Tectonic development of the Mid-Norway continental margin

The area reviewed covers the Mid-Norway continental margin between latitudes 62°N and 68°N. Main structural elements, as defined at the base Cretaceous level, are the Tröndelag Platform, underlying the inner shelf, the Möre and Vöring Basins, located beneath the outer shelf and slope, and the Möre Platform and the Outer Vöring Plateau, forming a base of slope trend of highs. Sediments contained in the Mid-Norway Basin range in age from Late Palaeozoic to Cenozoic. The basement was consolidated during the Caledonian orogenic cycle. Devonian and Early Carboniferous wrench movements along the axis of the Arctic-North Atlantic Caledonides are thought to have preceded the Namurian onset of crustal extension. Rifting processes were intermittently active for some 270 My until crustal separation between Greenland and Fennoscandia was achieved during the Early Eocene. During the evolution of the Norwegian-Greenland Sea rift system a stepwise concentration of tectonic activities to its axial zone (the area of subsequent continental separation) is observed. During the Late Palaeozoic to Mid-Jurassic a broad zone was affected by tensional faulting. During the Late Jurassic and Cretaceous the Tröndelag Platform was little affected by faulting whilst major rift systems in the Möre and Vöring Basins subsided rapidly and their shoulders became concomitantly upwarped. During the latest Cretaceous and Early Palaeogene terminal rifting phase only the western Möre and Vöring Basins were affected by intrusive and extrusive igneous activity. Following the Early Eocene crustal separation and the onset of sea floor spreading in the Norwegian-Greenland Sea, the Vöring segment of the Mid-Norway marginal basin subsided less rapidly than the Möre segment. During the Early and Mid Tertiary, minor compressional deformations affected the Vöring Basin and to a lesser degree the Möre Basin. Tensional forces dominated the Late Palaeozoic to Early Cenozoic evolution of the Mid-Norway Basin and effected strain mainly in the area where the crust was weakened by the previous lateral displacements. The lithosphere thinned progressively and the effects of the passively upwelling hot asthenospheric material became more pronounced. Massive dyke invasion of the thinned crust preceded its rupture.     

Sedimentary and structural patterns on the Iberian continental margin: an alternative view of continental margin sedimentation

A long-range side-scan sonar (GLORIA) survey of the entire West Iberian slope and rise has provided the first overview of the interrelationship between structure and sedimentation patterns on a continental margin. The results emphasize the importance of slope-following contour currents as a depositional mechanism in fashioning this continental rise. Terrigenous sediments transported down-canyon by-pass the rise which does not consist of a series of coalescing fans. The sedimentation patterns identified on the sonographs can be interpreted in terms of facies models and caution must be exercised against over-emphasis of downslope processes in models for the construction of lower slopes and rises.     

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

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.     

Particle flux across the mid-European continental margin

Results are presented from particle flux studies using sediment trap and current meter moorings along a transect at the European continental margin at 49°N within the EU-funded Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid-slope follow the slope contour, whereas seasonal off-slope flow was registered at the outer slope. At 600 m on the slope fluxes are similar to those in the abyssal North Atlantic. The flux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest fluxes were recorded at 1440 m at the outer slope, where off-slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral fluxes of particulate organic carbon exceed the primary flux by over a factor of 2 at 1440 m on the outer slope. Estimated lateral fluxes of suspended particulate matter in the water column and intermediate nepheloid layers at the outer slope are potentially large compared to sinking fluxes measured by sediment traps. A comparison is made of particle flux at three continental margin sites and two sites in the adjacent open North Atlantic, from which it is seen that bulk and organic matter flux increases exponentially with proximity to the shelf break. The percentage contribution of particulate organic carbon to biogenic fluxes increases from a mean of 5.7% in the abyssal N. Atlantic to 13.9% at the continental margins.     

