Sedimentary Prism Research Articles

Kaikoura Canyon, New Zealand: active conduit from near-shore sediment zones to trench-axis channel

Kaikoura Canyon is one of a few major, active conduits between a near-shore sediment transport system and a deep-ocean channel. It is the sink for mobile zones of gravel, sand and mud that migrate northwards off northeastern South Island, New Zealand. It is presently the primary source for the 1500 km long Hikurangi Channel, which supplies overbank turbidites to a filled trench, an ocean-plateau basin and a distal fan-drift. Swath data, seismic profiles, side-scan sonographs, aerial photographs, cores, grab samples and current meter data are used to define the shape and texture of the canyon and adjacent shelf in order to better understand how coastal processes and canyon interact to supply sediment to the deep-ocean. Kaikoura Canyon is 60 km long, up to 1200 m deep and generally U-shaped in profile. Its head is within 500 m of the shore, and within 200 m of rocky projections from the shore-platform, in a mountain-backed bay without large rivers. The canyon head incises the 18 m depth contour and boulders, pebble gravel and megarippled coarse sand reach the canyon rim. Fine sand migrating northward along the shelf under the influence of waves and currents is trapped in a southward-projecting, canyon-head gully, which incises the thickest part of the Holocene sediment prism. It is estimated that about 1.5×10 6 m 3 of sediment falls into the canyon head each year. Tensional fractures around the canyon rim suggest that sediments in the canyon-head gully are unstable. Gravel turbidites, with post last glacial age shells, are at or near the seabed in the lower canyon but are blanketed by many thin, silt and sand, possibly storm-generated, turbidites in the upper canyon. The top gravel contains a twig that is about 170 years old, suggesting that the last major collapse in the canyon head coincides with many onshore rockfalls triggered by rupture of a major, strike-slip, plate-boundary fault in about 1833. An underlying gravel is about 300 years old and may again coincide with fault rupture. Most of the large, earthquake-triggered, failures may “ignite” to form self-perpetuating, autosuspension flows, that feed a 1500 km long, deep-sea, turbidite channel.

Large-scale mass wasting on the north Spitsbergen continental margin, Arctic Ocean

Closely spaced, single-beam bathymetric and side-scan sonar investigations on the northern slope of the western Svalbard insular platform have revealed the presence of a Late Quaternary slump complex forming a hanging-wall slump canyon near the head of the Malene Bukta (Malene Bay) bathymetric embayment in the northern continental margin. Repeated slump erosion may be responsible for development of this young feature and the Malene Bukta Embayment. Focusing of the slumping may be due to the trapping of gas at shallow sea-floor depths by gas hydrate, with the consequent formation of subjacent gas-rich, low shear-strength decollement zones. Faults have likely controlled the upward migration of gas into the younger sedimentary prism.

Three‐dimensional velocity structure of Siletzia and other accreted terranes in the Cascadia forearc of Washington

Eocene mafic crust with high seismic velocities underlies much of the Oregon and Washington forearc and acts as a backstop for accretion of marine sedimentary rocks from the obliquely subducting Juan de Fuca slab. Arc‐parallel migration of relatively strong blocks of this terrane, known as Siletzia, focuses upper crustal deformation along block boundaries, which are potential sources of earthquakes. In a three‐dimensional velocity model of coastal Washington, we have combined surface geology, well data, and travel times from earthquakes and controlled source seismic experiments to resolve the major boundaries of the Siletz terrane with the adjacent accreted sedimentary prism and volcanic arc. In southern Washington and northern Oregon the Siletz terrane appears to be a thick block (∼20 km) that extends west of the coastline and makes a high‐angle contact with the offshore accreted sedimentary prism. On its east flank the high‐velocity Siletz terrane boundary coincides with an en echelon zone of seismicity in the arc. In northern Washington the western edge of Siletzia makes a lower‐angled, fault‐bound contact with the accretionary prism. In addition, alternating, east‐west trending uplifts and downwarps of the Siletz terrane centered on the antiformal Olympic Mountains may reflect focusing of north‐south compression in the northern part of the Siletz terrane. This compressional strain may result from northward transport and clockwise rotation of the Siletz terrane into the relatively fixed Canadian Coast Mountains restraining bend along the coast.

