examples suggest some identifying criteria. They are cored by continental crust that was created or last reworked during the previous collisional cycle. Judging from the examples listed, the early histories of the two flanks of a promontory will differ because separate continents or microcontinents drift away in different directions at different times. For example, the eastern flank of the >500-km-long South Tasman Rise formed when the Lord Howe Rise separated from Australia at ca. 85 Ma, whereas the western flank formed when Antarctica moved past at ca. 65-33 Ma. (Age spans of various passive margins quoted herein are from Bradley, 2008, Earth Sci. Rev. 91:1-26.) During ocean closure (typically, arc-passive margin collision), a promontory may be exposed to earlier and more intense tectonism than elsewhere along the margin. Unique events are also possible. For example, the tip of Florida experienced a glancing collision with Cuba during the Paleogene, an event that was not felt elsewhere along the Gulf or Atlantic margins of the southeastern U.S. Giant promontories are unlikely to have deep lithospheric keels and may be prone to being dislodged and rotated during collision. Thus, what starts as a promontory may end up as a microcontinent in an orogen. The case for giant promontories in the circum-Arctic has not been thoroughly assessed, but the shape of Laurentia and the ages of its Paleozoic margins suggest that promontories dating from breakup of Rodinia may have jutted from its NE and (or) NW corners. The NE corner lies at the junction of an eastern (Caledonian) passive margin that existed from ca. 815 to 444 Ma and a northern (Innuitian) passive margin that existed from ca. 620 to 444 Ma. The hypothetical NE promontory would have attached to northern East Greenland where early Paleozoic passive-margin deposits are notably lacking. Nearby remnants of the NE promontory might include the Yermak plateau off North Greenland, the Morris Jessup plateau off Svalbard, or parts of Svalbard itself. A hypothetical NW Laurentian promontory would have attached somewhere between Banks Island in the Canadian Arctic, where the 620-444 Ma Innuitian margin is truncated along the present-day rifted margin, and east-central Alaska, site of the most northerly rocks that can be confidently placed along the ca. 710-385 Ma Cordilleran passive margin. Remnants of this promontory might include older rocks of the Ruby terrane and (or) the northeastern Brooks Range, both in Alaska. Either hypothetical promontory would have been involved in orogenesis associated with the postulated extrusion of terranes through the gap between Laurentia and Siberia.

  15. An evaluation of 14C age relationships between co-occurring foraminifera, alkenones, and total organic carbon in continental margin sediments

    Microsoft Academic Search

    Gesine Mollenhauer; Markus Kienast; Frank Lamy; Helge Meggers; Ralph R. Schneider; John M. Hayes; Timothy I. Eglinton

    2005-01-01

    Radiocarbon age relationships between co-occurring planktic foraminifera, alkenones, and total organic carbon in sediments from the continental margins of southern Chile, northwest Africa, and the South China Sea were compared with published results from the Namibian margin. Age relationships between the sediment components are site-specific and relatively constant over time. Similar to the Namibian slope, where alkenones have been reported

  16. Journal of the Geological Society, London, Vol. 162, 2005, pp. 135146. Printed in Great Britain. Subsidence history of the north Indian continental margin, ZanskarLadakh

    E-print Network

    Watts, A. B. "Tony"

    to flexural loading of the north Indian margin by obduction of the Spontang ophiolite. The best fit model Cretaceous timing of obduction and the structure of the Spontang ophiolite, provide new constraints on the Te, continental margin, backstripping, flexure, ophiolites. The tectonic subsidence and uplift history

  17. 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.

  18. Sediment trap observations from the Gulf of St. Lawrence and the continental margin of eastern Canada

    NASA Astrophysics Data System (ADS)

    Romero, Nancy; Silverberg, Norman; Roy, Suzanne; Lovejoy, Connie

    2000-04-01

    Sediment trap samples have provided the first direct observations of the sinking particles that account for the export of material out of the photic zone in the Gulf of St. Lawrence, as well as their relationship to variations in the trophic regime. Particles were collected at several sites over 24-h periods using 0.03 m 2 (collecting surface) and 0.5 m 2 free-drifting sediment traps at 50 and 150 m. Total mass flux varied widely (80-1500 mg/m 2/d), as did carbon flux (16-300 mg C/m 2/d). Small cylinders consistently oversampled with respect to big cylinders, regardless of depth or drifter design. Also, 6-month time series were obtained with a moored, 0.125 m 2 trap at two sites. In the Anticosti Gyre, time-series fluxes were consistent with those obtained from the big drifting trap (means: 480 mg dry wt/m 2/d; 39 mg C/m 2/d), and with independently measured sediment accumulation rates. Numeric fluxes of phytoplankton cells were similar to moderately productive ocean margins during the April 1994 bloom, but otherwise resembled those from oligotrophic regimes. Fecal pellet numeric fluxes, in contrast, were always high, similar to other continental margins. The composition of the material collected by the small and big traps is a good indicator of the changing trophic regime in the water column. Relative numeric abundances suggest three distinctly different periods. A "bloom" period (represented by April 1994, but including a weaker late-fall bloom over a shelf valley), when a variety of centric and pennate diatom cells made up 70-95% of the particle numbers; a transitional or "post-bloom" period (June 1994), when phytoplankton were less abundant, pennate forms were scarce and a single species dominated the centric diatoms; and a "non-bloom" period (May to December, 1993) when fecal pellets and microzooplankton accounted for greater numbers than the phytoplankton cells, including abundant dinoflagellates. The time-series Anticosti Gyre trap showed continued large-particle settling throughout the winter with total mass and carbon fluxes similar to the ice-free seasons. The most frequent fecal pellets were 50-109 ?m diameter compact and loose rods, produced by the dominant calanoid copepods. Large macrozooplankton fecal pellets occurred only sporadically. Many pellets <49 ?m were collected in December 1993, probably produced by Microcalanus, which was unusually abundant at this time. Oval pellets occurred over a broad range of diameters, suggesting multiple origins. The bulk of the settling material produced by the pelagic food web in the Gulf appears to be of zooplanktonic origin (mainly fecal pellets and abundant microzooplankton). Diatom frustules were the most frequently encountered particles, numerically, but phytoplankton rarely made up more than 35% of the 2D projected area of all particles. Much of the organic matter produced photosynthetically must thus be transformed by heterotrophs before escaping from the surface and intermediate waters in this region.

  19. 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 Plateau basalts. The most striking feature is the sharp transition in crustal structure and thickness across the northern boundary of Walvis Ridge. Thin oceanic crust (6.5 km) of the Angola Basin lies next to the up to 35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. Both structures are separated by a very large transform fault zone. The velocity structure of Walvis Ridge lower crust is indicative of gabbro, and, in the lowest parts, of cumulate sequences. On the southern side of Walvis Ridge there is a smooth gradation into the adjacent 25-30 km thick crust underlying the ocean-continent boundary, with a velocity structure resembling that of Walvis Ridge The second profile shows a sharp transition from oceanic to rifted continental crust. The transition zone may be underlain by hydrated uppermost mantle. Below the Etendeka Plateau, an extensive high-velocity body, likely representing gabbros and their cumulates at the base of the crust, indicates magmatic underplating. We summarize by stating that rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magmatic-dominated segment to the south, and an amagmatic segment north of Walvis Ridge.

  20. 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.

  1. Dyke belt in North Western margin of Siberian platform

    NASA Astrophysics Data System (ADS)

    Ryabov, Victor

    2015-04-01

    The Early Triassic dyke swarm belt is strengthening at about 500 km (50-60 km width) along the northern margin of Siberian platform. Its locations is limited from the north by rift structure of the Yenisei-Khatanga trough, and from the south by Kystyktah-Ayan-Ambardah uprise. From west to east dyke belt is crossing Norilsk, Kamensky and Maimecha-Kotuiskaya province. In this direction the composition of dykes changing from basic rthrough alkali-basic to alkali-ultramafic varieties. The sickness of dykes varies from 0,5-10m up to 90 m and length - from 5-15 m to 140 km. The orientation of the bulk of dykes coincides with the direction of the major structural and tectonic lineaments of the region. Dykes are often refer to the fault planes. The composition of dykes may vary along the stretches. The density of hundreds of basaltic dykes and small intrusions in the dyke swarms is not permanent and sometimes essentially uneven. They form a compact dyke swarms of dykes, stocks and sub layering bodies veins. In dyke package on Huor-Uyallah river here are 20 subparallel dykes of different chemical composition at a distance of 2 km Lack of the evidence of the dykes crossing allows to assume their formation during the same magmatic cycle. Dykes cross cut through the entire incision of basaltic plateau. They did not created the flow effusions and refer to the final stage trap magmatism. Ar/Ar age of the youngest dykes in the province Kamenska 238-247 Ma. the age of lamproite dyke in Norilsk province is 235 Ma. Dykes are represented by dolerites trachydolerites, syenites, minettes, lamprophyres (camptonite, spessartite, vosgesite), avgitites, melanephelinite, alnoites, limburgites alkaline picrites, meimechites. Their content vary widely: SiO2 - 35,7-62,6; TiO2- 0,4-7,5; Al2O3- 4,4-17,5; Fe2O3- 4,6-20,6; MnO- 0,08-0,44; MgO- 0,8-31,5; CaO- 0,7-15,4; Na2O- 0,01-6,5; K2O 0,8-5,3 wt.%; P2O5 0,1-1,2 wt.%. The h Bolsheavamskaya volcanic basin in Kamensky province is most abundant in ighly alkali-basicdykes. There are undifferentiated and layered dykes, glassy and crystalline, single and multi-phase, with symmetric and asymmetric internal structure, with uniform or contrasting composition of the individual phases. Multiphase dykes along the. Namakan river have symmetrical "dyke dyke" structure. Glassy dykes are usually composed of olivine clinopyroxenite or picritic olivine porphyres. The number of dykes phases in the same locality may reaches 5. The internal structure of dykes are symmetric. The dyke in dyke consists of the quench zone in inner contact zone with silicate glass and carbonate globules and enriched by rounded K-feldspar, potassium feldspar with kaersutite +- mica glomeroporhyric intergrowth. In crystalline dykes contact zones are composed of variolitic clinopyroxenites and central by picritic porphyries. Contact zone composed of clinopyroxenites crystallized from a supercooled melt. Having elevated CaTi- and CaAl-tschermak components. This Dyke belt was formed by scattered spreading. Multiphase dykes were formed during multiple tectonic fracturing. Wide chemical variations c are caused by fluid-magmatic melt interaction in magma feeding channels. Multistage splitting of magmatic melts to different in composition liquids took place with the participation of CO2, H2, F, Cl, CH4.

  2. Seismic evidence of continental subduction and upper mantle deformation beneath the western Alps

    NASA Astrophysics Data System (ADS)

    Zhao, L.; Paul, A.; Solarino, S.; Guillot, S.; Malusa', M. G.; Zheng, T.; Aubert, C.; Salimbeni, S.; Dumont, T.; Schwartz, S.; Pondrelli, S.; Zhu, R.; Wang, Q.

    2014-12-01

    The finding of ultra-high pressure minerals in continental orogens like the Western Alps implies that continental crust can subduct to depths as great as 100 km and then be exhumed to the Earth's surface. The onset mechanism of continental subduction, including how the continental plate overwhelms the buoyancy resistance and how it is exhumed, however remains elusive. Using data of a new temporary seismic array deployed in the French-Italian Alps, we present here new evidence that in the Western Alps the European plate subducted deeply beneath the Adria plate. This study uses teleseismic P receiver functions and shear-wave splitting measurements from SKS phases. In the depth-migrated receiver function cross-section, the positive P to S (Ps) conversions (corresponding to velocity increase with depth) on the Moho interface can be continuously traced beneath the European plate. This Moho conversion fluctuates in depth, amplitude and dipping angle. Beneath the external zone, the Moho shows up strongly at depths of 25-40 km, exhibiting an eastward dip angle < 5°. Starting from beneath the outcrop of the Frontal Penninic Thrust (FPT) eastward, the dip of the European Moho strongly increases and Moho conversions can be traced to 70-80 km depth beneath the Adria plate. Shear wave splitting measurements demonstrate that fast polarization directions of seismic anisotropy are parallel to the strike of the orogen, which is consistent with previous studies. The most prominent new result is that the delay time increases rapidly from the external zone to the internal zone and then decreases rapidly from the FPT to the westernmost Po Plain. This rapid change of delay time suggests that the mantle lithosphere, partly serpentinized, has a major contribution to the observed SKS splitting. The largest delay times in the vicinity to the west of the Frontal Penninic Thrust may suggest localized strong shear in the lithospheric mantle beneath the boundary zone between the European and Adria plates. In summary, our receiver function cross-section is the first direct evidence of subduction of the European lower crust in the Adria mantle beneath the Western Alps; SKS splitting data demonstrate that the left-lateral shear due to the post-Miocene counterclockwise rotation of the internal zone may also be detected in the lithospheric mantle.

  3. Interaction of dipole eddies with the western continental slope of the Mozambique Channel

    NASA Astrophysics Data System (ADS)

    Roberts, Michael J.; Ternon, Jean-François; Morris, Tamaryn

    2014-02-01

    Sea Level Anomaly (SLA) data were used to track a southward propagating eddy dipole along the western slope of the Mozambique Channel over some 6 months. In April 2005, this dipole (with the cyclone to the south) was close to the continental slope off southern Mozambique. The contact zone between the contra-rotating vortices and the slope was surveyed by ship using onboard (S-)ADCP and CTD lines. The data showed strong (>1.4 m s-1) southward (geostrophic) currents over the slope adjacent to the anticyclone with horizontal divergence over the shelf edge. Significant slope upwelling between the dipole and the shelf was evident, concomitant with enhanced nutrient and chlorophyll levels enriching shelf near-surface waters. Satellite observations depicted a 300 km long surface chlorophyll filament extending offshore in the frontal zone between the contra-rotating vortices. A satellite-tracked drifter deployed at the coastal base of this filament confirmed the offshore advection of chlorophyll-enriched shelf water, which ultimately wrapped around the cyclone and filling its centre. The slope upwelling was also clearly evident in hourly temperature data collected by a recorder deployed on a nearby reef (Zambia Reef) in a depth of 18 m. According to the SLA data, the dipole took several weeks to pass Zambia Reef causing prolonged bouts of upwelling that finally ceased when it left the continental slope and moved southwards into the open ocean. Further analysis showed that lone anticyclones and cyclones against the Mozambique continental shelf also induce slope upwelling as a result of horizontal divergence created by the radial circulation of the vortex. In the case of cyclones, the divergence occurs north of the contact zone. Overall, this case study confirms that eddies moving southwards along the western side of the Mozambique Channel are the main mechanism for pumping nutrients into the otherwise oligotrophic surface waters, and moreover, provide a vigorous mechanism for shelf-open ocean exchange.

  4. 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.

  5. Continuous Mantle Exhumation at the Outer Continental Margin of the Santos, Campos and Espírito Santo Basins, Brazil

    NASA Astrophysics Data System (ADS)

    Zalan, P. V.; Severino, M. G.; Rigoti, C. A.; Magnavita, L. P.; Oliveira, J. B.; Viana, A. R.

    2011-12-01

    The interpretation of 12,000 km of very deep (PSTM to 16 sec., PSDM to 25 km) 2D seismic sections, coupled with gravimetric and magnetometric modeling line-by-line, and the integration of the results with the regional data bank of Petrobras, all together viewed in terms of the recent tectonic models developed for the rupturing and separation of mega-plates, led to a regional (500,000 km2), first-time ever, 3D-view of the deep structure underlying the prolific sedimentary basins of Santos, Campos and Espírito Santo in southeastern Brazil. The three basins are situated onto a continental margin that narrows gradually, from south to north, from a very wide (Santos), through an intermediate (Campos), and then to a narrow (Espírito Santo) passive margin. The seismic sections shows very well the dual rheological behavior of the continental crust, consisting of a deeper and plastic lower crust (with numerous short and strong reflections that display sub-horizontal ductile flow) and a shallower and brittle upper crust (represented by a mostly transparent and faulted seismic facies topped by the sedimentary sections of the rift and thermal subsidence phases). The crustal structure of the Santos Basin shows a zonation from west to east of alternating bands of NE-SW-trending thin (plastic basement terrains) and thick (resistant basement terrains) stretched continental crust. In vertical section this zonation is displayed as a series of necking zones, leading to a highly irregular, low to moderate crustal taper. Such zonation is less developed in the Campos Basin, where the crustal taper is moderate and regular, and practically non-existent in the Espírito Santo Basin, where the crustal taper is high. The most outstanding crustal feature shared in common by the three basins is the exhumation of mantle between the tip of the hyper-extended continental crust and the tabular-shaped oceanic crust. Although the crustal taper varies significantly from basin to basin their continental crust pinches out invariably on the flanks of exhumed mantle. This gives rise to a remarkable long (900 km along a N-S direction and 600 km in E-W direction), relatively narrow (15 to 70 km wide) and continuous belt of exhumed mantle that marks the passage from continental crust to oceanic crust in all three basins. The Santos, Campos and Espírito Santo Basins thus form a typical magma-poor passive margin. These are in sharp contrast with the adjacent basin to the south, the Pelotas Basin, that in turn is a typical volcanic passive margin displaying a long (1000 km in a N-S direction) and wide (100 to 220 km) belt of seaward-dipping reflectors at its outer margin and no exhumation of the mantle at the continent-ocean boundary.

  6. Neodymium, strontium, and oxygen isotopic variations in the crust of the western United States: Origin of Proterozoic continental crust and tectonic implications

    SciTech Connect

    Bennett, V.C.

    1989-01-01

    Initial Nd isotopic ratios of crystalline rocks from an area of about 1.5 {times} 10{sup 6} km{sup 2} of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. Three age provinces have been delineated. It is demonstrated that large regions of Early Proterozoic continental crust were formed with anomalous isotopic compositions ({sup 143}Nd/{sup 144}Nd ratios lower than Early Proterozoic depleted-mantle). The variations in the initial {epsilon}{sub Nd} and {delta}{sup 18}O values correlate with each other, and correspond to the previously determined Nd isotopic provinces. The Pelona, Rand, Chocolate Mountain and Orocopia Schists are represented by 15 lithologically and structurally similar schist bodies exposed along the San Andreas and Garlock faults in southern California. The grayschists have measured {epsilon}{sub Nd} values from -1.7 to -11.7 with depleted-mantle model ages of 0.9 to 1.7 Ga. The Nd isotopic compositions can be modeled as variable mixtures of Early Proterozoic continental crust with a Mesozoic are component. The measured {sup 87}Sr/{sup 86}Sr ratios are from 0.7087 to 0.7129 and reflect the presence of an old continental source. Independent of age, the high initial {epsilon}{sub Nd} values ({sup +}9 {plus minus} 1.5) are consistent with derivation at an oceanic spreading center, either at a MORB or in a back-arc basin environment. The presence of both Early Proterozoic continental detritus and a younger sedimentary component in the grayschist protolith, and the MORB affinity of the metabasalts are compatible with formation of the protoliths of the Pelona and related schists in a Mesozoic basin adjacent to the southwestern United States continental margin.

  7. Predicting OCT Location and Continental Extension for North Atlantic Rifted Margins Using Gravity Inversion

    Microsoft Academic Search

    A. Alvey; N. J. Kusznir

    2007-01-01

    Gravity inversion incorporating a lithosphere thermal gravity anomaly correction has been used to determine Moho depth, crustal basement thickness and lithosphere thinning factor for ~50 2D regional profiles across North Atlantic rifted margins including Labrador Sea, Baffin Bay, Iberian - Newfoundland and Norwegian - Greenland conjugate margins. Sediment thickness derived from seismic refraction\\/reflection data has been included in the gravity

  8. Permo-Triassic Collision, Subduction-Zone Metamorphism, and Tectonic Exhumation Along the East Asian Continental Margin

    NASA Astrophysics Data System (ADS)

    Ernst, W. G.; Tsujimori, Tatsuki; Zhang, Ruth; Liou, J. G.

    2007-05-01

    Convergent plate motion at 320-210 Ma generated the Tongbai-Dabie-Sulu (east-central China)-Imjingang-Gyeonggi (central Korea)-Renge-Suo (Southwestern Japan)-Sikhote-Alin orogen along the paleo-Pacific edge of cratonal Asia. This amalgamated belt reflects collision between the Sino-Korean and Yangtze cratons on the SW portion, and accretion of outboard oceanic arcs ± sialic fragments against the NE margin. Subducted Proterozoic-Paleozoic continental and oceanic crustal complexes underwent high- and ultrahigh-pressure metamorphism at low to moderate temperatures. Tectonic slices of sialic crust episodically disengaged from the downgoing plate and, driven by buoyancy, ascended rapidly to midcrustal levels from depths exceeding 90-200 km after continental collision in east-central China plus or minus Korea, and from 30-50 km after arrival of far-traveled oceanic terranes in SW Japan and the Russian Far East. On achieving neutral buoyancy and stalling out at 10-20 km depth, later doming, gravitational collapse, and erosion exposed parts of the high- and ultrahigh-pressure complexes. This curvilinear orogen has been segmented and offset by major and minor transverse faults. Also, regional backarc spreading opened marginal basins behind the Permo-Triassic convergent suture zone, further disturbing portions oceanward.

  9. 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 positive and have values between +0.5km and +1.5km. The positive measured RDAs within the OCT are consistent with anomalous subdued subsidence in these regions; however, dynamic uplift from the Afar plume cannot be discounted as an alternative explanation. Moho depth determined from gravity inversion and thinning factors determined from gravity inversion and subsidence analysis have been used to constrain OCT location. Gravity inversion, used to determine Moho depth and continental crustal basement thinning, incorporates a lithosphere thermal gravity anomaly correction and sediment thickness from the 2D seismic reflection data. Moho depths form gravity inversion have been calibrated against seismic refraction Moho depths where such data exists. Continental lithosphere thinning has been determined using flexural backstripping and subsidence analysis assuming the classical rift model of McKenzie (1978) with a corection for volcanic addition due to decompression melting based on White & McKenzie (1989). The OCT region of the Eastern Gulf of Aden, based on measured positive RDA, shows anomalously shallow bathymetry and decreased subsidence, consistent with a lithosphere thermal anomaly as proposed by Lucazeau et al.(2008), although the regional effects of Afar plume dynamic uplift cannot be ruled out.

  10. Recent sediment accumulation rates for the Western margin of the Barents Sea

    NASA Astrophysics Data System (ADS)

    Zaborska, Agata; Carroll, JoLynn; Papucci, Carlo; Torricelli, Leonardo; Carroll, Michael L.; Walkusz-Miotk, Jolanta; Pempkowiak, Janusz

    2008-10-01

    We investigate patterns, processes, and rates of sediment accumulation within the marginal ice zone of the western Barents Sea. The Barents Sea is among the most productive of the Arctic marginal seas, with approximately half of the sediment burial flux derived from marine rather than terrestrial sources. Sediment accumulation rates were quantified by 210Pb geochronology at 14 stations, ranging in water depths from 173 to 503 m, along a south-north latitudinal gradient of 6° (75-81°N). The average sediment accumulation rate for all stations is 0.7±0.4 mm/yr. In general, lower sediment accumulation rates are associated with coarser sediment fractions at shallower water depths (˜200 m) where currents remove fine-grained sediments and transport material to the deeper regions. Higher sediment accumulation rates are detected in deeper water stations and are largely associated with specific sedimentation events and/or features of the Barents seafloor. Although the C/N ratio of surface sediments (0-2 cm) from all stations indicates a predominance of sediments of marine origin (C/N=9.0±1.0), there is no distinct depositional pattern, indicating preferential burial in areas more heavily influenced by the marginal ice zone. We conclude that a combination of mixed sediment sources, large shifts in the location of the marginal ice zone over time, and the benthic boundary layer processes obscure the productivity signal preserved in these seafloor sediment deposits of the western Barents Sea.

  11. Elevated, passive continental margins: Not rift shoulders, but expressions of episodic, post-rift burial and exhumation

    NASA Astrophysics Data System (ADS)

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

    2012-06-01

    Many studies of elevated, passive continental margins (EPCMs) assume that their characteristic, large-scale morphology with high-level plateaux and deeply incised valleys has persisted since rifting and crustal separation, and that the absence of post-rift sediments is evidence of non-deposition. The high mountains in West Greenland, however, expose evidence of km-scale, post-rift subsidence, and recent studies showed that typical EPCM morphology with elevated plateaux formed c. 50 Myr after breakup through a process of uplift and dissection of a regional, post-rift erosion surface. Since the West Greenland margin shares all the morphological characteristics of EPCMs, the results from West Greenland lead us to question the common assumption that EPCMs have remained high since the onset of continental separation. We present published evidence of post-rift burial followed by uplift and exhumation from a number of EPCMs and their adjacent basins to support the notion that EPCMs are not permanent highs and that their morphology is unrelated to rifting and continental breakup. Geodynamic models that explain EPCMs as permanent highs since the time of rifting require either no lithospheric mantle extension below extending crust or effective elastic thicknesses > 100 km. Such models are, however, not consistent with the subsidence history inferred from actual rifts and their margins. Geodynamic models using low elastic thicknesses and a much more uniform distribution of strain within the lithosphere are more consistent with observations of early post-rift behaviour, but some additional process is needed to uplift the margins later. We suggest that EPCMs represent anticlinal, lithospheric folds formed under compression where an abrupt change in crustal or lithospheric thickness occurs between cratons and rift basins. We propose that EPCMs are expressions of episodes of post-rift burial followed by compression-induced uplift and exhumation; one episode of uplift results in erosion of the region to produce a low-relief surface near the level of the adjacent, opening ocean, and a second (or more) episode(s) raises the plateau to its present elevation, after which the plateau is dissected by fluvial and possibly glacial erosion.

