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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

Gas Hydrate Volume Estimations in the Western Continental Margin of India  

Microsoft Academic Search

Summary Multi-channel seismic data acquired on the Western Continental margin of India show some instances of promising BSR's. There are some localized occurrences, which have been established by the presence of BSR and some AVO studies that were carried out in those regions. The paucity of well-log data inhibits the computation of the volume estimates of these gas hydrate reserves.

N. Satyavani; D. Ramesh Kumar; N. K. Thakur

3

Late Cenozoic evolution of the western Barents Sea-Svalbard continental margin  

Microsoft Academic Search

Seven regionally correlatable reflectors, named R7 (oldest) to R1, have been identified in the Upper Cenozoic sedimentary succession along the western continental margin of Svalbard and the Barents Sea. Regional seismic profiles have been used to correlate between submarine fans that comprise major depocentres in this region. Glacial sediment thicknesses reach up to 3 seconds two-way time, corresponding to 3.5–4

Jan Inge Faleide; Anders Solheim; Anne Fiedler; Berit O. Hjelstuen; Espen S. Andersen; Kris Vanneste

1996-01-01

4

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

Microsoft Academic Search

The Mesozoic tectonic history of the western U.S. Cordillera records evidence for multiple episodes of accretionary and collisional orogenic events and orogen-parallel strike-slip faulting. Paleozoic-Jurassic volcanic arc complexes and subduction zone assemblages extending from Mexico to Canada represent an East-Pacific magmatic arc system and an accretionary-type orogen evolved along the North American continental margin. Discontinuous exposures of Paleozoic upper mantle

Y. Dilek

2001-01-01

5

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

NASA Astrophysics Data System (ADS)

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

Dilek, Y.

2001-12-01

6

NOTES ON DECAPOD AND EUPHAUSIID CRUSTACEANS, CONTINENTAL MARGIN, WESTERN ATLANTIC, GEORGES BANK  

E-print Network

and 1 species ofeuphausiid are reported from the outer continental shelf. submarine canyons. and nearby. transtridensl. Records of decapod crustaceans from the outer continental shelf, submarine canyons, and nearby continental shelf in the western At- lantic were reviewed by Squires (1965), Williams and Wigley (1977

7

Continent-ocean transition at the western Barents Sea/Svalbard continental margin  

SciTech Connect

The change in crustal type at the western Barents Sea/Svalbard margin takes place over a narrow zone related to primary rift and shear structures reflecting the stepwise opening of the Greenland Sea. Regionally, the margin is composed of two large shear zones and a central rifted-margin segment. Local transtension and transpression at the plate boundary caused the early Cenozoic tectonism in Svalbard and the western Barents Sea, and might explain the prominent marginal gravity and velocity anomalies.

Eldholm, O.; Faleide, J.I.; Myhre, A.M.

1987-12-01

8

Continent-ocean transition at the western Barents Sea\\/Svalbard continental margin  

Microsoft Academic Search

The change in crustal type at the western Barents Sea\\/Svalbard margin takes place over a narrow zone related to primary rift and shear structures reflecting the stepwise opening of the Greenland Sea. Regionally, the margin is composed of two large shear zones and a central rifted-margin segment. Local transtension and transpression at the plate boundary caused the early Cenozoic tectonism

Olav Eldholm; Jan Inge Faleide; Annik M. Myhre

1987-01-01

9

Major Ocean Features: Continental Margin  

E-print Network

of ocean earth sciences. It starts with features of the continental shelf associated with passive margins of continental margins, called the continental shelf. Continental shelves of the world vary greatly in places. The shelf, slope and rise together make up the entire continental margin. Many continental

10

Sedimentation Processes Along the Continental Margin of the Western Bellingshausen Sea, West Antarctica  

Microsoft Academic Search

Since the beginning of glaciation of West Antarctica, changes of glacial and interglacial periods affect the sediment supply across the shelf and onto the deep sea. Along the slope and rise of the continental margin of the Antarctic Peninsula, Cenozoic sediments were deposited which development was influenced by ice sheet fluctuation and mass transport processes. Thus, both the sediment stratigraphy

K. Gohl; C. Scheuer

2003-01-01

11

Late quaternary sedimentation and glacial history of the western Svalbard continental margin  

Microsoft Academic Search

Glacigenic sediments recovered in shallow cores from the western Svalbard continental slope, are subdivided into five facies associations based on grain-size, sedimentary structure, mineralogy, petrography and geochemistry. Two diamicton facies are recognised, one of which is interpreted as hemipelagic mud with variable amounts of ice-rafted debris (IRD), and the other as a product of mass-movement. The diamictons are associated with

Espen S. Andersen; Trond M. Dokken; Anders Elverhøi; Anders Solheim; Ingrid Fossen

1996-01-01

12

Late Miocene sedimentary architecture of the Ebro Continental Margin (Western Mediterranean): Implications to the Messinian Salinity Crisis  

NASA Astrophysics Data System (ADS)

The Messinian Salinity Crisis (MSC) resulted from a significant multi-phase drop and subsequent reflooding of the Mediterranean Sea during the Late Miocene. In a relatively short time span (5.96 to 5.33 Ma), partial desiccation of the basin and consequent subaerial exposure of the continental margins resulted in widespread erosion of continental shelves and slopes and regressive erosion along major fluvial valleys. Using 3D seismic reflection data from the Ebro Margin (Western Mediterranean), we provide new insights into the origin of the Messinian Erosional Surface (MES) and timing of the capture of the subaerial Ebro Basin. The observed sedimentary architecture of the Ebro Continental Margin indicates a sedimentary-active continental slope and delta progradation during Middle-Late Miocene, in a normal regressive context associated to a pre-Messinian proto-Ebro River. Configuration of the clinoforms below the MES suggests that deltaic sediments of the Messinian Paleo-Ebro River deposited during the Tortonian and initial Messinian sea-level drawdown. The MES formed at the top of the Tortonian Highstand, where a fluvial network was deeply carved, and in the topset region of the Messinian Falling Stage Systems Tract, where minor erosion occurred. The patterns of Messinian erosion and sedimentation produced a MES with a step-like profile. Significant Miocene progradation and the mature development attained by the Messinian Ebro River network during the MSC indicate that capture of the Ebro Basin occurred prior to the MSC. Fluvial deposits are outstandingly preserved on the main valleys of the MES indicating that re-flooding of the margin was extremely rapid. Therefore, the step-like profile of the MES was created during the latest stages of the main Messinian sea-level drawdown and lowstand.

Cameselle, Alejandra L.; Urgeles, Roger; De Mol, Ben; Camerlenghi, Angelo

2014-05-01

13

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

14

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

15

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

16

Gravity anomalies, crustal structure and rift tectonics at the Konkan and Kerala basins, western continental margin of India  

NASA Astrophysics Data System (ADS)

Litho-stratigraphic variation of sedimentary units constructed from seismic sections and gravity anomaly in the Konkan and Kerala basins of the western continental margin of India (WCMI) have been used to model processes such as lithospheric rifting mechanism, its strength, and evolution of flank uplift topography that led to the present-day Western Ghats escarpment. Based on the process-oriented approach, two lithospheric models (necking and magmatic underplating) of evolution of the margin were tested. Both, necking and underplating models suggest an effective elastic thickness (Te) of 5 km and 10 km along Konkan and Kerala basins, respectively and a deep level of necking at 20 km at both basins. Model study suggests that the necking model better explains the observed gravity anomalies in the southern part of the WCMI. A synthesis of these results along with the previously published elastic thickness estimates along the WCMI suggests that a low-to-intermediate strength lithosphere and a deeper level of necking explains the observed flank-uplift topography of the Western Ghats. Process-oriented gravity modelling further suggests that the lateral variations in the lithospheric strength, though not very significant, exist from north to south within a distance of 600 km in the Konkan and Kerala basins along the WCMI at the time of rifting. A comparison with previous Te estimates from coherence analysis along the WCMI indicates that the lithospheric strength did not change appreciably since the time of rifting and it is low both onshore and offshore having a range of 5-15 km.

Dev, Sheena V.; Radhakrishna, M.; Chand, Shyam; Subrahmanyam, C.

2012-06-01

17

Late Miocene sedimentary architecture of the Ebro Continental Margin (Western Mediterranean): implications to the Messinian Salinity Crisis  

NASA Astrophysics Data System (ADS)

The Messinian Salinity Crisis (MSC) resulted from a significant multi-phase drop and subsequent reflooding of the Mediterranean Sea from 5.96 to 5.33 Ma. Well-developed drainage networks, characterized by step-like profiles and abrasion platforms, are associated to this event. The Ebro Continental Margin (Western Mediterranean) presents an additional complexity since the capture of the drainage of the adjacent subaerial Ebro Basin took place sometime prior to the Messinian stage. Using 3D seismic reflection data, this work provides new insights into the origin of the step-like profile of the Messinian erosional surface (MES) and timing of the capture of the subaerial Ebro Basin. The results obtained indicate a sedimentary-active continental slope and delta progradation during Middle-Late Miocene, in a normal regressive context associated to a pre-Messinian proto-Ebro River. The mature development attained by the Messinian Ebro River network during the MSC corroborates that the capture of the Ebro Basin occurred prior to the MSC. The configuration of the clinoforms below the MES suggests that deltaic sediments of the Messinian Paleo-Ebro River deposited during the Tortonian and initial Messinian sea-level drawdown. The MES formed at the top of the Tortonian Highstand, where a fluvial network was deeply carved, and in the topset region of the Messinian Falling Stage Systems Tract, where minor erosion occurred. Fluvial deposits are outstandingly preserved on the main valleys of the MES. Therefore, the step-like profile of the MES was not created during Zanclean inundation, but during the latest stages of the main Messinian sea-level fall and lowstand.

Cameselle, Alejandra L.; Urgeles, Roger; De Mol, Ben; Camerlenghi, Angelo; Canning, Jason C.

2014-03-01

18

The marginal stability of continental margins  

NASA Astrophysics Data System (ADS)

Continental lithosphere is far thicker than its oceanic counterpart. The boundary between continent and ocean marks a transition in lithospheric thermal and chemical properties. This change in lithosphere structure can potentially nucleate downward travelling Rayleigh-Taylor instabilities that have the potential to generate melt, influence margin topography and sedimentary basin development. At the same time there exists a horizontal pressure gradient at this boundary where the continent may spread laterally. This lateral extension if slow may be responsible for large scale continental tilting and the initiation of intra-cratonic basin formation. It may also lead to the eventual initiation of subduction and the transition from a passive to active margin. The question is, does continental lithosphere spread laterally providing prolonged post-rift stretching, or is it destabilised downwards producing dynamic topography that may be transient, or both? We will address this key question by combining numerical and laboratory fluid mechanic experiments. The nature of continental lithosphere destabilisation is a function of its rheology, buoyancy and temperature. Early results indicate that for both Newtonian and non-Newtonian rheologies the style of destabilisation of continental lithosphere is primarily dependent on its buoyancy. For more buoyant continents there is a greater tendency for horizontal extension, while less buoyant lithosphere destabilises through vertical drips. The implication of these results is that margins between cratonic and oceanic lithosphere may tend to extend, producing very broad ramp-like subsidence down to the ocean, as observed in the North American continent. In contrast, the younger continental platform may tend to contain sag like sedimentary basins from a dynamic response to the vertical instabilities, as typified by the basin and swell nature of the African margins. It is also possible that these two modes may operate at different times during the secular evolution of a piece of continental lithosphere.

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

2011-12-01

19

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

Microsoft Academic Search

The nature of the crust in the Laxmi Basin, western margin of India, is an uncertain issue; more importantly, this has implications on paleogeographic reconstructions of the western Indian Ocean. We have analyzed three geophysical data sets and modeled gravity and magnetic anomalies for determining nature of the crust. Basement of the Laxmi Basin includes numerous highs, which make the

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

2006-01-01

20

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

21

East Africa continental margins  

SciTech Connect

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

Bosellini, A.

1986-01-01

22

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

USGS Publications Warehouse

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

Nelson, C.H.; Maldonado, A.

1990-01-01

23

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

24

Barium as a productivity proxy in continental margin sediments: a study from the eastern Arabian Sea  

Microsoft Academic Search

Barium, aluminum, cadmium and uranium were analyzed in 112 surface sediment samples from the western continental margin of India. Excess Ba (Baxs) concentrations are high at the SW margin of India, an area of high productivity and lower at the central western continental margin, an area of lower productivity. High organic carbon export coupled with high sedimentation rates and a

C. Prakash Babu; H.-J. Brumsack; B. Schnetger; M. E. Böttcher

2002-01-01

25

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

26

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

27

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

28

Gas hydrate on the continental margin  

Microsoft Academic Search

Because the stability of gas hydrate is constrained mainly by the temperature and pressure condition, gas hydrate on the continental margins had dissociated in response to the global climate changes during the earth's history. Marine gas hydrate hosts isotopically light carbon as gas (mostly methane) and heavy oxygen as water, dissociation of gas hydrate liberates these isotopes into the environment,

Hitoshi Tomaru

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

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

E-print Network

The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview MICHEL St (e-mail." seranne@dstu, univ-montp2.fr) Abstract: The Gulf of Lion margin is one of the Tertiary and the geodynamic setting of the margin within the Western Mediterranean. IBS-Gulf of Lion research was based

Demouchy, Sylvie

31

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

32

Marine geology and tectonic evolution of the northwestern margin of the California continental borderland  

Microsoft Academic Search

The geology and history of the northwestern margin of the California continental borderland are examined using data from seismic-reflection profiling and bottom sampling. It is suggested that the northwestern margin of the borderland along with the western Transverse Ranges was rifted from an original tectonic setting off northern Baja California begining about 18 m.y. ago. During the rifting event, the

J. K. Crouch

1979-01-01

33

Southeast Australia: A Cenozoic Continental Margin Dominated by Mass Transport  

E-print Network

to the Great Barrier Reef. Mass transport dominates the continental slope, which stretches from the shelf break continental margin (Boyd et al. 2004) stretches 1,500km north from Bass Strait (37°30 S) to the Great Barrier Reef (24° 15 S). This passive rifted margin fronts the Tasman Sea, and by world standards is nar- row

New Hampshire, University of

34

Structure of the North American Atlantic Continental Margin.  

ERIC Educational Resources Information Center

Offers explanations on the origin of the North American Atlantic continental margin. Provides an analysis and illustrations of structural and strategraphic elements of cross sections of the Atlantic continental margin. Also explains the operations and applications of seismic-relection profiles in studying ocean areas. (ML)

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

1986-01-01

35

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

36

Geotechnical characterization of sediments from Hydrate Ridge, Cascadia Continental Margin  

E-print Network

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

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

2004-01-01

37

Marginal stability of thick continental lithosphere E. Cottrell  

E-print Network

Marginal stability of thick continental lithosphere E. Cottrell Lamont-Doherty Earth Observatory a hotter but otherwise denser layer, analogous to continental mantle lithosphere over asthenosphere, of the lithosphere-like layer. Sufficient cooling at low buoyancy number results in an oscillatory convective

Mojzsis, Stephen J.

38

Ecological theory and continental margins: where shallow meets deep  

E-print Network

adjacent to the heavily exploited and trafficked coastal zone, the continental margins (100­4000 m depth a major sink for anthropogenically gener- ated carbon dioxide. Thus, climate remediation schemes increasing human dependence on the margins and accelerating levels of human-generated disturbance (Box 1

Levin, Lisa

39

Writing a Rosetta stone: insights into continental-margin sedimentary processes and strata  

E-print Network

and strata in time and space. Keywords Continental margin, continental shelf, continental slopeWriting a Rosetta stone: insights into continental-margin sedimentary processes and strata CHARLES, Charlottesville, VA 22904, USA ABSTRACT Continental margins are valuable for many reasons, including the rich

Lin, Andrew Tien-Shun

40

Subduction-driven recycling of continental margin lithosphere.  

PubMed

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

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

2014-11-13

41

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

E-print Network

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

Demouchy, Sylvie

42

Structure of the North American Atlantic Continental Margin.  

USGS Publications Warehouse

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

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

1986-01-01

43

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

44

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

E-print Network

. They reach 15 km in thickness off Angola and extend hundreds of kilometres across the continent-ocean-rift evolution of the margin and of adjacent continental Africa. Following hydrocarbon discoveries in the second

Paris-Sud XI, Université de

45

On isostasy at Atlantic-type continental margins  

NASA Technical Reports Server (NTRS)

The concept of isostasy describes the manner in which topographic features on the earth's surface are compensated at depth. The present investigation is concerned with the isostatic mechanism at Atlantic-type continental margins. Particular attention is given to the question whether the flexure model of isostasy, which has successfully been used at other geological features in oceans, is applicable at margins. Cross-spectral techniques are used to analyze the relationship between free air gravity and topography at Atlantic-type continental margins. The relatively old eastern North America is found to be associated with the highest value of the effective elastic thickness in the range 10-20 km, while the relatively young Coral Sea/Lord Howe rise is associated with the lowest value of less than 5 km. The differences in estimates of effective elastic thickness between margins can be explained by a simple model in which the flexural strength of the basement increases with age.

Karner, G. D.; Watts, A. B.

1982-01-01

46

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

47

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

SciTech Connect

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

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

2008-01-15

48

Ecological theory and continental margins: where shallow meets deep.  

PubMed

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 climate change. Continental margin ecosystems, with environments, constituents and processes that differ from those in shallow water, demand a new focus, in which ecological theory and experimental methods are brought to bear on management and conservation practices. Concepts of disturbance, diversity-function relationships, top-down versus bottom-up control, facilitation and meta-dynamics offer a framework for studying fundamental processes and understanding future change. PMID:19692143

Levin, Lisa A; Dayton, Paul K

2009-11-01

49

Cenozoic evolution of the Antarctic Peninsula continental margin  

SciTech Connect

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

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

1990-05-01

50

New continental margin magnetic anomalies of East Antarctica  

NASA Astrophysics Data System (ADS)

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

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

2013-02-01

51

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

52

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

53

Rifted continental margins: The case for depth-dependent extension  

NASA Astrophysics Data System (ADS)

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

Huismans, Ritske S.; Beaumont, Christopher

2014-12-01

54

Paleogeography and evolution of the Ordovician/Silurian (Whiterockian-Llandoverian) continental margin in central Nevada  

SciTech Connect

In central Nevada, stratigraphic successions of Whiterockian-Llandoverian lithofacies, transitional with autochthonous platform/shelf carbonates to the east, occur in isolated windows in outer slope to basinal lithotopes of the Roberts Mountains allochthon. Petrologic, chronostratigraphic and lithostratigraphic, and paleontologic comparison of those successions with platform/shelf facies to the east is integral for reconstruction of Ordovician-Silurian platform margin paleogeography and pre-Antler genesis of the western North American continental margin. Numerous facies changes and/or stratigraphic omissions in central Nevada can be related to sea level fluctuation and aggradation/progradation of the carbonate platform to the east, and not to a postulated, offshore geanticline (i.e., the Toiyabe Ridge). Stratigraphic omission of the Eureka Quartzite above Pogonip equivalents in transitional successions of the Toquima Range and the presence of correlative quartzite in outer slope/basinal parautochthonous facies of the Toiyabe Range suggest development of a possible bypass-margin during the Middle Ordovician. Deposition of Late Ordovician platform margin dolostones (Ely Springs Dolostone) and upper ramp limestones (Hanson Creek Formation and Martin Ridge strata) followed Late Ordovician transgression that drowned the margin and reestablished the carbonate factory. Glacioeustatic drawdown of Late Ordovician-earliest Silurian seas due to the Gondwanan glacial fluctuation can be recognized in strata along the platform margin and upper ramp. Rapid, Early Silurian transgression produced dark-gray carbonates and may have induced marginal flexure and regional, massive slope failure in central Nevada, generating stratigraphic hiatuses west of the platform margin.

Britt, L.W. (California State Univ., San Bernardino (United States))

1991-02-01

55

Paleogene continental margin truncation in southwestern Mexico: Geochronological evidence  

NASA Astrophysics Data System (ADS)

The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW-SE decreasing granitoid intrusion age trend (˜1.2 cm/yr in the 100 Ma-40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW-SSE trending Cretaceous-Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America-Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (˜7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco-Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40-25 Ma, with a velocity of 6.5-4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America-Farallon-Caribbean triple junction. Geological observations and paleomagnetic data do not give strong support for large-scale right-lateral displacements of crustal blocks like the Baja California. Given the isotopic data presented, the continental margin truncation in SW Mexico seems to be the consequence of an interaction of mechanisms. Of these, we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment. On the other hand, the lateral removal of material associated with the displacement of Chortis is more important in the southeastern segment.

Schaaf, Peter; MoráN-Zenteno, Dante; HernáNdez-Bernal, Maria Del Sol; SolíS-Pichardo, Gabriela; Tolson, Gustavo; KöHler, Hermann

1995-12-01

56

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

57

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

NASA Technical Reports Server (NTRS)

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

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

1985-01-01

58

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

E-print Network

- nian continental shelf as the source of pyrite to the water column. The speciation of suspended marineThe speciation of marine particulate iron adjacent to active and passive continental margins Phoebe and passive continental margins. Chemical- species mapping provides speciation information for heterogeneous

59

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

NASA Astrophysics Data System (ADS)

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

Cowie, Leanne; Kusznir, Nick; Leroy, Sylvie

2013-04-01

60

Predicting Rifted Continental Margin Subsidence History From Satellite Gravity Derived Crustal Thinning: Application to North Atlantic Margins  

NASA Astrophysics Data System (ADS)

3D spectral inversion of satellite derived gravity anomaly data (Smith and Sandwell 1997) and bathymetry data (Gebco 2003) has been used to determine oceanic and continental margin crustal thickness for the North Atlantic between 50 and 70 degrees N. The inverse technique incorporates a correction for the large negative thermal gravity anomaly present in the oceanic and stretched continental lithosphere. This correction can be determined using ocean isochron data for oceanic lithosphere, and margin rift age and beta stretching estimates derived iteratively from crustal basement thickness determined from the gravity inversion for the stretched continental lithosphere. A correction for the gravity anomaly contribution from sediments may be determined using thickness estimates derived from seismic reflection MCS data. Density depth variation within sediments is predicted assuming compaction. Crustal thicknesses determined using a thermal gravity correction derived from ocean isochron data give crustal thicknesses that are consistent with seismic observations. The resulting basement thickness determined from gravity inversion for the thinned continental margin lithosphere may be used to produce estimates of crustal thinning and stretching. Flexural backstripping and reverse post-breakup thermal subsidence modelling may be used to restore present 2D (or 3D) stratigraphic cross sections to earlier post-breakup times. Thermal subsidence arises from the cooling of stretched continental lithosphere and the recently formed oceanic lithosphere, and may be predicted from beta stretching factor (McKenzie 1978) and rift age. Beta stretching factors derived from gravity anomaly inversion have been used to predict reverse thermal subsidence for N Atlantic rifted margins. The resulting palaeo-bathymetric restorations show emergence of the Hatton Bank and NE Faroes rifted margins in early post-breakup times. The predicted palaeo-bathymetries are consistent with palaeo-bathymetry indicators observed in ODP and DSDP wells. Due to errors in the location of Continent-Ocean Boundary within the ocean isochron data set an alternative method may be used to determine the thermal gravity correction. This alternative method ignores the ocean isochron data and the thermal gravity correction is determined using only the rift age specified for continental breakup and lithosphere beta stretching factors derived from crustal basement thickness from gravity inversion. This method has the advantage that it does not assume the location of the COB but has the disadvantage that it fails to predict the increasing thermal gravity correction towards the ocean ridge, and hence overpredicts margin crustal thickness and underpredicts margin beta stretching factors. Failure to include volcanic addition in the gravity inversion for crustal basement thickness results in an overestimate of basement thickness, an underestimate of beta stretching factor and resulting overprediction of palaeo-bathymetry. Sensitivity to lithosphere flexural strength during flexural backstripping has been carried out. This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Schlumberger Cambridge Research and Badley Geoscience, supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, Conoco-Phillips, Shell, Statoil and WesternGeco.

Hurst, N. W.; Kusznir, N. J.; Roberts, A. M.; White, R. S.