Geology of the Argentine continental shelf and margin from aeromagnetic survey

New aeromagnetics show a complex of magnetic patterns and source depths of the north and central Argentine continental and adjacent oceanic crust. The 40 km wide, positive magnetic G-anomaly reflects an end-rift associated volcanic edifice at the continental margin. M4-MO and Cretaceous quiet zone oceanic crust are identified. The Colorado magnetic discontinuity forms the southern boundary of the Precambrian La Plata craton. To the south the continental crust of the Patagonia Platform is composed of amalgamated late Precambrian through end-Paleozoic weakly magnetic sedimentary prisms. The SW Patagonia Platform crust shows a strong high-frequency magnetic pattern associated with Mesozoic volcanics, while zones of mid-crustal level intrusives and some volcanics on the outer shelf are expressed as a broad, higher amplitude magnetic pattern.

Shallow structure of the eastern segment of the Mediterranean Ridge deduced from seismic and side-scan sonar data

The eastern Mediterranean Ridge reveals a peculiar feature called the “United Nations Rise”. It is notable for its complex morphology, interior structure, and mud volcanism. Its unusual structural–morphological characteristics are explained by its location at a junction of the western and eastern branches of the ridge and by the probable tectonic escape of accretionary prism sediments from the west. The geophysical data on the shallow structure of the eastern ridge branch showed some unusual structural trends, which could not be expected from the overall tectonic stress distribution. They are interpreted as resulting from the southward expansion of the Hellenic Arc.

Mechanical controls on collision-related compressional intraplate deformation

Intraplate compressional features, such as inverted extensional basins, upthrust basement blocks and whole lithospheric folds, play an important role in the structural framework of many cratons. Although compressional intraplate deformation can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the development of the most important compressional intraplate structures. These can occur at distances of up to ±1600 km from a collision front, both in the fore-arc (foreland) and back-arc (hinterland) positions with respect to the subduction system controlling the evolution of the corresponding orogen. Back-arc compression associated with island arcs and Andean-type orogens occurs during periods of increased convergence rates between the subducting and overriding plates. For the build-up of intraplate compressional stresses in fore-arc and foreland domains, four collision-related scenarios are envisaged: (1) during the initiation of a subduction zone along a passive margin or within an oceanic basin; (2) during subduction impediment caused by the arrival of more buoyant crust, such as an oceanic plateau or a microcontinent at a subduction zone; (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism, and convergence rates and directions; (4) during post-collisional over-thickening and uplift of an orogenic wedge. The build-up of collision-related compressional intraplate stresses is indicative for mechanical coupling between an orogenic wedge and its fore- and/or hinterland. Crustal-scale intraplate deformation reflects mechanical coupling at crustal levels whereas lithosphere-scale deformation indicates mechanical coupling at the level of the mantle-lithosphere, probably in response to collisional lithospheric over-thickening of the orogen, slab detachment and the development of a mantle back-stop. The intensity of collisional coupling between an orogen and its fore- and hinterland is temporally and spatially variable. This can be a function of oblique collision. However, the build-up of high pore fluid pressures in subducted sediments may also account for mechanical decoupling of an orogen and its fore- and/or hinterland. Processes governing mechanical coupling/decoupling of orogens and fore- and hinterlands are still poorly understood and require further research. Localization of collision-related compressional intraplate deformations is controlled by spatial and temporal strength variations of the lithosphere in which the thermal regime, the crustal thickness, the pattern of pre-existing crustal and mantle discontinuities, as well as sedimentary loads and their thermal blanketing effect play an important role. The stratigraphic record of collision-related intraplate compressional deformation can contribute to dating of orogenic activity affecting the respective plate margin.

