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1

Continental margins of the western North Atlantic  

Microsoft Academic Search

The US Geological Survey's interpretation of geological and geophysical data (seismic, magnetic, and gravimetric) provides a summary of the structural and sedimentary history of the North Atlantic margin. The shelf, slope, and rise are underlain by a sediment layer that reaches, in places, a thickness of 11 miles (17 km). Beneath the sedimentary accumulations on the margin lies a more

J. S. Schlee; D. W. Folger; W. P. Dillon; K. D. Klitgord; J. A. Grow

2009-01-01

2

Ooid turbidites from the central western continental margin of India  

Microsoft Academic Search

Gravity displaced debris flows\\/turbidites have been observed in five box cores collected between water depths of 649 and 3,627\\u000a m from the central western continental margin of India. Studies on grain size, carbonate content, and coarse fraction revealed\\u000a that the turbidites are mainly composed of ooids, shell fragments, and shallow water benthic foraminifera. Bioclastic sediments\\u000a of the outer shelf and

P. S. Rao

1989-01-01

3

Ooid turbidites from the central western continental margin of India  

NASA Astrophysics Data System (ADS)

Gravity displaced debris flows/turbidites have been observed in five box cores collected between water depths of 649 and 3,627 m from the central western continental margin of India. Studies on grain size, carbonate content, and coarse fraction revealed that the turbidites are mainly composed of ooids, shell fragments, and shallow water benthic foraminifera. Bioclastic sediments of the outer shelf and upper slope regions are considered the source of the debris flows/turbidity deposits. It appears that the flows were initiated by failure on the outer shelf and upper slope during late Pleistocene low stands of sea level.

Rao, P. S.

1989-06-01

4

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

5

Nature of the Crust in the Laxmi Basin, Western Continental Margin of India  

NASA Astrophysics Data System (ADS)

The nature of the crust in the Laxmi Basin, western margin of India is an uncertain issue; more importantly this has implications on paleo-geographic reconstructions of the western Indian Ocean. We have analysed three geophysical datasets and modelled gravity and magnetic anomalies for determining nature of the crust. Basement of the Laxmi Basin includes numerous highs, which make the basement uneven and shallower compared to the Western Basin. The Laxmi Basin is characterised by a broad gravity high and a narrower prominent gravity low within it, while within the basin the broad anomaly gradually increases towards north. The Panikkar Ridge is associated with the gravity low, which is comparable, at least in sign, to known negative gravity anomaly of Laxmi Ridge. Intrusive structures mapped in the Laxmi Basin coincide with significant magnetic anomalies, which were earlier interpreted as seafloor-spreading anomalies. Model studies reveal that the Laxmi Basin consists of ~14 km thick stretched continental crust, in which magmatic bodies have been emplaced, whereas Panikkar Ridge remains less altered stretched continental crust. The crust of the Laxmi Basin is mostly thinner than crust under Laxmi Ridge and continental margin. In addition to the rift-drift related stretching of the continental margin the Laxmi Basin possibly has undergone extra stretching in E-W direction during the pre-Tertiary period. At ~68 Ma Deccan volcanism on western India may have disrupted the initial conditions that were leading to onset of spreading in the basin. Subsequently the Réunion hotspot had emplaced the volcanic material within the stretched thinned continental crust. We interpret the Laxmi Basin as a failed rift, undergone stretching following intraplate kinematics prior to Deccan volcanism. Key words: Laxmi Basin, Laxmi Ridge, Panikkar Ridge, stretched continental crust, Deccan volcanism, northwest continental margin of India

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

2006-05-01

6

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

NASA Astrophysics Data System (ADS)

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

Rajaram, M.; S P, A.

2008-05-01

7

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

8

Continental Margins: Linking Ecosystems  

NASA Astrophysics Data System (ADS)

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

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

2008-02-01

9

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

10

Nature of the crust in the Laxmi Basin (14°-20°N), western continental margin of India  

NASA Astrophysics Data System (ADS)

The nature of the crust in the Laxmi Basin, western margin of India, is an uncertain issue; more importantly, this has implications on paleogeographic reconstructions of the western Indian Ocean. We have analyzed three geophysical data sets and modeled gravity and magnetic anomalies for determining nature of the crust. Basement of the Laxmi Basin includes numerous highs, which make the basement uneven and shallower compared to the Western Basin. The Laxmi Basin is characterized by a broad gravity high and a narrower prominent gravity low within it, while within the basin the broad anomaly gradually increases toward north. The Panikkar Ridge is associated with the gravity low, which is comparable, at least in sign, to known negative gravity anomaly of the Laxmi Ridge. Intrusive structures mapped in the Laxmi Basin coincide with significant magnetic anomalies, which were earlier interpreted as seafloor-spreading anomalies. Model studies reveal that the Laxmi Basin consists of ˜14 km thick stretched continental crust, in which magmatic bodies have been emplaced, whereas the Panikkar Ridge remains less altered stretched continental crust. The crust of the Laxmi Basin is mostly thinner than crust under the Laxmi Ridge and continental margin. In addition to the rift-drift-related stretching of the continental margin, the Laxmi Basin possibly has undergone extra stretching in E-W direction during the pre-Tertiary period. At ˜68 Ma Deccan volcanism on western India may have disrupted the initial conditions that were leading to onset of spreading in the basin. Subsequently the Réunion hot spot had emplaced the volcanic material within the stretched thinned continental crust. We interpret the Laxmi Basin as a failed rift, undergone stretching following intraplate kinematics prior to Deccan volcanism.

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

2006-02-01

11

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

NASA Astrophysics Data System (ADS)

We summarize the importance of great earthquakes (Mw ? 8) for hazards, stratigraphy of basin floors, and turbidite lithology along the active tectonic continental margins of the Cascadia subduction zone and the northern San Andreas Transform Fault by utilizing studies of swath bathymetry visual core descriptions, grain size analysis, X-ray radiographs and physical properties. Recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California margins are analyzed using two methods: (1) radiometric dating (14C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The H method provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that, on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great earthquakes are approximately 300 yr for the Cascadia subduction zone and 130 yr for the northern San Andreas Fault, which indicates both fault systems are in (Cascadia) or very close (San Andreas) to the early window for another great earthquake. On active tectonic margins with great earthquakes, the volumes of mass transport deposits (MTDs) are limited on basin floors along the margins. The maximum run-out distances of MTD sheets across abyssal-basin floors along active margins are an order of magnitude less (~100 km) than on passive margins (~1000 km). The great earthquakes along the Cascadia and northern California margins cause seismic strengthening of the sediment, which results in a margin stratigraphy of minor MTDs compared to the turbidite-system deposits. In contrast, the MTDs and turbidites are equally intermixed on basin floors along passive margins with a mud-rich continental slope, such as the northern Gulf of Mexico. Great earthquakes also result in characteristic seismo-turbidite lithology. Along the Cascadia margin, the number and character of multiple coarse pulses for correlative individual turbidites generally remain constant both upstream and downstream in different channel systems for 600 km along the margin. This suggests that the earthquake shaking or aftershock signature is normally preserved, for the stronger (Mw ? 9) Cascadia earthquakes. In contrast, the generally weaker (Mw = or <8) California earthquakes result in upstream simple fining-up turbidites in single tributary canyons and channels; however, downstream mainly stacked turbidites result from synchronously triggered multiple turbidity currents that deposit in channels below confluences of the tributaries. Consequently, both downstream channel confluences and the strongest (Mw ? 9) great earthquakes contribute to multi-pulsed and stacked turbidites that are typical for seismo-turbidites generated by a single great earthquake. Earthquake triggering and multi-pulsed or stacked turbidites also become an alternative explanation for amalgamated turbidite beds in active tectonic margins, in addition to other classic explanations. The sedimentologic characteristics of turbidites triggered by great earthquakes along the Cascadia and northern California margins provide criteria to help distinguish seismo-turbidites in other active tectonic margins.

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

2012-11-01

12

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

NASA Astrophysics Data System (ADS)

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

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

2012-07-01

13

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

NASA Astrophysics Data System (ADS)

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 nature of the crust below the Laxmi Basin we have analysed two new geophysical datasets in combination with previously published datasets, thereafter gravity and magnetic anomalies were modelled with the constraints from seismic reflection and refraction results. The sediments derived mostly from the Indus cone are generally thicker on west of the Laxmi Ridge than that in the Laxmi Basin. The basement in the Laxmi Basin includes numerous intrusive structures and faulted blocks, thus the basement topography becomes highly irregular and shallower compared to that of the Western Basin. The ship-borne and satellite gravity anomalies had enabled to map the regional extent of the structures in the basin especially the gravity lows associated with the Laxmi and Panikkar ridges. Intrusive structures mapped in the Laxmi Basin are seen coinciding with significant magnetic anomalies, but these anomalies were earlier considered as pre-Tertiary spreading-related anomalies. Free-air gravity anomalies of the Laxmi Ridge and Laxmi Basin are in general anomalous as they are associated with negative and positive gravity anomalies respectively. Within the basin the gravity anomaly is gradually rising toward north, reaches 40 mGal more, indicating the accretion of additional magmatic material toward north within crust. The velocity structures of the Laxmi Ridge, Laxmi Basin, Western Basin, Seychelles Bank and Indian continental shield reveal the Moho boundary only below the Seychelles Bank, Western Basin and Indian subcontinent. An anomalous velocity layer, 7.15 to 7.19 km/s is observed in the lower crust of the Laxmi Basin and Laxmi Ridge. The velocities are contrastingly deviating from the normal velocity structure of both continental and oceanic crust. On close observation of velocity structure we found that 6.2 - 6.3 km/s layer is conspicuously present in the upper crust of all the geological provinces except below the Western Basin. Instead the Western Basin bears about 6.7 km/s velocity layer, which is considered as a normal velocity for layer 3 of the oceanic crust. As the velocity layer 6.2 - 6.3 km/s in general represents the upper crustal rocks (granitic gneisses) of the continental crust, the similar velocity layer below the Laxmi Basin may be attributed to upper crustal rocks of the continental crust. The gravity and magnetic model studies have revealed that the Laxmi Basin consists of 11-14 km stretched continental crust, in which magmatic bodies have been emplaced, while the Panikkar Ridge in the middle of the basin has relatively less disturbed stretched continental crust. The seaward dipping reflectors (SDRs) on western margin of the Laxmi Ridge and sharp changes in regional gravity and magnetic fields across it lead to place the ocean-continent boundary on west of the Laxmi Ridge. The Reunion hotspot, when the basin was passing over it, had emplaced the volcanic material in the form of dykes and sills within existing stretched continental crust. We, therefore, believe that the Laxmi Basin basically consists of continental crust, which subsequently got modified by extensive stretching and volcanic outpourings of the Reunion hotspot.

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

2004-12-01

14

The ocean-continent boundary off the western continental margin of Iberia: Crustal structure west of Galicia Bank  

Microsoft Academic Search

A seismic refraction transect across the Galicia Bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin-parallel peridotite ridge (PR). Immediately west of the PR, oceanic crust is only 2.5-3.5 km thick, but farther west (oceanward) it thickens to 7 km. The PR caps a ~60-km-wide lens shaped serpentinized

Robert B. Whitmarsh; Robert S. White; Susan J. Horsefield; Jean-Claude Sibuet; Maurice Recq; Véronique Louvel

1996-01-01

15

Fine-scale analysis of shelf–slope physiography across the western continental margin of India  

Microsoft Academic Search

We attempt here to quantify and model physiographic features off the central west coast of India in terms of power spectral exponent, amplitude parameter. We demonstrate that statistical analysis of multi-beam echo-sounder grid bathymetry data is able to characterise the outer shelf, upper slope, shelf margin basin and several structural rises in the region. A scatter diagram analysis shows that

B. Chakraborty; R. Mukhopadhyay; P. Jauhari; V. Mahale; K. Shashikumar; M. Rajesh

2006-01-01

16

Continental margin drilling program  

Microsoft Academic Search

The initial responses from OMB, Congress, and industry have been overwhelmingly positive in support of a proposed major drilling program along the U.S. offshore coastal areas in deep water.The extensive sedimentary deposits located along the U.S. continental slopes are, as yet, unexplored. There have been numerous suggestions that there is a significant potential for extensive reserves of hydrocarbons, but these

Peter M. Bell

1980-01-01

17

What is the crustal architecture of hyper-extended domains at rifted continental margins: The Bay of Biscay-Western Pyrenees example  

NASA Astrophysics Data System (ADS)

Research into the formation of passive continental margins is currently evolving. The discovery of exhumed continental mantle and hyper-extended continental crust devoid of major normal faulting in deep-water rifted margins casts doubt on classical rift models. The development of new concepts that could explain passive margin formation is strongly limited by the resolution of offshore public seismic data and by the lack of drilling. However, the investigation of remnants of ancient margins or hyper-extended rift systems preserved in collisional belts gives important new insights on the study of rift structures and architecture. We use the Bay of Biscay-Western Pyrenees as a natural laboratory to study the processes and evolution related to passive continental margin formation. Offshore in the Bay of Biscay, this domain represents an embryonic oceanic basin that formed during Aptian time. At the south-eastern termination of the Bay of Biscay, several rift basins, likely to have formed in similar settings (as a consequence of Early Cretaceous rifting), show evidence of hyper-extended continental domains (e.g. Parentis, Arzacq-Mauléon, Cantabrian basins). The structural inversion of these basins during Pyrenean compression resulted in the reactivation of some of the rift structures as observed offshore on seismic sections and outcropping onland in the Western Pyrenees. We combine offshore studies, based on seismic reflection data and using gravity inversion and residual depth anomaly analysis, with onshore field observations to investigate the Bay of Biscay-Western Pyrenees rift system with the aim of better understanding the architecture of present-day deep-water rifted margins (in the Bay of Biscay) and of hyper-extended rift basins (Arzacq-Mauléon, Cantabrian basins). In order to compare the rift structures of the Bay of Biscay rifted margin with the onshore analog basins in the Western Pyrenees, we reconstruct several crustal scale cross-sections across the rift system. Initial results suggest that the rift architecture of the Arzacq-Mauléon basin is controlled by low-angle detachment fault systems. Interpretations indicate that the 3D evolution of the system is complex and that the basin is segmented by lateral ramps. In order to compare this hyper-extended domain onshore to a present-day passive margin, we have reinterpreted selected seismic lines across the North Biscay margin. First results show a well-defined hyper-extended domain between the continental proximal margin and distal oceanic domains. Two types of architecture can be distinguished across the margin within the hyper-extended domain. We believe that this change of architecture is linked with a switch from a lower to upper plate margin going from the northwest to the southeast. With this work, we aim to illustrate the architecture of hyper-extended domains and to investigate the processes responsible for the formation of hyper-extended rift systems.

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

2012-04-01

18

The ocean-continent boundary off the western continental margin of Iberia: Crustal structure west of Galicia Bank  

NASA Astrophysics Data System (ADS)

A seismic refraction transect across the Galicia Bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin-parallel peridotite ridge (PR). Immediately west of the PR, oceanic crust is only 2.5-3.5 km thick, but farther west (oceanward) it thickens to 7 km. The PR caps a ˜60-km-wide lens-shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust. When superimposed on a reflection time version of the velocity model, the S reflector is clearly intracrustal at its east end. Westward, S cuts down to lower crustal levels, eventually coinciding with the top of the serpentinized peridotite lens (original crust-mantle boundary). These observations render almost impossible the seafloor exposure of the PR by S acting as a top-to-the-west detachment fault. Numerical models of melting and borehole subsidence information constrain our rifting model. The easternmost continental crust experienced a total stretching factor of 4.3 (most likely in two stages); it probably occurred over ˜25 m.y., with the highest rate of stretching at the beginning of the main earlier rift phase (Valanginian; 141-135 Ma). The 3 (4.7) km thick continental crust (depending on whether serpentinized peridotite is assigned to crust or mantle), which may include melt products, requires stretching factors of more than 11 (7) and a rift duration of more than 25 (13) m.y. The thin oceanic crust immediately west of the PR is explained by conductive cooling of the mantle during the long prebreakup stretching phase, which temporarily caused reduced melting immediately after breakup.

Whitmarsh, Robert B.; White, Robert S.; Horsefield, Susan J.; Sibuet, Jean-Claude; Recq, Maurice; Louvel, VéRonique

1996-12-01

19

Margins: A new conceptual approach to continental margin research  

Microsoft Academic Search

The geology of continents is to a large degree the geology of continental margins. The margins mark the site where materials that form major components of the continental crust are distilled from Earth's mantle and where continents grow through plate interactions that progressively incorporate these margins into the continental mass.Much of the continents consists of the remnants of ancient margins

John Mutter

1990-01-01

20

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

21

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, H. A.

1989-01-01

22

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

Microsoft Academic Search

The continental-oceanic crust boundary and an incipient oceanic crust of the Red Sea opening are exposed within the Arabian plate along a narrow zone of the Tihama Asir coastal plain in SW Saudi Arabia. Dike swarms, layered gabbros, granophyres and basalts of the 22 Ma Tihama Asir (TA) continental margin ophiolite represent products of magmatic differentiation formed during the initial

Y. Dilek; H. Furnes; R. Schoenberg

2009-01-01

23

Fluid history in hyper-extended rifted margins: Examples from the fossil Alpine and western Pyrenean rift systems and the present-day Iberia rifted continental margin.  

NASA Astrophysics Data System (ADS)

The evolution of deep-water, magma-poor rifted margins is intimately linked with complex and polyphase fault structures. These structures, known as detachment faults, are responsible for extreme crustal thinning and mantle exhumation. During the evolution of detachment faults fluid-rock interaction plays an important role, changing the chemical and physical properties of rocks. These processes likely have major implications for the strain localization and structural evolution of the margin. The change in rock chemistry and rheology is best indicated by the breakdown of feldspars and olivine into clay and serpentine minerals and the pervasive cementation and precipitation of quartz along the fault zones. Although the chemical and mineral reactions are well known, it is still unclear to what extent these reactions lead to changes in the overall rheology of the extending lithosphere and how they can affect the thermal evolution of the hyper-extended rifted margins. In order to answer to these questions it is important to understand the origin, timing, pathways and composition of the fluids generated during rifting. Are fluids solely of marine origin or do they have a metamorphic- or mantle-derived component? Can we determine the range of temperature and consequently at what depth these fluids are formed? And can we constrain the age of their migration? These questions can be addressed in the well-known hyper-extended rift systems such as the Alpine Tethys margins exposed in the Alps, the Mauléon basin in the Western Pyrenees and the Deep Iberia margin drilled and seismically imaged offshore Portugal. All of these rift settings show evidence for detachment systems associated with hyper-extension and mantle exhumation. The aim of this ongoing study is to characterize the fluid signature in hyper-extended domain in magma-poor rifted margins. Including different sites with different degrees of compressional and metamorphic overprint enables us to compare results and to define the general importance of fluid systems in the development of hyper-extended rifts systems. The first results show that in all three geological settings fluid percolation can be recognized in fault rocks linked to the detachment systems. Evidence for the presence of fluids comes from the analyses of hydration reactions in fault zones. In the Alps the major and trace elements show a gain in elements typical from mantle rocks (Mg, Ni, Cu, Co, V). In the Pyrenees, microstructural studies show that detachment faulting crossed a range of crustal depths providing constraints on the depths of fluid migration. Future analyses will focus on additional major and trace elements and isotopic ratios (Sr and B) of hydrated rocks recovered from these hyper-extended domains, which will be linked with the temporal and spatial evolution of the major detachment structures.

Pinto, Victor Hugo; Manatschal, Gianreto; Karpoff, Anne Marie; Masini, Emmanuel; Lemarchand, Damien; Hayman, Nicholas; Trow, Rudolph; Viana, Adriano

2013-04-01

24

Late tertiary structure and stratigraphy of north Sinai continental margin  

SciTech Connect

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

Ben-Avraham, Z. (Oceanographic Inst., Haifa, Israel); Mart, Y.

1981-06-01

25

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

26

Pleistocene chronology of continental margin sedimentation  

Microsoft Academic Search

Commonly accepted models for the evolution of continental margins link sediment erosion, transport and deposition to eustasy. To test these models, we constructed an oxygen isotope record from 520 m of Pleistocene sediment recovered by the Ocean Drilling Program Leg 174A from the New Jersey continental slope. The ?18O record was calibrated to SPECMAP oxygen isotope time scale [Imbrie et

Cecilia M. G McHugh; Hilary Clement Olson

2002-01-01

27

Accretionary Orogenesis in the Active Continental Margins  

Microsoft Academic Search

This paper reviews the research history and progress in the study of accretionary orogenesis, and concludes that accretionary orogenesis is the fundamental process of continental evolution throughout Earth's history. According to the authors' point of view, the formation and evolution of orogenic belts can be explained by the evolution of composite island arc-basin systems along active continental margins. The formation

Sihua YUAN; Guitang PAN; Liquan WANG; Xinsheng JIANG; Fuguang YIN; Wanping ZHANG; Jiewen ZHUO

2009-01-01

28

Gas hydrates of outer continental margins  

SciTech Connect

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

Kvenvolden, K.A. (Geological Survey, Menlo Park, CA (USA))

1990-05-01

29

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

30

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

31

Polar continental margins: Studies off East Greenland  

NASA Astrophysics Data System (ADS)

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

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

32

Magmatic switch-on and switch-off along the South China continental margin since the Permian: Transition from an Andean-type to a Western Pacific-type plate boundary  

NASA Astrophysics Data System (ADS)

Detrital zircon provenance data for the Tananao schist in eastern Taiwan is consistent with its protolith being deposited on the South China continental margin at around, or soon after, 150 Ma, rather than being of an exotic origin and much older as previously suggested. The absence of ca. 200 Ma zircons agrees with the presence of a magmatic gap in the region after the orogenic and magmatic front migrated to central South China, due to a flat-slab subduction. The characteristic lack of input from interior South China (i.e., the lack of 1100-750 Ma and 470-420 Ma populations), and the immature nature of some of the schist units, suggest that they were sourced from the nearby coastal regions. On the other hand, they exhibit a dominant 190-150 Ma magmatic zircon population, suggesting the presence of abundant magmatic rocks of that age along the coastal regions. This, along with our newly discovered ca. 180 Ma I-type granites from eastern Zhejiang and other ca. 190-180 Ma magmatic rocks recently reported from the coastal regions, led us to propose that a new continental arc was initiated after ca. 190 Ma along the coastal region after a magmatic gap due to flat-slab subduction. This newly initiated arc likely persisted until ca. 90 Ma, and is represented by the I-type granitic rocks in eastern Taiwan. Slab roll-back likely caused the arc system to retreat towards the Pacific Ocean after 90 Ma, and ca. 60-17 Ma bimodal magmatism adjacent to the South China Sea signifies continental margin extension in the lead-up to, and during, the opening of the South China Sea. We thus argue that the continental margin of East Asia was transformed from an Andean-type plate margin at 280-90 Ma, to the present-day Western Pacific-type plate margin soon after 90 Ma.

Li, Zheng-Xiang; Li, Xian-Hua; Chung, Sun-Lin; Lo, Ching-Hua; Xu, Xisheng; Li, Wu-Xian

2012-04-01

33

Flexural and stratigraphic development of the west Indian continental margin  

NASA Astrophysics Data System (ADS)

Subsidence curves for 27 wells from the western continental margin of India show a characteristic late Oligocene to early Miocene (approximately 24 +/- 5 Ma) rapid increase in subsidence rate superposed on the long-lived, slow subsidence typical of the thermal subsidence phase of passive continental margins. By subtracting a best fit negative exponential subsidence from the observed subsidence curves, we obtain an estimate of the distribution and magnitude of the 'excess' subsidence affecting the Neogene development of the west Indian margin. The magnitude of this excess subsidence increases seaward from the coast, ranging from a few meters to greater than 2000 m near the shelf edge. We examine the following hypotheses to explain the distribution and timing of this excess subsidence: (1) modification of basin stratigraphy due to the effects of lithospheric in-plane compression, (2) creation of accommodation space on the margin by flexural effects associated with Indus fan loading, and (3) rapid growth of the continental margin and associated flexural effects.

Whiting, Brian M.; Karner, Garry D.; Driscoll, Neal W.

1994-07-01

34

Geologic structure and halokinesis on the continental margin of Angola  

SciTech Connect

The paper offers a brief overview of the structure in an area of considerable petroleum potential as interpreted from a viewpoint somewhat different from the standard Western model of the opening of the Atlantic. Seismoacoustic profiling data and geomorphological evidence obtained by the Sevmorgeologiya, as well as studies made by one of the authors in collaboration with the Sonangol national corporation and published data on the geology of Angola and deep-water drilling allow the authors to construct a seismogeological profile crossing all the subzones in the Angola continental margin and part of the eastern bathyabyssal region. A major feature of the sediment cover at the Angola continental margin is the presence of very extensive and thick salt-bearing beds, which are responsible for various forms of salt tectonics (halokinesis). Three types of section occur in the sedimentary cover. The paper describes all three types in detail. 13 references.

Dibner, V.D.; Mitin, N.E.; Rozhdestvenskaya, I.I.; Seryakov, M.M.; Ustinova, L.A.

1986-04-01

35

Repeated Pleistocene glaciation of the East Siberian continental margin  

NASA Astrophysics Data System (ADS)

During the Pleistocene glaciations, Arctic ice sheets on western Eurasia, Greenland and North America terminated at their continental margins. In contrast, the exposed continental shelves in the Beringian region of Siberia are thought to have been covered by a tundra landscape. Evidence of grounded ice on seafloor ridges and plateaux off the coast of the Beringian margin, at depths of up to 1,000m, have generally been attributed to ice shelves or giant icebergs that spread oceanwards during glacial maxima. Here we identify marine glaciogenic landforms visible in seismic profiles and detailed bathymetric maps along the East Siberian continental margin. We interpret these features, which occur in present water depths of up to 1,200m, as traces from grounding events of ice sheets and ice shelves. We conclude that the Siberian Shelf edge and parts of the Arctic Ocean were covered by ice sheets of about 1km in thickness during several Pleistocene glaciations before the most recent glacial period, which must have had a significant influence on albedo and oceanic and atmospheric circulation.

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

2013-10-01

36

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

Microsoft Academic Search

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

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

1986-01-01

37

Southern African continental margin: Dynamic processes of a transform margin  

NASA Astrophysics Data System (ADS)

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

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

2009-03-01

38

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

39

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

40

What Kind of Continental Margin am I? Active or Passive?  

NSDL National Science Digital Library

Volcanoes, earthquakes, and topography reveal whether a continental margin is active or passive. In this activity, students use the GeoMapApp tool to work with earthquake, volcano, and topographic data to identify active and passive margins.

Wetzel, Laura; Palinkas, Cindy; Bemis, Karen; Mcdaris, John

41

Some remarks on the evolution of sedimentary basins along the eastern Brazilian continental margin  

Microsoft Academic Search

This work provides a general overview of the tectono-stratigraphic evolution of the Eastern Brazilian Margin, discussing the main phases of subsidence and sedimentation and in particular, the structural styles and depositional megasequences of selected basins. The Mesozoic sedimentation along the Brazilian continental margin started with the breakup of Western Gondwana in the Late Juras- sic\\/Early Cretaceous. The rifting of the

César Cainelli; Webster U. Mohriak

42

Southern African continental margin: Dynamic processes of a transform margin  

Microsoft Academic Search

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

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

2009-01-01

43

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

44

Structure of Continental Margin off Sierra Leone, West Africa  

Microsoft Academic Search

Seismic-refraction data from the ocean are combined with geological data from the shore to determine the structure of the continental margin and the continent-ocean transition off West Africa. More than 5 km of Cretaceous-Quaternary sediments accumulated on the continental shelf and slope as the surface of the underlying continental crust subsided. On the shelf the pre-Mesozoic rocks consist of a

R. E. Sheridan; R. E. Houtz; C. L. Drake; M. Ewing

1969-01-01

45

Paleogene continental margin truncation in southwestern Mexico: Geochronological evidence  

Microsoft Academic Search

The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and\\/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW-SE decreasing granitoid intrusion age trend (~1.2

Peter Schaaf; Dante Morán-Zenteno; Maria del Sol Hernández-Bernal; Gabriela Solís-Pichardo; Gustavo Tolson; Hermann Köhler

1995-01-01

46

The instability of continental passive margins and its effect on continental topography and heat flow  

NASA Astrophysics Data System (ADS)

The long geological history of passive margin evolution is complex yet typified by an initial ramp-like tilting of the subaerial surface toward the continent-ocean boundary, followed by episodic uplift and subsidence at a smaller wavelength. We argue that this behavior is due to changes in margin structure brought about by buoyancy-driven lithospheric flow. Continental lithosphere is melt-depleted, buoyant, and thick. It will resist convective breakdown into the asthenosphere below, but will be prone to lateral flow due to horizontal density contrasts. Changes in lithosphere thickness at the transition between continent and ocean will nucleate convection cells. Using a numerical model of viscous upper mantle flow, we show that stability or instability of the continental lithosphere at a passive margin is a function of the lithospheric rheology and composition. Increased compositional buoyancy leads to oceanward lateral flow of the continental lithosphere whereas decreased buoyancy has the opposite effect, causing landward lateral flow of the continental lithosphere. In model simulations, a continental lithosphere thought typical of Phanerozoic continental platforms experiences first a margin-wide ramp-like tilting, followed by topographic fluctuations due to an evolving array of convection cells in the mantle. The timing and magnitude of predicted changes in topography are similar to those observed at the eastern North American margin, suggesting that the tilting and episodic uplift and subsidence at continental passive margins are a natural consequence of the evolution of continental lithosphere after breakup and during mature seafloor spreading.

Armitage, J. J.; Jaupart, C.; Fourel, L.; Allen, P. A.

2013-04-01

47

New magnetic anomaly map of the East Antarctic continental margin  

NASA Astrophysics Data System (ADS)

Marine magnetic survey coverage of the southern part of Indian Ocean is to a certain extent limited for defining the magnetic pattern of the continental margin of East Antarctica. The USA research vessels collected the bulk of the marine magnetic data in the beginning of 1960's. During the succeeding years Australian, German, Japanese, Russian and other international scientific programs made major contributions to the network of marine magnetic data. Since the beginning of new century only two nations (Russian and Australian) have acquired the marine magnetic data in the southern part of Indian Ocean. The marine surveys in the Cosmonaut Sea, the western part of the Cooperation Sea in the Davis and Mawson Seas were accomplished by the PMGRE in 2000-2009 field seasons. The marine magnetic data collected during two seasons (2001-2002) within the AASOPP Project which was established in early 2000 to define the outer limits of the continental shelf offshore of the Australian Antarctic Territory (AAT) covered the full length of the AAT from 40OE to 160OE. The new magnetic anomaly map of the East Antarctic continental margin incorporates all available data acquired by the international community since the IGY 1957-58 through to 2009. Results of the compilation do not radically alter recent models describing first-order motions between the Antarctic, Australian and Indian plates, but they help to resolve uncertainties in early break-up history of opening between these plates. The timing and direction of early seafloor spreading in the area off the Antarctic margin, once conjugate to part of the Southern Greater Indian margin and to Australian margin, along the largely unknown region of the Enderby Basin, Davis Sea and Mawson Sea has been analyzed by many authors using different data sets. It is highly likely that spreading in the Enderby Basin occurred around the same time as the well documented M-sequence (anomalies M10 to M0) off the Perth Basin, Western Australia (Powell et al. 1988). The history of the early spreading is complicated further by the likelihood of one or several ridge jumps in which most early seafloor crust was transferred to the Antarctic plate and the Elan Bank micro-continent was isolated from the Indian continent (Muller et al. 2001). Additionally, a large amount of the seafloor crust is now probably overprinted by igneous activity associated with the Kerguelen Plume, which began forming the Kerguelen LIP from about 120-110 Ma. However all available results of interpretations do not match to the magnetic anomaly pattern which can be distinguished by the newly compiled map. Our observations suggest that this is especially correct to the Enderby Basin and to lesser degree for the region that was conjugate to Australia. The prominent magnetic anomaly boundary signal and sharp basement step correlated with the MacRobertson Coast Anomaly or the Enderby Basin Anomaly (Golynsky et al., 2007) is not observed elsewhere in the Enderby Basin, Princess Elizabeth Trough or Davis Sea. In the central Enderby Basin there some evidences for an abandoned ‘fossil' spreading centre that might continue to the west of the Kerguelen Plateau, east of Gunnerus Ridge. The estimated timing of its extinction corresponding to the early surface expression of the Kerguelen Plume at the Southern Kerguelen Plateau around 120 Ma and the subsequent formation of the Elan Bank microcontinent. Alternatively, the ridge jump occurred only in the central Enderby basin, due to the proximity of the Kerguelen plateau, whereas seafloor spreading continued in the western Enderby basin and conjugate south of Sri Lanka basin.

Golynsky, Alexander; Ivanov, Sergey; Kazankov, Andrey

2010-05-01

48

Ocean Heat along the West Antarctic continental margin (Invited)  

Microsoft Academic Search

Published literature is converging to a consensus that the accelerated glacial melt in western Antarctica is due in part to ocean heat. This talk will present the history of ocean heat on the western continental shelves of the western Antarctic, likely contributing to this accelerated melt. The record is from nearly 20 years of gridded ocean data from the Palmer

D. G. Martinson

2010-01-01

49

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

50

Enhanced intraplate seismicity along continental margins: Some causes and consequences  

Microsoft Academic Search

In Australia, much of the seismic activity is restricted to zones up to several hundred kilometres wide inboard of continental margins at high angle to the trend in maximum horizontal compression (SHmax). Intriguingly, along the margin-side of one such zone, a near optimally oriented, crustal-scale structure – the Darling Fault – has no recorded historical seismicity. To explain these enigmatic

Mike Sandiford; David Lundbek Egholm

2008-01-01

51

The dynamics of continental extension and divergent margin formation  

SciTech Connect

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

Sawyer, D.S. (Rice Univ., Houston, TX (USA))

1990-05-01

52

Upper mantle viscosity and dynamic subsidence of curved continental margins.  