2004-05-01

61

Carbonate comparison of west Florida continental margin with margins of eastern United States  

SciTech Connect

Temperate carbonate margins may have as many similarities to clastic margins as to other carbonate systems. An example is the west Florida continental margin north of Florida Bay, a vast area of more than 150,000 km/sup 2/. The facies of this area differ from those of other Holocene carbonates, such as the Bahama Banks, the Great Barrier Reef, and the Caribbean and Pacific bioherms. The west Florida margin is analogous to the predominantly clastic southeastern US in both physiology and sedimentary processes. The shelf facies is a veneer of carbonate sand, primarily molluscan shell fragments, with low sedimentation rates. It is similar to the southeastern US sand veneer with the clastic component removed. Like the US system, the west Florida shelf has a ridge and swale topography replete with sedimentary structures, such as sand waves, with a series of drainage systems incised into its surface at lower stands of sea level. On the outer edge, it is commonly bounded by outcrops with considerable positive relief. The upper slope of the west Florida margin is a calcilutite, a Holocene chalk deposit accumulating at rates of tens of centimeters/1000 years, comparable to the clastic lutite depositional rates of the eastern US continental slope, and two orders of magnitude higher than deep-sea oozes of similar composition. These relatively high rates are probably caused by fines pumped from and across the coarser shelf-sand sheets in both systems.

Doyle, L.J.

1986-05-01

62

Structural evolution, metamorphism and restoration of the Arabian continental margin, Saih Hatat region, Oman Mountains  

Microsoft Academic Search

170 m.y. of relatively stable passive margin sedimentation along the northern continental margin of Arabia was abruptly terminated during the Cenomanian–Turonian (?95 Ma) when the Oman continental margin collapsed and subsided rapidly (Aruma basin) to accommodate obduction of the Semail ophiolite complex and underlying thrust sheets (Haybi and Hawasina complexes) in the Oman mountains. The ophiolite was emplaced at least

M. P Searle; C. J Warren; D. J Waters; R. R Parrish

2004-01-01

63

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

64

Ancient continental margin assemblage in the northern Coast Mountains, southeast Alaska and northwest Canada  

SciTech Connect

Geologic relations indicate that quartz-rich metasedimentary rocks in the northern Coast Mountains separate strata to the east that belong to the Stikine terrane from strata to the west of the Alexander, Wrangellia, and Taku terranes. The quartz-rich rocks structurally overlie western terranes along a mid-Cretaceous thrust fault and are overlain structurally (originally stratigraphically ) by strata of the Stikine terrace. These rocks are interpreted to be a continental margin assemblage that belongs to the Yukon Crystalline terrane. U-Pb and Nd isotopic data indicate that the metasedimentary rocks were shed from a source terrane consisting at least in part of Proterozoic rocks.

Gehrels, G.E.; McClelland, W.C.; Samson, S.D.; Pathcett, P.J.; Jackson, J.L. (Univ. of Arizona, Tucson (USA))

1990-03-01

65

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

66

Development of a Regional Seafloor Surficial Geologic (Habitat)Development of a Regional Seafloor Surficial Geologic (Habitat) Map for the Continental Margin of OregonMap for the Continental Margin of Oregon CChris Romsos, Chris Goldfinger, Rondi Robison,  

E-print Network

Macro Habitat EFH Composite Attribute Code* Continental Shelf Rocky Shelf Rocky Shelf She Sedimentary Surficial Geologic (Habitat) Map for the Continental Margin of OregonMap for the Continental Margin habitats and their surficial lithologies (Figure 1) along the continental margins of Washington and Oregon

Goldfinger, Chris

67

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

68

Effects of Trawling on Sedimentation of Continental Margins  

NASA Astrophysics Data System (ADS)

Human activities have become relevant control factors of sediment transfer in continental margins during the last decades. In the framework of several projects, different findings demonstrate that trawling has been one of the most relevant human-induced factors affecting the proprieties of the seabed during the last decades, since this activity increased dramatically. The direct physical effects of trawling include scraping and ploughing of the seabed and sediment resuspension. In mud prodeltas fishing grounds, it has been observed that trawling increases near-bottom turbidity (feeding constantly the bottom nepheloid layer), winnowing part of the finer sediment fraction and causing a slight coarsening upwards trend and an increase of organic matter content in the upper centimeters of the sediment column. On the continental slopes, it has been observed that trawling in canyon walls can trigger sediment gravity flows and increase sediment accumulation rates in deep canyon depositional areas. Thus, trawling is transferring sediment, changing the seabed and altering the habitat of marine organisms, and must be considered as a dynamic process whose effects should be taken into account in addition to natural processes.

Palanques, A.; Martín, J.; Puig, P.; Guillén, J.; Demestre, M.

2006-12-01

69

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

SciTech Connect

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

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

1988-12-31

70

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

SciTech Connect

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

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

1988-01-01

71

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

72

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

73

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

74

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

E-print Network

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

75

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

E-print Network

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

Bishop, James K.B.

76

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

E-print Network

and Fe from redox-mobilized labile sources at the continental shelf supplemented by a more variable #12;AGU Index Terms: 4875, Trace elements; 3002, Continental shelf and slope processes; 4808, ChemicalThe Continental Margin is a Key Source of Iron to the HNLC North Pacific Ocean Phoebe J. Lam1

77

The continental shelf and upper slope of the Oregon Cascadia margin are underlain by an  

E-print Network

ABSTRACT The continental shelf and upper slope of the Oregon Cascadia margin are underlain fans, unconformity. INTRODUCTION The Cascadia continental shelf is underlain by a thick sedimentary in seismic reflection profiles across the Oregon continental shelf; how- ever, it is less continuously

Goldfinger, Chris

78

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

79

Continental margin molybdenum isotope signatures from the early Eocene  

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

80

Recent carbonate slope development on southwest Florida continental margin  

SciTech Connect

The southwest Florida continental slope bordering the Florida Strait contains a thick sequence of seaward-prograding sediments. Sediments consist principally of a mixture of shallow water and pelagic carbonate sands and muds deposited rapidly on the upper slope. Sedimentary patterns are interpreted to be a function of high-frequency sea level fluctuations. Most vigorous off-shelf transport and highest sedimentation rates (exceeding 2.5 m/1000 years) occur during early transgressions and late regressions when water depths on the shelf are shallow. During sea level highstands, off-shelf transport is less vigorous and sedimentation rates decrease. During sea level lowstands, no off-shelf transport takes place and erosion of the previously deposited sequence occurs as a result of an increase in erosional capacity of the Florida Current. The presence of at least nine such sequences, all with similar characteristics, indicates that these processes have been occurring since at least the late Pleistocene in response to high-frequency glacial fluctuations. The location of the southwest Florida slope between the rimmed Bahama platform and the nonrimmed remainder of the west Florida margin, as well as similarities with ancient carbonate slope deposits formed during periods when shelf-edge reef-forming organisms were lacking, suggest that depositional patterns on the southwest Florida slope may be indicative of a transition between rimmed and nonrimmed carbonate platform environments. The southwest Florida slope may provide a valuable modern analog for identifying similar transitional environments in the geologic record.

Brooks, G.R.; Holmes, C.W.

1987-05-01

81

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

82

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

PubMed Central

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

Kirby, Michael Xavier

2004-01-01

83

Convective Removal of Continental Margin Lithosphere at the Edges of Subducting Oceanic Plates  

NASA Astrophysics Data System (ADS)

Although oceanic lithosphere is continuously recycled to the deeper mantle by subduction, the rates and manner in which different types of continental lithospheric mantle are recycled is unclear. Cratonic mantle can be chemically reworked and essentially decratonized, although the frequency of decratonization is unclear. Lithospheric mantle under or adjacent to orogenic belts can be lost to the deeper mantle by convective downwellings and delamination phenomena. Here we describe how subduction related processes at the edges of oceanic plates adjacent to passive continental margins removes the mantle lithosphere from beneath the margin and from the continental interior. This appears to be a widespread means of recycling non-cratonic continental mantle. Lithospheric removal requires the edge of a subducting oceanic plate to be at a relatively high angle to an adjacent passive continental margin. From Rayleigh wave and body wave tomography, and receiver function images from the BOLIVAR and PICASSO experiments, we infer large-scale removal of continental margin lithospheric mantle from beneath 1) the northern South American plate margin due to Atlantic subduction, and 2) the Iberian and North African margins due to Alboran plate subduction. In both cases lithospheric mantle appears to have been removed several hundred kilometers inland from the subduction zones. This type of ';plate-edge' tectonics either accompanies or pre-conditions continental margins for orogenic activity by thinning and weakening the lithosphere. These processes show the importance of relatively small convective structures, i.e. small subducting plates, in formation of orogenic belts.

Levander, A.; Bezada, M. J.; Palomeras, I.; Masy, J.; Humphreys, E.; Niu, F.

2013-12-01

84

Subduction and exhumation of continental crust in the Western Alps - A comparison of field-based and dynamic models  

Microsoft Academic Search

For more than a century, the Alps have served as a crucible for orogenic models. Most of these models, including all dynamic models so far, have assumed 2D subduction of the European lithosphere beneath the Apulian continental margin. We use the example of the Sesia Zone in the Western Alps to dispel misconceptions about Alpine subduction, and to compare assumptions

M. R. Handy; J. Babist; M. Konrad

2005-01-01

85

Distribution of oil and gas on active continental margins  

SciTech Connect

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

Barker, C.; Spyro, T.

1984-04-01

86

New Low-Temperature Thermochronology Reveals Contrasting Modes of Continental Extension Across the Sonoran Rifted Margin  

NASA Astrophysics Data System (ADS)

The Sonoran rifted margin extends 250 km from the western flanks of the Sierra Madre Occidental to the Gulf of California and contains a classic Basin and Range morphology that indicates "broad-rift" mode of continental extension. However, new low-temperature thermochronology reveals that the Sonoran rifted margin is also internally composed of at least two temporally and spatially distinct belts that display other distinct styles of extension. Mountain ranges that lie within a narrow belt (20 km wide) along the coast of the Gulf of California between Puerto Libertad and Bahia Kino yield highly discordant apatite fission track (AFT) ages that range from 5 to 54 Ma and likely reflect the strong tilting of these tectonic blocks. The widespread occurrence of AFT ages between 5 and 7 Ma, which are typically found in the deepest crustal levels of the tilt blocks, and the presence of Quaternary scarps indicate that extension in the coastal region largely occurred from late Miocene to recent times. We infer that this belt is dominated by a "narrow-rift" mode of extension where deformation has been focused to produce the Gulf depression. Well inland from the coast (175 km east) is a belt of metamorphic core complexes that extends more than 200 km from Magdalena to Mazatan and typically yields older and more concordant AFT ages from 14 to 23 Ma. However, the presence of ages as young as 8 to 11 Ma indicate that the "metamorphic-core-complex" mode of extension in this belt likely overlapped in time with the "narrow-rift" mode of extension in the Gulf of California. We conclude that the juxtaposition of major deformation belts each with different modes of continental extension reflects the diverse processes that have affected the Sonoran margin through time.

Kohn, B. P.; Fletcher, J. M.; Gleadow, A. J.; Calmus, T.; Nourse, J. A.

2003-12-01

87

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

88

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

E-print Network

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

Behn, Mark Dietrich, 1974-

2002-01-01

89

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

90

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

E-print Network

First Evidence for the Presence of Iron Oxidizing Zetaproteobacteria at the Levantine Continental that zetaproteobacterial populations were a dominant fraction of microbial community in the biofilm. We show for the first Evidence for the Presence of Iron Oxidizing Zetaproteobacteria at the Levantine Continental Margins. PLo

Cambridge, University of

91

Observations of intermediate nepheloid layers on the northern California continental margin  

Microsoft Academic Search

Conductivity–temperature–depth and transmissometer surveys were undertaken to investigate the characteristics and seasonal nature of intermediate nepheloid layers (INLs) over the outer shelf and upper slope of the northern California margin, near Eureka, CA. Observed INLs could generally be grouped into one of two categories: INLs that formed and spread seaward from the continental shelf, and INLs generated at continental slope

E. E. McPhee-Shaw; R. W. Sternberg; B. Mullenbach; A. S. Ogston

2004-01-01

92

How does continental crust thin in a young continental margin? Insights from Oman/Socotra conjugate margins in the eastern Gulf of Aden  

NASA Astrophysics Data System (ADS)

The discovery of hyper-thinned continental crust and exhumed mantle on continental margins has raised several key questions, such as how the crust thins until the breakup or what controls the locus of extreme crustal thinning, exhumation and final oceanic spreading. Reflection seismic lines (ENCENS-Sheba, Encens, Marges-Aden cruises) and seismological investigations (YOCMAL ANR project) across conjugate margins of the Oman/Socotra margins allow a detailed study of the crustal and sedimentary structure and a discussion on the structures and the age of the deformation. Structural analysis of new dataset enables mapping the area where the continental extension seems to be coupled to the mantle illustrating the exhumation phase. The crustal thinning is abrupt occurring mostly at the shoreline on both margins and shows along-margin variations.The thinning progressively migrates towards the locus of final breakup, which is interpreted by a progressive weakening of the mantle by lithospheric thinning and serpentinization. Then, a stage of uplift and erosion is observed in the proximal margins after the thinning phase. Uplift is usually higher where crustal thinning is more important in the deep basin, which could be interpreted by the onset of small-scale convection driven by the lateral temperature gradients at the necking zone.

Leroy, Sylvie; d'Acremont, Elia; Lucazeau, Francis; Poort, Jeffrey; Ahmed, Abdulhakim; Keir, Derek; Stuart, Graham; Khanbari, Khaled; Bellahsen, Nicolas; Nonn, Chloé

2013-04-01

93

Influence of the Iceland mantle plume on North Atlantic continental margins  

NASA Astrophysics Data System (ADS)

Early Tertiary breakup of the North Atlantic was accompanied by widespread magmatism. The histories of the Iceland mantle plume, of rifting and of magmatism are intimately related. The magmatism provides a challenge both to imaging structure, and to modelling the subsidence and development of the continental margins. We report new work which integrates state-of-the-art seismic imaging and new acquisition on the Atlantic volcanic margins with new techniques for modelling their evolution. We discuss the distribution of igneous rocks along the North Atlantic margins and discuss the temporal and spatial variations in the Iceland mantle plume in the early Tertiary, which have largely controlled this pattern of magmatism. Igneous rocks are added to the crust on rifted margins as extrusive lavas, as sills intruded into the sub-surface and as lower crustal intrusions or underplate. Each provide different, but tractable problems to seismic imaging. We show that many of these difficulties can be surmounted by using very long offsets (long streamers or two-ship methods) with a broad-band, low-frequency source, and by using fixed ocean bottom receivers. We report results from surveys on the North Atlantic continental margins using these methods. Imaging results are shown from the recent FLARE project and from the iSIMM project, which recorded new seismic data recorded in summer 2002. The iSIMM project acquired two seismic surveys, using 85 4-component ocean bottom seismometers with long streamers for wide-angle data, and vertical arrays for far-field source signature recording. One survey crosses the Faroes Shelf and adjacent continental margin, and a second the Hatton-Rockall Basin, Hatton Bank and adjacent oceanic crust. The Faroes wide-angle profiles were overshot by WesternGeco's Topaz using three single-sensor, Q-Marine streamers, 12km plus two 4km. We designed deep-towed, broad-band low-frequency sources tuned to enhance the bubble pulses, with peak frequencies at 8-11 Hz. The OBS survey used a 14-gun, 6,300 cu. in. array towed at 20 m depth, and the Q-marine survey used a 48-gun, 10,170 cu. in. array, with shot-by-shot signature recording. They provided excellent arrivals to ranges beyond 120 km, with penetration through the basalts and well into the upper mantle. iSIMM investigators are R.S. White, N.J. Kusznir, P.A.F. Christie, A.M. Roberts, N. Hurst, Z.C. Lunnon, C.J. Parkin, A.W. Roberts, L.K. Smith, R. Spitzer , V. Tymms, A. Davies and A. Surendra, with funding from NERC, DTI, Agip UK, BP, Amerada Hess Ltd., Anadarko, Conoco, Phillips, Shell, Statoil, and WesternGeco

White, R. S.; Isimm Team

2003-04-01

94

Trace metal (Cd, Cu, Ni and Pb) cycling in the upper water column near the shelf edge of the European continental margin (Celtic Sea)  

Microsoft Academic Search

This study investigates the relative importance of processes that affect trace metal (TM) cycling in the upper water column at the shelf edge of the Celtic Sea on the western European continental margin. The examined processes include external inputs (by atmosphere and river), physical factors (upwelling, winter mixing and water mass advection) and biological processes (in situ uptake, regeneration and

Marie-Hélène Cotté-Krief; Alain J Thomas; Jean-Marie Martin

2002-01-01

95

Listric Normal Faulting on the Cascadia Continental Margin  

NSDL National Science Digital Library

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

Mcneill, Lisa

96

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

97

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

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

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

SciTech Connect

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

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

1991-02-01

100

The Continental Margins of the Norwegian--Greenland Sea: Recent Results and Outstanding Problems: Discussion  

Microsoft Academic Search

Within a framework of plate tectonics the passive continental margins of the Norwegian--Greenland Sea may be classified as composed of rifted and sheared segments. An exception is the margin north of the Greenland-Senja Fracture Zone which appears to be of a combined sheared-rifted type. The margins south of the Greenland-Senja Fracture Zone are in part underlain by basement highs on

G. Wissmann; O. Eldholm

1980-01-01

101

Intermediate nepheloid layers observed over the continental margins off Oregon  

Microsoft Academic Search

Intermediate nepheloid layers were observed by a beam transmissometer operating at 650 nm during 3 to 8 November 1976 over the continental shelf and slope off Oregon. Two well defined intermediate nepheloid layers were observed. One was located at about 150 m depth and extended westward from a point 10 NM offshore. The second was at about 375 m depth

H. Pak; J. R. V. Zaneveld

1978-01-01

102

2008 Nature Publishing Group Fjord insertion into continental margins  

E-print Network

River Baffin Bay N A Walkerarm Sam Ford Fjord Baffin Island Barnes Ice Cap Baffin Bay Fox Basin Distance,200 ­1,200 1,600 2,000 0 100 200 300 Study area CS a b Figure 1 Baffin Island fjorded topography. a, The fjorded continental edge of northeastern Baffin Island showing the inland plateau, on which the Barnes Ice

Briner, Jason P.

103

179EARTHQUAKE CONTROL OF HOLOCENE TURBIDITE FREQUENCY, NORTHERN CALIFORNIA CONTINENTAL MARGINS EARTHQUAKE CONTROL OF HOLOCENE TURBIDITE FREQUENCY CONFIRMED  

E-print Network

intervals have been studied along the continental margins of the Cascadia Subduction Zone and northern San California margin. This difference in frequency of turbidites in a subduction zone compared to a transform ON THE CASCADIA AND NORTHERN CALIFORNIA ACTIVE CONTINENTAL MARGINS JULIA GUTIERREZ-PASTOR AND C. HANS NELSON

Goldfinger, Chris

104

Living Bulimina marginata in the SW Atlantic continental margin: Effect of the Subtropical Shelf Front and South Atlantic Central Water  

NASA Astrophysics Data System (ADS)

The western South Atlantic continental margin, between 27° and 37°S, is dominated by three main water masses: cold-fresh Subantarctic Shelf Water (SASW), warm-salty Subtropical Shelf Water (STSW) and Plata Plume Water (PPW). Despite the large seasonal variability of PPW extension along the shelf, an intense and relatively stable temperature-salinity gradient separates the SASW and the STSW forming the Subtropical Shelf Front (STSF) around 32°S. This article demonstrates that the distribution of Bulimina marginata, (shelf environment and deep-sea) a species of benthic foraminifera is influenced by the front location and it can be used as a proxy of the STSF in sediment core analysis.

Eichler, Patrícia P. B.; Pimenta, Felipe M.; Eichler, Beatriz B.; Vital, Helenice

2014-10-01

105

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

106

THE NORWEGIAN–GREENLAND SEA CONTINENTAL MARGINS: MORPHOLOGY AND LATE QUATERNARY SEDIMENTARY PROCESSES AND ENVIRONMENT  

Microsoft Academic Search

The continental margins surrounding the Norwegian–Greenland Sea are to a large degree shaped by processes during the late Quaternary. The paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features: slides, trough mouth fans and channels.Several large late Quaternary slides have been identified on the eastern Norwegian–Greenland Sea continental

TORE O. VORREN; JAN SVERRE LABERG; FRANK BLAUME; JULIAN A. DOWDESWELL; NEIL H. KENYON; JÜRGEN MIENERT; JAN RUMOHR; FRIEDRICH WERNER

1998-01-01

107

Origin and metamorphism of ultrabasic rocks associated with a subducted continental margin, Naxos (Cyclades, Greece)  

Microsoft Academic Search

Meta-peridotites outcropping at diVerent structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to re-examine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre-Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism

Y. K ATZIR; D. AV; A. M ATTHEWS; Z. G ARFUNKEL; B. W. E VANS

108

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

109

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

110

Internal tidal mixing as a control on continental margin ecosystems  

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

111

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

USGS Publications Warehouse

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

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

1987-01-01

112

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

Microsoft Academic Search

During the Palaeogene the NE Atlantic margin was subjected to a series of extension events immediately prior to and during continental break-up (at ca. 54 Ma). In the Faroes region of the margin, palaeostress analyses on faults exposed on the Faroe Islands indicate that the extension vector rotated in an anticlockwise sense from NE-SW to NW-SE. Remnants of the Faroe

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

2008-01-01

113

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

114

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

NASA Astrophysics Data System (ADS)

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

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

2009-04-01

115

The geodynamic province of transitional crust adjacent to magma-poor continental margins  

NASA Astrophysics Data System (ADS)

Two types of 'transitional crust' have been documented along magma-poor rifted margins. One consists of apparently sub-continental mantle that has been exhumed and serpentinized in a regime of brittle deformation during late stages of rifting. A second is highly thinned continental crust, which in some cases is known to have been supported near sea level until very late in the rift history and thus is interpreted to reflect depth-dependent extension. In both cases it is typically assumed that formation of oceanic crust occurs shortly after the breakup of brittle continental crust and thus that the transitional crust has relatively limited width. We here examine two representative cases of transitional crust, one in the Newfoundland-Iberia rift (exhumed mantle) and one off the Angola-Gabon margin (highly thinned continental crust). Considering the geological and geophysical evidence, we propose that depth-dependent extension (riftward flow of weak lower/middle continental crust and/or upper mantle) may be a common phenomenon on magma-poor margins and that this can result in a much broader zone of transitional crust than has hitherto been assumed. Transitional crust in this extended zone may consist of sub-continental mantle, lower to middle continental crust, or some combination thereof, depending on the strength profile of the pre-rift continental lithosphere. Transitional crust ceases to be emplaced (i.e., final 'breakup' occurs) only when emplacement of heat and melt from the rising asthenosphere becomes dominant over lateral flow of the weak lower lithosphere. This model implies a two-stage breakup: first the rupture of the brittle upper crust and second, the eventual emplacement of oceanic crust. Well-defined magnetic anomalies can form in transitional crust consisting of highly serpentinized, exhumed mantle, and they therefore are not diagnostic of oceanic crust. Where present in transitional crust, these anomalies can be helpful in interpreting the rifting history.

Sibuet, J.; Tucholke, B. E.

2011-12-01

116

Localized Deformation along an Inverted Rifted Margin: Example of the Northern Ligurian Margin, Western Mediterranean  

NASA Astrophysics Data System (ADS)

Along rifted margins, continental edges are heterogeneous systems that juxtapose lithospheres with different nature, mechanical behavior and structural inheritance. In this study, we focus on the northern Ligurian margin to examine how such complex systems might deform when they are submitted to a compressive stress field. The northern Ligurian margin, of Oligo-Miocene age, has been undergoing contraction over at least the past ~ 6 Ma. Active thrust faults and folds responsible for the regional uplift of the continental edge have previously been identified below the margin. Although seismicity extends as far as the axis of the basin, no recent or active crustal compressional structure has been identified so far in the oceanic domain. We used new 12-channel high-resolution seismic data (FABLES cruise, 2012) and other seismic reflexion lines from the last decades to image the sedimentary cover in the Ligurian oceanic basin, down to the bottom of the Messinian salt layer ~ 3 km below the seafloor. Because the Messinian event is well dated over the Mediterranean (5.96-5.32 Ma) and well identified in the seismic data, it forms a clear marker characterizing the recent deformation related to both salt and crustal tectonics. Noticeable deformation within the oceanic domain is restricted to large, SW-NE elongated salt walls located 10 to 40 km from the margin toe, over a 70-km length. The salt walls have a specific structure and arrangement that cannot result from salt tectonics only. We thus interpret them as resulting from combined deep-seated crustal and thin-skinned deformations. However, although the salt walls are well expressed in the seafloor morphology, their seismic images do not reveal any significant vertical throw across their trace, and they gradually disappear toward the SW. We thus interpret the salt walls as strike-slip structures with possibly very moderate compression. Overall, the post-Messinian deformation taken along these features is likely moderate as well. Thus, most of the contractional deformation would be focused along the margin since ~5 Ma. The synchronicity of the crustal deformation in the oceanic and the continental domains supports the idea that the lower deformation rates observed within the deep basin are related to somewhat different mechanical behaviors within the continental margin and the adjacent oceanic domain, rather than resulting from a recent basinward propagation of the deformation. Thermo-mechanical models suggest that mainly two factors could control the focused deformation along the margin: (1) the locus of highest topographic gradient of the main crustal interfaces, (2) the thermal contrast between the cooling subsiding oceanic domain and the warming uplifting margin. According to these models, the continental versus oceanic nature of the lithospheres would be of second order in the localization of the deformation.

