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1

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

2

Estimates of geothermal gradients and heat flow from BSRs along the Western Continental Margin of India  

Microsoft Academic Search

Geothermal gradients and heat flow were estimated from the position of Bottom Simulating Reflectors (BSRs) identified on seismic reflection sections from the Western Continental Margin of India (WCMI). The estimated geothermal gradients along the WCMI range between 35 to 65°C\\/km and heat flow varies from 50 to 130 mW\\/m2. The geothermal gradient structure of WCMI inferred from the BSRs, shows

Y. Hanumantha Rao; C. Subrahmanyam; S. R. Sharma; A. A. Rastogi; B. Deka

2001-01-01

3

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

USGS Publications Warehouse

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

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

2007-01-01

4

Deep continental margin reflectors  

USGS Publications Warehouse

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

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

1985-01-01

5

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

6

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

SciTech Connect

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

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

1986-07-01

7

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

8

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

9

Major Ocean Features: Continental Margin  

E-print Network

.noaa.gov 42 Continental margins may be geologically active or passive. Active margins occur along tectonic plate boundaries where earthquakes and/or volcanoes are common. Passive margins are not associated with plate boundaries, experiencing little volcanism and relatively fewer earthquakes. The Atlantic Ocean

10

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

11

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

USGS Publications Warehouse

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

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

1993-01-01

12

Slope failures along the western continental margin of India: a consequence of gas-hydrate dissociation, rapid sedimentation rate, and seismic activity?  

Microsoft Academic Search

Along the western continental margin of India (WCMI), several bottom simulating reflectors were identified on analogue single-channel seismic records, some of these located in areas where slumping and mass wasting were observed. The causes, consequences and degree of geographic variation of these geomorphic processes are assessed in terms of possible gas-hydrate dissociation during Pleistocene sea-level changes, high sedimentation resulting in

Hanumantha Y. Rao; C. Subrahmanyam; A. Rastogi; B. Deka

2002-01-01

13

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

14

The northern Egyptian continental margin  

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

15

Aeolian sands in continental red beds of the Middle Buntsandstein (Lower Triassic) at the western margin of the German Basin  

NASA Astrophysics Data System (ADS)

Aeolian sands occur widespread in continental red beds of the Middle Buntsandstein (Lower Triassic) at the western margin of the German Basin (Middle Europe), in the Eifel area. Cross-bedded sands were mainly deposited by grainfall on lee-slopes of barchanoid-type dunes in unimodal wind regime in both low-energy and high-energy environments. Occasionally, sets are internally truncated by reactivation surfaces. Crest height of solitary-set dunes varies in the range of some meters. Locally, sequences of cosets, both of tabular-planar and wedge-planar type, capped by flat or gently windward dipping topset beds, represent larger dune complexes of about 10-20 m in height and at least 80-100 m in lateral extent. Horizontal-laminated deposits originated as sand sheets in interdune areas. The dune sands were deposited by trade winds of the northern hemisphere in low palaeolatitude, predominantly by southeasterly and southwesterly trade winds in summer when the intertropical convergence zone was shifted to the north. Aeolian sands built up an extensive dune belt in the Eifel, intersected by braided to anastomosing rivers. Fluviatile incursions or heavy ephemeral rainfall led to aquatic redeposition of aeolian sediments and origin of shallow lakes in interdune depressions. Distribution of aeolian and fluviatile sediments in Karlstal-Schichten is different in the Southern, Western and Northern Eifel. Middle Buntsandstein sequence in the Southern Eifel reflects an evolution of the fluviatile depositional environment also incorporating the occurrence and distribution of aeolian sands. Evolution is characterized by increasing spacing and sinuosity of channels, weakening supply of coarse detritus from the source areas and decreasing braiding of the river systems. Aeolian sands occur in the final phase of this evolution which led from local alluvial fans via cobbly and pebbly braided rivers to sandy braided streams and finally to an intertonguing of aeolian dunes and braided to anastomosing rivers, matching with the climax of regional diversification of depositional milieu. The aeolian sand sea in the Eifel covered an area of nearly 2500 km 2 at the time of its largest extension. The dune belt ended in the Northern Eifel, the alternating sequence passed into an entirely alluvial succession which graded northwards by continuously diminishing grain size into the central playa/lacustrine to marine sediment series in Northwest Germany. Palaeogeography, sedimentology and genesis of the Middle Buntsandstein at the western margin of the Mid-European Basin and of Rotliegendes in Northwest Germany appear to be partially similar as is revealed by certain characteristics of the palaeogeographical reconstructions. Concurrence of Middle Buntsandstein palaeogeographical models for both Eifel and the whole German Basin most clearly delineates the key position of the Eifel for interpretation of the Buntsandstein sedimentary environment and the palaeogeography in Middle Europe.

Mader, Detlef

1982-04-01

16

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

17

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

18

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

19

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

E-print Network

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

New Hampshire, University of

20

Seismotectonics of the Norwegian continental margin  

Microsoft Academic Search

The Norwegian continental margin and surrounding areas are seismically less active than some other passive margins worldwide, indicating a potential earthquake deficit. The adjacent oceanic crust is mostly aseismic except for parts of the Lofoten and Norway Basins which have experienced rapid deposition of glacial sediments. The excess load has enhanced the local stress field and, in turn, the seismic

Unni Byrkjeland; Hilmar Bungum; Olav Eldholm

2000-01-01

21

The basins on the Argentine continental margin  

SciTech Connect

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

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

1996-08-01

22

Subsidence and tectonic controls on glacially influenced continental margins: examples from the Gulf of Alaska and the western Scotian Shelf and Slope  

Microsoft Academic Search

The glacial record on continental shelves and slopes at mid- to high latitudes is dominated by marine strata, but little is known regarding the large-scale architecture of such deposits. Models showing the gross architecture and facies successions of Late Cenozoic deposits in the Gulf of Alaska and on the western portion of the Nova Scotian Shelf and Slope of southeastern

Michael R Gipp

2003-01-01

23

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

24

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

25

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

E-print Network

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

Demouchy, Sylvie

26

Geologic structure and halokinesis on the continental margin of Angola  

SciTech Connect

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

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

1986-04-01

27

Evolution of Northeast Atlantic magmatic continental margins  

NASA Astrophysics Data System (ADS)

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

England, Richard; Cornwell, David; Ramsden, Alice

2014-05-01

28

Focused fluid flow in passive continental margins.  

PubMed

Passive continental margins such as the Atlantic seaboard of Europe are important for society as they contain large energy resources, and they sustain ecosystems that are the basis for the commercial fish stock. The margin sediments are very dynamic environments. Fluids are expelled from compacting sediments, bottom water temperature changes cause gas hydrate systems to change their locations and occasionally large magmatic intrusions boil the pore water within the sedimentary basins, which is then expelled to the surface. The fluids that seep through the seabed at the tops of focused fluid flow systems have a crucial role for seabed ecology, and study of such fluid flow systems can also help in predicting the distribution of hydrocarbons in the subsurface and deciphering the climate record. Therefore, the study of focused fluid flow will become one of the most important fields in marine geology in the future. PMID:16286294

Berndt, Christian

2005-12-15

29

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

30

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

E-print Network

South Atlantic margins of Africa. page 1 South Atlantic continental margins of Africa: a comparison of the tectonic vs climate interplay on the evolution of equatorial west Africa and SW Africa margins Michel / +(33) (0)467 54 48 11 Keywords: Continental margin, West Africa, Congo River, Orange River, deep

Paris-Sud XI, Université de

31

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

Microsoft Academic Search

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

Jean Mascle; Emmanuel Blarez

1987-01-01

32

Understanding Continental Margin Biodiversity: A New Imperative  

NASA Astrophysics Data System (ADS)

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

Levin, Lisa A.; Sibuet, Myriam

2012-01-01

33

Geophysical and geologic studies of Ross Sea continental margin, Antarctica  

SciTech Connect

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

Cooper, A.K.

1984-04-01

34

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

35

New magnetic anomaly map of the East Antarctic continental margin  

NASA Astrophysics Data System (ADS)

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

Golynsky, Alexander; Ivanov, Sergey; Kazankov, Andrey

2010-05-01

36

Ecological theory and continental margins: where shallow meets deep  

E-print Network

-0218, USA Continental margins, where land becomes ocean and plunges to the deep sea, provide valuable food on the resources that margins provide, and by warming, expanding hypoxia and acidification associated with climate for numerous protozoan and invertebrate species [4­7]. Given this significant abiotic and biotic heterogen

Levin, Lisa

37

Ecological theory and continental margins: where shallow meets deep  

Microsoft Academic Search

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

Lisa A. Levin; Paul K. Dayton

2009-01-01

38

Continental margin sedimentation: from sediment transport to sequence stratigraphy  

USGS Publications Warehouse

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

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

2007-01-01

39

Comparative biogeochemistry-ecosystem-human interactions on dynamic continental margins  

USGS Publications Warehouse

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

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

2014-01-01

40

Comparative biogeochemistry-ecosystem-human interactions on dynamic continental margins  

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

41

Continental transform margins : state of art and future milestones  

NASA Astrophysics Data System (ADS)

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

Basile, Christophe

2010-05-01

42

Antarctic Drilling Recovers Stratigraphic Records From the Continental Margin  

NASA Astrophysics Data System (ADS)

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

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

2009-03-01

43

Reactivation of Precambrian faults on the southwestern continental margin of India: Evidence from gravity anomalies  

NASA Astrophysics Data System (ADS)

Gravimetric and bathymetric studies on the southwestern continental margin of India confirm the extension of onshore NW-SE-, NNW-SSE-, N-S-, NE-SW-, ENE-WSW- and E-W-trending lineaments of Precambrian age over a considerable distance into the offshore region. The bight in the bathymetry off Coondapoor the offsets of Bessas de Pedro bank and the Cora Divh Island of the Laccadive group, the Prathap Ridge, and the inferred mid-shelf basement ridge suggest block movements on the southwestern continental margin. The physiographic expression on the Prathap Ridge (around 14° 20' N and 72° 50' E) is unaffected by some of the ENE-WSW lineaments, which probably indicates that these lineaments predate, the evolution of the topographic expression. As seafloor spreading advanced with respect to the Carlsberg Ridge, some of the ENE-WSW and NE-SW lineaments on the western continental margin appear to have been reactivated, and block movements took place. The presence of a basement ridge in the mid-shelf and the shelf margin basement high (Prathap Ridge) west of the slope resembles the structural style of a passive continental margin.

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

1993-03-01

44

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

45

Upper mantle viscosity and dynamic subsidence of curved continental margins  

NASA Astrophysics Data System (ADS)

Continental rifting does not always follow a straight line. Nevertheless, little attention has been given to the influence of rifting curvature in the evolution of extended margins. Here, using a three-dimensional model to simulate mantle dynamics, we demonstrate that the curvature of rifting along a margin also controls post-rift basin subsidence. Our results indicate that a concave-oceanward margin subsides faster than a convex margin does during the post-rift phase. This dynamic subsidence of curved margins is a result of lateral thermal conduction and mantle convection. Furthermore, the differential subsidence is strongly dependent on the viscosity structure. As a natural example, we analyse the post-rift stratigraphic evolution of the Santos Basin, southeastern Brazil. The differential dynamic subsidence of this margin is only possible if the viscosity of the upper mantle is >2-3 × 1019?Pa?s.

Sacek, Victor; Ussami, Naomi

2013-06-01

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

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

SciTech Connect

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

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

1987-05-01

48

Petroleum possibilities in continental margin off central Chile  

Microsoft Academic Search

The continental margin off central Chile, from Valparaiso to Valdivia, encompassing an area of 100,000 km², has been the target of exploratory activity by Empresa Nacional del Petroleo since 1970. Exploratory drilling began in 1972. By August 1984, total exploratory efforts had resulted in drilling 14 offshore wells and acquiring 12,130 km of seismic reflection lines. A biogenic gas accumulation

1986-01-01

49

Gas hydrate-filled fracture reservoirs on continental margins  

Microsoft Academic Search

Many scientists predicted that gas hydrate forms in fractures or lenses in fine-grained sediments, but only in the last decade were gas hydrates found in complex fracture systems on continental margins. Gas hydrate-filled fractures were captured on both in situ borehole images and in x-ray imaged pressure cores. These new discoveries of gas hydrate as fill in fractures have been

Ann Elizabeth Cook

2010-01-01

50

Triassic plutonism in southern California: Southward younging of arc initiation along a truncated continental margin  

USGS Publications Warehouse

Earliest Cordilleran magmatism in the southwestern United States is recorded by a belt of Triassic plutons that intrude Proterozoic basement of the Mojave crustal province and its cratonal/miogeoclinal cover.' The belt extends from the western Mojave Desert through the Transverse Ranges to the Colorado River trough. Triassic plutons are predominantly alkali-calcic, Fe- and Sr-enriched quartz monzodiorites and monzonites. The northern part of the belt is composed of two older plutonic suites (241-231 Ma) which are high K to shoshonitic; the southern part of the belt is a younger (218-213 Ma), sodic-alkalic suite. The plutonic record in southern California suggests a short-lived, southward younging continental margin arc setting for emplacement of Triassic plutons, superimposed on a continental margin modified by sinistral transform faulting Triassic plutonism in this region was followed by a magmatic lull prior to the onset of voluminous Middle to Late Jurassic Cordilleran arc magmatism.

Barth, A.P.; Tosdal, R.M.; Wooden, J.L.; Howard, K.A.

1997-01-01

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

A geophysical study of the northern Svalbard continental margin  

NASA Astrophysics Data System (ADS)

In the summer of 1999, the first systematic seismic profiles were acquired across the northern Svalbard continental margin east of 15°E. Approximately 1470 km of multi-channel seismic reflection data as well as sonobuoy wide-angle data were collected up to 82°N. With few exceptions the signals imaged the whole sedimentary cover down to the acoustic basement. The uppermost sedimentary deposits of the inner shelf yield P-wave velocities of 2 km s-1 and higher, indicating erosion and compaction due to a former ice load. The inner shelf east of Hinlopen Strait has only a thin veneer of over-consolidated sediments above the acoustic basement. Beneath the outer shelf, up to 3.5 km of sedimentary deposits cover the down-faulted acoustic basement. The continental slope is heavily eroded due to bottom current activity and slumping. At about 30°E the morphology of the continental slope has a smooth appearance. Shelf progradation only in the vicinity of glacial troughs crossing the shelf (associated with submarine fans) indicates main sediment transport by ice streams during former glacial periods. The maximum sedimentary thickness in the Sophia Basin is more than 9 km, and the Nansen Basin has a sediment thickness of 4.5 km close to the margin. Gravity modelling along the seismic profiles was performed to constrain the position of the continent-ocean transition. Existing sedimentary thickness and structural maps were extended over the area investigated. The new data provide no evidence for the presence of former extensive subaerial volcanic sequences (seaward-dipping reflectors), which would have been emplaced during the break-up along the margin. Thus, we consider this part of the margin as non-volcanic.

Geissler, Wolfram H.; Jokat, Wilfried

2004-07-01

54

Geophysical and morpho-tectonic study of the transition between seafloor spreading and continental rifting, western Woodlark Basin, Papua New Guinea  

Microsoft Academic Search

Two major morpho-tectonic domains, separated by a major transfer zone, are described at the transition between seafloor spreading and continental rifting in the western Woodlark Basin, off-shore eastern Papua New Guinea. The oceanic domain comprises new oceanic crust formed during the Bruhnes Epoch, older transitional crust and the rifted continental margins. Two rift branches are recognized within the continental domain.

Vladimir Benes; Natasha Bocharova; Eduard Popov; Steven D. Scott; Lev Zonenshain

1997-01-01

55

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

56

Tertiary evolution and petroleum potential of Oregon-Washington continental margin  

Microsoft Academic Search

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

Snavely; P. D. Jr

1986-01-01

57

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

E-print Network

Morphology of late Quaternary submarine landslides along the U.S. Atlantic continental margin David 2009 Accepted 30 January 2009 Keywords: landslides continental margin Atlantic Ocean sediments slope landslides along the margin and reassess the controls on their formation. Landslides can be divided into two

ten Brink, Uri S.

58

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

59

Antarctic glacial history from numerical models and continental margin sediments  

USGS Publications Warehouse

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

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

1999-01-01

60

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

NASA Astrophysics Data System (ADS)

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

Zimmermann, Udo; Niemeyer, Hans; Meffre, Sebastien

2010-10-01

61

Echo characters and sedimentary processes along a rifting continental margin, northeast of Taiwan  

NASA Astrophysics Data System (ADS)

The northeastern offshore of Taiwan, including the southern-most East China Sea continental margin, Ilan Shelf, Ilan Ridge and the western tip of the Okinawa Trough, is characterized by active rifting and an energetically complex hydrodynamic flow regime. In this study, sedimentary processes on the sea floor were inferred from regional mapping of 3.5 kHz echo characters. Eight distinct echo types were mapped, and based on echo type distribution, analysis of sediments and regional bathymetry, these were interpreted as deposits that had been formed under the influence of various local hydrodynamic processes. Different sedimentary processes, interpreted from the lithology and distribution pattern of sediments, were found to prevail on different physiographic provinces. In the southern East China Sea continental shelf margin, it is the outflow of Taiwan Strait Water and the on-shelf intrusion, upwelling and countercurrent induced by the impinging and turning of the Kuroshio Current that largely determine the distribution of sediments. On the narrow Ilan continental shelf, the deposition is mainly influenced by subaqueous deltaic and shallow marine processes. Over the rifting tip of the Okinawa Trough, including the Okinawa Trough Basin and its nearby slopes, the primary seafloor-shaping agents have been the mass-wasting processes and turbidity currents. Since the observed sediment data is in good consistency with other hydrographic data, the studies of transportation and deposition patterns of sediment can provide good constraints for the interpretation of physical oceanographic data.

Hong, Eason; Chen, I. Shih

2000-03-01

62

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

NASA Astrophysics Data System (ADS)

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

Kirillova, D.

2009-04-01

63

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

64

Fault reactivation within Avalonia: plate margin to continental interior deformation  

NASA Astrophysics Data System (ADS)

Lithotectonic terranes commonly have faults with movement histories that reflect their original tectonic setting and their subsequent re-activation during terrane accretion and post-accretionary dispersal. Since later movements tend to overprint evidence of earlier motions, documenting fault reactivation can be a difficult task. Avalonia, the largest terrane within the Canadian Appalachians preserves evidence for repeated episodes of movement along NE-trending fault zones in a variety of tectonic settings between the late Neoproterozoic and late Paleozoic. Evidence of Neoproterozoic motion is preserved in pre-final crystallization deformation fabrics in arc-related igneous complexes which intrude shear zones. These record strike-slip motion related to oblique subduction along the continental margin of Gondwana. In the Paleozoic, Avalonia migrated from its original peri-Gondwanan setting. Mid-Ordovician to earliest Silurian deformation and magmatism is attributed to the sinistral accretion of Avalonia to Laurentia. This reactivated Neoproterozoic shear zones and resulted in basin inversion in mainland Nova Scotia and the formation of mylonites and injection of dikes into brittle fractures in southern New Brunswick. Sinistral motion was accompanied by terrane dispersal and was followed in the Late Silurian and Early Devonian by further reactivation in the form of dextral strike-slip that reflects convergence between Laurentia and Gondwana. Following the accretion and dispersal of Avalonia, the fault zones became stranded within the continental interior. Subsequently, the fault zones accommodated local stresses which were far-field responses to collisional tectonics associated with the mid-Late Devonian Acadian and the Late Carboniferous Alleghanian orogenies. Hence, the generation of these fault systems along plate margins in the late Neoproterozoic and early Paleozoic profoundly influenced the tectonic evolution of late Paleozoic intra-continental deformation. Late Carboniferous deformation is recorded in (a) the mylonitic and cataclastic fabrics of the shear zones themselves, (b) the orientations of spatially related fold structures, (c) local controls on basin formation and sedimentary facies and (d) offsets in stratigraphy. This motion was predominantly dextral and records continued convergence between Laurentia and Gondwana during the amalgamation of Pangea.

Murphy, J. Brendan; Keppie, J. Duncan; Nance, R. Damian

1999-05-01

65

North Atlantic Margins: Case studies of Magmatic Continental Breakup  

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

66

Continental margin of eastern Canada: geologic framework and petroleum potential  

SciTech Connect

The Atlantic-type continental margin of eastern Canada is underlain by a series of Mesozoic-Cenozoic sedimentary basins separated by basement highs or areas of thinner sediments. Regional and/or salt tectonics have structured the Mesozoic sequence, which is masked by a less-deformed wedge of prograding uppermost Cretaceous and Cenozoic sediments. The basins have been targets of active hydrocarbon exploration for over 2 decades. Data from 138 exploratory wells and over 680,000 km (420,000 mi) of multichannel seismic coverage have indicated four major geologic/geochemical regions: Scotian Shelf, southern Grand Banks, northeastern Grand Banks, and Labrador-Southeast Baffin Shelf. The Geological Survey of Canada has developed hydrocarbon-generation models to explain the regional variation in oil and gas occurrence and to assess future potential in terms of the nature and thermal maturity of the source rocks, type of organic material, and time of trap formation. These factors are related to the geologic history of the margin, which is characterized regionally by diachronism in major basin inception and in the resultant stratigraphic record. We predict an exciting future for this vast petroleum province.

Grant, A.C.; McAlpine, K.D.; Wade, J.A.

