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1

Continental margin off Western India and Deccan Large Igneous Province  

Microsoft Academic Search

Massive, transient late syn-rift-to-breakup volcanism during separation between the Seychelles microcontinent and India formed the Deccan continental flood basalts and their equivalents on the Seychelles-Mascarene Plateau and on the conjugate continental margins, i.e. the Deccan Large Igneous Province. We estimate an original extrusive area of at least 1.8×106 km2, and a volume >1.8×106 km3, and suggest a plate tectonic model comprising: (1) development

Axel Todal; Olav Edholm

1998-01-01

2

The Continental Margins of the Western North Atlantic.  

ERIC Educational Resources Information Center

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

Schlee, John S.; And Others

1979-01-01

3

Morphology of turbidite systems within an active continental margin (the Palomares Margin, western Mediterranean)  

NASA Astrophysics Data System (ADS)

The Palomares Margin, an NNE-SSW segment of the South Iberian Margin located between the Alboran and the Algerian-Balearic basins, is dissected by two major submarine canyon systems: the Gata (in the South) and the Alías-Almanzora (in the North). New swath bathymetry, side-scan sonar images, accompanied by 5 kHz and TOPAS subbottom profiles, allow us to recognize these canyons as Mediterranean examples of medium-sized turbidite systems developed in a tectonically active margin. The Gata Turbidite System is confined between residual basement seamounts and exhibits incised braided channels that feed a discrete deep-sea fan, which points to a dominantly coarse-grained turbiditic system. The Alías-Almanzora Turbidite System, larger and less confined, is a good example of nested turbiditic system within the canyon. Concentric sediment waves characterize the Alías-Almanzora deep-sea fan, and the size and acoustic character of these bedforms suggest a fine-grained turbidite system. Both canyons are deeply entrenched on a narrow continental shelf and terminate at the base of the continental slope as channels that feed deep sea fans. While the Alías-Almanzora Turbidite System is the offshore continuation of seasonal rivers, the Gata Turbidite System is exclusively formed by headward erosion along the continental slope. In both cases, left-lateral transpressive deformation influences their location, longitudinal profiles, incision at the upper sections, and canyon bending associated with specific fault segments.

Perez-Hernandez, S.; Comas, M. C.; Escutia, C.

2014-08-01

4

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

NASA Astrophysics Data System (ADS)

The Mesozoic tectonic history of the western U.S. Cordillera records evidence for multiple episodes of accretionary and collisional orogenic events and orogen-parallel strike-slip faulting. Paleozoic-Jurassic volcanic arc complexes and subduction zone assemblages extending from Mexico to Canada represent an East-Pacific magmatic arc system and an accretionary-type orogen evolved along the North American continental margin. Discontinuous exposures of Paleozoic upper mantle rocks and ophiolitic units structurally beneath this magmatic arc system are remnants of the Panthalassan oceanic lithosphere, which was consumed beneath the North American continent. Pieces of this subducted Panthalassan oceanic lithosphere that underwent high-P metamorphism are locally exposed in the Sierra Nevada foothills (e.g. Feather River Peridotite) indicating that they were subsequently (during the Jurassic) educted in an oblique convergent zone along the continental margin. This west-facing continental margin arc evolved in a broad graben system during much of the Jurassic as a result of extension in the upper plate, keeping pace with slab rollback of the east-dipping subduction zone. Lower to Middle Jurassic volcanoplutonic complexes underlain by an Upper Paleozoic-Lower Mesozoic polygenetic ophiolitic basement currently extend from Baja California-western Mexico through the Sierra-Klamath terranes to Stikinia-Intermontane Superterranes in Canada and represent an archipelago of an east-facing ensimatic arc terrane that developed west and outboard of the North American continental margin arc. The Smartville, Great Valley, and Coast Range ophiolites (S-GV-CR) in northern California are part of this ensimatic terrane and represent the island arc, arc basement, and back-arc tectonic settings, respectively. The oceanic Josephine-Rogue-Chetco-Rattlesnake-Hayfork tectonostratigraphic units in the Klamath Mountains constitute a west-facing island arc system in this ensimatic terrane as a counterpart of the east-facing S-GV-CR system to the south. The Guerrero intra-oceanic island arc system in Mexico was also part of the ensimatic arc terrane. Incorporation of this super arc terrane into the North American continent occurred diachronously along the irregular continental margin in the Middle Jurassic (in the north) through Early Cretaceous (in the south) during an arc-continent collision, marking a collisional orogenic episode in the North American Cordilleran history. Rifting of this accreted arc in the Late Jurassic (155-148 Ma) might have resulted from a sinistral transtensional deformation associated with the rapid NW motion of North America. Magmas generated during this rifting event probably migrated through the accreted arc crust and the continental margin units in the tectonic lower plate. The Franciscan subduction zone dipping eastwards beneath the continent was established in the latest Jurassic, following the collisional event and restoring the North American Cordillera back into an accretionary-type, Andean-style orogen. Different episodes of orogen-parallel intra-continental strike-slip faulting facilitated lateral dispersion of accreted terranes and continental margin units during the Early Cretaceous and transpressional deformation and batholithic magmatism in the Sierra Nevada magmatic arc in the Late Cretaceous. A Jurassic-Cretaceous island arc system (Wrangellia-Insular Superterrane) that had developed west of the Jurassic archipelago collapsed into the edge of North America during Late Cretaceous-Tertiary time and underwent northward lateral translation along the continental margin. These observations and interpretations have strong implications for the tectonic evolution of Central America and the Caribbean region.

Dilek, Y.

2001-12-01

5

An integrated approach for determination of nature of the crust in the Laxmi Basin, western continental margin of India  

Microsoft Academic Search

The nature of the crust in the Laxmi Basin, northwest continental margin of India is an uncertain issue and more importantly this is limiting our understanding of the evolution of the lithosphere in the Arabian Sea. The aspect has become a key issue principally from the point of paleogeographic reconstructions of the western Indian Ocean. In order to determine the

K. S. Krishna; D. Gopala Rao; D. Sar

2004-01-01

6

Evolution of rifted continental margins: the case of the Gulf of Lions (Western Mediterranean Basin)  

NASA Astrophysics Data System (ADS)

The formation of rifted continental margins has long been explained by numerous physical models. However, field observations are still lacking to validate or constrain these models. This study presents major new observations on the broad continental margin of the Gulf of Lions, based on a large amount of varied data. Two contrasting regions characterize the thinned continental crust of this margin. One of these regions corresponds to a narrow rift zone (40-50 km wide) that was highly thinned and stretched during rifting. In contrast with this domain, a large part of the margin subsided slowly during rifting and then rapidly after rifting. The thinning of this domain cannot be explained by stretching of the upper crust. We can thus recognize a zonation of the stretching in both time and space. In addition, the Provencal Basin is characterized by a segmentation of the order of 100-150 km. These observations have important consequences on the formation and evolution of the Gulf of Lions margin. Independently of the geodynamic context, we can propose some general features that characterize the formation of rifted continental margins.

Bache, François; Olivet, Jean Louis; Gorini, Christian; Aslanian, Daniel; Labails, Cinthia; Rabineau, Marina

2010-05-01

7

Deep continental margin reflectors  

USGS Publications Warehouse

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.

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

1985-01-01

8

Heat flow variation from bottom simulating reflector in the Kerala-Konkan basin of the western continental margin of India  

Microsoft Academic Search

The base of the gas-hydrate stability field, representing the bottom simulating reflector or BSR, is observed over a closely spaced grid of multichannel seismic data in the Kerala-Konkan (KK) basin of the western continental margin of India (WCMI). The data reveal that gas-hydrates occur in the KK basin at places where water depth exceeds 1500 m. The thickness of the

Uma Shankar; Kalachand Sain

9

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

10

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

SciTech Connect

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

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

1986-07-01

11

Characterizing and identifying structural domains at rifted continental margins: application to the Bay of Biscay margins and its Western Pyrenean fossil remnants  

NASA Astrophysics Data System (ADS)

Over the past decade, the occurrence of hyperextended domains at rifted continental margins consisting of extremely thinned crust and/or exhumed mantle has been increasingly recognized both at present-day rifted margins and in fossils analogues preserved in collisional orogens. However, at present, most studies aiming to characterize rifted continental margin structure and the extreme thinning of the continental crust were either focused offshore relying on indirect geophysical methods, or onshore in deformed remnants offering direct access to geological observations. Marine and onshore examples provide complementary datasets, but their different scale and resolution of observations prevent direct correlations to be done. We use the Bay of Biscay and Western Pyrenees as a natural laboratory to develop and apply an innovative approach to characterize and identify distinctive rifted margin domains in offshore and onshore settings. The Bay of Biscay and Western Pyrenees offer the unique possibility to have access to seismically imaged, drilled and exposed parts of one and the same hyperextended rift margin system. Offshore, we use a gravity inversion technique and flexural backstripping combined with seismic interpretation to provide quantitative estimates of accommodation space, crustal thickness and lithosphere thinning. Onshore, we focus on key outcrops preserving remnants of the former rift domain to describe the nature of sediment and basement rocks and of their interface. This qualitative and quantitative characterisation provides the essential diagnostic elements for the identification of five distinct domains at magma-poor rifted margins and their fossil analogues. We name these 5 domains proximal, necking, hyperthinned, exhumed mantle and oceanic. This new approach can be used to reconcile offshore and onshore observations and aid interpretation especially when only local observations are available. Onshore remnants can be placed in an offshore rifted margin context, enabling the prediction of first order crustal architecture. For the interpretation of offshore seismic reflection sections, geological insights on rift structures and basement nature can be suggested based on onshore analogies. This combined onshore-offshore multidisciplinary approach enables us to identify and distinguish the distinct structural domains of rifted margins, resulting in a new paleogeographic map of the Bay of Biscay and Pyrenean rift. The approach underlying this mapping has general application to unravelling the spatial and temporal complexity of rifted margin structural domains.

Tugend, J.; Manatschal, G.; Kusznir, N. J.; Masini, E.

2013-12-01

12

The Brazilian continental margin  

NASA Astrophysics Data System (ADS)

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

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

1981-04-01

13

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

14

Major Ocean Features: Continental Margin  

E-print Network

41 Major Ocean Features: Continental Margin and Deep Sea This introduction summarizes ocean geology's continental shelf is a passive margin so the Ocean Exploration Hudson Canyon, Deep East and Islands as it relates to the Ocean Exploration expeditions. It is not a comprehensive examination of all aspects

15

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing - an overview and synthesis  

NASA Astrophysics Data System (ADS)

In this special issue we examine the biogeochemical conditions and marine ecosystems in the major marginal seas of the western North Pacific Ocean, namely, the East China Sea, the Japan/East Sea to its north and the South China Sea to its south. They are all subject to strong climate forcing as well as anthropogenic impacts. On the one hand, continental margins in this region are bordered by the world's most densely populated coastal communities and receive tremendous amount of land-derived materials. On the other hand, the Kuroshio, the strong western boundary current of the North Pacific Ocean, which is modulated by climate oscillation, exerts strong influences over all three marginal seas. Because these continental margins sustain arguably some of the most productive marine ecosystems in the world, changes in these stressed ecosystems may threaten the livelihood of a large population of humans. This special issue reports the latest observations of the biogeochemical conditions and ecosystem functions in the three marginal seas. The studies exemplify the many faceted ecosystem functions and biogeochemical expressions, but they reveal only a few long-term trends mainly due to lack of sufficiently long records of well-designed observations. It is critical to develop and sustain time series observations in order to detect biogeochemical changes and ecosystem responses in continental margins and to attribute the causes for better management of the environment and resources in these marginal seas.

Liu, K.-K.; Kang, C.-K.; Kobari, T.; Liu, H.; Rabouille, C.; Fennel, K.

2014-12-01

16

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing - an overview and synthesis  

NASA Astrophysics Data System (ADS)

In this special issue we examine the biogeochemical conditions and marine ecosystems in the major marginal seas of the western North Pacific Ocean, namely, the East China Sea, the Japan/East Sea to its north and the South China Sea to its south. They are all subject to strong climate forcing as well as anthropogenic impacts. On the one hand, continental margins in this region are bordered by the world's most densely populated coastal communities and receive tremendous amounts of land derived materials. On the other hand, the Kuroshio, the strong western boundary current, which is modulated by climate oscillation, exerts strong influences over all three marginal seas. Because these continental margins sustain arguably the most productive marine ecosystems, changes in these stressed ecosystems may threaten the livelihood of a large human population. This special issue reports the latest observations of the biogeochemical conditions and ecosystem functions in the three marginal seas. The studies exemplify many faceted ecosystem functions and biogeochemical expressions, but they reveal only a few long term trends mainly due to lack of long term records. It is critical to develop and sustain time series observations in order to detect biogeochemical changes and ecosystem responses in continental margins and to attribute the causes for better management of the environment and resources in these marginal seas.

Liu, K.-K.; Kang, C.-K.; Kobari, T.; Liu, H.; Rabouille, C.; Fennel, K.

2014-07-01

17

Seismic velocities and quantification of gas-hydrates from AVA modeling in the western continental margin of India  

NASA Astrophysics Data System (ADS)

The most commonly used marker for the investigation of gas-hydrates is the bottom simulating reflector (BSR), which is caused by gas-hydrate laden sediment underlain by either brine or gas-saturated sediment. A BSR has been identified by seismic experiment in the Kerala-Konkan Basin of the western continental margin of India. Here we perform AVA modeling of seismic reflection data from a BSR to investigate the seismic velocities for quantitative assessment of gas-hydrates and to understand the origin of the BSR. The result reveals a P-wave velocity of 2.245 km/s and an S-wave velocity of 0.895 km/s for the sediments above the BSR. This corresponds to a Poisson ratio of 0.406 and hydrates saturation of ˜30% in the study area. The comparison of estimated P-wave velocity (1.77 km/s) above the hydrates-bearing sediment to that (1.78 km/s) below the BSR implies that the origin of the BSR is mainly due to gas-hydrates, as the presence (even in small quantities) of free-gas reduces the P-wave velocity considerably.

Ojha, Maheswar; Sain, Kalachand

2007-06-01

18

Comparison of deep structure along three transects of the western North American continental margin  

USGS Publications Warehouse

Similarities in geology and potential field data that have in the past been noted among the regions of southern Alaska, southern Vancouver Island, and central California are now seen to be accompanied by similarities in deep crustal structure. A number of tectonic elements have been identified in the deep structure along transects in these three regions, although not all elements are present along each transect. These elements are A) an actively subducting oceanic plate and B) an overriding continental plate that consists of 1) a Cenozoic accretionary prism, 2) a Mesozoic accretionary prism, 3) a backstop to the Mesozoic prism, 4) a tectonically underplated body of oceanic rocks, and 5) a crustal root. -from Authors

Fuis, G.S.; Clowes, R.M.

1993-01-01

19

The northern Egyptian continental margin  

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

20

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

21

East Africa continental margins  

SciTech Connect

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

Bosellini, A.

1986-01-01

22

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

USGS Publications Warehouse

The Ebro continental margin sedimentation system originated with a Messinian fluvial system. This system eroded both a major subaerial canyon cutting the margin southeastward from the present Ebro Delta and an axial valley that drained northeastward down Valencia Trough. Post-Messinian submergence of this topography and the Pliocene regime of high sea levels resulted in a marine hemipelagic drape over the margin. Late Pliocene to Pleistocene glacial climatic cycles, drainagebasin deforestation, and sea-level lowstands combined to increase sediment supply, cause the margin to prograde, and create a regime of lowstand sediment-gravity flows in the deeper margin. The depositional patterns of regressive, transgressive and highstand sea-level regimes suggest that location of the sediment source near the present Ebro Delta throughout the late Cenozoic, southward current advection of sediment, and greater subsidence in the southern margin combined to cause generally asymmetric progradation of the margin to the southeast. Thicker, less stable deposits filling the Messinian subaerial canyon underwent multiple retrograde failures, eroded wide gullied canyons and formed unchanneled base-of-slope sediment aprons in the central margin area; other margin areas to the north and south developed a series of channel-levee complexes. On the basin floor, the formation of Valencia Valley over the Messinian subaerial valley and earlier faults led to draining of about 20% of the Ebro Pleistocene sediment from channel-levee complexes through the valley to prograde Valencia Fan as much as 500 km northeast of the margin. Thus, the Ebro margin has two growth directions, mainly southeastward during higher sea levels, and eastward to northeastward during lower sea levels. The northeastward draining of turbidity currents has produced unusually thin and widely dispersed turbidite systems compared to those on ponded basin floors. During the past few centuries, man's impact has exceeded natural controls on Ebro margin growth. Deforestation of the drainage basin more than doubled the normal Holocene sediment supply, and construction of dams then reduced the supply by 95%. This reduction of the past 50 years has caused erosion of the delta and contamination of bottom sediment because normal Holocene sediment discharge is not available to prograde the delta or help dilute pollutants. ?? 1990.

Nelson, C.H.; Maldonado, A.

1990-01-01

23

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

NASA Astrophysics Data System (ADS)

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.

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

2011-02-01

24

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

NASA Astrophysics Data System (ADS)

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.

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

2014-12-01

25

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

USGS Publications Warehouse

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

Karl, Herman A.

1989-01-01

26

Significance of indigenous Eocene larger foraminifera Discocyclina dispansa in Western Foothills, Central Taiwan: A Paleogene marine rift basin in Chinese continental margin  

NASA Astrophysics Data System (ADS)

The Western Foothills of Taiwan was known to be composed of Late Oligocene to Pleistocene shallow marine strata continuously deposited on the stable passive Chinese continental margin without significant stratigraphic break. Here we present multiple micropaleontological evidences, including occurrence of larger foraminifera Discocyclina dispansa ex. interc. sella-dispansa and calcareous nannoplanktons, to show that there are Middle Eocene marine strata (first named as the Chungliao Formation) exposed in the Tsukeng anticline of the Western Foothills, central Taiwan. Occurrences of intact tests with thin delicate outer rims and well-preserved embryonic chambers suggest that the Discocyclina dispansa ex. interc. sella-dispansa (Lutetian to Bartonian in the Tethys region) are buried indigenously on shallow inner shelf during an episodic transgression in the Early Middle Eocene. The conclusion is consistent with a biostratigraphy study of calcareous nannoplanktons (Zones NP14-15) in the shale/sandstone alternations overlying the Discocyclina-bearing bed of the Chungliao Formation and calcareous nannofossils of Zone NP16 integrated with an age dating of 38.8 ± 1 Ma (Late Middle Eocene) on zircon grains of the overlying Pinglin Tuff. The Middle Eocene syn-rift sequences (Chungliao Formation and Pinglin Tuff) exposed along the Tsukeng anticline are unconformably covered by the latest Oligocene-Miocene post-rift sequence, a scenario similar to what have been drilled in the East China Sea-Taiwan Strait-South China Sea. This rift basin (named as the Nantou Basin) is sitting on the Peikang Basement High margin which further extends southwestward to the Central Uplift of the Pearl River Mouth Basin in the northern slope of the South China Sea. The present work documents a hitherto unknown occurrence of the exposed early Tertiary marine rift basin sequence in the Western Foothills of Taiwan. The study extends our knowledge of the Western Foothills geohistory from the Late Oligocene downward to the Early Middle Eocene. The occurrence of the Paleogene Nantou rift basin in the Western Foothills may also suggest that there could have similar Paleogene rift sequences exposed in other parts of the Taiwan mountain belt like the Hsüehshan Range and the Central Range east of the Western Foothills.

Huang, Chi-Yue; Yen, Yi; Liew, Ping-Mei; He, Dai-Jie; Chi, Wen-Ron; Wu, Min-Shyan; Zhao, Meixun

2013-01-01

27

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

Microsoft Academic Search

Over the next decades, the shallow parts of continental margins in the Arctic are likely to experience warming of bottom-water. It is, therefore, important to evaluate how methane hydrate beneath the seabed in these margins will react to future increases in bottom-water temperature and whether release of methane from hydrate will have an impact on climate. As part of the

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

2009-01-01

28

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

E-print Network

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

New Hampshire, University of

29

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

Microsoft Academic Search

High-resolution multi-channel seismic data from continental slopes with minor sediment input off southwest Mallorca Island,\\u000a the Bay of Oran (Algeria) and the Alboran Ridge reveal evidence that the Messinian erosional surface is terraced at an almost\\u000a 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\\u000a terraces were eroded contemporaneously

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

2011-01-01

30

The basins on the Argentine continental margin  

SciTech Connect

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

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

1996-08-01

31

Age and geochemical characteristics of Paleogene basalts drilled from western Taiwan: Records of initial rifting at the southeastern Eurasian continental margin  

NASA Astrophysics Data System (ADS)

The southeastern Eurasian continental margin has been characterized by formation of rift basins associated with intraplate basaltic volcanism since early Cenozoic time. In contrast to Paleogene volcanic rocks that occur sporadically in the basins, Neogene basalts are more widespread on land as lava flows and pyroclastics in the Taiwan Strait (Penghu Islands) and northwestern Taiwan. To better understand the tectonomagmatic evolution, in particular the initial rifting record, this study reports new age, major- and trace-elemental, and Sr-Nd-Pb isotope data of volcanic rocks drilled from several locations in the Taiwan Strait and western Taiwan. 40Ar/39Ar dating results show two main episodes of volcanic activities: ~56-38 Ma (Eocene) and ~11-8 Ma (late Miocene). The volcanic rocks are composed dominantly of basalts and basaltic andesites, and subordinately of dacites and rhyolites of Eocene age. The two episodes of basaltic volcanism have distinct geochemical characteristics. Comparatively, the Eocene basalts are more depleted in basaltic components such as Ca, Fe and Ti, but have higher Al content. They are also more enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and show depletions in high field strength elements (HFSE). Sr-Nd-Pb isotope compositions of the late Miocene basalts are relatively more uniform and unradiogenic (?Nd = +6.0 to +3.8), similar to those of Miocene basalts from NW Taiwan and Penghu Islands, and broadly coeval OIB-type basalts from the South China Sea. However, the Eocene basalts have a wider range in isotope ratios (e.g., ?Nd(T) = +5.6 to -3.2) pointing towards an enriched mantle source. The overall geochemical characteristics suggest two distinct mantle sources: (1) a more refractory mantle source metasomatized by subduction-related processes to generate the Eocene basalts and (2) a fertile but isotopically depleted mantle source for the late Miocene basalts. These two source components are proposed to reside in the lithospheric mantle and asthenosphere, respectively. The change in magma sources with time reflects the evolution of an extensional regime within the Eurasian continental margin from an initial rifting to a well-established stage accomplished by thinning of the lithosphere and associated upwelling of the asthenosphere. The Eocene bimodal volcanism entails a transition from the latest Cretaceous magmatism in the western Taiwan Strait that not only signals incipient rifting in the region, but also records geochemical inputs from the subducted Paleo-Pacific plate to the southeastern Eurasian lithospheric mantle. As the preexisting, subduction-related component had been preferentially overprinted by the Eocene magma generation, there was a magmatic quiescence in the Oligocene before the onset of Miocene basaltic volcanism that resulted essentially from decompression melting of the ascended asthenospheric mantle.

Wang, K.; Chung, S.; Lo, Y.; Lo, C.; Yang, H.; Shinjo, R.; Lee, T.; Wu, J.; Huang, S.

2013-12-01

32

Age and geochemical characteristics of Paleogene basalts drilled from western Taiwan: Records of initial rifting at the southeastern Eurasian continental margin  

NASA Astrophysics Data System (ADS)

The southeastern Eurasian continental margin has been characterized by formation of rift basins associated with intraplate basaltic volcanism since early Cenozoic time. In contrast to Paleogene volcanic rocks that occur sporadically in the basins, Neogene basalts are more widespread on land as lava flows and pyroclastics in the Taiwan Strait (Penghu Islands) and northwestern Taiwan. To better understand the tectonomagmatic evolution, in particular the initial rifting record, this study reports new age, major- and trace-elemental, and Sr-Nd-Pb isotope data of volcanic rocks drilled from several locations in the Taiwan Strait and western Taiwan. 40Ar/39Ar dating results show two main episodes of volcanic activities: ~ 56-38 Ma (Eocene) and ~ 11-8 Ma (late Miocene). The volcanic rocks are composed dominantly of basalts and basaltic andesites, and subordinately of dacites and rhyolites of Eocene age. The two episodes of basaltic volcanism have distinct geochemical characteristics. Comparatively, the Eocene basalts are more depleted in basaltic components such as Ca, Fe and Ti, but have higher Al content. They are also more enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and show depletions in high field strength elements (HFSE). Sr-Nd-Pb isotope compositions of the late Miocene basalts are relatively more uniform and unradiogenic (?Nd = + 6.0 to + 3.8), similar to those of Miocene basalts from NW Taiwan and Penghu Islands, and broadly coeval OIB-type basalts from the South China Sea. However, the Eocene basalts have a wider range in isotope ratios (e.g., ?Nd(T) = + 5.6 to -3.2) pointing towards an enriched mantle source. The overall geochemical characteristics suggest two distinct mantle sources: (1) a more refractory mantle source metasomatized by subduction-related processes to generate the Eocene basalts and (2) a fertile but isotopically depleted mantle source for the late Miocene basalts. These two source components are proposed to reside in the lithospheric mantle and asthenosphere, respectively. The change in magma sources with time reflects the evolution of an extensional regime within the Eurasian continental margin from an initial rifting to a well-established stage accomplished by thinning of the lithosphere and associated upwelling of the asthenosphere. The Eocene bimodal volcanism entails a transition from the latest Cretaceous magmatism in the western Taiwan Strait that not only signals incipient rifting in the region, but also records geochemical inputs from the subducted Paleo-Pacific plate to the southeastern Eurasian lithospheric mantle. As the preexisting, subduction-related component had been preferentially overprinted by the Eocene magma generation, there was a magmatic quiescence in the Oligocene before the onset of Miocene basaltic volcanism that resulted essentially from decompression melting of the ascended asthenospheric mantle.

Wang, Kuo-Lung; Chung, Sun-Lin; Lo, Yi-Ming; Lo, Ching-Hua; Yang, Huai-Jen; Shinjo, Ryuichi; Lee, Tung-Yi; Wu, Jong-Chang; Huang, Shiuh-Tsann

2012-12-01

33

Continental margin tectonics - Forearc processes  

SciTech Connect

Recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of U.S. research from the period 1987-1990. Topics addressed include the geometry of accretionary prisms (Coulomb wedge taper and vertical motion in response to tectonic processes), offscraping vs underplating or subduction, the response to oblique convergence, fluids in forearc settings, the thermal framework and the effects of fluid advection, and serpentinite seamounts. Also included is a comprehensive bibliography for the period.

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

1991-01-01

34

Dynamics of the continental margins  

SciTech Connect

On 18--20 June 1990, over 70 oceanographers conducting research in the ocean margins of North America attended a workshop in Virginia Beach, Virginia. The purpose of the workshop was to provide the Department of Energy with recommendations for future research on the exchange of energy-related materials between the coastal and interior ocean and the relationship between the ocean margins and global change. The workshop was designed to optimize the interaction of scientists from specific research disciplines (biology, chemistry, physics and geology) as they developed hypotheses, research questions and topics and implementation plans. The participants were given few restraints on the research they proposed other than realistic time and monetary limits. The interdisciplinary structure of the meeting promoted lively discussion and creative research plans. The meeting was divided into four working groups based on lateral, vertical, air/sea and sediment/water processes. Working papers were prepared and distributed before the meeting. During the meeting the groups revised the papers and added recommendations that appear in this report, which was reviewed by an Executive Committee.

Not Available

1990-11-01

35

The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview  

E-print Network

The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview MICHEL St study for stretching models of 'Atlantic-type' margins. However, when the Inte- grated Basin Study (IBS and the geodynamic setting of the margin within the Western Mediterranean. IBS-Gulf of Lion research was based

Demouchy, Sylvie

36

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

ERIC Educational Resources Information Center

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

Poli, Maria-Serena; Capodivacca, Marco

2011-01-01

37

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

Microsoft Academic Search

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

Jean Mascle; Emmanuel Blarez

1987-01-01

38

Global distribution of naturally occurring marine hypoxia on continental margins  

NASA Astrophysics Data System (ADS)

Hypoxia in the ocean influences biogeochemical cycling of elements, the distribution of marine species and the economic well being of many coastal countries. Previous delineations of hypoxic environments focus on those in enclosed seas where hypoxia may be exacerbated by anthropogenically induced eutrophication. Permanently hypoxic water masses in the open ocean, referred to as oxygen minimum zones, impinge on a much larger seafloor surface area along continental margins of the eastern Pacific, Indian and western Atlantic Oceans. We provide the first global quantification of naturally hypoxic continental margin floor by determining upper and lower oxygen minimum zone depth boundaries from hydrographic data and computing the area between the isobaths using seafloor topography. This approach reveals that there are over one million km 2 of permanently hypoxic shelf and bathyal sea floor, where dissolved oxygen is <0.5 ml l -1; over half (59%) occurs in the northern Indian Ocean. We also document strong variation in the intensity, vertical position and thickness of the OMZ as a function of latitude in the eastern Pacific Ocean and as a function of longitude in the northern Indian Ocean. Seafloor OMZs are regions of low biodiversity and are inhospitable to most commercially valuable marine resources, but support a fascinating array of protozoan and metazoan adaptations to hypoxic conditions.

Helly, John J.; Levin, Lisa A.

2004-09-01

39

Understanding continental margin biodiversity: a new imperative.  

PubMed

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

Levin, Lisa A; Sibuet, Myriam

2012-01-01

40

Basement inheritance controls on continental margin architecture: steep basement fabrics and segmented margins  

NASA Astrophysics Data System (ADS)

The extent to which continental rifting and margin development is influenced by pre-existing anisotropies has been debated since Tuzo Wilson’s seminal 1966 paper on Atlantic opening and closing. Using digital fieldwork methodologies to acquire and analyse structural datasets, our investigation of the role of pre-existing structures on the development of basins and margins shows that, in general, steep belts seem to control inheritance during rifting. In contrast, gently inclined surfaces such as bedding and unconformities are more likely to be reactivated during crustal shortening,. In detail, the relative strain intensity, mineralogy, fabric dip and strike, and depth of reactivation are all likely to be important controls. Greenland’s margins provide an excellent example of how changes in margin trend and geometry commonly coincide with changes in basement structure. Thus, intra-cratonic rifting and break-up between Greenland and Labrador shows strong evidence for basement inheritance, with relatively simple orthogonal margins in the Archaean North Atlantic Craton, and complex oblique-margin segments in Proterozoic shear belts (i.e. Ketilidian, Nagssugtoqidian). Using a combination of structural geological mapping, geomorphology and new AFTA data, we have constructed a model for the tectonic evolution and influence of pre-existing structures on the development of this margin. The Nagssugtoquidian belt of central West Greenland is cut by at least two phases brittle faulting (early extension, late strike-slip), both of which are partitioned by steep discreet shear zones (Norder Stromfjord and Norder Isotoq). These shear zones may also be traced offshore where they also segment basin/rift patterns. A distinct contrast in fault style and geometry can be seen in steep belts and intervening fold belts, leading the interpretation that brittle fault patterns are partitioned in to domains of “basement influence” (steep belts) and “non-basement influence” (fold belts). In South Greenland, topography, gravity and aeromagnetic maps for the Ketilidian all show coincident changes, with particularly noticeable high elevations developed in the east and a distinct left lateral step on the western margin. Kinematic studies of the SW margin reveal a consistent kinematic story of NE-SW extension dominated by dextral shear and reactivation of steep Kelitidian fabrics. AFTA data provide an age constraint on footwall uplift of these faults which is comparable in age to the offshore faulting. In contrast, the SE margin appears to be unaffected by the sub-horizontal Ketilidian basement fabrics that predominate in the east. We suggest that both these examples emphasise the importance of steep basement shear zones and fabrics in continental rifting.

Wilson, R.; McCaffrey, K. J.; Holdsworth, R. E.; Japsen, P.

2009-12-01

41

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

E-print Network

-pressure Western Gneiss Region, Norway E. O. WALSH* AND B. R. HACKER Department of Geological Sciences, University of California, Santa Barbara, CA 93106-9630, USA (walshe@union.edu) ABSTRACT Thermobarometry suggests that ultrahigh-pressure (UHP) to high-pressure (HP) rocks across the Western Gneiss Region ponded at the Moho

Hacker, Bradley R.

42

Geotechnical characterization of sediments from Hydrate Ridge, Cascadia Continental Margin  

E-print Network

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

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

2004-01-01

43

Fluid Flow in Sediments along the Costa Rican Continental Margin  

Microsoft Academic Search

During expeditions with RVs Sonne and Meteor in 2002 along the active continental margin off Costa Rica a large number of mound-shaped structures was identified on the upper continental slope. Most of them are a few hundred metres in diameter with only low elevation above the surrounding seafloor. Preliminary sedimentological analyses of a number of gravity cores revealed that most

C. Hensen; K. Wallmann; S. Mau; U. Schacht; A. Deyhle; M. Schmidt; D. Garbe-Schönberg; E. Suess

2003-01-01

44

Continental margin sedimentation: from sediment transport to sequence stratigraphy  

USGS Publications Warehouse

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

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

2007-01-01

45

Ecological theory and continental margins: where shallow meets deep  

Microsoft Academic Search

Continental margins, where land becomes ocean and plunges to the deep sea, provide valuable food and energy resources, and perform essential functions such as carbon burial and nutrient cycling. They exhibit remarkably high species and habitat diversity, but this is threatened by our increasing reliance on the resources that margins provide, and by warming, expanding hypoxia and acidification associated with

Lisa A. Levin; Paul K. Dayton

2009-01-01

46

Comparative biogeochemistry-ecosystem-human interactions on dynamic continental margins  

USGS Publications Warehouse

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

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

2014-01-01

47

Comparative biogeochemistry-ecosystem-human interactions on dynamic continental margins  

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

48

Subduction-driven recycling of continental margin lithosphere.  

PubMed

Whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, the recycling of continental lithosphere appears to be far more complicated and less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings. We have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted Atlantic slab under northeastern South America and the Alboran slab beneath the Gibraltar arc region; the anomalies are under, and are aligned with, the continental margins at depths greater than 200 kilometres. Rayleigh wave analysis finds that the lithospheric mantle under the continental margins is significantly thinner than expected, and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores. Taking these data together, here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone. Subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins. This drives surface tectonics and pre-conditions the margins for further deformation by creating topography along the lithosphere-asthenosphere boundary. This can lead to development of secondary downwellings under the continental interior, probably under both South America and the Gibraltar arc, and to delamination of the entire lithospheric mantle, as around the Gibraltar arc. This process reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these subduction zones. PMID:25391963

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

2014-11-13

49

Subduction-driven recycling of continental margin lithosphere  

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

50

Stratigraphic turnover on west Africa margin page 1 Early Oligocene stratigraphic turnover on west Africa continental margin: a signature of  

E-print Network

Stratigraphic turnover on west Africa margin page 1 Early Oligocene stratigraphic turnover on west and evolution of the west African margin: The continental margin of west Africa resulted from the Neocomian profiles across #12;Stratigraphic turnover on west Africa margin page 2 the west African continental margin

Demouchy, Sylvie

51

Continental transform margins : state of art and future milestones  

NASA Astrophysics Data System (ADS)

Transform faults were defined 45 years ago as ‘a new class of fault' (Wilson, 1965), and transform margins were consequently individualized as a new class of continental margins. While transform margins represent 20 to 25 % of the total length of continent-ocean transitions, they were poorly studied, especially when compared with the amount of data, interpretations, models and conceptual progress accumulated on divergent or convergent continental margins. The best studied examples of transform margins are located in the northern part of Norway, south of South Africa, in the gulf of California and on both sides of the Equatorial Atlantic. Here is located the Côte d'Ivoire - Ghana margin, where the more complete data set was acquired, based on numerous geological and geophysical cruises, including ODP Leg 159. The first models that encompassed the structure and evolution of transform margins were mainly driven by plate kinematic reconstructions, and evidenced the diachronic end of tectonic activity and the non-cylindrical character of these margins, with a decreasing strike-slip deformation from the convex to the concave divergent-transform intersections. Further thermo-mechanical models were more specifically designed to explain the vertical displacements along transform margins, and especially the occurrence of high-standing marginal ridges. These thermo-mechanical models involved either heat transfer from oceanic to continental lithospheres across the transform faults or tectonically- or gravity-driven mass transfer in the upper crust. These models were far from fully fit observations, and were frequently dedicated to specific example, and not easily generalizable. Future work on transform continental margins may be expected to fill some scientific gaps, and the definition of working directions can benefit from the studies dedicated to other types of margins. At regional scale the structural and sedimentological variability of transform continental margins has to be emphasized. There is not only one type of transform margins, but as for divergent margins huge changes from one margin to another in both structure and evolution. Multiple types have to be evidenced together with the various parameters that should control the variability. As for divergent margins, special attention should be paid to conjugated transform margins as a tool to assess symmetrical / asymmetrical processes in the oceanic opening. Attention should also be focused on the three-dimensional structure of the intersections between transform and divergent margins, such as the one where the giant oil field Jubilee was recently discovered. There is almost no 3D data available in these area, and their structures still have to be described. An other key point to develop is the mechanical behavior of the lithosphere in and in the vicinity of transform margins. The classical behaviors (isostasy, elastic flexure) have be tested extensively. The localization of the deformation by the transform fault, and the coupling of continental and oceanic lithosphere across the transform fault have to be adressed to understand the evolution of these margins. Again as for divergent margins, new concepts are needed to explain the variations in the post-rift and post-transform subsidence, that can not always be explained by classical subsidence models. But the most remarkable advance in our understanding of transform margins may be related to the study of interactions between the lithosphere and adjacent envelops : deep interactions with the mantle, as underplating, tectonic erosion, or possible lateral crustal flow ; surficial interactions between structural evolution, erosion and sedimentation processes in transform margins may affect the topography and bathymetry, thus the oceanic circulation with possible effects on regional and global climate.

Basile, Christophe

2010-05-01

52

Structure of the North American Atlantic Continental Margin.  

USGS Publications Warehouse

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

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

1986-01-01

53

Antarctic Drilling Recovers Stratigraphic Records From the Continental Margin  

NASA Astrophysics Data System (ADS)

The Antarctic Geological Drilling (ANDRILL) program—a collaboration between Germany, Italy, New Zealand, and the United States that is one of the larger programs endorsed by the International Polar Year (IPY; http://www.ipy.org)-successfully completed the drilling phase of the Southern McMurdo Sound (SMS) Project in December 2007. This second drill core of the program's campaign in the western Ross Sea, Antarctica, complements the results of the first drilling season [Naish et al., 2007] by penetrating deeper into the stratigraphic section in the Victoria Land Basin and extending the recovered time interval back to approximately 20 million years ago. The primary objectives of ANDRILL (http://www.andrill.org/) were to recover stratigraphic records from the Antarctic continental margin that document key steps in Antarctica’s Cenozoic (0- to 65-million-year-old) climatic and glacial history, and in the tectonic evolution of the Transantarctic Mountains and the West Antarctic Rift System [Harwood et al., 2006]. These two ANDRILL stratigraphic drill cores are guiding the understanding of the speed, size, and frequency of the past 20 million years of glacial and interglacial changes in the Antarctic region. The drill cores will help to establish, through their correlation to existing records and their integration with climate and ice sheet models, how these local changes relate to regional and global events.

Harwood, David; Florindo, Fabio; Talarico, Franco; Levy, Richard; Kuhn, Gerhard; Naish, Tim; Niessen, Frank; Powell, Ross; Pyne, Alex; Wilson, Gary

2009-03-01

54

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

SciTech Connect

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

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

2008-01-15

55

Fluxes of dissolved organic carbon from California continental margin sediments  

Microsoft Academic Search

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

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

1999-01-01

56

The continental margin off Oregon from seismic investigations  

Microsoft Academic Search

In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was carried out aboard the German RV Sonne as a cooperative experiment between GEOMAR, the USGS and COAS. Offshore central Oregon, which is the subject of this study, the experiment involved the collection of wide-angle refraction and reflection data along three profiles across

M. Gerdom; A. M Trehu; E. R Flueh; D Klaeschen

2000-01-01

57

The world's offshore continental margins contain vast reserves of  

E-print Network

The world's offshore continental margins contain vast reserves of gas hydrate, a frozen form of nat- tor of 2 or more. Understanding marine hydrate systems has become critical for long-term worldwide-seafloor geology. Increasing use of marine multicomponent seismic technol- ogy by oil and gas companies now allows

Texas at Austin, University of

58

Canada basin: age and history of its continental margin  

SciTech Connect

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

Sweeney, J.F.

1985-02-01

59

The Effect of Temperature Dependent Rheology on a Kinematic Model of Continental Breakup and Rifted Continental Margin Formation  

Microsoft Academic Search

The effect of temperature dependent rheology has been examined for a model of continental lithosphere thinning by an upwelling divergent flow field within continental lithosphere and asthenosphere leading to continental breakup and rifted continental margin formation. The model uses a coupled FE fluid flow and thermal solution and is kinematically driven using a half divergence rate Vx and upwelling velocity

V. J. Tymms; N. J. Kusznir

2004-01-01

60

Cenozoic evolution of the Antarctic Peninsula continental margin  

SciTech Connect

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

Anderson, J.B. (Rice Univ., Houston, TX (USA))

1990-05-01

61

Composition of shelf methane seeps on the Cascadia Continental Margin  

NASA Astrophysics Data System (ADS)

Methane reservoirs and seeps are an active component of the continental margin carbon budget and represent a poorly characterized pathway for reduced carbon cycling and methane input to the atmosphere. Active gas seeps from three shelf settings on the Cascadia Continental Margin off Oregon and Northern California contain nearly pure methane with a heavy carbon isotope composition (-29 to -35‰). An extensive study of the gas seep at Coquille Bank, Oregon, revealed a warm, buoyant pore fluid associated with the pockmark. As methane enters the water column above these seeps in a steady gas stream, a fraction escapes directly to the atmosphere while the balance dissolves into local seawater. Measured oxidation rates are too slow for significant local oxidation within the water column near the seep. Large mats of pink and white bacteria, including Beggiatoa spp. are found around the vent, demonstrating the activity of sulfide oxidizers in this ecosystem.