SAGE, F.; Beslier, M.; Mercier De Lepinay, B. F.; Béthoux, N.; Gaullier, V.; Larroque, C.; Corradi, N.; Schenini, L.; Dessa, J.; Bigot, A.; Migeon, S.; Ruiz-Constán, A.

2013-12-01

117

LATE CENOZOIC SEISMIC STRATIGRAPHY AND GLACIAL GEOLOGICAL DEVELOPMENT OF THE EAST GREENLAND AND SVALBARD–BARENTS SEA CONTINENTAL MARGINS  

Microsoft Academic Search

The European program on the Late Cenozoic evolution of the Polar North Atlantic Margins (PONAM), running from 1990 to 1994, resulted in a wealth of seismic data and sediment cores, particularly from the Svalbard and Barents Sea continental margin, but also from areas of the East Greenland margin. Deep sediment coring was undertaken on both margins during ODP Legs 152

ANDERS SOLHEIM; JAN INGE FALEIDE; ESPEN S. ANDERSEN; ANDERS ELVERHØI; CARL FREDRIK FORSBERG; KRIS VANNESTE; GABRIELE UENZELMANN-NEBEN; JAMES E. T. CHANNELL

1998-01-01

118

The Mississippian Antler foreland and continental margin in southern Nevada: The Eleana Formation reinterpreted  

SciTech Connect

Rocks mapped as the Mississippian Eleana Formation at the type locality on the Nevada Test Site appear to comprise two completely different, but coeval, sedimentary units. In the Eleana Range (Western Eleana Formation), the strata are siliciclastic and carbonate turbidites of Mississippian age. From immediately east of the Eleana Range to Syncline Ridge (Eastern Eleana Formation), the strata are Devonian-Mississippian mudstone and quartzite conformably overlying Devonian limestone and underlying Pennsylvanian limestone. Although the contact between the two sedimentary packages is not exposed, small-scale structures document an east-dipping fault contact and reverse motion. Sandstone petrography and stratigraphic considerations support the age data in identifying two separate Mississippian units. Sandstones from the Western Eleana are chert litharenites with significant amount of feldspar and both volcanic and sedimentary lithic grains. These rocks are interpreted to be a submarine fan deposit; southwest-directed paleocurrent indicators suggest that they were deposited in an elongate trough, filled axially from the northeast. The source of the sediments was the antler allochthon and foreland basin. The authors tentatively correlate this section with the Dale Canyon-Chainman-Diamond Peak section near Eureka, Nevada. Sandstones from the Eastern Eleana are quartz arenites with rare chert and detrital heavy minerals. These strata are tentatively interpreted to be a shallow shelf deposit, with sediments derived from the continent to the east. They tentatively correlate this section with the Guilmette-Pilot-Scotty Wash-Chainman section of eastern Nevada. These sedimentary systems are initially separated an unknown distance across the late Paleozoic continental margin.

Cashman, P.H.; Trexler, J.H. Jr. (Univ. of Nevada, Reno (United States))

1991-02-01

119

Buried paleo-channels on the New Jersey continental margin: channel porosity structures from electromagnetic surveying  

Microsoft Academic Search

We report on a marine electromagnetic (EM) survey across two portions of the New Jersey continental margin that have been previously shown to contain buried paleo-channels. The EM method used provides bulk porosity estimates to depths of around 20 m below the seafloor and is thus able to place porosity constraints on the nature of the channel infill and the

Rob L. Evansa; L. K. Lawb; S. Cheesmand

120

Phosphorus cycling in marine sediments from the continental margin off Namibia  

Microsoft Academic Search

In this study we investigate benthic phosphorus cycling in recent continental margin sediments at three sites off the Namibian coastal upwelling area. Examination of the sediments reveals that organic and biogenic phosphorus are the major P-containing phases preserved. High Corg\\/Porg ratios just at the sediment surface suggest that the preferential regeneration of phosphorus relative to that of organic carbon has

Kathrin Küster-Heins; Ekkehard Steinmetz; Gert J. De Lange; Matthias Zabel

2010-01-01

121

Recent seismic investigations on gas hydrates at continental margins by BGR  

Microsoft Academic Search

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

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

2003-01-01

122

Recent seismic investigations on gas hydrates at continental margins by BGR  

Microsoft Academic Search

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

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

2002-01-01

123

Fluid flow and deformation at an active continental margin: The Eel River Basin, California  

Microsoft Academic Search

This dissertation addresses how active fluid flow and recent deformation are related to submarine morphology in the offshore Eel River basin along the northern California continental margin. The Eel River basin is an ideal location to study fluid flow and deformation because it is tectonically active, generates hydrocarbons at depth, and experiences rapid sediment loading. Five seismic reflection surveys of

Janet Wai Ngan Yun

2000-01-01

124

Wilson cycles, tectonic inheritance, and rifting of the North American Gulf of Mexico continental margin  

E-print Network

accommodated within a diffuse region adjacent to the oro- gen. This variation in location of rifting-Caledonian oro- gen (Ziegler, 1989). The association between the positions of continental breakup and older very little extensional deformation. Thus, the two margins differ in that the Ouachita oro- gen appears

Huerta, Audrey D.

125

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

Microsoft Academic Search

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

A. Milia; M. M. Torrente

2003-01-01

126

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

Microsoft Academic Search

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

Ingo Grevemeyer; Heinrich Villinger

2001-01-01

127

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

128

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

Microsoft Academic Search

The paper presents the molybdenum isotope data, along with the trace element content, to investigate the geochemical behavior of authigenic Mo during long-term burial in sediments in continental margin settings of the Yangtze block, as well as their indication to the burial of original organic carbon. The burial rate of original organic carbon were estimated on the basis of the

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

2007-01-01

129

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

E-print Network

bathymetry and backscatter imagery provides an opportunity to reevaluate the distribution of submarine continental slope and rise. Landslide distribution is in part controlled by the Quaternary history, 16% of the sea floor offshore of the fluvially dominated Middle Atlantic margin, and 13% of the sea

ten Brink, Uri S.

130

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

Microsoft Academic Search

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

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

2004-01-01

131

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

E-print Network

continental shelf, New Zealand Andrea J. Miller, Steven A. Kuehl Virginia Institute of Marine Science, 1208 Available online xxxx Keywords: Waipaoa River continental margin shelf sedimentation 210 Pb geochronology a sediment budget for the continental shelf, which is compared to long-term Holocene trends. 210 Pb and 239

Newman, Michael C.

132

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

NASA Astrophysics Data System (ADS)

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

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

2012-03-01

133

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

134

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

135

Erosion and tectonics at the margins of continental plateaus  

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

136

Evolution of continental slope gullies on the northern california margin  

USGS Publications Warehouse

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

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

2001-01-01

137

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

138

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

Microsoft Academic Search

Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment\\u000a failures on the low-angle (0.5°–2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps\\u000a restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80–150 m long\\u000a and are bounded by growth faults acting as

V. Lykousis; G. Roussakis; D. Sakellariou

2009-01-01

139

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

140

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

141

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

Federal Register 2010, 2011, 2012, 2013

...MMAA104000] Gulf of Mexico, Outer Continental Shelf (OCS), Western Planning Area...in the Proposed Final Outer Continental Shelf Oil & Gas Leasing Program...nautical miles north of the continental shelf boundary between the...

2013-07-29

142

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

143

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

NASA Technical Reports Server (NTRS)

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

Burke, Kevin

1988-01-01

144

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

145

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

146

Burial efficiency of phosphorus and the geochemistry of iron in continental margin sediments  

Microsoft Academic Search

We have examined the distributions of phosphorus and iron in sediments from well-oxygenated environments on the Atlantic Canadian and the Portuguese continental margins and from the anoxic region of the Chesapeake Bay. The measurements include total, citrate-dithionite-bicarbonate (CDB) extractable, ascorbate extractable, and dis- solved P and Fe; acid volatile sulfide; and pyrite. A surface layer (varying in thickness between 2

Pierre Anschutz; Shaojun Zhong; Bjørn Sundby; Alfonso Mucci; Charles Gobeil

1998-01-01

147

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

Microsoft Academic Search

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

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

2003-01-01

148

Nepheloid structure and hydrographic control on the Barcelona continental margin, northwestern Mediterranean  

Microsoft Academic Search

Contemporary processes controlling the off-shelf transport of sediment particles on the Barcelona continental margin were identified using CTD\\/transmissometer profiles during three hydrographic surveys. A well defined pattern of particulate matter distribution consisting in surface, intermediate and near-bottom nepheloid layers related with topographic and hydrographic structures was observed. Suspended sediment supplied by rivers or resuspended on the shelf is transferred to

Pere Puig; Albert Palanques

1998-01-01

149

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

150

Rift migration explains continental margin asymmetry and crustal hyper-extension  

PubMed Central

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

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

2014-01-01

151

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

152

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

153

Nature and distribution of the deformation front in the Luzon Arc-Chinese continental margin collision zone at Taiwan  

E-print Network

Nature and distribution of the deformation front in the Luzon Arc-Chinese continental margin, not a surface trace of the plate boundary between the Eurasian and Philippine Sea plates. Introduction The island of Taiwan was formed by oblique col- lision between the Luzon Arc and the Chinese continental

Lin, Andrew Tien-Shun

154

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

E-print Network

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

Pearce, Frederick Douglas

155

Disequilibrium response of permafrost in boreal continental western Canada to climate change  

Microsoft Academic Search

In the boreal forest of continental western Canada, permafrost is restricted toSphagnum-dominated peatlands on which air photo interpretation reveals the occurrence of five types of surface physiography. Concentrated in the northern part of the boreal forest, permafrost is present in peat plateaus with and without collapse scars. In the southern part of the boreal forest, continental bogs dominate, representing ombrotrophic

Linda A. Halsey; Dale H. Vitt; Stephen C. Zoltai

1995-01-01

156

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

157

Tectonic Implications of Canyon Directions Over the Northeast Atlantic Continental Margin  

NASA Astrophysics Data System (ADS)

The basis of this study is a new bathymetric map of the northeast Atlantic compiled from previously published maps made from conventional echosounder data, plus all Sea Beam data acquired on board the R/V JEAN CHARCOT since 1977. As most of the Sea Beam data have been obtained on the continental margin from Porcupine Seabight to the south of the Iberian Peninsula, a precise picture of the continental slope is given. A statistical analysis of the canyons, based on 750 measurements, reveals that many of the canyons present sharp changes in their direction, indicating a structural control mainly linked to the late Hercynian trends, especially around the Iberian Peninsula. Nevertheless, the paths of canyons may merely reflect recent gravity processes, as in the Porcupine Seabight. Canyons locally follow the directions of listric and associated transecting faults (Permian to Triassic and upper Jurassic to lower Cretaceous), as on the Celtic margin, and every type of tectonic lineament—for example, the North Pyrenean Paleogene thrust front which fringes the Gouf of Cap Breton. A comparison of diagrams for the northern and southern Bay of Biscay margin (especially trends predating the opening) is compatible with a 25° rotation of Iberia with respect to Europe.

Lallemand, Serge; Sibuet, Jean-Claude

1986-12-01

158

Sedimentary sequences of the Pacific-Indian Ocean sector of the Antarctic continental margin  

SciTech Connect

Seismic-reflection data across the Pacific-Indian Ocean sector of the Antarctic continental margin commonly reveal preglacial and glacial sedimentary sections up to 14 km thick. In this sector, diverse tectonic regimes have controlled the locations of preglacial rift deposits as well as glacial-till deltas. These regimes include major rift embayments, passive margins, formerly active and presently active margins, and active rifts. The sedimentary sections are principally of Mesozoic and Cenozoic age, although Paleozoic strata may exist at great depth. The upper parts of these sections commonly comprise prograding and aggrading sigmoidal sequences that are separated by unconformities and are up to 6 km thick. Where drilled in Prydz Bay and the Ross Sea, these upper sequences are solely glacial marine rocks of early Oligocene and younger age. The lower portions of the sections are commonly well-layered sequences that infill structural basins. The evolution of these sedimentary sequences is strongly controlled by extensional tectonic processes. Depocenters are located primarily within rift structures that formed initially during Gondwana breakup and later during magmatic-arc development. Rift-related deposits fill the basement grabens and are unconformably covered by glacial-till deltas. The till deltas apparently have been deposited beneath and at the front of former grounded ice sheets that selectively moved through rift embayments and over thermally subsiding margins. Since initial Cenozoic glaciation, these thick till deltas have prograded the continental shelf edge up to 70 km seaward to its present location. The sedimentary sequences underlying the Antarctic margin hold a record of Antarctic (Gondwana) rifting and glaciation - a record that would, if drilled, greatly improve their understanding of global climate and sea-level changes.

Cooper, A.; Eittreim, S. (Geological Survey, Menlo Park, CA (USA)); Anderson, J. (Rice Univ., Houston, TX (USA)); Stagg, H. (Bureau of Mineral Resources, Canberra (Australia))

1990-06-01

159

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

160

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

161

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

162

Submarine Landslides on the Mid-Atlantic and Southern New England Continental Margins, USA  

NASA Astrophysics Data System (ADS)

New multibeam bathymetry data, seismic reflection and sub-bottom profiles, and an extensive suite of piston cores are being used to investigate the failure history, and tsunami generating potential of submarine landslides on along the glacially-influenced southern New England (SNE) and fluvial-dominated mid-Atlantic sections of the U.S. Atlantic continental margin. Submarine landslides are ubiquitous features along the Atlantic continental margin of North America and are considered to be the most likely tsunami-generating source in the northwestern Atlantic Ocean. Multibeam bathymetry covering the SNE and mid-Atlantic sections of the margin reveals that the slope and upper rise from 200 m to greater than 2500 m water depth have been extensively evacuated by submarine landslides. While landslide scars abound on the upper and lower continental slopes, some of which have headwalls as much as 400 m high, several failures in these regions have resulted in the complete disintegration of the failed mass, while others have resulted in the deposition of well-defined blocky flows. Overall, these mass transport deposits (MTDs) have created an extensive, but diffuse, wedge of coalesced deposits that in places extend downslope for more than 100 km. Sediments recovered across the evacuation and MTD zones off southern New England contain a mixture of bioturbated, iron-rich hemipelagic layers, turbidites, chaotic debris composed of clay-balls within a heterogeneous gravel-sand-silt matrix, and convoluted silt and clay layers. Four- to five-fold increases in undrained shear strength (e.g., from 20 kPa to over 100 kPa) in cores from within landslide scars on the southern New England upper slope mark the location of failure planes. Preliminary age information based on the first abundant appearance of the planktonic foraminifera Globorotalia menardii in the piston cores suggests the bulk of the failures occurred prior to the Holocene, while layers within MTDs on the lower slope contain late Pleistocene shallow-water bivalve and gastropod macrofossils. Tighter age control of landslide features is expected with further radiocarbon dating of planktonic foraminifera and the development of oxygen isotope stratigraphies and biostraigraphic zonations based on planktonic and benthic foraminifera. As a complement to existing data, new piston cores and seafloor photographic images from submarine landslide evacuation zones and MTDs, to be collected in fall 2012, will provide the opportunity to further compare the failure histories of glacially-influenced and fluvial-dominated portions of the US Atlantic continental margin.

Chaytor, J. D.; Brothers, D. S.; Ten Brink, U. S.; Baxter, C.

2012-12-01

163

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

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

164

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

NASA Astrophysics Data System (ADS)

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

Müller, Claudia; Stein, Ruediger

2000-08-01

165

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

166

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

PubMed

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

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

2014-01-01

167

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

168

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

Federal Register 2010, 2011, 2012, 2013

...INTERIOR Minerals Management Service Gulf of Mexico, Outer Continental Shelf, Western...assessment (EA) for proposed Gulf of Mexico Outer Continental Shelf (OCS...2009, and was analyzed in the Gulf of Mexico OCS Oil and Gas Lease Sales:...

2010-04-05

169

Submarine erosion and karstification on the west Florida Continental margin: disparate environments yield similar features  

SciTech Connect

Thousands of kilometers of high resolution seismic profiles from the carbonate West Florida continental margin reveal two large bands of solution features. One band is found on the inner portion of the shelf and includes a variety of buried and filled karst features, the most spectacular of which are large solution valleys, paleodrainage extensions of the extent Tampa Bay and Charlotte Harbor estuaries. These features were probably formed subaerially during lower stands of sea level. This band of karst dies out in a line at mid-shelf between 75 m and 100 m water depth, marking a low stand of sea level. A second band of solution features is found on the upper slope at water depths of between 500 and 800 m. It is partially exposed in outcrop. Although many of the features are similar in appearance to those of the karst inner shelf band, their origin in submarine. They could have formed from dissolution by groundwater percolating down from the Florida mainland, from submarine erosion by the Loop Current which sweeps this portion of the slope or by a combination of the above processes. Although karstification is considered a continental process, extensive solution feature scan also form in the marine environment. Subsequently, these can be raised above sea level and be modified by continental processes, making it difficult to distinguish marine or partially marine solution features form those of traditional subaerial origin.

Doyle, L.J.; Brooks, G.; Herbert, J.H.

1985-01-01

170

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

171

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

172

Cenozoic prograding sequences of the Antarctic continental margin - What balance between structural and eustatic control  

SciTech Connect

Multichannel seismic reflection profiles across the Antarctic continental margin commonly reveal prograding sedimentary sequences that are bounded by unconformities. These sequences are as much as 5 km thick and, where sampled, are composed entirely of late Eocene( )-early Oligocene and younger glacial rocks. On nonpolar margins, prograding sequences generally are attributed to relative changes in sea level, sediment supply, and tectonism. Around Antarctica, ice sheets have also been important in controlling the geometry and location of prograding sequences. The Antarctic sequences may provide a proximal record of major Cenozoic ice volume changes and related sea level changes not obtainable from low-latitude continental shelves. Presently, the Antarctic record is poorly known because of limited core data. Two categories of prograding (P) and aggrading (A) sigmoidal sequences are observed around Antarctica: (1) P sequences that build principally outward (common) and (2) AP sequences that build largely upward and outward (less common). P sequences may result principally from grounded ice sheets, and AP sequences from open-marine basinal processes. Major rift embayments of Antarctica (e.g., eastern Ross Sea eastern Weddell Sea Lambert graben Wilkes basin) are also pathways for major ice movement. In general, most areas with P sequences lie within or adjacent to Mesozoic or older rift embayment, whereas the primary area with AP sequences (eastern Ross Sea) lies within a likely Cenozoic rift embayment. The Pacific side of the Antarctic Peninsula where Cenozoic ice sheets and Cenozoic tectonism have been active, is also marked by a P sequence. Scientific drilling on the Antarctic continental shelf has recovered openwater glacial deposits (Ross Sea) as well as glacial diamicts that were deposited beneath and in front of grounded glacier ice (Ross Sea and Prydz Bay).

Cooper, A.K. (Geological Survey, Menlo Park, CA (USA)); Barrett, P. (Victoria Univ., Wellington (New Zealand)); Hinz, K. (Federal Geosciences Institute, Hannover (West Germany)); Stagg, H. (Bureau of Mineral Resources, Canberra (Australia)); Traube, V. (Sevmorgeologia, Leningrad (USSR))

1990-05-01

173

Nature and Distribution of the Deformation Front in the Luzon Arc-Chinese Continental Margin Collision Zone at Taiwan  

NASA Astrophysics Data System (ADS)

Marine seismic reflection profiles from offshore SW Taiwan combined with onland geological data are used to investigate the distribution and nature of the deformation front west of Taiwan. Locations of the frontal structure west of Taiwan are generally connected in a linear fashion, although the alignment of frontal structures is offset by strike-slip faults. The deformation front begins from the northern Manila Trench near 21°N and continues northward along the course of the Penghu Submarine Canyon in a nearly N-S direction north of 21°N until it reaches the upper reaches of Penghu Canyon at about 22°15?N. The deformation front then changes direction sharply to the northeast. It connects to the Chungchou thrust fault or the Tainan anticline in the coastal plain and continues northwards along the outer Western Foothills to the northern coast of Taiwan near 25°N. Characteristics of structural style, strain regime, sedimentation and tectonics vary along the trend of the deformation front. Ramp anticlines, diapiric intrusion and incipient thrust faults are commonly associated with the deformation front. Variations in structural style along strike can be related to different stages of oblique collision in Taiwan. The deformation front (collision front) west of Taiwan can be considered as a boundary between contraction in the Taiwan orogen and extension west of the collision zone. The deformation front east of the Tainan Basin and its northward extension along the outer limit of the Western Foothills is the surface trace separating the foreland thrust belt from the nearby foredeep, not a boundary between the Chinese and Taiwan margins. The submarine deformation front off SW Taiwan is the surface trace separating the submerged Taiwan orogenic wedge from the Chinese passive continental margin, not a surface trace of the plate boundary between the Eurasian and Philippine Sea plates.

Yu, Ho-Shing

2004-03-01

174

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

Microsoft Academic Search

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

Karin Andreassen; Espen Glad Nilssen; Christian M. Ødegaard

2007-01-01

175

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

E-print Network

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

Pickart, Robert S.

176

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

NASA Astrophysics Data System (ADS)

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

Cowie, L.; Kusznir, N. J.

2012-12-01

177

Global Occurrences of Marine Gas Hydrate Beyond Active and Passive Continental Margins  

NASA Astrophysics Data System (ADS)

Natural gas hydrate occurs worldwide in oceanic sediment mainly in slopes and rises of active and passive continental margins. The Blake Outer Ridge, a passive margin in the Atlantic Ocean, and Hydrate Ridge, an active margin in the Pacific Ocean, are two regions of gas-hydrate occurrence that are particularly well studied. But gas hydrate occurs in other geologic settings where conditions of temperature, pressure, and amount and composition of gas and water are appropriate. For example, gas hydrate has been recovered from inland seas and lakes (Black Sea, Caspian Sea, and Lake Baikal). Gas hydrate has been inferred, based mainly on geophysical evidence, to be present in sediment of the Central Argentine Basin, the Bering Sea, and the Barents Sea. Submarine mud volcanoes such as Haakon Mosby in the Norwegian Sea and Kula in the Mediterranean Sea have yielded samples of gas hydrate, and near-surface sediment of the Gulf of Mexico contains gas hydrate associated with submarine gas vents. In fact, submarine gas hydrate was first observed by Russian scientists about thirty years ago when near-surface sediment recovered from the Black Sea was found to contain micro-crystalline aggregates of gas hydrate resembling hoarfrost. Most marine gas hydrate contains micorbially-generated methane; however, thermogenic methane has been observed in gas hydrate from the Gulf of Mexico and the Caspian Sea. Although most marine gas hydrate is found in sediment of active and passive margins, various occurrences beyond these margins demonstrate how globally extensive natural gas-hydrate phenomena actually are.

Kvenvolden, K. A.

2003-12-01

178

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

NASA Astrophysics Data System (ADS)

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

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

2004-11-01

179

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

180

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

181

Preliminary Insights to the Crust and Upper Mantle Structure of the northern Seychelles Continental Margin.  

