Multichannel Seismic Reflection Data Research Articles

Seismicity and tectonics of the South Shetland Islands and Bransfield Strait from a regional broadband seismograph deployment

We investigate the tectonics of the South Shetland Trench and Bransfield Strait by performing a detailed study of local seismicity. During 1997–1999 we deployed seven land seismometers and 14 ocean bottom seismometers in the South Shetland Island region. The data we obtained indicate a high level of local seismicity (mb 2–5), and we accurately located ∼150 earthquakes. Many of the earthquakes occur at locations and depths indicative of ongoing subduction in the South Shetland trench. A focal mechanism for the largest event in the forearc indicates shallow angle thrusting. The maximum depth of seismicity is ∼65 km, but the majority of the events are shallower than 30 km. These seismic results are consistent with recent magnetic, GPS, and multichannel seismic reflection data that suggest continued subduction at a very slow rate. The South Shetland trench thus represents an extreme end‐member of hot subduction resulting from slow convergence of young lithosphere, and the absence of intermediate depth earthquakes is consistent with thermal assimilation of the slab at shallow depths. We have located many earthquakes associated with volcanism and rifting in Bransfield Strait. A swarm of events near a submarine volcano suggests current magmatic activity. A normal faulting focal mechanism in the northeastern part of the strait gives evidence of extension. Earthquakes associated with rifting in the northeastern portion of the strait are clustered along well‐established rifts, but the seismicity is much more diffuse to the southwest. This observation is consistent with other evidence that extension has propagated from northeast to southwest.

Heat flow over the descending Nazca plate in central Chile, 32°S to 41°S: observations from ODP Leg 202 and the occurrence of natural gas hydrates

Bottom simulating reflectors (BSRs) were detected in multichannel seismic reflection data acquired in the vicinity of Isla Mocha across the southern Chile margin and near 33°S. Geothermal gradients were determined from the depth of the BSR that is interpreted to mark the thermally controlled base of a gas hydrate layer. Ground truth for the assessment and additional thermal constraints were provided by downhole measurements obtained during Ocean Drilling Program (ODP) Leg 202 in Site 1233 at 41°S and Sites 1234 and 1235 near 36°S. Both BSR-derived data and downhole temperatures were used to calculate heat flow anomalies and provide new constraints on the thermal regime of the continental slope and downgoing slab in Chile between 32°S and 41°S. Downhole chemical logs of Th, U, and K from Site 859 of ODP Leg 141 have been used to assess the radiogenic heat production in the margin wedge. Heat production is low (∼0.8 μW/m3). However, knowledge of this reduces the errors of estimating the contribution from frictional heating along the subduction thrust fault. With respect to the Eocene age of the incoming oceanic lithosphere, heat flow appears to decrease landward of the deformation front as expected due to the advective transport of heat into the subduction zone by the downgoing slab. Calculations of conductive fore-arc heat flow show that the modelled seafloor heat flow agrees with the measured heat flow only if there is negligible frictional heating. At 33°S, temperatures in the fault zone reach 100°C approximately 60 km landward of the deformation front and are coincident with the onset of earthquake activity and hence mark the up-dip limit of the seismogenic zone. The up-dip limit shifts seaward going to the south, reflecting the progressive southward decrease of lithospheric age of the subducting plate.

Depositional elements associated with a basin floor channel-levee system: case study from the Gulf of Mexico

The seismic geomorphology and seismic stratigraphy of a deep-marine channel-levee system is described. A moderate to high-sinuosity channel trending southeastward across the northeastern Gulf of Mexico basin floor, and associated depositional elements are well imaged using conventional 3D multi-channel seismic reflection data. Depositional elements described include channels, associated levees, a channel belt, avulsion channels, levee crevasses, frontal splays, sediment waves, and mass transport complexes. Distinguishing morphologic and stratigraphic characteristics of each depositional element are discussed. These deposits are presumed to be associated with repeated deep-marine turbidity flows and other mass transport processes.