Collier, Robert W.; Lilley, Marvin D.

2005-03-01

62

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

NASA Astrophysics Data System (ADS)

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 Late Mesozoic extrusion tectonics, the Cenozoic NW-directed crustal extension, and the regional far-field eastward flow of the western asthenosphere due to the India-Eurasia plate collision, accompanied by eastward jumping and roll-back of subduction zones of the Pacific Plate.

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

2014-07-01

63

Deep seismic profiling at the Argentinian and Uruguayan continental margin  

NASA Astrophysics Data System (ADS)

The opening of the southern South Atlantic resulted in the development of passive continental margins of the volcanic type. The transition of continental to oceanic crust at this type of margin is characterised by the presence of thickened crust showing elevated seismic velocities. We used coincident multichannel and refraction seismic data from two profiles offshore Argentina and Uruguay to derive the crustal structure. The wide-angle seismic data was recorded using 8 ocean bottom hydrophones. We identified refracted waves from the crust (Pg-phases) as well as reflected waves from the crust mantle boundary (PmP-phases). The thickness of the sedimentary cover was estimated from the multichannel seismic data. The application of a joint travel time inversion provided information about the seismic velocities within the crust, as well as about the depth of the crust mantle boundary. Tomography also allows assessing the resolution and non-uniqueness of the final model. Offshore Argentina at 44° S the lower crust has two distinct areas where the seismic velocity exceeds 7.0 km/s. Each of these area has a spatial elongation of 40 to 60 km. At the Uruguayan margin at 35° S the seismic velocities within the lower crust reaches also values of about 7.5 km/s. A comparison of both profiles will give insight into the distribution of magmatic products along the margin.

Schnabel, Michael; Dieter, Franke; Paterlini, Marcelo; Martinez, Alejandro

2010-05-01

64

South Atlantic margins of Africa. page 1 South Atlantic continental margins of Africa  

E-print Network

in the formation of the SW Africa volcanic margin that displays thick and wide intermediate igneous crust, adjacent to a thick unstretched continental crust. The non-volcanic mode of rifting north of the Walvis ridge, led were controlled by sedimentation rates. Regressive erosion across the prominent shoulder uplift of SW

Paris-Sud XI, Université de

65

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

Microsoft Academic Search

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

P. Hoffman

1973-01-01

66

Palynomorph assemblages from marine Eocene sediments on the west Tasmanian continental margin and the South Tasman Rise  

Microsoft Academic Search

Eocene sediments have been sampled at five localities on the continental margin off western Tasmania and on the South Tasman Rise. Palynomorph assemblages recovered from these show that carbonaceous, pollen?rich sediments in the Strahan Sub?basin sampled by gravity coring during the RV Sonne cruise of 1985, which were probably laid down in shallow water, belong to the Proteacidites asperopolus Zone

E. M. Truswell

1997-01-01

67

Specific character of the bottom simulating reflectors near mud diapirs: Western margin of India  

Microsoft Academic Search

Multi-channel seismic recording was carried out along the western continental margin of India in the early nineties for the exploration of hydrocarbons. Analysis of seismic data demonstrated a characteristic reflector, which usually coincides with the predicted base of methane hydrates stability field and mimics the sea- floor, known as the bottom simulating reflection (BSR) on marine seismic reflection data. Existence

Uma Shankar; Kalachand Sain

2007-01-01

68

Rifted continental margins: The case for depth-dependent extension  

NASA Astrophysics Data System (ADS)

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

Huismans, Ritske S.; Beaumont, Christopher

2014-12-01

69

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

E-print Network

The Continental Margin is a Key Source of Iron to the HNLC North Pacific Ocean Phoebe J. Lam1 ocean [Jickells et al., 2005]. Recent work, however, has shown that the continental margin surrounding and Fe from redox-mobilized labile sources at the continental shelf supplemented by a more variable

70

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

NASA Astrophysics Data System (ADS)

It has been proposed that some rifted continental margins have anomalous subsidence and that at break-up they were elevated at shallower bathymetries than the isostatic response predicted by classical rift models (McKenzie, 1978). The existence of anomalous syn- or early-post break-up subsidence of this form would have important implications for our understanding of the geodynamics of continental break-up and sea-floor spreading initiation. We have investigated subsidence of the young rifted continental margin of the eastern Gulf of Aden, focussing on the western Oman margin (break-up age 17.6 Ma). Lucazeau et al. (2008) have found that the observed bathymetry here is approximately 1 km shallower than the predicted bathymetry. In order to examine the proposition of an anomalous early post break-up subsidence history of the Omani Gulf of Aden rifted continental margin, we have determined the subsidence of the oldest oceanic crust adjacent to the continent-ocean boundary (COB) using residual depth anomaly (RDA) analysis corrected for sediment loading and oceanic crustal thickness variation. RDAs corrected for sediment loading using flexural backstripping and decompaction have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous subsidence of the Gulf of Aden rifted continental margin. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions of Crosby and McKenzie (2009). Non-zero RDAs at the Omani Gulf of Aden rifted continental margin can be the result of non standard oceanic crustal thickness or the effect of mantle dynamic topography or a non-classical rift and break-up model. Oceanic crustal basement thicknesses from gravity inversion together with Airy isostasy have been used to predict a "synthetic" gravity RDA, in order to determine the RDA contribution from non-standard oceanic crustal thickness. Gravity inversion, used to determine crustal basement thickness, incorporates a lithosphere thermal gravity anomaly correction and uses sediment thicknesses from 2D seismic data. Reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The difference between the sediment corrected RDA and the "synthetic" gravity derived RDA gives the component of the RDA which is not due to variations in oceanic crustal thickness. This RDA corrected for sediment loading and crustal thickness variation has a magnitude between +600m and +1000m (corresponding to anomalous uplift) and is comparable to that reported (+1km) by Lucazeau et al. (2008). We are unable to distinguish whether this anomalous uplift is due to mantle dynamic topography or anomalous subsidence with respect to classical rift model predictions.

Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie

2013-04-01

71

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

NASA Technical Reports Server (NTRS)

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

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

1985-01-01

72

An Assessment of Global Organic Carbon Flux Along Continental Margins  

NASA Technical Reports Server (NTRS)

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

Thunell, Robert

2004-01-01

73

Jurassic-Cretaceous continental margin of Southeastern Russia: Outcrop sequence stratigraphy, sedimentation and tectonics  

NASA Astrophysics Data System (ADS)

Data on the composition and distribution of the sedimentary complexes of the Late Mesozoic continental margin of Southeastern Russia and their boundaries permit reconstruction of the tectonic evolution stages using different units of outcrop sequence stratigraphy separated by unconformities. Four siliciclastic megasequences extending from the Bureya ancient massif (microcontinent) to Sakhalin have been recorded within the Jurassic-Cretaceous continental margin of SE Russia: Sinemurian-Oxfordian (MS-1), Volgian-Barremian (MS-2), Aptian-Cenomanian (MS-3), and Turonian-Maastrichtian (MS-4). Lateral facial profiles for each megasequence were constructed across the continental margin to a distance of 700 km. Megafossils (ammonites, buchiids and inocerams) and radiolarians were used for stratigraphic subdivision. During the Jurassic-Cretaceous, the studied region was affected by both Boreal and Tethys transgressions, as indicated by mixed faunal assemblages. The tectonic regime changed from passive continental margin in the Jurassic to active and transform in the Cretaceous. The first megasequence (Sinemurian-Oxfordian) includes five sequences on the margin of the Bureya Massif being separated by unconformities indicating regressions in the earliest Pliensbachian, Toarcian, in the earliest Aalenian, and at the end of the Bathonian. On the Bureya Massif margin in MS-1 a regressive succession is clearly demonstrated by changing from coastal-marine to continental coal-bearing in the latest Jurassic. To the east, in the direction of deeper parts of the basin, the sequences are divided indistinctly, unconformable boundaries are replaced by conformable, shallow-water shelf environments are replaced by the environments of a deeper shelf (siltstones and mudstones) and slope (turbidites), and finally by siliceous-clayey shales and cherts of a deep-water basin. The second sequence (Volgian-Barremian) comprises Volgian-Valanginian and Hauterivian-Barremian sequences, the first of which is subdivided into 3 parasequences which reflect transgressive-regressive cycles. The coastal line shifted to the east, although the character of facial changes from west to east remained the same in the Volgian-Barremian sequence. The Hauterivian-Barremian sequence is reliably defined only in the eastern part of the basin (turbidites). The western part of the basin was apparently elevated at that time due to intensive left-lateral strike-slip displacements and eroded. The third megasequence (Aptian-Cenomanian) involves two sequences: Aptian-mid-Albian and mid-Albian-Cenomanian. During the Aptian to mid-Albian, island arcs began to emerge both on the continental margin and in the sea basin, as supported by the volcanic material admixture in the sediments, including turbidites. Sedimentation took place in the back-arc, inter-arc and forearc basins (as in the case of the Philippine Sea). Mid-Albian-Cenomanian sequence formed in the complex conditions. On the one hand, it was a period of maximal global Cretaceous transgression and on the other Late Albian was a time of collision, formation of scaly-thrust structure in East Russia, onset of the formation of East Sikhote-Alin marginal-continental volcanic belt and as a consequence shrinking of the sea basin area in the belt back and its eastward migration. Such conditions caused a complex of volcanic sedimentary rocks in the back-arc (Priamurie) and forearc (West Sakhalin) basins to form.

Kirillova, D.

2009-04-01

74

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

NASA Astrophysics Data System (ADS)

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.

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

2011-03-01

75

Lower-crustal intrusion on the North Atlantic continental margin.  

PubMed

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

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

2008-03-27

76

Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska  

E-print Network

Tectonic Growth of a Collisional Continental Margin: Crustal Evolution of Southern Alaska Edited : crustal evolution of southern Alaska / edited by Kenneth D. Ridgway . . . [et al.]. p. cm. -- (Special, Structural--Alaska. 2. Plate tectonics--Alaska. 3. Continental margins--Alaska. 4. Geology, Stratigraphic

77

Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin  

E-print Network

Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin A. T and well data suggest that the West Taiwan basin developed by orogenic loading and flexure of a rift-type continental margin. The most likely source of the loading is Taiwan, where oblique convergence between

Watts, A. B. "Tony"

78

The Chukchi Borderland: a Sediment-starved Rifted Continental Margin  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

79

Integrated geophysical study of Newfoundland continental margin (east coast Canada)  

SciTech Connect

A synergetic approach is used to delineate the tectono-structural framework and establish a model of the geologic evolution of the Newfoundland continental margin. Over 100,000 mi of regional and detailed reflection seismic, various potential field maps and profiles, and selected well information constitute the data base of this study. More than two decades of exploratory effort was recently rewarded by significant oil discoveries contained in the Upper Jurassic-Early Cretaceous sands of the Jeanne d'Arc basin. Although a part of the East Coast North America chain (Florida to Baffin Bay) of passive margin basins, the Jeanne d'Arc basin had a unique intracratonic setting during its development. This prolific hydrocarbon-bearing basin is only one of the structural provinces identified on the Newfoundland Shelf. Other adjacent basins, sediment-covered ridges, platforms, and basement horsts were identified through integration of all available geophysical and geological data. All of the structural provinces defined in this paper are part of an aborted rift system which initially developed in the Late Triassic and was first activated at the end of the Jurassic and then again in Aptian time. Thermal subsidence prevailed during the Late Cretaceous and Tertiary. The extensional history of the Newfoundland passive margin was additionally complicated by intensive salt tectonics. A large 2-D and 3-D seismic base of more than 100,000 mi shows numerous and complex hydrocarbon traps are present, but up to now only the extensional sedimentary cycle has been found to be productive.

Enachescu, M.E.

1987-05-01

80

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

USGS Publications Warehouse

The continental margin of eastern North America was initiated when West Africa and North America were rifted apart in Triassic-Early Jurassic time. Cooling of the crust and its thinning by rifting and extension caused subsidence. Variation in amounts of subsidence led to formation of five basins. These are listed from south to north. (1) The Blake Plateau Basin, the southernmost, is the widest basin and the one in which the rift-stage basement took longest to form. Carbonate platform deposition was active and persisted until the end of Early Cretaceous. In Late Cretaceous, deposition slowed while subsidence persisted, so a deep water platform was formed. Since the Paleocene the region has undergone erosion. (2) The Carolina Trough is narrow and has relatively thin basement, on the basis of gravity modeling. The two basins with thin basement, the Carolina Trough and Scotian Basin, also show many salt diapirs indicating considerable deposition of salt during their early evolution. In the Carolina Trough, subsidence of a large block of strata above the flowing salt has resulted in a major, active normal fault on the landward side of the basin. (3) The Baltimore Canyon Trough has an extremely thick sedimentary section; synrift and postrift sediments exceed 18 km in thickness. A Jurassic reef is well developed on the basin's seaward side, but post-Jurassic deposition was mainly non-carbonate. In general the conversion from carbonate to terrigenous deposition, characteristics of North American Basins, occurred progressively earlier toward the north. (4) The Georges Bank Basin has a complicated deep structure of sub-basins filled with thick synrift deposits. This may have resulted from some shearing that occurred at this offset of the continental margin. Postrift sediments apparently are thin compared to other basins-only about 8 km. (5) The Scotian Basin, off Canada, contains Jurassic carbonate rocks, sandstone, shale and coal covered by deltaic deposits and Upper Cretaceous deeper water chalk and shale. ?? 1988.

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

1988-01-01

81

Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin  

NASA Astrophysics Data System (ADS)

The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ˜31 Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin are increasingly dominated by shallow decompressional melting of the ambient asthenosphere, the composition of which may in part be controlled by preferential channeling of plume material along the developing neo-oceanic axes of extension.

Rooney, Tyrone O.; Mohr, Paul; Dosso, Laure; Hall, Chris

2013-02-01

82

Characterizing, identifying and mapping structural domains at rifted continental margins: insights from the Bay of Biscay margins and its Pyrenean fossil analogue  

NASA Astrophysics Data System (ADS)

The occurrence of hyperextended domains at rifted continental margins consisting of extremely thinned crust and/or exhumed mantle has been increasingly recognized over the past decades, both at present-day rifted margins and in deformed remnants preserved in collisional orogens. At present, most studies aiming to characterize rifted continental margin structure and the extreme thinning of the continental crust and lithosphere are either focused offshore using geophysical methods, or onshore on fossil analogues relying on geological field observations. Marine and onland examples provide complementary datasets, but their different scale and resolution of observations prevent straightforward correlations to be done. In this contribution, we use the Bay of Biscay and Western Pyrenees to develop and apply a geological/geophysical approach to characterize and identify distinctive rifted margin domains both in offshore and onshore settings. The Bay of Biscay and Western Pyrenees represent a unique natural laboratory that offer the possibility to have access to seismically imaged, drilled and exposed parts of one and the same hyperextended rift system. Quantitative techniques (gravity inversion and flexural backstripping) are used on offshore examples (Western Approach margin and Parentis basin) to estimate accommodation space, crustal thickness and lithosphere thinning while seismic interpretations enable the recognition of extensional settings (low- and high-? settings). Field observations (Mauléon basin) and drill-hole data (Parentis basin) focused on key outcrops enables the description of the nature of sediment and basement rocks and of the structures forming fossil remnants of rifted margins. This qualitative and quantitative characterisation provides diagnostic elements to identify and map structural domains at magma-poor rifted margins and their fossil analogues. We name these 5 domains proximal, necking, hyperthinned, exhumed mantle and oceanic. This new geological/geophysical approach can be further used as an interface between onshore and offshore observations. Offshore seismic interpretations can take advantage of onshore observations on the nature of sediment, basement and of their interface. The large scale geometry and stratigraphic architecture imaged offshore can be used to restore onshore fossil remnants back into a rifted margin context. The application of this multidisciplinary approach to the Bay of Biscay margins and their onshore Pyrenean fossils remnants enables us to propose a new map of the different rift systems preserved at the transition between the European and Iberian plates. The approach underlying this mapping has general global application to unravelling the spatial and temporal complexity of rifted margin structural domains.

Tugend, Julie; Manatschal, Gianreto; Kusznir, Nick J.

2014-05-01

83

Subsidence and isostasy along a sheared margin—Cauvery Basin, Eastern Continental Margin of India  

NASA Astrophysics Data System (ADS)

We have applied a process-oriented method to a seismo-geological section across Cauvery Basin, a sedimentary basin along the southern part of the Eastern Continental Margin of India (ECMI) to examine the isostatic mechanism operating beneath the margin. The evolution of the ECMI appears to be within a shear-rift tectonic setting, mainly driven by the difference in the relative motions of India and East Antarctica and the subsequent direction of spreading and the Cauvery Basin seems to have formed in this pull-apart tectonic setting. Shearing along the southern ECMI is modeled with gravity anomalies and the resultant crustal configuration, through various models of isostasy. Backstripping analysis suggests a very low Te of 3 km beneath the basin. The shelf edge gravity anomaly reflects a strong contrast across a wrench fault parallel to the coast inferred from gravity forward modeling, denoting coast-parallel transform motion in the initial stages of continental separation prior to rotation and spreading.

Chand, Shyam; Subrahmanyam, C.

84

Rheology and Deformation of the Lithosphere at Continental Margins  

NASA Astrophysics Data System (ADS)

Rheology and Deformation of the Lithosphere at Continental Margins is a collection of well-written and interesting papers that provide a good overview of the current cutting edge in seismically oriented deformation studies. The book is, unfortunately, somewhat misnamed. It really should be titled something akin to ``A review of seismogenic behavior in different plate tectonic settings (and some partially molten mantle deformation experiments).'' Perhaps it sounds less impressive, but that is really what you get in this book: a series of excellent review papers, most of which focus on our current understanding of seismogenic behavior. The book had its genesis at a short course and workshop in 2000 held in Snowbird, Utah, associated with the MARGINS initiative. For this volume, there are three papers on divergent settings (Buck, Axen, Davis and Kuznir), three on convergent settings (Jackson, Hyndman, Willet and Pope), and two on transcurrent settings (Chester et al., Scholz and Hanks). In addition, there is an excellent overview on the seismogenic zone by L. Ruff (which should be read first and probably should have been placed there). All of these articles, with the exception of Willet and Pope, are primarily interested in seismogenic deformation. The above fit neatly into a volume that is focused on information gathered from seismic data. The papers that do not fit neatly into the above scheme are the last two papers in the volume, written by experimentalists working on partially molten behavior of mantle rocks: Xu et al. and Evans et al. Both papers are very good reviews of the subject and hopefully will not be overlooked because they are different from the dominant theme of the book.

Tikoff, Basil

2004-09-01

85

Continental margin molybdenum isotope signatures from the early Eocene  

NASA Astrophysics Data System (ADS)

Molybdenum (Mo) isotope compositions of marine sedimentary deposits that span the Paleocene-Eocene Thermal Maximum (PETM, approximately 56 Ma) are presented from two Tethys Ocean sites (Guru Fatima, Tajikistan and Kheu River, Georgia). Local redox indicators suggest that both locations experienced anoxia and intervals of euxinia (with hydrogen sulphide present in seawater) during the early part of the PETM. However, the Mo-isotope compositions (expressed as ?Mo98/95) for each site differ significantly. Local redox conditions were very stable at Guru Fatima, where ?Mo98/95 reached a maximum of 0.96‰, which is ?0.7‰ lower than for early Eocene seawater as recently inferred from euxinic Arctic Ocean deposits. This observation supports the argument that a ?0.7‰ difference between seawater and anoxic continental margin marine sediments documented at the present day might also be found in the paleo-record. In contrast, local redox conditions at Kheu River were not stable over the study interval, and ?Mo98/95 were much lower than at Guru Fatima. The low ?Mo98/95 values at Kheu River are attributed to the influence of post-depositional remobilization of Mo by repeated adsorption and dissolution of Fe-Mn oxides during brief intervals of bottom water oxidation. The data highlight the importance of obtaining multi-proxy constraints on both local redox and paleoceanographic setting before the Mo-isotope compositions of sedimentary deposits can be interpreted accurately.

Dickson, Alexander J.; Cohen, Anthony S.; Coe, Angela L.

2014-10-01

86

Tectonics of the West Iberia continental margin from seismic reflection data  

Microsoft Academic Search

Continental rifting is a fundamental component of the plate tectonic cycle. The West Iberia passive margin is a classic example of a nonvolcanic rifted margin. The West Iberia margin contains an enigmatic north-south ridge of serpentinized peridotite located within the ocean-continent transition. Interpretation of multichannel seismic data and tectonic subsidence analyses suggests that the ridge is located within a broad

Alison Teagan Henning

2005-01-01

87

Hyperextension, rifted margin geometry and its incorporation into mountain belts: Insights from the western Alps  

NASA Astrophysics Data System (ADS)

Collision orogens are the products of inversion of rifted margins, and components of these rifted margins can be recognised in mountain belts. The roles of pre-existing heterogeneities in localising contractional deformation in thrust belts are well-studied. However, the distribution of hyperextended continental lithosphere at rifted margins, especially the presence of substantial tracts of hydrated upper mantle, is likely to have a strong bearing on how orogens evolve. These issues are developed here using examples drawn from the western Alps. Quantifying the magnitude and distribution of crustal thinning prior to orogenesis is critical for crustal balancing and the related prediction of subduction of continental crust. By invoking pre-orogenic hyperextension it is now possible to balance crustal sections western Alps so that the seismically imaged crust is consistent with estimates of orogenic contraction. Hitherto the same sections have required long-term subduction of continental crust - although these inferred subducted tracts have resisted seismic imaging. Transitional parts of the ancestral rifted margin, where there are rapid changes in basin geometry (Dauphine-SubBrianconnais, Vocontian flanks), appear to focus weak inversion ahead of the developing orogen. Furthermore, the style of rifting (depth-heterogeneous vs quasi-uniform kinematics) with the relative importance of lithospheric detachments and hydrated, weak upper mantle can influence the extent to which continental crust can later be subducted. That the continental crust of the internal basement massifs (Dora Maira, Gran Paradiso) experienced transient subduction (HP peaks of 2-4 GPa) implies initially strong coupling with underlying mantle lithosphere (followed by decoupling for exhumation). These parts of the ancestral continental margin are thus likely to have stretched quasi-uniformly. However, those parts of the margin that stretched heterogeneously (rider blocks that rested on hydrated mantle) are less prone to subduction and thus show no UHP metamorphism (e.g. parts of the Brianconnais). The crust recycles without substantial burial, effectively concertinaing the rift blocks. The relative timing of contractional deformation in the subduction channel can therefore oscillate - without varying the far-field convergence rate. Lateral variations in margin geometry leads to complexly variable deformation timing but can still retain a broadly cylindrical orogenic structure.

Butler, R.

2012-12-01

88

Deep-sea Lebensspuren of the Australian continental margins  

NASA Astrophysics Data System (ADS)

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 untapped deep-sea datasets that can be used for biogeographical, evolutionary, behavioural, and ecological studies.

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

89

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

E-print Network

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

Behn, Mark Dietrich, 1974-

2002-01-01

90

Conditions of formation for carbonaceous silicites of the continental margins  

SciTech Connect

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.

Bazhenova, O.K.

1986-06-01

91

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

NASA Astrophysics Data System (ADS)

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 hot spots are ubiquitous in the upper 200 m at OSP, more than 900 km 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 hot spots 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.

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

2006-03-01

92

Permo-Triassic anatexis, continental rifting and the disassembly of western Pangaea  

NASA Astrophysics Data System (ADS)

Crustal anatectites are frequently observed along ocean-continent active margins, although their origins are disputed with interpretations varying between rift-related and collisional. We report geochemical, isotopic and geochronological data that define an ~ 1500 km long belt of S-type meta-granites along the Andes of Colombia and Ecuador, which formed during 275-223 Ma. These are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon U-Pb dates ranging between 240 and 223 Ma. A model is presented which places these rocks within a compressive Permian arc setting that existed during the amalgamation of westernmost Pangaea. Anatexis and mafic intrusion during 240-223 Ma are interpreted to have occurred during continental rifting, which culminated in the formation of oceanic crust and initiated the break-up of western Pangaea. Compression during 275-240 Ma generated small volumes of crustal melting. Rifting during 240-225 Ma was characterized by basaltic underplating, the intrusion of tholeiitic basalts and a peak in crustal melting. Tholeiitic intrusions during 225-216 Ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust, and we assign this period to the onset of continental drift. Dissected ophiolitic sequences in northern Colombia yield zircon U-Pb dates of 216 Ma. The Permo-Triassic margin of Ecuador and Colombia exhibits close temporal, faunal and geochemical similarities with various crustal blocks that form the basement to parts of Mexico, and thus these may represent the relict conjugate margin to NW Gondwana. The magmatic record of the early disassembly of Pangaea spans ~ 20 Ma (240-216 Ma), and the duration of rifting and rift-drift transition is similar to that documented in Cretaceous-Tertiary rift settings such as the West Iberia-Newfoundland conjugate margins, and the Taupo-Lau-Havre System, where rifting and continental disassembly also occurred over periods lasting ~ 20 Ma.

Cochrane, Ryan; Spikings, Richard; Gerdes, Axel; Ulianov, Alexey; Mora, Andres; Villagómez, Diego; Putlitz, Benita; Chiaradia, Massimo

2014-03-01

93

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

SciTech Connect

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

McCrory, P.A.; Arends, R.G. (Unocal Corp., Ventura, CA (United States)); Ingle, J.C. Jr. (Stanford Univ., CA (United States)); Isaacs, C.M.; Stanley, R.G. (Geological Survey, Menlo Park, CA (United States)); Thornton, M.L.C. (Unocal Corp., Ventura, CA (United States))

1991-02-01

94

Tectonic evolution at an early proterozoic continental margin: The Svecokarelides of eastern Finland  

NASA Astrophysics Data System (ADS)

The early Proterozoic history of the Baltic Shield in Fennoscandia provides evidence of fragmentation of a late Archaean craton, continental sedimentation and then back-arc spreading, interpreted as being associated with the uprise of a mantle diapir. Basin subsidence and infilling with flysch debris is explained on the basis of thermal decay, locking of a subduction zone and erosion of an uplifted arc. Compressive tectonism in the Karelian part of the Svecokarelian orogen resulted in obduction of the contents of the back-arc basin on to the continental foreland. Further compression caused further thickening of the supracrustal pile and interdigitation of tectonic slices of basement and cover due to thrusting. Subsequently movement was resolved along major NW-trending wrench-faults that generally follow the margin of teh craton and which represent zones of reactivation of planar features developed in late Archaean times. As the crustal pile was warped and uplifted in subsequent deformational phases, the deep levels of the wrench-faults acted as sites of granitoid emplacement. The tectonic activity in the Karelian part of the Svecokarelides is interpreted as the response to the northward movement of oceanic lithosphere in the Svecofennian part of the orogen where successively-formed early Proterozoic island arcs moved northwards and were intensely deformed, associated with the development and emplacement of large masses of igneous material. The resultant orogen, consisting of a stable craton, obducted nappes, exotic terrane, transcurrent faults and island arcs that moved obliquely to the margin of the craton, shows many similarities in development to that of the western Cordillera of North America.

Park, A. F.; Bowes, D. R.; Halden, N. M.; Koistinen, T. J.

1984-12-01

95

Listric Normal Faulting on the Cascadia Continental Margin  

NSDL National Science Digital Library

This article describes the occurrence of listric normal faults (those which gradually flatten out with depth) in the continental shelf offshore Oregon and Washington, as seen in seismic reflection profiles. There is also a discussion of the faulting mechanics, the timing of uplift on the continental shelf, and the separation of compressional and extensional tectonic regimes on the lower and upper slopes of the shelf. A link to a downloadable version of the complete article is provided.

Goldfinger, Chris

96

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

USGS Publications Warehouse

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

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

1985-01-01

97

Three-dimensional subsidence analysis and gravity modelling of the continental margin offshore Namibia  

Microsoft Academic Search

SUMMARY Seismic re£ection pro¢les and gravity anomaly data have been used to determine the structure and evolution of the Namibian continental margin. In comparison to other margins, the gravity anomaly at the Namibian margin shows a number of distinctive features. It lacks an o¡shore gravity 'low', and, despite the presence of up to 9 km of sediments, the gravity 'high'

J. Stewart; A. B. Watts; J. G. Bagguley

2000-01-01

98

Decomposition of dissolved organic matter from the continental margin  

NASA Astrophysics Data System (ADS)

Decomposition of dissolved organic carbon, nitrogen and phosphorus (DOC, DON, DOP) was measured for surface and bottom waters of the middle Atlantic bight (MAB) and deep slope water adjacent to the MAB on two occasions in March and August 1996. We used standard bottle incubation techniques to measure the decrease in dissolved organic matter (DOM) concentrations over a 180-day interval. Generally DOM concentrations in the MAB were elevated (125 ?M DOC, 10.2 ?M DON and 0.30 ?M DOP) relative to the surface ocean and deep slope water (46.7 ?M DOC, 2.76 ?M DON, 0.03 ?M DOP). On average the C:N:P ratio of shelf DOM (431:36:1) was substantially higher than the Redfield ratio, but not nearly as high for that of deep slope water (2700:215:1). Decomposition time course data were fit to a three-pool ( very labile, labile, and recalcitrant pools) multi-G model using a Marquardt fitting routine. The three-pool model was superior to a simple exponential decay model assuming a single pool of DOM. We observed no significant changes in concentration of DOM in deep-water samples, attesting to the old age of this material, its recalcitrant nature, and the cleanliness of our technique for measuring decomposition. There were major differences in the relative amount of very labile, labile and recalcitrant fractions of shelf-water DOC, DON and DOP as a result of preferential remineralization of P over N and N over C. Averaged over stations, the decomposable portion of the bulk DOC, DON and DOP pools increased from 30% to 40% to 81% for C, N and P. There was a wide range in decay coefficients for the very labile and labile DOM pools: average decay coefficient for the very labile pool was 0.219 d -1, and 0.018 d -1 for the labile pool. Average half-lives calculated from the decay coefficients were 4, 12 and 8 days for the very labile DOC, DON and DOP pools, and 54, 113 and 90 days for the labile DOC, DON and DOP pools. On the basis of pool turnover times relative to shelf-water residence time (˜100 days) we conclude that autochthonous algal production is the source of the very labile DOM pools. Its rate of production is sufficient to sustain estimated rates of bacteria C demand in continental margins. Our results for the MAB indicate that while substantial amounts of DOM are remineralized in the same time frame as shelf-water residence time, there is substantial DOM remaining that is depleted in N and P relative to C. Strong concentration gradients in DOM occur between shelf and ocean waters and between surface and deeper waters. Coupled with appropriate vertical and horizontal advective and eddy diffusive transports, DOM export from the MAB and other shelf systems may be a significant component of ocean C dynamics.

Hopkinson, Charles S.; Vallino, Joseph J.; Nolin, Amy

99

Holistic Approach Offers Potential to Quantify Mass Fluxes Across Continental Margins  

NASA Astrophysics Data System (ADS)

Most humans live on and utilize the continental margin, the surface of which changes continually in response to environmental perturbations such as weather, climate change, tectonism, earthquakes, volcanism, sea level, and human settlement and land use. Part of the margin is above sea level and the rest is submarine, but these land and seascape components are contiguous, and material transport from source to sink occurs as a seamless cascade. The margin responds to environmental perturbations by changing the nature and magnitude of a variety of important functions, including the distribution of soil formation and erosion; biogeochemical functioning (especially the storage and release of water, limiting nutrients and contaminants); and the form and behavior of geomorphic components from hill slopes and floodplains through the coastal zone to the continental rise. While some areas of the margin are eroding-for example, hill slopes-others accumulate sediment, such as tectonic basins and continental slope and rise. These areas record the history of surface changes. A major goal of the Earth science community is to provide quantitative explanations and predictions of the effects of environmental perturbations on surface changes and preserved sedimentary strata of continental margins. In past decades, margins have been investigated piecemeal by researchers who have tended to focus on a particular segment from one disciplinary perspective while eschewing the broader perspective of the margin as an interconnected whole. Recognizing this shortcoming, the U.S. National Science Foundation (NSF) has initiated the MARGINS Source-to-Sink (S2S) program, which, for the first time, will attempt to understand the functioning of entire margin systems through dedicated observational and community modeling studies. Following input from the Earth science community, the Waipaoa Sedimentary System (WSS) of the North Island, New Zealand, was chosen as one of the focus sites for possible study (see MARGINS Source-to-Sink science plan for selection criteria and rationale: http://www.ldeo.columbia.edu/margins/S2S/S2Ssciplan02.html).

Kuehl, Steven; Carter, Lionel; Gomez, Basil; Trustrum, Noel

100

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

NASA Astrophysics Data System (ADS)

RV "Polarstern" cruise ARK-XIII/3 (2008) and RV "Araon" cruise ARA03B (2012) investigated an area in the Arctic Ocean located between the Chukchi Borderland and the East Siberian Sea (between 165°W and 170°E). Based on swath bathymetry, sediment echosounding, seismic profiling and sediment coring we present evidence that the western Arctic Ocean had a glaciated continental margin during several glacial periods of the Pleistocene (Niessen et al. 2013). At the southern end of the Mendeleev Ridge and on the Chukchi and East Siberian continental slopes ice sheets and ice shelves grounded in up to 1200 m present water depth. We found mega-scale glacial lineations (MSGL) associated with deposition of glaciogenic wedges and debris-flow deposits indicative of sub-glacial erosion and deposition close to the former grounding lines. Glacially lineated areas are associated with large-scale erosion, capped with diamicton and draped by, in places, several metres of pelagic sediments. On the Arlis Plateau, a detailed bathymetric map exhibits several generations of MSGL, which we interpret as relicts of different Pleistocene glaciations. Traces of former grounding line positions suggest that an ice shelf of approximately 900 m in thickness has spread across the Southern Mendeleev Ridge in a north-easterly direction. According to our results, ice sheets of more than one km in thickness continued onto, and likely centered over, the East Siberian Shelf. A preliminary age model suggests that the youngest and shallowest grounding event of an ice sheet should be within Marine Isotope Stage (MIS) 3 and clearly predates the Last Glacial Maximum. The oldest and deepest event predates MIS 6. The youngest grounding event on the Arlis Plateau is tentatively dated to have occurred during MIS 4. These results have important implication for the former distribution of thick ice masses in the Arctic Ocean during the Pleistocene. They are relevant for albedo, ocean-atmosphere heat exchange, moisture supply to and freshwater export from the Arctic Ocean and the formation of submarine permafrost on the East Siberian Shelf. Niessen, F., Hong, J. K. , Hegewald, A. , Matthiessen, J. , Stein, R. , Kim, H. , Kim, S. , Jensen, L. , Jokat, W. , Nam, S. I. and Kang, S. H. (2013) Repeated Pleistocene glaciation of the East Siberian continental margin, Nature Geoscience, 6 (10), pp. 842-846.

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

2014-05-01

101

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

SciTech Connect

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

Johnson, K.S. (Moss Landing Marine Lab., CA (United States) Monterey Bay Aquarium Research Inst., Pacific Grove, CA (United States)); Berelson, W.M.; Iams, H.D.; Kilgore, T.E. (Univ. of Southern California, Los Angeles (United States)); Coale, K.H.; Coley, T.L.; Elrod, V.A.; Fairey, W.R.; Nowicki, J.L. (Moss Landing Marine Lab., CA (United States))

1992-08-28

102

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

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

103

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

NASA Astrophysics Data System (ADS)

Rifted continental lithosphere subsides as a consequence of combined crustal thinning and mantle lithosphere cooling yet basins on some continental margins experience anomalous subsidence events that postdate active extension. Deep basins on the northern margin of the South China Sea, notably the Baiyun Sag, show basement subsidence accelerating after ?21 Ma, postdating extension by several million years. We combine geophysical observations and numerical forward modeling to show that loading of the offshore basins by increased sediment flux caused by faster onshore erosion following Early Miocene monsoon intensification is a viable trigger for ductile flow after the cessation of active extension. This illustrates that offshore basin dynamics at continental margins with weak crust can be controlled by onshore surface processes in a newly recognized form of climate-tectonic coupling.

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

2015-06-01

104

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

E-print Network

1 Continental break-up history of a deep magma-poor margin based on seismic reflection data (north.autin@gmail.com Abbreviated title: Continental break-up history of the north-eastern Gulf of Aden SUMMARY Rifting between, where the continental break-up finally occurred. Initial seafloor spreading was followed by post

Paris-Sud XI, Université de

105

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

NASA Astrophysics Data System (ADS)

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

Ryang, Woo Hun; Kim, Seong Pil

2014-05-01

106

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

SciTech Connect

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

Biswas, S.K.

1982-10-01

107

Geologic development and characteristics of continental margins, Gulf of Mexico  

SciTech Connect

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

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

1986-09-01

108

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

109

Rifted continental margins: geometric control on crustal architecture and melting  

NASA Astrophysics Data System (ADS)

A new model is provided for the distribution of magma-poor and magma-rich rifted margins. The South Atlantic, Central Atlantic, North Atlantic - Arctic (Eurasia Basin), and Red Sea all are magma-rich at their distal ends and magma-poor at their proximal ends (with respect to their poles of rotation). The well-known architectural zonation across fully developed magma-poor margins (limited crustal stretching, hyperextension, exhumed mantle, oceanic crust) is also observed along the lengths of many margins at the super-regional scale. Zones of exhumed mantle, marking magma-poor margin, can be mapped for thousands of kilometers. Likewise can zones of seaward dipping reflectors (SDR) marking magma-rich margins. At this scale, the age of the oceanic crust becomes younger in the direction of the rotation pole, implying that the continents ruptured by rift tip propagation (and rotation pole propagation). Propagation is also manifested by the age of pre-break-up magmatism, break-up unconformity, and margin uplift. Hence, the classic cross-sectional depiction of margin evolution has a third dimension. The degree of melting follows the same pattern. At the distal end of e.g. the South Atlantic, SDR zones are wide and gradually thin toward the rotation pole. Eventually exhumed mantle takes over, marking the transition to the magma-poor margins, which remain to the proximal end of rifting. SDR zones also thin laterally from ca 10-15 km thickness at the continent-ocean boundary (COB) to ca 7 km thick oceanic crust beyond the SDRs. Outcrop data demonstrate that also exhumed mantle contains up to ca 12% melt, infiltrated in the peridotites. Thus, melting is largest at the distal ends near the COB, and decreases both laterally toward the evolving ocean and along strike toward the rift tip. Accepting that continents are rigid to a first order, the linear rate of extension at any given location along an evolving rift and ocean, is governed by the angular rate of opening, the distance to the rotation pole, and the rate of propagation of the pole. For a fixed angular rate, the linear extension rate increases away from the pole. Numerical models reveal that both mantle temperature and rate of extension can govern the degree of melting. However, the above empirical observations suggest that to a first order the rifted margin architecture, including the degree of melting, is governed by the linear rate of extension, which is a direct outcome of geometric rules of plate tectonics. Rapid pole propagation, or a pole jump, will induce a rapid increase in the linear rate. Magma-rich margins seem to form when continents break at a high extension/strain rate caused by rapid propagation; this occurs at the distal end of a rupturing plate. Our testable model questions the common ad hoc introduction of mantle plumes to explain "excess" melting along magma-rich margins. This does not rule out that mantle heterogeneities may exist, but such heterogeneities appear second order when it comes to generating magma-rich margins.

Lundin, Erik; Redfield, Tim; Peron-Pinvidic, Gwenn

2014-05-01

110

Imaging proto-oceanic crust off the Brazilian Continental Margin  

NASA Astrophysics Data System (ADS)

During the Sanba (Santos basin seismic transect) experiment in 2010-2011, a 380-km-long combined wide-angle and reflection seismic profile has been acquired using 30 ocean-bottom seismometers, a 4.5 km seismic streamer and a 8900 in.3 airgun array. The Sanba 3 profile crosses the southern flank of the Sao Paulo Plateau, the Sao Paulo Ridge and the easternmost Santos Basin in an east-west direction. Its eastern end is located on undisturbed oceanic crust. Tomographic and forward modelling of the wide-angle seismic data reveals that the sedimentary thickness is variable with only 1-2 km on top of the ridge and thickening to 4-5 km in the basin. Crustal thickness at the ridge is about 18 km and the relative layer thickness and velocity gradients indicate a continental origin of this ridge. The eastern Santos Basin is underlain by crust of only 5 km thickness, characterized by high seismic velocities between 6.20 km s-1 in the upper crust and 7.40 km s-1 in the lower crust. Three hypotheses for the nature of the crust in this region are tested here: (i) thinned continental crust, (ii) serpentinized upper mantle material, (iii) thin oceanic crust. As seismic velocity gradients seem to rule out a continental origin of this region, and clear Moho reflections argue against serpentinized upper mantle, we propose that the crust underlying the easternmost Santos Basin is of oceanic origin. Deviations from normal oceanic crustal velocities in the lower crust (6.70-7.00 km s-1) can be explained by accretion at slow spreading rates leading to the inclusion of serpentinite into the lower crust at the onset of organized seafloor spreading.