  12. 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 subtle modification by Holocene processes active during the present sea-level high-stand.

  13. Along-margin variations of magmatism at volcanic passive margins: Numerical models of continental extension with rift propagation barriers

    NASA Astrophysics Data System (ADS)

    Koopmann, Hannes; Brune, Sascha; Franke, Dieter; Breuer, Sonja

    2015-04-01

    Seaward-dipping reflectors (SDRs) constitute a first-order feature of volcanic rifted margins and are imaged in seismic reflection profiles of the North and South Atlantic. Recent studies describe distinct along-strike variations in the distribution of SDRs, where abundance of volcanic material could be spatially linked to transfer fault systems. These segmented the propagating rift that later developed into the ocean, and are interpreted as rift propagation barriers. Here we present 3d numerical forward models of a segmented rift system suggesting a causal link between segment boundaries and magmatic volume variations. Our results explain along-strike variations in magmatism by significant rift-parallel mantle flow across segment boundaries. This rift-parallel flow is caused by a lateral pressure gradient between sequentially opening segments. The along-strike flow of hot material near the segment boundary leads to elevated temperature and thus decompression melting if compared to the segment interior. This takes place without enhancing crustal thinning near the transfer zones and generates peaks in overall pre-break-up melt volumes. We conclude that delayed rift propagation at inherited structures can play an important part in enhancing and localizing volcanic activity by controlling the mantle flow beneath the rift axis.

  14. Late Quaternary sedimentation off the Queensland continental margin (northeast Australia) in response to sea level fluctuations 

    E-print Network

    Alexander, Ian T.

    1996-01-01

    Drilling during ODP Leg 133 offshore Cairns, northeast Queensland, provided a unique opportunity to document carbonate production and facies development on a mixed carbonate-siliciclastic margin. Recent studies have shown ...

  15. 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.

  16. Deep velocity structure of rifted continental crust, U.S. Mid-Atlantic Margin, from wide-angle reflection\\/refraction data

    Microsoft Academic Search

    W. Steven Holbrook; G. M. Purdy; J. A. Collins; R. E. Sheridan; D. L. Musser; L. Glover; M. Talwani; J. I. Ewing; R. Hawman; S. B. Smithson

    1992-01-01

    We present new ocean-bottom, wide-angle seismic data, collected during the 1990 EDGE Mid-Atlantic multichannel seismic experiment, which provide a measurement of the deep velocity structure of rifted Appalachian continental crust beneath the U.S. East Coast continental margin. Reflections from the entire crust and Moho are visible from offsets of zero to 100 km. One-dimensional inverse traveltime modeling reveals a 34-km-thick

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    The first conclusive evidence in support of the burial (and exhumation) of continental crust to depths larger than 90 km was provided by the discovery of coesite-bearing metamorphic rocks in the Dora Maira massif of the Western Alps (Chopin, 1984). Since then, even though similar outcrops of exhumed HP/UHP rocks have been recognized in a number of collisional belts, direct seismic evidences for subduction of continental crust in the mantle of the upper plate remain rare. In the Western Alps, the greatest depth ever recorded for the European Moho is 55 km by wide-angle seismic reflection (ECORS-CROP DSS Group, 1989). In an effort to image the European Moho at greater depth, and unravel the very complex lithospheric structure of the W-Alps, we have installed the CIFALPS temporary seismic array across the Southwestern Alps for 14 months (2012-2013). The almost linear array runs from the Rhône valley (France) to the Po plain (Italy) across the Dora Maira massif where exhumed HP/UHP metamorphic rocks of continental origin were first discovered. We used the receiver function processing technique that enhances P-to-S converted waves at velocity boundaries beneath the array. The receiver function records were migrated to depth using 4 different 1-D velocity models to account for the strongest structural changes along the profile. They were then stacked using the classical common-conversion point technique. Beneath the Southeast basin and the external zones, the obtained seismic section displays a clear converted phase on the European Moho, dipping gently to the ENE from ~35 km at the western end of the profile, to ~40 km beneath the Frontal Penninic thrust (FPT). The Moho dip then noticeably increases beneath the internal zones, while the amplitude of the converted phase weakens. The weak European Moho signal may be traced to 70-75 km depth beneath the eastern Dora Maira massif and the westernmost Po plain. At shallower level (20-40 km), we observe a set of strong amplitude negative-polarity converted phases (generated by downward velocity decreases) beneath the Dora Maira massif and the westernmost Po plain. Records in the Po plain display a strong but intricate converted signal from the Adria Moho between 10 and 35 km depth. We propose that the negative-polarity converted phases are generated by downward decreasing velocity between, from top to bottom, the Ivrea body of mantle origin, a thick wedge of HP/UHP metamorphic rocks and the European lower crust. Our receiver-function section thus displays the classical wedge-shaped image of the Alpine crust, but with the deepest European Moho ever recorded (70-75 km), and clear evidence of continental subduction of the European lower crust beneath the Ivrea mantle body (and possibly Adria mantle) as a negative-polarity converted phase indicative of an inverted Moho. Based on our seismic section, complemented with seismic and gravity modelling and geological arguments, we propose a new crustal-scale cross-section of the Western Alps.

  18. 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. (Univ. of Texas Institute for Geophysics, Austin (USA)); Oh, Jinyong (Univ. of Texas, Austin (USA)); Sawyer, D.S. (Rice Univ., Houston, TX (USA)); Purdy, G.M.; Reiter, E. (Woods Hole Oceanographic Institution, MA (USA)); Makris, J. (Universitaet Hamburg, Bundesstrasse, Hamburg (West Germany))

    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.

  19. Lateral variation of basalt magma type across continental margins and Island Arcs

    Microsoft Academic Search

    H. Kuno

    1966-01-01

    Quaternary basalt magmas in the Circum-Pacific belt and island arcs and also in Indonesia change continuously from less alkalic\\u000a and more siliceous type (tholeiite) on the oceanic side to more alkalic and less siliceous type (alkali olivine basalt) on\\u000a the continental side. In the northeastern part of the Japanese Islands and in Kamchatka, zones of tholeiite, high-alumina\\u000a basalt, and alkali

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

    Microsoft Academic Search

    Nicholas Eyles; James Daniels; Lisa E Osterman; Nicole Januszczak

    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 (<4.6Ma). 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

  1. Fine-grained sediment budget on the continental margin of the Bay of Biscay

    Microsoft Academic Search

    J. M. Jouanneau; O. Weber; M. Cremer; P. Castaing

    1999-01-01

    Under present-day conditions, rivers are the main source of fine sediments dispersed to the Bay of Biscay. They deliver about 2.5×106tyr?1 of continental fine sediments, 60% of which is derived from the Gironde estuary. Of this flux, 65% is believed stored on the shelf. Two kinds of mud fields can be found in the Bay of Biscay: coastal mud and

  2. Structural profile of pre-Mesozoic rocks along U. S. Atlantic Continental Margin

    SciTech Connect

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

    1986-05-01

    The US Geological Survey (USGS) seismic common-depth-point lines extending along the continental shelf from offshore New England to Florida transect several major geologic features, including the Cornwall-Kelvin fault near lat. 40/sup 0/N and a possible continental suture on the Georgia shelf near lat. 32/sup 0/N. Seismic reflectors in the Precambrian(.) to Paleozoic(.) terrane range from subhorizontal bands of significant contrast to disaggregated, greatly contorted reflectors. North of lat. 40/sup 0/N, on the shelf east of Long Island, USGS line 12 trends eastward and contains numerous reflectors indicating thrust faults. Near the Cornwall-Kelvin fault, reflectors are disaggreated and contorted. To the south, along the coast from New Jersey to North Carolina, reflectors are subhorizontal to broadly folded and wavy bedded to disaggregated. Farther south, off the coast of South Carolina and Georgia, subhorizontal reflectors generally occupy the upper part of the sequence, overlying more disaggregated patterns down to the Moho discontinuity. In contrast, to the south of the proposed continental suture, subhorizontral seismic reflectors of high contrast extend downward from the base of the Mesozoic sequence to the Moho.

  3. Recent and active deformation pattern off the easternmost Algerian margin, Western Mediterranean Sea: New evidence for contractional tectonic reactivation

    Microsoft Academic Search

    Abdelaziz Kherroubi; Jacques Déverchère; Abdelkarim Yelles; Bernard Mercier de Lépinay; Anne Domzig; Antonio Cattaneo; Rabah Bracène; Virginie Gaullier; David Graindorge

    2009-01-01

    We describe for the first time a set of large active thrusts and folds near the foot of the easternmost Algerian margin, Western Mediterranean, from swath bathymetry and high-resolution seismic data acquired in 2005 during the Maradja2\\/Samra cruise. This active system resumes a previous passive margin and creates growth strata deposition on the limbs of large folds, resulting in the

  4. Hydraulic sorting of foraminifers in visually undetected turbidites in Late Pleistocene sediments of Monterey Fan on the central California continental margin

    SciTech Connect

    Brunner, C.A.; Ledbetter, M.

    1985-01-01

    Turbiditic muds were distinguished from hemipelagic muds based on recognition of grading in silt particles in 5 turbidites from the abyssal western levee of Monterey Fan of the central Californian continental margin. Five-mg samples were taken at 1-cm intervals from beneath and above the megascopic portion of the turbidites. The silt fraction was analyzed in an electronic particle counter to determine the particle size distribution. The effect of hydraulic sorting on foraminiferal assemblages from visually undetected turbiditic muds was examined by comparing ten 10-cc samples from the turbiditic and hemipelagic muds of the 5 turbidites. Benthonic foraminifers were picked from the sand fraction, speciated and measured for length of long, intermediate, and short axes. Test shapes and sizes in graded turbiditic muds differ from those in hemipelagic muds. Hemipelagic muds contain tests that are large and variable in shape and size, whereas turbiditic muds contain specimens that are small and similar to one another in shape and size. Benthonic foraminifers are sorted by both shape and size when deposited in turbiditic muds, subtling biasing the relative species frequences in turbiditic assemblages compared to those in hemipelagic muds. The biasing can lead to paleoceanographic and biostratigraphic misinterpretation of faunas from undetected turbiditic muds.

  5. 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.

  6. Regional structure of the Southwest African continental margin based on results of lithosphere-scale 3D gravity and thermal modelling

    NASA Astrophysics Data System (ADS)

    Maystrenko, Y. P.; Scheck-Wenderoth, M.; Hartwig, A.; Anka, Z.; Watts, A. B.; Hirsch, K. K.; Fishwick, S.; Goetze, H.-J.; Reichert, C.

    2012-04-01

    A lithosphere-scale 3D structural model has been constructed in order to understand the structure of the Southwest African continental margin of South Africa and Namibia. This 3D model covers the marginal Cretaceous-Cenozoic Orange, Luderitz, Walvis and Namibe basins, the Walvis Ridge as well as two late-Proterozoic Owambo and Nama basins within the continent. The well-constrained 3D model has been used as a structural base for 3D gravity and thermal modelling. The first order configuration of the crystalline crust has been derived by 3D gravity modelling additionally constrained by available deep seismic information. According to the results of the gravity modelling, the Walvis Ridge is underlain by relatively thin continental crust. An important result of this study is related to the 3D distribution of a high-density layer within the lower crust of the study area. The thickness of this high-density lower crustal layer is largest beneath the Walvis Ridge, reaching more than 30 km. In addition to the high-density lower crust, high-density zones within the continental crystalline crust had to be included into the model to fit observed and calculated gravity. The thickness of this layer is locally up to 40 km. The obtained configuration of the Moho clearly correlates with the major tectonic units of the Southwest African continental margin where a deep Moho corresponds to Precambrian continental crust and a shallow one is located beneath the younger oceanic crustal domain. 3D thermal modelling has been carried out, assuming that the Southwest African continental margin has already reached steady-state conditions and that heat conduction is the dominant mechanism of heat transfer. The obtained results of the 3D thermal modelling demonstrate that there is a clear correlation between the location of thick sediments and areas with increased temperatures within the upper part of the 3D model. This implies that the low thermal conductivity of the sediments causes heat storage within the areas where sediments are thick. On the other hand, the main feature of the temperature distribution in the deeper part of the lithosphere is a gradual transition across the continental margin from a relatively cold oceanic part to a warmer continental part. This regional pattern is controlled by the large thickness of the continental crystalline crust, which is characterized by an increased radiogenic heat production in comparison to the upper mantle. Nonetheless, at a depth of 80-90 km, the temperature beneath the oceanic crustal domain is higher than beneath the continental domain, reflecting the configuration of the lower thermal boundary, which is represented by an isothermal lithosphere-asthenosphere boundary.

  7. 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.

  8. 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 ?m 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.

  9. Salt tectonics in the SW Alps (Italy-France): From rifting to the inversion of the European continental margin in a context of oblique convergence

    NASA Astrophysics Data System (ADS)

    Decarlis, A.; Maino, M.; Dallagiovanna, G.; Lualdi, A.; Masini, E.; Seno, S.; Toscani, G.

    2014-12-01

    The SW Alps result from the inversion of the European continental margin during the oblique convergence between Europe and Adria since the Cretaceous. The orogenic deformation is controlled by two factors: the inherited sedimentary and structural record and the geodynamic interaction between the two plates. In this paper we present a stratigraphic and structural analysis of the external SW Alps (Ventimiglia-Menton area) in order to define the sedimentary and deformational geometries of the chain and to reconstruct the evolutionary history. The field-data highlight the preeminent role played by inherited salt-structures, which derive from the depositional history experienced by the European margin since the Mesozoic onwards. From Late Triassic to Jurassic, evaporites and carbonates deposited as a response to the Thetyan rifting. The following emplacement of the Cretaceous flysch and of the Eocene foreland basin succession was strongly influenced by the extensionally-triggered salt diapirism and by the interactions with deformations connected to the Pyrenees dynamics. The resulting geologic discontinuities (i.e. diapir-induced highs and basins, inherited normal and trasform faults) strongly influenced the successive Oligo-Miocene evolution of the belt in the study area. Observed changes in the thrusts and folds kinematics are considered as the results of rotation during their approaching to inherited highs. Furthermore, the overturning of thrusts and folds in the front of the diapiric flanks are associated with the progressively salt squeezing into the anticlines cores promoted by ongoing Alpine compression. Finally, the kinematic data from the study area show radical differences in the tectonic transport direction with respect to the rest of the SW Alps (NW- to W-ward in the Ventimiglia-Menton area, S- to SW-ward in Provence and Ligurian Alps). This difference is interpreted to be caused by the relative motions of crustal blocks dominated by transpressive tectonics in the frame of the Oligo-Miocene western Mediterranean geodynamics.

  10. Recent distribution and accumulation of organic carbon on the continental margin west off Spitsbergen

    Microsoft Academic Search

    Daniel Winkelmann; Jochen Knies

    2005-01-01

    The study compiles the controlling factors for organic matter sedimentation patterns from a suite of organogeochemical parameters in surface sediments off Spitsbergen and direct seabed observations using a Remotely Operated Vehicle (ROV). In addition we assess its storage rates as well as the potential of carbon sinks on the northwestern margin of the Barents Sea with short sediment cores from

  11. 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 offset. ?? 1986.

  12. Turbiditic Sequences in Sediment Cores from the Continental Margin of Southern Chile as a Potential Record of Seismic Activity

    NASA Astrophysics Data System (ADS)

    Reichel, T.

    2004-12-01

    The tectonically active continental margin of Chile offers an excellent opportunity to study the relationship between local seismic activity and mass movements in the marine sedimentary record. The historical earthquake record of the past 500 years shows that even very strong events of magnitude > 7 (Richter scale) have been fairly common. Seismic events lead to remobilisation of sediments triggering downslope turbidites. Several deeply incised canyons channel most of the sediment into the trench, where the sediment sequence documents a regional record of events. Some intra-slope basins also bear a high potential for the preservation of a turbidite record of local significance. Sediment cores taken from different settings along the continental margin were analyzed for their succession of turbiditic layers. Age-dating - presently in progress - will be based on ? 18O-; AMS14C-, 210Pb-analyses in all cores. First results are: During the Pleistocene sedimentation rates were high and turbidite layers more frequent due to strong erosion in the hinterland associated with glacial activity and a colder and more humid climate. Even sediment cores in elevated and distant positions of the trench contain turbiditic-layers, although age-datings show that these layers are older than 10ka. An intra-slope basin near the foot of the slope gives a record of the Holocene turbiditic-sequences which contains 6 to 12 strong events. Further detailed age determination of the sequences - presently in progress - and comparison with the historical earthquake record promise an extension of the seismic record into the past beyond the last 500 years.

  13. 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.

  14. Epibenthic megacrustaceans from the continental margin, slope and abyssal plain of the Southwestern Gulf of Mexico: Factors responsible for variability in species composition and diversity

    NASA Astrophysics Data System (ADS)

    Escobar-Briones, Elva G.; Gaytán-Caballero, Adriana; Legendre, Pierre

    2008-12-01

    The community structure of megacrustaceans (orders Lophogastrida, Isopoda, and Decapoda) collected in trawls on the continental margin, upper slope and abyssal plain of the southern Gulf of Mexico was studied to determine to what extent broad-scale variation in community composition and diversity was influenced by geographic regions environmental variability and depth. Trawls were collected in the Mexican Ridges, the Campeche Bank, and the Sigsbee abyssal plain. There was variability in species composition, density and diversity among geographic regions and along the depth gradient. A total of 106 species were identified and grouped in three orders; five infraorders, 40 families, and 70 genera. This study extends the known geographic ranges of the species Homolodromia monstrosa and Ephyrina benedicti. The largest number of species was recorded in the Mexican Ridges and on the upper continental shelf; lower values were found on the continental margin and in the abyssal plain. The largest densities were recorded on the continental margin in the Mexican Ridges. Megacrustaceans show in general low frequencies and low abundances in trawls, characterizing them as rare components of benthic assemblages. Contrary to an accepted paradigm about deep-sea biodiversity, the highest H' diversity values were recorded in the Sigsbee abyssal plain, followed by values from the upper continental slope; diversity values were correlated with evenness. Canonical Redundancy analysis results showed a significant affinity to regions for 18 crustacean species; 33 species showed a significant affinity to both regions and depth zones within regions.

  15. Provenance and fate of organic carbon in three submarine canyons from the Portuguese Margin: Implications for transport processes of material in continental margins

    NASA Astrophysics Data System (ADS)

    Kiriakoulakis, Kostas; Wolff, George; Blackbird, Sabena

    2010-05-01

    Submarine canyons are key environments on the continental margin that are affected by unique and dynamic but often episodic and complex processes, and are difficult to study. Canyons are considered hotspots of biodiversity and enhancement of primary productivity at canyon heads has often been postulated to support this, although the evidence is sparse. Additionally canyons are considered to be fast-track corridors for material transported from the land to the deep sea and they are considered major pathways for the transportation and burial of organic carbon, acting as buffers for sediment and carbon storage. Organic geochemical and isotopic markers are often used as reliable indicators for the supply, quality and fate of organic matter in marine systems. In this study they have been used to test the above hypotheses in three contrasting submarine canyons (Nazaré, Setubal/Lisbon and Cascais) of the Portuguese Margin. The elemental and lipid biomarker composition of suspended particulate organic matter of surface waters close to the studied canyon heads had a fresh phytoplankton signal, however there was no clear evidence for enhanced primary productivity by comparison to the neighbouring open slope. By contrast, mid-depth waters (700-1600 m), that are dominated by the northward flowing Mediterranean Outflow Water, had high lipid content and abundant mesozooplankton biomarkers, perhaps reflecting zooplankton activity focused at the boundaries of distinct water masses. In the waters close to the floor of the Nazaré Canyon the presence of elemental sulphur (a product of sediment diagenesis) and high molecular weight hydrocarbons (recalcitrant, terrestrial markers) indicated high levels of resuspended material, particularly at the Upper section (

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

    E-print Network

    Buesseler, Ken

    in the upper 500 m of the Western Subarctic Pacific, an iron-limited High Nutrient Low Chlorophyll (HNLC of these profiles suggest a consistently strong lateral advection of labile Mn and Fe from redox-mobilized labile033294. 1. Introduction [2] The Subarctic Pacific is one of three major High Nutrient Low Chlorophyll

  17. Phenomena of pulsation tectonics related to the breakup of the eastern north American continental margin

    Microsoft Academic Search

    Robert E. Sheridan

    1983-01-01

    New data from the recent IPOD drilling at Deep Sea Drilling Project Site 534 in the Blake-Bahama Basin give a defi- nitive age for the spreading-center shift involved in the breakup of the North American Atlantic margin. An age of ba- sal Callovian (~ 155 m.y.) is determined for the Blake Spur magnetic anomaly marking this spreading-center shift that signals

  18. Benthic dynamics at the carbonate mound regions of the Porcupine Sea Bight continental margin

    Microsoft Academic Search

    Martin White; Geol Rundsch

    2007-01-01

    A brief review is given of some dynamical processes that influence the benthic dynamics within the carbonate mound provinces\\u000a located at the Porcupine Bank\\/Sea Bight margin, NE Atlantic. The depth range of the mounds in this region (600–1,000 m) marks\\u000a the upper boundary of the Mediterranean outflow water above which Eastern North Atlantic Water dominates. Both water masses\\u000a are carried northwards

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

    Microsoft Academic Search

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

    2007-01-01

    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

  20. 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…

  1. Journal of the Geological Society, London, Vol. 162, 2005, pp. 889892. Printed in Great Britain. Discussion on subsidence history of the north Indian continental margin,

    E-print Network

    Watts, A. B. "Tony"

    lithosphere (Spontang Ophiolite) on top of the Indian passive continental margin (Zanskar Shelf and supported the idea that the Spontang Ophiolite was obducted in the Late Cretaceous, making implicit or explicit correlation with the well-studied Semail Ophiolite of Oman (e.g, Searle 1983; 1986; Pedersen et al

  2. The imprint of sea-level changes in the Southeastern Iberian continental shelf, Western Mediterranean Sea

    NASA Astrophysics Data System (ADS)

    Pinna, Andrea; Lastras, Galderic; Acosta, Juan; Muñoz, Araceli; Canals, Miquel

    2014-05-01

    A detailed morphologic analysis of the Southeastern Iberian continental shelf, Western Mediterranean Sea, between the Mar Menor and the Gulf of Almería, based on swath bathymetry data, has revealed a number of seafloor features that we attribute to the imprint of sea-level changes since the last glacial maximum. The continental shelf has been divided in four different domains with contrasting characteristics: the Mar Menor sector, the Mazarrón and Vera sector, the Gata Cape shelf and the Gulf of Almería shelf. The Mar Menor sector displays an up to 40 km wide shelf with a very low slope gradient, which contrasts with the Mazarrón and Vera shelf, with a width ranging between 0.4 and 5 km, severely incised by the different branches of the Garrucha submarine canyon. On each of these sectors, a variety of morphologies such as crests and escarpments have been identified. Most of these crests and escarpments can be followed for distances exceeding 15 km, and are located at constant, characteristic water depths. We interpret these structures as the result of relatively short-lived sea-level still-stands and thus as palaeo-coastlines. Taking into account subsidence, we have correlated their bathymetric position with published post-MIS-5 Mediterranean sea-level evolution curves, allowing the attribution of an approximate age for each interpreted palaeo-coastline. The last sea-level regression is partially registered in the smooth Mar Menor shelf, where different crests and escarpments are cut by a LGM palaeo-channel, whereas all the sectors display structures related to the last sea-level transgression. The continuity of these structures along all the sectors has allowed reconstructing the evolution of the coastline during the last sea-level transgression, and thus inferring the palaeo-landscape of this sector of the Southeastern Iberian coast at different stages since 18 ka BP until the present.

  3. Tectonic Inversion of the Algerian Continental Margin off Great Kabylia (North Algeria) - Insights from new MCS data (SPIRAL cruise)

    NASA Astrophysics Data System (ADS)

    Beslier, M.; Aidi, C.; Yelles-Chaouche, A.; Ribodetti, A.; Bracene, R.; Schenini, L.; Djellit, H.; Sage, F.; Deverchere, J.; Medaouri, M.; Klingelhoefer, F.; Abtout, A.; Charvis, P.; Bounif, A.

