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Sample records for proterozoic continental margin

  1. Tectonics and sedimentation of late Proterozoic Damaran convergent continental margin, Khomas Hochland, central Namibia

    International Nuclear Information System (INIS)

    Kukla, P.A.

    1990-01-01

    The Late Proterozoic Khomas Trough comprises metasedimentary and metabasic rocks of the Kuiseb Formation and its evolution plays an important role in understanding the Damara orogenic belt as a whole. An attempt was made to characterize the metasedimentary and metabasic rocks encountered by means of sedimentological, structural, petrographical, mineralogical and geochemical methods. This led to the modelling of the geotectonic evolution of the Khomas Trough with particular focus on the sedimentary palaeoenvironments and the structural evolution of this area during Pan-African orogeny in Namibia. 251 refs., 81 figs., 14 tabs

  2. Deep continental margin reflectors

    Science.gov (United States)

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

    1985-01-01

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

  3. Reconstructing Rodinia by Fitting Neoproterozoic Continental Margins

    Science.gov (United States)

    Stewart, John H.

    2009-01-01

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

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

    International Nuclear Information System (INIS)

    Preciozzi, F

    2014-01-01

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

  5. Atlantic continental margin of the United States

    Science.gov (United States)

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

    1982-01-01

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

  6. Contributions to knowledge of the continental margin of Uruguay. Description of background samples in the continental margin of Uruguay

    International Nuclear Information System (INIS)

    Preciozzi, F

    2015-01-01

    This study provide data concerning of the background sediments of the continental margin of Uruguay. There were carried out different works with witnesses in order to extract various sediment samples from the continental shelf

  7. Glacier-influenced sedimentation on high-latitude continental margins

    National Research Council Canada - National Science Library

    Dowdeswell, J. A; Cofaigh, C. Ó

    2002-01-01

    This book examines the process and patterns of glacier-influenced sedimentation on high-latitude continental margins and the geophysical and geological signatures of the resulting sediments and landform...

  8. U.S. East Coast Continental Margin (CONMAR) Sediment Data

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The USGS/WHOI Continental Margin (CONMAR) Data set was compiled by the U.S. Geological Survey and the Woods Hole Oceanographic Institution as a joint program of...

  9. Exploration of the continental margins of India

    Digital Repository Service at National Institute of Oceanography (India)

    Siddiquie, H.N.; Hashimi, N.H.; Vora, K.H.; Pathak, M.C.

    impetus from sponsored surveys of other organizations, chiefly the oil industry, ports and harbours as well as industries disposing of their effluents in the marine environment. By now the entire western continental shelf and a large part...

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

    Science.gov (United States)

    Basile, Christophe

    2010-05-01

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

  11. Seaward dipping reflectors along the SW continental margin of India: Evidence for volcanic passive margin

    Digital Repository Service at National Institute of Oceanography (India)

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

    Multi-channel seismic reflection profiles across the southwest continental margin of India (SWCMI) show presence of westerly dipping seismic reflectors beneath sedimentary strata along the western flank of the Laccadive Ridge-northernmost part...

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

    Energy Technology Data Exchange (ETDEWEB)

    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.

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

    Science.gov (United States)

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

    1980-01-01

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

  14. Early diagenesis of phosphorus in continental margin sediments

    NARCIS (Netherlands)

    Slomp, C.P.

    1997-01-01

    Most of the organic material in the oceans that reaches the sea floor is deposited on continental margins and not in the deep sea. This organic matter is the principal carrier of phosphorus (P) to sediments. A part of the organic material is buried definitely. The other part decomposes,

  15. Ooid turbidites from the Central Western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, P.S.

    Gravity displaced debris flows/turbidites have been observed in five box cores collected between water depths of 649 and 3,627 m from the Central Western continental margin of India. Studies on grain size, carbonate content, and coarse fraction...

  16. Slumping on the western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Guptha, M.V.S.; Mohan, R.; Muralinath, A.S.

    continental margin is believed to have set in motion during the beginning of Holocene. Besides, it infers that after 6 k.y. B.P. the magnitude of slumping is minimal. The slumping may be attributed to the evolution of methane gas as one of the important causes...

  17. Contributions to knowledge of the continental margin of Uruguay. Uruguayan continental margin: morphology, geology and identification of the base of the slope

    International Nuclear Information System (INIS)

    Preciozzi, F.

    2014-01-01

    This work is about the morphology, geology and the identification of the base of the slope in the The Uruguayan continental margin which corresponds to the the type of divergent, volcanic and segmented margins. Morphologically is constituted by a clearly defined continental shelf, as well as a continental slope that presents configuration changes from north to south and passes directly to the abyssal plain

  18. Sapphirine granulites from Panasapattu, Eastern Ghats belt, India: Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin

    Directory of Open Access Journals (Sweden)

    C.V. Dharma Rao

    2012-01-01

    of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at p < 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the p–T path followed a near isobaric cooling stage to T < 900 °C. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subduction–accretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subduction–collision tectonics in a Proterozoic convergent plate margin.

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

    Science.gov (United States)

    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

  20. Geological features and geophysical signatures of continental margins of India

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.

    of crustal movement had been introduced to the scientific community; and it has laid the groundwork for the development of modern plate tectonics. As years passed, more and more evidences were uncovered to support the idea that the plates were moving... to build-up of the wide and low-relief (flat) continental shelf (covered by shelf seas), slope and rise. Initially passive margins form at divergent plate boundary following break-up of the continent, then they move away with the accretion of new...

  1. Preface: Biogeochemistry–ecosystem interaction on changing continental margins in the Anthropocene

    Digital Repository Service at National Institute of Oceanography (India)

    Liu, K-K.; Emeis, K.-C.; Levin, L.A.; Naqvi, S.W.A.; Roman, M.

    and hypercapnia in upwelling systems • Interactions between natural and social sciences for better steward- ship of continental margins. It has long been acknowledged (e.g., Doney, 2010; Liu et al., 2010) that marine ecosystems on continental margins, including... and possibly manage margin ecosystems in a changing world. Effective governance of social–ecological systems on continental margins is key to reducing the pervasive over- exploitation, depletion and destruction of marine resources and http://dx.doi.org/10...

  2. Tectonostratigraphy of the Passive Continental Margin Offshore Indus Pakistan

    Science.gov (United States)

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

    2017-12-01

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

  3. Anomalous heat flow belt along the continental margin of Brazil

    Science.gov (United States)

    Hamza, Valiya M.; Vieira, Fabio P.; Silva, Raquel T. A.

    2018-01-01

    A comprehensive analysis of thermal gradient and heat flow data was carried out for sedimentary basins situated in the continental margin of Brazil (CMB). The results point to the existence of a narrow belt within CMB, where temperature gradients are higher than 30 °C/km and the heat flow is in excess of 70 mW/m2. This anomalous geothermal belt is confined between zones of relatively low to normal heat flow in the adjacent continental and oceanic regions. The width of the belt is somewhat variable, but most of it falls within the range of 100-300 km. The spatial extent is relatively large in the southern (in the basins of Pelotas, Santos and Campos) and northern (in the basins of Potiguar and Ceará) parts, when compared with those in the central parts (in the basins of South Bahia, Sergipe and Alagoas). The characteristics of heat flow anomalies appear to be compatible with those produced by thermal sources at depths in the lower crust. Hence, magma emplacement at the transition zone between lower crust and upper mantle is considered the likely mechanism producing such anomalies. Seismicity within the belt is relatively weak, with focal depths less than 10 km for most of the events. Such observations imply that "tectonic bonding" between continental and oceanic segments, at the transition zone of CMB, is relatively weak. Hence, it is proposed that passive margins like CMB be considered as constituting a type of plate boundary that is aseismic at sub-crustal levels, but allows for escape of significant amounts of earth's internal heat at shallow depths.

  4. Archean and proterozoic continental crust in South America: Main building events

    International Nuclear Information System (INIS)

    Fuck, R.A; Brito Neves, B.B; Pimentel, M.M

    2001-01-01

    Available geochronological data reveal that the first building blocks of the South American continental crust were set up in the early Paleoarchean, ca. 3.4-3.5 Ga ago, although the presence of components as old as 3.7 Ga is indicated by Nd TDM model ages. The oldest rocks so far recognized are exposed in northeast Brazil and Uruguay. In the Sao Jose do Campestre block, Rio Grande do Norte, 3.45 Ga old tonalite, migmatized and intruded by granitoids between 3.3 and 3.0 Ga, is part of the basement to the Borborema Province (Dantas et al. 1998). In Bahia 3.42 Ga old tonalitic grey gneisses of Sete Voltas, Boa Vista, and Mairi form the basement of the Gaviao block, within the core of the Sao Francisco Craton (Nutman and Cordani, 1993, Martin et al., 1997). The Paleoarchean TTG suites as well as greenstone remnants of unknown age were involved in crust accretion events between 3.1 and 3.3 Ga ago (Teixeira et al. 2000 and references therein), which are also recorded in Campo Belo and Uaua (Teixeira et al., 1998, Oliveira et al., 1999), as attested by TTG intrusions and the ca. 3.1 Ga Pium-hi greenstone belt of W Minas Gerais (Machado and Schrank 1989). Microcontinents then formed were involved in deformation, metamorphism, and migmatization around 2.8-3.0 Ga ago, probably during amalgamation events. Widespread granite-greenstone associations in the Quadrilatero Ferrifero and other areas represent new crust built during the very important Neoarchean Rio das Velhas cycle, ca. 2.7-2.8 Ga ago (Machado and Carneiro 1992, Machado et al. 1992). Layered mafic-ultramafic and granite intrusions ca. 2.5-2.7 Ga old are recorded all over the Sao Francisco Craton, including the high-grade terrain of southern Bahia, formed during the late Archean Jequie Cycle (Teixeira et al. 2000 and references therein). Similar intrusions are recorded in many basement areas within Neoproterozoic fold belts (au)

  5. Proterozoic atmospheric oxygen

    DEFF Research Database (Denmark)

    Canfield, Donald Eugene

    2014-01-01

    This article is concerned with the evolution of atmospheric oxygen concentrations through the Proterozoic Eon. In particular, this article will seek to place the history of atmospheric oxygenation through the Proterozoic Eon in the context of the evolving physical environment including the history...... of continental growth and volcanic outgassing, as well as biogeochemical processing of elements within the oceans. The author will seek to explore constraints on the history of oxygenation and understand which processes have regulated oxygen through this eon....

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

    Digital Repository Service at National Institute of Oceanography (India)

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

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

  7. Mapping Mesophotic Reefs Along the Brazilian Continental Margin

    Science.gov (United States)

    Bastos, A.; Moura, R.; Amado Filho, G.; Ferreira, L.; Boni, G.; Vedoato, F.; D'Agostini, D.; Lavagnino, A. C.; Leite, M. D.; Quaresma, V.

    2017-12-01

    Submerged or drowned reefs constitute an important geological record of sea level variations, forming the substrate for the colonization of modern benthic mesophotic communities. Although mapping mesophotic reefs has increased in the last years, their spatial distribution is poorly known and the worldwide occurrence of this reef habitat maybe underestimated. The importance in recognizing the distribution of mesophotic reefs is that they can act as a refuge for corals during unsuitable environmental conditions and a repository for shallow water corals. Here we present the result of several acoustic surveys that mapped and discovered new mesophotic reefs along the Eastern and Equatorial Brazilian Continental Margin. Seabed mapping was carried out using multibeam and side scan sonars. Ground truthing was obtained using drop camera or scuba diving. Mesophotic reefs were mapped in water depths varying from 30 to 100m and under distinct oceanographic conditions, especially in terms of river load input and shelf width. Reefs showed distinct morphologies, from low relief banks and paleovalleys to shelf edge ridges. Extensive occurrence of low relief banks were mapped along the most important coralline complex province in the South Atlantic, the Abrolhos Shelf. These 30 to 40m deep banks, have no more than 3 meters in height and may represent fringing reefs formed during sea level stabilization. Paleovalleys mapped along the eastern margin showed the occurrence of coralgal ledges along the channel margins. Paleovalleys are usually deeper than 45m and are associated with outer shelf rhodolith beds. Shelf edge ridges (80 to 120m deep) were mapped along both margins and are related to red algal encrusting irregular surfaces that have more than 3m in height, forming a rigid substrate for coral growth. Along the Equatorial Margin, off the Amazon mouth, shelf edge patch reefs and rhodolith beds forming encrusting surfaces and shelf edge ridges were mapped in water depths greater

  8. The Role of the Submarine Channel Pernambuco in the Brazilian Continental Margin East

    International Nuclear Information System (INIS)

    Torres, L.; Villena, H.

    2010-01-01

    The Brazilian Continental Margin, which coastline measures more than 8,500km gives to Brazil continental dimensions. This huge region is conditioned by the action of process such as, sedimentals, tectonics, geomorphological and climatical, as example, which direct or in conjunction with other ones, since of continental break up between South America and Africa are going on and may be responsible for the current morphology of the margin. In accordance with this point of view, the Oriental part of the Brazilian Continental Margin, presents characteristics of a passive margin and fisiographically ''starved'', in which the continental break occur no more than 100km from de coastline and the sedimentary coverage is mainly carbonatic. The continental slope does not present great extension if compared with other parts of the Brazilian Margin and sharp gradient. The remark presence of the continental plateaus (Rio Grande Plateau and Pernambuco Plateau), which link with the continental rise and additionally the Paraiba, Pernambuco e Bahia seamounts, are the majors features in the morphology of the region between the slope and the continental rise. This paper will concentrate its focus on Bahia Seamount, with emphasis in the mainly erosive feature which cut transversally the seamounts, named Pernambuco Submarine Channel. It will be employed bathymetric multibeam and seismic data carried out by the Brazilian Continental Shelf Project (LEPLAC) in the current year and pieces of information from bibliographic researches in order to present a discussion by the hole of the Pernambuco Submarine Channel in the Occidental region of the Brazilian Continental Margin

  9. Geophysical studies over the continental margins of the east coast of India

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, T.C.S.; Rao, V.B.

    . Continuity of surface and subsurface features from profile to profile is indicated. Two major structural elements - viz. The "Marginal High", situated at the foot of the continental slope and extending along the east coast of India, and the "Marginal Basin...

  10. Particle flux across the mid-European continental margin

    CERN Document Server

    Antia, A N; Peinert, R

    1999-01-01

    Results are presented from particle flux studies using sediment trap and current meter moorings along a transect at the European continental margin at 49 degrees N within the Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid- slope follow the slope contour, whereas seasonal off-slope flow was registered at the outer slope. At 600 m on the slope fluxes are similar to those in the abyssal North Atlantic. The flux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest fluxes were recorded at 1440 m at the outer slope, where off- slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral fluxes of partic...

  11. Sedimentation on continental margins: An integrated program for innovative studies during the 1990s

    Science.gov (United States)

    Nittrourer, Charles A.; Coleman, James M.; Rouge, Baton; Flood, Roger D.; Ginsburg, Robert N.; Gorsline, Donn S.; Hine, Albert C.; Sternberg, Richard W.; Swift, Donald J. P.; Wright, L. Donelson

    Continental margins are of great scientific interest, and they represent the focus of human interaction with the ocean. Their deep structure forms the transition from continental to oceanic crust, and their surface expression extends from coastal environments of estuaries and shorelines across the continental shelf and slope to either the base of a continental rise or a marginal trough. Modern continental margins represent natural laboratories for investigation of complex relationships between physical, chemical, and biological phenomena, which are sensitive to environmental conditions both on the land and in the ocean. The history of these conditions is preserved within the sedimentary deposits of continental margins. The deposits form repositories for much of the particulate material transported off the world's land masses and produced from dissolved components in the world ocean. Past deposits of continental margins have been uplifted to form many mountain ranges and sedimentary terrains of the world, which record details of Earth history and contain valuable natural resources, such as petroleum and natural gas. Modern deposits of continental margins record the more recent events that have influenced Earth and also contain natural resources (for instance, minerals, sand, and gravel), as well as anthropogenic pollutants (for example, heavy metals and pesticides). The fates of many materials beneficial and deleterious to humans are dependent on the pathways followed by sedimentary particles on continental margins.

  12. The Pan-African continental margin in northeastern Africa: Evidence from a geochronological study of granulites at Sabaloka, Sudan

    International Nuclear Information System (INIS)

    Kroener, A.

    1987-01-01

    Ion microprobe zircon ages, a Nd model age and Rb-Sr whole-rock dates are reported from the high-grade gneiss terrain at Sabaloka on the River Nile north of Khartoum, formally considered to be part of the Archaean/early Proterozoic Nile craton. The granulites, which are of both sedimentary and igneous derivation, occur as remnants in migmatites. Detrital zircon ages range from ≅ 1000 to ≅ 2650 Ma and prove the existence of Archaean to late Proterozoic continental crust in the sedimentary source region. The Nd model age for one sedimentary granulite is between 1.26 (T CHUR ) and 1.70 (T DM ) Ga and provides a mean crustal residence age for the sedimentary precursor. Igneous zircons in enderbitic gneiss crystallized at 719±81 Ma ago, an age that also corresponds to severe Pb loss in the detrital zircons and whic probably reflects the granulite event at Sabaloka. The Rb-Sr data indicate isotropic homogenization at about 700 Ma ago in the granulites and severe post-granulite disturbance at ≅ 570 Ma in the migmatites. We associate this disturbance with hydration, retrograde metamorphism and anatexis that produced undeformed granites ≅ 540 Ma ago. The ≅ 700 Ma granulite event at Sabaloka suggests that this part of the Sudan belongs to the Pan-African Mozambique belt while the ancient Nile craton lay farther west. The gneisses studied here may represent the infrastructure of the ancient African continental margin onto which the juvenile arc assemblage of the Arabian-Nubian shield was accreted during intense horizontal shortening and crustal interstacking of a major collision event. (orig.)

  13. Climatic controls on arid continental basin margin systems

    Science.gov (United States)

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

    2016-04-01

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

  14. Organic carbon-sulfur relationships in sediment cores from the western and eastern continental margins of India

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, P.S.; Mascarenhas, A.; Paropkari, A.L.; Rao, Ch.M.

    Two sediment cores from the western continental margin (WMI, WM2) and one core from the eastern continental margin (EM) of India have been analysed to determine the relativ importance of factors such as oxidizing/reducing environment, mass...

  15. A Spatial Model of Erosion and Sedimentation on Continental Margins

    National Research Council Canada - National Science Library

    Pratson, Lincoln

    1999-01-01

    .... A computer model that simulates the evolution of continental slope morphology under the interaction of sedimentation, slope failure, and sediment flow erosion has been constructed and validated...

  16. Proterozoic intracontinental basin succession in the western margin of the São Francisco Craton: Constraints from detrital zircon geochronology

    Science.gov (United States)

    Martins-Ferreira, Marco Antonio Caçador; Chemale, Farid, Jr.; Dias, Airton Natanael Coelho; Campos, José Eloi Guimarães

    2018-01-01

    former members to formation status, representing an individual sag-type basin, different from the Araí rift-type basin. We also propose to combine the three Proterozoic intracontinental extensional pulses (which generated the Araí, Traíras and Paranoá groups) as the Veadeiros Supergroup, respectively chronocorrelated to the lower, middle and upper Espinhaço Supergroup, located at the SFC eastern margin.

  17. Scenario of gas-charged sediments and gas hydrates in the western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Karisiddaiah, S.M.; SubbaRaju, L.V.

    Echosounding, high-resolution shallow seismic data were collected along track lines spaced at 20 km interval across the western continental margin of India. A detailed analysis of the underway data revealed the occurrence of methane-bearing gas...

  18. Macrofaunal community structure in the western Indian continental margin including the oxygen minimum zone

    Digital Repository Service at National Institute of Oceanography (India)

    Ingole, B.S.; Sautya, S.; Sivadas, S.; Singh, R.; Nanajkar, M.

    (H`) showed a significant negative (P < 0.01) relationship between sediment Chl-a and C sub(org), suggesting food availability as a critical factor in species dominance. Results of multivariate analyses suggest that for continental margin fauna...

  19. Holocene sea level fluctuations on western Indian continental margin: An update

    Digital Repository Service at National Institute of Oceanography (India)

    Hashimi, N.H.; Nigam, R.; Nair, R.R.; Rajagopalan, G.

    A new Holocene curve is generated for the western Indian continental margin. While constructing this curve careful selection of the dates were made by giving due considerations to the genetic characteristics of the dated material. This new curve...

  20. Aeolian deposition of Arabia and Somalia sediments on the southwestern continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Chauhan, O.S.

    Kaolinite, smectite, illite and chlorite as major clay minerals and palygorskite and gibbsite in minor quantities have been recorded from the slope of southwestern continental margin of India. Contribution of kaolinite, smectite and gibbsite is from...

  1. Nutrient Distributions, Transports, and Budgets on the Inner Margin of a River-Dominated Continental Shelf

    Science.gov (United States)

    2013-10-02

    Nutrient distributions, transports, and budgets on the inner margin of a river-dominated continental shelf John C. Lehrter,1 Dong S. Ko,2 Michael C...and D. C. Biggs (1993), The influence of advec- tion on the spatial variability of nutrient concentrations on the Texas- Louisiana continental shelf

  2. Wintertime phytoplankton bloom in the Subarctic Pacific supported by continental margin iron

    International Nuclear Information System (INIS)

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

    2004-01-01

    Heightened biological activity was observed in February 1996 in the high-nutrient low-chlorophyll (HNLC) subarctic North Pacific Ocean, a region that is thought to be iron-limited. Here we provide evidence supporting the hypothesis that Ocean Station Papa (OSP) in the subarctic Pacific received a lateral supply of particulate iron from the continental margin off the Aleutian Islands in the winter, coincident with the observed biological bloom. Synchrotron X-ray analysis was used to describe the physical form, chemistry, and depth distributions of iron in size fractionated particulate matter samples. The analysis reveals that discrete micron-sized iron-rich hotspots are ubiquitous in the upper 200m at OSP, more than 900km from the closest coast. The specifics of the chemistry and depth profiles of the Fe hot spots trace them to the continental margins. We thus hypothesize that iron hotspots are a marker for the delivery of iron from the continental margin. We confirm the delivery of continental margin iron to the open ocean using an ocean general circulation model with an iron-like tracer source at the continental margin. We suggest that iron from the continental margin stimulated a wintertime phytoplankton bloom, partially relieving the HNLC condition

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

    Science.gov (United States)

    Mosher, David

    2016-04-01

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

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

    NARCIS (Netherlands)

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

    2010-01-01

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

  5. Gravity anomalies over a segment of Pratap ridge and adjoining shelf margin basin, western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Subrahmanyam, V.; Rao, D.G.; Ramprasad, T.; KameshRaju, K.A.; Rao, M.G.

    Bathymetric and gravity data totalling 2000 line km on the continental margin off Goa and Mulki, west of India have been studied. The free-air gravity anomalies vary between -60 to 25 mgals with prominent NNW-SSE trends in the outer shelf region...

  6. Influence of dynamic topography on landscape evolution and passive continental margin stratigraphy

    Science.gov (United States)

    Ding, Xuesong; Salles, Tristan; Flament, Nicolas; Rey, Patrice

    2017-04-01

    Quantifying the interaction between surface processes and tectonics/deep Earth processes is one important aspect of landscape evolution modelling. Both observations and results from numerical modelling indicate that dynamic topography - a surface expression of time-varying mantle convection - plays a significant role in shaping landscape through geological time. Recent research suggests that dynamic topography also has non-negligible effects on stratigraphic architecture by modifying accommodation space available for sedimentation. In addition, dynamic topography influences the sediment supply to continental margins. We use Badlands to investigate the evolution of a continental-scale landscape in response to transient dynamic uplift or subsidence, and to model the stratigraphic development on passive continental margins in response to sea-level change, thermal subsidence and dynamic topography. We consider a circularly symmetric landscape consisting of a plateau surrounded by a gently sloping continental plain and a continental margin, and a linear wave of dynamic topography. We analyze the evolution of river catchments, of longitudinal river profiles and of the χ values to evaluate the dynamic response of drainage systems to dynamic topography. We calculate the amount of cumulative erosion and deposition, and sediment flux at shoreline position, as a function of precipitation rate and erodibility coefficient. We compute the stratal stacking pattern and Wheeler diagram on vertical cross-sections at the continental margin. Our results indicate that dynamic topography 1) has a considerable influence on drainage reorganization; 2) contributes to shoreline migration and the distribution of depositional packages by modifying the accommodation space; 3) affects sediment supply to the continental margin. Transient dynamic topography contributes to the migration of drainage divides and to the migration of the mainstream in a drainage basin. The dynamic uplift

  7. Continental margin of Andhra Pradesh: Some new problems and perspectives

    Digital Repository Service at National Institute of Oceanography (India)

    Murthy, K.S.R.

    the Krishna - Godavari basin and the Nizampatnam Bay. Extensive bathymetric, magnetic, high resolution seismic reflection and gravity data were collected over the margin by the National Institute of Oceanography for the last one decade. Analysis of the data...

  8. Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

    KAUST Repository

    Laruelle, G. G.; Dü rr, H. H.; Lauerwald, R.; Hartmann, J.; Slomp, C. P.; Goossens, N.; Regnier, P. A. G.

    2013-01-01

    Past characterizations of the land-ocean continuum were constructed either from a continental perspective through an analysis of watershed river basin properties (COSCATs: COastal Segmentation and related CATchments) or from an oceanic perspective, through a regionalization of the proximal and distal continental margins (LMEs: large marine ecosystems). Here, we present a global-scale coastal segmentation, composed of three consistent levels, that includes the whole aquatic continuum with its riverine, estuarine and shelf sea components. Our work delineates comprehensive ensembles by harmonizing previous segmentations and typologies in order to retain the most important physical characteristics of both the land and shelf areas. The proposed multi-scale segmentation results in a distribution of global exorheic watersheds, estuaries and continental shelf seas among 45 major zones (MARCATS: MARgins and CATchments Segmentation) and 149 sub-units (COSCATs). Geographic and hydrologic parameters such as the surface area, volume and freshwater residence time are calculated for each coastal unit as well as different hypsometric profiles. Our analysis provides detailed insights into the distributions of coastal and continental shelf areas and how they connect with incoming riverine fluxes. The segmentation is also used to re-evaluate the global estuarine CO2 flux at the air-water interface combining global and regional average emission rates derived from local studies. © 2013 Author(s).

  9. Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

    Directory of Open Access Journals (Sweden)

    G. G. Laruelle

    2013-05-01

    Full Text Available Past characterizations of the land–ocean continuum were constructed either from a continental perspective through an analysis of watershed river basin properties (COSCATs: COastal Segmentation and related CATchments or from an oceanic perspective, through a regionalization of the proximal and distal continental margins (LMEs: large marine ecosystems. Here, we present a global-scale coastal segmentation, composed of three consistent levels, that includes the whole aquatic continuum with its riverine, estuarine and shelf sea components. Our work delineates comprehensive ensembles by harmonizing previous segmentations and typologies in order to retain the most important physical characteristics of both the land and shelf areas. The proposed multi-scale segmentation results in a distribution of global exorheic watersheds, estuaries and continental shelf seas among 45 major zones (MARCATS: MARgins and CATchments Segmentation and 149 sub-units (COSCATs. Geographic and hydrologic parameters such as the surface area, volume and freshwater residence time are calculated for each coastal unit as well as different hypsometric profiles. Our analysis provides detailed insights into the distributions of coastal and continental shelf areas and how they connect with incoming riverine fluxes. The segmentation is also used to re-evaluate the global estuarine CO2 flux at the air–water interface combining global and regional average emission rates derived from local studies.

  10. Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

    KAUST Repository

    Laruelle, G. G.

    2012-10-04

    Past characterizations of the land–ocean continuum were constructed either from a continental perspective through an analysis of watershed river basin properties (COSCATs: COastal Segmentation and related CATchments) or from an oceanic perspective, through a regionalization of the proximal and distal continental margins (LMEs: large marine ecosystems). Here, we present a global-scale coastal segmentation, composed of three consistent levels, that includes the whole aquatic continuum with its riverine, estuarine and shelf sea components. Our work delineates comprehensive ensembles by harmonizing previous segmentations and typologies in order to retain the most important physical characteristics of both the land and shelf areas. The proposed multi-scale segmentation results in a distribution of global exorheic watersheds, estuaries and continental shelf seas among 45 major zones (MARCATS: MARgins and CATchments Segmentation) and 149 sub-units (COSCATs). Geographic and hydrologic parameters such as the surface area, volume and freshwater residence time are calculated for each coastal unit as well as different hypsometric pro- files. Our analysis provides detailed insights into the distributions of coastal and continental shelf areas and how they connect with incoming riverine fluxes. The segmentation is also used to re-evaluate the global estuarine CO2 flux at the air–water interface combining global and regional average emission rates derived from local studies.

  11. Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

    KAUST Repository

    Laruelle, G. G.

    2013-05-29

    Past characterizations of the land-ocean continuum were constructed either from a continental perspective through an analysis of watershed river basin properties (COSCATs: COastal Segmentation and related CATchments) or from an oceanic perspective, through a regionalization of the proximal and distal continental margins (LMEs: large marine ecosystems). Here, we present a global-scale coastal segmentation, composed of three consistent levels, that includes the whole aquatic continuum with its riverine, estuarine and shelf sea components. Our work delineates comprehensive ensembles by harmonizing previous segmentations and typologies in order to retain the most important physical characteristics of both the land and shelf areas. The proposed multi-scale segmentation results in a distribution of global exorheic watersheds, estuaries and continental shelf seas among 45 major zones (MARCATS: MARgins and CATchments Segmentation) and 149 sub-units (COSCATs). Geographic and hydrologic parameters such as the surface area, volume and freshwater residence time are calculated for each coastal unit as well as different hypsometric profiles. Our analysis provides detailed insights into the distributions of coastal and continental shelf areas and how they connect with incoming riverine fluxes. The segmentation is also used to re-evaluate the global estuarine CO2 flux at the air-water interface combining global and regional average emission rates derived from local studies. © 2013 Author(s).

  12. Development of continental margins of the Atlantic Ocean and successive breakup of the Pangaea-3 supercontinent

    Science.gov (United States)

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

    2017-01-01

    Comparative tectonic analysis of passive margins of the Atlantic Ocean has been performed. Tectonotypes of both volcanic and nonvolcanic margins are described, and their comparison with other passive Atlantic margins is given. The structural features of margins, peculiarities of magmatism, its sources and reasons for geochemical enrichment of melts are discussed. The important role of melting of the continental lithosphere in the development of magmatism is demonstrated. Enriched EM I and EM II sources are determined for the lower parts of the volcanic section, and a depleted or poorly enriched source is determined for the upper parts of the volcanic section based on isotope data. The conclusions of the paper relate to tectonic settings of the initial occurrence of magmatism and rifting and breakup during the period of opening of the Mesozoic Ocean. It was found out that breakup and magmatism at proximal margins led only to insignificant structural transformations and reduction of the thickness of the ancient continental crust, while very important magmatic events happened later in the distal zone. New growth of magmatic crust at the stage of continental breakup is determined as a typical feature of distal zones of the margins under study. The relationship of development of margins with the impact of deep plumes as the source of magmatic material or a heat source only is discussed. Progradation of the zone of extension and breakup into the areas of cold lithosphere of the Atlantic and the formation of a single tectonomagmatic system of the ocean are under consideration.

  13. Comparative biogeochemistry–ecosystem–human interactions on dynamic continental margins..

    Digital Repository Service at National Institute of Oceanography (India)

    Levin, L.A.; Liu, K-K.; Emeis, K.-C.; Breitburg, D.L.; Cloern, J.; Deutsch, C.; Giani, M.; Goffart, A.; Hofmann, E.E.; Lachkar, Z.; Limburg, K.; Liu, Su-Mei; Montes, E.; Naqvi, S.W.A.; Ragueneau, O.; Rabouille, C.; Sarkar, S.K.; Swaney, D.P.; Wassman, P.; Wishner, K.F.

    aswitnessed by diverse case studies (Table 1, Fig. 1). Intensified nitrogen loading is widespread in coastal ecosystems receiving effluents from catchments with dense human populations (Glavovic et al., submitted for publication; Rabalais, 2004). This yields... margins high-frequency climate oscillations are the domi- nant driver of biogeochemical variation and consequently, ecosystem structure. In the Bay of Calvi in the Ligurian Sea of the NWMediterra- nean (Goffart et al., submitted for publication...

  14. Benthic assemblages of mega epifauna on the Oregon continental margin

    Science.gov (United States)

    Hemery, Lenaïg G.; Henkel, Sarah K.; Cochrane, Guy R.

    2018-01-01

    Environmental assessment studies are usually required by a country's administration before issuing permits for any industrial activities. One of the goals of such environmental assessment studies is to highlight species assemblages and habitat composition that could make the targeted area unique. A section of the Oregon continental slope that had not been previously explored was targeted for the deployment of floating wind turbines. We carried out an underwater video survey, using a towed camera sled, to describe its benthic assemblages. Organisms were identified to the lowest taxonomic level possible and assemblages described related to the nature of the seafloor and the depth. We highlighted six invertebrate assemblages and three fish assemblages. For the invertebrates within flat soft sediments areas we defined three different assemblages based on primarily depth: a broad mid-depth (98–315 m) assemblage dominated by red octopus, sea pens and pink shrimps; a narrower mid-depth (250–270 m) assemblage dominated by box crabs and various other invertebrates; and a deeper (310–600 m) assemblage dominated by sea urchins, sea anemones, various snails and zoroasterid sea stars. The invertebrates on mixed sediments also were divided into three different assemblages: a shallow (~100 m deep) assemblage dominated by plumose sea anemones, broad mid-depth (170–370 m) assemblage dominated by sea cucumbers and various other invertebrates; and, again, a narrower mid-depth (230–270 m) assemblage, dominated by crinoids and encrusting invertebrates. For the fish, we identified a rockfish assemblage on coarse mixed sediments at 170–370 m and another fish assemblage on smaller mixed sediments within that depth range (250–370 m) dominated by thornyheads, poachers and flatfishes; and we identified a wide depth-range (98–600 m) fish assemblage on flat soft sediments dominated by flatfishes, eelpouts and thornyheads. Three of these assemblages (the two

  15. Benthic assemblages of mega epifauna on the Oregon continental margin

    Science.gov (United States)

    Hemery, Lenaïg G.; Henkel, Sarah K.; Cochrane, Guy R.

    2018-05-01

    Environmental assessment studies are usually required by a country's administration before issuing permits for any industrial activities. One of the goals of such environmental assessment studies is to highlight species assemblages and habitat composition that could make the targeted area unique. A section of the Oregon continental slope that had not been previously explored was targeted for the deployment of floating wind turbines. We carried out an underwater video survey, using a towed camera sled, to describe its benthic assemblages. Organisms were identified to the lowest taxonomic level possible and assemblages described related to the nature of the seafloor and the depth. We highlighted six invertebrate assemblages and three fish assemblages. For the invertebrates within flat soft sediments areas we defined three different assemblages based on primarily depth: a broad mid-depth (98-315 m) assemblage dominated by red octopus, sea pens and pink shrimps; a narrower mid-depth (250-270 m) assemblage dominated by box crabs and various other invertebrates; and a deeper (310-600 m) assemblage dominated by sea urchins, sea anemones, various snails and zoroasterid sea stars. The invertebrates on mixed sediments also were divided into three different assemblages: a shallow ( 100 m deep) assemblage dominated by plumose sea anemones, broad mid-depth (170-370 m) assemblage dominated by sea cucumbers and various other invertebrates; and, again, a narrower mid-depth (230-270 m) assemblage, dominated by crinoids and encrusting invertebrates. For the fish, we identified a rockfish assemblage on coarse mixed sediments at 170-370 m and another fish assemblage on smaller mixed sediments within that depth range (250-370 m) dominated by thornyheads, poachers and flatfishes; and we identified a wide depth-range (98-600 m) fish assemblage on flat soft sediments dominated by flatfishes, eelpouts and thornyheads. Three of these assemblages (the two broad fish assemblages and the deep

  16. Organic geochemistry of continental margin and deep ocean sediments

    Energy Technology Data Exchange (ETDEWEB)

    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.

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

    Science.gov (United States)

    von Huene, Roland E.; 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

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

    Science.gov (United States)

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

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

  19. Deep-sea Lebensspuren of the Australian continental margins

    Science.gov (United States)

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

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

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

    DEFF Research Database (Denmark)

    Petersen, Kenni D.; Schiffer, Christian

    2016-01-01

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

  1. Morphology and tectonics of Mahanadi Basin, northeastern continental margin of India from geophysical studies

    Digital Repository Service at National Institute of Oceanography (India)

    Subrahmanyam, V.; Subrahmanyam, A.S.; Murty, G.P.S.; Murthy, K.S.R.

    the breakup of India from Antarctica. The model studies suggest that the northern part of the 85 degrees E Ridge abuts the coast at Chilika Lake. The shape of 2000 m isobath over the northeastern continental margin of India closely resembles to that of 2000 m...

  2. Recent sediment transport and deposition in the Nazaré Canyon, Portuguese continental margin.

    NARCIS (Netherlands)

    de Stigter, H.C.; Boer, W.; de Jesus Mendes, P.A.; Jesus, C.C.; Thomsen, L.; van den Bergh, G.D.; van Weering, T.C.E.

    2007-01-01

    Processes, pathways and fluxes of sediment transport and deposition in the Nazaré submarine canyon, Portuguese continental margin, were investigated by water column profiling of suspended particulate matter, recording of near-bottom currents and suspended particulate matter fluxes with benthic

  3. Early Eocene climatic optimum: Environmental impact on the North Iberian continental margin

    NARCIS (Netherlands)

    Payros, A.; Ortiz, S.; Millán, I.; Arostegi, J.; Orue-Etxebarria, X.; Apellaniz, E.

    2015-01-01

    The early Eocene climatic optimum, which constituted the peak of the long-term early Cenozoic global warming, had a significant impact on the environmental evolution of terrestrial and oceanic areas. Surprisingly, however, its influence on continental margins is poorly known. New insights are

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

    Energy Technology Data Exchange (ETDEWEB)

    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.

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

    Energy Technology Data Exchange (ETDEWEB)

    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.

  6. Potential of radioactive and other waste disposals on the continental margin by natural dispersal processes

    International Nuclear Information System (INIS)

    Ryan, W.B.F.; Farre, J.A.

    1983-01-01

    Mass wasting, an erosional process, has recently been active at deepwater waste disposal sites on the mid-Atlantic margin of the United States. On the continental slope there is a subsea drainage network consisting of canyons, gullies, and chutes, and there are meandering channels, erosional scars, and debris aprons present on the continental rise. Fresh-looking blocks of 40 to 45 million-year-old marl and chalk (from cobble to boulder size) are strewn among canisters of low-level radioactive wastes. Some of the blocks have traveled from their original place of deposition for distances in excess of 170 km. Waste containers on the continental slope and rise cannot be considered to be disposed of permanently. The drainage network of the slope provides a natural process for collecting wastes over a catchment area, and for concentrating it with interim storage in canyons. Erosion by slumping, sliding, and debris flows ultimately will transport the wastes from the continental slope and disperse it over potentially large areas on the continental rise and abyssal plain. If it is desirable that the wastes be buried in the seafloor and isolated from the environment, then the continental slope and rise are not attractive repositories. If, however, it is deemed beneficial that the wastes ultimately be dispersed over a wide area, then the continental slope could be used as a disposal site

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

    Science.gov (United States)

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

    2002-12-01

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

  8. Shallow gas in the Iberian continental margin; Gas somero en el margen continental Iberico

    Energy Technology Data Exchange (ETDEWEB)

    Garcia-Gil, S.; Cartelle, V.; Blas, E. de; Carlos, A. de; Diez, R.; Duran, R.; Ferrin, A.; Garcia-Moreiras, I.; Garcia-Garcia, A.; Iglesias, J.; Martinez-Carreno, N.; Munoz Sobrino, C.; Ramirez-Perez, A. M.

    2015-07-01

    The shallow gas reservoirs in marine sediments from the Iberian margin or their escapes can be detected by using direct methods: (1) the measurement of high concentrations of methane or other hydrocarbons in the water column or sediment cores, (2) the identification of chemosynthetic communities and/or authigenic car- bonates in the seafloor, and (3) identification (using underwater videos) of pockmarks or carbonate mounds and mud volcanoes associated with the fluid escapes; or by indirect technical characterization of anomalies in acoustic records such as: (1) the presence of acoustic plumes in echo-sounders records, (2) the identification of acoustic blanking and/or acoustic turbidity in the high resolution seismic records, (3) the interpretation of reflectivity and (4) morphologies of pockmarks or seamounts in sidescan sonar and multibeam echo sounder records. This article is a compilation of acoustic-seismic, sedimentologic and morphologic evidence associated to the presence of shallow gas (accumulations or escapes) that appear in the Iberian margin and hat have been published in various papers. The description is divided into geographical sectors, beginning in the north-eastern end of the Mediterranean margin and ending at the easternmost area of the Cantabrian margin, following a clockwise direction around the Iberian Peninsula. (Author)

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

    Science.gov (United States)

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

    2017-12-01

    Continental rifts evolve to seafloor spreading and are preserved in passive margins, or fail and remain as fossil features in continents. Rifts at different stages give insight into these evolutionary paths. Of particular interest is the evolution of volcanic passive margins, which are characterized by seaward dipping reflectors, volcanic rocks yielding magnetic anomalies landward of the oldest spreading anomalies, and are underlain by high-velocity lower crustal bodies. How and when these features form remains unclear. Insights are given by the Midcontinent Rift (MCR), which began to form during the 1.1 Ga rifting of Amazonia from Laurentia, but failed when seafloor spreading was established elsewhere. MCR volcanics are much thicker than other continental flood basalts, due to deposition in a narrow rift rather than a broad region, giving a rift's geometry but a LIP's magma volume. The MCR provides a snapshot of the deposition of a thick and highly magnetized volcanic section during rifting. Surface exposures and reflection seismic data near Lake Superior show a rift basin filled by inward-dipping flood basalt layers. Had the rift evolved to seafloor spreading, the basin would have split into two sets of volcanics with opposite-facing SDRs, each with a magnetic anomaly. Because the rift formed as a series of alternating half-grabens, structural asymmetries between conjugate margins would have naturally occurred had it gone to completion. Hence the MCR implies that many passive margin features form prior to seafloor spreading. Massive inversion of the MCR long after it failed has provided a much clearer picture of its structure compared to failed rifts with lesser degrees of inversion. Seismic imaging as well as gravity and magnetic modeling provide important insight into the effects of inversion on failed rifts. The MCR provides an end member for the evolution of actively extending rifts, characterized by upwelling mantle and negative gravity anomalies, to failed

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

    Science.gov (United States)

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

    2014-12-01

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

  11. Geomorphic response of a continental margin to tectonic and eustatic variations, the Levant margin during the Messinian Salinity Crisis

    Science.gov (United States)

    Ben Moshe, Liran; Ben-Avraham, Zvi; Enzel, Yehouda; Uri, Schattner

    2017-04-01

    During the Messinian Salinity Crisis (MSC, 5.97±0.01-5.33 Ma) the Mediterranean Levant margin experienced major eustatic and sedimentary cycles as well as tectonic motion along the nearby Dead Sea fault plate boundary. New structures formed along this margin with morphology responding to these changes. Our study focuses on changes in this morphology across the margin. It is based on interpretation of three 3D seismic reflection volumes from offshore Israel. Multi-attribute analysis aided the extraction of key reflectors. Morphologic analysis of these data quantified interacting eustasy, sedimentation, and tectonics. Late Messinian morphologic domains include: (a) continental shelf; (b) 'Delta' anticline, forming a ridge diagonal to the strike of the margin; (c) southward dipping 'Hadera' valley, separating between (a) and (b); (d) 'Delta Gap' - a water gap crossing perpendicular to the anticline axis, exhibiting a sinuous thalweg; (e) continental slope. Drainage across the margin developed in several stages. Remains of turbidite flows crossing the margin down-slope were spotted across the 'Delta' anticline. These flows accumulated with the MSC evaporate sequence and prior to the anticline folding. Rising of the anticline, above the then bathymetry, either blocked or diverted the turbidites. That rising also defined the Hadera valley. In-situ evaporates, covering the valley floor, are, in turn covered by a fan-delta at the distal end of the valley. The fan-delta complex contains eroded evaporites and Lago-Mare fauna. Its top is truncated by dendritic fluvial channels that drained towards the Delta Gap. The Delta Gap was carved through the Delta ridge in a morphological and structural transition zone. We propose that during the first stages of the MSC (5.97±0.01-5.59 ma) destabilization of the continental slope due to oscillating sea level produced gravity currents that flowed through the pre-existing Delta anticline. Subsequent folding of the Delta anticline

  12. Structural inversion in the northern South China Sea continental margin and its tectonic implications

    Directory of Open Access Journals (Sweden)

    Chin-Da Huang

    2017-01-01

    Full Text Available The northern South China Sea (SCS continental margin was proposed to be an active margin during the Mesozoic. However, only a few papers discussed the Mesozoic structural evolution in this region. Here, we provide information based on the seismic profile interpretations with age control from biostratigraphic studies and detrital zircon U-Pb dates of well MZ-1-1 in the western Dongsha-Penghu Uplift of the northern SCS continental margin. The industrial seismic profiles reveal evidence for structural inversion as represented by folds and high-angle reverse faults, formed by reactivation of pre-existing normal faults. The inversion event likely started after the Early Cretaceous, and developed in Late Cretaceous, but ceased before the Cenozoic. The areal extent of the structural inversion was restricted in the western Dongsha-Penghu Uplift and was approximately 100 km in width. Based on the paleogeographic reconstruction of SCS, the structural inversion was likely formed by a collision between the seamount (volcanic islands swarm of the current North Palawan block (mainly the Calamian Islands and the northern SCS continental margin around Late Cretaceous.

  13. {sup 210}Pb-Excess and Sediment Accumulation Rates at the Iberian Continental Margin

    Energy Technology Data Exchange (ETDEWEB)

    Carvalho, F. P.; Oliveira, J. M.; Soares, A. M. [Nuclear and Technological Institute, Sacavem (Portugal)

    2013-07-15

    Sediments from the continental shelf, slope, and rise at the continental margin of northern Portugal and the adjacent Iberian abyssal basin were analysed for 210Pb, {sup 226}Ra, {sup 137}Cs and {sup 14}C. Pb-210 derived sedimentation rates at the continental shelf off the Portuguese coast were 0.2-0.6 cm/a. In some cores from fine sediment deposits at the outer shelf, the {sup 210}Pb excess continuum was interrupted and sediment layers were missing, suggesting that events such as sediment slides could have occurred. Higher sedimentation rates were determined in locations at the rise of the continental slope, confirming enhanced deposition by sediment slides. In the deeper Iberian Abyssal Basin, using the {sup 14}C age of sediment layers the sedimentation rate was determined at 3.2 cm/ka, thus four orders of magnitude lower than at the continental shelf. The spatial distribution of sedimentation rates determined by radionuclide based chronologies, suggested that fine sediments from river discharges are deposited mainly at the outer continental shelf. These deposits may became unstable with time and, occasionally, originate sediment slides that are drained by the canyons and reach the deep sea. The Iberian abyssal basin receives some advective contribution of these sediment slides and the sedimentation rate is one order of magnitude higher than in other abyssal basins of the NE Atlantic Ocean. (author)

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

    Science.gov (United States)

    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

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

    Science.gov (United States)

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

    2007-01-01

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

  16. Reactivation of precambrian faults on the southwestern continental margin of India: Evidence from gravity anomalies

    Digital Repository Service at National Institute of Oceanography (India)

    Subrahmanyam, V.; Ramana, M.V.; Rao, D.G.

    con- fi~ration of the western continental margin, with various structural styles, has been considered as a single unit, even though there exists a difference of opinion about the origin and evolution of the Chagos-Laccadive Ridge complex...’tVATION OF PRECAMBRIAN FAULTS ON THE SW CON~~E~AL MARGIN OF INDIA 337 shelf break due to the steep topographic relief of shelf. Furthermore, the gradients might have played a dominant role in shaping the bathymetry when the block movement took place along the pre...

  17. Velocity Model for CO2 Sequestration in the Southeastern United States Atlantic Continental Margin

    Science.gov (United States)

    Ollmann, J.; Knapp, C. C.; Almutairi, K.; Almayahi, D.; Knapp, J. H.

    2017-12-01

    The sequestration of carbon dioxide (CO2) is emerging as a major player in offsetting anthropogenic greenhouse gas emissions. With 40% of the United States' anthropogenic CO2 emissions originating in the southeast, characterizing potential CO2 sequestration sites is vital to reducing the United States' emissions. The goal of this research project, funded by the Department of Energy (DOE), is to estimate the CO2 storage potential for the Southeastern United States Atlantic Continental Margin. Previous studies find storage potential in the Atlantic continental margin. Up to 16 Gt and 175 Gt of storage potential are estimated for the Upper Cretaceous and Lower Cretaceous formations, respectively. Considering 2.12 Mt of CO2 are emitted per year by the United States, substantial storage potential is present in the Southeastern United States Atlantic Continental Margin. In order to produce a time-depth relationship, a velocity model must be constructed. This velocity model is created using previously collected seismic reflection, refraction, and well data in the study area. Seismic reflection horizons were extrapolated using well log data from the COST GE-1 well. An interpolated seismic section was created using these seismic horizons. A velocity model will be made using P-wave velocities from seismic reflection data. Once the time-depth conversion is complete, the depths of stratigraphic units in the seismic refraction data will be compared to the newly assigned depths of the seismic horizons. With a lack of well control in the study area, the addition of stratigraphic unit depths from 171 seismic refraction recording stations provides adequate data to tie to the depths of picked seismic horizons. Using this velocity model, the seismic reflection data can be presented in depth in order to estimate the thickness and storage potential of CO2 reservoirs in the Southeastern United States Atlantic Continental Margin.

  18. Rollback of an intraoceanic subduction system and termination against a continental margin

    Science.gov (United States)

    Campbell, S. M.; Simmons, N. A.; Moucha, R.

    2017-12-01

    The Southeast Indian Slab (SEIS) seismic anomaly has been suggested to represent a Tethyan intraoceanic subduction system which operated during the Jurassic until its termination at or near the margin of East Gondwana (Simmons et al., 2015). As plate reconstructions suggest the downgoing plate remained coupled to the continental margin, this long-lived system likely experienced a significant amount of slab rollback and trench migration (up to 6000 km). Using a 2D thermomechanical numerical code that includes the effects of phase transitions, we test this interpretation by modeling the long-term subduction, transition zone stagnation, and rollback of an intraoceanic subduction system in which the downgoing plate remains coupled to a continental margin. In addition, we also investigate the termination style of such a system, with a particular focus on the potential for some continental subduction beneath an overriding oceanic plate. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-735738

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

    Science.gov (United States)

    Zuo, X.; Chan, L. S.

    2015-12-01

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

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

    Science.gov (United States)

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

    2013-12-01

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

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

    Science.gov (United States)

    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

  2. Great earthquakes along the Western United States continental margin: implications for hazards, stratigraphy and turbidite lithology

    Science.gov (United States)

    Nelson, C. H.; Gutiérrez Pastor, J.; Goldfinger, C.; Escutia, C.

    2012-11-01

    We summarize the importance of great earthquakes (Mw ≳ 8) for hazards, stratigraphy of basin floors, and turbidite lithology along the active tectonic continental margins of the Cascadia subduction zone and the northern San Andreas Transform Fault by utilizing studies of swath bathymetry visual core descriptions, grain size analysis, X-ray radiographs and physical properties. Recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California margins are analyzed using two methods: (1) radiometric dating (14C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The H method provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that, on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great earthquakes are approximately 300 yr for the Cascadia subduction zone and 130 yr for the northern San Andreas Fault, which indicates both fault systems are in (Cascadia) or very close (San Andreas) to the early window for another great earthquake. On active tectonic margins with great earthquakes, the volumes of mass transport deposits (MTDs) are limited on basin floors along the margins. The maximum run-out distances of MTD sheets across abyssal-basin floors along active margins are an order of magnitude less (~100 km) than on passive margins (~1000 km). The great earthquakes along the Cascadia and northern California margins cause seismic strengthening of the sediment, which

  3. Great earthquakes along the Western United States continental margin: implications for hazards, stratigraphy and turbidite lithology

    Directory of Open Access Journals (Sweden)

    C. H. Nelson

    2012-11-01

    Full Text Available We summarize the importance of great earthquakes (Mw ≳ 8 for hazards, stratigraphy of basin floors, and turbidite lithology along the active tectonic continental margins of the Cascadia subduction zone and the northern San Andreas Transform Fault by utilizing studies of swath bathymetry visual core descriptions, grain size analysis, X-ray radiographs and physical properties. Recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California margins are analyzed using two methods: (1 radiometric dating (14C method, and (2 relative dating, using hemipelagic sediment thickness and sedimentation rates (H method. The H method provides (1 the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2 the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that, on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great earthquakes are approximately 300 yr for the Cascadia subduction zone and 130 yr for the northern San Andreas Fault, which indicates both fault systems are in (Cascadia or very close (San Andreas to the early window for another great earthquake.

    On active tectonic margins with great earthquakes, the volumes of mass transport deposits (MTDs are limited on basin floors along the margins. The maximum run-out distances of MTD sheets across abyssal-basin floors along active margins are an order of magnitude less (~100 km than on passive margins (~1000 km. The great earthquakes along the Cascadia and northern California margins

  4. Interpretation of free-air gravity anomaly data for determining the crustal structure across the continental margins and aseismic ridges: Some examples from Indian continental margins and deep-sea basins

    Digital Repository Service at National Institute of Oceanography (India)

    Ramana, M.V.

    Content-Type text/plain; charset=UTF-8 202 Interpretation of free-air gravity anomaly data for determining the crustal structure across the continental margins and aseismic ridges: Some examples from Indian continental margins and deep... will undertake either regional, reconnaissance or detail gravity surveys. We generally deal with free air gravity anomalies in oceans. The free air gravity anomalies mostly mimic the seabed configuration and at times, the deviation observed in the free air...

  5. Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin

    Digital Repository Service at National Institute of Oceanography (India)

    Dewangan, P.; Ramprasad, T.

    The occurrence of gas hydrate has been inferred from the presence of Bottom-Simulating Reflectors (BSRs) along the western continental margin of India. In this paper, we assess the spatial and vertical distribution of gas hydrates by analyzing...

  6. Occurrence of gas hydrates along the continental margins of India, particularly the Krishna-Godavari offshore basin

    Digital Repository Service at National Institute of Oceanography (India)

    Ramana, M.V.; Ramprasad, T.; KameshRaju, K.A.; Desa, M.

    carried out in the Krishna-Godavari offshore area along the eastern continental margin of India, which is known for its hydrocarbon potential. Processed multibeam data provided a high-resolution seafloor mosaic with a fine scale geomorphology. Deep tow...

  7. Geomorphology and Neogene tectonic evolution of the Palomares continental margin (Western Mediterranean)

    Science.gov (United States)

    Gómez de la Peña, Laura; Gràcia, Eulàlia; Muñoz, Araceli; Acosta, Juan; Gómez-Ballesteros, María; R. Ranero, César; Uchupi, Elazar

    2016-10-01

    The Palomares continental margin is located in the southeastern part of Spain. The margin main structure was formed during Miocene times, and it is currently part of the wide deformation zone characterizing the region between the Iberian and African plates, where no well-defined plate boundary occurs. The convergence between these two plates is here accommodated by several structures, including the left lateral strike-slip Palomares Fault. The region is characterized by sparse, low to moderate magnitude (Mw shallow instrumental earthquakes, although large historical events have also occurred. To understand the recent tectonic history of the margin we analyze new high-resolution multibeam bathymetry data and re-processed three multichannel seismic reflection profiles crossing the main structures. The analysis of seafloor morphology and associated subsurface structure provides new insights of the active tectonic features of the area. In contrast to other segments of the southeastern Iberian margin, the Palomares margin contains numerous large and comparatively closely spaced canyons with heads that reach near the coast. The margin relief is also characterized by the presence of three prominent igneous submarine ridges that include the Aguilas, Abubacer and Maimonides highs. Erosive processes evidenced by a number of scars, slope failures, gullies and canyon incisions shape the present-day relief of the Palomares margin. Seismic images reveal the deep structure distinguishing between Miocene structures related to the formation of the margin and currently active features, some of which may reactivate inherited structures. The structure of the margin started with an extensional phase accompanied by volcanic accretion during the Serravallian, followed by a compressional pulse that started during the Latemost Tortonian. Nowadays, tectonic activity offshore is subdued and limited to few, minor faults, in comparison with the activity recorded onshore. The deep Algero

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

    Science.gov (United States)

    Voggenreiter, W.; Hötzl, H.