Klingelhoefer, F.; Evain, M.; Afilhado, A.; Rigoti, C.; Loureiro, A.; Alves, D.; Leprêtre, A.; Moulin, M.; Schnurle, P.; Benabdellouahed, M.; Baltzer, A.; Rabineau, M.; Feld, A.; Viana, A.; Aslanian, D.

2014-01-01

111

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

USGS Publications Warehouse

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

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

1987-01-01

112

Joint geophysical and petrological models for the lithosphere structure of the Antarctic Peninsula continental margin  

NASA Astrophysics Data System (ADS)

The Antarctic Peninsula (AP) is a composite magmatic arc terrane formed at the Pacific margin of Gondwana. Through the late Mesozoic and Cenozoic subduction has stopped progressively from southwest to northeast as a result of a series of ridge trench collisions. Subduction may be active today in the northern part of the AP adjacent to the South Shetland Islands. The subduction system is confined by the Shackleton and Hero fracture zones. The magmatic arc of the AP continental margin is marked by high-amplitude gravity and magnetic anomaly belts reaching highest amplitudes in the region of the South Shetland Islands and trench. The sources for these anomalies are highly magnetic and dense batholiths of mafic bulk composition, which were intruded in the Cretaceous, due to partial melting of upper-mantle and lower-crustal rocks. 2-D gravity and magnetic models provide new insights into crustal and upper-mantle structure of the active and passive margin segments of the northern AP. Our models incorporate seismic refraction constraints and physical property data. This enables us to better constrain both Moho geometry and petrological interpretations in the crust and upper mantle. Model along the DSS-12 profile crosses the AP margin near the Anvers Island and shows typical features of a passive continental margin. The second model along the DSS-17 profile extends from the Drake Passage through the South Shetland Trench/Islands system and Bransfield Strait to the AP and indicates an active continental margin linked to slow subduction and on-going continental rifting in the backarc region. Continental rifting beneath the Bransfield Strait is associated with an upward of hot upper mantle rocks and with extensive magmatic underplating.

Yegorova, Tamara; Bakhmutov, Vladimir; Janik, Tomasz; Grad, Marek

2011-01-01

113

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

USGS Publications Warehouse

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

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

2005-01-01

114

Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cap on the Newfoundland rifted margin  

Microsoft Academic Search

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin

John R. Hopper; Thomas Funck; Brian E. Tucholke; Hans Christian Larsen; W. Steven Holbrook; Keith E. Louden; Donna Shillington; Helen Lau

2004-01-01

115

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

Microsoft Academic Search

The present paper reports the first mineralogical description of green grains (verdine and glaucony) from sediments of the eastern continental margin of India. Only 24 of the 82 sediment samples studied, at depths between 18 and 247 m, contain green grains and their percentages in the coarse fraction (125–250 ?m) range from 6 to 40%. Of these, 13 samples from

Venigalla Purnachandra Rao; Meloth Thamban; Michel Lamboy

1995-01-01

116

Heat Flow Measurements at the Continental Margin off Peru Within the GEOPECO Project  

Microsoft Academic Search

The continental margin off Peru is dominated by the oblique subduction of the Nazca Plate and Nazca Ridge. The sedimentary wedge consists of folded and uplifted material. Along with compression, compaction, folding and faulting, processes of dewatering, material and energy migration occur. Bottom simulating reflectors (BSRs) are detectable in locations, indicating an abundant presence of methane. On the sea floor,

N. Kaul; R. Harris; M. Mueller

2001-01-01

117

The Hatton Basin and continental margin: Crustal structure from wide-angle seismic and gravity data  

Microsoft Academic Search

Results from a wide-angle seismic and gravity study between the Rockall Bank and the Iceland Basin in the North Atlantic are presented. Crustal and sedimentary structures are resolved in the Hatton Basin and across the Hatton continental margin (HCM) east of magnetic anomaly 24. The structure of the oceanic crust west of the anomaly is also determined. Gravity data support

Ulrike Vogt; Jannis Makris; Brian M. O'Reilly; Franz Hauser; Peter W. Readman; A. W. Brian Jacob; Pat M. Shannon

1998-01-01

118

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

E-print Network

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

Lin, Andrew Tien-Shun

119

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

E-print Network

Morphology of late Quaternary submarine landslides along the U.S. Atlantic continental margin David bathymetry and backscatter imagery provides an opportunity to reevaluate the distribution of submarine categories based on their source areas: those sourced in submarine canyons and those sourced on the open

ten Brink, Uri S.

120

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

Microsoft Academic Search

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

Robert W. Embley; Robert D. Jacobi

1977-01-01

121

Magmatism at rift zones - The generation of volcanic continental margins and flood basalts  

Microsoft Academic Search

A simple model is developed which explains the occurrence of volcanic continental margins and flood basalts as a consequence of their association with nearby plumes that were active at the time of rifting. In the model, asthenosphere temperatures are increased by 100-150 C over large regions of the earth by heat advected upward in mantle plumes. The amount of partial

Robert White; Dan McKenzie

1989-01-01

122

Regional tectonic framework, structure and evolution of the western marginal basins of India  

NASA Astrophysics Data System (ADS)

The Kutch-Saurashtra, Cambay and Narmada basins are pericontinental rift basins in the western margin of the Indian craton. These basins were formed by rifting along Precambrian tectonic trends. Interplay of three major Precambrian tectonic trends of western India, Dharwar (NNW-SSE), Aravalli-Delhi (NE-SW) and Satpura (ENE-WSW), controlled the tectonic style of the basins. The geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults. The Kutch basin opened up first in the Early Jurassic (rifting was initiated in Late Triassic) along the Delhi trend followed by the Cambay basin in the Early Cretaceous along the Dharwar trend and the Narmada basin in Late Cretaceous time along the Satpura trend. The evolution of the basins took place in four stages. These stages are synchronous with the important events in the evolution of the Indian sub-continent—its breakup from Gondwanaland in the Late Triassic-Early Jurassic, its northward drifting during the Jurassic-Cretaceous and collision with the Asian continent in the Early Tertiary. The most important tectonic events occurred in Late Cretaceous time. The present style of the continental margins of India evolved during Early Tertiary time. The Saurashtra arch, the extension of the Aravalli Range across the western continental shelf, subsided along the eastern margin fault of the Cambay basin during the Early Cretaceous. It formed an extensive depositional platform continuous with the Kutch shelf, for the accumulation of thick deltaic sediments. A part of the Saurashtra arch was uplifted as a horst during the main tectonic phase in the Late Cretaceous. The present high thermal regime of the Cambay-Bombay High region is suggestive of a renewed rifting phase.

Biswas, S. K.

1987-04-01

123

Organic geochemistry of continental margin and deep ocean sediments  

SciTech Connect

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.

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

1990-08-01

124

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

E-print Network

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

Sandiford, Mike

125

Oil and gas potential of continental margins of the Pacific Ocean  

Microsoft Academic Search

Oil and gas basins (OGB) of active and transform margins of the Pacific Ocean are discussed. Their western and eastern parts\\u000a differ substantially in the evolution, tectonic pattern, and scope of resources. In the west, marginal seas incorporated into\\u000a the Cenozoic geodynamic system of deep-water basins (marginal seas) and conjugate island arcs exhibit a greater oil and gas\\u000a potential (hereafter,

V. E. Khain; I. D. Polyakova

2008-01-01

126

Extension on rifted continental margins: Observations vs. models.  

NASA Astrophysics Data System (ADS)

Mapping the signature of extensional deformation on rifted margins is often hampered by thick sedimentary or volcanic successions, or because salt tectonics makes sub-salt seismic imaging challenging. Over the past 20 years the literature is witnessing that lack of mapable faults have resulted in a variety of numerical models based on the assumption that the upper crust takes little or no extensional thinning, while the observed reduction of crustal thickness is taken up in the middle and lower crust, as well as in the mantle. In this presentation two case studies are used to highlight the difference that 3D seismic data may have on our understanding. The small patches of 3D resolution data allow us to get a glance of the 'real' signature of extensional faulting, which by analogy can be extrapolate from one margin segment to the next. In the South Atlantic salt tectonics represents a major problem for sub-salt imaging. The conjugate margins of Brazil and Angola are, however, characterized by pronounced crustal thinning as documented by crustal scale 2D reflection and refraction data. Off Angola the 3D 'reality' demonstrates that upper crustal extension by faulting is comparable to the full crustal, as well as lithospheric thinning as derived from refraction data and basin subsidence analysis. The mapped faults are listric low angle faults that seem to detach at mid crustal levels. 2D seismic has in the past been interpreted to indicate that almost no extensional faulting can be mapped towards the base of the so-called 'sag basin'. The whole concept of the 'sag basin', often ascribed to as crustal thinning without upper crustal deformation, is in fact related to this 'lack of observation', and furthermore, have caused the making of different types of dynamic models attempting to account for this. In the NE Atlantic significant Paleocene extensional faulting is locally seen adjacent to the 50 to more than 200 km wide volcanic cover on each side of the breakup axis. The associated amount of lateral motion on these, mainly listric, normal faults represents several tens of km. These observations contrast with the general lack of observed faults along volcanic margins due to the overall problem with sub-basalt imaging. A variety of models with respect to mode and duration of extension, including narrow and fast breakup, melt generation by small scale convection, and different modes of mantle flow have been suggested. The interesting aspect is that it is all based on features we can't see. Both study areas clearly points towards the importance of improved seismic imaging, a need for revised understanding of strain rates and strain partitioning during rift development, and the necessity of moving from 2D cross section modeling to more realistic 3D spatial distribution of rift elements and subsequent break-up processes. One important aspect is that both volcanic and non-volcanic margins are rifted margins formed by a protracted rift development.

Skogseid, Jakob

2014-05-01

127

Diachronous evolution of Late Jurassic-Cretaceous continental rifting in the northeast Atlantic (west Iberian margin)  

NASA Astrophysics Data System (ADS)

Regional (2-D) seismic reflection profiles, outcrop, and borehole data are used to characterize the evolution of deep offshore sedimentary basins in southwest Iberia (Alentejo Basin). The interpreted data indicate the bulk of Late Jurassic-earliest Cretaceous subsidence occurred in the present-day continental slope area, as shown by (1) significant thickening of synrift strata basinward from a slope-bounding fault system (SFS), west of which the total thickness of sediment can reach more than 9.0 km, and (2) relatively thin Mesozoic strata east of the SFS, where thickening of synrift units against principal faults is limited. Five principal regressive events and their basal unconformities reflect tectonic uplift and relative emersion in proximal basins, which were located on the rift shoulder to subsiding tilt blocks west of the SFS. These regressive events are correlated with major rift-related events occurring on the deeper margin. Direct comparisons with the Peniche Basin of northwest Iberia reveal that significant portions of the Iberian lower plate margin were uplifted and eroded during the last stages of continental rifting. This process was repeated at different times (and in different areas) as the locus of rifting and continental breakup migrated northward. As a result, two distinct rift axes are recognized in west Iberia, a first axis extending from the Porto Basin to the Alentejo Basin and a second axis located on the outer proximal margin north of 38°30N. In addition, the SFS delimited (1) prograding deposits of Cretaceous-Paleogene age and (2) late Cenozoic deposits draping the modern continental slope. These latter facts demonstrate that on lower plate passive margins, the relative position of the continental slope is established during the final rifting episode(s) preceding continental breakup.

Alves, Tiago M.; Moita, Carlos; Cunha, Tiago; Ullnaess, Magnar; Myklebust, Reidun; Monteiro, José H.; Manuppella, G.

2009-08-01

128

Wintertime pytoplankton bloom in the Subarctic Pacific supportedby continental margin iron  

SciTech Connect

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.

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

2004-06-08

129

Data based 3D modelling of the southwest African continental margin  

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

130

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

131

Pre-glacial and Glacial Loading of the Western Barents Sea Transform Margin  

NASA Astrophysics Data System (ADS)

A distinctive feature of the western Barents Sea continental margin, between Norway and Svalbard, are the up to 6-km-thick Bear Island and Storfjorden fans. These fans have been deposited onto oceanic crust that developed in an Eocene segmented transform margin setting and later by slow, oblique seafloor spreading between Eurasia and Greenland. Approximately two thirds of this sedimentary load have been deposited since the late Cenozoic onset of glaciations (~2.6 Ma). At the same time, the Barents Sea shelf was uplifted and large glacially eroded troughs merge into the fan depocenters. The fans are associated with >50 mGal free-air anomalies, the peaks of which coincide with the maximum glacial sediment thickness, and similar high-amplitude gravity anomalies are seen along several Arctic and Antarctic margins. We present new interpretations of magnetic seafloor spreading anomalies, the continent-ocean boundary, and structural elements along the western Barents Sea margin, giving a detailed view of the Eocene transform margin development and new understanding of the crust loaded by the fan deposits. The interpretations are constrained by an extensive multi-channel seismic database as well as the most up-to-date gravity, magnetic and bathymetric maps of the region. We also present the results of a process-oriented backstripping and gravity modeling technique to determine the elastic thickness (Te) and, hence, strength of the lithosphere that best matches the observed gravity field. Our results suggest that the large fan-related free-air anomalies cannot be reproduced unless significant strength is assumed for the oceanic and rifted continental lithosphere, especially during the glacial loading event. Furthermore, they raise concerns about how the Te structure of the lithosphere should be computed. For example, the Eocene transform margin may have seen both spatially and temporally variable Te while rifting and seafloor spreading prograded along the transform fault zone. The computed Te structure also has implications on where the flexural bulges and moats associated with glacial deposition and erosion are located, with further implications for basin modeling and reconstructions of paleo-drainage patterns in the western Barents Sea.

Engen, O.; Watts, A. B.; Faleide, J. I.

2006-12-01

132

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

133

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

134

Earthquakes and crustal structure beneath the central Cascadia continental margin  

NASA Astrophysics Data System (ADS)

In the summer of 2004, two clusters of "repeating" earthquakes occurred beneath the continental shelf of the central Cascadia subduction zone near 44.5N, 124.5W where the subduction megathrust is thought to be locked or transitional. The largest event in each cluster reached moment magnitude M=4.8-4.9. Seismicity has continued since with small (M<3) earthquakes occurring in each cluster on August 23-25, 2007. Moment tensor analysis for the main shock in each cluster indicates a 6-15 degree eastward dipping fault plane, consistent with the plate boundary dip of ~12 degrees. One cluster is serendipitously occurring on a transect along which crustal structure is well known from active source seismic experiments, and raytracing through this crustal model to match observed relative arrival times of secondary phases indicates a source depth of 16 ± 1 km, within 1 km of the plate boundary. This segment of the forearc also displays several characteristics indicative of along-strike and down-dip variations in plate coupling including: a subducted ridge on the downgoing plate; a "bright spot" on the plate boundary at a depth of ~15-20 km; an along-strike change in the gravity field and basement depth; a transition in plate coupling indicated by inversion of GPS data; geologic indications of active folding in the upper plate; and anomalous deformation in the adjacent oceanic plate. On the other hand, no obvious correlation with ETS in this region is observed. In September, 2007, we deployed an array of ocean bottom seismometers to record microseismicity and distinguish among several possible models for the physical properties of the megathrust.

Trehu, A. M.; Braunmiller, J.; Nabelek, J. L.

2004-12-01

135

Earthquakes and crustal structure beneath the central Cascadia continental margin  

NASA Astrophysics Data System (ADS)

In the summer of 2004, two clusters of "repeating" earthquakes occurred beneath the continental shelf of the central Cascadia subduction zone near 44.5N, 124.5W where the subduction megathrust is thought to be locked or transitional. The largest event in each cluster reached moment magnitude M=4.8-4.9. Seismicity has continued since with small (M<3) earthquakes occurring in each cluster on August 23-25, 2007. Moment tensor analysis for the main shock in each cluster indicates a 6-15 degree eastward dipping fault plane, consistent with the plate boundary dip of ~12 degrees. One cluster is serendipitously occurring on a transect along which crustal structure is well known from active source seismic experiments, and raytracing through this crustal model to match observed relative arrival times of secondary phases indicates a source depth of 16 ± 1 km, within 1 km of the plate boundary. This segment of the forearc also displays several characteristics indicative of along-strike and down-dip variations in plate coupling including: a subducted ridge on the downgoing plate; a "bright spot" on the plate boundary at a depth of ~15-20 km; an along-strike change in the gravity field and basement depth; a transition in plate coupling indicated by inversion of GPS data; geologic indications of active folding in the upper plate; and anomalous deformation in the adjacent oceanic plate. On the other hand, no obvious correlation with ETS in this region is observed. In September, 2007, we deployed an array of ocean bottom seismometers to record microseismicity and distinguish among several possible models for the physical properties of the megathrust.

Trehu, A. M.; Braunmiller, J.; Nabelek, J. L.

2007-12-01

136

Erosion and tectonics at the margins of continental plateaus  

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

137

Seismic structure of the U.S. Mid-Atlantic continental margin  

NASA Astrophysics Data System (ADS)

Multichannel and wide-angle seismic data collected off Virginia during the 1990 EDGE Mid-Atlantic seismic experiment provide the most detailed image to date of the continent-ocean transition on the U.S. Atlantic margin. Multichannel data were acquired using a 10,800 cu inch (177 L) airgun array and 6-km-long streamer, and coincident wide-angle data were recorded by ten ocean bottom seismic instruments. A velocity model constructed by inversion of wide-angle and vertical-incidence travel times shows strong lateral changes in deep-crustal structure across the margin. Lower-crustal velocities are 6.8 km/s in rifted continental crust, increase to 7.5 km/s beneath the outer continental shelf, and decrease to 7.0 km/s in oceanic crust. Prominent seaward- dipping reflections comprise a 100-km-wide, 25-km-thick ocean- continent transition zone that consists almost entirely of mafic igneous material accreted to the margin during continental breakup. The boundary between rifted continental crust and this thick igneous crust is abrupt, occupying only about 20 km of the margin. Appalachian intracrustal reflectivity largely disappears across this boundary as velocity increases from 5.9 km/s to greater than 7.0 km/s, implying that the reflectivity is disrupted by massive intrusion and that very little continental crust persists seaward of the reflective crust persists seaward of the reflective crust. The thick igneous crust is spatially correlated with the East Coast magnetic anomaly, implying that the basalts and underlying intrusives cause the anomaly. The details of the seismic structure and lack of independent evidence for an appropriately located hotspot in the central Atlantic imply that nonplume processes are responsible for the igneous material.

Holbrook, W. Steven; Purdy, G. M.; Sheridan, R. E.; Glover, L., III; Talwani, M.; Ewing, J.; Hutchinson, D.

1994-09-01

138

Gravity anomalies and segmentation of the continental margin offshore West Africa  

NASA Astrophysics Data System (ADS)

The free-air 'edge effect' anomaly at passive continental margins provides constraints on the thermal and mechanical properties of the lithosphere in extensional regions. Like many margins, the edge effect 'high' offshore Gabon, West Africa is located at the shelf break in the region of maximum sediment thickness. Gabon differs, however, from other margins in that the edge effect 'low' is displaced from over the continental rise to the slope. The origin of the low has been investigated by backstripping the sediments at the margin. We have used the backstrip to calculate the amount of crustal thinning, assuming Airy isostasy. Such a model implies zero strength during rifting. Alternatively, this amount of crustal thinning can be explained by a strong margin and a depth of necking that exactly balances the upward and downward forces that act on the crust during rifting. We have developed a method, which takes into account variations in the strength during rifting and the depth of necking, to compute the thinning directly from the sediment backstrip. The backstrip and the crustal thinning have been used to calculate the rifting gravity anomaly. Differences between the backstrip and the present-day water depth give the geometry of the sediment load and hence, the contribution to the gaavity anomaly of sedimentation. By comparing the combined rifting and sedimentation anomaly to the observed free-air gravity anomaly, we have been able to constrain the flexural rigidity (and equivalent elastic thickness, T e) of the lithosphere during and after rifting. The best fitting model is one in which the 'low' is caused by a 125 km wide zone of relatively thin (i.e. ˜15 km) continental crust which was weak (i.e. 0 < T e < 10k ) during rifting and has remained so since then. We speculate that the weak zone extends for 350-400 km along-strike of the Gabon margin and that passive margins may be highly segmented as regards their long-term strength.

Watts, A. B.; Stewart, J.

1998-03-01

139

Evolution of continental slope gullies on the northern california margin  

USGS Publications Warehouse

A series of subparallel, downslope-trending gullies on the northern California continental slope is revealed on high-resolution seismic reflection profiles imaging the uppermost 50 m of sediment. The gullies are typically 100 m wide and have 1 to 3 m of relief. They extend for 10 to 15 km down the slope and merge into larger channels that feed the Trinity Canyon. In the lower half of the 50 m stratigraphic section, the gullies increase in both relief and number up section, to maxima at a surface 5 to 10 m below the last glacial maximum lowstand surface. Gully relief increased as interfluves aggraded more rapidly than thalwegs. Erosion is not evident in the gully bottoms, therefore gully growth was probably due to reduced sediment deposition within the gullies relative to that on interfluves. As the gullies increased in relief, their heads extended upslope toward the shelfbreak. At all times, a minimum of 10 km of non-gullied upper slope and shelf stretched between the heads of the gullies and the paleo-shoreline; the gullies did not connect with a subaerial drainage network at any time. Gully growth occurred when the gully heads were in relatively shallow water (??? 200 m paleo-water depth) and were closest to potential sediment sources. We suggest that prior to the last glacial maximum, the Mad River, then within 10 km of the gully heads, supplied sediment to the upper slope, which fed downslope-eroding sediment flows. These flows removed sediment from nearly parallel gullies at a rate slightly slower than sediment accumulation from the Eel River, 40 km to the south. The process or processes responsible for gully growth and maintenance prior to the last glacial maximum effectively ceased following the lowstand, when sea level rose and gully heads lay in deeper water (??? 300 m water depth), farther from potential sediment sources. During sea-level highstand, the Mad River is separated from the gully heads by a shelf 30 km wide and no longer feeds sediment flows down the gullies, which fill with sediment from the distal Eel River. Approximately one-half of the subsurface gullies have no expression on the seafloor, because they have completely filled with sediment following the last glacial maximum lowstand of sea level. Copyright ?? 2001, SEPM (Society for Sedimentary Geology).

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

2001-01-01

140

Crustal and upper mantle structure beneath south-western margin of the Arabian Peninsula from teleseismic tomography  

NASA Astrophysics Data System (ADS)

image the lithospheric and upper asthenospheric structure of western continental Yemen with 24 broadband stations to evaluate the role of the Afar plume on the evolution of the continental margin and its extent eastward along the Gulf of Aden. We use teleseismic tomography to compute relative P wave velocity variations in south-western Yemen down to 300 km depth. Published receiver function analysis suggest a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden, consistent with the velocity structure that we retrieve in our model. The mantle part of the model is dominated by the presence of a low-velocity anomaly in which we infer partial melting just below thick Oligocene flood basalts and recent off-axis volcanic events (from 15 Ma to present). This low-velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. Our new P wave velocity model beneath western Yemen suggests the young rift flank volcanoes beneath margins and on the flanks of the Red Sea rift are caused by focused small-scale diapiric upwelling from a broad region of hot mantle beneath the area. Our work shows that relatively hot mantle, along with partial melting of the mantle, can persist beneath rifted margins after breakup has occurred.

Korostelev, Félicie; Basuyau, Clémence; Leroy, Sylvie; Tiberi, Christel; Ahmed, Abdulhakim; Stuart, Graham W.; Keir, Derek; Rolandone, Frédérique; Ganad, Ismail; Khanbari, Khaled; Boschi, Lapo

2014-07-01

141

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

USGS Publications Warehouse

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

Kvenvolden, K.A.

1985-01-01

142

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

NASA Technical Reports Server (NTRS)

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

Burke, Kevin

1988-01-01

143

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

NASA Astrophysics Data System (ADS)

The architecture of magma-poor continental margins is remarkably variable. The width of highly thinned continental crust (with a thickness < 10 km) varies from 70 km off Iberia, and 200 km offshore Angola, to over 300 km in the Antarctic Enderby Basin. The respective conjugate margin, however, is restricted to few tens of kilometres resulting in large scale crustal asymmetry. Growing evidence from rifted continental margins in the North and South Atlantic, as well as from the East Australia/Lord Howe Rise margin pair supports the idea that rifts with a very wide margin and a narrow conjugate are rather the rule than the exception. In this study, we use numerical thermo-mechanical models to investigate the dynamics of rifting. Our simulations apply an elasto-visco-plastic rheology formulation that relies on laboratory-derived flow laws for crustal and mantle rock. The models are constrained by geophysical and geological observations like limited melt generation, cold initial geotherms, and mafic lower crustal rheology. We show that small-scale lateral rift migration simultaneously explains the observed margin asymmetry and the presence of highly thinned continental crust. Rift migration results from two fundamental processes: (1) Strain hardening in the rift centre due to cooling of upwelling mantle material; (2) Formation of a low viscosity exhumation channel adjacent to the rift centre that is generated by heat transfer from the upwelling mantle and enhanced by viscous strain softening. Rift migration takes place in a steady-state manner and is accomplished by oceanward-younging sequential faults within the upper crust and balanced through lower crustal flow. We demonstrate that the rate of extension has paramount control on margin width. Since higher velocities lead to elevated heat flow within the rift and hence to hot and weak lower crust, a larger low-viscosity exhumation channel is generated that facilitates rift migration leading to wider margins. The South Atlantic is an ideal test bed for the hypothesis of velocity-dependent margin width since rifting was fast in the south, but slow in the northern part. As predicted by our numerical models, the maximum present-day margin width increases almost linearly from the conjugate Equatorial margin segments to the Florianopolis/Walvis ridge. Even though the polarity of the magma-poor South Atlantic margins alternates, the asymmetry and the width of the wider margin are in very good agreement with our simulations. The described rift evolution has three fundamental implications: (1) It implies sustained transfer of material across the extensional plate boundary thereby predicting that large portions of a wide margin originate from its conjugate side. (2) Migration of the deformation locus causes faulting in the distal parts of the margin to postdate that of the proximal parts by as much as 10 million years. This means that syn-rift and post-rift phase are location-dependent. (3) Lateral movement of the rift centre generates drastically different peak heat flow and subsidence histories at the proximal and the distal margin.

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

2014-05-01

144

Gravity anomalies and segmentation at the East Coast, USA continental margin  

NASA Astrophysics Data System (ADS)

The free-air gravity `edge effect' anomaly at rifted continental margins has generally been attributed to the transition between thick continental and thin oceanic crust. While crustal thinning is a major contributor, recent studies suggest that sediment loading and magmatism may significantly modify the edge effect anomaly and cause it, at some margins, to be highly segmented along their strike. In this paper, we use a combined 3-D flexural backstripping and gravity anomaly modelling technique to determine the role that sediment loading has played in controlling the segmentation of Atlantic-type continental margins. We focus on the East Coast, USA since a substantial amount of high-quality seismic reflection and refraction, gravity, and magnetic data already exists for this margin. By calculating the gravity anomaly associated with rifting and sediment loading and iteratively comparing it to the observed free-air anomaly, we have determined the `best-fit' elastic thickness, Te, structure of the margin. We show that 0 < Te < 40 km and that Te varies across-strike and along-strike the margin. Since Te is a proxy for the long-term (>105 a) integrated strength of the lithosphere, these results imply that weak regions abut strong ones at the East Coast, USA margin. Te generally decreases with increase in the amounts of crustal thinning, ?, and the flexed basement curvature, K, suggesting it is controlled, at least in part, by the mechanical structure of the pre-rift lithosphere and yielding due to flexural loading. However, there is considerable scatter, suggesting other factors such as along-strike changes in crustal composition. Irrespective, we show that an isostatic anomaly that takes into account the `best-fit' Te distribution is significantly reduced in spectral power compared with one which is computed assuming only Airy compensation (i.e. Te = 0 km). This is not to imply that rifting and sediment loading completely accounts for the anomalies along-strike and across-strike the East Coast, USA margin. To the contrary, significant isostatic anomaly highs and lows persist, especially in inner and middle shelf regions. One of the most prominent is an arcuate, 670 km long, high with flanking lows offshore Carolina that we attribute to magmatism during the initial stages of continental break-up.

Wyer, P.; Watts, A. B.

2006-09-01

145

Deep structure of the western South African passive margin — Results of a combined approach of seismic, gravity and isostatic investigations  

NASA Astrophysics Data System (ADS)

The passive margin of the South Atlantic shows typical features of a rifted volcanic continental margin, encompassing seaward dipping reflectors, continental flood basalts and high-velocity/density lower crust at the continent-ocean transition, probably emplaced during initial seafloor spreading in the Early Cretaceous. The Springbok profile offshore western South Africa is a combined transect of reflection and refraction seismic data. This paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives. The velocity modelling revealed a segmentation of the margin into three distinct parts of continental, transitional and oceanic crust. As observed at many volcanic margins, the lower crust is characterised by a zone of high velocities with up to 7.4 km/s. The conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities, found to be 3100 kg/m 3. Both approaches identified the body to have a thickness of about 10 km. Yet, the gravity modelling predicted the transition between the high-density body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal. Finally, isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced. The results imply isostatic equilibrium over large parts of the profile, smaller deviations are supposed to be compensated regionally. The calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km.

Hirsch, Katja K.; Bauer, Klaus; Scheck-Wenderoth, Magdalena

2009-05-01

146

Quaternary contourite drifts of the Western Spitsbergen margin  

NASA Astrophysics Data System (ADS)

The study of contourite drifts is an increasingly used tool for understanding the climate history of the oceans. In this paper we analyse two contourite drifts along the continental margin west of Spitsbergen, just south of the Fram Strait where significant water mass exchanges impact the Arctic climate. We detail the internal geometry and the morphologic characteristics of the two drifts on the base of multichannel seismic reflection data, sub-bottom profiles and bathymetry. These mounded features, that we propose to name Isfjorden and Bellsund drifts, are located on the continental slope between 1200 and 1800 m depth, whereas the upper slope is characterized by reduced- or non-deposition. The more distinct Isfjorden Drift is about 25 km wide and 45 km long, and over 200 ms TWT thick. We revise the 13 years-long time series of velocity, temperature, and salinity obtained from a mooring array across the Fram Strait. Two distinct current cores are visible in the long-term average. The shallower current core has an average northward velocity of about 20 cm/s, while the deeper bottom current core at about 1450 m depth has an average northward velocity of about 9 cm/s. We consider Norwegian Sea Deep Water episodically ventilated by relatively dense and turbid shelf water from the Barents Sea responsible for the accumulation of the contourites. The onset of the drift growth west of Spitsbergen is inferred to be about 1.3 Ma and related to the Early Pleistocene glacial expansion recorded in the area. The lack of mounded contouritic deposits on the continental slope of the Storfjorden is related to consecutive erosion by glacigenic debris flows. The Isfjorden and Bellsund drifts are inferred to contain the record of the regional palaeoceanography and glacial history and may constitute an excellent target of future scientific drilling.

Rebesco, Michele; Wåhlin, Anna; Laberg, Jan Sverre; Schauer, Ursula; Beszczynska-Möller, Agnieszka; Lucchi, Renata Giulia; Noormets, Riko; Accettella, Daniela; Zarayskaya, Yulia; Diviacco, Paolo

2013-09-01

147

Subsidence of the Voring Basin and the influence of the Atlantic continental margin  

Microsoft Academic Search

The post-Cretaceous subsidence history of the Vøring Basin, part of the Atlantic passive margin oVshore mid-Norway, has been investigated. Extension and ‚-factors related to rifting and continental break-up during the Palaeocene have been quantified using both forward and reverse basin-modelling techniques. In the preferred geological model it is assumed that rifting occurred in the Vøring Basin during the Palaeocene (prior

ALAN M. ROBERTS; ERIK R. LUNDIN; NICK J. KUSZNIR

1997-01-01

148

Stratigraphy and tectonic significance of Lower Paleozoic continental margin strata in northeastern Washington  

NASA Astrophysics Data System (ADS)

Lower Paleozoic eugeoclinal strata in the Kootenay Arc in northeastern Washington and southeastern British Columbia are transitional between autochthonous lower Paleozoic miogeoclinal strata and outboard volcanic arc terranes of uncertain paleogeographic affinity. They provide a record of lower Paleozoic continental margin depositional and tectonic processes oceanward of the continental shelf. The southernmost stratigraphic unit, the Covada Group, is divided into two formations, the Daisy Formation, a mid fan sequence of arkosic and subarkosic wacke and arenite, and the Early Ordovician Butcher Mountain Formation, consisting of alkalic(?) pillow basalt and tuff of within-plate affinity. Another unit, formerly part of the Covada Group, is excluded and informally named the Bradeen Hill assemblage. It contains chert, chert-quartz arenite, quartz arenite, chert pebble conglomerate, shale, and basalt, and may be Ordovician to Devonian on the basis of stratigraphic evidence and regional correlations. The Covada Group and Bradeen Hill assemblage record (1) deposition of continentally derived sediments in a submarine fan setting, (2) relatively quiescent starved basin conditions, (3) local faulting; and (4) intermittent periods of volcanism, perhaps reflecting local extension. They can be correlated with other stratigraphic units in the Kootenay Arc and resemble units as far north as the Selwyn basin in northern Canada and as far south as the Roberts Mountains allochthon in central Nevada. This unites the stratigraphic record and implies a high degree of synchroneity of tectonic events along over 2500 km of the outer continental margin during early Paleozoic time.

Smith, Moira T.; Gehrels, George E.

1992-06-01

149

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

USGS Publications Warehouse

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

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

1998-01-01

150

Seismic investigation of the transition from continental to oceanic subduction along the western Hellenic Subduction Zone  

E-print Network

The western Hellenic subduction zone (WHSZ) exhibits well-documented along-strike variations in lithosphere density (i.e., oceanic versus continental), subduction rates, and overriding plate extension. Differences in slab ...

Pearce, Frederick Douglas

151

Rift migration explains continental margin asymmetry and crustal hyper-extension  

PubMed Central

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

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

2014-01-01

152

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

PubMed

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

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

2014-01-01

153

Rift migration explains continental margin asymmetry and crustal hyper-extension  

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

154

The deep structure of the east Oman continental margin: preliminary results and interpretation  

NASA Astrophysics Data System (ADS)

We describe preliminary results of a coincident normal incidence and wide-angle Seismic experiment across the east Oman continental margin just north of the Masirah Island ophiolite. This margin is affected by tectonic deformation and may be the site of margin-parallel shear in a zone parallel to the Owen Fracture Zone. The reflection profile is used to define upper crustal structure and shows a deep offshore basin dammed oceanwards by a ridge, interpreted here as a rotated fault block. Wide-angle data were collected using ten digital ocean-bottom Seismometers and 110 explosive shots. Preliminary raytracing of a crustal model based on the gravity model of Whitmarsh (1979) shows rapid changes in crustal thickness across the margin. A steep landward dipping reflector, probably the Moho, lies beneath the continental slope. The orientation of this reflector is exactly opposite to the direction of tectonic fabric predicted by a simple overthrust model of ophiolite emplacement from the ocean basin to the east.

Barton, P. J.; Owen, T. R. E.; White, R. S.

1990-02-01

155

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

Microsoft Academic Search

Major rivers (and associated deltaic environments) provide the dominant pathway for the input of terrestrial-derived organic carbon in sediments (TOCT) to the ocean. Natural watershed processes and land-use changes are important in dictating the amount and character of carbon being buried on continental margins. Seven core sites were occupied on the Louisiana continental margin aboard the R\\/V Pelican in July

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

2011-01-01

156

Gas hydrates on continental margins of the United States based on occurrence of bottom simulating reflectors on marine seismic records  

Microsoft Academic Search

Natural-gas hydrates are solid, ice-like mixtures of natural gas (mainly methane) and water that are found in continental margin sediments at depths of water greater than about 500 m. The presence of gas hydrates in continental margins is commonly inferred from marine seismic surveys. The base of the gas-hydrate zone often correlates with an anomalous acoustic reflector which approximately parallels

Kvenvolden

1983-01-01

157

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

E-print Network

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

Demouchy, Sylvie

158

Geometry of the neoproterozoic and paleozoic rift margin of western Laurentia: Implications for mineral deposit settings  

USGS Publications Warehouse

The U.S. and Canadian Cordilleran miogeocline evolved during several phases of Cryogenian-Devonian intracontinental rifting that formed the western mangin of Laurentia. Recent field and dating studies across central Idaho and northern Nevada result in identification of two segments of the rift margin. Resulting interpretations of rift geometry in the northern U.S. Cordillera are compatible with interpretations of northwest- striking asymmetric extensional segments subdivided by northeast-striking transform and transfer segments. The new interpretation permits integration of miogeoclinal segments along the length of the western North American Cordillera. For the U.S. Cordillera, miogeoclinal segments include the St. Mary-Moyie transform, eastern Washington- eastern Idaho upper-plate margin, Snake River transfer, Nevada-Utah lower-plate margin, and Mina transfer. The rift is orthogonal to most older basement domains, but the location of the transform-transfer zones suggests control of them by basement domain boundaries. The zigzag geometry of reentrants and promontories along the rift is paralleled by salients and recesses in younger thrust belts and by segmentation of younger extensional domains. Likewise, transform transfer zones localized subsequent transcurrent structures and igneous activity. Sediment-hosted mineral deposits trace the same zigzag geometry along the margin. Sedimentary exhalative (sedex) Zn-Pb-Ag ??Au and barite mineral deposits formed in continental-slope rocks during the Late Devonian-Mississippian and to a lesser degree, during the Cambrian-Early Ordovician. Such deposits formed during episodes of renewed extension along miogeoclinal segments. Carbonate-hosted Mississippi Valley- type (MVT) Zn-Pb deposits formed in structurally reactivated continental shelf rocks during the Late Devonian-Mississippian and Mesozoic due to reactivation of preexisting structures. The distribution and abundance of sedex and MVT deposits are controlled by the polarity and kinematics of the rift segment. Locally, discrete mineral belts parallel secondary structures such as rotated crustal blocks at depth that produced sedimentary subbasins and conduits for hydrothermal fluids. Where the miogeocline was overprinted by Mesozoic and Cenozoic deformation and magmatism, igneous rock-related mineral deposits are common. ??2008 Geological Society of America.

Lund, K.

2008-01-01

159

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

USGS Publications Warehouse

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

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

2001-01-01

160

Deposition processes from echo-character mapping along the western Algerian margin (OranTenes), Western Mediterranean  

E-print Network

on this margin on 2­5.2 kHz Chirp echo- sounder data (MARADJA 2003 cruise). The echo-character mapsDeposition processes from echo-character mapping along the western Algerian margin (Oran online 23 July 2008 Keywords: Algerian margin Sedimentary processes Echo character Seismic facies Mass

Déverchère, Jacques

161

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

NASA Astrophysics Data System (ADS)

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 next four years an unprecedented phase of surveying and analysis on margins will take place in order to prepare for the deadline. The international scientific community as a whole must recognise the potential for research in this work and ensure the data is made available as soon as practically possible for the scientific community. In conclusion, by way of a reality check, this presentation highlights the likely areas of most intense UNCLOS-driven research activity up to 2009, the type of data acquisition anticipated and their likely location, and speculates on the areas of understanding of margin evolution which will be most advanced by this process.

Parson, L.

2005-12-01

162

Nature of organic matter in surface sediments from the Pakistan continental margin and the deep Arabian Sea: amino acids  

NASA Astrophysics Data System (ADS)

The role of organic carbon input, sedimentation rate and bottom water oxygenation (BWO) for organic matter (OM) degradation was investigated using surface sediments collected from above, within and below the oxygen minimum zone (OMZ) within varying sedimentary regimes along the Pakistan continental margin. For this, samples from 47 box cores and multi cores were analyzed for their bulk component (organic carbon, total nitrogen, carbonate, opal) and amino acid concentrations. The results are compared to plankton net samples from the Arabian Sea, sinking particles collected at the Pakistan continental margin, cultured sedimentary deep-sea bacteria, and data from the deep Arabian Sea basins. High organic carbon and amino acid concentrations were found in the laminated sediments of the OMZ, in bioturbated sediments immediately below the OMZ, and below regions of high productivity in the western Arabian Sea. The contribution of amino acids to organic carbon and total nitrogen as well as the molar contribution of certain non-protein amino acids do not mirror these differences in OC contents. No clear trend in relation to sediment structure or water depth is observed. In contrast to these established indicators of OM degradation, the molar contribution of the aromatic amino acid tyrosine (Tyr) exhibits a distinct pattern. Its contribution is relatively high in sediments at depths above the OMZ and further increases towards the center of the OMZ. Below the OMZ, it decreases continuously with increasing water depth. In addition, for samples from similar depths and bottom water oxygen contents Tyr shows systematic differences between varying sedimentary regimes. The lack of a distinct trend for the established indicators does not support the idea of a predominant control of BWO on quantitative OM preservation. Nevertheless, the variability of Tyr in relation to water depth or the sedimentary regime suggests that (i) the productivity-related OC input, (ii) the bulk accumulation rate, and (iii) BWO influence the alteration of the sedimentary OM by controlling its oxygen exposure time.

Suthhof, A.; Jennerjahn, T. C.; Schäfer, P.; Ittekkot, V.