    2013-12-01

    Sub-marine active faulting threatens the coastline of Algeria, as shown by the major Mw 6.9 May 21, 2003 earthquake that occurred in Great Kabylia close to Boumerdes. We present here the structures associated to the Plio-Quaternary (P-Q) tectonic inversion of the central part of the Algerian margin offshore Great Kabylia using new deep multichannel seismic (MCS) lines. The large-scale structure of the margin deduced from wide-angle seismic (WAS) data modeling is presented in a companion abstract. Five MCS lines were acquired in the study area during the Algerian-French SPIRAL cruise (September 2009, R/V Atalante). Four lines were acquired using a 3040 cu. in. air-gun array and a 4.5 km 360 channel digital streamer and a 8350 cu. in. source favoring deep penetration was used for one coincident WAS profile and the fifth MCS line. All profiles are pre-stack time migrated and additional pre-stack depth migration was performed in key areas. The MCS lines crosscut the margin from the upper slope to the deep Algero-Provençal Basin either in a N-S direction sub-perpendicular to the structural trend of the margin, or in a NW-SE direction parallel to the actual convergence between Africa and Eurasia plates. Tectonic inversion is expressed on all profiles at the deep margin. The eastern line displays a flat-ramp compressive system in the deep sedimentary series, which emerges at the foot of the continental slope and marks the seaward limit of a P-Q basin perched at mid-slope. The south-dipping ramps are neo-formed structures, whereas the flats use inherited lithologic discontinuities (base of the Messinian evaporitic series, top of the acoustic basement). Westward in the Boumerdes area, the compressive deformation is expressed deeper in the acoustic basement where a southward dipping reflector is interpreted as a blind thrust on top of which all the sedimentary series (Miocene to P-Q) are bent in an antiform that uplifts the base of the Messinian series. A second antiform prolongates this uplift 20 km northward although no clear reverse structure is imaged underneath. These antiforms delimit two asymmetric sub-basins filled with a southward thickening P-Q wedge. As a whole, the geometry of the reverse structures supports fault-propagation or fault-bent fold models, as previously inferred from HR seismic interpretation. They are likely to participate to large uplifts such as the coastal one related to the Boumerdes earthquake. Reverse structures in the thinned continental crust co-exist with transtensional deformation at the transition with the oceanic domain 50 km northward of the margin toe, where a narrow asymmetric basin shows a downward offset of the base of the Messinian series and a southward thickening P-Q wedge. A transcurrent component on crustal faults playing since the P-Q may explain both basin geometry and lateral variations in width and depth.

  4. Structural features of the Southwest African continental margin according to results of lithosphere-scale 3D gravity and thermal modelling

    NASA Astrophysics Data System (ADS)

    Maystrenko, Yuriy P.; Scheck-Wenderoth, Magdalena; Hartwig, Alexander; Anka, Zahie; Watts, Antony B.; Hirsch, Katja K.; Fishwick, Stewart

    2013-09-01

    To understand the structure of the Southwest African continental margin, a lithosphere-scale 3D structural model has been developed, covering the marginal Cretaceous-Cenozoic Orange, Luderitz, Walvis and Namibe basins, the easternmost Walvis Ridge offshore. Onshore, the model includes two late-Proterozoic Owambo (Etosha) and Nama basins. This 3D model integrates published thickness maps (sediment isopach maps), shallow seismic and well data as well as published deep seismic information and has been additionally constrained by 3D gravity and thermal modelling. Using 3D gravity modelling, the first order configuration of the crystalline crust has been resolved with respect to the location of the continent-ocean boundary. The distribution of a high-density lower crustal layer indicates a continuous body extending below the Cretaceous-Cenozoic depocentres and aligned parallel to the coast line. In addition, high-density zones within the continental crystalline crust had to be included in the model to fit observed and calculated gravity. The obtained Moho topography correlates with the major tectonic units of this continental margin. The results of the 3D thermal modelling indicate that there is a clear relationship between the location of thickened sediments and areas with increased temperatures within the upper 10 km of the 3D model. This indicates that the low thermal conductivity of the sediments increases heat storage within the areas covered by thick sediments. Within the deeper crust, the main feature of the temperature distribution is the transition across the continental margin from the relatively cold oceanic part to the warm continental one. This regional pattern is controlled by the thickness of the crystalline continental crust, which is characterized by an increased radiogenic heat production. At a depth of 80-90 km, the temperature becomes higher beneath the oceanic domain than beneath the continent, reflecting the configuration of the lower thermal boundary which is represented by an isothermal lithosphere-asthenosphere boundary.

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

    DOE PAGESBeta

    Archer, D.

    2015-01-01

    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.more »There 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 zone. The implication is that any methane hydrate existing today will be insulated from anthropogenic climate change by hundreds of meters of sediment, resulting in a response time of thousands of years. The strongest impact of the glacial–interglacial cycles on the atmospheric methane flux is due to bubbles dissolving in the ocean when sea level is high. When sea level is low and the sediment surface is exposed to the atmosphere, the atmospheric flux is sensitive to whether permafrost inhibits bubble migration in the model. If it does, the atmospheric flux is highest during the glaciating, sea level regression (soil-freezing) part of the cycle rather than during deglacial transgression (warming and thawing). The atmospheric flux response to a warming climate is small, relative to the rest of the methane sources to the atmosphere in the global budget, because of the ongoing flooding of the continental shelf. The increased methane flux due to ocean warming could be completely counteracted by a sea level rise of tens of meters on millennial timescales due to the loss of ice sheets, decreasing the efficiency of bubble transit through the water column. The model results give no indication of a mechanism by which methane emissions from the Siberian continental shelf could have a significant impact on the near-term evolution of Earth's climate, but on millennial timescales the release of carbon from hydrate and permafrost could contribute significantly to the fossil fuel carbon burden in the atmosphere–ocean–terrestrial carbon cycle.« less

  6. Cenomanian-Turonian organic facies in the western Mediterranean and along the adjacent Atlantic margin

    SciTech Connect

    Kuhnt, W.; Herbin, J.P.; Thurow, J.; Wiedmann, J.

    1988-08-01

    Pre-Cenomanian sediments of the western Mediterranean and adjacent Atlantic margin are characterized by low total organic content (TOC) with an important terrestrial component. During the Cenomanian, TOC increased and the marine component became dominant, culminating around the Cenomanian-Turonian boundary with TOC up to 40%. After the Turonian, organic-rich sediments progressively disappeared and were replaced by more oxygenated sediments. Study methods include considering data from outcrops, DSDP/ODP sites, or petroleum wells. Detailed data from onshore locations allowed the development of high-resolution stratigraphy, analysis of depositional environment, and calculation of sedimentation rates. Analysis of these data indicates Cenoamnian-Turonian organic-rich sediments can be observed in a wide range of bathymetric settings. They are widespread in the western Mediterranean and Atlantic and have been especially studied in Italy (Apennines, southern Alps), Tunisia (Bahloul), Algeria, Morocco (Rif Mountains, Atlas Mountains, Tarfaya), Gibraltar arch, Spain (Betics, Bay of Biscay, Galicia margin), Senegal (Cape Verde basin, Casamance), and Nigeria (Benue, Calabar flank).

  7. Electrical conductivity images across the Namibian passive margin: Implications for tectonic processes along the Kaoko Belt, the western Kongo Craton and the Walvis Ridge

    NASA Astrophysics Data System (ADS)

    Weckmann, Ute; Meqbel, Naser; Kapinos, Gerhard; Jegen-Kulcsar, Marion; Ritter, Oliver

    2014-05-01

    The Special Priority Programme SAMPLE of the German Science Foundation DFG is focussed on investigating processes related to the breakup of supercontinent Gondwana and the post breakup evolution of the passive continental margins of Africa and South America. Within this framework an amphibian magnetotelluric (MT) experiment was conducted at the Southern African passive continental margin, starting at the Walvis Ridge in the Atlantic Ocean and crossing onshore the entire Kaoko Belt and the western boundary of the Kongo Craton in Northern Namibia. High-quality MT data at 167 onshore and xx offshore sites show a strong variability within short distances and indicate complex subsurface structures in parts of the Kaoko Belt and along some of the major thrust and fault zones. To identify the main conductivity features and resolve their properties in more spatial detail we started our modelling procedure with 2D inversion for a sub-set of the data where the 3D effects are less dominant along the amphibian profile. However, to account for 3D effects in the MT data and to assess robustness of conductivity anomalies revealed in the 2D model we used the entire data set for the 3D inversion using ModEM. 2D and 3D inversion models show zones of high electrical conductivity that correlate with surface expressions of prominent faults such as the Purros Mylonite Zone and the Three Palm Mylonite Zone of the Kaoko Belt. Outcropping Etendeka flood basalts in the Western Kaoko Zones are imaged by 10-15km deep reaching zones of high resistivity. Additionally, the inversion models reveal a spatial correlation of resistive zones with the cratonic Northern Platform; however, the geologically defined onset of the Kongo Craton appears as an area of high conductivity. Compared with other craton boundaries in Southern Africa this is very untypical.

  8. Lower Cretaceous organic-rich sediments drilled on Antarctic Continental Margin during ODP Leg 113

    SciTech Connect

    O'Connell, S.

    1988-02-01

    Lower Cretaceous organic-rich sediments were recovered below a major unconformity at two sites on the eastern Weddell Sea margin during ODP Leg 113. No age overlap exists between the two sites. A more continuous Cretaceous section was previously recovered at DSDP Site 511 on the Falkland plateau. Site 692 (2880 m water depth) is located on a mid-slope bench in Wegener Canyon. Early Cretaceous-age (pre-Albian) sediments extend from 53-98 m below sea floor (mbsf) and are dominated by organic-rich nannofossil claystone. Macrofossils (e.g., belemites and ammonites), thin carbonate lenses (<1 cm), and water escape structures are abundant. Thin beds of devitrified ash, bioturbation, and graded bedding are present. Site 693 (2360 m water depth), 30 km west of Site 692 on the canyon's outer rim, recovered Albian-age organic-rich claystones and mudstones from 416-483 mbsf. Site 693 sediments have lower organic contents than those at Site 692. Glauconite is common in the upper part of the unit. Well-preserved diatoms and diatomite layers suggest high productivity. These sediments differ from Albian-age sediments at Site 511, which consist of open-marine nannofossil ooze. The sediments were deposited in an anoxic or hypoxic upper bathyal (500-1000 mbsl) marine environment. Oxygenation occurred later along the Antarctic margin than the Falkland plateau, possibly as the inflow of Atlantic water proceeded south from an opening near the Falkland plateau.

  9. Pleistocene water cycle and eastern boundary current processes along the California continental margin

    NASA Astrophysics Data System (ADS)

    Lyle, Mitchell; Heusser, Linda; Ravelo, Christina; Andreasen, Dyke; Olivarez Lyle, Annette; Diffenbaugh, Noah

    2010-11-01

    Coastal marine sediments contain mixtures of terrestrial and marine paleoclimate proxies that record how the coastal water cycle has behaved over long time frames. We explore a 600 kyr marine record from ODP Site 1018, located due west of Santa Cruz, California, to identify coastal wet and dry periods and to associate them with oceanographic processes. Wet periods in central California, identified by increased tree pollen relative to pollen from grasslands and scrublands, are found on every major deglaciation in the last 600 kyr. Sea surface temperature (SST) data were collected for the last two deglaciations. Wet periods are associated with a rapid rise in SST off central California. SST gradients along the California margin and changes in biogenic deposition show that wet periods in central California are associated with a weakening of the California Current and weakened coastal upwelling. High carbonate production suggests that there was significant curl-of-wind stress upwelling offshore. We propose that wet periods in central California are associated with a meteorological connection to the tropical Pacific and weakened southward flow in the California Current that shunted temperate Pacific water northward into the Alaska gyre. We do not observe evidence for a south-shifted westerly storm track at the last glacial maximum but find that wet periods are diachronous along the California margin. The wettest period around the Santa Barbara Basin peaked at 16 ka, preceding the wet peak in central and northern California by 4 kyr.

  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)

    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 or only weak volcanism.

  11. Constraints on the thermal evolution of the Adriatic margin during Jurassic continental break-up: U-Pb dating of rutile from the Ivrea-Verbano Zone, Italy

    NASA Astrophysics Data System (ADS)

    Ewing, Tanya A.; Rubatto, Daniela; Beltrando, Marco; Hermann, Jörg

    2015-04-01

    The Ivrea-Verbano Zone (IVZ), northern Italy, exposes an attenuated section through the Permian lower crust that records high-temperature metamorphism under lower crustal conditions and a protracted history of extension and exhumation associated partly with the Jurassic opening of the Alpine Tethys ocean. This study presents SHRIMP U-Pb geochronology of rutile from seven granulite facies metapelites from the base of the IVZ, collected from locations spanning ~35 km along the strike of Paleozoic fabrics. Rutile crystallised during Permian high-temperature metamorphism and anatexis, yet all samples give Jurassic rutile U-Pb ages that record cooling through 650-550 °C. Rutile age distributions are dominated by a peak at ~160 Ma, with a subordinate peak at ~175 Ma. Both ~160 and ~175 Ma age populations show excellent agreement between samples, indicating that the two distinctive cooling stages they record were synchronous on a regional scale. The ~175 Ma population is interpreted to record cooling in the footwall of rift-related faults and shear zones, for which widespread activity in the Lower Jurassic has been documented along the western margin of the Adriatic plate. The ~160 Ma age population postdates the activity of all known rift-related structures within the Adriatic margin, but coincides with extensive gabbroic magmatism and exhumation of sub-continental mantle to the floor of the Alpine Tethys, west of the Ivrea Zone. We propose that this ~160 Ma early post-rift age population records regional cooling following episodic heating of the distal Adriatic margin, likely related to extreme lithospheric thinning and associated advection of the asthenosphere to shallow levels. The partial preservation of the ~175 Ma age cluster suggests that the post-rift (~160 Ma) heating pulse was of short duration. The regional consistency of the data presented here, which is in contrast to many other thermochronometers in the IVZ, demonstrates the value of the rutile U-Pb technique for probing the thermal evolution of high-grade metamorphic terrains. In the IVZ, a significant decoupling between Zr-in-rutile temperatures and U-Pb ages of rutile is observed, with the two systems recording events ~120 Ma apart.

  12. The bog landforms of continental western Canada in relation to climate and permafrost patterns

    SciTech Connect

    Vitt, D.H.; Halsey, L.A. (Univ. of Alberta, Edmonton, Alberta (Canada)); Zoltai, S.C. (Northern Forestry Centre, Edmonton, Alberta (Canada))

    1994-02-01

    In continental western Canada, discontinuous permafrost is almost always restricted to ombrotrophic peatlands (bogs). Bogs occur mostly as islands or peninsulas in large, often complex fens or are confined to small basins. Permafrost may be present in extensive peat plateaus (or more locally as palsas) and was preceded by a well-developed layer of Sphagnum that served to insulate the peat and lower the pore water temperatures. Air photo interpretation reveals the occurrence of bogs with five types of surface physiography. Concentrated to the south are bogs without internal patterns that have never had permafrost. Dominating the mid-latitudes are bogs with internal lawns and fens with internal lawns (mostly representing former bogs) that had permafrost lenses in the past that have recently degraded. Concentrated in the northwest are peat plateaus without internal lawns or distinct collapse scars, but with permafrost; dominating in the northernmost area are peat plateaus with extensive permafrost and collapse scars. Relationships are apparent between the current - 1[degrees]C isotherm and the southern occurrence of peat plateaus and between the 0[degrees]C isotherm and the southern edge of bogs and fens with internal lawns. We interpret bogs and fens with internal lawns to represent areas where permafrost degradation is currently occurring at a greater rate than aggradation, seemingly in response to warmer regional climate, although fire frequency may also be of local importance. 54 refs., 21 figs., 2 tabs.

  13. Preliminary report on geology along Atlantic Continental Margin of northeastern United States

    USGS Publications Warehouse

    Minard, J.P.; Perry, W.J.; Weed, E.G.A.; Rhodehamel, E.C.; Robbins, E.I.; Mixon, R.B.

    1974-01-01

    The U.S. Geological Survey is conducting a geologic and geophysical study of the northeastern United States outer continental shelf and the adjacent slope from Georges Bank to Cape Hatteras. The study also includes the adjacent coastal plain because it is a more accessible extension of the shelf. The total study area is about 324,000 sq km, of which the shelf and slope constitute about 181,000 sq km and the coastal plain constitutes 143,000 sq km. The shelf width ranges from about 30 km at Cape Hatteras to about 195 km off Raritan Bay and on Georges Bank. Analyses of bottom samples make it possible to construct a preliminary geologic map of the shelf and slope to a water depth of 2,000 m. The oldest beds cropping out in the submarine canyons and on the slope are of early ate Cretaceous age. Beds of Early Cretaceous and Jurassic age are present in deep wells onshore and probably are present beneath the shelf in the area of this study. Such beds are reported beneath the Scotian shelf on the northeast where they include limestone, salt, and anhydrite. Preliminary conclusions suggest a considerably thicker Mesozoic sedimentary sequence than has been described previously. The region is large; the sedimentary wedge is thick; structures seem favorable; and the hydrocarbon potential may be considerable.

  14. Geochemical index of trace metals in the surficial sediments from the western continental shelf of India, Arabian Sea

    Microsoft Academic Search

    C. M. Laluraj; S. M. Nair

    2006-01-01

    The present study focuses on the determination and abundance of trace metals (viz. Cu, Ni, Zn, Cr, Co, Cd, Mn and Fe) in the\\u000a surficial sediments of west coast of Arabian Sea along the Indian subcontinent. Sediment samples were collected from three\\u000a transects along the western continental shelf of Arabian Sea. The enrichment of Fe and Mn in coastal oxic-sediments

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

    To understand the evolution of the passive continental margin in Argentina low temperature thermochronology is an appropriate method, which will lead to new conclusions in this area. The Tandilia System, also called Sierras Septentrionales, is located south of the Río de la Plato Craton in eastern Argentina in the state of Buenos Aires. North of the hills Salado basin is located whereas the Claromecó basin is situated south of the mountain range. In contrary to most basins along the southamerican passive continental margin the Tandilia-System and the neighbouring basins trend perpendicular to the coast line. The topography is fairly flat with altitudes of. The igneous-metamorphic basement is pre-proterozoic in age and build up of mainly granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks and granitoid plutons it is overlain by a series of Neoproterozoic to early Paleozoic sediments (Cingolani, 2010), like siliciclastics, dolostones, shales and limestones (Demoulin et al., 2005). The aim of the study is to quantify the long-term landscape evolution of the passive continental margin in eastern Argentina in terms of thermal history, exhumation and tectonic activities. For that purpose, samples were taken from the Sierra Septentrionales and analyzed with the apatite fission-track method. Further 2-D thermokinematic modeling was conducted with the computer code HeFTy (Ketcham, 2005; Ketcham 2007; Ketcham et al., 2009). The results indicate apatite fission track ages between 101.6 (9.4) to 228.9 (22.3) Ma, what means all measured ages are younger as their formation age. That shows all samples have been reset. Six samples accomplished enough confined tracks and were used to test geological t-T models against the AFT data set. These models give a more detailed insight on the cooling history and tectonic activities in the research area. References: Cingolani C. A. (2010): The Tandilia System of Argentina as a southern extension of the Río de la Plata craton: an overview. Int. J. Earth Sci. (Geol. Rundsch.) (2011) 100:221-242, doi 10.1007/s00531-010-0611-5. Demoulin, A., Zarate, M., Rabassa, J.; (2005) Longterm landscape development: a perspective from the southern Buenos Aires ranges of east central Argentina; Journal of South American Earth Sciences, Vol. 19, pp. 193-204. Ketcham, R. A. (2005): Forward and inverse modeling of low-temperature thermochronometry data, in Low-Temperature Thermochronology: Techniques, Interpretations, and Applications, edited by P. W. Reiners and T. A. Ehlers, pp. 275-314, Mineralogical Society of America/Geochemical Society, Chantilly, Virginia. Ketcham, R. A., et al. (2007): Improved modeling of fission-track annealing in apatite, American Mineralogist, 92, 789-798. Ketcham, R. A., Donelick, R. A., Balestrieri, M. L., Zattin, M. (2009): Reproducibility of apatite fission-track length data and thermal history reconstruction, Earth and Planetary Science Letters 284 (2009), 504-515.

  16. Thrust tectonics along the north-western continental margin of Sabah\\/Borneo

    Microsoft Academic Search

    K. Hinz; J. Fritsch; E. H. K. Kempter; A. Manaf Mohammad; J. Meyer; D. Mohamed; H. Vosberg; J. Weber; J. Benavidez

    1989-01-01

    Zusammenfassung  Nach herkömmlichen plattentektonischen Vorstellungen soll eine inaktive Subduktionszone am nordwestlichen Kontinentalrand von Sabah liegen. Reflexionsseismische Meßdaten der BGR zeigen jedoch, daß hier autochthone kontinentale Kruste mit einer oligozänen-frühmiozänen Karbonatplattform progressiv von einem allochthonen Gesteinsverband überschoben wird. Fortschreitender Zusammenschub seit dem frühen Miozän führte zur Anlage von vier Deformationszonen: Tekonische Schuppen (Zone III); zwei übereinander geschobene Verschuppungssysteme (Zone IV); Gürtel mit

  17. Diverse Approaches USED to Characterize the Earthquake and Tsunami Hazards Along the Southern Alaska Continental Margin

    NASA Astrophysics Data System (ADS)

    Haeussler, P. J.; Witter, R. C.; Liberty, L. M.; Brothers, D. S.; Briggs, R. W.; Armstrong, P. A.; Freymueller, J. T.; Parsons, T.; Ryan, H. F.; Lee, H. J.; Roland, E. C.

    2014-12-01

    Earthquakes and tsunamis are the principal geohazards of southern Alaska. The entire margin has ruptured in megathrust earthquakes, including the M9.2 1964 event, and these earthquakes have launched deadly local and trans-Pacific tsunamis. Tsunamis have been by far the largest killer in these earthquakes. Moreover, the subduction zone displays a range in locking behavior from completely locked beneath Prince William Sound, to ­­­­nearly freely slipping beneath the Shumagin Islands. Characterizing earthquake-related tsunami sources requires a diverse set of methods, and we discuss several examples. One important source for tsunamis is from megathrust splay faults. The Patton Bay splay fault system ruptured during the 1964 earthquake and generated a tsunami that impacted coastlines tens of minutes after the earthquake. A combination of multibeam mapping, high-resolution and crustal-scale seismic data, thermochronology, and detrital zircon geochronology show focused exhumation along this splay fault system for the last 2-3 Ma. Moreover, this long term pattern of exhumation mimics the pattern of uplift in 1964. Submarine landslides are another example of a tsunami source. Numerous devastating slides were triggered by the 1964 earthquake. Multibeam bathymetry, bathymetry difference maps, high-resolution seismic data, and records of paleotsunamis in coastal marshes reveal a long history of submarine landsliding in the coastal fjords of Alaska. The Little Ice Age appears to have had a significant influence on the submarine landslides in the 1964 earthquake through increased sediment production, transport to fjord margins, and, locally, compaction by glacier advances. Glacial retreat before 1964 gave rise to over-steepened slopes susceptible to dynamic failure. Numerous blocks in the submarine landslides were particularly effective in generating high tsunami run up. Finally, regional tectonic displacements of the seafloor have launched trans-Pacific tsunamis. Coastal evidence of high tsunamis in the eastern Aleutians has helped us understand the frequency of megathrust earthquakes west of Kodiak Island. Recent studies of vertical displacements produced by tsunamigenic earthquakes has led to new insights about the persistence of rupture boundaries and long term constraints on locking behavior.

  18. Satellite-Based Investigations of the Transition from an Oceanic to Continental Transform Margin

    NASA Technical Reports Server (NTRS)

    Miller, M. Meghan

    1998-01-01

    Detailed characterization of neotectonics evolution of the Valle de San Felipe and Arroyo Grande regions in northern Baja California. Reoccupied GEOMEX GPS sites, and occupied a regional GPS (Global Positioning System) network. The Baja California peninsula in Mexico offers a unique setting for studying the kinematic evolution of a complex, active strike-slip/rift plate boundary. We are currently conducting remote sensing, geologic, and geodetic studies of this boundary. The combined data sets will yield instantaneous and time integrated views of its evolution. This proposal solicits renewed funding from NASA to support remote sensing and geologic studies. During the late Cenozoic, Baja California has been the locus of changing fault geometry that has accommodated components of the relative motion between the North America and Pacific plates. Contemporary slip between the two plates occurs in a broad zone that encompasses much of southern California and the Baja California Peninsula. The transfer of slip across this zone in southern California is relatively well understood. South of the border, the geometry and role of specific faults and structural provinces in transferring plate margin deformation across the peninsula is enigmatic. Results We use Landsat Thematic Mapper imagery of the Baja California Peninsula to identify recent and active faults, and then conduct field studies that characterize the temporal and spatial structural evolution of the plate margin. These data address questions concerning the neotectonic development of the Gulf of California, the Baja California Peninsula, and their role in evolution of the post-Miocene Pacific - North American plate boundary. Moreover, these studies provide constraints on the geometry of active faults, allowing more exact understanding of the results of ongoing NASA-supported geodetic experiments. In addition, anticipated publication of the TM scenes will provide a widely available geological data base for relatively little-known peninsula California. Achievements include development of an ArcInfo data base of Landsat and SPOT imagery, detailed field studies of Neogene structures in northeastern Baja California, and new constraint on Pacific - North America plate motion at Baja California latitudes. These results are reported in maps, manuscripts and data products which are published or near completion.

  19. Neotectonic reactivation of shear zones and implications for faulting style and geometry in the continental margin of NE Brazil

    NASA Astrophysics Data System (ADS)

    Bezerra, F. H. R.; Rossetti, D. F.; Oliveira, R. G.; Medeiros, W. E.; Neves, B. B. Brito; Balsamo, F.; Nogueira, F. C. C.; Dantas, E. L.; Andrades Filho, C.; Góes, A. M.

