West Continental Margin Research Articles

Submarine mass movements affecting contourites on the continental slope offshore of the Lofoten Islands, North Norway

The Norwegian continental margin has been shaped largely by glacial processes during the last c. 2.7 Ma, including high inputs of glacigenic sediments particularly beyond the mouths of ice streams (e.g. Dahlgren et al. 2005). The Lofoten Islands are located between 68° N and 69° N, west of the former Fennoscandian Ice Sheet that covered most of Scandinavia during glacial times (Fig. 1). The supply of glacigenic sediments to the continental slope west of the Lofoten Islands was relatively low because the alpine relief of the islands acted as a barrier guiding the large westward-flowing palaeo-ice streams draining the ice sheet to the shelf edge both south and north of the Lofoten Islands (e.g. Laberg et al. 2002; Fig. 1b). Sediment input to the continental margin west of the Lofoten Islands has been dominated by contour currents. This led to the development of the Lofoten Drift (Laberg et al. 1999). The drift sediments were deposited mainly during glacial maxima, and have been affected by several submarine mass movements occurring on the middle to lower slope in water depths of 1100–2500 m (Baeten et al. 2013). These failures have volumes of 0.06–8.7 …

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

Abstract. 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 water depth. Flares from this depth have 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 water depth, 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 water depth 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 δ13C = −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 for a large range of uncertainty due to our method (9 to 118 × 106 mol yr−1). First, these amounts show that gas emissions at the continental margin west of Svalbard were on the same order of magnitude as bubble emissions at other geological settings; second, they may be used to calibrate models predicting hydrate dissolution at present and in the future; and third, they may serve as a baseline (year 2012) estimate of the bubble flux that will potentially increase in the future due to ever-increasing global-warming-induced bottom water warming and hydrate dissociation.

Quaternary contourite drifts of the Western Spitsbergen margin

The study of contourite drifts is an increasingly used tool for understanding the climate history of the oceans. In this paper we analyse two contourite drifts along the continental margin west of Spitsbergen, just south of the Fram Strait where significant water mass exchanges impact the Arctic climate. We detail the internal geometry and the morphologic characteristics of the two drifts on the base of multichannel seismic reflection data, sub-bottom profiles and bathymetry. These mounded features, that we propose to name Isfjorden and Bellsund drifts, are located on the continental slope between 1200 and 1800m depth, whereas the upper slope is characterized by reduced- or non-deposition. The more distinct Isfjorden Drift is about 25km wide and 45km long, and over 200ms TWT thick. We revise the 13 years-long time series of velocity, temperature, and salinity obtained from a mooring array across the Fram Strait. Two distinct current cores are visible in the long-term average. The shallower current core has an average northward velocity of about 20cm/s, while the deeper bottom current core at about 1450m depth has an average northward velocity of about 9cm/s. We consider Norwegian Sea Deep Water episodically ventilated by relatively dense and turbid shelf water from the Barents Sea responsible for the accumulation of the contourites. The onset of the drift growth west of Spitsbergen is inferred to be about 1.3Ma and related to the Early Pleistocene glacial expansion recorded in the area. The lack of mounded contouritic deposits on the continental slope of the Storfjorden is related to consecutive erosion by glacigenic debris flows. The Isfjorden and Bellsund drifts are inferred to contain the record of the regional palaeoceanography and glacial history and may constitute an excellent target of future scientific drilling.

A note on the development of the bathymetry of the continental margin west of the Antarctic Peninsula from 65° to 71°S and 65° to 78°W

Multibeam data collected during the United States Southern Ocean Global Ocean Ecosystems Dynamics cruises in 2001 and 2002 aboard the R/VIB Nathaniel B. Palmer were augmented with available multibeam and centerline bathymetry data from a variety of sources to compile a bathymetric data set for the continental shelf west of the Antarctic Peninsula. This new bathymetry data set provides an improvement in the resolution of the bathymetry of the region. The improved data set was used to construct a high-resolution bathymetric map of the continental margin west of the Antarctic Peninsula from 65° to 71°S and from 65° to 78°W.

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

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.