PubMed

Continental rifting does not always follow a straight line. Nevertheless, little attention has been given to the influence of rifting curvature in the evolution of extended margins. Here, using a three-dimensional model to simulate mantle dynamics, we demonstrate that the curvature of rifting along a margin also controls post-rift basin subsidence. Our results indicate that a concave-oceanward margin subsides faster than a convex margin does during the post-rift phase. This dynamic subsidence of curved margins is a result of lateral thermal conduction and mantle convection. Furthermore, the differential subsidence is strongly dependent on the viscosity structure. As a natural example, we analyse the post-rift stratigraphic evolution of the Santos Basin, southeastern Brazil. The differential dynamic subsidence of this margin is only possible if the viscosity of the upper mantle is >2-3 × 10(19) Pa s. PMID:23774453

Sacek, Victor; Ussami, Naomi

2013-01-01

53

Drilling Gas Hydrates on hydrate Ridge, Oregon continental margin  

Microsoft Academic Search

During Leg 204, we cored and logged 9 sites on the Oregon continental margin to determine the distribution and concentration of gas hydrates in an accretionary ridge and adjacent slope basin, investigate the mechanisms that transport methane and other gases into the gas hydrate stability zone (GHSZ), and obtain constraints on physical properties of hydrates in situ. A 3D seismic

A. M. Trehu; G. Bohrmann

2002-01-01

54

Assessment of Global Organic Carbon Flux Along Continental Margins.  

National Technical Information Service (NTIS)

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 oc...

R. Thunell

2004-01-01

55

Magnetite diagenesis in marine sediments from the Oregon continental margin  

Microsoft Academic Search

The magnetochemistry of sediments from the Oregon continental margin is examined to determine the effects of iron-sulfur diagenesis on the paleomagnetic record. Magnetic mineral dissolution and transformation into iron sulfides are a common feature in these suboxic to anoxic lutites. These processes are evidenced in rapid decreases in natural remanent magnetization intensities and stabilities, systematic changes in other rock magnetic

Robert Karlin

1990-01-01

56

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

NASA Astrophysics Data System (ADS)

Here we show that labile particulate iron and manganese concentrations in the upper 500 m 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, Phoebe J.; Bishop, James K. B.

2008-04-01

57

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

58

Holocene subsurface temperature variability in the eastern Antarctic continental margin  

NASA Astrophysics Data System (ADS)

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

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

2012-03-01

59

The Conjugate Volcanic Continental Margins of the South Atlantic  

Microsoft Academic Search

In November-December 2004 the German Federal Institute for Geosciences and Natural Resources (BGR) undertook a marine geophysical cruise along the South Atlantic continental margin off Uruguay and Southern Argentina. In total more than 3,700 km of combined multi-channel reflection seismic (MCS), magnetic and gravity and an additional of 1,300 km with only magnetic and gravity were acquired. In combination with

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

2005-01-01

60

A geophysical study of the northern Svalbard continental margin  

Microsoft Academic Search

SUMMARY In the summer of 1999, the first systematic seismic profiles were acquired across the northern Svalbard continental margin east of 15°E. Approximately 1470 km of multi-channel seismic reflection data as well as sonobuoy wide-angle data were collected up to 82°N. With few exceptions the signals imaged the whole sedimentary cover down to the acoustic basement. The uppermost sedimentary deposits

Wolfram H. Geissler; Wilfried Jokat

2004-01-01

61

New continental margin magnetic anomalies of East Antarctica  

NASA Astrophysics Data System (ADS)

Over the past decade, Australian, Norwegian and Russian marine surveys have collected integrated seismic, gravity and magnetic data in the southern Indian Ocean. The more than 350,000 line-km of new airborne and marine magnetic observations for the East Antarctic continental margin have been compiled into an improved definition of crustal magnetic anomaly patterns. This compilation provides important new constraints on the breakup processes and igneous activity related to the formation of the passive margin of East Antarctica. The eastern sector of the map from Bruce Rise in the west to the D'Urville Sea in the east is largely dominated by seafloor spreading magnetic anomalies. The 'Adélie Rift Block' of highly stretched and extensively faulted continental crust is associated with a smooth anomaly fabric. Abrupt magnetic anomaly changes along the oceanic-continent transition in the Cooperation Sea including the Enderby Basin Anomaly extend for more than 1680 km from the Kerguelen Plateau towards the Cosmonaut Sea. Three sectors of the East Antarctic continental margin exhibit pronounced disparities in the anomaly patterns that strongly suggest different modes of seafloor formation. Strong positive seafloor magnetic anomalies mark the southern margin of the Kerguelen Plateau, the Maud Rise and adjacent areas in the Riiser-Larsen Sea. The new compilation suggests that at least 300 km of the Enderby Basin and Shackleton Basin may be part of the Cretaceous Kerguelen Volcanic Province and possibly maps an abandoned 'fossil' spreading center in the central Enderby Basin. The majority of the published age models for the Enderby Basin and "Australian sector" of the East Antarctic margin are not in agreement with the structural grain of magnetic anomalies in the newly compiled map.

Golynsky, A. V.; Ivanov, S. V.; Kazankov, A. Ju.; Jokat, W.; Masolov, V. N.; von Frese, R. R. B.

2013-02-01

62

Western Scotia Sea margins: Improved constraints on the opening of the Drake Passage  

NASA Astrophysics Data System (ADS)

We present a revised tectonic interpretation (from ˜28 Ma to 3.2 Ma) of the western sector of the Scotia Sea, incorporating new multichannel seismic reflection profiles and magnetic anomaly identifications for the continental margin off the Tierra del Fuego Island, and available complementary data for the conjugate margin of the northwestern flank of the South Scotia Ridge. Seismic profiles show a remarkable diversity of the pair of conjugate passive margins of the western Scotia Sea in both their morphology and structural framework. The Tierra del Fuego continental margin can be related to a classic rifted passive margin, while the southwestern margin of the Scotia Sea is characterized by steep slopes mostly generated by subvertical faults that abruptly separate the continental crust of the South Scotia Ridge from the oceanic crust of the western Scotia Sea. This structural difference was caused by intense strike-slip tectonism, mostly concentrated along the modern South Scotia Ridge since the early development of the western Scotia Sea. We find evidence for a previously unrecognized magnetic anomaly 10 (˜28 Ma) at the foot of the Tierra del Fuego continental margin; the same anomaly is present at the conjugate northern flank of the South Scotia Ridge. The timing of events leading to the earliest development of the western Scotia Sea, which determined the opening of the Drake Passage is important because this gateway opening had a profound effect on global circulation and climate. The thickness and the distribution of the sedimentary cover overall in the abyssal plain off the two western Scotia Sea margins is different. This is due to the different regimes of the bottom-current flows which affected the western Scotia Sea, both in the past and in the present time.

Lodolo, Emanuele; Donda, Federica; Tassone, Alejandro

2006-06-01

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

Tectonic evolution of Brazilian equatorial continental margin basins  

SciTech Connect

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

Azevedo, R.P. (Petrobas, Salvador, Bahia (Brazil))

1993-02-01

65

North Sinai-Levant rift-transform continental margin  

SciTech Connect

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

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

1985-01-01

66

Tectonic structure and evolution of the Atlantic continental margin  

SciTech Connect

The Atlantic continental margin developed across the boundary between continental and oceanic crust as rifting and then sea-floor spreading broke apart and separated the North American and African plates, forming the Atlantic Ocean Basin. Continental rifting began in Late Triassic with reactivation of Paleozoic thrust faults as normal faults and with extension across a broad zone of subparallel rift basins. Extension became localized in Early to Middle Jurassic along the zone that now underlies the large marginal basins, and other rift zones, such as the Newark, Hartford, and Fundy basins, were abandoned. Rifting and crustal stretching between the two continents gave way to sea-floor spreading Middle Jurassic and the formation of oceanic crust. This tectonic evolution resulted in formation of distinctive structural features. The marginal basins are underlain by a thinner crust and contain a variety of fault-controlled structures, including half-grabens, seaward- and landward-tilted blocks, faults that die out within the crust, and faults that penetrate the entire crust. This variable structure probably resulted from the late Triassic-Early Jurassic pattern of normal, listric, and antithetic faults that evolved from the Paleozoic thrust fault geometry. The boundary between marginal basins and oceanic crust is marked approximately by the East Coast Magnetic Anomaly (ECMA). A major basement fault is located in the Baltimore Canyon trough at the landward edge of the ECMA and a zone of seaward dipping reflectors is found just seaward of the ECMA off Georges Bank. The fracture zone pattern in Mesozoic oceanic crust can be traced landward to the ECMA.

Klitgord, K.D.; Schouten, H.; Hutchinson, D.R.

1985-01-01

67

Chronobiology of deep-water decapod crustaceans on continental margins.  

PubMed

Species have evolved biological rhythms in behaviour and physiology with a 24-h periodicity in order to increase their fitness, anticipating the onset of unfavourable habitat conditions. In marine organisms inhabiting deep-water continental margins (i.e. the submerged outer edges of continents), day-night activity rhythms are often referred to in three ways: vertical water column migrations (i.e. pelagic), horizontal displacements within benthic boundary layer of the continental margin, along bathymetric gradients (i.e. nektobenthic), and endobenthic movements (i.e. rhythmic emergence from the substrate). Many studies have been conducted on crustacean decapods that migrate vertically in the water column, but much less information is available for other endobenthic and nektobenthic species. Also, the types of displacement and major life habits of most marine species are still largely unknown, especially in deep-water continental margins, where steep clines in habitat factors (i.e. light intensity and its spectral quality, sediment characteristics, and hydrography) take place. This is the result of technical difficulties in performing temporally scheduled sampling and laboratory testing on living specimens. According to this scenario, there are several major issues that still need extensive research in deep-water crustacean decapods. First, the regulation of their behaviour and physiology by a biological clock is almost unknown compared to data for coastal species that are easily accessible to direct observation and sampling. Second, biological rhythms may change at different life stages (i.e. size-related variations) or at different moments of the reproductive cycle (e.g. at egg-bearing) based on different intra- and interspecific interactions. Third, there is still a major lack of knowledge on the links that exist among the observed bathymetric distributions of species and selected autoecological traits that are controlled by their biological clock, such as the diel rhythm of behaviour. Species evolved in a photically variable environment where intra- and inter-specific interactions change along with the community structure over 24 h. Accordingly, the regulation of their biology through a biological clock may be the major evolutionary constraint that is responsible for their reported bathymetric distributions. In this review, our aim is to propose a series of innovative guidelines for a discussion of the modulation of behavioural rhythms of adult decapod crustaceans, focusing on the deep waters of the continental margin areas of the Mediterranean as a paradigm for other marine zones of the world. PMID:20959158

Aguzzi, Jacopo; Company, Joan B

2010-01-01

68

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

69

The Conjugate Volcanic Continental Margins of the South Atlantic  

NASA Astrophysics Data System (ADS)

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

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

2005-12-01

70

The Mesozoic south Atlantic Ocean and evolution of its continental margins  

Microsoft Academic Search

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

Philip D. Rabinowitz; John LaBrecque

1979-01-01

71

Tertiary evolution and petroleum potential of Oregon-Washington continental margin  

Microsoft Academic Search

The Oregon-Washington continental margin was the site of a deep marginal basin in which more than 7000 m of Tertiary sedimentary and volcanic rocks accumulated. Oceanic basalts of Paleocene to early Eocene age form the basin floor and are interpreted to represent eruptions in an elongate trough formed by rifting of the continental margin. Middle Eocene turbidite sandstone overlapped both

Snavely; P. D. Jr

1986-01-01

72

The importance of continental margins in the global carbon cycle  

NASA Astrophysics Data System (ADS)

Approximately half of the world's net annual photosynthesis occurs in the oceans (~48 Pg C y-1). Areas bordering continents (bottom <2000 m) support 10-15% of this production. We used satellite data to compute annual global net primary production (1998-2001), and derived the global particulate organic carbon (POC) flux settling below the permanent thermocline and to the seafloor using an empirical model of POC remineralization. Approximately 0.68 Pg C y-1 sink below the thermocline on continental margins, compared to 1.01 Pg C y-1 in the deep ocean. Over 0.62 Pg C y-1 settles to the seafloor on margins, compared to 0.31 Pg C y-1 to deep ocean sediments. At least 0.06 Pg C y-1 may be buried in sediments on margins. Therefore, margins may be responsible for >40% of the carbon sequestration in the ocean. These regions must be accounted for in realistic models of the global carbon cycle and its linkages to climate change.

Muller-Karger, Frank E.; Varela, Ramon; Thunell, Robert; Luerssen, Remy; Hu, Chuanmin; Walsh, John J.

2005-01-01

73

From pull-apart basins to ultraslow spreading: Results from the western Barents Sea Margin  

Microsoft Academic Search

This paper describes results from a geophysical study in the area between the ultraslow Knipovich Ridge and Bear Island, western Barents Sea. The objective was to map the crustal structure along a profile crossing a pull-apart rifted continental margin and oceanic crust generated by ultraslow spreading. The results are based on modeling of wide-angle seismic and gravity data, together with

Audun Libak; Christian Haug Eide; Rolf Mjelde; Henk Keers; Ernst R. Flüh

74

Structure of Continental Margin between Cape Rhir and Cape Sim, Morocco, Northwest Africa.  

National Technical Information Service (NTIS)

Seismic reflection profiles and magnetic intensity measurements were made on the continental shelf and upper slope between Cape Rhir and Cape Sim, Morocco. Patterns of shelf and regional structures imply a downfaulted origin for this continental margin, m...

J. M. Robb

1970-01-01

75

Buried Mesozoic rift basins of Moroccan Atlantic continental margin  

SciTech Connect

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

Mohamed, N.; Jabour, H.; El Mostaine, M. [ONREP, Rabat (Morocco)

1995-08-01

76

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

77

Anomalous bathymetry, 3D edge driven convection, and dynamic topography at the western Atlantic passive margin  

Microsoft Academic Search

A series of topographic\\/bathymetric anomalies trend along the western passive plate margin of the North Atlantic and are characterized roughly by uplift offshore on the ocean plate and subsidence at the continental margin. The features—determined from two independent data sets—are not constant along the strike of the plate boundary, however. The anomalous topography\\/bathymetry is considered to be a surface dynamic

Travis Ramsay; Russell Pysklywec

2011-01-01

78

Crustal thinning and topography at passive continental margins  

NASA Astrophysics Data System (ADS)

There is a relationship between crustal thinning patterns and onshore topography at passive continental margins. This relationship appears to be governed principally by the crustal thinning gradient (taper) of the crystalline crust from unrifted crustal thickness down to a thickness less than 10 km. Surprisingly, the relationship appears to hold for very long time intervals (> 150 ma) after rifting and breakup. Offshore Norway, two end-member styles of crustal thinning are observed. Along the Møre margin, a basin-flank detachment complex thinned the crust dramatically from c. 40 to less than 10 km over a horizontal distance of < 100 km. Along the Trøndelag Platform, however, thinning down to less than 10 km was distributed between 2-3 large-magnitude normal faults over a much broader region, which evolved into platform and terrace areas. The location, displacement magnitude and lateral arrangement of faults that developed in the margi?s `thinnin? phase governed the position of the proximal-distal margin boundary and thus the gross-scale thinning gradient, or taper, of the crystalline crust. In the onshore areas, the effects on topography, landscape and fault reactivation patterns are profound. The highest escarpment and the most asymmetric margin topography developed inboard of sharply tapering crystalline crust. Inboard of sharp tapers, strong landscape contrasts developed across lineaments that were reactivated after the main phase of Mesozoic rifting, but prior to the glaciations. Glacial erosion enhanced tectonically induced drainage patterns, resulting in an asymmetric landscape distribution with high-relief alpine topography preferentially developed on the footwall sides of reactivated faults. This, in turn, pre-destined these landscapes to increased rockslide susceptibility because in the deeply incised escarpment topography, glacial incision undercut structures that had been reactivated in the brittle mode. The above relationships indicate that extensional faulting exerts a long-term control on escarpment topography, landscape contrasts, geohazard susceptibility and sediment routing patterns along passive margins, through the establishment of the taper. On more than 40 published profiles through passive margins, we have measured the distance from the taper break (where the crust is thinned to 10 km or less) to the point of maximum topographic elevation on the adjacent escarpment (apparent taper length) and plotted it against the maximum escarpment elevation measured on each profile. Our analysis indicates that breakup age, glaciations and calculated mantle effects are all subordinate to the taper in controlling escarpment topography. Also, substituting the taper break with the COB does not yield a particularly tight relationship. These observations indicate that it is the thinning of continental crystalline crust down to <10 km and not the replacement of continental with oceanic crust that matters for the topography of the escarpment. Moreover, sharp reductions in the thickness of crystalline crust from <30 km down to less than 10 km do not appear to produce long-standing escarpments.

Terje Osmundsen, Per; Redfield, Thomas F.; Ebbing, Jörg

2010-05-01

79

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

NASA Astrophysics Data System (ADS)

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

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

80

Some depositional patterns at continental margin of southeastern Mediterranean Sea  

SciTech Connect

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

Mart, Y. (National Oceanographic Inst., Haifa, Israel); Gai, Y.B.

1982-04-01

81

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

82

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

SciTech Connect

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

Agrawal, A. (Univ. of North Carolina, Chapel Hill (USA))

1990-05-01

83

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

SciTech Connect

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

Bartholomew, M.J. (ed.) (South Carolina Univ., Columbia, SC (United States). Inst. of Earth Sciences and Resources); Hyndman, D.W. (ed.) (Montana Univ., Missoula, MT (United States). Dept. of Geology); Mogk, D.W. (ed.) (Montana State Univ., Bozeman, MT (United States). Dept. of Earth Sciences); Mason, R. (ed.) (Queen's Univ., Kingston, ON (Canada). Dept. of Geological Sciences)

1988-01-01

84

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

SciTech Connect

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

Bartholomew, M.J. [ed.] [South Carolina Univ., Columbia, SC (United States). Inst. of Earth Sciences and Resources; Hyndman, D.W. [ed.] [Montana Univ., Missoula, MT (United States). Dept. of Geology; Mogk, D.W. [ed.] [Montana State Univ., Bozeman, MT (United States). Dept. of Earth Sciences; Mason, R. [ed.] [Queen`s Univ., Kingston, ON (Canada). Dept. of Geological Sciences

1988-12-31

85

Origin of volcanic seamounts at the continental margin of California related to changes in plate margins  

NASA Astrophysics Data System (ADS)

Volcanic samples collected with the Monterey Bay Aquarium Research Institute's ROV Tiburon from eight seamounts at the continental margin offshore central to southern California comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite. All samples experienced complex crystal fractionation probably near the crust/mantle boundary, based on the presence in some of mantle xenoliths. Incompatible trace elements, poorly correlated with isotopic compositions, suggest variable degrees of partial melting of compositionally heterogeneous mantle sources, ranging from MORB-like to relatively enriched OIB. High-precision 40Ar/39Ar ages indicate episodes of volcanic activity mainly from 16 to 7 Ma but document one eruption as recent as 2.8 Ma at San Juan Seamount. Synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland. Collectively, the samples from these seamounts have age ranges and chemical compositions similar to those from Davidson Seamount, identified as being located atop an abandoned spreading center. These seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction-dominated to a transform margin. They differ in composition, age, and origin from other more widespread near-ridge seamounts, which commonly have circular plans with nested calderas, and from age progressive volcanoes in linear arrays, such as the Fieberling-Guadalupe chain, that occur in the same region. Each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric. The process may be aided by transtensional tectonics related to continued faulting along the continental margin.

Davis, A. S.; Clague, D. A.; Paduan, J. B.; Cousens, B. L.; Huard, J.

2010-05-01

86

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

Microsoft Academic Search

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

P. Wyer; A. B. Watts

2006-01-01

87

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

88

Mass Wasting Morphology at Convergent Margins: Relations Between Subduction Parameters and Surface Expressions - the Southern Central Chilean Continental Margin  

Microsoft Academic Search

Offshore Southern Central Chile (35°S-42°S) large regions of the middle and lower continental slope are shaped by a variety of mass-wasting processes. The quantitative assessment of their morphology on the basis of swath bathymetry data allows the comparison with cases at the continental margin of Oregon and the Pacific margin of Central America (Nicaragua and Costa Rica). The size and

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

2008-01-01

89

Crustal Recylcing at Ocean Margin and Continental Subduction Zones and the Net Accumulation of Continental Crust  

NASA Astrophysics Data System (ADS)

CRUSTAL RECYCLING PROCESSES AND VOLUMES: At convergent ocean margins large volumes of rock and sediment are missing from the global length of submerged forearcs. Material is removed by the kindred tectonic process of sediment subduction and subduction erosion, both of which insert sediment and eroded crustal debris into the subduction channel separating the upper and lower plates. The channel transports entrained debris toward the mantle where it is ultimately recycled. Large tracks of exposed high P/T rocks are exposed remnants of subduction channels. Over the past 100-200 my, the average solid-rock volume of recycled crust is estimated to have averaged globally 2.5-3.0 km3/yr--or 2.5 to 3 Armstrong Units (AU). Exposed tracts of UHP rocks at collisional orogens document that crustal material is subducted deep into the mantle at continental subduction zones. Based on missing terranes of extended lower plate, a volume of recycled continental crust detached by slab failure can be estimated at ~5000 km3 for each km of the early Proterozoic Wopmay orogen of the NW Canadian Shield (Hildrenbrand and Bowring, 1999, Geology, v. 27, p.11-14). Averaged over an orogenic episode of ~40 my, the corresponding rate is ~125 km3/my/km of margin. Using the Wopmay- rate as a guide, and assuming that similar to the Cenozoic, collisional orogenic margins averaged 10-12,000 km in global length, then since the early Proterozoic crustal recycling at collisional subduction zones has averaged close to 1.5 AU (i.e., 1.5 km3/yr). Crustal losses from the upper plate have also been recognized for sectors of the Variscan orogen (Oncken, 1998, Geology, v. 26, p. 1975-1078). The missing crust is roughly 40 km3/my for each km of upper plate, thus globally tallying an additional ~0.5 AU. CRUSTAL GROWTH: New information implies that at intra-oceanic subduction zones the long-term (~50 my), global rate of arc magmatic productivity has averaged close to 5 AU, a much higher rate than formerly estimated (~1 AU). It is not clear that this rate, which is based on the growth of the Aleutian and Izu-Bonin-Mariana arc massifs corrected for subduction erosion losses, can be applied to continental or Andean arcs. But allowing that it can, then the combined global rate of additions of juvenile igneous rock to build continents ( 5 AU) is similar to that recycled at ocean margin (2-3 AU) and continental subduction zones (2 AU). Additional losses can arise from delamination of magmatically or tectonically thickened convergent-margin crust. The implication of these estimates and linked assumptions support the Armstrong posit that since the early Archean the yang of magmatic additions to the continents has been matched by the yin of recycling losses.

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

2004-12-01

90

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.

91

Gas hydrate-filled fracture reservoirs on continental margins  

NASA Astrophysics Data System (ADS)

Many scientists predicted that gas hydrate forms in fractures or lenses in fine-grained sediments, but only in the last decade were gas hydrates found in complex fracture systems on continental margins. Gas hydrate-filled fractures were captured on both in situ borehole images and in x-ray imaged pressure cores. These new discoveries of gas hydrate as fill in fractures have been a boon to the gas hydrate community, yet, very little is known about the features and dimensions of a gas hydrate-filled fracture reservoir. Geophysical prospecting techniques, such as exploration seismic and controlled source electromagnetic surveys have not been able to detect a gas hydrate-filled fracture reservoir. In this dissertation, I aim to define the marine gas hydrate-filled fracture reservoir. Three offshore drilling expeditions, known as the gas hydrate Joint Industry Project Expeditions 1 and 2 in the Gulf of Mexico and the Indian National Gas Hydrate Program Expedition 1 on the Indian continental margins, are the sources of the geophysical well log and core data used in this dissertation. In the following five chapters, I show that gas hydrate often forms in shallow, unconsolidated, fine-grained sediments in near-vertical fractures. Gas hydrate-filled fractures are planar features, but likely only extend a few meters in breath. Gas hydrate-filled fracture systems are likely controlled by in situ methanogenesis and or methane solubility. The near-vertical nature of the gas hydrate-filled fractures causes anisotropic conditions in geophysical logging measurements made in vertical boreholes. Measured resistivity is most affected by the anisotropy, producing high resistivities in near-vertical gas hydrate-filled fracture systems. Thus, using measured resistivity to calculate gas hydrate saturation produces unreliable results. Gas hydrate-filled fractures in the same hole usually have similar strike orientations. The fracture orientations are used to determine the shallow stress directions in hole. The stress directions orient with bathymetric contour lines showing shallow stress is chiefly affected by changes in seafloor topography.

Cook, Ann Elizabeth

92

Late Quaternary sedimentation on the North Aegean continental margin, Greece  

SciTech Connect

The late Quaternary seismic stratigraphy of the North Aegean continental shelf and adjacent basins has been interpreted from boomer and 3.5-kHz seismic profiles. Ages derived from shallow cores and offshore wells, and relative offsets on small synsedimentary faults, provide chronological control. Sea level history inferred from seismic stratigraphy correlates with the global eustatic sea level record based on oxygen isotopic curves. The present depth of the delta plain formed on the outer shelf during the late stage 6 lowstand provides a dated and originally horizontal marker for estimating rates of tectonic subsidence. Gross distribution of sediment facies is similar in both tectonically stable and active areas. The shell break formed by delta progradation, but is marked by faults in most places because of the accommodation provided by graben subsidence rates of 0.3-1.5 mm/yr. Standard sequence stratigraphic analysis can be applied to these sediments deposited during high-amplitude Quaternary sea level oscillations. High rates of subsidence result in the preservation an unusually complete record of sea level change. Major lowstand progradation is dependent on the duration, rather than the magnitude, of sea level lowstand. The long glaciations in isotopic stages 6, 12, 16, and 22 resulted in the most prominent seaward progradation on the margin. Sandy lowstand turbidite deposits formed only when there was rapid fall in sea level; otherwise sand was trapped on delta tops and silty muds were deposited in deep water.

Piper, D.J.W. (Bedford Inst. of Oceanography, Dartmouth, Nova Scotia (Canada)); Perissoratis, C. (Inst. of Geology and Mineral Research, Athens (Greece))

1991-01-01

93

An Amphibious Magnetotelluric Study at the South Chilean Continental Margin  

NASA Astrophysics Data System (ADS)

A large network of long-period magnetotelluric sites was operated in the southern Chilean Andes between latitudes 37.5--41°S. It consists of 3 profiles from the coast of the Pacific Ocean to the volcanic arc, several connecting sites and more detailed investigations around active Villarrica and Llaima volcanoes. In 2004/2005 one profile was extended with sea-bottom stations across the trench towards the Pacific plate as part of the multi-disciplinary TIPTEQ programme. While analysis of offshore data is still ongoing, the onshore data display an unexpected first-order effect: without exception real induction vectors at all sites point consistently NE for long periods over the whole study area, overprinting any other effect which might be connected to conductivity anomalies along the N10°E striking continental margin. While 3-D models with realistic geometries cannot account for this highly anomalous behavior of tippers, quite simple 2-D models incorporating anisotropy explain the data at least qualitatively. The ocean (modeled with crude bathymetry) accounts for the large tipper magnitudes, while an anisotropic layer in the upper-middle crust is responsible for the deflection of vectors. The direction of the highly conductive axis is NW-SE, in good agreement with the multitude of fault systems observed at the surface in the forearc and arc regions. The South Chilean crust has thus to be considered as deeply fractured with far-reaching consequences for migration of fluids and melts, where appropriate conditions exist.

Brasse, H.; Kapinos, G.; Li, Y.; Chave, A.

2005-12-01

94

Fishing down the coast: historical expansion and collapse of oyster fisheries along continental margins.  

PubMed

Estuarine ecosystems have changed dramatically from centuries of fishing, habitat disturbance, sedimentation, and nutrient loading. Degradation of oyster reefs by destructive fishing practices in particular has had a profound effect on estuarine ecology, yet the timing and magnitude of oyster-reef degradation in estuaries is poorly quantified. Here, I evaluate the expansion and collapse of oyster fisheries in 28 estuaries along three continental margins through the analysis of historical proxies derived from fishery records to infer when oyster reefs were degraded. Exploitation for oysters did not occur randomly along continental margins but followed a predictable pattern. Oyster fisheries expanded and collapsed in a linear sequence along eastern North America (Crassostrea virginica), western North America (Ostreola conchaphila), and eastern Australia (Saccostrea glomerata). Fishery collapse began in the estuaries that were nearest to a developing urban center before exploitation began to spread down the coast. As each successive fishery collapsed, oysters from more distant estuaries were fished and transported to restock exploited estuaries near the original urban center. This moving wave of exploitation traveled along each coastline until the most distant estuary had been reached and overfished. PMID:15326294

Kirby, Michael Xavier

2004-08-23

95

Fishing down the coast: Historical expansion and collapse of oyster fisheries along continental margins  

PubMed Central

Estuarine ecosystems have changed dramatically from centuries of fishing, habitat disturbance, sedimentation, and nutrient loading. Degradation of oyster reefs by destructive fishing practices in particular has had a profound effect on estuarine ecology, yet the timing and magnitude of oyster-reef degradation in estuaries is poorly quantified. Here, I evaluate the expansion and collapse of oyster fisheries in 28 estuaries along three continental margins through the analysis of historical proxies derived from fishery records to infer when oyster reefs were degraded. Exploitation for oysters did not occur randomly along continental margins but followed a predictable pattern. Oyster fisheries expanded and collapsed in a linear sequence along eastern North America (Crassostrea virginica), western North America (Ostreola conchaphila), and eastern Australia (Saccostrea glomerata). Fishery collapse began in the estuaries that were nearest to a developing urban center before exploitation began to spread down the coast. As each successive fishery collapsed, oysters from more distant estuaries were fished and transported to restock exploited estuaries near the original urban center. This moving wave of exploitation traveled along each coastline until the most distant estuary had been reached and overfished.

Kirby, Michael Xavier

2004-01-01

96

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

NASA Astrophysics Data System (ADS)

Geophysical measurements over the eastern end of the Tehuantepec Ridge and adjacent continental margins of southern Mexico and northern Guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces. A positive free-air gravity anomaly which extends northwestward along the outer continental shelf of Guatemala curves abruptly landward in the Gulf of Tehuantepec. The positive shelf anomaly is on trend with the positive anomaly of the Nicoya Peninsula, Costa Rica, and suggests that rocks genetically related to Cretaceous rocks of the Nicoya Complex extend northwestward along the continental shelf to the Gulf of Tehuantepec. A crustal and subcrustal cross section of the continental margin of Guatemala, constrained by gravity, magnetic, and seismic refraction data, indicates that the rock strata causing the outer shelf gravity high dip landward, consistent with imbricate thrusting of the oceanic crust beneath and into the continental margin. A model crustal cross section of the continental margin of southern Mexico, north of the Tehuantepec Ridge, shows a markedly different margin structure with a relatively small amount of continental accretion and a continental crustal block extending to within approximately 25 km of the trench axis.

Couch, Richard; Woodcock, Stephen

1981-03-01

97

Distribution of oil and gas on active continental margins  

SciTech Connect

Accumulation of oil and gas in an area depends on amount and type of organic matter, adequate temperature for generation, suitable trapping configuration, and correct timing of events. All these factors can vary considerably across active margins of both island arc and continental (Andean) type. Forearc areas are characterized by low geothermal gradient owing to subduction, poor reservoirs derived from volcaniclastics, and relatively low organic carbon content. Although tectonic complexity may offer a wide variety of trapping configurations, overall petroleum potential is low. Gas is present in some commercial (and many noncommercial) accumulations and is in part biogenic. What little oil is present is usually paraffinic with a low sulfur content (approx. = 0.1%) and an API gravity in the range 30/sup 0/-35/sup 0/. These facts suggest a major role for land-derived organic matter, and idea supported by the available geochemical data. Back arc areas are characterized by higher geothermal gradients and larger basins. These may show evidence of extension with the development of organic-rich lakes or marine embayments in the downthrown area. Here source rocks are shales with above average organic carbon content. They have generated high pour-point waxy crude oils with low sulfur content and API gravities around 35/sup 0/. Again, these characteristics suggest an important role for land-derived organic materials and this is supported by geothermal data for biomarker distributions, pristane/phytane ratios, etc. This simpler structural setting in back arc basins favors the development of larger fields, and several giants occur in this setting, for example, Minas in Sumatra.

Barker, C.; Spyro, T.

1984-04-01

98

Surficial sediments on the western Canadian continental shelf  

NASA Astrophysics Data System (ADS)

The active continental margin off western Canada is characterized in the south by convergence between the Explorer and Juan de Fuca Plates and the America Plate, and in the north by transform fault motion between the Pacific and America Plates along the Queen Charlotte Fault. Except in some of the deepest troughs and basins shelf sediments are dominated by immature lithic arenites reflecting this tectonic setting. The Vancouver Island shelf is from 5 to 75 km wide and displays complex topography on the inner shelf and a relatively featureless mid- and outer shelf. An exception is the area off southwestern Vancouver Island where large basins bounded by morainal deposits extend more than two-thirds of the distance across the shelf. The shelf edge varies from 180 to 225 m depth and is indented by numerous canyons. Nearshore sediments consist mainly of gravels and boulders and become finer offshore such that muds are slowly accumulating in depths greater than 100 m. Off northwestern Vancouver Island calcareous sediments are abundant with carbonate values often exceeding 75%. High wave and current energies and efficient sediment trapping in coastal fiords have resulted in low rates of accumulation. Olive, glauconitic, Holocene muds and muddy sands are generally less than 0.3 m thick and accumulate only on the outer shelf over an extensive stiff, gray, glaciomarine sandy mud. Queen Charlotte Sound exhibits three broad, shallow, glacially scoured troughs, filled mainly with clayey silts and fine sands and separated by sand and gravel covered banks. Hecate Strait, between the Queen Charlotte Islands and the mainland, consists of a prominent southward-opening trough along the east side of the Strait bounded by Dogfish and Laskeek Banks on the west adjacent to Graham and Moresby Islands, respectively. The trough below 200 m is filled by silts. The banks are covered by discontinuous sands and gravels of variable calcareous content. The bank edges often display megaripples and sand waves. Shore-parallel and oblique sand and gravel ridges occur along northernmost Dogfish Bank. Except for the inshore area (less than 100 m) off northern Graham Island little is known of the sediment distribution in Dixon Entrance. Off northwestern Graham Island the near shore zone is dominated by bedrock and discontinuous sands and gravels made up of up to 90% skeletal carbonate. Off northeastern Graham Island sandy sediments predominate in the nearshore. No studies have been undertaken on the narrow shelf off the western Queen Charlotte Islands.