    2014-02-01

    The eastern continental margin of South America comprises a series of rift basins developed during the breakup of Pangea in the Jurassic-Cretaceous. We integrated high resolution aeromagnetic, structural and stratigraphic data in order to evaluate the role of reactivation of ductile, Neoproterozoic shear zones in the deposition and deformation of post-rift sedimentary deposits in one of these basins, the Paraíba Basin in northeastern Brazil. This basin corresponds to the last part of the South American continent to be separated from Africa during the Pangea breakup. Sediment deposition in this basin occurred in the Albian-Maastrichtian, Eocene-Miocene, and in the late Quaternary. However, our investigation concentrates on the Miocene-Quaternary, which we consider the neotectonic period because it encompasses the last stress field. This consisted of an E-W-oriented compression and a N-S-oriented extension. The basement of the basin forms a slightly seaward-tilted ramp capped by a late Cretaceous to Quaternary sedimentary cover ~ 100-400 m thick. Aeromagnetic lineaments mark the major steeply-dipping, ductile E-W- to NE-striking shear zones in this basement. The ductile shear zones mainly reactivated as strike-slip, normal and oblique-slip faults, resulting in a series of Miocene-Quaternary depocenters controlled by NE-, E-W-, and a few NW-striking faults. Faulting produced subsidence and uplift that are largely responsible for the present-day morphology of the valleys and tablelands in this margin. We conclude that Precambrian shear zone reactivation controlled geometry and orientation, as well as deformation of sedimentary deposits, until the Neogene-Quaternary.

  20. Recent Inversion, Seismic Potential, and Neogene Kinematics of the Algerian Margin (Western Mediterranean) from Offshore Studies

    NASA Astrophysics Data System (ADS)

    Deverchere, J.; Yelles, K.; Bracene, R.; Mercier de Lepinay, B. F.; Cattaneo, A.; Medaouri, M.; Gaullier, V.; Babonneau, N.; Ratzov, G.; Boudiaf, A.; Graindorge, D.; Kherroubi, A.; Strzerzynski, P. H.; Authemayou, C.; Djellit, H.; Heddar, A.; Maradja'03; Maradja-Samra'05 Scientific Teams

    2011-12-01

    The reasons to study the Algerian margin (Western Mediterranean) are at least threefold: (1) the seismic hazard offshore is obviously present but unconstrained, (2) the way the opening of the Algerian basin occurred is highly debated, and (3) this margin represents one of the rare examples on Earth of an ongoing subduction inception. We present an overview of recent findings on the tectonic evolution of this margin, where most of the plate convergence between Africa and Europe is taken up today, mostly from cruises MARADJA and MARADJA2/SAMRA led by joint Algerian and French teams. Large, overlapping active thrust faults and folds apparently dominate the seismotectonic pattern from the Atlas domain on land to the foot of the margin offshore, with a clear segmentation. Strain is distributed across the whole area, with a significant part of the relative plate convergence taken up offshore. Fault activity offshore is tenuous and most often indirect (Plio-Quaternary growth strata, folds, uplifted basins, scars and slope breaks). Along the eastern margin, faults form stepwise, en-échelon systems on the slope and in the deep basin. Some thrusts identified turn to fault-propagation folds at the sub-surface. Thrusts interact with the sediment flux, Messinian salt and seafloor currents, forming complex structures at deep-sea fans and scarps or scars on the main slope breaks. The 2003 Mw 6.9 Boumerdes rupture is correlated segmented cumulative scarps on the slope and at the foot of the margin. Using various VHR seismic reflection and coring analyses, we show that the record of turbidite deposition since ca. 10.000 yrs can be identified and correlated over long distances within or across large segments of the margin affected by the 1954, 1980 and 2003 events. The consequences in term of earthquake size and recovery of their recurrences (identification of paleo-events) are explored and discussed. Although we cannot associate the triggering of large turbidity currents to a given fault, we find that the Algerian margin gathers favourable conditions to reconstruct times series of turbidites associated to significant earthquakes. Finally, we show that the structures inherited from the Algerian basin opening and from the Alpine belt building (AlKaPeCa blocks migration and collision) determine for a large part the size, style and location of this strain pattern. The overall geometry indicates the predominance of back thrusts, implying underthrusting of the young oceanic crust, although large dextral strike-slip structures may guide deformation at some places on land. The recent (probably less than 3 Ma) reactivation of the Algerian margin is strongly influenced by the subduction of the Tethyan Maghrebian ocean, implying not only an important roll-back of the slab, but also strong thermal, magmatic and isostatic effects of the slab evolution at depth.

  1. Lower cretaceous organic-rich sediments drilled on Antarctic continental margin during ODP Leg 113

    SciTech Connect

    O'Connell, S.

    1988-01-01

    Lower Cretaceous organic-rich sediments were recovered below a major unconformity at two sites on the eastern Weddell Sea margin during ODP Leg 113. No age overlap exists between the two sites. A more continuous Cretaceous section was previously recovered at DSDP Site 511 on the Falkland plateau. Site 692 (2,880 m water depth) is located on a mid-slope bench in Wegener Canyon. Early Cretaceous-age (pre-Albian) sediments extend from 53-98 m below sea floor (mbsf) and are dominated by organic-rich nannofossil claystone. Macrofossils (e.g., belemites and ammonites), thin carbonate lenses (<1 cm), and water escape structures are abundant. Thin beds of devitrified ash, bioturbation, and graded bedding are present. Site 693 (2,360 m water depth), 30 km west of Site 692 on the canyon's outer rim, recovered Albian-age organic-rich claystones and mudstones from 416-483 mbsf. Site 693 sediments have lower organic contents than those at Site 692. Glauconite is common in the upper part of the unit. Well-preserved diatoms and diatomite layers suggest high productivity.

  2. Regional Mesozoic basin development along the Irish continental margins: evidence from regional gravity studies

    NASA Astrophysics Data System (ADS)

    O'Reilly, B. M.; Readman, P. W.

    2003-04-01

    Sedimentary basins, which formed in response to multiphase rifting episodes during the early to late Mesozoic Era, comprise a global assemblage distributed across the entire North Atlantic region from eastern North America to the European Platform. This assemblage of basins has as its centre the region around and to the west of Ireland where tectonic re-activation of a strong NE-SW trending Caledonian basement fabric partly controlled the siting of the basins. The development of individual basins was contemporaneous with the onset of sea-floor-spreading within the Atlantic south of the Charlie Gibbs and Azores Fracture Zones. In this study a compilation of marine and satellite gravity data is primarily used to produce a regional interpretation of the tectonic fabrics in the region. A series of gravity models across the various sedimentary basins including the Rockall Basin, the Porcupine Basin and the Celtic and Irish Sea basins are used with vertical incidence and wide-angle seismic data to define crustal structure and its relationship with basin geometries. Two end-member sedimentary basin types are recognized in a regional preliminary model for Mesozoic basin development. The deep-water Rockall and Porcupine basins overlie North Atlantic lithosphere where large amounts of extensional strain were focused into the upper and mid-crust producing large amounts of syn-rift subsidence. In the shallow water shelf sea basin areas of the Celtic and Irish Sea generally smaller strains occur equally throughout all levels of the lithosphere and is distributed across wide regions of the lower crust and mantle lithosphere. This model for basin structuring requires that the strain field varies erratically within the upper to mid-crust across the North Atlantic. These changes in the pattern of strain are accommodated by rotation of continental crustal blocks (about vertical axes) and also by large-scale crustal transfer fault systems that penetrate at least to a lower crustal level. These fault systems may have been important in controlling fluid flow at a lithospheric scale.

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

    USGS Publications Warehouse

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

    2007-01-01

    The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. ?34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have ?34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/86Sr ratios for the barites vary over a narrow range of 0.70830–0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time of deposition of the host rocks for the PV samples. This indicates that the mineralizing fluids were not unaltered seawater. We develop a model in which the barites precipitated both below the sediment–water interface and at the seafloor from low-temperature fluids that circulated along faults. The isotopic, chemical, and textural data indicate that the barites formed by several processes. Mesozoic and Cenozoic basement rocks (crystalline and overlying sedimentary rocks), Quaternary basin fill, turbidite fans, and seawater provided various elements for the barites in different environments. The fluids had a deep-seated source and were hydrothermal in the deeper parts of the system for all the barite types, including the seafloor cold-seep deposit, based on Sr isotopes and regional geothermal gradients. These deep-seated fluids mixed with other fluids as they ascended, including seawater at and near the seafloor. The high ?34S values may have resulted from extreme Rayleigh fractionation during bacterially mediated (semi)closed-system sulfate reduction, probably driven by the flux of methane- and hydrocarbon-bearing fluids through basement rocks and basin sediments. Early diagenetic dolomite and pyrite in the host Monterey Formation in the PV Headland also formed in a zone of sulfate reduction, but within sediment buried only centimeters to a few meters and with a predominantly seawater source for the sulfur. Dolomite occurring with vein barite in the PV Headland formed at temperatures in the range of 40–90 °C. The cold-seep barites have ?34S values near that of modern seawater, although still somewhat fractionated. The barites that precipitated below the sediment–water interface have higher ?34S values, suggesting that the fluids were relatively reduced with molar dissolved barium in excess of dissolved sulfate. Those samples were exposed at the seafloor by uplift along faults and are composed predominantly of massive, brecciated, and vein barite.

  4. Gas hydrates, free gas distribution and fault pattern on the west Svalbard continental margin

    NASA Astrophysics Data System (ADS)

    Madrussani, Gianni; Rossi, Giuliana; Camerlenghi, Angelo

    2010-02-01

    A 3-D tomographic analysis of seismic velocity and attenuation fields is presented with the purpose to analyse the intimate relation of gas hydrates and free gas distribution with the fault pattern. The 3-D, four-component seismic data have been acquired offshore western Svalbard. The analysis of the subbottom topography of the base of the stability field of gas hydrates (indicated by the bottom simulating reflector) and the thickness of the underlying free gas bearing zone suggest a fault-induced compartmentalization of the gas reservoir. The proposed fluid and gas circulation scheme assumes deep sourced warm fluids moving upwards mainly along high permeability faults and fractures below the gas hydrate stability zone. Faults change from being a pathway of fluid flow to being barriers to fluid flow in the hydrate stability zone. Consequently, below the hydrate stability zone free gas is transferred from the permeable faults to non-faulted (but fault-bounded) sediments, where it accumulates below the base of the hydrate stability zone, producing a significantly thicker free gas layer. The tomographic approach is therefore proposed as an effective procedure to provide detailed information on 3-D P- and S-wave velocity and attenuation distribution in marine sediments and specifically to examine the details of the distribution of the gas hydrates and of the free gas, for which the dependence on the Vp, Vs and Qp is now fairly well known.

  5. X-ray Scanner for ODP Leg 204: Drilling Gas Hydrates on Hydrate Ridge, Cascadia Continental Margin

    SciTech Connect

    Freifeld, Barry; Kneafsey, Tim; Pruess, Jacob; Reiter, Paul; Tomutsa, Liviu

    2002-08-08

    An x-ray scanner was designed and fabricated at Lawrence Berkeley National Laboratory to provide high speed acquisition of x-ray images of sediment cores collected on the Ocean Drilling Program (ODP) Leg 204: Drilling Gas Hydrates On Hydrate Ridge, Cascadia Continental Margin. This report discusses the design and fabrication of the instrument, detailing novel features that help reduce the weight and increase the portability of the instrument. Sample x-ray images are included. The x-ray scanner was transferred to scientific drilling vessel, the JOIDES Resolution, by the resupply ship Mauna Loa, out of Coos Bay, Oregon on July 25. ODP technicians were trained in the instruments operation. The availability of the x-ray scanner at the drilling site allows real-time imaging of cores containing methane hydrate immediately after retrieval. Thus, imaging experiments on cores can yield information on the distribution and quantity of methane hydrates. Performing these measurements at the location of core collection eliminates the need for high pressures or low temperature core handling while the cores are stored and transported to a remote imaging laboratory.

  6. Organic matter quality and supply to deep-water coral/mound systems of the NW European Continental Margin

    NASA Astrophysics Data System (ADS)

    Kiriakoulakis, K.; Freiwald, A.; Fisher, E.; Wolff, G. A.

    2007-02-01

    Comparison of five deep-water coral (DWC)/mound ecosystems along the European Continental Margin shows that suspended particulate organic matter (sPOM), a potential food source, is lipid rich and of high quality. However, there are differences between the sites. The Darwin and Pelagia Mounds (N. Rockall Trough and N. Porcupine Bank, respectively) have higher proportions of labile particulate lipids (including high proportions of polyunsaturated fatty acids) in the benthic boundary layer than Logachev, Hovland and Belgica Mounds (Rockall Bank, S. Porcupine Bank and Porcupine Seabight, respectively). The high quality sPOM could be transported downslope from the euphotic zone. There is some evidence for inter-annual variability at some sites (e.g. Hovland and Logachev Mounds) as large differences in suspended lipid and particulate organic carbon concentrations were observed over the sampling period. Elevated total organic carbon contents of sediments at mound sites, relative to control sites in some cases (particularly Darwin Mounds), probably reflect local hydrodynamic control and the trapping of sPOM by the DWC. Fresh POM can be relatively rapidly transferred to significant depth (up to 8 cm) through bioturbation that is evident at all sites. There is no clear evidence of present day hydrocarbon seepage at any of the sites.

  7. Ebro margins sedimentary system in the western Mediterranean Sea, from delta to deep sea

    SciTech Connect

    Nelson, C.H.; Maldonado, A. (Geological Survey, Menlo Park, CA (USA))

    1988-08-01

    During Holocene high sea level, delta-front lobes of silty mud have deposited beside a lobate Ebro delta. Topset and foreset beds of these lobes extend up to 20 km offshore in up to 30 m of water. Geostrophic currents advect fine silt and clay from river discharge and storm wave resuspension in the delta front and deposit up to 20 m of bottomset beds in a distal prodelta clay belt formed on the inner to middle shelf for 70 km south from the delta. Intensified water circulation and increased bottom-current speeds inhibit prodelta progradation over the outer shelf and north of the delta and south of the clay belt, where the shelf narrows. Deposition of Holocene hemipelagic mud on the upper slope is restricted, but some modern Ebro sediment apparently bypasses to the deep margin. During Pleistocene low sea level, a series of shelf-edge deltas resulted in extensive progradation of foreset mud beds over the continental slope east of the modern delta and south to the Columbretes Islands. In the north, rapid sediment progradation has resulted in large canyons ({plus minus}5 km wide), unconfined sediment gravity flows, and deposition of large sediment aprons (50 km diameter) downslope from canyon mouths. In the south, narrow canyons ({plus minus}2 km wide) have funneled turbidity currents to side-by-side channel-levee complexes younger and smaller to the southwest. Subsidence of the Valencia trough has facilitated sediment transport from these channel-levee complexes into Valencia Valley and thence to the Valencia fan, 200 km to the northeast. Consequently, during low sea level stands a major portion of Ebro sediment is transported north to the Valencia fan, whereas the main progradational history of the Ebro margin has been offshore and to the south of the present delta.

  8. Geotechnical Properties of Submarine Sediments from Submarine Landslides on the Eastern Australian Continental Margin and Implications for Slide Initiation

    NASA Astrophysics Data System (ADS)

    Clarke, S. L.; Hubble, T.; Airey, D.

    2014-12-01

    Geomechanical test data are presented for 12 gravity cores, up to 5 m long, taken at sites from the upper slope (<1200 m) of the east Australian continental margin in or adjacent to five submarine landslide features. Sediments uniformly consist of olive grey to grey sandy silts (MH-ML), with clay content ranging from 2-12% (using the Unified Soil Classification System - USCS). Total unit weight varies between 14.1 to 17.4 kNm-3, bulk density 715-2065 kgm-3, water content 43-90+%, and specific gravity 2.5-2.74. Sediments present low plasticity, liquid limits 43-63%, and plasticity indices of 8.7-34%. Measured strength values, friction angle (?') and apparent cohesion (c'), vary between 30-40°, and 0-10 kPa respectively. One slide-adjacent core, and four within-landslide cores present boundary surfaces located at depths of 0.8 to 2.2 meters below the present-day seafloor that are identified by a sharp, colour-change boundary; small increases in sediment stiffness; slight increases in sediment bulk density of 0.1 gcm-3; and distinct gaps in AMS 14C age of at least 25 ka. Compression testing indicates that the sediment above and below the boundary surface is slightly overconsolidated. Triaxial tests indicate a significant increase in the brittleness of the shear response of the sediment with increasing vertical stress, which would cause a progressive increase of pore pressure if the sediment was subjected to cyclic (earthquake) loading. The boundary surfaces are interpreted to represent detachment surfaces or slide plane surfaces. Slope stability models based on classical soil mechanics and measured sediment shear-strengths indicate that the upper slope sediments should be stable. However, multibeam bathymetry data reveal that many upper slope landslides occur across the margin and that submarine landsliding is a common process. We infer from these results that: a) the margin experiences seismic events that act to destabilise the slope sediments, and/or b) an unidentified mechanism regularly acts to reduce the shear resistance of these sediments to the very low values required to enable slope failure.

  9. Reflection surveys conducted on the western side of the mid-continental gravity high

    SciTech Connect

    Taylor, R.W.; Fromm, A.J. (Fromm Applied Technology, Mequon, WI (United States)); Okita, P. (PHP Minerals, Herndon, VA (United States))

    1992-01-01

    The few spatially isolated deeper drill holes available on the western side of the mid-continental gravity high have established elevation changes in the Sioux quartzite that exceed 500m within a few hundred kilometers. Thirteen, 12-fold, CMP, reflection surveys were conducted within this area to supplement the limited drilling data. These surveys used an elastic wave generator as the energy source and a digital 24 channel IFP system for recording. The survey locations were selected to best supplement the existing drill hole data. Phone spacings and near offsets were selected on the basis of walk-out surveys conducted at each reflection site. No velocity control was available and the stacking velocities were selected based on graded velocity stacks. Interval velocities, constrained by general stratigraphic considerations, were calculated from the stacking velocities. For the near surface, interval velocities were extracted from the first arrivals. The lack of velocity control did not appear to seriously degrade the interpretation of gross structural features. Both the Sioux quartzite and a deeper interface, assumed to be the top of igneous basement, were reliably mapped. The two-way times of the basement reflector varied from 400m sec to 200m sec, approximately 500m to 300m respectively. The two-way times to the top of the quartzite varied from 300 m secs to 135m secs, approximately 350m to 160m respectively. The results suggest a major northeast, southwest trending basement fault with displacements exceeding 100m. The structure of both the basement and the quartzite appear to be a faulted anticline or dome. The reflection surveys provided a cost effective method for reconnaissance studies required to establish gross structural features.

  10. Ocean-continent transition and tectonic framework of the oceanic crust at the continental margin off NE Brazil: Results of LEPLAC project

    NASA Astrophysics Data System (ADS)

    Gomes, Paulo Otávio; Gomes, Benedito S.; Palma, Jorge J. C.; Jinno, Koji; de Souza, Jairo M.

    In 1992, Brazilian Navy and PETROBRAS carried out a geophysical survey along the continental margin off northeastern Brazil, as part of a governmental plan to delineate the "Legal Continental Shelf" according to the international Law of the Sea. This data set is leading to a better understanding of the crustal transition processes and on the evolution of the oceanic crust over that part of the Brazilian continental margin. On our seismic transects, we show a rifted marginal plateau (Pernambuco Plateau) where crustal extension was controlled by detachment faulting, possibly in a non-volcanic margin setting. Farther north, dealing with the ocean-continent transition nearby a major transform margin, we found a normal passive margin-style transition zone instead of transform-related structures. With the support of multichannel seismic profiles and gravity data derived from GEOSAT altimetry, several well-known oceanic fracture zones and structural lineaments were properly located and correlated. The relationship of these structures with volcanic ridges and extensional, compressive and strike-slip tectonic reactivations suggests that fracture zones at this area behaved either as zones of weakness or as locked transform fault scars. Striking lithospheric flexural deformation is also related to FZs in this region. In the surroundings of the Fernando de Noronha Ridge, lithospheric flexure represents an isostatic response to volcanic loading, while bending across Ascension FZ is likely to have been caused by differential subsidence in crustal segments of contrasting ages. We also correlate some other deformation of the oceanic crust with changes in spreading directions that possibly took place at the Upper Cretaceous.

  11. On the preservation of laminated sediments along the western margin of North America

    USGS Publications Warehouse

    VanGeen, A.; Zheng, Yen; Bernhard, J.M.; Cannariato, K.G.; Carriquiry, J.; Dean, W.E.; Eakins, B.W.; Ortiz, J.D.; Pike, J.

    2003-01-01

    Piston, gravity, and multicores as well as hydrographic data were collected along the Pacific margin of Baja California to reconstruct past variations in the intensity of the oxygen-minimum zone (OMZ). Gravity cores collected from within the OMZ north of 24??N did not contain laminated surface sediments even though bottom water oxygen (BWO) concentrations were close to 5 ??mol/kg. However, many of the cores collected south of 24??N did contain millimeter- to centimeter-scale, brown to black laminations in Holocene and older sediments but not in sediments deposited during the Last Glacial Maximum. In addition to the dark laminations, Holocene sediments in Soledad Basin, silled at 290 m, also contain white coccolith laminae that probably represent individual blooms. Two open margin cores from 430 and 700 m depth that were selected for detailed radiocarbon dating show distinct transitions from bioturbated glacial sediment to laminated Holocene sediment occurring at 12.9 and 11.5 ka, respectively. The transition is delayed and more gradual (11.3-10.0 ka) in another dated core from Soledad Basin. The observations indicate that bottom-water oxygen concentrations dropped below a threshold for the preservation of laminations at different times or that a synchronous hydrographic change left an asynchronous sedimentary imprint due to local factors. With the caveat that laminated sections should therefore not be correlated without independent age control, the pattern of older sequences of laminations along the North American western margin reported by this and previous studies suggests that multiple patterns of regional productivity and ventilation prevailed over the past 60 kyr. Copyright 2003 by the American Geophysical Union.

  12. Marine ice sheets of Pleistocene age on the East Siberian Continental Margin (Invited)

    NASA Astrophysics Data System (ADS)

    Niessen, F.; Hong, J.; Hegewald, A.; Matthiessen, J. J.; Stein, R. H.; Kim, H.; Kim, S.; Jensen, L.; Jokat, W.; Nam, S.; Kang, S.

    2013-12-01

    Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present results of the RV "Polarstern' cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA03B (2012), which investigated an area between the Chukchi Borderland and the East Siberian Sea between 165°W and 170°E. At the southern end of the Mendeleev Ridge, close to the Chukchi and East Siberian shelves, evidence is found for the existence of Pleistocene ice sheets/ice shelves, which have grounded several times in up to 1200 m present water depth. We found mega-scale glacial lineations 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, accentuated by a conspicuous truncation of pre-glacial strata typically capped with mostly thin layers of diamicton draped by pelagic sediments. Our tentative age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3. The oldest and deepest event predates MIS 6. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. They were possibly linked to previously suggested ice sheets on the Chukchi Borderland and the New Siberian Islands. We propose that the ice sheets extended northward as thick ice shelves, which grounded on the Mendeleev Ridge to an area up to 78°N within MIS 5 and/or earlier. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for global sea-level variations, albedo, ocean-atmosphere heat exchange, freshwater export from the Arctic Ocean at glacial terminations and the formation of submarine permafrost. The existence of km-thick Pleistocene ice sheets in the western Arctic Ocean during glacial times predating that of the Last Glacial Maximum (LGM) also implies significantly different atmospheric circulation patterns, in particular availability and distribution of moisture during pre-LGM glaciations.

  13. Fluid outflow from the accretionary wedge of the Pacific continental margin of Colombia and southern Panama modelled from the variation in heat flow

    Microsoft Academic Search

    G. K. Westbrook; P. S. Oneill

    2003-01-01

    Heat flow from the accretionary wedge that forms the Pacific continental margin of Colombia and southern Panama increases to its maximum value at the toe of the wedge, where it reaches values of up to 120 mW\\/m2 in the south [4 degrees N] and more than 200 mW\\/m2 in the north [6.5 degrees N]. Heat flow diminishes landward to about

  14. The Norwegian–Barents–Svalbard (NBS) continental margin: Introducing a natural laboratory of mass wasting, hydrates, and ascent of sediment, pore water, and methane

    Microsoft Academic Search

    P. R. Vogt; J. Gardner; K. Crane

    1999-01-01

    Side-scan sonar mapping and ground-truthing of the Norwegian–Barents–Svalbard continental margin shed new light on shelf\\u000a glaciation, mass wasting, hydrates, and features like the Håkon Mosby mud volcano (HMMV), reflecting upward mobility of gas,\\u000a pore fluids, and sediments. Detailed HMMV examination revealed thermal gradients to 10°\\/m, bottom-water CH4 and temperature anomalies, H2S- and CH4-based chemosynthetic ecosystems, and subbottom methane hydrate (to

  15. Evolving subduction zones in the Western United States, as interpreted from igneous rocks

    USGS Publications Warehouse

    Lipman, P.W.; Prostka, H.J.; Christiansen, R.L.

    1971-01-01

    Variations in the ratio of K2O to SiO4 in andesitic rocks suggest early and middle Cenozoic subduction beneath the western United States along two subparallel imbricate zones dipping about 20 degrees eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate. Mesozoic subduction apparently occurred along a single steeper zone.

  16. Systematics of smooth leaf margin Juniperus of the western hemisphere based on leaf essential oils and RAPD DNA fingerprinting

    Microsoft Academic Search

    Robert P. Adams

    2000-01-01

    The composition of the leaf essential oils of 13 taxa of the smooth leaf margin Juniperus in sect. Sabina from the western hemisphere are reported and compared. In addition, DNA fingerprinting revealed similar patterns among these species. Based on these data, a new species, very similar to J. blancoi and J. scopulorum, is recognized from northern Mexico: Juniperus mucronata sp.

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

    NASA Astrophysics Data System (ADS)

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

    2014-12-01

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

  18. Platform-margin and marginal slope relationships and sedimentation in Devonian reef complexes of Canning basin, Western Australia

    SciTech Connect

    Playford, P.E.; Kerans, C.; Hurley, N.F.