Magnetic anomalies over the Central Levant continental margin

The magnetic field over the central Levant continental margin, off northern Israel and southern Lebanon, and the adjacent Levant Basin has two distinct trends. Mount Carmel and its offshore continuation (Carmel Nose), which are the surface expression of a large subbottom structure that extends from the land area across the continental shelf to the continental slope, form a dividing zone between the two magnetic trends. South of the Carmel structure the magnetic field trends east-west, while north of the Carmel structure it trends northeast and north-northeast.Several pronounced magnetic anomalies exist mainly north of the Carmel structure, the majority of which trend north-northeast and northeast, parallel and sub-parallel to the trend of the magnetic field in this area. Some also trend northwest, perpendicular to the trend of the magnetic field. In several cases the magnetic anomalies indicate large lithological elements which continue from land to sea.Gravity and seismic refraction data show that the two magnetic domains north and south of the Carmel structure are associated with areas of different crustal structure. South of the Carmel structure the continetal-oceanic crustal transition zone is located beyond the continental margin at the base of the continental slope, while north of the Carmel structure it is located under the continental shelf, near the shore. On land, there are also differences in the structure of the crust north and south of the Carmel structure, the crust being much thinner north of the structure than south of it.We suggest that some of the large magnetic anomalies off the Central Levant were formed during the rifting phase of the eastern Mediterranean.     

Turbidity of waters over the Northwest Iberian continental margin

In OMEX-II-II, 9 cruises gathered optical data, principally by transmissometer. The distribution of optical turbidity caused by concentration of particulate matter (PMC) in the water column over the northern Iberian margin shows several features related to hydrography. It is concluded that a signal of PMC seen in Mediterranean Water (MW) found north of 42°N is not carried from its source at the Gibraltar Sill and Gulf of Cadiz because it is shown, using intermediate stations, that this turbid plume decays, mainly by fall out but also partly by mixing, to very low levels around southern Portugal. PMC maxima sometimes seen in MW on the northern Iberian margin are thus most likely to result from intermittent local resuspension by MW interacting with slope sediments. The highest turbidity is found over the upper slope and is the result of (i) shelf edge resuspension and off-shelf flow of turbid plumes, mainly between 100 and 300 m depth, and (ii) resuspension under the slope current aided by internal waves, in the depth range 500–800 m where the density gradient between ENACW and MW is maximal. Below the MW, flows are generally slow, and turbidity is low. The bottom nepheloid layer in deep water is also weak with PMC values <100 mg m-3. The focus of resuspension activity on the upper slope means that the region is an efficient exporter to the ocean of sediment that either escapes from the shelf or sinks to the bed from surface production. This accounts for upper slope sediments recorded in other studies as sandy or in places as rocky bottom.     

Pleistocene glacial history of the NW European continental margin

In this paper new and previously published data on the Pleistocene glacial impact on the NW European margin from Ireland to Svalbard (between c. 48°N–80°N) are compiled. The morphology of the glaciated part of the European margin strongly reflects repeated occurrence of fast-moving ice streams, creating numerous glacial troughs/channels that are separated by shallow bank areas. End-moraines have been identified at several locations on the shelf, suggesting shelf-edge glaciation along the major part of the margin during the Last Glacial Maximum. Deposition of stacked units of glacigenic debris flows on the continental slope form fans at a number of locations from 55°N and northwards, whereas the margin to the south of this is characterised by the presence of submarine canyons. Glaciation curves, based primarily on information from the glacial fed fan systems, that depict the Pleistocene trends in extent of glaciations along the margin have been compiled. These curves suggest that extensive shelf glaciations started around Svalbard at 1.6–1.3 Ma, while repeated periods of shelf-edge glaciations on the UK margin started with MIS 12 (c. 0.45 Ma). The available evidence for MIS 2 suggest that shelf-edge glaciation for the whole margin was reached between c. 28 and 22 ¹⁴C ka BP and maximum positions after this were more limited in some regions (North Sea and Lofoten). The last glacial advance on the margin has been dated to 15–13.5 ¹⁴C ka BP, and by c. 13 ¹⁴C ka BP the shelf areas were completely deglaciated. The Younger Dryas (Loch Lomond) advance reached the coastal areas in only a few regions.     

The bathymetry of the North Victoria Land continental margin

Abstract

The very unique continental margin of North Victoria Land, Antarctica, is characterized by complex bathymetry, reflecting control by glacial, tectonic, and marine processes. The abnormally shallow shelf can be divided into a deep, rugged, glacially dominated inner shelf and a smoother, shallower outer shelf, which is dominated by marine and glacial marine processes. Deep u‐shaped glacial troughs incise the shelf, while relict v‐shaped canyons incise the upper slope. Trending northwest‐southeast along the eastern edge of the area lies a rugged chain of seamounts representing the southern extension of the Balleny Fracture Zone. The continental slope is dominated by strong contour currents and gravity processes.     