2000-01-01

163

Prominent submarine mass wasting structures at the Southern Central Chilean continental margin: the Roca Slide  

NASA Astrophysics Data System (ADS)

Offshore Southern Central Chile (35 °S-42 °S), morphological expressions of voluminous submarine mass-wasting events are documented in swath bathymetry data. The variety in shape, water depth, runout direction and volume of these landslides points to a number of different processes at different phases of the evolution of the Chilean continental margin. A blocky Olistolith deposited in the Chile Trench, probably of Holocene age was targeted during RRS JAMES COOK Cruise JC23. The size of this feature, which was termed Roca Slide, as well as properties such as the very steep and high headscarp, the large runout distance and the cohesive nature of the slumped masses make it interesting in terms of the geotechnical issue of the continental lower slope stability and tsunami hazard. These questions are adressed by a morphometrical description of the Roca Slide as well as calculations of the volumes of both the missing and the dumped slump masses. Single channel seimic reflection profiles are used to elucidate the internal structure. Gravity core samples were retrieved in order to better date the mass wasting event, and relate it to the subduction dynamics and climate history of this particular segment of the Chilean continental margin. In particular, we discuss ideas regarding the nature of the possible weak layer which formed the gliding plane of this slump. Another, much larger and apparently older structure is observed at the middle continental slope. This arcuate depression is interpreted as a huge rotational slide. Remarkably, there is no present-day surface expression of slumped masses visible in the sediment-filled trench. However, a distinct change in the reflection character of the trench fill, observed in seismic reflection profiles across the trench may be explained by the sudden input of a very large sediment volume, possibly due to the slump event which caused the described mid-slope depression.

Voelker, D.; Weinrebe, W.; Behrmann, J.; Bialas, J.; Klaeschen, D.

2009-04-01

164

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

NASA Astrophysics Data System (ADS)

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

Müller, Claudia; Stein, Ruediger

2000-08-01

165

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

SciTech Connect

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

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

1990-05-01

166

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

167

The benthic manganese cycle along the Oregon-California continental margin  

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

168

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

NASA Astrophysics Data System (ADS)

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

Ragueneau, O.; Si-Webs Team

2003-04-01

169

Storm-induced upwelling of high pCO2 waters onto the continental shelf of the western Arctic Ocean  

E-print Network

Storm-induced upwelling of high pCO2 waters onto the continental shelf of the western Arctic Ocean system of the western Arctic Ocean is undergoing a rapid transition as sea ice extent and thickness in open water along the continental shelf of the Beaufort Sea in the western Arctic Ocean. During

Pickart, Robert S.

170

Crustal structure of the rifted volcanic margins and uplifted plateau of Western Yemen from receiver function analysis  

NASA Astrophysics Data System (ADS)

We analyse P-wave receiver functions across the western Gulf of Aden and southern Red Sea continental margins in Western Yemen to constrain crustal thickness, internal crustal structure, and bulk seismic velocity characteristics in order to address the role of magmatism, faulting and mechanical crustal thinning during continental breakup. We analyse teleseismic data from 21 stations forming the temporary Young Conjugate Margins Laboratory (YOCMAL) network together with GFZ and Yemeni permanent stations. Analysis of computed receiver functions shows that (1) the thickness of unextended crust on the Yemen plateau is ~35 km; (2) this thins to ~22 km in coastal areas and reaches less than 14 km on the Red Sea coast, where presence of a high velocity lower crust (HVLC) is evident. The average Vp/Vs ratio for the western Yemen Plateau is 1.79, increasing to ~1.92 near the Red Sea coast and decreasing to 1.68 for those stations located on or near the granitic rocks. Thinning of the crust, and by inference extension, occurs over a ~130 km wide transition zone from the Red Sea and Gulf of Aden coasts to the edges of the Yemen plateau. Thinning of continental crust is particularly localized in a <30-km-wide zone near the coastline, spatially co-incident with addition of magmatic underplate to the lower crust, above which at the surface we observe the presence of seaward dipping reflectors (SDRs)_and thickened Oligo-Miocene syn-rift basaltic flows. Our results strongly suggest the presence of high velocity mafic intrusions in the lower crust, which are likely either synrift magmatic intrusion into continental lower-crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the SDRs. Our results also point toward a regional breakup history in which the onset of rifting was synchronous along the western Gulf of Aden and southern Red Sea volcanic margins followed by a second phase of extension along the Red Sea margin.

Ahmed, Abdulhakim; Tiberi, Christel; Leroy, Sylvie; Stuart, Graham; Keir, Derek; Sholan, Jamal; Khanbari, Khaled; Al-Ganad, Ismeal; Basuyau, Clemence

2013-04-01

171

Crustal structure of the rifted volcanic margins and uplifted plateau of Western Yemen from receiver function analysis  

NASA Astrophysics Data System (ADS)

We analyse P-wave receiver functions across the western Gulf of Aden and southern Red Sea continental margins in Western Yemen to constrain crustal thickness, internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism, faulting and mechanical crustal thinning during continental breakup. We analyse teleseismic data from 21 stations forming the temporary Young Conjugate Margins Laboratory (YOCMAL) network together with GFZ and Yemeni permanent stations. Analysis of computed receiver functions shows that (1) the thickness of unextended crust on the Yemen plateau is ˜35 km; (2) this thins to ˜22 km in coastal areas and reaches less than 14 km on the Red Sea coast, where presence of a high-velocity lower crust is evident. The average Vp/Vs ratio for the western Yemen Plateau is 1.79, increasing to ˜1.92 near the Red Sea coast and decreasing to 1.68 for those stations located on or near the granitic rocks. Thinning of the crust, and by inference extension, occurs over a ˜130-km-wide transition zone from the Red Sea and Gulf of Aden coasts to the edges of the Yemen plateau. Thinning of continental crust is particularly localized in a <30-km-wide zone near the coastline, spatially co-incident with addition of magmatic underplate to the lower crust, above which on the surface we observe the presence of seaward dipping reflectors (SDRs) and thickened Oligo-Miocene syn-rift basaltic flows. Our results strongly suggest the presence of high-velocity mafic intrusions in the lower crust, which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the SDRs. Our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western Gulf of Aden and southern Red Sea volcanic margins followed by a second phase of extension along the Red Sea margin.

Ahmed, Abdulhakim; Tiberi, Christel; Leroy, Sylvie; Stuart, Graham W.; Keir, Derek; Sholan, Jamal; Khanbari, Khaled; Al-Ganad, Ismael; Basuyau, Clémence

2013-06-01

172

The influence of tectonics on flank margin cave formation on a passive continental margin: Naracoorte, Southeastern Australia  

NASA Astrophysics Data System (ADS)

Intensive cave development within the highly porous and permeable Eocene-Middle Miocene Gambier Limestone in southeastern South Australia is restricted to a 1 × 11 km area at Naracoorte. The caves are overwhelmingly horizontal, consisting of large solutional domes connected by smaller passages, with bell holes, small pendants and large non-directional scallops on the walls and ceilings. Orientation is strongly controlled by NW/SE joints. Cave entrances have been opened by subsequent collapse, and breakdown is common. The caves are located on the Kanawinka Fault escarpment, which was uplifted in the Late Miocene-Early Pliocene and then overlain by a series of Pleistocene carbonate strandline dunes, deposited as the sea retreated following a Late Miocene transgression. The coastline lay along the Kanawinka escarpment at ~ 0.9-1.1 Ma, when the caves formed just inland of the shoreline within the zone of enhanced dissolution at the seaward margin of the freshwater lens. They have the typical flank margin cave morphology, except that joint development adjacent to the fault caused the strong linear orientation parallel to the coastline. The beach dune deposited to the west of the Kanawinka escarpment shows that sea-level had dropped sufficiently to completely drain the caves at 780-880 ka. Cave development was therefore confined to a period spanning ~ 0.8-1.1 Ma. Continuing gradual uplift through the Pleistocene means that the caves are now > 100 km inland, obscuring the essentially coastal nature of the Naracoorte karst. Speleogenesis at Naracoorte reflects the interaction of neotectonics with coastal dissolution, and emphasizes the role that tectonism can play in Pleistocene karst development even on passive continental margins, where glacio/eustatic sea-level fluctuations are generally ascribed as the dominant role.

White, Susan; Webb, John A.

2015-01-01

173

Three-dimensional subsidence analysis and gravity modelling of the continental margin offshore Namibia  

NASA Astrophysics Data System (ADS)

Seismic reflection profiles and gravity anomaly data have been used to determine the structure and evolution of the Namibian continental margin. In comparison to other margins, the gravity anomaly at the Namibian margin shows a number of distinctive features. It lacks an offshore gravity `low', and, despite the presence of up to 9km of sediments, the gravity `high' is displaced landwards of the maximum sediment thickness. In an attempt to explain these features, the Early Cretaceous-Recent stratigraphic record is analysed using a combined 3-D backstripping and gravity modelling tech-nique that enables constraints to be placed on the long-term mechanical properties of the lithosphere. A neutral depth of necking and a relatively high flexural rigidity (or equivalent elastic thickness, Te, of ~ 25km) can explain part of the anomaly, at least in the south of the margin. However, large residual anomalies exist that can be reduced by the presence of pre-rift low-density sediments within the zone of maximum stretching, south of 22° S, high-density volcanic material at the eastern pinch-out of the lower rift sequence and a lateral sediment density variation across the shelf, slope and rise. In addition, the presence of a magmatic body at the base of the crust, provided that the flexural strength of the margin is high, significantly improves the fit between observed and calculated gravity anomalies. Recovered stretching factors enable predictions of the geometry of the Moho to be made. Palaeobathymetry is estimated along the entire margin through time by comparing the subsidence determined from backstripping to that predicted by simple models of rifting. These estimates are within the errors of observed palaeowater depths where well control exists. The requirement of high Te rather than Te that increases with time suggests that stretched continental crust extends offshore for some distance and the continent-ocean boundary occurs in relatively deep water near magnetic anomaly M4. Reconstruction of the gravity field of the South Atlantic at 100Ma illustrates that, in terms of their gravity signature, the South Atlantic margins are asymmetric.

Stewart, J.; Watts, A. B.; Bagguley, J. G.

2000-06-01

174

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

PubMed

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

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

2013-01-01

175

First Evidence for the Presence of Iron Oxidizing Zetaproteobacteria at the Levantine Continental Margins  

PubMed Central

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

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

2014-01-01

176

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

PubMed

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

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

2014-01-01

177

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

178

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

PubMed Central

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

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

2013-01-01

179

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

SciTech Connect

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

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

1988-03-01

180

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

SciTech Connect

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

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

1985-01-01

181

Tertiary Formation of the Faroes-Shetland Basin, NE Atlantic Margin, by Failed Continental Breakup  

NASA Astrophysics Data System (ADS)

The Faroes-Shetland Basin, an apparent intracontinental rift basin located to the north of the UK, is co-axial with the Moere rifted margin to the north-east which formed by sea-floor spreading initiation on the Aegir ridge at ~54 Ma. Although the Faroes-Shetland Basin experienced Late Jurassic and Early Cretaceous rifting and subsequent thermal relaxation, well logging and paleoenvironment mapping indicates that the basin became emergent in the Late Paleocene, before experiencing anomalously high rates of subsidence in the Early Eocene. Flexural backstripping and decompaction to 54 Ma has been carried out on regional stratigraphic cross sections to quantify the water loaded subsidence since the basin emergence at top Paleocene times. If a depth uniform intracontinental rifting model is used, lithospheric beta factors greater than three are required to restore the post-Paleocene subsidence of the basin. However fault heave estimates indicate that there was little stretching in the upper crust of the Faroes-Shetland Basin at Paleocene and Late Cretaceous times, and substantially less than that indicated by post-Paleocene thermal subsidence assuming depth-uniform lithospheric stretching. Residual post-rift thermal subsidence from Late Jurassic and Early Cretaceous rifting, or Palaeocene mantle plume dynamic uplift also cannot explain the Tertiary subsidence of the Faroes-Shetland Basin. The subsidence history of the Faroes-Shetland Basin since the Early Tertiary can be accounted for by thinning of the lithospheric mantle and lower crust beneath the Faeroe-Shetland Basin, contemporaneous with continental breakup and formation of the Moere rifted margin to the north. At ~54Ma, the Faeroe-Shetland Basin lay at the southern tip of the newly forming Aegir ocean ridge. Post-Palaeocene subsidence has been successfully modelled using a new model of continental lithosphere breakup and sea-floor spreading initiation in which thinning and rupture of continental lithosphere occurs due to an upwelling divergent flow field within continental lithosphere and asthenosphere. The model predicts that an ascending flow field, propagating upwards from the base of the lithosphere, first thinned the lithospheric mantle under the Faroes-Shetland Basin causing thermal uplift before thinning the lower crust, leading to rapid basin subsidence. While to the north the ascending flow field reached the surface and successfully ruptured the lithosphere giving continental breakup and sea-floor spreading initiation on the Aegir Ridge, under the Faroes-Shetland Basin the ascending flow field appears to have `died out' before reaching the surface. Upper Palaeocene basin collapse was followed by lithosphere thermal re-equilibration and thermal subsidence, which continues to the present day.

Fletcher, R. J.; Kusznir, N. J.; Roberts, A. M.

2005-12-01

182

Evolution of the SW African passive continental margin during the post-rift phase  

NASA Astrophysics Data System (ADS)

The tectonic evolution of the SW African margin and the breakup of the South Atlantic Ocean are still under debate. Furthermore, there are economic interests in terms of hydrocarbon resources. In particular, the understanding of the subsidence history at the SW African passive continental margin can help to investigate the evolution of this margin. For this reason, we aim to reconstruct paleotopographies for three time steps during the post-rift phase (112 Ma to present day). These three time steps are: Cretaceous-Tertiary boundary (67 Ma), Cenomanian-Turonian boundary (93 Ma) and start post-rift (112 Ma). We use a recent regional scale 3D structural model (Maystrenko et al., 2013) as base for our subsidence analysis. This model includes the upper mantle, the crystalline crust, four sedimentary units as well as the water column. The sedimentary units comprise sediments of the (1) Cenozoic, (2) base Turonian-base Cenozoic, (3) base Aptian-base Turonian and (4) pre-Aptian sediments. Therefore, our subsidence reconstruction has the particular advantage that we include as much present day information as possible. In order to reconstruct paleotopographies we calculate the subsidence components separately. On the one hand we determine the thermal subsidence due to cooling of the lithosphere. On the other hand, the load induced subsidence exerted by the preserved sedimentary cover is calculated by applying a backstripping method which considers local isostatic rebound and decompaction. Both the amount of thermal subsidence and the amount of load induced subsidence are then subtracted from the total subsidence which is nowadays observed. Subtracting these individual subsidence components leads to the paleotopographies. The paleotopographies provide information about the long-term behavior of the margin area since the beginning of the post-rift phase. Moreover, the paleotopographies provide the opportunity to estimate vertical movements which have occurred during the post-rift phase across the entire margin area and which can be compared to recent findings on e.g. vertical movements from onshore SW Africa or to results from subsidence analysis in local sub-basins. We find indications for two phases of uplift with a minimum estimate of the vertical movements in a range of a few hundred meters each. Maystrenko, Y. P., Scheck-Wenderoth, M., Hartwig, A., Anka, Z., Watts, A. B., Hirsch, K. K. Fishwick, S. (2013): Structural features of the Southwest African continental margin according to results of lithosphere-scale 3D gravity and thermal modelling. Tectonophysics 604, 104-121.

Dressel, Ingo; Scheck-Wenderoth, Magdalena; Götze, Hans-Jürgen; Reichert, Christian

2014-05-01

183

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

184

Shear wave velocity structure of Reed Bank, southern continental margin of the South China Sea  

NASA Astrophysics Data System (ADS)

The shear wave velocity structure of a wide angle seismic profile (OBS973-2) across Reed Bank in the southern continental margin of the South China Sea (SCS) is simulated by 2-D ray-tracing method, based on its previous P-wave model. This profile is 369-km-long and consists of fifteen three-component ocean bottom seismometers (OBS). The main results are as follows.(1) The model consists of seven layers and the shear wave velocity increases from 0.7 km/s at the top of sediment layer to 4.0 km/s in the lower crust. (2) The Moho depth decreases from 20-22 km at the Reed Bank to 9-11 km at the deep oceanic basin with the shear wave velocity of 4.2 km/s below the Moho. (3) The Vp/Vs ratio decreases with depth through the sedimentary layers, attributed to increased compaction and consolidation of the rocks. (4) In the continental upper crust (at model distance 90-170 km), S-wave velocity (2.5-3.2 km/s) is relatively low and Vp/Vs ratio (1.75-1.82) is relatively high compared with the other parts of the crust, corresponding to the lower P-wave velocity in the previous P-wave model and normal faults revealed by MCS data, indicating that a strong regional extensional movement had occurred during the formation process of the SCS at the Reed Bank area. (5) The S-wave structures indicate that Reed Bank crust has different rock compositions from that in the east section of the northern margin, denying the presence of conjugate relationship of Reed Bank with Dongsha islands. According to P-wave models and other data, we inferred that Reed Bank and Macclesfield were separated from the same continental crust during the rifting and break-up process.

Wei, Xiaodong; Ruan, Aiguo; Zhao, Minghui; Qiu, Xuelin; Wu, Zhenli; Niu, Xiongwei

2015-03-01

185

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

NASA Astrophysics Data System (ADS)

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

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

2003-04-01

186

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

NASA Astrophysics Data System (ADS)

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.

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

2014-05-01

187

Timing of Depth-dependent Lithosphere Stretching on the Lofoten Continental Margin: Pre-breakup or post-breakup?  

NASA Astrophysics Data System (ADS)

Depth-dependent stretching has been observed at both non-volcanic and volcanic rifted continental margins (Driscoll and Karner 1998, Davis and Kusznir 2003). Stretching estimates, independently determined from upper crustal faulting, whole crustal thinning and post-rift lithosphere thermal subsidence, show that extension increases with depth within ˜100 km of the COB such that whole-crustal and whole-lithosphere extension is significantly greater than upper-crustal extension. A key question is whether depth-dependent stretching occurs during pre-breakup rifting or during early sea-floor spreading. Flexural backstripping and forward structural-and-stratigraphic modelling show that depth-dependent lithosphere stretching occurs on the outer part of the Norwegian volcanic rifted margin. The Lofoten segment of the margin shows very large thinning of continental lithosphere within 100 km of the COB at continental breakup time (at ˜54 Ma), while the upper crust shows no significant faulting and extension at breakup, or in the Palaeocene preceding breakup. For the southern Lofoten Margin, beta stretching-factors approaching infinity are required for the 54 Ma rift event west of the Utrøst Ridge to restore Top Basalt and the Top Tåre (˜54 Ma) to presumed sub-aerial depositional environments in the area of the marginal high. In contrast Palaeocene upper crustal beta stretching-factors are no greater than 1.1. For the mid-Lofoten margin, post-breakup subsidence with a beta stretching-factor of infinity is insufficient to generate observed margin subsidence, implying that an additional Eocene crustal thinning event younger than 54 Ma is required. The absence of significant Palaeocene extension on the Lofoten margin prior to continental breakup, and the additional Eocene subsidence supports the hypothesis that depth-dependent stretching of continental rifted margin lithosphere occurs during early sea-floor spreading rather than during pre-breakup rifting. Fluid-flow models of ocean ridge processes using analytical or finite-element solutions show divergent motion of upwelling mantle beneath the ocean ridge, which when viewed in the reference frame of the young continental margin, predict oceanward flow of lower continental crust and lithospheric mantle of the young rifted margin, so giving depth dependent stretching.

Kusznir, N. J.; Hunsdale, R.; Roberts, A. M.

2003-04-01

188

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

USGS Publications Warehouse

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 subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid-volume sediment subduction at convergent ocean margins (???43,500 km) is calculated to be 1.5 km3/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long-term rate of sediment subduction at 1.0 km3/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km3/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long-term rates, which range from 25 to 50 km3/m.y. per kilometer of margin. A conservative long-term rate applicable to many sectors of convergent margins is 30 km3/km/m.y. If applied to the length of type 2 margins, subduction erosion removes and transports approximately 0.6 km3/yr of upper plate material to greater depths. At various places, subduction erosion also affects sectors of type 1 margins bordered by small- to medium-sized accretionary prisms (for example, Japan and Peru), thus increasing the global rate by possibly 0.5 km3/yr to a total of 1.1 km3/yr. Little information is available to assess subduction erosion at margins bordered by large accretionary prisms. Mass balance calculations allow assessments to be made of the amount of subducted sediment that bypasses the prism and underthrusts the margin's rock framework. This subcrustally subducted sediment is estimated at 0.7 km3/yr. Combined with the range of terrestrial matter removed from the margin's rock framework by subduction erosion, the global volume of subcrustally subducted materia

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

1991-01-01

189

Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin  

E-print Network

fissuring, diking and voluminous eruption of the Ethiopian- Yemen flood basalt pile. The dikes which fed the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood

Paris-Sud XI, Université de

190

ARCHITECTURE OF RIFTED CONTINENTAL MARGINS AND BREAKUP EVOLUTION: INSIGHTS FROM THE SOUTH ATLANTIC, NORTH ATLANTIC AND RED SEA – GULF OF ADEN CONJUGATE MARGINS  

E-print Network

The tectono-sedimentary development of the South Atlantic is compared with the Central Atlantic margins, which are associated with major episodes of magmatism during the Mesozoic continental breakup. Subsequently, the Cenozoic breakup in the North Atlantic led to the formation of the volcanic Norwegian-Greenland conjugate margins. However, the DSDP boreholes in the magma-poor Iberian-Newfoundland margins have confirmed the occurrence of exhumed mantle at the ocean-continent transition. This possibility has been suggested for the South Atlantic margins, but still lacks confirmation by drilling. The Red Sea and the Gulf of Aden may be considered as natural laboratories to study breakup processes and formation of divergent continental margins. Using key geological and geophysical data, we compare some of the structures observed in incipient stages of basin formation between the African and the Arabian plates with the structures observed in older sedimentary basins associated with the Gondwana breakup. We also analyze deep seismic reflection profiles and potential field data at the continent-ocean boundary of these conjugate margins, using palinspastic reconstructions to define the corresponding seismic pairs. We conclude the Red Sea and the Gulf of Aden display remarkable differences from the Iberian-Newfoundland margins and notable similarities with the South Atlantic margins.

unknown authors

2013-01-01

191

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

NASA Astrophysics Data System (ADS)

It has been proposed that some continental rifted margins have anomalous subsidence histories and that at breakup they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn or post breakup subsidence of this form would have important implications for our understanding of the geodynamics of continental breakup and rifted continental margin formation, margin subsidence history and the evolution of syn and post breakup depositional systems. We have investigated three rifted continental margins; the Gulf of Aden, Galicia Bank and the Gulf of Lions, to determine whether the oceanic crust in the ocean-continent transition of these margins has present day anomalous subsidence and if so, whether it is caused by mantle dynamic topography or anomalous oceanic crustal thickness. Residual depth anomalies (RDA) corrected for sediment loading, using flexural backstripping and decompaction, have been calculated by comparing observed and age predicted oceanic bathymetries in order to identify anomalous oceanic bathymetry and subsidence at these margins. Age predicted bathymetric anomalies have been calculated using the thermal plate model predictions from Crosby & McKenzie (2009). Non-zero sediment corrected RDAs may result from anomalous oceanic crustal thickness with respect to the global average, or from mantle dynamic uplift. Positive RDAs may result from thicker than average oceanic crust or mantle dynamic uplift; negative RDAs may result from thinner than average oceanic crust or mantle dynamic subsidence. Gravity inversion incorporating a lithosphere thermal gravity anomaly correction and sediment thickness from 2D seismic data has been used to determine Moho depth and oceanic crustal basement thickness. The reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. The gravity inversion crustal basement thicknesses together with Airy isostasy have been used to predict a "synthetic" gravity derived RDA. Sediment corrected RDA for oceanic crust in the Gulf of Aden are positive (+750m) indicating anomalous uplift with respect to normal subsidence. Gravity inversion predicts normal thickness oceanic crust and a zero "synthetic" gravity derived RDA in the oceanic domain. The difference between the positive sediment corrected RDA and the zero "synthetic" gravity derived RDA, implies that the anomalous subsidence reported in the Gulf of Aden is the result of mantle dynamic uplift. For the oceanic crust outboard of Galicia Bank both the sediment corrected RDA and the "synthetic" gravity derived RDA are negative (-800m) and of similar magnitude, indicating anomalous subsidence, which is the result of anomalously thin oceanic crust, not mantle dynamic topography. We conclude that there is negligible mantle dynamic topography influencing the Galicia Bank region. In the Gulf of Lions, gravity inversion predicts thinner than average oceanic crust. Both sediment corrected RDA (-1km) and "synthetic" gravity derived RDA (-500m) are negative. The more negative sediment corrected RDA compared with the "synthetic" gravity derived RDA implies that the anomalous subsidence in the Gulf of Lions is the result of mantle dynamic subsidence as well as thinner than average oceanic crust.

Cowie, L.; Kusznir, N. J.

2012-12-01

192

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

Microsoft Academic Search

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

P. de Deckker; William M. Last

1988-01-01

193

Thermal history of the Rio Muni (West Africa) NE Brazil margins during continental breakup  

NASA Astrophysics Data System (ADS)

We document the thermal record of breakup of the conjugate Rio Muni (West Africa) and NE Brazil margins using apatite fission track analysis, vitrinite reflectance data and stratigraphic observations from both margins. These results permit determination of the timing of four cooling episodes, and the temperature of samples at the onset of each episode. All samples are interpreted to have experienced higher temperatures in the geological past due to i) elevated basal heatflow (palaeogeothermal gradient in Rio Muni-1 well decaying from 58 °C/km during the Mid Cretaceous to 21.5 °C/km in the Late Cenozoic) and ii) progressive exhumation from formerly greater burial depth. A well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section (exhumation) than if a constant heatflow is assumed. Cooling episodes identified from the palaeotemperature data at 110-95 Ma (both margins) and 85-70 Ma (Rio Muni only) coincide with major unconformities signifying, respectively, the cessation of rifting (breakup) and compressional shortening that affected the African continent following the establishment of post-rift sedimentation (drift). The interval between these separate unconformities is occupied by allochthonous rafts of shallow-water carbonates recording gravitational collapse of a marginal platform. The rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent Ascension Fracture Zone, and thermal doming due to the coeval St Helena and Ascension Plumes. The data also reveal a c.45-35 Ma cooling episode, attributed to deep sea erosion at the onset of Eo-Oligocene ice growth, and a c.15-10 Ma episode interpreted as the record of Miocene exhumation of the West African continental margin related to continent-wide plume development. Integration of thermal history methods with traditional seismic- and stratigraphy-based observations yields a dynamic picture of kilometre-scale fluctuations in base level through the breakup and early drift phases of development of these margins. Major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy, but also regional plate margin-generated deformation, local 'intra-basinal' reorganization, and the amplifying effect of negative feedbacks between these processes.

Turner, Jonathan P.; Green, Paul F.; Holford, Simon P.; Lawrence, Stephen R.

2008-06-01

194

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

NASA Astrophysics Data System (ADS)

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

Handwerger, David A.; Jarrard, Richard D.

2003-12-01

195

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

196

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

NASA Astrophysics Data System (ADS)

In North Africa, a large amount of Mesozoic terrigenous sedimentary rocks are deposited in most of the basins along the continental margin indicating a major episode of erosion occurred during the rift and early post-rift period in the Central Atlantic. In the Tarfaya-Dakhla Basin, Morocco the sedimentary cover reaches thicknesses of up to 9000 m. The presence of high surface elevations in the Anti-Atlas mountain belt (2500 m) indicates a potential source area for the surrounding basins. The NE-SW oriented Anti-Atlas of Morocco is located at the northwestern fringe of the West African Craton and south of the High Atlas and represents the Phanerozoic foreland of the Late Paleozoic North African Variscides and the Cenozoic Atlas Belt. Variscan deformation affected most of Morocco. Paleozoic basins were folded and thrusted, with the major collision dated as late Devonian to Late Carboniferous. Zircon fission-track ages of 287 (±23) to 331 (±24) Ma confirmed the main exhumation referred to the Variscan folding, followed by rapid exhumation and the post-folding erosion. Currently, phases of uplift and exhumation in the Anti-Atlas during the Central Atlantic rifting and places where the associated erosion products are deposited are poorly constrained and there is little quantitative data available at present. The objective of the study is to determine the thermal and exhumation history of the Anti-Atlas and the connected Tarfaya-Dakhla Basin at the Moroccan passive continental margin. Besides zircon fission-track dating, apatite and zircon (U-Th-Sm)/He and apatite fission-track analyses and furthermore 2-D modelling with 'HeFTy' software has been carried out at Precambrian rocks of the Western Anti-Atlas and Cretaceous to Neogene sedimentary rocks from the Northern Tarfaya-Dakhla Basin. The apatite fission-track ages of 120 (±13) to 189 (±14) Ma in the Anti-Atlas and 176 (±20) to 216 (±18) Ma in the Tarfaya Basin indicate very obvious a Central Atlantic opening signal and confirm the Anti-Atlas as a potential source area of the Mesozoic basins along the passive continental margin. Young apatite (U-Th-Sm)/He ages of 49 (±3) Ma to 89 (±5) Ma in the Anti-Atlas and 64 (±4) to 73 (±4) Ma in the Tarfaya Basin are related to the interplay between the African and Eurasian plates. The time-temperature models of samples from the AA indicate that the main exhumation in the Anti-Atlas occurred during the Variscan folding, the post-folding erosion and besides the Central Atlantic rifting phase until the Upper Triassic. After this event large parts of the Western Anti-Atlas hold a stable position without significant movements during the Jurassic and Cretaceous, followed by an exhumation phase during the Atlasian orogeny.

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

2012-04-01

197

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

USGS Publications Warehouse

Sediment inventories of the cosmogenic radionuclide 7Be (t1/2=53 d) were measured on the Eel River shelf and slope (northern California continental margin) to investigate sedimentation processes associated with coastal river flooding. Seabed coring shortly after major riverflow events in 1995 and 1997 documented a shelf-wide flood deposit, and subsequent radionuclide studies determined 7Be to be a powerful tracer of fine-grained river sediment. In addition, distinctive signatures of 234Th and 210Pb were observed in oceanic flood deposits and provided additional information regarding depositional processes. During the 1995–1997 monitoring period, 7Be was present (2–35 dpm cm-2) in shelf and slope sediments only after periods of high rainfall and river runoff during the winter months. It is suggested that fluvial input was the primary source of 7Be in shelf sediments after the floods. 7Be sediment inventories and sediment-trap fluxes determined after the 1997 flood revealed that fine-grained fluvial sediments were rapidly (within one month) broadcast over the continental margin, to the 500 m isobath. Dispersal was apparently facilitated by energetic storm waves, which resuspended and redistributed some fraction of the suspended load residing on the shelf prior to accretion as flood deposits. These observations illustrate that floods are an important sedimentary process for modern environments of the Eel shelf and slope, and perhaps for other fluviomarine sedimentary systems of the northern California continental margin. Ratios of the 210Pb sediment-accumulation rate (100 yr average) to the 7Be deposition rate (1–2 month average) for shelf sites illustrate the episodic nature of shelf sedimentation, and suggest that a minimum of 3–30 depositional events complete the most recent stratigraphic record. This observation is consistent with the magnetude and frequency of fluvial sediment input, as Eel River floods with return periods of 3–33 yr (3% of the time of record) have supplied >80% of the total 85 yr suspended load. Based on radionuclide and hydrologic data, it can be concluded that a small number of flood depositional events have had a disproportionate impact on the sedimentary record of the Eel shelf.

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

1999-01-01

198

Study on the slope break belts in the Jurassic down-warped lacustrine basin in western-margin area, Junggar Basin, northwestern China  

Microsoft Academic Search

The Junggar Basin is one of the large-scale petroliferous basins in northwestern China. During the Jurassic Age it was a down-warped continental basin. Different types of slope break belts with different origins have been identified in the basin, including the fracture, flexure, erosional and depositional slope break belts. Fracture and flexure slope break belts were mainly developed in the western-margin

Liu Hao; Wang Yingmin; Xin Renchen; Wang Yuan

2006-01-01

199

ICLO Subduction; modelling differences in styles of continental extension between the Hellenic - W.Anatolian, the Pannonian - Carpathian, and the W. Mediterranean convergent margins  

NASA Astrophysics Data System (ADS)

ICLO (intra-collisional landlocked ocean) subduction (proposed by Edwards and Grasemann 2009) provides a mantle dynamics mechanism by which to explain key differences in the nature of continental extension at convergent margin settings. We present a new model that identifies why salient differences are present in the three greater Africa-Eurasia collision components of (1) The Hellenic - Western Anatolian subduction collisional system, (2) the Pannonian - Carpathian subduction collisional system and (3) the Western Mediterranean system. Key differences between these three systems are found in the presence or absence of: (A) core complexes: major in (1), i.e. Rhodope, Aegean, Menderes, Lycian; rare in (2), i.e. portions of Corsica, Calabria and Tuscany are present as scraps ripped off the Alps during the rotation and retreat of Italy. (B) sedimentary basins: major in (2), i.e. the Pannonian Basin; minor in (1), i.e. small trough areas in the Aegean; minor in (3), i.e. remnant shelf areas of the Ligurian-Provençal Basin before total failure of continental crust and oceanization due to extreme crustal stretching. (C) melt generation: significant I- and S-type plutonism in (1), mainly mantle melting in (2). Whereas the Hellenic - Western Anatolian system represents true "post-orogenic" collapse of continental collision (in that multiple continental crust elements collided over time to generate the present day, now massively extended back arc continental crust), the Pannonian - Carpathian and the W. Mediterranean systems were almost exclusively accretionary orogenesis convergent margins. This accretionary orogenesis however differs fundamentally from (e.g.) Andean type accretionary orogenesis (where hot crust with extensional collapse also exists) in that subducting slab geometries for this study's Mediterranean examples are extremely narrow and steep; mantle dynamics in Mediterranean type land-locked collision settings (thereby comprising limited subductable lithosphere) do not encourage widespread buoyed up subducted lithosphere (i.e. Farallon type flat slab subduction) and accordingly, the back arc is relatively heat-starved (at least until oceanization a la the Western Mediterranean). For the Hellenic - Western Anatolian system, the residual heat from the "true" continental collision alone is sufficient to obtain the observed extension-related phenomena in the presence of narrow and steep subducting slab geometry. Nevertheless, different slab geometry (dip angle) may have provided some heat over the past; this is especially likely during brief periods before or after discrete terrane collisions in order to elegantly generate the now-notorious spatio-temporally restricted pulses in Hellenic - Western Anatolian melting and / or extensional periods. This obviates the need to explain these pulses through changes in slab retreat velocity, albeit that ICLO subduction as a whole predicts accelerating subduction retreat velocity as a product of steadily narrowing subducting slab width. A linearly increasing retreat velocity, however, does elegantly obtain the observed rates of back arc extension (oceanization) in the Western Mediterranean as well as the migration of sedimentation depocentre loci in the Pannonian stretched continental crust. The model also predicts key features common to all three systems such as prolonged absence of high topography (in comparison to lofty examples of core complex and extension such as Southern Tibet or the Central Andes) and a climate parameter (e.g. precipitation) is not needed for the observed landscape evolution.

Edwards, M.; Grasemann, B.

2009-04-01

200

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

201

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

NASA Astrophysics Data System (ADS)

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

Clift, Peter D.; Turner, Jonathan

1995-12-01

202

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

PubMed Central

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

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

2011-01-01

203

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

USGS Publications Warehouse

Authigenic carbonates from five continental margin locations, the Eel River Basin, Monterey Bay, Santa Barbara Basin, the Sea of Okhotsk, and the North Sea, exhibit a wide range of mineralogical and stable isotopic compositions. These precipitates include aragonite, low- and high-Mg calcite, and dolomite. The carbon isotopic composition of carbonates varies widely, ranging from -60??? to +26???, indicating complex carbon sources that include 13C-depleted microbial and thermogenic methane and residual, 13C-enriched, bicarbonate. A similarly large variability of ??18O values (-5.5??? to +8.9???) demonstrates the geochemical complexity of these sites, with some samples pointing toward an 18O-enriched oxygen source possibly related to advection of 18O-enriched formation water or to the decomposition of gas hydrate. Samples depleted in 18O are consistent with formation deeper in the sediment or mixing of pore fluids with meteoric water during carbonate precipitation. A wide range of isotopic and mineralogical variation in authigenic carbonate composition within individual study areas but common trends across multiple geographic areas suggest that these parameters alone are not indicative for certain tectonic or geochemical settings. Rather, the observed variations probably reflect local controls on the flux of carbon and other reduced ions, such as faults, fluid conduits, the presence or absence of gas hydrate in the sediment, and the temporal evolution of the local carbon reservoir. Areas with seafloor carbonates that indicate formation at greater depth below the sediment-water interface must have undergone uplift and erosion in the past or are still being uplifted. Consequently, the occurrence of carbonate slabs on the seafloor in areas of active hydrocarbon seepage is commonly an indicator of exhumation following carbonate precipitation in the shallow subsurface. Therefore, careful petrographic and geochemical analyses are critical components necessary for the correct interpretation of processes related to hydrocarbon seepage in continental margin environments and elsewhere. ?? 2007 Elsevier Ltd. All rights reserved.

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

2007-01-01

204

The CARIACO Time Series: Carbon fluxes in a tropical, continental upwelling margin  

NASA Astrophysics Data System (ADS)

Continental margins play an important role in the global carbon cycle, accounting for about 10-15% of the carbon produced in oceanic systems, and 40% of carbon deposition on the global ocean floor. In an effort to understand some of the processes operating along a wind-driven coastal upwelling margin, the CARIACO time-series project (CArbon Retention In A Colored Ocean) has carried out monthly oceanographic cruises to the Cariaco Basin on the continental shelf off Eastern Venezuela to collect hydrographic, biogeochemical and bio-optical observations since November 1995. Subtropical Underwater (SUW) provides new nutrients to the Basin, which support annual primary production of ~500 mgCm-2. These estimates are comparable to Monterrey Bay and higher than other upwelling sites such as off West Africa and the Oregon shelf. However, there is a disconnect between vertical, sinking Particulate Organic Carbon fluxes and primary production in the Basin, possibly due to lateral advection of particles. The POC budget between the surface and 275m indicates that about ~2.8x1011 mol C yr-1 are either remineralized or available for export as POC out of the Basin. Most of the carbon fixed by primary producers in the upper 20-50 m of the water column is exported out of the basin. Remineralization can contribute between 1-10% to the DIC values between the surface and 275 m per day. Rivers play a minor role in nutrient input. The data obtained from the CARIACO time-series helps assess the carbon budget and the seasonality of hydrographic variables in the South-Eastern Caribbean. Ultimately, these data are critical to understand the paleoclimate record of changes in the Atlantic Ocean stored in the laminated sediments at the bottom of the Cariaco Basin.

Muller-Karger, F.; Varela, R.; Thunell, R. C.; Scranton, M. I.; Taylor, G. T.; Astor, Y. M.; Benitez-Nelson, C.; Lorenzoni, L.; Tappa, E. J.; Goni, M. A.; Rueda, D. T.; Hu, C.

2007-12-01

205

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

NASA Astrophysics Data System (ADS)

sources and distribution of organic matter (OM) in surface sediments of the eastern continental margin of India, including the region influenced by river discharge, were investigated using content, molar C:N ratios and stable isotopes of carbon and nitrogen. Despite relatively high water column integrated chlorophyll-a concentrations were found in the continental shelf than the slope; however, the lower sediment organic carbon (SOC) was found in the former than the latter region suggesting that in situ production did not play significant role on preservation of SOC in the coastal Bay of Bengal. The broad range of ?13C of SOC (-23.2 to -16.7‰) suggests that OM is a broad mixture of terrestrial and marine OM. Relative contributions from terrestrial C3 and C4 plants and marine sources are quantified as 34%, 23%, and 43%, respectively, indicating that dominant source of allochthonous OM (~57%) in the coastal Bay of Bengal. Relatively higher contribution of OM from C4 plants was found in the sediments at off river Krishna indicating that this region received detritus of agricultural crops such as jowar, bajra, and sugar cane, which are dominant in its drainage basin, during SW monsoon. This study revealed that relatively high OM preserved in the slope than shelf region along the coastal Bay of Bengal and the composition of OM is primarily controlled by the type of agricultural crops and vegetation in the drainage basin of the river.

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

2013-12-01

206

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

SciTech Connect

Microcrystalline dolomite forms a major constituent of Holocene sediments of numerous continental, saline playa lakes in southeastern Australia. The lake waters are highly supersaturated with respect to dolomite as well as other Mg carbonates, but undersaturated or near saturation with respect to calcite and aragonite. The dolomite shows no replacement textures and most likely formed by direct precipitation. Conditions in these lakes that appear conducive to the precipitation of dolomite are (1) high salinity, (2) high Mg/Ca ratios, and (3) high alkalinity. The moderate sulfate levels of the brines do not seem to be inhibiting carbonate precipitation.

De Deckker, P.; Last, W.M.

