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1

The Continental Margins of the Western North Atlantic.  

ERIC Educational Resources Information Center

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

Schlee, John S.; And Others

1979-01-01

2

Structure of shumagin continental margin, Western Gulf of Alaska  

SciTech Connect

The Shumagin margin is characterized by five major structural features or trends: (1) Shumagin basin, containing about 2.5 km of late Miocene and younger strata above acoustic basement; (2) Sanak basin, containing as much as 8 km of dominantly late Cenozoic strata in two subbasins separated by a basement high; (3) cenozoic shelf-edge and upper-slope sedimentary wedges that are 3-4 km thick and possibly as thick as 6 km; (4) a midslope structural trend, Unimak ridge, that is characterized by numerous surface and subsurface structural highs; and (5) a 30-km wide accretionary complex at the base of the slope. A thin (less than 1-2 km) sediment cover of Miocene and younger age covers the continental shelf areas outside of Shumagin and Sanak basins. The tectonic history of the margin includes: (1)j late Cretaceous or early Tertiary removal of the seaward part of the Cretaceous Alaska Peninsula margin along the Border Ranges fault and accretion of the Shumagin Formation against the truncated margin; (2) Miocene uplift and erosion of the shelf; (3) middle or late Miocene uplift of Unimak ridge; and (4) late Miocene and younger subsidence and infilling of Sanak and Shumagin basins, and subduction-accretion along the Aleutian Trench.

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

1985-04-01

3

CDP seismic sections of the western Beaufort continental margin  

USGS Publications Warehouse

The continental rise, slope, and shelf in the Beaufort Sea off northern Alaska were surveyed with 5600 km of common-depth-point (CDP) seismic data by the U.S. Geological Survey in 1977. The lower continental rise consists of a wedge of at least 4.5 km of low-velocity, generally flat-lying, parallel-bedded sediments. Slump-related diapiric folds, probably cored by shale, occur on the upper rise and lower slope. The observed minimum depth to oceanic basement in the Canada Basin requires an age for this basin of at least 120 m.y., assuming it to be floored by oceanic crust with a subsidence history similar to that of the Atlantic and Pacific oceans. ?? 1979.

Eittreim, S.; Grantz, A.

1979-01-01

4

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

5

Mesozoic tectonics and volcanism of Tethyan rifted continental margins in western Sicily  

NASA Astrophysics Data System (ADS)

The paleotectonic and volcanic features of the Jurassic-Cretaceous carbonate successions, outcropping in central-western Sicily, allow us to restore the tectono-sedimentary evolution of a sector of the African continental margin. These successions consist of shallow-to-deep-water Mesozoic deposits that have formed in the carbonate platform-to- pelagic plateau depositional setting of the so-called Trapanese paleogeographic domain. Fieldwork, including structural analyses, has indicated the occurrence of lateral facies changes, resedimented materials, volcanic products (pillow lavas and tuffitic deposits), unconformity surfaces and paleofaults of different trends and age. These data, combined with facies and a physical-stratigraphic analysis, allow one to distinguish between the different tectono-stratigraphic settings. A structural low, filled with thick pillow lavas and reworked deposits, appears bordered by two main structural highs: an articulate carbonate-pelagic platform with a stepped fault margin, and the flank of a structural high, where the volcanoclastic deposits are interlayered with reworked reef materials, suggest the occurrence of a submarine volcano, that evolved into an atoll-type carbonate shelf setting. Tectonic and magmatic events punctuated the sedimentary evolution during the Early Jurassic, Middle-Late Jurassic, Early and Late Cretaceous. The horst and graben structures connected by steeped margins and the occurrence of distinctive types of volcanic products suggest that the study areas could represent a sector of the Jurassic-Cretaceous southern Tethyan passive continental margin, situated at the edge of the continental platform bordering those basinal areas where rifting and possible spreading processes were active during the Mesozoic.

Basilone, Luca; Morticelli, Maurizio Gasparo; Lena, Gabriele

2010-04-01

6

Tectonic Configuration of the Western Arabian Continental Margin, Southern Red Sea  

NASA Astrophysics Data System (ADS)

The young continental margin of the western Arabian Peninsula is uplifted 3.5 to 4 km and is well exposed. Rift-related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18° N and its intensity increases gradually from east to west. Extension is negligible near the crest of the Arabian escarpment, but it reaches a value of 8 to 10% in the western Asir, a highly dissected mountainous region west of the escarpment. There is an abrupt increase in extensional deformation in the foothills and pediment west of the Asir (about 40 km inland of the shoreline) where rocks in the upper plate of a system of low-angle normal faults with west dips are extended by 60 to 110%. The faults were active 23 to 29 Ma ago and the uplift occurred after 25 Ma ago. Tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 Ma old are concentrated in the foothills and pediment as well. The chemistry of the dikes suggests (1) fractionation at 10 to 20 kbar, (2) a rapid rise through the upper mantle and lower crust, and (3) differentiation and cooling at 1 Atm to 5 kbar. Structural relations between dikes, faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval, but that most of the extension preceded the expansion. A tectonic reconstruction of pre-Red Sea Afro/Arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide. Steep Moho slopes probably developed during rift formation as indicated by published gravity data, two published seismic interpretations and the surface geology.

Bohannon, Robert G.

1986-08-01

7

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

8

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

9

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

10

Mud Volcanoes revealed and sampled on the Western Moroccan Continental Margin  

NASA Astrophysics Data System (ADS)

We surveyed and sampled two active and 3 inactive mud volcanoes on the Atlantic continental margin of Morocco. The active mud volcanoes (named Yuma and Ginsburg) are each about 4 km in diameter, rise between 150-250 meters above the seafloor and are the first active methane-related mud volcanoes to be identified in this region. The inactive volcanoes range in diameter from 1-3 km, with relief between 50 and 80 meters. Gravity cores from the crest of the active mud volcanoes yielded methane hydrates and mud breccia deposits. Chemosynthetic communities of Pogonophora worms and the bivalve Solemya were found on the surface of the active mud volcanoes. Sediment cores from the inactive volcanoes contained hemipelagic sediments, rich in foraminifera and ahermatypic coral debris, overlying mud breccia

Gardner, Joan M.

11

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

12

The Brazilian continental margin  

NASA Astrophysics Data System (ADS)

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

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

1981-04-01

13

Inversion of the Paleogene Chinese continental margin and thick-skinned deformation in the Western Foreland of Taiwan  

NASA Astrophysics Data System (ADS)

New structural data, available seismicity data together with mechanical constraints on the Eurasian continental lithosphere and reconstruction of paleostress trajectories are combined in order to re-assess and discuss the dominant deformation mode (thin-skinned vs thick-skinned) in the Western fold-and-thrust belt of the active Taiwan collision zone. Serial balanced cross-sections and computed paleostress tensors suggest that structural styles and stress trajectories at the front of the Taiwan mountain belt may vary rapidly along-strike depending on the presence of pre-orogenic Paleogene troughs in the Chinese continental margin (Taihsi and Tainan basins) that are favourably oriented to be reactivated and inverted. In localities of the western foreland, where basin inversion and thick-skinned basement-involved shortening predominate, the crustal seismic activity is important and characterized by strike-slip faulting. In these domains of the fold-thrust belt, the stress deviations with respect to the regional transport direction are also important. By contrast, in domains of significant syn-orogenic subsidence, a thin-skinned style of deformation may be prominent due to the lack of available pre-existing features. The seismic activity is limited to few major faulted boundaries such as the active Chelungpu-Sani thrust, and the stress deviations are limited. The timing of deformation at the belt front seems to be independent of the structural styles, so that frontal folds are active since probably 0.5 Ma. However, in inner parts of the Western Foothills of Taiwan, where superimposed thick-skinned and thin-skinned deformation are required, out-of-sequence thrusting occurs to maintain the topographic profile. Finally, we suggest that kinematics and structural styles of the western fold-thrust belt are controlled by the mechanics of the Eurasian continental margin in agreement with a recent study. At the scale of the orogen, the limited shortening amounts across the WF and in the Central Range as well as the absence of continental HP-rocks better support a thick-skinned collision model for Taiwan.

Mouthereau, Frédéric; Lacombe, Olivier

2006-11-01

14

Edge-driven Rayleigh Taylor Instabilities at Transtensional Continental Margins: Western North Island, New Zealand  

NASA Astrophysics Data System (ADS)

At a continental transform system with an element of extension, regular and thinned lithosphere are juxtaposed. Such a system will be gravitationally unstable as negative buoyancy is created by the regular mantle lithosphere terminating at an abrubt edge with less dense asthenospheric mantle. Finite element experiments with dimensionless ratios of viscosity and density show that such a gravitational instability can grow, migrate and eventually drip off into the lower density asthenosphere providing two criteria are met: lithospheric thinning across the edge is at least 30% or more; and that viscosity at the top of the mantle lithosphere is no more than 2.4 or 4.5 x1021 Pa s, for lithospheric thicknesses of 100 and 200 km respectively. These are low values for regular mantle lithosphere, but are in keeping with mantle lithosphere found adjacent to plate boundaries, or paleo subduction zones. As the mantle lithosphere deforms and migrates away from an edge it both thins and thickens different portions of the overlying crust. At the surface regions of subsidence and uplift migrated in concert with the subjacent gravitational instability. A dimensionless variable analysis of instability development shows that 3 critical dimensionless ratios control the shape, migration speed and form of the instability: ?' = ratio of viscosity between crust and mantle; m' = ratio of crust to lithosphere thickness; and d' = portion of mantle lithosphere thinning to initiate the instability. Western North Island, New Zealand, displays characteristics of uplift and subsidence in the past 10-12 Ma, which can be explained by a migrating instability that initiated from the Auckland-Hauraki area around 10 Ma. Transtensional faults developed in mid-North Island from 5 Ma as back-arc spreading from the oceanic Lau Havre trough penetrated into the continental lithosphere of New Zealand. It is this transtensional phase that we argue started the edge-driven instability. The present position of the proposed migrating instability is now beneath the east-west line joining the active volcanoes of Mts Taranaki and Ruapehu. Here seismic, gravity, MT and seismicity evidence show a major, near vertical step in both the Moho and mantle lithosphere. A stacked receiver function profile shows a 7-10 km step in the Moho, and to the north of the step low upper-mantle P and S wave velocities suggests the mantle lithosphere is either very thin or missing. A migrating edge-instability provides a credible explanation for much of the phenomena observed in western North Island providing the mantle lithosphere is of the order of 5 x 1020 Pa s. We propose additional geophysical tests to check on the present position of the instability and to also extend the numerical experiments into three dimensions.

Stern, T. A.; Houseman, G. A.; Evans, L.

2011-12-01

15

Sediments of the East Atlantic Continental Margin.  

National Technical Information Service (NTIS)

This is a preliminary report on the study of sediments of the East Atlantic Continental Margin. The location of sediment samples from the northern and equatorial portions of the western African continental shelf and upper slope are presented, along with a...

J. D. Milliman

1972-01-01

16

Neotectonics seismicity in the south-eastern Beaufort Sea polar continental margin of north-western Canada  

NASA Astrophysics Data System (ADS)

A two-dimensional numerical model is presented that links inferences about recent rates of onshore uplift in the area of the Mackenzie Delta to rapid subsidence of the offshore Beaufort-Mackenzie Basin and the present-day upper lithospheric structure across the continent-ocean transition in this region of north-western Canada. The model results comprise a self-consistent set of internal displacements (velocities) and boundary conditions which predict a concentration of shear stress in the oceanic crust (and overlying sedimentary basin) in a region of the south-eastern Beaufort Sea featuring a concentration of earthquake epicentres. The mode of faulting implied by the calculated stress tensors is normal, consistent with a published fault-plane solution and in-situ stress measurements suggesting the axis of maximum compression to be perpendicular to the model cross-section. The results indicate that the uplift?subsidence pattern and seismicity are tectonically related but do not support models invoking tectonic compression, which have been previously proposed here and on other seismically active continental margins with rapidly subsiding sedimentary basins. Rather, a key role for dynamic processes in the upper mantle (including the asthenosphere), related to the continent-ocean lithosphere transition at passive continental margins, is implied.

Stephenson, R. a.; Smolyaninova, E. i.

1998-10-01

17

Long-term landscape evolution, cooling and exhumation history at the Moroccan passive continental margin, Western Anti-Atlas  

NASA Astrophysics Data System (ADS)

The ENE-trending Anti-Atlas of Morocco is located at the northwestern fringe of the West African Craton and south of the High Atlas and represents the Phanerozoic foreland of the Late Palaeozoic North African Variscides and the Cenozoic Atlas Belt. The Anti-Atlas mountain belt extends from the Atlantic Ocean over 500 km into the Moroccan interior and shows a rugged topography with elevations of about 2700 m. The exhumation of the Precambrian basement and the deformation and erosion of the Palaeozoic cover is mainly related to the Variscan orogeny in the Upper Carboniferous-Lower Permian. Subsequently, exhumation of the inliers occurred in the Triassic-Jurassic, as the Anti-Atlas formed the shoulder of the Atlantic rift and finally in the Upper Eocene-Pleistocene, contemporaneously with the uplift of the Atlas belt. In Morocco, a large amount of Mesozoic terrigenous sedimentary rocks are deposited in most of the basins along the continental margin indicating a major episode of erosion during the rift and early post-rift period in the Central Atlantic. In the Tarfaya-Laâyoune-Dakhla Basin, south of the Anti-Atlas, the sedimentary cover reach a thickness of up to 12 km. The presence of high surface elevations in the Anti-Atlas mountain belt indicates a potential source area for the surrounding basins. Currently, phases of exhumation in the Anti-Atlas during the Central Atlantic rifting and places where the associated erosion products are deposited are poorly constrained and there is little quantitative data available at present. The present study was focused on the thermal and exhumation history of the Western Anti-Atlas, the burial and inversion history of the Tarfaya-Laâyoune-Dakhla Basin and on provenance analysis of the Meso-Cenozoic sedimentary rocks in the basin. In order to characterize the t-T history, apatite and zircon fission-track dating, apatite and zircon (U-Th-Sm)/He dating and furthermore 2-D modelling with 'HeFTy' software has been carried out at Precambrian rocks of the Western Anti-Atlas and Cretaceous to Neogene sedimentary rocks from the Northern Tarfaya-Laâyoune-Dakhla Basin. Thermochronological data and t-T path modelling indicate exhumation in the Western Anti-Atlas between Upper Carboniferous and Lower Cretaceous, whereby 9 km of Precambrian-Palaeozoic overburden has been eroded.

Sehrt, Manuel; Glasmacher, Ulrich A.

2014-05-01

18

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

19

East Africa continental margins  

SciTech Connect

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

Bosellini, A.

1986-01-01

20

High-temperature deformation during continental-margin subduction & exhumation: The ultrahigh-pressure Western Gneiss Region of Norway  

NASA Astrophysics Data System (ADS)

A new dataset for the high-pressure to ultrahigh-pressure Western Gneiss Region allows the definition of distinct structural and petrological domains. Much of the study area is an E-dipping homocline with E-plunging lineations that exposes progressively deeper, more strongly deformed, more eclogite-rich structural levels westward. Although eclogites crop out across the WGR, Scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area. The Scandian reworking increases westward, culminating in strong Scandian fabrics with only isolated pockets of older structures; the dominant Scandian deformation was coaxial E-W stretching. The sinistrally sheared Møre-Trøndelag Fault Complex and Nordfjord Mylonitic Shear Zone bound these rocks to the north and south. There was moderate top-E, amphibolite-facies deformation associated with translation of the allochthons over the basement along its eastern edge, and the Nordfjord-Sogn Detachment Zone underwent strong lower amphibolite-facies to greenschist-facies top-W shearing. A northwestward increase in exhumation-related melting is indicated by leucosomes with hornblende, plagioclase, and Scandian sphene. In the western 2/3 of the study area, exhumation-related, amphibolite-facies symplectite formation in quartzofeldspathic gneiss postdated most Scandian deformation; further deformation was restricted to slip along biotite-rich foliation planes and minor local folding. That the Western Gneiss Region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation, implies that continental crust in general need not undergo pervasive deformation during subduction.

Hacker, Bradley R.; Andersen, Torgeir B.; Johnston, Scott; Kylander-Clark, Andrew R. C.; Peterman, Emily M.; Walsh, Emily O.; Young, David

2010-01-01

21

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

NASA Astrophysics Data System (ADS)

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

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

2011-02-01

22

Geodynamic models for the Neoproterozoic Yenisey Ridge orogenic belt (western Siberian Craton continental margin) and evolution of granitoid and alkaline magmatism  

NASA Astrophysics Data System (ADS)

We assume that the genesis of the Yenisey Ridge accretional orogen is a result of three events: a) syn-collisional events (probably outside the Siberian craton), which resulted in the forming of the S- and I-type 880-860 Ma Teya granites in the Central Angara terrane; b) the collision between the Central Angara terrane and the Siberian craton and the forming of the syn- and post-collisional S- and A- type 760-720 Ma Ayakhta and Glushikha granites; c) the forming of island arcs and ophiolites along the margin of the Siberian craton, their accretion and obduction onto the continent in the interval of 700-630 Ma. The last event is of special interest because at the same time in the Tatarka-Ishimba suture zone of the Yenisey Ridge, which is subparallel to the continental margin, the forming of intrusive and volcanic rocks of various composition and heightened alkalinity was taking place, including alkaline syenites as well as carbonatites and A-type granites, whose magmatic sources vary from mantle to mantle-crustal and crustal. These rocks form small plutons of round or oval shape, dikes, bedded bodies, tracing this tectonic zone along its entire length. Some of the A-type granites may be considered as hybrid rocks as suggested by mineralogical-petrographical features: inclusions of rocks of basic composition and magma-mingling structures. These granites are slightly peraluminous rocks. In contrast to the 760-720 Ma A-type granites, they have higher REE and Nb and Ta contents. They formed synchronously with the rocks of the island arc complex and their accretion and obduction onto the continental margin of Siberia in the interval of 700-630 Ma. It is quite probable that their forming in the back-arc suprasubduction zone was taking place at the same time that the oceanic plate was subducting below the continent from the western margin of the Siberian craton and reached the asthenospheric layer. The obtained data and the developed models uncover the geodynamic evolution of the forming of accretional orogens in the western margin of the Siberian craton in the Neoproterozoic.

Vernikovsky, Valery; Vernikovskaya, Antonina

2010-05-01

23

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

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

24

Toarcian–Kimmeridgian depositional cycles of the south-western Morondava Basin along the rifted continental margin of Madagascar  

Microsoft Academic Search

After rifting and final breakup of Gondwana along the former East-African-Antarctic Orogen during the Toarcian–Aalenian, passive\\u000a margins formed around the Proto-Indian Ocean. Sedimentological and stratigraphic studies in the southern Morondava Basin contribute\\u000a to an improved reconstruction of palaeoenvironmental changes during the syn-rift and post-rift margin formation. Depositional\\u000a models based on outcrop and literature data in combination with subsurface data sets

Markus Geiger; Günter Schweigert

2006-01-01

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

Structural discontinuity within the southern California continental margin: Seismic and gravity models of the western Transverse Ranges  

NASA Astrophysics Data System (ADS)

Crustal models of P wave velocity structure and bulk density along a north-south profile across the western Transverse Ranges of California are based on published seismic reflection profiles, a seismic refraction experiment using shots at sea to receivers on land and at sea, Pn and teleseismic arrivals at permanent seismic stations, and published gravity data on land and at sea. The seismic ray trace model is further constrained by two intersecting east-west crustal refraction profiles. The density model is constrained by an auxiliary profile tying the outer Borderland to oceanic lithosphere. A range of density models is evaluated based on the range of densities reported for particular P wave velocities and the preferred model is selected by regional seismic criteria. Lateral variations in the crust are resolved to greater than 10 km depth. Variations observed in teleseismic arrival times are substantially accounted for by the crustal velocity model. The western Transverse Ranges are not in isostatic balance, the south side being heavier than the north side.

Keller, B. R.

27

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

28

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

29

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

30

Crustal anatomy of a transform continental margin  

NASA Astrophysics Data System (ADS)

This paper describes new deep Seismic reflection results across the major transform margin southwest of the Grand Banks, off eastern Canada. An interpretation of these results is presented, supplemented with previous Seismic refraction, gravity and magnetic measurements. The results show a fairly sharp transition between oceanic and continental regions at the transform margin. A narrow zone, about 40 km wide, separates the two regions and is interpreted to be a zone in which shearing has destroyed the original fabric of the continental crust. Faults in this region extend throughout the crust and there is some evidence that a major strike-slip fault may extend some tens of kilometres into the upper mantle. Magmatism may have further modified the crust in this shear zone, which is bounded on the oceanic side by a small seamount. The continental crust is thinned by a factor of 3 or more in this zone, which may be due to erosion or to flow in the lower crust during shearing. The oceanic crust does not appear to thin significantly as the transform margin is approached. Crustal-scale dipping reflectors over a confined region below the shelf are interpreted to represent the contact between two Appalachian terranes: the Meguma and Avalon terranes. This supports earlier interpretations of a large magnetic lineament in the region as marking this boundary. These crustal Seismic data crossing a large transform margin are the first of their kind and will help us to constrain evolutionary models of transform margins. The picture they provide is different in many respects from that across rifted margins and similar in others. In some cases it might be difficult to distinguish between these two classes of Atlantic-type margins on the basis of crustal structure.

Keen, C. E.; Kay, W. A.; Roest, W. R.

1990-02-01

31

Evolution of Northeast Atlantic magmatic continental margins  

NASA Astrophysics Data System (ADS)

One of the major problems interpreting the evolution of magmatic continental margins such as those which dominate the Irish, UK and Norwegian margins of the NE Atlantic is that the structure which should record the pre-magmatic evolution of the rift and which potentially influences the character of the rifting process is partially or completely obscured by thick basalt lava flows and sills. A limited number of deep reflection seismic profiles acquired with tuned seismic sources have penetrated the basalts and provide an image of the pre-magmatic structure, otherwise the principle data are lower resolution wide-angle/refraction profiles and potential field models which have greater uncertainties associated with them. In order to sidestep the imaging contraints we have examined the Ethiopian ñ Afar rift system to try to understand the rifting process. This magmatic rift system provides, along its length, a series of ësnapshotsí into the possible tectonic evolution of a magmatic continental margin which are associated with different amounts of extension. The Main Ethiopian rift contains an embryonic magmatic passive margin dominated by faulting at the margins of the rift and en-echlon magmatic zones at the centre. Further north toward Afar the rift becomes infilled with extensive lava flows fed from fissure systems in the widening rift zone. Deep seismic profiles crossing the NE Atlantic margins reveal ocean dipping reflector sequences (ODRS) of basaltic lavas overlying extended crust and lower crustal sill complexes of intruded igneous rock, often referred to as underplate, which extend back beneath the continental margin. The ODRS show a variety of morphologies and settings but frequently occur in fault bounded rift structures along the margins. We suggest, by analogy to the observations that can be made in the Ethiopia Afar rift that these fault bounded basins largely form at the embryonic rift stage and are then partially or completely filled with lavas fed from fissures which are now observed as the ODRS. The oceanward dip on the ODRS is predominantly the result of post-eruption differential subsidence, as opposed to syn-eruption extension. The timing of intrusion of the lower crustal sill complexes remains unclear but they are most likely to have been emplaced as the supply of magma increased, which implies they are a late stage addition. The structure of the Main Ethiopian rift appears to have been influenced by the pre-existing basement structure at an early stage in the rift process, defining the geometry of the rift and providing a control on the later magmatic phase and modification of the crust. This early influence of existing structure is less clear on the NE Atlantic margins and in the UK and Irish sectors it is difficult to link substantial along strike variations in the properties of the margin to variations in basement structures which can be traced across the continental shelf. It is possible that such variations are completely overprinted by magmatic additions to the crust to the point at which they no longer influence the break-up mechanism.

England, Richard; Cornwell, David; Ramsden, Alice

2014-05-01

32

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

33

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

34

Crustal structure and formation of the southeast Newfoundland continental margin  

NASA Astrophysics Data System (ADS)

The non-volcanic continental passive margin of southeast Newfoundland offers an ideal place to study rifting, because of the modest amount of sediment cover through which basement structure can be determined. Another advantage is that the Iberia margin has a wide range of data collected over it and offers an intensively studied conjugate margin that can be compared with the Newfoundland margin. This doctoral thesis is a detailed study of the structures on the southeast Newfoundland margin. It focuses on multi-channel seismic reflection (MCS) data collected during the summer of 2000 with SCREECH (Study of Continental Rifting and Extension on the Eastern Canadian sHelf). The reflection data is processed using standard methods such as filtering, deconvolution, stacking and migration. This data set is supplemented by multi-channel seismic data provided by WesternGeco and GSI, and other industry data that has been released to the Canada Newfoundland and Labrador Offshore Petroleum Board (CNLOPB). Gravity, magnetic and wide angle data were used to enhance and constrain interpretations. The thesis examines the structural variation and degree of segmentation in the upper crust along the slope area of the margin. The two shelf edge basins (Carson and Bonnition) exhibit a change in half graben polarity and three transfer faults are shown to segment the margin. A major basement ridge extending ˜170 km near the present day shelf break separates the shelf basins from the Salar slope basin. The Moho is found to be relatively flat along the continental shelf and shallows towards the shelf break. Extension values extracted from normal fault restoration and crustal thickness are different, with values based on crustal thickness appearing much larger. It is hypothesized that the unextended crustal thickness was less than the estimated 35 km to account for the discrepancies. To account for the differences in features observed on the Newfoundland and Iberia margins, a wide rift stretching model is favoured with break up occurring at the Iberia end of the rift. The petroleum potential of the area is assessed and the Bonnition and Salar basins are found to be the most prospective, while the Carson Basin is the least prospective.

Solvason, Krista L. M.