1984-09-01

67

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

Microsoft Academic Search

Data on the composition and distribution of the sedimentary complexes of the Late Mesozoic continental margin of Southeastern Russia and their boundaries permit reconstruction of the tectonic evolution stages using different units of outcrop sequence stratigraphy separated by unconformities. Four siliciclastic megasequences extending from the Bureya ancient massif (microcontinent) to Sakhalin have been recorded within the Jurassic-Cretaceous continental margin of

Kirillova

2009-01-01

68

Seismic investigation of the continental margin off- and onshore Valparaiso, Chile  

Microsoft Academic Search

At the latitude of Valparaiso, Chile, a fundamental change in the configuration of the Benioff zone, volcanic activity, and the structure of the continental margin occurs opposite the subducting Juan Fernandez Ridge. Three legs of the German RVSonne (cruises SO101, SO103 and SO104) surveyed the continental margin and oceanic plate offshore Valparaiso, aiming at studying the crustal structure and investigating

E. R. Flueh; N. Vidal; C. R. Ranero; A. Hojka; R. von Huene; J. Bialas; K. Hinz; D. Cordoba; J. J. Dañobeitia; C. Zelt

1998-01-01

69

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

E-print Network

The speciation of marine particulate iron adjacent to active and passive continental margins Phoebe We use synchrotron-based chemical-species mapping techniques to compare the speciation of suspended and passive continental margins. Chemical- species mapping provides speciation information for heterogeneous

70

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.

71

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

72

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

73

Seismic refraction shooting on the continental margin west of the outer Hebrides, Northwest Scotland  

SciTech Connect

Seventeen sonobuoy refraction profiles have been shot to determine the nature of the basement and the broad pattern of sedimentation on the continental margin west of the outer Hebrides, NW Scotland. Under much of the shelf, crystalline rocks (V/sub p/ >5.1 km/s) lie within 100 m of the seafloor, the basemet being largely an extension of the Precambrian (Lewisian) metamorphic complex of western Scotland. V/sub p//V/sub s/ gives Poisson's ratios (sigma) of 0.26--0.30 for the Lewisian, values which are significantly higher than sigma in the deep crust under northern Britain, implying important compositional differences. Comparisons with ultrasonic velocities in rocks from the Scourian (approx.2700 Ma) and Laxfordian (approx.2200--1500 Ma) belts of the Scottish mainland suggest that the Lewisian on the inner continental shelf is predominantly Laxfordian (V/sub p/ approx.5.5 km/s). Higher-velocity rocks, probably Scourian with only a moderate degree of Laxfordian reworking (V/sub p/ approx.5.9 km/s), and Cenozoic intrusions occur locally. Two seismic profiles indicate that the outer continental shelf may be underlain by a zone of dense Scourian/early Laxfordian granulites, whose presence possibly influenced the siting of the continental slope. The sediments covering the basement are generally thin. Thickness exceeding 1 km are restricted to a faultbounded trough off the Isle of Lewis and to the outer shelf and continental slope. The deposits can be divided into Cenozoic (1.7--1.9 km/s) and Mesozoic (3.0--4.4 km/s) units, velocity variations in the latter probably reflecting the abundance of early Cenozoic basic intrusions. The distribution of the Mesozoic is partly controlled by faults which appear to be related to early Precambrian shear zones in the basement. These highly foliated belts seem to have facilitated stress relief by normal faulting during Permo-Triassic rifting activity.

Jones, E.J.W.

1981-12-10

74

Late Mesozoic North African continental margin: Sedimentary sequences and subsidence history  

SciTech Connect

Cretaceous facies types and subsidence history have been studied along two well outcropping and almost complete transversals through the Tellian units of the Mesozoic North African margin, the Western Rif (Morocco), and the Babors (Algeria). Sedimentologic observations and characteristic foraminiferal assemblages enabled estimates for Late Cretaceous paleobathymetries. Both palinspastic reconstruction and sedimentologic and biofacies analyses led to the following results. (1) The morphology and evolution of the Cretaceous North African margin, which in general represents a classic passive continental margin, were complicated by various factors such as Late Cretaceous compressional and lateral movements, the onset of (tectonically controlled ) diapirism, and the existence of intramarginal highs and basins. (2) The Cretaceous subsidence history of both areas can be divided into four stages which are accompanied by characteristic sedimentary formations: (I) distension and subsidence of the margin (Early Cretaceous); (II) a first compressional phase with uplift and slight metamorphism in the Albian/early Cenomanian which affected mainly the northerly paleogeographic zones, accompanied by first diapiric movements and resedimentation of Triassic saliferous material; (III) a Late Cretaceous stage of subsidence (Cenomanian-Santonian); and (IV) a second compressional phase starting with the Campanian and reflected by the formation of sedimentary klippes and olistostromes. (3) As a general trend, sedimentary basins deepened from south to north during Campanian/Maastrichtian time, giving rise to a characteristic succession of bathymetric zones which have been observed on both transversals.

Kuhnt, W.; Obert, D.

1988-08-01

75

Gas hydrate-filled fracture reservoirs on continental margins  

NASA Astrophysics Data System (ADS)

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

Cook, Ann Elizabeth

76

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

Microsoft Academic Search

The Marginal basins of Western and Northwestern Australia extend approximately 3,800 km along the coast and comprise both continental shelf and adjacent deepwater plateau areas. From north to south, the principal basins are the Sahul\\/Malita, Browse, Carnarvon, and Perth basins. The stratigraphic sequence within each basin is broadly similar, with initial widespread Triassic-Early Jurassic deposition in broad regional pre-rift sags.

1991-01-01

77

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

78

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

79

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

80

Petroleum possibilities in continental margin off central Chile  

SciTech Connect

The continental margin off central Chile, from Valparaiso to Valdivia, encompassing an area of 100,000 km/sup 2/, has been the target of exploratory activity by Empresa Nacional del Petroleo since 1970. Exploratory drilling began in 1972. By August 1984, total exploratory efforts had resulted in drilling 14 offshore wells and acquiring 12,130 km of seismic reflection lines. A biogenic gas accumulation was discovered in the F well. Because these attempts to find oil were unsuccessful and because drilling costs have escalated, exploratory activities have been curtailed. Forearc basins off central Chile are characterized by low geothermal gradient and a sedimentary filling of Cretaceous and Tertiary strata. Tertiary sequences are characterized by low organic carbon content, immature humic-type organic matter, and a biogenic gas potential. Cretaceous sequences are characterized by higher organic carbon content, good reservoir rocks, and fair to good source rocks. The organic matter is sapropelic, with vitrinite and liptinites, and is favorable for oil and gas generation. Seismic and well data suggest that Mesozoic and Cenozoic sedimentary rock sequences filling the basins (more than 4000 m thick at the shelf edge) extend 40-70 km beyond the present shelf edge. Mesozoic rocks deposited on the slope may generate petroleum and gas that could migrate upslope and accumulate in traps associated with the faulted basement highs and graben-type depressions existing at the shelf edge. This geologic setting favors the development of large petroleum accumulations along the shelf edge and graben on the sedimentary basins off central Chile.

Gonzalez, E.

1986-07-01

81

Deep-sea Lebensspuren of the Australian continental margins  

NASA Astrophysics Data System (ADS)

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

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

82

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

83

A Fresh Look at the Evolution of the Continental Margin Off Nova Scotia  

Microsoft Academic Search

The margin off Nova Scotia is characterized by a wide continental shelf and deep sedimentary basins. Rifting initiated in the Late Triassic - Early Jurassic, producing synrift basins that later filled with thick salt accumulations (Klitgord & Schouten 1986). Seismic data and subsidence studies (e.g. Keen & Potter 1995; Dehler & Keen 1993) show that the continental crust was thinned

Sonya A. Dehler; Charlotte E. Keen

84

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

Microsoft Academic Search

The United Nations Convention on the Law of the Sea (UNCLOS) requires coastal states intending to secure sovereignty over continental shelf territory extending beyond 200 nautical miles to submit geological\\/geophysical data, along with their analysis and synthesis of the relevant continental margin in support of their claim. These submissions are scrutinised and assessed by a UN Commission of experts who

L. Parson

2005-01-01

85

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

86

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

NASA Astrophysics Data System (ADS)

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

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

2006-03-01

87

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

88

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

89

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

90

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

PubMed

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

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

2001-09-13

91

Seismic-reflection signature of cretaceous continental breakup on the wilkes land margin, antarctica.  

PubMed

The passive (rifted) continental margin of Wilkes Land, Antarctica, is characterized on seismic reflection records by (i) in the south, a block-faulted sequence of highly stratified continental beds overlain by two distinct unconformities; (ii) a transitional, greatly thinned continental crust overlain by material interpreted to be flood basalt; and (iii) in the north, oceanic crust with a boundary ridge at its edge. The Mohorovici? discontinuity can be followed across the continent-ocean boundary and shows a progressive thinning of continental crust to a minimum of 2.5 kilometers at its northern edge. PMID:17753282

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

1985-09-13

92

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

Microsoft Academic Search

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

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

2000-01-01

93

Sediment Distribution and Volcanic Activities in the Northern South China Sea Continental Margin  

NASA Astrophysics Data System (ADS)

Multi-channel seismic reflection data and magnetic anomaly data have been used to probe the distribution of slope sediments and igneous bodies in the northern South China Sea (SCS) continental margin and deep sea basin south of the Dongsha Island. We have identified faults, volcanic bodies and basement structures in the study area. Most of the normal faults are distributed in the continental slope and abyssal plain; they were formed in the syn-rift crust when seafloor spreading of the South China Sea was active. From the compiled isopach map, the thicknesses of the continental margin sediments vary between 250~3400 m, and the sediment distribution is largely affected by the development of small basins formed due to normal faulting and volcanic activities. Three areas in the study area where thick sediments are present: the South China Sea abyssal plain; the Pearl River Mouth Basin; and a series of slope basins developed on the continental slope south of the Dongsha Island. Many igneous bodies have been identified from the seismic reflection profiles. We utilize borehole data from ODP Leg 184 to provide age constraints of the sedimentary layers. Then the ages of various igneous activities are proposed based on the contact relationships of igneous bodies with their surrounding sediment layers, respectively. We suggest that the continental margin of the northern South China Sea is a "volcanic rifted margin", the high velocity materials in the lower crust reported from seismic refraction studies were formed during the South China Sea expansion period through underplating processes, and distributed beneath the continent slope along the whole northern South China Sea continental margin. After the seafloor spreading ceased, another magmatic event occurred in the eastern portion of the continental margin that thickened high velocity material of the lower crust south of the Dongsha Island.

Liu, C.; Chuang, S.; Tsai, Y.; Hsu, H.

2011-12-01

94

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

95

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

NASA Astrophysics Data System (ADS)

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

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

2015-01-01

96

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

Microsoft Academic Search

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

Stefan Bünz; Jürgen Mienert; Christian Berndt

2003-01-01

97

Pockmarks and seafloor instability in the Olbia continental slope (northeastern Sardinian margin, Tyrrhenian Sea)  

Microsoft Academic Search

The seafloor morphology and the subsurface of the continental slope of the Olbia intraslope basin located along the eastern,\\u000a passive Sardinian margin (Tyrrhenian Sea) has been mapped through the interpretation of high-resolution multibeam bathymetric\\u000a data, coupled with air-gun and sparker seismic profiles. Two areas, corresponding to different physiographic domains, have\\u000a been recognized along the Olbia continental slope. The upper slope

Giacomo Dalla Valle; Fabiano Gamberi

2011-01-01

98

Alkalinity distribution in the western North Atlantic Ocean margins  

Microsoft Academic Search

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

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

2010-01-01

99

Geologic development and characteristics of continental margins, Gulf of Mexico  

SciTech Connect

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

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

1986-09-01

100

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

SciTech Connect

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

Biswas, S.K.

1982-10-01

101

Imaging proto-oceanic crust off the Brazilian Continental Margin  

NASA Astrophysics Data System (ADS)

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

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

2014-01-01

102

Fluid seepage at the continental margin offshore Costa Rica and southern Nicaragua  

NASA Astrophysics Data System (ADS)

A systematic search for methane-rich fluid seeps at the seafloor was conducted at the Pacific continental margin offshore southern Nicaragua and northern central Costa Rica, a convergent margin characterized by subduction erosion. More than 100 fluid seeps were discovered using a combination of multibeam bathymetry, side-scan sonar imagery, TV-sled observations, and sampling. This corresponds, on average, to a seep site every 4 km along the continental slope. In the northwestern part of the study area, subduction of oceanic crust formed at the East Pacific Rise is characterized by pervasive bending-induced faulting of the oceanic plate and a relatively uniform morphology of the overriding continental margin. Seepage at this part of the margin typically occurs at approximately cone-shaped mounds 50 - 100 m high and up to 1 km wide at the base. Over 60 such mounds were identified on the 240 km long margin segment. Some normal faults also host localized seepage. In contrast, in the southeast, the 220 km long margin segment overriding the oceanic crust formed at the Cocos-Nazca Spreading Centre has a comparatively more irregular morphology caused mainly by the subduction of ridges and seamounts sitting on the oceanic plate. Over 40 seeps were located on this part of the margin. This margin segment with irregular morphology exhibits diverse seep structures. Seeps are related to landslide scars, seamount-subduction related fractures, mounds, and faults. Several backscatter anomalies in side-scan images are without apparent relief and are probably related to carbonate precipitation. Detected fluid seeps are not evenly distributed across the margin but occur in a roughly margin parallel band centered 28 ± 7 km landward of the trench. This distribution suggests that seeps are possibly fed to fluids rising from the plate boundary along deep-penetrating faults through the upper plate.

Sahling, Heiko; Masson, Douglas G.; Ranero, CéSar R.; Hühnerbach, Veit; Weinrebe, Wilhelm; Klaucke, Ingo; Bürk, Dietmar; Brückmann, Warner; Suess, Erwin

2008-05-01

103

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

104

GEOLOGIC STRUCTURE AND HALOKINESIS ON THE CONTINENTAL MARGIN OF ANGOLA  

Microsoft Academic Search

The paper offers a brief overview of the structure in an area of considerable petroleum potential as interpreted from a viewpoint somewhat different from the standard Western model of the opening of the Atlantic. Seismoacoustic profiling data and geomorphological evidence obtained by the Sevmorgeologiya, as well as studies made by one of the authors in collaboration with the Sonangol national

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

1986-01-01

105

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

NASA Astrophysics Data System (ADS)

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

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

106

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

NASA Astrophysics Data System (ADS)

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

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

2011-01-01

107

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

108

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

109

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

110

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

Microsoft Academic Search

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

Robert W. Embley; Robert D. Jacobi

1977-01-01

111

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

Microsoft Academic Search

More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150-400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane.

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

2009-01-01

112

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

Microsoft Academic Search

More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150–400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane.

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

2009-01-01

113

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

114

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

Microsoft Academic Search

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

G. K. Westbrook

2009-01-01

115

Salt diapirs bordering the continental margin of northern kenya and southern somalia.  

PubMed

The presence of newly discovered diapirs of presumed salt origin is documented for the continental margin of northeastern Kenya and southeastern Somalia. These structures are probably a manifestation of a significant thickness of Lower Jurassic evaporites deposited during the rift and early-drift stages of the separation of Madagascar from the African continent. PMID:17842401

Rabinowitz, P D; Coffin, M F; Falvey, D

1982-02-01

116

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

117

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

118

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 trace separating the submerged Taiwan orogenic wedge from the Chinese passive conti- nental margin The island of Taiwan was formed by oblique col- lision between the Luzon Arc and the Chinese continental

Lin, Andrew Tien-Shun

119

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

E-print Network

Crater as the source for New Jersey continental margin ejecta is provided by fine-grained tektites American strewn field, occurs on the New Jersey continental margin at Ocean Drilling Program (ODP) Sites of Mexico, Barbados, and New Jersey; and (2) the equatorial Pacific Field [1­4]. The Pacific ejecta

120

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

E-print Network

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

Sandiford, Mike

121

Extension on rifted continental margins: Observations vs. models.  

NASA Astrophysics Data System (ADS)

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

Skogseid, Jakob

2014-05-01

122

66 FR 38319 - Outer Continental Shelf, Western Gulf of Mexico, Oil and Gas Lease Sale 180  

Federal Register 2010, 2011, 2012, 2013, 2014

...Outer Continental Shelf, Western Gulf of Mexico, Oil and Gas Lease Sale 180 AGENCY...blocks offered in Sale 180, Western Gulf of Mexico, pursuant to the Outer Continental...referenced herein, from the MMS Gulf of Mexico Region's Public Information...

2001-07-23

123

67 FR 47394 - Outer Continental Shelf, Western Gulf of Mexico, Oil and Gas Lease Sale 184  

Federal Register 2010, 2011, 2012, 2013, 2014

...Outer Continental Shelf, Western Gulf of Mexico, Oil and Gas Lease Sale 184 AGENCY...blocks offered in Sale 184, Western Gulf of Mexico, pursuant to the Outer Continental...referenced herein from the MMS Gulf of Mexico Region's Public Information...

2002-07-18

124

Energetic plumes over the western Ross Sea continental slope  

NASA Astrophysics Data System (ADS)

Rapid descent of dense Drygalski Trough (western Ross Sea, Antarctica) shelf water over the continental slope, within 100 to 250 m thick benthic plumes, is described. Speeds of up to 1.0 m/s are recorded flowing at an average angle of 35° to the isobaths, entraining ambient Lower Circumpolar Deep Water en route. This process is predominant in determining the concentration and placement of the shelf water injected into the deep sea as a precursor Antarctic Bottom Water. Nonetheless, a 4-hour duration pulse of undiluted shelf water was observed at depth (1407 m) directly north of the Drygalski Trough, moving at around 90 degrees to isobaths, and at a speed of 1.4 m/s. Thus the export of Ross Sea shelf water to the deep sea is accomplished within plumes descending at moderate angle to isobaths, punctuated by rapid downhill cascades.

Gordon, Arnold L.; Zambianchi, Enrico; Orsi, Alejandro; Visbeck, Martin; Giulivi, Claudia F.; Whitworth, Thomas; Spezie, Giancarlo

2004-11-01

125

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

126

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

127

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

128

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

129

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

130

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

131

Subduction of continental margins and the uplift of high-pressure metamorphic rocks  

NASA Astrophysics Data System (ADS)

The mechanism by which high-pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis. We suggest that the emplacement results from partial subduction of the continental margin which, because of its high flexural rigidity, produces a rapid change in the trajectory of the descending slab. We assume a two-fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone, and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity. We assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate. The wedges overlying the subduction zones, and the subducting slab itself, experience substantial extra compressional stress at depths of around 100 km, and extensional stress at shallower depths, as the continental margin passes through the zone of maximum curvature. The magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and/or the descending plate, and are experienced in a time interval of less than 5 m.y. for typical subduction rates. The spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab, which is the weakest region in the deeper parts of the subduction zone. This may result in rapid upward migration of the crust of the partially subducted continental margin, against the flow of subduction. High-pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above. Movement on the faults would have been coeval, and would have resulted in rapid unroofing of the high-pressure terranes, synchronous with arrival of the continental margin at the subduction zone and, therefore, relatively early in the history of a collisional orogen.

Hynes, Andrew; Arkani-Hamed, Jafar; Greiling, Reinhard

1996-05-01

132

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

133

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

134

Meiofauna of the western continental shelf of India, Arabian Sea  

NASA Astrophysics Data System (ADS)

Meiofaunal standing stock and nematode community structure were investigated in the western continental shelf of India by collecting samples from every degree square of the shelf during two cruises of the FORV ( Fishery and Oceanographic Research Vessel) Sagar Sampada, conducted in 1998 and 2001. Samples were collected from 30, 50, 100 and 200 m depths using a Smith Mc Intyre grab. Meiofaunal density ranged from 8 Ind. 10 cm -2 to 1208 Ind. 10 cm -2 and biomass from 0.07 mg 10 cm -2 to 6.11 mg 10 cm -2. Nematodes were the dominant meiofaunal group, contributing 88% of the density and 44% of the biomass. Harpacticoid copepods were the second important taxa, contributing 8% of both biomass and density. Altogether, 154 species of nematodes belonging to 28 families were recorded from the study area. Numerically, Desmodora spp., Dorylaimopsis sp. , Tricoma spp., Theristus spp. and Halalaimus spp. were the dominant species. In general, there was a decrease in biomass and density of meiofauna and species diversity of nematodes with increase in depth. There was a 67% drop in species number from 51 to 100 m (106 species) to the shelf edge (35 species). Species richness and diversity indices showed consistent decrease with depth. The species dominance index was higher below 150 m depth. ANOSIM (from PRIMER) showed a significant difference between the nematodes of the near shore and shelf edge. Latitudinal variation was observed only in the number of nematode species. Biomass and abundance of nematodes were found to increase from coarse to fine sediment, while copepods showed an opposite trend. Multivariate analyses of nematode communities did not reveal any latitudinal or substratum differences. Variables such as depth, latitude, organic matter (OM) and amount of clay were the most relevant parameters influencing the biomass and density of meiofauna, while depth and temperature were the important parameters explaining the distribution of the nematode communities along the western Indian shelf.

Sajan, S.; Joydas, T. V.; Damodaran, R.

2010-03-01

135

Molybdenum isotope composition from 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\\u000a of authigenic Mo during long-term burial in sediments in continental margin settings of the Yangtze block, as well as their\\u000a indication to the burial of original organic carbon. The burial rate of original organic carbon was estimated on the basis\\u000a of the

Lian Zhou; Junhua Huang; Corey Archer; Chris Hawkesworth

2007-01-01

136

Seabottom Backscatter Studies in the Western Continental Shelf of India  

NASA Astrophysics Data System (ADS)

In this paper, a study is initiated to observe the interaction effect of the sound signal with three different sediment bottoms in the shelf area between Cochin and Mangalore in the western continental shelf of India. An echo signal acquisition system has been designed and interfaced with the 12 kHz echosounder installed onboard ORV Sagar Kanya. The reflection coefficients including attenuation at the seawater/bottom interface are computed in the three different sediment areas based on the sediment mean grain size. The experimental coherent reflection coefficients are calculated using the attenuation corrected reflection coefficients and the normalized cross-correlation between successive backscatter echo signal waveforms in those areas. Further, analyses conducted by determining the echo peak Probability Density Function (PDF) and matching them with the experimental echo peak histograms provide root mean square (rms) roughness amplitude in the three different survey areas. The rms roughness values are used to compute the coherent reflection coefficients. An attempt to establish concurrence between the coherent reflection coefficients based upon the rms roughness amplitude and the experimental coherent reflection coefficients using the backscatter echo signals, reveals the importance of seawater/bottom interface roughness in the coarse grained sediment bottoms like sand and silty sand. The existence of microtopographic features are responsible for the seawater/bottom interface roughness. However, in the fine grained sediment area, the bottom does not contain any such feature.