1988-01-01

207

Crustal Structure, Gravity Anomalies and Flexure of the Continental Margin Offshore North-Eastern Brazil  

NASA Astrophysics Data System (ADS)

During November/December 2003 we carried out a marine geophysical survey of the continental margin offshore north-eastern Brazil onboard RRS Discovery. The survey included 340 line km of coincident multi-channel seismic reflection and wide-angle seismic refraction data that were acquired along a transect of the margin in the region of the Amazon deep-sea fan. The wide-angle data, which were recorded by 16 ocean bottom instruments deployed at ~ 15 km intervals, have been used to determine a P wave velocity model of the crust and upper mantle. The model shows that the middle Amazon fan is underlain by > 12 km of sediments which can be divided into an upper and lower layer with seismic velocities between 1.6-4.0 and 3.6-4.6 km s-1 respectively. The sediments overlie a relatively thin, ~5 km thick, oceanic crust with seismic velocities between 5.0-7.5 km s-1. We have used the velocity model, together with existing commercial seismic reflection data, to estimate the distribution of sediments beneath the adjacent upper and lower Amazon fan. The data show two main sediment depocentres: an early one associated with the Cretaceous to mid-Miocene margin and a later one associated with the mid-Miocene to Recent superposition on the margin of the Amazon fan. Our data show that the maximum sediment thickness of the later depocentre is displaced by ~ 50 km seaward of the early depocentre. We have used a sediment thickness grid, together with combined 3-D flexural backstripping and gravity anomaly modelling techniques, to estimate the temporal and spatial distribution of the elastic thickness of the lithosphere, Te, in the Amazon fan region. Preliminary results show that the early margin sediments loaded a relatively weak lithosphere with a Te ~ 10 km while the later Amazon fan sediments loaded a relatively strong lithosphere with a Te ~ 35 km. We discuss here the significance of these results and their implications for the structure, evolution and thermal and mechanical properties of rifted margins.

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

2005-12-01

208

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

NASA Astrophysics Data System (ADS)

The origin of the forces that produce elevated, passive continental margins (EPCMs) is a hot topic in geoscience. It is, however, a new aspect in the debate that episodes of uplift coincide with changes in plate motion. This has been revealed, primarily, by studies of the burial, uplift and exhumation history of EPCMs based on integration on stratigraphic landscape analysis, low-temperature thermochronology and evidence from the geological record (Green et al., 2013). In the Campanian, Eocene and Miocene, uplift and erosion affected the margins of Brazil and Africa (Japsen et al., 2012b). The uplift phases in Brazil coincided with main phases of Andean orogeny which were periods of relatively rapid convergence at the Andean margin of South America (Cobbold et al., 2001). Because Campanian uplift in Brazil coincides, not only with rapid convergence at the Andean margin of South America, but also with a decline in Atlantic spreading rate, Japsen et al. (2012b) suggested that all these uplift events have a common cause, which is lateral resistance to plate motion. Because the uplift phases are common to margins of diverging plates, it was also suggested that the driving forces can transmit across the spreading axis; probably at great depth, e.g. in the asthenosphere. Late Eocene, Late Miocene and Pliocene uplift and erosion shaped the elevated margin of southern East Greenland (Bonow et al., in review; Japsen et al., in review). These regional uplift phases are synchronous with phases in West Greenland, overlap in time with similar events in North America and Europe and also correlate with changes in plate motion. The much higher elevation of East Greenland compared to West Greenland suggests dynamic support in the east from the Iceland plume. Japsen et al. (2012a) pointed out that EPCMs are typically located above thick crust/lithosphere that is closely juxtaposed to thinner crust/lithosphere. The presence of mountains along the Atlantic margin of Brazil and in East and West Greenland, close to where continental crust starts to thin towards oceanic crust, illustrates the common association between EPCMs and the edges of cratons. These observations indicate that the elevation of EPCMs may be due to processes operating where there is a rapid change in crustal/lithosphere thickness. Vertical motion of EPCMs may thus be related to lithosphere-scale folding caused by compressive stresses at the edge of a craton (e.g. Cloetingh et al., 2008). The compression may be derived either from orogenies elsewhere on a plate or from differential drag at the base of the lithosphere by horizontal asthenospheric flow (Green et al., 2013). Bonow, Japsen, Nielsen. Global Planet. Change in review. Cloetingh, Beekman, Ziegler, van Wees, Sokoutis, 2008. Geol. Soc. Spec. Publ. (London) 306. Cobbold, Meisling, Mount, 2001. AAPG Bull. 85. Green, Lidmar-Bergström, Japsen, Bonow, Chalmers, 2013. GEUS Bull. 2013/30. Japsen, Chalmers, Green, Bonow 2012a, Global Planet. Change 90-91. Japsen, Bonow, Green, Cobbold, Chiossi, Lilletveit, Magnavita, Pedreira, 2012b. GSA Bull. 124. Japsen, Green, Bonow, Nielsen. Global Planet. Change in review.

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

2014-05-01

209

Phanerozoic history of Western Australia related to continental drift  

Microsoft Academic Search

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

J. J. Veevers

1971-01-01

210

Trough-mouth-fan Evolution on the Pacific-Margin of the Antarctic Peninsula Outer Continental Shelf and its Relation to Sediment Drifts on the Adjacent Continental Rise  

NASA Astrophysics Data System (ADS)

Rebesco et al. (1998) propose a general depositional model that relates sediment drift evolution on the Antarctic Peninsula Pacific-margin abyssal plain to glacial processes on the continental shelf. In their model, the terrigenous sediment were directly delivered to the rise and contributed to the construction of large sediment drifts when grounded ice sheets extended to the shelf edge. In this scenario, large volumes of sediment by-passed the margin at the mouth of ice streams (i.e., fast flowing ice), whereas prograding slopes were constructed on those portions of the shelf margin between major ice streams. This model relies heavily on the modern geomorphology of the margin. In contrast, an evaluation of the subsurface stratigraphy suggested that there may have been significant lateral shifts of ice-stream locations and associated trough-mouth-fan depositional systems through time (Bart and Anderson, 1995). New seismic data acquired along the strike of the Antarctic Peninsula shelf during the 2002 season aboard the NBP R/VIB confirm that slope progradation between the modern troughs was indeed associated with large ice streams. Moreover, the data illustrate that the last several glacial cycles did not produce significant slope progradation anywhere along the margin which signifies a major change in the stratal-stacking pattern on the outer continental shelf. This change in stacking pattern is roughly coincident with a major reduction in sedimentation rates on at least two of the drifts (i.e., those drilled at ODP Sites 1095 and 1101).

Holloman, J.; Bart, P. J.

2002-12-01

211

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

SciTech Connect

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

Miki, Takashi (Kyushu Univ., Fukuoka (Japan))

1994-07-01

212

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

NASA Astrophysics Data System (ADS)

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

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

2002-12-01

213

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

E-print Network

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

Beaumont, Christopher

214

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

NASA Astrophysics Data System (ADS)

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

Liu, C.; Chiu, J.

2008-12-01

215

Estimates of conductive heat flow through bottom-simulating reflectors on the Hikurangi and southwest Fiordland continental margins, New Zealand  

Microsoft Academic Search

Bottom-simulating reflectors (BSRs) represent the base of the stability field for gas hydrates in shallow oceanic sediments. A simple conductive model is used to calculate surface heat flow through the Hikurangi and southwest Fiordland continental margins of New Zealand, based on the depths of BSRs. The results indicate mean uncorrected heat flows through the two regions of 37 ± 8

John Townend

1997-01-01

216

Sediment accretion against a buttress beneath the Peruvian continental margin at 12° S as simulated with sandbox modeling  

Microsoft Academic Search

Reflection seismic data from the Peruvian continental margin at 12° S clearly reveal an accretionary wedge and buttress. Sandbox experiments applying the physical concept of the Coulomb theory allow the systematic investigation of the growth and deformation of such an accretionary structure. The style of deformation of the buttress and the internal structure of the wedge is observed in the

N. Kukowski; R. von Huene; J. Malavieille; S. E. Lallemand

1994-01-01

217

ELSEVIER Earth and Planetary Science Letters 160 (1998) 353367 Upper Eocene ejecta of the New Jersey continental margin reveal  

E-print Network

Crater as the source for New Jersey continental margin ejecta is provided by fine-grained tektites I contains a 12-cm-thick sedimentary deposit containing rare silt-sized tektites and evidence Science B.V. All rights reserved. Keywords: impact craters; tektites; tsunamis; Chesapeake Bay; ejecta

1998-01-01

218

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

NASA Astrophysics Data System (ADS)

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

Faugères, J. C.; Pujol, Cl.

219

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

USGS Publications Warehouse

During the past ten years of the Association of American State Geologists-Mineral Management Service Continental Margins Program, the Maryland Geological Survey investigated the sedimentological, paleontological, stratigraphical and geophysical character of Maryland's inner continental shelf. Based on seismic records and sedimentological analyses completed during the first four years, a late Quaternary stratigraphic model was developed. Five distinct stratigraphic units were identified and described on the Maryland inner shelf. These units represent late Pleistocene interglacial deposits, the oldest of which corresponds to pre-Illinoian (oxygen-isotope stages 7 and/or 9) transgressive shelf sands. Overlying the Q1 unit, the Q2 unit is a 6-meter thick mud sequence of oxygen-isotope stage 5 (128-75 ka) age. Units Q3 and Q4 representing fluvial and leading edge estuarine deposits (oxygen-isotope stages 4, 3 and 2) filled numerous paleochannels that were incised into units Q2 and Q1. Modern trailing-edge transgressive shelf shoals (Unit Q5) discontinuously cap the sequence. The 5th and 6th years studies reported on the economic minerals of surficial and cored sediments. Vibracores collected off the Maryland's shelf during previous studies were analyzed for mineral types and abundances, weight percent of general size fractions, and heavy mineral (HM) content. Mineralogic maturity indices were compiled to correlate the THM and economic heavy minerals (EHM) abundances with position offshore, sediment type, and the indices themselves. For the 7th year, the Maryland Geological Survey re-examined geophysical records and lithological data originally collected by the Army Corps of Engineers to locate and assess beach fill borrow areas for the Ocean City Beach Replenishment Project. Data from 163 vibracores and over 300 kilometers of high-resolution seismic profile records collected off Ocean City, Maryland, supported the stratigraphic model developed by MGS during the first four years of the AASG-MMS program. The Maryland coastal bays became the focus of study during the 8th and 9th years during which seismic records, cores and surficial sediment were collected in Isle of Wight and Assawoman Bays. Shallow pretransgression surface was mapped, relating the existing streams to offshore paleochannels. The tenth year study focused on developing a repository for vibracores collected on Maryland's inner continental shelf.During the past ten years of the Association of American State Geologists-Mineral Management Service Continental Margins Program, the Maryland Geological Survey investigated the sedimentological, paleontological, stratigraphical and geophysical character of Maryland's inner continental shelf. Based on seismic records and sedimentological analysis completed during the first four years, a late quaternary stratigraphic model was developed. Five distinct stratigraphic units were identified and described on the Maryland inner shelf. These units represent late Pleistocene interglacial deposits, the oldest of which corresponds to pre-Illinoian transgressive shelf sands.

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

1999-01-01

220

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

NASA Astrophysics Data System (ADS)

A two-dimensional model of a passive continental margin was adapted to the simulation of the methane cycle on Siberian continental shelf and slope, attempting to account for the impacts of glacial/interglacial cycles in sea level, alternately exposing the continental shelf to freezing conditions with deep permafrost formation during glacial times, and immersion in the ocean in interglacial times. The model is used to gauge the impact of the glacial cycles, and potential anthropogenic warming in the deep future, on the atmospheric methane emission flux, and the sensitivities of that flux to processes such as permafrost formation and terrestrial organic carbon (Yedoma) deposition. Hydrological forcing drives a freshening and ventilation of pore waters in areas exposed to the atmosphere, which is not quickly reversed by invasion of seawater upon submergence, since there is no analogous saltwater pump. This hydrological pump changes the salinity enough to affect the stability of permafrost and methane hydrates on the shelf. Permafrost formation inhibits bubble transport through the sediment column, by construction in the model. The impact of permafrost on the methane budget is to replace the bubble flux by offshore groundwater flow containing dissolved methane, rather than accumulating methane for catastrophic release when the permafrost seal fails during warming. By far the largest impact of the glacial/interglacial cycles on the atmospheric methane flux is attenuation by dissolution of bubbles in the ocean when sea level is high. Methane emissions are highest during the regression (soil freezing) part of the cycle, rather than during transgression (thawing). The model-predicted methane flux to the atmosphere in response to a warming climate is small, relative to the global methane production rate, because of the ongoing flooding of the continental shelf. A slight increase due to warming could be completely counteracted by sea level rise on geologic time scales, decreasing the efficiency of bubble transit through the water column. The methane cycle on the shelf responds to climate change on a long time constant of thousands of years, because hydrate is excluded thermodynamically from the permafrost zone by water limitation, leaving the hydrate stability zone at least 300 m below the sediment surface.

Archer, D.

2014-06-01

221

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

USGS Publications Warehouse

We interpret seismic-reflection data, which were collected in Santa Monica Bay using a 70-in3 generator-injector air gun, to show the geologic structure of the continental shelf and slope and of the deep-water, Santa Monica and San Pedro Basins. The goal of this research is to investigate the earthquake hazard posed to urban areas by offshore faults. These data reveal that northwest of the Palos Verdes Peninsula, the Palos Verdes Fault neither offsets the seafloor nor cuts through an undeformed sediment apron that postdates the last sea level rise. Other evidence indicates that this fault extends northwest beneath the shelf in the deep subsurface. However, other major faults in the study area, such as the Dume and San Pedro Basin Faults, were active recently, as indicated by an arched seafloor and offset shallow sediment. Rocks under the lower continental slope are deformed to differing degrees on opposite sides of Santa Monica Canyon. Northwest of this canyon, the continental slope is underlain by a little-deformed sediment apron; the main structures that deform this apron are two lower-slope anticlines that extend toward Point Dume and are cored by faults showing reverse or thrust separation. Southeast of Santa Monica Canyon, lower-slope rocks are deformed by a complex arrangement of strike-slip, normal, and reverse faults. The San Pedro Escarpment rises abruptly along the southeast side of Santa Monica Canyon. Reverse faults and folds underpinning this escarpment steepen progressively southeastward. Locally they form flower structures and cut downward into basement rocks. These faults merge downward with the San Pedro Basin fault zone, which is nearly vertical and strike slip. The escarpment and its attendant structures diverge from this strike-slip fault zone and extend for 60 km along the margin, separating the continental shelf from the deep-water basins. The deep-water Santa Monica Basin has large extent but is filled with only a thin (less than 1.5-km) section of what are probably post-Miocene rocks and sediment. Extrapolating ages obtained from Ocean Drilling Program site 1015 indicates that this sedimentary cover is Quaternary, possibly no older than 600 ka. Folds and faults along the base of the San Pedro Escarpment began to form during 8-13 ka ago. Refraction-velocity data show that high-velocity rocks, probably the Catalina Schist or Miocene volcanic rocks, underlie the sedimentary section. The San Pedro Basin developed along a strike-slip fault, widens to the southeast, and is deformed by faults having apparent reverse separation and by folds near Redondo Canyon and the Palos Verdes Peninsula.

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

2003-01-01

222

Geology and exploration in southwest Pacific Australian region: Western and Northwestern Margin basins of Australia  

SciTech Connect

The Marginal basins of Western and Northwestern Australia extend approximately 3,800 km along the coast and comprise both continental shelf and adjacent deepwater plateau areas. From north to south, the principal basins are the Sahul/Malita, Browse, Carnarvon, and Perth basins. The stratigraphic sequence within each basin is broadly similar, with initial widespread Triassic-Early Jurassic deposition in broad regional pre-rift sags. Middle Jurassic to Late Jurassic rifting on the Northwest Shelf and early Cretaceous rifting in the Perth basin varies in intensity across the region, and provides the primary control of hydrocarbon distribution through trap type, reservoir, and source rock quality. The best-developed oil source rocks of the region are found in these Jurassic rifted sequences. Although drilling to date has been generally sparse, with only some 824 wells throughout the entire area, significant accumulations of gas and oil have been found in the offshore Carnarvon basin and the Vulcan Graben area of the Browse basin. Recent discoveries of commercial oil at Jabiru in the Vulcan graben and Griffin in the Carnarvon basin have again focused activity on the area's oil potential. Many structural traps remain to be drilled and the potential for stratigraphic trapping has been only slightly explored. Large, poorly explored frontier areas, between the established producing areas and in the Perth basin, as well as adjacent deepwater, are available. However, present data suggest that not all key success factors, notably mature oil source rocks and good reservoirs, may be present in these areas.

Woollands, M.A. (BHP Petroleum Pty Ltd., Melbourne (Australia))

1991-03-01

223

The structural features and petroleum resource potential of basins in the western and northwestern Australian margins  

NASA Astrophysics Data System (ADS)

The oil and gas basins of Australia are confined to its western and northwestern margins. They are typical pericontinental depressions in the continent-ocean transition zone with a passive tectonic regime. The following oil and gas basins are definable from the south to northward: the Perth, Carnarvon, Canning, Browse, and Bonaparte. All these basins are well studied. Among them, the Carnarvon basin is the most productive. Despite the discovery of approximately a hundred oil and gas fields in this basin, its continental slopes are still insufficiently known. In this connection, the morphostructural features of the productive areas were analyzed using a specialized GIS technique. The performed analysis of the Carnarvon hydrocarbon-bearing basin demonstrated the efficiency of this technique and allowed several promising zones located west, north, and south of the discovered oil and gas fields and forming a single trend with them to be outlined. The total reserves of the country are as high as 2.1 × 109 t of oil and 840 × 109 m3 of gas. The annual oil production in Australia by January 1, 2008 was 22.25 × 106 t of oil and 14 × 109 m3 of gas. Approximately 95% of the oil and 80% of the gas produced in Australia by the beginning of 2008 were obtained from offshore parts of its basins.

Zabanbark, A.

2010-04-01

224

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

SciTech Connect

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

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

1989-03-01

225

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

226

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

NASA Astrophysics Data System (ADS)

During the Palaeogene the NE Atlantic margin was subjected to a series of extension events immediately prior to and during continental break-up (at ca. 54 Ma). In the Faroes region of the margin, palaeostress analyses on faults exposed on the Faroe Islands indicate that the extension vector rotated in an anticlockwise sense from NE-SW to NW-SE. Remnants of the Faroe Islands Basalt Group (FIBG) exposed on the islands were emplaced at or around sea-level, to a total stratigraphic thickness in excess of 6.6 km, requiring a comparable magnitude of subsidence; to date, the structures preserved on the Faroe Islands have been inferred as being concurrent with subsidence. However, no onshore studies have accounted for the uplift events that must have occurred to bring the Faroe Islands to their current elevation (the highest peak at 882 m a.s.l.). The purpose of this study is to constrain the relative timings and kinematics of structures exposed on the Faroe Islands in order to investigate the regional tectonic deformation regime during continental break-up and sea-floor spreading; processes that have not hitherto been resolved using geophysical techniques. For the first time, we provide structural evidence that suggests uplift was accommodated by reactivation of pre-existing structures in the period immediately following emplacement of the FIBG. Structures on the islands provide clear evidence for a 3-phase tectonic evolution: (1a) anticlockwise rotation from E-W to NE-SW extension, facilitated first on N-S (dip-slip) faults, followed by NW-SE (dip-slip) faults. NE- SW extension (1b) continued with emplacement of a NW-SE- and NNE-SSW-oriented dyke swarm. Event-1 began prior to the deposition of the coal-bearing Prestfjall Formation, and was sustained through to emplacement of the Enni Formation, resulting in notable thickness variations across the Judd, Brynhild and Westray fault-zones. Further anticlockwise rotation of the extension vector led to (2a) the emplacement of ENE-WSW and ESE-WNW conjugate dykes. This magmatic input resulted in a N-S dilation of the crust that was followed by (2b) N-S extrusion and E-W shortening facilitated by recurrent slip on ENE-WSW (dextral) and ESE-WNW (sinistral) conjugate strike-slip faults. A component of the E-W shortening was accommodated by the development of numerous minor thrusts and low-angle normal faults. The final stages of this event show a rotation of the extension vector into a NNW-SSE orientation, which is taken up predominantly on NW- SE oriented dextral-oblique-slip faults. Event-2 began in immediate succession to Event-1, and continued through to the end of magmatism associated with the FIBG. Both events 1 and 2 display multiple generations of (predominantly calcite and zeolite) mineralisation as tensile (mode-I) and shear hydraulic veins. Finally, (3) uplift occurred with reactivation of some existing faults, characterised by the entrainment of clastic material, and an absence of mineralisation. Event-3 orientations are controlled by the pre-existing anisotropy; the predominantly N-S and NW-SE orientated faults of event-1 were inverted - in the north, the E-W conjugate- fault set and associated thrust faults of event-2 were reactivated. In all observed cases, motion-sense puts the Islands in the (relatively up-thrown) footwall block. These observations emphasise the necessity of carrying out detailed field studies, in addition to the more usual margin-scale modelling studies, in order to fully constrain the kinematics of continental break-up.

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

2008-12-01

227

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

NASA Astrophysics Data System (ADS)

New marine geophysical data are used to describe the structure and history of the Gulf of Aden. Magnetic anomaly data shows seafloor spreading magnetic anomalies on Sheba Ridge from the axial anomaly to anomaly 5 (10 m.y. B.P.) between the Owen fracture zone and 45°E and to anomaly 2' (3 m.y. B.P.) or anomaly 3 (4 m.y. B.P.) west of 45°E. The data does not support the two episodes of seafloor spreading recently proposed. Landward of the seafloor spreading magnetic anomalies is a magnetic quiet zone of uncorrelatable anomalies. The magnetic quiet zone boundary is also a structural boundary effectively marking the edge of Sheba Ridge, with deeper basement lacking a significant topographic gradient found on the landward side. A magnetic quiet zone is found not only where Sheba Ridge splits continental lithosphere but also on East Sheba Ridge where the ridge splits the old oceanic lithosphere of the Owen and Somali basins. There the position occupied by the continental margin within the gulf is marked by nonmagnetic ridge complexes that stretch from the continents to the Owen fracture zone. The magnetic quiet zone boundary is not an isochron in either the Gulf of Aden or the Red Sea, suggesting that significant horizontal motions can occur prior to the initiation of seafloor spreading. The offset on the Dead Sea Rift is used to estimate that from 80 to 160 km of opening, amounting to between 65% and 200% extension of the initial rift valley, occurred in the Gulf of Aden and Red Sea prior to the establishment of a mid-ocean ridge. It is suggested that the development of a new ocean basin occurs in two stages. The first involves diffuse extension over an area perhaps 100 km wide in a rift valley environment without an organized spreading center. This is followed by concentration of the extension at a single axis and the beginning of true seafloor spreading.

Cochran, James R.

1981-01-01

228

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

229

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

NASA Astrophysics Data System (ADS)

Active margins are important sites of new continental crust formation by magmatic processes related to the subduction of oceanic plates. We investigate these phenomena using a three-dimensional coupled petrological-geochemical-thermomechanical numerical model, which combines a finite-difference flow solver with a non-diffusive marker-in-cell technique for advection (I3ELVIS code, Gerya and Yuen, PEPI,2007). The model includes mantle flow associated with the subducting plate, water release from the slab, fluid propagation that triggers partial melting at the slab surface, melt extraction and the resulting volcanic crustal growth at the surface. The model also accounts for variations in physical properties (mainly density and viscosity) of both fluids and rocks as a function of local conditions in temperature, pressure, deformation, nature of the rocks, and chemical exchanges. Our results show different patterns of crustal growth and surface topography, which are comparable to nature, during subduction at active continental margins. Often, two trench-parallel lines of magmatic activity, which reflect two maxima of melt production atop the slab, are formed on the surface. The melt extraction rate controls the patterns of new crust at different ages. Moving free water reflects the path of fluids, and the velocity of free water shows the trend of two parallel lines of magmatic activity. The formation of new crust in particular time intervals is distributed in finger-like shapes, corresponding to finger-like and ridge-like cold plumes developed atop the subducting slabs (Zhu et al., G-cubed,2009; PEPI,2011). Most of the new crust is basaltic, formed from peridotitic mantle. Granitic crust extracted from melted sediment and upper crust forms in a line closer to the trench, and its distribution reflects the finger-like cold plumes. Dacitic crust extracted from the melted lower crust forms in a line farther away from the trench, and its distribution is anticorrelated with the finger-like plumes. We demonstrate the potential applicability of our model to clustering of arc magmatism in several subduction zones, such as Baja California (Ramos-Velázquez et al., Revista Mexicana de Ciencias Geológicas,2008), North Island of New Zealand (Booden et al., J. Volcanol. Geotherm. Res., 2010), Northeast Japan (Kimura and Yoshida,Journal of Petrology, 2006); Ecuador (Schütte et al., Tectonophysics,2010) and Lesser Antilles (Labanieh et al., EPSL,2010).

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

2011-12-01

230

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

NASA Astrophysics Data System (ADS)

We mapped, sampled, and quantified gas emissions at the continental margin west of Svalbard during R/V Heincke cruise He-387 in late summer 2012. Hydroacoustic mapping revealed that gas emissions were not limited to a zone just above 396 m water depth. Flares from this depth have gained significant attention in the scientific community in recent years because they may be caused by bottom-water warming-induced hydrate dissolution in the course of global warming and/or by recurring seasonal hydrate formation and decay. We found that gas emissions occurred widespread between about 80 and 415 m water depth, which indicates that hydrate dissolution might only be one of several triggers for active hydrocarbon seepage in that area. Gas emissions were remarkably intensive at the main ridge of the Forlandet moraine complex in 80 to 90 m water depths, and may be related to thawing permafrost. Focused seafloor investigations were performed with the remotely operated vehicle (ROV) "Cherokee". Geochemical analyses of gas bubbles sampled at about 240 m water depth as well as at the 396 m gas emission sites revealed that the vent gas is primarily composed of methane (> 99.70%) of microbial origin (average ?13C = -55.7‰ V-PDB). Estimates of the regional gas bubble flux from the seafloor to the water column in the area of possible hydrate decomposition were achieved by combining flare mapping using multibeam and single-beam echosounder data, bubble stream mapping using a ROV-mounted horizontally looking sonar, and quantification of individual bubble streams using ROV imagery and bubble counting. We estimated that about 53 × 106 mol methane were annually emitted at the two areas and allow for a large range of uncertainty due to our method (9 to 118 × 106 mol yr-1). First, these amounts show that gas emissions at the continental margin west of Svalbard were on the same order of magnitude as bubble emissions at other geological settings; second, they may be used to calibrate models predicting hydrate dissolution at present and in the future; and third, they may serve as a baseline (year 2012) estimate of the bubble flux that will potentially increase in the future due to ever-increasing global-warming-induced bottom water warming and hydrate dissociation.

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

2014-11-01

231

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

NASA Astrophysics Data System (ADS)

The Paleoproterozoic Sokoman Formation (ca. 1.88 Ga) of the Labrador Trough, eastern Canada, is a ca. 100-m-thick succession of interbedded iron formation and fine-grained, terrigenous clastic sedimentary rocks. Detailed examination of drill cores and outcrops indicates a dynamic paleoshelf where an oxygen-stratified water column, coastal upwelling of hydrothermally derived Fe and Si, as well as tide- and storm-generated currents controlled lithofacies character. Vertical and lateral facies stacking patterns record deposition through two relative sea-level cycles that produced seven distinct lithofacies comprising two unconformity-bounded sequences. Sequence 1 reflects deposition of hematitic peritidal iron formation as deep as the upper shoreface. Sequence 2 is truncated by later erosion and encompasses the change to deeper-water accumulation of magnetite and Fe silicate-rich iron formation. The character and lateral distribution of redox-sensitive facies indicate that iron formation accumulation was controlled as much by shelf hydraulics as oxygen levels. The development of a suboxic surface ocean is interpreted to reflect photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that flourished in nutrient-rich, upwelled waters offshore. Deposition of other continental margin iron formations also occurred on Paleoproterozoic shelves that were favorably positioned for coastal upwelling. Variability between iron formations reflects intrinsic factors such as shelf profile, fluvial contribution, eolian input, evaporation rates, and coastal current systems, which influenced upwelling dynamics and the delivery of Fe, Si, and nutrients. Aridity onshore was a primary depositional control since it governed the transport and type of diluting terrigenous clastics as well as evaporative precipitation along the coastline. As in the Phanerozoic, unconformities, and transgressive and maximum flooding surfaces frame iron formation sequences, but with important differences. The absence of trace and body fossils as well as lack of terrestrial vegetation can make the recognition of these surfaces difficult. Transgressive surfaces can also be easily mistaken for Phanerozoic-style maximum flooding surfaces since stratigraphic condensation was restricted to inboard environments during ravinement. Outboard the accumulation of fresh precipitates increased sedimentation to produce a maximum flooding surface not usually marked by a prominent depositional hiatus. Understanding these differences is essential for establishing an accurate sequence stratigraphic framework. Such context is the backdrop for properly interpreting the sedimentology, oceanography, microbial ecology, and geochemistry of continental margin iron formations.

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

2014-07-01

232

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

NASA Astrophysics Data System (ADS)

We mapped, sampled, and quantified gas emissions at the continental margin west of Svalbard during R/V Heincke cruise He-387 in late summer 2012. Hydroacoustic mapping revealed that gas emissions were not limited to a zone just above 396 m below sea level (m b.s.l.). Flares from this depth gained significant attention in the scientific community in recent years because they may be caused by bottom water-warming induced hydrate dissolution in the course of global warming and/or by recurring seasonal hydrate formation and decay. We found that gas emissions occurred widespread between about 80 and 415 m b.s.l. which indicates that hydrate dissolution might only be one of several triggers for active hydrocarbon seepage in that area. Gas emissions were remarkably intensive at the main ridge of the forlandet moraine complex in 80 to 90 m water depths, and may be related to thawing permafrost. Focused seafloor investigations were performed with the remotely operated vehicle (ROV) "Cherokee". Geochemical analyses of gas bubbles sampled at about 240 m b.s.l. as well as at the 396 m gas emission sites revealed that the vent gas is primarily composed of methane (> 99.70%) of microbial origin (average ?13C = -55.7‰ V-PDB). Estimates of the regional gas bubble flux from the seafloor to the water column in the area of possible hydrate decomposition were achieved by combining flare mapping using multibeam and single beam echosounder data, bubble stream mapping using a ROV-mounted horizontally-looking sonar, and quantification of individual bubble streams using ROV imagery and bubble counting. We estimated that about 53 × 106 mol methane were annually emitted at the two areas and allow a large range of uncertainty due to our method (9 to 118 × 106 mol yr-1). These amounts, first, show that gas emissions at the continental margin west of Svalbard were in the same order of magnitude as bubble emissions at other geological settings, and second, may be used to calibrate models predicting hydrate dissolution at present and in the future, third, may serve as baseline (year 2012) estimate of the bubble flux that will potentially increase in future due to ever-increasing global-warming induced bottom water-warming and hydrate dissolution.

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

2014-05-01

233

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

NASA Astrophysics Data System (ADS)

Recent research on the Mississippi margin indicates notable seasonal variation in seabed dynamics. During years with minimal tropical-system activity, sediments initially deposited from late spring to early fall are remobilized by wind-driven currents and wave energy during extra-tropical weather systems in the winter. This research reveals the profound significance of tropical cyclones on Louisiana Shelf sedimentation. The amount of material delivered to and advected across the shelf by recent tropical cyclones is considerably larger than that related to winter storm systems. In Fall 2004, the river-dominated shelf of Louisiana was impacted by three tropical systems in less than a month, including Hurricane Ivan. Ivan, with maximum sustained winds in excess of 74 m s -1 (144 knots) and a minimum measured central pressure of 910 mbar, was the eighth most intense Atlantic hurricane on record at the time. In order to assess the impact these tropical systems had on the continental margin west of the Mississippi delta, seabed samples were collected from box cores in October 2004 and analyzed for particle-reactive radionuclides 234Th, 7Be, and 210Pb. Radiochemical data and observations from X-radiographs indicate event-driven sediment deposits ranged from 4 to 30 cm on the shelf and 2-6 cm in the Mississippi Canyon. These deposits exhibit distinct radiochemical signatures and differ visually and texturally from the underlying sediment. The well-developed physical stratification and graded nature of the deposits observed in core X-radiographs suggests that the sediment could have been deposited from sediment-gravity flows. Inventories of 7Be and 7Be/ 234Th xs ratios reveal this series of cyclones transported considerably more material to the outer shelf and slope than periods of minimal tropical-system activity. When compared to seasonal depositional rates created by winter storms, tropical-cyclone-related event deposits on the middle and outer shelf are up to an order of magnitude greater in thickness. The number and thickness of these event deposits decrease with distance from the delta and suggest that only the most severe tropical systems are likely capable of redistributing significant quantities of sediment to more distal portions of the shelf and slope. These severe-event-driven deposits may account for as much as 75% of the sediment burial budget on decadal time scales within Mississippi Canyon. Higher than average tropical cyclone activity, predicted by the National Hurricane Center over the next decade, may be the major mechanism controlling sediment transport and deposition on the Mississippi River continental shelf and in Mississippi Canyon.

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

2007-08-01

234

Non-linear processes affecting the deposition and accumulation of mud on continental margins  

NASA Astrophysics Data System (ADS)

The deposition and accumulation of mud is affected by nonlinear processes associated with particle adhesion. Specifically, particle adhesion in suspension produces aggregate particles that increase the deposition rate of muds. Particle adhesion in the seabed reduces erosion rates and limits the sorting of fine particles, which enhances accumulation rate of muds. This talk brings together results from a decade of work on these processes, synthesizing how they affect the position of mud depo-centers on river-influenced continental margins. While in suspension, silts and clays collide and adhere in a process called "flocculation". The resulting aggregate particles, called "flocs", sink faster than their component grains and are the primary vector for delivery of clays, very fine silts and fine silts to the seabed. The rate of aggregation scales generally with the square of particle concentration, so deposition rates of the finest sediment particles are greater where concentrations are larger. This general trend is modulated by turbulence, which can break flocs when it is energetic. The ideal conditions for flocculation, therefore, are large sediment concentrations and weak turbulence, conditions which typically do not co-occur. Riverine sediment plumes entering quiet receiving basins and bottom boundary layers where turbulence has been damped by suspended sediment density stratification are two important environments where large concentrations and weak turbulence co-occur. Deposition of sediments rich in fines occurs when there is rapid delivery of riverine sediment to sections of the seabed where the stress is imparted primarily by waves rather than currents. The deposition of clays, very fine silts and fine silts to the seabed is important to accumulation of mud because the presence of these finest fractions decreases the erodibility and sorting of a sediment deposit. Once the clay content of a deposit exceeds 5-10%, the erosion rate at a given applied stress decreases, but more importantly, particles finer than 16 micrometers adhere to one another, making it challenging to winnow these fine sediments from the bed. In summary, clays, very fine silts and silts deposit due to flocculation. The presence of these sizes in sediment decreases the erodibility and sortability of that sediment, leading to the accumulation of mud. These non-linear processes must be considered when predicting eventual depo-centers of mud on continental margins.

Hill, P. S.

2012-12-01

235

Causes of long-term landscape evolution of "passive" margins and adjacent continental segments at the South Atlantic Ocean.  

NASA Astrophysics Data System (ADS)

During the last 10 years research efforts have been devoted to understand the coupling between tectonic and surface processes in the formation of recent topography. Quantification of the rate at which landforms adapt to a changing tectonic, heat flow, and climate environment in the long term has become an important research object and uses intensively data revealed by low-temperature thermochronology, terrigenous cosmogenic nuclides, and geomorphological analyses. The influence of endogenic forces such as mantle processes as one of the causes for "Dynamic Topography Evolution" have been explored in a few studies, recently. In addition, the increased understanding how change in surface topography, and change in the amount of downward moving cold surface water caused by climate change affects warping isotherms in the uppermost crust allows further interpretation of low-temperature thermochronological data. "Passive" continental margins and adjacent continental segments especially at the South Atlantic ocean are perfect locations to quantify exhumation and uplift rates, model the long-term landscape evolution, and provide information on the influence of mantle processes on a longer time scale. This climate-continental margin-mantle process-response system is caused by the interaction between endogenic and exogenic forces that are related to the mantle-process driven rift - drift - "passive" continental margin evolution of the South Atlantic, and the climate change since the Early/Late Cretaceous climate maximum. Furthermore, the influence of major transform faults (also called: transfer zones, Fracture Zones (FZ)) on the long-term evolution of "passive" continental margins is still very much in debate. The presentation will provide insight in possible causes for the differentiated long-term landscape evolution along the South Atlantic Ocean.

Glasmacher, Ulrich Anton; Hackspacher, Peter C.

2013-04-01

236

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

SciTech Connect

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

Anderson, J.B.

1987-05-01

237

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

USGS Publications Warehouse

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

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

1991-01-01

238

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

239

Late Mesozoic magmatism and Cenozoic tectonic deformations of the Barents Sea continental margin: Effect on hydrocarbon potential distribution  

NASA Astrophysics Data System (ADS)

The paper is focused on the two tectonic-geodynamic factors that made the most appreciable contribution to the transformation of the lithospheric and hydrocarbon potential distribution at the Barents Sea continental margin: Jurassic-Cretaceous basaltic magmatism and the Cenozoic tectonic deformations. The manifestations of Jurassic-Cretaceous basaltic magmatism in the sedimentary cover of the Barents Sea continental margin have been recorded using geological and geophysical techniques. Anomalous seismic units related to basaltic sills hosted in terrigenous sequences are traced in plan view as a tongue from Franz Josef Land Archipelago far to the south along the East Barents Trough System close to its depocentral zone with the transformed thinned Earth's crust. The Barents Sea igneous province has been contoured. The results of seismic stratigraphy analysis and timing of basaltic rock occurrences indicate with a high probability that the local structures of the hydrocarbon (HC) fields and the Stockman-Lunin Saddle proper were formed and grew almost synchronously with intrusive magmatic activity. The second, no less significant multitectonic stress factor is largely related to the Cenozoic stage of evolution, when the development of oceanic basins was inseparably linked with the Barents Sea margin. The petrophysical properties of rocks from the insular and continental peripheries of the Barents Sea shelf are substantially distinct as evidence for intensification of tectonic processes in the northwestern margin segment. These distinctions are directly reflected in HC potential distribution.

Shipilov, E. V.

2015-01-01

240

Timing and Magnitude of Depth-dependent Lithosphere Stretching on the Lofoten Segment of the Norwegian Rifted Continental Margin  

NASA Astrophysics Data System (ADS)

Flexural backstripping and forward structural-and-stratigraphic modelling show that depth-dependent lithosphere stretching occurs on the outer part of the Norwegian rifted margin. Subsidence analysis on the Lofoten segment of the margin shows substantial thinning of the continental lithosphere within 100 km of the COB at continental breakup time (at approx. 54 Ma), while the upper crust shows no significant faulting and extension at breakup or immediately preceding breakup in the Palaeocene. For the Lofoten Margin beta stretching-factors approaching infinity are required at 54 Ma west of the Utroest Ridge to restore Top Basalt and the Top Taare to presumed sub-aerial depositional environments. Breakup age beta stretching-factors are predicted to rapidly reduce towards the east of the Utroest Ridge. For the mid-Lofoten margin, an additional Eocene crustal thinning event younger than 54 Ma is required to explain observed margin subsidence; post-breakup subsidence with a beta stretching-factor of infinity is insufficient to generate observed post-breakup subsidence. The absence of significant Palaeocene extension on the Lofoten margin, and the additional Eocene subsidence and faulting, implies that depth-dependent stretching of the Norwegian rifted margin occurred during early sea-floor spreading rather than during pre-breakup intra-continental rifting. For the Voering segment of the Norwegian rifted margin, south of the Bivroest Transform and Lineament System, smaller b stretching-factors of ~ 1.8 to 2.5 are needed to restore Top Basalt and Top Taare to sea level. No similar magnitude of extension by faulting is observed in the upper crust (Roberts et al.1997). Depth dependent stretching of margin lithosphere is also observed in the northern Moere Basin. Depth-dependent stretching has been observed at other rifted continental margins including the Galicia, Goban Spur, NW Australian and South China Sea rifted margins (Driscoll and Karner 1998, Davis and Kusznir 2002). Stretching estimates, independently determined from upper crustal faulting, whole crustal thinning and post-rift lithosphere thermal subsidence, show that extension increases with depth within 100 to 150 km of the COB such that upper-crustal extension is significantly smaller than whole-crustal or whole-lithosphere extension. Finite-element models of early sea-floor spreading predict depth-dependent lithosphere stretching and imply that depth dependent stretching of young rifted margin lithosphere is an inevitable consequence of early sea-floor spreading. The timing of depth dependent stretching on the Lofoten margin supports the hypothesis that depth dependent stretching of continental rifted margin lithosphere occurs during early sea-floor spreading rather than during pre-breakup rifting. Depth dependent stretching may have a similar causal mechanism to that responsible for observed mantle exhumation on the Iberian rifted margin (Pickup et al 1996, Whitmarsh et al 2001). *current address Teknologi, Tektonikk og Stratigrafi, Statoil ASA, Grensveien 21, 4035 Stavanger, Norway

Kusznir, N.; Roberts, A.; Hunsdale, R.