35

Magnetic Polarity Dating of Tectonic Events at Passive Continental Margins  

Microsoft Academic Search

Palaeomagnetic studies of continental margin sediments drilled during Ipod Legs 47a (NW African margin), 47b (NW Portugal margin) and 48 (NE Bay of Biscay and SW Rockall margins) have provided magnetic polarity records for the late Neogene, early Palaeogene and Cretaceous periods. The general pattern of geomagnetic field polarity reversals during late Neogene times is now well established, and the

E. A. Hailwood; G. E. Morgan

1980-01-01

36

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

NASA Astrophysics Data System (ADS)

The effect of temperature dependent rheology has been examined for a model of continental lithosphere thinning by an upwelling divergent flow field within continental lithosphere and asthenosphere leading to continental breakup and rifted continental margin formation. The model uses a coupled FE fluid flow and thermal solution and is kinematically driven using a half divergence rate Vx and upwelling velocity Vz. Viscosity structure is modified by the evolving temperature field of the model through the temperature dependent Newtonian rheology. Continental lithosphere and asthenosphere material are advected by the fluid-flow field in order to predict crustal and mantle lithosphere thinning leading to rifted continental margin formation. The results of the temperature dependent rheology model are compared with those of a simple isoviscous model. The temperature dependent rheology model predicts continental lithosphere thinning and depth dependent stretching, similar to that predicted by the uniform viscosity model. However compared with the uniform viscosity model the temperature dependent rheology predicts greater amounts of thinning of the continental crust and lithospheric mantle than the isoviscous solutions. An important parameter within the kinematic model of continental lithosphere breakup and rifted continental margin development is the velocity ratio Vz/Vx. For non-volcanic margins, Vz/Vx is thought to be around unity. Applying a velocity ratio Vz/Vx of unity gives a diffuse ocean-continent transition and exhumation of continental lithospheric mantle. For volcanic margins, Vz/Vx is of order 10, falling to unity with a half-life of order 10 Ma, leading to a more sharply defined ocean-continent transition. While Vx during continental breakup may be estimated, Vz can only be inferred. FE fluid flow solutions, in which Vz is not imposed and without an initial buoyancy driven flow component, predict a velocity ratio Vz/Vx of around unity for both temperature dependent rheology and isovisous fluid-flow solutions. The effect of incorporating a lithology dependent continental lithosphere rheology (quartz-feldspar crust, olivine mantle) with temperature dependence is also being investigated. The work forms part of the Integrated Seismic Imaging and Modelling of Margins (iSIMM*) project. This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Schlumberger Cambridge Research & Badley Geoscience, supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, Conoco-Phillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, R Spitzer, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & D. Healy.

Tymms, V. J.; Kusznir, N. J.

2004-12-01

37

The Morton-Black hypothesis for the thinning of continental crust—revisited in Western AFAR  

NASA Astrophysics Data System (ADS)

Geological observations along the western margin of Afar show that the Morton-Black model, in which thinning of upper continental crust at a nascent continental margin is accomplished through block faulting and tilting, is not wholly applicable. Rather, dikes are concentrated into swarms, suggestive of important direct dilatation. Faults are concentrated into sets, within which no block-tilting steeper than 45 g is observed. Therefore attenuation of the Afar margins requires a significant role for processes other than block tilting, and in some margin sectors the transition from continental to neo-oceanic crust may be relatively abrupt.

Mohr, Paul

1983-05-01

38

Tectonic evolution of Brazilian equatorial continental margin basins  

Microsoft Academic Search

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

1993-01-01

39

Geophysical and geologic studies of Ross Sea continental margin, Antarctica  

SciTech Connect

In February 1984, the US Geological Survey conducted geophysical and geologic investigations of the Ross Sea Shelf and outer continental margin of Antarctica aboard the research vessel S.P. Lee. The geophysical data included 24-channel seismic-reflection, high-resolution seismic-reflection, sonobuoy seismic, gravity, magnetic-gradiometer, bathymetry, and heat-flow measurements. Sea-floor samples were collected for geologic and geochemical studies, using 3-m gravity corer, box corer, and rock dredge. Principal survey areas were along the southern and western Ross Shelf, near Cape Adare and Islin Bank, and along the central outer continental margin. These areas lie adjacent to those covered by earlier multichannel-seismic-reflection surveys made by France, West Germany, and Japan. Three north-south-trending sedimentary basins, containing as much as 5 km (16,404 ft) of Cenozoic sediment, lie beneath the Ross Sea Shelf and extend seaward beyond the continental shelf edge. These basins are separated by basement ridges and are bounded on the west by the Transantarctic mountains and on the east by mountain ranges of Marie Bydr Land. Seismic stratigraphy, crustal-thickness measurements, and rock samples from the Ross Sea region indicate that the three basins may have formed initially by crustal rifting during the middle and Late Cretaceous time and have subsequently filled with early Tertiary (.) as well as Oligocene and younger glacial marine sediment. Sedimentary thickness, heat-flow values, and geochemical analyses indicate that some parts of the Ross Sea Shelf may have favorable conditions for the generation of hydrocarbons.

Cooper, A.K.

1984-04-01

40

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

Microsoft Academic Search

Thermobarometry suggests that ultrahigh-pressure (UHP) to high-pressure (HP) rocks across the Western Gneiss Region ponded at the Moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust. Eclogite across the c. 22 000 km2 study area records minimum pressures of c. 8-18 kbar and temperatures of c. 650-780 ? C. One

E. O. WALSH; B. R. HACKER

2004-01-01

41

The Peruvian Continental Margin: Results from wide angle seismic Data  

Microsoft Academic Search

Within the scope of the GEOPECO (Geophysical Experiments at the Peruvian Continental Margin) project, seismic investigations along the Pacific margin of Peru were carried out using ocean bottom hydrophones (OBH) and seismometers (OBS) recording marine airgun shots. The structure and the P- wave velocity of the oblique subducting Nazca and overriding South-American Plates from 8°S to 15°S were determined by

A. Krabbenhoeft; J. Bialas; H. Kopp; N. Kukowski; C. Huebscher

2003-01-01

42

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

43

Comparative biogeochemistry-ecosystem-human interactions on dynamic continental margins  

USGS Publications Warehouse

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

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

2014-01-01

44

Western Ross Sea continental slope gravity currents  

NASA Astrophysics Data System (ADS)

Antarctic Bottom Water of the world ocean is derived from dense Shelf Water that is carried downslope by gravity currents at specific sites along the Antarctic margins. Data gathered by the AnSlope and CLIMA programs reveal the presence of energetic gravity currents that are formed over the western continental slope of the Ross Sea when High Salinity Shelf Water exits the shelf through Drygalski Trough. Joides Trough, immediately to the east, offers an additional escape route for less saline Shelf Water, while the Glomar Challenger Trough still farther east is a major pathway for export of the once supercooled low-salinity Ice Shelf Water that forms under the Ross Ice Shelf. The Drygalski Trough gravity currents increase in thickness from ˜100 to ˜400 m on proceeding downslope from ˜600 m (the shelf break) to 1200 m (upper slope) sea floor depth, while turning sharply to the west in response to the Coriolis force during their descent. The mean current pathway trends ˜35° downslope from isobaths. Benthic-layer current and thickness are correlated with the bottom water salinity, which exerts the primary control over the benthic-layer density. A 1-year time series of bottom-water current and hydrographic properties obtained on the slope near the 1000 m isobath indicates episodic pulses of Shelf Water export through Drygalski Trough. These cold (<-1 °C), salty (>34.75) pulses correlate with strong downslope bottom flow. Extreme examples occurred during austral summer/fall 2003, comprising concentrated High Salinity Shelf Water (-1.9 °C; 34.79) and approaching 1.5 m s -1 at descent angles as large as ˜60° relative to the isobaths. Such events were most common during November-May, consistent with a northward shift in position of the dense Shelf Water during austral summer. The coldest, saltiest bottom water was measured from mid-April to mid-May 2003. The summer/fall export of High Salinity Shelf Water observed in 2004 was less than that seen in 2003. This difference, if real, may reflect the influence of the large iceberg C-19 over Drygalski Trough until its departure in mid-May 2003, when there was a marked decrease in the coldest, saltiest gravity current adjacent to Drygalski Trough. Northward transport of cold, saline, recently ventilated Antarctic Bottom Water observed in March 2004 off Cape Adare was ˜1.7 Sv, including ˜0.4 Sv of High Salinity Shelf Water.

Gordon, Arnold L.; Orsi, Alejandro H.; Muench, Robin; Huber, Bruce A.; Zambianchi, Enrico; Visbeck, Martin

2009-06-01

45

Modelling of sea floor spreading initiation and rifted continental margin formation  

NASA Astrophysics Data System (ADS)

Recent observations of depth dependent (heterogeneous) stretching where upper crustal extension is much less than that of the lower crust and lithospheric mantle at both non-volcanic and volcanic margins plus the discovery of broad domains of exhumed continental mantle at non-volcanic rifted margins are not predicted by existing quantitative models of rifted margin formation which are usually based on intra-continental rift models subjected to very large stretching factors. New conceptual and quantitative models of rifted margin formation are required. Observations and continuum mechanics suggest that the dominant process responsible for rifted continental margin formation is sea-floor spreading of the young ocean ridge, rather than pre-breakup intra-continental rifting. Simple fluid flow models of ocean ridge processes using analytical iso-viscous corner-flow demonstrate that the divergent motion of the upwelling mantle beneath the ocean ridge, when viewed in the reference frame of the young continental margin, shows oceanward flow of the lower continental crust and lithospheric mantle of the young rifted margin giving rise to depth dependent stretching as observed. Single-phase fluid-models have been developed to model the initiation of sea-floor spreading and the thermal, stretching and thinning evolution of the young rifted continental margin. Finite element fluid-flow modelling incorporating the evolving temperature dependent viscosity field on the fluid flow also show depth dependent stretching of the young continental margin. Two-phase flow models of ocean ridges incorporating the transport of both solid matrix and melt fluid (Spiegelman &Reynolds 1999) predict the divergent motion of the asthenosphere and lithosphere matrix, and the focusing of basaltic melt into the narrow axial zone spreading centre at ocean ridges. We are adapting two-phase flow models for application to the initiation of sea-floor spreading and rifted continental margin formation. iSIMM investigators are V Tymms, NJ Kusznir, RS White, AM Roberts, PAF Christie, N Hurst, Z Lunnon, CJ Parkin, AW Roberts, LK Smith, R Spitzer, A. Davies and A. Surendra, with funding from NERC, DTI, Agip UK, BP, Amerada Hess Ltd., Anadarko, Conoco, Phillips, Shell, Statoil, and WesternGeco.

Tymms, V. J.; Isimm Team

2003-04-01

46

Preglacial and Glacial Loading of the Western Barents Sea Transform Margin  

Microsoft Academic Search

A distinctive feature of the western Barents Sea continental margin, between Norway and Svalbard, are the up to 6-km-thick Bear Island and Storfjorden fans. These fans have been deposited onto oceanic crust that developed in an Eocene segmented transform margin setting and later by slow, oblique seafloor spreading between Eurasia and Greenland. Approximately two thirds of this sedimentary load have

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

2006-01-01

47

Geomorphic characterization of the U.S. Atlantic continental margin  

USGS Publications Warehouse

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

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

2013-01-01

48

Electromagnetic study of the active continental margin in northern Chile  

NASA Astrophysics Data System (ADS)

Magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic arc and forearc regions of northern Chile between 19.5° and 22°S to study the electrical conductivity structures of this active continental margin. The instruments used covered a very broad period range from 10 -4 s to approx. 2 × 10 4 s and thus enabled a resolution of deep as well as shallow structures. In this paper we focus on the interpretation of data from an east-west profile crossing Chile from the Pacific coast to the Western Cordillera at 20.5°S. A decomposition of the impedance tensors using the Groom-Bailey decomposition scheme shows that a two-dimensional interpretation is possible. The resulting regional strike direction is N9°W. Two-dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the Andes. The final model shows a generally high resistivity in the forearc and a very good conductor below the Precordillera. Unlike earlier models from areas further south, a good conductor is not observed below the magmatic arc itself. This correlates with the so-called Pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas.

Echternacht, Friedrich; Tauber, Sebastian; Eisel, Markus; Brasse, Heinrich; Schwarz, Gerhard; Haak, Volker

1997-06-01

49

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

50

Seismic Studies at the Northern Namibian Passive Continental Margin  

NASA Astrophysics Data System (ADS)

Passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up and the role of hot-spot related magmatism. We conducted a combined on- and offshore seismic experiments designed to characterize the Southern African passive margin at the Walvis Ridge. Main aim of the project is to assess the interaction of the presumed plume with continental lithosphere and to determine the deep structure of the transition from the coastal fold belt to the stable craton, where the Walvis Ridge hits the African continent. The project integrates three experiments, (A) an onshore, coast-parallel refraction seismic profile, (B) two onshore-offshore wide-angle seismic transects, and (C) a combined on- and offshore seismic imaging of the sub-Moho velocities at the ocean-continent transition. The knowledge of the lithospheric structure of the margin together with results from other geoscientific studies will help to address fundamental questions such as, how continental crust and plume head interact, what the extent and volumes of magmatic underplating is, and how and which inherited (continental) structures might have been involved and utilized in the break-up process. First results of the subproject A + B which show a characteristic high-velocity body in the lower crust will be presented.

Ryberg, T.; Haberland, C. A.; Haberlau, T.; Weber, M. H.; Behrmann, J. H.; Jokat, W.; Hutchins, D.

2011-12-01

51

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

52

The bathymetry of the North Victoria Land continental margin  

Microsoft Academic Search

The very unique continental margin of North Victoria Land, Antarctica, is characterized by complex bathymetry, reflecting control by glacial, tectonic, and marine processes. The abnormally shallow shelf can be divided into a deep, rugged, glacially dominated inner shelf and a smoother, shallower outer shelf, which is dominated by marine and glacial marine processes. Deep u?shaped glacial troughs incise the shelf,

Christopher French Brake; John B. Anderson

1983-01-01

53

Canada basin: age and history of its continental margin  

SciTech Connect

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

Sweeney, J.F.

1985-02-01

54

Marine Geology of the Atlantic Continental Margin of Europe.  

National Technical Information Service (NTIS)

A geological reconnaissance has been made of the continental slope of western Europe between the Faeroe Isles and Lisbon by means of a 60 kJ reflexion profiler, supplemented by Boomer profiles and extensive Asdic (side-looking sonar) coverage of the conti...

A. H. Stride J. R. Curray D. G. Moore R. H. Belderson

1968-01-01

55

Gravity and magnetic investigations along the Peruvian continental margin  

NASA Astrophysics Data System (ADS)

This work presents the first three-dimensional gravity and magnetic investigation along the convergent Peruvian margin. Three-dimensional magnetic modelling is still a relatively untried and challenging technique. The gravity and magnetic models image nearly the whole margin which has been only partly resolved with geophysical methods up to now. The gravity and magnetic models are constructed for three areas between 7.25°S and 16.75°S and are based on the available wide-angle seismic velocity models (Hampel et al., 2002a; Broser et al., 2002). The continental margin is characterised by positive free-air anomalies of varying amplitudes, indicating that the margin has been shaped by the subduction of different features on the Nazca Plate. A comparison of the shipboard gravity measurements with the satellite data ensures that the data compiled from different marine surveys are compatible. In the Yaquina Area (7.25°S to 11°S) gravity anomalies caused by the Trujillo Trough and the Mendaña Fracture Zone are successfully modelled with remarkable undulations in the layer geometry of the oceanic crust. Along the continental margin, especially in the Lima Area (10.50°S to 14.40°S), strong undulations of the lower continental crust influence the upper sedimentary layers and support the development of basins along the Peruvian margin. The theory stating that the Peruvian margin is uplifted by the subducting Nazca Ridge (Kulm et al., 1988; Hagen &Moberly, 1994) is supported by gravity modelling. Consequently the buoyant Nazca Ridge is, at least partly, responsible for the extended region of flat subduction. The thickened and slightly asymmetrical crust of the Nazca Ridge is envisaged in gravity modelling. In the Nazca Ridge Area (14.25°S to 16.75°S) no accretionary prism is modelled. We conclude that the ridge is eroding the continental margin; furthermore the subduction of eroded sediments is probable. Gravity modelling suggests that the Nazca Ridge has fractured the continental margin. North of the ridge, in the Lima Area, a rather uniform accretionary complex is observed. This indicates that, after the margin was eroded by the southwards moving Nazca Ridge, the prism rapidly reached its stable size. In the Yaquina Area an accretionary prism is modelled in the whole research area but local variations of its location and structure indicate the former erosive influence on the continental margin of subducting features on the Nazca Plate. The layers of the oceanic crust show increasing densities implying they possess an originally high degree of porosity before actually subducting. The lineations 13 to 18 are traced from the southern part of the studied region as far north as the Mendaña Fracture Zone. Their orientation indicates that the Nazca Plate has not significantly changed its convergence direction between 12°S and 17.5°S in the past 33 Ma. In the Nazca Ridge Area a different anomaly pattern compared to the surrounding areas is observed. We conclude that the ridge is younger than the respective Nazca Plate and that it was formed on the already existing Nazca Plate and obliterated the original magnetic anomalies. In all models the basaltic layer sheeted dykes is considerably weaker magnetised than the upper pillow lavas and the lower gabbros. Generally the rocks of the continental margin possess a weak magnetisation. The Königsberg ratio is small for all layers of the oceanic crust, i.e. the induced magnetisation is partly higher than the remanent magnetisation. When it has subducted to a minimum depth of 25 km the magnetised oceanic crust exerts almost no influence on the observed total intensity field.

Heinbockel, R.; Dehghani, G. A.; Huebscher, Ch.

2003-04-01

56

ODP Leg 107 results from continental margin east of Sardinia (Mediterranean Sea): a transect across a very young passive margin  

SciTech Connect

A 200-km wide zone east of Sardinia, characterized by thin continental crust with tilted, listric(.)-fault-bounded blocks, has been interpreted as a passive continental margin formed during back-arc opening of the Tyrrhenian Sea. Leg 107 of the Ocean Drilling Project drilled a transect of four sites across this margin plus three sites in the basaltic basin. Site 654, closest to Sardinia, recovered a transgressive sequence attributed to basin subsidence: coarse-grained, iron-oxide rich, subaerial conglomerates underlie oyster-bearing sands followed upsection by open-water Tortonian marine marls. The synrift sequence, as inferred from seismic reflection profiles, correlates with sediments of Tortonian to Messinian age. Farther east the synrift sediments are younger: site 652, near the continental/oceanic transition, recovered an inferred synrift sequence of Messinian to early Pliocene age. The pan-Mediterranean Messinian desiccation event is represented at the western two sites (654 and 653) by a basinal facies including laminated gypsum, whereas at the eastern two sites the Messinian facies are terrestrial (lacustrine at 652 and subaerial at 656). They therefore infer that subsidence was more advanced at the western sites than at the eastern sites as of 5 Ma. Leg 107 results suggest that subsidence and stretching were diachronous across the passive margin, beginning and ending several million years earlier in the west than in the east. This asynchroneity may result from the inherent asymmetry of back-arc basin opening, or it may be a common characteristic of passive margins which has been revealed by the unusually precise time resolution of this data set.

Kastens, K.A.; Mascle, J.; Auroux, C.; Bonatti, E.; Broglia, C.; Channell, J.; Curzi, P.; Emeis, K.; Glacon, G.; Hasegawa, S.; Hieke, W.

1987-05-01

57

Early diagenesis of phosphorus in continental margin sediments  

Microsoft Academic Search

Most of the organic material in the oceans that reaches the sea floor is deposited on continental margins and not in the deep sea. This organic matter is the principal carrier of phosphorus (P) to sediments. A part of the organic material is buried definitely. The other part decomposes, resulting in a release of dissolved HP0 4<\\/sub>2-<\\/SUP>to the pore water.

C. P. Slomp

1997-01-01

58

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

NASA Astrophysics Data System (ADS)

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

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

2012-01-01

59

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

60

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

61

A geophysical study of the northern Svalbard continental margin  

NASA Astrophysics Data System (ADS)

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 of the inner shelf yield P-wave velocities of 2 km s-1 and higher, indicating erosion and compaction due to a former ice load. The inner shelf east of Hinlopen Strait has only a thin veneer of over-consolidated sediments above the acoustic basement. Beneath the outer shelf, up to 3.5 km of sedimentary deposits cover the down-faulted acoustic basement. The continental slope is heavily eroded due to bottom current activity and slumping. At about 30°E the morphology of the continental slope has a smooth appearance. Shelf progradation only in the vicinity of glacial troughs crossing the shelf (associated with submarine fans) indicates main sediment transport by ice streams during former glacial periods. The maximum sedimentary thickness in the Sophia Basin is more than 9 km, and the Nansen Basin has a sediment thickness of 4.5 km close to the margin. Gravity modelling along the seismic profiles was performed to constrain the position of the continent-ocean transition. Existing sedimentary thickness and structural maps were extended over the area investigated. The new data provide no evidence for the presence of former extensive subaerial volcanic sequences (seaward-dipping reflectors), which would have been emplaced during the break-up along the margin. Thus, we consider this part of the margin as non-volcanic.

Geissler, Wolfram H.; Jokat, Wilfried

2004-07-01

62

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

Microsoft Academic Search

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

P. Hoffman

1973-01-01

63

How a Curvilinear Continental Margin Influences Its Subsidence History  

NASA Astrophysics Data System (ADS)

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

Sacek, V.; Ussami, N.

2012-12-01

64

Segmentation of the Levant continental margin, eastern Mediterranean  

NASA Astrophysics Data System (ADS)

The Levant continental margin is divided into two major segments by the Carmel structure, which extends from the Dead Sea fault into the eastern Mediterranean. New seismic reflection data over the unexplored northern segment are used for completing the structural framework of the Levant area, together with existing data south of it. Inclusive depth structural maps of the area were produced for the base Pliocene and base Messinian evaporites. Previous studies indicate that differences between the two segments are well expressed in the deep crustal structure. The present study, which focuses mainly on the shallow section, shows that these differences are maintained throughout the accumulation of young sedimentary units, and even in the bathymetry. This preservation of segmentation, both in the shallow and in the deep structure, insinuates that the two segments were formed through different continental breakup processes, which continue to dictate the style of sediment accumulation.

Ben-Avraham, Z.; Schattner, U.; Lazar, M.; Hall, J. K.; Ben-Gai, Y.; Neev, D.; Reshef, M.

2006-10-01

65

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

66

An Assessment of Global Organic Carbon Flux Along Continental Margins  

NASA Technical Reports Server (NTRS)

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

Thunell, Robert

2004-01-01

67

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

68

The Conjugate Volcanic Continental Margins Of Argentina And Southern Africa  

NASA Astrophysics Data System (ADS)

In April-June 2003 the German Federal Institute for Geosciences and Natural Resources (BGR) undertook a new marine geophysical cruise along the South Atlantic continental margin off South Africa. In total more than 3,000 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 (>20,000 km) data set from the Argentine margin we are able to interpret seven conjugate traverses available over almost exact conjugate parts of these volcanic passive margins. A first interpretation of the raw stacked seismic data from the South African side shows that the volcanic features seem to be mirror images to the structures found on the Argentine margin. 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), and a distinct high-velocity (average value 7.3 km/s) lower crustal body beneath the SDRS. The data from the Argentine margin already suggested that the emplacement of the deeply buried, 60-120km wide SDRS was 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 new data a detailed correlation of timing of these individual pulses is intended. Furthermore, it is 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 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. In the new data from South Africa only one half graben was identified up to now.

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

2003-12-01

69

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

70

Antarctic glacial history from numerical models and continental margin sediments  

USGS Publications Warehouse

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

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

1999-01-01

71

The character of the glaciated Mid-Norwegian continental margin  

NASA Astrophysics Data System (ADS)

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

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

2010-05-01

72

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

73

Revealing the continental margin of Gondwana: the Ordovician arc of the Cordón de Lila (northern Chile)  

NASA Astrophysics Data System (ADS)

The tectonic evolution of the proto-Andean margin of western Gondwana has been commonly seen in terms of terrane accretion processes, requiring the existence of early Palaeozoic terrane boundaries and associated sutures. A new study of the Cordón de Lila Ordovician volcano-sedimentary successions in northern Chile reveals for the first time an arc assemblage deposited on thin crust within a continental arc system, having regional implications. Primitive basalts, rhyolites, volcanogenic wackes and siltstones are associated, bearing not only debris from mainly arc sources but also basement rocks; the latter is only accessory in the form of metamorphic lithoclasts and detrital zircons with ages around 1.0 Ga. Magmatic zircons in rhyolites reveal an eruptive age of ca. 478 Ma, concordant with Upper Arenigian to Lower Llanvirnian ages of brachiopods in overlying conglomerates. Bimodal volcanic associations, including low-K tholeiites, characterize the magmatic rocks, with evolved rhyolitic rocks showing pronounced arc-like geochemical signatures (negative anomalies in Ti, Nb and Ta). Some of the basaltic rocks are tholeiitic and display Ce/Y ratios below 1 and might point to a Moho depth of less than 10 km, hence a thin continental crust, coinciding with depletions in Zr and Hf concentrations. Associated volcaniclastic rocks display generally low Th/Sc (0.4-1), La/Sc (mostly <3.5), Zr/Sc (6-20) and high Ti/Zr (~10-60) ratios. The rock succession resembles the same geochemical and lithostratigraphical trends as retro-arc basin deposits further east in the Argentinean Puna of similar age. However, in the Cordón de Lila, the intercalated mafic rocks are less evolved, and the percentage of arc debris is higher, while the percentage of metamorphic lithoclasts and rounded quartz grains is much lower, indicating the existence of a thinned continental margin underpinning. We propose a transition in the Ordovician of the Central Andes from trench-ward fore-arc deposits to a dominantly intra-arc basin (Cordón de Lila), transitioning further eastwards towards a retro-arc basin (Puna) and foreland basin deposits of the Cordillera Oriental of northwestern Argentina. The Cordón de Lila intra-arc assemblage and associated fore-arc basin deposits therefore defined the western margin of Gondwana during the Ordovician. The absence of any terrane boundaries and sutures across strike is consistent with an evolving continental margin arc constructed on attenuated crust of the proto-Andean margin.

Zimmermann, Udo; Niemeyer, Hans; Meffre, Sebastien

2010-10-01

74

Cretaceous–Tertiary convergence and continental collision, Sanandaj–Sirjan Zone, western Iran  

Microsoft Academic Search

The Sanandaj–Sirjan Zone contains the metamorphic core of the Zagros continental collision zone in western Iran. The zone has been subdivided into the following from southwest to northeast: an outer belt of imbricate thrust slices (radiolarite, Bisotun, ophiolite and marginal sub-zones, which consist of Mesozoic deep-marine sediments, shallow-marine carbonates, oceanic crust and volcanic arc, respectively) and an inner complexly deformed

M. Mohajjel; C. L Fergusson; M. R Sahandi

2003-01-01

75

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

76

North Atlantic Margins: Case studies of Magmatic Continental Breakup  

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

77

The Chukchi Borderland: a Sediment-starved Rifted Continental Margin  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

78

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

USGS Publications Warehouse

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

Dillon, W. P.; Schlee, J. S.; Klitgord, K. D.

1988-01-01

79

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

SciTech Connect

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

Enachescu, M.E.