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

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

    Science.gov (United States)

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

    2014-07-01

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

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

    Science.gov (United States)

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

    2017-04-01

    In northwestern Namibia the Kaoko Belt is one of the most important Precambrian crustal segments that have stored the subsidence, cooling, and exhumation history of Namibia since the Neoproterozoic. ZFT-ages, with ages between 292.7 (46.0) and 436.8 (45.9) Ma, are giving new insights on this early evolution. Paleozoic to Mesozoic sedimentary rocks of the Karoo Supergroup and the Lower Cretaceous volcanic rocks of the Etendeka sequence overlay the Proterozoic metamorphic and intrusive rocks (1). New apatite fission-track (AFT) ages range from 390.9 (17.9) Ma to 80.8 (6.0) Ma. Along the coast apatites of Proterozoic rock samples reveal the youngest ages. Further inland the ages increase significantly. In addition, rapid change of AFT-ages occurs on both sides of major thrust and shear zones. Using the oldest thermochronological data the revealed t-T paths indicate a long era of exhumation, starting at the end of the Pan-African Orogeny in the Neoproterozoic and continuing into the Permo-Carboniferous. The subsequent sedimentation of the Karoo Supergroup initiates a new era of subsidence until the end of Triassic (2).The subsequent period of denudation ends abruptly with the rapid deposition of the Etendeka basalts in the Early Cretaceous (3). The maximum thickness of the Etendeka volcanic suite has been estimated, using the apatite fission-track data, to about 3.2 (1.2) km. With the ongoing opening of the South Atlantic and the formation of the continental margin the Kaoko Belt went through a rapid cooling event starting 130 Ma and ending 80 Ma, at a mean rate of 0.034 km/Ma for the western, and 0.018 km/Ma for the northern and eastern Kaoko Belt. This cooling event was accompanied by a reactivation of major fault zones, like the Purros Mylonite Zone (4). Thereafter, stable conditions were established, with denudation rates generally lower than 0.010 km/Ma, until the Neogene, where a second cooling event led to increased exhumation rates around 0.042 km/Ma. The total

  11. Basement tectonics and flexural subsidence along western continental margin of India

    Directory of Open Access Journals (Sweden)

    D.K. Pandey

    2017-09-01

    Full Text Available The Paleocene-recent post-rift subsidence history recorded in the Mumbai Offshore Basin off western continental margin of India is examined. Results obtained through 2-D flexural backstripping modelling of new seismic data reveal considerable thermo-tectonic subsidence over last ca. 56 Myr. Reverse post-rift subsidence modelling with variable β stretching factor predicts residual topography of ca. 2000 m to the west of Shelf Margin Basin and fails to restore late Paleocene horizon and the underlying igneous basement to the sea level. This potentially implies that: (1 either the igneous basement formed during the late Cretaceous was emplaced under open marine environs; or (2 a laterally varying cumulative subsidence occurred within Mumbai Offshore Basin (MOB during ca. 68 to ca. 56 Ma. Pre-depositional topographic variations at ca. 56 Ma across the basin could be attributed to the extensional processes such as varied lower crustal underplating along Western Continental Margin of India (WCMI. Investigations about basement tectonics after unroofing of sediments since late Paleocene from this region support a transitional and heavily stretched nature of crust with high to very high β factors. Computations of past sediment accumulation rates show that the basin sedimentation peaked during late Miocene concurrently with uplift of Himalayan–Tibetan Plateau and intensification of Indian monsoon system. Results from basin subsidence modelling presented here may have significant implications for further studies attempting to explore tectono–climatic interactions in Asia.

  12. The role of deep-water sedimentary processes in shaping a continental margin: The Northwest Atlantic

    Science.gov (United States)

    Mosher, David C.; Campbell, D.C.; Gardner, J.V.; Piper, D.J.W.; Chaytor, Jason; Rebesco, M.

    2017-01-01

    The tectonic history of a margin dictates its general shape; however, its geomorphology is generally transformed by deep-sea sedimentary processes. The objective of this study is to show the influences of turbidity currents, contour currents and sediment mass failures on the geomorphology of the deep-water northwestern Atlantic margin (NWAM) between Blake Ridge and Hudson Trough, spanning about 32° of latitude and the shelf edge to the abyssal plain. This assessment is based on new multibeam echosounder data, global bathymetric models and sub-surface geophysical information.The deep-water NWAM is divided into four broad geomorphologic classifications based on their bathymetric shape: graded, above-grade, stepped and out-of-grade. These shapes were created as a function of the balance between sediment accumulation and removal that in turn were related to sedimentary processes and slope-accommodation. This descriptive method of classifying continental margins, while being non-interpretative, is more informative than the conventional continental shelf, slope and rise classification, and better facilitates interpretation concerning dominant sedimentary processes.Areas of the margin dominated by turbidity currents and slope by-pass developed graded slopes. If sediments did not by-pass the slope due to accommodation then an above grade or stepped slope resulted. Geostrophic currents created sedimentary bodies of a variety of forms and positions along the NWAM. Detached drifts form linear, above-grade slopes along their crests from the shelf edge to the deep basin. Plastered drifts formed stepped slope profiles. Sediment mass failure has had a variety of consequences on the margin morphology; large mass-failures created out-of-grade profiles, whereas smaller mass failures tended to remain on the slope and formed above-grade profiles at trough-mouth fans, or nearly graded profiles, such as offshore Cape Fear.

  13. Western Continental Margin of India - Re-look using potential field data

    Science.gov (United States)

    Rajaram, M.; S P, A.

    2008-05-01

    The Western Continental Margin of India (WCMI) evolved as a result of rifting between India and Madagascar that took place during mid Cretaceous (~88Ma).The WCMI is equally important in terms of natural resources as well as research point of view. The major tectonic elements in the western offshore includes the Laxmi and Chagos- Laccadive ridge dividing the WCMI and the adjoining Arabian sea into two basins, Pratap Ridge, Alleppey platform etc. Different theories have been proposed for the evolution of each of these tectonic elements. In the current paper we look at geopotential data on the west coast of India and the western off-shore. The data sets utilized include Satellite derived High Resolution Free Air Gravity data over the off-shore, Bouguer data onland, Champ Satellite Magnetic data, published Marine Magnetic data collected by ONGC, NIO, ground magnetic data over west cost collected by IIG and available aeromagnetic data. From the free air gravity anomaly the structural details of the western offshore can be delineated. The Euler depths of FAG depict deep solutions associated with Pratap Ridge, Comorin Ridge, the west coast fault and the Laxmi Ridge. These may be associated with continental margin and continental fragments. From the aeromagnetic and marine magnetic data it is evident that the West Coast Fault is dissected at several places. The shallow circular feature associated with Bombay High is evident both on the FAG and the analytic signal derived from satellite Magnetic data. The crustal magnetic thickness from MF5 lithospheric model of the Champ appears to suggest that the continental crust extends up to the Chagos- Laccadive ridge. Based on the analysis of these geopotential data sets the various theories for the evolution of the WCMI will be evaluated and these results will be presented.

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

    Science.gov (United States)

    Parson, L.

    2005-12-01

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

  15. Thermochronological constraints on the Cambrian to recent geological evolution of the Argentina passive continental margin

    Science.gov (United States)

    Kollenz, Sebastian; Glasmacher, Ulrich A.; Rossello, Eduardo A.; Stockli, Daniel F.; Schad, Sabrina; Pereyra, Ricardo E.

    2017-10-01

    Passive continental margins are geo-archives that store information from the interplay of endogenous and exogenous forces related to continental rifting, post-breakup history, and climate changes. The recent South Atlantic passive continental margins (SAPCMs) in Brazil, Namibia, and South Africa are partly high-elevated margins ( 2000 m a.s.l.), and the recent N-S-trending SAPCM in Argentina and Uruguay is of low elevation. In Argentina, an exception in elevation is arising from the higher topography (> 1000 m a.s.l.) of the two NW-SE-trending mountain ranges Sierras Septentrionales and Sierras Australes. Precambrian metamorphic and intrusive rocks, and siliciclastic rocks of Ordovician to Permian age represent the geological evolution of both areas. The Sierras Australes have been deformed and metamorphosed (incipient - greenschist) during the Gondwanides Orogeny. The low-temperature thermochronological (LTT) data (history of the Gondwanides and the Mesozoic and Cenozoic South Atlantic geological evolution. Upper Carboniferous zircon (U-Th/He)-ages (ZHe) indicate the earliest cooling below 180 °C/1 Ma. Most of the ZHe-ages are of Upper Triassic to Jurassic age. The apatite fission-track ages (AFT) of Sierras Septentrionales and the eastern part of Sierras Australes indicate the South Atlantic rifting and, thereafter. AFT-ages of Middle to Upper Triassic on the western side of the Sierras Australes are in contrast, indicating a Triassic exhumation caused by the eastward thrusting along the Sauce Grande wrench. The corresponding t-T models report a complex subsidence and exhumation history with variable rates since the Ordovician. Based on the LTT-data and the numerical modelling we assume that the NW-SE-trending mountain ranges received their geographic NW-SE orientation during the syn- to post-orogenic history of the Gondwanides.

  16. 210Pb and 210Po as tracers of particle transport mechanisms on continental margins

    International Nuclear Information System (INIS)

    Radakovitch, O.; Heussner, S.; Biscaye, P.; Abassi, A.

    1997-01-01

    The natural radionuclides 210 Po and 210 Pb, members of the 238 U decay chain, are particularly helpful to the understanding of particle transport processes in the ocean. These isotopes were analysed on sediment trap particles collected during 3 one-year experiments on continental margins. In the Bay of Biscay (Northeastern Atlantic) and in the Gulf of Lion (Northwestern Mediterranean Sea) both as part of the French ECOMARGE programme, and in the Middle Atlantic Bight (Northwestern Atlantic) as part of the SEEP programme. They yielded great insights into scenarios of particle transfer at each site, mainly based on the spatial and temporal distribution of 210 Pb particulate concentrations and fluxes. (author)

  17. Relationship between radionuclides and sedimentological variables in the South Atlantic Continental Margin

    International Nuclear Information System (INIS)

    Ferreira, Paulo A.L.; Figueira, Rubens C.L.

    2015-01-01

    There is a lack of information regarding marine radioactivity in sediments of the Continental Margin of the South Atlantic. "1"3"7Cs and "4"0K radioactivity and sedimentological variables were determined in superficial sediment samples. It was demonstrated that "4"0K is a good indicator for sediment granulometry, whilst "1"3"7Cs presents a good correlation with its chemical composition. Moreover, it was identified through the radiometric data the occurrence of input of allochtonous matter to the Brazilian southernmost compartment from the Rio de La Plata estuary, as previously reported in the literature. (author)

  18. Regional gravity and magnetic studies over the continental margin of the Central West Coast of India

    Digital Repository Service at National Institute of Oceanography (India)

    SubbaRaju, L.V.; KameshRaju, K.A.; Subrahmanyam, V.; Rao, D.G.

    Institute of Oceanography, Dona Paula, Goa 403 004, India Abstract Gravity studies over the continental margin of the central west coast of India show a sediment thickness of 2-3 km on the shelf associated with deeper hoest and graben structures, of 6 km... sequence ranges from Palaeocene to Recent. Stratigraphy as obtained from the explor atory wells BH-I, DCS-IA, HI2-1, and R6-110cated in the Bombay offshore basin is shown in Figure 2. Figure 3 depicts the seismogeological section of the Bombay offshore...

  19. A proterozoic tectonic model for northern Australia and its economic implications

    International Nuclear Information System (INIS)

    Rossiter, A.G.; Ferguson, J.

    1980-01-01

    It is argued that at the end of Archaean time the Australian continent was confined to the area now occupied by the Yilgarn, Pilbara, Gawler, and Musgrave Blocks, and the southern part of the Arunta Block. During the Early Proterozoic, sedimentary and volcanic rocks were laid down in an extensive depositional zone trending roughly east-west along the northern margin of the Archaean continent. Copper and gold mineralization, commonly showing stratigraphic control, is widespread in this belt. Following deformation and metamorphism of the Early Proterozoic rocks, felsic and mafic igneous activity, and accumulation of platform sediments on the newly stabilized crust, a predominantly north-south depositional zone developed along the eastern margin of the continent during the Middle Proterozoic. Lead and zinc assume much more importance in the mineral deposits of this belt. It is postulated that the present positions of rocks of the Pine Creek and Georgetown regions are due to horizontal displacements of several hundred kilometres along major fault zones. Apparent rifting of these blocks away from palaeo-continental margins may be related to the occurrence of uraniferous granitic rocks and uranium mineralization within them via a mantle plume mechanism. Although current data are limited, tectonic environments suggested for Proterozoic mafic igneous rocks of northern Australia by their geochemistry are compatible with the geological settings of these rocks and with the tectonic model put forward. (author)

  20. Stratigraphic landscape analysis, thermochronology and the episodic development of elevated, passive continental margins

    Directory of Open Access Journals (Sweden)

    Green, Paul F.

    2013-12-01

    Full Text Available The continental margin of West Greenland is similar in many respects to other elevated, passive continental margins (EPCMs around the world. These margins are characterised by extensive regions of low relief at elevations of 1–2 kilometres above sea level sloping gently inland, with a much steeper, oceanward decline, often termed a 'Great Escarpment', terminating at a coastal plain. Recent studies, based on integration of geological, geomorphological and thermochronological evidence, have shown that the high topography of West Greenland was formed by differential uplift and dissection of an Oligo-Miocene peneplain since the late Miocene, many millions of years after continental break-up between Greenland and North America. In contrast, many studies of other EPCMs have proposed a different style of development in which the high plateaux and the steep, oceanward decline are regarded as a direct result of rifting and continental separation. Some studies assume that the elevated regions have remained high since break-up, with the high topography continuously renewed by isostasy. Others identify the elevated plains as remnants of pre-rift landscapes. Key to understanding the development of the West Greenland margin is a new approach to the study of landforms, stratigraphic landscape analysis, in which the low-relief, high-elevation plateaux at EPCMs are interpreted as uplifted peneplains: low-relief surfaces of large extent, cutting across bedrock of different age and resistance, and originally graded to sea level. Identification of different generations of peneplain (re-exposed and epigene from regional mapping, combined with geological constraints and thermochronology, allows definition of the evolution leading to the formation of the modern-day topography. This approach is founded particularly on results from the South Swedish Dome, which document former sea levels as base levels for the formation of peneplains. These results support the view

  1. First discovery of a cold seep on the continental margin of the central Red Sea

    KAUST Repository

    Batang, Zenon B.

    2012-06-01

    A new cold brine seep system with microbial mats and metazoan assemblages was discovered by a remotely operated vehicle (ROV) on the Saudi continental margin of central Red Sea. Now named as Thuwal Seeps, it has a shallow brine pool between 840 and 850. m water depths that is formed by focused brine expulsions from two sites (Seep I: 22°17.3\\'N, 38°53.8\\'E; Seep II: 22°16.9\\'N, 38°53.9\\'E). The seep is located at the base of a steep wall rock closer to the shore (20. km) than to the axial trough (120. km). The brine pool does not exhibit a significant thermal anomaly (<. 0.3°C) and is so far the coldest (21.7°C) and least saline (74‰) among brine pools in the Red Sea. This discovery provides the first direct evidence of a cold seep with associated biota on the continental margin of the Red Sea. © 2011 Elsevier B.V.

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

    Science.gov (United States)

    Schiffer, C.; Petersen, K. D.

    2016-12-01

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

  3. Ocean-Continent Transition Structure of the Pelotas Magma-Rich Continental Margin, South Atlantic

    Science.gov (United States)

    Harkin, Caroline; Kusznir, Nick; Roberts, Alan; Manatschal, Gianreto; McDermott, Ken

    2017-04-01

    Rifted continental margins in the southern South Atlantic are magma-rich showing well developed volcanic extrusives known as seaward dipping reflectors (SDRs). Here we examine the magma-rich continental rifted margin of the Pelotas Basin, offshore Brazil. Deep seismic reflection data displays a large package of seaward dipping reflectors with an approximate width of 200 km and a varying thickness of 10 km to 17 km that have previously been interpreted as volcanic SDRs. We examine these SDRs to explore if they are composed predominantly of basaltic or sedimentary-volcaniclastic material. We also study the thickness of the crustal basement beneath the SDRs. Additionally we investigate if these SDRs are underlain by thin 'hyper-extended' continental crust or if they have been deposited on new magmatic basement. The answers to these questions are important in understanding the structure and formation processes of magma-rich continental margins. We use gravity inversion to investigate SDR composition by varying the proportion of basalt to sediments-volcaniclastics (basalt fraction) which determines the SDR densities in the gravity inversion. By matching the Moho depth and two-way travel time from gravity inversion and deep seismic reflection data, we determine the lateral variation in basalt fraction of the SDRs. Our analysis suggests: 1) There is an overall pattern of SDR basalt fraction and bulk density decreasing oceanward. This could be due to increasing sediment content oceanward or it could result from the change in basalt flows to hyaloclastites as water depth increases. 2) The SDR package can be split into two distinct sub packages based on the basalt fraction results, where the proximal side of each package has a higher basalt fraction and density. 3) The inner SDR package contains reflectors that bear a resemblance to the SDRs described by Hinz (1981) corresponding to syn-tectonic volcanic eruptions into an extensional basin, while the outer SDR package has

  4. Seismic investigation on the Littoral Faults Zone in the northern continental margin of South China Sea

    Science.gov (United States)

    Sun, J.; Xu, H.; Xia, S.; Cao, J.; Wan, K.

    2017-12-01

    The continental margin of the northern South China Sea (SCS) had experienced continuous evolution from an active continental margin in the late Mesozoic to a passive continental margin in the Cenozoic. The 1200km-long Littoral Faults Zone (LFZ) off the mainland South China was suggested to represent one of the sub-plate boundaries and play a key role during the evolution. Besides, four devastating earthquakes with magnitude over 7 and another 11 destructive events with M>6 were documented to have occurred along the LFZ. However, its approximity to the shoreline, the shallow water depth, and the heavy fishing activities make it hard to conduct a marine seismic investigation. As a result, understandings about the LFZ before 2000 were relatively poor and mostly descriptive. After two experiments of joint onshore-offshore wide-angle seismic surveys in the 1st decade of this century, several cruses aiming to unveil the deep structure of the LFZ were performed in the past few years, with five joint onshore-offshore wide-angle seismic survey profiles completed. Each of these profiles is perpendicular to the shoreline, with four to five seismometers of campaign mode deployed on the landside and over ten Ocean Bottom Seismometers (OBSs) spacing at 20km deployed on the seaside. Meanwhile, multi-channel seismic (MCS) data along these profiles were obtained simultaneously. Based on these data, velocity models from both forward modeling and inversion were obtained. According to these models, the LFZ was imaged to be a low-velocity fractured zone dipping to the SSE-SE at a high-angle and cutting through the thinned continental crust at some locations. Width of the fractured zone varies from 6km to more than 10km from site to site. With these results, it is suggested that the LFZ accommodates the stresses from both the east side, where the Eurasia/Philippine Sea plate converging and mountain building is ongoing, and the west side, where a strike-slip between the Indochina

  5. Morphology and sedimentology of glacigenic submarine fans on the west Greenland continental margin

    Science.gov (United States)

    O'Cofaigh, Colm; Hogan, Kelly A.; Dowdeswell, Julian A.; Jennings, Anne E.; Noormets, Riko; Evans, Jeffrey

    2014-05-01

    Along the West Greenland continental margin adjoining Baffin Bay, bathymetric data show a series of large submarine fans located at the mouths of cross-shelf troughs. Two of these fans, the Uummannaq Fan and the Disko Fan are trough-mouth fans built largely of debris delivered from ice sheet outlets of the Greenland Ice Sheet during past glacial maxima. On the Uummannaq Fan glacigenic debris flow deposits occur on the upper slope and extend to at least 1800 m water depth in front of the trough-mouth. The debris flow deposits are related to the remobilisation of subglacial debris that was delivered onto the upper slope at times when an ice stream was positioned at the shelf edge. In contrast, sedimentary facies from the northern sector of the fan are characterised by hemipelagic and ice-rafted sediments and turbidites; glacigenic debris flows are notably absent in cores from this region. Further south along the Greenland continental margin the surface of the Disko Fan is prominently channelised and associated sediments are acoustically stratified. Although glacigenic debris flow deposits do occur on the upper Disko Fan, sediments recovered in cores from elsewhere on the fan record the influence of turbidity current and meltwater sedimentation. The channelised form of the Disko fan contrasts markedly with that of the Uummannaq Fan and, more widely, with trough mouth fans from the Polar North Atlantic. Collectively these data highlight the variability of glacimarine depositional processes operating on trough-mouth fans on high-latitude continental slopes and show that glacigenic debris flows are but one of a number of mechanisms by which such large glacially-influenced depocentres form.

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

    Science.gov (United States)

    von Huene, Roland E.; Scholl, D. W.

    1991-01-01

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

  7. Geomorphological and sedimentary processes of the glacially influenced northwestern Iberian continental margin and abyssal plains

    Science.gov (United States)

    Llave, Estefanía; Jané, Gloria; Maestro, Adolfo; López-Martínez, Jerónimo; Hernández-Molina, F. Javier; Mink, Sandra

    2018-07-01

    The offshore region of northwestern Iberia offers an opportunity to study the impacts of along-slope processes on the morphology of a glacially influenced continental margin, which has traditionally been conceptually characterised by predominant down-slope sedimentary processes. High-resolution multibeam bathymetry, acoustic backscatter and ultrahigh-resolution seismic reflection profile data are integrated and analysed to describe the present-day and recent geomorphological features and to interpret their associated sedimentary processes. Seventeen large-scale seafloor morphologies and sixteen individual echo types, interpreted as structural features (escarpments, marginal platforms and related fluid escape structures) and depositional and erosional bedforms developed either by the influence of bottom currents (moats, abraded surfaces, sediment waves, contourite drifts and ridges) or by gravitational features (gullies, canyons, slides, channel-levee complexes and submarine fans), are identified for the first time in the study area (spanning 90,000 km2 and water depths of 300 m to 5 km). Different types of slope failures and turbidity currents are mainly observed on the upper and lower slopes and along submarine canyons and deep-sea channels. The middle slope morphologies are mostly determined by the actions of bottom currents (North Atlantic Central Water, Mediterranean Outflow Water, Labrador Sea Water and North Atlantic Deep Water), which thereby define the margin morphologies and favour the reworking and deposition of sediments. The abyssal plains (Biscay and Iberian) are characterised by pelagic deposits and channel-lobe systems (the Cantabrian and Charcot), although several contourite features are also observed at the foot of the slope due to the influence of the deepest water masses (i.e., the North Atlantic Deep Water and Lower Deep Water). This work shows that the study area is the result of Mesozoic to present-day tectonics (e.g. the marginal platforms

  8. The Continental Margin of East Asia: a collage of multiple plates formed by convergence and extension from multiple directions

    Science.gov (United States)

    Mao, J.; Wang, T.; Ludington, S.; Qiu, Z.; Li, Z.

    2017-12-01

    East Asia is one of the most complex regions in the world. Its margin was divided into 4 parts: Northeast Asia, North China, South China and Southeast Asia. During the Phanerozoic, continental plates of East Asia have interacted successively with a) the Paleo Tethyan Ocean, b) the Tethyan and Paleo Pacific Oceans and c) the Pacific and Indian. In the Early Mesozoic, the Indosinian orogeny is characterized by the convergence and extension within multiple continental plates, whereas the Late Mesozoic Yanshanian orogeny is characterized by both convergence and compression due to oceanic subduction and by widespread extension. We propose this combination as "East Asia Continental Margin type." Except in Northeast Asia, where Jurassic and Cretaeous accretionary complexes are common, most magmatic rocks are the result of reworking of ancient margins of small continental plates; and oceanic island arc basalts and continental margin arc andesites are largely absent. Because South China is adjacent to the western margin of the Pacific Plate, some effects of its westward subduction must be unavoidable, but juvenile arc-related crust has not been identified. The East Asian Continental Margin is characterized by magmatic rocks that are the result of post-convergent tectonics, which differs markedly from the active continental margins of both South and North America. In summary, the chief characteristics of the East Asian Continental Margin are: 1) In Mesozoic, the periphery of multiple blocks experienced magmatism caused by lithospheric delamination and thinning in response to extension punctuated by shorter periods of convergence. 2) The main mechanism of magma generation was the partial melting of crustal rocks, due to underplating by upwelling mafic magma associated with the collapse of orogenic belts and both extension and compression between small continental blocks. 3) During orogeny, mostly high Sr/Y arc-related granitoids formed, whereas during post-orogenic times, A

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

    Science.gov (United States)

    Shahraki, Meysam; Schmeling, Harro; Haas, Peter

    2018-01-01

    Isostatic equilibrium is a good approximation for passive continental margins. In these regions, geoid anomalies are proportional to the local dipole moment of density-depth distributions, which can be used to constrain the amount of oceanic to continental lithospheric thickening (lithospheric jumps). We consider a five- or three-layer 1D model for the oceanic and continental lithosphere, respectively, composed of water, a sediment layer (both for the oceanic case), the crust, the mantle lithosphere and the asthenosphere. The mantle lithosphere is defined by a mantle density, which is a function of temperature and composition, due to melt depletion. In addition, a depth-dependent sediment density associated with compaction and ocean floor variation is adopted. We analyzed satellite derived geoid data and, after filtering, extracted typical averaged profiles across the Western and Eastern passive margins of the South Atlantic. They show geoid jumps of 8.1 m and 7.0 m for the Argentinian and African sides, respectively. Together with topography data and an averaged crustal density at the conjugate margins these jumps are interpreted as isostatic geoid anomalies and yield best-fitting crustal and lithospheric thicknesses. In a grid search approach five parameters are systematically varied, namely the thicknesses of the sediment layer, the oceanic and continental crusts and the oceanic and the continental mantle lithosphere. The set of successful models reveals a clear asymmetry between the South Africa and Argentine lithospheres by 15 km. Preferred models predict a sediment layer at the Argentine margin of 3-6 km and at the South Africa margin of 1-2.5 km. Moreover, we derived a linear relationship between, oceanic lithosphere, sediment thickness and lithospheric jumps at the South Atlantic margins. It suggests that the continental lithospheres on the western and eastern South Atlantic are thicker by 45-70 and 60-80 km than the oceanic lithospheres, respectively.

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

    Science.gov (United States)

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

    2011-07-01

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

  11. {sup 210}Pb and {sup 210}Po as tracers of particle transport mechanisms on continental margins

    Energy Technology Data Exchange (ETDEWEB)

    Radakovitch, O.; Heussner, S. [Perpignan Univ., 66 (France). Lab. de Sedimentologie et Geochimie Marines; Biscaye, P.; Abassi, A. [Columbia Univ., Palisades, NY (United States). Lamont Doherty Earth Observatory

    1997-12-31

    The natural radionuclides {sup 210}Po and {sup 210}Pb, members of the {sup 238}U decay chain, are particularly helpful to the understanding of particle transport processes in the ocean. These isotopes were analysed on sediment trap particles collected during 3 one-year experiments on continental margins. In the Bay of Biscay (Northeastern Atlantic) and in the Gulf of Lion (Northwestern Mediterranean Sea) both as part of the French ECOMARGE programme, and in the Middle Atlantic Bight (Northwestern Atlantic) as part of the SEEP programme. They yielded great insights into scenarios of particle transfer at each site, mainly based on the spatial and temporal distribution of {sup 210}Pb particulate concentrations and fluxes. (author) 11 refs.

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

    Science.gov (United States)

    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.

  13. Crustal structure and development of the SW Barents Sea and the adjacent continental margin

    Energy Technology Data Exchange (ETDEWEB)

    Breivik, Asbjoern Johan

    1998-12-31

    Because of its expected petroleum potential, the western Barents Sea has been extensively mapped and investigated. The present thesis deals with many aspects of the geological development of this area. The emphasis is on Late Paleozoic structuring, Late Mesozoic basin formation, and early Tertiary margin formation including geodynamical response to the late Cenozoic sedimentation. The thesis begins with a review of the literature on the Late Palaeozoic structural development of the south-western Barents Sea, Svalbard and eastern Greenland. A structural map is developed for the Upper Carboniferous rift system in the southwestern Barents Sea that shows the interference of the northeasterly and the northerly structural grain. A discussion of the Ottar Basin uses a combination of seismic interpretation and gravity modelling to investigate this important structural element of the Upper Palaeozoic rift system. Previous work on Late Mesozoic basin formation in the southwestern Barents Sea is extended by incorporating new seismic reflection data and gravity modelling. Finally, the focus is shifted from the Barents Sea shelf to the continental-ocean transition and the oceanic basin. Gridded free-air gravity data from the ERS-1 enables the construction of a Bouguer gravity map of unprecedented resolution. The relationship between isostacy and gravity was resolved by modelling the thermal structure across the margin. Admittance analysis of the relationship between bathymetry and free-air gravity indicates an elastic thickness of the oceanic Lithosphere of 15-20 km, which is compatible with the depth to the 450{sup o}C isotherm obtained from thermal modelling. It is concluded that the southwestern Barents Sea margin does not deviate in any significant respects from passive rifted margins, except for a very straight and narrow continent-ocean transition zone. 332 refs., 55 figs., 7 tabs.

  14. Subduction of Proterozoic to Late Triassic continental basement in the Guatemala suture zone: A petrological and geochronological study of high-pressure metagranitoids from the Chuacús complex

    Science.gov (United States)

    Maldonado, Roberto; Ortega-Gutiérrez, Fernando; Ortíz-Joya, Guillermo A.

    2018-05-01

    Many continental subduction complexes contain abundant granitic rocks coexisting with minor volumes of eclogite-facies rocks. Characterization of granitic protoliths is crucial to decipher the origin of subducted continental crust, whereas knowledge of its metamorphic evolution is required to constrain the mechanisms of burial and exhumation. In this work we present geochronological and petrological evidence that demonstrate the occurrence of a subducted Proterozoic to Late Triassic granitic basement in the Chuacús complex of central Guatemala. Metagranitoids exposed in this area are interlayered with eclogite and other high-pressure rocks, and their structure is considerably variable due to strain partitioning during deformation. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon data from two ferroan metagranites yield protolith crystallization ages of ca. 1.1 Ga and their trace-element abundances suggest an origin related to intraplate magmatism, while a high-silica, peraluminous metagranite is dated at 1.0 Ga and was probably originated by partial melting of a high-grade continental crust. On the other hand, two megacrystic to augen metagranitoids yield protolith crystallization ages of ca. 224 Ma, which are identical within errors to the protolith age of hosted eclogitic metabasites. Their high incompatible trace element abundances together with the observed spatial-temporal relationships with mafic protoliths suggest that Late Triassic bimodal magmatism in the Chuacús complex was probably originated in a within-plate setting. Regardless of their age or structure, the studied metagranites preserve evidences for high-pressure metamorphic equilibration, such as the occurrence of Ca-rich garnet (XCa up to 0.52) in association with phengite (Si contents of up to 3.4 pfu) and rutile. The integration of Zr-in-rutile thermometry and phengite barometry allows the peak metamorphic conditions to be constrained at 640-680 °C and 13 kbar. This

  15. Crustal structure of the North Iberian continental margin from seismic refraction/wide-angle reflection profiles

    Science.gov (United States)

    Ruiz, M.; Díaz, J.; Pedreira, D.; Gallart, J.; Pulgar, J. A.

    2017-10-01

    The structure and geodynamics of the southern margin of the Bay of Biscay have been investigated from a set of 11 multichannel seismic reflection profiles, recorded also at wide angle offsets in an onshore-offshore network of 24 OBS/OBH and 46 land sites. This contribution focuses on the analysis of the wide-angle reflection/refraction data along representative profiles. The results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the Iberian Peninsula. Offshore, the crust has a typical continental structure in the eastern tip of the bay, which disappears smoothly towards the NW to reach crustal thickness close to 10 km at the edge of the studied area ( 45°N, 6°W). The analysis of the velocity-depth profiles, altogether with additional information provided by the multichannel seismic data and magnetic surveys, led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic. Typical oceanic crust has not been imaged in the investigated area. Onshore, the new results are in good agreement with previous results and document the indentation of the Bay of Biscay crust into the Iberian crust, forcing its subduction to the North. The interpreted profiles show that the extent of the southward indentation is not uniform, with an Alpine root less developed in the central and western sector of the Basque-Cantabrian Basin. N-S to NE-SW transfer structures seem to control those variations in the indentation degree.

  16. The De Long Trough: a newly discovered glacial trough on the East Siberian continental margin

    Directory of Open Access Journals (Sweden)

    M. O'Regan

    2017-09-01

    Full Text Available Ice sheets extending over parts of the East Siberian continental shelf have been proposed for the last glacial period and during the larger Pleistocene glaciations. The sparse data available over this sector of the Arctic Ocean have left the timing, extent and even existence of these ice sheets largely unresolved. Here we present new geophysical mapping and sediment coring data from the East Siberian shelf and slope collected during the 2014 SWERUS-C3 expedition (SWERUS-C3: Swedish – Russian – US Arctic Ocean Investigation of Climate-Cryosphere-Carbon Interactions. The multibeam bathymetry and chirp sub-bottom profiles reveal a set of glacial landforms that include grounding zone formations along the outer continental shelf, seaward of which lies a  >  65 m thick sequence of glacio-genic debris flows. The glacial landforms are interpreted to lie at the seaward end of a glacial trough – the first to be reported on the East Siberian margin, here referred to as the De Long Trough because of its location due north of the De Long Islands. Stratigraphy and dating of sediment cores show that a drape of acoustically laminated sediments covering the glacial deposits is older than ∼ 50 cal kyr BP. This provides direct evidence for extensive glacial activity on the Siberian shelf that predates the Last Glacial Maximum and most likely occurred during the Saalian (Marine Isotope Stage (MIS 6.

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

    Science.gov (United States)

    ZUO, Xuran; CHAN, Lung

    2015-04-01

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

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

    OpenAIRE

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

    2018-01-01

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

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

    Science.gov (United States)

    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

  20. DELP Symposium: Tectonics of eastern Asia and western Pacific Continental Margin

    Science.gov (United States)

    Eastern Asia and the western Pacific make up a broad region of active plate tectonic interaction. The area is a natural laboratory for studying the processes involved in the origin and evolution of volcanic island arcs, marginal basins, accretionary prisims, oceanic trenches, accreted terranes, ophiolite emplacement, and intracontinental deformation. Many of our working concepts of plate tectonics and intraplate deformation were developed in this region, even though details of the geology and geophysics there must be considered of a reconnaissance nature.During the past few years researchers have accumulated a vast amount of new and detailed information and have developed a better understanding of the processes that have shaped the tectonic elements in this region. To bring together scientists from many disciplines and to present the wide range of new data and ideas that offer a broader perspective on the interrelations of geological, geochemical, geophysical and geodetic studies, the symposium Tectonics of Eastern Asia and Western Pacific Continental Margin was held December 13-16, 1988, at the Tokyo Institute of Technology in Japan, under the auspicies of DELP (Dynamics and Evolution of the Lithosphere Project).

  1. Seamounts along the Iberian continental margins; Los montes submarinos en los margenes continentales de Iberia

    Energy Technology Data Exchange (ETDEWEB)

    Vazquez, J. T.; Alonso, B.; Fernandez-Puga, M. C.; Gomez-Ballesteros, M.; Iglesias, J.; Palomino, D.; Roque, C.; Ercilla, G.; Diaz-del-Rio, V.

    2015-07-01

    Seamounts are first-order morphological elements on continental margins and in oceanic domains, which have been extensively researched over recent decades in all branches of oceanography. These features favour the development of several geological processes, and their study gives us a better understanding of their geological and morphological domains. The seamounts around Iberia are numerous and provide excellent examples of the geo diversity of these morphological elements. Here we present a compilation of 15 seamounts around the Iberian Peninsula. These seamounts have different origins related to the geodynamic evolution (volcanism, extensional or compressive tectonics, and diapirism) of the domains where they are located. The current configuration of their relief has been influenced by Neogene-Quaternary tectonics. Their positioning controls the current morpho-sedimentary processes in the basins and on the margins, and high- lights the fact that downslope processes on seamount flanks (mass flows, turbidite flows, and landslides) and processes parallel to seamounts (contouritic currents) correspond to the major geological features they are associated with them. Biogenic structures commonly develop on the tops of seamounts where occasionally isolated shelves form that have carbonate-dominated sedimentation. (Author)

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

    Science.gov (United States)

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

    2011-04-01

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

  3. Quantitative distribution of metazoan meiofauna in continental margin sediments of the Skagerrak (Northeastern North Sea)

    Science.gov (United States)

    De Bovée, F.; Hall, P. O. J.; Hulth, S.; Hulthe, G.; Landén, A.; Tengberg, A.

    1996-02-01

    A quantitative survey of metazoan meiofauna in continental-margin sediments of the Skagerrak was carried out using virtually undisturbed sediment samples collected with a multiple corer. Altogether 11 stations distributed along and across the Norwegian Trench were occupied during three cruises. Abundance ranged from 155 to 6846 ind·10 cm -2 and revealed a sharply decreasing trend with increasing water depth. The densities were high on the upper part of the Danish margin (6846 ind·10 cm -2 at 194 m depth) and low in the central part of the deep Skagerrak (155 ind·10 cm -2 at 637 m depth). Also body lengths were significantly shorter on the Danish margin then elsewhere in the Skagerrak, indicating a greater importance of juveniles in this area. We suggest that the high densities may be explained by a stimulated renewal of the fauna, possibly induced by an adequate food supply. The low abundances found in sediments from the deepest part of the Norwegian Trench cannot be attributed to any lack of oxygen. We suggest that the low meiofaunal abundances are caused by a decrease in the food supply (accentuated in this area by lower sedimentation rates) and/or by the very high concentrations of dissolved manganese in the pore water of these sediments. The metazoan meiofauna was largely dominated by nematodes. Comparison of the respiration rates of the nematode population with the total benthic respiration (0.5 to 14%) suggests that the relative importance of metazoan meiofauna decreased with water depth.

  4. Structural interpretation of the Konkan basin, southwestern continental margin of India, based on magnetic and bathymetric data

    Digital Repository Service at National Institute of Oceanography (India)

    Subrahmanyam, V.; Krishna, K.S.; Murty, G.P.S.; Rao, D.G.; Ramana, M.V.; Rao, M.G.

    Magnetic and bathymetric studies on the Konkan basin of the southwestern continental margin of India reveal prominent NNW-SSE, NW-SE, ENE-WSW, and WNE-ESE structural trends. The crystalline basement occurs at about 5-6 km below the mean sea level. A...

  5. Influence of marginal highs on the accumulation of organic carbon along the continental slope off western India

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, B.R.; Veerayya, M.

    and 743E (after De Sousa, unpublished data). by the gasometric method using the &karbonate-bombe’ (Muller and Gastner, 1971) and organic carbon by the wet oxidation method (El Wakeel and Riley, 1957). Some samples from the upper continental slope, marginal...

  6. Late-Quaternary variations in clay minerals along the SW continental margin of India: Evidence of climatic variations

    Digital Repository Service at National Institute of Oceanography (India)

    Chauhan, O.S.; Sukhija, B.S.; Gujar, A.R.; Nagabhushanam, P.; Paropkari, A.L.

    Down-core variations in illite, chlorite, smectite and kaolinite (the major clays) in two sup(14)C-dated cores collected along the SW continental margin of India show that illite and chlorite have enhanced abundance during 20-17, 12.5, 11-9.5, and 5...

  7. Bathymetric highs in the mid-slope region of the western continental margin of India - Structure and mode of origin

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, D.G.; Paropkari, A.L.; Krishna, K.S.; Chaubey, A.K.; Ajay, K.K.; Kodagali, V.N.

    Analysis of the multi- and single beam bathymetric, seismic, magnetic and free-air gravity (ship-borne and satellite derived) data from the western continental margin of India between 12 degrees 40 minutes N and 15 degrees N had revealed...

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

    Science.gov (United States)

    Robertson, Alastair

    2016-04-01

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

  9. A Comparison of Continental Extension Estimates Across the Margins of the Woodlark Basin, Papua New Guinea

    Science.gov (United States)

    Nazlim, B.; Goodliffe, A. M.

    2016-12-01

    Previous studies have shown that depth dependent extension is commonly observed across rifted margins. This has resulted in a discrepancy between the estimates of extension made through whole lithosphere/crust vs fault heave calculations (for example northwest Australia, South China Sea, Galicia). In the Woodlark Basin, the amount of extension estimated from observed subsidence and brittle extension also do not match. Taking into account sub-seismic resolution and poly-phase faulting reduces this mismatch. In the Woodlark Basin continental extension can also be estimated by extending Euler pole kinematics from the oceanic domain. Previous studies show that this predicts almost double the extension calculated from subsidence and brittle extension. Extension in the Woodlark Basin began at 8.4 Ma and transitioned to sea-floor spreading in the east at 6 Ma. The basin is an ideal place to study the extension discrepancy because of its young age and thin sediments. Seismic reflection easily images basement and fault structures. High resolution bathymetry permits tracing of major faults on the seafloor. A previous study focused on the extension discrepancy at the rifting to spreading transition. This study will focus on the discrepancy further east where seafloor spreading began just after 2 Ma and opening rates are faster. Data used in this study include bathymetry, magnetics, gravity, and low-fold 2-D seismic reflection data. Using the available data, extension estimates have been calculated through brittle extension and subsidence. Euler pole derived extension rates from previous studies were used for comparison. Results indicate that Euler pole kinematics predict far more extension than estimates calculated through subsidence and brittle extension. This provides important insights into processes in the low crust and supports earlier hypotheses that the mantle lithosphere and upper crust may be moving at different rates prior to continental breakup.

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

    Science.gov (United States)

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

    2011-12-01

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

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

    Science.gov (United States)

    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

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

    Science.gov (United States)

    Lang, Guy; Lazar, Michael; Schattner, Uri

    2017-04-01

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

  13. Potential tsunamigenic hazard associated to submarine mass movement along the Ionian continental margin (Mediterranean Sea).

    Science.gov (United States)

    Ceramicola, S.; Tinti, S.; Praeg, D.; Zaniboni, F.; Planinsek, P.

    2012-04-01

    Submarine mass movements are natural geomorphic processes that transport marine sediment down continental slopes into deep-marine environments. Type of mass wasting include creep, slides, slump, debris flows, each with its own features and taking place over timescale from seconds to years. Submarine landslides can be triggered by a number of different causes, either internal (such as changes in physical chemical sediment properties) or external (e.g. earthquakes, volcanic activity, salt movements, sea level changes etc.). Landslides may mobilize sediments in such a way as to form an impulsive vertical displacement of a body of water, originating a wave or series of waves with long wavelengths and long periods called tsunamis ('harbor waves'). Over 600 km of continental margin has been investigated by OGS in the Ionian sea using geophysical data - morpho-bathymetry (Reson 8111, 8150) and sub-bottom profiles (7-10 KHz) - collected aboard the research vessel OGS Explora in the framework of the MAGIC Project (Marine Geohazard along the Italian Coasts), funded by the Italian Civil Protection. The objective of this project is the definition of elements that may constitute geological risk for coastal areas. Geophysical data allowed the recognition of four main types of mass wasting phenomena along the slopes of the ICM: 1) mass transport complexes (MTCs) within intra-slope basins. Seabed imagery show the slopes of all the seabed ridges to be marked by headwall scarps recording widespread failure, multiple debris flows in several basins indicate one or more past episodes of failure that may be linked to activity on the faults bounding the structural highs. 2) submarine landslide - a multiple failure event have been identified (Assi landslide) at about 6 km away from the coastline nearby Riace Marina. Headwall scars up to 50 m high across water depths of 700 to 1400 m, while sub-bottom profiles indicate stacked slide deposits at and near seabed. 4) canyon headwalls - in the

  14. The tectonic significance of the Cabo Frio Tectonic Domain in the SE Brazilian margin: a Paleoproterozoic through Cretaceous saga of a reworked continental margin

    Directory of Open Access Journals (Sweden)

    Renata da Silva Schmitt

    Full Text Available ABSTRACT: The Cabo Frio Tectonic Domain is composed of a Paleoproterozoic basement tectonically interleaved with Neoproterozoic supracrustal rocks (Buzios-Palmital successions. It is in contact with the Neoproterozoic-Cambrian Ribeira Orogen along the SE Brazilian coast. The basement was part of at least three continental margins: (a 1.97 Ga; (b 0.59 - 0.53 Ga; (c 0.14 Ga to today. It consists of continental magmatic arc rocks of 1.99 to 1.94 Ga. Zircon cores show a 2.5 - 2.6 Ga inheritance from the ancient margin of the Congo Craton. During the Ediacaran, this domain was thinned and intruded by tholeiitic mafic dykes during the development of an oceanic basin at ca. 0.59 Ma. After the tectonic inversion, these basin deposits reached high P-T metamorphic conditions, by subduction of the oceanic lithosphere, and were later exhumed as nappes over the basement. The Cabo Frio Tectonic Domain collided with the arc domain of the Ribeira Orogen at ca. 0.54 Ga. It is not an exotic block, but the eastern transition between this orogen and the Congo Craton. Almost 400 m.y. later, the South Atlantic rift zone followed roughly this suture, not coincidently. It shows how the Cabo Frio Tectonic Domain was reactivated as a continental margin in successive extensional and convergent events through geological time.

  15. Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

    KAUST Repository

    Laruelle, G. G.; Dü rr, H. H.; Lauerwald, R.; Hartmann, J.; Slomp, C. P.; Regnier, P. A. G.

    2012-01-01

    files. Our analysis provides detailed insights into the distributions of coastal and continental shelf areas and how they connect with incoming riverine fluxes. The segmentation is also used to re-evaluate the global estuarine CO2 flux at the air–water interface combining global and regional average emission rates derived from local studies.

  16. Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins

    NARCIS (Netherlands)

    Laruelle, G.G.; Dürr, H.H.; Lauerwald, R.; Hartmann, J.; Slomp, C.P.; Goossens, N.; Regnier, P.A.G.

    2013-01-01

    Past characterizations of the land–ocean continuum were constructed either from a continental perspective through an analysis of watershed river basin properties (COSCATs: COastal Segmentation and related CATchments) or from an oceanic perspective, through a regionalization of the proximal and

  17. Comparative organic geochemistry of Indian margin (Arabian Sea) sediments: estuary to continental slope

    Science.gov (United States)

    Cowie, G.; Mowbray, S.; Kurian, S.; Sarkar, A.; White, C.; Anderson, A.; Vergnaud, B.; Johnstone, G.; Brear, S.; Woulds, C.; Naqvi, S. W.; Kitazato, H.

    2014-02-01

    Surface sediments from sites across the Indian margin of the Arabian Sea were analysed for their carbon and nitrogen compositions (elemental and stable isotopic), grain size distributions and biochemical indices of organic matter (OM) source and/or degradation state. Site locations ranged from the estuaries of the Mandovi and Zuari rivers to depths of ~ 2000 m on the continental slope, thus spanning nearshore muds and sands on the shelf and both the semi-permanent oxygen minimum zone (OMZ) on the upper slope (~ 200-1300 m) and the seasonal hypoxic zone that impinges on the shelf. Source indices showed mixed marine and terrigenous OM within the estuaries, and overwhelming predominance (80%+) of marine OM on the shelf and slope. Thus, riverine OM is heavily diluted by autochthonous marine OM and/or is efficiently remineralised within or immediately offshore of the estuaries. Any terrigenous OM that is exported appears to be retained in nearshore muds; lignin phenols indicate that the small terrigenous OM content of slope sediments is of different origin, potentially from rivers to the north. Organic C contents of surface shelf and slope sediments varied from winnowing and/or dilution) on the shelf and progressive OM degradation with increasing oxygen exposure below the OMZ. Reduced oxygen exposure may contribute to OM enrichment at some sites within the OMZ, but hydrodynamic processes are the overriding control on sediment OM distribution.

  18. Age and composition of the UHP garnet peridotites in the Dabie orogenic belt (central China) record complex crust-mantle interaction in continental margin

    Science.gov (United States)

    Zhao, Y.; Zheng, J.; Wang, B.

    2017-12-01

    The Dabie-Sulu UHP belt was created by the collision between the North and South China cratons in Middle Triassic time (240-225 Ma). There are lots of garnet-bearing ultramafic body occurs as a lens in the belt. Age and composition of the Maowu garnet peridotites in the Dabie orogenic belt are reported. The garnet harzburgites are main moderately refractory (Mg#Ol=92) and minor fertile (Mg#Ol=88) with high Ni (2344-2603 ppm) and low Al2O3 (0.35-0.54 wt.%), CaO (0.76-2.19 wt.%) and TiO2 (˜0.01 wt.%). Zircons in the harzburgites mainly document metamorphism at 230 ± 2 Ma, 275 ± 5 Ma, 357 ± 4 Ma, and complex minor populations of ages including: 1.8 Ga, 1.3 Ga, and Neoproterozic-early Paleozoic ages (901-420 Ma). The early Meszosic and late Paleozoic zircons have similar trace-element patterns and ranges in ɛHf(t) (+0.6 to +3.4), Th/U ratio (0.2-0.7) and Hf depleted-mantle model ages (TDM ) mainly cluster in the interval 1.2-0.9 Ga. In contrast, the Paleo-Mesoproterozoic zircons have negative ɛHf(t) (-24.9 to -2.7) and oldest Hf TDM = 3.4Ga. Zircons of Neoproterozic-early Paleozoic have a wide range of Hf depleted-mantle model ages (2.4-0.7Ga) and ɛHf(t) (-15.3 to +9.5). Above of the all, we suggest that the Maowu garnet harzburgites are interpreted as a fragment of the metasomatized ancient lithospheric mantle beneath the southern margin of the North China Craton. They experienced the Proterozoic thermal event (1.9-1.8Ga), which is coeval with the assembly of the supercontinent Columbia. And then 1.3Ga mantle metasomatism with asthenospheric materials resulted in the final breakup of the Columbia supercontinent. Neoproterozic-early Paleozoic (901-420 Ma), deep parts of the south margin of the craton were metasomatized during the assembly and breakup of the Rodinia supercontinent. Then, the southern margin of the craton occurred oceanic crust subduction ( 357 Ma), subsequent continental deep subduction and final continent-continent collision in Triassic.

  19. Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

    DEFF Research Database (Denmark)

    Boyle, R. A.; Clark, J. R.; Poulton, Simon W.

    2013-01-01

    Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic similar to 0.55 Ma, despite oxygenation of Earth's atmosphere nearly 2 Gyr earlier. Marine sediments from the intervening period suggest predominantly ferruginous (anoxic Fe(II)-rich) waters, interspersed with euxinia...... (anoxic H2S-rich conditions) along productive continental margins. Today, sustained biotic H2S production requires NO3- depletion because denitrifiers outcompete sulphate reducers. Thus, euxinia is rare, only occurring concurrently with (steady state) organic carbon availability when N-2-fixers dominate...... the production in the photic zone. Here we use a simple box model of a generic Proterozoic coastal upwelling zone to show how these feedbacks caused the mid-Proterozoic ocean to exhibit a spatial/temporal separation between two states: photic zone NO3- with denitrification in lower anoxic waters, and N-2...

  20. Lithosphere/asthenosphere interaction during continental breakup: preliminary isotopic date on the passive Galicia margin (North-Atlantic)

    International Nuclear Information System (INIS)

    Charpentier, S.; Kornprobst, J.; Chazot, G.; Cornen, G.

    1998-01-01

    The Galicia Margin ultramafic ridge has been cross-cut by diorites, pyroxenites and gabbros before the end of the rifting stage, and then by dolerites, after the continental break-full; it has been further overlaid by basaltic lava flows. The younger the rocks, the higher the initial ξ Nd (2.2-8.8). This evolution would be the result of the contamination of liquids extracted from the asthenosphere, by the enriched (ξ Ndi =4.0) and partially melted previous continental lithosphere. Time-decreasing contamination is related to progressive lithospheric thinning from the end to the beginning of oceanic spreading. (authors)

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

    Science.gov (United States)

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

    2015-04-01

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

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

    Science.gov (United States)

    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

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

    Science.gov (United States)

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

    2009-01-01

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

  4. Relationship between radionuclides and sedimentological variables in the South Atlantic Continental Margin; Relacoes entre radionuclideos e variaveis sedimentologicas na Margem Continental do Atlantico Sul

    Energy Technology Data Exchange (ETDEWEB)

    Ferreira, Paulo A.L.; Figueira, Rubens C.L., E-mail: paulo.alves.ferreira@usp.br, E-mail: rfigueira@usp.br [Universidade de Sao Paulo (IO/USP), SP (Brazil). Instituto Oceanografico

    2015-07-01

    There is a lack of information regarding marine radioactivity in sediments of the Continental Margin of the South Atlantic. {sup 137}Cs and {sup 40}K radioactivity and sedimentological variables were determined in superficial sediment samples. It was demonstrated that {sup 40}K is a good indicator for sediment granulometry, whilst {sup 137}Cs presents a good correlation with its chemical composition. Moreover, it was identified through the radiometric data the occurrence of input of allochtonous matter to the Brazilian southernmost compartment from the Rio de La Plata estuary, as previously reported in the literature. (author)

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

    Science.gov (United States)

    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

  6. Geomorphic Thresholds of Submarine Canyons Along the U.S. Atlantic Continental Margin

    Science.gov (United States)

    Brothers, D. S.; ten Brink, U. S.; Andrews, B. D.; Chaytor, J. D.

    2011-12-01

    Vast networks of submarine canyons and associated channels are incised into the U.S. Atlantic continental slope and rise. Submarine canyons form by differential erosion and deposition, primarily from sedimentary turbidity flows. Theoretical and laboratory studies have investigated the initiation of turbidity flows and their capacity to erode and entrain sedimentary material at distances far from the shelf edge. The results have helped understand the nature of turbidite deposits on the continental slope and rise. Nevertheless, few studies have examined the linkages between down-canyon sediment transport and the morphology of canyon/channel networks using mesoscale analyses of swath bathymetry data. We present quantitative analysis of 100-m resolution multibeam bathymetry data spanning ~616,000 km2 of the slope and rise between Georges Banks and the Blake Plateau (New England to North Carolina). Canyons are categorized as shelf-indenting or slope-confined based on spatial scale, vertical relief and connection with terrestrial river systems during sea level low stands. Shelf-indenting canyons usually represent the trunk-canyon of submerged channel networks. On the rise, shelf-indenting canyons have relatively well-developed channel-levees and sharp inner-thalwag incision suggesting much higher frequency and volume of turbidity flows. Because of the similarities between submarine canyon networks and terrestrial river systems, we apply methods originally developed to study fluvial morphology. Along-canyon profiles are extracted from the bathymetry data and the power-law relationship between thalwag gradient and drainage area is examined for more than 180 canyons along an ~1200 km stretch of the US Atlantic margin. We observe distinct thresholds in the power-law relationship between drainage area and gradient. Almost all canyons with heads on the upper slope contain at least two linear segments when plotted in log-log form. The first segment along the upper slope is flat

  7. Sulfur Cycling in an Iron Oxide-Dominated, Dynamic Marine Depositional System: The Argentine Continental Margin

    Directory of Open Access Journals (Sweden)

    Natascha Riedinger

    2017-05-01

    Full Text Available The interplay between sediment deposition patterns, organic matter type and the quantity and quality of reactive mineral phases determines the accumulation, speciation, and isotope composition of pore water and solid phase sulfur constituents in marine sediments. Here, we present the sulfur geochemistry of siliciclastic sediments from two sites along the Argentine continental slope—a system characterized by dynamic deposition and reworking, which result in non-steady state conditions. The two investigated sites have different depositional histories but have in common that reactive iron phases are abundant and that organic matter is refractory—conditions that result in low organoclastic sulfate reduction rates (SRR. Deposition of reworked, isotopically light pyrite and sulfurized organic matter appear to be important contributors to the sulfur inventory, with only minor addition of pyrite from organoclastic sulfate reduction above the sulfate-methane transition (SMT. Pore-water sulfide is limited to a narrow zone at the SMT. The core of that zone is dominated by pyrite accumulation. Iron monosulfide and elemental sulfur accumulate above and below this zone. Iron monosulfide precipitation is driven by the reaction of low amounts of hydrogen sulfide with ferrous iron and is in competition with the oxidation of sulfide by iron (oxyhydroxides to form elemental sulfur. The intervals marked by precipitation of intermediate sulfur phases at the margin of the zone with free sulfide are bordered by two distinct peaks in total organic sulfur (TOS. Organic matter sulfurization appears to precede pyrite formation in the iron-dominated margins of the sulfide zone, potentially linked to the presence of polysulfides formed by reaction between dissolved sulfide and elemental sulfur. Thus, SMTs can be hotspots for organic matter sulfurization in sulfide-limited, reactive iron-rich marine sedimentary systems. Furthermore, existence of elemental sulfur and iron

  8. Land-ocean tectonics (LOTs) and the associated seismic hazard over the Eastern Continental Margin of India (ECMI)

    Digital Repository Service at National Institute of Oceanography (India)

    Murthy, K.S.R.; Subrahmanyam, V.; Subrahmanyam, A.S.; Murty, G.P.S.; Sarma, K.V.L.N.S.

    , Subrahmanyam AS, Murty GPS, Murthy KSR (2009) Tectonic significance of Gundlakamma river (Krishna Basin) over Eastern Continental Margin of India – A qualitative appraisal (Communicated to Current Science) Subrahmanya K (1996) Active Intraplate deformation... for his suggestions to improve the MS. Thanks are also due to Miss.T.Madhavi for her help in preparing the illustrations This is N.I.O. (C.S.I.R.) contribution No. References Banerjee PK, Vaz VV, Sengupta BJ, Bagchi A (2001) A qualitative...