2002-12-01

241

Paleozoic archipelagic tectonic evolution of Western Junggar, NW China: implications for continental growth of southern Altaids  

NASA Astrophysics Data System (ADS)

The Western Junggar, NW China, a dominant site for continental growth in Southern Altaids, bridges the Circum-Balkhash and Junggar belts and exposes ophiolite, igneous rocks and strata from Cambrian to Carboniferous. Recent updated data on structure, geochronology, geochemisty and paleomagnetism, integrated with previous data, present a newly Paleozoic spatial and temporal framework of Western Junggar. In Cambrian, the Western Junggar begins to birth at the Tangbale area to south, where occurs Ordovician blueschist and top-to-south vergence structures, indicating north-dipping subduction. This event triggers intra-arc extension to generate Ordovician island arc in the Hongguleleng-Xiemisitai area to north and seamount in the Mayile area, middle of Western Junggar. Until Silurian, a southeastward subduction begins in the extended back-arc basin to west of Mayile, occurring blueschist at the Barleik trench and the Nalunsuo magmatic arc, at the rear of which generates Devonian back-arc basin around the Durbut area. Meanwhile, a Silurian Xiemisitai magmatic arc has been developed at the northern part of Western Junggar, along which a northward subduction has emplaced the Tarbahatai ophiolite and generates the Carboniferous Sawur magmatic arc. At the middle part of Western Junggar, the coeval adakite and sanukitic dykes, charnockite, multiple properties of ophiolite and plutons, SSZ-like andesite, dacite and rhyolite and regional structures suggest that there develop double-subduction systems with ridge-trench interaction in Carboniferous. These features suggest that the Western Junggar experiences rollback, intra-oceanic extension and subduction polarity reversal/flip in back-arc basin settings. Furthermore, positive ?Nd(t) values and no huge movements of blocks suggest that the Western Junggar is amalgamated by juvenile elements with different orientations. Therefore, we conclude that the Western Junggar enlarges from an island arc to Paleozoic tectonic regime with island arcs and subduction-accretion complexes via continuous accretion presented as episodic events and it significantly contributes to continental growth in southern Altaids.

Zhang, Jien; Xiao, Wenjiao; Han, Chunming; Ma, Chong; Song, Dongfang

2013-04-01

242

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

SciTech Connect

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

Moustafa, A.R.; Khalil, M.H. (Ain Shams Univ., Cairo (Egypt))

1988-08-01

243

Controlling factors of interannual variability of downward particle fluxes on a NW Mediterranean continental margin  

NASA Astrophysics Data System (ADS)

Downward transfer of particulate matter, currents and water temperature have been measured on the Gulf of Lions margin since October 1993, at 500 and 1000 m depth within two submarine canyons. At the global scale, principal component and cross-correlation analysis of the long term time series underlines two features: (i) transfer of particulate matter to the deep ocean is neither correlated with atmospheric inputs nor forced by the wave induced bottom stress on the continental shelf; (ii) mass fluxes are likely controlled by the meandering of the alongslope current (Northern Current) and by dense water formation in winter. At the local scale, i.e. when each canyon is considered separately, statistical analysis reveals the influence of Rhône river inputs at intermediate depth on the slope (500 m) and of intense dense water formation that occurred during winter in 1999 and 2000. Numerical simulations have been carried out to answer the questions of particulate transfer of riverine origine down to 500 m as well as of the frequency and location of the Northern Current intrusions and dense water formation events. This modelling approach examines the impact of local atmospheric forcing (wind stress, heat fluxes, precipitation - evaporation budget) on the variability of the alongslope circulation and of total mass fluxes within the canyons. Preliminary results show a positive correlation between anomalies of dense water formation rates and interannual variability of total mass fluxes.

Guarracino, M.; Durrieu de Madron, X.; Heussner, S.; Marsaleix, P.; Barnier, B.

2003-04-01

244

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

NASA Astrophysics Data System (ADS)

We are investigating sediments from the fjords and continental margin of southern Alaska to develop high-resolution climatic and oceanographic records for the Late Quaternary. Our goal is to better understand linkages between climatic, terrestrial and oceanic systems in this tectonically active and biologically productive region. A field program was conducted aboard the R/V Maurice Ewing in August/September 2004 utilizing geophysical surveys (high-resolution swath bathymetric and backscatter imaging, shallow sub-bottom profiling, and where permitted, high-resolution seismic reflection profiling), piston and multi-coring, and CTD/water sampling at about 30 sites in this region. Cores are being analyzed for sedimentological, microfossil, geochemical and stable isotopic proxies, with chronologies constrained by Pb-210, AMS radiocarbon, tephrochronolgic and paleomagnetic dating. Our preliminary results demonstrate that these rapidly accumulating sedimentary archives can resolve environmental changes on annual to decadal timescales. Records of recent changes in lithogenic sediment accumulation and biological productivity on the Gulf of Alaska shelf track historical climatic data that extends to the early 20th century in this region. The records also correlate with multi-decadal climate regimes during the Little Ice Age as suggested by tree-ring, glacial advance and salmon abundance records from nearby coastal sites. Jack Dymond's enthusiasm for collaborative, interdisciplinary research will help guide us in unraveling the fingerprints of key processes in this relatively unexplored region.

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

2004-12-01

245

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

NASA Astrophysics Data System (ADS)

Surface sediments from sites across the Indian margin of the Arabian Sea were analysed for their elemental and stable isotopic organic carbon (Corg) and total nitrogen compositions, 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 oxygen minimum zone (OMZ) on the upper slope (~ 200-1300 m) and the seasonal hypoxic zone that appears on the shelf. Source indices showed mixed marine and terrigenous OM within the estuaries, but consistent predominance (80-100%) of marine OM on the shelf and slope. Thus, riverine terrigenous OM is diluted or replaced by autochthonous marine OM and/or is efficiently re-mineralised, within or immediately offshore of the estuaries. Organic C contents of surface shelf sediments varied from < 0.5 wt% in relict shelf sands to up to ~ 4 wt% for nearshore muds, while upper slope sites within the OMZ showed a wide range (~ 2 to 7 + wt%), progressively decreasing below the OMZ to ? 1 wt% at 2000 m. Thus, major variability (~ 5 wt%) was found at slope sites within the OMZ of similar depth and near-identical bottom-water O2 concentrations. A strong relationship between %Corg and sediment grain size was seen for sediments within the OMZ, but lower relative Corg contents were found for sites on the shelf and below the OMZ. Further, Corg loadings, when related to estimated sediment surface area, indicated distinct enrichment of Corg in the OMZ sediments relative to sites above and below the OMZ and to sediments from normoxic margins. Diagenetic indices confirmed that lower Corg content below the OMZ is associated with more extensive OM degradation, but that shelf sediment OM is not consistently more degraded than that found within the OMZ. Together, the results indicate that OM distribution across the margin is controlled by interplay between hydrodynamic processes and varying preservation associated with O2 availability. This inference is supported by multiple regression analysis. Hydrodynamic processes (expressed as %Silt) followed by O2 availability, can explain the large majority of %Corg variability when the shelf and slope are considered as a whole. However, while O2 becomes the primary influence on %Corg for sediments below the OMZ, %Silt is the primary influence across the OMZ and, apparently, the shelf. Thus, reduced O2 exposure is responsible for OM enrichment within the OMZ, but hydrodynamic processes are the overriding control on sediment OM distributions across both the shelf and the OMZ.

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

2014-12-01

246

Polychaete community structure in the South Eastern Arabian Sea continental margin (200-1000 m)  

NASA Astrophysics Data System (ADS)

Macrofaunal polychaete communities (>500 ?m) in the South Eastern Arabian Sea (SEAS) continental margin (200-1000 m) are described, based on three systematic surveys carried out in 9 transects (at ~200 m, 500 m and 1000 m) between 7°00?and 14°30?N latitudes. A total of 7938 polychaetes belonging to 195 species were obtained in 136 grab samples collected at 27 sites. Three distinct assemblages were identified in the northern part of the SEAS margin (10-14°30?N), occupying the three sampled depth strata (shelf edge, upper and mid-slope) and two assemblages (shelf edge and slope) in the south (7-10°N). Highest density of polychaetes and dominance of a few species were observed in the shelf edge, where the Arabian Sea oxygen minimum zone (OMZ) impinged on the seafloor, particularly in the northern transects. The resident fauna in this region (Cossura coasta, Paraonis gracilis, Prionospio spp. and Tharyx spp.) were characteristically of smaller size, and well suited to thrive in the sandy sediments in OMZ settings. Densities were lowest along the most northerly transect (T9), where dissolved oxygen (DO) concentrations were extremely low (<0.15 ml l-1, i.e.<6.7 ?mol l-1). Beyond the realm of influence of the OMZ (i.e. mid-slope, ~1000 m), the faunal density decreased while species diversity increased. The relative proportion of silt increased with depth, and the dominance of the aforementioned species decreased, giving way to forms such as Paraprionospio pinnata, Notomastus sp., Eunoe sp. and lumbrinerids. Relatively high species richness and diversity were observed in the sandy sediments of the southern sector (7-9°N), where influence of the OMZ was less intense. The area was also characterized by certain species (e.g. Aionidella cirrobranchiata, Isolda pulchella) that were nearly absent in the northern region. The gradients in DO concentration across the core and lower boundary of the OMZ, along with bathymetric and latitudinal variation in sediment texture, were responsible for differences in polychaete size and community structure on the SEAS margin. Spatial and temporal variations were observed in organic matter (OM) content of the sediment, but these were not reflected in the density, diversity or distribution pattern of the polychaetes.

Abdul Jaleel, K. U.; Anil Kumar, P. R.; Nousher Khan, K.; Correya, Neil S.; Jacob, Jini; Philip, Rosamma; Sanjeevan, V. N.; Damodaran, R.

2014-11-01

247

Subglacial geomorphology reveals connections between glacial dynamics and deeper hydrocarbon reservoir leakages at the Polar north Atlantic continental margin  

NASA Astrophysics Data System (ADS)

Three-dimensional (3D) seismic data from the Barents Sea continental shelf and margin reveal spatial links between subsurface distributions of inferred glacitectonic geomorphic landforms and seismic indications of fluid flow from deeper hydrocarbon reservoirs. Particularly 3D seismic techniques allow detailed mapping and visualization of buried glacial geomorphology and geophysical indications of fluid flow and gas accumulations. Several subsurface glacitectonic landforms show pronounced depressions up to 200 m deep and several km wide. These appear in many locations just upstream from hills of similar sizes and volumes, and are inferred to be hill-hole pairs. The hills are interpreted as thrusted and compressed slabs of sediments and bedrock which have been removed from their original location by moving glaciers during the last glacial, leaving the holes as depressions. The mapped depressions seem often to appear in sediments of different lithology and age. The appearance of mega-scale glacial lineations indicates that fast-flowing ice streams, draining the former Barents Sea and Fennoscandian ice sheets were the main agents of these glacitectonic landforms. Mapped fluid flow migration pathways from deeper reservoirs and shallow gas accumulations show evidence of active fluid migration systems over longer time periods, and their spatial relationship with the glacitectonic landforms is documented for several areas of the Barents Sea continental shelf. A conceptual model is proposed for the depressions, where brittle glacitectonic deformation takes place along a weak layer at the base of gas-hydrate cemented sediments. Fluid flow from deeper hydrocarbon reservoirs is inferred to be associated with cycles of glaciations and unloading due to glacial erosion and ice retreat, causing gas to expand, which in turn potentially breaks the traps, reactivates faults and creates new faults. Gas hydrate stability modeling indicates that the south-western Barents Sea is today outside the stability area for methane gas hydrates of structure I, but hydrates of this type would have been stable when grounded ice covered the area. Structure II hydrates, with a few percent of heavier hydrocarbons are likely stable within the area today. Acknowledgements. This research is part of the Centre of Excellence for Gas Hydrate, Environment and Climate (CAGE) funded by the Research Council of Norway (RCN) grant 223259. It is also a contribution to the project "Glaciations in the Barents Sea area (GlaciBar)" RCN grant 20067 and to the Research Centre for Arctic Petroleum Exploration (ARCEx) RCN grant 228107.

Andreassen, Karin; Deryabin, Alexey; Rafaelsen, Bjarne; Richarsen, Morten

2014-05-01

248

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

NASA Astrophysics Data System (ADS)

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

Cowie, Leanne; Kusznir, Nick

2014-05-01

249

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

NASA Astrophysics Data System (ADS)

We develop a synoptic model for the breakup and drift of a major sheared continental margin system: the Rio Muni basin and its NE Brazilian counterpart. It relates the evolution of crustal structure determined from seismic interpretation (including gravity-modelling of the deep-imaging PROBE dataset) to the cooling history of this margin system yielded from thermal history data (mainly apatite fission track analysis - AFTAr - and vitrinite reflectance data). Shear margins initiate as a leaky transform fracture system accommodating the oblique (i.e. non-orthogonal) divergence of opposing rifted continental margins. As such, the transition from continental breakup (i.e. rupture) to continental drift (i.e. ocean opening) at shear margins exhibits significant differences from that of the much better understood normally divergent rifts, where the spreading vector is normal to the strike of the opposing rift margins. For example, unlike at normally divergent rifts, continental breakup and drift are separate episodes in the early evolution of shear margins. In Rio Muni-NE Brazil, they are recorded by separate breakup and drift unconformities spanning a 15-20Ma. interval, the time taken for the ocean ridge to traverse the length of the margin before a continuous arm of oceanic crust separated Rio Muni from its Brazilian counterpart. In the Rio Muni basin, the c.70km-wide Ascension Fracture Zone (AFZ) exhibits oblique-slip faulting and synrift half-graben formation that accommodated oblique extension during the period leading up to and immediately following whole-lithosphere failure and continental breakup 117Ma. Gravity-modelling of PROBE seismic profiles reveals a land-locked precursor oceanic basin that preceded full ocean opening and which subsequently was stranded on the African margin. Its existence supports the idea of multiple rift suture lines accommodating episodic breakup and it emphasizes the significance of separate breakup and drift episodes at shear margins. Oblique extension is recorded also by strike- and oblique-slip fault geometry within the AFZ and buckling of Aptian synrift rocks in response to block rotation and local transpression. Principal cooling episodes affecting both margins identified from the AFTAr occurred in the Mid-Cretaceous 110-90Ma., following a period of high heatflow during which the geothermal gradient was more than twice that of the present day 25°C/km gradient, and 45-35Ma. coinciding with a major change in the pole of rotation at Chron 34 some 84Ma. Additional cooling episodes between 85-75Ma. and 10-2Ma. are restricted to the African side. Given that this margin system underwent breakup some 117Ma., it appears that thermal equilibrium was maintained throughout the initial breakup meaning that no significant uplift and erosion is recorded by the thermal history data during this period. Instead, Mid-Cretaceous cooling is interpreted as a response to the influence of i) the c.105Ma. St Helena Plume and ii) transpressional shortening and inversion that accompanied the breakup to drift period.

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

2005-12-01

250

Structural Controls on the Evolution of the Southeastern Brazilian Continental Margin  

NASA Astrophysics Data System (ADS)

The South Atlantic passive margins show considerable variation along strike in terms of both structural style and margin width. Much of this change is thought to be due to variations in basement structure. Previous studies have shown that the influence of pre-existing structures can range from metre-scale local variations in basement fabrics to tens of kilometre-scale lithospheric heterogeneities relating to past deformation events. The Santos basin (offshore Rio de Janeiro, Brazil) is an increasingly important target for hydrocarbon exploration. The basin is thought to be underlain by thinned continental crust, possibly part of the Neoproterozoic Ribeira mobile belt, whose structures onshore lie parallel or sub-parallel to the continental margin. The formation of structures in the Santos basin and onshore southeastern Brazil has previously been thought to have been controlled by reactivation of these basement structures. Recent discoveries such as the giant Tupi oil field and recently drilled dry wells in the basin (Guaraní, Corcovado-2), highlight the importance of understanding the sub-salt structure in the basin. A remote sensing- and field-based study of structures formed during Cretaceous and Tertiary rifting events was carried out. We identify two generations of structures: ~120Ma Cretaceous tholeiitic dykes and associated faults; and ~60Ma Tertiary faults, showing silicified breccias associated with further alkaline magmatism. A strong northeast - southwest structural trend is identified from remote sensed imagery. At outcrop scale, sinistral-oblique and normal faults appear to have formed during the Cretaceous and Tertiary, and Cretaceous dykes show sinistral-oblique emplacement kinematics. These datasets are consistent with sinistral transtension during repeated phases of regional east-west extension. Basement fabrics often show strike parallel to the northeast-southwest trend of the brittle structures, but show a wide variation in dip angle. The identification of Cretaceous faults and fractures has shown for the first time that the early rifting can be studied onshore, as well as offshore. Data from outcrops and remote sensing can be combined with maps of pre- and post-rift structures interpreted from seismic and other geophysical data to provide an integrated onshore-offshore view of the evolution of the Santos basin. This study also highlights that, whilst we see a consistent trend of the brittle structures on both regional and outcrop scales, the basement fabrics display a great deal more heterogeneity. Where the basement is not oriented parallel to the northeast-southwest regional trend of brittle structures, we do not see an influence on the trend or structural style of the brittle faults. This suggests that whilst the local reactivation of exposed basement fabrics may lead to the development of complex fault systems at sub-seismic scales, the ultimate control on the formation of brittle structures was at a larger scale relating either to the initiation of the South Atlantic and/or to the development of pre-existing fabrics in the upper mantle.

Ashby, David; McCaffrey, Ken; Holdsworth, Bob; Almeida, Julio

2010-05-01

251

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

E-print Network

convergent-margin basins of Baja California, Mexico Cathy Busby* Department of Geological Sciences Abstract Mesozoic rocks of the Baja California Peninsula form one of the most areally extensive, best must be constructed from well-preserved ancient examples like Baja California. This convergent margin

Lee, Cin-Ty Aeolus

252

76 FR 82319 - Gulf of Mexico (GOM), Outer Continental Shelf (OCS), Western Planning Area (WPA) and Central...  

Federal Register 2010, 2011, 2012, 2013, 2014

...Continental Shelf (OCS), Western Planning Area (WPA) and Central Planning Area (CPA), Oil and Gas Lease Sales for 2012-2017 AGENCY: Bureau...The BOEM has prepared a Draft EIS on oil and gas lease sales tentatively scheduled in...

2011-12-30

253

77 FR 40081 - Gulf of Mexico, Outer Continental Shelf (OCS), Western Planning Area (WPA) and Central Planning...  

Federal Register 2010, 2011, 2012, 2013, 2014

...Continental Shelf (OCS), Western Planning Area (WPA) and Central Planning Area (CPA), Oil and Gas Lease Sales for 2012-2017 AGENCY: Bureau...BOEM has prepared a Multisale FEIS on oil and gas lease sales tentatively scheduled during...

2012-07-06

254

Geology of the Continental Margin of Enderby and Mac. Robertson Lands, East Antarctica: Insights from a Regional Data Set  

Microsoft Academic Search

In 2001 and 2002, Australia acquired an integrated geophysical data set over the deep-water continental margin of East Antarctica from west of Enderby Land to offshore from Prydz Bay. The data include approximately 7700 km of high-quality, deep-seismic data with coincident gravity, magnetic and bathymetry data, and 37 non-reversed refraction stations using expendable sonobuoys. Integration of these data with similar

H. M. J. Stagg; J. B. Colwel; N. G. Direen; P. E. O’Brien; G. Bernardel; I. Borissova; B. J. Brown; T. Ishirara

2004-01-01

255

Mixed glauconitic-carbonate-siliciclastic surficial sediments on the north Kaipara continental margin, northwestern North Island, New Zealand  

Microsoft Academic Search

A mosaic of siliciclastic and mixed carbonate-siliciclastic sediments and authigenic minerals occurs at shelf and slope depths (30–1015 m water depth) on the open, wave-dominated north Kaipara continental margin (NKCM) off northern New Zealand. Texture and composition define five surficial sediment facies. Facies 1 (siliciclastic sand) comprises generally well-sorted fine sands that extend to outer shelf depths. Facies 2 (glauconitic

DS Payne; CS Nelson; SD Hood

2010-01-01

256

Upper Carboniferous retroarc volcanism with submarine and subaerial facies at the western Gondwana margin of Argentina  

NASA Astrophysics Data System (ADS)

During Late Carboniferous times a continental magmatic arc developed at the western margin of Gondwana in South America, as several marine sedimentary basins were formed at the same time in the retroarc region. North of 33°S, at Cordón Agua del Jagüel, Precordillera of Mendoza, Argentina, a volcanic sequence crops out which was emplaced in a submarine environment with some subaerial exposures, and it is intercalated in marine sediments of Agua del Jagüel Formation, which fills of one of these retroarc basins. This paper presents, for the first time, a facies analyses together with geochemical and isotopic data of this volcanic suite, suggesting its deposition in an ensialic retroarc marine basin. The volcanic succession comprises debris flows with either sedimentary or volcanic fragments, base surge, resedimented massive and laminated dacitic-andesitic hyaloclastite, pillow lava, basic hyaloclastite and dacitic-andesitic lavas and hyaloclastite facies. Its composition is bimodal, either basaltic or dacitic-andesitic. The geochemistry data indicate a subalkaline, low K calk-alkaline and metaluminous affinity. The geochemistry of the basalts points to an origin of the magmas from a depleted mantle source with some crustal contamination. Conversely, the geochemistry of the dacites-andesites shows an important participation of both crustal components and subduction related fluids. A different magmatic source for the basalts than for the dacites-andesites is also supported by Sr and Nd isotopic initial ratios and Nd model ages. The characteristics of this magmatic suite suggest its emplacement in an extensional setting probably associated with the presence of a steepened subduction zone at this latitude during Upper Carboniferous times.

Koukharsky, M.; Kleiman, L.; Etcheverría, M.; Quenardelle, S.; Bercowski, F.

2009-04-01

257

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

USGS Publications Warehouse

Because of the extensive data base of seismic profiles, radiometric ages, and stratigraphic time markers such as the subaerial Messinian surface, sedimentation rates and Ebro River sediment discharge can be estimated for different periods and environments of the Ebro continental margin. New values for sediment discharge (i.e., 6.2 versus previous estimates of 2-3.5 million t/yr) for the Holocene highstand are more reliable but remain minimum estimates because a small proportion of Ebro sediment advected to the Balearic Rise and Abyssal Plain cannot be accounted for, especially during lowstands. The general highstand conditions of the Pliocene, which were similar to those of the Holocene, resulted in a low discharge of Ebro River sediment (ca. 6.5 million t/yr) and an even thickness of sediment across the margin that deposited at rates of about 24-40 cm/ky. In contrast, sediment supply increased two-three times during the Pleistocene, the margin prograded rapidly and deposition occurred at rates of 101-165 cm/ky on the outer shelf and slope, but basin floor rates remained anomalously low (21-26 cm/ky) because sediment was drained and broadly dispersed eastward in Valencia Trough. During the late Pleistocene rise of sea level, the main depocenters progressively shifted shoreward and sedimentation rates greatly decreased from 175 cm/ky on the upper slope during the early transgression to 106 cm/ky on the outer shelf and then to 63 cm/ky on the mid-shelf during the late transgression as the river sediment discharge dropped to half by Holocene time. Maximal sedimentation rates occurred in active depocenters of sediment dispersal such as the Holocene delta (370 cm/ky) or the youngest Pleistocene Oropesa channel-levee complex (705 cm/ky) where deposition rates increased by an order of magnitude or more compared to average Ebro shelf (38 cm/ky) or base-of-slope rates in the Pleistocene (21 cm/ky). The sedimentation rates verify the importance of sea-level control on the progressive change in location of depocenters and amount of sediment supply, but Pleistocene climatic change and deforestation alone can be observed to double river sediment discharge. The latter observation helps explain the anomalously high deposition rates in Pleistocene turbidite systems compared with older systems that may be controlled more by tectonic and sea-level changes alone. During the past 2000 years, in contrast, man has controlled deposition in the Ebro margin system, first by deforestation that more than doubled river sediment discharge and shelf deposition rates to equal those of Pleistocene time; and second by dam contruction that reduced sediment discharge to less than 5% of the normal Holocene discharge. Similar recent discharge reductions from the Nile and Rhone Rivers suggest that loss of the majority of the river sediment supply in the Mediterranean Sea may result in significant erosion of biologically and agriculturally important lobate delta areas. ?? 1990.

Nelson, C.H.

1990-01-01

258

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

NASA Astrophysics Data System (ADS)

Within subduction zones of active continental margins, large amounts of methane can be mobilized by dewatering processes and transported to the seafloor along migration pathways. A recently discovered seep area located off Concepción (Chile) at water depth between 600 to 1100 mbsl is characterized by active methane vent sites as well as massive carbonates boulders and plates which probably are related to methane seepage in the past. During the SO210 research expedition "Chiflux" (Sept-Oct 2010), sediment from the Concepción Methane Seep Area (CSMA) at the fore arc of the Chilean margin was sampled to study microbial activity related to methane seepage. We sampled surface sediments (0-30cm) from sulfur bacteria mats, as well as clam, pogonophoran, and tubeworm fields with push cores and a TV-guided multicorer system. Anaerobic oxidation of methane (AOM) and sulfate reduction rates were determined using ex-situ radioisotope tracer techniques. Additionally, porewater chemistry of retrieved cores as well as isotopic composition and age record of surrounding authigenic carbonates were analyzed. The shallowest sulfate-methane-transition zone (SMTZ) was identified at 4 cm sediment depth hinting to locally strong fluid fluxes. However, a lack of Cl- anomalies in porewater profiles indicates a shallow source of these fluids, which is supported by the biogenic origin of the methane (?13C -70‰ PDB). Sulfide and alkalinity was relatively high (up to 20 mM and 40 mEq, respectively). Rates of AOM and sulfate reduction within this area reached magnitudes typical for seeps with variation between different habitat types, indicating a diverse methane supply, which is affecting the depths of the SMTZ. Rates were highest at sulfur a bacteria mats (20 mmol m-2 d-1) followed by a large field of dead clams, a pogonophoran field, a black sediment spot, and a carbonate rich clam field. Lowest rates (0.2 mmol m-2 d-1) were measured in close vicinity to these hot spots. Abundant massive carbonate blocks and plates hint to a very old seep system with a probably much higher activity in the past. The U-Th age record of these authigenic carbonates reach back to periods of venting activity with more than 150 ka ago. Carbon isotopic signatures of authigenic carbonates (?13C -50 to -40‰ PDB) suggest a biogenic carbon source (i.e. methane), also in the past. We found several indications for the impact of recent earthquakes within the seep area (cracks, shifted seafloor), which could be an important mechanism for the triggering of new seepage activity, change in fluid expulsion rates and colonization patterns of the cold seep fauna.

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

2012-04-01

259

Relict sand waves in the continental shelf of the Gulf of Valencia (Western Mediterranean)  

NASA Astrophysics Data System (ADS)

The presence of fossil or relict bedforms is common in the Quaternary fill of modern continental shelf due to sea level oscillations, tectonic subsidence and migration of associated sedimentary facies. The continental margin of the Gulf of Valencia has been strongly influenced by glacio-eustasy and neotectonics. High-resolution multibeam bathymetry data, seismic reflection profiles and box core samples were collected across the continental shelf of the Gulf of Valencia during the DERIVA cruises carried out in 2010 and 2011. The integrated analysis of this data set and high-resolution mapping of the relict bedforms on the Valencian continental shelf, ranging between 50 and 90 m allowed the study of previously identified system of sand waves located in front of the present-day Albufera de Valencia lagoon. The system is composed of 27 ridges with a NNE-SSW orientation, i.e. oblique to the present shoreline, in which the lateral horns point backwards. These sand waves can reach 10 m in height and 3 km in length resulting in a maximum slope of 6°. According to seismic stratigraphic and relative sea level curve reconstructions, these sand waves were formed during the Younger Dryas (~ 12-10 ky BP). Consequently, they have been classified as Holocene sand waves associated with coastal sedimentary evolution.

Albarracín, Silvia; Alcántara-Carrió, Javier; Montoya-Montes, Isabel; Fontán-Bouzas, Ángela; Somoza, Luis; Amos, Carl L.; Salgado, Jorge Rey

2014-10-01

260

Will the French Riviera disappear? Ancient and present day submarine landslides along the north-western Ligurian Margin (NW Mediterranean) under the microscope  

NASA Astrophysics Data System (ADS)

The integration between seismic-reflection and bathymetric data highlighted more than one thousand and three hundred ancient and recent scars and Mass Transport Deposits (MTDs) related to submarine failures along the whole northern margin of the Ligurian Basin. Several types of failures are pointed out along the margin according to their morphology, distribution and origin: (1) Numerous superficial landslides affected largely the inner walls of canyons and the upper continental slope at the vicinity of the main mountain-supplied rivers where the sedimentation rates are the strongest and thus contribute to the progressive stressing of the slope. This type of scars is mainly observed in the western margin where the thickness of plio-quaternary sediment is the lowest. (2) Large-scale scars and successive MTDs affecting the upper part of interfluves in the Gulf of Genova (Finale Slide and Portofino Slide) could have been triggered by earthquakes as well as by indirect effects of the last sea level drop. (3) At the north-eastern margin, offshore of Imperia, there is a large promontory bounded by a network of N60°E faults on its southern side and characterized by the presence of many landslides of variable sizes associated to unconformities could correspond to the different phases of recent, and perhaps present-day, uplift of the Imperia Promontory. (4) Deep scars along the base of the continental slope and possibly related to the seismic activity of a neighbouring fault, as the Cirque Marcel located near the Marcel Fault which appears on the seafloor as a 10-km long scarp trending N60°E. (5) In the western basin, near of the foot of the continental slope, the bathymetry is mark by a convexity which corresponds on the seismic data to the superposition of several thick MTDs. (6) In the deep basin and the eastern part of the Ligurian Basin, the thick plio-quaternary deposits are constitute by an alternation between continuous reflectors corresponding to hemipelagic deposits and numerous wide-transparent bodies being turbidite deposits. These deposits are cutting by many saliferous faults. (7) Some thick MTDs affect the messinian erosion surface and the deposits set up right after this erosional period. The water handing-over of the basin is probably the main triggering factor of these failures. Thus, only the association of tectonic deformation and earthquake activity, sea-level fluctuations, high sedimentation rates and steep continental slope could explain the distribution and types of submarine instabilities along a steep margin showing a complex tectonic setting together with a complex topography as it is the case for the north-western Ligurian Margin.

Hassoun, V.; Migeon, S.; Larroque, C.; Cattaneo, A.; Mercier De Lepinay, B. F.

2011-12-01

261

CaCO 3 dissolution in California continental margin sediments: The influence of organic matter remineralization  

NASA Astrophysics Data System (ADS)

In situ benthic flux chamber and oxygen microelectrode and shipboard porewater results have been used to quantify sea floor dissolution of CaCO 3 on the continental rise adjacent to central California, USA. The porewater distributions and benthic fluxes of O 2, NO 3-, TA, Ca 2+, ? 13C, and TIC are interpreted using a numerical simulation of organic matter remineralization and CaCO 3 dissolution in marine sediments. The processes considered in the simulation include: organic matter oxidation by O 2, NO 3, and SO 42-; CaCO 3 dissolution and precipitation; HS - and NH 4+ oxidation; and sediment mixing and sediment accumulation. Calculated benthic fluxes of O 2, NO 3-, TA, TIC, ? 13C, and Ca 2+; porewater concentrations of O 2, NO 3-, and NH 4+; and sediment distributions of organic carbon, CaCO 3, excess 210Pb, and 14C agree well with the measurements. Benthic fluxes of alkalinity and inferred CaCO 3 dissolution rates cannot be explained on the basis of dissolution driven solely by bottom water undersaturation. If the influence of metabolically-produced CO 2 is included, benthic fluxes are fully reconciled, however. This is in agreement with benthic chamber Ca 2+ and ? 13C results that independently imply substantial CaCO 3 dissolution in these sediments. The above observations are in contrast to those reported by Jahnke et al. (1994) for the west African continental rise and the western equatorial Pacific where 1-G diagenetic models predict dissolution fluxes larger than observed with benthic flux chambers. We conclude that the extent of metabolic CaCO 3 dissolution may vary regionally. Numerous factors, such as the depth of metabolic CO 2 production and CaCO 3 dissolution kinetics, are known or predicted to influence metabolic dissolution. Among the factors that should be considered in reconciling these observations are: (l) the extent to which sulfate reduction and reoxidation reactions may influence acid-base properties in surface sediments and (2) the total sedimentary CaCO 3 content of the sites that may influence porewater acidity through surface exchange reactions at the mineral surface.

Jahnke, Richard A.; Craven, Deborah B.; McCorkle, Daniel C.; Reimers, Clare E.

1997-09-01

262

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

USGS Publications Warehouse

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

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

1992-01-01

263

3D seismic analysis of sedimentary processes on deepwater continental margins  

NASA Astrophysics Data System (ADS)

Two 3D seismic reflection datasets from the West African and Brazilian continental margins were analysed to determine their architectural elements and to further the understanding of the sedimentary processes that control their morphology. The results suggest a strong influence of local slope variations on the sedimentary processes and depositional and erosional products within these complex deep water settings. The Niger Delta dataset is characterised by large channel-levee systems and thrust-related folds. The folds degrade by channel erosion and slope failure, which creates laterally discontinuous erosional surfaces on the crests and flanks of the anticlines and chaotic deposits at their bases. The type of slope failure depends on the length and morphology of the local slope, sediment properties and the presence of anisotropics, such as faults. The location and morphology of the channel-levee systems on the Niger Delta are affected by topographical effects associated with thrust-related folding. Fold-induced local changes in gradient cause turbidity currents to deposit sediment upstream of the folds and erode the seafloor downstream of them. This results in the formation of knickpoints along the present- day thalweg of a channel-levee system. A model for the formation and evolution of the knickpoints predicts that they migrate upstream and leave internal erosion surfaces and terraces with coarse sediments in the sedimentary record. They may be an important process by which channels cut through uplifting fold belts. The Espirito Santo Basin dataset is characterised by salt diapirs, slope failures, channels and canyons. Interaction of turbidity currents with variations in topography has led to the formation of large depressions, which occur above abrupt breaks in slope and in trails that follow underlying erosional channels. They are inferred to form by Froude-supercritical currents that become unstable as they encounter topographical irregularities, such as scarps and knickpoints. This leads to the formation of erosional scours and deposits similar to sediment waves, which, when confined within channels, appear as roughly circular depressions.

Heinio, Paivi Tuuli

264

Generation of granitic batholiths along a proposed proterozoic continental margin in east-central Minnesota  

SciTech Connect

The 1800 m.y. igneous terrane of East-Central Minnesota is composed of mafic plutons and dikes, diorites (54% SiO2), and a range of granodiorites to granites (63-75% SiO2). Basalts through granite appear to be genetically related because they show similar enrichment of LILE, have similar incompatible element ratios, similar Nd isotope initial values which average -1 epsilon, and lie about the same 1800 m.y. /sup 207/Pb//sup 204/Pb vs. /sup 206/Pb//sup 204/Pb isochron. /sup 18/O isotope values for the felsic rocks lie between 8.2 and 9.6, for the mafic rocks between 6 and 9.2. The Reformatory Granite has the lowest SiO/sub 2/ (ca. 64%) of the granitic rocks, has Mgnumber's of 0.40-0.50, 557-652 ppm Sr, and 18-22 ppm Ni. Its Sr content and REE pattern are not consistent with its derivation from the exposed basic rocks, but may be by differentiation of mantle derived alkali basalt or high- Mg andesite. The major and trace element chemistry for the more siliceous granitic rocks is compatible with their being more extensively differentiated from a similar melt. These granites have apatite and zircon saturation temperatures of 800/sup 0/-940/sup 0/ C, which is higher than what might be expected for partial melting of crustal rocks. A convergent continental margin has been proposed for the tectonic setting for the terrane. The parent melts or their sources could have mixed with a component that provided the high /sup 18/O relative to mantle values and negative epsilon Nd.

Spencer, K.J.; Hanson, G.N.; Horan, M.F.

1985-01-01

265

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

USGS Publications Warehouse

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

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

1980-01-01

266

Influence of the Afar plume on the deep structure of Aden and Red Sea margins - Insight from teleseismic tomography in western Yemen  

NASA Astrophysics Data System (ADS)

Continental rupture processes under mantle plume influence are still poorly known although extensively studied. The Afar plume has been largely investigated in Ethiopia to study early stages of continental break-up. Here we imaged the lithospheric structure of western continental Yemen to evaluate the role of the Afar plume on the evolution of the continental margin and its extent towards the East. A part of the YOCMAL project (YOung Conjugate MArgins Laboratory) permitted the deployment of twenty-three broadband stations in Yemen (from 2009 to 2010). Using a classical teleseismic tomography (Aki et al., 1974) on these stations together with a permanent GFZ station, we image the relative velocity variations of P-waves in the crust and lithosphere down to 300 km depth, with a maximum lateral resolution of about ~20 km. The model thus obtained shows (1) a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden (2) the presence of magmatic underplating related to seaward dipping reflectors under those two volcanic margins (3) two granitic syn-rift intrusions on the border of the great escarpment (4) a low velocity anomaly in which with evidence of partial melting, just below thick Oligocene trapps series and other volcanic events (from 15 Ma to present). This low velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. (5) Finally, we infer the presence of hot material under the Southwestern corner of Yemen that could be related to Miocene volcanism in Jabal an Nar.

Korostelev, Félicie; Basuyau, Clémence; Leroy, Sylvie; Ahmed, Abdulhakim; Keir, Derek; Stuart, Graham; Rolandone, Frédérique; Ganad, Ismail Al; Khanbari, Khaled

2013-04-01

267

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

NASA Astrophysics Data System (ADS)

It has been a long held view that oceanic fracture zones play an important role in the segmentation of continental margins and therefore provide a major structural control on their evolution and the development of associated petroleum systems. The geometry of fracture zones reflects the spreading history of the seafloor: subtle changes in plate motion causes stress-field reorientation, which in turn results in changes in the orientation of the fracture zone. These changes can introduce strike-perpendicular compression or extension across transform faults; the latter may lead to increased ridge segmentation and the initiation of new spreading centres. We present two examples of secondary fracture zone initiation and disappearance within the Atlantic Ocean between 1) the Atlantis and Kane major fracture zones in the Central Atlantic and 2) the Ascension and Rio de Janeiro fracture zones in the South Atlantic. We investigate the discontinuous nature of these fracture zones by exploring their relationship with major plate re-organisation events and seafloor spreading geometry. Using a series of stage reconstruction poles that represent the motion of both North and South America relative to Africa since the initiation of Atlantic seafloor spreading, we have performed a quantitative analysis of spreading directions along major Atlantic fracture zones. Our results demonstrate a notable correlation between the timing of major plate reorganisation events and the initiation and disappearance of secondary fracture zones. Such events are clearly recorded in the Atlantic margin stratigraphic record as major unconformities. We are therefore able to interpret fracture zone abundance in terms of palaeo-spreading geometry and the opening history of the Atlantic Ocean. This allows us to make important inferences about the influence of fracture zones on the segmentation and structural control of continental margins. Specifically, in our South Atlantic case study, where secondary fracture zones do not extend up to the offshore Angolan and conjugate Brazilian margins, we conclude that small offset transform faulting did not influence the evolution of the continental margin as has been previously suggested. On a regional scale, the evolution of the Africa-wide Mesozoic rift system is intimately linked to global plate tectonics and to changes in plate interactions. On a basinal scale, changes in the orientation of the dominant stress field resulting from plate reorganisation have had a clear impact on the deformation history and fault geometries of rift basins. We demonstrate this relationship by correlating the timing of changes in South Atlantic fracture zone geometries and African margin unconformities with major unconformities that are observed in a unified stratigraphy chart for the West and Central African Rift System. We propose a controlling mechanism in which changes in plate stress control the effective elastic strength of a plate, resulting in a focused change in isostatic response over continental margins.

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

2012-12-01

268

New apatite fission-track data reflecting the landscape evolution using the example of the southeastern passive continental margin in Central Brazil  

Microsoft Academic Search

Low-temperature thermochronolgy like AFT yield a well established tool to understand and reconstruct the rift to post-rift evolution of the passive continental margin in Brazil. The aim of the study is to quantify the temperature, exhumation, uplift, and long-term dynamic evolution of the topography of the southeastern passive continental margin in the states of Paraná and Santa Catarina (Central Brazil)

Markus Karl; Ulrich A. Glasmacher; Peter C. Hackspacher; Ana O. B. Franco-Magalhaes

2010-01-01

269

Tectonic relations between shallow and deep crust in the southeastern Brazilian continental margin: low temperature thermochronology, gravimetry and seismic reflection  

NASA Astrophysics Data System (ADS)

Low-temperature thermochronology studies, gravimetric and seismic reflection modeling, developed on the southeastern Brazil has been approached independently and without apparent connection. This paper correlates data from shallow and deep crust in the region that includes the Serra do Mar and Mantiqueira. This region is formed by Precambrian rocks with steep topography resulted of intense reworking during the Mesozoic and Cenozoic. Fission tracks data on zircon, apatite and U-Th/He methodology record a polycyclic history with tectonic peaks at temperatures below 240oC in 90, 60 and 45 Ma. Uplift and exhumation alternated heterogeneously along the margin, related to a E-W extensional process with strong vertical movements. Associated with the history of the Eocene, Precambrian rocks, forms structure of the Southeastern Brazilian Continental Rift of totaling approximately 2 000 km along the continental and submerged margin of the southeastern South America Gravimetric modeling shows an alignment of denser rocks at the base of the crust along the Rift. Interpretation of reflexion seismic section in the Campos Basin, shows syn-rift, post-rift stratigraphic sequences and Precambrian basement, postulating an tectonic evolution with an crustal stretching (Cainelli, C., Mohriak, W.U.,1998; Macedo, J.M., 1989). This process would be associated with the drift phase (Post-Albian) responsible for the large amount of clastic sediments to the marginal basins and can be observed in the interpretations of seismic profiles and wells. The correlation of the thermochronological, seismic and gravimetric tools allows us to consider an E-W stretching with thinning of the continental, until the oceanic crust, in SE Brazil, with uplift of the lithospheric mantle and consequent formation of the Serra do Mar and Mantiqueira, erosion and deposition of sediments of the Southeastern Brazilian Continental Rift, all occurring, after the drift phase of the South Atlantic Rifting. Concepts of plume and delamination can be attributed to these modeling.