NASA Astrophysics Data System (ADS)

Marine geophysical profiles were collected over the conjugate Seychelles - Laxmi Ridge pair of rifted continental margins during Charles Darwin cruise 144 in Jan/Feb 2003. The aim was to obtain data from an initially fast-spreading rift (half rate 59 mm/yr) to constrain modelling of strain rate and its effect on passive margin structure. The margins were restored using seafloor spreading isochrons and a single transect was designed across the entire reconstructed rift avoiding fracture zones and seamounts. Here we present preliminary results from the Seychelles margin. The Seychelles islands consist principally of pre-Cambrian granite surrounded by a carbonate platform. The transect extends north of Mahé, over the shallow water plateau and into the deep-water Eastern Somali Basin (1.5-5° S, 55-57.5° E). The line extends beyond magnetic anomaly A27 onto undisputed oceanic crust. Wide angle and multi-channel seismic (MCS) data were obtained along a 300 km NNE/SSW line, and an additional 800 km of MCS data were also collected in the area. These MCS profiles were acquired using a 96-channel, 2.4 km-long streamer and a 6920 cu in airgun array fired every 60 s for the main line and a 3890 cu in, 30 s array for the extra MCS lines. 32 GEOMAR OBS were deployed along the main line and 21 Leeds/Potsdam land seismometers on the islands for the wide-angle work. Gravity, magnetics, swath bathymetry and dredge samples were also collected. The Moho is widely imaged on brute stacks of the MCS profiles beneath the deep-water part of the transect. Its character changes from flat and smooth on the ocean side to a more complex and layered form towards the continent. The crustal thickness appears to thicken from about 6 km to 14 km over a horizontal distance of 8 km, before the Moho image is lost beneath the rough platform edge. A narrow package of seaward-dipping reflectors is evident under sediments off the edge of the platform with 15-20° dip and 16 km length. Several prominent seamounts were imaged using swath bathymetry within the ocean-continent zone. Three of these were dredged, and yielded basalts erupted from shallow marine or subaerial environments. The wide-angle data, from both OBS and landstations, recorded Pn arrivals at the furthest distance possible (300 km). First arrival travel times were picked and used in FAST to create a preliminary velocity model of the continental margin.

Sansom, V.; Collier, J. S.; Minshull, T. A.; Whitmarsh, R. B.; Kendall, M. J.; Lane, C. I.; Ryberg, T.; Rumpker, G.

2003-12-01

182

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

183

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

USGS Publications Warehouse

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

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

1999-01-01

184

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

NASA Astrophysics Data System (ADS)

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

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

2009-10-01

185

Fuerteventura palaeomagnetism and the evolution of the continental margin off Morocco  

NASA Astrophysics Data System (ADS)

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

Storetvedt, K. M.

1980-03-01

186

Processes in the benthic boundary layer at continental margins and their implication for carbon mineralization  

NASA Astrophysics Data System (ADS)

The presentation evaluates the role of biogeochemical processes in the cycling of organic matter within the benthic boundary layer (BBL). The data suggest an important additional mechanism of carbon mineralization at hydrodynamically energetic continental margins: that the near-bed fluid layer of the BBL is a major region for organic carbon minerlisation and the amount of carbon finally buried depends on the BBL exposure time of aggregates. Long term data reveal a tidally modulated flow field withe repetitive cycling of particles between seabed and suspension, representing altogether a situation of prolonged particle resuspension time. Data on the bioavailability of organic carbon within the BBL suggest fast mineralization rates but also a carbon protection mechanism so that labile organic carbon can still reach areas far offslope.

Thomsen, L.; Hedges, J.; Gust, G.; van Weering, T.

2003-04-01

187

A synopsis of the circulation in the Gulf Of Mexico and on its continental margins  

NASA Astrophysics Data System (ADS)

This article is a synopsis of the state-of-the-art knowledge and understanding of the circulation in the deep Gulf of Mexico as well as in the coastal flow regimes on its continental margins. The primary purpose is to review the ideas and results in this special new volume on the circulation in the Gulf of Mexico and integrate them with material from selected previous publications. This overview therefore contains more repetition and expository discussion than normally found in journal articles. An extensive reference list is provided for interested readers, a special feature for students. This volume is also meant to appeal to an audience beyond the general population of physical oceanographers, to include a more diverse community of Marine Scientists and Engineers, along with social interests of a scientific/technical nature. The article in this volume by Biggs et al. is focused on a study of some specific biogeochemical implications of on- and off-margin flow (on- and off- continental shelf/slope regions) in the northern Gulf of Mexico. The articles by Chassignet et al. [this volume] and Morey et al. [this volume], for example, note particular applications. Potential influences by the currents in the Gulf of Mexico on processes of interest in other branches of Marine Science are occasionally considered in this synopsis (for example, there is a special section on the effects of current patterns on coral reefs). The reader may also refer to a comparatively recent review of some of the physical oceanographic influences that regulate the biology of the Gulf of Mexico by Wiseman and Sturges [1999]. Finally, this synopsis is meant to complement and expand upon the Introduction to this volume by Sturges et al., which includes consideration of some general societal interests motivating marine research in the Gulf of Mexico.

Schmitz, W. J., Jr.; Biggs, D. C.; Lugo-Fernandez, A.; Oey, L.-Y.; Sturges, W.

188

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

USGS Publications Warehouse

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

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

2007-01-01

189

Late Cretaceous - Cenozoic development of outer continental margin, southwestern Nova Scotia  

SciTech Connect

The growth pattern for the outer continental margin of Nova Scotia during the Late Cretaceous and Cenozoic was studied using seismic stratigraphy and well data. Sediment accumulation was broadly controlled by temporal changes in relative sea level, but significant spatial and temporal changes in accumulation patterns were caused by changes in sediment supply rate, morphology, erosion by abyssal currents, and salt tectonics. A Jurassic-Early Cretaceous carbonate platform remained exposed until the Late Cretaceous and controlled the location and steepness of the paleoslope until the late Miocene. Local erosion of the outer shelf and slope in the late Paleocene-early Eocene produced chalky fans on the upper rise. The relationship between erosion of the shelf in the late Eocene and early Oligocene, and abyssal current erosion of the upper rise in the Oligocene, is unclear. Seaward extensions of Tertiary shelf-edge canyons are poorly defined except for the Eocene fans. In the Miocene, abyssal currents eroded a bench on the upper continental rise. Subsequently, sediments lapped onto and buried the paleoslope. The lower rise above horizon A/sup u/ (Oligocene) is composed of fans and olistostromes shed from halokinetic uplift of the upper rise. Current eroded unconformities are common in the rise sequence, but the only current deposit is a Pliocene interval (< 300 m) restricted to the lowermost rise. Pleistocene turbidity currents eroded the present canyon morphology. 15 figures, 2 tables.

Swift, S.A.

1987-06-01

190

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

191

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

192

New High-Resolution Mapping of Submarine Canyons in the Mid-Atlantic Continental Margin  

NASA Astrophysics Data System (ADS)

During June 2011, a BOEMRE/NOAA/USGS -funded multibeam bathymetry survey mapped the upper reaches (<900-m depths) of the Norfolk, Washington, and Baltimore canyons. Combined with existing multibeam bathymetry of the continental slope and rise, the new data provide a detailed view of the sedimentary processes that shaped the mid-Atlantic margin. The shelf-breaching canyon heads are surrounded by two terraces at depths of 95-100 m and 115-125 m in the Norfolk and Washington canyons and at depths of 115-125 m and 135-145 m in the Baltimore canyon. These terraces may represent paleo-shorelines formed during sea level stillstands. The canyon thalwegs within the shelf appear to be filled with sand in accord with old core results. The gradient of their thalweg profile is variable and relatively low across the shelf, slope, and upper rise, in contrast to the concave gradient of most non-shelf breaching canyons in the region. A few of the non-shelf breaching canyons in the mid-Atlantic margin also have relatively low and variable gradients suggesting that they once breached the shelf but are now completely filled. The seaward extensions of the Norfolk, Washington, and Baltimore canyons onto the continental rise are characterized by channels bordered by 100-200 m high levees. In places, these channels meander tightly. The extensions of other canyons onto the rise are either defined by subtle, linear depressions or cannot be traced. Channel-capture by adjacent canyons and channel abandonment originate in the lower slope and were prompted by either landslides or levee breaching. These observations indicate dynamic outer shelf deltas fed by large rivers, which were active at the last glacial maximum (LGM). The river channels on the shelf have been progressively filled during the Holocene. The clear expression of levied channels on the continental rise that extend from shelf-breaching canyons suggests that these canyons were the last ones to deliver turbidity flows to the rise. The terraces, assuming they are former shorelines, indicate that shelf-breaching canyon heads were incised into the shelf during or prior to the LGM. Some of the channels on the rise are partially filled by landslide deposits, suggesting that landslides have continued to shape the rise during the Holocene.

ten Brink, U. S.; Chaytor, J. D.; Brothers, D. S.; Twichell, D. C.; Ross, S. W.; Brooke, S.

2011-12-01

193

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

194

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

195

Subsidence and eustasy at the continental margin of eastern North America  

NASA Technical Reports Server (NTRS)

Biostratigraphic data from the COST B-2 well off New York and four deep commercial wells off Nova Scotia have been used to remove the effect of sediment loading at the Atlantic-type continental margin off the East Coast of North America. The resulting subsidence contains terms due to both 'tectonic' and 'eustatic' effects. By assuming the tectonic subsidence is thermal in origin these effects can be separated. The 'eustatic' effects have been isolated by least squares fitting an exponential curve to the subsidence data. The resulting sea-level curve shows a maximum rise in sea level during the Late Cretaceous era which probably does not exceed 150 m. The tectonic subsidence has been interpreted in terms of a simple thermal model for the cooling lithosphere. Based on this model the thermal thickness of the lithosphere and the total amount of crustal thinning are estimated. These estimates which are consistent with surface ship gravity and GEOS-3 altimeter measurements are used to define the structural elements which control the tectonic evolution of the margin.

Watts, A. B.; Steckler, M. S.

1979-01-01

196

Benthic remineralization in the northwest European continental margin (northern Bay of Biscay)  

NASA Astrophysics Data System (ADS)

We report a dataset of sediment characteristics and biogeochemical fluxes at the water-sediment interface at the northwest European continental margin (northern Bay of Biscay). Cores were obtained in June 2006, May 2007 and 2008, at 18 stations on the shelf break (120-180 m), and at 2 stations on the continental slope (520 and 680 m). Water-sediment fluxes of dissolved oxygen (O 2), total alkalinity (TA), nitrate ( NO3-), and dissolved silicate (DSi) were measured at a total of 20 stations. Sediment characteristics include: grain size, chlorophyll- a (Chl- a) and phaeopigment (Phaeo) content, particulate organic (POC) and inorganic (PIC) carbon content, and lead-210 ( 210Pb) and thorium-234 ( 234Th) activities. Sediments were sandy (fine to coarse) with organic matter (OM) (1.0-4.0%) and Chl- a (0.01-0.95 ?g g -1) contents comparable to previous investigations in the same region, and a relatively high PIC fraction (0.8-10.2%). Water-sediment O 2 fluxes (-2.4 to -8.4 mmol O 2 m -2 d -1) were low compared to other coastal environments and correlated well with OM and Chl- a content. 234Th activity profiles indicated that Chl- a sediment content was mainly controlled by physical mixing processes related to local hydrodynamics. The correlation between water-sediment fluxes of O 2 and NO3- indicated a close coupling of nitrification/denitrification and total benthic organic carbon degradation. Dissolution of biogenic silica (0.05-0.95 mmol m -2 d -1) seemed uncoupled from organic carbon degradation, as characterized by water-sediment O 2 fluxes. The link between water-sediment fluxes of TA and O 2 indicated the occurrence of metabolic driven dissolution of calcium carbonates (CaCO 3) in the sediments (˜0.33±0.47 mmol m -2 d -1), which represented ˜1% of the pelagic calcification rates due to coccolithophores measured during the cruises. These CaCO 3 dissolution rates were below those reported in sediments of continental slopes and of the deep ocean, probably due to the high over-saturation with respect to CaCO 3 of the water column overlying the continental shelf sediments of the northern Bay of Biscay. Rates of total benthic organic carbon degradation were low compared to water column rates of primary production and aphotic community respiration obtained during the cruises.

Suykens, K.; Schmidt, S.; Delille, B.; Harlay, J.; Chou, L.; De Bodt, C.; Fagel, N.; Borges, A. V.

2011-04-01

197

Benthic remineralization in the northeast European continental margin (northern Bay of Biscay)  

NASA Astrophysics Data System (ADS)

We report a data-set of sediment characteristics and biogeochemical fluxes at the water-sediment interface at the northeast European continental margin (northern Bay of Biscay). Cores were obtained in June 2006, May 2007 and 2008, at 8 stations on the shelf break (120 to 180 m), and at 2 stations on the continental slope (520 m and 680 m). Sediment-water fluxes of dissolved oxygen (O2), total alkalinity (TA), nitrate (NO3-), and dissolved silicate (DSi) were measured at a total of 20 stations. Sediment characteristics include: grain size, chlorophyll-a (Chl-a) and phaeopigment (Phaeo) content, particulate organic (POC) and inorganic (PIC) carbon content, and 234Th and 210Pb activities. Sediments were sandy (fine to coarse) with organic matter (OM) (1.0 - 4.0 %) and Chl-a (0.01 - 0.95 µg g-1) contents comparable to previous publications in the same region, and a relatively high PIC fraction (0.8 - 10.2 %). Sediment-water O2 fluxes (-2.4 to -8.4 mmol O2 m-2 d-1) were low compared to other coastal environments and correlated well with OM and Chl-a content. 234Th activity profiles indicated that Chl-a sediment content (apparently the main driver of total benthic organic carbon degradation) was mainly controlled by physical mixing processes related to local hydrodynamics. The correlation between sediment-water fluxes of O2 and NO3- indicated a close coupling of nitrification/denitrification and total benthic organic carbon degradation. Dissolution of biogenic silica (0.05 to 0.95 mmol m-2 d-1) was uncoupled from organic carbon degradation, characterized by sediment-water O2 fluxes. The link between sediment-water fluxes of TA and O2 indicated metabolic driven dissolution (~ 0.33 ± 0.47 mmol m-2 d-1) of calcium carbonates (CaCO3) in the sediments which represented ~ 1 % of the pelagic calcification rates due to coccolithophores. These rates were below those reported in sediments of continental slopes and of the deep ocean, probably due to the high over-saturation with respect to CaCO3 of the water column overlying the continental shelf sediments of the northern Bay of Biscay. Rates of total benthic organic carbon degradation and CaCO3 dissolution were low compared to water column rates of primary production, aphotic community respiration and CaCO3 production obtained during the cruises.

Suykens, Kim; Schmidt, Sabine; Delille, Bruno; Chou, Lei; de Bodt, Caroline; Harlay, Jérôme; Fagel, Nathalie; Borges, Alberto V.

2010-05-01

198

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

USGS Publications Warehouse

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

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

2003-01-01

199

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

200

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

NASA Astrophysics Data System (ADS)

A prominent feature across some continental margins is a bottom-simulating reflector (BSR). This seismic reflection generally coincides with the depth predicted for the base of the gas hydrate stability field. Because the occurrence of gas hydrates is controlled by temperature and pressure conditions, it has been suggested that BSRs mark an isotherm and they have therefore been used to estimate the heat flow through continental margins; crucial parameters are the temperature at BSR depth and at the seafloor and the thermal conductivity structure between the BSR and the seabed. However, very often the required parameters are not available and therefore they have been derived from models for gas hydrate stability and empirical relationships to obtain thermal conductivities from seismic velocities. Here, we use downhole temperature, thermal conductivity, porosity and logging data from 10 Ocean Drilling Program (ODP) sites drilled into and through the gas hydrate field to investigate the quality of estimates. Our analyses and application of constraints to the Makran margin off Pakistan indicate the following. (i) The temperature at BSR depth could be approximated by a seawater-methane system, although capillary forces, chemical impurities or non-equilibrium conditions can lower (or increase) the temperature. If calibration by heat probe measurements is possible, errors of geothermal gradients are less than 10 per cent, otherwise uncertainties of 20 per cent (or even higher) may arise. In addition, seasonal variations of bottom water temperature have to be considered, because they may affect thermal gradients by up to ~10 per cent. (ii) The impact of typical quantities of low-thermal-conductivity gas hydrate on the bulk thermal conductivity is insignificant. (iii) The thermal conductivity profile between the BSR and the seabed can generally be approximated by a mean value. Thus, (iv) seabed measurements should be used instead of empirical relationships, which may produce errors of 5-30 per cent. Consequently, in addition to high-quality seismic data, a prerequisite should be a large data set of thermal conductivities and oceanographic data. Heat probe measurements are recommended to constrain geothermal gradients. In this case the uncertainty of heat flow is 5-10 per cent of the estimated heat flow. If these data are not available errors/uncertainties can reach 50-60 per cent of the calculated value.

Grevemeyer, Ingo; Villinger, Heinrich

2001-06-01

201

Seismic Structure of the Continental Margin Offshore French Guiana and North-Eastern Brazil  

NASA Astrophysics Data System (ADS)

During November/December 2003 we carried out a marine geophysical survey of the continental margin offshore French Guiana and north-eastern Brazil onboard RRS Discovery. The survey comprised 1800 line km of coincident multi-channel seismic (MCS) reflection and wide-angle seismic refraction data which were acquired along 3 'transects' of the margin. In addition, more than 3000 line km of underway gravity and magnetic anomaly data were acquired. Preliminary processing of the MCS data has revealed the main stratigraphic units and the nature of the underlying basement at the margin. Up to 4 stratigraphic units have been identified, the uppermost of which is separated from lower units by an angular unconformity. We interpret the unconformity as of mid-Miocene age that corresponds to a major influx of sediment to the margin associated with uplift in the Bolivian Andes and the development of the Amazon deep-sea fan. The lower units beneath the fan offshore north-eastern Brazil comprise of gently landward-dipping reflectors that we interpret as post-rift sediments. Beneath the Demerara plateau offshore French Guinea, however, there is evidence of more steeply dipping reflectors that we attribute to Early Cretaceous and older syn-rift sediments. Basement has been imaged along each transect, most clearly beneath the middle and lower Amazon fan and the deep-water to the north of the Demerara plateau where we interpret it to be of oceanic-type. Preliminary processing of the wide-angle data recorded by ocean bottom instruments deployed at 10 km interval along each transect reveal high velocity arrivals at source-receiver offsets greater that 200 km. Ray-modelling indicate that primary intra-crustal first arrivals originate at major discontinuities within the stratigraphic column. Additional wide-angle land recording data contain arrivals at offsets greater than 400 km. We discuss here the results of the new seismic data, especially as they relate to the early opening history the Equatorial Atlantic and the structure and evolution of both its rift- and transform-type margins.

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

2004-12-01

202

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

E-print Network

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

Beaumont, Christopher

203

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

Microsoft Academic Search

Mass-physical properties of the surficial (upper 5 m) sediments on the Gulf of Lions continental margin were analysed, from more than 100 short (1 m) and longer (5 m) cores obtained during several cruises. Data include water content, unit weight, Atterberg limits (liquid limit, plastic limit, plasticity index), shear strength and compression index, and are used to determine: first, the

Bernard Chassefiere

1990-01-01

204

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

205

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

Microsoft Academic Search

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

John Townend

1997-01-01

206

Late quarternary glacioeustatic sea-level fluctuations: What are the sedimentologic processes and stratigraphic responses on continental margins  

SciTech Connect

Published stable isotope data from deep-sea sediments clearly show that the earth's climatic cycles have oscillated through at least 10 major glacial and interglacial episodes during the last million years. These high-frequency, orbitally-forced events should have resulted in major glacioeustatic sea-level fluctuations on the continental margins with dramatic sedimentologic effects and stratigraphic responses. However, such high-frequency events have proven difficult to resolve. Are they too short-lived to be recorded, to complex to decipher, or have traditional stratigraphic tools not been adequate to recognize them in continental margin sequences A detailed, multidisciplinary study of various continental margins is necessary to test the sensitivity of sedimentologic systems and response in stratigraphic records. This study must utilize (1) high-resolution event stratigraphy to define the depositional and erosional sediment sequences; (2) sediment analyses to delineate depositional environments and characterize lithofacies of specific system tracts; and (3) biostratigraphic and geochronologic analyses to place the depositional sequences in time. Integration of these data sets will (4) determine the resolving power of sequence stratigraphy; (5) develop working stratal models for recognizing short-pulsed, glacioeustatic sea-level events within the stratigraphic record; and (6) define a chronostratigraphy of changing paleoclimatic and paleoceanographic events operating on continental margin systems during the late Quaternary.

Riggs, S.R. (East Carolina Univ., Greenville, NC (United States)); Snyder, S.W. (North Carolina Univ., Raleigh (United States)); Hine, A.C. (Univ. of South Florida, St. Petersburg (United States))

1990-01-09

207

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

Microsoft Academic Search

In continental western Canada, discontinuous permafrost is almost always restricted to ombrotrophic peatlands (bogs). Bogs occur mostly as islands or peninsulas in large, often complex fens or are confined to small basins. Permafrost may be present in extensive peat plateaus (or more locally as palsas) and was preceded by a well-developed layer of Sphagnum that served to insulate the peat

D. H. Vitt; L. A. Halsey; S. C. Zoltai

1994-01-01

208

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

209

Tectonic features associated with the overriding of an accretionary wedge on top of a rifted continental margin: An example from Taiwan  

Microsoft Academic Search

Off southwest Taiwan, a west-advancing orogenic wedge has obliquely impinged on the northern continental slope of the South China Sea (SCS) margin. We analyzed a dense grid of multi-channel seismic profiles to reveal the tectonic features in this oblique collision setting. In the upper SCS slope and adjacent to the accretionary wedge, the rifted continental margin is characterized by a

Andrew T. Lin; Char-Shine Liu; Che-Chuan Lin; Philippe Schnurle; Guan-Yu Chen; Wei-Zhi Liao; Louis S. Teng; Hui-Ju Chuang; Ming-Shyan Wu

2008-01-01

210

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

211

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

NASA Astrophysics Data System (ADS)

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

Hill, P. S.

2012-12-01

212

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

213

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

214

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

NASA Astrophysics Data System (ADS)

The continental margin of West Iberia lacks significant synrift magmatism and exhibits a zone up to 100 km wide thought to consist mainly of serpentinized and exhumed mantle between the thinned continental and the oceanic crust. However, the existence of linear magnetic anomalies pose an ambiguity regarding the exact amount of synrift magmatism produced during mantle exhumation at the surface. We investigate how the thinned continental crust gives way to a broad zone of exhumed and serpentinized mantle with little synrift magmatism. For this we use a finite element code that includes brittle and ductile deformation in both crust and mantle, production of serpentine and melt. Serpentinisation is only allowed to occur when the entire crust has become brittle so that large amounts of water can reach the mantle through brittle faults. The increase in temperature due to the exothermic nature of serpentinisation and the decrease in the coefficient of friction where serpentinisation occurs is also taken into account. Melt production includes the effect of increased depletion in mantle temperatures. In a first approximation, melt is assumed to migrate instantaneously upwards and accumulate at crustal levels. We present tests with a range of extension velocities and asthenospheric temperatures. Preliminary model runs shows how the entire crust becomes brittle after it is has reached a thickness of less than 10 km. For slow rifting velocities (< 5 mm/yr), serpentinisation occurs prior to melting, whereas for faster rifting velocities the opposite is true. In all models, crustal separation and the exposure of mantle at the continent-ocean transition (COT) occurs after the entire crust has become brittle. The relative amount of serpentinite and melt in the COT depends on the rifting velocity, with slower velocities promoting the production of more serpentinite than melt. However, for a normal mantle temperature (1300 C), even for the slow extension rate of 5 mm/yr, 3-4 km of melt should be found at shallow levels in the COT, mixed with serpentinised mantle. In order, to shut off the production of melt a lower asthenospheric temperature is required, e.g. 1200 C.

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

2003-12-01

215

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.

216

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

NASA Astrophysics Data System (ADS)

Submarine mass movements are natural geomorphic processes that transport marine sediment down continental slopes into deep-marine environments. Type of mass wasting include creep, slides, slump, debris flows, each with its own features and taking place over timescale from seconds to years. Submarine landslides can be triggered by a number of different causes, either internal (such as changes in physical chemical sediment properties) or external (e.g. earthquakes, volcanic activity, salt movements, sea level changes etc.). Landslides may mobilize sediments in such a way as to form an impulsive vertical displacement of a body of water, originating a wave or series of waves with long wavelengths and long periods called tsunamis ('harbor waves'). Over 600 km of continental margin has been investigated by OGS in the Ionian sea using geophysical data - morpho-bathymetry (Reson 8111, 8150) and sub-bottom profiles (7-10 KHz) - collected aboard the research vessel OGS Explora in the framework of the MAGIC Project (Marine Geohazard along the Italian Coasts), funded by the Italian Civil Protection. The objective of this project is the definition of elements that may constitute geological risk for coastal areas. Geophysical data allowed the recognition of four main types of mass wasting phenomena along the slopes of the ICM: 1) mass transport complexes (MTCs) within intra-slope basins. Seabed imagery show the slopes of all the seabed ridges to be marked by headwall scarps recording widespread failure, multiple debris flows in several basins indicate one or more past episodes of failure that may be linked to activity on the faults bounding the structural highs. 2) submarine landslide - a multiple failure event have been identified (Assi landslide) at about 6 km away from the coastline nearby Riace Marina. Headwall scars up to 50 m high across water depths of 700 to 1400 m, while sub-bottom profiles indicate stacked slide deposits at and near seabed. 4) canyon headwalls - in the upper parts of all canyons, numerous headwall scarps are consistent with retrogressive activity of the canyons. 3) possible gravity sliding -elongate seabed features oriented subparallel to contours are observed, associated with diapiric structures suggest that the elongate seabed features may record a form of downslope sediment sliding above salt. The aim of this work is to reconstruct the dynamics of different type of submarine mass movements on the tectonically active Ionian Calabrian margin (ICM), calculate the volume of sediment mobilized and assess the potential tsunamigenic hazard associated to different type of mass movements. Assessments of tsunami arrival time in adjacent coastal areas, period and wavelength of the tsunami and implication for coastal geohazards have been formulated for the Calabrian margin (small scale) and extrapolated to adjacent margins of the Mediterranean basin (large scale).