Tectonic erosion of the Peruvian forearc, Lima Basin, by subduction and Nazca Ridge collision

Subsidence of Lima Basin, part of the Peruvian forearc, is controlled by tectonic erosion by the subducting Nazca plate. Multichannel seismic reflection data coupled with age and paleowater depth constraints derived from Ocean Drilling Program (ODP) coring now allow the rates of erosion to be reconstructed through time. In trenchward locations the forearc has experienced limited recent relative uplift (700–850 m) likely due to preferential basal erosion under the center of Lima Basin. Long‐term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench. Since 47 Ma (Eocene) up to 148 km of the plate margin have been lost at an average rate of up to 3.1 km myr−1. Appoximately 110 km of that total appears to be lost since 11 Ma, implying much faster average rates of trench retreat (10 km myr−1) since collision of the Nazca Ridge with the Lima Basin at 11 Ma. Although there is no clear subsidence event at ODP Site 679 during the time at which Nazca Ridge was subducting beneath this part of the forearc (4–11 Ma), the more trenchward ODP Sites 682 and 688 show significant deepening after 11 Ma indicating that subduction of the ridge accelerates tectonic erosion. Long‐term rates of crustal erosion in the region of Lima Basin are greater than estimates of regional arc magmatic productivity, implying that such margins are net sinks of continental crust.

Geophysical signatures of mud mounds at hydrocarbon seeps on the Louisiana continental slope, northern Gulf of Mexico

Mud mounds are common features at cold seeps in many areas of the world. They are abundant in the northern Gulf of Mexico, where they occur at the numerous hydrocarbon seeps on the continental margin. Although the mounds are widely known, their geophysical signatures and geology have not been widely studied in detail. In this study, we examined mounds in two areas of the upper Louisiana slope using geophysical data of several types: 11–12 kHz side-scan sonar, 3.5 kHz echo-sounder profiles, near-bottom 2–12 kHz chirp sonar profiles, and three-dimensional (3D) multichannel seismic data. In side-scan images the mud mounds appear as ovoid high backscatter spots associated with larger, irregular high backscatter zones along faults and fault complexes. Three different sonar responses were noted: (1) a more-or-less uniform appearance, (2) a central higher backscatter spot surrounded with a lower return halo (bulls-eye), and (3) a more-or-less uniform high backscatter area with an acoustic dead-spot near its center. Near-bottom chirp sonar records indicate high lateral variability in seafloor reflection characteristics, implying that the long-range acoustic signature of a given mound results from the integration of seafloor characteristics. In these records, anomalous seafloor echo types include strong seafloor reflectors (hardbottom) or buried hardbottom, acoustic wipe-out (attenuation), and acoustic turbidity (prolonged echoes). Cores from acoustically anomalous seafloor areas indicate that a variety of seep-related material has been introduced into the near-seafloor sediments. The prevalence of strong bottom return signatures implies that the formation of hard grounds or crusts is an integral part of hydrocarbon seep evolution and suggests that much of the ovoid high backscatter signatures may result from such features. Multichannel seismic reflection data from several of the active mounds show columnar subsurface chaotic reflection zones and strong reflections in sediments directly underneath. Seafloor seismic amplitude maps from these mounds are either generally uniform, with a positive or negative anomaly, or display a ring of higher reflectivity around the mound periphery. Positive or negative anomalies may depend on the degree of seafloor lithification and sediment gas charging. The ring feature is thought to result from the preferential concentration of seep-debris in the mound periphery relative to the center. Some mounds, which show no seismic reflection anomaly from the seafloor, appear inactive. Altogether, observations from this study are consistent with the mud mounds being formed by sedimentary volcanism. The data imply they are Holocene features that have a life-cycle that includes hardbottom formation, which progressively crusts over the fluid vent, ending its activity.

Seismo-stratigraphic analysis of the Wilkes Land continental margin (East Antarctica): influence of glacially driven processes on the Cenozoic deposition