1987-05-01

80

Tectonic structure of the continental Rifting of the northern margin of the South China Sea  

NASA Astrophysics Data System (ADS)

We present multichannel seismic reflection images of about 2250 km of reprocessed seismic records collected during Sonne cruise 49 across the northern margin of the South China Sea. The data were collected in four seismic lines striking perpendicular to the strike of the margin. The lines cross the outer continental shelf and slope, and two of them continue across the central region of the basin. The four lines have been carefully reprocessed to improve signal to noise ratio including pre-stack statistical predictive deconvolution, pre-stack multiple attenuation by radon filtering and by FK filtering. Semblance-based velocity picking was before and after processing with analyses every 5 km or closer in areas of rough basement topography. Constant velocity stacks were also used in areas of interest. The data were stacked and subsequently post-stack time migrated using an FD algorithm and time and space variant smooth velocity models. The resulting sections display in detail the structure of post-rift and syn-rift sediment, the basement structure of fault-bounded blocks, often fault reflections, and very often clear and fairly continuous reflections from crust - mantle boundary. The four seismic images show the tectonic structure formed during -now inactive- rifting. From west to east the tectonic structure changes considerably as the amount of extension attained increases towards the east. In the western region the lines show the structure of the Xisha Trough, from the region where continental rifting stopped before continental-crust break up and separation to the east where a extension may have led to break up and subsequent to seafloor spreading. The images show clearly how the continental crust-mantle boundary progressively shallows to reach close to the top of the basement. The conjugate continental flanks of the rift display rather different styles of faulting producing a very asymmetric structure. The conjugate flanks display rather different amounts of fault dimensions and associated extension. The area of intense necking is < 50 km wide and in one of the flanks is not associated to apparent faulting. There is evidence of important volcanism in the images, with the most spectacular evidence displayed as large conical volcanoes formed over continental crust. The continent ocean transition is characterized by complex faulting in some lines, and seems to occur relatively abruptly in < 30 km distance in all lines. The crust next to the transition appears oceanic in nature with crust-mantle boundary reflections at roughly 2 seconds below top basement (i.e. about 5-6 km thick crust). The data support a model of rapid transition from continental crust rifting to seafloor spreading.

Cameselle, A. L.; Ranero, C. R.; Franke, D.; Barckhausen, U.

2012-12-01

81

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

NASA Astrophysics Data System (ADS)

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

Roure, FrançOis; Casero, Piero; Addoum, Belkacem

2012-06-01

82

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

83

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.

2007-12-01

84

Rheology and Deformation of the Lithosphere at Continental Margins  

NASA Astrophysics Data System (ADS)

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

Tikoff, Basil

2004-09-01

85

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

86

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

87

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

NASA Astrophysics Data System (ADS)

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

Butler, R.

2012-12-01

88

Post-rift km-scale uplift of passive continental margins can be caused by compressive stresses within continental crust  

NASA Astrophysics Data System (ADS)

Many passive continental margins are flanked by a mountain range up to more than 2 km high (Elevated Passive Continental Margins; EPCMs), e.g. Norway, east and west Greenland, East Brazil, eastern Australia and other margins elsewhere, that have been uplifted long after continental break-up. Explanations for these uplifted margins have been ad hoc, but there has hitherto been no explanation that accounts for their presence at both volcanic and non-volcanic margins and in both polar and tropical climatic environments. A continent breaks up by extension and thinning of the continental crust. Thinning varies from small amounts in the proximal rift to perhaps a factor of 5 or more adjacent to oceanic crust. Continental crust > ca. 25 km thick contains two weak layers, one between strong upper (quartz-rich) and lower (dioritic) crust and the other between strong lower crust and strong mantle. Continental crust < ca. 20 km thick is too thin for there to be weak layers and the strong layers are effectively annealed to one another and to the underlying strong mantle. Rifting of a passive continental margin must take place under tension. After rifting ceases, however, the margin can come under compression from forces originating elsewhere on or below its plate, e.g. collision between continental plates. The World Stress Map (www.world-stress-mp.org) shows that, where data exists, all EPCMs are currently under compression. Continental crust responds to moderate compression stress in two modes; flow in the weak lower crust and by forming gentle buckle-folds with a wavelength of 200-400 km and an amplitude of ca. 0.5 km. Under moderate compression, material in the crust's weak layers starts to flow towards the rift from under the adjacent continent. The lack of weak layers under the thinned, distal rift basin means, however, that flow cannot continue towards the ocean. Mid- and lower crustal material therefore accumulates under the proximal rift, thickening the crust there and lifting it by isostatic response to the thickening. Material flows into the rift until the crust under it is once more as thick as it was prior to extension, but no thicker. This thickened layer extends gradually further and further below the rift, at speeds of a few km per million years, uplifting it and exposing post-rift sediments. At higher stress, buckling may enhance this uplift, and it may be enhanced even more by the isostatic response to the erosion of deep valleys in the rising mountains. Both the thickening and folding continues until there is a reduction in imposed far-field compressive stress, after which the thickened crust 'freezes' in place.

Chalmers, J. A.

2012-12-01

89

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.

90

First amphibian magnetotelluric experiment at the passive continental margin in northern Namibia  

NASA Astrophysics Data System (ADS)

An amphibian magnetotelluric (MT) study across the passive continental margin of northern Namibia was conducted in December/January 2010/2011 and October/November 2011 to image the subsurface electrical conductivity structure. The MT experiment is part of the interdisciplinary SAMPLE project (South Atlantic Margin Processes and Links with onshore Evolution) which focusses on imaging and understanding processes related to rifting and the breakup history of the supercontinent Gondwana, in particular the opening of the South Atlantic and the post breakup evolution of the continental passive margins of Africa and South America. The onshore MT data were acquired in the Kaoko Mobile Belt at 167 sites in a ~140 km wide and ~260 km long EW extending corridor, from the Atlantic Ocean onto the Congo Craton. The Kaoko Mobile Belt is a transpressional strike slip orogen with NNW striking sinistral shear zones, folds and thrusts, which was formed during the Pan-African orogeny and the amalgamation of West Gondwana. This onshore network is extended offshore with MT measurements along 2 transects parallel and perpendicular to the Walvis Ridge - an approximately 3400km long seamount volcanic chain, trending NE-SW, from Africa to the Middle Atlantic Ridge, thought to be formed by the volcanic activity of the Tristan da Cunha Plume since the early Cretaceous. The onshore impedances and vertical magnetic transfer functions are generally of excellent quality but indicate significant three-dimensional structures in the crust and upper mantle, particularly in the Western Kaoko Zone, in the vicinity of the prominent shear zones. 2-D inversion of a sub-section of the entire data set, where two-dimensional modeling is consistent with the MT data revealed spatial correlations of a resistive zone and the Archean Congo Craton as well as of conductive structures and surface expressions of prominent faults.

Kapinos, G.; Weckmann, U.; Ritter, O.; Jegen, M. D.

2012-12-01

91

Seismic Profile Showing Cenozoic Development of the New England Continental Margin.  

National Technical Information Service (NTIS)

A new seismic profiling system consisting of a hydroacoustic transducer with a correlation receiver has yielded a detailed profile of the outer continental margin off New England. The profile shows new evidence of Cenozoic sedimentation and shallow geolog...

D. C. Krause

1966-01-01

92

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

93

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

94

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

NASA Astrophysics Data System (ADS)

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

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

2006-03-01

95

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

96

Elevated Passive Continental Margins may form much Later than the time of Rifting  

NASA Astrophysics Data System (ADS)

Many current models of the development of elevated passive continental margins assume that they are either the remains of foot-wall uplift at the time of rifting or due to underplating by magma from a plume or other mantle source. We have studied the rift and post-rift history of such a passive margin in West and South Greenland and have concluded that the present-day elevations developed 25-60 million years after cessation of rifting and local volcanism, suggesting that additional factors need to be considered when modelling such margins. The morphology of West Greenland is similar to that of other elevated passive margins ion many parts of the world. There are high-level, large-scale, quasi-planar landscapes (planation surfaces) at altitudes of 1-2 km cut by deeply incised valleys. The gradient from the highest ground to the coast is much steeper than that away from the coast. We combined analysis of the morphology of the landscape with studies of fission tracks and the preserved stratigraphic record both on- and off-shore. Rifting and the commencement of sea-floor spreading in the Early Paleogene was accompanied by voluminous high-temperature volcanism. Kilometer-scale uplift at the time of rifting was followed shortly afterwards by kilometer-scale subsidence and possibly by transgression of marine sediments across the rift margin. The present elevated margin formed during three episodes of uplift during the Neogene, 25-60 million years after the cessation of rifting and local volcanism. The quasi-planar planation surfaces presently at 1-2 km altitude are the end-products of denudation to near sea-level in the mid- and late Cenozoic and these surfaces were uplifted to their present altitudes during the Neogene events. Rivers then incised the summit surface to form valleys that were further enlarged and deepened by glaciers. Similar elevated margins exist all around the northern North Atlantic and in many other parts of the world; eastern North America, on both sides of the South Atlantic, western India, eastern Australia, and possibly in Antarctica. Our results show that we cannot simply assume that these elevations were produced either at the time of rifting or as underplating at the time of plume impact. There is, however, no general agreement as to what caused them and we suggest that the history of these margins need to be re-assessed in the light of our results.

Chalmers, J. A.; Japsen, P.; Green, P. F.; Bonow, J.; Lidmar-Bergstrom, K.

2007-12-01

97

Elevated Passive Continental Margins may form much Later than the time of Rifting  

NASA Astrophysics Data System (ADS)

Many current models of the development of elevated passive continental margins assume that they are either the remains of foot-wall uplift at the time of rifting or due to underplating by magma from a plume or other mantle source. We have studied the rift and post-rift history of such a passive margin in West and South Greenland and have concluded that the present-day elevations developed 25-60 million years after cessation of rifting and local volcanism, suggesting that additional factors need to be considered when modelling such margins. The morphology of West Greenland is similar to that of other elevated passive margins ion many parts of the world. There are high-level, large-scale, quasi-planar landscapes (planation surfaces) at altitudes of 1-2 km cut by deeply incised valleys. The gradient from the highest ground to the coast is much steeper than that away from the coast. We combined analysis of the morphology of the landscape with studies of fission tracks and the preserved stratigraphic record both on- and off-shore. Rifting and the commencement of sea-floor spreading in the Early Paleogene was accompanied by voluminous high-temperature volcanism. Kilometer-scale uplift at the time of rifting was followed shortly afterwards by kilometer-scale subsidence and possibly by transgression of marine sediments across the rift margin. The present elevated margin formed during three episodes of uplift during the Neogene, 25-60 million years after the cessation of rifting and local volcanism. The quasi-planar planation surfaces presently at 1-2 km altitude are the end-products of denudation to near sea-level in the mid- and late Cenozoic and these surfaces were uplifted to their present altitudes during the Neogene events. Rivers then incised the summit surface to form valleys that were further enlarged and deepened by glaciers. Similar elevated margins exist all around the northern North Atlantic and in many other parts of the world; eastern North America, on both sides of the South Atlantic, western India, eastern Australia, and possibly in Antarctica. Our results show that we cannot simply assume that these elevations were produced either at the time of rifting or as underplating at the time of plume impact. There is, however, no general agreement as to what caused them and we suggest that the history of these margins need to be re-assessed in the light of our results.

Chalmers, J. A.; Japsen, P.; Green, P. F.; Bonow, J.; Lidmar-Bergstrom, K.

2004-12-01

98

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

99

Listric Normal Faulting on the Cascadia Continental Margin  

NSDL National Science Digital Library

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

Goldfinger, Chris; Kulm, LaVerne D., 1936-; McNeill, L. C. (Lisa C.); Piper, Kenneth A.; Yeats, Robert S.

2011-02-04

100

Detachment models for the formation of passive continental margins  

NASA Astrophysics Data System (ADS)

The inherent asymmetry of extension by detachment leads to contrasting and conjugate classes of passive margins. Upper-plate margins comprise crust above a deeper detachment. Lower-plate margins comprise the footwall of the detachment, overlain by faulted upper plate remnants. Such margins have distinctive architectures, structural styles, uplift-subsidence paths and thermal histories. The wide range in structural styles on passive margins is predicted by five models which incorporate detachment faults linked to flat ductile shear zones, and ductile stretching of the thermal lithosphere below the shear zones. These models provide explanations for enigmatic structural and morphological features of passive margins such as marginal plateaux, outer highs, unstructured synrift sag basins, and perched rift basins. Numerical modelling of isostatic uplift-subsidence histories shows that different patterns of uplift-subsidence behaviour can be explained by variations in detachment geometry and change in the amount of lithospheric stretching. Voluminous igneous underplating is predicted if anomalously hot asthenosphere is uplifted. The arrival of such mantle derived melts may cause significant additional uplift. Upper-plate margins undergo thermally induced uplift, with permanent uplift due to igneous underplating. This uplift may be the origin of passive margin mountains in the adjacent hinterlands. Marginal plateaux are emergent or very shallowly submerged throughout the extension history, with postrift subsidence to intermediate water depths. The lithosphere is extended below a midcrustal detachment, but with little extension of the upper-plate. The pattern of subsidence on an Atlantic margin requires an extended upper plate superimposed on progressively more stretched subdetachment lithosphere. Conjugate margins are described from the Tasman Sea, the Atlantic Ocean and the Great Southern Ocean, illustrating both the principle of complementary asymmetry and the different patterns of uplift or subsidence on opposing passive margins.

Lister, G. S.; Etheridge, M. A.; Symonds, P. A.

1991-10-01

101

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

National Technical Information Service (NTIS)

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 relation...

M. D. Behn

2002-01-01

102

Dependence of Mantle Exhumation at Rifted Continental Margins on the Deformation Mode of Breakup Lithosphere Thinning  

NASA Astrophysics Data System (ADS)

Mantle exhumation at rifted continental margins requires that rupture of continental crust and the unroofing of mantle occurs before the start of significant melt production. The relative timing of the onset of ocean ridge melt production is sensitive not only to extension rate, mantle temperature and mantle depletion but also the deformation mode of continental lithosphere thinning leading to continental breakup. Two end-member modes of continental lithosphere thinning deformation have been examined: depth-uniform (pure-shear) lithosphere stretching and thinning, and lithosphere thinning by upwelling divergent flow. Horizontal tensile plate forces provide the driving force for the pure-shear deformation. Upwelling divergent flow is assumed to be driven by a combination of horizontal plate boundary forces and thermal and melt buoyancy initiated by pure-shear lithosphere stretching, and predicts a simple transition from pre-breakup lithosphere thinning to sea-floor spreading. For the N. Iberian - N. Newfoundland margins, pure-shear breakup lithosphere thinning model predicts that the onset of melt generation occurs prior to breakup rupture of the continental crust for normal mantle temperature and chemical composition. In contrast the upwelling divergent flow model predicts the onset of melt generation after continental crust rupture leading to ~ 100 km mantle exhumation on each margin. Continental lithosphere thinning leading to continental breakup and sea-floor spreading initiation is most likely achieved by a simultaneous combination of pure-shear and upwelling divergent flow within continental lithosphere and asthenosphere. The relative importance of these deformation modes is dependent on depth, pre-breakup extension rates and mantle temperature. We proposes that beneath 10-15 km depth the dominant mode of continental lithosphere thinning leading to breakup is upwelling divergent flow, while for depths shallower than 10-15 km (corresponding to the cooler upper lithosphere) the dominant thinning mode is pure-shear in the form of brittle faulting.

Kusznir, N. J.; Fletcher, R. J.; Manatschal, G.

2008-12-01

103

Case for Upper Miocene continental break-up in western Afar, southern Red Sea rift  

NASA Astrophysics Data System (ADS)

We describe the distribution, nature and timing of extension in the youthful Western Afar in order to infer mechanisms for continental break-up. New and existing remote sensing, fieldwork and structural analyses calibrated by geochronology were integrated to define rift stages and the distribution and nature of volcanic units and structures that accommodate extension. Oligo-Miocene continental rift basins of the Southern Red Sea rift, Ethiopia are typical steep-sided continental rift graben, but contain almost exclusively volcanic fill. Rift segmentation was defined by border faults that were the locus of strain, but strain migrated to magmatic centres within the basins in the Upper Miocene. Widespread basaltic volcanism in the Upper Miocene led to extension, burial, riftward rotation and seismic reactivation of Oligo-Miocene rift basins. These basalts were sourced from linear magmatic segments that are independent of the older border fault segmentation. Cycles of subsidence and volcanism gave the basalt successions a concave-down profile similar to seaward-dipping reflectors (SDRs) on buried volcanic margins. Mid-Miocene propagation of the Main Ethiopian rift (MER) arm into the Afar Depression led to overprinting of the early Red Sea rift structures. We suggest that the locus of strain in western Afar jumped from Upper Miocene Southern Red Sea magmatic segments to Plio-Quaternary MER magmatic segments. The distribution of strain across the western Afar margin, and age of SDR-like basalts and the intersecting Southern Red Sea and Main Ethiopian rifts suggests that the Southern Red Sea magmatic segments were abandoned in the Upper Miocene leaving SDRs stranded above Oligo-Miocene rift basins, possibly due to the northward propagation of the seismically and volcanically active MER.

Wolfenden, E.; Ebinger, C.; Yirgu, G.; Kelley, S.; Deino, A.; Renne, P.

2003-04-01

104

Slope instability along the northeastern Iberian and Balearic continental margins  

Microsoft Academic Search

This paper gathers the available information on submarine landslides identified in the northeastern Iberian con- tinental margin and presents new data on both already known landslides and new, previously unknown ones. The 2,000 km 2 , 26 km 3 resulting deposit of the BIG'95 debris flow in the Ebro margin; the 4 up to 16 km 2 , 0.4 km

Carrer Martí

2007-01-01

105

Transience and persistence in the depositional record of continental margins  

Microsoft Academic Search

Continental shelves and coastal plains are large persistent depositional landforms, which are stationary (nonmigrating) at their proximal ends and characterized by relatively steady long-term growth. In detail, however, their surface form and stratigraphic record is built of transient freely migrating landscape elements. We derive the timescales of crossover from transient to persistent topographic forms using empirical scaling relations for mean

Douglas J. Jerolmack; Peter Sadler

2007-01-01

106

1. Physical properties and age of mid-slope sediments dredged from the Eastern Australian Continental Margin and the implications for continental margin erosion processes  

NASA Astrophysics Data System (ADS)

A large number of submarine landslides were identified on the continental slope on the Eastern Australian margin during voyages aboard the RV Southern Surveyor in 2008. Preliminary sedimentological analysis as well as geotechnical and biostratigraphic data determined for mid-slope dredge samples are reported. The dredge samples are normally-consolidated, calcareous sandy-muds of Neogene age and were recovered from submarine scarps located on the mid-continental slope. These scarps probably represent submarine landslide failure surfaces. Slope stability modelling using classical soil mechanics techniques and measured sediment shear-strengths indicates that the slopes should be stable; however, the ubiquity of mid-slope slides on this margin indicates that their occurrence is relatively common and that submarine-sliding should probably be considered to be a normal characteristic of the margin as suggested by Boyd et al (2010). While this presents something of an interpretational paradox, it nevertheless indicates that an, as yet, unidentified mechanism acts to reduce the shear resistance of these sediments to very low values which enables the slope failures to occur. It is suspected that the expansion of the Antarctic Icesheet in Mid-Miocene time and the consequent large-scale production of cold, equator-ward migrating, bottom water has caused significant erosion and removal of material from mid-slope and lower slope of the Australian continental margin in the Tasman Sea since the Mid-Miocene. Such a process would help to explain the exposure of hard Palaeozoic basement rocks along much of the southern part of the Eastern Australian continental margin. It is also suspected that erosion due to equator-ward moving bottom water effectively and progressively removed material from the toe of the continental slope sediment wedge. This rendered the slope sediments that were deposited throughout the Tertiary more susceptible to mass failure than would have otherwise been the case. Reference: Boyd, R., Keene, J., Hubble, T., et al. (2010). Southeast Australia: A Cenozoic Continental Margin Dominated by Mass Transport. Submarine Mass Movements and Their Consequences, Advances in Natural and Technological Hazards Research. D. C. Mosher, et al. 28: 491-502.

Hubble, T.; Yu, P.; Airey, D.; Clarke, S. L.; Boyd, R.; Keene, J.; Exon, N.; Gardner, J. V.

2010-12-01

107

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

108

Alkalinity distribution in the western North Atlantic Ocean margins  

Microsoft Academic Search

Total alkalinity (TA) distribution and its relationship with salinity (S) along the western North Atlantic Ocean (wNAO) margins from the Labrador Sea to tropical areas are examined in this study. Based on the observed TA-S patterns, the mixing processes that control alkalinity distribution in these areas can be categorized into a spectrum of patterns that are bracketed by two extreme

Wei-Jun Cai; Xinping Hu; Wei-Jen Huang; Li-Qing Jiang; Yongchen Wang; Tsung-Hung Peng; Xin Zhang

2010-01-01

109

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

110

Flow of material under compression in weak lower continental crust can cause post-rift uplift of passive continental margins  

NASA Astrophysics Data System (ADS)

There are mountain ranges up to more than 2 km high along many passive continental margins (e.g. Norway, eastern Australia, eastern Brazil, SE and SW Africa, east and west Greenland etc.), dubbed Elevated Passive Continental Margins (EPCMs). EPCMs contain several features in common and observations indicate that uplift of these margins took place after continental break-up. There are many explanations for their formation but none that satisfy all the observations. Lack of a geodynamical mechanism has meant that there has been difficulty in getting the community to accept the observational evidence. Formation of a passive continental margin must take place under conditions of tension. After rifting ceases, however, the margin can come under compression from forces originating elsewhere on or below its plate, e.g. orogeny elsewhere in the plate or sub-lithospheric drag. The World Stress Map (www.world-stress-mp.org) shows that, where data exists, all EPCMs are currently under compression. Under sufficient compression, crust and/or lithosphere can fold, and Cloetingh & Burov (2010) showed that many continental areas may have folded in this way. The wavelengths of folding observed by Cloetingh & Burov (2010) imply that the lower crust is likely to be of intermediate composition; granitic lower crust would fold with a shorter wavelength and basic lower crust would mean that the whole lithosphere would have to fold as a unit resulting in a much longer wavelength. Continental crust more than 20 km thick would be separated from the mantle by a weak layer. However, crust less thick than that would contain no weak layers would become effectively annealed to the underlying strong mantle. Under sufficient horizontal compression stress, material can flow in the lower weak layer towards a continental margin from the continental side. The annealed extended crust and mantle under the rift means, however, that flow cannot continue towards the ocean. Mid- and lower crustal material therefore accumulates in the proximal rift and rift margin, thickening them and lifting them by isostatic response to the thickening. Flow into the rift margin is opposed by uplift and folding of the upper, strong crust, which imposes an additional normal stress, until crust thickens no more. However, flow continues through this thickened crust, thickening and uplifting the area "downstream", so widening the thickened area. Flow and uplift can continue until a reduction in imposed far-field compressive stress causes a consequent large reduction in inflow, thereby 'freezing' the thickened crust in place. Erosion of the uplifted area will lead to further uplift of the uneroded material because of the isostatic response to the erosion. Reference Cloetingh, S. & Burov, E. 2010: Lithospheric folding and sedimentary basin evolution: a review and analysis of formation mechanisms. Basin Research 22, 1365-2117. doi:10.1111/j.1365-2117.2010.00490.x.

Chalmers, James

2014-05-01

111

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

Microsoft Academic Search

A high-resolution paleomagnetic and rock magnetic study has been carried out on sediment cores collected in glaciomarine silty-clay sequences from the continental shelf and slope of the southern Storfjorden trough-mouth fan, on the northwestern Barents Sea continental margin. The Storfjorden sedimentary system was investigated during the SVAIS and EGLACOM cruises, when 10 gravity cores, with a variable length from 1.03

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

2011-01-01

112

Late Pliocene margin development and mega debris flow deposits on the Antarctic continental margins: Evidence of the onset of the modern Antarctic Ice Sheet?  

Microsoft Academic Search

Major global climatic changes in the Cenozoic coincided with major changes in volume of polar ice sheets, which are reflected in the sedimentary regime of polar continental margins. We compile and review the most recent evidences of the Neogene architectural change in the sedimentary units of the Antarctic continental margins to provide a comprehensive interpretation of timing and circumstances of

M. Rebesco; A. Camerlenghi

2008-01-01

113

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

114

Rifted continental margins: geometric control on crustal architecture and melting  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

115

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.

116

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

NASA Astrophysics Data System (ADS)

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

Ryang, Woo Hun; Kim, Seong Pil

2014-05-01

117

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

118

Extension of continental crust at the margin of the eastern Grand Banks, Newfoundland  

NASA Astrophysics Data System (ADS)

Seismic and gravity observations from the rifted margin of the eastern Grand Banks, Newfoundland, support a new model for extension of the continental crust from the shelf edge to ODP Site 1277, where mantle rocks are exhumed. We find that the largest decrease in crustal thickness, from about 28 km to 6 km, occurs beneath the continental slope of the Grand Banks over a distance of just 20 km. This rapid decrease in crustal thickness coincides with anomalously high seismic velocities (7.0-7.2 km·s - 1 ) in the lower crust of the shelf edge. The thin crust of the continent-ocean transition (COT) in this area has a smooth basement surface, void of upper crustal blocks and prerift sediments. We compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere. Both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust. The geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins: Beneath the continental shelf of the Grand Banks, the Moho and the strong lower crust rotated upwards toward to a 50° dip without visible internal deformation. The presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward. With ongoing extension, some high-angle normal faults may have rotated to a sub-horizontal orientation, which would explain the lack of brittle deformation visible in the seismic reflection data. The two geodynamic models produce different amounts of extension of continental crust in the distal margins. The hot rifting model localizes strain much more rapidly, leaving narrow zones of extended continental crust, and it produces a relatively large amount of melt (> 30%) in the final stages of rifting. Continental breakup may occur rapidly in hot lithosphere (< 5 Myr). On the other hand, a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin, similar to what we inferred at the eastern Grand Banks. The cold lithospheric model requires about 23 Myr of extension before continental breakup, and it predicts much less melting in the mantle (13%). The long rift duration, wide zones of thinned continental crust, and small amount of magmatism make the cold rifting model the most applicable to Newfoundland-Iberia rift.

Van Avendonk, Harm J. A.; Lavier, Luc L.; Shillington, Donna J.; Manatschal, Gianreto

2009-04-01

119

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

120

Structure and evolution of the northern Barents-Kara Sea continental margin from integrated analysis of potential fields, bathymetry and sparse seismic data  

NASA Astrophysics Data System (ADS)

The northern Barents-Kara Sea continental margin is a poorly investigated area because of a permanent ice cover hampering seismic exploration. The available geological and geophysical data show that the magma-poor margin developed in response to early Cenozoic break-up and subsequent opening of the Arctic Eurasia Basin. In this study, a series of crustal-scale geotransects illustrating the architecture of the continental margin are constructed using sparse seismic reflection profiles and a gravity inversion method incorporating a thermal model of rifting. The continental side of the northern Barents Sea margin is underlain by Palaeozoic-Early Mesozoic deep sedimentary basins separated from the oceanic side by the marginal uplift. A bathymetry analysis complements low-resolution seismic data to predict the sedimentary depocenters beyond the shelf break. These depocenters are associated with troughs, perpendicular to the shelf edge. The depocenter in front of the St. Anna Trough may contain a sedimentary section more than 4 km thick. The gravity correction for the effect of sedimentary cover was added to the inversion. This correction used an exponential density-depth function. The inversion supports a narrow and steep continent-ocean transition (COT; ca. 100 km). The conjugate Lomonosov Ridge margin is modelled using the same technique. Palaeoreconstructions were made to predict the break-up setting. The northern Barents Sea-Lomonosov Ridge rift system can be described as an initially narrow symmetric rift. A transitional zone of extreme thinning is assumed between the oldest spreading magnetic anomaly and the stretched continental crust. The free-air gravity anomaly in the western part of the margin can be predicted by the upwelling divergent flow model implying the exhumation of the lower crust and the continental upper mantle within the COT. It is suggested that an episode of shear or oblique extension before breakup is required to explain the observed narrow symmetric conjugate margins in the Eurasia Basin.