Chakraborty, B.; Pathak, D.

1999-01-01

137

Sea level rise and submarine mass failures on open continental margins  

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

138

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

139

Submarine canyon and slope processes of the U.S. Atlantic continental margin  

USGS Publications Warehouse

Two regions on the U.S. Atlantic continental margin were surveyed using single-channel, seismic-reflection profiling techniques: the Mid-Atlantic Continental Slope and Rise seaward of New Jersey in the vicinity of Baltimore Canyon and the Continental Slope and upper Rise just north of Cape Hatteras. Submarine canyons are the dominant morphologic feature in both areas. The Continental Slope in the Baltimore Canyon area has a general sea-floor gradient of 3?-4? and a width of approximately 40 km, whereas the study area north of Cape Hatteras has a general sea-floor gradient of approximately 9? and a width of 20 km. The dominant slope process differs in each area. In the Baltimore Canyon area, subbottom reflectors suggest that sediment deposition with progradation of the slope is related to canyon processes. In the study area north of Cape Hatteras, the canyons appear erosional and mass wasting is the dominant erosional process. Dominant slope processes appear to be correlated with the width and sea-floor gradient of the Continental Slope. Although the absolute age of the canyons is difficult to determine without rotary-drill cores for stratigraphic control, Baltimore Canyon is suggested to be older than the shelf-indenting canyon just north of Cape Hatteras. An anomalously large ridge flanking Baltimore Canyon on the upper rise appears to be related to canyon depositional and erosional processes.

McGregor, B.A.

1983-01-01

140

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

141

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

142

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

143

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

E-print Network

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

Demouchy, Sylvie

144

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

NASA Astrophysics Data System (ADS)

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

Osler, John C.

2003-10-01

145

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

USGS Publications Warehouse

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

Lund, K.

2008-01-01

146

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

NASA Astrophysics Data System (ADS)

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

Parson, L.

2005-12-01

147

Transition from rifted continental to oceanic crust at the southeastern Korean margin in the East Sea (Japan Sea)  

Microsoft Academic Search

The southeastern Korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the East Sea (Japan Sea). Two-dimensional crustal structure of the southeastern Korean margin was computed from ocean bottom seismometer data by tomographic inversion and iterative forward modeling. The crustal structure shows the emplacement of high-velocity (>7

Hyun-Moo Cho; Han-Joon Kim; Hyeong-Tae Jou; Jong-Kuk Hong; Chang-Eob Baag

2004-01-01

148

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

149

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

150

Continental Margin Tectonics Along the Convergent Plate Boundary of Central Chile  

NASA Astrophysics Data System (ADS)

Multibeam bathymetry along central Chile provides a detailed map of recent tectonic deformation of the margin and incoming oceanic plate from about 28? S to 36? S. The data were collected during R/V SONNE cruises 101, 102, 104 and 161 and a cruise with R/V Vidal Gormaz. Individual pings were edited and cleaned and the different surveys have been merged after depth calculations using a different measured velocity function for each of them. The oceanic Nazca plate is covered by about 100 m of pelagic sediment and the morphology of the igneous basement is displayed well in the bathymetric maps. The oceanic plate topography changes markedly along the subduction zone and exerts a first order control in the distribution of trench sediment infill and in the tectonic style of deformation of the margin. A major boundary occurs at latitude 32?-33? S where the hotspot volcanic chain of Juan Fernadez is currently subducting. The chain subducts oblique to the margin strike and thus the tectonic boundary has been migrating along the subduction zone through time. South of the area of ridge subduction the trench is filled with turbidites and a 20-40 km wide accretionary prism occurs at the front of the continental slope. The upper slope has a smooth morphology indicative of a quiet tectonic domain. At the current area of ridge subduction and north of it (28?-33?S) the trench has a reduced turbiditic infill. The trench infill seems to be at minimum at 31-32S and slightly larger to the north as the trench axis becomes deeper. Here, a small ridge at the slope toe may indicate that reduced accretion is active. The continental slope is deeper and more rugged that to the south displaying a series of small midslope basins. Here, the continental slope morphotectonic structure is the product of tectonic erosion due to the passage of the volcanic ridge.

Weinrebe, W.; Ranero, C. R.; Diaz, J.; Reichert, C.; Vera, E. E.

2003-12-01

151

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

152

Origins and fates of DOM along the New England continental margin  

SciTech Connect

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

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

1993-01-01

153

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

SciTech Connect

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

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

1993-06-01

154

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

NASA Astrophysics Data System (ADS)

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

Ragueneau, O.; Si-Webs Team

2003-04-01

155

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

NASA Astrophysics Data System (ADS)

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

White, Susan; Webb, John A.

2015-01-01

156

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

157

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

PubMed Central

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

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

2014-01-01

158

Paleomagnetic and stratigraphic techniques for identifying sediment processes on continental margins  

NASA Astrophysics Data System (ADS)

Understanding the origin of stratigraphic and morphologic features, whether they are formed by primary deposition or are the consequence of post-depositional alteration, is the critical first step to determining the history and evolution of continental margins. I have developed a new approach that when combined with prior geological and geophysical data, provides new constraints on the origin of stratigraphic sequences. This approach can detect high water content that could evolve into overpressure. Such zones have the potential to develop into slope failures. In addition, this approach can determine if morphologic features observed on many continental margins are current-controlled bedforms or retrogressive slides. The method combines anisotropy of magnetic susceptibility (AMS) with CHIRP seismic data, which provides new insights into the processes responsible for strata formation. First, I explore the limb of a slump in the Ardath Shale in the cliffs of La Jolla, CA. Laterally adjacent to the observed slump, AMS is able to identify a "crypto-slump" that is not directly identifiable in outcrop. Next, a study of the Santa Barbara basin explores the rapid deposition and deformation occurring on the northern slope, where AMS combined with other rock magnetic techniques distinguished diagenesis from zones of excess water and overconsolidation. Finally, I use the AMS approach to address the controversy regarding the formation of the Humboldt Slide. Based on morphology observed in CHIRP seismic data and the AMS signature of collocated piston cores, the Humboldt Slide is composed of a series of sediment waves, not a thin skinned deformation feature.

Schwehr, Kurt

159

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

SciTech Connect

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

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

1988-03-01

160

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

PubMed Central

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

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

2013-01-01

161

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

PubMed

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

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

2013-01-01

162

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

163

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

164

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

165

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

166

Recent seismic investigations on gas hydrates at continental margins by BGR  

NASA Astrophysics Data System (ADS)

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

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

2003-04-01

167

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

168

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

169

Thermal evolution of a sheared continental margin: Insights from the Ballenas transform in Baja California, Mexico  

NASA Astrophysics Data System (ADS)

The Ballenas transform margin in central Baja California offers an unparalleled opportunity to study the thermal behaviour of a sheared continental margin during various stages of its evolution. Apatite fission track and (U-Th)/He results from two transects perpendicular to the coast reveal a pronounced latest Pliocene to Pleistocene (~ 1.8 Ma) heating event related to the Neogene opening of the Gulf of California. Proximity to a regional pre-rift unconformity indicates that samples remained at near-surface levels since Paleogene unroofing, despite having experienced reheating to maximum paleotemperatures within or above the fission track partial annealing zone. In general, maximum paleotemperatures during overprinting decrease from > 100-120 °C near the coast to below 60 °C ca. 5-8 km further inland, suggesting lateral heat flow from a source within the Gulf of California. Heat sources related to the structural development of the Ballenas transform fault, located approximately 1.5-4.5 km offshore from the two sample transects, most likely controlled the observed reheating. Overprinting patterns do not support conductive reheating due to reburial, magmatism or frictional shear. Instead, a pronounced thermal spike in between much less overprinted neighbouring samples strongly favours convective heating by hydrothermal fluids as the dominant overprinting process. Hydrothermal activity may be caused by either deep fluid circulation along newly formed shear zones of the transform fault or, more likely, magmatic leaking along the transform fault. Latest Pliocene to Pleistocene (~ 1.8 Ma) activity on the Ballenas transform fault is closely linked to extension in the Lower and Upper Delfín basins and provides a minimum age for the structural reorganisation and the relocation of extension in the northern Gulf of California. This study shows that hydrothermal activity can cause significant thermal events in a transform margin before the passage of the spreading centre. Paleotemperatures from the Ballenas transform are similar to those of other transform margins after passage of a spreading centre, which suggests that hydrothermal fluids may have an important thermal buffering effect, moderating the maximum temperatures of the ridge segments near their intersection with a continental transform margin.

Seiler, Christian; Gleadow, Andrew J. W.; Fletcher, John M.; Kohn, Barry P.

2009-07-01

170

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

171

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

172

Journal of the Geological Society, London, Vol. 162, 2005, pp. 135146. Printed in Great Britain. Subsidence history of the north Indian continental margin, ZanskarLadakh  

E-print Network

. 135 Subsidence history of the north Indian continental margin, Zanskar­Ladakh Himalaya, NW India R. I of these techniques, however, to highly deformed margins in the rock record. In the Zanskar­Ladakh Himalaya, NW India

Watts, A. B. "Tony"

173

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

174

Authigenic apatite formation and burial in sediments from non-upwelling, continental margin environments  

SciTech Connect

Evidence for precipitation of authigenic carbonate fluorapatite (CFA) in Long Island Sound and Mississippi Delta sediments suggests that formation of CFA is not restricted to environments of active coastal upwelling. The authors present porewater data suggestive of CFA formation in both these areas. Application of a sequential leaching procedure, designed specifically to separate authigenic carbonate fluorapatite from other phosphorus-containing phases, including detrital apatite of igneous or metamorphic origin, provides strong supporting evidence for authigenic apatite formation in these sediments. The size of the authigenic apatite reservoir increases with depth, indicating continued formation of CFA during early diagenesis. This depth increase is mirrored by a decrease in solid-phase organic P at both sites, suggesting that CFA is forming at the expense of organic P. Mass balance considerations, application of diagenetic models to intersitital water nutrient data, and the saturation state of the interstitial water are consistent with this interpretation. Diagenetic redistribution of phosphorus among the different solid-phase reservoirs is observed at both sites, and results in near perfect retention of P by these sediments over the depth intervals sampled. Formation of CFA in continental margins which do not conform to the classically defined regions of phosphorite formation renders CFA a quantitatively more important sink than has previously been recognized. Including this reservoir as a newly identified sink for reactive P in the ocean, the residence time of P in the modern ocean must be revised downward. The implication for ancient oceans of CFA formation in continental margin sediments other than phosphorites is that phosphorite formation may be less a representation of episodicity in removal of reactive P from the oceans than of localized concentration of CFA in phosphatic sediments by secondary physical processes. 90 refs., 5 figs., 2 tabs.

Ruttenberg, K.C.; Berner, R.A. (Yale Univ., New Haven, CT (United States))

1993-03-01

175

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

USGS Publications Warehouse

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

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

2007-01-01

176

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

NASA Astrophysics Data System (ADS)

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

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

2005-12-01

177

Average QLg, QSn, and observation of Lg blockage in the Continental Margin of Nova Scotia  

NASA Astrophysics Data System (ADS)

term "Lg blockage" refers to the sudden disappearance of the Lg phase along a particular propagation path which is commonly seen at continental-oceanic transition zones. In this paper we present observational evidence of Lg blockage across the continental margin of Nova Scotia in eastern Canada. Regional Lg and Sn spectra from 91 events with epicentral distances between 100 and 1200 km and magnitudes between 2.5 and 4.7 are inverted simultaneously for the source spectrum, site amplification, and average attenuation. The vertical displacement spectra were estimated between 0.9 and 10.75 Hz. The assumptions include a fixed frequency-independent geometric spreading rate for Lg and a frequency-dependent spreading model for the Sn. Estimates for the apparent regional attenuations are QLg (f) = 615(±25) f0.35(±0.04) and QSn (f) = 404(±23) f0.45(±0.03). Results from this study provide an accurate parameterization of observed amplitude spectra and are valuable for representing wave propagation in the region. Based on the observation of a strong trade-off between Sn and Lg amplitudes which have different attenuation characteristics, we conclude any attenuation study based on measuring amplitude of a package of several different phases, without taking into consideration the propagation characteristics of individual waveforms at the region of study, may bias the estimation of average regional Q.

Mousavi, S. Mostafa; Cramer, Chris H.; Langston, Charles A.

2014-10-01

178

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

179

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

180

Cretaceous source rock characterization of the Atlantic Continental margin of Morocco  

SciTech Connect

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

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

1993-02-01

181

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

SciTech Connect

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

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

1994-07-01

182

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

183

Structure and evolution of the Møre Basin - mid-Norwegian continental margin  

NASA Astrophysics Data System (ADS)

The Norwegian continental margin experienced multiple rift episodes since the collapse of the Caledonides, eventually leading to break-up in the Early Eocene. Information on the tectonic development is documented in the stratigraphic record of sedimentary basins. The Møre Basin is one of the deep sedimentary basins at the Norwegian Margin that developed during successive episodes of stretching, starting in the Devonian. In this study we present new geophysical data of the Møre Basin, which have been integrated with existing deep seismic reflection and refraction data. The basin can be divided into several sub-basins and structural highs, which formed prior to the final rift phase, mainly during extension in the Late Jurassic-Early Cretaceous. The deepest depocenters comprise of at least 15 km sedimentary infill. The crystalline crust thins from about 40 km onshore to around 10-12 km with local minima of ca. 5 km thickness beneath the deepest parts of the basin. One key profile across the Møre Basin has been chosen for structural and thermal reconstruction. We will compare different basin modelling techniques for the structural reconstruction, estimation of sedimentation and stretching factors. We will reconcile our results with temperature, vitrinite, and paleo waterdepth data and make comparison with plate reconstruction models.

Theissen-Krah, S.; Schmid, D. W.; Faleide, J.; Planke, S.; Gabrielsen, R. H.; Myklebust, R.

2013-12-01

184

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

185

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

186

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

187

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

NASA Astrophysics Data System (ADS)

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

Liu, C.; Chiu, J.

2008-12-01

188

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

Microsoft Academic Search

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

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

1995-01-01

189

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

190

Phanerozoic history of Western Australia related to continental drift  

Microsoft Academic Search

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

J. J. Veevers

1971-01-01

191

Evidence for Rifting Above Hotter Than Normal Mantle : Deep Seismic Sounding at the Continental Margin of Korea in the East Sea (Japan Sea)  

Microsoft Academic Search

The continental margin of the Korean Peninsula is little known in its crustal structure, although various opening models of the southwestern part of the East Sea (Japan Sea) have been presented. Accordingly, continental rifting and subsequent seafloor spreading processes in the East Sea have not been adequately addressed. The crustal and sedimentary velocity structures were investigated across the Korean margin

H. Jou; H. Kim; H. Cho; H. Bijwaard

2003-01-01

192

Tectonic isolation of the Levant basin offshore Galilee-Lebanon – effects of the Dead Sea fault plate boundary on the Levant continental margin, eastern Mediterranean  

Microsoft Academic Search

The continental margin of the central Levant, offshore northern Israel and southern Lebanon is characterized by a sharp continental-oceanic crustal transition, exhibited on the bathymetry as a steep continental slope. At the base of the slope a narrow zone of faulting deforms the upper Messinian-recent sedimentary sequence. Further into the basin no major deformations are observed. However, onland a restraining

U. Schattner; Z. Ben-Avraham; M. Lazar; C. Hüebscher

2006-01-01

193

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

SciTech Connect

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

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

1989-03-01

194

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

NASA Astrophysics Data System (ADS)

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

Ernesto, M.; Marques, L. S.

2011-12-01

195

Geomorphology and sedimentary features in the Central Portuguese submarine canyons, Western Iberian margin  

NASA Astrophysics Data System (ADS)

The Central Portuguese submarine canyons (Nazaré, Cascais and Setúbal-Lisbon canyons) dissect the Western Iberian margin in an east-west direction from the continental shelf, at water depths shallower than 50 m, down to the Tagus and Iberian abyssal plains, at water depths exceeding 5000 m. We present an analysis of the geomorphology of the canyons and of the sedimentary processes that can be inferred from the observed morphology of the three canyons, based on a compilation of swath bathymetry data and TOBI deep-towed side-scan sonar imagery. This first complete detailed mapping of the Central Portuguese canyons reveals substantial differences in their morphologies and downslope evolution. The canyons are divided into three sections: 1) canyon head and upper reach, 2) middle canyon, and 3) canyon mouth and distal part. The canyon heads and upper reaches are severely indented into the continental shelf, and they are characterised, in the Nazaré and Setúbal-Lisbon canyons, by sinuous V-shaped valleys entrenched within high canyon walls occupied by rock outcrops dissected by gullies. The Cascais upper canyon is complex, with multiple branches with high axial gradients and signs of mass wasting. Middle canyon sections, indented in the slope, display axial incisions with perched, stacked terraces, and are affected by debris avalanches originating from the canyon walls. At the base of slope, the distal Cascais and Setúbal-Lisbon canyons show many characteristics of channel-lobe transition zones: erosional features such as isolated to amalgamated chevron scours, and depositional bedforms such as mud to gravel waves. Pervasive scouring occurs up to 95 km beyond the canyon mouths. By contrast, the Nazaré canyon opens into a 27 km wide and 94 km long channel, whose flat-bottomed thalweg is occupied by sediment waves, irregular, comet-shaped and crescentic scours, and a second-order channel. Transverse, kilometre-scale sediment waves occupy the overbank area of the southern channel margin. The present morphology of the Central Portuguese canyons is the result of erosive processes, subsequent sediment transport and deposition, and sediment instability, whereas inherited tectonic fabric controls their location. Morphological differences between the canyons are explained by the main mechanisms driving their activity. Overall, these morphological features suggest that these canyons have acted as an efficient conduit of sediment to the deep basin, transporting large quantities of material to the deep sea during high-energy events.

Lastras, G.; Arzola, R. G.; Masson, D. G.; Wynn, R. B.; Huvenne, V. A. I.; Hühnerbach, V.; Canals, M.

2009-02-01

196

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

197

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

NASA Astrophysics Data System (ADS)

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

Westbrook, G. K.

2009-04-01

198

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

199

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

NASA Astrophysics Data System (ADS)

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

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

2007-08-01

200

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

201

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

SciTech Connect

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

Anderson, J.B.

1987-05-01

202

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

Federal Register 2010, 2011, 2012, 2013, 2014

...Mexico, Outer Continental Shelf, Western Planning Area, Oil and Gas Lease Sale 215 (2010...and gas Lease Sale 215 in the Western Planning Area (WPA) (Lease Sale 215) scheduled...Gas Lease Sales: 2007-2012; Western Planning Area Sales 204, 207, 210, 215,...

2010-04-05

203

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

204

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

SciTech Connect

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

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

1988-08-01

205

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

206

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

207

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

208

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

209

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

210

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

211

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

212

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

213

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

Microsoft Academic Search

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

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

2005-01-01

214

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

NASA Astrophysics Data System (ADS)

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

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

2004-09-01

215

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

NASA Astrophysics Data System (ADS)

The evolution of the Pan-African ancient continental margin in northeastern Africa was investigated, using an Nd model age, ion-microprobe data on zircon ages, and Rb-Sr whole-rock dates on the high-grade gneiss terrain at Sabaloka, Sudan, a region which is formally considered to be part of the Archaean/early Proterozoic Nile craton. The analysis of these data indicates that the Sabaloka granulites and gneisses are not Archaen in age. Instead, they reflect Pan-African metamorphic events. The gneisses studied may represent the infrastructure of the ancient African continental margin onto which the juvenile arc assemblage of the Arabian-Nubian shield was accreted during intense horizontal shortening and crustal interstacking of a major collision event.

Kroener, A.; Stern, R. J.; Dawoud, A. S.; Compston, W.; Reischmann, T.

1987-09-01

216

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

SciTech Connect

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

Prather, B.E.

1988-02-01

217

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

218

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

SciTech Connect

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

Cochran, J.R.

1981-01-10

219

Palaeo-ice streams on the west Greenland continental margin during the last glacial cycle  

NASA Astrophysics Data System (ADS)

The Greenland Ice Sheet is currently experiencing short-term mass-balance and dynamic changes at low elevations. These changes may reflect recent climate/ocean warming, or alternatively they may be part of the natural cycle of ice sheet growth and decay. Key to resolving this question is an understanding of long-term changes in Greenland Ice Sheet behaviour during the Late Pleistocene and the Holocene. However, our understanding of the long-term changes in the dynamic behaviour of the ice sheet is still poor, and major outstanding questions remain regarding past ice-sheet extent, and the timing and controls on initial ice retreat in many areas of Greenland. This is particularly the case on the west Greenland continental shelf bordering Baffin Bay. In this region, several major fast flowing outlets, including Jakobshavns Isbrae, currently drain the ice sheet. Marine geophysical and geological data collected in 2009 from the continental shelf and slope on the central west Greenland margin provide a detailed record of the landform and sediment record of these outlets during the last glacial cycle. Multibeam swath bathymetric data show the presence of streamlined bedforms focused along cross-shelf troughs. These streamlined bedforms record the former presence of major fast-flowing ice sheet outlets emanating from Disko Bugt (into which Jakobshavns Isbrae currently drains) and the Umanak fjord system. Geophysical and core evidence indicate that these outlets were grounded as far as the outer shelf/shelf edge. Major submarine fans are present at the mouths of the cross-shelf bathymetric troughs and cores and acoustic records from these fans show that they are composed of a variety of glacially-related mass flow sediments including turbidites and debris flow deposits. The landform-sediment record of these former ice sheet outlets will be discussed as will the results of radiocarbon dating investigations of the timing of ice sheet advance and retreat on the shelf. Dates indicate significantly later deglaciation of the ice sheet from the outer shelf than previous reconstructions have suggested.