Abstract Multichannel seismic reflection data were collected from the Wilkes Land continental margin and particularly focusing on the lower rise area, by the international WilkEs basin GlAcial history (WEGA) project. Existing, lower-resolution seismic data also are used to understand the relationship between the basement and the sedimentary cover and to correlate the evolution of the rise and the shelf. On the basis of the acoustic character and the internal geometry, we recognise nine different sequences (named WL-S1–WL-S9, from the deepest to the shallowest) within the sedimentary section. Sequence WL-S1 represents syn-rift sediments deposited within tilted blocks of sedimentary, crystalline and volcanic basement. Sequences WL-S2–WL-S9 represent the post-rift sediments that show quite a complex architecture: sequence WL-S2 is a well-developed wedge downlapping onto the break-up unconformity; sequences WL-S3–WL-S7 are parts of a margin prograding/aggrading sequence during which large sediment ridges were deposited on the slope and rise. In particular, sequences WL-S5–WL-S7 began with the deposition of a turbidite fan system (sequence WL-S5), that developed in a braided channel–levee complex (WL-S6–WL-S7), dominated by downslope flows. Indirect correlation of the sequences WL-S4–WL-S6 with DSDP 269 about 600 km north of the study area suggests an early late Miocene age. Sequences WL-S8 and WL-S9 represent the smoothing and filling of the pre-existing morphology, in a generally low-energy environment with reduced bottom current activity. During this time, most of the sediment is deposited on the shelf and slope, while the rise is generally starved. Differences in downslope current energy and in sediment load are interpreted to reflect variation of the terrigenous input from the shelf to the rise through time. We suggest that channel–levee systems on the rise reflect an increase in sediment supply from the continental shelf during the Early Miocene, marking the growth of wet-based, temperate ice sheet on the near continent. The youngest sequences on the rise and shelf reflect decreased sediment supply to the rise that may be related to the transition from a temperate to a polar glacial system.

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.

Eurasia spreading basin to Laptev Shelf transition: structural pattern and heat flow

New geophysical data have become available from shipborne and satellite measurements allowing a re-evaluation of the largely unknown junction of the Arctic spreading centre and the northeastern Siberian continental margin where the transpolar mid-ocean Gakkel Ridge abuts against the continental slope of the Laptev Sea. Based on multichannel seismic reflection and gravity data, this sediment-covered spreading axis can be traced to the continental rise where it is cut-off by a transcurrent fault. Further continuation of the extensional axis into the continental slope can be attributed to two asymmetric grabens, which terminate against the prominent Khatanga–Lomonosov Fracture. Remnants of hydrothermal fauna and high heat-flow values of approximately 100 mW m−2 documented around these grabens in the up-slope area are typical for an oceanic spreading axis. Thus we consider these grabens to be morphotectonic termination of the global Atlantic–Arctic spreading system with plate motions shifting to the Khatanga–Lomonosov Fracture. The high heat flow and the distribution of earthquake epicentres allow us to assume that the present-day divergent plate tectonic boundary passes from the Gakkel Ridge to the eastern part of Laptev Sea with an offset of initial rifting along the Bel'kov–Svyatoi Nos Rift to the projected prolongation of the buried spreading axis by 140–150 km.

Direct seismic detection of methane hydrate on the Blake Ridge

Seismic detection of methane hydrate often relies on indirect or equivocal methods. New multichannel seismic reflection data from the Blake Ridge, located approximately 450 km east of Savannah, Georgia, show three direct seismic indicators of methane hydrate: (1) a paleo bottom‐simulating reflector (BSR) formed when methane gas froze into methane hydrate on the eroding eastern flank of the Blake Ridge, (2) a lens of reduced amplitudes and high P‐wave velocities found between the paleo‐BSR and BSR, and (3) bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas. Velocities within the lens (∼1910 m/s) are significantly higher than velocities in immediately adjacent strata (1820 and 1849 m/s). Conservative estimates show that the hydrate lens contains at least 13% bulk methane hydrate within a 2‐km3 volume, yielding 3.2 × 1010kg [1.5 TCF (4.2 × 1010 m3] of methane. Low seismic amplitudes coupled with high interval velocities within the lens offer evidence for possible methane hydrate “blanking.” Hydrate bright spots yield velocities as high as 2100 m/s, with bulk hydrate concentrations predicted as high as 42% in an approximately 15‐m thick layer. Our results show that, under certain circumstances, hydrate in marine sediments can be directly detected in seismic reflections but that quantification of hydrate concentrations requires accurate velocity information.