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

2012-04-01

217

Particle export during a bloom of Emiliania huxleyi in the North-West European continental margin  

NASA Astrophysics Data System (ADS)

Coccolithophores, the dominant pelagic calcifiers in the oceans, play a key role in the marine carbon cycle through calcification, primary production and carbon export, the main drivers of the biological CO2 pump. In May 2002 a cruise was conducted on the outer shelf of the North-West European continental margin, from the north Bay of Biscay to the Celtic Sea (47.0°-50.5°N, 5.0°-11.0°W), an area where massive blooms of Emiliania huxleyi are observed annually. Biogeochemical variables including primary production, calcification, partial pressure of CO2 (pCO2), chlorophyll-a (Chl-a), particle load, particulate organic and inorganic carbon (POC, PIC) and 234Th, were measured in surface waters to assess particle dynamic and carbon export in relation to the development of a coccolithophore bloom. We observed a marked northward decrease in Chl-a concentration and calcification rates: the bloom exhibited lower values and may be less well developed in the Goban Spur area. The export fluxes of POC and PIC from the top 80 m, determined using the ratios of POC and PIC to 234Th of particles, ranged from 81 to 323 mg C m- 2 d- 1 and from 30 to 84 mg C m- 2 d- 1, respectively. The highest fluxes were observed in waters presenting a well-developed coccolithophore bloom, as shown by high reflectance of surface waters. This experiment confirms that the occurrence of coccolithophores promotes efficient export of organic and inorganic carbon on the North-West European margin.

Schmidt, S.; Harlay, J.; Borges, A. V.; Groom, S.; Delille, B.; Roevros, N.; Christodoulou, S.; Chou, L.

2013-01-01

218

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

USGS Publications Warehouse

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

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

1991-01-01

219

Formation of modern and Paleozoic stratiform barite at cold methane seeps on continental margins  

USGS Publications Warehouse

Stratiform (bedded) Paleozoic barite occurs as large conformable beds within organic- and chert-rich sediments; the beds lack major sulfide minerals and are the largest and most economically significant barite deposits in the geologic record. Existing models for the origin of bedded barite fail to explain all their characteristics: the deposits display properties consistent with an exhalative origin involving fluid ascent to the seafloor, but they lack appreciable polymetallic sulfide minerals and the corresponding strontium isotopic composition to support a hydrothermal vent source. A new mechanism of barite formation, along structurally controlled sites of cold fluid seepage in continental margins, involves barite remobilization in organic-rich, highly reducing sediments, transport of barium-rich fluids, and barite precipitation at cold methane seeps. The lithologic and depositional framework of Paleozoic and cold seep barite, as well as morphological, textural, and chemical characteristics of the deposits, and associations with chemosymbiotic fauna, all support a cold seep origin for stratiform Paleozoic barite. This understanding is highly relevant to paleoceanographic and paleotectonic studies, as well as to economic geology.

Torres, M.E.; Bohrmann, G.; Dube, T.E.; Poole, F.G.

2003-01-01

220

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

NASA Astrophysics Data System (ADS)

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

Alves, Tiago M.; Elliott, Claire

2014-11-01

221

Phosphate regeneration from sediments of the Peru continental margin by dissolution of fish debris  

NASA Astrophysics Data System (ADS)

Generally, oxidative regeneration of phosphate from anoxic sediments is by microbially mediated sulfate reduction processes. Stoichiometric modelling of such reactions takes into consideration varying proportions of 'decomposable' organically bound P to account for the ratios among nutrients in depth-concentration profiles of near-surface sediments. New results of interstitial water composition from sediments underlying the water masses influenced by coastal upwelling of the eastern boundary current system off Peru indicate that dissolution of phosphatic fish debris represents a mechanism for remineralization of phosphate comparable to or larger in magnitude than that by oxidative regeneration of organically bound P. Dissolved interstitial phosphate from fish debris is revealed by an excess amount of phosphate over that predicted from a simple stoichiometric oxidative regeneration model and by anomalously high dissolved interstitial fluoride concentrations. Phosphate flux estimates based on diffusion from the sediment suggest that this mechanism may generate up to 10% of the nutrient pool in the waters of the Peru undercurrent. Partitioning of P among the two sources reveals further that fish debris phosphate is about four times more important than organically bound P in nutrient generation from sediments of the Peru continental margin. Not only does this mechanism of regeneration affect the nutrient cycling but may also control widespread phosphorite formation in this area.

Suess, Erwin

1981-04-01

222

Subsidence and sedimentation on Jurassic passive continental margin, southern Alps, Italy  

SciTech Connect

The southern Alps of Italy preserve a tectonically intact array of Jurassic facies that record the evolution of a part of the margin of the Apulian plate from its ancestrial beginnings in a complex of Permian and Triassic rifted continental basins through the initial stages of breakup and stepwise foundering of a carbonate platform. Breakup was accompanied first by the rapid accumulation of thick prisms of carbonate turbidites in newly formed fault troughs. Then, as the new Ligurian oceanic basin began to open farther west and, as subsidence gradually slowed, accumulation of a succession of slowly deposited biogenous pelagic sediments recorded not only the increasing depths of the seafloor but also fluctuations in oceanographic conditions of fertility, carbonate dissolution levels, and the strength of bottom currents. Estimates of the history of seafloor depths, based on a simple subsidence law of the form Subsidence = K(Age) /sup 1/2/, provide a basis for the construction of a set of curves showing the changing depths of significant carbonate dissolution surfaces during the Jurassic in this region. The rapid 1-km deepening of the compensation depth for calcite during the Late Jurassic may be due to a change in regional oceanic vertical circulation patterns from upwelling (fertile, silica-rich, carbonate dissolving) to downwelling (less fertile, silica-poor, carbonate preserving).

Winterer, E.L. (Scripps Institution of Oceanography, La Jolla, CA); Bosellini, A.

1981-03-01

223

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

NASA Astrophysics Data System (ADS)

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

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

2004-12-01

224

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

225

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

226

Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin  

USGS Publications Warehouse

Wide-angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Francisco Bay Area Block and across the offshore continental margin. San Francisco Bay is underlain by a veneer (<300 m) of sediments, beneath which P wave velocities increase rapidly from 5.2 km/s to 6.0 km/s at 7 km depth, consistent with rocks of the Franciscan subduction assemblage. The base of the Franciscan at-15-18 km depth is marked by a strong wide-angle reflector, beneath which lies an 8- to 10-km-thick lower crust with an average velocity of 6.75??0.15 km/s. The lower crust of the Bay Area Block may be oceanic in origin, but its structure and reflectivity indicate that it has been modified by shearing and/or magmatic intrusion. Wide-angle reflections define two layers within the lower crust, with velocities of 6.4-6.6 km/s and 6.9-7.3 km/s. Prominent subhorizontal reflectivity observed at near-vertical incidence resides principally in the lowermost layer, the top of which corresponds to the "6-s reflector" of Brocher et al. [1994]. Rheological modeling suggests that the lower crust beneath the 6-s reflector is the weakest part of the lithosphere; the horizontal shear zone suggested by Furlong et al. [1989] to link the San Andreas and Hayward/Calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust. A transect across the continental margin from the paleotrench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seismic velocity and wide-angle reflectivity suggests that crustal composition changes across major transcurrent fault systems. Pacific oceanic crust extends 40-50 km landward of the paleotrench but, contrary to prior models, probably does not continue beneath the Salinian Block, a Cretaceous arc complex that lies west of the San Andreas fault in the Bay Area. The thickness (10 km) and high lower-crustal velocity of Pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby Pioneer seamount. The Salinian Block consists of a 15-km-thick layer of velocity 6.0-6.2 km/s overlying a 5-km-thick, high-velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceous arc-derived lower crust, or a magmatically underplated layer. The strong structural variability across the margin attests to the activity of strike-slip faulting prior to and during development of the transcurrent Pacific/North American plate boundary around 29 Ma. Copyright 1996 by the American Geophysical Union.

Holbrook, W. S.; Brocher, T. M.; Ten, Brink, U. S.; Hole, J. A.

1996-01-01

227

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

228

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

229

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

230

76 FR 14040 - Outer Continental Shelf (OCS), Central and Western Gulf of Mexico, Oil and Gas Lease Sales for...  

Federal Register 2010, 2011, 2012, 2013

...Continental Shelf (OCS), Central and Western Gulf of Mexico, Oil and Gas Lease Sales...covering all lease sales in the Central and Western Gulf of Mexico (GOM) Planning Areas...Central GOM Planning Area and five in the Western GOM Planning Area. Concurrent with...

2011-03-15

231

Sedimentary environments and gas hydrates on the Vøring Plateau, mid-Norwegian continental margin  

NASA Astrophysics Data System (ADS)

A variety of geological settings exists globally where gas hydrates occur, however with respect to possible future exploration most of these are beyond reach with existing technologies. According to the gas hydrate resource pyramid (Boswell and Collett, 2006) the most promising resource is represented by relatively limited occurrences of Arctic sandy deposits. Besides factors unique to gas hydrate systems (i.e., temperatures, pressures, and geochemical regimes) many of the parameters that industry is using to explore for conventional resources are also relevant for gas hydrate resources. The Norwegian continental margin represents a large conventional resource. Recently, gas hydrate formation has been identified at the seafloor of the Vøring Plateau, mid-Norwegian margin. Furthermore, a large number of fluid expulsion features are found in an area of the Vøring Plateau called Nyegga. These features are both relict and active fluid flow locations. The sedimentary facies connected with gas hydrate occurrence along this margin is however not well known yet. The sedimentary environment of the mid-Norwegian margin was highly influenced by the Fennoscandian Ice Sheet throughout the Pleistocene. Seismic and sedimentary records assume that the ice sheet reached the shelf edge several times. A 32 m long composite sediment record, which is recovered from the southern Vøring Plateau, is documenting continuous sedimentation within this area for the last ca. 300 ka. Detailed geophysical, geochemical and sedimentological results indicate different lithofacies as a result of alterations in prevailing sedimentary processes adherent to climate and ice sheet variability (e.g., IRD supply, current transport). The record indicates that various sandy layers of 10-15 % sand in average and a well-sorted layer of up to 40 % of sand are intercalated within otherwise predominantly silty deposits. As inferred from the age model, processes related to these sandy layers have often lasted for a couple of thousand years. Using seismic profiles the different sedimentary facies identified in the sediment record can be tied up to the Nyegga area and correlated to sediment cores in this area. Furthermore, extremely high sedimentation rates during the last deglaciation were recently proposed to be relevant for hydraulic fracturing and the formation of fluid escape chimneys at Nyegga (Hustoft et al., 2009). Within this context the long record documents that extremely high sedimentation rates have moreover also occurred during the previous deglaciation. References: Boswell, R. and Collett, T. (2006) The gas hydrates resource pyramid. Fire in the Ice, Methane newsletter, U.S. Depart. of Energy, Office of Fossil Energy, National Energy Techn. Lab. 5-7. Hustoft, S., Dugan, B. and Mienert, J. (2009) Effects of rapid sedimentation on developing the Nyegga pockmark field: Constraints from hydrological modeling and 3-D seismic data, offshore mid-Norway. Geochem. Geophys. Geosyst., 10.1029/2009GC002409.

Zühlsdorff, C.; Haflidason, H.; Sejrup, H. P.; Brendryen, J.; Grasmo, K. J.

2009-12-01

232

Crustal structure anomalies detected with Lg waves in grabens near continental margins in Greenland and in the North Sea  

Microsoft Academic Search

Anomalies of Lg-wave attenuation have been noticed in two areas near continental margins, in eastern Greenland and in the North Sea. In both areas are grabens with mainly Mesozoic sediments, and crustal stretching and thinning has been suggested. The structure difference between the Fennoscandian bedrock area and the Norwegian-Danish basin, with 8-10 km of sediments, shows almost no effect on

Søren Gregersen

1984-01-01

233

The effect of early diagenesis on the Fe isotope compositions of porewaters and authigenic minerals in continental margin sediments  

Microsoft Academic Search

Iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the California continental margin, where isotope compositions range from ?56Fe=?3.0‰ to +0.4‰. At one site near Monterey Canyon off central California, organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction (DIR) and bacterial sulfate reduction,

Silke Severmann; Clark M. Johnson; Brian L. Beard; James McManus

2006-01-01

234

Advances in our understanding of the gem corundum deposits of the West Pacific continental margins intraplate basaltic fields  

Microsoft Academic Search

Recent discoveries over the last decade of new gemfields, exploitation of new and existing deposits, and application of relatively new techniques have greatly increased our knowledge of the basalt-derived gem sapphire–ruby–zircon deposits. In this paper we focus on the Late Mesozoic to Cenozoic intraplate basaltic fields of the West Pacific continental margins. We review advances made in understanding the genesis

Ian Graham; Lin Sutherland; Khin Zaw; Victor Nechaev; Alexander Khanchuk

2008-01-01

235

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

Microsoft Academic Search

Comparison of five deep-water coral (DWC)\\/mound ecosystems along the European Continental Margin shows that suspended particulate\\u000a organic matter (sPOM), a potential food source, is lipid rich and of high quality. However, there are differences between\\u000a the sites. The Darwin and Pelagia Mounds (N. Rockall Trough and N. Porcupine Bank, respectively) have higher proportions of\\u000a labile particulate lipids (including high proportions

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

2007-01-01

236

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

237

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

238

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

NASA Astrophysics Data System (ADS)

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

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

2009-01-01

239

Lithium isotope systematics of deep-sourced pore fluids at continental margins  

NASA Astrophysics Data System (ADS)

In the past two decades, the behavior of lithium (Li) isotopes has been studied in various marine systems, including mid-ocean ridge and sediment-hosted hydrothermal systems, subduction zone settings and normal coastal and deep-sea sediments recovered by the Ocean Drilling Program (ODP). Major processes identified to cause deviations from the seawater isotopic composition are adsorption/desorption reactions, formation and transformation of silicate minerals, and leaching of Li from sediments or underlying crust at high temperature. As a result of the accomplished work, Li isotopes are considered a promising tracer for the diagenetic evolution and provenance of pore fluids in overpressured sedimentary environments. Here, we present Li concentration and isotope data of 18 cold seep locations and reference fluids from shallow marine sediments, a sediment-hosted hydrothermal system and two Mediterranean brine basins. The new reference data and literature data of hydrothermal fluids and pore fluids from the ODP follow an empirical relationship reflecting increasing Li release and decreasing isotope fractionation during clay mineral authigenesis with increasing temperature. Lithium concentration and isotope data of cold seep fluids are mostly in agreement with this empirical relationship. Ubiquitous diagenetic signals of clay dehydration in all cold seep fluids indicate that authigenic smectite-illite is an important sink for light pore water Li in deeply buried continental margin sediments. Deviations from the general relationship are attributed to the varying proportion of weatherable (e.g. volcanogenic) components and to transport-related fractionation trends. A simple transport-reaction model was applied to simulate Li isotope fractionation during upwelling of pore fluids to the seafloor. It is demonstrated that slow pore water advection (order of mm a-1) suffices to convey much of the deep-seated diagenetic Li signal into shallow sediments. If carefully applied, Li isotope systematics may, thus, provide a valuable record of fluid/mineral interaction that has been inherited several hundreds or thousands of meters below the seafloor.

Scholz, Florian; Hensen, Christian; de Lange, Gert J.; Haeckel, Matthias; Liebtrau, Volker; Meixner, Anette; Reitz, Anja; Romer, Rolf L.

2010-05-01

240

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

NASA Astrophysics Data System (ADS)

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

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

2008-12-01

241

Evidence for local volcanism in Mississippian continental-margin strata in southwestern Nevada, Nevada test site  

SciTech Connect

A thick section of siltstone, shale, and quartzite at NTS records continental-margin sedimentation coeval with Antler foreland-basin clastic fill to the west. These strata are Osagean and Meramecian (palynology) through upper Chesterian and Morrowan (ammonoids) in age, and appear to contain no sediment of sand or coarser size derived from the Antler allochthon. These rocks have been referred to by previous workers as part of the Eleana Fm., but the authors regard them as more properly equivalent to the Chainman Shale. Their composition has important tectonic implications. Preliminary data suggest that swelling clays in core drilled in these strata may originally have been wind-blown bentonitic ash. A continuously cored section at the north end of Syncline Ridge (hole UE17e) contains abundant swelling clays, as well as dolomite, anhydrite, and other evaporitic minerals. This mineral assemblage, combined with a paucity of fauna and bioturbation, indicates restricted circulation. Rare quartzite intervals near the top of the section are clean, current-reworked arenites with sedimentary structures indicating shallow marine, possibly intertidal conditions. This association suggests a restricted, shallowing-upward marine basin, probably anoxic at times. The clay content indicates a significant sediment input from air-fall ash. Present plate-reconstructions place southern Nevada near 2--8[degree] N latitude, with trade winds prevailing from the paleonortheast (near present north). Volcanic detritus in Late Mississippian sediments of southern Nevada is evidence that volcanism in the Antler orogenic terrane west of the foreland basin is recorded in shelf sediments east of the foreland.

Trexler, J.H. Jr.; Cashman, P.H. (Univ. of Nevada, Reno, NV (United States)); Herring, D.M.

1993-04-01

242

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

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

243

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

244

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

245

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

246

Volcanism along the Western Indian Margin - elements for plate organisation and dynamic topography within the Indian Ocean evolution  

NASA Astrophysics Data System (ADS)

The Indian Plate has been the focus of intensive research concerning the flood basalts of the Deccan Traps. Here I document using modern seismic reflection images a volcanostratigraphic analysis of the offshore segment of the western Indian volcanic large igneous province, between the shoreline and the first magnetic anomaly (An 28 ~63 Ma). I have mapped the different crustal domains of the NW Indian Ocean from stretched continental crust through to oceanic crust, using seismic reflection and potential field data. Volcanic ridges and seaward dipping reflectors sequences are identified along the margin and conjugated blocks. The geometry and internal seismic facies within the volcanic basement suggest a tholeiitic composition and subaerial to shallow marine emplacement. At the scale of the western Indian Plate, the emplacement of volcanic platform is constrained by structural lineation associated with rifting. I use these features to document the arrangements of the rifts geometry and the vertical movements at the transition from Mesozoic to Cenozoic along the western part of the Indian Plate.

Calvès, G.

2012-12-01

247

Pleistocene deformations in the contexte of the Rharb foredeep basin (north western Atlantic Moroccan margin)  

NASA Astrophysics Data System (ADS)

This study relates to the Cenozoic post rift deformations of Rharb foredeep basin in response to the Europe-Africa convergence. Here we are going to retail the tectonic structures of the Rharb basin, in particular the active front of the Prerifaine nappe in the area of Lalla Zahra. The method is based on the interpretations of the high resolution seismic reflection data acquired during the Protit2 (2003) and the Nomads cruises (2007). The surveys were conducted by the University of Brest in France and the Faculté des Sciences d'El Jadida in Morocco. They allowed to record more than 2000 km of seismic lines through the Rharb continental shelf. The integration of new data with industrial seismic lines provided by ONHYM and field observations collected along the coastline allows us to identify the formation and the recent evolution of the western termination of the Southern Rif Corridor. This coastal basin corresponds to the foredeep basin linked to the Rif Cordillera and extends southwards through the northern Moroccan Meseta that defines the foreland region of the Western Rif (Flinch,93). The integrated study clarifies the post-nappe evolution of the offshore Rharb basin during Neogene and quaternary times. A succession of deformations affect the Rharb basin with separating episodes of relaxation and quiescence. Their ages are based on chronostratigraphical attribution of mean unconformities. A Lower Pliocene episode is characterized by reactivation of faults affecting the Nappe. The uplift of the basin and the individualization of the Lallah Zarah ridge increases and controls the terrigenous fluxes. A Middle Pleistocene still active episode and corresponds to a new uplift of the two margins of the basin. Faulting remains more active in the North along the Lallah Zarah ridge and offshore Larache where large active listric faults are observed. The progressive segmentation of the basin determinates the sedimentary filling with cyclic sequences extending progressively Southward. Seismic analysis allows to describe successive episodes of faults reactivation with uplift of the southern forebulge and individualization of the northern topographic front. The detailed analysis of the post-nappe sedimentary evolution underlines the tectonic control of the Rharb basin with important lateral shift of the depocenter during Upper Pliocene and Quaternary.

Maad, N.; Le Roy, P.; Sahabi, M.; Gutscher, M. A.; Dakki, M.; Hssain, M.; van Vliet-Lanoë, B.; Brahim, L. Ait; M'hammdi, N.; Trenteseaux, A.

2009-04-01

248

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

NASA Astrophysics Data System (ADS)

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

McClellan, Elizabeth; Gazel, Esteban

2014-10-01

249

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

Microsoft Academic Search

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

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

2010-01-01

250

Pathways of carbon oxidation in continental margin sediments off central Chile.  

PubMed

Rates and oxidative pathways of organic carbon mineralization were determined in sediments at six stations on the shelf and slope off Concepcion Bay at 36.5 degrees S. The depth distribution of C oxidation rates was determined to 10 cm from accumulation of dissolved inorganic C in 1-5-d incubations. Pathways of C oxidation were inferred from the depth distributions of the potential oxidants (O2, NO3-, and oxides of Mn and Fe) and from directly determined rates of SO4(2-) reduction. The study area is characterized by intense seasonal upwelling, and during sampling in late summer the bottom water over the shelf was rich in NO3- and depleted of O2. Sediments at the four shelf stations were covered by mats of filamentous bacteria of the genera Thioploca and Beggiatoa. Carbon oxidation rates at these sites were extremely high near the sediment surface (>3 micromol cm-3 d-1) and decreased exponentially with depth. The process was entirely coupled to SO4(2-) reduction. At the two slope stations where bottom-water O2 was > 100 microM, C oxidation rates were 10-fold lower and varied less with depth; C oxidation coupled to the reduction of O2, NO3-, and Mn oxides combined to yield an estimated 15% of the total C oxidation between 0 and 10 cm. Carbon oxidation through Fe reduction contributed a further 12-29% of the depth-integrated rate, while the remainder of C oxidation was through SO4(2-) reduction. The depth distribution of Fe reduction agreed well with the distribution of poorly crystalline Fe oxides, and as this pool decreased with depth, the importance of SO4(2-) reduction increased. The results point to a general importance of Fe reduction in C oxidation in continental margin sediments. At the shelf stations, Fe reduction was mainly coupled to oxidation of reduced S. These sediments were generally H2S-free despite high SO4(2-) reduction rates, and precipitation of Fe sulfides dominated H2S scavenging during the incubations. A large NO3- pool was associated with the Thioploca, and the shelf sediments were thus enriched in NO3- relative to the bottom water, with maximum concentrations of 3 micromol cm-3. The NO3- was consumed during our sediment incubations, but no effects on either C or S cycles could be discerned. PMID:11540503

Thamdrup, B; Canfield, D E

1996-12-01

251

A water column study of methane around gas flares located at the West Spitsbergen continental margin  

NASA Astrophysics Data System (ADS)

In the Arctic Seas, the West Spitsbergen continental margin represents a prominent methane seep area. In this area, free gas formation and gas ebullition as a consequence of hydrate dissociation due to global warming are currently under debate. Recent studies revealed shallow gas accumulation and ebullition of methane into the water column at more than 250 sites in an area of 665 km2. We conducted a detailed study of a subregion of this area, which covers an active gas ebullition area of 175 km2 characterized by 10 gas flares reaching from the seafloor at~245 m up to 50 m water depth to identify the fate of the released gas due to dissolution of methane from gas bubbles and subsequent mixing, transport and microbial oxidation. The oceanographic data indicated a salinity-controlled pycnocline situated ~20 m above the seafloor. A high resolution sampling program at the pycnocline at the active gas ebullition flare area revealed that the methane concentration gradient is strongly controlled by the pycnocline. While high methane concentrations of up to 524 nmol L-1 were measured below the pycnocline, low methane concentrations of less than 20 nmol L-1 were observed in the water column above. Variations in the ?13CCH4 values point to a 13C depleted methane source (~-60‰ VPDB) being mainly mixed with a background values of the ambient water (~-37.5‰ VPDB). A gas bubble dissolution model indicates that ~80% of the methane released from gas bubbles into the ambient water takes place below the pycnocline. This dissolved methane will be laterally transported with the current northwards and most likely microbially oxidized in between 50 and 100 days, since microbial CH4 oxidation rates of 0.78 nmol d-1 were measured. Above the pycnocline, methane concentrations decrease to local background concentration of ~10 nmol L-1. Our results suggest that the methane dissolved from gas bubbles is efficiently trapped below the pycnocline and thus limits the methane concentration in surface water and the air-sea exchange during summer stratification. During winter the lateral stratification breaks down and fractions of the bottom water enriched in methane may be vertically mixed and thus be potentially an additional source for atmospheric methane.