Bornhold, Brian D.; Barrie, J. Vaughn

1991-08-01

99

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.

100

The crustal structure across the transform continental margin off Ghana, eastern equatorial Atlantic  

Microsoft Academic Search

Forward modeling of a suite of wide-angle seismic lines across the transform continental margin at the eastern end of the Romanche fracture zone off Ghana has shown the transition from continental to oceanic crust to be confined to a narrow, 6 to 11-kin-wide zone located at the foot of the steep continental slope. The structure of the adjacent oceanic and

Rosemary A. Edwards; Robert B. Whitmarsh; Roger A. Scrutton

1997-01-01

101

The initiation of subduction by crustal extension at a continental margin  

NASA Astrophysics Data System (ADS)

We investigate how subduction may be triggered by continental crust extension at a continental margin. The large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement. Subduction requires the oceanic plate to get submerged in mantle, so that negative buoyancy forces may take over and drive further descent. This is promoted by two mechanisms. Loading by continental crust bends the oceanic plate downwards. Extension in the continental domain induces crustal thinning, which acts to raise mantle above the oceanic plate. In this model, the width of the continental region undergoing extension is an important control parameter. The main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading. Three governing dimensionless parameters are identified. One involves the poorly constrained oceanic plate buoyancy. We find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant, enabling metamorphic reactions and density increase in the oceanic crust. Another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate. Initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong. The third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness, such that a weak plate and a thick crust do not favour subduction. Thus, the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere. It is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides. Evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed.

Lévy, F.; Jaupart, C.

2012-03-01

102

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

Microsoft Academic Search

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

Richard Couch; Stephen Woodcock

1981-01-01

103

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

Microsoft Academic Search

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

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

1998-01-01

104

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

Microsoft Academic Search

The continental margin of West Iberia lacks significant synrift magmatism and exhibits a zone up to 100 km wide thought to consist mainly of serpentinized and exhumed mantle between the thinned continental and the oceanic crust. However, the existence of linear magnetic anomalies pose an ambiguity regarding the exact amount of synrift magmatism produced during mantle exhumation at the surface.

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

2003-01-01

105

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

106

Crustal structure of the Labrador Sea conjugate margin and implications for the formation of nonvolcanic continental margins  

NASA Astrophysics Data System (ADS)

Wide-angle seismic studies have determined the detailed velocity structure along a 350-km-long profile across the Labrador margin. Combination of this model with a previously published cross section for the southwestern Greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure. The results indicate three distinct zones across the Labrador margin, similar to the structure of the conjugate Greenland margin. Zone 1 represents 27 to 30-km-thick continental crust thinning gradually seaward over ˜100 km distance. Farther seaward, zone 2 is 70-80 km wide, characterized by a distinct lower crust, 4-5 km thick, in which velocity increases with depth from 6.4 to 7.7 km/s. Interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under-plating. Zone 3 represents two-layered, normal oceanic crust. The cross sections from both margins are reconstructed to an early drift stage at Chron 27. This demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of Chrons 31 and 33 on both margins. Zone 1 shows a marked asymmetry, with a gradual thinning of continental crust off Labrador contrasted with a rapid thinning off Greenland. The abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt.

Chian, Doping; Louden, Keith E.; Reid, Ian

1995-12-01

107

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.

Mcneill, Lisa

108

The geomorphology of a glaciated continental shelf, Western Scotland, UK  

NASA Astrophysics Data System (ADS)

We present recently collected swath bathymetry and legacy seismic data from two regions of the north-west UK continental shelf: the Sea of the Hebrides; and the Firth of Lorn, western Scotland. Both regions have experienced extensive Pleistocene ice sheet glaciation and both provide abundant geomorphological evidence of subglacial and postglacial processes. The Sea of the Hebrides bathymetry data cover 2200 km2 and provide new geomorphological evidence for an ice stream flowing from western Scotland and the Inner Hebrides focusing towards a trough-mouth fan (the Barra Fan) at the continental shelf break during the height of the last glaciation. Notably, bedrock structures provide a control on the location and orientation of glacially overdeepened basins and troughs on the inner shelf. Whilst around the Islands of Canna and Rum, convergent seabed glacial lineations and other subglacially streamlined features eroded in bedrock preserve the direction of ice sheet movement - indicating ice streaming in a south-westerly direction across the continental shelf. We propose that this fast-flow zone formed part of a larger convergent ice stream system draining much of western Scotland and the north of Ireland. The Firth of Lorn bathymetry acquisition comprises 553km2 of data, collected as part of the INIS Hydro program (Ireland, Northern Ireland and Scotland Hydrographic Survey). This region of nearshore continental shelf is revealed as predominantly bedrock-dominated seabed, characterised by a series of narrow, strongly fault-controlled troughs, part of the Great Glen Fault Zone complex. Evidence for glaciation is widespread and well preserved in the Firth of Lorn and surrounding seabed with moraines, bedrock lineations (?megagrooves?) and overdeepened basins common across the area. Initial mapping shows that our understanding of the configuration and style of deglaciation in these sectors of the former British-Irish Ice Sheet can be greatly improved by the collection of these new high-resolution bathymetric datasets.

Howe, John; Dove, Dayton; Bradwell, Tom

2013-04-01

109

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

PubMed

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

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

2001-09-13

110

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

NASA Astrophysics Data System (ADS)

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

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

2001-09-01

111

Mesozoic and Cenozoic granitoid complexes in the stucture of the continental margin of northeast Asia  

NASA Astrophysics Data System (ADS)

The integral data on structural position, age, and paleo-geodynamic setting of Mesozoic and Cenozoic granitoid complexes in northeast Asia make it possible to divide them into preaccretionary, accretionary, and postaccretionary groups participating in the structure of the accretionary-type continental margin. The preaccretionary granitoids are members of volcanic-plutonic associations of ensimatic island arcs or suprasubduction ophiolitic complexes, which mark the onset of growth of the granitic-metamorphic layer in the future continental crust. The accretionary granitoids emplaced during the accretion of diverse rock complexes to the continental margin and are localized in its frontal zone, where granitic-metamorphic layer grows further. The postaccretionary granitoid plutons of the marginal continental volcanic-plutonic belts seal up fold-nappe structures, determining the upper age limit of accretion and deformation. The origin of postaccretionary granitoids is related to remelting of older heterogeneous accretionary-island arc crust.

Luchitskaya, M. V.

2013-09-01

112

Holistic Approach Offers Potential to Quantify Mass Fluxes Across Continental Margins  

Microsoft Academic Search

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

Steven Kuehl; Lionel Carter; Basil Gomez; Noel Trustrum

2003-01-01

113

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

Microsoft Academic Search

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

Peter D. Clift; Jonathan Turner

1995-01-01

114

Age and significance of sequence boundaries on Jurassic and early Cretaceous rifted continental margins  

Microsoft Academic Search

The age and significance of sequence boundaries on Jurassic to Early Cretaceous rifted continental margins in three ocean basins have been documented. The margins are the Santos basin in the South Atlantic, the Grand Banks in the North Atlantic, and the Beaufort Sea in the Arctic Ocean. Large industry data bases were used for the interpretation of each area. Megasequence

Hubbard

1988-01-01

115

Morpho-tectonic Evolution of the Western Afar Margin (southern Red Sea, Ethiopia)  

NASA Astrophysics Data System (ADS)

The western Afar margin represents the southern end of the Red Sea rift system. It developed in the heart of the Afar plume related volcanic province which lies at the famous triple junction connecting the East African, the Gulf of Aden and the Red Sea rifted systems. This margin is presently separating the Afar depression from the Ethiopian Highlands and is topographically expressed by an impressive altitudinal gradient (more than 3000 m in less than 50 km). This topographical passive margin has been developed during Miocene times from the top of an uplifted dome, which has been related principally to the Afar plume impingement and the associated extrusion and differentiation of Ethiopian Continental Flood Basalts 30 Ma ago (Pik et al. 2003). This margin is also typically separating the edge of a thick continental block (below the Oligocene CFB) from an extremely thinned domain (the Afar depression) which most probably represents one of the rare worldwide rifting step corresponding to the Ocean Continent Transition zone at the rift to drift transition. Mechanisms invocated to explain this geodynamical phenomenon of extreme lithospheric thinning, predating continental break-up and sea floor spreading, are up to know controversial and do not really satisfy geological and geophysical observations along old passive margins bounding well developed oceanic domains. In this context, the on-shore western Afar margin is an ideal case study to try to precise geodynamic and structural thinning processes because time lapse since the initiation of rifting is so reduce (less than 30 Ma) that outcropping morphological expression of implicated geological events has been preserved and can be easily documented and investigated with traditional approaches. A particular geological characteristic of this southern part of the Red Sea margin resides in the well developed marginal basins, morphologically expressed (but at various altitude) all along the topographical gradient from the Eritrean margin to the heart of the volcanic province at the transition with the East African Rift branch. Such morphological steps have been variously described and interpreted in the past. In this study we will present a new geomorphological view and interpretation of these marginal basins development, as well as the potential structural and geodynamic implications of this new interpretation in the margin development and thinning.

Pik, R.; Lave, J.; Foeken, J.; Carignan, J.; Ayalew, D.; Yirgu, G.

2009-12-01

116

Numerical modeling of the development of southeastern Red Sea continental margin  

Microsoft Academic Search

The Red Sea continental margin (RSCM) corresponds to a wide hinge zone between Red Sea and Arabian plate. This margin has\\u000a been studied through geological and geophysical observations primarily in regard to the evolution of Red Sea rift. This margin\\u000a is characterized by occurrence of thin sediments, significant onshore uplift, tectonic subsidence of the offshore sedimentary\\u000a basin, active faulting and

Sunil Kumar Dwivedi; Daigoro Hayashi

2009-01-01

117

Plate Tectonic Models for Orogeny at Continental Margins  

Microsoft Academic Search

IN the theory of plate tectonics convergent plate junctures are the loci of orogeny1, marked surficially by arc-trench systems2, and the margin of one plate is consumed in a trench subduction zone. The overriding or consuming plate margin is characterized by a magmatic belt supporting both arc volcanism and batholithic intrusion at crustal levels. For rock assemblages formed within active

William R. Dickinson

1971-01-01

118

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

119

Manganese and copper fluxes from continental margin sediments  

Microsoft Academic Search

Total dissolvable Cu and Mn have been measured in seawaters collected from the continental shelf of the eastern Bering Sea. Copper concentrations of 4 nmole kg -1 inshore of a hydrographie front over the 100 m isobath. Manganese concentrations also were low over the shelf break, 10 nmole kg -1 inshore of the hydrographic front. Depth distributions of Mn at

David Heggie; Gary Klinkhammer; Douglas Cullen

1987-01-01

120

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

121

Geological controls on the Storegga gas-hydrate system of the mid-Norwegian continental margin  

Microsoft Academic Search

The geologic setting of the formerly glaciated mid-Norwegian continental margin exerts specific controls on the formation of a bottom-simulating reflector (BSR) and the inferred distribution of gas hydrates. On the continental slope the lithology of glacigenic debris flow deposits and pre-glacial basin deposits of the Kai Formation prevent gas-hydrate formation, because of reduced pore size, reduced water content and fine-grained

Stefan Bünz; Jürgen Mienert; Christian Berndt

2003-01-01

122

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

SciTech Connect

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

Pratson, L.; Ryan, W. (Lamont-Doherty Geological Observatory, Palisades, NY (USA)); Twichell, D. (Geological Survey, Woods Hole, MA (USA))

1990-05-01

123

The lateral flux of biogenic particles from the eastern North American continental margin to the North Atlantic Ocean  

NASA Astrophysics Data System (ADS)

Sediment trap samples from two field programs on the continental margin of the northeast coast of the United States, which constituted the Shelf Edge Exchange Program (SEEP), were analyzed for phytoplankton taxonomic composition and the fluxes of organic carbon, nitrogen and opaline silica. The traps, with a rotating carousel collection system, were located on taut-wire moorings between 150 and 2700 m below the surface and extended from the 500 m isobath on the upper continental shelf to the 2750 m isobath at the edge of the abyssal plain of the western North Atlantic Ocean. The temporal and spatial distributions of phytoplankton in the azide-poisoned trap samples revealed a general increase of intact cells with depth, which is consistent with lateral transport from the margins to the ocean interior. Taxonomic analysis of the phytoplankton indicated that >90% of the intact cells (containing identifiable intracellular structures) consisted of diatoms. The distribution of the species further supports the lateral transport origin of the particles, and indicates that the particulate materials are delivered to the ocean interior primarily in pulses of rapidly sinking aggregates. However, quantitative analysis suggests that intact phytoplankton contribute only 0.8 ± 0.7% and (mean and S.D.) and 0.9 ± 0.7% of the total particulate carbon and nitrogen fluxes, respectively. Using silica-to-carbon ratios to budget the remaining trap organic carbon fluxes, it would appear that between 17 and 100% of the sedimenting particles were originally diatomaceous, but that the organic carbon became solubilized and/or oxidized in the water column during descent. A simple two-dimensional model was developed to quantify the contribution of the flux of particulate organic carbon to the interior of the North Atlantic Ocean. The results suggest that north of Cape Hatteras, the mean lateral flux of particulate organic carbon sinking through the upper 500 m of the water column into the western edge of the basin is 4.8 × 10 12 g C y -1, which is about 6% of the primary production on the shelf. This flux represents the lateral export of carbon from the continental margin to the interior of the North Atlantic Ocean. Based on estimates of vertical export production for the basin of about 4.2 × 10 14 g C y -1, we estimate that the export of carbon from the western margin, north of Cape Hatteras, represents about 1% of the new production of the entire basin. This export is a significant source of energy which fuels the high benthic respiration on the continental slope.

Falkowski, P. G.; Biscaye, P. E.; Sancetta, C.

124

Lomonosov Ridge---A double-sided continental margin  

Microsoft Academic Search

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

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

1992-01-01

125

Total Tectonic Subsidence Analysis Provides Constraint on Basement Composition Near the Peridotite Ridge on the West Iberia Continental Margin  

Microsoft Academic Search

The West Iberia continental margin is a nonvolcanic, sediment-starved rifted margin located off the west coasts of Spain and Portugal. The margin formed in the Early Cretaceous during the opening of the North Atlantic Ocean, and continental break-up occurred around 120 Ma. In 1997, over 4000 km of two-dimensional seismic reflection data were collected along this margin as part of

A. T. Henning; D. S. Sawyer

2002-01-01

126

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

SciTech Connect

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

Cooke, H.E. (Geological Survey, Menlo Park, CA (United States)); Taylor, M.E. (Geological Survey, Denver, CO (United States)); Zhemchuzhnikov, S.V.; Apollonov, M.K.; Ergaliev, G. Kh.; Sargaskaev, Z.S. (Inst. of Geological Sciences, Alma-Ata (Kazakhstan)); Dubinina, S.V. (Geological Inst., Moscow (Russian Federation))

1991-02-01

127

The crustal structure across the transform continental margin off Ghana, eastern equatorial Atlantic  

NASA Astrophysics Data System (ADS)

Forward modeling of a suite of wide-angle seismic lines across the transform continental margin at the eastern end of the Romanche fracture zone off Ghana has shown the transition from continental to oceanic crust to be confined to a narrow, 6 to 11-km-wide zone located at the foot of the steep continental slope. The structure of the adjacent oceanic and continental crusts has also been resolved. These results are confirmed and enhanced by gravity and magnetic models. The crust of the ocean-continent transition zone is characterized by high velocities (5.8-7.3 km s-1), a high density (3.10 Mg m-3), and high magnetizations (1.1-1.25 A m-1). This rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin. The oceanic crust within 70 km of the margin is abnormally thin compared to normal Atlantic oceanic crust and shows an average thickness of just 4.4 km. We suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded, on at least three sides, by cold continental lithosphere. The preferred seismic model also shows a new fracture zone, 30 km southeast of the ocean-continent transition, which is characterized by low velocities in the upper crust, a lens-shaped layer with velocities of 7.2-7.4 km s-1 at the base of the crust, and a crustal thickness of just 3.4 km. The continental crust appears largely unaffected by the proximity of the adjacent oceanic crust. There is no evidence for underplating of the continental crust adjacent to the ocean-continent transition zone.

Edwards, Rosemary A.; Whitmarsh, Robert B.; Scrutton, Roger A.

1997-01-01

128

Geologic history of the continental margin of North America in the Bering Sea  

USGS Publications Warehouse

The North American continental margin beneath the Bering Sea is nearly 1,300 km long and extends from Alaska to eastern Siberia. The margin is a canyon-scarred 3,200-3,400-m high escarpment separating one of the world's largest epicontinental seas (the shallow Bering Sea) and the Aleutian Basin (the deep-water Bering Sea), a marginal oceanic basin distinguished by having its southern boundary formed by the Aleutian Ridge. Three geomorphic provinces can be recognized: a southeastern province characterized by a gentle continental slope (lacking V-shaped canyons) and an outlying continental borderland (formed by Umnak Plateau); a central province distinguished by a steep canyon-scarred slope, and a northwestern province having a gentler and, apparently, less eroded continental slope. Continuous seismic reflection profiles show that the margin is constructed of three major structural-stratigraphic units: (1) an acoustic basement underlying the outer shelf and upper slope; (2) an overlying main layered sequence; and (3) a stratified rise unit underlying and forming the continental rise at the base of the slope. The existing margin evolved with downbowing and faulting of the acoustic basement, an older margin probably of Late Mesozoic age, consisting in part of well-indurated siltstone and mudstone, in Early Tertiary time. Concomitant with subsidence as much as 1,500 m of main-layered-sequence strata were draped over the basement. Intense canyon cutting, presumed to have been caused by the rapid deposition of unstable masses of riverborn sediment over the outer shelf and upper slope, is thought to have begun in Late Tertiary and Quaternary time. Concurrent with canyon cutting, submarine fans, consisting of turbidites forming the rise unit, accrued at the base of the continental slope. Subsidence of the continental margin during the Tertiary may be related to foundering ("oceanization") of a continental block to form the Aleutian Basin, or to simple isostatic depression of a former segment of the North Pacific oceanic floor in response to sediment infilling north of the Aleutian Ridge. ?? 1968.

Scholl, D. W.; Buffington, E. C.; Hopkins, D. M.

1968-01-01

129

Molecular Diversity of Sulfate-Reducing Bacteria from Two Different Continental Margin Habitats  

Microsoft Academic Search

This study examined the natural diversity and distributions of sulfate-reducing bacteria along a natural carbon gradient extending down the shelf-slope transition zone of the eastern Pacific continental margin. Dissimilatory (bi)sulfite reductase gene sequences (dsrAB) were PCR amplified and cloned from five different sampling sites, each at a discrete depth, from two different margin systems, one off the Pacific coast of

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

2003-01-01

130

Continental margin evolution of the northern Arabian platform in Syria  

SciTech Connect

Synthesis of available geological and geophysical data in the Syrian Arab Republic permits a descriptive account of the pre-Cenozoic geologic history of the northern Arabian platform. The northern Arabian platform appears to be a composite plate similar up to that interpreted in the rocks of the Arabian shield. The structural and stratigraphic relationships of the Paleozoic and Mesozoic sedimentary sections in Syria record the transformation of an eastward-facing Gondwana passive margin in the early Paleozoic into a westward-facing Levantine margin in the Mesozoic, at which time the northern platform was closely associated with the creation of the eastern Mediterranean basin. Timing of the margin transformation is inferred from the orientation and thickness variations of Lower Triassic rocks, but the transformation may have initiated as early as the Permian. The diversity and timing of geological features in Syria suggest that the northern Arabian platform did not behave as a rigid plate throughout its geological history. The present-day Palmyride mountain belt, located within the northern Arabian platform in Syria and initiated in the early Mesozoic as a northeast-trending rift nearly perpendicular to the Levantine margin, subsequently was inverted in the Cenozoic by transpression. The location of the rift may be associated with the reactivation of a zone of crustal weakness, i.e., a Proterozoic suture zone previously proposed from modeling of Bouguer gravity data. Thus, the northern and southern parts of the Arabian platform are similar in their respective geologic histories during the Proterozoic and Paleozoic; however, the northern Arabian platform was greatly affected by Mesozoic rifting and the creation of the eastern Mediterranean basin during the Mesozoic. 13 figs.

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

1993-02-01

131

Differential loading by prograding sedimentary wedges on continental margins: An arch-forming mechanism  

NASA Astrophysics Data System (ADS)

Rapid deposition of prograding sedimentary wedges on continental margins will cause excess pore pressure, fluid flow, and compaction as they load the substratum. They may also cause faulting and structural deformation in the sedimentary succession below and in front of the load. Large depositional units are in addition associated with pronounced isostatic subsidence. The magnitude and effects each component has on the basin as a whole is often debated. To address this problem, we use a quantitative approach with a coupled hydromechanical mathematical model based on linear isotropic elasticity. The equations are solved by the finite element method (FEM). In the modeling, a wedge-shaped load emplaced successively over a period of 1.5 Myr is used to simulate the progradation of a thick sedimentary wedge. Results show that the differential load generates excess pressure below the prograding depocenter with lateral fluid flow in front and a tail of draining pore pressure behind. The sediments are pushed downward and laterally in front of the wedge and vertical and horizontal compressional and shear stresses are generated below the wedge. The formation of the Helland Hansen Arch, located in the Vøring Basin off mid-Norway, is genetically associated with differential loading. Our results indicate that the whole eastern flank of the arch is a result of differential loading, whereas the western flank is mainly related to thermal subsidence since Paleogene times. Thus, alternative explanations like intraplate stress seem to be irrelevant as a genetic mechanism.

Kjeldstad, A.; Skogseid, J.; Langtangen, H. P.; BjøRlykke, K.; HøEg, K.

2003-01-01

132

Internal tidal mixing as a control on continental margin ecosystems  

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

133

The Late Paleozoic Southern Margin of the Siberian paleocontinent: transformation from an active continental margin to intracontinental rifting  

NASA Astrophysics Data System (ADS)

The large volcanoplutonic belt was formed on the southern margin of Siberian paleocontinent in the Early Carboniferous-Early Permian. Now it's stretched through whole Mongolia and the adjacent region of China. In the belt structure there are defined the successive rock complexes: the older one represented by differentiated basalt-andesite-rhyodacite series and younger bimodal complex of basalt-comendite-trachyrhyolite composition. The granodiorite-plagiogranite and diorite-monzonite-granodiorite plutonic massifs are associated with the former, while peralkaline granite massifs are characteristic of the latter. Geochronological results and geological relations between rocks of the bimodal and differentiated complexes showed first that rocks of the differentiated complex originated 350 to 330 Ma ago at the initial stage of forming of the marginal continental belt, linked with development active continental margin. This is evident from geochronological dates obtained for the Adzh-Bogd and Edrengiyn-Nuruu massifs and for volcanic associations of the complex. The dates are consistent with paleontological data. The bimodal association was formed later, 320 to 290 Ma ago. The time span separating formation of two igneous complexes ranges from several to 20-30 m.y. in different areas of the marginal belt. The bimodal magmatism was interrelated with rifting responsible for development of the Gobi-Tien Shan rift zone in the belt axial part and the Main Mongolian lineament along the belt northern boundary. Loci of bimodal rift magmatism likely migrated with time: the respective magmatic activity first initiated on the west of the rift system and then advanced gradually eastward with development of rift structures. Normal granitoids untypical but occurring nevertheless among the products of rift magmatism in addition to peralkaline massifs are assumed to have been formed, when the basic magmatism associated with rifting stimulated crustal anatexis and generation of crustal granitoid magmas under specific conditions of rifting within the active continental margin.

Kozlovsky, A. M.; Yarmolyuk, V. V.; Sal'Nikova, E. B.

2009-04-01

134

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

135

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

136

The Geodynamics Models of the Active Continental of Eurasia-Pacific Margins Transition Zone  

Microsoft Academic Search

The active continental margins of the transition zone from Eurasian continent to the Pacific Ocean are characterized by high seismicity, volcanic eruption and natural cataclysms hazardous for the people living there. It is also area of recent intense tectonic movements and hydrothermal processes, place of accumulation of different kinds of useful minerals. The task of the research is (1) to

Alexander Rodnikov; Natalia Sergeyeva; Ludmila Zabarinskaya

2010-01-01

137

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

Microsoft Academic Search

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

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

2009-01-01

138

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

Microsoft Academic Search

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

Roland von Huene; David W. Scholl

1991-01-01

139

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

Microsoft Academic Search

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

F. M. Gradstein; U. Von Rad

1990-01-01

140

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

141

Geological evolution of the late Mesozoic continental margin of Northwestern Anatolia  

Microsoft Academic Search

The Armutlu peninsula is a composite tectonic entity made up of sections of the Sakarya continent, the Rhodope-Pontide fragment and an ophiolite. These were assembled following a continental collision between Gondwanaland and Laurasia during the Late Cretaceous. The northern margin of the Sakarya continent underwent progressively increasing deformation prior to and during the advancing collision, due to continued convergence between

Y. Yilmaz; ?. C. Genç; E. Yi?itba?; M. Bozcu; K. Y?lmaz

1995-01-01

142

Late-Quaternary volcanism and transtensional tectonics in the Bay of Naples, Campanian continental margin, Italy  

Microsoft Academic Search

Summary ¶The Campanian continental margin is characterised by asymmetric half grabens and large-volume volcanic deposits. Vesuvius and Campi Flegrei are active volcanoes located along the coast of Naples Bay along one of these half grabens. The interpretation of an extensive set of seismic reflection data allowed to reconstruct the stratigraphy and structural pattern in Naples Bay and their relationships with

A. Milia; M. M. Torrente

2003-01-01

143

Plumes of bubbles release methane gas from the seabed along the West Spitsbergen continental margin  

Microsoft Academic Search

Over 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, at and above the upper limit of the gas hydrate stability zone (GHSZ), at depths of 150-400 m. Some plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane, and seismic reflection data indicate free

G. K. Westbrook

2009-01-01

144

Differential seafloor spreading of the North Atlantic and consequent deformation of adjacent continental margins  

Microsoft Academic Search

One of the main assumptions of the theory of plate tectonics is that all plates are rigid. However, in some plate reconstructions, the fits improve if the continents deform. Moreover, along parts of the North Atlantic continental margins, there is good evidence for post-rift deformation, in the form of inverted basins and compressional domes. Possible causes of these features are

Eline Le Breton; Peter R. Cobbold; Pierrick Roperch; Olivier Dauteuil

2010-01-01

145

Differential loading by prograding sedimentary wedges on continental margins: An arch-forming mechanism  

Microsoft Academic Search

Rapid deposition of prograding sedimentary wedges on continental margins will cause excess pore pressure, fluid flow, and compaction as they load the substratum. They may also cause faulting and structural deformation in the sedimentary succession below and in front of the load. Large depositional units are in addition associated with pronounced isostatic subsidence. The magnitude and effects each component has

A. Kjeldstad; J. Skogseid; H. P. Langtangen; K. Bjørlykke; K. Høeg

2003-01-01

146

A new Look at Gas Hydrate Occurrences on the Peruvian Continental Margin - ODP Site 1230  

Microsoft Academic Search

During Ocean Drilling Program (ODP) Leg 201, gas hydrates were recovered at Site 1230 on the Peruvian outer continental margin. Previous work at ODP Site 685, which is located in close proximity to Site 1230, suggested that gas hydrates occurred below 40 meter below sea floor (mbsf) in this region, but were concentrated in two intervals at about 107 and

T. H. Naehr

2002-01-01

147

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

Microsoft Academic Search

The East Siberian Arctic Continental Margin (ESAM) represents a rare case of rifting to spreading transition. Present-day geodynamics of this plate tectonic interplay is characterized by a very slow plate divergence in the Laptev Sea as this regions is located just landward of the slowest spreading center worldwide (the Gakkel Ridge), close to the pole of North American\\/Eurasian plate rotation.

S. S. Drachev

2003-01-01

148

Effect of thermal contrasts on gravity modeling at passive margins: Results from the western Barents Sea  

NASA Astrophysics Data System (ADS)

The western Barents Sea passive margin is a key locality to demonstrate the effect of the thermal structure of the lithosphere on forward gravity modeling. This margin developed by shear motion between the Eurasian and Greenland plates during the early Tertiary, and it is a significant border zone between young, hot oceanic lithosphere and cooler continental lithosphere. We construct two-dimensional gravity models of 125 km thick lithosphere based on expansion of mantle rocks determined from thermal modeling. The approach has a substantial impact over traditional shallow gravity models, here demonstrated on a previously published model. On the basis of a 140 mGal free-air anomaly, the old model proposes an anomalous, high-density oceanic crust emplaced in a leaky transform adjacent to the continent during early margin development. However, the lithospheric models predict a homogeneous oceanic crust, while preserving regional isostasy at base lithosphere from continent to ocean. Two further tests agree with this conclusion: A map of Bouguer corrected ERS-1 satellite data reveals no residual anomalies originating from the oceanic crust at the margin. Admittance analysis shows a strong oceanic lithosphere, and the high coherence between bathymetry and free-air gravity discounts a significant subsurface load. The high gravity anomalies at the margin are thus an edge effect, enhanced by sedimentation onto the strong oceanic lithosphere, and shaped by the effect of the lithospheric thermal field. Other results of this work include a new continent-ocean boundary map and two crustal transects across the margin.

Breivik, AsbjøRn Johan; Verhoef, Jacob; Faleide, Jan Inge

1999-07-01

149

Cenozoic transtensional to transpressional basin development at the sheared Western Barents Sea - Svalbard Margin  

NASA Astrophysics Data System (ADS)

The Western margin of the Barents Sea evolved as a sheared margin in response to early Cenozoic continental breakup and subsequent opening of the Norwegian-Greenland Sea. An Oligocene change in spreading direction led to oblique opening of the Fram Strait between Greenland and Eurasia and ended the transform margin stage. In such a sheared margin setting, sedimentation is strongly controlled by tectonic uplift and subsidence which vary in space and in time. The tectonic evolution of sedimentary basins associated with a sheared margin is determined by (1) the orientation of the margin with respect to the relative displacement of the plates, as well as (2) changes in spreading direction and (3) earlier tectonic events affecting the crustal thickness and shaping the basement grain. The Western Barents -- Svalbard margin may be subdivided in different segments based on its geometry. South of Bjørnøya, the margin forms a releasing bend with right stepping dextral shear zones and is characterized by transtension and pull-apart basin formation (Tromsø and Sørvestsnaget basins and Vestbakken volcanic province). Transpression was dominant in the restraining bend to the north, leading to the formation of the Paleocene-Eocene West Spitsbergen fold and thrust belt and its associated 'foreland basin'. Before it evolved into a sheared margin, the southern part of the margin experienced a series of post-Caledonian rift episodes which caused crustal thinning and formed the basement grain of major horst and graben structures trending in a generally NE-SW to N-S direction. The northern part, in contrast, has been largely unaffected by rifting since Carboniferous time. Besides the two main stages in the first order plate tectonic model for the region, recent refined models (e.g. Gaina et al., 2009) propose local complexities due to minor changes in the spreading direction. The margin segments are thus characterized by distinct structural and magmatic styles, expressed in the geometry and kinematics of the Cenozoic sedimentary basins along the margin. Not only the style, also the age of the basin forming events changes along strike due to the factors mentioned above: different areas are in the same tectonic setting at different points in time. Interpretation of seismic reflection and refraction data combined with insight from analogue and numerical modelling allowed us to highlight the differences and similarities in the evolution of the sedimentary basins in the different margin segments. Reference: Gaina, C., Gernigon, L. and Ball, P., 2009. Palaeocene-Recent plate boundaries in the NE Atlantic and the formation of the Jan Mayen microcontinent. Journal of the Geological Society, 166: 601-616.

Leever, Karen; Faleide, Jan Inge; Gabrielsen, Roy H.

2010-05-01

150

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

151

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

Microsoft Academic Search

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

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

2008-01-01

152

The Levant Slumps and the Phoenician Structures: collapse features along the continental margin of the southeastern Mediterranean Sea  

Microsoft Academic Search

Two distinct series of slumps deform the upper part of the sedimentary sequence along the continental margin of the Levant.\\u000a One series is found along the base of the continental slope, where it overlies the disrupted eastern edge of the Messinian\\u000a evaporites. The second series of slumps transects the continental margin from the shelf break to the Levant Basin. It

Yossi Mart; William Ryan

2007-01-01

153

The speciation of marine particulate iron adjacent to active and passive continental margins  

NASA Astrophysics Data System (ADS)

We use synchrotron-based chemical-species mapping techniques to compare the speciation of suspended (1-51 ?m) marine particulate iron collected in two open ocean environments adjacent to active and passive continental margins. Chemical-species mapping provides speciation information for heterogeneous environmental samples, and is especially good for detecting spectroscopically distinct trace minerals and species that could not be detectable by other methods. The average oxidation state of marine particulate iron determined by chemical-species mapping is comparable to that determined by standard bulk X-ray Absorption Near Edge Structure spectroscopy. Using chemical-species mapping, we find that up to 43% of particulate Fe in the Northwest Pacific at the depth of the adjacent active continental margin is in the Fe(II) state, with the balance Fe(III). In contrast, particulate iron in the eastern tropical North Atlantic, which receives the highest dust deposition on Earth and is adjacent to a passive margin, is dominated by weathered and oxidized Fe compounds, with Fe(III) contributing 90% of total iron. The balance is composed primarily of Fe(II)-containing species, but we detected individual pyrite particles in some samples within an oxygen minimum zone in the upper thermocline. Several lines of evidence point to the adjacent Mauritanian continental shelf as the source of pyrite to the water column. The speciation of suspended marine particulate iron reflects the mineralogy of iron from the adjacent continental margins. Since the solubility of particulate iron has been shown to be a function of its speciation, this may have implications for the bioavailability of particulate iron adjacent to passive compared to active continental margins.