Deep structure of the Armorican Basin (Bay of Biscay): a review of Norgasis seismic reflection and refraction data

The Bay of Biscay is bounded to the north by the North Biscay margin, which comprises the Western Approaches and Armorican segments. In the 1970s and 1980s, most researchers considered this margin typical of a non-volcanic passive margin: it is characterized by a striking succession of tilted blocks beneath which occurs the S reflector and the continent–ocean boundary is abrupt. This paper examines the Armorican segment and is based on a study of all early seismic profiles together with new multichannel reflection and refraction seismic data (Norgasis cruise). An important result is the discovery of a 80 km wide ocean–continent transition zone that coincides with the Armorican Basin (a deep sedimentary basin). It is characterized by a high-velocity lower-crustal layer (7.4–7.5 km s −1 ) overlain by sediments. The other results are: (1) the main crustal thinning occurs exclusively under the narrow continental slope; (2) the tilted blocks and the S reflector are observed only at the base of the continental slope in the narrow domain called the ‘neck area’; (3) the North Biscay Ridge is a large oceanic plateau present only off the NW Armorican margin rather than a long ridge elongated off the whole North Biscay margin.

Tectonic evolution of the Pacific margin of Antarctica 2. Structure of Late Cretaceous–early Tertiary plate boundaries in the Bellingshausen Sea from seismic reflection and gravity data

Interpretations of multichannel seismic (MCS) reflection and potential field data suggest that some prominent gravity anomalies in the Bellingshausen Sea are associated with plate boundaries that were active during the Late Cretaceous and early Tertiary. Between 83° and 93°W, a belt of negative anomalies extends along the West Antarctic continental slope, which we term the continental slope gravity anomaly (CSGA). MCS profiles show that the CSGA coincides with an acoustically opaque structural high imaged beneath the lower slope. We interpret this structure as the upper part of an accretionary prism which formed during southward subduction of the Phoenix and Charcot plates, before Chatham Rise separated from West Antarctica. MCS profiles crossing the same margin to the northeast show no evidence of an extensive buried accretionary prism, but instead reveal an abrupt northeastward steepening of the continental slope near 78°W. We attribute this change in tectonic style, at least in part, to subduction erosion resulting from subduction of rough oceanic basement which formed at the Antarctic‐Phoenix ridge after an abrupt decrease in spreading rate at chron 23r (52 Ma). Near 95°W, the Bellingshausen gravity anomaly (BGA) consists of a prominent low‐high gravity couple which crosses the West Antarctic continental shelf, slope, and rise. The BGA corresponds to a buried asymmetric basement trough, where Cretaceous oceanic basement dips beneath more elevated basement to the east. The trough probably formed after subduction of Charcot plate ocean floor stalled at the nearby Antarctic Peninsula margin, near the end of the Cretaceous Normal Superchron. Ocean floor to the east of the BGA became attached to the Antarctic Peninsula, and the BGA trough subsequently accommodated a small amount of convergent motion between the Antarctic Peninsula and the ocean floor to the west (initially part of the Marie Byrd Land plate and later part of the Bellingshausen plate). Tectonism probably ceased at the BGA at chron 27 (61 Ma), as a result of a general plate reorganization in the South Pacific.

Crustal thickness along the western Galápagos Spreading Center and the compensation of the Galápagos hotspot swell

Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Galápagos Spreading Center (GSC) between 97°W and 91°25′W thickens by 2.3 km as the Galápagos plume is approached from the west. This crustal thickening can account for ∼52% of the 700 m amplitude of the Galápagos swell. After correcting for changes in crustal thickness, the residual mantle Bouguer gravity anomaly associated with the Galápagos swell shows a minimum of −25 mGal near 92°15′W, the area where the GSC is intersected by the Wolf–Darwin volcanic lineament (WDL). The remaining depth and gravity anomalies indicate an eastward reduction of mantle density, estimated to be most prominent above a compensation depth of 50–100 km. Melting calculations assuming adiabatic, passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of ∼30°C. The associated thermal expansion and increase in melt depletion reduce mantle densities, but to a degree that is insufficient to explain the geophysical observations. The largest density anomalies appear at the intersection of the GSC and the WDL. Our results therefore require the existence of compositionally buoyant mantle beneath the GSC near the Galápagos plume. Possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the Galápagos Islands that is later transported to the GSC. Our estimate for the buoyancy flux of the Galápagos plume (700 kg s−1) is lower than previous estimates, while the total crustal production rate of the Galápagos plume (5.5 m3s−1) is comparable to that of the Icelandic and Hawaiian plumes.