New data on the age of lamproite-lamprophyre magmatism in the Urals

Several occurrences of lamproites on the eastern slopeof the Urals are objects of our investigation (Fig. 1). Thelamproite occurrence situated in the central and easternpart of the Paleozoic Magnitogorsk island-arc system ata few tens of kilometers northeast from Magnitogorskwas described in [4, 9]. These lamproites make up apetrogenetic series with lamprophyres (monchiquite,camptonite, minette, and kersantite) identified as theKalymbai Complex [9]. The second lamproite occur-rence (Pervomaiskii area) is located in the eastern partof the East Ural Uplift 40 km south of Chelyabinsk nearPervomaiskii Settlement [3, 7]. This area includes anumber of pipe-shaped magnetic anomalies, as well asdiatremes and dikes that crosscut the Devonian rocks. Inaddition, we previously studied the lamproite occurrencenear Skalistyi Settlement 25 km west of the town ofTroitsk in the Transural region [6], where lamproite dikescrosscut adamellites of the Lower Sanarka pluton.Based on Rb–Sr and K–Ar datings, the ages of thelamproites mentioned above vary from 198 to 295 Ma[1–4]. Lamproites of this age are related to the postcol-lision extension of the Earth’s crust [9], tectonomag-matic reactivation of the young platform along latitudi-nal transverse faults [4], or manifestation of an epicon-tinental hot spot [11].Lamprophyres have been investigated in the Urals toa lesser extent than lamproites, although they occur inmany places and vary in age. The best studied and mosteasily accessible lamprophyre related to the alkaline–ultrabasic potassic magmatism is represented by a dikein the Shartash granitic pluton on the outskirts of Yeka-terinburg [5]. This occurrence is situated at the conti-nental margin west of the Murzinka–Adui Block of theEast Ural Uplift. The K–Ar age of the Shartash dike,which is appreciably altered at the contact with granite,ranges from 261 Ma (reaction amphibole at the contact)to 274 Ma (phlogopite) [5].In this communication, we present the first results of

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

The study compiles the controlling factors for organic matter sedimentation patterns from a suite of organogeochemical parameters in surface sediments off Spitsbergen and direct seabed observations using a Remotely Operated Vehicle (ROV). In addition we assess its storage rates as well as the potential of carbon sinks on the northwestern margin of the Barents Sea with short sediment cores from a selected fjord environment (Storfjord). While sedimentation in the fjords is mainly controlled by river/meltwater discharge and coastal erosion by sea ice/glaciers resulting in high supply of terrigenous organic matter, Atlantic water inflow, and thus enhanced marine organic matter supply, characterizes the environment on the outer shelf and slope. Local deviations from this pattern, particularly on the shelf, are due to erosion and out washing of fine‐grained material by bottom currents. Spots dominated by marine productivity close to the island have been found at the outer Isfjord and west off Prins Karls Forland as well as off the Kongsfjord/Krossfjord area and probably reflect local upwelling of nutrient‐rich Atlantic water–derived water masses. Accumulation rates of marine organic carbon as well as reconstructed primary productivities decreased since the middle of the last century. Negative correlation of the Isfjord temperature record with reconstructed productivities in the Storfjord could be explained by a reduced annual duration of the marginal ice zone in the area due to global warming. Extremely high accumulation rates of marine organic carbon between 5.4 and 17.2 g m−2 yr−1 mark the Storfjord area, and probably high‐latitude fjord environments in general, as a sink for carbon dioxide.

Lateral variations in tectono-magmatic style along the Lofoten–Vesterålen volcanic margin off Norway