Lam, Phoebe J.; Ohnemus, Daniel C.; Marcus, Matthew A.

2012-03-01

154

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

155

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

156

Molecular Diversity of Sulfate-Reducing Bacteria from Two Different Continental Margin Habitats  

PubMed Central

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

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

2003-01-01

157

Rifting to Spreading Process along the Northern Continental Margin of the South China Sea  

Microsoft Academic Search

Understanding the development from syn-rift to spreading in the South China Sea (SCS) is important in elucidating the western Pacific's tectonic evolution because the SCS is a major tectonic constituent of the many marginal seas in the region. This paper describes research examining the transition from rifting to spreading along the northern margin of the SCS, made possible by the

Yukari Kido; Kiyoshi Suyehiro; Hajimu Kinoshita

2001-01-01

158

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

159

The Formation of Non-Volcanic Rifted Margins by the Progressive Extension of the Continental Lithosphere  

NASA Astrophysics Data System (ADS)

Rifted margins include two main end-members: those termed "Volcanic Rifted Margins - VRMs" where magmatism is much more voluminous than predicted by passive asthenospheric upwelling (e.g. White et al., 1989), and those where magmatism is consistent or even less than the same predictions. The latter are termed "Non-Volcanic Rifted Margins - NVRMs" to emphasise the contrast with the VRMs: the name does not exclude the presence of minor amounts of magmatic activity. The NVRMs are typified by the North Biscay, south Australian, SW Greenland, and the West Iberian margins, which share a number of common characteristics: - extreme crustal thinning, increasing towards the ocean; - presence of well-defined rotated fault blocks. However at the feather edge of the continent there is an extension discrepancy: the amount that can be inferred from the geometry of these faults is far less than that indicated by the crustal thinning observed; - presence in places of a detachment fault at the base of the fault blocks; - little evidence for synrift magmatism; - the presence of a broad zone of partially serpentinised mantle (Boillot et al., 1988; Whitmarsh et al., 1996; Krawczyk et al., 1996; Pickup et al., 1996), both occurring beneath the highly thinned and faulted continental crust, and as a zone of exhumed continental mantle, now largely buried by postrift sediments. We show that such margins are the logical result of progressive extension of continental lithosphere above cool sub-lithospheric mantle. The key factors controlling the development of the margin are the rheological evolution of the crust (explaining the serpentinisation of the mantle), the occurrence of multiple phases of faulting (explaining the apparent extension discrepancy), and the temperature structure of the sub-continental mantle (explaining the lack of magmatism).

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

2003-12-01

160

From pull-apart basins to ultraslow spreading: Results from the western Barents Sea Margin  

NASA Astrophysics Data System (ADS)

This paper describes results from a geophysical study in the area between the ultraslow Knipovich Ridge and Bear Island, western Barents Sea. The objective was to map the crustal structure along a profile crossing a pull-apart rifted continental margin and oceanic crust generated by ultraslow spreading. The results are based on modeling of wide-angle seismic and gravity data, together with interpretation of multichannel reflection data. Our results show a two layered oceanic crust in the western part of the profile. The thickness of the oceanic crust is variable in the western 130 km, ranging from 3.5 to 5.5 km. East of km 130 the crustal thickness is relatively constant, with values close to the global average for oceanic crust. The oceanic crust is buried by a thick package of Cenozoic sedimentary rocks. The continent-ocean transition (COT) is placed in the interval 207-255 km, between unequivocal oceanic crust and the foot of the westernmost fault in the Hornsund Fault Zone. It is not possible to conclude whether this interval is oceanic crust or thinned and intruded continental crust, but we favor the latter interpretation, at least for the eastern part of the COT. Stretched continental crust is observed between Hornsund Fault Zone and the Knølegga Fault. Here the sedimentary rocks have high velocities and are interpreted to be mainly of Mesozoic and Late Paleozoic age. In this interval Moho depths increase abruptly from 15 km in the west to 27 km in the east. Crystalline basement velocities are observed close to the seafloor east of the Knølegga Fault. We suggest that continental breakup north of Greenland-Senja Fracture Zone occurred around 33 Ma, after a period of pull-apart tectonics. The spreading rate of the earliest seafloor spreading may have been higher than the present day spreading, creating thicker oceanic crust close to the COT.

Libak, Audun; Eide, Christian H.; Mjelde, Rolf; Keers, Henk; Flüh, Ernst R.

2012-01-01

161

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

Microsoft Academic Search

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

Karin Andreassen; Espen Glad Nilssen; Christian M. Ødegaard

2007-01-01

162

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

163

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

NASA Astrophysics Data System (ADS)

Submarine gullies are common features of high latitude continental slopes and, over the last decade, have been shown to play a key role in continental margin evolution, submarine erosion, downslope sediment transport, slope deposits, and the architecture of petroleum reservoirs. However, the processes that form these gullies, the timescales over which they develop, and the environmental controls influencing their morphology remain poorly constrained. We present the first systematic and comparative analysis between Arctic and Antarctic gullies with the aim of identifying differences in slope character, from which we infer differences in processes operating in these environments.

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

2013-11-01

164

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

NASA Astrophysics Data System (ADS)

The continental margin off northwestern Svalbard (between 79° and 80° N) has been affected by repeated mass wasting, as well as from northward-flowing contourite currents. One on the most striking morphological features is the approximately 120 km long, SE-NW oriented Kongsfjord Channel System (KCS) that can be traced from the shelf break at 250-300 m water depth to a maximum water depth of about 4000 m. Several channels/gullies starting at the shelf break merge into the main channel at around 1400 m. The main channel is at maximum 500 m wide and 80 m deep. Repeated interaction of the channel with contourite deposition has occurred. This includes relocation of the channel's axis related to the northward migration of the contourites, as well as renewed incision of contourites by activitiy within the channel. Two sediment cores were retrieved from either side of the main channel at 1430 m water depth, and two from about 1560 m water depth, respectively. The stratigraphy of these cores can easily be correlated to dated cores from the west Spitsbergen continental margin (Jessen et al., 2010). Preliminary results reveal that the sedimentation rates (SRs) at the shallower locations are generally higher than at the deeper stations during the past c. 30,000 years. At the sites from 1430 m water depth, the SRs were rather uniform during the past c. 10,000 years. However, prior to c. 10,000 cal. years BP, the SRs were significantly higher at the location of the core that was retrieved northeast of the channel. We assume that this is related to enhanced down-slope sediment transport within the channel and deposition to the right (northeast) of the transport direction - due to the Coriolis force and/or the northward-flowing contourite currents. The sources for sediment transport within the channel are most probably 1) enhanced sediment supply from the Kongsfjorden ice stream during the last glacial, and 2) winnowing of fine-grained material in the upper parts of the KCS and on the continental shelf following the retreat of the ice-stream front from the shelf break around 20,000 years ago. At the 1560 m water-depth sites, the SR was higher to the south of the channel prior to c. 14,000 cal. years BP, but higher to the north after c. 14,000 cal. years BP. This indicates that the northward migration of a contourite drift had the largest impact on the southern location prior to c. 14,000 cal. years BP. After c. 14,000 cal. years BP, down-slope sediment transport probably exceeded along-slope sediment transport, resulting in higher rates in the north. Reference: Jessen, S.P., Rasmussen, T.L., Nielsen, T. & Solheim, A., 2010. A new Late Weichselian and Holocene marine chronology for the westerns Svalbard slope 30,000-0 cal years BP. Quaternary Science Reviews 29, 1301-1312.

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

2011-12-01

165

Ordovician continental margin terranes in the Lachlan Orogen, Australia: Implications for tectonics in an accretionary orogen along the east Gondwana margin  

NASA Astrophysics Data System (ADS)

Four continental margin turbidite ± black shale terranes of the Lachlan Orogen in the southern Tasmanides of eastern Australia formed in two major systems along the east Gondwana margin and constrain the Ordovician assembly of this accretionary orogen. Key features are the dissimilar stratigraphies of the adjacent Bendigo and Melbourne terranes in the western system; the dissimilar stratigraphies of the adjacent Melbourne and Albury-Bega terranes that reflect juxtaposition of the two systems during the Middle Devonian, and the presence of the Albury-Bega Terrane both west and east of the Macquarie Arc in the eastern system that also includes the ocean floor Narooma Terrane and igneous ocean crust terrane(s). Repetition of the Albury-Bega Terrane either side of the arc requires either rifting or orogen-parallel, strike-slip duplication of a once contiguous package. Terrane interactions began in the earliest Gisbornian with early docking, uplift, deformation, and exchange of detritus. Amalgamation occurred in the earliest Silurian Benambran Orogeny with accretion in the Middle Devonian. Over 40 Myr, discrete turbidite terranes aligned along the Gondwana margin in two systems were converted into a very wide orogen characterized by the along-strike juxtaposition of superficially similar terranes.

Glen, R. A.; Percival, I. G.; Quinn, C. D.

2009-12-01

166

Necking of continental crust in magma-poor rifted margins: Evidence from the fossil Alpine Tethys margins  

NASA Astrophysics Data System (ADS)

Studies conducted in present-day magma-poor rifted margins reveal that the transition from weakly thinned continental crust (˜30 km) in proximal margins to hyper-extended crust (?10 km) in distal margins occurs within a narrow zone, referred to as the necking zone. We have identified relics of a necking zone and of the adjacent distal margin in the Campo, Grosina and Bernina units of the fossil Alpine Tethys margins and investigated the deformation and sedimentary processes associated with extreme crustal thinning during rifting. Within the basement rocks of the necking zone, we show that: (1) Grosina basement represents pre-rift upper/middle crust, while the underlying Campo unit consists of pre-rift middle/lower crust that was exhumed and cooled below ˜300°C by ca. 180 Ma, when rifting started to localize within the future distal margin; (2) the juxtaposition of the Campo and Grosina units was accommodated by the Eita shear zone, which is interpreted as a decollement/decoupling horizon active at mid-crustal depth at 180-205 Ma; (3) the Grosina unit hosts a large-scale brittle detachment fault. Our observations suggest that crustal thinning, accommodated through the necking zone, is the result of the interplay between detachment faulting in the brittle layers and decoupling and thinning in ductile quartzo-feldspatic middle crustal levels along localized ductile decollements. The excision of ductile mid-crustal layers and the progressive embrittlement of the crust enables major detachment faults to cut into the underlying mantle, exhuming it to the seafloor. This structural evolution can explain the first-order crustal architecture of many present-day rifted margins.

Mohn, G.; Manatschal, G.; Beltrando, M.; Masini, E.; Kusznir, N.

2012-02-01

167

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

NASA Astrophysics Data System (ADS)

The thermohaline circulation in the Argentine Basin today is characterized by the interaction of northward flowing Antarctic water masses (Antarctic Intermediate Water, AAIW; Circumpolar Deep Water, CDW; Antarctic Bottom Water, AABW) and southward flowing North Atlantic Deep Water (NADW). The transfer of heat and energy via both AABW and NADW constitutes an important component in maintaining the global conveyor belt. We aim at a better understanding of both paths and intensity of this current system in the past by investigating an extensive (> 11000 km) set of high quality seismic reflection profiles from the Argentine continental margin. The profiles show a significant contourite system containing both erosive and depositional features that formed through the evolution of water masses and their modifications (path, physical and chemical properties) due to plate tectonic events such as the opening of the Drake Passage or the extensive emplacement of volcanic flows at the Rio Grande Rise. Overall the depositional features indicate that along slope (contour current) transport dominates over down slope (turbiditic) processes at the southern Argentine margin south of 45° S. Further to the North down slope transport was more extensive as indicated by the presence of submarine canyons crossing the slope down to a depth of ~3500 m. Here we present preliminary results from the southern part of the continental margin (42°-50° S) where we focus on a set of ~50 km wide terraces on the slope and rise separated by contouritic channels. The terraces developed over time in alternating constructional (depositional) and erosive phases. An initial age frame was developed by mapping regional reflectors and seismic units known from previous studies. The sedimentary layer between regional reflectors AR 4 and AR 5 spanning roughly the time interval from the Eocene/Oligocene boundary to the early middle Miocene is thin (0.1 - 0.4 s TWT) below the Valentine Feilberg Terrace but thickens towards the East forming a giant buried drift and also towards the West building a unit of plastered drifts below the Piedra Buena Terrace. Here, the maximum thickness of this unit is ~1.4 s (TWT). In contrast to this the sediments of late Miocene to recent age are very thin or completely eroded over the Piedra Buena terrace but form drifts at the Valentin Feilberg terrace that can be further divided into subunits whose reflections have stratified facies with good lateral continuity. Mounded drift structures on the western and eastern edges of the terrace are bounding an onlap fill structure possibly associated with bottom currents of reduced activity. With an assumed age of ~15 Ma for reflector AR5 the average sedimentation rate since the middle Miocene is estimated to be > 10 cm/ka and thus would make a drill site on the terrace suitable for high resolution palaeoclimate studies.

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

2010-05-01

168

The Geodynamics Models of the Active Continental of Eurasia-Pacific Margins Transition Zone  

NASA Astrophysics Data System (ADS)

The active continental margins of the transition zone from Eurasian continent to the Pacific Ocean are characterized by high seismicity, volcanic eruption and natural cataclysms hazardous for the people living there. It is also area of recent intense tectonic movements and hydrothermal processes, place of accumulation of different kinds of useful minerals. The task of the research is (1) to study the deep structure and processes in the subduction and paleosubduction zones, beneath seismic hazardous zones, volcanic areas, mineragenetic fields, and sedimentary basins; (2) to determine the role of the deep processes going on in the upper mantle in the formation of major structural units of active continental and oceanic margins; (3) the study of the deep causes of geological phenomena to assess, predict and mitigate the natural disasters, specifically earthquakes, and volcanic eruptions. The distinctive feature of the deep structure of the continental margins is the presence of an asthenosphere in the upper mantle. The tectonically active regions, such as the island arcs and the rifts of the marginal seas, correlate with a thick asthenosphere. The asthenosphere resides in a depth of 50-80 km under the old Paleogene deep basins of the marginal seas, at about 30 km under the Neogene basins, and at a depth of 10-20 km under the Pliocene-Quaternary and recent inter-arc basins, causing the breaks of the lithosphere, the formation of rifts, basalt lava eruptions, and hydrothermal activity. The asthenospheric diapirs are marked on the surface by rift formations and mainly tholeiitic magma eruption. Rifts in the marginal seas and island arcs may by accompanied by intense mineralization. The asthenospheric diapirs represent the channels by which hot mantle fluids from the asthenosphere penetrate to the geological structures of the transition zone from Eurasian Continent to the Pacific Ocean. The work was supported by Russian Foundation for Basic Research. No 09-05-00406-a

Rodnikov, Alexander; Sergeyeva, Natalia; Zabarinskaya, Ludmila

2010-05-01

169

Prebreakup sedimentary and volcanic sequences on rifted Continental Margin of Wilkes Land, Antarctica  

SciTech Connect

The transition zone from continental crust to oceanic crust on the Wilkes Land, Antarctica, margin was surveyed with 24-fold multichannel seismic reflection along 1800 line-km in 1984. The northward transition from extended continental crust to normal oceanic crust is characterized by a progression through three distinct volcanic sequences. The first, deposited on extremely thin continental crust, is interpreted to be a flood basalt, presumably deposited during the late rifting stage. The second, which overlaps the first at its northern end, is a seaward-dipping reflector sequence that forms anomalously thick oceanic crust. The third, which in turn onlaps the second, is normal oceanic crust with a rough upper surface and no internal reflectors.

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

1986-07-01

170

Neogene rotations and quasicontinuous deformation of the Pacific Northwest continental margin  

SciTech Connect

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

England, P. (Oxford Univ. (England)); Wells, R.E. (Geological Survey, Menlo Park, CA (United States))

1991-10-01

171

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

NASA Astrophysics Data System (ADS)

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

Batang, Zenon B.; Papathanassiou, Evangelos; Al-Suwailem, Abdulaziz; Smith, Chris; Salomidi, Maria; Petihakis, George; Alikunhi, Nabeel M.; Smith, Lloyd; Mallon, Francis; Yapici, Tahir; Fayad, Nabil

2012-06-01

172

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

173

pCO2 Dynamics on the Western Canadian Coastal Margin  

NASA Astrophysics Data System (ADS)

There is large uncertainty in sea-air CO2 fluxes from coastal margins due to sparse data sets that inadequately resolve the extreme temporal and spatial variability inherent in these regions. The western Canadian shelf is no exception to this, and it is thought to be part of an important high latitude sink region for atmospheric CO2, balancing lower latitude source regions along the North American continental margin. Model results suggest that autumn and winter are likely important outgassing seasons that counterbalance summertime drawdown of CO2 in this region. Here we present new summertime, and the first autumn and winter pCO2 measurements from the western Canadian coastal margin. The highest levels of oversaturated pCO2 observed on the shelf were during autumn, although outgassing was also evident during winter. Summer measurements were below atmospheric levels over most of the west Canadian shelf. pCO2 was heavily oversaturated within the tidally mixed straits around Vancouver Island during all seasons. It is likely important to include fluxes from inland waterways in annual estimates of sea-air CO2 flux from this geographically complex coastal region because they, in addition to non-upwelling season fluxes, may act to counterbalance summertime drawdown on the shelf.

Evans, W.; Hales, B. R.; Ianson, D. C.; Strutton, P. G.

2010-12-01

174

Lithospheric thermal-rheological structures of the continental margin in the northern South China Sea  

Microsoft Academic Search

Thermal structures of three deep seismic profiles in the continental margin in the northern South China Sea are calculated,\\u000a their “thermal” lithospheric thicknesses are evaluated based on the basalt dry solidus, and their rheological structures are\\u000a evaluated with linear frictional failure criterion and power-law creep equation. “Thermal” lithosphere is about 90 km in thickness\\u000a in shelf area, and thins toward

Xiaobin Shi; Di Zhou; Yixiang Zhang

2000-01-01

175

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

Microsoft Academic Search

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

Taras Gerya; Bernhard Stöckhert

2006-01-01

176

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

Microsoft Academic Search

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

Konstantinos Skourlis; Theodor Doutsos

2003-01-01

177

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

Microsoft Academic Search

We present the first comprehensive study of mass wasting processes in the continental slope of a convergent margin of a subduction zone where tectonic processes are dominated by subduction erosion. We have used multibeam bathymetry along ˜1300 km of the Middle America Trench of the Central America Subduction Zone and deep-towed side-scan sonar data. We found abundant evidence of large-scale

Rieka Harders; César R. Ranero; Wilhelm Weinrebe; Jan H. Behrmann

2011-01-01

178

Mineralization, Magmatism, and Continental Margin Transtension in the Jiaodong Gold Province, Eastern China  

Microsoft Academic Search

The Jiaodong gold province is the largest gold repository in China. Both mineralization and granitoid hosts are spatially related to the crustal-scale Tan-Lu strike-slip fault system, which developed along the Mesozoic continental margin in eastern China. A series of 40 Ar\\/ 39 Ar laser incremental heating analyses of hydrothermal sericite\\/muscovite from three major gold deposits (Jiaojia, Xincheng, and Wangershan) and

Jian-Wei Li; Paulo M. Vasconcelos; Jun Zhang; Mei-Fu Zhou; Xiao-Jun Zhang; Feng-Hua Yang

179

Tectonics and crustal structure of the Campania continental margin: relationships with volcanism  

Microsoft Academic Search

Summary ¶The crustal structure of the Campania continental margin is synthesized from outcrop, seismic reflection and gravimetric data. Outcrop and subsurface geological data reveal the presence of NE–SW faults, E–W faults and NW–SE faults. An older extensional event occurred along NW–SE faults and was followed by the main extensional event linked to the activity of NE–SW normal faults. The latter

A. Milia; M. M. Torrente; M. Russo; A. Zuppetta

2003-01-01

180

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

Microsoft Academic Search

Dolomite is presently forming at rates less than 500 m\\/m.y. in continental margin marine sediments having organic carbon contents greater than 0.5 wt. %. At higher sedimentation rates, the dolomite content of the sediments is greatly diluted. At lower organic carbon content, the pore-water chemistry precludes dolomite formation. The reactants for dolomite formation have the following sources: magnesium supplied from

P. A. Baker; S. J. Burns

1985-01-01

181

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

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

182

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

183

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

184

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

NASA Astrophysics Data System (ADS)

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

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

2011-06-01

185

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

186

From Crustal Thinning to Continental Break-up in Magma-Poor Rifted Margins: How Important are Detachment Faults?  

NASA Astrophysics Data System (ADS)

Detachment faults are widely regarded as playing an important role in crustal thinning and mantle exhumation at magma-poor rifted margins. However, how important are these structures and what is their role in crustal thinning and mantle exhumation? In our presentation, we review the pertinent observations made along the Iberia-Newfoundland, Western Pyrenees-Bay of Biscay and the Alpine margins. A reconstruction of the future distal margins 10 to 20 myr before continental break-up shows that: (1) extension was localized and the crust was thinned to less than 10 km despite subdued high-angle normal faulting, (2) detachment faults responsible for mantle unroofing are late and shallow crustal structures, (3) the mantle lithosphere was locally omitted and replaced by infiltrated asthenospheric mantle, and (4) shallow marine sediments were deposited over thinned crust suggesting a retardation of tectonic subsidence. These observations scrutinize our present knowledge of rift processes and ask for a paradigm shift in the way to interpret rifted margins. To further investigate the role of detachment faulting during crustal thinning and mantle exhumation, we developed a numerical model that: (1) uses initial conditions, strain distribution, rheology and deformation modes constrained from the study of Alpine margins, and (2) is able to replicate the well-documented tectonic evolution of the Iberia and Alpine margins, starting with pure-shear dominated crustal stretching and ending with mantle exhumation along detachment faults. The most interesting result of the experiment is the way the model evolves from the prescribed initial distributed stretching (e.g. pure-shear) to final localized exhumation (e.g. simple shear). During this transitional phase, referred to as thinning phase, extension is accommodated by a system of superimposed but decoupled concave-downward faults that simultaneously exhume middle crust to the seafloor and deeper mantle at the base of the crust. The two rolling hinges eventually merge to form one concave downward fault that unroofs the mantle to the seafloor. These results suggest that exhumation associated with detachment faulting is an important process. It can explain crustal thinning to less than 10 km; the lack of major fault bounded topography; and the emplacement of deep and infiltrated mantle rocks beneath thinned crust 10 to 20 myr before continental break-up. This has important consequences for the isostatic, thermal and rheological evolution of deep magma-poor margins that need to be constrained by data. At present, the thinning mode is neither understood nor constrained by geological or geophysical data. Because the proximal and distal margins are mainly preserving structures of the initial stretching and the final exhumation modes, we started to investigate the transition zone between the proximal and distal margins in the Alps and the zone ahead of a propagating ocean in the Western Pyrenees - Bay of Biscay. In our presentation, we present evidence for the existence of the thinning mode within these two domains and show how these results help to develop new conceptual ideas to re-interpret seismic data from present-day magma-poor rifted margins.

Manatschal, G.; Lavier, L. L.; Péron-Pinvidic, G.; Jammes, S.; Mohn, G.; Muntener, O.

2007-12-01

187

Non-Energy Resources and Shallow Geological Framework of the Inner Continental Margin Off Ocean City, Maryland.  

National Technical Information Service (NTIS)

As part of the seventh year of the Minerals Management Service -Association of American State Geologists Continental Margin Program, the Maryland Geological Survey examined over 300 kilometers of high resolution seismic profile records and lithological lo...

D. V. Wells

1994-01-01

188

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

189

The Acatlan Complex, Southern Mexico: Ordovician-Devonian Passive Margin on the Southern Rheic Ocean Margin and Carboniferous-Permian Subduction on Western Pangea  

NASA Astrophysics Data System (ADS)

New structural, geochronological and geochemical data from the Acatlan Complex of southern Mexico shows it preserves a history of Cambro-Ordovician rifting along the southern margin of the Rheic Ocean, followed by Devono-Carboniferous subduction and exhumation and development of a Permo-Triassic transtensional arc on the eastern Pacific margin. The oldest rocks are represented by low-grade, siliciclastics intruded by bimodal, rift- related, mafic and felsic plutons and continental rift tholeiitic dikes ranging in age from ca.480 to 440 Ma. Greenschist facies psammites and pelites containing detrital zircons as young as ca.360 Ma represent continental rise deposits that contain tectonic slices of mafic rocks with oceanic tholeiitic and alkalic affinities. Tectonically interleaved with these greenschist facies rocks are slices of high pressure eclogite with a continental rift tholeiitic protolith, blueschist mafic rocks, high grade rift-passive margin, metasedimentary rocks, ca.470-420 Ma granitoid rocks, and periarc serpentinite. Concordant U-Pb zircon data on one of these eclogites yields an age of ca.346 Ma interpreted as the age of metamorphism that was closely followed by rapid decompression migmatization and cooling through ca.350°C by ca.350 Ma (40Ar/39Ar muscovite data). Carboniferous to Middle Permian exhumation of these high grade rocks occurred after the amalgamation of Pangea and during: (1) deposition of continental to shallow marine rocks with two pulses of fault-related conglomerates in the lower Mississippian (Kinderhookian) and Lower Permian (Leonardian); (2) listric normal deformation under greenschist facies conditions dated at ca.347 Ma and ca.300 Ma (40Ar/39Ar muscovite and amphibole data, respectively), and associated with either core complex development or extrusion; (3) Permian arc magmatism that extends along the western margin of Pangea through Mexico; and (4) Permian, dextral transtensional deformation. The western margin of Pangea during the Triassic changes from rift and passive margin in northern Mexico to S-vergent thrust and associated clastic wedge in the Acatlan Complex consistent with oblique dextral convergence between the Pacific and Pangean plates.

Keppie, J. D.; Ramos-Arias, M.; Morales-Gamez, M.; Nance, R. D.; Miller, B. V.; Murphy, J. B.; Dostal, J.; Ortega-Rivera, A.; Lee, J. W.

2007-05-01

190

Seismic velocity structure across the Eastern Newfoundland (Grand Banks) and Iberia Abyssal Plain Continental Margin Conjugates  

NASA Astrophysics Data System (ADS)

We acquired coincident wide-angle and multi-channel seismic (MCS) data over three transects across the Eastern Grand Banks/Newfoundland Basin (NB) in July-August, 2000. This non-volcanic continental margin is conjugated to the Galicia Bank/Iberia Abyssal Plain (IAP) margins. The objective was to determine structures reaching from full thickness continental crust seaward to unequivocal oceanic crust, and to compare these with their conjugates in order to better constrain the nature of the extension. Reported here is the southernmost transect (Transect 3), which is conjugated to the southern IAP near profile IAM-9. Over this 565km-long profile, we recorded wide-angle reflection/refraction data on 23 ocean bottom seismometers. The seismic velocity structures from the beginning of the profile eastward to the landward limit of the Carson Basin can be interpreted as full thickness (35km) typical Newfoundland Appalachian continental crust. The crust thins abruptly beneath the Carson Basin to ~7km (extension factor = 5) over a distance of 50km and then gradually to < 3km thickness over the next 60km, beneath the NB. This thinning is dominated by the removal of the lower continental crust (extension factor = ~ 8.0). Crustal thinning of the IAP margin is relatively gentler (from 28km to 8km thickness over a distance of 90km). Similar to the IAP margin, a wide layer (~200km) of low velocity mantle (7.5-8.0km/s) is observed beneath the extremely thin (< 5km) crust of the NB up to 15km below sea level. These velocities are interpreted as of serpentinized mantle. A transition zone ~80km wide is found seaward of the extremely thin continental crust (beginning at ~150km from the shelf break) where the serpentinized mantle is observed very close to the basement. The thin 'crust' (~ 2km) over this zone is poorly resolved but is consistent with a high gradient layer similar to that of the IAP transition interpreted as exhumed mantle. The transition zone ends at ~230km from the shelf break without evidence of a ridge province similar to that observed at the seaward end of IAP transition zone. The presence of oceanic layer 2 (4.7-6.3km/s) and layer 3 (6.3-7.4km/s) seaward defines the oceanic crust, and the serpentinized-mantle layer pinches out. The MCS reflection data also show changes in reflection character of basement that match the changes in velocity structure outlined above. This velocity model, therefore, further supports observations in reflectivity to show that the conjugates are similar in gross scale but different within individual structural zones.

Lau, K. W. H.; Louden, K. E.; Funck, T.; Hall, J.; Deemer, S.; Tucholke, B.; Holbrook, W. S.; Larsen, H. C.; Hopper, J.

2003-04-01

191

Quantifying Continental Margin Deformation North and South of the Opening of the Gulf of California—Evidence for Subduction Erosion?  

NASA Astrophysics Data System (ADS)

A striking contrast exists in the geology of the continental margin north and south of the opening of the Gulf of California. For 1200km north of the Gulf opening, a reasonably well-preserved forearc region is associated with strongly magnetic 130-100 Ma plutonic rocks, volcanics and ophiolitic basement. This package of rocks is also coupled with a weakly magnetic eastern belt of 100-90 Ma tonalite and trondhjemite. In contrast, the southern continental margin bound by the Acapulco Trench lacks both a forearc basin and the 130-100 Ma plutonic belt. Instead, Late Cretaceous and Early to Middle Cenozoic granitoids crop out both along the coast and in the offshore region east of the Acapulco Trench, extending 1300 km south to 16°N. The geologic contrast between the northern and southern segments has been attributed to Cenozoic subduction erosion of the latter. The transition region between these two regimes occurs ~50 km northwest of the Gulf opening at 23°N along the Pacific coast of Baja California. North of Todos Santos, Late Cretaceous and younger strata depositionally overlie the western margin of the 130-100 Ma plutonic belt. The plutonic rocks are well-expressed in aeromagnetic images, even where buried. South of Todos Santos, 100-90 m.y. granitoids crop out along the coast and underlie the offshore Todos Santos basin; the strongly magnetic basement and undeformed forearc rocks are absent. Near Todos Santos, 94 Ma orthogneiss intercalated with upper amphibolite facies (garnet ± andalusite) Late Triassic(?) wallrocks are structurally juxtaposed above epidote amphibolite calc-schist of the mid-Cretaceous accretionary complex, suggesting significant forearc deformation. Previous 40Ar/39Ar data gathered south of Todos Santos reveal 5-10 m.y. of discordance between hornblende and biotite in the 100-90 m.y. granitoids. The age discordance could be related to either transient heating from 83-68 Ma granitoids located to the east or subduction erosion deformation similar to that described along the Acapulco margin. To determine the cause of discordance, we are integrating monazite (U-Th)/He thermochronology and geochemical analysis with K-feldspar multi-domain diffusion modeling and other low-temperature thermochronology to assess the magnitude of denudation. An increase in the magnitude of denudation towards the continental margin would indicate that subduction erosion is continuous along the southern continental margin for 1300 km. Alternatively, if the discordance were caused by thermal heating, the magnitude of denudation would be more homogeneous. To further understand the contrast in batholith denudation across the north-south boundary, these data will be compared with thermal history results from the westernmost batholith exposures in the La Paz region, ~100 km to the north, where the forearc is still preserved.

Peterman, E. M.; Grove, M.; Kimbrough, D. L.

2010-12-01

192

The Indian Ocean-type isotopic signature in western Pacific marginal basins: Origin and significance  

NASA Astrophysics Data System (ADS)

Basalts forming the floor of the western Pacific marginal basins sampled by DSDP and ODP legs and dredging cruises over the past 20 years have isotopic characteristics like those of basalts erupted at Indian Ocean spreading ridges, distinct from those of basalts from Pacific Ocean ridges. The basins, which include the Yamato and Japan Basins of the Sea of Japan, the West Philippine, Shikoku and Parece Vela Basins and Mariana Trough of the Philippine Sea Plate, the South China Basin, the Celebes Sea, the Sulu Sea and the Lau Basin, range in age from >60 to O Ma and have differing tectonic histories. The origin of the Indian Ocean-type isotopic signature in basalts from these basins has been investigated by examining and comparing geochemical patterns found within each basin in the light of the tectonic processes, such as subduction and continental rifting, involved in basin development. Results suggest that the basins tapped a shared asthenospheric mantle source that is related to the source of Indian Ocean ridge basalts, but which is modified by local processes in some cases. Based on this conclusion, it is likely that the Indian Ocean asthenospheric domain extends beneath the western Pacific marginal basins. Flow of Indian Ocean-type asthenosphere into this region may have occurred in two stages: initial flow to the north of Australia, followed by displacement to the northwest, north, northeast, and southeast during the rapid northward motion of the Australian continent after 50 Ma.

Hickey-Vargas, Rosemary; Hergt, Janet M.; Spadea, Piera

193

Kinematic and thermal evolution of the Moroccan rifted continental margin: Doukkala-High Atlas transect  

NASA Astrophysics Data System (ADS)

The Atlantic passive margin of Morocco developed during Mesozoic times in association with the opening of the Central Atlantic and the Alpine Tethys. Extensional basins formed along the future continental margin and in the Atlas rift system. In Alpine times, this system was inverted to form the High and Middle Atlas fold-and-thrust belts. To provide a quantitative kinematic analysis of the evolution of the rifted margin, we present a crustal section crossing the Atlantic margin in the region of the Doukkala Basin, the Meseta and the Atlas system. We construct a post-rift upper crustal section compensating for Tertiary to present vertical movements and horizontal deformations, and we conduct numerical modeling to test quantitative relations between amounts and distribution of thinning and related vertical movements. Rifting along the transect began in the Late Triassic and ended with the appearance of oceanic crust at 175 Ma. Subsidence, possibly related to crustal thinning, continued in the Atlas rift in the Middle Jurassic. The numerical models confirm that the margin experienced a polyphase rifting history. The lithosphere along the transect preserved some strength throughout rifting with the Effective Elastic Thickness corresponding to an isotherm of 450°C. A mid-crustal level of necking of 15 km characterized the pre-rift lithosphere.

Gouiza, M.; Bertotti, G.; Hafid, M.; Cloetingh, S.