Hackspacher, P. C.; Souza, I. A.; Almeida, S. H.; Glasmacher, U. A.

2012-04-01

270

Inland jump of the Arabian northwest plate boundary from the Levant continental margin to the Dead Sea Transform  

NASA Astrophysics Data System (ADS)

While the breakup of Arabia and Africa began in the early Oligocene, the northwestern boundary of the Arabian Plate along the Dead Sea Transform (DST) formed 10-15 Myr later (early-middle Miocene). During the early stage of breakup the Red Sea Rift continued propagating northwestward, forming the Suez Rift. The present ˜45° northward twist of the plate boundary from the Red Sea toward the Gulf of Eilat (Aqaba) still did not exist. What happened at the northern tip of the Suez Rift at that time? How was strain distributed to its surroundings, and where did the plate boundary continue from there? Here we describe an abandoned segment of the Arabian northwestern plate boundary that extended from the northern tip of the Suez Rift northeastward, along the Levant margin. Seismic data collected offshore Israel support a subsurface, deep-rooted fault zone running along the base of the continental slope. These faults indicate Oligocene transpressional lateral shearing. We propose that during the early stage of continental breakup a left-lateral strike-slip motion of ˜10 km took place along this embryonic plate boundary. Such deep-rooted tectonism implies that unlike the passive situation of the Israel-Sinai continental margin witnessed presently and before the Oligocene, during the early stage of the Africa-Arabia breakup this part of the continental margin was reactivated. We further suggest that the inland jump of the plate boundary toward the DST was not immediate and that during the transitional period the Israel-Sinai triangular block was an independent subplate with deformation all around it.

Gvirtzman, Zohar; Steinberg, Josh

2012-08-01

271

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

E-print Network

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

Yang, Zong-Liang

272

Sedimentology of seismo-turbidites off the Cascadia and northern California active tectonic continental margins, Pacific Ocean  

NASA Astrophysics Data System (ADS)

Holocene turbidites from turbidite channel systems along the active tectonic continental margins of the Cascadia subduction zone (offshore Vancouver Island to Mendocino Triple Junction) and the northern San Andreas Transform Fault (the Triple Junction to San Francisco Bay), have been analyzed for sedimentologic features related to their seismic origin. Centimeter thick silt/sand beds (turbidite base) capped by mud layers (turbidite tail) and interbedded with hemipelagic silty clay intervals with high biogenic content have been characterized by visual core descriptions, grain-size analysis, X-ray radiographs and physical properties. Along the northern California margin in upstream single tributary canyons and channels, most turbidites are uni-pulsed (classic fining up) whereas downstream below multiple tributary canyon and channel confluences, most deposits are stacked turbidites. Because each set of stacked turbidites has no hemipelagic sediment between each turbidite unit and each unit has a distinct mineralogy from a different tributary canyon, we interpret that a stacked turbidite is deposited by several coeval turbidity currents fed by multiple tributary canyons and channels with synchronous triggering from a single San Andreas Fault earthquake. The Cascadia margin is characterized by individual multi-pulsed turbidites that contain multiple coarse-grained sub-units without hemipelagic sediment between pulses. Because the number and character of multiple coarse-grained pulses for each correlative multi-pulsed turbidite is almost always constant both upstream and downstream in different channel systems for 600 km along the margin,we interpret that the earthquake shaking or aftershock signature is usually preserved, for the much stronger Cascadia (?9 Mw) compared to weaker California (?8Mw) earthquakes, which result in upstream uni-pulsed turbidites and downstream stacked turbidites. Consequently, both the strongest (?9 Mw) great earthquakes and downstream confluences cause multi-pulsed and stacked turbidites that are typical for seismo-turbidites generated by a single great earthquake. Along both margins, earthquake triggering and multi-pulsed or stacked turbidites become an alternative explanation for amalgamated turbidite beds in active tectonic margins and show that multiple grain-size pulses and reverse grading are not unique criteria for hyperpycnites, thalweg levee turbidites, or mini-basin margin turbidites. Analyses of the turbidites along the Cascadia and northern California margins reveal common sedimentologic characteristics of turbidites triggered by great earthquakes that can be used to distinguish seismo-turbidites in other active tectonic margins around the world. Gutierrez-Pastor, J., Nelson C. H., Goldfinger, C., and Escutia, C., Sedimentology of seismo-turbidites off the Cascadia and northern California active tectonic continental margins, northwest Pacific Ocean, Marine Geology 336 (2013) 99-119. http://dx.doi.org/10.1016/j.margeo.2012.11.010

Gutierrez Pastor, Julia; Nelson, Hans; Goldfinger, Chris; Escutia, Carlota

2013-04-01

273

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

USGS Publications Warehouse

The Neoproterozoic Ar Rayn terrane is exposed along the eastern margin of the Arabian shield. The terrane is bounded on the west by the Ad Dawadimi terrane across the Al Amar fault zone (AAF), and is nonconformably overlain on the east by Phanerozoic sedimentary rocks. The terrane is composed of a magmatic arc complex and syn- to post-orogenic intrusions. The layered rocks of the arc, the Al Amar group (>689 Ma to ???625 Ma), consist of tholeiitic to calc-alkaline basaltic to rhyolitic volcanic and volcaniclastic rocks with subordinate tuffaceous sedimentary rocks and carbonates, and are divided into an eastern and western sequence. Plutonic rocks of the terrane form three distinct lithogeochemical groups: (1) low-Al trondhjemite-tonalite-granodiorite (TTG) of arc affinity (632-616 Ma) in the western part of the terrane, (2) high-Al TTG/adakite of arc affinity (689-617 Ma) in the central and eastern part of the terrane, and (3) syn- to post-orogenic alkali granite (607-583 Ma). West-dipping subduction along a trench east of the terrane is inferred from high-Al TTG/adakite emplaced east of low-Al TTG. The Ar Rayn terrane contains significant resources in epithermal Au-Ag-Zn-Cu-barite, enigmatic stratiform volcanic-hosted Khnaiguiyah-type Zn-Cu-Fe-Mn, and orogenic Au vein deposits, and the potential for significant resources in Fe-oxide Cu-Au (IOCG), and porphyry Cu deposits. Khnaiguiyah-type deposits formed before or during early deformation of the Al Amar group eastern sequence. Epithermal and porphyry deposits formed proximal to volcanic centers in Al Amar group western sequence. IOCG deposits are largely structurally controlled and hosted by group-1 intrusions and Al Amar group volcanic rocks in the western part of the terrane. Orogenic gold veins are largely associated with north-striking faults, particularly in and near the AAF, and are presumably related to amalgamation of the Ar Rayn and Ad Dawadimi terranes. Geologic, structural, and metallogenic characteristics of the Ar Rayn terrane are analogous to the Andean continental margin of Chile, with opposite subduction polarity. The Ar Rayn terrane represents a continental margin arc that lay above a west-dipping subduction zone along a continental block represented by the Afif composite terrane. The concentration of epithermal, porphyry Cu and IOCG mineral systems, of central arc affiliation, along the AAF suggests that the AAF is not an ophiolitic suture zone, but originated as a major intra-arc fault that localized magmatism and mineralization. West-directed oblique subduction and ultimate collision with a land mass from the east (East Gondwana?) resulted in major transcurrent displacement along the AAF, bringing the eastern part of the arc terrane to its present exposed position, juxtaposed across the AAF against a back-arc basin assemblage represented by the Abt schist of the Ad Dawadimi terrane. Our findings indicate that arc formation and accretionary processes in the Arabian shield were still ongoing into the latest Neoproterozoic (Ediacaran), to about 620-600 Ma, and lead us to conclude that evolution of the Ar Rayn terrane (arc formation, accretion, syn- to postorogenic plutonism) defines a final stage of assembly of the Gondwana supercontinent along the northeastern margin of the East African orogen. ?? 2007 Elsevier B.V. All rights reserved.

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

2007-01-01

274

Continental growth at convergent margins facing large ocean basins: a case study from Mesozoic convergent-margin basins of Baja California, Mexico  

NASA Astrophysics Data System (ADS)

Mesozoic rocks of the Baja California Peninsula form one of the most areally extensive, best-exposed, longest-lived (160 my), least-tectonized and least-metamorphosed convergent-margin basin complexes in the world. This convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins: a progression from highly extensional (phase 1) through mildly extensional (phase 2) to compressional (phase 3) strain regimes. This trend is largely due to the progressively decreasing age of lithosphere that is subducted, which causes a gradual decrease in slab dip angle (and concomitant increase in coupling between lower and upper plates), as well as progressive inboard migration of the arc axis. This paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin. Phase 1 consists of Late Triassic to Late Jurassic oceanic intra-arc to backarc basins that were isolated from continental sediment sources. New, progressively widening basins were created by arc rifting and sea floor spreading, and these were largely filled with progradational backarc arc-apron deposits that record the growth of adjacent volcanoes up to and above sea level. Inboard migration of the backarc spreading center ultimately results in renewed arc rifting, producing an influx of silicic pyroclastics to the backarc basin. Rifting succeeds in conversion of the active backarc basin into a remnant backarc basin, which is blanketed by epiclastic sands. Phase 1 oceanic arc-backarc terranes were amalgamated by Late Jurassic sinistral strike slip faults. They form the forearc substrate for phase 2, indicating inboard migration of the arc axis due to decrease in slab dip. Phase 2 consists of Early Cretaceous extensional fringing arc basins adjacent to a continent. Phase 2 forearc basins consist of grabens that stepped downward toward the trench, filled with coarse-grained slope apron deposits. Phase 2 intra-arc basins show a cycle of (1) arc extension, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions, followed by (2) arc rifting, characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites. This extensional-rifting cycle was followed by mid-Cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent, caused by a decrease in slab dip as well as a possible increase in convergence rate. Phase 2 fringing arc terranes form the substrate for phase 3, which consists of a Late Cretaceous high-standing, compressional continental arc that migrated inboard with time. Strongly coupled subduction resulted in accretion of blueschist metamorphic rocks, with development of a broad residual forearc basin behind the growing accretionary wedge, and development of extensional forearc (trench-slope) basins atop the gravitationally collapsing accretionary wedge. Inboard of this, ongoing phase 3 strongly coupled subduction, together with oblique convergence, resulted in development of forearc strike-slip basins upon arc basement. The modern Earth is strongly biased toward long-lived arc-trench systems, which are compressional; therefore, evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California. This convergent margin is typical of many others, where the early to middle stages of convergence (phases 1 and 2) create nonsubductable arc-ophiolite terranes (and their basin fills) in the upper plate. These become accreted to the continental margin in the late stage of convergence (phase 3), resulting in significant continental growth.

Busby, Cathy

2004-11-01

275

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

NASA Astrophysics Data System (ADS)

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

McClellan, Elizabeth; Gazel, Esteban

2014-10-01

276

75 FR 17156 - Gulf of Mexico, Outer Continental Shelf, Western Planning Area, Oil and Gas Lease Sale 215 (2010...  

Federal Register 2010, 2011, 2012, 2013, 2014

...THE INTERIOR Minerals Management Service Gulf of Mexico...Continental Shelf, Western Planning Area, Oil and Gas Lease...Assessment AGENCY: Minerals Management Service, Interior...SUMMARY: The Minerals Management Service (MMS) is issuing...Sale 215 in the Western Planning Area (WPA)...

2010-04-05

277

Tectonic entrapment and its role in the evolution of continental lithosphere: An example from the Precambrain of western Canada  

Microsoft Academic Search

New geophysical data acquired over the buried crystalline basement of western Canada provide constraints on the history of tectonic assembly of the western Canadian Shield in the interval 1.75-1.85 Ga. Specifically, these data provide new perspectives on the evolution of an Archean continental fragment (Hearne province) that was trapped in a tectonic ``vise'' between coeval orogenic belts that dipped beneath

Gerald M. Ross; David W. Eaton; David E. Boerner; Warner Miles

2000-01-01

278

Salt tectonics and crustal tectonics along the Eastern Sardinian margin, Western Tyrrhenian: New insights from the " METYSS 1 " cruise  

NASA Astrophysics Data System (ADS)

The Tyrrhenian Sea is usually interpreted to be a Neogene back-arc basin that opened by continental rifting and oceanic spreading caused by the eastward migration of the Apennine subduction system from Tortonian to Pliocene times. Rifting of the Tyrrhenian Sea started first along the Eastern Sardinian margin during the Tortonian-Messinian times. The " METYSS " project aims at better constraining the relations between crustal tectonics, salt tectonics and sedimentation from Messinian times to present-day. The " METYSS 1 " cruise (2009, R/V "Téthys II"-INSU) allowed us to obtain about 1200 km of high-resolution seismic reflection profiles along the SE Corsican margin, and along the upper and middle parts of the Eastern Sardinian margin, Western Tyrrhenian. This new dataset clearly illustrates that this area has been highly dissected during the rifting stage by N-S trending normal faults delineating ridges and basins, as previously described. The Messinian seismic markers (UU and MU) locally display fan-shaped stratal geometries, but the mechanism responsible for such geometries, i.e., salt tectonics or rifting, has yet to be carefully deciphered. We also mapped the spatial distribution of the mobile salt (MU). The highly variable thickness of the confined salt basins may be related to the initial pre-Messinian basin geometry or to the fact that sediment deposition was syn-rift. Southeastward, salt tectonics appears highly vigorous. More surprisingly, our data demonstrate that some of the major faults have been significantly reactivated during the post-rift period, up to late Quaternary time. Such recent fault activity has been recorded by bathymetric scarps and associated footwall debris flows. Moreover, some of this post-rift deformation can be evidenced within the Plio-Quaternary sequence by a regional unconformity. The post-Miocene deformation style varies greatly between different areas of the margin, and can also be strongly influenced by the presence or absence of mobile Messinian evaporites (MU).

Gaullier, V.; Chanier, F.; Lymer, G.; Maillard, A.; Sage, F.; Thinon, I.; LOFI, J.; Vendeville, B.; Loncke, L.

2011-12-01

279

Strain partitioning along the western margin of North America  

NASA Astrophysics Data System (ADS)

This paper describes an elastic block model for the interseismic horizontal crustal velocity field occurring in that part of the United States located west of longitude 100° W and between latitudes 31°N and 49°N. We developed the model by simultaneously inverting 6873 GPS-derived velocity vectors and 166 geological fault slip rates for the angular velocities (i.e. the Euler poles relative to the North America plate) of 46 elastic blocks, horizontal strain rate tensors for 38 of these blocks, and the spatially variable elastic coupling coefficients on faults that bound adjacent blocks. While the model covers all of the western United States located between Canada and Mexico, this paper focuses on the region residing south of Cape Mendocino where plate boundary deformation is accommodated predominantly by slip on the San Andreas fault system. Block strain rates (which account for deformation associated with distributed faults within blocks) are systematically higher in blocks located in the western part of the model and adjacent to the plate boundary. Strain rate magnitudes range from over 10-7/yr for some blocks adjacent to the San Andreas fault system to values of about 10-9/yr for blocks located in eastern Nevada and western Utah. Blocks adjacent to the San Andreas fault system are characterized by strain rate tensors that correspond to uniaxial contraction perpendicular to the local strike of the San Andreas. The highest rates of fault normal contraction are associated with the northern end of the fault (north of San Francisco) and in the southern end (south of Los Angeles). The central San Andreas (including the creeping segment of the fault) is characterized by strain rate tensors more consistent with dextral shear. Thus the northern and southern ends of the fault are consistent with a transpressional strain partitioning model with strike slip occurring on the San Andreas fault system and distributed shortening occurring within the blocks adjacent to this fault system. There is no evidence of strain partitioning in the central San Andreas.

Pearson, Christopher F.; Snay, R. A.

2014-07-01

280

Methane flux and sulfate reduction variations in the continental margin offshore southwestern Taiwan  

NASA Astrophysics Data System (ADS)

Methane gas is an important greenhouse gas. It can affect global climate if large amounts of methane gas release to the atmosphere. During migration, methane will be consumed via anaerobic methane oxidation (AOM) before reaching seawater or the atmosphere (Hinrichs et al, 1999). High methane concentration associated with gas hydrate will enhance sulfate reduction and AOM, resulting in steep sulfate gradients and shallow sulfate-methane interface (SMI) (Borowski et al., 1999). Therefore, sulfate profile is useful to estimate methane flux if AOM become the predominant process in sediment. During our TowCam survey, methane seep-related seafloor features, especially authigenic carbonate buildups, chimney structures and chemosynthetic communities, were found in the continental margin offshore southwestern Taiwan. In order to understand methane flux and sulfate reduction variations in this study area, piston cores and gravity cores were collected on board the r/v Ocean Research I. Sediment samples collected were analyzed for methane, sulfate, dissolved sulfide, pyrite-sulfide, organic carbon, and carbonate content. Spatial variations of methane concentration and sulfate reduction were found in this study area. Shallow SMI and high methane concentration were found on the front of the accretional wedge close to the deformation front (Yung-An lineament). Some SMI depth is shallower than 1m. Methane and dissolved sulfide concentration were as high as 10mM at some locations. Concentration of methane and dissolved sulfide decreased while SMI depth increase away from the accretional wedge front from west to east. C-13 isotope depleted authigenic carbonate and mussel tissues found on gas seep area showed that methane is the major carbon source and the AOM is an important biogeochemical process in the study area. The range of sulfate and methane flux calculated by the Fick's law were 1.80-219 mmole/m2/yr and 0.01-87 mmole/m2/yr, respectively. Sulfate flux, however, was higher than methane flux, indicating that sulfate reduction was not controlled only by the AOM, but also through oxidizing sedimentary organic carbon. Sulfate flux in this study area was higher than those of the Blake Ridge (Dickens, 2001, Borowski et al., 1999), the Namibia (Niewohner et al., 1998) and the West Argentine Basin (Hensen et al., 2003), probably reflecting methane flux seeping from beneath in this study region.

Lim, Y.; Lin, S.; Yang, T. F.; Chen, Y.; Liu, C.; Wang, Y.; Chung, S.

2008-12-01

281

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

NASA Astrophysics Data System (ADS)

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 < 0.5 wt % in relict shelf sands to over 7 wt % at slope sites within the OMZ, decreasing to ? 1 wt % at 2000 m. Major variability (~ 5 wt %) was found at slope sites within the OMZ of similar depth and near-identical bottom-water oxygen concentration. A strong relationship between organic C and sediment grain size was seen for sediments within the OMZ, but lower C loadings were found for sites on the shelf and below the OMZ. Diagenetic indices confirmed that lower C content below the OMZ is associated with greater extent of OM degradation, but that C-poor shelf sediments are not consistently more degraded than those within the OMZ. Together, the results indicate that OM enrichment on the upper slope can be explained by physical controls (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.

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

282

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

USGS Publications Warehouse

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

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

2002-01-01

283

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

NASA Astrophysics Data System (ADS)

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

Bradley, D. C.

2013-12-01

284

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

E-print Network

Denudation of the Côte d'Ivoire-Ghana transform continental margin from apatite fission tracks Apatite fission track analysis of samples from the shoulder (marginal ridge) of the Côte d'Ivoire-Ghana of the Gulf of Guinea, the Côte d'Ivoire-Ghana (CIG) ridge is 25 km-width (Figure 1), 130 km­long structure

285

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

Microsoft Academic Search

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

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

2005-01-01

286

ARCHITECTURE OF RIFTED CONTINENTAL MARGINS AND BREAKUP EVOLUTION: INSIGHTS FROM THE SOUTH ATLANTIC, NORTH ATLANTIC AND RED SEA GULF OF ADEN  

E-print Network

ATLANTIC, NORTH ATLANTIC AND RED SEA ­ GULF OF ADEN CONJUGATE MARGINS Mohriak, Webster Ueipass1 Leroy of magmatism during the Mesozoic continental breakup. Subsequently, the Cenozoic breakup in the North Atlantic Atlantic is compared with the Central Atlantic margins, which are associated with major episodes

287

The Dauki Fault in NE India: A crustal scale thrust-fold reactivating the continental margin  

NASA Astrophysics Data System (ADS)

New structural data along the central part of the Dauki topographic front supports the hypothesis that the Shillong Plateau is a highly asymmetric south-verging Quaternary anticline driven by a north-dipping blind thrust fault that projects into Bangladesh, south of the topographic front. This thrust-fold is tectonically more important than it appears from the relatively modest accumulated deformation, and may represent a reorganization of the eastern Himalayan front. The Dauki Fault is the most likely source of the 1897 Great Indian Earthquake and poses a hazard to densely populated areas on the Ganges-Brahmaputra Delta region. The sharp linear topographic feature often mapped as the Dauki fault is instead a contact between competent Eocene limestone and much less competent younger clastic units. This contact may be depositional or locally a secondary back thrust. While the Sylhet basin has been rapidly subsiding in the Late Quaternary, the topographic front is marked by raised and eroded river fanglomerates, thus still on the hangingwall side of the fault. Samples from these raised terraces will be dated using optically stimulated luminescence. The exposed structural relief is primarily accounted for by folding, very broad at the culmination on the "plateau," but much sharper at the southern front. In the central and steepest Cherrapunji segment of the Dauki front, the fold is marked by the erosion resistant Cretaceous-Paleocene passive-margin sequence overlying the Sylhet Traps with evidence that the Cretaceous rifting was parallel to the Dauki front. The Dauki fault, therefore, could be a passive margin-related normal fault reactivated as a thrust. The part of the forelimb exposed in the ~20 km Cherrapunji segment exhibits two sharp kinks, suggesting blind imbricates above the main blind fault. The Shillong Plateau is characterized by a two-level drainage morphology. The well-preserved Precambrian surface and its Cretaceous cover along the southern edge of the plateau have been folded and uplifted up to 1.8 km and are only beginning to be dissected by deep canyons along its southern margin despite the high relief and rainfall. This immature morphology coupled with ongoing subsidence in the foredeep basin suggests active and rapid uplift, postdating the Miocene exhumation event inferred from available Miocene cooling ages. Samples of sediment collected from the deeply incised valleys are being dated using cosmogenic nuclides in order to quantify erosion rates in the south-central region of the Shillong Plateau. Tilting associated with the forelimb is manifested in asymmetric erosion along strike-parallel river valleys on the plateau and gravitational collapse of the saprolitic cover down the forelimb. Future work will investigate the structure associated with the eastern and western segments of the Dauki fault to determine the continuity along strike and the interaction between the Dauki Fault and the Burma fold belt.

Ferguson, E. K.; Seeber, L.; Akhter, S. H.; Steckler, M. S.; Biswas, A.; Mukhopadhyay, B. P.

2011-12-01

288

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

NASA Astrophysics Data System (ADS)

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, and West Africa Margin) world-wide by using the substantial archive of satellite synthetic aperture radar (SAR) images. An automated system for satellite image interpretation will make it possible to process hundreds of SAR images to increase the geographic and temporal coverage. Field programs will quantify the flux and fate of hydrate methane in sediments and the water column.

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

2008-12-01

289

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

290

Tectonic entrapment and its role in the evolution of continental lithosphere: An example from the Precambrain of western Canada  

NASA Astrophysics Data System (ADS)

New geophysical data acquired over the buried crystalline basement of western Canada provide constraints on the history of tectonic assembly of the western Canadian Shield in the interval 1.75-1.85 Ga. Specifically, these data provide new perspectives on the evolution of an Archean continental fragment (Heame province) that was trapped in a tectonic "vise" between coeval orogenic belts that dipped beneath the Hearne province. The Trans-Hudson orogen developed along the southeastern margin of the Hearne province as a series of ocean floor, oceanic arc, and arc marginal basins were telescoped and thrust obliquely beneath the Hearne. Along the northwest edge of the Hearne, collapse and subduction of a narrow marginal basin, now marked by the subsurface extension of the Snowbird Tectonic Zone, led to formation of magmatic arc and collision of older Proterozoic terranes. The Hearne province itself is characterized by regional granulite-grade metamorphism and evidence of extensive and pervasive partial melting of the crust. The internal character of the Hearne province seen on crustal seismic reflection profiles is that of a crustal-scale structural fan with reflection fabrics that verge toward the bounding orogens. The deformation of the Hearne is predominantly of Paleoproterozoic age and constitutes a thorough reworking of this formerly Archean crustal domain over a distance of more than 600 km across strike. Entrapment and thermal weakening of the Hearne resulted from mechanical coupling of inferred buoyant subduction-collision zones and removal of or modification of Archean lithospheric mantle that may have originally formed the keel to the Hearne. Long-period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day Hearne province, which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination/convective removal of thickened lithosphere in the Proterozoic. Tectonic entrapment, as illustrated by the Proterozoic structural and thermal evolution of the Hearne province crust and subcontinental mantle, may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents.

Ross, Gerald M.; Eaton, David W.; Boerner, David E.; Miles, Warner

2000-02-01

291

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

USGS Publications Warehouse

Six computer-generated seismic depth sections over the outer continental shelf and upper slope reveal that subhorizontal Lower Cretaceous reflectors continue 20 to 30 km seaward of the present shelf edge. Extensive erosion on the continental slope has occurred primarily during the Tertiary, causing major unconformities and retreat of the shelf edge to its present position. The precise age and number of erosional events is not established, but at least one major erosional event is thought to be Oligocene and related to a marine regression in response to a worldwide eustatic lowering of sea level. Velocities derived from the multichannel data reveal distinctive ranges and lateral trends as functions of sediment age, depth of burial, and distance from the coastline. Seismic units beneath the shelf and slope of inferred Tertiary age range from 1.7 to 2.7 km/sec, increasing with age and depth of burial. Units interpreted as Upper Cretaceous rocks beneath the shelf range from 2.3 to 3.6 km/sec and show a distinct lateral increase across the shelf followed by a decrease beneath the present continental slope. Inferred Lower Cretaceous and Upper Jurassic rocks beneath the shelf increase from 3.7 to 4.8 km/sec from nearshore to offshore and indicate a change in facies from clastic units below the inner shelf to carbonate units beneath the outer shelf and upper continental slope. Both reflection and refraction data suggest that thin, high-velocity limestone units (5.0 km/sec) are present within the Lower Cretaceous and Upper Jurassic units beneath the outermost shelf edge, but that these change lithology or pinch out before reaching the middle shelf. Although lateral changes in velocity across the shelf and local velocity inversions appear, the interval velocities along the length of the margin show excellent continuity between Cape Hatteras and Cape Cod. The high-velocity horizons within the Lower Cretaceous and Upper Jurassic shelf-edge complex indicate the presence of a carbonate bank or reef. The continental shelf off New Jersey is underlain by a trough approximately 150 km wide with up to 12 km of sedimentary fill. The oceanic basement beneath the upper continental rise is usually at a depth of 10 to 11 km and is overlain by 6 to 8 km of sediments. The rise sediment trough is separated from the shelf trough by an acoustic basement ridge 25 to 75 km wide where penetration never exceeds 6 km beneath sea level, although faulting and carbonate bank or reef complexes frequently limit penetration to 3 to 4 km in this zone. The acoustic basement ridge coincides with the East Coast Magnetic Anomaly and is interpreted as thick oceanic crust formed during the initial phase of seafloor spreading between North America and Africa. Rapid differential subsidence occurred on opposite sides of the basement ridge during the Jurassic and Early Cretaceous. Differential subsidence beneath the shelf also occurred along the margins, with narrower and shallower shelf troughs occurring off platform areas such as Cape Cod and Cape Hatteras.

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

1979-01-01

292

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

NASA Astrophysics Data System (ADS)

Although the older record and successive tectonic scenarios experienced by a continental margin is commonly fragmentary, integrated field, petrological and geochronological analysis can reconstruct the long term tectonic evolution of continental margins and characterized major controls on the orogenic style. We present new geochronological constraints from igneous and low to very low grade metasedimentary rocks from the Caribbean continental margin of northeastern Colombia (Guajira region) in order to reconstruct the different tectonic events recorded by the margin before, during and following the arc-continent collision with the front of the Caribbean plate. Zircon U-Pb LA-ICP-MS geochronology results from leucogranites associated with garnet amphibolites, tonalites and volcanic rocks that made the continental basement of northeastern Colombia reveals and Early to Middle Mesozoic tectonic activity with peaks at ca. 220-230 Ma and 170-180 Ma. This magmatic record is related to a collisional belt link to the final agglutination of Pangea and was followed by an overimposed far field back-arc setting associated to the subduction of the Pacific (Farrallon) plate under the Pangea supercontinent. Muscovite and biotite Ar-Ar geochronology from basement rocks and low grade Mesozoic metasediments also reveals the existence of Middle Jurassic to Early Cretaceous thermal events link to the final opening of the proto-Caribbean ocean. The South American continental margin was subsequently affected by an arc-continent collisional event with the front of the Caribbean plate. This event is recorded by the growth of a Banda-type collisional melange that mixed South American continental margin sediments with mafic and ultramafic blocks of intra-oceanic arc origin, the formation of a coherent metasedimentary belt also made of South American margin sediments, and the mylonitization of the continental basement. Ar-Ar temporal constraints on the low grade metasedimentary rocks and detrital apatite fission track ages from younger sedimentary sequences suggest a Late Campanian age for this deformational event. Continuous convergence and the formation of a new subduction zone in the South American margin were responsible for the remobilization of inland extensional structures and the associated growth of an Early Paleocene mylonitic belt. During the Eocene the installation of a short duration magmatic arc and a widespread cooling event record the final installation of an oblique subduction setting. We argue that the pre-collisional tectonic evolution of the South American continental margin have prepare a warm continental margin with significant weakness zone that determined an arc-continent collisional style characterized by frontal accretion of the South American plate over the intra-oceanic Caribbean domain, and in which the younger compressional and thermal events are link to the remobilization of older structures.

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

2013-04-01

293

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

USGS Publications Warehouse

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.

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

2014-01-01

294

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

NASA Astrophysics Data System (ADS)

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

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

2014-02-01

295

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

296

Sediment provenance and dispersal on tropical starved continental margins: Example of the Potiguar Basin margin, NE Brazil  

NASA Astrophysics Data System (ADS)

Mesozoic and Cenozoic rocks from the Potiguar Basin have been investigated to determine the provenance of terrigenous and carbonate sediments on the Brazilian Continental shelf offshore of the Acu River. Located in the northern coast of the Rio Grande do Norte State (NE Brazil), the Acu River is submitted to highly dynamic coastal processes, such as mesotidal regime (up to 2.6 m), waves, winds and currents. It is inserted in the geological context of Potiguar Basin and is the largest source of freshwater in this area, as the biggest hydrographic basin. Bottom sediments characterization, including heavy mineral and Nd isotopes studies, associated with an integrated shallow water geophysical investigation was carried out on this area, The integration between currents velocity, particles transported by the current of water and other physical parameters with bedforms characterization and different sedimentary textures in the study area allows a better knowledge of the active sedimentary processes, which are responsible for the formation of morphologic features of this area. The distribution patterns of the principal heavy minerals indicate the major point sources of sediment supply onto the shelf and reveal intensive mixing, sorting processes and that a westerly (E-W) longshore current plays an important role in the sediment dispersal. As a result, the mineral distribution E of the Acu River is rich in stable heavy minerals (mainly zircon and tourmaline) whilst of the Acu River is rich in unstable ones (mainly hornblende and epidote.). In addition to provenance-induced variability, mineralogical differences between E and W provinces could indicate that environmental processes have contributed to the total variability of the inner continental shelf sediments. The Sm-Nd isotopic signatures of the rocks yielded model ages (TDM) in the range of 2,19-2,88 Ga, indicating archean to paleoproterozoic sources from the basement. The terrigenous sediments yielded model ages (TDM) in the range of 2,31-2,26 Ga, from 17,5 to 0 cm depth. Limited variations of provenance ages indicates the homogenization of the sediments, probably due to the strong influence of the basement, as the main source of sediments to the shelf. The results also indicate that the shelf sediments are mainly derived from the Acu River or other small rivers from the Setentrional Sector of Rio Grande do Norte State.

Vital, H.; Gomes, M. P.; Dantas, E. L.; Soares, C. H.

2013-12-01

297

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

NASA Astrophysics Data System (ADS)

Several highly dynamic and spatially extended cold seeps were found and analyzed on the Makran accretionary wedge off Pakistan during R/V Meteor cruise M74-3 in 2007. In water depths of 550m to 2870m along the continental slope nine different gas escape structures were examined some of which are situated within a stable oxygen minimum zone (OMZ) between 150m and 1100m water depth (von Rad et al., 1996, 2000). Echosounder data indicate several gas bubble streams in the water column. The gas seepage presumably originates from squeezing of massive sediment packages being compressed by subduction at the continental margin off Pakistan. Gas- and fluid venting and associated surface-near anaerobic oxidation of methane (AOM) feed several cold seepage systems in the seabed. The seep sites show strong inter- and intraspecific variability of benthic chemosynthetic microhabitats. Singular seeps are often colonized by different chemosynthetic organisms in a concentric fashion. The seep-center, where active bubble ebullition occurs, is often colonized by large hydrogen sulfide-oxidizing bacteria, which are surrounded by a rim inhabited by small chemosynthetic clams and tube worms. These different habitats and the associated sediments show distinct geochemical zonations and gradients. Geochemical analyses of pore water and sediment samples obtained via ROV (push corer) show that concentrations of hydrogen sulfide and alkalinity rapidly increase to >15 mmol/l and >35 mmol/l respectively several cm below the seafloor in the center of the cold seep. In places, sulfate is depleted to concentrations below detection limit at the same depth (ROV push core GeoB 12313-6). Ammonium concentrations in this core on the other hand show a different pattern: In the center of the cold seep, which is colonized by bacterial assemblages, ammonium concentrations fluctuate around 100 µmol/l and peak with 274.4 µmol/l just above the aforementioned sulfide maximum values at 5 cm followed by a rapid decrease to near zero below that depth. A feature of a number of Makran cold seeps within the OMZ is that the central orifice of gas ebullition is solely surrounded by white to rose or yellow colored chemosynthetic bacteria which colonize the seepage spots in concentric rims that are in places elongated towards one direction and hence display a comet tail-like shaped bacterial mat on the sea floor. In contrast to the cold seep centers, the outer rim around the seep sites, which is colonized by chemosynthetic clams and tubeworms, is characterized by ammonium concentrations that stay below detection limit and hydrogen sulfide and alkalinity concentrations are as well lower here than in the central part with values >8 mmol/l and >25mmol/l respectively at a depth of 13 cm (ROV push core GeoB 12313-12). Sulfate concentrations fluctuate around 15 mmol/l here and hint to lower sulfate reduction rates compared to the central part of the seep. The low contents of ammonium in the pore water of the outer seep sections may originate from processes initially proposed by Tryon et al. (2002): Gas and fluids constantly emanating from a central orifice cause the formation of a small-scale, local fluid-flow system comparable to convectional or belt-like flow dynamics. These convectional fluid-flow characteristics would cause an outflow of the actual gas/fluid composite from the central orifice and an inflow of bottom-near sea water poor in ammonium into the surrounding sediment where it would cause the very low pore water concentrations detected here. References: Tryon, M.D., Brown, K.M. and Torres, M.E. (2002). Fluid and chemical flux in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, II: Hydrological processes. Earth and Planetary Science Letters, 201, 541-557 von Rad, U., Rösch, H., Berner, U., Geyh, M., Marching, V. and Schulz, H. (1996) Authigenic carbonates derived from oxidized methane vented from the Makran accretionary prism off Pakistan. Marine Geology, 136, 55-77 von Rad, U., Berner, U., Delisle, G., Doose-Rolinski, H., Fech

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

2009-04-01

298

Salt tectonics and crustal tectonics along the Eastern Sardinian margin, Western Tyrrhenian: New insights from the “METYSS 1” cruise  

NASA Astrophysics Data System (ADS)

The Tyrrhenian Sea is usually interpreted to be a Neogene back-arc basin that opened by continental rifting and oceanic spreading caused by the eastward migration of the Apennine subduction system during Miocene and Pliocene times. Rifting of the southern Tyrrhenian Sea started first along the Eastern Sardinian margin during the middle to late Miocene times, including the Messinian. The “METYSS” project aims at better constraining the relationships between crustal tectonics, salt tectonics and sedimentation from Messinian times to present-day. The “METYSS 1” cruise (2009) allowed to acquire about 1200 km of HR seismic-reflection profiles along the Eastern Sardinian margin, Western Tyrrhenian Sea. This data set clearly illustrates that this area has been highly dissected during the rifting stage by N-S trending normal faults delineating ridges and basins, as previously described. The Messinian seismic markers (UU and MU) locally display fan-shaped stratal geometries, but the mechanism responsible for such geometries, salt tectonics or rifting, has yet to be carefully deciphered. We also mapped the spatial distribution of the mobile salt. The highly variable thickness of the small confined salt basins may be related to the initial pre-Messinian basin geometry, to the fact that salt deposition was syn-rift or to salt movement. Southeastward, salt tectonics is vigorous in unconfined basins and appears to have started early during the UU deposition. More surprisingly, our data demonstrate that some of the major faults have been significantly reactivated during the post-rift period, up to late Quaternary time, as shown by bathymetric scarps and associated debris flows. Moreover, some of this post-rift deformation can be evidenced within the Plio-Quaternary sequence by a regional unconformity. The post-Miocene deformation style varies greatly between different areas of the margin, and can also be strongly influenced by the presence or absence of mobile Messinian salt.

Gaullier, V.; Chanier, F.; Lymer, G.; Vendeville, B. C.; Maillard, A.; Thinon, I.; Lofi, J.; Sage, F.; Loncke, L.

2014-03-01

299

Basement controls on rifting and the associated formation of ocean transform faults—Cretaceous continental extension of the southern margin of Australia  

NASA Astrophysics Data System (ADS)

The initial stage of continental extension between Australia and Antarctica was associated with lateral changes in extension direction along the margin that reflects the three-dimensional nature of strain during continental rifting. In the Cretaceous Otway Basin, this change in extension direction was related to substantial rheological differences in the lithosphere across the boundary between two Paleozoic fold belts, the Lachlan and Delamerian, with the net extension direction at a high angle to this boundary. The initial Early Cretaceous rifting preserved within the onshore Otway Basin has two main structural subdomains in the eastern and western Otway Basins distinguished by different structural trends of Early Cretaceous normal faulting. This is not controlled by a variation in preexisting structural weaknesses within the underlying Paleozoic basement because the same geometry of extensional structures also occurs within the basement to the north irrespective of the preexisting structural grain. The eastern Otway Basin is dominated by NE-striking NW-dipping normal faults. In the western Otway Basin, the faults define arrays of predominantly NE-dipping or SW-dipping faults separated by wide accommodation zones defined by folding and variably striking and dipping faults. The partitioning of strain along the boundary between the eastern and western Otway Basins is accommodated by a progressive change in strike of faults and not via a transfer fault. Younger rifting in the Late Cretaceous had a similar extension direction in the western Otway basin, but had a dominant seaward dip, extension appears to have been hindered in the eastern Otway basin by a Proterozoic/Paleozoic basement feature. These factors produced a region of diverging extension along the lithospheric boundary between the Delamerian and Lachlan Fold Belts that lead to failure along this boundary and the formation of a localized sinistral trans-tensional graben, the Shipwreck trough, in the early Late Cretaceous. As a result, the younger rifting stepped south of the eastern Otway Basin leaving Bass Strait (the strait between the Australian mainland and Tasmania) a failed rift. The formation of oceanic crust in the Mid to Late Eocene followed the boundary of Late Cretaceous rifting, which led to the formation of the Tasman Fracture Zone.

Miller, John McL.; Norvick, Martin S.; Wilson, Christopher J. L.

2002-11-01

300

Long-term landscape evolution and post-rift reactivation in the southeastern Brazilian passive continental margin: Taubaté basin  

NASA Astrophysics Data System (ADS)

Zircon (ZFT) and apatite (AFT) fission-track low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct the post-rift evolution since the breakup of southwestern Gondwana. The thermochronological data obtained from samples of both the Precambrian basement and the Paleogene to Neogene sedimentary rocks from the continental rift of southeastern Brazil provided ZFT ages between 148 (15) and 64 (6) Ma, and AFT ages of 81 (8)-29 (3) Ma. These data clearly indicate syn- and post-rift reactivations during the Early Cretaceous, with great emphasis on Paleogene to Neogene times. Integrating the results of older thermochronological studies, the reactivation of the southeastern Brazilian margin can be described in three main phases related to the rift to post-rift evolution of SE Brazil. In general, ZFT and AFT data yield spread values that become younger as samples are closer to the reactivated Neoproterozoic shear zones and might reflect source area exhumation. The analysis of ZFT and AFT data allowed interpretations regarding the main phases that occurred in the study area related to the thermotectonic and tectono-stratigraphic evolution in southeastern Brazil.

Franco-Magalhaes, A. O. B.; Cuglieri, M. A. A.; Hackspacher, P. C.; Saad, A. R.