The ∼400km-long passive continental margin west of the Lofoten–Vesterålen archipelago, off northern Norway, links the volcanic rifted Vøring margin and the sheared W Barents Sea margin. Multi-channel seismic reflection profiles, supplemented with crustal velocity, gravity and magnetic anomaly data are used to outline the regional setting and main tectono-magmatic features. A well-defined along-strike margin segmentation comprising three segments characterized by distinct crustal properties, structural and magmatic styles, sediment thickness, and post-opening history of vertical motion is revealed. The margin segments are governed by changes in fault polarity on Late Jurassic–Early Cretaceous border faults and are separated by coeval cross-margin transfer zones which acted as persistent barriers to rupture propagation and reflect the trend and character of older structural heterogeneities. The transfer zones spatially correlate to small-offset, early opening oceanic fracture zones, implying a structural inheritance from one rift episode to another culminating with lithospheric breakup at the Paleocene–Eocene transition. The pre-seafloor spreading margin structural evolution is governed by the older, predominantly Late Jurassic–Early Cretaceous structural framework. However, the margin also provides evidence for mid- and Late Cretaceous extension events that are poorly understood elsewhere off Norway. Furthermore, the Lofoten–Vesterålen post-breakup subsidence history contrasts with the adjacent margins reflecting breakup in thicker crust and a diminishing volume of high-velocity lower crust emplaced during breakup.

Seabed photography, environmental assessment and evidence for deep-water trawling on the continental margin west of the Hebrides

A photographic survey in 1998 of the seabed along depth transects from 700 to 1300 m across the N.E. Atlantic continental slope off north-west Scotland shows clear depth-related change in sediment type and megabenthic community in an environment where biological communities and species distributions are poorly known. Small-scale features, such as trawl marks and dense fields of xenophyophores, were resolved that may have remained unknown using conventional sampling or lower resolution imaging techniques. Because xenophyophores accumulate barite, a constituent of some drilling muds, their local-scale occurrences will be important to baseline environmental survey prior to hydrocarbon prospecting in deep water. Our results indicate that deep-sea trawling is physically impacting the seabed to depths of more than 1000 m. The persistence and biological consequence of this impact is unknown, but may depend on sediment type and natural physical disturbance. Comparison with similar seabed photographs taken from a neighbouring area in 1988, which show a high incidence of trawl marks, indicates that such impacts have been taking place over at least 10 years.

Geologic processes of accretion in the Cascadiasubduction zone west of Washington State

The continental margin west of Oregon and Washington undergoes a northwardtransition in morphology, from a relatively narrow, steep slope west of Oregon to a broad,midslope terrace off Washington. Multichannel seismic (MCS) reflection data collected over theaccretionary complex show that the morphologic transition is accompanied by significant changein accretionary style: West of Oregon the direction of thrust vergence in the wedge toe flip-flopsbetween landward and seaward, whereas off Washington, thrust faults in the toe vergeconsistently landward, except near the mouth of the Columbia River where detachment folding ofaccreted sediment is evident. Furthermore, rocks under the broad midslope terrace west ofWashington appear to be intruded by diapirs. The combination of detachment folding, diapirs, andlandward- vergent thrust faults all suggest that nearly as far landward as the shelf break, couplingalong the interplate decollement is, or has been, low, as suggested by other lines of evidence. ©1998 Elsevier Science Ltd. All rights reserved.