Sediment slides on the northwestern continental margin of Africa

A sediment slide complex has been mapped on the West African continental margin north of Dakar, Senegal. Four major slides covering approximately 44,300 km² were delineated by seismic reflection profiles, 3.5 and 12 kHz echograms and piston cores. Although the slide areas have been altered by later erosion and deposition by turbidity flows, the major components of the slides — slide scar, zones of hummocky and blocky slide material and zones of debris flow — are recognizable. Cores containing flow folds with horizontal axial surfaces substantiate the echogram interpretations of debris flow. Morphology and depositional areas of the slides indicate that several major slide movements have occurred in each of the various slide areas. The triggering mechanism for these slides is perhaps earthquakes associated with the Cape Verde Islands, Cape Verde Plateau, and adjacent fracture zones.     

Gravity results from the continental margin of south-western Africa

Gravity, magnetic and bathymetric data were collected over the continental margin of south-western Africa by H.M.S. Hecla in 1966. A study of the gravity measurements shows that the positive free-air anomalies of the continental edge effect are unusually large, and in excess of those calculated for an isostatic model of the Earth's crust. Taking into account the available seismic and magnetic evidence, a two-dimensional crustal model has been designed incorporating a body of relatively high density in the upper crust to account for the unusually large values.     

南极大陆边缘研究进展

本文综合国外有关资料,介绍了70年代以来南极大陆边缘地球物理调查、尤其是多道反射地震调查,深海钻探,地震地层研究,以及面积大、赋有油气远景的两个海区——威德尔海和罗斯海研究的进展情况。     

Fluid escape structures in the north Sicily continental margin

High resolution and multichannel seismic profiles coupled with multibeam echosounder (seafloor relief) data, acquired along the northern Sicily continental margin (southern Tyrrhenian Sea), document the occurrence of mound and pockmark features, revealing fluid escape processes. Along this margin, morphology of the high-gradient continental slope is irregular due to the presence of structural highs, slope failures and canyons, and is interrupted by flat areas at a mean depth of 1500 m.Seismostratigraphic analysis tools and methods were used to identify fluid escape structures and to work out a classification on the basis of their morpho-acoustic characteristics. The detailed 3D bathymetric chart was used to define the top view morphologic features and their areal distribution. With the aim to evaluate the geochemical content of fluids, we collected a 2.3 m long sediment core in correspondence of a pockmark at a depth of 414 m. Pore waters were sampled every 10 cm and analysed in relation to their conductivity (EC) and composition (δ¹⁸O, δD, Li, Na, K, Mg, F, Cl, Br, NO₃, SO₄).The new data show the occurrence of different types of structures with highly contrasting seismic and morphologic signatures, both dome-type and concave-upward structures. The latter have a characteristic circular shape and are known as pockmarks. Morphobathymetric, stratigraphic and structural data suggest that these structures occur along fault planes, mainly associated with diagenetic carbonates and fluid venting activity. Pockmarks could be the result of both fault and landslide structures, as they appear aligned along a straight direction and occur in proximity of the slope, and are associated with slope instabilities. The structural features are possibly associated with the recent tectonics mapped on-land as well as the widespread seismicity of the margin.Geochemical features reveal that pore water is slightly enriched in heavy isotopes with respect to Mediterranean seawater, while the distribution profiles of EC, ion concentration (Cl, SO₄, Na, K, Mg, Ca), ion/Chloride ratios (Na/Cl, K/Cl, Ca/Cl, Mg/Cl and Alk/Cl) seem to indicate the existence of an external source of fluids and the occurrence of sediment-fluids interaction processes. A possible mechanism causing pore water freshening could be the destabilisation of gas hydrates.     

Geohazards surveys in the North Carolina upper continental margin

Geophysical surveys on the upper continental slope and shelf-break area off North Carolina confirm previous suggestions that this highly dissected region is characterized by deposition of sediment swept from the adjacent shelf. Seismic reflection profiling, echo sounding and side-scan sonar studies failed to reveal conclusive evidence for slumping or other gravity processes which have been described for similar areas to the north. Outcrops appear to be lacking and the upper slope is a region of sediment accumulation and/or periodic bypassing.     