O'Cofaigh, Colm; Dowdeswell, Julian; Kilfeather, Aoibheann; Jennings, Anne; Evans, Jeffrey; Noormets, Riko; Walton, Mariah

2010-05-01

220

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

Microsoft Academic Search

Three ancient impact craters (Chesapeake Bay—35.7Ma; Toms Canyon—35.7Ma; Montagnais—51Ma) and one multiring impact basin (Chicxulub—65Ma) are currently known to be buried beneath modern continental shelves. All occur on the passive Atlantic margin of North America in regions extensively explored by seismic reflection surveys in the search for oil and gas reserves. We limit our discussion herein to the three youngest

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

2002-01-01

221

Late precambrian volcanism at Wadi Allaqi, south Eastern Desert, Egypt: evidence for transitional continental arc\\/margin environment  

Microsoft Academic Search

The Dokhan volcanics at Wadi Allaqi, situated in the south Eastern Desert of Egypt, range in composition from basaltic-andesite to dacite. Geochemically, they have a transitional character from low K-tholeiite to calc-alkaline with a relatively high ZrY ratio that characterises a continental arc\\/margin setting. The most basic sample has extremely low Mg# (40) and Ni (55 ppm) values, indicating significant

Said A. El-Nisr

1997-01-01

222

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

Microsoft Academic Search

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

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

1981-01-01

223

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

Microsoft Academic Search

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

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

2004-01-01

224

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

USGS Publications Warehouse

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

Nelson, C.H.

1990-01-01

225

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

NASA Astrophysics Data System (ADS)

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

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

226

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

USGS Publications Warehouse

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

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

1992-01-01

227

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

USGS Publications Warehouse

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

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

1980-01-01

228

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

SciTech Connect

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

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

1991-08-01

229

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

230

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

SciTech Connect

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

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

1985-01-01

231

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

232

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

233

Evaporitic constraints on the southward drifting of the western Gondwana margin during Early Cambrian times  

Microsoft Academic Search

Lower Cambrian evaporites and carbonates are reported from nearly all the platforms of the western Gondwana margin, which comprises the Souss, Ossa–Morena, Cantabro-Iberian, Armorican and Montagne Noire–Sardinian Basins. Both lithologies were deposited in climatically restricted belts and their changing palaeogeographic distributions, according to recent biostratigraphic correlations, are used to infer the latitudinal motion of this margin. As a result, the

J. J. Álvaro; J. M. Rouchy; T. Bechstädt; A. Boucot; F. Boyer; F. Debrenne; E. Moreno-Eiris; A. Perejón; E. Vennin

2000-01-01

234

Structural evolution of the accretional continental margin of the Paleoproterozoic Svecofennian orogen in southern Sweden  

NASA Astrophysics Data System (ADS)

Volcanic and sedimentary rocks of the Västervik and Valdemarsvik groups along the southern margin of the 1.90-1.85 Ga south Svecofennian crustal province in Sweden have been migmatised and intruded by granitoids and gabbros during extension (deformation phase D 1), at a depth of at least 10 km. Peak metamorphic conditions were reached around 1825 Ma. Extension switched to compression before cooling. The partially molten middle crust facilitated shortening and thickening by isoclinal and sheath folds on scales from a decimetre to 10 km (D 2). A pervasive mylonitic S 2 foliation developed. These structures were refolded into eastward plunging crustal scale upright folds with E(SE)-W(NW) trending axial planes (D 3). D 2-D 3 deformation structures are colinear and developed during top-to-the-WNW slip along S 2. Locally, zones of high ductile strain in garnet-bearing migmatite assemblages record considerable vertical displacement (from 850 to 300 MPa), possibly in nappe roots. Some major D 2 sheath folds are embraced by D 3- like, but NNW-trending megafolds (D 4), apparently resulting from shortening around the former. D 2-D 4 deformations are interpreted to be broadly coeval and related to transpressive crustal shortening against the northern Svecofennian continental back-stop. After cooling to high greenschist-facies conditions, early structures underwent renewed transpression in north-south direction, expressed in localized, retrogressive shear zones (phase D 5). Much D 5 strain was accommodated by the 10 km wide, NW-SE striking, right-lateral Loftahammar-Linköping Deformation Zone (LLDZ), operative around 1800-1780 Ma. Regional strain partitioning caused transverse shortening in 15-20 km wide border zones of the LLDZ. Later overprints (D 6) involve narrow, subparallel, NW-SE striking and steeply NE dipping low greenschist-facies mylonite zones, some of which merge into the regional, polyphase Åsbro-Norrköping Deformation Zone. D 6 was transpressive; oblique, left-lateral, NE-block-up slip suggests a W(SW)-E(NE) orientation of ?1. Peak metamorphism and partial melting in the supracrustal rocks was approximately coeval with ?1800-1835 Ma old subduction-related magmatism in the Oskarshamn-Jönköping Belt (OJB), 100 km to the south-west. Seismic reflection profiling off the Baltic coast (BABEL line B) suggests northward polarity of subduction beneath the OJB and a back-arc environment of the Västervik and Valdemarsvik groups between the OJB arc and the margin of the >1850 Ma old Svecofennian orogen. We suggest that D 1-D 5 deformation was caused by intracontinental back-arc extension and subsequent closure of the back-arc basin by oblique accretion of the OJB marginal arc onto the northern continent. Accretion involved northward indentation by mid-crustal wedges, one of which coincides with and possibly caused the nucleation of the D 5 stage LLDZ. 40Ar/ 39Ar data from hornblende and white mica identify a prolonged cooling history of the area (1810-1490 Ma), influenced by the nearby, late- and postorogenic Transscandinavian Igneous Belt and, possibly, by largely hidden, c. 1530 Ma old anorogenic rapakivi-type intrusions.

Beunk, F. F.; Page, L. M.

2001-09-01

235

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

NASA Astrophysics Data System (ADS)

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

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

2009-04-01

236

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

NASA Astrophysics Data System (ADS)

We present the aeromagnetic compilation of the Norwegian mainland and its shelf area and its importance for geological models and tectonic studies. The combined data-set reveal that the bedrock structures are continuous from the Baltic Shield under the Caledonian orogen into the continental shelf and that the late-Caledonian collapse of the Caledonian orogene has influenced the style of extension on the Norwegian shelf. On the margin, modern high-resolution aeromagnetic surveys with small line-spacing, more accurate navigation and sensitive magnetometers have revealed the existence of significant magnetic anomalies arising from sedimentary layers. Sub-cropping Late Paleozoic to Tertiary sedimentary units along the Trøndelag-Nordland coast produce a very distinct anomaly pattern. The asymmetry of the anomalies, with a steep gradient and a negative anomaly to the east and a more gentle gradient to the west, relate the anomalies to a strata gently dipping westward. Susceptibility measurements on core samples, hand specimens and in situ on bedrock exposures are essential for the interpretation of these anomalies. Remapping of the oceanic crust has also improved our under-standing of the Tertiary opening of the North Atlantic as previously interpreted oceanic fracture zones zones do not exist; these were artefacts of poor navigation and wide line spacing of the vintage datasets. Tectonic reconstruction has shown that the opening of the Norwegian-Greenland Sea between the Jan Mayen and Senja fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies and without jumps in spreading axis. Transfer zones have previously been associated with oceanic fracture zones along the Mid-Norwegian and East-Greenland margins. Transfer zones are important entry points for sedimentary drainage systems, a relationship that has also been suggested for the transport of Cretaceous sands to the mid-Norwegian margin. Our new interpretation has consequently implications for evaluating the petroleum potential in the Vøring Basin, Mid-Norway. A high-resolution survey of the Oslo Graben changed our understanding of this Permian Rift. The magnetic field data of the Oslo Graben area are unique since two surveys recorded at different flight altitudes (50 m above ground and 3400 m above sea level) exist. The magnetic anomaly reaches values up to 1500 nT over an elongated area of 75 by 30 km. Local structures, such as calderas, ring fracture zones and their associated circular-shaped magnetic anomalies are a first order feature in the aeromagnetic data. Analysis of the depth-to- bottom of the magnetic sources and forward modeling indicates the presence of a more than 15 km thick intrusion beneath the Oslo Graben. The intrusion is interpreted to indicate a differentiation series from base (gabbro) to top (granite). This petrological variation with depth is not detectable in the gravity anomaly because of the small density contrast to the surrounding crust.

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

2007-12-01

237

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

238

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

NASA Astrophysics Data System (ADS)

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 2003 along two major sediment transport pathways south and west of the Mississippi River mouth. Lignin profiles in these age-dated cores (210Pb geochronology) indicate artificial reservoir retention as a primary control on organic carbon quantity and quality reaching the margin post-1950, whereas pre-1950 sediments may reflect soil erosion due to land clearing and farming practices. Lignin (?8) concentrations (range 0.2 to 1.7) also indicate that TOCT delivery rates/decay processes have probably remained relatively consistent from proximal to distal stations along transects. The down-core profile at the Canyon station seems to be temporally linked and connected to inner shelf deposition, suggestive of rapid cross-shelf transport. Sources of terrestrially derived organic carbon were reflective of mixed angiosperms over the last 150 years in cores west and south of the Mississippi River delta. The lignin-phenol vegetation index (LPVI) (range 130.0 to 510) proved to be a sensitive indicator of source changes in these sediments and eliminated some of the variability compared to C/V (range 0.01 to 0.4) and S/V (range 0.9 to 2.1) ratios. Stochastic events such as hurricanes and large river floods have a measurable, albeit ephemeral, effect on the shelf TOCT record. Burial of TOCT on the river-dominated Louisiana continental margin is largely driven by anthropogenic land-use alterations in the last 150 years. Land-use changes in the Mississippi River basin and river damming have likely affected carbon cycling and TOCT burial on the Louisiana continental margin over a large spatial extent as observed by similar trends in cores from across and along the margin.

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

2011-03-01

239

Nitrogen exchange at the continental margin: A numerical study of the Gulf of Mexico  

NASA Astrophysics Data System (ADS)

A two-layered baroclinic circulation model and a 21-layered biochemical model are used to explore the consequences of Loop Current-induced upwelling and terrestrial eutrophication on “new” production within the Gulf of Mexico. During a quasi-annual penetration and eddy-shedding cycle of the Loop Current, the simulated seasonal changes of incident radiation, wind stress, and surface mixed layer depth induce an annual cycle of algal biomass that corresponds to in situ and satellite time series of chlorophyll. The simulated nitrate fields match those of shipboard surveys, while fallout of particulate matter approximates that caught in sediment traps and accumulating in bottom sediments. Assuming an f ratio of 0.06-0.12, the total primary production of the Gulf of Mexico might be 105-210g C m -2y -1 in the absence of anthropogenic nutrient loadings, i.e. 2-3 fold that of oligotrophic regions not impacted by western boundary currents. Less than 25% of the nitrogen effluent of the Mississippi River may be stored in bottom sediments, with most of this input dispersed in dissolved from beneath the pycnocline, after remineralization of particulate detritus within several production cycles derived from riverine loading. At a sinking rate of 3m d -1, however, sufficient phytodetritus survives oxidation in the water column to balance estimates of bottom metabolism and burial at the margins.

Walsh, John J.; Dieterle, Dwight A.; Meyers, Mark B.; Müller-Karger, Frank E.

240

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

E-print Network

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

Yang, Zong-Liang

241

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

242

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

243

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

244

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

245

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

246

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

NASA Astrophysics Data System (ADS)

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

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

2006-10-01

247

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

NASA Astrophysics Data System (ADS)

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

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

2006-12-01

248

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

249

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

250

Sources and transport of dissolved iron and manganese along the continental margin of the Bay of Biscay  

NASA Astrophysics Data System (ADS)

Dissolved iron (DFe; <0.2 µm) and dissolved manganese (DMn; <0.2 µm) concentrations were determined in the water column of the Bay of Biscay (eastern North Atlantic Ocean) in March 2002. The samples were collected along a transect traversing from the European continental shelf over the continental slope. The highest DFe and DMn concentrations (2.39 nM and 6.10 nM, respectively) were observed in the bottom waters on the shelf at stations closest to the coast. The release of trace metal from resuspended particles and the diffusion from pore waters were probably at the origin of elevated DFe and DMn concentrations in the Bottom Boundary Layer (BBL). In the slope region, the highest total dissolvable iron (TDFe), DFe and DMn values (24.6 nM, 1.58 nM and 2.12 nM, respectively) were observed close to the bottom at depth of ca.~600-700 m. Internal wave activity and slope circulation are thought to be at the origin of this phenomenon. These processes were also very likely the cause of elevated concentrations (DFe: 1.27 nM, DMn: 2.34 nM) measured in surface waters of stations located in the same area. At stations off the continental slope, the vertical distribution of both metals were typical of open ocean conditions, indicating that inputs from the continental margin did not impact the metal distributions in the offshore waters.

Laës, A.; Blain, S.; Laan, P.; Ussher, S. J.; Achterberg, E. P.; Tréguer, P.; de Baar, H. J. W.

2007-03-01

251

Sources and transport of dissolved iron and manganese along the continental margin of the Bay of Biscay  

NASA Astrophysics Data System (ADS)

Dissolved iron (DFe; <0.2?m) and dissolved manganese (DMn; <0.2?m) concentrations were determined in the water column of the Bay of Biscay (eastern North Atlantic Ocean) in March 2002. The samples were collected along a transect traversing from the European continental shelf over the continental slope. The highest DFe and DMn concentrations (2.39 nM and 6.10 nM, respectively) were observed in the bottom waters on the shelf at stations closest to the coast. The release of trace metal from resuspended particles and the diffusion from pore waters resulted in elevated DFe and DMn concentrations in the Bottom Boundary Layer (BBL). In the slope region, the highest total dissolvable iron (TDFe), DFe and DMn values (24.6 nM, 1.58 nM and 2.12 nM, respectively) were observed close to the bottom at depth of ca. 600-700 m. Internal wave activity and slope circulation are thought to be at the origin of this phenomenon. These processes were also very likely the cause of elevated concentrations (DFe: 1.27 nM, DMn: 2.34 nM) measured in surface waters of stations located in the same area. At stations off the continental slope, the vertical distribution of both metals were typical of open ocean conditions, indicating that inputs from the continental margin did not impact the metal distributions in the offshore waters.

Laës, A.; Blain, S.; Laan, P.; Ussher, S. J.; Achterberg, E. P.; Tréguer, P.; de Baar, H. J. W.

2006-09-01

252

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

253

Fission track thermochronology in South-Central Chile (37°S-42°S): implications for mass transfer and denudation along an active continental margin  

NASA Astrophysics Data System (ADS)

Seismicity, faulting and uplift point to active deformational processes along the South-Central Chilean continental margin and make this area a natural laboratory for investigations of long-term mass transfer patterns at convergent margins. Understanding the factors controlling the tectonic behaviour of the continental margin has some general implications for continental growth and the mass transfer processes in "subduction factories". In order to identify and quantify mass transfer processes we applied apatite and zircon fission track thermochronology along two N-S oriented, trench-parallel sections. The western profile is located within the Permo-Triassic accretionary complex of the Coastal Cordillera and provides information about the timing and magnitude of forearc uplift and/or subsidence. The eastern profile runs through the Main Cordillera and aims towards quantifying the denudation history and understanding the mechanisms of exhumation within the magmatic arc. Early Cretaceous to Early Tertiary fission track ages of the metamorphic and granitoid rocks of the Coastal Cordillera indicate that exhumation of less than 4 km occurred since about the Early Cretaceous. The exhumation rates were variable with time: while geomorphology and sedimentary cover on the basement rocks indicate uplift of several 100m during Pliocene and Quaternary, sedimentary basins suggest subsidence during Miocene times. Therefore, most of the recorded exhumation must be Early Cretaceous to Paleogene in age. The slow and discontinuous exhumation suggests that processes leading to vertical displacements, like basal underplating or tectonic erosion, were not active to any major extent here. In the Main Cordillera, apatite fission track ages at the southern end of the profile (40°S) range from Late Miocene to Late Pliocene, revealing that exhumation there is recent and rapid, with rates of nearly 1 km/Ma during the last 4 Ma. Fission track ages tend to get older along the profile until Eocene fission track ages at the northern end of the profile (38°S) indicate that exhumation there was more than an order of magnitude less efficient. Although variations of the geothermal gradient and tectonics may have influenced the age distribution pattern, climate seems to be an important factor. Younger ages towards the south correlate with drastic increases of precipitation rates and intensity of Pleistocene glaciation.

Gräfe, K.; Glodny, J.; Seifert, W.; Rosenau, M.; Echtler, H.

2003-04-01

254

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

255

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

Microsoft Academic Search

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

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

2005-01-01

256

The Triassic Cobquecura Pluton (Central Chile): An example of a fayalite-bearing A-type intrusive massif at a continental margin  

NASA Astrophysics Data System (ADS)

The Late Triassic Cobquecura Pluton is a small epizonal intrusive body that was emplaced into the Paleozoic Basement at the western side of the Cordillera de la Costa, near to the town of Cobquecura (35°57'-36°12'S, central Chile). The pluton displays a wide spectrum of compositions that include gabbro and fayalite-granite, and less abundant hybrid rocks. SiO 2-content ranges from 48 to 76 wt.% with a compositional gap between 57 and 63 wt.% SiO 2. SiO 2-poor rocks have Mg-Fe minerals olivine (Fa 33-34), diopside-augite (Mg# = 56-91; Mg# = (Mg/(Mg + Fe 2+) × 100) and pargasite (Mg# = 4-8), which with progressive differentiation become Fe-rich olivine (Fa 89- 98), hedenbergite-ferroaugite (Mg# = 10-43) and ferroedenite (Mg# = 8-29). Similar K/Ar isotopic ages, mingling structures, and coherent arrays in Harker diagrams, as well as mineral compositions show that the SiO 2-poor and SiO 2-rich rocks are comagmatic. The Fe-rich mineralogy and Ga-content of the Cobquecura Pluton point to an anorogenic A-type magmatism. The compositional similarities, as well as the bimodal character of the magmatism of Cobquecura and other Upper Triassic granitoids and lavas north of 30°S at the Chilean continental margin indicate a common origin of the magmatism with anorogenic signature and coeval processes along the southwestern margin of Gondwana in the Triassic.

Vásquez, Paulina; Franz, Gerhard

2008-11-01

257

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

USGS Publications Warehouse

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

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

1979-01-01

258

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

259

Hot regions and the origin of marginal basins in the western Pacific  

NASA Astrophysics Data System (ADS)

Volcanic provinces, showing no genetic relationship to plate boundaries, may be classified into two categories: hot spots which are a few hundred kilometers across and may be nearly fixed in position with respect to deep mantle, and hot regions which are a few thousand kilometers across and can migrate more freely. The mantle upwellings which presumably give rise to them may originate at different depths by different causes. It is observed that the marginal basins, older than 15 Ma, of the western Pacific become progressively younger northward (with the exception of the South Fiji Basin). This leads to the hypothesis that the spreading of marginal basins was caused by the mantle upwelling associated with a migrating hot region, and that a hot region which started from the Australia region in the Late Cretaceous, migrated northward successively creating the Tasman, Coral Sea, Caroline, South China, Parece Vela, Shikoku, and Japan Basins through Paleocene, Eocene, Oligocene, and Early Miocene time. This hot region presently underlies northern China, Mongolia and adjacent areas. Another hot region is assumed to have caused the spreading of the South Fiji Basin and all the currently spreading basins in the southwestern Pacific. Spreading of individual marginal basins occurred during the short period when a hot region was passing the pertinent area. The main merit of this hypothesis is that it can explain the relationship between the location and age as well as the short duration of spreading, of western Pacific marginal basins. Marginal basins are formed commonly (though not always) in the overriding plates near subduction zones, because in such a setting a spreading marginal basin is readily accommodated either by an oceanward movement of the nearby subduction zone or by a continentward movement and subduction of the overriding oceanic plate. There are a few marginal basins that were formed in other tectonic settings and accommodated by some other mechanism of compensative subduction. Presumably the splitting of the supercontinent Pangea was also caused mainly by the effect of the mantle upwelling associated with the same hot region that later migrated northward, creating marginal basins in the western Pacific. The main basis for this presumption is the fact that the splitting of Pangea was completed in the Australia region in Middle Cretaceous time, whereas the spreading of marginal basins in the western Pacific began in the same Australia region in the Late Cretaceous immediately following.