GLACIAL AND OCEANIC HISTORY OF THE POLAR NORTH ATLANTIC MARGINS: AN OVERVIEW

The five-year PONAM (Polar North Atlantic Margin: Late Cenozoic Evolution) programme was launched by the European Science Foundation in 1989. Its aim was to study the major climate-driven environmental variations in the Norwegian-Greenland (also Nordic) Sea and its continental margins over the last 5 milliion years. The programme has provided substantial new insights into the contrasting behaviour of the ice sheets covering the Svalbard-Barents Sea and East Greenland over the last glacial-interglacial cycle in particular. The highly dynamic Svalbard - Barents Sea Ice Sheet, after reaching the shelf edge during each stadial, almost vanished during subsequent interstadials. By contrast, the East Greenland Ice Sheet showed only minor advances confined to fjord basins or ending on the inner shelf. Although there is a striking correspondence in the timing and duration of the first post-Eemian ice advance in East Greenland and on Svalbard, their chronology and dynamics have been very different since about 65 ka. The Svalbard-Barents Sea Ice Sheet showed well-defined Middle and Late Weichselian ice advances, whereas the East Greenland Ice Sheet was characterised by a 55 kyr-long period with a relatively stable ice margin located in fjords or the inner shelf. The contrasting behaviour of the two ice sheets is probably linked to the palaeoceanographic circulation pattern in the Polar North Atlantic. East Greenland is under the influence of the cold East Greenland Current, whereas the development and behaviour of ice in the Barents Sea is influenced by the continuous, but highly variable. North Atlantic meridional current system that has resulted in a northward inflow of relatively warm waters of Atlantic origin on the eastern side of the Polar North Atlantic. Of particular interest are the so-called “Nordway events” in glacial stages 6 and 4 to 2. These represented periods of pronounced inflow of temperate waters from the south and an associated increase in seasonally open waters, providing moisture for ice-sheet growth. The largest of these events ended in major glaciations, which were reflected in terrestrial glacial sequences and in deep-sea records of ice-rafted debris. Differences in ice extent and dynamics around the Polar North Atlantic are expressed in the evolution and architecture of its east and west continental margins. The Svalbard-Barents Sea Ice Sheet developed much later than the East Greenland Ice Sheet, in the Late Pliocene as compared with the Middle/Late Miocene. The Svalbard-Barents Sea margin is characterised by major prograding fans, built mainly of stacked debris flows. These fans are interpreted as products of rapid sediment delivery from fast-flowing ice streams reaching the shelf break during full glacial conditions. Such major submarine fans are not found north of the Scoresby Sund Fan off East Greenland, where ice seldom reached the shelf break, sedimentation rates were relatively low and sediment transport appears to have been localised in several major deep-sea submarine channel systems. Few debris flows are present and more uniform, acoustically-stratified sediments predominate. In general, the Greenland Ice Sheet has been more stable than those on the European North Atlantic margin, which reflect greater variability in heat and moisture transfer at timescale varying from 100,000 year glacial cycles to millennial-scale fluctuations.

Ferromanganese crust and nodule deposits from the continental margin south and west of Tasmania

West of Tasmania and on the South Tasman Rise, from 41°S to 50°S, rocky outcrops in water depths of 1500–4500 m are commonly coated with ferromanganese crusts up to 20 cm thick. These crusts are unusually thick as compared to those in most parts of the world ocean. Manganese nodules of similar chemical composition are often associated with the crusts. The results from 43 samples analysed here build on previous work on a few samples that showed most crusts and nodule deposits to be of low grade. Both types of deposits had been shown previously to be dominantly ferruginous vernadite and X‐ray amorphous iron oxyhydroxide, and of hydrogenetic origin. The Tasmanian nodules are exceptionally large in places and roughly spherical (up to 12 cm in diameter). Unusually, many irregular nodules are cored by granite or other basement rocks. The nodules, excluding nuclei, average 12.1% Mn, 16.1% Fe, 0.14% Cu, 0.29% Ni and 0.20% Co. The three valuable metals (Cu, Ni, Co) total 0.63% on average. These contents are not commercially interesting as compared to those of the deep‐ocean nodules from the central Indian Ocean and the northeast Pacific Ocean (high Ni and Cu), and the Cook Islands Exclusive Economic Zone (high Co). The crusts average 16.2% Mn, 17.2% Fe, 0.08% Cu, 0.32% Ni and 0.34% Co, the latter three valuable metals together averaging 0.74%, grades that are not of commercial interest. Calculated growth rates apparently vary from about 0.5 mm/106 y above 2000 m water depth to about 20 mm/106 y below 3000 m. Six bulk analyses of crusts taken at three stations within the lower part of the oxygen minimum zone (1500–2000 m water depth) average 0.79% cobalt, contents comparable to those in high‐grade crusts from the Marshall Islands, indicating that these shallow‐water crusts have long‐term economic potential. But the shallow‐water crusts sampled are only 2–4 cm thick (approximately the cut‐off grade for economic deposits). Further seabed sampling in the oxygen‐minimum zone might reveal thicker high‐grade crusts.