2010-10-01

194

Deformation Kinematics of Rifted Continental Margin Lithosphere From Measured Bathymetry, Gravity and Upper Crustal Extension Using a New Model of Sea Floor Spreading Initiation  

Microsoft Academic Search

We apply inverse methods to a two-dimensional coupled fluid-flow thermal model of sea floor spreading initiation and rifted continental margin formation in order to determine the rifted margin lithosphere deformation history and structure. The model assumes that stretching of continental lithosphere leading to breakup and sea floor spreading initiation is generated by an upwelling divergent flow field within continental lithosphere

D. Healy; N. Kusznir

2004-01-01

195

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

Microsoft Academic Search

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

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

2003-01-01

196

Recent sediments, sediment accumulation and carbon burial at Goban Spur, N.W. European Continental Margin (47–50°N)  

Microsoft Academic Search

To quantify recent sediment accumulation, carbon fluxes and cycling, three N.W. European Continental Margin transects on Goban Spur and Meriadzek Terrace were extensively studied by repeated box- and multicore sampling of bottom sediments. The recent sediment distribution and characteristics appear directly related to the near-bed hydrodynamic regime on the margin, which at the upper slope break on the Goban Spur

Tj. C. E. van Weering; I. R. Hall; H. C. de Stigter; I. N. McCave; L. Thomsen

1998-01-01

197

Role of carbonate platforms, continental margin sediments/volcanics and oceanic units in Tethyan assembly: evidence from the Eastern Taurides, Turkey  

NASA Astrophysics Data System (ADS)

We focus here on the palaeotectonic development of the Eastern Tauride region in its wider regional setting related to the opening and closure of Neotethys. Continental margin-type, ophiolitic and melange units are widely exposed in eastern central Turkey (Gürün, Darende, Hekimhan and Pinarba?? areas). These units restore as a Triassic rifted continental margin that underwent passive margin subsidence during Jurassic-Early Cretaceous. Ophiolitic rocks (e.g. P?narba??, Gürün area, Kuluncak, Hekimhan, Divri?i) formed by spreading above a northward-dipping intra-oceanic subduction zone during Late Cretaceous time. Melanges associated with the emplaced ophiolites are interpreted as parts of the former rifted margins and the early formed (Triassic) oceanic crust that were assembled into an accretionary prism during latest Cretaceous time. The emplacement of the continental margin units, melanges and ophiolites onto the Eastern Tauride platform (e.g. Gürün Autochthon) was driven by trench-margin collision during latest Cretaceous time (Campanian-Maastrichtian). The allochthonous units were re-thrust further south during Early to Mid-Eocene time related to regional continental collision. Eocene thrusting strongly affected the western (P?narba??) and central (Gürün) areas of the region studied, whereas areas further east mainly experienced folding. Specifically, there is little evidence of Eocene thrusting Darende Basin in the east. This suggests that the thrusting was focused by the collision of irregularly shaped microcontinents while intervening areas remained less affected. The complex present-day Eastern Tauride outcrop then reflects post-collisional suture tightening (Miocene) and Plio-Quaternary strike-slip (transpression/transtension) related to the westward 'escape' of Anatolia. As a result of this, different levels of the thrust stack are commonly juxtaposed complicating reconstruction. After taking account of several alternative tectonic models, we infer that the Eastern Taurides experienced a common tectonic development with the Central and Western Taurides during Mesozoic-Early Cenozoic time. However, the geological history diverged after Eocene ocean closure mainly as a result of the contrasting neotectonic deformation in different areas. We conclude that all of the oceanic units (melanges and ophiolites) in the region studied relate to the rifting and closure one particular strand of Neotethys, namely the Inner (Intra) Tauride ocean, while the wider ?zmir-Ankara-Erzincan ocean was located further north adjacent to Eurasia. The Eastern Tauride oceanic and continental units therefore evolved separately from the better known Southern Neotethys that opened and closed further south adjacent to Arabia

Robertson, A. H. F.; Metin, Y.; Parlak, O.; Vergili, O.; Tasl?, K.; Inan, N.; Soycan, H.

2012-04-01

198

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

199

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

NASA Astrophysics Data System (ADS)

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

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

2010-02-01

200

Seismic imaging of a convergent continental margin and plateau in the central Andes (Andean Continental Research Project 1996 (ANCORP'96))  

NASA Astrophysics Data System (ADS)

A 400-km-long seismic reflection profile (Andean Continental Research Project 1996 (ANCORP'96)) and integrated geophysical experiments (wide-angle seismology, passive seismology, gravity, and magnetotelluric depth sounding) across the central Andes (21°S) observed subduction of the Nazca plate under the South American continent. An east dipping reflector (Nazca Reflector) is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km. We interpret parts of the Nazca Reflector as a fluid trap located at the front of recent hydration and shearing of the mantle, the fluids being supplied by dehydration of the oceanic plate. Patches of bright (Quebrada Blanca Bright Spot) to more diffuse reflectivity underlie the plateau domain at 15-30 km depth. This reflectivity is associated with a low-velocity zone, P to S wave conversions, the upper limits of high conductivity and high V p /V s ratios, and to the occurrence of Neogene volcanic rocks at surface. We interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during Neogene shortening and plateau growth. The imaging properties of the continental Moho beneath the Andes indicate a broad transitional character of the crust-mantle boundary owing to active processes like hydration of mantle rocks (in the cooler parts of the plate margin system), magmatic underplating and intraplating under and into the lowermost crust, mechanical instability at Moho, etc. Hence all first-order features appear to be related to fluid-assisted processes in a subduction setting.

ANCORP Working Group,

2003-07-01

201

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

202

Subduction of spreading ridges as a factor in the evolution of continental margins  

NASA Astrophysics Data System (ADS)

The subduction of spreading ridges creates a special geodynamic setting distinguished by the interference of convergent and divergent boundaries between lithospheric plates and their long-term interaction accompanied by the formation of characteristic geological complexes and structures. The available data on subduction of the contemporary Chile Ridge make it possible to reconstruct such settings in the geological past. The subduction of the spreading ridge leads to uplift of the continental margin, cut off the accretionary wedge by means of tectonic erosion, emplacement of a fold-thrust structure and longitudinal strike-slip faults, and creates settings favorable for obduction of the young oceanic lithosphere. A lithospheric window expressed in geological and geophysical features opens beneath the continental margin at the continuation of the ridge axis. The subduction-related volcanic activity ceases above this window, giving way to specific proximal magmatism close to the boundary with the ocean and distal magmatism at a distance from this boundary. The proximal bimodal magmatism was related to the sources of tholeiitic basalts characteristic of the ridge involved in subduction and to the partial melting of its oceanic crust and sediments. The distal basaltic magmatism is a product of melting of the fertile oceanic asthenosphere ascending through the lithospheric window with subsequent transformation of magma in the mantle wedge and the continental crust. The use of the Chilean tectonotype for paleoreconstructions is limited by the diverse settings of ridge subduction. The Paleogene magmatism at the Pacific margin of Alaska, where the kinematics of subduction was close to the Chilean subduction, is similar to the proximal igneous rocks of Chile in composition and zoning, retaining some geological differences. Another aspect of the paleoreconstruction is discussed on the basis of Jurassic and Cretaceous granitoids of the Ekonai Terrane of the Anadyr-Koryak System and terranes of southern Alaska. These localities are known for a special, accretionary type of granitoids in the forearc region related to anatectic magma formation without participation of the plunging ridge. Proceeding from comparison with the Chilean tectonotype, the criteria for the identification of granitoids varying in their origin are considered. The effect of subducting ridges on continental margins changed over geologic time and was subject to the rhythm of supercontinental cycles.

Lomize, M. G.; Luchitskaya, M. V.

2012-01-01

203

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

204

Seismic stratigraphy of the central Scotian Rise: A record of continental margin glaciation  

NASA Astrophysics Data System (ADS)

3.5-k Hz profiles from low channel levees on the Scotian Rise show transparent Holocene acoustic facies overlying stratified glacial facies, dated by Carbon-14 in cores. Corresponding acoustic facies in seismic records are correlated with glaciations by “counting back” from the present sea floor using sedimentation rates from Carbon-14 dating and biostratigraphy from wells as a guide. The regional thickness and character of the seismic units correlate with the duration and intensity of glacial periods inferred from the global isotopic record. Changes in glacial supply to the continental margin are interpreted using this chronology, which shows stage 12 as the first widespread erosional glacial event.

Berry, J. A.; Piper, D. J. W.

1993-12-01

205

Tectonic significance of Synrift sediment packages across the Congo continental margin  

SciTech Connect

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

McGinnis, J.P.; Karner, G.D.; Driscoll, N.W. (Lamont-Doherty Geological Observatory, Palisades, NY (United States)); Brumbaugh, W.D. (Conoco, Worldwide Exploration Services, Houston, TX (United States)); Cameron, N. (Conoco Ltd., London (United Kingdom))

1993-09-01

206

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

207

Speciation and export of particulate iron from the Northwest African continental margin into the water column  

NASA Astrophysics Data System (ADS)

The tropical North Atlantic receives the highest inputs of mineral dust in the world oceans, and this aeolian supply is thought to be the primary source of iron to this region. Both the path and magnitude of atmospheric dust delivery change seasonally, however, allowing the possibility that lateral input of iron from the continental margin may be important at some times of the year. Here we use a new technique called chemical mapping that allows a quick and statistically meaningful assessment of the speciation of marine particulate iron in a sample. Chemical mapping is a powerful synchrotron x-ray technique that incorporates the chemical speciation insight from X-ray Absorption Spectroscopy with the wealth of spatial and distribution information from XRF mapping. We mapped African dust aerosol samples and sediment samples from the African continental margin, the two putative end-member sources of iron to the Eastern Tropical North Atlantic. The resultant chemical maps show that 40% of the particulate iron in a sediment sample was pyrite, with the balance consisting of Fe(III)-compounds (40%) and Fe(II)-silicates (20%). In contrast, pyrite was absent in African dust, which was composed of Fe(III)-compounds (85%) and Fe(II) silicates (15%). The presence of pyrite in water column marine suspended particles thus indicates a margin source of iron. We determined the speciation of marine particulate iron in a coastal to open ocean transect between the Mauritanian upwelling region to the Tropical Eastern North Atlantic Time-Series Observatory (TENATSO) in September 2008. We found the presence of pyrite in suspended marine particles as far as the TENATSO station, ~900km from the Mauritanian coast, indicating long-distance influence of Fe from the margin. Pyrite was not a significant component of total particulate Fe, however, which was instead dominated by Fe(III)-compounds (80-90%), and some Fe(II)-silicates (10-20%).

Lam, P. J.; Ohnemus, D.; Marcus, M. A.; Fakra, S.

2010-12-01

208

The Evolution of Lithospheric Deformation and Crustal Structure from Continental Margins to Oceanic Spreading Centers  

NASA Astrophysics Data System (ADS)

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 Margin and investigates the relationship between incipient margin structure and segmentation at the modern Mid-Atlantic Ridge. The second part (Chs. 3-5) focuses on the mechanics of faulting in extending lithosphere. In Ch. 3, 1 show that the incorporation of a strain-rate softening rheology in continuum models results in localized zones of high strain rate that are not imposed a priori and develop in response to the rheology and boundary conditions. I then use this approach to quantify the effects of thermal state, crustal thickness, and crustal rheology on the predicted style of extension deformation. The mechanics of fault initiation and propagation along mid-ocean ridge segments is investigated in Ch. 4. Two modes of fault development are identified: Mode C faults that initiate near the center of a segment and Mode E faults that initiate at the segment ends. Numerical results from Ch. 5 predict that over time scales longer than a typical earthquake cycle transform faults behave as zones of significant weakness. Furthermore, these models indicate that Mode E faults formed at the inside-corner of a ridge-transform intersection will experience preferential growth relative to faults formed at the conjugate outside-corner due to their proximity to the weak transform zone. Finally, the last part of this thesis (Ch. 6) presents a new method to quantify the relationship between the seismic velocity and composition of igneous rocks. A direct relationship is derived to relate Vp to major element composition and typical velocity-depth profiles are used to calculate compositional bounds for the lower continental, margin, and oceanic crust.

Behn, Mark D.

2002-06-01

209

Crustal Structure of the Northern Continental Passive Margin of the Eastern Gulf of Aden (Dhofar, Southern Oman) Inferred From Teleseismic Receiver Function Analysis  

Microsoft Academic Search

The process of strain localization preceding the onset of seafloor spreading is still poorly understood, although extensively studied along passive continental margins worldwide. Current models of continental breakup processes differ in the geometry and distribution of strain, and the presence or absence of magmatism along conjugate margins. One key constraint needed to differentiate between models for passive margin formation is

C. Tiberi; S. Leroy; C. Ebinger; G. Stuart; A. Al-Lazki; E. D'Acremont; N. Bellahsen; A. Pointu

2006-01-01

210

Warm-water flow above deep water methane hydrates at an Arctic continental margin - A view on climate impacts  

NASA Astrophysics Data System (ADS)

We detected geophysical evidence for the base of the gas hydrate stability zone (GHSZ) in form of a bottom simulating reflector (BSR) at mid slope of the continental margin of NW-Svalbard and collected acoustic and video-footage evidence for major methane release at the shelf (flares). Flares are acoustic expressions of methane bubbles emanating from the seabed. Warming of upper ocean water masses that impinge on sediments of the upper continental slope may reduce the methane hydrate stability zone. As a result, it can cause melting of gas hydrates, overpressure build up, abrupt releases of methane from the seabed, and geohazards. However, the warm-water core of the W-Spitsbergen current does not intercept with the continental slope (July 2009). There is also no evidence for a BSR pinch out on the upper continental slope. Accordingly, warming of the W-Spitsbergen current at this location may have less or no effects on the release of methane from the seabed by reducing the GHSZ. Alternatively, fluids may be released by reactivation of faults due to seismic activity (< 7 magnitude) around Svalbard. Earthquakes occur frequently along the divergent plate boundary and the nearby continental-ocean boundary (COB) of the W-Svalbard continental margin. We therefore consider two major ways to release fluids: 1) an upward migration of free gas beneath the GHSZ towards the projected outcrop zone at the upper continental margin, and/ or 2) a migration and release of methane at active fault zones.

Mienert, J.; Bünz, S.; Greinert, J.

2009-12-01

211

The 3D magnetic structure beneath the continental margin of the northeastern South China Sea  

NASA Astrophysics Data System (ADS)

Understanding the continental margin of the Northeastern South China Sea is critical to the study of deep structures, tectonic evolution, and dynamics of the region. One set of important data for this endeavor is the total-field magnetic data. Given the challenges associated with the magnetic data at low latitudes and with remanent magnetism in this area, we combine the equivalent-source technique and magnetic amplitude inversion to recover 3D subsurface magnetic structures. The inversion results show that this area is characterized by a north-south block division and east-west zonation. Magnetic regions strike in EW, NE and NW direction and are consistent with major tectonic trends in the region. The highly magnetic zone recovered from inversion in the continental margin differs visibly from that of the magnetically quiet zones to the south. The magnetic anomaly zone strikes in NE direction, covering an area of about 500 km × 60 km, and extending downward to a depth of 25 km or more. In combination with other geophysical data, we suggest that this strongly magnetic zone was produced by deep underplating of magma associated with plate subduction in Mesozoic period. The magnetically quiet zone in the south is an EW trending unit underlain by broad and gentle magnetic layers of lower crust. Its magnetic structure bears a clear resemblance to oceanic crust, assumed to be related to the presence of ancient oceanic crust there.

Li, Shu-Ling; Li, Yao-Guo; Meng, Xiao-Hong

2012-06-01

212

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

NASA Astrophysics Data System (ADS)

P-wave azimuthal seismic anisotropy of the uppermost mantle has been shown to indicate directions of extension, both recent and fossil, in oceanic and continental settings. We have determined the P-wave anisotropy of a zone of exhumed continental mantle beneath the southern Iberia Abyssal Plain, as a possible indication of the initial direction of separation of Iberia from North America in a region where there are no magnetic lineations or fracture zone lineations adjacent to the margin with which to constrain this direction. Over 10 000 airgun shots were recorded by 11 ocean-bottom seismometers/hydrophones from which 24 900 arrivals that had traversed unaltered/weakly serpentinized upper mantle were picked. Complete 360° azimuthal coverage was attained between the shots and receivers for ranges up to 55 km. P-wave raypaths through the zone of exhumed continental mantle were ray-traced through a velocity model that incorporated the 3-D relief of the seabed and acoustic basement. Time residuals with respect to this model were fitted using an azimuthally dependent function. Small anisotropy (<~7 per cent) with a fast direction of 143° was determined to exist between 3.1 and 6.7 km below the top of the basement. Although the fast direction suggests that the post-breakup mantle stretching between Iberia and North America was northwest-southeast, this hypothesis is hard to reconcile with other observations. Instead, we conclude that motion along concave-downward faults during the exhumation of the upper mantle could have been sufficient to change an initially roughly east-west margin-normal fast direction to the 143° azimuth observed today. The degree of serpentinization was estimated to decrease from ~20 to 0 per cent between 3 and 7 km depth into the basement in the zone of exhumed continental mantle. The degree of upper-mantle anisotropy is similar to that observed in other oceanic and continental settings where similar degrees of serpentinization are inferred from mean P-wave velocities.

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

2002-11-01

213

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

214

Numerical Models of Salt Tectonics and Associated Thermal Evolution of Rifted Continental Margins  

NASA Astrophysics Data System (ADS)

Salt tectonics at rifted continental margins reflects the interplay between the geometry of the initial evaporite basin and subsequent mobilization of the salt which is partly controlled by the density and strength of the overburden. Salt mobility is also influenced by the overall thermo-mechanical evolution of the margin which includes factors such as: initial seaward tilt of the margin basement owing to crustal thinning; an initial thermal anomaly owing to the rifting and the subsequent long-term postrift thermal subsidence; and the flexural isostatic response to sedimentation which may reverse the basal tilt. The high thermal conductivity of salt also has a significant impact on the thermal evolution of rifted margin sedimentary basins. We present two-dimensional thermo-mechanical finite element models designed to assess salt mobility and its impact on the thermal evolution of the surrounding sediments and underlying crust in the context of an evolving rifted margin, that includes the processes listed above. Model experiments include: the initial geometry of the rifted margin and the embedded autochthonous salt basin, and its subsequent thermal subsidence, sedimentation and water loading and their flexural response, erosion and, sediment compaction. Salt is mobilized by aggrading sediments with a sinusoidally perturbed surface that represents natural bathymetric unevenness. The model results indicate that the presence of a highly conductive salt layer perturbs the initial thermal structure of the rifted margin resulting in a negative thermal anomaly beneath the autochthonous salt basin. For a given thickness of the salt layer, the depth of this perturbation increases with the width of the salt basin. Flow of salt initially occurs by gliding owing to the initial seaward tilt of the margin which is enhanced by thermal subsidence as the margin cools and, subsequently by a combination of gliding and gravitational spreading when loaded by aggrading sediments. Uneven sediment loading results in well-developed minibasins and intervening salt diapirs. The seaward flow of salt, and down-dip contraction owing to the evolving margin tilt, result in wider diapirs at the seaward end of the basin; a geometry consistent with observations in rifted margins (e.g., south Atlantic margins). Salt flow results in the migration of the associated thermal perturbation. The expulsion of salt by the sediment loading results in higher temperatures beneath the grounded minibasins. However, owing to the thermal focusing of heat by salt, the minibasins remain significantly cooler than the sediments outside the salt basin where standard geothermal gradients are established as the margin cools. Under these circumstances hydrocarbons generation will be delayed until sediments of significant thickness are deposited in salt minibasins. With all other parameters being equal, the down-dip flow of salt, maximum temperature (thickness) attained in the basin are higher for narrower margins. The thermal refraction of heat owing to the salt basin observed in our models has important implications for thermal and petroleum systems modeling of rifted margin sedimentary basins.

Goteti, R.; Beaumont, C.; Ings, S. J.

2011-12-01

215

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

Microsoft Academic Search

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

Karin Andreassen; Espen Glad Nilssen; Christian M. Ødegaard

2007-01-01

216

North Atlantic Volcanic Rifted Continental Margins: Further insight from S- waves.  

NASA Astrophysics Data System (ADS)

The North Atlantic provides classic examples of volcanic rifted continental margins but in such regions flood basalt sequences provide a challenge to deep seismic imaging. High-quality, wide-angle, ocean-bottom seismometer (OBS) data have been acquired with a low frequency (9 Hz) seismic source across two margins. S-wave arrivals, which are dominantly converted from P- to S-waves at the sediment-top basalt interface, were recorded at 170 four-component OBS locations. Tomographic inversion of over 70,000 converted S- wave crustal diving waves and Moho reflections was successfully performed to produce S-wave velocity models using two methods. A flexible layer-based approach was used for initial modelling and quality control while a more automated grid-based approach, which required correction of the S-wave travel-times to effectively symmetric ray paths, provided more robust models and measures of uncertainty through Monte Carlo analyses. The Vp/Vs ratio structures of the margins were estimated by combining P- and S-wave velocity models. The Vp/Vs ratio provides insight to crustal composition. Across the continent-ocean transition, P-wave velocities change from 6.8 km/s to 7.3 km/s and Vp/Vs ratios increase from 1.70 to 1.78 indicating increased levels of mafic intrusion into the crystalline lower continental crust oceanward. The oldest oceanic crust beneath the profiles has high lower-crustal P-wave velocities (up to 7.5 km/s) and low Vp/Vs ratios (1.75- 1.80) caused by the melt produced from abnormally hot mantle immediately after continental break-up. A sub-basalt P- and S-wave low velocity zone was modelled on the Faroes margin and comparison of the modelled properties with measurements from relevant lithologies reveals that this zone is likely to contain largely sedimentary rock rather than igneous hyaloclastites similar to those found beneath the basalt in the Lopra-1/1A borehole. The presence of a sedimentary basin prior to volcanism, beneath the top of what is now the Fugloy Ridge, is also indicated by the properties of the unit immediately beneath the low velocity zone with Vp/Vs ratios of 1.80-1.85 and P-wave velocities of 5.5-6.0 km/s, consistent with sill-intruded Mesozoic sedimentary rock.

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

2008-12-01

217

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

NASA Astrophysics Data System (ADS)

Passive margins have been characterized as magma-dominant (volcanic) or magma-poor (non-volcanic). However, the conditions under which margins might switch states are not well understood as they typically have been studied as end member examples in isolation to each other. The Nova Scotia (NS) continental margin, however, offers an opportunity to study the nature of such a transition between the magma-dominant US East Coast margin to the south and the magma-poor Newfoundland margin to the north within a single rift segment. This transition is evidenced by a clear along-strike reduction in features characteristic of syn-rift volcanism from south-to-north along the NS margin, such as the weakening of the East Coast Magnetic Anomaly (ECMA) and the coincident disappearance of seaward dipping reflector sequences (SDRS) on multichannel seismic (MCS) reflection profiles. Results from recent industry MCS profiles along and across the margin suggest a potentially narrow magma-dominant to magma-poor along-strike transition between the southern and the central NS margin. Such a transition is broadly consistent with results of several widely-spaced, across-strike ocean bottom seismometer (OBS) wide-angle profiles. In the southern region, the crustal structure exhibits a narrow (~120-km wide) ocean-continent transition (OCT) with a high velocity (7.2 km/s) lower crust, interpreted as a gabbro-rich underplated melt, beneath the SDRS and the ECMA, similar to crustal models across the US East Coast. In contrast, profiles across the central and northern margin contain a much wider OCT (150-200-km wide) underlain by a low velocity mantle layer (7.3-7.9 km/s), interpreted as partially serpentinized olivine, which is similar to the magma-poor Newfoundland margin to the north. However, the central-to-northern OBS profiles also exhibit significant variations within the OCT and the along-strike continuity of these OCT structures is not yet clear. In November 2010, we acquired, in the OCTOPUS survey, wide-angle seismic data along a 240-km-long margin parallel profile extending from the central to the northern margin segments along an existing industry MCS profile (Ion/GX Technology NovaSPAN 5100). Twenty OBSs at 10-km spacing were analysed. A preliminary p-wave velocity model along the profile indicates that the cross-strike structures are continuous within the OCT. However, a substantial anisotropy in velocity (~8% lower parallel to the margin) is observed within the OCT. This result is consistent with an interpretation of partially serpentinized mantle that flowed perpendicular to the margin during its extension. In addition, along strike variations are also observed along the profile, which suggest a higher degree of volcanism and a thinner layer of serpentinized mantle to the southwest. These results provide a framework for future studies to the southwest to further investigate the transition to a magma-dominant regime towards the US East Coast.

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

2011-12-01

218

Crustal structure of the Peruvian continental margin from wide-angle seismic studies  

NASA Astrophysics Data System (ADS)

Active seismic investigations along the Pacific margin off Peru were carried out using ocean bottom hydrophones and seismometers. The structure and the P-wave velocities of the obliquely subducting oceanic Nazca Plate and overriding South American Plate from 8°S to 15°S were determined by modelling the wide-angle seismic data combined with the analysis of reflection seismic data. Three detailed cross-sections of the subduction zone of the Peruvian margin and one strike-line across the Lima Basin are presented here. The oceanic crust of the Nazca Plate, with a thin pelagic sediment cover, ranging from 0-200 m, has an average thickness of 6.4 km. At 8°S it thins to 4 km in the area of Trujillo Trough, a graben-like structure. Across the margin, the plate boundary can be traced to 25 km depth. As inferred from the velocity models, a frontal prism exists adjacent to the trench axis and is associated with the steep lower slope. Terrigeneous sediments are proposed to be transported downslope due to gravitational forces and comprise the frontal prism, characterized by low seismic P-wave velocities. The lower slope material accretes against a backstop structure, which is defined by higher seismic P-wave velocities, 3.5-6.0 km s-1. The large variations in surface slope along one transect may reflect basal removal of upper plate material, thus steepening the slope surface. Subduction processes along the Peruvian margin are dominated by tectonic erosion indicated by the large margin taper, the shape and bending of the subducting slab, laterally varying slope angles and the material properties of the overriding continental plate. The erosional mechanisms, frontal and basal erosion, result in the steepening of the slope and consequent slope failure.

Krabbenhöft, A.; Bialas, J.; Kopp, H.; Kukowski, N.; Hübscher, C.

2004-11-01

219

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

220

Evolution of the East Greenland passive continental margin: new evidence from single-grain-age low-temperature thermochronology  

Microsoft Academic Search

Neogene uplift has been widely used to explain the present high topography of many North Atlantic continental margins, but a contrary view exerts that such landscapes developed by protracted exhumation of topography since the Caledonian Orogeny as a result of gravitational collapse, continental rifting and erosion (e.g. Nielsen et al., 2009). We have obtained a single-grain apatite (U-Th)\\/He age dataset

Darrel Swift; Cristina Persano; Finlay Stuart; Kerry Gallagher; Andrew Whitham; Valerie Olive

2010-01-01

221

Basalt magmatism along the passive continental margin of SE Brazil (Campos basin)  

NASA Astrophysics Data System (ADS)

The SE-Brazil passive continental margin is characterized by tholeiitic magmatism that is particularly widespread in the marginal Campos basin, facing the inland flood basalts of the adjacent Paranà basin. Campos magmatism is represented by Early Cretaceous (EC; 134 122 Ma) flood basalts and minor Upper Cretaceous-Early Tertiary basalt flows and intrusives, which were emplaced in a basin with attenuated crustal thickness (20 km). Petrography, mineral chemistry, wholerock geochemistry and Sr-Nd isotope composition emphasize that the EC-Campos basalts have suffered extensive seawater interaction which caused enrichment in MgO, FeO total, K2O, Rb and Ba, and depletion in SiO2 and CaO, while Zr, Nb, Y and REE remained virtually constant in samples with loss-on-ignition values less than 4 wt%. In general, Campos basalts have bulkrock chemistry similar to those of the inland Parana tholeiites (140 130 Ma) with relatively low concentrations of incompatible elements and TiO2 (<2 wt%). Batch-melting calculations suggest that Campos basalt genesis requires a garnet-peridotite source and variable degree of melting (9 25%) in order to explain the rare-earth-element (REE) patterns with chondrite normalised La/Yb(N) ratio ranging from 0.9 1.0 to 4.4 7.1. The Sr-Nd isotopic data for the slightly altered Campos basalts, of both Early Cretaceous and Upper Cretaceous-Early Tertiary age, plot close to bulk earth, or in the enriched quadrant of the mantle array. Inter-element (La, Zr, Nb, Ba) ratios preclude for the Campos (and Paranà) basalt genesis any significant participation of N-type (Zr/Nb>16) MORB (mid-ocean-ridge basalt) mantle in simple binary mixing models. On the whole, the Early Cretaceous Campos basalts appear as an easterly, younger extension of the northern Paranà volcanism and probably erupted during early stages of the major riftingprocesses which caused continental thinning. It is notable that in the Campos marginal basin both the basalt magmatism contemporaneous with the continental break-up, as well as that which occurred after the S. America-Africa separation, appears substantially related to subcontinental lithosphere and a Dupal-like OIB (ocean-island basalt) (e.g. Tristan da Cunha) source components.

Mizusaki, A. M. P.; Petrini, R.; Bellieni, P.; Comin-Chiaramonti, P.; Dias, J.; de Min, A.; Piccirillo, E. M.

1992-06-01

222

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

223

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

SciTech Connect

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

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

1993-02-01

224

Structural and depositional styles of Gulf Coast tertiary continental margins: application to hydrocarbon exploration  

SciTech Connect

The interrelationship of the structure and genetic stratigraphy of the Gulf of Mexico continental margin is the basis of this course, which provides a summary of the key concepts, models, and tools needed for successful hydrocarbon exploration. It recognizes that, as hydrocarbon exploration and exploitation advance to and beyond the present shelf edge and into the deeply buried Tertiary basin fill, interpretation of the complex depositional and structural styles of the outer shelf and upper slope setting will be increasingly challenging to interpreters. The course presents material in a series of short chapters, each dealing with a specific topic and containing illustrations and citations of key references for further reading. Although the units are self-contained, they are arranged in a logical sequence leading to a general synthesis of exploration concepts. 54 references, 165 figures, 5 tables.

Jackson, M.P.A.; Galloway, W.E.

1984-01-01

225

Fuerteventura palaeomagnetism and the evolution of the continental margin off Morocco  

NASA Astrophysics Data System (ADS)

Palaeomagnetic results from Fuerteventura (Canary Islands) suggest that the oldest subaerial lava succession (Series I) originated at around the Cretaceous/Tertiary boundary, implying that the basal, post-Albian, complex of submarine volcanics, sheeted dikes and plutonics was emplaced sometime in the late Cretaceous. This time-range for early Fuerteventura magmatism and related tectonic movements is contemporaneous with the major sedimentary hiatus encountered in IPOD drill sites along the continental margins of northwest Africa and southwest Europe. This extensive late Cretaceous unconformity is probably directly related to tectonic uplift which locally formed the island of Fuerteventura. The close timing of these tectonomagmatic processes with the suggested final break-up between Africa and South America may indicate that both the North and South Atlantic Oceans had an intimately connected initial spreading history.

Storetvedt, K. M.

1980-03-01

226

Age and significance of sequence boundaries on Jurassic and early Cretaceous rifted continental margins  

SciTech Connect

The age and significance of sequence boundaries on Jurassic to Early Cretaceous rifted continental margins in three ocean basins have been documented. The margins are the Santos basin in the South Atlantic, the Grand Banks in the North Atlantic, and the Beaufort Sea in the Arctic Ocean. Large industry data bases were used for the interpretation of each area. Megasequence boundaries separate the major phases of basin evolution, for example syn-rift and post-rift. Boundaries developed with an average periodicity of 49 m.y. Sequence boundaries define the component parts of each megasequence and developed with a modal periodicity of 10-15 m.y. Out of 27 total boundary ages, most (16) are developed on just one margin. Only two possible age ranges overlap on all three margins. 80% of the megasequence boundaries and 50% of the sequence boundaries show a direct causal connection with coveal faulting and/or folding. The rest of the boundaries appear as unstructured surfaces separating transgressive and/or regressive sedimentary wedges and are interpreted to result from changes in the rate of basin subsidence, sediment input, and long-term eustatic sea level. These data do not support theories advocating synchronous worldwide boundary development resulting from periodic, short-term falls in global eustatic sea level. Only in like basins of the same age, with identical subsidence and sediment input rates, are boundaries likely to develop synchronously. Hence, the concept of global synchroneity of sequence boundary development may well be an illusion created by the similarity in age of the majority of basins studied. As a result of this study, it seems wise to discard the global approach to basin analysis. 13 figures, 1 table.

Hubbard, R.J.

1988-01-01

227

Magnetic Evidence for Volcanism at Eastern Continental Margin of India: Juxtaposition with Elan Bank (Southern Indian Ocean)  

NASA Astrophysics Data System (ADS)

The rifted Eastern Continental Margin of India (ECMI) has evolved as a result of breakup of East Gondwanaland. Previous geophysical studies of the continental margin have not elucidated upon its volcanic nature. Magnetics plays a useful role in the study of continental margins, particularly in identifying the volcanic units. The aeromagnetic map of the offshore Mahanadi basin of ECMI displays a conspicuous linear anomaly along the continental shelf. A comprehensive study of the published aeromagnetic, marine magnetic and gravity data of the offshore Mahanadi basin reveals the existence of a seaward dipping volcanic unit in the offshore Mahanadi basin bordering the Hinge zone. This inference suggests that the ECMI is a volcanic rifted margin. The study further indicates the deepening of the basement towards the sea. In addition, the existing geological studies on the ECMI demarcated the probable limit of the continental crust by studying the basement detached tectonic style of the sedimentation in sub-surface configuration of the East coast basins of India. The probable continental crustal limit, the Hinge zone, and the inner edge of the presently inferred volcanic unit conform to one another spatially in the offshore Mahanadi region. These features characterize the inferred volcanic body as seaward dipping reflectors (SDRs) that usually occur at the rifted continental margins. The deepening of the basement towards the sea and the presence of the volcanic body on the continental margin are indicative of the transitional nature of the crust. It is generally accepted that Antarctica and India were juxtaposed before the breakup of Gondwanaland. But the microcontinents in the southern Indian Ocean are neglected in the reconstruction of Gondwanaland continents. The recent studies of the discovery of continental crust within the Elan Bank (EB) microcontinent show that the EB was contiguous with the East coast of India before the breakup of Gondwanaland. Moreover, it is reported that the upper igneous crust of the EB consists of a 2 3 km thick layer of accumulated lava flows originating from the Kerguelen hotspot. An estimate shows that the total volume of volcanic and plutonic component of the Elan Bank is about 0.3 million cubic kilometers. The present inference of a volcanic body from the offshore Mahanadi basin is in agreement with the above observations of the juxtaposition of EB with ECMI.