    1984-04-01

    Devonian limestone platforms in the Canning basin were generally rimmed by reef-margin and reef-flat deposits, constructed by stromatoporoids, algae, and corals in the Givetian and Frasnian, and by algae in the Famennian. However, some platforms were low-relief banks with little or no reef development. The reefs and slowly deposited parts of the marginal-slope facies were subject to pervasive early submarine cementation by fibrous high-magnesium calcite (now radiaxial spar). The strongly cemented reef limestones formed rigid wave-resistant rims to the platforms. Fracturing of these limestones, probably largely associated with earthquake shaking, gave rise to extensive networks of neptunian dikes and sills, and to the collapse of some sections of the margins. Such collapses in turn initiated debris flows and the deposition of allochthonous reef blocks on the adjoining marginal slopes. The reef complexes are being explored extensively for lead-zinc deposits in outcrop and oil in the subsurface. A significant oil discovery was made in a Famennian platform margin (the Blina field) in 1981.

  19. Strong depth-related zonation of megabenthos on a rocky continental margin (?700-4000 m) off southern Tasmania, Australia.

    PubMed

    Thresher, Ronald; Althaus, Franziska; Adkins, Jess; Gowlett-Holmes, Karen; Alderslade, Phil; Dowdney, Jo; Cho, Walter; Gagnon, Alex; Staples, David; McEnnulty, Felicity; Williams, Alan

    2014-01-01

    Assemblages of megabenthos are structured in seven depth-related zones between ?700 and 4000 m on the rocky and topographically complex continental margin south of Tasmania, southeastern Australia. These patterns emerge from analysis of imagery and specimen collections taken from a suite of surveys using photographic and in situ sampling by epibenthic sleds, towed video cameras, an autonomous underwater vehicle and a remotely operated vehicle (ROV). Seamount peaks in shallow zones had relatively low biomass and low diversity assemblages, which may be in part natural and in part due to effects of bottom trawl fishing. Species richness was highest at intermediate depths (1000-1300 m) as a result of an extensive coral reef community based on the bioherm-forming scleractinian Solenosmilia variabilis. However, megabenthos abundance peaked in a deeper, low diversity assemblage at 2000-2500 m. The S. variabilis reef and the deep biomass zone were separated by an extensive dead, sub-fossil S. variabilis reef and a relatively low biomass stratum on volcanic rock roughly coincident with the oxygen minimum layer. Below 2400 m, megabenthos was increasingly sparse, though punctuated by occasional small pockets of relatively high diversity and biomass. Nonetheless, megabenthic organisms were observed in the vast majority of photographs on all seabed habitats and to the maximum depths observed--a sandy plain below 3950 m. Taxonomic studies in progress suggest that the observed depth zonation is based in part on changing species mixes with depth, but also an underlying commonality to much of the seamount and rocky substrate biota across all depths. Although the mechanisms supporting the extraordinarily high biomass in 2000-2500 m depths remains obscure, plausible explanations include equatorwards lateral transport of polar production and/or a response to depth-stratified oxygen availability. PMID:24465758

  20. Strong Depth-Related Zonation of Megabenthos on a Rocky Continental Margin (?700–4000 m) off Southern Tasmania, Australia

    PubMed Central

    Thresher, Ronald; Althaus, Franziska; Adkins, Jess; Gowlett-Holmes, Karen; Alderslade, Phil; Dowdney, Jo; Cho, Walter; Gagnon, Alex; Staples, David; McEnnulty, Felicity; Williams, Alan

    2014-01-01

    Assemblages of megabenthos are structured in seven depth-related zones between ?700 and 4000 m on the rocky and topographically complex continental margin south of Tasmania, southeastern Australia. These patterns emerge from analysis of imagery and specimen collections taken from a suite of surveys using photographic and in situ sampling by epibenthic sleds, towed video cameras, an autonomous underwater vehicle and a remotely operated vehicle (ROV). Seamount peaks in shallow zones had relatively low biomass and low diversity assemblages, which may be in part natural and in part due to effects of bottom trawl fishing. Species richness was highest at intermediate depths (1000–1300 m) as a result of an extensive coral reef community based on the bioherm-forming scleractinian Solenosmilia variabilis. However, megabenthos abundance peaked in a deeper, low diversity assemblage at 2000–2500 m. The S. variabilis reef and the deep biomass zone were separated by an extensive dead, sub-fossil S. variabilis reef and a relatively low biomass stratum on volcanic rock roughly coincident with the oxygen minimum layer. Below 2400 m, megabenthos was increasingly sparse, though punctuated by occasional small pockets of relatively high diversity and biomass. Nonetheless, megabenthic organisms were observed in the vast majority of photographs on all seabed habitats and to the maximum depths observed - a sandy plain below 3950 m. Taxonomic studies in progress suggest that the observed depth zonation is based in part on changing species mixes with depth, but also an underlying commonality to much of the seamount and rocky substrate biota across all depths. Although the mechanisms supporting the extraordinarily high biomass in 2000–2500 m depths remains obscure, plausible explanations include equatorwards lateral transport of polar production and/or a response to depth-stratified oxygen availability. PMID:24465758

  1. Pb and Sr isotopic constraints on lithospheric magma sources during Mesozoic continental margin arc initiation, southern California

    SciTech Connect

    Barth, A.P. (Indiana/Purdue Univ., Indianapolis, IN (United States). Dept. of Geology); Tosdal, R.M.; Wooden, J.L. (Geological Survey, Menlo Park, CA (United States))

    1993-04-01

    Initiation of the Mesozoic Cordilleran arc in the southwestern US is marked by plutonism from about 241--213 Ma, exposed in the Granite Mountains in the southwestern Mojave Desert, through the Transverse Ranges to the Colorado River trough in southeastern California and southwestern Arizona. Plutons range in composition from diorite to granite, but quartz monzonite and monzodiorite predominate. Plutons intruded 1,700 to 1,100 Ma rocks of the Mojave crustal province [as defined isotopically by Wooden and Miller (1990) and Bennett and DePaolo (1987)], and episomal plutons locally intruded its deformed cratonal/miogeoclinal cover. Plutons emplaced during arc initiation overlap isotopically with local Proterozoic basement rocks, but typically have less radiogenic [sup 207]Pb/[sup 204]Pb, [sup 208]Pb/[sup 204]Pb and [sup 87]Sr/[sup 86]Sr than adjacent, more voluminous Middle-Late Jurassic and Late Cretaceous arc plutons. Among early arc plutons, an inter-suite trend toward more radiogenic Pb and [sup 87]Sr/[sup 86]Sr at similar silica contents reflects regional basement isotopic variability. Generally high Sr contents, low Rb/Sr and limited REE data suggest this inter-suite variations records heterogeneity in eclogite/garnet amphibolite facies mafic lithospheric magma sources, corresponding to mafic lower crust and/or upper mantle formed during 1,700 Ma orogenesis or rift-related magmatism at 1,100 Ma. Intra-suite trends toward less radiogenic Pb at constant or more radiogenic Sr reflect involvement of a U and Th depleted, silicic intracrustal contaminant. Distinctive magma sources and limited crustal interaction during emplacement probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin arc.

  2. Shear wave azimuthal anisotropy in the transition zone from oceanic to continental subduction along the western Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Evangelidis, Christos

    2015-04-01

    In western Greece, the Hellenic subduction system is separated by the Cephalonia Transform Fault (CTF), a dextral offset of ~100 km, into the northern and southern segments, which are characterized by different convergence rates and slab composition. Recent seismic data show that north of CTF there is a subducted continental lithosphere in contrast to the region south of CTF where the on-going subduction is oceanic. Shear wave splitting of SKS phases provide useful information for the upper mantle anisotropy. Its direction and strength is caused by the ongoing upper mantle flow that constrain the subduction geodynamics. I have now measured SKS splitting parameters from all broadband stations of the Hellenic Unified Seismic Network (HUSN) and some selected stations of the National Strong Motion Network, specially concentrated in the transitional area from oceanic to continental subduction. These measurements, combined with previously published observations, provide the most complete up-to-date spatial coverage for the area. Generally, the pronounced zonation of seismic anisotropy across the subduction zone, as inferred from other studies, is also observed here. Fast SKS splitting directions are trench-normal in the region nearest to the trench. The fast splitting directions change abruptly to trench-parallel above the corner of the mantle wedge and rotate back to trench-normal over the back-arc. Additionally, beneath western Greece, between the western Gulf of Corinth in the south and the Epirus-Thessaly area in the north a toroidal pattern emerges that possibly depicts a slab tear between the oceanic and the continental subducted slabs and a consequent toroidal asthenospheric flow.

  3. Seabeam and seismic reflection imaging of the tectonic regime of the Andean continental margin off Peru (4°S to 10°S)

    NASA Astrophysics Data System (ADS)

    Bourgois, J.; Pautot, G.; Bandy, W.; Boinet, T.; Chotin, P.; Huchon, P.; Mercier de Lepinay, B.; Monge, F.; Monlau, J.; Pelletier, B.; Sosson, M.; von Huene, R.

    1988-01-01

    Marine geophysical surveys employing Seabeam, multi- and single-channel seismic reflection, gravity and magnetic instruments were conducted at two locations along the continental slope of the Peru Trench during the Seaperc cruise of the R/V "Jean Charcot" in July 1986. These areas are centered around 5°30'S and 9°30'S off the coastal towns of Paita and Chimbote respectively. These data indicate that (1) the continental slope off Peru consists of three distinct morpho-structural domains (from west to east are the lower, middle and upper slopes) instead of just two as previously reported; (2) the middle slope has the characteristics of a zone of tectonic collapse at the front of a gently flexured upper slope; (3) the upper half of the lower slope appears to represent the product of mass wasting; (4) thrusting at the foot of the margin produces a continuous morphologic feature representing a deformation front where the products of mass-wasting are overprinted by a compressional tectonic fabric; (5) a change in the tectonic regime from tensional to compressional occurs at the mid-slope-lower slope boundary, the accretionary prism being restricted to the very base of the lower slope in the Paita area. The Andean margin off Peru is an "extensional active margin" or a "collapsing active margin" developing a subordinated accretionary complex induced by massive collapse of the middle slope area.

  4. Overview on the Plate Boundaries Along the Western Mexican Pacific Margin

    NASA Astrophysics Data System (ADS)

    Mortera-Gutierrez, C. A.; Bandy, W. L.; Michaud, F.; Ortega Ramírez, J.

    2013-05-01

    The cinematic of the Pacific, Rivera and Cocos oceanic plates have a significant impact on the subduction process and seismic cycles occurring along the western Mexican Pacific margin of the North American and Caribbean plates. Sections of Pacific (PAC), Rivera (RIV), Cocos (COC), North American (NAM) and Caribbean (CAB) plate boundaries along the western margin of Mexico are not well constrained. From north to south: the transform-rift system at Gulf of California has been generally considered as part of PAC-NAM plate boundary. However results of the FAMEX cruise at 2002 evidenced that Tosco-Abreojos Fault System along the western margin of Baja California Peninsula is active. Should this tectonic structure be considered as a plate boundary? At the RIV plate northern corner (including Mazatlan Basin), the scatter seismicity recorded between Tamayo FZ and the Marias Islands restricts the characterization of the plate boundary between the RIV and NAM plates. Some authors have proposed that Tamayo FZ and Marias I. Escarpment are the RIV-NAM plate boundary. Recently other authors have called that RIV-NAM boundary is a geomorphology lineament that runs from a Rivera Rise transform at 23N to the northern end of Marias I. Escarpment. Even so this concept is not sustained with seismic activity. Further this thought would imply that the oceanic lithosphere of Mazatlan Basin would form part of NAM plate. Other thoughts are either that there is a diffuse RIV-NAM plate boundary to the north of the Maria Archipelago, or Middle America Subduction Zone is gradually extending northward of the Maria Is. While the plate boundary at SE corner of the RIV plate is neither well defined morphologically nor seismically constraint, offshore Colima Coast. Some authors have proposed that this zone is a diffuse plate boundary between RIV and COC plates, result of a NE-SW shear plate motion. Other authors have proposed that the RIV-COC boundary extends E-W from the El Gordo Graben (EGG) at the Middle American Trench (MAT) to northern tip of the East Pacific Rise (EPR). Results of recently multibeam and magnetic surveys indicate that this boundary is possible segmented as an echelon E-W structure, north of EGG. Clearly these hypotheses on the RIV-COC plate boundary show that its configuration is neither well seismic nor morphology constrained. To the south, the triple junction point of COC, NAM, and CAB plate boundaries is also another case where the boundaries are poorly constrained seismically and morphologically. Traditionally, the COC-NAM-CAB triple junction point has been positioned where the MAT trend bends by the Tehuantepec Ridge (TR) collision, but no offshore geophysical data sustain that NAM-CAB plate boundary extends to MAT-TR point. In the last decade, the Servicio Sismológico Nacional (SSN) has extended its seismic station network at the southern Mexican territory. From this data, the distribution of offshore earthquakes covers a broad marine zone in front the Chiapas and Guatemala coastline and does not show a defined earthquake concentration associated to the proposed offshore extension of the Polochic-Motogua Fault through Guatemala and Mapastepec Fault through Chiapas, Mexico.

  5. Tectonics of Precambrian basement along the Pacific margin of Antarctica and relation to western North America

    SciTech Connect

    Goodge, J.W.; Hansen, V.L. (Southern Methodist Univ., Dallas, TX (United States). Dept. of Geological Sciences); Walker, N.W. (Univ. of Texas, Austin, TX (United States). Dept. of Geological Sciences)

    1993-02-01

    High-grade metamorphic rocks of the Precambrian Nimrod Group (NG) constitute one of few cratonal basement exposures in the Transantarctic Mountains. These rocks represent an outlier of the East Antarctic craton, evolved as part of Gondwana and pre-Gondwana (Rodinia) supercontinents. Despite pervasive, high-strain ductile deformation at T [>=] 650 C, they preserve petrologic and geochronologic evidence of an earlier history. Sm-Nd model ages from several NG lithologies, including that of a [approximately]1.7 Ga orthogneiss, range from about 2.7--2.9 Ga; these ages reflect both sedimentary and magmatic derivation from Archean crust. Individual detrital zircon U-Pb ages (about 1.7--2.6 Ga) from NG quartzites indicate clastic input from Archean to Paleoproterozoic source terrains. The Sm-Nd and U-Pb ages are reminiscent of both the Yavapai-Mazatzal (1.6--1.8 Ga) and Wyoming (> 2.5 Ga) provinces in western North America. U-Pb ages from syn-tectonic metaigneous and pelitic NG tectonites indicate that this basement complex was re-worked by the major ductile deformation in latest neoproterozoic to Early Cambrian time. Supracrustal assemblages that lie outboard of the Nimrod craton include Neoproterozoic graywacke, impure carbonate, and minor mafic volcanics (Beardmore Group), and Cambrian to Lower Ordovician carbonate and siliciclastic rocks (Byrd Group). Neoproterozoic ([approximately]750 Ma) rifting along the proto-Pacific margin of East Antarctica is reflected by deposition of Beardmore turbidites and coeval mafic magmatism. Latest Neoproterozoic to early Paleozoic orogenesis occurred along a left-oblique convergent plate margin of East Antarctica is reflected by deposition of Beardmore turbidites and coeval mafic magmatism.

  6. Seismicity of the Koyna region and regional tectonomagmatism of the Western Margin (India)

    NASA Astrophysics Data System (ADS)

    Raval, U.

    1995-10-01

    Recent findings on the Meso-Cenozoic tectonomagmatism and deep-seated anomalous geophysical structures suggest a close linkage between the seismicity of the Koyna region, the Westernghat uplift (WG-U) and associated thermomechanical and fluid activities. The WG-U seems to be the result of late Cretaceous thermal mobilization, erosion of the Deccan trap cover and superposition of compressional stress. The association of seismicity with uplift seems to result from movement of deep-seated heat and fluids/volatiles along the edges (or boundary faults) of the uplift; because the force required for crustal deformation depends on the relief. Observed gradients in relief may be attributed to the differential erosion-rates and heat inputs, due to the time gap of ˜ 50 Ma in the break-ups and plume activities on the eastern and western sides and consequence magmatism. Further, the geology and tectonics strongly indicate that the western margin (WM) is a relic of a mobile arm (MA), that included Madagascar, and which formed a part of the Proterozoic mobile belt of ‘greater India’ (for t>85 Ma). The mobile nature of the WM facilitates mantle upwellings and transient elevation of isotherms at depth, raising the possibility of intermittent metamorphism and greater deformation. Superposition of the ongoing compression and uplift-induced forces make local permeability and pore-fluid pressure vital in triggering the seismic slip over the Peninsular shield. Certain representative model calculations have been carried out to estimate change in the e.m. induction characteristics caused by an intermittent hydraulic connectivity. The results show a drop in the resistivity which could be a useful monitoring index. The close connection of uplift and fluid activity as discussed here seems applicable for other active parts of the South Indian Shield (SIS) also.

  7. Pliocene-Pleistocene coastal events and history along the western margin of Australia

    NASA Astrophysics Data System (ADS)

    Kendrick, George W.; Wyrwoll, Karl-Heinz; Szabo, Barney J.

    Coastal deposits along the western coastal margin of Australia, a region of relative tectonic stability, record Plio-Pleistocene events and processes affecting the inner shelf and adjacent hinterland. Tectonic deformation of these deposits is more apparent in the Carnarvon Basin, and rather less so in the Perth Basin. The most complete record comes from the Perth Basin, where units of Pliocene and Pleistocene ages are well represented. In the Perth Basin, the predominantly siliciclastic Yoganup Formation, Ascot Formation and Bassendean Sand represent a complex of shoreline, inner shelf and regressive-dune facies equivalents, the deposition of which began at an undetermined stage of the Pliocene, through to the Early Pleistocene. The deposition of this sequence closed with a major regression and significant faunal extinction. Bioclastic carbonates characterize the Middle and Late Pleistocene of the Perth and Carnarvon basins. Fossil assemblages include a distinct subtropical element, unknown from the Ascot Formation and suggesting a strengthening of the Leeuwin Current. The estuarine arcoid bivalve Anadara trapezia characterizes assemblages of Oxygen Isotope Stages 5 and 7 in the Perth and Carnarvon basins, where it is now extinct. Deposits of Substage 5e (Perth Basin) also record a southerly expansion of warm-water corals and other fauna consistent with shelf temperatures warmer than present. New uranium-series ages on corals from marine sequences of the Tantabiddi Member, of the Bundera Calcarenite of the western Cape Range are consistent with the 'double peak' hypothesis for levels of Substage 5e but the evidence remains less than conclusive. Initial uranium-series dates from the Bibra and Dampier formations of Shark Bay indicate that both derive from the Late Pleistocene. These numerical ages contradict previous interpretations of relative ages obtained from field studies. The age relationship of the units requires further investigation.

  8. Pliocene-Pleistocene coastal events and history along the western margin of Australia

    USGS Publications Warehouse

    Kendrick, G.W.; Wyrwoll, K.-H.; Szabo, B. J.

    1991-01-01

    Coastal deposits along the western coastal margin of Australia, a region of relative tectonic stability, record Plio-Pleistocene events and processes affecting the inner shelf and adjacent hinterland. Tectonic deformation of these deposits is more apparent in the Carnarvon Basin, and rather less so in the Perth Basin. The most complete record comes from the Perth Basin, where units of Pliocene and Pleistocene ages are well represented. In the Perth Basin, the predominantly siliciclastic Yoganup Formation, Ascot Formation and Bassendean Sand represent a complex of shoreline, inner shelf and regressive-dune facies equivalents, the deposition of which began at an undetermined stage of the Pliocene, through to the Early Pleistocene. The deposition of this sequence closed with a major regression and significant faunal extinction. Bioclastic carbonates characterize the Middle and Late Pleistocene of the Perth and Carnarvon basins. Fossil assemblages include a distinct subtropical element, unknown from the Ascot Formation and suggesting a strengthening of the Leeuwin Current. The estuarine arcoid bivalve Anadara trapezia characterizes assemblages of Oxygen Isotope Stages 5 and 7 in the Perth and Carnarvon basins, where it is now extinct. Deposits of Substage 5e (Perth Basin) also record a southerly expansion of warm-water corals and other fauna consistent with shelf temperatures warmer than present. New uranium-series ages on corals from marine sequences of the Tantabiddi Member, of the Bundera Calcarenite of the western Cape Range are consistent with the 'double peak' hypothesis for levels of Substage 5e but the evidence remains less than conclusive. Initial uranium-series dates from the Bibra and Dampier formations of Shark Bay indicate that both derive from the Late Pleistocene. These numerical ages contradict previous interpretations of relative ages obtained from field studies. The age relationship of the units requires further investigation. ?? 1991.

  9. Ship and satellite observations of chlorophyl stocks in interacting cyclone-anticyclone eddy pairs in the western Gulf of Mexico

    Microsoft Academic Search

    Douglas C. Biggs; F. E. Mueller-Karger

    1994-01-01

    When anticyclonic eddies shed by the Loop Current of the Gulf of Mexico reach the western margin of the gulf, they influence the surface circulation over the continental slope and rise. Of particular interest is the generation of cyclone (cold-core)- anticyclone (warm-core) pairs when aging Loop Current eddies interact with the continental margin. In this paper the authors describe the

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  11. Distribution and sedimentary characteristics of tsunami deposits along the Cascadia margin of western North America

    USGS Publications Warehouse

    Peters, R.; Jaffe, B.; Gelfenbaum, G.

    2007-01-01

    Tsunami deposits have been found at more than 60 sites along the Cascadia margin of Western North America, and here we review and synthesize their distribution and sedimentary characteristics based on the published record. Cascadia tsunami deposits are best preserved, and most easily identified, in low-energy coastal environments such as tidal marshes, back-barrier marshes and coastal lakes where they occur as anomalous layers of sand within peat and mud. They extend up to a kilometer inland in open coastal settings and several kilometers up river valleys. They are distinguished from other sediments by a combination of sedimentary character and stratigraphic context. Recurrence intervals range from 300-1000??years with an average of 500-600??years. The tsunami deposits have been used to help evaluate and mitigate tsunami hazards in Cascadia. They show that the Cascadia subduction zone is prone to great earthquakes that generate large tsunamis. The inclusion of tsunami deposits on inundation maps, used in conjunction with results from inundation models, allows a more accurate assessment of areas subject to tsunami inundation. The application of sediment transport models can help estimate tsunami flow velocity and wave height, parameters which are necessary to help establish evacuation routes and plan development in tsunami prone areas. ?? 2007.

  12. 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 correlation between break-up age and LIP age. We interpret this to indicate that plumes can aid the factual continental break-up. However, plumes may have been guided towards the rift for margins that experienced a long rift history (e.g., Norway-Greenland), to then trigger the break-up. This could offer a partial reconciliation in the debate of a passive or active role for mantle plumes in continental break-up. (Wilson, J.T., 1966. Did the Atlantic close and then re-open? Nature 211, 676-681)

  13. The Palaeozoic geography of Laurentia and western Laurussia: A stable craton with mobile margins

    NASA Astrophysics Data System (ADS)

    Cocks, L. Robin M.; Torsvik, Trond H.

    2011-05-01

    The large Palaeozoic continent of Laurentia was largely in North America, but included parts of modern Europe. It was independent from late Neoproterozoic times at about 570 Ma until it merged with Avalonia-Baltica in the 430-420 Ma Silurian Caledonide Orogeny, after which it formed the major western sector of the combined Laurussia Supercontinent. Laurussia in turn became part of the even larger Pangea Supercontinent in the Late Carboniferous, as documented by the oblique Laurussia-Gondwana collision seen in the Laurentian sector in the Ouachita Orogeny. Laurentia's margins and the many peri-Laurentian terranes are reviewed. Those parts of northeast Siberia which today form parts of the North America Plate, but were not part of Laurentia or Laurussia in the Palaeozoic, are also reviewed. A revised Apparent Polar Wander Path (APW) for the Laurentian Craton is presented for all of the Palaeozoic. Laurentia was at equatorial palaeolatitudes throughout and rotated little, apart from shortly after its collision with Avalonia-Baltica in the Silurian Caledonide Orogeny; however, in contrast, its position and orientation were much less affected in the Ouachita Orogeny at the time of Pangean assembly. The Laurentian Craton was variably flooded at many times with epeiric seas, which formed optimal numbers of ecological niches which in turn encouraged animal speciation and evolution. A summary is presented of the Palaeozoic geological history of Laurentia and its surrounding areas, and the Laurentian sector of Laurussia during and after its integration within Pangea, together with new palaeogeographical maps from the Cambrian to the end of the Permian. On those maps there are plotted areas of land, shallow shelf, deeper shelf and oceans derived from much pre-existing data, as well as reefs, volcanic and plutonic rocks and some selected faunas and floras. The substantial number of terranes at the margins of the continent through time are briefly reviewed, notably those in Mexico, the Appalachians, and northwestern parts of Europe which were once parts of Laurentia. The many terranes containing Palaeozoic rocks in the northwestern part of the North American Plate, and forming much of the Cordillera in northwest Canada and Alaska as well as northeast Siberia, are itemised: some were peri-Laurentian, some peri-Siberian, and others originally oceanic in the Palaeozoic. The concept of an Arctida Microcontinent is discussed. That microcontinent had originally been postulated as existing from the Neoproterozoic to the Devonian, and to have consisted of the composite Arctic Alaska and the Seward, York and Farewell terranes in Alaska, and the Pearya Terrane of Ellesmere Island, as well as the Chukotka Peninsula, New Siberian Islands, Severnaya Zemlya, northern Taimyr and adjacent areas now in the northeast of modern Siberia. Many parts of that area contain faunas of both Siberian and Laurentian aspect, which are reviewed and analysed. It is concluded that there was a smaller independent continent in the Lower Palaeozoic, which was originally somewhere between Siberia and Laurentia in the Cambrian, but which did not include the New Siberian Islands, Kolyma and Omolon (which were parts of Siberia), Severnaya Zemlya and northern Taimyr (the independent Kara Microcontinent), or the Farewell Terrane (independent until the Mesozoic). The eastern end of that Arctic Alaska-Chukotka Microcontinent docked with northwestern Laurussia (Ellesmerian margin) in the Devonian, but it did not reach its present position within North America until after rotation in the Cretaceous. The Cordilleran terranes of Wrangellia, Alexander and some smaller units are confirmed as having existed as another microcontinent independent from North America until the Mesozoic. However, there appear to be no terranes now in western North America which originated from Baltica. The Pearya Terrane, now forming northern Ellesmere Island, was probably involved in the most northerly sector of the Silurian Caledonide Orogeny.