  9. Assessment of Submarine Slope Stability on the Continental Margin off SW Taiwan

    Science.gov (United States)

    Hsu, Huai-Houh; Dong, Jia-Jyun; Cheng, Win-Bin; Su, Chih-Chieh

    2017-04-01

    The abundant gas hydrate reservoirs are distributed in the southwest (SW) off Taiwan. To explore this new energy, geological methods were systematically used and mainly emphasized on the storage potential evaluation. On the other hand, the correlation between gas hydrate dissociation and submarine slope stability is also an important issue. In this study, three submarine profiles on the active and passive continental margin were selected and assessed their slope stabilities by considering two influence factors (seismic forces and number of sedimentary layers). The gravity corers obtained from these three sites (Xiaoliuqiu, Yuan-An Ridge, and Pointer Ridge) to conduct soil laboratory tests. The physical property tests and isotropically consolidated undrained (CIU) triaxial tests were carried out to establish reference properties and shear strength parameters. Before the stability analysis is performed, it is also necessary to construct the seabed profile. For each submarine profile, data from P-waves and from S-waves generated by P-S conversion on reflection from airgun shots recorded along one line of ocean bottom seismometers were used to construct 2-D velocity sections. The seabed strata could be simplified to be only one sedimentary layer or to be multilayer in accordance with the velocity structure profile. Results show the safety factors (FS) of stability analysis are obviously different in considering the number of sedimentary layers, especially for a very thin layer of sediments on a steep slope. The simplified strata condition which treated all seabed strata as only one sedimentary layer might result in the FS lower than 1 and the slope was in an unstable state. On the contrary, the FS could be higher than 10 in a multilayer condition.

  10. Calcareous nannoplankton and benthic foraminiferal assemblages from the Nazare Canyon (Portuguese continental margin): Preliminary results

    International Nuclear Information System (INIS)

    Guerreiro, C; Oliveira, A; Rodrigues, A; Rosa, F; Cachao, M; Fatela, F

    2009-01-01

    Submarine canyons are assumed to play an important role in oceanic/neritic circulation, marine productivity and sedimentary processes, acting as preferential conduits between the littoral and deep oceanic domain. Here we present first results of a comparative micropalaeontological study on calcareous nannoplankton and benthic foraminifera from surface sediments from the surroundings of the upper Nazare Canyon (Portuguese continental margin) and from the shelf north of the canyon. Regardless of the difficulty to distinguish taphonomical from (palaeo)ecological effects in such a complex and still poorly known marine system, the first results suggest that the canyon's hydro-sedimentary dynamic regime act as a prolongation of the shore/inner shelf hydrodynamic conditions towards west, preventing deposition and/or preservation of the smaller and fragile species of calcareous nannoplankton (e.g. E. huxleyi and G. ericsonii) and enhancing the record of the larger and more opportunistic ones (e.g. G. oceanica); and disturbing benthic foraminiferal productivity and/or diversity, or their preservation in the fossil record. Both calcareous nannoplankton and benthic foraminifera are more abundant off the canyon's domain, suggesting that its highly energetic thalweg conditions are probably filtering the fossil record in the sediment. Still, preliminary results suggest that the occurrence of persistent physical phenomena related with the canyon's morphology and proximity to the coast (e.g. solitary internal waves) may be locally promoting favourable conditions for calcareous nannoplankton, as shown by high values of nannoliths, chlorophyll a and 19' hexanoyloxyfucoxantine (unpublished data) north of the canyon's head. It is our goal to test this hypothesis in the near future by (a) studying multicore and surficial sediments from more recent surveys, and (b) calibrating the sediment results with water column data presently in process at the Institute of Oceanography (IO).

  11. Calcareous nannoplankton and benthic foraminiferal assemblages from the Nazare Canyon (Portuguese continental margin): Preliminary results

    Energy Technology Data Exchange (ETDEWEB)

    Guerreiro, C; Oliveira, A; Rodrigues, A [Division of Marine Geology, Portuguese Hydrographic Institute (IH), Rua das Trinas 49, 1249-093 Lisboa (Portugal); Rosa, F [CIACOMAR, Algarve University, Av. 16 de Julho s/n 8700-311 Olhao (Portugal); Cachao, M; Fatela, F [Geology Center and Geology Department, FCUL, Bloco C6, 3o Piso, sala 6.3.57 Campo Grande 1749-016 Lisboa (Portugal)], E-mail: catarina.guerreiro@hidrografico.pt

    2009-01-01

    Submarine canyons are assumed to play an important role in oceanic/neritic circulation, marine productivity and sedimentary processes, acting as preferential conduits between the littoral and deep oceanic domain. Here we present first results of a comparative micropalaeontological study on calcareous nannoplankton and benthic foraminifera from surface sediments from the surroundings of the upper Nazare Canyon (Portuguese continental margin) and from the shelf north of the canyon. Regardless of the difficulty to distinguish taphonomical from (palaeo)ecological effects in such a complex and still poorly known marine system, the first results suggest that the canyon's hydro-sedimentary dynamic regime act as a prolongation of the shore/inner shelf hydrodynamic conditions towards west, preventing deposition and/or preservation of the smaller and fragile species of calcareous nannoplankton (e.g. E. huxleyi and G. ericsonii) and enhancing the record of the larger and more opportunistic ones (e.g. G. oceanica); and disturbing benthic foraminiferal productivity and/or diversity, or their preservation in the fossil record. Both calcareous nannoplankton and benthic foraminifera are more abundant off the canyon's domain, suggesting that its highly energetic thalweg conditions are probably filtering the fossil record in the sediment. Still, preliminary results suggest that the occurrence of persistent physical phenomena related with the canyon's morphology and proximity to the coast (e.g. solitary internal waves) may be locally promoting favourable conditions for calcareous nannoplankton, as shown by high values of nannoliths, chlorophyll a and 19' hexanoyloxyfucoxantine (unpublished data) north of the canyon's head. It is our goal to test this hypothesis in the near future by (a) studying multicore and surficial sediments from more recent surveys, and (b) calibrating the sediment results with water column data presently in process at the Institute of

  12. IODP Expedition 307 Drills Cold-Water Coral Mound Along the Irish Continental Margin

    Directory of Open Access Journals (Sweden)

    Trevor Williams

    2006-03-01

    Full Text Available Introduction Over the past decade, oceanographic and geophysical surveys along the slope of the Porcupine Seabight off the southwestern continental margin of Ireland have identified upwards of a thousand enigmatic mound-like structures (Figs. 1 and 2. The mounds of the Porcupine Seabight rise from the seafl oor in water depths of 600–900 m and formimpressive conical bodies several kilometers wide and up to 200 m high. Although a few mounds such as Thérèse Mound and Galway Mound are covered by a thriving thicket of coldwater corals, most mound tops and fl anks are covered by dead coral rubble or are entirely buried by sediment (De Mol et al., 2002; Fig. 2, Beyer et al., 2003. Lophelia pertusa (Fig.3 and Madrepora oculata are the most prominent cold-water corals growing without photosynthetic symbionts. The widespread discovery of large and numerous coral-bearing banks and the association of these corals with the mounds have generated signifi cant interest as to the composition, origin and development of these mound structures.Challenger Mound, in the Belgica mound province, has an elongated shape oriented along a north-northeast to south-southwest axis and ispartially buried under Pleistocene drift sediments. In high-resolution seismic profiles the mounds appear to root on an erosion surface (van Rooij et al., 2003. During IODP Expedition307 the Challenger Mound in the Porcupine Seabight was drilled with the goal of unveiling the origin and depositional processes withinthese intriguing sedimentary structures. Challenger Mound, unlike its near neighbors the Thérèse and Galway mounds, has little to no livecoral coverage and, therefore, was chosen as the main target for drilling activities, so that no living ecosystem would be disturbed.

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

    Directory of Open Access Journals (Sweden)

    E. Gholamrezaie

    2018-02-01

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

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

    Science.gov (United States)

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

    2018-02-01

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

  15. Quaternary development of resilient reefs on the subsiding kimberley continental margin, Northwest Australia

    Directory of Open Access Journals (Sweden)

    Lindsay B. Collins

    2010-01-01

    Full Text Available The Kimberley region in remote northwest Australia has poorly known reef systems of two types; coastal fringing reefs and atoll-like shelf-edge reefs. As a major geomorphic feature (from 12ºS to 18ºS situated along a subsiding continental margin, the shelf edge reefs are in a tropical realm with warm temperatures, relatively low salinity, clear low nutrient waters lacking sediment input, and Indo-West Pacific corals of moderate diversity. Seismic architecture of the Rowley Shoals reveals that differential pre-Holocene subsidence and relative elevation of the pre-Holocene substrate have controlled lagoon sediment infill and reef morphology, forming an evolutionary series reflecting differential accommodation in three otherwise similar reef systems. The Holocene core described for North Scott Reef confirms previous seismic interpretations, and provides a rare ocean-facing reef record. It demonstrates that the Indo-Pacific reef growth phase (RG111 developed during moderate rates of sea level rise of 10 mm/year from 11 to about 7-6.5 ka BP until sea level stabilization, filling the available 27 m of pre-Holocene accommodation. Despite the medium to high hydrodynamic energy imposed by the 4m tides, swell waves and cyclones the reef-building communities represent relatively low-wave energy settings due to their southeast facing and protection afforded by the proximity of the South Reef platform. This study demonstrates the resilience of reefs on the subsiding margin whilst linking Holocene reef morphology to the relative amount of pre-Holocene subsidence.Kimberly é uma região remota e pouco conhecida, localizada no noroeste da Austrália, ali são encontrados dois sistemas recifais: recifes costeiros de franja e os tipo-atois localizados na margem da plataforma continental. Esses recifes formam a feição geomórfica mais importante entre 12ºS a 18ºS estando localizados ao longo de uma margem continental em subsidência. Esses recifes encontram

  16. Extensional tectonics and sedimentary response of the Early–Middle Cambrian passive continental margin, Tarim Basin, Northwest China

    Directory of Open Access Journals (Sweden)

    Zhiqian Gao

    2012-09-01

    Full Text Available The fact that several half-grabens and normal faults developed in the Lower–Middle Cambrian of Tazhong (central Tarim Basin and Bachu areas in Tarim Basin, northwest China, indicates that Tarim Basin was under extensional tectonic setting at this time. The half-grabens occur within a linear zone and the normal faults are arranged in en echelon patterns with gradually increasing displacement eastward. Extensional tectonics resulted in the formation of a passive continental margin in the southwest and a cratonic margin depression in the east, and most importantly, influenced the development of a three-pronged rift in the northeast margin of the Tarim Basin. The fault system controlled the development of platform – slope – bathyal facies sedimentation of mainly limestone-dolomite-gypsum rock-saline rock-red beds in the half-grabens. The NW-SE trending half-grabens reflect the distribution of buried basement faults.

  17. Unravel Spurious Bathymetric Highs on the Western Continental Margin of India

    Science.gov (United States)

    Mahale, V. P.

    2017-12-01

    Swath mapping multibeam echosounder systems (MBES) have become a de-facto-standard component on today's research vessel (RV). Modern MBES provide high temporal and spatial resolution for mapping the seabed morphology. Improved resolution capabilities requires large hull mounted transceivers, which after installation undergoes calibration procedure during the sea acceptance test (SAT). To accurately estimate various vessel offsets and lever-arm corrections, the installer runs calibration lines over a prominent seabed feature. In the year 2014, while conducting SAT for the RV Sindhu Sadhana and calibrate the ATLAS make MBES system, a hunt was on to find suitable bathymetric highs in the region of operation. Regional hydrographic charts published by the National Hydrographic Office, in India were referred to locate such features. Two bathymetric highs were spotted on the chart that are 20 km apart and 40 km west of the shelf-edge on the Western Continental Margin of India. The charted depth on these highs are 252 m and 343 m on a relatively even but moderately sloppy seabed, representing an isolated elevations of 900 m. The geographic locations of these knolls were verified with the GEBCO's 30-arc second gridded bathymetry, before heading out for the waypoints. There were no signs of knolls at those locations, indicating erroneous georeferencing. Hence, the region was subsequently revisited in the following years until an area of 3000 sq. km was mapped. Failing to locate the bathymetric highs they are referred to as 'spurious'. Investigation was planned to unravel the rationale of existence and sustenance of these knolls in the hydrographic charts since historic time. Tweaking the MBES settings reveals existence of strong acoustic scattering layer, to which even the depth tracking gate gets locked-on and is documented. Analogically, in the past, ships transecting the region equipped with single beam echosounder tuned for shallow depth operations might have charted the

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

    Science.gov (United States)

    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

  19. Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

    Science.gov (United States)

    Davy, R. G.; Morgan, J. V.; Minshull, T. A.; Bayrakci, G.; Bull, J. M.; Klaeschen, D.; Reston, T. J.; Sawyer, D. S.; Lymer, G.; Cresswell, D.

    2018-01-01

    Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterized by a complex pattern of faulting, thin continental fault blocks and the serpentinization, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Traveltime tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here, we demonstrate the application of 2-D time-domain acoustic full-waveform inversion (FWI) to deep-water seismic data collected at the Deep Galicia Margin, in order to attain a high-resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modeled waveforms, checkerboard tests, testing of parameter and inversion strategy and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector, there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinization. This study supports the hypothesis that normal faulting acts to hydrate the upper-mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinization. Our results confirm the feasibility of applying the FWI method to sparse, deep-water crustal data sets.

  20. Peridotites and mafic igneous rocks at the foot of the Galicia Margin: an oceanic or continental lithosphere? A discussion

    Energy Technology Data Exchange (ETDEWEB)

    Korprobst, J.; Chazot, G.

    2016-10-01

    An ultramafic/mafic complex is exposed on the sea floor at the foot of the Galicia Margin (Spain and Portugal). It comprises various types of peridotites and pyroxenites, as well as amphibole-diorites, gabbros, dolerites and basalts. For chronological and structural reasons (gabbros were emplaced within peridotites before the continental break-up) this unit cannot be assigned to the Atlantic oceanic crust. The compilation of all available petrological and geochemical data suggests that peridotites are derived from the sub-continental lithospheric mantle, deeply transformed during Cretaceous rifting. Thus, websterite dykes extracted from the depleted MORB mantle reservoir (DMM), were emplaced early within the lithospheric harzburgites; subsequent boudinage and tectonic dispersion of these dykes in the peridotites, during deformation stages at the beginning of rifting, resulted in the formation of fertile but isotopically depleted lherzolites. Sterile but isotopically enriched websterites, would represent melting residues in the peridotites, after significant partial melting and melt extraction related to the thermal erosion of the lithosphere. The latter melts are probably the source of brown amphibole metasomatic crystallization in some peridotites, as well as of the emplacement of amphibole-diorite dykes. Melts directly extracted from the asthenosphere were emplaced as gabbro within the sub-continental mantle. Mixing these DMM melts together with the enriched melts extracted from the lithosphere, provided the intermediate isotopic melt-compositions - in between the DMM and Oceanic Islands Basalts reservoir - observed for the dolerites and basalts, none of which are characterized by a genuine N-MORB signature. An enriched lithospheric mantle, present prior to rifting of the Galicia margin, is in good agreement with data from the Messejana dyke (Portugal) and more generally, with those of all continental tholeiites of the Central Atlantic Magmatic Province (CAMP

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

    Science.gov (United States)

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

    2008-12-01

    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: "Remote sensing and sea-truth measurements of methane flux to the atmosphere." The project will address this problem with a combined effort of satellite remote sensing and data collection at proven sites in the Gulf of Mexico where gas hydrate releases gas to the water column. Submarine gas hydrate is a large pool of greenhouse gas that may interact with the atmosphere over geologic time to affect climate cycles. In the near term, the magnitude of methane reaching the atmosphere from gas hydrate on continental margins is poorly known because 1) gas hydrate is exposed to metastable oceanic conditions in shallow, dispersed deposits that are poorly imaged by standard geophysical techniques and 2) the consumption of methane in marine sediments and in the water column is subject to uncertainty. The northern GOM is a prolific hydrocarbon province where rapid migration of oil, gases, and brines from deep subsurface petroleum reservoirs occurs through faults generated by salt tectonics. Focused expulsion of hydrocarbons is manifested at the seafloor by gas vents, gas hydrates, oil seeps, chemosynthetic biological communities, and mud volcanoes. Where hydrocarbon seeps occur in depths below the hydrate stability zone (~500m), rapid flux of gas will feed shallow deposits of gas hydrate that potentially interact with water column temperature changes; oil released from seeps forms sea-surface features that can be detected in remote-sensing images. The regional phase of the project will quantify verifiable sources of methane (and oil) the Gulf of Mexico continental margin and selected margins (e.g. Pakistan Margin, South China Sea

  2. Reactivation of the Archean-Proterozoic suture along the southern margin of Laurentia during the Mazatzal orogeny: Petrogenesis and tectonic implications of ca. 1.63 Ga granite in southeastern Wyoming

    Science.gov (United States)

    Jones, Daniel S.; Barnes, Calvin G.; Premo, Wayne R.; Snoke, Arthur W.

    2013-01-01

    The presence of ca. 1.63 Ga monzogranite (the “white quartz monzonite”) in the southern Sierra Madre, southeastern Wyoming, is anomalous given its distance from the nearest documented plutons of similar age (central Colorado) and the nearest contemporaneous tectonic margin (New Mexico). It is located immediately south of the Cheyenne belt—a ca. 1.75 Ga Archean-Proterozoic tectonic suture. New geochronological, isotopic, and geochemical data suggest that emplacement of the white quartz monzonite occurred between ca. 1645 and 1628 Ma (main pulse ca. 1628 Ma) and that the white quartz monzonite originated primarily by partial melting of the Big Creek Gneiss, a modified arc complex. There is no evidence that mafic magmas were involved. Open folds of the ca. 1750 Ma regional foliation are cut by undeformed white quartz monzonite. On a regional scale, rocks intruded by the white quartz monzonite have experienced higher pressure and temperature conditions and are migmatitic as compared to the surrounding rocks, suggesting a genetic relationship between the white quartz monzonite and tectonic exhumation. We propose that regional shortening imbricated the Big Creek Gneiss, uplifting the now-exposed high-grade rocks of the Big Creek Gneiss (hanging wall of the thrust and wall rock to the white quartz monzonite) and burying correlative rocks, which partially melted to form the white quartz monzonite. This tectonism is attributed to the ca. 1.65 Ga Mazatzal orogeny, as foreland shortening spread progressively into the Yavapai Province. Mazatzal foreland effects have also been described in the Great Lakes region and have been inferred in the Black Hills of South Dakota. We suggest that the crustal-scale rheologic contrast across the Archean-Proterozoic suture, originally developed along the southern margin of Laurentia, and including the Cheyenne belt, facilitated widespread reactivation of that boundary during the Mazatzal orogeny. This finding emphasizes the degree to

  3. Terrigenous sediment supply along the Chilean continental margin: modern regional patterns of texture and composition

    Science.gov (United States)

    Lamy, F.; Hebbeln, D.; Wefer, G.

    The regional patterns of texture and composition of modern continental slope and pelagic sediments off Chile between 25°S and 43°S reflect the latitudinal segmentation of geological, morphological, and climatic features of the continental hinterland. Grain-size characteristics are controlled by the grain-size of source rocks, the weathering regime, and mode of sediment input (eolian off northern Chile vs fluvial further south). Bulk-mineral assemblages reveal a low grade of maturity. Regional variations are governed by the source-rock composition of the different geological terranes and the relative source-rock contribution of the Coastal Range and Andes, as controlled by the continental hydrology. The relative abundance of clay minerals is also predominantly influenced by the source-rock composition and partly by continental smectite neoformation. Latitudinal variations of illite crystallinities along the Chilean continental slope (and west of the Peru-Chile trench) clearly reflect modifications of the weathering regime which correspond to the strong climatic zonation of Chile.

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

    International Nuclear Information System (INIS)

    Harben, P.; Rodgers, A.

    1999-01-01

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

  5. Bottom current processes along the Iberian continental margin; Procesos sedimentarios por corrientes de fondo a lo largo del margen continental iberico

    Energy Technology Data Exchange (ETDEWEB)

    Llave, E.; Hernandez-Molina, F. J.; Ercilla, G.; Roque, C.; Van Rooij, D.; Garcia, M.; Juan, C.; Mena, A.; Brackenridge, R.; Jane, G.; Stow, D.; Gomez-Ballesteros, M.

    2015-07-01

    The products of bottom current circulation around the Iberian continental margin are characterised by large erosional and depositional features formed under a variety of geological and oceanographic contexts. The Iberian margins are influenced by several water masses that mainly interact along the upper and middle con- tinental slopes, as well as along the lower slope with the abyssal plains being influenced to a lesser extent. The main depositional features occur along the Ceuta Contourite Depositional System (CDS) within the SW Alboran Sea, in the Gulf of Cadiz (the most studied so far), the western margins of the Portugal/Galician mar- gin, the Ortegal Spur and the Le Danois Bank or Cachucho. Moreover, erosional contourite features have also been recently indentified, most notably terraces, abraded surfaces, channels, furrows and moats. The majority of these features are formed under the influence of the Mediterranean water masses, especially by the interaction of the Mediterranean Outflow Water (MOW) with the seafloor. The MOW is characterized as relatively warm (13 degree centigrade) and with a high salinity (∼36.5), giving it a high density relative to the surrounding water masses, hence constituting an important contribution to the global thermohaline circulation, making it one of the most studied water masses surrounding Iberia. The development of both depositional and ero- sional contourite features does not only depend on the bottom-current velocity but also on several other important controlling factors, including: 1) local margin morphology affected by recent tectonic activity; 2) multiple sources of sediment supply; 3) water-mass interphases interacting with the seafloor; and 4) glacioeustatic changes, especially during the Quaternary, when the increasing influence of the bottom cur- rent has been observed during the cold stages. The main objective of this special volume contribution is to provide a review and description of the regional along

  6. 137Cs as tracer of the origin of allochthonous sediments in the Southeast Continental Margin of Brazil

    International Nuclear Information System (INIS)

    Ferreira, Paulo A.L.; Mahiques, Michel M.; FIgueira, Rubens C.L.

    2015-01-01

    The distribution of 137 Cs, artificial radionuclide for which there is no current source, can inform on the origin and destination of sediments. This study analyzed about 60 samples of surface sediment to generate a model of spatial distribution of 137 Cs in the Southeast Continental Margin of Brazil and surroundings for evaluating possible sediment sources for this region. The model showed that the levels of 137 Cs in the southern compartment of the Southeast Brazilian Margin (south of Sao Sebastiao Island) are statistically similar to those of the Rio de la Plata river mouth region, indicating sediment entry due to the seasonal intrusion of the plume of Rio de la Plata, a phenomenon already studied by other authors

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

    Science.gov (United States)

    Brothers, D.S.; Ruppel, C.; Kluesner, J.W.; ten Brink, Uri 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.

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

    Science.gov (United States)

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

    2014-10-01

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

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

    Science.gov (United States)

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

    2016-04-01

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

  10. The potential for satellite and marginal field developments on the Norwegian continental shelf

    International Nuclear Information System (INIS)

    Raustein, O.; Abrahamsen, L.E.; Einang, G.

    1994-01-01

    Norway is faced with decreasing field sizes in hostile waters. On the other hand, approximately 620 billion 1993-NOK have been invested in field installations and transport systems. These installations will have significant available processing and transport capacity in the future, and thus represent a valuable infrastructure. This paper describes the resource situation and the installed infrastructure on the Norwegian Continental Shelf. Then the potential of still maintaining a high activity level in field developments is outlined

  11. Indian continental margin gas hydrate prospects : results of the Indian National Gas Hydrate Program (NGHP) expedition 01

    Energy Technology Data Exchange (ETDEWEB)

    Collett, T [United States Geological Survey, Denver, CO (United States); Riedel, M. [McGill Univ., Montreal, PQ (Canada). Dept. of Earth and Planetary Sciences; Cochran, J.R. [Columbia Univ., Palisades, NY (United States). Lamont Doherty Earth Observatory; Boswell, R. [United States Dept. of Energy, Morgantown, WV (United States). National Energy Technology Lab; Kumar, P. [Pushpendra Kumar Oil and Natural Gas Corp. Ltd., Mumbai (India). Inst. of Engineering and Ocean Technology; Sathe, A.V. [Oil and Natural Gas Corp. Ltd., Uttaranchal (India). KDM Inst. of Petroleum Exploration

    2008-07-01

    The geologic occurrence of gas hydrate deposits along the continental margins of India were investigated in the first expedition of the Indian National Gas Hydrate Program (NGHP). The objective was to determine the regional context and characteristics of the gas hydrate deposits through scientific ocean drilling, logging, and analytical activities. A research drill ship was the platform for the drilling operation. The geological and geophysical studies revealed 2 geologically distinct areas with inferred gas hydrate occurrences, notably the passive continental margins of the Indian Peninsula and along the Andaman convergent margin. The NGHP Expedition 01 focused on understanding the geologic and geochemical controls on the occurrence of gas hydrate in these 2 diverse settings. The study established the presence of gas hydrates in Krishna-Godavari, Mahanadi and Andaman basins. Site 10 in the Krishna-Godavari Basin was discovered to be the one of the richest gas hydrate accumulations yet documented, while site 17 in the Andaman Sea had the thickest and deepest gas hydrate stability zone yet known. The existence of a fully-developed gas hydrate system in the Mahanadi Basin was also discovered. Most of the gas hydrate occurrences discovered during this expedition appeared to contain mostly methane which was generated by microbial processes. However, there was also evidence of a thermal origin for a portion of the gas within the hydrates of the Mahanadi Basin and the Andaman offshore area. Gas hydrate in the Krishna-Godavari Basin appeared to be closely associated with large scale structural features, in which the flux of gas through local fracture systems, generated by the regional stress regime, controlled the occurrence of gas hydrate. 3 refs., 1 tab., 2 figs.

  12. Geomorphic characterization of four shelf-sourced submarine canyons along the U.S. Mid-Atlantic continental margin

    Science.gov (United States)

    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

  13. Tectono-Magmatic Evolution of the South Atlantic Continental Margins with Respect to Opening of the Ocean

    Science.gov (United States)

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

    2018-03-01

    The history of the opening of the South Atlantic in Early Cretaceous time is considered. It is shown that the determining role for continental breakup preparation has been played by tectono-magmatic events within the limits of the distal margins that developed above the plume head. The formation of the Rio Grande Rise-Walvis Ridge volcanic system along the trace of the hot spot is considered. The magmatism in the South Atlantic margins, its sources, and changes in composition during the evolution are described. On the basis of petrogeochemical data, the peculiarities of rocks with a continental signature are shown. Based on Pb-Sr-Nd isotopic studies, it is found that the manifestations of magmatism in the proximal margins had features of enriched components related to the EM I and EM II sources, sometimes with certain participation of the HIMU source. Within the limits of the Walvis Ridge, as magmatism expanded to the newly formed oceanic crust, the participation of depleted asthenospheric mantle became larger in the composition of magmas. The role played by the Tristan plume in magma generation is discussed: it is the most considered as the heat source that determined the melting of the ancient enriched lithosphere. The specifics of the tectono-magmatic evolution of the South Atlantic is pointed out: the origination during spreading of a number of hot spots above the periphery of the African superplume. The diachronous character of the opening of the ocean is considered in the context of northward progradation of the breakup line and its connection with the northern branch of the Atlantic Ocean in the Mid-Cretaceous.

  14. Aedes (Stegomyia) aegypti in the continental United States: a vector at the cool margin of its geographic range.

    Science.gov (United States)

    Eisen, Lars; Moore, Chester G

    2013-05-01

    After more than a half century without recognized local dengue outbreaks in the continental United States, there were recent outbreaks of autochthonous dengue in the southern parts of Texas (2004-2005) and Florida (2009-2011). This dengue reemergence has provoked interest in the extent of the future threat posed by the yellow fever mosquito, Aedes (Stegomyia) aegypti (L.), the primary vector of dengue and yellow fever viruses in urban settings, to human health in the continental United States. Ae. aegypti is an intriguing example of a vector species that not only occurs in the southernmost portions of the eastern United States today but also is incriminated as the likely primary vector in historical outbreaks of yellow fever as far north as New York, Philadelphia, and Boston, from the 1690s to the 1820s. For vector species with geographic ranges limited, in part, by low temperature and cool range margins occurring in the southern part of the continental United States, as is currently the case for Ae. aegypti, it is tempting to speculate that climate warming may result in a northward range expansion (similar to that seen for Ixodes tick vectors of Lyme borreliosis spirochetes in Scandinavia and southern Canada in recent decades). Although there is no doubt that climate conditions directly impact many aspects of the life history of Ae. aegypti, this mosquito also is closely linked to the human environment and directly influenced by the availability of water-holding containers for oviposition and larval development. Competition with other container-inhabiting mosquito species, particularly Aedes (Stegomyia) albopictus (Skuse), also may impact the presence and local abundance of Ae. aegypti. Field-based studies that focus solely on the impact of weather or climate factors on the presence and abundance of Ae. aegypti, including assessments of the potential impact of climate warming on the mosquito's future range and abundance, do not consider the potential confounding

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

    Science.gov (United States)

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

    2015-04-01

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

  16. Structure of the Lithosphere-Asthenosphere Boundary Onshore and Offshore the California Continental Margin from Three-Dimensional Seismic Anisotropy

    Science.gov (United States)

    Gomez, C. D.; Escobar, L., Sr.; Rathnayaka, S.; Weeraratne, D. S.; Kohler, M. D.

    2016-12-01

    The California continental margin, a major transform plate boundary in continental North America, is the locus of complex tectonic stress fields that are important in interpreting both remnant and ongoing deformational strain. Ancient subduction of the East Pacific Rise spreading center, the rotation and translation of tectonic blocks and inception of the San Andreas fault all contribute to the dynamic stress fields located both onshore and offshore southern California. Data obtained by the ALBACORE (Asthenospheric and Lithospheric Broadband Architecture from the California Offshore Region Experiment) and the CISN (California Integrated Seismic Network) seismic array are analyzed for azimuthal anisotropy of Rayleigh waves from 80 teleseismic events at periods 16 - 78 s. Here we invert Rayleigh wave data for shear wave velocity structure and three-dimensional seismic anisotropy in the thee regions designated within the continental margin including the continent, seafloor and California Borderlands. Preliminary results show that seismic anisotropy is resolved in multiple layers and can be used to determine the lithosphere-asthenosphere boundary (LAB) in offshore and continental regions. The oldest seafloor in our study at age 25-35 Ma indicates that the anisotropic transition across the LAB occurs at 73 km +/- 25 km with the lithospheric fast direction oriented WNW-ESE, consistent with current Pacific plate motion direction. The continent region west of the San Andreas indicates similar WNW-ESE anisotropy and LAB depth. Regions east of the San Andreas fault indicate NW-SE anisotropy transitioning to a N-S alignment at 80 km depth north of the Garlock fault. The youngest seafloor (15 - 25 Ma) and outer Borderlands indicate a more complex three layer fabric where shallow lithospheric NE-SW fast directions are perpendicular with ancient Farallon subduction arc, a mid-layer with E-W fast directions are perpendicular to remnant fossil fabric, and the deepest layer

  17. Verdine and glaucony facies from surficial sediments of the eastern continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, V.P.; Thamban, M.; Lamboy, M.

    -trioctahedral Fe-rich 1:l clay mineral. Clay Miner., 23: 237-247. Murthy, K.S.R., 1989. Seismic stratigraphy of Ongole-Paradip continental shelf East coast of India. Indian J. Mar. Sci., 16: 47-58. Murthy, K.S.R., Rao, T.C.S., Subramanyam, AS., Rao, M... at the continent- ocean boundary: the verdine facies. Clay Miner. 25: 477-483. Rao, K.M. and Rao, T.C.S., 1994. Holocene sea levels of Visakhapatnam shelf, East Coast of India. J. Geol. Sot. India, 44: 685-689. Rao, V.P., 1991. Clay mineral distribution...

  18. Echinodermata, Ophiuroidea, Gorgonocephalus Leach, 1815: First report of the genus for the Brazilian continental margin

    Directory of Open Access Journals (Sweden)

    Barboza, C. A. M.

    2010-01-01

    Full Text Available The Gorgonocephalidae includes 38 genera, five of them reported for Brazilian waters. Gorgonocephalus chilensishas a wide distribution throughout Antartica and Subantartican regions and its northern limit was restricted to the coast ofUruguay. This work aims to report the first occurrence of the Gorgonocephalus genus for the Brazilian continental marginand extend the northern limit of distribution of G. chilensis to the coast of Santa Catarina. Tolerance to a large temperatureand bathimetric range are crucial to understand the distributions patterns of ophiuroids from the polar circle that alsooccur at southern South America.

  19. Characterizing slope morphology using multifractal technique: a study from the western continental margin of India.

    Digital Repository Service at National Institute of Oceanography (India)

    Chakraborty, B.; Karisiddaiah, S.M.; Menezes, A.A.A.; Haris, K.; Gokul, G.S.; Fernandes, W.A.; Kavitha, G.

    margin of India. Geo-Mar Lett 26: 114–119. doi: 10.1007/s00367-006-0022-6 Cheng Q, Agterberg FP (1996) Comparison between two types of multifractal modeling. Math Geol 28 (8): 1001–1015 Dandapath S, Chakraborty B, Maslov N, Karisiddaiah SM...

  20. Magnetic anomalies of offshore Krishna–Godavari basin, eastern continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Swamy, K.V.; Murthy, I.V.R.; Krishna, K.S.; Murthy, K.S.R.; Subrahmanyam, A.S.; Rao, M.M.M.

    with the volcanic material. Inversion of the magnetic and gravity anomalies was also carried out to establish the similarity of anomalies of the two geological features (structural high on the margin and the 85 degrees E Ridge) and their interpretations. In both...

  1. Structure and petroleum potential of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska

    Science.gov (United States)

    Bruns, T.R.

    1982-01-01

    Major structural features of the Yakutat segment, the segment of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska, are delineated by multichannel seismic reflection data. A large structural high is centered on Fairweather Ground and lies generally at the edge of the shelf from Cross Sound to west of the Alsek Valley. A basement uplift, the Dangerous River zone, along which the seismic acoustic basement shallows by up to two kilometers, extends north from the western edge of Fairweather Ground towards the mouth of the Dangerous River. The Dangerous River zone separates the Yakutat segment into two distinct subbasins. The eastern subbasin has a maximum sediment thickness of about 4 km, and the axis of the basin is near and parallel to the coast. Strata in this basin are largely of late Cenozoic age (Neogene and Quaternary) and approximately correlate with the onshore Yakataga Formation. The western subbasin has a maximum of at least 9 km of sediment, comprised of a thick (greater than 4.5 km) Paleogene section overlain by late Cenozoic strata. The Paleogene section is truncated along the Dangerous River zone by a combination of erosion, faulting, and onlap onto the acoustic basement. Within the western subbasin, the late Cenozoic basin axis is near and parallel to the coast, but the Paleogene basin axis appears to trend in a northwest direction diagonally across the shelf. Sedimentary strata throughout the Yakutat shelf show regional subsidence and only minor deformation except in the vicinity of the Fairweather Ground structural high, near and along the Dangerous River zone, and at the shoreline near Lituya Bay. Seismic data across the continental slope and adjacent deep ocean show truncation at the continental slope of Paleogene strata, the presence of a thick (to 6 km) undeformed or mildly deformed abyssal sedimentary section at the base of the slope that in part onlaps the slope, and a relatively narrow zone along the slope or at

  2. Methane Migration and Its Influence on Sulfate Reduction in the Good Weather Ridge Region, South China Sea Continental Margin Sediments

    Directory of Open Access Journals (Sweden)

    Saulwood Lin

    2006-01-01

    Full Text Available Bacteria sulfate reduction is a major pathway for organic carbon oxidation in marine sediments. Upward diffusion of methane from gas hydrate deep in the sedimentary strata might be another important source of carbon for sulfate reducing bacteria and subsequently induce higher rates of sulfate reduction in sediments. Since abundant gas may migrate upward to the surface as a result of tectonic activity occurring in the accretionary wedge, this study investigates the effect of methane migration on the sulfate reduction process in continental margin sediments offshore southwestern Taiwan. Piston and gravity core samples were taken in order to evaluate vertical and spatial variations of sulfate and methane. Pore water sulfate, sulfide, methane, sediment pyrite, and organic carbon were extracted and analyzed.

  3. Structural lineaments from the magnetic anomaly maps of the eastern continental margin of India (ECMI) and NW Bengal Fan

    Digital Repository Service at National Institute of Oceanography (India)

    Murthy, K.S.R.; Rao, T.C.S.; Subrahmanyam, A; Rao, M.M.M.; Lakshminarayana, S.

    extension of 85 degrees E ridge, abutting the continental shelf off Chilka Lake and (3) trend 3, locted over the continental shelf/slope between Visakhapatnm and Paradip represents a folded (ridges and depressions) nature of the continental basement...

  4. Deposition and Burial Efficiency of Terrestrial Organic Carbon Exported from Small Mountainous Rivers to the Continental Margin, Southwest of Taiwan

    Science.gov (United States)

    Hsu, F.; Lin, S.; Wang, C.; Huh, C.

    2007-12-01

    Terrestrial organic carbon exported from small mountainous river to the continental margin may play an important role in global carbon cycle and it?|s biogeochemical process. A huge amount of suspended materials from small rivers in southwestern Taiwan (104 million tons per year) could serve as major carbon source to the adjacent ocean. However, little is know concerning fate of this terrigenous organic carbon. The purpose of this study is to calculate flux of terrigenous organic carbon deposited in the continental margin, offshore southwestern Taiwan through investigating spatial variation of organic carbon content, organic carbon isotopic compositions, organic carbon deposition rate and burial efficiency. Results show that organic carbon compositions in sediment are strongly influenced by terrestrial material exported from small rivers in the region, Kaoping River, Tseng-wen River and Er-jan Rver. In addition, a major part of the terrestrial materials exported from the Kaoping River may bypass shelf region and transport directly into the deep sea (South China Sea) through the Kaoping Canyon. Organic carbon isotopic compositions with lighter carbon isotopic values are found near the Kaoping River and Tseng-wen River mouth and rapidly change from heavier to lighter values through shelf to slope. Patches of lighter organic carbon isotopic compositions with high organic carbon content are also found in areas west of Kaoping River mouth, near the Kaoshiung city. Furthermore, terrigenous organic carbons with lighter isotopic values are found in the Kaoping canyon. A total of 0.028 Mt/yr of terrestrial organic carbon was found in the study area, which represented only about 10 percent of all terrestrial organic carbon deposited in the study area. Majority (~90 percent) of the organic carbon exported from the Kaoping River maybe directly transported into the deep sea (South China Sea) and become a major source of organic carbon in the deep sea.

  5. Submarine geo-hazards on the eastern Sardinia-Corsica continental margin based on preliminary pipeline route investigation

    Science.gov (United States)

    Cecchini, S.; Taliana, D.; Giacomini, L.; Herisson, C.; Bonnemaire, B.

    2011-03-01

    The understanding of the morphology and the shallow geo-hazards of the seafloor is a major focus for both academic and private industry research. On November and December 2009 a geophysical pipeline survey was carried out by Fugro Oceansismica S.p.A. (FOSPA) and FUGRO France (FFSA) for DORIS Engineering on behalf of GRTgaz (Engineering centre, Transmission Pipe Department; http://www.grtgaz.com) which are currently investigating the possibility of laying a pipeline between Sardinia and Corsica as a spur line from the planned GALSI Project. The Project, "Alimentation de la Corse en gaz naturel", consists of a corridor 100 km long and 1.0 km wide along the Corsica-Sardinia shelf. The integration of the multibeam, sidescan sonar and sparker data provided a high resolution seafloor mapping for geo-hazard assessment. In this article the data acquired along a break of slope section (approximately 20 km × 1.5 km), in the eastern sector of the Strait of Bonifacio are described. The area was abandoned during the survey, because of its unsuitability. Indeed, in this area the continental shelf, approximately 100 m deep and deepening gently eastward, is characterized by an uneven morphology, with different seabed features such as Beach- rocks mainly NNW-SSE oriented. Also, the continuity of the continental margin, identified around -110/-115 m, is interrupted by four canyon heads which incise the slope and are associated with glide deposits.

  6. 3D thermal modelling within the Lofoten-Vesterålen segment of the Mid-Norwegian continental margin

    Science.gov (United States)

    Maystrenko, Yuriy P.; Gernigon, Laurent; Gradmann, Sofie; Olesen, Odleiv

    2017-04-01

    A lithosphere-scale 3D structural model has been constructed based on the available structural data to reveal a deep structure of the Lofoten-Vesterålen segment and the northern part of the Vøring segment of the Mid-Norwegian continental margin. The constructed model covers the Vestfjorden, Ribban and Røst basins, the northern parts of the Vøring Basin and the Trøndelag Platform. The model also extends from the Fennoscandian Shield to the north-eastern part of the North Atlantic Ocean. The initial 3D structural model has been refined using a 3D gravity modelling over the whole study area. The final gravity-consistent model has been used as a structural base for a further 3D thermal modelling, which has been made by use of commercial software package COMSOL Multiphysics. As an upper thermal boundary condition, time-dependent temperature at the Earth's surface and sea bottom has been set, considering palaeoclimatic changes due to the last two Europe-scale glaciations (the Saalian and Weichselian glacial periods). The lithosphere-asthenosphere boundary has been used as a lower thermal boundary which corresponds to the 1300 °C isotherm. In addition to the above-mentioned paleoclimatic scenario, the effects of late Cenozoic erosion onshore and sedimentation offshore have been taken into account during the 3D thermal modelling. Results of this thermal modelling indicate that the continent is generally colder than the basin areas within the upper part of the 3D model. In particular, considering the transient perturbations in the near-surface thermal regime, as a result of the post-Paleogene erosion and sedimentation, helps us to understand additional details of subsurface temperature distribution within the study area. The thermal effects of the simultaneous erosion over the mainland and deposition within the basin areas indicate that a positive thermal anomaly should exist onshore, whereas the negative one must occur in the offshore part. These two thermal

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

    Digital Repository Service at National Institute of Oceanography (India)

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

    of the northern and western Gulf of Mexico, the north coast of Alaska and the Niger Delta [Gearing et al., 1977], surface sediments from the Beaufort shelf, Beaufort Sea [Goni et al., 2000], coastal sediments from the Gulf of Trieste, N Adriatic Sea [Ogrinc et... the chemical composition of SOC off the Washington margin [Keil et al. 1994; Prahl et al. 1994], Amazon [Goni, 1997], Bengal fans [Lanord and Derry, 1994], and Gulf of Mexico [Goni et al. 1997]. Other possible reason could be preferential removal of N...

  8. Basement configuration of Visakhapatnam - Paradip continental margin from inversion of magnetic anomalies

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, M.M.M.; Rao, S.J.; Venkateswarlu, K.; Murthy, K.S.R.; Murthy, I.V.R.; Subrahmanyam, A.S.

    . References 1 Curray J R, Emmel F J, Moore D G & Raitt R W, in: Ocean basins and margins, 6 (Plenum, New York) 1982, pp. 399-450. 2 Rao T C S & Murthy K S R, Magnetic surveys over the con- tinental shelf off Visakhapatnam, Mahasagar - Bull Nat Inst... Visakhapatnam, east coast of India, Indian J Earth Sci, 14(1987) 109-113. 8 Murthy K S R, Rao M M M, Rao T C S & Subrahmanyam A S, A comparative study of Werner deconvolution and con- ventional modelling of marine magnetic data, Geophy Res Bull, 25(1987) 152...

  9. Cenozoic sedimentation in the Mumbai Offshore Basin: Implications for tectonic evolution of the western continental margin of India

    Science.gov (United States)

    Nair, Nisha; Pandey, Dhananjai K.

    2018-02-01

    Interpretation of multichannel seismic reflection data along the Mumbai Offshore Basin (MOB) revealed the tectonic processes that led to the development of sedimentary basins during Cenozoic evolution. Structural interpretation along three selected MCS profiles from MOB revealed seven major sedimentary sequences (∼3.0 s TWT, thick) and the associated complex fault patterns. These stratigraphic sequences are interpreted to host detritus of syn- to post rift events during rift-drift process. The acoustic basement appeared to be faulted with interspaced intrusive bodies. The sections also depicted the presence of slumping of sediments, subsidence, marginal basins, rollover anticlines, mud diapirs etc accompanied by normal to thrust faults related to recent tectonics. Presence of upthrusts in the slope region marks the locations of local compression during collision. Forward gravity modeling constrained with results from seismic and drill results, revealed that the crustal structure beneath the MOB has undergone an extensional type tectonics intruded with intrusive bodies. Results from the seismo-gravity modeling in association with litholog data from drilled wells from the western continental margin of India (WCMI) are presented here.

  10. Recent Sedimentary Processes Along the Western Continental Margin of the South Korea Plateau, East Sea of Korea

    Science.gov (United States)

    Cukur, D.; Um, I. K.; Bahk, J. J.; Chun, J. H.; Lee, G. S.; Soo, K. G.; Horozal, S.; Kim, S. P.

    2017-12-01

    The continental margins of the marginal seas is largely shaped by a complex interplay of sediment transport processes directed both downslope and along-slope. Factors influence the sediment transport from shelf to the deep basin include: (i) seabed morphology, (ii) climate, (iii) sea level changes, (iv) slope stability, (v) oceanographic regime, and (vi) sediment sources. In order to understand the recent sedimentary processes along the western margin of the South Korea Plateau in the East Sea, we collected multiple geophysical datasets including the subbottom profiler and multibeam echosounder as well as geological sampling. Twelve echo types have been defined and interpreted as deposits formed by shallow marine, hemipelagic sedimentation, bottom currents, combined- (mass-movement/hemipelagic and hemipelagic/turbidites) and mass-movement-processes. Hemipelagic sedimentation, which is reflected as undisturbed layered sediments, appears to have been the primary sedimentary process throughout the study area. Two major slope-parallel channels appear to have acted as major conduits for turbidity currents from shallower shelf into the deep basins. Bottom current deposits, which is expressed as undulating seafloor morphology, are prevalent in the southern mid-slope at water depths between 250 to 450 m. Mass-transport deposits, consisting of chaotic seismic facies, occur in the upper and lower parts of the continental slope. Piston cores confirm the presence of MTDs that are characterized by mud clasts of variable size and shape. Multibeam bathymetry data show that these MTDs chiefly initiate on lower-slopes (400-600 m) where the gradient is up to 3°. In addition, subbottom profiles suggest the presence of numerous faults in close vicinity of headwall scarps; some are extending to the seafloor suggesting their recent activity. Earthquakes associated with tectonic activity are considered as the main triggering mechanism for these MTDs. Overall, the acoustic facies

  11. Authigenic carbonates in the sediments of Goa offshore basin, western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Kocherla, M.

    in all the studied locations. The methane con- centration varied from 0.45–5.25 nM (top) to 11.25 nM (middle) and 0.64–13.23 nM (bottom). The CaCO 3 content of the sediments ranged from 33.6% to 69.9% and TOC varied from 0.21% to 0.86%. The porosity.... G. and Almeida, F., Detection of gas charged sediments and gas hydrate horizons in high resolution seismic profiles from the western con- tinental margin of India. Spec. Publ. Geol. Soc. Lond., 1998, 137, 239–253. 15. Satyavani, N., Thankur, N. K...

  12. The biogeochemistry of carbon in continental slope sediments: The North Carolina margin

    Energy Technology Data Exchange (ETDEWEB)

    Blair, N.; Levin, L.; DeMaster, D.; Plaia, G.; Martin, C.; Fornes, W.; Thomas, C.; Pope, R.

    1999-12-01

    The responses of the continental slope benthos to organic detritus deposition were studied with a multiple trace approach. Study sites were offshore of Cape Fear (I) and Cape Hatteras (III), N.C. (both 850 m water depth) and were characterized by different organic C deposition rates, macrofaunal densities (III>I in both cases) and taxa. Natural abundances of {sup 13}C and {sup 12}C in particulate organic carbon (POC), dissolved inorganic carbon (DIC) and macrofauna indicate that the reactive organic detritus is marine in origin. Natural abundance levels of {sup 14}C and uptake of {sup 13}C-labeled diatoms by benthic animals indicate that they incorporate a relatively young component of carbon into their biomass. {sup 13}C-labeled diatoms (Thalassiorsira pseudonana) tagged with {sup 210}Pb, slope sediment tagged with {sup 113}Sn and {sup 228}Th-labeled glass beads were emplaced in plots on the seafloor at both locations and the plots were sampled after 30 min., 1-1.5 d and 14 mo. At Site I, tracer diatom was intercepted at the surface primarily by protozoans and surface-feeding annelids. Little of the diatom C penetrated below 2 cm even after 14 months. Oxidation of organic carbon appeared to be largely aerobic. At Site III, annelids were primarily responsible for the initial uptake of tracer. On the time scale of days, diatom C was transported to a depth of 12 cm and was found in animals collected between 5-10 cm. The hoeing of tracer from the surface by the maldanid Praxillela sp. may have been responsible for some of the rapid nonlocal transport. Oxidation of the diatom organic carbon was evident to at least 10 cm depth. Anaerobic breakdown of organic matter is more important at Site III. Horizontal transport, which was probably biologically mediated, was an order of magnitude more rapid than vertical displacement over a year time scale. If the horizontal transport was associated with biochemical transformations of the organic matter, it may represent an

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

    Science.gov (United States)

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

    2007-01-01

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

  14. New interpretations based on seismic and modelled well data and their implications for the tectonic evolution of the west Greenland continental margin

    DEFF Research Database (Denmark)

    Mcgregor, E.D.; Nielsen, S.B.; Stephenson, R.A.

    Davis Strait is situated between Baffin Island and Greenland and forms part of a sedimentary basin system, linking Labrador Sea and Baffin Bay, developed during Cretaceous and Palaeocene rifting that culminated in a brief period of sea-floor spreading in the late Palaeocene and Eocene. Seismic...... reflection profiles and exploration wells along the Greenland margin of Davis Strait have been analysed in order to elucidate uplift events affecting sedimentary basin development during the Cenozoic with a focus on postulated Neogene (tectonic) uplift affecting the west Greenland continental margin...... tectonic event. An interpretation in which the inferred onshore cooling is related to erosion of pre-existing topography is more consistent with our new results from the offshore region. These results will have important implications for other continental margins developed throughout the Atlantic...

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

    Science.gov (United States)

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

    2014-12-01

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

  16. Benthic-pelagic coupling: effects on nematode communities along southern European continental margins.

    Directory of Open Access Journals (Sweden)

    Ellen Pape

    Full Text Available Along a west-to-east axis spanning the Galicia Bank region (Iberian margin and the Mediterranean basin, a reduction in surface primary productivity and in seafloor flux of particulate organic carbon was mirrored in the in situ organic matter quantity and quality within the underlying deep-sea sediments at different water depths (1200, 1900 and 3000 m. Nematode standing stock (abundance and biomass and genus and trophic composition were investigated to evaluate downward benthic-pelagic coupling. The longitudinal decline in seafloor particulate organic carbon flux was reflected by a reduction in benthic phytopigment concentrations and nematode standing stock. An exception was the station sampled at the Galicia Bank seamount, where despite the maximal particulate organic carbon flux estimate, we observed reduced pigment levels and nematode standing stock. The strong hydrodynamic forcing at this station was believed to be the main cause of the local decoupling between pelagic and benthic processes. Besides a longitudinal cline in nematode standing stock, we noticed a west-to-east gradient in nematode genus and feeding type composition (owing to an increasing importance of predatory/scavenging nematodes with longitude governed by potential proxies for food availability (percentage of nitrogen, organic carbon, and total organic matter. Within-station variability in generic composition was elevated in sediments with lower phytopigment concentrations. Standing stock appeared to be regulated by sedimentation rates and benthic environmental variables, whereas genus composition covaried only with benthic environmental variables. The coupling between deep-sea nematode assemblages and surface water processes evidenced in the present study suggests that it is likely that climate change will affect the composition and function of deep-sea nematodes.

  17. Using crustal thickness, subsidence and P-T-t history on the Iberia-Newfoundland & Alpine Tethys margins to constrain lithosphere deformation modes during continental breakup

    Science.gov (United States)

    Jeanniot, L.; Kusznir, N. J.; Manatschal, G.; Mohn, G.; Beltrando, M.

    2013-12-01

    Observations at magma-poor rifted margins such as Iberia-Newfoundland show a complex lithosphere deformation history and OCT architecture, resulting in hyper-extended continental crust and lithosphere, exhumed mantle and scattered embryonic oceanic crust before continental breakup and seafloor spreading. 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 kinematic lithosphere deformation mode leading to continental breakup and sea-floor spreading cannot explain observations. We have determined the sequence of lithosphere deformation events, using forward modelling of crustal thickness, subsidence and P-T-t history calibrated against observations on the present-day Iberia-Newfoundland and the fossil analogue Alpine Tethys margins. Lithosphere deformation modes, represented by flow fields, 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 topmost upper 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). Buoyancy enhanced upwelling is also included in the kinematic model as predicted by Braun et al (2000). We predict melt generation by decompressional melting using the parameterization and methodology of Katz et al., 2003. We use a series of numerical experiments, tested and calibrated against crustal thicknesses and subsidence observations, to determine the distribution of lithosphere

  18. Subduction and exhumation of a continental margin in the Scandinavian Caledonides: Insights from ultrahigh pressure metamorphism, late orogenic basins and 3D numerical modelling

    Science.gov (United States)

    Cuthbert, Simon

    2017-04-01

    The Scandinavian Caledonides (SC) represents a plate collision zone of Himalayan style and scale. Three fundamental characteristics of this orogen are: (1) early foreland-directed, tectonic transport and stacking of nappes; (2) late, wholesale reversal of tectonic transport; (3) ultrahigh pressure metamorphism of felsic crust derived from the underthrusting plate at several levels in the orogenic wedge and below the main thrust surface, indicating subduction of continental crust into the mantle. The significance of this for crustal evolution is the profound remodeling of continental crust, direct geochemical interaction of such crust and the mantle and the opening of accommodation space trapping large volumes of clastic detritus within the orogen. The orogenic wedge of the SC was derived from the upper crust of the Baltica continental margin (a hyper-extended passive margin), plus terranes derived from an assemblage of outboard arcs and intra-oceanic basins and, at the highest structural level, elements of the Laurentian margin. Nappe emplacement was driven by Scandian ( 430Ma) collision of Baltica with Laurentia, but emerging Middle Ordovician ages for diamond-facies metamorphism for the most outboard (or rifted) elements of Baltica suggest prior collision with an arc or microcontinent. Nappes derived from Baltica continental crust were subducted, in some cases to depths sufficient to form diamond. These then detached from the upper part of the down-going plate along major thrust faults, at which time they ceased to descend and possibly rose along the subduction channel. Subduction of the remaining continental margin continued below these nappes, possibly driven by slab-pull of the previously subducted Iapetus oceanic lithosphere and metamorphic densification of subducted felsic continental margin. 3D numerical modelling based upon a Caledonide-like plate scenario shows that if a continental corner or promontory enters the subduction zone, the continental margin

  19. PROMESS 1: Past Global Changes Investigated by Drilling Mediterranean Continental Margins

    Science.gov (United States)

    Berne, S.

    2004-12-01

    Between June, 24th and July, 22nd, 2004, a team of European scientists embarked from Brindisi (Italy) to Barcelona (Spain) onboard the Russian vessel "Bavenit", operated by the Dutch geotechnical company FUGRO, for a drilling expedition in the Adriatic Sea and the NW Mediterranean Sea. The purpose of this cruise was to collect long sediment sections and in situ measurements from two deltaic margins where the history of global changes during the last ca. 400 kyr is particularly well preserved. In the Adriatic, two boreholes were drilled at site PRAD1 (water depth 184 m), where the objective was to study the record of the last 4 glacial cycles. A pilot hole was first drilled for assessing the risk of shallow gases, a downhole logging was carried out in this borehole. A second site allowed continuous coring to the targeted depth (71m below sea-floor) with excellent recovery (better than 95%). Very preliminary interpretation indicates that seismic sequences previously identified correspond to 100 kyr glacial cycles. Downhole logging and physical properties of cores allow to identify magnetic events, and tephras. Site PRAD2 was devoted to the study of the recent most sediments (last 12,000 yrs) near the coastline, at a water depth of 56m. The targeted depth was 32 m below sea floor, sufficient to obtain a good record for the last ca 12,000 years. All together, six boreholes were drilled at PRAD2, including a pilot hole, one for continuous sediment recovery, and additional holes for in situ geotechnical tests and sampling. One of the objectives of these tests is to determine whether the wavy features shaping the sedimentary sequences are caused by near-bottom currents or result from liquefaction of unstable sediments triggered by earthquakes or storms. Site PRGL1 in the Gulf of Lion is at 298 m water depth, and the targeted depth below sea floor was 300 m, allowing to reach an expected age of about 430 kyr BP. A pilot hole was drilled down to 310 mbsf, and logged. Two

  20. Fission track dating: methodology and thermo-chronological applications in alpine and continental margin contexts

    International Nuclear Information System (INIS)

    Sabil, N.

    1995-06-01

    the thermal history of the massifs considered since the last cooling below 120 C of the samples analysed. In the works done before 1994, we have shown that, on the one hand, the transform margin of Cote d'Ivoire-Ghana had known a heating period between 250 C and 60 C post-dating by far its scanning by an oceanic ridge and on the other hand, in the Elbe Island, we have brought the first 'fission track' data in the cooling history of the Monte Capanne granodiorite. In the beginning of the 90's, the fission track method still lacked good reference samples for volcanic glass dating. At the suggestion of the Geochronology Commission of the International Union of Geological Sciences we have studied macusanites, obsidians of the SE Peru. The results of this work suggest that these glasses are not convenient as potential age standards, even if they keep some value as a material for laboratory intercalibration purposes. (author)

  1. Petrology and isotopic composition of Quaternary basanites dredged from the Bering Sea continental margin near Navarin Basin

    Science.gov (United States)

    Davis, A.S.; Gunn, S.H.; Gray, L.-B.; Marlow, M. S.; Wong, F.L.