Sarma, B. S. P.

2008-01-01

228

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, III, W.; Lorenson, T. D.; Greene, H. G.

2007-01-01

229

Seismic reflectivity of detachment faults of the Iberian and Tethyan distal continental margins based on geological and petrophysical data  

Microsoft Academic Search

Low-angle detachment faults are key to our understanding of the tectonic evolution of magma-poor rifted continental margins. In seismic images of present-day rifted margins the identification and interpretation of such features is, however, notoriously difficult and ambiguous. We address this problem by studying the structure and seismic response of such faults through a synoptic interpretation of petrophysical data and geological

Andreas B. Holker; Klaus Holliger; Gianreto Manatschal; Flavio Anselmetti

2002-01-01

230

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

231

Parallel computing of multi-scale continental deformation in the Western United States: Preliminary results  

NASA Astrophysics Data System (ADS)

Lithospheric deformation in the western United States is one of the best examples of diffuse continental tectonics that deviate from the plate tectonics paradigm. Conceptually, diffuse continental deformation is known to result from (1) weak and heterogeneous rheology of continents and (2) driving forces that arise from plate boundaries as well as within the continental lithosphere. However, the dynamic interplay of continental rheology and driving forces, hence the geodynamics of continental tectonics, remains poorly understood. The heterogeneous rheology and multiple driving forces cause continents to deform over different spatiotemporal scales with different physical processes, yet most geodynamic models for continental tectonic avoid dealing with such multiphysics partly because of (1) the limited observational constraints of lithospheric structure and deformation, and (2) high demands on computing algorithms and resources. These constraints, however, have relaxed significantly in recent years to permit exploration of some of the multi-scale physics governing continental tectonics. Here we present preliminary results of modeling multi-scale tectonics in the western United States using parallel finite element computation. In a 3D subcontinental-scale model, we used fine numerical meshes to incorporate all major tectonic boundaries and rheological heterogeneities in the model to explore their interplay with tectonic driving forces in controlling active tectonics in the western US. In another model for the entire San Andreas Fault system, we explored strain localization and simulated fault behavior at multi-timescales ranging from rupture in seconds to secular fault creep in tens of thousands of years. These models can help to integrate data grids with distributed high-performance computing resources in the emerging geosciences cyberinfrastructure.

Liu, Mian; Yang, Youqing; Li, Qingsong; Zhang, Huai

2007-08-01

232

Effect of Depleted Continental Lithosphere Counter-flow and Inherited Crustal Weakness on Rifting of the Newfoundland-Iberia and Nova Scotia-Morocco Continental Margins  

NASA Astrophysics Data System (ADS)

In the past two decades, significant advances have been made in understanding the present-day structure of rifted continental margins using reflection and refraction seismic techniques. Despite these advances, we have only a rudimentary understanding of the processes involved in the development of non-volcanic rifted margins, particularly the role of depth-dependant crustal extension, inherited crustal weaknesses, flow of lower continental mantle lithosphere, and syn-rift sedimentation. In this study we use 2D thermo-mechanical finite element modeling to investigate the evolution of upper-mantle scale systems (1200 km wide, 600 km deep). The results are compared with the Newfoundland - Iberia and Nova Scotia - Morocco conjugate margin pairs. The models include thick (200 km) chemically depleted mantle lithosphere, inherited crustal weaknesses, and syn-rift sedimentation. Depending on the properties of the crust and mantle, two types of two-layer two-stage rift systems develop: Type I margins where the crust remains coupled to the mantle lithosphere during rifting producing narrow margins in which the crust necks before the mantle lithosphere; Type II margins where the crust is weaker allowing it to decouple from the mantle during extension producing wide margins in which the mantle necks before the crust finally rifts. When the lithosphere is thick and chemically depleted (e.g., cratonic as an end-member), the hotter buoyant lower mantle lithosphere flows toward to rift axis during rifting allowing the formation of exhumed continental mantle lithosphere. The result is the formation of wide tracts of exhumed mantle lithosphere, subsequently serpentinized owing to hydration, which forms transitional crust between extended continental crust and oceanic crust, consistent with observations of serpentinized continent-derived mantle rocks in the transitional crust region of the Newfoundland - Iberia conjugates, and similar interpretations from the northern Nova Scotia margin. When offset weak zones are included in the crust, these zones become the focus of early extension, but later become abandoned as rifting progressively shifts to the central rift axis. The degree to which the offset weak zones accommodate extension is dependant on the position of the weak zones, the strength of the crust, and syn-rift sedimentation. When the crust is relatively strong, the offset weak zones evolve into narrow (<50 km) syn-rift sedimentary basins. When the crust is relatively weak, or if syn-rift sedimentation is more pronounced, the offset basins remain active longer, and evolve into significantly wider basins. Model results will illustrate how offset weak zones and thick depleted mantle lithosphere flow can result in complex multi-stage rifting that has implications for the timing and structural evolution of syn-rift sediment depocenters.

Ings, S. J.; Beaumont, C.

2011-12-01

233

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

234

Magnetic anomalies of offshore Krishna-Godavari basin, eastern continental margin of India  

NASA Astrophysics Data System (ADS)

The marine magnetic data acquired from offshore Krishna-Godavari (K-G) basin, eastern continental margin of India (ECMI), brought out a prominent NE-SW trending feature, which could be explained by a buried structural high formed by volcanic activity. The magnetic anomaly feature is also associated with a distinct negative gravity anomaly similar to the one associated with 85°E Ridge. The gravity low could be attributed to a flexure at the Moho boundary, which could in turn be filled with the volcanic material. Inversion of the magnetic and gravity anomalies was also carried out to establish the similarity of anomalies of the two geological features (structural high on the margin and the 85°E Ridge) and their interpretations. In both cases, the magnetic anomalies were caused dominantly by the magnetization contrast between the volcanic material and the surrounding oceanic crust, whereas the low gravity anomalies are by the flexures of the order of 3-4 km at Moho boundary beneath them. The analysis suggests that both structural high present in offshore Krishna-Godavari basin and the 85°E Ridge have been emplaced on relatively older oceanic crust by a common volcanic process, but at discrete times, and that several of the gravity lows in the Bay of Bengal can be attributed to flexures on the Moho, each created due to the load of volcanic material.

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

2009-08-01

235

Cretaceous source rock characterization of the Atlantic Continental margin of Morocco  

SciTech Connect

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

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

1993-02-01

236

Middle Miocene marine and continental climate and environments at the Wilkes Land margin, Antarctica (IODP 318)  

NASA Astrophysics Data System (ADS)

Integrated Ocean Drilling Program (IODP) Expedition 318 accomplished successful drilling of the Wilkes Land margin (East Antarctica) in early 2010. Understanding the development and the dynamics of the cryosphere during the Cenozoic and obtaining high-resolution records of climate variability during the Neogene and the Quaternary were among the main targets. Samples from Site U1356 Hole A, between ~400 (across unconformity U5) and ~100 mbsf are analysed for dinoflagellate assemblages, pollen and spores, TEX86 and MBT in order to unravel marine and terrestrial climate variability during the early to middle Miocene. Results show that dinoflagellate assemblages, dominated by autotrophic species, are indicative of warm ice-free surface waters during the Middle Miocene Climatic Optimum (MMCO). TEX86-derived Sea Surface Temperatures (SSTs) confirm this interpretation. Continental temperatures based on pollen and Mean Annual Temperatures (MATs) as derived from MBT organic proxy indicate a vegetated Antarctic margin with temperate conditions. A clear climate deterioration occurs during the Mid Miocene Climate Transition (MMCT), when dinocyst and pollen assemblages indicate (year-round) sea-ice development and ice-sheets advance, respectively. Notably, SSTs and MATs markedly decrease.

Sangiorgi, F.; Schouten, S.; Bendle, J. A.; Salzmann, U.; Brinkhuis, H.; Escutia, C.; IODP Expedition 318 Science Party

2011-12-01

237

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

238

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

239

Seismic Patterns of the Guerrero-Oaxaca, Mexico Region and its Relationship to the Continental Margin Structure  

Microsoft Academic Search

The purpose of this article is to make aware seismic evidences that suggest a possible segmentation of the continental margin at the Guerrero-Oaxaca, Mexico region. The analysis of the aftershocks of relevant earthquakes and a survey of microseismicity using a portable seismographs network permit to infer the characteristics of the seismic patterns of the Acapulco-Ometepec segment of the Middle America

J. Yamamoto; T. Gonzalez; Z. Jimenez; L. Quintanar

2005-01-01

240

Seismic Pattern of the Guerrero-Oaxaca, Mexico Region and its Relationship to the Continental Margin Structure  

Microsoft Academic Search

The purpose of this article is to make aware seismic evidences that suggest a possible segmentation of the continental margin at the Guerrero-Oaxaca, Mexico region. A survey of microseismicity using a portable seismographs network and the analysis of the aftershocks distribution of 1982 and 1995 major earthquakes permit to infer the characteristics of the seismic patterns of the Acapulco-Pinotepa segment

J. Yamamoto; T. Gonzalez-Moran

2007-01-01

241

Benthic metazoan biomass, community structure and bioturbation at three contrasting deep-water sites on the northwest European continental margin  

Microsoft Academic Search

Size structure, taxonomic composition and bioturbation potential of benthic metazoan communities were examined at three contrasting sites on the northwest European continental margin as part of a wider study of biogeochemical cycling in the deep-sea benthic boundary layer. Sampling was conducted in the Hatton–Rockall Basin (1100 m depth), on the North Feni Ridge (1920 m) and in the South Rockall

D. J. Hughes; J. D. Gage

2004-01-01

242

Benthic metazoan biomass, community structure and bioturbation at three contrasting deep-water sites on the northwest European continental margin  

Microsoft Academic Search

Size structure, taxonomic composition and bioturbation potential of benthic metazoan communities were examined at three contrasting sites on the northwest European continental margin as part of a wider study of biogeochemical cycling in the deep-sea benthic boundary layer. Sampling was conducted in the Hatton Rockall Basin (1100 m depth), on the North Feni Ridge (1920 m) and in the South

D. J. Hughes; J. D. Gage

2004-01-01

243

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

Microsoft Academic Search

The Indian National Gas Hydrate Program (NGHP) Expedition 01 presented the unique opportunity to constrain regional heat flow derived from seismic observations by using drilling data in three regions on the continental margin of India. The seismic bottom simulating reflection (BSR) is a well-documented feature in hydrate bearing sediments, and can serve as a proxy for apparent heat flow if

Anne Trehu; Peter Kannberg

2011-01-01

244

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

Microsoft Academic Search

Organic and inorganic nitrogen and their isotopic signatures were studied in continental margin sediments off Spitsbergen. We present evidence that land-derived inorganic nitrogen strongly dilutes the particulate organic signal in coastal and fjord settings and accounts for up to 70% of the total nitrogen content. Spatial heterogeneity in inorganic nitrogen along the coast is less likely to be influenced by

Jochen Knies; Steven Brookes; Carsten J. Schubert

2007-01-01

245

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

246

Shelf sedimentation on a tectonically active margin: A modern sediment budget for Poverty continental shelf, New Zealand  

Microsoft Academic Search

The combination of high sediment yields and the prevalence of tectonically controlled accommodation on collision margins such as that adjacent to the Waipaoa River, NZ, create the potential for these areas to contain high-resolution records of natural and anthropogenic signals. This study describes modern (100-yr) sedimentation patterns off the Waipaoa and quantifies a sediment budget for the continental shelf, which

Andrea J. Miller; Steven A. Kuehl

2010-01-01

247

56. MORPHOLOGY AND BASEMENT STRUCTURES OF THE GOBAN SPUR CONTINENTAL MARGIN (NORTHEASTERN ATLANTIC) AND THE ROLE OF THE PYRENEAN OROGENY1  

Microsoft Academic Search

A new bathymetric map, based mainly on Seabeam data, has been established for the Goban Spur area during a postcruise survey of DSDP Leg 80. Using both Seabeam data and a new series of eight seismic profiles obtained perpen- dicular to the continental margin, we have constructed a new detailed structural map of the Goban Spur continental margin which clearly

Jean-Claude Sibuet; Benoit Mathis; Léo Pastouret; Jean-Marie Auzende; Jean-Paul Foucher; Peter M. Hunter; Pol Guennoc; Pierre-Charles de Graciansky; Lucien Montadert; Douglas G. Masson

248

A comparison between the megafauna communities on the N.W. Iberian and Celtic continental margins—effects of coastal upwelling?  

Microsoft Academic Search

Megafauna biomass and feeding guilds were studied on the NW Iberian upwelling Continental Margin in order to determine the presence of enriched zones pointing to enhanced particle input. We compare these findings with similar data obtained from a transect across the Celtic Continental Margin that represents a regime without coastal upwelling. Additionally sediment concentrations of phytopigments (chlorophyll-a, phaeophorbides) representing recent

M. S. S Lavaleye; G. C. A Duineveld; E. M Berghuis; A Kok; R Witbaard

2002-01-01

249

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

NASA Astrophysics Data System (ADS)

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

Ernesto, M.; Marques, L. S.

2011-12-01

250

Holocene sponge bioherms on the western Canadian continental shelf  

NASA Astrophysics Data System (ADS)

Bioherms dominated by hexactinellid sponges occur in the three major troughs which cross the north-central continental shelf of British Columbia, Canada. The preferred substrate of the bioherms is till, glaciomarine sediments or bedrock in 150 to 250 m water depth. These bioherms began to develop in the early Holocene and have expanded since that time to discontinuously mantle more than 700 km2 today. The sponge colonies form mounds and sheet-like structures up to 15 m thick and several kilometres wide and accrete new material to the bioherm through the removal of silty clay from suspension by the baffling of bottom currents. The sedimentologic and morphologic characteristics of the bioherms are in part due to fused siliceous mainframe skeletons of the sponges that provide the framework for the construction of the bioherm. The locus of sponge habitation on the surface of the bioherms changes with time in response to sediment accretion and microtopographic changes which affect delivery of sediment and nutrients entrained in bottom currents.

Conway, K. W.; Barrie, J. V.; Austin, W. C.; Luternauer, J. L.

1991-08-01

251

Plumes of bubbles release methane gas from the seabed along the West Spitsbergen continental margin  

NASA Astrophysics Data System (ADS)

Over 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, at and above the upper limit of the gas hydrate stability zone (GHSZ), at depths of 150-400 m. Some plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane, and seismic reflection data indicate free gas beneath the plume field. A 1°C warming of the northward-flowing West Spitsbergen current over the last thirty years is likely to have increased the release of methane from the seabed by reducing the extent of the GHSZ, causing the liberation of methane from decomposing hydrate. If this process is widespread along Arctic continental margins, the methane released could be a large proportion of global methane flux. Methane released from gas hydrate in submarine sediments has been invoked as an agent of past climate change, yet comparatively little is known about methane fluxes in the present-day marine environment. Global atmospheric methane concentration continues to rise, following a period of stability between 1998 and 2006. A multidisciplinary marine geological, geophysical, and geochemical expedition was undertaken with the Royal Research Ship James Clark Ross between 23 August and 24 September 2008, as part of the International Polar Year, to investigate the role of the GHSZ in the release and retention of methane from geological sources along the West Spitsbergen continental margin, between 78° and 80° N. The techniques employed in the expedition included: detailed (10-m resolution) mapping of sea-floor morphology; detailed acoustic imaging of sea-floor stratigraphy and of features extending into the water column; seismic portrayal of geological features to depths of several hundreds of metres beneath the seabed, such as depositional and tectonic structures and the bottom-simulating reflector (BSR, the boundary between free-gas-containing sediment and hydrate-containing sediment); sediment coring to obtain sequences for geochemical and palaeoceanographic investigations; water-column sampling for chemical analyses of the water and dissolved gases; and atmospheric sampling for gas concentration (notably methane). In the Arctic, the GHSZ is especially sensitive to climate-induced changes in temperature, because the degree of temperature change is greater than at lower latitudes. The GHSZ for a specific gas or gases and salinity of water is defined by conditions of temperature and pressure (dependent on water depth plus depth beneath seabed), both of which have varied greatly in this area over the past 15 kyr. At present, the GHSZ (for pure methane gas and water with 3.5 wt % NaCl) is expected to taper out at its landward limit where water temperature is 3°C at a depth of about 396 m. It is in water just shallower than this depth that most of the bubble plumes occur.

Westbrook, G. K.

2009-04-01

252

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

253

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

254

Role of differential erosion in uplifting continental margins: examples from Antarctica, New Zealand and Norway  

NASA Astrophysics Data System (ADS)

Uplifted and back-tilted continental margins are common but the processes that drive uplift are not always obvious. Isostatic rebound as a response to incisional erosion is an important process that can produce rock uplift of the order of kilometer or two on a time scale of millions of years. Here we compare contrasting continental margins where glacial (and fluvial) erosion have produced fiord structures of the order of a kilometre or two deep, and 1-40 kilometres wide. The uplift response is a function of the depth of erosion, the width of erosion and the spacing of the eroded fiords, relative to the flexural parameter. We calculate the 3D response with a flexural model that allows either a continuous or free-edged plate. By far the largest fiord system we know of is that created by the outlet glaciers of the East Antarctic ice cap as they pass through the Transantarctic mountains (TAM). Here we calculate a rebound response for mountain peaks of ~ 2000 m, or about 50% of the peak heights. In Fiordland, New Zealand, incision is not as deep or as wide as in the TAM and the rebound for Te =20 km is less at ~ 150 m. Peak heights in Fiordland are typically ~ 1500 m so the rebound is about 10% of the peak height. In both the TAM and Fiordland the regions are bounded by major plate-boundary-type faults that suggest a flexural uplift model with a free-edge. The Norwegian margin, in contrast, sits on a continuous plate, with elastic thicknesses that likely decrease towards the Atlantic margin. Here the fiords cut to depths of up to 1300 m but the calculated rebound response is still significant at ~ 500-700 m (for constant Te=20 km)). Peak heights are up to 2500 m and thus predicted rebound is about 25 % of the peak height. Based on these examples we conclude that magnitude of incision and subsequent rebound is related to climate. For alpine type glaciations of Fiordland over the last 2-3 myr the rebound as a function of the peak height is a minimum. At the other end of the scale some 34 my of glacial history, of which 15 my was in polar conditions, created a condition in the TAM of wet based glaciers efficiently carving deep troughs that are bounded by frozen and thus preserved peaks. The second consequence of rebound from incision is that an apparent isostatic imbalance can be created whereby the mountain peak height implies a crustal thickness which is greater than that observed. This is particularly evident at the TAM where the highest mountain peaks are ~ 4500m high but the crustal thickness is only 35-40 km. An important contribution of the rebound analysis is partitioning of an observed rock-uplift history into that due to tectonics versus the isostatic response to incisional erosion. This is well illustrated in Fiordland where a flight of uplifted marine terraces can be ascribed to both subduction processes and isostatic rebound.

Stern, T. A.; Stratford, W. R.; Davies, R.

2010-05-01

255

Magmatism at the west Iberia non-volcanic rifted continental margin: evidence from analyses of magnetic anomalies  

NASA Astrophysics Data System (ADS)

We discuss the magmatic development of the west Iberia non-volcanic rifted continental margin in the North Atlantic Ocean. So-called `non-volcanic' rifted continental margins are characterized by a lack of syn-rift magmatism and are considered to be largely amagmatic. However, this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously. We concentrate our attention on the recently described zone of exhumed continental mantle (ZECM) to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the ZECM leading to the onset of seafloor spreading. We use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide-angle seismic experiments presented elsewhere. Forward and inverse modelling of a sea-surface magnetic anomaly chart and of surface and deep-towed magnetometer profiles shows that anomalies within the ZECM differ in trend, amplitude and source type from those in the adjacent oceanic crust and thinned continental crust. The ZECM anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin. We interpret such bodies as syn-extensional intrusions that increase in volume oceanward. They eventually merge in the vicinity of a margin-parallel, basement peridotite ridge to give rise to a continuous crust that records reversals in the Earth's magnetic field from the time of anomaly M4(N)-M5(R), i.e. to mark the onset of seafloor spreading. We find no evidence for anomalies formed by seafloor spreading, at either slow or ultraslow rates, before M5(R) (128 Ma).

Russell, S. M.; Whitmarsh, R. B.

2003-09-01

256

GAS HYDRATES ON THE MID-NORWEGIAN CONTINENTAL MARGIN: PACEMAKERS OF SLOPE STABILITY THROUGH TIME  

NASA Astrophysics Data System (ADS)

Gas hydrate is stable in marine sediments on many Arctic continental slopes under present temperature and pressure fields. Yet, changes in the physical conditions have been shown to trigger dissolution and emanation of methane into the ocean. Access to a huge database of 2D and 3D seismic records, covering the entire mid-Norwegian margin, now provide an exceptional opportunity to test the relationship between methane release and slope stability. On the mid-Norwegian margin wedges of thick glacigenic units were deposited during past glacial intervals and covers older sequences of fine-grained hemipelagic siliceous ooze. This stratigraphic architecture combined with subsidence, large amount of biogenic methane, deep thermogenic methane reservoirs and thermal processes, provide a natural laboratory where to study the development and dynamics of methane hydrates and other digenetic processes through Cenozoic time. Gas hydrate bearing sediments are commonly detected in our seismic profiles by the presence of cross-cutting bottom simulating reflectors (BSR’s). We also recognize the presence of a second, deeper BSR. This reflector has previously been interpreted as a fossil base of the gas hydrate stability zone caused by hydrate dissociation during postglacial sea level rise and increase bottom water temperature. Several submarine slides confine the spatial distribution of present day gas hydrates, whereas the occurrence of the second BSR is patchy and discontinuous, but appears to be detached from the mass wasting structures. This observation indicates the possible link between methane dissociation and migration from the deeper BSR to the present-day gas hydrate stability depth with ocean floor destabilization at different temporal scales. The presence of a diagenetic-related BSR deeper in the stratigraphical sequence may also suggest thermal gradient increase at depth, thus providing a complementary scenario for methane hydrates dynamics through time.

Waldmann, N.; Haflidason, H.; Hjelstuen, B.; Sejrup, H. P.

2009-12-01

257

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

SciTech Connect

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

Paull, C.K.; Buelow, W.J.; Ussler, W. III; Borowski, W.S. [Univ. of North Carolina, Chapel Hill, NC (United States)

1996-02-01

258

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

259

Seismic structure, gravity anomalies, and flexure of the Amazon continental margin, NE Brazil  

NASA Astrophysics Data System (ADS)

Seismic and gravity data have been used to determine the structure of the sediments, crust, and upper mantle that underlie the Amazon continental margin, offshore NE Brazil. Seismic reflection profile data reveal a major unconformity at ˜7 s two-way travel time (TWTT) which we interpret as marking the onset of the transcontinental Amazon River and the formation of the Amazon deep-sea fan system in the late Miocene. Seismic refraction data show mean sediment velocities that decrease by >1.5 km s-1 in a seaward direction. We attribute this decrease to facies changes associated with sediment progradation and the development of topset, foreset, and bottomset beds. Seismic refraction data show that the sediments are underlain by oceanic crust that has a similar velocity structure compared to elsewhere in the Atlantic Ocean but is unusually thin (˜4.2 km). We attribute the thin crust to either slow seafloor spreading or a limited magma supply during the initial rifting of South America and Africa in the Early Cretaceous. The seismic data have been used to construct a new sediment thickness grid that together with gravity anomaly data, suggests the Amazon fan loaded lithosphere with an unusually high flexural strength. While a high-strength lithosphere explains the overall depth of the seismic Moho, there are discrepancies (of up to 2 km) beneath the upper fan, where the modeled flexed Moho is shallower than the seismic Moho, and beneath the middle fan, where it is deeper. Gravity and seismic modeling suggest these discrepancies are caused by lateral changes in subcrustal density such that the mantle underlying the upper fan is denser than it is beneath the middle fan. We attribute these lateral density differences to proximity to the Ceara Rise, which is believed to have formed during the Late Cretaceous in a mid-ocean ridge setting. Fan loading of a relatively strong, dense, and, hence, cold lithosphere predicts stress orientations that are consistent with borehole breakout data and the location and height of the Gurupé Arch onshore. Despite its proximity to "leaky" transform faults, the margin that underlies the Amazon fan appears to be of nonvolcanic origin. The main differences with other nonvolcanic margins, such as West Iberia and Newfoundland, are a greater sediment accumulation, a narrower zone of transitional crust, and a lack of any evidence for extreme extension and mantle serpentinization.

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

2009-07-01

260

Evolution of a Lower Paleozoic continental margin carbonate platform, northern Canadian Appalachians  

SciTech Connect

The western margin of the northern Appalachian orogene and adjacent craton is composed of a lower Paleozoic, low-latitude carbonate platform which originally lay along the northern margin of Iapetus Ocean. Parts of the platform interior are now exposed in Quebec, much lies beneath the Gulf of St. Lawrence, and the outer shelf and deep-water deposits crop out in western Newfoundland. The shelf break and upper slope are nowhere exposed, but their nature has been determined from numerous clasts in sediment gravity flows redeposited on the lower slope and now stacked in allochthonous thrust complexes. Four separate phases recording platform growth and demise can be differentiated, reflecting the interplay between tectonics, eustasy and the evolving lower Paleozoic biota. Phase 1, Preplatform Shelf, reflects initial siliciclastic deposition on rifted crystalline basement followed by a short phase of carbonate sedimentation dominated by archeocyathan buildups and ooid shoals that was terminated by offlap of thick quartzarenites. Phase 2, Narrow, High-Energy Platform, is characterized by mixed siliciclastic-carbonate peritidal sedimentation throughout, manifest as three grand cycles. Contemporaneous deep-water sediments comprise basal welded conglomerates overlain by quartzose carbonate turbidites. Phase 3, Wide Low-Energy Platform is an onlap package of muddy fossiliferous subtidal and peritidal carbonates arranged in the form of two unconformity-bounded megacycles. The adjacent deep-water slope was a narrow belt of debris flows and wide carbonate-shale apron, deposited in the lower part of an oxygen minimum zone. Phase 4, Foundered Platform, documents the initial uplift, faulting, subsidence, and fragmentation of the platform in a sequence of peritidal to subtidal to deep-water carbonates, reflecting the initial stages of the Taconic orogeny.

James, N.P.; Barnes, C.R.; Stevens, R.K.; Knight, I.

1987-05-01

261

Testing mantle dynamics models beneath the southeastern North America passive continental margin  

NASA Astrophysics Data System (ADS)

A variety of models for mantle flow beneath southeastern North America have been proposed, including those that invoke westward-driven return flow from the sinking Farallon slab, small-scale convective downwelling at the edge of the continental root, or the upwards advective transport of volatiles from the deep slab through the upper mantle. However, due to the paucity of seismic stations located in the eastern United States, constraints on the character of the mantle flow beneath this passive margin are very sparse. A recent temporary deployment of 9 broadband stations (the Test Experiment for Eastern North America, or TEENA) provides new data along a transect from coastal North Carolina across Virginia and West Virginia stretching into Ohio. We use shear wave splitting observations and receiver function analysis at permanent broadband seismic stations in the southeastern US and stations of the TEENA array to test several proposed mantle flow geometries beneath the passive margin. Near the coast, permanent broadband stations exhibit well-resolved null (no splitting) behavior for SKS phases over a range of backazimuths, consistent with either isotropic upper mantle or with a vertical axis of anisotropic symmetry, which would be expected for vertical mantle flow. Further inland we identify significant shear wave splitting with mainly NE-SW fast directions, consistent with asthenospheric shear due to absolute plate motion (APM), lithospheric anisotropy aligned with Appalachian tectonic structure, or a combination of these. P-to-S receiver functions at three of the permanent stations examined resolved unambiguous conversions from the base of the upper mantle. Arrivals from the 410-km velocity discontinuity are generally consistent with the predictions of the Preliminary Reference Earth Model (PREM). We find that the 410-km discontinuity conversion arrives ~0.7 sec early at a station in northern West Virginia compared to the corresponding arrival at a station in coastal South Carolina. We observe striking variations in the crustal response of the receiver functions with respect to azimuth, related to the strong variation in near-surface structure across the region. In addition to permanent stations, approximately 6 months worth of data from the TEENA array were examined for evidence of SKS splitting. Preliminary splitting measurements agree very well with the results from the permanent stations: stations located closer to the coast are dominated by null measurements, while inland stations exhibit generally NE-SW fast directions. Our results are relevant for testing different models for mantle dynamics beneath the southeastern US and support a model in which mantle flow is primarily vertical (either upwelling or downwelling) beneath the southeastern edge of the North American continent, in contrast to the likely horizontal, APM-driven flow beneath the continental interior. This work demonstrates that the mantle flow field beneath the North American passive margin may be more complex than previously thought and highlights the likely importance of vertical mantle flow.

Long, Maureen; Chapman, Martin; King, Scott; Benoit, Margaret

2010-05-01

262

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

263

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

NASA Astrophysics Data System (ADS)

From Permo-Carboniferous to Mid Jurassic northern Namibia was affected by deep erosion of the Damara Orogen, Permo-Triassic collisional processes along the southern margin of Gondwana and eastern margin of Africa (Coward and Daly 1984, Daly et al. 1991), and the deposition of the Nama Group sediments and the Karoo megasequence. The lithostratigraphic units consist of Proterozoic and Cambrian metamorphosed rocks with ages of 534 (7) Ma to 481 (25) Ma (Miller 1983, Haack 1983), as well as Mesozoic sedimentary and igneous rocks. The Early Jurassic Karoo flood basalt lavas erupted rapidly at 183 (1) Ma (Duncan et al. 1997). The Early Cretaceous Paraná-Etendeka flood basalts (132 (1) Ma) and mafic dike swarms mark the rift stage of the opening of the South Atlantic (Renne et al. 1992, Milner et al. 1995, Stewart et al. 1996, Turner et al. 1996). The "passive" continental margin in northern Namibia is a perfect location to quantify exhumation and uplift rates, model the long-term landscape evolution and provide information on the influence of mantle processes on a longer time scale. The poster will provide first information on the long-term landscape evolution and thermochronological data. References Coward, M. P. and Daly, M. C., 1984. Crustal lineaments and shear zones in Africa: Their relationships to plate movements, Precambrian Research 24: 27-45. Duncan, R., Hooper, P., Rehacek, J., March, J. and Duncan, A. (1997). The timing and duration of the Karoo igneous event, southern Gondwana, Journal of Geophysical Research 102: 18127-18138. Haack, U., 1983. Reconstruction of the cooling history of the Damara Orogen by correlation of radiometric ages with geography and altitude, in H. Martin and F. W. Eder (eds), Intracontinental fold belts, Springer Verlag, Berlin, pp. 837-884. Miller, R. M., 1983. Evolution of the Damara Orogen, Vol. 11, Geological Society, South Africa Spec. Pub.. Milner, S. C., le Roex, A. P. and O'Connor, J. M., 1995. Age of Mesozoic igneous rocks in northwestern Namibia, and their relationship to continental breakup, Journal of the Geological Society of London 152: 97-104. Renne, P.R., Ernesto, M., Pacca, I.I., G. Coe, R.S., Glen, J. M., Prévot, M., Perrin, M., 1992. The age of Paraná flood volcanism, rifting of Gondwanaland, and the Jurassic -Cretaceous boundary. Science 258, 975 - 979. Stewart, K. S., Turner, S., Kelly, S., Hawkesworth, C. J., Kirstein, L. and Mantovani, M. S. M., 1996. 3D 40Ar-39Ar geochronology in the Para?a flood basalt province, Earth and Planetary Science Letters 143: 95-110. Turner, S., Hawkesworth, C., Gallagher, K., Stewart, K., Peate, D. and Mantovani, M., 1996. Mantle plumes, flood basalts, and thermal models for melt generation beneath continents: Assessment of a conductive heating model and application to the Parana, Journal of Geophysical Research 101: 11503- 11518.

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

2013-04-01

264

Seafloor classification using artificial neural network architecture from central western continental shelf of India  

Microsoft Academic Search

Seafloor classification studies are carried out at the central western continental shelf of India employing two frequency normal incidence single beam echo-sounder backscatter data. Echo waveform data from different seafloor sediment areas are utilized for present study. Three artificial neural network (ANN) architectures, e.g., Self-Organization Feature Maps (SOFM), Multi-Layer Perceptron (MLP), and Learning Vector Quantization (LVQ) are applied for seafloor

Vasudev Mahale; Bishwajit Chakraborty; Gajanan S. Navelkar; R. G. Prabhu Desai

2005-01-01

265

Tectonomagmatic Evolution of the Neo - Tethyan Region in the Iranian Continental Margin  

NASA Astrophysics Data System (ADS)

The tectonic history of Neo - Tethyan realm in Iran began with the rifting of the Central Iranian Block (CIB) separated from Arabia and Gondwana during Late Permian - Early Triassic time. This realm travelled to the north to creation of the Neo-Tethyan oceanic lithosphere. The subduction of the Neo-Tethys could start to the south of the Central Iranian Block at Late Triassic to Plio-Quaternary time. The subduction of the Neo - Tethyan ocean beneath the active continental margin of the Iranian block was established by arc magmatism and back - arc spreading. These magmatic activities are marked from SW to NE by the presence of: calc-alkaline arc magmatism from Late Triassic to Late Jurassic in the Sanandaj-Sirjan Zone (SSZ), back - arc spreading with Late Cretaceous in the Esfandagheh Colour Melange Zone (ECMZ), back-arc spreading with Late Cretaceous - Palaeocene Nain-Baft Ophiolitic Belt (NBOB) and calc-alkaline arc magmatism from Eocene to Plio-Quaternary in the Urumieh-Dokhtar Magmatic Zone (UDMZ). Urumieh-Dokhtar magmatic zone has been considered as a place for the main magmatic activities in the Central Iranian continent in the Cenozoic age. This magmatic arc is situated to the North of the Mesozoic arc of the Sanandaj-Sirjan zone and the back-arc basin of the Central Iranian Block of Cretaceous age. During Oligocene-Miocene time the magmatic activity favored to alkaline magmatism. Geochemical data confirm the presence of transtensional tectonic setting along the Urumieh-Dokhtar magmatic zone, opened during Paleogene and early Neogene due to the collision of the Arabia platform and Central Iranian continent. These magmatic activities are linked to the subduction of the Neo-Tethys to the North below the CIB, followed by the Paleogene collision and continental subduction of the Gondwana (Arabia) beneath the CIB along the Main Zagros Thrust (MZT). Keywords: Neo - Tethys; Gondwana; Central Iranian Block (CIB); Sanandaj-Sirjan Zone (SSZ); Esfandagheh Colour Melange Zone (ECMZ); Nain-Baft Ophiolitic Belt (NBOB); Urumieh-Dokhtar Magmatic Zone (UDMZ)

Monsef, R.; Monsef, I.; Rahgoshay, M.; Emami, M. H.; Shafaii Moghadam, H.