Flow processes and sediment deformation in the Canary Debris Flow on the NW African Continental Rise

The Canary Debris Flow formed an extensive deposit on the NW African Continental Margin west of the Canary Islands. Sidescan sonar images and 3.5-kHz profiles show that the middle part of the debris flow deposit consists of complex channel systems separated by banks and ridges of debris. Channels are typically up to 10 km wide and 10 to 20 m deep, and have little or no debris fill. They appear to feed a more laterally continuous debris flow sheet which is seen further downslope. Interchannel banks and ridges are composed of 5 to 20 m thick debris deposits. This morphology is remarkably similar to that seen in subaerial debris flows, and we therefore infer that the observed submarine debris flow morphology is a primary flow fabric, rather than the result of the debris flow interacting with or exploiting pre-existing channels. High-resolution Sidescan sonar images show that the debris flow surface is covered by sediment blocks up to 300 m in diameter. A single core collected from the flow shows that most of the 4.6-m flow thickness at the core site is composed of a single clast. The clast has been folded, with its upper part consisting of an inverted minor image of the lower part. The same sequence occurs again, in situ, beneath the debris flow, suggesting that the clast may have a local source, rather than having been derived from the debris flow source area, some 200 km upslope. This indicates that the debris flow was capable of substantial seabed erosion in the middle part of its course. In these middle reaches, erosion within the channelled areas probably occurred simultaneously with deposition in the interchannel areas. Interchannel deposits may contain both locally derived and original source area material.

Bottom reflection coefficient measurement and geoacoustic inversion at the continental margin near Vancouver Island with the aid of spiking filters

This paper presents the results of an experiment to measure the reflection coefficient versus grazing angle at the continental margin west of Vancouver Island. The measured reflection coefficients are inverted and estimates of the surficial sediment geoacoustic properties obtained. The receiver used in this experiment was a long towed array with the source in aft-endfire aspect. The acoustic sources for this experiment were explosives that produced direct-path arrivals approximately 0.1 s in length. Bottom and sub-bottom arrivals continued for several seconds duration. Explosives were detonated at intervals such that bottom reflected energy received at the array sampled grazing angles in the interval 10°–80°. Source–receiver ranges were computed by measuring arrival time differences at each of the array elements, comparing these time differences with those computed using a ray-tracing propagation model, and globally minimizing the accumulated squared time difference residuals. To aid in the measurement of arrival times, spiking filters were employed to produce a deconvolution-like output that separates arrivals overlapped in the original time series. The reflection coefficient estimates were determined from the linear spiking filter outputs by forming the ratio of the direct-path and bottom-reflected arrival amplitudes. This technique is efficient and provides estimates of the surficial sediment reflection coefficient by excluding energy returned from sub-bottom layers.

The crustal structure of the Rockall Trough: Differential stretching without underplating

The crustal structure along the axis of the Rockall Trough, in the North Atlantic, has been studied along a 600‐km refraction/wide‐angle reflection transect, containing three lines each 200 to 250 km long, using explosives and ocean bottom seismometers. One‐dimensional inversions of each section were made using the τ – p method and forward modeling of the observed travel times. In the next stage, travel times and amplitudes were modeled using ray tracing techniques through two‐dimensional heterogeneous structures. The results indicate that there are three sedimentary layers with velocities ranging from 2 km/s to 4.5 km/s. The whole sedimentary section is up to 6 km thick and interpreted as late Paleozoic to Tertiary in age. A two‐layer continental crust, 5 to 7 km thick, occurs along the length of the profile. The upper crust (6.0–6.3 km/s), is circa 2 km thick and the lower crust (6.6–6.9 km/s), is circa 3 km thick. A Moho transition zone, approximately 1 km thick, lies at the base of the crust. Velocities in this transition zone increase from 6.9 km/s up to 7.8 km/s along the profile. The underlying upper mantle has a laterally variable velocity between 7.6 and 7.8 km/s. Unstretched crust onshore in Ireland comprises a three‐layered crust, with each layer approximately 10 km thick, and a Moho transition zone, which is about 3 km thick. The two upper layers in the onshore region are interpreted as corresponding to the upper crust in the Rockall Trough and indicate a stretching factor (β) of 8–10. The velocity pattern in the lower crust in the Rockall Trough and under Ireland are similar, suggesting significantly less stretching (β = 2 ‐ 3). The differential stretching model is supported by the presence of the Moho transition zone which is stretched by a similar factor to the lower crust. The bulk stretching factor for the crust as a whole is in the range of 4–6. If this represents the lithospheric stretching factor, significant underplating would be expected. However, if the stretching factor for the lower crust in the differential stretching model is more representative of overall lithospheric stretching, little or no underplating is predicted. The velocity patterns observed in the Rockall Trough indicate the absence of any significant underplating at the base of the crust, such as that observed at the continental margin west of the Hatton Basin.