    1993-01-01

    Quaternary basanites were recovered from the continental margin of the Bering Sea near Navarin Basin. The basanites are highly vesicular flow rock and hyaloclastites similar to other alkalic volcanic rocks erupted repeatedly during the last Cenozoic on islands in the Bering Sea region and in mainland Alaska. K-Ar ages for the basanites indicate at least two episodes of volcanism at about 1.1 and 0.4 Ma. Trace-element data indicate these alkalic lavas have been generated by small, but variable, amounts of partial melting of a metasomatized lherzolite source. The relativley primitive compositions (MgO >9%), presence of mantle-derived xenoliths in some alkalic lavas, and presence of forsteritic olivine with low CaO and high NiO suggest that magma rose rapidly from great depth without spending time in large, long-lived magma chambers. Alkalic volcanism apparently resulted from upwelling and decompressional melting of small isolated mantle diapirs in response to local lithospheric attenuation associated with jostling of blocks during adjustment to regional stresses. -from Authors

  2. Re-assessing the nitrogen signal in continental margin sediments: New insights from the high northern latitudes

    Science.gov (United States)

    Knies, Jochen; Brookes, Steven; Schubert, Carsten J.

    2007-01-01

    Organic and inorganic nitrogen and their isotopic signatures were studied in continental margin sediments off Spitsbergen. We present evidence that land-derived inorganic nitrogen strongly dilutes the particulate organic signal in coastal and fjord settings and accounts for up to 70% of the total nitrogen content. Spatial heterogeneity in inorganic nitrogen along the coast is less likely to be influenced by clay mineral assemblages or various substrates than by the supply of terrestrial organic matter (TOM) within eroded soil material into selected fjords and onto the shelf. The δ15N signal of the inorganic nitrogen ( δ15N inorg) in sediments off Spitsbergen seems to be appropriate to trace TOM supply from various climate- and ecosystem zones and elucidates the dominant transport media of terrigenous sediments to the marine realm. Moreover, we postulate that with the study of sedimentary δ15N inorg in the Atlantic-Arctic gateway, climatically induced changes in catchment's vegetations in high northern latitudes may be reconstructed. The δ15N org signal is primarily controlled by the availability of nitrate in the dominating ocean current systems and the corresponding degree of utilization of the nitrate pool in the euphotic zone. Not only does this new approach allow for a detailed view into the nitrogen cycle for settings with purely primary-produced organic matter supply, it also provides new insights into both the deposition of marine and terrestrial nitrogen and its ecosystem response to (paleo-) climate changes.

  3. Conference on Continental margin mass wasting and Pleistocene sea-level changes, August 13-15, 1980

    Science.gov (United States)

    Folger, David W.; Hathaway, J.C.

    1987-01-01

    A conference on Continental Margin Mass Wasting and Pleistocene Sea-Level Changes was held in Woods Hole, Mass., August 13-15, 1980. Forty-seven participants, representing many government, academic, and industrial organizations, discussed the current state of knowledge of the features of marine mass wasting and of the interrelations of factors influencing them. These factors include sediment source, composition, textures, sedimentation rates, climatic and sea-level changes, gas and gas hydrate (clathrate) contents of sediments, geotechnical characteristics, oceanographic and morphological factors, ground-water processes, and seismic events. The part played by these factors in the processes and features of mass movement and the engineering considerations imposed by the emplacement of manmade structures on the sea floor were considered vital to the evaluation of hazards involved in offshore exploration and development. The conference concluded with a call for bold programs to establish the probability of occurrence and the quantitative importance of these factors and to devise more reliable means of measurement, particularly in place, of the characteristics of the sediment and features involved.

  4. The George V Land Continental Margin (East Antarctica): new Insights Into Bottom Water Production and Quaternary Glacial Processes from the WEGA project

    Science.gov (United States)

    Caburlotto, A.; de Santis, L.; Lucchi, R. G.; Giorgetti, G.; Damiani, D.; Macri', P.; Tolotti, R.; Presti, M.; Armand, L.; Harris, P.

    2004-12-01

    The George Vth Land represents the ending of one of the largest subglacial basin (Wilkes Basin) of the East Antarctic Ice Sheet (EAIS). Furthermore, its coastal areas are zone of significant production of High Salinity Shelf Water (HSSW). Piston and gravity cores and high resolution echo-sounding (3.5 kHz) and Chirp profiles collected in the frame of the joint Australian and Italian WEGA (WilkEs Basin GlAcial History) project provide new insights into the Quaternary history of the EAIS and the HSSW across this margin: from the sediment record filling and draping valleys and banks along the continental shelf, to the continuous sedimentary section of the mound-channel system on the continental rise. The discovery of a current-lain sediment drift (Mertz Drift, MD) provides clues to understanding the age of the last glacial erosive events, as well as to infer flow-pathways of bottom-water masses changes. The MD shows disrupted, fluted reflectors due to glacial advance during the LGM (Last Glacial Maximum) in shallow water, while undisturbed sediment drift deposited at greater water depth, indicates that during the LGM the ice shelf was floating over the deep sector of the basin. The main sedimentary environment characterising the modern conditions of the continental rise is dominated by the turbiditic processes with a minor contribution of contour currents action. Nevertheless, some areas (WEGA Channel) are currently characterised by transport and settling of sediment through HSSW, originating in the shelf area. This particular environment likely persisted since pre-LGM times. It could indicate a continuous supply of sedimentary material from HSSW during the most recent both glacial and interglacial cycles. This would be consistent with the results obtained in the continental shelf suggesting that the Ice Sheet was not grounding over some parts of the continental shelf. Furthermore, the comparison of the studied area with other Antarctic margins indicate that, contrary

  5. Ore-controlling mechanism of carbonaceous-siliceous-pelitic rock type uranium deposits with down-faulted red basins in the southeast continental margin of Yangtze plate

    International Nuclear Information System (INIS)

    Zhang Zilong; Qi Fucheng; He Zhongbo; Li Zhixing; Wang Wenquan; Yu Jinshui

    2012-01-01

    One of the important ore-concentrated areas of carbonaceous-siliceous-pelitic rock type uranium deposits is the Southeast continental margin of Yangtze plate. Sedimentary-exogenously transformed type and sedimentary- hydrothermal superimposed transformed type uranium deposits are always distributed at or near the edge of down-faulted red ba sins. In this paper, the distributions of the deposits are analyzed with the relation to down-faulted red basins. The connective effect and ore-controlling mechanism are proposed of carbonaceous-siliceous-pelitic rock type uranium deposits with marginal fractures of red basins. (authors)

  6. Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean.

    Science.gov (United States)

    Boyle, R A; Clark, J R; Poulton, S W; Shields-Zhou, G; Canfield, D E; Lenton, T M

    2013-01-01

    Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic ~0.55 Ma, despite oxygenation of Earth's atmosphere nearly 2 Gyr earlier. Marine sediments from the intervening period suggest predominantly ferruginous (anoxic Fe(II)-rich) waters, interspersed with euxinia (anoxic H(2)S-rich conditions) along productive continental margins. Today, sustained biotic H(2)S production requires NO(3)(-) depletion because denitrifiers outcompete sulphate reducers. Thus, euxinia is rare, only occurring concurrently with (steady state) organic carbon availability when N(2)-fixers dominate the production in the photic zone. Here we use a simple box model of a generic Proterozoic coastal upwelling zone to show how these feedbacks caused the mid-Proterozoic ocean to exhibit a spatial/temporal separation between two states: photic zone NO(3)(-) with denitrification in lower anoxic waters, and N(2)-fixation-driven production overlying euxinia. Interchange between these states likely explains the varying H(2)S concentration implied by existing data, which persisted until the Neoproterozoic oxygenation event gave rise to modern marine biogeochemistry.

  7. Exploration of the Canyon-Incised Continental Margin of the Northeastern United States Reveals Dynamic Habitats and Diverse Communities.

    Directory of Open Access Journals (Sweden)

    Andrea M Quattrini

    Full Text Available The continental margin off the northeastern United States (NEUS contains numerous, topographically complex features that increase habitat heterogeneity across the region. However, the majority of these rugged features have never been surveyed, particularly using direct observations. During summer 2013, 31 Remotely-Operated Vehicle (ROV dives were conducted from 494 to 3271 m depth across a variety of seafloor features to document communities and to infer geological processes that produced such features. The ROV surveyed six broad-scale habitat features, consisting of shelf-breaching canyons, slope-sourced canyons, inter-canyon areas, open-slope/landslide-scar areas, hydrocarbon seeps, and Mytilus Seamount. Four previously unknown chemosynthetic communities dominated by Bathymodiolus mussels were documented. Seafloor methane hydrate was observed at two seep sites. Multivariate analyses indicated that depth and broad-scale habitat significantly influenced megafaunal coral (58 taxa, demersal fish (69 taxa, and decapod crustacean (34 taxa assemblages. Species richness of fishes and crustaceans significantly declined with depth, while there was no relationship between coral richness and depth. Turnover in assemblage structure occurred on the middle to lower slope at the approximate boundaries of water masses found previously in the region. Coral species richness was also an important variable explaining variation in fish and crustacean assemblages. Coral diversity may serve as an indicator of habitat suitability and variation in available niche diversity for these taxonomic groups. Our surveys added 24 putative coral species and three fishes to the known regional fauna, including the black coral Telopathes magna, the octocoral Metallogorgia melanotrichos and the fishes Gaidropsarus argentatus, Guttigadus latifrons, and Lepidion guentheri. Marine litter was observed on 81% of the dives, with at least 12 coral colonies entangled in debris. While

  8. Multiphase Structural Evolution of a Continental Margin During Obduction Orogeny: Insights From the Jebel Akhdar Dome, Oman Mountains

    Science.gov (United States)

    Grobe, A.; Virgo, S.; von Hagke, C.; Urai, J. L.; Littke, R.

    2018-03-01

    The structural evolution of the carbonate platform in the footwall of the Semail ophiolite emplaced onto the passive continental margin of Arabia helps to better understand the early stages of obduction-related orogens. These early stages are rarely observable in other orogens as they are mostly overprinted by later mountain building phases. We present an extensive structural analysis of the Jebel Akhdar anticline, the largest tectonic window of the Oman Mountains, and integrate it on different scales. Outcrop observations can be linked to plate motion data, providing an absolute timeframe for structural generations consistent with radiometric dating of veins. Top-to-S overthrusting of the Semail ophiolite and Hawasina nappes onto the carbonate platform during high plate convergence rates between Arabia and Eurasia caused rapid burial and overpressure, generation and migration of hydrocarbons, and bedding-confined veins, but no major deformation in the carbonate platform. At reduced convergence rates, subsequent tectonic thinning of the ophiolite took place above a top-to-NNE, crustal-scale ductile shear zone, deforming existing veins and forming a cleavage in clay-rich layers in early Campanian times. Ongoing extension occurred along normal- to oblique-slip faults, forming horst-graben structures and a precursor of the Jebel Akhdar dome (Campanian to Maastrichtian). This was followed by NE-SW oriented ductile shortening and the formation of the Jebel Akhdar dome, deforming the earlier structures. Thereafter, exhumation was associated with low-angle normal faults on the northern flank of the anticline. We correlate the top-to-NNE crustal-scale shear zone with a similar structure in the Saih Hatat window to develop a unified model of the tectonic evolution of the Oman Mountains.

  9. Uranium-polymetallic ore-forming system and mechanism of the black rock series in the southeast continental margin of Yangtze plate

    International Nuclear Information System (INIS)

    Qi Fucheng; Zhang Zilong; He Zhongbo; Li Zhixing; Wang Wenquan; Su Xiangli; Zhang Chao

    2011-01-01

    It is the indisputable fact that the large scale uranium-polymetallic mineralization had happen in the southeast continental margin of Yangtze plate. The rations of Fe/Ti, (Fe+Mn)/Ti, Al/(Al+Fe+Mn), and Ni-Co-Zn diagrams show that the siliceous rocks and phosphorites are the products of hydrothermal sedimentation in these area. In La/Yb-REE diagrams the data of phosphorites are clustered in the area of basales. Obviously, the source of phophorite's ore-forming materials is closely related with the geological processes at depth. REE patterns are characterized by a low content in total and a gradually increasing with the increasing of REE atomic number, and the NASC-normalized value ranging between the upper and the lower limits of typical hydrothermal deposits. The large scale uranium-polymetallic mineralization was controlled by the environment of continental margin rifting. The authors propose that thermal sedimentation or exhalation-sedimentation is the mechanism of the large scale uranium-polymetallic mineralization in the southeast continental margin of the Yangtze plate. (authors)

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

    Science.gov (United States)

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

    2014-12-01

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

  11. Syn-collisional felsic magmatism and continental crust growth: A case study from the North Qilian Orogenic Belt at the northern margin of the Tibetan Plateau

    Science.gov (United States)

    Chen, Shuo; Niu, Yaoling; Xue, Qiqi

    2018-05-01

    The abundant syn-collisional granitoids produced and preserved at the northern Tibetan Plateau margin provide a prime case for studying the felsic magmatism as well as continental crust growth in response to continental collision. Here we present the results from a systematic study of the syn-collisional granitoids and their mafic magmatic enclaves (MMEs) in the Laohushan (LHS) and Machangshan (MCS) plutons from the North Qilian Orogenic Belt (NQOB). Two types of MMEs from the LHS pluton exhibit identical crystallization age ( 430 Ma) and bulk-rock isotopic compositions to their host granitoids, indicating their genetic link. The phase equilibrium constraints and pressure estimates for amphiboles from the LHS pluton together with the whole rock data suggest that the two types of MMEs represent two evolution products of the same hydrous andesitic magmas. In combination with the data on NQOB syn-collisional granitoids elsewhere, we suggest that the syn-collisional granitoids in the NQOB are material evidence of melting of ocean crust and sediment. The remarkable compositional similarity between the LHS granitoids and the model bulk continental crust in terms of major elements, trace elements, and some key element ratios indicates that the syn-collisional magmatism in the NQOB contributes to net continental crust growth, and that the way of continental crust growth in the Phanerozoic through syn-collisional felsic magmatism (production and preservation) is a straightforward process without the need of petrologically and physically complex processes.

  12. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    Energy Technology Data Exchange (ETDEWEB)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.

  13. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    Science.gov (United States)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

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

    Science.gov (United States)

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

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

    Directory of Open Access Journals (Sweden)

    Alexandre Cunha Ribeiro

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

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

    Science.gov (United States)

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

    2006-06-01

    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 is proposed on the basis of the morphology and nature of deposits: (1) mud volcanoes, (2) methane-derived authigenic carbonates (MDAC) mounds, and (3) crater-like pockmarks. Mud volcanoes are, topographically, cone-shaped sea-floor edifices, built up from catastrophic mud and fluid degassing, intercalated with periods of inactivity. So far more than 25 mud volcanoes have been discovered in the Gulf of Cádiz, named in memory of deceased colleagues (e.g., Ginsburg and Baraza), or researchers' birth places (e.g. Faro, Cibeles, Almazán, San Petersburgh, Yuma, Rabat, Bonjardim, Coruña, Gades). These structures range from 800 to 2500 m in diameter and tower 150-300 m above the seabed. The volcanoes consistently feature a well-defined outer ring or circular terrace and an inner dome. All mud volcanoes are built up of episodes of mud-breccia flows, intercalated with deep-current deposits, with evident indications of gas saturation: degassing structures, a strong H 2S smell, and chemosynthetic fauna (such as Pogonophora sp. tube worms and Calyptogena sp.). Commonly observed carbonate crusts and slabs overlying some mud volcanoes are thought to have been formed by slow, diffuse venting during periods of inactivity or slower rates of fluid venting following the ejection of mud. A "fermentation" process, the result of microbial-mediated oxidation of hydrocarbon-enriched fluids, seems to play an important role in the growth of large deep-water carbonate mounds and chimneys during periods of low methane-seep fluid pressure. More than 400 crater

  17. Structure and function of nematode communities across the Indian western continental margin and its oxygen minimum zone

    Science.gov (United States)

    Singh, R.; Ingole, B. S.

    2016-01-01

    We studied patterns of nematode distribution along the western Indian continental margin to determine the influence of habitat heterogeneity and low oxygen levels on the community's taxonomic and functional structure. A single transect, perpendicular to the coast at 14° N latitude was sampled from 34 to 2546 m depth for biological and environmental variables during August 2007. The oxygen minimum zone extended from 102 to 1001 m. Nematodes (described and undescribed) were identified to species and classified according to biological and functional traits. A total of 110 nematode species belonging to 24 families were found along the transect. Three depth zones were identified: the shelf (depth range: 34-102 m; highest nematode mean density: 176.6 ± 37 ind 10 cm-2), the slope (525-1524 m; 124.3 ± 16 ind 10 cm-2), and the basin (2001-2546 m; 62.9 ± 2 ind 10 cm-2). Across the entire study area, the dominant species were Terschellingia longicaudata, Desmodora sp. 1, Sphaerolaimus gracilis, and Theristus ensifer; their maximum density was at shelf stations. Nematode communities in different zones differed in species composition. Chromadorita sp. 2 (2.78 %) and Sphaerolaimus gracilis (2.21 %) were dominant on the shelf, whereas Terschellingia longicaudata (4.73 %) and Desmodora sp. 1 (4.42 %) were dominant on the slope, but in the basin, Halalaimus sp. 1(1.11 %) and Acantholaimus elegans (1.11 %) were dominant. The information in a particular functional group was not a simple reflection of the information in species abundance. Ecological information captured by adult length, adult shape, and life-history strategy was less site-specific and thus differed notably from information contained in other taxonomic groups. The functional composition of nematodes was strongly linked to the organic-carbon and dissolved-oxygen concentration. Seven species were found exclusively in the oxygen minimum zone: Pselionema sp. 1, Choanolaimus sp. 2, Halichoanolaimus sp. 1, Cobbia dentata

  18. 16 Years, 16 Cruises, 1.6 Billion Soundings: a Compilation of High-Resolution Multibeam Bathymetry of the Active Plate Boundary Along the Chilean Continental Margin

    Science.gov (United States)

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

    2011-12-01

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

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

    Science.gov (United States)

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

    2013-11-01

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

  20. Uplift history of a transform continental margin revealed by the stratigraphic record: The case of the Agulhas transform margin along the Southern African Plateau

    Science.gov (United States)

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

    2018-04-01

    The south and southeast coast of southern Africa (from 28°S to 33°S) forms a high-elevated transform passive margin bounded to the east by the Agulhas-Falkland Fracture Zone (AFFZ). We analysed the stratigraphic record of the Outeniqua and Durban (Thekwini) Basins, located on the African side of the AFFZ, to determine the evolution of these margins from the rifting stage to present-day. The goal was to reconstruct the strike-slip evolution of the Agulhas Margin and the uplift of the inland high-elevation South African Plateau. The Agulhas transform passive margin results from four successive stages: Rifting stage, from Late Triassic to Early Cretaceous ( 200?-134 Ma), punctuated by three successive rifting episodes related to the Gondwana breakup; Wrench stage (134-131 Ma), evidenced by strike- and dip-slip deformations increasing toward the AFFZ; Active transform margin stage (131-92 Ma), during which the Falkland/Malvinas Plateau drifts away along the AFFZ, with an uplift of the northeastern part of the Outeniqua Basin progressively migrating toward the west; Thermal subsidence stage (92-0 Ma), marked by a major change in the configuration of the margin (onset of the shelf-break passive margin morphology). Two main periods of uplift were documented during the thermal subsidence stage of the Agulhas Margin: (1) a 92 Ma short-lived margin-scale uplift, followed by a second one at 76 Ma located along the Outeniqua Basin and; (2) a long-lasting uplift from 40 to 15 Ma limited to the Durban (Thekwini) Basin. This suggests that the South African Plateau is an old Upper Cretaceous relief (90-70 Ma) reactivated during Late Eocene to Early Miocene times (40-15 Ma).

  1. Uranium exploration target selection for proterozoic iron oxide/breccia complex type deposits in India

    International Nuclear Information System (INIS)

    Dwivedy, K.K.; Sinha, K.K.

    1997-01-01

    Multimetal iron oxide/breccia complex (IOBC) type deposits exemplified by Olympic Dam in Australia, fall under low grade, large tonnage deposits. A multidisciplinary integrated exploration programme consisting of airborne surveys, ground geological surveys, geophysical and geochemical investigations and exploratory drilling, supported adequately by the state of the art analytical facilities, data processing using various software and digital image processing has shown moderate success in the identification of target areas for this type of deposits in the Proterozoic terrains of India. Intracratonic, anorogenic, continental rift to continental margin environment have been identified in a very wide spectrum of rock associations. The genesis and evolution of such associations during the Middle Proterozoic period have been reviewed and applied for target selection in the (i) Son-Narmada rift valley zone; (ii) areas covered by Dongargarh Supergroup of rocks in Madhya Pradesh; (iii) areas exposing ferruginous breccia in the western part of the Singhbhum Shear Zone (SSZ) around Lotapahar; (iv) Siang Group of rocks in Arunachal Pradesh; (v) Crystalline rocks of Garo Hills around Anek; and (vi) Chhotanagpur Gneissic complex in the Bahia-Ulatutoli tract of Ranchi Plateau. Of theses six areas, the Son-Narmada rift area appears to be the most promising area for IOBC type deposits. Considering occurrences of the uranium anomalies near Meraraich, Kundabhati, Naktu and Kudar and positive favourability criteria observed in a wide variety of rocks spatially related to the rifts and shears, certain sectors in Son-Narmada rift zone have been identified as promising for intense subsurface exploration. 20 refs, 4 figs, 1 tab

  2. Mineralogy and Origin of Sediments From Drill Holes on the Continental Margin Off Florida, 1965-1969 (NODC Accession 7100714)

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — Drill cores obtained during the Joint Oceanographic Institutions' Deep Earth Sampling Program from the continental shelf, the Florida-Hatteras Slope, and the Blake...

  3. EX1204: Northeastern Canyons and Continental Margins Exploration on NOAA Ship Okeanos Explorer between 20120529 and 20120613

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — During the Okeanos Explorer (EX) mission EX1204, the vessel will sail from Norfolk, VA, along the continental shelf break of the U.S. East Coast from Virginia to...

  4. Deep structure of the continental margin and basin off Greater Kabylia, Algeria - New insights from wide-angle seismic data modeling and multichannel seismic interpretation

    Science.gov (United States)

    Aïdi, Chafik; Beslier, Marie-Odile; Yelles-Chaouche, Abdel Karim; Klingelhoefer, Frauke; Bracene, Rabah; Galve, Audrey; Bounif, Abdallah; Schenini, Laure; Hamai, Lamine; Schnurle, Philippe; Djellit, Hamou; Sage, Françoise; Charvis, Philippe; Déverchère, Jacques

    2018-03-01

    During the Algerian-French SPIRAL survey aimed at investigating the deep structure of the Algerian margin and basin, two coincident wide-angle and reflection seismic profiles were acquired in central Algeria, offshore Greater Kabylia, together with gravimetric, bathymetric and magnetic data. This 260 km-long offshore-onshore profile spans the Balearic basin, the central Algerian margin and the Greater Kabylia block up to the southward limit of the internal zones onshore. Results are obtained from modeling and interpretation of the combined data sets. The Algerian basin offshore Greater Kabylia is floored by a thin oceanic crust ( 4 km) with P-wave velocities ranging between 5.2 and 6.8 km/s. In the northern Hannibal High region, the atypical 3-layer crustal structure is interpreted as volcanic products stacked over a thin crust similar to that bordering the margin and related to Miocene post-accretion volcanism. These results support a two-step back-arc opening of the west-Algerian basin, comprising oceanic crust accretion during the first southward stage, and a magmatic and probably tectonic reworking of this young oceanic basement during the second, westward, opening phase. The structure of the central Algerian margin is that of a narrow ( 70 km), magma-poor rifted margin, with a wider zone of distal thinned continental crust than on the other margin segments. There is no evidence for mantle exhumation in the sharp ocean-continent transition, but transcurrent movements during the second opening phase may have changed its initial geometry. The Plio-Quaternary inversion of the margin related to ongoing convergence between Africa and Eurasia is expressed by a blind thrust system under the margin rising toward the surface at the slope toe, and by an isostatic disequilibrium resulting from opposite flexures of two plates decoupled at the continental slope. This disequilibrium is likely responsible for the peculiar asymmetrical shape of the crustal neck that may thus

  5. The Thermal Evolution of the Southeast Baffin Island Continental Margin: An Integrated Apatite Fission Track and Apatite (U-Th)/He Study

    Science.gov (United States)

    Jess, S.; Stephenson, R.; Brown, R. W.

    2017-12-01

    The elevated continental margins of the North Atlantic continue to be a focus of considerable geological and geomorphological debate, as the timing of major tectonic events and the age of topographic relief remain controversial. The West Greenland margin, on the eastern flank of Baffin Bay, is believed by some authors to have experienced tectonic rejuvenation and uplift during the Neogene. However, the opposing flank, Baffin Island, is considered to have experienced a protracted erosional regime with little tectonic activity since the Cretaceous. This work examines the thermal evolution of the Cumberland Peninsula, SE Baffin Island, using published apatite fission track (AFT) data with the addition of 103 apatite (U-Th)/He (AHe) ages. This expansion of available thermochronological data introduces a higher resolution of thermal modelling, whilst the application of the newly developed `Broken Crystals' technique provides a greater number of thermal constraints for an area dominated by AHe age dispersion. Results of joint thermal modelling of the AFT and AHe data exhibit two significant periods of cooling across the Cumberland Peninsula: Devonian/Carboniferous to the Triassic and Late Cretaceous to present. The earliest phase of cooling is interpreted as the result of major fluvial systems present throughout the Paleozoic that flowed across the Canadian Shield to basins in the north and south. The later stage of cooling is believed to result from rift controlled fluvial systems that flowed into Baffin Bay during the Mesozoic and Cenozoic during the early stages and culmination of rifting along the Labrador-Baffin margins. Glaciation in the Late Cenozoic has likely overprinted these later river systems creating a complex fjordal distribution that has shaped the modern elevated topography. This work demonstrates how surface processes, and not tectonism, can explain the formation of elevated continental margins and that recent methodological developments in the field of

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

    Science.gov (United States)

    Santantonio, Massimo; Fabbi, Simone; Aldega, Luca

    2016-01-01

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

  7. {sup 137}Cs as tracer of the origin of allochthonous sediments in the Southeast Continental Margin of Brazil; {sup 137}Cs como tracador da origem de sedimentos aloctones na Margem Continental Sudeste do Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Ferreira, Paulo A.L.; Mahiques, Michel M.; FIgueira, Rubens C.L., E-mail: paulo.alves.ferreira@usp.br, E-mail: mahiques@usp.br, E-mail: rfigueira@usp.br [Universidade de Sao Paulo (IO/USP), SP (Brazil). Instituto Oceanografico; Franca, Elvis J., E-mail: ejfranca@cnen.gov.br [Centro Regional de Ciencias Nucleares do Nordeste (CRCN-NE/CNEN-PE), Recife, PE (Brazil)

    2015-07-01

    The distribution of {sup 137}Cs, artificial radionuclide for which there is no current source, can inform on the origin and destination of sediments. This study analyzed about 60 samples of surface sediment to generate a model of spatial distribution of {sup 137}Cs in the Southeast Continental Margin of Brazil and surroundings for evaluating possible sediment sources for this region. The model showed that the levels of {sup 137}Cs in the southern compartment of the Southeast Brazilian Margin (south of Sao Sebastiao Island) are statistically similar to those of the Rio de la Plata river mouth region, indicating sediment entry due to the seasonal intrusion of the plume of Rio de la Plata, a phenomenon already studied by other authors.

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

    Energy Technology Data Exchange (ETDEWEB)

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

    2015-07-01

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

  9. Oligocene to Holocene sediment drifts and bottom currents on the slope of Gabon continental margin (west Africa). Consequences for sedimentation and southeast Atlantic upwelling

    Science.gov (United States)

    Séranne, Michel; Nzé Abeigne, César-Rostand

    1999-10-01

    Seismic reflection profiles on the slope of the south Gabon continental margin display furrows 2 km wide and some 200 m deep, that develop normal to the margin in 500-1500 m water depth. Furrows are characterised by an aggradation/progradation pattern which leads to margin-parallel, northwestward migration of their axes through time. These structures, previously interpreted as turbidity current channels, display the distinctive seismic image and internal organisation of sediment drifts, constructed by the activity of bottom currents. Sediment drifts were initiated above a major Oligocene unconformity, and they developed within a Oligocene to Present megasequence of general progradation of the margin, whilst they are markedly absent from the underlying Late Cretaceous-Eocene aggradation megasequence. The presence of upslope migrating sediment waves, and the northwest migration of the sediment drifts indicate deposition by bottom current flowing upslope, under the influence of the Coriolis force. Such landwards-directed bottom currents on the slope probably represent coastal upwelling, which has been active along the west Africa margin throughout the Neogene.

  10. Multifractal detrended fluctuation analysis to compare coral bank and seafloor seepage area-related characterization along the central Western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Chakraborty, B.; VishnuVardhan, Y.; Haris, K.; Menezes, A.A.A.; Karisiddaiah, S.M.; Fernandes, W.A.; Kurian, J.

    ], [9]. In the western continental margin off Malpe, Karnataka coast, India, two living coral banks were identified using an EM 1002 multibeam echo sounder (MBES), namely, 1) the Gaveshani bank [10] and 2) an unnamed coral bank [11]. The other important.... MATERIALS AND METHODS The MBES data system for the present study was used to acquire a dataset during a survey (in 2006) onboard coastal research vessel (CRV) Sagar Sukti. The EM 1002 system operates at a frequency of 95 kHz and acquires the bathymetry 1545...

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

    Science.gov (United States)

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

    2009-12-01

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

  12. A review of the sedimentology of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting

    Science.gov (United States)

    Eriksson, P. G.; Schreiber, U. M.; van der Neut, M.

    The sedimentary rocks of the Early Proterozoic Pretoria Group form the floor rocks to teh 2050 M.a. Bushveld Complex. An overall alluvial fan-fan-delta - lacustrine palaeoenvironmental model is postulated for the Pretoria Group. This model is compatible with a continental half-graben tectonic setting, with steep footwall scarps on the southern margin and a lower gradient hanging wall developed to the north. The latter provided much of the basin-fill detritus. It is envisaged that the southern boundary fault system migrated southwards by footwall collapse as sedimentation continued. Synsedimentary mechanical rifting, associated with alluvial and deltaic sedimentation (Rooihoogte-Strubenkop Formations) was followed by thermal subsidence, with concomitant transgressive lacustrine deposition (Daspoort-Magaliesberg Formations). The proposed half-graben basin was probably related to the long-lived Thabazimbi-Murchison and Sugarbush-Barberton lineaments, which bound the preserved outcrops of the Pretoria Group.

  13. Acoustic wipeouts over the continental margins off Krishna, Godavari and Mahanadi river basins, East coast of India

    Digital Repository Service at National Institute of Oceanography (India)

    Murthy, K.S.R.; Rao, T.C.S.

    wipeouts. Behrens (1988) reported occurrence of gas/oil seepages beneath such ridges associated with underlying salt diapir ofT Gulf of Mexico. Acoustic wipeouts in the sub-surface layers due to a faulted continental slope are also observed of... the figures. References BEHRESS, E. W., (1988) Geology of a continental slope oil seep, Northern Gulf of Mexico. Amer. Assoc. Petrot Geo!. Bulletin. v. 72, pp. 105-114. 568 K. S. R. MURTHY AND T. C. S. RAO BRYANT, W. R. and L. B. ROEMER, (1983) Structure...

  14. Pb-Sr-Nd-O isotopic characterization of Mesozoic rocks throughout the northern end of the Peninsular Ranges batholith: Isotopic evidence for the magmatic evolution of oceanic arc–continental margin accretion during the Late Cretaceous of southern California

    Science.gov (United States)

    Kistler, Ronald W.; Wooden, Joseph L.; Premo, Wayne R.; Morton, Douglas M.

    2014-01-01

    contaminated magmatic arc. The Peninsular Ranges batholith magmatic arc was initially an oceanic arc built on Panthalassan lithosphere that eventually evolved into a continental margin magmatic arc collision zone, eventually overriding North American cratonic lithosphere. Our Pb-Sr-Nd data further suggest that the western arc rocks represent a nearshore or inboard oceanic arc, as they exhibit isotopic signatures that are more enriched than typical mid-ocean-ridge basalt (MORB). Isotopic signatures from the central zone are transitional and indicate that enriched crustal magma sources were becoming involved in the northern Peninsular Ranges batholith magmatic plumbing. As the oceanic arc–continental margin collision progressed, a mixture of oceanic mantle and continental magmatic sources transpired. Magmatic production in the northern Peninsular Ranges batholith moved eastward and continued to tap enriched crustal magmatic sources. Similar modeling has been previously proposed for two other western margin magmatic arcs, the Sierra Nevada batholith of central California and the Idaho batholith.Calculated initial Nd signatures at ca. 100 Ma for Permian–Jurassic and Proterozoic basement rocks from the nearby San Gabriel Mountains and possible source areas along the southwestern Laurentian margin of southern California, southwestern Arizona, and northern Sonora strongly suggest their involvement with deep crustal magma mixing beneath the eastern zones of the Peninsular Ranges batholith, as well as farther east in continental lithospheric zones.Last, several samples from the allochthonous, easternmost upper-plate zone, which are considerably younger (ca. 84 Ma) than any of the rocks from the northern Peninsular Ranges batholith proper, have even more enriched average Sri, 206Pbi, 208Pbi, and εNdisignatures of 0.7079, 19.344, 38.881, and −6.6, respectively, indicative of the most-evolved magma sources in the northern Peninsular Ranges batholith and similar to radioisotopic

  15. Oblique strike-slip motion off the Southeastern Continental Margin of India: Implication for the separation of Sri Lanka from India

    Science.gov (United States)

    Desa, Maria Ana; Ismaiel, Mohammad; Suresh, Yenne; Krishna, Kolluru Sree

    2018-05-01

    The ocean floor in the Bay of Bengal has evolved after the breakup of India from Antarctica since the Early Cretaceous. Recent geophysical investigations including updated satellite derived gravity map postulated two phases for the tectonic evolution of the Bay of Bengal, the first phase of spreading occurred in the NW-SE direction forming its Western Basin, while the second phase occurred in the N-S direction resulting in its Eastern Basin. Lack of magnetic data along the spreading direction in the Western Basin prompted us to acquire new magnetic data along four tracks (totaling ∼3000 km) to validate the previously identified magnetic anomaly picks. Comparison of the synthetic seafloor spreading model with the observed magnetic anomalies confirmed the presence of Mesozoic anomalies M12n to M0 in the Western Basin. Further, the model suggests that this spreading between India and Antarctica took place with half-spreading rates of 2.7-4.5 cm/yr. The trend of the fracture zones in the Western Basin with respect to that of the Southeastern Continental Margin of India (SCMI) suggests that SCMI is an oblique transform margin with 37° obliquity. Further, the SCMI consists of two oblique transform segments separated by a small rift segment. The strike-slip motion along the SCMI is bounded by the rift segments of the Northeastern Continental Margin of India and the southern margin of Sri Lanka. The margin configuration and fracture zones inferred in its conjugate Western Enderby Basin, East Antarctica helped in inferring three spreading corridors off the SCMI in the Western Basin of the Bay of Bengal. Detailed grid reconstruction models traced the oblique strike-slip motion off the SCMI since M12n time. The strike-slip motion along the short northern transform segment ended by M11n time. The longer transform segment, found east of Sri Lanka lost its obliquity and became a pure oceanic transform fault by M0 time. The eastward propagation of the Africa

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

    Energy Technology Data Exchange (ETDEWEB)

    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

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

    Energy Technology Data Exchange (ETDEWEB)

    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

  18. Geochemical characteristics of Proterozoic granite magmatism from ...

    Indian Academy of Sciences (India)

    57

    spectrometry, Ind. Inst. Petroleum, Dehradun, Abst.vol. pp.480-482. ... range of settings including volcanic areas, continental shields and orogenic belts. .... represents the northern margin, the Palghat-Cauvery Shear Zone marks the southern ...... due to feldspar accumulation or assimilation of feldspar rich material (Ragland ...

  19. Final Scientific/Technical Report: Characterizing the Response of the Cascadia Margin Gas Hydrate Reservoir to Bottom Water Warming Along the Upper Continental Slope

    Energy Technology Data Exchange (ETDEWEB)

    Solomon, Evan A. [Univ. of Washington, Seattle, WA (United States); Johnson, H. Paul [Univ. of Washington, Seattle, WA (United States); Salmi, Marie [Univ. of Washington, Seattle, WA (United States); Whorley, Theresa [Univ. of Washington, Seattle, WA (United States)

    2017-11-10

    The objective of this project is to understand the response of the WA margin gas hydrate system to contemporary warming of bottom water along the upper continental slope. Through pre-cruise analysis and modeling of archive and recent geophysical and oceanographic data, we (1) inventoried bottom simulating reflectors along the WA margin and defined the upper limit of gas hydrate stability, (2) refined margin-wide estimates of heat flow and geothermal gradients, (3) characterized decadal scale temporal variations of bottom water temperatures at the upper continental slope of the Washington margin, and (4) used numerical simulations to provide quantitative estimates of how the shallow boundary of methane hydrate stability responds to modern environmental change. These pre-cruise results provided the context for a systematic geophysical and geochemical survey of methane seepage along the upper continental slope from 48° to 46°N during a 10-day field program on the R/V Thompson from October 10-19, 2014. This systematic inventory of methane emissions along this climate-sensitive margin corridor and comprehensive sediment and water column sampling program provided data and samples for Phase 3 of this project that focused on determining fluid and methane sources (deep-source vs. shallow; microbial, thermogenic, gas hydrate dissociation) within the sediment, and how they relate to contemporary intermediate water warming. During the 2014 research expedition, we sampled nine seep sites between ~470 and 520 m water depth, within the zone of predicted methane hydrate retreat over the past 40 years. We imaged 22 bubble plumes with heights commonly rising to ~300 meters below sea level with one reaching near the sea surface. We collected 22 gravity cores and 20 CTD/hydrocasts from the 9 seeps and at background locations (no acoustic evidence of seepage) within the depth interval of predicted downslope retreat of the methane hydrate stability zone. Approximately 300 pore water

  20. Cretaceous–Eocene provenance connections between the Palawan Continental Terrane and the northern South China Sea margin

    NARCIS (Netherlands)

    Shao, Lei; Cao, Licheng; Qiao, Peijun; Zhang, Xiangtao; Li, Qianyu; van Hinsbergen, Douwe J.J.

    2017-01-01

    The plate kinematic history of the South China Sea opening is key to reconstructing how the Mesozoic configuration of Panthalassa and Tethyan subduction systems evolved into today's complex Southeast Asian tectonic collage. The South China Sea is currently flanked by the Palawan Continental Terrane

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

    Science.gov (United States)

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

    2014-12-01

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

  2. Meso-Cenozoic tectonic evolution of the SE Brazilian continental margin: Petrographic, kinematic and dynamic analysis of the onshore Araruama Lagoon Fault System

    Science.gov (United States)

    Souza, Pricilla Camões Martins de; Schmitt, Renata da Silva; Stanton, Natasha

    2017-09-01

    The Ararauama Lagoon Fault System composes one of the most prominent set of lineaments of the SE Brazilian continental margin. It is located onshore in a key tectonic domain, where the basement inheritance rule is not followed. This fault system is characterized by ENE-WSW silicified tectonic breccias and cataclasites showing evidences of recurrent tectonic reactivations. Based on field work, microtectonic, kinematic and dynamic analysis, we reconstructed the paleostresses in the region and propose a sequence of three brittle deformational phases accountable for these reactivations: 1) NE-SW dextral transcurrence; 2) NNW-SSE dextral oblique extension that evolved to NNW-SSE "pure" extension; 3) ENE-WSW dextral oblique extension. These phases are reasonably correlated with the tectonic events responsible for the onset and evolution of the SE onshore rift basins, between the Neocretaceous and Holocene. However, based on petrographic studies and supported by regional geological correlations, we assume that the origin of this fault system is older, related to the Early Cretaceous South Atlantic rifting. This study provides significant information about one of the main structural trends of the SE Brazilian continental margin and the tectonic events that controlled its segmentation, since the Gondwana rifting, and compartmentalization of its onshore sedimentary deposits during the Cenozoic.

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

    Science.gov (United States)

    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

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

    Digital Repository Service at National Institute of Oceanography (India)

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

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

  5. Deep structure of the Mid-Norwegian continental margin (the Vøring and Møre basins) according to 3-D density and magnetic modelling

    Science.gov (United States)

    Maystrenko, Yuriy Petrovich; Gernigon, Laurent; Nasuti, Aziz; Olesen, Odleiv

    2018-03-01

    A lithosphere-scale 3-D density/magnetic structural model of the Møre and Vøring segments of the Mid-Norwegian continental margin and the adjacent areas of the Norwegian mainland has been constructed by using both published, publically available data sets and confidential data, validated by the 3-D density and magnetic modelling. The obtained Moho topography clearly correlates with the major tectonic units of the study area where a deep Moho corresponds to the base of the Precambrian continental crust and the shallower one is located in close proximity to the younger oceanic lithospheric domain. The 3-D density modelling agrees with previous studies which indicate the presence of a high-density/high-velocity lower-crustal layer beneath the Mid-Norwegian continental margin. The broad Jan Mayen Corridor gravity low is partially related to the decreasing density of the sedimentary layers within the Jan Mayen Corridor and also has to be considered in relation to a possible low-density composition- and/or temperature-related zone in the lithospheric mantle. According to the results of the 3-D magnetic modelling, the absence of a strong magnetic anomaly over the Utgard High indicates that the uplifted crystalline rocks are not so magnetic there, supporting a suggestion that the entire crystalline crust has a low magnetization beneath the greater part of the Vøring Basin and the northern part of the Møre Basin. On the contrary, the crystalline crust is much more magnetic beneath the Trøndelag Platform, the southern part of the Møre Basin and within the mainland, reaching a culmination at the Frøya High where the most intensive magnetic anomaly is observed within the study area.

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

    Science.gov (United States)

    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

  7. New insights on the geological evolution of the continental margin of Southeastern Brazil derived from zircon and apatite (U-Th-Sm)/He and fission-track data

    Science.gov (United States)

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

    2017-04-01

    New insights on the geological evolution of the continental margin of Southeastern Brazil derived from zircon and apatite (U-Th-Sm)/He and fission-track data Krob, F.C.1, Stippich, C. 1, Glasmacher, U.A.1, Hackspacher, P.C.2 (1) Institute of Earth Sciences, Research Group Thermochronology and Archaeometry, Heidelberg University, INF 234, 69120, Heidelberg, Germany (2) Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Av. 24-A, 1515 Rio Claro, SP, 13506-900, Brazil Passive continental margins are important geoarchives related to mantle dynamics, the breakup of continents, lithospheric dynamics, and other processes. The main concern yields the quantifying long-term lithospheric evolution of the continental margin between São Paulo and Laguna in southeastern Brazil since the Neoproterozoic. We put special emphasis on the reactivation of old fracture zones running into the continent and their constrains on the landscape evolution. In this contribution, we represent already consisting thermochronological data attained by fission-track and (U-Th-Sm)/He analysis on apatites and zircons. The zircon fission-track ages range between 108.4 (15.0) and 539.9 (68.4) Ma, the zircon (U-Th-Sm)/He ages between 72.9 (5.8) and 427.6 (1.8) Ma whereas the apatite fission-track ages range between 40.0 (5.3) and 134.7 (8.0) Ma, and the apatite (U-Th-Sm)/He ages between 32.1 (1.52) and 92.0 (1.86) Ma. These thermochronological ages from metamorphic, sedimentary and intrusive rocks show six distinct blocks (Laguna, Florianópolis, Curitiba, Ilha Comprida, Peruibe and Santos) with different evolution cut by old fracture zones. Furthermore, models of time-temperature evolution illustrate the differences in Pre- to post-rift exhumation histories of these blocks. The presented data will provide an insight into the complex exhumation history of the continental margin based on the existing literature data on the evolution of the Paraná basin in Brazil and the latest

  8. Shyok Suture Zone, N Pakistan: late Mesozoic Tertiary evolution of a critical suture separating the oceanic Ladakh Arc from the Asian continental margin

    Science.gov (United States)

    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 (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

  9. A multi-factor approach for process-based seabed characterization: example from the northeastern continental margin of the Korean peninsula (East Sea)

    Science.gov (United States)

    Cukur, Deniz; Um, In-Kwon; Chun, Jong-Hwa; Kim, So-Ra; Lee, Gwang-Soo; Kim, Yuri; Kong, Gee-Soo; Horozal, Senay; Kim, Seong-Pil

    2018-04-01

    This study investigates sediment transport and depositional processes from a newly collected dataset comprising sub-bottom chirp profiles, multibeam bathymetry, and sediment cores from the northeastern continental margin of Korea in the East Sea (Japan Sea). Twelve echo-types and eleven sedimentary facies have been defined and interpreted as deposits formed by shallow-marine, hemipelagic sedimentation, bottom current, and mass-movement processes. Hemipelagic sedimentation, which is acoustically characterized by undisturbed layered sediments, appears to have been the primary sedimentary process throughout the study area. The inner and outer continental shelf (shallow-marine sedimentary processes. Two slope-parallel canyons, 0.2-2 km wide and up to 30 km long, appear to have acted as possible conduits for turbidity currents from the shallower shelf into the deep basins. Bottom current deposits, expressed as erosional moats immediately below topographic highs, are prevalent on the southern lower slope at water depths of 400-450 m. Mass-movements (i.e., slides/slumps, debris flow deposits) consisting of chaotic facies characterize the lower slope and represent one of the most important sedimentary processes in the study area. Piston cores confirm the presence of mass-transport deposits (MTDs) that are characterized by mud clasts of variable size, shape, and color. Multibeam bathymetry shows that large-scale MTDs are chiefly initiated on the lower slope (400-600 m) with gradients up to 3° and where they produce scarps on the order of 100 m in height. Sandy MTDs also occur on the upper continental slope adjacent to the seaward edge of the shelf terrace. Earthquakes associated with tectonic activity and the development of fluid overpressure is considered as the main conditioning factor for destabilizing the slope sediments. Overall, the sedimentary processes show typical characteristics of a fine-grained clastic slope apron and change down-slope and differ within each

  10. Verdine and other associated authigenic (glaucony, phosphate) facies from the surficial sediments of the southwestern continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, V.P.; Lamboy, M.; Dupeuble, P.A.

    spaced contorted clay blades and globules. X-ray mineralogy suggests that these grains are a mixture of verdine dominated minerals. Phyllite C is the principal verdine mineral in the shelf zone. On the continental slope phyllite V dominates between 100... grains from the shelf are mostly homogeneous at low magnification (Fig. 4A). However, at high magnification the authigenic clays are characterized by small con- torted blades (1 Ixm long) at some places and compact clays (Fig. 4B and C) at others...

  11. How the structural architecture of the Eurasian continental margin affects the structure, seismicity, and topography of the south central Taiwan fold-and-thrust belt

    Science.gov (United States)

    Brown, Dennis; Alvarez-Marron, Joaquina; Biete, Cristina; Kuo-Chen, Hao; Camanni, Giovanni; Ho, Chun-Wei

    2017-07-01

    Studies of mountain belts worldwide show that along-strike changes are common in their foreland fold-and-thrust belts. These are typically caused by processes related to fault reactivation and/or fault focusing along changes in sedimentary sequences. The study of active orogens, like Taiwan, can also provide insights into how these processes influence transient features such as seismicity and topography. In this paper, we trace regional-scale features from the Eurasian continental margin in the Taiwan Strait into the south central Taiwan fold-and-thrust belt. We then present newly mapped surface geology, P wave velocity maps and sections, seismicity, and topography data to test the hypothesis of whether or not these regional-scale features of the margin are contributing to along-strike changes in structural style, and the distribution of seismicity and topography in this part of the Taiwan fold-and-thrust belt. These data show that the most important along-strike change takes place at the eastward prolongation of the upper part of the margin necking zone, where there is a causal link between fault reactivation, involvement of basement in the thrusting, concentration of seismicity, and the formation of high topography. On the area correlated with the necking zone, the strike-slip reactivation of east northeast striking extensional faults is causing sigmoidal offset of structures and topography along two main zones. Here basement is not involved in the thrusting; there is weak focusing of seismicity and localized development of topography. We also show that there are important differences in structure, seismicity, and topography between the margin shelf and its necking zone.

  12. Record of Permian-Early Triassic continental arc magmatism in the western margin of the Jiamusi Block, NE China: petrogenesis and implications for Paleo-Pacific subduction

    Science.gov (United States)

    Yang, Hao; Ge, Wenchun; Dong, Yu; Bi, Junhui; Wang, Zhihui; Ji, Zheng; Yang, H.; Ge, W. C.; Dong, Y.; Bi, J. H.; Wang, Z. H.; Ji, Z.

    2017-09-01

    In this paper, we report zircon U-Pb ages, Hf isotopes and whole-rock geochemical data for the Permian to Early Triassic granitoids from the western margin of the Jiamusi Block (WJB), NE China. The intermediate to felsic (SiO2 = 59.67-74.04 wt%) granitoids belong to calc-alkaline series and are characterized by enrichments in light rare earth elements and large ion lithophile elements with pronounced negative Nb, Ta and Ti anomalies, revealing typical continental magmatic arc geochemical signatures. The zircon U-Pb determinations on the granodiorite, monzogranite, syenogranite and quartz diorite samples yielded ages between ca. 275-245 Ma, which, together with the published coeval intrusive rocks, indicates that Permian to Early Triassic continental arc magmatism occurred extensively in the WJB. The low and mainly negative zircon ɛ Hf( t) values between -7.6 and +1.6 and the zircon Hf model ages of 1.2-1.8 Ga, which are significantly older than their crystallization ages, suggest that they were mainly derived from reworking of ancient crustal materials with a limited input of juvenile components. The geochemical systematics and petrogenetic considerations indicate that the studied granitoids were generated from a zone of melting, assimilation, storage, and homogenization, i.e., a MASHed zone at the base of Paleo- to Mesoproterozoic continental crust, where large portions of igneous rocks and minor clay-poor sediments involved in the source region. In combination with regional geological data, we argue that the Jiamusi Block was unlikely the rifted segment of the Songliao Block and two possible geodynamical models were proposed to interpret the formation of the ca. 275-245 Ma granitoids in the WJB. In the context of Permian global plate reconstruction, we suggest that Paleo-Pacific plate subduction was initiated in the Permian to Early Triassic beneath the Jiamusi Block, and even whole eastern NE China.

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

    Science.gov (United States)

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

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

  14. Mass-physical properties of surficial sediments on the Rhoˆne continental margin: implications for the nepheloid benthic layer

    Science.gov (United States)

    Chassefiere, Bernard

    1990-09-01

    Mass-physical properties of the surficial (upper 5 m) sediments on the Gulf of Lions continental margin were analysed, from more than 100 short (1 m) and longer (5 m) cores obtained during several cruises. Data include water content, unit weight, Atterberg limits (liquid limit, plastic limit, plasticity index), shear strength and compression index, and are used to determine: first, the mass property distribution, according to the main parameters influencing mass-physical properties; the relationships between these properties and the nepheloid layer on the shelf. The shoreline (lagoons) and inner shelf are characterized by low density and shear strength and high water content deposits, due to electrochemical flocculation of the sediment. The outer shelf is blanketed by higher density and shear strength and lower water content deposits generated by normal settling of suspended particles. On the inner shelf, during river peak discharges, a short-term thin bottom layer of "yogurt-like" [ FASS (1985) Geomarine Letters, 4, 147-152; FASS (1986) Continental Shelf Research, 6, 189-208] fluid-mud (unit weight lower than 1.3 mg m -3) is supplied, by a bottom nepheloid layer. During stormy periods, this "yogurt-like" layer (about 10 cm thick) partly disappears by resuspension of suspended particulate matter; this is advected, in the bottom nepheloid layer, over the shelf and the canyons within the upper slope.

  15. Lower Brioverian formations (Upper Proterozoic) of the Armorican Massif (France): geodynamic evolution of source areas revealed by sandstone petrography and geochemistry

    Science.gov (United States)

    Dabard, Marie Pierre

    1990-11-01

    Formations with interbedded cherts constitute an important part of the Lower Brioverian succession (Upper Proterozoic age) in the Armorican Massif (northwest France). These formations are composed of shale-sandstone alternations with interbedded siliceous carbonaceous members. Petrographic and geochemical study of the detrital facies shows that these rocks are compositionally immature. The wackes are rich in lithic fragments (volcanic fragments: 3-20% modal; sedimentary and metamorphic fragments: 0-7% modal) and in feldspar (5-16%). From the geochemical point of view, they are relatively enriched in Fe 2+MgO (about 5.5%) and in alkalis with {Na 2O }/{K 2O } ratios greater than 1. The CaO contents are low (about 0.3%). Slightly negative Eu anomalies are observed ( {Eu}/{Eu ∗} = 0.8 ). Their chemical compositions are in agreement with a dominantly acidic source area with deposition in a continental active margin setting. Compared with other Upper Proterozoic deposits of the Armorican Massif, the interbedded-chert formations appear rather similar to other deposits in North Brittany which accumulated in an intra-arc or back-arc basin environment. The formations with interbedded cherts are interpreted as having been deposited during an early stage of magmatic arc activity (around 640-630 Ma ago) in an immature marginal basin. The clastic supply to these formations is derived in part from early volcanic products (acidic to intermediate) which are linked to subduction beneath the North Armorican Domain. Another component is inherited from the reworking of 2000 Ma old basement relics. The opening of the back-arc domain, with associated basaltic volcanism, would bring about a progressive displacement of the interbedded-chert depositional basin towards the continental margin.

  16. 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

    Science.gov (United States)

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

    2007-01-01

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

  17. Ostracoda and Foraminifera associated with macrofauna of marginal marine origin in continental sabkha sediments of Tayma (NW Saudi Arabia)

    Science.gov (United States)

    Pint, Anna; Frenzel, Peter; Engel, Max; Plessen, Birgit; Melzer, Sandra; Brückner, Helmut

    2016-04-01

    The oasis Tayma in northwestern Saudi Arabia (27°38'N, 38°33'E) is well known for its rich archaeological heritage and also hosts a key sedimentary record of Holocene environmental change.The palaeontologically investigated material comes from two 5.5 m long sediment cores taken in the northeastern and central part of the sabkha and two outcrops of shoreline deposits at the northeastern and southwestern margin of a large lake. Microfossil-rich layers have an age of about 9.2-ca. 8 ka BP. The sandy and carbonate-dominated sediments contain autochthonous balanids, the gastropods Melanoides tuberculatus and hydrobiids as well as the foraminifers Ammonia tepida (Cushman, 1926), Quinqueloculina seminula (Linnaeus, 1758), and Flintionoides labiosa (d'Orbigny, 1839). This brackish water association is completed by partially mass-occurrence of Cyprideis torosa (JONES, 1850), an euryhaline and generally widely tolerant ostracod species. Only the smooth shelled morphotype littoralis occurs. The association indicates a large brackish water lake with temporary freshwater inflows. All species documented originate in the marginal marine environment of the Red or Mediterranean Sea within the intertidal zone and hence they are adapted for strong environmental changes. We assume negative water balance under arid climatic conditions as cause for the high salinity of this athalassic lake. Sieve-pore analyses and shell chemistry suppose a trend of increasing salinity towards the top of the studied microfossil-bearing sections. This pattern is confirmed by increasing test malformation ratios of foraminifers. The marine origin of the fauna is surprising in this area 250 km away from the sea in an altitude of about 800 m a.s.l. We assume an avian-mediated transport of eggs, larvae or even adult animals to this site. The brackish water character of the lake enabled a permanent settling of marginal marine foraminifers, ostracods and even macrofauna as gastropods and balanids. The studied

  18. Latest Proterozoic stratigraphy and earth history

    Science.gov (United States)

    Knoll, Andrew H.; Walter, Malcolm R.

    1992-01-01

    Novel biostratigraphic and chemostratigraphic data furnish an improved framework for stratigraphic correlation of the Proterozoic Eon as well as tools for a chronostratigraphic division of the late Proterozoic. It is argued that, in conjunction with geochronometric data, protistan microfossils and isotope geochemistry can furnish a means for an eventual integration of the latest Proterozoic Eon. Attention is given to the emerging methodologies of fossil protists and prokaryotes and of isotopic chemostratigraphy.

  19. Palaeomagnetism and the continental crust

    Energy Technology Data Exchange (ETDEWEB)

    Piper, J.D.A.

    1987-01-01

    This book is an introduction to palaeomagnetism offering treatment of theory and practice. It analyzes the palaeomagnetic record over the whole of geological time, from the Archaean to the Cenozoic, and goes on to examine the impact of past geometries and movements of the continental crust at each geological stage. Topics covered include theory of rock and mineral magnetism, field and laboratory methods, growth and consolidation of the continental crust in Archaean and Proterozoic times, Palaeozoic palaeomagnetism and the formation of Pangaea, the geomagnetic fields, continental movements, configurations and mantle convection.

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

    Science.gov (United States)

    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.

  1. Ocean Drilling Program Leg 112, Peru continental margin: Part 2, Sedimentary history and diagenesis in a coastal upwelling environment

    Science.gov (United States)

    Suess, E.; von Huene, R.