2009-04-01

266

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

SciTech Connect

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

Cochran, J.R.

1981-01-10

267

Quantifying Continental Margin Deformation North and South of the Opening of the Gulf of California---Evidence for Subduction Erosion?  

Microsoft Academic Search

A striking contrast exists in the geology of the continental margin north and south of the opening of the Gulf of California. For 1200km north of the Gulf opening, a reasonably well-preserved forearc region is associated with strongly magnetic 130-100 Ma plutonic rocks, volcanics and ophiolitic basement. This package of rocks is also coupled with a weakly magnetic eastern belt

E. M. Peterman; M. Grove; D. L. Kimbrough

2010-01-01

268

Rift to post-rift evolution of a ``passive'' continental margin: the Ponta Grossa Arch, SE Brazil  

Microsoft Academic Search

Low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct the post-rift evolution since the break-up of southwestern Gondwana. Thermochronological data obtained from apatite fission-track analysis of Neoproterozoic metamorphic and Paleozoic to Mesozoic siliciclastic rocks as well as Mesozoic dikes and alkaline intrusions from the Ponta Grossa Arch provided ages between 66.2 (1.3) and

A. O. B. Franco-Magalhaes; P. C. Hackspacher; U. A. Glasmacher; A. R. Saad

2010-01-01

269

Rift to post-rift evolution of a “passive” continental margin: the Ponta Grossa Arch, SE Brazil  

Microsoft Academic Search

Low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct\\u000a the post-rift evolution since the break-up of southwestern Gondwana. Thermochronological data obtained from apatite fission-track\\u000a analysis of Neoproterozoic metamorphic and Paleozoic to Mesozoic siliciclastic rocks as well as Mesozoic dikes and alkaline\\u000a intrusions from the Ponta Grossa Arch provided ages between 66.2 (1.3) and

A. O. B. Franco-MagalhaesP; P. C. Hackspacher; U. A. Glasmacher; A. R. Saad

2010-01-01

270

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

Microsoft Academic Search

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

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

2007-01-01

271

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

Microsoft Academic Search

Three ancient impact craters (Chesapeake Bay—35.7Ma; Toms Canyon—35.7Ma; Montagnais—51Ma) and one multiring impact basin (Chicxulub—65Ma) 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

C. Wylie Poag; Jeffrey B Plescia; Phillip C Molzer

2002-01-01

272

Oil-Spill Risk Analysis: Central and Western Gulf of Mexico Outer Continental Shelf Lease Sales 152 and 155.  

National Technical Information Service (NTIS)

The Federal Government has proposed to offer Outer Continental Shelf lands in Central and Western Gulf of Mexico for oil and gas leasing. Because oil spills may occur from activities associated with offshore oil production, the Minerals Management Service...

J. M. Price E. M. Lear

1994-01-01

273

Volume transport of the western boundary current penetrating into a marginal sea  

Microsoft Academic Search

The amount of penetration of a western boundary current into a marginal sea which is connected to an open ocean by two narrow straits is estimated from a linear, steady and barotropic theoretical model. In this model the western boundary current in the open ocean is driven by a wind stress imposed at the sea surface. The inflow of the

Shinya Minato; Ryuji Kimura

1980-01-01

274

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

NASA Astrophysics Data System (ADS)

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

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

275

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

NASA Astrophysics Data System (ADS)

Vast networks of submarine canyons and associated channels are incised into the U.S. Atlantic continental slope and rise. Submarine canyons form by differential erosion and deposition, primarily from sedimentary turbidity flows. Theoretical and laboratory studies have investigated the initiation of turbidity flows and their capacity to erode and entrain sedimentary material at distances far from the shelf edge. The results have helped understand the nature of turbidite deposits on the continental slope and rise. Nevertheless, few studies have examined the linkages between down-canyon sediment transport and the morphology of canyon/channel networks using mesoscale analyses of swath bathymetry data. We present quantitative analysis of 100-m resolution multibeam bathymetry data spanning ~616,000 km2 of the slope and rise between Georges Banks and the Blake Plateau (New England to North Carolina). Canyons are categorized as shelf-indenting or slope-confined based on spatial scale, vertical relief and connection with terrestrial river systems during sea level low stands. Shelf-indenting canyons usually represent the trunk-canyon of submerged channel networks. On the rise, shelf-indenting canyons have relatively well-developed channel-levees and sharp inner-thalwag incision suggesting much higher frequency and volume of turbidity flows. Because of the similarities between submarine canyon networks and terrestrial river systems, we apply methods originally developed to study fluvial morphology. Along-canyon profiles are extracted from the bathymetry data and the power-law relationship between thalwag gradient and drainage area is examined for more than 180 canyons along an ~1200 km stretch of the US Atlantic margin. We observe distinct thresholds in the power-law relationship between drainage area and gradient. Almost all canyons with heads on the upper slope contain at least two linear segments when plotted in log-log form. The first segment along the upper slope is flat (constant gradient, low area). The second segment dips (exponentially decreasing gradient with increasing area). We interpret the transition between the two segments to be either diffusive creep/landslide processes that evolve into turbidity flows or the boundary that separates up-canyon infilling from relic, lower-canyon incision. Furthermore, the threshold occurs at a nearly constant drainage area regardless of location and morphology of the drainage network. This suggests that time-averaged erosion rate in submarine canyons depends on frequency of turbidity flows, which in turn depends on the volume of unstable sediments deposited near canyon heads and along canyon walls. We find that the gradient-area relationship does not follow a power-law in shelf-indenting canyons, most likely due to allogenic processes of the continental shelf and linkage to terrestrial river discharge.

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

2011-12-01

276

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

277

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

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

278

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

279

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

280

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

NASA Astrophysics Data System (ADS)

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

Guerreiro, C.; Rosa, F.; Oliveira, A.; Cachão, M.; Fatela, F.; Rodrigues, A.

2009-01-01

281

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

SciTech Connect

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

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

1991-08-01

282

An interdisciplinary investigation of a recent submarine mass transport deposit at the continental margin off Uruguay  

NASA Astrophysics Data System (ADS)

Assessing frequency and extent of mass movement at continental margins is crucial to evaluate risks for offshore constructions and coastal areas. A multidisciplinary approach including geophysical, sedimentological, geotechnical, and geochemical methods was applied to investigate multistage mass transport deposits (MTDs) off Uruguay, on top of which no surficial hemipelagic drape was detected based on echosounder data. Nonsteady state pore water conditions are evidenced by a distinct gradient change in the sulfate (SO42-) profile at 2.8 m depth. A sharp sedimentological contact at 2.43 m coincides with an abrupt downward increase in shear strength from ˜10 to >20 kPa. This boundary is interpreted as a paleosurface (and top of an older MTD) that has recently been covered by a sediment package during a younger landslide event. This youngest MTD supposedly originated from an upslope position and carried its initial pore water signature downward. The kink in the SO42- profile ˜35 cm below the sedimentological and geotechnical contact indicates that bioirrigation affected the paleosurface before deposition of the youngest MTD. Based on modeling of the diffusive re-equilibration of SO42- the age of the most recent MTD is estimated to be <30 years. The mass movement was possibly related to an earthquake in 1988 (˜70 km southwest of the core location). Probabilistic slope stability back analysis of general landslide structures in the study area reveals that slope failure initiation requires additional ground accelerations. Therefore, we consider the earthquake as a reasonable trigger if additional weakening processes (e.g., erosion by previous retrogressive failure events or excess pore pressures) preconditioned the slope for failure. Our study reveals the necessity of multidisciplinary approaches to accurately recognize and date recent slope failures in complex settings such as the investigated area.

Henkel, Susann; Strasser, Michael; Schwenk, Tilmann; Hanebuth, Till J. J.; Hüsener, Johannes; Arnold, Gail L.; Winkelmann, Daniel; Formolo, Michael; Tomasini, Juan; Krastel, Sebastian; Kasten, Sabine

2011-08-01

283

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

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

284

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. ?? 1980.

Dillon, W. P.; Robb, J. M.; Greene, H. G.; Lucena, J. C.

1980-01-01

285

Multiproxy characterization and budgeting of terrigenous end-members at the NW African continental margin  

NASA Astrophysics Data System (ADS)

Grain-size, terrigenous element and rock magnetic remanence data of Quaternary marine sediments retrieved at the NW African continental margin off Gambia (gravity core GeoB 13602-1, 13°32.71'N, 17°50.96'W) were jointly analyzed by end-member (EM) unmixing methods to distinguish and budget past terrigenous fluxes. We compare and cross-validate the identified single-parameter EM systems and develop a numerical strategy to calculate associated multiparameter EM properties. One aeolian and two fluvial EMs were found. The aeolian EM is much coarser than the fluvial EMs and is associated with a lower goethite/hematite ratio, a higher relative concentration of magnetite and lower Al/Si and Fe/K ratios. Accumulation rates and grain sizes of the fluvial sediment appear to be primarily constrained by shore distance (i.e., sea level fluctuations) and to a lesser extent by changes in hinterland precipitation. High dust fluxes occurred during the Last Glacial Maximum (LGM) and during Heinrich Stadials (HS) while the fluvial input remained unchanged. Our approach reveals that the LGM dust fluxes were ˜7 times higher than today's. However, by far the highest dust accumulation occurred during HS 1 (˜300 g m-2 yr -1), when dust fluxes were ˜80 fold higher than today. Such numbers have not yet been reported for NW Africa, and emphasize strikingly different environmental conditions during HSs. They suggest that deflation rate and areal extent of HSs dust sources were much larger due to retreating vegetation covers. Beyond its regional and temporal scope, this study develops new, in principle, generally applicable strategies for multimethod end-member interpretation, validation and flux budgeting calibration.

Just, Janna; Heslop, David; Dobeneck, Tilo; Bickert, Torsten; Dekkers, Mark J.; Frederichs, Thomas; Meyer, Inka; Zabel, Matthias

2012-09-01

286

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

287

Miocene to Present evolution of the Calabria Tyrrhenian continental margin (Southern Tyrrhenian Sea)  

NASA Astrophysics Data System (ADS)

The Miocene to Present evolution of the Calabria Tyrrhenian Continental Margin (CTCM, Southern Tyrrhenian Sea) are reconstructed using two ENE-WSW oriented, near-vertical seismic profiles (CROP-M27 and SISTER 11 lines). The interpreted profiles were time-to-depth converted, merged and translated in a geological section, which was also extended to the Tyrrhenian bathial plain and the Calabrian arc using wide-angle seismic data [Scarascia et al., 1994], and tested with gravity modelling. Across the CTCM, top of KCU is laterally variable in depth forming basins filled by Oligo-Miocene clastic to terrigenous deposits up to 1500m thick. Basins are separated by major structures with contractional or transcurrent kinematics, where faults are arranged in a positive flower structure fashion, affecting the KCU as well as lower Oligocene to Miocene deposits. The Messinian evaporites display essentially a constant thickness of ~-400m with the exception of the Paola Basin where deep-water Messinian evaporites are up to 1000 m thick. Plio-Quaternary deposits display a remarkable variation in thickness from ~-4.5 km in the Paola Basin to less then 400m in the central sector of the margin. Plio-Quaternary sediments are internally sub-divisible into four sub-units separated by tectonics enhanced angular unconformities. W-ward vergent reverse faults with limited vertical displacement offset the top of KCU as well as the Oligo-Miocene sedimentary and evaporitic units in the eastern side of the Paola basin and in the distal part of the CTCM where a number of closely spaced, W-vergent thrust faults are also observed in the Plio-Pleistocene deposits. Along the CTCM, the only significant normal fault which was identified is located around its central sector, dips to the W and has a displacement of ~-580m. Across the margin, the Moho was inferred at ~-35 km beneath the Calabria Arc and shallows up to 24 km in correspondence with the coastline. Moho deepens again to a depth of ~-28 km in correspondence with the depocenter of the Paola Basin and then climbs gently and regularly reaching a depth of ~-15 km at the continent-ocean transition. Westward, the ~-8-9 km thick oceanic crust of the Marsili basin is recognised. The CTCM crust has undergone substantial thinning that starts becoming important in correspondence with the W coast of Calabria where thinning is up to delta=1.5 and, on the whole, shows then a fairly gradual increase from the E to the W where thinning reaches up to delta=3.2 at the continent-ocean transition. The disaggregated analysis of thinning factors for the upper (including KCU, Oligo-Miocene and Plio-Pleistocene deposits) and lower crust identify a long wavelength trend which is essentially similar to that of the entire crust thereby suggesting that regional thinning affected in equal amounts the upper and lower crust. Two important deviations are observed, underneath the Paola Basin and towards the zone of the continent-ocean transition where upper crustal thinning is much larger than the crustal one. On the basis of tectonic features recognised in the KCU, the CTCM may be partitioned into three segments characterized by different post Late-Messinian tectonic deformation and separated by localised strike-slip fault zone. References Finetti, I. R., (2005). The Calabrian Arc and subduction Ionian slab from new CROP seismic data. In: CROP Project, Deep seismic Exploration of the Central Mediterranean and Italy (I.R Finetti, ed.), pp. 393-412, Atlas in GeoScience 1, Elsevier. Scarascia, S., Lozej, A., Cassinis, R., (1994). Crustal structures of the Ligurian, Tyrrhenian and Ionian Sea and adjacent onshore areas interpreted from wide-angle seismic profile. Boll. Geofis. Teor. Appl. 36, 5 -19.

Pepe, F.; Sulli, A.; Bertotti, G.; Cella, F.

2009-04-01

288

Geology and ages of porphyry and medium- to high-sulphidation epithermal gold deposits of the continental margin of Northeast China  

Microsoft Academic Search

The continental margin of Northeast China, an important part of the continental margin-related West Pacific metallogenic belt, hosts numerous types of gold-dominated mineral deposits. Based on ore deposit geology and isotopic dating, we have classified hydrothermal gold–copper ore deposits in this region into four distinct types: (1) gold-rich porphyry copper deposits, (2) gold-rich porphyry-like copper deposits, (3) medium-sulphidation epithermal copper–gold

Shi-Jiong Han; Jing-Gui Sun; Ling-An Bai; Shu-Wen Xing; Peng Chai; Yong Zhang; Fan Yang; Lan-Jing Men; Yi-Xin Li

2012-01-01

289

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

Microsoft Academic Search

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

John Mc L. Miller; Martin S. Norvick; Christopher J. L. Wilson

2002-01-01

290

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

291

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

292

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

USGS Publications Warehouse

Sedimentary sections up to 6-14 km thick lie beneath many areas of the Antarctic continental margin. The upper parts of the sections contain up to 6 km of Cenozoic glacial and possibly non-glacial sequences that have prograded the continental shelf up to 85 km. We describe the Cenozoic sequences using two general categories based on their acoustic geometries. Type IA sequences, which account for most prograding of the Antarctic continental shelf, have complex sigmoidal geometries and some acoustic characteristics atypical of low-latitude margins, such as troughs and mounds lying parallel and normal to the shelf edge and high velocities (2.0-2.6 km/s) for flat layers within 150 m of the seafloor. Type IIA sequences, which principally aggrade the paleoshelf, lie beneath type IA sequences and have mostly simple geometries and gently dipping reflections. The prograding sequences are commonly located near the seaward edges of major Mesozoic and older margin structures. Relatively rapid Cenozoic subsidence has occured due to the probable rifting in the Ross Sea, thermal subsidence in the Antarctic Peninsula, and isostatic crustal flexure in Wilkes Land. In Prydz Bay and the Weddell Sea, prograding sequences cover Mesozoic basins that have undergone little apparent Cenozoic tectonism. Grounded ice sheets are viewed by us, and others, as the principal mechanism for depositing the Antarctic prograding sequences. During the initial advance of grounded ice the continental shelf is flexurally overdeepened, the inner shelf is heavily eroded, and gently dipping glacial strata are deposited on the shelf (i.e type IIA sequences). The overdeepened shelf profile is preserved (a) during glacial times, by grounded ice sheets episodically crossing the shelf, eroding sediments from onshore and inner shelf areas, and depositing sediments at the front of the ice sheet as outer shelf topset-banks and continental slope foreset-aprons (i.e. type IA sequences), and (b) during interglacial times, like today, by little or no clastic sedimentation on the continental shelf other than beneath retreated ice shelves lying far from the continental sheld edge. Ice streams carve broad depressions across the shelf and carry abundant basal sediments directly to the continental shelf edge, thereby creating troughmouth fans and sheet-like prograding sequences (i.e. type IA sequences). Numerous acoustic unconformities and multiple overcompacted layers within the prograding sequences suggest major fluctuations of the Antarctic Ice Sheet. The available drilling and seismic interpretations provide the following history: (1) Cenozoic ice sheets have existed in places near the continental shelf since middle to late Eocene time. (2) A grounded Antarctic ice sheet first expanded to the continental shelf edge, with probable overdeepening of the outer shelf, in late Eucene to early Oligocene time in Prydz Bay, possibly in early Miocene time in the Ross Sea, and at least by middle Miocene time in the Weddell Sea. (3) The relative amounts of shelf prograding and inferred ice-volume variations (and related sea-level changes) have increased since middle to late Miocene time in the eastern Ross Sea, Prydz Bay, and possibly Weddell Sea. Our analysis is preliminary. Further acoustic surveys and scientific drilling are needed to resolve the proximal Antarctic record of glacio-eustatic, climatic, and tectonic events recorded by the prograding sequences. ?? 1991.

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

1991-01-01

293

Crustal structure across the Grand Banks-Newfoundland Basin Continental Margin - I. Results from a seismic refraction profile  

NASA Astrophysics Data System (ADS)

A P-wave velocity model along a 565-km-long profile across the Grand Banks-Newfoundland Basin rifted margin is presented. Continental crust ~36 km thick beneath the Grand Banks is divided into upper (5.8-6.25 km s-1), middle (6.3-6.53 km s-1) and lower crust (6.77-6.9 km s-1), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6-7 km thick occur in two primary layers within the Jeanne d'Arc and the Carson basins (~3 km s-1 in upper layer; ~5 km s-1 in lower layer). Abrupt crustal thinning (Moho dip ~35°) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ~60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5-1.5 s-1) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6-7.9 km s-1) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5-6.3 km s-1) and layer 3 (6.3-7.2 km s-1) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side.

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

2006-10-01

294

Subduction evolution and mantle dynamics at a continental margin: Central North Island, New Zealand  

NASA Astrophysics Data System (ADS)

Central North Island, New Zealand, provides an unusually complete geological and geophysical record of the onset and evolution of subduction at a continental margin. Whereas most subduction zones are innately two-dimensional, North Island of New Zealand displays a distinct three-dimensional character in the back-arc regions. Specifically, we observe "Mariana-type" subduction in the back-arc areas of central North Island in the sense of back-arc extension, high heat flow, prolific volcanism, geothermal activity, and active doming and exhumation of the solid surface. Evidence for emplacement of a significant percent of new lithosphere beneath the central North Island comes from heat flux of 25 MW/km of strike (of volcanic zone) and thinned crust underlain by rocks with a seismic wave speed consistent with underplated new crust. Seismic attenuation (Qp-1) is high (˜240), and rhyolitic and andesitic volcanism are widespread. Almost complete removal of mantle lithosphere is inferred here in Pliocene times on the basis of the rock uplift history and upper mantle seismic velocities as low as 7.4 ± 0.1 km/s. In contrast, southwestern North Island exhibits "Chilean-type" back-arc activity in the sense of compressive tectonics, reverse faulting, low-heat-flow, thickened lithosphere, and strong coupling between the subducted and overriding plates. This rapid switch from Mariana-type to Chilean-type subduction occurs despite the age of the subducted plate being constant under North Island. Moreover, stratigraphic evidence shows that processes that define the extensional back-arc area (the Central Volcanic Region) are advancing southward into the compressional system (Wanganui Basin) at about 10 mm/yr. We link the progression from one system to another to a gradual and viscous removal of thickened mantle lithosphere in the back-arc regions. Thickening occurred during the Miocene within the Taranaki Fault Zone. The process of thickening and convective removal is time- and temperature-dependent and has left an imprint in both the geological record and geophysical properties of central North Island, which we document and describe.

Stern, T. A.; Stratford, W. R.; Salmon, M. L.

2006-12-01

295

Methane release from hydrate dissociation on the West Svalbard continental margin  

NASA Astrophysics Data System (ADS)

Sonar data from the West Svalbard continental margin in August-September 2008 have shown the presence of plumes of methane bubbles emanating from the seabed up-slope from the upper limit of the methane hydrate stability zone. In the same area, there is evidence for an increase in bottom water temperatures of 1oC over the last 30 years. Numerical modelling has been used to evaluate the effect of such warming of the seabed on the release of methane from dissociation of hydrate within the gas hydrate stability zone. The amount of methane released at the seabed, derived from dissociating hydrate, depends on a number of often poorly constrained variables. The time history of seabed temperature, the concentration of hydrate and its distribution in the sediment beneath the seabed and the effective permeability of the sediments affect the amount of gas produced and the time it takes for the gas to flow to the seabed. Using a numerical model, many different scenarios have been examined, derived from the uncertainty in the data describing the system. Over the 30-year warming period, numerical models show that gas reaches the seabed within this period if the hydrate is less than 5 metres below the seabed with a permeability of >9x10-14 m2 and an initial hydrate concentration of >10% of pore space. Deeper hydrate layers undergo less dissociation as it takes time for seabed warming to move down through the sediments. Small amounts of dissociation and less concentrated hydrate layers will produce less gas and gas flow will be slower due to reduced relative permeability. Lower absolute permeability also slows the gas flow to the seabed. The time history of temperature change at the seabed over the last 1000 years could have redistributed hydrate within the hydrate stability zone. Warming associated with the Medieval warm period would shoal the base of the hydrate stability zone (BHSZ) and increase hydrate concentration at this depth. The subsequent cooling would deepen the BHSZ leaving a layer of high hydrate concentration above the BHSZ. Such a process could be responsible for moving hydrate close enough to the seabed that it has released gas to the seabed in the last 30 years. Our results show a time lag between the seabed warming and the release of gas at the seabed caused by the time taken for the hydrate to dissociate and the time for gas to flow to the seabed. The numerical models have been run into the future and the duration of gas release due to 30 years of warming is presented.

Thatcher, Kate; Westbrook, Graham

2010-05-01

296

4-D Numerical Modeling of Crustal Growth at Active Continental Margins  

NASA Astrophysics Data System (ADS)

Crustal growth and topography development in subduction-related arcs are intimately related to magmatic processes and melt production above subducting slabs. Lateral and temporal variations in crustal thickness and composition have been observed in nature, but until now no integrated approach has been developed to comprehensively understand magmatic activity in subduction-related arcs. Here we investigate the 4-D spatial, temporal and compositional character of continental crustal growth at active margins using a new 4-D (space-time) petrological-thermomechanical numerical model of a subduction-related magmatic arc. Based on a series of numerical experiments we demonstrate that crustal growth inside the arc is inherently clustered in both space and time. The characteristic wavelength of variations in crustal thickness and topography along the arc is defined mainly by plate convergence velocity: faster subduction favors longer wavelength. The clusters of new crust are formed mainly by basaltic melt episodically extracted from partially molten peridotite due to lateral variation of water release and transport in the mantle wedge. Melts derived from subducted oceanic crust and sediments could contribute up to 15-50 vol% to the arc crust growth and their relative proportion is maximal at the onset of subduction. The total amount of newly formed crust correlates mainly with the amount of convergence since the beginning of subduction and is not strongly influenced by the plate convergence velocity. Indeed, slower subduction and lower melt extraction efficiency helps partially molten sediments and oceanic crust to be transported into the mantle wedge by hydrated, partially molten diapiric structures. For our modeled type of stable subduction, the maximum crustal additional rate (25-40 km**3/km/Myr) occurs when amount of convergence reaches around 700 km. Mantle wedge structures developed in our models correlate well with available geophysical (seismological) observations for the Alaskan subduction zone. In particular, partially molten mantle plumes found in our models could explain low seismic anomalies in the mantle wedge, whereas mobile water and water release patterns could reflect paths and sources for magmatic activity evidenced by seismic b-value and Vp/Vs ratio analysis.

Zhu, Guizhi; Gerya, Taras; Tackley, Paul; Kissling, Eduard

2013-04-01

297

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

NASA Astrophysics Data System (ADS)

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

Zhao, Z.

2011-12-01

298

Mining the Western North American Volcanic and Intrusive Rock Database (NAVDAT) for Insights Into the Origin of Continental Intraplate Magmatism  

Microsoft Academic Search

Space-time-composition patterns in Late Cretaceous and Cenozoic igneous activity in southwestern North America are critical to models of the evolution of the deep continental lithosphere in this region. In the 1970's, for example, consideration of igneous rocks found some 1,000km inland from the continental margin in the southern Rocky Mountains led directly to suggestions that significant variations in the dip

G. L. Farmer; K. Fornash

2008-01-01

299

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

NASA Astrophysics Data System (ADS)

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

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

2000-04-01

300

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

301

78 FR 45557 - Gulf of Mexico, Outer Continental Shelf (OCS), Western Planning Area (WPA) Oil and Gas Lease Sale...  

Federal Register 2010, 2011, 2012, 2013

...Western Planning Area (WPA) Oil and Gas Lease Sale 233 AGENCY: Bureau of...Sale 233 on the Gulf of Mexico OCS Oil and Gas Lease Sales: 2013-2014; Western...Proposed Final Outer Continental Shelf Oil & Gas Leasing Program: 2012-2017....

2013-07-29

302

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

303

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

304

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

305

Occurrence of Marine Gas Hydrates in the Indian Continental Margin: Results of the Indian National Gas Hydrate Program (NGHP) Expedition 01  

Microsoft Academic Search

Studies of geologic and geophysical data from the offshore of India have revealed two geologically distinct areas with seismically inferred gas hydrate occurrences: the passive continental margins of the Indian Peninsula and along the Andaman convergent margin. The Indian National Gas Hydrate Program (NGHP) Expedition 01 was designed to study the gas hydrate occurrences both spatially and temporally off the

T. S. Collett; N. Scientific Party

2007-01-01

306

Halocline water modification and along slope advection at the Laptev Sea continental margin  

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

307

A north to south transect of Holocene southeast Atlantic continental margin sediments: Relationship between aerosol transport and compound-specific ?13C land plant biomarker and pollen records  

NASA Astrophysics Data System (ADS)

We examined near-surface, late Holocene deep-sea sediments at nine sites on a north-south transect from the Congo Fan (4°S) to the Cape Basin (30°S) along the Southwest African continental margin. Contents, distribution patterns and molecular stable carbon isotope signatures of long-chain n-alkanes (C27-C33) and n-alkanols (C22-C32) are indicators of land plant vegetation of different biosynthetic types, which can be correlated with concentrations and distributions of pollen taxa in the same sediments. Calculated clusters of wind trajectories and satellite Aerosol Index imagery afford information on the source areas for the lipids and pollen on land and their transport pathways to the ocean sites. This multidisciplinary approach on an almost continental scale provides clear evidence of latitudinal differences in lipid and pollen composition paralleling the major phytogeographic zonations on the adjacent continent. Dust and smoke aerosols are mainly derived from the western and central South African hinterland dominated by deserts, semi-deserts and savannah regions rich in C4 and CAM plants. The northern sites (Congo Fan area and northern Angola Basin), which get most of their terrestrial material from the Congo Basin and the Angolan highlands, may also receive some material from the Chad region. Very little aerosol from the African continent is transported to the most southerly sites in the Cape Basin. As can be expected from the present position of the phytogeographic zones, the carbon isotopic signatures of the n-alkanes and n-alkanols both become isotopically more enriched in 13C from north to south. The results of the study suggest that this combination of pollen data and compound-specific isotope geochemical proxies can be effectively applied in the reconstruction of past continental phytogeographic developments.

Rommerskirchen, Florian; Eglinton, Geoffrey; Dupont, Lydie; Güntner, Ute; Wenzel, Claudia; RullköTter, Jürgen

2003-12-01

308

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

309

Eastern margin of the Ross Sea Rift in western Marie Byrd Land, Antarctica: Crustal structure and tectonic development  

NASA Astrophysics Data System (ADS)

The basement rock and structures of the Ross Sea rift are exposed in coastal western Marie Byrd Land (wMBL), West Antarctica. Thinned, extended continental crust forms wMBL and the eastern Ross Sea continental shelf, where faults control the regional basin-and range-type topography at ˜20 km spacing. Onshore in the Ford Ranges and Rockefeller Mountains of wMBL, basement rocks consist of Early Paleozoic metagreywacke and migmatized equivalents, intruded by Devonian-Carboniferous and Cretaceous granitoids. Marine geophysical profiles suggest that these geological formations continue offshore to the west beneath the eastern Ross Sea, and are covered by glacial and glacial marine sediments. Airborne gravity and radar soundings over wMBL indicate a thicker crust and smoother basement inland to the north and east of the northern Ford Ranges. A migmatite complex near this transition, exhumed from mid crustal depths between 100-94 Ma, suggests a profound crustal discontinuity near the inboard limit of extended crust, ˜300 km northeast of the eastern Ross Sea margin. Near this limit, aeromagnetic mapping reveals an extensive region of high amplitude anomalies east of the Ford ranges that can be interpreted as a sub ice volcanic province. Modeling of gravity data suggests that extended crust in the eastern Ross Sea and wMBL is 8-9 km thinner than interior MBL (? = 1.35). Gravity modeling also outlines extensive regions of low-density (2300-2500 kg m-3) buried basement rock that is lighter than rock exposed at the surface. These regions are interpreted as bounded by throughgoing east-west faults with vertical separation. These buried low-density rocks are possibly a low-density facies of Early Paleozoic metagreywacke, or the low-density epizonal facies of Cretaceous granites, or felsic volcanic rocks known from moraines. These geophysical features and structures on land in the wMBL region preserve the record of middle and Late Cretaceous development of the Ross Sea rift. Thermochronology data from basement rocks and offshore stratigraphy suggest that the wMBL rift margin formed and most extension occurred in mid- and Late Cretaceous time, before seafloor spreading initiated between wMBL and the Campbell Plateau. The Cretaceous tectonic record in wMBL contrasts with the Transantarctic Mountains that form the western rift margin, where significant rift-flank relief developed in middle Tertiary time.

Luyendyk, Bruce P.; Wilson, Douglas S.; Siddoway, Christine S.

2003-10-01

310

Western Tasman Sea Floor  

Microsoft Academic Search

The western Tasman Sea floor is characterised by physiographic units that tend to parallel the coastline of south-eastern Australia. The margin has a narrow continental shelf and a steep continental slope. Sediment derived mainly from northern New South Wales and Queensland in the north, and from Tasmania and Victoria in the south, has built a system of fans forming the

John R. Conolly

1969-01-01

311

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

312

Active faulting within the offshore western Gulf of Corinth, Greece: Implications for models of continental rift deformation  

NASA Astrophysics Data System (ADS)

Discrimination between different lithospheric extension models focusing on the roles of low-angle vs. high-angle faulting, and how strain is distributed, requires high-fidelity imaging of brittle deformation. High-resolution seismic reflection and multibeam bathymetric data in the western Gulf of Corinth continental rift were collected to establish the contribution of offshore faults to extension. Onshore fault slip here is significantly less than expected from geodetic strain rates. The rift at this location is a half-graben tilted to the north by S-dipping faults within the uppermost crust. A basement horst on the northern margin is uplifted by the North and South Eratini faults, and the axial channel is fault controlled. Subsided lowstand shorelines in the hanging wall of the North Eratini and the well-studied Aigion fault suggest that the faults have similar displacements. Summed extension from the four major faults across this part of the rift (Eliki, Subchannel, South Eratini, North Eratini) is ˜8 16 mm/yr, thereby reconciling geologic and geodetic data sets. Distributed deformation across isolated multiple faults can model this part of the rift without recourse to, and potentially incompatible with, an underlying low-angle detachment.

McNeill, L. C.; Cotterill, C. J.; Henstock, T. J.; Bull, J. M.; Stefatos, A.; Collier, R. E. Ll.; Papatheoderou, G.; Ferentinos, G.; Hicks, S. E.