Minakov, A.; Faleide, J. I.; Glebovsky, V. Yu.; Mjelde, R.

2012-01-01

121

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

NASA Astrophysics Data System (ADS)

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

Bell, C. M.

122

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

USGS Publications Warehouse

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

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

1987-01-01

123

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

124

Crustal Structure of The Goban Spur Continental Margin From Wide-angle Modelling  

NASA Astrophysics Data System (ADS)

The structure of rifted margins can provide an insight into the processes operating during continental breakup and separation. They are commonly divided into `volcanic' where substantial igneous activity ocurred around breakup and `non-volcanic' where there has been little or none. The Goban Spur continental margin, southwest of the UK, has previously been interpreted as a simple `non-volcanic' margin with a relatively abrupt transition from continental fault blocks to normal oceanic crust. This contrasts with `volcanic' margins to the north, where there is a transitional region of heavily intruded continental crust and `non-volcanic' margins to the south where broad zones consisting largely of upper mantle rocks have been identified. During September 2000 a 100 km wide-angle seismic line was shot colinear with existing normal incidence and sparsely sampled wide-angle profiles. Data was collected on seven ocean-bottom hydrophones at a spacing of 15-30 km and on six sonobuoys with a 15 km separation. Here we present an initial interpretation of the wide-angle velocity model across the margin. A 120 km wide transition zone between continental and unequivocal oceanic crust is observed and displays seismic velocities of 7.2-7.6 kms-1 at its base. These velocities are not typical of normal oceanic crust or mantle and may be interpreted as a zone of exhumed upper mantle emplaced at the end of rifting or oceanic crust formed by ultra-slow sea-floor spreading processes. The presence of a very low relief basement immediately overlying this region strongly supports the first hypothesis.

Bullock, A. D.; Minshull, T. A.

125

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

NASA Astrophysics Data System (ADS)

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

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

2011-01-01

126

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

127

Extension on rifted continental margins: Observations vs. models.  

NASA Astrophysics Data System (ADS)

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

Skogseid, Jakob

2014-05-01

128

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

NASA Astrophysics Data System (ADS)

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

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

129

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

Microsoft Academic Search

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

Robert White; Dan McKenzie

1989-01-01

130

Sources, transport, and partitioning of organic matter at a highly dynamic continental margin  

Microsoft Academic Search

Continental shelves play a major role as transition zone during transport of multiply-sourced organic matter into the deep sea. In order to obtain a comprehensive understanding of the origin and fractionation processes of organic matter at the NW Iberian margin, 40 surface sediment samples were analyzed for a structurally diverse range of lipid biomarkers, lignin phenols, grain size distribution, organic

Frauke Schmidt; Kai-Uwe Hinrichs; Marcus Elvert

2010-01-01

131

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

Microsoft Academic Search

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

Robert W. Embley; Robert D. Jacobi

1977-01-01

132

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

SciTech Connect

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

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

1993-04-01

133

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

134

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

135

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

136

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

SciTech Connect

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

Chen, P.P.H. (Conoco Inc., Lafayette, LA (United States)); Zhi Yuong Chen; Qi Min Zhang (Nanhai West Oil Corp., Guangdong (China))

1993-05-01

137

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

NASA Astrophysics Data System (ADS)

The continental margin offshore NW Sabah/Borneo (Malaysia) has been investigated with reflection and refraction seismics, magnetics, and gravity during the recent cruise BGR01-POPSCOMS. A total of 4000 km of geophysical profiles has been acquired, thereof 2900 km with reflection seismics. The focus of investigations was on the deep water areas. The margin looks like a typical accretionary margin and was presumably formed during the subduction of a proto South China Sea. Presently, no horizontal movements between the two plates are being observed. Like in major parts of the South China Sea, the area seaward of the Sabah Trough consists of extended continental lithosphere which is characterised by a pattern of rotated fault blocks and half grabens and a carbonate platform of Early Oligocene to Early Miocene age. We found evidence that the continental crust also underlies the Sabah Trough and the adjacent continental slope, a fact that raises many questions about the tectonic history and development of this margin. The tectonic pattern of the Dangerous Grounds' extended continental crust can be traced a long way landward of the Sabah Trough beneath the sedimentary succession of the upper plate. The magnetic anomalies which are dominated by the magnetic signatures of relatively young volcanic features also continue under the continental slope. The sedimentary rocks of the upper plate, in contrast, seem to generate hardly any magnetic anomalies. Based on the new data we propose the following scenario for the development of the NW Sabah continental margin: Seafloor spreading in the present South China Sea started at about 30 Ma in the Late Oligocene. The spreading process separated the Dangerous Grounds area from the SE Asian continent and ceased in late Early Miocene when the oceanic crust of the proto South China Sea was fully subducted in eastward direction along the Borneo-Palawan Trough. During Lower and/or Middle Miocene, Borneo rotated counterclockwise and was thrusted onto the edge of the rifted continental block of the Dangerous Grounds. The subducted oceanic crust of the proto South China Sea must today be located below the Eastern part of Sabah and not along the present NW Sabah Trough.

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

2002-12-01

138

Earthquakes and crustal structure beneath the central Cascadia continental margin  

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

139

Earthquakes and crustal structure beneath the central Cascadia continental margin  

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

140

Emplacement of the Peridotite Ridge Within the Ocean-Continent Transition on the West Iberia Continental Margin  

NASA Astrophysics Data System (ADS)

The West Iberia continental margin began to form in the Late Triassic with the breakup of Pangea and culminated with the opening of the North Atlantic Ocean in the Early Cretaceous. While it is perhaps the best-studied continental margin in the world, the formation of a north-south segmented ridge of serpentinized peridotite located within the ocean-continent transition is not fully understood. The relatively thin sediment cover and lack of obvious volcanics make the Iberia margin an ideal place to study crustal features using seismic methods, and in 1997 the Iberia Seismic Experiment (ISE 97) collected over 4000 km of two-dimensional seismic reflection data off Iberia. Nine lines were recorded normal to the margin at a 10-25 km spacing and therefore provide cross-sections of the margin at regular intervals over a 200 km north-south distance. We prestack depth migrated the western portions of the nine margin-normal ISE 97 seismic lines. Interpretation of the depth sections suggests that the peridotite ridge is located within a broad zone of exhumed upper mantle that has been serpentinized. Total tectonic subsidence analyses provide minimum bounds on the thickness of the serpentinized layer, which extends from the zone of exhumed mantle landward under thinned continental crust. Exhumed upper mantle has previously been identified in the wider ocean-continent transition to the south in the southern Iberia Abyssal Plain, and we conclude that similar but narrower exposures occur to the north in the Galicia Bank area. While upper mantle appears to have been exhumed along all the ISE 97 lines, it does not always form a distinct basement ridge. Three ISE 97 profiles do not image a well-defined basement ridge, offering the first seismic evidence of a gap in the otherwise continuous peridotite ridge. Where it is well-developed, the peridotite ridge parallels a deeply-penetrating normal fault that developed near the end of rifting. This fault appears to penetrate into the upper mantle. We propose that this fault served as a conduit for water to localize serpentinization and cause the peridotite ridge to form diapirically within a broader zone of tectonically exhumed mantle.

Henning, A. T.; Sawyer, D. S.

2003-12-01

141

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

SciTech Connect

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

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

1990-05-01

142

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

SciTech Connect

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

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

1985-01-01

143

Understanding the Development of Passive Continental Margins: the Value of Detailed Heat Flow Surveys  

NASA Astrophysics Data System (ADS)

The Earth Science community has spent much effort trying to understand the development of passive continental margins within the framework of plate tectonics. In the span of the last thirty years, we have recorded detailed geological and geophysical observations on more than 25 such margins with little success in providing a generic model that can form the basis for understanding these margins. Orthogonal to spreading margins appear to form differently from transform margins and, despite excellent measurements and well developed experiments, there are still almost as many models for how these margins are created as the number of margins that have been studied. In addition, not one of the younger margins, when studied in detail offers a good analogue for the development of most mature margins. Before we give up attempts to find some generic concept underlying their development and attribute everything to a differing initial geology, we believe that we should utilize all possible observational techniques that could yield relevant information on the margins. One such technique is heat flow where, over that past 20 years, detailed measurements and multichannel seismic surveys have yielded critical information on the flow of fluids within the ocean crust and sediment and the effects of sedimentation on the conductive heat flow. We believe that detailed heat flow measurements can now address a number of important scientific questions on the both young and mature margins. We outline a few below: (a) fluid flow though both sediment and basement and the loss of heat by advection, (b) the conductive heat loss and fluid flow due to shallow sills, (c) constrain between models of crustal extensions and massive dyke intrusion and perhaps determine the onset of extension knowing (a) and (b), (d) determine the ocean continent transition on mature margins where the sedimentation rate is known, and, (e) identify areas of recent massive slumping on unstable sedimentary margins using detailed 3D shallow seismic surveys and heat flow.

Sclater, J. G.; Hasterok, D. P.

2011-12-01

144

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

PubMed

The Neogene stratigraphic section of the southeastern U.S. continental shelf-coastal plain system is characterized by (i) a series of major regional phosphogenic episodes; (ii) a strong spatial relationship between the structural or topographic framework and phosphate deposition; and (iii) distinct cyclical and regional patterns of deposition of the terrigenous, carbonate, and phosphate lithofacies. The complex depositional patterns are explained by a paleoceanographic model based upon the interaction of glacial eustatic sea-level fluctuations, associated changes in climate, and the dynamics of the Gulf Stream in response to the bathymetric configurations of the continental margin during the past 20 million years. PMID:17733785

Riggs, S R

1984-01-13

145

Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps  

NASA Astrophysics Data System (ADS)

The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65-80 Ma) subduction to different depths at high-pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0-1.2, 1.0-1.5, and 1.5-2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal-scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E-W trending mylonitic shear zone under retrograde blueschist- to greenschist-facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15-20 km occurred in the footwall of a greenschist-facies, top-SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb-Sr mineral ages constrain D2 to have occurred at about 60-65 Ma and D3 at about 45-55 Ma. Thus top-SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro-Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra-HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo-Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism is absent or was overprinted by Tertiary temperature-dominated metamorphism. Most exhumation of continental HP rocks in the Sesia Zone therefore preceded Tertiary Alpine collision and coincided with Late Cretaceous to Early Tertiary subduction of the Adriatic and Tethyan lithosphere. The transition from D2 to D3 in the Sesia Zone is interpreted to mark a change from high-stress, oblique SE directed subduction and accretion of the distal Adriatic continental margin to NW retreating, low-stress subduction of the Liguro-Piemont oceanic lithosphere.

Babist, J.; Handy, M. R.; Konrad-Schmolke, M.; Hammerschmidt, K.

2006-12-01

146

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

147

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

USGS Publications Warehouse

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

Kvenvolden, K. A.

1985-01-01

148

How does the continental crust thin in a hyper-extended rifted margin: insights from the Iberia margin  

NASA Astrophysics Data System (ADS)

During the last twenty years the knowledge of rifted margins was strongly challenged by new observations in particular related to ODP drilling and seismic refraction surveys. The discovery of hyper-thinned continental crust and exhumed mantle in deep water rifted margins results in two fundamental questions: How does the crust thin and what controls extreme crustal thinning and mantle exhumation? Reflection and refraction seismic lines indicate that distinct domains in rifted margins are separated by structures that cut decoupling levels in the crust and eventually transfer deformation to mantle levels. This evolution from decoupled to coupled deformation on a crustal level may explain the crustal architecture of magma-poor rifted margins. A comparison of seismic sections across the Iberia rifted margin shows that decoupled extension was distributed in the North and more localized in the South. In contrast coupled extension resulted in similar structures and width of the extended domain along the whole margin. Based on drill hole data the transition from decoupled to coupled deformation occurred during the Tithonian (±145Ma). A delay or retardation of subsidence observed over the hyper-extended coupled domain may indicate that crustal thinning had to occur simultaneously with lithospheric necking and may be controlled by deeper mantle processes. Based on our observations we suggest that the decoupled deformation affecting the crust seems to be mainly controlled by crustal structure and rheology, both of which are dependent on inheritance. In contrast the coupled deformation cuts the entire crust and occurs in a completely embrittled crust and is therefore much less dependent on the initial structure of the margin.

Sutra, E.; Manatschal, G.

2011-12-01

149

Tectonostratigraphic evolution of Cenozoic marginal basin and continental margin successions in the Bone Mountains, Southwest Sulawesi, Indonesia  

NASA Astrophysics Data System (ADS)

The Bone Mountains, located in Southwest Sulawesi along the SE margin of Sundaland, are composed of Oligocene to possibly lower Miocene marginal basin successions (Bone Group) that are juxtaposed against continental margin assemblages of Eocene-Miocene age (Salokalupang Group). Three distinct units make up the latter: (i) Middle-Upper Eocene volcaniclastic sediments with volcanic and limestone intercalations in the upper part (Matajang Formation), reflecting a period of arc volcanism and carbonate development along the Sundaland margin; (ii) a well-bedded series of Oligocene calc-arenites (Karopa Formation), deposited in a passive margin environment following cessation of volcanic activity, and (iii) a series of Lower-Middle Miocene sedimentary rocks, in part turbiditic, which interfinger in the upper part with volcaniclastic and volcanic rocks of potassic affinity (Baco Formation), formed in an extensional regime without subduction. The Bone Group consists of MORB-like volcanics, showing weak to moderate subduction signatures (Kalamiseng Formation), and a series of interbedded hemipelagic mudstones and volcanics (Deko Formation). The Deko volcanics are in part subduction-related and in part formed from melting of a basaltic precursor in the overriding crust. We postulate that the Bone Group rocks formed in a transtensional marginal basin bordered by a transform passive margin to the west (Sundaland) and by a newly initiated westerly-dipping subduction zone on its eastern side. Around 14-13 Ma an extensional tectonic event began in SW Sulawesi, characterized by widespread block-faulting and the onset of potassic volcanism. It reached its peak about 1 Ma year later with the juxtaposition of the Bone Group against the Salokalupang Group along a major strike-slip fault (Walanae Fault Zone). The latter group was sliced up in variously-sized fragments, tilted and locally folded. Potassic volcanism continued up to the end of the Pliocene, and locally into the Quaternary.

van Leeuwen, Theo M.; Susanto, Eko S.; Maryanto, Sigit; Hadiwisastra, Sapri; Sudijono; Muhardjo; Prihardjo

2010-06-01

150

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

151

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

152

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

USGS Publications Warehouse

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

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

2009-01-01

153

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

154

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

USGS Publications Warehouse

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

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

1987-01-01

155

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

NASA Astrophysics Data System (ADS)

More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150-400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane. Warming of the northward-flowing West Spitsbergen current by 1°C 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 becomes widespread along Arctic continental margins, tens of Teragrams of methane per year could be released into the ocean.

Westbrook, Graham K.; Thatcher, Kate E.; Rohling, Eelco J.; Piotrowski, Alexander M.; Pälike, Heiko; Osborne, Anne H.; Nisbet, Euan G.; Minshull, Tim A.; Lanoisellé, Mathias; James, Rachael H.; Hühnerbach, Veit; Green, Darryl; Fisher, Rebecca E.; Crocker, Anya J.; Chabert, Anne; Bolton, Clara; Beszczynska-Möller, Agnieszka; Berndt, Christian; Aquilina, Alfred

2009-08-01

156

Gas-hydrate stability thickness map along the Indian continental margin  

Microsoft Academic Search

The gas-hydrate stability thickness (GHST) map along the Indian continental margin is prepared from available bathymetry, sea-bottom temperature and geothermal gradient data. The bottom-simulating reflector (BSR) often marks the base of gas-hydrate stability zone. The prior information about the stability thickness in a particular area will help in identifying BSR on seismic data. The map is also useful to the

Kalachand Sain; V. Rajesh; N. Satyavani; K. V. Subbarao; C. Subrahmanyam

2011-01-01

157

Gas hydrates and active mud volcanism on the South Shetland continental margin, Antarctic Peninsula  

Microsoft Academic Search

During the Antarctic summer of 2003–2004, new geophysical data were acquired from aboard the R\\/V OGS Explora in the BSR-rich area discovered in 1996–1997 along the South Shetland continental margin off the Antarctic Peninsula. The\\u000a objective of the research program, supported by the Italian National Antarctic Program (PNRA), was to verify the existence\\u000a of a potential gas hydrate reservoir and

U. Tinivella; F. Accaino; B. Della Vedova

2008-01-01

158

Hydrogen sulfide hydrates and saline fluids in the continental margin of South Australia  

Microsoft Academic Search

During the drilling of the southern Australian continental margin (Leg 182 of the Ocean Drilling Program), fluids with unusually high salinities (to 1060\\/00) were encountered in Miocene to Pleistocene sediments. At three sites (1127, 1129, and 1131), high contents of H2S (to 15%), CH4 (50%), and CO2 (70%) were also encountered. These levels of H2S are the highest yet reported

P. K. Swart; U. G. Wortmann; R. M. Mitterer; M. J. Malone; P. L. Smart; D. A. Feary; A. C. Hine

2000-01-01

159

On the heat flux related to stretching in the NW-Mediterranean continental margins  

Microsoft Academic Search

Summary  The surface thermal flux of the continental margins of the northwestern Mediterranean Sea is interpreted on the basis of a 1-D instantaneous pure shear stretching model of the lithosphere in terms of three components: the background heat flowing out from the asthenosphere (38 mW m–2), the transient contribution depending on the rift age and extension amount (35 mW m–2 at

V. Pasquale; M. VERDOYAAND; P. Chiozzi

1995-01-01

160

Role of the continental margin in the global carbon balance during the past three centuries  

NASA Astrophysics Data System (ADS)

The global continental margins are important sites of deposition and regeneration of terrestrial organic carbon and of calcium carbonate produced in situ. The material balance of organic carbon in the coastal zone determines to a large extent its role as either a source or sink of CO2 relative to the atmosphere. We explore the role of the continental margin in the global cycling of carbon during the recent geologic past using a new model, TOTEM (Terrestrial Ocean aTmosphere Ecosystem Model). We conclude that during the past 300 yr, the coastal zone has been a site of relatively stable calcium carbonate deposition, as well as a site of greater input, recycling, and storage of terrestrial organic carbon. As a result, more organic carbon in the coastal zone is remineralized than produced in situ by photosynthesis (i.e., the coastal zone is net heterotrophic), and this reduces its sink strength for anthropogenic CO2. Continuation of this trend in the future will weaken the ability of the coastal zone to act as a sink for the rising anthropogenic CO2 in the atmosphere. Reversal of the carbon biogeochemical balance from net remineralization (heterotrophy) to net production (autotrophy) may occur in those continental margin sections where bioproduction is enhanced by elevated inorganic nutrient inputs (e.g., coastal upwelling zones) and/or where there is efficient carbon storage.

MacKenzie, Fred T.; Lerman, Abraham; Ver, Leah May B.

1998-05-01

161

Sea level rise and submarine mass failures on open continental margins  

NASA Astrophysics Data System (ADS)

Submarine mass failures (which include submarine slides or submarine landslides) occur widely on open continental margins. Understanding their cause is of great importance in view of the danger that they can pose both to coastal populations through tsunamis and to the exploitation of ocean floor resources through mass movement of the sea floor. Present knowledge of the causes of submarine mass failures is briefly reviewed, focussing on the role of sea level rise, a process which has previously only infrequently been cited as a cause. It is argued that sea level rise could easily have been involved in at least some of these events by contributing to increased overpressure in sediments of the continental margin whilst causing seismic activity. The Holocene Storegga Slide off South West Norway may have been partly caused by the early Holocene sea level rise in the area, accentuated by meltwater flux from the discharges of Lake Agassiz-Ojibway in North America. Relative sea level rise increased water loading on the Norwegian continental margin, increasing overpressure in the sediments and also causing seismic activity, triggering the Holocene Storegga Slide. Given that some forecasts of future sea level rise are not greatly different from rises which obtained during the early Holocene, the implications of rising sea levels for submarine mass failures in a global warming world are considered.

Smith, D. E.; Harrison, S.; Jordan, J. T.

2013-12-01

162

Sediment storage and stability along the western Tibetan plateau margin  

NASA Astrophysics Data System (ADS)

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

Blöthe, J. H.; Munack, H.; Korup, O.

2012-04-01

163

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

164

Rift migration explains continental margin asymmetry and crustal hyper-extension  

PubMed Central

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

Brune, Sascha; Heine, Christian; Perez-Gussinye, Marta; Sobolev, Stephan V.

2014-01-01

165

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

PubMed

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

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

2014-01-01

166

Tectonics and sea-level changes recorded in Late Triassic Sequences at rifted margins of eastern and western Tethys (Northwest Australia, Leg 122; Western Europe)  

NASA Astrophysics Data System (ADS)

During ODP Leg 122 upper Triassic (Carnian to Rhaetian) sediments were recovered at the sediment-starved passive continental margin off Northwest Australia [Haq et al., 1990]. The early-rift series of the Wombat Plateau, a northern sub-plateau of the Exmouth Plateau, consists of upper Triassic fluviodeltaics and shallow-marine carbonates including reefal facies. These sequences are capped by an erosional "post-rift unconformity" with a 70 m.y. hiatus during the Jurassic. The Wombat Plateau bears only a thin pelagic post-rift sedimentary cover of Cretaceous to Cenozoic age. Detailed investigations of microfacies, wireline logs and high-resolution seismics allow the reconstruction of paleoenvironments and identification of the depositional sequences. The Rhaetian paleoenvironments and facies are very similar to those of the Western Tethys (Western and Eastern Alps). Some of the Australian sequences fit well with the sequences of the Haq et al. [1987] global cycle chart, and with the sequences in Western Europe. Their boundaries seem to correspond to global events (especially the Norian/Rhaetian boundary), suggesting that the most important causal factor was eustasy. However, short-lived tectonic or volcanic events have affected the Australian and European continental margins at the same time (Carnian, Triassic/Jurassic boundary). This unexpected synchronism suggests that these short tectonic events are not confined to the Australian margin and may have played a significant role in the globally synchronous deposition of third-order sequences in addition to eustasy. The erosional "post-rift unconformity" which overlies the Rhaetian series corresponds to several superimposed events related to the multi-stage rifting and opening of the oceanic domains surrounding the Northwest Australian shelf. These events began with a tectonic reorganization around the Trias sic/Jurassic boundary, which also affected the Exmouth Plateau and the Rankin Platform, and which is known from other plate margins (Western Tethys, Central Atlantic).

Dumont, Thierry; Röhl, Ursula

167

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

168

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

169

Carbonate platforms of passive (extensional) continental margins: Types, characteristics and evolution  

NASA Astrophysics Data System (ADS)

Carbonate platforms of extensional margins may be grouped into several major categories. Homoclinal ramps have gentle slopes into deep water, may have skeletal or ooid/pellet sand shoal complexes, that grade without break in slope into deep-ramp nodular limestone, and then into pelagic/hemipelagic basin facies they generally lack significant slump and sediment gravity flow deposits in the deeper-water facies. Distally steepened ramps differ from the above in having a marked increase in slope at the seaward edge of the deep ramp, and abundant slumps, slope breccias and turbidites. However, clasts of shallow platform margin facies are generally absent from breccias. Rimmed shelves have linear trends of shelf edge lime sands and reefs, a marked increase in slope into deep water, and foreslope and slope sands, breccias (with clasts of platform margin rocks) and turbidites, grading seaward into basin hemipelagic/pelagic muds. They may be divided into accretionary, bypass and erosional margins. Isolated platforms are broad flat-topped shallow platforms surrounded by deeper water (few hundred meters to 4 km deep); many are bypass margins but accretionary and erosional margins also occur. Finally, incipiently to completely drowned or open platforms may develop by rapid submergence of ramps, shelves or isolated platforms; shallow platform margin facies are shifted landward and the earlier shallow-water platform is covered with transgressive lags, and deeper-water blankets of hemipelagic or pelagic facies or open marine, whole fossil wackestone. The carbonate prisms commonly develop above a basal clastic phase. Ramps develop early, and evolve into rimmed shelves. However, with drowning, rimmed shelves may evolve up into ramps. Rimmed shelves may also develop into ramps where miogeoclinal faces prograde into filling marginal basins. Carbonate miogeocline deposition terminates when the shelf becomes a prograding clastic shelf, or where it becomes part of a collisional orogen. The various platform types may be recognized from continental margin sequences ranging from Proterozoic to Holocene in age.

Read, J. F.

1982-01-01

170

Clathrate Smoking Gun Hypothesis: Ocean Gas Hydrate Melting Causes Cracking of Upper Continental Margin Slopes Creating Major Gas Release Pathways  

Microsoft Academic Search

Despite the potential for large releases of methane from the upper continental margin by hydrate dissociation, geophysical investigations have not included the critical part of the hydrate stability zone (HSZ) outcrop. If these HSZ outcrop areas at the shelf edge and upper continental slope are exposing dynamic \\

J. Mienert; S. Buenz; M. Vanneste; K. Andreassen

2003-01-01

171

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

NASA Astrophysics Data System (ADS)

It is commonly assumed that elevated, passive continental margins have remained uplifted since the time of rifting. In many areas, e.g. Scandinavia, the timing and extent of uplift movements are difficult to determine because the uplifted area consists almost exclusively of ancient metamorphic rocks. However, the preserved Mesozoic-Cenozoic sedimentary and volcanic record of West Greenland makes this a key area for studying the uplift of passive continental margins. We have combined apatite fission-track analysis data with landform analysis to investigate the development of West Greenland landscapes across areas with substantially different geology. We show that the present-day mountains of West Greenland (65-71°N) are the end result of three Cenozoic phases of uplift and erosion. The first phase that began between 36 and 30 Ma led to the formation of a planation surface during the Oligocene-Miocene. This surface was offset by reactivated faults, resulting in megablocks that were tilted and uplifted to present-day altitudes of up to 2 km in two phases that began between 11 and 10 and between 7 and 2 Ma. These late Neogene uplift phases postdate rifting by c. 50 million years and sea-floor spreading west of Greenland by c. 30 million years, while the first of these phases predates onset of glaciation in Greenland by c. 3 million years. The similarity in morphology between West Greenland and other rifted continental margins suggests that uplift of rift flanks to present-day elevations may often post-date rifting significantly. The regional nature of the uplift movements along passive margins and their considerable distance from active plate boundaries suggest that the causal mechanisms must be located in the deep crust or the upper mantle where the thickness of the crust and lithosphere changes substantially over a short distance.