Cenozoic tectonics of Amazon Mouth Basin

The Cenozoic shelf margin of the Amazon Mouth Basin is characterized by a thick prograding prism of siliciclastic sediments. This prism, composed mainly of Upper Miocene and younger sediments, overlies a Lower Tertiary carbonate shelf. Two tectonic–sedimentary models for the area were developed with the aid of new deep-reflection seismic data. Gravitational tectonics dominate the regional geological framework. Tensional stresses are created near the shelf margin, and compressional features dominate at the base of the slope. The morphology of this compressional zone is closely associated with the St. Paul Fracture Zone and the boundary between continental and oceanic crusts.

Collisional intraplate deformation

Intraplate compressional structures, such as inverted extensional basins, upthrusted basement blocks and whole lithospheric folds, play an important role in the tectonic framework of many cratons. Although compressional intraplate structures can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the most important compressional intraplate deformations. Such structures can occur at distances of up to 1600 km from a collision front, both in back-arc (hinterland) and fore-arc (foreland) positions with respect to the subduction system controlling the respective orogen. The stratigraphic record of collision-related intraplate compressional deformation can contribute to the dating of orogenic activity affecting the respective plate margin. Four collision-related scenarios are envisaged for the build-up of intraplate compressional stresses: (1) during the initiation of subduction zones along passive margins or within oceanic basins, (2) during impedance of subduction processes caused by the arrival of an oceanic plateau, transform ridge or microcontinent at a subduction zone, (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism and convergence rates and direction, (4) during post-collisional over-thickening of an orogenic wedge. Crustal-scale foreland deformation reflects mechanical coupling between the orogenic wedge and its foreland at crustal levels; build-up of high fluid pressures in subducted sediments may account for their decoupling. Lithosphere-scale foreland deformation reflects mechanical coupling between the orogenic wedge and its forelands at the level of the mantle-lithosphere, possibly related to collisional lithospheric over-thickening and the development of mantle back-stops. Processes governing mechanical coupling/decoupling of an orogen and its forelands are still poorly understood.

Serial reverse and strike slip on imbricate faults: The Coastal Range of east Taiwan

Holocene marine chronologies for three segments of the Coastal Range of east Taiwan indicate that there has been broadly uniform uplift of about 3–5 mm/yr during the past 8000 yr. Previous reports have cited local Holocene uplift rates as high as 14 mm/yr, and geodetic measurements indicate uplift of as much as 20 mm/yr. Furthermore, the seismic record reflects contraction of 26–54 mm/yr, whereas the Luzon volcanic arc is here being subducted beneath Eurasia at an average rate of 68 mm/yr. We attribute the discrepancies in both uplift and contractional measurements to distributed deformation between upthrust imbricate slices of an accreted sediment prism, the vertical component being taken up by serial reverse slip on the thrusts that bound the slices and the horizontal component being taken up by serial strike slip along the thrusts.

The seismogenic zone of subduction thrust faults

Abstract Subduction thrust faults generate earthquakes over a limited depth range. They are aseismic in their seaward updip portions and landward downdip of a critical point. The seaward shallow aseismic zone, commonly beneath accreted sediments, may be a consequence of unconsolidated sediments, especially stable‐sliding smectite clays. Such clays are dehydrated and the fault may become seismogenic where the temperature reaches 100‐‐150°C, that is, at a 5‐‐15 km depth. Two factors may determine the downdip seismogenic limit. For subduction of young hot oceanic lithosphere beneath large accretionary sedimentary prisms and beneath continental crust, the transition to aseismic stable sliding is temperature controlled. The maximum temperature for seismic behavior in crustal rocks is ∼ 350°C, regardless of the presence of water. In addition, great earthquake ruptures initiated at less than this temperature may propagate with decreasing slip to where the temperature is ∼ 450°C. For subduction beneath thin island arc crust and beneath continental crust in some areas, the forearc mantle is reached by the thrust shallower than the 350°C temperature. The forearc upper mantle probably is aseismic because of stable‐sliding serpentinite hydrated by water from the underthrusting oceanic crust and sediments. For many subduction zones the downdip seismogenic width defined by these limits is much less than previously assumed. Within the narrowly defined seismic zone, most of the convergence may occur in earthquakes. Numerical thermal models have been employed to estimate temperatures on the subduction thrust planes of four continental subduction zones. For Cascadia and Southwest Japan where very young and hot plates are subducting, the downdip seismogenic limit on the subduction thrust is thermally controlled and is shallow. For Alaska and most of Chile, the forearc mantle is reached before the critical temperature, and mantle serpentinite provides the limit. In all four regions, the seismogenic zones so defined agree with estimates of the extent of great earthquake rupture, and with the downdip extent of the interseismic locked zone.