    1988-10-01

    On the shelf and upper slope off Peru the signal of coastal upwelling productivity and bottom-water oxygen is well preserved in alternately laminated and bioturbated diatomaceous Quaternary sediments. Global sea-level fluctuations are the ultimate cause for these cyclic facies changes. During late Miocene time, coastal upwelling was about 100 km west of the present centers, along the edge of an emergent structure that subsequently subsided to form the modern slope. The sediments are rich in organic carbon, and intense microbially mediated decomposition of organic matter is evident in sulfate reduction and methanogenesis. These processes are accompanied by the formation of diagenetic carbonates, mostly Ca-rich dolomites and Mg-calcites. The downhole isotopic signatures of these carbonate cements display distinct successions that reflect the vertical evolution of the pore fluid environment. From the association of methane gas hydrates, burial depth, and low-chloride interstitial fluids, we suggest an additional process that could contribute to the characteristic chloride depletion in pore fluids of active margins: release of interlayer water from clays without a mineral phase change. The shelf sediments also contain a subsurface brine that stretches for more than 500 km from north to south over the area drilled. The source of the brine remains uncertain, although the composition of the oxygen isotopes suggests dissolution of evaporites by seawater.

  2. Footwall degradation styles and associated sedimentary facies distribution in SE Crete: Insights into tilt-block extensional basins on continental margins

    Science.gov (United States)

    Alves, Tiago M.; Cupkovic, Tomas

    2018-05-01

    Depositional facies resulting from footwall degradation in extensional basins of SE Crete are studied based on detailed geological maps, regional transects, lithological columns and outcrop photos. During an extensional episode affecting Crete in the late Miocene-early Pliocene, depocentres trending N20°E and N70°E were filled with fan deltas, submarine mass-wasting deposits, sandy turbidites and fine-grained hemipelagites sourced from both nearby and distal sediment sources. Deposition of proximal continental and shallow-marine units, and relatively deep (marine) turbidites and mass-transport deposits, occurred within a complex mosaic of tectonically controlled depocentres. The new geological maps and transects in this work reveal that depositional facies in SE Crete were controlled by: a) their relative proximity to active faults and uplifting footwall blocks, b) the relative position (depth and relative height above sea level) of hanging-wall basins, and c) the nature of the basement units eroded from adjacent footwall blocks. Distal sediment sources supplied background siliciclastic sediment ('hemipelagites'), which differ markedly from strata sourced from local footwalls. In parallel, mass-transport of sediment was ubiquitous on tectonically active slopes, and so was the presence of coarse-grained sediment with sizes varying from large blocks > 50 m-wide to heterolithic mass-transport deposits and silty-sandy turbidites. We expect similar tectono-sedimentary settings to have predominated in tectonically active Miocene basins of the eastern Mediterranean, in which hydrocarbon exploration is occurring at present, and on rifted continental margins across the world.

  3. U-Pb thermochronology of rutile from Alpine Corsica: constraints on the thermal evolution of the European margin during Jurassic continental breakup

    Science.gov (United States)

    Ewing, T. A.; Beltrando, M.; Müntener, O.

    2017-12-01

    U-Pb thermochronology of rutile can provide valuable temporal constraints on the exhumation history of the lower crust, given its moderate closure temperature and the occurrence of rutile in appropriate lithologies. We present an example from Alpine Corsica, in which we investigate the thermal evolution of the distal European margin during Jurassic continental rifting that culminated in the opening of the Alpine Tethys ocean. The Belli Piani unit of the Santa Lucia nappe (Corsica) experienced minimal Alpine overprint and bears a striking resemblance to the renowned Ivrea Zone lower crustal section (Italy). At its base, a 2-4 km thick gabbroic complex contains slivers of granulite facies metapelites that represent Permian lower crust. Zr-in-rutile temperatures and U-Pb ages were determined for rutile from three metapelitic slivers from throughout the Mafic Complex. High Zr-in-rutile temperatures of 850-950 °C corroborate textural evidence for rutile formation during Permian granulite facies metamorphism. Lower Zr-in-rutile temperatures of 750-800 °C in a few grains are partly associated with elongate strings of rutile within quartz ribbons, which record recrystallisation of some rutile during high-temperature shearing. Zr thermometry documents that both crystallisation and re-crystallisation of rutile occurred above the closure temperature of Pb in rutile, such that the U-Pb system can be expected to record cooling ages uncomplicated by re-crystallisation. Our new high-precision single-spot LA-ICPMS U-Pb dates are highly consistent between and within samples. The three samples gave ages from 160 ± 1 Ma to 161 ± 2 Ma, with no other age populations detected. The new data indicate that the Santa Lucia lower crust last cooled through 550-650 °C at 160 Ma, coeval with the first formation of oceanic crust in the Tethys. The new data are compared to previous depth profiling rutile U-Pb data for the Belli Piani unit1, and exploited to cast light on the tectonothermal

  4. 3-D thermal effect of late Cenozoic erosion and deposition within the Lofoten-Vesterålen segment of the Mid-Norwegian continental margin

    Science.gov (United States)

    Maystrenko, Yuriy Petrovich; Gernigon, Laurent; Olesen, Odleiv; Ottesen, Dag; Rise, Leif

    2018-05-01

    A 3-D subsurface temperature distribution within the Lofoten-Vesterålen segment of the Mid-Norwegian continental margin and adjacent areas has been studied to understand the thermal effect of late Cenozoic erosion of old sedimentary and crystalline rocks and subsequent deposition of glacial sediments during the Pleistocene. A lithosphere-scale 3-D structural model of the Lofoten-Vesterålen area has been used as a realistic approximation of the geometries of the sedimentary infill, underlying crystalline crust and lithospheric mantle during the 3-D thermal modelling. The influence of late Cenozoic erosion and sedimentation has been included during the 3-D thermal calculations. In addition, the 3-D thermal modelling has been carried out by taking also into account the influence of early Cenozoic continental breakup. The results of the 3-D thermal modelling demonstrate that the mainland is generally colder than the basin areas within the upper part of the 3-D model. The thermal influence of the early Cenozoic breakup is still clearly recognizable within the western and deep parts of the Lofoten-Vesterålen margin segment in terms of the increased temperatures. The thermal effects of the erosion and deposition within the study area also indicate that a positive thermal anomaly exists within the specific subareas where sedimentary and crystalline rocks were eroded. A negative thermal effect occurs in the subareas affected by subsidence and sedimentation. The erosion-related positive thermal anomaly reaches its maximum of more than +27 °C at depths of 17-22 km beneath the eastern part of the Vestfjorden Basin. The most pronounced deposition-related negative anomaly shows a minimum of around -70 °C at 17-20 km depth beneath the Lofoten Basin. The second negative anomaly is located within the northeastern part of the Vøring Basin and has minimal values of around -48 °C at 12-14 km depth. These prominent thermal anomalies are associated with the subareas where

  5. Early Jurassic Volcanism in the South Lhasa Terrane, Southern Tibet: Record of Back-arc Extension in the Active Continental Margin

    Science.gov (United States)

    Wei, Y.; Zhao, Z.; Zhu, D. C.; Wang, Z.; Liu, D.; Mo, X.

    2015-12-01

    Indus-Yarlung Zangbo Suture Zone (IYZSZ) represents the Mesozoic remnants of the Neo-Tethyan Ocean lithosphere after its northward subduction beneath the Lhasa Terrane. The evolution of the Neo-Tethyan Ocean prior to India-Asia collision remains unclear. To explore this period of history, we investigate zircon U-Pb geochronology, geochemistry and Nd-Hf isotopes of the Early Jurassic bimodal-like volcanic sequence around Dagze area, south Tibet. The volcanic sequence comprises calc-alkaline basalts to rhyolites whereas intermediate components are volumetrically restricted. Zircons from a basaltic andesite yielded crystallization age of 178Ma whereas those from 5 silicic rocks were dated at 183-174Ma, which suggest that both the basaltic and the silicic rocks are coeval. The basaltic rocks are enriched in LREE and LILE, and depleted in HFSE, with Epsilon Nd(t) of 1.6-4.0 and zircon Epsilon Hf(t) of 0.7-11.8, which implies that they were derived from a heterogenetic mantle source metasomatized by subduction components. Trace element geochemistry shows that the basaltic rocks are compositionally transitional from normal mid-ocean ridge basalts (N-MORB) to island arc basalts (IAB, e.g. Zedong arc basalts of ~160-155Ma in the south margin of Lhasa Terrane), with the signature of immature back-arc basin basalts. The silicic rocks display similar Nd-Hf isotopic features of the Gangdese batholith with Epsilon Nd(t) of 0.9-3.4 and zircon Epsilon Hf(t) of 2.4-17.7, indicating that they were possibly generated by anatexis of basaltic juvenile lower crust, instead of derived from the basaltic magma. These results support an Early to Middle Jurassic (183-155Ma) model that the back-arc extension tectonic setting were existing in the active continental margin in the south Lhasa Terrane.

  6. New Zircon U-Pb Age Constrain of the Origin of Devil's River Uplift (SW Texas) and Insights into the Late Proterozoic and Paleozoic Evolution of the Southern Margin of Laurentia

    Science.gov (United States)

    Rodriguez, E.; Dickerson, P. W.; Stockli, D. F.

    2017-12-01

    The Devils River Uplift (DRU) in SW Texas records the evolution of the southern Laurentian margin from Grenvillian orogenesis and assembly of Rodinia, to its fragmentation by rifting, and to the amalgamation of Pangaea. It was cored by a well (Shell No. 1 Stewart), penetrating Precambrian gneisses and Cambrian metasediments and sandstones. New zircon LA-ICP-MS data from a total of 10 samples elucidate the crystallization and depositional ages, as well as the detrital provenance, of Precambrian and Cambrian rocks from the DRU. Zircons from five Precambrian crystalline basement samples (6000-9693') yield uniform U-Pb crystallization ages of 1230 Ma that are similar to ages for young gneisses of the Valley Spring Domain (Llano uplift) in central Texas, where they mark the cessation of arc magmatism within the Grenville orogenic belt. The 1230 Ma igneous basement is overlain by L.-M. Cambrian metasedimentary rocks ( 4000-6000') with maximum depositional ages of 533-545 Ma. Detrital zircons from Cambrian strata are dominated by a 1070-1080 Ma population, likely derived from basement units exposed in Texas (Llano uplift, Franklin Mts.), with minor contributions from local 1230 Ma Precambrian basement and the 1380-1500 Ma Granite Rhyolite Province. The L.-M. Cambrian interval is dominated (>80%) by Neoproterozoic detrital magmatic zircons with two major distinct age clusters at 570-700 Ma and 780-820 Ma, supporting a two-stage Rodinia rift model and providing strong evidence for major Cryogenian-Eocambrian intraplate magmatism along the southern margin of Rodinia. Moreover, detrital zircon signatures for L.-M. and U. Cambrian strata strongly correlate with those from the Cuyania terrane of W. Argentina - notably the W. Sierras Pampeanas (Sa. Pie de Palo, Sa. de Maz): 1230 Ma from metasandstones (PdP); 1081-1038 Ma from metasiliciclastics (PdP, SdM); Cryogenian-Eocambrian [774 & 570 Ma] plutons (SdM, PdP). In summary, these new zircon U-Pb data from DRU in SW Texas show

  7. Satellite-Derived Distributions, Inventories and Fluxes of Dissolved and Particulate Organic Matter Along the Northeastern U.S. Continental Margin

    Science.gov (United States)

    Mannino, A.; Hooker, S. B.; Hyde, K.; Novak, M. G.; Pan, X.; Friedrichs, M.; Cahill, B.; Wilkin, J.

    2011-01-01

    Estuaries and the coastal ocean experience a high degree of variability in the composition and concentration of particulate and dissolved organic matter (DOM) as a consequence of riverine and estuarine fluxes of terrigenous DOM, sediments, detritus and nutrients into coastal waters and associated phytoplankton blooms. Our approach integrates biogeochemical measurements, optical properties and remote sensing to examine the distributions and inventories of organic carbon in the U.S. Middle Atlantic Bight and Gulf of Maine. Algorithms developed to retrieve colored DOM (CDOM), Dissolved (DOC) and Particulate Organic Carbon (POC) from NASA's MODIS-Aqua and SeaWiFS satellite sensors are applied to quantify the distributions and inventories of DOC and POC. Horizontal fluxes of DOC and POC from the continental margin to the open ocean are estimated from SeaWiFS and MODIS-Aqua distributions of DOC and POC and horizontal divergence fluxes obtained from the Northeastern North Atlantic ROMS model. SeaWiFS and MODIS imagery reveal the importance of estuarine outflow to the export of CDOM and DOC to the coastal ocean and a net community production of DOC on the shelf.

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

    Science.gov (United States)

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

    2014-11-01

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

  9. Differences in meiofauna communities with sediment depth are greater than habitat effects on the New Zealand continental margin: implications for vulnerability to anthropogenic disturbance

    Directory of Open Access Journals (Sweden)

    Norliana Rosli

    2016-07-01

    Full Text Available Studies of deep-sea benthic communities have largely focused on particular (macro habitats in isolation, with few studies considering multiple habitats simultaneously in a comparable manner. Compared to mega-epifauna and macrofauna, much less is known about habitat-related variation in meiofaunal community attributes (abundance, diversity and community structure. Here, we investigated meiofaunal community attributes in slope, canyon, seamount, and seep habitats in two regions on the continental slope of New Zealand (Hikurangi Margin and Bay of Plenty at four water depths (700, 1,000, 1,200 and 1,500 m. We found that patterns were not the same for each community attribute. Significant differences in abundance were consistent across regions, habitats, water and sediment depths, while diversity and community structure only differed between sediment depths. Abundance was higher in canyon and seep habitats compared with other habitats, while between sediment layer, abundance and diversity were higher at the sediment surface. Our findings suggest that meiofaunal community attributes are affected by environmental factors that operate on micro- (cm to meso- (0.1–10 km, and regional scales (> 100 km. We also found a weak, but significant, correlation between trawling intensity and surface sediment diversity. Overall, our results indicate that variability in meiofaunal communities was greater at small scale than at habitat or regional scale. These findings provide new insights into the factors controlling meiofauna in these deep-sea habitats and their potential vulnerability to anthropogenic activities.

  10. The evolution of the passive continental margin of Norway and its adjacent mainland - using the sub-Cambrian peneplain as a reference surface.

    Science.gov (United States)

    Gabrielsen, R. H.; Faleide, J. I.; Jarsve, E. M.

    2012-04-01

    The structuring, uplift and subsidence of the passive margin and shelf of Norway and its adjacent mainland were affected by several profound geological processes, including inherited basement structural grain related to the Proterozoic and Caledonian orogens and also including the extensional collapse of the Caledonides (Gabrielsen et al. 2000). This has been followed by several stages of late Palaoezoic - Cenozoic rifting and associated thermotectonic activity, Cenozoic accelerated uplift of uncertain origin of the hinterland, creating an irregular pattern of upheaval and, finally Pleistocene - Holocene glacial loading and unloading (Gabrielsen et al. 2010). These processes have strongly influenced the topography of the hinterland, thus causing and acting in concert with climate fluctuations (Nielsen et al. 2009). The correlation of erosional surfaces of regional significance on the shelf and on the mainland is a key to the evaluating the total topography of the margin. Because of the lack of datable surfaces on the mainland, this is problematic. The so-called Paleic surface has been used in this context, but its age and nature is not well constrained and the absence of post-Caledonian rocks in the western and central mainland of southern Norway adds to this complexity. In contrast, the sub-Cambrian peneplain, which is found in larger parts of Scandinavia, is well established when it comes to dating and development (e.g. Strøm 1948). It is generally accepted that this surface had only a minor topography, if any, throughout Scandinavia at the earliest Cambrian. Hence, its present relief is the result of the accumulated vertical displacements from the Caledonian to the Present. Still, even though it was well established through regional mapping already in the late 19th century, much remains in the detailed documentation of this important surface. To improve the topographic accuracy in its characterization, fieldwork has been initiated to establish a detailed WNW

  11. The Two Subduction Zones of the Southern Caribbean: Lithosphere Tearing and Continental Margin Recycling in the East, Flat Slab Subduction and Laramide-Style Uplifts in the West

    Science.gov (United States)

    Levander, A.; Bezada, M. J.; Niu, F.; Schmitz, M.

    2015-12-01

    The southern Caribbean plate boundary is a complex strike-slip fault system bounded by oppositely vergent subduction zones, the Antilles subduction zone in the east, and a currently locked Caribbean-South American subduction zone in the west (Bilham and Mencin, 2013). Finite-frequency teleseismic P-wave tomography images both the Atlanic (ATL) and the Caribbean (CAR) plates subducting steeply in opposite directions to transition zone depths under northern South America. Ps receiver functions show a depressed 660 discontinuity and thickened transition zone associated with each subducting plate. In the east the oceanic (ATL) part of the South American (SA) plate subducts westward beneath the CAR, initiating the El Pilar-San Sebastian strike slip system, a subduction-transform edge propagator (STEP) fault (Govers and Wortel, 2005). The point at which the ATL tears away from SA as it descends into the mantle is evidenced by the Paria cluster seismicity at depths of 60-110 km (Russo et al, 1993). Body wave tomography and lithosphere-asthenosphere boundary (LAB) thickness determined from Sp and Ps receiver functions and Rayleigh waves suggest that the descending ATL also viscously removes the bottom third to half of the SA continental margin lithospheric mantle as it descends. This has left thinned continental lithosphere under northern SA in the wake of the eastward migrating Antilles subduction zone. The thinned lithosphere occupies ~70% of the length of the El Pilar-San Sebastian fault system, from ~64oW to ~69oW, and extends inland several hundred kilometers. In northwestern SA the CAR subducts east-southeast at low angle under northern Colombia and western Venezuela. The subducting CAR is at least 200 km wide, extending from northernmost Colombia as far south as the Bucaramanga nest seismicity. The CAR descends steeply under Lake Maracaibo and the Merida Andes. This flat slab is associated with three Neogene basement cored, Laramide-style uplifts: the Santa Marta

  12. A comparison of geochemical exploration techniques and sample media within accretionary continental margins: an example from the Pacific Border Ranges, Southern Alaska, U.S.A.

    Science.gov (United States)

    Sutley, S.J.; Goldfarb, R.J.; O'Leary, R. M.; Tripp, R.B.

    1990-01-01

    The Pacific Border Ranges of the southern Alaskan Cordillera are composed of a number of allochthonous tectonostratigraphic terranes. Within these terranes are widespread volcanogenic, massive sulfide deposits in and adjacent to portions of accreted ophiolite complexes, bands and disseminations of chromite in accreted island-arc ultramafic rocks, and epigenetic, gold-bearing quartz veins in metamorphosed turbidite sequences. A geochemical pilot study was undertaken to determine the most efficient exploration strategy for locating these types of mineral deposits within the Pacific Border Ranges and other typical convergent continental margin environments. High-density sediment sampling was carried out in first- and second-order stream channels surrounding typical gold, chromite and massive sulfide occurrences. At each site, a stream-sediment and a panned-concentrate sample were collected. In the laboratory, the stream sediments were sieved into coarse-sand, fine- to medium-sand, and silt- to clay-size fractions prior to analysis. One split of the panned concentrates was retained for analysis; a second split was further concentrated by gravity separation in heavy liquids and then divided into magnetic, weakly magnetic and nonmagnetic fractions for analysis. A number of different techniques including atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry and semi-quantitative emission spectrography were used to analyze the various sample media. Comparison of the various types of sample media shows that in this tectonic environment it is most efficient to include a silt- to clay-size sediment fraction and a panned-concentrate sample. Even with the relatively low detection limits for many elements by plasma spectrometry and atomic absorption spectrometry, anomalies reflecting the presence of gold veins could not be identified in any of the stream-sediment fractions. Unseparated panned-concentrate samples should be analyzed by emission

  13. Instability and deformation in the sedimentary cover on the upper slope of the southern Aquitaine continental margin, north of the Capbreton canyon (Bay of Biscay

    Directory of Open Access Journals (Sweden)

    Eliane Gonthier

    2006-06-01

    Full Text Available Acoustic and core data have recently been collected on the shelf break and the upper part of the slope of the south Aquitaine continental margin. They reveal the major role played by mass-flow gravity processes in deposit erosion and redistribution, modelling of the sea-bed, and transfer of sediment toward the deep-sea. The study region is bounded in the south by the Capbreton canyon. The northern area, which shows a smooth morphology, is characterised by small-scale deformations due to sediment creep or low-amplitude slide processes. The deformations are associated with mini listric-like faults that bound packets of sediments in which the deposit geometry is typical of constructional sediment waves. These sediment waves result from the interaction of depositional and gravity deformation processes. In the southern area, closer to the canyon, wave-like structures are still present but mostly of smaller size. They only result from gravity deformation processes without any evidence of constructional processes. In the vicinity of the Capbreton canyon, the shelf break and upper slope have a much more uneven morphology with sedimentary reliefs, escarpments and depressions directed toward the canyon thalweg. The depressions look like slide scars, and could be the result of regressive slides initiated at the top of the canyon flank. The age of the sliding event responsible for the formation of the depression observed today could be middle to upper Quaternary. Since their formation, these depressions act as conduits that channel the transfer of shelf sediment into the canyon, as demonstrated by the occurrence of a meandering channel on the sea-floor of one depression.

  14. The impact of proto- and metazooplankton on the fate of organic carbon in continental ocean margins. Final progress report, May 1992--July 1995

    Energy Technology Data Exchange (ETDEWEB)

    Paffenhofer, G.A.; Verity, P.G.

    1995-12-31

    Three fates potentially consume primary production occurring on ocean margins: portions can be oxidized within the water column, portions can sediment to shelf/slope depots, and portions can be exported to the interior ocean. Zooplankton mediate all three of these processes and thus can alter the pathway and residence time of particulate organic carbon, depending on the size structure and composition of the zooplankton (and phytoplankton). To achieve the long-term goal of quantifying the role of proto- and metazooplankton in removing newly formed POC (primary production), the authors must accomplish two major component objectives: (a) determine plankton carbon biomass at relevant temporal and spatial scales; and (b) measure zooplankton carbon consumption rates and (for metazoan zooplankton) fecal pellet production. These measurements will specify the importance of different zooplankton groups as consumers and transformers of phytoplankton carbon. During Phase 1, they concentrated on methodological and technological developments prerequisite to an organized field program. Specifically, they proposed to develop and test an optical zooplankton counter, and to fully enhance the color image analysis system. In addition, they proposed to evaluate a solid-phase enzyme-linked immunospot assay to quantify predation by metazoan zooplankton on protozoans; and to improve methodology to determine ingestion and growth rates of salps, and accompanying pellet production rates, under conditions which very closely resemble their environment. The image analyzer data provide insights on basic ecosystem parameters relevant to carbon flux from the continental ocean to the deep ocean. Together these approaches provide a powerful set of tools to probe food web relationships in greater detail, to increase the accuracy and speed of carbon biomass and rate measurements, and to enhance data collection and analysis.

  15. Continental Growth and Recycling in Convergent Orogens with Large Turbidite Fans on Oceanic Crust

    Directory of Open Access Journals (Sweden)

    Ben D. Goscombe

    2013-07-01

    Full Text Available Convergent plate margins where large turbidite fans with slivers of oceanic basement are accreted to continents represent important sites of continental crustal growth and recycling. Crust accreted in these settings is dominated by an upper layer of recycled crustal and arc detritus (turbidites underlain by a layer of tectonically imbricated upper oceanic crust and/or thinned continental crust. When oceanic crust is converted to lower continental crust it represents a juvenile addition to the continental growth budget. This two-tiered accreted crust is often the same thickness as average continental crustal and is isostatically balanced near sea level. The Paleozoic Lachlan Orogen of eastern Australia is the archetypical example of a tubidite-dominated accretionary orogeny. The Neoproterozoic-Cambrian Damaran Orogen of SW Africa is similar to the Lachlan Orogen except that it was incorporated into Gondwana via a continent-continent collision. The Mesozoic Rangitatan Orogen of New Zealand illustrates the transition of convergent margin from a Lachlan-type to more typical accretionary wedge type orogen. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of turbidite and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism recorded in these rocks reflects the crustal thickening phase, whereas post-tectonic magmatism constrains the timing of chemical maturation and cratonization. Cratonization of continental crust is fostered because turbidites represent fertile sources for felsic magmatism. Recognition of similar orogens in the Proterozoic and Archean is important for the evaluation of crustal growth models, particularly for those based on detrital zircon age patterns, because crustal growth by accretion of upper oceanic crust or mafic underplating does not readily result in the addition of voluminous zircon

  16. Provenance, tectonics and palaeoclimate of Proterozoic Chandarpur ...

    Indian Academy of Sciences (India)

    the analysis of time- or environment-related changes, a major emphasis of .... The environments of deposition for different units ...... Hill, and Keefer Formations, Lower and Middle Silurian ... architecture of the Proterozoic succession in the east-.

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

    Science.gov (United States)

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

    2017-12-01

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

  18. Long-lasting Microbial Methane Release at the Aquitaine Shelf Break (Bay of Biscay): Relation with the (Plio)-Pleistocene Sedimentary Progradation of the Continental Margin

    Science.gov (United States)

    Dupré, S.; Michel, G.; Pierre, C.; Ruffine, L.; Scalabrin, C.; Ehrhold, A.; Loubrieu, B.; Gautier, E.; Baltzer, A.; Imbert, P.; Battani, A.; Deville, E.; Dupont, P.; Thomas, Y.; Théréau, E.

    2017-12-01

    The recent identification of acoustic and visual gas release in the water column at the Aquitaine Shelf (140 and 220 m water depths) led to the discovery of a 200 km2 fluid system at the seafloor with 3000 bubbling sites associated with microbial methane (Dupré et al 2014; Ruffine et al. 2017). The moderate methane fluxes (measured in situ, on average 200 mLn/min per bubbling site) contribute to the formation of small-scale sub-circular authigenic carbonate mounds (with reliefs < 1 m in height) (Pierre et al. 2017). The emitted gases have neither a genetic link with thermogenic hydrocarbons from the Parentis Basin beneath, nor are issued from gas hydrate dissociation, but originate from microbial CO2 reduction. Based on estimated thickness and growth rate of authigenic carbonates, this system has lasted for at least several tens to possibly hundreds of kyears with a volume of escaping methane reaching 3.1012 Ln per 10 kyr. Seismic evidences for gas-charged layers and fossil authigenic carbonates point to organic matter source levels within the sedimentary deposits of the Late Pleistocene progradation system. The Aquitaine Shelf fluid system highlights the edge of continental shelves as preferential areas for bio-geological processes. The GAZCOGNE project is co-funded by TOTAL and IFREMER as part of the PAMELA (Passive Margin Exploration Laboratories) scientific project. References Dupré S, Berger L, Le Bouffant N, Scalabrin C, Bourillet J-F (2014) Fluid emissions at the Aquitaine Shelf (Bay of Biscay, France): a biogenic origin or the expression of hydrocarbon leakage? Cont. Shelf Res. 88:24-33 Pierre C, Demange J, Blanc-Valleron M-M, Dupré S (2017) Authigenic carbonate mounds from active methane seeps on the southern Aquitaine Shelf (Bay of Biscay, France): Evidence for anaerobic oxidation of biogenic methane and submarine groundwater discharge during formation. Cont. Shelf Res. 133:13-25 Ruffine L, Donval J-P, Croguennec C, Bignon L, Birot D, Battani A, Bayon

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

    Science.gov (United States)

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

    2014-12-01

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

  20. Reliability of AMS and ARM/KARM to reconstruct the relative vigor and variability of sea-bottom currents during the Last Glacial Period in the North-West Iberian Continental Margin.

    Science.gov (United States)

    Rey, D.; Plaza-Morlote, M.; Mohamed Falcon, K. J.; Pares, J. M.; Bernabeu, A. M.; El Mekadem, N.; Rubio, B.

    2017-12-01

    We explore the potential and reliability of the anisotropy of magnetic susceptibility (AMS) to reconstruct the relative vigor of the sea-bottom currents by comparison with the widely-used paleo current proxy `sortable silt' mean (SS) grain size. SS and AMS are used as two independent proxies to determine relative changes in bottom-current speed, and their consistency tested against each other. The suitability of the magnetic grainsize proxy ARM/KARM ratio as a proxy for bottom-current variability is also investigated. The results are based on CI12PC3 core from the Galicia Interior Basin (North West Iberian Continental Margin) spanning the last 80 ka. The majority of AMS ellipsoid minimum axes (K3) in core CI12PC3 are close to vertical (INCmean= 80o) and nearly orthogonal to the bedding plane. Maximum axes (K1) are well-grouped marking two main magnetic lineation directions. Their orientations are consistent with present-day bottom-currents flow directions along the continental margin. Down-core variations in the degree of anisotropy (Pj) showed significantly higher values during Heinrich Stadials (cold) than during interstadials (warm). CI12PC3 also shows systematically lower ARM/KARM values during the stadials than during interstadials, indicating provenance induced coarsening of the magnetic fraction during the cold periods. We have noted that Pj down-core changes are independent of magnetic grain size, and interpreted them as the result of differences in the degree of grain alignment. These changes can be attributed to the variability in the strength of the bottom currents, indicating that they are stronger during stadials, which is consistent with the SS results. Magnetic properties and AMS of core CI12PC3 are climatically modulated on a millennial time scale reflecting the evolution in the near bottom-currents of the North West Iberian Continental Margin. Their variability exposes unchartered links with the AMOC and with the North Atlantic climate variability

  1. Late Carboniferous to Early Permian magmatic pulses in the Uliastai continental margin linked to slab rollback: Implications for evolution of the Central Asian Orogenic Belt

    Science.gov (United States)

    Chai, Hui; Wang, Qingfei; Tao, Jixiong; Santosh, M.; Ma, Tengfei; Zhao, Rui

    2018-05-01

    The Paleo Asian Ocean underwent a protracted closure history during Late Paleozoic. Here we investigate the magmatic evolution during this process based on a detailed study in the Baiyinwula region along the Uliastai continental margin. The major rock types in this area are Late Carboniferous-Early Permian volcanic sequences and coeval intrusions. We identified four stages of magmatic evolution based on the diverse assemblages and their precise isotopic ages. The first stage is represented by andesites with a zircon 206Pb/238U age of ca. 326 ± 12 Ma. These rocks are metaluminous to weakly peraluminous, high-K calc-alkaline, and possess high Na2O/K2O ratios in the range of 1.23 to 2.45. They also display enrichment of large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE), with markedly positive zircon εHf (t) varying from 8.1 to 15.6.The geochemical features of these andesites are similar to those of typical arc volcanic rocks. The second stage includes granodiorites emplaced at 318.6 + 1.8 Ma. The rocks are high-K calc-alkaline with A/CNK values ranging from 0.95 to 1.06, and show enrichment in LILE and depletion in HFSE. They show geochemical affinities to adakites, with high Sr and low Y and Yb contents, indicating magma derivation from thickened lower crust. Zircon grains from these rocks display positive initial εHf (t) values ranging from 11.1 to 14.6 with corresponding two-stage Hf model ages (TDM2) of 394-622 Ma. The third stage consists of syenogranite together with a volcanic suite ranging in composition from rhyolite todacite, which formed during 303.4 ± 1.2 to 285.1 ± 2.2 Ma. They possess elevated silica and alkali contents, high FeOt/MgO and Ga/Al ratios, low Al2O3, MgO and CaO contents, and high Rb, Y, Nb, Ce, Zr, Y, and Ga contents, strong negative Ba, Sr and Eu anomalies, showing I- to A-type granitic affinities. Zircons in these rocks show elevated Hf isotopic compositions (εHf (t) = 9.9 to 14.6) with TDM2

  2. Geochemistry and geochronology of the Archean and palaeo-Proterozoic formations of southern Cameroon (Ntem group, Congo craton)

    International Nuclear Information System (INIS)

    Rchameni, R.

    1997-01-01

    The aim of this work is to understand the crustal evolution of the NW margin of the Congo craton using structural, petrography, isotopic, geochemical and geochronological studies of the Archean and palaeo-Proterozoic formations of the Ntem group of southern Cameroon. The synthesis of these studies allows to propose a diapir-type gravity model linked with the genesis of granitoids to explain the geodynamical evolution of this part of the craton during the Archean. A convergence model with the collision of the Congo and Sao-Francisco cratons and with crust thickening followed by a relaxation phase is proposed for the palaeo-Proterozoic. (J.S.)

  3. The influence of El Niño-Southern Oscillation (ENSO) cycles on wave-driven sea-floor sediment mobility along the central California continental margin

    Science.gov (United States)

    Storlazzi, Curt D.; Reid, Jane A.

    2010-01-01

    Ocean surface waves are the dominant temporally and spatially variable process influencing sea floor sediment resuspension along most continental shelves. Wave-induced sediment mobility on the continental shelf and upper continental slope off central California for different phases of El Niño-Southern Oscillation (ENSO) events was modeled using monthly statistics derived from more than 14 years of concurrent hourly oceanographic and meteorologic data as boundary input for the Delft SWAN wave model, gridded sea floor grain-size data from the usSEABED database, and regional bathymetry. Differences as small as 0.5 m in wave height, 1 s in wave period, and 10° in wave direction, in conjunction with the spatially heterogeneous unconsolidated sea-floor sedimentary cover, result in significant changes in the predicted mobility of continental shelf surficial sediment in the study area. El Niño events result in more frequent mobilization on the inner shelf in the summer and winter than during La Niña events and on the outer shelf and upper slope in the winter months, while La Niña events result in more frequent mobilization on the mid-shelf during spring and summer months than during El Niño events. The timing and patterns of seabed mobility are addressed in context of geologic and biologic processes. By understanding the spatial and temporal variability in the disturbance of the sea floor, scientists can better interpret sedimentary patterns and ecosystem structure, while providing managers and planners an understanding of natural impacts when considering the permitting of offshore activities that disturb the sea floor such as trawling, dredging, and the emplacement of sea-floor engineering structures.

  4. Indian Ocean margins

    Digital Repository Service at National Institute of Oceanography (India)

    Naqvi, S.W.A

    in the latter two areas. Some of these fluxes are expected to be substantial in the case of Indonesian continental margins and probably also across the eastern coasts of Africa not covered in this chapter. However, a dearth of information makes these margins...

  5. Submarine mass movements around the Iberian Peninsula. The building of continental margins through hazardous processes; Inestabilidades sedimentarias submarinas alrededor de la Peninsula Iberica. Construccion de margenes a traves de procesos peligrosos

    Energy Technology Data Exchange (ETDEWEB)

    Casas, D.; Casalbore, D.; Yenes, M.; Urgeles, R.

    2015-07-01

    Submarine mass movements, such as those which occur in all environments in every ocean of the world, are widely distributed across the Iberian continental margins. A lack of consistent data from various areas around the Iberian Peninsula makes it difficult to precisely understand their role in the sedimentary record. However, all the studies carried out over the past two decades reveal that they are a recurrent and widespread sedi- mentary process that may represent a significant geohazard. The majority of submarine mass movements observed in both the Mediterranean and Atlantic margins of the Iberian Peninsula have been generically identified as Mass Transport Deposits, but debris flows, slides, slumps and turbidites are common. Only a few remarkable examples involve huge volumes of sediment covering large areas (such as ∼500 km{sup 3} and ∼6x10{sup 4} km{sup 2}), but more moderate deposits (<200 km{sup 2}) are frequently found on the seafloor or embedded in the sedi- mentary sequences, building margins and basins. (Author)

  6. Benthic megafaunal and demersal fish assemblages on the Chilean continental margin: The influence of the oxygen minimum zone on bathymetric distribution

    Science.gov (United States)

    Quiroga, Eduardo; Sellanes, Javier; Arntz, Wolf E.; Gerdes, Dieter; Gallardo, Victor A.; Hebbeln, Dierk

    2009-07-01

    Benthic megafaunal and demersal fish assemblages were sampled in three areas off Chile during the German-Chilean Expedition PUCK (SO-156) onboard the R/V Sonne from March to May 2001, at depths ranging from 120 to 2201 m. These samples, taken with an Agassiz trawl, are among the deepest ever taken in Chilean waters. A total of 147 species were recorded, mainly decapod crustaceans (Galatheidae, Pandalidae, Crangonidae), gastropods (Trochidae, Muricidae, Volutidae), ophiuroids (Asteronychidae, Gorgonocephalidae, Ophiolepididae, Ophiurinae), asteroids (Pterasteridae, Solasteridae, Goniopectinidae), polychaetes (Onuphidae, Aphroditidae, Maldanidae), and demersal fish (Macrouridae, Ipnopidae, Squalidae). Species richness and rarefaction analyses suggest that the fauna was undersampled. From the 147 species identified in this study, 36 species (24.5%) occurred only once and another 24 species occurred only twice (16.3%). Depth and dissolved oxygen levels were found to be the main factors influencing megafaunal changes along the continental shelf and in bathyal areas, as indicated by principal component and Pearson's correlation analyses. Some species appear to be limited to distinct areas in the upper and lower bathyal zones, whereas other species have a wider range, extending from the continental shelf to lower bathyal zones. Biogeographic relations exist with the Pacific, South Atlantic, and Southern Oceans, but the latter seem to be weaker than would be expected considering the connection by Antarctic intermediate water.

  7. Geochronology and geochemistry of upper proterozoic granites from Southern Benin

    International Nuclear Information System (INIS)

    Cordani, U.G.; Kawashita, K.; Vancini, K.R.B.; Cadoppi, P.; Sacchi, R.

    1993-01-01

    The Upper Proterozoic basement of Benin, like that of nearby Nigeria and like the polycyclic basement of Central Hoggar, belongs to the hinterland of the Pharusian Chain (Pan-African Trans-Saharan Belt) generated by the collision between the (passive) margin of the West African craton and the (reactivated) margin of the Tuareg Shield and its southern extension. Rb-Sr dating of sub alkaline, meta-aluminums, syn-Kinematic granite forming tabular bodies near Dassa-Zoume and near Save yielded two WR isochron ages of 650 ± 35 Ma (I.R. = 0.7043) and 705 ± 70 Ma (I.R. = 0.7045). Emplacement of these bodies was clearly controlled by trans current movements along the Kandi Fault System. The analyzed granites are comparable with those of Central Hoggar and North-Central Nigeria on the ground of field, geochronological and geochemical data; they also display some affinities with the late-tectonic granites of the Adrar des Iforas. They are expected to find their Brazilian continuation in the Chaval Granitoids west of Fortaleza, but data for comparison are inadequate. (author)

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

    Science.gov (United States)

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

    2017-12-01

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

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

    Science.gov (United States)

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

    1983-01-01

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

  10. Determination of the Anthropogenic Carbon Signal to the Total Change in Dissolved Carbon in the Coastal Upwelling Region Along the Washington-Oregon-California Continental Margin

    Science.gov (United States)

    Feely, R. A.

    2016-02-01

    The continental shelf region off the Washington-Oregon-California coast is seasonally exposed to water with a low aragonite saturation state by coastal upwelling of CO2-rich waters. To date, the spatial and temporal distribution of anthropogenic CO2 (Canthro) contribution to the CO2-rich waters is largely unknown. Here we use an adaptation of the linear regression approach described in Feely et al (2008) along with the GO-SHIP Repeat Hydrography data sets from the northeast Pacific to establish an annually updated relationship between Canthro and potential density. This relationship was then used with the NOAA Ocean Acidification Program west coast cruise data sets from 2007, 2011, 2012 and 2013 to determine the spatial variations of Canthro in the upwelled water. Our results show large spatial differences in Canthro in surface waters along the coast with the lowest surface values (40-45 µmol kg-1) in strong upwelling regions of off northern California and southern Oregon and higher values (50-70 µmol kg-1) to the north and south. Canthro contributes an average of about 70% of the increased amount of dissolved inorganic carbon in the upwelled waters at the surface. In contrast, at 50 m the Canthro contribution is approximately 31% and at 100 m it averages about 16%. The remaining contributions are primarily due to respiration processes in the water that was upwelled and transported to coastal regions or underwent respiration processes that occurred locally during the course of the upwelling season. The uptake of Canthro has caused the aragonite saturation horizon to shoal by approximately 30-50 m since preindustrial period so that the undersaturated waters are well within the regions that affect the biological communities on the continental shelf.

  11. Crustal structure and sedimentation history over the Alleppey platform, southwest continental margin of India: Constraints from multichannel seismic and gravity data

    Directory of Open Access Journals (Sweden)

    P. Unnikrishnan

    2018-03-01

    Full Text Available The Alleppey Platform is an important morphological feature located in the Kerala-Konkan basin off the southwest coast of India. In the present study, seismic reflection data available in the basin were used to understand the sedimentation history and also to carry out integrated gravity interpretation. Detailed seismic reflection data in the basin reveals that: (1 the Alleppey Platform is associated with a basement high in the west of its present-day geometry (as observed in the time-structure map of the Trap Top (K/T boundary, (2 the platform subsequently started developing during the Eocene period and attained the present geometry by the Miocene and, (3 both the Alleppey platform and the Vishnu fracture zone have had significant impact on the sedimentation patterns (as shown by the time-structure and the isochron maps of the major sedimentary horizons in the region. The 3-D sediment gravity effect computed from the sedimentary layer geometry was used to construct the crustal Bouguer anomaly map of the region. The 3-D gravity inversion of crustal Bouguer anomaly exhibits a Moho depression below the western border of the platform and a minor rise towards the east which then deepens again below the Indian shield. The 2-D gravity modelling across the Alleppey platform reveals the geometry of crustal extension, in which there are patches of thin and thick crust. The Vishnu Fracture Zone appears as a crustal-scale feature at the western boundary of the Alleppey platform. Based on the gravity model and the seismic reflection data, we suggest that the basement high to the west of the present day Alleppey platform remained as a piece of continental block very close to the mainland with the intervening depression filling up with sediments during the rifting. In order to place the Alleppey platform in the overall perspective of tectonic evolution of the Kerala-Konkan basin, we propose its candidature as a continental fragment.

  12. Milankovitch cyclicity in modern continental margins: stratigraphic cycles in terrigenous shelf settings; El registro de la ciclicidad de Milankovitch en margenes continentales actuales: ciclos estratigraficos en plataformas terrigenas

    Energy Technology Data Exchange (ETDEWEB)

    Lobo, F. J.; Ridente, D.

    2013-06-01

    We present a synthesis of the sedimentary responses to Late Quaternary Milankovitch-type sea-level cycles (100 and 20 kyr periodicities) as a basis for our investigations into the patterns and concepts of composite sequences in shallow-shelf settings. We describe the record of both 100 and 20 kyr cycles as documented worldwide and discuss the pattern of composite cyclicity mainly on the basis of previously published data from the Adriatic Sea and Gulf of Cadiz margins. Cycles of 100 kyr are those most frequently documented in Quaternary margins; they occur in the form of unconformity-bounded depositional sequences dominated by fairly uniform pro gradational-regressive units and more variable, though less well developed, transgressive deposits. Sequence boundaries correspond to prominent polygenic (regressive-transgressive) erosional surfaces that bear witness to considerable transgressive reworking of the original sub-aerial unconformity. Although the progradational units making up the greater part of these sequences have usually been interpreted as a record of a falling sea-level stage, recent evidence is pointing towards a more complex stratigraphic picture, including a distinction between relative highstand and lowstand deposits. The 20-kyr stratigraphic motifs show greater variation compared to that displayed by the more common 100-kyr sequences, particularly in the basic structure of systems tracts and the nature of bounding surfaces. The two case studies described here, the Adriatic Sea and Gulf of Cadiz margins, highlight the fact that, concomitantly with an increase in frequencies of cycles and sequences, sediment supply and the dynamics of their dispersal significantly affected the stratigraphic response to the main controlling factor, which was sea-level, thus determining the variety of expression in the 20 kyr cycles. (Author)

  13. Distribution and transport of particle-bound polycyclic aromatic hydrocarbons in a river-influenced continental margin: the northern Gulf of Mexico

    Science.gov (United States)

    Adhikari, P. L.; Maiti, K.

    2017-12-01

    Polycyclic aromatic hydrocarbons (PAHs) are particle-reactive and get preferentially sorbed on particulate organic carbon (POC), thus, the transport and fate of POC in aquatic systems plays an important role in biogeochemical cycling of PAHs. In this study, we examine POC and PAHs in finer suspended particulate matter collected from the Louisiana coast, shelf and slope - progressively south-west transect along the direction of the Mississippi River plume, and also from a transect of Atchafalaya River. The concentrations of total particulate PAHs (ΣPAH43) varied between 0.92 to 7.04 ng/L, while POC varied between 4 to 131 µg/L. The concentrations of total particulate ΣPAH43 as well as individual PAH analytes were significantly positively correlated to the concentrations of POC which indicates that the concentrations and transport of POC plays an important role in distribution of PAHs in marine systems. The river influence, characterized by the change in salinity, had significant negative correlation with both the concentrations of particulate PAHs and POC. These results show that the Mississippi River derived particle influx can be an important vector in delivering particle-reactive hydrophobic organic pollutants such as PAHs into the river dominated continental ecosystems in the northern Gulf of Mexico. The underlying seafloor sediment PAHs' concentration and accumulation rates were not correlated to the water column particulate PAH and POC concentrations, which is attributed to re-mineralization during vertical transport, sediment resuspension/redistribution and different timescales of comparison.

  14. Paleoenvironmental Reconstruction of the North Atlantic Current Variations from MIS 3 to Holocene Based on Multiproxy Record from the North-East Scotland Continental Margin.

    Science.gov (United States)

    Ovsepyan, Y.; Tikhonova, A.; Novichkova, E.; Gupta, R. M.; Korsun, S.; Matul, A.

    2017-12-01

    In order to reconstruct the history of water mass interaction between the North Atlantic and the Nordic Seas since MIS 3 to the present, the sediment core from the North-East Scotland continental slope was investigated. The site of core AI-3521 (59°30.009 N, 7°20.062 E) from the 1051 m water depth is located beneath the pathway of the North Atlantic current which transports warm and saline Atlantic surface water to the Norwegian Sea. The age model of the sequence is based on stable isotope record of benthic Cassidulina neoteretis and planktic Neogloboquadrina pachyderma sin. and Globigerina bulloides. The Holocene interval of the upper 1.5 m is characterized by high sedimentation rates and the high biodiversity of microfauna. The distribution of ice rafted debris and CaCO3 content; benthic and planktic foraminiferal assemblages; oxygen, carbon and boron isotopes, Mg/Ca ratio were used to reconstruct the regional paleoceanographic conditions (bioproductivity, temperature, salinity) and to compare with the paleoclimatic events in the subpolar North Atlantic in the frame of the global environmental changes during the Late Pleistocene and Holocene. The research was supported by Russian Science Foundation projects 16-47-02009 and 14-50-00095.

  15. Preservación y abundancia de escamas de peces en sedimentos del margen continental de Chile (21-36° S Fish scale preservation and abundance in sediments from the continental margin off Chile (21-36° S

    Directory of Open Access Journals (Sweden)

    JAVIER A DÍAZ-OCHOA

    2008-12-01

    Full Text Available Con el objetivo de evaluar la relación entre la preservación de escamas de peces y las variaciones en las condiciones redox en sedimentos del margen continental de Chile, este trabajo presenta recuentos de escamas de peces y concentraciones normalizadas de elementos redox sensibles (Mo/Al, S/Al, Fe/Al en ocho testigos de sedimento recolectados en la zona de minimo oxígeno frente a Chile (Iquique: 20°15' S, bahía de Mejillones: 23° S y Concepción: 36° S. En el norte de Chile (Iquique y Mejillones predominan las escamas de anchoveta (Engraulis ringens y de peces de la familia Myctophidae (media = 90 y 120 escamas 1.000 cm-3, respectivamente mientras que en Chile centro-sur (Concepción son más abundantes las escamas de jurel (Trachurus murphy; media = 140 escamas 1.000 cm-3. La abundancia de escamas de merluza común (Merluccius gayi gayi y de sardina (Sardinops sagax es aproximadamente un orden de magnitud más baja que la de anchoveta o jurel. En general, los valores más altos y los rangos más amplios de variación en las razones Mo/Al, S/Al y Fe/Al se encuentran en Mejillones (Mo/Al ~0,8-12 mg g-1, S/Al 0,2-4,6 g g-1, Fe/Al 0,3-0,7 g g-1 seguidos por Iquique (Mo/Al -0,2-1,8; S/Al 0,2-0,7, Fe/Al 0,5-0,8, mientras que Concepción revela valores más bajos y poco variables (Mo/Al ~0,07, S/Al ~0,15 y Fe/Al ~0,5. La razón Mo/Al, utilizada como indicador de paleo-oxigenación, permite inferir condiciones reductoras relativamente más intensas en los sedimentos de la Bahía de Mejillones e Iquique que en Concepción. En las tres localidades de muestreo se evidencia una asociación estadísticamente significativa entre la abundancia de escamas de anchoveta y el logaritmo de la razón Mo/Al (r²= 0,46; P 1 mg g-1.The relationship between fish scale preservation and variations in the sediment redox conditions on Chile's continental shelf are evaluated herein through fish scale counts and normalized concentrations of redox sensitive elements

  16. Techniques for the non-destructive and continuous analysis of sediment cores. Application in the Iberian continental margin; Tecnicas para el analisis no destructivo y en continuo de testigos de sedimento. Aplicacion en el Margen Continental de Iberia

    Energy Technology Data Exchange (ETDEWEB)

    Frigola, J.; Canals, M.; Mata, P.

    2015-07-01

    Sediment sequences are the most valuable record of long-term environmental conditions at local, regional and/or global scales. Consequently, they are amongst the best archives of the climatic and oceanographic his- tory of the Earth. In the last few decades a strong effort has been made, both in terms of quantity and quality, to improve our knowledge regarding the evolution of our planet from marine and lake sediment records, and also from other records such as ice cores. Such an effort requires reinforcing the geographical coverage and achieving the highest possible robustness in the reconstruction of past environments. Such a target requires the optimization of the time resolution of the records and reconstructions so that fast, high frequency shifts, such as those occurring nowadays due to the on-going global warming, can be disentangled. Beyond paleoenvironmental research, other disciplines have also contributed significantly to the fast growing number of sediment cores already available worldwide. Knowing the physical state and the chemical composition of sedimentary deposits is essential for land management purposes and for many industrial applications. A number of key technological developments are now allowing the acquisition for the first time of massive amounts of multiple parameters from sediment cores in a non-destructive, fast, continuous, repetitive and high-resolution form. In this paper we provide an overview of the state-of-the-art continuous and non-destructive analytical techniques used by the geo scientific community for the study of sediment cores and we present some examples of the application of these methods in several studies carried out around the Iberian Margin. (Author)

  17. NOAA Office of Ocean Exploration and Research'sOkeanos Explorer Program 2014 Discoveries - U.S. Atlantic Continental Margin and Gulf of Mexico

    Science.gov (United States)

    Lobecker, E.; McKenna, L.; Sowers, D.; Elliott, K.; Kennedy, B.

    2014-12-01

    NOAA ShipOkeanos Explorer, the only U.S. federal vessel dedicated to global ocean exploration, made several important discoveries in U.S. waters of the North Atlantic Ocean and Gulf of Mexico during the 2014 field season. Based on input received from a broad group ofmarine scientists and resource managers, over 100,000 square kilometers of seafloor and associated water column were systematically explored using advanced mapping sonars. 39 ROV diveswere conducted, leading to new discoveries that will further ourunderstanding of biologic, geologic, and underwater-cultural heritage secrets hidden withinthe oceans. In the Atlantic, season highlights include completion of a multi-year submarine canyons mapping effort of the continental shelf break from North Carolina to the U.S.-Canada maritime border;new information on the ephemerality of recently discovered and geographically extensive cold water seeps; and continued exploration of the New England Seamount chain; and mapping of two potential historically significant World War II wreck sites. In the Gulf of Mexico, season highlights includecompletion of a multi-year mapping effort of the West Florida Escarpment providing new insight into submarine landslides and detachment zones;the discovery of at least two asphalt volcanoes, or 'tar lilies'; range extensions of deep-sea corals; discovery of two potential new species of crinoids; identification of at least 300 potential cold water seeps; and ROV exploration of three historically significant19th century shipwrecks. In both regions, high-resolution mapping led to new insight into the geological context in which deep sea corals develop,while ROV dives provided valuable observations of deep sea coral habitats and their associated organisms, and chemosynthetic habitats. All mapping and ROV data is freely available to the public in usable data formats and maintained in national geophysical and oceanographic data archives.

  18. Continental weathering as a driver of Late Cretaceous cooling: new insights from clay mineralogy of Campanian sediments from the southern Tethyan margin to the Boreal realm

    Science.gov (United States)

    Chenot, Elise; Deconinck, Jean-François; Pucéat, Emmanuelle; Pellenard, Pierre; Guiraud, Michel; Jaubert, Maxime; Jarvis, Ian; Thibault, Nicolas; Cocquerez, Théophile; Bruneau, Ludovic; Razmjooei, Mohammad J.; Boussaha, Myriam; Richard, James; Sizun, Jean-Pierre; Stemmerik, Lars

    2018-03-01

    New clay mineralogical analyses have been performed on Campanian sediments from the Tethyan and Boreal realms along a palaeolatitudinal transect from 45° to 20°N (Danish Basin, North Sea, Paris Basin, Mons Basin, Aquitaine Basin, Umbria-Marche Basin and Tunisian Atlas). Significant terrigenous inputs are evidenced by increasing proportions of detrital clay minerals such as illite, kaolinite and chlorite at various levels in the mid- to upper Campanian, while smectitic minerals predominate and represented the background of the Late Cretaceous clay sedimentation. Our new results highlight a distinct latitudinal distribution of clay minerals, with the occurrence of kaolinite in southern sections and an almost total absence of this mineral in northern areas. This latitudinal trend points to an at least partial climatic control on clay mineral sedimentation, with a humid zone developed between 20° and 35°N. The association and co-evolution of illite, chlorite and kaolinite in most sections suggest a reworking of these minerals from basement rocks weathered by hydrolysis, which we link to the formation of relief around the Tethys due to compression associated with incipient Tethyan closure. Diachronism in the occurrence of detrital minerals between sections, with detrital input starting earlier during the Santonian in the south than in the north, highlights the northward progression of the deformation related to the anticlockwise rotation of Africa. Increasing continental weathering and erosion, evidenced by our clay mineralogical data through the Campanian, may have resulted in enhanced CO2 consumption by silicate weathering, thereby contributing to Late Cretaceous climatic cooling.

  19. Cuddapah basin and its environs as first-order uranium target in the Proterozoics of India

    International Nuclear Information System (INIS)

    Sharma, Mithilesh; Rai, A.K.; Nagabhushana, J.C.; Vasudeva Rao, M.; Sinha, R.M.

    1995-01-01

    In peninsular India the middle Proterozoic intracratonic Cuddapah basin and its environs possess good geological favorability for several types of uranium deposits. Investigations so far have revealed the strata bound carbonate-hosted uranium mineralization in the Vempalle dolomitic limestone (e.g. Tummallapalle) and in the Pulivendla quartzite, confined to the lower part of the Cuddapah supergroup, and the structurally controlled uranium mineralization in the late Archaean/early Proterozoic granitoids and metamorphics along eastern (e.g. Kasturigattu), south-western (e.g. Sanipaya and T-Sanipaya and T-Sundapalle), and northern margins (e.g. Lambapur-Yellapur) of the Cuddapah basin. Based on the present level of work within the Cuddapah Basin and its environs, the following favourable locales and prospecting techniques have been suggested to identify the unconformity/vein-type high grade uranium deposits. (i) Detailed geological examination of the contact of basement with mid-Proterozoic Gulcheru/Nagari quartzite for locating unconformity-type uranium mineralisation. (ii) Extensive ground radiometric survey along the unconformity between basement granite and outliers of Srisailam formation, Banganpalle formation, Cumbum/Pullampet formation and Bairenkonda formation along northern and eastern margins of Cuddapah basin. (iii) Examination of the contact zone of the igneous intrusives (syenite and granite) into the Cumbum formation of central and northeastern parts of the basin e.g. Chelima - Giddalur area. (iv) Geophysical survey like resistivity (viz. SP, IP, TEM) to (a) delineate the concealed sulphide-rich zones along the prominent structures of the basinal margins and (b) study the possible existence under cover of quartzite and their subsurface behaviour for the fracture zones identified in the T. Sundapalle-Sanipaya, Pincha, Maddireddigaripalle, Chakrayapeta and Vepamanipeta areas. (author). 22 refs., 1 fig., 3 tabs

  20. Geology of uranium vein deposits (including Schwartzwalder Mine) in Proterozoic metamorphic rocks, Front Range, Colorado

    International Nuclear Information System (INIS)

    Voto, R.H. de; Paschis, J.A.