Gas-hydrate stability thickness map along the Indian continental margin

The gas-hydrate stability thickness (GHST) map along the Indian continental margin is prepared from available bathymetry, sea-bottom temperature and geothermal gradient data. The bottom-simulating reflector (BSR) often marks the base of gas-hydrate stability zone. The prior information about the stability thickness in a particular area will help in identifying BSR on seismic data. The map is also useful to the exploration scientists to set a depth window within which proxies for gas-hydrate can be looked into. A GHST map was initially prepared in 1998 based on the-then available data. A lot of new data has been generated by various organizations under the Indian National Gas Hydrate Programs for the advancement of exploration and exploitation activities. By incorporating the new data from the published and available documents, we have modified the GHST map along the Indian margin. Besides filling the data gap, the new map shows the gas-hydrate stability zone in the Andaman offshore. In addition, we show maps of sea-bottom temperature, sediment thickness, geothermal gradient and heat flow to provide a bird’s eye view of these parameters along the continental margin of India.     

Diversity and distribution of methane-oxidizing microbial communities associated with different faunal assemblages in a giant pockmark of the Gabon continental margin

A giant 800-m-diameter pockmark named REGAB was discovered on the Gabon continental margin actively emitting methane at a water depth of 3200 m. The microbial diversity in sediments from four different assemblages of chemosynthetic organisms, Mytilidae, Vesicomyidae, Siboglinidae and a bacterial mat, was investigated using comparative 16S rRNA gene sequence analysis. Aggregates of anaerobic methanotrophic archaea (ANME-2) and bacteria of the Desulfosarcina/Desulfococcus cluster were found in all four chemosynthetic habitats. Fluorescence in situ hybridization targeting the ANME-2/Desulfosarcina/Desulfococcus aggregates showed their presence few centimeters (3–5 cm) below the surface of sediment. 16S rRNA gene sequences from all known marine ANME groups were detected in the pockmark sediments, as well as from both known bacterial partners. The archaeal diversity was limited to the ANME cluster for all investigated samples. The bacterial diversity included members of the Proteobacteria, Bacilliales, Cytophaga/Flavobacteria, Verrucomicrobia, JS1 and Actinobacteria clusters. Bacterial 16S rRNA gene sequences related to those of known sulphide-oxidizing symbionts were recovered from tissues of several invertebrates including vesicomyid clams and siboglinid tubeworms of REGAB.     

Ooid turbidites from the central western continental margin of India

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

The continental margin of the Mesozoic Tethys in the Western Alps

Most of the structural units of the Western Alps were derived from the European Continental margin of the Ligurian Ocean, a segment of the Mesozoic Tethys ocean. Their Mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages: (1) Deposition of the Triassic platform carbonates, essentially prerift but nevertheless bearing the imprint of some extensional movements. (2) The Liassic-Middle Jurassic rifting corresponds to the creation of a horst-and-graben system with especially tilted blocks and hence a shoal-and-basin palaeogeography, but without a preliminary doming event. (3) The late Jurassic-Early Cretaceous opening and spreading of the Ligurian ocean began with a general collapse of the continental margin (‘thermal’ subsidence, latest Middle Jurassic and Earliest Late Jurassic).In this paper, emphasis is given to the refting-derived structures, more especially to the major tilted crustal blocks, a few tens of kilometres wide, which can be either reconstructed or directly observed. The rifting stage lasted roughly 40 m.y., with the alternation of extensional tectonic phases and of relatively ‘quiet’ periods. During the extensional phases, movements along fault-planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence: the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere. The tectonic evolution of the margin continued, but decreased, during the late Jurassic-Early Cretaceous oceanspreading stage. Tectonic activity resumed in the late Cretaceous: this probably resulting from the beginning of contraction of both the ocean and the continental margin, leading progressively to the continental collision in the Tertiary.     

Infaunal benthic communities of the New Zealand continental shelf

Seventeen infaunal benthic communities are recognised from sampling on the New Zealand continental shelf and upper continental slope. These are characterised in terms of geographic distribution, substrate and bathymetric range, and commoner infaunal bivalves and echinoderms.     

New dredge samples from the continental margin bordering Rockall Trough

Rockall Trough may represent the oldest part of the Atlantic Ocean north of latitude 51°N. Stratigraphical evaluation of this hypothesis has been hindered by the lack of dateable samples of the Trough's sedimentary succession. A recent programme of dredging, aimed at sampling the sediment prism along the Trough's upper continental slope, yielded five hauls of in-situ carbonate rocks. Microfaunal analysis of these rocks suggests that the deposition of the marginal sediment prism spans the Late Cretaceous—Recent interval, tending to confirm that Rockall Trough was already in being in Late Cretaceous times.