The modern foreland basin system adjacent to the Central Andes

Regional variations in sediment thickness, internal structures, average elevation, and Bouguer gravity define a four-component foreland basin system adjacent to the Central Andes. In the most proximal part of the foreland basin system, the eastern Subandean zone and westernmost Chaco Plain, 1–3 km of Cenozoic deposits overlies active folds and thrusts of the frontal Andean orogenic wedge. These wedge-top deposits pass cratonward into a foredeep depozone containing a 3–4-km-thick sedimentary prism that tapers toward (and locally pinches out against) a broad-wavelength forebulge in the central-eastern Chaco Plain. The forebulge is underlain by Precambrian–Mesozoic rocks and is largely covered by a thin veneer of Quaternary alluvium. East of the forebulge, a thin (0.5 km) saucer-shaped accumulation of sediment beneath the Pantanal Wetland represents a back-bulge depozone. Ancient counterparts of these four depozones can be identified in the Central Andes, suggesting that modern basin architecture is the result of continuous, eastward migration of the coupled orogenic wedge and foreland basin system since the Late Cretaceous–Paleocene.

Tectonics of the southeastern Australian Lachlan Fold Belt: structural and thermal aspects

Abstract The Lachlan Fold Belt of southeastern Australia is a 700 km wide belt of deformed, Palaeozoic deep and shallow marine sedimentary rocks, cherts, and mafic volcanic rocks. Characterized by large areas of chevron-folded turbidite sequences, linked contractional and strike-slip faults, and superposed thrust-belts of different age and vergence, it has large volumes of granite and low pressure/high temperature metamorphic rocks. Surface structural elements suggest that it formed by massive telescoping and strike-slip translation within a continental margin sediment prism along the former margin of Gondwanaland during the mid-Palaeozoic. Migrating, sporadic compressional and extensional events over the 100 Ma deformational history of the belt produced a crustally thickened, high geothermal gradient orogen, involving non-craton directed thrusting in an inferred convergent margin setting. Compressional periods were marked by eastward-migrating deformation zones involving detachment-related folding and reverse faulting in the largely turbidite dominated sequences. High-grade metamorphic rocks are not part of a metamorphic hinterland, but are largely confined to a shear zone-bounded crustal wedge in the central part of the fold-belt. Extension was marked by localized development of rift basins and half grabens accompanied by extensive granitic magmatism and silicic volcanism. Subsequent basin collapse was by reactivation/inversion of the extensional faults and folding of the basinal sequence. Magmatic activity and high temperature/low pressure metamorphism are linked to a convergent margin setting over much of the Middle Palaeozoic, but with major convergence in the Silurian.

Electrokinetic signature of the Nankai Trough accretionary complex: preliminary modelling for the Kaiko-Tokai program