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

2006-08-01

172

Modelling of Continental Lithosphere Breakup and Rifted Margin Formation in Response to an Upwelling Divergent Flow Field Incorporating a Temperature Dependent Rheology  

NASA Astrophysics Data System (ADS)

We numerically model continental lithosphere deformation leading to breakup and sea floor spreading initiation in response to an imposed upwelling and divergent flow field applied to continental lithosphere and asthenosphere. The model is used to predict rifted continental margin lithosphere thinning and temperature structure. Model predictions are compared with observed rifted margin structure for four diverse case studies. Prior to application of the upwelling divergent flow field the continental lithosphere is undeformed with a uniform temperature gradient. The upwelling divergent flow field is defined kinematically using boundary conditions consisting of the upwelling velocity Vz at the divergence axis and the half divergence rate Vx . The resultant velocity field throughout the continuum is computed using finite element (FE) code incorporating a Newtonian temperature dependent rheology. The flow field is used to advect the continental lithosphere material and lithospheric and asthenospheric temperatures. Viscosity structure is hence modified and the velocities change correspondingly in a feedback loop. We find the kinematic boundary conditions Vz and Vx to be of first order importance. A high Vz/Vx (greater than10), corresponding to buoyancy assisted flow, leads to minimal mantle exhumation and a well defined continent ocean transition consistent with observations at volcanic margins. For Vz/Vx near unity, corresponding to plate boundary driven divergence, mantle exhumation over widths of up to 100 km is predicted which is consistent with observations at non-volcanic margins. The FE method allows the upwelling velocity Vz to be propagated upwards from the top of the asthenosphere to the Earth's surface without the requirement of imposing Vx. When continental breakup is achieved the half divergence velocity Vx can be applied at the lithosphere surface and the upwelling velocity Vz left free. We find this time and space dependent set of boundary conditions is more plausible than a constant corner flow type solution and predicts levels of depth dependent stretching and continent ocean transitions consistent with observation. Depth dependent lithosphere stretching, which is observed at rifted continental margins, is predicted to occur before continental breakup and sea-floor spreading initiation. The model may be used to predict surface heat flow and bathymetry, and to provide estimates of melt production rates and cumulative thickness. We compare model predictions with observed margin structure for four diverse rifted margins: the Lofoten Margin (a mature volcanic margin), Goban Spur (a mature non-volcanic margin), the Woodlark Basin (a neotectonic young ocean basin) and the Faroe-Shetland Basin (a failed attempt at continental breakup). This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Badley Geoscience & Schlumberger Cambridge Research supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, Conoco¬Phillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, A Chappell, J Eccles, R Fletcher, D Healy, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & R Spitzer.

Tymms, V. J.; Kusznir, N. J.

2005-05-01

173

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

USGS Publications Warehouse

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

Lund, K.

2008-01-01

174

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

NASA Astrophysics Data System (ADS)

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

Voggenreiter, W.; Hötzl, H.

175

Are buried river channels sources of geoclutter on the New Jersey Continental Margin?  

NASA Astrophysics Data System (ADS)

Geological features on a continental shelf may be responsible for anomalous acoustic scatter that are identified as (false) targets, or GeoClutter, on active sonar systems. Features on the New Jersey Continental Margin include a drainage system that formed when sea-level was much lower, ran across the shelf, and incised channels approximately 10 meters deep into the surrounding seabed. These channels have since been filled with sediments that are not apparent on bathymetric maps. The potential for these channels to create GeoClutter depends in part on the contrast in geoacoustic properties between the sediments filling the channels and the adjacent flanks. To study this matter, an experiment was conducted to measure the reflection loss from 1 to 10 kHz of channel fill and flank sediments in an area where GeoClutter has been observed and where there is supporting geophysical data. The measurements were made using the WARBLE technique [C. W. Holland and J. C. Osler, J. Acoust. Soc. Am. 107, 1263-1279 (2000)], adapted for use in rapid environmental assessment using modified sonobuoys. Results from the experiment will be presented and the role of buried channels acting as sources of GeoClutter on the New Jersey Continental Margin will be discussed.

Osler, John C.

2003-10-01

176

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

USGS Publications Warehouse

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

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

1997-01-01

177

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

USGS Publications Warehouse

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

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

2001-01-01

178

Changes in plate motion and vertical movements along passive continental margins  

NASA Astrophysics Data System (ADS)

The origin of the forces that produce elevated, passive continental margins (EPCMs) has been a hot topic in geoscience for many years. Studies of individual margins have led to models, which explain high elevations by invoking specific conditions for each margin in question. We have studied the uplift history of several margins and have found some striking coincidences between episodes of uplift and changes in plate motion. In the Campanian, Eocene and Miocene, pronounced events of uplift and erosion affected not only SE Brazil (Cobbold et al., 2001), but also NE Brazil and SW Africa (Japsen et al., 2012a). The uplift phases in Brazil also coincided with three main phases of Andean orogeny (Cobbold et al., 2001, 2007). These phases, Peruvian (90-75 Ma), Incaic (50-40 Ma), and Quechuan (25-0 Ma), were also periods of relatively rapid convergence at the Andean margin of South America (Pardo-Casas and Molnar, 1987). Because Campanian uplift in Brazil coincides, not only with rapid convergence at the Andean margin of South America, but also with a decline in Atlantic spreading rate, we suggest that all these uplift events have a common cause, which is lateral resistance to plate motion (Japsen et al., 2012a). Because the uplift phases in South America and Africa are common to the margins of two diverging plates, we also suggest that the driving forces can transmit across the spreading axis, probably at great depth, e.g. in the asthenosphere (Japsen et al., 2012a). Similarly, a phase of uplift and erosion at the Eocene-Oligocene transition (c. 35 Ma), which affected margins around the North Atlantic, correlates with a major plate reorganization there (Japsen et al., 2012b). Passive continental margins clearly formed as a result of extension. Despite this, the World Stress Map shows that, where data exist, all EPCMs are today under compression. We maintain that folds, reverse faults, reactivated normal faults and strike-slip faults that are typical of EPCMs are a result of post-rift compression that leads to the formation of EPCMs and that the necessary forces build up during changes in plate motion (e.g. Leroy et al., 2004; Cobbold et al., 2010; Japsen et al., 2012a,b).

Japsen, P.; Cobbold, P. R.; Chalmers, J. A.; Green, P. F.; Bonow, J. M.

2012-04-01

179

Early Cenozoic tectonic quiescence at the southern Levant continental margin, eastern Mediterranean  

NASA Astrophysics Data System (ADS)

The geological record of the easternmost Mediterranean shows almost continuous tectonic activity across the Levant continental margin since its formation during the early Mesozoic until the Recent. The margin developed as part of the Gondwana super continent breakup. Since mid Cenozoic onwards Gondwana descendants, the African and Arabian plates, progressively collided with Eurasia. This collision along with the initiation activity of the Afar plume led to the Cenozoic reactivation of the Levant margin. We explore the geodynamic development of the Levant region between its formation and reactivation through one of the poorly understood time periods in its history. Geological evidence shows that tectonomagmatic processes associated with the formation of the Levant margin and later geodynamic events lasted until the Maastrichtian. During the following ~25 Myr exclusively, thick pelagic and deep sea sediments accumulated on the submerged northwestern Arabian plate. We interpret this early-to-middle Paleogene time window as a unique interval of tectonic and magmatic quiescence. During this mature post continental, breakup stage, thermal equilibrium and isostatic compensation were achieved. A three-dimensional layered lithosphere model was constructed to describe this Middle-Late Eocene geodynamic scene of the Levant area before its reactivation. Layers of the model include the Moho, top of the basement interfaces and the top Avedat Group (Upper Eocene) interfaces. The model was established after a 100 km horizontal restoration along the Dead Sea transform and vertical correction by isostatic compensation to achieve the paleo structure which prevailed in the study area at the end of the Eocene. The reconstructed elevation defines a ramp-shaped structure compatible with independent geological evidence. Results show that most parts of the central Levant margin were submerged ~200 m to ~1800 m, while the paleo bathymetric slopes ranged from ~2° (shelf) to ~6° (slope). The early Cenozoic (early-to-middle Paleogene) tectonic quiescence, which resulted in increased subsidence, also induced the maximal transgression of the Mesotethys Ocean over the Sinai and northwestern Arabian plates.

Schattner, Uri; Segev, Amit; Lyakhovsky, Vladimir

2010-05-01

180

The three scales of submarine groundwater flow and discharge across passive continental margins  

USGS Publications Warehouse

Increased study of submarine groundwater systems in recent years has provided a wealth of new data and techniques, but some ambiguity has been introduced by insufficient distinguishing of the relevant spatial scales of the phenomena studied. Submarine groundwater flow and discharge on passive continental margins can be most productively studied and discussed by distinct consideration of the following three spatial scales: (1) the nearshore scale, spanning approximately 0-10 m offshore and including the unconfined surficial aquifer; (2) the embayment scale, spanning approximately 10 m to as much as 10 km offshore and including the first confined submarine aquifer and its terminus; and (3) the shelf scale, spanning the width and thickness of the aquifers of the entire continental shelf, from the base of the first confined aquifer downward to the basement, and including influences of geothermal convection and glacioeustatic change in sea level. ?? 2010 by The University of Chicago. All rights reserved.

Bratton, J. F.

2010-01-01

181

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

NASA Astrophysics Data System (ADS)

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

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

2009-04-01

182

Sediment Characteristics of Submarine Landslides On the Upper East Australian Continental Margin - Preliminary Findings  

NASA Astrophysics Data System (ADS)

A large number of recent submarine landslides on the Eastern Australian continental slope were investigated during voyages aboard the RV Southern Surveyor in 2006 and 2008. Preliminary sedimentological analysis, geotechnical and radiocarbon data resulting from the examination of twelve gravity cores recovered from upper-slope slides showed that at least three of the twelve cores penetrated large, geologically-recent, submarine landslide failures. The failure surfaces lay within slide scars at distances of between 60 cm and 200 cm beneath the present-day seabed. Sediment present on the upper slope comprises mixtures of calcareous and terrigenous sand and mud. Distinct differences in physical properties (bulk unit density, water content, grain-size distribution) were recorded across the slide-plane boundaries. Slope stability modelling using classical soil mechanics techniques and measured sediment shear-strengths indicates that the slopes should be stable. However, the ubiquity of slides on this margin indicates that their occurrence is a relatively common event and that submarine-sliding should be considered to be a normal characteristic of this continental margin. While this presents something of an interpretational paradox, it nevertheless indicates that an unidentified mechanism acts to reduce the shear resistance of these sediments to very low values which enables the slope failures to occur. Preliminary bulk dates confirm Boyd et al's (2009) conclusion based on sedimentation rates, that some of the landslide masses were mobilised during the most recent glacial-interglacial cycle. Penecontemporaneous dates recorded for separate but adjacent slides are consistent with the slides being triggered by a single event such as an earthquake. Boyd, R., J. Keene, Hubble T.C.T et al. (2009). Southeast Australia: A Cenozoic Continental Margin Dominated by Mass Transport. Submarine Mass Movements and Their Consequences, Advances in Natural and Technological Hazards Research. D. C. Mosher, et al. 28: 491-502.

Clarke, S. L.; Boyd, R.; Hubble, T.; Airey, D.; Keene, J.; Exon, N.; Gardner, J. V.; Shipboard Party Ss12/2008

2010-12-01

183

Tertiary evolution and petroleum potential of Oregon-Washington continental margin  

SciTech Connect

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

Snavely, P.D. Jr.

1986-07-01

184

Structure and development of the southeast Newfoundland continental passive margin: derived from SCREECH Transect 3  

NASA Astrophysics Data System (ADS)

New seismic reflection data from the Grand Banks of Newfoundland and the Newfoundland Basin add to the growing knowledge of the composition, structure and history of this non-volcanic margin. Geophysical imaging is now approaching the extent of that done previously on the conjugate margin along Iberia, providing a valuable database for the development of rifting models. Two parallel profiles over the shelf platform image deep crustal fabric representing Precambrian or possibly Appalachian deformation as well as Mesozoic extension. Progressively more intense extension of continental crust is imaged oceanwards without the highly reflective detachments frequently seen on profiles off Galicia. A landward-dipping event `L' is imaged sporadically and appears to be analogous to a similar event on the Iberian IAM9 profile. The transition zone is probably exposed serpentinized mantle as interpreted off the Iberian margin although there appears to be a difference in the character of ridge development and reflectivity. The distinctive `U' reflection identified previously at the base of the Newfoundland Basin deep water sedimentary section and recently identified as one or more thin basalt sills is imaged on newly presented profiles that connect previously published profiles SCR3 and SCR2 showing that `U' is highly regular and continuous except where interrupted by basement highs. `U' is also seen to have a major impact on the ability to image underlying basement. A full transect beginning over completely unextended continental crust through to oceanic crust has provided a data set from which estimates of extension and the pre-rifting location of the present continental edge can be made. Two estimates were obtained; 85 km based on faulting and 120 km based on crustal thickness.

Deemer, Sharon; Hall, Jeremy; Solvason, Krista; Lau, K. W. Helen; Louden, Keith; Srivastava, Shiri; Sibuet, Jean-Claude

2009-08-01

185

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

SciTech Connect

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

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

1990-05-01

186

Origins and fates of DOM along the New England continental margin  

SciTech Connect

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

Fry, B.; Hopkinson, C. (Marine Biological Lab., Woods Hole, MA (United States). Ecosystems Center); Altabet, M. (Woods Hole Oceanographic Institution, MA (United States))

1993-01-01

187

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

SciTech Connect

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

Fry, B.; Hopkinson, C. [Marine Biological Lab., Woods Hole, MA (United States). Ecosystems Center; Altabet, M. [Woods Hole Oceanographic Institution, MA (United States)

1993-06-01

188

Submarine mass wasting features at the southern central Chilean continental margin - a new database  

NASA Astrophysics Data System (ADS)

Based on an extensive set of swath bathymetry data that was obtained on 12 cruises and cover about 72% of the Chilean continental margin between 33°S and 43°S, up to now more than 60 submarine mass wasting features were detected, mapped and described. They form a wide spectrum in size, apparent slide mechanism and volume. Most of them are small if compared to mass wasting features on passive continental margins. We analyze the database with the aim to determine the main tectonic preconditioning factors for mass wasting along this densely populated partof the Chilean coastline. Major factorsare (1) slope undercutting by the incision of erosive submarine canyons, (2) slope oversteepening by active folding and faulting,and (3) accretionary wedge collapse due to localized sediment underplating and/or frontal accretion. A major stratigraphic control on mass wasting may be in the form of offshore deposition of thick volcanic ash fallout of the Southern Volcanic Zone of Chile. Also fluid seepage from overpressured faults tapping dewatering subducted sediments at depth may contribute to destabilization of sediments on the continental slope. Obvious triggers are the large thrust earthquakes that shake the region with a historical recurrence time of ~ 150 y. About half of the features are directly related to the active submarine canyons on the forearc. The collapse of canyon walls partly impinges on the open slope where thin translational slides detach from discrete horizons . Apart from the slides directly related to canyons, we identified medium-sized (5-25 km3) blocky slides that affect the lowermost continental slope, and small spoon-shaped headscarps of slides, which deliver their material into slope basins. A particular feature is given by Valdes Slide which developed on an upper slope thrust ramp , with mass wasting directed landward. There is no straight-forward relationship between simple geomorphic parameters such als slope gradient or curvature, and the spatial distribution of slump features, as was previously observed e.g. offshore the Pacific coast of Nicaragua. The database will serve as comparison to other regional datasets of convergent margins such as for Central America, but also to the COSTA database of mass wasting features on the North Atlantic passive margin.

Voelker, David; Geersen, Jacob; Weinrebe, Willi R.; Behrmann, Jan H.

2010-05-01

189

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

190

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

191

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 Vp/Vs 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.

Oncken, O.; Asch, G.; Haberland, C.; Metchie, J.; Sobolev, S.; Stiller, M.; Yuan, X.; Brasse, H.; Buske, S.; Giese, P.; GöRze, H.-J.; Lueth, S.; Scheuber, E.; Shapiro, S.; Wigger, P.; Yoon, M.-K.; Bravo, P.; Vieytes, H.; Chong, G.; Gonzales, G.; Wilke, H.-G.; Lüschen, E.; Martinez, E.; RöSsling, R.; Ricaldi, E.; Rietbrock, A.

2003-07-01

192

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

193

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

PubMed

We studied the relationship between viral particle and microbial cell abundances in marine subsurface sediments from three geographically distinct locations in the continental margins (offshore of the Shimokita Peninsula of Japan, the Cascadia Margin off Oregon, and the Gulf of Mexico) and found depth variations in viral abundances among these sites. Viruses in sediments obtained offshore of the Shimokita and in the Cascadia Margin generally decreased with increasing depth, whereas those in sediments from the Gulf of Mexico were relatively constant throughout the investigated depths. In addition, the abundance ratios of viruses to microbial cells notably varied among the sites, ranging between 10(-3) and 10(1) . The subseafloor viral abundance offshore of the Shimokita showed a positive relationship with the microbial cell abundance and the sediment porosity. In contrast, no statistically significant relationship was observed in the Cascadia Margin and the Gulf of Mexico sites, presumably due to the long-term preservation of viruses from enzymatic degradation within the low-porosity sediments. Our observations indicate that viral abundance in the marine subsurface sedimentary environment is regulated not only by in situ production but also by the balance of preservation and decay, which is associated with the regional sedimentation processes in the geological settings. PMID:24308555

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

2014-04-01

194

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

Microsoft Academic Search

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

P. de Deckker; William M. Last

1988-01-01

195

Normal Faulting at the Western Margin of the Altiplano Plateau, Southern Peru  

Microsoft Academic Search

Although the western margin of the Altiplano Plateau is commonly used to illustrate the marked differences in the evolution of a mountain range with strong latitudinal and longitudinal precipitation gradients, the nature of tectonism in this semi-arid region is poorly understood and much debated. The western margin of the Altiplano in southern Peru and northern Chile marks an abrupt transition

T. F. Schildgen; K. V. Hodges; K. X. Whipple; M. Perignon; T. M. Smith

2004-01-01

196

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

PubMed

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

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

2013-01-01

197

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

SciTech Connect

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

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

1988-03-01

198

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

199

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

SciTech Connect

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

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

1987-09-01

200

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

201

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

NASA Astrophysics Data System (ADS)

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

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

2003-04-01

202

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

USGS Publications Warehouse

At ocean margins where two plates converge, the oceanic plate sinks or is subducted beneath an upper one topped by a layer of terrestrial crust. This crust is constructed of continental or island arc material. The subduction process either builds juvenile masses of terrestrial crust through arc volcanism or new areas of crust through the piling up of accretionary masses (prisms) of sedimentary deposits and fragments of thicker crustal bodies scraped off the subducting lower plate. At convergent margins, terrestrial material can also bypass the accretionary prism as a result of sediment subduction, and terrestrial matter can be removed from the upper plate by processes of subduction erosion. Sediment subduction occurs where sediment remains attached to the subducting oceanic plate and underthrusts the seaward position of the upper plate's resistive buttress (backstop) of consolidated sediment and rock. Sediment subduction occurs at two types of convergent margins: type 1 margins where accretionary prisms form and type 2 margins where little net accretion takes place. At type 2 margins (???19,000 km in global length), effectively all incoming sediment is subducted beneath the massif of basement or framework rocks forming the landward trench slope. At accreting or type 1 margins, sediment subduction begins at the seaward position of an active buttress of consolidated accretionary material that accumulated in front of a starting or core buttress of framework rocks. Where small-to-mediumsized prisms have formed (???16,300 km), approximately 20% of the incoming sediment is skimmed off a detachment surface or decollement and frontally accreted to the active buttress. The remaining 80% subducts beneath the buttress and may either underplate older parts of the frontal body or bypass the prism entirely and underthrust the leading edge of the margin's rock framework. At margins bordered by large prisms (???8,200 km), roughly 70% of the incoming trench floor section is subducted beneath the frontal accretionary body and its active buttress. In rounded figures the contemporary rate of solid-volume sediment subduction at convergent ocean margins (???43,500 km) is calculated to be 1.5 km3/yr. Correcting type 1 margins for high rates of terrigenous seafloor sedimentation during the past 30 m.y. or so sets the long-term rate of sediment subduction at 1.0 km3/yr. The bulk of the subducted material is derived directly or indirectly from continental denudation. Interstitial water currently expulsed from accreted and deeply subducted sediment and recycled to the ocean basins is estimated at 0.9 km3/yr. The thinning and truncation caused by subduction erosion of the margin's framework rock and overlying sedimentary deposits have been demonstrated at many convergent margins but only off northern Japan, central Peru, and northern Chile has sufficient information been collected to determine average or long-term rates, which range from 25 to 50 km3/m.y. per kilometer of margin. A conservative long-term rate applicable to many sectors of convergent margins is 30 km3/km/m.y. If applied to the length of type 2 margins, subduction erosion removes and transports approximately 0.6 km3/yr of upper plate material to greater depths. At various places, subduction erosion also affects sectors of type 1 margins bordered by small- to medium-sized accretionary prisms (for example, Japan and Peru), thus increasing the global rate by possibly 0.5 km3/yr to a total of 1.1 km3/yr. Little information is available to assess subduction erosion at margins bordered by large accretionary prisms. Mass balance calculations allow assessments to be made of the amount of subducted sediment that bypasses the prism and underthrusts the margin's rock framework. This subcrustally subducted sediment is estimated at 0.7 km3/yr. Combined with the range of terrestrial matter removed from the margin's rock framework by subduction erosion, the global volume of subcrustally subducted materia

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

1991-01-01

203

Variations in sediment transport at the central Argentine continental margin during the Cenozoic  

NASA Astrophysics Data System (ADS)

The construction of the sedimentary cover at most passive continental margins includes gravitational downslope transport and along-slope contourite deposition, which are controlled by tectonics, climate and oceanography. At the eastern continental margin of Argentina the history of deposition and erosion is intimately linked to the evolution of the South Atlantic and its water masses. Here we present a detailed seismic investigation of the mixed depositional system located between 41°S and 45°S. The study provides a northward complement to prior investigations from the southern Argentine margin and together with these may be used as background information for future ocean drilling in the region. Prominent features in our seismic cross sections are submarine canyons, mass wasting deposits, contourite channels, and sediment drifts. Four major seismic units above regional reflector PLe (˜65 Ma) are separated by distinct unconformities of regional extent. Using a dense grid of reflection seismic profiles, we mapped the depocenter geometries of the seismic units and derived a chronology of the depositional processes during the Cenozoic. While the Paleocene/Eocene (˜65-34 Ma) is characterized by hemipelagic sedimentation under relatively sluggish bottom water conditions, strong Antarctic bottom water (AABW) circulation led to widespread erosion on the slope and growth of a detached sediment drift during the Oligocene and early Miocene (˜34-17 Ma). After deposition of an aggradational seismic unit interpreted to represent the Mid-Miocene climatic optimum (˜17-14 Ma), gravitational downslope sediment transport increased during the middle to late Miocene (˜14-6 Ma) possibly related to tectonic uplift in South America. The Pliocene to Holocene unit (<˜6 Ma) is very heterogeneous and formed by interactions of downslope and along-slope sediment transport processes as indicated by the evolution of canyons, slope plastered drifts and channels.

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

2012-10-01

204

Recent seismic investigations on gas hydrates at continental margins by BGR  

NASA Astrophysics Data System (ADS)

In the last years all marine seismic cruises of BGR on continental margins revealed deposits of gas hydrates. The standard analysis of these data begins with the mapping of BSRs in the processed reflection seismic data to estimate the minimal extension of gas hydrates. This is followed by derivation of heat flow from BSR depths at selected locations. The work of BGR with these data has a variety of objectives: reservoir investigations, structural studies, comparative studies to understand the origin of the gas and to assess the role of gas hydrates and free gas beneath as a possible future energy resource. Data from four areas are presented. The Sunda subduction zone formed the Mentawai and the Java forearc basins. Gas hydrates are observed predominantly in boundary parts of the basins and in the anticlinal structures which run nearly parallel to the subduction zone. Gas hydrate occurrence off Sabah appears to be linked to structural and tectonic units and to be focused mainly in the folded, thrusted, and uplifted structures. The BSRs occur mainly in the hanging walls of the individual thrust sheets which form anticline-like structures. Due to the tectonically controlled morphology of the seafloor the distribution of BSRs appear mainly as elongated bodies which run parallel to each other. At the active margin of middle Chile gas hydrate has only been observed in the southern part. They occur mainly on the middle slope and form lengthy patches parallel to the coast. The convergent continental margin of Costa Rica is an area with large known gas hydrate occurrences. The mapping of BSRs from these data reveals different areas of gas hydrates and indications for strong variability of the heat flow. One area is subject of an ongoing detailed seismic reservoir study, where long-offset seismic data open the way for pre-stack analyses with methods such as amplitude variation with angle.

Boennemann, C.; Mueller, C.; Behain, D.; Meyer, H.; Neben, S.

2003-04-01

205

Sedimentary processes on the Storfjorden trough-mouth fan during last deglaciation phase: the role of subglacial meltwater plumes on continental margin sedimentation  

Microsoft Academic Search

The continental margin of the Southern Storfjorden trough-mouth fan was investigated within the SVAIS project (BIO Hesperides cruise, August 2007) as a Spanish contribution to IPY Activity N. 367 (Neogene ice streams and sedimentary processes on high- latitude continental margins - NICE STREAMS). The objectives were to investigate the glacially-dominated late-Neogene-Quaternary sedimentary architecture of the NW Barents Sea continental margin

Renata G. Lucchi; Angelo Camerlenghi; Elena Colmenero-Hidalgo; Francisco J. Sierro; José-Abel Flores; Roger Urgeles; Patrizia Macri; Leonardo Sagnotti; Andrea Caburlotto

2010-01-01

206

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

NASA Astrophysics Data System (ADS)

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

Carrivick, Jonathan L.; Quincey, Duncan J.