Integrated crustal structure across the south central California Margin: Santa Lucia escarpment to the San Andreas Fault

The continental margin of south central California has undergone different styles of deformation over the past 30 m.y. as the North America‐Pacific plate boundary has evolved from subduction to transform tectonics. Results presented here identify strike‐slip and compressional structures in the lower crust that probably developed during this period of tectonic transition. An integrated model of crustal structure along deep seismic line PG&E‐3 crosses the entire continental margin west of the San Andreas fault near the latitude of San Luis Obispo. A compressional wave velocity model was developed using coincident marine multi‐channel seismic and onshore/offshore wide‐angle reflection/refraction seismic data and was tested independently with gravity modeling. A high‐velocity layer (6.8–7.0 km/s, about 6 km thick) in the lower crust that extends east possibly as far as the San Andreas fault is interpreted as tectonically underplated oceanic crust. The upper and middle crust of the Patton and Sur‐Obispo Franciscan terranes extends to depths of 14 to 20 km. The velocity and density modeling indicate lower average velocities and densities for the Patton terrane, supporting earlier interpretations that it is a distinct terrane within the Franciscan complex. Abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the Santa Lucia Escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 Ma following the subduction of the Pacific‐Arguello ridge. An imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust. Convergence between the Monterey and Arguello microplates during the final stages of Farallon subduction (about 24 to 16 Ma) probably caused the imbrication.

Integrated geophysical modelling of terranes and other structural features along the western Canadian margin

An integrated geophysical data set has been used to develop structural models across the continental margin west of Vancouver Island, Canada. A modern accretionary complex underlies the continental slope and shelf and rests against and below the allochthonous Crescent and Pacific Rim terranes. These terranes in turn abut against the pre-Tertiary Wrangellia terrane that constitutes most of the island. Gravity and magnetic anomaly data, constrained by seismic reflection, seismic refraction, and other data, were interpreted to determine the offshore positions of these terranes and related features. Iterative 2.5-dimensional forward models of anomaly profiles were stepped laterally along the margin to extend areal coverage over a 70 km wide swath oriented normal to the tectonic features. An average model was then developed to represent this part of the margin. The Pacific Rim terrane appears to be continuous and close to the coastline along the length of Vancouver Island, consistent with emplacement by strike-slip motion along the margin. The Westcoast fault, the boundary between the Pacific Rim and Wrangellia terranes, is interpreted to be 15 km farther seaward than in previous interpretations in the region of Barkley Sound. The Crescent terrane forms a thin landward-dipping slab along the southern half of the Vancouver Island margin, and cannot be confirmed along the northern part. Model results suggest the slab has buckled into an anticline beneath southern Vancouver Island and Juan de Fuca Strait, uplifting high-density lower crustal or upper mantle material close to the surface to produce the observed intense positive gravity anomaly. This geometry is consistent with emplacement of the Crescent terrane by oblique subduction.

Quaternary Sedimentary Processes on the Northwestern African Continental Margin--An Integrated Study Using Side-Scan Sonar, High-Resolution Profiling, and Core Data