    1980-01-01

    The Schwartzwalder uranium deposit is one of many uranium vein occurrences in the Lower Proterozoic metamorphic rocks of the Front Range, Colorado. The principal veins of significant uranium content occur marginal to the Colorado Mineral Belt; are localized by structural dilation zones, vein junctions, fault deflections or branching; and occur dominantly within or at the contact of certain preferred metamorphic-stratigraphic units, particularly the siliceous, garnetiferous gneisses, where these rock units are broken by faults and fractures associated with the north-northwest-trending throughgoing faults. Uranium at the Schwartzwalder mine occurs primarily as open-space brecciated vein filling along the steeply west-dipping Illinois vein and numerous east-dipping subsidiary veins where they cut preferred metamorphic host rocks that are tightly folded. Uraninite occurs with molybdenite, adularia, jordisite, ankerite, pyrite, base-metal sulphides, and calcite in vein-filling paragenetic sequence. Minor wall-rock alteration is mainly hematite alteration and bleaching. Vertical relief on the developed ore deposit is 900 metres and still open-ended at depth. No vertical zonation of alteration, vein mineralogy, density of the subsidiary veins, or ore grade has been detected. The Schwartzwalder uranium deposit is of substantial tonnage (greater than 10,000 metric tons of U 3 O 8 ) and grade (averaging 0.57% U 3 O 8 ). Structural mapping shows that the Illinois vein-fault is a Proterozoic structure. Discordant Proterozoic (suggested) and Laramide dates have been obtained from Schwartzwalder ore. The data suggest, therefore, a Proterozoic ancestry of this heretofore presumed Laramide (Late Cretaceous-Early Tertiary) hydrothermal uranium deposit. The authors suggest a polygenetic model for the origin of the Schwartzwalder uranium deposit

  1. Retrospective of fossil dinoflagellate studies in Brazil: their relationship with the evolution of petroleum exploration in the Cretaceous of continental margin basins; Historico do estudo de dinoflagelados fosseis no Brasil: sua relacao com a evolucao da exploracao petrolifera no Cretaceo das bacias da margem continental

    Energy Technology Data Exchange (ETDEWEB)

    Arai, Mitsuru; Lana, Cecilia Cunha [PETROBRAS, Rio de Janeiro, RJ (Brazil). Centro de Pesquisas. Gerencia de Bioestratigrafia e Paleoecologia]. E-mail: arai@cenpes.petrobras.com.br

    2004-05-01

    Palynostratigraphy of the Brazilian Cretaceous was mainly based on terrestrial palynomorphs (mostly spores and pollen grains) until the end of the nineteen eighties. This was so because, for several decades, petroleum exploration in Brazil was focused on the essentially non marine sediments of the Reconcavo Basin. During the nineteen sixties, PETROBRAS extended oil exploration to the Brazilian continental shelf (offshore areas). Nevertheless, at that time the company did not invest immediately in marine palynostratigraphy, because it was believed that bio stratigraphic schemes based on foraminifera and calcareous nanno fossils would be more efficient than palynology in marine sequences. This belief changed only in the seventies, when commercial oil reservoirs were discovered in the Campos Basin, within the Macae Formation, a rock unit deposited in a carbonate shelf environment. Its environmental conditions were certainly hostile to the development and post-mortem preservation of foraminifera and calcareous nanno fossil-producing algae. As a result, no more than two or three bio zones could be identified in the Macae carbonate section on the basis of such organisms. Besides, carbonate shelf sediments, subject to only minor terrigenous input, are usually poor in terrestrial palynomorphs. On the other hand, the palynological content of some Macae strata consists mainly (up to 100%) of such marine palynomorphs as dino flagellates, acritarchs e palynoforaminifera. Consequently, PETROBRAS recognized the importance of developing a zonal framework based on these organisms. The first Cretaceous dinoflagellate zonation in Brazil was erected in 1976, and since the nineteen eighties, marine Cretaceous palynostratigraphy has made significant advances mainly due to the use of dino flagellates. Hundreds of Cretaceous dinoflagellate index species have been introduced into PETROBRAS databanks, becoming widely applicable to the bio stratigraphy of all Brazilian continental margin

  2. Precambrrian continental crust evolution of southeastern Sao Paulo state-Brazil: based on isotopico evidences

    International Nuclear Information System (INIS)

    Tassinari, C.C.G.; Campos Neto, M.C.

    1988-01-01

    The focussed area comprises five major different tectonic terranes separated by faults, which are named Alto Rio Grande Belt, Socorro-Guaxupe Nappe, Sao Roque, Embu and Costeiro Domains. The geological and geochronological history of these terranes show that the metamorphic episodes of crust-forming occurred involving both mantle-derived magmas and reworking of continental material since 3.4 Ga until 600 Ma. The post-tectonic granitic activities occurred within 1000-500 Ma range and in general, the rocks are progressively younger from the Socorro-Guaxupe Nappe (1000-850 Ma) in the NW towards the Costeiro Domain (550 Ma) in the SE. The Sr and Pb isotopic evidences, together with geological and geophysical informations, suggest that the proportions of the rock-forming processes through the geological time are: Archean, 10%; Lower Proterozoic, 10%; Middle Proterozoic, 38%; Late Proterozaic, 42%. Although the Mid and Late Proterozoic time were a period of a large amount of rocks were formed, they were not a major crustforming period, because these rocks are mainly constituted by recycled continental crust material. In our view, at end of the Early Proterozoic time, at least 85% of continetal crust, in this area, has accreted and differentiate. During the Middle and Late Proterozoic the continental crust grew at small rate. (author) [pt

  3. The continental lithosphere: a geochemical perspective

    International Nuclear Information System (INIS)

    Hawkesworth, C.J.; Person, G.; Turner, S.P.; Calsteren, P. Van; Gallagher, K.

    1993-01-01

    The lithosphere is the cool strong outler layer of the Earth that is effectively a boundary layer to the convecting interior. The evidence from mantle xenoliths and continental basalts is that the lower continental crust and uppermost mantle are different beneath Archaen and proterozoic areas. Mantle xenoliths from Archaen terrains, principally the Kaapvaal craton in southern Africa, are significantly depleted in Fe and other major elements which are concentrated in basalts. Nd and Os isotope data on inclusions in diamonds and peridoties respectively, indicate that such mantle is as old as the overlying Archaen crust. Since it appears to have been coupled to the overlying crust, and to have been isolated from the homogenising effects of convection for long periods of time, it is inferred to be within the continental lithosphere. The mantle lithosphere beneath Proterozoic and younger areas is less depleted in major elements, and so it is more fertile, less buoyant, and therefore thinner, than the Archaen mantle lithosphere. (author). 136 refs, 14 figs

  4. Geochemistry and petrogenesis of Proterozoic granitic rocks from ...

    Indian Academy of Sciences (India)

    Geochemistry and petrogenesis of Proterozoic granitic ... This study presents the geochemical characteristics of granitic rocks located on the northern ... Frost and Frost 2013). ...... King P L, White A J R, Chappell B W and Allen C M 1997.

  5. Real-Time PCR Quantification and Diversity Analysis of the Functional Genes aprA and dsrA of Sulfate-Reducing Prokaryotes in Marine Sediments of the Peru Continental Margin and the Black Sea.

    Science.gov (United States)

    Blazejak, Anna; Schippers, Axel

    2011-01-01

    Sulfate-reducing prokaryotes (SRP) are ubiquitous and quantitatively important members in many ecosystems, especially in marine sediments. However their abundance and diversity in subsurface marine sediments is poorly understood. In this study, the abundance and diversity of the functional genes for the enzymes adenosine 5'-phosphosulfate reductase (aprA) and dissimilatory sulfite reductase (dsrA) of SRP in marine sediments of the Peru continental margin and the Black Sea were analyzed, including samples from the deep biosphere (ODP site 1227). For aprA quantification a Q-PCR assay was designed and evaluated. Depth profiles of the aprA and dsrA copy numbers were almost equal for all sites. Gene copy numbers decreased concomitantly with depth from around 10(8)/g sediment close to the sediment surface to less than 10(5)/g sediment at 5 mbsf. The 16S rRNA gene copy numbers of total bacteria were much higher than those of the functional genes at all sediment depths and used to calculate the proportion of SRP to the total Bacteria. The aprA and dsrA copy numbers comprised in average 0.5-1% of the 16S rRNA gene copy numbers of total bacteria in the sediments up to a depth of ca. 40 mbsf. In the zone without detectable sulfate in the pore water from about 40-121 mbsf (Peru margin ODP site 1227), only dsrA (but not aprA) was detected with copy numbers of less than 10(4)/g sediment, comprising ca. 14% of the 16S rRNA gene copy numbers of total bacteria. In this zone, sulfate might be provided for SRP by anaerobic sulfide oxidation. Clone libraries of aprA showed that all isolated sequences originate from SRP showing a close relationship to aprA of characterized species or form a new cluster with only distant relation to aprA of isolated SRP. For dsrA a high diversity was detected, even up to 121 m sediment depth in the deep biosphere.

  6. Recrutamento do peixe-rei, Atherinella brasiliensis (Quoy & Gaimard (Atheriniformes, Atherinopsidae, na margem continental da Baía de Sepetiba, Rio de Janeiro, Brasil Recruitament of the silverside, Atherinella brasiliensis (Quoy & Gaimard (Atheriniformes, Atherinopsidae, in continental margin of Sepetiba Bay, Rio de Janeiro, Brazil

    Directory of Open Access Journals (Sweden)

    André Luiz Machado Pessanha

    2001-12-01

    Full Text Available The silverside, Atherinella brasiliensis (Quoy & Gaimard, 1824, a resident species of the shallows of bays, estuaries and coastal lagoons, was studied during two annual cycles, using beach seines (July 1996 to June 1998, in the continental margin of the Sepetiba Bay, aiming to assess recruitment and distribution patterns. A wide recruitment period, with young-of-the-year appearing from early spring to early autumn, was shown, with peaks of juveniles (Total Length-LT = 10-30 mm being recorded in two periods of the year; one in November and another in March. The highest abundances, of both juvenile and adults, were recorded in Coroa Grande and Itacuruçá, sites located in the outer Bay, during the whole summer, with significant differences were found. In spite of not having been found any statistically significant relationship between the environmental factors and the occurrence of this species, a trend of higher abundance in colder and more saline waters was detected. High values of growth coefficient (K = 0.93 and low asymptotic lenght (L∞ = 11.6 cm indicated that this species shows a high growth rate and short life cycle, with life span of 3.09 years, in this system.

  7. Sedimentary petrography of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting

    Science.gov (United States)

    Schreiber, U. M.; Eriksson, P. G.; van der Neut, M.; Snyman, C. P.

    1992-11-01

    Sandstone petrography, geochemistry and petrotectonic assemblages of the predominantly clastic sedimentary rocks of the Early Proterozoic Pretoria Group, Transvaal Sequence, point to relatively stable cratonic conditions at the beginning of sedimentation, interrupted by minor rifting events. Basement uplift and a second period of rifting occurred towards the end of Pretoria Group deposition, which was followed by the intrusion of mafic sill swarms and the emplacement of the Bushveld Complex in the Kaapvaal Craton at about 2050 Ma, the latter indicating increased extensional tectonism, and incipient continental rifting. An overall intracratonic lacustrine tectonic setting for the Pretoria Group is supported by periods of subaerial volcanic activity and palaeosol formation, rapid sedimentary facies changes, significant arkosic sandstones, the presence of non-glacial varves and a highly variable mudrock geochemistry.

  8. Authigenic minerals from the continental margins

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, V.P.

    . Chemistry and environment of dolomitization – a reappraisal. Earth-Sci. Reviews, 23, 175-222. Melim, LA., Scholle, PA., 2002. Dolomitization of the Capitan formation forereef facies (Permian, west Texas and New Mexico): seepage reflux revisited...

  9. Timing and composition of continental volcanism at Harrat Hutaymah, western Saudi Arabia

    Science.gov (United States)

    Duncan, Robert A.; Kent, Adam J R; Thornber, Carl; Schliedler, Tyler D; Al-Amri, Abdullah M

    2016-01-01

    Harrat Hutaymah is an alkali basalt volcanic field in north-central Saudi Arabia, at the eastern margin of a large Neogene continental, intraplate magmatic province. Lava flow, tephra and spatter cone compositions in the field include alkali olivine basalts and basanites. These compositions contrast with the predominantly tholeiitic, fissure-fed basalts found along the eastern margin of the Red Sea. The Hutaymah lava flows were erupted through Proterozoic arc-associated plutonic and meta-sedimentary rocks of the Arabian shield, and commonly contain a range of sub-continental lithospheric xenoliths, although the lavas themselves show little indication of crustal contamination. Previous radiometric dating of this volcanic field (a single published K–Ar age; 1.8 Ma) is suspiciously old given the field measurement of normal magnetic polarity only (i.e. Brunhes interval, ≤ 780 Ka). We report new age determinations on 14 lava flows by the 40Ar–39Ar laser step heating method, all younger than ~ 850 Ka, to better constrain the time frame of volcanism, and major, trace and rare earth element compositions to describe the chemical variation of volcanic activity at Harrat Hutaymah. Crystal fractionation was dominated by olivine ± clinopyroxene at a range of upper mantle and crustal pressures. Rapid ascent and eruption of magma is indicated by the array of lower crustal and lithospheric xenoliths observed in lava flows and tephra. Modeling suggests 1–7% melting of an enriched asthenospheric mantle source occurred beneath Harrat Hutaymah under a relatively thick lithospheric cap (60–80 km).

  10. Proterozoic microbial reef complexes and associated hydrothermal mineralizations in the Banfora Cliffs, Burkina Faso

    Science.gov (United States)

    Álvaro, J. Javier; Vizcaïno, Daniel

    2012-07-01

    The Proterozoic Guena-Souroukoundinga Formation of the Mopti arm (Gourma Aulacogen, southerm Taoudeni Basin) consists of a shale-dominated succession, up to 200 m thick, with scattered microbial reef complexes. Quarry exposures of the Tiara reef complex allow reconstruction of a transect across back-reef peritidal laminites, reef margin and peri-reef ooidal shoals, and fore-reef slope strata. Microbial carbonate productivity nucleated on isolated palaeohighs during transgression, whereas its end was controlled by two tectonically induced drowning pulses that led to the successive record of onlapping kerogenous limestones and pelagic shales. Reef carbonates are crosscut by fractures and fissures occluded by hydrothermal mineralizations, which are related to the rifting activity of the Gourma Aulacogen. The Tiara reef complex is similar to other Proterozoic reefs in being composed nearly entirely of stromatolites, although calcimicrobial (filamentous) and thromboid textures are locally abundant, which contrast with their scarcity or absence in coeval stable-platform microbial reefs of the northern Taoudeni Basin.

  11. Continental tectonics and continental kinetics

    International Nuclear Information System (INIS)

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

    1985-01-01

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

  12. Spongicoloides sp. aff. a Spongicoloides galapagensis (Decapoda: Stenopodidea: Spongicolidae: una nueva especie para la carcinofauna chilena y primer registro de un estenopodido en aguas del margen continental de Chile Spongicoloides sp. aff. to Spongicoloides galapagensis (Decapoda: Stenopodidea: Spongicolidae: a new species for Chilean carcinofauna and the first record of aStenopodid for the Chilean margin

    Directory of Open Access Journals (Sweden)

    Guillermo L Guzmán

    2011-11-01

    Full Text Available Se cita por primera vez para el margen continental de Chile una especie de camarón Stenopodidae, asociado a una esponja hexactinelida. Los especímenes fueron recolectados en dos sitios de Chile central (31°12'S, 71°52'W y 36°00'S, 73°38'W; 922 a 1000 m de profundidad. Las esponjas estaban adheridas al sustrato con probable origen en emanaciones de metano. Spongicoloides sp. aff. a S. gala-pagensis, es el primer registro de la familia en aguas del margen continental del Pacífico suroriental. Los ejemplares coinciden con los rangos de la variación morfológica de S. galapagensis, no obstante difieren en el número de branquias descrito.This is the first record of a species of Stenopodid shrimp along the Chilean margin, associated with a hexactinellid sponge. The specimens were collected at two sites in central Chile (31°12'S, 71°52'W and 36°00'S, 73°38'W; 922 to 1000 m depth. The sponges were attached to the substrate with probable origins in methane seepage. Spongicoloides sp. aff. to S. galapagensis constitutes the first record of the family for the eastern south Pacific continental margin. The specimens coincide with the ranges of morphological variation of S. galapagensis, although they differ in the number of brachia described.

  13. Arachania, A neo proterozoic magmatic arc and its fragments in south America and Africa

    International Nuclear Information System (INIS)

    Gaucher, C.; Bossi, J.; Frimmel, H.

    2010-01-01

    The name Arachania has been recently proposed for the block that comprises the Cuchilla Dionisio-Pelotas, Marmora, Tygerberg and correlative terranes at both sides of the south Atlantic, which is considered a fragment of the Kalahari Craton that a later stage (660-550 Ma) evolved into an active margin. The block played a key role in the amalgamation of southwestern Gondwana, which has been only recently recognized. Arachania is composed of three different lithotectonic elements: (1) a high-grade metamorphic basement of Namaquan age with evidence of older, Eburnean components that crop out mainly in southern Uruguay; (2) a voluminous calc alkaline granitic batholith s mostly within the 660-550 Ma age range, representing the roots of a Neo proterozoic magmatic arc; and (3) deep-water, turbiditic, Ediacaran sedimentary successions marking the eastern border of Arachania, often associated with mafic to ultramafic rocks

  14. Folds in multilayered rocks of Proterozoic age, Rajasthan, India

    Indian Academy of Sciences (India)

    R. Narasimhan (Krishtel eMaging) 1461 1996 Oct 15 13:05:22

    Johnson and Johnson 2002 etc) shows that the fold shape modification may be brought about by buckling and flattening operating simultaneously throughout the development of fold. In the present paper a series of F1 folds devel- oped in slates with interlayered alternations with quartzite of Proterozoic age and unaffected ...

  15. Radiochronological age and correlation of proterozoic sediments in Brazil

    International Nuclear Information System (INIS)

    Bonhomme, M.G.; Macedo, M.-H.F.; Filho, A.T.

    1982-01-01

    A review of available Rb-Sr and K-Ar datings obtained on sedimentary sequences, metamorphosed or not, interbedded volcanics and cross-cutting intrusives of the Precambrian of Brazil yields the following conclusions: (1) The Roraima and Rio Fresco Formations, resting upon the Amazonian craton, have been affected by the Trans-Amazonian orogeny and are of Lower Proterozoic age. (2) The Beneficiente Group, in the same region, seems to be of Middle Proterozoic, Lower Riphean age. (3) Upon the Sao Francisco craton, and upon the Lower Proterozoic Rio dos Remedios complex, the Paraguacu Group might be of middle Riphean age and the glacial sequences, Macaubas Group and Bebedouro Formation, date back to between 950 and 1000 Ma. (4) The age of the Bambui and Una Groups, in the same region, remains undetermined. It might be either Upper Riphean, or Upper Riphean and Vendian. (5) The molassic series associated with the Brazilian orogeny are dated back to between the Proterozoic-Phanerozoic boundary and the Ordovician. (Auth.)

  16. Empirical Records of Environmental Change across the Archean-Proterozoic Transition

    Science.gov (United States)

    Kaufman, A. J.

    2011-12-01

    Time-series geochemical analyses of scientific drill cores intersecting the Archean-Proterozoic transition suggest a coupling of environmental and biological change that culminated in the pervasive oxygenation of Earth's atmosphere and oceans. Elemental and multiple isotope measurements of sedimentary archives, including carbonate, shale, and banded iron-formation from Western Australia, South Africa, Brazil, and southern Canada, indicate important changes in the carbon, sulfur, and nitrogen cycles that monitor the redox state of the oceans and the cyanobacterial buildup of atmospheric oxygen and ozone. In response, continental weathering would have increased, resulting in the enhanced delivery of sulfate and nutrients to seawater, further stimulating photoautotrophic fluxes of oxygen to surface environments. The positive feedback may additionally be responsible for the decline of atmospheric methane and surface refrigeration, represented by a series of discrete ice ages beginning around 2.4 billion years ago, due to the loss of greenhouse capacity during a time of lower solar luminosity. While speculative, the linkage of surface oxidation with enhanced nutrient supply and development of stratospheric sunscreen soon after the Archean-Proterozoic boundary suggests that the earliest perturbation in the carbon cycle may be associated with the rapid expansion of single-celled eukaryotes. Both sterol synthesis in eukaryotes and aerobic respiration require significant levels of oxygen in the ambient environment. Hence, Earth's earliest ice age(s) and onset of a modern and far more energetic carbon cycle may have been directly related to the global expansion of cyanobacteria that released oxygen to the environment, and of eukaryotes that respired it.

  17. Precambrian continental crustal evolution of Southeastern Sao Paulo state - Brazil: based on isotopic evidences

    International Nuclear Information System (INIS)

    Tassinari, C.C.G.

    1988-01-01

    The isotopic studies on granitic intrusions, orthogneissic rocks and migmatitic terranes in the Southeastern Sao Paulo are presented, indicating the age and the geochemical nature of the continental crust of this area. Approximately 300 Ar, Sr and Pb isotopic age determinations are included in this paper, categorized as to their reliability and significance. Looking for the continental crust growth related to the geological time, at the end of the lower Proterozoic, at least 85% of the continental crust has already accreted and differentiated. (C.G.C.)

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

    Directory of Open Access Journals (Sweden)

    André Pouclet

    2017-01-01

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

  19. Continental Rifts

    Science.gov (United States)

    Rosendahl, B. R.

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

  20. Late Proterozoic island-arc complexes and tectonic belts in the southern part of the Arabian Shield, Kingdom of Saudi Arabia

    Science.gov (United States)

    Greenwood, William R.; Stoeser, D.B.; Fleck, R.J.; Stacey, J.S.

    1983-01-01

    Sr ratios are not included in the appendix, but all rocks more than 660 m.y. old have initial ratios in the range 0.7021-0.7035, with only two greater than 0.7030. Thus, nothing in the Rb-Sr data suggests involvement of an older continental crust during the evolution of the southern Shield. A lead isotope study of ore minerals and potassium feldspars of the Arabian Shield by Stacey and others (1980) also suggests that no older (Archean to early Proterozoic) evolved continental-type crust underlies the southern Shield. An early summary of mapping (Schmidt and others, 1973) suggests that older sialic basement underlies the late Proterozoic layered rocks in the southern Shield. However, subsequent-mapping and the isotopic studies cited above have established that all of these rocks are of late Proterozoic age and that all rocks of the southern Shield that are more than 660 m.y. old have ensimatic or mantle isotopic characteristics. Figure 2 shows, with only two exceptions, that rocks more than 800 m.y. old are present west of the boundary separating the Tayyah and Khadra belts. The exceptions are two poorly controlled Rb-Sr ages obtained by Fleck (1980) on two quartz diorite plutons in the Malahah region (appendix 1, localities 26 and 27). Preliminary uranium-thorium zircon data of Stacey now suggest that one of these quartz diorite plutons (locality 26) has an age of approximately 640 m.y. Therefore, we prefer to discount the two dates of Fleck until further information is available. As noted earlier and as described below, most of the rocks of the southern Arabian Shield have characteristics typical of those formed in the island-arc environment by subduction-related processes. We shall refer to the group of rocks in the western part of the southern Shield, which formed from 1100 to 800 m.y. ago, as the 'older ensimatic-arc complex' and those in the eastern and northwestern parts, which formed from 800 to 690 m.y. ago, as the 'younger marginal-arc compl

  1. Some problems of geologic relations between the Amazon craton and east margins fold belts

    International Nuclear Information System (INIS)

    Almeida, F.F.M. de

    1986-01-01

    This paper deals with some geologic problems related to the limits between the Amazon craton and the fold belts developed at its margins during the Precambrian. These limits are diversified but clearly recognized. To the north, the Araguaia-Tocantins fold belt, of presumed Middle Proterozoic age, is separated from the cratonic block by a deep marginal fracture zone permeated by mafic and ultramafic rocks. The geologic, magmatic and aeromagnetic characteristics of this zone point out the presence of deep faults, supposed to be of Middle Proterozoic age. The southern Paraguay fold belt constitutes and accurated zone of linear structures supposed to be of Late Proterozoic development. Despite the great increase of knowledge during the last ten years many tectonic, stratigraphic and geochronologic problems remain unsolved. The aim of this paper is to point out some of these problems and suggest specific studies to solve them. (author)

  2. Continental divide

    International Nuclear Information System (INIS)

    Quinn, F.J.

    1991-01-01

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

  3. Late Devonian and Triassic basalts from the southern continental ...

    Indian Academy of Sciences (India)

    continental margin of the East European Platform, tracers of a single heterogeneous ... Areas of Precambrian consolidation within the Late Paleozoic orogen; 3. Areas of ...... and hydrocarbon accumulations; J. Petroleum Geology. 16 183–196.

  4. Association of the Purana basins and the middle Proterozoic mobile belts in peninsular India: implications on targeting uranium deposits

    International Nuclear Information System (INIS)

    Kale, V.S.

    1995-01-01

    The disparate Archaean Cratonic Nuclei of the Indian peninsular shield coalesced together through late Archaean - Palaeoproterozoic accretionary tectonic events. The subsequent Mesoproterozoic and Neoproterozoic sequences are preserved either in the Purana basins or in the middle Proterozoic mobile belts (MPMB). The latter contain deformed and metamorphosed supracrustal sequences; and can be ascribed to compressive tectonic regimes. The Purana basins on the other hand represent shallow marine, epicratonic, passive-margin sequences deposited in an extensional tectonic regime. Major deformational events and metamorphism of the MPMB are known to have taken place around 1600 ±200 Ma and 900 ± 100 Ma. These two periods coincide with the ages of initiation and major intrabasinal breaks in the growth of the Purana basins. The contemporary juxtapositioning of these two dissimilar tectonic regimes in peninsular India, is examined within the framework of the available data on them and the current models of Proterozoic tectonics. Its implications on uranium mineralization and possible regions for targeting exploration activities are discussed on this basis. (author). 112 refs., 4 figs

  5. Seismites in a Proterozoic tidal succession, Singhbhum, Bihar, India

    Science.gov (United States)

    Bhattacharya, H. N.; Bandyopadhyay, Sandip

    1998-08-01

    Early Proterozoic metasediments of the Chaibasa Formation (Galudih-Ghatsila-Dhalbhumgarh region, Singhbhum, Bihar, India) comprise a number of cyclic fining-upward prograding successions of tidalites. The tidalites show indications for earthquakes in the form of synsedimentary deformation features, apart from the structures due to high-energy wave action. Deformed cross-bedding, convolute laminations, synsedimentary faults, graben-like structures, sandstone dykes, pseudonodules and slump folds record the seismic activity. A gradual decline in the frequency of seismites and tsunami-related depositional features, in combination with an upward increase in thickness of the tidal cycles, are attributed to gradual diminishing of tectonic activity within the basin.

  6. Ted Irving and the Precambrian continental drift of (within?) the Canadian Shield

    Science.gov (United States)

    Hoffman, P. F.

    2014-12-01

    Ted Irving was no stranger to the Precambrian when he began paleomagnetic studies in the Canadian Shield (CS) that would dominate his research in the early and mid-1970's. Twenty years before, his graduate work on billion-year-old strata in Scotland established paleomagnetic methodologies applicable to sedimentary rocks generally. In 1958, he and Ronald Green presented an 'Upper Proterozoic' APW path from Australia as evidence for pre-Carboniferous drift relative to Europe and North America (the poles actually range in age from 1.2 to 2.7 Ga). His first published CS poles were obtained from the Franklin LIP of the Arctic platform and demonstrate igneous emplacement across the paleoequator. Characteristically, his 1971 poles are statistically indistinguishable from the most recent grand mean paleopole of 2009. His main focus, however, was on the question of Precambrian continental drift. He compared APW paths with respect to Laurentia with those obtained from other Precambrian shields, and he compared APW paths from different tectonic provinces within the CS. He was consistently antagonistic to the concept of a single long-lived Proterozoic supercontinent, but he was on less certain ground regarding motions within the CS due to inadequate geochronology. With Ron Emslie, he boldly proposed rapid convergence between parts of the Grenville Province and Interior Laurentia (IL) ~1.0 Ga. This was controversial given the uncertain ages of multiple magnetic components in high-grade metamorphic rocks. With John McGlynn and John Park, he developed a Paleoproterozoic APW path for the Slave Province from mafic dikes and red clastics, encompassing the time of consolidation of IL during 2.0-1.8 Ga orogenesis. Before 1980, he constructed Paleoproterozoic APW paths for IL as a whole, finding little evidence for significant internal displacement. He recognized that the Laurentian APW path describes a series of straight tracks linked by hairpins, the latter corresponding in age to

  7. MARGINS: Toward a novel science plan

    Science.gov (United States)

    Mutter, John C.

    A science plan to study continental margins has been in the works for the past 3 years, with almost 200 Earth scientists from a wide variety of disciplines gathering at meetings and workshops. Most geological hazards and resources are found at continental margins, yet our understanding of the processes that shape the margins is meager.In formulating this MARGINS research initiative, fundamental issues concerning our understanding of basic Earth-forming processes have arisen. It is clear that a business-as-usual approach will not solve the class of problems defined by the MARGINS program; the solutions demand approaches different from those used in the past. In many cases, a different class of experiment will be required, one that is well beyond the capability of individual principle investigators to undertake on their own. In most cases, broadly based interdisciplinary studies will be needed.

  8. Real-time PCR quantification and diversity analysis of the functional genes aprA and dsrA of sulfate-reducing bacteria in marine sediments of the Peru continental margin and the Black Sea

    OpenAIRE

    Axel eSchippers; Anna eBlazejak

    2011-01-01

    A quantitative, real-time PCR (Q-PCR) assay for the functional gene adenosine 5´-phosphosulfate reductase (aprA) of sulfate-reducing bacteria (SRB) was designed. This assay was applied together with described Q-PCR assays for dissimilatory sulfite reductase (dsrA) and the 16S rRNA gene of total Bacteria to marine sediments from the Peru margin (0 – 121 meters below seafloor (mbsf)) and the Black Sea (0 – 6 mbsf). Clone libraries of aprA show that all isolated sequences originate from SRB...

  9. Continental Divide Trail

    Data.gov (United States)

    Earth Data Analysis Center, University of New Mexico — This shapefile was created to show the proximity of the Continental Divide to the Continental Divide National Scenic Trail in New Mexico. This work was done as part...

  10. Carboniferous granite basement dredged from a site on the southwest margin of the Challenger Plateau, Tasman Sea

    International Nuclear Information System (INIS)

    Tulloch, A.J.; Kimbrough, D.L.; Wood, R.A.

    1991-01-01

    Discordant zircon fractions from a granite sample dredged off a basement horst on the western margin of the Challenger Plateau yield a 335 ± 7 Ma lower intercept date interpreted as the crystallisation age of the granite. This age, and the modal composition of the granite, is similar to that of the Karamea Suite of Westland and Nelson, New Zealand, and some Tasmanian granites. The concordia upper intercept date of 1747 ± 300 Ma implies the presence of Proterozoic continental crustal material in the source region of the granite. The Challenger granite is distinct from older S-type granites of southeastern Australia and I-type granites of northern Victoria Land and Marie Byrd Land, Antarctica. Subsequent to emplacement, the granite was brecciated and hydrothermally altered. A K-Ar age of 95 Ma on hydrothermal sericite indicates that this event overlapped with a major crustal extension event recorded in Westland and Nelson, and predates the oldest known sea floor in the Tasman Basin by at least 11 Ma. (author). 23 refs., 3 figs., 2 tabs

  11. Marginal Matter

    Science.gov (United States)

    van Hecke, Martin

    2013-03-01

    All around us, things are falling apart. The foam on our cappuccinos appears solid, but gentle stirring irreversibly changes its shape. Skin, a biological fiber network, is firm when you pinch it, but soft under light touch. Sand mimics a solid when we walk on the beach but a liquid when we pour it out of our shoes. Crucially, a marginal point separates the rigid or jammed state from the mechanical vacuum (freely flowing) state - at their marginal points, soft materials are neither solid nor liquid. Here I will show how the marginal point gives birth to a third sector of soft matter physics: intrinsically nonlinear mechanics. I will illustrate this with shock waves in weakly compressed granular media, the nonlinear rheology of foams, and the nonlinear mechanics of weakly connected elastic networks.

  12. Study of the particulate matter transfer and dumping using 210 Po et le 210 Pb. Application to the Gulf of Biscary (NE Atlantic Ocean) and the Gulf of Lion (NW Mediterranean Sea) continental margins

    International Nuclear Information System (INIS)

    Radakovitch, O.

    1995-01-01

    210 Po and 210 Pb activities and fluxes were measured on seawater, sediment-trapped material collected during one year and sediment. Focalization of 210 Pb is clearly noticed on the Cap-Ferret canyon (Gulf of Biscary) and the Lacaze-Duthiers canyon (western part of the Gulf of Lion). In both sites, 210 Pb fluxes in traps and sediment are always higher than 210 Pb flux available from atmospheric and in situ production. On the contrary, Grand-Rhone canyon and its adjacent open slope exhibit a 210 Pb budget near equilibrium in the near-bottom sediment traps, but focalization is important in the sediment. For the entire Gulf of Lion margin, focalization of 210 Pb in the sediment occurred principally between 500 and 1500 m water depth on the slope, and on the middle shelf mud-patch. 210 Po and 210 Pb have been used in the Cap Ferret and Grand-Rhone canyons to characterize the origin of the particulate trapped material. Two main sources feed the water column. The first source, localized in surface waters, is constituted by biogenic particles from primary production and lithogenic material. The second source, deeper, is due to resuspension at the shelf break and/or on the open slope. In each site, 210 Po and 210 Pb activities of the trapped particles did not show any relations with the major constituents. Quantity of particles appeared to be the main factor regulating adsorption processes of these nuclides. Sedimentation rates based on 210 Po profiles decreased with increasing water depth, from 0.4 ti 0.06 cm y-1 on the Cap Ferret canyon (400 to 3000 m water depth) and from 0.5 to 0.05 cm y-1 for the entire Gulf of Lion margin (50 to 2000 m water depth). (author)

  13. Instability of the southern Canadian Shield during the late Proterozoic

    Science.gov (United States)

    McDannell, Kalin T.; Zeitler, Peter K.; Schneider, David A.

    2018-05-01

    Cratons are generally considered to comprise lithosphere that has remained tectonically quiescent for billions of years. Direct evidence for stability is mainly founded in the Phanerozoic sedimentary record and low-temperature thermochronology, but for extensive parts of Canada, earlier stability has been inferred due to the lack of an extensive rock record in both time and space. We used 40Ar/39Ar multi-diffusion domain (MDD) analysis of K-feldspar to constrain cratonic thermal histories across an intermediate (∼150-350 °C) temperature range in an attempt to link published high-temperature geochronology that resolves the timing of orogenesis and metamorphism with lower-temperature data suited for upper-crustal burial and unroofing histories. This work is focused on understanding the transition from Archean-Paleoproterozoic crustal growth to later intervals of stability, and how uninterrupted that record is throughout Earth's Proterozoic "Middle Age." Intermediate-temperature thermal histories of cratonic rocks at well-constrained localities within the southern Canadian Shield of North America challenge the stability worldview because our data indicate that these rocks were at elevated temperatures in the Proterozoic. Feldspars from granitic rocks collected at the surface cooled at rates of histories suggest that at ca. 1.1-1.0 Ga these rocks were still near ∼200 °C - signaling either reheating, or prolonged residence at mid-crustal depths assuming a normal cratonic geothermal gradient. After 1.0 Ga, the regions we sampled then underwent further cooling such that they were at or near the surface (≪60 °C) in the early Paleozoic. Explaining mid-crustal residence at 1.0 Ga is challenging. A widespread, prolonged reheating history via burial is not supported by stratigraphic information, however assuming a purely monotonic cooling history requires at the very least 5 km of exhumation beginning at ca. 1.0 Ga. A possible explanation may be found in evidence of

  14. Basins in ARC-continental collisions

    Science.gov (United States)

    Draut, Amy E.; Clift, Peter D.; Busby, Cathy; Azor, Antonio

    2012-01-01

    Arc-continent collisions occur commonly in the plate-tectonic cycle and result in rapidly formed and rapidly collapsing orogens, often spanning just 5-15 My. Growth of continental masses through arc-continent collision is widely thought to be a major process governing the structural and geochemical evolution of the continental crust over geologic time. Collisions of intra-oceanic arcs with passive continental margins (a situation in which the arc, on the upper plate, faces the continent) involve a substantially different geometry than collisions of intra-oceanic arcs with active continental margins (a situation requiring more than one convergence zone and in which the arc, on the lower plate, backs into the continent), with variable preservation potential for basins in each case. Substantial differences also occur between trench and forearc evolution in tectonically erosive versus tectonically accreting margins, both before and after collision. We examine the evolution of trenches, trench-slope basins, forearc basins, intra-arc basins, and backarc basins during arc-continent collision. The preservation potential of trench-slope basins is low; in collision they are rapidly uplifted and eroded, and at erosive margins they are progressively destroyed by subduction erosion. Post-collisional preservation of trench sediment and trench-slope basins is biased toward margins that were tectonically accreting for a substantial length of time before collision. Forearc basins in erosive margins are usually floored by strong lithosphere and may survive collision with a passive margin, sometimes continuing sedimentation throughout collision and orogeny. The low flexural rigidity of intra-arc basins makes them deep and, if preserved, potentially long records of arc and collisional tectonism. Backarc basins, in contrast, are typically subducted and their sediment either lost or preserved only as fragments in melange sequences. A substantial proportion of the sediment derived from

  15. Uranium deposits in Proterozoic quartz-pebble conglomerates

    International Nuclear Information System (INIS)

    1987-09-01

    This report is the result of an effort to gather together the most important information on uranium deposits in Proterozoic quartz-pebble conglomerates in the United States of America, Canada, Finland, Ghana, South Africa and Australia. The paper discusses the uranium potential (and in some cases also the gold potential in South Africa, Western Australia and Ghana) in terms of ores, sedimentation, mineralization, metamorphism, placers, geologic formations, stratigraphy, petrology, exploration, tectonics and distribution. Geologic history and application of geologic models are also discussed. Glacial outwash and water influx is also mentioned. The uranium deposits in a number of States in the USA are covered. The Witwatersrand placers are discussed in several papers. Refs, figs, tabs

  16. An example of Lower Proterozoic sediments: the Francevillian in Gabon

    International Nuclear Information System (INIS)

    Bonhomme, M.G.; Gauthier-Lafaye, F.; Weber, F.

    1982-01-01

    New mineralogical and isotopic data have been obtained on diagenetic fine fractions of the Francevillian sediments. Together with the Rb-Sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products, they allow us to present a more complete geological history of the Francevillian. The main steps are: deposition of the FA Formation before 2140 Ma deposition of the FB-FE Formations between 2140 and 2050 Ma, siliceous diagenesis followed by nuclear reactions between 2050 and 1930 Ma, carbonate diagenesis at nearly 1870 Ma, end of cooling at 1700 Ma and dolerite intrusion at ca. 970 Ma. The Francevillian sediments are, to date, the oldest true sediments known; they are diagenetic, Lower Proterozoic sediments. (Auth.)

  17. The proterozoic Georgetown Province - a Broken Hill analogue

    International Nuclear Information System (INIS)

    Carr, G.R.

    2000-01-01

    Collaborative work between CSIRO and AGSO has resulted in the development of a Pb isotope model that attempts to place relatively precise (∼5 Ma) age constraints on Proterozoic mineralisation in the Mount Isa and McArthur River terrains (Sun et al., 1994). Although this model was developed for sediment hosted mineralisation in low grade metamorphic terrains, the CSIRO-AGSO model ages for other mineralisation in high-grade terrains such as Broken Hill appear to be consistent with the U-Pb zircon ages obtained for the high-grade host sequences. Without independent evidence that the model is applicable to such terrains, the observations cannot be used to indicate the age of the mineralisation. Lead isotope data obtained on potassium feldspar separates from five felsic intrusive samples in the Georgetown terrain show a wide range of Pb isotope ratios. The lowest 206 Pb/ 204 Pb analyses are considered to approximate to the Proterozoic initial ratio and indicate a model age of ∼1510 Ma based on the CSIRO-AGSO model. This age is 45 Ma younger than the crystallisation age of the granite, but must be considered a minimum as the initial 206 Pb/ 204 Pb ratio may well prove to be lower after more comprehensive analysis. Sulfide mineralisation within the Einasleigh Metamorphics has a wide range of 206 Pb/ 204 Pb ratios that lie between this granite value and the relatively homogeneous population from Railway Flat. The Railway Flat data are very similar to values for Broken Hill and also the Broken Hill-type Pegmont mineralisation in the Mount Isa Eastern Succession. These data all have significantly lower 207 Pb/ 204 Pb ratios than the CSIRO-AGSO model, suggesting a significantly different source rock environment for this style of mineralisation from that for the sediment hosted deposits

  18. Proterozoic Milankovitch cycles and the history of the solar system.

    Science.gov (United States)

    Meyers, Stephen R; Malinverno, Alberto

    2018-06-19

    The geologic record of Milankovitch climate cycles provides a rich conceptual and temporal framework for evaluating Earth system evolution, bestowing a sharp lens through which to view our planet's history. However, the utility of these cycles for constraining the early Earth system is hindered by seemingly insurmountable uncertainties in our knowledge of solar system behavior (including Earth-Moon history), and poor temporal control for validation of cycle periods (e.g., from radioisotopic dates). Here we address these problems using a Bayesian inversion approach to quantitatively link astronomical theory with geologic observation, allowing a reconstruction of Proterozoic astronomical cycles, fundamental frequencies of the solar system, the precession constant, and the underlying geologic timescale, directly from stratigraphic data. Application of the approach to 1.4-billion-year-old rhythmites indicates a precession constant of 85.79 ± 2.72 arcsec/year (2σ), an Earth-Moon distance of 340,900 ± 2,600 km (2σ), and length of day of 18.68 ± 0.25 hours (2σ), with dominant climatic precession cycles of ∼14 ky and eccentricity cycles of ∼131 ky. The results confirm reduced tidal dissipation in the Proterozoic. A complementary analysis of Eocene rhythmites (∼55 Ma) illustrates how the approach offers a means to map out ancient solar system behavior and Earth-Moon history using the geologic archive. The method also provides robust quantitative uncertainties on the eccentricity and climatic precession periods, and derived astronomical timescales. As a consequence, the temporal resolution of ancient Earth system processes is enhanced, and our knowledge of early solar system dynamics is greatly improved.

  19. Real-time PCR quantification and diversity analysis of the functional genes aprA and dsrA of sulfate-reducing bacteria in marine sediments of the Peru continental margin and the Black Sea

    Directory of Open Access Journals (Sweden)

    Axel eSchippers

    2011-12-01

    Full Text Available A quantitative, real-time PCR (Q-PCR assay for the functional gene adenosine 5´-phosphosulfate reductase (aprA of sulfate-reducing bacteria (SRB was designed. This assay was applied together with described Q-PCR assays for dissimilatory sulfite reductase (dsrA and the 16S rRNA gene of total Bacteria to marine sediments from the Peru margin (0 – 121 meters below seafloor (mbsf and the Black Sea (0 – 6 mbsf. Clone libraries of aprA show that all isolated sequences originate from SRB showing a close relationship to aprA of characterised species or form a new cluster with only distant relation to aprA of isolated SRB. Below 40 mbsf no aprA genes could be amplified. This finding corresponds with results of the applied new Q-PCR assay for aprA. In contrast to the aprA the dsrA gene could be amplified up to sediment depths of 121 mbsf. Even in such an extreme environment a high diversity of this gene was detected. The 16S rRNA gene copy numbers of total Bacteria were much higher than those of the functional genes at all sediment depths and used to calculate the proportion of SRB to the total Bacteria. The aprA and dsrA copy numbers comprised in average 0.5 - 1 % of the 16S rRNA gene copy numbers of total Bacteria in the sediments up to a depth of ca. 40 mbsf. Depth profiles of the aprA and dsrA copy numbers were almost equal for all sites. Gene copy numbers decreased concomitantly with depth from around 108 / g sediment close to the sediment surface to less than 105 / g sediment at 5 mbsf. In the zone without detectable sulfate in the pore water from ca. 40 – 121 mbsf (Peru margin ODP site 1227, only dsrA (but not aprA was detected with copy numbers of less than 104 / g sediment, comprising ca. 14 % of the 16S rRNA gene copy numbers of total Bacteria. In this zone sulfate might be provided for SRB by anaerobic sulfide oxidation.

  20. Stratigraphy, sedimentology and petrology of neogene rocks in the Deschutes Basin, Central Oregon: a record of continental-margin volcanism and its influence on fluvial sedimentation in an arc-adjacent basin

    International Nuclear Information System (INIS)

    Smith, G.A.

    1986-07-01

    Neogene rocks of the Deschutes basin include the middle Miocene Columbia River Basalt Group and Simtustus Formation, and late Miocene to early Pliocene Deschutes Formation. Assignment of Prineville chemical-type flows to the Grande Ronde Basalt of the Columbia River Basalt Group is based on correlation of these lavas from their type area through the Deschutes basin and onto the Columbia Plateau, where they have been previously mapped as Grande Ronde Basalt. Simtustus Formation is a newly defined unit intercalated with and conformable upon these basalts, and is unconformably overlain by Deschutes Formation. Burial of mature topography by middle Miocene basalts raised local base levels and initiated aggradation by low-gradient streams within the basin represented by the tuffaceous sandstones and mudstones of the Simtustus Formation. These sediments are enriched in pyroclastic constituents relative to contemporaneous Western Cascades volcanics, reflecting preferential incorporation of easily eroded and more widespread pyroclastic debris in distal sedimentary sequences compared to epiclastic contributions from lavas. The abundance of basalts, combined with the paucity of hydrous minerals and FeO and TiO 2 enrichment in intermediate lavas, characterizes early High Cascade volcanics as atypical for convergent-margin arcs. These petrologic characteristics are consistent with high-level fractionation in an extensional regime. Extension culminated in the development of an intra-arc graben, which ended Deschutes Formation deposition by structurally isolating the basin from the High Cascade source area

  1. A quantitative analysis of transtensional margin width

    Science.gov (United States)

    Jeanniot, Ludovic; Buiter, Susanne J. H.

    2018-06-01

    Continental rifted margins show variations between a few hundred to almost a thousand kilometres in their conjugated widths from the relatively undisturbed continent to the oceanic crust. Analogue and numerical modelling results suggest that the conjugated width of rifted margins may have a relationship to their obliquity of divergence, with narrower margins occurring for higher obliquity. We here test this prediction by analysing the obliquity and rift width for 26 segments of transtensional conjugate rifted margins in the Atlantic and Indian Oceans. We use the plate reconstruction software GPlates (http://www.gplates.org) for different plate rotation models to estimate the direction and magnitude of rifting from the initial phases of continental rifting until breakup. Our rift width corresponds to the distance between the onshore maximum topography and the last identified continental crust. We find a weak positive correlation between the obliquity of rifting and rift width. Highly oblique margins are narrower than orthogonal margins, as expected from analogue and numerical models. We find no relationships between rift obliquities and rift duration nor the presence or absence of Large Igneous Provinces (LIPs).

  2. Earth history. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals.

    Science.gov (United States)

    Planavsky, Noah J; Reinhard, Christopher T; Wang, Xiangli; Thomson, Danielle; McGoldrick, Peter; Rainbird, Robert H; Johnson, Thomas; Fischer, Woodward W; Lyons, Timothy W

    2014-10-31

    The oxygenation of Earth's surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth's surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans. Copyright © 2014, American Association for the Advancement of Science.

  3. New proposal for the Piedras de Afilar lithostratigraphic formation, upper Neo proterozoic of Uruguay

    International Nuclear Information System (INIS)

    Pamoukaghlian, K.; Gaucher, C.; Poire, D.

    2010-01-01

    This work is about the Piedras de Afilar lithostratigraphic formation which is part of Tandilia Precambrian terrain of Uruguay. This sedimentary sequence of Neo proterozoic age is supported by a Paleoproterozoic basement

  4. PRE-RIFT COMPRESSIONAL STRUCTURES AS A CONTROL ON PASSIVE MARGIN FORMATION

    DEFF Research Database (Denmark)

    Schiffer, Christian; Petersen, Kenni Dinesen

    Passive margins are commonly separated into volcanic and non-volcanic modes, each with a distinct formation mechanism and structure. Both form the transition from continental to oceanic crust. Large amounts of geophysical data at passive margins show that the tapering continental crust is often u...

  5. Microfossils' diversity from the Proterozoic Taoudeni Basin, Mauritania

    Science.gov (United States)

    Beghin, Jérémie; Houzay, Jean-Pierre; Blanpied, Christian; Javaux, Emmanuelle

    2014-05-01

    Prokaryotes and microscopic eukaryotes are known to have appeared well before the Cambrian's adaptative radiation which flourished perceptibly as a generalized macroscopic world. What do we know about the trigger events which stimulated eukaryotic diversification during the Proterozoic? Biological innovations or environmental changes, and indeed probably both (Knoll et al., 2006), played a fundamental role controlling this important step of life's evolution on Earth. Javaux (2011), proposed a diversification pattern of early eukaryotes divided into three steps and focusing on different taxonomic levels, from stem group to within crown group, of the domain Eukarya. Here, we present a new, exquisitely preserved and morphologically diverse assemblage of organic-walled microfossils from the 1.1 Ga El Mreiti Group of the Taoudeni Basin (Mauritania). The assemblage includes beautifully preserved microbial mats comprising pyritized filaments, prokaryotic filamentous sheaths and filaments, microfossils of uncertain biological affinity including smooth isolated and colonial sphaeromorphs (eukaryotes and/or prokaryotes), diverse protists (ornamented and process-bearing acritarchs), as well multicellular microfossils interpreted in the literature as possible xanthophyte algae. Several taxa are reported for the first time in Africa, but are known worldwide. This study improves microfossil diversity previously reported by Amard (1986) and shows purported xanthophyte algae contrary to a previous biomarker study suggesting the absence of eukaryotic algae, other than acritarchs, in the basin (Blumenberg et al., 2012). This new microfossil assemblage and others provide, all together, evidences of early and worldwide diversification of eukaryotes. Thereby, those first qualitative results also provide a basis for further and larger quantitative studies on the Taoudeni Basin. To better understand the palaeobiology (stem or crown group, aerobic or anaerobic metabolism) and

  6. Geochronology of La Tinta Upper Proterozoic sedimentary rocks, Argentina

    International Nuclear Information System (INIS)

    Cingolani, C.A.; Bonhomme, M.G.

    1982-01-01

    Olavarria-Sierras Bayas, Barker-San Manuel and Balcarce-Mar del Plata fine-grained sedimentary rocks from La Tinta Formation, the pre-Cenozoic cover of the Tandilia region, were studied using the Rb-Sr and K-Ar geochronology. The mineralogical study of the fine fraction has shown that only the Olavarria-Sierras Bayas area presents suitable material comprising typical sedimentary clays, affected only by diagenetic processes. Two Rb-Sr isochrons were obtained from Olavarria-Sierras Bayas rocks. They show: (1) an age of 769 +- 12 Ma with ( 87 Sr/ 86 Sr) 0 = 0.7121 +- 0.0005, for Aust Quarry rocks; and (2) an age of 723 +- 21 Ma with ( 87 Sr/ 86 Sr) 0 = 0.7171 +- 0.0012 for Cerro Negro and Losa Quarries rocks. Considering the above-mentioned isochron data and the mineralogy of the clays studied, the conclusion is drawn that the ages obtained reflect the isotopic setting of a late diagenetic process, dated back to nearly 720 Ma. K-Ar data also support the Rb-Sr isochrons and the late diagenetic clay origin. The lower section of La Tinta sequence in the Sierras Bayas area must then be considered as Upper Proterozoic in age. These new data support the recently reported stratigraphical divisions and ages. (Auth.)

  7. Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age

    Science.gov (United States)

    Johnston, D. T.; Wolfe-Simon, F.; Pearson, A.; Knoll, A. H.

    2009-01-01

    Molecular oxygen (O2) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580–550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic). Here we illustrate how contributions to primary production by anoxygenic photoautotrophs (including physiologically versatile cyanobacteria) influenced biogeochemical cycling during Earth's middle age, helping to perpetuate our planet's intermediate redox state by tempering O2 production. Specifically, the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained euxinia in the OMZ. On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated sulfide availability and water column oxygenation. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced oceanic redox and the Proterozoic O2 budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe2+ rich) subsurface waters set in motion Earth's transition from its prokaryote-dominated middle age, removing a physiological barrier to eukaryotic diversification (sulfide) and establishing, for the first time in Earth's history, complete dominance of oxygenic photosynthesis in the oceans. This paved the way for the further oxygenation of the oceans and atmosphere and, ultimately, the evolution of complex multicellular organisms. PMID:19805080

  8. Proterozoic strata-bound uranium deposits of Zambia and Zaire

    International Nuclear Information System (INIS)

    Meneghel, L.

    1984-01-01

    The Katanga System, host to uranium and copper mineralisation, is several thousands of metres thick and rests unconformably on an older complex of crystalline rocks and metasediments and is locally covered by Karoo sandstones or Kalahari sands. The deposition of the Katanga System took place during the Late Proterozoic in a wide complex basin extending from Shaba province in Zaire through a large part of Zambia and into eastern Angola. The sediments were affected by different grades of metamorphism, tectonic events, and by thermal events associated with post-tectonic metamorphism. At the base of Katanga system there are 84 known copper deposits and 42 uranium occurrences. It is suggested that all the known uranium and copper occurrences are of an essentially syngenetic sedimentary origin. The mineralisation is found in the Lower Roan Formation near the base of the Katanga System occurring in rocks produced in similar environmental conditions and thus being stratigraphic controlled, however, their areal distribution is localised producing a regional metal zonation. Many of the uranium occurrences have a typical vein aspect. These transgressive relationships are not inconsistent with a syngenetic origin as evidenced by the vein morphology. (author)

  9. Late Archaean-early Proterozoic source ages of zircons in rocks from the Paleozoic orogen of western Galicia, NW Spain

    Energy Technology Data Exchange (ETDEWEB)

    Kuijper, R P; Priem, H N.A. [Laboratorium voor Isotopen-Geologie, Amsterdam (Netherlands); Den Tex, E [Rijksuniversiteit Utrecht (Netherlands). Inst. voor Aardwetenschappen

    1982-08-01

    U-Pb data are reported for nine suites of zircons and three monazites from the Paleozoic orogen in western Galicia: one paragneiss and six orthogneisses from the early Paleozoic basement, and two Carboniferous (ca. 310 Ma old) intrusions of two-mica granite. New whole-rock Rb-Sr analyses, along with earlier data, indicate an age of ca. 470-440 Ma (Ordovician) for the emplacement of the granitic precursors of the orthogneisses. Monazite from the paragneiss also yields an U-Pb age of ca. 470 Ma. From the high initial /sup 87/Sr//sup 86/Sr ratios an involvement of Precambrian continental crust material is evident in the generation of the early Paleozoic suite of granites, while the zircon U-Pb data give evidence of the presence of about 3.0-2.0 Ga old (late Archaean-early Proterozoic) components in the source material. Zircons from the oldest sedimentary rocks in the area, now present as catazonal paragneisses and a likely source for the granites, likewise reveal a provenance age of 3.0-2.0 Ga.

  10. Late Proterozoic glacially controlled shelf sequences in western Mali (west Africa)

    Science.gov (United States)

    Deynoux, M.; Prousti, J. N.; Simon, B.

    The Late Proterozoic deposits of the Bakoye Group (500 m) in western Mali constitute a remarkable example of a glacially influenced sedimentary record on an epicratonic platform. They are composed of alternating marine and continental formations which represent accumulation in a basin located in the vicinity of upland areas covered by ice sheets. One of these formations (the Ba4 Formation), which is the focus of this study, is composed of three major units. The basal Unit 1 is made up of carbonaceous coarse to fine grained sandstones which are organized in fining upward sequences and which comprise lenticular diamictite intercalations. This Unit is considered to represent the fore slope gravity flows of a subaqueous ice-cootact fan fed by meltwater streams (≪glacioturbidites≫). Unit 2 is made up of coarse to fine grained sandstones in a highly variable association of facies. This Unit is characterized by the abundance of wave ripples associated with convolute beddings. planar or wavy beddings and tabular or hummocky crossbeddings in a general shallowing upward trend. It also comprises evidence of gravity processes including debris flows and large slumped sandstone bodies. Unit 2 represents the progressive filling of the Ba4 basin and reflects the combined effect of glacially induced eustatism and isostacy during a phase of glacial retreat. The basal part of Unit 3 is made up of a succession (a few meters thick) of conglomerates, diamictites, sandstones, siltstones or carbonates lying on an erosional unconformity marked by periglacial frost wedges. The upper part of Unit 3 is thicker (100-150 m) and onlaps on these basal facies with a succession of sandstone bars exhibiting swaley and hummocky crossbeddings, large cut and fill structures, and planar laminations. Unit 3 is strongly transgressive, the lower shoreface and backshore deposits include algal mats and are onlapped by sand ridges emplaced in a high energy upper to middle shoreface environment. Overall

  11. Holocene phosphorites of the western continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Nair, R.R.

    , francolite (carbonate flourapatite), quartz, feldspar and aragonite. P2 O5 concentration varies between 0.8% and 10.8% indicating a wide variation in chemical composition with the area. Age determined by the radiocarbon method is 11,040 years. It appears...

  12. Structure and tectonics of the southwestern continental margin of India

    Digital Repository Service at National Institute of Oceanography (India)

    Subrahmanyam, V.; Rao, D.G.; Ramana, M.V.; Krishna, K.S.; Murty, G.P.S.; Rao, M.G.

    , b and c) have been interpreted indepen- dently from gravity, magnetic and seismic reflection data (Subrahmanyam, 1992). The Prathap Ridge is reflected well on the seismic records. The ridge stands out as a discrete feature on the different... boundary of the ridge could not be mapped on the shelf due to inadequate data. The western boundary of mid-shelf basement ridge (basement high) towards the offshore is also evident as a discrete feature in Fig. 8c. In this paper we show, using gravity...