Opening geometry of the Kurile Basin (Okhotsk Sea) as inferred from structural data

Abstract The Kurile Basin in the Okhotsk Sea, northwestern Pacific, is a back‐arc basin located behind the Kurile Island Arc. It is underlain by oceanic crust and its origin has been attributed to back‐arc spreading. Two models for the opening of the Kurile Basin exist, for which the spreading axis is oriented northeast–southwest and northwest–southeast, respectively. New data are presented here on the morphostructure of the slope of the northern Kurile Basin and of the central Kurile Basin which support a strike of the spreading axis in the latter direction. Bathymetric as well as single‐channel and multichannel seismic reflection data demonstrate the existence of dominant northwest‐striking normal faults on the northern slope of the Kurile Basin. In the central Kurile Basin a basement rise striking north‐northwest–south‐southeast (here named the Sakura Rise) was mapped. The rise morphology has the distinct imprint of a rift structure with symmetrical volcanic edifices on the rise axis and faulted blocks that tilt in opposite directions on the flanks. These data suggest that the Kurile Basin opened in a northeast–southwest direction. In the generally accepted plate tectonic reconstructions, northwest–southeast spreading associated with dextral strike–slip along the north–south‐striking shear zone of Sakhalin and Hokkaido islands has been assumed. In the present model, spreading in the Kurile Basin was presumably connected with dextral displacement along a northeast‐striking shear zone on the southern segment of the Okhotsk Sea.

Tectonic setting of the southern Marmara Sea region: based on seismic reflection data and gravity modelling

On the basis of gravity modelling on the southern Marmara Sea shelf and conventional multichannel seismic reflection data, the tectonic setting of the region was studied. The relation of the middle strand of the North Anatolian Fault Zone with the Thrace–Eskişehir Fault Zone was examined. The southern Marmara Sea region was tectonically uplifted in the Late Pliocene. Gemlik Bay was developed during Late Pliocene–Early Pleistocene under the control of right lateral strike–slip faults along the middle strand of the North Anatolian Fault. The northwest trending Thrace–Eskişehir Fault is the second important tectonic element in this evolution.

Back arc extension, tectonic inheritance, and volcanism in the Ligurian Sea, Western Mediterranean

The Ligurian basin, western Mediterranean Sea, has opened from late Oligocene to early Miocene times, behind the Apulian subduction zone and partly within the western Alpine belt. We analyze the deep structures of the basin and its conjugate margins in order to describe the tectonic styles of opening and to investigate the possible contributions of forces responsible for the basin formation, especially the pulling force induced by the retreating subduction hinge and the gravitational body force from the Alpine wedge. To undertake this analysis, we combine new multichannel seismic reflection data (Malis cruise, 1995) with other geophysical data (previous multichannel and monochannel seismic sections, magnetic anomalies) and constrain them by geological sampling from two recent cruises (dredges from Marco cruise, 1995, and submersible dives from Cylice cruise, 1997). From an analysis of basement morphology and seismic facies, we refine the extent of the different domains in the Ligurian Sea: (1) the continental thinned margins, with strong changes in width and structure along strike and on both sides of the ocean; (2) the transitional domain to the basin; and (3) a narrow, atypical oceanic domain. Margin structures are characterized by few tilted blocks along the narrow margins, where inherited structures seem to control synrift sedimentation and margin segmentation. On the NW Corsican margin, extension is distributed over more than 120 km, including offshore Alpine Corsica, and several oceanward faults sole on a relatively flat reflector. We interpret them as previous Alpine thrusts reactivated during rifting as normal faults soling on a normal ductile shear zone. Using correlations between magnetic data, seismic facies, and sampling, we propose a new map of the distribution of magmatism. The oceanic domain depicts narrow, isolated magnetic anomalies and is interpreted as tholeitic volcanics settled within an unroofed upper mantle, whereas calcalkaline volcanism appears to be discontinuous but massive and has jumped in space and time, from the beginning of rifting on the Ligurian margin (∼30 Ma), toward the Corsican margin at the end of the Corsica‐Sardinia block rotation (∼16 Ma). This space and time shift reveals the importance of the rollback of the Apulian slab and of the migration of the Alpine‐Apennines belt front toward the E‐SE for driving basin formation. We also state that initial rheological conditions and inherited crustal fabric induce important changes in the styles of deformation observed along margins and between conjugate margins. In the NE Ligurian basin the prerift Alpine crustal thickening together with slow rollback velocity likely contribute to distribute strain across the whole NW Corsican margin, whereas farther south the inherited Hercynian structural pattern combined with a faster rollback of the subducting plate tend to focus the extension at the foot of the margin, up to the Sardinian rift which ends within the SW Corsican margin. Therefore the mode of opening and the margin structures mainly depend on the balance between intrinsic, inherited crustal heterogeneity (fabric and rheological changes) and external conditions imposed by rollback of the subducting lithosphere.