  14. Location of deeply buried, offshore Mesozoic transform fault along the western margin of the Gulf of Mexico inferred from gravity and magnetic data

    NASA Astrophysics Data System (ADS)

    Nguyen, L. C.; Mann, P.; Bird, D. E.

    2013-12-01

    Several workers have proposed that a Jurassic age, 500-km-long, right-lateral transform fault along the western margin of the Gulf of Mexico, possibly extending southward and onshore for another 500 km onto the isthmus area of southern Mexico, was formed as the ocean basin opened. This proposed transform fault plays a critical role in the most widely accepted tectonic model for the Mesozoic opening of the Gulf of Mexico by a ~40 degree, CCW rotation of the Yucatan block about a pole near southern Florida. Previously proposed names for the fault include the Tamaulipas-Chiapas transform fault and the Western Main transform fault for the offshore fault and the Orizaba transform fault for the southern, onland continuation of the fault into southern Mexico. There are few direct geologic or geophysical observations on the location or characteristics of the proposed offshore transform because it is buried beneath an over 10-km-thick sedimentary wedge along the continental margin of eastern Mexico. To better define this offshore fault, we identify a 500-km-long, 40-km-wide gravity anomaly, concentric with, and located about 60-70 km off the eastern coast of Mexico. Two east-west 200/1200-km-long gravity models constructed to cross the anomaly at right angles are parallel to existing multi-channel seismic lines with age-correlated stratigraphy. Both gravity models reveal an abrupt crustal thickness change beneath the gravity anomaly: from 27 km to 12 km over a distance of 65 km in the southern profile, and from 23 km to 16 km over a distance of 30 km in northern profile. The linearity of the anomaly in map view combined with the abrupt change in thickness inferred from gravity modeling is consistent with the tectonic origin of a right-lateral transform fault separating continental rocks of Mexico from Mesozoic seafloor produced by the opening of the Gulf of Mexico. Magnetic profiles were analyzed using a Werner depth-to-magnetic source technique, coincident with the gravity models, estimate the depth to top of crystalline basement for the northern (9 km) and southern (11 km) transects. Subsidence analysis along both transects shows that sedimentation rates sharply peaked during the Laramide orogeny in the latest Cretaceous-Eocene, but otherwise conform to steady thermal subsidence of oceanic crust in the deep Gulf of Mexico that formed during the Jurassic CCW rotation of the Yucatan block. The more precisely defined offshore fault aligns well with the onland right-lateral Orizaba transform fault of southern Mexico that is thought to have been active in Mesozoic time.

  15. Evidence of low density sub-crustal underplating beneath western continental region of India and adjacent Arabian Sea: Geodynamical considerations

    NASA Astrophysics Data System (ADS)

    Pandey, O. P.; Agrawal, P. K.; Negi, J. G.

    1996-07-01

    The known high mobility of the Indian subcontinent during the period from 80 to 53 Ma has evoked considerable interest in recent times. It appears to have played an important role in shaping the subcontinental structures of western India and the adjoining Arabian Sea. During this period, a major catastrophic event took place in the form of Deccan volcanism, which coincides with the biological mass extinction at the K-T boundary, including the death of dinosaurs. The origin of Deccan volcanism is still being debated. Geophysically, western India and its offshore regions exhibit numerous prominent anomalies which testify to the abnormal nature of the underlying crust-lithosphere. In this work, we develop a two-dimensional structural model of these areas along two long profiles extending from the eastern basin of the Arabian Sea to about 1000 km inland. The model, derived from the available gravity data in the oceanic and continental regions, is constrained by seismic and other relevant information in the area, and suggests, for the first time, the presence of an extensive low-density (2.95-3.05 g/cm 3) sub-crustal underplating. Such a layer is found to occur between depths of 11 and 20 km in the eastern basin of the Arabian Sea, and betweeen 45 and 60 km in the continental region where it is sandwiched in the lower lithosphere. The low density may have been caused as a result of serpentinization or fractionation of magma by a process related in some way to the Deccan volcanic event. Substantial depletion of both oceanic and continental lithosphere is indicated. We hypothesize that the present anatomy of the deformed lithosphere of the region at the K-T boundary is the result of substantial melt generated owing to frictional heat possibly giving rise to a hot cell like condition at the base of the lithosphere, resulting from the rapid movement of the Indian subcontinent between 80 and 53 Ma.

  16. Origin of a Cretaceous low-18O granitoid complex in the active continental margin of SE China

    NASA Astrophysics Data System (ADS)

    Li, Ye; Ma, Chang-Qian; Xing, Guang-Fu; Zhou, Han-Wen; Zhang, Hang; Brouwer, Fraukje M.

    2015-02-01

    Low-18O felsic igneous rocks (with ?18O values of < 5‰) are a group of volumetrically minor rocks that in their genesis require involvement of oxygen from seawater or meteoric water at high temperature. Most low-18O felsic rocks are reported in caldera or rift settings, where high-temperature hydrothermal exchange of surface waters with source rocks readily imprints low-18O water values. Some low-18O felsic rocks occur in other settings, and their genesis remains controversial. In this paper we report a Cretaceous low-18O granitoid complex at Xiaocuo in SE China Coast. This complex consists of granites, granodiorites and diorites; all these rocks dated at ~ 116 Ma have zircon ?18O values between 3.8 and 5.2‰. They are metaluminous to weakly peraluminous (A/CNK = 0.84-1.13), and characterized by homogenous isotopic compositions (i.e., initial 87Sr/86Sr (ISr) = 0.7054-0.7056, ?Nd(T) = - 1.6 to - 2.6, zircon ?Hf(T) = - 0.6 to + 3.5 and zircon ?18O = 3.8-5.2‰), high Na2O/K2O (> 1), low Sr/Y (7.23-49.5) and La/Yb ratios (6.87-46.0) and relatively low Zr saturation temperatures (~ 730 °C). In addition, the diorites and granodiorites contain a variety of inherited high-Ca plagioclase (An = 60-87). We demonstrate that the Xiaocuo granitoid rocks were derived from preexisting, subduction-related metabasic source rocks. Their low ?18Ozrc values (3.8-5.2‰) are reflective of their source compositions. The formation of the low ?18O source rocks is most likely formed in a subduction-induced lithospheric extension regime during Early Jurassic to Early Cretaceous time when a Basin and Range-like province developed in SE China. Intensive lithospheric extension could have increased the fracture permeability and facilitated deep-circulated surface waters for exchanging with the source rocks at high temperature at depth, making the metabasic source rocks depleted in 18O. Remelting of the 18O-depleted metabasic intrusive source rocks produced the Xiaocuo low-18O granitoid complex at ~ 116 Ma. Our work indicates that, in addition to the caldera and rift settings, extensional active continental margin could also be a favorable setting for the formation of low-18O granitoid rocks.

  17. Tectonically induced methane seepage into a nearly anoxic water column at the Costa Rican continental margin (Quepos Slide)

    NASA Astrophysics Data System (ADS)

    Rehder, G. J.; Schleicher, T.; Linke, P.

    2011-12-01

    The continental margin off Cost Rica is characterized by active cold venting induced by the subduction of the Cocos Plate underneath the Caribbean Plate. Submarine landslides, often triggered by the subduction of seamounts, have been shown to considerably contribute to the fluid discharge in the area. At the same time, the hydrographic conditions are characterized by very low oxygen conditions in the oxygen minimum zone centred around 400m, as a result of the reinforcement of the already low oxygen content in the Eastern Tropical Pacific by the local upwelling of the Costa Rica Dome. Here we report on the injection of methane-rich fluids into nearly oxygen-free waters at Quepos Slide. The slide resulted in the formation of a plateau at approximately 400 m water depth, with walls in the NW and NE. In the northern part of the slide, the seafloor is paved with bacterial mats along an elongated, weakly pronounced elevation oriented in NW-SE direction, dominated by filamentous Beggiatoa, often covering more than 80% of the seafloor for more than 200m. The colour of the bacterial assemblages shows strong zoning from white to yellow-orange, while grey assemblages were often associated with bathymetric elevations and smaller, circular- shaped patches. A remarkable characteristic in this unique settin is the almost complete lack of all other forms of vent-specific fauna. A quantitative description of the benthos fauna was achieved using quantitative video analysis based on ROV-based video mapping. The methane inventory in the water column within the embayment defined by the landslide was investigated with a grid of 17 hydrocast stations, verifying the highest methane emission in the northern corner of the slope, with concentrations more than two orders of magnitude above local background. Measurements of the stable carbon isotopic ratio on most of the methane samples were used to assess mixing and oxidation processes within this water body. Together with current meter data from ADCP deployments, these will be used to estimate the integrated methane flux from Quepos slide.

  18. The crustal structure of the Central Mozambique continental margin — Wide-angle seismic, gravity and magnetic study in the Mozambique Channel, Eastern Africa

    NASA Astrophysics Data System (ADS)

    Leinweber, Volker Thor; Klingelhoefer, Frauke; Neben, Sönke; Reichert, Christian; Aslanian, Daniel; Matias, Luis; Heyde, Ingo; Schreckenberger, Bernd; Jokat, Wilfried

    2013-06-01

    The continental margin of Mozambique formed during the initial dispersal of Gondwana about 180 Ma. Due to the lack of deep seismic and dense potential field data, many details of the timing and geometry of the early breakup in this region remained unknown to date. To close this gap, a research project (MoBaMaSis ("Mozambique Basin Marine Seismic Survey") with the French research vessel R/V Marion Dufresne II was conducted in 2007. This paper presents the results of P-wave, magnetic and 2D-gravity modelling along two parallel seismic refraction profiles between 37° and 41° E, crossing the Mozambique rifted margin. The crust shows the characteristics of normal to slightly thickened oceanic crust. A lower crustal high-velocity-body with P-wave-velocities of 7.0-7.5 km/s is observed along both profiles. Its origin is discussed in the context of upper mantle convection and thermal properties. The existing magnetic anomaly identifications have been extended to older ages. We postulate that the oldest oceanic crust near the Central Mozambique continental margin has been formed around M41n (166 Ma). Closer to the coast a pronounced negative magnetic anomaly exists that we interpret to coincide with the continent-ocean-transition. This implies that the position of the continent-ocean-transition is located significantly closer to the shoreline than proposed before.

  19. Geomicrobiology of Meltwater From the Western Margin of the Greenland Ice Sheet

    NASA Astrophysics Data System (ADS)

    Hagedorn, B.; Dieser, M.; Choquette, K.; Cameron, K. A.; Sletten, R. S.; Liu, L.; Junge, K.; Christner, B. C.

    2014-12-01

    Subglacial environments are cold, dark, and possess a range of redox conditions. These environments are gaining attention in global biogeochemical cycles as to their role in releasing bioavailable micronutrients such as Fe and the production of greenhouse gases. However, there is uncertainty about how the microbial communities interact with lithology and mediate geochemical reactions under glacial conditions. We examined the microbial communities and their influence on elemental cycling in two glacial environments along the western Greenland Ice Sheet margin: Thule in the north (76ºN, 68ºW) and Kangerlussuaq in the south (67ºN, 51ºW). The north is dominated by supraglacial melting with considerable contribution from the periglacial environment; the south has a well-developed subglacial drainage system. The lithology is sedimentary rocks in the north and crystalline rocks in the south and this difference was reflected in the geochemistry of the drainages. Runoff in the north was oxygen saturated throughout the season. A change from Na and Cl dominance in spring to Ca and SO4 and overall increase in solute concentration marked a stronger contribution from active layer thawing. In the south, waters were undersaturated in oxygen at times, presumably due to biological and chemical sinks of subglacial origin. The meltwater here was dominated by HCO3, SO4 and Ca. In subglacial outflows Fe (oxyhydr)oxide concentrations increased with decreasing oxygen concentration suggesting their formation under oxygen limiting conditions. The high abundance of sulfate implies oxidation of iron sulfides which is consistent with inverse modeling of subglacial weathering processes under anoxic conditions. Meltwater in both locations transported reactive particulate iron which in the north consisted mainly of Fe oxides while Fe(oxyhydr)oxides dominated in the south. DNA and RNA signatures indicate microbial phylotypes that are active in iron reduction, sulfidic mineral weathering, sulfur oxidation, and sulfide reduction supporting the assumption that FeS2 oxidation and formation of particulate reactive Fe(oxyhydr)oxide is microbial mediated. Our data suggest the presence of active microbially iron and sulfur cycling in ice-covered and ice-free zones near the margin of the Greenland Ice Sheet.

  20. Shelf Clinoforms and Suspended Sediments at the Northern and Western Margin of the South China Sea

    NASA Astrophysics Data System (ADS)

    Stattegger, K.; Unverricht, D.; Szczucinski, W.; Liu, Z.; Heinrich, C.; Nguyen, T. C.; Lahajnar, N.

    2013-12-01

    Sediment plumes and shelf clinoforms can be traced almost continuously from the Yangtse until the Mekong Delta as a unique morphodynamics feature of the northern and western marginal parts of the South China Sea. Major sediment plumes originate at the mouths of large rivers, namely the Yangtse, Pearl, Red and Mekong Rivers and extend several hundred kilometers in downdrift direction which is governed mainly by the monsoonal system with the prevailing winter-monsoon winds from NE. Outside the zone of influence of these large rivers we find numerous small mountainous rivers between the Yangtse and Pearl Rivers in SE China and between the Red and Mekong Rivers in central Vietnam providing high amounts of detrital sediments as well. Age control provides clear evidence that the shelf clinoforms developed after 8200 calendar years BP together with a re-organization of the coastal zone and the onset of major modern river deltas after the last phase of accelerated sea-level rise in the Early Holocene, 9000 - 8200 calendar years BP. For the last century 210-Pb activity profiles reveal complex sedimentary processes with event deposition and frequent erosion and re-deposition with average accumulation rates up to 1.5 cm/yr. Suspended sediment contents (SSC) were measured on the Vietnam Shelf. Key regions like the Gulf of Tonkin and the Mekong delta show a strong monsoonal signal as well as a strong tidal signal in the inter-monsoonal period by significant areas of SSCs greater than 25 ?l/l even in the inter-monsoonal season. Suspended sediments exhibit distinct seasonal variability while the deposits of shelf clinoforms reflect the average long-term accumulation patterns of terrigenous sediments sometimes winnowed by marine sedimentation and/or masked by bioturbation.

  1. Structural and tectonic interpretation of geophysical data along the Eastern Continental Margin of India with special reference to the deep water petroliferous basins

    NASA Astrophysics Data System (ADS)

    Bastia, Rabi; Radhakrishna, M.; Srinivas, T.; Nayak, Satyabrata; Nathaniel, D. M.; Biswal, T. K.

    2010-11-01

    The study area encompasses the Eastern Continental Margin of India (ECMI) and the adjoining deep water areas of Bay of Bengal. The region has evolved through multiple phases of tectonic activity and fed by abundant supply of sediments brought by prominent river systems of the Indian shield. Detailed analysis of total field magnetic and satellite-derived gravity data along with multi channel seismic reflection sections is carried out to decipher major tectonic features, basement structure, and the results have been interpreted in terms of basin configuration and play types for different deep water basins along the ECMI. Interpretation of various image enhanced gravity and magnetic anomaly maps suggest that in general, the ENE-WSW trending faults dominate the structural configuration at the margin. These maps also exhibit a clear density transition from the region of attenuated continental crust/proto oceanic crust to oceanic crust based on which the Continent Ocean Boundary (COB) has been demarcated along the margin. Basement depths estimated from magnetic data indicate that the values range from 1 to 12 km below sea level and deepen towards the Bengal Fan in the north and reveal horst-graben features related to rifting. A comparison of basement depths derived from seismic data indicates that in general, the basement trends and depths are comparable in Cauvery and Krishna-Godavari basins, whereas, in the Mahanadi basin, basement structure over the 85°E ridge is clearly revealed in seismic data. Further, eight multichannel seismic sections across different basins of the margin presented here reveal fault pattern, rift geometries and depositional trends related to canyon fills and channel-levee systems and provide a basic framework for future petroleum in this under explored frontier.

  2. New insights on shear margin gravitational evolution through time. The case of the equatorial margins

    NASA Astrophysics Data System (ADS)

    Loncke, L.; Basile, C.; Gaullier, V.; Maillard, A.; Patriat, M.; Sage, F.; Roest, W.

    2009-04-01

    30% of passive margins in the world correspond to shear margins. Unlike divergent margins, those margins present a very sharp ocean-continent boundary which is expressed by steep surface slopes and complex rift structures. In addition of tilted blocks, wrench and strike-slip faults frequently deform the continental crust. High marginal ridges, rising 1-3 km over the adjacent margin typically form along the continental side of the margin. The best known example of transform margin is the Côte d'Ivoire-Ghana margin, highly investigated in the 1980's. New observations along the French-Guiana shear margin (GUYAPLAC survey, 2003) have evidenced massive early (immediately after rifting) and late collapses of the margin. These collapses concern huge volumes: remobilized masses that reach nearly 15000 km3 have been identified in the abyssal plain. No marginal ridge has been observed there. These observations have been compared to results published for the Surinam prolongation of this shear segment (Gouyet, 1988; Erbacher et al., 2004). There also, collapses and slope instabilities are evident, though part of a marginal ridge remains present. Finally, published data from the western Côte d'Ivoire transform margin (De Caprona, 1992) show wide collapses, some deep-seated, and other shallow. Sinking of entire parts of shear margins by gravity collapses appears thus rather common. These observations show that the post-rift gravity collapse of shear margins has been largely underestimated, and has even not been considered in evolutional models of transform margins, despite the fact this has important implications on the geometry and balance of those margins. On the basis of these observations, we propose a tentative scenario for the equatorial Atlantic shear margin gravitational evolution. References: Gouyet, S., 1988. Evolution tectono-sédimentaire des marges guyannaise et Nord-Brésilienne au cours de l'ouverture de l'Atlantique Sud. PhD Thesis, univ Pau et des pays de l'Adour, 374 pp. Erbacher, J., Mosher, D.C., Malone, M.J. et al. 2004. Proceedings of the Ocean Drilling Program, Initial Reports, Volume 207. De Caprona G., 1992. The continental margin of western Côte d'Ivoire: structural framework inherited from intra-continental shearing, 150p, Publ A 69., ISBN 91-7032-685-1.

  3. PTtD evolution of continental crust during subduction-collision processes : example of the Briançonnais domain (Western Alps, France).

    NASA Astrophysics Data System (ADS)

    Strzerzynski, P.; Guillot, S.; Leloup, P. H.; Arnaud, N.; Vidal, O.; Ledru, P.; Courrioux, G.; Darmendrail, X.

    2009-04-01

    Understanding exhumation processes of high to ultra-high pressure (HP to UHP) terrains during plate convergence is a major challenge for the comprehension of plate convergence processes and mountain building. Contemporaneous contraction and extension coupled with erosion are frequently proposed to drive the exhumation process. In the internal alpine belt, HP and UHP metamorphism are recorded both in the Piemont oceanic unit and in the continental units such as the Internal Crystalline Massifs and the Briançonnais zone. In the western Alps, the consensus is to relate the top to NW then W directed D1 thrusting phase to nappe stacking and exhumation of the HP to UHP units within a subduction channel. Although there is an agreement on the occurrence of a top to the east D2 tectonic phase, its significance in terms of shortening or extensional deformation is controversial. On one hand, top to the east D2 shear zones and associated folds are interpreted as backthrusts or backfolds active during the Oligocene syn-collisional shortening phase, post-dating the exhumation of the HP units. On the other hand, D2 structures are interpreted as top the east normal faults that are active during the exhumation of the HP unit within a subduction channel. To decipher between these two different interpretations, we conducted a combined structural, petrological and geochronological study of the Modane-Aussois area in order to build a PTtD path of the Briançonnais zone. The current construction of a high velocity railway tunnel between the Maurienne and Susa valleys provides the opportunity to gather a large amount of geological data in the internal Western Alps and to extend surface observation at depth. We provide a structural analysis on ductile and brittle domains. New PT estimates are obtained using pseudosection and multiequilibra methods. Ar/Ar step heating on phengite provide time constraint on tectonic phases. Then, in light of our data and available literature, we focus on the significance of the D2 structures in the internal Western Alps. Results indicate that polyphased tectonic occurs during exhumation. The first deformation phase (D1) is characterized by nappe stacking in a context of top to the NW shearing, between 37 and 35 Ma deformation occurs between 1.0 and 0.5 GPa and 360-350°C. Top to the East deformation phase (D2) is associated with decompression up to 0.1 GPa and cooling down to 280°C. D2 deformations end at 31Ma. Following these phases of ductile deformation, two successive brittle deformation phases are evidenced: the first one is characterized by a N-S direction of extension and produce the overall tilting toward the south of the studied zone. The second one is characterized by E-W direction of extension. In the Internal Alps, the transition between Oceanic-continental subduction and continental collision occurred at 32Ma. In this context, D1 deformations that are dated between 37 and 35 Ma are clearly related to continental subduction. In the same way, brittle deformation phases are the expression of continental collision. The D2 tectonic phase took place at the transition between subduction and collision. Its attribution to one of these two processes remains ambiguous and will be discussed at the light of these new results.

  4. The crustal structure and tectonic development of the continental margin of the Amundsen Sea Embayment, West Antarctica: implications from geophysical data

    NASA Astrophysics Data System (ADS)

    Kalberg, Thomas; Gohl, Karsten

    2014-07-01

    The Amundsen Sea Embayment of West Antarctica represents a key component in the tectonic history of Antarctic-New Zealand continental breakup. The region played a major role in the plate-kinematic development of the southern Pacific from the inferred collision of the Hikurangi Plateau with the Gondwana subduction margin at approximately 110-100 Ma to the evolution of the West Antarctic Rift System. However, little is known about the crustal architecture and the tectonic processes creating the embayment. During two `RV Polarstern' expeditions in 2006 and 2010 a large geophysical data set was collected consisting of seismic-refraction and reflection data, ship-borne gravity and helicopter-borne magnetic measurements. Two P-wave velocity-depth models based on forward traveltime modelling of nine ocean bottom hydrophone recordings provide an insight into the lithospheric structure beneath the Amundsen Sea Embayment. Seismic-reflection data image the sedimentary architecture and the top-of-basement. The seismic data provide constraints for 2-D gravity modelling, which supports and complements P-wave modelling. Our final model shows 10-14-km-thick stretched continental crust at the continental rise that thickens to as much as 28 km beneath the inner shelf. The homogenous crustal architecture of the continental rise, including horst and graben structures are interpreted as indicating that wide-mode rifting affected the entire region. We observe a high-velocity layer of variable thickness beneath the margin and related it, contrary to other `normal volcanic type margins', to a proposed magma flow along the base of the crust from beneath eastern Marie Byrd Land-West Antarctica to the Marie Byrd Seamount province. Furthermore, we discuss the possibility of upper mantle serpentinization by seawater penetration at the Marie Byrd Seamount province. Hints of seaward-dipping reflectors indicate some degree of volcanism in the area after break-up. A set of gravity anomaly data indicate several phases of fully developed and failed rift systems, including a possible branch of the West Antarctic Rift System in the Amundsen Sea Embayment.