2014-05-01

207

Grounding-zone wedges (GZWs) on high-latitude continental margins  

NASA Astrophysics Data System (ADS)

The grounding-zone of marine-terminating ice sheets is the area at which the ice-sheet base ceases to be in contact with the underlying substrate. The grounding-zone is a key site at which ice, meltwater and sediment are transferred from ice sheets to the marine environment. GZWs are asymmetric sedimentary depocentres which form through the rapid accumulation of glacigenic debris along a line source at the grounding-zone largely through the delivery of deforming subglacial sediments, together with sediment remobilisation from gravity flows. The presence of GZWs in the geomorphological record indicates an episodic style of ice retreat punctuated by still-stands in the grounding-zone position. GZWs may take decades to centuries to form. Moraine ridges and ice-proximal fans may also build up at the grounding-zone during still-stands or re-advances of the ice margin, but these require either considerable vertical accommodation space or are derived from point-sourced subglacial meltwater streams. We present an inventory of GZWs which is compiled from available studies of bathymetric, shallow acoustic and reflection seismic data from high-latitude continental margins. The objectives are to present locations of and morphological data on GZWs from the Arctic and Antarctic, alongside a synthesis of their key architectural and geomorphic characteristics. We use, for example, newly-available two-dimensional seismic reflection data to show the approximate locations of GZWs off northwest and northeast Greenland. Controls on GZW formation are considered in relation to shelf topography and ice-sheet internal dynamics. A total of 129 GZWs are described from high-latitude continental shelves. GZWs are only observed within cross-shelf troughs and major fjord systems, which are the former locations of ice streams and fast-flowing outlet glaciers. Typical high-latitude GZWs are less than 15 km long and 15 to 100 m thick. A positive correlation between GZW length and thickness is inferred for GZWs on the Greenland, Norwegian, Canadian and Barents Sea margins. However, no significant relationship between GZW length and thickness exists for the GZWs described from the Antarctic margin. GZWs typically possess a semi-transparent to chaotic acoustic character, which reflects the delivery of diamictic subglacial debris. Many GZWs contain low-amplitude, seaward-dipping internal reflections, which indicate sediment progradation and wedge-growth through continued delivery of basal sediments from the flow of active ice. The formation of GZWs is inferred to require high rates of sediment delivery to a relatively stable, fast-flowing ice margin. Ice-margin stabilisation, and consequently GZW formation, is dependent on a number of factors, including the ice-sheet mass balance, sea-level fluctuations, and the rate of inland-ice delivery to the grounding-zone. GZWs may be formed preferentially by glaciers with termini ending as floating ice shelves, which restrict vertical accommodation space and prevent the build-up of high-amplitude moraine ridges. The basal topography of the continental shelf can also act as a control on GZW formation. The majority of high-latitude GZWs are located at topographic or lateral pinning points within cross-shelf troughs, which encourage ice-margin stabilisation through reducing iceberg calving and increasing basal and lateral drag.

Batchelor, Christine; Dowdeswell, Julian

2014-05-01

208

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

NASA Astrophysics Data System (ADS)

Pyrophaeophorbide- a, a degradation product of chlorophyll- a, is predominantly formed by grazing processes in sediments as well as in the water column. Water column profiles of pyrophaeophorbide- a/suspended particulate organic carbon (SPOC) concentrations, at an abyssal site in the northeast (NE) Pacific (Sta M, 34°50'N, 123°00'W; 4100 m water depth), show low concentrations (0.01-0.1 ng/?g SPOC) at surface and mesopelagic depths, and increasing concentrations with closer proximity to the sea floor (0.05-0.6 ng/?g SPOC). However, in June 1992, the deep maximum of pyrophaeophorbide- a/SPOC in the water column of Sta M extended higher into the water column, as much as 1600 m above the bottom (mab) (2500 m water depth); in other seasons they only extended up to 650 mab (3450 m water depth). Previous studies have demonstrated lateral transport of particulate matter from the continental shelf to the deep ocean off the coast of northern California. Recent work suggests that the benthic boundary layer (BBL) extends to 50 mab, based on sediment trap and transmissometry measurements (Smith, K.L., Kaukmann, R.S., Baldwin, R.J., 1994. Coupling of near-bottom pelagic and benthic processes at abyssal depths. Limnology and Oceanography 39, 1101-1118.), and that lateral transport is significant only during summer, which is consistent with our observations. A partial vertical profile of pyrophaeophorbide- a/SPOC from the north central (NC) Pacific provides some evidence that the deep maximum may be absent due to the distance of this site from the continental margin. Thus, the observed deep maximum of pyrophaeophorbide- a/SPOC at Sta M is likely due mainly to lateral transport from the continental slope rather than to local vertical resuspension in the BBL exclusively. Pyrophaeophorbide- a concentrations in SPOC at Sta M were negatively correlated with ? 14C values of SPOC (SPOC samples from Druffel, E.R.M., Bauer, J.E., Williams, P.M., Griffin, S.A. and Wolgast, D., 1996, Seasonal variability of particulate organic radiocarbon in the northeast Pacific Ocean. Journal of Geophysical Research 101, 20543-20552), further supporting our contention that pyrophaeophorbide- a peaks in the deep water column are derived from "older" resuspended sediments that were laterally transported from continental margin sediments. Our molecular biomarker (pyrophaeophorbide- a) data are reflective of a specific fraction of SPOC that likely remains in the water column for long periods of time or is derived from resuspended sediments.

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

209

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

210

Link between Mid-Ocean Ridge kinematics and uplift of passive continental margins  

NASA Astrophysics Data System (ADS)

Tectonic models predict post-rift subsidence of rift margins after initial flexural rebound and transgression of a sedimentary wedge over the subsiding mar¬gin as the lithosphere cools with time. However, studies of North Atlantic rifted margins show that thermal subsidence following breakup at the Paleocene-Eocene transition was interrupted by signi?cant uplift movements. These vertical movements represent a long-standing enigma and they have been linked to sea-level ?uctuations, climate deterioration and tectonics but as yet they remain unexplained. Here we combine regional Multi-Channel Seismic re?ection data across the NE Greenland Shelf, the Greenland Fracture Zone (GFZ) and continental East Greenland Ridge (EGR) in the northern NE Atlantic and stratigraphic data from a drill core. We show that a mid-Miocene change from down-faulting to uplift along the GFZ-EGR correlates with signi?cant uplift of the NE Greenland margin. This tectonic change is associated with a regional unconformity that marks the first occurrences of mass-¬wasted deposits in the deep sea off the NE Greenland Shelf and the development of prograd¬ing mega-sequences and angular truncation of hemipelagic sediments below the unconformity, respectively, on the outer and inner NE Greenland Shelf. We attribute the tectonic changes at the GFZ to the development of a modern, continuous spreading system along the Mohns-Knipovich Ridge segments that led to an opening of the Fram Strait corridor, to large-scale changes in ocean circulation and climate and possibly to medium-scale (20-30 m) sea-level ?uctuations. While these consequences of the tectonic changes may have affected the amplitude of uplift in NE Greenland, they cannot explain the uplift at the GFZ-EGR in deep sea. We therefore find that plate-tectonic changes produced the driving force for the mid-Miocene uplift in NE Greenland.

Døssing, A.; Japsen, P.; Nielsen, T.; Thybo, H.; Dahl-Jensen, T.

2012-04-01

211

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

NASA Astrophysics Data System (ADS)

The continental slope and the upper rise centered on Hudson Canyon offshore New York and New Jersey lie within a major gas-hydrate province. This region exhibits evidence of gravitational mass movements and possible methane expulsion, as inferred from our bathymetric and water-column surveys conducted in 2002 with support from NOAA/OE, and prior data. The bathymetric data cover our study area (200 km by 110 km; 37\\deg40'N to 39\\deg50'N, 70\\deg00'W to 72\\deg30'W) from the inner edge of the continental slope (depth 200 m) seaward to the middle rise (c.3500 m). The world's largest hub of submarine telecommunications cables partially passes through this area. Evidence of gravitational mass movements and of probable gas release is extensive. Examples of the former include: (1) blocks of landward-dipping strata up to 2-km wide and 150-m high that lie at the base of the continental slope (water depth 2100-2200 m) seaward of an over-pressured zone beneath the continental slope (639 mbsf in ODP Hole 1073A; water depth 650 m; Dugan and Flemings, 2000); (2) boulders of Eocene chalk that litter the lower slope and upper rise; (3) a semicircular, tabular glide block, about 20 km in diameter, which thickens to about 150 m at its seaward margin; the block is centered at 39\\deg23.5'N, 71\\deg10.0'W between 2450 and 2600 m depth on the upper rise, about 15 km downslope from a congruent scarp at 2200 m on the lower slope; (4) apparent penecontemporaneous faulting and gliding in strata inclined sub-parallel to the seafloor along the upper rise; 5) apparent clogging of Hudson Canyon with hummocky sediment at a right-angle turn of the axis (depth 3368 m; 38\\deg39.6'N, 71\\deg01.8'W); 6) changes in stratification from the upper to middle rise; uneven layering beneath the upper rise (seafloor mean inclination 0.75\\deg down to 2700 m) is inferred to reflect disturbance by gravitational mass movements; even layering parallel to the seafloor beneath the middle rise (inclination increase seaward from 0.25\\deg to 0.76\\deg) may reflect less disrupted hemipelagic sedimentation. Evidence of gas release includes: (1) a zone of irregular pits, each up to 500-m in diameter and spaced kilometers apart, that extends along the upper rise (2600-2650 m); (2) a line of eight depressions each 50-70 m in diameter with several meters relief and spaced 100-120 m apart that trends NNW (2150 m; near 39\\degN, 71\\deg52'W)and that may indicate gas or porewater escape along a local fault; 3) a plume of sediment suspended in fluid that is discharging through the seafloor and rising about 1 m at 2625 m depth on the SW margin of Hudson Canyon (38\\deg52.4'N, 71\\deg31.0'W) recorded on deep-towed video; and 4) three zones of regional, water-column, methane anomalies, which exceed background values, deeper than 2500-m depth, centered at 1200-m depth, and near 200-m depth. These features indicate past and present dissociation of gas hydrates and/or venting of free gas. They also indicate that coherent gravitational mass movements over low seafloor slopes (< 1\\deg) have been facilitated by excess fluid (water and/or gas) pressures. The continental slope and rise in the Hudson Canyon region comprise a natural laboratory in which to study slope instability in a gas hydrate province and to assess the hazards to telecommunications cables. Dugan, B. and P.B. Flemings (2000), Overpressure and fluid flow in the New Jersey continental slope: implications for slope failure and cold seeps, Science, 289, 288-291.

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

2004-12-01

212

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

PubMed Central

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

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

2011-01-01

213

Geological and Sediment Thickness Data Sources From the U.S. Continental Margins  

NASA Astrophysics Data System (ADS)

Although the United States has not yet ratified the United Nations Convention on the Law of the Sea (UNCLOS), work has begun to assess the geophysical and geological data sources that might be applied to an extended continental shelf submission under Article 76 of the UNCLOS. The U.S. Geological Survey, as a follow-up to the Congressional Report published by the University of New Hampshire on data relevant to a potential U.S. submission (Mayer and others, 2002), has identified existing seismic reflection, seismic refraction, and drill-hole data on the U.S. margins for the areas where an extended continental shelf submission could be considered. This work complements ongoing NOAA efforts to map the foot-of-the-slope. The USGS compilation includes more than 80,000 km of multichannel seismic data, 70,000 km of single-channel seismic reflection data, 25 refraction experiments, and 12 drill holes that penetrate to basement. Data quality varies according to year collected and acquisition system used. Data coverage is generally excellent within the 200-nm EEZ boundary, but new data will be required to adequately assess sediment thickness in the area beyond 200-nm in some of the poorly surveyed regions (e.g., the Arctic). Velocity and drill-hole control for deeper sedimentary units is generally poor; this deficiency will also need to be addressed in new data gathering efforts. Subsea mineral resources that might exist in the region of an extended continental shelf vary by region and include conventional hydrocarbons, gas hydrate, ferro-manganese crusts and nodules, and possibly phosphorite deposits. On-going efforts are directed at interpreting these data with reference to UNCLOS criteria and guidelines, as well as evaluating how recent submissions to the United Nations by other States might affect a possible U.S. submission.

Hutchinson, D. R.; Childs, J. R.; Edgar, N. T.; Barth, G.; Hammar-Klose, E.; Dadisman, S. V.; Rowland, R.

2005-12-01

214

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

215

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

USGS Publications Warehouse

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

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

1989-01-01

216

New discoveries of mud volcanoes on the Moroccan Atlantic continental margin (Gulf of Cádiz): morpho-structural characterization  

NASA Astrophysics Data System (ADS)

During the MVSEIS-08 cruise of 2008, ten new mud volcanoes (MVs) were discovered on the offshore Moroccan continental margin (Gulf of Cádiz) at water depths between 750 and 1,600 m, using multibeam bathymetry, backscatter imagery, high-resolution seismic and gravity core data. Mud breccias were recovered in all cases, attesting to the nature of extrusion of these cones. The mud volcanoes are located in two fields: the MVSEIS, Moundforce, Pixie, Las Negras, Madrid, Guadix, Almanzor and El Cid MVs in the western Moroccan field, where mud volcanoes have long been suspected but to date not identified, and the Boabdil and Al Gacel MVs in the middle Moroccan field. Three main morphologies were observed: asymmetric, sub-circular and flat-topped cone-shaped types, this being the first report of asymmetric morphologies in the Gulf of Cádiz. Based on morpho-structural analysis, the features are interpreted to result from (1) repeated constructive (expulsion of fluid mud mixtures) and destructive (gravity-induced collapse and submarine landsliding) episodes and (2) interaction with bottom currents.

León, Ricardo; Somoza, Luis; Medialdea, Teresa; Vázquez, Juan Tomás; González, Francisco Javier; López-González, Nieves; Casas, David; del Pilar Mata, María; del Fernández-Puga, María Carmen; Giménez-Moreno, Carmen Julia; Díaz-del-Río, Víctor

2012-12-01

217

Zooplankton Distribution and Dynamics in a North Pacific Eddy of Coastal Origin: I. Transport and Loss of Continental Margin Species  

Microsoft Academic Search

Zooplankton from coastal\\/continental margin environments can be transported long distances seaward into the subarctic North\\u000a Pacific by the large (100–200 km diameter) anticyclonic eddies that form annually in late winter along the eastern margin\\u000a of the Alaska Gyre. One recurrent region for eddy formation is off the southern tip of the Queen Charlotte Islands (near 52°N\\u000a 132°W). Eddies from this

David L. Mackas; Moira D. Galbraith

2002-01-01

218

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

219

Gas hydrates and active mud volcanism on the South Shetland continental margin, Antarctic Peninsula  

NASA Astrophysics Data System (ADS)

During the Antarctic summer of 2003 2004, new geophysical data were acquired from aboard the R/V OGS Explora in the BSR-rich area discovered in 1996 1997 along the South Shetland continental margin off the Antarctic Peninsula. The objective of the research program, supported by the Italian National Antarctic Program (PNRA), was to verify the existence of a potential gas hydrate reservoir and to reconstruct the tectonic setting of the margin, which probably controls the extent and character of the diffused and discontinuous bottom simulating reflections. The new dataset, i.e. multibeam bathymetry, seismic profiles (airgun and chirp), and two gravity cores analysed by computer-aided tomography as well as for gas composition and content, clearly shows active mud volcanism sustained by hydrocarbon venting in the region: several vents, located mainly close to mud volcanoes, were imaged during the cruise and their occurrence identified in the sediment samples. Mud volcanoes, vents and recent slides border the gas hydrate reservoir discovered in 1996 1997. The cores are composed of stiff silty mud. In core GC01, collected in the proximity of a mud volcano ridge, the following gases were identified (maximum contents in brackets): methane (46 ?g/kg), pentane (45), ethane (35), propane (34), hexane (29) and butane (28). In core GC02, collected on the flank of the Vualt mud volcano, the corresponding data are methane (0 ?g/kg), pentane (45), ethane (22), propane (0), hexane (27) and butane (25).

Tinivella, U.; Accaino, F.; Della Vedova, B.

2008-04-01

220

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

221

Tectonic and eustatic controls on sedimentary successions at passive continental margins  

SciTech Connect

The configuration and internal characteristics of sedimentary successions at passive continental margins are controlled largely by patterns of tectonically driven subsidence and deformation by eustasy, by the character and flux of available terrigenous sediment, and by the rate of in-situ production of carbonate and other chemical sediments, Rates of subsidence and sediment accumulation are also influenced by the effects of sediment loading and deformation and by the manner in which sediments become lithified through compaction and diagenesis. The purpose of this presentation is to explore, with reference to passive margins of Mesozoic and Cenozoic age, some of the complex interconnections and feedbacks between these controls, their characteristic time scales and rates, and the leads and lags that may be involved. In particular, the authors draw attention to the controversy concerning the roles of tectonics and eustasy in the development of unconformities. A practical application of this ongoing research is the promise of new concepts and tools for the exploration and production of oil and gas. An inherent difficulty is that none of the important variables can be isolated completely, and simplifying assumptions, although necessary for analysis, unfortunately tend to take on the aura of established fact, Models have already outstripped the data available to test them, and at best they can place bounds only on what is possible. The apparent success of a stratigraphic simulation is no guarantee that all (or even any) of the important variables are included within the starting assumptions.

Christie-Blick, N. (Lamont-Doherty Geological Observatory, Palisades, NY (USA))

1990-05-01

222

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

223

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

NASA Astrophysics Data System (ADS)

The Chilean Andes extend north-south for about 3000 km over the subducting Nazca plate, and show evidence of local rheological controls on first-order tectonic features. Here, rheological parameters are tested with numerical models of a subduction driven by slab-pull and upper plate velocities, and which calculate the development of stress and strain over a typical period of 4 Myr. The models test the effects of subduction interface strength, arc and fore-arc crust rheology, and arc temperature, on the development of superficial near-surface faulting as well as viscous shear zones in the mantle. Deformation geometries are controlled by the intersection of the subduction interface with continental rheological heterogeneities. Upper plate shortening and trench advance are both correlated, and favored, to a first-order by upper plate weakness, and to a second-order by interface strength. In cases of a strong interface, a weak fore-arc crust is dragged downward by “tectonic erosion”, a scenario for which indications are found along the northern Chilean margin. In contrast for a resistant fore-arc, the slab-pull force transmits to the surface and produces topographic subsidence. This process may explain present-day subsidence of the Salar de Atacama basin and/or the persistence of a Central Depression. Specific conditions for northern Chile produce a shear zone that propagates from the subduction zone in the mantle, through the Altiplano lower crust into the Sub-Andean crust, as proposed by previous studies. Models with a weak interface in turn, allow buoyant subducted material to rise into the continental arc. In case of cessation of the slab-pull, this buoyant material may rise enough to change the stress state in the continental crust, and lead to back-arc opening. In a case of young and hydrated oceanic plate forced by the slab-pull to subduct under a resistant continent, this plate is deviated and indented by the continental mantle, and stretches horizontally at ˜100 km depth. This situation might explain the flat Wadati-Benioff zone of Central Chile.

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

2009-12-01

224

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

NASA Astrophysics Data System (ADS)

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

Archer, D.

2014-06-01

225

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

226

Recent seismic investigations on gas hydrates at continental margins by BGR  

NASA Astrophysics Data System (ADS)

In the last years all marine seismic cruises of BGR on continental margins revealed deposits of gas hydrates. The standard analysis of these data begins with the mapping of BSRs in the processed reflection seismic data to estimate the minimal extension of gas hydrates. This is followed by derivation of heat flow from BSR depths at selected locations. The work of BGR with these data has a variety of objectives: reservoir investigations, structural studies, comparative studies to understand the origin of the gas and to assess the role of gas hydrates and free gas beneath as a possible future energy resource. Data from four areas are presented. The Sunda subduction zone formed the Mentawai and the Java forearc basins. Gas hydrates are observed predominantly in boundary parts of the basins and in the anticlinal structures which run nearly parallel to the subduction zone. Gas hydrate occurrence off Sabah appears to be linked to structural and tectonic units and to be focused mainly in the folded, thrusted, and uplifted structures. The BSRs occur mainly in the hanging walls of the individual thrust sheets which form anticline-like structures. Due to the tectonically controlled morphology of the seafloor the distribution of BSRs appear mainly as elongated bodies which run parallel to each other. At the active margin of middle Chile gas hydrate has only been observed in the southern part. They occur mainly on the middle slope and form lengthy patches parallel to the coast. The convergent continental margin of Costa Rica is an area with large known gas hydrate occurrences. The mapping of BSRs from these data reveals different areas of gas hydrates and indications for strong variability of the heat flow. One area is subject of an ongoing detailed seismic reservoir study. High-resolution and long-offset seismic data open the way for pre-stack analyses with methods such as amplitude variation with angle (AVA). First results indicate the possibility to differentiate between locations characterized by hydrate only and locations with free gas beneath the hydrate stability zone.

Boennemann, C.; Mueller, C.; Behain, D.; Meyer, H.; Neben, S.

2002-12-01

227

Responses of deep-water shrimp populations to intermediate nepheloid layer detachments on the Northwestern Mediterranean continental margin  

Microsoft Academic Search

A clear link between the distribution of intermediate nepheloid layer detachments on the Northwestern Mediterranean continental margin and the population structure of five congeneric megafaunal species of deep-water benthic shrimps inhabiting different depth ranges between 100 and 1100m was found. The results of the multidisciplinary approach presented in this study provide evidence for the ecological conditions that affect the spatial

Pere Puig; Francesc Sardà; Albert Palanques

2001-01-01

228

Seismic studies of a bottom simulating reflection related to gas hydrate beneath the continental margin of the Beaufort Sea  

Microsoft Academic Search

The upper continental margin of the Beaufort Sea, north of Alaska, is underlain by a strong bottom simulating reflection (BSR) that lies 300 to 700 m beneath the seafloor and corresponds to the phase boundary between interstitial water and natural gas below and solid gas hydrate above. BSRs of similar origin are common worldwide, where they are usually interpreted to

K. Andreassen; P. E. Hart; A. Grantz

1995-01-01

229

History of faulting and magmatism in the Galilee (Israel) and across the Levant continental margin inferred from potential field data  

Microsoft Academic Search

The Galilee study area, northern Israel, is at present an uplifted, steep continental margin that formed mainly during the Jurassic and has a large positive isostatic anomaly. Since the Jurassic, it was modified by several tectonomagmatic events, which this study attempts to define and classify by updating, reprocessing and reinterpreting gravity, aeromagnetic and geological data. The prominent Rehovot-Carmel N–S positive

Amit Segev; Michael Rybakov

2011-01-01

230

Influence of rock strength properties on escarpment retreat across passive continental margins  

NASA Astrophysics Data System (ADS)

We examine the influence of bedrock strength properties, imparted by jointing and fracturing, on the retreat of erosional escarpments across passive continental margins. Two types of escarpment are identified, and focus is placed on the gorge head knickpoint form of escarpment, where upland streams drain across the escarpment. Along the Macleay River system of eastern Australia, major knickpoints associated with escarpment retreat are currently located about 200 km upstream from the river mouth, despite differences in bedrock lithology and order-of-magnitude differences in drainage areas upstream of the gorge heads, implying an average escarpment retreat rate of 2 km/m.y. since the margin formed 85 m.y. ago. Field observations from Apsley River gorge, a Macleay River tributary, indicate that bedrock jointing profoundly affects the evolution of the gorge headwall and sidewalls. Where dominant jointing dips into hillslopes, failure occurs by block and column toppling. Conversely, where jointing dips out of hillslopes, mass wasting occurs by joint-parallel rock sliding, allowing slopes to remain steep. As material is shed from the gorge sidewalls, the toppling slopes develop lower overall gradients, the gorge becomes asymmetric in cross section, and the channel remains nearer to the steeper sidewall affected by joint-parallel sliding. Coupling between fluvial and hillslope processes is essential to maintain headward propagation of the knickpoint gorge head and to facilitate mass wasting from the gorge sidewalls. Peak stream discharge across the gorge head must exceed a threshold sufficient to transport debris away from the gorge head to keep the knickpoint rock face free of talus, to incise the stream channel, and to erode the toes of the sidewalls. If these conditions on fluvial processes are met, then hillslope processes, including rockslope failure mechanisms, are the rate controlling processes for knickpoint propagation and therefore determine the rate of escarpment retreat across the passive margin.

Weissel, Jeffrey K.; Seidl, Michele A.

1997-07-01

231

Eastern Atlantic Continental Margin Program of the International Decade of Ocean Exploration (GX-28193), Report for Period March 1971 to July 1972.  

National Technical Information Service (NTIS)

During the first half of 1972 R/V ATLANTIS II made a 50,000 line-km cruise as part of the Eastern Atlantic Continental Margin program of the International Decade of Ocean Exploration. The area of investigation was the continental margin between Port Eliza...

K. O. Emery

1972-01-01

232

Seasonal anoxia Over the Western Indian Continental Shelf  

NASA Astrophysics Data System (ADS)

The eastern Arabian Sea contains the only eastern-boundary-type upwelling environment in the entire Indian Ocean, albeit on a seasonal basis. During the southwest monsoon, when the surface current flows equatorward, upwelling brings oxygen-poor, nutrient-rich subsurface waters to the Indian continental shelf that turn anoxic (sulfate-reducing) by late summer due to exhaustion of oxygen and nitrate by heterotrophic microorganisms. This natural oxygen-deficient system, by far the world's largest in the coastal ocean, is apparently more intense now than it was three decades ago. This is consistent with the sedimentary record, which indicates that productivity over the past few decades has been the highest ever in the last seven centuries. However, a trend of ongoing intensification is not seen in the data collected during the last 10 years at a coastal time series station off Goa. These data, nevertheless, show considerable interannual variability with the most severe anoxia having occurred in 2001, adversely impacting local fisheries (especially demersal fish catch). While increased nutrient loading to the coastal zone largely through atmospheric deposition of nitrogen most likely occurred in the last few decades, contributing to a shift to fully anoxic conditions, the observed interannual variability suggests that subtle changes in local hydrography/circulation could also modulate coastal anoxia in the region.

Naqvi, S. Wajih A.; Naik, Hema; Jayakumar, Amal; Pratihary, Anil K.; Narvenkar, Gayatri; Kurian, Siby; Agnihotri, Rajesh; Shailaja, M. S.; Narvekar, Pradip V.