Side-scan sonar data, cores, and high-resolution profiles have been used to produce an integrated model of sedimentation for the continental margin west of the Canary Islands. Long-range side-scan sonar (GLORIA) data and a grid of 3.5-kHz profiles, covering some 200,000 km{sup 2} allow a regional appraisal of sedimentation. More detailed studies of selected areas have been undertaken using a new 30 kHz deep-towed side-scan sonar (TOBI) developed by the U.K. Institute of Oceanographic Sciences. Sediment cores have been used both to calibrate acoustic facies identified on sonographs and for detailed stratigraphic studies. The most recent significant sedimentation event in the area is to Saharan Sediment Slide, which carried material from the upper continental slope off West Africa to the edge of the Madeira Abyssal Plain, a distance of some 1000 km. The authors data shows the downslope evolution of the debris flow. Near the Canaries, it is a 20-m-thick deposit rafting coherent blocks of more than 1 km diameter; side-scan records show a strong flow-parallel fabric on a scale of tens of meters. On the lower slope, the debris flow thins to a few meters, the flow fabric disappears, and the rafted blocks decrease to meters in diameter. Side-scan datamore » from the lower slope show that the Saharan Slide buries an older landscape of turbidity current channels, typically 1 km wide and 50 m deep. Evidence from the Madeiran Abyssal Plain indicates a history of large but infrequent turbidity currents, the emplacement of which is related to the effects of sea level changes on the northwest African margin.« less

Geology of Porcupine Bank and Goban Spur, Northeastern Atlantic — Preliminary results of the Cyaporc submersible cruise

During the Cyaporc cruise, five submersible traverses and four dredge hauls were made up the steep western slopes of Porcupine Bank and Goban Spur on the continental margin west of Ireland. The major aims of the study were to examine the syn- and post-rift evolution of the margin during the Mesozoic and Cenozoic, and to compare the “basement” rocks of Porcupine Bank and Goban Spur with those of the Hercynian and Caledonian orogens of the western British Isles. On Porcupine Bank, sampling and observational data show a metamorphic basement overlain by a sequence of Upper Palaeozoic siltstones and sandstones. Mesozoic sediments are restricted to isolated patches of Lower Cretaceous shallow-marine deposits. On Goban Spur, the oldest rocks are weakly metamorphosed sandstones of Late Palaeozoic age (Hercynian basement). In contrast to Porcupine Bank, these are overlain by a thick sequence of shallow-water Barremian limestones deposited during the late Early Cretaceous rifting event. Upper Cretaceous and Tertiary sediments have only a patchy development in both areas. Comparison with onshore areas suggest that the Porcupine Bank Palaeozoic sequence is comparable with that of the Hercynian foreland seen in Ireland. The Palaeozoic sequence at Goban Spur, with granites intruded along its southern flank, is less easily compared with onshore sequences, but most closely resembles that seen in onshore southwest England. The development of a thick syn-rift sequence on Goban Spur resulted from faulting and subsidence related to crustal thinning during the Early Cretaceous rifting event. In contrast, seismic refraction results show little crustal thinning beneath Porcupine Bank, offering an explanation for the absence of Mesozoic subsidence and the lack of preserved Mesozoic sediments.

Structure and Development of Porcupine Seabight Sedimentary Basin, Offshore Southwest Ireland

The Porcupine basin is a deep sedimentary basin on the continental margin west of Ireland. All available geologic and geophysical data have been used to interpret the history of the basin. It contains up to 8 km of Cretaceous and younger sediments, underlain by an unknown thickness of Jurassic and Triassic rock. Gravity and magnetic anomaly models have been used to investigate the nature of the crust beneath the Porcupine basin. These models suggest a thinned continental crust rather than the oceanic crust previously proposed by some authors, although the nature of a narrow zone of crust (< 10 km thick) in the central southern basin cannot be determined conclusively. A high-amplitude elongate gravity high associated with a negative magnetic anomaly in the axis of the n rthern Porcupine Seabight correlates with a volcanic ridge, probably of Early Cretaceous age, seen on seismic reflection profiles. Regional geologic and geophysical studies show that the Porcupine basin was originally structured in the early Mesozoic as part of a complex northeast to north-northeast-trending intracontinental rift system extending from the Grand Banks of Newfoundland to the present northwest Europe continental shelf. The basin can be divided into a sequence of segments bounded by north-northeast-trending normal faults and northwest-trending strike-slip faults. The degree of extension within the basin decreases northward across each strike-slip fault, accounting for the overall northward narrowing of the basin. The early Mesozoic rift fabric of the Porcupine basin was reactivated during the Early Cretaceous when the basin became caught up in the northwest-trending rift that later evolved into the N rth Atlantic. This second phase of extension led to subsidence and the accumulation of extreme thicknesses of Cretaceous and younger sediments.