Gentz, Torben; Damm, Ellen; Schneider von Deimling, Jens; Mau, Susan; McGinnis, Daniel Frank; Schlüter, Michael

2014-01-01

252

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

253

Continental margin magmatism and migmatisation in the west-central Fennoscandian Shield  

NASA Astrophysics Data System (ADS)

The Ljusdal Batholith (LjB) is a major component of the central Svecofennian Domain in Sweden. It is separated from the Bothnian Basin to the north by the 1.82-1.80 Ga crustal-scale Hassela Shear Zone (HSZ). The LjB has emplacement ages of 1.86-1.84 Ga, is mainly alkali-calcic, metaluminous, has ?Nd values between - 0.3 and + 1.2 and was formed in a magmatic arc setting. During the Svecokarelian orogeny the LjB was affected by at least three fold episodes. Large-scale folded screens of migmatised metasedimentary rocks occur in the eastern part of the batholith, and to the north of the HSZ, there is a 50 km wide diatexite belt. The Transition Belt (TrB), consisting of 1.88-1.85 Ga granitoids, is located at the northwestern extension of this belt. A calc-alkaline and peraluminous composition combined with negative ?Nd values (- 1.7 to - 0.8) indicates a large proportion of metasediments in the source for these granitoids. U-Pb SIMS data on zircon rims from migmatites and leucogranites to the north and east of LjB yield ages of 1.87-1.86 Ga, i.e. coeval with the granitoids of the LjB and the TrB. There is thus a close relationship between the LjB, the TrB and the migmatites in both space and time. Syn-migmatitic shearing along the HSZ indicates that a proto-HSZ was initiated already at c. 1.86 Ga, and the location of the proto-HSZ is inferred to be controlled by two older nuclei present in the lower parts of the crust. As crustal-scale shear zone systems are known to act as ascent pathways for sheet-like flow in active orogenies the TrB may represents accumulations of melts that were attracted and extracted by the proto-HSZ and intruded in a block that was not pervasively affected by subsequent shear along the HSZ. An active continental margin setting for the LjB implies subduction at c. 1.86 Ga, and provides a heat source for both the migmatites and the TrB. A later migmatisation at 1.82 Ga has been recorded to the south of the HSZ. Within the LjB the 1.82 Ga stromatic migmatites are folded by F 2 folds, and the fabric is truncated by 1.80 Ga pegmatites.

Högdahl, Karin; Sjöström, Håkan; Andersson, Ulf B.; Ahl, Martin

2008-05-01

254

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

255

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

256

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

257

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

258

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

259

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

260

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

Microsoft Academic Search

The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites

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

2007-01-01

261

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

Microsoft Academic Search

The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults.Barite samples from two sea-cliff sites and four offshore sites in

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

2007-01-01

262

Stratigraphic and isotopic link between the northern Stikine terrane and an ancient continental margin assemblage, Canadian Cordillera  

SciTech Connect

Geologic and isotopic data strongly imply a Late Triassic depositional link between a juvenile volcanic arc (northern Stikine terrane) and an outboard ancient continental margin assemblage (Nisling terrane) in the Canadian Cordillera. Two sandstone samples and a schist clast from a conglomerate layer at the base of the Upper Triassic Stuhini Group (northern Stikine terrane) have Nd-depleted mantle model ages of 1400-1430 and 1600 Ma, respectively; other Stuhini Group rocks have model ages of 390,660 and 690 Ma. Three samples of Nisling terrane schist and gneiss yield Nd model ages of 910, 1770, and 2450 Ma and highly radiogenic {sup 87}Sr/{sup 86}Sr ratios. These isotopic data corroborate stratigraphic evidence that detritus at the base of northern Stikine was shed from the Nisling terrane and strengthen interpretations that these terranes became linked by Late Triassic time. Thus, Upper Triassic strata of the northern Stikine terrane may have accumulated on top of or adjacent to an exotic continental fragment, a rifted fragment of the North American margin, or the in situ North American margin.

Jackson, J.L.; Gehrels, G.E.; Patchett, P.J. (Univ. of Arizona, Tucson (United States)); Mihalynuk, M.G. (British Columbia Ministry of Energy, Victoria (Canada))

1991-12-01

263

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

USGS Publications Warehouse

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

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

2002-01-01

264

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

265

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

266

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

267

Structural and stratigraphic analyses along the western margin of the Salton  

E-print Network

ABSTRACT Structural and stratigraphic analyses along the western margin of the Salton Trough show fault zones (the San Andreas fault and the West Salton detachment fault) to a network of dextral faults- ate, the older West Salton detachment fault, and Cretaceous mylonitic rocks below the detachment

Dorsey, Becky

268

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

269

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

270

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

271

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

272

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

273

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

274

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

275

LARISSA: Benthic Foraminiferal Analysis from Barilari Bay, western Antarctic Peninsula margin  

NASA Astrophysics Data System (ADS)

The LARISSA (LARsen Ice Shelf System, Antarctica) Project is an interdisciplinary project funded by the US National Science Foundation bringing together marine and Quaternary geology, cryosphere, ocean and marine ecology. The goal of LARISSA is to use the Larsen Ice Shelf System to study climate change effects on ice shelf systems and the oceanic and ecological responses to ice shelf collapse. This study focuses on jumbo piston core (JPC) 127 collected during cruise NBP 10-01. The 8.5 meter core was recovered from Barilari Bay (65.917°S 64.700°W) on the western side of the Antarctic Peninsula. The core site at the outer edge of the bay is of particular interest due to its location between the inner fjord high deposition rate sites and the more distal middle shelf sites, its exposure to open ocean circulation within the bay, and its proximity to the ice core site drilled on the Bruce Plateau during the LARISSA project. The abundance of calcareous taxa throughout the core makes it a target for isotopic studies. The recovered section was sampled for a variety of proxy measurements with samples collected for benthic foraminiferal analysis at 10 cm intervals. Correlation of the magnetic susceptibility data, as well as a peak in the diatom Eucampia antarctica from JPC 127 and other core records from the western Antarctic Peninsula indicate recovery of at least mid-Holocene through recent. Foraminiferal analyses resulted in the identification of 92 species of foraminifera. Three of the abundant species are indicators of specific water masses. Bulimina aculeata is indicative of the warmer, less saline Circumpolar Deep Water (CDW). Marked high abundance of Bulimina aculeata in the uppermost 600 cm indicates incursions of CDW onto the continental shelf and into Barilari Bay. This period of increased Bulimina aculeata abundance is also seen in other cores taken around the peninsula such as kasten core (KC) 11 in the Hugo Island Trough and in the Palmer Deep. Fursenkoina is a common taxa found in fjords throughout the world and an opportunist that increases in abundance when other species decrease. The presence of Fursenkoina has also been correlated with the presence of Weddell Sea Transitional Waters in the Bransfield Strait. Nonionella spp. are indicative of Bransfield Strait northern waters. The opportunistic nature of Fursenkoina is manifested in the inverse relationship between the abundance of Fursenkoina and Nonionella spp. throughout the core. The foraminiferal data are correlated to the magnetic susceptibility record, indicating times of vacillating terrestrial input and high diatom productivity. The foraminiferal data from Barilari Bay ultimately will be used in conjunction with the oceanographic, cryospheric, and biologic data to gain a better understanding of climate change along the western margin of the Antarctic Peninsula during the Holocene.

Matulaitis, I.; Ishman, S. E.; Leventer, A.; Brachfeld, S. A.; Jeong, S.; Domack, E. W.

2011-12-01

276

Late Neogene and Quaternary evolution of the northern Albemarle Embayment (mid-Atlantic continental margin, USA)  

Microsoft Academic Search

Seismic surveys in the eastern Albemarle Sound, adjacent tributaries and the inner continental shelf define the regional geologic framework and provide insight into the sedimentary evolution of the northern North Carolina coastal system. Litho- and chronostratigraphic data are derived from eight drill sites on the Outer Banks barrier islands, and the Mobil #1 well in eastern Albemarle Sound. Within the

David Mallinson; Stan Riggs; E. Robert Thieler; Stephen Culver; Kathleen Farrell; David S. Foster; D. Reide Corbett; Benjamin Horton; John F. Wehmiller

2005-01-01

277

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

Microsoft Academic Search

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

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

1990-01-01

278

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

E-print Network

of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold without visible internal deformation. The presence of these strong lower crustal rocks at shallow depth of brittle deformation visible in the seismic reflection data. The two geodynamic models produce different

Shillington, Donna J.

279

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

280

Survival Of Magnetic Paleoclimatic Signals From Shallow To Deep Water Marine Redoxomorphic Sediments Across The Northwest Iberian Continental Margin  

NASA Astrophysics Data System (ADS)

The magnetic properties of marine sediments on the North Atlantic Iberian continental Margin are strongly dependent on the organic matter input to the sediments and the onset of reductive diagenesis. An onshore-offshore gradient in the intensity of early diagenesis was recently described for the Ría de Vigo, matched by similar patterns in the adjacent rias of Pontevedra and Muros. In the ria environments of NW Iberia, early diagenetic dissolution of magnetic minerals can lead to magnetite half-lives of a few decades, and virtually obliterates any paleoenvironmental signal carried by magnetic minerals, rendering magnetic properties especially useful for the study of early diagenesis dynamics. Early diagenesis has also been identified in sediments of the adjacent continental shelf and deeper environments of the Galician Bank and Iberian Abyssal Plain. However, in these settings, slower dissolution of magnetic minerals allows the preservation of paleoclimatic signatures on different temporal scales. For instance, magnetic properties of continental shelf sediments reveal periods of enhanced rainfall and continental sediment input to the shelf, coincident with the Roman Warm Period and Medieval Climatic Optimum. On the contrary, cold periods are associated with less detrital input. Furthermore, levels of intensified diagenesis are also recorded during cold periods, which have been interpreted as periods of intensified coastal upwelling probably related to long-term North Atlantic Oscillation positive state. At the Galician Bank and Iberian Abyssal Plain sediments early diagenesis is also pervasive, although a paleoceanographic record of changes in the concentration of magnetic minerals transported by water masses flowing from the Portuguese Margin can still be identified. In addition to the progressive dissolution of magnetic minerals with depth, bulk magnetic properties in these deep marine settings show strong dependence on the pelagic carbonate sedimentation and low-magnetic turbiditic sediments that originate in the Galician Bank. Thin levels of highly magnetic sediments have been identified as Heinrich events, which provide an easily identifiable chronostratigraphic marker and a very fast and cost-effective tool for core correlation on a regional scale. These case-studies on the magnetic properties of sediments from the NW Iberian Margin provide examples of the range of applications of environmental magnetism in marine sediments with variable degrees of early diagenetic alteration of their magnetic mineral assemblage.

Mohamed Falcon, K. J.; Rey, D.; Rubio, B.

2013-05-01

281

The Dynamics of fluid flow and associated chemical fluxes at active continental margins  

E-print Network

convergent margin and for plate tectonic models. J. Geophys.Revised tectonic boundaries in the Cocos Plate off Costatectonic erosion and is speculated to be controlled by the roughness of the incoming plate.

Solomon, Evan Alan

2007-01-01

282

The Dynamics of Fluid Flow and Associated Chemical Fluxes at Active Continental Margins  

E-print Network

convergent margin and for plate tectonic models. J. Geophys.Revised tectonic boundaries in the Cocos Plate off Costatectonic erosion and is speculated to be controlled by the roughness of the incoming plate.

Solomon, Evan A

2007-01-01

283

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

NASA Astrophysics Data System (ADS)

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

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

2014-10-01

284

Impact of lithospheric heterogeneities on continental rifting evolution: Constraints from analogue modelling on South Atlantic margins  

NASA Astrophysics Data System (ADS)

Lithospheric-scale experiments integrated with restored crustal transects are used to study the evolution of the Central Segment (confined between the Rio Grande Fracture Zone to the south and the Chain Fracture Zone to the north) of the South Atlantic margin. The presence of crustal inhomogeneities, located within the Brazilian Santos and Campos basins, have been analysed and modelled in order to better understand their effects on the rift evolution and resulting structural architecture of the conjugate rifted margins. The results show that heterogeneities located within the lower crust can have a remarkable impact on the along-margin segmentation promoting articulated basins with horsts and grabens in response to a relative “strong” rheology, and focused and deeper basins related to a relatively “weak” rheology on the equivalent parts of the conjugate pairs. In particular, at the early-stage rift evolution the deformation is concentrated at the inner margin where, in the presence of a weak lower crust rheology, a main deep listric half-graben fault and associated thick and wedge-shaped syn-rift basin sequences are developed. A strong lower crust rheology, instead, gives rise to more planar, rotated, domino-type faulted basins with thinner sequences directly controlled by the individual fault-blocks. At the late-stage rift evolution, once the effects of the initial crustal rheology inhomogeneities are reduced due to the lithosperic thinning process, the outer margin records a late syn-rift sequence which shows comparable thicknesses for both cases of lower crust rheologies. This tectono-stratigraphic evolution of the rifting process gives rise to along-margin alterations in symmetry versus asymmetry of the width and structural architecture. The performed analogue modelling experiments also indicate that during the rifting evolution pieces of brittle mantle are preserved and could be elevated beneath the developed upper crustal structures, giving rise to complicated predictions for the along-margin heat-flow.

Cappelletti, A.; Tsikalas, F.; Nestola, Y.; Cavozzi, C.; Argnani, A.; Meda, M.; Salvi, F.

2013-11-01

285

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

USGS Publications Warehouse

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

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

2013-01-01

286

Reproductive biology and recruitment of the deep-sea fish community from the NW Mediterranean continental margin  

NASA Astrophysics Data System (ADS)

Temporal patterns in deep-sea fish reproduction are presently unknown for the majority of deep continental margins. A series of seasonal trawling surveys between depths of 300 to 1750 m in the Blanes submarine canyon and its adjacent open slope (NW Mediterranean) were conducted. The bathymetric size distributions and reproductive cycles of the most abundant species along the NW Mediterranean margin were analyzed to assess the occurrence of (i) temporal patterns in reproduction (i.e., spawning season) along a bathymetric gradient and (ii) preferential depth strata for recruitment. The fish assemblages were grouped in relation to their bathymetric distribution: upper slope, middle slope and lower slope species. Middle-slope species (i.e., 800-1350 m) showed short (i.e., highly seasonal) reproductive activity compared to the upper (300-800 m) and lower (1350-1750 m) ones. Our results, together with those previously published for megabenthic crustacean decapods in the area, suggest a cross-phyla depth-related trend of seasonality in reproduction. In the middle and lower slope species, the reproductive activity reached a maximum in the autumn-winter months and decreased in the spring. The observed seasonal spawning patterns appear to be ultimately correlated with changes in the downward transport of organic particles and with seasonal changes in the physicochemical characteristics of the surrounding water masses. The distribution of juveniles was associated with the bathymetric stratum where intermediate nepheloid layers interact with the continental margins, indicating that this stratum acts as a deep-sea fish nursery area.

Fernandez-Arcaya, U.; Rotllant, G.; Ramirez-Llodra, E.; Recasens, L.; Aguzzi, J.; Flexas, M. M.; Sanchez-Vidal, A.; López-Fernández, P.; García, J. A.; Company, J. B.

2013-11-01

287

Subduction and exhumation of continental crust in the Western Alps - A comparison of field-based and dynamic models  

NASA Astrophysics Data System (ADS)

For more than a century, the Alps have served as a crucible for orogenic models. Most of these models, including all dynamic models so far, have assumed 2D subduction of the European lithosphere beneath the Apulian continental margin. We use the example of the Sesia Zone in the Western Alps to dispel misconceptions about Alpine subduction, and to compare assumptions and results of field-based and dynamic models. The Sesia Zone comprises subducted slivers of Apulian passive continental margin that were initially exhumed by thrusting and oblique-slip shearing during late Cretaceous, dextral transform motion of Apulia with respect to Europe. Deformation of these slivers under eclogite- to blueschist- to greenschist-facies conditions in the presence of excess water enhanced their density contrast with the surrounding mantle. This facilitated buoyancy-driven exhumation from about 60 to 20 km at the base of the upper plate. Further exhumation of these slivers to near the surface involved SE-directed extensional shearing under greenschist-facies conditions and was broadly coeval with Eocene, SE-subduction of the Piemont-Liguria oceanic lithosphere. These kinematics are consistent with the idea that the Sesia basement nappes were wedged upward and laterally thinned above a NW-retreating hinge in the subducting oceanic plate. Oligo-Miocene exhumation of the NE Sesia Zone involved dextral transpressional shearing, backfolding and magmatism in the retro-wedge of the Alpine orogen. The validity of dynamic models is often judged by their ability to (re)produce first-order structures and PTt paths of exhumed crust. Insufficient regard for available 3D kinematic and thermobarometric constraints, especially on the pre-orogenic history of the plates' lithospheres, can lead to models that look realistic, but are misleading from thermal and/or kinematic standpoints. On the other hand, field-based models draw on diverse, sometimes tenuously compatible data sets whose interpretation can be biased by the desire to produce visibly attractive pictures. Nowadays, such pictures are influenced by published dynamic models. This feedback is fruitful if orogenic models are recognized as tests of ideas rather than simulations, and if geologists and modelers spend time understanding each other's Nature.

Handy, M. R.; Babist, J.; Konrad, M.

2005-12-01

288

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

Microsoft Academic Search

Ocean Drilling Program Leg 178 (February–April 1998) drilled two sites (Sites 1097 and 1103) on the outer Antarctic Peninsula Pacific continental shelf. Recovered strata are no older than late Miocene or early Pliocene (<4.6Ma). Recovery at shallow depths in loosely consolidated and iceberg-turbated bouldery sediment was poor but improved with increasing depth and consolidation to allow description of lithofacies and

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

2001-01-01

289

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

Microsoft Academic Search

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

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

2009-01-01

290

Ocean acidification research alongside extended continental shelf exploration in the western Arctic Ocean  

NASA Astrophysics Data System (ADS)

Research investments funded to fulfill the requirements of the UN Convention on the Law of the Sea in the western Arctic have allowed simultaneous acquisition of marine chemistry data, including baseline monitoring of changes in ocean acidification. Our participation in the Extended Continental Shelf cruises on the USCGC Healy in the western Arctic have allowed us to collect data focused on understanding processes driving rapid changes in seawater chemistry that result from increased oceanic uptake of CO2 (ocean acidification), increased freshwater runoff, changes in sea ice growth and decay processes and changes in biogeochemical processes. Carbonate mineral saturation data collected during HLY1002, HLY1102, and HLY1202 (summers 2010-2012) document undersaturation with respect to aragonite (?aragonite) in ~20% of the surface waters of the Canada and Makarov Basins, in direct association with areas of recently accelerated sea ice loss. Conservative tracer studies using salinity, stable oxygen isotopic composition, dissolved silica and barium augment this work by elucidating contributions from distinct water sources. These data show that while surface water in this entire area retains abundant freshwater from meteoric sources, it is freshwater additions from melting of multiyear sea ice which is most closely linked to the areas of aragonite undersaturation. Depth profiles from 20 oceanographic stations taken during the cruises show a ~100 m thick lens of ?aragonite undersaturated water at ~150 m depth in the western Arctic, but not further north than 85°N. The surface waters in the Canada and Makarov Basins have pCO2 values much lower than the atmospheric pCO2 (~390 uatm), ranging between 350 ?atm and 100 ?atm, and are a strong sink for atmospheric CO2. The strong sink areas are found in the Chukchi Sea and western Beaufort shelf areas. These studies represent the frontiers of ocean acidification research in the western Arctic, in which baseline data have been greatly expanded with the support of the ECS surveys.

Wynn, J. G.; Robbins, L. L.; Knorr, P. O.; Byrne, R. H.; Takahashi, T.; Onac, B. P.

2013-12-01

291

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

NASA Astrophysics Data System (ADS)

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

Gaullier, Virginie; Lofi, Johanna

2010-05-01

292

Recent sedimentology and ocean dynamics of the Western Nigerian continental shelf and coastline  

NASA Astrophysics Data System (ADS)

The Western Nigerian continental shelf lies approximately between longitudes 2°42 ' and 5°00 ' E. It is a relatively gently sloping and narrow shelf, incised by two canyons, Avon and Mahin. The beaches between longitudes 2°42 ' and 4°30 ' E are sandy while Mahin Mud Beach is further to the east. The western sand deposits extend about 12 km offshore and are separated into two coast-parallel sand bodies by a narrow dark grey silt-mud facies, 1-2 km wide. Lekki lagoon is situated inland of the sandy beach-barrier systems near Lagos. This low energy tidal and freshwater lagoon is slowly silting up with river sediment; bottom sediment characteristics reflect influence of both fluvial transport and weak tidal current patterns. Mahin Mud Beach is starved of sand due to a lack of longshore drift as a result of the two canyons siphoning off arenaceous material. It is thus highly vulnerable to erosion and flooding. The two coast-parallel sand bodies in the west, combined with four preserved drowned coral reef systems, suggest that the post-Pleistocene sea level rise along the Western Nigerian shelf was punctuated by a number of sea level stillstands.

Ihenyen, A. E.

2003-04-01

293

Rifted continental margins are formed by progressive extension of the lithosphere.  

E-print Network

,a major seismic experi- ment was conducted in February­March 2005 in the eastern Black Sea Basin (Figure 1 cannot be well constrained.However,the Black Sea differs from these sediment-starved rifted margins in that the surrounding continent has provided an ongoing sediment supply. The Eastern Black Sea Basin The Black Sea

Shillington, Donna J.

294

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

SciTech Connect

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

Poag, C.W.; Ward, L.W.

1987-02-01

295

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

Microsoft Academic Search

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

Daniel Winkelmann; Jochen Knies

2005-01-01

296

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

297

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

USGS Publications Warehouse

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

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

1995-01-01

298

Data file, Continental Margin Program, Atlantic Coast of the United States: vol. 2 sample collection and analytical data  

USGS Publications Warehouse

The purpose of the data file presented below is twofold: the first purpose is to make available in printed form the basic data relating to the samples collected as part of the joint U.S. Geological Survey - Woods Hole Oceanographic Institution program of study of the Atlantic continental margin of the United States; the second purpose is to maintain these data in a form that is easily retrievable by modern computer methods. With the data in such form, repeate manual transcription for statistical or similar mathematical treatment becomes unnecessary. Manual plotting of information or derivatives from the information may also be eliminated. Not only is handling of data by the computer considerably faster than manual techniques, but a fruitful source of errors, transcription mistakes, is eliminated.

Hathaway, John C.