  5. Heat Flow and Gas Hydrates on the Continental Margin of India: Building on Results from NGHP Expedition 01

    SciTech Connect

    Trehu, Anne; Kannberg, Peter

    2011-06-30

    The Indian National Gas Hydrate Program (NGHP) Expedition 01 presented the unique opportunity to constrain regional heat flow derived from seismic observations by using drilling data in three regions on the continental margin of India. The seismic bottom simulating reflection (BSR) is a well-documented feature in hydrate bearing sediments, and can serve as a proxy for apparent heat flow if data are available to estimate acoustic velocity and density in water and sediments, thermal conductivity, and seafloor temperature. Direct observations of temperature at depth and physical properties of the sediment obtained from drilling can be used to calibrate the seismic observations, decreasing the uncertainty of the seismically-derived estimates. Anomalies in apparent heat flow can result from a variety of sources, including sedimentation, erosion, topographic refraction and fluid flow. We constructed apparent heat flow maps for portions of the Krishna-Godavari (K-G) basin, the Mahanadi basin, and the Andaman basin and modeled anomalies using 1-D conductive thermal models. Apparent heat flow values in the Krishna-Godavari (K-G) basin and Mahanadi basin are generally 0.035 to 0.055 watts per square meter (W/m2). The borehole data show an increase in apparent heat flow as water depth increases from 900 to 1500 m. In the SW part of the seismic grid, 1D modeling of the effect of sedimentation on heat flow shows that ~50% of the observed increase in apparent heat flow with increasing water depth can be attributed to trapping of sediments behind a "toe-thrust" ridge that is forming along the seaward edge of a thick, rapidly accumulating deltaic sediment pile. The remainder of the anomaly can be explained either by a decrease in thermal conductivity of the sediments filling the slope basin or by lateral advection of heat through fluid flow along stratigraphic horizons within the basin and through flexural faults in the crest of the anticline. Such flow probably plays a role in bringing methane into the ridge formed by the toe-thrust. Because of the small anomaly due to this process and the uncertainty in thermal conductivity, we did not model this process explicitly. In the NE part of the K-G basin seismic grid, a number of local heat flow lows and highs are observed, which can be attributed to topographic refraction and to local fluid flow along faults, respectively. No regional anomaly can be resolved. Because of lack of continuity between the K-G basin sites within the seismic grid and those ~70 km to the NE in water depths of 1200 to 1500 m, we do not speculate on the reason for higher heat flow at these depths. The Mahanadi basin results, while limited in geographic extent, are similar to those for the KG basin. The Andaman basin exhibits much lower apparent heat flow values, ranging from 0.015 to 0.025 W/m2. Heat flow here also appears to increase with increasing water depth. The very low heat flow here is among the lowest heat flow observed anywhere and gives rise to a very thick hydrate stability zone in the sediments. Through 1D models of sedimentation (with extremely high sedimentation rates as a proxy for tectonic thickening), we concluded that the very low heat flow can probably be attributed to the combined effects of high sedimentation rate, low thermal conductivity, tectonic thickening of sediments and the cooling effect of a subducting plate in a subduction zone forearc. Like for the K-G basin, much of the local variability can be attributed to topography. The regional increase in heat flow with water depth remains unexplained because the seismic grid available to us did not extend far enough to define the local tectonic setting of the slope basin controlling this observational pattern. The results are compared to results from other margins, both active and passive. While an increase in apparent heat flow with increasing water depth is widely observed, it is likely a result of different processes in different places. The very low heat flow due to sedimentation and tectonics in the Andaman basin is at the low end of glob

  6. Heat Flow and Gas Hydrates on the Continental Margin of India: Building on Results from NGHP Expedition 01

    SciTech Connect

    Anne Trehu; Peter Kannberg

    2011-06-30

    The Indian National Gas Hydrate Program (NGHP) Expedition 01 presented the unique opportunity to constrain regional heat flow derived from seismic observations by using drilling data in three regions on the continental margin of India. The seismic bottom simulating reflection (BSR) is a well-documented feature in hydrate bearing sediments, and can serve as a proxy for apparent heat flow if data are available to estimate acoustic velocity and density in water and sediments, thermal conductivity, and seafloor temperature. Direct observations of temperature at depth and physical properties of the sediment obtained from drilling can be used to calibrate the seismic observations, decreasing the uncertainty of the seismically-derived estimates. Anomalies in apparent heat flow can result from a variety of sources, including sedimentation, erosion, topographic refraction and fluid flow. We constructed apparent heat flow maps for portions of the Krishna-Godavari (K-G) basin, the Mahanadi basin, and the Andaman basin and modeled anomalies using 1-D conductive thermal models. Apparent heat flow values in the Krishna-Godavari (K-G) basin and Mahanadi basin are generally 0.035 to 0.055 watts per square meter (W/m{sup 2}). The borehole data show an increase in apparent heat flow as water depth increases from 900 to 1500 m. In the SW part of the seismic grid, 1D modeling of the effect of sedimentation on heat flow shows that {approx}50% of the observed increase in apparent heat flow with increasing water depth can be attributed to trapping of sediments behind a 'toe-thrust' ridge that is forming along the seaward edge of a thick, rapidly accumulating deltaic sediment pile. The remainder of the anomaly can be explained either by a decrease in thermal conductivity of the sediments filling the slope basin or by lateral advection of heat through fluid flow along stratigraphic horizons within the basin and through flexural faults in the crest of the anticline. Such flow probably plays a role in bringing methane into the ridge formed by the toe-thrust. Because of the small anomaly due to this process and the uncertainty in thermal conductivity, we did not model this process explicitly. In the NE part of the K-G basin seismic grid, a number of local heat flow lows and highs are observed, which can be attributed to topographic refraction and to local fluid flow along faults, respectively. No regional anomaly can be resolved. Because of lack of continuity between the K-G basin sites within the seismic grid and those {approx}70 km to the NE in water depths of 1200 to 1500 m, we do not speculate on the reason for higher heat flow at these depths. The Mahanadi basin results, while limited in geographic extent, are similar to those for the K-G basin. The Andaman basin exhibits much lower apparent heat flow values, ranging from 0.015 to 0.025 W/m{sup 2}. Heat flow here also appears to increase with increasing water depth. The very low heat flow here is among the lowest heat flow observed anywhere and gives rise to a very thick hydrate stability zone in the sediments. Through 1D models of sedimentation (with extremely high sedimentation rates as a proxy for tectonic thickening), we concluded that the very low heat flow can probably be attributed to the combined effects of high sedimentation rate, low thermal conductivity, tectonic thickening of sediments and the cooling effect of a subducting plate in a subduction zone forearc. Like for the K-G basin, much of the local variability can be attributed to topography. The regional increase in heat flow with water depth remains unexplained because the seismic grid available to us did not extend far enough to define the local tectonic setting of the slope basin controlling this observational pattern. The results are compared to results from other margins, both active and passive. While an increase in apparent heat flow with increasing water depth is widely observed, it is likely a result of different processes in different places. The very low heat flow due to sedimentation and tectonics in the Andaman basi

  7. Gas hydrates (clathrates) causing pore-water freshening and oxygen isotope fractionation in deep-water sedimentary sections of terrigenous continental margins

    USGS Publications Warehouse

    Hesse, R.; Harrison, W.E.

    1981-01-01

    The occurrence of gas hydrates in deep-water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep-sea drilling results. On the Pacific continental slope off Guatemala gas hydrates were brought up for the first time from two holes (497, 498A) drilled during Leg 67 of the DSDP in water depths of 2360 and 5500 m, respectively. The hydrates occur in organic matter-rich Pleistocene to Miocene terrigenous sediments. In the hydrate-bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10-20 m subbottom depth. Pore waters at the bottom of the holes (near 400 m subbottom) have as little as half the chlorinity of seawater (i.e. 9???). Similar, but less pronounced, trends were observed during previous legs of the DSDP in other hydrate-prone segments of the continental margins where recharge of fresh water from the continent can be excluded (e.g. Leg 11). The crystallization of hydrates, like ice, excludes salt ions from the crystal structure. During burial the dissolved salts are separated from the solids. Subsidence results in a downward motion of the solids (including hydrates) relative to the pore fluids. Thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation. The volume of the latter decreases downhole thus causing downward decreasing salinity (chlorinity). Hydrate formation is responsible for oxygen isotope fractionation with 18O-enrichment in the hydrate explaining increasingly more positive ??18O values in the pore fluids recovered (after hydrate dissociation) with depth. ?? 1981.

  8. Sedimentary evolution of the fan systems of the northern Mediterranean margin (Gulf of Lions and western Provence) since the Pliocene

    SciTech Connect

    Droz, L.; Bellaiche, G.; Coutellier, V.

    1988-08-01

    The continental rise of the northern Mediterranean margin is dominated by a coalescing fan-type sedimentation characterized by the complex stacking of interbedded sedimentary bodies, including symmetrical channel-levee complexes organized off the mouth of major canyons, curved ridges probably representing underdeveloped levees of asymmetrical systems, and widespread mass-movement deposits. Fan-type sedimentation in the Mediterranean basin began during the Pliocene. It first appeared ubiquitous on the deep margin and related to the Messinian paleodrainage system. Later, possibly in the early Quaternary, the sedimentation became more focused and organized off major canyons of the margin. The construction of the channel-levee complexes is inferred to have been active mainly during the drops of sea level linked with the Quaternary glaciations. In contrast, mass-movement processes are thought to have been favored during the rise of sea level characterizing the interglacial periods. On the grounds of the relative chronology of the construction of the different types of sedimentary bodies, we tentatively correlate the major active periods of the fan systems with the paleohydrographic network draining, at those times, the southern part of France. These correlations indicate that various successive regions of the Alps provided detritus to the basin through shifting feeding paths (rivers and canyons).

  9. Seismic imaging of small horizontal scale structures of the shallow thermocline on the western Brittany continental shelf (North-East Atlantic)

    NASA Astrophysics Data System (ADS)

    Piete, H.; Marié, L.; Marsset, B.; Gutscher, M.

    2012-12-01

    The recent development of the seismic oceanography technique has made possible the imaging of a variety of deep oceanographic structures (Holbrook et al., 2003); however, until now this method has remained ill suited for the study of shallow (<200m) thermohaline structures. This difficulty is partly due to the fact that both important seismic trace lengths and large offsets that characterize the acoustic receiver device (seismic streamer) cause significant signal attenuations through an induced antenna filter effect. Further difficulties are related to limitations of currently employed seismic sources, which do not conciliate 1- high power (essential to the imaging of weakly reflective structures in a noisy environment) and 2- spectral contents offering high vertical resolutions (relevant to the mapping of small vertical wavelength structures). In this study we defined and tested a new experimental seismic acquisition system capable of imaging the ~10 m thick seasonal thermocline on the western Brittany continental shelf. To accomplish this task, we pursued two complementary approaches: 1. Analysis of legacy seismic data (multi-channel seismic reflection profiles acquired on the East-Corsican margin, Bahamas Plateau and Gulf of Cadiz in various oceanographic environments) featuring reflectors at depths between 25 and 150 m, in order to identify and quantify the influence of acquisition parameters (seismic trace length, offsets, emission level and frequency content). 2. Incorporation of new oceanographic data acquired during the FROMVAR cruise (July 28th to August 10th 2010) on the western Brittany shelf in thermally stratified waters for use in the simulation of the seismic acquisition, in order to further define the optimal parameters for the system. Finally a 3D seismic system has emerged and was tested during the ASPEX scientific cruise led from June 17th to 19th 2012 across the western Brittany shelf. The device featured: i- four seismic streamers, each consisting of 6 traces at a spacing of 1.80 m; ii- a 1000 J SIG Sparker producing a 400 Hz signal with a 220 dB re 1?Pa @1m level of emission, towed at a 8 m distance of the first seismic trace. This survey provided high lateral resolution images of the seasonal thermocline located at a 30 m depth with vertical displacements induced by internal waves. References Holbrook, W.S., Paramo, P., Pearse, S. and Schmitt, R.W., 2003. Thermohaline Fine Structure in an Oceanographic Front from Seismic Reflection Profiling. Science, 301(5634): 821.

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

    NASA Astrophysics Data System (ADS)

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

    2015-04-01

    The aim of the research is to quantify the long-term landscape evolution of the South Atlantic passive continental margin (SAPCM) in SE-Brazil and NW-Namibia. Excellent onshore outcrop conditions and complete rift to post-rift archives between Sao Paulo and Porto Alegre and in the transition from Namibia to Angola (onshore Walvis ridge) allow a high precision quantification of exhumation, and uplift rates, influencing physical parameters, long-term acting forces, and process-response systems. Research will integrate the published and partly published thermochronological data from Brazil and Namibia, and test lately published new concepts on causes of long-term landscape evolution at rifted margins. The climate-continental margin-mantle coupled process-response system is caused by the interaction between endogenous and exogenous forces, which are related to the mantle-process driven rift - drift - passive continental margin evolution of the South Atlantic, and the climate change since the Early/Late Cretaceous climate maximum. Special emphasis will be given to the influence of long-living transform faults such as the Florianopolis Fracture Zone (FFZ) on the long-term topography evolution of the SAPCM's. A long-term landscape evolution model with process rates will be achieved by thermo-kinematic 3-D modeling (software code PECUBE1,2 and FastScape3). Testing model solutions obtained for a multidimensional parameter space against the real thermochronological and geomorphological data set, the most likely combinations of parameter rates, and values can be constrained. The data and models will allow separating the exogenous and endogenous forces and their process rates. References 1. Braun, J., 2003. Pecube: A new finite element code to solve the 3D heat transport equation including the effects of a time-varying, finite amplitude surface topography. Computers and Geosciences, v.29, pp.787-794. 2. Braun, J., van der Beek, P., Valla, P., Robert, X., Herman, F., Goltzbacj, C., Pedersen, V., Perry, C., Simon-Labric, T., Prigent, C. 2012. Quantifying rates of landscape evolution and tectonic processes by thermochronology and numerical modeling of crustal heat transport using PECUBE. Tectonophysics, v.524-525, pp.1-28. 3. Braun, J. and Willett, S.D., 2013. A very efficient, O(n), implicit and parallel method to solve the basic stream power law equation governing fluvial incision and landscape evolution. Geomorphology, v.180-181, 170-179.

  11. CETACEAN HIGHUSE HABITATS OF THE NORTHEAST UNITED STATES CONTINENTAL SHELF

    E-print Network

    CETACEAN HIGH·USE HABITATS OF THE NORTHEAST UNITED STATES CONTINENTAL SHELF 1 RoBERT D. KENNEY offthe northeast United States. which is used most intensively as cetacean habitat, is the western margin AND HOWARD E. WINN2 ABSTRACT Results of the Cetacean and 'furtle Assessment Program previously demonstrated

  12. Deep Crustal Structure of the Continental Margin off the Explora Escarpment and in the Lazarev Sea, East Antarctica

    Microsoft Academic Search

    Wilfried Jokat; Oliver Ritzmann; Christian Reichert; Karl Hinz

    2004-01-01

    This study presents the results of a seismic refraction experiment that was carried out off Dronning Maud Land (East Antarctica) along the Explora Escarpment (14° W–12° W) and close to Astrid Ridge (6°E). Oceanic crust of about 10 km thickness is observed northwest of the Explora Escarpment. Stretched continental crust, observed southeast of the escarpment, is most likely intruded by volcanic

  13. Deep crustal structure of the continental margin off the Explora Escarpment and in the Lazarev Sea, East Antarctica

    Microsoft Academic Search

    Wilfried Jokat; Oliver Ritzmann; Christian Reichert; Karl Hinz

    2005-01-01

    This study presents the results of a seismic refraction experiment that was carried out off Dronning Maud Land (East Antarctica) along the Explora Escarpment (14? W-12? W) and close to Astrid Ridge (6? E). Oceanic crust of about 10 km thickness is observed northwest of the Explora Escarpment. Stretched continental crust, observed southeast of the escarpment, is most likely intruded

  14. What do Great Subduction Earthquakes tell us About Continental Deformation of the Upper Plate in the Central Andes Forearc? Insights From Seismotectonics, Continental Deformation and Coulomb Modelisation Along Southern Peru Margin

    NASA Astrophysics Data System (ADS)

    Audin, L.; Perfettini, H.; Tavera, H.

    2007-05-01

    Subduction of the Nazca plate beneath the Peruvian margin has produced numerous megathrust earthquakes during the last century and still constitutes mature seismic gaps in some places such as in between Ilo (Peru) and Arica (Chile). The rupture zones of the 1604, 1784 and 1868 southern Peru events were partially reactivated by the Arequipa 2001 (Mw = 8.5) seismic event, whose rupture zone was about 350km-long and stopped its propagation towards the south on Ilo Peninsula. Just after the occurrence of 2001 event, some reactivation of continental fault systems are identified and monitored thanks to the Peruvian seismic network and describe continental deformation processes occurring perpendicularly to the trench or parallel to the trench, traducing the continental plate response to major subduction earthquakes and some partitioning of the deformation. The Chololo and associated ( perpendicular to the trench) fault systems define some 80-km-long margin crustal blocks and the major one coincides with the 2001 earthquake southern limit of the rupture zone as it propagated to the south. These blocks are made from Late Jurassic and Cretaceous plutonic rocks from the Coastal Batholith; these are outcropping in some places and are evidenced by the aeromagnetic mapping elsewhere around the area. Northward along the subduction zone, another boundary between two rupture zones of major subduction earthquake was reactivated recently, perpendicularly to the trench, by the seismic crisis of October 2006, M=6.4, near Lima, right at the southern end of the rupture zone of the 1974 event (Mw=8.1).Those boundaries corresponding to discontinuities (lithospheric fault systems) in the upper plate, trending nearly perpendicular to the trench, act as earthquake barriers during rupture of large seismic events. Additionally occurred on 20 of November 2006 another seismic event (Mw=5.6 Neic, Ml=5.3) in Tacna region, showing a reverse focal mechanism compatible with the trend of the Sama Calientes Fault system (parallel to the trench) and a crustal depth of about 20km. Such a magnitud and crustal depth in the area correlates perfectly with the Quaternary geomorphic evidences of tectonic activity along the Sama-Calientes thrust fault in the forearc in Southern Peru. Some questions are raised by the occurrence of such continental seismicity, just after a major subduction event, as none has been registered in the area since more than 40 years. Continental fault systems constitute a key to the understanding of the forearc deformation in the Arica Elbow, where the Andes obliquity with respect to the Nazca plate convergence direction. Also these results suggest that continental deformation should give us clues to define the pattern of segmentation of the subduction zone by studying seismotectonics and its relation to the segmentation of the upper continental plate.

  15. 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.

  16. Abbot Ice Shelf, structure of the Amundsen Sea continental margin and the southern boundary of the Bellingshausen Plate seaward of West Antarctica

    NASA Astrophysics Data System (ADS)

    Cochran, James R.; Tinto, Kirsty J.; Bell, Robin E.

    2015-05-01

    Inversion of NASA Operation IceBridge airborne gravity over the Abbot Ice Shelf in West Antarctica for subice bathymetry defines an extensional terrain made up of east-west trending rift basins formed during the early stages of Antarctica/Zealandia rifting. Extension is minor, as rifting jumped north of Thurston Island early in the rifting process. The Amundsen Sea Embayment continental shelf west of the rifted terrain is underlain by a deeper, more extensive sedimentary basin also formed during rifting between Antarctica and Zealandia. A well-defined boundary zone separates the mildly extended Abbot extensional terrain from the deeper Amundsen Embayment shelf basin. The shelf basin has an extension factor, ?, of 1.5-1.7 with 80-100 km of extension occurring across an area now 250 km wide. Following this extension, rifting centered north of the present shelf edge and proceeded to continental rupture. Since then, the Amundsen Embayment continental shelf appears to have been tectonically quiescent and shaped by subsidence, sedimentation, and the advance and retreat of the West Antarctic Ice Sheet. The Bellingshausen Plate was located seaward of the Amundsen Sea margin prior to incorporation into the Antarctic Plate at about 62 Ma. During the latter part of its independent existence, Bellingshausen plate motion had a clockwise rotational component relative to Antarctica producing convergence across the north-south trending Bellingshausen Gravity Anomaly structure at 94°W and compressive deformation on the continental slope between 94°W and 102°W. Farther west, the relative motion was extensional along an east-west trending zone occupied by the Marie Byrd Seamounts. The copyright line for this article was changed on 5 JUN 2015 after original online publication.

  17. Flexural expression of European continental lithosphere under the western outer Carpathians

    Microsoft Academic Search

    Reini Zoetemeijer; Cestmir Tomek; Sierd Cloetingh

    1999-01-01

    We present the results of an analysis of the flexural down bending of the European lithosphere under the western outer Carpathians for a grid of five profiles with a lateral spacing of about 100 km crossing the Carpathians and its foreland. Seismic sections, including two deep seismic reflection lines, and numerous wells allowed us to quantify the flexural curvature of

  18. A new species of Bolitoglossa (Caudata, Plethodontidae) from the continental divide of western Panama.

    PubMed

    Hertz, Andreas; Lotzkat, Sebastian; Köhler, Gunther

    2013-01-01

    We describe the new salamander species Bolitoglossa jugivagans from the Atlantic slopes of the Fortuna depression in western Panama on the basis of morphological and molecular data. Based on mtDNA data, the new species seems to be closely related to B. aureogularis and B. robinsoni, with which it forms a subclade within the subgenus Eladinea. PMID:26042305

  19. Tectonic and provenance history of the Neotethyan margin in NE Africa recorded by detrital zircon (U-Th)/He thermochronometry from a borehole in the Western Desert, Egypt

    NASA Astrophysics Data System (ADS)

    Stockli, D.; Glauser, T.; Bosworth, W.; Maher, T.; Clare, A.

    2009-04-01

    The subsurface of the Western Desert of Egypt contains multiple stacked sedimentary basin deposits separated by major unconformities reflecting the long-lived tectonic evolution of the Neotethyan continental margin in eastern North Africa. In this study, zircon (U/Th)/He (ZHe) data were collected from cuttings from a ~15000 ft borehole that penetrated Tertiary and Cretaceous strata and a major erosional unconformity at 13000 ft that juxtaposes Cretaceous and Cambro-Ordovician strata. A total of 56 samples spanning the borehole from 750-15400 ft yielded >200 single-grain ZHe ages in order to elucidate the thermal evolution of the borehole and constrain the thermal history of detrital provenance. ZHe ages above the unconformity are significantly older than the depositional age, suggesting detrital ZHe ages that were not reset subsequent to deposition. ZHe ages from Cambro-Ordovician strata below the unconformity are substantially younger than the minimum depositional age suggesting major cooling and resetting of zircon (>200C) during the Hercynian orogeny. In detail, ZHe ages form Cretaceous strata above the unconformity show the following trends. (1) ZHe ages from 6000-9000 ft (Aptian-Early Cenomanian) are characterized by a ZHe age peak at ~450 Ma and a minor Albian peak, (2) samples from 9000-12000 ft (Late Hauterivian-Barremian) show two major detrital ZHe age peaks at ~450 and 350 Ma, while (3) samples from 12000-13000 ft (Early Hauterivian) exhibit three dominant ZHe age components at ~450 Ma, 350 Ma, and 170-200 Ma. Additional cuttings from an offset containing complete stratigraphy yielded ZHe ages that mainly represent a strong Hercynian input as well as Late Triassic and Early Jurassic components of Tethyan related input. These ZHe age peaks display provenance characteristics typical for cooling signatures of rocks from the eroding Arabian-Nubian Shield, a North-African Hercynian source, and eroded material from exhumed fault blocks along the Triassic-Jurassic Neotethyan rifted margin. While immediately above the unconformity Hercynian ZHe ages dominate, the occurrence of Triassic or Early Jurassic suggest the presence of eroding rapidly cooled and exhumed Tethyan normal fault blocks. At decreasing depth, first Jurassic-Triassic, and then Hercynian source input disappears and the arrival of detritus from the Arabian-Nubian Shield begins to dominate the North African passive continental margin in the Western Desert in the middle to late Cretaceous. This unique data set illustrates the power of ZHe thermochronometry as a thermochronometer in boreholes with temperatures in excess of other low-temperature dating techniques and as a detrital provenance tool, not constraining crystallization ages, but rather shedding light on the cooling and exhumation history of the source terrane and the tectonic/geological environment of the basin deposits.

  20. Continental ultra-deep drilling locating research status and progress in the Jinchuan Ni-Cu ore-concentrated area,Western China

    NASA Astrophysics Data System (ADS)

    Yan, H.; Tang, Z.; Yang, J.

    2010-12-01

    The abstract is the initial achievements of "Continental ultra-deep drilling locating pre-study in the Jinchuan nickel-copper ore-concentrated area,Western China". 1 Some scientific problems faced by Jinchuan scientific drilling 1.1 Tectonic research of plates convergent margins Jinchuan ore-concentrated area locate in intersection of the south China plate, the north China plate, the tarim plate and convergent orogenic belts. Carrying out deep drilling could obtain informations such as deep material composition, fluid composition, rock fabric, structural features of the special region,etc.Then we can discuss tectonic evolution of the area between the plates. 1.2 Metallogenic theory research Ore-forming material source and extension of the deep rock situation are the key of resolving and verifying metallogenic theory. Carrying out scientific drilling research which will make some issues, that reasons of the huge amount of metal accumulation, formation mechanism of Cu-Ni-PGE sulfide deposits, relationships of metals accumulation process and the crust-mantle interaction and geological background, in particular, fluid role in this process, be resolved. And we could research mantle-derived magmatism and mineralization. 1.3 Perfect Jinchuan metallogenic model Jinchuan deposit is typical for the world's "formation of large deposit in a small intrusion". That establishing the metallogenic model is predominant in international field of mafic-ultramafic magmatic sulfide deposits. Scientific deep drilling could obtain informations which will enrich the theory system. 2 New achievements of continental ultra-deep drilling locating pre-study (1) Previous studies shown that Mesoproterozoic and Neoproterozoic strata,to south of Ore-bearing rock, are monoclines, but this research reveals that which are incomplete complex anticlines, which controlled and destructed by several major regional faults. And there are several ductile shear zones in Sinian strata that increased complexity of regional structure,deformation and metamorphism. These findings have a significant impact on studies of regional diagenesis, mineralization,magmation and tectonic evolution. (2) Systematic sampling analysis revealed that the formation of copper-rich,,PGE-rich ore bodies related to the latest magma (Su et al.,2010). The magma mineralization concentrated in the midwestern portion of No.1 ore body, this part was probably the location of the occurrence of magma channel. It's worthy of exploring at depth with other factors. (3)The application of controlled source audio-frequency magnetotelluric sounding shown that spatial distribution of electromagnetic geological bodies of the mining area over 3200m under the surface.

  1. Shyok Suture Zone, N Pakistan: late Mesozoic Tertiary evolution of a critical suture separating the oceanic Ladakh Arc from the Asian continental margin

    NASA Astrophysics Data System (ADS)

    Robertson, Alastair H. F.; Collins, Alan S.