Seismic investigation of the Lake Bosumtwi impact crater: preliminary results

The Lake Bosumtwi impact crater in Ghana, West Africa, has a diameter of 10.5 km and is one of the youngest ( 1.07 Ma ) well-preserved large craters on Earth. It has a total dynamic range of topography of more than 400 m , and it is the source crater of tektites and microtektites of the Ivory Coast strewn field. The crater was excavated in early Proterozoic rocks. According to its size, the Bosumtwi impact crater should be a complex impact structure, with a central peak. Multichannel seismic (MCS) reflection and wide angle data, using Ocean-Bottom-Hydrophones (OBHs), were acquired in order to investigate the structure's subsurface, image the presumed central uplift and determine the thickness of impact-related formations and the post-impact sediments. An integrated interpretation of the seismic data sets, and modelling and inversion of the OBH data yield an initial 2D velocity-depth model, which shows indications for a central peak feature. Due to the relatively low seismic velocity ( 3.0 km s −1 ) of the corresponding layer, the top of the uplifted structure is interpreted to consist of allochthonous breccia. The central peak has a width of ∼1.8 km and a maximum height of 120 m above the top of the breccia away from the center. Fracturing may be responsible for the relatively low velocity of 3.8 km s −1 in the crater floor. The post-impact sediments covering the crater structure are 180– 300 m thick. The apparent crater depth, defined as the difference between the original target surface and the top of the breccia layer, is ∼550 m and thereby slightly deeper than some other larger complex impact structures on Earth. The results indicate that the Lake Bosumtwi impact structure provides an interesting setting for scientific drilling of a young large impact crater and will be supplemented by complimentary recent geophysical (potential field) and possibly future drilling studies.

Mechanical decoupling and basal duplex formation observed in sandbox experiments with application to the Western Mediterranean Ridge accretionary complex

Sandbox experiments of accretionary wedges were performed incorporating a thin weak layer of micro glass beads. The impact of heterogeneous sedimentary input on wedge mechanics, evolution and mass transfer was investigated. We report the first experimentally documented growth of basal duplexes. These occurred for high basal friction conditions, with restricted output of the lower section. The upper and lower sections were completely decoupled due to the intervening layer of glass beads, with frontal accretion occurring in the upper section simultaneously with basal duplex formation and underplating of subsequent generations of duplexes. IMERSE multichannel seismic reflection data from the Western Mediterranean Ridge (WMR) image Tertiary clastics beneath a thick section of Messinian evaporites. The base of the evaporites is identified as the primary décollement for deformation in the frontal part of the accretionary complex. Constriction of the channel of subducting Tertiary sediments, as well as internal deformation observed as arcward-dipping reflectors argue for basal underplating and/or two different active décollements. We propose an evolution of the WMR in accordance with the sandbox experimental results. A weak mid-level detachment (base of evaporites) combined with a strong basal detachment produce mechanical decoupling and basal accretion of toeward-verging duplexes.