2014-03-01

301

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

NASA Astrophysics Data System (ADS)

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

Ernst, W. G.; Tsujimori, T.; Zhang, R.; Liou, J. G.

2006-12-01

302

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

NASA Astrophysics Data System (ADS)

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

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

2007-05-01

303

Continental margins and Ocean-Continent Transitions of the Gulf of Aden: how Africa and Arabia broke up ? (Invited)  

NASA Astrophysics Data System (ADS)

Fundamental characters of rifted-margins are commonly related to the large-scale processes during rifting, but variations at the scale of 10-20 kilometers segments can be equally significant. The presence or absence of magmatism during continental rifting appears to control the subsequent geometry and duration of continental breakup within discrete rift sectors. Here we show, from new geophysical and geological observations in the Gulf of Aden, that rift sectors between transforms have a large internal variability attributed usually to large-scale processes: over short distances (~10 kilometers), the OCT evolves from a narrow magmatic transition (few kms to 15km wide) to wider zones (~50km) where continental mantle is probably exhumed. We suggest that this small-scale variability is caused (1) by the long-offset Fracture Zones, which can enhance magma production on its young side, (2) by channeled flow of Afar plume material along the nascent OCT of the Gulf of Aden and (3) by along-strike differences in time-averaged extension rate in the oblique rift system.

Leroy, S.; Lucazeau, F.; D'Acremont, E.; Sloan, H.; Razin, P.; Robinet, J.; Autin, J.; Watremez, L.

2010-12-01

304

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

NASA Astrophysics Data System (ADS)

It has been proposed that some continental rifted margins have anomalous early subsidence histories and that at break-up they were elevated at shallower bathymetries than the isostatic response of classical rift models (McKenzie 1978) would predict. The existence of anomalous syn- or early post-breakup subsidence, of this form, would have important implications for our understanding of the geodynamics of continental breakup and sea-floor spreading initiation and important consequences for syn- and post-breakup depositional systems. Possible explanations for anomalous subsidence during continental breakup could include transient effects as the continental geotherm evolves towards an oceanic form, or small scale convection. Lucazeau et al. (2008) have reported anomalously high heat-flows in the ocean continent transition (OCT) of the young rifted margin of the Eastern Gulf of Aden which would have implications for its subsidence history. In order to verify (or otherwise) the proposition of an anomalous early post-breakup subsidence history in the Eastern Gulf of Aden, we have determined anomalous oceanic subsidence using residual depth anomaly (RDA) analysis and have compared lithosphere thinning across the OCT measured using subsidence analysis with continental crustal basement thinning from gravity inversion. Both 3D regional and localised 2D analyses have been carried out. The localised studies focus on published seismic reflection lines (Autin et al, 2010; D'Acremont et al, 2005; Fournier et al, 2007; Leroy et al, 2004; Leroy et al, 2010; Lucazeau et al 2008; Lucazeau et al 2010). RDAs have been calculated by comparing observed and predicted oceanic bathymetries. Regional 3D RDAs for the Gulf of Aden, without a sediment correction, show positive RDAs between 3km and 4km at the rifted margins decreasing to 0.5km at the ocean ridge axis. Localised 2D sediment corrected RDA profiles determined within and adjacent to the OCT of the Eastern Gulf of Aden are also positive and have values between +0.5km and +1.5km. The positive measured RDAs within the OCT are consistent with anomalous subdued subsidence in these regions; however, dynamic uplift from the Afar plume cannot be discounted as an alternative explanation. Moho depth determined from gravity inversion and thinning factors determined from gravity inversion and subsidence analysis have been used to constrain OCT location. Gravity inversion, used to determine Moho depth and continental crustal basement thinning, incorporates a lithosphere thermal gravity anomaly correction and sediment thickness from the 2D seismic reflection data. Moho depths form gravity inversion have been calibrated against seismic refraction Moho depths where such data exists. Continental lithosphere thinning has been determined using flexural backstripping and subsidence analysis assuming the classical rift model of McKenzie (1978) with a corection for volcanic addition due to decompression melting based on White & McKenzie (1989). The OCT region of the Eastern Gulf of Aden, based on measured positive RDA, shows anomalously shallow bathymetry and decreased subsidence, consistent with a lithosphere thermal anomaly as proposed by Lucazeau et al.(2008), although the regional effects of Afar plume dynamic uplift cannot be ruled out.

Cowie, L.; Kusznir, N. J.

2011-12-01

305

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

NASA Astrophysics Data System (ADS)

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

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

306

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

SciTech Connect

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

Bennett, V.C.

1989-01-01

307

Salt tectonics and crustal tectonics along the Eastern Sardinian margin, Western Tyrrhenian : New insights from the « METYSS » cruise (June 2009)  

NASA Astrophysics Data System (ADS)

The « METYSS » cruise was carried out in June 2009 onboard the R/V « Téthys II » along the eastern Sardinian and south-eastern Corsican margins, western Tyrrhenian Sea, in order to better constrain the potential links between deformation related to either crustal tectonics or salt tectonics and sediment accumulation, especially during the Messinian and Plio-Quaternary times. We acquired 15 high-resolution seismic reflection profiles (about 1200 km in cumulative length) along the south-eastern Corsican margin, immediately north of the Bonifacio Strait and along the upper and middle parts of the eastern Sardinian margin, from the continental slope to the Cornaglia Terrace. The Tyrrhenian Sea is considered as a Neogene back-arc basin that opened during continental rifting and oceanic spreading related to the eastward migration of the Apennine subduction system from Tortonian to Pliocene times (Jolivet et al., 2006). Rifting of the Tyrrhenian Sea started first along the Eastern Sardinian margin during the Tortonian-Messinian times and therefore the series of that age should be considered as syn-rift sediments (Sartori et al., 2004). The « METYSS » seismic profiles clearly illustrate that this part of the Tyrrhenian was highly segmented during the rifting stage by N-S trending normal faults delineating ridges (e.g., Baronie Ridge) and basins (e.g., Sardinian Basin and Cornaglia Terrace), as previously described for example by Thommeret (1999) and Sartori et al. (2004). The Messinian sedimentary units and especially the « Upper Unit » (UU, Lofi et al., this congress, corresponding to the « Upper Evaporites » in the previous literature) are, without any doubt, of syn-rift age, as they display a fan-shaped stratal geometry. The Mobile Unit (MU, Lofi et al., this congress), i.e., the Messinian halite, is clearly imaged in the study area and its spatial repartition can be outlined. The highly-variable thickness of the confined salt basins could be due to the initial basin geometry (i.e., before the Messinian salinity Crisis) or to the syn-rift character of the deposition. Southeastward of the study area, in the vicinity of the Cornaglia Seamount, salt tectonics appears surprisingly vigorous. More surprisingly, several normal faults seem to have remained active in recent times, if not even at present time. Fault slip has been recorded by bathymetric scarps and associated footwall debris flows interfingered within the Plio-Quaternary sequence, even though the eastern Sardinian margin is usually considered to be passive now. Moreover, some amount of tectonic inversion is visible on some normal faults that show contractional or transpressional components of late slip. In addition, this "post-rift" deformation can be illustrated within the Plio-Quaternary sequence by a regional unconformity. Consequently, numerous mass-transport deposits and channel-levees systems observed in the Plio-Quaternary cover could be partly controlled by tectonic activity. These very preliminary results require further investigations in order to better decipher the role of crustal tectonics and salt tectonics, salt-related structures being very efficient markers to discriminate between the respective contribution of gravity-driven, salt tectonics and deep-seated, crustal tectonics (Gaullier et al., 2010). Finally, we aim to precisely determine the relative vertical movements (tilting, subsidence, magmatism…) and geodynamical history of the different segments of the area since 6 Ma. References Gaullier V., Loncke L., Vendeville B., Déverchère J., Droz L. et al., 2010. Interactions between salt tectonics and deep-seated tectonics. Part I: Examples from the western Mediterranean. International Conference SEPM-The Geological Society: "Salt tectonics, sediments and prospectivity", 20-22 January 2010, London, United Kingdom, Abstract volume, 84. Jolivet, L., Augier, R., Robin, C., Suc, J.-P., Rouchy, J.-M., 2006. Lithospheric-scale geodynamic context of the Messinian salinity crisis. Sedimentary Geology, 188-189, 9-33. Sartori, R.

Gaullier, Virginie; Lofi, Johanna

2010-05-01

308

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

NASA Astrophysics Data System (ADS)

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.

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

2014-10-01

309

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

USGS Publications Warehouse

Shelf-sourced submarine canyons are common features of continental margins and are fundamental to deep-sea sedimentary systems. Despite their geomorphic and geologic significance, relatively few passive margin shelf-breaching canyons worldwide have been mapped using modern geophysical methods. Between 2007 and 2012 a series of geophysical surveys was conducted across four major canyons of the US Mid-Atlantic margin: Wilmington, Baltimore, Washington, and Norfolk canyons. More than 5700 km2 of high-resolution multibeam bathymetry and 890 line-km of sub-bottom CHIRP profiles were collected along the outer shelf and uppermost slope (depths of 80-1200 m). The data allowed us to compare and contrast the fine-scale morphology of each canyon system. The canyons have marked differences in the morphology and orientation of canyon heads, steepness and density of sidewall gullies, and the character of the continental shelf surrounding canyon rims. Down-canyon axial profiles for Washington, Baltimore and Wilmington canyons have linear shapes, and each canyon thalweg exhibits morphological evidence for recent, relatively small-scale sediment transport. For example, Washington Canyon displays extremely steep wall gradients and contains ~100 m wide, 5–10 m deep, v-shaped incisions down the canyon axis, suggesting modern or recent sediment transport. In contrast, the convex axial thalweg profile, the absence of thalweg incision, and evidence for sediment infilling at the canyon head, suggest that depositional processes strongly influence Norfolk Canyon during the current sea-level high-stand. The north walls of Wilmington, Washington and Norfolk canyons are steeper than the south walls due to differential erosion, though the underlying cause for this asymmetry is not clear. Furthermore, we speculate that most of the geomorphic features observed within the canyons (e.g., terraces, tributary canyons, gullies, and hanging valleys) were formed during the Pleistocene, and show only subtle modification by Holocene processes active during the present sea-level high-stand.

Obelcz, Jeffrey; Brothers, Daniel S.; Chaytor, Jason D.; ten Brink, Uri S.; Ross, Steve W.; Brooke, Sandra

2013-01-01

310

Continental Break-up and the dynamics of rifting in backarc basins : the Gulf of Lions margin  

NASA Astrophysics Data System (ADS)

Deep seismic profiles and subsidence history of the Gulf of Lions margin reveal a non-classical evolution with intense stretching of the distal margin and delayed subsidence, despite a rather weak extension of the onshore and shallow offshore portion of the margin. The interpretation of an unpublished MCS profile (TGS-NOPEC) and published geophysical data leads us to revisit this evolution. The 70 km-long domain of extremely thinned continental crust, the GoL MCC, has been extracted from below the margin by the south-eastward flow of hot asthenosphere in the backarc region during rollback of the Apennines slab. The combination of Eocene crustal thickening related to formation of the Pyrenees and the nearby volcanic arc and associated hot asthenosphere makes the upper mantle and the lower crust weak enough to flow south-eastward entrained by the underlying asthenospheric flow due to slab retreat. The upper crust, more resistant, has been left behind and was only moderately thinned. The overall hot geodynamic environment also explains the subaerial conditions during most of the rifting stage and the delayed subsidence after the breakup. The efficiency of such a basal drag is not ascertained and it should certainly be further tested but, in the Mediterranean backarc regions, the coupling between asthenospheric and lower crustal deformation seems quite strong as suggested by the comparison of stretching directions in MCCs and seismic anisotropy of SKS waves which suggests that shear stresses due to asthenospheric flow toward retreating subduction zones can be transmitted up to the lower crust. This model cannot be simply used for Atlantic-type passive margins because they usually do not show exhumed lower crust within the continent-ocean transition but the role that an astheospheric flow could play during rifting should be looked at.

Jolivet, L.; Gorini, C.; Bache, F.; Smit, J.; Leroy, S.

2012-04-01

311

Cenozoic East Antarctic Ice Sheet Evolution From Wilkes Land Continental Margin Sediments and IODP Drilling in 2009  

NASA Astrophysics Data System (ADS)

The long-term history of glaciation along the East Antarctic Wilkes Land margin is inferred using an integrated geophysical and geological approach. We postulate that the first arrival of the ice to the Wilkes Land continental shelf resulted in the development of a large unconformity (WL-U3) between 33.42 and 30 Ma. Above WL-U3, substantial margin progradation takes place with early glacial strata (e.g., outwash deposits) deposited as low- angle prograding foresets by temperate glaciers. The change in geometry of the prograding wedge across regional unconformity WL-U8 is interpreted to represent the transition, during the late Miocene-Pliocene, from a glacial regime characterized by a warm dynamic ice sheet (i.e., ice sheets come and go) to a regime dominated by a cold-based and persistent ice sheet. The steep foresets above WL-U8 likely consist of ice proximal sediments (i.e., water-lain till and debris flows) deposited when grounded ice-sheets extended into the shelf. On the continental rise, shelf progradation above WL-U3 results in an up-section increase in the energy of the depositional environment (i.e., seismic facies indicative of more proximal turbidite and of bottom contour current deposition from the deposition of the lower WL-S5 sequence to WL-S7). Maximum rates of sediment delivery to the rise occur during the development of sequences WL-S6 and WL-S7, inferred to be of middle Miocene age. During deposition of the two uppermost sequences (WL-S8 and WL-S9), there is a marked decrease in the sediment supply to the lower continental rise and a shift in the depocenters to more proximal areas of the margin. We believe WL-S8 records sedimentation during the final transition from a dynamic to a persistent but oscillatory ice sheet in this margin during the late Miocene. Sequence WL-S9 forms under polar regime during the Pliocene-Pleistocene, when most sediment delivered to the margin is trapped in the outer shelf and slope forming steep prograding wedges. During the warmer but still polar, Holocene, biogenic sediment accumulates quickly in deep inner-shelf basins during the high-stand intervals. These sediments contain an ultrahigh resolution (annual to millennial) record of climate variability. Drilling of the Wilkes Land margin by IODP scheduled for 2009 is designed to test the above inferred ages and history of glaciation. Drilling of the Wilkes Land margin will be a contribution by the ACE (Antarctic Climate Evolution) Program of SCAR (Scientific Committee on Antarctic Research) to the forthcoming International Polar Year.

Escutia, C.; Cooper, A.; Eittreim, S.; Tanahashi, M.; Ishihara, T.; de Santis, L.; O'Brien, P.; Domack, E.; Dunbar, R.

2007-12-01

312

Middle-Late Eocene structure of the southern Levant continental margin — Tectonic motion versus global sea-level change  

NASA Astrophysics Data System (ADS)

During the Paleogene greenhouse episode Earth experienced the warmest period of the Cenozoic while global sea level rose by more than 100 m. However, geological evidence from the Levant margin, northwestern Arabian plate, indicates that throughout this period seabed deepening exceeded 1000 m. Lithology from Israel, Syria, Lebanon and Jordan is mainly pelagic and neritic, interfered by occasional fossil sub-marine slumps. In order to understand this dissimilarity we quantify the vertical tectonic motion of the Levant continental margin through the Paleogene. The margin began to take shape during the Late Permian and it was reactivated during the Oligocene. Based on information from outcrops, drillholes, seismic reflection and refraction, gravity, and previous publications, a multi-layered model of the Levant lithosphere was established. Layers include the Moho, top of the crystalline basement and covering sediments up to the Late Eocene. The model was restored horizontally by 100 km along the younger Dead Sea transform. Assuming local isostatic compensation, vertical restoration yielded the paleo-bathymetry which prevailed across northwestern Arabia during the Middle-Late Eocene. Results show that following the margin subsidence the Cretaceous Levantine platform became ramp shaped during the Eocene. Most parts of the central Levant were submerged under ~ 200 to ~ 1800 m of water, while the paleo-bathymetric gradients ranged from ~ 2° at the shelf to ~ 6° at the slope. The apparent dissimilarity between sea level and our tectonic-based calculations is up to an order of magnitude. These differences may be resolved by accounting for vertical tectonic motions and sediment supply rates. Our results stress the importance of the presented crustal structure. As opposed to the backstripping procedure, the structural map of the top Eocene interface was constructed upwards from the well established top Turonian (Judea Group) interface since only scarce and sporadic outcrops of the target horizon are available. We suggest that a similar approach should be applied to re-evaluate the depositional environments across the entire continental margin of the eastern Mediterranean, an area with proven hydrocarbon prospects.

Segev, Amit; Schattner, Uri; Lyakhovsky, Vladimir

2011-03-01

313

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

Microsoft Academic Search

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

H. Kuno

1966-01-01

314

Methylmercury production in sediments of Chesapeake Bay and the mid-Atlantic continental margin  

Microsoft Academic Search

Methylmercury (MeHg) concentration and production rates were studied in bottom sediments along the mainstem of Chesapeake Bay and on the adjoining continental shelf and slope. Our objectives were to 1) observe spatial and temporal changes in total mercury (HgT) and MeHg concentrations in the mid-Atlantic coastal region, 2) investigate biogeochemical factors that affect MeHg production, and 3) examine the potential

T. A. Hollweg; C. C. Gilmour; R. P. Mason

2009-01-01

315

Climatic changes during the last deglaciation recorded in sediment cores from the northeastern Brazilian Continental Margin  

Microsoft Academic Search

Detailed 14C AMS data and isotope based stratigraphies from high-resolution paleoceanographic records for the last 22?ka of cores from\\u000a the upper continental slope off NE Brazil reveal sedimentation rates of up to 100?cm per 1000 yr. Variations in the sediment\\u000a composition relate to changes in the input of terrigenous material. The sedimentation is controlled by sea level and by the

H. W. Arz; J. Pätzold; G. Wefer

1999-01-01

316

Geologic development and characteristics of the continental margins, Gulf of Mexico. Research report, 1983-1986  

SciTech Connect

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

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

1986-01-01

317

Mantle Exhumation at Magma-Poor Rifted Margins due to Melt Suppression During Continental Break-up and Seafloor Spreading Initiation.  

NASA Astrophysics Data System (ADS)

Rifted continental margins exhibit large variations in magmatic activity. Non-volcanic margins may display a broad ocean-continent transition, up to 150 km wide, of exhumed mantle separating oceanic crust from thinned continental crust, whilst voluminous volcanism accompanies break-up at volcanic margins. Previous studies have shown the importance of asthenospheric temperature (White and McKenzie, 1989), lithosphere thinning rate (Bown and White, 1995; Pérez-Gussinyé et al., 2006) and initial continental geotherm (Reston and Morgan, 2004) on melt production at rifted margins, assuming that continental break-up occurs by pure-shear stretching of the lithosphere. Pure-shear models of continental lithosphere thinning generally predict melt generation before continental break-up, unless anomalously cool asthenosphere temperatures (Minshull et al., 2001) or depleted mantle source (Pérez-Gussinyé et al., 2006) are invoked. As a consequence pure-shear models have difficulty explaining mantle exhumation at non-volcanic margins. We model the onset and development of melt production during rifting of continental margins and seafloor spreading initiation using a model of depth- dependent lithospheric thinning and extension. For a mantle potential temperature of 1333°C and a half- spreading rate of 10mm/yr we predict that approximately 100km of lower crust and mantle is exhumed prior to melt production. Melt production reaches a steady state as the model reaches thermal equilibrium and seafloor spreading proceeds. The predicted width of exhumed mantle increases if the modelled spreading rate is decreased, if melt is retained in the mantle (e.g. as gabbroic intrusions), or if the subcontinental mantle is initially cool or depleted. In this model the thinning of continental lithosphere leading to break-up and sea-floor spreading initiation occurs in response to an upwelling and divergent flow-field. The melt parameterisations of Katz et al. (2003) are used to calculate melt fractions in the model space. The ability of the model to predict mantle exhumation prior to the onset of volcanism and normal seafloor spreading suggests that an upwelling and divergent flow-field may provide a better kinematic representation of continental break-up at non-volcanic margins (e.g. Iberia) than a pure shear model.

Fletcher, R.; Kusznir, N.; Cheadle, M.

2007-12-01

318

77 FR 31037 - Outer Continental Shelf, Central and Western Gulf of Mexico Planning Areas, Oil and Gas Lease...  

Federal Register 2010, 2011, 2012, 2013, 2014

...Bureau of Ocean Energy Management Outer Continental Shelf...Western Gulf of Mexico Planning Areas, Oil and Gas...Bureau of Ocean Energy Management (BOEM), Interior...within the Eastern GOM Planning Area and are within...Bureau of Ocean Energy Management, Gulf of Mexico...

2012-05-24

319

Methanogenesis and Methanogen Diversity in Three Peatland Types of the Discontinuous Permafrost Zone, Boreal Western Continental Canada  

Microsoft Academic Search

Because recent patterns of permafrost collapse in boreal peatlands appear to enhance emissions of CH4 to the atmosphere, we examined methanogenesis and methanogen diversity in peat soil from peatlands with and without permafrost in two peatland complexes situated in continental western Canada. Peat soil from the active layer of permafrost bogs had very low rates of CH4 production (ca. 10

Joseph B. Yavitt; Nathan Basiliko; Merritt R. Turetsky; Anthony G. Hay

2006-01-01

320

76 FR 70473 - Outer Continental Shelf (OCS) Western Planning Area (WPA) Gulf of Mexico (GOM) Oil and Gas Lease...  

Federal Register 2010, 2011, 2012, 2013, 2014

...Continental Shelf (OCS) Western Planning Area (WPA) Gulf of Mexico (GOM) Oil and Gas Lease Sale 218 AGENCY: Bureau of...of bids affecting blocks in this area. Areas Offered for Leasing: In WPA Oil and Gas Lease Sale 218, BOEM is...

2011-11-14

321

GIANT DUNE MORPHOLOGIES AND DYNAMICS IN A DEEP CONTINENTAL SHELF ENVIRONMENT: EXAMPLE OF THE BANC DU FOUR (WESTERN BRITTANY,  

E-print Network

1 GIANT DUNE MORPHOLOGIES AND DYNAMICS IN A DEEP CONTINENTAL SHELF ENVIRONMENT: EXAMPLE OF THE BANC-temporal variability of very large to giant dunes in deep tide dominated environment. Their growth mechanisms bathymetry surveys across the wide dune field of the Banc du Four located offshore the western Brittany

Paris-Sud XI, Université de

322

Cenozoic unconformities and depositional supersequences of North Atlantic continental margins: testing the Vail model  

USGS Publications Warehouse

Integrated outcrop, borehole, and seismic reflection stratigraphy from the U.S. and Irish margins of the North Atlantic basin reveals a framework of Cenozoic depositional supersequences and interregional unconformities that resembles the Vail depositional model. Paleo-bathymetric and paleoceanographic analyses of associated microfossil assemblages indicate a genetic link between the depositional framework and the relative position of sea level.

Poag, C. Wylie; Ward, Lauck W.

1987-01-01

323

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

Microsoft Academic Search

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

Daniel Winkelmann; Jochen Knies

2005-01-01

324

Thermal evolution of rifted continental margins: new evidence from fission tracks in basement apatites from southeastern Australia  

NASA Astrophysics Data System (ADS)

The Palaeozoic granitic basement of southeastern Australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin. Apparent ages away from the margin vary between about 230 and 360 Myr but drop sharply to minimum values of around 80 Myr near the coast just south of the Sydney Basin and 150-175 Myr elsewhere along the coast. Fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80-100 Myr. This event accompanied the rift phase of passive margin development and peaked just before the onset of sea-floor spreading in the adjacent Tasman Sea. The zone of reduced apatite ages is deeply embayed around the Gippsland Basin supporting the suggestion that it originated as a Cretaceous "failed" arm of the Tasman rift system. The thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages. The bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions. The youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre-existing tracks in these samples went almost to completion. Maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110-120°C. Thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface. Post-breakup erosion was generally greater towards the new coastline, with near-coastal samples being previously buried a probable 1.5-2.5 km.

Moore, Marilyn E.; Gleadow, Andrew J. W.; Lovering, John F.

1986-06-01

325

Analysis of Submarine Landslides and Canyons along the U.S. Atlantic Margin Using Extended Continental Shelf Mapping Data  

NASA Astrophysics Data System (ADS)

U.S. Geological Survey (USGS) studies of the U.S. Atlantic continental slope and rise aim to understand the: 1) the role of submarine landslides in tsunami generation, and 2) the linkages between margin morphology and sedimentary processes, particularly in and around submarine canyon systems. Data from U.S. Extended Continental Shelf (ECS) and numerous subsequent mapping surveys have facilitated the identification and characterization of submarine landslides and related features in fine detail over an unprecedented spatial extent. Ongoing analysis of USGS collected piston cores, sub-bottom and multichannel seismic (MCS) reflection profiles, and an extensive suite of legacy MCS data from two landslides, the Southern New England landslide zone and the Currituck Landslide, suggest that the most recent major landslide events are pre-Holocene, but that failures were complex and most likely multi-phase, at times resulting in extensive overlapping debris deposits. Piston core records plus visual observations of the seafloor from recent TowCam deployments and NOAA Ship Okeanos Explorer ROV dives reveal ongoing development of colluvial wedge-style debris aprons at the base of scarps within these landslides, showing that these regions continue to evolve long after the initial failure events. Multibeam bathymetry data and MCS profiles along the upper slope reveal evidence for vertical fluid migration and possible seabed gas expulsion. These observations underscore the need to reevaluate the sources of pore fluid overpressure in slope sediments and their role in landslide generation. ECS and more recent multibeam mapping have provided the opportunity to investigate the full extent of submarine canyon morphology and evolution from Cape Hatteras up to the US-Canadian EEZ, which has led to better understanding of the important role of antecedent margin physiography on their development. Six submarine canyon systems along the margin (Veatch, Hydrographer, Hudson, Wilmington-Baltimore, Norfolk-Washington, and Hatteras) are being investigated from the canyon heads down to their deep-water submarine fans in an effort to characterize their sediment transport history and constrain the influences of external processes on their morphology. Each canyon-fan system is morphologically unique and is strongly controlled by source region, antecedent margin morphology, landslide and debris flow processes, and the long-term influence of deep-water (along-slope) currents.

Chaytor, J. D.; Brothers, D. S.; Ten Brink, U. S.; Hoy, S. K.; Baxter, C.; Andrews, B.

2013-12-01

326

High-pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east- central Alaska  

USGS Publications Warehouse

Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska: the northern, structurally higher Taylor Mountain terrane and the southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane. The pressure, temperature, kinematic and age data are interpreted to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. -from Authors

Dusel-Bacon, C.; Hansen, V.L.; Scala, J.A.

1995-01-01

327

Deep structure of the U.S. Atlantic continental margin, offshore South Carolina, from coincident ocean bottom and multichannel seismic data  

Microsoft Academic Search

We present the results of a combined multichannel seismic reflection (MCS) and wide-angle, ocean bottom seismic profile collected in 1988 across the Carolina Trough on the U.S. Atlantic continental margin. Inversion of vertical-incidence and wide-angle travel time data has produced a velocity model of the entire crust across the continent-ocean transition. The margin consists of three structural elements: (1) rifted

W. Steven Holbrook; E. C. Reiter; G. M. Purdy; D. Sawyer; P. L. Stoffa; J. A. Austin; J. Oh; J. Makris

1994-01-01

328

The TEENA experiment: a pilot project to study the structure and dynamics of the eastern US continental margin  

NASA Astrophysics Data System (ADS)

During the summer of 2009, a quasi-linear transect of 9 broadband seismic stations was deployed from Knotts Island, North Carolina across Virginia and West Virginia to Marietta, Ohio, comprising the TEENA (Test Experiment for Eastern North America) array. Very little is known about the detailed seismic structure of the crust and mantle beneath this region, and while several models for mantle dynamics beneath the eastern US passive continental margin have been proposed, the paucity of available seismic data has made it difficult to discriminate among them. The TEENA array traverses several physiographic provinces, including the Atlantic coastal plain, Appalachian Piedmont, Blue Ridge Mountains, Appalachian Valley and Ridge, and Appalachian Plateau. Data recorded from this array will be used to examine variations in crust and mantle structure across these different provinces and will help elucidate how the lithosphere of this region has evolved throughout its complex tectonic history. We also expect to obtain constraints on upper mantle seismic anisotropy beneath the region, which will place constraints on mantle dynamics beneath the passive continental margin. We present preliminary crustal thickness measurements from h-K stacking of receiver functions, SKS splitting measurements of anisotropic mantle structure, and Rayleigh wave dispersion measurements of ambient noise based on the first few months of data from TEENA. Constraints on the structure and dynamics of the crust and mantle gleaned from TEENA and similar pilot projects will be useful in guiding future seismic studies in the Appalachian geologic province of eastern North America, particularly as the Transportable Array and Flexible Array components of the Earthscope USArray move east. Station locations for the TEENA broadband seismic array.

Benoit, M. H.; Long, M. D.

2009-12-01

329

Thermal history and long-term evolution of the South Atlantic passive continental margin, Kaoko belt, NW Namibia  

NASA Astrophysics Data System (ADS)

From Permo-Carboniferous to Mid Jurassic the Kaoko belt in northwestern Namibia was affected by deep erosion of the Damara Sequence, Permo-Triassic collisional processes along the southern margin of Gondwana (Coward & Daly 1984), and the deposition of the Karoo Supergroup. The lithostratigraphic units consist of Proterozoic and Cambrian metamorphosed rocks with ages of 534 (7) Ma to 481 (25) Ma (Miller 1983), as well as Mesozoic sedimentary and igneous rocks. The Early Jurassic Karoo flood basalt lavas erupted rapidly at 183 (1) Ma (Duncan et al. 1997). The Early Cretaceous Paraná-Etendeka flood basalts (132 (1) Ma) and mafic dike swarms mark the rift stage of the opening of the South Atlantic (Stewart et al. 1996). The 'passive' continental margin in northern Namibia is a perfect location to quantify exhumation and uplift rates, model the long-term landscape evolution and provide information about the major processes controlling the landscape evolution in this region. The poster will present thermochronological data, t-T-models and exhumation rates for the Kaoko belt, NW Namibia. References Coward, M. P. and Daly, M. C., 1984. Crustal lineaments and shear zones in Africa: Their relationships to plate movements, Precambrian Research 24: 27-45. Duncan, R., Hooper, P., Rehacek, J., March, J. and Duncan, A., 1997. The timing and duration of the Karoo igneous event, southern Gondwana, Journal of Geophysical Research 102: 18127-18138. Miller, R. M., 1983. Evolution of the Damara Orogen, Vol. 11, Geological Society, South Africa Spec. Pub.. Stewart, K. S., Turner, S., Kelly, S., Hawkesworth, C. J., Kirstein, L. and Mantovani, M. S. M., 1996. 3D 40Ar-39Ar geochronology in the Paraná continental flood basalt province, Earth and Planetary Science Letters 143: 95-110.

Menges, D. P.; Glasmacher, P. A.

2013-12-01

330

Last Glacial to recent sea ice coverage in the northern North Atlantic (Fram Strait and East Greenland Continental Margin)  

NASA Astrophysics Data System (ADS)

Sea ice is a critical component of global climate since it plays a crucial role in terms of heat reduction and deep-water formation - a driving mechanism of the global thermohaline circulation. The currently observed retreat of Arctic sea ice provokes the question as to whether past variations in sea ice coverage in this climate-sensitive area can be firstly identified, and secondly, linked to climatic fluctuations. Since the major export of Arctic sea ice to the world‘s oceans occurs through Fram Strait and along the eastern continental margin of Greenland, marine sediments from this area serve as climate archives providing useful information about past changes in this efflux system. The presence of the sea ice proxy IP25 (a biomarker molecule synthesised by sea ice algae) in Arctic Ocean sediments, for instance, may be used as a direct hint for sea ice coverage and its variability through time. With the investigation of a high-resolution sediment core on its IP25 content, we reveal extreme variations in sea ice cover for Fram Strait during the past 30 ka. By combining IP25 and common open-water phytoplankton data, we have even been able to reconstruct different sea ice scenarios, which align well with known climatic fluctuations, i.e. cooling and warming events, such as the Last Glacial Maximum or the Bølling warm period. Besides this long-term sea ice record for northern Fram Strait, we present IP25 data obtained on marine surface sediments from the continental margin of East Greenland. IP25 concentration profiles suggest a prolonged sea ice cover at the proximal shelf area, whereas the annual concentration/duration of sea ice coverage seems to decrease with increasing distance from the shelf as reflected by lower IP25 abundances. This sea ice distribution pattern likely mirrors the course of the East Greenland Current carrying polar water southwards through Fram Strait.

Mueller, J.; Masse, G. G.; Stein, R. H.; Belt, S.

2009-12-01

331

Ascension Submarine Canyon, California - Evolution of a multi-head canyon system along a strike-slip continental margin  

USGS Publications Warehouse

Ascension Submarine Canyon, which lies along the strike-slip (transform) dominated continental margin of central California, consists of two discrete northwestern heads and six less well defined southeastern heads. These eight heads coalesce to form a single submarine canyon near the 2700 m isobath. Detailed seismic stratigraphic data correlated with 19 rock dredge hauls from the walls of the canyon system, suggest that at least one of the two northwestern heads was initially eroded during a Pliocene lowstand of sea level ???3.8 m.y. B.P. Paleogeographic reconstructions indicate that at this time, northwestern Ascension Canyon formed the distal channel of nearby Monterey Canyon and has subsequently been offset by right-lateral, strike-slip faulting along the San Gregorio fault zone. Some of the six southwestern heads of Ascension Canyon may also have been initially eroded as the distal portions of Monterey Canyon during late Pliocene-early Pleistocene sea-level lowstands (???2.8 and 1.75 m.y. B.P.) and subsequently truncated and offset to the northwest. There have also been a minimum of two canyon-cutting episodes within the past 750,000 years, after the entire Ascension Canyon system migrated to the northwest past Monterey Canyon. We attribute these late Pleistocene erosional events to relative lowstands of sea level 750,000 and 18,000 yrs B.P. The late Pleistocene and Holocene evolution of the six southeastern heads also appears to have been controlled by structural uplift of the Ascension-Monterey basement high at the southeastern terminus of the Outer Santa Cruz Basin. We believe that uplift of this basement high sufficiently oversteepened submarine slopes to induce gravitational instability and generate mass movements that resulted in the erosion of the canyon heads. Most significantly, though, our results and interpretations support previous proposals that submarine canyons along strike-slip continental margins can originate by tectonic trunction and lateral offset. ?? 1986.

Nagel, D.K.; Mullins, H.T.; Greene, H. Gary

1986-01-01

332

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

NASA Astrophysics Data System (ADS)

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.

Chassefiere, Bernard

1990-09-01

333

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

NASA Astrophysics Data System (ADS)

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.

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

2011-03-01

334

Two modes of Weddell Sea Bottom Water Production: continental margin gravity currents and open ocean convection, which wins and when?  

NASA Astrophysics Data System (ADS)

There are 2 processes by which Southern Ocean surface waters may reach into the deep ocean: gravity currents over the continental slope and convection within the open ocean. In February 1977 the Islas Orcadas found clear evidence of the latter process, when it observed the remnants of a convective 'chimney' near Maud Rise, in the Weddell Sea. This observation was key in linking deep ocean convective processes to the "Great Weddell Polynya", a 250,000-km2 area virtually free of sea ice during the winters of 1974-1976. Further research from AWI research vessel Polarstern revealed the vulnerability of central Weddell gyre, particularly in the Maud Rise region, to breakdown of water column stability. Climate forcing related to prolonged period of negative or neutral Southern Annular Mode, as was the situation before the "Great Weddell Polynya", acts to reduce freshwater input to the Weddell Sea and thus serves as a trigger for open ocean convection and Polynya development. Similar condition may be occurred during the1912 Deutschland expedition into the Weddell Sea. We speculate that during glacial times, with sea level 130 m lower and the glacial ice extended to shelf break, with the Southern Annular Mode very much in a prolonged negative mode, open ocean production of Weddell Sea Bottom Water [and perhaps that of the Ross Sea too] was prevalent. The bottom water product during the open ocean convection mode may be expected to be saltier than that produced along the continental margin, which would incorporate glacial melt.

Gordon, Arnold L.

2014-05-01

335

Vertical distribution of benthic invertebrate larvae during an upwelling event along a transect off the tropical Brazilian continental margin  

NASA Astrophysics Data System (ADS)

Abundance and composition of marine benthic communities have been relatively well studied in the SE Brazilian coast, but little is known on patterns controlling the distribution of their planktonic larval stages. A survey of larval abundance in the continental margin, using a Multi-Plankton Sampler, was conducted in a cross-shelf transect off Cabo Frio (23°S and 42°W) during a costal upwelling event. Hydrographic conditions were monitored through discrete CDT casts. Chlorophyll- a in the top 100 m of the water column was determined and changes in surface chlorophyll- a was estimated using SeaWiFS images. Based on the larval abundances and the meso-scale hydrodynamics scenario, our results suggest two different processes affecting larval distributions. High larval densities were found nearshore due to the upwelling event associated with high chlorophyll a and strong along shore current. On the continental slope, high larval abundance was associated with a clockwise rotating meander, which may have entrapped larvae from a region located further north (Cabo de São Tomé, 22°S and 41°W). In mid-shelf areas, our data suggests that vertical migration may likely occur as a response to avoid offshore transport by upwelling plumes and/or cyclonic meanders. The hydrodynamic scenario observed in the study area has two distinct yet extremely important consequences: larval retention on food-rich upwelling areas and the broadening of the tropical domain to southernmost subtropical areas.

Yoshinaga, Marcos Y.; Sumida, Paulo Y. G.; Silveira, Ilson C. A.; Ciotti, Áurea M.; Gaeta, Salvador A.; Pacheco, Luiz F. C. M.; Koettker, Andréa G.

2010-01-01

336

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

337

Mesoproterozoic bimodal volcanism in SW Norway, evidence for recurring pre-Sveconorwegian continental margin tectonism  

Microsoft Academic Search

Between ca. 1.3Ga and the onset of the Sveconorwegian orogeny (ca. 1.0Ga) in the Baltic Shield, the margin of Baltica was subjected to repeated episodes of bimodal magmatism. A new TIMS U–Pb date of 1259 +\\/? 2Ma for magmatic zircon from metarhyolite of the Trossovdal Formation is presented. At least two distinct episodes of bimodal volcanism in southern Norway: the

T. S. Brewer; K.-I. Åhäll; J. F. Menuge; C. D. Storey; R. R. Parrish

2004-01-01

338

ROV study of a giant pockmark on the Gabon continental margin  

Microsoft Academic Search

A giant, 800-m wide pockmark, called Regab, was discovered along the Equatorial African margin at 3160-m water depth and was\\u000a explored by remote operated vehicle (ROV) as part of the Zaiango (1998–2000) and Biozaire (2001–2003) projects carried out\\u000a conjointly by TOTAL and a number of French research institutes. A microbathymetric map obtained using the ROV sensors shows\\u000a that the pockmark

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

2005-01-01

339

Transfer of organic carbon on the Moroccan Atlantic continental margin (NW Africa): productivity and lateral advection  

Microsoft Academic Search

Land—ocean transfer of sediment and organic matter along the Moroccan Atlantic margin (NW Africa) seems to have been very\\u000a effective during the last 130 ka. In a marine core from this region, we found total organic carbon (TOC) values ranging from\\u000a 0.3 to 1.7 dry wt% of bulk sediments. These relatively high values are fairly unusual, as the core was

Graziella Bozzano; Belén Alonso

2009-01-01

340

Soil organic matter as an important contributor to Late Quaternary sediments of the tropical West African continental margin  

NASA Astrophysics Data System (ADS)

The contribution of soil organic matter (SOM) to continental margins is largely ignored in studies on the carbon budget of marine sediments. Detailed geochemical investigations of late Quaternary sediments (245-0 ka) from the Niger and Congo deep-sea fans, however, reveal that C org/N tot ratios and isotopic signatures of bulk organic matter (? 13C org) in both fans are essentially determined by the supply of various types of SOM from the river catchments thus providing a fundamentally different interpretation of established proxies in marine sciences. On the Niger fan, increased C org/N tot and ? 13C org (up to -17‰) were driven by generally nitrogen-poor but 13C-enriched terrigenous plant debris and SOM from C 4/C 3 vegetation/Entisol domains (grass- and tree-savannah on young, sandy soils) supplied during arid climate conditions. Opposite, humid climates supported drainage of C 3/C 4 vegetation/Alfisol/Ultisol domains (forest and tree-savannah on older/developed, clay-bearing soils) that resulted in lower C org/N tot and ? 13C org (< -20‰) in the Niger fan record. Sediments from the Congo fan contain a thermally stable organic fraction that is absent on the Niger fan. This distinct organic fraction relates to strongly degraded SOM of old and highly developed, kaolinite-rich ferallitic soils (Oxisols) that cover large areas of the Congo River basin. Reduced supply of this nitrogen-rich and 12C-depleted SOM during arid climates is compensated by an elevated input of marine OM from the high-productive Congo up-welling area. This climate-driven interplay of marine productivity and fluvial SOM supply explains the significantly smaller variability and generally lower values of C org/N tot and ? 13C org for the Congo fan records. This study emphasizes that ignoring the presence of SOM results in a severe underestimation of the terrigenous organic fraction leading to erroneous paleoenvironmental interpretations at least for continental margin records. Furthermore, burial of SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO 2.

Holtvoeth, Jens; Kolonic, Sadat; Wagner, Thomas

2005-04-01