1971-01-01

299

ROV study of a giant pockmark on the Gabon continental margin  

Microsoft Academic Search

A giant, 800-m wide pockmark, called Regab, was discovered along the Equatorial African margin at 3160-m water depth and was\\u000a explored by remote operated vehicle (ROV) as part of the Zaiango (1998–2000) and Biozaire (2001–2003) projects carried out\\u000a conjointly by TOTAL and a number of French research institutes. A microbathymetric map obtained using the ROV sensors shows\\u000a that the pockmark

H. Ondréas; K. Olu; Y. Fouquet; J. L. Charlou; A. Gay; B. Dennielou; J. P. Donval; A. Fifis; T. Nadalig; P. Cochonat; E. Cauquil; J. F. Bourillet; M. Le Moigne; M. Sibuet

2005-01-01

300

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

301

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

302

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

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

303

Gas hydrates, free gas distribution and fault pattern on the West Svalbard continental margin  

Microsoft Academic Search

A three dimensional tomographic analysis of seismic velocity and attenuation fields is presented with the purpose to analyze the intimate relation of gas hydrates and free gas distribution with the fault pattern. The 3-dimensional, 4-component seismic data have been acquired offshore western Svalbard. The analysis of the sub-bottom topography of the base of the stability field of gas hydrates (indicated

G. Madrussani; G. Rossi; A. Camerlenghi

2009-01-01

304

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

Microsoft Academic Search

A 3-D tomographic analysis of seismic velocity and attenuation fields is presented with the purpose to analyse the intimate relation of gas hydrates and free gas distribution with the fault pattern. The 3-D, four-component seismic data have been acquired offshore western Svalbard. The analysis of the subbottom topography of the base of the stability field of gas hydrates (indicated by

Gianni Madrussani; Giuliana Rossi; Angelo Camerlenghi

2010-01-01

305

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

NASA Astrophysics Data System (ADS)

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

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

2010-12-01

306

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

NASA Astrophysics Data System (ADS)

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

Chassefiere, Bernard

1990-09-01

307

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

Federal Register 2010, 2011, 2012, 2013

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

2012-05-24

308

Major changes in ice stream dynamics during deglaciation of the north-western margin of the Laurentide Ice Sheet  

NASA Astrophysics Data System (ADS)

Victoria Island lies at the north-western limit of the former North American (Laurentide) Ice Sheet in the Canadian Arctic Archipelago and displays numerous cross-cutting glacial lineations. Previous work suggests that several ice streams operated in this region during the last (Wisconsinan) glaciation and played a major role in ice sheet dynamics and the delivery of icebergs into the Arctic Ocean. This paper produces the first detailed synthesis of their behaviour from the Last Glacial Maximum through to deglaciation (˜21-9.5 cal ka BP) based on new mapping and a previously published radiocarbon-constrained ice sheet margin chronology. Over 70 discrete ice flow events (flow-sets) are 'fitted' to the ice margin configuration to allow identification of several ice streams ranging in size from large and long-lived (thousands of years) to much smaller and short-lived (hundreds of years). The reconstruction depicts major ice streams in M'Clure Strait and Amundsen Gulf which underwent relatively rapid retreat from the continental shelf edge at some time between ˜15.2 and 14.1 cal ka BP: a period which encompasses climatic warming and rapid sea level rise (meltwater pulse-1a). Following this, overall retreat was slower and the ice streams exhibited asynchronous behaviour. The Amundsen Gulf Ice Stream continued to operate during ice margin retreat, whereas the M'Clure Strait Ice Stream ceased operating and was replaced by an ice divide within ˜1000 years. This ice divide was subsequently obliterated by another short-lived phase of ice streaming in M'Clintock Channel ˜13 cal ka BP. The timing of this large ice discharge event coincides with the onset of the Younger Dryas. Subsequently, a minor ice divide developed once again in M'Clintock Channel, before final deglaciation of the island shortly after 9.5 cal ka BP. It is concluded that large ice streams at the NW margin of the Laurentide Ice Sheet, equivalent in size to the Hudson Strait Ice Stream, underwent major changes during deglaciation, resulting in punctuated delivery of icebergs into the Arctic Ocean. Published radiocarbon dates constrain this punctuated delivery, as far as is possible within the limits imposed by their precision, and we note their coincidence with pulses of meltwater delivery inferred from numerical modelling and ocean sediment cores.

Stokes, Chris R.; Clark, Chris D.; Storrar, Robert

2009-04-01

309

Did Upper Cretaceous Intrusions reactivate Pre-Cretaceous structures at the South Atlantic passive continental margin of Brazil?  

NASA Astrophysics Data System (ADS)

"Passive" continental margins especially of the South Atlantic Ocean are perfect locations to quantify exhumation, rock uplift, and surface uplift rates, model the long-term landscape evolution and provide information on the influence of mantle processes on a longer time scale. Furthermore, these passive margin allow to study the influence of large intrusions on the reactivation of Pre-Intrusion structures. In Southern Brazil, the Poços de Caldas intrusion (83 Ma) took place in Neoproterozoic para-metamorphic rocks of amphibolite facies, which are deformed and metamorphosed during the Central Brazilian Orogeny (630 Ma - 510 Ma). The compressional deformation caused major N-S trending transform faults, and related perpendicular structures. In the Serra da Mantiqueira and Serra do Mar to the East, these N-S trending transform structures are reactivated at about 120 Ma. Together with the surrounding Precambrian metamorphic rocks the Poços de Caldas Intrusion forms the Poços de Caldas Plateau reaching elevatiosn between 900 m.a.s.l. and 1300 m.a.s.l. The intrusion covers an area of 800 km². The presentation will provide data and discuss the influence of the large intrusion on Pre-Intrusion structures within the surrounding metamorphic basement.

Glasmacher, Ulrich Anton; De Souza Silva, Jaqueline; Hackspacher, Peter Christian; Doranti-Tiritan, Carolina

2014-05-01

310

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

311

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

Microsoft Academic Search

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

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

2009-01-01

312

Submarine weathering of silicate minerals and the extent of pore water freshening at active continental margins  

NASA Astrophysics Data System (ADS)

In order to investigate how submarine weathering processes may affect the water balance of sediments at convergent plate margins, six sediment cores were retrieved off Central Chile at water depth between ˜800 and 4000 m. The sediment solid phase was analyzed for its major element composition and the pore fluids were analyzed for dissolved sulfate, sulfide, total alkalinity, major cations, chloride, bromide, iodide, hydrocarbons as well as the carbon isotopic composition of methane. Because of negligible weathering on land, surface sediments off Central Chile are rich in reactive silicate minerals and have a bulk composition similar to volcanic rocks in the adjacent Andes. Deep-sourced fluxes of alkalinity, cations and chloride indicate that silicate minerals are subject to weathering in the forearc during burial. Comparison of deep-sourced signals with data from nearby Ocean Drilling Program Sites reveals two different types of weathering processes: In shallow (tens of meters), methanic sediments of slope basins with high organic carbon burial rates, reactive silicate minerals undergo incongruent dissolution through reaction with CO2 from methanogenesis. At greater burial depth (hundreds of meters), silicate weathering is dominated by authigenic smectite formation. This process is accompanied by uptake of water into the clay interlayers thus leading to elevated salinities in the surrounding pore water. Deep-seated smectite formation is more widespread than shallow silicate dissolution, as it is independent from the availability of CO2 from methanogenesis. Although solute transport is not focused enough to form cold seeps in the proper sense, tectonically induced, diffuse fluid flow transfers the deep-seated signal of smectite formation into the shallow sediments. The temperature-controlled conversion of smectite to illite is considered the most important dehydration process in marine forearc environments (depth of kilometers). However, in agreement with other studies at active margins (e.g. Aleutians, Cascadia, Nankai Trough) and despite ubiquitous evidence for smectite formation, little evidence for seafloor seepage of dehydration fluids could be found off Central Chile. We argue that the circular process of pore water uptake during smectite formation and release upon illitization implies a balanced freshwater budget and therefore a rather limited potential for net pore water freshening on a margin-wide scale. According to this rationale, pore water freshening at seafloor seeps preferentially occurs at lower latitudes (Central America, Barbados, Mediterranean Ridge) where terrestrial weathering is more intense thus leading to external (i.e. detrital) smectite and thus freshwater inputs to the subduction system.

Scholz, Florian; Hensen, Christian; Schmidt, Mark; Geersen, Jacob

2013-01-01

313

Bulletin of the Seismological Society of America, Vol. 93, No. 5, pp. 19551983, October 2003 Geology of the Continental Margin beneath Santa Monica Bay,  

E-print Network

Geology of the Continental Margin beneath Santa Monica Bay, Southern California, from Seismic Abstract We interpret seismic-reflection data, which were collected in Santa Monica Bay using a 70-in3 and of the deep-water, Santa Monica and San Pedro Basins. The goal of this research is to investigate

314

Principles of Geological Mapping of Marine Sediments (with Special Reference to the African Continental Margin). Unesco Reports in Marine Science No. 37.  

ERIC Educational Resources Information Center

Designed to serve as a complement to the Unesco Technical Papers in Marine Science, this report concentrates on theoretical and practical problems of geological mapping of the sea floor. An introduction is given to geological mapping procedures at continental margins as well as some practical recommendations taking as an example the African region…

Lisitzin, Alexandre P.

315

Banda Forearc Metamorphic Rocks Accreted to the Australian Continental Margin in Timor: Detailed Analysis of the Lolotoi Complex of East Timor  

Microsoft Academic Search

Petrologic, structural and age investigations of the Lolotoi Complex of East Timor indicate that it is part of a group of thin metamorphic klippe found throughout the region that were detached from the Banda forearc and accreted to the NW Australian continental margin during Late Miocene to Present arc-continent collision. Metamorphic rock types are dominated by (in order of abundance),

C. R. Standley

2006-01-01

316

NOAA Ship Okeanos Explorer 2013 Field Season on the U.S. Atlantic Continental Margin  

NASA Astrophysics Data System (ADS)

During the 2013 field season, Okeanos Explorer used its suite of state-of-the-art sonars to systematically map and explore our nation's waters off the Atlantic seaboard, specifically the Atlantic Canyons and New England Seamounts. High resolution three dimensional maps created from the ship's sonars were used to select sites for fine-resolution exploration with the new 6000 meter remotely operated vehicle (ROV) Deep Discoverer. Scientists onboard and onshore around the world were actively engaged in all steps of the exploration process, from the identification of broad-scale mapping targets, to the planning and real-time refining of 300 to 1500 meter long planned ROV dive tracks. Live video feeds were available to the general public through the award winning website www.oceanexplorer.noaa.gov. Important exploration milestones during the Okeanos Explorer 2013 Field Season include: the completion of comprehensive, high-resolution multibeam mapping of the continental shelf break from Cape Hatteras to the northern U.S. Atlantic offshore border, totaling over 100,000 square kilometers of new seafloor data within the U.S. Exclusive Economic Zone; the first successful field season of the ROV Deep Discoverer, which conducted over 40 successful dives; and initial mapping site characterization of eight of the outer seamounts of the New England Seamount Chain. All data collected by Okeanos Explorer are available via the NOAA public archives with metadata records within 60 to 90 days of the end of each cruise.

Lobecker, E.; Malik, M.; Skarke, A. D.

2013-12-01

317

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

USGS Publications Warehouse

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

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

1974-01-01

318

Methylmercury accumulation in plankton on the continental margin of the northwest Atlantic Ocean.  

PubMed

Accumulation of monomethylmercury (MMHg) by plankton is a key process influencing concentrations of this toxic mercury species in marine food webs and seafood. We examined bioaccumulation and biomagnification of MMHg in microseston and four size fractions of zooplankton on the continental shelf, slope, and rise of the northwest Atlantic Ocean. The bioaccumulation factor (BAF, L/kg) for MMHg in microseston averaged 10(4.3±0.3) among 21 locations, and concentrations were unrelated to those in colocated, filtered surface water. Instead, concentrations and the BAF of MMHg in microseston were related inversely with total suspended solids in surface water, a proxy for planktonic biomass at these remote locations. MMHg was biomagnified by a factor of 4 from microseston to zooplankton, and both concentrations of MMHg and the fraction of total mercury as MMHg increased with larger size fractions of zooplankton. These results suggest that the initial magnitude of MMHg uptake into pelagic marine food webs is influenced by the degree of primary production in surface waters and propagated up through large zooplankton. Accordingly, biological productivity, in addition to inputs of MMHg to surface waters, must be considered when predicting how MMHg bioaccumulation will vary spatially and temporally in the ocean. PMID:23488773

Hammerschmidt, Chad R; Finiguerra, Michael B; Weller, Robert L; Fitzgerald, William F

2013-04-16

319

Morphological and sedimentological characters of the East Sea (Japan Sea) continental margin  

NASA Astrophysics Data System (ADS)

Consecutive geophysical and geological surveys were conducted in the southwestern margin of the East Sea (Japan Sea) in Korea in order to investigate the complex patterns of morphological and sedimentological characters as well as to understand their relationship with various oceanographic agents. A total of 4,200 line-km seismic records and 172 samples were analyzed so that a series of maps could be made and compared. Based on the results it is revealed that the study area could be divided into five unique subenvironments according to slope changes and surface sediments; flat monotonous shelf (inner and outer), steep and irregular slope (upper and lower) and gentle basin plain. Tectonic movement, sea level change, earthquake, and regional current system are seemingly the major controlling factors to formulate the regional scale morphology and sediment distribution pattern.

Kim, Seong-Pil; Koo, Bon Young; Kong, Gee Soo; Um, Inkwon; Lee, Gwang Soo; Chung, Gong Soo; Choi, Jin-Yong

2014-05-01

320

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

321

A Seismic Refraction Analysis of the Flemish Cap Continental Margin (E Canada): New Evidence for Asymmetric Rifting from Goban Spur (NW Europe)  

NASA Astrophysics Data System (ADS)

The Flemish Cap - Goban Spur conjugate margin was one of the first margin pairs where deep seismic reflection data were used to determine the style of non-volcanic rifting. These studies supported of a symmetric pure shear model of extension followed by an asymmetric breakup. In contrast, a more recent seismic refraction study of Goban Spur indicates that extension is more complex and includes a wide transition zone interpreted as serpentinized mantle. In order to determine a complete conjugate section, the Flemish Cap margin has been re-examined with a 460-km-long refraction seismic profile, including dense airgun shots to 21 OBS receivers along the original deep MCS reflection profile. A P-wave velocity model has been developed by forward and inverse methods to define the crustal thickness, structure and composition of the crust and uppermost mantle along the line. The velocity model displays continental crust with a maximum thickness of 30 km. The crust thins rapidly to a 6-km-thick, 30-km-wide zone of highly extended continental crust. Farther seaward, the velocities in the lower part of the crust increase and this 85-km-wide transition zone is interpreted as partially serpentinized mantle. A sharp boundary separates the transition zone from 6-km-thick oceanic crust. The thin continental crust, transition zone and oceanic crust are overlain by sediment layers of up to 3 km thickness on the thin continental crust, decreasing to 1 km thickness on the oceanic crust. The transition zone on Flemish Cap displays somewhat higher velocities in the lower crust and Moho is a bit deeper than in the zone of serpentinized mantle on Goban Spur, but the transition zones have approximately similar width. On both margins changes in the velocity structure are observed at about the same place relative to magnetic anomaly 33-34. However, the thinning of the continental crust occurs differently on the two margins. The crust thins more rapidly for Flemish Cap than for Goban Spur. Furthermore, a zone of thin continental crust, as observed on Flemish Cap, is not observed on Goban Spur. These new results from Flemish Cap, together with the recent results from Goban Spur, indicate that rifting of the Flemish Cap - Goban Spur conjugate margin is asymmetric and includes transitional zones of serpentinized mantle between continental and oceanic crust, which is similar to results from other non-volcanic margins.

Gerlings, J.; Louden, K. E.; Jackson, H. R.

2009-04-01

322

The crustal structure of the Flemish Cap Continental Margin (E Canada): New Evidence for Asymmetric Rifting from Goban Spur (NW Europe)  

NASA Astrophysics Data System (ADS)

The Flemish Cap - Goban Spur conjugate margin pairs were formed by continental extension starting in Barremian (126 -128 Ma) with final breakup leading to formation of oceanic crust in Albian (~110 Ma). These conjugate margin pairs have previously been studied by deep seismic reflection profiles and used in support of a symmetric pure shear model of extension followed by an asymmetric breakup. In contrast, a more recent seismic refraction study of Goban Spur indicates that extension is more complex and includes a wide transition zone interpreted as serpentinized mantle. In order to determine a complete conjugate section, the Flemish Cap margin has been re-examined with a 460-km-long refraction seismic profile, including dense airgun shots to 21 OBS receivers along the original deep MCS reflection profile. The refraction profile extends more than 50 km seaward of magnetic anomaly 34 and hence well on to oceanic crust. P-wave and S-wave velocity modeling by forward and inverse methods were carried out to define the thickness, structure and composition of the crust and uppermost mantle along the line. A comparison of the Flemish Cap and Goban Spur velocity models gives new insight into the rifting style of the two margins. The continental crust on the Flemish Cap margin thins from a 31 km thick crust to about 7 km over a zone of 40 km. This thinning is more rapid than for the crust on the conjugate Goban Spur margin, which thins from ~27 km thick crust to 7 km over a zone of ~100 km. Furthermore, the two margins display different crustal characteristics in the transition zones. S-wave modeling indicated that the transition zone consists of thin continental crust on the Flemish Cap margin, in contrast to the serpentinized mantle on the Goban Spur margin. The Flemish Cap velocity model also shows a layer of partly serpentinized mantle below the thin continental. A landward dipping reflection, observed on the MCS profile, is coincident with the landward termination of the serpentinized layer. These new results from Flemish Cap, together with the recent results from Goban Spur show that the transition zones are of similar widths but have different characteristics. These differences indicate asymmetric extension and asymmetric breakup between the Flemish Cap - Goban Spur conjugate margin, including asymmetric transitional zones between continental and oceanic crust.

Gerlings, J.; Louden, K. E.; Jackson, H. R.

2009-12-01

323

The impact of U.S. continental outflow on ozone and aerosol distributions over the western Atlantic  

NASA Technical Reports Server (NTRS)

Aircraft measurements of selected trace gas species, aerosols, and meteorological parameters were performed in the lower troposphere off the U.S. east coast during August and September 1989 as part of the NASA Global Tropospheric Experiment (GTE) Chemical Instrumentation Test and Evaluation (CITE 3) expedition. In this paper, we examine these data to assess the impact of continental outflow on western Atlantic O3 and small aerosol budgets. Results show that mixed layer (ML) O3 concentrations and small aerosol number densities (Np) were enhanced by factors of 3 and 6, respectively, within air masses of predominantly continental origin compared with clean maritime background air. These enhancements exhibited a marked altitude dependence, declining rapidly above the ML to the point where only slight to moderate differences in O3 and Np, respectively, were notable above 2.4 km. Within continentally influenced ML's, both O3 and Np were correlated with CO, exhibiting linear regression slopes averaging 0.4 ppbv (O3)/ppbv(CO) for O3 and 7.7 (particles/cc)/ppbv(CO) for Np and indicating a primarily anthropogenic origin for the observed enhancement of these species. Comparisons between profiles in continental and background maritime air masses suggest that photochemical production below 1.4-km altitude adds over 10% to western Atlantic tropospheric column O3 abundance in continental outflow regimes. For aerosols, eastward advection of low-level continental air contributes an average net flux of 2.8 metric tons of submicron (accumulation mode) particles per kilometer of shoreline per day to the western Atlantic troposphere.

Anderson, B. E.; Gregory, G. L.; Barrick, J. D. W.; Collins, J. E., Jr.; Sachse, G. W.; Bagwell, D.; Shipham, M. C.; Bradshaw, J. D.; Sandholm, S. T.

1993-01-01

324

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

NASA Astrophysics Data System (ADS)

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

Aidi, Chafik; Beslier, Marie-Odile; Yelles-Chaouche, Karim; Ribodetti, Alessandra; Bracene, Rabah; Schenini, Laure; Djellit, Hamou; Sage, Françoise; Déverchère, Jacques; Medaouri, Mourad; Klingelhoefer, Frauke; Abtout, Abdeslam; Charvis, Philippe; Bounif, Abdallah

2014-05-01

325

Late Quaternary paleohydrology deduced from new marine sediment cores taken on the proximal Amazon continental margin  

NASA Astrophysics Data System (ADS)

Throughout the late Quaternary the Amazon Basin has been influenced by abrupt North-South climate forcing and has undergone several large climate variations as recorded in previously reported speleothem records. Despite its importance in the global carbon cycle there are few continuous, high-resolution records of the Amazon Basin that date back to and beyond the last glacial period. In this study, we report the first results of a marine geological expedition to the Amazon continental shelf and fan region. During this expedition we collected eight ~30 meter piston cores along with gravity, box and multicores. At both sites we undertook complementary multibeam and high resolution seismic reflection profiling. Analyses will be presented from two sets of box/gravity/piston cores. One core (32m) is from a high sedimentation site on the northern flank of the main submarine canyon within the Amazon Fan complex at 1700m water depth. The other core (30m) is located on a seamount to the south of the Amazon Fan complex at 3100m water depth. A mixed assemblage of foraminifera is used for 14C dating to obtain an age model and bulk organic geochemistry is analyzed to determine percent organic carbon, C/N ratios, ?13C and ?15N. The cores were continuously measured shipboard for magnetic susceptibility and gamma density using a GEOTEK logger. These findings uncover the contribution of pelagic and terrestrial organic matter, whether the terrigenous carbon is derived from C3 versus C4 vegetation, and whether the marine organic matter is composed of phytoplankton or marine algae.

Nace, T.; Baker, P. A.; Dwyer, G. S.; Hollander, D. J.; Silva, C. G.

2010-12-01

326

Nutrient distributions, transports, and budgets on the inner margin of a river-dominated continental shelf  

NASA Astrophysics Data System (ADS)

Physical and biogeochemical processes determining the distribution, transport, and fate of nutrients delivered by the Mississippi and Atchafalaya river basin (MARB) to the inner Louisiana continental shelf (LCS) were examined using a three-dimensional hydrodynamic model and observations of hydrography, nutrients, and organic carbon collected during 12 cruises. Two aspects of nutrient transport and fate on the inner LCS (<50 m depth) were evaluated: (1) along-shelf and cross-shelf transports were calculated and (2) nutrient sinks and sources were inferred. On average, 47% of the lower Mississippi River freshwater traveled westward on the LCS, but this percentage was reduced during summer when currents reversed to a predominately upcoast direction. Changes from mainly inorganic to organic nutrients were observed at salinity between 20 and 30, and above 30, organic nutrients were the dominant forms. Westward transport of dissolved inorganic nitrogen (DIN) was about 25% of the combined DIN load from the MARB, whereas westward transport of dissolved organic nitrogen (DON) was 2.8-fold larger than the MARB DON load. Different from dissolved inorganic nutrients, for which the rivers were the primary source, the dominant source of organic nutrients was advection from offshore. Overall, the inner LCS was estimated to be a net sink for total nitrogen in the amount of -3.14 mmol N m-2 d-1 and a net sink for total phosphorus in the amount of -0.28 mmol P m-2 d-1. These sinks were approximately 33% and 59% of the total N and P sources, respectively, to the inner LCS.

Lehrter, John C.; Ko, Dong S.; Murrell, Michael C.; Hagy, James D.; Schaeffer, Blake A.; Greene, Richard M.; Gould, Richard W.; Penta, Bradley

2013-10-01

327

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

NASA Technical Reports Server (NTRS)

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

Miller, M. Meghan

1998-01-01

328

Deep structure and structural inversion along the central California continental margin from EDGE seismic profile RU-3  

SciTech Connect

Deep-penetration seismic reflection profile RU-3 reveals a subducted oceanic plate, a modified accretionary prism, and complex structures of the overlying sedimentary basins. This structural framework was established by subduction processes during Paleogene and earlier time and subsequently was modified by Neogene transform motion combined with apparent components of extension and compression. Subducted rocks are indicated by deep, gently dipping reflectors that extend beneath the continental margin for at least 38 km at a depth of about 15 km. The authors interpret the subducted crust as either a part of the Pacific plate or, more likely, a subducted fragment derived from the Farallon plate. A set of more steeply dipping, deep events may indicate faulting within the subducted plate or its boundary with a no-slab zone. The overlying, largely nonreflective layer of accreted material rapidly reaches 10 km in thickness landward of the paleotrench and increases to 15 km in thickness near the coast. The Santa Lucia Basin, landward of the steep continental slope, originated as a slope basin during Paleogene subduction. The lower strata of this basin were deposited onto and partially incorporated into the accretionary complex. The offshore Santa Maria Basin exhibits a variety of compressional structures that formed in the last 3.5 m.y. and whose locations correspond to an earlier framework of extensional faults. Structural inversion has occurred in Miocene depocenters adjacent to the Santa Lucia Bank fault and at the Queenie structure. Miocene and lower Pliocene strata also thicken toward the Hosgri fault zone where subsequent compression is characterized by low-angle thrusts and folding.