  13. Pathways of organic carbon oxidation in three continental margin sediments

    DEFF Research Database (Denmark)

    Canfield, Donald Eugene; Jørgensen, Bo Barker; Fossing, Henrik

    1993-01-01

    We have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of Denmark and Norway. Rates of NH4+ and Sigma CO2 liberation sediment incubations were used with O2 penetration depths to conclude...... that O2 respiration accounted for only between 3.6-17.4% of the total organic carbon oxidation. Dentrification was limited to a narrow zone just below the depth of O2 penetration, and was not a major carbon oxidation pathway. The processes of Fe reduction, Mn reduction and sulfate reduction dominated...... organic carbon mineralization, but their relative significance varied depending on the sediment. Where high concentrations of Mn-oxide were found (3-4 wt% Mn), only Mn reduction occurred. With lower Mn oxide concentrations more typical of coastal sediments, Fe reduction and sulfate reduction were most...

  14. Continental margin neotectonics in South Africa: a nuclear siting approach

    International Nuclear Information System (INIS)

    Andreoli, M.A.G.; Hambleton-Jones, B.B.; Anderson, N.J.B.; Faurie, J.N.; Raubenheimer, E.; Antoine, L.; Von Veh, M.W.; McMillan, I.K.

    1990-01-01

    Neotectonic activity in South Africa has only recently received attention despite the fact that a rigorous knowledge of this phenomenon could have a major impact on industrial development within certain areas. Since the early 1980s the Atomic Energy Corporation of South Africa has addressed this specific problem in the process of detailed site investigations for the licensing of six facilities related to the nuclear industry. The data indicates that there is a general correspondence in five selected areas along the South African coast between the development of Quarternary joints/faults and the degree of seismic activity recorded in each geological domain. Finally, the results provide new data that may add to a recent survey of global tectonic stresses. 13 refs., 1 fig

  15. A potential phosphorite deposit on the continental margin off Chennai

    Digital Repository Service at National Institute of Oceanography (India)

    Rao, V.P; Rao, K.M.; Vora, K.H.; Almeida, F.; Subramaniam, M.M.; Souza, C.G.A.

    into four types as (1) light brown friable phosphorites (2) grey dense phosphorites (3) phosphorite sands and (4) phosphatized algal nodules/limestones. Major element composition of phosphorites indicate that the P@d2@@ O@d5@@ content is highest (31...

  16. Seismic structure and tectonics of the continental margins of India

    Digital Repository Service at National Institute of Oceanography (India)

    Krishna, K.S.; Chaubey, A.K.; Rao, D.G.; Reddy, P.R.

    continent from East Antarctica and subsequent detachments of Madagascar, Seychelles and Elan Bank micro-continents from India are comprehensively discussed. Evolution of Ninetyeast, Laccadive and 85 degrees E ridges, and nature of the crust beneath the Laxmi...

  17. Evolution of passive continental margins and initiation of subduction zones

    NARCIS (Netherlands)

    Cloetingh, Sierd

    1982-01-01

    The initiation of subduction is a key element in plate tectonic schemes for the evolution of the Earth's lithosphere. Nevertheless, up to present, the underlying mechanism has not been very well understood (e.g. Dickinson and Seely, 1979; Hager, 1980; Kanamori, 1980). The insight into the initiation

  18. Evolution of passive continental margins and initiation of subduction zones

    NARCIS (Netherlands)

    Cloetingh, S.A.P.L.

    1982-01-01

    The initiation of subduction is a key element in plate tectonic schemes for the evolution of the Earth's lithosphere. Nevertheless, up to present, the underlying mechanism has not been very well understood (e.g. Dickinson and Seely, 1979; Hager, 1980; Kanamori, 1980). The insight into the

  19. Architectural elements from Lower Proterozoic braid-delta and high-energy tidal flat deposits in the Magaliesberg Formation, Transvaal Supergroup, South Africa

    Science.gov (United States)

    Eriksson, Patrick G.; Reczko, Boris F. F.; Jaco Boshoff, A.; Schreiber, Ute M.; Van der Neut, Markus; Snyman, Carel P.

    1995-06-01

    Three architectural elements are identified in the Lower Proterozoic Magaliesberg Formation (Pretoria Group, Transvaal Supergroup) of the Kaapvaal craton, South Africa: (1) medium- to coarse-grained sandstone sheets; (2) fine- to medium-grained sandstone sheets; and (3) mudrock elements. Both sandstone sheet elements are characterised by horizontal lamination and planar cross-bedding, with lesser trough cross-bedding, channel-fills and wave ripples, as well as minor desiccated mudrock partings, double-crested and flat-topped ripples. Due to the local unimodal palaeocurrent patterns in the medium- to coarse-grained sandstone sheets, they are interpreted as ephemeral braid-delta deposits, which were subjected to minor marine reworking. The predominantly bimodal to polymodal palaeocurrent trends in the fine- to medium-grained sandstone sheets are inferred to reflect high-energy macrotidal processes and more complete reworking of braid-delta sands. The suspension deposits of mudrocks point to either braid-delta channel abandonment, or uppermost tidal flat sedimentation. The depositional model comprises ephemeral braid-delta systems which debouched into a high-energy peritidal environment, around the margins of a shallow epeiric sea on the Kaapvaal craton. Braid-delta and tidal channel dynamics are inferred to have been similar. Fine material in the Magaliesberg Formation peritidal complexes indicates that extensive aeolian removal of clay does not seem applicable to this example of the early Proterozoic.

  20. Basement and climate controls on proximal depositional systems in continental settings

    NARCIS (Netherlands)

    Ventra, D.

    2011-01-01

    This doctoral dissertation discusses the sedimentology and dynamics of selected, modern and ancient clastic depositional systems (alluvial fans and colluvial aprons) at continental basin margins. The focus on single depositional systems gave the opportunity to devote particular attention to

  1. Examples from the Rewa Group of Proterozoic Vindhyan bas

    Indian Academy of Sciences (India)

    The Rewa Group of the Vindhyan Supergroup in the Son valley begins with a thick (∼200 m) dominantly shaly, shelfal succession, occurring between the Dhandraul Formation of the Kaimur. Group (fluvial sandstone) below and Drammondganj Formation of the Rewa Group (marginal marine sandstone) above. Such a ...

  2. Proterozoic orogenic belts and rifting of Indian cratons: Geophysical constraints

    Directory of Open Access Journals (Sweden)

    D.C. Mishra

    2014-01-01

    Full Text Available The Aravalli–Delhi and Satpura Mobile Belts (ADMB and SMB and the Eastern Ghat Mobile Belt (EGMB in India form major Proterozoic mobile belts with adjoining cratons and contemporary basins. The most convincing features of the ADMB and the SMB have been the crustal layers dipping from both sides in opposite directions, crustal thickening (∼45 km and high density and high conductivity rocks in upper/lower crust associated with faults/thrusts. These observations indicate convergence while domal type reflectors in the lower crust suggest an extensional rifting phase. In case of the SMB, even the remnant of the subducting slab characterized by high conductive and low density slab in lithospheric mantle up to ∼120 km across the Purna–Godavari river faults has been traced which may be caused by fluids due to metamorphism. Subduction related intrusives of the SMB south of it and the ADMB west of it suggest N–S and E–W directed convergence and subduction during Meso–Neoproterozoic convergence. The simultaneous E–W convergence between the Bundelkhand craton and Marwar craton (Western Rajasthan across the ADMB and the N–S convergence between the Bundelkhand craton and the Bhandara and Dharwar cratons across the SMB suggest that the forces of convergence might have been in a NE–SW direction with E–W and N–S components in the two cases, respectively. This explains the arcuate shaped collision zone of the ADMB and the SMB which are connected in their western part. The Eastern Ghat Mobile Belt (EGMB also shows signatures of E–W directed Meso–Neoproterozoic convergence with East Antarctica similar to ADMB in north India. Foreland basins such as Vindhyan (ADMB–SMB, and Kurnool (EGMB Supergroups of rocks were formed during this convergence. Older rocks such as Aravalli (ADMB, Mahakoshal–Bijawar (SMB, and Cuddapah (EGMB Supergroups of rocks with several basic/ultrabasic intrusives along these mobile belts, plausibly formed during

  3. Post-Panafrican late Proterozoic basins in the Central Anti-Atlas (Morocco): their influence on the Variscan contractional structures.

    Science.gov (United States)

    Guimerà, Joan; Arboleya, María. Luisa

    2010-05-01

    Located South of the High Atlas, in Morocco, The Anti-Atlas is a 700 km-long chain trending NE-SW. In the Central Anti-Atlas region, between Warzazat and Taznakht, the Proterozoic Pan-African basement (X1 to X2-3) crops out in isolated areas (boutonnières), where it is overlaid by late to post Pan-African Upper Proterozoic and Palaeozoic rocks. Late to post Pan-African Upper Proterozoic rocks (X3) have been classically divided into three units (X3i, X3m and X3s) which include volcanic rocks — mainly rhyolites— and continental siliciclastic rocks, the older units intruded by late granites (Choubert, 1952 and Choubert et al., 1970). Rocks belonging to the upper unit of post Pan-African Upper Proterozoic rocks (X3s) were deposited in basins bounded by faults with a dominant dip-slip normal motion; as a result, this unit have a variable thickness, being locally absent in the uplifted blocks. Uppermost Proterozoic (Adoudounian) and Palaeozoic rocks deposited unconformable on the older rocks in the Anti-Atlas. The Central Anti-Atlas was slightly deformed during the Variscan orogeny by folds and high-angle thrusts. Two areas are selected to study the post Pan-African to Variscan evolution of the area: the Tiwiyyine basin and the Anti-Atlas Major Fault. Tiwiyyine basin This basin is delimited by kilometric-scale normal faults. Three of them can be observed in the field: two striking NE-SW (NW and SE boundaries) and one striking NW-SE (SW boundary), while the NE boundary is covered by Cenozoic rocks. The basin fill reaches 725 m and has been divided into three units: 1. X3s1: Coarse conglomerates with basal breccias. 2. X3s2: Laminated dolomites at the base, red pelites and conglomerates. 3. X3s3: Conglomerates with interbedded andesites. Unit X3s2 passes laterally to the SW to unit X3s1. The thickness of the basin fill diminishes to the SE. This is specially visible at the basal X3s1 unit. At both sides of the two NE-SW-striking faults, only the upper X3s3 unit is

  4. Regional magnetic anomaly constraints on continental rifting

    Science.gov (United States)

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

    1985-01-01

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

  5. Root zone of a continental rift

    DEFF Research Database (Denmark)

    Kirsch, Moritz; Svenningsen, Olaf

    2016-01-01

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

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

    DEFF Research Database (Denmark)

    Artemieva, Irina; Shulgin, Alexey

    2015-01-01

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

  7. Proterozoic rifting and major unconformities in Rajasthan, and their implications for uranium mineralisation

    International Nuclear Information System (INIS)

    Sinha-Roy, S.

    2004-01-01

    Evolution of the Precambrian terrain in Rajasthan has taken place via crustal consolidation of the basement at ca. 2.9 Ga, its cratonisation at ca. 2.5 Ga, through protracted tectonostratigraphic evolution of the Proterozoic cover sequences, following repeated rifting and Wilson cycles in the Aravalli and Delhi foldbelts. Consequently, the Proterozoic rift basins are characterised by growth faults and pull-aparts, and multitier volcanose dimentary sequences that contain a number of unconformities and stratigraphic breaks. The Archaean basement of the Mewar terrain that witnessed end-Archaean K-magmatism and ductile shearing, led to the creation of a possible uranium province, namely uranium enriched basement. This province acted as the source of remobilised uranium and its concentration at suitable multilevel structural and stratigraphic traps within the Proterozoic rift basins to give rise to unconformity-related syngenetic uranium mineralisation. Late Neoproterozoic to Pan-African tectonothermal reworking of the basement rocks produced fracture zones and caused Na-metasomatism giving rise to albitite-related uranium mineralisation. Based on an analysis of Proterozoic rift kinematics and lithofacies characteristics, five possible uranium-enriched stratigraphic horizons have been identified in the Aravalli and its equivalent sequences as well as in the North Delhi foldbelt sequences. From a regional synthesis, ten possible uranium metallogenic events, spanning ca. 2.5-0.5 Ga, are recognised in Rajasthan. These uranium events have predictive value for delineation of target areas for exploration. (author)

  8. Revised stratigraphic nomenclature and correlation of early proterozoic rocks of the Darwin - Katherine region, Northern Territory

    International Nuclear Information System (INIS)

    Needham, R.S.; Stuart-Smith, P.G.

    1984-01-01

    New stratigraphic names and correlations are given for parts of the Early Proterozoic Pine Creek Geosyncline metasedimentary sequence and overlying felsic volcanics of the Darwin-Katherine region. They have significant implications for the stratigraphic distribution of uranium mineralisation in the Rum Jungle, Alligator Rivers and South Alligator Valley uranium fields

  9. Geochemistry and petrology of mafic Proterozoic and Permian dykes on Bornholm, Denmark:

    DEFF Research Database (Denmark)

    Holm, Paul Martin; Pedersen, Lise E.; Højsteeen, Birte

    2010-01-01

    More than 250 dykes cut the mid Proterozoic basement gneisses and granites of Bornholm. Most trend between NNW and NNE, whereas a few trend NE and NW. Field, geochemical and petrological evidence suggest that the dyke intrusions occurred as four distinct events at around 1326 Ma (Kelseaa dyke...

  10. Continental Mathematics League.

    Science.gov (United States)

    Quartararo, Joseph

    2002-01-01

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

  11. Outer Continental Shelf Lands Act

    Data.gov (United States)

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

  12. Evolution of the Southern Margin of the Donbas (Ukraine) from Devonian to early Carboniferous Times.

    NARCIS (Netherlands)

    McCann, T.; Saintot, A.N.

    2003-01-01

    A Devonian-Early Carboniferous succession comprising thick clastic and carbonate sediments with interbedded volcanics was examined along the southern margin of the Donbas fold belt. Ukraine. Following initial rifting and subsidence, a continental (fluvial, lacustrine) succession was established.

  13. The Myanmar continental shelf

    Digital Repository Service at National Institute of Oceanography (India)

    Ramaswamy, V.; Rao, P.S.

    reveal a minimum of 18 m thick strata of modern muds (Fig. 2g). At the outer boundary of the Gulf of Myanmar Continental Shelf 8 Martaban (15oN Latitude), brown muds overlie coarse sands indicating that modern deltaic sediments... on the Myeik Bank (Rodolfo, 1969a). Modern sediments on the Ayeyarwady shelf General composition, Texture and Grain-size: The distribution and sediment texture on the Ayeyarwady shelf shows fine-grained sediments comprising silty-clay and clayey...

  14. Eclogite-facies metamorphism in impure marble from north Qaidam orogenic belt: Geodynamic implications for early Paleozoic continental-arc collision

    Science.gov (United States)

    Chen, Xin; Xu, Rongke; Schertl, Hans-Peter; Zheng, Youye

    2018-06-01

    In the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, impure marble and interbedded eclogite represent a particular sedimentary provenance and tectonic setting, which have important implications for a controversial problem - the dynamic evolution of early Paleozoic subduction-collision complexes. In this contribution, detailed field work, mineral chemistry, and whole-rock geochemistry are presented for impure marble to provide the first direct evidence for the recycling of carbonate sediments under ultrahigh-pressures during subduction and collision in the Yuka terrane, in the North Qaidam UHP metamorphic belt. According to conventional geothermobarometry, pre-peak subduction to 0.8-1.3 GPa/485-569 °C was followed by peak UHP metamorphism at 2.5-3.3 GPa/567-754 °C and cooling to amphibolite facies conditions at 0.6-0.7 GPa/571-589 °C. U-Pb dating of zircons from impure marble reveals a large group with ages ranging from 441 to 458 Ma (peak at 450 Ma), a smaller group ranging from 770 to 1000 Ma (peak at 780 Ma), and minor >1.8 Ga zircon aged ca. 430 Ma UHP metamorphism. The youngest detrital zircons suggest a maximum depositional age of ca. 442 Ma and a burial rate of ca. 1.0-1.1 cm/yr when combined with P-T conditions and UHP metamorphic age. The REE and trace element patterns of impure marble with positive Sr and U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti), and Ce anomalies imply that the marble had a marine limestone precursor. Impure marble intercalated with micaschist and eclogite was similar to limestone and siltstone protoliths deposited in continental fore-arc or arc setting with basic volcanic activity. Therefore, the Yuka terrane most likely evolved in a continental island arc setting during the Paleozoic. These data suggest that metasediments were derived from a mixture of Proterozoic continental crust and juvenile early Paleozoic oceanic and/or island arc crust. In addition, their protoliths were likely

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

    Energy Technology Data Exchange (ETDEWEB)

    Van Wijk, Jolante W [Los Alamos National Laboratory

    2008-01-01

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

  16. Magmatism and deformation during continental breakup

    Science.gov (United States)

    Keir, Derek

    2013-04-01

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

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

    Science.gov (United States)

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

    2015-10-07

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

  18. Earth's evolving subcontinental lithospheric mantle: inferences from LIP continental flood basalt geochemistry

    Science.gov (United States)

    Greenough, John D.; McDivitt, Jordan A.

    2018-04-01

    Archean and Proterozoic subcontinental lithospheric mantle (SLM) is compared using 83 similarly incompatible element ratios (SIER; minimally affected by % melting or differentiation, e.g., Rb/Ba, Nb/Pb, Ti/Y) for >3700 basalts from ten continental flood basalt (CFB) provinces representing nine large igneous provinces (LIPs). Nine transition metals (TM; Fe, Mn, Sc, V, Cr, Co, Ni, Cu, Zn) in 102 primitive basalts (Mg# = 0.69-0.72) from nine provinces yield additional SLM information. An iterative evaluation of SIER values indicates that, regardless of age, CFB transecting Archean lithosphere are enriched in Rb, K, Pb, Th and heavy REE(?); whereas P, Ti, Nb, Ta and light REE(?) are higher in Proterozoic-and-younger SLM sources. This suggests efficient transfer of alkali metals and Pb to the continental lithosphere perhaps in association with melting of subducted ocean floor to form Archean tonalite-trondhjemite-granodiorite terranes. Titanium, Nb and Ta were not efficiently transferred, perhaps due to the stabilization of oxide phases (e.g., rutile or ilmenite) in down-going Archean slabs. CFB transecting Archean lithosphere have EM1-like SIER that are more extreme than seen in oceanic island basalts (OIB) suggesting an Archean SLM origin for OIB-enriched mantle 1 (EM1). In contrast, OIB high U/Pb (HIMU) sources have more extreme SIER than seen in CFB provinces. HIMU may represent subduction-processed ocean floor recycled directly to the convecting mantle, but to avoid convective homogenization and produce its unique Pb isotopic signature may require long-term isolation and incubation in SLM. Based on all TM, CFB transecting Proterozoic lithosphere are distinct from those cutting Archean lithosphere. There is a tendency for lower Sc, Cr, Ni and Cu, and higher Zn, in the sources for Archean-cutting CFB and EM1 OIB, than Proterozoic-cutting CFB and HIMU OIB. All CFB have SiO2 (pressure proxy)-Nb/Y (% melting proxy) relationships supporting low pressure, high % melting

  19. Mantle Earthquakes in Thinned Proterozoic Lithosphere: Harrat Lunayyir, Saudi Arabia

    Science.gov (United States)

    Blanchette, A. R.; Klemperer, S. L.; Mooney, W. D.; Zahran, H. M.

    2017-12-01

    Harrat Lunayyir is an active volcanic field located in the western Arabian Shield 100 km outside of the Red Sea rift margin. We use common conversion point (CCP) stacking of P-wave receiver functions (PRFs) to show that the Moho is at 38 km depth, close to the 40 km crustal thickness measured in the center of the craton, whereas the lithosphere-asthenosphere boundary (LAB) is at 60 km, far shallower than the 150 km furthest in the craton. We locate 67 high-frequency earthquakes with mL ≤ 2.5 at depths of 40-50 km below the surface, located clearly within the mantle lid. The occurrence of earthquakes within the lithospheric mantle requires a geothermal temperature profile that is below equilibrium. The lithosphere cannot have thinned to its present thickness earlier than 15 Ma, either during an extended period of rifting possibly beginning 24 Ma or, more likely, as part of the second stage of rifting following collision between Arabia and Eurasia.

  20. To reactivate or not to reactivate: nature and varied behavior of structural inheritance in the Proterozoic basement of the Eastern Colorado mineral belt over 1.7 billion years of earth history

    Science.gov (United States)

    Caine, Jonathan S.; Ridley, John; Wessel, Zachary R.

    2010-01-01

    The eastern central Front Range of the Rocky Mountains in Colorado has long been a region of geologic interest because of Laramide-age hydrothermal polymetallic vein-related ores. The region is characterized by a well-exposed array of geologic structures associated with ductile and brittle deformation, which record crustal strain over 1.7 billion years of continental growth and evolution. The mineralized areas lie along a broad linear zone termed the Colorado Mineral Belt. This lineament has commonly been interpreted as following a fundamental boundary, such as a suture zone, in the North American Proterozoic crust that acted as a persistent zone of weakness localizing the emplacement of magmas and associated hydrothermal fluid flow. However, the details on the controls of the location, orientation, kinematics, density, permeability, and relative strength of various geological structures and their specific relationships to mineral deposit formation are not related to Proterozoic ancestry in a simple manner. The objectives of this field trip are to show key localities typical of the various types of structures present, show recently compiled and new data, offer alternative conceptual models, and foster dialogue. Topics to be discussed include: (1) structural history of the eastern Front Range; (2) characteristics, kinematics, orientations, and age of ductile and brittle structures and how they may or may not relate to one another and mineral deposit permeability; and (3) characteristics, localization, and evolution of the metal and non–metal-bearing hydrothermal systems in the eastern Colorado Mineral Belt.

  1. Understanding and modelling Neo-proterozoic glaciations and their associated phenomena

    International Nuclear Information System (INIS)

    Le Hir, Guillaume

    2007-01-01

    The objective of this research thesis is to provide a consistent image of extreme glaciations which occurred during the Neo-proterozoic era. By using climate and carbon cycle models (or model of bio-geochemical cycles), the author aims at answering various scientific questions raised by the Snowball Earth hypothesis. After a description of the main geological features which characterize the Proterozoic, scientific problems are presented. The author then reports the study of carbon cycle during glaciation in order to understand its operation. Based on this constraint, a consistent scenario of exit from glaciation is defined. The physical-chemical evolution of the ocean during and after a global glaciation is then quantified in order to assess its potential effects on the environment and on the Precambrian biosphere. The last part focuses on the post-glacial evolution to establish the delay for a return to equilibrium of climate after such an extreme event [fr

  2. Middle proterozoic supra crustal and brazilian orogeny in the southeast Ceara state: a mono cyclic evolution

    International Nuclear Information System (INIS)

    Sa, J.M.; Bezerra, F.H.R.; Freitas Macedo, M.H. de; Pereira, R.

    1988-01-01

    The Oros belt is situated in the southeastern part of Ceara state, Brazil, and geologically pertains to the Borborema province. This belt comprises a volcano-sedimentary sequence of middle proterozoic age resting unconformably upon basement of Archean/low proterozoic age. In the geological map of Ceara state, this belt displays an elongate shape towards N-S, turning to ENE-WSW in the south, and reaches 12 km wide in the central park. This paper describes the relationships between the country rocks and the supra crustal sequence, as well as the plutonic intrusions and their tectonic metamorphic evolution. New Rb-Sr whole-rock dates are presented which are very important to separate anorogenic and syn-orogenic granites. (author)

  3. Proterozoic kimberlites and lamproites and a preliminary age for the Argyle lamproite pipe, Western Australia

    International Nuclear Information System (INIS)

    Skinner, E.M.W.; Bristow, J.W.; Smith, C.B.; Scott Smith, B.H.; Dawson, J.B.

    1985-01-01

    The Argyle pipe occurring in the East Kimberley Province of Western Australia is a unique, highly-diamondiferous lamproite. Although it resembles other lamproites located in the West Kimberley Province with respect to its setting, structure, petrography and geochemistry, it is probably Proterozoic in age and hence substantially older than Tertiary occurrences of the West Kimberley Province. Rb-Sr measurements on whole rock and phlogopite samples from magmatic olivine-phlogopite lamproite, reveals a two point model age of 1126 +- 9 Ma for the Argyle pipe. This age is consistent with ages of other, similar volcanic igneous rocks occurring in several localities worldwide. The widespread occurrence of Proterozoic kimberlites and lamproites suggests that this was an important period of worldwide alkalic intrusive activity

  4. Evidences of a tangential proterozoic tectonic from Quadrilatero Ferrifero, Minas Gerais state, Brazil

    International Nuclear Information System (INIS)

    Belo de Oliveira, O.A.; Teixeira, W.

    1990-01-01

    Radiometric Rb/Sr ages of 2,1 - 2,2 Ga determined for milonites of the Caete complex, combined with tectonic relationships among the sequences of the Espinhaco, Minas and Rio das Velhas Supergroups, suggest that the thrust and fold tectonic style observed around Caete results from two deformation episodes, with similar vergence and style. The parautochthonous domain in Caete Region has been affected by both deformations episodes (Early Proterozoic and Upper Proterozoic) whereas the allochthonous domain apparently was affected only by the younger episode. A preliminary analysis of the Quadrilatero Ferrifero as a whole, considering these two major deformation episodes, is compatible, at least in part, with the large scale features observed in maps. In an effort to understand the tectonic framework of Q.F. an speculation is made on av evolutive model, considering also the existence of two district extensional events (Late Archean and Middle Proterozoic), respectively related to the deposition of Minas and Espinhaco Supergroups in a rift/aulacogen systems. (author)

  5. Lesser Himalayan sequences in Eastern Himalaya and their deformation: Implications for Paleoproterozoic tectonic activity along the northern margin of India

    Directory of Open Access Journals (Sweden)

    Dilip Saha

    2013-05-01

    Full Text Available Substantial part of the northern margin of Indian plate is subducted beneath the Eurasian plate during the Caenozoic Himalayan orogeny, obscuring older tectonic events in the Lesser Himalaya known to host Proterozoic sedimentary successions and granitic bodies. Tectonostratigraphic units of the Proterozoic Lesser Himalayan sequence (LHS of Eastern Himalaya, namely the Daling Group in Sikkim and the Bomdila Group in Arunachal Pradesh, provide clues to the nature and extent of Proterozoic passive margin sedimentation, their involvement in pre-Himalayan orogeny and implications for supercontinent reconstruction. The Daling Group, consisting of flaggy quartzite, meta-greywacke and metapelite with minor mafic dyke and sill, and the overlying Buxa Formation with stromatolitic carbonate-quartzite-slate, represent shallow marine, passive margin platformal association. Similar lithostratigraphy and broad depositional framework, and available geochronological data from intrusive granites in Eastern Himalaya indicate strikewise continuity of a shallow marine Paleoproterozoic platformal sequence up to Arunachal Pradesh through Bhutan. Multiple fold sets and tectonic foliations in LHS formed during partial or complete closure of the sea/ocean along the northern margin of Paleoproterozoic India. Such deformation fabrics are absent in the upper Palaeozoic–Mesozoic Gondwana formations in the Lesser Himalaya of Darjeeling-Sikkim indicating influence of older orogeny. Kinematic analysis based on microstructure, and garnet composition suggest Paleoproterozoic deformation and metamorphism of LHS to be distinct from those associated with the foreland propagating thrust systems of the Caenozoic Himalayan collisional belt. Two possibilities are argued here: (1 the low greenschist facies domain in the LHS enveloped the amphibolite to granulite facies domains, which were later tectonically severed; (2 the older deformation and metamorphism relate to a Pacific type

  6. Convexity and Marginal Vectors

    NARCIS (Netherlands)

    van Velzen, S.; Hamers, H.J.M.; Norde, H.W.

    2002-01-01

    In this paper we construct sets of marginal vectors of a TU game with the property that if the marginal vectors from these sets are core elements, then the game is convex.This approach leads to new upperbounds on the number of marginal vectors needed to characterize convexity.An other result is that

  7. "We call ourselves marginalized"

    DEFF Research Database (Denmark)

    Jørgensen, Nanna Jordt

    2014-01-01

    of the people we refer to as marginalized. In this paper, I discuss how young secondary school graduates from a pastoralist community in Kenya use and negotiate indigeneity, marginal identity, and experiences of marginalization in social navigations aimed at broadening their current and future opportunities. I...

  8. Clay sediment accumulation rates on the monsoon-dominated western continental shelf and slope region of India

    Digital Repository Service at National Institute of Oceanography (India)

    Borole, D.V.

    Clay accumulation rates shown in sediment cores from the nearshore to outer continental shelf and slope regions in water depths of 10-1246 m on the western continental margins of India were determined by the 210Pb dating technique. The 210Pb excess...

  9. The continental waters pollution

    International Nuclear Information System (INIS)

    Marsily, G. de

    1996-01-01

    This work deals with the continental water pollution. The sewage affect considerably the quality of some rivers water and of some basins. Moreover, a slow and general damage of natural waters has been established. The direct effects on men and on the natural medium (climatic change, aquatic ecosystems, water cycle) are given as well as the protection means (waste processing, the water-bearing bed and underground water protection, the aquatic ecosystems protection and planning) used and future to abate the water pollution. (O.L.). 17 refs., 6 tabs

  10. U-Pb ages and geochemistry of zircon from Proterozoic plutons of the Sawatch and Mosquito ranges, Colorado, U.S.A.: Implications for crustal growth of the central Colorado province

    Science.gov (United States)

    Moscati, Richard J.; Premo, Wayne R.; Dewitt, Ed; Wooden, Joseph L.

    2017-01-01

    southern portion of the Sawatch and Mosquito ranges.(2) Calc-alkalic to alkali-calcic magmas intruded this region approximately 55 m.y. after the Roosevelt Granite with emplacement of pre-deformational plutons at ca. 1,710 Ma (e.g., Henry Mountain Granite and diorite of Denny Creek), and this continued for at least 30 m.y., ending with emplacement of post-deformational plutons at ca. 1,680 Ma (e.g., Kroenke Granodiorite, granite of Fairview Peak, and syenite of Mount Yale). The timing of deformation can be constrained to sometime after intrusion of the diorite of Denny Creek and likely before the emplacement of the undeformed granite of Fairview Peak. Geochemistry of both whole-rock and zircon indicates that the older group of ca. 1,710-Ma plutons formed at shallower depths, and then they intruded the younger group of more deeply generated, commonly peraluminous and sodic plutons. Although absent in the Sawatch and Mosquito ranges, Mazatzal-age (ca. 1,680–1,620 Ma) plutonic rocks are present regionally. Inherited zircon components of Mazatzal-age were found as cores in some 1.4-Ga Sawatch and Mosquito Range zircons, indicating the likelihood of a relatively local source. These combined data suggest the possibility that all were produced within a continental-margin magmatic arc created as a result of southward-migrating (slab rollback?), north-dipping subduction to the south of the region.(3) Widespread Mesoproterozoic plutonism—with emplacement at various depths and exhibiting bimodal geochemistry—is recognized in 16 different samples. An older group of predominantly peraluminous, yet magnesian granitoids (e.g., granodiorite of Sayers, granite of Taylor River, and the St. Kevin Granite) were emplaced between ca. 1,450 and 1,425 Ma. These geochemical parameters suggest moderate degrees of partial melting in a low-pressure environment. Three younger metaluminous, but ferroan plutons (diorite of Grottos, diorite of Mount Elbert, and granodiorite of Mount Harvard

  11. Controls on Atmospheric O2: The Anoxic Archean and the Suboxic Proterozoic

    Science.gov (United States)

    Kasting, J. F.

    2015-12-01

    Geochemists have now reached consensus that the Archean atmosphere was mostly anoxic, that a Great Oxidation Event (GOE) occurred at around 2.5 Ga, and that the ensuing Proterozoic atmosphere was consistently oxidized [1,2]. Evidence for this broad-scale change in atmospheric composition comes from a variety of sources, most importantly from multiple sulfur isotopes [3,4]. The details of both the Archean and Proterozoic environments remain controversial, however, as does the underlying cause of the GOE. Evidence of 'whiffs' of oxygen during the Archean [5] now extend back as far as 3.0 Ga, based on Cr isotopes [6]. This suggests that O2 was being produced by cyanobacteria well before the GOE and that the timing of this event may have been determined by secular changes in O2 sinks. Catling et al. [7] emphasized escape of hydrogen to space, coupled with progressive oxidation of the continents and a concomitant decrease in the flux of reduced gases from metamorphism. But hydrogen produced by serpentinization of seafloor could also have been a controlling factor [8]. Higher mantle temperatures during the Archean should have resulted in thicker, more mafic seafloor and higher H2 production; decreasing mantle temperatures during the Proterozoic should have led to seafloor more like that of today and a corresponding decrease in H2 production, perhaps by enough to trigger the GOE. Once the atmosphere became generally oxidizing, it apparently remained that way during the rest of Earth's history. But O2 levels in the mid-Proterozoic could have been as low at 10-3 times the Present Atmospheric Level (PAL) [9]. The evidence, once again, is based on Cr isotopes. Possible mechanisms for maintaining such a 'suboxic' Proterozoic atmosphere will be discussed. Refs: 1. H. D. Holland, Geochim. Cosmochim. Acta 66, 3811 (2002). 2. H. D. Holland, Philosophical Transactions of the Royal Society B-Biological Sciences 361, 903 (Jun 29, 2006). 3. J. Farquhar, H. Bao, M. Thiemans, Science

  12. Oceanic-type accretion may begin before complete continental break-up

    Science.gov (United States)

    Geoffroy, L.; Zalan, P. V.; Viana, A. R.

    2011-12-01

    Oceanic accretion is thought to be the process of oceanic crust (and lithosphere) edification through adiabatic melting of shallow convecting mantle at oceanic spreading ridges. It is usually considered as a post-breakup diagnostic process following continents rupturing. However, this is not always correct. The structure of volcanic passive margins (representing more than 50% of passive continental margins) outlines that the continental lithosphere is progressively changed into oceanic-type lithosphere during the stage of continental extension. This is clear at least, at crustal level. The continental crust is 'changed' from the earliest stages of extension into a typical -however thicker- oceanic crust with the typical oceanic magmatic layers (from top to bottom: lava flows/tuffs, sheeted dyke complexes, dominantly (sill-like) mafic intrusions in the lower crust). The Q-rich continental crust is highly extended and increases in volume (due to the magma) during the extensional process. At the continent-ocean transition there is, finally, no seismic difference between this highly transformed continental crust and the oceanic crust. Using a large range of data (including deep seismic reflection profiles), we discuss the mantle mechanisms that governs the process of mantle-assisted continental extension. We outline the large similarity between those mantle processes and those acting at purely-oceanic spreading axis and discuss the effects of the inherited continental lithosphere in the pattern of new mafic crust edification.

  13. Do Continental Shelves Act as an Atmospheric CO2 Sink?

    Science.gov (United States)

    Cai, W.

    2003-12-01

    Recent air-to-sea CO2 flux measurements at several major continental shelves (European Atlantic Shelves, East China Sea and U.S. Middle Atlantic Bight) suggest that shelves may act as a one-way pump and absorb atmospheric CO2 into the ocean. These observations also favor the argument that continental shelves are autotrophic (i.e., net production of organic carbon, OC). The U.S. South Atlantic Bight (SAB) contrasts these findings in that it acts as a strong source of CO2 to the atmosphere while simultaneously exporting dissolved inorganic carbon (DIC) to the open ocean. We report pCO2, DIC, and alkalinity data from the SAB collected in 8 cruises along a transect from the shore to the shelf break in the central SAB. The shelf-wide net heterotrophy and carbon exports in the SAB are subsidized by the export of OC from the abundant intertidal marshes, which are a sink for atmospheric CO2. It is proposed here that the SAB represents a marsh-dominated heterotrophic ocean margin as opposed to river-dominated autotrophic margins. To further investigate why margins may behave differently in term of CO2 sink/source, the physical and biological conditions of several western boundary current margins are compared. Based on this and other studies, DIC export flux from margins to the open ocean must be significant in the overall global ocean carbon budget.

  14. Tectonics and sedimentary process in the continental talud in Uruguay

    International Nuclear Information System (INIS)

    De Santa Ana, H.; Soto, M.; Morales, E.; Tomasini, J.; Hernandez-Molina, F.; Veroslavsky, G.

    2012-01-01

    The morphology and evolution of the continental margin of Uruguay is due to the interaction of an important set of sedimentary processes. The contourite and turbiditic are the most significant processes which are associated with the development of submarine canyons as well as the gravitational mass respect to major landslides. These processes generate erosional and depositional features with a direct impact on different areas of application, which have potential environmental risks (gravitational landslides, earthquakes, tsunamis) and potential economic resources

  15. Influence of the Nazaré Canyon, central Portuguese margin, on late winter coccolithophore assemblages

    NARCIS (Netherlands)

    Guerreiro, C.; Sá, C.; de Stigter, H.; Oliveira, A.; Cachão, M.; Cros, L.; Borges, C.; Quaresma, L.; Santos, A.I.; Fortuño, J.-M.; Rodrigez, A.

    2014-01-01

    This paper presents a first attempt to characterize coccolithophore assemblages occurring in the context of an active submarine canyon. Coccolithophores from the upper-middle sections of the Nazaré Canyon (central Portuguese margin) – one of the largest canyons of the European continental margin

  16. Structure of the Gabon Margin from integrated seismic reflection and gravity data

    NARCIS (Netherlands)

    Dupre, S.; Cloetingh, S.A.P.L.; Bertotti, G.V.

    2011-01-01

    In the South Gabon Basin, deep multi-channel seismic reflection and gravity modeling analysis have shed light on key features of the structure of the margin. The thinned continental crust beneath the Gabon Margin appears to be composed of two distinct layers, separated by a clear, strong and more or

  17. Deep Structures of The Angola Margin

    Science.gov (United States)

    Moulin, M.; Contrucci, I.; Olivet, J.-L.; Aslanian, D.; Géli, L.; Sibuet, J.-C.

    1 Ifremer Centre de Brest, DRO/Géosciences Marines, B.P. 70, 29280 Plouzané cedex (France) mmoulin@ifremer.fr/Fax : 33 2 98 22 45 49 2 Université de Bretagne Occidentale, Institut Universitaire Europeen de la Mer, Place Nicolas Copernic, 29280 Plouzane (France) 3 Total Fina Elf, DGEP/GSR/PN -GEOLOGIE, 2,place de la Coupole-La Defense 6, 92078 Paris la Defense Cedex Deep reflection and refraction seismic data were collected in April 2000 on the West African margin, offshore Angola, within the framework of the Zaiango Joint Project, conducted by Ifremer and Total Fina Elf Production. Vertical multichannel reflection seismic data generated by a « single-bubble » air gun array array (Avedik et al., 1993) were recorded on a 4.5 km long, digital streamer, while refraction and wide angle reflection seismic data were acquired on OBSs (Ocean Bottom Seismometers). Despite the complexity of the margin (5 s TWT of sediment, salt tectonics), the combination of seismic reflection and refraction methods results in an image and a velocity model of the ground structures below the Aptian salt layer. Three large seismic units appear in the reflection seismic section from the deep part on the margin under the base of salt. The upper seismic unit is layered with reflectors parallel to the base of the salt ; it represents unstructured sediments, filling a basin. The middle unit is seismically transparent. The lower unit is characterized by highly energetic reflectors. According to the OBS refraction data, these two units correspond to the continental crust and the base of the high energetic unit corresponds to the Moho. The margin appears to be divided in 3 domains, from east to west : i) a domain with an unthinned, 30 km thick, continental crust ; ii) a domain located between the hinge line and the foot of the continental slope, where the crust thins sharply, from 30 km to less than 7 km, this domain is underlain by an anormal layer with velocities comprising between 7,2 and 7

  18. The continental lithosphere

    DEFF Research Database (Denmark)

    Artemieva, Irina

    2009-01-01

    The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle...... are consistent with modern geophysical data. In the lithospheric mantle of the continents, seismic velocity variations of a non-thermal origin (calculated from global Vs seismic tomography data [Grand S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philosophical Transactions...... and evolution of Precambrian lithosphere: A global study. Journal of Geophysical Research 106, 16387–16414.] show strong correlation with tectono-thermal ages and with regional variations in lithospheric thickness constrained by surface heat flow data and seismic velocities. In agreement with xenolith data...

  19. Gold-bearing fluvial and associated tidal marine sediments of Proterozoic age in the Mporokoso Basin, northern Zambia

    Science.gov (United States)

    Andrews-Speed, C. P.

    1986-07-01

    The structurally defined Mporokoso Basin contains up to 5000 m of continental and marine clastic sediments and minor silicic volcanics which together form the Mporokoso Group. These rocks overlie unconformably a basement of silicic-intermediate igneous rocks and accumulated within the interval 1830-1130 Ma. This sedimentological study was restricted to the eastern end of the basin and was part of an assessment of the potential for palaeoplacer gold in the Mporokoso Group. At the base of the Mporokoso Group, the Mbala Formation consists of 1000-1500 m of purple sandstones and conglomerates deposited in a braided-stream system overlain by 500-1000 m of mature quartz arenites deposited in a tidal marine setting. A general coarsening-upward trend exists within the fluvial sediments. Sandy, distal braided-stream facies passes upwards into more proximal conglomeratic facies. In proximal sections, poorly sorted conglomerates form the top of the coarsening-up sequence which is 500-700 m thick. The overlying fluvial sediments fine upwards. The tidal marine sandstones at the top of the Mbala Formation resulted from reworking of fluvial sediments during a marine transgression. Well-exposed sections with fluvial conglomerates were studied in detail. Individual conglomerate bodies form sheets extending for hundreds of metres downstream and at least one hundred metres across stream, with little sign of deep scouring or channelling. They are generally matrix-supported. The whole fluvial sequence is characterised by a paucity of mud or silt. These conglomerates were deposited by large velocity, sheet flows of water which transported a bed-load of pebbles and sand. Most fine material settling out from suspension was eroded by the next flow. The great lateral and vertical extent and the uniformity of the fluvial sediments suggest that the sediments accumulated over an unconfined alluvial plain and that the tectonic evolution of the source area was relatively continuous and not

  20. Refining margins and prospects

    International Nuclear Information System (INIS)

    Baudouin, C.; Favennec, J.P.

    1997-01-01

    Refining margins throughout the world have remained low in 1996. In Europe, in spite of an improvement, particularly during the last few weeks, they are still not high enough to finance new investments. Although the demand for petroleum products is increasing, experts are still sceptical about any rapid recovery due to prevailing overcapacity and to continuing capacity growth. After a historical review of margins and an analysis of margins by regions, we analyse refining over-capacities in Europe and the unbalances between production and demand. Then we discuss the current situation concerning barriers to the rationalization, agreements between oil companies, and the consequences on the future of refining capacities and margins. (author)

  1. Marginalization of the Youth

    DEFF Research Database (Denmark)

    Jensen, Niels Rosendal

    2009-01-01

    The article is based on a key note speach in Bielefeld on the subject "welfare state and marginalized youth", focusing upon the high ambition of expanding schooling in Denmark from 9 to 12 years. The unintended effect may be a new kind of marginalization.......The article is based on a key note speach in Bielefeld on the subject "welfare state and marginalized youth", focusing upon the high ambition of expanding schooling in Denmark from 9 to 12 years. The unintended effect may be a new kind of marginalization....

  2. The rifted margin of Saudi Arabia

    Science.gov (United States)

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

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

  3. Initiation of continental accretion: metamorphic conditions

    Science.gov (United States)

    Clement, Conand; Frederic, Mouthereau; Gianreto, Manatschal; Adbeltif, Lahfid

    2017-04-01

    The physical processes involved at the beginning of the continental collision are largely unknown because they are transient and therefore hardly identifiable from the rock record. Despite the importance of key parameters for understanding mountain building processes, especially the formation of deep mountain roots and their impacts on earthquakes nucleation, rock/fluid transfers and oil/gas resources in the continental crust, observations from the earliest collision stages remain fragmentary. Here, we focus on the example of Taiwan, a young and active mountain belt where the transition from oceanic subduction, accretion of the first continental margin to mature collision can be followed in space and time. We present preliminary results and provide key questions regarding the reconstruction of time-pressure-temperature paths of rocks & fluids to allow discriminating between rift-related thermal/rheological inheritance and burial/heating phases during convergence. Previous studies have focused on peak temperatures analyzed by Raman Spectrometry of Carbonaceous Matter from the deeper structural layers exposed in the Central Range of Taiwan. In the pre-rift sediments, these studies reported a positive gradient from West to Est, and values from geothermal gradients (up to 60°C/km) known in the region, and higher temperature closer to the pre-rift units. Cross sections and maps with high resolution peak temperatures are in process as well as pressure estimations to determine how the sediments were metamorphosed. In addition to this work, we report a few inherited temperatures in the 390-570 °C range, indicating recycling of organic matter from metasediments that recorded HT events, likely originated from higher grade metamorphic units of mainland China, which have been eroded and deposited in the post-rift sediments.

  4. Gas hydrate formation and dissipation histories in the northern margin of Canada: Beaufort-Mackenzie and the Sverdrup Basins

    Czech Academy of Sciences Publication Activity Database

    Majorowicz, J.; Osadetz, K.; Šafanda, Jan

    2012-01-01

    Roč. 2012, č. 1 (2012), 879393/1-879393/17 ISSN 1687-8833 Institutional research plan: CEZ:AV0Z30120515 Keywords : gas hydrates * Canadian Arctic continental margin * permafrost Subject RIV: DC - Siesmology, Volcanology, Earth Structure

  5. Model photoautrophs isolated from a Proterozoic ocean analog - aerobic life under anoxic conditions

    Science.gov (United States)

    Hamilton, T. L.; de Beer, D.; Klatt, J.; Macalady, J.; Weber, M.; Lott, C.; Chennu, A.

    2016-12-01

    The 1-2 billion year delay before the final rise of oxygen at the end of the Proterozoic represents an important gap in our understanding of ancient biogeochemical cycling. Primary production fueled by sulfide-dependent anoxygenic photosynthesis, including the activity of metabolically versatile cyanobacteria, has been invoked as a mechanism for sustaining low atmospheric O2 throughout much of the Proterozoic. However, we understand very little about photoautotrophs that inhabit Proterozoic-like environments present on Earth today. Here we report on the isolation and characterization of a cyanobacterium and a green sulfur bacterium that are the dominant members of pinnacle mats in Little Salt Spring—a karst sinkhole in Florida with perennially low levels of dissolved oxygen and sulfide. The red pinnacle mats bloom in the anoxic basin of the sinkhole and receive light that is of very poor quality to support photosynthesis. Characterization of the isolates is consistent with observations of oxygenic and anoxygenic photosynthesis in situ—both organisms perform anoxygenic photosynthesis under conditions of very low light quality and quantity. Oxygenic photosynthesis by the cyanobacterium isolate is inhibited by the presence of sulfide and under optimal light conditions, rates of anoxygenic photosynthesis are nearly double that of oxygenic photosynthesis. The green sulfur bacterium is tolerant of oxygen and has a very low affinity for sulfide. In Little Salt Spring, oxygenic photosynthesis occurs for only four hours a day and the water column remains anoxic because of a continuous supply of sulfide. Isolation and characterization of these photoautotrophs combined with our high resolution microsensor data in situ highlight microbial biogeochemical cycling in this exceptional site where aerobic microorganisms persist in a largely anoxic ecosystem.

  6. The role of biology in planetary evolution: cyanobacterial primary production in low‐oxygen Proterozoic oceans

    Science.gov (United States)

    Bryant, Donald A.; Macalady, Jennifer L.

    2016-01-01

    Summary Understanding the role of biology in planetary evolution remains an outstanding challenge to geobiologists. Progress towards unravelling this puzzle for Earth is hindered by the scarcity of well‐preserved rocks from the Archean (4.0 to 2.5 Gyr ago) and Proterozoic (2.5 to 0.5 Gyr ago) Eons. In addition, the microscopic life that dominated Earth's biota for most of its history left a poor fossil record, consisting primarily of lithified microbial mats, rare microbial body fossils and membrane‐derived hydrocarbon molecules that are still challenging to interpret. However, it is clear from the sulfur isotope record and other geochemical proxies that the production of oxygen or oxidizing power radically changed Earth's surface and atmosphere during the Proterozoic Eon, pushing it away from the more reducing conditions prevalent during the Archean. In addition to ancient rocks, our reconstruction of Earth's redox evolution is informed by our knowledge of biogeochemical cycles catalysed by extant biota. The emergence of oxygenic photosynthesis in ancient cyanobacteria represents one of the most impressive microbial innovations in Earth's history, and oxygenic photosynthesis is the largest source of O 2 in the atmosphere today. Thus the study of microbial metabolisms and evolution provides an important link between extant biota and the clues from the geologic record. Here, we consider the physiology of cyanobacteria (the only microorganisms capable of oxygenic photosynthesis), their co‐occurrence with anoxygenic phototrophs in a variety of environments and their persistence in low‐oxygen environments, including in water columns as well as mats, throughout much of Earth's history. We examine insights gained from both the rock record and cyanobacteria presently living in early Earth analogue ecosystems and synthesize current knowledge of these ancient microbial mediators in planetary redox evolution. Our analysis supports the hypothesis that anoxygenic

  7. Impacts of continental arcs on global carbon cycling and climate

    Science.gov (United States)

    Lee, C. T.; Jiang, H.; Carter, L.; Dasgupta, R.; Cao, W.; Lackey, J. S.; Lenardic, A.; Barnes, J.; McKenzie, R.

    2017-12-01

    On myr timescales, climatic variability is tied to variations in atmospheric CO2, which in turn is driven by geologic sources of CO2 and modulated by the efficiency of chemical weathering and carbonate precipitation (sinks). Long-term variability in CO2 has largely been attributed to changes in mid-ocean ridge inputs or the efficiency of global weathering. For example, the Cretaceous greenhouse is thought to be related to enhanced oceanic crust production, while the late Cenozoic icehouse is attributed to enhanced chemical weathering associated with the Himalayan orogeny. Here, we show that continental arcs may play a more important role in controlling climate, both in terms of sources and sinks. Continental arcs differ from island arcs and mid-ocean ridges in that the continental plate through which arc magmas pass may contain large amounts of sedimentary carbonate, accumulated over the history of the continent. Interaction of arc magmas with crustal carbonates via assimilation, reaction or heating can significantly add to the mantle-sourced CO2 flux. Detrital zircons and global mapping of basement rocks shows that the length of continental arcs in the Cretaceous was more than twice that in the mid-Cenozoic; maps also show many of these arcs intersected crustal carbonates. The increased length of continental arc magmatism coincided with increased oceanic spreading rates, placing convergent margins into compression, which favors continental arcs. Around 50 Ma, however, nearly all the continental arcs in Eurasia and North America terminated as India collided with Eurasia and the western Pacific rolled back, initiating the Marianas-Tonga-Kermadec intra-oceanic subduction complex and possibly leading to a decrease in global CO2 production. Meanwhile, extinct continental arcs continued to erode, resulting in regionally enhanced chemical weathering unsupported by magmatic fluxes of CO2. Continental arcs, during their magmatic lifetimes, are thus a source of CO2, driving

  8. The role of biology in planetary evolution: cyanobacterial primary production in low?oxygen Proterozoic oceans

    OpenAIRE

    Hamilton, Trinity L.; Bryant, Donald A.; Macalady, Jennifer L.

    2015-01-01

    Summary Understanding the role of biology in planetary evolution remains an outstanding challenge to geobiologists. Progress towards unravelling this puzzle for Earth is hindered by the scarcity of well?preserved rocks from the Archean (4.0 to 2.5?Gyr ago) and Proterozoic (2.5 to 0.5?Gyr ago) Eons. In addition, the microscopic life that dominated Earth's biota for most of its history left a poor fossil record, consisting primarily of lithified microbial mats, rare microbial body fossils and m...

  9. Characterization of organic matter associated with uranium deposits in the Francevillian formation of Gabon (Lower Proterozoic)

    International Nuclear Information System (INIS)

    Cortial, F.; Gauthier-Lafaye, F.; Weber, F.; Oberlin, A.

    1990-01-01

    Elemental analysis, organic petrography, and high-resolution transmission electron microscopy were used to study organic matter in Lower Proterozoic rocks of the Francevillian Series in Africa. Results show a convincing relationship between solid bitumens derived from thermal alteration of crude oil, and deposition of uraninite ores. Evidence is presented that suggests the presence of migration paths for crude oil in associated sandstones. Moreover, the solid bitumens appear to have been further altered by radiation damage as a consequence of oxidation and uranium mineralization. (author)

  10. Geotectonic aspects of the proterozoic triple junction in the center-south part of Goias state

    International Nuclear Information System (INIS)

    Valente, C.R.

    1986-01-01

    The scope of this paper is to made up, in a regional synthesis the tectonical framework of intracontinental proterozoic rifts, from the point of view of an evolutive model through plate tectonic mechanism. based upon lithoenvironment and geotectonics. In this context, this analysis take into account the tectonical interpretation and typification of Canastra, Cuiaba, Estrondo and Tocantins Groups. Structurally these geological entities are found to be settled in rifts of triple junction, in the center-south part of Goias State, individualized among the Oriental Plate (Sao Francisco Craton and Goias Central Massif) Occidental Plate (Amazonic Craton) and Meridional Plate (Paramirim Craton and Parana Block). (author)

  11. On marginal regeneration

    NARCIS (Netherlands)

    Stein, H.N.

    1991-01-01

    On applying the marginal regeneration concept to the drainage of free liquid films, problems are encountered: the films do not show a "neck" of minimum thickness at the film/border transition; and the causes of the direction dependence of the marginal regeneration are unclear. Both problems can be

  12. Matthew and marginality

    Directory of Open Access Journals (Sweden)

    Denis C. Duling

    1995-12-01

    Full Text Available This article explores marginality theory as it was first proposed in  the social sciences, that is related to persons caught between two competing cultures (Park; Stonequist, and, then, as it was developed in sociology as related to the poor (Germani and in anthropology as it was related to involuntary marginality and voluntary marginality (Victor Turner. It then examines a (normative scheme' in antiquity that creates involuntary marginality at the macrosocial level, namely, Lenski's social stratification model in an agrarian society, and indicates how Matthean language might fit with a sample inventory  of socioreligious roles. Next, it examines some (normative schemes' in  antiquity for voluntary margi-nality at the microsocial level, namely, groups, and examines how the Matthean gospel would fit based on indications of factions and leaders. The article ,shows that the author of the Gospel of Matthew has an ideology of (voluntary marginality', but his gospel includes some hope for (involuntary  marginals' in  the  real world, though it is somewhat tempered. It also suggests that the writer of the Gospel is a (marginal man', especially in the sense defined by the early theorists (Park; Stone-quist.

  13. Fixing soft margins

    NARCIS (Netherlands)

    P. Kofman (Paul); A. Vaal, de (Albert); C.G. de Vries (Casper)

    1993-01-01

    textabstractNon-parametric tolerance limits are employed to calculate soft margins such as advocated in Williamson's target zone proposal. In particular, the tradeoff between softness and zone width is quantified. This may be helpful in choosing appropriate margins. Furthermore, it offers

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

    Science.gov (United States)

    Gouiza, M.; Paton, D.

    2017-12-01

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

  15. Self-Consistent Generation of Primordial Continental Crust in Global Mantle Convection Models

    Science.gov (United States)

    Jain, C.; Rozel, A.; Tackley, P. J.

    2017-12-01

    We present the generation of primordial continental crust (TTG rocks) using self-consistent and evolutionary thermochemical mantle convection models (Tackley, PEPI 2008). Numerical modelling commonly shows that mantle convection and continents have strong feedbacks on each other. However in most studies, continents are inserted a priori while basaltic (oceanic) crust is generated self-consistently in some models (Lourenco et al., EPSL 2016). Formation of primordial continental crust happened by fractional melting and crystallisation in episodes of relatively rapid growth from late Archean to late Proterozoic eras (3-1 Ga) (Hawkesworth & Kemp, Nature 2006) and it has also been linked to the onset of plate tectonics around 3 Ga. It takes several stages of differentiation to generate Tonalite-Trondhjemite-Granodiorite (TTG) rocks or proto-continents. First, the basaltic magma is extracted from the pyrolitic mantle which is both erupted at the surface and intruded at the base of the crust. Second, it goes through eclogitic transformation and then partially melts to form TTGs (Rudnick, Nature 1995; Herzberg & Rudnick, Lithos 2012). TTGs account for the majority of the Archean continental crust. Based on the melting conditions proposed by Moyen (Lithos 2011), the feasibility of generating TTG rocks in numerical simulations has already been demonstrated by Rozel et al. (Nature, 2017). Here, we have developed the code further by parameterising TTG formation. We vary the ratio of intrusive (plutonic) and extrusive (volcanic) magmatism (Crisp, Volcanol. Geotherm. 1984) to study the relative volumes of three petrological TTG compositions as reported from field data (Moyen, Lithos 2011). Furthermore, we systematically vary parameters such as friction coefficient, initial core temperature and composition-dependent viscosity to investigate the global tectonic regime of early Earth. Continental crust can also be destroyed by subduction or delamination. We will investigate

  16. Structural and metamorphic evolution of the Mid-Late Proterozoic Rayner Complex, Cape Bruce, East Antarctica

    International Nuclear Information System (INIS)

    Dunkley, D.J.; Clarke, G.L.; White, R.W.