233

Unravelling the process of continental breakup: a case study of the Australia-Antarctica conjugate margins  

NASA Astrophysics Data System (ADS)

Recent studies of rifted margins, in particular of the North Atlantic, resulted in the development of new concepts and models. However, these studies also revealed a number of new questions concerning: the nature and composition of hyper-extended crust and the processes related to crustal thinning; the origin, volume and timing of magmatic rocks; and also how magmatic and tectonic processes are interrelated during rifting. A probably even more fundamental question that remains to be answered is: How do plate boundaries form in divergent systems and how, when and where does the transition from rifting to oceanic seafloor spreading occur? This statement seems at odds with the fact that on most global maps the limit between continents and oceans is mapped as an Ocean Continent Boundary (OCB) using magnetic lineations and other geophysical proxies. Therefore, one could assume that the problem of localizing the initial plate boundary in present-day oceans should be resolved. We will consider different margins including the N- and S- Atlantic sites, however, we give priority to the example of the Australia-Antarctica conjugate margins to discuss location, nature and age of the lithospheric breakup. The aim of this work is to gather new observations and to develop new methods to determine timing, location, and processes related to the formation of a plate boundary in a magma-poor rift system. In this presentation we focus on preliminary results on the Australia-Antarctica rift system. These results, obtained after a synthesis of geological (dredges, wells) and geophysical (seismic, gravity, magnetic, P-wave velocities) data, show that the conjugate Australia-Antarctica margins are the result of a polyphase rift evolution. Moreover, the architecture and the evolution of the main structural domains display a strong lateral variability. These results suppose a complex temporal and spatial evolution (including different stages of thinning, exhumation and seafloor spreading) with variations occurring across and along the margin. The results also provide new constraints on the proposed East-West diachronous opening and on the two main directions of extension (first NW-SE then N-S). Moreover, we suggest that polyphase detachment faulting may play an important role, in particular during the mantle exhumation phase potentially leading to the breakup and onset of steady state seafloor spreading. Another important observation is that the current interpretations of magnetic anomalies for the breakup identification may not work. Indeed, these interpretations are based on a symmetric model of accretion, whereas in our assumption, the first magnetic anomalies have been recorded during an asymmetric phase related to continental mantle exhumation. The comparison with other magma-poor rifted margins such as the central segment of the South Atlantic or the southern North Atlantic, will allow determining if these observations result from similar processes in magma-poor rifted margins or if they are specific and restricted to the Australian-Antarctic margins.

Gillard, Morgane; Autin, Julia; Karpoff, Anne-Marie; Manatschal, Gianreto; Munschy, Marc; Sauter, Daniel; Schaming, Marc

2013-04-01

234

The breakup sequence and associated lithospheric breakup surface: Their significance in the context of rifted continental margins (West Iberia and Newfoundland margins, North Atlantic)  

NASA Astrophysics Data System (ADS)

Regional (2D) seismic-reflection profiles and borehole data are used to characterise the syn- to post-rift transition in the shallow offshore Porto Basin, and in deep-offshore regions of West Iberia and Newfoundland (East Canada). The interpreted data highlight the development of a regional stratigraphic surface at the time of complete lithospheric breakup between West Iberia and Newfoundland. This surface, usually called "breakup unconformity", is renamed in this work as Lithospheric Breakup Surface (LBS), on the basis that: (1) it is not always developed as an unconformity and (2) all lithosphere is involved on the breakup process, not only the continental crust. Depositional changes occur across the LBS in association with Late Aptian lithospheric breakup, which is marked by the deposition of a breakup sequence (BS) rather than a single stratigraphic surface. Stratigraphic correlations between strata in shallow and deeper parts of the two margins lead us to propose the breakup sequence (BS) as representing the transitional period between lithospheric breakup and the establishment of thermal relaxation as the main process controlling subsidence on divergent continental margins. The results in this work are important for other continental margins as they demonstrate that during lithospheric breakup significant quantities of sediment bypassed the inner proximal margins of West Iberia and Newfoundland on their way to the outer proximal margin. In addition, the interpreted data show that complete lithospheric breakup between conjugate margins is recorded by similar tectono-stratigraphic events. In Iberia and Newfoundland, these events are associated with reservoir successions in sediment overfilled basins and with carbon-rich strata ('black shales') in sediment-starved basins.

Soares, Duarte M.; Alves, Tiago M.; Terrinha, Pedro

2012-11-01

235

Distribution of coccolithophores in marginal seas along the western Pacific Ocean and in the Red Sea  

Microsoft Academic Search

The distribution of coccolithophores was studied in the neritic environment along the western margin of the Pacific Ocean: the Inland Sea of Seto, Yellow Sea, East China Sea, South China Sea, Java Sea, Timor Sea, Arafura Sea and Gulf of Carpentaria. The coccolithophore community in the Red Sea was also studied for comparison with the Pacific marginal seas. With minor

H. Okada; S. Honjo

1975-01-01

236

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

NASA Astrophysics Data System (ADS)

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

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

2011-11-01

237

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

238

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

239

Constraining the carbon and hydrological cycles of North American continental margins across the Paleocene-Eocene thermal maximum  

NASA Astrophysics Data System (ADS)

The impacts of the Paleocene-Eocene Thermal Maximum (PETM, ca 55 Ma), one of the most rapid and extreme warming events in Earth history, are well characterized in open marine and terrestrial environments but less so on continental margins, a major carbon sink. Continental margins are also ideally located to record changes in sediment fluxes, for instance in response to a change in the degree of continental weathering. Here we present results from an interdisciplinary study based on outcrop sections in California and ODP cores drilled on the New Jersey margin. Our goal is to characterize the PETM interval on continental margin locations, with an emphasis on the duration and magnitude of the Carbon Isotope Excursion. We also explore potential changes in sedimentation rates as a mean to assess sediment flux and associated changes in the hydrological cycle and continental weathering rates. For this, we generated stable isotope, carbonate content, organic matter content, C:N ratio and mineralogy data. Stable isotope data was measured on carbonate material, organic matter, and the siliciclastic clay-sized fraction. Foraminifer oxygen isotope data suggest that mid-latitude shelves warmed by a similar magnitude as the open ocean (5-8°C), while the Carbon Isotope Excursion (CIE), recorded both in carbonate and organic carbon isotope records, is slightly larger (3.3-4.5 per mil) than documented in open ocean records. Sediment accumulation rates increase dramatically during the CIE in marked contrast to the open ocean sites. In parallel, mass accumulation rates of both organic and inorganic carbon also increased by an order of magnitude. The estimated total mass of accumulated carbon in excess of pre-CIE rates suggests that continental margins, at least along North America, became carbon sinks during the CIE, mainly due to weathering feedbacks and rising sea-level. This result is significant, because it implies that the negative feedback role of carbon burial on continental margins was greater than previously recognized. Furthermore, preliminary oxygen isotope results on the siliciclastic clay-sized fraction show a marked excursion during the PETM. This could suggest important changes in the amount of precipitation during the PETM, but the stable isotope composition of individual clay species needs to be measured before a conclusive interpretation can be reached.

John, C. M.; Zachos, J. C.; Bohaty, S. M.; Sluijs, A.; Brinkhuis, H.; Gibbs, S.; Bralower, T. J.; Banerjee, N.; Longstaffe, F. J.

2009-04-01

240

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

NASA Astrophysics Data System (ADS)

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

Sohn, B.; Choi, M.

2013-12-01

241

Seismic study of the '41st Parallel' Fault System offshore the Campanian Latial continental margin, Italy  

NASA Astrophysics Data System (ADS)

A set of seismic reflection lines, collected in the Southern Tyrrhenian Sea offshore the Campanian-Latial continental margin has been selected and interpreted. The aim is to characterize: (1) the structural features of the 41st Parallel Line (41PL), an E-W elongated magnetic anomaly zone separating the Northern Tyrrhenian domain from the Southern Tyrrhenian domain; and (2) to study the relationships between the 41PL and the Ortona-Roccamonfina Line (ORL), a tectonic structure transversal to the Apennines along which the Northern Apennine Arc merges the Southern Arc. The interpretation of the seismic lines is discussed in light of the available geological (stratigraphic and structural) and geophysical (gravimetric and magnetic) information. Results show that the Campanian-Latial continental margin is characterized by a series of structural lows and highs that match with the main structures on the mainland (Mt. Massico horst, the Volturno and Garigliano depressions). The study area is characterized by ESE-WNW to E-W and NE-SW striking faults. The activity of these faults developed during Pliocene-Early Pleistocene times. ESE-WNW to E-W faults display structures consistent with strike-slip movements. These faults, which are located on the maximum gradient of the E-W elongated magnetic alignment of 41PL, are responsible for the SSE translation of the offshore sector of the Mt. Massico horst. The horizontal dislocation of Mt. Massico suggests left-lateral movements for the ESE-WNW to E-W faults. The NE-SW faults that affect the Mt. Massico horst, which represents the southern tip of ORL, show seismic features consistent with normal movements, as also revealed by inland data. Since: (1) background seismicity is virtually absent along the study area; and (2) the uppermost seismic reflectors seem unaffected by faults, it is very likely that both the ORL and 41PL fault zones are now inactive, at least in the Campanian-Latial area. Seismic data indicate that ORL is older than 41PL and support the interpretation that the 41PL represents a deep-seated transfer fault system formed in response to the different rates of opening of the Tyrrhenian Sea. The strike-slip movements along the 41PL faults and the normal movements along the ORL faults are consistent with a NW-SE extension, which is responsible for the longitudinal extension in the Southern Apennines belt.

Bruno, Pier Paolo; Di Fiore, Vincenzo; Ventura, Guido

2000-09-01

242

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

NASA Astrophysics Data System (ADS)

High-quality, wide-angle, ocean bottom seismometer (OBS) data have been acquired with a low frequency (9 Hz) seismic source across the Faroes and Hatton Bank volcanic rifted continental margins in the North Atlantic. In these regions thick Tertiary flood basalt sequences provide a challenge to deep seismic imaging. S-wave arrivals, which are dominantly converted from P- to S-waves at the sediment-top basalt interface, were recorded at 170 4-component OBS locations. Variation in the conversion efficiency was observed along the profiles. Tomographic inversion of over 70,000 converted S-wave crustal diving waves and Moho reflections was performed to produce S-wave velocity models and hence, when combined with pre-existing P-wave velocity models, a measure of the Vp/Vs ratio structure of the crust. Resolution testing shows the structure of the oceanic crust and continent-ocean transition is generally well resolved on both profiles. Lateral and vertical changes in Vp/Vs resolves changing crustal composition within, and between, oceanic and continental crust, including regions in the lower crust at the continent-ocean transition with high P-wave velocities of up to 7.5 km/s and low Vp/Vs ratios of ~ 1.75 associated with intense high-temperature intrusion at the time of break-up. Vp/Vs ratios of 1.75-1.80 at the base of the thickened oceanic crust are also lower than generally reported in normal oceanic crust. The P-wave travel-time tomography revealed a low velocity zone (LVZ) beneath the basalt on the Faroes margin and additional constraint on the Vp/Vs of the LVZ beneath the Fugloy Ridge has been gained by analysing the relative travel-time delays between basalt and basement refractions for P- and S-waves. This approach is less subject to the velocity-depth ambiguity associated with velocity inversions than is the determination of P- or S- wave velocity alone. Comparison of the calculated Vp/Vs ratio and P-wave velocity with measurements from relevant lithologies reveals that the LVZ is likely to contain sill-intruded Paleocene sedimentary rock rather than igneous hyaloclastites similar to those found beneath the basalt in a nearby well. Immediately beneath the LVZ, a unit with Vp/Vs ratios of 1.80-1.85 and P-wave velocities of 5.5-6.0 km/s is interpreted as sill-intruded sedimentary rock of a pre-breakup Mesozoic basin. We thank C.J. Parkin, A.W. Roberts and L.K. Smith for their contributions.

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

2007-12-01

243

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

244

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

NASA Astrophysics Data System (ADS)

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

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

2007-08-01

245

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

246

Modelling the composition of melts formed during continental breakup of the Southeast Greenland margin  

NASA Astrophysics Data System (ADS)

We have developed a generic dynamic model of extension of the lithosphere, which predicts major element composition and volume of melt generated from initial extension to steady state seafloor spreading. Stokes equations for non-Newtonian flow are solved and the mantle melts by decompression. Strengthening of the mantle due to dehydration as melting progresses is included. The composition is then empirically related to depletion. Using a crystallisation algorithm, the predicted primary melt composition was compared with mean North Atlantic mid-ocean ridge basalt (MORB). At steady state, using half spreading rates from 10 to 20 mm yr - 1 and mantle potential temperatures of 1300 to 1325 °C we predict a major element composition that is within the variation in the mean of North Atlantic MORB. This model is applied to the Southeast Greenland margin, which has extensive coverage of seismic and ODP core data. These data have been interpreted to indicate an initial pulse of magmatism on rifting that rapidly decayed to leave oceanic crustal thickness of 8 to 11 km. This pattern of melt production can be recreated by introducing an initial hot layer of asthenosphere beneath the continental lithosphere and by having a period of fast spreading during early opening. The hot layer was convected through the melt region giving a pulse of high magnesian and low silica melt during the early rifting process. The predicted major element composition of primary melts generated are in close agreement with primary melts from the Southeast Greenland margin. The observed variations in major element composition are reproduced without a mantle source composition anomaly.

Armitage, John J.; Henstock, Timothy J.; Minshull, Timothy A.; Hopper, John R.

2008-05-01

247

Epi-benthic megafaunal zonation across an oxygen minimum zone at the Indian continental margin  

NASA Astrophysics Data System (ADS)

The Arabian Sea oxygen minimum zone (OMZ) impinges upon the Indian continental margin at bathyal depths (150-1500 m) producing changes in ambient oxygen availability and sediment geochemistry across the seafloor. The influence of these environmental changes upon the epi-benthic megafaunal assemblage was investigated by video survey at six stations spanning the OMZ core (540 m), lower boundary (800-1100 m) and below the OMZ (2000 m), between September and November 2008. Structural changes in the megafaunal assemblage were observed across the six stations, through changes in both megafaunal abundance and lebensspuren (biogenic traces). Most megafauna were absent in the OMZ core (540 m), where the assemblage was characterised by low densities of fishes (0.02-0.03 m -2). In the lower OMZ boundary, megafaunal abundance peaked at 800 m, where higher densities of ophiuroids (0.20-0.44 m -2) and decapods (0.11-0.15 m -2) were present. Total abundance declined with depth between 800 and 2000 m, as the number of taxa increased. Changes in the megafaunal assemblage were predicted by changes in abundance of seven taxonomic groups, correlated to both oxygen availability and sediment organic matter quality. Lebensspuren densities were highest in the OMZ boundary (800-1100 m) but traces of large infauna (e.g., echiurans and enteropneusts) were only observed between 1100 and 2000 m station, where the influence of the OMZ was reduced. Thus, changes in the megafaunal assemblage across the Indian margin OMZ reflect the responses of specific taxa to food availability and oxygen limitation.

Hunter, William R.; Oguri, Kazumasa; Kitazato, Hiroshi; Ansari, Zakir A.; Witte, Ursula

2011-06-01

248

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

249

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

NASA Astrophysics Data System (ADS)

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

Glasmacher, Ulrich Anton; Hackspacher, Peter C.

2013-04-01

250

Very large dune formation along the Ebro outer continental shelf (Western Mediterranean)  

NASA Astrophysics Data System (ADS)

Large and very large subaqueous dunes have been observed in a number of outer shelf regions around the world, tipically developing on fossil sand bodies and ridges. Dunes observed on outer shelves usually display large dimensions with maximum wavelength reaching up to 500 m and heights up to 20 m. Forcing mechanisms able to induce their formation have been described as strong bottom currents related to tidal variations and water masses flowing under geostrophic conditions, generally controlled and enhanced by local geomorphologic configurations. In this study, such bed features have been recognized, mapped and measured around the Columbretes Islands (Ebro continental shelf - Western Mediterranean) with the aim to reconstruct which are the potential forcing processes that could generate them in relation to the local settings of the area. Swath-bathymetry around the Columbretes Islands was collected using 30 kHz and 180 kHz Multi Beam echo-sounders for a 50-400 m water depth range. Bathymetric data revealed the presence of three main relict sand bodies along the outer shelf, for a 80-116 m depth range, above which asymmetrical, slightly asymmetrical and symmetrical large and very large 2D and 3D subaqueous dunes were observed. Dunes range from 150 to 760 m in wavelength and from tens of cm to 6 m in height. These bedforms are composed of sandy sediments, presumably coming from the degraded relict sand bodies on which they developed, mixed to the fine fractions coming from the recent draping holocenic sediments. The orientation of the dunes is SSW and progressively turns to W directions moving towards the southernmost sector of the area, following the trend of the shelf-edge. Observed dunes display a strong asymmetric profile for those occurring along the shelf-edge (Symmetry Index (SI): 2.6) and lose progressively their asymmetry towards the inner portion of the shelf (SI: 0.5), being 0.6 the minimum SI value to classify the dunes as asymmetric. The subaqueous dunes observed along the studied region are amongst the largest ever recognized on an outer shelf setting. Morphologic characters and the orientation towards SW and W directions suggest the Liguro-Provenzal-Catalan geostrophic current as the primary forcing factor in their formation. Contemporary hydrodynamic measurement at the Ebro continental shelf-edge show that near-bottom wave action is negligible in this area, whereas maximum shear stresses induced by currents are able to resuspend fine sand particles and prevent the relict transgressive deposits from being covered by mud. However, recorded nearbottom currents generate shear stresses below the critical value for transport the relict coarse sands found in the study area and form large bedforms. The comparison of successive bathymetric images and the relation wavelength/height suggest that the described very large dunes are inactive features over long periods, as observed in similar environments along several continental margins. Thus, the morphological configuration of the Columbretes outer shelf must have played a crucial role in enhancing the southward flowing bottom currents during energetic hydrodynamic events, giving them the potential to generate such bedforms.

Lo Iacono, Claudio; Guillén, Jorge; Puig, Pere; Ribó, Marta; Ballesteros, Maria; Palanques, Albert; Farrán, Marcelli; Acosta, Juan

2010-05-01

251

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

SciTech Connect

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

Anderson, J.B.

1987-05-01

252

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

NASA Astrophysics Data System (ADS)

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

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

2008-06-01

253

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

254

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

255

Observations of the internal tide on the California continental margin near Monterey Bay  

NASA Astrophysics Data System (ADS)

Observations of the semidiurnal internal tide on the California continental margin between Monterey Bay and Point Sur confirm the existence of northward energy flux predicted by numerical models of the region. Both a short-duration tide-resolving survey with expendable profilers and a multi-week timeseries from FLIP measured northward flux in the mean, supporting the hypothesis that topographic features off Point Sur are the source of the strong internal tides observed in Monterey Canyon. However, the observed depth-integrated semidiurnal flux of 450±200 W m-1 is approximately twice as large as the most directly-comparable model and FLIP results. Though dominated by low modes with O(100 km) horizontal wavelengths, a number of properties of the semidiurnal internal tide, including kinetic and potential energy, as well as energy flux, show lateral variability on O(5 km) scales. Potential causes of this spatial variability include interference of waves from multiple sources, the sharp delineation of beams generated by abrupt topography due to limited azimuthal extent, and local generation and scattering of the internal tide into higher modes by small-scale topography. A simple two-source model of a first-mode interference pattern reproduces some of the most striking aspects of the observations.

Terker, Samantha R.; Girton, James B.; Kunze, Eric; Klymak, Jody M.; Pinkel, Robert

2014-07-01

256

Dextral transpression in Late Cretaceous continental collision, Sanandaj–Sirjan Zone, western Iran  

Microsoft Academic Search

The Sanandaj–Sirjan Zone of western Iran is a metamorphic belt (greenschist–amphibolite) that was uplifted during Late Cretaceous continental collision between the Afro-Arabian continent and the Iranian microcontinent. In the June area, 300 km southwest of Tehran, the Late Palaeozoic–Mesozoic succession was affected by two major episodes of deformation. The first deformation formed tight folds and axial plane schistosity. These are

Mohammad Mohajjel; Christopher L Fergusson

2000-01-01

257

Late quaternary transgressive\\/regressive sequences from Taranaki continental shelf, western New Zealand  

Microsoft Academic Search

Interpretation of seismic reflection profiles (3.5 kHz, airgun), and piston core and borehole stratigraphic logs, collected from mid-outer shelf depths (50–130 m) on the Taranaki continental shelf, western New Zealand, have delineated at least 4 alternating late Quaternary seismic and lithologic couplets. Each couplet comprises an acoustically transparent, upper silty unit that overlies a highly reflective sandy unit, characterised by

Scott D. Nodder

1995-01-01

258

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

259

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

260

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

261

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

262

Upper Jurassic and Lower Cretaceous facies relationships in a passive margin basin, western North Atlantic  

SciTech Connect

Correlation of facies from hydrocarbon-bearing continental and transitional marine sandstones to time-equivalent high-energy shelf-margin carbonates provide insight into hydrocarbon habitats of the Baltimore Canyon basin. These facies occur within a thick (> 10,000 ft) prograded wedge of shelf sediments in this passive margin basin. Wells drilled to test structural closures in shallow-water (< 600 ft) areas of Baltimore Canyon penetrate clastic facies which are time-equivalent to the downdip carbonate facies tested in deep-water wells. Numerous hydrocarbon shows, including a noncommercial gas and gas-condensate accumulation, occur with sandstone units that were deposited in prograding continental/fluvial and transitional marine environments located updip of the Oxfordian/Kimmeridgian carbonate shelf edge. The continental and transitional facies are overlain by a fine-grained deltaic complex which forms a regionally extensive top seal unit. The deltaic complex was deposited during aggradation of the Kimmeridgian through Berriasian shelf-margin carbonates penetrated by the deep-water wells. Deep-water wells (> 5000 ft) drilled off the continental shelf edge to test large structural closures along the downdip termination of the Upper Jurassic/Lower Cretaceous carbonate shelf edge encountered no significant hydrocarbon shows. Reservoir rocks in these wells consist of (1) oolite grainstone which was deposited within a shoal-water complex located at the Aptian shelf edge, and (2) coral-stromatoporoid grainstone and boundstone which formed an aggraded shelf-margin complex located at the Kimmeridgian through Berriasian shelf edge. Structural closures with reservoir and top seals are present in both updip and downdip trends. The absence of hydrocarbon shows in downdip carbonate reservoirs suggests a lack of source rocks available to charge objectives at the shelf margin.

Prather, B.E.

1988-02-01

263

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

NASA Astrophysics Data System (ADS)

In 2001 and 2002, Australia acquired an integrated geophysical data set over the deep-water continental margin of East Antarctica from west of Enderby Land to offshore from Prydz Bay. The data include approximately 7700 km of high-quality, deep-seismic data with coincident gravity, magnetic and bathymetry data, and 37 non-reversed refraction stations using expendable sonobuoys. Integration of these data with similar quality data recorded by Japan in 1999 allows a new regional interpretation of this sector of the Antarctic margin.

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

2004-09-01

264

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

265

Reconstruction Of Eocene To Miocene Antarctic Surface Temperature And Aridity From Bulk Sediment Geochemistry Of Continental Margin Drillholes  

NASA Astrophysics Data System (ADS)

Documenting the Cenozoic evolution of Antarctic continental climate and glaciation has been a primary objective of drilling efforts on the Antarctic margin. Here we evaluate the major element geochemistry of detrital sediment from three drilling projects to reconstruct Antarctica's continental surface conditions. The dataset includes analyses of mudstone samples from IODP Site U1356 on the Wilkes Land margin, ODP Site 1166 in Prydz Bay, and three drillholes of the Cape Roberts Drilling Project in the Ross Sea; taken together, these drillcores provide a record of sediments shed from the East Antarctic craton from the early Eocene to the late Miocene. In our conceptual model of erosion and sediment transport, pre-glacial and interglacial mudstones are primarily derived from erosion of soils under transport limited conditions (weathering exceeds erosion), whereas glacial mudstones are derived from continental surfaces that are weathering-limited (erosion exceeds weathering). Within this framework, we interpret the oxide molar ratio of K+Na/Al for pre-glacial and interglacial mudstones on the continental margin as an indicator of paleo surface temperatures and the chemical index of alteration (CIA) as an aridity indicator. Using transfer functions derived for B horizons of modern, active, soils in a range of climate conditions, we are able to show that Antarctica continuously cooled and became progressively more arid from the Early Eocene to the late Miocene with large shifts in continental conditions at the Eocene-Oligocene boundary and the Miocene climate transition. These results will be discussed in the context of other proxies for terrestrial paleoclimates, including paleosols, pollen, clay mineralogy, and organic geochemical proxies.

Passchier, S.; Bohaty, S. M.; Henao, V.; Jiménez, F.; van de Flierdt, T.; Tauxe, L.; Expedition 318 scientists

2011-12-01

266

A multiproxy approach to reconstruct the environmental changes along the Eurasian continental margin over the last 150?000 years  

Microsoft Academic Search

Sediment cores located along the Eurasian continental margin (Arctic Ocean) have been studied to reconstruct the environmental changes in terms of waxing and waning of the Barents\\/Kara Sea ice-sheets, Atlantic water inflow, and sea-ice distribution over the last 150 kyr. The stratigraphy of the cores is based on stable oxygen isotopes, AMS 14C, and paleomagnetic data. We studied variations in

Jochen Knies; Norbert Nowaczyk; Claudia Müller; Christoph Vogt; Ruediger Stein

2000-01-01

267

Synoptic observations of the three-dimensional structure and propagaton of Gulf Stream meanders along the Carolina continental margin  

Microsoft Academic Search

Simultaneously measured Eulerian currents and spatially extensive subsurface temperatures have provided a time series of eigth synoptic, three-dimensional views of the Gulf Stream frontal zone along the Carolina continental margin. Two large-amplitude meanders were observed to progress through the study area between Charleston and Cape Hatteras during February 1979. Each meander had a vertically coherent, skewed wave-like subsurface structure. The

John M. Bane; David A. Brooks; Karen R. Lorenson

1981-01-01

268

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

269

New gravity and magnetic data measured at the continental margin off Israel and at the Nile Cone off Egypt  

Microsoft Academic Search

During the Meteor expedition M52\\/2 GEMME (February 4th to March 7th 2002) gravity and magnetic data were collected. The main research area was located in the south-eastern Mediterranean at the continental margin off Israel and at the Nile cone off Egypt. The aim of gravity and magnetic investigations was to understand the tectonic and geodynamical processes such as crustal structure,

G. A. Dehghani; Ch. Huebscher; Z. Ben-Avraham; D. Gajewski

2003-01-01

270

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

USGS Publications Warehouse

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

Nelson, C. H.

1990-01-01

271

Comparative geochemistry of Indian margin (Arabian Sea) sediments: Estuary to continental slope.  

NASA Astrophysics Data System (ADS)

Factors controlling the distribution of organic matter in the Arabian Sea have been the subject of much research and debate ever since organic-rich slope deposits were associated with the mid-water oxygen minimum zone (OMZ) However, the debate remains open, and numerous interacting factors have been invoked as important controls. A limitation of most previous studies is that they have been restricted to limited portions of the margin, and have not included molecular-level tracers that allow distinction of organic matter (OM) source and degradation state as factors in OM distribution. We report results from sites across the Indian margin of the Arabian Sea, which were analysed for carbon and nitrogen compositions (elemental and isotopic), grain size and indices of OM source and degradation state. Site locations ranged from the Mandovi/Zuari estuaries to depths of ~2000m on the continental slope, thus spanning both the semi-permanent OMZ on the upper slope (~200-1300m) and the seasonal hypoxic zone that impinges on the shelf. Source indices showed mixed marine and terrigenous OM within the estuaries, but overwhelming predominance (80%+) of marine OM on the shelf and slope, even in nearshore deposits. Thus, riverine OM is heavily diluted or efficiently remineralised within or immediately offshore of the estuaries. Any terrigenous OM that is exported appears to be retained in nearshore muds; lignin phenols indicate that the small terrigenous OM content of slope sediments is of different origin, potentially from rivers to the north. Organic C contents of surface shelf and slope sediments varied from <0.5 wt% in relict shelf sands to a maximum of >7 wt% at upper slope sites within the OMZ, then decreasing to ?1wt% at 2000m. However, major variability (~5 wt%) occured within the OMZ at sites with near-identical depths and bottom-water oxygen. A strong relationship between organic C and grain size was seen for OMZ sediments, but lower C loadings were found for sites on the shelf and below the OMZ. Diagenetic indices confirmed that lower C content below the OMZ is associated with greater extent of OM degradation, but that C-poor shelf sediments are not consistently more degraded than those within the OMZ. Together, results indicate that OM enrichment on the upper slope, where it occurs, can be explained by winnowing or other physical processes on the shelf combined with progressive OM degradation with increasing oxygen exposure below the OMZ. Reduced oxygen exposure may contribute to observed OM enrichment with the OMZ, but hydrodynamic processes are the overriding control on sediment OM distribution, even within the OMZ.