McIntosh, K.D.; Reed, D.L.; Silver, E.A. (Univ. of California, Santa Cruz (USA)); Meltzer, A.S. (Rice Univ., Houston, TX (USA))

1991-04-10

329

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

Microsoft Academic Search

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

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

1989-01-01

330

Particulate barium fluxes on the continental margin: a study from the Alboran Sea (Western Mediterranean)  

Microsoft Academic Search

Particulate biogenic barium (bio-Ba) fluxes obtained from three instrumented arrays moored in the Alboran Sea, the westernmost basin in the Mediterranean Sea, are presented in this study. The mooring lines were deployed over almost 1 year, from July 1997 to May 1998, and were equipped with sediment traps at ?500–700 m depth, ?1000–1200 m depth and 30 m above the

Anna Sanchez-Vidal; Robert W. Collier; Antoni Calafat; Joan Fabres; Miquel Canals

2005-01-01

331

Early Cretaceous »events« in the evolution of the eastern and western North Atlantic continental margins  

Microsoft Academic Search

Zusammenfassung  Ein bedeutender »stratigraphischer Wendepunkt« kennzeichnet die frühneokome Entwicklung in vielen nordatlantischen Kontinentalrändern: auf eine Karbonatbankbzw. pelagische Karbonat-Fazies (Oberjura bis Berrias) folgt eine mehr hemipelagische Sedimentation mit mächtigen »Wealden«-ähnlichen Deltas und benachbarten Tiefseefächern (Valendis bis Barrême).Die Stratigraphie und Tektonik des sehr alten, »verhungerten«, passiven Kontinentalrandes im Gebiet des Mazagan-Plateaus und -Steilhanges vor Marokko wurde während der deutsch-französischen CYAMAZ-Tieftauchexpedition untersucht. Die »Ertränkung«

Ulrich von Rad; Massimo Sarti

1986-01-01

332

Relationship Between Subduction Erosion, Seamount Subduction, Fluid Venting and Mound Formation on the Slope of the Costa Rican Continental Margin  

NASA Astrophysics Data System (ADS)

The oceanic crust off central Costa Rica northwest of the Cocos Ridge is dominated by chains of seamounts rising 1-2 km above the seafloor with diameters of up to 20 km. The subduction of these seamounts leads to strong indentations, scars and slides on the continental margin. A smoother segment of about 80 km width is located offshore Nicoya peninsula. The segment ends at a fracture zone which marks the transition of oceanic crust created at the Cocos-Nazca spreading center (CNS) and at the East Pacific Rise (EPR). Offshore Nicaragua the incoming EPR crust is dominated by bending related faults. To investigate the relationship between subduction erosion, fluid venting and mound formation, multibeam bathymetry and high-resolution deep-tow sidescan sonar and sediment echosounder data were acquired during R/V Sonne cruises SO163 and SO173 (2002/2003). The deep-tow system consisted of a dual-frequency 75/410 kHz sidescan sonar and a 2-12 kHz chirp sub-bottom profiler. The connection of the observed seafloor features to deeper subduction related processes is obtained by analysis of multi-channel streamer (MCS) data acquired during cruises SO81 (1992) and BGR99 (1999). Data examples and interpretations for different settings along the margin are presented. Near the Fisher seamount the large Nicoya slump failed over the flank of a huge subducted seamount. The sidescan and echosounder data permit a detailed characterization of fault patterns and fluid escape structures around the headwall of the slump. Where the fracture zone separating CNS and EPR crust subducts, the Hongo mound field was mapped in detail. Several mounds of up to 100 m height are located in line with a scar possibly created by a subducting ridge of the fracture zone. MCS data image a topographic high on the subducting oceanic crust beneath the mound field which lead to uplift and possibly enabled ascent of fluids from the subducting plate. The combined analysis of geoacoustic and seismic MCS data confirms that fracturing of the continental slope by subducting oceanic relief is a major mechanism which causes the opening of pathways for fluids to migrate upwards.

Petersen, C.; Klaucke, I.; Weinrebe, W.

2006-12-01

333

Benthic-Pelagic Coupling: Effects on Nematode Communities along Southern European Continental Margins  

PubMed Central

Along a west-to-east axis spanning the Galicia Bank region (Iberian margin) and the Mediterranean basin, a reduction in surface primary productivity and in seafloor flux of particulate organic carbon was mirrored in the in situ organic matter quantity and quality within the underlying deep-sea sediments at different water depths (1200, 1900 and 3000 m). Nematode standing stock (abundance and biomass) and genus and trophic composition were investigated to evaluate downward benthic-pelagic coupling. The longitudinal decline in seafloor particulate organic carbon flux was reflected by a reduction in benthic phytopigment concentrations and nematode standing stock. An exception was the station sampled at the Galicia Bank seamount, where despite the maximal particulate organic carbon flux estimate, we observed reduced pigment levels and nematode standing stock. The strong hydrodynamic forcing at this station was believed to be the main cause of the local decoupling between pelagic and benthic processes. Besides a longitudinal cline in nematode standing stock, we noticed a west-to-east gradient in nematode genus and feeding type composition (owing to an increasing importance of predatory/scavenging nematodes with longitude) governed by potential proxies for food availability (percentage of nitrogen, organic carbon, and total organic matter). Within-station variability in generic composition was elevated in sediments with lower phytopigment concentrations. Standing stock appeared to be regulated by sedimentation rates and benthic environmental variables, whereas genus composition covaried only with benthic environmental variables. The coupling between deep-sea nematode assemblages and surface water processes evidenced in the present study suggests that it is likely that climate change will affect the composition and function of deep-sea nematodes. PMID:23565176

Pape, Ellen; Jones, Daniel O. B.; Manini, Elena; Bezerra, Tania Nara; Vanreusel, Ann

2013-01-01

334

Transfer of organic carbon on the Moroccan Atlantic continental margin (NW Africa): productivity and lateral advection  

NASA Astrophysics Data System (ADS)

Land—ocean transfer of sediment and organic matter along the Moroccan Atlantic margin (NW Africa) seems to have been very effective during the last 130 ka. In a marine core from this region, we found total organic carbon (TOC) values ranging from 0.3 to 1.7 dry wt% of bulk sediments. These relatively high values are fairly unusual, as the core was recovered from an open-ocean environment that is currently oligotrophic. In order to explain this trend, more typical of an upwelling eutrophic setting, three processes were evaluated: (1) in situ primary production associated with the extension of the Cape Ghir upwelling filament, (2) bottom water conditions that may favour organic carbon preservation and (3) lateral organic carbon advection. The site occasionally experienced more eutrophic conditions, especially during termination I; here, we recorded a relative high abundance of the planktonic foraminifer Globigerina bulloides, suggesting high primary production. However, given the absence of correlation between TOC and G. bulloides records, high TOC storage cannot be attributed exclusively to primary production. Preservation factors such as bottom water ventilation are also ruled out. Lateral TOC advection seems to be the most plausible process. Today, lateral advection and offshore transport of nutrients and organic matter characterize the study region. However, the triggering mechanisms deserve further investigation. Different controlling factors influencing the mobilization and advection of organic carbon from coastal upwelling sites to the deep basin are discussed. The correlation found between down-core TOC and sea-level changes suggests sea-level fluctuations as the most effective mechanism driving nepheloid layer detachment and seaward material transport.

Bozzano, Graziella; Alonso, Belén

2009-10-01

335

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

SciTech Connect

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

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

1988-08-01

336

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

SciTech Connect

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

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

1994-02-01

337

Magnetic characterization of distal IRD layers at the NW Iberia Continental margin  

NASA Astrophysics Data System (ADS)

Deep marine environments are a sink for diverse materials from very distinct sources. The magnetic signal retrieved from these sediments reflect a combination of magnetic carriers, arriving as IRD (ice rafted debris), transported as nepheloid layers or as result of aeolian contribution (Thompson and Oldfield, 1986; Verosub and Roberts, 1995; Dekkers, 1997; Maher and Thompson, 1999; Evans and Heller, 2004). IRD layers are widelly distributed along the Northern Atlantic, representing a distal input transported by icebergs released from the major continental ice caps during the Heinrich events (eg. Robinson, 1986; Heinrich, 1988; Bond et al., 1992; Oppo et al., 1998; Kissel et al., 1999). At latitudes ranging the Rudimann belt (40-55N) (Rudimann, 1977; Rudimann and McIntire, 1981), IRD layers can be identified by the rapid increase in magnetic susceptibility values (?) up to 400x10-6SI, from background values lower than 100x10-6 SI (Robinson et al., 1995), providing key information on climatically forced events and allowing a tighter chronostratigraphic control, as demonstrated by other authors on nearby areas (eg. Lebreiro et al., 1996; Zahn et al., 1997; Moreno et al., 2002). The mixing of these materials with local/regional components may difficult their depiction, and also the occurence of diagenetic processes that alter their original magnetic composition, to the point of undetection by standard magnetic analysis (susceptibility). Particularly, that was the case on the Galicia Bank half-graben sediment cores, dominated by local biogenic and detrital turbiditic levels during MIS2, in which IRDs are interbedded, topped by hemipelagic sediments deposited during the last 14 ka (Alonso et al, 2008, Rey et al, 2008). Original low concentration, influence of diamagnetic carbonate materials, and /or elimination of magnetic carriers by diagenesis masked some of the IRD levels, only recognizable by detail magnetomineralogical characterization of the materials transported during the Heinrich events. H-events in the study area have been assigned to those horizons that exceed 0.5 times the standard deviation background value recalculated to Ln(k) of a given core after detail magnetic characterization of "IRD-suspicious" layers. The robustness of the method have been demonstrated against increasing grain size trends loops on the ARM vs. K plots, the identification of larger size magnetite grains by low temperature techniques, EDX assisted BS-SEM, consistency with elemental FRX derived composition down-core trends, and 14C ages in hemipelagic layers. The method allowed fast recognition of events H1 to H5 at the study area and subsequently the creation of a robust and inexpensive geochronological framework. Further to this, we have been able to identify the presence of H-layer derived diagenetic greigite in some of the horizons. Bond, G. et al., 1992. Nature, 360, 245- 249. Heinrich, 1988. Quaternary Research, 29, 142-152 . Kissel, 2005. C. R. Geoscience 337, 908-918 Lebreiro, et al.,1996. Marine Geology 131, 47-56. Moreno et al. 2002. EPSL, 202, 465-480. Rey et al., 2008. Mar. Geol, 249, 64-92 . Robinson et al. 1995. Paleoceanography, 10, 221-250 . Ruddiman, W.F. (1977). Geol. Soc. Am. Bull. 88, 1-12 . Thouveny, et al. 2000. EPSL, 180, 61-75. Cont. Proj. CTM2007-61227/MAR, GCL2010-16688, 09MMA012312PR & 10MMA312022PR

Rey, D.; Mohamed, K. J.; Andrade, A.; Rodríguez-Germade, I.; Coimbra, R. L.; Rubio, B.; Bernabeu, A. M.; Alvarez-Iglesias, P.; Frederichs, T.

2012-12-01

338

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

NASA Astrophysics Data System (ADS)

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

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

2014-02-01

339

Three-dimensional inversion of magnetotelluric data from the Central Andean continental margin  

NASA Astrophysics Data System (ADS)

Magnetotelluric data were collected in the late 1990s in the Central Andes of Chile and Bolivia, with the aim to delineate the electrical conductivity distribution in the subsurface and its relations to subduction processes. In previous studies, these data were interpreted based on 2-D models. The principal result was a vast conductivity zone beneath the Altiplano high plateau at mid and lower crustal depths and a much smaller, though significant conductor associated with the Precordillera Fault System. However, there are some significant 3-D effects in the investigation area, in particular near the coast and on the eastern Altiplano. The aim of this work is to give a reinterpretation based on new 3-D inversion of these data. The 3-D inversion not only provides a better fit to the data compared to 2-D results but furthermore allows to include sites with strong telluric distortion which were ignored in previous studies. We are now able to explain anomalous phases above 90° and induction arrows pointing subparallel to the coast as observed at several sites in the Coastal Cordillera. These strongly distorted data are caused by highly conductive near-surface structures that are partly connected to the Pacific Ocean, forcing currents to flow around the sites. The lower crust beneath the Coastal Cordillera resembles a poorly conductive, nearly homogeneous half-space and is electrically unremarkable. Besides, we can now image the Precordillera conductor as a continuous, elongated feature. The volcanic arc of the Western Cordillera is highly resistive with the exception of a few conductive spots which may be associated with certain individual volcanoes or geothermal resources, respectively. The Altiplano conductor is again the dominant electrical feature in the Central Andes, indicating widespread melting of the middle and lower back-arc crust.

Kühn, Christine; Küster, Jonas; Brasse, Heinrich

2014-12-01

340

Early carboniferous transgression on a passive continental margin: Deposition of the Kekiktuk Conglomerate, northeastern Brooks Range, Alaska  

SciTech Connect

The base of the Ellesmerian sequence in the northeastern Brooks Range is a widespread, terrigenous, clastic succession assigned to the Kekiktuk Conglomerate, a potential hydrocarbon reservoir across much of the North Slope of Alaska and adjacent offshore regions of the Beaufort and Chukchi Seas. The Kekiktuk is situated above a regional angular unconformity and is the base of a transgressive succession that is thought to record coastal onlap associated with the formation of a passive continental margin. Six depositional units have been recognized in the Kekiktuk Conglomerate. A systematic description of the sedimentologic organization of the Kekituk Conglomerate as developed in outcrop in the northeastern Brooks Range is presented and the regional stratigraphic distribution of depositional units is shown. From the data, it is suggested that thermal subsidence landward of the tectonic hinge zone produced the regional accommodation to which the Kekiktuk dispersal system responded. In addition, it is proposed that the primary control on the distribution of reservoir-quality strata within this setting was a regional variation in erosional relief along the sub-Mississippian unconformity. A paleographic reconstruction of the evolution of Kekiktuk depositional environments is presented which we believe will serve as a predictive model for the distribution of lithologies and potential reservoir strata in the subsurface. 101 refs., 9 figs., 1 tab.

LePain, D.L. (Univ. of Alaska, Fairbanks (United States) Shell Offshore, Inc., New Orleans, LA (United States)); Crowder, R.K.; Wallace, W.K. (Univ. of Alaska, Fairbanks (United States))

1994-05-01

341

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

SciTech Connect

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

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

2002-08-08

342

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

NASA Astrophysics Data System (ADS)

Between June, 24th and July, 22nd, 2004, a team of European scientists embarked from Brindisi (Italy) to Barcelona (Spain) onboard the Russian vessel "Bavenit", operated by the Dutch geotechnical company FUGRO, for a drilling expedition in the Adriatic Sea and the NW Mediterranean Sea. The purpose of this cruise was to collect long sediment sections and in situ measurements from two deltaic margins where the history of global changes during the last ca. 400 kyr is particularly well preserved. In the Adriatic, two boreholes were drilled at site PRAD1 (water depth 184 m), where the objective was to study the record of the last 4 glacial cycles. A pilot hole was first drilled for assessing the risk of shallow gases, a downhole logging was carried out in this borehole. A second site allowed continuous coring to the targeted depth (71m below sea-floor) with excellent recovery (better than 95%). Very preliminary interpretation indicates that seismic sequences previously identified correspond to 100 kyr glacial cycles. Downhole logging and physical properties of cores allow to identify magnetic events, and tephras. Site PRAD2 was devoted to the study of the recent most sediments (last 12,000 yrs) near the coastline, at a water depth of 56m. The targeted depth was 32 m below sea floor, sufficient to obtain a good record for the last ca 12,000 years. All together, six boreholes were drilled at PRAD2, including a pilot hole, one for continuous sediment recovery, and additional holes for in situ geotechnical tests and sampling. One of the objectives of these tests is to determine whether the wavy features shaping the sedimentary sequences are caused by near-bottom currents or result from liquefaction of unstable sediments triggered by earthquakes or storms. Site PRGL1 in the Gulf of Lion is at 298 m water depth, and the targeted depth below sea floor was 300 m, allowing to reach an expected age of about 430 kyr BP. A pilot hole was drilled down to 310 mbsf, and logged. Two geotechnical boreholes were drilled, allowing tests and measurements to a depth of 150 mbsf. Another borehole was drilled for continuous coring to the depth of 300 mbsf. The recovery was excellent (>95%). Preliminary estimations of coccolithophore assemblages provide a general time-frame for this site. Marine isotope stage (MIS) 12 was reached at the bottom of the hole. We have also good estimates of the position of the intervals corresponding to MIS 2-3, MIS 4, MIS 5a-d, and the transition between MIS 8 and 7. This shows that, as in the Adriatic Sea, seismic bounding surfaces are linked to 100 kyr cycles, that modify lithology and sedimentation rates on the upper slope. The presence of coarser sediment at the end of each "forced regression", and the occurrence of some biogenic gas, trapped by the overlying clayey sediments deposited during the ensuing warm period, is likely at the origin of seismic anomalies. Site PRGL2 is at 103 m water depth, an area where glacial shorelines that formed duringthe last ca. 500 kyr are the best preserved. A CPTU borehole was first drilled, followed by a sampling borehole, down to 100 mbsf. As expected, many sandy intervals were encountered, but the overall recovery was however quite good, in the order of 82%. Gamma ray downhole logging was performed in the drill pipe afterward. PROMESS 1 is an European Community funded project of the 5th framework programme (EVR1-2001-41). It belongs to the OMARC cluster of projects. It is a companion project of the joint Euro-US "EUROSTRATAFORM" project. The "PROMESS 1" shipboard party: S. Berne, M. Canals, A. Cattaneo, E. Colmenero, G. Floch, B. Dennielou, J. Frigola, R. Gelfort, J. Gravalosa, D. Ridente, T. Schoolmeester, N. Sultan, G. Tulloch, H.J. Wallrabe-Adams

Berne, S.

2004-12-01

343

On the origin and flow behavior of submarine slides on deep-sea fans along the Norwegian–Barents Sea continental margin  

Microsoft Academic Search

Debris lobes with characteristic lengths, widths, and thickness of 30–200?km, 2–10?km, and 10–50?m, respectively, represent\\u000a the main building blocks of deep-sea fans along the Norwegian–Barents Sea continental margin. Their formation is closely related\\u000a to the input of clay-rich sediments to the upper continental slope by glaciers during periods of maximum ice advance. It is\\u000a likely that slide release was a

A. Elverhøi; H. Norem; E. S. Andersen; J. A. Dowdeswell; I. Fossen; H. Haflidason; N. H. Kenyon; J. S. Laberg; E. L. King; H. P. Sejrup; A. Solheim; T. Vorren

1997-01-01

344

Pervasive evidence for seabed fluid expulsion along upper slope of the U.S. Atlantic continental margin  

NASA Astrophysics Data System (ADS)

Warming-induced hydrate dissociation along the US Atlantic margin (USAM) is poorly understood due to an absence of direct evidence for both in situ methane hydrate and seabed gas venting. Using high-resolution multibeam bathymetric data collected on the shelf-edge and upper slope from North Carolina to Canada, we map more than 5000 pockmarks in water depths of 120 to 700 m. The pockmarks are semicircular, ranging from 50-500 m in diameter, with the vast majority being 100-200 m wide, and 5-15 m in relief. Pockmarks are concentrated in and around canyon heads just seaward of the shelf-edge rollover, but are not found farther downslope. We utilize a dense grid of high-resolution multichannel seismic reflection profiles along the Southern New England stretch of the margin to examine the relationships among the pockmarks, substrate gas/fluid migration, and Pleistocene stratigraphy. By calculating seismic energy and gas-chimney meta-attributes along both profiles we are able to detect high-energy zones and identify probable fluid-migration pathways below the outer shelf and slope. Pockmarks overlie highly disrupted substrate containing abundant evidence for gas pockets (high-amplitude, inverse polarity reflectors) and high probability for fluid chimneys. Approximately coincident with the downslope extent of the pockmark fields (450-600 m depth), a series of enhanced reflectors that were delineated based on attribute analyses, appear within the gas hydrate stability zone (GHSZ) or near its base. The continuation of some of these reflectors upslope beyond the present-day GHSZ, the onset of pockmarks near the upslope extent of the GHSZ, and the widespread occurrence of gas chimneys and other fluid flow features in this same area implies that the GHSZ on this margin may be metastable. As the GHSZ oscillates up and down the slope, gas is released and reformed as new hydrate; associated fluids are inferred to migrate upslope and are expelled within the pockmark field. These results have implications for the spatial distribution of hydrate-bearing sediments along the upper continental slope of the USAM, the development of vertically oriented substrate permeability pathways, and the relationships among ocean warming, hydrate dissociation and slope failure.

Brothers, D. S.; Ruppel, C. D.; Kluesner, J. W.; Chaytor, J. D.; Ten Brink, U. S.; Hill, J. C.

2013-12-01

345

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

346

Timing and Composition of Continental Volcanism, Harrat Hutaymah, Western Saudi Arabia  

NASA Astrophysics Data System (ADS)

One of the largest alkali basalt provinces in the world (area: 180,000 km2) is located in the western part of the Arabian Peninsula. These extensive Cenozoic basaltic lava fields ('harrats') erupted from N-S oriented volcanic centers that lie within 200 km of the NW-trending eastern margin of the Red Sea. The origin of these fields is partially related to regional extension of thick, Pan-African craton that began ~30 Ma, but there is no reflection of these volcanic centers on the western margin (African Plate). Lava compositions in the region include alkali ol-basalts, ol-transitional basalts, hawaiites, and more evolved compositions, which contrast with the tholeiitic basalts of the Red Sea. In the harrat province early, Red Sea-parallel volcanism (>12 Ma) was dominated by tholeiitic to transitional compositions, but has become more alkalic with younger, N-S oriented eruptive centers (<12 Ma). A prominent volcanic structure, the Makkah-Madinah-Nafud (MMN) line, appears to coincide with the axis of uplift of the Arabian shield, beginning ~15 Ma. Therefore flexure and/or asthenospheric flow may explain the timing, distribution and variable depth and degree of partial melting across the region. We selected the lava field of Harrat Hutaymah, on the northeastern-most edge of the harrat province for high-resolution age and compositional investigation. Previous dating at this harrat (a single K/Ar age; 1.8 Ma) is suspiciously old given the common occurrence of xenolithic material in the lava flows and the field measurement of only normal magnetic polarity. We report new age determinations by the 40Ar-39Ar incremental step heating method and major, trace and rare earth element compositions to better constrain the time frame of volcanism and chemical variation at Harrat Hutaymah. We use these new data to model the depth and degree of partial melting beneath Harrat Hutaymah (65-80 km; ~5%). This compares with contemporaneous Harrat Rahat (20-40 km; 10-15%), part of the central MMN axis, Harrat Lunayyir (60-75 km; 7-12%), 100 km east of the Red Sea, and the Red Sea spreading axis (0-10 km; 25%). This variability in mantle melting is explained by regional lithospheric extension and mantle decompression melting coupled with northward asthenospheric flow from the Afar hot spot.

Schlieder, T.; Duncan, R. A.; Al-Amri, A. M.; Al-Shaltoni, S.

2013-12-01

347

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

348

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

NASA Astrophysics Data System (ADS)

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

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

349

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

SciTech Connect

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

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

1992-01-01

350

Determining OCT structure and COB Location of the Omani Gulf of Aden Continental Margin from Gravity Inversion, Residual Depth Anomaly and Subsidence Analysis.  

NASA Astrophysics Data System (ADS)

Knowledge and understanding of the ocean-continent transition (OCT) structure and continent-ocean boundary (COB) location, the distribution of thinned continental crust and lithosphere, its distal extent and the start of unequivocal oceanic crust are of critical importance in evaluating rifted continental margin formation and evolution. In order to determine the OCT structure and COB location for the eastern Gulf of Aden, along the Oman margin, we use a combination of gravity inversion, subsidence analysis and residual depth anomaly (RDA) analysis. Gravity inversion has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning; subsidence analysis has been used to determine the distribution of continental lithosphere thinning; and RDAs have been used to investigate the OCT bathymetric anomalies with respect to expected oceanic bathymetries at rifted margins. The gravity inversion method, which is carried out in the 3D spectral domain, incorporates a lithosphere thermal gravity anomaly and includes a correction for volcanic addition due to decompression melting. Reference Moho depths used in the gravity inversion have been calibrated against seismic refraction Moho depths. RDAs have been calculated by comparing observed and age predicted oceanic bathymetries, using the thermal plate model predictions from Crosby and McKenzie (2009). RDAs have been computed along profiles and have been corrected for sediment loading using flexural back-stripping and decompaction. In addition, gravity inversion crustal basement thicknesses together with Airy isostasy have been used to predict a synthetic RDA. The RDA results show a change in RDA signature and may be used to estim