Fluid flow in the Nankai Trough accretionary complex has been observed during the previous phases of the Kaiko project to be of order 100 m year −1 in small-scale high-permeability conduits, and 10 m year −1 where shallow detachment faults intersect the seafloor. The rate of flow was also variable, increasing up to 30% over a month. Monitoring the fluid flow will be one of the principal objectives of the next phase of the Kaiko program, known as Kaiko-Tokai. One method is to measure the electric and magnetic fields generated by the electrokinetic phenomena (or streaming potential), resulting from fluid flow in the porous sediments. This paper describes modelling of fluid flow in the accretionary prism and the resulting electrokinetic signals which may be observed from sensors on the seafloor or in shallow boreholes. A two-dimensional finite-difference model is used to study the hydrogeology of a realistic section of the Nankai Trough, with constraints from seismic reflection data and Ocean Drilling Program boreholes. Fluid sources owing to porosity reduction in the prism and of distant external origin are modelled. We find that porosity reduction alone is insufficient to reproduced the observational estimates of excess pore pressure and fluid flux. Additional fluid source terms are therefore required along a low-angle décollement, or detachment fault, which separates the deformed prism sediments from the undeformed underthrust sediments beneath. The décollement must be significantly more permeable than the adjacent sediments (our models suggest by approximately four orders of magnitude), and is probably anisotropic parallel to the main structural trends. From the hydrogeological model, the electric potential and electric and magnetic fields within the ocean and sediments were calculated from the electrokinetic phenomena. Seafloor electric potentials are found to be of the order of a few millivolts, with seafloor horizontal and vertical electric fields of the order of 0.5 μVm −1 close to the deformation front. As the electric potential rises significantly with depth through the sediments, boreole measurements may be particularly useful. We conclude with some comments about noise sources and experimental logistics for the Kaiko-Tokai program.

Transtensional tectonics along the south Scotia Ridge, Antarctica

Multichannel seismic reflection profiles recorded aboard B/O Hespérides during the austral summer of 1991–1992 were used to identify the tectonic style of the South Scotia Ridge along the Scotia/Antarctica plate boundary. The ridge is composed of continental crustal fragments transported eastward from the South America-Antarctic isthmus 28 to 6 Ma during the opening of Drake Passage. It is made up of two highs (north and south branches of the South Scotia Ridge) separated by a central depression that contains four narrow deeps. Fragmentation of the ridge during and since its transport to its present position is due to transtensional sinistral motion along the Scotia-Antarctic plate boundary. This fragmentation of the ridge appears to have been in two phases. During an early phase of transtension, which probably took place in the Oligocene, a half graben fronted by a high along its northern edge was formed along the southern flank of the ridge. Concurrent with this transtension episode an extensive sediment prism was deposited north of the ridge. The second tectonic episode during which the present plate boundary was established in its current location along the central depression may have begun about 4 Ma. Transtensional tectonics along the sinistral transform fault plate boundary during this phase led to the creation of the present tectonic geomorphology of the South Scotia Ridge. Extension during this phase is characterized by listric faults dipping both north and south which root into a northerly dipping basal detachment surface. Motion along these faults caused the block above the detachment surface (upper block—north branch of the South Scotia Ridge) to undergo some degree of tilting. Differences in morphology along the north branch suggest that this block tilting varies along strike being the least on its eastern and western ends and maximum in the center. This suggests that continuity of the listric faults parallel to the plate boundary is disrupted by transverse structures, structures which may have been produced by bends along the plate boundary. As the blocks were transported laterally along the transtensional sinistral plate boundary they experienced some degree of counterclockwise rotation along the right lateral transverse structures creating local zones of compression and extension at their corners. It is this compression created by the rotating blocks which led to the deformation of the sediments north of the eastern end of the north branch of the South Scotia Ridge and within the central depression itself.