2005-04-01

313

ALONG STRIKE VARIABILITY IN THE CRUSTAL STRUCTURE OF THE WEST INDIAN VOLCANIC CONTINENTAL MARGIN  

Microsoft Academic Search

An interpretation of the crustal structure in the area of the Indus Submarine fan, offshore India and Pakistan is presented. This case study combines regional satellite gravity interpretation with detailed mapping of high quality seismic data. The area shows many classic characteristics of a volcanic-type margin. There is major variation in the structure and style of volcanism across the margin

Richard I. Corfield; Scott Carmichael; John Bennett; Shakeel Akhter; Mansoor Fatimi; Richard Tozer; Tim Craig

314

Slope failures and stability analysis of shallow water prodeltas in the active margins of Western Greece, northeastern Mediterranean Sea  

NASA Astrophysics Data System (ADS)

Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment failures on the low-angle (0.5°-2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80-150 m long and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10-15 m below seafloor. Shear planes develop in the lower part of muddy and/or gas charged HST foresets. Deeper basal transparent muddy layers of the early HST bottomset, together with the late Pleistocene transgressive systems tract sequences (TST), are mostly unaffected. On the steeper (2°-6°) fan delta slopes of the western Gulf of Corinth debris flows and avalanches with a significant retrogressive component dominate slope destabilisation. Sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments thereby indicating that failure planes are in the late HST foresets/upper part of the early HST bottomsets gas charged zone. The foresets of the HST prodelta deposits display high water content (30-80%), low bulk density (1.4-1.9 g cm-3) and relatively low values of undrained shear strength (3-20 kPa). The water content of the HST distal muddy bottomsets is relatively higher (50-110%) and bulk density relatively lower (1.3-1.7 g cm-3) with low values of shear strength (2-10 kPa). The shear strength of the gas releasing sediment layer displays lower values (2-9 kPa) relative to the overlying, post failure, muddy sediments of the late 100-300 years. Slope stability was calculated using the normalised soil parameter (NSP) method under undrained conditions for normally consolidated prodelta sediments. This analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26.6-29.6% g for the HST foresets and 12.4-14.1% g for the basal transparent layer belonging to the early HST bottomsets. Consequently the early HST bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations (PGAs) for the next 50 years are expected to range from 19 to 30% g. Moreover, our results show that new glide planes in the prodeltaic sediment bodies of the seismically active continental margins of Western Greece will likely develop from the gas charged sediments of the lower part of the HST foresets to the upper part of early HST bottomsets.

Lykousis, V.; Roussakis, G.; Sakellariou, D.

2009-06-01

315

Seismic Patterns of the Guerrero-Oaxaca, Mexico Region and its Relationship to the Continental Margin Structure  

NASA Astrophysics Data System (ADS)

The purpose of this article is to make aware seismic evidences that suggest a possible segmentation of the continental margin at the Guerrero-Oaxaca, Mexico region. The analysis of the aftershocks of relevant earthquakes and a survey of microseismicity using a portable seismographs network permit to infer the characteristics of the seismic patterns of the Acapulco-Ometepec segment of the Middle America subduction region. Two different seismic regimes are apparent, one in the Acapulco-Copala area and another in the Ometepec-Pinotepa area. In the Acapulco-Copala area the seismicity is disperse and broader it starts at the trench up to 80 km inland. Seismicity in the Ometepec-Pinotepa area on the other hand, is narrower and concentrates near the coast. The seismic zones are separated by narrow bands of low seismic activity nearly perpendicular to the coast and trench axis. These low activity strips are interpreted as corresponding to disruptions of the continental margin. This result is important to understand the mechanics of the major earthquakes that frequently occur in this region.

Yamamoto, J.; Gonzalez, T.; Jimenez, Z.; Quintanar, L.

2005-12-01

316

Sm--Nd Garnet Geochronology Demonstrates Wholesale Transformation of Continental Crust During UHP Subduction---Western Gneiss Region, Norway  

NASA Astrophysics Data System (ADS)

Determining the extent to which ultrahigh-pressure (UHP) terranes transform to high-pressure (HP) minerals during subduction is central to understanding the processes attending the subduction of continental margins. The Western Gneiss Region (WGR) of Norway is one of two giant UHP terranes on Earth, and as such constitutes an important natural laboratory for investigating these processes. The distribution of UHP and HP eclogites shows that a 60,000 km2 area was subducted, but these eclogites are sparse (1--2 vol%) blocks in a sea of chiefly quartzofeldspathic, amphibolite-facies orthogneiss. Aside from the eclogites, the only widespread indicator of (U)HP metamorphism in this enclosing orthogneiss is garnet. This study uses high- precision Sm--Nd geochronology of these garnets to evaluate the degree of transformation of the WGR to high- pressure minerals during subduction and back to low-pressure minerals during exhumation. We compare these garnet ages to the 420--400 Ma Sm--Nd ages reported for WGR eclogites (e.g., Kylander-Clark et al., 2007). Eight garnet samples dated using the garnet--whole-rock isochron technique yielded the following results: 1) 921.7 ± 1.3 Ma for a sample from the easternmost section of the WGR; 2) 418.1 ± 1.7 Ma, 417.3 ± 1.2 Ma and 403.9 ± 0.8 Ma for three UHP samples; 3) 410.3 ± 2.5, 406.9 ± 1.5 Ma and 398.5 ± 0.8 Ma for three HP samples; 4) 587.3 ± 4.3 Ma for a fourth HP sample. These ages are interpreted to represent 1) the Proterozoic granulite-facies metamorphism of ca. 950 Ma; 2 & 3) (U)HP prograde metamorphism of the continental crust; and 4) a mixed age derived from the granulite-facies and (U)HP metamorphic events. The Proterozoic garnet age implies that the easternmost WGR did not transform to eclogite-facies minerals. In contrast, the good agreement between the 420--400 Ma eclogite ages (e.g., Kylander-Clark et al., 2007) and the 418--398 Ma gneiss ages reported here, indicates that the bulk of the WGR continental crust underwent wholesale transformation at (U)HP conditions. That the garnets are the only HP silicate mineral (other than quartz) surviving within a quartzofeldspathic gneiss composed of otherwise amphibolite-facies minerals indicates wholesale retrogression during exhumation.

Peterman, E. M.; Hacker, B. R.; Baxter, E. F.

2007-12-01

317

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

318

Particle fluxes and ecosystem response on a continental margin: the 1985 1988 Mediterranean ECOMARGE experiment  

Microsoft Academic Search

The first experiment of the ECOMARGE programme (ECOsystèmes de MARGE continentale) was initiated in 1983 1984, in the Gulf of Lions (northwestern Mediterranean Sea). The objectives of the ECOMARGE---I experiment were: to quantify the transfer of particulate matter, in general, and of organic carbon, in particular, from its introduction to and formation in the waters of the continental shelf---to its

Andre Monaco; Pierre Biscaye; Jacques Soyer; Roger Pocklington; Serge Heussner

1990-01-01

319

Global distribution of large submarine canyons: Geomorphic differences between active and passive continental margins  

Microsoft Academic Search

The aim of this study is to assess the global occurrence of large submarine canyons to provide context and guidance for discussions regarding canyon occurrence, distribution, geological and oceanographic significance and conservation. Based on an analysis of the ETOPO1 data set, this study has compiled the first inventory of 5849 separate large submarine canyons in the world ocean. Active continental

Peter T. Harris; Tanya Whiteway

2011-01-01

320

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

Microsoft Academic Search

We present the aeromagnetic compilation of the Norwegian mainland and its shelf area and its importance for geological models and tectonic studies. The combined data-set reveal that the bedrock structures are continuous from the Baltic Shield under the Caledonian orogen into the continental shelf and that the late-Caledonian collapse of the Caledonian orogene has influenced the style of extension on

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

2007-01-01

321

Origin of Volcanic Seamounts Offshore California Related to Interaction of Abandoned Spreading Centers with the Continental Margin  

NASA Astrophysics Data System (ADS)

The numerous NE-SW trending volcanic seamounts at the continental margin offshore central to Southern California owe their existence to the complex tectonics that resulted when small spreading ridge segments intersected and partly subducted beneath the continental margin during the Miocene plate reorganization. A limited number of dredged samples had indicated multiple episodes of coeval, alkalic volcanism at geographically widely separated sites (Davis et al., 2002, GSA Bull. 114, 316-333). 450 new samples were collected from 8 seamounts from 37. 5°N to 32.3°N with MBARI's ROV Tiburon. Ar-Ar ages for 50 of these samples extend the ages of volcanism from 18 Ma to 2.8 Ma. The dominant whole rock compositions are differentiated alkalic basalt, hawaiite, and mugearite, but include minor benmoreite, trachyte, and rare tholeiitic basalt. This entire range of compositions is also present in glassy margins or in volcaniclastic breccias, except for the trachyte, which had no glassy margins. Trace element abundances and ratios (e.g. REE, Zr, Nb, Ta, Th, Ba, etc.) are typical for ocean island basalt, whether the seamount is located on the Pacific plate (e.g. Pioneer, Gumdrop, Guide, Davidson, San Juan, San Marcos) or on the continental slope (Rodriguez) or within the Southern Continental Borderland (Northeast Bank). Nine samples, predominantly from Rodriguez Seamount, show a calc-alkaline trend with lower Nb, Ta, and higher Th. These samples may be erratics (Paduan et al., 2007, Marine Geology, in press). Sr, Nd, and Pb isotopic compositions plot within the Pacific N-MORB field for the northern seamounts (Pioneer, Gumdrop, Guide) but suggest progressively more radiogenic sources southward. There is considerable scatter at each site, especially with regard to 87Sr/86Sr, despite severe acid-leaching of the samples. Isotopic and trace element compositions indicate sources that are heterogeneous at a small scale. Chondrite-normalized Ce/Yb suggest smaller degree of melting and more alkalic compositions with decreasing age, although there is again considerable scatter. Chondrite-normalized La/Sm versus Zr/Nb form a continuum from the seamount lavas to depleted N-MORB and E-MORB suggesting a common origin by decompression melting of a mantle source with randomly distributed enriched heterogeneities, which are incorporated to a greater degree with decreasing degree of melting. Based on symmetric magnetic anomalies, only Davidson Seamount has been identified as straddling a fossil spreading center (Lonsdale, 1991, AAPG Mem. 47, 87-125). However, the other seamounts along the continental margin with the same NE-SW orientation and similar geochemical characteristics probably originated in a similar setting, erupting lavas along zones of weakness in the ocean floor fabric related to past seafloor spreading. Small volumes of magma can apparently rise long after spreading ceases if there is enough enriched source component to facilitate melting combined with zones of weakness in the underlying ocean crust fabric and/or extensional tectonics.

Davis, A. S.; Clague, D. A.; Paduan, J. B.; Cousens, B. L.; Huard, J.

2007-12-01

322

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.

323

Continental break-up history of conjugate poor magmatic margins from seismic reflection (oriental Gulf of Aden)  

NASA Astrophysics Data System (ADS)

The Gulf of Aden is an oceanic basin separating Arabia from Somalia. The rifting started 35 Ma ago followed by oceanic spreading from 17.6 Ma. The gulf orientation (N75°E) and the kinematics (about N30°E divergence) mark an oblique rifting where normal faults striking between N70°E (rift axis parallel) and N110°E (perpendicular to the divergence), are due to an extension direction probably evolving from N20°E to N160°E. The accurate 3D structure of the margins and the influence of structural inheritance or thermal and rheological evolution need to be better constrained. In order to answer this question, we mapped the tectonic features of the first-order segment between Alula-Fartak and Socotra Fracture Zones of the eastern Gulf of Aden continental margin. The Encens cruise (Leroy et al., 2006) take place in this area where the syn-rift structures are well exposed. Multibeam bathymetry, 360 channels seismic reflection (10 km spaced profiles), gravity and magnetism data were gathered. Furthermore one reflection seismic profile was processed with a pre-stack depth migration method. This excellent-quality dataset will permit us to image the structure of the margin and to propose an evolution from rifting to the onset of oceanic spreading. These results complement the field work realized onshore on conjugate margins (Oman and Socotra). Thus the land evolution can be correlated to the distal evolution. The style of deposit seems completely different in the proximal and in distal parts of the margin. Indeed fault controlled syn-rift carbonate systems, well developed onshore, are not really well expressed offshore. After the major syn-rift structuration in grabens and horsts, the deformation localised where the crust is the thinnest. This occurred in the distal margin graben (DIM) at the northern boundary of the Ocean-Continent Transition (OCT) represented by the OCT ridge. At the onset of the OCT formation, a differential uplift induces a landslide on the top of the deepest tilted block and the crustal deformation is localised in the southern part of the DIM graben, where the continental break-up finally occurs. The OCT ridge may be exhumed serpentinised mantle intruded by magma material confusing the OCT geometry. Indeed, the spreading is followed by magmatic events during post-rift time (flows, sills and volcano-sedimentary wedge), whose timing is constrained by analysis of sedimentary cover of the OCT ridge, correlated with onshore stratigraphy. The structural scheme of the area provides a faults organization, which can be compared to analogic models of oblique rifting. Furthermore refraction and seismological studies, providing MOHO depth, will constrain the lithospheric scale analogue models.

Autin, J.; Leroy, S.; D'Acremont, E.; Beslier, M.-O.; Ribodetti, A.; Bellahsen, N.; Razin, Ph.; Robin, C.

2009-04-01

324

The ancient continental margins of the North American and South American plates and regularities in the occurrence of oil and gas accumulations in them  

NASA Astrophysics Data System (ADS)

Various stages of the development of sedimentary basins along the ancient margins of the North American and South American plates are considered. It is shown that the potential of the oil-and-gas bearing is related to a certain stage of evolution of the basins. For the margins of the North American plate, it is the first stage of development in the structure of the ancient Paleozoic continental margins that developed under passive tectonic conditions. For the basins along the ancient margins of the South American plate, it is the second stage, which is the stage of the formation and development of foredeeps overlaid on the earlier structures. An interesting regularity is displayed: than younger the folding-mountain structures that originated in the distal parts of the continental margins, than greater the age range of source rocks in the sedimentary basins preserved there.

Zabanbark, A.; Lobkovskii, L. I.

2012-02-01

325

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

326

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

SciTech Connect

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

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

1993-01-01

327

Structural Controls on the Evolution of the Southeastern Brazilian Continental Margin  

Microsoft Academic Search

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

David Ashby; Ken McCaffrey; Bob Holdsworth; Julio Almeida

2010-01-01

328

Assessing lateral plume drainage and thermal craton-edge convection as possible causes of continental margin basaltic volcanism  

NASA Astrophysics Data System (ADS)

The origin(s) of basaltic volcanism along passive continental margins have yet to reach a state of a preferred conventional wisdom hypothesis. The idea that lateral temperature differences between continents and suboceanic mantle leads to efficient edge-convection has been championed by Anderson and King. However the lateral density anomalies that drive this mode of flow are so small that any effect of partial melt-extraction on decreasing the density of the residual mantle will effectively stop this hypothesized process for convective melt generation. (This conclusion was previously reached by John Hopper and coworkers; here we present additional 2-D numerical experiments that support this null result.) Lateral plume drainage has also been proposed to lead to continental margin volcanism. A major technical difficulty in assessing this process with 2-D numerical experiments is that mantle plumes are intrinsically a 3-D pipe instead of a 2-D sheet. The consequence is that in 2-D experiments a deep-sourced 'plume' will separate surrounding mantle into two isolated regions, while a 3-D pipe would still allow mantle to flow around plume structures. We will discuss several numerical experiments that explore the consequences of lateral drainage of Iceland Plume-fed asthenosphere switching from the W- to E-edge of the Greenland craton at ~58Ma. We first show how 2-D experiments with a deep-rooted plume have intrinsic artifacts due to the 2-D realization of plume upwelling. Then we discuss a suite of experiments where we idealize the plume as a source at the base of the asthenosphere below which mantle can readily flow without a disrupting 2-D plume 'sheet'. In contrast to edge-driven convection, these experiment better reproduce the timing and volume of Greenland margin basaltic volcanism as Greenland migrated over the Iceland Plume.

Liu, T.; Morgan, J. P.

2011-12-01

329

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

330

Magnetomineralogical approach to distal IRD layers detection and characterization. The case study of the Galician Iberian Continental Margin  

NASA Astrophysics Data System (ADS)

Ice Rafted Debris (IRD) layers are widely distributed along the Northern Atlantic, representing the discharge of detrital material transported by icebergs released from the major continental ice caps during the Heinrich events (Heinrich et al. 1988, Bond et al. 1992). They provide key information on climatically forced events and are commonly used as tracers holding some chronostratigraphic value. At latitudes ranging the Rudimann belt (40-55 N) (Ruddiman et al. 1977) IRD layers can be identified by magnetic susceptibility values (?) up to 400x10-6 SI, emerging from background values lower than 100x10-6 SI. At the Galician Continental Margin, thousands of km away from their Laurentide, Greenland and Fenoscandian source areas, the distance-weakened icecap detrital signal is mixed within the continental shelf inputs and depleted and modified by upwelling related redoxomorphic diagenetic processes making their recognition very difficult, when not impossible, by the standard susceptibility measurements and/or mineralogical counting. IRD layers were first detected at the Galician Continental Margin in turbidite-dominated sequences off the Galician Bank deposited during MIS2 by Rey et al. (2008). We propose a magnetomineralogical method based on systematic measurement of the magnetic susceptibility and Anhysteretic Remanent Magnetization to detect IRD-like horizons that are subsequent measurement of hysteresis properties, and low temperature remanence and susceptibility will unequivocally demonstrate or reject the occurrence distally sourced materials transported during the Heinrich events. Furthermore, coercivity spectra analysis of detected IRD layers is used to characterize each Heinrich event by their distinct magnetic signature. This have important consequences to asses their ages and provenance. Bond et al, 1992. Nature 360, 245-249. Heinrich, 1988. Quaternary Res. 29, 142-152. Kissel, 2005. C. R. Geosciences 337, 908-918. Rey et al, 2008. Mar. Geol. 249, 64-9 Robinson, 1986. Phys. Earth. Planet In., 42, 22-47. Rudimann, 1977. Geol. Soc. Am. Bull. 88, 1-12. Contribution to the projects CTM 2007-61227/MAR, PGDIT06TAM31201PR and OILDEBEACH

Rey, D.; Coimbra, R. L.; Mohamed-Falcon, K. J.; Frederichs, T.; Vilas, F.

2009-12-01

331

Morphogenesis of the SW Balearic continental slope and adjacent abyssal plain, Western Mediterranean Sea  

NASA Astrophysics Data System (ADS)

We present the seafloor morphology and shallow seismic structure of the continental slope south-east of the Balearic promontory and of the adjacent Algero-Balearic abyssal plain from multibeam and chirp sonar data. The main purpose of this research was to identify the sediment pathways from the Balearic promontory to the Algero-Balearic deep basin from the Early Pliocene to the Present. The morphology of the southern Balearic margin is controlled by a SW-NE structural trend, whose main expressions are the Emile Baudot Escarpment transform fault, and a newly discovered WSW-ENE trend that affects the SW end of the escarpment and the abyssal plain. We relate the two structural trends to right-lateral simple shear as a consequence of the Miocene westward migration of the Gibraltar Arc. Newly discovered steep and narrow volcanic ridges were probably enabled to grow by local transtension along the transform margin. Abyssal plain knolls and seahills relate to the subsurface deformation of early stage halokinetic structures such as salt rollers, salt anticlines, and salt pillows. The limited thickness of the overburden and the limited amount of deformation in the deep basin prevent the formation of more mature halokinetic structures such as diapirs, salt walls, bulbs, and salt extrusions. The uppermost sediment cover is affected by a dense pattern of sub-vertical small throw normal faults resulting from extensional stress induced in the overburden by subsurface salt deformation structures. Shallow gas seismic character and the possible presence of an active polygonal fault system suggest upward fluid migration and fluid and sediment expulsion at the seafloor through a probable mud volcano and other piercement structures. One large debris flow deposit, named Formentera Debris Flow, has been identified on the lower slope and rise of the south Formentera margin. Based on current observations, we hypothesize that the landslide originating the Formentera Debris Flow occurred in the Holocene, perhaps in historical times.

Camerlenghi, Angelo; Accettella, Daniela; Costa, Sergio; Lastras, Galderic; Acosta, Juan; Canals, Miquel; Wardell, Nigel

2009-06-01

332

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

SciTech Connect

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

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

1990-10-01

333

Comparative study of the moment and graphic size parameters of the sediments of the western continental shelf of India  

Microsoft Academic Search

A comparative study of the moment and graphic size parameters was undertaken on about two hundred surficial sediment samples varying from sand to clay sizes from the western continental shelf of India. The studies reveal that the mean size and standard deviation may be calculated either by the moment method or by graphic methods (Inman; Folk and Ward) without any

N. H. Hashimi

1981-01-01

334

Collision and Extension at Continental Margins: Example of the Sea of Okhotsk  

Microsoft Academic Search

The crustal structure of the northern Sea of Okhotsk region was reworked after the relict Okhotsk Sea plate collided with Eurasia about 55 Ma. Post-collisional extension and magmatic processes were likely initiated within the Okhotsk Sea plate and Asian margin along the remnant convergent plate boundary. The crustal structural pattern differs from the north and from the south by the

E. Konstantinovskaya

2004-01-01

335

Sediment Flux in a Fiord/Shelf Transect of an Ice-Dominated Continental Margin.  

National Technical Information Service (NTIS)

Piston cores from fiords along the eastern margin of Baffin Island, N>W>T> are being studied in order to compute variations in sediment flux on time scales of decades to thousands of years. Radiocarbon dates indicate that the sedimentation rates in outer ...

J. T. Andrews

1987-01-01

336

Cretaceous source rock characterization of the Atlantic Continental margin of Morocco  

Microsoft Academic Search

Characterization of the petroleum potential for the Atlantic margin of Morocco has been based primarily on limited, antiently acquired organic geochemical data. These indicate the area of drilling behind the paleoshelf edge to be only fair in organic carbon and C15+ extract values with predominantly terrestrial kerogen types. Recently acquired geochemical data obtained from relatively recent drilling both behind and

1993-01-01

337

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

338

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

339

Seismic patterns of the Guerrero-Oaxaca, Mexico region, and its relationship to the continental margin structure  

NASA Astrophysics Data System (ADS)

The main purpose of this paper is to enhance awareness on the seismic evidences that suggest a possible segmentation of the continental margin at the Guerrero-Oaxaca, Mexico region. Data from a recent 7 months survey of microseismicity carried out from 2008 December to 2009 June at Ometepec, Guerrero area, using a portable broad-band digital seismographs network added with data of a previous survey and the aftershocks distribution of the 1982 and 1995 major earthquakes permit to infer the characteristics of the seismic patterns of the Acapulco-Pinotepa Nacional portion of the southern Mexico subduction region. Two different seismic regimens are apparent, one in the Acapulco-Marquelia and the other in the Marquelia-Pinotepa Nacional areas. In the Acapulco-Marquelia portion, the seismicity is broader and dispersed starting at the coast up to 160 km inland approximately. Seismicity in the Marquelia-Pinotepa portion, on the other hand, is narrower and concentrates near the coast. The two seismic regimens are separated by a narrow band or strip of low seismic activity, nearly perpendicular to the coast and trench axis. The apparent low seismicity strip that separates the seismic regimens may trace the position either of a seismically inactive fracture zone or a seismic gap. Moreover, careful observation of the epicentres distribution of the Marquelia-Pinotepa segment reveals two clusters of events separated by another low seismicity strip. Thus, the two observed low activity strips, located near the northern tip of the Ometepec submarine canyon and Punta Maldonado, respectively, are interpreted in this paper as corresponding to disruptions of the continental margin. Other low seismic activity strips probably exist but these two are the most conspicuous. Supplementary information on fault mechanisms available for this area seems to substantiate additionally this interpretation. The observations reported are important to understand the mechanics of the major earthquakes that frequently occur in this region.

Yamamoto, Jaime; González-Moran, Tomas; Quintanar, Luis; Zavaleta, Ana B.; Zamora, Araceli; Espindola, Victor H.

2013-01-01

340

Using ammonium pore water profiles to assess stoichiometry of deep remineralization processes in methanogenic continental margin sediments  

NASA Astrophysics Data System (ADS)

many continental margin sediments, a deep reaction zone exists which is separated from remineralization processes near the sediment surface. Here, methane diffuses upward to a depth where it is oxidized by downwardly diffusing sulfate. However, the methane sources that drive this anaerobic oxidation of methane (AOM) in the sulfate-methane transition zone (SMT) may vary among sites. In particular, these sources can be thought of as either (i) "internal" sources from in situ methanogenesis (regardless of where it occurs in the sediment column) that are ultimately coupled to organic matter deposition and burial, or (ii) "external" sources such as hydrocarbon reservoirs derived from ancient source rocks, or deeply buried gas hydrates, both of which are decoupled from contemporaneous organic carbon deposition at the sediment surface. Using a modeling approach, we examine the relationship between different methane sources and pore water sulfate, methane, dissolved inorganic carbon (DIC), and ammonium profiles. We show that pore water ammonium profiles through the SMT represent an independent "tracer" of remineralization processes occurring in deep sediments that complement information obtained from profiles of solutes directly associated with AOM and carbonate precipitation, i.e., DIC, methane, and sulfate. Pore water DIC profiles also show an inflection point in the SMT based on the type of deep methane source and the presence/absence of accompanying upward DIC fluxes. With these results, we present a conceptual framework which illustrates how shallow pore water profiles from continental margin settings can be used to obtain important information about remineralization processes and methane sources in deep sediments.

Burdige, David J.; Komada, Tomoko

2013-05-01

341

Quantifying the distribution and abundance of rippled scour depressions (RSDs) on the seafloor of California's continental margin using autoclassfication models  

NASA Astrophysics Data System (ADS)

The California Seafloor Mapping Project (CSMP) is a cooperative initiative creating a comprehensive, high-resolution (2-5m) coastal/marine geologic and habitat base map for all of California’s State waters (Mean high water to three nautical miles). This massive dataset covering > 8500 sq. km of coastal seafloor is enabling researchers to study patterns and distribution of near shore habitats and geomorphology on a scale never before possible. Data from CSMP reveal the presence of rippled scour depressions (RSD) as the most prominent features on the continental shelf. These features are found worldwide and are characterized as depressions (.4m-1m) of coarse grain sediment and long period sand waves surrounded by a fine sediment plateau. While previous studies have described the geomorphologies of RSDs and speculated on their origin, this is the first regional study describing their patterns of abundance and distribution on a scale of 1000s of km. The purpose of this study is to use auto classification methods to quantify the spatial extent and distribution of three benthic habitats (rock, sediment, RSD) within the state waters of California. Using CSMP acoustic backscatter imagery and derived bathymetric products (rugosity, bathymetric position index, and slope), we developed a habitat classification model in ArcGIS to assign benthic habitat into one of these three classes. These results will then be used to quantify and characterize spatial patterns in the distribution and abundance of these habitats along the California continental margin.

Davis, A. C.; Mueller, C.; Hallenbeck, T.; Carrillo, J.; Gomez, J.

2010-12-01

342

Uplift along the western margin of the Deccan Basalt Province: Is there any geomorphometric evidence?  

Microsoft Academic Search

In line with the passive margin landscape evolutionary model in vogue, sustained erosion and long-distance retreat of the\\u000a Western Ghat escarpment are widely considered to be the results of erosionally-driven isostatic uplift since Tertiary by many\\u000a workers. Others have postulated or adduced evidence for strong neotectonic activity in the Ghat region. An obvious question\\u000a in this regard is whether there

Vishwas S. Kale; Nikhil Shejwalkar

2008-01-01

343

Rift flank uplift and isostatic response to glacial erosion: Creation of a high-elevation continental margin.  

NASA Astrophysics Data System (ADS)

Southern Baffin Island lies to the west of Davis Strait, which is part of a sedimentary basin system, linking Labrador Sea and Baffin Bay. It developed during Cretaceous and Palaeocene rifting that culminated in a brief period of sea-floor spreading in the late Palaeocene and Eocene. To date the cause of the high elevation southeastern margin of Baffin Island has not been a focus of much research, whereas the origin and age of elevated topography on its conjugate west Greenland margin is a matter of lively debate. For west Greenland it has been argued by some authors, from interpretations of on- and offshore data (fission track, seismic and well), that onshore topography was created by tectonically-driven uplift in the Neogene. However, we have previously demonstrated that offshore seismic and well data along the Greenland margin of Davis Strait are consistent with a model of rifting followed by thermal subsidence and fjord excavation by glaciers, where Neogene tectonic uplift is not required. For southeastern Baffin Island, we have analysed offshore seismic reflection profiles, exploration well and gravity data along the western margin of Davis Strait and conclude that rift flank uplift of older remnant topography and subsequent isostatic response to glacial erosion have produced the present-day high elevation onshore. This interpretation of the offshore evolution conforms with an onshore evolution for which elevated topography is related to erosion of pre-existing topography.

McGregor, E. D.; Nielsen, S. B.; Stephenson, R. A.; Clausen, O. R.; Petersen, K. D.; Macdonald, D. I. M.

2012-04-01

344

MTD distribution on a ‘passive’ continental margin: The Espírito Santo Basin (SE Brazil) during the Palaeogene  

Microsoft Academic Search

Mass-wasting on the Brazilian margin during the Mid-Eocene\\/Oligocene resulted in the accumulation of recurrent Mass Transport Deposits (MTDs) offshore Espírito Santo, SE Brazil. In this paper, we use three-dimensional seismic data to characterize a succession with stacked MTDs (Abrolhos Formation), and to assess the distribution of undeformed stratigraphic packages (i.e. turbidites) with reservoir potential separating the interpreted MTDs. High-amplitude strata

Davide Gamboa; Tiago Alves; Joe Cartwright; Pedro Terrinha

2010-01-01

345

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

346

Sidescan sonar imagery of widespread fossil and active cold seeps along the central Chilean continental margin  

NASA Astrophysics Data System (ADS)

The central Chilean subduction zone between 35°S and 37°S was investigated in order to identify, document and possibly understand fluid flow and fluid venting within the forearc region. Several areas were mapped using multibeam bathymetry and backscatter, high-resolution sidescan sonar, chirp subbottom profiling and reflection seismic data. On a subsequent cruise ground-truthing observations were made using a video sled. In general, this dataset shows surprisingly little evidence of fluid venting along the mid-slope region, in contrast to other subduction zones such as Central America and New Zealand. There were abundant indications of active and predominantly fossil fluid venting along the upper slope between 36.5°S and 36.8°S at the seaward margin of an intraslope basin. Here, backscatter anomalies suggest widespread authigenic carbonate deposits, likely the result of methane-rich fluid expulsion. There is unpublished evidence that these fluids are of biogenic origin and generated within the slope sediments, similar to other accretionary margins but in contrast to the erosional margin off Central America, where fluids have geochemical signals indicating an origin from the subducting plate.

Klaucke, Ingo; Weinrebe, Wilhelm; Linke, Peter; Kläschen, Dirk; Bialas, Jörg

2012-12-01

347

Models of the development of the West Iberia rifted continental margin at 40 deg 30 min N deduced from surface and deep-tow magnetic anomalies  

NASA Astrophysics Data System (ADS)

The ocean-continent transition (OCT) on nonvolcanic continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading. The nature of the OCT off western Iberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by Leg 149 of the Ocean Drilling Program. Here we concentrate on the interpretation of a new digital magnetic anomaly for the Iberia Abyssal Plain between 39.5 deg and 42.2 deg N. The most striking anomaly is a trough which exists immediately east of, and locally parallel to, anomaly J and is closely coincident with a basement peridotite ridge. We conclude from modeling surface magnetic anomalies and a deep-tow magnetometer profile that seafloor spreading began about 129.9 m.y. ago (Barremian) at a rate of 10.0 mm/yr, consistent with drilling and seismostratigraphic results from the southern Iberia Abyssal Plain.

Whitmarsh, Robert B.; Miles, Peter R.

1995-03-01

348

The continental margin in Iceland — A snapshot derived from combined GPS networks  

Microsoft Academic Search

The tectonical setting in Iceland is quite complex due to the interaction of the Iceland hot spot and the Mid Atlantic Ridge. While in the north of the island one active spreading zone exists, the divergent motion in the centre and the south is distributed over at least two volcanic rift zones. The spreading rate increases linearly along the Western

J. Perlt; M. Heinert; W. Niemeier

2008-01-01

349

Eastern margin of the Ross Sea Rift in western Marie Byrd Land, Antarctica: Crustal structure and tectonic development  

Microsoft Academic Search

The basement rock and structures of the Ross Sea rift are exposed in coastal western Marie Byrd Land (wMBL), West Antarctica. Thinned, extended continental crust forms wMBL and the eastern Ross Sea continental shelf, where faults control the regional basin-and range-type topography at ~20 km spacing. Onshore in the Ford Ranges and Rockefeller Mountains of wMBL, basement rocks consist of

Bruce P. Luyendyk; Douglas S. Wilson; Christine S. Siddoway

2003-01-01

350

Hydrothermal Petroleum in Active Continental Rift: Lake Chapala, Western Mexico, Initial Results.  

NASA Astrophysics Data System (ADS)

Lake Chapala in western Mexico is located partially in the Citala Rift, which belongs to the well-known neotectonic Jalisco continental triple junction. The region is characterized by active volcanism (Ceboruco, Volcan de Fuego), tectonic (1995 earthquake, M=8, 40-50 mm to SW) and hydrothermal (San Juan Cosala & Villa Corona spas and La Calera sinter deposit) activities. Hydrothermal petroleum has been described in active continental rift (East African Rift) and marine spreading zones (Guaymas Basin, Gulf of California). In 1868 the Mexican local press reported that manifestations of bitumen were appearing in front of the Columba Cap on the mid south shore of Lake Chapala. This bitumen is linked to the lake bottom and when the water level decreases sufficiently it is possible to access these tar bodies as islands. Because of these manifestations the Mexican oil company (PEMEX) drilled an exploration well (2,348m) at Tizapan El Alto without success. Hydrothermal activity is evident in the tar island zone as three in-shore thermal springs (26.8 m depth, 48.5° C, pH 7.8 and oriented N-S). The preliminary analyses by GC-MS of the tar from these islands indicate hydrothermal petroleum derived from lake sedimentary organic matter, generated at low temperatures (150° -200° C). The tars contain no n-alkanes, no PAH or other aromatics, but a major UCM of branched and cyclic hydrocarbons and mature biomarkers derived from lacustrine biota. The biomarkers consist of mainly 17? (H),21? (H)-hopanes ranging from C27 to C34 (no C28), gammacerane, tricyclic terpanes (C20-C26), carotane and its cracking products, and drimanes (C14-C16). The biomarker composition indicates an organic matter source from bacteria and algae, typical of lacustrine ecosystems. 14C dating of samples from two tar islands yielded ages exceeding 40 kyrs, i.e., old carbon from hydrothermal/tectonic remobilization of bitumen from deeper horizons to the surface. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be included as a target in exploration for future energy resources in such regions.

Zarate-del Valle, P. F.; Simoneit, B. R.; Ramirez-Sanchez, H. U.

2003-12-01