Cowie, Greg; Mowbray, Stephen; Kurian, Siby; Sarkar, Amit; White, Carol; Anderson, Amy; Vergnaud, Bianca; Johnstone, Gisele; Brear, Samuel; Woulds, Clare; Naqvi, Wajih; Kitazato, Hiroshi

2014-05-01

272

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

273

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

274

Conductivity model of the passive continental margin derived from an amphibian magnetotelluric study on the Walvis Ridge and at the Kaoko Belt in Northern Namibia  

NASA Astrophysics Data System (ADS)

The magnetotelluric (MT) study is a part of the interdisciplinary SAMPLE project investigating processes related to the breakup of supercontinent Gondwana and the post breakup evolution of the passive continental margins of Africa and South America. We present an electrical conductivity image from the Southern African passive continental margin, derived from an amphibian MT experiment crossing the entire Kaoko Belt in Northern Namibia and the Walvis Ridge in the Atlantic Ocean. The MT data at 167 onshore sites are generally of a high quality but large diagonal components of impedance tensor, phases over 90° at some sites and a strong variability of transfer functions within short distances indicate three-dimensional structures in the crust and upper mantle. Such 3D effects are observed particularly in the Western Kaoko Zone in the vicinity of the prominent Neoproterozoic shear zones. Thus, we apply a two-part inversion strategy: In areas and frequency ranges where the 3D effects are not dominant, we apply 2D inversion of data sub-sets in order to identify the prominent conductivity features and assess their resolution and robustness; however, the entire data set can only be explained by 3D inversion. The 2D models of the crust beneath the profile from the Walvis Ridge onto the Congo Craton reveal a spatial correlation of resistive zones with the cratonic Northern Platform. Zones of high electrical conductivity seem to correlate with surface expressions of prominent faults such as the Purros Mylonite Zone and the Three Palm Mylonite Zone of the Kaoko Belt. Outcropping Etendeka flood basalts in the Western Kaoko Zones correlate with zones of high resistivity. The offshore part of the models shows a thin conductive layer corresponding to sea floor sediments. Interestingly, the Walvis Ridge exhibits a more resistive and deeper reaching seafloor compared to what typically is expected from oceanic crust. These results are complemented by 3D inversion, which generally confirms the 2D results but reveals some additional off-profiles structures.

Weckmann, U.; Kapinos, G.; Ritter, O.; Jegen, M.

2013-12-01

275

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

276

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

277

Using the Silica Cycle to Re-assess the Role of Continental Margin Systems in the Deep Biological Carbon Pump  

NASA Astrophysics Data System (ADS)

Predictions of future climate require that the long-term partitioning of carbon dioxide between the oceans and atmosphere be estimated. Deep sea sediments receive only a small amount of organic carbon and waters above the base of the main thermocline exchange with the atmosphere on time scales of generally less than a few decades. Therefore, the most important oceanic reservoir for carbon storage on climate-change time-scales is oceanic deep water. Studies of the biological pump must focus on those ecosystems that supply the majority of the flux to the deep ocean, i.e. to below approximately 1000 m. Previous studies of deep organic fluxes based on benthic flux compilations have indicated that regions near continental boundaries, particularly those that experience significant, wind-driven coastal upwelling, supply a disproportionately large proportion of the organic carbon to the deep ocean. Margin systems in general were estimated to account for approximately 1/2 of the total deep biological pump. Despite this potential role, process studies of carbon fluxes have not focused on these environments. Because processes such as benthic exchange, iron input from shelf sediments, denitrification, and nepheloid layer transport are unique to margin systems, open ocean process studies may not provide an accurate assessment of carbon dynamics in margin settings. Recent publications report that 1. POC and opal fluxes are not strongly correlated in open ocean sediment trap samples, 2. measured opal fluxes in traps are less than those inferred from a global circulation model (GCM) and 3. silica burial is greater in continental margin sediments and less in the southern ocean than previously reported. Here we present a compilation of benthic flux chamber results from margin systems that confirm a strong correlation between POC and opal fluxes, consistent with ecological models. These results support the conclusion that there is a strong link between organic carbon and Si cycles and suggest that the discrepancy between GCM and open ocean trap fluxes may be balanced by an opal flux along continental margins. Taken together, these results support previous benthic flux studies that suggest that continental margin systems significantly contribute to the deep biological carbon pump.

Jahnke, R. A.; Jahnke, D. B.

2004-12-01

278

Fluids in hyper-extended rifted margins: Examples from the paleomargins in the eastern Alps and west Pyrenees and present-day Iberia rifted continental margin.  

NASA Astrophysics Data System (ADS)

The evolution of deep-water rifted margins is intimately linked with complex and poly-phase 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 play an important role, changing the chemical and physical properties of rocks with 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 clays and serpentine minerals, and the pervasive cementation and precipitation of quartz within the fault rocks and veins along detachment faults. Although the chemistry reaction is well known it is still unclear to what extent those reactions can lead to changes in the rheology of the lithosphere and how they can affect the thermal evolution of deep water, hyper-extended rifted margins. Another important question arise about the origin, timing, pathways and composition of these fluids. Are they mantle-derived fluids and/or of marine origin? In this work we present preliminary results from fossil rift-related detachment faults exposed in the Alps and Pyrenees and discuss the role of fluids during lithospheric thinning. We compare the results obtained from these fossil rifted margins with those from the present-day Iberia rifted continental margin.

Pinto, V. H.; Manatschal, G.; Viana, A.; Karpoff, A. M.; Masini, E.; Szatmari, P.; Penteado, H.; Svartman, A.; Lemarchand, D.

2012-04-01

279

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

USGS Publications Warehouse

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

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

1989-01-01

280

Strain partitioning along the western margin of North America  

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

281

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

282

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

283

Compilation of Geophysical Data on the East Greenland Continental Margin and Its Use in Gravity Modelling Across the Continent-Ocean Transition.  

National Technical Information Service (NTIS)

Magnetic, gravity, and seismic reflection data on the East Greenland continental margin were collected. The data were used to model the ocean-continent transition, and to study the nature and distribution of dipping, subacoustic basement (sub B) seismic r...

C. I. Uruski L. M. Parson

1985-01-01

284

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

285

Impact of seasonal oxygen deficiency on the phosphorous geochemistry of surface sediments along the Western Continental Shelf of India  

Microsoft Academic Search

The intensification of the natural coastal hypoxic zone over the western Indian shelf in the recent years and its impact on the biogeochemistry and marine life is a matter of concern. This study examines the influence of the seasonal oxygen deficiency on the phosphorus geochemistry of the surface sediments along the western continental shelf of India (WCSI). Speciation of phosphorus

Josia Jacob; Prosenjit Ghosh; K. K. Balchandran; Rejomon George

2010-01-01

286

Sub-Milankovitch variability in the terrigenous sediment input to the southern Chilean continental margin during the last glacial and Holocene (ODP Site 1233)  

Microsoft Academic Search

Sediments recovered at Ocean Drilling Project (ODP) Site 1233 (drilled during Leg 202 at the southern Chilean continental margin) provide an outstanding potential for unprecedented ultra high resolution reconstructions of surface and deep ocean conditions as well as continental paleoclimates during the last glacial\\/interglacial cycle. Shipboard biostratigraphic data indicate that the base of Site 1233 is younger than 260 ka.

F. Lamy

2003-01-01

287

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

SciTech Connect

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

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

1991-03-01

288

Post-early cretaceous landform evolution along the western margin of the banca~nnia trough, western nsw  

USGS Publications Warehouse

Previously undated post-Devonian sediments outcropping north of Fowlers Gap station near the western margin of the Bancannia Trough are shown by plant macro- and microfossil determinations to be of Early Cretaceous (most likely Neocomian and/or Aptian) age, and thus part of the Eromanga Basin. They are assigned to the previously defined Telephone Creek Formation. Study of the structural configuration of this unit and the unconformably underlying Devonian rocks suggests that the gross landscape architecture of the area results from post-Early Cretaceous monoclinal folding along blind faults at the western margin of the trough, combined with the effects of differential erosion. This study shows that, while landscape evolution in the area has been dynamic, the major changes that have occurred are on a geological rather than human timescale.

Gibson, D. L.

2000-01-01

289

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

USGS Publications Warehouse

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

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

2007-01-01

290

Clathrate Smoking Gun Hypothesis: Ocean Gas Hydrate Melting Causes Cracking of Upper Continental Margin Slopes Creating Major Gas Release Pathways  

NASA Astrophysics Data System (ADS)

Despite the potential for large releases of methane from the upper continental margin by hydrate dissociation, geophysical investigations have not included the critical part of the hydrate stability zone (HSZ) outcrop. If these HSZ outcrop areas at the shelf edge and upper continental slope are exposing dynamic "hydrate reservoirs", then it potential to affect the Earth's climate by releasing methane to the ocean and atmosphere has to be considered. Our investigations on the high latitude Norwegian Margin show the instability of methane hydrates driven mainly by ocean warming and not of sea level change. Widespread cracks and faults at the outcrop zone of the HSZ point towards a relation to dissociating hydrates causing cracking of outer shelf and upper continental slope sediments in the shallow geosphere. Such recognition of cracks on the outer shelf will become important in the evaluation of methane releases as a powerful contributer to the Greenhouse Warming associated with millenia-scale climate change. Our geophysical observations will be discussed in context with ocean warming and hydrate melting scenarios during the Holocene.

Mienert, J.; Buenz, S.; Vanneste, M.; Andreassen, K.

2003-04-01

291

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

USGS Publications Warehouse

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

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

1999-01-01

292

Four-dimensional numerical modeling of crustal growth at active continental margins  

NASA Astrophysics Data System (ADS)

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 on the order of 30-80 km and is comparable to volcano clustering in natural arcs. 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 the modeled regime of stable subduction, the maximum crustal additional rate (25-40 km3/km/Myr) occurs when the 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 V.; Tackley, Paul J.; Kissling, Eduard

2013-09-01

293

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

294

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

NASA Astrophysics Data System (ADS)

Surface sediments from sites across the Indian margin of the Arabian Sea were analysed for their carbon and nitrogen compositions (elemental and stable isotopic), grain size distributions and biochemical indices of organic matter (OM) source and/or degradation state. Site locations ranged from the estuaries of the Mandovi and Zuari rivers to depths of ~ 2000 m on the continental slope, thus spanning nearshore muds and sands on the shelf and both the semi-permanent oxygen minimum zone (OMZ) on the upper slope (~ 200-1300 m) and the seasonal hypoxic zone that impinges on the shelf. Source indices showed mixed marine and terrigenous OM within the estuaries, and overwhelming predominance (80%+) of marine OM on the shelf and slope. Thus, riverine OM is heavily diluted by autochthonous marine OM and/or is efficiently remineralised within or immediately offshore of the estuaries. Any terrigenous OM that is exported appears to be retained in nearshore muds; lignin phenols indicate that the small terrigenous OM content of slope sediments is of different origin, potentially from rivers to the north. Organic C contents of surface shelf and slope sediments varied from < 0.5 wt % in relict shelf sands to over 7 wt % at slope sites within the OMZ, decreasing to ? 1 wt % at 2000 m. Major variability (~ 5 wt %) was found at slope sites within the OMZ of similar depth and near-identical bottom-water oxygen concentration. A strong relationship between organic C and sediment grain size was seen for sediments within the OMZ, but lower C loadings were found for sites on the shelf and below the OMZ. Diagenetic indices confirmed that lower C content below the OMZ is associated with greater extent of OM degradation, but that C-poor shelf sediments are not consistently more degraded than those within the OMZ. Together, the results indicate that OM enrichment on the upper slope can be explained by physical controls (winnowing and/or dilution) on the shelf and progressive OM degradation with increasing oxygen exposure below the OMZ. Reduced oxygen exposure may contribute to OM enrichment at some sites within the OMZ, but hydrodynamic processes are the overriding control on sediment OM distribution.

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

2014-02-01

295

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

296

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

297

Gravity field and deep structure of the Bengal Fan and its surrounding continental margins, northeast Indian Ocean  

NASA Astrophysics Data System (ADS)

A revised gravity anomaly map for the northeast Indian Ocean shows that the shelf edge underlying the eastern continental margin of India is a rather narrow but extensively linear gravity low (minimum free-air = -149 mGal). The Bengal Fan seaward of the shelf has a depressed gravity field (average free-air = -20 to -30 mGal) in spite of the enormous thickness of sediments of as much as 10-15 km. The two buried ridges below the Bengal Fan—the 85° East and 90° East Ridges—have a large negative (-75 mgal) and a substantial positive (40 mGal) free-air anomaly, respectively. The Andaman and Burmese arcs lying along the east margin of the Bengal Fan are active subduction areas which have typical bipolar gravity signatures with a maximum amplitude of 300 mGal. Gravity interpretation for three regional traverses across the central and northern parts of the Bengal Fan and their surrounding continental margins suggests that a thickened oceanic crustal wedge juxtaposes the transitional crust under the eastern continental slope of India; the 85° East Ridge, that was created when the Indian Ocean lithosphere was very juvenile, appears to underlie a nearly 10 km thick and 120 km wide oceanic crustal block consisting of the ridge material embedded in the upper lithosphere; while the 90° East Ridge submarine topography/buried load below the Bengal Fan is probably isostatically compensated by a low-density mass acting as a cushion at the base of the crust. The Bengal Fan crust, with its thick sediment layer, is carried down the Andaman subduction zone to a depth of about 27 km where, possibly, phase transition takes place under higher pressure. The maximum sediment thickness at the Andaman-Burmese subduction zone is of the order of 10-12 km. The gravity model predicts a low density zone about 60 km wide below the Andaman-Burmese volcanic arc, penetrating from crustal to subcrustal depths in the overriding Burma plate. A more complex density distribution is however, envisaged for the Andaman volcanic arc that is split by the Neogene back arc spreading ridge. The ocean-continent crustal transition possibly occurs farther east of the volcanic arc; below the Shan plateau margin in Burma or below the Mergui terrace at the Malayan continental margin east of the Andaman Sea.

Mukhopadhyay, Manoj; Krishna, M. R.

1991-02-01

298

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

299

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

300

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

NASA Astrophysics Data System (ADS)

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, Jeffrey B.; Molzer, Phillip C.

301

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

302

Crustal architecture and deep structure of the Namibian passive continental margin around Walvis Ridge from wide-angle seismic data  

NASA Astrophysics Data System (ADS)

The opening of the South Atlantic ocean basin was accompanied by voluminous magmatism on the conjugate continental margins of Africa and South America, including the formation of the Parana and Entendeka large igneous provinces (LIP), the build-up of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDR), as well as the formation of paired hotspot tracks on the rifted African and South American plates, the Walvis Ridge and the Rio Grande Rise. The area is considered as type example for hotspot or plume-related continental break-up. However, SDR, and LIP-related features on land are concentrated south of the hotspot tracks. The segmentation of the margins offers a prime opportunity to study the magmatic signal in space and time, and investigate the interrelation with rift-related deformation. A globally significant question we address here is whether magmatism drives continental break-up, or whether even rifting accompanied by abundant magmatism is in response to crustal and lithospheric stretching governed by large-scale plate kinematics. In 2010/11, an amphibious set of wide-angle seismic data was acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The experiments were designed to provide crustal velocity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, constrain the velocity signature of SDR sequences, as well as the extent of magmatic addition to the lower crust near the continent-ocean transition. Sediment cover down to the igneous basement was additionally constrained by reflection seismic data. Here, we present tomographic analysis of the seismic data of one long NNW oriented profile parallel to the continental margin across Walvis Ridge, and a second amphibious profile from the Angola Basin across Walvis Ridge and into the continental interior, crossing the area of the Etendeka Plateau basalts. The most striking feature is the sharp transition in crustal structure and thickness across the northern boundary of Walvis Ridge. Thin oceanic crust (6.5 km) of the Angola Basin lies next to the up to 35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. Both structures are separated by a very large transform fault zone. The velocity structure of Walvis Ridge lower crust is indicative of gabbro, and, in the lowest parts, of cumulate sequences. On the southern side of Walvis Ridge there is a smooth gradation into the adjacent 25-30 km thick crust underlying the ocean-continent boundary, with a velocity structure resembling that of Walvis Ridge The second profile shows a sharp transition from oceanic to rifted continental crust. The transition zone may be underlain by hydrated uppermost mantle. Below the Etendeka Plateau, an extensive high-velocity body, likely representing gabbros and their cumulates at the base of the crust, indicates magmatic underplating. We summarize by stating that rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magmatic-dominated segment to the south, and an amagmatic segment north of Walvis Ridge.

Behrmann, Jan H.; Planert, Lars; Jokat, Wilfried; Ryberg, Trond; Bialas, Jörg; Jegen, Marion

2013-04-01

303

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

NASA Astrophysics Data System (ADS)

HyFlux - Part I: Regional modeling of methane flux from near-seafloor gas hydrate deposits on continental margins MacDonald, I.R., Asper, V., Garcia, O., Kastner, M., Leifer, I., Naehr, T.H., Solomon, E., Yvon-Lewis, S., and Zimmer, B. The Dept. of Energy National Energy Technology Laboratory (DOE/NETL) has recently awarded a project entitled HyFlux: "Remote sensing and sea-truth measurements of methane flux to the atmosphere." The project will address this problem with a combined effort of satellite remote sensing and data collection at proven sites in the Gulf of Mexico where gas hydrate releases gas to the water column. Submarine gas hydrate is a large pool of greenhouse gas that may interact with the atmosphere over geologic time to affect climate cycles. In the near term, the magnitude of methane reaching the atmosphere from gas hydrate on continental margins is poorly known because 1) gas hydrate is exposed to metastable oceanic conditions in shallow, dispersed deposits that are poorly imaged by standard geophysical techniques and 2) the consumption of methane in marine sediments and in the water column is subject to uncertainty. The northern GOM is a prolific hydrocarbon province where rapid migration of oil, gases, and brines from deep subsurface petroleum reservoirs occurs through faults generated by salt tectonics. Focused expulsion of hydrocarbons is manifested at the seafloor by gas vents, gas hydrates, oil seeps, chemosynthetic biological communities, and mud volcanoes. Where hydrocarbon seeps occur in depths below the hydrate stability zone (~500m), rapid flux of gas will feed shallow deposits of gas hydrate that potentially interact with water column temperature changes; oil released from seeps forms sea-surface features that can be detected in remote-sensing images. The regional phase of the project will quantify verifiable sources of methane (and oil) the Gulf of Mexico continental margin and selected margins (e.g. Pakistan Margin, South China Sea, and West Africa Margin) world-wide by using the substantial archive of satellite synthetic aperture radar (SAR) images. An automated system for satellite image interpretation will make it possible to process hundreds of SAR images to increase the geographic and temporal coverage. Field programs will quantify the flux and fate of hydrate methane in sediments and the water column.

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

2008-12-01

304

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

NASA Astrophysics Data System (ADS)

New multichannel seismic reflection data were collected over a 565 km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350 km of the profile: (1) continental crust; (2) transitional basement and (3) oceanic crust. Continental crust thins over a wide zone (~160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastwards beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landwards by a basement high that may consist of serpentinized peridotite and seawards by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landwards of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ~138 Ma (Valanginian) in the south (southern Newfoundland Basin) to ~125 Ma (Barremian-Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.

Lau, K. W. Helen; Louden, Keith E.; Deemer, Sharon; Hall, Jeremy; Hopper, John R.; Tucholke, Brian E.; Holbrook, W. Steven; Christian Larsen, Hans

2006-10-01

305

Melt initiation and mantle exhumation at the Iberian rifted margin: Comparison of pure-shear and upwelling-divergent flow models of continental breakup  

NASA Astrophysics Data System (ADS)

Observations of a wide (up to 170 km) zone of exhumed continental mantle on the Iberian non-volcanic rifted margin have questioned our understanding of the processes involved in continental breakup and seafloor spreading initiation. Models of continental lithosphere thinning by pure-shear predict melt generation before continental breakup, and thus do not predict the exhumation of mantle. Whilst the paucity of volcanism during breakup on the Iberian margin may be explained by invoking a cooler or depleted mantle, or by poor melt extraction, other lithosphere scale processes may play an important role during continental breakup. We compare melt production predicted by pure-shear models of continental lithosphere thinning to that predicted by an upwelling-divergent flow model, using kinematic constraints appropriate to the Iberian margin. The upwelling-divergent flow model predicts exhumation of a more than 50 km-wide zone of continental mantle prior to melt initiation, and we suggest that this mode of lithosphere deformation may play an important role in the formation of rifted margins.

Fletcher, Rosie; Kusznir, Nick; Cheadle, Mike

2009-05-01

306

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

NASA Astrophysics Data System (ADS)

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

Bradley, D. C.

2013-12-01

307

Very-High-Pressure Metamorphism in the Western Alps: Implications for Subduction of Continental Crust  

NASA Astrophysics Data System (ADS)

The widespread occurrence of coesite and pure pyrope, the presence of several new mineral assemblages and new rock-forming minerals in common lithologies, but also crystal--chemical features such as the existence of silicate--phosphate solid solutions or the repeated occurrence among the new minerals of a new dense structure with face-sharing octahedra, are evidence that unusual pressures have been attained in part of the Dora Maira massif, western Alps, during Alpine regional metamorphism. Mineral assemblages suggest minimum pressures well in excess of 25 kbar (1 bar = 105 Pa) which, according to preliminary experimental data, may have reached 35 kbar in this at least 5 × 10 km2 continental terrain clearly of supra-crustal origin. Obviously even continental material may be buried to depths of the order 100 km and then uplifted to the surface. Whereas the burial in a low temperature regime is readily explained by subduction of the continental lithosphere (of a peninsula?) 110 Ma ago or earlier, the uplift is much more problematic because it is petrologically constrained to proceed without significant temperature increase. The progressive migration 'in-plate' of intra-continental thrusts once subduction was blocked, accounts for the stepwise decrease in age and grade of high-pressure metamorphism toward the external part of the chain; by the repeated underthrusting of cold material it might also have prevented a temperature increase in the most internal, early subducted zones. The widespread occurrence of coesite and pure pyrope, the presence of several new mineral assemblages and rock-forming minerals in common lithologies, but also more subtle crystal- chemical details such as the existence of silicate-phosphate solid solutions or the repeated occurrence among the new minerals of a dense, otherwise unknown structure, constitute compelling evidence that the metamorphic terrain considered must have endured exceptionally high pressures for continental crust. The mineral assemblages encountered suggest minimum pressures that are in excess of 25 kbar and may reach 35 kbar, according to preliminary experimental work. We therefore cannot escape the issue that a continental terrain may be buried to depths of the order of 100 km and then uplifted to reach the surface. Since this is a new result and often considered to be at variance with concepts prevailing in geodynamics, an attempt must be made at reconstructing the successive stages of this evolution. In the following, before envisaging consequences, we will shortly consider the few constraints imposed by the metamorphic records and geochromological data on the conditions and timing of burial and uplift.

Chopin, C.

1987-01-01

308

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

SciTech Connect

Seismic stations at islands and continental margins will be an essential component of the International Monitoring System (IMS) for event location and identification in support of Comprehensive Nuclear-Test-Ban Treaty (CTBT) monitoring. Particularly important will be the detection and analysis of hydroacoustic-to-seismic converted waves (T-phases) at island or continental margins. Acoustic waves generated by sources in or near the ocean propagate for long distances very efficiently due to the ocean sound speed channel (SOFAR) and low attenuation. When ocean propagating acoustic waves strike an island or continental margin they are converted to seismic (elastic) waves. We are using a finite difference code to model the conversion of hydroacoustic T-waves at an island or continental margin. Although ray-based methods are far more efficient for modeling long-range (> 1000 km) high-frequency hydroacoustic propagation, the finite difference method has the advantage of being able to model both acoustic and elastic wave propagation for a broad range of frequencies. The method allows us to perform simulations of T-phases to relatively high frequencies ({>=}10 Hz). Of particular interest is to identify factors that affect the efficiency of T-phase conversion, such as the topographic slope and roughness at the conversion point and elastic velocity structure within the island or continent. Previous studies have shown that efficient T-phase conversion occurs when the topographic slope at the conversion point is steep (Cansi and Bethoux, 1985; Talandier and Okal, 1998). Another factor impacting T-phase conversion may be the near-shore structure of the sound channel. It is well known that the depth to the sound channel axis decreases in shallow waters. This can weaken the channeled hydroacoustic wave. Elastic velocity structure within the island or continent will impact how the converted seismic wave is refracted to recording stations at the surface and thus impact the T-phase amplitudes. For this paper we will focus on validating the finite difference method for modeling T-phases in the ocean and land environments and on modeling T-phases observed by the May 1999 Ascension Island Experiment. A network of broadband seismometers on Ascension Island recorded a large number of offshore airgun shots. The shots occurred at all azimuths around the island and at ranges from 1-45 km. Measurements of signal amplitude and duration will be made to understand the variability of T-phase behavior on Ascension Island. The sensitivity to topographic slope and island structure will also be investigated.

Harben, P.; Rodgers, A.

1999-07-26

309

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

310

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

NASA Astrophysics Data System (ADS)

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

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

2014-01-01

311

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

USGS Publications Warehouse

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

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

2014-01-01

312

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

313

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

314

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

315

Crustal structure and evolution of the mid-Norwegian continental margin in a conjugate setting  

Microsoft Academic Search

The database on the extensively studied volcanic margin off mid-Norway comprises a regional grid of deep wide-angle seismic data (OBS and ESP), deep and conventional multichannel seismic (MCS) reflection profiles, potential field data, heat flow, and scientific and commercial boreholes. Off Greenland, deep seismic data only exist from the fjords and on the outer margin south of 76 degrees N,

J. I. Faleide; F. Tsikalas; O. Eldholm; R. Mjelde; A. J. Breivik; T. Raum

2003-01-01

316

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

NASA Astrophysics Data System (ADS)

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