Rock-magnetic signature of gas hydrates in accretionary prism sediments

Sediments from two Ocean Drilling Program Leg 146 sites from the Cascadia margin of western North America have magnetic properties indicating diagenesis of magnetic minerals associated with the presence of gas hydrates. Two indices combining coercivity, remanence, and susceptibility parameters, D JH ( = {J rs/J s}{H cr/H c”) and D S ( = {J rs/k}H cr), when combined with thermo-magnetic data, can be diagnostic of these changes. At Site 892, D S values are distinctly higher and more scattered above the bottom simulating seismic reflector (BSR), which marks the base of the hydrate stability zone. Within the hydrate stability zone at Site 892, D JH shows two trends: an increase from about 50 mbsf to the BSR at 73 mbsf, corresponding to an expected increase in hydrate concentration near the BSR; and a second increase upwards from 50 mbsf to peak values at less than 21 mbsf, associated with hydrate recovered in cores above 19 mbsf. At Site 889/890 D JH increases downhole to about 285 mbsf, substantially below the BSR at 225 mbsf. High D JH sediments within a low Cl − zone at this site have magnetic mineralogies which are dominated by fine-grained magnetic sulfides, whereas sediments from above and below this zone are characterised by magnetite-magnetic sulfide mixes. This trend at Site 889/890 is consistent with an interpretation based on pore-water geochemistry (low Cl −) and bottom-water temperature that a ‘fossil gas hydrate zone’ extended downwards to about 295 mbsf during the last glacial. The observed changes in magnetic properties can be attributed to steps in the reduction series from magnetite through SD greigite to pyrite (or to overgrowth of SD greigite to MD size). Diagenetic growth of magnetic iron sulfides (greigite and/or pyrrhotite) has been reported in other accretionary wedge sediments. Thermal demagnetization of multi-component isothermal remanent magnetization (mIRM) indicates the presence of a low-coercivity magnetic mineral with an unblocking temperature (T ub) between 310° and 350°C. High J rs/k ratios suggest that the low-coercivity, low unblocking temperature mineral is predominantly greigite rather than pyrrhotite. A low- to medium-coercivity mineral with T ub ca. 580°C — magnetite — is also present in varying amounts. Hydrate apparently controls the presence of greigite by incorporating H 2S, shown to be present as a hydrate phase together with methane in hydrate recovered at Site 892. Release of H 2S below the base of the hydrate layer allows overgrowth of greigite grains to MD size or the conversion of some of the greigite to pyrite.

Palaeozoic basins of southern South Australia: New insights into their structural history from regional seismic data

Interpretation of three recently recorded offshore seismic lines provides a regional picture of the geology from the Gawler Craton across the Stansbury and Troubridge Basins to the Otway Basin in South Australia. The 300 km transect crosses most of the Cambrian Stansbury Basin, which consists of a marginal platform in the west and the Kanmantoo Trough in the east. The Kanmantoo Trough is filled by an eastward deepening sedimentary prism formed by the Kanmantoo Group. The little‐deformed platformal Cambrian sedimentary rocks onlap the Gawler Craton and underlie the Gulf St Vincent. The eastern margin of the platform is separated from the western Kanmantoo Trough by a northeast‐trending zone of intense deformation. This transition zone comprises numerous southeast‐dipping faults which are correlated to faults and shear zones with reverse displacement that are mapped in outcrop on Kangaroo Island and Fleurieu Peninsula. These faults constitute contractionally reactivated former extensional faults, which cont...

Strain decoupling across the decollement of the Barbados accretionary prism

The interrelation between deformation styles and behavior of fluids in accretionary prisms is under debate, particularly the possibility that overpressuring within the basal decollement may enable mechanical decoupling of the prism from the subducting material. Anisotropy of magnetic susceptibility (AMS) data from sediments spanning the basal decollement of the Barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is markedly overpressured. In the accretionary prism, above the decollement, the minimum AMS axes are subhorizontal and oriented nearly east-west, whereas the maximum AMS axes are oriented nearly north-south and shallowly inclined. At the top of the decollement, the minimum AMS axes orientations abruptly change to nearly vertical; this orientation is maintained throughout the decollement and in the underthrust sediments below. The AMS orientations in the prism sediments above the decollement are consistent with lateral shortening due to regional tectonic stress, as the minimum axes generally parallel the convergence vector of the subducting South American plate and the maximum axes are trench-parallel. Because the orientations of the AMS axes in deformed sediments usually parallel the orientations of the principal strains, the AMS results indicate that the incremental strain state in the Barbados prism is one dominated by subhorizontal shortening. In contrast, the AMS axes within and below the decollement are consistent with a strain state dominated by vertical shortening (compaction). This abrupt change in AMS orientations at the top of the decollement at Site 948 is a direct manifestation of mechanical decoupling of the off-scraped prism sediments from the underthrust sediments. The decoupling horizon occurs at the top of the decollement zone, coinciding with the location of flowing, high-pressure fluids.