Miyashiro, Akiho

1986-02-01

260

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

NASA Astrophysics Data System (ADS)

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

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

2014-02-01

261

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

NASA Astrophysics Data System (ADS)

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

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

2006-12-01

262

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

NASA Astrophysics Data System (ADS)

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

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

2007-05-01

263

A comparison of Holocene fluctuations of the eastern and western margins of the Greenland Ice Sheet  

NASA Astrophysics Data System (ADS)

Determining how the Greenland Ice Sheet (GrIS) responded to past temperature fluctuations is important for assessing its future stability in a changing climate. We present a record of the Holocene extents of the western GrIS margin near Kangerlussuaq (67.0°N, 50.7°W) and compare this with the past fluctuations of Bregne ice cap (71°N, 25.6° W), a small ice cap in the Scoresby Sund region 90 km from the eastern GrIS margin, to examine the mechanisms that influenced past ice margin fluctuations. The past extents of the Bregne ice cap are a proxy for the climatic conditions that influenced the nearby GrIS margin. We used glacial geomorphic mapping, 10Be dating of boulders and bedrock, and sediment cores from proglacial and non-glacial lakes. In western Greenland, 10Be ages on the Keglen moraines, 13 km west of the current GrIS margin and the Ørkendalen moraines, ?2 km west of the current ice margin date to 7.3 × 0.1 ka (n=6) and 6.8 × 0.3 ka (n=9), respectively. Fresh moraines, ?50 m from the current ice margin date to AD 1830-1950 and are likely associated with advances during the Little Ice Age (LIA). In some areas, the LIA moraines lie stratigraphically above the Ørkendalen moraines, indicating the GrIS was inboard of the Ørkendalen limit from 6.8 ka to the 20th century. In eastern Greenland, 10Be ages show that Bregne ice cap retreated within its late Holocene limit by 10.7 ka. A lack of clastic sediment in a proglacial lake suggests the ice cap was smaller or completely absent from ~10-2.6 ka. A snowline analysis indicates that temperatures ~0.5°C warmer than present would render the entire ice cap into an ablation zone. Glacial silts in the proglacial lake at ~2.6 and ~1.9 cal kyr BP to present indicate advances of Bregne ice cap. Fresh moraines ?200 m of Bregne ice cap were deposited ?2.6 cal kyr BP and mark the largest advance of the Holocene. Both the western GrIS margin and Bregne ice cap were influenced by Northern Hemisphere summer insolation during the Holocene. The western GrIS margin retreated significantly and Bregne ice cap likely disappeared during the warm early to middle Holocene. 10Be ages (10.7 ka) outboard of the late Holocene moraines at Bregne ice cap compared to those outside of the LIA moraines near Kangerlussuaq (6.8 ka) differ by ~4 kyr. This disparity in ages may have been caused by a large late Holocene advance in eastern Greenland, or perhaps the western GrIS margin retreated farther inland during the middle Holocene. Decreasing Northern Hemisphere summer insolation during the late Holocene, combined with a strong, cold East Greenland Current near Scoresby Sund may have influenced a significant ice cap advance. The temporal pattern of the responses of the eastern and western ice margins to Holocene climate changes may be indicative of how the GrIS will respond to future changes.

Levy, L.; Kelly, M. A.; Lowell, T. V.; Hall, B. L.; Applegate, P. J.; Howley, J.; Axford, Y.

2013-12-01

264

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

NASA Astrophysics Data System (ADS)

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

Zabanbark, A.; Lobkovskii, L. I.

2012-02-01

265

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

266

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

NASA Astrophysics Data System (ADS)

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; Brink, Uri ten; Ross, Steve W.; Brooke, Sandra

2014-06-01

267

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

268

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

269

Gas hydrates (clathrates) causing pore-water freshening and oxygen isotope fractionation in deep-water sedimentary sections of terrigenous continental margins  

Microsoft Academic Search

The occurrence of gas hydrates in deep-water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep-sea drilling results. On the Pacific continental slope off Guatemala gas hydrates were brought up for the first time from two holes (497, 498A) drilled during Leg 67 of the DSDP in water depths of

Reinhard Hesse; William E. Harrison

1981-01-01

270

41. REGIONAL CORRELATION OF MINERALOGY AND DIAGENESIS OF SEDIMENT FROM THE EXMOUTH PLATEAU AND ARGO BASIN, NORTHWESTERN AUSTRALIAN CONTINENTAL MARGIN1  

Microsoft Academic Search

Correlation of mineral associations from sediment recovered on the northwestern Australian continental margin document the juvenile-to-mature evolution of a segment of the Indian Ocean. Lower Cretaceous sediments contain sandy-to-silty radiolarian claystone that consists of highly smectitic mixed-layered illite\\/smectite (I\\/S) in addition to minor amounts of diagenetic pyrite, barite, and rhodochrosite. These immature, poorly sorted sediments were derived from nearby continental

John S. Compton; David Mallinson; Toedsit Netratanawong; Stanley D. Locker

271

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

Microsoft Academic Search

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

H. Kuno

1966-01-01

272

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

273

Shelf-to-canyon sediment-transport processes on the Eel continental margin (northern California)  

Microsoft Academic Search

To investigate the processes by which sediment is supplied to the head of a submarine canyon, an instrumented tripod and a mooring were deployed in the northern thalweg of the Eel Canyon during autumn and winter 1999–2000. This was done as part of the STRATAFORM program, and in combination with a long time-series benthic-tripod data collection on the Eel continental

P. Puig; A. S. Ogston; B. L. Mullenbach; C. A. Nittrouer; R. W. Sternberg

2003-01-01

274

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

SciTech Connect

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

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

1986-01-01

275

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

SciTech Connect

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

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

1990-10-01

276

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

E-print Network

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

Paris-Sud XI, Université de

277

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

278

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

279

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

USGS Publications Warehouse

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

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

1986-01-01

280

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

NASA Astrophysics Data System (ADS)

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

Burdige, David J.; Komada, Tomoko

2013-05-01

281

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

282

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

Microsoft Academic Search

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

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

1994-01-01

283

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

Microsoft Academic Search

The Shyok Suture Zone (Northern Suture) of North Pakistan is an important Cretaceous-Tertiary suture separating the Asian continent (Karakoram) from the Cretaceous Kohistan–Ladakh oceanic arc to the south. In previously published interpretations, the Shyok Suture Zone marks either the site of subduction of a wide Tethyan ocean, or represents an Early Cretaceous intra-continental marginal basin along the southern margin of

Alastair H. F. Robertson; Alan S. Collins

2002-01-01

284

Recent sediment transport and deposition in the Lisbon-Setúbal and Cascais submarine canyons, Portuguese continental margin  

NASA Astrophysics Data System (ADS)

Recent sediment transport and deposition in the Lisbon-Setúbal and Cascais submarine canyons, Portuguese continental margin, were investigated on the basis of water column profiles of suspended particulate matter, records of near-bottom currents and settling fluxes of particulate matter obtained with benthic landers, and analysis of surface sediments. The results show that fine-grained predominantly lithogenic sediment derived from adjacent shelf areas accumulates in the upper reaches of the canyons. Sediment deposited further down in the middle and lower reaches of the canyons is essentially similar to the hemipelagic sediment found on the adjacent continental slope, indicating that down-canyon transport of sediment from the upper to the lower canyon is limited. Tidal currents measured at various depths in the Lisbon-Setúbal Canyon appear sufficiently strong to resuspend and transport sediment, but net up-canyon flow of the bottom water may retain shelf-derived lithogenic sediment in the upper canyon. Sediment gravity flows, which in the nearby Nazaré Canyon are an effective mechanism for down-canyon sediment transport, appear rare in the Lisbon-Setúbal Canyon and probably also in the Cascais Canyon. Turbidity current events recorded in the sedimentary record of the canyons may correspond to seismic events of 1969 and 1755 AD.

de Stigter, Henko C.; Jesus, Carlos César; Boer, Wim; Richter, Thomas O.; Costa, Ana; van Weering, Tjeerd C. E.

2011-12-01

285

Vertical distribution of benthic invertebrate larvae during an upwelling event along a transect off the tropical Brazilian continental margin  

NASA Astrophysics Data System (ADS)

Abundance and composition of marine benthic communities have been relatively well studied in the SE Brazilian coast, but little is known on patterns controlling the distribution of their planktonic larval stages. A survey of larval abundance in the continental margin, using a Multi-Plankton Sampler, was conducted in a cross-shelf transect off Cabo Frio (23°S and 42°W) during a costal upwelling event. Hydrographic conditions were monitored through discrete CDT casts. Chlorophyll- a in the top 100 m of the water column was determined and changes in surface chlorophyll- a was estimated using SeaWiFS images. Based on the larval abundances and the meso-scale hydrodynamics scenario, our results suggest two different processes affecting larval distributions. High larval densities were found nearshore due to the upwelling event associated with high chlorophyll a and strong along shore current. On the continental slope, high larval abundance was associated with a clockwise rotating meander, which may have entrapped larvae from a region located further north (Cabo de São Tomé, 22°S and 41°W). In mid-shelf areas, our data suggests that vertical migration may likely occur as a response to avoid offshore transport by upwelling plumes and/or cyclonic meanders. The hydrodynamic scenario observed in the study area has two distinct yet extremely important consequences: larval retention on food-rich upwelling areas and the broadening of the tropical domain to southernmost subtropical areas.

Yoshinaga, Marcos Y.; Sumida, Paulo Y. G.; Silveira, Ilson C. A.; Ciotti, Áurea M.; Gaeta, Salvador A.; Pacheco, Luiz F. C. M.; Koettker, Andréa G.

2010-01-01

286

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

287

Salt tectonics in the SW Alps (Italy-France): From rifting to the inversion of the European continental margin in a context of oblique convergence  

NASA Astrophysics Data System (ADS)

The SW Alps result from the inversion of the European continental margin during the oblique convergence between Europe and Adria since the Cretaceous. The orogenic deformation is controlled by two factors: the inherited sedimentary and structural record and the geodynamic interaction between the two plates. In this paper we present a stratigraphic and structural analysis of the external SW Alps (Ventimiglia-Menton area) in order to define the sedimentary and deformational geometries of the chain and to reconstruct the evolutionary history. The field-data highlight the preeminent role played by inherited salt-structures, which derive from the depositional history experienced by the European margin since the Mesozoic onwards. From Late Triassic to Jurassic, evaporites and carbonates deposited as a response to the Thetyan rifting. The following emplacement of the Cretaceous flysch and of the Eocene foreland basin succession was strongly influenced by the extensionally-triggered salt diapirism and by the interactions with deformations connected to the Pyrenees dynamics. The resulting geologic discontinuities (i.e. diapir-induced highs and basins, inherited normal and trasform faults) strongly influenced the successive Oligo-Miocene evolution of the belt in the study area. Observed changes in the thrusts and folds kinematics are considered as the results of rotation during their approaching to inherited highs. Furthermore, the overturning of thrusts and folds in the front of the diapiric flanks are associated with the progressively salt squeezing into the anticlines cores promoted by ongoing Alpine compression. Finally, the kinematic data from the study area show radical differences in the tectonic transport direction with respect to the rest of the SW Alps (NW- to W-ward in the Ventimiglia-Menton area, S- to SW-ward in Provence and Ligurian Alps). This difference is interpreted to be caused by the relative motions of crustal blocks dominated by transpressive tectonics in the frame of the Oligo-Miocene western Mediterranean geodynamics.

Decarlis, A.; Maino, M.; Dallagiovanna, G.; Lualdi, A.; Masini, E.; Seno, S.; Toscani, G.

2014-12-01

288

Structural evolution of the accretional continental margin of the Paleoproterozoic Svecofennian orogen in southern Sweden  

Microsoft Academic Search

Volcanic and sedimentary rocks of the Västervik and Valdemarsvik groups along the southern margin of the 1.90–1.85Ga south Svecofennian crustal province in Sweden have been migmatised and intruded by granitoids and gabbros during extension (deformation phase D1), at a depth of at least 10km. Peak metamorphic conditions were reached around 1825Ma. Extension switched to compression before cooling. The partially molten

F. F. Beunk; L. M. Page

2001-01-01

289

19. SEDIMENTARY RESPONSE TO PALEOCLIMATE FROM DOWNHOLE LOGS AT SITE 693, ANTARCTIC CONTINENTAL MARGIN 1  

Microsoft Academic Search

The first well logs collected below the Antarctic circle were obtained during Leg 113 at Site 693 on the Dronning Maud Land Margin (Antarctica) in the Weddell Sea. Gamma-ray, resistivity, and sonic logs were collected between 108.0 and 439.0 mbsf. The downhole logs show good agreement with the data collected from cores and provide a con­ tinuous measurement of the

Xenia Golovchenko; Suzanne B. O'Connell; Richard Jarrard

290

Provenance and fate of organic carbon in three submarine canyons from the Portuguese Margin: Implications for transport processes of material in continental margins  

NASA Astrophysics Data System (ADS)

Submarine canyons are key environments on the continental margin that are affected by unique and dynamic but often episodic and complex processes, and are difficult to study. Canyons are considered hotspots of biodiversity and enhancement of primary productivity at canyon heads has often been postulated to support this, although the evidence is sparse. Additionally canyons are considered to be fast-track corridors for material transported from the land to the deep sea and they are considered major pathways for the transportation and burial of organic carbon, acting as buffers for sediment and carbon storage. Organic geochemical and isotopic markers are often used as reliable indicators for the supply, quality and fate of organic matter in marine systems. In this study they have been used to test the above hypotheses in three contrasting submarine canyons (Nazaré, Setubal/Lisbon and Cascais) of the Portuguese Margin. The elemental and lipid biomarker composition of suspended particulate organic matter of surface waters close to the studied canyon heads had a fresh phytoplankton signal, however there was no clear evidence for enhanced primary productivity by comparison to the neighbouring open slope. By contrast, mid-depth waters (700-1600 m), that are dominated by the northward flowing Mediterranean Outflow Water, had high lipid content and abundant mesozooplankton biomarkers, perhaps reflecting zooplankton activity focused at the boundaries of distinct water masses. In the waters close to the floor of the Nazaré Canyon the presence of elemental sulphur (a product of sediment diagenesis) and high molecular weight hydrocarbons (recalcitrant, terrestrial markers) indicated high levels of resuspended material, particularly at the Upper section (

Kiriakoulakis, Kostas; Wolff, George; Blackbird, Sabena

2010-05-01

291

Pleistoceneholocene Sedimentation Pattern in the Southeastern Brazilian Continental Margin Interplay Between Sea Level Climate Changes and Regional Ocean Circulation  

NASA Astrophysics Data System (ADS)

The general conclusions emerging from Southeastern Brazilian continental margin over the past decade is in general in agreement that the current dynamics, sea level coupled with climate system developed over the Pleistocene suggest changing in dynamics closely follow the precession-driven insolation cycle. The physical mechanism invoked by most authors involves the intensification of trade NE wind, which are linked principally with the southward migration of the Intertropical Convergence Zone (ITCZ) and the intensification of the South American Monsoon. This model yields several hypotheses about the nature of sedimentation along the Southeastern Brazilian margin that are yet to be regionally tested. Here, we provide high-resolution sedimentation data from 16 cores distributed along the Cabo Frio upwelling zone, off the southeastern coast of Brazil (23°11'24"S, 41°47'59.9"W). The first results show low sedimentation rate during late glacial, an increase during the mid-Holocene followed by a decrease to present day. More than the climate changes, these results suggest a combination between sea level and regional circulation impacts, which controls the regional pattern of sedimentation in this region

Sifeddine, A.; Albuquerque, A.; Belem, A. L.; Meyers, P. A.; Gurgel, M.; Mendoza, U.; Turcq, B.

2011-12-01

292

Organic matter budget in the Southeast Atlantic continental margin close to the Congo Canyon: In situ measurements of sediment oxygen consumption  

Microsoft Academic Search

A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and

C. Rabouille; J.-C. Caprais; B. Lansard; P. Crassous; K. Dedieu; J. L. Reyss; A. Khripounoff

2009-01-01

293

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.

294

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

295

Role of tectonic-sedimentary melange and Permian-Triassic cover units, central southern Turkey in Tethyan continental margin evolution  

NASA Astrophysics Data System (ADS)

Melanges play a key role in the interpretation of orogenic belts, including those that have experienced deformation and metamorphism during continental collision. This is exemplified by a Palaeozoic tectonic-sedimentary melange (part of the Konya complex) that is exposed beneath a regionally metamorphosed carbonate platform near the city of Konya in central Anatolia. The Konya complex as a whole comprises three units: a dismembered, latest Silurian-Early Carboniferous carbonate platform, a Carboniferous melange made up of sedimentary and igneous blocks in a sedimentary matrix (also known as the Hal?c? Group or S?zma Group), and an overlying Volcanic-sedimentary Unit (earliest Permian?) . The Palaeozoic carbonates accumulated on a subsiding carbonate platform that bordered the northern margin of Gondwana, perhaps as an off-margin unit. The matrix of the melange was mainly deposited as turbidites, debris flows and background terrigenous muds. Petrographic evidence shows that the clastic sediments were mostly derived from granitic and psammitic/pelitic metamorphic rocks, typical of upper continental crust. Both extension- and contraction-related origins of the melange can be considered. However, we interpret the melange as a Carboniferous subduction complex that formed along the northern margin of Gondwana, related to partial closure of Palaeotethys. Blocks and slices of Upper Palaeozoic radiolarian chert, basic igneous rocks and shallow-water carbonates were accreted and locally reworked by gravity processes. Large (up to km-sized) blocks and slices of shallow-water limestone were emplaced in response to collision of the Palaeozoic Carbonate Platform with the subduction zone. The overlying Volcanic-sedimentary Unit (earliest Permian?) comprises alkaline lava flows, interbedded with volcaniclastic debris flows and turbidites, volcanogenic shales and tuff. The complex as a whole is overlain by shallow-water, mixed carbonate-siliciclastic sediments of mainly Late Permian age that accumulated on a regional-scale shelf adjacent to Gondwana. Successions pass transitionally into Lower Triassic rift-related shallow-water carbonates and terrigenous sandstones in the southwest of the area. In contrast, Triassic sediments in the southeast overlie the melange unconformably and pass upwards from non-marine clastic sediments into shallow-marine calcareous sediments of Mid-Triassic age, marking the base of a regional Mesozoic carbonate platform. During the latest Cretaceous-Early Cenozoic the entire assemblage subducted northwards and underwent high pressure/low temperature metamorphism and polyphase folding as a part of the regional Anatolide unit.

Robertson, Alastair H. F.; Ustaömer, Timur

2011-01-01

296

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

297

Morphology of central California continental margin, revealed by long-range side-scan sonar (GLORIA)  

SciTech Connect

Leg 2 of the 4-leg USGS EEZ-SCAN 84 program used GLORIA long-range side-scan sonar to survey the region from Pt. Conception to just south of Pt. Arena, from the shelf break to the 200-nmi coverage. The overlapping digital sonographs were slant-range and anamorphically corrected, and a photomosaic of the sonographs was constructed at a scale of 1:375,000 (1 in. = 11.1 km). The underlying bed rock appears to be an important control in shaping the morphology of this margin. Several faults have sea-floor expression and lie subparallel to the margin. The density of canyons and gullies on the slope varies from south to north, probably because of variations in the characteristics of the bed rock. The slope west of San Francisco is the most dissected segment of the central California slope. Monterey Fan is covered by large-scale bed forms (5-15 m amplitude and 1.5-2.0 km wavelength) over much of its surface. Monterey channel crosses southwestward across the fan, but abruptly turns south along a 40-km long surface fault that coincides with a well-mapped meander loop. The channel loops to the north then turns southward crossing the entire Monterey Fan, at its distal reaches, changes to a broad, braided pattern. Major slumps on the margin have long (> 30 km) scarps, some have slump folds, and one has a debris-flow deposit that can be acoustically traced for more than 75 km. Seventeen new seamounts were mapped. Taney Seamounts are large, rimmed, calderas with diameters of about 15 km each; these appear to be very large explosive or summit-collapse features.

Gardner, J.V.; McCulloch, D.S.; Eittreim, S.L.; Masson, D.G.

1985-02-01

298

The Peru continental margin: High petroleum potential in a modern fore-arc setting  

SciTech Connect

Strata within modern fore-arc basins commonly are characterized as (1) containing little or no oil-prone source rocks and poor, volcaniclastic-rich reservoir rocks and (2) being submature due to low geothermal gradients. Hence, many explorationists may view modern fore-arc basin settings as having little oil potential or, at best as only marginally prospective. The Talara fore-arc basin of Peru is a striking exception to this generally held belief. Situated along the northwestern part of Peru's active convergent margin, it extends offshore to within 50 km of the Peru-Chile Trench. Unlike the typical modern fore arc, the Talara basin is a prolific oil producer. From onshore and offshore fields, it has already produced over 1.3 billion bbl of oil averaging 38{degree} API gravity. Moreover, the lightly explored offshore, which constitutes more than half of the Talara basin, probably holds an additional 2 billion bbl of undiscovered recoverable reserves. The Talara basin encloses an area of about 17,000 km{sup 2} yet 70% of the production (> 900 million bbl) has come from coastal onshore fields that encompass an area about one-tenth this total size (1,750 km{sup 2}). Production has come chiefly from Paleocene and Eocene sandstones enclosed within a thick (composite thickness of 10,000 m) lower Tertiary marine clastic section. Pervasive normal block faulting has persisted across this and other fore-arc basins situated along the Peru margin from the Late Cretaceous through much of the Tertiary. This distinctive structural style, along with younger detachment faults, provides numerous structural traps whose complexities will, no doubt, challenge explorationists for years to come. A number of other Peru fore-arc basins. which are both geomorphically and structurally on trend with the Talata basin, also contain thick lower Tertiary sections and exhibit similar extensional histories.

Crouch, J.K.; Bachman, S.B.; Zucker, C.L. (Crouch Bachman and Associates, Inc., Santa Barbara, CA (USA))

1990-05-01

299

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

NASA Astrophysics Data System (ADS)

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

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

2003-12-01

300

Structural features of the Southwest African continental margin according to results of lithosphere-scale 3D gravity and thermal modelling  

NASA Astrophysics Data System (ADS)

To understand the structure of the Southwest African continental margin, a lithosphere-scale 3D structural model has been developed, covering the marginal Cretaceous-Cenozoic Orange, Luderitz, Walvis and Namibe basins, the easternmost Walvis Ridge offshore. Onshore, the model includes two late-Proterozoic Owambo (Etosha) and Nama basins. This 3D model integrates published thickness maps (sediment isopach maps), shallow seismic and well data as well as published deep seismic information and has been additionally constrained by 3D gravity and thermal modelling. Using 3D gravity modelling, the first order configuration of the crystalline crust has been resolved with respect to the location of the continent-ocean boundary. The distribution of a high-density lower crustal layer indicates a continuous body extending below the Cretaceous-Cenozoic depocentres and aligned parallel to the coast line. In addition, high-density zones within the continental crystalline crust had to be included in the model to fit observed and calculated gravity. The obtained Moho topography correlates with the major tectonic units of this continental margin. The results of the 3D thermal modelling indicate that there is a clear relationship between the location of thickened sediments and areas with increased temperatures within the upper 10 km of the 3D model. This indicates that the low thermal conductivity of the sediments increases heat storage within the areas covered by thick sediments. Within the deeper crust, the main feature of the temperature distribution is the transition across the continental margin from the relatively cold oceanic part to the warm continental one. This regional pattern is controlled by the thickness of the crystalline continental crust, which is characterized by an increased radiogenic heat production. At a depth of 80-90 km, the temperature becomes higher beneath the oceanic domain than beneath the continent, reflecting the configuration of the lower thermal boundary which is represented by an isothermal lithosphere-asthenosphere boundary.