    2002-02-01

    The Shyok Suture Zone (Northern Suture) of North Pakistan is an important Cretaceous-Tertiary suture separating the Asian continent (Karakoram) from the Cretaceous Kohistan-Ladakh oceanic arc to the south. In previously published interpretations, the Shyok Suture Zone marks either the site of subduction of a wide Tethyan ocean, or represents an Early Cretaceous intra-continental marginal basin along the southern margin of Asia. To shed light on alternative hypotheses, a sedimentological, structural and igneous geochemical study was made of a well-exposed traverse in North Pakistan, in the Skardu area (Baltistan). To the south of the Shyok Suture Zone in this area is the Ladakh Arc and its Late Cretaceous, mainly volcanogenic, sedimentary cover (Burje-La Formation). The Shyok Suture Zone extends northwards (ca. 30 km) to the late Tertiary Main Karakoram Thrust that transported Asian, mainly high-grade metamorphic rocks southwards over the suture zone. The Shyok Suture Zone is dominated by four contrasting units separated by thrusts, as follows: (1). The lowermost, Askore amphibolite, is mainly amphibolite facies meta-basites and turbiditic meta-sediments interpreted as early marginal basin rift products, or trapped Tethyan oceanic crust, metamorphosed during later arc rifting. (2). The overlying Pakora Formation is a very thick (ca. 7 km in outcrop) succession of greenschist facies volcaniclastic sandstones, redeposited limestones and subordinate basaltic-andesitic extrusives and flow breccias of at least partly Early Cretaceous age. The Pakora Formation lacks terrigenous continental detritus and is interpreted as a proximal base-of-slope apron related to rifting of the oceanic Ladakh Arc; (3). The Tectonic Melange (<300 m thick) includes serpentinised ultramafic rocks, near mid-ocean ridge-type volcanics and recrystallised radiolarian cherts, interpreted as accreted oceanic crust. (4). The Bauma-Harel Group (structurally highest) is a thick succession (several km) of Ordovician and Carboniferous to Permian-Triassic, low-grade, mixed carbonate/siliciclastic sedimentary rocks that accumulated on the south-Asian continental margin. A structurally associated turbiditic slope/basinal succession records rifting of the Karakoram continent (part of Mega-Lhasa) from Gondwana. Red clastics of inferred fluvial origin ('molasse') unconformably overlie the Late Palaeozoic-Triassic succession and are also intersliced with other units in the suture zone. Reconnaissance further east (north of the Shyok River) indicates the presence of redeposited volcaniclastic sediments and thick acid tuffs, derived from nearby volcanic centres, presumed to lie within the Ladakh Arc. In addition, comparison with Lower Cretaceous clastic sediments (Maium Unit) within the Northern Suture Zone, west of the Nanga Parbat syntaxis (Hunza River) reveals notable differences, including the presence of terrigenous quartz-rich conglomerates, serpentinite debris-flow deposits and a contrasting structural history. The Shyok Suture Zone in the Skardu area is interpreted to preserve the remnants of a rifted oceanic back-arc basin and components of the Asian continental margin. In the west (Hunza River), a mixed volcanogenic and terrigenous succession (Maium Unit) is interpreted to record syn-deformational infilling of a remnant back-arc basin/foreland basin prior to suturing of the Kohistan Arc with Asia (75-90 Ma).

  2. The Close Connection Between the Subduction Processes, the Morphology of the Continental Margin and the Marine Basins on the accretionary segment of the Chilean Margin

    NASA Astrophysics Data System (ADS)

    Contardo, X. J.; Cembrano, J. M.

    2007-05-01

    The differences observed in the morphology of the accretionary segment of the Chilean margin and the architecture of the associated marine basins, are indicative of variations in the degree of erosion-accretion and uplift of the prism at different latitudes of the margin. The geometry of the slope basins as wedge-shaped half graben basins is controlled by differential subsidence and uplift of the upper levels of the slope. The dynamics of the deposition and deformation of the sequences suggest alternating tectonic events of extension, compression (or transpression) and relative stability. The main geodynamic elements controlling the tectonic evolution of the Chilean margin can be grouped into factors associated with the subducting oceanic Nazca Plate and those related to mass transfer processes. They influence the geometry and deformation of slope basins. Accretionary and non-accretionary or erosional episodes are linked to temporal variations in sediment thickness of the trench, which is mainly associated with climatic fluctuations. Glacial material flux contributed significantly to the trench infill, and has also likely increased accretion rates. In some cases an accelerated sediment supply to the trench may cause unsustainable rates. This can induce subsidence in the trench and sediment subduction to the base of the prism, which drives more basal underplating and strong uplift localized near surface extension in the entire convergent system. The examination of the seismic lines acquired during a Chilean expedition in the RV Vidal Gormaz suggests differential subsidence controlled by displacement on flanking faults as well as pre-kinematic, syn-kinematic and post-kinematic deposition. The thickness of basins also will influence the amount of sediment which is either distributed from the shelf to the trench, or trapped on the slope basins. The analyzed slope basins, between 33°30' - 36°50' S, exhibit variations along the margin. The deepest basin is observed near the Juan Fernandez Ridge (JFR) intersection with the margin, on seismic line VG02-18 (~1 km). While thinner sequences are observed to the south, along VG02-10 (~0,27 km and ~0,45 km) and VGO2-5 (~0 4 km and ~0,5 km). In proximity to the JFR, the high subsidence rate on a slope basin of the seismic line VG02-18, is influenced by the tectonic erosion at the base of the accretionary wedge linked with the progressive positions of subducting seamounts. The alternation between syn-tectonic and pre-post tectonic deposition documents cyclic tectonic events which can be associated with the accretionary and the non-accretionary, or erosive episodes for the development of the south Chile margin. The most probable mass transfer mode in the South Chilean accretive system is partly frontal accretion and partly underthrusting of sediment entering the trench with potential underplating. Frontal and basal accretion leads to characteristic growth patterns of the forearc area. In this study, episodes of frontal accretion can be associated with pre-post tectonic sequences along the slope, which evidences episodes of stability without deformation of sequences and parallel continuous deposition of sediment. The basal accretion of large volumes of underplated sediment beneath the buttress undergoes deformation and tilting and can be associated with the syn-tectonic units.

  3. Downslope Eulerian mean flow associated with high-frequency current fluctuations observed on the outer continental shelf and upper slope along the northeastern United States continental margin: implications for sediment transport

    USGS Publications Warehouse

    Butman, B.

    1988-01-01

    Eulerian current measurements made 5-7 m above bottom at six stations along the United States east coast continental margin show a net downslope flow of 1-5 cm s-1. Although the scalar current speed decreases with water depth and toward the bottom, fluctuations in the cross-isobath flow were stronger and increasingly asymmetric near the bottom. Maximum downslope flow exceeded maximum upslope flow by a factor of two to three. The strength of the low-passed downslope flow was proportional to the upslope Reynolds flux of density as well as to the amplitude of the current fluctuations that have periods shorter than 30 h. These flow characteristics may be caused by differential vertical mixing in the bottom boundary layer where a stratified fluid flows upslope (unstable) and downslope (stable). The asymmetry in current strength clearly favors net downslope transport of sediments that move as bedload. ?? 1988.

  4. Seabed photography, environmental assessment and evidence for deep-water trawling on the continental margin west of the Hebrides

    Microsoft Academic Search

    J. M. Roberts; S. M. Harvey; P. A. Lamont; J. D. Gage; J. D. Humphery

    2000-01-01

    A photographic survey in 1998 of the seabed along depth transects from 700 to 1300 m across the N.E. Atlantic continental slope off north-west Scotland shows clear depth-related change in sediment type and megabenthic community in an environment where biological communities and species distributions are poorly known. Small-scale features, such as trawl marks and dense fields of xenophyophores, were resolved

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

    To understand the evolution of the passive continental margin in Argentina apatite fission track dating is an appropriate method, which will lead to new conclusions in this area. The Tandilia System, also called Sierras Septentrionales, is located south of the Río de la Plato Craton in eastern Argentina in the state of Buenos Aires. North of the hills Salado basin is orientated whereas the Claromeó basin is located south of the mountain range. In contrary to most basins along the southamerican passive continental margin the Tandilia-System and the neighbouring basins trend perpendicular to the coast line. The topography ranges between 50 and 250m within the study area and is therefore fairly flat. The igneous-metamorphic basement is pre-proterozoic in age build up of mainly granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks and granitoid plutons and is overlain by a series of Neoproterozoic to early Paleozoic sediments (Cingolani, 2010). The aim of the study is to evaluate the long-term landscape evolution of the passive continental margin in eastern Argentina in terms of thermal history and exhumation. For that purpose samples were taken from the Sierra Septentrionales basement analyzed for the apatite-FT method. The results so far indicate apatite fission track ages between 146.2 (10.1) Ma and 200.4 (12.7) Ma, which shows all samples have been reseted. Still ongoing length measurements will lead to 2D thermo kinematic Hefty (Ketcham, 2005; Ketcham et al., 2009; Ketcham, 2007) models. This will leads to further more insights on the cooling history and tectonic activities in the research area. References: Cingolani C. A. (2010): The Tandilia System of Argentina as a southern extension of the Río de la Plata craton: an overview. Int. J. Earth Sci. (Geol. Rundsch.) (2011) 100:221-242, doi 10.1007/s00531-010-0611-5. Ketcham, R. A. (2005): Forward and inverse modeling of low-temperature thermochronometry data, in Low-Temperature Thermochronology: Techniques, Interpretations, and Applications, edited by P. W. Reiners and T. A. Ehlers, pp. 275-314, Mineralogical Society of America/Geochemical Society, Chantilly, Virginia. Ketcham, R. A., et al. (2007): Improved modeling of fission-track annealing in apatite, American Mineralogist, 92, 789-798. Ketcham, R. A., Donelick, R. A., Balestrieri, M. L., Zattin, M. (2009): Reproducibility of apatite fission-track length data and thermal history reconstruction, Earth and Planetary Science Letters 284 (2009), 504-515.

  6. A comparison between the megafauna communities on the N.W. Iberian and Celtic continental margins—effects of coastal upwelling?

    NASA Astrophysics Data System (ADS)

    Lavaleye, M. S. S.; Duineveld, G. C. A.; Berghuis, E. M.; Kok, A.; Witbaard, R.

    Megafauna biomass and feeding guilds were studied on the NW Iberian upwelling Continental Margin in order to determine the presence of enriched zones pointing to enhanced particle input. We compare these findings with similar data obtained from a transect across the Celtic Continental Margin that represents a regime without coastal upwelling. Additionally sediment concentrations of phytopigments (chlorophyll-a, phaeophorbides) representing recent inputs of algal production and of nucleic acids (DNA, RNA) are used as proxies for microbial biomass, to assess if there was a relation between these parameters and the megafauna distribution. The sediment on the upper slope (<1600 m) of the Iberian Margin was found to be inhabited by filter-feeding megafauna (26-73% of total invertebrate density, and 1-35% of biomass), and contained relatively low levels of phytopigments (3-6 ng/cm 3 chlorophyll-a) and nucleic acids (12-16 ?g -1 DNA, 1.5-3.5 ?g -1 RNA). In contrast, on the upper slope of the Celtic Margin the dominant component of the megafauna were deposit-feeders (57-92% of total invertebrate density, and 23-90% of biomass) and the sediments contained higher concentrations of phytopigments and nucleic acid. These observations, supplemented by video records revealing the presence of current ripples on the Iberian upper slope, show that these upper slope regions are non-depositional, high energy environments. Conditions at the lower slope and the abyssal plain on the Iberian transect were more quiescent with large deposit-feeding holothurians dominating the megafauna (72-94% of invertebrate biomass), and with relatively high sediment concentrations of phytopigments (7-9 ng/cm 3 chlorophyll-a, 157-170 ng/cm 3 phaeophorbides) and nucleic acids (21-38 ?g -1 DNA, 2.4-5.5 ?g -1 RNA). On the basis of our data we argue that the benthic food for the deepest stations on the Iberian transect does not consist of shelf derived organic matter. More likely, fast sinking offshore blooms, possibly associated with filaments of upwelling water, form the major contribution to the annual food supply of the deep living megafauna.

  7. Applying high-resolution sequence stratigraphic tools to the Texas continental margin to fine-tune conventional sequence stratigraphic models and improve reservoir prediction

    SciTech Connect

    Anderson, J.A.; Banfield, L.A.; Eckles, B.J. (Rice Univ., Houston, TX (United States)) (and others)

    1996-01-01

    A regional sequence stratigraphic study of the Texas continental margin resulted in an improved understanding of Pleistocene-Holocene eustatic sea level fluctuations and their influence on depositional environments associated with several distinct systems: the Colorado/Brazos system, the Texas interfluve region, and the Rio Grande system. The dataset consists of approximately 7500 kilometers of high-resolution seismic profiloes, 200 borehole descriptions, and 12 sediment cores. The study involved analysis of three independent variables to better constrain Pleistocene-Holocene sea level history: coastal onlap derived from high-resolution seismic profiles; oxygen isotope data determined from selected sediment cores; and paleoenvironmental data developed from high-resolution micropaleontologic analyses. Combined lithologic data and seismic facies analysis is used to map the distribution of major depositional systems on the shelf during one complete glacial eustatic cycle. Our study provided anopportunity to fine tune conventional stratigraphic models with regard to the distribution of potential reservoirs relative to regionally mappable stratigraphic surfaces.

  8. Applying high-resolution sequence stratigraphic tools to the Texas continental margin to fine-tune conventional sequence stratigraphic models and improve reservoir prediction

    SciTech Connect

    Anderson, J.A.; Banfield, L.A.; Eckles, B.J. [Rice Univ., Houston, TX (United States)] [and others

    1996-12-31

    A regional sequence stratigraphic study of the Texas continental margin resulted in an improved understanding of Pleistocene-Holocene eustatic sea level fluctuations and their influence on depositional environments associated with several distinct systems: the Colorado/Brazos system, the Texas interfluve region, and the Rio Grande system. The dataset consists of approximately 7500 kilometers of high-resolution seismic profiloes, 200 borehole descriptions, and 12 sediment cores. The study involved analysis of three independent variables to better constrain Pleistocene-Holocene sea level history: coastal onlap derived from high-resolution seismic profiles; oxygen isotope data determined from selected sediment cores; and paleoenvironmental data developed from high-resolution micropaleontologic analyses. Combined lithologic data and seismic facies analysis is used to map the distribution of major depositional systems on the shelf during one complete glacial eustatic cycle. Our study provided anopportunity to fine tune conventional stratigraphic models with regard to the distribution of potential reservoirs relative to regionally mappable stratigraphic surfaces.

  9. Continental arc magmatism in a Mesoproterozoic convergent margin: Petrological and geochemical constraints from the magmatic suite of Kondapalle along the eastern margin of the Indian plate

    NASA Astrophysics Data System (ADS)

    Rao, C. V. Dharma; Santosh, M.

    2011-09-01

    The magmatic suite at Kondapalle represents a Mesoproterozoic (~ ca. 1634 Ma) magmatic arc emplaced in the southern sector of the Eastern Ghats Belt (EGB). Here we present new geological, mineralogical and geochemical data on the various lithological units in this complex including anorthosites, gabbronorites and pyroxenites. The major mineral constituents in these rocks are plagioclase (An 98-57), amphibole (X Mg 0.93-0.52), orthopyroxene (X Mg 0.94-0.51), clinopyroxene (X Mg 0.93-0.63) and chromite (X Mg 0.20-50). The near-absence of plagioclase in the orthopyroxenites, early and abundant crystallization of orthopyroxene, and formation of gabbronorites rather than gabbro or olivine gabbro in the Kondapalle suite are correlated with the features of arc cumulates. The chemistry of chromian spinel and clinopyroxene also displays the trend for arc cumulates. The variations in the anorthite content of plagioclase vs. the Mg# of olivine attest to an arc-related magma source. The rocks display low abundance of incompatible trace elements (Ba, Rb, K and Zr) comparable to the values typically observed in subduction-related magmatic arcs. In trace element N-MORB normalized diagrams, all the rock units show Nb-Ta-Ti-Zr troughs reflecting the features characteristic of arc magmas. We interpret the Kondapalle rocks to represent the root zone of a deeply eroded magmatic arc built during the Mesoproterozoic associated with the subduction of an oceanic lithosphere in a long-lived convergent margin. We identify that the Mesoproterozoic subduction along the eastern margin of the Indian plate generated a wide arc-accretionary complex with an extruded high P-T metamorphic orogen during the final stage of collision. The subduction-accretion process is also supported by recent findings of Mesoproterozoic ophiolite mélanges from this zone, marking the history from the break-up of the Paleoproterozoic Columbia supercontinent to the assembly of the Neoproterozoic Rodinia supercontinent.

  10. Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid-Atlantic continental margin

    E-print Network

    ten Brink, Uri S.

    Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid Washington Canyon Norfolk Canyon Turbidity flow a b s t r a c t Shelf-sourced submarine canyons are common of major submarine canyons. Submarine canyons on passive margins are primarily the result of erosion

  11. Detailed investigation of continental shelf morphology using a high-resolution swath sonar survey: the Eel margin, northern California

    Microsoft Academic Search

    John A. Goff; Daniel L. Orange; Larry A. Mayer; John E. Hughes Clarke

    1999-01-01

    The Eel shelf, northern California, lies within an active compressional tectonic margin subject to abundant terrigenous sediment input from the Eel River. A recent high-resolution swath sonar survey provides us with the opportunity to investigate seafloor morphology and acoustic backscatter patterns within this dynamic region. Our analysis of the statistical character of bathymetry demonstrates a clear separation into large- and

  12. Disintegration of the continental margin of northwestern Gondwana: Late Devonian of the eastern Anti-Atlas (Morocco)

    Microsoft Academic Search

    Jobst Wendt

    1985-01-01

    The Devonian early Carboniferous sequence in the eastern Anti-Atlas represents a complete record of the last stage of the depositional and tectonic evolution along the northwestern margin of Gondwana. As a consequence of early Variscan block faulting, a platform and basin topography was established during the Middle and Late Devonian. Platforms were covered by condensed cephalopod limestones; sedimentation in the

  13. 41. TECTONISM AND VOLCANISM AT THE SOUTHEAST GREENLAND RIFTED MARGIN: A RECORD OF PLUME IMPACT AND LATER CONTINENTAL RUPTURE 1

    Microsoft Academic Search

    H. C. Larsen; A. D. Saunders

    During Ocean Drilling Program Leg 152, Sites 914 through 919 were drilled on the southeast Greenland Margin along a transect from the middle shelf into the adjacent deep-water Irminger Basin 500 km south of the Iceland hot-spot track (Iceland- Greenland Ridge). Sites 915 through 918 penetrated the entire cover of postrift sediments, and three of these four sites sample d

  14. Using subsidence and P-T-t history on the Alpine Tethys margin to constrain lithosphere deformation modes during continental breakup

    NASA Astrophysics Data System (ADS)

    Jeanniot, Ludovic; Kusznir, Nick; Manatschal, Gianreto; Mohn, Geoffroy; Beltrando, Marco

    2014-05-01

    Mantle exhumation and hyper-extended crust, as observed on the Iberia-Newfoundland conjugate margins, are key components of both present-day and fossil analogue magma-poor rifted margins. Conceptual models of the Alpine Tethys paleogeography evolution show a complex subsidence history, determined by the nature and composition of sedimentary, crustal and mantle rocks in the Alpine domains (Mohn et al., 2010). The relative timing of crustal rupture and decompressional melt initiation and inherited mantle composition control whether mantle exhumation may occur; the presence or absence of exhumed mantle therefore provides useful information on the timing of these events and constraints on lithosphere deformation modes and composition. A single mode of lithosphere deformation leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere deformation modes for the fossil Alpine Tethys margin using a numerical model of the temporal and spatial evolution of lithosphere deformation; the model has been calibrated against observations of subsidence and P-T-t history for the Alpine Tethys margin. A 2D finite element viscous flow model (FeMargin) is used to generate flow fields for a sequence of lithosphere deformation modes, which are used to advect lithosphere and asthenosphere temperature and material. FeMargin is kinematically driven by divergent deformation in the topmost 15-20 km of the lithosphere inducing passive upwelling beneath that layer; the upper lithosphere is assumed to deform by extensional faulting and magmatic intrusions, consistent with observations of deformation processes occurring at slow spreading ocean ridges (Cannat, 1996). We also include buoyancy enhanced upwelling in the kinematic model as proposed by Braun et al. (2000). We generate melt by decompressional melting using the parameterization and methodology of Katz et al. (2003). In the modelling of the Alpine Tethys margin, lithosphere deformation starts with simultaneous lithosphere extensional events to create both Valais and Piemont-Liguria rift basins. Buoyancy enhanced upwelling is required to generate uplift and emergence of the Briançonnais which is located between the Valais and Piemont-Liguria basins. Rifting in the Valais then ceases and the axis of extension and buoyant upwelling is focussed on the Piemont-Liguria basin leading to hyper-extension (e.g. Err and Bernina domains) and then to mantle exhumation (e.g. Platta domain). We explore the cause of the rapid post-uplift subsidence of the Briançonnais and show that this cannot be explained by thermal subsidence alone; crustal thinning (possibly depth-dependent) is also required. Particle tracking is used to predict P-T-t histories, which are tested against observations. Initial continental crust thickness and lithosphere temperature structure are important in controlling initial elevation and subsequent subsidence and depositional histories.

  15. Palaeomagnetism of flood basalts in the Pilbara Craton, Western Australia: Late Archaean continental drift and the oldest known

    E-print Network

    Utrecht, Universiteit

    continental drift and the oldest known reversal of the geomagnetic field Geert Strik,1 Tim S. Blake,2 Tanja E continental drift and the oldest known reversal of the geomagnetic field, J. Geophys. Res., 108(B12), 2551 of terrestrial continental flood basalts, mafic tuffs, felsic volcanic rocks, and clastic sedimentary rocks

  16. Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America

    NASA Technical Reports Server (NTRS)

    Patino Douce, Alberto E.; Humphreys, Eugene D.; Johnston, A. Dana

    1990-01-01

    This paper presents a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening exemplified by the Sevier hinterland in western North America, and uses the model to examine the geological and physical processes leading to crustally derived magmatism. The results of numerical experiments show that anatexis was an inevitable end-product of Barrovian metamorphism in the thickened crust of the late Mesozoic Sevier orogenic belt and that the advection of heat across the lithosphere, in the form of mantle-derived mafic magmas, was not required for melting of metasedimentary rocks. It is suggested that, in the Sevier belt, as in other intracontinental orogenic belts, anatexis occurred in the midcrust and not at the base of the crust.

  17. OCEAN, ICE, AND ATMOSPHERE: INTERACTIONS AT THE ANTARCTIC CONTINENTAL MARGIN ANTARCTIC RESEARCH SERIES, VOLUME 75, PAGES 215-240

    E-print Network

    Gordon, Arnold L.

    Ice Shelf. It snakes northward near the 3000 m isobath, descending to depths >4000 m near 65°S, 50°W of the warm core in this sector and the absence of strong mixing inhibits vertical heat flux in the western of the coldest, densest ocean water masses of global importance:Weddell Sea Bottom Water (WSBW), Copyright 1998

  18. Deep seismic reflection data of EDGE US mid-Atlantic continental-margin experiment: Implications for Appalachian sutures and Mesozoic rifting and magmatic underplating

    SciTech Connect

    Sheridan, R.E.; Musser, D.L. [Rutgers Univ., New Brunswick, NJ (United States)] [Rutgers Univ., New Brunswick, NJ (United States); Glover, L. III [Virginia Polytechnic Institute and State Univ., Blacksburg, VA (United States)] [and others] [Virginia Polytechnic Institute and State Univ., Blacksburg, VA (United States); and others

    1993-06-01

    The EDGE seismic experiment across the Virginia continental margin delineated a Paleozoic suture, buried Appalachian terranes, and Mesozoic rifting and magmatic events. The seismic grid revealed that the Mesozoic Norfolk rift basin exists only in the northern one-third of the previously mapped area. The north-striking listric border fault of the Norfolk basin half-graben parallels seismic laminations in the basement. The Jurassic volcanic wedge pinches out just landward of the Baltimore Canyon trough hinge zone and downlaps on the hummocky oceanic basement under the continental rise. Under the continental slope, the volcanic wedge reaches depths >9 s (20 km). Two distinct intracrustal reflections at 4.0-5.0 s and at 7.0 s TWITT (two-way traveltime) dip southeastward at low angles ({approximately}15{degrees}). The Moho reflection is disrupted where it is intersected by the 7.0 s reflection. Northwest of this point the Moho dips landward; seaward it is horizontal. Seaward of this point, the lower-crustal boundary laminations exist in a narrow interval (10.5-11.0 s) and are of strong amplitude. These changes in the Moho and lower crust represent the seaward edge of the Grenville-age North American crust and the landward edge of Jurassic magmatic underplating. A northwest-dipping reflection observed for the first time on the US Atlantic margin may be the top of the Jurassic magmatic-underplating layer; the northwest-dipping reflection truncates the southeast-dipping 7.0 s TWITT reflection. Landward projection of the 7.0 s reflection yields a north-south trace on the postrift unconformity under the center of lower Chesapeake Bay. This trace is near a basement fault between low-grade metamorphic rocks (Carolina slate-Avalonia) on the east and high-grade rocks (Goochland terrane) on the west. This fault boundary and the southeast-dipping 7.0 s reflection probably represent the Taconic suture. 23 refs., 5 figs.

  19. Evolution of the South Atlantic passive continental margin<