Evidence for short term retreat of the barrier shorelines

In many places on the present shoreface, offshore bar or beach sands with a sharp-basal contact overlie thin-bedded fine sands and muds. Ancient analogues are commonly interpreted as lowstand shoreface deposits lying on an erosional surface cut into the shelf during a relative sealevel fall. New observations may challenge this interpretation by providing evidence that such sequences develop during a recent shoreline retreat under highstand conditions without any significant change in sea level. Two examples of such a superimposition are provided by wave-dominated clastic shorefaces in the microtidal environment from the Senegal coast (Saloum delta, Sangomar spit) and in a non-tidal environment on the French Mediterranean coast in the Gulf of Lions. In the Saloum, a littoral cross-section across the Sangomar beach barrier shows a dune sand deposit overlying a man-made shell accumulation, dated 600 BP, and a layer of green muds formed in a lagoon at the back of an ancient beach barrier whose remnants are dated 3150 BP. The retreat of this ancient beach barrier is demonstrated by mapping the former offshore extension of a littoral sand unit which is defined clearly in the region by a grain-size signature. Other observations in the region of the Saloum delta give evidence of the widespread occurrence of coastal recession along the whole Sangomar sedimentary spit. In the Gulf of Lions, vibrocores were used to sample the beach and the shoreface of the Thau lagoon lido. The geological record of recent and present sedimentation exhibits varied littoral sands overlying ancient deposits of fine muddy sand and, at the very bottom of the sections, shoreface sands including fragments of beachrock. Radiocarbon datings give ages of 2050–6700 BP for the underlying muddy fine sand which has been interpreted as a lagoonal deposit enriched in organic matter, typical of lagoon environments in the region. An evolution similar to that proposed for the Sangomar region is supported by additional evidences of a very recent (some centuries) landward displacement of the beach barrier. Lines of wrecks of different ages are observed along this part of the Gulf of Lions, showing that the shoal determined by the offshore bars has migrated successively landwards as the littoral sedimentary prism has receded.

Dynamic Permeability, Consolidation and Deformation Structures in Accretionary Prism Sediments: ABSTRACT

Dynamic Permeability, Consolidation and Deformation Structures in Accretionary Prism Sediments: ABSTRACT

The Lithoprobe corridor across the Vancouver Island continental margin: the structural and tectonic consequences of subduction

The deep structure and tectonic history of the continental margin of southwestern Canada have been determined by phase I of the Lithoprobe program across south-central Vancouver Island and associated marine studies across the continental shelf and slope. This article reviews results from the marine portion of the corridor but also presents continuous onshore–offshore data and interpretation. The geophysical data include multichannel seismic reflection, seismic refraction, magnetics, gravity, bathymetry, sea-floor acoustic imagery, heat flow, and seismicity. There has been Ocean Drilling Program (ODP) coring and downhole measurements on the continental slope. The margin structure and Cenozoic tectonic history are dominated by the consequences of subduction. Two narrow terranes, the Eocene volcanic Crescent and the Mesozoic mainly sedimentary Pacific Rim, were emplaced along the coast at the time of north Pacific plate reorganization at 43 Ma. They provide the landward-dipping backstop to a large accretionary sedimentary prism formed from the sediments scraped off the incoming oceanic plate. The prism exhibits active folding, thrusting, and tectonic consolidation, and it provides a model for the formation of ancient fold and thrust belts. An extensive gas hydrate layer was detected beneath the continental slope by a seismic BSR (bottom-simulating reflector) and was penetrated by the ODP drilling. The surface heat flow decreases landward from high values over the young oceanic plate to low values just seaward of the volcanic arc as a consequence of the heat sink provided by the underthrusting oceanic plate. The margin seismicity includes continental crust-events and earthquakes in the downgoing oceanic plate. No earthquakes have been detected on the subduction thrust fault but great thrust events are inferred to occur with an average interval of 600 years. The seismic source zone for such events is restricted to a narrow region beneath the continental shelf because of the high temperatures over the young Juan de Fuca plate.