Maystrenko, Yuriy P.; Scheck-Wenderoth, Magdalena; Hartwig, Alexander; Anka, Zahie; Watts, Antony B.; Hirsch, Katja K.; Fishwick, Stewart

2013-09-01

301

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

302

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

303

Forms and deposits of the continental quaternary of the Saharan margin of Eastern Maghreb (tentative synthesis)  

NASA Astrophysics Data System (ADS)

The Saharan margin of Eastern Maghreb extends from Aurès in the West to the Gulf of Gabès in the East. The Plio-Quaternary boundary is very complex in the West where it is characterized by the persistence of Tertiary sedimentation with the development of ablation forms, presence of crusts and the formation of a lacustrine system. This is followed by the development of accumulation forms during the Middle Pleistocene and mostly during the Upper Pleistocene to Holocene with contemporaneous diminution in the granulometry of the deposits. The evolution of gypcretes at the expense of calcretes crust suffered a partial reverse during the Middle Pleistocene. At this time a lacustrine phase settled down in the Algero-Tunisian Chotts (sebkhas). The end of the Middle-Pleistocene corresponded with a major climatic break with the disappearance of the calcretes. By the end of the Upper Pleistocene, the gypcretes attained their maximum development and latter disappeared during the Holocene.

Ballais, J. L.; Ouezdou, H. Ben

304

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

305

Geochemistry of Oceanic Island-Arc and Active Continental Margin Volcanic Suites: Some Statistical Evaluations and Implications Using the Database GEOROC  

NASA Astrophysics Data System (ADS)

The major- and trace-element and the isotopic compositions of volcanic rocks from island arcs and active continental margins are compared using the comprehensive dataset available in the geochemical database GEOROC (http://georoc.mpch-mainz.gwdg.de). The data surveyed include the Aleutian, Kurile, IBM, Indonesian, Vanuatu, Tonga, Kermadec, Lesser Antilles, and Scotia island arcs. Active continental margins are the Andean, theCentral American, the Mexican, and the Cascade arcs. Average major-element compositions are tabulated for basalts, basaltic andesites, andesites, dacites, and rhyolites from the individual oceanic and continental arcs. Major-element variation diagrams show similar patterns for volcanic rocks from oceanic island arcs and active continental margins. Low-K series magmas are poorly represented, while high-K and shoshonitic magmas are more common in the active continental margin settings compared with the island-arc volcanic suites. With the GEOROC database, the well known overall depletion of high-field strength elements such as Nb, Ta, Ce, Zr, and Hf relative to large-ion-lithophile elements such as Rb, Sr, Ba, Pb, U, and Th can be verified for arcs from both oceanic and active coninental margin settings. It also allows more detailed insights into the geochemical signature of individual arcs. The Sr, Nd, and Pb isotopic composition of most oceanic island-arc volcanic rocks shows an overlap with the composition of OIB and MORB, with the exception of samples from the Indonesian and Lesser Antilles arcs, consistent with the involvement of subducted upper-crustal material in these arcs. Among the active continental margin settings, the Cascades, the Mexican, and the Central American arcs as well as the Southern and Northern Volcanic Zones of the Andean Arc overlap with the composition of OIB, whereas volcanic rocks from the central Andean Arc show high 87Sr/86Sr and low 143Nd/144Nd ratios. There appears to be no distinctive grouping of arc compositions in any "island-arc" versus "continental" type, rather a continuous range from Tonga-Kermadec and Scotia at one extreme with Indonesia and the Andes at the other.

Sarbas, B.

2002-12-01

306

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

NASA Astrophysics Data System (ADS)

To understand the evolution of the passive continental margin in Argentina low temperature thermochronology is an appropriate method, which will lead to new conclusions in this area. The Tandilia System, also called Sierras Septentrionales, is located south of the Río de la Plato Craton in eastern Argentina in the state of Buenos Aires. North of the hills Salado basin is located whereas the Claromecó basin is situated south of the mountain range. In contrary to most basins along the southamerican passive continental margin the Tandilia-System and the neighbouring basins trend perpendicular to the coast line. The topography is fairly flat with altitudes of. The igneous-metamorphic basement is pre-proterozoic in age and build up of mainly granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks and granitoid plutons it is overlain by a series of Neoproterozoic to early Paleozoic sediments (Cingolani, 2010), like siliciclastics, dolostones, shales and limestones (Demoulin et al., 2005). The aim of the study is to quantify the long-term landscape evolution of the passive continental margin in eastern Argentina in terms of thermal history, exhumation and tectonic activities. For that purpose, samples were taken from the Sierra Septentrionales and analyzed with the apatite fission-track method. Further 2-D thermokinematic modeling was conducted with the computer code HeFTy (Ketcham, 2005; Ketcham 2007; Ketcham et al., 2009). The results indicate apatite fission track ages between 101.6 (9.4) to 228.9 (22.3) Ma, what means all measured ages are younger as their formation age. That shows all samples have been reset. Six samples accomplished enough confined tracks and were used to test geological t-T models against the AFT data set. These models give a more detailed insight on the cooling history and tectonic activities in the research area. References: Cingolani C. A. (2010): The Tandilia System of Argentina as a southern extension of the Río de la Plata craton: an overview. Int. J. Earth Sci. (Geol. Rundsch.) (2011) 100:221-242, doi 10.1007/s00531-010-0611-5. Demoulin, A., Zarate, M., Rabassa, J.; (2005) Longterm landscape development: a perspective from the southern Buenos Aires ranges of east central Argentina; Journal of South American Earth Sciences, Vol. 19, pp. 193-204. Ketcham, R. A. (2005): Forward and inverse modeling of low-temperature thermochronometry data, in Low-Temperature Thermochronology: Techniques, Interpretations, and Applications, edited by P. W. Reiners and T. A. Ehlers, pp. 275-314, Mineralogical Society of America/Geochemical Society, Chantilly, Virginia. Ketcham, R. A., et al. (2007): Improved modeling of fission-track annealing in apatite, American Mineralogist, 92, 789-798. Ketcham, R. A., Donelick, R. A., Balestrieri, M. L., Zattin, M. (2009): Reproducibility of apatite fission-track length data and thermal history reconstruction, Earth and Planetary Science Letters 284 (2009), 504-515.

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

2014-05-01

307

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

NASA Astrophysics Data System (ADS)

Seafloor classification studies are carried out at the central western continental shelf of India employing two frequency normal incidence single beam echo-sounder backscatter data. Echo waveform data from different seafloor sediment areas are utilized for present study. Three artificial neural network (ANN) architectures, e.g., Self-Organization Feature Maps (SOFM), Multi-Layer Perceptron (MLP), and Learning Vector Quantization (LVQ) are applied for seafloor classifications. In case of MLP, features are extracted from the received echo signal, on the basis of which, classification is carried out. In the case of the SOFM, a simple moving average echo waveform pre-processing technique is found to yield excellent classification results. Finally, LVQ, which is known as ANN of hybrid architecture is found to be the efficient seafloor classifier especially from the point of view of the real-time application. The simultaneously acquired sediment sample, multi-beam bathymetry and side scan sonar and echo waveform based seafloor classifications results are indicative of the depositional (inner shelf), non-depositional or erosion (outer shelf) environment and combination of both in the transition zone. [Work supported by DIT.

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

2005-04-01

308

Vertical flux of biogenic matter during a Lagrangian study off the NW Spanish continental margin  

NASA Astrophysics Data System (ADS)

Lagrangian experiments with short-term, drifting sediment traps were conducted during a cruise on RRS Charles Darwin to the NW coast of Spain to study the vertical flux and composition of settling biogenic matter. The cruise was split into two legs corresponding to (i) a period of increased production following an upwelling event on the continental shelf (3-10 August 1998) and (ii) an evolution of a cold water filament originating from the upwelled water off the shelf (14-19 August). The export of particulate organic carbon (POC) from the upper layer (0-60m) on the shelf was 90-240mgC.m -2.d -1 and off the shelf was 60-180mgC.m -2.d -1. Off shelf the POC flux at 200m was 50-60mg.m -2.d -1. A modest sedimentation of diatoms (15-30mgC.m -2.d -1) after the upwelling was associated with increased vertical flux of chlorophyll a (1.8-2.1mg.m -2.d -1) and a decrease of the POC:PON molar ratio of the settled material from 9 to 6.4. Most of the pico-, nano-, and microplankton in the settled material were flagellates; diatoms were significant during the on shelf and dinoflagellates during the off shelf leg. Off shelf, the exponential attenuation of POC flux indicated a strong retention capacity of the plankton community between 40 and 75m. POC:PON ratio of the settled particulate matter decreased with depth and the relative portion of flagellates increased, suggesting a novel, flagellate and aggregate mediated particulate flux in these waters. Export of POC from the euphotic layer comprised 14-26% of the integrated primary production per day during the on shelf leg and 25-42% during the off shelf leg, which characterises the importance of sedimentation in the organic carbon budget of these waters.

Olli, Kalle; Wexels Riser, Christian; Wassmann, Paul; Ratkova, Tatjana; Arashkevich, Elena; Pasternak, Anna

2001-11-01

309

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

310

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

311

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

NASA Astrophysics Data System (ADS)

The Galilee study area, northern Israel, is at present an uplifted, steep continental margin that formed mainly during the Jurassic and has a large positive isostatic anomaly. Since the Jurassic, it was modified by several tectonomagmatic events, which this study attempts to define and classify by updating, reprocessing and reinterpreting gravity, aeromagnetic and geological data. The prominent Rehovot-Carmel N-S positive reduced-to-pole (RTP) magnetic anomaly caused by the Gevim Volcanics, as well as the coexisting Helez-Gaash high Bouguer gravity and the Pleshet low Bouguer gravity, represent the deep (>5 km) Permo-Triassic dominant horst and graben structure of Israel. The Jonah Ridge and Beirut high SW-NE RTP magnetic anomalies in the Levant basin delineate the Levant continental edge that is marked by a deeply buried horst covered by a Late Cretaceous volcanic complex. The Asher and Devora Jurassic volcanics appear to be responcible for the Atlit and Galilee negative magnetic anomalies and for significant negative gravity anomalies which became clear after removing gravity effect of the upper (post-Turonian) light density sediments from the observed gravity. The volcanics extend along a SW-NE belt parallel to the strike of the Moho. It is suggested here that the Carmel-Gilboa fault propagated during the Late Cretaceous from the Levant basin across the Galilee area southeastward to form the Azraq-Sirhan graben in Jordan. As such, it forms a right-step, en echelon, dextral strike-slip fault with associated tectonic basins of various shapes. During the Oligocene and before formation of the Dead Sea transform (DST), the reactivation of the Azraq-Sirhan graben was accompanied by tectonic driven rift propagation in the opposite direction, from Azraq-Sirhan to northwest. It dispersed into many faults and terminated ˜10 km west of the present DST. During the Miocene it propagated in the same direction and includes internal volcanic activity. The numerous Miocene-Pliocene volcanic centers on the margins of the DST indicate that the preferred pathway for magmas at that time was not within the deep basins of the DST.

Segev, Amit; Rybakov, Michael

2011-04-01

312

Gas hydrate volume estimations on the South Shetland continental margin, Antarctic Peninsula  

USGS Publications Warehouse

Multi-channel seismic data acquired on the South Shetland margin, northern Antarctic Peninsula, show that Bottom Simulating Reflectors (BSRs) are widespread in the area, implying large volumes of gas hydrates. In order to estimate the volume of gas hydrate in the area, interval velocities were determined using a 1-D velocity inversion method and porosities were deduced from their relationship with sub-bottom depth for terrigenous sediments. Because data such as well logs are not available, we made two baseline models for the velocities and porosities of non-gas hydrate-bearing sediments in the area, considering the velocity jump observed at the shallow sub-bottom depth due to joint contributions of gas hydrate and a shallow unconformity. The difference between the results of the two models is not significant. The parameters used to estimate the total volume of gas hydrate in the study area were 145 km of total length of BSRs identified on seismic profiles, 350 m thickness and 15 km width of gas hydrate-bearing sediments, and 6.3% of the average volume gas hydrate concentration (based on the second baseline model). Assuming that gas hydrates exist only where BSRs are observed, the total volume of gas hydrates along the seismic profiles in the area is about 4.8 ?? 1010 m3 (7.7 ?? 1012 m3 volume of methane at standard temperature and pressure).

Jin, Y.K.; Lee, M.W.; Kim, Y.; Nam, S.H.; Kim, K.J.

2003-01-01

313

Constraining uplift and denudation of west African continental margin by inversion of stacking velocity data  

NASA Astrophysics Data System (ADS)

An inverse model has been developed to determine the magnitude of denudation at seabed and subsurface unconformities by using root-mean-square (RMS) stacking velocity data derived from processing a set of seismic reflection profiles. This approach provides superior spatial coverage in comparison to other methods, such as vitrinite reflectance, apatite fission track, and sonic velocity modeling, which are restricted to borehole locations. The model assumes exponential porosity decay with depth and a standard velocity-porosity relationship in order to compute a synthetic RMS velocity profile. Denudation values at two levels in the stratigraphy are then adjusted until the fit between the model and the data is optimized. Successful modeling is dependent upon independent estimates of the initial porosity of sediment since significant trade-off occurs between initial porosity and denudation. Application to the west African shelf shows that 0.5-1 km of denudation occurred along the entire margin, probably during late Neogene times. The amount of denudation decreases oceanward and was probably triggered by regional tilting associated with initiation and/or regeneration of continent-wide mantle convective upwelling, which is thought to have affected much of subequatorial Africa. A subsurface Oligocene unconformity represents as much as 2.5 km of denudation and was probably produced by initiation of an oceanic current.

Walford, H. L.; White, N. J.

2005-04-01

314

Regional Mesozoic basin development along the Irish continental margins: evidence from regional gravity studies  

NASA Astrophysics Data System (ADS)

Sedimentary basins, which formed in response to multiphase rifting episodes during the early to late Mesozoic Era, comprise a global assemblage distributed across the entire North Atlantic region from eastern North America to the European Platform. This assemblage of basins has as its centre the region around and to the west of Ireland where tectonic re-activation of a strong NE-SW trending Caledonian basement fabric partly controlled the siting of the basins. The development of individual basins was contemporaneous with the onset of sea-floor-spreading within the Atlantic south of the Charlie Gibbs and Azores Fracture Zones. In this study a compilation of marine and satellite gravity data is primarily used to produce a regional interpretation of the tectonic fabrics in the region. A series of gravity models across the various sedimentary basins including the Rockall Basin, the Porcupine Basin and the Celtic and Irish Sea basins are used with vertical incidence and wide-angle seismic data to define crustal structure and its relationship with basin geometries. Two end-member sedimentary basin types are recognized in a regional preliminary model for Mesozoic basin development. The deep-water Rockall and Porcupine basins overlie North Atlantic lithosphere where large amounts of extensional strain were focused into the upper and mid-crust producing large amounts of syn-rift subsidence. In the shallow water shelf sea basin areas of the Celtic and Irish Sea generally smaller strains occur equally throughout all levels of the lithosphere and is distributed across wide regions of the lower crust and mantle lithosphere. This model for basin structuring requires that the strain field varies erratically within the upper to mid-crust across the North Atlantic. These changes in the pattern of strain are accommodated by rotation of continental crustal blocks (about vertical axes) and also by large-scale crustal transfer fault systems that penetrate at least to a lower crustal level. These fault systems may have been important in controlling fluid flow at a lithospheric scale.

O'Reilly, B. M.; Readman, P. W.

2003-04-01

315

Inorganic geochemical indicators of glacial-interglacial changes in productivity and anoxia on the California continental margin  

NASA Astrophysics Data System (ADS)

Evidence from sediments in cores collected from within the present oxygen-minimum zone (OMZ; 600-1200 m) on the central and northern California margins record several episodes during the last interstadial (OIS-3, ca. 60-24 ka) of deposition of laminated sediments containing elevated concentrations of several trace elements indicative of anoxic conditions (e.g., Mo, Ni, Zn, and Cu). The presence of abundant well-preserved organic matter, as well as lack of bioturbation and the presence of elevated concentrations of Mo and other trace elements, all support the theory that the OMZ in the northeastern Pacific Ocean was more intense, possibly anoxic, at several times during the late Pleistocene. Sediments of all ages in cores from the southern California margin contain elevated concentrations of Mo, suggesting that this area has always had higher rates of sulfate reduction than either the central or northern California areas. Most of the Ba in sediments in all cores collected on the upper continental slope (200-2700 m) off California and southern Oregon is derived from detrital clastic material, and this source did not change much in time. However, the amount of biogenic Ba did vary with time, and these variations closely follow the temporal variations in organic C (Corg) mass accumulation rate. Using Ba and Corg mass accumulation rates as proxy variables for productivity, all cores show that organic productivity under the California Current upwelling system was highest during OIS-3 and the Holocene, and lowest during the last glacial interval (LGI, ca. 24-10 ka). All paleoproductivity proxy variables indicate that the southern California area has always experienced higher productivity than other areas under the California Current, at least over the last 50 ky.

Dean, Walter E.; Gardner, James V.; Piper, David Z.

1997-11-01

316

Rift to Post-rift evolution of a "passive" continental margin: The Ponta Grossa Arch, SE Brazil  

NASA Astrophysics Data System (ADS)

Low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct the post-rift evolution since the break-up of southwestern Gondwana. Thermochronological data obtained from apatite fission-track analysis of Neoproterozoic metamorphic and Paleozoic to Mesozoic siliciclastic rocks as well as Mesozoic dikes from the Ponta Grossa Arch provided ages between 66.2 (1.3) and 5.9 (0.8) Ma. These data clearly indicate a post-rift reactivation during the Late Cretaceous and Paleogene. Integrating the results of older thermochronological studies, the reactivation of the southeastern Brazilian margin could be described in three main phases. Furthermore, the spatial distribution of age data indicate a NE-age group (NE of Curitiba) of about 20 Ma and a SW-age group (Curitiba and NW) of about 50 Ma. The change of ages follows the NW-SE trending São Jerônimo-Curiúva fault zone that can be traced offshore into the southern end of the Santos basin. Within the Santos basin these lineament terminates the salt occurrence in the south. It seams to play a major role in the structural evolution of the Santos basin and the Rio Grande Rise. Sedimentological studies in the Santos basin evidenced that the transport direction changed in Miocene time. During the Oligocene and earlier the sediments were transported mainly from the direction of the "Curitiba area" into the Santos basin. Within the Miocene an additional transport direction from an area north of Curitiba developed.

Franco-Magalhaes, Ana. O. B.; Hackspacher, Peter C.; Glasmacher, Ulrich A.; Saad, A. R.

2010-05-01

317

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

NASA Astrophysics Data System (ADS)

Low-temperature thermochronology was applied at the Brazilian passive continental margin in order to understand and reconstruct the post-rift evolution since the break-up of southwestern Gondwana. Thermochronological data obtained from apatite fission-track analysis of Neoproterozoic metamorphic and Paleozoic to Mesozoic siliciclastic rocks as well as Mesozoic dikes and alkaline intrusions from the Ponta Grossa Arch provided ages between 66.2 (1.3) and 5.9 (0.8) Ma. These data clearly indicate a post-rift reactivation during Late Cretaceous and Paleogene times. Integrating the results of older thermochronological studies, the reactivation of the southeastern Brazilian margin could be described in three main phases related to the rift to post-rift evolution of SE Brazil. Furthermore, the spatial distribution of age data indicates the presence of two age groups: a NE age-group (NE of Curitiba), with ages around 20 Ma and a SW age-group (Curitiba and NW) with ages of around 50 Ma. The change of ages follows the NW-SE trending São Jerônimo-Curiúva fault zone that can be traced offshore into the southern end of the Santos basin. Within the Santos basin, this lineament ends up to the salt occurrence in the south and seams to play a major role in the structural evolution of the Santos basin and the Rio Grande Rise. Sedimentological studies in the Santos basin evidenced that the transport direction changed in Miocene from WNW to WNW/NNW. During the Oligocene and earlier, the sediments were transported mainly from southeastwards to the direction of the “Curitiba area” into the Santos basin. Within the Miocene, an additional transport direction from an area north of Curitiba developed.

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

2010-10-01

318

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

NASA Astrophysics Data System (ADS)

The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO 4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. ?34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have ?34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/ 86Sr ratios for the barites vary over a narrow range of 0.70830-0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time of deposition of the host rocks for the PV samples. This indicates that the mineralizing fluids were not unaltered seawater. We develop a model in which the barites precipitated both below the sediment-water interface and at the seafloor from low-temperature fluids that circulated along faults. The isotopic, chemical, and textural data indicate that the barites formed by several processes. Mesozoic and Cenozoic basement rocks (crystalline and overlying sedimentary rocks), Quaternary basin fill, turbidite fans, and seawater provided various elements for the barites in different environments. The fluids had a deep-seated source and were hydrothermal in the deeper parts of the system for all the barite types, including the seafloor cold-seep deposit, based on Sr isotopes and regional geothermal gradients. These deep-seated fluids mixed with other fluids as they ascended, including seawater at and near the seafloor. The high ?34S values may have resulted from extreme Rayleigh fractionation during bacterially mediated (semi)closed-system sulfate reduction, probably driven by the flux of methane- and hydrocarbon-bearing fluids through basement rocks and basin sediments. Early diagenetic dolomite and pyrite in the host Monterey Formation in the PV Headland also formed in a zone of sulfate reduction, but within sediment buried only centimeters to a few meters and with a predominantly seawater source for the sulfur. Dolomite occurring with vein barite in the PV Headland formed at temperatures in the range of 40-90 °C. The cold-seep barites have ?34S values near that of modern seawater, although still somewhat fractionated. The barites that precipitated below the sediment-water interface have higher ?34S values, suggesting that the fluids were relatively reduced with molar dissolved barium in excess of dissolved sulfate. Those samples were exposed at the seafloor by uplift along faults and are composed predominantly of massive, brecciated, and vein barite.