    2002-01-01

    Granulite to transitional granulite facies gneisses exposed at Cape Bruce, Rayner Complex, East Antarctica, record three main orogenic/magmatic phases: (1) intrusion of c. 1000-980 Ma felsic orthogneisses into Mid-Proterozoic metasediments, contemporary with the development of north-trending reclined to recumbent folds; (2) extensive c. 980-900 Ma felsic magmatism, including equivalents of the Mawson Charnockite, which accompanied the development of upright, east-northeast-trending folds; and (3) ultramylonite zones of uncertain age. The first two phases are known as the Rayner Structrual Episode, the effects of which are similar in rocks to the east of Cape Bruce, at Mawson, and in the northern Prince Charles Mountains. Archaean rocks immediately to the west of Cape Bruce were tectonically reworked during the Rayner Structural Episode. The first orogenic phase is inferred to represent the collision between a wedge-shaped Proterozoic block comprising rocks of the Mawson Coast and Eastern Ghats Province, with the Archaean Napier Complex. The second orogenic phase included a major period of crustal growth through emplacement of the Mawson Charnockite and equivalents. (author). 41 refs., 6 figs., 1 tab

  17. Stable isotope, chemical, and mineral compositions of the Middle Proterozoic Lijiaying Mn deposit, Shaanxi Province, China

    Science.gov (United States)

    Yeh, Hsueh-Wen; Hein, James R.; Ye, Jie; Fan, Delian

    1999-01-01

    The Lijiaying Mn deposit, located about 250 km southwest of Xian, is a high-quality ore characterized by low P and Fe contents and a mean Mn content of about 23%. The ore deposit occurs in shallow-water marine sedimentary rocks of probable Middle Proterozoic age. Carbonate minerals in the ore deposit include kutnahorite, calcite, Mn calcite, and Mg calcite. Carbon (−0.4 to −4.0‰) and oxygen (−3.7 to −12.9‰) isotopes show that, with a few exceptions, those carbonate minerals are not pristine low-temperature marine precipitates. All samples are depleted in rare earth elements (REEs) relative to shale and have negative Eu and positive Ce anomalies on chondrite-normalized plots. The Fe/Mn ratios of representative ore samples range from about 0.034 to deep ocean-floor during the Cenozoic. Because the Lijiaying precursor mineral formed in a shallow-water marine environment, the atmospheric oxygen content during the Middle Proterozoic may have been lower than it has been during the Cenozoic.

  18. Modern and ancient geochemical constraints on Proterozoic atmosphere-ocean redox evolution

    Science.gov (United States)

    Hardisty, D. S.; Horner, T. J.; Wankel, S. D.; Lu, Z.; Lyons, T.; Nielsen, S.

    2017-12-01

    A detailed understanding of the spatiotemporal oxygenation of Earth's atmosphere-ocean system through the Precambrian has important implications for the environments capable of sustaining early eukaryotic life and the evolving oxidant budget of subducted sediments. Proxy records suggest an anoxic Fe-rich deep ocean through much of the Precambrian and atmospheric and surface-ocean oxygenation that started in earnest at the Paleoproterozoic Great Oxidation Event (GOE). The marine photic zone represented the initial site of oxygen production and accumulation via cyanobacteria, yet our understanding of surface-ocean oxygen contents and the extent and timing of oxygen propagation and exchange between the atmosphere and deeper ocean are limited. Here, we present an updated perspective of the constraints on atmospheric, surface-ocean, and deep-ocean oxygen contents starting at the GOE. Our research uses the iodine content of Proterozoic carbonates as a tracer of dissolved iodate in the shallow ocean, a redox-sensitive species quantitatively reduced in modern oxygen minimum zones. We supplement our understanding of the ancient record with novel experiments examining the rates of iodate production from oxygenated marine environments based on seawater incubations. Combining new data from iodine with published shallow marine (Ce anomaly, N isotopes) and atmospheric redox proxies, we provide an integrated view of the vertical redox structure of the atmosphere and ocean across the Proterozoic.

  19. Continental United States Hurricane Strikes

    Data.gov (United States)

    National Oceanic and Atmospheric Administration, Department of Commerce — The Continental U.S. Hurricane Strikes Poster is our most popular poster which is updated annually. The poster includes all hurricanes that affected the U.S. since...

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

    Science.gov (United States)

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

    2016-04-01

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

  1. Petrology and geochemistry of Late Proterozoic hornblende gabbros from southeast of Fariman, Khorasan Razavi province, Iran

    Directory of Open Access Journals (Sweden)

    Seyed Masoud Homam

    2015-04-01

    Full Text Available Introduction Hornblende-bearing gabbroic rocks are quite common in subduction-related magmatic suites and considered to represent magmatic differentiation process in arc magmas (Heliker, 1995; Hickey-Vargas et al., 1995; Mandal and Ray, 2012. The presence of hornblende as an important mineral phase in gabbroic rocks of subduction zone has been considered either as an early crystallizing mineral from water-bearing mafic magmas (Beard and Borgia 1989; Mandal and Ray, 2012 or as a product of reaction of early crystallized minerals (olivine, pyroxene and plagioclase and water-rich evolved melt/aqueous fluid (Costa et al., 2002; Mandal and Ray, 2012. The careful study of petrology and geochemistry of hornblende-bearing gabbroic rocks from Chahak area, of Neoproterozoic age, can provide important information about their petrogenesis. Because of the special characteristics of Chahak hornblende gabbros according to their age and their situation in the main structural units of Iran, their study can present critical keys for the knowledge of geological history of Iran specially central Iran zone. Material and Methods This study carried out in two parts including field and laboratory works. Sampling and structural studies were carried out during field work. Geological map for the study area was also prepared. 65 thin and polished thin sections for petrographical purpose were studied. Major oxides, rare earth elements and trace elements were analyzed for 4 samples (92P-1, 92P-3, B1and B6 from hornblende gabbros on the basis of 4AB1 method using ICP-MS of ACME Laboratory from Canada. In addition, major oxides of three hornblende gabbro samples (89P-62, 89P-59 and 89P-46 were used from Partovifar (Partovifar, 2012. Results and discussion Fariman metamorphic terrains, of Proterozoic age, consist of metamorphosed sedimentary and igneous (plutonic and volcanic rocks. Hornblende gabbros of the study area include plagioclase, hornblende, biotite pyroxene and

  2. Continental crust formation: Numerical modelling of chemical evolution and geological implications

    Science.gov (United States)

    Walzer, U.; Hendel, R.

    2017-05-01

    Oceanic plateaus develop by decompression melting of mantle plumes and have contributed to the growth of the continental crust throughout Earth's evolution. Occasional large-scale partial melting events of parts of the asthenosphere during the Archean produced large domains of precursor crustal material. The fractionation of arc-related crust during the Proterozoic and Phanerozoic contributed to the growth of continental crust. However, it remains unclear whether the continents or their precursors formed during episodic events or whether the gaps in zircon age records are a function of varying preservation potential. This study demonstrates that the formation of the continental crust was intrinsically tied to the thermoconvective evolution of the Earth's mantle. Our numerical solutions for the full set of physical balance equations of convection in a spherical shell mantle, combined with simplified equations of chemical continent-mantle differentiation, demonstrate that the actual rate of continental growth is not uniform through time. The kinetic energy of solid-state mantle creep (Ekin) slowly decreases with superposed episodic but not periodic maxima. In addition, laterally averaged surface heat flow (qob) behaves similarly but shows peaks that lag by 15-30 Ma compared with the Ekin peaks. Peak values of continental growth are delayed by 75-100 Ma relative to the qob maxima. The calculated present-day qob and total continental mass values agree well with observed values. Each episode of continental growth is separated from the next by an interval of quiescence that is not the result of variations in mantle creep velocity but instead reflects the fact that the peridotite solidus is not only a function of pressure but also of local water abundance. A period of differentiation results in a reduction in regional water concentrations, thereby increasing the temperature of the peridotite solidus and the regional viscosity of the mantle. By plausibly varying the

  3. Refining margins: recent trends

    International Nuclear Information System (INIS)

    Baudoin, C.; Favennec, J.P.

    1999-01-01

    Despite a business environment that was globally mediocre due primarily to the Asian crisis and to a mild winter in the northern hemisphere, the signs of improvement noted in the refining activity in 1996 were borne out in 1997. But the situation is not yet satisfactory in this sector: the low return on invested capital and the financing of environmental protection expenditure are giving cause for concern. In 1998, the drop in crude oil prices and the concomitant fall in petroleum product prices was ultimately rather favorable to margins. Two elements tended to put a damper on this relative optimism. First of all, margins continue to be extremely volatile and, secondly, the worsening of the economic and financial crisis observed during the summer made for a sharp decline in margins in all geographic regions, especially Asia. Since the beginning of 1999, refining margins are weak and utilization rates of refining capacities have decreased. (authors)

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

    Science.gov (United States)

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

    2016-04-01

    The low-temperature thermochronology has been an important tool to quantify geological process in passive continental margins. In this context, the Angolan margin shows evidence of a polycyclic post-rift evolution marked by different events of uplift, basin inversion and changes in sedimentation rates to the marginal basins, which have controlled the salt tectonics and the hydrocarbon deposits (1,2,3,4). To understand the post break-up evolution of the southwestern Angola margin, it were collected outcrop samples for apatite fission track (AFT) and (U-Th)/He analysis ranging in elevation from 79 m to 1675 m from the coast toward the interior plateau in a profile between Namibe and Lubango cities. The area lies on the edge of Central and Southern Atlantic segments a few kilometers northward the Walvis ridge and encompasses the Archean and Proterozoic basement rocks of the Congo craton. The AFT ages ranging from 120.6 ± 8.9 Ma to 328.8 ± 28.5 Ma and they show a trend of increasing age toward the Great Escarpment with some exceptions. The partial mean track lengths (MTLs) vary between 11.77 ± 1.82 μm to 12.34 ± 1.13 μm with unimodal track length distributions (TDLs). The partial (U-Th)/He ages ranging from 104.85 ± 3.15 Ma to 146.95 ± 4.41 Ma and show the same trend of increasing ages landward, little younger than the AFT ages, which could be interpreted as a fast exhumation episode in Late Jurassic - Early Cretaceous times. The thermal histories modelling has been constrained with the kinetic parameters Dpar (5) and c-axis angle (6) by the software Hefty (7). Both AFT and (U-Th)/He thermal histories modelling indicate three episodes of denudation/uplift driven cooling: (a) from Late Jurassic to Early Cretaceous, (b) a smallest one in the Late Cretaceous and (c) from Oligocene-Miocene to recent, which are compatible with geophysical data of the offshore Namibe basin that estimate the greater thickness of sediments formed in the first and third episodes

  5. SOCIAL MARGINALIZATION AND HEALTH

    Directory of Open Access Journals (Sweden)

    Marjana Bogdanović

    2007-04-01

    Full Text Available The 20th century was characterized by special improvement in health. The aim of WHO’s policy EQUITY IN HEALTH is to enable equal accessibility and equal high quality of health care for all citizens. More or less some social groups have stayed out of many social systems even out of health care system in the condition of social marginalization. Phenomenon of social marginalization is characterized by dynamics. Marginalized persons have lack of control over their life and available resources. Social marginalization stands for a stroke on health and makes the health status worse. Low socio-economic level dramatically influences people’s health status, therefore, poverty and illness work together. Characteristic marginalized groups are: Roma people, people with AIDS, prisoners, persons with development disorders, persons with mental health disorders, refugees, homosexual people, delinquents, prostitutes, drug consumers, homeless…There is a mutual responsibility of community and marginalized individuals in trying to resolve the problem. Health and other problems could be solved only by multisector approach to well-designed programs.

  6. Pickering seismic safety margin

    International Nuclear Information System (INIS)

    Ghobarah, A.; Heidebrecht, A.C.; Tso, W.K.

    1992-06-01

    A study was conducted to recommend a methodology for the seismic safety margin review of existing Canadian CANDU nuclear generating stations such as Pickering A. The purpose of the seismic safety margin review is to determine whether the nuclear plant has sufficient seismic safety margin over its design basis to assure plant safety. In this review process, it is possible to identify the weak links which might limit the seismic performance of critical structures, systems and components. The proposed methodology is a modification the EPRI (Electric Power Research Institute) approach. The methodology includes: the characterization of the site margin earthquake, the definition of the performance criteria for the elements of a success path, and the determination of the seismic withstand capacity. It is proposed that the margin earthquake be established on the basis of using historical records and the regional seismo-tectonic and site specific evaluations. The ability of the components and systems to withstand the margin earthquake is determined by database comparisons, inspection, analysis or testing. An implementation plan for the application of the methodology to the Pickering A NGS is prepared

  7. Strongly seasonal Proterozoic glacial climate in low palaeolatitudes: Radically different climate system on the pre-Ediacaran Earth

    Directory of Open Access Journals (Sweden)

    George E. Williams

    2016-07-01

    Full Text Available Proterozoic (pre-Ediacaran glaciations occurred under strongly seasonal climates near sea level in low palaeolatitudes. Metre-scale primary sand wedges in Cryogenian periglacial deposits are identical to those actively forming, through the infilling of seasonal (winter thermal contraction-cracks in permafrost by windblown sand, in present-day polar regions with a mean monthly air temperature range of 40 °C and mean annual air temperatures of −20 °C or lower. Varve-like rhythmites with dropstones in Proterozoic glacial successions are consistent with an active seasonal freeze–thaw cycle. The seasonal (annual oscillation of sea level recorded by tidal rhythmites in Cryogenian glacial successions indicates a significant seasonal cycle and extensive open seas. Palaeomagnetic data determined directly for Proterozoic glacial deposits and closely associated rocks indicate low palaeolatitudes: Cryogenian deposits in South Australia accumulated at ≤10°, most other Cryogenian deposits at 54° during Proterozoic low-latitude glaciations, whereby the equator would be cooler than the poles, on average, and global seasonality would be greatly amplified.

  8. Uraniferous concentrations in the Francevillian of Gabon: their association with fossil hydrocarbon-bearing ore deposits of the lower Proterozoic

    International Nuclear Information System (INIS)

    Gauthier-Lafaye, Francois; Weber, Francis

    1981-01-01

    The Francevillian U deposits of Gabon are located in one of the oldest non metamorphic sedimentary series of the Lower Proterozoic 2150 M.Y. old. The uraniferous mineralization 2050 M.Y. old is strongly related to organic matter concentrated in oil-bearing structures after migration. No geological event remobilized the mineralizations since their settling [fr

  9. The role of biology in planetary evolution: cyanobacterial primary production in low-oxygen Proterozoic oceans.

    Science.gov (United States)

    Hamilton, Trinity L; Bryant, Donald A; Macalady, Jennifer L

    2016-02-01

    Understanding the role of biology in planetary evolution remains an outstanding challenge to geobiologists. Progress towards unravelling this puzzle for Earth is hindered by the scarcity of well-preserved rocks from the Archean (4.0 to 2.5 Gyr ago) and Proterozoic (2.5 to 0.5 Gyr ago) Eons. In addition, the microscopic life that dominated Earth's biota for most of its history left a poor fossil record, consisting primarily of lithified microbial mats, rare microbial body fossils and membrane-derived hydrocarbon molecules that are still challenging to interpret. However, it is clear from the sulfur isotope record and other geochemical proxies that the production of oxygen or oxidizing power radically changed Earth's surface and atmosphere during the Proterozoic Eon, pushing it away from the more reducing conditions prevalent during the Archean. In addition to ancient rocks, our reconstruction of Earth's redox evolution is informed by our knowledge of biogeochemical cycles catalysed by extant biota. The emergence of oxygenic photosynthesis in ancient cyanobacteria represents one of the most impressive microbial innovations in Earth's history, and oxygenic photosynthesis is the largest source of O2 in the atmosphere today. Thus the study of microbial metabolisms and evolution provides an important link between extant biota and the clues from the geologic record. Here, we consider the physiology of cyanobacteria (the only microorganisms capable of oxygenic photosynthesis), their co-occurrence with anoxygenic phototrophs in a variety of environments and their persistence in low-oxygen environments, including in water columns as well as mats, throughout much of Earth's history. We examine insights gained from both the rock record and cyanobacteria presently living in early Earth analogue ecosystems and synthesize current knowledge of these ancient microbial mediators in planetary redox evolution. Our analysis supports the hypothesis that anoxygenic photosynthesis

  10. A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

    Science.gov (United States)

    Sheen, Alex I.; Kendall, Brian; Reinhard, Christopher T.; Creaser, Robert A.; Lyons, Timothy W.; Bekker, Andrey; Poulton, Simon W.; Anbar, Ariel D.

    2018-04-01

    Emerging geochemical evidence suggests that the atmosphere-ocean system underwent a significant decrease in O2 content following the Great Oxidation Event (GOE), leading to a mid-Proterozoic ocean (ca. 2.0-0.8 Ga) with oxygenated surface waters and predominantly anoxic deep waters. The extent of mid-Proterozoic seafloor anoxia has been recently estimated using mass-balance models based on molybdenum (Mo), uranium (U), and chromium (Cr) enrichments in organic-rich mudrocks (ORM). Here, we use a temporal compilation of concentrations for the redox-sensitive trace metal rhenium (Re) in ORM to provide an independent constraint on the global extent of mid-Proterozoic ocean anoxia and as a tool for more generally exploring how the marine geochemical cycle of Re has changed through time. The compilation reveals that mid-Proterozoic ORM are dominated by low Re concentrations that overall are only mildly higher than those of Archean ORM and significantly lower than many ORM deposited during the ca. 2.22-2.06 Ga Lomagundi Event and during the Phanerozoic Eon. These temporal trends are consistent with a decrease in the oceanic Re inventory in response to an expansion of anoxia after an interval of increased oxygenation during the Lomagundi Event. Mass-balance modeling of the marine Re geochemical cycle indicates that the mid-Proterozoic ORM with low Re enrichments are consistent with extensive seafloor anoxia. Beyond this agreement, these new data bring added value because Re, like the other metals, responds generally to low-oxygen conditions but has its own distinct sensitivity to the varying environmental controls. Thus, we can broaden our capacity to infer nuanced spatiotemporal patterns in ancient redox landscapes. For example, despite the still small number of data, some mid-Proterozoic ORM units have higher Re enrichments that may reflect a larger oceanic Re inventory during transient episodes of ocean oxygenation. An improved understanding of the modern oceanic Re

  11. Quaternary nanofossils on the Brazilian continental shelf; Nanofosseis calcarios do quaternario da margem continental brasileira

    Energy Technology Data Exchange (ETDEWEB)

    Antunes, Rogerio Loureiro [PETROBRAS S.A., Rio de Janeiro, RJ (Brazil). Centro de Pesquisas (CENPES). Gerencia de Bioestratigrafia e Paleoecologia], E-mail: rlantunes@petrobras.com.br

    2007-07-01

    The study of calcareous nanofossils occurring in the deposits on the Brazilian continental margin began in the late 1960s, undertaken solely by PETROBRAS. Instead of presenting an academic outlook, the purpose of these investigations is first to formulate a biostratigraphic framework to apply to oil well samples. The initial result was the first zoning for the Brazilian continental margin, which considered the deposits formed between the Aptian and Miocene series. Since the 1960s to date, many papers have been written either with details of that original zoning or applying nanofossil biostratigraphy to solve stratigraphic problems. Regardless of all the papers and studies undertaken, little attention has been paid to the Quaternary, since these deposits are normally of no interest to petroleum geology stricto sensu, especially in a large part of the Brazilian margin. On the other hand, there are a few articles and some Master's dissertations and PhD theses that were written and/or are in progress in Brazilian universities. On the other hand, elsewhere in the world, Quaternary nanofossils have been thoroughly investigated in terms of biostratigraphy and paleoceanography. It is, therefore, very clear that there is a gap between what is being done elsewhere in the world and what has been done in Brazil. In fact, this gap is not larger simply because of a few researchers in Brazilian universities who are studying this topic. The intention of this paper is to contribute toward a richer study of Quaternary nanofossils. It, therefore, contains illustrations and taxonomic descriptions of many species observed in the younger strata of the Brazilian margin basins. This article not only aspires to portray and disseminate the potential of nanofossils for the marine Quaternary study but is also an invitation to students (under and post-graduates) and university researchers - an invitation to learn a little more about the subject and spend some time studying these real gems

  12. Quaternary nanofossils on the Brazilian continental shelf; Nanofosseis calcarios do quaternario da margem continental brasileira

    Energy Technology Data Exchange (ETDEWEB)

    Antunes, Rogerio Loureiro [PETROBRAS S.A., Rio de Janeiro, RJ (Brazil). Centro de Pesquisas (CENPES). Gerencia de Bioestratigrafia e Paleoecologia], E-mail: rlantunes@petrobras.com.br

    2007-07-01

    The study of calcareous nanofossils occurring in the deposits on the Brazilian continental margin began in the late 1960s, undertaken solely by PETROBRAS. Instead of presenting an academic outlook, the purpose of these investigations is first to formulate a biostratigraphic framework to apply to oil well samples. The initial result was the first zoning for the Brazilian continental margin, which considered the deposits formed between the Aptian and Miocene series. Since the 1960s to date, many papers have been written either with details of that original zoning or applying nanofossil biostratigraphy to solve stratigraphic problems. Regardless of all the papers and studies undertaken, little attention has been paid to the Quaternary, since these deposits are normally of no interest to petroleum geology stricto sensu, especially in a large part of the Brazilian margin. On the other hand, there are a few articles and some Master's dissertations and PhD theses that were written and/or are in progress in Brazilian universities. On the other hand, elsewhere in the world, Quaternary nanofossils have been thoroughly investigated in terms of biostratigraphy and paleoceanography. It is, therefore, very clear that there is a gap between what is being done elsewhere in the world and what has been done in Brazil. In fact, this gap is not larger simply because of a few researchers in Brazilian universities who are studying this topic. The intention of this paper is to contribute toward a richer study of Quaternary nanofossils. It, therefore, contains illustrations and taxonomic descriptions of many species observed in the younger strata of the Brazilian margin basins. This article not only aspires to portray and disseminate the potential of nanofossils for the marine Quaternary study but is also an invitation to students (under and post-graduates) and university researchers - an invitation to learn a little more about the subject and spend some time studying these real gems of

  13. Geophysical imaging of buried volcanic structures within a continental back-arc basin

    DEFF Research Database (Denmark)

    Stratford, Wanda Rose; Stern, T.A.

    2008-01-01

    Hidden beneath the ~2 km thick low-velocity volcaniclastics on the western margin of the Central Volcanic Region, North Island, New Zealand, are two structures that represent the early history of volcanic activity in a continental back-arc. These ~20×20 km structures, at Tokoroa and Mangakino, fo...

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

    Directory of Open Access Journals (Sweden)

    Joann M. Stock

    2007-09-01

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

  15. River Valley pluton, Ontario - A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    Science.gov (United States)

    Ashwal, Lewis D.; Wooden, Joseph L.

    1989-01-01

    This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.

  16. Stages of material transformations of Archean-Proterozoic rocks (Central-Karelian domain)

    International Nuclear Information System (INIS)

    Vinogradov, V.I.; Buyakajte, M.I.; Kolodyazhnyj, S.Yu.; Leonov, M.G.; Orlov, S.Yu.

    2001-01-01

    The age of the Archean-Proterozoic rocks from the south-east part of the Central-Karelian domain was determined by the method of Rb-Sr dating. It was ascertained that the age of the least tectonized rocks of granite-greenstone Archean foundation makes up 2800±70 mln. years at initial strontium isotopic ratio of 0.7022±0.0007. Gneisses of mainly plagiogranite composition, their age 1930±118 mln. years and strontium isotopic ratio 0.7170±0.0026, constitute the second group of the rocks. It is shown that isotopic age defined for the two groups of rocks agrees well with major geological events on the Baltic shield and planet as a whole [ru

  17. Upper Proterozoic (1800-570 Ma) stratigraphy: a survey of lithostratigraphic, paleontological, radiochronological and magnetic correlations

    International Nuclear Information System (INIS)

    Trompette, R.

    1982-01-01

    Classical stratigraphy which was conceived in relation to the Phanerozoic, can be extended into the Precambrian, particularly the upper part. Stromatolite biostratigraphy provides natural subdivisions of the Upper Proterozoic, as indicated by studies on the Riphean of the U.S.S.R. The correlation techniques are identical to those used in the Phanerozoic. Imperfections in the biostratigraphic record, however, have encouraged the development of lithostratigraphic, radiochronological and paleomagnetic techniques. The possibility of dating argillaceous sedimentary rocks largely depends on the care taken in the selection of samples. The samples should have the necessary mineralogy and should not contain inherited 40 Ar or 87 Sr. Good results have been obtained on glauconites rich in K 2 O and particularly on illite and smectite, using the Rb-Sr method. Geochronology is the most precise of the four methods of correlation considered. It also provides a framework on which to hang the results of lithostratigraphy and magnetostratigraphy. (Auth.)

  18. Distribution and sedimentary arrangement of carbon in South African proterozoic placer deposits

    International Nuclear Information System (INIS)

    Minter, W.E.L.

    1981-01-01

    Carbon, which occurs as grains, films, and thin seams in Witwatersrand Proterozoic placer deposits, is generally confined to carbon-seam reefs that were deposited in distal environments. The distribution of carbon on paleosurfaces, on sedimentary accumulation surfaces like pebble layers, on trough-shaped bedforms of pi-crossbedded units and foresets, and on the winnowed top of placer sediments implies that its growth took place contemporaneously with placer deposition in an aquatic fluvial environment. The areal distribution of carbon seams in distal environments is patchy, and its sparsity or total absence in some areas does not affect either the gold or the uranium content of the placer. High gold and uranium contents that appear to be associated with carbon seams are at the base of the reef because that position represents both the stable consolidated paleosurface upon which the plant material anchored itself and also the surface of bedload concentration

  19. Ocean Margins Programs, Phase I research summaries

    Energy Technology Data Exchange (ETDEWEB)

    Verity, P. [ed.

    1994-08-01

    During FY 1992, the DOE restructured its regional coastal-ocean programs into a new Ocean Margins Program (OMP), to: Quantify the ecological and biogeochemical processes and mechanisms that affect the cycling, flux, and storage of carbon and other biogenic elements at the land/ocean interface; Define ocean-margin sources and sinks in global biogeochemical cycles, and; Determine whether continental shelves are quantitatively significant in removing carbon dioxide from the atmosphere and isolating it via burial in sediments or export to the interior ocean. Currently, the DOE Ocean Margins Program supports more than 70 principal and co-principal investigators, spanning more than 30 academic institutions. Research funded by the OMP amounted to about $6.9M in FY 1994. This document is a collection of abstracts summarizing the component projects of Phase I of the OMP. This phase included both research and technology development, and comprised projects of both two and three years duration. The attached abstracts describe the goals, methods, measurement scales, strengths and limitations, and status of each project, and level of support. Keywords are provided to index the various projects. The names, addresses, affiliations, and major areas of expertise of the investigators are provided in appendices.

  20. Iron-dependent nitrogen cycling in a ferruginous lake and the nutrient status of Proterozoic oceans

    Science.gov (United States)

    Michiels, Céline C.; Darchambeau, François; Roland, Fleur A. E.; Morana, Cédric; Llirós, Marc; García-Armisen, Tamara; Thamdrup, Bo; Borges, Alberto V.; Canfield, Donald E.; Servais, Pierre; Descy, Jean-Pierre; Crowe, Sean A.

    2017-01-01

    Nitrogen limitation during the Proterozoic has been inferred from the great expanse of ocean anoxia under low-O2 atmospheres, which could have promoted NO3- reduction to N2 and fixed N loss from the ocean. The deep oceans were Fe rich (ferruginous) during much of this time, yet the dynamics of N cycling under such conditions remain entirely conceptual, as analogue environments are rare today. Here we use incubation experiments to show that a modern ferruginous basin, Kabuno Bay in East Africa, supports high rates of NO3- reduction. Although 60% of this NO3- is reduced to N2 through canonical denitrification, a large fraction (40%) is reduced to NH4+, leading to N retention rather than loss. We also find that NO3- reduction is Fe dependent, demonstrating that such reactions occur in natural ferruginous water columns. Numerical modelling of ferruginous upwelling systems, informed by our results from Kabuno Bay, demonstrates that NO3- reduction to NH4+ could have enhanced biological production, fuelling sulfate reduction and the development of mid-water euxinia overlying ferruginous deep oceans. This NO3- reduction to NH4+ could also have partly offset a negative feedback on biological production that accompanies oxygenation of the surface ocean. Our results indicate that N loss in ferruginous upwelling systems may not have kept pace with global N fixation at marine phosphorous concentrations (0.04-0.13 μM) indicated by the rock record. We therefore suggest that global marine biological production under ferruginous ocean conditions in the Proterozoic eon may thus have been P not N limited.

  1. Polyphase Rifting and Breakup of the Central Mozambique Margin

    Science.gov (United States)

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

    2017-04-01

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

  2. Marginal kidney donor

    Directory of Open Access Journals (Sweden)

    Ganesh Gopalakrishnan

    2007-01-01

    Full Text Available Renal transplantation is the treatment of choice for a medically eligible patient with end stage renal disease. The number of renal transplants has increased rapidly over the last two decades. However, the demand for organs has increased even more. This disparity between the availability of organs and waitlisted patients for transplants has forced many transplant centers across the world to use marginal kidneys and donors. We performed a Medline search to establish the current status of marginal kidney donors in the world. Transplant programs using marginal deceased renal grafts is well established. The focus is now on efforts to improve their results. Utilization of non-heart-beating donors is still in a plateau phase and comprises a minor percentage of deceased donations. The main concern is primary non-function of the renal graft apart from legal and ethical issues. Transplants with living donors outnumbered cadaveric transplants at many centers in the last decade. There has been an increased use of marginal living kidney donors with some acceptable medical risks. Our primary concern is the safety of the living donor. There is not enough scientific data available to quantify the risks involved for such donation. The definition of marginal living donor is still not clear and there are no uniform recommendations. The decision must be tailored to each donor who in turn should be actively involved at all levels of the decision-making process. In the current circumstances, our responsibility is very crucial in making decisions for either accepting or rejecting a marginal living donor.

  3. Crustal structure and inferred extension mode in the northern margin of the South China Sea

    Science.gov (United States)

    Gao, J.; Wu, S.; McIntosh, K. D.; Mi, L.; Spence, G.

    2016-12-01

    Combining multi-channel seismic reflection and satellite gravity data, this study has investigated the crustal structure and magmatic activities of the northern South China Sea (SCS) margin. Results show that a broad continent-ocean transition zone (COT) with more than 140 km wide is characterized by extensive igneous intrusion/extrusion and hyper-extended continental crust in the northeastern SCS margin, a broader COT with 220-265 km wide is characterized by crustal thinning, rift depression, structural highs with igneous rock and perhaps a volcanic zone or a zone of tilted fault blocks at the distal edge in the mid-northern SCS margin, and a narrow COT with 65 km wide bounded seawards by a volcanic buried seamount is characterized by extremely hyper-extended continental crust in the northwestern SCS margin, where the remnant crust with less than 3 km thick is bounded by basin-bounding faults corresponding to an aborted rift below the Xisha Trough with a sub-parallel fossil ridge in the adjacent Northwest Sub-basin. Results from gravity modeling and seismic refraction data show that a high velocity layer (HVL) is present in the outer shelf and slope below extended continental crust in the eastern portion of the northern SCS margin and is thickest (up to 10 km) in the Dongsha Uplift where the HVL gradually thins to east and west below the lower slope and finally terminates at the Manila Trench and Baiyun sag of the Pearl River Mouth Basin. The magmatic intrusions/extrusions and HVL may be related to partial melting caused by decompression of passive, upwelling asthenosphere which resulted primarily in post-rifting underplating and magmatic emplacement or modification of the crust. The northern SCS margin is closer to those of the magma-poor margins than those of volcanic margins, but the aborted rift near the northwestern continental margin shows that there may be no obvious detachment fault like that in the Iberia-Newfoundland type margin. The symmetric aborted

  4. From Borders to Margins

    DEFF Research Database (Denmark)

    Parker, Noel

    2009-01-01

    of entities that are ever open to identity shifts.  The concept of the margin possesses a much wider reach than borders, and focuses continual attention on the meetings and interactions between a range of indeterminate entities whose interactions may determine both themselves and the types of entity...... upon Deleuze's philosophy to set out an ontology in which the continual reformulation of entities in play in ‘post-international' society can be grasped.  This entails a strategic shift from speaking about the ‘borders' between sovereign states to referring instead to the ‘margins' between a plethora...

  5. Biogeochemistry of southern Australian continental slope sediments

    International Nuclear Information System (INIS)

    Veeh, H.H.; Crispe, A.J.; Heggie, D.T.

    1999-01-01

    Sediment cores from the middle to lower slope of the southern continental margin of Australia between the Great Australian Bight and western Tasmania are compared in terms of marine and terrigenous input signals during the Holocene. The mass accumulation rates of carbonate, organic carbon, biogenic Ba. and Al are corrected for lateral sediment input (focusing), using the inventory of excess 230 Th in the sediment normalised to its known production rate in the water column above each site. The biogenic signal is generally higher in the eastern part of the southern margin probably due to enhanced productivity associated with seasonal upwelling off southeastern South Australia and the proximity of the Subtropical Front, which passes just south of Tasmania. The input of Al, representing the terrigenous signal, is also higher in this region reflecting the close proximity of river runoff from the mountainous catchment of southeastern Australia. The distribution pattern of Mn and authigenic U, together with pore-water profiles of Mn ++ , indicate diagenetic reactions driven by the oxidation of buried organic carbon in an oxic to suboxic environment. Whereas Mn is reduced at depth and diffuses upwards to become immobilised in a Mn-rich surface layer. U is derived from seawater and diffuses downward into the sediment, driven by reduction and precipitation at a depth below the reduction zone of Mn. The estimated removal rate of U from seawater by this process is within the range of U removal measured in hemipelagic sediments from other areas, and supports the proposition that hemipelagic sediments are a major sink of U in the global ocean. Unlike Mn, the depth profile of sedimentary Fe appears to be little affected by diagenesis, suggesting that little of the total Fe inventory in the sediment is remobilised and redistributed as soluble Fe. Copyright (1999) Blackwell Science Pty Ltd

  6. Splenic marginal zone lymphoma.

    Science.gov (United States)

    Piris, Miguel A; Onaindía, Arantza; Mollejo, Manuela

    Splenic marginal zone lymphoma (SMZL) is an indolent small B-cell lymphoma involving the spleen and bone marrow characterized by a micronodular tumoral infiltration that replaces the preexisting lymphoid follicles and shows marginal zone differentiation as a distinctive finding. SMZL cases are characterized by prominent splenomegaly and bone marrow and peripheral blood infiltration. Cells in peripheral blood show a villous cytology. Bone marrow and peripheral blood characteristic features usually allow a diagnosis of SMZL to be performed. Mutational spectrum of SMZL identifies specific findings, such as 7q loss and NOTCH2 and KLF2 mutations, both genes related with marginal zone differentiation. There is a striking clinical variability in SMZL cases, dependent of the tumoral load and performance status. Specific molecular markers such as 7q loss, p53 loss/mutation, NOTCH2 and KLF2 mutations have been found to be associated with the clinical variability. Distinction from Monoclonal B-cell lymphocytosis with marginal zone phenotype is still an open issue that requires identification of precise and specific thresholds with clinical meaning. Copyright © 2016 Elsevier Ltd. All rights reserved.

  7. Komorbiditet ved marginal parodontitis

    DEFF Research Database (Denmark)

    Holmstrup, Palle; Damgaard, Christian; Olsen, Ingar

    2017-01-01

    Nærværende artikel præsenterer en oversigt over den foreliggende væsentligste viden om sammenhængen mellem marginal parodontitis og en række medicinske sygdomme, herunder hjerte-kar-sygdomme, diabetes mellitus, reumatoid arthritis, osteoporose, Parkinsons sygdom, Alzheimers sygdom, psoriasis og...

  8. Marginally Deformed Starobinsky Gravity

    DEFF Research Database (Denmark)

    Codello, A.; Joergensen, J.; Sannino, Francesco

    2015-01-01

    We show that quantum-induced marginal deformations of the Starobinsky gravitational action of the form $R^{2(1 -\\alpha)}$, with $R$ the Ricci scalar and $\\alpha$ a positive parameter, smaller than one half, can account for the recent experimental observations by BICEP2 of primordial tensor modes....

  9. Marginalization and School Nursing

    Science.gov (United States)

    Smith, Julia Ann

    2004-01-01

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

  10. Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

    Science.gov (United States)

    Sauer, K. B.; Gordon, S. M.; Miller, R. B.; Vervoort, J. D.; Fisher, C. M.

    2017-12-01

    The metasupracrustal units within the north central Chelan block of the North Cascades Range, Washington, are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U-Pb and Hf-isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complex metasedimentary rocks have (1) a wide range of zircon U-Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and Southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (circa 134-79 Ma). Sediment was likely underthrust into the active arc by circa 74-65 Ma, as soon as 7 Ma after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (circa 79-60 Ma) and may indicate a link between the burial of sediments and pluton emplacement.

  11. Isotopic data from proterozoic sediment-hosted sulfide deposits of Brazil: Implications for their metallogenic evolution and for mineral exploration

    International Nuclear Information System (INIS)

    Misi, Aroldo; Coelho, Carlos E.S.; Franca Rocha, Washington J.S.; Gomez, Adriana S.R.; Cunha, Iona A.; Iyer, Sundaram S.; Tassinari, Colombo C.G.; Kyle, J. Richard

    1998-01-01

    Geological, petrographic, fluid inclusions studies and isotopic data of seven Proterozoic sediment-hosted Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition of the mineral deposits. The studies suggest that they were formed during periods of extensional tectonics: Growth faults or reactivated basement faults were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. In most cases, sulfides were formed by the reduction of sedimentary sulfates. Linear structures are important controls for sulfide concentration in these Proterozoic basins. (author)

  12. Correlation of proterozoic sediments of Western and Central Africa and South America based upon radiochronological and paleontological data

    International Nuclear Information System (INIS)

    Bonhomme, M.G.

    1982-01-01

    Nearly 70 new Rb-Sr isochron ages and many K-Ar conventional ages have been determined between 1975 and 1980 on Proterozoic sedimentary or metasedimentary sequences in western and Central Africa and South America. Some stratigraphic results have been established: (1) five formations have been dated of the Lower Proterozoic; (2) a long sedimentation gap occurs, mainly in western Africa and in some regions of Central Africa and South America between nearly 1600 and 1100 Ma; (3) the upper Riphean assemblages of stromatolites have been dated and compared to those of the Eurasian craton; (4) two main glacial events have been dated, the first one placed at ca. 950 Ma, the second during the Vendian, at ca. 650-620 Ma; (5) it can be stated that, when applied to Precambrian sequences, all stratigraphic methods must be used together. (Auth.)

  13. How Strong is the Case for Proterozoic Low-Latitude Glaciation?

    Science.gov (United States)

    Evans, D. A.

    2004-05-01

    The most recent global compilations of paleomagnetic depositional latitudes for Proterozoic glaciogenic formations indicate a dominant mode near the paleo-equator (Evans 2000 AJS; Evans 2003 Tectonophysics). This result would therefore support either the snowball Earth or the large-obliquity hypotheses for Precambrian ice ages, but would reject the uniformitarian comparison to polar-temperate-restricted Phanerozoic glaciogenic deposits. The most reliable low-latitude results come from the Australian Marinoan succession, but a recent summary of these units has suggested that a glaciogenic origin is not yet demonstrated (Eyles and Januszczak 2004 Earth-Sci Reviews). It becomes useful, then, to review the global evidence for Proterozoic low-latitude glaciation. Eyles and Januszczak (ibid.) identified 13 Neoproterozoic deposits with "demonstrated" glacial influence. Among these, poor age constraints and lack of paleomagnetic data prohibit estimation of depositional paleolatitudes for the Fiq, Sturtian, Vreeland, Taoudeni, East Greenland, Port Askaig, and Zhengmuguan units. Moderate paleolatitudes are reasonably well supported for the South China, Gaskiers, Smalfjord, and Moelv units. Among the three remaining units, the Rapitan Group can be assigned a near-equatorial paleolatitude indirectly through use of the Galeros and Franklin-Natkusiak paleomagnetic results, as long as the Rapitan age lies within 750-720 Ma as generally expected. The Moonlight Valley Formation in northern Australia may be assigned a tropical paleolatitude according to high-quality paleomagnetic results from compellingly correlated Marinoan strata in southern Australia. Those strata, including the famous Elatina Formation, have yielded a robust paleomagnetic signature that is commonly interpreted to imply frigid climate (manifest in part by frost-wedge polygons) at near-equatorial latitudes. Concerns that the Neoproterozoic geomagnetic field was either nonaxial or nondipolar are valid in principle

  14. Middlemen Margins and Globalization

    OpenAIRE

    Pranab Bardhan; Dilip Mookherjee; Masatoshi Tsumagari

    2013-01-01

    We develop a theory of trading middlemen or entrepreneurs who perform financing, quality supervision and marketing roles for goods produced by suppliers or workers. Brand-name reputations are necessary to overcome product quality moral hazard problems; middlemen margins represent reputational incentive rents. We develop a two sector North-South model of competitive equilibrium, with endogenous sorting of agents with heterogenous entrepreneurial abilities into sectors and occupations. The Sout...

  15. Containment safety margins

    International Nuclear Information System (INIS)

    Von Riesemann, W.A.

    1980-01-01

    Objective of the Containment Safety Margins program is the development and verification of methodologies which are capable of reliably predicting the ultimate load-carrying capability of light water reactor containment structures under accident and severe environments. The program was initiated in June 1980 at Sandia and this paper addresses the first phase of the program which is essentially a planning effort. Brief comments are made about the second phase, which will involve testing of containment models

  16. Marginalized Youth. An Introduction.

    OpenAIRE

    Kessl, Fabian; Otto, Hans-Uwe

    2009-01-01

    The life conduct of marginalized groups has become subject to increasing levels of risk in advanced capitalist societies. In particular, children and young people are confronted with the harsh consequences of a “new poverty” in the contemporary era. The demographic complexion of today’s poverty is youthful, as a number of government reports have once again documented in recent years in Australia, Germany, France, Great Britain, the US or Scandinavian countries. Key youth studies have shown a ...

  17. The Seismicity of Two Hyperextended Margins

    Science.gov (United States)

    Redfield, Tim; Terje Osmundsen, Per

    2013-04-01

    A seismic belt marks the outermost edge of Scandinavia's proximal margin, inboard of and roughly parallel to the Taper Break. A similar near- to onshore seismic belt runs along its inner edge, roughly parallel to and outboard of the asymmetric, seaward-facing escarpment. The belts converge at both the northern and southern ends of Scandinavia, where crustal taper is sharp and the proximal margin is narrow. Very few seismic events have been recorded on the intervening, gently-tapering Trøndelag Platform. Norway's distribution of seismicity is systematically ordered with respect to 1) the structural templates of high-beta extension that shaped the thinning gradient during Late Jurassic or Early Cretaceous time, and 2) the topographically resurgent Cretaceous-Cenozoic "accommodation phase" family of escarpments that approximate the innermost limit of crustal thinning [See Redfield and Osmundsen (2012) for diagrams, definitions, discussion, and supporting citations.] Landwards from the belt of earthquake epicenters that mark the Taper Break the crust consistently thickens, and large fault arrays tend to sole out at mid crustal levels. Towards the sea the crystalline continental crust is hyperextended, pervasively faulted, and generally very thin. Also, faulting and serpentinization may have affected the uppermost parts of the distal margin's lithospheric mantle. Such contrasting structural conditions may generate a contrasting stiffness: for a given stress, more strain can be accommodated in the distal margin than in the less faulted proximal margin. By way of comparison, inboard of the Taper Break on the gently-tapered Trøndelag Platform, faulting was not penetrative. There, similar structural conditions prevail and proximal margin seismicity is negligible. Because stress concentration can occur where material properties undergo significant contrast, the necking zone may constitute a natural localization point for post-thinning phase earthquakes. In Scandinavia

  18. Lower-crustal xenoliths from Jurassic kimberlite diatremes, upper Michigan (USA): Evidence for Proterozoic orogenesis and plume magmatism in the lower crust of the southern Superior Province

    Science.gov (United States)

    Zartman, Robert E.; Kempton, Pamela D.; Paces, James B.; Downes, Hilary; Williams, Ian S.; Dobosi, Gábor; Futa, Kiyoto

    2013-01-01

    Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/€204Pb vs 207Pb/€204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/€204Pb vs 206Pb/€204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a

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

    Science.gov (United States)

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

    2014-12-01

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

  20. Study of southern CHAONAN sag lower continental slope basin deposition character in Northern South China Sea

    Science.gov (United States)

    Tang, Y.

    2009-12-01

    Northern South China Sea Margin locates in Eurasian plate,Indian-Australia plate,Pacific Plates.The South China Sea had underwent a complicated tectonic evolution in Cenozoic.During rifting,the continental shelf and slope forms a series of Cenozoic sedimentary basins,including Qiongdongnan basin,Pearl River Mouth basin,Taixinan basin.These basins fill in thick Cenozoic fluviolacustrine facies,transitional facies,marine facies,abyssal facies sediment,recording the evolution history of South China Sea Margin rifting and ocean basin extending.The studies of tectonics and deposition of depression in the Southern Chaonan Sag of lower continental slope in the Norther South China Sea were dealt with,based on the sequence stratigraphy and depositional facies interpretation of seismic profiles acquired by cruises of“China and Germany Joint Study on Marine Geosciences in the South China Sea”and“The formation,evolution and key issues of important resources in China marginal sea",and combining with ODP 1148 cole and LW33-1-1 well.The free-air gravity anomaly of the break up of the continental and ocean appears comparatively low negative anomaly traps which extended in EW,it is the reflection of passive margin gravitational effect.Bouguer gravity anomaly is comparatively low which is gradient zone extended NE-SW.Magnetic anomaly lies in Magnetic Quiet Zone at the Northern Continental Margin of the South China Sea.The Cenozoic sediments of lower continental slope in Southern Chaonan Sag can be divided into five stratum interface:SB5.5,SB10.5,SB16.5,SB23.8 and Hg,their ages are of Pliocene-Quaternary,late Miocene,middle Miocene,early Miocene,paleogene.The tectonic evolution of low continental slope depressions can be divided into rifting,rifting-depression transitional and depression stages,while their depositional environments change from river to shallow marine and abyssa1,which results in different topography in different stages.The topographic evolvement in the study

  1. Sporulation and ultrastructure in a late Proterozoic cyanophyte - Some implications for taxonomy and plant phylogeny

    Science.gov (United States)

    Cloud, P.; Moorman, M.; Pierce, D.

    1975-01-01

    Electron microscopical studies of a morphologically diverse, coccoid, presumably late Proterozoic blue-green alga are here reported. They show, together with light microscopy, that the form studied is widespread in the Cordilleran geosyncline, extend the record of well-defined endosporangia perhaps 700 million years into the past, and reveal previously unrecorded ultrastructural details. Coming from northeastern Utah, southwestern Alberta, and east central Alaska, these minute fossils belong to the recently described, morphologically diverse taxon Sphaerocongregus variabilis Moorman, are related to living entophysalidaceans, and have affinities with both the chroococcalean and chamaesiphonalean cyanophytes. Included in the morphological modes displayed by this alga are individual unicells, coenobial clusters of unicells, and a range of endosporangia comparable to those described for living entophysalidaceans. Scanning and transmission electron microscopy reveal that the endospores are commonly embedded in a vesicular matrix, that some of them show what appears to be a bilaminate or perhaps locally multilaminate wall structure, and that some remain together to mature as coenobial clones or 'colonies'. Taxonomic classification and phylogeny are discussed.

  2. Identifying high-grade uranium deposits in the Proterozoic basins of India- a challenge to exploration

    International Nuclear Information System (INIS)

    Mahadevan, T.M.

    1995-01-01

    The favorability factors that bestow on the Proterozoic formation of India, a potential to host high grade uranium deposits are discussed in the light of the known features of the new class of unconformity- related and strata bound uranium deposits. The need to reorient several past approaches is emphasised and it is suggested that future programmes must avail of the constraining benefits of a spectrum of geophysical, geochemical, and sedimentological studies in the choice of target areas for detailed exploration and development. A synthesis of geological and geochemical data with such geophysical features as magnetic and gravity anomalies, velocity structure, seismic reflectivity, electrical conductivity, and radioactivity can effectively lead to relatively more favourable exploration targets. Such efforts may lead to the generation of more than one model of the deep basinal features, which then provide wider options for drilling and proving of ore bodies. The alternative to the above approach is saturation drilling, which is a costly and time-consuming process and, therefore, very often self-defeating. (author). 28 refs., 2 figs., 2 tabs

  3. New paleomagnetic data from Siberia: Non-uniformitarian geomagnetic field around the Proterozoic-Phanerozoic boundary?

    Science.gov (United States)

    Pavlov, V.; Shatsillo, A.; Kouznetsov, N.; Gazieva, E.

    2017-12-01

    There is a range of evidence, mainly from sedimentary and volcanic rocks of the Laurentia and Baltica cratons, that argue for the anomalous character of the Ediacaran-Early Cambrian paleomagnetic record. This feature could be linked either to some peculiarities of the paleomagnetic record itself or to some unusual geophysical event that would have taken place around the Proterozoic-Phanerozoic boundary (e.g., true polar wander or nonuniformitarian geomagnetic field behavior). In the latter case, the traces of this event should be observed in Ediacaran-Early Cambrian rocks anywhere there is a possibility to observe a primary paleomagnetic signal. In previous work, we reported results that suggested an anomalous paleomagnetic record in Siberian Ediacaran-Lower Cambrian rocks. Here we present new Siberian data that indicate a very high geomagnetic reversal frequency during this period and the coexistence of two very different paleomagnetic directions. We speculate that these features could be due either to a near-equatorial geomagnetic dipole during the polarity transitions or to alternation between axial and near equatorial dipoles not directly linked with polarity reversals.

  4. Some aspects of the genesis of heavy mineral assemblages in Lower Proterozoic uranium-gold conglomerates

    International Nuclear Information System (INIS)

    Clemmey, H.

    1982-01-01

    Some genetic models for Lower Proterozoic gold- and uranium-bearing pyritic conglomerates favour a modified placer origin in which low levels of atmospheric oxygen are used to account for the survival of uraninite and pyrite. There are many difficulties with such models. New evidence concerning the genesis of the deposits is derived from a clast of ferric iron clay thought to represent a precursor sediment of the Witwatersrand Basin. Reworking of such clays and transport of a magnetite and ferric clay assemblage with subsequent sulphidation, could account for the porous pyrites, the absence of magnetite and the lack of hydraulic equivalence of the mineral grains in the conglomerates. The presence of oxygen, as indicated by the ferric iron clasts, would account for the evidence of mobility of uranium and of gold and would enhance their extraction from source rocks; particularly the release of gold from a precursor auriferous iron formation source. It is suggested that some aspects of the genesis of uranium deposits of the Witwatersrand and Elliott Lake may be similar to those of the Phanerozoic 'Roll Front' ores involving interaction between oxidizing uraniferous groundwaters and previously sulphidized and reduzate facies sediments. (author)

  5. Stratigraphy and uranium potential of early proterozoic metasedimentary rocks in the Medicine Bow Mountains, Wyoming

    International Nuclear Information System (INIS)

    Karlstrom, K.E.; Houston, R.S.

    1979-01-01

    The Medicine Bow Mountains of southeastern Wyoming contain an eight mile (13 km) thick section of Early Proterozoic (2500 to 1700 My b.p.) metasedimentary rocks which is subdivided into three successions: the Phantom Lake Metamorphic Suite (oldest), Deep Lake Group, and Libby Creek Group. The most promising units are the basal conglomerate of the upper Phantom Lake Suite, which appears to unconformably overlie metavolcanics of the lower Phantom Lake Suite, and the Magnolia Formation, which unconformably overlies the upper Phantom Lake Suite. Outcrops of the former have yielded assays of up to 141 ppM U and 916 ppM Th, with no appreciable gold. Outcrops of the Magnolia Formation have yielded up to 8.4 ppM U and 38 ppM Th. Several factors indicate that these units deserve further study. First, the lithologies of the radioactive and nonradioactive units are remarkably similar to those found in known uranium fossil-placers. Second, the paleogeography was favorable for placer accumulation if the conglomerates are fluvial sediments in an epicontinental clastic succession which was deposited during several transgressive-regressive cycles, as interpreted to be, Third, the age of the conglomerates may be similar to the age of other known uranium placers-i.e., more than 2000 My b.p. And fourth, geological and geochemical studies indicate that both uranium and pyrite have been strongly leached from outcrops and that subsurface rocks contain more uranium than surface rocks do

  6. Uranium in early proterozoic phosphate-rich metasedimentary rocks of east-central Minnesota

    International Nuclear Information System (INIS)

    McSwiggen, P.L.; Morey, G.B.; Weiblen, P.W.

    1986-01-01

    Exploration for unconformity-type uranium deposits in the late 1970s in east-central Minnesota led to the discovery of several uranium-bearing phosphorite occurrences in rocks of early Proterozoic age. In this report the authors use the term phosphorite for a rock or specimen that contains substantial sedimentary apatite (Altschuler et al., 1958). The deposits in Minnesota are especially interesting because of their high uranium content but low metamorphic grade. These occurrences characteristically contain 0.025 to 0.085 percent U and locally as much as 0.157 percent U (Ullmer, 1981), whereas typical primary marine phosphorites have uranium contents of 0.005 to 0.02 percent U (Altschuler et al., 1958). The presence of uranium in a marine phosphorite generally is explained by either the replacement of calcium in the apatite crystal structure or the adsorption of uranium in admixed organic matter and cryptocrystalline apatite. In east-central Minnesota the uranium is closely associated with the finely crystalline apatite, but the uranium has also been involved in several episodes of remobilization and redeposition. Thus, even though the phosphorite deposits are an interesting geologic phenomenon in themselves, they also are important as a possible source for epigenetic uranium deposits that may occur in the area

  7. Constraining the redox landscape of the mid-Proterozoic oceans: new insights from the carbonate uranium isotope record

    Science.gov (United States)

    Gilleaudeau, G. J.; Kaufman, A. J.; Luo, G.; Romaniello, S. J.; Zhang, F.; Kah, L. C.; Azmy, K.; Bartley, J. K.; Sahoo, S. K.; Knoll, A. H.; Anbar, A. D.

    2017-12-01

    The redox landscape of the global oceans during the prolonged period between the Great Oxidation Event (GOE) and the Neoproterozoic Oxygenation Event (NOE) is a topic of considerable debate. Data from local redox proxies such as iron speciation suggest largely ferruginous conditions in the subsurface oceans (with the exception of one report of oxic subsurface waters) and a variable degree of euxinia in shallow shelf and epeiric sea environments. There is general consensus that anoxia was more widespread than in the modern ocean, but quantifying the degree of seafloor anoxia is challenging given that most redox proxies are inherently local and/or based on the relatively sparse black shale record. Here, we present new uranium (U) isotope data from carbonate rocks than span the mid-Proterozoic Eon. U-isotopes operate as a proxy for seafloor anoxia because the δ238U value of seawater is largely controlled by the size of the anoxic/euxinic U sink, which preferentially removes isotopically heavy 238U, leaving the oceans enriched in 235U. Our compilation of data from mid-Proterozoic successions reveals δ238U values similar to modern seawater (-0.39 ± 0.19 ‰ [1 s.d.] for the Gaoyuzhuang, Angmaat, El Mreiti, Vazante, and Turukhansk successions spanning 1.5 to 0.9 Ga). Given the potential for an isotopic offset between carbonate minerals and seawater of up to 0.3 ‰, we suggest that mid-Proterozoic seawater had a δ238U value generally between -0.4 and -0.7 ‰, which is lower than modern seawater, but higher than has been inferred for intervals of expanded anoxia elsewhere in Earth history. These results are consistent with recently published U-isotope data from the 1.36 Ga Velkerri Formation, and suggest that large portions of the seafloor may have been covered by at least weakly oxygenated waters during the mid-Proterozoic Eon. Uncertainty remains, however, because the isotopic effects of the non-euxinic anoxic sink are poorly constrained. Nonetheless, our data

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

    Science.gov (United States)

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

    2014-05-01

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

  9. Marginal Models for Categorial Data

    NARCIS (Netherlands)

    Bergsma, W.P.; Rudas, T.

    2002-01-01

    Statistical models defined by imposing restrictions on marginal distributions of contingency tables have received considerable attention recently. This paper introduces a general definition of marginal log-linear parameters and describes conditions for a marginal log-linear parameter to be a smooth

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

    Science.gov (United States)

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

    2017-04-01

    Continental extension, subsequent rifting and eventual breakup result in the development of passive margins with transitional crust between extended continental crust and newly created oceanic crust. Globally, passive margins are typically classified as either magma-rich or magma-poor. Despite this simple classification, magma-poor margins like the West Orphan Basin, offshore Newfoundland, do exhibit some evidence of localized magmatism, as magmatism to some extent invariably accompanies all continental breakup. For example, on the Newfoundland margin, a small volcanic province has been interpreted near the termination of the Charlie Gibbs Fracture Zone, whereas on the conjugate Irish margin within the Rockall Basin, magmatism appears to be more widespread and has been documented both in the north and in the sout