Analyses of multichannel seismic reflection, gravity and magnetic data along a regional profile across the central-western continental margin of India

Analyses of multichannel seismic reflection, gravity, magnetic and bathymetry data along a regional profile across the central-western continental margin of India have revealed the depositional pattern of sediments, crustal structure and tectonics. The four most distinct and varied crustal regions of the margin are palaeo-shelf edges, shelf margin basin, Prathap and Laccadive Ridges and the Arabian Basin. The shelf margin basin is carpeted by ∼4.5 km maximum thick aggraded and prograded Paleocene to Holocene sediments. Six major seismic sequences of the sediments of the margin are identified and their ages are assigned on correlation with drill-well results. Development of the sequence boundaries is attributed to the events of rifting of western India, eustatic sea-level changes, Indian and Eurasian plate collision and Himalayan orogeny. Tilted fault blocks (half-grabens) located almost equi-distance from the igneous construct of the ‘Prathap Ridge’ in the shelf margin basin suggest a failed rift associated with stretched continental crust of the basin. 2-D model studies of gravity and magnetic anomalies, constrained by the seismic results, have revealed 6 to 27 km thick crust across the margin. The Laccadive Ridge crust limited by two volcanic intrusives and a steep scarp at its western end is ∼16 km thick. It gradually thins towards offshore and juxtaposed with early Tertiary normal oceanic crust ∼6 km thick of the Arabian Basin. The crustal thickness and velocity and density structure of the ridge are comparable to that of the Laxmi Ridge, a continental sliver. The inferences and abrupt change in magnetic and gravity anomaly signatures across the western end of the Laccadive Ridge mark the zone of transition from continental to oceanic crust.

Structure and tectonic evolution of the Southern Eurasia Basin, Arctic Ocean

Multichannel seismic reflection data acquired by Marine Arctic Geological Expedition (MAGE) of Murmansk, Russia in 1990 provide the first view of the geological structure of the Arctic region between 77–80°N and 115–133°E, where the Eurasia Basin of the Arctic Ocean adjoins the passive-transform continental margin of the Laptev Sea. South of 80°N, the oceanic basement of the Eurasia Basin and continental basement of the Laptev Sea outer margin are covered by 1.5 to 8 km of sediments. Two structural sequences are distinguished in the sedimentary cover within the Laptev Sea outer margin and at the continent/ocean crust transition: the lower rift sequence, including mostly Upper Cretaceous to Lower Paleocene deposits, and the upper post-rift sequence, consisting of Cenozoic sediments. In the adjoining Eurasia Basin of the Arctic Ocean, the Cenozoic post-rift sequence consists of a few sedimentary successions deposited by several submarine fans. Based on the multichannel seismic reflection data, the structural pattern was determined and an isopach map of the sedimentary cover and tectonic zoning map were constructed. A location of the continent/ocean crust transition is tentatively defined. A buried continuation of the mid-ocean Gakkel Ridge is also detected. This study suggests that south of 78.5°N there was the cessation in the tectonic activity of the Gakkel Ridge Rift from 33–30 until 3–1 Ma and there was no sea-floor spreading in the southernmost part of the Eurasia Basin during the last 30–33 m.y. South of 78.5°N all oceanic crust of the Eurasia Basin near the continental margin of the Laptev Sea was formed from 56 to 33–30 Ma.

Pronounced central uplift identified in the Bosumtwi impact structure, Ghana, using multichannel seismic reflection data

The Lake Bosumtwi impact structure is the youngest and best-preserved complex terrestrial impact crater and serves as an important reference site for the study of cratering processes. Because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform, no significant backwash processes have modified the crater morphology. Not only may Bosumtwi contain the best-preserved central uplift structure on Earth, but it is the most accessible relatively large, young crater in the solar system generated in a large gravity field. There is a well-established link between the Lake Bosumtwi impact structure and the Ivory Coast tektite field, and the lacustrine sediments within the crater contain a unique 1 m.y. record of paleoclimate in the continental tropics south of the Sahel. Eight profiles of marine-type multichannel seismic reflection (MCS) data were acquired from the 8-km-diameter, similar to75-m-deep lake that fills much of the crater. These were augmented by wide-angle seismic data acquired with ocean-bottom hydrophones. MCS data reveal a well-defined central uplift near the northwest-central part of the lake and a maximum postimpact lacustrine sediment thickness of similar to310 m. The central uplift structure has a diameter of 1.9 km and a maximum height of 130 m above the annular moat inside the crater. An intermediate velocity layer (3200 m/s) beneath the lacustrine sediment is interpreted as fallback breccia or a breccia-melt horizon. The measured apparent depth of the crater (d(a)) is 500 m, implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships. The Bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws, perhaps because of the effects, of a large gravity field.