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

2007-06-01

319

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

USGS Publications Warehouse

The Southern California Continental Borderland (SCCB) is part of the broad San Andreas transform-fault plate boundary that consists of a series of fault-bounded, petroleum-generating basins. The SCCB has high heat flow and geothermal gradients produced by thinned continental crust and Neogene volcanism. Barite deposits in the SCCB occur along faults. Barite samples from two sea-cliff sites and four offshore sites in the SCCB were analyzed for mineralogy, chemical (54 elements) and isotopic (S, Sr) compositions, and petrography. Barite from Palos Verdes (PV) Peninsula sea-cliff outcrops is hosted by the Miocene Monterey Formation and underlying basalt; carbonate rocks from those outcrops were analyzed for C, O, and Sr isotopes and the basalt for S isotopes. Cold-seep barite from Monterey Bay, California was analyzed for comparison. SCCB offshore samples occur at water depths from about 500 to 1800 m. Those barites vary significantly in texture and occurrence, from friable, highly porous actively growing seafloor mounds to dense, brecciated, vein barite. This latter type of barite contrasts with cold-seep barite in being much more coarse grained, forms thick veins in places, and completely replaced rock clasts in breccia. The barite samples range from 94 to 99 wt% BaSO4, with low trace-element contents, except for high Sr, Zr, Br, U, and Hg concentrations compared to their crustal abundances. ?34S for SCCB offshore barites range from 21.6‰ to 67.4‰, and for PV barite from 62‰ to 70‰. Pyrite from PV sea-cliff basalt and sedimentary rocks that host the barites averages 7.8‰ and 2.2‰, respectively. Two offshore barite samples have ?34S values (21.6‰, 22.1‰) close to that of modern seawater sulfate, whereas all other samples are enriched to strongly enriched in 34S. 87Sr/86Sr ratios for the barites vary over a narrow range of 0.70830–0.70856 and are much lower than that of modern seawater and also lower than the middle Miocene seawater ratio, the time of deposition of the host rocks for the PV samples. This indicates that the mineralizing fluids were not unaltered seawater. We develop a model in which the barites precipitated both below the sediment–water interface and at the seafloor from low-temperature fluids that circulated along faults. The isotopic, chemical, and textural data indicate that the barites formed by several processes. Mesozoic and Cenozoic basement rocks (crystalline and overlying sedimentary rocks), Quaternary basin fill, turbidite fans, and seawater provided various elements for the barites in different environments. The fluids had a deep-seated source and were hydrothermal in the deeper parts of the system for all the barite types, including the seafloor cold-seep deposit, based on Sr isotopes and regional geothermal gradients. These deep-seated fluids mixed with other fluids as they ascended, including seawater at and near the seafloor. The high ?34S values may have resulted from extreme Rayleigh fractionation during bacterially mediated (semi)closed-system sulfate reduction, probably driven by the flux of methane- and hydrocarbon-bearing fluids through basement rocks and basin sediments. Early diagenetic dolomite and pyrite in the host Monterey Formation in the PV Headland also formed in a zone of sulfate reduction, but within sediment buried only centimeters to a few meters and with a predominantly seawater source for the sulfur. Dolomite occurring with vein barite in the PV Headland formed at temperatures in the range of 40–90 °C. The cold-seep barites have ?34S values near that of modern seawater, although still somewhat fractionated. The barites that precipitated below the sediment–water interface have higher ?34S values, suggesting that the fluids were relatively reduced with molar dissolved barium in excess of dissolved sulfate. Those samples were exposed at the seafloor by uplift along faults and are composed predominantly of massive, brecciated, and vein barite.

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

2007-01-01

320

Microbial communities of deep-sea methane seeps at Hikurangi continental margin (New Zealand).  

PubMed

The methane-emitting cold seeps of Hikurangi margin (New Zealand) are among the few deep-sea chemosynthetic ecosystems of the Southern Hemisphere known to date. Here we compared the biogeochemistry and microbial communities of a variety of Hikurangi cold seep ecosystems. These included highly reduced seep habitats dominated by bacterial mats, partially oxidized habitats populated by heterotrophic ampharetid polychaetes and deeply oxidized habitats dominated by chemosynthetic frenulate tubeworms. The ampharetid habitats were characterized by a thick oxic sediment layer that hosted a diverse and biomass-rich community of aerobic methanotrophic Gammaproteobacteria. These bacteria consumed up to 25% of the emanating methane and clustered within three deep-branching groups named Marine Methylotrophic Group (MMG) 1-3. MMG1 and MMG2 methylotrophs belong to the order Methylococcales, whereas MMG3 methylotrophs are related to the Methylophaga. Organisms of the groups MMG1 and MMG3 are close relatives of chemosynthetic endosymbionts of marine invertebrates. The anoxic sediment layers of all investigated seeps were dominated by anaerobic methanotrophic archaea (ANME) of the ANME-2 clade and sulfate-reducing Deltaproteobacteria. Microbial community analysis using Automated Ribosomal Intergenic Spacer Analysis (ARISA) showed that the different seep habitats hosted distinct microbial communities, which were strongly influenced by the seep-associated fauna and the geographic location. Despite outstanding features of Hikurangi seep communities, the organisms responsible for key ecosystem functions were similar to those found at seeps worldwide. This suggests that similar types of biogeochemical settings select for similar community composition regardless of geographic distance. Because ampharetid polychaetes are widespread at cold seeps the role of aerobic methanotrophy may have been underestimated in seafloor methane budgets. PMID:24098632

Ruff, S Emil; Arnds, Julia; Knittel, Katrin; Amann, Rudolf; Wegener, Gunter; Ramette, Alban; Boetius, Antje

2013-01-01

321

The North American Atlantic outer continental margin landslides data base: Summary and observations  

SciTech Connect

A compilation of published data from 179 Quaternary mass movement features was analyzed to determine the common attributes of the slides, to reveal general trends, and to classify and compare slide types. The data set was derived primarily from high-resolution, seismic-reflection data and sidescan-sonar images. In general, evidence of slope failure is found throughout the length of the margin and in all water depths. Slides have occurred on slope angles ranging from 1{degree} to 30{degree} (avg.{approximately}5{degree}); they vary in width from 0.2 to 50 km (avg. {approximately}4 km) and in length from 0.3 to 380 km (avg. {approximately}10 km) and have been reported to be as thick as 650 m. They are slightly more prevalent on open slopes than in other physiographic settings (e.g., canyons, ridges, spurs) and more commonly translational than rotational (i.e., slumps). The slides show no striking affinity for a particular depth range, either in the data set as a whole or when analyzed in terms of physiographic setting, size, slope angle, or other basis for classification. Comparison of slides found on the open slope with those found within canyons shows that the average open slope slide tends to occur at lower slope angles and is much larger (by an order of magnitude) than the average canyon slide. Regardless of the physiographic setting or other characteristic, large-scale slides (area >100 km{sup 2}) rather than small-scale slides (area <10 km{sup 2}) tend to be associated with gentle slopes ({approximately}3-4{degree}) Similarly, slides generated on steep slopes ({>=}10{degree}), regardless of other attributes, tend to be small (avg. area <5 km{sup 2}). With few exceptions, comparisons between slide categories show only minor differences.

Booth, J.S.; O'Leary, D.W. (Geological Survey, Woods Hole, MA (USA))

1990-06-01

322

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

323

Microbial Communities of Deep-Sea Methane Seeps at Hikurangi Continental Margin (New Zealand)  

PubMed Central

The methane-emitting cold seeps of Hikurangi margin (New Zealand) are among the few deep-sea chemosynthetic ecosystems of the Southern Hemisphere known to date. Here we compared the biogeochemistry and microbial communities of a variety of Hikurangi cold seep ecosystems. These included highly reduced seep habitats dominated by bacterial mats, partially oxidized habitats populated by heterotrophic ampharetid polychaetes and deeply oxidized habitats dominated by chemosynthetic frenulate tubeworms. The ampharetid habitats were characterized by a thick oxic sediment layer that hosted a diverse and biomass-rich community of aerobic methanotrophic Gammaproteobacteria. These bacteria consumed up to 25% of the emanating methane and clustered within three deep-branching groups named Marine Methylotrophic Group (MMG) 1-3. MMG1 and MMG2 methylotrophs belong to the order Methylococcales, whereas MMG3 methylotrophs are related to the Methylophaga. Organisms of the groups MMG1 and MMG3 are close relatives of chemosynthetic endosymbionts of marine invertebrates. The anoxic sediment layers of all investigated seeps were dominated by anaerobic methanotrophic archaea (ANME) of the ANME-2 clade and sulfate-reducing Deltaproteobacteria. Microbial community analysis using Automated Ribosomal Intergenic Spacer Analysis (ARISA) showed that the different seep habitats hosted distinct microbial communities, which were strongly influenced by the seep-associated fauna and the geographic location. Despite outstanding features of Hikurangi seep communities, the organisms responsible for key ecosystem functions were similar to those found at seeps worldwide. This suggests that similar types of biogeochemical settings select for similar community composition regardless of geographic distance. Because ampharetid polychaetes are widespread at cold seeps the role of aerobic methanotrophy may have been underestimated in seafloor methane budgets. PMID:24098632

Ruff, S. Emil; Arnds, Julia; Knittel, Katrin; Amann, Rudolf; Wegener, Gunter; Ramette, Alban; Boetius, Antje

2013-01-01

324

Tectonic isolation of the Levant basin offshore Galilee-Lebanon effects of the Dead Sea fault plate boundary on the Levant continental margin, eastern Mediterranean  

NASA Astrophysics Data System (ADS)

The continental margin of the central Levant, offshore northern Israel and southern Lebanon is characterized by a sharp continental-oceanic crustal transition, exhibited on the bathymetry as a steep continental slope. At the base of the slope a narrow zone of faulting deforms the upper Messinian-recent sedimentary sequence. Further into the basin no major deformations are observed. However, onland a restraining bend along the Dead Sea fault plate boundary results in the formation of the Lebanon and anti-Lebanon mountain ranges, which exhibit a large positive isostatic anomaly not compensated at depth. All these geologic features follow a NNE-SSW trend. A dense network of multi-channel and single-channel seismic profiles, covering 5000 km of ship-track offshore northern Israel and southern Lebanon, was analyzed for the purpose of characterizing the continental margin. Additional seismic surveys covering the area between the Levant margin and the Cyprean arc were examined. Data were then incorporated with magnetic, gravity and earthquake measurements to reveal the deep crustal structure of the area and integrated with bathymetry data to describe the behavior of the young sedimentary basin fill. Results indicate that the Levant basin, offshore northern Israel and southern Lebanon (up to Beirut) is more-or-less unaffected by the intense tectonic deformation occurring onland. The transition between the deformed area onland and the undeformed Levant basin occurs along the base of the continental slope. Along the base, the upper Messinian-recent sedimentary sequence is cut by two sets of faults: shallow growth faults resulting from salt tectonics and high angle faults, marking the surface expression of a deeper crustal discontinuity - the marine extension of the Carmel fault zone. The central Levant continental margin is being reactivated by transpressional faulting of the marine continuation of the Carmel fault, at the base of the continental slope. This fault system coincides with the sharp continental-oceanic crustal transition, and acts as an isolator between the Levant basin and its land counterpart. To the north, this feature may initiate the formation of a new triple junction, with the Latakia ridge (part of the eastern Cyprean arc) and the East Anatolian fault.

Schattner, U.; Ben-Avraham, Z.; Lazar, M.; Hüebscher, C.

2006-11-01

325

Hyper-extended rifted margins in Alpine-type orogenic belts: evidences from the Valaisan Domain (Western Alps)  

NASA Astrophysics Data System (ADS)

The Valaisan Domain, in the Petit St. Bernard Pass area (Punta Rossa unit, Western Alps), consists of largely serpentinized sub-continental mantle juxtaposed with Paleozoic basement, meta-pillow lavas and Mesozoic to Tertiary meta-sediments. The complex lithostratigraphy was largely acquired during rift-related extensional tectonics, when mantle peridotites were exhumed at the bottom of the North Penninic basin. Extensional faulting resulted in widespread cataclasis of continental basement rocks, which rested above serpentinized mantle as extensional allochthons. The serpentinite-Paleozoic basement pair was sealed by locally sourced polymictic breccias, prior to the deposition of radiolaria bearing gray micaschists, followed by other basinal meta-sediments, including calcschists. Despite subsequent Alpine deformation and metamorphism, resulting in multi-stage folding and high Pressure-low Temperature metamorphism, the rift-related relationships between the different rock types can still be observed or inferred in several localities. The Punta Rossa unit preserves evidence of a multi-stage Alpine evolution. Post-high pressure isoclinal folding (Fctd) is associated with a pervasive axial planar cleavage (Sctd), defined by chloritoid and white mica. Following re-heating to ~400 °C, Sctd was statically overgrown by garnet and chloritoid, prior to large-scale recumbent folding at greenschist facies conditions (Frec). Interference between Fctd and Frec is responsible for the regional occurrence of basement rocks resting upon Mesozoic metasediments. Following Frec, shear zones with top-to-the-south kinematics dissected the tectonic pile, prior to the formation of upright folds with NNE-SSW trending fold axes. Therefore, the Punta Rossa unit preserves evidence of complete crustal excision in the Valaisan basin, with exhumation of ultramafics and minor mafic magmatism. Multi-stage deformation and a laterally discontinuous pre-Alpine architecture, typical of hyper extended rifted margins, are responsible for the complex outcrop pattern observed in the field. This study adds evidences indicating that a large part of the apparent complexity of Alpine-type orogens is related to inheritance from the rifting history, rather than to complex subduction dynamics.

Frasca, Gianluca; Beltrando, Marco; Compagnoni, Roberto; Vitale-Brovarone, Alberto

2013-04-01

326

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

NASA Astrophysics Data System (ADS)

Organic and inorganic nitrogen and their isotopic signatures were studied in continental margin sediments off Spitsbergen. We present evidence that land-derived inorganic nitrogen strongly dilutes the particulate organic signal in coastal and fjord settings and accounts for up to 70% of the total nitrogen content. Spatial heterogeneity in inorganic nitrogen along the coast is less likely to be influenced by clay mineral assemblages or various substrates than by the supply of terrestrial organic matter (TOM) within eroded soil material into selected fjords and onto the shelf. The ?15N signal of the inorganic nitrogen ( ?15N inorg) in sediments off Spitsbergen seems to be appropriate to trace TOM supply from various climate- and ecosystem zones and elucidates the dominant transport media of terrigenous sediments to the marine realm. Moreover, we postulate that with the study of sedimentary ?15N inorg in the Atlantic-Arctic gateway, climatically induced changes in catchment's vegetations in high northern latitudes may be reconstructed. The ?15N org signal is primarily controlled by the availability of nitrate in the dominating ocean current systems and the corresponding degree of utilization of the nitrate pool in the euphotic zone. Not only does this new approach allow for a detailed view into the nitrogen cycle for settings with purely primary-produced organic matter supply, it also provides new insights into both the deposition of marine and terrestrial nitrogen and its ecosystem response to (paleo-) climate changes.

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

2007-01-01

327

Response of infaunal macrobenthos to the sediment granulometry in a tropical continental margin southwest coast of India  

NASA Astrophysics Data System (ADS)

Surficial sediment samples, collected from the continental margin of the southwest coast of India in July 2004, were examined for the grain size and soft-bottom macrobenthic fauna, to understand the sediment granulometry and its effect on the faunal distribution. Samples were collected using Smith-McIntyre Grab, from 20 to 200 m depth range, consisting of mid-shelf, outer shelf and slope. Fine-grained sediment located in the mid shelf and supported low faunal abundance. Polychaetes constituted the bulk of the fauna. Feeding guild changed with depth and sediment granulometry. Coexistence of deposit feeders and carnivores in outer shelf and deposit feeders and filter feeders in the slope region indicated the effective utilization of different food resources. In general, richness and diversity were high in the southern region. Depth wise, the diversity and abundance were relatively high in the 50-75 m depth range. Correlation and BIO-ENV analysis showed that combination of different factors such as sediment texture, sediment sorting and depth were found to influence the distribution of macrobenthos. Hence, spatial variations observed in benthic community were presumably linked to the variations in sediment granulometry and the energy level conditions prevailing in the area.

Jayaraj, K. A.; Sheeba, P.; Jacob, Josia; Revichandran, C.; Arun, P. K.; Praseeda, K. S.; Nisha, P. A.; Rasheed, K. A.

2008-05-01

328

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

329

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

330

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

NASA Astrophysics Data System (ADS)

Neogene uplift has been widely used to explain the present high topography of many North Atlantic continental margins, but a contrary view exerts that such landscapes developed by protracted exhumation of topography since the Caledonian Orogeny as a result of gravitational collapse, continental rifting and erosion (e.g. Nielsen et al., 2009). We have obtained a single-grain apatite (U-Th)/He age dataset for East Greenland to test and refine theories of passive continental margin evolution in this region. Furthermore, multiple single-grain ages (up to 20 per sample) were obtained to enable investigation of (U-Th)/He age reliability and the factors that influence age reproducibility. We have focussed on East Greenland because stratigraphical evidence demonstrates unambiguously that the first-order topography, which consists of a series of deeply dissected quasi-planar high-level surfaces that are separated by well-defined escarpments, evolved prior to the onset of flood volcanism that coincided with lithospheric breakup at ~ 57 Ma. The maximum age of this ancestral topography is constrained by our (U-Th)/He age dataset, which consists of eighteen samples from four topographic profiles located in the inner fjord-system and spanning elevations from sea-level to ~ 1100 m. Importantly, the data demonstrates a rapid period of exhumation at ~ 60-70 Ma that indicates that the first-order topography formed rapidly in response to a single erosional event stimulated either by climatic change or the renewed onset of rifting. However, many individual samples exhibit poor single-grain age reproducibility, with standard deviations (calculated over the arithmetic mean of the corrected single-grain age) approaching 40%. Such variability has been presumed to indicate either ‘excess He' arising from the presence of undetected fluid and/or mineral inclusions, or the anomalous He retention as a result of radiation damage that affects He diffusivity. Nevertheless, statistical analysis of the dataset has demonstrated no clear systematic pattern behind the wide age variation. Notably, analysis of three Caledonian Basement samples from one particular topographic profile, each of which has > 15 age determinations, demonstrated that: (1) age variation was normally distributed, indicating that age variability cannot be ascribed to ‘excess He'; and (2) ages demonstrated only weak correlation with [eU] for high-[eU] samples (r2 = 0.17; p = 0.05; n = 19), whilst many low-[eU] and high-[eU] samples demonstrate similarly weak reproducibility, indicating that the samples may be insufficiently old for radiation damage to have influenced He diffusivity. Furthermore, inverse modelling of sample cooling histories using the uncorrected AHe age, the AFT age and the AFT track length distribution supports an age close to the weighted-mean age as being the most plausible. The single-grain age data therefore supports our previously-published conclusions based on multiple-grain age data from the same topographic profiles (Swift et al., 2008) that the East Greenland topography formed rapidly between ~ 70 and ~ 55 Ma. Furthermore, though further work is needed to understand the causes of the observed age variation and how such ages should be interpreted, it is possible that single-grain (U-Th)/He age variation could contain important information that can be further used to constrain thermal histories. We therefore advocate that non-reproducible samples should not be discounted on the basis of an inferred poor quality of the picking procedure or suspected radiation damage without rigorous analysis of a reasonable number of single-grain age determinations.

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

2010-05-01

331

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

332

Geodynamic Models of the Active Continental Margins of the Sea of Okhotsk  

NASA Astrophysics Data System (ADS)

The deep structure models of the lithosphere on the Okhotsk Sea Region and the region of Neftegorsk earthquake which has occurred on May 28, 1995 in the northern part of Sakhalin, caused victims and destructions are examined. The geodynamic model shows that North Sakhalin consists of the North Sakhalin basin, the Deryugin basin and the ophiolite complex located between them. The Deryugin basin was formed in position of an ancient deep trench after subducting the Okhotsk Sea Plate under Sakhalin in Late Cretaceous-Paleogene. The North Sakhalin basin was formed on a place of back-arc basin at that time. The ophiolite complex combined by the ultrabasic rocks, fixes position of ancient subduction zone acting about 100-60 million years ago. It is probably that the Deryugin basin and the North Sakhalin basin have been formed in the following way. Approximately 100 million years ago, the oceanic lithosphere of the Sea of Okhotsk subsided under Sakhalin, the eastern part of which was an andesite island arc. Behind it, in western Sakhalin there was a basin where sandy - clayey deposits accumulated in the Late Cretaceous, which subsequently formed the basement of Cenozoic North Sakhalin oil and gas basin. Approximately 10 - 15 million years ago subduction of the lithosphere of the Sea of Okhotsk apparently ceased. The remains of subduction zone, in the form of an ophiolite complex were revealed by geological and geophysical data. On a surface the subduction zone is shown as deep faults stretched along Sakhalin. It is probable that the manifestation of the Neftegorsk earthquake was a result of activization of this ancient subduction zone. As a result of mobile components along ancient subduction zone under Sakhalin, considerable displacements in earth crust along numerous faults and deformation of an earth surface go on. From a position of the ancient subduction zone under Sakhalin, which is a cause of strong earthquakes here, it follows that the region is one of seismic dangerous in Russia. The work was supported by Russian Foundation for Basic Research. No: 09-05-00406-a

Rodnikov, Alexander; Sergeyeva, Natalia; Zabarinskaya, Ludmila

2010-05-01

333

Tectonic denudation of upper mantle along passive margins: a model based on drilling (ODP Leg 103) and diving (Galinaute cruise) results, western Galicia Margin, Spain  

SciTech Connect

During ODP Leg 103 (April-June 1985) and the Galinaute cruise (June-July 1986), serpentinized peridotite (clinopyroxene-spinel harzburgite) was recovered within the basement approximately at the boundary between the North Atlantic ocean crust to the west and the thinned continental crust of the Galicia passive margin (Spain) to the east. The exposure of mantle-derived peridotite on the sea floor occurred at the end of the period of rifting, roughly 110 Ma. Ductile shear zones observed in the peridotite are consistent with movements along a deep, low-angle normal fault rooted within the upper mantle and dipping eastward beneath the Galicia margin. To explain the tectonic denudation of the mantle at the ocean-continent boundary, they use a nonuniform stretching model for the lithosphere, set up from Wernicke's model.

Boillot, G.; Winterer, E.L.; Recq, M.; Girardeau, J.; Kornprobst, J.; Loreau, J.P.; Malod, J.; Mougenot, D.

1987-05-01

334

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.

335

Development of sediment drifts approaching an active plate margin under the SW Pacific Deep Western Boundary Current  

NASA Astrophysics Data System (ADS)

The abyssal Pacific Ocean is fed by a 1000 km wide, deep western boundary current (DWBC) that flows northward along the continental margin, east of New Zealand. Between the passive margin of Chatham Rise and the subduction zone of Kermadec Trench, a distance of 1200 km, the DWBC has formed a suite of sediment drifts over a depth range of 2200-5700 m. Airgun and 3.5-kHz profiles record a variety of drift types that reflect regional variations in bathymetry, sediment supply, and the tectonic/volcanic framework. On Chatham Rise the DWBC has deposited a sinuous, linear body along the south flank (3000 m), an extensive apronlike drift on the north flank (2200-4500 m), and a ridgelike drift about the rise base (4500-5200 m). The flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby Louisville Seamount Chain. Further downcurrent, the 250 km long Rekohu Drift (3600-4190 m) has developed northward to 39°S. South of this latitude, drifts comprise mainly reworked pelagic/hemipelagic material and sediment transported from distant southerly sources. In contrast, drifts north of 39°S have received a major injection of terrigenous sediment from Hikurangi Channel which runs 1400 km from New Zealand, eastward across the Hikurangi Plateau to disgorge on to the abyssal floor at the plateau edge. En route, turbidity current overspill from the channel has moved north under the influence of the shallow DWBC to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500-4200 m. Off Hikurangi Channel mouth, a large fan has accumulated. The DWBC has extended the fan into a drift running over 250 km along the base of Hikurangi Plateau (5150-5770 m) toward Kermadec Trench. Here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area. The seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the DWBC in the region with the late Oligocene opening of the Australian-Antarctic seaway. Drift construction commenced during the Miocene but was punctuated in the late Miocene by another period of erosion that coincided with increased bottom water production in Antarctica. Deposition resumed in Plio-Pleistocene times when large quantities of sediment from the rapidly rising landmass of New Zealand were injected into the boundary current. The modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of Hikurangi Plateau and widespread scour zones and sediment wave fields.

Carter, L.; McCave, I. N.

1994-12-01

336

Ebro margins sedimentary system in the western Mediterranean Sea, from delta to deep sea  

SciTech Connect

During Holocene high sea level, delta-front lobes of silty mud have deposited beside a lobate Ebro delta. Topset and foreset beds of these lobes extend up to 20 km offshore in up to 30 m of water. Geostrophic currents advect fine silt and clay from river discharge and storm wave resuspension in the delta front and deposit up to 20 m of bottomset beds in a distal prodelta clay belt formed on the inner to middle shelf for 70 km south from the delta. Intensified water circulation and increased bottom-current speeds inhibit prodelta progradation over the outer shelf and north of the delta and south of the clay belt, where the shelf narrows. Deposition of Holocene hemipelagic mud on the upper slope is restricted, but some modern Ebro sediment apparently bypasses to the deep margin. During Pleistocene low sea level, a series of shelf-edge deltas resulted in extensive progradation of foreset mud beds over the continental slope east of the modern delta and south to the Columbretes Islands. In the north, rapid sediment progradation has resulted in large canyons ({plus minus}5 km wide), unconfined sediment gravity flows, and deposition of large sediment aprons (50 km diameter) downslope from canyon mouths. In the south, narrow canyons ({plus minus}2 km wide) have funneled turbidity currents to side-by-side channel-levee complexes younger and smaller to the southwest. Subsidence of the Valencia trough has facilitated sediment transport from these channel-levee complexes into Valencia Valley and thence to the Valencia fan, 200 km to the northeast. Consequently, during low sea level stands a major portion of Ebro sediment is transported north to the Valencia fan, whereas the main progradational history of the Ebro margin has been offshore and to the south of the present delta.

Nelson, C.H.; Maldonado, A. (Geological Survey, Menlo Park, CA (USA))

1988-08-01

337

Thin and layered subcontinental crust of the great Basin western north America inherited from Paleozoic marginal ocean basins?  

USGS Publications Warehouse

The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton. ?? 1974.

Churkin, M., Jr.; McKee, E.H.

1974-01-01

338

Organic geochemistry of sediments from the continental margin off southern New England, U.S.A.--Part I. Amino acids, carbohydrates and lignin  

NASA Technical Reports Server (NTRS)

Total organic carbon (TOC), lignin, amino acids, sugars and amino sugars were measured in recent sediments for the continental margin off southern New England. The various organic carbon fractions decreased in concentration with increasing distance from shore. The fraction of the TOC that was accounted for by these major components also decreased with increasing distance from shore. The concentration of lignin indicated that only about 3-5% of the organic carbon in the nearshore sediment was of terrestrial origin. The various fractions were highly correlated, which was consistent with a simple linear mixing model of shelf organic matter with material form the slope and rise and indicated a significant transport of sediment from the continental shelf to the continental slope and rise.

Steinberg, S. M.; Venkatesan, M. I.; Kaplan, I. R.

1987-01-01

339

Western U. S. continental climate record since 30 Ma as recorded in alunite: Comparison with the marine record  

SciTech Connect

Supergene alunite is an important benchmark indicator of continental climates. There is little or no D/H fractionation between supergene alunite and water from which it forms. Therefore, variations in delta D of this mineral reflect the isotopic composition of regional meteoric waters, which in turn can be used to infer paleoclimatic information. Because alunite also contains K, absolute ages can be easily and accurately determined by K/Ar methods. delta D values of Tertiary alunite from the Great Basin were measured for comparison to other paleoclimatologic proxies. Because tectonic arguments effectively rule out significant changes in altitude and latitude since 30 Ma, any observed variations in delta D must reflect climatic changes in the western US, and possibly globally. Latitude-corrected delta D values of meteoric waters in Nevada as recorded by alunite increased from about [minus]150 permil 30 million years ago to about [minus]105 permil in mid-Miocene time. This change is a reflection of the mid-Miocene global warming trend that has been inferred from both oceanic and other continental paleoclimatologic indicators. After mid-Miocene time, delta D values decreased rapidly to present-day values of near [minus]130 permil. This change is congruent with changes in other paleoclimatic indicators, both continental and oceanic. The alunite ages, however, are better constrained. The large changes in the isotopic compositions of alunite confirm that climatic indicators for continental samples, particularly from sites in continental interiors, may be much more sensitive to global changes than are oceanic indicators. This sensitivity is probably a function of the thermal buffering effect of the large oceanic mass. Caution must be taken, however, to account for possible variations in isotopic compositions of continental proxies that may have been caused by local or regional climatic factors.

Arehart, G.B. (Argonne National Lab., Argonne, IL (United States)); O'Neil, J.R. (Univ. of Michigan, Ann Arbor, MI (United States))

1992-01-01

340

Gabbro and related rock emplacement beneath rifting continental crust: U Pb geochronological and geochemical constraints for the Galicia passive margin (Spain)  

Microsoft Academic Search

The thinned continental crust of the west Galicia margin is bound by a belt of serpentinized peridotites (‘peridotite ridge’) lying about 300 km off the coast in the North Atlantic ocean. From this ridge, a gabbro and a chlorite rock were studied in an attempt to substantiate rift-related subcontinental magmatism, occurring prior to sea-floor spreading. U-Pb dating of 13 different

Urs Schärer; Jacques Kornprobst; Marie-Odile Beslier; Gilbert Boillot; Jacques Girardeau

1995-01-01

341

Glacial\\/interglacial changes in southern Africa: Compound-specific ?13C land plant biomarker and pollen records from southeast Atlantic continental margin sediments  

Microsoft Academic Search

This is part 2 of a study examining southwest African continental margin sediments from nine sites on a north-south transect from the Congo Fan (4°S) to the Cape Basin (30°S) representing two glacial (MIS 2 and 6a) and two interglacial stages (MIS 1 and 5e). Contents, distribution patterns, and molecular stable carbon isotope signatures of long-chain n-alkanes (C27-C33) and n-alkanols

Florian Rommerskirchen; Geoffrey Eglinton; Lydie Dupont; Jürgen Rullkötter

2006-01-01

342

Deep seismic reflection data of EDGE US mid-Atlantic continental-margin experiment: Implications for Appalachian sutures and Mesozoic rifting and magmatic underplating  

Microsoft Academic Search

The EDGE seismic experiment across the Virginia continental margin delineated a Paleozoic suture, buried Appalachian terranes, and Mesozoic rifting and magmatic events. The seismic grid revealed that the Mesozoic Norfolk rift basin exists only in the northern one-third of the previously mapped area. The north-striking listric border fault of the Norfolk basin half-graben parallels seismic laminations in the basement. The

R. E. Sheridan; D. L. Musser; L. Glover

1993-01-01

343

Deep seismic reflection data of EDGE U.S. mid-Atlantic continental-margin experiment: Implications for Appalachian sutures and Mesozoic rifting and magmatic underplating  

Microsoft Academic Search

The EDGE seismic experiment across the Virginia continental margin delineated a Paleozoic suture, buried Appalachian terranes, and Mesozoic rifting and magmatic events. The seismic grid revealed that the Mesozoic Norfolk rift basin exists only in the northern one-third of the previously mapped area. The north-striking listric border fault of the Norfolk basin half-graben parallels seismic laminations in the basement. The

Robert E. Sheridan; Douglas L. Musser; Lynn Glover III; Manik Talwani; John I. Ewing; W. Steven Holbrook; G. Michael Purdy; Robert Hawman; Scott Smithson

1993-01-01

344

Complexities of the Western Margin of the Trans-Hudson Orogen in Saskatchewan, Canada.  

NASA Astrophysics Data System (ADS)

The north-northwest trending margin of the Trans-Hudson Orogen, from latitude 52W to 59W, in Saskatchewan, is delineated by an 810 km long, mainly positive magnetic anomaly. On the surface this anomalous zone marks the boundary between the Archean Rae/Hearne craton and the Paleoproterozoic accreted terrane of the Reindeer Zone. Three widely separated, variable length regional deep sounding reflection surveys crossed this anomalous trend and determined a consistency along the strike of the crustal images of the western margin of the orogen. Several phases of tectonic development, including multi-stage subduction and continent-to-continent collision could be inferred from the seismic images. On all seismic sections, imbricate set of well-defined thrust sheets delineate the remnants of an orogenic wedge of a near complete convergence with its characteristic pro and retro-shear zones. South of the 52W latitude the north trending magnetic signatures are suddenly terminated in a triangular shaped magnetic low, and followed by the regionally characteristic magnetic features of the Archean Wyoming craton. These complex regional magnetic images clearly indicate that the convergence of the Rae/Hearne and Superior cratons, in the south, was directly and intensively affected by the interference of the Wyoming craton

Hajnal, Z.; Pandit, B. I.; Closson, J.

2004-05-01

345

The Cinder Lake Intrusive Complex, Knee Lake area, Central Manitoba: a Syenite- Carbonatite Association from a Neoarchean Continental Margin  

NASA Astrophysics Data System (ADS)

The Cinder Lake intrusive complex is the only known occurrence of feldspathoid rocks in Manitoba. These rocks were initially mapped in the southeastern part of the Lake by Elbers (in Gilbert, 1985) and Lenton (1985), but have not been adequately studied. On the basis of new field, petrographic and geochemical evidence acquired in 2008, three discrete intrusive phases can be presently identified at Cinder Lake: fine-grained aegirine-nepheline syenite, fine-grained biotite-vishnevite syenite and syenitic pegmatite. There is also convincing mineralogical and geochemical evidence for the presence of unexposed clinopyroxenite and carbonatitic units genetically associated with the alkaline syenitic rocks. The evidence for the presence of unexposed carbonatite includes pervasive calcitization of the syenitic rocks, occurrence of rare-earth minerals (britholite, monazite and REE-rich apatite) in association with Sr-rich calcite in metasomatised pegmatite, and andradite veins crosscutting the syenites. The geochemistry of the Cinder Lake rocks is most consistent with the HFSE-depleted, potassic, high-Ba/La and high-Th/Nb signature of arc magmas (Edwards et al., 1994). In common with island-arc and continental-margin phonolites, the Cinder Lake syenites are potassic rocks with a chondritic Zr/Hf ratio, strong enrichment in Ba relative to La and Th relative to Nb. Uranium-lead dating of zircon crystals recovered from the biotite-vishnevite syenite yielded an age of 2705±2 Ma, interpreted as the timing of syenite emplacement. This value is close to the age of the incipient accretion of subprovinces in the northwestern Superior province at 2.70-2.71 Ga (Davis et al. 2005). Given this age relationship, the Cinder Lake complex is probably derived from magmas produced in a Neoarchean subduction zone underlying the North Caribou microcontinent. The regional geological setting of the complex (abundance of tonalite and granodiorite among the plutonic rocks and the predominance of subaerially deposited sediments and calc- alkaline volcanics in the local stratigraphy) is more consistent with an active continental margin, rather than an island-arc environment.The economic potential of the Cinder Lake complex remains undetermined. According to Müller et al. (2001), over 20% of the world's largest gold deposits are associated with alkaline volcanics in subduction-zone settings. The unexposed carbonatite unit may also be of economic interest as a source of rare earth elements, Ba and Sr. Carbonanites in collisional settings are known for their enrichment in these elements and have been exploited for REE in the United States and China (Xu et al., 2004). Further work is required to ascertain the extent and petrographic makeup of the Cinder Lake complex and to gain a better understanding of its resources. References: Davis, D.W., Amelin, Y., Nowell, G.M. and Parrish, R.R. 2005: Precambrian Research, v. 140, p. 132-156. Edwards, C.M.H., Menzies, M.A., Thirlwall, M.F., Morris, J.D., Leeman, W.P. and Harmon, R.S. 1994: Journal of Petrology, v. 35, p. 1557-1595. Gilbert, H.P. 1985: Manitoba Energy and Mines, Geological Services, Geological Report GR83-1B, 76 p. Lenton, P.G. 1985: Report of Field Activities, Manitoba Energy and Mines, Geological Services, Mines Branch, p. 203-208. Müller, D., Franz, L., Herzig, P.M. and Hunt, S. 2001: Lithos, v. 57, p. 163-186. Xu, C., Zhang, H., Huang, Z., Liu, C., Qi, L., Li, W. and Guan, T. 2004: Geochemical Journal, v. 38, p. 67-76.

Chakhmouradian, A. R.; Böhm, C. O.; Kressall, R. D.; Lenton, P. G.

2009-05-01

346

Timing of crust formation and recycling in accretionary orogens: Insights learned from the western margin of South America  

NASA Astrophysics Data System (ADS)

Accretionary orogens are considered major sites of formation of juvenile continental crust. In the central and southern Andes this is contradicted by two observations: siliciclastic fills of Paleozoic basins in the central Andean segment of the accretionary Terra Australis Orogen consist almost exclusively of shales and mature sandstones; and magmatic rocks connected to the Famatinian (Ordovician) and Late Paleozoic magmatic arcs are predominantly felsic and characterized by significant crustal contamination and strongly unradiogenic Nd isotope compositions. Evidence of juvenile crustal additions is scarce. We present laser ablation (LA)-ICPMS U-Pb ages and LA-MC-ICPMS Hf isotope data of detrital zircons from seven Devonian to Permian turbidite sandstones incorporated into a Late Paleozoic accretionary wedge at the western margin of Gondwana in northern Chile. The combination with Nd whole-rock isotope data permits us to trace the evolution of the South American continental crust through several Proterozoic and Paleozoic orogenic cycles. The analyzed detrital zircon spectra reflect all Proterozoic orogenic cycles representing the step-wise evolution of the accretionary SW Amazonia Orogenic System between 2.0 and 0.9 Ga, followed by the Terra Australis Orogen between 0.9 and 0.25 Ga. The zircon populations are characterized by two prominent maxima reflecting input from Sunsas (Grenville) age magmatic rocks (1.2-0.9 Ga) and from the Ordovician to Silurian Famatinian magmatic arc (0.52-0.42 Ga). Grains of Devonian age are scarce or absent from the analyzed zircon populations. The Hf isotopic compositions of selected dated zircons at the time of their crystallization ( ?Hf ( T) ; T = 3.3-0.25 Ga) vary between - 18 and + 11. All sandstones have a significant juvenile component; between 20 and 50% of the zircons from each sedimentary rock have positive ?Hf ( T) and can be considered juvenile. The majority of the juvenile grains have Hf-depleted mantle model ages (Hf TDM) between 1.55 and 0.8 Ga, the time of the Rondonia-San Ignacio and Sunsas orogenic events on the Amazonia craton. The corresponding whole-rock ?Nd ( T) values fot these same rocks are between - 8 and - 3 indicating a mixture of older evolved and juvenile sources. Nd-depleted mantle model ages (Nd TDM*) are between 1.5 and 1.2 Ga and coincide broadly with the zircon Hf model ages. Our data indicate that the Paleo- and Mesoproterozoic SW Amazonia Orogenic System, and the subsequent Neoproterozoic and Paleozoic Terra Australis Orogen in the region of the central and southern Andes, developed following two markedly different patterns of accretionary orogenic crustal evolution. The SW Amazonia Orogenic System developed by southwestward growth over approximately 1.1 Ga through a combination of accretion of juvenile material and crustal recycling typical of the extensional or retreating mode of accretionary orogens. In contrast, the central Andean segment of the Terra Australis Orogen evolved from 0.9 to 0.25 Ga in the compressional or advancing mode in a relatively fixed position without the accretion of oceanic crustal units or large scale input of juvenile material to the orogenic crust. Here, recycling mainly of Mesoproterozoic continental crust has been the dominant process of crustal evolution.

Bahlburg, Heinrich; Vervoort, Jeffrey D.; Du Frane, S. Andrew; Bock, Barbara; Augustsson, Carita; Reimann, Cornelia

2009-12-01

347

Spatial and temporal patterns of benthic macrofaunal communities on the deep continental margin in the Gulf of Guinea  

NASA Astrophysics Data System (ADS)

Density, taxonomic composition at higher taxon level and vertical distribution of benthic macrofaunal communities and sediment characteristics (pore water, nitrogen, organic carbon, sulfur, C/N ratio, n-alcohol biomarkers) were examined at three deep sites on the Congo-Gabon continental margin. This study was part of the multidisciplinary BIOZAIRE project that aimed at studying the deep benthic ecosystems in the Gulf of Guinea. Sampling of macrofaunal communities and of sediment was conducted during three cruises (January 2001, December 2001 and December 2003) at two downslope sites (4000 m depth), one located near the Congo submarine channel (15 km in the south) and the other one far from the channel (150 km in the South). The third area located 8 km north of the Congo channel in the surroundings of a giant pockmark at 3160 m depth was sampled during one cruise in December 2003. At these three locations the macrofaunal communities presented relatively high densities (327-987 ind. 0.25 m -2) compared with macrofaunal communities at similar depths; that is due to high levels of food input related to the Congo river and submarine system activities that affect the whole study area. The communities were different from each other in terms of taxonomic composition at higher taxon level (phylum, class, order for all the groups except for the polychaetes classified into families). The polychaetes dominated the communities and were responsible for the increase in densities observed at both deep sites (4000 m) between January 2001 and December 2003 whereas the tanaidaceans, the isopods and the bivalves were the other most abundant taxa responsible for the spatial differences between these sites. The community at 3150 m differed from the two deep communities by higher abundances in bivalves, nemerteans and holothuroids. The composition of the polychaete community also differed among sites. In the vicinity of the Congo channel, the expected positive effect of the additional organic matter transported through the turbiditic currents on to the surrounding benthic communities was not observed, as the increase in densities during the study period was higher at the site located away from the Congo channel than near the channel (80% vs 30%). That may be due to the low food value of the organic matter of terrestrial origin carried through the turbidites, and/or to the disturbance caused by these turbidites. Conversely, far from the channel the macrofaunal communities benefit from organic matter of higher energetic value originating mainly from marine sources, but also from continental sources, carried by the Congo plume or by near-bed currents across or along the continental slope. Spatial and temporal variability in trophic and physical characteristics of the sediment habitat at both deep sites also affected the vertical distribution of the macrofaunal communities. The activities of the very active Congo system structure the deep macrofaunal communities on a large area in terms of densities, composition and vertical distribution. The food input is enhanced at regional scale as well as the heterogeneity of the sediment characteristics, mainly in terms of organic matter quality (marine vs terrigenous). In turn, the densities are enhanced as well as the regional diversity of the macrofaunal communities in terms of taxonomic composition and distribution.

Galéron, J.; Menot, L.; Renaud, N.; Crassous, P.; Khripounoff, A.; Treignier, C.; Sibuet, M.

2009-12-01

348

Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt  

NASA Astrophysics Data System (ADS)

The collision between the North and South China cratons in Middle Triassic time (240-225 Ma) created the world's largest belt of ultrahigh-pressure (UHP) metamorphism. U-Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ?3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (?3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.

Zheng, J. P.; Tang, H. Y.; Xiong, Q.; Griffin, W. L.; O'Reilly, S. Y.; Pearson, N.; Zhao, J. H.; Wu, Y. B.; Zhang, J. F.; Liu, Y. S.

2014-07-01

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